EDITED BY
DONALD L. WISE
Northeastern University Boston, Massachusetts
DEBRA J. TRANTOLO
Cambridge Scientific Inc. Belmont, Massachusetts
Marcel Dekker, Inc.
New YorkaBaselaHongKong
Library of Congress Cataloging-in-Publication Data
Process engineering for pollution control and waste minimization / edited by Donald L. Wise, Debra J. Trantolo. p. cm. -- (Environmental science and pollution control: 7) Includes bibliographical references and index. ISBN 0-8247-9161-4 (alk. paper) 1. pollution.2.Wasteminimization. I. Wise,DonaldL.(DonaldLee). U. Trantolo, Debra J. III. Series. TD191.5.W6 1994 628.5-dc20 93-4601 CIP
The publisher offers discounts on this book when ordered in bulk quantities. For more information, write to Special Sales/Professional Marketing at the addressbelow. This book is printed on acid-free paper. Copyright 0 1994 by Marcel Dekker, Inc. All Rights Reserved. be reproduced or transmitted in anyform or by any means, electronic Neither this book nor any part may or mechanical, including photocopying, microfilming, and recording, or by an information storage and retrieval system, without permission in writing from the publisher.
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Current printing (last digit): l0987654321 PRINTED IN THE UNITED STATES OF AMERICA
Preface
A clean environment is a goal to which we all strive. However, we have been the victims of activities. The severe environmental damage as a result of industrial growth and defense-related damage to our environment is substantially affecting our overall health and welfare. It is a credit to our human spirit that we remain optimistic and share an enthusiasm about environmental issues. The numbers of registered wastesites are alarming, and continue to grow daily. No longer can we casually consider wastean acceptable by-product of our everydayactivities. While the consumer hasbegun to embrace the concept of waste reductionas, for example,in the practice of recycling, the large-scaleindustrialconcern has also turned to wastecontrolmethods. Whether driven by governmentmandate,socialresponsibility,economics, or other forces, waste control and waste minimization practices are increasingly welcomed. Process Engineeringfor Pollution Control and Waste Minimization provides an up-to-date source of technical information relating to current and potential pollution control and waste minimization practices. Overfifty recognized experts provide an in-depth treatmentof this rapidly growing field that draws its resources frommany disciplines. We have deliberately solicited input from governmental,industrial, and academic specialiststo ensure a multidimensional presentation of the pollution control and waste minimization schemes that are shaping our environmental outlook. The text is divided into five parts. It begins with the presentation of general engineering considerations and the regulatory, ethical, and technical framework within which these processes are managed, then enters into specific wastelwastewater pollution control technologies that are used throughout industry. Models for potential control and minimization techniques are offered, and industry-specific case studies complete the text. Throughout, we have attempted to provide a sense that the scopeof waste control and minimizationmay be immense, but it is not overwhelming. We trust that this book will provide a contribution to this important field and emphasize the need for continued progress. One way to better our environment is to eliminate or reduce iii
iv
Preface
pollution at the source. Potentially great benefits await us if we can develop economical, effective, and efficient solutions to our waste generation problems. All readers of this text will contribute something to the environment of tomorrow. Donald L. Wise Debra J. Trantolo
Contents
Preface Contributors
Part I: Engineering Issues In Pollution Control and Waste Minimization Process Engineering for Pollution Control and Waste Minimization John Hanna and Osawaru A. Orumwense
iii ix
3
Selection of Least Hazardous Material Alternatives Alvin F. Meyer
17
Multiple Approaches to Environmental Decisions Douglas M . Brown
25
Introduction to Engineering Evaluation for Contaminated Sites David S. Wilson, Alan C . Funk, Ronald G . Fender, and Marilyn Hewitt
47
Innovative Approaches to Cleanup Level Development Ronald J. Kotun, Richard F. Hoff, Robert J. Jupin, Diane McCauslund, and Patrick B. Moroney
87
Designing to Prevent Pollution James Lounsbury
145
Biochemical, Genetic, and Ecological Approaches to Solving Problems During 171 in Bioremediation situ and Off-site 0. A. Ogunseitan V
vi 8
9
allenges Competitive
10
Contents
Commandments of Waste Management Donald K. Walter A Proactive Approach to Environmental Management: Meeting and Environmental William E. Schramm and Stella S. Schramm HealthHazardsAssociatedwithPollutionControlandWasteMinimization Patrick D . Owens
193
213
227
Part 11: Methodologies of Waste Control 11 Techniques for Controlling Solid and Liquid Wastes Hsai-Yang Fang and Jejhrey C. Evans
247
l2
Solidification and Stabilization Techniquesfor Waste Control A. Samer Ezeldin and George P. Korj?atis
271
Soil Remediation with Environmentally Processed Asphalt @PATM) M. Testa and D. L. Patton
297
14
Lead Decontamination of Superfund Sites Ann M . Wethington, Agnes Y. Lee, and Vernon R. Miller
311
15
A Secure Geologic Repository for Hazardous Waste Residuals Thomas R . Klos
331
Photocatalytic Degradation of Hazardous Wastes
363
l3
S.
16
M. S. Chandrasekharaiah, S. S. Shukla, J. L. Margrave, and S. C. Niranjan 17
Photocatalytic Oxidation of Organic Contaminants Allen P. Davis
377
18
Biodegradation of Organic Pollutants in Soil Paul D . Kuhlmeier
405
19
Siallon: The Microencapsulationof Hydrocarbons Withina Silica Cell Tom McDowell
425
20
Remediation of Heavy Metal Contaminated Solids Using Polysilicates George J. Trezek
441
21
Fluidized Bed Combustion for Waste Minimization: Emissions andAsh Related Issues E. J. Anthony and F. Preto
Part 111: Wastewater Treatment 22 An Overview of Physical,Biological, andChemicalProcesses for Wastewater Kanti L. Shah
467
489
vii
Contents 23
FreezeConcentration: Its Application in HazardousWastewaterTreatment Ray Ruemekorf
24
OrganoclaySorbents for Selective Removalof OrganicsfromWater and Wastewater Steven K. Dentel, Ahmad I. Jamrah, and Michael G. Stapleton
513
525
25
Removal of Chromate,Cyanide,and Heavy MetalsfromWastewater Klaus Schwitzgebel and David M . Manis
535
26
NeutralizationTacticsforAcidicIndustrialWastewater Christopher A. Hazen and James I. Myers
557
Part Iv:Modeling for Pollution Control 27 IntroducingUncertainty ofAquifer Parametersinto an OptimizationModel Robert L. Ward 28
29
30
Application of Total QualityManagement(TQM)Principles Prevention Programs Prasad S. Kodukula
569
to Pollution 591
PC Software for OptimizingGroundwaterContaminantPlumeCapture and Containment Richard C . Peralta, Herminio H. Suguino, and Alaa H. Aly
597
HorizontalWellsforSubsurfacePollutionControl George Losonsky and Milovan S. Berjin
619
Part V Industry-Specific Pollution Control Pollution Control and Waste Minimization in Military Facilities 31 Merrit R Drucker
637
32
Waste Reduction Strategies for Small Businesses Dan A. Philips
643
33
Contaminated Soils in Highway Construction Namunu J. Meegoda
663
34
Management of Waste Compressed Gases Dan Nickens
685
35
Pollution Control in the Dairy Industry T. Viraraghavan
705
36
Landfill Gas Collection and Destruction Systems: Evaluating Toxic Emissions and Potential Health Risk Karnig Ohannessian, Anna Peteranecz,and Thomas Kear
Index
715 727
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Contributors
AlaaH.Aly Utah State University,Logan,Utah E.J. Anthony CANMET, Ottawa, Canada Milovan S. Beljin University of Cincinnati,Cincinnati,Ohio Douglas M. Brown TheLogistics Management Institute,Bethesda, Maryland M. S. Chandrasekharaiah Houston Advanced Research Center,TheWoodlands, T~xas Allen F? Davis University of Maryland,CollegePark, Maryland Steven K. Dentel University of Delaware,Newark, Delaware Merrit F? Drucker Army Management Staff College, Fort Belvoir,Virginia JeffreyC.Evans Bucknell University,Lewisburg, Pennsylvania A. Samer Ezeldin Stevens Institute of Technology,Hoboken, New Jersey Hsai-YangFang LehighUniversity,Bethlehem, Pennsylvania Ronald G. Fender Environmental Resources Management Group,Exton, Pennsylvania Alan C. Funk Environmental Resources Management Group,Exton, Pennsylvania John Hanna TheUniversity of Alabama,Tuscaloosa, Alabama Christopher A. Hazen MilesInc., New Martinsville, West Virginia Marilyn Hewitt Environmental Resources Management Group,Exton, Pennsylvania Richard F. Hoff ChesterEnvironmental,Monroeville, Pennsylvania Ahmad 1. Jamrah University of Delaware, Newark, Delaware Robert J. Jupin ChesterEnvironmental,Monroeville, Pennsylvania ThomasKear OP&L,Inc., San Diego, California Thomas R. Klos Envirovest Management,Houston, Texas Prasad S. Kodukula Woodward-Clyde Consultants, Overland Park, Kansas George F? Korfiatis Stevens Institute of Technology,Hoboken, New Jersey RonaldJ.Kotun ChesterEnvironmental,Monroeville, Pennsylvania
X
Contributors
Paul D. Kuhlmeier Consulting Environmental Engineer,Boise,Idaho Agnes Y. Lee US.Bureau of Mines,Rolla,Missouri George Losonsky Eastman Christensen Environmental Systems,Houston, Texas James Lounsbury National Roundtable of State Pollution Prevention Programs, Silver Spring, Maryland David M. Manis EET,Austin, Texas J. L. Margrave Houston Advanced Research Center,TheWoodlands, Texas DianeMcCausland Chester Environmental,Monroeville, Pennsylvania TomMcDowell SiallonCorporation, Laguna Niguel, California Namunu J. Meegoda New Jersey Institute of Technology, Newark, New Jersey Alvin F. Meyer A. F. Meyer and Associates,Inc.,MeLean,Virginia Patrick B.Moroney Chester Environmental,Monroeville, Pennsylvania Vernon R. Miller U.S.Bureau of Mines,Rolla, Missouri James 1. Myers MilesInc., New Martinsville, West Virginia DanNickens Earth Resources Corporation,Ocoee, Florida S. C. Niranjan RiceUniversity,Houston, Texas 0.A. Ogunseitan University of California,Irvine, California KarnigOhannessian OPdiL,Inc., San Diego,California Osawaru A. Orumwense The University of Alabama,Tuscaloosa, Alabama Patrick D. Owens Tosco Refining Company,Martinez,California D. L. Patton Applied Environmental Services, Inc., San JuanCapistrano, California Richard C. Peralta Utah State University,Logan,Utah AnnaPeteranecz OPdiL,Inc., San Diego, California Dan A. Philips Pensacola JuniorCollege,Pensacola, Florida F. Preto CANMET, Ottawa, Canada RayRuemekorf NIRO,Inc.,Columbia, Maryland Stella S. Schramm University of Tennessee,Knoxville, Tennessee William E. Schramm Oak Ridge National Laboratory, Oak Ridge, Tennessee KlausSchwitzgebel EET,Austin, Texas KantiL.Shah OhioNorthernUniversity, Ada, Ohio S. S. Shukla* Houston Advanced Research Center,TheWoodlands, Texas Michael G. Stapleton University of Delaware,Newark, Delaware Herminio H. Suguino Utah State University,Logan,Utah S. M. Testa Applied Environmental Services, Inc., San JuanCapistrano,California George J. Trezek Greenfield Environmental, Carlsbad, Californiaand University of California at Berkeley, Berkeley,California T. Viraraghavan University of Regina,Regina, Canada Donald K.Walter US.Department of Energy,Washington, D.C. Robert L. Ward Ohio Northern University, Ada, Ohio Ann M. Wethington US.Bureau of Mines,Rolla, Missouri David S. Wilson Environmental Resources Management Group,Exton, Pennsylvania _________
*Current affiliation: Lamar University, Beaumont, Texas
'
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Part I
ENGINEERING ISSUESIN POLLUTION CONTROL AND WASTE MINIMIZATION
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Process Engineering for Pollution Control and Waste Minimization
John Hanna and Osawaru A. Orumwense The University of Alabama Tuscaloosa, Alabama
1.
INTRODUCTION
As a part of the material cycle, ores and fossil fuels are extracted from the earth, processed, and converted into metals, chemicals, and other processed (high value added) materials. Hence, any expansion in the world economy increases the demand for minerals and metals with subsequent increases in the amount of waste generated. Wastes are generated by the mining, mineral processing, metallurgical, and chemical industries at an estimated annual rate of over 2.3 billion tons. The accumulated solid wastes at both active and inactive mining sites approach a whopping 30 billion tons [l]. These wastes include gases, dusts, sludges or solutions, ashes, and a variety of massive solid materials such as overburden, waste rocks, tailings, and slags that must be disposed of at low cost with a minimum of environmental degradation. A large volume of the wastes is normally disposed of at locations close to either the mining sites or processing plants. Evidence of these can be found in Minnesota, Utah, Alabama, California, Tennessee, Idaho, Montana, and other states having high mining and industrial activities. A significant amount of the tailings is disposed of in impoundments, which range in size from a few acres to large ponds covering thousands of acres. Wastes from processing plants pose the most difficult disposal and environmental problems in view of the physical and chemical properties of the wastes as well as the enormous volumes involved, and consequently a large expanse of land must be used for the disposal [2]. A typical example is the Florida phosphate slimes. The overburden and waste rocks with characteristic high contents of pyrite, heavy metals, and radioactive materials also present potential environmental and health problems. Unfortunately, most of the mineral processing wastes have been excluded from the Resource Conservation Recovery Act of 1976 (RCRA). However, this situation is changing with the stringent environmental regulations introduced in recent years, which have necessitated that precautions be taken to both minimize and control waste disposal. Hence, the mineral or metal
3
Orumwense 4
and
Hanna
constituents in wastes, whether of little or no economic value, must be amendedfor their environmental impact or as a source of resource supply.
II. SOURCE OF WASTES A. Mining and Processing Solid Wastes Most operations in the extraction and processing cycle generate wastes (refer to Figure l), but the extent to which a material can be classified as a waste depends on a number of factors. These include 1. Sources andvolumesofwasteproduced 2. Potential dangers to health and the environment 3. Long-term reactivity with air and/or water and mobilization to the environment 4. Present disposal practices and alternative disposal methods 5 . Cost of disposal and potential use of the waste Overburden Sub-grade mlnerals -Slurries - cI Flnes
RAW MATERINS EXTRACTION mlnlng, quanylng. dredglng. exploratlon
""
I I
-
I
I RAW MATERINS BENEFICIATION
I
I I
I I
Concentrates. refined 011.gas. minerals
U
Dusts
stags Smoke Fumes Muds -Dresses solutlons Resldues Ash
t
Ingots. plgs. chemicals. energy
1
"S1urrlea
Chemlcals
I
7
-
I +I
"
SolUUOns
I
$
Talllngs Sands
--
mllllng. washlng. concentratJon. upgmdlng
MANUFACTURING AND SERVICES assembly, packing. transportation.energydlstrlb.
I
vI
I I
-
Pulp Dust Smoke Fumes solutlons
-- -
Coods and servias
I I I I I
I I
Ores. crude 011. coal. etc.
I
I I
-
Spoils
-
Drosses Grlndlng CIlpplngS
I
4
"
'3
I
I
Fumes Dusts
s01uu0ns Metals. glass. -paper. p~asuc.etc.
L-
Figure 1 Mineralwastematerialssupply,utilization,anddisposalsystem.
Smokes
I
--J
Process Engineering Control for Pollution
5
The following are typical examples of the waste generatedby mining and related processing industries. It has been reported that in the production of about 1.6 million tons of copper in 1976, 1 billion tonsof materials were processed. This breaks down to684 million tonsof overburden, 264 million tonsof tailings, 5 million tons of slag, 3.3 million tons of sulfur dioxide,and about 100 billion gallons of process water [3]. The iron industry is one source of enormous amounts of waste, since most of the iron concentrates used in the manufacture of iron and steel are derived from relatively low grade ores. spically, raw ores assaying25-33% Fe are mined and beneficiated to producehigh quality pellets assaying 60-65% Fe and 5% Si for the manufacture of iron and steel. About 330 million long tons of iron ore was mined in the United States in 1976, and the amount of wastes generated was about 200 million tons [4], excluding the slag and dust wastes from the steelmaking step. Conventional magnetic and gravity separation processing of magnetic and nonmagnetic taconites of the Lake Superior Region resulted in substantial iron losses of about 20-30% in the tailing products [5]. On the other hand, the more advanced beneficiation of the tailings from techniques such as flocculation and flotation processes reduced the iron lost in the rejectsto as low as 10%. The loss is partly due to the mineralogical compositionof the ores and the grain size of both iron and gangue. High iron losses are observed for Birmingham red hematite ore, for instance, because it produces more slimes than taconite ores. This is oneof the factors responsible for the relatively poor recovery of iron from run-of-mine material and the generation of large tonnages of wastes, particularly in large-scale beneficiation operations. The Florida phosphate industry is another sourceof a tremendous volumeof wastes. In the production of phosphate, the soft minerals in the matrix, particularly clays and the very fine phosphate aggregates,are dispersed readily in water, forming slimes duringthe hydraulic mining, transportation, and separation steps. These slimes are difficult to recover, and in addition they impair the beneficiationoperation. About one-third of the phosphate contentof the matrix is lost in the slimes, which are generally discarded as wastes. The Florida phosphate slimes are characterized byveryslow settling and trap a highvolume ofwater. Currently, impounded slimes are stored behind earth dams and pose a serious threat to the environment. The reclamation of the land and enormous volumes of water are important for resource conservation and in order to comply with stringent environmental regulations. The recovery of the phosphate values discarded in the slime and tailingfractions containing about 30%-40% of the phosphate present in the mined matrix would enhance the economy of the phosphate industryand expand the available resources. The impact of this on reducing potential environmental hazards is enormous. The phosphate losses at thecurrent rate of rock production of about 40 million tons per year includes over 11 million tons of high grade phosphate that is lost in the slimes annually. Vasan [6] has estimated that about1.5 billion tons of phosphate slimes is accumulated over the years in dams together with about 4.5 billion tons of water. The coalmining industry is another sourceof a large volume of solid wastes. The methods used in the past for cleaning coal were highly inefficient and resulted in high coal losses in waste streams duringthe mining and washing operations [2]. The washer waste fines are normally storedin above-ground impoundments. Quitea number of processing plants still indulge in the practice of discarding coal fines.As a result, about 25% of the coal mined is disposed of as waste.Based on the current rate of coal production of about 1 billion tons, about250 million tons of wastes is produced annually, and out of this, about 200 million tons is coarse particles and 50 million tons is fines. The amountof coal in coarse waste particles is more than 30 million tons of carbon per year, while the corresponding amount in the fine fraction is about 30 million tons on an annual basis. The disposal of coarse waste particles is not a serious problem
6
Hanna and Orumwense
in most coal preparation plants,as they are usedas landfill. The fine-size wastes, on the other hand, are a problem because of the difficulties experienced in dewatering and the characteristic relatively low structural strengthof fine particles, which prevent fines from being used as landfill [7].
B. Mining and ProcessingLiquidWastes Effluents from coal preparation plants and drainage from waste disposal sites have a characteristic dark color and have high concentrations of suspended particles that cause not only siltation due to the settling of coarse particles, but also water pollution, both of which have negative effects on aquatic lives[2,7]. Effluents from coal cleaning plantsand mines are also reputed to have a great impact on the environment through the phenomenon as known acid mine drainage (AMD). This is oneof the causes of the destruction of forestsand vegetation today. Acid drainage is reportedto be causedby the reaction,between oxygen, water, and iron sulfides such as pyrite and marcasite. Microorganisms are known to enhance the rate of this reaction. The most common techniques for mitigating acid drainage are neutralization using either lime, limestone, soda ash, or caustic soda; reverse osmosis; and treatment involving silicates [7]. These techniques are discussed in detail later. The highly acidic solutions produced dissolve several heavy metals in the waste pilesor impounded material and become loaded with a host of environmentally undesirable heavy metal species, sulfates, and other anions. On the other hand, the water discharged from some mines contains valuable metals such as copper and uranium that couldbe recovered economically. Copper is usually extracted from such discharges by either cementation or liquid ion exchange. Mine drainage containing9-12 ppm U,O, is stripped by ion exchange as exemplified by the operation in the Ambrosia Lake district [8]. The acid mine drainage containing300-600 ppm A1,0, and 10-20 ppm U,O,, on the other hand, is stripped by a combination of ion and liquid ion exchanges [9].
C.Coal
UtilizationWastes
As a result of burning coal in boilersand electric power plants, a large quantityof ash is produced. The amount of ash generated by power plants in 1977 is estimated to have been about 67.8 million tons, of which 48.5 million tons wasfly ash, 14.1 million tons bottom ash, and the remainder boiler slag. During that year, about6.3 million tons of the fly ash, 4.6 million tons of the bottomash, and 3.1 million tonsof the boiler slag,or approximately 21%of the total ash generated, was recycled in such products as concrete blocks, asphalt, and roofing materials [lo-121.
D. MetallurgicalWastes The production of alumina by the Bayer process each year is accompanied by simultaneous formation of about 7 million tons of red mud that consists of a substantial amount of valuable minerals and dissolved salts. These wastes are estimated to contain a large amount of caustic soda, 1.2 million tons of alumina, 1.7 million tons of iron, and about 450,000 tons of titania [13]. These pose severe environmentaland health hazards. In steelmaking, over 2 million tons of dust and gases is generated by electric and basic oxygen furnaces annually. The dust contains a substantial quantity of lead (0.4-2.6%), zinc (6.3-24.8%), manganese (0.5-5.3%), and copper (0.03-0.27%) in addition to iron [14]. Similarly, in manufacturing stainless steel, a large amount of metals is as lost wastes. Powell et al. [l51 estimated that approximately5 million poundsof molybdenum is lost in stainless steel furnace dust each year.
Process Engineering Control for Pollution
7
Table 1 Characterization of FoundryDust Analysis (wt. %) locationSample Alabama Ohio Michigan New Hampshire Pennsylvania Massachusetts West Virginia
cuPb
Zn
13.52 0.50 7.50 0.24 0.79 0.75 4.90
65.34 63.70 56.70 44.70 54.80 65.04 78.25
Fe 3.01 1.10 6;30 0.58 5.80 0.30 2.95 0.15 5.80 460 0.06 6.86 0.15 2.40 39 0.61
Toxicity
c1
(mg Pb/L)
0.06 0.54
530 440 764 188
0.66 0.40 1.30 0.5 1 0.05
6
A large amount of dust is producedby brass and bronze foundries and secondary smelters annually in the United States. The baghouse dusts vary in composition, but the main constituents are zinc(40-78%), copper (10-15%), and small amountsof lead and tin. Most of the zinc is present at ZnO, while the remainder is in the form of brass or bronze alloys. A typical characterization of dust from some foundries is presented in Table1. These wastes are considered to be hazardous because of the high lead contents. During the production of elemental phosphorus using an electric furnace, a large amount of toxic wastes such as sludge, slag, gases, and phossy water are also generated. It is known that between 5 and 10% of the elemental phosphorus that is produced is left behind in the sludge. The compositionof the other solid constituentsof the sludge is4040% SO,, 5-15% CaO, 2-4% Fe,O,, and 2-5% P,O, [16]. In general, the ratio of phossy water and sludge that are formed to the amount of elemental phosphorus produced is about5: 1. Phosphorus wastes pose both environmental and fire hazards,and these wastes are producedat a rateof 1.5 million tons annually. 111.
METHODS OF CONTROL AND TREATMENT OF BULK SOLID WASTES
A number of measures are taken to minimize or render bulk solid wastes safe for disposal. These include the extraction of heavy metals or toxic constituents from the waste materials using either physical, chemical,or bioremediation techniques.On the other hand, some wastes are either recycledor used directly, but more often a combination of these techniques is applied to achieve maximum process efficiency. The following methods are classified according to the source of the solid wastes.
A. Copper Mine Wastes Copper mine wastes are increasingly important because of thelow very grade of most available copper ores. Rule and Siemens [l71 have shown that the bulk flotation method is effective in extracting such metal values as copper, cobalt, and nickel from copper mine wastes with recoveries in the range of 54-95%. The primary problem in using the flotation method for this purpose is the intimate association of the valuable minerals or metals (minor) with the predominant gangue materials. Consequently, a high degree of fineness is necessary in order to ensure liberationand subsequent separationof the metal values. However, reagent consumption is also expected to be high. In most instances, the residues still contain fairly high levels of valuable minerals or heavy metals and as a result mustbe subjected to further treatment. Pressure leaching or bacterial leaching (bioleaching) is often used for this purpose.
Hanna and Orumwense
8
B. IronOre Wastes In the past, many iron tailing ponds were subjected to gravity concentration[l81 to recover the iron contents. Jones and Laughlin Steel Corporationin Calmet, Minnesota, is an example of a company that at one time combined flotation and gravity concentrationfor treating iron wastes to recover the metal values. The presence of a large amount of slimes and the high impurity contents of either the initial ores or the wastes impaired the recovery of iron from the wastes. In contrast, selective flocculation and high-intensity and high-gradient magnetic separations [l91 are some of the other techniques that canbe used effectively totreat such materials. Waste materials can also be subjected to reductive roastingand magnetic separation to reduce the energy required for processing.
C.Phosphate
Rock Wastes
Phosphate slimes are known to be not only difficult to recover but also economically unsound. However, the associated adverse environmental impact necessitates treatment. Laboratory tests on waste pond materials, low-grade washer products, and some raw Tennessee phosphate ores have shownthat some of the phosphate can be recovered. Market grade phosphate concentrates assaying 60-82% P205can be obtained in substantial amounts using the anionic flotation method [20]. Direct digestion of the phosphate matrix with sulfuric acid is an alternative approach for the minimization of slime disposal problems.This process producesa simple waste consisting of gelatinous slime, sand, and gypsum. The composite is a compact sandy cake that could be 95%of the P205is recovered as useful used as a filling materialin mined-out areas while about material [2l ,221.
D. Fine CoalWastes TWO major
techniqueshave been proposedfor treating coal wastes. Theseare gravity separation and flotation [23,24]. The use of Humphreys spirals to treat coal wastes has been established. Although such treatmentsare capable of yielding high-grade coalconcentrates, the recovery is relatively low. Also these techniquesare only applicableto feeds withparticle size coarser than 200 mesh. Besides,a substantial amountof the coal is lostin the tailings-about 10-71% [24]. Therefore, techniques thatare suitable for fine particles processing are required to supplement the spirals in order to improve coal recovery.This has led to the development of a process that is based on a combination of gravity separation and froth flotation. In this process, Humphreysspirals are used to recover the coarse coalparticles while column flotation is employed for the minus 200 mesh size fractions [24]. Mechanical flotation can also be used in place of spirals to separate the coarse particles. In this manner, both the quality and the recovery of coal are improved significantly. Similarly, thepyrite present in the wastes can be removed, and by doing so, acid drainage problems can alsobe mitigated. It is also possible to employ a bioleaching techniqueto eliminate the pyrite constituents from coal wastes. This can be achieved by allowing bacteria to oxidize the pyrite in coal wastes as feed.
E. Phosphorus Wastes The methods of treating phosphorus waste include physical, ch’emical, and bioleaching techniques. The physical methods include sizing, sedimentation, centrifugation, cycloning, and flotation [25-27, 311, while air oxidation, chlorine oxidation, electrolytic oxidation, catalytic
gineeringProcess
for Pollution Control
9
oxidation, distillation, CS2 extraction[28-331, and ion exchangeconstitute the chemical methods. Most of these processes either partially separate or oxidize phosphorus from the impurities.Therefore, a combination of twotreatmenttechniques is necessary for complete remediation of phosphorus wastes. Another factor necessitating this methodology is the associated low operating costs for such schemes. A combination of clarification and chlorination techniques has been developed for extracting elemental phosphorus from phossy water [26]. However, the associated residual chlorine has an adverse environmental impact, and this renders the technique impractical, The ERCO process is based on the use of nascent oxygen to oxidize elemental phosphorus prior to subsequent separation [33]. Another method uses distillation as the basis for the remediation of phosphorus from sludges [29]. The high operating costs associated with these methods have limited their application. In many cases a major part of the phosphorus wastes are present in the coarse particles. Anazia et al. [31] have shown that between 26 and 29% by weight of the particles in the tested sludge samples (obtained from FMC Corporation of Pocatello, Idaho, andthe TVA at Muscle Shoals, Alabama) are coarse phosphorus particles containing 82-91% P4. It was also demonstrated in the same study that about61-88% of the coarse phosphorusparticles can be recovered by screening. The fine fractions represent 71-74% byweightof the sludge and assay 5-21% P4. The as-received unsized sludge can also be subjected to flotation to separate phos61 and 78%. P4 with a recovery in the range of 71-79% phorus concentrates assaying between depending on the characteristics of the wastes. The tailings assayed between9 and 18% P4and constitute about 59-68% of the sludge [31]. It is obvious, therefore, that the fine fractions or tailings must likewise be subjected to further treatment using other methods. The phosphorus remainingin the physical separation rejects can be extracted after air oxidation treatment at ambient temperatures. These form the basis for the proposed two-step method comprising either flotation or screening and conventional air oxidation for the treatment of phosphorus sludges [31]. However, the P4 concentrations in the refuse from the oxidation step can be as much as 4%, which is still high in terms of toxicity. A long air oxidation periodof several days or weeks may be necessary to achieve 90-95% conversion of P4 to H3P04 at an ambient temperatureof 30°C. Under these conditions the oxidation rate of P4 in water is slow and is influenced by many factors such as pH, oxygen content, temperature, presence of metal ions, and degree of dispersion of colloidal material [34]. Therefore, an incomplete conversion of P4 to oxyphosphorus compounds occurs during the conventional oxidation process because the reaction kinetics appear to be influenced by other factors such as agitation, particle size, and surface coating [34]. This process has been further developed at the University of Alabama such that the oxidation and conversion of P4 to soluble oxyphosphorus compoundsare enhanced significantly [32, 351. In the new process, the insoluble P4 is converted to highly soluble and nontoxic compounds thatare easy to extract from the rest of the sludge. This improvement has been achieved by employing a novel reactor design known as HSAD to expedite the remediation operation. Thus, depending on the P4 contentof a sludge, an almost complete oxidationof phosphorus is achieved in about 1-3 hr, and the resultant acid solution can be employed in the manufacture of either phosphoric acid or fertilizer by-product by neutralization. The chemically inactive solid waste can be dried and safely disposed of as nonhazardous landfill product. Someof the results obtained employing this processare given in Table 2. The advantagesof the HSAD technique includeshort processing duration, high efficiency, simple configuration, low cost, and applicability to various phosphorus wastes. The process requires no catalysts, chemical oxidants, or high temperatures [35].
Hanna and Orumwense
10
Table 2 Test No.
mid Results of HSAD West Oxidation of Phosphorus Sludge [32] Product Weight
(g)
Weight (%)
P4 Analysis (%) ~
1
2
Solution Residue 35.40
50.20 27.51
Feed
77.71
63.44 Solution 36.56 Residue
49.76 26.68
Feed
78.44
64.60
100.00
100.00
P4 Removal (%)
~~
53.39" 0.02
99.97 0.03
34.50
100.00
54.35" 0.05
99.94 0.06
34.50
100.00
"Equivalent P4analysis of oxyphosphorus compounds.
F. 'Brass and Bronze Foundries Dust Baghouse zinc dust is processed by using sulfuric acid leaching and electrolysis or crystalli[36,37]. The zinc extractionattainable with this method zation to recover zinc and other metals is in the range of 89-99%. Basically, the method involves the use of strong sulfuric acid and intense aeration to dissolve the zinc oxide and metallic copper from the dust. The lead, tin, and zinc metal alloys present in the dust are not dissolved by sulfuric acid and remain with the solid residues. The leach residues, which account for 20-50% by weight of the dust and are rich in a number of metals, can be further treated to extract the metallic components [36]. The pregnant leach liquor is subjected to electrowinning to produce metallic zinc. However, the zinc electrowinning operation is adversely affectedby the presenceof chloride ions or some metals. Hence, additional measuresare required to eliminate chloride ions andother impurities in order to produce high-grade metallic zinc. This can render the whole process expensive. Alternatively, the crystallization technique is employed to recover the zinc from the leach liquor as zinc sulfate salt.
IV. ADVANCEDREMEDIATIONTECHNOLOGY A. Leaching The most common method of recovering the metal values from low-grade sources such as waste dumps or heaps is leaching. Leaching is a process in which a solid material is contacted with a solvent in order to selectively dissolve some of the components. The objectives of leaching metals from sludge include the dissolution of the metal valuesfor recycling or subsequent separation by other methods, to render wastes nonhazardous,or to render wastes amenable to further treatment. Leaching is known to account for about 10% of the yearly copper production. The commonly used leaching agents are sulfuric acid, hydrochloric acid, ferric chloride, nitric acid, ferric sulfate,ammonia or ammonium carbonate, hydroxide, and microorganisms such as bacteria, yeast, and fungi. Unfortunately, many factors concerning leaching such as thesize and heightof dumps and factors affecting solution percolation andthe kinetics and recoveryof the valuable metals from the leach of pregnant liquors in general still require detailed studies and information dissemination. The fact that many of the minerals in wastes canbe recovered inexpensivelyby leaching implies that some of the problems associated with the disposal of fine wastes canbe alleviated. Biological remediation of wastes is accomplished by using naturally occurring microorganisms such as bacteria, yeast, and fungi to treat contaminants. Its use is rapidly increasing. However, the microorganisms requirea wide rangeof macro and micro nutrientsfor their met-
Process Engineering Control for Pollution
I1
abolic activities and growth. The environment is generally poor in the nutrients such as nitrogen, phosphorus,andcarbonrequired by the microorganismsforsustenance,andsome contaminants exhibit a certain degree of resistance to different microorganisms. These are the primary causeof the slow rate of breakdown of contaminants. Therefore,a successful bioleachingoperationrequiresthegrowth of appropriatemicroorganismsthatcan be inducedby manipulating conditions suchas the availability of nutrients, temperature, electron acceptors, and aeration.
B. Precipitation Precipitation is one of the common means of remediating wastewater. In this method, chemicals are used to alter the physical state of dissolved or suspended metals and to enhance subsequent separation using sedimentation techniques. Chemicals such as caustic soda, lime, soda ash, sodium borohydroxide, sodium phosphate, ferrous sulfide, and sodium sulfides are used to induce precipitation. It is sometimes necessary to subject wastewater to some form of pretreatment such as filtration, destruction or organic matter and cyanides, metal reduction, neutralization, and/or oil separation prior to precipitation. Some metals as typifiedby hexavalent chromium are difficult to precipitate in the form in which they occur and must be reduced if the operation is to be successful. Reducing agents commonly used include sulfur dioxide, sodium bisulfite, sodium metabisulfite, and ferrous sulfate. Similarly, to effect sedimentation and subsequent separation of precipitates, flocculants are sometimes required. Lime,alum, and polyelectrolytes are used for this purpose. The major characteristics of wastes that have an impact on precipitation operations are the type and concentration of metals, amount of total dissolved solids, concentration of residual complexing agents, and amount of oil and grease present in the wastes. Metal-laden wastewater resulting from electroplating, pigment manufacture, the photographic industry, battery manufacture,and nonferrous metal industriesare usually subjected to precipitation treatment.
C.IonExchange In the ion-exchangeprocess,metalions in a dilute solution are substituted for identically charged ions electrostatically bonded to the surface of an immobile solid medium. The solid medium can be either a naturally occurring inorganic zeolite or a synthetic organic resin. Ion exchange is a reversible chemical reaction. Therefore,the loaded resin or exchange medium is placed in a pure solution of appropriate pH and the trapped metal ions are released. This method is applicable only to liquid wastes or pregnant solutions. The performanceof the process depends on(1) the concentration and valenceof the metal constituents, (2) the presence of competing ion species, and (3) the presence of dissolved or suspended solids and organic compounds. Therefore, the feed toan ion-exchange system must be subjected to pretreatment. Thismethod results in about 95% metal recovery and high purity products. This method is fully developed and is used commercially to remove chromium, copper, nickel, cadmium, silver, and zinc from wastewater.
D. ElectrolyticRemediation Electrolytic cell is the primary device used in electrolytic remediation. It consists of an anode and a cathode immersed in an electrolyte. When an electric current is applied to an electrolyte solution, the dissolved metals are reduced and subsequently deposited at the cathode. The electrolytic remediation technique isalso known as electrowinning because the metals recovered are of high purity. This is one of the most effective methods for remediating chelated
12
Hanna and Orurnweme
metals, which are difficult to retrieve by other techniques. This method has the advantage of producing metal-laden free sludge, but it is limited to solutions containing a fewtypesof elements. Electrolysis can be used to remediate cadmium, chromium, copper, lead, tin, and zinc. However, such treatments involve a high energy expenditure. Wastes containing copper and certain other elements must be leached with hydroxides before being subjected to electrolytic treatment. A variation of the electrolytic technique known as electrodialysis is obtained when a membrane is placed between the anode and cathode such that the mobility of some ions throughthe membrane is obstructed. Electrodialysis can be used for remediating wastes from such sources as gold-, chromium-, silver-, zinc-, nickel-, and tin-plating operations where the ion concentration is low and would not be economical for electrowinning. Most feeds for electrodialysis is a compulsory treatment must be filtered to remove suspended solids. Besides, pH control pretreatment measure because of the effect on metalseparation. When electrodes having a high surface area are employed, metals removalof about 98% can be achieved.
E. MembraneSeparation The membrane separation method encompasses such techniquesas filtration (microfiltration, ultrafiltration, etc.), reverse osmosis, and electrodialysis. Thefiltration technique is used after the sludge hasbeen pretreated for the removal of metals. The techniqueis also usedto pretreat feeds destined for subsequent treatmentby both reverse osmosis and electrodialysis. Reverse osmosis and electrodialysis are used to retrieve metalsor plating compounds from wastewater. The electrodialysis method is described in the preceding subsection. Reverse osmosis (RO) systemsare characterized by having a number of modular units connectedeither in parallel or series or a combination of the two. The applicationof this method to the remediation of metal-laden sludgeis limited by the pH range in which the membrane can be used. Cellulose acetate membranes are not suitable for use at pH above 7, while amide and polysulfone membranes can be used in the pH range between 1 and 12. The performance of R 0 systems is impaired by the presence of colloidal matter, dissolved organics, and insoluble constituents. It is recommended that the feeds to R 0 systems be subjected to such pretreatments as pH adjustment, carbon adsorption, and filtration. The method is used commercially to remove brass, chromium, copper, nickel, and zinc from metal-finishing wastes. These techniques canbe used to produce effluents with very low metal constituents, provided, of course, that adequate pretreatmentshave been carried out. Metal removal onthe order of 99% can be achieved by making use of a combination of precipitation and filtration.
F. Evaporation Evaporation is a simple method for remediationof mixed materials based onthe difference in volatility. Hence, the concentration of metals is brought about by the reduction in the volume of the waste. The primary instrumentation used for this purpose includes rising film, flash, and submerged tube evaporators. Cadmium, chromium, nickel, zinc, copper, and silver from platingbaths are retrieved in theelectroplatingindustry by using this method.However, this method of remediating wastes is cost-effective only when a very small volume of waste is involved.
G. Encapsulation Soluble silicates and their derivatives are very effective for the stabilization and fixation of hazardous wastes.Silicates are used in waste treatment becauseof their inherent characteristics
ngineering Process
Control for Pollution
13
such as alkaline nature (pH 20-14), ability to form gels, and reactivity with multivalent cations, and because their disposal poses no potential dangerto the environment. Soluble silicates are polymeric and condense on aging to form anions having a siliconoxygen-silicon linkage that are complex and exist in various chain lengths and cyclic structures. Silicates react with metal ionsto form insoluble amorphous metal silicates. These metal pH range compared with simple metal hydroxides. silicate complexesare insoluble over a large This is responsible for the increased resistance to leaching of metals in solidified wastes and is perhaps the main feature of silicates in waste treatment. Soluble silicates are made by fusing sodiumcarbonate or potassiumcarbonateandsandinafurnace at 1450°F. Theresultant nSiO,Na,O compound has silica (SiO,) to alkalinity (Na,O) ratio in the rangeof 1.6:3.9. The SiO,Na,O ratio has great significancein subsequent useof silica because only compounds having high ratios are employed in the manufacture of products such as gels, precipitated silica, and zeolites and in the treatmentof wastes. Setting agents commonly used in waste treatment include Portland cements, pozzolanic fly ashes, and cement or lime kiln dust. The active components in setting agents are such derivatives as the mono-, di-, and tricalcium silicates formed when the agent is mixed with water. The physical properties and behavior of setting agents are strongly influencedby the calcium silicate content, as this is directly related to the number and strength of the resultant bonds formed. Silicates also reduce the permeability by reducing calcium hydroxide inclusion formation or the presence of voids in the structure of the material. The treatment of hazardous waste with setting agents can be subdivided into two categories, stabilization and fixation. Stabilization is a chemical processof transforming a liquid waste into a solid. The setting agents are mixed with the waste, and when they “set up” or harden, the waste material is entrapped in the structure. The procedure used in the stabilization operation involves premixing the waste and setting agents before introducing soluble silicate. The role of silicates in the stabilization process includes the reduction of setting time, decreasing of the permeability, increasingofthecompressivestrength,andreductionofboththeamountofsettingagents employed and the volume of the treated waste. Fixation, on the other hand, is similar to stabilization in many respects, but rather than merely entrapping the wastes as inclusions, the wastesare modified and bonded into a cementlike matrix. Hence the solubility or leachability of hazardous components is reduced dramatically. In this manner, the toxicity and mobility ofheavymetalwastesarechangedbythe treatment. The treatment steps involve mixing the waste with cement or kiln dust as a setting agent and water. Thereafter, a soluble silicate is introduced and mixed thoroughly. The procedure is recommendedif a good result mustbe obtained, and cementmust be used when a waste canused with silicates is tobe fixed. Portland cement is the most effective setting agent that be for this purpose. The reason is that during hydration cement produces gels that help to encapso sulate waste. Lime-based materialsdo not produce a large amount of gels during hydration, the amount of bonded wastes is reduced. Therefore, lime-based setting agents should not be used for waste fixation.
REFERENCES Hill, R. D., and Auerbach, J. L., Solid waste disposal in the mining industry, in Fine Particles Processing, Vol. 2 (€?Somasundran, e d . ) , SME-AIME,NewYork, 1980, pp. 1731-1753. 2. Hanna, H. S., and Rampacek, C., Resources potential of mineral and metallurgical wastes, in Fine Particles Processing,Vol. 2 (F?Somasundaran. ed.), SME-AIME, New York, 1980, pp. 1709-1730. 3. Mineral Trends and Forecasts, US. BureauofMines, 1979. 4. Klinger, F. L., Mineral facts and problems-iron, Bull. 667, U.S. Bureau of Mines, 1976, pp. 5251.
545.
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14
5 . Rampacek, C., The impact of R&D on the utilization of low-grade resources,
Chem. Eng. Prog.,
February, 57-68, 1977. 6. Vasan S., Utilization of Florida phosphate slimes, Proc. 3rdMineral Waste UtilizationSymp., Chicago, 1972, pp. 171-177. 7. Moudgil, B. M., Handling and disposal of coal preparation plant refuse, in Fine Particles Processing, Vol. 2 (F!Somasundran, ed.), SME-AIME, New York, 1980, pp. 1754-1779. 8. Spendlove, M. J., Bureau of Mines research on resource recovery, IC 8750, U.S. Bureau of Mines, 1977. 9.
IO. 11. 12. 13. 14.
15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.
Ross, J. R., and George D. R., Recovery of uranium from mine waters by countercurrent ion ex-
change, RI 7471, U.S. Bureau of Mines, 1971. Faber,J.H., Coal Technology Conference, Houston, Texas, 1978. Brackett, G . E., Production and utilization of ash in the United States, Proc. 3rd. Int. Ash Utilization Symp., Pittsburgh, Penna., IC 8640, U.S. Bureau of Mines, 1974, pp. 12-18. Jackson, J., Total utilization of fly ash, Proc. 3rd Miner. Waste Utilization Symp., Chicago, Ill., 1972, pp. 85-93. Dean, K. C.,Utilizationofminemillandsmelterwastes, Proc. 2nd MineralWaste Utilization Symp., Chicago, Ill., 1978, pp. 138-141. Dressel, W. M.. Barnard, F! G., and Fine, M. M.. Removal of lead and zinc and the production of prereduced pellets from iron and steel making wastes, RI 7027. U.S. Bureau of Mines, 1974. Powell, H. E., Dressel, W. M., and Crosby, R. L.,Converting stainless steel furnace flue dust and wastes to a recyclable alloy, RI8039, U.S. Bureau of Mines, 1975. Slack, A. V. (ed., Phosphoric Acid, Parts 1 and 2, MarcelDekker,NewYork, 1965. Rule, A. R., and Siemens, R. E., Recovery of copper, cobalt and nickel from waste mill tailings, Proc. 5th Mineral Waste Utilization Symp., 1976, pp. 62-67. Fine, M. M., and Heising, L. F., Iron ore waste occurrence, beneficiation and utilization,Proc. 1st Mineral Waste Utilization Symp., Chicago, Ill., March 1968. pp. 67-72. Colombo, A. F., Jacobs, H. D., and Hopstock, D. M.,Beneficiation of Western Mesabi Range oxidized taconite, RI 8325, U.S. Bureau of Mines, 1978. Lamont, W. E.,etal.,LaboratoryflotationstudiesofTennesseephosphatesinthepresenceof slimes, RI 7601, U.S. Bureau of Mines, 1972. White, J. C., Fergus, A. J., and Goff, T. N., Phosphoric acid by didect sulfuric digestion of Florida land pebble matrix, RI 8086, U.S. Bureau of Mines, 1975. White, J. C., Goff, T. N., and Good, I? C., Continuous circuit preparation of phosphoric acid from Florida phosphate matrix, U.S. Bureau of Mines, 1978. Browning, J. S . , Recovery of fine-size waste coal, Final Rep. U.S.Dept. of Energy, Contract ET76-G-01-9005, Univ. Alabama, May 1978. Hanna, J., and Kalathur, R., Recovery of fine size coal from impounded wastes, Miner. Metall. Processing, November, 174-179. (1992). Fleming, J. D.,Removalofphosphorus,aliteraturesurvey,TennesseeValleyAuthority,Muscle Shoals, Alabama, 1970. Barber, J. C., Recovery of phosphorus from dilute waste streams, U.S. Patent 4,595,492 (July 17. 1969).
27. 28. 29. 30. 3 1. 32.
Crea, D.A., et al., Recovery of phosphorus from electric furnace sludge, U.S. Patent 3,615,218 (October 1986). Post, L. B., et al., Recovery of phosphorus from electric furnace sludge, U.S. Patent 3,615,218 (October 1971). Holmes, W. S., Lowe, E. J., and Brazier, E. R., Phosphorus distillation, U.S. Patent 4,081,333 (Mar. 28, 1978). Hinkebein, J. A. Recovering phosphorus from sludge, U.S.Patent 3,436,184 (April 1969). Anazia, I., Jung, J., and Hanna J., Recovery and removal of elemental phosphorus from electrical furnace sludge, Min. Metall. Processing, May, 64-68 (1992). Hanna, J., and Jung, J., Phosphorus removal by dispersed air oxidation, Miner. Metall. Processing November, 200-205 (1992).
Process Engineering Control for Pollution
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33. Deshpande, A. K., Oxidation of phosphorus in aqueous medium, U.S. Patent 3,971,707 (July 27, 1976). 34. Sullivan, J. H., Jr., et al., A summary and evaluation of aquatic environmental data in relation to establishing water quality criteriafor munitions-unique compounds. Part 3. White phosphorus,Final Report, Water and Air Research, Inc., Gainesville, Ha., April 1979. streams, Proc. Haz35. Hanna, J., and Jung,J., Remediation of phosphorus from electric furnace waste ardous Materials Control, HMC-South '92. New Orleans, La., Feb. 26-28, 1992, pp. 34-39. 2nd Annual Environmental 36. Hanna,J.,andRampacek.C.,Recoveringzincfrombaghousedust, Society, Milwaukee,Wisc.,Aug. 23-24,1989, Affairs Conf. of theAmericanFoundrymen's pp. 119-126. 36. CommodityDataSummaries,Phosphates, U.S. Bureau of Mines, 1978. 37. Powell, H. E.,et al., Recoveryof zinc, copper and lead-tin mixtures from brass smelter flue dusts, RI 7637, U.S.Bureau of Mines, 1972. 38. Proceedings 3rd International Symposium, IC 8640, U.S.Bureau of Mines, 1974.
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2 Selection of Least Hazardous Material Alternatives
Alvin F. Meyet A. E Meyer and Associates, Inc. Mckan, Virginia
1.
INTRODUCTION
A. Summary This chapter addresses the underlying purposesof accomplishing measures to select the least hazardous of alternative materials usedor planned for use by industrial and governmental entities. It then discusses the relationship of substitution processes to other considerations in the decisionprocess. An overview of someapproaches for selectionmethodsispresented.A “methodology” originally developed for the U.S. Navy is described, along with examples.
B. Substitution Methods as an Element of Pollution Prevention 1. Substitutionvs.Controls A longstanding principle of environmental control and industrial hygiene is that the first and basic consideration in control of hazards is the elimination of the hazardous component, or if that is not feasible, then the substitution of a lesser hazard[l]. The concept of eliminating the source or reducing the amount of toxic materials includes in addition to substitution of materials such other measuresas process or operation2 changes, properdesign of operations, and housekeeping. The importance of eliminating the need for costly environmental control measures by substituting less dangerous or less offensive materials also is of longstanding recognition[2]. Likewise, the economic advantages of industrial waste recovery is not a new concept. Nonetheless the primary approachto environmental control, until the late198% was that of using &‘end of the pipe” control measures. This was primarily in response to the focus of environmental regulations specifying limits to be metby treatment or other control measures. In the early 1990s there emerged a steady increase in recognition both in regulatory agenties [e.g., u.S. Environmental Protection Agency (EPA),the Department of Defense1 and in the private sector that a comprehensive, cost-effective approach to environmental quality in17
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Meyer
cludes source reduction, recovery, and reutilization, with treatment and ultimate disposal as the last resort. The Pollution Prevention Act of 1990 established these concepts in a national policy that “pollution should be prevented or reduced at the source whenever feasible.”
II. HAZARDOUS MATERIAL SUBSTITUTION ACTION A. Statutory and Regulatory Requirements There are many statutory and regulatory requirements that directly or indirectly create the need for hazardous material assessment. These include federal statutes and their implementing regulations or standards, executive orders requiring federal agency compliance, and state and local codes, standards, and regulations. The Department of Defense (DOD) and the military departments and agencies have requirements that implement the federal mandates and some requirements that predate them. 1 . Federal Codes, Standards, and Regulations The primary federal statutes and their implementing regulations regarding environment, safety, and health are the Clean Air Act, Resource Conservation and Recovery Act (RCRA), Clean Water Act, Safe Drinking Water Act, Toxic Substances Control Act (TSCA), Emergency Planning and Community Right to Know Act (EPCRA), Pollution Prevention Act of 1990, Occupational Safety and Health Act (OSHA), Hazardous Materials Transportation Act, and the National Environmental Policy Act (NEPA). These acts taken together impose a need to examine the feasibility of using materials that are less hazardous, are less costly, or impose fewer administrative or other regulatory compliance resource requirements.
2. Possible Application of Department of Defense Methodologies A Risk Assessment Code (RAC) procedure was developed by the Department of Defense in the early 1960s [3]. Initially, it was designed to provide a means of ranking hazards associated with new weapon systems. Subsequently, the procedure was adapted in 1981 to rate occupational safety and health deficiencies. In its simplest form, the procedure provides a rating scheme based on a matrix to estimate the severity of effects of the hazard and the probability of occurrence, with the results stated as a risk assessment code. The range is from RAC 1 (catastrophic impact) to RAC 5 (negligible) (see Table 1). The later (1981) procedure, which is still in use, includes a cost effectiveness index and an abatement priority ranking. The revised procedure takes into account, with a numerical algorithm, such circumstances of exposure and resultant effects as the OSHA Permissible Exposure Limits (PEL), number of employees involved, effects of exposure (ranging from death to minimal lost time, disease, or injury), and duration of exposure. It is firmly established in the military services procedures for evaluating and prioritizing occupational safety and health hazard abatement requirements. The RAC schemes deal primarily with chemical and safety hazards ratings, with no consideration for environmental ramifications. Recently there has been an increased focus on the environmental aspect of hazardous materials use on the decision-making process. Unlike the long history of chemical and safety hazards rating schemes, there are no universally accepted systems for environmental hazards and risk acceptance. One possible method is a European model, described below. 3. A European Method for Priority Selections and Risk Assessment A study requested by the European Community Commission to provide a practical approach for priority setting among existing chemicals was prepared by Sampaolo and Binetti [4]. Using this
Least Hazardous Material Alternatives
19
Table 1 Risk Assessment Code Rating Schemea Hazard Severity (HHSC)
I I1 111 IV
Mishap probability (MPC) A 1 1
2 3
B
C
D
1
2 3
4
2 3 4
4 5
3 5 5
aInterpretation of HM selection Risk Assessment Code: RAC 1 = high risk (imminent danger of life or property; possible civil or criminal action) RAC 2 = serious risk (may result in severe injury or illness on or off site, potential for major damage to environment, and resulting notice of violation) RAC 3 = moderate risk (may cause few illnesses or injuries or significant property damage or environmental impact on or off site) R A C 4 = low risk (can result in only minor impact on or off site or only violation of a standard without damage) RAC 5 = negligible (insignificant impact)
model, an individual property or a number of properties of a given chemical can be evaluated and then ranked with those of other substances. This flexibility allows for evaluating different relationships. For example, one might want to compare only the intrinsic properties with respect to direct personal exposure in a particular circumstance or with respect to environmental exposure. Certain chemicals might have different relative rankings for these two categories. This model offers a number of advantages: The system is simple and flexible enough to be adapted to different and specific needs (i.e., personal exposure to general exposure, risk from domestic exposure vs. professional exposure, etc.). It is a self-improving system because new information can be input and the result can be refined further. This model uses three sets of parameters to evaluate risk and the priority of a given chemical: assessment of intrinsic properties, risk assessment or potentiality, and priority assessment.
Intrinsic Properties. Intrinsic properties of a substance are based on the set of physicochemical , toxicological, and ecotoxicological properties that are considered fundamental (or intrinsic) to the first evaluation of the substance. Each element of the intrinsic property (e.g., molecular weight under the physicochemical category) is assigned a numerical value that corresponds to its level of danger. From this information a score is developed for each intrinsic property, which also addresses the availability or nonavailability of the data. These intrinsic properties are considered additive and determine the intrinsic danger of the substance independently of external agents or factors that may influence it. External Factors. Risk assessment or potentiality includes not only the intrinsic danger of a substance but also the external factors that can influence the danger. These external factors include the quantity of the substance on the market, the plurality of possible exposures, and the size of the risk population. As an example, a substance may be highly dangerous, but if it is not on the market it will not pose any effective risk, and thus its intrinsic risk will be minimal. Priority Meusurement. Priority assessment involves both the known or presumed danger of the chemical and the degree of the lack of knowledge of the substance’s properties. A priority measurement can be made by calculating the ratio of the weighted figures for properties without data to those figures with available data. Both the risk assessment and priority assessment parameters can affect the intrinsic properties of a substance by multiplying or canceling them.
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Meyer
LIFECYCLE AND MANAGEMENT CONSIDERATIONS
111.
A. LifeCycleConsiderations Regardless of the methodology usedin rating hazardous properties of a material, the selection process for the “least hazardous” involves more than environmental, safety, and health considerations. In addition to assessmentsto determine the least harmful material based ona hazardassessment(suchasthealgorithmdescribed later in this chapter), there are major considerations that must be carefully assessed. 1. BasicFeasibilityandEngineeringConsiderations A fundamental question that should be addressed is: Does the substitute perform adequately for its intended use? This requires determination of the following: 1.
2. 3.
4. 5. 6.
“Favorable” vs. “adverse” effects on required performance of the material(s) in production, operations, and maintenance situations. Creation of new or different hazards (suchas substitution of a less toxic material witha fire hazard potential for one that is highly toxic but has a low hazard or no hazard). Durability and life cycle times to failure (as with a low volatile organic compound(VOC) paint that may or may not last as long in a very hot or very cold climate). Maintainability of equipment involved in using a substitute. Possible process or equipment changes that may be needed. Environmental and/or OSHA controls required even if it is the lesser hazard.
2. LifeCycleCostConsiderations There are costs and benefits associated with the engineering and feasibility considerations that need to be assessed. In addition, there are many other costs associated with the life cycle of hazardous material. That life cycle extends from the time of concept through procurement, storage, use, and disposal. It is beyond the scopeof this chapterto do more than highlight such costs. It is also beyond its scope to describe the economic analyses required to evaluate the relative costs and benefits of two or more candidate materials for selection. Among the many costs that should be taken into account in the selection process are those shown in Table 2. A very useful guide for comparing alternatives is the EPA Pollution Benefits Manual [5]. The EPA Pollution Benefits Manualprovides for a financial analysis approachto compare alternatives for pollution prevention. It involvesa four-tier cost analysis from which economy feasibility of alternatives can be evaluated. The four tiers are as follows: Tier 0,Usual costs. The alternatives are identified, and all normal costs associated with each are determined. These include investment (depreciable capital, expenditures), operating costs, and operating revenues. Table 2 Life Cycle HazardousMaterialsCostsandCost Avoidance Considerations Acquisition Supply and storage Use
Waste treatment Other disposal Emission control Inventory control Engineering/process controlkhange Training
Safety Hazard/risk assessment ENEIS Permits Personal protection Medical monitoring Spill prevention and control Regulatory overhead Liability
Least
21
Tier l , Hidden Costs. These include such investments as monitoring equipment, protective equipment, control technology, and operating costs such as reports, monitoring training, and medical costs. Tier 2, Liability Costs. Included are penalties, fines, and future liabilities. Tier 3 , Less TangibleCosts. These include costs such as those associated with labor relations, and public relations. The results of the tier analyses are thenincorporatedintocostsummariesandfinancial worksheets, which result in an assessment and/or comparison of any cost savings for each alternative. This procedureallowsforcomparison of relative costs and benefits of selected alternatives.
B. Management Decisions and Actionsfor Selection of Least Hazardous Material 1. DrivingForces The basic driving forces for managementto consider in the selection of less hazardous materials, in addition to regulatory requirements, include the life cycle cost considerations discussed above and such needs as planning for new products or processes (or changes to existing ones), avoidance of new and long-term liabilities, and the possible benefitsof participation in such voluntary programs as the U.S. EPA Industrial Toxics Project (ITP). Other benefits include improved employee and community relations.
2. “Closing the Loop” Once decisions are made for substitution, a large number of follow-on actions are needed. These include planning for phase-out of the existing in-usematerials, development of new specifications and technicaldata documents, trainingof personnel, provision of any necessary controls, and compliance with any new permit or similar requirements.
W. DESCRIPTION OF METHODOLOGY A. Overview of the Navy Substitution Algorithm The hazardous material substitution algorithm developed for the U.S.Navy also had wide POtential for selection or substitution of least hazardous materials in civilian applications [6]. 1. The NavyModel The hazardous material substitution algorithm is sufficiently flexible that it can either serve as a preliminary screeningto identify the most likely candidatefor further study or become a part of a much more detailed and sophisticated decision model. In the first instance, the model would be useful to an industrial or commercial concern or to a military installation comparing materials proposedby vendors as substitutesfor existing materials not conforming to regulatory requirements. The second application would be for screening as part of an in-depth decision process for changes to production operations, or comparison of newly synthesized materials for possible large-scale application throughout a major industry. For maximum utility, the Navy model is adaptable to either simple manual computations or computer applications.
2. Description of the Algorithm The algorithmis used to assign numerical “points” for various hazard descriptor elements such as toxicity, duration of personnel exposures, number of persons exposed, related medical effects, fire and explosion potential, requirements for personal protective equipment, anda limited assessmentof environmental impact and control requirements. These latter include volatile
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Meyer
organic chemicals, EPA Reportable Quantities, and EPA’s List of Lists materials (40 CFR 302,4) among others. The “points” assigned are totals that providea numerical score anda risk assessment code (RAC). This provides for determination of a hazardous material selection factor (HMSF), which allows for materials to be compared with one anotherin numerical terms. The resultcan then be used as an entry point into the foregoing overall decision process. The input data are readily available. Principal among these are the Material Safety Data Sheets (MSDS) required by 29 CFR 1910.1000, OSHAPermissible Exposure Limits (Table 2 29 CFR 1910.100), and EPA Publication 56014-90-011“Title 111 List of Lists.” The RAC procedure is based on a commonly used system safety analysis method (MIL-STD-882), and the basic approach to the “point” algorithm is the previously described DoD system for rating occupational, safety, and health hazards.
B. Understanding the Basis for the “Points” The following brief information providesan understanding of the basis for selecting the range of numerical values for the algorithm’s points. 1.
Toxic Effects The evaluation should include the frequency and duration of possible worker exposure. This includes whether the material presents toxic hazardson brief, short-term exposures associated with high concentrations and accidental releases or primarily causes harm from extended exposure to relatively low concentrations. Materials that are irritants skin or sensitizers or that are suspect or known carcinogens, teratogens, or mutagens require special attention even if the projected quantities are small. In many instances, the MSDS will only summarize the toxicity data of the individual components of the mixture andwill not provide information concerning specific toxicological studies on the material itself. In such cases, judgments will have to be based on consultation with also must be given such approved sourcesas the Navy Environmental Health Center. Attention to any information indicating that the material is a known skin sensitizer or possesses allergenic is the National Institute of properties. A suggested source of reference regarding toxic hazards Occupational Safety and Health (NIOSH) Pocket Guide to Chemical Hazards available from the US. Government Printing Office. 2. Characteristics Physical Characteristics. Materials with a high vapor pressure are more likely to be easily dispersed into the environment than those with lower vapor pressures. Those with low flash point and low boiling point (flash point lower than 73°F and boiling point below 100°F) are extremely hazardous froma fire and explosion viewpoint compared with those with flash points greater than 100°F. Liquids with specific gravities less than 1.O present fire-spreading hazards because such materials float on water. A “toxic material” with a high vapor pressure is more of a hazard in a confined work area than one with the same toxicproperties but a much lower vapor pressure. This is because the higher vapor pressure will afford a greater risk of room atmospheric contamination. ChemicalCharacteristics. Wheremixtures are involved, it is importanttounderstandthat those that include aromatic organic chemicals are generally more toxic (and often pose greater fire and explosion hazards) than those classed as aliphatic chemicals. Among the chemical characteristics that must be considered are stability, reactivity with other chemicals (forexample, is the material an oxidizer or corrosive?), and solubility, not only in water but in other media.
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Circumstancesof Exposure. In addition to the specifics of probable work areas, questions on the distribution of material throughout theweapon system life cycle or on-shore activity need to be considered. Localized use (in a single work area) of a material determined to be highly hazardous presentsa different setof concerns with respectto approval decisions than those that apply to a material with moderate hazard potential that is widely used. Among the considerations that should be examined are size of the work force or number of persons at a work site, present and/or needed engineering or other controls, and work area environmental conditions that affectthe hazard (temperature, humidity, the presence of other chemicalsthat may be synergistic or additive, etc.). During a general review and evaluation of a proposed material, questions need to be examined with respect to the interactionof the proposed material with others already approved and its use in the system or work areas and with nearby operations. For example, it would be a mistake to approvea new cleaning solvent with a high vapor pressure and low flash point for use in shops in which arc welding is conducted. EnvironmentalImplications. The potential for hazardous waste (HW) generation and compliance with various federal, state, and local codes, standards, and regulations must be evaluated. Insomegeographical areas, regulationson useand/or release of volatileorganic compound air pollutants are very severe and may require special controls if a material is approved. Similar concerns must be examined with regard to air quality and water permits. Because of the, widevariety ofsuch requirements,the “points” used in thismethod are simplified. More detailed ratings may have to be developed by the user for some analyses.
V. CONCLUSIONS AS TO UTILITY OF THE METHODOLOGY The hazardous material substitution algorithm developed for the U.S. Navy has been tested extensively and found to be a useful first screening tool.It also fills the need for a wide variety of applications in the civil sector. As indicated earlier in this chapter, itis only one element of the decision process. It is also essential to note that although one goalmay be the elimination of hazardous materials that affect people or the environment, in many instances complete elimination is not feasible. The selection method, and other considerationsin the decision process, provide fora rational and cost-effective determinationof the most suitable material. As stated by EPA (Pollution Prevention 1991, EPA 21P-3003) and the Pollution Prevention Act of 1990, when pollution cannot be prevented, reduced at the source, or recycled, it “should be treated in an environmentally safe manner . . . and disposal or other release to the environment should be employed only as a last resort and should be conducted in an environmentally safe manner.”
ACKNOWLEDGMENTS This chapter is based in part on AFMA-TR-91001, Development of Guidance for Selection/ Substitution of Less Hazardous Materials, for the U.S. Naval Supply Systems Command, underUSAFcontractF3361589-D-4003,Order16,A. F. MeyerandAssociates,Inc.with Engineering-Science, Inc. Publication rights to this chapter are retained by the U.S. Government. Copies of the basic technical report canbe obtained from Defense Technical Information Center.
REFERENCES 1. Patty, F. T.,IndustrialHygieneToxicology, Vol. 1 , Inter-SciencePublishing Co., Chicago, 1948. 2. Meyer, A. F., Jr., Engineeringbiotechnologyinoccupationalhealth, Trans. Am. Soc. Civil Eng., 121: Paper No. 2798 (1956).
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U.S. Department of Defense, Deparfment of Defense Occupational Safety and Health (OSH) Programs, DODI 6055.1A. 9 Sep 87. 4. Sampaolo, A., and Binetti. R., Regulatory Toxicology and Pharmacology, Vol. 6, 1986. 5. U.S. EnvironmentalProtectionAgency, Pollution Benefits Manual, October1989. 6. U.S. Navy, Naval Supply Systems Command, Development of guidance for selection substitution of less hazardous materials, Tech. Rep. AFMA-TR-91001. 1992. 3.
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Multiple Approaches to Environmental Decisions
Douglas M. Brown The Logistics Management Institute Bethesda, Maryland
1.
THE IMPORTANCE OF DECISION MAKING
It would be difficult to overstate the importance of the environment as a policy issue. Aside from the ecological implications of decisions in many “nonenvironmental” policy fields, environmental policies have impacts on other policyfields. The recent controversy over whether jobs of timber protecting the spotted owl should weigh more or less heavily than protecting the industry workers is not going to be solved here. The important thing is to realize that environmental policies, often considered to be based on scientific analysis, must include consideration of nonscientific issues such as fiscal realities, economic growth policies, and cultural values. Even race has surfaced as an issue in this field [l]. Because of the weakness of the current state of the art in fundamental measurability of environmental policies, an appreciation of the impact of such policies can only be hintedat by using other proxy measures. While environmentalactivists prefer to see environmental protection as a universally superior good not subject to such comparisons,the fact is that protective activities incur costs. Whether theyare continuing expenses or just investments that will result in lower costs later on is a matter of interest, but it does not relieve societyof the obligationto pay the bills until the investments mature. In the end, all policy costs are experienced by society’s consumers and taxpayers. Individual firms, of course, can be punished with criminal sanctions or forced into bankruptcy over environmental breaches; but as a rule governments and entire sectors of industry simply pass the costs along in the form of coerced tax hikes or industry-wide price hikes. Thus, neither government or industry (in a wide sense) “pays” for environmental protection except to the extent that when consumers or taxpayers find themselves with no more money to spend, the popular taste for government or the industrial product may evaporate. Those who believed thatthe 1970s’ Great Society programsor the 1980s’ defense buildups nearly achieved this national policy bankruptcy point should look closely at the environment. The cost to the taxpayer of dealing with environmental problems is expected to exceed 6%of 25
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the Gross National Productby mid-decade. That is the size of the entire defense budget during the peak of the Reagan buildup. Simply cleaningup known hazardous waste sitesat federally owned facilities is expected to cost over$125 billion [2], with the figurebeing adjusted upwards every year. Newly recognized threats are being discussed that will only add to the potential size of this burden of this burden[3,4]. In addition, consumers will bear anadditional burden. Some is hidden in the priceof consumer goods, as those private-sector firms that continue in business rather than declaring bankruptcy either pay for required cleanups, self-insure against the need to clean up in the future, or develop new processes to avoid becoming a party to a future cleanup. In some cases, businesses will choose to go out of business rather than risk personal or corporate liabilities of staggering proportions. At the least, this will reduce the number of choices available to consumers, and at the worst, employees will be thrown out of work and further impact on other taxpayers. All in all, there are compelling fiscal and social reasons for ensuring that our very real environmental problemsare identified and dealt with in prudent and responsible ways. Money spent on the environmenteither directly by governments on behalf of consumersor indirectly by consumers through higher prices chargedby producers as a result of regulations, cannot be spent on other worthy causes such as consumer and national savings and debt reduction, urban issues, transportation networks, and national security: whatever your policy preferences are, environmental spending competes with it. And, if not properly thought out, environmental policy can compete with itself. For instance, the EPA has spent years convincing the public that toxic wastesites are a tremendous sourceof health risks and must be dealt with promptly whatever the cost. The publication of the Unfinished Business report [4] requires the EPA to reeducate the public that in its new view many other threats are more risky; EPA competes for funding for those higher risks against an established, costly effort that the EPA itself established and plansto continue. A micro version of the same argument canbe made at the level of the individual producer facility. Statutory responsibility or not, an organization cannot devoteso many resources to environmental protection that it can no longer afford to remain in business. Environmental actions, even where deemed socially worthy, must compete for funding with other programs, and where the available funding does not cover all perceived needs, then environmental spending itself must be prioritized. In short, for regulator, policy analyst, and facility manager,a sound basis for making environmental decisionsis essential to the development and effective execution of a holistic, complex, and credible program for the protection of health and resources. While some may argue for a policy based strictly on scientific evidence, others argue for environmental policies based onemotion, and yet others argue thatthe costs of delay on the one hand and regulation on the other are socially destructive, environmental managers are faced with a situation where something hasto be done that will satisfy all sides without bankruptcy. Thus, while decision theoryis not the cornerstone of environmental science, it may well be the keystone of environmental management.
II. ENVIRONMENTALROLES Environmental threatsare produced and dealt with by organizations whose missionsare broader than simply protectionof the environment. Environmental agencies, however, havethe mission of ensuring that producers do not forget their environmental responsibilities. Those responsibilities are to the third player in this process: the public. Through the political process, the public caused an environmental policy to be put in place to protect healthand the environment, and at the grass-roots level the public maintains oversight onthe specific actions of both reg-
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Table 1 PolicyRoles Characteristic PolicyviewpointSinglefocus on narrow portion of environmental issues. Mission pollution Prevent all and punish all polluters. Objectives Focus resources worst on Resources Cleanup
Approach Policy effect
threat. costs do not detract from primary mission. Extract payment for pollution. Deters, does not repair, problem.
Complete operation with environment as part of the whole. Produce products and pay for environmental protection. Minimize resource drain. Cleanup costs are taken from funds otherwise available for mission. Focus on avoidance, then cleanup. Avoids, evades, or repairs problem.
Does not generally under-
stand or get involved.
Seeks products and protection. No perceived threat is acceptable. Pay in either case, through taxes or prices. Passivity until aroused; then paranoia. Suffers consequences.
ulators and producers. The differencesin the roles of regulators, producers, and the publicare summarized in Table 1. The most familiar enforcement agency is a police department. It has a single focus on a statutory area (in thiscase, public order). The primary responsibilitiesare (preferably) to deter criminals, which itself deters further crime or, when that fails, toseekoutandapprehend crimes. In addition to fines levied through the punishment process, such departments may collect fees to recoup their costs of doing business, thereby reducing the burden on their budget (and, in theory, on the taxpayer). Frequently, however, there is no payor, either because the guilty party hasnot been apprehended or because the enormityof the crime makes financial restitution, even with damages, unacceptable or so high as to be unpayable. Such cases, which are the norm more than the exception, make it necessary for the department to absorb the cost of enforcement; but such expenses are budgeted for and appropriated over and abovethe cost of normal operations, not at the expense of those operations. We expect them to deter and apprehend, not to repair, the problem of crime. In those few jurisdictions where victim compensation is considered, it does not come out of the police operating budget. It is not generally expected that the enforcer will have to police itself (although such occasions do arise, and have arisen in most of our major cities over the past 15 years, and are generally poorly handled). In some jurisdictions, we see a cooperative enforcement approach [5]: “community policing” or the “cop on the beat,” tomatch the environmental metaphor witha police example. Nonetheless, thosecooperative approaches are part of an overall enforcementstrategy; the regulator coaches the producer toward compliance rather than taking over the operating responsibility itself. Finally,we expect the police to focus on the worst problem-catching murderers rather than staking out shoplifters. The producers have a completely different set of responsibilities, the foremost of which is the factthat they must continue on with their production task in order to survive; environmental issues are a secondary concern. When a cleanup does become necessary, the producer must pay, and its payment comes out of its normal operating expenses. To some degree, consumers will absorb someof this cost, but in general, passing on too much of the cost will simply drive
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the producer out of business (unless the producer happens to be a government agency). With limited discretionary funding available for environmentalrestoration, then, producers need to accomplish as much as possible with the resources they have. Normally, government regulatory agencies act as enforcers. Once enforcement has occurred, however, one is faced with the need to restore the situation. Then, and especially in the case of the Superfund, the government becomesa “producer” and needs to act and think like one. Finally, there is the public. The public tends to be easily excited over health and safety issues, although a much smaller (but more active) group maintains vigilance overnon-human health and natural resources, anda very small groupis both active in and knowledgeable about global ecology issues. Inaddition, the public is concerned aboutjobs, general economic issues, property values, and the quality of life in communities. Thus, the public concerns tend to be more diffuse than the single focus enjoyed by enforcers and producers. Because of that diffuseness, the public seldom speaks with a coherent voice, which makes it easier for activists and extremists on all sides of an issue to misrepresent or override the public will. While the federal government’s National Environmental PolicyAct provides processes for public involvement, as do a number of state statutes, there is at present no real requirementto go along with public preferences as long as the pro forma requirementsare met. Thus, on any given decision, the public can be and often is ignored. The more this happens, of course, the more the public comes to see the regulator as well as the producers as its enemy, especially as these parties will be on different sides at different times. Given this disparity in roles, it may come as no surprise that there are different perspectives on what the general objectives of the environmental effort shouldbe. Ihave identified seven primary goals; others may exist. While most of them appear desirable, at least in isolation, they are not all consistent. They are presented here in no particular order; indeed, the ordering process itself is one of the most important facets of the environmental decisionmaking process. Risk. Eliminate risks to human health and the environment. Cost. Engage in environmental projects that achieve organizational objectives without posing an unacceptable risk to the organization’s economic competitivenessor viability. Time. Accomplish objectives rapidly, if in fact there are risks to the environment and especially to our health. Acceptability. Satisfy publicandorganizationalexpectations. All environmentaldecisions, whether takenby public or private organizations, occur underthe observation of a political structure and still remain consistent with the organization’s own value system.’ Deterrence. Ensure that environmental offenders, and particularly the worst offenders, are caught and prosecuted. Administration. Minimize the debate over the intent and application of environmental laws and regulations. Practicality. Develop policy alternatives that are executable, compromising the ideal to maximize what can be accomplished. If the budget is unlimited, decision making or prioritization is unnecessary: one Simply does everything that is wanted as soon as it is feasible to do so. However, increasing sophistication in regulations and increasing effectiveness of environmental compliance efforts are ‘This objective was titled“Politics” in earlier work [l]. Some managers objectto the idea that “politics” intrudes on the pristine pursuit of public and ecological health but feel quite comfortable with the need to tolerate public value systems. “Acceptability” seems less threatening.
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Table 2 DecisionApproaches Knowledge Multiple-rule solution; (one-rule) Universal Certainty solutions (e.g., cost-benefit) Statistical uncertainty Static Dynamic Outcome uncertainty
Expected value Simulation Aspiration level Subjective decisions Minimax
uncertainty Total allocation Political
combining to produce requirements estimates for environmental work that are increasing exponentially. For federal agencies alone, workingoff existing project requirementsand dealing with a number of pending interagency agreements with the EPA or stateagencies could require three- to fivefold increasesin current environmental spending, even assuming that the historical inflation of environmental costs can be controlled. But as soon as we say that some projects expectations, the question of priorities will have tobe delayed to meet more reasonable funding emerges: which projects should be delayed, and why? In short, under reasonable resource conditions, not all of these objectives can be accomA total focus modated in full simultaneously. A blind emphasis on speed usually wastes money. on scientifically assessed risk may be impractical if the necessary science is incomplete. Administrative simplicity may be translated into rigidity, resulting in the carrying out of regulations blindly, resulting in large costs with no appreciable improvement in the environment. And so on. It is in just such cases-multiple, conflicting objectives-that the use of decision methodologies is needed.
111.
DECISION-MAKINGMETHODOLOGIES
This chapter is not intended as a single referencefor explaining the details of decision theory. Such textsare available-indeed required-in every college’s management course work.’This chapter proposes rather to explain why decision analysis is needed in environmental decisions as much as any other. In general, decision-making methodologiesare selected based on the degree of uncertainty surrounding a condition or situation; Table 2 shows that alignment.
A. Certainty Under the condition of certainty, we know eachof the outcomes; it is simply a matter of choosing the programthat benefits us the most. In that case, there is a universally superior solution or decision rule. Even where such a rule is deemed to exist, it must be tested to determine its universality and the existence of underlying rule structures, which may require reversion to a more complex decision approach. Forinstance, the accepted rule may be that projects that reduce the most human health risks will take priority over all others.The real world presents us %ere are too many such texts to list, and each university has its own preferred texts. Each manager has, or should have, several. On my bookshelf, perhaps the most frequently used is Fleischer’s Engineering Economy [6]. Others include Lapin’s Quanrirarive Methods for Business Decisions [71, Quade’s Analysis for Public Decisions [g], and Douglas’s Managerial Economics [9].
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with resource constraints, not necessarily limited to constraints on ready operating funds: the total capacity of administrative, technical, industrial,capital, and labor resources formsan effective obstacle to unlimited activity levels. Given a clear goal and normal capacity constraints, the basic rule can be expected to meet a quick challenge: if two projects are equally effective in risk reduction but there are not enough resources to accomplish both, anothercriterion must be applied as a tie-breaker. And, as a result of limited resources, at some point the residual funding may be adequate only for a low-cost but also low-priority project while a higher priority but more expensive activity must be forgone. Given a high degree of certainty, we know all potential cases and all potential outcomes (and the relationships between the two). Then rule-based decisions are the appropriate solution to ensure that the best possible outcome is achieved from each possible situation. The decision makers can incorporate multiple factors simply by adding more complex rules, but in a rule-based system the rules cannot be waived.Norshould there be anyreasonto do so, because all possible cases and outcomes can be predicted and the best course of action identified. Generally, we do not have such perfect information. Nonethelessmany environmental decisions are based entirely on methods that address certainty. This occurs for two reasons: the real information is unknown or the real information is too complex to be usedin its full detail. When the real information is unknown, decision makers should apply appropriate decision approaches under uncertainty, suchas those displayed in Table2. However, environmental decisions are frequently made by bureaucracies (governmental organizationsor large industries) that do not subscribe to subjectivity or political acceptability as an explanation for how decisions were reached [lo]. Thus, where the real information is too complex to be analyzed effectively (as is often the case), managerssimplifyittoits essentials [ll]. If a modewill describe the behavior of a natural phenomenon adequately(e.g., saying thatthe prevailing wind is at 5 mph from the east, when in fact over the courseof a year it blows from mostdirections at various speedsfor some periodof time), and if the resulting policiesdo not appear to be too badly flawed, the approach is validated in terms of the value of the time and effort requiredof the manager. Indeed, contrast this behavior with the warning to remain consciousof the value of perfect information issued later in this chapter. Again, however, true certainty seldom prevails.And in acting “as if,” we are simply making assumptions: takinga mode to be the only possible outcomerather than the most frequent one. The real situation may take oneof four alternative forms, representing increasing degrees of uncertainty. Statistical uncertainty means that wehave a good idea of what might happen and that we have some probabilistic statements of how, when, or how often. Technically speaking, this is called decision making under conditions of risk, but because of the extensive use of the word “risk” in the environmental sense of a threat, I have called it statistical uncertainty instead.
B. StatisticalUncertainty When statistical uncertainty exists, we must use our statistical knowledge to make decisions “as if” the outcome were to be as predicted by the probabilities, with appropriate consideration for the fact that it might not. This is basically done using an expected value approach, which some of you may know as a weighted average approach. Rule-based approaches become meaningless ifwehave uncertainty, because we do not know what situation exists or which rule should be applied. Despite the masses of data collected to support science’s continuing assault on the mysteries of the ecosystem, we remain extremely uncertain in our understanding both of the system as a whole and of most of our
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individual “facts.” There is actually nothing wrong with an uncertain situation; in fact, our economic system and our national security systems function tolerably well under admitted uncertainty. The Apollo missions reached themoon and returned safely under conditions of uncertainty. Decisions under uncertainty are quite possible: the recognition of uncertainty simply requires some adjustments to our decision pattern. Indeed, the bulk of decision theories address this uncertainty directly, either in factoring in the possibility that multiple outcomes may occur (through weighting) or through safety factors. The more serious error is to conduct business under uncertain conditions as if there were no uncertainty. Unfortunately, in many cases, environmental policies have been devised and implemented in exactly that manner.For instance, one of the primary“rules” that dominated implementation of the Superfund program was the “worst-first”rule, under which priority of effort was givento the most hazardous known contaminated sites. Thisrule was implemented through a detailed process resulting in a numeric score that was deemed at the time to be somehow a measure of the site’s risk. As detailed site investigations were pursued, it began to appear that in many cases the scores did not reflect risk at all [12]. Part of the reason for this is that so that the worst are probably not first[13]. the scoring system used is mathematically skewed Subsequently, the EPA issued a revision of its scoring systems, but EPA is refusing to rescore the sites now found on the National Priorities List. Although the reasonsfor that decision are more related to politics and face saving, it forms a good example of the fact that under uncertainty the desired or statistically most likely outcome may not in fact occur. In addition to using expected value methodologies to overcome statistical uncertainty problems, the technique for calculating the valueof perfect information is one with which all environmental decision makers should be familiar (at least in concept). The specific equation is not particularly relevant, because there is often no “variance” (one of the terms in the perfect information equation) in environmental data, which tend to be highly site-specific.But a great portion of the funds in environmental activities are expended on “just one more round of testing,” a round that generally proves as inconclusive as the original round. There are a numberof activities that are worthwhileof themselves but very much subject to the question of perfect information. Facility managers are asked to comb large tracts of land looking for endangered species that might be there but often are not. Products are banned or consigned to expensive disposal programs because they might be dangerous. Although protective measures are laudatory, all environmental professionals (whether they are regulators, fato be conscious of the fact thatat some cility managers,or taking the public’s perspective) need point enough is enough. Part of any action discussion should be an explicit understanding of what “enough” is, what (specifically) is expectedto be gained by achieving “enough” as opposed to some lower level of effort, and what the costs and other implications of getting to “enough” will be.
C. DynamicUncertainty Many managerial texts address uncertainty as if it remained constant, albeit unknown. In the environmental world, things do not remain the same. Technology advances or is discovered to be ineffective, regulatory requirements change, the ecosystem changes, and specific activities evolve. Additionally, many decision theory models assume a single decision, even if a protracted and complex one. Environmental compliance activities often do not fit this mold. They occur within a continually evolving process where the outcomeat any point is dictated As a result, more complex modonly inpart by the objective facts of the original circumstance. els than the basic decision tree are needed to deal with the statistical uncertainty facedby environmental managers.
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As an example, compliance requirementsfor wastewater treatment facilities have changed only superficially over the past two decades. Therefore, techniques of static statistical analysis are useful when looking at a large population(the view enjoyed by the EPA of a large number of wastewater plants, for instance) because the baseline for sucha view has been well established and changes slowly. And EPA-level decisions, because of the mission of regulation and enforcement, addresshow to regulate entire populations andhow to scan those populations to identify recalcitrants. For a single facility, the domainof most environmental managers, this aggregate information is of little value. Decisions are not made on how to comply; that has already been spelled out. Generally, they are not made on whether to comply. The question is how to maintain the operation in compliance with no deficiencies, or at least as few as are practical, in the face of any number of unpredictable events. The ability to comply with regulatory requirementsis only partially a result of the original adequacy of a plant’s technical design (the only static aspect of the operation). Much more important are a myriad of specificcircumstances arising, or not arising, onanygiven day. What is needed is a model of the entire system in operation, from water flow production to effluent disposal, so that each point wherea problem might occur can be identified and dealt with. Even wherea violation may occur, itdoes not follow that an enforcement action will result. That too isa dynamic process, dependentin part on theplant’s record from the past, on teambuilding efforts by the regulated facilitystaff, on the sheer coincidence of inspection scheduling on days when the facility does or does not suffer a reversal of fortune, and on whether or not the facility has implemented anyof the improvements recommendedby the regulators on their previous visits. This situation, in which the probabilistic variables themselves vary over time, can only be of a scenario over multiple addressed by simulation tools. Such tools can represent the running iterations to represent the effects of the passage of time or to try out the effect of assigning different probabilities to variables.Thus, in addition to the decision trees and contingency tables often found in managerial textbooks, simulation techniques must be employed. Generally, managerial texts restrict their discussion (if any) of simulations to the Monte Carlo technique. This does providea very powerful tool. However, even in its most basic form it requires recomputationof known equations in a number of iterations, which effectivelydemands automated tools. As simulationsare being applied to increasingly challenging problems in industry and commerce, the computational power of simulations is being enhanced with graphics to provide comprehensibility to problems and solutions thatwould otherwise be nothing but piles of computer printout. For environmental purposes,which tend to address problemsan order of magnitude more complex than industrial process modeling, dynamicsimulations that incorporate visual effects are needed both to complete a reasonably accurate representation and to enable the functional manager to see what the computer is trying to communicate. Itis also important to understand that elaborate graphical presentations, even though they may be very data-intensive, are not the useful to a rational decision processunless they canbe used to develop relationships among data. High-end geographic information systems generally display informationin multiple layers, making it accessible to intuitive analysis. They may provide a database management system that permits the display of selected information, but usually they have no capacity for mathematical analysis. Simulation tools are very rarely found in environmental use, despitetheir obvious utility. One reason is the complexity of environmental issues,which often forcesa long tool develop-
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ment time. Also, the policy system under which simulations are developed (asmany withother areas of environmental activity) allows for endless researchand refinement rather than product fielding. Another reason is that a circular logic is in place. There is understandable reluctance at the EPA to certify any software products that have not been thoroughly validated by the EPA. However, the experience of many producers with the EPA’s unwillingness to accept anyway but the EPA way has made them wary of attempting to present EPA with any new products or approaches, and warier still of investing in the creation of new suchways until the EPA approves the product for general use. As a result, managerial innovation is restricted. The final reason is that formany environmental managers on the job today, the computer remains a fearsome tool. But the very complexity of environmental issues and the increasing cost of environmental solutions will soon make an effective decision-making process dependent on the ability to exploit the power of automation. More familiarity with the use, applicaof environtions, and abuse of computers in general and simulations in particular is required mental professionals.
D. OutcomeUncertainty Outcome uncertaintyis technical talk that means basically we have no idea of whether, how, or when things might happen, although we may be able to make some guesses as to what the things that might happen are. Under outcome uncertainty, we are forced into more subjective decision-making processes. The “aspiration level” approach [6] is seldom acknowledged but frequently seen. In such an approach, not having confidencein predictions of what may happen, and finding the risk unacceptable, we decide t6 guard against the worst possible event. U.S. defensestrategy:anuclearattack by theformerSovietUnion Thisislikethe wasalwaysconsideredextremelyunlikely,butthepotentialdamagethatsuchanattack could cause was so great that we spent enormous sums protecting ourselvesas best we could from such an event, even though we knew that by doing so many other needs would simply have to go unaddressed. The present worst-first policy is also an aspiration-level policy: it assumes that we know little about costs and remedies of cleanups, but we believe that we have identified the worst situations and we are committed to removing those situations, beginning with the worst. Even under such conditions, however, cost-’benefit considerations are at work, although less obviously; they are squeezed in through the back door using the potential for public outrage as the vehicle. We accepted the cost of the defense programas necessary in conceptand generally affordable. And our national policy is to save endangered species. However, where a human community must give up its current livelihood in order to save a species (as is threatened in the effort to preserve the spotted owl habitat), the regulation enters the political arena where the EPA may win or it may lose. The contest willbe presented to the public on the one hand as preservation of the quality of human life in preference to unproven allegations of harm to what is onlyone of millions of species, and on the other hand as preservation of defenseless creatures against greedand callousness. The essence of this argument is the cost-effectiveness of the effort: is the public willing topay the price for a particular environmental project? Another methodof dealing with uncertainty is subjective decision making using group processes. There are any number of approaches available, from public meetings and roundtables to expert opinions eitheras individual contributionsor controlled through a Delphic process.An example of the weighting of subjective preferences is seen in the U.S. Department of Energy (DOE) approachto its overall capital facilities investment strategy[14]. Successful results (as
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proven by the subsequent accuracy of the predictions) have been experienced with using an even more structured approach known as the analytical hierarchy process (for instance, our work on community fiscal impact analysis [15]). TheEPA has begun looking into that process in its negotiations withthe U.S. Department of Energy over howDOEwill conduct its cleanup operations [16]. The process in a nutshell is that stakeholders and/or experts come to agreement over the relative significance of factors taken two at a time;at the end of making many such comparisons, the relative significance of each of the factors can be mathematically arrayed into a numerical weight table. Another perfectly valid approach under uncertainty is problem avoidance, an extreme form of the minimax principle in which we minimize the maximum cost without much concern for probable benefits. If the issues and the costs are imperfectly defined, or if the impact of the events (including enforcement) is seen as unlikely or highly arbitrary, the whole system may not be worth worrying about. It is probably cheaper to be dragged intocourt from timeto time precisely the strategy than to go around solving problems that may not exist. In our view,is this in place among many producers today. We have named it “problem avoidance,” although one could also characterize it as foot dragging or passive resistance.
E.TotalUncertainty The final possible approach, under total uncertainty (if we have little confidence in the risk data or the cost data), is to simply allocate the available funds on some arbitrary or politically acceptable basis and hope for the best. That division may occur on a social basis or on a geographical basis,and there are some suggestions that this is exactly what is occurring, either deliberately or as a consequence of the problem avoidance strategy. However, using similar arguments to achieve the opposite result (as in the present enthusiasm over racial equity in environmental issues) is analytically no more pure than the original failure. The approach is not covered as fully as others in this text because such issues are not amenable to environmental management and because there is some evidence that, at least in the Superfund probe gram, fundsdo flow to higher scoringsites [17]. However, the environmental manager must aware that such a schoolof thought exists and that when skillfully used it can awaken powerful political pressures.
W. HOW CERTAIN IS ENVIRONMENTAL SCIENCE? The range of approaches notedin Section I11 varies according to the amount of certainty that is attached to the situation. Clearly, it varies or there would be no need for so many approaches. In this section, we briefly consider some of the issues that lend uncertainty tomany cases. to considerable controversy. Each of the major objectives to be accomplished is still subject Research needs to be applied in each of those areas if we are to move forward effectively in managing the environment andto move from the imaginary certaintyof acting “as if” (when really acting under outcome uncertainty if not total uncertainty) to at least acting under statistical Uncertainty. The following brief summary is provided hereto allow the environmental manager a glimpse of all that we do not know.
A. Risk The definition of chemicals as hazardous is done through a series of separate environmental regulations, and there is little comparability of threat. In many cases, two chemicals have overlapping effects levels: the question becomes whether it is worse to die immediatelyor to get
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lung cancer. In such cases, how do we establish “worse”? Even when we stick with a single chemical class, there is a great deal of doubt as to what risk, if any, a regulated material poses. While the chemical data are confusing and contradictory (see, e.g., reference 18, from which the original Hazard Ranking System (HRS) factors were derived,or reference 19), we cannot always rely on the accepted approximation systems: current measurement systems (the original and revised HRS) cannot be shown to approximate risk. In our earlier paper [13], we showed that the original HRS was mathematically inconsistent, leading notto the potential but to the actuality of score inversion. While the’revised system corrected several glaring deficiencies of the original system, there are enough departures from the generally accepted threat-pathwaydose-receptor paradigm to raise doubts as to whether risk is related to cost under the revised scoring scheme.
B. Cost Given a reasonably accurate description of the problem, a reasonably accurate cost estimate A common criticismof current modshould be possible for the engineering cost of the remedy. els is that they are usually off by 50% or more and usually understate the final cost. This is partly because of the lackof data points, partly a resultof inaccurate input, and partly dueto the continuing ballooning of legal intervention costs. However, it should be noted that a 50% error may be less than the current errors in risk estimation.
C. Time Time has interesting impacts on environmental decision making thatto need be explored. Time, for instance, is tied to risk issues. Givenlow risks now and high long-term risks, or moderate transient risks now, how do we choose? What about the tendencyof pollution problems to expand over time? Time has extensive impacts on the cost side of our decision model. What does the slope of the environmental learning curve look like? What is the real rate of environmental inflation, and can it be separated into its components: engineering, legal, procedural, and so on? Even more significantly, what would be the most appropriate way to integrate the concerns into a decision model, even if the answers were known?
D. Acceptability A great deal of work remains to be done in the integration of legitimate political and public concerns into the decision process. If political distribution of benefits is a real show-stopping issue, how can that be integrated into a decision process in an open manner so that it can be assigned a proportionate role?Is the distribution of pollution problems consistent with industrialization and hence with population density, and does this ensure an acceptably proportionate share in the program?How do we distinguish between self-interested “NIMBY-ism” and valid public safety issues, or does it not really matter?
E. Deterrence Generally, deterrence is achieved through a combinationof other objectives: effective definition of the risk factors, approaches that maximize the benefits of compliance, and a smooth A great deal of research is going administrative process that makes apprehension more likely. on today on the question of the right amount of fees to charge polluters. Less research addresses and what the costs of checking the questionof how to make producers stop polluting altogether on it are.
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F. Administration Research into the other bureaucratic goal-administrative smoothness-would be well served. The primary attraction of the “as-if‘’ approaches are that they are very easy to administer. HRS, for instance,is self-operated by polluters or agency contractors;sites simply go on a list. No effort has to be expended by the agency on the messy questions of cost, time, risk assessment, and so on; all of that is up to the polluter. From the producers* perspective, problem avoidance has advantages administratively because one hasto fund only whatever the agency requires, so there is little need for decision making and a good probability that the major exof time. But the primary pense of restoration canbe spread out overan almost indefinite period reason for such behavior is that the process required to do anything else is so convoluted that it is not worththe cost. The simple actof declaring that a site should not be listed as dangerous becomes a very expensive process. Research is needed to address the valueof each of the existing steps of the process and todetermine whether it is possible to design a less cumbersome administrative process with less reliance litigation on that still protectsthe interestsof the agencies and the producers.
G. Practicalities Practical problems include the capacityof the environmental industry to handlethe workload, to handle projects above a the availability of technology, the capability of project managers certain size and speed, and the limited experience at all levels with actually executing many types of environmental activities. of today’s feasible actions. Are There are plenty of other practical issues within the scope existing contract vehicles adequate? To what standards should contractors be held, and how effectively can they be managed? What are the lessons learned from past programs, andhow are they being disseminated? Answersto any of these questions would improve the process of planning and executing environmental cleanup.
V. DECISION APPROACHES
AND ENVIRONMENTAL OBJECTIVES
We noted earlier that there are several possible objectives for environmental programs and the people and organizations charged with carrying out those programs.How do the approaches we have discussed meetthose objectives? Table 3 compares the major approaches in terms of the objectives of the environmental program. The table shows a plus sign where the approach enhances that objective, a minus sign where it detracts from an objective, and a zero where it has no effect. The single-rule approach (exemplifiedby the Superfund worst-firstrule) addresses the risk and acceptability issues by focusing onthe worst cases. In ignoring certain other objectives, the single-rule case in particular engenders a number of practical problems. Most notably, ignoring cost issues results in a number of cases of self-protective evasion by responsible parties. In addition, an oversimplified approach ignoresmany other practical limitations, such as the capacity of the environmental industry to undertake many projects or the availability of reliable technology with which to do the work. And it ignores the value of time in terms of inflation, opportunity costs, and continued public exposure to hazards. More recent policies (if not many efforts) have focused on maximizing risk reduction[20], a multiple-rule policy based on both risk and cost. Cost as a consideration at least forces inestimates will need direct considerationof some unspecified issues. Effectively performed cost to consider practicality andthe value of time, as a minimum. Such policies will necessarily be
Management 37
Environmental Table 3 Decisions and EnvironmentalObjectives Static
Multiple (expected Aspiration Single Subjective mle rule Risk reduction
cost Time Acceptability Deterrence Administration
Practicalities
+ -
+
+ +
-
value)
+ + + 0 + + +
Simulation weighting level
+ + +
+ + + + + 0 +
0 0
-
+
Maximin Politics
+-
+ + +-
-
-
0
+-
0
+ 0
-
+
0
0
+ +
+
0
-
-
Key: +, enhances: - deqacts; 0, no impact.
more complexto administer and may, because of their complexity, be less acceptable. Indeed, in the chart shown in Table 3, the multiple-rule-based approach appears almost ideal, with many pluses and no minuses. But this assumes that the rules can be developed in such a way as to enhance the objective, in short, that we are operating with certainty, which as we have noted above is usually not the case at all. Operating under statistical uncertainty,we find that a more detailed understanding of the variables should result in a more targeted approach to the problemto be solved; thus risk, cost, time, and practicality are well addressed. The added complexity of the approach makes it less acceptable until it proves its worth. Likewise, deterrence may be stronger after the approach has been proved but will initially be poorer than a straightforward policy until it proves effective. And of course such a policy will always be complicated to administer. Introducing the dynamic aspect of uncertaintywould be a policy disaster without the appropriate tools. While an academic case might be made for the power of formal automated modeling and simulation in enhancing almost every objective except administration, the lack of appeal of a massive computer printout (often the only form of output available) would make it untenable to either politicians or bureau executives. However, given the right tool (a powerful simulation engine with an attractive graphic interface), this method becomes very useful. The scoring in Table3 reflects this latter case. The effectiveness of a good simulation can helpto account for practical constraints and can maximize risk reductions while minimizing time and money spent. This also maximizes the policy's deterrent effect. Effective graphics can make the conclusions more acceptable to both politiciansand the public, and a well-programmed tool will ease the administrative taskby making the decision rationale cleareror perhaps providing a recommendation. So, why is everybody not using such tools? As noted earlier, the EPA often refuses to endorse them, and wisely so because the state of the science is such thatone can barely be said to be operating under statistical uncertainty. A case of outcome uncertainty is more often the case. Earlier, three typical approaches were noted under outcome uncertainty. The worstfirst policy is an aspiration-level choice if we confess that not much is known about risks or that we care little for costs. This has the principal advantage of addressing the mission directly: every effort will be expended to meet it. This may also be well accepted until it starts to cost too much, which in the case of the environment it will. In fact, in every way, its strengths and weaknesses (as shown in Table3) parallel those of the single-rule-based decision; indeed it differs only in that the single rule assumes that the outcome is known, whereas the aspiration level accepts the risk that the outcome may not transpire but feels it to be worth guarding against. '
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Environmental management today is taking a close look at subjective decision making. The principal vehicle for this in recent efforts has been the analytic hierarchy process (pairwise comparisons). In addition to being used with some success (in terms of acceptability to activists and regulators alike)at the state level, it is under development now between DOE and EPA. It maximizes participationearly in the processby the groups that most delay action through legal and regulatory maneuvering. Because all parties have an opportunity to participate in the decision-making process, and a properly designed process will leave few completely disgruntled, all are co-opted into the solution. Those few who continue their challenges will have limited credibility given the ecumenical composition of those who have agreedto the solution. In addition, the process can create a rule-based framework with which to make decisions, giving the illusion of a quantitative universal solution under certainty. This capability also presents the biggest trap presentedby the method. As a rule, the process is conducted in conjunction with computer programs to present the questions, record the opinions, and perform the weighting calculations. The programs sell themselves with a pleasing and simple interface that encourages their use by novices, but the comparison structure must be set up very carefully. This has often been overlooked, with the result that the comparisons are inherently flawed. Even where the analysis is properly performed, the illusion of quantified objectivity remains. Decisionsby subjective processes necessarily mean that objective information is lacking or inadequate. The form placed on the structure simply provides a facade of quantifiability. In fact, however, the process is structuring a political decision, made in this case by consensus among expertsor activists rather than duly elected or appointed authority. While there is nothing wrong with that, it is important to recognize it for the political process that it is. In short, subjective decisions maximize political acceptanceand basically admit a lack of hard knowledge in other areas, especially the core objective. Finally, within outcome uncertainty, there is the minimax approach, principally represented in environmental policyby problem avoidance. This approach is characterized by conthe spread of definite tinually seekingnew facts while making temporary investments to control risks. Since nothing is actually accomplished by this, the final cost of a solution is unaffected, and becauseof the tendency for environmental problems to diffuse themselves that cost may be much higher althoughthe payment date is set further into the future. The primary positive outputs of delay are that it can be given an acceptable facade, through continuing studies (especially ifnobody isconcernedaboutcosts), and itminimizestheopportunitycosttothe organization, which is free to go about its business with minimal budgetary impact. In terms of our objective function, problem avoidance deals with neither risk nor effectiveness, but in a perverse sort of way it does keep an eye on costs, at least in current years, by simply refusing to do the needed work. Finally, there is the straightforward political approach. As far as we know fromthe limited studies available, thisis seldom seen (environmental equity authors might differ with that statement). An arbitrary decision by an administrator has the same effect. In suchcases, the basis for the decisionrules is very different from thosewe have noted so far and is unlikely tomaximize any value except political acceptability.
VI. APPLICATIONS OF THE DECISION MODELS In the remainder of this chapter we review five applications of decision theoryto a single enare the use of a singlerule and vironmental problem: investment decisions. Those applications the use of multiple rules under certainty, the useof an expected value approach in a dynamic statistical uncertainty mode, and the use of public opinion under uncertainty. However, the
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state of management science within the environmental field is such that only decisions under certainty can be reviewed in any detail: the other approaches are still being considered but do not as yet represent any official policy.
A. Certainty The first cases to be analyzed use rules to solve the capital allocation question. Theobjective of this exercise is to conduct remediationwork on a group of toxic waste sites that exceeds our ability to pay. One approach is to use a single rule: worst-first. Under that rule, all available resources will be focused on the site posing the worst risk to public health. Remaining funds will be applied to the next-worst site, and so on. An aIternative approachappliesmultiple rules, or at least a morecomplexrule.In this case, the rules are that all available funds will be devotedin sequential priority to the site where the risk reduction per dollar is the maximum. Once work ona site is started under this formula it must be finished. Thisrule is called the “results-first” rule [l]. The situation to be addressed is displayed in Table 4, where a third approach is also displayed for convenience. That approach, problem avoidance, is in a way a rule-based approach (do the minimum necessary for as long as possible) but, as was described earlier,is a manifestation of an approach under uncertainty.
1. Applying the Rules to a Hypothetical Problem Table 4 shows a notional five-site universe given a budget of 100 money units (call them millions of dollars, if you will). It provides the agreed risk (in whatever units) posedby each site and the estimated cost to restore each. Additionally, the table shows the cost of “doing nothing” at each site:these are the costs of the minimum essential security measures,studies, monitoring, and so on (not to mention legal fees) that must be carried out even when no action is possible or desired. Under the worst-firstapproach, only the site risk needbeknown.Wewould attempt to fund site A first; with luck, the sumof the studies over the next 12 years would result in a partial resolution of the problem, but it is more likely that at the end of the period we would have to spend another 800 units of money to execute the remedy (requiring several else could have yearstoaccomplish, at a rate of 100 unitsperyear).Meantime,nothing been accomplished because the balance of our funds each year (20 units) is insufficient to remediate the next site, B. Note also that remediation at either site C or E, which could have been completed with the residual funds, willnotbeaddressed because these sites are not “worst first.”
Table 4 Example of Approaches” Project
Risk
Cleanup cost
Risk/$
Avoidance cost
A
100 70
800 35
0.12 2 3 0.5 5
80 8 3
B
C
Db
E
60
40 20
20 80 4
Total budget available = IOO/yr. bPmject D is already under way. Source: Brown et al. [l].
40 1
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The table shows the risk reduction per dollar ratio, which isthemeasureneededfor a results-first decision. This is quite simply the risk dividedby the cost. There is one twist to this: project D was already started, andwe assume that 40 money units has already been expended on it. That is, its total cost was originally greater than 80 units and so its ratio was evenlessthanone-half.Unfortunately, we haveauniqueenoughsituation at Dthatsimply keeping it alive through studies is up to one-half as expensive as finishing it up (a situation that occurs more frequently than one might think). Using results first, we see that the best risk reduction per dollar gives the highest ratio, making our order of preference E, C, B, D, and E, C, and then A. Withthis preference, wesee that within our budgetwe can complete projects B. We also have 41 units left over to fund some of the work at A and D. By the end of the second year we would have completed all the sites except A, and we would be left with the same problem as in the worst-first case-the inability to restore A-except that while only being 2 years further behind at A (which had not really gone anywhere anyway), we have completed all our other work and in the process we may have learned something abouthow to deal with site A. us to fund studiesat all sitesand to finishD, since work Problem avoidance would require there was already started and it is too much trouble to stop. In either case, we do not have enough money to do the full studies requiredat A, so we skimp along on whatwe do have after funding everything else. This does happen in practice, since the durationand scope of tests and studies are fairly arbitrary and highly negotiable. The result of this strategy is that site D, which offers one of the lowest returns in risk reduction per dollaras well as being a relatively low ranking site, is the only one cleaned up, because every year the entire budget is consumed in studies. After4 years, we have spentas much in studiesat site E as we will eventually spend in cleaning it up-another event that is not infrequent in reality. Witness the fact that most of the sites removed from the National Priorities List have not been the “worst” sites that expensive studiesto have posed should have been first, but rather sites that proved after ofyears little risk.
2. ApplyingRealData Setting up Table 4 for illustrative purposeswas easy enough butmay be considered to be misleading. Would the superiority of a results-first approach be demonstrated with real data?An analysis was performed using the scoring data from1600sites containedin the EPA’s National Priorities List Technical Data Files database, and estimated remediation costs were extracted from the 523 then-existing records of decision (RODs).The result was a joint database of 412 unique sites with cost data (some RODs are for operable units within sites or are updates of earlier RODs). These data are acknowledged to be less than perfect. Aside from the fact that the HRS score may not really be a measure of risk, in some cases the factor valuesmay have been improperly assigned during the site assessment and scoring processes. The costs are quite the for operable units are fluid. More important, itis not always clear in the ROD whether data cumulative or separate (one can sometimes get enough of an impression to know that either is used on occasion), so that the decision to add to or replace the previously recorded cost is subject to error. The model uses an arbitrary cleanup budget ranging from 1 to 3 billion dollars per year over a 30-year period, reflecting the reasonable expected range of Superfund appropriations $2 billion). In addition, no more than an arbitrary limit of $5 (presently being scaled back from million per year maybe spent onany one site; this limitation was designed to preventmodel the from miraculously fixing a complex site suchas Commencement Bay or Rocky Mountain Arof cash.In fact, theenvironmentalprogram senalinoneyearwithamassiveinfusion
Environmental Management
41
Table 5 ResultsUsing NPL Data Worst-first Results-first Results ratio Sites cleaned HRS points reduced Total risk reduction including time effect
5 365 5,608
69 2,877 69,989
1 41 8:I 12:1
Source: Brown et al. [l].
has shown someof the same tendency to throwaway money underoverstimulation aswas seen in the Defense Departmentin the 1980s: sometimes thereis a limit to how much money one can spend wisely in a given period. Finally,the model recognizes the continuationof risk incurred by failure to remediate. Because of the funding limitby time period, and because of the life-cycle risk approach, the model requires an iterative solution based on the initial decision rules. It is important for environmental managersto note, however, that this relatively complex calculation canbe (and was, for this exercise) accommodated with plain old dBase 111 Plus rather than using a more complex simulationtool. The working of the model is described in more detail in reference 1. The simulation determined which projects were to be funded in each year using the onerule and multiple-rule methodologies. The results are shown in Table 5 . Clearly, a more sophisticated rule system that acknowledges the effect of realistic constraints (i.e., resource limitations) outperforms onethat adheres more closely tothe original goal. One might argue that for the specific sites we have selected, the HRS scores do not represent risk and do not represent toxicity or time effects. However, the purpose of this analysis is not to show how many HRS points can be cleaned up or what strategic approach is superior. Rather, it is to point out that effective management depends on effective identification of objectives, requirements, resources, and constraints. The use of tools because they are simpler (such as a one-rule system)will often be less productivethan the use of tools that more closely reflect real conditions. The manager’sjob is to identify the relevant aspectsof any decision and to ensure that all aspects are properly considered.
B. StatisticalUncertainty The rule-based system just described was fairly complex, even when operating with the assumption of certainty. Incorporating probabilities or ranges of values into such a model as the multiple-rule model would require automated systems support. There are many models in existence that assist in the solution of particular problems. There are few, however, that provide information to a manager before an event occurs.The EPA’s CAMEO model may be the best known; designed for emergency response use, it can be used for “what-if” analysis under selected circumstances. (Contact theUSEPA Office of Solid Waste and Emergency Response in Washington, DC., for additional informationon CAMEO.) Even with CAMEO, however, the situation needs to be specified quite closely. Other “screening” models have been produced under various development initiatives by the EPA and by private vendors; those models generally addressa specific pollution mode (air, soil, surface water, or groundwater) and usually address fate and transport issues rather than providing impact analyses. In addition, many site-specific models have been developedfor individual environmental projects,and in a number of cases those models have been placed inthe
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public domain in case they may be found useful. In other words, there are literally hundreds of dynamic models available; some address known a situation over time,and some aremuch more s~phisticated.~ What is needed is a simulation capability that can portray sources and receptors and model (GIS) systems pathway behavior. Unfortunately,at present the Geographic Information System that contain the needed data cannot be interfaced with the programs that replicate the pathways. Some systems development is under way that may help in this area [21]. The requirement is for a system that will represent the behavior not of a single site, but of a typical environmental situation with multiple potential sources and receptors. Such a model must include the possibility that currently contained sources may fail, must allow for changes to be made to reflect proposed upgradesor corrective actions, and must be able to correlate these activities to costs. Finally, the systemmust be visually interpretable and preferably simple to use. These requirements will not be simple to meet, which is why there are no such systems today.
C. Outcome Uncertainty It was demonstrated earlier that the problem avoidance (minimax) strategyis self-defeating in environmental operations because in many cases it costs almost as much to do the minimum (once the effectof time is considered) as it does to go ahead and take careof the problem. We have also seen, thanks to the “worst-first” model under certainty, that aspiration-level approaches (which may be expressed as “just do it”) may well fall apart when faced with the reality of resource or capacity constraints. In fact, the very insistence on mission focus at the expense of realism results in less of the actual mission being accomplished than in the more realistic multiple-rule or expected value approach. This leaves the subjective decision-making approach. This method of analysis is receiving increasing attention. Two effortsaddressingcapitalfacilitiesdemonstratedifferentapEPA’s effort to develop policies for DOE facilities. EPA DOE’S and use proaches. The first is the of such an approach was encouraged by the successof earlier efforts in developing underground storage tank replacement strategies at the state level, with New Mexico being the leader in this regard. 1. AnalyticHierarchy
The analytic hierarchy process(AHP), often known as “pairwise comparison,” works by considering all of the factors involved in reaching a decision and comparing their relative importance in that decision. For instance, when trying to select an item of equipment, one may consider purchase cost, reliability, and efficiency. Then these elements are compared. One could just assign weights directly; AHP assumes that effective quantitative judgments on these qualitative issues wouldbe illusory. Instead, the participants are asked to assess the relative importanceof one factor over another. Based on the amount by which one element is said to be more important than another, a weight is established (unseen to the participants) for each factor. Factors can be tiered: thus, each factormay have subordinate indicators (in the example to above, reliability may be assessed in terms of both manufacturer’s advertised mean time failure and the corporate experience with similar machines, with one rated more heavily than the other).
’A helpful referenceis the USEPA Informution Resources Directory, publishedby the Information and Resources Management Division of the USEPA. My copy is document number OPA 003-89. March 1989, but newer versions may subsequently be published under different document numbers.
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The apparent simplicity of the AHP method is deceptive.It leads people who are not very familiar withit to attempt its use, and because the result is produced by the computer to three decimal places it has the aura of authenticity. But if the factors are improperly constructed, the result willbe meaningless. While the computer can assess the technical consistency of the rankings with each other based on the pairwise comparisons, it cannot answer whether the comparisons were meaningful. The best test of structure is to ask the same question the software asks; using the example above,“In assessing whatequipment to buy, which is more important: reliability or flciency?” If the sentence with the blanks filled in seems nonsensical, the structure may be improper. Another indication of a problem with the structure of the model, or a lack of general understanding of the issues, is the finding of generally very high or very low scores (near 0% and 100% weighting) for numerous factors. In theEPA’s implementation of the Risk Information System (RIS), the major factors were health impacts and environmental impacts. Tiered beneath these were various intervals within which pollution would be expected to arrive. These in turn were constructed in terms of the individual and population risks, which in turn had three layers of other indicators. In early efforts, the panels returned numerous zero-weight responses; where decision trees have only two branches, eliminationof one branch renders the decisiontree useless. To the EPA’s credit, they recognized the problem this created and went back to review the model structure; at the time of writing, the revision has not been published. This servesto reinforce the point made earlier in offering multiple decision models: not all will work in every situation. When the model doesn’t work, one has to make assumptions to develop the best possible outcome. Criticalto that process, of course, is to recognize the fact that assumptions have beenmade.
2. WeightingWithoutAHP An alternative formulationof the same problem came from DOE‘SResource Allocation Support System (RASS) project. In this case, DOE did not use the AHP approach, electing instead to rate the need for facilities based on a series of scales, each scale having a weight of its own and the raw data being used to construct the scales being weighted within each scale. Raw data supplied by local managers to describe their proposed project were converted to scales presented as “letter grades.” The siting strategy considered four primary factors: cost, health issues, the volume of waste that an alternative could handle, and the degree to which the alternative complied with applicable regulations. An extensive amountof preparation would be required to obtain the raw data needed to make the system work. However, it was not intended as a detailed engineering analysisas much as a first-tier estimate to allow general project decisions and funding allocations tobe made. This process in still in the pilot project phase, and a report on the results is not yet available. The item that environmental managers should consider in using such a system is whether it is in fact appropriate to the problem to be solved. The difference between the RASS application and the AHP-based RIS application is that RIS attempted to capture nonquantifiable values and norms (the relative significanceof carcinogenic and noncarcinogenic pollution, for instance). RASS deals exclusively with highly quantifiable values (waste tons, costs, etc.). Considering that the compliance question is really fairly irrelevant,as no facility will be permitted that does not comply with all applicable regulations, one could ask whether the objective function could have been simplified to maximize the waste or risk reduction (as was demonstrated in the “results-first” model) without need the for a weighting system that distorts the true dimensional valuesof the data. Nonetheless, the RASS approach is a relatively complete attack on a multifaceted problem and is worthy of consideration as an approachto solving environmental problems.
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VII. CONCLUSION This chapter does not answer any questions about the environment. has,Ithopefully, stimulated a number of questions about how decisions are madeand what basis is offered for those decisions. Environmental managers needto understand the full range of tools available for decision support. More important, they need to understand the degree to which the way in which the the solution objective is stated caninfluence the choice of decisionapproachandhence adopted. And they need to be aware of the strengthsand limitations both of the decision methods employed and of the data used to support those decisions. Few decisions will ever satisfy all parties involved in such a complex and controversial policy area as the environment. Elaborate mathematical proofs, eye-catching simulations, or cutely worded weighting scales will neither improve the decision nor overcomethe reluctance of an opponent to concede that a position has some merit. But a better understanding of the premises used to arrive at a decision can ensurethat discussions of the decision are based on the same understandingof the problem,an essential first step in arriving at a sustainable public policy decision.
ACKNOWLEDGMENTS This chapter represents a significant advancementfrom, but nonetheless capitalizes onmany of the frameworks and concepts found inan earlier article [l]. The permissionof Hazardous Material Control to reproduce major portions of the text of that article are gratefully acknowledged.
REFERENCES 1. Brown, D. M., Dienemann, P. F.,and Kline, R. C., Results first: an enhancement of the worst-first approach, Hazardous Mater. Control, 5(4): 20-33 (1992). 2. Hembra, R. L. (U.S. EPA), The need to establish environmental priorities, presentedat the Southeastern Conference on Public Administration, Am. Soc. Public Administration, Charlotte, N.C. Oct.16,1991. 3. U.S. Environmental Protection Agency,Comparing Risks and Setting Environmental Priorities, Office of Policy, Planning and Education (PM-220). August 1989. 4. U.S. Environmental Protection Agency, Unfinished Business: A Comparative Assessment of Environmental Problems, 1987. 5 . Hawkins, K. and Thomas, J. (eds.), Enforcing Regulation, Kluwer-Nijhoff, Boston, Mass. 1984. 6. Fleischer, G. A. Engineering Economy: Capital Allocation Theory, Wadsworth, Belmont, Calif., 1984. 7. Lapin, L. L., Quantitative Methodsfor Business Decisions, 2nd e d . , Harcourt Brace Jovanovich, NewYork,1981. 8. Quade, E. S., Analysis for Public Decisions, 2nd ed., North-Holland, New York, 1982. 9. Douglas, E. J., Managerial Economics: Theory, Practice and Problems, 2nd ed., Prentice-Hall, Englewood Cliffs, N.J., 1983. 10. Weber, Max,c. 1922, Economy and Society: An Outline of Interpretive Society, edited and translated by G . Roth and K. Wittich, 2nd e d . , University of California Press, Berkeley, Calif., 1955. 11. Simon, H. A., Administrative Behavior, 3rd ed., Free Press, New York, 1976. 12. Doty, C. B.andTravis,C.C., Is EPA's national priorities list correct? Environ. Sci. Technol., 24( 12): 1778-1780 (1990). 13. Brown, D. M.,and Kline, R. C., Are the worst first? A review of national priority list sites, Proc. Region VI Conf. Am. Soc. Public Administration.Dayton, Ohio, 3 Oct. 1991, pp. 87-104, Political Science Department, Univ. Dayton, October 1991.
Environmental 14.
15.
16.
17. 18. 19. 20.
Management 45
U.S. Department of Energy, Objectives, scales and scoring instructions for a pilot study of waste management's resource allocation support system (RASS), draft, U.S. Dept. of Energy, Germantown, Md., June 1992. Contact Mr. Kevin Donovan of DoE for more information on the status of the RASS study. Moore, W. B., Brown, D. M., and Hutchinson. R. A., Updatedfiscal impact analysisfor the Naval Submarine Base, Kings' Bay, Georgia, LMIRept.No.FF'605R1,LogisticsManagementInst., Bethesda, Md., December 1986. Walsh, W., Susel, I., and Ronayne, A., Setting risk-based priorities: a method for ranking sites for response, Proc. HMCRI R&D Conf. Anaheim, Calif., February 1991, HMCRI, Silver Spring, Md., 1991, pp. 112-123. Hird, J. A., Superfund expenditures and cleanup priorities: distributive policiesor the public interest? J . Policy Anal. Manage., 4455-483 (1990). Sax, I. N., Dangerous Properties of Industrial Materials, 5th ed., Van Nostrand Reinhold. New York, 1979. Weiss, G . , Hazardous Chemicals Data Book, 2nd ed., Noyes Data Corp., Park Ridge, N.J. 1986. Fiorino, D. J., Can problems shape priorities? The case of risk-based environmental planning, Public Admin. Rev. 50( l): 82-90 (1989).
21.
Brown, D. M., Integrated environmental management: a GIS system you can afford, Proc. HMC Superfund 92 Conf. (Hazardous Materials Control and Research Institute), Washington, D.C., Dec. 1-3.1992.
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4 Introduction to Engineering Evaluation for Contaminated Sites
David S. Wilson, Alan C. Funk, Ronald G. Fender, and Marilyn Hewitt Environmental Resources Management Group Exton, Pennsylvania
1. CHARACTERISTICS OF ENVIRONMENTAL CONTAMINANTS There are literally thousands of substances that can contaminate the environment. In some cases, this contamination is related to natural conditions and is thus a “background” condition. Most often, however, environmental contaminants are related to human activity. Current regulatory programs governing contaminated sites generally focus on a class of contaminants called hazardous substances. These regulated substances have been identified by the United States Environmental Protection Agency (USEPA) on the basis of their relative toxicity in the environment. Many other substances may be released to the environment; some are unregulated, and others may be regulated by agencies other than the USEPA. Any given contaminated site is usually characterized by one primary contaminantor suite of contaminants related to activities conducted at the site. This contaminant profile is often referred to as the site “fingerprint.” Once this fingerprint hasbeen established by general environmental sampling and comprehensive chemical analysis, detailed studies of the site are generally focused on the fingerprint compounds. The contaminants present at any given site are highly dependent on the natureof the acof ortivities at the site. For example, at chemical industry manufacturing facilities, a variety ganic compoundsmay be present, whereasat a facility that does substantial electroplating, the contaminants may be principally heavy metals. The electronics industry and metal products industry will also commonly be characterized by the presence of organic solvents as environmental contaminants. A wood-preserving operation, by contrast, would be characterized by phenols and creosote-related compounds. A commercial hazardous waste disposal facility could be characterized by any combination of organic and inorganic contaminants. A municipal waste disposal landfill is generally characterized by some amount of organic compounds and metals, but mostly by liquid leachate containing organic acids, iron, and ammonia from the decomposition of trash and garbage. Environmental contaminants are generally divided into three classes: 47
Wilson et al.
48
Organic Compounds. These are human-produced compounds that do not occur naturally in the environment to any significant degree. Inorganic Compounds. Heavy metal ions are the principal concerns. Although these are naturally occurring elements, human activity tends to concentrate them to levels that have higher potential toxicity than natural levels. Other contaminants such as the anions chloride and sulfate are also inorganic contaminants. Inorganic contamination can also result in environmentally unacceptablepH levels in the environment at some sites. Biological. The principal concerns for biological contaminants are bacteria and pathogens associated with sewage. These are not generally concerns at contaminated sites related to industrial or disposal activities. A brief summary of the nature of common organic and inorganic contaminants follows.
A. Organic Contaminants The USEPA divides organic contaminants intoclasses related to chemical characteristics that drive required analytical methods. These classes are discussed below. Volatile Organics By far the most common environmental contaminants are volatile organic compounds (VOCs). This is due to their presence in petroleum hydrocarbons that are widely used as fuel in our society (gasoline, fuel oil, etc.) and to the almost universal use by American industryof volatile organic solvents for degreasing during manufacturing operations. Volatile organics are also used as raw materials in some industrial processes, such as those of the pharmaceutical industry. Table l presents USEPA's Target Compound List for VOCs. Volatile organic compounds are characterized principally by their high vapor pressure, that is, their tendency to volatilize to the gaseousstate at standard temperature and pressure. These compounds are also characterizedby relatively low levels of water solubility, generally froma few hundred parts per million 1% in water. However, due to toxicological characteristics, these solubilities, although limited, are significant in terms of potential environmental and human health impact. A third majorcharacteristic of volatile organic compounds is their specific gravity. Those compounds without chlorine atoms tend to be lighter than water and therefore will float. Compounds that have been chlorinatedare generally heavier than water and will sink in water under the influence of gravity. This characteristic is very important in the dynamics of migration of VOCs in the environment. Chlorinated aliphatic compounds are among the most common environmental contaminants. These are the Compounds that have been most commonly usedby industry as degreasing solvents. The principal compounds of concern are tetrachloroethene, which is used in industrial applications and as commercialdry cleaning fluid; trichloroethene, widely used for metal degreasing; l,l,l-trichloroethane, also widely used for metal degreasing; and methylene chloride, whichhas a variety of industrialuses,includingitsuseastheprincipalsolvent for decaffeination of coffee and for wood-stripping operations. Of these compounds, trichloroethene, also knownas TCE, is perhaps the most ubiquitous and notorious. Depending upon environmentalconditions, this compound undergoes reductive dehalogenation to form dichloroethenes,and finally vinyl chloride, the most toxic of the chlorinated aliphatic compounds. These chlorinated compounds are heavier than water and thus tend to sink under the influence of gravity in water systems. Also very common in the environment are aromatic compounds, which are used as solvents and are present in petroleum products. Perhaps the most common class of these aromatics is referred to as the BTEX series-benzene, toluene, ethylbenzene, and xylene. These com-
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Table 1 TargetCompoundlAnalyteList Volatiles ~~
~~
Acetone Benzene Bromochloromethane Bromodichloromethane Bromoform Bomomethane/methyl bromide ZButanone/MEK Carbon disulfide Carbon tetrachloride Chlorobenzene Chloroethane Chloroform Chloromethane/methyl chloride Dibromochloromethane
1,2-Dibromo-3-chloropropane 1 ,ZDibromoethane 1 ,2-Dichlorobenzene 1,3-Dichlorobenzene 1 ,4-Dichloroethane 1.1-Dichloroethane l ,2-Dichloroethane 1, I-Dichloroethene cis-l ,2-Dichloroethene trans- 1,2-Dichloroethene 1 ,ZDichloropropane cis-l ,3-Dichloropropene trans- 1,3-Dichloropropene Ethylbenzene
2-Hexanone Methylene chloride 4-Methyl-2-pentanonelMIBK
Styrene 1,1,2,2-Tetrachloroethane Tetrachloroethene Toluene 1,1,1 -Trichloroethane 1,1,2-Trichloroethane lkichloroethane Vinyl chloride Xylenes (total)
Semivolatiles Acenaphthene Acenaphthylene Anthracene Benzo[a]anthracene Benzo[b]fluoranthene Benzo[k]fluoranthene Benzo[ghi]perylene Benzo[a]pyrene bis(2-Ch1oroethox)methane bis(2-Chloroethy1)ether bis(2-Ethylhexy1)phthalate CBromophenyl phenyl ether Butyl benzyl phthalate p-Chloroaniline p-Chloro-m-CreSOl 2-Chloronaphthalene 2-Chlorophenol CChlorophenyl phenyl ether Chrysene o-Cresol/2-methylphenol
p-CresollCmethylphenol Di-n-butyl phthalate Dibenz[a,h]anthracene Dibenzofuran 3,3’-Dichlorobenzidine 2,CDichlorophenol Diethyl phthalate 2,CDimethylphenol Dimethyl phthalate 4,6-Dinitro-o-cresol 2,CDinitrophenol 2,4-Dinitrotoluene 2,6--Dinitrotoluene Di-n-octyl phthalate Fluoranthene Fluorene Hexachlorobenzene Hexachlomyclopentadiene Hexachloroethane Hexachlorobutadiene
Indeno[ 1,2,3-cd]pyrene Isophorone 2-Methylnaphthalene Naphthalene o-Nitroaniline m-Nitroaniline p-Nitroaniline Nitrobenzene o-Nitrophenol p-Nitrophenol n-Nitrosodiphenylamine n-Nitrosodi-n-propylamine 2,2‘-0xbil( I-chloropropane) Pentachlorophenol Phenanthrene Phenol F‘yrene 1,2,4-Trichlorobenzene 2,4,5-TrichlorophenoI 2,4,6-Trichloropheno1
PesticidesPCBs Aldrin a-BHC p-BHC y-BHC (Lindane) &BHC a-Chlordane y-Chlordane 4,4’-DDT 4.4”DDE 4,4‘-DDED
Dieldrin Endosulfan I Endosulfan I1 Endosulfan sulfate Endrin Endrin aldehyde Endrin ketone Heptachlor Heptachlor epoxide Methoxychlor
Aroclor 1016 Aroclor 1221 Aroclor 1232
Aroclor 1242 Aroclor 1248 Aroclor 1254 Aroclor 1260 Toxaphene
de,
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Table 1 (Continued) Inorganics ~
Nickel
Aluminum Antimony Arsenic Barium Beryllium Cadmium Calcium Chromium
~~
~
~
~
~~~
Cobalt
pounds are present in refined petroleum products, principally in gasoline, and are thus found widely distributed through the environment due to leaking subsurface gas tanks and surface spills and discharges of petroleum products. In pure form they are also used by certain industries as raw materials and/or solvents. These compounds, being unchlorinated,are lighter than water and therefore float. Semivolatile Organics Semivolatile organicsare by-products of many types of industrial and nonindustrial processes. Table 1 shows the USEPA's Target Compound List of semivolatile compounds. Examples of common compounds found in the environment are phthalates (used as plasticizers), polynuclear hydrocarbons (PAHs) (produced by the combustion offossil fuels), and phenol compounds (which havemany industrial applications inthe chemical industry andare used as wood preservatives). The characteristics of semivolatile organic compounds includelower vapor pressure than the volatile organics. These compounds are therefore of low volatility and tend to exist in the solid or liquid state at standard temperature and pressure. They may be present in the environment either in liquid form or in some cases solidparticulate form (e.g., PAHs in soot produced by incompletecombustion in residential wood stoves). Many semivolatileorganic compounds are heavier than water in their liquid state and thus will tend to sink in water. In general, semivolatile organics alsohave relatively low solubility in water. Phenol compounds are the general exception,with solubilities in the tens ofparts per million range; PAHs, by contrast, have solubilities limited to the low parts per billion range. Pesticides and PCBs Pesticides and polychlorinated biphenyls (PCBs) are also semivolatile compounds. They are lumped togetherby the USEPA on the basis of similar analytical protocol requirements. These compounds have been found to be fairly ubiquitousin the environment at very low levels (i.e., low parts per million to parts per billion). Table 1 shows USEPA's Target Compound List of pesticides and PCB compounds of concern. Pesticides enter the environment through both agricultural use and heavy historical residential use for pest control and weed control. PCBs, on the other hand, have had principally industrial applications. Because of excellent heat resistance properties, PCBs were used for many years in electrical transformers and as hydraulic fluids in heavy equipment. PCBs were generally dissolved in oil for their application. The manufactureof PCBs was of use in the halted by the USEPA in the 1970s. They have been almost completely phased out United States today. Pesticides may enter the environment either in particulate form as powdersor dissolved in solvents. In their liquidform, pesticides are usually lighter than water, as they tend to be dis-
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solved for commercial application in BTEX solvents. PCBs are also lighter than water when present at percent levels or less in oil; however, pure PCB liquids are heavier than water. Perhaps the most controversial semivolatile organic compounds are dioxin and its related compounds, the furans. Dioxin is principally a by-product of the manufacture of certain other substances, the most well known processes being the production of defoliants such as Agent Orange, and the bleaching of paper in the commercial paper industry. Dioxinand furans can also be produced as combustion by-products of some organic materials. Small amountsof dibe present in uncontained emissions from municipal waste incinerators. oxin have been found to As these compoundsare by-products of other processes, they are not found in concentratedor by pure form; rather, they may be found in a wastewater effluent stream, in media affected incinerator emission fallout, or as contaminants in certain waste streams produced from the manufacture of other substances.
B. InorganicContaminants As previously noted, inorganic contaminants are naturally occurring substances that can become concentrated in the environment due tohuman activities. The principal contaminants of concern in the environment are theheavy metals. Metals on the USEPA's Target Analyte List are listed in Table 1. Heavy metals are of relatively low vapor pressure and are thus effectively nonvolatile at standard temperature and pressure. However, some of them, for example, lead and mercury, become very volatile at elevated temperatures. This is very significant to environmental engineers considering thermal treatment techniques for wastes or contaminated media containing such metals. In addition to the more toxic heavy metals, less toxic metals such as iron and manganese are common environmental contaminants. These metals in particular can create difficulties for environmental engineers in water collection and treatment systems at contaminated sites. In addition, salt ionsin solution (e.g., calcium, sodium, magnesium, and potassium chlorides and sulfates) are often present at contaminated sites.At some sites, theremay be elevated levelsof nutrient compounds (i.e., nitrogen and phosphorus compounds). Both the salts and nutrients present significant difficulties and costs for environmental engineers when they must be removed from water.
C. Summary In summary, of the thousands of substances that can contaminate environmental media, the USEPA has developed a list of hazardous substances that are of primary concern. Volatile organic compounds, semivolatile organic compounds, and inorganics, principally heavy metals, are the classesofhazardous substances commonly dealtwith by environmentalengineers at contaminated sites. The following discussion addresses the occurrence of these classes of compounds in various environmental media and the implications for remediationof contaminated sites.
II. CHARACTERISTICS OF CONTAMINATED MEDIA The media that may require remediationat any given contaminated site may include waste materials that were disposed or of spilled in the environment, soils contaminated either directly by spillage and discharges of liquids or indirectly by infiltrating contaminated water, groundwater contaminated either directly by nonaqueous-phase liquids (NAPLs) or by contaminants in solution, surface water containing nonaqueous-phase liquids and/or contaminants in solutionor
52
Wilson et al.
suspension, and air,which may transport contaminated dustor Contaminants in gaseous form. Each of these media is described briefly in this section.
A. Waste Materials There are a great variety of types of waste that can cause environmental contamination. Some of the materials commonly encounteredare listed below. LiquidChemicals. Raw materials,waste,and/oroff-specliquidchemicals mayhave been spilled at a site, disposed of in bulk on the ground, or placed in containers such as drums or tanks. Such spillage or disposal can contaminate soil, groundwater, and surface water. Thus, these types of waste may have to be remediated either as bulk materials still in containers or as constituents absorbed in soil or water. Solid Materials. Solid waste materials are usually residuals generated by a manufacturing or treatment process. Examples include sludgesfrom treatment of electroplating wastewaters and refinery sludges containing oilsand BTEX compounds. Sludges to be remediated either may be very low in solids content or may have been dried to essentially solid form. or are present as precipitates in treatment These typesof materials m often found in drums or holding lagoons that must be remediated. Other types of industrial solids may include set organic resins, powders, and materials such as slag or mine spoils. MunicipalSolid Waste. This class represents materials disposedofin municipallandfills. Older landfills requiring remediation commonly contain principally household refuse, usually mixed with at least some wastes generatedat industrial facilities. Low levels of hazardous substances may be present in leachate from these landfills, originating both from industrial disposal and from the disposalof household products containing hazardous substances. Interestingly, codisposal of industrial and municipal wastes at many such landfills has resulted in biodegradation of most hazardous organic compounds. Also, the essentially neutral pH of municipal landfill leachate does not favor migration of heavy metals from many landfills. Thus, at many such facilities, the principal problemis leachate containing iron and ammonia and having a high biological oxygen demand.
B. Soil Contaminated soil must be remediated principally for two reasons: 1.
Potential adverse health affects on humans and/or animals coming into direct contact with the contaminants 2. The fact the contaminants in soil often provide a continuing source of contamination to underlying groundwater and/or surface water The degree to which soil at a site has been contaminated is dependentupon the nature of the disposal activities, the quantity of materials disposed of, and the chemical nature of those materials. When selecting a remedial action forsoil, several key characteristics of that material must be taken into consideration: 1. 2.
Soil grain size and cohesiveness. It is generally much easier to extract contaminants from a permeable sandy soil than froma low permeability clay soil. Organic carbon and clay content. Certain classes of contaminants, such as heavy metals, PAHs, PCBs, and pesticides, readily adhere to soils that have either high organic carbon content or significant cation-exchange capacity relatedto clay mineralogy. Other contaminants, such as the volatile organics, have very low absorption coefficients on soil materials regardless of organic carbon or clay content.
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3. Soil moisture content. This can be an important factor both for handling soils during ex-
cavation and for treating soils, either in situ or after excavation. For example, effective the removal of VOCs from soilby volatilization, either in situ or above ground, depends on rate at which the soil can be dried and disaggregated. 4. BTU content. If soils are to be incinerated, the BTU content is important to determining the feed rate to the incinerator. Since most soilsare very low BTU materials, the degreeto which the contamination itself contributes BTUs is often thekey factor. Another very important factor, unrelated to the soils themselves, is the required cleanup levels for the contaminants of concern. Where very low cleanup levels (generallylow ppm to ppb levels) are required, it is fairly unrealistic to expect that in situ technologies will reach those levels. Although significant contaminant massmay be removed (for example, by soil vapor extraction of volatile organics or in situ biodegradation of biodegradable organics), these methods will not generally achieve verylow cleanup levels. This is due to the heterogeneityof the subsurface environment,which prevents in situ processes from effectively reachingareas all of contamination. Excavation and carefully planned above-ground treatmentof soils, although they can become very expensive, are generally required to meet very low cleanup levels.
C. Groundwater Remediation of groundwater requirestwo key components: 1. Interception and/or collection of the contaminated water in the subsurface 2. Treatment of the contaminated water that has been intercepted and/or collected
Interception and/or collection of the contaminant “plume” is designed by a hydrogeologist or hydrologist with assistance from geotechnical engineers. The hydrogeologist must determine aquifer characteristics such as mechanisms of flow, directions of flow, extent of the contaminant plume, aquifer permeability, and subsurface stratigraphy in order to design a collection system. For example, selection of collectiodinterception technologies such as pumping wells, interception trenches,barrier walls, and potentialfor in situ treatment are all highly dependent upon the aquifer characteristics. Full remediation of contaminated groundwater to cleanup levels below regulatory standards cannot generallybe achieved with existing technology. The degreeto which contamination can be removed from an aquifer system depends upon two main factors: 1. The nature of the contaminants released-their degree of solubility and the aquifer material’s absorption capacity for them. 2. The nature of the release. If water materials or contaminated soilsin the unsaturated zone are not completely removed,they will serve as continuing sourcesof contamination to the groundwater below.If liquids were released in sufficientquantity to physicallyreach the groundwater table in free form, then either a light nonaqueous-phase liquid (LNAPL) or a dense nonaqueous-phase liquid (DNAPL) willbe present in the groundwater system. LNAPLs can be well defined in many hydrogeologic systems and removed to a significant degree. However,DNAPLs migrate downwardbeneath the water table, in response to gravity, along natural pathsof permeability contrast withinan aquifer. It is commonly difficult if not impossible to track the migration of all DNAPLs. In addition, if found, they cannot usually be completely removed.
Groundwater that is collected for treatment above ground is usually readily treatable using well-developed, proven treatment technologies. However, common pitfalls for recovery/treatare ment systems are the effects of iron and manganese on those systems. Iron and manganese
54
Wilson et al.
often presentat contaminated sites at elevated levels,either naturally or related to the sitecontamination. Well screens, trenches, and treatment systemsin media may be fouled by corrosion andlor bacteria related to iron and manganese. This can greatly affect operation and maintenance requirements andcosts for groundwater remediation facilities.For this reason, it is imperativethatthese characteristics of thegroundwater be determinedbeforerecoveryand treatment systems are designed. To assess the potential forin situ treatment, thecharacteristics of an aquifer must be well defined. To date, in situ treatment of groundwater has been limited becauseof the limited biodegradability of many compounds and heterogeneitiesin the subsurface aquifer materials.The one exception to this generality is the use of biological degradation in homogeneous unconsolidated permeable aquifers where nonchlorinated petroleum product contaminants are present.
D. SurfaceWater Direct discharges of contaminants flowing to surface water are dispersed rapidly; therefore, remediation of surface water problems usually requires that the source of the contaminationbe mitigated. Direct treatment of surface water at contaminated sites is not commonly required except in the case of a large-scale emergency spillage. At a contaminated site, surface water is generally affected via three pathways:
1. Contaminants in solution or suspension inrunoff 2. Discharge of contaminants via the groundwater system 3. Contaminant accumulation in bottom sediments, providing a continuous source of release of soluble contaminants intothe surface water Remediation of surface water requires knowledge of the mass balance of contaminant input from these various sources.Only then can the proper combination of runoff and erosion conbe selected for meeting trols, groundwater interception, and/or sediment treatment and removal surface water cleanup requirements. Due to the low solubilities and high adsorption coefficients, contaminants such as metals, PAHs, pesticides, and PCBs can accumulatein bottom sediments. Volatile organics, and sometimes metals, can enter surface water in solution via groundwater. Oils and other NAPLs can also be discharged directly into surface water via groundwater.
E. Air Air quality issues at contaminated sites are generally associated with two potential concerns: (1) airborne dust emission containing semivolatilesor metals, and (2) direct gaseous emissions of volatile organics by volatilization from othersite media. These pathways are sometimes of concern under ambient site conditions, that is, at the existing unremediated site. This is particularly true if heavy surface soil contamination is present and there is little or no vegetation to prevent fugitive dust emissions fromoccurring. It also may be true where NAPLs discharge from subsurface via seepsor to surface water bodies. Even more commonly, however,these two pathways are of concern during remedial action itself. Excavation of soils, movement of heavy equipment, and exposure of volatile contaminants deep in the subsurface by excavation can create air emission problems during remediation. Also, emissionsmay be created by treatment processes. For example, any air stripping of volatile organics will result in the generation of gaseous state compounds that may require treatment. Soil vacuum extraction systemsfor volatile organics commonly require treatmentof the air stream, usually by carbon absorption or fume incineration. The use of heat for treating environmental mediamay also result in the emission ofsuch substances as volatile metals, PAHs, dioxins, and furans. The potential emissions depend upon the natureof the contaminants at the site and the nature of the treatment method selected.
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F. Summary In summary, media requiring remediationat contaminated sites most commonly include waste materials, soil, groundwater, surface water, and air. The characteristicsof the media themselves as well as the characteristics of the contaminants present must be well known before effective remedial measures can be taken. The degree to which remediation of the site is practical is often a function of the degree of cleanup desired or required. Removalof trace levels of environmental contaminants from environmental media is often beyond the means of available technology.
111. REGULATORY STANDARDS FOR SITE REMEDIATION To mitigate risks to human health and the environment andto define levels for cleanupof existing contaminated sites, the USEPA and state regulatory agencieshave established standards for environmentalmedia discussed respectively in Sections I and 11. Because the list of chemical parameters subject to regulatory standards is extensive, our presentation of standards is limited to some of the most common contaminants in environmental media. These contaminants and the applicable standards are listed in Tables 2-5. The standards discussed herein include those from federal regulations suchas the Federal Drinking Water Criteria (FDWC) and the Resource Recovery and Conservation Act (RCRA) action levels. As an example of state regulatory standards, New Jersey cleanup standards are included because they are some of the most stringent and most recently updated cleanup criteria. At the time of writing, the New Jersey standards were proposed but not yet promulgated on all ongoing as law. However, NewJersey hasbeen using the proposed standards for guidance and proposed cleanups.
A. SolidWastes The standards for cleanup of solid wastes will,in most cases, be the same as the standards for cleanup of contaminated soils(see Table 2). Handling of solid wastes is dependenton whether they are classified as hazardousor nonhazardous. They may be classified as hazardous on the basis of being a listed waste (per 40 CFR 261 Subpart D) or on the basis of being hazardous by characteristic (per 40 CFR Subpart C). Characteristics of hazardous wastes include ignitability, corrosiveness, reactivity, and toxicity, as defined by the results of standard tests. In particular, the Toxicity Characteristic Leaching Procedure (TCLP) is currently used to characterize the toxicity of waste.If the waste is classified as hazardous, it will usually trigger land ban restrictions (LDRs), which call for treatment of the wastes to RCRA cleanup levels prior to disposal at an RCRA landfill and within 90 days of removal using the RCRA best demonstrated available technology (BDAT) or other technologies capableof reaching the same goals.
B. SoilsandSediments Table 2 presents RCRA and New Jersey soil standards. RCRA action levels dictate when a corrective measures study (CMS) is to be performed to evaluate solutions for a site cleanup. RCRA cleanup levelsare determined ona case-by-case basisby USEPA based on potential risk to human health and the environment. When determining levels of constituents in soils, it is important to examine the natural levels (i.e., background concentrations)of constituents forthe area being investigated. Possible sources of interference that can produce elevated background conditions mayinclude runoff from roadways (which can contribute lead and other metals, semivolatile compounds, and
mg/kg)
Wilson et al.
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Table 2 SoilandSedimentStandards New Jersey cleanup standardsb
Subsurface Nonresidential Residential RCRA soil soil surface soil surface levels' action Parameter
(mag) ~
Arsenic Benzene Benzo[a]pyrene' Cadmium Chlordane Chromium 4,4-DDT Endrin Lead Lindane Nickel PCBs Selenium Tetrachloroethylene Toluene Trichloroethylene 1,1,1 -Trichloroethane Vinyl chloride Xylenes Zinc
8.00E + 01
4.00E + 01
5.00E - 01 4.00E + 02(Cr6) 2.00 + 00 2.00E + 01 5.00E - 01 2.00E + 03 9.00E - 02
-
-
1.00E + 01 2.00E + 04 6.00E + 01 7.00E + 03
2.00E + 05 -
2.00E + 01 3.00E + 00 6.60E - 01 1.00E + 00
2.00 + 01 1.30E + 01 2.50E - 01 1.00E + 02
2.00E + 00 1.70E + 01 1.00E + 02 5.20E - 01 2.50E + 02 4.50E - 01 1.00E + 00 9.00E + 00 1.00E + 03 2.30E + 01 2.10E + 02 2.00E + 00 3.60E + 02 1.50E + 03
9.00E + 00 3.10E + 02 6.00E + 02 2.20E + 00 2.40E + 03 2.00E + 00 1.00E + 03 3.70E + 01 1.00E + 03 1.00E + 02 3.80E + 03 7.00E + 00 6.30E + 03 1.50E + 03
-
-
1.00E + 00 1.00E + 02
1.00E + 02 5.00E + 01 -
1.00E + 00
-
1.00E + 02
-
1.00E + 00 5.00E + 02 1.00E + 00 5.00E + 01 1.00E + 00 1.00E + 01
'USEPA, ECRA Corrective Action Proposed Rules; FR30798/27 July 1990. bNJDEPE Cleanup Standardsfor Contaminated Sites-Proposed Rule; N.J.A.C. 7:26D NJ Register 3 February 1992. CBenze[a]pyrene (BaP) is presented to be representative for semivolatiles.
chlorides), roof drains (semivolatiles), and small amounts of debris on the ground surface (a small piece of plastic can result in a positive result for phthalates).
C. Groundwater and Surface Water
Table 3 presents FDWC (40CFR 141 et al), RCRA, and New Jersey water standards.As with soils and sediments, runoff from roadways can cause interferences with surface water results and some groundwater results. Other possible interferences include septic tanks, sewer lines, and water lines, which can sometimes contribute chloroformand lesser amounts of other organics that canbe formed during the chlorination of surface water for municipal water systems. One common pitfall is use to municipal waterfor drilling and installing a monitoring well without testing the municipal water along with other background samples. Many false positives for chloroform have been overlooked thisway.
D. Air Since theearly years of air quality management, air qualityrules and regulations inthe United States have been based on a set of air quality standards known as the National Ambient Air Quality Standards,or NAAQS. The NAAQS represent a maximum concentration or "threshold
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forSites Contaminated
Table 3 GroundwaterandSurfaceWaterStandards Federal drinking water criteria Parameter (m@.)) Amnic Benzene Benzo[a]pyrene' Cadmium Chlordane Chromium 4.4-DDT Endrin Lead Lindane Nickel PCBs Selenium Tetrachloroethylene Toluene Trichloroethylene 1,l ,l-Trichloroethane Vinyl chloride Xylenes Zinc
MCUj (mg/kg) m (& )$
-
OBOE + OOb OBOE + 00" 5.00E - 03' O.WE + 00" 1.00E - 01'
-
2.00E - 03d OBOE + 00' 2.00E - 0 4 '
-
0.00E + 00" 5.00E - 02' OBOE + 00" 1.00E + 00" OBOE + 0Ob 2.00E - O l b O.WE + 0Ob 1.00E + 01'
-
New Jersey standards cleanup RCRA
levelsg action MCL (mg/L)
for Class IIA GWh
5.00E - 02" 5.00E - 03b 2.00E - 04d 5.00E - 03' 2.00E - 03' 1.00E - 01'
5.00E - 02 5.00E - 03
-
-
-
5.00E - 03 3.00E - 05 5.00E - 02 (CR6) 1.00E - 04 2.00E - 03d 2.00E - 03 1.50E - 02 1.50E - 02' 2.00E - 04 4 ' 2.00E - 0 7.00E - 01 5.00E - 06 5.00E - 0 4 ' 5.00E - 02' 5.00E - 02 5.00E - 03' 7.00E - 04 1.00E + 01 1.00E + 00" 5.00E - 03 5.00E - 03b 2.00E - Olb 3.00E - 00 2.00E - 03 2.00E - 03b 7.00E + 01 1.00E + 01'
8.00E - 03 1.00E - 03 2.00E - 02 4.00E - 03 5.00E - 04 1.00E - 01 1.00E - 04 2.00E - 03 1.00E - 02 2.00E - 04 1.00E - 01 5.00E - 04 5.00E - 02 1.00E - 03 1.00E + 00 1.00E - 03 3.00E - 02 2.00E - 03 4.00E - 02 5.00E + 00
'USEPA, National Primary and Secondary Drinking Water Regulation; FR 5956924 December 1975. bUSEPA, National Primary and Secondary Drinking Water Regulations; FR 256 90/8 July 1987. 'Benzo[a]pyrene (BaP) is presented to be representative for semivolatiles. dUSEPA, National Primary and Secondary Drinking Water Regulation; F R 31776/17 July 1992. 'USEPA, National Primary and Secondary Drinking Water Regulation; FR 3526/30 January 1991. 'USEPA, National Primary and Secondary Drinking Water Regulation FR/7 May 1991. WSEPA, RCRA Corrective Action Proposed Rules; FR 30798/27 July 1990. hNJDEPE, Cleanup Standards for Contaminated Sites-Proposed Rule; N.J.A.C.7:26D NJ Register 3 February 1992.
level" of a pollutant in the air above which humans or the environmentmay experience some adverse effects. The actual threshold levelsare based on years of epidemiological,health, and environmental effects research conductedby the USEPA. The USEPA has developed twotypes of NAAQS: primary standards, which are set at levels that are designed to protect the public health, and secondarystandards, which are designed to protect the public welfare (such as vegetation, livestock, building materials, and other elements of the environment). TheNAAQS differentiate between the effects from short-term exposureandthosefromlongertermexposure to air pollutants. Thus, there are short-term NAAQS based on l-hr or 8-hr average concentrations and long-termNAAQS based on annual concentrations. Because the NAAQS representnumerical criteria, the reports inwhich the USEPA presentsinformation on thedevelopment of an NAAQS are called criteria documents. The pollutantsfor which the USEPA hasdeveloped standards are thus known as criteria pollutants.
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Table 4 National AmbientAir Quality Standards Primary
Secondary
(pg/m3)
NAAQS @@m3)
NAAQS Averaging period pollutant Criteria PM10 (particulate matter) SO2 (sulfur dioxide)
NO, (nitrogen dioxide) Ozone CO (Carbon monoxide) Lead
Annual 24-hr Annual 24-hr 3-hr Annual 1-hr 8-hr l-hr
50 150 80 365
-
100 235 10,Ooo 40O , oo 1.5
50 150
1,300 100 235 10,Ooo 40,Ooo 1.S
The USEPA has establishedNAAQS for six compounds sincethe concept of NAAQSwas established in the CAA amendments of 1970. Table 4 lists these six criteria pollutants and their standards. In Table 4, the NAAQS concentrations are expressed in terms of micrograms per cubic meter (pg/m3);however, you will sometimes see them expressed in terms of parts per million (ppm). This table also presents some additional information regarding thecriteria pollutants.
IV. SUMMARY OF REMEDIAL ENGINEERING TECHNOLOGIES This section presents the identification and evaluation of many available remedial engineering technologies for various environmental media. This presentation is not intended to be comprehensive, but it can serve as a general summary of common remedial technologies. The technologies are grouped into the following broad categories:
No Action. Monitoring and inspection technologies that do not contribute to actual remediation of site conditions. Institutional Actions. Indirect methods of reducing exposure to site hazards. Containment. Physical isolation of solid waste, groundwater, and/or other affected media. Removal. Physical removal of solid waste, groundwater, and/or other affected media. Treatment. Alteration of solid waste, groundwater, and/or other affected media to reduce the toxicity, mobility, or volume of site constituents. Disposal. Placement of solid waste, treatment residuals, and/or affected media into a secure disposal facility, or discharge of treated water to the environment. If site remediation is required, two or more technologies may be used in combination to provide a comprehensive approach to site cleanup. An example of combining technologies would be the use of a treatment technology to reducethe toxicity and volume of affected material combined with a containment technologyto reduce the mobility of residual constituents in the treated product. Remedial technologiesare identified for each of the categories listed abovefor the various environmental media found at contaminated sites. The identified technologiesare then evaluated to present some of the major advantages and disadvantages associated with each one. Where appropriate, specific data to be collected to facilitate further evaluation andselection of a remedial technology are also presented. c
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Table 5 Notes on the Criteria Pollutants
PM 10 is composed of solid particles, less than 10pm in diameter, that are small enough to be inhaled by humans. The particles can be composed of any number of compounds, from road dust to heavy metals, depending on the source. Sulfur dioxide(SO,) is a compound formed both naturally and by the combustion of fossil fuels. Exposure to higher concentrations ofSO, can cause respiratory problems for some people. When combined with water, SO, forms sulfur compounds that are one of the main components of acid rain. Nitrogen dioxide (NO,) is another compound formed by the combustion of fossil fuel that contributes to the formation of both acid rain and smog. NO, is one of several nitrogen oxide (NO,) compounds present in the atmosphere. The NAAQS is established for NO,, but NO, is more commonly measured. Ozone itself is not emitted directly into the air but rather is formed through a series of complex physical and chemical reactions in the atmosphere. Therefore, discussions about the criteria pollutant ozone often focus on a group of gaseous pollutants known as volatile organiccompounds or VOCs, which are carbon-based, organic compounds that tend to evaporate into the air easily. Solvents, cleaners, and paints are among the hundreds of compounds in use that contain VOCs. In the presence of sunlight, VOCs and other chemical compounds, including NO,, react to form ozone. Carbon monoxide (CO) is a colorless, odorless, and tasteless gas that occurs naturally in the atmosphere and is also formed in the combustion of fossil fuel. teud (Pb) is a common metal that is released into the atmosphere from a number of sources, including burning of leaded fuels. Lead has the lowest NAAQS, since exposure to low levels can present health problems.
A list of the technologies presented in this section and the applicable environmental media is presented in Table 6. A brief description and evaluation of the potentially applicable technologies identified in this section are presented in Table 7 . A summary of the major data requirements identified for each technology is presented in table 8.
A. No ActionTechnologies “No action’’ implies that no remedial actions are to be conducted on the media of concern. Actions such as groundwater monitoring and site inspections are included as no action technologies because theyare intended to detect changes in site conditions rather than to actually remediate existing contamination. Monitoring Description. Long-term, periodic groundwater and/or surface water monitoring to detect changes in site conditions, such as the migration of constituents in groundwater, due to natural processes. Evaluation. Although groundwater monitoring does not reduce site constituents, it is a proven method of detecting changes in site conditions and is commonly required as a component of remediation. Site Inspection Description. Long-term, periodic site inspections to detect visible changes in site conditions due to natural processes. Evaluation. Although site inspections do not reduce site constituents, they are effective in site conditions andare commonly requiredas a component of for identifying visible changes remediation.
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Table 6 Potential Corrective Measure Technologies and Applicable Environmental Media Applicable Media ~ _ _ _ ~
Technology No Action Monitoring Site inspections Institutional Actions Physical barriers Deed restrictions Containment Storm water controls Capping Vertical barriers Filter barriers Subsurface drains Removal Excavation Dredging Recovery wells Interceptor trench Vacuum extraction Treatment Air stripping (soil) Biological (soil) Asphalt batching Soil flushing Stabilization Incineration Air stripping (water) Biological (water) Chemical precipitation GAC adsorption Ion exchange Oxidation-reduction Steam stripping Filtration Neutralization Off-site water treatment Disposal On-site landfill Off-site landfill Surface water discharge Reinjection
Solid waste
Soil
Sediment
Groundwater
Surface water
X
X X
X X
X X
X
X X X
X
X
X X X X X
X
X
X X
X X
X
X X X X
X
X
X X X X
X
X
X X
X X X X X X X X
X X
X X
X X
X X X
X
X
X X X X
X X X X
X X
X
X X
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Table 7 Summary of Potential Corrective Measure Technologies Technology
No Action Monitoring
Site inspections
Institutional actions Physical barriers
Deed restrictions
Containment Storm water controls
Capping
Subsurface vertical barriers
Filter barriers
Long-term periodic groundwater and/or surface water monitoring. Long-term periodic inspection of the site.
Does not reduce conDetect changes in site stituents. Long-term conditions. Monitor expense. effectiveness of corrective measures. Can help detect changes D o e s not reduce constituents. Long-term in site conditions. Enexposure. sure continued effectiveness of corrective measures.
Physical barriers such as a chain-li& fence or vegetation around waste areas.
Periodic maintenance Reduce risk of exposure to site constituents and inspection required. Does not reby restricting site acduce constituents. cess. Reduce risk of site disturbanceby unauthorized intruders. Restricts future usability Reduce hypothetical of the site. risks from future land use and/or groundwater use. Reliability is dependent upon continued enforcement.
Legal limitations placed on future property and/or groundwater use.
May require some Reduces the potential waste disturbance. for erosion of coversoils. Can reduce infiltration of water and migration of constituents. Does not reduce conMinimizes surface waConstruction of a soil stituents. Could ter infiltration and conor multilayer cap to restrict future stituent migration. contain waste areas. development at Reduces risk of contact the site. with waste. Reduces watedwind erosion. Could disturb adjacent Low-permeability verti- Reduces mobility of wetland areas. Effecconstituents and subsecal subsurface barrier tiveness could be quent risk of exposure. to groundwater flow low. Relatively high Could increase the efsuch as a slurry wall. cost. fectiveness of groundwater recovery. Effectiveness is not Could reduce cation A subsurface wall or proven. Could concentrations in surface blanket of require periodic groundwater. Could glauconitic sands replacement. reduce migration of with a high capacity constituents without for adsorbing metal affecting wetlands. cations.
Improve storm water drainage by regrading, vegetation, swales, or pipes.
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Table 7 (Continued) Technology Subsurface drains
Removal Excavation
Dredging
Groundwater recovery wells
Interceptor trench
Vacuum extraction
Treatment (soil/waste) Air stripping
Subsurface drain or trench upgradient of disposal facilities to reroute clean groundwater.
Can be used to lower groundwater levelsto below waste levels to reduce groundwater infiltration and constituent migration.
Limited effectiveness. Could require significant disturbance of the site for construction.
Removal of solid waste and/or affected soils.
Required for subsequent Treatment and/or distreatment and/or disposal Would be reposal of solid waste. quired. Risks Removes potential associated with waste sources of constituexcavation and transents from the site. portation. Removal of affected Reduces potential risks Dredged material would sediments from water associated with afrequire disposal and fected sediments. courses. may require treatReduces potential ment. Dredging acfor migration of contivities would disturb stituents and risk of aquatic life and could exposure. increase constituent migration. Reduces potential miCollection of affected Requires long-term gration of constitugroundwater with operation and mainents via groundwater. recovery wells. tenance. Site condiFacilitates groundtions must be water treatment. suitable for this technology to be effective. Requires long-term A subsurface trench to Reduces potential mioperation and maingration of constitufacilitate groundwater tenance. Site condicollection. ents via groundwater. tions must be Facilitates groundsuitable for this techwater treatment. nology to be effective. Economically limited to shallow depth. Effective for the recov- Not effective for recovExtraction of VOCs ery of heavy metals. from pore spaces in ery of VOCs from Requires high porossoils. Does not resoils. quire excavation of ity and low moisture waste or affected content. Requires air material. treatment. Mechanical screening of soils to increase effective surface area.
Effective for theremoval of VOCs from soils. Implementation is fairly easy. Proven technology.
Not effective for heavy metals or semivolatiles. Air emission controls would be required.
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Engineering Evaluationfor Contaminated Sites Table 7 (Continued)
Advantages
Technology organic Biological Breakdown of
Not effective for removal of heavy metals. Effectiveness can be lowered by a number of site conditions. Limited to certain types of petroleum of constituents. Aphydrocarbonplicable only for containing soils in coarse-grain soils. asphalt pavements. Regulatory agencies may restrict applications. Unrecovered solution Proven to be effective In situ injection of can contribute to for the removalof flushing solution to groundwater degradaheavy metals. Does facilitate collection tion. Only practical not require waste of constituents. for a limited range of excavation. soil conditions. Increases waste volStabilization of constit- Proven to be effective ume. Not effective for reducing the uents by mixing with for volatile organics. leachability of heavy stabilizing agents. Does not destroy metals. Can be conconstituents. ducted in situ. Readily available. Combustion of organic Proven effective for the Does not destroy heavy metals. Residual ash destruction of organic constituents with may require further constituents. Would high-temperature oxitreatment before disremove constituents dation, on-site or posal. Very expenfrom the site. off-site. sive. constituents by microorganisms.
Asphalt batching Reuse
Soil flushing
Stabilization
Incineration
Treatment (water) Air stripping
Disadvantages
Effective for the removal of many organic solvents and petroleum hydrocarbons. Beneficial reuse of affected media. Relatively low cost.
Removal of VOCs from liquids with an airstripping column or other mechanical facilities
Effective for the removal of VOCs from water. Easily implemented and well proven.
Biological
Breakdown of organic constituents by microorganisms.
Effective for the removal of many organic solvents.
Chemical precipitation
Commonly used for the Alteration of pH to reremoval of metals duce the solubility of from water. Proven constituents and facileffectiveness. Could itate precipitation. be used in combination with other treatment technologies.
Not effective for the removal of heavy metals. Air discharge permit would be required. Air treatment may be required. Not effective for removal of heavy metals. Effectiveness can be lowered by a number of site conditions. Recovered sludge may require treatment. Not effective for removal of organic constituents.
cription
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Table 7 (Continued) Technology GAC adsorption
Passing a waste stream through activated carbon to remove constituents by adsorption.
Ion exchange
Removal of toxic metal ions from waste streams.
Oxidation-reduction
Alteration of waste Could be effective for stream to reduce toxthe removal of mercury from waste icity or solubility or to create a waste that streams or affected is easier to handle. waters. May be applicable to the organic constituents at the site. Use of superheated More effective than air steam to stripVOCs stripping for less volatile organics. More from water. cost-effective than air-stripping for highVOC concentrations. Proven effective for the Removal of suspended removal of suspended solids by passing solids. Could be used through a porous in combination with medium. other treatment technologies. Easily implemented and Adjustment of pH to well proven. Relareduce corrosiveness tively inexpensive. and acidity. Could be used in combination with other treatment technologies. Off-site facilities exist Treatment of affected for the treatment of a water at an off-site wide variety of contreatment facility. stituents. A new treatment facility would not have to be built. Best for small quantities of liquids.
Steam stripping
Filtration
Neutralization
Off-site water treatment
Well suited for the removal of VOCs, and has some effectiveness for mercury removal. Readily available and well proven technology. Proven effective for removal of heavy metals.
May not be effective for all site constituents. GAC unit requires regeneration and/or disposal. May not be costeffective. Spent reagent solutions may require treatment. Appropriate reactions must be determined. May not be effective for some combinations of constituents.
Not effective for the removal of heavy metals. Stripped effluent may require treatment before discharge. Not effective for dissolved constituents. Filter backwash would require treatment. Limited effectiveness.
Off-site treatment may not be practical for large volumes of waste. Some risks involved with waste transportation.
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Table 7 (Conrinued) Technology Disposal On-site landfill Placement treated of
or
untreated wastes in a secure disposal facility on site. Off-site landfill
Transportation and disposal of untreated waste or treatment residues at an approved off-site landfill.
Surface water discharge
Discharge of treated water to an existing surface water body.
Reinjection
Injection of treated water into the ground through injection wells or infiltration galleries.
Could provide for secure containment of wastes and/or treatment residues. Reduces mobility of constituents and subsequent risk of exposure.Removes constituents from the site. Increases potential for future use of the site. Proven means of disposing treated water. Surface water body is readily available. Has been proven to be effective.
Construction would involve high costs and site disturbance. Long-term maintenance would be required. Limited off-site landfill capacity. Potentially high costs. Potential long-term liability. Untreated materials may be restricted from land disposal. NPDES permit would be required. Periodic sampling and maintenance would be required. May not be practical in low-permeability aquifers. Groundwater modeling may be required. Could increase constituent migration.
B. InstitutionalActions Institutional action technologies reduce potential exposures to site constituents by indirect methods rather than by containment or treatment of the affected media. Institutional actions include physical barriers and deed restrictions. Physical Barriers Description. Physical barriers provide an easily implemented, low-cost method for restricting pedestrian and animal traffic across areas of concern, thus decreasing the potential for exposure to site media or damage to on-site storageor containment structures. Physicalbarriers could range from chain-link security fencing to closely grouped rows of obstructive vegetation. be required to maintain the integrity of the barrier. Periodic inspection and maintenance would Evaluation. Physical barriers do not reduce site constituent levels, but they canbe effective for protecting human health and the environment by preventing exposureto affected media. Deed Restrictions Description. Deed restrictions place legal limitations on future property use. These restrictions can prohibit future property and/or groundwater uses that could result in increased exposure to site constituents. Deed restrictions can be easily implemented, but their effectiveness is dependent upon continued enforcement.
Wilson et al.
66 Table 8 Potential Corrective Measure Technologies and Applicable Data Requirements Required data items"
No Action Monitoring Site inspections Institutional Actions barriers Physical restrictions Deed Containment Storm water controls Capping barriers Vertical Filter barriers Subsurface drains Removal Excavation Dredging Recovery wells Interceptor trench
I
Air stripping (water) Chemical precipitation GAC adsorption Ion exchange Oxidation-reduction Steam stripping Filtration Neutralization Off-site water treatment Disposal landfill On-site Off-site landfill Surface water discharge Reinjection
.. . 0
* I
l
1 . 1 .
.
.
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Table 9 Description of Data Items Groundwater chemistry(Conr.) Topographylsetting Conductivity Ridges Dissolved oxygen Valleys Nutrients Hills Chemical oxygen demand Surface water bodies Total organic carbon Forests Constituent concentrations Wetlands Heavy metals Drainage patterns Source area characterization Buildings Location Vegetation “ypeldesign Climatological data Operating practices and Evaporation history Evapotranspiration Physical condition Temperature Wind speed and direction Age Method of closure Precipitation Type of wastelclassification Atmospheric pressure Volume/extent Relative humidity Constituent chemical properties Regionallsite geology Geologic units PH Molecular weight Strata Hydrolysis Strike and dip Chemical class Folding Viscosity Faulting Solubility Depositional history Oxidation/reduction Soillrock types potential Aquifer characteristics Vapor pressure Hydraulic conductivity Sorption Porosity Biodegradability Grain size distribution Photodegradability Saturated and unsaturated Chemical transformations zones Migration potentiall Attenuation characteristics leachability Extent Constituent physical properties Depth Physical form Thickness Potential migration pathways 5pe Temperature Rechargeldischarge areas Density and amounts Boiling point Aquifer leakagelinteractions Soil physical properties Groundwater Flow Soil classification Water level contours Grain size distribution Vertical and horizontal flow Soil profilelstratigraphy TidaVseasonal influences Permeability Man-made influences Density Groundwater chemistry Porosity PH Moisture content Total dissolved solids Infiltration Total suspended solids Storage capacity Biological oxygen demand Mineral content Alkalinity
Soil physical properties(Cont.) Settlement potential Soil index properties Erosion potential Soil chemical properties PH Organic content Sorptive capacity Ion-exchange capacity Constituent types Constituent concentrations Surface water characteristics Location Elevation Area Depth Velocity Width Inflowloutflow Temperature Seasonal fluctuations Flood plain Stream cross sections Surface water chemistry PH Total dissolved solids Total suspended solids Biological oxygen demand Alkalinity Conductivity Dissolved oxygen Nutrients Chemical oxygen demand Total organic carbon Constituent concentrations Heavy metals Sediment characteristics Depositional area Thickness Grain size distribution Density Organic carbon Ion exchange capacity PH Constituent extentlvolume Horizontal and vertical extent in waste Horizontal and vertical extent in soil Horizontal and vertical extent in sediments
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Table 9 (Continued) Constituent extent/volume (Cont.) Horizontal and vertical extent in groundwater Horizontal and vertical extent in surface water Total volume Concentration profiles Constituent migration Horizontal and vertical migration direction
Contituent migration (Cont.) Horizontal and vertical migration rate Factors influencing migration Potential future movement Potential receptors Age Location Use Population
Potential receptors (Cont.) Ecology Endangered species 'Iteatability Study Bench-scale tests Residual characteristics TCLP Waste samples SoiYsediment samples Water samples
Evaluation. If enforced, deed restrictions can be effective in reducing the potential for disturbance of affected site media.
C. Containment Containment technologies reducethe potential for direct exposure to site constituents and the potential for their migration by physically isolating the affected media or wastes. Storm Water Controls Description.Stormwatercontrolssuch as surfaceregrading,increasedvegetation, drainage swales, and drainpipes can be used to improve the drainage of surface water away from waste disposal facilities such as landfills and impoundments. Improved drainage can reduce the potentialfor erosion of cover materials, reduce infiltration of surface water, and minimize the potential for ponding of surface water. Evaluation. This technology can be effective for reducing the potential volume of leachate produced by infiltration of surface water. Surface water controlsalso minimize erosion and subsequent migrationof constituents. Data Requirements. Data needs include: topography (extent of wetlands, existing drainage patterns, drainage area), soilphysical properties (erosion potential), andclirnatological data. Caps and Liners Description. Capping is a common containment technology thatis used to prevent direct contact with wastes, reduce the infiltration of surface water and subsequent leaching of constituents, prevent erosionof waste materials, and control surface runoff. Low-permeability caps such as clay caps and multilayer capsare the most effective caps for waste containment. Multilayer caps typically consistof an upper vegetative layer underlainby a drainage layer, an impermeable synthetic membrane liner, and a low-permeability clay layer and are usually more effective than clay caps at restricting surface water infiltration. Evaluation. This technology is one of the most common technologies for the containment of hazardous waste, and it has been proven to be effective. Multilayer caps are appropriate for improving waste containment and reducing the potential for leaching of constituents. Data Requirements. Data requirements include topography (extent of wetlands, surface slopes, vegetation conditions), climatological data, source areacharacterization (extent, depth, and volume of waste), constituent chemicaland physical properties, and soil physical properties (infiltration, permeability of existing covers and waste, bearing capacity of waste, potential settlement).
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Vertical Barriers Description. Subsurface vertical barriers (e.g., slurry walls, membrane walls, grout curtains, sheet piling) can contain land-disposed wastesby restricting the lateral flow of groundwater through the waste, thereby reducing the potential for migrationof constituents into the groundwater. Vertical barriers can also allow pumping within the contained area without significant water level drawdown of nearby water bodies (rivers, lakes, etc.). Evaluation. Vertical barriers have been used successfully in a number of waste containment applications. Vertical barriersare most effectiveat shallow depths and when they canbe keyed (or tied) into a confining substratum that can restrict the flow of groundwater beneath the barrier. Because vertical barriers restrict the flow of groundwater, they can impact nearby ecosystems such as wetlands, rivers, and streams. Data Requirements. Data requirements include topography (extent of wetlands), aquifer rate characteristics (physical propertiesof confining layer, Columbia Aquifer profile, discharge to surface water), groundwater flow (depth to water table, groundwater flow rates and direction, hydraulic gradient), and constituent extent and migration. Filter Barriers or ablanketfilterbarriercould be created Description. A verticalwallfilterbarrier by the placement of natural sediments having high cation-exchange capacity (e.g., glauconitic greensand) in a vertical trench or as a blanket, respectively. Glauconitic greensands are sediments with a high capacity for adsorbing cations (e.g., lead, mercury) from liquid solutions. Evaluation.Bench-scaleexperimentshavedemonstratedthatgreensanddepositsfrom the Delaware Coastal Plain can remove heavy metals from spiked water samples and landfill leachates. Greensands havebeen shown to retain more mercury from basic (high pH) solutions than from acidic (low pH) solutions.To date, the effectivenessof natural greensand filter barriers has not been proven in field applications, and a pilot field study wouldbe required prior to full-scale implementation. Data Requirements. Data needs include sources of greensand or other media with high cation exchange capacity; extent of groundwater degradation, if any; extent of wetlands; depth to water table; groundwater flow rates and direction; hydraulic gradient; aquifer profile; disbe required to charge to surface water; and surface water flow rates. Bench-scale testing would evaluate this technology on a site-specific basis. Subsurface Drains Description. A subsurface drain or drains could be constructed hydraulically upgradient of any disposal facility impacting groundwater to lower the groundwater table to a level below the bottomof the impoundment(s). This technology is considered a containment action because it physically isolates the waste from the underlying groundwater and reduces the potential for leachateproductionand/orleachatemigration.Groundwatercollectedintheupgradient drain(s) could be discharged to a nearby stream or storm water drainage system. Subsurface drains can also be used to collect groundwater affected by site constituents. This technology is further discussed in Section D under groundwater removal. Evaluation. If the groundwater table is periodically above the bottom of a landfill or disof posal impoundment, this technology couldbe effective for reducing the potential migration constituents into groundwater. Data Requirements. Data requirements include depth to groundwater table, groundwater flow rates, depth of impoundment bottoms, soil and waste hydraulic conductivities or transmissivities, groundwater gradient, and aquifer profile.
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D. Removal Removal technologies focus on the physical removal of affected media from the site, usually to facilitate treatment and/or disposal. Solid Waste, Soil,andlor Sediment Removal Excavation Description. Excavation is applicable for the removal of solid wastes and affected soils and sediments. If materials must be removed, they can typically be excavated using standard methods andpractices, although the volume of material to be removed could necessitate staged excavation or other special handling requirements. Evaluation. Although excavation alone is not a remedial technology, it may be required in conjunction with other treatment and/or disposal actions. Fugitivedust and volatilization of constituents into theair during excavation could pose short-term risks to human health and the environment. Data Requirements. Data requirements include extent, depth, and volume of waste and/or affected soils and sediments; soil physical properties; and constituent concentrations. Dredging Description. Dredging is the removal of sediments from under water, which may be accomplished using hydraulic systems, clamshells, or other means of excavation. Evaluation.Dredgingcaneffectively remove affectedsedimentsfromsurfacewater courses. However, dredging operations can disturb aquatic life and potentially result in increased mobilization of constituents downstream. DataRequirements.Datarequirementsincludestreamflow rates, stream profile,and sediment characteristics (extent, depth, and volume of affected sediments). GroundwaterRemoval Recovery Wells Description. Affected groundwater can be collected with pumping wells located in the affected aquifer, hydraulically downgradient of the constituent source area(s). By intercepting groundwater, the migration of constituents from the source areas can be restricted. Collected groundwater would probably require some type of treatment prior to discharge. Evaluation. Recovery wells can be effected for long-term groundwatercollection in certain aquifers, although this technology may not be appropriate under all site conditions. Recovery well systems are most effective when measures are taken to remediate the source(s) of constituents migration. DataRequirements. Data requirementsincludedepth to water table, groundwater flow rates and direction, hydraulic gradient, aquifer characteristics, and extent of affected groundwater. interceptor Trench Description. An interceptor trench is typically constructedby excavating a narrow trench into a stratum of concern, placing a perforated drainpipe alongthe bottom of the trench, and backfilling the trench with some type of drainage aggregate. The drainpipe conveys waterby gravity to a collection sump, and the water collectedin the sump is pumped out for treatment and/or discharge to surface water. Evaluation. Interceptor trenches havebeenfound to be effective for the collection of shallow groundwater in unconsolidated aquifers, but they are not appropriate for all site conditions. At shallow depths(10-20 ft), interceptor trenchesare typically more cost-effective and perform better than recovery wells in unconsolidated aquifers.
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Data Requirements. Data requirements include depth to water table, groundwater flow rates and direction, hydraulic gradient, and aquifer characteristics. Vapor Extraction Description. A vacuum extraction system consists of a network of vapor withdrawal (or vacuum) wells installed in the vadose (unsaturated) zone to remove volatile organic compounds (VOCs). A vacuum is applied to promote movement of air and VOCs to the extraction well points. An in-line water removal system can be provided to remove condensate and limited quantities of recovered groundwater. Recovered vapors are treated in an in-line vapor-phase carbon adsorption system (forVOC capture) or an exhaust fume incineration system (forVOC destruction). Steam can be injected through injection wells to enhance the vacuum extraction process. Vacuum extraction is considered for the removal of VOCs from wastes and affected soils. Evaluation. Vacuum extraction has been provento be effective for the removal ofVOCs from the vadose zone. This technology can be used to remediate soils in place, thereby minimizing disturbanceto waste facilities that result from excavation and removal operations. This technology is only effective for removal of volatile constraints. Data Requirements. Data requirements include depth to groundwater, physical and chemical properties of soils and wastes, soil and groundwater quality, and extent of affected soil and waste.
E. Treatment Treatment technologies reducethe toxicity, mobility, or volume of affected media or wastes, thus reducing the potential for constituent exposure to human health and the environment. Removal and disposal technologies may be required in conjunction with treatment alternatives, although some treatment technologies can be implementedin situ. Solid Waste, Soil, andlor Sediment Treatment Air Stripping Description. Air stripping of volatile organic compounds (VOCs) in soils can be accomplished by a process of mechanical screening that involves the passage of excavated soils through oneor more vibrating screens commonly used in construction projects. In this process, the rate of volatilization is maximizedby soil disaggregation and the resulting increase of effective soil particle surface area. Evaluation. The removal of VOCs from wastes and/or affected soils can reduce the potential for migrationof VOCs into groundwater. The equipment, materials, and labor required to perform VOC stripping of soils are readily available. Excavation of materials is required prior to implementation of this technology, which would disturb the site and potentially result in short-term exposure. Air emission and erosion and sedimentation controls and permits would likely be required. DataRequirements.Datarequirementsincludewastevolume,chemicalandphysical characteristics of constituents, and soil physical properties. Biological Treatment Description.Biologicaltreatment,sometimesreferred to as bioremediation,generally refers to the breakdown of organic constituents by microorganisms. The most common processes are based on aerobicor anaerobic bacteria, suchas those processes used in the treatment of muncipal wastewaters. In situ, pump-and-treat, solid-phase, slurry phase, and soil heaping biological treatment techniques have been used to remediate contaminated sludges.
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Effectiveness. The effectiveness of biological treatment can be influenced by a number of parameters, including pH, temperature, availability of nutrients, and the presence ofheavy metals. Some biological treatment methods have been successful for the treatment of certain organic solvents,but this technology is not effective for the removal of heavy metals from solid waste or soils. Data Requirements. Data requirements include physical chemical characteristics of soils and physical and chemical properties of constituents. Asphalt Batching Description. Asphalt batching can be used as an alternative to landfillingor on-site treatment of soil containing petroleum hydrocarbons to recycle it into useful products such as asphalt paving material. After testing for petroleum hydrocarbon content (and the presence of hazardous substances), the soil is delivered to the batch plant, where it is crushed and sieved through screens to remove wood, metal, or other undesirable debris. Next, the soil is passed through a gas- or oil-fired rotary kiln, where it is heated to approximately 350°F, which evaporates all the water and burns the petroleum hydrocarbon components. Thereafter, the soil is blended with other aggregates and asphalt and delivered to the paving location. Evaluation. Recycling by asphalt batching removes the contaminants from the original site, destroys undesirablematerials, and providesa valuable constructionmaterial. Reasonable safety cautions during the initial haulingand storage and adequate quality testingare the only major technical difficulties related to asphalt batching. Local regulatory agencies may have administrative control and shouldbe contacted prior to using this process. This process is best for sandy soils rather than clay soils, because a high percentage of clay is not desirable in asphalt paving materials. Data Requirements. Data requirements include chemical types and concentrations, physical properties of the soil, particularly grain size analysis, and the extent of constituents. Soil Flushing Description. Soil flushing involves the in situ injection or percolation of large volumes of flushing solution to an area of waste and/or soil requiring remediation and the subsequent collection or recovery of the flushing solution. The solution is intended to dissolve or precipitate constituents as it passes through the affected media. Water is a common flushing solution, although aqueous surfactant solutions and organic solvents have also been used. Well points, subsurface drains, or anothertype of collectionsystemmustbe installed to collect the constituent-laden solution. The recovered solution requires treatment. Evaluation. Soil flushing has been proven to be effectivefor the removalof heavy metals from solid wastes and soils. Soil flushingmay not be effective for all organic constituents,but the recovered groundwater and flushing solution can typically be treated with an alternative method for the removal of organics. Cloggingof soil pores can limit the flushing of the soils, thereby reducing the effectiveness of the treatment and preventing recoveryof the flushing solution. Unrecovered flushing solution can contribute to increased groundwater degradation. Data Requirements. Data requirements include chemical and physical properties of soil, aquifer characteristics, groundwater flowrate and direction, physical and chemicalproperties of constituents, and the extent of constituents. Stabilization Description. Stabilization technologies have been used to immobilize organic and inorganic compounds in wet or dry media, using reagents to produce a stable mass. Stabilization methods include cement-based methods, silicate-based (pozzolanic) methods, thermoplastic methods, organic polymer methods, and others. Waste materials and/or affected soils can be mixed in place with shallow or deep soil mixing systems, or the affected materials can be ex-
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cavated and consolidated into oneor more locations before mixing. 'I)lpically, this technology does not destroy constituents but incorporates them into a dense, homogeneous, low-porosity structure that reduces their mobility. Stabilization has been proven to be effective for immobilizing inorganic constituents (e.g., metals) but hashad limited success for volatile organics. Because a reagent must be added to the soil, the volumeof treated soilmay be greater than the original soil volume by as much as 20-100%. Evaluation. This technology can restrict the migration of constituents from the waste impoundments and reduce the potential for subsequent exposure. It can be implemented in place. However, its applicability to heterogeneous wastesmay be limited, as the proper selectionof a stabilizing reagent for amixed waste may be difficult. DataRequirements.Datarequirementsincludebench-scale or pilot-scaletestresults, leachability of unstabilizedandstabilizedmasses,physicalandchemicalcharacteristics of waste, and the extent and volume of waste. Incineration Description. Incineration is a thermal treatment method that uses high-temperature oxidation under controlled conditions to degrade waste materials into by-products that include carbon dioxide, water vapor, ash, nitrous oxide, sulfur dioxide, and hydrochloric acid gases. Air pollution controlsare typically needed to ensure that air quality standards are not violated. Residual incinerator ashmay require stabilization priorto disposal due to high concentrations of metals. Typesof incinerators that are commonly used for the remediation of solid waste and transsoils include rotary kiln, fluidized bed, and infrared incinerators. Excavated soils can be ported off-site for incineration,or they can be treated with a mobile incinerator assembled on site. Incineration is not effective for the destruction of metalsin solid media. Evaluation. Incineration is a feasible and well-developed technology for the destruction of organic constituents, although residual ash would require stabilization prior to disposal if high metals concentrations are present. Due to cost considerations, off-site incineration is most appropriate for smallto moderate volumes of materials; on-site incineration would only be appropriate for the incineration of large volumes of material. Waste handling and stack emissions could present risks to human health and the environment unless appropriate healthand safety equipment and emission controls are employed. Limited availability and capacity of off-site incinerators can result in lengthy remediation schedules if large volumes of waste are present. Public oppositionand permitting requirements could hinder the sitinganofon-site incinerator. Data Requirements. Data requirements include a treatability study (test burn); ash content, heat value, and halogen content; and physical and chemical characteristics of waste. Liquid Treatment The liquid treatment technologies presented here may be applicable to the treatment of affected groundwater, surface water, or water generated or collected during solid waste treatment. Off-Site Water Treatment Description. Affected groundwater recovered from the siteor water resulting from some type of on-site treatment process is treated at an off-site treatment facility. Water is pumped or hauled via tank trucksto an off-site treatment facilityor discharged to a publicly owned treatment works (FQTW) upon authorization by the POTW. Evaluation. Off-site facilities have been used successfully for the treatment of water containing various constituents.If a large volume of water requires treatment, hauling the to water an off-site facility can be extremely expensive and impractical. If the water is hazardous, a permit is required for off-site transportation. Data Requirements. Data requirements include identification of a facility, pilot-scaletest, ground and/or surface water chemistry, and physical and chemical properties of constituents.
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Air Stripping Description. Air stripping to remove organics from groundwateris most often performed by passing the water through a countercurrent air-stripping column tofacilitate the transfer of volatile organics fromthe liquid phase to the gas(air) phase. Off gas treatmentmay be required for vapor-phase VOC removal. Other methods of air stripping are employed that do not entail countercurrent air andliquidflow(diffused aeration andmechanical aeration); however, packed tower stripping is most widely used. Evaluation. Air stripping has been proven to be effective for the removal ofVOCs from groundwater, although it is not effective for the removal of metals. Stripping efficiency is related to the air-to-water ratio, system temperature, tower packed depth, and volatility of the constituents. An air discharge permit is required, and offgas treatment may be required. Air stripping can be prone to fouling, depending on mineral and bacterial levels in the water. Data Requirements. Data requirements include groundwater and/or surface water chemistry, physical and chemical properties of constituents, and effluent target levels. Biological Description. In situ biotreatment is the aerobic or anaerobic microbial degradationof organic constituents in groundwater. The complete biodegradation process converts organics to carbon dioxide and water (aerobic) or carbon dioxide and methane (anaerobic). In biotreatment, a microbial population (biomass)is maintained, by injection of nutrients and oxygenor methane, to feed on the organic substances present in the water. The microbial populations available for forming the biomass are numerous, and many alternative designs are available for both aerobic and anaerobic systems. Effectiveness. Biotreatment is well documented in attaining high organic removal efficienciesmeasured as COD or BOD reduction. However,some organicsubstances do not biodegrade but are stripped to the atmosphere as the water is treated. Biotreatment methods are most effective for water that has high VOC concentrations. High levels of inorganic cations (specifically calcium, iron, and magnesium) can exert toxic effects on anaerobic treatment processes. Data Requirements. Data requirements include groundwater and/or surface water chemistry, physical and chemical properties of constituents, and effluent target levels. Chemical Precipitation Description.Chemicalprecipitationprocesses are commonlyused to removemetals from water. Chemical precipitation is pH dependent in that acid or base is added to a solution to adjust the pH to a point where the constituents to be removed have their lowest solubility. Frequently a bulking agent such as lime or a soluble salt that forms insoluble metalprecipitates is employed as a precipitant. The precipitant (lime, caustic, or sulfide) is added to the metalcontaining water, and thenthe solution is mixed with a flocculating agent (polymer)to promote solidsagglomeration andsettling.Someorganic or inorganicsubstances (e.g., chelants) present in the water may inhibit precipitation. Evaluation. This technology could be effective for the removal of metals from collected groundwater and/or process streams from other treatment technologies. Sludge produced during the process would have to be dewatered or treated before disposal. Data Requirements. Data requirements include bench-scale (iar) test results (precipitant and/or surface water chemistry; physdoses, setting rates, and sludge production); groundwater ical and chemical properties of constituents; and effluent target levels. Granular Activated Carbon Adsorption Description. Carbon adsorption involves contact of a waste stream with activated carbon, usually by downward flow througha series of packed bed reactors. Molecular adsorption
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onto granular activated carbon (GAC) occurs through physical and/or chemical forces in which molecules are held on the surface of the carbon particle. A compound’s affinity for carbon depends on its molecular size and water solubility. Constituents with high molecular weights and low water solubilities exhibit a high affinity for carbon. Evaluation. Granular activated carbon adsorption is well suited forthe removal of VOCs, with the exception of vinyl chloride and chloromethane compounds, and some inorganic constituents such as mercury have also shown a limited potential for adsorption onto GAC. AlthoughGACadsorptiondoesnotproduceanyoff-gases, the spentcarbon would require regeneration or treatment prior to disposal. Adsorption of metals onto GAC may limit the regenerability of the carbon. GAC is also subject to bacterial fouling. Data Requirements. Data requirements include groundwater and/or surface water chemistry and physical and chemicalproperties of constituents. Also, a pilot test would be required to determine the adsorption capacity of the carbon for the organics to be removed and to estimate the carbon bed life. Ion Exchange Description. GAC adsorption is commonlyused for the removal of toxic metal ions from solutions when recoveryof the concentrated metal isfeasible and/or a relatively low proportion of competing non-heavy metal ions are present. The design of an ion-exchange system must consider the specific ionsto be exchanged, the controlof influent bacteria and suspended solids, and the treatment of spent regenerant solutions. Evaluation. Ion exchange could be effective for the removal of metals from collected groundwater and/or process waste streams. Highly concentrated waste streams can usually be separated with more cost-effective technologies. Data Requirements. Data requirements include pilot-scale test results, groundwater and/ or surface water chemistry, and physicaland chemical properties of constituents. Oxidation-Reduction Description. Chemical oxidation-reduction reactions are used to reduce toxicity or solubility or to transform a substance to one that is more easily handled. Chemical reduction is primarily used for the treatment of wastes containing hexavalent chromium. Common reducing agents include sulfurdioxide, sulfite salts, and ferrous sulfate. Chemical oxidationcan be used for the treatment of oxidizable organics. Chlorine (hydrochloride), hydrogen peroxide, and ozone are common oxidizing agents. Ultraviolet light is used as a catalyst to facilitate chemical oxidation reactions. Evaluation. Chemical reduction could be effective for the removal of reducible organics from collected groundwater and/or process waste streams. Chemical oxidation may be applicable to some organic constituents. DataRequirements.Datarequirementsincludepilot-scaletest results, groundwater and/or surface water chemistry, physical and chemical properties of constituents, and benchscale testing. Steam Stripping Description. Steam stripping consists of passing superheated steam countercurrent to a preheated groundwater stream in a packed or tray tower to strip VOCs and other organics into the vapor phase. The off-gases from the columnare routed to a condenser, which can be cooled using groundwaterin a non-contact mode. From the condenser, the condensate flows to a decanter, where the organic layer is drawn off and the bottoms are routed to the stripping tower influent. Effluent from the stripping tower can be routed to a polishing treatment or discharged. The efficiency of stream stripping can be increased by operating the column at a negative pressure.
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Evaluation. Steam stripping will treat less volatile and more soluble wastes than air strip be effective ping andis more cost-effectivefor high VOC concentrations. Steam stripping could for the treatment of collected groundwater and/or process water containing highoflevels VOCs. Stripped effluent may require treatment prior to discharge, and air pollution controls would likely be required to reduce toxic emissions. DataRequirements.Datarequirementsincludepilot-scale test results, groundwater and/or surface water chemistry, physical and chemical properties of constituents, and benchscale testing. Filtration Description. Filtration is a process of separating and removing suspended solids from a liquid by passing the liquid througha porous medium. Filtrationis a proven technology for the removal oflow levels of suspended solids from liquid waste streams. In particular, granular media filters have been demonstratedto be effective as partof mobile treatment systems as well as on-site systems for hazardous wastewater treatment. filter The backwash would require treatment prior to disposal due to high solids levels. Evaluation. This technology could be effective for the treatment of collected groundwater and/or process waters that have constituents, such as metals, in the form of suspended as ion exchange solids. Filtration canbe an effective pretreatment process for technologies such and carbon adsorption. Data Requirements. Data requirements include groundwater and/or surface water chemistry and physical and chemical properties of constituents. Neutralization Description.Neutralization is used to treat wasteacids and wastealkalies(bases) to eliminate or reduce their corrosiveness. More frequently, neutralization is used as a pretreatment step prior to other liquid waste treatment processes. Care should be taken to avoid the formation of hazardous compounds. Evaluation. Neutralization may be required prior to the treatment or disposal of collected groundwater and/or process waste streams. Data Requirements. Data requirements include pilot-scale test results, groundwater and/ or surface water chemistry, and physical and chemical properties of constituents.
F. Disposal Disposal technologies provide secure, permanent containment of affected media or wastes, thus reducingthe potential for exposure to or migration of constituents. A removal action would be required prior to the implementation of any disposal action.
Disposal of Solids On-Site Landfills Description. On-site landfilling is the placement of treated or untreated wastes in a disposal unit (such as a landfill, surface impoundment, or vault) constructed on site to meet the relevant standardsof RCRA and anyother applicable federal and state requirements. An on-site landfill would provide containment and secure storageof affected solid media. Therisk of exposure to constituents would be reduced by minimizing the mobility of the waste. Evaluation. The construction of a new, approved, on-site landfillmay be impractical due to the required disturbance of the waste, limited site area, and shallow groundwatertable. The construction of an on-site landfill would involve high capital costs and long-term maintenance. Data Requirements. Data requirements include site characteristics, permit requirements, and physical and chemical properties of the waste.
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Off-Site Landfill Description. Off-site landfilling is the transportation and disposal of untreated wastes or treatment residues at an approved off-site landfill. An off-site landfill could provide for the secure containmentof affected mediaor wastes. Wastesor treatment residues (e.g., incinerator ash) would have to meet certain analytical parameters (e.g., TCLP) before off-site disposal would be permitted. The risk of exposure to site constituents would be eliminated by removing the affected mediaor wastes from the site. Excavationwould be required prior to the disposal of the waste. Evaluation. This technology could be a component of remediation for alternative corrective measures that generate wastes or treatment residues that cannot be contained on site. However, off-site disposalof waste would be relatively expensive, and existing landfill capacity is limited. Permits would be required for transportation of wastesto a permitted facility. Data Requirements. Data requirements include identification of a facility, permit requirements, and physical and chemical properties of the waste. Disposal of Liquids Surface Water Discharge Description. Surface water discharge is the discharge of treated water to an existing body of surface water. Priorto discharge, the water would have to meet all applicable environmental standards including Surface Water Quality Criteria (SWQC) and National Pollution Discharge Elimination System (NPDES) standards. Evaluation. Discharge to surface water is a proven means of disposing of treated water. Because discharged water would have to meet all applicable environmental standards, there would be no adverse impacts on human health or the environment. The equipment, materials, and labor requiredto construct a surface water discharge line are readily available. An NPDES permit and other permitsmay be required prior to discharge, or a current NPDES permit may require modification to include the new discharge. Periodic water quality sampling and maintenance of the discharge line would be required. Data Requirements. Data requirements include surface water characteristics and effluent limitations. Reinjection Description. Reinjection refers to the pumping or gravity flow of treated groundwater back into the ground through injection wells or infiltration galleries. Reinjection could be used for the disposalof treated waterif surface water discharge is not possible. Treated water would have to meet all applicable environmental standards before reinjection. Reinjection can also be used to provide more control during groundwater collection. Evaluation. Reinjection has been used successfully at a number of sites for the disposal of treated groundwater. Clogging of reinjection wells from silting, mineralization, and/or biological growth can reduce the effectiveness of reinjection. Reinjection of water into aquifers with low permeabilities may not be practical. Groundwater modeling couldbe required to determine the implementability of reinjection and to determine which reinjection method (injection wells or infiltration galleries) would be the most appropriate. Data Requirements. Data requirements include aquifer and groundwater flow characteristics and effluent limitations.
V. DEVELOPMENT AND EVALUATION OF REMEDIAL ALTERNATIVES This section is intended to serve as a primer on the process of remedy selection. It focuses on the identification, development, evaluation, and selection of appropriate remedial measures,
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technologies, techniques, and procedures. The term “remedy selection” is used in the broad sense of referring to the series of evaluations and assessments that take place inthe practice of determining the most appropriate remedial action fora contaminated site. The reader can and should read the various guidance documents issued by the USEPA and other regulatory agencies that outline in detail the proper procedures for evaluating remedial technologies and alternatives.Thesedocumentseffectivelypresent the agencies’requirements.This chapter attempts to present, froma practitioners’ point of view, how the process really worksor sometimes doesn’t work. In this section we review the basics of the remedy evaluation and selection process from an engineeringperspective. The properengineeringperspectiveis essential tomaintain a solution-oriented focus, as it is easy for the practitioner to become overwhelmed and sidetracked in the regulatory procedural requirements of remedy selection and losethe focus of the real goal-identification and selection of a balanced, protective, and cost-effective approach to remediation of a given site. Obviously, this must be achieved in a manner that satisfies the regulatory requirements, but it remains essential to keep the proper technical perspective. It is important for the engineering practitioner to understand that remedy selection, like many technical endeavors, is an art as well as a science. We firmly believe that in most cases there is not just one “technically correct” remedy for a site or that a singular correct remedy will be identified by any qualified engineer if one follows the specified procedures as defined in various guidance documents prepared by state and federal regulatoryauthorities. Rather, the appropriate remedy for a site is a balance of several factors that include technical, business, and financial considerations as well as environmental protectiveness, compliance with standards, and effectivenessand other criteria prescribed by the USEPA in the National Contingency Plan (which serves as the main regulatory document outliningthe requirements for proper remedy selection under the federal Superfund program andwill be addressed later).
A. Setting Remedial Objectives The fmt step in selecting an appropriate remedyfor a site is to set realistic objectives. Remediation objectives should be focused on ensuring a suitable level of protectiveness of human health and the environment. Political, regulatory, and financial considerations should come into play when assessing how to achieve the objectives, not in setting them. Some of the key factors that must be integrated into setting the objectives are as follows: 1. Real environmental and human health risk (based on realistic exposure scenarios) Current andplanned site use Useof surroundingproperty Impact of site on surrounding areas Assessment of active versus historical contaminant migration 6. Public opinion/support/opposition 7. Technicalfeasibility of achieving objectives
2. 3. 4. 5.
With these factors in mind, there is one overriding questionthat frames the core of any set of objectives regarding remedy selection: What remedy should be implemented? This may seem like an obvious question, but very often the remedy selection process focuses on Whatremedy can be implemented? Whenthe focus is on what is possible rather than and cost-effective,an unnecessarily expensive remedy what is necessary, appropriate, feasible, will almost certainly be selected. The possible remedy, which often attempts to remediate to background or preexisting conditions, often imparts little or no added risk reduction but costs substantially more andimparts an increase in short-term risk duringthe implementation period
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due to the inevitable site disruption, which is typically proportional to the aggressiveness of remediation. Therefore, like most endeavors, there is a point of diminishing returns beyond which additional beneficial effectsare not obtained and in fact negative effects are probable. The assessment of should versus can goes to the most basic, yet controversial issues dominating site remediation projects after nearly 12 years of implementing Superfund and other remediation programs: How clean is clean? The answer to this fundamental question is different if you assess it from a technical or scientific perspective than from a political, cost-benefit, regulatory, or financial perspective. Therefore, by definition, the final answer is site-specific and must properly integrate and bafance the many factors that drive the remedy selection process at each site. A negotiation process must therefore take place between the parties that have genuine interests at each site. Federal and state environmental regulatory agencies usually allow and even facilitate this negotiationprocess by offeringtothe parties having a financialliability and responsibility to remediate the site the privilegeof taking the lead technicalrole in defining and evaluating alternative remedies. The financially liable parties may include the current owner/operator, all past owners/operators, or a party that sent any waste to the site (e.g., a company that formerly used a permitted landfill); these are known as potentially responsible parties (PRPs).
B. RemedySelectionCriteria The National Contingency Plan (NCP),as revised in March 1990, sets forth the selection criteria to be used in the evaluation of remediation alternatives. Remedies are evaluated in three steps in accordance with the NCP and USEPA guidance documents: 1. Identification and evaluation of remedial technologies-usually targeted at one or more specific site problems 2. Combining of suitable technologiesintoremedialalternatives-usuallyintendedtoaddress the entire site 3. Detailed analysis of alternatives-intended to support subsequent remedy selection The first two steps are initially evaluated according to three criteria: effectiveness, implementability, and cost. Upon review of the definitions of these three criteria, as stated in the NCP, the reader will see that they are essentially a repackaging of the expanded set of nine criteria prescribed for the third step. Therefore, in this text, the focus is on the nine criteria for three reasons: ( l ) The step-by-step procedureis already described in the NCP; (2) the focus of this section is on the overall concept and thought process of remedy selection, not on strict procedures; and (3) the practitioner is well served to keep all nine criteria in mind from the beginning. The nine criteria are listed below in the prioritized order the NCP has assigned to them. Each listing includes an explanationof what the criterion means, or should mean, in practical application. Threshold Criteria These two criteria must be met. Protectiveness of Human Health andtheEnvironment. Attaining an acceptableresidual risk presented by the site. The target residual risk must be within the range of one additionalcancerriskper 10,OOO population (1 X to oneadditionalriskper1 O , OOO , OO (1 X Thesecalculatedrisklevelsareverysensitiveto the set of exposure asare sumptions made and the toxicological parameters and factors applied. These factors
Wilson et al. continuously being updated; however, itoften takes years for new information to be accepted in practice, especially if it shows less risk than previously assumed. Compliance with Applicable or Relevant and Appropriate Requirements (ARARs). Compliance with promulgated laws and standards. Often, proposed standards, guidelines, and agency policiesare incorrectly “applied” as if they were promulgated standards. This criterion is often the focus of controversy between the agency and the owner or PR€! In addition, there are waiver provisions in the NCP that are sometimes granted when ARARs are particularly burdensome or expensive or when compliance actually increases site risk (usually short-term risk). Balancing Criteria These five criteria should optimize the remedy. The first three are relatively more important: The ability of the remedy to achieve and mainLong-Term Effectiveness and Permanence. tain the target protectiveness on a long-term and permanent basis.It is important to note thatoftentheconcept of permanence is incorrectlyinterpretedtomean “destruction.” Treatment and containment remedies can also be permanent with proper control and maintenance. Reduction in Toxicity, Mobility, or Volume. The degree to which the contaminantsare altered or recycled to render them less toxic or less likely to migrate or reduce their volume. Zmplementability. An assessment of whether the remedy can be constructed and executed in a technicallyfeasiblemanner to achievetheremediationobjectives.Availabilityand proven performance are addressed under this criterion. The other two balancing criteria are considered to be relatively less important: Short-Term Effectiveness. The risk and impacts imparted by the implementation of the remedy,such as risk to remediation workers and local residents, including transportationrelated risks for off-site remedies. Cost. The capital and operations and maintenance costsof implementing the remedy and the net present value ofthe remedy. It is interesting to note that for government-led Superfund (lead-fund) cleanups, the cost criterion often becomes relatively more important than if an owner or PRP is the funding entity. Modifying Criteria State Acceptance. The special concerns requiredor requested by the state agency in the case of a federally led Superfund project. Community Acceptance. The special concerns required or requested by the local community or local agency in the case of a federally led or state-led Superfund project. These criteria are used to evaluate remedial alternatives prior to actual remedy selection. This evaluation effort may be completed by either the regulatory agencyor the owner/operator or a PR€? Only the agency can select the remedy. However, when an owner has accepted the responsibility to conduct the evaluation, it should also anticipate remedy selection and in fact go thmugh the process of “selecting” the remedy to facilitate effective interchange with the regulatory agency. We propose that the following additionalconsiderations be integrated withthe nine selection criteria stated above: 1. Efficacy of the remedy. Does it achieve the objectives? It is not a success to efficiently solve the wrong problem.
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Availability of target remedy. Is the technology commercially available or just in the laboratory development phase? This consideration is often sidestepped, even though it is included in the implementability criterion, in the name of promoting innovative technologies. We support the development and useof innovative technology. However,this must be balanced with the needs of the specific site. Necessary pre-or post-treatment or other treatmenthandling requirements. These requirements add cost and often add time to the remediation schedule.Too often the focus is only on the core technology, when in reality the sum of supplemental unit processesmay cost more or impart higher short-term risk than the core process. Total cost of the remedy overthe life of the remedy (capital, operation and maintenance, and monitoring). Use of innovative technology. Should be selected only when conventional technology cannot achieveobjectives, or innovative technology can achieve equal or superior results at a lower cost, or innovative technologycan achieve equalor superior resultsat the same cost but in less time or with less site disruption. Cash flow demand-the rate at which the remedy must be funded. This is critical in determining affordability. Technical and regulatory precedent. Has the remedy been implemented previously? Degree of uncertainty in problem definition, especially in terms of the volume or area requiring remediation. Unfortunately, many projects advance to the remedy selection stage before the “problem” has been adequately defined. Agency objectives, predispositions, and national or logical policies. Legal situation. The negotiate/remediate/litigatedilemma. Corporate commitment of owner or PRF!
These additional considerations are consistent with the concept, as proposed in this section, of targeting a remedy that is necessary rather than one that is merely possible. It is important to note that the “cost-benefit” concept is not often adopted by the regulatory agencies in the remedy selection process in any formal manner (except in the case of a fund-led cleanup). Cost is delegated to a low-priority criterion, which often is used only to select from among remedies that are essentially equal in all other criteria. This can lead to a circumstance whereany marginal reduction in residual riskis considered appropriate regardless of the incremental cost increase. This text proposes that properly balanced remediation can be achieved only through a concept of risk-based remedy selection.
C. ”Selecting”theRemedy A formal three-phase procedure for identification and evaluation of remedies is presented in “Guidance for Conducting Remedial Investigationsand Feasibility Studies Under CERCLA” issued by the USEPA in October 1988. This procedure can be overly prescriptive and rigid if misinterpreted; however, it does facilitate the development of appropriate remedial alternatives for a given site. Unfortunately, in the absence of the proper context, criteria, and objectives, it is easy to follow this procedure throughthe development and selection of extremely expensive and unnecessarily complex and burdensome remedies. The first step in the remedy selection process is a basic understandingof the problem-the nature and extentof contamination. It is not necessary to have completed the problem definition phase, known as the Remedial Investigation (RI), to begin to develop and evaluate solutions, that is, remedies. In fact, it is beneficial to beginto focus onthe remedy as soon as there is a basic understanding of the site contamination and remedial objectives. This “early focus on the remedy” allows the problem to be defined in the context of the possibldprobable
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solutions. This serves to streamline the RI and ensure that those conducting it obtainthe site and contaminant information necessaryto properly assess and select an appropriate remedy. The initial information should definethe media affectedby contamination-groundwater, soil, sludges; the basic geologyand hydrogeology of the site, and the type of contaminantsvolatiles, semivolatiles, pesticides/PCBs,and inorganics. A preliminary assessmentof whether . or not there are active source areas of ongoing contaminant migrationis very important at this early stage, as it will be one of the main factors to consider in the remedy evaluation process. In addition, one will need a preliminary assessment of the real risk imposed by the site contaminants. This will form the beginning steps of what is known as a “baseline” risk assessment, which will prove essential in proper and informed remedy selection. Evaluating Remedial Technologies Once you know these factors, you can begin to target the types of remedial technologies that will abate specific unacceptable conditions and satisfy remedial objectives. Each typeof problem can be addressed by a technology or linked technologies. Itis vitally important to buildan array of remedial technologies beginning with one that ensures satisfaction of the protectiveness criterion for the targeted problem. This is often the least controversial criterion, as all parties in a remediation project are interestedin ensuring protection of human health and the environment. Assumptions madein calculating risk to assess protectivenessare sometimes the subject of disagreement between the agencies and the owner, but the concept of protectiveness is rarely in dispute. Many of the other evaluation criteria are the subject of greater disagreement. Therefore, it is a good practice to identify a limited set of technologies that arguably achieve the protectiveness goal. The potential remedies may fall into one or more of the following categories of remedial technologies presented in Section I V no action, institutional actions, containment, removal, treatment, and disposal. Initially, one or more of the remedial technologies from these categories should be considered. At the early stages of the remedial investigation, one can afford to be somewhat broadbased in assessing which remedies show real promise in achieving the remedial objectives, including cost.However, it is a good idea to streamline the target remedies as soon as possible. Concurrent progress onthe RI effort will help inthe streamlining process. Once a focused set of technologies and remedies have been tentatively identified that satisfy the protectiveness criterion, they must be compared to the additional criteria to see if the remedies are still in compliance. If the remedies do not comply with successive criteria, they must be modified or additional remedies developedas necessary to ensuresatisfaction of the criteria. After a targeted subset of remediation categories are identified, the next step is to define the basic method,or subcategory, within that category. That is to say, if treatment is one of the targeted categories, one must then identify whether that is best achieved through physicall chemical, chemical, biological, or thermal treatment technologies. The next step is to define the particular technology or process within the subcategoryjust defined. For example, within the physicakhemical subcategory for treating groundwater, there are several choices: air stripping, steam stripping, vacuum-enhanced stripping, carbon adsorption, resin adsorption, or other. For soil remediation within the physical containment subcategory, choices include clay cap, RCRA (layered) cap, soil cover with synthetic membrane, simple soil cover, and others. Some practitioners will take the position that refinement of the remedy to the level displayed above should be deferred untilthe remedial design (RD) stage. However, the RD stage comes after the remedy is selected. At that point the flexibilityto refine or modify the remedy is minimal and is burdened by regulatory procedural limitations. Therefore, it is advantageous
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to evaluate remedies down to the specific technology stage prior to remedy selection. This will sometimes require that treatability studies be conducted. This effort should also precede remedy selection. It is more difficult and requires more skill and knowledge to evaluate specific technologies in the feasibility study (FS) stage and still maintain a focused and limited set of alternative technologies; however, that is the best way to lead to the selection of the optimal remedy; one that meetsthe remedial objectives including cost-effectiveness. It is often true that a wide array of technologies can be “made to work” at many sites. This reflects the concept of possible versus necessary. The goal of the technology evaluation and selection process is not just to identify any workable technology but to define the most balanced remedy. Evaluating Remedial Alternatives A remedial alternative is the combination of remedial technologies that have been targeted to address the sum of the individual problems at a site. Many times, it is appropriate to modify one or more component remedial technologies or partial remedies becauseof beneficial or negative “side effects” resulting from the combination of component remedies. For example,s u p pose there are two types of sludges on a site and the optimal technology for each is different when viewed separately. However, the volume of sludge A is five times that of sludge B. If sludge B is amenable to treatment via the targeted technology for sludge A, it may be more cost-effective overall to combine sludges A and B and treat them bythe sludgeA method, even though there is a more efficient technology available for sludgeB. Overall, the elimination of the sludge B technology is likely to more than pay for the increased volume to be handled by the sludge A technology. To put remedy selection in a logical context, it is useful to array the remedial alternatives as a function of residual site risk versus total cost. Thisis shown graphically in Figure 1. This “cost-risk” curve, when calibrated to an actual site, is a powerful tool that clearly shows the choices available and their implications. If implementation of alternative A yields an acceptable residual site risk, then it should be chosen unless there are compelling and rational reasons to implementone of the other alternatives. Thegraph clearly demonstratesthat to the residual risk curve approaches an asymptote at some point while total cost continues increase dramatically with respect to risk. This is the point of diminishing returns concept previously mentioned. After protectiveness is ensured, there are two of the remaining eight criteria that tend to move the “acceptable” remedy from left to right on Figure 1: compliance with ARARs and long-term effectiveness andpermanence. An oftenquoted rule of thumb is that if it costs X dollars to achieve protectiveness, it will cost 3 X 5X dollars to achieve compliance with ARARs and 10-15X dollars to achieve “permanence” (when is viewed as “destruction” of contaminants). This is why it is essential to begin by developing remedies that achieve the initial threshold criterion, protectiveness. Then expand or modify the remedy, only as necessary, to progressively comply with the additional criteria. This “bottom-up” approach is the best way to ensure compliance in the most cost-effective manner achievable. Unfortunately, many remedies are selected onthe basis of a “top-down” approach, which tendstoresult in unnecessarilyexpensiveremedies.If you define a 15-20X remedy first, nearly all other remedies could be viewed as relatively cost-effective. One of the fundamental areas of controversy in remediation projects between owners or PRPs and regulatory agencies is this cost escalationafter protectiveness hasbeen ensured. The disagreements essentiallycenter around the argument that what is protective today may not be so in the future, especially if proper controls are not maintained.
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Predominant Remedial Action
Predominant Remedial Action
Containment
Excavatioflreatment
/
Degree of Cost Uncertainiy
Al. 1
0
5
10
15
20
25
30
Estimated RemediationCosts, Millions of Dollars
Figure 1 Representativecost-residualriskrelationship.
D. Typical Problems with Remedy Evaluation and Selection The most pervasive problem regardingremedy selection in the years since the Superfund program began is the dominance of a “define the problem” mentality instead of a “define the solution” focus. By farthe greatest amountof effort, dollars, and timeat remediation sites has, to date, been spent in the investigation of contamination, with RIs often taking years and the feasibility study effort being limited to a few months. Unfortunately, this has led to situations where the selected remedy was unable to achieve the objectives, was extremely and unnecessarily expensive, or was commercially unavailable,or to combinations of these andother factors. Obviously, this has been partly responsible for the poor reputation and extensive criticism that plagues the Superfund program as well as other remediation programs. This situation is gradually changing. Public demand and frustrated agencies and corporations are bringing pressures, sometimes opposing pressures, on the process. In the last few years, the remedy selection process has gone from being very sequentialto showing some increased degreeof integration regarding the definitionof the problem and the definition of the solution. Additional necessary changesare expected. Although overstudyingthe problem (the “Study it to death” mentality) is a real shortfall, it is also possible and equally disadvantageous to “under-define” the contaminant situation and prematurely proceed tothe remedial design and remedial action(RDRA) stage. Often public or political pressures forcethe RUFS effort to be unrealistically acceleratedto the point that the engineers who facethe task of identifying and evaluating remedies find that the problem is
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not well defined. This inevitablyleads to a situation where allor most of the site-specific thinking is deferred to the subsequent remedial design phase. This may lead to the implementation of a remedy that is substantially less than optimal. Premature remedy selection can lead not only to a waste of funds but also to a situation where thewrong problem is “solved,” leaving the real problem unremediated. Sometimes the problem that remains unsolved is due to the contaminants that pose the “principal threats” at the site. In such a case (and unfortunately, there are several), the contaminants that causethe lesser concernmay have been “efficiently” abated, giving the illusion of remediation and protectiveness while the true site risk is minimally affected. One of the main deficiencies arising from a poor understanding of the site problems is a high degree of uncertainty regarding the volume or area of contaminated media requiring remediation. This is a major burden in the effort to define a protective, balanced, and costeffective remedy because remedy selection is not a linear process. The optimal remedy for 40,000 cubic yardsof soil contaminated with volatile and semivolatile organics may not be the optimal remedy for 4OOO cubic yards of soil with the same level of the same contaminants. Many factors prevent the simple adoption of a remedy for a similar application but a site of substantially different magnitude.
VI. SUMMARY This chapter has presented some of the issues that face the site remediation engineer in professional practice andhas suggested someof the techniques applicablein the endeavorto clean up the nation’s hazardous waste sites with a knowledge-based, cost-effective, and systematic but integrated approach. The key concept presented in this chapter is one of balance. There are no perfect remedies;they all have disadvantages and limitations. Each site presents a unique set of trade-offs. The challenge is to balance the pros and cons in a manner that satisfies the essential objectives, is implementable, and is affordable to the funding entity. Without this balance, a strategy of either stall or litigate will likely result instead of a remediation strategy. The business ofremedy evaluation and selection is extremely complex and requires a working knowledgeof environmental regulation, environmental law, process engineering, cost estimating, and construction engineering.No single chapter,or even a full volume, can attempt to convey more than the very basics of this profession. To compound the situation, it is very dynamic and changes continuously,so what was optimal in the past may not be acceptable in the future. This makes the practice very challenging, but also very exciting.
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Innovative Approaches to Cleanup Level Development Ronald J. Kotun, Richard F. Hoff, Robert J.Jupin, Diane McCausland, and Patrick B. Moroney Chester Environmental Monroeville, Pennsylvania
1.
INTRODUCTION
Recently proposed regulations on the federal and state levels establish uniform cleanup standards to provide objectives for remediating contaminated sites. The purpose of these standards is to restore contaminated sites to levels that ensure protectionof human health and the environment. These regulations may result in consistent cleanup decisions and may expedite the remediation of contaminated sites by eliminatinglengthynegotiationsovertheextent of cleanup. However, this “consistency” may result in overconservative remediations and excessive expense or, more important, inadequate remediation. As part of a remedial investigation, a baseline risk assessment is conducted to determine if there is any potential for adverse health effects due to hazardous substances released from a site in the absence of controls. The risk assessment identifies constituents of interest (COIs) exposure scenarios, and likely receptors (residents, construction workers, etc.) and quantifies the amount of chemical intakeby a receptor at the site. The amount of intake can be translated into a risk value that indicates the extent to which public health may be affected. The results of the baseline risk assessment document the magnitude of risk and identify its primary cause. The results also provide a basis for a decision as to whether remedial action is necessary. The U.S.Environmental Protection Agency (EPA) has established guidelinesthat provide a consistent process for evaluating and documenting threats to the public health and the environment. Consequently, risk assessment provides a means to determine site-specific cleanup goals that are adequately protective of human health. Other scenarios consider the environmental fate and transport of chemicals to determine whether chemicalsin one medium might have an impact on another medium. These approaches consider factors suchas the spatial distribution of chemicals, interim measures currentlyin place to control chemicaltransport, plausible exposure scenarios, likely receptors to chemical exposure,and the physical and chemical properties of the chemicals. Therefore, the results generated from these approachesmay span several orders of magnitude yet still resultin protection of human health and the environment. More important, these approaches derive levels that are technologically and economically feasible to attain. 87
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In this chapter we present four case studies that consider the factors previously discussed to establish cleanup levels. These cases illustrate various means to derive a cleanup level that is technically defensible. However, these levels may not necessarily be the optimum for site remediation. Usually, more than one approach should be explored to establish the optimum cleanup level. Case I explores an innovative approachto the derivation of risk-based cleanup levels for carcinogenic polycyclic aromatic hydrocarbons (cPAHs). Using relative potency factors and specific exposure scenarios and considering apparent remedial strategies, a cleanup level for cPAHs in pond sediments was determined. The target cleanup level was developed by establishing a correlation between the cPAH concentration in a pond sediment sample and the corresponding risk. Plotting thesedata to generatea regression equation andselecting a target risk level allows one to identify a cleanup level. Case I1 employs a similar statistical and graphical approachto deriving cleanup levels for cPAHs, but the approach is more complex because cleanup levels are also being derived for pentachlorophenol (PCP) and arsenic. Risk-based cleanup levels can be derived for exposures to single constituents by back-calculating a soil constituent concentration corresponding to a target risk level using risk assessment methodology. However, when the risks result from exposure to more than one constituent, the target risk level should be apportioned among the individual constituents. The availablesite data were evaluated to determine the relative distribution of the chemicals across the site. Target risk levels were then apportioned among the individual chemicals according tothese relative distributions. Case I11 uses EPA's recently developed Multimedia Exposure AssessmentModel (Multimed) to derive soil cleanup levels for a source area of chemical release that would be protective of groundwater at a designated downgradient receptor location. In this study, Multimed was used to model the unsaturated and saturated zone fate and transport of PAHs. Groundwater investigations revealed that the PAHs may leach from source soils and migrate horizontally through an underlying aquifer toa downgradient well location and an adjacentestuarine river. The cleanup levelswere developed to be protective of humans consuming waterat the receptor well location, organisms inhabiting the river, and humans consuming organisms from the river. Soil cleanup levels were calculated byusing the dilution-attenuation factors derived by the model in combination with partition coefficients to meet applicable and appropriate performance standards in the water. Finally,Case IV employs other environmental fate and transportmodels,specifically EPA's Organic Leachate Model (OLM) and Vertical and Horizontal Spreading (VHS)model, PAHs that is protectiveof underlying groundto determine a soil cleanup goal for carcinogenic water quality. These two models account for the leaching of cPAHs into groundwater and their dilution in the shallow saturated zone affordedby dispersion. Use of these analytical solutions to derive a soil cleanup level providesa timely, cost-effectivealternative to complex numerical modeling. Furthermore, the conservatism of the results may account for some of the temporal and spatial variations inherent in even the simplest groundwater flow domainsnot accounted for in complex models.
II. CASE STUDY I: REGRESSION ANALYSIS AND APPARENT REMEDIAL STRATEGIES TO DERIVE CLEANUP LEVELSFOR CARCINOGENIC PAHs A. Introduction The Comprehensive Environmental Response, Compensation, and Liability Actof 1980, as amended (Superfund),is a national program that establishesa means to respond to releases of
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hazardous substances.The National Oil and Hazardous Substance Pollution Contingency Plan (NCP) is theregulationthatimplementsSuperfund and establishesthegeneralapproach for remediation of hazardous waste sites. The objective of the program is to protect human health and the environment fromthe potential harm posed by the constituents presentat these sites. The U.S. Environmental Protection Agency (EPA), administrator of Superfund, has established a framework for assessing risks to human health that ultimately determines whether remedial action is necessary at a site. The risk assessment process, as set forth in EPA's Risk Assessment Guidancefor Superfund, Volume I, Human Health Evaluation Manual(Part A) [l], provides an analysis of baseline risks, determines theneed for remedial action, and establishes a basis for calculating cleanup levels. This process provides an alternative to meeting regulatory cleanup criteria and allows for the use of site-specific information to establish practical remedial alternatives. Cleanup levelsare being established for a pond that is adjacent to a former wood-treating facility that operatedfor 18 years. The facility used creosote asthe preserving chemical.Of the 27 shallow sediment samples collected from the pond, cPAHs were detected in all samples with total concentrations ranging from 0.807 to 144.82 mgkg (Table 1). In many cases, the high degree of contamination is limited to a small area and is notcharacteristic of the entire site. A frequency distribution of the cPAHs in the shallow pond sediments indicates that only fourof the 27 samples had total cPAHs exceeding 30 mg/kg and seven had total cPAHs exceeding 10 mgkg (Table 2). A risk assessment was prepared in accordance with Superfund for this pond to establish baseline risks for all potential receptors and exposure scenarios. Based on the data, children with the cPAH-impacted sedimentsare those wading in the pond and coming into direct contact at the greatest potential risk. Therefore, development of cleanup levels for pond sediments is based on this most sensitive receptor. The primary objective of establishing cleanup levelsat this site is to be protective of human health, specifically the health of children wading in the pond. These children may come in contact with the cPAHs in the sediments through dermal contact and incidental ingestion. Baseline risks associated with exposure to the cPAH-contaminated sediments are based on the application of Chu andChen's relative potency factor approach [2]. Using the site-specific data and the relative potency factors,a cleanup level canbe calculated. Sediments withcPAH concentrations above a certain concentration would be removed, thus resulting in a reduced average concentration of cPAHs and a concomitant reduction in risk.
B. Chu and Chen's Relative Potency Factor Approach
.
Originally, risks attributed to exposure to cPAHs were derivedby assuming that all cPAHsare equipotent to benzo[a]pyrene. Realizing that there is much scientific evidence that demonare strates that benzojalpyrene is regardedas one of the most potent cPAHs and that all cPAHs not equipotent [2], alternative methods have been developed that assess risks more fairly. A study conducted by Margaret Chu and Chao Chen of EPA's Carcinogenic Assessment Group provides evidence to support these contentionsand also provides relative potency factorsthat allow for a more appropriate evaluation of risks associated with these less potent cPAHs. Relative potency factors (RPFs) for six cPAHs are presented in Table3. These cancer slope factors are derived by multiplying benzo[a]pyrene'soral cancer slope factorof 11S 3 [mg/(kg/ day)]" [3] by the RPF. Ultimately, potential risks are calculated by summing the products of the cPAH concentrations and the corresponding cancer slope factors[l]. The total risk can be defined by the equation
2
l l l l $ l l l l 0
Kotun et al.
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Table 2 Frequency Distribution of Total cPAHs in Pond Sediments" Concentration range
(mag)
Number of samples
BCLDL DL5
0 9 8 4 2 1 0 0 0 0 2 0 0 0 0 0 1 0
5- 10 10-20 20-30 30-40 40-50
50-60 60-70 70-80 80-90
90- 100 100-1 10 110-120 120-130 130-140 140-150 >150 "BDL = below detection limits; DL = detection limit.
x n
RL =
-
(CSi CSFi EXP)
i= 1
where RL is the risk level from exposure to cPAHs, CS, is the concentration of cPAH,, CSF, is the cancer slope factor for PAH,, and EXP is the product of intake parameters to establish long-term daily intakeof cPAH,. EXP is defined as EXP =
CR EF ED
BW-AT
where CR is the contact rate (ingestion rate or surface contact), EF the exposure frequency, ED the exposure duration, BW the average body weight, and AT is the averaging time. For children, an exposed skin surface area of 3768 cm2 was assumed to account for 50% of the total body surfacearea. The exposed surface areais limited to 50% because itis unlikely that the total surfacearea would be covered with sediment [4]. Absorption factorswere used to reflect the desorption ofchemicalfrom the sedimentand the absorption of thechemical through the skin to the bloodstream. A value of 5% was used for cPAHs[4]. Adherence to skin was estimated at 1.45 mg/cm2 [4]. An exposure frequency of24 days per year(2 days per week during the summer) for an exposure duration of 13 years was used. The average body weight was 41.5 kg [5]. The averaging time was 70 years [l]. There are no recommended values for the amount of sediment a child may incidentally ingest; therefore,the recommended incidental ingestion rate of 100 mg/day was used [6], with only 50% of the sediment coming from the site. The exposure frequency, exposure duration, body weight, and averaging timeare the same parameters thatwere used to assess risks due to dermal exposure.
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Table 3 Chu and Chen Relative Potency Factors for cPAH factorpotency Relative Compound 1 .o 0.69
Benzo[a]pyrene anthracene Dibenzo[a,h] fluoranthene Benzo[b] Chrysene Indeno[ 1,2,3-cd]pyrene anthracene Benzo[a] pyrene Benzo[k] Source: Chu and
0.08
0.00122 0.0171
0.0134 0.00444
Chen [2].
With no remedial action taking place, the average cPAH concentration in the pond sediment is 20 mgkg. The risk to children exposedto these sediments through dermal contact and incidental ingestion using the previously stated assumptions is 7.3 X
C. Site-Specific Development
of Risk-Based Cleanup Levels
Each sediment sample consists of a different compositionof cPAHs that results in a correspondingly different probability of risk. For example, two sediment samples may have equivalent total cPAH concentrations. However, the one sample that has a greater percentage of benzo[a]pyrene, one of the most potent cPAHs, will pose the greater probability of risk. To illustrate this relationship among the sediment samples, a log-log plot of the probability of risk 1). Cleanup levels corresponding to versus the total cPAH concentration was developed (Figure
1E44
1E47 0.1
1
10
100
Total Carcinogenic PAHs (rngwg)
Figure 1 Risks to children associated with total cPAHs resulting from exposure
to pond sediments.
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Table 4 Total cPAHConcentrations
and Their
Corresponding Target Risk Levels Total cPAH concentration (mgncg)
Risk
280 28
10~-4 IOE-~
10E-6 target risk levels can be determined by simply locating them on the regression line. For example, a target risk level of lo-’ corresponds to an average total cPAH concentration of 28 mg/kg (Table 4). This pond has only fourof its 27 samples with totalcPAH concentrations exceeding a concentration of 30 mgkg (approximately the target risk of lo-’)). Remediating areas with elevated levels of total cPAH to this concentration or “pickup level” would reduce the average total cPAH concentration, or cleanup level,in the pond and consequently reduce the associated risk. Table 5 presents various pickup levels and the risks that correspondto the cleanup levels for children who wade in the pond. A pickup level of 30 mgkg results in a reduction of the cleanup level for total cPAH concentration from 20 mgkg to 12 mgkg with a corresponding reductioninriskfrom 7.3 X loF6to 4.3 X This approachresultsinlimitingtheremediation to the heavily impacted areas and minimizing unnecessary cleanup.
D. Conclusion Risk assessment provides a means to determine whether chemicals present on a site exist in concentrations that pose a potential threat to public health or the environment. The risk assessment performed for this pond indicated that exposure to sediments from thepond presented a significantrisk, with children wading in the pond being the most sensitive receptor. There are be impractical isolated areas in the pond that obviously require remediation; it would certainly to remediate the entire pond to pristine conditions. Using risk assessment principles, acPAH level can be estimated to which one can be exposed and still have a negligible probability of developing cancer. Our assessment of the site indicated that remediation to an average concentration of 30 mgikg would result in a risk level of l X lo-’. Without any remedial action, exposure to the Remediating averagecPAHconcentrationthroughoutthesiteresultsinarisk of 7.3 X areas in excess of 30 mgkg, the so-called pickup level, will lower the site average concentra-
Table 5 Pickup Levels, ResultingCleanupLevels,andTheir Corresponding Risks to Children Wading in the Pond and Exposed to Impacted Sediments Pickup level (mg/kg)
Site cleanup level ( m a g )
No action 7.3E-06
20 18 16 14 12 10 7
90
70 6.OE-06 50 5.2E-06 30 4.3E-06 20 3.7E-06 10 2.6E-06
Risk level
6.68-06
Kotun et al.
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tion to a cleanup level of 12 mgkg and concomitantly reduce the risk by 41%. In essence, determination of a site pickup level for cPAHs through regression analysis to meet a target risk level can result in a practical remediation that is protective of public health, minimizes the volume of sediments to be removed, and is most likely cost-effective.
111.
CASE STUDY II: STATISTICAL METHODS TO DERIVE CLEANUP GOALS FOR A SITE IMPACTED BY MORE THAN ONE CHEMICAL
A. Introduction Cleanup levelsare being established to address contaminated surface and subsurfacesoils at a 65 years. The facility used creosote and penwood-treating site that operated for approximately tachlorophenol as wood-preserving chemicals.In general, polynuclear aromatic hydrocarbons (PAHs) and pentachlorophenol were detected in nine areas of the site, ranging in concentration from 1 to 10,800 mgkg and from 0.1 to 240 mgkg, respectively, The areas showingthe highinadjacent to the treating area est concentrationsof PAHs or pentachlorophenol were located or and in a former disposal area. Although wood was never treated with chromated copper arsenate (CCA) at this site, arsenic and copper were detected above background levels in the wood storage yard, at concentrations ranging from 24 to 369 mgkg and from 0.005 to 1.13 mg/kg, respectively. A risk assessment was prepared .in accordance with Superfund for this site to establish baseline risks for all potential receptors and exposure scenarios. Based on the data, on-site workers and construction workers coming into direct contact with the impacted surface and subsurface soilswould be at the greatest potentialrisk. Therefore, development ofcleanup levels for surface and subsurface soilsis based on these most sensitive receptors. The primary objective of establishing cleanup levels at thissite is to be protective of human health, specifically the health of on-site workers and construction workers. These individuals may come into contact with the constituents in the surfaceand subsurface soils through dermal contact, incidental ingestion, and inhalation. Using site-specific the data and targetrisk levels, a cleanup level can be calculated. Constituents detected at the site exhibit both carcinogenic and noncarcinogenic adverse health effects. Potential adverse health effects due to carcinogenic substances are derived differently from those due to noncarcinogenic substances. Therefore, cleanup levels have to be developed to address both carcinogenic and noncarcinogenic adverse health effects. Theseare dealt with in turn in the following sections.
B. Cleanup Levels for Potentially Carcinogenic Constituents Results of the risk assessment indicated that potential risks for exposureof on-site workers and construction workers to surface and subsurface soils were above the U.S. EPA's target risk range of 10-4-10"j [7];therefore, risk-based levels were derived for these potential human receptor groups. Constituents evaluatedin the risk assessment included PAHs, pentachlorophenol, volatile organic compounds (VOCs), and metals. Cleanup levels can be developed for all of the evaluated constituents of interest (COIs), but this is not necessary if the potential risks from the individual COIs are insignificant. Following EPA Region 111 guidelines, cleanup levels were developedfor only those constituents that contribute to 99% of the total potentialrisk. Tables 6 and 7 present a summary of the potential risks and the percent contribution of the individual constituentsto the total risk for the evaluated human receptor groups. As can be seen from these tables, PAHs, pentachlorophenol, and arsenic were the COIs responsiblefor 99% of
95
Cleanup Level Development
Table 6 Summary of Potential Risks of Surface Soils to On-Site Workers E Area
D
B Area Constituent A Area Potential risk PAHs Pentachlorophenol Benzene Styrene Arsenic Chromium Total potential risk
4.7E-04 8.6E-06 O.OE+OO O.OE+OO 2.8E-05 O.OE+OO 5.1E-04
4.4E-03 1.4E-06 1.5E-10 7.3E-11 1.OE-04 O.OE+OO 4.5E-03
1.7E-03 1.4E-04 2.3E-09 5.5E-10 O.OE+OO O.OE+OO 1.9E-03
7.8E-05 8.8E-06 4.6E-09 8.9E-10 5.8E-05 3.1E-07 8.5E-04
15e-03
1.5E-06 O.OE+OO O.OE+OO 1.1E-05 3.5E-07 1.6E-03
Percent contribution to total potential risk ~~~
99.14% 92.04% 92.66% 97.73% 92.75% PAHs %7.34%0.03% Pentachlorophenol 1.69% .00% Benzene 00% Styrene 0.00% Chromium 56% Arsenic
. 0.04%
0.10% 0.02%
Table 7 Summary of Potential Risks of Surface and Subsurface Soils to Future Construction Workers B Area Constituent A Area
D
E Area
Potential risk PAHs Pentachlorophenol Benzene Styrene Arsenic Chromium Total potential risk
2.8E-05 l .4E-07 O.OE+OO 4.6E- 13 7.OE-07 4.2E-08 2.9E-05
4.6E-04 1.8E-08 7.9E- 12 1.6E- 12 2.8E-06 4.2E-07 4.6E-04
2.3E-05 1.7E-06 3.8E- 11 1.4E-09 O.OE+OO O.OE+OO 2.5E-05
1.8E-05 1.6E-07 4.7E- 11 9.3E- 12 1.6E-06 5.OE-08 1.9E-05
6.2E-05 1.OE-07 2.2E- 10 1.9E- 11 3.8E-06 8.OE-08 6.6E-05
Percent contribution to total potential risk PAHs Pentachlorophenol Benzene Styrene Arsenic Chromium
96.99% 0.47% 0.00% 0.00% 2.40% 0.14% 100.00% 100.00%
99.30% 0.00% 0.00% 0.00% 0.61% 0.09%
93.25% 6.74% 0.00% 0.01% 0.00% 0.00% 100.00%
90.48% 0.85%
0.00% 0.00% 8.42% 0.26% 100.00%
93.98% 0.16% 0.00% 0.00% 5.75% 0.12% 100.00%
the potential risk to on-site workers and construction workers; thus cleanup levels were derived only for these COIs. Potential risks were estimated in the risk assessment by evaluating exposuresto site surface and subsurface soils via dermal contact, incidental ingestion, and inhalation exposure pathways. Applying the 99% contribution criteria mentioned aboveto each exposure pathway indicated that cleanup levels shouldbe derived using eachof the evaluated exposure pathways.
Kotun er aZ.
96
Risk-based cleanup levels were derived using the procedures and exposures assumptions presented in the risk assessment forestimating potential risk. Instead of using the sampled CO1 concentration in soil and solving the risk equations for potential risk, a target risk level was assumed and the risk equations were solved for the concentration of the CO1 in soil. The following equation was used in the risk assessment to estimate potential risk (PR):
-
PR = C, CSF EXP where C, is concentrationof COI, CSFis cancer slope factor, andEXP is the product of intake parameters used to establish long-term daily intakeof COI. EXP is defined as
EXP = (CR EF ED)/(BW AT) where CR is contact rate, EF is exposure frequency, ED is exposure duration, BW is average body weight, and AT is averaging time. Solving Equation (3) to yield a soil concentration results in
C, = PR/(CSF EXP)
(4)
Soil cleanup levels were derived for target risk levels and of as required by the U.S. EPA. Using the above method and the same intake assumptionsas were used in the risk assessment, cleanup levels corresponding to were and calculated pentachlofor rophenol and arsenic and are presented in Table 8. A different methodwas used for deriving cleanup levels for PAHs. PAHs comprise several constituents, and itis more desirable to express a cleanup level for PAHs as total PAHs than to derive cleanup levels for each individual constituent. First, the potential risk resulting from exposure to only PAHs for on-site workers and construction workers was calculated at each soil-sampling location. Second, the concentrationsof total PAHs were plotted against the corresponding potential risks as shown in Figures 2 and 3. Having statistically significant correlov5,and risklevels were located lations (R2), the cleanup goalscorresponding to in Table8 are appropriate on the plot and are presented in Table8. The cleanup levels presented for those areas of the site that contain only PAHs, pentachlorophenol, or arsenic and not a combination of the three COIs.
Table 8 CleanupLRvels (mgkg)with No Apportioning of Target Risk Levels Target risk level Constituent
10-6 10-~
10-5
On-site workers
Pentachlorophenol Arsenic Total PAHs Pentachlorophenol Arsenic Total PAHs
700 314
70 31
229 16 Construction workers
7 3 1
30,500
3050
305
6,620 20,000
662
1300
66 85
97
Cleanup
Total PAHs (mg/Kg)
Figure 2 Potentialrisk vs. total PAHs-on-site workers.
1. o E a
Y
.v, 1.oE-05 a
l.OE-07
1. o E a
Figure 3 Potentialrisk vs. total PAHs-construction workers.
Kotun et al.
98
Table 9 Distribution of Risk to On-site Workers from Exposure to Surface Soils Arithmetic Standard
Minimum mumdeviation mean Constituent Potential risk PAHs Pentachlorophenol Arsenic Total
4.7E-04 3.8E-06 3.7E-05 5.1E-04
1.2E-03 8.9E-06 3.5E-05 1.2E-03
2.OE-06 O.OE+00 1.2E-06 8.OE-06 Percent contribution
PAHs Pentachlorophenol Arsenic
64.88% 1.75% 33.37%
30.31% 2.93% 30.04%
4.51% 0.00% 0.37%
6.3E-03 4.1E-05 1.5E-04 6.5E-03
7.8E-05 8.2E-07 2.6E-05 1.1E-04
99.09%
71.16%
12.40% 95.28%
25.51%
0.46%
Since the total potential riskfor most areas of the site is the result of exposure to several COIs, the target risk levels must be apportioned among the COIs. The target risk level was apportioned basedon the contributions of the risk levelsof the individualCOIs. For example, and the site average potential risk from PAHs if the site average potential risk level is 1 X is 7.5 X itwould be assumedthat (7.5 X 10-5)/(1 X or 75%, of thetargetrisk level would be apportioned amongthe PAHs. This is the preferred method for apportioning the target risk level among the COIs of the site, since potential risk is based on both the toxicity and the concentration of the COI. Polycyclic aromatic hydrocarbons, pentachlorophenol, and arsenic were identified above as the major contributors tothe potential risk at the site. Eighty-seven surface samples and65 subsurface samples were collected at the site and analyzed for PAHs and pentachlorophenol, but only 32 of the surface soil samples and 27 of the subsurface samples were analyzed for arsenic. Only those soil samples analyzed for all three COIs were used in determining the apportionment of the risk levels. The potential risk of exposure to PAHs, pentachlorophenol, and arsenic for on-site workers and construction workers was calculated for each of the selected soil samples. The average contribution of each CO1 to the total risk for on-site workers is presented in Table 9. Table 9 illustrates that 65% of the total risk for on-site workers is due PAHs, to 33%is due to arsenic, and 2% is due to pentachlorophenol. Table 10 shows that 75% of the potential risk for construction workers is due to PAHs, 23% is dueto arsenic, and 1.4%is due to pentachlorophenol. The target risk levels were apportioned according to these distributions. As an example, asfor on-siteworkers,cleanuplevelswouldbederived suming a targetrisklevel of 1 X or a 6.5 X lo-’ targetrisklevel. A for PAHs corresponding to 65% of the 1 X 3.3 X lo-’ target risk level would be used for arsenic,and a 2 X targetrisklevelfor pentachlorophenol. Cleanup levels for on-site workers and construction workers based on the apportioning method described above are presented in Table 11.As can be seen from thetable, cleanup goals for on-site workers are lower than the corresponding goals for construction workers.
C. Cleanup Levels For Noncarcinogenic Constituents Results of the risk assessment indicated that total hazard indices for on-site workers and construction workers were above the “acceptable” level of 1.O for some areas of the site. Tables 12 and 13 present a summary of the hazard indices and the percent contribution of the indi-
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99
Table 10 Distribution of Risk to Construction Workers from Exposure to Surface and Subsurface Soils Arithmetic Standard
tion mean Constituent
Minimum
Maximum
Median
8.2E-01 7.5E-03 1.9E-03 8.2E-01
1.1E-03 1.4E-04 1.2E-05 2.2E-03
99.%%
94.36% 1.78% 0.65%
Potential risk ~~
PAHs Arsenic Pentachlorophenol Total
2.6E-02 9.6E-04 1 .OE-04 2.7E-02
3.6E-05 2.OE-08 1.9E-07 3.7E-05
l.lE-01 1.6E-03 3.OE-04 l.lE-01
Percent contribution PAHs Arsenic Pentachlorophenol
31.77% 31.78% 2.16%
75.64% 22.94% 1.42%
2.49% 0.00% 0.02%
97.43% 9.25%
Table 11 CleanupLevels ( m a g ) with Apportion Apportionment of Target Risk Levels Target risk level Constituent
10-4
10-5
10-6
On-site workers Pentachlorophenol Arsenic Total PAHs
14 104 140
Pentachlorophenol 15 Arsenic 152 PAHs Total
427 1515 5350
1.4 10 10
0.14 1 .o 0.7
Construction workers 43
4
445
35
vidual constituents to the total hazard index for the evaluated human receptor groups. As can be seen from these tables, PAHs, pentachlorophenol, arsenic, copper, chromium, and zinc are the COIs thatare responsible for 99%of the hazard index for on-site workers and construction workers. When the total hazard index exceeds 1.0, EPA guidelines [l] recommend that COIs be segregated by target organ effects and mechanism of action to derive separate hazard indices for each group. The COIs were segregated by effect and mechanismof action, and the resulting hazard index still exceeded unity, indicating that the cleanup levels would also have to be derived for noncarcinogenic COIs. Past experience has shown that when cleanup levels are developed for constituents thatr exhibit both carcinogenic and noncarcinogenic effects, those developed to address the carcinogenic effects are always lower than those developed to address the noncarcinogenic effects. If the cleanup levels developed for the carcinogenic effects are also protective of the noncarcinogenic effects, then cleanup levels to address the noncarcinogenic effectsneed not be separately developed. Plots of the total hazard indices (for exposures to allCOIs, not just PAHs) versus total PAHconcentrations are shown in Figures4 and 5. The targetcleanup level fortotal PAHs corresponding to the target risk range presented in Table 8 is 140 mgikg for on-site workers and 5350 mgikg for construction workers. The loF4cleanup level of 140 mgkg for
Kotun et al.
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Table 12 Summary of Hazard Indices-Surface Soils and On-site Workers BArea C Area
Area A Constituent Area
D
Area E
Hazard Index PAHs Pentachlorophenol Phenol Toluene Ethylbenzene Styrene Xylenes (total) Arsenic Chromium Copper Zinc Hazard index0.29
2.3E-02 6.7E-03 O.OE+OO O.OE+OO O.OE+OO O.OE+OO O.OE+OO 4.5E-02 3.5E-03 4.1E-03 2.3E-03 1.22 0.09
2.6E-01 1. IE-03 3.9E-05 2.2E-07 2.9E-07 5.1E-07 l.lE-07 1.6E-01 3.3E-02 8.lE-02 3.1E-02
1.1E+00 l.lE-01 2.6E-06 4.5E-06 1.4E-05 3.8E-06 1 .OE-04 O.OE+OO O.OE+OO O.OE+OO 1.4E-03
1.7E-01 6.88-03 1.OE-05 7.1E-06 1.2E-05 6.2E-06 4.8E-06 9.OE-02 6.6E-03 7.OE-03 1.9E-03
0.57
3.7E-01 1.2E-03 1.9E-06 O.OE+OO O.OE+OO O.OE+OO O.OE+OO 1.8E-02 7.6E-03 2.4E-03 l.lE-03 0.40
Percent contribution to total hazard index PAHs 92.54% 60.70% 91.08% 45.87% 26.90% Pentachlorophenol 2.39% 8.79% 0.20% 7.89% 0.01% Phenol 0.00% Toluene 0.00% 0.00%0.00% 0.00% 0.00% 0.00% Ethylbenzene 0.00% Styrene 0.00% Xylenes0.01% (total) 0.00% 0.00% 4.40%Arsenic 31.46% 0.00% 28.37%53.48% Chromium 1.89% 2.32% 0.00% 5.78% 4.14% 14.35% 4.86% Copper Zinc 0.12% 5.43% 2.72% 0.28% 0.67%
0.29%
0.00%
0.00% 0.00%0.00%
0.00%
0.00%
0.00%
0.00% 0.00%
0.00% 0.00%
0.00%0.61%
0.00%
2.45%
on-site workers corresponds to a total hazard index of 0.01 on Figure 4. The cleanup level to a total hazard index of 1.O on Figure5 . of 5350 mgkg for construction workers corresponds Remediating the site to the cleanup levels based on potential risk will lower the total hazard indices for all the Cols to 1.O or lower; therefore, separate cleanup levelswere not derived for noncarcinogenic compounds.
D. Conclusion Risk assessment provides a means to determine whether constituents presentat a site exist in concentrations that pose a potential threat to public health or the environment. The risk assessment performed as part of the remedial investigation study for the site indicated that exposuretosurfaceandsubsurfacesoilspresented a significant risk to on-site workersand construction workers. Risk assessment methodology along withstatistical and graphical methods were used to derive cleanup levels for the site. risk level were 140 For areas that contain all COIs, the risk-based cleanup goals at the mg/kg for total PAHs, 104 mgkg for arsenic, and 14 mgkg for pentachlorophenol. For areas of the site that contained only a single constituent, the risk-based cleanup goals at the risk levelwere 230 mg/kg for totalPAHs, 314 mgkg for arsenic,and 700 mg/kg for pentachlorophenol.
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Table 13 Summary of Hazard Indices4urface and Subsurface Soils, Future Construction Workers
ea A Constituent Area
E Area D Area B C Area Hazard Index
PAHs 9.5E-01 5.3E-03 Pentachlorophenol 2.OE-06 Phenol 7.6E-08 Tolulene Ethylbenzene 2.5E-07 1.6E-07 Styrene l.lE-07 Xylenes (total) 4.68-02 Arsenic 2.9E-02 Chromium 4.2E-03 Copper 5.9E-03 Zinc 1.3E-04 4-Methylphenol 8.3E-05 2-Methylphenol 6.9E-04 2,4-Dimethylphenol 1.88 0.28 1.040.76 Hazard Index
1.3E+00 7.OE-04 1 SE-05 6.8E-07 4.6E-07 5.7E-07 1.4E-07 1.8E-01 2.8E-01 9.8E-02 3.5E-02 1.4E-05 O.E+00 2.4E-05 1.93
7.OE-01 6.5E-02 61.E-06 4.7E-06 1.1E-05 2.1E-06 5.3E-05 O.OE+OO O.OE+OO O.OE+OO 1.9E-03 9.9E-05 9.9E-05 2.5E-04
1.2E-01 6.4E-03 3.7E-06 3.8E-06 6.1E-06 3.2E-06 2.5E-06 l.lE-01 3.4E-02 1.4E-02 3.OE-03 O.OE+OO O.E+00 O.OE+00
1.6E+00 4.1E-03 1.9E-05 7.7E-06 1.8E-05 6.6E-06 3.8E-06 2.5E-01 5.4E-02 1.2E-02 3.3E-03 3.7E-04 3.7E-04 1.OE-03
Percent contribution to total hazard index PAHs Pentachlorophenol Phenol Phenol Tolulene Ethylbenzene Styrene Xylenes (total) Arsenic Chromium Copper Zinc CMethylphenol 2-Methylphenol 2,CDimethylphenol
91.26% 0.51%
0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 4.41% 2.77% 0.40% 0.57% 0.01% 0.01% 0.07%
68.85% 0.04%
0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 9.50% 14.71% 5.08% 1.82% 0.00% 0.00% 0.00%
91.21% 8.47% 0.00% 0.00% 0.00% 0.00% 0.00% 0.01%
0.00% 0.00% 0.00% 0.25% 0.01% 0.01%
0.03%
41.61% 2.27% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 37.97% 12.06% 5.03% 1.05% 0.00% 0.00% 0.00%
82.79% 0.22% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 13.16% 2.89% 0.66% 0.17% 0.02% 0.02% 0.06%
IV. CASE STUDY 111: DERIVATION OF SOIL PAH CLEANUP GOALS USING THE MULTIMED MODEL A. Introduction A remedialinvestigatiodfeasibilitystudy (RUFS) was conductedat a former wood-treating and storage facility in Virginia that is currently included onEPA's National Priority List (NPL) of Superfund sites. A finalRI report, which characterized the natureand extent of contamination of surface and subsurface soils at the site as well as groundwater in the shallow, unconfined Columbia Aquiferand the deep Yorktown Aquifer beneath the site, was submittedto the EPA and VDWM and was subsequently approved. Analytical groundwater data acquired for the RI revealed that detectable concentrationsof polycyclic shallow, unconfined aquifer during the aromatic hydrocarbons (PAHs) were present in the groundwater. Analytical data acquired during the RI also revealed the presenceof the samePAHs in the unsaturated zone soils above the
102
Kotun et al. 10
1
v
2
0.1
U
0.001
o.Ooo1 10
100
1
Total PAHs (mg/Kg)
Figure 4 Totalhazardindex vs. total PAHs-on-site workers.
100
10
1
0.1
0.01
0.001
1
10
100
1O , oo
Total PAHs (mg/Kg)
Figure 5 Totalhazardindex vs. totalPAHs-constructionworkers.
Cleanup Level Development
103
Table 14 Summaryof Polycyclic Aromatic Hydrocarbons (PAHs) Detected in Soil and Groundwater Samples During Remedial Investigation Potentially carcinogenic PAHs
Noncarcinogenic PAHs
Benzo[u]anthracene Benzo[u]pyrene Benzo[b]fluoranthene Benzo[k]fluoranthene Chrysene Dibenzo[a,h]anthracene Indeno[ 1,2,3-cdlpyrene
Acenaphthene
Acenaphthylene Anthracene Benzo[g,h,qperylene Fluoranthene Fluorene Naphthalene Phenanthrene Pyrene
shallow aquifer. Both potentially carcinogenic and noncarcinogenic PAHs (cPAHs and nPAHs, respectively) that are commonly associated with wood-treating and storage activities were identified as contaminants in a public health and environmental assessment (PHEA) that was performed as part of the final RI for the site (Table 14). l h o areas, areas A and B, were delineated as potential source areas on the western and eastern portions of the site, respectively (Figure 6). The presence of PAHs in the soil and groundwater samples collected from these areas implies the possibility of downward vertical movement of organic leachate from the unsaturated zoneto the groundwater of the Columbia Aquifer. Once in the groundwater, the potential exists for the transport of PAHs from beneath source areas A and Bto a receptor domestic welland an adjacent estuarine river, respectively. be used as Although the aquifer is not currently being used as a potable water supply, it could such in the future. Hypothetical off-site domestic wells in areas west of the site are therefore considered to be potential receptors in a future scenario. The adjacent river is considered be to a potential environmental receptor for those PAHs that have leached from the soil into the shallow aquifer at the site. For the purpose of conducting a focused feasibility study, it became necessary to develop soil cleanup goals for PAHs that were protective of (1) humans consuming groundwater at a receptor well location impacted by area A, (2) organisms inhabiting the river, and(3) humans consuming organisms from the river impacted by area B. It was suggested by EPA that this task be performed by applying EPA's Multimedia Exposure AssessmentModel (Multimed) to designated source area(s) at the site [8]. The monitoring well designated MW-l02 (Figure6 ) , installed in the Columbia Aquifer in the southwestern portion of the site, was selected as the receptor welllocationfor PAHs migrating fromareaA, sinceitissituateddowngradient of source area Aand PAHs weredetected in groundwater samples collected from this location. In addition, property west of the site is more likely to undergo residential development than property at any other on-site or offsite location. The adjacent river, as stated previously, was determined to be the potential environmental receptor for constituents migrating from source area B. The purpose of Multimed in the development of soil cleanup goals was the derivationof dilution-attenuation factors (DAFs) that are used as multipliers for selected applicableand appropriate performance standards at the receptor location of interest. This will be explained in greater detail in a later discussion.
104
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I
a CI
Kotun et al.
d W
B
Cleanup Level Development
105
B. Description of Multimed Multimed is a recently developed user-friendly computer model that is capable of simulating the release of chemicals in leachate form from a source (or designatedarea) at the site to soils directly beneath the source. In addition, Multimed can be used to further simulate chemical fate and transport in the unsaturated and saturated zones, followedby possible interceptionof the subsurface plume by a specified receptor (e.g., a well or surface stream). The fate and transport of a chemical released from a source is simulated by incorporating the known responses of the chemical toa number of complexphysical, chemical,and biological processes the chemical encounters as moves it in the multimedia environment. These responses are incorporated as chemical-specificvariable input data by the model user. Other variable input data, such as source-specific and aquifer-specific data, must also be incorporated by the user. For some of the variable input data, the model provides the user with an optionto either manually specify values for the variable input data (constant input) or have the model mathematically derive the variable input data from other constant inputs (derived input). After all relevant input data have been defined and the type of output desired has been specified, the multimedia transport of each contaminant is mathematically simulated by the model. An output file is then generated showing final concentrations of specific constituents and any other pertinent information(i.e., times of concentration Occurrences or statistical distributions resulting from multiple iterations). The final downgradient concentration(s) calculated by the model can be used to represent potential toxic exposureconcentration(s)to which human and/or environmental receptors may be subjected. For this site, deterministic and Monte Carlo simulations of steady-state unsaturated and saturated zone flowand transport were performed using Multimed.A Gaussian boundary condition was applied to the saturated zone transport of the contaminants away from the source, with the maximum concentration occurring at the source. Steady-state conditions in the model were used for the approximation of a system mass balance in which water entering the flow systemis balanced by the water leaving the system. There is no significant temporal variationin the system. Thus, the assumption of a steady-state system basically simplifies the mathematical equations used in describing the flow and transport processes and reduces the amount of input data, since no information on temporal variability is necessary. The primary assumption of steady-state flow and transport is that the source is of a sufficiently large chemical mass to ensure that the final downgradient contaminant concentration in the groundwater is maintained at the receptor location. The source is assumed to be continuous and constant, without decay or any other temporal variation. In the deterministic model of steady-state conditions, each inputvariable is of fixed value and is assumed to havea fixed mathematical relationship with the other variables. Each run of a deterministic model can result in either the output of one maximum concentration or timestepped concentrations occurring over a specified time interval. For this site, the output selected was the maximum concentration that wouldOccur over an arbitrarily selected 500-year period. The deterministic mode of the Multimed model should only be applied to one or more particular modeling situations in which all values for the input variables are known or can be assumed with a high level of confidence. If there is uncertainty as to the values of input variables, then the simulation(s)may be performed within the MonteCarlo framework, wherethe randomness and uncertaintiesof values inherent in the modeled system can be evaluated. Input values in the deterministic model were either constant or derived values. Tables 15-17 present
its
Kotun et al.
106
Table 15 Summary of Multimed Input Parameters Used in Modeling PAH Leachate Flow and Transport Through the Unsaturated Zone for Modeling to MW-l02 and the River-Deterministic Model of Steady-State Conditions variable
Input Unsaturated zone material variables m Depth of unsaturated zone Number of layers 4.42" cmlhr Saturated hydraulic conductivity Unsaturated zone porosity Unsaturated zone function variables cm" Alfa coefficient Residual water content Van Genuchten exponent Unsaturated zone transport variables Bulk density of soil for layer @cm3 0.042' m Longitudinal dispersivity of layer Percent organic matter m Thickness of layer
type
1 1 0.38b
0.075' 0.065* 1 .89'
1.4P 0.59
1
Constant Constant Constant Constant Constant Constant Constant Constant Derived Constant Constant
"Literature value obtained from Mulrimed User's Manual, Table 6-2. for sandy loam. bValue obtained from Mulrimed User's Manual, Table 6-3. Represents average porosity for sand (fine and coarse), gravel (fine and coarse), silt, and clay. 'Literature value obtained from Mulrimed User's Manual, Table 6-5, for sandy loam. dLiterature value obtained from Mulrimed User's Manual, Table 6 - 4 , for sandy loam. =Literature obtainedfrom Mulfimed User's Manual, Table 6-8, for sandy loam. 'Value obtained from the calculation av = 0.02+ O.O22L, where UY is longitudinal dispersivity (unsaturated flow in the vertical direction) and L is the depth of the unsaturated zone = Im. Xiterature value obtained fromMultimed User's Manual. Table 6-7,for group B soils.
all values used in defining the unsaturated and saturated zone input variable parameters assumed for the site in the deterministic model. The Monte Carlo method provided a means of applyingthe known uncertainty associated with an input variable to that variable. This uncertainty is expressed as a cumulative probability distribution. For each uncertain inputvariable, a probability distribution must be specified that best describes the frequenciesof Occurrence of measured valuesfor that variable.As the Monte Carlo simulation is run overa large number of iterations (the number of iterations is specified by the user), random values generated froma specified probability distribution are assigned to the variable. For this site, 500 Monte Carlo simulations were performed by Multimed. The probability distribution may be specified as uniform, log,, uniform, normal, log,, normal, exponential, empirical, or the Johnson system of distributions, Relating the input variableto any one cumulative probabilitydistribution may be difficult. The difficulty arises from the fact that the specification of a distribution for an input variable requires a large amount of site-specific data that may not be available. The types of values assigned to the Monte Carlo, steady-state input variables were either constant, derived, or ranges of uniform distribution. For this site, a uniform distribution of values was specified for each select input variable due to the lack of site-specific data for those variables. All values used in defining the unsaturated and saturated zone inputvariable parameters assumed forthe site in the Monte Carlo steady-statemodel are presented in Tables 18-2 1.
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Cleanup
Table 16 Summary of Multimed Input Parameters Used in Modeling PAH Leachate Flow and Transport Through the Saturated Zoneof the Columbia Aquifer to Receptor Well Location MW-102Deterministic Model of Steady-State Conditions Input Chemical-specific variables Acid-catalyzed hydrolysis Base-catalyzed hydrolysis rate Biodegradation coefficient (sat. zone) Dissolved phase decay coefficient Distribution coefficient, Kd Neutral hydrolysis rate constant Normalized distribution coefficient, Koc Overall chemical decay coefficient Overall first-order decay coefficient Reference temperature Solid-phase decay coefficient Source-specific variables Area of western source Duration of pulse Infiltration rate Initial concentration of leachate Length scale of source Near-field dilution Recharge rate Source decay constant Spread of contaminant source Width scale of source Aquifer-specific variables Angle off centerline of plume Aquifer porosity Aquifer thickness Bulk density Distance to receptor,X, Groundwater seepage velocity Hydraulic conductivity Hydraulic gradient Longitudinal dispersivity Mixing zone depth f, Organic carbon content (fraction), Particle diameter Retardation Coefficient Temperature of aquifer Transverse dispersivity Vertical dispersivity pH
0" 0"
0" 0"
-
0"
-a
v 0" 25
0" 8260'
-
0.3178 l00h
90.88'
>lg 0.317g
v
15.1' 90.88' 0
0.38' 5.34 1 .49k 183'
-m
442 0.006
18.3"
-m
0.005"
0.03p
-
18 6.1" 1.02" 6.5
Constant Constant Constant Constant Derivedb Constant Constant Constant Derived Constanf Constant
'
Constant Constant' Constant Constant Derived Derived Constant Constant Derived Derived Constant' Constant Constant Constant Constant Derived Constant Constant Constant Derived Constant Constant Derived Constant' Constant Constant Constant'
.Literature value obtained from Ref. 11. bDistributioncoefficient (not presented by model output) was derived from the calculation K,, = K, f,, where K, is the normalized distribution coefficient and f, is the organic carbon content (fraction). 'Conservative input value assumed. dValue will be zero since it is derived from solid and dissolved phase coefficients, which themselves were assigned a value of zero.
Kotun et al.
108
Table 16 Continued Temporal factors are ignored under steady-state conditions. 'Area was designated as shown in Figure 7. Model assumed area is square, approximated area at 8260 mz with length = width = 90.88 m. %filtration and recharge rates selected for model represents minimum values derived from Hydrological Evaluation of Landfill Performance (HELP) model. hAssumedvalue. 'Spread of Gaussian contaminant source = (width of source)/6. jValue represents mean of the mean porosity values for materials ranging from fine sand to clay-Table 6-3, Multimed User's Manuul. 'Literature value obtained from Multimed User's Manual, Table 6.8. for sandy loam. 'Determined from Figure 7. "'Modelderived value unknown to user. "Longitudinal dispersivity,a, = 0. I X,;transverse dispersivity = aU3; vertical dispersivity = 0.056 d. "No site-specificf, data available. Input value of0.005 is a model default value that falls within range off, values derived fromfom values obtained fromMulrimed User's Manual, Table 6-7,for group B soils using the equationf, = foJ172.4. Wean particle diameter assumed from range givenfor medium sand in Table6-10 of Multimed User's Manual. SActual modelderived R,, value unknown to user.
Table 17 Summary of Multimed Input Parameters Used in Modeling PAH Leachate Flow and Transport Through the Saturated Zone of the Columbia Aquifer to the Adjacent River-Deterministic Model of Steady-State Conditions
Variable
Input
type
Chemical-specific variables Acid-catalyzed hydrolysis rate Basecatalyzed hydrolysis rate Biodegradation coefficient (sat. zone) Dissolved phase decay coefficient Distribution coefficient, Kd Neutral hydrolysis rate constant Normalized distribution coefficient. K, Overall chemical decay coefficient Overall first-order decay coefficient Reference temperature Solid-phase decay coefficient Source-specific variables rea of eastern sourcef Duration of pulse Infiltration rate Initial concentration of leachate Length scale of sourcef Near-field dilution Recharge rate Source decay constant Spread of contaminant source Width scale of sourcef Aquifer-specific variables Angle off centerline of plume Aquifer porosity
Input Units
Value 0" 0'
v
oe -
0"
-a 0'
Od 25 0'
100
Constant Constant' Constant Constant Derived Derived Constant Constant Derived' Derived
0 0.38'
Constant' Constant
10,Ooo
-
0.317g 100h 100 >1g d y r
0.317g
m m
16.7
yr-'
Constant Constant Constant Constant Derivedb Constant Constant Constant Derived Constant' Constant
0'
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Cleanup
Table 17 Continued Input
Aquifer Bulk X, to Distance velocityseepage Groundwater ivity Hydraulic Hydraulic rsivity Longitudinal Mixing fraction), content carbon Organic Particle Retardation coefficient Temperature of aquifer vity Transverse Vertical PH
m
m d y r
-
5.34 1.49k 244'
-
820 0.006
24.4
-
f,
-
'
"C m
-
0.005
0.03
-
18 8.13 1.37 6.5
Constant Constant Constant
Derived"'
Constant Constant Constant"' Derived'" Constanto ConstantP Derivedq Constante Constant" Constant" Constant'
'Literature value obtained from Ref. 1 I . bDistribution coefficient (not presented by model output) was derived from the calculation Kd = K,f,, where K, is the normalized distribution coefficient andf, is the organic carbon content(fraction). cConservative input value assumed. dValue will zero since it is derived from solid and dissolved phase coefficients, which themselves W- assigned a value of zero. Temporal factors are ignored under steady-state conditions. 'Area was designated as shown in Figure 7. Model assumed area is square, approximated area at 10,ooO m* with length = width = IOOm. %filtration and recharge rates selected for model represents minimum value derived from Hydrological Evaluation of Landfill Performance (HELP) model. hAssumed value. 'Spread of Gaussian contaminant source = (width of sourceY6. jValue represents mean of the mean porosity values for materials ranging from fine sand to clay-Table 6-3, Mulrimed User's Manual. 'Literature value obtained from Uulrimed User's Manual, Table 6-8, for sandy loam. 'Determined from Figure 7 . "Modelderived value unknown to user. "Longitudinal dispersivity, aL = O . l X r ; transverse dispersivity = d 3 ; vertical dispersivity = 0.056oL. "No site-specificf, data available. Input value of 0.005 is a model default value that falls within range off, values derived fromf, values obtained fromUulrimed User's Manual, Table 6-7,for group B soils using the equationf, = fJ172.4. Wean particle diameter assumed from range given for medium sandin Table 6-10 of Multimed User's Manual. qActual modelderived Rd value unknown to user.
C. Use of Output Data For Derivation of Site-Specific Soil Cleanup Goals
As stated previously,the purpose of Multimed inthe development of site-specificcleanup goals is the derivation of dilution-attenuation factors.These derived factors are then used as multipliers for selected performance standardsat the receptor locations of interest. Rather than dePAH compound individually, one soil cleanup goal veloping a soil cleanup level for each for total PAHs was calculated that will be protective of the groundwater at monitoring well MW-102,marine organisms in the river, and humans consuming organisms from the river.
Kotun et al.
110
Table 18 Summary of Multimed Input Parameters Used in Modeling PAH Leachate Flow and Transport Through the unsaturated Zone for Modeling to MW- 102-Monte Carlo Model of Steady-State Conditions variable
Input ~
~~~~~
~
Unsaturated zone material variables Depth of unsaturated zone Number of layers Saturated hydraulic conductivity Unsaturated zone porosity Unsaturated zone function variables Alfa coefficient Residual water content Van Genuchten exponent Unsaturated zone transport variables Bulk density of soil for layer Longitudinal dispersivity of layer Percent organic matter Thickness of layer '
@cm3 m
0.613-1.33 1 l .O-150 0.250-0.500
Uniform Constant Uniform Uniform
0.005-0.145' 0.034-0. 100b 1.09-2.68'
Uniform Uniform Uniform
1.25-1.76'
Uniform Derivedd Uniform Uniform
-
0.180-1.30'
m
0.500-2.00'
'Literature values obtained from Multimed User's Manual, Table 6-5, for sandy loam. kiteramre values obtained from Multimed User's Manual, Table 6 - 4 , for sandy loam. 'Literature values obtained from Multimed User's Manual, Table 6-8, for sandy loam. dDerived values obtainedfrom the calculation av = 0.02+0.022L, where av is the longitudinal dispersivity (unsaturated flow in the vertical direction) and L is the depth of the unsaturated zone = lm. 'Literature values obtained from Multimed User's Manual, Table 6-7, for group B soils.
Table 19 Summary of Multimed Input Parameters Used in Modeling PAH Leachate Flow and Transport Through the Saturated Zone of the Columbia Aquifer to Receptor Well Location MW-102-Monte Carlo Model of Steady-State Conditions Value(s)
Input
Chemical-specific variables Acid-catalyzed hydrolysis rate yr)" Base-catalyzed (Mhydrolysis rate Biodegradation coefficient (sat. zone) coefficient Dissolved decay phase coefficient, Distribution of K,, constant hydrolysis Neutral rate Normalized distribution coefficient, Overall chemical decay coefficient yrOverall first-order decay coefficient temperature Reference coefficient decay Solid-phase Source-specific variables source'western of Area rate Infiltration concentration leachate mg/L Initial of ourcef of scale Length dilution Near-field te Recharge constantdecaySource
(MY r r l
Yr- I Yr" Yr" K,
&g I
Yr- ' Yr" dYr
0" 0" 0' 0"
-
08 14.2-5,500,0008 0' Od 25' 0"
8260 0.317-0.587'
100"
dYr
Yr- I
90.88 l' 0.308-0.744 OC
Constant Constant Constant Constant Derivedb Constant Uniform Constant Derived Constant Constant Constant Uniform Constant Derived Derived Uniform Constant
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Table 19 Continued Input
Spread Width Aquifer-specific variables e off Angle Aquifer Aquifer Bulk X, o Distance WYrvelocity Groundwater seepage ctivity Hydraulic Hydraulic gradient ersivity Longitudinal ne Mixing Organic carbon content (fraction),f, Particle Retardation coefficient Temperature16-25' of aquifer Transverse dispersivity Vertical dispersivity PH
type
of ale
v Uniform
183'
m
-
-
Uniform 0.0011-0.0100 18.3"'
m Constant
-
-
Uniform 0.0010-0.0076"
-
-
"C m m
-
Constant
0.26-0.57j
Constant
6.1"' 1.02"' 6.00-9.00'
Constant Derived
Derived Uniform Derived Uniform Constant Constant
"Literature values obtained from Ref. 11. bDistribution coefficient was derived from the equation Kd = K,f,, where K, is the normalized distribution coefficient andf, is the organic carbon content (fraction). 'Conservative input value assumed. dValue will be zero since it is derived from solid and dissc!ved phase coefficients, which themselves were assigned a value of zero. 'Assumed value(s). ' h a was designated as in Figure 7. Model assumed area is square, approximated area at 8260 m*, with length = width = 90.88 m. %filtration rates selected for model represent the minimum and maximum values derived from Hydrological Evaluation of Landfill Performance (HELP) model. hAssumed value. 'Spread of Gaussian contaminant source = (width of source)/6. @orosity values represent range fromfine sand to Clay-Mulrimed User's Manual, Table 6-3. 'Literature values obtained from Mulrimed User's Manual, Table 6-8. for sandy loam. 'Determined from Figure7. "'Longitudinal dispersivity, OL. = O.lX,; transverse dispersivity = oU3; vertical dispersivity = 0.056aL. "No site-specificf, data available. Input value range obtained fromMulrimed User's Manual, Table 6-7, for group B soils using the equationf, = f0,,,/172.4. OParticle diameter range assumed for particle types ranging from fine silt to coarse gravel, given in Table6-10 of the Mulrimed User's Manual.
The groundwater performance standard assumed at well location MW-l02 for the derivation of a soil cleanup goal for total PAHs at area A was the proposed maximum contaminant level (MCL) for benzo[a]anthracene,O.OOO1 mg/L [9]. Although there are various MCLs and DWELs established for the individual PAH compounds, this MCL was selected for the total PAHs since it represents the most health-conservative drinking water standard. The perforof soil cleanup goals for total PAHs that mance standards assumed at the river for the derivation were protectiveof marine organisms and humans consuming organisms from the river impacted by area B were 0.3 and O.oooO311 mg/L, respectively [lo].
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Table 20 Summary of Multimed Input Parameters Used in Modeling PAH Leachate Flow and Transport Through the Unsaturated Zone for Modeling to the Adjacent River-Monte Carlo Model of Steady-State Conditions
variable
Input
type ~
Unsaturated zone material variables Depth of unsaturated zone Number of layers Saturated hydraulic conductivity Unsaturated zone porosity Unsaturated zone function variables Alfa coefficient Residual water content Van Genuchten exponent Unsaturated zone transport variables Bulk density of soil for layer Longitudinal dispersivity of layer Percent organic matter . Thickness layer of
0.500-2.00 1 1.0-150 0.250-0.500
Uniform Constant Uniform Uniform
0.005-0.145"
Uniform Uniform Uniform
0.034-0.l00b 1.09-2.68'
@cm3 m m
1.25-1.76' 0.180-1 .30' 0.500-2.00"
Uniform Derivedd Uniform Uniform
'Literature values obtained from Mulrimed User's Manual, Table 6-5, for sandy loam. bLiterature values obtained from Mulrimed User's Manual, Table 6-4, for sandy loam. 'Literature values obtained from Mulrimed User's Munual. Table 6-8, for sandy loam. dDerived values obtained from calculation m = 0.02+0.022L, where av is longitudinal dispersivity (unsaturated flow in the vertical direction) and L is depth of the UnSaNrated zone = 1 m. 'Literature values obtained from Mulrimed User's Manual, Table 6-7,for group B soils.
Table 21 Summary of Multimed Input Parameters Used in Modeling PAH Leachate Flow and Transport Through the Saturated Zone of the Columbia Aquifer to the Adjacent River-Monte Carlo Model of Steady-State Conditions ~
Input Variable
type
Input Value(s) Units
Chemical-specific variables Acidcatalyzed hydrolysis rate Basecatalyzed hydrolysis rate Biodegradation coefficient (sat. zone) Dissolved phase decay coefficient Distribution coefficient, K,, Neutral hydrolysis rate constant Normalized distribution coefficient.K, Overall chemical decay coefficient Overall firstader decay coefficient Reference temperature Solid-phase decay coefficient Source-specific variables k e a of eastern sourcef Infiltration rate Initial concentration of leachate Length scale of sourcef Near-field dilution Recharge rate
Od 25" 0"
Constant Constant Constant Constant Derivedb Constant Uniform Constant Derived Constant Constant
10,Ooo 0.317-0.587' 100h 100 'l 0.308-0.744
Constant Uniform Constant Derived Derived Uniform
0"
0" 0" 0"
-
0" 14.2-5,500,0008
W
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Table 21 Continued Input
Source decay constant Spread of contaminant source Width scale of sourcef Aquifer-specific variables off Angle Aquifer Aquifer Bulk o Distance X, velocity Groundwater seepage uctivity Hydraulic ent Hydraulic ersivity Longitudinal e Mixing (fraction), content carbon Organic r Particle cient Retardation Temperature of aquifer Transverse dispersivity Vertical dispersivity PH
type 0'
p-'
m m
16.7 100
v
.76k
m
mm
0.26-0.57' 4.57-6.10 1.25-1 244'
-
dYr
142-3.780 0.0059-0.0068 24.4"
-
0.0010-0.0076" 0.0004-0.2000"
-
-
f,
"C m m
-
-
16-25" 8. 13" 1.37" 6.00-9.00'
Constant Derived' Derived Constant Derived Uniform Uniform Constant Derived Uniform Uniform Constant Derived Uniform Uniform Derived Uniform Constant Constant Constant
'Literature values obtained from Ref. 11. bDistribution coefficient was derived from the equationKd = K, f,, where K, is the normalized distribution COefficient andf, is the organic carbon content (fraction). CConservative input value assumed. ''Value will be zero since it is derived from solid- and dissolved-phasecoefficients, which themselves were assigned a value of zero. 'Assumed value(s). fh was designated as in Figure 7. Model assumed m is square. approximated area at 1O.ooO m*, with length = width = 100 m. Infiltration rates selected for model represents the minimum and maximum values derived from the Hydrological Evaluation of Landfill Performance (HELP) model. hAssumed value. 'Spread of Gaussian contaminant source = (width of sour~e)/6. jPorosity values represent range from fine sand to clay-Table 6.3. Mulrimed User's Manual. 'Literature value obtained from Multimed User's Manual, Table 6-8, for sandy loam. 'Determined from Figure 7. mLongitudinal dispersivity, al. = 0. I X,;transverse dispersivity = aLJ3; vertical dispersivity = 0.056aL. "No site-specific f, data available. Input value range obtained fromMultimed User's Manual. Table 6-7,for group B soils using the equation f, = fJ172.4. OParticle diameter range assumed for particletypes ranging from fine silt to coarse gravel, given in Table 6-10 of the Multimed User's Manual.
Prior to discussing the actual calculation process, Table 22 presents and defines the parameters that were used in the development of soil cleanup goals. 1. GroundwaterApproach of groundwater at the designated The calculations for developing soil cleanup goals protective receptor well location MW-l02 are as follows.
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Table 22 Parameters Used for Derivation of Soil Cleanup Goals
Riverconcentrationofacontaminant,equaltotheappropriate AWQC DAFDilution-attenuationfactorderivedfromMultimedastheratiobetweentheinitialleachate concentration at the source and the modeled downgradient groundwater concentration at the receptor RM Rivermixing(dilution)factorforcontaminantsintheriver Dd Soilwaterequilibriumpartitioningcoefficientusedforderivinginterimsoilcleanup'goals from steady-state modeling ci Initial contaminant leachate concentration at source Cf Finaldowngradientgroundwaterconcentrationatreceptorlocation CS, Contaminantconcentrationingroundwateratpointofdischargeintoriver,back-calculated from river concentration,C,,, C,, Contaminantconcentrationingroundwateratmonitoringwell MW-102,equaltotheappropriate groundwater performance standard Cl Leachateconcentrationatsource,back-calculatedfromdowngradienttargetgroundwater concentration (performance standard) at receptor location CS Soilconcentrationcorrespondingtointerimsoilcleanupgoal C,,,,
1. Initial PAH concentrations (Ci)are modeled from the designated source area through the unsaturated and saturated zones to the receptor location at monitoring well MW-102. A groundwater concentration at this location (C') is output by the model. 2. The following relationship is then used to calculate a dilution-attenuation factor: DAF -*Ci/Cf 3. Assuming that the target PAH groundwater concentration at location MW-l02 is mul-
tiplying by the model-derivedDAF(derived in step 2) givesthe PAH leachateconcentration (C,) at the source, or C/ = C,, 4.
*
DAF
Finally, multiplication of C, by the distribution coefficient (Kd) results in a soil concentration at the source corresponding to the soil cleanup goal. This partitioning is expressed as
c,
= C&
2. River Approach The calculations for developing soil cleanup goals protective of the river are as follows.
Initialleachate PAH concentrations (Ci) are modeledfrom the designatedsourcearea through the unsaturated and saturated zones to the point of groundwater discharge at the river. A groundwater concentration at this location (C/>is output by the model. 2. The following relationship is then used to calculate a dilution-attenuation factor: 1.
DAF = CiCf 3. Assuming that the target river concentration is Caw,,
the targeted PAH concentration in the groundwaterat the point of discharge intothe river (C,) can be calculated by multiplying with RM, or C,
= Cawv RM
I15
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4.
Multiplying Cgwby the derived DAF from the model gives the leachate concentration at the source (Cl): Cl =
c,
*
DAF
5 . Finally, multiplication of Cl by the distribution coefficient (Kd) results in a soil concentration at the source correspondingto the soil cleanup goal. Thispartitioning is expressed as CS=
c&
3. Soil Cleanup Goals for TotalPAHs The calculation for developing soil cleanup goals for total PAHs that are protective of the groundwater at monitoring well location MW-l02 and the river is as follows. 1. The leachate concentration of total PAHs (C,,TpAH) at each sourceis calculated in the same manner as follows. For protection of groundwater at MW-102, C1,TPAH
=
c s td
DAF
For protection of the river, C~,~= AH C,,,,
RM DAF
Since the MCLs and DWELs are different for each PAH, the lowest groundwater perforis being used for mance standard (O.OOO1 mg/L - proposed MCL for benzo[a]anthracene) totals PAHs as a conservative approach. ( C s , p A H ) must take into account 2. Since the calculationof a soil cleanup goal for total PAHs the Kdof each PAH (Kd,pAH) as well as the percent distribution of each individual PAH (%DpAH) across the area of interest, the following assumptions can be made regarding the derivation of the soil cleanup goal. For each PAH, C1,PAH
=
Cs.PAH/Kd.PAH
(5)
where C~,PAH = C~.TPAH %DPAH
(6)
Substitution of Equation (6) into Equation ( 5 ) yields the relationship c/,PAH
=
(Cs.TPAH
%DPAH)/Kd,PAH
(7)
For total PAHs, Equation (7) becomes
By moving Cs H ,. outside the summation and rearranging, the following expression for the calculation of a soil cleanup goal for total PAHs is obtained
Table 23 presents a summary of the mean percent distribution of each PAH in soil at the site (%D),the distribution coefficient (Kd)of each PAH, and the adjusted distribution coefficient obtained for each PAH by dividing each Kd value by the corresponding %D.
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Table 23 Summary of Diskibution Coefficients for Polycyclic Aromatic Hydrocarbons ~
Distribution Mean percent Adjusted coefficient distribution," Polycyclic aromatic hydrocarbon (cm3/g) %D Benzo[a]anthracene Benzo[a]pyrene Benzo[b]fluoranthene Benzo[k]fluoranthene Chrysene Dibenzo[a,h]anthracene Indeno[l,2,3-cdjpyrene Acenaphthene Acenaphthylene Anthracene Benzo[g,h,i]perylene Fluoranthene Fluorene Naphthalene Phenanthrene Pyrene
Kd
6.33 5.33 10.0
9.72 7.66 1.S5 3.01 2.96 1.45 5.75 2.97 15.6 3.23 2.74 9.83 11.8
value,b K; (cm3/g)
109,005 515,947 27,500 28.292 13,055 1,064,516 8OOO 265,78 1 23 777 12.5 862 70 1,217 8o00 269,360 190 1,218 36.5 1.130 4.7 172 70 712 190 1,610 Sum K: = 2,301,155
6900 27500 2750 2750 loo0 16500
'%D for each PAH = (mean PAH conc./total mean PAH conc.) 100%. bKd =
K,J(%D)/100].
D. Results 1. Dilution-Attenuation Factors Table 24 presents a summary of the dilution-attenuation factors (DAFs) that were derived from the final downgradient groundwater concentrations estimated at two receptor locations (MW-102 and the river) by the Multimed simulationsof steady-state flow and transport from
Table 24 Summary of Dilution-Attenuation Factors from Flow andTransport Models Through Unsaturated and Saturated Zones-U.S. EPA Exposure Assessment Multimedia Model, Steady-State Conditions Modeled groundwater finalModeled concentration dilutionAssumed for total PAHs attenuation initial leachate receptor facto? at for concentration location (mg/Ll total PAHs for total PAHs conditions source" at (mg/L) MW-IO2 River MW-102 River Model type for steady-state PAHs of Deterministic models 9.8Carlo 45 10.2 2.23 100 of PAHs' models Monte
11.71 0 011.9 8.52 8.38 ~~~
'Initial leachate concentration at source location is in the unsaturated zone. 100
m& was assumed for modeling
presentation.
bDilution-attenuation factors (dimensionless)are used for calculations of interim soil cleanup levels. 'Values presented for Monte-Carlo simulations representthe 95th percentile.
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Table 25 Summary of Interim Soil Cleanup Goals for PAHs Protective of Groundwater and the Adjacent River Protective of humans consuming groundwater from Well MW-102 (mgkg) Deterministic model 10,355
2738
Monte Carlo model"
Protective of humans consuming aquatic organisms from river (mgkg) Deterministic model
Monte Carlo model"
Protective of marine organisms in river (mgkg) Deterministic model
403,852,703 35,067 41,866
Monte Carlo model" 338,269,785
'Interim soil cleanup goals presented for the Monte Carlo model represent the 95th percentile.
the source areas of the site, through the saturatedand unsaturated zones. The table shows that two DAFs were determined for each receptor location. One DAF was derived based on the results of a deterministic model, the other based on the 95th percentile results of a Monte Carlo model of 500 iterative simulations. The groundwater concentration of all PAHs estimated at receptor location MW-102by the deterministic and Monte Carlo simulations of steady-state conditions were 8.38 and 2.23 mg/L, respectively. Sincethe original leachate concentration was arbitrarily assumed to be 100 mg/L (for ease of presentation), the corresponding DAFs derived from the results of the deterministic and Monte Carlo modeling efforts to receptor location MW-102 are l l.9 and 45, respectively. Thegroundwaterconcentrations of all PAHs estimated at thepoint of groundwater discharge to the river by the deterministic and Monte Carlo simulations of steady-state conditions were 8.52 and 10.2 mg/L, respectively. Since the original leachate concentration was 100 mg/L, the corresponding DAFs derived from the results of the deterministic and Monte Carlo modeling efforts to the river are 11.7 and 9.8, respectively. 2.SoilCleanupGoals the The DAFs derived and summarizedin Table 24 were usedto calculate soil cleanup goals for site that are protective of (1) the groundwater at monitoring well MW-102, (2) humans consuming aquatic organisms from the river, and (3) marine organisms in the river. These soil cleanup goals are presented in Table 25. Table 25 shows that the soil cleanup goals for totalPAHs in area A, derived from the results of the deterministic and Monte Carlo simulations, for protection of humans consuming groundwater at the receptor well location were 2738 and 10,355 mgkg, respectively. The soil cleanup goals for total PAHs in area B,derived fromthe results of the deterministic and Monte Carlo simulations, for protection of the humans consuming river organisms were 41,866 and PAHs in area B, derived from the 35,067 mgkg, respectively. The soil cleanup goals for total results of the deterministic and MonteCarlo simulations, for protection of river organismswere , OOO , OO parts per million. Based on these soil cleanup goals and the exboth greater than 1 O isting PAH concentrationsdetected at the site, no soil remediationof PAHs would be necessary in either area A or B.
E. Conclusion Environmental fate and transport modeling of contaminants in the multimedia environment provides an alternative means of developing and establishing cleanup goals for potential source
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areas at hazardous waste sites. As was shown in this case, cleanup goals canbe derived from modeling outputs that protect potential human and/or environmental receptors from contaminants as they become mobilized following release into the environment. The soilcleanup goals derived for thissite were to protect both human and environmental receptors fromPAHs originating from two sourcelocations, with the most conservativesoil cleanup goals being derived from both deterministic and Monte Carlo models for the protection of humans consuming groundwater as drinking water. These soilcleanup goals were 2738 and 10,355 mgkg, respectively. The uncertainties associated with applying Multimed to the derivation of soil cleanup goals for this site are discussed in the following paragraphs. In the deterministic models of steady-state conditions, literature valueswereused for chemical and physicalproperties of individual PAHs. These values may not be appropriate for the actual existing conditionsat the site. Many of the literature values were obtained from laboratory conditions or field conditions different from those atthe site. Many site-specific conditions may cause the chemical and physical properties and behaviors of the PAHs to deviate from values reported in the literature. In addition, the model evaluates chemicals separately. The behavior in the environment of chemicals that are constituents of mixtures, such as PAHs in creosote, may be different from what their behaviorwould be if they were interacting individually with the environment. The MonteCarlo model of steady-stateconditions assumesa constant, nondecaying source of large area and sufficient chemical mass to forcethe modeled system into steady-state conditions and equilibrium, such thata constant downgradient groundwaterPAH concentration is maintained at all times. In reality, however, the source strength may decay over time as PAHs migrate away (downgradient) from the sourceor degrade naturally. An uncertainty associated withthe Monte Carlo mode exists in the random generation of values from a specified distribution. It is uncertain whether the model considers interdependencies thatmay exist betweenor among many of the input variables. For example, the organic carbon partition coefficient (K,) may, in reality, change with the changing pHof a system. This is probably ignored by the model, especially when K, values are entered as constant input. Another consideration for uncertainty also existsin Monte Carlo simulations. Since there was a very limited base of site-specific data for each input variable, the uniform probability distribution was best suited for the input variables because of the degree of uncertainty associated with them. Hydraulic conductivity,for example, is estimated tofollow a log-normal distribution, and application of a uniform distributionmay not be appropriate, but due to the lack of data for this parameter, it was the only option available. Other overall uncertainties were associated with the use of the Multimed model for this site. These include (1) the uncertainty resulting froma lack of sufficient aquifer-specificdata for calibration of the model to actual conditions beneath the site; (2) the uncertainties that exist in parameter estimation from literature values, especially for values presented fora particular variable for differenttypes or classifications of unsaturated andsaturated zone materials (i.e., soils), noneof whichmay adequately match the materials in the unsaturated and saturated zones at the site; (3) uncertainty associated with theselection of a representative location and area geometry, since the size of each source area;and (4) the uncertainty associated with source model assumed that the geometry of each source area at this site was square, which may not represent the actual geometryof the area. Selection of the area geometry will affect how the plume is modeled.
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V. CASE STUDY W: DETERMINATION OF SOIL CLEANUP GOALS PROTECTIVE OF GROUNDWATER QUALITY USING ANALYTICAL SOLUTIONS A. Introduction During the 1980s, a large number of hazardous wastesites were investigated to determine the potential impactsof these sites on human health and the environment. The detail with whichthe investigations were conducted varies greatly fromsite to site. Databases for sites can be composed of several to several thousand sample analyses and observations. Despite the reliability of a given database, emphasis is currently on the remediation of sites having U.S. Environmental Protection Agency (U.S.EPA) promulgated Records of Decision (RODS). Within a ROD, remedial goals are established for those media presenting unacceptable risk upon exposure or for any medium representing sources or potential sources of continuing contaminant release. Remedial goals designed to abate human health risks can be derived once the rate and duration of exposure to a given medium have been determined. Rate andduration of exposure are based onsite-specificinformation such as area demography and land use patterns. Exposure is defined in the Public Health and Environmental Assessment (PHEA)section of the remedial investigation (RI) report for any site. However, establishing cleanup goals protectiveof actual or potential contaminant releaseis more difficult and may require somefate and transportmodeling, particularly in the case of soil contamination and groundwaterprotection. Soil cleanup goals protectiveof underlying groundwater canbe derived by developing simple, logical models of site-specific contaminant migration from affected soils to groundwater and, if necessary, to an alternate point of compliance (APC). An APC is a hypothetical boundary beyond which contamination cannot extend at concentrations exceeding some minimum performance standard. APCs are established byEPA and can extend to the nearest potential household or be limited to the downgradient source area boundary. Once the APC is established, the site-specific conceptof contaminant migration can then be approximated by the use of site data and/or mathematical models. The use of mathematical models to describe the movement of groundwater is not new. Mathematical models representativeof groundwater flow regimes have been used by engineers and scientists since the late 1800s [12]. Application of groundwater models to pollution transport and management problems has become increasingly popular over the last twenty years due to the enactment of legislation such as the Federal Water Pollution Control Act of 1972, the Safe Drinking WaterAct (SDWA) of 1976, the Clean Water Act of 1977 (CWA), andthe Comprehensive Environmental Response Compensation and LiabilityAct of 1980 (CERCLA). The complexity of the available groundwater models varies greatly. Selection of a model for use at a site is therefore based on factors such as the complexity of the groundwater flow regime,the importance of modeling resolution, and the data available to the engineer or scientist doing the modeling. Under relatively simple hydrogeological conditions or when simplifying assumptions about the flow regime are appropriate, contaminant migration can be approximated by the useof mathematical models such as analytical solutions or analytical models. The distinction between the two is more than semantic. Analytical solutions solve a very simple process equation by hand calculations. They require few site-specific analytical data and provide conservative estimates of contaminant transport. Analytical models solve more complex process equations
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with the aid of a computer program. The data needs and the need to develop mathematical solutions that describe boundary conditions for analytical models are more extensive and are open to conjecture. This section discusses the useof analytical solutions in the determination of soil cleanup goals protectiveof underlying groundwater quality accepted by EPA, RegionV (EPA V), at an active CERCLA site in Texas. The site is a former wood-treating facility that operated for over 50 years. Data necessary for the use of more complex analytical modelsare not available. All process-related structures have been removed, andportions of the site are currently occupiedby other industries. The ROD issued for this particular site by EPA V specifies that all soils containing in excess of 700 ppm of carcinogenic polynuclear aromatic hydrocarbons(cPAHs) must be remediatedtoprotectindividualspotentiallyexposed by directcontact(dermalcontactand accidental ingestion). The ROD also specifies that the remaining soils must be remediated to achieve the “no leaching potential” in order to protect underlying groundwater quality. Since the underlying shallow aquifer is not currently usedas a drinking water supply and Texas legislation currently prohibits the further development of groundwater for potable use in the area, the attainment standard for cPAHs in groundwater is less than 10 ppb (not detected) at the APC, designatedas the downgradient property boundary. Possible impacts on the deep aquifer of an improperly abandonedwell thought to exist inthe southern portion of the site must also be addressed. The deep aquifer is a potential drinking water source; therefore, an attainment standard of 0.2 pg/L corresponding to the proposed maximum contaminant level (MCL) for benzo[a]pyrene is warranted. The site conceptual model of contaminant migration (site conceptual model) at the site uses EPA’s Organic Leaching Model (OLM) [l31 to account for cPAH soil leaching andEPA’s Vertical and Horizontal Spreading (VHS) model [l41 to account for dilution in the shallow saturated zone afforded by dispersion. A two-dimensional horizontal flow model representing the presence of a continuous solute line source representsthe potential migration (and subsequent dilution) of cPAHs in the deep aquifer emanating from the improperly abandoned well. Contaminant attenuation is afforded primarilyby the leaching portion of the site concep tual model. Dispersion in the saturated zone is minimal becauseof the relatively shallow saturated zone under consideration and proximity of the soil sourceareas to the APC. Attenuation by leaching is also a controlling factorfor the potential impacts on groundwater quality inthe deeper aquifer. Derivation of soil cleanup goalsby these methods provided a timely, cost-effective alternative to complex numerical modeling. Furthermore, the conservatism of the results may account for someof the temporal and spatial variations inherent in even the simplest groundwater flow domains not accounted for in complex models. Upon completion of the modeling effort and a qualitative sensitivity analysis, site-specific data were obtained to verify the findings of the leachateportion of the model that provided the majority of contaminant attenuation. These data will be summarized and analyzed to ensure that the theoretical site concept of cPAH migration does not underestimate the site-specific potential for continuing release of cPAHs from soils. If site data do show that the modeling resultsare underestimates or overestimates of actual conditions at the site, the model will be modified to account for the difference.
B. Purpose The ROD issued in September 1988 for the site stipulates that during the initial stages of the remedial design, contaminated soilareas will be remediated if they exceedeither the risk-based soil cleanup goal of 700 ppm of cPAHs or the leaching potential-based cleanup goal (“no
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leaching potential”). The leaching potential-based goal is defined as the residual cPAH soil concentration that will not leach and impact groundwater beneath the site. Although a detailed hydrogeological assessment was conducted at the site during the remedial investigation, chemical analytical data and data requirements for groundwater flow modeling are limited. A visual soil investigation was attempted to determine the extent of contaminated soils, and only select samples of the visually impacted soils were analyzed for the presence of cPAHs. The remedial investigation was concluded in 1986, and nofurther studies were conducted. The development of a remedial design work plan was initiated by one of the potentially responsible parties (PRPs) in 1989 in responseto the promulgation of the ROD by EPA V. In early 1991, acceptance of the Remedial Design Work Plan byEPA V was contingent upon addressing the “no leaching potential.”) Meetings with the PRP and EPA V resulted in defining the no leaching potentialas a total cPAH concentration in leachate emanating from contaminated soils not exceeding 10 pg/L at the downgradient property boundary. A detection is not categorized as a potential drinklimit of 10 pg/L was chosen because the shallow aquifer ing water unit because of its low yield and general water quality.If the aquifer were categorized been selected as a potential potablesource, the proposed MCL for benzo[a]pyrene would have as the minimum attainment standard for cPAHs. MCLsare enforceable standards for drinking water suppliesor potential potable sources.EPA V was concerned not only with protecting the shallow water-bearing unit fromfurther deterioration but also with the possible impact on the deep aquifer, in which an improperly abandoned wellis thought to exist. An improperly abandoned deep well could act as a conduit fromthe shallow aquifer tothe deep aquifer. The deep well was installed around 1912 but is no longer in service and has not been used since the late 1950s. However, records of proper abandonment do not exist. EPA V therefore stipulated that soil cleanup goals protective of shallow groundwater qualitymust also be protective of the deep aquifer, which is used as a supply of potable water in the immediate area of the site. A minimum attainment standardof 0.2 pg/L corresponding to the proposed MCLfor benzo[a]pyrene was established as the minimum attainment standard for this water-bearing zone. EPA V also concluded thatthe use of models if applicable would be acceptable but subject to EPA’s review and final approval. The purposeof this section is to present the rationale for the development of a soil cleanup goal protective of groundwater quality in the shallow and deep aquifers using available site information and analytical solutions that will withstand an EPA V review. Acceptance of the soil cleanup goal by EPA V would provide the ROD-specifiedno leaching potential and result in the completion of the remedial design work plan for the site.
C. Application 1. Development of the Site Conceptual Model The conceptual model for the site is based upon information about the source areas and hydrogeology at the site presented in the final remedial investigation report. The regional geologic setting for the site is the Quaternary Gulf Coastal Plain of Texas. This region comprises a series of sedimentary depositional plains, the youngest of which is of recent, postglacial deposition (Holocene deposits). Sediments of the Holoceneare deposited alongthe coast and in the alluvial flood plains of existing river systems. The site is located specifically in the surface sediments of the Beaumont Formation. The average depthof the Beaumont Formation is 0-20ft below grade. The Lissie Formation lies below the Beaumont and extends to approximately200 ft below the surface. Themajority of the historical site soil borings were drilled to a depth of 65 ft, although deeper borings
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were drilled to depths of200 ft below the surface. Sediments making up these units consist of a top stratum of cohesive soils (sandy claysand silty clays) and a substratum of cohesionless soils (silty sands, clayey sands, and poorly graded sands). Primary water-bearing zones and their corresponding thickness encountered at the site are organized as follows: shallow zone (10-21 ft), intermediate zone (115-127 ft), and deep zone (174-200 ft).A generalized cross section of these zones is presented in Figure 7. The 1986 RI report states that the shallow water-bearing units consist predominantly of silty and clayey sands with occasional gradations to sand and clayey silts.The base of this unit is irregular and slopes gently to the east. Shallow zone groundwater trends at the site generally slope to the westat a gradient averaging about 20 ft/mi. Site surveys indicated that the shallow zone extends continuously offsite to the west, toward a bayou. This drainage feature may as act a groundwater discharge area, thereby influencing westward-trending gradients. A surface impoundmentlocatedoffsitealongtheeastpropertylinemaycauselocalizedgroundwater mounding, which also would produce westward gradients in the shallow zone. The hydraulic conductivityof the shallow zone was evaluated by conducting falling-head field permeability tests at selected well locations. Table 26 presents the field hydraulic conductivity test results for wells screenedin the shallow water-bearing zone. Soilsof the waterbearingunit are alsotypedforfutureconsideration.Measuredfieldhorizontalhydraulic 2.4 X to 2.2 X c d s e c andaverconductivitiesfortheshallowzonerangedfrom aged 8.3 X low4cdsec. The next significant water-bearing unit is the intermediate water-bearing zone, which is overlain by a clay layer and is not considered to be affected by historical site activities. Hydraulic conductivities for this zone are not currently available. The continuous nature of the intermediate aquitard excludes this zone from the modeling effort. The deep zone is also considered to be a confined water-bearing unit. However, historical data indicate that a deep well was screened in this unit, and records of the well’s abandonment are not available. Physicaldata are not available for this particular zone. It is believed that the deep zone gradient slopes to the south-southwestat approximately 6 ft/mi. Vertical seepage rates between shallow and intermediate water-bearing units are extremely low because of the nature of the confining strata. However, a potential conduit between the shallow zone and the deep may exist in the form of a leaking improperly abandoned well in the southern portion of the site. For this reason, the deep water-bearing unit is a concern EPA of V and is considered in the modeling effort. Given the natureof the site and the characteristics of the underlying strata, contamination of the shallow zone by infiltration of precipitation is the most obvious groundwater contami-
Table 26 Field Hydraulic Conductivity Test Results4hallow Groundwater Zone Field horizontal hydraulic Monitoring well MW0 1 MW02 OW07 OW08
Soilconductivity Screen type interval (ft)
Sand Silty sand Sand
Sand
9.0-21 .O 9.0-21 .O 11.0-16.0 14.0-19.0
(cdsec) 2.4 4.5 4.5 2.2
X 10-~ X IO-^ X 10-~ X 10-~
Avg. 8.3 X
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nation mechanism. Contamination reaching the shallow zone would in all likelihood move in the westward direction of shallow zone groundwater migration. Vertical and horizontal dispersion would then occur, withvertical movement bounded below by the intermediate aquitard. If the improperly abandoned wellexists, contaminants might alsoenter the deep water-bearing zone in which the well was screened. Groundwaterin the deep water-bearing zonein which the well was screened. Groundwater in the deep water-bearing zone trends toward the southern property boundary. The site conceptual model consists of three elements: (1) leaching of constituents of interest (COIs) from affected soils, (2) migration and subsequent dispersion of the COIs with respect to shallow groundwater flowdirection, and (3) migration from the shallow zone tothe deep water-bearing unit throughthe open conduit and subsequent dispersion. The site conceptual model is presented in Figure 8. An important aspect of this conceptual model isthat it is chemically conservative, assuming that the cPAHs are not biodegraded or adsorbed to soils before, during, or after migration.
2. ConstituentMobility Polynuclear Aromatic Hydrocarbons (PAHs), in general, are immobile constituents in environmental media. Recent research shows that PAHs bind to soil surfacesas a result of their van der Waals forces [15]. Van der Waals forces act solely between molecules within close proximity of each other. As a rule, the larger the molecular size, the greater the van der Waals forces. cPAHs, being generally larger than noncarcinogenic nPAHs, are even less mobile in environmental media. Similarly,relatively low water solubilitiesand vapor pressures add to the inherent environmental immobility of cPAHs. A semiquantitative assessment of theoretical mobility developed by Laskowski et al. [l61 can be used to describe the immobility of cPAHs in the environment based onknown physical and chemical constants. The basis for the assessment is an algorithm that utilizes water solubility (S), vapor pressure (VP), and the organic carbon partition coeffkient (K,) of the constituents to determine a relative mobility index (MI). The MI is defined as MI = log [(S VP)&]
(10)
A relative scale is then used to evaluate the MI derived for eachcPAH [17]. The scale is a descriptive one, comparing a numerical MI to the categories extremely mobile, very mobile, slightly mobile, immobile, and very immobile.
Description indexMobility >S
0 to 5 -5 to 0 -10 to -5
<- 10
mobile Extremely Very mobile Slightly mobile Immobile Very immobile
Mobility index values and the physical-chemical constants for the cPAH are presented in Table 27. The MIS for the cPAHs range from valuesof -8.9 (immobile) for benzo[b]fluoranthene to - 11.7 (very immobile) for chrysene. PAH mobility may be enhanced if these constituents bind onto dissolved organic macromolecules such as humic acids or suspended particulate organic matter in groundwaters. It is important to note that in most soil-water systems, these macromolecules are, themselves, not
IMPROPERL Y ABANDONED NELL
III. 2-D HURIZONTAL LINE SOURCE /CUNTINUOUS SOLUTE LINE SOUACE/
\
Figure 8 Conceptual model of potential containment migration. I, Organic Leachate Model (OLM); 11, Vertical and Horizontal Spreading (VHS) model; 111, two-dimensional horizontal line source model (continuous solute line source).
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Table 27 Physical-Chemical Constants and Relative Mobility Indices for cPAHs Vapor mobility pressure solubility Mol wt. Constituent (g/mol) Benzo[a]pyrene Benzo[a]anthracene Benzo[b]fluoranthene Benzo[k]fluoranthene Chrysene Dibenzo[a,h]anthracene Indenoil ,2,3-cd]pyrene
252.0 228.4 252.3 252.3 228.3 278.4 276.3
Water
Relative
(ppm Hg) (mm @ 25°C) 0.0038 0.0057 (20°C) 0.014 0.0043
0.0018 0.0005 0.00053
5.6 2.2 5 5 6.3 7 7
log K,
index
6.74 5.30 ( 2 0-8.9 0 ~ ) 5.74 ( 2 0-9.4 ~ ) 5.74 (25°C)-11.7 5.30 6.52 -11.2 6.20
-10.5
X (25°C) X lo-' (20°C) -9.6 X 10-7 X 10-7
X lo-' x 10-8 x 10-8
-11.3
Source: U.S. EPA Aquatic Fate Process Data for Organic Priority Pollutants [ll].
mobile. Nevertheless, this site conceptual model considers that the cPAHs could potentially migrate advectively, boundto mobile dissolved organic matter in the underlying shallow aquifer and the deeper zones. Retardation will not be considered in order to account for the maximum cPAH mobility in groundwater at the site. 3. GoverningEquations
Element I . Contaminant Leaching. The potential concentrationof an organic constituent that will leach from source area soils [C(l)] is estimated using the 95% confidence versionof EPA's OLM. The OLMis a multiple regressionequation that is derived froma database of measured leachate concentrations. The database is composed of TCLP data, EP-Tox data, and field lysimeter results formany of EPA's Target Compound List (TCL) compounds. The log form of the equation used to calculate the 95% percent C1 is 95% C1 =
t0.05,~-2 *
MSE [X', (X'X)" X/,]
(11)
where to.o5is the factor for the 95% CI, q - 2 is the number of degrees of freedom, MSE is mean square error, and X,X', X,, X,' are various versions of the waste and leachate concentration data matrix. The OLM equation is C(I) = 0.0021I(Cw)0.678(Sw)0.373
(12)
C, is the estimated concentration of an organic constituent in source area soils, andS, is the water solubility of the constituent. Since benzo[b]fluoranthene is the most relatively mobile cPAH as a result of its water solubility, the modeling effort will focus on this constituent. Element I I . Dispersionin the Shallow Aquver. Thetransport of a constituent by shallow groundwater to the APC results in dilution by vertical and horizontal dispersion processes.The VHS model [l31 provides a conservative estimate of the dilution provided by these dispersive transport phenomena and is used as the final element in the site conceptual model. The VHS model equation is CAE
= C,,[erf
[(Y'/~Y)'.~] erf {X/[~(U,Y)~*~]}]
(13)
where cAPCis the estimated constituent concentrationat the APC, c, is equal to C(l)developed by OLM since dilution prior to transport is not a consideration, Y' is the width of the source
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area parallelto the directionof groundwater flow,Y is the distance from the source area to the APC, a,is the transverse dispersivity, and x is the length of the source area perpendicular to groundwater flow. The error function of some number z, erf(z), can be estimated using an approximation method. The equation is an integral part of the VHS algorithm and can be expressed as
erf(z) =
Z
[Z' + (22/28)exp ( - 2 ~ ~ / 3 ) ] ' . ~
The calculated error function canbe checked against publishederror function values such as those presented inEPA's Water Quality Assessment Document[l81 to ensure its legitimacy. The VHS model is based on the dispersion-convection equation
In essence, a continuous source-contaminated parcel moving at a steady one-dimensional D, is the transverse dispersion coefficient, velocity is subjectto a transverse spreading process. y is the spatial coordinate collinear with the velocityof the contaminant V,,, and x and z represent the horizontaland vertical spatial coordinates perpendicularto groundwater flow direction. The problem is viewed as a two-dimensional semiinfinite medium bounded at the topby z = 0, the flux boundary 6C/6Z = 0. At y = 0,
where 2 and X represent the penetration depth at the waste area boundary and the width of the waste unit perpendicular to groundwater flow direction, respectively. The solution to Equation (14) given the boundary conditions is C(x,y,z)=
c(x7yJ) =
erf
[
Z
X
2(D,y/Vy)'.') erf ( 4 ( D , y / v ~ ) ' . ~ ) ]
This solution was first provided by Domenico and Palciauskas [l91 and was used in obtaining exclusions for solid wastes on a generator-specific basis. Applying for an exclusion involvesapubliccommentperiodduringwhichmodification of the model issuggested. Modifications to themodel were due to a change in the manner which in penetration depth(Q is calculated. EPA agreed with the comments regarding the modificationof the 2 term. Since Z is calculated differently, notation in the equation changes slightly. Modification in the cal(13) culation of the Z term and notation changes account for the differences between Equations and (18). The Z term and its importance in this modeling effort are discussed in subsequent sections of this chapter.
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Element III. Dispersion in the Deep Aquifer. A direct conduit to the deep aquifer in the form of an improperly abandoned well could exist in the south-centralportion of the site. Historical records indicatethe presence and approximate locationof a deep well, and although U.S. Geological Services (USGS) boring logs exist, records ofthe well’s abandonment were not located. A number of studies were conducted in 1986 to locate the improperly abandoned well. Geophysical testing and excavation were undertaken in the area of the site described in the company records. These investigations failed to locate the deep well conduit. However, the study did reveal several magnetic anomalies that could correspond to the presence of the deep well casing. Excavation was undertakenas a result of the magnetometric survey, but the exact location of the well was never found. It is unlikely that this is a major pathway of cPAH migration at the site, and the uncertainty in the existence of the open conduit makes modeling this particular pathway difficult. If the well does exist, it could be a conduit to the deep aquifer. The approach used in estimating the potential impact on the deep aquifer considers two[18]. The aquiferis considered dimensional horizontal flowwith continuous solute line sources to be homogeneous and isotropic. Benzo[b]fluorantheneis assumed to discharge continuously and uniformly within the deep aquifer. Steady-state conditions are assumed, to maintain consistency with other portions of the site conceptual model. The equation is
where C, is the acceptable concentration of COIs emanating from a source area that is protective of the shallow aquifer (pglL); Q is the flow emanating from the shallow aquifer as a b is the saturated thickness of the deep aquifers (m); result of the hypothetical conduit (m3/day); p is the porosity of the deeper aquifers (dimensionless);D,, D, are the dispersion coefficients in the x and y directions, respectively (m2/day);B = 2DJVd; V,, is the Darcy velocity (&day); f3 is an estimate of retardation (dimensionless); V, is the seepage velocity (Darcy velocity) of regional flow in the x direction (&day); x the distance from the source area to the APC (m); y the distance between the centerline of the plume and the APC (m); and
4.
Site-Specific ModelingInputs The site conceptual model comprises three elements describedby analytical solutions predictive of an approximate groundwater concentrationat an APC resulting from a soil concentration of cPAHs at a source area. Given the performance standard (CAK), the VHS model is solved for each source area capable of producing an impact on the shallow zone groundwater quality. The VHS reduces to (20)
CAR = CODF, DFH
where CAE is the performance standardin the shallow aquifer(10 pg/L); and DF, and DF, are the dilution factors derived from vertical and horizontal dispersion, respectively. The equation is then rearranged and solved forC,, which is derived for eachsource area of interest at the site. Then C, is entered into the rearranged OLM equation, which becomes I .4149
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C,, the concentration of the constituent in the soil, becomes the soil cleanup goal protective of the shallow aquifer at the APC and meets cAK. The performance standard associated with the established soil leachingcriteria isthen assessed with respectto the deep aquifer. Ifthe groundwater qualityin the deep unitis unaffected at the APC, the performance standard established for the shallow zone is also protective of the deep aquifers and no modification tothe soil leachingcriteria is necessary. If the groundwater quality at the deeper aquifers' APC is affected, the no leaching potentialcriteria for the shallow zone may be modified to also be protective of the deep aquifer. The performance standardfor the deep aquifer is assumed to be the practical quantitation limit (PQL) ofany individual cPAH. The two source areas located in the southern portion of the study area are the most influential source areas (with respect to the shallow aquifer) by virtue of their size and their proximity to the APC. The western property boundary was chosen as the APC for the shallow aquifer because it is hydrogeologically downgradient of both potential source areas. The southern property boundary was chosen as the APC for the deep aquifer for the same reason. The source areas in the northern portion of the site seem relatively small and too far from either APC to produce a significant impact on groundwater quality. Since itis possible that cPAHs emanating from the northern sourceareas are additive, the two small areas are combined and considered to be a single source of cPAHs to simplify the modeling effort. Combined, the northern source areas may have some impact at the APC. As discussed previously, the most conservative approachto modeling a mixture of cPAHs is to assume that the total cPAH soil value is attributable to the occurrence of the single most water-soluble cPAH and not a mixture of the seven constituents (e.g., the concentration of benzo[b]fluoranthene (C,,,) is 700 m a g , and no other cPAHs are detected). This is an extreme worst-case scenario, since the concentration of cPAHs in groundwater at the APC is directly proportional to the constituent soil concentration at the source area of interest and cPAHs typically occuras a mixture at the site as a result of past practices, notjust as individual components. Figure 9 presents the source areas, the location of the hypothetical abandoned well, and the locations of the shallow zone and deep zone APCs used in the conceptual modeling effort. The sourceareas were delineated duringthe 1986 site remedial investigationas those soils having visible stainingbetween the 0 . 5 4 and 6.0-ft depths,an indication of cPAH contamination. The 0.5-6.0-ft interval was selected because of its proximity to the shallow zone and its potential to affect groundwater quality in that zone. Furthermore, the location of the hypothetical improperly abandoned deep well is approximate, as excavation failed to identify its exact location. The remainderof this section discusses the numerical inputsto the algorithms and the subsequent groundwater results at the APC as a function of constituent concentrations in soils at the source areas.
5. ShallowAquifer Three general areas were evaluated to determine soilcPAH concentrations that are protective of the shallow aquifer at the APC. These are the southeastern, southwestern, and northern areas. Each area its corresponding modeling inputs are discussed below. Regional shallow groundwater trends indicate that groundwater beneath the site flows from (l"), east to west, perpendicular to the southwestern source area. Estimated source width source length (X), and distance to the APC (Y)are 25, 275, and 230 m, respectively. Only unpaved areas were considered in calculating the source area dimensions, because pavement would prevent infiltration of precipitation and subsequent leachingof cPAHs.
0
Figure 9 Approximate location of source areas and points of compliance. Hatching indicates areas in which visual staining was observed in
subsurface soils.
3 P rp
.%
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The source locatedin the southwestern portion of the site is closer to the APC than those in the southeastern areas and should therefore have a greater effect on groundwater qualityat the APC.Input estimates for the southwestern source area includewidth (Y’ = 45.7 m), length (X = 69 m), and distance to the APC (Y = 47 m). The central area of the source is pavedand is not considered part of the overall source area since migration of the cPAHs by leaching from soils beneath the paved area will be negligible without the infiltration of precipitation. Groundwater inthe northern portion of the site also flows from east to west perpendicular to the two source areas. The two source areas are combined to account for the potential cocontribution of cPAHs to the shallow zone and simplify theestimation of a soil cleanup goal. The source inputs for width (Y‘),length (X), and distance to the point of compliance (Y)are 111, 78, and 91 m, respectively. 6.DeepAquifer To determine the potential impact of shallow zone groundwater concentrations on the deeper aquifer, a number of assumptions must be made. For the sake of conservatism, it is assumed that the improperly abandonedwell exists as a conduit to the deep water-bearing zone. Impact on the deep aquiferis influenced by the integrity of the abandoned well and the physical characteristics of the aquifers. Little site-specific information pertainingthe to deep well conduit or the deep water-bearing zones is available; therefore, hydraulic conductivity, horizontal gradient, porosity, and aquifer thickness must be estimated from the available literature. The 5304 water-bearing unit corresponds to the base of the Chicot Aquifer in the Alta Loma Sands.This layer is approximately 200-400 ft thick in the vicinity of the site and is fairly homogeneous, being 60-80% sands. Hydraulic conductivities of lo-’ c d s e c and a porosity of 35% are considered to be typical of the formation [21]. The estimated Darcy velocity for boththe deep zones used in the modeling effortis therefore 1 X lo-’ cdday. Steady-state flow from the hypothetical conduitis a function of the integrity of the well and the availability of groundwater present in the shallow aquifer. A characteristic of the shallow aquifer at the site is its relatively low yield. Yields of 1-1.5 gallmin (gpm) are common. Pump tests conducted in 1988 used a pumping rate of 1.25 gpm, and significantdrawdown was observed in each of the observed wells. If shallow zone groundwater is flowing to the deeper zones through the conduit, a trend toward the conduit would be apparent in the shallow zone groundwater contours. Shallow zone groundwater contours presented in the RI report do not indicate the presence of a significant trend associated with an open conduit losing 1.25 gpm to deeper aquifers. Therefore,a conservative estimate of flow rate (Q)equal to one-tenth of the 1.25 gpm rate (0.125 gpm) is used as the maximum potential flow from the shallow zone through the hypothetical conduit. The exact location of the hypothetical conduit is also unknown. U.S.Geological Survey records and magnetometric surveys indicate that the general location is believed to be in the south-central portionof the site, approximately 55 m north of the southern property boundary.
D. Modeling Results and Conclusions 1. No LeachingPotential The following paragraphs present the modeling results and conclusions for cPAH concentrations in soil that are protectiveof the shallow water-bearing unitas well as the deep zone.The results are presented along with the intricaciesof the modeling effort and a discussion of the validity of the results from an analytical solution standpoint. Assumptions concerning the mobility of cPAHs are formulated from a review of the respective MIS. Benzo[b]fluorantheneis the most relatively mobile cPAH because of its higher water solubility. It has the greatest po-
Kotun et al.
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tential to leach andaffect water qualityat a point of compliance. As stated previously, all modeling efforts will therefore use benzo[b]fluoranthenephysicalandchemical data in the determination of a total cPAH soil cleanup goal. 2.ShallowAquifer The VHS model is used to estimate the dispersion of cPAHs in shallow zone groundwater.The shallow aquifer is defined as the first 3 m of the saturated zone and is hydraulically connected to the intermediate aquifer. The shallow zone and the upper intermediate water-bearing zone communicate through the upper intermediateaquitard, which consists of an 11-30-ft layer of interbedded clays, silty clays, and sandy clays. Since the VHS model assumes isotropic homogeneous conditions, it cannot be extended to describe constituent movement in the upper intermediate aquitard; therefore, vertical dispersion cannot be extended beyond 3 m in the shalZ by monitoring the contaminant plume at low aquifer. Domenico and Palciauskas [l91 defined the waste area boundary. This typeof monitoring datais not always availableor possible, and commenters on the model suggest that a less arbitrary estimation of the Z parameter can be calculated using vertical dispersivity and the width of the source area parallel to groundwater flow directions. Constituent penetration at the downgradient source boundary is estimated with the equation
z
=
(U~Y’)O.~
(22)
where Z is the penetration depth, uzthe vertical dispersivity, andY’the width of the sourcearea parallel to the groundwater flow direction. The value of U, is estimated as 0.2 m [13]. Penetration depths of 2.24, 3.02, and 4.71 m were derived for the Southeastern, southwestern, and northern source areas, respectively. Penetration depths in the southwestern and northern source areas exceed the effective depth of the shallow aquifer. Therefore, dilutionby vertical dispersion is applicable only for the southeastern source area. Furthermore, vertical dispersion for constituents emanating from the southeastern source area is complete after 20 m of travel. Horizontal spreading provideslittle dispersion forCOIs from the southeastern source area (due to the length of the source area perpendicular to groundwater flow direction) and the southwestern source area (due to its proximity to the APC). Horizontal dispersion provides some dilution for the combined northern source areas.The dilution associated with vertical and horizontal dispersion is applied to the performance standard of 10 pg/L to determine the CO1 concentration in leachate from the source area. The leachate concentration is then substituted into the OLM equation to determinethe soil concentrationthat is protective of shallow groundwater at the APC. Soil cleanup goalsfor cPAHs calculated forthe southeastern, southwestern, and northern source areas are 250, 100, and 125 mgkg, respectively. 3. DeepAquifers The potential impact of the lO-pg/L individual cPAH performance standard on the deep aquifer can be estimated by assuming that the hypothetical deepwell conduit acts asa continuing line source to the deep water-bearing zones. The approximate locationof the abandoned wellis not directly downgradientof the southeastern source area, and since horizontal spreading does not occur to any significant extent relative to the southeastern source area’s width, this area should not affect groundwater near the hypothetical conduit. Also, the hypothetical conduit is upgradient of the southwestern source area and is not located near the northern source areas; therefore, it should not be af-
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fected by the leachate emanating from either of these source areas. Since 10 pg/L is the performance standard for shallow groundwater, this concentration is used to represent the cPAH concentrations entering the conduit. Shallow zone groundwater entering the deep aquifer is limited by availability. As stated previously, the flow rate used in the pumping test should be modified by a factor of 10 to account for the fact that shallow groundwater contours do notreflect an influence in the area of the hypothetical conduit. Well logs indicatethat the well was screened overa 15-m interval. Although the aquifer in the vicinity of the site is thought to be about200 ft thick, dilution will be considered over only the 15 m that represent the screened interval. The results of modeling indicate that the lo$@ performance standard for the shallow zone is sufficiently protective of the performance standard of 0.2 pg/L in the deeper waterbearing unit. Uncertainties The site conceptual model consists of a series of assumptions and algorithms derived fromthe 1988 RI report and current EPA databases. As with any model,uncertainties in the output stem from the variability in selected input parametersand the mathematicallimitations of describing environmental transport phenomena. Every effort has been made in this modeling effort to ensure that when uncertainties arise any error in the output will be conservative. This section describes the major sourcesof uncertainty inherent to the site conceptual model for the site and possible implications for the modeling outcome. The site conceptual model is limited to describing transport phenomena associated with Darcian flow characteristics of the underlying aquifer and subsequent Fickian transport of the cPAHs in thesaturated soil matrix. If conditions other than these exist, the assumptionsof the VHS model are violated. For example, if preferential flow pathways or zones of high permeability exist in the saturated substrata, dispersion does not occur geometrically with respectto the groundwater flowdirection. The applicabilityof the VHS model is therefore limited to describing constituent movement in the first 3 m of the saturated zone. Although thereis an interconnection between the shallowzoneandtheupperintermediateaquifer, the model is limited in its ability to describe potential dispersion of cPAHs through the upper intermediate aquitard that separates the two zones. To this extent, the model assumes thata no-flow boundary exists between the shallow zone and the deeper aquifers. This is a reasonable assumption given site hydrogeological data and will not lead to over- or underestimation of contaminant transport and subsequent attenuation. The shallow aquiferis characterized in the RI report as being comprised of predominantly silty materialsand clayey sands. This materialis relatively homogeneous, and preferential flow pathways do not exist, with the exception of slickenslides, which occur regionally. The RI report also indicates that the shallow zone is continuous beneath the site. The use of the.VHS model should provide an adequate quantitative estimate of the dispersive processesat the site in the shallow zone. Limited dilution is afforded by the use of the VHS model, due to the size of the potential source areas, their proximity to the AFCs, and the thickness of the shallow aquifer. This is particularly evident in the southern portion of the site, where no dilution by vertical or horizontal spreading is produced when cPAHsare modeled from the southwestern source area to the western property boundary. The assumption that cPAHs are chemically conservative and are not retarded by the organic carbon contentof the saturated zone soils producesa very significant level of conserva-
4.
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Kotun et al.
tism in the estimation ofcPAH transport ingroundwater. Similarly,high estimates of permeability and porosity reduce the potential for underestimating migration potential from the source areas. The site conceptual model depends upon the size of the potential source area used in the calculations of groundwater at the APC. Theseareas are approximated, and insufficient chemical data are available to better estimate the time and the size and shapeof the areas.The source areas were estimated by a visual inspection of the soils (i.e., if a soil is stained, it is assumed to contain cPAHs) completed in 1988 as part of the RI. Overestimation or underestimation of the size of the potential source area could lead toan over- or underestimation of the source’s effect on the underlying groundwater quality. Due to the conservative nature of the modeling efforts, the underestimation would probably not be significant in the determination of the no leaching potential. The most significant sourceof uncertainty in the site conceptual model arises from the use of the OLM to estimate leaching potential. Actual leaching of cPAHs could be greater or less than leaching predicted by the OLM. Verificationof site-specific leaching potential by a comparison ofOLM estimates to actual TCLP analytical results will reduce this source of uncertainty by providing site-specific soil leachate concentrations or a level of certainty to the OLM values. No site-specific information exists for the deep aquifer. Information on the zone is either gathered from recent literature or inferred from trends occurring in overlying aquifers. The greatest uncertainty in modeling potential constituent migrationto the deep aquifer is the estimation of flow through the hypothetical conduit from the shallow zone to the deep unit. The existence of thispathway is questionable, as historical investigations failed to determine whether the conduit exists, and trends in shallow groundwater do not indicate the presence of a significant shallow groundwater sink in the approximate location of the well. If the conduit does exist, in all likelihood is it not open fromthe shallow zone to the deep zone,and flow will be impeded by subsurface materials inside the former well or by the integrity of the remaining well casing. 5 . Validationof the Site Conceptual Model Examination of the site conceptual model indicates that the total cPAH action level is greatly influenced by each constituent’s abilityto leach from site soils, not its dilution by dispersion. Vertical and horizontal spreading account for a dilution factor of approximately 1.8 from the southeastern sourcearea to the fence line. Vertical and horizontal spreading do not accountfor dilution from the southwestern source area to the fenceline, suggesting that leaching fromunsaturated soils is most influential in determining the soil cleanup goal. As a result, EPA suggested that samples below the established direct-contact soil cleanup goal of 700 mgkg should be analyzed for cPAHs and subjected to TCLP analysis. Results obtained from the OLM portion of the site conceptual model could then compare to sitespecific soil leachate data. In addition, OLM values will be generated for the samples subbe compared jected to TCLP The OLM value and the TCLP result for each cPAH can then for each sample collected from the site. The average difference between theOLM and TCLP data could thenbe subjected to a paired t test at the 95% confidence levelto determine whether the site-specific TCLP results are less than, equal to, or greater than the OLM results used in the modeling effort for the determinationof the soil cleanup goal [22]. If the TCLP results are less than or equal to the OLM results, then modification of the model is not required. If the TCLP results exceed the OLM results, then the leaching portion of the site conceptual model will be revisited to include site-specific leachingdata, and a new soil cleanup goal will be developed.
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E. SiteFieldInvestigation As a result of the uncertainties associated with the modeling results (inparticular the leaching portion of the site conceptual model), a Supplemental Field Investigation was implemented. This investigation focused on better defining the source areas with respectto soil contamination and determining site-specific leaching potential to compare to OLM results.The following sections present an overview of the field investigation. 1. Development of Area-Specific Soil Boring Locations The f i t task associated with the soil boring program was to establish a sampling grid over each potential source area. The locations of the soil boring grids were approved by EPA V in 1986 RI reportand in the siteSeptember 1991 basedonthesourceareasdefinedinthe modeling effort. The original grid' spacing for all four investigativeareas was approximately 1 0 0 ft by 100 ft. However, some sampling was done in areas not corresponding to grid loci, as a result of field limitations or greater data needs in a given area. Limitations for this spacing were introduced by the railroad right-of-way and paved areas. Boring locations were established at less than the 100 ft by 100 ft grid due to asphalt paving (along the west side of the southeastern area), site buildings, and property linesor fence lines.The boring affected by paving and a site building was G-17. Borings G-51, G-52, and G-53 were located only 60 ft from brings G-13, G-26, and G-28, respectively, becauseof the property or fence line. BoringsG-l through G-l3 and G-51 are located along the eastern property line. Figure 10 shows the four investigative areas and their respective sampling locations.
2. DrillingMethodology Each soil boring was drilled using the hollow-stem auger method of drilling with a truckmounted rig. Soil sampleswere collected continuously usinga 24-in. split-spoon device as the hole was advanced to the termination depth of 6 l? at each boring location. Samples were collected continuouslyso the soils could be classified with respect to textural compositionand the presenceor absence of visually identifiable potential site-related constituents. Samples were collected using a 3-in. OD split-spoon sampler, logged in the field by the site hydrogeologist, cornposited for the individual spoon sample interval, and placed in clean laboratory-preparedjars with Teflon-lined lids for submittalto the analytical laboratory. All soil samples were classified according to the Unified Soil Classification System. Followingcompletion of each boring, the borehole was sealed to thesurfaceusing a cementhntonite grout. The split-spoon sampler was cleaned after collection of each sample using a soapy water bath followed by a clean water rinse. Between boreholes, and prior to leaving the site, the drilling rig, hollow-stem augers,drilling rods, and associated drilling tools were steam-cleaned at the on-site decontamination area. The cuttings from the drilling operations were contained on site in roll off boxes for eventual treatment or disposal. 3. ChemicalAnalysis The soil samples collected from each depth interval of the grid boring locationswere submitted to a certified laboratory for chemical analysis. The samples were analyzed for cPAHs using EPA Method SW846 8270, a high-resolution gas chromatographic, low-resolution mass spectrometric (GCIMS) method of analysis. Mass spectrometric detection allows for nearly absolute identification of the cPAHs as opposed to other methods of detection. Samples that exhibited concentrations of less than 700 mgkg of cPAHs were also submittedfor TCLP extraction and subsequent leachate and analysis for cPAH by Method SW846 8270 to determine the actual leaching potential of the sample. An analytical sampling summary is presented in Table 28.
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Table 28 Analytical Sampling Summary TCLP/cPAH analysis
Soil cPAH analysis Area Southeastern Southwestern Northeastern Northwestern
No. of brings
Total No. of samples
Rinsate blanks
Lab blanks
53
162
16
48
13 5
39 15
8 2 2 1
11 4 1 1
No.ofTCLP samples
Lab blanks
MSMSD
Duplicates
9 3
4 4
158
9
8
1
46
4
4
4
2 I
39
3 1
3 4 2
2
15
MS/MSD
Duplicates
0 1
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As part of the study, a quality assurance/quality control(QNQC) sampling program was completed that consisted of collecting matrix spikdmatrix spike duplicate samples and field blanks. A laboratory auditwas also conductedby EPA to ensure the legitimacyof the analyses. Site soil samples subjectedto TCLP extraction didnot produce detectable concentrations of any cPAH inthe corresponding leachate. The majority of soil samples contained total cPAHs in excess ofthe soil cleanup goals obtained by the modeling effort. Soils containing asmuch as 1200 mgkg of total cPAHs did not leach, suggesting that site leaching potential is at least a factor of 4-12 times lower than predicted by the site conceptual model. Samples containing more than 1200 mgkg total cPAHs were not subjected to TCLP extraction. The following paragraphs and Figures 11-13 present a spatial presentation of source area are related to the direct-contact soil cPAH concentrations. Discussions concerning remediation soil cleanup goal of 700 mgkg total cPAHs since site-specific data suggest that this value is also protective of the no leaching potential. Areas requiring remediation were delineated onthe assumption that the extent of impact is midway between a “clean” point and an adjacent impacted point. The volume of soils exceeding the remedial goal of 700 mgkg was calculated based on the depth at which the exceedance was noted. The analytical resultsof the samples collected from53 soil boring locations in the southeastern area were used to delineate the horizontal and vertical extents of soils for remediation (Figure 11).The locations in this area that exhibited total cPAH concentrations greater than 700 ppm were grid locations G-60, G-5,and G-61; G-64,G-67, G-9, G-22, G-23, G-24, G-25; and G-73. Soils delineated for remediation are confined tothe upper 2 ft, except for three locations. Locations G-73 and G-22 show soils exceeding700 ppm total cPAHs inthe 2-443 interval, and G-61 showed total cPAHs exceeding 700 ppm in the 0 - 4 4 interval. Approximately 24662868 cu yd of soil in this area exceed the remedial goal. A total of 16 soil borings were drilled in the southwestern area. The analytical results are presented in Figure 12. Grid sampling locations G-80, G-81, G-82, G-83, G-35, and G-36 exhibited totalcPAH concentrations above 700 ppm. Soil intervals delineated for remediationare limited to a small portion of this area and are generally from 2 to 6 ft. Boring location G-83 the exhibited cPAH concentrations above700 ppm in the 0-2 ft interval, boring G-81 exceeded remedial goal in the 2-4-ft interval, and borings G-80, G-82, G-35, and G-36 exceeded the goal in the 4-64? interval. Approximately 626-1588 cu yd of soils in this area exceed the remedial goal. In the northern area, grid samplings locations G-46 and G-87, between 0 and 2 ft, exceeded the risk-based criteria of 700 ppm for total cPAHs (Figure 13). Approximately 5581177 cu yd of soils present in this area exceed remedial goals. No grid sampling locations in the northwestern area exhibited total cPAH concentrations above 700 ppm (Figure 13). Remediation of soils in this area is therefore not necessary.
F. Conclusions To determine soil cleanup goals for the cPAHs protective of underlying groundwater, a conceptual model of potential constituent migration was developed. The site conceptual model consists of two elements:(1) leaching of constituents from the potentially affected soils and (2) migration and dispersionof the constituents with respect to groundwater flow direction. The OLM was used to describe the leaching potential of the cPAHs present in the soil (Element I), while the VHS model was selected to describe dilution associated with dispersion of the cPAHs in groundwater (Element 11). The conceptualmodel is conservative (froma modeling standpoint) in that biodegradation andretardation in soils and groundwaters are not considered.
SCALE (FEET1
0
50
100
1%.
Figure 11 Southeastern source area-total cPAH analytical results. (0)Soil sampling location (approximate). Hatching indicates estimated area with soils exceeding 700 m a g total cPAHs. Total cPAH concentrations: 0-2 ! I 500,2-4 , ft, 685; 4-6 ft, 191.6. ND, not detected.
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Figure 13 Northern source area-total cPAH analytical results. (0) Soil sampling location. Hatching indicates soils exceeding 700 rnglkg total cPAHs. Total cPAH concentrations: 0-2 ft, 102; 2-4 ft, 81.8; 4-6 ft, 188.9. ND, not detected.
s
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A semiquantitative sensitivity analysiswas performed on the results of thesite conceptual model. Element I, leaching as predicted by the OLM, was the most influential portion of the modeling effort. Element I1 had little impact on the final modeled results because of the shallowness of the uppermost water-bearing zoneand the proximity of the potential sourceareas to the alternative point of compliance (APC). As a result, leachate testing was performed to determine whether the use of the OLM was representative of site conditions. EPA suggested the use of TCLP analyses of soil samples taken from each of the potential source areas. Element I, represented by the OLM, is a multiple regression equation:
C[ = 0.000211 x
,F678(23) x
WF373
where C, is the estimatedleachate concentration, C, is soil concentration, and W,is water solubility. TCLP data collected to confirm the validity of Equation (23) with respect to site leaching potential indicates that for cPAH soil concentrations below 1200 ppm (C, < 1200 mgkg) the leachate concentration is below the limit of detection. Cl can therefore be expressed as
c, = 0
(24)
The first element of the conceptual model is therefore undefined from a practical standpoint using site-specific leaching data, given that
where {x} is any number. Site soils containing in excessof 1200 mgkg total cPAHs (C,) did not produce detectable concentrations in their respective leachate(C,), and mathematicalcorrelation between soil concentration and site-specific cPAH leachate values beyond the previously stated relationship could not be established. Soils with cPAH levels greater than 1200 mgkg were not submitted for TCLP analysis. Based on the site-specific conditions, the use of the OLM to describe leaching in the site conceptual model of constituent transport was a conservative assumptionin lieu of site-specific informationregardingconstituentleaching.Therefore, the ROD-specifieddirect-contact cleanup goal of 700 mgkg total cPAHs is also protective of the no leaching potential for this particular site. In this particular case, spending the additional money to derive a site-specific estimate of leaching potential resultedin a significant cost savingsfor the potentially responsible partyas opposed to removal of soils based solely on the modeled results. However, it should not be overlooked that the modeled results werealso adequately protectiveof the no leaching potential established in the site ROD. Using the analytical solutions in determining a soil cleanup goal at sites having smallersource areas or where less costly remedial alternatives are an option will produce protectiveand cost-effective results.In any case, thedevelopmentof soil cleanup goals using analytical solutions protective of underlying groundwater quality is necessary to focus additional field investigations and data collection efforts when the protection of groundwater quality from continued soil releasesis a priority.
REFERENCES 1.
U.S. Environmental Protection Agency, Risk Assessment Guidance for Superfund. Vol. I , Human Health Evaluation Manual, Washington, D.C., 1989.
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2. Chu,M.M. L., and Chen, C. W., Evaluation and estimation of potential (Carcinogenic risks of polynuclear aromatic hydrocarbons, Symposium on Polynuclear Aromatic Hydrocarbons in the Workplace, International Chemical Congress of Pacific Basis Societies, 1984. (HEAST). 1991. 3. U.S. Environmental Protection Agency,Health Effects Assessment Summary Table I), Supplemental Risk Assessment Guidance for the 4. U.S. Environmental Protection Agency (Region Superfund Program, 1989. Development ofStatistical Distributionsof Ranges of Stan5. U.S. Environmental Protection Agency, dard Factors Used in Exposure Assessments,1985. 6. U.S. Environmental Protection Agency, Exposure Factors Handbook, 1989. 7. U.S. Environmental Protection Agency, National Oil and Hazardous Substance Pollution Contingency Plan, 1990. for Evaluating the 8. U.S. Environmental Protection Agency,Multimedia Exposure Assessment Model Land Disposal of Wastes: Model Theory and Application,1991. 9. U.S. Environmental Protection Agency, Office of Water, Drinking Water Regulations and Health Advisories, April1992. 10. U.S.Environmental Protection Agency, Office of Water Regulations and Standards, Quality Criteriafor Water, 1986. 11. U.S. Environmental Protection Agency, Mice of Water Regulations and Standards,Aquatic Fate Process Data for Organic Priority Pollutants,1982. 12. Wang, H. F., and Anderson, M. F!, Introduction to Groundwater Modeling: Finite Difference and Finite Element Methodr, W. H. Freeman, San Francisco, 1982. 13. U.S. EPA, VHS model, Fed. Reg., 50(229), 48897-48901 (Nov. 27, 1985). Fed. Reg., 51(145), 27061-27064 (July 29, 1986). 14. U.S.EPA,OLMmodel, 15. Lkagun, The Soil Chemistryof Hazardous Materials,Hazardous Materials Control Research Institute, Silver Spring, Md., 1988. 16. Laskowski, D. A., Goring, C. A., McCall, F! J., and Swann, R. L.,Terrestrial environment in environmental risk analysis for chemicals, in Environmental Risk Analysis Chemicals (R. A. Conwaya, Ed.), Van Nostrand Reinhold New York, 1983. 17. Ford and Gurba, Methods of determining relative contaminant mobilities and migration pathways using physicalchemical data, 1984. for Toxic and Conventional Pollutants 18. US. EPA, Water Quality Assessment: A Screening Procedure in Surface and Groundwater,Part I1 EPA/600/6-85/0026, 1985. V. V., Alternateboundariesinsolidwastemanagement, 19. Domenico, I? A.,andPalciauskas, Groundwater,20, 301-311 (1982). 20. Morgenau, M., and Murphy, G . M., The Mathematics of Physics and Chemistry, Van Nostrand, Princeton, N.J., 1956. 21. Freeze, R. A., and Cherry F. A., Groundwater,Prentice-Hall, Englewood Cliffs, N.J., 1979. 22. Rosner, B., Fundamentals ofBiostatistics, 2nd d.,PWS, 1982. 23. Kreitler, C. W., Guevara, E., Granata, G., and McKalips, D., Hydrogeology of Gulf Coast aquifers, Housbn-Galvestonarea, Texas, reprinted fromTrans. GulfCoast Assoc.Geol. Sci., Vol. 27, Geologic Circular 77-4, 1977.
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6
Designing to Prevent Pollution
James Lounsbury National Roundtable of State Pollution Prevention Programs Silver Spring, Maryland
1.
INTRODUCTION: ENGINEERS IN A CHANGING ENVIRONMENT
In the annals of engineering folklore, it’s been said that Henry Ford required his suppliers to ship raw materials on pallets of specified dimensions-an unusual requirement at the time since pallets were always dumped or burned after use. Henry’s genius saw the opportunity: dismantling the pallets provided made-to-order hardwood floorboards for his ModelT’s. This simple idea eliminated the cost of floorboards and of the men and equipment needed to dispose of used pallets. Environmental benefits were not explicitly considered-but they were there. Ford reduced the amount of raw materials (trees) extracted from the environment and the human and energy resources neededto convert trees into pallets, and he also reduced the amount be burned) and the amount of ash to be of smoke emittedto Detroit’s air (if the pallets were to disposed of. When the industrial revolution began in the 1850s, the United States was a land of abundant resources, diverse markets, and transportation systems capableof moving raw materials and finished products. Engineers were concerned with performance, quality, and maximum yield. Their main objective was to make a profit by producing more and better goods and services. Environmental considerations were not a significant factor in the manufacturing equation. We had seemingly infinite land and water resources capable of assimilating whatever amounts of wastes were dumped. Adverse environmental impacts were not perceived as a serious problem (although adverse impacts were, in fact, present). Therefore, there were few or no environmental coststo consider. The advance of manufacturing and the engineering sciences that supported the American industrial base proceeded for about a hundred years before public awareness of environmental pollution and health-related issues began to catch up. It’s been only during the past 30 years that environmental quality has become a national watchword. In the 1960s, “Keep American Beautiful” was a widely publicized effort to keep our country litter-free. As urbanites inhaled more smog, and as rural citizens became more 145
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aware of dying rivers and contaminated drinking water wells,they began to seek solutions to these problems throughthe legislative process. Beginning in the 1970s, the U.S.Congress embarked on a legislative agenda that brought environmental management to the forefront and changed the ground rules for manufacturers forever. Forthe next two decades, industry andthe government focused on managing wastes and emissions from factories, autos, and sewage treatment plants by building bigger and better pollution abatementand control equipment designed to remove pollutants from “end-of-pipe” discharges.As the economy has expanded,so has the price tag for pollutioncontrol-now over $1 15 billion dollars per year. Kenneth Chilton, in his 1991 study Environmental Dialogue:Setting Priorities for EnvironmentalProtection, is quoted capita price for by Mason [l] as stating that “to put these (EPA) costs into perspective, the per pollution controls was more than $450 in 1990. A family of four, thus, spent more than $1800 in the form of taxes or higher prices for manufactured goods to pay for pollution abatement.” As the costs of pollution control have continued to rise, both industry and government have begun to respond to this startling price tag by finding innovativeways of reducing these costs. Government is looking to expand beyond its traditional role of ensuring compliance with endof-pipe treatment and disposal regulations and is instead seeking nontraditional approaches to prevent or reduce pollution generationup front by providing industry with technical information and assistance outside the traditional regulation and enforcement realm. Government is also looking for ways to modify its regulatory and enforcement processesto include better incentives for pollution prevention. At the same time, industry is looking for ways to work with government in both of these areas to reduce the use of toxic materials in products anddesigning production processes thatgenerate less or no waste. These concepts,known as “pollution prevention,” and “green design,” are the keys to integrating environmental protection and competitive manufacturing in the 1990s and the next century and are the subject of this chapter.
Organization of This Chapter This chapter provides a basic road map for engineering students to better understand The evolution of environmental laws and regulations facingthe engineer “End-of-pipe” pollution control versus “pollution prevention” and “green design” Where to go for information on chemicals, pollution prevention, and green design It is designed to provide the reader with general information andis presented with the understanding that the author is not engaged in renderinglegal, accounting, or other professional services. If professional advice is needed, the services of a competent professional should be sought. It should also be noted that some of the statutes and regulations discussed here are regularly revised. As a result, the information in this chapter may be incomplete or subject to contrary opinion. It would be wise for the reader to check on thecurrent status of any particular statute or regulation before proceeding with related work.
II. EVOLUTION OF ENVIRONMENTAL LAWS AND REGULATIONS By about 1970, the federal government responded to citizens’ concerns about environmental problems by making environmental legislation a priority. The federal government acknowledgedthatbettertoolswereneededtomanageenvironmentalproblems. Starting in1970, Congress embarked on a decade of ambitious legislative activity inwhich it would enact landmark legislation to improve water quality andair quality and protect our land and groundwater resources. This section provides a brief summary of the major statutes enacted since 1970. From the summaries, the readerwill hopefully get a sense of the types of approaches pursued inthe law
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and see that there has been a gradual shift in focus on how to best manage the environment. This shift began with medium-specific (air, water, land, etc.) end-of-pipe pollution control and has shifted slowly toward up-front pollution prevention and green design. be obtained from various sources, including public libraries Full texts of these statutes can and publishers that specialize in environmental publications. A brief discussion of the regulations promulgated by EPA that implement these laws is providedin the next section.
A. Summary of Key Federal Environmental Statutes Enacted Since 1970' In 1970, Congress established the U.S. Environmental Protection Agency (EPA) with the enactment of the Reorganization Act of 1970. This act consolidated the various environmental management responsibilities that were previously dispersed throughout the executive branch agencies, for example, Department of Interior (federal land management, mining), the Department of Agriculture (pesticide registration), and the Federal Water Pollution ControlAdministration (development of scientific and technical information for protecting water quality). EPA is charged with implementing most of the environmental laws passed by Congress, although some functions still remain with other agencies. For example, the Department of Interior manages federally owned lands andthe Occupational, Safety and Health Administration is responsible for workplace exposure to toxic chemicals. In general, EPA's job includes ( l ) developing the scientific and technical tools for assessing environmental impacts and risks that may accompany society's activities (e.g., disposal of manufacturing wastes, disposal of municipal sewage and solid waste, assessmentof automobile and other air pollutants affectingurban populations), (2) publishing regulations that restrict actions that pose unacceptable risks to humans and the environment, and (3) enforcing its regulations through civil and criminal legal actions. In 1970, Congress also enacted the National Environmental Policy Act (NEPA).NEPA effectively required all federal agencies to consider the impacts on the environment before taking any major federal action. NEPA was a key force, philosophically and legally, for all future environmental decision making in the United States. It created anew decision-making process and provided the opportunity for the public to be involved in reviewing all major federal actions. NEPA requires federal agenciesto analyze the environmental impacts of all proposed major federal actions suchas building dams, leasing federal land for oil exploration, and building interstate highways. NEPA further requires agencies to consider alternatives to the proposed action, analyze the impacts of each, publish the agency's analysisof alternatives and recommended course of action in an Environmental Impact Statement (EIS), and ensure that there is an opportunity for public participation in the review of the EIS through written comments and/ or testimony in public hearings.NEPA specifies a process for reaching a decision. It does not dictate any particular decision. The Federal Water Pollution Control Act of 1972 (FWPCA) established goals for restoring andmaintainingtheintegrity ofwaters. The FWPCA,now theCleanWater Act (CWA), has been amended several times to put in place several major programs, including (1) technology-basedwastewatertreatmentregulations, (2) requirements for plant-specific wastewater discharge permits and chemical-specific water quality criteria to use as a guide-in writing the permits, (3) technology-based requirements for municipal sewage treatment plants and a federally sponsored funding program to assist municipal governments in the construction 'Source material from Environment Srarures, 1990 d., Government Institutes. Inc., Rackville, Md., 1990.
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of sewage treatment plants, (4) area-wide water quality planning programs to ensure that all pollution sourcesand institutions are considered in water quality protection,and (5) a specially designed program to protect the nation's wetlands. Congress amended earlier air quality legislation with the Clean Air Act in 1972. These requirements, combined with amendments enacted in 1977 and 1990, cover a very broad array of programs and requirements, includingthe establishment of ambient air quality standards and the development of state implementation plans for a variety of pollutants, national emissions standards for stationary sources of hazardous pollutants, requirements for ozone protection, motor vehicle emission and fuel standards, and the prevention of significant deterioration of air use of market-based quality. The 1990 amendments are significant because they encourage the principles, including performance-based standards and emission banking and trading, provide a framework for finding a cost-effective combinationof fuels and technologyto develop alternative clean fuels, promote the use of clean low-sulfur coal and natural gas as well as innovative technologies to clean high-sulfur coal through the acid rain program, sufficiently reduce energy waste andcreate enough of a market for clean fuels derived from grain and natural gas to cut dependency on oil imports by 1million barrels a day, promote energy conservation' through an acid rain program that gives utilities the flexibility to obtain needed emission reductions through programs that encourage customers to conserve energy, and establishan important innovative approachto pollution prevention. The 1990 CAA amendments require that EPA conduct a basic engineering research and technology program to develop, evaluate, and demonstrate nonregulatory strategies and technologies for air pollution prevention. This signals that Congress had added prevention measures to the end-of-pipe treatment approach previously addressed in the statute. The 1990 CAA amendments also offer other pollution prevention incentives. One of the most widely discussed and challenging is the opportunity for facilities to receive extended compliance periodsin exchange for early and increased reductions of their hazardousair pollutants. As a result, industry may find that source reduction measures are a more cost-effective approach to achieving emission standards for hazardous air pollutants. The Safe Drinking Water Act was amended several times during the 1970s and 1980sto improve the drinking water standard-setting process for consumptionof drinking water at the tap. Standards are set considering both the cost and effectiveness of available treatment technologies. The SWDA amendments of1986required EPA to promulgate a total of 108 standards for toxic chemicalsby 1991. They also require states to develop a program to protect the area around wells (wellheads). This provision was enacted in response to findingsby EPA that 8000 drinking water wells in the nation were contaminated by thousands of sources, including land disposal facilities, underground storage tanks,septic systems, underground injection wells, and pesticide andfertilizer applications. The chemicals foundare widely used in common products, including plastics, solvents, pesticides, paints, dyes, varnishes, and ink. The Federal Insecticide, Fungicide and Rodenticide Act (FIFRA) was originally enacted in 1947 and has been amended several times to factor in new information and policies on the effects of chemicals on human health and the environment. FIFRA requires manufacturers of pesticides, herbicides, and related chemicals to comply with a registration process (or reregistration process for pesticides registered before 1972). inwhich the manufacturer must complete batteries of tests to ensure that when used properly a pesticide presents no unreasonable healthor environmental risks.EPA is authorized to review test results and prohibit, ban the manufacture of, or limit the use of existing or proposed products. Over 50,000 pesticides have been registered since FFRA was originally enacted. However, significant advancesin the sciences of toxicology and risk assessment have occurred over the past decade. These advances have led to concern over potentialrisks from previously reg-
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istered pesticides. Amendments to FIFRA in 1988 require previously registered pesticidesto be reregistered under current, stricter procedures. The Toxic Substances Control Act (TSCA) of 1976 includes very broad authorities that require EPA to control the risks from over65,000 existing chemical substances on the market (other than chemicals regulated under FIFRA) and to regulate the manufacture, processing, distribution, and use of proposed new chemicals. Manufacturers must complete laboratory tests and provide detailed information on the processes used to manufacture each chemical EPA to for review. Based on its review, EPA has the authority to prohibit production, limit use,or ban production of the chemical. The Solid Waste Disposal Act (SWDA)of 1976, later renamed the Resource Conservation and Recovery Act (RCRA),radically changed the role of the federal government in solid waste management. EPA promulgated guidelines in 1979 that prohibited the previously widely accepted practice of open dumping and required the use of sanitary landfill standards to reduce the potential for disease and restrict open access to garbage by humans and animals. Under RCRA, EPA created the nation’s first “cradleto grave” hazardous waste management system 2% of all industrial solid waste)and in which EPA identified the worst industrial wastes (about set standards for tracking these wastes from generation to treatment to disposal and for disposal of wastes in landfills. In 1984, RCRAwasamendedwiththe HazardousandSolidWasteAmendments (HSWA). HSWA prohibited the disposal of hazardous wastes on land unless they were first treated using standards setby EPA for best demonstrated available treatment (BDAT) technologies. HSWA also directed EPA to launch a major new program to establish standards for replacing approximately 2 million leaking underground storage tanks containing gasoline and chemical products. HSWA was the first statute that explicitly shifted national priorities for waste management from end-of-pipe pollution abatement and control to pollution prevention. (Note:The term used in HSWA was “waste minimization.”) HSWA directed EPA to study the incentives and barriers to a prevention-based approach because the Congress realized that the end-of-pipe approach of the past was not providing any significant reduction in the amount of waste generated and managed even though the standards for end-of-pipe environmental controls were becoming more strict. The progress EPA made in the next several years in studying avenues to encourage pollution prevention provided some of the basis for pollution prevention legislation passed 1990. in This is discussed in more detail at the end of this section. The ComprehensiveEnvironmentalResponseCompensationandLiabilityAct (CERCLA or Superfund) of 1990 established a landmark environmental program for the cleanup of hazardous substances improperly disposed of on land. The enactment of Superfund was a rude awakening for many U.S. companies. It has led to the discovery of 30,000 poten$9 billion fundto pay for cleanup tially contaminated chemical dumps. Superfund established a of sites where responsible parties cannot be foundor where they are recalcitrant. However,$9 billion is enough to pay for only a small fraction of the total sites eligible for cleanup. Superfund made companies liable for cleanup costs and damages, including wastes that the companiesbelievedweresafelydisposedofbutthathadneverthelessresultedinenvironmental damages. CERCLA required companies to pay taxes on feedstock raw materials (which were to support the the fundamental source of wastes discoveredto be causing damage in the ground) federal cleanup fund. Companies involved in Superfund cleanup found that they pay for improper waste disposal in four ways. They pay for raw materials that became waste, the cost of waste disposal, a feedstock tax to pay for cleanup,and cleanup costs (plus punitive damages in some cases).
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Superfund was amended in 1986 with the Superfund Amendments and Reauthorization Act (SARA). SARA strengthened all of the major portions of the cleanup program, forced federal facilities to comply with the law to the same extent as any other facility, and reauthorized the cleanup fund and the feedstock tax on industry to support the fund. The Emergency Planning and Community Right-&Know Act (EPCRA, also known as Title I11 of SARA) was also enacted in 1986. The enactmentof EPCRA stems directly from an incident involving the chemical release of methyl isocyanate from a pesticide plant in Bhopal, India, in 1984 chat killed thousands of nearby residents and the release of methylene chloride and aldicarb froma chemical plant in Institute, West Virginia, in 1985. EPCRA mandates that states and local communities prepare for chemical emergencies, requires facilities to notify their states and communities of the presenceof an extremely hazardous substance andto report spills or releases of such substances immediately, and requiresfacilities to report annually on the amounts of certain hazardous chemicals produced, used, and stored within the facility if that amount exceedsa specified amount. Facilities must also report to EPA annually on the amount of certain toxic chemicals they release to the air, land, and water. EPA compiles this information in a publicly accessible database called theToxic Release Inventory (TRI) and publishes an annual report describing the reported releases. The TRI has had a very significant impact on the way industry views its environmental responsibilities and accountability to the public. The Pollution Prevention Act (PPA) of 1990 moved the government significantly forward in its commitment to change the way it manages environmental protection. The PPA established a national policy that pollution should be prevented or reduced at the source whenever feasible; pollution that cannot be prevented should be recycled in an environmentally safe manner whenever feasible; pollution that cannot be prevented or recycled should be treated in an environmentally safe manner whenever feasible; and disposal or other release into the environment should be employed only as a last resort and should be conducted in an environmentally safe manner. The PPA requires EPA to considerpollutionprevention in itsrule-making activities, provide grants to states for technical assistance to businesses seeking pollution prevention opportunities, andmanage a computerizedtechnicalinformationclearinghouse on pollution prevention technologies that is available to the public. The PPA also requires facilities that are required to report releases to the environment under EPCRA (Title I11 of SARA) to also reporttheirprogress in pollutionpreventionto EPA eachyear,andrequires EPAto issue a report to Congresseverytwoyears that describes the nation’sprogress inpollution prevention. EPA is also charged with administering several other environmental statutes, which for the sake of brevity are not described here. These include The Marine Protection, Research and Sanctuaries Act of 1972 The Noise Control Act of 1972 The Radon Gas and Indoor Air Quality Research Act of 1986 The Asbestos Information Act of 1988
B. EnvironmentalRegulations Most federal environmental statutes set broad goals and timetables for achieving these goals. EPA is charged with the task of developing regulations that implement the law.
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The regulatory development process is describedin this chapter to emphasize the important opportunity for the engineering community andfor individuals and companies to participate in the process and potentially contribute to the outcome of individual regulations. The overall process for developing, proposing, and promulgating regulations isforth set in the Administrative Procedures Act,a statute that applies to all federal agencies. Thereare two basic but complex steps in the process. The first step covers the development of a proposed regulation. EPA first determines what policy, economic, and technical factors are relevant to the decision at hand and then generally proceeds with extensive engineering, toxicological, economic, and other studies necessaryto arrive at a proposed approach. At the completion of its studies, EPA proposes a regulation and solicits public comment on the methods and rationale used in its proposal. The second major step includes completing whatever additional studies may be necessary to adequately respond to public comments, incorporating relevant new information that may affect the outcome, and promulgatinga final regulation. It generally takesat least two yearsto develop and promulgate a regulation, depending on the complexity of the issues involved. Each proposed and final regulationis published in the Federal Regisrer (FR),a daily government publication dedicated entirely to notifying the public of regulatory developments. A compendium of all environmental regulations promulgatedby EPA is found in Title 40 of the Code of Federal Regulations (40 Cm). In addition to the regulations, EPA frequently publishes supplemental guidancethat provides additional technicalor procedural information relevant to understanding the compliance requirements of the regulation.
C. States and Local Laws and Regulations States and local governments also enact environmental laws and regulations. In many cases, Congress directs EPA to establish federal regulations under a particular statute, then authorizes EPA to delegate responsibility for implementing and enforcing a program to states that have statutes and regulations that are at least as stringent as the federal program. In some cases, state programs are more stringent than the federal program. It is not surprising to find differences in the ways states implement federal environmental laws. Municipal governments also pass environmental laws and should be consulted regarding matters particular to the municipality.
111.
"END-OF-PIPE POLLUTION CONTROL VERSUS "POLLUTION PREVENTION" AND "GREEN DESIGN"
End-of-pipe pollution control, which began with media-specific legislation of the 1970s and 1980s, has begun a shift in the 1990s to up-front pollution prevention and "green design.'' As an expanding setof scientific, economic, and a result of this shift, engineers today must address technological factors if they wish to build better products and remain competitive, Several initiatives under way in government and industry to foster this approach are discussed in this section.
A. Assumptions Underlying the Green Design
Shift to Pollution Prevention and
Much of the environment legislation enacted in the 1970s and 1980s was media-specific; that is, each statute focused on either emissions to air, dischargesto water, or disposal on land.This
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approach arose froma general consensusthat industry shouldbe in charge of deciding how to manufacture its products. The government's role wash to protect human health and the environment and therefore should be limited to regulating emissions, discharges, and wastes after they leave the manufacturing facility. There were several assumptions built into this approach, however. It was assumed that if the adverse impacts of waste disposal on the public increased, the public marketplace would resist by making it more difficult for industries to site disposal facilities, which would in turn drive up the costsof waste management. As the cost of waste managementrose, industry would find an equilibrium point at which it was no longer profitable to generate and manage waste. Consequently, business managers would be forced to find more efficient ways of producing goods and services in order to stay in business. In general, these wereprobablygoodassumptions,butseveralunanticipatedfactors slipped into theequation. First, the study of the impactsof contaminants on human health and the environmentwas not as advanced as manufacturing science and technology. Therefore, neither the government, the public, nor industry could fully evaluate the costs of the impacts of waste management on the environment and assign a true cost to that activity. Second, no one anticipated some of the outcomes of media-specific statutes. In general, each of these statutes sets standards withoutconsidering relationships to other media. In some cases, media-specific statutes have inadvertently promoted the shifting of pollutant releases from one medium to another without reducing the overall generation of pollutants. For example, some of EPA's air quality emissions'standards are based on end-of-pipe pollution control technologies such as stack gas scrubbing. In the past, in order to achieve compliance quickly and with certainty, manufacturers often chose to add a stack gas scrubber to the smokestack rather than redesign their products and/or the production process to eliminate the source of the toxic emission. Using a scrubber achieved compliance with air regulations by removing pollutants from stack gases andtransferring these pollutants to scrubber wastewaters. What were once air emissions, therefore, become water pollutants. Similarly, to comply with Clean Water Act regulations, most manufacturers chose toadd wastewater treatment plantsat the end of the pipe rather than redesign productsor processes to reduce pollutants at the source. Thus, while wastewater treatment removed pollutants from wastewater, it transferred these pollutants, in the form of sludge, to a waste that was usually incincerated (thereby creating an air emissions problem) and/or was disposed of in landfills. There is no doubt that the United States has made significant headwayin cleaning up the environmentusing a media-specific approach. However,two significant problems arose as a result: 1.
End-of-pipe pollution control doesn't really solve the problem-it just moves it from medium to medium. 2. Managing waste costs money and adds to the cost of production-often at the expense of other opportunities.
As noted earlier, the cost of the end-of-pipe approach has risen to an estimated $115 billion annually. Figure 1 illustrates the amount of solid waste generated annually in the United Statesestimated at 13.2 billion tons per year. In addition to generating solid waste, industry releases billions of pounds of toxic chemicals to the air and to deep underground injection wells each year. Some of these releases are legally permitted releases, and some are not. The extent to which the unpermitted releases cause a risk, if any, is not known. However, in all cases managing releases costs money.
Pollution Designing to Prevent
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Figure 1 “Solid” wastes as defined under theResource Conservation andRecovery Act (RCRA). (a) All RCRA wastes (billions of tons); (b) nonhazardous RCRA wastes (billions of tons). Much of the solid waste produced in the United States is not directly generated by consumers. Municipal solid waste, the focus of much public concern, represents less than 2% of all solid waste regulated under RCRA. In contrast, industrial activities produce about700 million tons of hazardous waste (a) and about 11 billion tons of nonhazardous wastes (b). Note: All numbers are estimates. The nonhazardous waste total has been rounded to reflect uncertainty. Much of the “solid” waste defined under RCRA, perhaps as much as 70%, consists of wastewater. The termshazardous and nonhazardous refer to statutory definitions of Subtitles C and D of RCRA, respectively. The mining wastes shown in (b) exclude mineral processing wastes; the oiYgas wastes in (b) exclude produced waters used for enhanced oil recovery; the “other” category in (b) includes wastes from utility coal combustion. (Adapted from U.S. Congress, Office of Technology Assessment, Managing Industrial Solid Wastes From Manufacturing, Mining, Oil and Gas Production, andUtility Coal Combustion,OTA-BP-0-82, U.S. Govt. Printing Office, Washington, D.C., February 1992.) A recent Chemical Manufactures Association survey indicates that about 20% ofnew capital expenditures are now for pollution abatement and control [l]. Furthermore, as a result of the escalating costs associated with federal environmental regulations, a number of petroleum refineries in the United States have closed. Increasingly, companiesare sacrificing less profitable refineries in favor of those capable of absorbing the costs of complying with the United States in the past environmental requirements. No new refineries have been built in 15 years [2]. By the mid-l980s, it had become clearto government and industry that the key to increasingeconomiccompetitivenessandenvironmentalimprovementwaspollutionprevention. There was compelling evidence that companies who were pioneers in pollution prevention were saving money on waste management, generating less pollution, and improving their competitive position as a result. In 1984, Congress began a serious search for a preventive approach that would lessen the costs of end-of-pipe management and reduce the amount of waste generated. In 1986, both EPA [3] and the Congressional Office of Technology Assessment [4] published reports that explored this issue. Both reports pointed to the need for government and industryto find new incentives to reduce the amountof pollution generated and suggested specific approaches that should be explored and implemented. At thesametime,CongressbeganworkonCommunity Right-To-Know Legislation (EPCRA), which helpedto spur citizen concern about chemical usein factories and chemical releases to the environment. Citizens and public interest groups have become wiser in their purchasing choicesand have greater accessto information on the effectsof toxic chemicals on human health and the environment. Citizen groupsare placing increased pressure on industry and to recycle used productsto preto reduce the useof toxic chemicals in consumer products ventthemfromcausingharmandalsofromtakinguppreciouslandfillspace.Consumer
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sure has causedmajorcompaniestochangeproduct lines, use alternative packaging,and design products composedof less toxic materials to meet public demand. The combined effect of the government’s shift in priorities (from end-of-pipe pollution control to up-front pollution prevention) and enhanced citizen awareness and involvement is influencing the way the United States will conduct business in the future. The next sections provide the reader with an overview of the types of programs that are under way.
B. PollutionPreventionPrograms Pollution prevention is often discussed within the context of a widely accepted notion of a “waste management hierarchy.” The waste management hierarchy places the highest priority on reducing waste at the source of generation through the use of less toxic raw materials, equipment changes, process redesign, and better housekeeping and materials management. The second preference inthe hierarchy is reuse and recyclingof wastes that cannot be reduced at the source. The third preference in the hierarchy is waste treatment, and the least preferred alternative is disposal. It is important to note here that prior to the mid-l980s, federal environmental legislation focused heavily on prescribing and regulating waste treatment and disposal (the bottom of the hierarchy). In the mid-l980s, in response to rising concern over the costs of environmental compliance, some companies began seeking ways to reduce the amount of waste generated by manufacturing operations withoutcompletelyabandoningandredesigningexistingproduction processes. Many companies have implemented pollution prevention programs over the past10 of waste disposaland environmental compliance. years and have significantly reduced the costs There are numerous successstories of firms with successful pollution prevention programs that have found itcheaper and more publiclyacceptable to sell products undera pollution prevention banner. The following are from Preventing Pollution in the Chemical Industry [5]. Amoco’s facility in Natchez, MS no longer uses xylene as process solvent. A team composed of process engineers, researchers, andoperations personnel successfully founda way to replace xylene with an aliphatic solvent. The resulting product quality met all analytical specifications, the new process met all plant requirements, and the product exhibited improved aesthetics and incorporated raw materials more efficiently, thus increasing product yield. By making this change in the process, the plant realized direct solvent savings, decreased energy requirements, and decreased hazardous waste oil generation. The resultis 2 million pounds per year of toxic waste. The that the Natchez team has eliminated nearly team continues to work on its goal of reducing toxic emissions to 1% of the 1987 level by 1995. The next project will focus on eliminating ammonia wastes. Dow Chemical Company’s chlorinated ethane products facility in Freeport, TX, produces raw materials usedin Saran Wrap plastic film, herbicides, and latex paints. The waste reduction team realizedthat they could reduce the amount of product lost in a byproductstream,aswell as improvethequality of theirby-producthydrochloric acid stream. The team changed the production process to eliminate the use of excess ethylene, which contaminated a hydrogen chloride stream during production.This reduction allowed Dow to produce high quality hydrochloric acid. At this same facility, Dow improved the separation of a by-product fromthe vinylidene chloride recovered during separation. These waste reduction changes produceda high quality hydrochloric acid foruse at other facilities. Ethylene and vinylidenechloride, which had been previously sent to a thermal oxidizer as waste, are now recoverable products. Many more companies are still in the early stages of identifying opportunities that will reducewastegenerationandsavemoney or havenotyetbegun a program.Thefocus of
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Designing to Prevent Pollution
COST CATEGORIES OTHER
81092
SIC20
W
7
SKiP
510211
81050
-
0
LABOR
0
TRANSPORTATION DISPOSAL RAW MATERIAL
/
STANDARD INDUSTRIAL CATEGORIES
Figure 2
True Costs of Waste Generation and Management for 70 “typical” manufacturing plants in
the United States.
government efforts, described below, is to provide better incentives for firms to begin pollution prevention programs and to provide them with technical information and hands-on assistance to get started. Understanding how to reduce pollutant generation and save money is the key to building a successful pollution prevention program in a company. Andthe first step to saving money is to understand how to calculate the true costs of waste management. In many cases, companies include only the costs of waste treatment, disposal, and transportation in their estimates of waste management costs. The true costs of waste management, however, must also include the value of materials contained in each waste stream generated (i.e., materials in waste streams are raw materials that were not turned into product) and the labor, management, and energy costs associated with the generation and management of “wastes.” The difference between conventional calculationsof waste management (treatment, disposal, and transportation costs) and the true costs of waste management can be dramatic. Figure 2 illustrates this difference. Figure 2 shows that the true costs of waste management for70 “typical” manufacturing facilities in the United States2are 85-95% greater than conventional cost estimates that focus ~~~
~
~
*In this analysis[6]. 10 “typical” plants weredrawn from eachof seven different Standard IndustrialCategories (SICS) contained in the database of a private engineeringfirm that specializes in completing pollution prevention opportunity assessments. The seven SICs included in the anlaysis were 20-food and kind& products; 22-textile mill products; 28“chemicals and allied products; 30-rubber and miscellaneous plastics products; 32“stone, clay, glass, and concrete products; ”fabricated metal products, except machinery and transportation equipment; and 37-transportstion equipment.
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only on the costs of treatment, disposal, and transportation. In conventional estimates, companies do not consider the valueof the materials in the waste stream or other costs related to managing wastes. to A look at the true cost of waste generation and management often compels companies complete an analysis of options for reducing these costs-a pollution prevention opportunity assessment. A pollution prevention opportunity assessment consistsof 1. Identification of individual waste streams, their composition, and their origin in the pro-
duction process Identification of theamountof raw materialpurchasedthatwaseventuallydiscarded as waste 3. Estimating the value of the materials disposed of, using original purchase prices 4. Identification of allother costs associated with generating and managing each waste stream 5. Evaluation of options that would reduce the use and generation of the waste materials 2.
After completing a pollution prevention opportunity assessment, many companies find they have been using more materials than necessary, not managing inventory carefully, not managing quality control carefully, using highly toxic raw materials when a safer substitute could be used instead,or using and wasting costly materials “because we have always done it that way.” In many cases companies find more efficient and cheaper ways to manufacture their products and generate less waste with little or no investment of capital. Figure 3 illustrates a true cost calculation for a typical waste stream. To encourage more companies to pursue pollution prevention, EPA began building apollution prevention program in 1987. At that time EPA focused its efforts on the prevention and recycling of industrial solid hazardous waste.By 1988, EPA expanded its efforts to all agency Treatment, storage, and disposal facility cost: The plant generates 24,000 gal of waste coolantper year. Disposal of the waste coolant at a treatment, storage, and disposal facility costs $0.20/gal 24,000 gaYyr X $0.20/gal Waste transportation cost: The waste coolant costs $0.15 per gallon to transport to the treatment, storage and disposal facility. 24,000 gaYyr x $0. Wgal Wasted raw material cost: The coolant becomes contaminated with oil and grease under current operations and can no longer be used. The coolant was purchased for $1 .00/gal. 24,000 gaYyr x $1.00/gal Labor cost: 100 hours per year of plant labor time ($lO.OO/h& 100 hr X $lO.OO/hr 10 hours per year of management time ($20.00/hr). 10 hr x $20.00/hr Other costs Future waste disposal liability costs Total coolant waste generation costs
$4,800
$3,600
$24,000 $ 1.000
$200 none unknown $33,600
Figure 3 Analysis of costs for waste stream 1: Waste coolants. (From Ref. 6.)
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programs and began working with states and industry to seek out new opportunities. EPA developed a publicly accessible computerized Pollution Prevention Information Clearinghouse (free of charge to users) that contains hundredsof process-oriented examplesof pollution prevention and recycling techniques. Information on contacting the clearinghouse is provided in Section IV. EPA developed a series ofuser guides to helpcompaniesbegintheirownpollution prevention efforts. EPA's Facility Pollution Prevention Guide [7]is a field-tested handbook for manufacturers that describes how to form internal teams, gather necessary process and waste data, identifyin-plant opportunities, analyze costs andsavings,andset priorities among projects. In 1991, EPA initiated the 33/50 program-a program that encourages companiesto voluntarily commit to reducing pollutant generation 33% by the year 1992 and 50% by the year 1995. To date, over 600 companies have joined the program. This enthusiastic response indicates that pollution prevention is an important opportunity to save money while gaining favorable public recognition by being good corporate citizens. About 45 states have pollution prevention assistance programs in place. These state programs provide on-site technicalassistance, telephone information, and written guidebooks on how to conduct an assessment of a plant's pollution preventionopportunities for many specific waste streams. (These guidebooksare also listedin the Pollution Prevention InformationClearinghouse.) Many states provide an invaluable and uniqueservice to manycompanies that is free and nomgulatory, saves money, and improves their public image. The American Institute of Chemical Engineers and the American Institute for Pollution Prevention have published an engineering workbookof ptactical pollution prevention problems [8] that has been distributed to chemical engineering departments at universities throughout the country. The workbook provides problems in six areas: Life cycle analysis (an analysis of a product life from conception through design, production, use, and eventual disposal) Identifying and setting priorities for managing pollutants from industrial sites Selection of environmentally compatible materials Design of unit operations for minimizing waste Economics of pollution prevention Process flowsheeting for minimizing waste One of the problems is reprinted below. Problem 153 ChemicalEngineeringTopics. Mass balances, engineering Economics Pollution PreventionConcepts. Design of unit operations for minimizing waste, reducing unit size to reduce waste. Background. A facilitymanufacturessheetsofcompositematerialforuseinthe aerospace andsporting goods industries.The composites are made by coating fiberglass or Kevlar fabrics with the liquefied resin. As shown in Figure 4, the coating process takes place in a pan containing the resin. The resin is dissolved in solvent, and a heat curing process drives off excess solvent from the compositeto make the finished product. At the end of each run, the resin pan must be emptied, rinsed, and cleaned. This results in a 'Allen et al. [S], p. 118.
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scrap prepreg
fiber
solvated resin waste rinse solvent waste
Figure 4
hazardous waste (rinsate, leftover solvent, and resin), which is then either partially recycled or incinerated. Treater pans must be at least 10 in. wider than the product being coated to provide clearance for machinery, but when the facility’s operating records were examined, it was discovered that the pans were excessively wide. This results in unnecessary waste generation. Plant data for the percentageof production at each fabricheater pan width combination are given in the following table:
FabricwidthTreater pan widthPercent of (in.) (in.) production 32
60
44 50
84 86
60
86
38 50
78 84
40 20 15 5 9 11
It is proposed that blocks molded to fit into the endsof the pans be used to ensure that the effective treater pan width be exactly 10 in. wider than the fabric being coated. The blocks would havea one-time costof $lo00 and would notrequire removalof the pans. An
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operator would only have to remove the last block and insert the block appropriate forthe next run. Your task is to consider the economic viability of this option, given that in a 9-month period there are 1369 resin treater cleanouts. The panshave a wettedcrosssectional area of 0.22 ft2, and the specific gravityof the resin is 1.1. The cost of the resin in $1.64/lb; the cost of the incinerating resin waste is $0.14/lb. Problem Statement. Neglect interest and calculate how long it would take for the molded blocks to pay for themselves. Note that when the blocks are used, some incineration cost is avoided and less resin is thrown away. Ignore the volume of resin in the recirculation reservoir, and assumethat the amount of rinsing required is not changedby the blocks. Also assume that all the waste is incinerated. Solution to Problem 15 Average treater pan width (before reduction):
60
X 0.4
+ 86" X
+
+
78 X 0.2 84" X 0.15 0.11 = 73.6 in.
+ 84" X
0.05
+ 86" X
0.09
Minimum possible average treater pan width:
+
+
42" X 0.4 48" X 0.2 60" X 0.15 70" X 0.11 = 51.2 in.
+
+ 54" X
+ 60" X
0.09
(E)+
0.14)
0.05
Cost of resin and incineration without the use of blocks: 9 mo (73.6 in.) (0.22 m
ft2)
= $225,800/9 mo
(1.1)
(-)
X
$(1.64
Ib
Cost of resin and incineration with the use of blocks:
m 9 mo (51.2 in.) (0.22 ft?) (1.1) (-) = $157,000/9 mo
(E)+ X
$(1.64
0.14)
lb
Daily savings: 12 mo 1 9mo 1 yr (365 gys) = $251/day Payback period would be about 4 days. 225,800 - 157,000
(
(
(Note:The material for this problem resulted froma waste minimization audit sponsored by a grant from the California Department of Health Services.) EPA began developing a more focused approach to pollution prevention, known the as Design for the Environment (DFE) program, in 1992. The program has several main facets that focuson particular industries andprofessionalareas.Forexample,EPAhas initiated two industry-specific projects: 1. The dry cleaning project exploresalternatives for dry cleaning processes. Industry groups are active participants and are involved in comparative risk assessment, performance evaluations, and cost analysis of the various alternatives explored. 2. The printing project brings together several hundred printing companies in an effort to find safer substitutes for the chemicals usedin the printing industry(e.g., inks, press washes).
The DFE programis also exploring design opportunities in crosscutting professional areas. For example, an effort is under way to standardize "true cost" waste management accounting
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for accountants in companies of all types. Another project looks at initiatives the insurance industry could pursue to better factor pollution prevention opportunities into underwriters’ evaluations of corporate risks. Contacts for the DFE programare listed in Section IV.
C. Green Product Design Several yearsafter the surge of pollution prevention programs,a growing numberof companies have lookedto a more expanded viewof pollution prevention-“green product design.” These companies are finding that “green” products-products that reduce the burden on the environment during use and disposal-have additional marketing appeal to consumers. In contrast to past practices, in which product performance and environmental compatibility were managed at different points in the production process, manufacturing executives have targeted the design stage of product development to satisfy both of these consumer demands. A recent report published by the congressional Officeof Technology Assessment (OTA)[9] explores the benefits of green designin depth andtargets the product design stage as the perfect leveraging point for determining how to reduce environmental impact and compliance costs and improve product quality and performance. that the qualOTA cites a 1991 report by the National Research Council (NRC) that found ity of U.S.engineering design is generally poor. The report recommended that the federal government make engineering design a national priority to improve competitiveness. The NRC concluded that the design stage determines 70% or more of the cost of product development, manufacture, and use [IO]. The key elements of green design are simple to comprehend but challengingto implement and consequentlyare the fundamental challenge to engineers of the future. OTA focuses on four objectives of green design in its report [lo]: Design for pollution prevention. Examples include reducing the use of toxic materials, increasing energy efficiency, using less material to perform the same function, or designing products so that they have a longer useful life. Design for better materials management. Examples include making products that can be remanufactured, recycled, composted, or safely incinerated with energy recovery. Design for remanufacturing and recycling. Recycling can reduce virgin material extraction rates, wastes generated from raw material separation and processing, and energy use associated with manufacturing. It can also divert residual material from municipal waste, relieving pressure on overburdened landfills. Designfor cornposting and incineration. Designers canfacilitate composting by making products entirely out of biodegradable materials. For example, starch-based polymers (which are inherently biodegradable)and easily composted and films can substitute for plastic in a variety of applications. Each of these design objectives represents an engineering challenge. Designing products that meet all four elevates the challengeeven more. A look at the life cycle of products helps explain the scope of green design. Products affect the environment at many points in their life cycle (see Figure 5). It does not take much imagination to see that engineering work is integral to each step of this process. However, in the past, each step in the processwas often viewed independently from other steps in the process. Engineers did not often communicate with each other along the way.Many companies are now integrating the work of their product designers, raw materials suppliers, purchasing department, manufacturing and maintenance operations, and public affairs departments and their customers to identify opportunities for improving product designs. OTA found that design trade-offs are a major challenge.
Designing to Prevent Pollution
161
“Stages of the Product Llfe Cycle
Materlalextraction
Material processing
t
R W 8
Manutacturlng
t
R8,llaflUfedWe
L
USe
t
Waste management
Rem8
I
Figure 5 Stages of the product life cycle. Environmental impacts occur all at stages of a product’s life by changing the amount and type of materials used cycle. Design canbe employed to reduce these impacts in the product, by creating more efficient manufacturing operations,by reducing the energy and materials consumed during use, and by improving recovery of energy and materials during waste management. (Adapted from D. Navin Chandra, The Robotics Institute, Carnegie Mellon University, personal communication, March 1992.) These choices often involve environmental dilemmas. Tradeoffs may be required, not only between traditional design objectives and environmental objectives, but even among environmental objectivesthemselves-for example, waste prevention versus recyclability. As an illustration, consider the cross sectionof a modern snack chip bag [Figure 61. The combination of extremely thin layersof several different materials produces a lightweight package that meets a variety of needs (e.g., preserving freshness, indicting tampering, and providing product information). The use of so many materials effectively inhibits recycling. On the other hand, the package has waste prevention attributes: it is much lighter than an equivalent package made of a single material and provides a longer shelf life, resulting in less food waste. Even this relatively simple product demonstrates the difficulties of measuring green design [lo, p. 81. Some foreign competitors are making huge advances in green design. For example, OTA points out in their report [lo, p. 121 that several German auto companies, includingBMW and Volkswagen, have begun to explore this system oriented approach. BMW recently built a pilot plant in Bavaria to study disassembly and recyclingof recovered materials, andVolkswagen AG has constructed a similar facility. The goal of the BMW facility is to learn to make an automobile out of 1 0 0 percent reusable/recyclable parts by the year 2000. In 1991, BMW introduced a two seat roadster model with plastic body panels designed for disassembly and labeled as to resin type so they may be collected for recycling. [See Figure 7.1
D. OtherFederal Efforts Other federal agenciesare developing new approaches to green design and environmental protection. In 1992, the Federal Trade Commission issued guidelines to help reduce consumer confusion and prevent the false or misleading use of environmental terms such as “recyclable,” degradable,” and “environmentally friendly” in the advertisingandlabeling of productsinthemarketplace. Theguidelines are also
...
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Copolymer
Molsture barrier Polypropylene
Stillness Clarhy Puncture resistance
Copolymer
Ink adheslon
Inks Polyethylene
quallly
Oraphlcs lnteriamlnar adhesion
Alumlnum metallzatlon
Barrier lo oxygen, moisture. and l!ght
Copolymer
Metal adhesion
Polypropylene
Copolymer
Stiffness Molsture barrier
Seal Integrlty Hot seal strength Easy opening Tamper evidence
Figure 6 The cross section of a snack chip bag illustrates the complexity of modern packaging. The bag is approximately 0.002 in. thick and consists of nine different layers, each with a specific function. While such complexity can inhibit recycling efforts, it also can reduce the overall weight of the bag and keep food fresher, thus providing waste prevention benefits. (Source:Council on Plastics and Packaging in the Environment.)
Figure 7 BMW roadster constructed of plastic body parts that can be dismantled and recycled.
ollution Designing to Prevent
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intendedtoreducemanufacturers' uncertainty about which claimsmightlead to FTC law-enforcement actions, thereby encouraging marketersto produce and promote products that are less harmful to the environment. . . . The guides do not rigidly define environmental terms. Instead, through specific guidance and a seriesof examples of both acceptconveyed able and deceptive claims, the guides set out the different meanings thatbemight by the useor omission of particular language describing environmental features. The types of claims addressed by the guides include recyclable, degradable, compostable, recycled content, source reduction, refillable, and ozone safe[l 13. The guides are not themselves legally enforceable. However, the laws theyare intended to support are. Table 1 provides a brief summary of some of the key programs under way at the federal level.
W. WHERE TO GET INFORMATION ON TOXIC CHEMICALS, POLLUTION PREVENTION,AND GREEN DESIGN
In designing any project, engineers need to knowwhich chemicals to be concerned about. Thousands of chemicals are in use in the marketplace today. Hundreds of those chemicals are regulated by the federal andstate governments. Although engineers need notbe expert on the environmental impactsof materials used in engineering projects, they should be aware of the chemical make-upand properties of materials in use and whether or not the chemicals used are regulated or of some environmental concern. Information on the human health and environmental effects of toxic chemicals is obtained from animal studies, controlled epidemiological investigations of exposed populations, and clinical studiesor case reports of exposed humans. Other information available on the adverse effects of human exposure comes from experimental studies in systems other than whole animals (e.g., isolated organs, cells, subcellular components). EPA's Integrated Risk Information System (IRIS), preparedand maintained by the U.S. EPA, is an electronic database containing health risk and U.S. regulatory information on specific chemicals. IRIS contains descriptive and numerical informationin several forms, including(1) chemical fileson long-term oral and inhalation and noncarcinogenic health effects, (2) EPA regulatory action summaries, (3) supplementary data on acute health hazardsand physicalkhemical properties, and (4) background documents describing the rationales and methods used in arrivingat the results shown in the chemical files. Further information on IRIS canbe obtained by contacting User Support EPA Office of Environment Criteria and Assessment Cincinnati, Ohio Telephone: (5 13) 569-7254 A list of chemicals currently subject to regulation under the Community Right to Know may or maynot be Act ispublished byEPA [12]. Chemicalsnotincludedonthislist of concern. Information concerning the amounts of toxic chemicals released to the environment on a national, state, or facility-specific basis can be found in EPA's Toxic Release Inventory. Inquiries can be made by contacting
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Designing to Prevent Pollution TRI Representative Specialized Information Services National Library of Medicine 8600 Rockville Pike Bethesda, MD 20894 Telephone: (301) 496-6531
Information on pollution prevention, including case studies, publications, training and educational materials, programs, and legislation, and industry-specific fact sheets, can be obtained by contacting Pollution Prevention Information Clearinghouse U.S. EPA Pollution Prevention Division 401 M Street SW Washington, D.C. 20460 Telephone: (202) 260-2602 Information on EPA's Design for the Environment Program canbe obtained by contacting Office of Pollution Prevention and Toxics U.S. EPA 401 M Street SW Washington, D.C. 20406 Telephone: (202)
260-0981
or EPA's Pollution Prevention Information Clearinghouse (PPIC) (see address above). Information on pollution prevention canalso be obtained by contacting state pollution prevention programs. These contacts are listed below.
State Pollution Prevention Contacts Alabama HAMMARR 241 Mineral Ind. Bldg University of Alabama PO. Drawer G Tbscaloosa, AL 34587-9644 (205)
348-4878
Alaska Pollution Prevention Program Department of Environmental Conservation PO. Box 0 Juneau, AK 99811-1800
Hazardous Waste Division PO.Box 8913 Little Rock, AR 72219-8913 (501)
570-2861
California Toxic Substances Control Program Department of Health Services Alternative Technology Division PO.Box 942732 Sacramento, CA 94234-7320 (916)
322-2822
Alaska Health Project 431 W. 7th, Suite 101 Anchorage, AK 99501
Colorado Waste Minimization Assessment Center Colorado State University Mechanical Engineering Department Fort Collins, CO 80523
(907)
(303)
(907)
465-2671
276-2864
Arkansas Arkansas Department of Pollution Control and Ecology
491-5317
Pollution Prevention Office Office of Health and Environmental Protection
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Colorado Department of Health
4210 E 11th Avenue, Room 350 Denver, CO 80220 (303) 331-4510
Connecticut Connecticut Hazardous Waste Management Service 900 Asylum Avenue, Suite 360 Hartford, CT 06105-1094 (203) 244-2007
Delaware Pollution Prevention Program Department of Natural Resources and Environmental Control PO. Box 1401 Dover,DE 19903 (302) 739-3822
Florida Hazardous Waste Reduction Assistance Program Department of Environmental Regulations h i n Towers Building 2600 Blair Stone Rd. Tallahassee, FL 32399-2400 (904) 488-0300
Georgia Hazardous Waste Management Program Land Protection Branch Environmental Protection Division Georgia Department of Natural Resources Floyd Tower East, Suite 1154 205 Butler Street SE Atlanta, GA 30334 (404)656-2833 Hazardous Waste Technical Assistance Program Evironmental Health and Safety Division Georgia Technical Research Institute Atlanta, GA 30332 (404) 894-3806
Hawaii Solid and Hazardous Waste Branch Hawaii State Department of Health 5 Waterfront Plaza, Suite 250 500 Ala Moana Blvd. Honolulu, HI 96813 (808) 543-8226
Idaho Hazardous Materials Bureau 1410 N. Hilton Street Boise, ID 83706 (208) 334-5879
Illinois Office of Pollution Prevention Illinois EPA 2200 Churchill Rd. Springfield, IL 62706 (2 17) 782-8700
Hazardous Waste Research and Information Center One East Hazelwood Drive Champaign, IL 61820 (217) 333-8940
Indiana Office of Pollution Prevention and Technical Assistance Indiana Departmentof Environmental Management 105 S . Meridian St. PO. Box 6015 Indianapolis, IN 46206-6015 (317) 232-8172
Iowa Waste Management Authority Division Department of Natural Resources Waste Management Authority Division Wallace State Office Bldg. Des Moines, IA 50319 (515) 281-8489
Iowa Waste Reduction Center 75 BRC University of Northern Iowa Cedar Falls, IA 50614-0185 (319) 273-2079
Kansas Department of Health and the Environment Forbes Field, Building 740 Topeka, KS 66620 (913) 296-1603
Kentucky Kentucky Partners Waste Reduction Center Ernst Hall, Room 312
167
Designing to Prevent Pollution University of Louisville Louisville, KY 40292 (502) 588-7260 Louisiana Alternative Technologies Research and Development Department of Environmental Quality PO. Box 44066 Baton Rouge, LA 70804 (504) 342-1254 Maine Bureau of Oil and Hazardous Materials Control Department of Environmental Protection State House Station #l7 Augusta, ME 04333 (207) 289-2651 Maine Waste Management Agency Office of Economic and Community Development State House Station #l30 Augusta, ME 04333 (207) 289-6800 Maryland Hazardous Waste Program Maryland Department of the Environment 2500 Broening Highway, Bldg. 40 Baltimore, MD 21224 (301) 631-3343 Technical Extension Service Engineering Research Center University of Maryland College Park, MD 20742 (301) 454-1941 Michigan Office of Waste Reduction Service Department of Commerce 309 N. Washington St., Suite 103 Lansing, MI 48909 (517) 335-1178 Minnesota Minnesota Pollution Control Agency Hazardous Waste Division 520 Lafayette Road St. Paul, MN 55155 (612) 643-3497
Minnesota Technical Assistance Program Box 197, Mayo Bldg. 420 Delaware St. SE University of Minnesota Minneapolis, MN 55455 (612) 625-9471 Minnesota Office of Waste Management 1350 Energy Lane St. Paul, MN 55108 (612) 649-5494 Mississippi Mississippi Technical Assistance Program Mississippi State University Department of Chemical Engineering PO. Drawer CN Mississippi State, MS 39762 (601) 325-8454 Waste ReductionWaste Minimization Program Department of Environmental Quality PO.Box 10385 Jackson, MS 39289-0385 (601) 961-5241 Missouri Environmental Improvement and Energy Resource Authority 225 Madison Street PO.Box 744 Jefferson City, MO 65102 (314) 751-4919 Department of Natural Resources Waste Management Program PO.Box176 Jefferson City, MO 65102 (314) 751-3176 Montana Department of Health & Environmental Sciences Cogswell Bldg., Room B-102 Helena, MT 59620 (406) 444-2821 Nebraska Pollution Prevention Office Department of Environmental Resources PO. Box 98922
168
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Lincoln, NE 68509-8922 (402) 471-4217 Nevada Small Business Development Center Room 41 1 Department of Business Administration University of Nevada Reno, NV 89557 (702) 784-1717 New Hampshire Waste Management Division Department of Environmental Services 6 Hazen Drive Concord, NH 03301 (603) 271 -290l New Mexico Hazardous Waste and Remediation Bureau 1190 St. Francis Drive Santa Fe, NM 87503 (505) 827-2926 New York Bureau of Pollution Prevention NYDEC 50 Wolf Road Albany, NY 12233-7253 (518) 457-7267 North Carolina Office of Waste Reduction North Carolina Departmentof the Environment, Health, and Natural Resources PO. Box 27687 Raleigh, NC 27611 (919) 571-4100 Ohio Ohio Technology Transfer Organization 77 S. High Street, 26th Floor Columbus, OH 43215 (614) 466-4286
Columbus, OH 43266-0149 (614) 644-3492 Oklahoma Pollution Prevention Technical Assistance PrOgraIIl Oklahoma Departmentof Health lo00 NE 10th Street Oklahoma City, OK 73152 (503) 229-5913 Oregon Hazardous Waste Reduction Program Department of Environmental Quality 811 SW 6th Ave. Portland, OR 97204-1390 (503) 229-5913 Pennsylvania Center for Hazardous Materials Research University of Pittsburgh Applied Research Center 320 William Pitt Way Pittsburgh, PA 15238
Pollution Prevention Section Division of Solid and Hazardous Waste Management Ohio EPA PO. Box 1049 1800 Watermark Drive
Hazardous Waste Management Research Fund Institute of Public Affairs University of South Carolina Columbia, SC 29208 (803) 777-8157
Department of Environmental Resources PO. Box 2063 Harrisburg, PA 17 105-2063 (717) 787-7382 Rhode Island Hazardous Waste ReductionSection Office of Environmental Coordination Department of Environmental Management 83 Park Street Providence, RI 02903 (401) 277-3434 South Carolina Center for Waste Minimization Department of Health and Environmental Control 2600 Bull Street Columbia, SC 29201 (803) 734-4715
169
Designing to Prevent Pollution South Dakota Waste Management Program Department of Environment and Natural Resources Division of Environmental Regulation 523 East Capitol Avenue Pierre, SD 57501-3153 (605) 773-3153 Tennessee Bureau of Environment Department of Health and Environment 150 9th Ave. N Nashville, TN 37219-3657 (615) 741-3657 TexaS Waste Minimization Unit Texas Water Commission PO. Box 13087, Capitol Station Austin, TX 78711-3087 (512) 463-7761 Texas Hazardous Waste Research Center Lamar University PO. Box 10613 Beaumont, TX 77710 (409)880-8768 Center for Hazardous and Toxic Waste Studies Texas Tech University PO. Box 4679 Lubbock, TX 79409-3121 (806) 724-1413 Utah Department of Environmental Quality 288 North 1460 West Salt Lake City, UT 84116 (801) 538-6121 Vermont Vermont Waste Minimization Program Hazardous Waste Management Section
Agency of Natural Resources 103 S . Main Street Waterbury, VT 05676 (802) 244-8702 Virginia Waste Minimization Program Monroe Building, 1lth Floor 101 N. 14th Street Richmond, VA 23219 (804) 37 1-87 16 Washington Waste Reduction, Recycling, and Litter Control Program Department of Ecology Mail Stop PV-l1 University of Washington Olympia, WA 98504-7541 (206) 438-7541 West Virginia Generator Assistance Program Waste Management Section West Virginia Department of Natural Resources 1356 Hansford Street Charleston, WV 25301 (304) 348-6350 Wisconsin Bureau of Solid and Hazardous Waste Management Department of Natural Resources Box 7921 (TS/3) Madison, W1 53707-7921 (608) 266-9259 Wyoming Solid Waste Management Program Department of Environmental Quality 122 W. 25th Street Herschler Building Cheyenne, WY 82002 (307) 777-7752
V. SUMMARY The challenge for engineers of the future is to ask the right questions about what chemicals are in the products that industry designs and sells. How will productsbe used? Have we considered the safest possible materials for this use?How will products be reused or recycled after their useful life? How will products eventually be disposed of?
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REFERENCES 1. Mason, A. M., The New Environmental Age, ENR, Pittsfield, Mass., 1992. 2. U.S. Department of Energy, Petroleum Supply Annual 1990, DOE-EIA-0340 (90-91), p. 82. 3. U.S. EPA, Office of Solid Waste, Report to Congress: Minimization of Hazardous Waste, EPA/ 530-SW-86-033,Washington,D.C.,1986. 4. Congress of the United States, Office of Technology Assessment, Serious Reduction of Hazardous Waste, OTA-ITE-318, Washington, D.C., 1986. 5. Anon., Preventing Pollution in the Chemical Industry, Chemical Manufacturers Association, Washington, D.C., 1992. 6. EPA: Waste Generation Costs, Office of Solid Waste (Waste Advantage, Inc., contractor), Washington, D.C., 1991. 7. U.S. EPA, Office of Research and Development, Facility Pollution Prevention Guide, EPA/600/ R-92/088, Cincinnati, Ohio, 1992. Problemfor Engineering 8. Allen, D. T., Bakshani,N.,Pollution Prevention: Homework and Design Curricula, Rosselot, K. S., and Department of Chemical Engineering. University of California. Los Angeles, Calif., 1992. 9. U.S. Congress, Officeof Technology Assessment,Green Productsby Design: Choicesfor a Cleaner Environment, OTA-E-541, U.S.Govt. Printing Office, Washington, D.C., October 1992. for Competitive Advantage, IO. National Research Coouncil,Improving Engineering Design: Designing National Academy Press, Washington, D.C., 1991. 11. Federal Trade Commission, Washington, D.C., F7C News, 1992. 12. U.S. EPA, Office of Pesticides and Toxic Substances, Title I11 List of Lists: Consolidated List of Chemicals Subject to Reporting Under the Emergency Planning and Community Right to Know 500-B-92-002,1992; epa 13. U.S.. EPA, EPA PollutionPrevention1991:ProgressonReducingIndustrialPollutants, 21:3003, Washington, D C , 1991.
7 Biochemical, Genetic, and Ecological Approaches to Solving Problems During irt sitzt and Off-site Bioremediation 0.A. Ogunseitan Universiry of California Irvine, California
1.
INTRODUCTION
The manipulation of biological organismsor their components for complete or partial purification of contaminated ecosystems is an old technology the application of which has increased tremendously due to elucidation made possibleby recent advances in molecular genetics, biochemistry, and the ecologyof microorganisms. In orderto derive maximum environmental and industrial ecological benefits from current remediation biotechnology, it is important to identify and solve specific problems that remain both for basic research forand field practice. This chapter presents realized and potential problems confronting current attempts to implement environmental bioremediation schemes, and research approaches to solve these problems. The focus of the chapter is on problemsof molecular and organismic dimensions. The biodegradation of many natural and anthropogenic chemicals present in the environment is inevitable dueto the versatility of microbial metabolic capabilities and the ubiquity of microbial populations in the global ecosystem. It is mainly because of these two factors that thebiogeochemicalcycling ofmany elementson earth continues. However, advancesin chemistry-based and industrial engineering-based technologies in this century resulted in the manufacture and wide distribution of several hazardous and recalcitrant chemical compounds. The reliance of society on domestic waste purificationby microorganisms appeared to be misplaced when it came to dealing with the new wave of industrial chemicals. Because the polluting chemicals accumulated in unchanged form, biomagnified, andremainedtoxicinthe environment, specific limitations placed on natural chemical cycling processes canbe identified. First, it seemed that the rate of evolutionary processes responsible for the development of new metabolic functions in microbial communities lagged behind the rate and sophisticationwithwhichnovelchemicalcompoundswererecombinedfornewindustrialproducts. Second, the rate of enzymatic functioning in situ (intracellular, or secreted into the contaminated milieu) is not rapid enough to catalyze the degradationof chemical compounds for which a biochemical pathway already exists. Finally, these polluting compounds are extremely toxic and therefore prevent microbial growth and significant catabolic activity. 171
172
Ogunseitan
Seminal research on the environmental fate of hazardous industrial chemicals concentrated on gathering data on recalcitrance and half-lives in various ecological milieux [1,2] and on ecotoxic effects [3,4]. Data on the capacity of soil bacteria to degrade polynuclear aromatic hydrocarbons and several pesticides began to appear about 30 years ago [5,6]. Most of those studies were conducted to demonstrate the occurrence of biodegradation in natural microbial communities, partly by using sterilized controls and by elucidating the degradative mechanisms in pure cultures of bacteria [7]. Approximately 20 years ago, biodegradation became established as a promising mechanism through whichat least the concentration of recalcitrant compounds in the environment may be lowered. Following that period, research activity focused on identifyingthe responsible enzymes and intermediate compoundsin the degradative pathway of many polluting compounds [8,9]. Those studies used a combination of sophisticated techniques, including chromatography and radiolabel tracing [10,1l]. Concurrent with the period of intense biochemical analysisof degradative pathways was the availability of tools for genetic analysisof degradative strains. Itwas recognized that several degradative pathways were encoded on extrachromosomal genetic elements (plasmids), a fact that facilitated intensive genetic analyses[12]. Restriction maps were produced, degradative operonswere cloned, and the mechanisms linking gene expression, enzyme activity, and chemical degradation were established [ 121. It became possibleto recruit degradative genes from various microorganisms into single “broad-spectrum” degradative strains [13,14]. Genetically engineered strains are now available for the degradation of compounds previously thought to be beyond microbial degradative potential, but government regulation issues concerned with ecological fate and long-term environmental safety prevent the application of genetically engineered organismsfor hazardous waste bioremediation in the open environment [15]. With the extensive documentation of biodegradative processes and the availability of powerful degradative strains, the stage was set for application and commercialization of biodegradation technology. Initial attempts in this direction met with limited successmainly because of inadequate understandingof ecological interactions that control microbial catabolic processes [16]. In addition, genetically engineered organisms could notbe used because only limiteddata were available on the ecological fate of such organisms [16]. The last decade was then characterized by a flurry of research projects dealing with the ecological aspects of biodegradation, including limiting concentrations of pollutants [17], survival of degradative microbial populations[18], genetic interactions in the degradative community and ecological fate assessment of natural and engineered degradative organisms [19, 201, and the effects of physical-chemical environmental parameters on an agenda designedto biodegradation rates [21]. Most of these research projects were based on answer specific questions concerning the limits of biodegradation and policy formulation by regulatory agencies [22]. The implementationof environmental bioremediation using microorganisms or their products in situ or off-site (in reactors or landfills) necessitates that problemsposed by three major disciplines be surmounted. These disciplinesare biology, engineering and geology, and legislation. This chapter deals with issues based only on the biological issues, namely, biochemical, genetic, and ecological parameters. It is anticipated that provision of comprehensive solutions to these problems will facilitate the tackling of engineering and biogeochemical problems as well as lighting the path toward the solution of government regulatory problems. Environmental bioremediation now encompasses the treatment of contaminated soils, groundwater, river and lake sediments, stabilization ponds, and seawater and proximal shoreline environments. In most cases where proposals for bioremediation are being considered, cost, time, ease of implementation, and compatibility with physicalor chemical remediation technologies are among the factors to be considered.
Approaches to Problem Solving
I73
II. BIOCHEMICAL LIMITATIONS AND SOLUTIONS For practical purposes, there are ten major classes of chemicals that constitute environmental problems amenable to biotechnical solutions. These ten classes are pesticides, halogenated aliphatic hydrocarbons, halogenated ethers, polychlorinated biphenyls, monocyclic aromatic hydrocarbons,phthalate esters, polycyclicaromatichydrocarbons,nitrosamines,metals,and inorganic compounds suchas nitrates, phosphates, and sulfates (see Table 1). Several chemical and mathematical modelsas well as empirical data have been generated to predict the biodegradative fate of many chemicals in environmental systems. The functions of a well-integrated bioremediation scheme include increasing the rates of biodegradation, containmentof pollutant and degradative product mobility, and achievement of low residual levels. In many situations, the site to be remediated contains a heterogeneous mixture of several groups of chemicals. Before a decision is reached to implement bioremediationof a contaminated environment, it is important to conduct an initial biochemical feasibility study. Analysis of the chemical componentsof the waste site or effluent can be conducted using oneor more chromatographic techniques. The in situ biological componentsof the waste samplescan usually be analyzed by microscopic and plating techniques to determine whether the site or samples are sterile. More specific analysis ofmicrobial populations can be done by selective cultivation on chemicals known to be present in the waste material. It is during this period that potential biochemical impedimentsto successful bioremediation can be identified. Perhaps the most important biochemical impedimentof bioremediation is representedby the species identityof the contaminating chemical inthe target environment. There are an estimated 86,000 chemicals in international commerce [23], with an annual increment in this number of about 1000 new chemicals [23]. Partial toxicological data exist for only 2195 of these chemicals[23]. Various combinations of these chemicalsmay be present in the 260 million metric tons of hazardous waste generated annuallyin the United States [24]. The problem is compounded by the fact that data on potentialand rates for environmental biodegradation of organic compounds exist for only about 350 chemicals (see Table 1). Once the chemical components to be targeted for bioremediation are known, several factors concerningthe biochemical properties of the chemicalsmust be considered before thepotential rate of bioremediation can beestimated. 1. Water solubility, which directly affects access to microbial enzymes or cellular uptake. 2. OctanoVwater partitioning coefficient (Kw), which affects chemical mobility andsorption
3.
4. 5.
6.
to soil particles, which may or may not enhance biodegradation depending on the partitioning properties of degradative organisms. Hydrolysis under varying conditions of pH, temperature, moisture levels, and ionic exchange capacity. (Initial physical-chemical hydrolysis of certain compounds may enhance biodegradation by increasing the affinity of microbial enzymes.) Potential for photolysis, which may act in concert with biodegradative enzymes for efficient chemical degradation, for instance, in the case of halogenated polynuclear aromatic hydrocarbons [23]. Potential for volatilization, which may determine the accessibility of chemicals to degradative bacteria and suggest design plans for the bioremediation facility. Toxicity, radioactivity, and reactivity of the chemicals, which may select for nonbiodegradative microbial populations and therefore render the management of bioremediation facilities difficult.
Text resumes on page 188.
s
Table 1 Environmentally Important Chemicals of Anthropogenic Origin or Redistribution with Potential for Biological TreatmenP Maximum measured or estimated aqueous biodegradation half-life (hr) in unacclimated samples Chemical
Aerobic
Anaerobic
Biodegradation pathway
Acenaphthene Acenaphthylene Acephate Acetamide Acetone Acetonitrile Acetophenone
168
4320 5760 ND 672 672 2688 ND 17280 ND 2880 4320 2208 2688 ND 15240 168 672 2688 2688 672 2688 16128 672 17280 2688 44160
Established Deducible Deducible Established Established Deducible Deducible Deducible Deducible Deducible Established Deducible Deducible Dechlorination Established Established Deducible Hydrolases Deducible Hydroxylase Established Deducible Established Deducible Deducible Established
2-Acety laminofluorene
Acridine Acrolein Acrylic acid Acrylonitrile Aflatoxin B1 Alachlor Aldicarb Aldrin Ally1 alcohol 2-Aminoanthraquinone CAminoazobenzene Chinobiphenyl 1-Amino-2-methylanthraquinone Amitrole Aniline o-Anisidine p-Anisidine Anthracene
1440
ND 168 168 672
ND 4320 ND 672 168 552 672
ND 8664 14200 168 672 672 168 672 4032 312 4320 672 1lOQ0
Enzymes Oxygenases Oxygenase Demethylases Oxidases Dehydrogenases ND ND Oxygenases ND ND Dehydrogenases ND Hydrolases Hydroxylation Demethylases Dehalogenases ND Oxygenases ND Oxygenases Hydrolases ND Oxygenases ND ND Oxygenases
Genetic determinants Cloned Cloned Cloned Cloned Mapped ND ND Mapped ND ND Mapped ND Cloned ND Mapped Cloned Cloned Cloned ND Cloned Map@ ND Cloned ND ND Cloned
Organismsb
0
Do
E
1
2. c.L
n
3
Approaches to Problem Solving
175
Table 1 Continued Maximum measured or estimated aqueous biodegradation half-life (hr) in unacclimated samples Chemical
Aerobic
Anaerobic
Bromodichloromethane Bromoethylene Bromoform CBromophenyl phenyl ether Bromoxynil octanoate 1,3-Butadiene Bufencarb 1-Butan01 Butralin Butyl acrylate Butyl benzene phthalate Butyl4chlorodiphenyl oxide Butyraldehyde Cadmium Camphor Captan Carbaryl Carbon tetrachloride Catechol Chloramben Chlorbromuron Chlordane Chloroacetic acid 2-C hloroacetophenone Chlorobenzene(s) Chlorobenzilate Chlorobenzoate
ND 4320 4320
ND 17280 17280
ND
ND 21 12
528 672
ND 168 ND 168 168 ND 168 ND 168 1440
720 8640 168 ND ND 33264 168 672 3600 840 144
2688 ND 1296 ND 672 4320 ND 672 Bioprecipitation 672 5760 2880 672 672 ND ND 168 672 2688 14400 2688 672
Biodegradation pathway Established Established Deducible Deducible Deducible Deducible Deducible Established Deducible Established Deducible Deducible Established Established Established Established Established Established Established Deducible Deducible Established Established Deducible Established Deducible Established
Enzymes Monooxygenases Monooxygenases Monooxygenases ND Dehalogenases Oxidases ND Dehydrogenases ND Dehydrogenase ND Oxidoreductase Dehydrogenase Phosphatases Oxygenases Dehalogenases Hydrolases Dehalogenases Oxygenases Dehalogenases Dehalogenases Dehalogenases Dehydrogenase ND Oxidoreductases Hydrolases Oxygenases
Genetic determinants Cloned Cloned Cloned ND Map@ Cloned ND Cloned ND Mapped ND ND Cloned Cloned Cloned Cloned Cloned Cloned Cloned Cloned Cloned Cloned Cloned ND Cloned ND Cloned
Organismsb B B B
ND B B
ND B
ND B
ND ‘
ND B B B B, F F, B B B B B B B
ND B ND B, F
0
2
F
2.
5
Tabfe 1 Continued Maximum measured or estimated aqueous biodegradation half-life (hr) in unacclimated samples Chemical
Aerobic
Anaerobic
DDT Dalapon Decabromophenyl ether Diallate Dialifor 4,4'-Diaminodiphenyl ether Diamidofos 2,CDiaminotoluene Diazinon Dibenz[a,h]anthracene Dibenzofuran 1,2,7,8-Dibenzopyrene Dibromochloromethane
137000
2400 5760 35040 8640
1,2-Dibromo-3-chloropropane Dibutylnitrosamine Di-n-butyl phthalate Dichlofenthion 1,2-Dichlorobenzene 3,3-Dichlorobenzidine 4,4'-Dichlorobiphenyl Dichlorodifluoromethane(Freon- 12) I ,ZDichloroethane l,%Dichloroethylene
1440
8760 2160 ND 4320 ND 4320
ND 17280
ND 17280
ND
ND
22560 672 8664 4320 4320 4320 552
90240 2688 34656 4320 17280 17280 552
ND
ND
4320 4320 4320 4032 4320 4320
17280 17280 17280 16128 17280 17280
Biodegradation pathway Established Established Deducible Deducible Deducible Deducible Deducible Established Deducible Deducible Established Deducible Established Deducible Deducible Established Deducible Established Deducible Established Deducible Deducible Established
Enzymes Dehydrodehalogenases Dehalogenases ND Demethylases Thiolase Oxygenases Amidases Hydroxylases Thiolases Oxygenases Hydroxylases 0xygenases Monooxygenases Monooxygenases Deaminases Oxygenases Thiolases Dehalogenases Hydroxy lases Dehalogenases Monooxygenases Monooxygenases Monooxy genases
Genetic determinants Organismsb Mapped Mapped ND ND ND ND ND Cloned ND ND Map@ ND Cloned ND Mapped Cloned Mapped Cloned ND Cloned Cloned Cloned Cloned
B, F B
ND ND F ND F B F
ND B, F ND B
ND B, F B F B B B B B B
Approaches to Problem Solving
Y m
179
Table 1 Continued ~
Maximum measured or estimated aqueous biodegradation half-life (hr) in unacclimated samples Chemical
Aerobic
Anaerobic
2,4Dimethylphenol Dimethyl phthalate Dimethyl sulfate Dimethyl terephthalate Dimethyl tetrachloroterephthalate Dimilin 1,3-Dinitrobenzene 4,6-Dinitro-o-cresol 2,CDinitrophenol Dinitmtduene(s) Dinoseb Di(n-octyl) phthalate 1 ,CDioxane Diphenylamine Diphenyloxide Disodium methanearsonate Disulfoton Diuron Dursban Endothall Endrin Ethion Endosulfan Epichlomhydrin Ethanol 2-Ethoxyethanol
168 168 672 672 2208 4320 504 63 12 4320 2952 672 4320 672 ND
672 672 2688 2688 8832 ND 300 170 170 300 360 8760 17280 2688 ND
ND
ND
504 ND
2016 ND ND ND ND ND
ND
ND ND ND ND 336 672 26 672
1344
2688 I04 2688
Biodegradation pathway Established Established Established Deducible Deducible Deducible Deducible Deducible Deducible Established Deducible Deducible Deducible Established Established Deducible Deducible Deducible Deducible Deducible Deducible Deducible Deducible Deducible Established Deducible
Enzymes Demethylases Demethylases
Sulfurylase Oxygenases Dehalogenases Amidases Oxygenases Dehydrogenases Dehydrogenase Monooxygenases Dehydrogenases Oxygenases ND Deaminases Oxygenases Dehydrogenases Demethylases Urease, Oxygenases Thiolases Dehydrogenases Dehalogenases Thiolases Dehalogenases ND Dehydrogenases Dehydrogenase
Genetic determinants Mapped Mapped Mapped Cloned Cloned ND ND Cloned Cloned Cloned Cloned Cloned ND Cloned Cloned Cloned M> Mapped Mapped Mapped Cloned Mapped Cloned ND Cloned ND
Organismsb
B
B B B B
ND ND
B B B
B B ND B
B B
ND B, F F B, F B F B ND B ND
0
23 51
3
Approaches to Problem Solving
181
Table 1 Continued Maximum measured or estimated aqueous biodegradation half-life (hr) in unacclimated samples Chemical
Aerobic
Anaerobic
Hexachloronaphthalene Hexachlorophene Hydrazine Hydrazobenzene Hydrocyanic acid Hydroquinone Imidan Indeno[l,2,3-~d]pyrene Ipanzine Isobutyl alcohol Isobutyraldehyde Isocil Isophrone Isoprene Isopropalin Isopropanol 4,4'-Isopropylidenediphenol Isosafrole Kepone Lasiocarpine Lead Leptophos Linuron Malathion Mecoprop
8760 7872 168 8640 4032 168 ND 17520
35040 31488
ND 173 168 ND 672 672 2520 168 4320 672 17280 672 Bioprecipitation ND 4272 1236 240
672 4320 16128 672 ND 70080 ND 692 672 ND 2688 2688 360 672 17280 2688 69 120 2688 ND 17088 4944 4320
Biodegradation pathway Deducible Deducible Deducible Deducible Deducible Established Deducible Deducible Deducible Established Established Deducible Deducible Established Deducible Established Deducible Deducible Unknown Deducible Established Deducible Deducible Established Established
Enzymes ND Dehalo(hydro)genases Oxidases Oxygenases Oxidases Dehydrogenases Thiolases Oxygenases ND Dehydrogenases Dehydrogenases ND Demethylases Oxidoreductases ND Dehydrogenases Dehydrogenases ND Oxidoreductases phosphates Thiolase, dehalogenase Dehalogenses Thioiases Dehydrogenases
Genetic determinants
ND ND ND Cloned Mapped Cloned Mapped ND ND Cloned Cloned ND MaPM Cloned ND Cloned ND ND ND ND Cloned Cloned
ND Mapped Cloned
Organismsb
ND B, F ND B B B
F ND ND B, F
B, F ND
B B
ND B ND ND ND
B B
Approaches to Problem Solving
C 0
.-
183
z
Table 1 Continued Maximum measured or estimated aqueous biodegradation half-life (hr) in unacclimated samples Chemical
Aerobic
Anaerobic
Mirex Michler’s ketone Mitomycin C Monolinuron Monuron Mustard gas Naphthalene 1-Naphthol a Naphthylamine Nickel Nitral i n Nitrapyrin Nitrates Nitrilotriacetic acid 5-Nitro-o-anisidine Nitrobenzene CNitrobiphenyl Nitrogen mustard Nitroglycerin ZNitropbenol 2-Nitropropane N-Nitrosodiethanolamine N-Nitrosodiethylamine N-Nitrosodirnet hylamine N-Nitrosodiphenylamine N-Nitrosodipropy lamine N-Nitroso-N-ethylurea
ND 672 672 ND ND 672 480 168 4320
ND 2688 2688 ND ND 2688 6192 672 17280 Bioaccumulation
ND
m
ND Biomass uptake 672 672 4728 672 4320 168 672 4320 4320 4320 4320 816 4320 4320
ND Denitrification 2688 240 300 240 17280 672 672 17280 17280 17280 17280 3264 17280 17280
Biodegradation pathway Established Deducible Deducible Deducible Deducible Deducible Established Established Established Established Deducible Deducible Established Established Deducible Deducible Deducible Deducible Deducible Established Established Deducible Deducible Deducible Deducible Deducible Deducible
Enzymes Bioaccumulation Dehydrogenases Mutagenldehydrogenases Oxygenase, urease Oxygenase, urease Dehalogenase Dioxygenase dehydrogenase Oxygenases Capsule-binding Peroxidases Dehalogenase Nitrate reductase Dehydrogenase Deaminase, oxygenase Oxygenases Oxygenases Dehalogenases Dehydrogenses Dehydrogenases Monooxy genase Dehydrogenases Monooxygenases Monooxygenases Dioxygenases Monooxygenases Ureases
Genetic determinants
ND
Organismsb A B
ND ND Map@ Mapped Cloned Cloned Cloned Cloned Mapped Mapped ND Cloned Cloned Mapped ND Cloned ND
B B B B B B
ND
ND
Mapped Cloned Mapped Cloned Cloned Cloned Cloned Mapped
B B B B B B B B, F
ND B, F B. F B B, F
B, F B B
F F, B
Approaches to Problem Solving
a
a
185
186 Ogunseitan
Thiabendazole Thioacetamide 4,4Thiodianiline Thiourea Tillam Toluene Toluene-2,Wiisocyanate o-Toluidine
672 Biosorption ND 168 672 168 ND 528 672 168
Toxaphene
ND
Triaziquone I ,2,4-Trichlorobenzene 1,l ,I-Trichloroethane Trichlororethylene Trichlorofluoromethane Trichlorofon 2,4,5-TrichlorophenoI
672 4320 6552 8640 8640 1080 16560 480 8640 8640 ND ND 1422 672 4320 168 672 Bioaccumulation 4320 672 672 4320 Bioaccurnulation
Tetraethyllead Thallium
2,4,5-Trichlorophenoxyaceticacid 1,2,3-TrichIoropropane 1,1,2-Trichloro- 1,2,2-trifluoroethane Triclopyr Trietazine Trifluralin 1,2,3-Trimthylbeneme 2.4.6-Trinitrotoluene Tris (2,3-dibromopropyl) phosphate Uracil mustard Uranium Vinyl chloride Warfarin Xylenes 2,6-Xylidine Z i C
2688
ND 672 2688 672 ND 5040
2688 672 ND 2688 17280 26208 39672 34560 4320 43690 4320 34560 34560 ND ND 672 2688 4320 672 17280 17280 2688 8640 17280
Deducible Deducible Deducible Deducible Deducible Deducible Established Established Deducible Established Deducible Deducible Established Established Established Deducible Deducible Established Established Established Deducible Deducible Deducible Established Established Established Deducible Deducible Established Established Deducible Established Deducible Established
ND ND Oxygenases Hydrolases Oxygenases Ureases Ester hydrolases Oxygenases Oxygenases Oxygenases Dehalogenases
ND
ND ND ND
A
Dehalogenases Monooxygenases Monooxygenases Monooxygenases Dehalogenases Dehalogenases Oxygenases Monooxygenases
ND ND ND Mapped Cloned Cloned Cloned ND ND Cloned Cloned Cloned Cloned Cloned Cloned Cloned Cloned
ND
ND
Dehalogenases Hydrolases Dehydrogenase Demethylases Oxygenases Dehalogenases Dehalogenases Cytochrome oxidase Monooxygenase Decarboxydases Dealkyl. oxygenase Oxygenases Exopolyrners
ND ND Mapped ND Cloned ND ND Cloned Cloned
ND
ND Cloned ND Cloned ~~
ND ND ND
ND
F
B B B
ND ND B B B B B B B B ND ND ND B, F
b
8
s
22 s 'a
a f
3 T
S'
b
ND B
ND ND B BF, A ND B
ND B ~~
'Chemicals representing typical classes of toxic environmental pollutants (65 classes) are set in bddface type. Inorganic chemical pollutants are set in bo&ifbce i t d c s . bB = bacteria; F = fungi; A = algae. ND = no data available. Source: Compiled from Refs. 12. 23, and 28-35.
zv
Ogunseitan
188
Table 2 Biochemical Limitations and Potential Solutions in Environmental Bioremediation Potential
Limitations or chemicalmeansbefore bib HighlyheterogeneouschemicalSeparatechemicalsbyphysical remediation off-site. species component Target individual chemicals for biodegradationor detoxification, starting with the most biologically available. Explosive,radioactive, or other-Dilutechemicalstobiologicallytolerablelevels. and solid-phase systems for containment. wise toxic chemicals Use composting Use immobilized cell or cell-component slurry reactors. Chemical species Biochemical of enzyme andcatalysis modeling. Pilot biodegradation assessment studies utilizing sterile sample undocumented biodegradative controls and chemical analysis to characterize degradative pathways products. Vary pH for solubility. Insoluble chemicals Physical fragmentationfor better distribution and accessibility to microbial populations. Increase moisture levels and consider using wetting agents. Increase temperature. Sorbed chemicals Desorbing agents, e.g., surfactants. Chemicals impermeable to cells Immobilized enzyme reactors. using intracellular enzymes,or limiting membrane transport
Solutions to some of thesebiochemicalimpedimentsto successful implementation of bioremediation are presented in Table 2. Several problems that arise from the biogeochemical properties of the polluted ecosystem targeted for bioremediation can be solved by integrating other physical and chemical remediation technologies with biological treatment. In some cases, solutions may include deciding betweenin situ or off-site bioremediation, betweenthe use of naturally occurring organisms and exogenous strains, and between treatment with living cells and treatment withcellular components such as free enzymes, membranes, or heat-killed cells. The implementation of some of these solutions requires considerably more research, for example, in the use of immobilized enzymes for treatment of heterogeneous wastes.
111.
GENETICLIMITATIONS AND SOLUTIONS
Most microorganismshave short generation timesand reach high populationdensities in small spaces. Therefore the rate of evolution of new traits is rapid relative to that of multicellular organisms. Bacteria especiallyare metabolically versatile andcan catabolize or resist the toxic effects of a wide variety of environmental chemical contaminants. Within the past three decades, the tools became available for detailed analysis of the genetic basis of the metabolic versatility of bacteria. This resulted in the cloning of a wide variety of enzymatic capabilities in various groups of bacteria and some fungi (see Table 1). As the genetic clones became available, fragmentsof DNA able to identify (through hybridizations) specific groups of degradative bacteria were generated [25,26]. As part of the initial assessment study usually conducted prior to full implementation of bioremediation, a gene probe analysis is recommended, in addition to phenotypic assays for specific biodegradation reactions. l)q)ically, samples from the contaminated ecosystemare serially diluted and plated on selective media to determine the populationdensities of total heterotrophicand bio-
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degradative bacterial populations.A gene probe analysis (usually colony hybridizationto a radiolabeled DNA fragment from the biodegradative operon) is then performed on the isolated colonies. The proportion of total bacteria that exhibitthe degradative phenotypeis then compared to the proportion of bacteria containing the characterized biodegradative genes. ' b o types of important information can be obtained from such genetic potential assessment study. First, a correlation between the proportion of bacteria exhibiting chemical degradation and the proportion of bacteria containing the gene(s) of interest indicates an already selected population of degraders, and this will simplify attempts to maintain the biodegradative population and enhance degradative activityby nutritional or inducive stimulation. On the other hand, if there is a wide discrepancy betweenthe phenotypic assessment data and the proportion of bacteria containing the specific genes, it may be that there is considerable diversityin the biodegradative population or that the biodegradation is mediated by a consortium of organisms and cometabolic activities encoded by heterologous genetic operons. In the latter case, a detailed laboratory investigation of the ecological interactions between the biodegradative organisms will be helpful in designing a strategy for maintaining the composition and efficiency of the biodegradative consortium. In cases where microorganisms showing phenotypic biodegradative activity cannot be isolated and the gene probe analysis indicates that there is significant genetic potential, then two conclusions may be reached. Oneis that the biodegradative genes are present in the population but the genes are not being expressed intoa catabolic phenotype. Sucha lack of gene expression may be due to lack of induction by necessary catabolite chemicals (e.g., salicylate and naphthalene for naphthalene degradation). Inductionof gene expression may therefore be activated by the addition of intermediate chemicalsof the degradative pathway thathave been shown to be capable of genetic induction[27]. Alternatively, chemical or physical means may be usedto make the parent substrate more biologically available to induce enzyme production. Finally, there may also be cases where neither genotypic nor phenotypic biodegradative activities can be demonstrated. This may occur where the polluting chemical is too toxic to microorganisms, not availablefor biological metabolism, or simply too exotic for biodegradaor seeding with exogenousbacteria, becomes tion. It is in such instances that bioaugmentation, necessary. There are currently many strains of bacteria registered for their biodegradative or detoxifying capabilityin commerce. Thesestrains of microorganisms usually must be grown to high cell densities in the laboratory and released intothe contaminated environment toinitiate bioremediation. Qpically, the process hasto be repeated frequently, with intermittent addition of nutrients to help maintain the cell density, because thereare several ecological factors that may limit the population of exogenous species in stressed environments. In additionto seeding with naturally occurring biodegradative bacteria isolated from a different habitat, the use of genetically engineered organisms may be the final choice in bioremediation. The use of genetically engineered organisms requires several preliminary investigations of ecological and public health safety but remains a viable environmental remediation strategy. In cases where contained bioreactors canbe installed (e.g., in groundwater pump-and-treat processes), immobilized genetically engineered organismsmay be used. Limitations thatmay arise from genetic insufficiency and potential solutions to these limitations are summarized in Table 3.
W. ECOLOGICALLIMITATIONS AND SOLUTIONS The technology of environmental bioremediation depends on ecological investigations of bacterial metabolic diversity. However, research on ecological approachesto solving problems that may arise during in situ bioremediation has focused on ecological risk assessment of the en-
ns
Ogunseitun
190
Table 3 Genetic Limitations and Potential Solutions in Environmental Bioremediation Potential
Limitations ~
~~
~
~~~
Heterogeneous chemicals requiring diverse degradative operons Biodegradative bacteria and genes absent from contaminated site Biodegradative genes present but rate of bioremediation slow because genes are not expressed at optimal
~~~
~
~
~
~~
~~
Molecular breeding of versatile microorganisms using sample of contaminated materials. Direct cloning for recruitment of operons in single strain. Seeding with genetically characterized biodegradative bacteria. Off-site bioremediation with genetically engineered bacteria. Add chemical inducers of genetic operons. Chemically or biologically remove biodegradation intermediates possibly involved in feedback inhibition.
vironmental release of genetically engineered organisms. In the process of assessing risk, important aspects of microorganism dispersal, mechanisms of gene transfer, and organism survival in natural environments were investigated [19,20]. Problems of ecological significance thatmay arise during bioremediation andthe potential solutions to these problemsare listed in Table4. On an ecosystem scale, serious contamination problems result because of chemical mobility inthe environment. Contaminant migration in the natural ecosystem may occur by transport into groundwater, seepage into coastal regions, and partitioning into river, lake, or ocean sediments. Although these problems are tightly linked to biochemical properties of the contaminating chemicals, they are also attributable to physicalchemical characteristics of the ecosystem surroundingthe source of polluting chemicals. The repercussions of such chemical mobilitymay bring about extensive ecological damage and render the implementation of bioremediation difficult. In cases where chemicals are suspected to be mobile in receptive ecosystems, the immediate solution would rely on engineering impermeable barriers. In both in situ and off-site bioremediation, ecological impedimentsmay arise concerning the organisms involved in the environmental bioremediation. There are three classes of ecological impediments to be dealt with: (1) the initial population density of biodegradative organisms and applicable genotypes, (2) the nutritional and aeration status of the contaminated environment, and (3) the stability of naturally occurring or seeded populationsof biodegrada-
Table 4 Ecological Limitations and Potential Solutions to Environmental Bioremediation Potential
Limitations ~
~~~
~
~
~
Highly mobile chemicals in the environment rates Degradative bacteria absent or present at too low population densities Degrader population low Minimal aerobic zones limiting bioremediation Predation of degradative bacterial population by protozoa or phages
Install physical barriers prior to deliberate bioremediation activity. Bioaugmentation (seed with laboratory-grown degradative bacteria). Degradative bacteria can be isolated preferably from the contaminated site for reintroduction in high cell densities. Off-site bioremediation using genetically engineered organisms. Biostimulate with nutrients. Add hydrogen peroxide or other electron acceptors. Monitor and replenish degradative population frequently.
Approaches to Problem Solving
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tive organisms. It is important to monitor these three factors before and during the implementation of bioremediation. Population densities of microorganisms are usually monitored by plating and microsocopic techniques, while oxygen levels are determined by electrochemicalprobes and adjusted with hydrogen peroxide additions provided the biodegradationor biotransformation process is carried outby aerobic organisms. The levels of nutrients are usually monitored by analyzing the concentrationsof nitrogen sources (ammoniaor nitrates) and phosphates using colorimetric of chromatographic techniques. Nutrient levelsmay be adjusted by adding commercially available nutrient preparations to reach an optimum balance between the components. Biodegradative populations can also be depleted through predation by larger microorganisms or viruses. Predation canbe better controlledin contained reactors during off-site bioremediation. Where specific degradative strains are employed, virus-resistant varieties can be selected and used.However, in most cases, a constant supply of degradative organisms may have to be added to maintain the population density at levels not affected by predation.
1 . Alexander, M., Biodegradation: problems of molecular recalcitrance and microbial fallibility,Adv. Appl. Microbiol., 7, 35-80 (1965). 2. Alexander, M.,Biotechnological report: non-biodegradable and other recalcitrant molecules,Biotech. Bioeng., 15, 611-647 (1973). 3. Autian, J., Structure-toxicityrelationship of acrylicmonomers, Environ.Health Perspect., 11, 141-152 (1975). 4. Ogunseitan, 0. A., and Odeyemi, O., Effects of lindane, captan, and malathion on nitrification, sulphur oxidation, phosphate solubilization, and respiration in a tropical soil, Environ. Pollution (Ser. A), 37. 343-354 (1985). 5. Davies, J. I., and Evans, W. C., Oxidative metabolism of naphthalene by soil pseudomonads. The ring fission mechanism, Biochem. J . , 91, 251-261 (1964). 6. Evans, W. C., Fernley, H. N., and Griffith, E., Oxidative metabolism of phenanthrene and anthracene by soil pseudomonads. The ring fission mechanism, Biochem. J . , 95, 819-831 (1965). 7. Sugiyama, S., Yanao, K., Tanaka, H., Komagata, K., and Arima, K., Metabolismofaromatic Pseudomonas compounds by bacteria. I. Gentisic acid oxidase and protocatechuic acid oxidase of ovalis S-5, J . Gen. Appl. Microbiol., 4 , 223-240 (1958). 8. Kiyohara, H., and Wagao, K., Enzymatic conversion of I-hydroxy-2-naphthoate in phenanthrenegrown Aeromonas sp. S45P1, Agric. Bio. Chem., 41, 705-707 (1977). 9. Furukawa, K., and Tonomura, K., Enzyme system involved in the decomposition of phenyl mercuric acetate by mercury-resistantPseudomonas,Agric. Biol. Chem., 35, 604-610 (1971). 10. Seidman, M.M.,Toms, A., and Wood, J. M., Influence of side chain substituentson the position of cleavage of the benzene ring byPseudomonasfluorescens, J . Bacteriol., 97, 1192-1197 (1969). on the biodegradability of1.1 ,l11. Subba-Rao, R. V., and Alexander, M., Effect of chemical structure trichloro-1,l-bis@-chlorophenyl) ethane (DDT), J . Agric. Food Chem.. 25, 327-329 (1977). 12. Omenn, G. S., and Hollaender A., (eds.), Genetic Control of Environmental Pollutants, Plenum, NewYork, 1984. 13. Reinecke, W., and Knackmuss,H.-J., Construction of haloaromatic utilizing bacteria,Nature, 277, 385-386 (1979). 14. Kellog, S. T., Chattejee, D. K., and Chakrabarty. A. M., Plasmid assisted molecular breeding. Newtechniqueforenhancedbiodegradationofpersistentchemicals, Science, 214, 1133-1135 (1981). 15. Kovalick, W. W., Removing impediments to the use of bioremediation and other innovative technologies, in Environmental Biotechnology for Waste Treatment (G. S. Sayler, R. Fox, and J. W. Blackburn, eds.), Plenum, New York, 1991, pp. 53-60. Genetic Controlof Environmental 16. Alexander, M.,Ecological Fonstraints on genetic engineering, in Pollutants (G. S. Omenn and A. Hollaender, eds.), Plenum, New York, 1984, pp. 151-168.
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17. Rubin, H.E., Subba-Rao, R. V., and Alexander, M., Rates of mineralization of trace concentra-
tions of aromatic compounds in lakewater and sewage samples,
Appl. Environ. Microbiol., 43,
1133-1 138 (1982). 18. Levin, M., and Segal, M., Engineering organisms to survive, inGenetic Controlof Environmental Pollutants (G. S. Omenn and A. Hollaender, eds.), Plenum, New York, 1984, pp. 91-96. 19. O’Morchoe, S . , Ogunseitan, O., Sayler, G . S., and Miller, R. V., Conjugal transfer of R68.45 and
FP5 between Pseudomonas aeruginosa in a natural freshwater environment, Appl. Environ. Microbiol., 54, 1923-1929 (1988). 20. Ogunseitan, 0.A., Sayler, G . S., and Miller, R. V., Dynamic interactions of Pseudomonas aeruginosa and bacteriophages in lakewater, Microb. Ecol., 19, 171-185 (1990). 21. Focht, D. D., Performance of biodegradative microorganisms in soil: xenobiotic chemicals as unexploitedmetabolicniches,in EnvironmentalBiotechnology:ReducingRisks from Chemicals Through Biotechnology (G. S. Omenn, ed.),Plenum, New York, 1988, pp. 15-29. of biological 22. Day, S . M., Federal regulations: how they impact research and commercialization treatment, in Environmental Biotechnologyfor Waste Treatment (G.S. Sayler, R. Fox, and J. W. 23.
24. 25.
26.
27. 28.
Blackburn, eds.), Plenum, New York, 1991, pp. 217-232. Samiullah, Y.,Prediction of the Environmental Fate of Chemicals, Elsevier Applied Science, London, 1990. Postel, S., Stabilizing chemical cycles, in State of the World (L.R. Brown, d.), Norton, New York, 1987, pp. 172-201. Sayler. G . S., Shields, M. S., Tedford, E. T., Breen, A., Hooper, S. W., and Davis, J. W., A p plication of DNA-DNA colony hybridization to the detection of catabolic genotypes in environmental samples, Appl. Environ. Microbiol., 49, 1295-1303 (1985). Atlas, R. M., and Sayler, G . S., Tracking microorganisms and genes in the environment, in Environmental Biotechnology: Reducing Risksfrom Chemicals Through Biotechnology (G. S. Omenn, ed.), Plenum, New York, 1988, pp. 31-43. Ogunseitan, 0.A., and Olson, B. H.,Effect of 2-hydroxybenzoate on naphthalene mineralization in soil, Appl. Microbiol. Biotechnol.. 38, 799-807 (1993). Kearney, P. C.,andKaufmann,D.D., Degradation of Herbicides, MarcelDekker,NewYork, 1969.
29. 30. 31. 32. 33. 34.
35.
Matsumura, E, and Krishna Murti, C.R., Biodegradation of Pesticides, Plenum, New York, 1982. Kobayashi, H.,and Rittman, B. E., Microbial Removal of Hazardous Organic Compounds, EPA Pub.No. 82-6, Cincinnati, Ohio, 1982. Gottschalk, G., Bacterial Metabolism, Springer-Verlag, New York, 1986. Rochkind, M., Blackburn, J. W.,and Sayler, G . S., Microbial Decomposition of Chlorinated Aromatic Compounds, EPA h b l. No. 600/2-86/090, Cincinnati, Ohio, 1986. Ney, R. E., Jr., Where Did That ChemicalGo? A Practical Guide to Chemical Fate and Tmnsport in the Environment, Van Nostrand Reinhold, New York, 1990. Sayler. G . S., Fox, R., and Blackburn J.W.,(eds.), Environmental Biotechnologyfor Waste neatment, Plenum, New York, 1990. Howard F! H., Boethlng, R. S . , Jarvis, W. F., Meylan, W. M., and Michalenko, E. M., Handbook of Environmental Degradation Rates, Lewis, Chelsea, Mich., 1991.
8 Commandments of Waste Management
Donald K. Walter U.S. Department of Energy
Washington,D.C.
1.
INTRODUCTION Waste: v. to use, consume, or expend carelessly or thoughtlessly without need;n. a worthless or useless byproduct. (Webster’s, 1984)
Wastesfrommanufacturing,agriculture,mining, oil and gas production,andmunicipal sources (residential, commercial, and institutional) must be managed. The preferred management technique, all other things being equal, is source reduction; however, wastes are inevitable in any activity.A waste management system is usedto collect, transport, and manage the disposal of waste. The management system may include any combination of disposal on land, in air, or in water, or utilization as fuel or material. In each case the economic, environmental, and energy issues mustbe considered in a life cycle analysis. Today, most people believe that the common method of waste managementbyis discard to the environment, usuallyto a landfill, although incineration (discardto the air) and discardto water are also practiced.Yet, historically humans have used wastes for productive purposes. Cities are, in part, constructed from older structures. Even today estimates are 50% that of the primary metal used annually in the United States is produced from scrap and wastes.A limited database of information exists for evaluating the relative merits of these options, particularly as they relate to industrial wastes. The largest body of knowledge is for municipal solid waste (MSW). There are significant environmental advantages to the productive use of wastes. Using wastes productively reduces their discardto the environment and displaces other fuels or raw materials with their associated concerns. Despite such environmental advantages, permitting is often delayed or projects rejected because of misperception, the lack of comparative fuel cycle based data, the need for improved technology, and high first cost versus excellent life cycle cost. The consequences of any decision relating to economics,environmentalconcerns, or energysystemsmustconsider the fullbreadth of theimpactsincludingmining,refining, 193
194
Walter 7.8
Leaend
1.4
Figure 1 Waste disposal in the United States (in
billions of tons).
collecting, transporting, utilizing, and, ultimately, waste disposal.For example, the conversion of MSW into energy reducesthe amount of waste goingto landfills by W%,reduces landfill gas emissions identified as greenhouse gasesby a factor of 15, reduces the need for other conventional energy sources and their associated emissions, and provides an indigenous energy source while producing air emissions (at lower than the rates for criteria pollutants for some displaced alternatives) and benign ashfor disposal. Recycling reduces the amount of waste going to landfills, eliminates landfill gas emissions associated with biodegradablematerials, and may conserve conventional energy sources and their associated emissions but produces air emissions from added transportation and manufacturing.
11. THE RESOURCE There are a variety of sources of information on waste generatedin the United States although they are generally inadequatefor estimating potential for reducing or using the waste resource. Basedonan analysis of available data from a variety of documents, Americans generate By far the greatest portion of these wastes are roughly 14.7 billion tonsof solid wastes per year. classified as nonhazardous manufacturing wastes, accounting for just under 60% of the total waste stream by weight. Hazardous waste, which is the focus of most data collection efforts, represents roughly 6% of the total. The balance of industrial wastes are agricultural, oil and gas, and mining wastes. Figure 1 presents the estimated amount of each waste stream and its contribution to the total. The total of 14.7 billion tons representsa reasonable conjectureof the total industrial and municipal wastes generatedin the United States, but it is by no means a precise figure because of a number of imperfections in the data collection and analysis.
A. IndustrialWastes There are two impediments to calculating the total industrial solid waste stream. First, waste is not easily defined. A waste at one firm may be a raw material at another. Federal and state governments have adopted regulations that specifycertain materials (generally those that are classified as toxic or hazardous) as wastesor pollutants even though they have utility and are reused. The principal regulation for solid wastes, the Resource Conservation and Recovery Act (RCRA), defines solid wasteas solid, semisolid, liquid, or gaseous with a few exceptions such as municipal sewage sludges. In addition, RCRA splits wastes into hazardous and nonhazardous categories. While perhaps adequate for regulating wastes, these definitions may omit or
Commandments Management of Waste
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exclude large groups of wastes (such as agricultural and “nonhazardous” wastes). They also lack consistency, as substances are often added to or subtracted from these lists. Still, these collect definitions are commonly used by the trade associations and government agencies that data. Their surveys usually concentrate on waste materials that are deemed hazardous or are suspected of being so. A second obstacle is that the collection of waste data is not centralized or comprehensive. To calculate the total amount of waste, several data sources must be consulted. One reason for this is that the Environmental Protection Agency (EPA), whoseefforts to collect waste data are the most comprehensive in the United States, collects information on waste generation and disposal along program lines established by legislative edict. The Office of Air and Radiation, Office of Water, Office of Solid Waste, and Office of Toxic Substances individually and independently track the wastes governed by their programs, as follows: Air emissions governed under the Clean AirAct are tracked in the Officeof Air and Radiation. Water discharges regulatedby the Clean Water Act are recorded by the Office of Water. Information on the solid wastes governedby RCRA can be found in the Officeof Solid Waste. Data on toxic chemical releases reported under the Emergency Planning and Community Rightto-Know Act are collected in the Office of Toxic Substances. These officesdo not normallycoordinate their data collection and analysisefforts, though data on some materialsare collected by more than one office.The data on RCRA hazardous wastes may be gathered on a wet weight basis, so a significant amount of water is included in those totals. The waste generation information collectedby other officesis on a dry weight basis, so comparison and coordinationof the data are difficult. An effort to determine the amount of waste generated in the United States must begin by selecting the wastes to include. The working definition of industrial waste can be confined primarily to solid wastes regulated under RCRA. The contributions of other wastes are less significant ona weight basis. For example, wastes emitted to air thetotal about 20 million tons, not including the normal constituents suchas carbon dioxide, oxygen, and nitrogen, less than are significant in terms 0.2% of the total estimated waste stream. Although discharges to water of volume and weight, the actual amount of waste is a very small fraction of the total; the discharges are essentially water with relatively minor amounts of contaminants. In addition, most, if not all, of the toxic substances for which disposal data are recorded by the Office of Toxic Substances are also regulated under RCRA and total about 6 million tons. Therefore, wastes found in air and water and toxic wastes were not included in calculating the annual generation of industrial waste. In addition, there are other factors that inhibit the calculation of the industrial waste stream. The following information is important to understanding how the 14.7 billion ton estimate was derived. In a 1988 report to Congress on solid waste disposalin the United States [l], EPA reports an In its estimated annual generationrate of 7.8 billion tons ofindustrial nonhazardous waste. analysis, EPA includes wastes generated by utilities, namely electric power generating and water treatment facilities. The amount of oil and gas wastes generatedin the United States is reported in another report to Congress [2]. This document reports estimates made by EPA and by the American Petroleum Institute. EPA’s estimates of the amount of drilling waste generated in the exploration, development, andproductionof crudeoilandnatural gas werechosenfor inclusion. The amountof water generatedin these activities is also reported, but was omitted fromthe calculation as it is predominantly water witha high saline content andis often
1%
Walter
Legend
0
Rubber and Leather
Textiles wood
~00dwa~te
Yard Waste Glass Metals Miscellaneous
Figure 2 Municipal solid wastecomponents. discharged to surface waters, used for agriculhral purposes, or consumed in the production process. Each of the surveys fromwhich data were taken was conducted once, so the estimates recorded in Figure 1 are snapshots of the waste generationin a single year. A complicating factoris that the data refer to practices in different years. In addition, much of the data are now close to 10 years old. The total 1.4 billion tons of mining wastes excludes coal mining wastes, which were not included in Ref. 3. In Ref. 1 , EPA reports that 2 billion tons of feedlot wastes are generated annually. The amount of these wastes that were used as fertilizer on fields in unknown. No estimate of crop residues is included. The 750 million tons of hazardous waste reported in Ref. 4 includes wastes regulated under RCRA as well as wastes regulated under the Clean Water Act. Much of this waste is mixed with water; however, the relative percentages of wastes and water are unknown. The 196.7 million tons of residential, commercial, and institutional waste reported in Ref. 5 represents 1990 data. Some of these limitations are a result of the data collection methods used. EPA's goal is to protect human health and the environment; data on the characteristics and amountof waste are collected in its efforts to determine risk. Thus, waste generationdata are largely a by-product of EPA's attempts to meet its goals. EPA's methods and priorities limit the breadth of the data and its usefulness for quantifying the amount of waste generated in the United States.
B. Municipal Solid Wastes EPA has funded studies of MSW generation for at least 20 years. In 1960, each person in the United States produced approximately2.7 lb of trash per day. Today, this numberhas increased to approximately4.2 lb per person per day fromresidences, commercial businesses,and public institutions such as schools and hospitals. Reportsare that the average Japanesediscards 2.5 lb per day and the average Norwegian l .7 lb per day, although it is not clear that these data have the same basis as for the United States. There is evidence, particularly from Europe in the 1970s, that MSW generation changes with the change in Gross Domestic Product (GDP). These estimates are based on industry output of consumer goods multiplied by an estimated
Commandments Management of Waste
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discard rate for each commodity. The estimated production from these sectors was just under 200 million tons per year in 1991. As noted, industry produces large quantities of waste eachyear. The majority of this waste istreatedanddiscarded in landfills,generallyonindustrialproperty,althoughsignificant amounts are reused in industrial processesand are not recognized as waste. A fraction of industrial waste is discarded in the same sanitary landfills that receive residential, commercial, and public waste. There has been no estimate of the quantity of this industrial waste since EPA's 1973 first reportto Congress which indicated that a total of80 million tons per year of industrial waste was discarded. There have been significant changes in the industrial sector since that report, suchas the diversionof hazardous and toxic wastes from industry to control technology and special landfills. The industrial waste that is discarded in municipal landfills is estimated at 70 million tons per year. Not all of the 200 million tonsof residential, commercial,and institutional waste discarded is recoverable for productive use. From existing data, currently50 tons per day (TPD) is the smallest facility size that is considered economic. Some smaller facilities(as small as 6 TPD) have been built and operated successfully in special circumstances such as in prisons and other government installations. A 50-TPD plant serves a population of 25,about 000.The available population 'data of the Bureau of the Census report standard municipal statistical areas (SMSAs) with populations of 50,000 or more. The latest available census data are for 1990, when about75% of the U.S. population lived within SMSAs. Given the steady increase popin ulation in SMSA from 1950 (56.1%)to 1980 (74.6%), the 1990 SMSAdata should show that about 80% of the population lives in SMSAs. Municipal solid waste is very heterogeneous and varies in composition from day to day, season to season, and location to location. Further, the quantity produced varies throughout the year, with most produced in the summer. Figure 2 shows typical constituents found in the waste stream.
C. Thou Shalt Have the Waste at Thy Disposal The first of the commandments of waste management seems to be unnecessary after the discussion of the very large quantities of waste that exist in the United States. However, those wastes can be dispersed and not available in the quantities required to construct an economic project. The U.S. Department of Energy planned and was about to enter into an agreementto test a technology in an industrial plant. The project was canceled when the industry determined that it had less than half of the waste required. In another case, a municipality planned and constructed a waste-to-energy plant that was sized at 1000 TPD. Only after operation began did the city discover that it had only about 400 V D . Therefore the plant cost2.5 times as much to operate as the city had expected. The city implemented a number of measures to increase the amount of waste available. Several undesirable events occurredas a direct result, including lawsuits, fatal accidents, unexpected costs, and poor technical performance.
D. Thou Shalt Not Covet Thy Neighbor's Waste me second of the commandments seems as unnecessary as the first, yet every material believed to be worthless acquires valueas soon as the owner believes that someone else desires it. Basically this commandment states that, all other issues being equal, the productive use of the available waste should not be delayed while attempting to negotiate an arrangement for added resources.
Walter
198
111.
ECONOMICS
The increasing volume of waste, declining capacity of disposal systems, and increased regulation and cost have contributedto significant increasesin the cost of waste disposal. Various reports, including those published in Waste Age by the National Solid Waste Manufacturers Association, indicate that disposal costs have increased dramatically from about $lO/ton in 1978 to over $150/ton today in some parts of the country.
A. IndustrialWaste Little is published on the cost of the disposal of industrial wastes. The cost of toxic and hazardous waste disposal isknown to be high and increasing. In addition, the personal liability of owners and managers of firms is increasing. Each proposed new environmental law imposes even more stringent requirements. As a result, capital expenditures and operating costs for industry have increased dramatically. One estimate is that every inch of paper added to regulations has increased industrial expenditures for environmental control by $10 million per year [7]. In general, industry does not assess the costs of environmental control but rather installs whatever end-of-the-pipe control is specified as the best available control technology by regulation. This limits their liability and negative press. A few companies search for more economic waste reduction technologies or new waste utilization systems. For example, when faced with the need to reduce emissions of volatile organic carbons (VOCs), 3M entered into an agreement with the US. Department of Energy (DOE) to develop a Brayton cycle heat pump system that condenses the solvent from the waste. The VOC then is reused in the process. While incinerationof the VOC and recovery cost the same today, anticipated future regulations will make recovery the more economic solution.
B. MunicipalSolid Waste
A municipal solid waste disposal management system may include anycombination of sanitary landfill, energy production, and recycling. Historically the collection and transportation of MSW accounts for 75%of the total cost of the waste management system. Landfill' costs are increasing dramatically as more stringent standards are imposed. Further, the costof transportation of waste is increasing dramatically as large cities run out of landfill space and the will to site new landfills and, as a result, resort to long haul for disposal. The actual cost of, and accounting for, any element of the management system varies widely. Some jurisdictions include the chargesin the general tax baseand indicate zero costfor waste management. Others operate as a utility and include all charges in their costs. Most operate with a combination of the two systems. Collection seems to cost in the vicinity of $100 per ton. EMW disposal facilities charge a tipping fee that is designed to cover the costs of the disposal system. As for collection, the accounting methods vary. Frequently, the cost of capital amortization and formation, postclosure expenses,and similar costsare not included in landfill fees. Typically, all of the costs of a waste-to-energy plant are included in the budget. Unlike most municipal capital improvements, energy-from-waste facilitiesare typically financed from tax exempt revenue bonds that cover all the costs related to the project. The tipping fee in the 'In 1976. when RCRA amended the Solid Waste Act, an estimated 30,000 landfills were in operation. By 1988, that number had been reduced to 6034, with EPA projecting that 2000 of these would be closed within 5 years. In fact, that has not occurred, and the capacity of landfills has remained relatively constant. While fewer new landfills have opened than old ones have closed, the new ones tend to be large high-capacity regional unitswhile those closing tend to be small.
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plant, energy and materials sales, and interest pay all operating, bond amortization, debt service and other reserves, and other costs of the facility. Tipping fees in energy-from-waste plants vary from$6 to $66 per ton with an average of $33 according to GAA 1991 data. One landfill has published tipping fee (disposal) rates of ($21.40/ton for district residents, $63.30 for state residents out of the district, and $850 for out-of-state trash. Recently EPA reported the annual cost of disposal of a ton of waste in a landfill as $46/ton and for disposal in an EMW plant as W/ton. The added cost of collecting and transporting recyclable materials, particularly where separate curbside collection is used, is highly variable. Costs as high as $1100/ton have been estimated, although an average of $100-200/ton seems to be more reasonable. Where sourceseparated materials are delivered to a central location, there is little increase in cost, if the cost of the counto the individualis ignored. The valueof recycled materials varies with the region try and the specific recyclable. Some, such as aluminum beverage containers, have had positive values for muchof their history. Others, such as used paper, have very volatile markets. Some proponents of recycling suggest that the cost of collection and the tipping fee in a landfill should be subtracted to arrive at a net cost of recycling. However, few if any locations have managed to recycle sufficient wasteto reduce the cost of the collection or landfill operations. In such instances, the use of offsetting costs is not appropriate. Currently, the value of the energy produced or conserved by MSW utilization is approximately $1.8 billion per year(oil at $20/barrel). Additional economic and energy savings would be realized by reducing the amount of raw materials and feedstocks used in production of recycled materials. The energy conservedby recycling is valued at about $0.9 billion. National security can be enhanced by providing an augmented domestic energy source.
C. Markets There is a large variety of markets for the productive use of wastes. However, these can be roughly divided into materials markets for recycling and markets for energy. Materials markets economics depends upon the value of the raw material displaced and to a significant extent the value of the energy displaced by the raw material. Thus one of the better recyclable materials is aluminum, where recycling displaces 281 GJ/ton [8]. As noted, aluminum can stock currently sells for $480/tOn in the northeast. In general, the more pure the recyclable, the higher the price. Thus, can stock, which is at most two alloys, is more valuable than other aluminum scrap, which is a mixture of alloys. To reuse mixed scrap, it must be assayed and blended, and other elements, such as pure aluminum, must be added to achieve the desired alloy. Some recyclable materials have had very volatile markets throughout their history. Old newsprint went from $40 ($ 1974) per ton in July 1974 to no value in September. In November 1989 it was selling for $40 ($ 1989) to zero in February 1990. Some selected current prices are listed in Table 1. Because of the volumes of elements in municipal waste, used paper is the most recycled product. There is not enough known about industrial waste recycling to make any specific comments. However, 5040% of the energy used in the pulp and paper industry is from its waste, and the steel industry uses scrap for about 60% of its U.S.output. There are three feasible energy markets; fuel, steam, and electricity. The most common market is for electricity. The most economic market is for steam. Steam priced at a total of fuel andcapitalcostsisworthabout$0.0095/mJ($9/MBtu),while electricity is $0.0215/MJ ($O.O75/kWh). However, the capital costs for a steam plant are 10% less, and three times as much steam is produced per unit of input energy. As fuels, natural gas and oil are worth about $0+002/MJand coal about $0.001/MJ. Wasteis a solid fuel and as such can only be used in a
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Table 1 Average RecyclableMaterialPrices for Three Regions ($/ton) Commodity West
South
Old newsprint Used aluminum beverage containers Other aluminum scrap Glass Steel cans
Northeast -$ 25
$ 5
$480 $400
$490 $480
$ 13 $ 15
$ 37 $ 15
$10 $450 $400 $ 28 $ 21
Source: Recycling Times [9].
coal-fired boiler with today’s technology. Therefore, steam is the preferred energy product from waste, but matching to an industrial steam load is difficult. Except where an industry is using its own wasteor a city has a district heating system,electricity is the preferred energy product. Even where the waste is recovered as biogas froma landfill and fuelor natural gas markets are feasible, most of the gas is used to produce electricity. Today about 80% of all waste used for energy produces electricity.
D. Thou Shalt Not Produce Anything No One Wants To Buy The third of the commandments is essential even for businesses. One firm won the right to construct a municipal waste plant producing refuse-derived fuel but did not have a market for the fuel. As a result the design was changed to a fuel technology that was developmental but for which there was a market. The technology demonstration could not be upgraded rapidly enough to correct the problems, and ultimately the project failed.
E. Thou Shalt Honor Thy Economics The fourth of the commandments pertains primarily to municipal waste. One city used municipal resources to collect recyclables and private collectors for nonrecyclable trash. Each home owner selects and directly pays a private collector. The city provides landfillservices to the private collectors. The city pays for collecting newspaperand must pay a processor to take the recyclable paper. In addition, the removal of the recyclable paper fromthe landfill did not change the cost of operating the landfill, so that the cost of disposal in the landfill increased. Any city trash sent to the landfill was chargeda higher tippingfee, as were the citizens through the private collectors. The situationwas made worse when some of the collectors determined that it was cheaper to haul waste to a distant landfill.
F. Thou Shalt Not Expect Gold in Garbage The fifth of the commandments recognizes that wastes have no valueor even have a negative value until the producer believes that someone else might want the material. Inaddition, many people in cities believe that they will make money from their garbage.
IV. ENVIRONMENT W a s h can be managed three ways: They can be reduced at the source, they can be used, or they can be discarded. With all things equal, the best solution is to avoid producing them (source reduction). This implies that raw materials are used more efficiently. Any waste disposal system has the potential to make discharges to the air, land, or water. For example, a natural organic waste discarded on the land will degrade to acids and eventually
Commandments of Waste Management
Table 2 Ultimate Analysis of Selected Fuels MSW
Wood
Carbon Hydrogen Oxygen Sulfur Nitrogen Chlorine
48.9 5.9 43.3 0.01 0.03 0.01
201 (%
dry weight) coal
39.5 5.0 29.5 O.l(O.1-0.2) 0.5 0.4(0.2-0.5)
74.3 4.9 7.8 3.7 (0.5-4) 1.4 0.06(0.06-2)
to biogas, stoichiometrically, a mixture of equal molecules of carbon dioxideand methane. The acids will increase the solubilityof heavy metals, while bothof the latter are gases that affect the global climate. The same organic material could be burned to produce air emissions including carbon dioxide and sulfur, chlorine, and nitrogen compounds. In water, organics rot, consume oxygen, and produce acids that have adverse environmental impacts.
A. IndustrialWaste A significant amount of the waste discarded by industry is reused. The wastes that are reused save the environmental degradation caused by input material acquisition, reduce energy use and its associated impacts, and reduce the impacts of end-of-the-pipe control and final disposition. For example, when paper is produced by chemical pulping, lignin is dissolvedto free cellulose and hemicellulose fibers. In addition, the pulping chemicals and lignin combine to produce black liquor. Originally, black liquor was dischargedto the nearest stream. A combinationof the increased costof chemicals and energy and environmental regulation sparked the development of recovery boilers. These units use lignin as a fuel to produce energy and recover chemicals for reuse. Since ligninand waste wood and bark (from paper making and burning)are low sulfur, chlorine, and ash fuelsand are renewable, theyare less polluting than the fossil fuels displaced. Tables 2 and 3 list the ultimate and proximate composition of some wastes and fossil fuels. As noted previously, relatively little is known of industrial waste management, and the types of wastes between and within industries are very varied. Therefore, detailed discussion is difficult. Industry has long used home wastes (producedin the plant or factory, e.g., leftover metal from ingots) and prompt wastes (produced in the next level manufacturing facility, e.g., aluminum can scraps) when it was economical to do so. Within the context of the industrial waste program of the Departmentof Energy, industry is displaying great interest in the reduction or substitution of hazardous and toxic wastesto simplify and reduce environmental costs. are sharing equally the development of a replacement for the For example, Motorola and DOE volatile organic compounds (VOCs) used to clean circuit boards. By using a nitrogen blanket and adipic acidas a combined flux and cleaning agent, VOCs have been displaced and cleanliness standards retaihed. In a separate program, DOE and EPA are cooperating to apply existing technology to industrial waste minimization. For example, the federal government, the
Table 3 ProximateAnalysis of Selected Fuels (%) MSW Wood Moisture Volatiles Fixed carbon
Ash
50.0 40.3 9.7 .02
coal
25.1 43.8 6.1 25.O
13.1 27.9 51.1 7.9
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State of Texas, and FMC Company cooperated to demonstrate the recovery of methanol from hydrogen peroxide production. The methanol is used to clean catalyst and is contaminated with dilute quantities of hydrogen peroxide, which makes it hazardous. By steam stripping the hydrogen peroxide, 98% of the methanol can be returned to the process instead of being sent to a hazardous waste facility for destruction. Another example from the same program is PPG, which applied reverse osmosis to clean machinery wash water and reduced hazardous waste 4 0 O ,O O Vyear, and improved the effidisposal by 95%, reduced fresh water consumption by 4 ciency of its operations.
B. MunicipalWaste As for industrial wastes, source reduction the is preferred waste management technique for municipal postconsumer wasteif all other things are equal. Municipal waste is very complex. An increase in one wastemay result in the reductionof another. The Chamberof Commerce of the United States estimates that for every pound of packaging added to the municipal waste stream in the 1980s, 2.7 lb of food waste was eliminated. Some of the added packaging was plastic, which does not degrade ina landfill but does not produce acids and therefore does not increase and reduces highly degradable food waste. However, there are attempts to reduce the amount and type of packing. For example, recently, under public pressure, some fast food stores have replaced Styrofoam serving boxes with multilayer paper and plastic boxes on the basis that Styrofoam does not degrade in landfills and takes excessive space. However, the disposal of nonbiodegradable materialsin landfills reduces the toxicity of leachates (decreases metal solubility) and reduces the need for landfill space (Styrofoam is weaker and less likely to bridge than the multilayerpapedplastic replacement). In addition, Styrofoam can be recycled into styrene or burned, whereas the multilayer paper can only be burned. The comparison of the environmental emissions from landfill, recycling, and fuel is not the simple. For example, glass can be landfilled with no resulting net emissions except for transport of the glass to the landfill.When recycled, the transportation emissions are dramatically increased, but energy consumption (generally in the form of natural gas)is reduced, thus reducing associated air emissions. Landfills emit biogas and many trace chemicals through their surface to the atmosphere. Stoichiometrically, biogas is a mixture of carbon dioxide and methane in equal numbers of molecules. In practice, carbon dioxideis more soluble in water, forming carbonicacid, and therefore more methane is emitted. These two gases are the largest contributorsto global warming emissions. Various estimates have been made of the comparative methanekarbon dioxide global warming gas effect. Currently EPA is using a 25 times estimate. Rainfall and other water that penetrates the landfill produces leachate, which then enters groundwater. In the degradation process the natural organic waste landfills form acids. Most heavy metals are more soluble at pH less than 5 or greater than 12. Mixed waste placed in a landfill is neutral to slightly acidic. The combination of acid rain and degradation of the organics can reduce the pH of a mixed waste landfill to less than 5 and result in leachate that occasionally exceeds EPA standards for lead and cadmium. MSW combustion ash has a pH of about 11. Leachate from an MSWcombustion ash landfill has been tested for 4 years. Leachate samples have rarely exceeded drinking water standards (100 times more stringent than leachate standards). In addition, the pH has stabilized, and the amount of heavy metals in the leachate has begun to decrease. The effect of recycling on the environment is not well understood. The majority of the available data are anecdotal. It is known that, most often, recycling increases vehicular emissions over the other forms of MSW disposal management even compared to raw material ac-
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Table 4 GreenhouseGas Emissions CO,
Landfillb Fuel Recycle
1
CH,'
Equiv. CO,
1
26 2
2 0.15
0.15
3.9
'CH, is 25 times as reactive as CO2 bFifteen percent ofshort fiber is landfilled.
quisition. Wood used for paper manufacture is50% moisture and about2.5% extractives (sap, etc.) that do not become part of the product paper. When kraft paper is produced, the lignin (12.5% of wet wood) is dissolved to free the cellulose and hemicellulose fiber for paper production. Thus less than 35% to no more than 47% of the raw wood feed becomes paper, and industry naturally locates closerto the wood source to minimize transportation costs. One notable exception is boxboard, which is virtually 100% manufactured from waste paper and therefore tendsto have plants near cities. Recycled paper results in deinking slops (again except for boxboard and cardboard, where color is immaterial) and short fiber for landfill. Considering the 15% short fiber that results from remanufactureof paper and is landfilled, recycling paper produces more net greenhouse gas emissions than burning allof the paper (see Table4). The net environmental effects of recycling plastics and inorganic materials are almost unknown. There are some environmentaldata available to indicate concern for zinc from galvanized steel and VOCs from paint on cans. Combustion of MSW produces air emissions. With modern pollution control equipment, including electrostatic precipitators, bag houses, and acid gas scrubbers, these emissions are reduced to meet control standards. At the present time, EMW emission standards are more stringent thanthoseforcoal or Therefore, modern MSW combustionplantsproduceless criteria air emissions than fossil fuel plants producing the same energy. Particulates are 10 times lower, sulfur oxides 15 times lower, and nitrogen oxides about the same, with a small increase in chloride emissions compared to most coal. With some coals, the chlorine content is high and chloride emissions exceed thoseof MSW. Similarly coals haveheavy metal contents that on occasion exceed those of MSW. MSW combustion does result in measurable dioxins in the stack gas; however, those emissions are lo00 times lower than state allowable limits, are less than the dioxin measured in the input waste, and degrade to below detection limits in 8 hr or less in sunlight. Where checks have been made, there has been no detectable dioxin at ground level around operating plants. There are no specific standards for chloride, carbon dioxide, and dioxin emissions from fossil fuels. Measurement of the chlorine content of coal is just being initiated. Prior U.S. measurements did not detect dioxin in the air emissions of coal 'For example, an Alexandria, Virginia, plant usesdirect injection of limestone into the boiler box for acid gas control and electrostatic precipitators for particulate control. The resultant emissions include0.063 Ib of particulate, 0.26 Ib of sulfur oxides, 0.10 Ib of nitrogen oxides, 0.47 Ib of hydrogen chloride and 0.05 Ib of carbon monoxide, all per million Btu. In addition, the plant releases0.006 Ib of dioxin per year per million Btu. These are all currently or about to be criteria pollutants from EMW plants. The plants with the moreefficient scrubber systems for sulfur and chlorine control increasesulfur and chloride removal efficiency by 10 percentage points (from60 to 70% and from 80 to 90%. respectively). The comparative standards for emissions from fossil plants are 0.2 Ib of particulate, 2.6 Ib of sulfur oxide, and 0.9 Ib of nitrogen oxide emissions per million Btu. Carbon monoxide and non-methane hydrocarbon emissions from waste-toenergy processes are comparable or lower. In one plant in Commerce City, California. the nitrogen oxide emissions from the plantand the hauling of the ash to a landfill are less than the nitrogen oxide emissions from the trucks required to haulthe raw waste to the landfill. The air in an Indianapolis plantis cleaner since an EMW plant was put into operation.
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plants, although dioxinhas been detected in the emissions from coal combustionin the United Kingdom. There has been very little comparison of the emissions of EMW and those of fossil fuel plants. When emissions are compared for the landfill, recycling, and energy production from MSW, combustion is more attractive than presumed.
C. Thou Shalt Honor All Thy Environment The sixth commandment demandsthat all of the effects on the environment be considered, not just those in the immediate vicinity of an activity. In effect, the replacement of “not in my backyard” (NIMBY) by “in someone else’s backyard” (ISEBY) is not acceptable.
V. ENERGY AND TECHNOLOGY A large variety of thermal, physical, chemical, and biological technologiesare available to use wastes productively. The appropriate technology depends onthe state of the waste (solid, liquid, or gas), the type of waste (organic or inorganic), and the end product (material or fuel). Organic wastes canbe used economically as fuels or as raw materials; that is, they can produce be used as raw materials. Ingeneral, the thermal or conserve energy. Inorganic wastes can only technologies produceeither energy or gaseous or liquid fuels. The physical and chemical technologies produce raw materials for recycling, products, or solid fuels. The biochemical technologies produce gaseous or liquid fuels or chemicals.
A. IndustrialWastes Significant amounts of industrial waste have been reused for generations. About 60% of our domestic production of steel is from scrap, which saves an estimated 1.3 X 10l8 J (1.2 X 10’’ Btu) per year. Also, significant quantities of scrap iron and steel are exported and returned as finished products. The closer the scrap is to the point of production, the more likely itis to be used. In contrast, U.S. junkyards have a 17-year supply of scrap steel, mostly from discarded automobiles and appliances. The precise technologyfor reusing scrap is highly dependent onthe nature of the scrap. If it is organic likewood pallets andplastics, it may be burned or reused as material. For example, to the extent possible, industry reuses wooden pallets and shipping materials. At some point these can no longerbe used, and if sufficient quantities are available they are burned for plant energy. If there is not enough material for economic use as a fuel, they are discarded. In a program in the late 1970s, DOE developed a technology to burn atactic polypropylene, a waste from the production of polypropylene. A commercial-scale plantwas constructed andoperated successfully. Later, changes to the process increased the output of product per unit of input. There was no longer sufficient waste to warrant investment in a recovery unit. Sincethe early 1980s when utilities were required by law to purchase excess electricity, industries have increased their use of organic wastes to cogenerate steam and electricity for process use, with excess electricity sold to utilities. In fact, a new business has developedthat takes wastes and either cogenerates steam for sale to industry and electricity for sale to utilities or simply generates electricity for sale. In terms of the efficiency of fuel use, cogeneration is the preferred technology sinceelectricity generation is 2 0 4 0 % efficient (depending on the fuel and thesize of the facility) and cogeneration can approach 80% fuel efficiency. As discarded, waste paper has an energy content of 17.4-20.8 &/kg (7500-9000 BNlb). Using postconsumer paperas a feedstock to make new paper saves 0-4.4 &/kg (0-2000 Btu/
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lb). Plastics have an energy contentof about 46 d l k g (20,000 BNlb). There are no consistent data on the energy savings from recycling plastics. Inorganic materials can only be used as raw materials for a process. The technology used and the energy conservedare dependent on boththe type of material andits relative purity. For example, U.S.automobile aluminumis alloyed with strontium to make it ductile. Foreign car makers use antimony for the same purpose. When melted for reuse, the two alloying metals combine to cancel the desiredductile effects. A method to neutralize one is in development to protect this valuable scrap market. Although the energy savings fromthis particular scrap have not been traced, they can be assumedto approach the IO-MJ savings from recycling aluminum can stock.
B. Municipal Waste Municipal solid waste can be converted to energy by thermal or biochemical technologies. The commercial thermal technologyis combustion to produce steamfor industry, hot water for district heat, or electricity for the grid. MSW may be burned as received in specially designed facilities on a hearth (typically 25-50 metric tons/day per unit)or on inclinedgrates (typically 250-1000 metric tons/day per unit). In addition, solid fuel canbe produced mechanically. This refuse-derived fuel (RDF) can be burned in a specially designed boiler on a traveling grate (typically 400-1000 metric tons/day). RDF is also used as a substitute for coal, preferably in a grated boiler but typically in a suspension-fired boiler. Limited research is developing new technologies that produce fuelgas or liquid transportation fuels or new forms of RDF such as densified and powdered RDF. The former can be stored for longer periods and marketed to multiple users; the latter can be used as a substitute for oil in boilers designed for heavy oil. Current commercial combustionfacilities are 65-796 efficient in producing steam (fossil fuel content. In contrast, a low facilities are 8 8 4 0 % efficient) mainly dueto moisture and inorganic moisture and low ash waste such as paper or dry wood has a calculated efficiency of 89%. Biochemical conversion technologies use organisms to produce specific fuels. For example, a complex consortium of organisms found in nature will reduce cellulose and other carbohydrates to simple sugars, organic acids, and finally biogas. Currently, commercial biochemical conversion is limited to the recovery of biogas from landfills and sewage treatment. Future research could completethe development of technology to produce biogas or ethanol from municipal waste or selected components. Recent measurements of the energy content of as-received MSW indicate that approximately 11.6 d l k g (5000 BNlb) of energy is contained in each poundof MSW [up from 10.4 &/kg (4500 BNlb) in recent years]. Municipal solid waste management can conserve energy by remanufactureof specific components into new materials. Certain inorganic components(steel, aluminum, and glass) canbe separated by source separation of mechanically, magnetically, by density, size, shape, electrostatics, etc.) and recycled into new materials. Certain organic components (specific gradesof clean paper and specific plastics) can be recovered and remanufactured. Since high purity is necessary for materials to be recycled, source separation is the preferred recovery technique. Research is required to economically recover componentsother than steel from mixed waste. The amount of energy conserved is different for different commodities. For eximple, the recycling a pound of aluminum conserves 242 &/kg (100,000 BNlb) while recyclinga pound of glass conserves about 3.8 &/kg (1500 BNlb) [lo]. In addition, the conservation of energy within a commodity may be different. For example, recycling old newsprint into new newsprint conserves about 4.6 d l k g (2000 BNlb) while recycling cardboard into new cardboard saves about 2.3 &/kg (1000 BNlb).
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Currently thereare approximately 150 waste-to-energy facilities in the United States. They vary from mass-burn facilities where MSW is burned as received, through facilities that prepare a fuel for use in existing boilers, to those that recover recyclables. Over90%of the energy generated from these facilities is from the burning of waste. There are a large number of recycling programs and 50 or more materials recovery facilities (MRF). Based on GAA, EPA, and DOE databases, currently about 0.29 X 10l8joules (0.28 X lo’’ Btu) of primary energy is produced by burning, about 0.16 X lo’*joules (0.15 X lo’’ Btu) 0.15 quad is conserved by recycling, and about 0.05 quad from landfillgas (LFG) is being recovered from about 100 landfills. About 80% of the energy produced from burning is electricity, with the remainder being steam or hot water for industry or district heating. The recycling mostly conserves electricity in aluminum production. About 80% of LFG is used to produce electricity, while the rest is used as boiler fuel or cleaned to pure methane for use as substitute natural gas.
C. Thou Shalt Honor All Thy Technology The seventh commandment notes that there are a number of ways to use wastes. In many ways the productive use of waste is more important than the way the waste is reused.
VI. LIFECYCLE For sound waste management decisions, the life cycle of the systemmust be considered. Waste must be collected fromthe point of generation, transported to a point of reuse or disposal, and used as fuel or raw material, with any resulting waste discarded. It is difficult to determine the appropriate bounds for a life cycle analysis forthe disposal of waste and the appropriate comparative data. Should the life cycle be bounded by the acquisition of the raw material (by miningor recovery of recyclable)or by the manufactureof the mining equipment, or even earlier in the recovery of raw materials for the mining equipment? Experience indicates that, given the accuracy of the data available, going beyond the acquisition of the raw materials does not improve accuracy.
A. An Example Since much of the data are either missing or anecdotal, an example is provided toillustrate the complexities of the analysis, provide a method of consideration, and indicate missing data. Since most data are available for MSW, the example concerns thelandfill, recycling, and burning of paper. With few data available on a methodology to trade off impacts, the economic, environmental, and energy effects of each option are considered separately. The appropriate start for MSW management would seemto be at the point of discard for waste paper andin the forest for virgin paper. But in keeping with a necessary themethat there seemto be exceptions to any rule for MSW management, one special consideration is needed. Should the waste have entered the system at all? Again, this is an issue that has life cycle connotations but is one where few data exist. For example, a current belief is that reducing packaging will reduce the amount of waste. However, the estimate is that for every pound of packaging added in the 1980s, 2.7 Ib of food waste was avoided. Therefore, it may not be advisable to avoid packaging, plus packaging can be recycled or used as fuel. Landfill Profile. Waste paperis collected from the generator at the door as part of MSW management. There is no separation, and thereis a single collection vehicle with a crew of one to three.The collection vehicle transports the mixed waste either to a landfill for disposal or, if more eco1.
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nomic, to a transfer station for repacking into a larger vehicle to be transported to a landfill. Landfill is considered the baseline against which the other options are compared since most MSW and postconsumer waste paper is landfill. As a result, certain costs and impactsare not discussed under landfill. Expected changes in the system for the other options are discussed. For example, the emissions fromtransportation fuels are significant and include carbonmonoxide nitrogen oxides, particulates, volatile organiccarbons, carbon dioxide, and, from diesel fuel, sulfur oxides. Transportationis a major pollution sourceand therefore is important. Any significant increase in miles driven results in a significant increase in air pollution. Economics. The costs of collection, transportation, and landfill vary from location to location. Past estimates indicate that 75% of the cost of waste management is in collection and transportation with the remaining 25% for landfill. The cost of landfill has been increasing rapidly as new environmental requirements to reduce the impact of emissions are implemented. The highest reported cost of landfill of mixed waste is $150/ton. EPA recently estimated the cost of landfill at $45/ton. Landfill of waste paper has no advantage over mixed waste and should cost the same or potentially more since paper occupies more volume per tonthan mixed waste. Environment. Landfills emitair emissions including carbon dioxide and methane (greenhouse gases) and trace organics. In one DOE study, over 50 different trace organic emissions were measurable in landfill gas. The long-term, difficult-to-estimate environmental concern is leachate, potentially with increased heavy metal content, entering groundwater. Modern landfills require liners and leachate treatment for control;however, science indicates that landfill liners will breech, the only question is when. Energy. In 1976, DOE examined the total energy consumptionin the collection of trash. The study indicated that the energy used by MSW collection was minor. Landfills do degrade naturally to produce a gas that is half carbon dioxide and half methane. Thisgas can be captured and used as a boiler fuelto produce steam and/or electricity, used in an engine to produce electricity, or cleaned to remove the carbon dioxide and used assubstitute natural gas for the pipeline system or as a vehicle fuel. Theefficiency of theconversion of organics to LFG is estimated to be 32%; that is, 5.8-7.0 d / k g (2500-9000 Btu/lb) would be recovered as biogas.
2. Use for Recycling Profile. Paper is collected separately from other wastes. In general,as for all recyclables, the more pure the paperis, the more valuable itis. Thus, to maximize value, used newsprint,cardboard, and mixed paper should be kept separate although the same truck can beused to collect them. The collection must be by curbside pickup,in the same or a separate truck, for maximum recovery, although deliveryto a central point by each resident is also feasible. No matter which is used, vehicles tendto travel added miles. Transportation to the remanufacturing plantis generally long in comparison to the landfill or use as energy options. Economics. Curbside collection is costly. There are few careful estimates of the cost of sep$100 arate collection for paper recycling. It appears that separate curbside collection costs from to over $200 per ton. A method to describe the costof separate delivery of recyclable material to a recycling center has not been developed.The current price for used paper is listed in Table 1. In 1990, one city burned half of its used paper to earn enough to recycle the other half. At the same time the expansion of markets for used newspaper is capital-intensive. A deinking plant alone costs $60-90 million for a 250 todday capacity. A full-scale plant built from the ground up will cost over $1billion. Environment. Transportation is important when considering environmental issues, and very little is known about recycling. Two examples are available. Both Wilmington, Delaware and
Walter Hartford, Connecticut ship their newsprint to ThunderBay, Ontario, a distance of 1500 miles, for recycling. In contrast, a boxboard plant inSpringfield, Massachusetts obtains its waste paperand cardboardsupplylocally.Intoto, transportation emissions canbe expectedtobe greater for recycled paper than for either landfill or use as a fuel and greater than those from obtaining wood to make virgin paper. A second issue is the value of the wood used for virgin paper. Often, paper recycling is advertised on the basisthat recycling a ton of paper saves 17 trees. But what 17 trees are saved? They are not from virgin or primeval forest but rather from tree farms or from forest management. In the formercase, the tree farm has completed growth andis beginning to recycle carbon dioxide; that is, the loss of tree weight in branches and leaves is balanced by the annual growth of the tree. In addition, seven trees are replanted for every one harvested. In thelatter case, the removal of trash trees results in a more vigorous forest. It is difficult to compare MSW disposal management environmental impacts. There is relatively little consistently gathered comparable information. Transportation effects are not includedsincereasonableinformation is not available; however,they are major. If paper is of carbon dioxide(CO,) landfilled, each two molecules of carbon are degraded to one molecule and one moleculeof methane (CH,). Theseare the two most significant greenhouse gases, with a molecule of CH, 20-100 times as reactive as a molecule of CO,. If landfill gas is used as a fuel or flared, then the molecule of CH, is converted toa molecule of CO2 and four molecules of water. In addition, some volatile organic carbonsare emitted. The length of time needed for the degradation to occur is not well defined but is less than 100 years, which is very short in comparison to fossil fuels, which represent carbon captured eons ago. If paper is burned, then every two molecules of carbon become two molecules of CO,. If paper is recycled, several changes to the system occur. Fifteen to twenty percent of the fiber is too short to reuse and is landfilled. Here it degradesto COz andCH,. In addition, 65% of the energy to produce paper is obtainedfrom wood waste, which is no longer available and is replaced with fossil fuel. The wood that would otherwise be used to produce paper remains in the forest. However, this wood is either from forest managementor from a tree farm that is replanted after harvest. In either case, the trees will use more CO2 than the forest primeval, since the latter has completed its growth. Table 4 summarizes greenhouse gas information. Transportation effectsare not included. Energy. Since the paper must travel a longer distance, energy consumption is higher. The energy savings from recycling paper depends upon the type of paper used and the product. For example, when used newsprint is recycledinto new newsprint, about 4.6 d l k g (2000 Btu/lb) is saved. When the same used newsprint is recycled into boxboard, there is no net savings. When cardboard is recycled intonew cardboard, there is a savings of 2.3 mJkg (1000 Btu/lb). These savings are all in the manufacturing plant. 3. Use as a Fuel Profile. Paper is collected as part of mixed waste, and there are no changes to the collection system. Thetransportation distance to an energy plantis generally shorter than or equal to that for a landfill. As paper is burned, ash is produced and must be transported to either a landfill or a site to be utilized. There is no significant leachate, since there is no organic material to degrade and produce the acid necessary to dissolve heavy metals. Economics. There is a significant savings in vehicle maintenance costs since the vehicles use a concrete tippingfloor rather than dirt landfill roads. The oneestimate is from theearly 1980s when New Orleans reported a $500,000 annual savings. MSW energy plants have high capital cost. However, between the tipping fee and the income from energy and material sales, the plant earns enough to pay all the costsof construction and operation, including amortized cap-
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ital costs, interest during construction, debt reserve,operations, maintenance and repair, etc. If financed with revenue bonds, the financing has no effect onthe city’s general obligationbond authority and rating. EPA estimates the average tipping fee in an energy plant as $42/ton including ash disposal. Environment. There maybe a change in vehicle emissions depending on the total vehicle miles driven; however, most energy plants are at the site of the landfill or closer to the center of collection so that transportation emissions are either the same or at best slightlyless than the baseline. However, in one case, a net decrease in NO, emissions was reported as a result of transportation. That plant was between the landfill and the collection district. The ash weighed about 75% less than the raw waste. Removal of three fourths of the trash trucks from theroad between the energy plant and the landfill reduced more NO, emissions than the energy plant produced. If only the paper fraction is considered, then only about 1% of ash would remain as waste. When burned, MSW produces emissions of particulates; sulfur oxides (SO,), nitrogen oxides (NO,), and hydrogen chloride (HCl) (acid gases); volatile organiccarbons, including dioxins and furans;and metals. All of these canbe andare controlled. About80% of MSW plants produce electricity, and since55% of overall electricity production inthe United States is from coal, the latter is the dominant fuel displaced. When MSW is burned to produce electricity, it produces 22 tons less pollution per megawatt per year than coal using current environmental regulations for each fuel for comparison. The majority of the pollution displaced is SO, and particulates. The amount of NO, produced is about the same. HCl and dioxin are increased compared to those for many coals. However, there are some coals with relatively high chlorine contents thatwould be expectedto exceed MSW dioxin and HClemissions. To put dioxin emissions in perspective, the 20-MW Alexandria, Virginia plant produces about 3 g of all isomers of dioxin3 annually. The same basic statements are true for heavy metals. Although the discussion aboveis for coal combustion,if all the facts wereknown the same could be said for oil. Energy. The energy costs of transportation are either equal to or slightly less than for the landfill disposal option; however, any savings would be negligible. When burned, paper proSO00 Btdlb. Since it haslow ash and low moisture, it burns duces energy. Paper contains about efficiently. There are no furnaces burning pure paper to produce energy; however, from calculations developed to estimate the efficiency of fossil fuel combustion processes, if pure paper were burned, it would be about 89% efficient. Therefore, burning 1 Ib of paper will produce about 15.5-18.6 r d k g (6700-8000 Btullb) of energy.
B. Thou
Shalt Not Believe in Total Resource Recovery
The eighth commandment, simply stated, is that waste is bound to occur in any system, and true waste must be discarded.
C. Thou Shalt Consider the Life Cycle The last of the commandments is that life cycle economic, environmental, and energy costs must be considered when decisionsare made for the management of municipal solid waste. If %hlorinated dioxin has 72 isomers that vary in the number of chlorine atoms and their placement in the molecule. 2.3.7,8-Tetradibed,ioxin is the most dangerousof the chlorinated dioxin molecules. The next most significant molecule is 100 times less dangerous. Further, dioxin is degraded by sunlight, with concentrationsdropping below detection limits in 6-8 hr.
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Table 5 EnergytoRemanufactureVersusFuelEnergy
[ d l k g (BWlb)]
Energy saved in Energy to manufacture remanufacture energy Fuel Paper Newsprint Cardboard Boxboard Plastic Steel Aluminum Glass
4.6(2000) 62.8(27,100) 2.3( 1ooO) 0 (0)
49.0 281.0(100,000) 2.9
19.7(8500) 17.4(7500) 17.4(7500) 0 (0) 0 (0) 0 (0)
there is one thing to remember, it is that everything goes somewhere.For energy, economics, and the environment, we must think in very broadterms. Few understood when very tallsmoke stacks were constructed onelectric generation plants in the Ohio River valley to disperse pollution as far as possible, that those plants would be later found to be responsible for acid gas depositions on the eastcoast and in Canada. We must honorofallour environment and not just the part in our backyard. There's a significant energy supply and a significant energy demand reduction opportunity available to the nation. Supply and demand are not opposites; they are complements. Both must be pursued. Experience is provingthat the use of waste as fuel and as a raw material are complementary. Introducing both intothe solid waste management system improves each. Removing grass and other wet yard waste for composting and cans and bottles for recycling actuallyimprovescombustion.Where there are recycling and energy markets for paperand plastic, the recovery for both uses is enhanced. The construction of oversized energy plants seen in the late 1970s has not been repeated, and the conflict between energy and recycling does not exist. Table 5 summarizes energy values. Should we bum all our waste? No. Should we bum some of our waste? Yes. Should we recycle all of our waste? No. Should we recycle what we can? Yes. Can we do without a landfill? No. The most useful system combinesthe advantages of use as fuel and use as raw material to minimize discards.
REFERENCES 1. EPA, Report to Congress: Solid Waste Disposal in the United States, Vols. 1 and 2, EPN530-SW88-011, Office of Solid Waste, U.S. Environmental Protection Agency, Washington, D.C., 1988. 2. EPA, Report to Congress: Management of Wastesfrom the Exploration, Development, urd Production of Crude Oil, Natural Gas, and Geothermal Energy, Vol. 1 , Oil and Gas, EPN530-SW88-003A, preparedbyVersar, Inc., forthe U.S. EnvironmentalProtectionAgency,Washington, D.C., 1987. 3. EPA, Report to Congress: Wastesfromthe Extraction andBenefication of Metallic Ores, Phosphate Rock, Asbestos, Overburden from Uranium Mining, and Oil Shale, EPN530-SW-85433, Office of Solid WasteandEmergency Response, U.S. EnvironmentalProtectionAgency,Washington, D.C., 1985. 4. EPA, National Screening Survey of Hazardous Waste "keatment,Storage, Disposal, and Recycling Facilities, 1986, EPN530-SW-88411, Office of Solid Waste, U.S. EnvironmentalProtection Agency, Washington, D.C., 1988. 5. EPA, Characterization of Municipal Solid Waste in the United States 1960-2000 (Update 1990). 1992, PB88-232780; EPN530-SW-88433, Franklin Associates, Ltd., Prairie Village, Kansas.
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6. EPA, First Report to Congress, Resource Recovery and Source Reduction, Feb. 22, 1973. Los AlamosNationalLaboratory, Los Alamos, NewMexico,personalcom7.Benson,R.A., munication. 8. Alter,H.,andDunn,J.J., Jr., SolidWasteConversiontoEnergy, MarcelDekker,NewYork, 1980. 9. Recycling Times,Biofuels: renewable fuels for the future, The Markets Page, Recycling Times,Nov. 17, 1992, Solar Energy Research Institute, Golden, Colo., 1992. 10. DOE, A Review of Comparative Energy Use in Materials Potentially Recoverablefrom Municipal Solid Waste, DOE/CS/20167-12, U.S. Dept. of Energy, Washington, D.C., 1982.
ADDITIONAL READING ANL (1983). Institutional Issues,Concerning Energyfrom Municipal Waste: A Status Report,Rep. No. ANUCNSV-TM-123, Argonne National Laboratory, Argonne, 111. Bogner, X. X. (1988). The US.Londfill Gas Resource: h - C o s t Biogasfrom Municipal Solid Waste, ANUCNSV-TM-206, Argonne Natl. Laboratory, Argonne, 111. DOE (1988). Five Year Research Plan, Biofuels and Municipal Waste, 1988-1992. EL4(1992). ManufacturingEnergyConsumptionSurvey:Consumption of Energy,1991, DOE/ EIA-0512(91), Energy Information Administration, U.S. Dept. of Energy, Washington, D.C. EPA(1985a). ReporttoCongress on theMinimization of HazardousWaste, EPN530-SW-86433, Office of Solid Waste and Emergency Response, U.S. Environmental Protection Agency, Washington, D.C. EPA (1985b). Summary of Data on Industrial Non-Hazardous Waste Disposal Practices, EPA Contract No. 68-01-7050, Work Assignment No. 11, Office of Solid Waste and Emergency Response, U.S. Environmental Protection Agency, Washington, D.C. EPA (198%). Waste Generators and Treatment, Storage, and Disposal Facilities Regulated Under RCRA in 1981, Office of Solid Waste, U.S. Environmental Protection Agency, Washington, D.C. EPA (1986). Waste Minimization: Issues and Options,EPA Contract No. 68-01-7053, Task No. 17, Prepared for theU.S.Environmental Protection Agency by Versar, Inc. and Jacobs Engineering Group, Washington, D.C. ofHazardous Waste Generators and Treatment, Storage, and EPA ( 1989a). 1985 National Biennial Report Disposal Facilities Regulated Under RCRA,Office of Solid Waste,U.S. Environmental Protection Agency, Washington, D.C. EPA (1989b). Office of Research and Development, Office of Environmental Engineering and Technology Demonstration, Dec. 22, 1989, Drafr Municipal Solid Waste Research Agenda, unpublished, U.S.Environmental Protection Agency, Washington, D.C. andReEPA (1991).National Survey of Hazardous Waste Generators and Treatment, Storage, Disposal, cycling Facilities in 1986,EPN530-SW-91-060, Office of Solid Waste and Emergency Response, U.S. Environmental Protection Agency, Washington, D.C. GAA (1991a). Resource Recovery Yearbook, Directory & Guide, Governmental Advisory Associates, Inc., New York. GAA (1991b).Materials Recovery and Recycling Yearbook, Directory& Guide, Governmental Advisory Associates, Inc., New York. Hasselriis, X. X. (1984). Refuse-Derived Fuel Processing,Butterworth, Boston. Hocking, X. X. (1991). Paper versus polystyrene: a complex choice, Science, 251, 504 (Feb. 1, 1991). Mishkin, X.,and Friedland, X. (1990). What the new EPA rules mean, Solid Waste Power, 4(1), 10. The Mitre Corporation (1978).Energy Conservation Waste Utilization Research and Development Plan, Mitre Corporation, Bedford, Mass. Office of Technology Assessment (19XX).Energyfrom Biological Processes,Vol. 2, Technical and Environmental Analyses,Superintendent of Documents, Stock No. 052-003-00782-7, U.S. Govt. Printing Office, Washington, D.C. K.,Director, Sussman, D., Director, Washington o f f i c e , Ogden Martin Corporation, and Walter, D. Waste Material Management Division, U.S. Dept. of Energy (1990). Personal communication. The
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experience at Ogden Martin plants is that the energy content of waste at Ogden facilities is incr to SO00 BWIb and higher.As a result, the Alexandria plant(seefootnote 2) now receives 800 tons/ day in lieu of the design 900. U.S. Bureau of the Census (1990). Stutisticul Abstructs of the United Stntes, 110th e d . Walter, X. X., Goodman, X. X., and Thomas, X. X. (1989). Thermal and Biological Optionsfor Advanced RDF Systems, AlChE Symp. Ser. No. 265, Vol. 8 4 . Williams, X. X., and Porter, X. X. (1989). Power Plays: Profiles of America’s Independent Renewable Electricity Developers, Investor Responsibility Research Center, Washington, D.C. Webster’s (1984).New Riverside University Dictionary,Reference Division, Houghton-MifflinCo., Boston, MA.
9 A Proactive Approach to Environmental Management: Meeting Environmental and Competitive Challenges William E. Schramm Oak Ridge National Laboratory Oak Ridge, Tennessee
Stella S. Schramm University of Tennessee Knoxville, Tennessee
1.
INTRODUCTION
American industry faces bothnew constraints and new opportunities. Over the past 20 years, Americans have become increasingly concerned about environmental degradation and more supportive of effortsto protect the environment. Government has responded to public pressure with far-reaching regulation restricting industry’s access to collectively owned resources such as air, surface water, and groundwater. Practices that threaten individuals or these collectivelyowned resources may be restricted even when conducted on private property. Such practices include the generation, use, treatment, storage,or disposal of inherently dangerous materials. Concurrent with these heightened environmental constraints, new international competitive challenges and limits on resourcesare increasing the performance pressure on firms. Conventional approaches to these challenges are proving to be limited in their effectiveness. Political lobbyingto block environmental regulationsor to raise barriers to foreign goods has become less effective as the environmental movement has become more politicallypowerful and as the concept of free trade has become more widely accepted. In addressing environmental objectives, industry has historically focused on end-of-pipe control (i.e., pollution control via the additionof treatment equipmentto waste streams at the end of the production process) rather than prevention. Yet, end-of-pipe control becomes an increasingly expensive means of meeting environmental standardsat higher reduction levels, limiting both the ability to comply with escalating environmental requirements and the abilityto compete with foreign producers.
The submitted manuscript has been authored by a contractor of the U.S. Government under contract No. DE-ACO5-
840121400.Accordingly, theU.S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for U.S. Government purposes.
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Although some suggest that firms may move operations abroad to reduce costs and avoid environmental regulation, the actual effectivenessof such a strategy is questionable. A recent analysis by Cole et al. [l] examines competition between firms based on different industrial organization paradigms, the mass production/scientific management (MP/SM) paradigm typical of the United States and the Japanese continuous improvement firm. The mass production/ scientific management form of organization uses specialization and hence functional division within the firm as a means of gaining greater efficiencyand reducing costs. The Japanese continuous improvement organization, in contrast, focuses on overall systemsof delivering product astherelevantscopeforanalysisandimprovement.Costreductions are achieved by improving process, product, and overall delivery performance rather than greater specialization in activities. Often continuous improvement firm improvements and their benefits are in the interrelationshipof activities within the firm, rather than falling withinthe scope of individual departments. Theseopportunities are likely to be deemphasizedor obscured by the functionally segmented analysis indicated by a scientific management approach. In contrpsting these two approaches, Cole et al. [l] predict a widening gap between MP/ SM and continuous improvement firm performance and point to a need to go beyond a narrow cost minimization and functional specialization approach to compete successfully.In terms of relocation, these findings suggest that if a change in focus is not made, a one-time reduction in costs from moving operations overseas may prove insufficient for an MP/SM firm to maintain competitiveness. The Cole et al. findings imply that in order to compete, firms must develop the abilityto continuously increase value provided to customers by improving all facetsof their operations. This chapter considers one aspect of operations, environmental management,where some U.S. firms may be developing such an ability. Recently, various firms have initiated environmental management programs thatgo beyond mere reaction to regulatory dictates, treating environmentally responsible corporate policies as not only economically beneficial but essential to corporate survival in a changing legal and social environment. Pollution prevention and waste minimization efforts are integral parts of these proactive environmental management programs.Pollutionpreventionusesproductreformulation,processmodification,chemical substitution, and equipment redesign to eliminate the generation of pollution at its source. Where it is not possible to completely eliminate waste streams, waste minimization uses these techniques as well as recovery systems, waste recycling, and reuse to reduce the amount of waste generated. firms toreevaluate their Thischapterconsiders (1) the changesthat haveledsome environmental management goals, (2) the basic elements and outcomes of proactive environ(3) theimplications of theseprogramsforindustrial mentalmanagementprograms,and competitiveness.
II. CHANGINGCIRCUMSTANCES This section considers changes that have influenced the business environment over the past 20 years. These changes include societal views on environmental issues, governmental responses to publicenvironmentalconcern (i.e., regulation), theavailabilityof data on pollutiongenerating activities, and the level of global competition. Although public opinion on the subject of the environment isdiverse, it is not inaccurate to suggest that the level and nature of public interest on environmental matters have changed. Generally, Americans have become less tolerant of pollution from all sources, particularly from industrial sources. In a democracy, industry, like government, ultimately operates with the consent of the people. The importance of retaining public confidence has been demon-
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strated as industries failing to adequately consider public sentiment have had their right to operate questioned. The chemical industry in North America experienced this. Surveys in the late 1980s indicated profound public distaste for the chemical industry[2]. Many industry leaders concede that the industry’s failure to meaningfully address public opinion contributed significantly to the passage of the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA) in 1980. This experience hasbeen repeated in other countries as well. In Canada, a public opinion poll in 1986 indicated that 48% of Canadians felt that the risks from the chemical industry’s activities in Canada outweighed the benefits. In Japan, pollution abatement is viewed as a moral obligation.Governmentholds an antibusiness attituderegardingpollution,although [3]. In western Europe, government in Japanis not inimical toward business on other accounts “green” parties, once considered fringe groups, have developed into a significant political force. Nor is growing environmental concern just a featureof industrialized countries. A recent [4] foundthatmajoritiesinbothdevelopedandless GallupInternationalInstitutesurvey developedcountriesgaveahigherpriority to increasedenvironmentalprotectionthan to economicgrowth.Respondentsalsoindicatedawillingnessto pay higherpricesforsuch protection. Governmentshaveresponded to publicdemandforenvironmentalsafeguardswith regulation.’ While environmental regulation in the United States has often meant increased operatingcostsanduncertainty, ithas also provided an improvedenvironmentalinformation base. Under the early U.S. environmental laws [the National Environmental Policy Act (1969), the Clean Air Act (1970), the 1977 amendments to the Clean Air Act, the Federal Water Pollution ControlAct (1972), and the 1977 amendmentsto the Federal Water Pollution Control Act that, among other things, changed that act’s name to the Clean Water Act], industry could readily and reliably estimate the costs associated with meeting environmental requirements, allowing decision making to proceed in a familiar manner. While the requirements of increasingly stringent environmental regulation led to rising costs for end-of-pipe treatment, these cost increases were small in comparison to the costs that resulted from the of passage CERCLA (the Superfund act) in 1980. CERCLA and SARA (the 1986 Superfund Amendments and Reauthorization Act) changed the economicsof U.S. environmental regulation and industry’s view of environmental responsibility and liability. Under CERCLA, the government imposed retroactive strict liability for actions that were legal (and in some cases common practice) at the time they occurred. Strict liability imposes legal responsibility for injuries even though the liable parties were not negligent and did not intend to cause injury. Courts have also held that landowners keeping potentially dangerous substances on their land that are certain to cause injury ‘Command mechanisms whichdominate environmental regulation inthe United States [5], mandate that all polluters achieve some specified standard of control on releases. The United States is not alone in its emphasis on command mechanisms; numerous other countries, including Japan [3,6], France, Germany, and The Netherlands [7],rely primarily on command mechanisms to achieve pollution control objectives. Many economists view command mechanisms as inefficient and wasteful of society’s resources because they require all emitters to achievethe same controlstandard regardless of cost to the individual firm or the cost per unit of pollution reduction. In contrast, market mechanisms suchas taxes, subsidies, and tradable permits provide economic incentives for desired behaviorsrather than mandating them. Economists generally prefer market mechanisms. However, the sharp contrast frequently made between command mechanisms and market mechanisms in economic discussions may be somewhat misleading because command mechanisms encompass a broad range of techniques, someof which are, like market mechanisms, fairly sophisticated and cost-sensitive[7].Market mechanisms, while theoretically more efficient than command mechanisms, represent an unknown. Governments havelittle experience with market techniques, and, unlike command mechanisms, market mechanisms lack a recordclearly demonstrating their effectiveness [7].
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to others or to others’ property if released, must make good the damages resulting from the escape of such substances. The potential environmental liabilities and uncertainty faced hy industry increased with CERCLA. While it was understood in 1980 that CERCLA’s imposed liability represented a substantial cost, its true magnitude was unclear. Time has clarified the cost; investigating and cleaning up old waste sites has proved so expensive that involvement in a Superfund site as a “potentially responsible party” may represent a liability sufficient to threaten the existence of many firms. Cropper and Oates [7] argue that the imposition of strict liability places an expected value on pollutingactivities. The expected valuewould be the discounted costof a potential future cleanup or compensation effort multiplied by the probability (<1.0) that the action would be conducted. Such an estimate may be incalculable within reasonable limits of certainty.* What can be ascertained, however, is that the potential liability may be extremely large. of liabilities in response to CERCLA, In addition toan implicit expected value assessment other factors, particularly uncertainty,appear to be important. Very largeliabilitieslimit its decision promanagement’s abilityto plan because of significantly increased uncertainty in cess. Firms frequently cannot accurately determine the probability of their involvement with CERCLA litigation, and it is difficult to effectively allocate resources when potential liabilities (recognized or unrecognized) may exist that could dwarf investment plans. Uncertainty negatively affects decision making because decision tools, such as cost-benefit analysis, cease to provide reliable information when necessary inputs (e.g., costs and cash flows) cannot be estimated within reasonable confidence limit^.^ The retroactive natureof liability imposition under CERCLA is particularly significant because it suggests that compliance withcurrent laws and regulations is not adequate protection against future retroactive laws [8]. Thus, firms do not know what standards their current actions may be held to in the future. The difficultyof managing this kind of uncertainty suggests that the potential for liability should be avoided. Some firms have determined that this can be done by minimizing releases of potentially offensive materials into the environment regardless of current regulatory requirements. The information available on industrial pollution in the United States has improved significantly in the past two decades. Regulation, particularly SARA Title 111, has been responsible for much of the improvement in data. This is because the statute requires the collection of information onthe volume of materials that industrialfacilities release into the environment or ship off-site. While the legislation does not require any pollution control activity on the part of industry, it sheds light on releases for two important groups: the public and boards of directors of firms. Public access to information has led to increased public pressure to curtail %e expected valuecalculation would require estimates of (1) the future release or emissions standard, (2) the probability that the firm would violate the unknown future standard, (3) the probability that the violation of the standard would be recognized. (4) the probability that the firm would be held responsible for the waste of others that may be commingled with that of the fii, (5) the future cleanup standard, and (6) the cost of meeting the cleanup standard. %is type of uncertainty differs from normalrisk. Risk as addressed in insurance actuarial tables is associated with a reasonably well defined probabilitydistribution. While uncertainty exists regarding an individual event’s probability of occurrence under normal risk, sufficient information exists to limit uncertainty regarding the overall probability distribution. This, in effect, allows the quantification of risk (via a probability) andenables managers to make reasoned provision for occurrences. However, significant uncertainty regarding an entire probability distribution is moredifficult to This uncertainty may occur because the information availableto define a probability distribution may be inadequate or unreliable. In the case of CERCLA or future CERCLA-like laws, uncertainty regarding the probability distribution for CERCLA site involvement limitsthe ability of fums to estimate their involvement risk and therefore to anticipate and plan for potential liabilities.
address.
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releases. Equally important, however, is the fact that data now exists that fully disclose the magnitude of releases to the leaders of firms. Boards have begun to realize not only that current practices may be setting firms up for future liabilities, but also that substantial quantities of valuable resources are, in effect, being thrownaway. These environmental constraints come at a time of increasing international competition. In addition to demands for greater environmental protection, the public also demands greater choice and quality in the products offered. Thus, industry must meet the challenge posed by public environmental expectations in the context of other public demandsand growing international competition.
111.
PROACTIVEENVIRONMENTALMANAGEMENT
Firms that have responded to the changed circumstances described above by implementing proactive environmental management programs are moving from a focus on problem management to a focus on problem elimination. These firms have set goals to eliminate both the environmental problem and associated costs. F m s have demonstrated that proactive efforts to eliminate pollution and reduce waste generation do not necessarily represent added costsand may enhance earnings and efficiency. Problems, in effect, become opportunities to gain further insight into operations and to suggest process improvements. Experiences of these firms offer insights concerning the specific requirements of successful environmental management efforts. This section describes the basic features of environmental management programs that successfully respondto both new environmental and competitive challenges. The essence of the approach will be familiar to those are proacquainted with pollution prevention and waste minimization. These “guidelines” vided by the experience of proactive firms. The implications of these proactive programsare discussed in later sections. The suggested guidelines include Treat the task seriously. Invest the time needed to learn processes in intricate detail. Set and clearly communicate goalshargets, and measure progress quantitatively. Empower people to uncover and fix problems. These guidelines are not revolutionary. Many would state “We do this now” and question whether such guidelines offer much insight. As with many undertakings, success in environmental management appears to be a matter of degree. It is likely that no firm applies (or ignores) these guidelines in its operations100% of the time. Rewards will likely be proportional to the consistent application of sincere effort. Successful programs will be those that obtain and sustain a strong commitment from the broadest spectrumof employees, particularly management. In other words, successful programs will involve a significant cultural change. The remainder of this section considers the guidelines listed above in more detail.
A. Treat the Task Seriously Success in the elimination of pollution depends on the effective efforts of all employees. If or environmental issuesare viewed as solely the responsibility of an environmental department if input to production process modifications are provided solely by process engineers, the needed commitment from other departmentsmay not be achieved. Further, if the firm fails to maximize its use of available information, it may miss important insights. To a large extent, the degree to which individuals within the firm accept the oftask eliminating pollution will depend onthe“signals”delivered by uppermanagement.Topmanagementmust be absolutely
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committed in word and deed and must be prepared to invest time andthe firm’s resources in the effort. It is the responsibility of management to (1) increase the environmental awareness of and (3) move the activitiesof the the firm’s personnel,(2) demonstrate the firm’s commitment, firm from discussion to analysis and action. Increasing awarenessof the problem means communicating with personnel frequently and describing a clear “vision” of the course the firm intends to follow. Minnesota Mining and Manufacturing (3M) states that the corporation’s ultimate environmental goal is to reduce emissions to as close to zero as possible. With regard to corporately owned and operated facilities, Martin Marietta has told its employees, “We are going to get out of the business of [generating]hazardouswaste.” The ChemicalManufacturersAssociation(CMA)hasannounced that with regard to the reduction of emissions, “Improvement will be continuous, there will be no cut-off in sight where companies can say we finished.” CMA notes that this effort is intended to be“a fundamental changein the way the chemical industry will do business” and has asked the publicto check on their progress. Theseare simple, vivid statements of intent. What they share is a commitment to move beyond regulatory requirements. This moves the focusof activity from oneof reacting to regulatory change to proactively addressing the source of pollution problems within the firm and eventually allowing the firm to regain control of its environmental agenda. Awareness is not a one-way street, however. Officials with responsibility for environmental management of several firms stress the importance of “putting light on problems” and indicate that this requires a willingness on the part of personnel to bring problems forward. Awareness of potential process modifications must, of necessity, move from the bottom up within the firm. For many employees, participation in a proactive environmental management program will involve a work activity, the contributionof their ideas, that isnew to them. Because the activity is new, workers maysee it as a risk, and it is upto management to provide an environment that to listen and management ensures that this contribution is possible. This demands a willingness that stresses problem analysisand the search for solutions rather than blame.If problem analysis is directed at finding out who is responsible rather than what process changes are needed to ensure that problems (including human error) do not recur, bottom-up communication is undermined.Some firms haveestablished“hotlines” or ombudsmansystems to provide paths for this information when employees do not feel they can communicate through their supervisors. The firm’s commitment to an environmental vision must be demonstrated by the active involvement of top management, the commitment of appropriate levels of resources, and other signals. A number of firms have taken opportunitiesto indicate that the issueof pollution reduction transcends economic considerations. For example, 3”s top management decided that credits earned for “excess” emission reductions (beyond regulatory requirements) that are not needed for future 3M expansion would not be sold, as allowed by existing legislation, but donated back to regulatory agencies. Obtaining credits provides thefirm with greater flexibility in meeting future production needs. The donation of excess credits sends a strong signal to employees and others concerning the firm’s commitment. At times, this commitmentmay appear to challenge short-term profit maximization goals. In Florida, Martin Marietta Corporation decided to terminate a printed circuit line’s daily 100,OOO-gal discharge to a private utility. Although discharge to a local water body probably would have been permissible, the company chose to eliminate most of the effluent through a closed-loop system using ion-exchange columns. The change not only eliminated the effluent concern but also improved process stability, significantly reducing wastage previously associated with the line. While savings as a resultof lowered wastage covered installation costs of the closed loop system within18 months, the potential for the high level of process stability even-
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tually achieved was notfully appreciated at first. Consequently, the short-term financial benefit was not initially recognized. It is this demonstration of a clear goal and the firm’s commitment that enables management and employees to focus their energies onaction. Time spent discussingthe relative merits of environmental statutes or regulations is time, energy, and focus that are not applied to the analysis of operational processes. Finally, the active participation of employees is encouraged by the knowledge that past employee input has been considered seriously and has to change. led Firms have used a variety of approaches to communicate this messageto employees, including posting descriptions of successful process changes along production linesor near the workstations of the responsible individuals, printing and circulating such successdescriptions among workers, or formally recognizing individuals involvedin successful changes.
B. Learn the Process The word “process” here is very broadly defined. It incorporates all of a firm’s activities, including management systems, product development, and means of dealing with customers and suppliersin addition to whatare typically considered process engineeringactivities. Learning the firm’s processes means becoming intimately familiar with all phases of operations, beginning with those process phases that involve targeted substances. This learning must cross all phases of firm operations. For example, the solution tothe costly disposalof expired chemicals may be better communication between users and purchasers anda change in purchasing procedures(e.g.,buyingchemicals in smaller quantities). Assessingprocessesnecessitates a process-by-process inventory and cataloging of procedures, inputs, products, and wastes and the development of a quantitative process baseline. Developingthe quantitative process inventory requires a significant upfront “investment,” primarily in the form of time. The CMA made the development of “quantitative inventories” the heart of its Code on Pollution Prevention, and it is anticipated that this will be the most difficult part of implementing the code for many firms [9]. The developmentof the process inventoryinitiates the firm’s learning process and is essential to pollution preventiodwaste minimization efforts. Time invested in the development of the inventory, however, is not an investment in the environment alone, since this information baseis fundamental to sound management and to process improvement efforts regardless of the specific goal.
C. Set Goals and Targets and Measure Progress Quantitatively Deadlines are effective motivators. History providesa myriad of examples, large and small,of seemingly impossible tasks accomplishedin limited time frames because a dedicated group of people were given a clearly defined goal, adequate resources, anda deadline. Goals for environmental performance have been put forth voluntarily and publicly by numerous firms. In mid-1988, 3M stated that, using 1987 as a baseline, it intended to cut all hazardous and nonhazardous releases to the air, water, and land by 90% and reduce the generation of waste by 50% by the year 2000. Martin Marietta committed itself to reducing emissions of 17 targeted toxic chemicalsby 50% by 1992 and 75% by 1995. (Martin Marietta met its 1992 goal in 1990 and anticipates beating the 1995 target as well.) Corporate officials of various firms indicate that such targets are essential as activity drivers and reminders of commitment. Measuring progresstoward a deadline is valuable not only because itserves as a reminder of the deadline but also because quantitative measurement provides feedback onthe direction future activities should take and aids in the definition of priorities. Tom Peters, a well-known management consultant, has noted simply that what gets measured, gets-done. Quantitative measurements support analytic thinking about relationships among problems and other factors.
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Quantitative measurement is also the root of statistical process control and total quality management. By examining the specific circumstances in which a problem emerges, insight can be gained into possibilities for change. In some cases, just measuring the problem is enough to precipitate change by giving employees feedback on operations. By clearly identifying the source and magnitude of undesirable effects, all involved have the opportunity to consider and recommend possible improvements.
D. Empower People to Fix Problems Empowering employees involves both communication and support. Communication includes defining the firm’s goals, as previously discussed, and communicating detailed objectives and process performance information. Information regarding processes is important to both the uncovering of potential areas of improvement and the development of solutions. Empowerment requires that this process-related information be available to all those affected by the problem or by the potential solution. This information sharing is vital if those who have direct insight into the processes being evaluated (such as line personnel and process engineers) are to give informed consideration to problem assessments and proposed solutions. Since it is extremely rare that one individual has all the information relevant to the evaluation of a situation and because solutions do not always fall neatly within departmental areas of responsibility, pooled efforts are required. Seeking the input of others and collectively assessing information and information needs supports employee efforts by limiting information gaps, oversights, miscommunications, and wasted time. Additionally, communication and input from the full spectrum of those affected is required to prevent situations in which different groups addressing related problems work at cross-purposes. As change that has not been properly considered may be disruptive, firms equip employees for proactive change by supporting the development of a discipline of thoroughness, preparation , coordination , and inclusion. Firms have also attempted to support employees by establishing clear procedures for raising suggestions and by training workers in analytic tools and methods, such as statistical process control, for uncovering and examining problem sources.
IV. Results and Implications of Proactive Environmental Management for Firm Competitiveness The experience of proactive firms indicates that significant reductions in the generation or release of wastes are not only possible but may be achieved within limited periods of time. With its voluntary Pollution Prevention Pays program, 3M cut its hazardous waste generation by nearly 50% between 1980 and 1990. The member companies of the CMA reduced total releases (air, water, land, and underground injection) by 35% between 1987 and the end of 1990. In a period of 3 years, Martin Marietta Corporation cut hazardous waste generation by 80% and cut releases of both ozone-depleting and toxic chemicals by 65%. In fact, in the first year of its hazardous waste reduction program, Martin Marietta cut hazardous waste generation by 47%. Numbers such as these suggest that substantive reductions are not dependent upon the development and implementation of complex new technologies. It is unlikely that such technologies could have been developed and implemented in one year. These results further suggest that the payback period on these efforts is brief, a suggestion confirmed by an EPA study of 28 firms that found that 54% of the waste reduction efforts undertaken paid back the initial investment in less than one year [lo]. While the solutions used to achieve these results were often innovative and imaginative, they frequently represented modest improvements in the way things were done, and surpris-
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ingly few were high tech. This suggests that long before high tech solutions can be developed, less technical solutions (e.g. , operational changes) are developed that are simpler, cheaper, and more readily implemented. It appears to many in industry with responsibility for environmental management that state-of-the-arttechnological capabilities are simply not a major limiting factor in determining the success of efforts to reduce waste generation. Further, although outside consultants may offer insights, the experience of firms seems to indicate that the identification and solution of most pollution problems are situation-specific and are therefore best developed internally by those with the most direct and detailed knowledge of the situation. The message appears to be “Have faith in your people.” The economic results, like the environmental results, of proactive environmental management have been impressive. Dow Chemical’s Louisiana Division reduced waste production by 250 million pounds and saved $5.2 million between 1984 and 1990. During the period 19751989, 3M Corporation not only cut its pollution per unit of production in half, but by its own accounting realized cumulative cost savings of $500 million. Cost savings occurred in the form of cost reductions in meeting environmental and worker safety objectives and reduced operational costs associated with more efficient resource use. For example, in Cottage Grove, Minnesota, 3M discharged an 8% ammonium sulfate solution into the facility’s wastewater treatment facility. Regulations required a reduction in the ammonia content of effluent. The company installed a system that removed ammonium sulfate from the waste stream before dilution in the wastewater treatment facility made recovery impractical. The compression evaporation equipment that was installed strips ammonia until a 40% solution, which is marketable as fertilizer, is achieved. The $1.5 million removal system eliminated the need for $1 million worth of downstream control equipment and produced annual fertilizer sales of $150,000. Positive economic results have been achieved by smaller firms as well. In 1986, CleoWrap, a producer of gift wrapping paper in Memphis, Tennessee, completed a conversion from solvent-based inks to water-based inks. The conversion reduced waste disposal costs by $35,000 annually, resulted in lower fire insurance premiums, and eliminated the need for underground storage tanks (USTs) for solvents. The elimination of USTs allowed the firm to avoid involvement with federal UST regulations. Additionally, firms may recognize increased revenues as a result of product improvements and the development of new market niches. For example, when Martin Marietta decided to remove chromium from various products produced for the steel industry, it was predicted that revenues would decline as a result of the action. Revenues did decline initially, until the steel industry recognized that products that met their specification without chromium content were available. Sales and revenues then rebounded and surpassed previous levels. Numerous corporate officials with responsibility for environmental management suggest that their firms would have been foolish not to make environmentally motivated process changes for purely economic reasons. However, the need for and benefits of change were not recognized until environmental concerns necessitated a search for process improvements and brought the opportunities to top management’s attention. This raises the question, which we consider later, as to why such large potential economic benefits went unrecognized for so long. The goal of merely controlling pollution is being questioned as firms address liability issues; recognize that pollution control, at best, transfers contaminants from one medium to another; and evaluate the waste of valuable resources under current practices. This is spurring many firms to move beyond a focus on regulatory compliance by establishing internal goals with the intent of exceeding regulatory requirements. In this way, firms may lift many regulatory requirements on operations and enhance their flexibility in planning and decision making. Efforts under way at 3M are explained this way: “Conventional control only constrains the [pollution] problem temporarily; it does not eliminate the problem.’’ With the objective of
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eliminating the pollution problem, 3”s “ultimate goal is to reduce emissions to as close to zero as possible.” In accordance with this goal, efforts have been initiated that “will take 3M from a position of compliance with government regulations to being substantially under the limitations established by the regulations.” DuPont Corporation, too, has stated that the “ultimate goal is zero pollution in all activities” [ll]. DuPont’s chief executive officer, EdgarS. Wollard, Jr., stated. The environmental challengeis not one of responding to the next regulatory proposal.Nor is it making the environmentalists see things our way. Nor is it educating the public to appreciate the benefitsof our products and thus tolerate to their environmental impacts and those of the processes usedto make them. Our continued existence asa leading manufacturer requires that we excel in environmental performance and that we enjoy the nonobjection-indeed the support-of the people andgovernmentsin societies where we operate around the world. I’m calling fora corporate environmentalism, which I define as an attitude and a performance commitmentthat places corporate environmental stewardship fully in line with public desires and expectations. In fact, entire industries have adoptedthis approach. The CMAinitiated their Responsible Care program in 1988, in part due to a recognition of the need to address the public’s perception of the industry. In this program, member firms commit themselves to “continually improve performance in the areas of health, safety, and environmental quality,” to do a better job of eliciting and respondingto public concerns about products and operations, and to make available to the public detailed performance measurements forthe purpose of judging industry progress. The actions that constitute proactive environmental management programs have much in common with the efforts of certain Japanese firms to improve product quality and process efficiency. The Japanese practices have been referred to as a “continuous improvement” approach to production. Similarities between proactive environmental management and continuous improvement involve establishing and committing to goals, integrating both improvement efforts and normaloperations across functional divisions,and continually analyzingoperations for appropriate modifications. Given the documented role of continuous improvement in improving the qualityof Japanese productsand the efficiencyof Japanese industry, itshould, perhaps, not be surprising that proactive environmental management has yielded benefits in areas of product quality and process efficiency as well as in achieving environmental goals. Theexperience of proactive environmental management indicatesthat improvements in process and resource use efficiency andin product quality that accompany proactive efforts change the economics of environmental management. Achieving environmental goals does not necessarily represent an additional cost and need not, in all cases, reduce othercorporate investments. This is significant because the opposite concept, that environmental management necessarily represents an added cost, has long been assumed by the management of many firms. With regard to international competition, Piasecki and Asmus [l21 note that many European firms combine economic motives and environmental goals through an emphasis on three principles: conserve energy, save materials, and reduce waste. In fact, some European managers “see waste reduction as a comprehensive strategy to cut production costs, spur innovation and promoteinternational competitiveness” [lo]. There is some precedence for thisview. Japan realized major benefits from actions its industry took in response to the oil price shocks and environmental legislation of the 1970s. In the early 1970s, the Japanese parliament passed antipollution legislationthat is among the strictest in the world. At the same time, Japan imported 99.8% ofits oil [6]. The oilprice increasesandinvestmentsforpollutioncontrol of the 1970srepresentedmassivecostsfor Japanese industry that threatened the profitability of firms and, indeed, their very existence.
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Japanese industry was forced to seriously address both energyuse and environmental quality, and the response of industry to these demands is credited with playing a significant role in inducing the rapid progress of Japan’s industrial technology [13]. These efficiency and technological gains positioned the Japanese to capitalize on changing market demands. As a result, Japan realized direct gains in competitiveness from their response to energy and environmental constraints. For example, Japanese standards for NO, emissions from automobiles, when initially established, were generally considered “unmeetable.” Nonetheless,in a matter of years, several automobile manufacturers had developed the technology to meetthe standards [3]. Japan’s domestic efforts to reduce air pollution gave ita competitive edge in the world automobile market [14]. This and the fuel efficiency of Japanese cars, which improved as a result of the energy constraint at home, contributed to Japan’s successful entry into the U.S. auto market.
V. OBSTACLES TO ADOPTING A PROACTIVE APPROACH Traditional management approaches and corporate cultures can present substantial obstacles to finding and implementing changes to existing methods. An emphasis on short-term financial indicators to the detriment of more fundamental measures of operational excellence, a culture of blame, and a rigid segmentation along functional or hierarchical lines can easily obscure opportunities to advance corporate performance. Short-term financial indicators by themselves are inadequate to provide insight into the opportunities for improvingfirm operations, and a focus on these indicatorsto the exclusion of other performance measures is one possible reason thatthe potential economic benefits of some environmentally related process changes went unrecognized for years and in some cases dean understanding cades. Yet the meaningful utilization of other performance measures requires by top management of the importance of operational improvement to the long-term financial stability and competitiveness of the firm. A focus on blame,rather than understanding how and why problems arise, precludes discovering inherent flaws in procedures. Firms need to determine, regardless of individual culpability, what in the management system allows problems to occur. Root causes are a matter of system characteristics that allow problems to develop and individuals to makeerrors. Evaluation that assigns an individual responsibility for a problem may provide a sense of having solved the problem while, in fact, the root cause remains undetermined and the flawed system unchanged. A lack of shared responsibility and cooperation resulting from organizational barriers between levels and stages of firm activities stands in the way of improving operations. Process success fromthe standpoint of overall system effectivenessmay be quite low even with superlative performance on the basis of individual departmental criteria, if those criteria fail to reflect overall system success. For example, implementing solutions to problems that are uncovered requires flexibility in the use of operational funds. If financial management systems place excessive restrictions on the application (or redirection) of funding, the motivation to implement improvementsmay falter. This may be a problem in any organization, but it is particularly problematic in government organizations. Environmental considerations are more effectively dealt with when integrated into every activity of the firm. A functionally restricted approach leaves environmental improvementas an activity apart from normal operations and gives no incentive forothers to take responsibilityor initiative in finding opportunities to eliminate problems before they begin. Overcoming these culturalbarriers depends on theattitudes and commitment demonstrated by top management within a firm. Efforts by others in the firm will not be sustained if cor-
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porate actions appearto contradict, downplay,or otherwise fail to support the task. The leadership needed to overcome these obstacles requires genuine commitment and vision by management in order to guide a rethinking of firm practices at the most fundamental levels.
VI. CONCLUSION In addressing environmentalobjectives, industry has historically focused on end-of-pipe treatment and compliance with governmentally imposed pollution standards. Changesin corporate legal responsibilitiesas a result of environmental legislation(particularly CERCLA) and evolving social views toward environmental issues worldwide have led many firms to question the premises on which they previously based their environmental management programs. A new premise-that environmentally responsible corporate policies are notmerely economically beneficial but essential to corporate survival in a changing legal and social environment-is emerging. As a result, these firms have focused not on regulatory compliance but onthe goal of eliminating waste generation and pollution resulting from their operations. The results of these proactive environmental management programs suggest that internalizing responsibility for the reduction of pollution and establishing stringent voluntary targets for reductions encourages actions that broadenthe environmental protection optionsinvestigated, enhance environmentalmanagementeffectiveness,reducecompliance costs, andlimit the uncertainty firms face in the environmental arena. Proactive environmental management programsstrive to use the skills and knowledge of all employeesby increasing employee awareness viaclearly defined environmental goals, analyzing all firm activities quantitatively, setting and measuring progress toward targets based on overall goals, and equipping employeesto uncover and solve problems. A growing body of evidence suggests not only that this proactive approach to environmental management improves environmental performance, butalso that the continuous process review employed improves understanding of operations, thereby contributing to more efficient resource use, reduced costs, and improved product quality. Thus, proactive environmental initiatives appear to have beneficial spin-off effects that improve a firm’s competitive capabilities.Thesenonenvironmentalbenefitsseemtoresultfrom characteristics of a proactive environmental approachthat are similar to the Japanese continuous improvement methodology for improving product quality and industrial efficiency.
REFERENCES 1.
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Cole, W. E., Mogab, J. W., and Sanders, R. D., The continuous improvement fwm: implications for economicanalysis,internationalcompetition, and technology transfer,WorkingPaper No.269, Univ. Tennessee, May 1992. Coombes, P,Responsible c m : a journey of profound cultural change, Chem. Week, July 17, 1991, p. 5 . hud’homme, R., Appraisal of environmental policies in Japan, inEconomic Growth and Resources, Vol. 5 , Problems Related to Japan (S. Tsuru, e d . ) , St. Martin’s Press, New York, 1978, pp. 193208. Dunlap, R. E.,Gallup, G. H., Jr., and Gallup, A. M., The Healzh ofthe Planet Survey, George H. Gallup International Institute, Princeton, N.J., July, 1992. Butler, W. A., Incentives for conservation, inEcology, Economics, Ethics:The Broken Circle (F. H. Bormann and S. R. Kellert, eds.), Yale Univ. Press, New Haven, Conn., 1 9 9 1 , pp. 180-195. Barrett, B. F. D., and Therivel, R., Environmental Policy and Impact Assessment in Japan, Routledge, New York, 1991. Cropper, M. L., and Oates, W. E., Environmental economics: a survey, J . Econ. Lit., 30(2),June 1992, pp. 675-740.
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8. Freeze, R. A., and Cherry, J. A., What has gone wrong?, Groundwater, 27(4) (1989). 9. Rotman, D.,Pushingpollutionprevention, Chem. Week. July 17, 1991, p. 14. 10. Hirshhorn, J. S., Cutting production of hazardous waste, Technol. Rev., 91, 52-61 (1988). 11.Kirkpatrick, D.,Environmentalism:thenewcrusade. Fortune, Feb. 12, 1990, pp. 44-55. 12. Piasecki, B., and Asmus, F!, In Search of Environmental Excellence: Moving Beyond Blame, Simon & SchusterlTouchstone, New York, 1 9 9 0 . 13. Watanabe, C., Santoso, I., and Widayanti, T.,The Inducing Paver of Japanese Technological Innovation. Pinter, London, 1991. 14. The Economist, The endless road a survey of the car industry, Oct. 17-23, 1992.
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10 Health Hazards Associated with Pollution Control and Waste Minimization Patick D.Owens Tosco Rejlning Company Martinez, California
I. INTRODUCTION This chapter focuses onthe need for engineering controlsto reduce the risk of adverse health effects due to occupational exposures at hazardous waste treatment, storage, and disposal facilities. From a management perspective, the best opportunity for preventing adverse health effects lies with engineers who can recognize the potential risks while designing processes for waste minimization. Oneof the first steps toward reducing riskis the awarenessof the etiology of the adverse health effect. In some cases, once an engineer becomes aware of a potential problem, the solution to reducing the risk is apparent. Therefore this chapter begins with a discussion of identification and prevention of health hazards. Examplesare presented of workrelated hazards found in general industry and in several emerging hazardous waste treatment technologies.
II. HEALTHHAZARDRECOGNITION In any remediation, pollution control, or waste minimization activity there exists the need to recognize and prevent occupational disease. When designing for chemical process pollution control and waste minimization, the engineer must consider the human element. Those designing processes for hazardous waste handling should consider the potential exposures of personnel operating the process. If engineers knowof the potential adverse health effects of the hazardous materials and other agents, then the design can incorporate methods to reduce these effects on workers and nearby residents. In many cases, the design stage is the critical time to incorporate safety measures. During the design stage of a Superfund remediation project,the of29 CFR designer is responsibleforunderstanding andcomplyingwiththeboundaries 1910.120 and for describing the reasons for the boundaries’ locations [l]. In addition, retrofitting a process is costly and time-consuming. Two examples of costly mistakes are the improvement of local exhaust ventilation in a chemical lab to reduce worker exposureto volatile 227
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organics each time the exposure limit decreases and the purchase of a less expensive, noisier motor that then requires a motor intake silencerto reduce the sound pressure level. In somecases, the etiology of a disease is attributed to a known agent. If a process includes that agent, the design goal should be to reduce all exposures to the agent. By identifying the specific agents present at a site and including ventilation, material substitution, or isolation, the designer may prevent the possibility of occupational disease. The recognitionof occupational diseaseand the steps leading to it is the key element in the prevention of the disease. Many occupational diseases are attributed to specific etiology and therefore are relatively easyto identify. Physical agents suchas electron beam sources thatlead to mutations are easily identifiable. Other hazardsarise from known exposures to identified air contaminants. For instance, eye and respiratory tract irritation arise from acute exposure to hydrogen sulfide in the petroleum industry. Asbestosis or lung cancer can result from yearsof or handling asbestos insulation. Other work-related adverse health effects have multiple causes unknown causes. For instance, increased renal cancerdeaths in the petroleum and steel industries have not been related to a specific agent [2]. The fields of epidemiology and occupational medicine provide guidance in determining specific causes of disease. However, when dealing with hazardous material and emerging technology, there is a need to recognize potential sourcesof exposures at a specific site. Once the potential sources of exposure and disease are identified, prevention is the next step toward a safer workplace. However, identifying the potential dangers for hazardous waste workers is difficult because hazardous waste streams vary from site to site and may vary from day to day at any one site. Of utmost importancein recognizing occupationaldisease is the identification of all agents present in the workplace that, alone or with other materials, are capable of causing adverse health effects. Agents are divided into two broad categories: physical and chemical. Common agents include noise, solvents, heavy metals, and temperature extremes. The list of harmful agents also includes ionizing radiation, infectious agents, and musculoskeletal stressors. At a typical hazardous wastesite a single physical agent, such as vibration, may not pose a significant risk; however, in combination withother stressors it may have an additive or synergistic effect. For instance, workers requiredto wear personal protective equipmentmay be more susceptible to heat illnesses, rashes, and allergies. For each physical task, answer the following questions: How close, how long, how often, and under what conditions? For each task or job, examine the workers’ interaction with their equipment. Jobs that typically lead to physical ailments include those that require repetitive motions, awkward positions, prolonged lack of movement, and/or exposure to extreme temperatures. The combinationof low temperature and repetitive motion have led torepetitive motion syndrome, for example.
A. Identification Identification of potential health hazardsa learned is skill, and proficiency in it can be attained by evaluating work practices. Many adverse health effects are due to acute exposure to contaminants,especiallyconcentratedchemicalagents.Whenever a concentratedchemical is present, there isa potential for harmful exposure. It is importantbe toespecially cautiouswith chemicals that require periodic replacement because of evaporation. Even light solvent evaporation may result in personnel exposure and adverse health effects. Solid material lost in processes involving cutting, grinding, drilling, or pulverizing may also cause adverse health effects. For instance, invisible carbide steel grinding fines may result in overexposure to agents that produce pulmonary sensitization and cancer. One way to check
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for airborne dusts is to observe surfaces for accumulated dust; however, this only signifies that a dust source is present, not the degree of hazard. Also, any chemical that visibly state, changes for example, from a liquidto a solid, may be an exposure point. Even if the material does not visibly change, heating a solid can liberate fumes. Off-gassing of new carpets, particleboard, and plywood has been implicated as the source of formaldehyde in indoor air quality complaints. Stirred, heated, or agitated liquidsmay generate mists, fumes,or fogs that are invisible to the naked eye. These and other processes are keyto identifying potential exposure sources.
B. Prevention Prevention of occupational disease should beone of the engineer’s objectives when designing a hazardous waste handling facility. The purpose of handling the hazardous waste in the first place is to reduce the riskof harm to humans. Therefore, it isof utmost importanceto protect those workers who typically work closely with the material. The design of these advancing technologies should intentionally and actively incorporate a concern for human health and safety. Asking facility operators for their input on new processes may go a long way toward maintaining a happy and healthy work force. In addition, the EPA “strongly endorses an open communication policy in which all health and safety inquiries receive a prompt, professional to adopt a new process when their response”[l]. Workers and facility operators are more likely ideas are incorporated into the design. Some people are skeptical of new processes because of experiences associated with past “wonder products” such as asbestos. Workers may adopt a new facility if they can examine the proposed design for hidden risks. And these risks, if any are found, should be addressed by the engineer. Newly legislated and anticipated occupational regulations pose challenges to the engineer who is designing a facility that should operate for several years. new A amendment soon to be law or an amendment to an existing regulation may instate or lower an exposure limit and thereby require retrofitting of existing processes or relocation of site boundariesand zones of contamination.
C. OccupationalRegulationsandRecommendations The Occupational Safety and Health Administration (OSHA) regulates occupational hazards. In doing so, OSHA defines exposure limits for substances, monitoring requirements, control methods, andmany other items. Engineers shouldbe aware of changing exposure limitsas well as control methods for hazardous environments. As an example of the former, consider the recent changes for the OSHA 8-hr Permissible Exposure Limit (PEL) for benzene, known a car10 parts per million (pprn) limit to l ppm. cinogen. Several years ago, OSHA lowered its Presently, OSHA is considering a 0.1 ppm limit, which represents a significant drop from the original exposure limit. OSHA also requires specific control methods for workers exposedto one-half the exposure limit, and the first choice lies with engineering measures. Ventilation, by the engineto reduce isolation, substitution, etc., mustbe the first line of defense employed exposure. However, an engineer who designs a complete ventilation system to reduce exposure from 50 ppm to 10 ppm could be wasting time and money if the exposure limit drops later. Therefore, to plan properly, engineers need to stay current with OSHA regulatory requirements, International Agency for Research on Cancer (IARC) classifications, toxicologic data, and epidemiologic studiesto determine to reduce the PEL. Undoubtedly, the safest engineering designs will reduce all carcinogenic exposures to as low as reasonably achievable. The Occupational Health and Safety Administration (OSHA), which guarantees the right to a safe and healthful workplace, mandates that the first step taken to minimize occupational may provide a exposure be “engineering protective measures.” A caretidly engineered facility
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low-risk workplace that leads to the prevention of occupational disease. On such case study in the proper designof a hazardous waste processing unit involved metals in a caustic stream[3]. In this design, ventilation successfully reduced worker exposure to hazardous waste.
1. Hazwoper In addition, OSHA requires safety and health activities for hazardous waste operations, including treatment, storage,and disposal facilities, uncontrolled waste sites, and also any emergency responses for hazardous materials. The regulations cover employee protection during initial site characterization, air monitoring for employees, precautions for material handling, training, and emergency response capabilities [l]. For instance, the employer must write a sitespecific healthand safety plan;however, a general plan may be used for all but the site-specific activities if the owner has multiple locations. Site-specific activities include analyzing tasks and warning workers of both the known and potential hazards presentat that site. This training requirement includes contractors. The final design should include (1) an estimateof increased hazards (over background),(2) degree of hazard based on the contamination,(3) standards for protection of workers and the public, (4) “safe” contaminant concentrations and levels measured at the site, and ( 5 ) emergency responseand evacuation plans. Also, the employer should provide an emergency response plan with written procedures for responding to on- and off-site accidents. Some of the specific requirements that can be included in facility plans are eyewash and shower equipment, spill and overflow control, storage location for hazardous substances, and specific processes for treatment and handling of hazardous substances. Facilities in which employees handle eye corrosives, irritants,or toxins absorbable through the skin must be provided with eyewash and shower equipment. The eyewash station must be located where an affected employee can get to it within 10 s e c . In addition, if an employee needs both the eyewashand shower simultaneously, the station mustbe designed so that the employee can use both simultaneously. The American National Standards Institute (ANSI)Standard 2358.1-1981 provides detailed information on eyewash station design and placement. Zoning remediation sites provides a clear indication of the locations of hazards. And, depending on the toxins present, zoning defines the level and type of personal protective equip ment. Also, it helps reduce the spread of contaminants into support zones where workerseat, be far enough fromthe source to resmoke, or drink. The contaminant reduction zone should duce exposure potential to decontamination workers. The contaminant reduction zone may encompass several hot zones provided that the workers follow the decontamination procedures for each agent [4]. Of course, workers exiting a hot zone must remove protective clothing in an orderly fashion to prevent hand or face contact, and the used clothing should be bagged and labeled to warn other personnel of the contentsof the bags. The Superfund Remedial Design (RD) and Remedial Action (RA) Guidance Document provides assistance to privateparties who are designing remediation measures. Currently, the a health and safety chapter. The pertinent items to address document is being revised to include at any remediation site include identifying potential worker exposures, making agreements with community emergency responders, and defining roles and responsibilities of all the involved personnel. The central theme is the identification of hazards via task analysis. One key design feature of a safer site is the regulatory site-specific health and safety plan, which includesmeteorologicalmonitoring,downwindandperimeterairmonitoring,personnelair monitoring, dust suppression, zoning, decontamination areas, and specific health and safety controls. are confusing. For Some of the regulations that pertain to hazardous material handling instance, two separate regulations applyto site remediation; these areOSHA‘s General Indus-
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try Standard (Part 1910) and Construction Standard(Part 1926). Both regulations apply during remediation projects; however, they cover similar points. When deciding which regulation applies, use the more protective standard. As another example,EPA fugitive emission standards presently require abatement of emissions from equipment that emits10,OOO parts of hydrocarbon per million parts of air (ppm). In contrast, the Occupational Safety and Health Administration (OSHA) regulates the workplace exposure to benzene at 1 ppm, andCalOSHAhas proposed a limit of 0.1 ppm. In some industries, workers required to abate the EPA “air toxics” emissions may be overexposed to benzene. In this case, the environmental and occupational acceptable concentrations differby four orders of magnitude.
ExposureLimits While OSHA Permissible Exposure Limits (PELS) carry the weight of law,the American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Values (TLVs) do in industry or who have not. A panel of scientists, physicians, and engineers who have worked knowledge of industrial processes develop the TLVs. OSHA has adopted many of the TLVs in the past, and OSHA’s 1910.120 regulation specifically defersto the use of the TLV when it is stricter than the PEL [5]. There are certain contaminants for which the PEL is more stringent than the TLV,andOSHAusually regulates these Contaminants as carcinogens. Hexavalent chromium compounds, formaldehyde, and benzene are a few of these chemicals.The best management practice consists of designing processes that attempt to reduce exposures to carcinogensto as low as achievable. The National Institute for OccupationalSafetyandHealth (NIOSH) also publishes recommended occupational exposure limits called RELs. addition, In the American Industrial Hygiene Association (AIHA) publishes a list of exposure limits called Workplace Environmental Exposure Levels (WEELs). Theselimits, developed and published by professionals in the field of industrial hygiene, do not carry the weight of law; however, similar to the TLVs, the WEELs cover some contaminants not addressed by OSHA. For instance, methyl tert-butyl ether (MTBE), one of the “clean gasoline” additives, has a WEEL but no PEL. When handling, concentrating, or treating a variety of solvents, one must be aware of certain potentiating health hazards. For instance, toluene and some chlorinated hydrocarbons act synergistically to induce liver disease. In these cases, extreme caution must be taken in comparing exposure monitoring results for each compoundto its respective occupational exposure limit. For agents acting on the same target organ with similar modes of action, a weighted average of the exposure limitsfor both compounds usually adequately representsthe degree of health hazard present. In addition to engineering control methods, OSHA requires special control of areas where known exposures to carcinogenic or regulated materials exceedthe exposure limits. Suchareas require posting of contaminant-specific warning signs and control of the personnel working in the area. The design of a work area should include clearly marked entrances and exits and locate personnel and equipment decontamination stations at exits from “hot” zones. In addition, areas not subject to remediation should beisolated, thus preventing exposure to workers outside the hot or contaminated zone. This may not always be possible, especially with airborne contaminants, but there are other methods for controlling these contaminants. Fixedpoint air-monitoring stations positioned at the edge of the hot zone or site perimeter with alarms set at appropriate levels have been used successfully at hazardous waste sites [6]. The latest technology for vapor or gas continuous monitoring is open-path Fourier-transform infrared spectrometry,which may be used as a short-term or hourly concentration-averaging device. When workers handle a chemical with an established PEL, OSHA requires personal air monitoring data to define each potentially exposed worker’s exposure concentration. If no site-
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specific data are obtained, an employer may be liable when specifying what level of personal protective equipment workers mustwear. When deciding how to design a waste treatment process, one must consider the toxicity of the material. If the material is regulated as a carcinogen by OSHA, then specific regulations apply. These require engineering controlsto reduce exposuresto as low as reasonably achievable. Although OSHA may not classify an agent as carcinogenic, other regulatory agencies, such as the International Agency for Research on Cancer (IARC), publish lists of suspected, probable, or possible carcinogens. If IARC identifies the agent as a potential or probable human carcinogen, the wise engineer takes steps to reduce exposures before OSHA promulgates regulations requiring engineering controls. A contaminant’s toxicityand physical and chemical characteristics determine how workers and engineers handleit to reduce possible exposure.If at all possible, engineers should design facilities requiring no additional chemical treatment. Engineers designing chemical treatment processes should consider using the most economical material with the highest occupational exposure limit. Often, the higher the exposure limit, the less toxicthe chemical. The engineer must also consider the route of intake by the chemical. If the route is inhalation, using a less volatile chemical may reduce exposure. Many exposure limit concentrations are so low that 0.1% lead in paint has overexposure may occur in unexpected cases. For instance, less than been shown to overexpose welders, cutters, or grinders to lead fumes and dust. Other contaminants may be organic compounds, pesticides,or minerals. Anothercase involved heating polyethylene floor covering, which liberated enough formaldehyde to approach the recently reduced exposure limit. The contaminants frequently found on hazardous waste sites include benzene, arsenic, asbestos, beryllium, cadmium, carbon tetrachloride, chloroform, chromates, cobalt, ethylene dibromide, formaldehyde, methylene chloride, B-naphthalene, nickel, vinyl chloride, and vinyl bromide. 3. HazardCommunication
OSHA’s Hazard Communication Standard (HAZCOM) requires a written program to describe and identify all hazardous chemicals in the workplace except Resource Conservationand Recovery Act listed waste. The facility must have a listof all the hazardous chemicals used in the workplace. There mustbe training methods for handling all these chemicals and suggested protective measures for possible exposure areas. In addition, all containers leaving the workplace must be labeled with the chemical identity, appropriate hazard warnings, and the name of the responsible party, Installing computers that have hazard material warning information stored i a database is one method of providing information about material stored in vessels or tanks. Material safety data sheets (MSDSs) for purchased chemicals must be kept in a readily accessible location for each worker. Engineers who subcontract work on a site should identify all contamination on the site and water, and/or inform workers of potential exposure. This may require taking samples soil, of the or other written informaair long before any work occurs. Hazardous material warning sheets be distributed to the workers or supervisors. In adtion on the site-specific contamination can dition, an emergency evacuation plan mustbe developed and discussed with the subcontractor.
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A. ChemicalAgents Chemical agents suchas solvents or hydrocarbons may have multiple effects on many different body systems. Solvents are ubiquitous at hazardous waste sites; therefore, there is a good chance that workers handling hazardous waste will be exposed to one or more solvents. For
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many of these agents, the adverse effect is seen relatively soon, so the cause of the effect is identifiable; with some carcinogenic agents that may cause cancer years later, the link between the agentand the disease is less clear. When workers handle mixtures of solvents, the etiology of the disease may be difficult to determine. In some cases, epidemiologic studies provide information on the etiology of the disease.
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1. Solvents Organic chemicals can cause many different adverse health effects. Solvents or hydrocarbons present at a site may cause specific symptoms suchas dizziness, eczema, and chloracne. One may see respiratory tract irritation from inhalation,or skin irritation from skin contact. Therefore, the routeof exposure determines,to some extent, the chemical’s effect and site of action. Skin diseases canbe reduced by designing equipment that reduces skin contact. Designs should eliminate the need for manual contact with material and avoid equipment that unnecessarily heats, agitates, or stirs a material. However, if engineering precautions such as local exhaust ventilation are implemented, workers should be adequately protected. For inhalation hazards such as air sparging of contaminated soil, engineering solutions may include ventilation of the off-gases. When engineering control methods fail to reduce exposures to below the OSHA PEL, workers must wear personal protective equipment. OSHA requires the use of properly fitted respirators only when an engineering solution is being developed or if all other engineeringor administrative controls fail. In addition to inhalation hazards, diseases occurwhen workers ingest hydrocarbons, metals, and other agents. Likely times include breaks when workers eat, drink, or smoke. Engineers should design facilities with break rooms or areas that are separated from the workplace by a washing station and some means of preventing dust, fumes, or mist from being carried by convection currents into the break area. Besides the organic agents posing ingestion hazards, some metallic agents can pose ingestion or skin contact hazards; examples are silver, arsenic, beryllium, cadmium, chromium, mercury, nickel, lead, selenium, tin, and thallium. In addition, insecticides, herbicides, and pesticides pose health hazards from food contamination. Workers handling incompatible substances have risks in addition to the potential toxicity. be stored far enough apart to eliminate the hazardof accidental Incompatible substances should mixing. Incompatible substances are those that, when mixed, react violently or evolve toxic vapors or gases, or in combination become hazardousby reason of toxicity, oxidizing power, flammability, explosiveness, or other properties. These chemicals should be stored in chemically compatible containers, and the location chosen to prevent accidental damage to the conto the tainer, for instance, avoiding high-temperature areas. Improper storage can leadevolution of toxic gases to which employees can be exposed when they open the container. Some emerging hazardous waste treatment processes such as reverse osmosis may potentially concentrate the hazardous material. Concentrated hazardous material poses health risks, and if improperly handled there is an increased risk of overexposure to the workers. In general, themoreconcentratedthematerial,thesmallerthequantitynecessaryforoverexposure.Filtering waste streams generates another problem. One potential exposure point occurs when workers replace contaminated filters. In addition, if the filters are air-dried, there is a greater risk of particulate exposure once the collected material dries. Also, workers’ potential exposures include the volatile chemicals offgassing from an overloaded filter. For that matter, any particulate matter, with the exceptionof reactive metals, when kept damp with water, should pose little inhalation risk. Where feasible, all solid hazardous material should be kept damp. When mixing substances that may splatter, splash, spray, boil over,or produce heat, shielding air or steam is used,as in spargis the first control priority for worker protection. If pressurized be used to prevent excess airor steam thatmay induce a violent ing, engineering controls must
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reaction. When the engineer lays out the controls for a process, he must locate them so that the employee will not be exposed to or come into contact with a hazardous substance. If the substance is a liquid, care must be taken to ensure that no employee will be exposed to liquid accidentally released from gaugesor other measuring instruments thatmay be under pressure.
2. Allergens A variety of airborne agents resultin occupational asthma. These include plant matter, animal products, and organic chemicals. The chemicals thought to produce an immune-type asthmatic and ethylenediresponse that may be presenting hazardous waste include formaldehyde, urea, amine [ 7 ] . Chemicals known to be respiratory tract irritants and that cause or contribute to asthma include ammonia, chlorine, sulfur dioxide, and hydrochloric acid. Some individualsmay develop an allergic dermatitis response to a chemical after being in contact with it or another chemical once. The initial exposure may be by an route-skin absorption, ingestion, or inhalation-as long as the chemical is absorbed in sufficient quantity. After the initial contact, the individual is said to be “sensitized” to the chemical. Thereafter, the chemically allergic individual experiences an adverse effectafter a brief exposure to a lower concentration than would normally result in an adverse effect. The sensitization reaction may occur at extremely low doses; however, a dose-response curve applies-a certain concentration a reaction. A number of chemicals can cause sensitization of chemical must be present to cause by external contact; these include formaldehyde, mercury(metallic, ammoniated, and bichloride), cobalt chloride, ethylenediamine hydrochloride, epoxy amine hardener, and alcohol [7]. 3. IonizingRadiation
Ionizing radiation at a remediation site may be a significant health hazard owing to its poor warning properties and nonspecificity of target organ. Radioactive waste on abatement sites at a site must be surveyed must be located andidentified, and all unknown materials discovered for radioactivity. Then the engineer should draw a plot plan that identifies all radioactive sources by zones indicating the degree of hazard. Some hazardous waste sites that may have radioactive materials presentare those with wastes from hospitals and dental offices, petroleum production pipe scale, or certain types of petroleum distillation scale and sludge, nuclear test sites, nuclear power generators, and some types of instrument manufacturers [S]. Instruments that haveemployedradioactivesourcesincludetelevision sets, smoke detectors, electronmicroscopes, and luminous painted dials. The survey meters most commonly used are scintillation counters and Geiger-Muller counters, both of which detect beta and gamma rays, and alpha counters for alphaparticles. Airborne alpha particle emitters pose inhalation and ingestion hazardsbut less of an external (i.e., skin) hazard. Beta rays, X-rays, and gamma rays may cause internal injury from external exposure becauseof their penetratingpower. The actual health effects due to radiation exposure depend on the quality factor,i.e., penetrating and linear energy transfer,of the source. Different sources, for example, betaor alpha emitters, have different quality factors and hence different degrees of health hazard for a given route of exposure. The dose equivalentof radiation, or the absorbed dose multiplied by the quality factor and other modifying factors, expresses theirradiation that directly causesthe adverse health effects[8]. When prioritizing risk from radiation exposure to design protective measures, the dose equivalent shouldbe used because it expresses the absorbed dose on a common scale. Shielding radiation sources from workers is one way to reduce or eliminate exposure. beta interact with the However, the type of radiation must be known because alpha and particles shield differently. Beta particles that strike other atoms, have a portion of their energy converted into X-rays, which are more penetrating [8]. A shield for high-energy beta emitters
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should be of medium density, i.e., water, plastic, or aluminum, to reduce the production of X-rays. It is possible for an engineer to design a shield specifically for that emitter’s energy and to successfully prevent most radiation exposure. On other the hand, gamma rays have no charge and therefore pass easily through matter, being stopped onlyby chance encounters with electrons. Gamma ray shielding calls for dense material such as lead or leaded glass. However, some portion of a large source of gamma photons will penetrate shielding, and the key to shielding is to reduce the concentration penetrating toan acceptable level. On the other hand, alpha particles usually do not require shielding becauseof their short penetrating powerin any material. However, inhaled or ingested alphaparticles pose a high risk of cancer. Ifengineering controls fail to protect workers from inhaling alpha particles, workers must wear respirators equippedwithhigh-efficiency particulate (HEPA) cartridges whoseefficiency for 0.3-pm aerodynamic diameter particles is 99.97%. Ionizing radiation at high dose rates has caused cancer, pulmonaryfibrosis, and pneumonitis[g]. Low dose rates may lead to adverseeffects on cell reproduction, andcells that undergo mitosis frequently, suchas bone marrow cells, sperm cells, and oocytes, are more susceptible to adverse effects than long-lived cells. One indicator of radiation exposure at moderate dose rates, i.e., 25 rem (radiation equivalent, man) is the frequency of chromosome aberrations in the lymphocytes of human peripheral blood. Acute effects may be observed at absorbed doses as low as 50 rad (radiation absorbed dose) if exposures occur rapidly, for example, at several rad per hour. Qpically, by the time the adverse health effects occur, it is too late for medical intervention. 4.
Dusts,Mists, andFumes Vibration of any dry material will generate some dust. The quantity of dust generated depends on how dry the material is and on how much force is applied in vibrating, cutting, or sanding the material. Invisible particles may penetrate deep into the lungto the alveoli andmay remain there for a considerable length of time. The majority of these particles are less than 5 pm in aerodynamic diameter. Fumes are solid particles having particle diameters of less than 1 pm, and they may remain airborne, and hence in the worker’s breathing zone, for hours. These fumes may increase in concentration in the breathing zone throughout theday unless there is adequate dilution ventilation. The potential health hazard posed by dusts or mists depends not only on the mass concentration but also on the particle size. Because not all dust clouds contain the same size particles, there are twoindices for dustconcentrations,total and respirable. The total dust concentration is the mass of dust collected on a filter divided by the volume of air sampled. However, the respirable dust index represents the concentration of dust that can penetrate through the nose and trachea to the alveoli. This index is the fraction of the total respirable particles that pass through an aerodynamic size-selective sampler.For a thorough analysis of the hazard potentialof a specific dust source, the engineer measures the size distribution using a size-selective sampler,which collects particles in specific size ranges and therefore allows the industrial hygienist to determine the percentage of the total dust that is respirable and other peramaters. Given two identical dust concentrations, the greater potential health hazard lies with the concentration consistingof a larger quantity of smaller aerodynamic diameters. The health hazardmay be greater if the dust containscrystalline silica, metals, or organics that have absorbed ontothe dust. For instance, charcoalused to absorb organics posesa greater risk than clean activated charcoal. Also, one must be careful when handling dusts that may contain low percentages of heavy metals. There existsa risk of workers taking the metal-laden dust home with them, thereby exposing children and other adults the to hazard. Good personal hygiene, including after-work showering and the use of disposable coveralls, should reduce the risk of taking toxins home.
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There are two forms of silica, crystalline and amorphous silica, and they differ in their toxicity. Amorphous silicais similar to the sand on beaches, in cement, and in ordinary soil and has a relatively low toxicity. Crystalline silica has a different molecular structure and is more toxic via inhalation. Crystalline silica may be formed by heating amorphous silicates such as kawool wool. Silica that has been heated above 1400"for a considerable length of time forms crystalline silica in varying percentages. However, thereis some evidence that heating to higher temperatures produces amorphoussilica, which is much less toxic than crystalline silica [lo]. Crystalline silica causes chronicsilicosis, a disease that usually occurs inthe upper lobesof the lungs from long-term exposure to low concentrations. Short-term, extremely high concentrations from worksuch as sand blasting have caused acute silicosis. Unlike typical silicosis, which develops into measurable health effects years later, acute silicosis may be diagnosed within 6 months to 2 years after exposure. The adverse health effectsare similar for acute and chronic silicosis. When handling a mixture of crystalline silica and amorphoussilica, one must adjust the exposure limit to account for the proportion of crystalline silica in the dust. The greater the percentage of crystalline silica, the lower the concentration thatmay cause adverse health effects, and therefore the lower the exposure limit. Crystalline silica is not usually found in most typical earthen materials such as diatomaceous earth. However, when diatomaceous earth is heated to remove organic material such as absorbed hydrocarbons, crystalline silica forms. It is considered fibrogenic and may lead to silicosis. Furnaces lined with fire brick made of silica have the potential for crystalline silica exposures. The same is true ofany refractory lining material in kilns or thermal oxidation chambers. When the temperature of refractory ceramicfiber insulation rises, such as in an incinerator, furnace, or heat exchanger, some percentage of the insulation converts from amorphous to crystalline silica. Thus, when workers mustenter equipment that was heated to above to dust. about 1400" for any length of time, they must take precautions to reduce their exposure This can be done by wetting, isolating, or ventilating the material. In addition to dust from silica, workers must avoid exposure to wood dusts, especially wood treated with fungicides or bactericides. Ordinary wood dust has been associated with oak, mahogany, California redwood, asthma in some susceptible workers. These woods include andWestern red cedar [9]. Inhaled wood impregnatedwithfungicides or otherorganics may pose a health hazard greater than the hazard for wood alone. The wood dust acts as the carrier for the organic chemical, whichmay remain in the lungs fora long period of time. As the chemical leaches from the wood pores, the lung absorbs it, and it then enters the blood. Any abrading of wood may release particles small enoughto penetrate to the alveoli and remain there. poWhen designing crushing processes, one must always consider the combustible hazard tential and the associated control measures requiredby OSHA. If the concentration of a flammable or combustible liquid or powder has the potential to reach 25% of its lower explosive limit, then specific control measures are designed into the process. First, there must be no source of ignition, and the potential for static charges must be eliminated by bonding and grounding all equipment. This includes all machines, air hoses, cleaning systems, and air nozzles. In addition, equipment expected to contain an explosive concentrationmust be designed to explode away from personnel performing normal duties. This same equipment should be ventilated by a permanently installed groundedvacuum cleaning system. 5 . Catalysts The destruction of organics by catalysts poses potential health hazards due to the catalysts or dust liberated duringcatalyst transport. Nickel, cobalt, chromium, and vanadium catalystsmay be oxidized, thus releasing gasesor fumes that may cause a variety of responses including res-
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piratory diseaseand skin sensitization.The toxicity of metals from catalyst dust depends on the dust’s lipid solubility and the rate of reaction of the toxicant in the body. Metal compounds that are lipid-soluble are more readily absorbed into the body to cause internal reactions [9]. However, catalysts dusts that are not lipid-soluble may also lead to adverse health effects. For instance, a high concentration of nickel dust results in a skin reaction that looks like chronic eczema, i.e., redness, itching, and scaling. fipically, only the Ioading and unloading of catalyst from sealed reactors poses an inhalation and dermal exposure risk to catalyst dust.A vacuum truck transfer from the reactor reduces the inhalation risk. In addition, the workers should be protected from inhalation and dermal contactby the use of supplied-air respiratory equipment and full body suits. 6. Metals Although thereare metals inmany hazardous wastestreams, the potential for exposureto hazardous concentrations varies depending on the matrix. Metals may be bound in an inorganic matrix and pose little riskor exist as a fume in the oxidized state with the potential for carcinogenicity. For a specific metal, the more soluble it is in the body, the greater the toxicity. A metal is soluble in the human body only if it is in a specific oxidation state or bound to an organic molecule. When the element is bound to an alphatic hydrocarbon or nonpolar hydrocarbon, such as tetraethyllead, the potential for skin absorption is greater than if the element exists alone or is bound in a polar molecule. In some instances, its oxidationstate determines to a great extent an element’s toxicity in the body. For instance, hexavalent chromium is a known carcinogen, but the trivalent and divalent states cause no cancer[9]. There are a few known potential exposure points for metal intoxication, and inhalation of metal oxides poses a health risk. Ingestion of high enough concentrations of a metal to cause This is especially true an effect, although rare,may occur at sites with poor washing facilities. for metals that tend to bioaccumulate, such as lead. Whenever lead-contaminated waste contacts the skin, the potential for ingestion exposure exists. Burning any material containing metalsleads to theformation of metaloxides,whicharerespirablebecause of theirsmall aerodynamic diameter. Incinerationof metal-containing waste may generate nonhazardousfly ash, which canbe recycled; however, care should be taken when handling the feedstream. The site of transfer of the waste from the tank trucks to the incineratorand the vent openingsare two potential, albeit usually insignificant, exposure points.
B. Physical Agents 1. Noise In almost all industrial facilities, noise is a common agent. However, engineers can reduce noise exposure easilyif they address itin the design stage.The engineering design stage is the crucial timeto plan for a quiet process unit. The noisy process unit usually contains sources of noise that could havebeen much more easily fixed before construction than after.In designing are quieted by (1) designing gradual fluidflow,especiallyturbulentflow,vibratingpipes bends, (2) placing control valves as far as possible from bends, (3) using pipe with gradual expansions and contractions, and (4) using oversized pipingto reduce fluid turbulence[ll]. A simple means of quieting an electric motor to is install silencers on the motor’sair intake and exhaust. Motorsare highly directional noise sources, and in some cases an operator’s exposure is reduced by simply pointing the cooling-air intake toward an unpopulated area. Steam-driven motors exhaust steam in bursts that produce pulsed noise. A steam silencer, similar to an exhaust muffler on a car, disperses the pressure wave and hence reduces the noise level. Overall, the f i t choice in abating noise is to silence the source;however, isolation may be beneficial,
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especially if the source generates toxic dusts, mists, fumes, or gases. If engineering controls fail to reduce noise levels, personal protective equipment such as earplugs and muffs may be recommended. Vibration 2. The potential of overexposure to excessive vibrations at hazardous waste treatmentfacilities is rare. However, the engineer designing equipment shouldbe aware of the hazards of excessive vibration. There are specific ranges of frequencies of vibration that induce adverse health effects. Depending on the frequency and source, the effects may be whole-body or segmental, e.g., affecting only the hands. As a rule of thumb, the smaller the body part, the higher the frequency needed to produce an effect. For instance, allof the body organs tend to resonatein the range of 2-100 Hz; however, the torso resonates predominantly in the lower frequencies. The human eye resonatesat 60-90 Hz,and the head tends toward this higher frequency range. Excessive exposure of the eye to vibratibns in the 60-90-H~ range is associated with vision disorders [2]. Whole-body vibration exposure may cause digestion problems and changes in nerve conduction and bonestructure. In addition, segmental, i.e., hand and foot, exposure to low frequencies of vibration has led to tenosynovitis, bone cysts, and Raynaud’s phenomenon, i.e., vibration white finger [12]. Engineering control measuresare the first line of defense against excessive vibration exposure, and administrative controls are second. Equipping heavy machinery with seats that ride on a cushion of air or water helps dampen the motor vibration. Operators of heavy tractor trailers hope that these and other engineering solutions will reduce back, neck, and shoulder distools, such as pneumatic hammers, that orders associated with the trade. The use of hand vibrate in the 40-300-Hzrange may lead to Raynaud’s phenomenon. It is a reversible disease in the early stages, but continued exposure can leadto permanent hand disability. In addition, simultaneous exposure to cold can enhance the progression of the disease. Engineering measures to cancel or dampen the source of the vibration are the most appropriate. Personal probe effective in all cases. tective equipment suchas antivibration gloves have not been shown to If the temperature is cool, the firststep should be to increasethe ambient working temperature. In the new groundwater vibrorecovery process, engineers can reduce worker exposure to whole or segmental vibration and thereby reduce the risk of adverse health effects. 3. ConfinedSpaces
The definition of a confined space takes into account the potentialfor an oxygen-deficient or displaced atmosphere. Thusthe conventional definition includesmany areas not typically considered confined. Oxygen-deficient atmospheres may be present dueto displacement by another gas source or by an oxidation reaction, and both of these may not be readily apparent. OSHA’s confined space standard required employersto evaluate confined spaces for oxygen deficiency or other hazards prior to entry of personnel. A potentially oxygen-deficient worksite is evaluatedwithdirect-readingoxygensensors. A confinedspaceshouldbeequippedwith continuous-reading oxygen sensorsif there is a possibility of toxic gas releases. The monitor should be set to alarm when the workplace oxygen concentration dropsto 19.5%. Typical adverse health effects, including inattention and increased heart and ventilation rates, begin to develop in oxygen-deprived atmospheresof 16% [9]. Symptoms progressto nausea, vomiting, unconsciousness, and deathwhen the oxygencontent drops below 16%. Examples of confinedspace hazards foundat hazardous wastesites include vessels, scrubber towers,tanks, trenches, sewers, drains, and building basements. Normal oxygen content ranges from 19.5 to 21%, but in the presence of some hazardousmaterials, oxygen is displaced orits concentration reduced by oxidation.
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In addition to oxygen deficiency, confined spaces pose other health hazards. For instance, stabilization reactions may leave a caustic material in the vessel. If soil has been ground in a vessel, workers may be exposed to metals, silica, or fibers from the fine powder remains. Dusts from soil vitrification processesmay create an explosive, if not oxygen-deficient, atmosphere. Some solid materials usedas catalysts or additives in soil stabilization will explode at certain concentrations. Hydrogen sulfide may be generated in some soil treatment processes that use acids mixed with sulfur compounds. This toxic gas has good warning properties at first due to its odor, but the sense of smell is diminished after prolonged exposure, at which time the concentration may increase enoughto lead to respiratory center paralysis.Any vessel or tank that needs cleaning after use poses hazardsbeyond those associated with the contained material. The safest method for preventing health hazards due to oxygen deprivation isto ventilate the space until the oxygen content is above 19.5% and to monitor the air continuously. If work must be done in an oxygen-deficient atmosphere, supplied air-breathing equipment operating in the positive pressure modemust be worn. In addition, workers must wear emergency respiratory breathing equipment to prevent asphyxiation in the event of the failure of supplied air equipment. Ventilation of the confined space is the key to providing a working space safe from oxygen deprivationor other inhalation hazards, provided the ventilationrate reduces the contaminant concentration in the vessel to a safe level and prevents exposure at the exhaust of the ventilation. One recommendation is to provide direct communication between workers inside the confined spaceand attendants outside. Also, all workers should wear a harness with a line attached so the attendant can remove a worker without entering the vessel. There have been numerouscases of fatalitiesofrescuerswhoprematurelyenteredaconfinedspace.One confined-spaces improvement is the use of structural support for pulleys above manholes, the idea being that the attendant can easily remove a worker by using a block and tackle. Heat Stress Heat stress may cause illnesses ranging from behavioral disorders to heat stroke and possibly to death [13]. Heat stress can leadto a range of reactions, and workers who are acclimatized are more heat-tolerant. The mildest form of heat stress is transient heat fatigue, which manifests itself as impaired mental and sensorimotor performance. Workers exposed to temperatures to the heat referred to above normal body temperature over several weeks develop a tolerance as heat acclimatization. This increases the heat exposure a worker can tolerate before experiof acencing disorders. A few days without heat exposure can substantially reduce the degree climatization, but even after weeks without heat exposure, some heat tolerance remains[13]. Skin reactions to heat include rashes, i.e., “prickly heat,” and anhidrotic heat exhaustion, or when the which is a rare disease. Rashesmay occur more frequently in humid environments skin is constantly damp, as when chemical protective clothing is worn. Workers wearing personal protective clothing that does not allow passage of moisture are more susceptible to all forms of heat stress than workers wearing water-penetrable clothing. Heat cramps and heat body moisture and minexhaustion aretwo reversible illnesses that result from excessiveofloss erals. When the body failsto regulate its temperature, heat strokeor heat syncope may occur. Heat syncope, or fainting while standing, may occur in physically fit individuals who are not acclimatized because blood flows away from the brain toward the extremities to cool thebody. Symptoms of heat stroke include hot dry skin and an elevated core body temperature, which may cause loss of consciousness leadingto coma. Effective engineering means of reducing these effects include shielding, cooling the surrounding air, increasing the workspace air velocity, dehumidifying the surrounding air, and wetting the skin surface. Engineering a cooler environment, which lessens the evaporative sweat loss, is one solution to reduce these adverse effects. Provided atmospheric humidity is rela-
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tively low and air temperature is less than skin temperature, increasing air velocity provides increased evaporative cooling. In environments in which radiant heat loadingis a contributing factor, such as the area adjacent to a furnace, shielding consisting of reflective material may significantly reduce heat stress. Radiant heating arises from objects that are hotter than skin temperature. Shielding designed to reflect light provides the most efficient protection from radiant heat. One exampleof a material that provides radiant heat protectionis aluminized Kevtar. In humid environments, dehumidifiers reduce ambient moisture content and effectively increase the body’s evaporative sweat rate, thus providing cooling. When engineering controls fail to provide a safe environment, administrative controls, such as task rotation or mandatory rest periods, are the second means of hazard control. If administrative controls failor cannot be implemented, the last resort is the use of personal protective equipment. This includes coolvests, vortex coolers, icepacks,andotherdevices. also wear personal Workers wearing level A,B, or C chemical protective equipment frequently protective equipment such as cool vests, vortex coolers, and ice packs.
IV. HEALTH HAZARDS ASSOCIATED WITH EMERGING TECHNOLOGY In this emerging field, new technologies may cause occupational diseases that may not have readily identifiable causes. In addition, hazardous waste contains toxins from many different industries simultaneously; therefore worker exposure can include exposure to toxins from multiple sources. And the waste stream that workers handle often varies from day to day, making it difficult to identify the toxic agent(s). Besides the usual chemical hazards, since emerging technologies use engineering froma variety of industries, workers need to be protected from the various physical agents discussed in the preceding section. For instance, the use of vibrating tools to liquefy soils for remediation may lead to Raynaud’s syndrome and is in contact with or loweringthedesignfrethe worker [12,14]. However, a simplemodification-raising quency-can significantly reduce the risk of adverse worker health reactions.
A. Trenching Trenching at remediation sites poses hazards in addition to the potential oxygen deprivation. The major concernis a cave-in, but other concerns include uncovering contamination,electric lines, gas lines, etc. OSHA regulates trench design in Title 29 CFR 1926 Subpart P [15]. The conditions likely to lead toa cave-in include loose soil, previously excavated soil, moist soils, vibrating equipment placed close to the trench, and trench sloping inappropriate for the soil tYpen
B. AirStripping Air stripping of groundwater may expose workers to hydrocarbons when work is carried out in the vicinity of the effluent. There are several potential exposure points, including sampling points, leakage from lines, monitoring wells, the insides of vessels during maintenance, and effluent vents. The design of sparging wells should include assurances that the extraction well pressure will always be less than that of the injected air well. If the extraction line includesa dilution port, a check valve should be added to prevent vapor release when the blower cycles be conon and off. Air emissions from the extraction well pose inhalation hazards that should trolled by engineering means. If activated charcoal is the absorbent used, one should consider the exposure of the workers to carbon dust saturatedwith hydrocarbons. Respirable dust thatis saturated with hydrocarbons may be more hazardous than carbon dust. In addition, the worker is potentially exposed when disconnecting the inlet hose tothe absorbent container. Exposure
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risk should be reduced by installing a block valve on the upstream side of the inlet flange. Measurements of the effluent should be obtained to determine whether carbon breakthrough has occurred. If the vapors are released directly to the air, measurements should be taken to ensure that the concentrations are well below occupational exposure limits. Also, fittings at pumps, valves, and flanges should be monitored for leaking hydrocarbon streams because theseleaks may become a source of operator exposure. Volatile hydrocarbon process units that include valves, flanges, and pumps must be maintained to reduce emissions of what the EPA calls “air toxics” The EPA refers to these emissions fromidentifiable sources as “fugitive emissions” and limits the concentration emitted to 10,OOO parts of contaminant per million parts of air. However, the occupational exposure limit for some toxic air contaminants is 1 ppm. The EPA requires certain pumps and seals to be installed for specific process streams to reduce emissions. These and other measures should reduce operator exposure to fugitive emissions. Engineers should be aware of the new pump systems designed to reduce process stream losses through the pump seals. Also, selection of appropriate flange gasket material may reduce operator exposure because a gasket that slowly dissolvesmay leak product. Engineers designing new processes should consider the possible presence of chemicals that were in limited use until recently, such as methyl tert-butyl ether (MTBE). For instance, MTBE swells Viton gaskets, which may lead to leakage and failure at flange seals. Designing towers, vessels, and drums with large manholes reduces worker exposure by providing better ventilation. Maintenance may consist of welding, chemical washing, and/or packing replacement. OSHA requires confined space ventilation to provide an atmosphere free of toxic gases and fumes and to supply sufficient oxygen. The typical method for providing a large volume of air is by general dilution ventilation. An air-stripping tower should have many manholes, and, if possible, they should be spaced evenly throughout its length to offer ventilation options. The ideal ventilation is local exhaust or supply ventilation for each worker, but this usually requires ductworkentering the manholes. The manhole should be large enough that the worker can make an emergency escape while the ductwork is inthe manhole. Ventilation supplied todry a tower after chemical washing requireslarge volumes of air to be drawn through the tower. As long as all the air is flowing in the same direction, for example, top to bottom, the tower should dry quickly. The larger the volume of air, the faster the tower will dry, and once it is dry the maintenance workers can begin work inside. When the air flows from the ground up, the air laden with chemical cleaner exhausts higher above ground and, ideally, away from personnel. However, if the tower contains puddles of cleaner at the bottom, bottom-to-top air flow may spread the cleaner throughout the tower, thus increasing drying time. When designing the sample points, it is important to provide more than adequate local exhaust ventilation to eliminate operator exposure. This is a good idea if carcinogens are present, because their exposure limits are gradually being reduced to “as low as feasible.” Therefore, the design of sample pointsin processes should be based onthe toxicity of the specific chemical(s). For instance, samples of highly flammable chemicals must have ventilation provided by nonsparking means, and sampling points for acutely hazardous chemicals should have closed-loop line purging and valves that spring closed upon release.
C.PhotolyticDegradation Photolytic degradation uses sunlight or artificial sources of ultraviolet (UV) light to initiate [16]. These reactions occur faster when the process photochemical reactions in hazardous waste uses shorter wavelengths or more energetic light. One concern when using high-energy light is
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the possibility of eye exposure and subsequent injury. Sunlight contains a variety ofwavelengths of light, some of which may cause biological damageif viewed for extended periodsor if concentrated. Focused sunlight may form energy concentrations of up to 10 MW/m2. l", naked sunlight can cause photochemical blue light If the viewing angle is less than injury. Photochemical blue light injury to the retinaof the eye is primarily the result of exposure to wavelengths of 400-550 nm. One proposed method of detoxifying hazardous waste is to concentrate sunlight into a narrow beam. There are two potential health problems associated with concentrating the sunlight on Earth: (1) the increase in the effective vewing angle,which potentially increases the absorbed irradiance; and (2) the greater intensity associated with increasing concentration. For photochemical blue light injury, the lowest irradiance known to MW/m2. A retinal burn may result if the sun is magnified so the viewcause injury is 1 X ing angle subtended by the sun is 4" or greater. For instance, viewing the sun through a binocular or other magnifying lensmay cause permanent damage to the retina. Viewing a source of magnitude greater than 0.01 MW/m2 overa viewing angleof 4" has caused retinalbums, and a viewing angle of only 5 minutes (5") with a source greater than 1 MW/m2 leadsto the same effect. Thus, the engineer should consider adding eye-protective mechanisms to systems that use magnified UV sources. Besides the thermal injury that canresult, skin exposuremay cause bums andlead to skincancer. Protective equipmentcouldincludepolycarbonateeyewear, which absorbs more than 99% of the incident UV light and most of the infrared. Complete enclosure of the concentrated sourceis an option, and a fail-open switch should prevent accidental exposure when the enclosure is opened.
D. UltravioletOxidation Ultraviolet (UV) oxidation is gaining popularity for its ability to destabilize organicsand catalyze oxidation [17]. However, equipment must be carefully designed to reduce the riskof acUV. Sources usedby photosensitiveindividuals or in cidentaleye or skinexposureto conjunctionwithphotosensitizingagentsproduceadversehealtheffects.Basal cell and squamous cell skin cancers result from long-term exposure toUV theportion of sunlight. However, they can also be caused by exposure to specificportions of the UV spectrum produced by a UV lamp. Mercury vapor discharge lamps usedas UV sources are associated with increased risks. The wavelength of the light emitted is directly related to the potential for adverse health effects. Radiation having wavelengths near 270 nm have a greater hazard potential; therefore these exposures should be limited as much as possible. As the wavelength increases, the average daily exposure limitto skin or eyes increases;e.g., with an increase from300 to 320 nm, wavethe limit increases by a factor of about 2. The 8-hr time-weighted average exposure for lengths around245 nm, the primary emission froma mercury vapor discharge lamp, should be limited to about 0.002 J/cm2. When designing treatment technologies, one should be cautious when the sourceemits more than this. If the exposure periodis half as long, the exposure limit is approximately 2 times as great. Therefore, if the exposure lasts for only1 sec, the exposure limit would be about 3000 times that for the 8-hr exposure. One lamp undergoingtests to detoxify organic wasteis a type of mercury vapor lamp with spectral emissions at 254 nm, which can be a significant health concern if handled unsafely. This wavelength has a relatively high spectral effectiveness, and if changed to a lower wavelength, i.e., below 180 nm, or to a higher wavelength, i.e., above 315 nm, would pose less of an eyeor skin hazard. If at all possible, the source shouldbe sealed to eliminate all chancesof radiation being emitted exceptin the directionof the waste stream. Thereis always therisk that a maintenance worker will open the cover of a UV source. For all UV sources, regardless of
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strength, the cover should be equipped with a fail-open switch that will turn off the power to the lamp when the cover is opened. The relation between an acceptable exposure leveland the exposure period is linear until the exposure period is less than 0.1 sec. OSHA requires that machinery producing UV radiation, either direct or reflected, be shieldedto protect employees’ eyes. This is because a worker may accidentally look directly intothe source of radiation for more than 0.1 sec. Where such shielding is not practicable, employees must be given goggles that prevent radiation injury to the eyes.
E. Stabilization A common treatment practice among hazardous waste disposal facilities is the stabilization of contaminated soil or sludge. The process involves mixing the waste with some type of solid matrix, typically cementor kiln dust. However, this mixing occurs in open pitsor bins, both of which are open to the atmosphere. In somecases, the workers mixingthe dust doso with heavy equipment such as backhoes. These generate dust concentrations that could overexpose the workers. The waste could contain metals, asbestos, and solvents. Worker exposure can be reduced by mixing the waste in a closed or ventilated container such asa cement mixer. An additional concern is the stabilization reaction by-products such as heat.
V. CONCLUSION This chapter addressed some of the health hazards associated with emerging technologiesfor to general hazardous waste minimization.Much of the information focused on hazards inherent industries, because many health issues pertainto both production and treatment facilities. Hazardous waste handling facilities and sites pose unique health hazards, some of which were focused on with examples. Engineering and design solutions are the best methods to reduce worker exposure, and some examples of specific solutions were included. In designing processes for hazardous material handling, engineers must incorporate ways to reduce occupational health hazards. With knowledge of the health hazards associated with processes and sites and a concern for the human element, engineers can significantly reduce the risk of adverse health effects for hazardous material workers.
ACKNOWLEDGMENTS I am grateful to Paul Borenstein for providingcase studies and to Cathy Aaron for editing.
REFERENCES 1. EPA, Health and Safety Roles and Responsibilities at Remedial Sites, U.S. EPA, Office of Solid
Waste and Emergency Response, 9285.142. July 1 9 9 1 . Disease. Little, 2. Levy, B. S. (ed.), Occupational Health: Recognizing and Preventing Work-Related Brown, New York, 1988. 3. Oransky,J. J., et al., Implementation of industrial hygiene engineering principles to reduce employee exposure at ahazardouswasteprocessingunit,PresentedatAlHAConference.Boston, Mass.,1992. 4. Andrews, L. F? (ed.), Worker Protection During Hazardous Waste Remediation, Van Nostrand Reinhold. New York, 1990. 5. ACGIH, Threshold Limit Values for Chemical Substancesand Physical Agents and Biological Exposure Indices with Intended Changes for 1991-1992, American Conference of Governmental Industrial Hygienists, Cincinnati, Ohio, 1991-1992.
244 6. Mickunas, D. B., The
Owens
use of remote optical sensing for fenceline monitoring during cleanup operations at uncontrolled hazardous waste sites, Presented at AIHA Conference, Boston, 1992. 7. American Industrial Hygiene Biohazards Committee, Biohazards Reference Manual, American Industrial Hygiene Association, Akron, Ohio, 1986. 8. Shapiro, J., Radiation Protection:A Guidefor Scientists and P hysicians, Harvard Univ. Press, Cambridge, Mass., 1990. 9. Cassarett, C. D., and Doull, J. (eds.), Toxicology:The Basic Science of Poisons, Macmillan, New York,1986. 10. Young, J., Health-related aspects of the heating of refractory ceramic fibre,Mechanisms in in Fibre Carcinogenisis. Plenum, New York, 1991. 11. U.S. Department of Labor, Noise Control:A Guidefor Workersand Employers, OSHA 3048, 1980. 449. 12. Chaffin, D.B., Occupational Biomechanics, Wiley,NewYork,1991,p. 13. U.S. Dept. of Health Services, The Industrial Environment: Its Evaluation and Control, Washington,D.C.,1973. 14. Reddi, L. N., Integrated vibrorecovery process (IVP) for remediation of NAPL-contaminated sites, HMCRI Proc. R&D92 Natl. Research and Development Conf. on the Control of Hazardous Materials, San Francisco, 1992, pp. 305-309. 15. U.S.OSHA,ExcavatingandTrenchingOperations,Pub.No.2226,U.S. Govt. Printing Ofice, Washington,D.C.,1975. 16.Sutton, M. M., Solar-photochemicaldestructionofhazardousorganicchemicalwastes,HMCRI Proc. R&D92 Natl. Research and Development Conf. on the Control of Hazardous Materials, San Francisco,1992,pp.122-126. 17. Camp, D. W., Effect of lampcoating mineral deposits on oxidation of groundwater contaminants, HMCRI Proc. R&D92 Natl. Research and Development Conf. on the Control of Hazardous Materials, San Francisco, 1992, pp.111-116.
Part I1 METHODOLOGIES OF WASTE CONTROL
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11
Techniques for Controlling Solid and Liquid Wastes
Hsai-Yang Fang k h i g h University Bethlehem, Pennsylvania
Jeffrey C . Evans Bucknell Universiw Lewisburg. Pennsylvania
1. INTRODUCTION Wastes canbe grouped intosolid and liquid forms.The majority of solid wasteis urban refuse. This refuse will decompose with time and generate gases and liquid waste, generally referred as leachate. According to the U.S.EPA [l] about 90% of wastes (which includes industrial hazardous wastes) are in liquid form. Therefore, control of solid and liquid wastes requires special emphasis on the control of liquid wastes. The understanding of the interaction between pore fluids and clay behavior requires knowledge of environmental aspects of geotechnical engineering. Itis essential to the engineering use of naturally occurring materials for the containment of hazardous and toxic wastes. Without this knowledge of clay behavior in responseto hazardous wastes, engineering systemsdesign, such as remedial action programs, can have no sound basis on which to project the long-term behavior of that system [2]. Solid and liquid waste control facilities are complex systems [3], requiring interdisciplinary knowledge fromgeotechnical, hydrogeological, and environmentalfields to analyze, design, and construct waste containment systems. Conventional passive hydraulic barriers are frequentlyadapted asbarriers forwastecontainment; however, special considerations are needed in dealing with these barriers. In this chapter, discussion will focus on the following points: 1. Characteristics ofurban refbse including decomposition processes 2. Basic considerations for analysis and design of control systems 3. Techniques of wastecontrol 4. Precautions and protections on these controlling facilities
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II. CHARACTERISTICS OF URBAN REFUSE A. General Discussion There are several ways to dispose of municipal refuse, including incineration and landfilling or, where the nature of the waste permits, by reuse as a substitute construction material. Landfilling is presently the most expedientsimple way to manage refuse. Historically, the refuse was referred to as garbage and the landfill was known as the dump. Waste disposal material consists of anything that cannot be further used or recycled economically. As a result, its composition varies from country to country and from community to community as well as from season to season. The density varies from 50 to 400 lb/ft3 depending on the amount of metal and debris. Table 1 gives the average composition of solid waste based onU.S.national survey results and the cities of New York, New York andOsaka, Japan. In general, refuse can be divided into two categories: degradable and relatively nondegradable. Among refusematerials, some are hazardous and/or toxic and some are not. Depending on environmental conditions in the landfill, the wastes are found in each of the states of matter-solid, liquid, and gaseous forms. Many wastes, when mixed with other wastes or materials at a facility, can produceeffects that are harmful to human health andthe environment, such as heat, pressure, fire, explosion, violent reaction, dust, mist, and gas. Generators of waste materials are required to determine whether the waste is a hazardous wastein one of two ways: 1. They are either waste and spent materials that are hazardous by definition and contained
in specific lists (such as U.S. EPA lists), or 2. They exhibit one of four hazardous characteristics-ignitability, reactivity, corrosiveness, or toxicity.
B. Basic Considerations for Analysis and Design of Landfill Facilities Municipal refuse or any other industrial waste dumpedinto a landfill site requires that proper precautions be taken. Technical aspects such as hydrogeological, geohydrological, climatological, and geotechnical must be examined. However, from the geotechnical engineering viewpoint, the ability of control facilities to mitigate contaminant migration is most important and includes the following considerations:
Table 1 Comparison of Composition of Urban Refuse ~~
Spe Paper Food wastes Metal 9.9 Glass Wood 2.7 Textiles 1.9 Rubber,leather Plastic Garden wastes
New York City
U.S. (national)
Osaka, Japan
(1968)
(1973)
(1980)
51.6 19.3 10.2
37.1
58.8 13.2 9.2 7.6 8.6 2.5 0.8 0.8 15.2 0.8 10.1
Source: Data from Remson et
al. [4]. Sowers [S],
3.0
5.5 12.3 2.5 4.0
1.4
-
and Yarnamura [6].
2.5
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1. Control of any polluted water (leachate or contaminated runoff) in the landfill to prevent it from seeping into the groundwater aquifer 2. Control of the top of the landfill so no additional water (i.e., rainwater, surface water) or vectors (i.e., burrowing animals, flies) disturb the landfill materials 3. Environmental pressures (loads) and their effect on stability 4. Hydraulic conductivity and mass transport phenomena
C. Processes of Decomposition of Refuse in Landfills Landfilled material will gradually change in form and will exhibit different engineering and physical properties as a result of various environmental factors. Witmer and coworkers[7,8] reported the garbage decomposition process for a 10-year period as reflected on a particle size distribution curve. The aging process from fresh trash to aged trash along with the environmental and mechanical factors are illustrated in Figure 1. 1. MechanicalProcesses
Mechanical processes do not directly contributeto the decomposition process. Compression of landfill materials due to the weight of the overlying landfill materials (self weight) may result in significant settlement in a landfill area. Additional settlement results from additional loads such as surcharge loading from landfill cover materials, snow, and downward percolating rainwater. Since the refuse is nonhomogeneous and contains organic matter, the settlement in the landfill areas is also nonuniformly distributed. Becauseof this differential settlement, the top seal (the landfill cover) is subject to cracking.
H Mechanical Loads
Environmental Factors Hazardous wastes Toxic wastes
Surcharge Compaction Rainwater Snow, etc.
Polluted pore fluids Acid rain Freezing-thawing Wet-dry b
Mechanical Process Physico-chemical Process Chemical Process
I G Pe ro om ci ec sr so b i o l o g i c a l I Time ~~
t Factor ~
1
I
~~~~~~~
Figure 1 Aging process (geomorphic)stages between fresh and aging garbage.
Fang and Evans
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2. ChemicalandPhysicochemicalProcesses Leaching and ion-exchange reactions may significantly affect soil properties. Such soil property changes occur when rainwateror drainage processesremove some soluble clay minerals in the clay liners or around the landfill site by leaching. During this leaching, mineral elements suchascalcium,magnesium,nitrogen,potassium,andphosphorus may be removed.The leaching process in turn reducesthe cementation of the soil matrix and/or changes the ion concentration withinthe soil-water-electrolyte system andmay result in significant changesin the properties of soil. 3. BiologicalDecompositionProcess
During the biological decomposition processin the landfill, the temperature increases. At elevated temperatures, sulfate concentrations increase. The problems result from the reduction of sulfates to hydrogen sulfide (H,S). Hydrogen sulfide is the cause of corrosion of various underground structural members. In sanitary landfill areas, the refuse is nonhomogeneous and contains a large amount of organic matter, generating biological changes causedby decomposition for many years. There are five stages relatedto the biological activityin the landfill [9]. Figure 2 shows a schematic diagram illustrating the cumulative settlement due to decomposition of organic matter versus time.
D. Contaminated Water and Pressures Produced in the Landfill Site 1. m s of Contaminants Leachate produced in a landfill site generally results from the interaction of infiltrating liquids with the landfilled material and from the liquids contained within the landfill material at the time of disposal. The properties of leachate change from communityto community as well as from seasonto season. The parameters and their range of concentrations are shown on Table2.
Time
Figure 2 Schematic diagram illustrating cumulative settlement of waste due to decomposition of organic matter versustime. Decomposition stages:(1) aerobic, (2) anaerobic (nonmethanogenic), (3) anaerobic (methanogenic), (4) anaerobic decline, (5) return to anaerobic growth. (After Wardwell et al. [g].)
Controlling Solid and Liquid Wastes
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Table 2 Range of Composition of Leachate from Municipal Solid Waste Component PH Hardness, CaC03 Alkalinity, CaCO, Ca Mg Na
K Fe Ferrous ion Chloride Sulfate Phosphate Organic N NH4 nitrogen BOD COD Zn Ni Suspended solids
Range 3.7-8.5 200-7600 720-9500 240-2400 64-410 85-3800 281700 0.15-1640 8-9 50-2400 20-750 0.5-130 3.0-490 0.3-480 22,ooO-30,ooO 800-50,ooO 0.02-130 0.15-0.9 13-27,ooO
BOD, biological oxygen demand; COD, chemical oxygen demand.
For purposes of examining the effect of leachate on the landfill liner system components, leachate can be grouped into inorganic and organic pore fluids. Aqueous inorganic fluids are those in which water is the solvent and the solute is mostly inorganic. Aqueous organicfluids are those in which water is the solvent and solutesare predominantly organic.The organic part covers organic wastes and organic fluids. Organic wastes are those in which an organic fluid is the solvent and the solutes are other organic chemicals dissolved in the organic solvent. Organicfluids can be classified as organic acids and organic bases. Organic acids include those organicfluids that react with bases and include proton donors; and organic bases include any organic fluid capable of accepting a proton to become an ionized cation [IO]. 2.Acidity at the Linear Interface Pore fluids produced from landfillsare generally acidic. The acidic leachate seeps through from the landfill site into the liner; regardless of the type of liner used, the polluted fluid will interact with soil at various stages. All soils contain H+ ions in their aqueous phases since wateritself is dissociated into H + and OH- ions. Higher H+ concentrations are due to exchangeable H+ ions on the soil particles and/or the presence of mineral or organic acids such as H2S04 from oxidation of pyrites and other organic acidsfrom the decompositionof vegetation. Soilacidity varies with the season [ll]. Soil acidity and acidic pore fluid affects many geostructural members. For examples, they lead to corrosion of metals and other construction materials. On the other hand, acidity may have a desirable impact such as the catalytic effect on certain reactions employed in soil stabilization [121.
Evans
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and
Fang
Table 3 Hydraulic Conductivity and Mass Transport Phenomena in Fine-Grained Clay Liner Material Mechanical energy field Hydraulic conductivity due to mechanical (hydrostatic) potential Multimedia energy fields Energy conductivity (environmental) (a) Hydration energy due to the hydration energy of ions, related to the heat of wetting (b) Osmotic energy due to the osmotic energy of ions either held in a kind of Donnan equilibrium on the solid particle surfaces or free in the aqueous solution (c) Capillary potential due to the surface tension of water and the size and geometry of the soil pores (d)Electricpotential;electroosmosis;electrokineticphenomena (e)Thermalpotential;thermoosmosis.thermalelectriceffect (f) Magneticpotential;electromagneticforce (g)Vaporpressurepotential ~~
Source: Based on Fang [14].
3. Stresses fromLandfills
The stresses or pressures developed with the soil-water system are caused mainly by changes in the overlying stress conditions. There are at least three types of pressures to be considered, including external loads such as surcharge weight from landfill, lateral earth pressures, and loads due to construction activities around the landfill area [13]. Conductivity Phenomena in Landfills and Liner Systems Flow through fine-grained liner material is at low velocity, unsteady, nonuniform, and sometimes discontinuous and contains contaminants insolid or gas forms. There are at least eight possible causes (potentials) of flow from one place to another. These are summarized in Table 3. In most cases, the potentials (and thus the flow) are small in magnitude. However, the accumulation of these small amounts canbe significant, affecting the performance of waste con3, bacteria, chemical corrosion, and erosion trol systems. In addition to factors shown on Table may contribute to barrier leakage as previously indicated. Case studies indicate that liners or barrier walls may leak because of these factors, which are not commonly considered in transport calculations. 4.
E. Effect of Pore Fluid on Liner Behavior Pore fluid effects on liner behavior havebeen discussed in various publicationsin recent years. Many of these studies considered construction impacts, settlement effects, and pore fluidhydraulic conductivity interactions. In the following sections, selected additional factors such as changes insoil cracking patterns, swelling, shrinking, and volumeas well as changes in soil particle distribution.due to pollution are discussed. 1. Soil Cracking Soil cracks are frequently observed in natural and constructedearthen structures such as clay liners, Cracks can be a result of an internal energy imbalance in the soil mass caused by nonuniform stress distribution from compaction energyduring construction. These cracks, usually small and considered insignificant,may be the initiation point for progressive erosion and pre-
Controlling Solid and Liquid Wastes
253
e
0
20
40
ElapsedTime,
60
80
100
3
(x10 minutee)
Figure 3 Effect of pore fluid on permeability of sand-bentonite mixture.
mature failures. Cracking patterns developed in the presence of various fluids are significantly different between noncontaminated (water) and contaminated pore fluids [E]. Contaminated inorganic pore fluids (such as those studied with a pH < 2 or pH > 11) produce significant cracking. Certain organic pore fluids such as aniline (C,H,NH,), acetic acid (C,H,O,), and carbon tetrachloride (CCI,) show greater influence on cracking than others. 2. Particle Size Distribution Effects of pH value on soil-water structures indicate that the higher the pH value, the smaller the effective grain size and the greater the uniformity coefficient as determined using standard procedures for particle size distribution (ASTM D422-72). The effect of changes in porefluid chemistry on the measured grainsize distribution may result in wincorrect assessment offilter requirements since the filter design criteria are based solely on grain size distribution.
3. HydraulicConductivity The effects of pore fluid on hydraulic conductivity are of considerable interest to geotechnical engineers concerned with the construction of hazardous and toxic material control systems. Several technical symposia encompassing this topic, both national andinternational, have been organized in recent years. Hydraulic conductivity is generally correlated with void ratio of the soil and properties of the pore fluid (such as pH value anddielectric constant). The porefluids can include organic and inorganic acids along with landfill leachate. Significant variability in hydraulic conductivity and changes in hydraulic conductivity have been measured by various researchers using various types of pore fluids, such as those shown in Figures 3-5. Various explanations are presented by many of the investigators as reported by Evans [2].
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Brown S a r n i a S o i l f r o m Canada Void Ratio
10"
-
1.0
,
10-5p d , n e
Propanol
"Acetone
0-
tn-6
I
I
I
.'Methanol
I
I
I
40
20
0
I
60
I
I
I
I
l00
80
D i e l e c t r i c Constant
Figure 4 Effect of dielectric constant on hydraulic conductivity of Brown-Sarnia soil from Canada, void ratio 1.0. (After Fkrnandez and Quigley [16].)
t
Water
1
Leachate
CO ' l
W 0
r)
1
I I
I
I
m
I
I
I
1 23
0.000
T i m e (x~OGmlnutes)
Figure 5 Log permeability versus time for clay liner material. (After Fang and Evans [17].)
255
Controlling Solid and Liquid Wastes
111.
BASIC CONSIDERATIONS FOR DESIGN OF WASTE CONTROL SYSTEMS
Waste disposalfacilities can be grouped intothree catagories as shown in Table4. Facilities can be classified as past disposal/waste management sites, active waste management facility sites, and future landfillsites. A systematic approach to the controlof wastes requires the engineer to fully assess both the site and subsurface conditions and evaluate the applicability of containment alternatives. Basic requirements for controllingthe wastes are given by Fang et al. [19]. There are four steps involved, the steps being closely interrelated.
A. CollectHistoricalData Data collection often begins witha review of existing information including historical site data as well as geotechnical, geological and hydrological, and geohydrological data. For past disposal sites, it is necessary to obtain as much information as possible on the types of waste disposed, the timetable of waste disposal, and the previous disposal practices-drum, solid waste, or lagoons. Aerial photos have been found to be quite useful in reconstruction of the site development history. Historic drainageways,landfilling practices, haul roads, and liquidimpoundments all show up quite well on photos. Information regarding the subsurface conditions can be obtained from past records and borings, from the site construction history, and from geological information.
B.PerformInitialSiteAssessment It is necessary to assess, by gathering site-specific data, the existing site conditions including geological conditions, groundwater conditions, and contamination distribution. Insitu investigations are required at this stage. The use of geophysical instrumentsprior to test boringor the monitoring of well installations can provide valuable insight into the subsurface conditions. Details regarding subsurface investigations are discussed by Lowe and Zaccheo [20].
C. Perform Detailed Quantification of Site Conditions It is typically necessary to quantify site conditions including the direction, volume, and velocity of groundwater flow;the interaction of groundwater with surface water; thedistribution Table 4 WasteDisposalFacility Classification
y Facility es names description category I
disposal Past
I1
disposal Active
I11
disposal Future
Source: Evans and Fang [H].
site
none abandoned toLittle inactive retired midnight dump uncontrolled site orphaned secure landfill sanitary landfill waste treatment complex recycling facility
Some Well controlled
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Fang and Evans
of contamination in the groundwater system; and the contaminant loading.The degree of sophistication of this quantification phase may vary froma simple conceptualmodel to a complex computer model. For example, the recently developed GEOTOX knowledge-based expert system can be used for this purpose [21,22].
D. Develop the Containmentrrreatment Program The development of the containment and/or treatmentportion of the program is where the application of environmental geotechnology receivesthe major emphasis. In many cases, investigations described in the first three steps (above) are primarily geohydrologic investigations. Geotechnical properties of soil may not normally be includedin the routine investigation. It is desirable to have geotechnical engineering input during the site investigation phase to avoid future data gaps. Thus, testing should include tests for both physicochemical and engineering properties. [23-251. Waste control systems to be considered at this stage are described in the following section.
IV. WASTE CONTROL SYSTEM COMPONENTS Solid and liquid waste control systems can consist of a wide rangeof components. For purposes of discussion these components canbe classified in two general categories: active and passive [W.
A. ActiveComponents Active components of a containment system are those that require ongoing energy input. Examples of active components include disposal wells, pumping wells, and treatment plants. Disposal wells include injection wells; pumping wells include pumping create to a pumping ridge; and treatment processes include activated carbon specialty processes.
B. PassiveComponents Passive componentsof a containment system are those that do not require ongoing energy input. m i c a 1 examples of passive components include drain tile collection systems, barrier walls, liners, and covers. While an active system requires ongoing energy input, the passive components typically require maintenance. For example, itis necessary to keep vegetationtaproots from penetrating a clay cap. (Treehegetation effects on liner stabilityare discussed further in Section VI.)
C. Passive Waste Control Systems Passive components canbe further divided into three major subsystems. 1.
Vertical barrier walls, such as soil-bentonite cutoff walls, cement-bentonite walls, composite vertical cutoff systems, and vibrating-beam cutoff walls 2. Top seals (cap or cover barrier layers) such as natural clay caps, bentonite admixed caps, and polymeric membrane caps 3. Bottom seals (liner barrier layers) such as natural clay liners, bentonite clay liners, and polymeric membrane liners The structures of these systems are illustrated in Figures 10 and 11, and a brief discussion of each system follows.
Controlling Solid and Liquid Wastes
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V. ENGINEERING STRUCTURE OF WASTE CONTROL SYSTEM COMPONENTS A. Barrier Walls The control of contaminant migration from existing disposal sites or impoundments may necessitate a vertical subsurfacebarrier to horizontal groundwater flow. Barriers commonly constructed include soil-bentonite slurry trench cutoff walls and cement-bentonite slurry trench cutoff walls. Vibrating beam cutoff walls, grout curtains, sheet piling, and composite walls are also used. For vertical barrier walls to be effective, they are generally keyed into an impermeablestratum of naturalmaterialsbeneaththesite,althoughthisisnotrequiredin all cases. Soil-BentoniteSlurryTrenchCutoffWall Design and construction methods for soil-bentonite slurry trench cutoff walls are well documented [18,26,27]. As shown in Figure6, a trench is excavated below the ground surface using slurry exerts the slurryof bentonite and water to maintain trench stability. The bentonite-water a hydrostatic pressure, and a thin filter cake of bentonite forms on the walls of the trench to maintain trench stability in much the same way as drilling fluid maintains borehole stability. The bentonite-water slurry is designedby the Figure 7 is a photograph of an actual installation. geotechnical engineerto have proper density, viscosity, and filtrate loss properties, which allow 1.
lm (3')
WATER TABLE
\
+ -
t
. . .
BENTONITE-WATER SLURRY ( 5 % TYPICAL)
AOUl FER
77777
/AOUlCLUDE IAOUITARD
/
,
... ... . . .... . '. .' . . . . '
F I L T E RC A K E Irnul.5m
DURING EXCAVATION
E X C A V A T IAOFNT E R
Figure 6 Schematic drawing of section of slurry trench excavation and backfill.
SOIL-BENTONITE BACK F ILL
1.2 % BENTONITE [TYPICAL)
258
Fang and Evans
Figure 7 The excavation of a slurry trench.
for the formation of a filter cake along the walls of the trench and for sufficient fluid pressure to result in a computed factor of safety greater than 1 for trench stability [2,28,29]. Trench depths can generally reach about 10 m using conventional backhoes. To achieve greater depths (up to about 17 m), a modified dipper stick is required and can be provided by specialty slurry wall contractors. To go deeper than 17 m usually requires the use of a clamshell, and an extended backhoe capable of excavating 22 m has been developed [30]. m i c a 1 cross sections of a slurry wall during excavation andafter backfilling are shown in Figure 8. Upon the completion of excavation under the head of bentonite-water slurry, the trench is backfilled with a mixture of soil and bentonite (addedby way of the bentonite-water slurry). The mixture of soil and bentonite-water slurry typically has a consistency similar to that of high slump concrete. Soil-bentonite slurry trench cutoff walls can generally developa hydraulic conductivity of 1 X 10” c d s e c or less. For the design of a soil-bentonite slurry wall for waste containment, studies are required beyond those typically required for conventional dewatering applications. Chemical analysis of samples of on-site materials considered as potential backfill materials may be required. Leachate compatibility tests must be conducted using the site liquid as a permeant. 2. Cement-Bentonite Slurry TrenchCutoffWalls An alternative to a soil-bentonite cutoff wall is a cement-bentonite cutoff wall. The trench is excavated ina manner similar to soil-bentonite walls as described above, usinga slurry to maintaintrenchstability.However,cementisadded to the slurry. Thecement-bentonite-water slurry is left in the trench and allowedto harden. A strength equivalentto that of stiff clay can be obtained after a period of about a month. Design considerations include the cement and bentonite content andtype and their relationship to the strengthand permeability of the backfill 1251. The overall permeability of cement-bentonite cutoff walls (typically about 1 X lO-‘cml sec) is generally higher thanthat of soil-bentonite walls. Laboratory testing of a cement-bentonite mix for a plastic diaphragm wall is discussed by Gill and Christopher [31].
Controlling Solid and Liquid Wastes
C
I n 1
I
INTO CLAY LAYER
Y
259
-
BACKFILL PLACED
HERE
B E N T O N I T E SLURRY
Aquiclude Aquitard (Impervious Zone)
3. CompositeVerticalCutoff Barrier Systems A concept for constructing a composite vertical impermeable barrier to prevent the migration of contaminated groundwateror leachate from a hazardous waste site or waste disposal area is described by Druback and Arlotta [32]. The composite system is a hybrid cutoff wall constructed with high-density polyethylene (HDPE), and sand backfill and is installed using the slurry trench construction method. When properly installed, a low-permeability composite vertical barrier results that has unique engineering properties, including improved chemical resistance, leak detection, and groundwater migration control. A full-scale construction test project of the system was performedat an existing sanitary landfill in New Jersey to demonstrate the overall fabrication and construction procedures. Detailed procedures on this system, including design, construction, and performance data, are provided by Druback and Arlotta in their 1985 paper [32]. 4.
Vibrating-BeamWalls Vertical barriers to horizontal groundwater flow have been designed and constructed using the vibrating-beam injection method as shown in Figure 9. This technique uses a vibratory-type pile driver to cause the penetration of a beam of specified dimensions to the design depth. Slurry is added through injection nozzles as the beam penetrates the subsurface soils and as the beam is withdrawn. or The slurry used with the vibrating-beam technique is generally either cement-bentonite bituminous grout. Mix design considerations for cement-bentonite were discussed above. Bituminous grouts are prepared as a homogeneous blend of asphalt emulsions, sand, portland cement, and water. Fly ash may also be included. It is reported that this bituminous grout can resist strong acids and high saline content wastes.The use of thin slurry cutoff walls installed by the vibrating-beam method is discussed by Leonards et al. [33] and McLay [34].
B. Top Seals (Cap or Cover Barrier Layers) Top seals (i.e., surface seals, caps, covers), as shown in Figure 10, function to control surface water so as to minimize infiltration, maximize runoff, prevent direct contact (suchas by burrowing animals, whichmay dig into the landfill areas), and thereby reduce leachate production
260
Fang and Evans
MIX I
NC P L A N T SLURRY PUMP
V i b r a t i n g Bean S l u r r y SLURRY INJECTlNC NOZZLES AT BEAM
P e r v i o u s Zone
1 1 1 1 1 1 1 1 1
p e r v i o u s 20
I I I I
IIIIII
Figure 9 Schematicdrawing of avibrating-beam slurry wall. (1) Topseal; (2) landfillmaterial; bottom seal.
(3)
I m p e r v i o u s Zone
@A.’
Figure 10 Qpical cross section of a solid waste landfill witha native clay top seal. (1) Surface erosion; ( 2 ) nonuniform settlement; (3) bottom erosion.
Solid
Controlling
and Liquid Wastes
261
and/or contaminant transport potential. Several types of materials can be used for the top seal barrier layer, including compacted natural clay, bentonite clay, and synthetic membrane. Each of these is discussed briefly in the following. 1.
CompactedNaturalClayCaps
The most cost-effective top sealbarrier layer usually isone of compacted native clay material from locally available sources. If suitable native claysare not available, imported materialsor synthetic membranes mustbe used. In general, clays are thought to last longer than synthetic materials, and, where feasible, clay caps rather than synthetic caps may be chosen. However, to avoid the “bathtub effect” (more water entering the facility than can drain out), the use of a synthetic membrane cap is required whenever the bottom liner is also a synthetic membrane W]. Selection of clay materials for clay caps is based on the compacted hydraulic conductivity of available materialas discussed by Johnson et al. [27]. As shown in Figure 10, the thickness of the clay cap is typically0.6-0.8 m. The following are some of the factorsto be considered in the design of a clay barrier layer. The top several inchesof clay cannot beas well compacted as the remainder of the thickness owing to the lack of confinement. Further, it may be difficult, in the long term, to maintain the clay density in the top few inches due to potential desiccation cracking,wetdry cycles, and freeze-thaw cycles. The bottomof the claycap barrier layer may become somewhat intermixed with subgrade material during construction. Therefore,the effective thicknessof a nominal 0.6-m cap is likely to be less than 0.6 m. The compaction procedures for the clay barrier layers are, in many ways, similar to those followed for standard compacted fills as discussed by Hilf [36]. Protection of the claybarrier layer from degradation due to erosion by surface water runoff, cracking due to drying, rutting from moving vehicles, and penetration by tree roots and other vegetation mustbe provided. Some of these factors willbe further discussed in Section 4 below. Bentonite Clay Caps Where local natural clay is not available in sufficient quantity or at an acceptable quality or price, processed clay is a common alternative. Processed clay is typically bentonite from sodium montmorillonitic clay deposits. Bentonite clay is a strongly hydrophilic colloidal clay that swells (expands) in water. The construction of a processed claycap requires the applicationof the bentonite in powdered or granular form at a controlled rate (e.g., about 2 lb/ft2) followed by adequate mixing with the in-place soilto a predetermined loose thickness. The mixture of soil and bentonite is then compacted. The main advantages of this method are the low cost coupled with relatively low hydraulic conductivity. The disadvantages are (1) it is difficult to (2) bentonite clay swelling is very sensitivepore to obtain uniform application and blending and fluid chemistry. Thus if contaminated water permeates the bentonite clay barrier layer, significant increases in the hydraulic conductivitymay occur. 3. SyntheticMembraneCaps
Synthetic membranes (Figure11) can be used as the barrier layer in covers for waste containment [37,38]. The principal advantages of geosynthetics are their low hydraulic conductivity and their ability to deform and remain intact. Thus geosynthetics can often accommodate the large total and differential settlements that may occur in the landfill beneath the cover layer.
262
Fang and Evans Polymeric Membrane Seal Vegetated
ion
WASTE
(Garbage)
LBottom
Seal
Figure 11 vpical cross section of a waste control system with a polymeric membrane seal. (1) Surface erosion; (2) nonuniform settlement; (3) bottom erosion.
Membranes are available in a wide range of materials from numerous manufacturers. Additional information regarding geosynthetic materials is provided later in this chapter. OtherSurfaceControls In addition to surface sealing with barrier layers, surface water diversion and collection systems can provide short- and long-term measures to isolate waste disposal sites from surface water inputs. Techniques used to control flooding and off-site erosion transport of cover and surface seal materials include dikes andberms, interceptor ditches, diversion dikes and berms, terraces and benches, sheets and downpipes, levees, seepage ditches, and sedimentation basins and ponds [39]. In summary, surface seals provide multiple functionsin the overall liquid and solid waste containment control system.Their main function has been discussed as the control of infiltration by minimizing water infiltration and/or maximizing surface runoff.
4.
C. Bottom Seals (Liners) In newwaste containmentfacilities, it is usually necessaryto provide a liner system beneath the waste disposalarea. The major functionof a liner is to prevent leachate or waste from migrating downward and entering the groundwater flow regime. The barrier layer for liner systems can consist of native clays, prucessed clays, or geosynthetic membranesas previously discussed for covers. It is importantto note that under the presentU.S.regulations, the use of a synthetic membrane liner is considered the best technology to “prevent” migration of wastes, whereas a clay liner will “minimize” migration of wastes. 1. CompactedNativeClayLiners The compatibility between the compacted natural clays and the waste is an important design consideration for the use of natural clays as liners. It is important to ascertain the volume change and permeabilitycharacteristics of the proposed clay linermaterial. The bulk transport of liquid waste throughcracks as discussed in Section I1 must be precluded. Bulk transport of liquid through clay liners could occur due to differential settlement of the foundation basema-
tes
Controlling Liquid Solid and
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terials. Compacted natural clay liners are often used as the final barrier layer in a liner system composed of multiple leachate collection and geosyntheticbarrier layers. In this configuration the capacity of natural clays to sorb contaminants is acknowledged. 2.
Bentonite Clay Liners Design and construction considerations for the use of processed clay for liners must include waste-linercompatibility as well as the considerations previouslydiscussed. The volume change characteristics of the processed clay are especially important. Generally, a bentonitic processed clay is mixed with the subgrade material to form the low-permeability liner. The impedance to groundwater flow is primarily due to the processed clays, especially when the matrix soilis relatively free of natural fines. Hence,if the processed clayshrinks upon exposure to the waste or leachate, large increases in hydraulic conductivity can occur. The hydrationof a processed clay liner with uncontaminated water prior to waste disposalis recommended [M]. Triaxial permeability tests using the actual proposed clay subgradematerial, groundwater, and leachate should be conducted as part of the design studies. 3. GeosyntheticMembraneLiners
As with other liner types, waste compatibility is a major design consideration. However, the permeability of a polymeric liner canalso increase dueto liner stretching. Thus, total and differential foundation settlement can impact the liner design. Close construction control is essential to the overall system performance. The “permeability” of an installed membrane liner system is generally a function of bulk transport through seams, joints, tears, holes, and pinholes. The long-term durability of geomembranes is discussed by Koerner et al. [41]. Studies include ultraviolet radiation and chemical degradation as well as swelling, oxidation, and temperature. In addition, predictive methods for the evaluation of long-term durability are proposed and discussed.
D. Double-CompositeLinerSystem A double-composite liner system proposed by Daniel and Koerner [42] is based on the combination of clay and geotextile structures. This system is the minimum standard of care to which future landfills should be held. It requires careful design, testing, and construction. As shown in Figure 12, the double-composite liner system consists of four major parts: Leachate collection systems (layers 1, 2, and 3) Primary liners (layers 4, 5 , and 6 ) Leak detection system (layer 7) Secondary liners (layers 8 and 9) These major parts form nine layers. A tenth layer, the soil or rock subgrade, completes the double-composite liner system. 1. Leachate Collection System The leachate collection system is the uppermost segment, as shown in Figure 12. The system is composed of a highly permeable granular material on the base of the disposal unit and a high-transmissivity geocomposite materialfor side slopes, provided timely cover andadequate protection are provided. Layer I :Filters (Soil or Geotextile). A filter (soil or geotextile) to separate the lowest portion of select waste or initial operations layer of soil from the leachate collection and drainage medium is essential. If suspended particles from the leachate travel into the drainage system, it
Fang and Evans
264 Individual components
/
/
Cover geotextile) Filter or (soil
A
/
f
Waste
//
(geocomposite)
Drain or other) Protector (geotextile Barrier (geomembrane) Leachate collection system Primary composite liner
Leak detection system Secondary composite liner
Jbgradf3
Figure 12 Double-composite liner system. (After Daniel and Koerner [42].)
may clog. To minimize this problem, it is suggested that(1) filters be used only when they are truly needed and that (2) high-permeability filters be employed. A geotextile filter is part of the geocomposite drainage layer along side slopes. Geotextiles are sensitive to ultraviolet light degradation if left exposed and therefore require protectionto avoid this failure mode. Layer 2: Drain (Gravel for Base, Geocomposite for Side Slopes). The leachate collection layer requires high in-plane transmissivity and pores sized to resist plugging. In this way the hydraulic head on the underlying barrier layer can be minimized. Layer 3: Protector (Geotextife or Other). This layer prevents materials in the drainage layer from puncturingthe primary geomembrane liner.Protectors are usually thick, needle-punched, nonwoven geotextiles. 2.
Primary CompositeLiner Layer 4: Barrier (Geomembrune). This component of the primary liner can be made from polymeric materials, includingpolyvinyl chloride (PVC), chlorinated polyethylene (CPE), chlorosulfonated polyethylene (CSPE), ethylene interpolymer alloy (EIA), high-density polyethylene (HDPE), and verylow density polyethylene (VLDPE).At present, HDPE geosynthetics offer the most versatility with respect to contaminant resistance and overall engineering properties such as strength and permeability. Layer 5: Barriers (GeosyntheticCfayLiner). For the soil component of the primary liner, the geosynthetic clay liner (GCL) is recommended. GCLs, previously called prefabricated clay blankets or referred to by other terms, are factory-manufactureddry bentonite clay layers sandwiched between geotextiles or attached to a geomembrane. This barrier offers low hydraulic conductivity along with adsorption capacity and reduces the rate of contaminant transport, particularly transport due to diffusion. Layer 6 Separator (Geotextifeor Other). To avoid migration of clay particles from the GCL into the underlying geonet, adequate separation is needed. Best results in experimental tests
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have been obtained with (1) nonwoven, needle-punched geotextile with small apparent openings; (2) nonwoven, heat-bonded geotextile; and (3) geomembranes. 3. LeakDetectionSystem
This leak detection system is the third major part of the system. It is also known as the secondary leachate collection system. It identifies leakage from the primary lining system and enables it to be collected and removed. Its design is similar to that of the primary leachate collection system. Layer 7 Drain (Geonef). A geonet is preferable to granular materials for the leak detection layer because itis much easier to place onside slopes and canbe placed with lightweight equipment, and because granular materials can puncture the underlying geomembrane. Geonets also offer fasterdetection of leaks thanmost granular materials. Hazardous waste facilities typically must detect leaks within 24 hr, and often a geonet is the only material than can do this. 4.
SecondaryCompositeLiner m e r 8: Barriers (Geornembrane). Technical requirements for the secondary geomembrane liner are generally the same as for the primary layer (layer5), so the same type and thickness of material is usually used. Layer 9 Barriers (Cornpacred Soil Liner). A compacted soilliner can be constructed without risk of damaging any underlying liner system components. 5 . Subgrade (Soil or Rock)(Layer 10) For large-scale facilities, a complete subsurface soil investigationis necessary. The large areal extent of landfills resultsin a significant depthof stress influence. The strength and compressibility of the underlying soils must be fully investigated to ensure foundation stability.To correct any disturbance due to construction, the site should be proofrolled after final grading.
VI. MAINTAINING PERFORMANCE OF WASTE CONTROL SYSTEMS A. GeneralDiscussion It is necessary to ensure that the waste control system willcontinue to operate as designedfor many years. The landfilled waste may remain toxic for many hundreds of years. As engineers we have frequently been gearedto a design life of 40 or 50 years whereas the landfills we are presently buildingwill be here for generations to come. It is therefore necessaryto design and construct waste control systems that acknowledge future uncertainties and properly account for long-term impacts on the system performance.
B. ChemicalAttackonLiners Long-term durability evaluations for liners of both clay and geosynthetic membranes require studies of the impact of chemicals on the engineering properties of the liners. The change in hydraulic conductivityof clay barriers due to changes in pore fluid chemistry must be studied if the liner is to successfully perform over the long term. The “compatibility” of geosynthetics with the landfill leachate must be evaluated. In allof these studies it is necessary to accelerate time in order to complete the testing within a reasonable time period. For hydraulic conductivity testing, this acceleration of time is accomplished through the use of high hydraulic gradients. In other studies, more concentrated forms of the contaminants are used. In anycase, our ability to predict barrier performance for long time periods is limited as we do not have performance data to verify our models.
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Figure 13 Mechanisms of soil-root-pore fluid (leachate) interaction at landfill area. (1) hre fluid (leachate) in landfillseepsoutthroughroothairsbysuctionforce. (2) Pore fluid (leachate) in landfill seeps out between root and soil by capillary action. (3) Cross section of liner.
C. Vegetative Root Attack on Liners Landfill sites are generally planted with vegetation to enhance evapotranspiration and for the purpose of beautification of unattractive landfill sites. Further, without regular maintenance, woody vegetationwill begin to appear. Trees and vegetation also have a detrimental side effect. l k o important phenomena related to how tree roots attack liners are discussed below. 1.
Tree roots lookingfor nutrition (food) and water will grow deeper intothe subsurface of the landfill area. Roots can and will penetrate barrier layers. 2. Pore fluid (leachate) in landfill can migrate through and along roots and root hairs.
Roots penetrate the subsurface to acquire food and water, and landfill sites readily provide these necessities for growing trees and other vegetation. The penetrating force producedby tree roots is strong enough to penetrate clay liners or synthetic membranes. For example, bamboo roots can penetrate into a 6-in. thick concrete wall. Many roadways, pavements, and drainage facilities have been damaged by roots of common trees that grow around landfill areas. Some typesof vegetation (such as squashes) spread theirroots laterally and widely in the near-surface (less than 1 ft in depth) zone. Some roots systems are coarse and open, whereas others consist of masses of fine fibrous roots. The abilityof roots to obtain nutrient substances
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also varies greatly. Even though we cannot explain wholly the quantity of nutrients produced in the landfill environment that is utilized by roots, the forces within the plants and aroundtheir feeding roots may also have a controlling influence. Plants also require immense quantities of water, practically all of which is absorbed from the soil through their root systems. Special plant features that provide for absorption includethe tips of roots and innumerable root hairs.A mechanism by which pore fluid (leachate) in landfills may migrate out through the root system is illustrated schematically in Figure 13. This phenomenon of moisture migration within the root hair and between root hair and soil is due to suction force and capillaryaction. Plant species differ not only in their water requirements but also in their adaptation to water conditions. Some plants are so constituted that they can tolerate an abundance of soil water, whereasothers can endure limited moisture. Since our present regulations requirethat a landfill site be maintained for a period of 30 years after closure, it is necessary to determine the impact of vegetation on the waste control system forthe reminder of the landfill life while the contained contaminants are still toxic.
D. Other Factors Construction quality assurance (CQA)is required to correctly transfer the design as contained in reports and drawings into the real systems as constructed in the field. Careless or incompetent construction can defeat the best design.
VII. SUMMARYANDCONCLUSIONS Almost 90% of hazardous wastes are in liquid form. An understanding of the effect of pore fluids on liner behavior is essential to the design of various components of waste control facilities. Without a good understanding of the liner-pore fluid interaction, there is no sound basis on which to project the long-term behavior of these systems; Passive techniques presented in this chapter can and have been used to mitigate contaminant migration. Consideration must be givento the identification of all contaminant pathways and selection of the most appropriate control technique. Each technique then must be evaluated as to its effectiveness, and the design and construction must incorporate all site-specific and technique-specific considerations. Contaminants will eventually migrate into the environment, despite our best efforts. With this premise, our control systems must reduce the rate of migration to an imperceptible rate or to a rate at which our environment can assimilate the contaminants without damage.
REFERENCES 1. 2. 3. 4.
5. 6. 7.
U.S. EnvironmentalProtection Agency, “1987 NationalBiennialRCRAHazardousWaste Report,” Office of Solid Waste, Washington, D.C.,July 8. 1991. Evans, J. C., Geotechnics of hazardous waste control systems, in Foundution Engineering Handbook, 2nd ed., Van Nostrand Reinhold, New York, 1991, Chapter 20, pp. 750-777. Turner, C. F., and McCreery, J. W., The Chemistry of Fire and Hazardous Materials, Allyn and Bacon, Boston, 1981. Remson, I., Fungaroli. A. A., and LawrenceA., Water movement in an unsaturated sanitary landfill, J. Sanitary Eng. Div. Proc. ASCE, 94(SA2), (1968). Sowers, G . F., Settlement of waste disposal fills, Proc. 9th Int. Soil Mechanics Foundation Eng. Conf., Moscow, 4 , 297-3 10 (1973). Yamamura, K.,Current statusof waste management in Japan,Waste Manage.Res., I , 1-15 (1983). Witmer, K.,Geomorphic process: from fresh garbage to organic soil, Proc. 1st Int. Symp. Environ. Geotechnol., 2, 389 (1987).
-
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hoc. In?. Witmer, K., Volk, J., and Naik, D., Lowcost ground improvement for landfill area, Symp. Low-Cost and Energy Saving Construction Materials, Rio de Janeiro, Brazil, July 1984, pp. 497-5 15.
9. Wardwell, R. E., Charlie, W. A., and Doxtader, K. A., Test Method for Determining the Potential for Decompositionin Organic Soils, American Society of Testing and Materials, Spec. Tech. Publ. 820, 1983,pp.218-229. IO. Manaham, S. E., Environmental Chemistry, 5th e d . , Lewis, Boca Raton, Ha., 1991. 11. Faust, S. D., and Aly, 0. M., Chemistry ofNatural Waters,Ann Arbor Science, Ann Arbor, Mi&., l98 1. S . , Soil stabilization and grouting, in Foundation Engineering 12. Winterkorn, H. F., and Pamukcu, Handbook, 2nd e d . , Van Nostrand Reinhold, New York, 1991, pp. 317-378. 13. Fang, H. Y.,Pamukcu, S., and Chaney, R. C., Soil-Pollution Interaction Effects on the Stability of GeosyntheticCompositeWalls,AmericanSociety ofTesting and Materials, Spec. Tech. Publ. 1129,1992. Proceedings, 1st h r . 14. Fang, H. Y.,Mass transport phenomena and stability of waste control system, Symp. Environ. Geotechnol., 2, Envo Publishing Company, Inc. Allentown, PA, 1987, pp. 290294. 15. Alther, G. R., et al., Inorganic Permanent Effects upon Bentonite, American Society of Testing and Materials, Spec. Tech. Publ. 874, 1985, pp. 64-74. 16. Fernandez, F., and Quigley, R. M., Hydraulic conductivity of natural clays permeated with simple liquid hydrocarbons, Can. Geotech. J . , 22, 205-214 (1985). 17. Fang, H. Y., andEvans, J. C., Long-Term Permeability Tests Using Leachate on a Compacted Clayey Liner Material, American Society of Testing and Materials, Spec. Tech. Publ. 963, 1988, pp. 397-404. 18. Evans, J. C., and Fang, H. Y., Geotechnical aspects of the design and construction of waste contaminant systems, Proceedings, National Conference on Management of Uncontrolled Hazardous Waste Sites, Washington, D.C., 1982, pp. 175-182. Solid 19. Fang, H. Y.,Evans, J. C., and Kugelman, I. J.. Solid and liquid waste control techniques, in and Liquid Wastes: Management, MethorLF and Socioeconomic Considerations, (S. K. Majumdar and E. W. Miller, eds.), Pennsylvania Academy of Science, Philadelphia, 1984, pp. 104-118. 20. hwe,J., 111and Zaccheo, P. F., Subsurface explorations and sampling, inFoundation Engineering Handbook, 2nd ed., Van Nostrand Reinhold, New York, 1991, pp. 1-71. 21. Mikroudis, G. K., and Fang, H. Y., GEOTOX-PC:a new hazardous waste management tool, in Microcomputer Knowledge-Based Expert Systems in Civil Engineering, American Society of Civil Engineers,New York,1988,pp.101-117. 22. Fang, H. Y., Mikroudis, G. K., and Pamukcu, S., Multidomain expert systems for hazardous waste site investigations, inExpert Systemsfor Environmental Applications, ACS Symp. Ser. 431, American Chemical Society, Washington, D.C., 1990, pp. 146-161. H a m , A., Perket, C. L., and Lacy, W.J.,Haz23. Lorenzen, D., Conway, R. A., Lackson, L. ardous and Industrial Solid Waste Testing and Disposal, American Society of Testing and Mater Spec. Tech. Publ. 933, 1986. 24. Collins, A. G., and Johnson, A. I., Ground-water Contamination: Field Methods, American Society of Testing and Materials, Spec. Tech. Publ. 963, 1988. 25. Evans, J. C., and Fang, H. Y.,Triaxial Permeability and Smngth Testing of Contaminated Soils, American Society for Testing and Materials, Spec. Tech. Publ. 977, 1988, pp. 387-404. 26. Xanthakos, P,Slurry Walls, McGraw-Hill, New York, 1979. 27. Johnson, A. J., Frobel, R. K., Cavalli, N. J., and Petterson, C. B., eds. Hydraulic Barriers in Soil and Rock, ASTM STP 874, 1985. 28. D'Appolonia, D. J., Soil-bentonite slurry trench cutoffs, J . Geotech. Eng. Div. h o c . ASCE, 106 (GT4), 399-417 (1980). 29. Anderson, D. C., Crawley, W., and Zabcik, J. D., Effects of Various Liquids on Clay Soil: Bentonite Slurry Mixtures, American Society of Testing and Materials, Spec. Tech. Publ. 874, 1985, pp.93-101.
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30. Case International CO, Case Study No. 5 , Case Slurry Wall Note Book, 1981. 31. Gill and Christopher,Hydraulic Barriers in Soil and Rock, ASTM STP 874, 1985, pp. 64-74. 32. Druback, G. W., and Arlotta, S. V., Jr., Subsurface Pollution Containment Using a Composite System Vertical Cutoff Barrier, Hydraulic Barriers in Soil and Rock, A. I. Johnson, R. K. Frobel, N. J. Cavalli, and C. B. Pettersson, eds., American Society for Testing and Materials, Spec. Tech. hbl. 874, 1985, pp. 24-33. 33. Leonards, G . A., Schmednecht, E. Chameau, J. L., and Diamond, S., Thin Slurry Cutoff Walls Installed by the Vibrated Beam Method, American Society of Testing and Materials, Spec. Tech. Publ. 784, 1985, pp. 34-44. 34. McLay, D., Installation of a cement-bentonite slurry wall using the vibrating beam method-a case history, Proceedings ofthe Nineteenth Mid-Atlantic Industrial Waste Conference(J. C. Evans, d.). Technomic, Lancaster,Penna.,1987. 35. U.S. Environmental Protection Agency. Design and Construction of RCRNCERCLA Final Covers, Rep.EPN625/4-91/025,May,1991. 36. Hilf, J. W., Compacted fill, in Foundation Engineering Handbook, 2nd ed., Van Nostrand Reinhold, New York, 1991, Chapter 8, pp. 249-316. Pro37. Emrich, G . H., and Beck, W., Top sealing to minimize leachate generation-status report, ceedings, 7th Annual Research Symposium Land Disposal: Hazardous Waste, U.S. Environmental Protection Agency, Rep. 600/9-81-002b, 1981, pp. 291-297. 38. Koerner,R. M., Designing with Geosynthetics, 2nd ed., Prentice-Hall, Englewood Cliffs, N.J., 1990. 39. Cederpn, H. R., Seepage, Drainage, and Flow Nets, Wiley, New York, 1967. 40. Hughes, J., Use of Bentonite as a Soil Sealant for Leachate Control in Sanitary Landfills, Volclay Soil Engineering Rep. Data 280-E, 1975 41. Koerner, R. M., Yick, H. H., and Lord, A. E., Jr., Long-term durability of geomembranes, Civil Engineering, ASCE, New York, April 1991, pp. 56-58. in Engineering, ASCE, Civil 42. Daniel, D. E., and Koerner, R. M., Landfill liners from top to bottom, NewYork,1991,pp.46-49. 43. Koerner, R. M., Geosynthetics in geotechnical engineering, inFoundation Engineering Handbook, 2nd ed., Van Nostrand Reinhold, New York, 1991, Chapter 22, pp. 796-813. 44. Evans, J. C., and Fang, H. Y., Triaxial equipment for permeability testing with hazardous and toxic permeants, ASTM Geotech. Testing J . , 9(3), 126-132 (1986). 45. Fong, M. A., and Haxo, H. E., Jr., Assessment of liner materials for municipal solid waste landfills, Proc. 7thAnnual Research Symposium, LandDisposal: Municipal Solid Waste, U.S.EPA Rep. 600/9-81-002a,1981,pp.138-162. 46. Rogowski, A. S., Relationship of laboratory and field determined hydraulic conductivity in compacted clay layer, U.S. Environmental Protection Agency, Rep. EPN600/2-90/025, Cincinnati, June 1990. p e on Consolidation of Clay-Liquid 47. Weidelich, W.C., Influence of Liquid and Clay Mineral 5 Systems, Highway Research Board Special Rep. 40, 1958, pp. 24-42.
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12 Solidification and Stabilization Techniques for Waste Control A. Samer Ezeldin and George I? Korfiatis Stevens Institute of Technology Hoboken, New Jersey
1.
INTRODUCTION
Industrial activities inthe United States and other developed countries generate large volumes to avoid of by-product waste. These by-products mustbe disposed of in a safe manner in order adverse impacts on the environmentand public health. In recent years, solidification and stabilization techniques have emerged as viable alternatives for safe waste disposal. Currently a large number of research organizations and professional companies are engaged in waste stabilization and solidification processes. The terms stabilization and solidification have been traditionally used interchangeably. The fact is, they are two separate processes. Stabilization refers to a chemical fixation technique used to render a waste less toxic or less harmful to the surrounding environment. In general, stabilization reduces the hazard potential of the waste. Examples of stabilization techniques include ion exchange of heavy metals in the aluminosilicate matrix of a cementitious stabilization agent and sorption of heavy metals on fly ash in aqueous systems. Solidification is used to transform the waste into a stable and durable matrix that is more compatible for storage, landfill, or reuse. Solidification can be accomplished with or without chemical fixation. This process creates barriers between waste components and the environment by either or reducing the effective surface area available for diffusion, reducing permeability of the waste or both.
II. NONCHEMICALSOLIDIFICATIONTECHNIQUES Nonchemical solidification includes dewatering, mixing with absorbents, and vegetative stabilization. Dewatering is the removal of water by thermal drying, filtration, or centrifugation. The dried waste is usually either left in place or transported to a suitable landfill location. 271
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Ezeldin and Koflatis
In the process of mixing with absorbents, the purposeis to absorb the water phase of the waste using agents with high absorptive capacity. The most commonlyabsorptive used material is soil. Becauseof the absence of chemical reactions betweenthe waste and the absorbent, the absorbed water can be pressurized from the waste-absorbent system. This process is usually followed by chemical solidification, which solidifies the waste-absorbent matrix. In order not to oversaturate the absorbent (resulting in a larger quantity of waste to be disposed of), the addition of absorbent must be carefully controlled. Vegetative stabilizationinvolves growing plants whose roots stabilize the soil. This method has been used for decades in the erosion control of unconsolidated soil systems. This process can be applied for waste stabilization provided that the waste constituentsare compatible with the vegetative growthand that the long-termuse of the landdoes not include productionof food crops or fodder.
111. CHEMICAL SOLIDIFICATION AND STABILIZATION PROCESSES Chemical solidification and stabilization processes are used to treat industrial and hazardous wastes. These processes represent alternatives to Ocean disposal or conventional landfilling. Most of these processes originatedin the field of radioactive waste control and management. Chemical solidification and stabilization refer to waste treatment that results in the combined effect of
1. Improvement of physical properties (mechanical stabilization) 2. Encapsulation of pollutants (immobilization by fixation) 3. Reduction of solubility and mobility of the toxic substances (immobilization by isolation) Mechanical stabilization reduces the bulk mobility of the waste and makes it durable and dimensionally stable, The final product should be capableof resisting deterioration caused by mechanical or environmental stresses. The extent of mechanical stabilization is evaluated by conducting strength and durability tests. Immobilization by fixation minimizes the local mobility of individual contaminant components. Leaching tests are conducted to determine the effectiveness of fixation. Immobilizationby isolation limits the risk of the contaminants reaching the boundaries of the deposited massby providing protection againstinternal contaminant transport (avoiding internal and external cracking) and by restricting internal transport pathways (low permeability). There are many solidificationand stabilizationtechniques used inthe industry for different types of waste materials.Each of these techniquesis usually recommended for use with specific waste constituents. Hence, process selection involves weighing advantages and disadvantages for each particular project. Generally, stabilizatiodsolidificationtechniques can be divided into the following seven major categories: 1. Cement-basedtechniques 2. Pozzolanictechniques(silicate-basedtechniques) 3. Thermoplastic techniques (including the incorporation of bitumen, paraffin, and polyethylene) 4. Sorbenttechniques 5 . Organicpolymertechniques 6 . Encapsulationtechniques(jacketing) 7. Vitrification
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Solidification and Stabilization Techniques
A. Cement-BasedTechniques Portland cement is obtained by heating limestone and clay or other silicate mixtures at high temperatures. The resulting clinker is ground to a highly uniformpowder. fine Anhydrous portlandcementconsistsmainly of C3S,C2S,C3A,and C,AF, whereC = CaO, S = SiOz, A = A1203, and F = Fez03. In ordinary portland cements, the respective amounts rangebetween 35 and 65%, 10 and 40%, 4 and 15%, and 5 and 15%. Table l presents selected characteristics of these materials. Addition of water to portland cement initiates the cementation process. Needle-like crystals of calcium sulfoaluminate hydrate (ettringite) start forming within a few minutesof cement hydration. After a period of time, the ettringite eventually transforms to the monosulfate hydrate. A few hours after initiation of the cementation process, large prismatic crystals of calcium hydroxide (CH) and very small crystals of calcium silicate hydrates (C-S-H) beginto fill the empty spaces previously occupied by water and the dissolving cement particles. There are three major components of the hydrated cement paste.
Calcium silicate hydrate forms about 60% of the solids volume and is the most important component. It is composed of layer structures with a very high surface area (about 500 m2/g). Its strength is attributed mainly to van der Waals physical adhesion forces. Calcium hydroxide constitutes about 25% of the solids volume. It is formed of large crystals with low surface area. Its limited van der Waals forces and its higher solubility compared to C-S-H make the concrete reactive to acidic solutions. Calcium sulfoaluminate forms about 15% of the solids volume. It plays a minor role in the cementitious structure properties. Due to the presence of the monosulfate hydrate, concrete chemical durabilityto sulfate attack is usually of concern.
be incorporated in a waste-cement system. The suspended polMost hazardous wastes can lutants would be incorporated into the final hardened concrete. During the solidification process, the concrete formation binds and strengthens the mass, coats and incorporates some
Table 1 Principal Compounds of Portland Cement and Their Characteristics Ca0.Al2O3FezO3 Ca0.AIz03 fJ-CaO.SiOz CaO.SiOz Abbreviated formula Common name Principal impurities
c3s
Alite MgO.AI2037
form Avg. amount present (%) (range) in ordi-
nary cement Rate of reaction with water Contribution to strength Early age Ultimate Typical heat of hydration (calk)
C4AF Ferrite phase, Fss SiO2. MgO
50 (35-65)
SO2, MgO, alkalies Cubic, orthorhombic 8 (0-15)
Medium
Fast
Medium
Good
Good Medium 320 (high)
Good
b 0 3
Common crystalline
C3A
-
Monoclinic
Good 120 (medium)
Orthorhombic 8 (5-15)
Medium 100 (medium)
274
Ezeldin and KoMatis
contaminant moleculesin the siliceous solids, and blocks pathways between pores. Due to the elevated pH of cement mixtures (in the range of 9-11), most multivalent cations are converted into insoluble hydroxides or carbonates. Thus, this process is highly effective for waste components with high levels of toxic metals. However,the presenceof certain inorganic compounds and organic componentsin the waste can create an interference mechanism, delaying the setting and curing of the final product. Biczok[l] presented twolists of industrial wastesthat are harmless or are deemed to be aggressive to cementitious products. The list of wastes that can be tolerated includes Brines containing basesbut not sulfates Potassium permanganate, occurring at fermenting and purification installations Sodium carbonate and potassium carbonate Bases, provided that their concentrations are not excessively high Oxalic acid occurring at tanneries Mineral oils and petroleum products (benzene, kerosene, cut-back oil, naphtha, paraffin, tar) as long as they contain no acids that can continue to remain in the products after chemical treatment The list of wastes considered aggressiveto cementitious products includes Water containing gypsum Ammonia salts Hydrochloric, nitric, acetic, and sulfuric acids Chlorine and bromine All sulfur and magnesium salts Salts of strong acids 'High sulfate content in the waste can be tolerated by using Vpe I1 or Vpe V portland cement rather than the typically used "Qpe I cement. There is reasonable correlation between sulfate resistance of cement and its tricalcium aluminate (C3A)content. Hence, use of 1s.pe I1 cement (C3A is limited to8%) or "Qpe V cement (C3A is limited 5%) to can providea moderate or good sulfate resistance, respectively. Additives havebeen developed to overcome the effects of several of the other interfering agents. Some of these additives are clay, vermiculite, soluble silicates, and some proprietary products. The advantage of cement-based techniques is the use of low-cost materials and common commercial concrete processing equipment with no need for specially skilled operators. The main disadvantage isthat most of the wastesare not chemically bonded. Hence, once subjected to acidic leaching, resolubilization of the metal hydroxides and carbonates becomes a major concern.
B. Pozzolanic Techniques Pozzolanictechniquesinvolvemixing a siliceous (pozzolanic) material with other alkaline earths such as lime or gypsum in the presence of water to produce a concrete-like mass. This technique generally involves pozzolanic reactions between SiO,, A1203, F%03, and available calcium in lime. The results of these reactions are very stable and strong calcium silicates and aluminates that can be considered the equivalent of portland cement in initiating the cementation process. The most common pozzolanic materials used in stabilizatiodsolidificationof of these materials, wastes are fly ash, ground blast-furnaceslag, and cement kiln dust. The use themselvesconsideredby-products of little commercialvalue, to stabilindsolidify another
Solidification and Stabilization Techniques
275
waste may offer economic advantages. The vulnerability of the final product to acidic leaching is the major disadvantage of this technique. Combined cement-pozzolanic processes can be used to give a better and more economical final waste containment.
C. ThermoplasticTechniques In thermoplastic processes, the waste is dried, heated, and then dispersed through a heated plastic system. Bitumen is themost common matrix material. However, paraffin and polyethylene have alsobeen employed. The final mixture is then cooled and is usually embedded in a containment system. Organic chemicals that could react with the plastic matrix, leading to physical deterioration, cannot be disposed of with this technique. The process employs specialized equipment, is energy-intensive, and requires trained operators. The elevated temperatures of the process limit the typesof materials that canbe incorporated into the matrix (e.g., citrates). Typically, the associated leach rates are low. Also, the binders tend to perform well in resisting water and microbial attack.
D. SorbentTechniques The sorbent techniques involve the use of certain clay minerals characterized with high specific surfaces to fix and immobilize hydrocarbon molecules. This immobilization usually takes place on the outer and inner surfacesof clay particles. Typical clay minerals used in this technique include sodium montmorilloniteand vermiculite.
E. OrganicPolymerTechniques Originally, organic polymer techniques were introduced to solidify and/or stabilize radiological (UF)system is considered the most wastes to allow for transportation. The urea-formaldehyde frequently employed organic polymer solidification technique. However, this system has been, criticized becauseof its high cost, water loss during reaction, and environmental concerns with formaldehyde. Generally, in this technique, the wet or dry waste is mixed with a prepolymer in a waste secondary containment system. After the two components have been thoroughly mixed, a catalyst is introduced into the mixture. This process forms a mass that traps the waste. be disposed of after polymerization. After curing, Any water associated with the waste should the polymerized mass becomes highly impermeable. The product is characterized by a slow leaching rate even when crushed owing to its hydrophobic nature.
F. EncapsulationTechniques
1
In this type of technique, wastes that have been compacted or bonded togetherare encapsulated in a coating of inert materials. These coating materials are chemically stable, with good resistance to biodegradation and adhesionto enclosed wastes. High-density polyethylene (HDP) and polybutadiene have been used to encapsulate wastes.The process is energy-intensive and requires skilled labor.
G. Vitrification Vitrification is a thermal treatment process that uses high temperatures to melt waste material. The melted waste is then fused in glass or another form of synthetic mineral. Organic conare intaminants are volatilized during the melting process, while the inorganic contaminants
276
Ezeldin and Kolfiatis L
corporated intothe impermeable matrixof the final product. Glass andsynthetic silicates offer a safe means of waste disposal because they are very slowly leached by water. The process mandates highly skilled labor and is extremely energy-intensive. Vitrification requires an electric source to produce high temperatures (1500-2OOO"C)to melt the waste material. Electrodes made of molybdenum or graphite are commonly used for this purpose. An electric current is then applied to the electrodes and travels throughthe waste material. Typically a 12,500-V three-phase current is used. The melted waste becomes more conductive, and the current causes the melt to continue downward and outward. Heat willeither destroythe organics or cause volatilization. A gas collection and treatment systemis usually required to address the problem of emissions. Because of melting, the pore spaces are eliminated, resulting in a volume reduction of about 20-40% depending on the porosity ofthe treated waste. The melted material cools to a glassy microcrystalline material commonly referred to as a monolith. The glassy product, which has a random crystalline structure, will fracture colloidally instead of along planes. The end product has an estimated life expectancy of 10,OOO to 20 million years.
IV. SPECIFICATIONS FOR SOLIDIFIEDISTABILIZED WASTES The final form of stabilized/solidified waste mustbe stable, environmentally compatible, and, if possible, of commercial value. A great deal of consideration should be given to waste and process compatibility to avoid detrimental effectsof the final product suchas heat generation, release of toxic materials, and loss of strength. Table 2, from Martin and Johnson [2], summarizes compatibility considerations of selected waste categories. The most important characteristics of the stabilizedkolidified waste that must be evaluated and found acceptable are the following. Leachability. Leaching tests are designed to determine the maximum concentration level of contaminants that water can remove from the hardened waste. The most used leaching test is the EPAToxicity Contaminant Leachate Procedure (TCLP). It requires grinding the waste to ensure maximum surface contact area with an acidic extractionfluid. The ratio of waste to extraction fluid is set to achieve a saturated solution. The contaminant concentration is obtained and compared against the toxicity characteristics set by the EPA (see Table 3). Free-liquid conrenr. Current EPA regulations allow no free water in the waste final form. h-permeability. The treated waste should have relatively low permeability (typical acceptable permeabilityrangesfrom to 10"' cdsec). Higherpermeability wouldresultin increased leaching of contaminants. Biodegradation. Biological activity is undesirable and should be avoided as it can produce acids that could dissolve and leach pollutants from the waste. Tests suchas ASTM G-21 and G-22 are used to determine the ability of wastes to withstand biological attack. Strength. The unconfined compressive strength of the hardened waste depends on its final use. Usually, only 20 psi is required for burial. However, if the solid waste is to be used for other purposes, for example, as a construction material, greater strength may be required (up to 4OOO psi). ASTM C-39 is usually employed to evaluate the compressive strength of cementitious stabilizedkolidified wastes. Durabiliq. If it willbeexposedtofreeze-thawandwet-dry cycles, stabilized/solidified waste should be tested for durability. For moderate exposure,ASTM D-560(freeze-thaw durability) and ASTM D-554 (wet-dry durability) are used. However, for harsh exposure, ASTM C-666 is recommended.
Table 2 Compatibility of Waste Categories with Solidification/Stabilk.ationTechniques -
~
~~
~
Treatment type Waste component Organics Organic solvents and oils Solid organics (e.g., plastic, resins, tars) Inorganics Acid wastes Oxidizers
Organic polymer (UF)
Cement-based
Lime-based
Thermoplastic
Surface encapsulation
May impede setting; may escape as vapor Good;often increase durability
May impede setting; may escape as vapor Good; often increases durability
Organics may vaporize on heating Possible use as binding agent
May retard setting of P O l Y ~ ~ May retard setting of polymers
Must first be absorbed on solid matrix Compatible; many encapsulation materials are plastic
Cement will neutralize acids Compatible
Compatible
Can be neutralized be-
Compatible
Compatible
fore incorporation May cause matrix breakdown
May cause matrix breakdown
Compatible
May dehydrate and rehydrate, causing splitting
Compatible
Can be neutralized before incorporation May cause deterioration of encapsulating materials Compatible
Easily leached from cement; may retard setting Compatible
May retard setting; most are easily leached Compatible
May dehydrate
Compatible
Compatible
Compatible
Compatible
Compatible
Compatible
Compatible
Acid pH solubilizes metal hydroxides Compatible
Sulfates
May retard setting and cause spalling unless special cement is
Halides
Ussd
Heavy metals Radioactive materials
Source: Martin and Johnson 121.
Compatible
278
and
Ezeldin
Koflatis
Table 3 Toxicity Characteristics of Constituents and Regulatory Levels Chronic toxicity Regulatory EPA number Constituent HW number WO4 WO5 DO18 WO6 DO19 DO20 DO21 W22 m 7 DO23 DO24 DO25 DO26 DO16 DO27 DO28 DO29 DO30 DO12 DO3 1 DO32 DO33 DO34 DO08 DO13 Do09 DO 14
DO35 DO36 DO37 DO38 DO10 DO1 1
DO39 DO 15 Do40
DO41 m 2
DO17 Do43
Arsenic Barium Benzene Cadmium Carbon tetrachloride Chlordane Chlorobenzene Chloroform Chromium 0-Cresol mCresol p-Cresol Cresol 2,4-D 1 &Dichlorobenzene 1 ,2-Dichloroethane 1,1-Dichloroethylene 2,CDinitrotoluene Endrin Heptachlor (and its hydroxide) Hexachlorobenzene Hexachloro-l , 3-butadiene Hexachloroethane Lead
Lindane Mercury Methoxychlor Methyl ethyl ketone Nitrobenzene Pentachlorophenol Pyridine Selenium Silver Tetrachloroethylene Toxaphene Trichloroethylene 2,4,5-Trichlorophenol 2,4,6-TrichlorophenoI 2,4,5-TP (Silvex) Vinyl chloride
7440-38-2 7440-39-3 7 1-43-2 7440-43-9 56-23-5 57-74-9 108-90-7 67-66-3 7440-47-3 95-48-7 108-39-4 106-44-5 94-75-7 106-46-7 107-06-2 75-35-4 121-14-2 72-20-8 76-44-8 118-74-1 87-68-3 67-7 -1 7439-92-1 58-89-9 7439-97-6 72-43-5 78-93-3 98-95-3 87-86-5 110-86-1 7782-49-2 7440-22-4 127-18-4 8001-35-2 79-01-6 95-95-4 88-06-2 93-72-1 75-01-4
0.05
1.o
0.005 0.01 0.005 0.0003 1 0.06 0.05
2 2 2 2 0.1 0.075 0.005 0.007 0.0005 0.0002 O.ooo08
0.0002 0.005 0.03 0.05 0.004
0.002 0.1 2 0.02 1
0.04
0.01 0.05
0.007 0.005 0.005 4 0.02 0.01 0.002
5.0
100.0 0.5
1.o 0.5 0.03 100.0 6.0 5.0
200.0 200.0 200.0 200.0 10.0 7.5 0.5
0.7 0.13 0.02 0.008 0.13 0.5
3.0 5.0
0.4 0.2 10.0 200.0 2.0 100.0 5.0 1 .o 5.0
0.7 0.5
0.5 400 2.0 1.o 0.2
V. EFFECT OF ORGANIC AND INORGANIC WASTES ON PORTLAND CEMENT Several studies have examined the effects of typical organic waste components upon cementbased solidification/stabilizationtechniques. Most of these studies investigate the setting and durability of the waste product. Fewer studies have presented extensive testing the for strength
Solidificationand Stabilization Techniques
279
Table 4 - Compatibility of Portland Cement with Organic Compounds Portland cement groupChemical
Durability: decrease (destructive action occurs over a long time period) Aldehydes and ketones DIU Aliphatic and aromatic Set time: increase (lengthen or prehydrocarbons vent from setting) Durability: no significant effect DIU Amides and amines Chlorinated hydrocarbons Set time: increase (lengthen or prevent from setting) Durability: decrease (destructive action occurs over a long time period) Ethers and epoxides DIU Heterocyclics DIU Nitriles DIU Set time: no significant effect Organic acids and acid Durability: decrease (destructive chlorides action occurs over a long time period) D/U Organometallics Set time: no significant effect Phenols Durability: decrease (destructive action occurs over a long time period) DIU Organic esters Alcohols and glycols
Types I1 and V
I
Durability: decrease (destructive action occurs over a long time period) DIU Set time: increase (lengthen or prevent from setting) DAJ Set time: increase (lengthen or prevent from setting) Durability: decrease (destructive action occurs over a long time period) DIU DIU DIU Set time: no significant effect Durability: decrease (destructive action occurs over a long time period) DIU DIU
DN
DRI = dataunavailable. Source: Spooner et al [3].
and leachabilityof the hardened waste product. In general, strength and leachability properties vary withtime, waterkement ratio, and interaction with individual waste components. Spooner et al. [3] compiled existing information about the compatibility of cement-based stabilization and solidification techniques with different classes of organic chemicals. Table4, from Spooner et al. [3],includes a matrix that summarizes the compatibility of Type I, Type 11, and Type V cements with different groups representing thetypes of organic compounds most likelyto be found in hazardous wastes. Spooner et al. [3] also reviewed existing literature about the known effects of inorganic compounds on stabilizatiodsolidification cement-based processes. Table 5 illustrates that inorganics have a pronounced effect on setting time and durability of the final product.
VI. EFFECT OF ORGANIC AND INORGANIC WASTES ON POZZOLANIC BINDERS During the last decade, there has been increasing interest in the use of pozzolanic bindersto stabilize/solidify wastes. Selected research projectsin which pozzolanic binders are used as a binder for waste stabilizatiodsolidification are presented in Table 6 (Eklund [4]). This table
280
and
Ezeldin
Koeatis
Table 5 Compatibility of Portland Cement with Inorganic Compounds Portland cement group Chemical
QpeI
HeavymetalsaltsandSettime:increase(lengthen or presetting)from vent complexes Durability: decrease (destructive action begins within a short time period) Inorganic acids time: Set significant no effect Durability: decrease (destructive action occurs over a long time period) Inorganic bases time: Set significant no effect Durability: no significant effect
Inorganic salts
Set time: increase (lengthen or prevent from setting) Durability: decrease (destructive action begins within a short time period)
and
Qpes I1
V
Set time: increase (lengthenor prevent from setting) Durability: no significant effect Set time: no significant effect Durability: no significant effect KOH and NaOHSet time: no significant effect Durability: decrease (destructive action occurs over a long time period) OthersSet time: no significant effect Durability: no significant effect Set time: increase (lengthen or prevent from setting) Durability: no significant effect
Source: Spooner et al. [3].
provides an overview of the types of inorganic and organic wastes that have been studied for waste stabilization. These research projects indicate that the useof pozzolans is becoming widely accepted in research studies to stabilizelsolidify both organic and inorganic wastes. There is, however, a need for a better understanding of fundamental chemical and physicalreactions that controlthe pozzolans-waste interaction and better predictive models for determining short- and long-term performancecharacteristics of the hardened waste.
VII. EFFECT OF ORGANIC AND INORGANIC WASTES ON ASPHALT AND POLYMERIC BINDERS Spooner et al. [3] compiled existing studies (up to 1984) on the interaction of organic and in7-9 from that reference represent the organic wastes with asphalt and polymeric binders. Tables available information in matrix form. The effect on setting time and durability was found to vary significantly depending on the chemical group and the binder type.
VIII. REUSE OF PETROLEUM-CONTAMINATED SOILS IN CONCRETE PRODUCTION An experimentalinvestigationdesigned to evaluatetheeffectiveness of usingpetroleumcontaminated soils (PCS)as fine aggregate replacement in concrete was conductedat Stevens Institute of Technology. The details of the experimental programand the resulting findingsare
e
281
solidification and Stabilization Techniques
Table 6 Research on Stabilization using Pozzolanic Binders Binder
Organization type description Waste Acid hydrocarbon sludge, spent clay Solvent extraction raffinate
Drexel Univ.
Methanol, xylene, benzene, adipic acid, oil and grease Organic sludges
Corps of Engineers
Radioactive waste alkaline salt solution Radioactive soluble salts waste MO processing waste, Zn refining sludge, electroplating sludge Zinc nitrate, cadmium nitrate, mercury chloride, mercury nitrate solutions Neutralized sludge from titanium processing
E.I. DupontAJ.S. DOE Pennsylvania State Univ. American Resources Corp.
Inorganic
Iron. steel
Inorganic
Electroplating sludge
Pennsylvania State Univ. Univ. of Missouri
Inorganic
Synthetic metal hydroxide sludges Water-based drilling muds Garbage and chemical incinerator waste, gasification ash and slag, sewer sludge, kiln waste, gypsum, ash from Lonox burners, fluidizedbed ashes
Fly ash, lime
Organic
Class C ash, lime, and cement mixtures with Class F ash Fly ash, lime
Organic
Portland cement, fly ash, calcium sulfate dihydrate, lime, recycled rubber, asphaltene, adsorbent material Class C fly ash, hydraulic blast furnace slag Portland cement, Class C and Class F fly ash Class C fly ash
Organic
Fly ash, portland cement
Inorganic
Fly ash, cement, lime, hemihydrated calcium sulfate, dehydrated calcium sulfate 'Qpe I cement with ClassF fly ash Portland cement, Class C fly ash 'Qpe 2 portland cement
Inorganic
Class C fly ash Class C fly ash, lime
Inorganic Organic and inorganic
Organic
Inorganic Inorganic Inorganic
ORNIfDOE
Velsicol Chemical Corp.
Oxford Univ. Imperial College State Univ. of New York
Univ. of New Hampshire Univ. of Oklahoma Aardelite USA
Source: Eklund [4].
discussed by Ezeldin [5-71. In this section a general description of the experimental program and highlights of results are presented. Five PCS types with different levels of heating oil and gasoline contamination were investigated. Three PCSlsand replacementratios were incorporated. Setting times, strength, durability, and leachability of benzene to water were evaluated. 111portland cement meeting ASTM C-l50standard specifications was used during the entire experimental program. Fine
npe
p
282
and
Ezeldin
Kolfiatis
Table 7 Interaction of SolidificationlStabilization Binders and Organic Chemical Groups Chemical Alcohols and glycols Aldehydes and ketones
Durability: no significant effect Durability: decrease (destructive action occurs over a long time period) Aldehydes only-Set time and durability: no significant effect Aliphatic and aromatic Durability: decrease (destructive hydrocarbons action occurs over a long time period) Amides and Amines DAJ Chlorinated hydrocarbons D/U Ethers and epoxides DN Heterocyclic DIU Nitriles DN Organic acids and acid Durability: no significant effect chlorides Organometallics Phenols Organic esters
DIU Durability: decrease (destructive action occurs over a long time period) DN
Durability: decrease Durability: no significant effect
Durability: no significant effect DN Durability: no significant effect Durability: no significant effect Durability: no significant effect DIU Set time: increase (lengthen or prevent from setting) Durability: no significant effect DIU D/U D/U
D/U = data unavailable. Source: Spooner et al. [3].
aggregate was a natural sand, and coarse aggregate consistedof 3/8-in. crushed stone.The soil classification, moisture content, contaminant type, and contamination concentration are shown in Table 10. Soils were sieved through a No. 4 sieve before use to discard any debris or large unwanted particles. A concrete control mixture with no PCS was used as reference and had a compressive strength of 6OOO psi after 7 days. For each soiltype, three mixtures were obtainedby replacing sand with PCS (PCS/sand ratio of lo%, 20%, and 40% by weight). Table 11 shows the mixture proportions and designations. After mixing,the following specimens were cast: 3 X 6 in. and 4 X 8 in.cylinders for compression tests, 4 X 4 X 14 in.prismsfor flexural tests, 2 X 2 X 10 in. prisms for durability tests, and 2 X 2 X 2 in. cubes for leachability tests. All concrete specimenswere left at room temperature for24 hr and then removed from their molds. They were then immersed in water until testing, with the exception ofthe leachability test specimens, which were left uncovered at room temperature. The 3 X 6 in. compression cylinderswere tested in duplicate at an early age (2 days; see Table 12). The 4 X 8 in. compression cylinders were tested in duplicateat a later age (7 days; in accordancewith ASTM procedureC-39.The see Table 12). Testswereconducted 4 X 4 X 14 in. prisms were tested after 7 days accordingto A S T M C-78 as indicated in Table 12. The durability tests consisted of two parts: freeze-thaw testing and wet-dry testing. The wet-dry testing was performedin accordance with ASTM D-559, while the freeze-thaw testing was performed according to ASTM D-560. The leachability testwere performed in dupliC3 for eachof the five cate on monolithic2 X 2 X 2 in. concrete cubes obtained using mixture soil types (see Table 11).
2 z
$
Table 8 Interaction of SolidificatiodStabilization Binders and Organic Chemical Groups Chemical group Alcohols and glycols
Aldehydes and ketones
Aliphatic and aromatic hydrocarbons
Amides and amines
phenolic
DN
Set time: decrease Durability: no significant effect Durability: decrease (destructive action occurs over a long time period) Low molecular weight polymers only DN
Chlorinated hydrocarbons
Durability: decrease (destructive action occurs over a long time period)
Ethers and epoxides
DN
Urethane Set time: decrease Durability: no significant effect Durability: decrease (destructive action occurs over a long time period) Durability: no significant effect
Durability: decrease (destructive action begins within a short time period) Durability: no significant effect
Durability: no significant effect
Urea-formaldehyde
DN
EPOXY
Polyester
Durability: no significant effect
Durability: no signifi-
DN
DN
cant effect
2. 0 3
% Cll
ii
z
* h
DN
2. -
g. 3
3 Set time: increase Durability: no significant effect
Durability: decrease (destructive action occurs over a long time period)
Durability: decrease (destructive action occurs over a long time period)
DN
DN
DN
set time: increase (lengthen or prevent from setting) Durability: no significant effect Durability: decrease (destructive action occurs over a long time period)
Durability: decrease (destructive action occurs over a long time period)
Durability: decrease (destructive action occurs over a long time period)
DN
DN
5
2.
9 E rg il
Table 8 Continued .~
Chemical group
Phenolic
Polyester
EPXY
D/U DIU
DN DN
DN D/U
Set time: increase (lengthen or prevent from setting) Durability: no significant effect
Set time: no significant effect Durability: no significant effect
Durability: decrease (destructive action occurs over a long time period)
Durability: decrease (destructive action occurs over a long time period)
DN
DN
Set time: increase (lengthen or prevent from setting) Durability: no significant effect
Durability: decrease (destructive action begins within a short time period)
DN DN
D/U DN
DIU DN
DN
DN
DN
DN
DIU
DN DN DN
Organometallics Phenols
DRI = data unavailable. Source: Spooner et al. [3].
Urea-formaldehyde
DN DN
Heterocyclics Nitriles Organic acids and acid chlorides
Organic esters
Urethane
Table 9 Interaction of Solidification/StabilizationBinders and Inorganic Chemical Groups ~
Chemical Group Heavy metal salts and complexes
UreaBitumen Durability:
decrease
Silicate Set time: decrease
(destructive action occurs over a long time period) Set time: Inorganic acids Durabiiity: no significant decrease effect Durability: no Nonoxidizing, significant except coneffect centrated acids
Acrylamide
Phenolic
Urethane
formaldehyde
Set time: increase (lengthen or prevent from setting)
D/U
Set time: increase (lengthen or prevent from setting) Durability: decrease (destructive action begins with in a short time period)
Durability: no Set time: Set time: no significant increase significant effect. Nonox(lengthen or effect idizing prevent from Durability: desetting) crease (deDurability: destructive crease (deaction occurs structive over a long action begins time period) within a short time period)
DN
Durability: no significant effect
EPOXY
DN
Polyester
DnJ
Durability: no Durability: no significant significant effect. Nonoxeffect. Nonoxidizing idizing, except NF
Table 9 Continued Chemical Group
Bitumen
Inorganic bases Durability: no significant effect
Inorganic salts
Durability: decrease (destructive action occurs over a long time period)
DAJ = data unavailable. Source: Spooner et al. 131.
Silicate Set time: increase (lengthen or prevent from setting) Durability: decrease (destructive action begins with in a short time period) Set time: decrease
Acrylamide
Phenolic
Urethane
Ureaformaldehyde
EPOXY
Polyester
Set time: decrease Durability: decrease (destructive action occurs over a long time period)
Durability: decrease (destructive action occurs over a long time period)
Durability: decrease (destruction action occurs over a long time period)
Set time: Durability: no increase significant (lengthen or effect prevent from setting) Durability: decrease (destruction action begins within a short time period)
Durability: decrease (destructive action occurs over a long time period)
Set time: decrease Durability: decrease (destructive action occurs over a long time period)
Set time: decrease Durability: no significant effect BleachesSet time: decrease Durability: decrease (destructive action occurs over a long time period)
Durability: decrease (destruction action occurs over a long time period)
Durability: no significant effect
Durability: no significant effect
Durability: no significant effect
287
Solidijkation and Stabilization Techniques Table 10 SoilDescription
(a)
Classification Soil Moisture content 1
ay-silty 2 sand Silty 3 4 5
Type of contaminant Concentration
Well-graded sand
oil Heating 7.3 oil Heating 14.3 oil Heating24.7 Poorly graded sand Gasoline 14.4 clay Silty Gasoline 19.6
0.11% by weight 0.12% by weight 0.66% by weight 25PPm 1500 ppm
The initial and final setting times of concrete are commonly defined by the ASTM C403 test method. Setting times are determined from the rate of solidification curve. The solidification curve of the control mixtureis shown in Figure 1. The recorded initial and final setting times were about 1.5 and 6 hr, respectively. Figure 2 indicates the effect of introducing PCS (soil ' I & 1) on the initial and final setting times of concrete. The initial setting time seems to increase with the increase of PCS/sand replacement ratio. The final setting time is found to be less affected by the increase in the PCSlsand replacement ratio. For instance, at a PCS/sand replacement ratio of 40%, the initial setting time increasesby about 30% compared to the control mix, while the final setting time increasesby only 20%. A similar trend is observed for all five soil types included in this investigation. During this experimental program, all concrete mixtures reached their final setting time well before the specimens were removed from the molds, that is, in less than24 hr (usually within9 hr). The effectof contaminant concentration on setting time can be seen in Figure 3. When comparing the setting time of soil 1 (0.11% contaminant concentration)to soil 3 (0.66% contaminant concentration), it is found that both the initial and final setting times are consistently greater for the soil with higher contaminant concentration. These results indicate that the inclusion of €'CS and the contaminant concentration affect the setting time of concrete. However, for the soil types included in this invesTable 11 MixtureProportions Sand/
Cement Mix cement"sand typecement" (Iblyd')design Soilcement" Control 1 2
20% 0.91
800 0.56 . 0.31 800
1c1 1.37 1.21 1c 2 1C3 2c1 10% 1.21 2c2 2C3
800 800 0.61 800 0.15
.
PCSI
Water/
-
-
0.50
0.15 0.31
10% 20%
0.45
40%
0.45
40%
0.56
20% 40%
0.47 0.47
PCSl
0.91 1.37
0.45
800
3
1.21
800 0.61
3C2 0.91 3c3
800
4 0.495 1.21 40%
0.61 800 0.61
4c2 0.91 4c3 10% 5C1 5C2 0.91 5c3
800
800 0.15 800
0.3
I
1.21
1 0.48
20% 40%
0.48
1.37
800
'Ratio by weight. Note: Ratio of coarse aggregate to cement (by weight) = 1.52. Waterhinder ratio was adjusted to maintain slump valueat 6 l-in. slump.
+
288
Ezeldin and Korfiatis
Table 12 ExperimentalStrengthResults Compressive strength (psi)
desig.Mix
2 days
7 days
C
4950 4950 4950 4450 3258 2691 2124 3536 2404 2263 3395 3395 3253 2688 2826 2826
6160 5569 5171 5370 5728 5171 3262 4379 3941 3503 4658 4419 4140 4928 4805 38 19
1c1 1c2 1C3 2 c1 2c2 2C3 3C1 3c2 3c3 4C 1 4c2 4c3 5C1 5C2 5c3
Unit weight
Flexural strength (psi), 7 days
(1bm3) 151.6 152.5 148.4 146.5 143.6 141.6 137.4 144.3 142.6 138.4 148.9 149.3 154.3 151.6 149.9 148.6
975 975 956 843 750 731 656 693 675 487 881 788 788 693 656 637
tigation and for the contaminant concentrations used, this effect is not major; with a 0.66% contaminant concentrationand a PCS/sand replacement ratio of M%, the initial and final setting times recorded were only 50% and 30% higher, respectively. The compressive and flexural resultsfor all five soils are given in Table 12. Figures 4-8 indicate that, irrespective of the soil type, concrete with a higher PCS/sand replacementratio develops lower compressive and flexural strengths at both early and late stages. The presence of contaminants seems to interfere with the water-cement binding reactions, delaying or preventing the full hydration of the cement particles. The increase of PCS content (increase of PCSlsand replacement ratio) yields to the presence of more petroleum contaminants, which
4
a
6000 4600 --_-I.. 4000 "-.
600
0
-....-....-..".......-.
..
-c
1
I
1
I I
............ I
"
-__-
-"-.
_.I_.
.......................... ...".._
"
I
3
TIME, HOURS
Figure 1 Solidification curve of control mixture.
-
....._" ....."_. . -..."
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I
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I
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Solidification and Stabilization Techniques W
zc-
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8
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"
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._."...._..
I
P
E v)
1' 10%
1
1
20%
PCS replacement percentage
40%
Figure 2 Effect of PCS replacement percentage on setting time.
separate the cement particles from water. Hence, for the same total content of cement, less is actually available to react with water to produce the hardened binder. This results in the concrete being weaker than the control. The strength reductionat each PCS/sand replacementratio level depends on contaminant concentration, contaminant type, and soil type.The increase in contaminant concentration hasan adverse effect onthe concrete strength.When comparing the strength resultsof concrete containing soil1 (sandy soil with0.11% oil contamination)and soil 3 (sandy soil containing0.66% oil contamination),the results indicatethe following. While the presence of soil 1 with a PCS/sand replacement ratio of 40% reduces the concrete compressive strength by 10% after 2 days and 13% after 7 days, the presence of soil 3 with the same PCS/ sand replacement ratio yields concrete 54% weaker than the control after 2 days and 43% weaker after 7 days. A strength comparison for concrete containing soil 1 (sandy with 0.11% oil contamination) and concrete containing soil 4 (sandy with 0.003% gasoline contamination) reflects the effect of contaminant type on strength; sandy soil contaminated with gasoline produces concreteof lower strength than concrete containing a higher concentrationof heating oil.
" "
lW
01
20%
40%
PCS replacement percentage INITIAL SET TIME(S1)
0 FINAL SET TIME (81)
m INITIAL SET TIME(Sl)
m FINAL SET TIME (a31
Figure 3. Effect of contamination concentration on setting times.
290
E
Ezeldin and Korjiatis
N
6000
G
4000
;
9000
,
E 3
I
2000
m
l000 "
20%
10%
-
40%
PCS replacementpercentage LATE STRENGTH (10)
EARLY STRENGTH (2DI CONTROL
(ID)
FLEXURAL
I
S lzool T
I
'
40% 10%
20%
PCS replacementpercentage mLATE STRENGTH (70) CONTROL (70) Figure 4 Effect of K S replacement percentage on strength soil 1.
This is observed forearly and late stages. When comparingthe contaminant type effect on the strength of concrete containingfine-particle soil, no conclusive observationswere obtained for soil 2 (clay-silt with 0.12% oil contamination) vs. soil 5 (clay-silt with 0.15% gasoline contamination). The effectof soil type onthe strength of concrete containingPCS was found to be very important. The presence of soil with fine contaminated particles (soil 2) yields a lower strength than concrete mixed with sandy contaminated soil (soil 1). Whereas for soil 1 an average of 13% strength reduction is observed at a PCS/sand replacementratio of 40%, the inclusion of soil 2 results in a strength reduction of about 45%. The results of the wet-dry and freeze-thaw tests are presented in Table 13. The tests were conducted on the control mixture andthe C3 mixture of all five soils. These results presentthe percentage of weight loss fora 2 X 2 X 10 in. prism subjected to thewet-dry and freeze-thaw cycles. In general, the percentageof loss for all tested specimens was less than 2%.No visible cracking or surface deterioration could be observed on any of the tested specimens at the end of the test procedure. These preliminary results indicate that up to 40% PCS/sand replacement ratio could be used without seriously affecting the integrity of concrete members. However, more tests are required before such a conclusion can be confirmed. Leaching tests were conducted in duplicate for soils 1-5 using mixture C3 (20 samples) after 24 hr for a test duration of 96 hr and after 10 days fora test duration of 24 hr. All of these samples were nondetectable for benzene. Thedetection limit was0.01 ppm. It is apparent that
291
Solidification and Stabilization Techniques COMPRESSION
7000
R E N
6000
G
4000
T H ,
g
5000 9000
2000
l000 0000
I
10%
EARLY STRENGTH CONTROL
S
1200
R
1000
T E
I
40%
208
PCS replacementpercentage
'
m LATE MRENQTH
(201
(ID1
e01
-"---FLEXURAL
" "
" " _ _ _ I " "
10%
20%
40%
PCS replacementpercentage mLATE STRENGTH ( 7 0 )
CONTROL
(m)
Figure 5 Effect of PCS replacement percentage on strength soil 2.
the amount of benzene leaching from concretewas not sufficient to producea measurable response. To increase the sensitivity of the leachingtest, it wasdecided tocreate a soil artificially contaminated with neat benzene. Samples from soils 1 and3 were left in an oven at 105°C for 4 hr to drive off VOCs. The samples were left uncovered overnightto allow rehydration. Contaminant concentration of 0.5% and 3% benzene (C, in Table 14) were added by weight basis. Samples free of contamination were used as a control. Concrete cubes were made using C3 mixture proportions. The leachability results, after 24. hr, for concrete containing artificially contaminated soilsand for loose soilsare shown in Table 14.The loose soil results (designated mix L) indicated that soil 3 leached about twicethe benzene at 0.5% benzene contaminationas indicated by the benzene concentration in the water (C,,,)at the end of the test. At 3.0% benzene concentration, little difference was observed between the behaviorof the two soils. Since the soils were not in a solid matrix, mass transport in this system is relatively nonlimiting. It is likely that the concentrations observedare more indicative of the adsorption behaviorof the soils Thus it can be concluded that soil 1 is a stronger absorber than soil 3.A possible reason why there was little difference at the higher concentration is that the concentration may be approaching the solubility limit in this system. The leachate concentration, C,, for the PCS concrete cubes ranged from0.55 to 40 ppm. The leaching increased with the increaseof contaminant concentration for both soil types. The cube surface leaching flux (M) varied from 0.003 to 0.217 g/(hr-m*). The fraction of benzene in the cubes that leached &) was 0.111.27%. These levels are about 99% lower than the values for loose soil in the case of 0.5% contamination, and about 95% lower for the case of 3% contamination.
292 S T R
Ezeldin and Kofiatis COMPRESSION
7000 8000
E N
a
" "~
~
5000
G 4000 T aooo H
.
2000
1000 nnnn ""
P S I
mm
40%
20%
PCS replacementpercentage
m EARLY STRENQIH (201 CONTROL
FLEXURAL
1200
S T
R E N
m LATE STRENGTH (ID)
(101
1000
_- "
.
G T
H
P S I
20%
lo%
40%
PCS replacementpercentage =LATE
STRENGTH ( 7 0 )
-CONTROL
(70)
Figure 6 Effect of PCS replacement percentage on strength: soil 3. COMPRESSION
7000
;
8000
E so00 N G 4000 T sow
H
2000
6
1000
s o
20%
io%
PCS replacementpercentage EARLY STRENGTH (201 CONTROL
S T
1200
l000
t
N G
T
40%
m LATE BTRENGTH
(7D)
(701
FLEXURAL
.
-.
"
800
WO
'r'
400
p
200 10%
40%
20%
PCS replacementpercentage
mLATE STRENGTH (70)
CONTROL
(70)
figure 7 Effect of PCS replacement percentage on strength: soil 4.
293
Solidijkation and Stabilization Techniques COMPRESSION
R
7000
ln"n
l
PCS replacementpercentage
S T
R
5.-
". . -
-
" "
m LATE 8TTRENOTH
EARLY 8TRENOTH (2D) CONTROL
(ID)
(ID)
FLEXURAL
1200
__ .____._
__._
1000 3
:
800
H
400
p
200
.."
I I
~
800
10%
40%
20%
PCS replacementpercentage LATE STRENQTH ( 7 0 )
CONTROL
(70)
Figure 8 Effect of PCS replacement percentage on strength: soil 5.
ACKNOWLEDGMENTS We gratefully acknowledge the valuable assistance of several graduate students who contributed to the experimental studies.This investigation was supported by the Division of Science and Research, NJDEPE, by the Hazardous Substance Management Research Center, Newark, New Jersey, and by the PSE&G Company, Newark, New Jersey. Their contribution is gratefully acknowledged.
Table 13 DurabilityTestResults Control
c3
Soil F-Ttype W-D F-T W-D 1 2 3 4 5
0.30 0.30 0.30 0.30 0.30
0.20 0.20 0.20 0.20 0.20
0.90 0.70 - 0.70 1.80 2.00
W-D, wet-dry testing; F-T,freeze-thaw testing.
0.50 1.10 0.30 0.90 1S O
294
Ezeldin and Koflatis
Table 14 ExperimentalLeachabilityResults Mix Soil 1
C, (%,Run w/w)
3 3 1
c3 c3 c3 c3 c3 c3 c3 c3 c3 c3 c3 c3 c3 c3 c3 L L L L L
1
L
3 3
L L L L L L
1 3 3 3 3 1 1 3 3 1 1
3 3 3 1 1
1 1
3 3
C , (ppm)
0.0 0.0 0.0 0.0 0.0 0.0 0.5 0.5 0.5 0.5
A B A B C
3.0 3.0 3.0 3.0 3.0
A B A B C A B A
0.0 0.0 0.0 0.0 0.5 0.5 0.5 0.5
3.0 3.0 3.0 3.0
D
A B A B
B
A B
A B
A B
A B
fL
(%)
O.OOO4 O.OOO4
0.074 0.072 0.115 0.118 0.126 0.112 0.868 0.952
0.170 0.186
0.550
0.107
0.558 8.467 9.519 39.235 25.737 26.686 CO.01 <0.01
0.109 0.276 0.310 I .277 0.838 0.869
0.0006 0.0006 0.0007 0.0006 0.0048
0.0053 0.0030 0.0031 0.0467 0.0525 0.2166 0.1421 0.1473 O.oo00 O.oo00 O.oo00 O.oo00
<0.01 ,<0.01
101.4 101.9 193.0 203.5 562.7 587.1 599.7 605.7
Flux M [g/(hr.m*)]
26.122 26.271 49.083 51.751 24.178 25.229 25.425 25.679
0.7381 0.7423 1.3869 1.4623 4.0989 4.2770 4.3103 4.3535
REFERENCES 1.
2. 3. 4. 5.
6. 7.
Biczok, I., Concrete Corrosionand Concrete Protection,Chemical Publishing Company, New York, 1967. Martin, E., and Johnson, J., Jr., (eds.),Hazardous Waste Management Engineering.Reinhold, New York,1986,pp.140-141. Spooner, F! A., et al., Compatibilityof Grouts with Hazardous Wastes, EPA-600/2-84-015, Municipal Environmental Research Laboratory, U.S. Environmental Protection Agency, Cincinnati, Ohio, 1984. Proceedings of AmerEklund, A. G., Survey of the useof coal ash in hazardous waste stabilization, ican Coal Ash Association, 9th Int. Coal Ash Utilization Symp., Orlando, Fla., 1991, pp. 28.128.20. Proceedings Ezeldin, A. S., Use of coal ash in productionof concrete containing Contaminated sand, of American Coal Ash Association, 9th Int. Coal Ash utilization Symp., Orlando, Ha., 1991, pp. 17.1-17.9. Ezeldin, A. S., et al., Stabilization and solidificationof hydrocarbon contaminatedsoils in concrete, J. Soil Contamination, 1(1), 61-79 (1992). Ezeldin, A. S., Vaccari, D. A., and Mueller, R. T.,Fly Ash Concrete Containing Hydrocarbon Contaminated Soils, American Concrete Institute, Spec. Publ. SP 132-38, 1993, pp. 693-712.
Solidification and Stabilization Techniques
295
ADDITIONAL READING ASTM, Annual Book of ASTM Standards, Concrete and Mineral Aggregates, 1989. Britton, C. L., NewJerseyGround-waterContaminationIndex,Sept.1974-April1984,NewJersey Geological Survey, Open File Rep. 84-1. Calabrese, E. J., and Kostecki, I? T.,Petroleum Contaminated Soils, Vol. 2, Lewis, Chelsea, Mich., 1990. Conner, J. R., Fixation and solidification of hazardous wastes, Chem. Eng., Nov. 10, 1986, pp. 79-85. Conner. J. R., Chemical Fixation and Solidificationof Hazardous Wastes,Van Nostrand Reinhold, New York, 1990. Dowd, R. M.,Leaking underground storage tanks, Environ. Sci. Technol., 18(10), (1984). Jones, L. W,, Interference Mechanisms in Waste StabilizatiodSolidificationProcesses, U.S.Army Engineer Waterways Experiment Station, Vicksburg, Miss., EPA/600/2-89/067, January 1 9 9 0 . Kostecki, F! T., and Calabrese, E. J., Petrofeum Contaminated Soils, Vol. 3, Lewis, Chelsea, Mich., 1991. AmerMalhotra, V. M. (ed.), Fly Ash, Silica Fume, Slag, and Other Mineral By-Products in Concrete, ican Concrete Inst., SP-79, 1983. Malhotra. V. M. (ed.), Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete,American Concrete Inst., SP-114, 1989, 2 volumes. Martin, J. I?,Biehl, F. J.,andRobinson,W. T., Stabilized petroleum waste interaction with silty clay subgrade,in PetroleumContaminatedSoils, Vol.2,(E. J. Calabreseand P. T. Kostecki,eds.), Lewis, Chelsea, Mich., 1990, pp.177-197. Mehta, l? K.,Concrete: Structures, Properties, and Materials, Prentice-Hall, Englewood Cliffs, N.J., 1986. hndergast, J., Fear of trying, Civil Eng., Proc., 61 (4). 52-55 (1991). Suprenant, B. A., Lahrs, M. C., and Smith, R. L.,OilCrete, Civil Eng., Proc., 60(4), 61-63 (1990). Sehgal, S. B., and Gronowicz, A., Solidifying and stabilizing waste at a hazardous landfill, Concrete Int., Proc., 12(7), 41-44 (1%).
This Page Intentionally Left Blank
13
Soil Remediation with Environmentally Processed Asphalt (EPA" ) S. M. Testa and D. L. Patton Applied Environmental Services, Inc. San Juan Capistrano, California
1.
INTRODUCTION
A major consideration when formulating remedial action plans is the determination of what technology will best serve the specific project needs. No two projects are exactly alike, and each has site-specific limitations. However, three objectives must always be met if the remediation project is to be successful: The remedial method must be cost-effective, time-efficient, and environmentally sound. The production of asphalt is conventional in nature and well established [l]. Recycling soil containing petroleum hydrocarbons and metals affected soil via incorporation into cold-mix asphalt is a proven remedial technology [2-61. In addition, the incorporation of affected soil into asphalt products by the process refer to as environmentally processed asphalt (EPA'") is viewed as environmentally sound and has proved to be costeffective while providing the minimal amount of long-term liability in comparison with other soil remediation options and alternatives. Affected soils, formerly classified as hazardous waste, are incorporated with asphalt emulsion and specified grades of aggregate to produce a range of cold-mix asphalt products that fulfill therequirements of a variety of end uses. Notable structures; parkamong these uses are landfill caps andliners; tank farm dikes and containment ing lots, truck terminal and salvage yard pavements; road construction material; and port facility container shipping yard surfacing. This chapter discusses the methodology, regulatory framework, and certain properties of EPA pertaining to durability, chemical resistance, aging, biological resistance, permeability, leachability, chemical aspects, and product use.
II. METHODOLOGY The incorporation of affected soils to produce a commekially viable product can be accomplished with either of two processes: mixed-in-place for large quantities and windrowing for smaller quantities. For large-quantity projects, mixed-in-place processing is accomplished us297
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Testa and Patton
ing a portable asphalt batch plant. Providing production averaging 150 tons of cold-mix asphalt per hour, the plant operation consists of a mechanical screening plant, transfer conveyor,electric generator (all equipment except the rolling stock is electrically powered), the asphalt plant or pug mill, and asphalt emulsion truck (Figure 1). Materials are introduced into the process through the screening unit, which serves to separate all deleterious materials (i.e., trash, plastic, large rocks, etc.) from the soil and to size material in accordance with the design criteria. For example, asphalt sub-base would use lV2-in. aggregate, whereas pavement may call for to the batch plant’s %-in. or less. From the screen, the materials travel on the transfer conveyor soil hopper. There are two hoppers on the plant unit: one for soil and the other for aggregate. These materials are fed at a predetermined rate from each hopperby variable-speed conveyors and adjustable feed gates. Mix design suchas 50% soil and 44% aggregate are maintained to within 1% of total mix. The feed hoppers discharge onto the plant unit’s transfer conveyor to the mixing chamber. The materials discharge from the conveyor into a fluffer wheel compartment, where they are further mixed. Inside this compartment the asphalt spray bar applies the required amount of emulsion. From the asphalt compartment the material discharges into the mixing chamber. Inside this chamber are two counter rotating paddle-wheel mixers that have adjustable rotating speeds. This providesthe proper retention time to ensure a complete blending of all materials. The product is then transferred into trucks or to a stockpile ready for application. Theproductcan be. stockpiled formonths until needed.Forsmaller quantities, windrowing involves coating the soil with a proprietary emulsion and mixingthe materials in place. Product specifications are in accordance with or exceed those used by county and state road departments to produce road-mixed asphalt.
111. REGULATORY FRAMEWORK The regulatory frameworkof the federal environmental laws (and such states asCalifornia) do not deem everything as “hazardous” and mandate its disposal in a Class I landfill or by incineration. A review of current regulations proves quite the contrary. The letter, spirit, and intent of current hazardous materials legislation is to promote and develop alternative technology that encourages the use, reuse, and recycling of materials rather than the archaic load, haul, and dump remediation techniques that have produced more environmental problems than they ever solved. Specifically, the recyclingof affected soil into EPA is, for example, carried out in California under the following enabling legislation: California Code of Regulations (CCR) Title 22, Section 66262.11, Hazardous Waste Determination CCR Title 22, Section 66261.2, Definition of Waste CCR Title 22, Section 66261.3, Definition of Hazardous Waste CCR Title 22, Section 66261.4, Exclusions California Health and Safety Code (CHSC) Chapter 6.5, Article 4, Section 25143.2(b), Recyclable Material Code of Federal Regulations (CFR) Title 40, Part 261, Section 2 (40 CFR 261.2), Definition of Solid Waste 40 CFR 261,2(e), Materials that are not solid waste when recycled The above are the main sections that deal withthe use, reuse, and recycling of materials. There are a myriad of subsections and cross references to othersections that the reader will note upon review of these main listed regulations. Briefly stated, to paraphrase CHSC 25143.2(b), recyclable material that is or will be (1) used or reused as an ingredient in an industrial process to make a product or (2) used or reused as a safe and effectivesubstitute for commercial products
Soil
299
with Asphalt .
.
Figure 1 Portable asphalt batch plant processing cold-mix EPA via mixed-in-place process.
,
c
Figure 2 Grader producing cold-mix EPA via the windrowing process.
_
:
300
Testa and Patton
is excluded from classification as a waste. Hence, if the regulations do not classify recyclable materials as “waste” and these materials are not regulated as “hazardous waste,” their use, reuse, and recycling are within the letter, spirit, and intent of environmental legislation. Thus, the objectives of EPA methodology as a soil remediation option are
To effectively reuse affected soil as an ingredient in a stable, nonhazardous cold-mix asphalt that would be used on the property of origin as paving material To reduce generator liability to a minimum by complying with pertinent federal and state regulations To reduce the costof remediation by reusing affected soil as an ingredient in cold-mixasphalt, thereby eliminating many of the hazardous waste taxes and pretreatment and landfill disposal costs To demonstrate that the EPA method effectively stabilizesthe hazardous constituents comprising affected soil To demonstrate that EPA is a cost-effective, time-efficient, and environmentally sound remediation alternative to landfill disposal of hazardous wastes
IV. DURABILITY The best indicationthat asphalt liners andstructures have long-term durability and performance is the existence of surviving asphaltic structures from antiquity [7]. Asphalt was in general use in western civilizations from about 2000 B.C. to the first century A.D., when its use was superseded by more economic methods of working wood,tar and pitch, andasphalt deposits were no longer availableto existing mining technologies. Surprisingly, the ancient mixesare not that dissimilar to modem ones. Besides the obvious uses as mortars, pavements, revetments, and foundations, at many Mesopotamian sites asphalt liners from 0.1 mm to several centimeters thick were commonly used in drainage, water tanks, and plumbing fixtures. Inmany of these cases, the asphaltic structures that have escaped intentional or accidental destruction are still adequately performing their primaryfunction. Samples taken from thesestructures often show favorable properties, including low permeabilities, but it is uncertain how long exposureto UV light and surface conditions have affected the asphalt relative to the general aging processes expected in the subsurface environment.
V. CHEMICALRESISTANCE The testing and performance assessment of asphalt has traditionally focused on its structural performance as pavement and buildingmaterial. However, when evaluating the long-term performance of asphalt liners produced with affected soilas part of the aggregate, the focus is on their chemical performance. Becauseof the great immiscibility of petroleum products withiespect to the aqueous phases expected under impoundment conditions, large favorable freeenergy change exists for preventing the release of contaminants fromthe asphalt. Therefore,the chemical behavior and performance of the petroleum contaminant should parallel the behavior and performance ofthe asphalt itself. Detailed chemical testshave been performed on asphalt liners for disposal sites for uranium mill tailings [8] and for land disposal of radioactive waste [9]. These studies can be used to make a preliminary evaluation of the use of affected soil for cold-mix EPA incorporation. The resistance of asphalt to many reagents at atmospheric temperaturesis well documented [7,8,10]. Prolonged contact with dilute acidic solutions can result in hardening of the asphalt due to formation of asphaltenes. Nitric acid is very reactive with asphalt even in dilute solu-
Soil
301
tions, whereas hydrochloric acid does not affect asphalt. Asphalt reaction with sulfuric acid is intermediate. Asphalts are generally more resistant to alkaline solutions than to acidic solutions, a favorable characteristic for asphalt liners. However, alkaline solutions can react to form salts such as sodium naphthenates that form excellent emulsifying agents. Theoretically, this could be a problem for affected soilif contaminants were mobilized in the emulsified solution. However, the emulsification depends on the degree of alkalinity and the diffusion and hydraulic 10"' cm2/sec and resistances of the asphalt, which are generally extremely low, less than c ds e c , respectively [l l]. Without further experimental verification, emulsification of an asphalt liner is not expectedto be important, nor is it leachableto the extent of releasing hydrocarbonconstituents inexcessof regulatorylimits. The resistanceofasphalt to selected chemicals under a varietyof conditions is listed by MRM Partnership [7].
VI. AGING Of most importance to an asphalt liner are the effects of aging. Although not documented, hardening and other aging affects might increase mobilization of the petroleum contaminants from theEPA by supplying pathways out of the asphalt by and causing separation of petroleum constituents from the asphalt phases during aging. However, these effects would have to be excessive and affect a large proportionof the asphalt to mobilize the small amount of contaminants in the 10% fines of the aggregate. Physical hardeningdue to peptization, paraffin crystallization,and volatilization occursto differentdegreesinalltypes of asphaltandisunaffected by thepresenceofpetroleumcontaminated soil as a small part of the aggregate. Chemical hardening,however, may be imand varies portant. The rate of reaction of asphalt with oxygen is very temperature-dependent with asphalt type at high temperatures, but below 50°C the reactions are independent of temperature and asphalt type, are restricted to the asphalt surface, and should not be affected by affected soil. The hardening rate is higher in the presence of light, but because of the dark conditions of the subsurface environment, only the aging reactions that occur in the are darkof importance to an asphalt liner. In the dark, oxygen is bound into SO groups after short aging times and into CO groups after long aging times. Petroleum contaminants are not present in great enough quantities to affect the rate or degree of these reactions. Experiments show that the maximum depth of oxygen penetration is in the range of 2.5-5 mm [7], but the rate of hardening reduces considerably with time.
VII. BIOLOGICALRESISTANCE Microorganisms can degrade certain asphaltic components under ideal conditions [12]. search into microbial degradation of asphalt canbe summarized as follows [9,13,14]:
Re-
There is no single microorganism that will oxidize all asphaltic components. Microbial degradation occurs onlyat the outermost surfaces. The higher the molecular weight of the asphalt component, the more resistant it is to microbial degradation. Most soil asphalt-oxidizing microbesgrow best at pH 6-8. (0.7 mm Evenunderidealconditions,microbialdegradationrates do notexceedcmlday penetration per 1 0 0 years) and are usually an order of magnitudes less. Anaerobic degradation is much slower than aerobic degradation. Microbial attack is fastest for stream-refined bitumens, followedby air-blown and finally coal tar pitches.
Parton
302
and
Testa
Table 1 AnticipatedFieldLinerPermeabilities Effectiveness Assumed field final Avg permeability. K (cm/sec)thickness,
Liner
L (cm)factor,
KIL (sec”) ~
7 2
concrete Asphalt, Hypalon Asphalt membrane rubber 4 Catalytic air-blown membrane Sodium bentonite Saline seal 100 GSR-60
a
(as
Soil
x
10 0.12
x 10“O x
0.9 10 10 10
1 X 10-~
8x 6X
~~
7 X 10-9 2 X 10-9 5 x 10-6 8 X 10-9
0.8
7 X 10-9
~~
8 X 8 X 10-7
6 X 10” 1 x 10-6
10 X 10-5
Microbial inhibitors are ineffective over long time periods. Environmentalfactors(e.g.,temperature,pH, state of hydrocarbons,nutrientandoxygen concentrations) haveto be perfect for a very long time to result in any noticeable asphalt degradation. Overall, microbial degradation will be unimportant for all practical purposes.
VIII. PERMEABILITY Permeability tests have been performed ona variety of liners that were first subjected to aging is presented in Table 1. Permeability results tests [g]. The permeability obtained for each liner generated as part of this duty on five samplesof cold-mix EPA incorporating affected soilare presented in Table 2. Accelerated aging testsof an asphalt liner at 20°C under oxygen partial pressures of 0.21, 1, and 1.7 atm, with continuous exposure toan acidic leachate at 20°C under varying oxygenpartial pressures havebeen performed [g]. Solution pH values of 2.5,2.0, and 1.5 were designated as normal, intermediate, and highly accelerated conditions, respectively. Acidity levels were shown to havean unmeasurable effect on asphalt aging. Permeabilitywas used as a means to measure the immediate effectivenessof the asphalt liner. The permeability appears to be relatively unaffected under these exposure conditionsas shown in Figure 3.
IX. LEACHABILITY A normal asphaltic concrete or cold-mix EPA paving material is somewhat acceptable as an environmentally safe product even though there may be volatile organic compound (VOC)
Table 2 Results of Permeability Testing on Cold-Mix Environmentally Processed Asphalt Sample Sample Asphalt sample No. A-B- 1 A-B-2 5.06 A-B-5 4.98 A-B-7 5.06 A-B-10
Bulk
Hydraulic Effective conductivity permeability
diameter length (cm) volume (cm) (mdarcy) (cm’) 6.02 6.81 4.87 6.42 7.52
(cdsec) 5.05
5.05
120.58 136.94 94.86 129.10 150.62
0.135 0.013 0.034
0.096
0.142
1.42 X 1.37 X 3.58 X 1.01 X 1.50 X
10-7 lo-’ lo-* 10-~
10-7
Soil Remediation with Asphalt
303
U
W
a
m A
INTERMEDIATE
m NORMAL
l-
I
1
10-11 30 20
40
50
60
EXPOSUREPERIOD
70
00
90
(Days)
Figure 3 Permeability of an asphaltlineratnormal,intermediate, tions [g].
and highlyacceleratedcondi-
emissions during manufacture and placement, notably in regardsto the hot-mix process. The question of contamination is not frequently associated with asphaltic concrete even though certain halogenated volatile organics may be present in association with hot-mix bituminous prodor other solvents in their ucts, notably those containing cut-back asphalt using kerosene, diesel, dilution process [6]. Asphalt concreteor cold-mix EPA may be accepted by Class I11 landfill and is commonly crushed and used as a component for road base material.The fact that thereare few questions about the safety of asphaltic concreteis justified by the existence of thousands of miles of paved on asphalt show roadways constructed every year. In addition, the performance of leachate tests a definite lack of contaminants that might leach out under extreme conditions. Leachate analytical results for certain petroleum constituents in both hot-mix and cold-mix asphalt incorporating affected soil are presented in Table 3.
X. GEOCHEMICAL ASPECTS The chemical aspects associated with the incorporation of metals-affected soil (and other contaminants) has been extensively studied with respect to pavement properties, leaching behavior, sensitivities to moisture damage, and functional group analysis[15,16]. These studies provide in soils information that can be used to evaluate the stability of metals and other contaminants that have been asphalted. The studies indicate that asphalted contaminated soil willbe highly stable and will perform adequately as an end product.
Table 3 Leachate Analysis of Qpical Asphaltic Concrete Samples Concentrationa Unit method Analytical Parameter
Cold-mix asphalt (EPA) Cyanide, point Flash PH Sulfide mg/L Gasoline ND Diesel Hot-mix asphalt SM412 total Cyanide, point Flash PH Sulfide Gasoline Diesel ND
SM412
Testa and Patton
304
limit Detection
total
(')
EVD 1010 9045
EPA 376.1 EPA 8015 EPA 418.1 E/D
9085 EPA 376.1 EPA 8015 EPA 418.1
ND >220 7.62 ND
mag
1
"F
5
-
ND
mg/L
.01 1 .5 .5
ND
mag "F
1 5
>220 8.43
ND ND
mag
-
mag mglL mgIL
0.01
1 .5 .5
ND = not detected above the respective dection limit.
For the best chemical performance, the asphalt should have high contents of pyridinic, phenolic, and ketone groups, which can be achieved by carefully choosing the source material. If the situation requires special stability or redundancy, small amounts of shale oil and lime can be used as additives. Situations and conditions that favor the presence of inorganic sulfur, monovalent salts, and high ionic strength solutions in the asphalt should be avoided, because these conditions decrease the chemical stabilityof the asphalt cementby disruption of the functional groupaggregate bonds and by increasing the overall permeability. However, these conditions are not expected in the anticipated uses of asphalt cement to stabilize contaminantsin metals-affected soil using environmentally processed remedial technology.
XI. DISCUSSION OF USE Environmentally processed asphalt can at best be described as user-friendly. There currently exist a multitude of uses for cold-mixEPA incorporating affectedsoil. One of the more viable and creative is keeping contaminated soils outof landfills as a waste and placing itin landfills as an end product [7]. The imminent closure of many of the nation's Class 111 and municipal landfills creates the potential use for hundreds of thousands of tons of contaminated soils incorporated into asphalt for use as a landfill liner or cap. The cost effectivenessof this method of capping landfills is very attractive to financially strained municipalities.Prior to the advent of EPA for useas a liner or a cap, clay wasthe specified material. In addition to environmental concerns associated with mining vast quantities of clay for these uses,the cost of landfill closure had no cost recovery options. By using EPA, the municipalities and landfill owners can charge attractive fees for the acceptance of affected soil.In mostcases, this acceptance fee pays for the cost of on-site processing of the affected soil into the asphaltend product. The effectiveness of the cost recovery is obvious as the capping materials production process becomes a profit center. By the use of on-site material, not only is the cost of obtaining the clay canceled, but transportation costs also are eliminated. In essence, the capping processof landfill closure is more affordable, makesuse of a product far superior to the traditional clay method, and reduces a broad spectrum of environmental concernsby keeping affected soil outof landfills as a waste. Instead it places affected soilas environmentally sound end products such as caps or liners.
Soil Remediation
305
Studies of asphalt, clay, and other membrane liners subjected to a variety of aging tests in exposure columns at various temperatures, pH conditions, oxygen concentrations, and hydroand static pressures havebeen discussed [ 8 ] . The conclusions were that the asphalt liners membranes were extremely stable chemically and physically. An aging period equivalentto 7 years produced penetration of reaction products to only 0.5 mm (0.5% of the 10-cm liner thickness). 6%, these linerswould The results showed that if the asphalt content of the liner exceeded about perform adequately under impoundment conditions for over 1000 years, conditions that are similar to those expected for cold-mix EPA [17]. Catalytically blown asphalt was considered the best liner material and was selected for long-term field testing. Field testsof catalytically blown asphalt over a 2-year period showed superior performance of the asphalt liners compared be especially true for the petroleum constituents in cold-mix to thatof the clay liners. This will EPA’s liner; overall, asphalt is a much better liner material for this application than clay. Its use as a cap or liner is only one exampleof this product’s cost effectiveness and versatility, but what of its more traditional useas a pavement? To best describe “user-friendly,” one should visualize a typical multilane high-traffic-volume freeway and the load-bearing capability and durability that must be designed into the asphalt product used in its construction. Now visualize the typical bicycle path windingway itsthrough our urban areas. The point being that both the freeway and the bicycle path are asphalt pavements, but their end uses are drastically different. Cold-mixEPA pavement is certainly nothingnew. There are very few, if any, state and county road departments that do not use variations of cold-mix EPA. The end use of asphalt dictates its specifications, or better said, if the asphalt mix will perform its required function, from freewayto bicycle path,it is within specifications. In fact, the ASTM procedure for cold-mix asphalt design includes a section that states that the mix must fulfill the requirements of its intended application. Recalling the term “user-friendly” it becomes apparent that the function of the end product will determine the asphalt mix design. Pavement for a heavy equipment yard has been constructed from EPA made with affected soil recovered from leaking underground tanks. By producing parking lot pavement for on-site of their contaminated soil in a use, the generator eliminated the inherent liability of disposing dump site. Approximately $80.00/ton of disposal taxes were saved as the materials were recycled and not disposed of. The pavement produced not only kept the project’s pricing below any other option but created a paved parking lot of extremely low permeabilityto prevent further adverse subsurface impact. The mix design was not the same as that required to construct a freeway, but then a freeway was not the intended use. The intended use was for low traffic volume but required extremely high load-bearing strength. Another project used affected soil from an oil tank spill for paving loading and unloading facilities at an oil refinery. Again, the affected soil was not disposed as of hazardous waste but was recovered and used in a cold-mix asphalt pavementto remediate the affected soil and prevent further contamination, and the mix design was consistent with the end use. Stability or strength (as measured by the Marshall test) achievedby various mix designs of cold-mix EPA is presented in Table 4. The minimum Marshall stability required for paving mixtures, for example, is 2224 [18]. Mix designs used for actual applications range 95% from a contaminated soil (native silt, sand, and gravel contaminated with diesel fuel to 32,000 ppm total petroleum hydrocarbons) with a 5% emulsion, to a 5% contaminated soil (heavy black clay contaminated with machine cutting oils to 55,000 ppm total petroleum hydrocarbons) plus 90% Class I1Y4-in. or less base rock and 5% emulsion. To date, EPA has been successfully used on projects ranging from road base and road pavement to containment dikes and drain channels. The procedure wasto determine the requirements, then design the EPA mix to fit the use. AS the equipment used to produce EPA is portable and certainly not complex, field test field batches of20 tons or more are used rather than bench-scale tests. In this manner the actual
306
Patton
Testa and
Table 4 Summary of Marshall Test Results for EPA Stability number:5 Sample
4
3
1
2
6
62-64 5 5.2
62-64 5 5.2
62-64 5 5.2
62-64 6 5.2
62-64 6 5.2
62-64 6 5.2
2.08 1116.1 584.4 1120.1 2 5/8" 3100 2880 30 NTW
2.07 1125.1 586.5 1129.0 2 11/16" 2350 2090 27.5 2520
2.11 1130.0 599.0 1134.1 2 518" 2800 2600 31
2.05 1120.1 577.2 1124.0 2 11/16" 2450 2180 31
2.05 1120.7 577.5 1124.9 2 11/16" 2200 1960 24
NT
NT
2.07 1122.6 584.4 1126.5 2 11/16" 2250 2000 30 2050
X
X
75/25 Blend; 314-in. Class I1
base and contaminated soil Asphalt in emulsion" Residual asphalt in mixtureasb Total mix watef Compacted specimen data (emulsion in percent) Bulk density Weight in air Weight in water Weight SSD Thickness Stability Adjusted stability Flow Average stability number:
Sample 9
8
7
85/15 Blend; 314-in. Class I1 base and contaminated soil Residual asphalt in mixtureasb 6.5 6.5 6.5 Bulk density 2.002.022.00 3548 Stability 24-Hour soak NT 14number:
total
Sample 13
12
85/15 Blend; 314-in. Class I1 base and contaminated soil mixtureamb in Residual asphalt 2.03 Bulk density2.01 2.03 Stability 24-Hour soak absorbedMoisture Maximum
' In percent Emulsion e
Not tested
NT
10
6.5 2.02
NT
3260 NT
1410
NT 1056
11
6 . 2.01 3158 NT X
6
6
6
NT
2640
NT
1577
NT
1201
X
X X
'
Soil Remediation with Asphalt
307
3000
I
0
z -
2500
-
2000
-
W
$
Moximumdrydensity = 122pcf Optimummoisturecontent = 10.47,
0 CK
1500
X Standard
W
Q
v,
100.4
1000
n
% Standard 95.5
Z
1 500
2
0 0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
PENETRATION INCHES Figure 4 California bearing ratio for Class I1 base using cold-mix EPA.
mix is tested rather than a small hand-mixed batch. The bearing ratio for processed EPA for Class I1 base is presented in Figure 4.
XII. CONCLUSIONS In consideration of certain factors including durability, chemical resistance andaging, biological resistance, permeability, and leachability, cold-mix EPA is anticipated to perform more than adequately under normal conditions fora long period of time, probably over lo00 years. The use of EPA as a liner, cap, or any number of other site-specific applications has vast potential. Hazardous waste cleanup projects become cold-mixasphalt production projects. Contaminated soil becomes a recoverable resource and is within the letter, spirit, and intent of current regulations. Under Californiaregulations, for example, non-RCRA regulated recyclablematerials used in a manner such that they are not considered to be “used in a manner constituting disposal” are not subject to the provisions of Health and Safety Codes of the State of California, subsection 25143.2(b). Thus, if the recyclable materials satisfy the conditions of subsection 25143.2, then (1) they are not considered hazardous waste and (2) they are conditionally exempt from the California Departmentof Health Services hazardous waste regulations. Providing the conditions summarized above are met, permanent fixation of petroleum- and metalscontaminated soils via asphalt incorporation is a viable, cost-effective soil remediation option that can be accomplished withina relatively short period of time with minimal long-term liability [18]. Furthermore, although highway-type paving material as a resultant product is limited, multiple secondarymarketsexist,includinglinersandlandfill caps, roadbase, dust abatement, bank stabilization, and paved storage areas, among other uses.
308
Testa and Patton
Table 5 EPA Costs vs. Disposal Costs for Non-RCRA Hazardous Wastea ~
Task description
~
~~
costs Disposal costsEPA
Difference
Site investigation Characterization and work Plan Profiling and agency approval Excavation Transportation
Same Same
Same Same
0 0
Same Same 0 for on-site processing $5.00 per ton for central plant processing
Same Same
0 0 0
NA
$25.00 per ton $65.00 per ton
$ 10.00
Disposal costs Processing costs Taxes and fees' Superfund (HS) HAMY County tax (10% of disposal costs) Site restorationd Product cost recovery
$35.00 per ton (avg)
NA
$ 30.00
0 0 0
$7.88 $26.25 $6.50
$ 7.88 $ 26.25 $ 6.50
Same
Same
$20.00 per ton (avg)
0
NA
0 $ 20.00
Total per ton
$100.63
All costs in 1992 dollars. NA = notapplicable. State taxes and fees shown are from a recap of hazardous waste fees for fiscal year 1991-92 compiled by the State Board of Equalization. Disposal costs shown above do not includegenerator fees or hazardouswastereporting surcharge. Site restoration costs where cold-mix EPA was used on site would be the same; however, the product cost recovery would be deducted from the gross site restoration costs, thereby providing a much lower actual net cost.
REFERENCES 1. Asphalt Institute, Principles of Construction of Hot-Mix Asphalt Pavements, The Asphalt Institute, College Park, Md., Manual Ser. No. 22, 1982. 2. Preston, R. L., and Testa, S. M., Permanent fixation of petroleumcontaminated soils. Proc. Nut. Res. Develop. Conf. on the Control of Hazardous Materials, Anaheim, Calif., 1991, pp. 4-10. 3. Testa, S. M., and Patton, D. L., Paving market shows promise, Soils, Nov.-Dec. 1991, pp. 9-11. 4. Testa, S. M., Patton. D. L., and Conca, J. L., The use of environmentally processed asphalt as a contaminated soils remediation method,Proc. Hazardous Materials Control Res. Inst. Conf. (Hazardous Materials Control South), New Orleans. La., 1992. 5. Testa, S. M., Patton, D. L., and Conca, J. L., The use of environmentally processed asphalt as a contaminated soil remediation method,Petroleum in Conmminated Soils, Vol. 4, E. J. Clarence and F? T. Kostecki, eds., Lewis, Chelsea, Mich., 1992. 6. Testa, S. M., Patton, D. L., and Conca, J. L., Fixation of petroleum contaminated soils via cold-mix asphalt for use as a liner, Proc. Hazardous Materials Control Res. Inst. Conf., HMC South, New Orleans, La., 1992, pp. 30-33. on 7. MRM Partnership, Bituminous and Asphaltic Membranes for Radioactive Waste Repositories Land, Report to Dept. of the Environment, DOuRW187.009. Bristol, England, 1988. 8. Buelt, J. L., Liner Evaluationfor Uranium Mill Tailings, Final Report,PHL-4842, Pacific Northwest Laboratory, Richland, Wash., 1983. 9. Eschrich, H., PropertiesandLong-TermBehavior of BitumenandRadioactiveWaste-Bitumen Mixtures, SKBF KBS Tech. Rep. 80-14, Swedish Nuclear Fuel and Waste Management Company,
Stockholm.
Soil
Asphalt
309
10. Benedetto, A. T., Lottman, R. F?,Cratin, F? D., and Ensley, E. K. Asphalt Adhesion and Interfacial Phenomena, Highway Research Record No.340, Nat. Research Council Highway Research Board,
Washington, D.C., 1980. 11. Hickle, R. D., Impermeable asphalt concrete pond liner, Civil Eng., 1976, 56-59. 12. Atlas, R. M. Microbial degradation of petroleum hydrocarbons,an environmental perspective,Microbiol. Rev. 1981, 180-209. 13. Harris, J. O., Preliminary studies on the effect of micro-organisms on the physical properties of asphalt, Trans. Kansas Acad. Sci., 61, 110-113 (1958). 14. Jones, T. K., Effects of bacteria and fungi on asphalt,Material Protection, 4 , 39 (1965). 15. Testa, S . M., and Conca, J. L., When contaminated soil meets the road, Soils, December 1992, pp.32-38. 16. Conca, J. L.,and Testa, S. M., Chemical aspects of environmentally processed asphalt, Int. Symp. Asphaltene Particles in Fossil Fuel Exploration, Recovery, Refining and Production Process, Las Vegas,Nev.,1992. 17. Haxo, H. E., Jr., Assessing synthetic and admixed materials for liner landfills, including gas and leachate from landfills, in Formation, Collection and Treatment (E. J. Genetelli and J. Circello. eds.), EPA Rep. 600/9-76-004,U.S. Environmental Protection Agency,MTIS Rep. PB251161, Cincinnati, Ohio, 1976, pp.130-158. Soils, May-June 18. Testa, S. M., and Patton, D. L. (1992). Add zinc and lead to pavement recipe, 1992, pp, 22-35.
Additional Reading Asphalt Institute (1990). Asphalt Cold Mix Manual, 3rd d.,Asphalt Inst. Manual Ser. No. 14 (MS-14), The Asphalt Institute, College Park, Md. of Hazardous Waste, Van Nostrand Reinhold, Conner. J. R. (1991). Chemical Fixation and Solidification New York. U.S. Environmental Protection Agency (1991). Treatment of lead-Contaminated Soils, Superfund Engineering Issue.
This Page Intentionally Left Blank
14
Lead Decontamination of Superfund Sites
Ann M. Wethington, Agnes Y. Lee, and Vernon R. Miller US. Bureau of Mines Rolla. Missouri
1.
INTRODUCTION
The U.S. Bureau of Mines and the EPA entered into a Memorandum of Understanding in June 1987, that provides for the Bureau to supply technical assistance to the EPA in the area of treatment of inorganic wastes on Superfund sites. Bureau researchers had already developed an award-winning processto reclaim lead fromscrap batteries electrolytically [l-31. On the basis of this experience, interagency agreements were signed to conduct the treatability studies of waste at five battery-recycling Superfund sites. In recent years, the numberof lead-acid battery recycling plants in the United States has been reduced from over 1 0 0 to about 20. When some of the plants closed, they left behind hundreds of thousands of tons of lead-contaminated wastes and soil. To date, 23 such sites, ranging in size from 26 acres to about 4 acres, have been designated as Superfund sites by the EPA [4]. Environmental engineers have investigated new techniques to excavate, stabilize, vitrify, incinerate, biodegrade, and encapsulate materials considered to be hazardous. Most of these treatment methods will leave heavy metals in the waste body, which will require subsequent monitoring, and do not result in a permanent solutionto the problem. None of these methods can remove lead or lead compounds from the lead battery casing wastes, the adjunct sulfateoxide sludge, or the contaminated soils that surround the casing wastes. This chapter discusses the studyon a site in Ohio, but material from other lead-acid battery sites has been treated by the Bureau-developed process. Although the battery casing wastes
The Comprehensive Environmental Response Compensation and Liability Act or Superfund, as it is now commonly to help pay for cleanup of hazardous known, was passed in1980. This trust fund, administered by the EPA, was created waste sites that threatened the public health or environment.
311
312
Wethingtun et al.
from various sites are very similar,the soil composition and level of contamination vary widely from site to site. The 26-acre Ohio site is located in a rural area with farmland to the north and south. The north boundary of the site is bordered by a gravel road. The south boundary is a tributary of the Miami river witha small industrial plantto the south of it. A railroad is adjacent to the east boundary with woods and undeveloped land beyond. To the west, the site is bounded by four residential or business properties and a county road. There is one residence across the road, directly west of the site. The site drains to the southeast into the tributary, through a swampy area overgrown withtrees and understory materials. Theoffice, loading dock, and a couple of sheds remain on the site from the original operation. The estimated amount of material to be cleaned is55,000 yd3 of waste batter casings and the waste pile. approximately 85,000-100,000 yd3 of lead-contaminated soil under and around These battery wastes consist of ebonite battery cases, lead sulfate-oxidesludge, metallic lead, and the associated soils and debris.Lead contamination of the cases ranged from 900 to 3000 ppm Pb, the lead content of the adherent sludge rangedfrom 20 to 36%, and the lead content of the soils ranged from 0.05 to 2%. Initially, EPA's criteria for success were that the battery waste material and soils had to pass the EP toxicitytest and contain less thanlo00 ppm Pb after cleanup. Later, the criteria for the battery casings (since they will be shipped off-site) were changedso that the casings had to pass the TCLP test instead of the EP toxicity test. The EP toxicity was test retained forthe soils, and the standard for lead content in the soil and casing residue was reduced to 500 ppm. The criteria were accepted by the State of Ohio. Generally, state laws cannot be less strict than federal laws or conflict with the intent and purpose of the federal law. After initial treatability studies, a process was developedto decontaminate the ebonite battery casings and lead-contaminated soils. The process consists of physical separation of the different components, carbonation to convert PbSO, into PbC03, and acid leaching to solubilize the PbC03. After a solid-liquid separation and rinsing, the lead levels in the casings and soils meet the EPA requirements. For the carbonation, two reagents [(NH,),CO, and Na2C03] were investigated. For the acid leach, two effectiveacids (HNO, and HzSiF6) were investigated and compared.An electrowinning method was used to strip lead fromthe H,SiF, leachate prior to acid recycling. The cleaning battery chips, with a heating value of 13.78 X lo6 Jikg, may be suitable as a fuel, and the decontaminated soils canbe replaced on the site. The metallic lead and sludge removed from the battery casings pile may be recycled by a secondary smelter.
II.CHARACTERIZATIONSTUDIES A. ScreenAnalysis Casing and soil materials shipped from the Ohio site were split and sampled. Throughout the treatment process,the casings and soilwere handled separately. The lead contentof the sludge clinging to the battery casings, the different handling characteristics of the soil, and the subsequent changein the criteria for the casings were all factorsin that decision. Screen analyses were conducted onthe as-received materials to determine the relative weight of each constituent and the lead concentration in each fraction. The plus 18 mesh fractions from battery casing wastes were mixtures of casing chips, rocks or gravel, metallic lead grids, and twigs. A screen analysis on battery casing material is listed in Table 1. (In a screen analyses, standard nomenclature is to refer to material that remains on a given screening surface as the plus size and material that passes through the screening surface as the minus size.)
Lead Decontamination of Superfund Sites
313
Table 1 ScreenAnalysis of As-ReceivedBatteryCasings Size
wt %
Plus 318 in. Minus 318 in.,17.1 mesh plus 8 Minus 8, plus 18 mesh Minus 18 mesh
78.0 0.3 4.6
Table 2 BatteryCasingsSample-LeadContentandDistribution Casings Rocks Sludge
(%)
metal Pb
~
Sample weight distribution Pb weight distribution Pb analyses
60.55 0.53 0.13
4.95 0.08 0.25
30.7 74.1 35.8
3.75 25.3
96"
'kur to six percent antimony is associated with the lead metal.
Table 3 Screen Analysis of As-Received Soil Sample Size
Wt %
Plus 318 in. 318 Minus mesh in.,8 plus Minus 8, plus 18 mesh Minus 18 mesh
9.4 16.9 2.0 71.7
Table 4 SoilSampleDistribution (%) Size
Other
Plus 318 in. Minus 318 in., plus 18 mesh Minus18mesh
WtRocks %
Casings
9.4 94.3 18.9 71.7
2.7 1
-
3"
99
-
-
-
b
'Wood, tramp iron, and metallic lead. No lead metal was discerned in any other mesh
size. Samples from other sites contained significant amounts of metallic lead in the other mesh sizes, depending upon the individual battery recycling operation. bo.71-0.85% Pb compounds; 15-20% moisture.
A typical lead analysis and the distribution of the different fractions in the casing wastes are listed in Table 2. The total lead distribution in the sludge and metal fractions was 99.4%. The screen analyses of a composite of as-received soil waste sample is listed in Table 3, and the distribution of this composite sample is shown in Table 4.
B. MicroscopicExamination One set of casing samples was studied using an electron probe X-ray microanalyzer[7]. These studies were performed to (1) determine what mineral phases, particularly the lead phases, were present in the soils and casings; (2) quantify the various lead phases; and (3) determine complete chemical composition. The casing sample was washed free of sludge, ground, cast in epoxy, and polished before being examined with the electron probe. These studies showed the presence of PbSO,, PbO,, Pbo, and metallicPb in all fractions. The characterization studies of the sludge removed from the battery casings indicated almost
314
Wethington et al.
Table 5 Analysis of theWashedChips (13/8-in. Chips) Element
PPm
W As Ba
3000 11.1 44 1.1 22.2
Cd Cr
55
Sb Se
0.001
60% of the lead compounds were PbSO, and the remainder was a mixture of oxides and metallic Pb. The examinationof the casing fragments underthe electron probe revealed numerous small fractures throughout that were filled with PbSO, and, occasionally, metallic Pb. This was a significant finding sinceany successful lead treatment would be dependent on removal of the PbSO, from these cracks.
C.ChemicalAnalyses A partial chemical analysis of the washed 3/8-in. battery cases is listed in Table 5 . A partial chemical analysis of the sludge washed from the casing material is shown in Table 6 . The partial chemical analysis of as-received soil samples is given in Section V.B.
111.
PRELIMINARYTREATABILITYSTUDIES
Initially, studies were done on the battery casings and soils to determine if physical methods could be used to concentrate the contaminants and reducethe volume of material that had to be treated. Since lead compounds have a relatively high density, gravity separation was a logical choice. The finenessof the soils (<37 pm) suggested that a Bartles-Mosely vannerbe used for the gravityseparation since it is the only gravity separator that has had any success on<37 pm materials. The soils containeda high percentage of fines (approximately 15% was <2 pm) that the vanner could not separate, so there was little or no upgrading. Next, a series of flotation tests were done using a xanthate collector, mercaptobenzothiazole, andan amine. The best en-
Table 6 Analysis of BatterySludge ~~
~
~~
Element Ag As
Ba Cd Cr Hg Pb Sb Se
ND
PPm
Element
%
6 122 245
AI
6
Mg K Na
52,000 858
C S
0.52 2.8 1.3 0.72 0.09 0.02 21.2 0.22
( 5
Si
ND
24
ND
= not determined.
Ca
Fe
Lead Decontamination of Superfund Sites
315
richment ratio was 2:l at a recovery of 20% Pb. Feed grades were reduced, on average, from 0.7% to 0.5% in one pass. Although this test work showed that there was some selectivity toward the lead minerals present, better reduction of the feed grade wouldbe necessary to consider flotation as a removal method for the lead. Tests using carrier flotation or an air-sparged hydrocyclone were also applied but showed no improvement. A series of hydrometallurgical approaches were investigated next. Preliminary tests on the casings included a water soak. The battery chips were first broken into minus 3/8-in. pieces and then soaked-stirred in water for 4 days. Various chemical additions were made to the water along with more aggressive scrubbing methods. An ultrasonic cleaner was used with various surfactants, detergents, and known lead solubilizers [disodium ethylenediamine tetraacetate (EDTA), ammonium acetate, sodium citrate, acetic acid,H,SiF,, and HNO,]. One approach consisted of soaking the material in EDTA for 3 days. After this treatment, the cleaned battery casings passed theEPA EP toxicity test with <5 ppm Pb in the extract and also met the standard of <500 ppm Pb in the residue. However, the chips disintegrated badly, making solid-liquid separation very difficult, and lead removal from the spent EDTA was not achieved. Based on previous scrapbattery research, it was known that converting the sulfates to carbonates [8] and reducing the dioxides to oxides would form readily soluble compounds of all lead except the metallic Pb, which is slightly acid-soluble when finely divided [9].An acid wash could then be usedto remove the lead. Size reduction would helpto expose the lead trappedin the cracks and fissuresof the casings. The larger pieces of metallic lead would be removed by a gravity separation method,and the finely divided (minus18 mesh and smaller) pieces would be soluble in the time frame used for acid leaching.
IV. CLEANING PROCEDURE, TESTS, AND RESULTS A.BatteryCasingWastes The bench-scale cleaning process developed (see Figure 1) consisted of prewashing, gravity separation, granulation and sizing, carbonation, and acid leaching. 1. Prewashing Prewashing the battery casings removed most of the adherent sludge and fine metallic lead. This prevented the sludge and metallic lead from becoming embedded in the casings during subsequent size reduction. The as-received battery wastes weremixed with water, tumbled in a ball mill without a grinding medium for 1 hr, and then screened through an 18 mesh screen to remove the freed sludge and small pieces of metallic lead. This sludge, containing 20-36 wt % lead compounds and metallic lead, was set aside withoutfurther treatment since it was assumed that it could be sent to a secondary smelter for lead recovery.
2.GravitySeparation The second step entailed separation of the casing material from the metallic lead, rocks, and foreign matter by screening and elutriation. Since metallic lead has a much higher density than the rocks and casings, a gravity separation is possible. The separation technique used in this work was water elutriation, but air or other density separation deviceswould be effective. The feed material was screened through 3/4-and 3/8-in. screens, and in both plus fractions the metallic lead and rocks were separated from the pieces of battery casings by elutriation. The minus 3/8-in. material was rescreened through 4 and 8 mesh screens. Because the suspension of the different materials is a function of the surface area as well as density, it was preferable to screen the material into similar size fractions to provide a consistent feed forthe laboratory-scale water elutriation system. A schematic diagram of the equipment used is shown in Figure 2.
et
316
m
WATER
I
CARBONATE
al.
SPRAYS
rT1-p I
Wethington
PbS04
SLUDGE
JI
ROCKS, Pb METAL, ETC.
SEPARATION
Na ,SO,
CARBONATION
+RECYCLE
1 ' i *
T
ELECTROWINNING ELECTROWINNING
1I
Pb METAL CLEANED CASING WASTES
Figure 1 Flowdiagram for decontamination of casing wastes.
3. GranulationandSizing All the separated casings over 3/8 in. in size were reduced to less than 3/8 in. by granulation the cracks. Othersize to facilitate carbonation and leachingof the lead compounds entrained in reduction equipment, such as a hammer mill or shredder, could also be used. 4.
Carbonation The prewashed minus 3/8-in. casings from the separation and size reduction steps were combined, and the residual PbSO, was carbonated at room temperature for 30 min to 1 hr. The carbonate solution contained 4 g/L carbonate, to ensure excess above the stoichiometric amount of (NH4)*C03needed to convert the PbSO, to PbC03. A reducing agent, am-
317
Lead Decontamination of Superfund Sites UATERIAL FEED
TO C A T M CHIPS
LEAD/ R X K
TRAP
monium bisulfite (47% NH4HS03), wasadded to reduce anyPbOz to PbSO,andthen PbCO, according to the equations
+ (NH&CO3 "* PbCO3 + (NH4)2S04 PbO2 + NhHS 03 + PbSO4 + N&OH
PbS04
to (W (1b)
The carbonate solution was recycledfive times and then used to neutralize the rinse water before discarding. Although it is an effective carbonating agent, there are drawbacks in using (NH4)&03, such as odor, disposal problems, and highercost. Sodium carbonate and NaHSO, were substituted without any problem in the carbonation of the chips. Any carbonate should be equally effective; however, CaC0, would be the least desirable because the CaS04 produced has little value and is difficult to dewater and diicard.
Wethington et al.
318
5 . AcidLeachwithNitricAcid After a solid-liquid separation, the chips were rinsed and then leached with5.0 g/L HNO, at ambient temperaturefor 1 hr. The reaction occurring during the leaching operation can be represented as
The cleaned battery casings were well below the EPA requirement of S500 ppm residual lead the TCLP test was designated as the standard, the and 5 5 ppm in the EP toxicity extract. When HNO, acid concentration of the leachate had to be increased fourfold to pass the 5-ppm extraction portion of the test. The conditions and results of several tests are shown in Table 7. To minimize the volumes of leachate and rinse, recycling of all waste streams is critical. The Na2CO3 solution, water rinse, and HN03 leach solutions were each recycled five times. Makeup reagents were addedto the carbonate and leach solutions. There was no reduction in cleaning efficiency even by the fifth recycle. Since there are few contaminants except lead in the chips, this recycling should be feasible until the lead levels are high enough to warrant precipitation as a lead sludge. 6. Disposal of Nitric Acid Solution and Lead Removal by Lead Sulfate Precipitation
Attempts were made to remove the lead by precipitating as PbSO, with H2S04. The solubility of PbSO, in water is 38 ppm, with solubility increasing as pH decreases. The drinking water standard is 0.05 ppm; therefore, before discarding the HNO, it had to be treated to remove the residual lead. After an H2S04 precipitation, the lead remaining in solution was 250 ppm. By raising the pH to -8 with a hydroxide, adding sodium borohydride anda flocculant, and mixing for -1 hr before filtering, the lead content was reduced to <0.2 ppm. The small amount of sludge produced by this procedure would have to be sent to a hazardous waste landfill. The spent HNO, is more troublesome todiscard. Nitrates must meet drinking water standards of 10 ppm or less before being discarded to a waterway; possibilities include regeneration by a bipolar membrane water-splitting system. This processmay be too expensive for the volume of acid to be cleaned.
7. AcidLeachusingFluosilicicAcid An alternative to HNO, for the leach is fluosilicic acid (H2SiF6), whichis produced as a byproduct of the fertilizer industry. The major advantage of using H2SiF6 is that the lead can be recovered directly as pure metallic lead by the Bureau’s patented electrowinning process [3]. Additionally, the acid is regenerated during electrowinning and thereby made available for recycling. The problems associated with the disposal of nitrates are also eliminated. The preTable 7 Battery Casings Leached with HN03-Results
m03 (&)
20.2 30.2 40.4 50.4 100.8
Leach residue 213 160 128 125 137
of TCLP Tests
TCLP filtrate 2 1.4 1
< l 4.1
Note: AI1 tests were on carbonated chips at m m temperature for 1 hr, leach solution was 1 U125 g of chips.
319
Lead Decontamination Sites of Superjknd
liminary steps are the same as with the HNO,; the battery casings are prewashed, separated from the sludge and metalliclead, granulated, and carbonated. The NaHSO, is omitted in the carbonation step. Instead, the Pb02 is reduced to PbO by additions of hydrogen peroxidein the acid leach. The process reactions for using H2SiF6 as the leach reagent are described as follows: Carbonation: PbS04 + Na2C03 --$ PbCO3
+ Na2S04
Acid leach:
+
"f PbSiF6 + H20 + c 02 PbCO, Pb02 + Pbo "* 2 PbO PbO2 H202 4H+ + PbO + 3H20 PbO H2SiF6 + PbSiF6 + H20
+ +
+
Electrowinning:
+ 4 H+ + 4 e- 2 Pbo + 2 H2SiF6, O2 + 4 H+ + 4 e- + 2 H20, 2 PbsiF6 + 2 H20 + 2 Pb + 2 H2SiF6 + 02,
2 PbSiF6
E,:
€: = -0.126 V
€F
(5)
= +L23 V = €:
+ E:
= - 1.36 V
(6) (7)
cathode cell potential
E ~ anode : E,:
"f
cell potential
total cell potential
The cathodic and anodic reactionsoccurring during electrowinningare given in Equations (5) and (6), respectively, and the overall reaction is given in Equation (7). As an alternative to electrowinning for cleaning and regenerating the acid, H2S04 was used to precipitate PbSO, from the lead-rich H2SiF6 leachate.At ambient temperature,the solubility of PbSO, is high enough to leave -IO00 ppm Pb in the leachate. This lead level is excessive, and, in addition, the sulfate ions remaining in solution will form PbSO, when the solution is recycled. This option was not pursued further. 8. Results of Decontamination of Casing Wastes Using Fluosilicic Acid Several tests using H2SiF6 were completed onthe battery casings. After leaching, TCLPtests were done on the residues. For each test, enough fresh acid was diluted to the noted concentration to make 0.5 L lixivantll25 g granulated, carbonated chips. Leaches were at ambient temperature for 1 hr. The acid concentration, amount of 30% H202, and TCLP results are shown in Table8. All residues metthe - 3 0 0 ppm Pb standard. However, using 80 g/L acid was considered only marginally successful.
B. Soil Wastes Tests on the soil samples were conducted simultaneously with the research on the battery casings. Since upgrading by gravity separation, flotation, etc. were not effective, the hydrometallurgical approach used for the casings was followed for the soils (see Figure 3). First, the as-receivedsoilsamples were characterized andchemicalanalyseswereperformed. The amount of lead in the soils varied widely, depending on the specific site and the area within that site. Partial analyses on one shipment of six samples are listed in Table 9. In addition to the analyses, sample 3 was identified as vegetation, and samples 1, 2,4, 5, and 6 were identified
320
Wethington et al.
(g/L)
H,SiF6 Test leach
Test No. l First leach Second Test No. 2, single leach Test No. 3, single leach
30% H202 ( m L ) Filtrate Residue
80 80 80
2 2
ND
2
468
200
2
129
116
ND = not determined.
CONTAMINATED SOIL WATER Ne2C0,
“ 1 7 -
N a $ O , ~ RECYCLE
RECYCLE
I
ACID MAKEUP
+
l
I WtETAL
ACID LEACH
=b
ACID TO RECYCLE
CLEANED SOIL, TO SITE
Figure 3 Flow diagram for decontamination of
soil wastes.
-
ND
<1
2.3 1.3
Lead Decontamination of Superfund Sites
321
Table 9 Partial Soil Analyses Sample No. 2
Elements, pprn Ag As Ba 492 Cd Cr Hgc5 477 Pb Sb Se < 5 Elements, pct AI
Ca 2.1 Fe 1.1 Mg Si 27.7
5
1
4.3
<5 6.6 442 <S
C5
11 725
c IO c5
40
43
c5
517 47 <5 <5 5.7 2.2 3.5 1.5 28.3
87 <S
<S 109 c5 34< 20 c5
c5 6.2 3.9
c5
c5
c5
5.6
.8
3
.41
6
c5 <5
<5
8.7
8.1 698 < 10
c5
c5
<5
2200 11 <5
22 <5
44 <5 4067 <5
1
6.2 1 1.4.82 25.6 30.6
419
<5 21
3.5 5.2 1.5
.82 29.5
Table 10 Results of EPToxicityTests
Filtrate, ppm Element
Soil 1
soil 2
As
< 0.5
Ba
1.1 c 0.1 C 0.5 3.2 < 0.2 C 0.5 < 0.5
< 0.5 < 0.5 < 0.1
Cd
Cr Pb Hg Sb Se
0.21 1.o c 0.2 1.5 < 0.5
as soil. One significant feature observed in samples1, 2, and 6 was the presence of dolomite [CaMg(C03)2] and, in sample 6 only, bassanite (CaSO,-!h H20). Sample 6 also had more rocks than the other samples. Since lead in samples 1 and 2 analyzed below the standard of 500 ppm, only an EP toxicity test was completed to verify that lead content would not exceed the 5 ppm extraction standard. Table 10 outlines the results. Approximately 100 g of the vegetation sample(No. 3) was cut into smaller pieces, washed with 1.6 L of room temperature tap water for0.5 hr, drained, and then rewashed for a total of five times. The EP toxicity test results were< l .O ppm Pb in the filtrate and
b
322
Wethington et al.
Table 11 Characterization of Soil by Size,Weight, and Material Distribution Wt %
Plus 318 in., 9.4 wt % Rocks
Casings .Wood
Metallic Minus 3/8, plus 18 mesh, 18.9 wt % Rocks Wood and casings Metallic Pb Minus 18 mesh, 71.7 wt % Soil
94.3 2.7 3.0 0
99.0
1.o
0.0 100.0
Nore: The minus 18 mesh soil included “35-208 moisture.
1. SoilCarbonation
The following procedurewas used with soil samples 4-6. The sample was wet-screened, and the plus fraction, which was mostly rock, was set aside and studied with the subsurface samples. The minus 18 mesh fraction of the sample was mixed with tap water and a carbonate in a ball mill to give a pulp density of 25 wt %. Since preliminary testing proved that PbSO, in the soil could notbe leached to pass the standard andhad to be converted toPbCO, before acid leaching, all subsequenttesting was on carbonated materials. The soil slurry was tumbled without a grinding medium at room temperature for 1 hr with more than the stoichiometric amount of the chosen carbonate. Initially, the carbonate used was (NH4),C03; however, as with the chips, there were objections to the cost and odor. Of the other carbonates used, K2C03 and MgCO, were too expensive and CaC0, was ineffective; therefore, Na2C03 was chosen. Sodium carbonate at room temperature for l hr in concentrations of 8-100 glL was an effective carbonating agent. The amount of bisulfite added dependedupon the amount of PbO,, usually about 30% of the lead analysis. When H,SiF, was used as the lixiviate, bisulfite was omitted and, instead, H202was added with the acid to reduce the PbO,.
2. Soil Dewatering-Trommeland Flocculant After carbonation, the minus 18 mesh fraction of the soil sample was dewatered and rinsed. However, with the soils, extreme difficulties were encountered with the solid-liquid separations. Pressure filtration at 60 psi, vacuum filtration, settling and decanting, and centrifuging were all tried with little or no success. The filtration problems may have been caused by the formation of a sodium silicate or theextremely fine particle size of the soils. This problem was finally solved by using a flocculant, polyethylene oxide (PEO)[lo]. PE0 is a nonionic, watersoluble polymer with a molecular weight of 5 million. Published studies have confirmed that PE0 and its degradation products are nontoxic and do not have an adverse effect on the environment [l l]. The dewatering process using PE0 was developed at the Bureau’s lhscaloosa Research Center, Tuscaloosa, Alabama, where it was used for dewatering phosphate slimes and other fine-grained wastes. The process consistsof mixing a small amountof PE0 with the carbonate or leachate solutions, allowing theflocs to form, and dewateringthe flocs on a trommel. The trommel (Figure 4) consisted of an 8-in.-diameter pipe, 32 in. long, fitted on the inside with a slightly smaller diameter 35 mesh screen that was 4 in. longer than theouter pipe. A %-hp motor supplied the power to rotate the trommel. A variable controller was used to
Lead Decontamination of Superfund Sites
323
Wethington et al.
324
change the rotationspeed, and the trommel was mounted ona frame so the angle of inclination could also be altered. The dewatering efficiency of the system is affected by the slope and speed of rotation and the feed rate of the flocculant. The soil slurry (12 kg soiY60 L lixiviate or rinse) was pumped into a small tank, mixed with sufficient PE0 solution to create flocs, and then overflowed into the rotating trommel. The PE0 solution concentration used was 2.5 g PEO/L H20. In the 12-kgtest, -3.5 L of PE0 solution was needed in each solid-liquid separation. This created a product that dewatered rapidly, forming loose rolls containing 50 wt % solids. A drier solid could be generated by increasing the quantity of PE0 added [lo]. The amount of PE0 needed depended upon the variables in the process, including soil characteristics, amount of carbonate used, acid concentration of the leachate, temperature, and solid/ liquid ratio. 3.
AcidLeach-NitricAcid
The soil (0.8%Pb) was carbonated with Na2C03and NaHSO, and leached twice with HN03. Studies were conducted at ambient temperature,70"C, and 90°C. Thefirst leach used 50.4 g/L HNO,, followed by a solid-liquid separation and a second 20.2 glL acid leach. There was no rinse between the two acid leaches. After the last acid leach and solid-liquid separation, the slurry was rinsed with water until neutrality was reached. The conditions and results of tests on minus 18 mesh soil are given in Table 12. As the cost of heating is high, lower temperatures were preferred for all steps. The leaches at ambient temperaturehad to have twice the leach volumes and greatly increased leachtimes (2-3 hr per leach) to achieve the objective of <500 ppm Pb in the residue. The increase in leach solutions required a corresponding increase in rinse volume. The volumes of leachate and rinse water used require recyciingto make this a viable process. In cleaning the chips, there was no difficulty in recycling, but in the soil, HN03 solubilizes Ca, Fe, Mg, organics, etc., leaving no free acid to be recycled. Fresh acid equal to the original concentration had to be added before any new soils could be leached. This procedure quickly built up the NO3- ion concentration in the leachate and in the rinse. Since nitrogen levels have tobe reduced to<10 ppm before discharge of the waste stream, reducing the nitrate level in the wastewater would have a major impact on the cost of site cleanup. 4.
AcidLeach-FluosilicicAcid
Similar acid leaching tests were performed using H2SiF6 based onits successful use in the recycling of scrap batteries [2]. The samples were carbonated using established conditions [8] and leached with various combinationsof time and acid concentration. For each test, the soils were treated in two stages. First a leach, using the given parameters, then a solid-liquid s e p aration and a releach. Tests were conducted at 50"C, 70"C, and ambient temperature. Results of tests using minus 18 mesh carbonated soil (0.8% Pb), 80 g/L HzSiF6, and varying time,
Table 12 Leachingwith HNO, Temp, ("C) 25 25' 70
90
First leach (hr)'
Second leach
(hr)b Residual
1
l 2
2
1 1
1 1
"Acid concentration, 50.4 @L. bAcid concentration, 20.2 @L. Tests used I L leach solutionll25 g soil instead of 0.5 U125 g.
Pb (ppm)
1058 166 229 133
'
325
Lead Decontamination of Superfund Sites Table 13 Effect of Temperature, Time, and H20z on Soil Cleaning Using 80 g/L H2SiF6, Two-Stage Leach
Time (hr) 4" 4 lb 1
Temp. ("C)
70 70 50 50
1
room room
lb
1
5332 1210
0 0 0 4 0 4 0 4
mom room
lb
Residual Pb (ppm)
H202 (mUL)
ND = notdeterminednotdeterminedonthe "125g soiV0.5 L leach solution. b125 g soil/L leach solution.
ND
180
ND
168
ND
337
first stage.
Table 14 Effect of H2SiF6Concentration on Soil Leaching, Two-Stage Leach ~
~~
(gL) 10" 10
40" 40
100" 100 2Oob
200
H202 (mUL)
Residual Pb (ppm)
1
1 1 1 2 2 4 4
ND
3580
ND
983
ND
387
ND
317
ND = not determined on first stage. '125 g soil/L leach solution. b125 g soiV0.5 L leach solution.
temperature, and amount of 30% H,02 are givenin Table 13. Tests using ambient temperature, 1 hr leach time, and varying concentrations of H2SiF6 and amount of 30% H202are shown in Table 14. Using H,SiF6 as the lixiviate, the conditions adopted as a standard with125-g soil samples were (1) a two-step leach using1 L of 80-100 g/L H2SiF6 each leach, (2) ambient temperature, and (3) 2-5 mL/L H202. The amount of H202 required for cleaning the soil is more than the of soil stoichiometric amount forPbO, and had to be determined experimentally for each type waste. Organic materialsas well as iron and magnesium in the soil catalyze the decomposition of the H202. All wash, rinse, and acid solutions used in the process are recycled in a closed loop. Before recycling the leachate, the lead was removed from thebyacid electrowinning, and any acid loss was replenished. 5 . LeadStripping by Electrowinning
The acid leachates, containing600-1200 pprn lead, were strippedto <100 ppm using aPm 2 coated titanium anode and a stainless steel cathode. Figure 5 shows the rate of lead removal from 1 L of PbSiF, (80 g/L H2SiF6) by electrowinning at ambient temperature for 2 hr at a
Wethington et al.
326
l4O0C
1000 -
E si 2c
.-
*
600-
n
a
Current Density 300 A/rn
800-
400
-
200
-
0
0.6
l .6
1
I
2
2.6
Time, h
Figure 5 Leadstripping by electrowinning.
0'
0.6 l
I
t
l.6
1
I
l
2
2.6
Time, h "CD. 1 6 0
f-CD.200
-%+C.D.400
Figure 6 Effect of current density on the rate of lead stripping. Current density: (W) 150; (+) 200; (*) 400.
current density of300 Mm2. After 2 hr, the lead level was reduced from 1240 ppm to 11 ppm. The effect on strippingrate of varying the current density from 150 to 400 A/m2 was studied (Figure 6). In these tests the acid was-95 g/L and the leachate had been recycled four times. As impurity levels increased with the continued recycling of the leachate, therate of lead stripping was degraded. After 2 hr, the lead level was reduced from 1100 ppm to 220 ppm. In the studies shown(see Figure 6), the current efficiency was about15-20% at all current densities. During these tests, no attemptwas made to optimize current efficiency. For mostof the recycling tests, a current density of 300 Mm2 was used for stripping the lead.
Lead Decontaminationof Superfund Sites
327
Table 15 Results of Using Recycled H,SiF6to clean soils No. of recyclesa Test No. 1 2 3
HzSiF6 (g/L) 4
13
2
80
720 520 369
10oOb 640
393 100
1086b 585 476
2mb 593 489
‘Lead in cleaned soil residue, ppm. bNo makeup acid added during recycling leach.
6. Soil Cleaning with Recycled Fluosilicic Leachate Results from soil cleaningtests using recycled H2SiF6 with concentrations adjusted to 80 and 100 g/L are shown in Table 15. Asindicated in tests 1 and2, 80g/L acid did not decontaminate the soil to meet the criteria.All tests were at ambient temperature on minus 18 meshcarbonated soilcontaining0.8% lead. The acid was replenished as necessarybeforeeachrecycling leach. Test 3 (see Table 15) was continued by recycling the 100 glL H2SiF6 solution to clean eight carbonated soil samples. After the eighth recycle, the soil still met the residual lead standard by containing only 448 ppm Pb. The test procedure was (1) acid leaching a carbonated soil sample, a solid-liquid separation; (2) electrowinning to remove lead from the acid leachate, followed by makeup additions to replenish the acid to 100 g/L, the desired concentration; and (3) a second leaching of the same soil sample using the replenished acid. These three steps were repeated to continuously leach carbonated soil samples. Experiments indicated that the most important criterion for acid recyclingwas to reduce the lead level in the acid leachate to
-
’
328
Wethington et al.
H,SiF,
I
H,SiF6
1
Figure 7 H,SiF6 recycling test:Procedureandresults.
EW = electrowinning; F = filtering;
R = rinsing.
Table 16 FirstLeachate:ImpurityBuildupandH,SiF6Concentration Number of recycles
H,SiF6 Ca 1.34 2.34 3.02 3.78 4.51 4.44 4.87 5.24
ND
AI Fe 1.10 2.10 3.08 3.47 3.89 3.79 3.87 4.21
Rinse H!P
(@L)
Mg .72 1.27 1.79 2.08 2.34 2.32 2.56 2.91
1.04 2.13 3.04 3.48 3.87 3.74 4.22 4.82
84.5 86.9 84.5 77.8 76.8 77.3
ND ND
= not determined.
In later tests, instead of using pure Na2C03, the spent 10 g/L Na2C03 solution from the carbonation step was used to adjust thepH in the rinsewater. The results were comparable and furnished an opportunity to utilizethiswastestream.However,theadditionof the impure Na,CO, solution to the rinse. water formed a gelatinous compound that would not filter. The parameters investigatedto improve the lead removal and filterability included pH, additions of alum and flocculants, and reduction of the metals with sodium borohydride. Optimum filtration was achieved by adjusting the pH to 5.0 and using 0.5 mL of Superfloc 320 per liter of solution. Maximum lead removal was achieved by adjusting the pH from -5 to 8, and adding 0.75 g of alum and 0.3 mL of Superfloc 320 per liter of solution.
.83 .41 .88
e
329
Lead Decontaminationof Superfund Sites
Table 17 SecondLeachate:ImpurityBuildupandHzSiF6 Concentration (glL) Number of Ca recycles 1 2
2.15
3 4 5
3.54
Fe
Mg
AI
1.69 2.90 3.43 3.76 3.81
0.77 1.35 1.73 1.93 2.06
1.57 2.73 3.32 3.47 3.66
6 7 8
HzSiF6 107.0 104.0
96.0 93.1 96.0 ND ND
ND = not determined.
Table 18 LeadContentVersus pH PH
content
Pb
l 2.0 3.0 4.0 5.0 5.8
(ppm) 50
14
Table 19 ScreenAnalyses of Subsurface Samples (wt 95) Sample no. Mesh Plus318 in. Minus26.6 318 plus 4 6.5 4 plus 8 4.2 Minus13.4 Minus23.2 8 plus 18 4.5 Minus18 36.3 14.5
A
B
46.0 0.5 10.1 7.7 7.8 28.4
70.3
C
C. SubsurfaceSamples Portions of boring samples were submittedby the EPA contractors for testing. These samples were from depths of 0.5-25 ft, both under the casing pile and around the perimeter. These lower level samples consisted of mostly gravel and sand, with very little clay, and varied widely in size distribution. Of the samples received, screen analyses for three are shown in Table 19. Lead was not determinedon every fraction.EP toxicity tests (no leaches) were done on the (<5 ppm Pb) and residue plus 38, plus 4, and plus 8 sizes. These passed both the filtrate (C500 ppm Pb) standards. Leadin the minus 8 plus18 fraction ranged from970 ppm down to 60 ppm. In the minus 18 mesh fractions, lead content ranged from0.12% to over 1S % . Limited tests were completed on these fractions to determine the effect of usingH,SiF, on a high 4 plus 8 and minus 8 plus 18 fractions, calcium and silicon medium. When leaching the minus a silica gel formed that greatly inhibited rinsing and dewatering. A sample of the minus 18
330
Wethington et al.
mesh material was carbonated, then leached twice with 100 g/L of H2SiF6. This fraction fiitered rapidly. There was a 44% weight loss, but the residue still analyzed 850 ppm Pb-too high to meet the criteria of <500 ppm. Depending onthe amount of material and lead content, these fractions may need to be ground and blended with the surface soils for treatment.
V. CONCLUSION The Bureau of Mines, under several interagency agreements with the EPA, has successfully developed a process to removethe lead contamination from lead-acid battery breaker sites. The cleaned materials can pass the requirements ofthe EPA’s EP toxicity or TCLP test with ( 5 0 0 ppm Pb remaining in the soil and battery casings. The choice of acid would depend on the economics, the ability to clean the waste acid streamsto the discharge limits, and the type of lead product desired. At completion of the cleanup at a particular site, lead would be removed from the waste streams prior to discharge to meet the limit of 50 ppb, f i t by pH adjustment to remove most of the lead and then by polishing methods such as ion exchange or precipitation.
ACKNOWLEDGMENTS The suggestionsand assistance of Dr. E. R. Cole, Jr., retired Research Supervisor,U.S. Bureau of Mines, Rolla Research Center, Rolla,Missouri, have been most valuable.Thanks are due to Steve Paulson and staff of the analytical services group at l b i n Cities Research Center, Minneapolis, Minnesota, for the characterization studies and analyses performed.
REFERENCES 1. Cole, E. R., Jr., Lee,A. Y., and Paulson. D. L., Electrolytic method for recovery of lead from scrap batteries, BuMines Report of Investigations, Vol8602, 1-19 (1981). A. Y., and Paulson, D. L., Recovery of lead from battery sludge,J. Met., 35 2. Cole, E. R., Jr., (8). 42-46 (1983). L., Electrowinning of lead from H,SiF6 solutions, 3. Cole, E. R., Jr., Lee, A. Y., and Paulson, D. U.S. Patent 4,272,340 (June 9, 1981). Proc. 10th Natal.Conf., Hazardous 4. Tetta, D. A., Recycling of battery casings at a Superfund site, Materials Control Research Institute, Washington, D.C., 1989, pp. 301-305. 5 . U.S. EPA, 40 CFR Ch. 1, Part 261, App. 11, Extraction Procedure Toxicity Test (EP). 6. U.S. EPA, 40 CFR Ch. 1 (7-1-89 Ed), Pt. 261, App. I, Toxicity Characteristic Leaching procedure (TCLP). 7. Paulson, S. E., Petrie, L. M., and Mamas, D.C.,Geologic-geochemicalcharacterization of heavy metal contamination in soil, Proc. 1992 Fed. Environ. Restoration Con$ & Exhibition, Vienna, va., Apr.15-17, 1992, pp.363-367. 8. Gong, Y.,Dutrizac, J. E., and Chen, T. T., The conversion of lead sulfate to lead carbonate insodium carbonate media, Hydrometallurgy, 28, 1-22 (1992). 9. Mellor, J. W., A Comprehensive Treatise on Inorganic and Theoretical Chemistry, Vol. 7, Longmans, Green, London, 1947. 10. Smelley, A. G., and Feld, I. L., Flocculation dewatering of florida phosphatic clay wastes, BuMines Report of Investigations, Vol8349, 26 (1979). 11. Zatko, J. R., An environmental evaluation of polyethylene oxide when used as a flocculant for clay dwastes, BuMines Report of Investigations, Vol8438, 1-13 (1980). 12. Gomer. J. S., S. W. Yopps, S. E? Sandoval, and A. L. Clark, Copper exchange capacityof clays and potentialeffectonin-situcopperleaching, BuMinesReport of Investigations, Vol 9396, 1-10 (1992).
Lee.
15
A Secure Geologic Repository for Hazardous Waste Residuals
Thomas R. Nos Envirovest Management Houston, Texas
1. INTRODUCTION The Resource Conservation and Recovery Act of 1976 (RCRA) and the more recent Hazardous and Solid Waste Amendments of 1984 (HSWA) have mandated the development of innovative technologies for the destruction, treatment, andlor secure containment of hazardousas waste an alternative to land disposal methods currently in use. The Hunter Industrial Facilities, Inc. (HIFI) North Dayton Containment Facility is designed to use innovative technology that will meet the objectivesand demands of the hazardous waste regulatory programand the needs of industry for a secure waste containment facility. HIFI has developed and engineered a facility suitable for treated wastesand raw waste streams that cannot be further treated, recycled, incinerated, or eliminated by improvements in the manufacturing process. The concept of the North Dayton Containment Facility is to isolate treated and solidified waste by containment in a stable geologic salt formation, preventing the release of hazardous constituents into the environment. The need for secure, long-term hazardous waste disposal facilities that ensure protection of human health and the environment has increased in recent years for several reasons.The enforcement of new federal and state pollution control laws and regulations has expanded the universe of wastes regulated as hazardous and has provided for the impositionof fines to remove hazardous compounds from wastewater discharges and air emissions. The intensified regulation and treatment of such discharges has resulted in increasing the volume of controlled hazardous waste. The regulation of a greater number of generators HSWA under and the cleanup of abandoned disposal sites under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) amended by the Superfund Amendments and Reauthorization Act of hazardous waste requiring disposal and the of 1986 (SARA) have also increased the volume resulting demand for environmentally secure disposal facilities. The disposal method developed by HIFI involves the solidification and stabilization of industrial and commercially generated regulated waste and its placement in caverns constructed 331
332
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deep within a geologically securesalt dome. This waste management concept uses the following engineered and natural geologic protection systems. 1. Waste is solidified to reduce the mobilityof regulated constituents and improve handling characteristics. 2. Solidified waste is deposited in a dry, geologically stable, solution-mined repository,a salt cavern. 3. The repository is isolated from all groundwater aquifers by the impermeable salt formation and is constructed belowthe deepest potable aquifer and underground sources of drinking water in the area. 4. Solidified waste in caverns is permanently contained away from the erosional effects of rain, wind, and freeze-thaw cycles. 5 . Solidified waste is permanently isolated from humanactivities and inadvertent intrusion. 6. Salt is self-healing, thereby providing its own inherent mechanism for perpetual maintenance.
Salt formations, both domal and bedded strata, originated from saltwaterseas an estimated 150-300 million years ago. Repeated cycles of evaporation and recharge createdthe regional accumulations of bedded salt with which weare all acquainted (Figure1). Eventually the cycles of salt deposition ended andwere followed by cycles of sediment deposition. After millionsof years this sediment deposition and other geologic andclimatic influences culminatedin the salt formations encountered today. Throughout this period, salt strata experienced a variety of temperature and pressure influences determined in part by their depth and their proximity to interior or coastal areas of continents. Influencesoccurring at the relatively stable interior areas tended to consolidate the salt into horizontally stratified formations termed bedded salt. In the less stable coastal areas, buoyancydifferentialscausedverticalmovement of the salt formation, forcingitupward
Figure 1 U.S. salt deposits.
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through the overlying sediments. Salt beds of the coastal plains tend to be deeper than those encountered in interior areas, and at this additional depth, temperatures as high as 400°F greatly reduce the viscosity of the formation, addingto its relativebuoyancy and vertical flow. This flow, arising from the salt’s plasticity, functions also as a mechanism that expels impurities, including liquids (brine) and solids (rock). Over geologic time this resulted in the formation of underground mountains of homogeneous pure salt (Figure 2). The North Dayton 3), extends to a depth of45,000 ft below land dome, which actually resembles a column (Figure surface and has anm a of more than 4 mi2 at its top (Figure4). It is estimated that there may be 500 similar structures, termed salt domes, in the Gulf Coast region. Proven solution-mining techniques are employed to create the caverns. Fresh water is injected into thesalt formation under controlled conditions to dissolve salt and leach the caverns. The water and salt combine to form a brine that ispumped from the cavern and injected into 125 ft in a saline aquifer using conventional deep well injection techniques. Cylindrical caverns diameter will be completed approximately 600 ft below the surface of the salt and extend an additional 1800 ft deep (Figure5). The approximate depthbelow land surface to the top of the salt is about900 ft. Therefore, the topsof the caverns willbe created about 1500 ft below land surface, with the bottoms of the caverns 3300 ft below land surface. All waste will be solidified, cured, then pulverized on-site prior to disposal. During the solidification process, waste willbe mixed with fly ash, cement, and other additives. The solidified, pulverized waste will be pneumatically conveyed into the underground caverns. After a cavern is filled with solidified waste, it will be sealed permanently with cement grout, the open bore hole filled with salt, and the cased hole cementedto the surface. Solution-mined caverns in salt formations are currently being used by the U.S. Government to store crude oil. Since1978, the U.S. Department of Energy has stored more than 750 million barrels of crude oil in solution-mined salt dome caverns for the Strategic Petroleum Reserve. Contractors engaged by HIFI have been responsible for the development of these crude oil storage caverns for DOE. These same contractors, who will be developing HIFI’s caverns, have well over 1000 man-years of experience in this process. In addition to the government’s use of salt dome caverns for secured storage, private industry operates over 1800 caverns in the UnitedStates for the storageof liquid petroleum products, petrochemicals, and natural gas. The North Dayton salt dome already contains two naturalgas storage caverns that have been in operation for over 4 years for the benefit of a local utility.
Figure 2 Salt structures.
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SOUTH
NORTH SEA LEVEL -1 000 "
-2000
-4000
-6000 -7000 -8000
-10000 Figure 3 Cross section of North Dayton dome area. European countries also use caverns in salt formations as containmentsites for hazardous and nuclear wastes. Based on the fourth amendment to the lawof waste disposalof the Federal Republic of Germany,certain West German states now require the disposal of irreducible solid waste in salt caverns. Domal salt is attractive as a containment medium because ofthe following characteristics. 1.
2. 3. 4.
5.
6.
7.
Salt in salt domes is uniquely homogeneous. During salt dome formation, portions of the surroundingbedded materials, notablyclay, carbonate, andanhydride, are selectively sorted and displaced by buoyancy and plasticity mechanisms as development of the salt dome proceeds toward the surface. Domal salt generally has no intrabedded sedimentary layers of impurities to serve as pathways for water toenter the body of the dome or asdiscontinuitiesalong which cavern roof cave-ins may occur. Domal salt is geologically stable and has been stable for millions of years. During formation, salt domes evolved through repeated stages recrystallization of that left these structures dense and impermeable to fluids. A relatively impermeable cap rock layer often develops between the salt dome and the overlying sediments, thereby further isolating the salt stock from shallow groundwater influences. square miles, that lateral and Salt domes are sufficiently large, typically occupying several vertical salt buffers exist around the actual repository. A cavern repository wouldbe substantially separated from the nearest aquifer, typicallyby hundreds of feet of salt. Salt in dome structures is a nonreactive material and will not react with solidified hazardous waste.
335
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4000 2000 SCALE
"_
IMPROVED ROAD PROPERM BOUNDARY
-70'-
CONTOUR WITH ELEVATION (FEET MSL)
--1000-
SALT DOME CONTOUR WITH ELEVATION (FEET MSL)
Figure 4 Structure map.
0
4000 FEET
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*h%Y>%my>xmay~$
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24" 1ST CASING SALT
28" HOLE SECONDARYSEAL OF CEMENT
13.38" CASING 17.50" HOLE
.- .
BLANKEf FLUID
10.75" LEACHING (BLANKET) STRING
7 LEACHING (WASH) STRING
NOT TO SCALE
Figure 5 Caverncompletion.
The designof the HIFI disposal cavernsis in compliance withthe Texas Injection Well Act and regulations and guidelines of the Texas Water Commission Underground Injection Control program relativeto well construction, siting, and monitoring.Multiple cemented steel casings prevent any contact between waste and the upper freshwater aquifers. Monitoring of operational are parameters ensuresthe mechanical integrity of casings and verifies that excessive pressures not experienced during cavern development and disposal operations.
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Figure 6 Site area map.
II. SITESELECTIONCRITERIA HIFI has developed an innovative approach to hazardous waste management that meets the environmental objectives of the Texas Water Commission, U.S. EPA, and other agencies as well as the needs of the Texas economy. The waste management facility located in Liberty County, Texas (Figure6 ) will permanently isolate solidified waste deep within the North Dayton salt dome.
A. Location The proposedsite is about40 mi northeastof Houston, Texas. Dayton, the nearest municipality, is about 5 mi southeast of the proposed site. In 1986, approximately 48,000 people lived in Liberty County, 6OOO of them in Dayton.
B. Market The HIFI facility is in close proximity to the Texas coastal area, a major center of hazardous waste production within Texas and within the Gulf Coast region. This location will offer a significant long-term market for the waste disposal facility while minimizing transportation distances and costs for the greatest volume of wastes. Minimizing shipping distances also reduces the potential for accidental releaseof waste during transportation. The entire capacity of this facility could be used by in-state demands alone. HIFI expects that an increasingly stringent
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regulatory environment and an increased scope of waste subject to disposal regulations will further enhance the existing east Texas market for HIFI services. Even with waste minimizationby industry, expanding regulationof industrial and commercial waste has resulted in greater numbers of regulated generators and of wastes regulated as hazardous. In addition, Congress has enacted increased restraints on handling certain wastes in landfills, substantiallyrestricting landfills as a viable methodof hazardous waste disposal. EPA estimated that in 1984, 1.5billion gallonsof hazardous wastewas disposed of in off-site facilities. A review of Texashazardous wasterecords, after elimination of large one-time shipments, indicates that in 1984,359 million gallons of hazardous waste was shipped from industrial fiims in Texas for off-site disposal.Of this 359 million gallons, 85% was generated within east Texas. In 1987, Texas generated 1,783,480 tons of liquid non-water-based hazardous waste. The federal Superfund Amendments and Reauthorization Act (SARA) of 1986as amended (CERCLA, also known as Superfund), Section 104 (C) (9), requires the President to withhold Superfund monies fromthose states not providing disposal capacity for hazardous waste generated in-state. HIFI’s NorthDayton Containment Facility will help Texas meet the demands of SARA.
C. Area Geology The facility is located overan outcropping of the Beaumont Formation. Thisarea consists primarily of calcareous clays withlow permeability and limited recharge potential. The salt dome itself is essentially impermeable. The major saline aquifer, Frio, theis separated from the freshwater systemby 800-1OOO ft of aquitards and aquicludes consisting primarilyof multicolored shales, mark, redeposited cretaceous shells, and lenticular sands. Major freshwater aquifersin the North Dayton dome area are the Chicot, Evangeline, and Jasper. Most wells tap the Chicot and uppersection of the Evangeline aquifers. Water levels in from 40 to 130ft below wells completed in the Chicot and Evangeline aquifers generally range land surface. Regional groundwater flow in the Chicot and Evangeline aquifers is generally southwestward. The reasonable upper limit for groundwater flow within the Chicot aquifer is less than 2 Wday. Because the aquifers do not outcrop over the dome and the overlying clays are largely impermeable, there is no potential for groundwater rechargein the facility area.
D. SurfaceFacilities The surface and subsurfacefacilities, when evaluated in lightof the proposed design, construction, and operational features, minimize any potential for contamination of air, surface water, andgroundwater.Design, construction, andoperationaldetails to accomplish this include plans for zero dischargeof wastewater and storm water; redundant controls for potentially conof waste operations in buildings, secondary containments, taminated storm water; containment and curbing; RCRA training and contingency planning; inspection; tank designs; and many other aspects.
E. 100-Year Flood Plain
+
The 100-year flood plain, as delineated by the FederalEmergencyManagementAgency (FEMA), does notintrude into the facility area. Therefore, all surface facilities and caverns will be outside the 100-year flood plain.
F. Wetland Areas A survey of soils and plant species in the area has determined that no wetlands exist on the facility site.
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G. AquiferRechargeAreas The facility is not located withinthe recharge area of any sole source aquifer. Nonetheless,the facility design incorporates multiple containments to preclude migration of waste to surface water or groundwater.
H. RegionalAquifer The regional aquifer is separated from the solidified waste by nearly lo00 ft of impermeable salt. In addition, the surface facility is isolated from the regional aquifer by design and site geology.
1.
Soil Units
The entire North Dayton dome area is overlain by the Beaumont Formation of the Houston Group. Five soil series have been identified at the North Dayton dome site: Beaumont, Bernard, Morey, Lake Charles, and the YatodMocarey series. Although eachof these soils exists within the overallsite boundary, the Morey and YatodMocarey series cover the major portion of the actual facility. These series are composed of easily weathered calcareous clay, silt, clayey silt, and silty clay. These soils are known to have very low permeability.
J. LocalBodiesofWater The North Dayton dome is located in the Cedar Bayou watershed, a small coastal basin that drains to TrinityBay. There are no surface waters adjacent to the facility except for some ephemeral or constructed ponds. A canal that drains to Cedar Bayou is the only transmissive water body located closer than 2.5 mi from the proposed facility. The drainage area does not include a public drinking water supply.
K. ActiveGeologicalProcesses The North Dayton dome is not undergoing active geological processes. No major low-lying areas or areas with high subsidence rates exist on the site.
L.EndangeredSpecies No critical habitat of endangered species is known to exist in thearea. Field studiesof the site did not locate any such species or related critical habitat.
M. Faults There is no evidenceof any active fault within the facility area that would adversely affectthe operation of the HIFI North Dayton facility.
111.
RELATED OPERATIONS IN SALT FORMATIONS
There is a substantial body of technical literature that is related to storage and/or disposal operations in salt formations. Of this, a large portion pertains to operations in salt domes of the Gulf Coast. Cavernsin U.S. salt domes were first used to store liquefiedgases in 1951; it follows that many issues affecting containmentproperties of caverns in domes have been previously addressed over the past 40 years. The largest bodyof written information about salt domes has been derived from “energy”-related studies. This includes reports, papers, etc.,
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emanating mainly from national programs funded by the U.S. Department of Energy (DOE)or one of its predecessors. Oneearly Atomic Energy Commission-funded study dealt with design principles for the disposal of nuclear waste in salt, and it was performed at The University of Texas over 30 years ago. Long-term isolation of waste in salt formations has been recommended by the National Academy of Sciences-National Research Council. More recent national programs relating to energy include the Strategic Petroleum Reserve (SPR), which is sited entirely in coastal basin salt domes. In addition, from around the early 1970s through the late 1980s, many studies relating to nuclear waste disposal inU.S. salt domes were performed through the Office of Nuclear Waste Isolation (ONWI), managed for the DOEby Battelle Memorial Institute, and its predecessor, the Office of Waste Isolation (OWI), managed by Oak Ridge National Laboratory(ORNL). These studies focused mainly on domes in the inland basins of the Gulf Coast as potential sites for nuclear waste repositories; however, considerable research was also performed in several of the rock salt mines of the Texas-Louisiana Coastal Basin. Another national program that incorporated field tests in salt mines in domes dealt with compressedair energy storage (CAES)in salt domes. This program was managed by the Pacific Northwest Laboratories (PNL) and operated for the DOEby Battelle Memorial Institute. A major national program in salt research that has been maintained for some time is the Waste Isolation Pilot Plant (WIPP) near Carlsbad, New Mexico. The objective of WIPP is to demonstrate the feasibility of disposing of nuclear wastesof the Defense Departmentin bedded salt. Despite the significant differences between domes and bedded salt formations, the numerical modeling, tests, and field measurements that have been used to study the mechanical behavior of salt in the WIPP facility are thorough, and the associated reports and papers are essential referencesfor describing the current state of the art in the analysisof salt containment. This work is applicable to both domal and bedded salt structures. Additionally, the number of publications on energy-related national programs involvingsalt formations is also very large. For example, the list of SPR-related reportsand papers produced solely by Sandia National Laboratories exceeds 120 items, and other contractors for the DOE involved in this program also have published papers and reports. The number of publications related to the disposal of nuclear wastes in salt domes probably exceeds those of the SPR program by an order of magnitude. Considerable experience hasbeen accumulated in the private sector in the practice of mining and storage in salt domes. As noted previously,the first use of salt dome cavernsfor storage and/or containment was by private industry around 40 years ago. Because of the proprietary character of commercial enterprise, advances madein cavern storage technology by industrial operators have not always been reported as promptly or with as much detail as those made in national programs. However, the many working examplesof containment provided by properly designed and operated salt dome storage cavernsin the private sector provide verification for much of the technology proposed for the disposal of solidified wastes in salt dome caverns. Many storage caverns in salt domes have safely contained materials for decades that are hazfrom such operations ardous in character (e.g., hydrocarbons), and thus the experience gained is directly related to the HIFI project. Rock salt mines in Gulf Coast salt domes provide the opportunity to make first-hand observations of certain characteristics of salt domes thatare issues for storage/disposaloperations in caverns. The first rock salt mine in the United States was at Avery Island, located west of New Iberia in southern Louisiana, and is it still operatingtoday. This mine figured prominently in the national program for nuclear waste disposalin salt. Three other operating mines in the Texas-Louisiana coastal basin include Hockley, northwest of Houston, and Weeks Island and Cote Blanche in Louisiana. Another rocksalt mine is the Grand Saline minein the salt domes of the northeast Texas basin of the Gulf coast.
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Regulations for hydrocarbon storage caverns insalt domes were promulgated in Louisiana as early as 1978. These regulations were formulated initially by a committee whose members were selected mainly on the basis of their expertise in solution mining andstorage operations in Gulf Coast salt domes. This committee representedmany decades of accumulated first-hand experience in utilization of Gulf Coast salt dome caverns. Regulationsfor SPR caverns in salt domes were set forth in a Boeing PetroleumServices (BPS) documentin 1983. The SPR guidelines incorporate some of the early considerations used in designing cavern fields of large capacities. The Underground Injection Control (UIC) department of the Railroad Commissionof Texas also has in place regulations pertaining to operations of “brine wells” and storage caverns in salt formations in Texas. In summary, there exist a very large body of technical literature and considerable experience related to the construction and operations of salt dome caverns and the solidification of hazardous waste. This literature and experience was used as guidance for the HIFI project.
W. CAVERN-LEACHINGTECHNIQUES The fundamental technique of cavern development involvesdrilling and cementing concentric casings into the salt dome, then drilling an uncased hole to expose thesalt for dissolution.W O concentric leaching strings of pipe providecirculation of water throughthe well to dissolvethe salt (Figure 7). Water is injected through either the inner leach string or the annulus, depending upon the leach phase. As water is injected and cavern development proceeds, a flow of brine is circulated back to the surface. The result is the development of a cavern by dissolution. The brine is filtered for solids removal, then pumped through a pipeline to brine disposal wells located offthe dome. Alternatively, thishigh purity brine may be used in chemical production activities. The rate at which the cavern enlarges depends on two parameters, the flow rate and the brine concentration achieved within the cavern. These parameters are essentially a function of the water injection rate, existing cavern volume, contact surface between salt and water, and method of leaching. The two basic leaching methods are direct circulation and reverse circulation. The direct circulation leaching method is used in the first phase of the leaching process to enlarge the initial bore hole and formthe cavern chimney and sump. Directcirculation involves the injection of raw water through the inner leach tubing suspended near the bottom of the cavern and the withdrawal of brine through the annulus positioned above the raw water injection point. With this method, maximum diametersoccur near the bottom of the cavern, and the diameters decrease toward the cavern top. The reversecirculation method is used in the final stage for developmentof the upper area of the disposal cavern. In reverse circulation, water is injected down the annulus between the inner andouter suspended strings, causing brineto circulate into the inner string below the raw water injection point. Reverse circulation causes cavern enlargementin the top half of the cavern. Cavern shapes resulting from reverse circulation depend on the positions of the suspended strings within the cavern. By raising or lowering the strings, salt dissolution can be controlled to achieve favorable shapes. In this manner, direct and reverse leaching result in a nearly cylindrical cavern. Both direct and reverse leaching methods will dissolve salt on all exposedsalt surfaces. To prevent salt dissolution above the planned caverntop, the salt in this region is protected from leaching by injection of a material that is immiscible and lighter than water. This material blankets the exposedsalt at the cavern top, prevents leachingof the salt from aroundthe cemented casing, and protects the casing from internal corrosion. The blanket is injected in the annulus
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NOT TO SCALE
Figure 7 Directinjectionprocess.
between the last cemented casing and the leaching string. Careful control of blanket withdraw near the end of the leaching program permits the development of an arched roof in the caverns to enhance roof stability.
A. Operational Controls Maintaining the levelof the blanket and careful monitoring of injection pressures are critical in 0.9 psi per leaching a cavern. Pressures imposed upon the final casing shoe should not exceed foot of depth.An alarm system is installed to prevent overpressuringand possible fracturing of the salt. A high-pressure actuator will close the valve to the well and shut down the pumps automatically.
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Control of the leaching program consists of control of flow rates, brine salinity, blanket injection, and periodic downhole geophysical logs and sonar surveys in conjunction with repositioning of leaching strings and changes of circulation patterns.
B. DailyControls The daily routine is oriented toward maintaining a constant flow rate of fresh water of20002500 gpm and equivalent brine return.
C. Flow Meters Flow rates are monitored by in-line flow meters and controlled by control valves. Over a period be used to calculate the mass balance of the of days, flow rates and brine density readings can process and thereby the progress of the leaching operation. The blanket used to protect the cavern roof is measured as it is injected into, and recovered from, the cavern. This allows for an approximation of the depth of the interface level between the brine and blanket layer and therefore of the cavern roof. The exact interface level is checked by a measurement survey such as a density log.
D. PressureIndicators The pressure on both the brine line and the raw water injection is line monitored and recorded. A slight increase in water pressuremay be indicative of an increase in brine salinity and specific gravity, thus causing a higher differential pressure to exist between the raw water and brine. The maximum fluid pressure that will be allowed at the last cemented casing shoe at 1500 ft will be no more than0.9 psi per foot of depthto avoid the possibilityof fracturing the salt-cement bond. A maximum of 1180 psig leaching pressure has been specified for this application.
E. HydrometerMeasurements The specific gravity of the brine produced from the cavern is monitored and provides accurate measurements of the volume of salt dissolved, which can be related to the volume of the cavems at any point in the process.
F. PeriodicControls During each repositioning of the leaching string, sonar surveys are conducted in the cavern to determine the cavern shape. If sonar surveys indicate any anomalous development, adjustments to the leaching plan may be necessary. These adjustments could include a temporary change in the leaching rate or a changein the depth settingof the leaching strings for subsequent leaching phases. During the final days of leaching, the blanket is withdrawn in stages in order to form a stable arched roof with a top about100 ft below the long string casing.
G. Cavern ModelSimulations The physical basis for cavern construction is the solubility of salt in water. Through development of a patented solution-mining laboratory simulator, it is possibleto quantitatively identify the chemical solubility of different salt minerals. This simulator has been used andtoleach plan caverns for the Strategic Petroleum Reserve, and the results have been verifiedby sonar surveys. HIFI will use these simulator profiles for establishment of the solution-mining plan. If
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any changes are required in the plan, additional computer simulations of cavern growth are performed. The computer simulationsincorporate new casing setting depths and salt properties as determined through brine analysis and sonar surveys.
H. Schedule of Leaching Each cavernis developed through two phases of leaching. These phasesare engineered to provide a cylindrical cavern at the completion of the solution mining. The development program 340 leaching days to complete. Variationsin the is the samefor each cavern and requires about salt body may require changesin the duration of each leaching phase and modifications of the leaching plan. Phase I consists of about 240 days of solution miningby the direct-injection method. This phase provides for initial mining and development ofa sump forcollection of insolubles during a sonar survey is conducted the remainderof the solution-mining process. At the end of phase I, in the cavern to compare the actual cavern configuration tothe simulation plan. to develop the cavern to its final Phase I1 lasts for about100days and uses reverse injection design shape. Another sonar survey is conducted at the end of phase 11. Dewatering and dryingthe caverns and retrofittingthe wellhead in preparation for disposal operations requires an additional 12 months.
V. WASTEPROCESSINGFACILITY The facility (Figure 8) includes five distinct waste storage or processing areas (Figure 9). This section identifies these areas and the process flow sequence from the receipt of waste to the disposal of solidified material (Figure 10).
A. Waste Receiving, Handling, and Temporary Storage The process involved in the handling and storage of regulated waste can be summarized as follows. 1. Trucks enter the facility through the regulated waste entranceand remain in the iegulated waste truck holding area until an unloading bay in the appropriate storage building be-
comes available. 2. As space becomes available, the trucks 3.
4.
5. 6.
7.
are directed to the appropriate building for unloading and testing. Waste is unloaded and tested according to the waste acceptance procedure establishedat the facility and in the permit. Waste accepted for disposalis allocated space in the chemically compatible storage area within the storage building. Waste not accepted will be managed appropriately. Unloaded trucks are taken to the truck wash station and washed before being permittedto leave the facility. Oversized debris and drums are collected for shredding andseparate disposal at thefacility. Separate disposal is required for all solid waste that is not suitable for feed to the solidification process.Oversizedmaterialandshreddeddrums is sized as necessary for disposal into the salt caverns. The wastesolids are sized and dewateredas necessary before conveyanceto the wastesolids hoppers for temporary storage and subsequent processing.
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I X
X
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Y
r
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LEGEND
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PEDESTRIANCROSSING CONTROLLED ACCESS GATE -X-X-
-- -Figure 8 Wasteprocessingfacility.
PERIMETER FENCE PROPERTY BOUNDARY
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NO. 200
201 202
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IDENTIFICATION GATE AND GUARD HOUSE (REGULATED ENTRANCE) TRUCK SCALES (REGULATED ENTRANCE) REGULATED WASTE TRUCK HOLnlNG AND SAMPUNC A R U (COWFR
203
204 205 206 207 208 209
210 300
301-310
NONFLAMMABLE DRUM BUILDING BULKLlOUlDS AREA (COVERED) PROCESSING BUILDING SOLIDIFIED HATERIAL BUILDING TRUCK WASH FLAMMABLE DRUM BUILDING PLANT OPERATIONS, PROCESS MATERIALS SUPPLY DISPOSAL CAVERNS
400 40 l
STORMWATER TANKS STORMWATER PONDS
500
TRUCK SCALES (NONREGULATED ENTRANCE) OIL STORAGE TANK MAINTENANCE EUllOlNG WAREHOUSE BUILDING TRUCK HOLDING AREA No. 1 (NONREGUUTED)
50l 503 504 506
507
508 509 510
51 l 512
TRUCK HOLDING AREA No. 2 (NONRECUUTED) DRIVER WAmNC AREAS FIRE PROTECTION AND RAW WATERSTORAGE WATERTREATMENT P M ANDFIREPUMPHOUSE ELECTRICAL SUBSTATION BACKUP POWER
513 514
515
600 60 l 602 603 604
605
606 608 70 1
702 7a3
WATER HEATER/BOILER GATE AND GUARD HOUSE (NONREGULATED ENTRANCE) VISITOR PARKING ADMINISTRATION BUILDING EHPLOYEE CLOCK ROOM LOUNGE.AND CAJTTERIA EMPLOYEE SHOWER AND EOUlPMENT ROON LABORATORY EMERGENCY RESPONSE AND FIRST-AID STATION EHPLOYEE PARKING RAWWATER A N 0 BRINE DISPOSAL PUMPHOUSE DESANDER AND LINED BRINEPOND I”NG OPERATIONS BUllDlNC
Figure 9 Functional areas.
B.Blending,Neutralization,andHolding A waste process formulationfor liquids, sludges, and slurries is established by laboratory testing based on the following criteria. Available waste feedstocks Forecast of waste to be received in the next 4-10 days Chemical compatibilityof available feedstock Prior formulation experience The liquids, sludges, and slurries are pumped from their respective bulk storage tanks into the blending tank. Thereare two processing systemsthat can simultaneously processthe waste mixtures and feedtwo solidification lines. The equipmentin each of the two systems has been sized to processa batch formulation every24 hr. The process design allocates time in the holding tanks for batch formulation and adjustments and allows for solidification curing. The use of two processing trains provides the needed flexibility for the batch processor. The blended waste mixture is pumped into the neutralization tank, adjusted forpH control, then pumped into the holding tank. Each of the five holding tanks is sized for a 16-hr batch.
C. Solidification The solidification process involves adding portland cement, fly ash, lime, and other solidifyingreagentstothewaste.There are two solidification processingtrains,eachproviding
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$1
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one-half of the total processing capacity. The sequence of activities in the solidification process is described below. 1.Reagentand Solids Feed Solidification reagents are conducted from elevated hoppers and combined with waste solids in proportions to ensure appropriate blends. The combinationis mixed to achieve a homogeneous, dry batch, then held awaiting addition of liquid waste. 2.
LiquidSolids Mixing In the liquid-solids mixer, the liquid wastes are combined with the dry solids and mixed as necessary for the cemenVpozzolanic reactions to occur. The liquid-solids mixers are sized for 50% void space to provide thorough mixing and initial solidification. 3. Curing Reaction
The curing reaction occurs in curing processors. Feed is introduced into the curing processor from the liquid-solids mixer by a screw conveyor; additives may be used to supplement the reactions. The processor provides for agitation of the reactants during the curing and assists in the discharge of the solidified mass when the cure is complete. The curing processor dischargesby a screw conveyor located at its base that subsequently feeds the lump breaker. The 10 curing processors are each sized to hold a batch that represents approximately 16 hr of normal processing flow. The design allows forup to 72 hr of curing time in the processor for each batch at normal production capacity. 4. Post-SolidificationDrying The solidified material that exits the curing processor is a relatively dry solid with a free moisture content of approximately 5-10% by weight. To further dry the solid to a sufficiently low free moisture content for pneumatic conveyance, the product is fed into a rotary dryer,if needed. The solidmaterial passes througha lump breaker to provide a free-flowing and reduced particle size mixture before entering the two separate rotary dryers. The two rotary dryers are consistent with the dual trains used elsewhere throughout the process. The solid material flows through the rotary dryer to bring the free moisture content of the dried solid to within the desired range neededfor pneumatic conveyance. Thedrying occurs in a nitrogen-enriched atmosphere.
5. FinalSizing The dry solid that exits the rotary dryer passes through a final size-reduction lump breaker to provide a finely divided, free-flowing material that can be conveyed efficientlyin a pneumatic conveyance system. The waste product has reached its final form and is conveyed to the solidified material storage building.
D. Temporary Storage Waste in the solidified material storage building has been solidified in a cement matrix and sized for final conveyance to the caverns. Temporary storage of this material allows for operating flexibility in loading caverns and provides additional residence time for curing if needed. Storage allowsfor material testing prior to disposal or return to the waste solids handling and storage building for reprocessing. In most cases, the material in the building is sufficiently cured and in the proper form for direct discharge to the salt cavern.
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E. Disposal The solid material is pneumatically conveyed into the salt cavern. The pneumatic conveyor system is composed of dual trains that operate independently. Each pneumatic conveying line can discharge the final product into a separate cavern. The pneumatic conveyance system is based on solids being conveyed by a nitrogengas stream. The nitrogen blanketing of the cavern is established after each cavern is dewatered. Nitrogen is used in the conveyance system because its use minimizes the presence of humidity in the system and it is a nonreactive gas, ensuring an inherently explosion-proof atmosphere. Each pneumatic conveyance system is composed of the following major equipment items: Three blow tanks Three gas receiver surge tanks Three gas blowers One waste disposal well head assembly One mobile blowdown tank One return gas filter The following procedures describe the basic process flow pneumatic conveyance system.
of solids and nitrogen in the
1. 2.
The solids are charged into a blow tank by a screw conveyor. The blow tank is charged with system discharge pressure gas from the gas receiver tank. 3. The blow tank discharges to the conveying pipe and the salt cavern being filled. 4. The solids and gas mixture flows through the conveying pipe and well head assembly down into the salt cavern. 5. The solidsaredepositedinto thecavern, and the gas returns to thesurfacethrougha nitrogen-return annulus formed between the two hanging casings. 6. The return gas passes through a filter for particulate removal, then to the main blower suction header, where it combines with makeup nitrogen as necessary. 7. The blow tank that just discharged its solids is now depressurized. 8. The next blow tank is charged with solids by the screw conveyor, and the sequence is repeated until the current conveying requirement is satisfied. The design allows blow for two tanks on each trainto be used continuously during a conveying transfer while the remaining blow tank and its ancillary receiver surge tank and blower are on standby. Each blow tank is designed to hold and convey approximately a 15-min charge of solids based upon the normal design flow rate of one processing train. Each of the pneumatic conveying trains consists of three pipelines. These lines are the conveying line, the return gas line, and the nitrogen booster line. Provisionally, a mobile blowdown tank will be located adjacent to the two caverns being filled. This tank is designed to receive and contain the solids blowdown in the event that a conveying line needs to be evacuated because of a plug be evacuated into the or leak. If this should occur, the conveying line with the problem will blowdown tank. Automatic valve sequencing will divert the flow from the salt cavern being filled into the blowdown tank. Ten disposal caverns are planned for the current well field. Each cavern is designed contain approximately 800,000 yd3 of solidified material.
to
VI. SOLIDIFICATION OF WASTE The process of solidificatiodstabilizationor chemical fixation of liquid waste prior to landfill disposal is a commonly accepted practice in the field. The process has reached a sufficient
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state of maturity that extensive experience and research literature are available. Since the bans on land disposal of hazardous wastes that were included in the 1984 amendments to RCRA, both research and practical useof the technologyhave greatly expanded.As generally understood, the technology includes the use of organic polymers in addition to cementitious and pozzolanic materials. In practice, cementitious solidification is the only form commonly used in the field. Research into use of the technology most often focuses on the question of whether hazardous constituents in the solidified material have been immobilized. In a landfill situation, the solidified waste will be in contact with groundwater. The most likely means by which the hazardous constituents will present a threat to human health or the environment is through migration into groundwater and transport to an aquifer that is used for drinking water. In the case of salt dome disposal,the salt itself presents an effectivebarrier to migration to groundwater. The solidifying agents in common use are portland cement, alone or mixed with fly ash, lime, or cement kiln dust. The basic chemistry of the process is the same for each. The cementitious materials are combined with wastein the presence of moisture or free water, which carries out hydration reactionsto produce the solidified end product. The solid acquires some degree of additional structural strength through the formation of calcium carbonate. There is a history of practical experiencewith lime mortars and cements extendingback to ancient times. Late Greek constructionand that used throughout Roman times commonly employed lime mortars. Eventually the Romans developeda very close approximationof modem portland cementby adding crushed volcanic ash (from the town of Pozzuoli-hence the modern term “pozzolanic”) to lime. This effort culminated in the development of modern portland cement in England. The important point is that there is over 2000 years of experience with cements that can be used to judge the durability of the material under a wide variety of circumstances and varying technologies. Although a wide variety of wastes will be received for disposal at the facility, the vast majority willbelarge-volume,commonindustrialwastestreams, ash, contaminated soils, and sludges. Materialsthat are gaseous will not be handled, nor will ignitables, explosives, or radioactive materials. Chemical analysis of the waste stream will be required of the generator of the waste, and this will be checked periodically on-site. Upon receipt, wastes will be categorized in termsof compatibilities, reactive potential, and treatment required. The treatment train will be established according to the following steps. Bench-scale solidification studies. Bench-scale solidification studies willbe carried out on individual waste streamsor on blended wasteto ensure that satisfactory solidification can be achieved. The determinationof mix design will draw on bench studies, literature, and previous experience. The criteria to be used for establishing satisfactorysolidification are the developmentof an unconfined compressive strength of more than50 psi, no free liquids present in the solidified wasteas measured by the EPA Paint Filter Test, and no release of liquids under compression as measuredby the EPA Liquids Release Test. pH adjustment. Cement- or pozzolanic-based solidificationprocesses involve additionof very alkaline materialsto the waste. If the waste is acidic, the reaction canbe very exothermic. Therefore, pH adjustment under controlled conditions must be accomplished to prepared the waste for subsequent treatment. The extent to which pH is adjusted will be assessed in bench-scale studies. Reagent mixing. The appropriate solid and/or liquid reagents are mixed with the waste in a batch mixer. The output from this step is a semisolid materialwith a consistency similar to that of concrete prior to setting.
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Curing. The material is allowed to cure at ambient temperatures for a period up to 3 days. This allows a hydration reaction to take place in the binding agents in order to produce a completely solid material. Drying. A drying process is carried out by heating the solidified material in a rotary dryer in the presence of a stream of dry nitrogen-enriched air. The temperature in the dryer can range from 70 to 160°C. The drying step reduces “free” moisture content to approximately 3%. Sizing. In order to be pneumatically conveyed into the cavern, the dried material is passed through a lump breaker to achieve approximately pea-sizeparticles.
VII. CAVERNCLOSURE Salt cavern repositories are ideal for safe isolation of solidified hazardous waste. The subsurface disposal facilities can be designed and constructed in sucha way that there is virtually no possibility of escape of the contaminants withinthe time frameto perpetuity (defined as 10,OOO years by EPA). Solution-mined caverns in dry domal salt meet the requisite criteria for this demonstration and provide the only plausible solution to this problem, for the foreseeable future. Fortunately, dry stable salt structures, of which the North Dayton dome is an excellent example, are plentiful and providethe requisite qualities of strength, stability, flow properties, andimperviouscontainment capabilities for ultimate waste isolation. Thus, a salt cavern repository provides an excellent natural systemthat excludes waste from contact with the biosphere. The bore hole from the land surface to the top of the salt cavern is the only pathway connecting the waste to the biosphere. For permanent isolation, this avenue must be eliminated, not by means of an engineered solution, but by permitting natural isolationto occur. To achieve a natural seal, the 100-ft section of uncased hole in salt (Figure 11) extending upward from the top of the cavern, will be sealed by salt that has recrystallizedto a salt rock possessing physical properties as good as or better than the in situ salt of the confining zone. This canbe accomplished; however, several yearsmay be requiredto achieve a permanent naturalsalt seal, during which time an impermeable salt-saturated expansive concrete and chemical and asphalt seals (Figure 12) placed above the waste at closure will provide a reliable interim shield. Closure of bore holes in salt has been the focus of considerable researchin the last decade, spurred primarilyby the need to develop methodsfor sealing openingsin bedded salt following storage of military-generated radioactive waste at the Waste Isolation Pilot Plant (WIPP) and openings in salt structures elsewhere including the HIFI project. In situ rocksalt has nearly ideal containment characteristics, including very low, saturated, porosity and permeability in m/s range. This verylow permeabilitycan also beachievedsynthetically by conthe solidation of implaced salt particles. The effects on consolidation resulting from particle size, water content (inherent and added), temperature, pressure, and stress and of the processes responsible for them have been addressed in numerous studies. Figure 11 shows a schematic diagram of the seal, which is the only pathway from the waste-filled caverns at the North Dayton dome to the surface. The seal is placed between the cavern roof and the lowermost cemented steel casing at 1500 ft below the land surface. Properties of the salt-saturated expansive concrete and componentsof the plug other than salt will provide an excellent sealfor the interim period, during which time the permanent salt seal will become effective. For the long-term seal, a thick section of salt has been provided for in the plug design. Initially, the uncased hole to be filled with salt will be 1.46 ft in diameter. The hole diameter Wyr.If closure is willbe reduced by creep at an averageclosure rate of about 2.25 X
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l-[. . ...",.. ..
i SWA
62.5'
RP91115
____c1
NOT TO SCALE
Figure 11 Cavern closure sed. unimpeded, the hole diameter will change from 1.46 ft initially to 1.235 ft in 100 years. The closure/creep will result in a 29% reduction in volume over this period. Resistance to closure of the hole will be provided by compression of the contained salt aggregate. The time required for the salt to consolidate to >95% density depends only on the initial consolidation state. There are several methods for maximizing this consolidation; however, regardless of the procedure implemented,if the average void space in the salt column is less than
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Figure 12 Cavern closure. 30% initially, an effective seal will be established in less than 100 years. On the basis of ob-
servations pertinent to consolidation rates, little difficulty is anticipated in achieving a preclosure average salt particle density greater than 80%. Accordingly, the permanent salt seal will achieve optimum consolidationin less than 60 years.
VIII. WASTE CONTAINMENT IN DOMAL SALT The most widespread concern about hazardous waste containment is the perceived threat of contamination of groundwater. This concern is warranted by the record of groundwater contamination associated with traditional near-surface disposal technologies. Therefore, the key question is whether a salt dome has a particular set of attributes that will prevent the release of contaminants to the environment in both short-term and very long term time frames. From a regulatory perspective,a “no migration” petition must be approved by the Environmental Protection Agency for the containment facility. By “no migration,” it is implied that the waste
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, -I
Injection Zone , I
II
II
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Figure 13 Wastecontainmentzone. must be contained for 10,OOO years. A demonstration that this condition will be met will require model calculations, and such models must be based on.the physical and chemical characteristics of thewasteand the geologicenvironment. A unique characteristic here is that no-migration petitions are routinely conducted for liquid waste injection wells, whereas the waste form to be disposed at the Dayton facility is in solid form. As fluid flow in deep formations is well understood, the tasks required in a no-migration petition for liquid wastes are routinely performed by specialists in hydrogeology and by petroleum engineers. Wellan injection zone and a confining zone (Figdefined geologic units in such studies include ure 13). Injection zones have a reasonably high permeability, and the confining zones should be of low permeability. The tasks required in the no-migration petition for a domal salt facility are, however, not routine, for several reasons. First, solid (not liquid) waste willbe disposed of in solution-mined caverns thatwill be on the order of 1800 ft in height and 125 ft in diameter. For migration to take place, the solid waste would first have to be converted to a liquid form. Second, the question of how this might take place in a salt dome is not a simple one. Salt, particularly in salt domes, is not normally considered a flow medium because its permeability, porosity, and water content are very low. Tests at the North Dayton dome, for example, indicate that the total water content of in situ salt varies from 0.001 to 0.002 wt%. For comparative purposes, the bedded salt at the WIPP site contains 0.1-1 wt% water. Even if we assume that the wastes take on a liquid form some time after placement, the question remains as to how they would be transported beyond the walls of the salt cavern. ’ h 0 mechanisms may be available to do this: fluid flow and molecular diffusion throughthe “porous” salt rock. Last, the concepts of injection zones, injection intervals, and confining zones as conceived for liquid injection wells have no geologic identity in an extensive salt environment. Some relief is provided here, however, as definitions have been provided by the Texas Waste Commission.
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Based on Figure 13, the injection interval is taken as the cavern measuring 38 m in diameter and approximately 610 m in height plus an additional 15.24 m radially, making a circular cylinder 68.5 m in diameter. The injection zone extends another15.24 m in all directions from the injection interval, and the confining zone is the remainder of the salt-rock environment. The task of the “no migration” petition is to demonstrate that the site conditions are such that the waste will not migrate out of the injection zone. This constraint permits waste 30.5 m radially from the cavern wall, or approximately transport over a distance not exceeding 49 m from the cavern axis. The following issues must be addressed in the petition. 1. The potential for brine infiltration, infiltration rate, and how quickly an immobile solid waste could potentially convert to a more mobile liquid state. 2. The potential for migration of dissolved wasteby advective transport and how far the waste could be transported over the 10,OOO-year containment period. 3. The potential for migration of dissolved waste by molecular diffusion andhow far the waste could be transported over the 10,OOO-year containment period. 4. The potential for gas transport out of the cavern and how far gas or gaseous waste could be transported over the 10,OOO-year containment period. 5. The potential for transportof the waste components in the solid state and how far the waste could be transported by solid-state diffusion over the 10,OOO-year containment period.
A. BrineInfiltration Many studies of brine migration in salt bodies focus on the movement of intracrystalline As the solid waste fluids driven by a thermal field associated with disposed radioactive wastes. is not expected to affect the existing low-temperature gradientat a domal facility, movement of this type would be insignificant. However, the possibility exists for brine transport into the cavern by flow in the interconnected pore space, driven by a pressure gradient. The pressure gradients can be caused by the condition of atmospheric pressure within the solution-mined cavern and pore fluid pressures in the rock, which can vary between hydrostatic and lithostatic. Mobilization of thesolid’wastecannot occurin the absence of transformation to fluid a (liquid or gas) form. %o conceptual models havebeen proposed to describe this flow condition. The first assumes that there isat least some interconnected porosityin the salt massand that this porosity contains some liquid brine. The grain boundary fluidsare likely held so tightly that flow can occur only in response to large pressure gradients,as in the vicinity of the cavern. However, no remote flow would occur because of an absence of strong driving forces on a localized scale. The conceptual model predicts a limited inflow of brine that converges to near zero within a short time period. Very likely, the only brine available to enter the cavern would be that held in the mined or disturbed zone at the cavern wall. A second conceptual model assumes remote field saturation and continuity of the effective porosity in the salt stock. The pressure gradient is readily transmitted from the localized field to the remote field, resulting in remote field flow directed toward the cavern. This porousmedia model predicts greater inflow volumes. Cavern closure also affects this modeling. When the solid waste is initially disposed of within the cavity, its porosity may be as high as 44% and the pressure is atmospheric. With plastic creep, this porositywould be reduced considerably, perhaps to as little as lo%, and the pressures within the cavern will eventually of bethe same magnitudeas in the undisturbed salt rock. This means that the driving force for fluid movement into the cavernbewill eliminated.
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B. AdvectiveTransport Another important parameter is the permeability of the salt rock, which affects the estimates of fluid diffusion. There are two values of permeability to be concerned with. The first is the permeability of the localized disturbed zonein the immediate vicinity of the cavern wall. The second is the permeability of the remote salt-rock body outside the disturbed zone. Disturbed zone permeabilities in salt are not well documented although it has been suggested that the “weeps” on the salt face at the WIPP site are due exclusivelyto the increased permeabilityof the disturbed zone. As previously stated, bedded salt similar to that at the WIPP site contains a “wet” salt, with 0.1-1 wt% unbound water. The disturbed zoneat the Asse Mine in Germany has been determined to be about 3 m, where the permeability decreases from 10“’ m/s at the rock facetoitsambientvalue of m/s at 3 m. The AsseMinehasbeenmechanically mined, whereasthe HIFIcaverns wouldbe solution-mined. It is anticipated that solution mining should result in a smaller disturbed zone than would be produced by mechanical mining. Substantial informationon salt permeability comes from the WIPP site, where the concern is the disposal of nuclear wastesin bedded salt. Domal salt, which is the proposed host rockat this project, is noted for being considerably drier and of a lower permeability than beddedsalt. In salt domes containing onthe order of 0.002 wt% total water(free and chemically bound),we would anticipate a permeability somewhat less than lo-*’ m/s. It has been demonstrated that mobilization of the solid waste by contact with the brine is not a serious problem in domal salt. Even if the calculated inflow entered the chamber for 1OOO years, the cumulative fluid volume would be of the order of 912 m3, and it would be contained within slightly more than 1% of the available pore space. Eventually, because of host rock creep, the pressure conditions established in the cavern would achieve equilibrium with thosein the outlying salt rock, and the driving force forfluid movement would beeliminated. As mentioned previously,calculations suggest a time frame of 100 years. During this time, as long as there is inflow toward the cavern, there can be no advective outflow of solutes, and the diffusional transport out would go counter to an advective brine inflow, greatly reducing the effectiveness of diffusion as an outward transport mechanism.
C. Molecular Diffusion Molecular diffusion is one of the possible mechanisms of transport of chemical wastes outof the salt caverns. In order for diffusion to occur, brine must first infiltrate the cavern and dissolve at least part of the solid waste. Additionally, outward diffusion would encounter an inward flow of interstitial brine for some decades after waste disposal, which would reduce the effectivenessof any diffusional flow. Thus more aggressive assumptions were needed to assess this mechanism. A numerical model was developed that assumed that molecular diffusionof waste intothe salt mass was not affected by the counterflow of molecular brine intothe waste-filled cavern. It was also assumed that a sufficient quantity of brine surrounded the cavern to transform solid waste into an aqueous state immediately after disposal, thereby allowing diffusion to occur throughout the entire 10,OOO-year containment period.And finally it was assumedthat the 3-m disturbed zone became saturated with contaminants immediately, that is to say, at time zero, and that the pore space in the adjoining salt rock was liquid-filled. Although this model approached the subject of long-term containment froma different and more aggressive perspective than the previous demonstration, it reached essentially the same conclusion: Disposal cavernsin salt provide strong containment, evenfor dissolved wastes for time periods far beyond conventional consideration.
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D. GasTransport Gas transport of waste out of the cavern’s unfilled pore space into the brine-saturated pore space of the surrounding salt rock is an additional mechanism for containment migration. The gas transport models dealt with gas a that might form within the cavern from any source whatever, including organic and inorganic processes. The main condition for containment of dissolved contaminants migrating out of a salt cavern is their attenuation within the injection zone boundary.Three modes of gas migration were modeled: may dissolve in the brine from the 1. Dissolved gas diffusion. A soluble gas within the cavern cavern wall to the injection zone boundary. For a safe-side model, no allowance was made for counterflow of the brine into the cavern. 2. Volume expansion of gas. For an insoluble gas, the pressure difference between the gas may cause the in the cavern and the hydrostatic pressure of the brine in the salt rock gas to expand and drive the brine outward, in the direction of the injection zone boundary, until the pressure of the expanding gas has declined to the level of the hydrostatic pressure. At this point, there would be no driving force for further flow of gas out of the cavern. 3. Expansion and d i m i o n of gas. In a combination of the two preceding processes, some expansion of thegas takes placebeyond the cavern wall, and dissolution of thegas in the brine transports it by molecular diffusion further toward the injection zone boundary.
E. Dissolved Gas Diffusion Gas filling the pore space within the cavern may dissolve in the salt brine at the cavern wall and diffuse in the brine-filled interconnected pore space of the salt rock outwardto the injection zone boundary. The attenuation of dissolved gas at the injection zone boundary after 10,OOO years can be estimated by the same procedure that was used for modeling dissolved contaminants. In either case, however, the attenuation of the dissolved gas at the injection zone boundary is as strong as the attenuation of other dissolved species that were modeled and discussed previously. We conclude, therefore, that sufficient containment of water-soluble gases is providedby the salt cavern model system, consistent with the no-migration guideline conditions.
F. Volume Expansion of Gas A gas occupying the pore space within the cavern at some pressure higher than the hydrostatic pressure of the brine in the surrounding salt rock may expand through the cavern wall and displace the brine in the direction of the injection zone boundary. For such a process to occur, a be exceeded to overcome the capillary resistance of the brine in certain threshold pressure must the salt rock. Displacementof the brine by gas expansion would continue until the initial gas pressure within the cavern decreased to the hydrostatic pressure. The pressure decreases with distance from the cavern wall and is affected by the porosities of the cavern and salt rock. Models show that the expected travel distancesof an expanding gas in 10,OOO years would be less than 4 m. From this we have concluded that the salt cavern system can provide the regulatory guideline containment for gases that are either weakly or strongly soluble in salt brines over a period of 10,OOO years.
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G.Solid-stateDiffusion Solid-state diffusion at temperatures encounteredin salt domes is expected to be extremely low, particularly for the time frame and distances associated with the project. Solid-state migration has been ruled out over the 10,OOO-yearcontainment period.
H. Conclusions HIFI has examined the containment capabilities of domal salt and has reached the following conclusions. 1. A nominal volume of brine will seep into the cavern. 2. Most voids between the injected solidified hazardous waste pellets will remain unsatur3.
4. 5. 6. 7.
ated. Very small quantities of hazardous constituents will be leached from the saturated waste pellets. Hazardous constituents in a liquid state will not breach the injection zone boundary, Hazardous constituents in a gaseous state will not breach the injection zone boundary. Hazardous constituents in a solid waste will not breach the injection zone boundary. The containment capabilities of domal salt are exceptional.
IX. FAULTING, FRACTURING, AND SELF-HEALING PROPERTIES The faulting and fracturing of overlying sedimentary strata associated with a salt dome are related primarily to volume reduction of the upper portion of the salt mass as it invades zones be measured by of fresh groundwater. This volume reduction occurs over periods that can only geologic time. Secondarily, faulting and fracturing may be related to upward geologic movement of the salt mass. In either case, there is no reason to assume that fractures in rock peripheral to or overlying the dome would propagate fractures in the salt mass itself. Generally, the salt rock is surrounded by a circumferential fault that is the focusof differential movement between the salt and surrounding strata. At most domes, including the one at North Dayton, fault, usually with a thick gouge there is no surface expressionof this fault. This circumferential or brecciated zone, forms a sheath of limited permeability aroundthe dome. Fractures createdby stress relief creep withina cavern are of an exfoliation type. Similarly, fractures created by reloading stress in mine pillars are of this type. Brittle response of the skin of salt is common when strain dictates movement at a free face. Exfoliation is parallel to the strain or open wall and results in dislocated laminae that may fall into the opening. In an evacuated cavern, the pressure differential at the wall is atmospheric versus lithostatic,,whereas in the brine-filled cavern, it is hydrostatic versus lithostatic. Exfoliation might be enhanced by dewatering the cavern. The processwill rapidly diminish as the cavern is filled with granular solids and stop when the cavern is plugged and abandoned. Cavern integrity is not compromised or threatened. The cavern spacing and the timing of Cavern construction, dewatering, and filling relative to construction of the next cavern preclude the possibility of cavern coalescence due to wall spalling. To appreciate the phenomenon of self-healing of salt at intergrain or intercrystallineboundaries, the operation of pellet, block, or tablet presses in use bymost salt processors can be reviewed. These synthesized forms of granular salt have nearly the density and compressive strength of in situ rocksalt, even though short-term pressure, no heat, and no moistureis used to form the pellets.
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X. COMMERCIALIZATION A. Overview of Hazardous Waste Regulations Since its establishment in 1970, the U.S.Environmental Protection Agency (EPA) has been air and water resources. To the primary federal entity charged with protecting the nation’s date, this mission has been guided by more than a dozen federal statutes, including the Resource Conservation and Recovery Actof 1976 (RCRA) and the Comprehensive Environmental Response Compensation and Liability Act of 1980 (CERCLA). These two acts embody whatisreferred to as the“cradleto graveand beyond”perspectiveonhazardouswaste management. RCRAprimarilyaddressesthemanagementanddisposal of wastesproduced as byare commonly categorized products of current production and consumption activities. They according to characteristics of ignitability, corrosivity, reactivity, and toxicity. These are the wastes produced daily by numerous chemical plants, automobile manufacturers, pharmaceutical producers, and households. CERCLA,commonly known as theSuperfund program,providesformanagementof waste produced in the past, much of which was originally stored or disposed of improperly. Superfundfocusesonremediatingenvironmentalcontaminationresultingfrominadequate management practices.
B. Market Size and Description The scopeoftheRCRAmanagementprogramcan be assessedfrom many perspectives and levels of detail. From a national perspective, America produces annually approximately 1 ton of RCRA-regulated industrial hazardous waste for every man, woman, and child in the nation, at a cost of management and disposal exceeding $20 billion per year. Texas produces 20% of the national total, but hazardous wastesare produced in every city and state in the country. These wastes must be managedin compliance with federal RCRA standardsand at facilities permitted under federal or state authority. As a result of complex permitting standards and economic considerations, the market for commercial waste management services has evolved from what was previously a localor regional marketto a market that is effectively national in scope. No individual state has the capabilitiesto manage all of the typesor quantities of waste produced within its jurisdiction. All states participate in the interstate importation and exportation of waste, and, on average, each state exports hazardous wasteto 19 states. The Superfund program identifies and responds to environmental contamination resulting from abandonedor improperly controlled waste sites. In most instances, the materials managed at Superfund sites is RCRA waste, suitable for treatment or disposal at appropriately permitted commercial facilities. However, the contaminants may also be non-RCRA waste such as radioactive material and require different handling. Currently there are more than 1200 sites on the Superfund National Priority List (NPL). These sites have gone through environmental impact assessments, as a result of which each has been characterizedas posing an imminent risk to human health and the environment. Each NPL site will undergo additional, more detailed assessment including a determination of remedy, sources of funding, and implementation scheduling. In addition to the 1200 listed sites, there are an estimated 20,000 locations nationwide awaiting characterization. In 1990, the federal funding for this program was reauthorized through the end of fiscal 1994 with a budget of $5.1 billion. This amount is in addition to the billions spent on remediation by private industry.
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I(
U
INCINERATIONU
-e
U
LANDFILL
U
Figure 14 Matrix of technologies for liquids.
C. Pretreatment Technologies and Residuals Management RCRA waste, regardless of whether it is generated as a by-product of current production and consumption or reenters the waste management arena viaa Superfund cleanup, must by law be managed at an appropriately permitted facility. For the majority of wastes this dictates waste management by either recycling, waste water treatment,incineration, or landfilling (Figures 14 and 15). It is important to realize that each of these technologies, with the exception of landfilling, functions in part as a volume reduction or chemical separation process. Recycling recovers a reusable by-productby separating components in a waste stream; wastewater treatment removes organics and inorganicsin an effort to restore water’s natural characteristics; and incineration destroys certain types of waste while simultaneously achievinga volume reduction. Each technology yields a by-product residue, chemicallyor physically altered in the process but still ultimately requiring disposal.In today’s hierarchy of waste management alternatives, the “end-of-the-pipeline” disposal technology is landfilling.
D.CommercialFacilities The number of commercialfacilities available for managing hazardous waste has been declining for the past decade (Figure 16). This is the result of regulatory pressures brought about by the passage of increasingly stringent regulations. Owners of many facilities either could not afford the increasing cost of compliance or were incapable of meeting the new technical standards. Having fewerfacilities available for management of waste has pushed costs higher for treatment and disposal while also increasing transportation distances.Today, a haul distance of 500 mi is notuncommon.This attrition hasresulted in significantcapacity shortfalls nationwide.
E. Waste Disposal Application of Salt Dome Technology HIFI has received draft permits to construct andoperate a waste treatment facility and a series the North Daytonsalt dome in Texas. Each cavern of 10 solution-mined disposal caverns within will be capable of containing approximately 800,000 yd3 of material. This equates to two to four times the capacity of a typical hazardous waste landfill. As is the case with other states, Texas does not have the capacity to manage the types or quantities of waste produced by its industries. As a result Texas is a net exporter of hazardous
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Figure 15 Matrix of technologies for solids.
waste, shipping an estimated 235,000 tons per year to out-of-state facilities for commercial for Texas to manage its own disposal. TheHIFI facility will provide additional in-state capacity waste for an estimated 10-20 years.
F. Dayton HlFl TechnologyPark Adjacent to the waste containment facility, HIFI is developing the Dayton HIFI Technology Park, a 6600-acre masterplanned industrial complex. Park development is envisionedas a 20year project designedto accommodate a wide range of heavy industrial, manufacturing, office,
L - LANDFILL I - INCINERATOR
Figure 16 Location of commercialfacilities.
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Table 1 PlannedLand Use forDayton HIFI TechnologyPark Land use
Acreage
Heavy industrial General industriahanufacturing Manufacturing Light industrial Industriallcommercial Servicehetail Waste treatment and disposal Recreatiodpreservdreserve Road, rail, utilities, ponds, drainage
3606 644 335 421 116 56 197 618 607
Total land area
6600
and commercial uses. In addition to the waste disposal and chemical storage capabilities provided by the existence of the salt dome, the park is directly served by interstate pipeline sys3 mi of frontage on the Missouri Pacific Railroad. Highway and water tems and includes nearly transportation are readily available as well as access to international markets through the Port of Houston. The planned utilization program for the DaytonHIFI Technology Park is summarized in Table 1.
XI. CONCLUSION Disposal of hazardous wastein solution-mined salt caverns will be cost-competitive withother technologies-more expensive than landfill disposal but lower in cost than incineration. However, the superior features of salt caverns in terms of the relative permanenceof the repository, the impermeability of the salt, and its abilityto isolate waste from humansand our environment provide this technology with incomparable advantages. Problems associated with near-surface disposal in landfillsis common knowledge, and the costs in terms of both human health and site remediation are tragic. Near-surface disposalin landfills is not capableof achieving truly longterm isolation unless the landfill is continually monitored and perpetually maintained, both above and below grade. The relative size of salt domes is a unique feature for long-term recordkeeping purposes. A history of activities conducted at a particular dome can be archived to indicate precise locations of past activities. Future generations can be cautioned as to appropriate uses of certain dome areas. Occurrences at Love Canal inNewYork and elsewhere evidence the failure of current practices in this regard. Disposal in salt domes provides a unique mechanism for protecting against inadvertent intrusion forcenturies into the future. Disposal caverns constructed within a dome utilize a minimal amount of surface land and to achieve much moreefficient land use than current technologies. A near-surface landfill sized contain a quantity of material equal to one of the HIFI caverns would require 10-20 times the amount of land needed for a cavern. The HIFI project brings numerous favorable economic benefits to the host community. When the containment facility reachesfull operation, it will employ an estimated 170 people, and the host community will receive millions of dollars in taxes, fees, and contributions. Development of the industrial park will provide the host community with a large and diversified economic base. In total, the benefits provided to Dayton and Liberty County will be a significant boost to the area’s fiscal health.
16 Photocatalytic Degradation of Hazardous Wastes
M. S. Chandrasekharaiah, S. S. Shukla*, and J. L. Margrave Houston Advahced Research Center The Woodlands, Texos
S. C. Niranjan Rice University Houston, Texas
I. INTRODUCTION Nature has many built in checks and balances to maintain a healthy environment. But the industrial growth of the past twocenturies has been adversely affecting this balance.Thus, there is an urgent need to regulate our environment-pollutingactivities before irreversible damage is done to the environment. The generation of wastes (hazardous and nonhazardous) is inevitable in our technological society. Until very recently, no serious thought was given to the proper management of wastesfrom the manufacturingsector.Large-scalelandfilldisposal or oceandumping are two examples of the previousimpropermanagementofhazardouswastes.Thesemethods do not remove the pollutants from the environment but only delay their catastrophic effects. Pragmatic wisdom dictates the necessity of using waste disposal methodsin which the pollutants are degraded into environmentally benign species prior to their discharge into the environment. Increased public awareness of the consequences of improper disposal of hazardous materials has placed the responsibility for safe disposal of such wastes squarely on the industries that produce them. An ideal waste treatment process will completely mineralize all the toxic species present in the waste stream without leaving behind any hazardous residues. It should also be costeffective. None of the treatment technologies at present approach this ideal situation. There are a number of waste disposal methods currently in practice with varying degrees of success. Figure 1 is a schematic representation of different treatment technologies either currently available or in varied stages of development. At present, the disposal of the bulk of the industrialwastesisbasedon the processesdevelopedonphase-separationprinciples [l-31, eventhoughnoneofthem is completelysatisfactory. The incineration of organic wastes is the other widely practiced method.This in principle should destroy the toxic pollut*Currenfaffiliafion:Lamar University, Beaumont, Texas
363
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Chandrasekharaiah et al. WASTESTREAM
mm Photocatalvtic IDegradatidn
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p gEi%-l Activated Carbon Figure 1 ‘Iteatment methods for hazardous wastes.
ant completely, but the incineration ofmany hazardous organic wastes releases other toxic species into the air [4]. An example of this is the incineration of PCBs, which has resulted in the release of dioxins that are as toxic as the PCBs themselves. Incineration as practiced todayisthus also notan ideal waste treatment process. Biotreatment ofmunicipalwastes has been practiced, but similar biotreatment of industrial wastes are not common methods of waste management. Photochemical degradation of wastes containing toxic species has many attractive features. The process does not require exogenous chemical additions, thus eliminating the consequence of residual chemical species in the effluents. Ultraviolet irradiation as an alternative to chlorination for the disinfection of drinking water is probably the best example of a photochemical detoxification process. Though there have been some attempts to treat toxic wastes with direct UV irradiation [5,6], their success has not been such that large-scale commercial waste treatment facilities using direct UV light irradiation have been built. One reason for this may be the absence of a fully developed photochemical treatment process for the management of large volumes of hazardous industrial wastes. The other reason may be the incommensurate cost of the direct photochemical degradation processes. If the process can be made less expensive and more specific, then toxic wastes could be processed through photochemical degradation. Photocatalytic degradation of wastes rather than direct photochemicaldecomposition shows promise. Recently,a number of investigators have reported the degradation of hazardous chemical species in aqueous medium using microheterogeneous media as a photocatalytic agent rather than direct photolysis [7-121. The potentialfor the development of waste treatment processes based on photocatalytic reactions in microheterogeneous media appears to be very promising. Here, we describe the use of two such microheterogeneous media, namely, colloidalsemiconductor sols and micellar systems, as photocatalytic agents.
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II. PHOTOCATALYSISINMICROHETEROGENEOUSMEDIA A. Photocatalysis Photochemical reactions in solution media have been extensively studied. There are a number of references on this subject, and the reader may refer to them for details [e.g., 131. Photocatalysis is a distinct subsection of photochemistry [14, 151. The term “photocatalysis” implies a combination of photochemical reactions and a catalytic process. A chemical transformation brought about in a molecular entity as a result of initial absorption of radiation in another molecular entity may be the simplest exampleof a photocatalytic process. Photocatalysis in heterogeneous media will be exclusively considered. The catalytic photoreduction of water to hydrogen in the presence of a reducing agent and semiconductingtitanium dioxide is a good example of a photocatalytic reactionin a microheterogeneous medium. The titania particle initially absorbs the radiation and thencatalyzes the transfer of electrons to water adsorbed on the surface of the particle. Capture of an electron from the reducing agent on the surface regenerates the catalyst. Serpone and Pelizzetti [l21 provide a basic in-depth discussion of photocatalysis. The remainder of this chapter will concentrate on the application of photocatalysis in microheterogeneous media for hazardous waste management.
B. MicroheterogeneousMedia Photochemistry in homogeneous solutions is a subject familiar to most chemists. Microheterogeneous photochemistry is less so. The system here is somewhat homogeneous on a macro scale, but it is heterogeneous on a microscopic level. Many of the processes in these systems are taking placeat the interfaceof two phases. Thecolloidal semiconductor sols and surfactant micellar systems are good examples of microheterogeneous media. Thus, all the microheterogeneous systems are at least two-phase systems in the true thermodynamic sense butmay appear to be a singlephaseonmacroscopicobservation [16,17]. Figure 2 is a schematic representation of some of the microheterogeneous mediathat have been studied in detail. Some of the implications of stating that a system is microheterogeneousare listed below. Many physicochemical properties, e.g., the bulk density, concentrations, or chemical ptentials, are nonuniform throughout the volume. 2. There may be charged interfaces, and hence the local space-charge effects may influence the reactivity rather significantly. 3. Most of these systems are generally optically transparent to light. (This aspect is of paramount importance in the photocatalysis.) 1.
In this presentation, micellar media and colloidal semiconductor media will be used for the discussion.
111.
PHOTOCATALYTIC DEGRADATION OF HAZARDOUS WASTES IN COLLOIDAL SEMICONDUCTOR SOLS
The results of the studies devoted to the splitting of water by solar light in the presence of semiconductors formthe basis for the photocatalytic degradation of the toxic pollutantspecies in aqueous solutions in the presence of semiconductor particles [18]. Stated briefly, when selected semiconductor particles are illuminated with the proper light source, the absorption of light is followed by the formation of electron-hole pairs at the surface of the semiconductor particle. The excited electron-hole pairs thus created will react with the adsorbed species on
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the surface of the semiconductor. It has been shown that under certain circumstances adsorbed water will also oxidize to oxygen. The efficiency of this photodecomposition of water is insufficient to develop an economically viable process for the conversion of solar energy into hydrogen fuel. However, the photogenerated electron-hole pairsat the surface of the semiconductors can oxidizea number of species present in the solution rather efficiently [7-9,19-371. Many of the species observed to degrade under theseconditions are included inthe EPA list of priority pollutants. Thus, the scientific basis for the photocatalytic degradation of hazardous species was established. Thougha toxic waste treatment process based on this phenomenon has not yet been built, the photocatalytic degradationof toxic pollutantsat the surface of semiconductor particles has the potential for an energy-efficient and safe waste management process commensurate with the economy of disposal.
A. Photocatalysis at the Semiconductor Surface: Principles A necessary condition for the absorption of the photons by the semiconductor particle is that the energy of the photon should exceed the energy of the bandgap of the concerned semiconductor [38]. This threshold wavelength for the absorption, Abg, can be expressed according to the equation (nm) = 1240/Ebg(eV)
(1)
where is the threshold wavelength of a photon and Ebs is the bandgap energy. Table 1 lists the and Ebg values for some important semiconductors as determined by the flat band condition. These values for the bandgap energies may be altered when the semiconductor surface is in contact with an electrolyte solution [18,38]. The band edge positions and the bandgap energies of a few semiconductors of importance in the photocatalytic degradation studies are shown in Figure 3. The data refer to the conditions where the semiconductors are in contact
367
Photocatalytic Degradation Table 1 ThresholdWavelengthsandBandgapEnergies for Some Semiconductors
ThresholdBandgap Semiconductor Energy (eV) SnO, ZnO
SrTiO, TiO, CdS Si
3.5 3.2 3.2 3.0 2.4 2.1 1.1
wavelength (nm) 354 388 388 413 517 590 1128
Source: Finklea [38].
withanaqueoussolution ofpH 1. These values are derived from the flat band potential measurements [181. The knowledge of the band edge position is particularly useful in the discussion of photocatalysis [18,21,39]. In Figure 3, the standard potentials for several redox systems are also listed. The relative positions of the standard potentials and the band edge positionsare indicative of the thermodynamic limits for the photochemical reactions at the surface of the illuminated semiconductor particles. For example, if an oxidation of the species in the electrolyte is to be performed, the valence band edge position of the semiconductor mustbe positioned below the relevant redox level. Thus it can be seen that colloidal TiO, will be a strong oxidizing system. The free energy of the charge carriers generated by photoexcitation of semiconductors is directly related to the chemical potential. In the dark, under thermal equilibriumthe chemical potential of the electron is equal to that of the hole and corresponds to the Fermi level of the solid. But under illumination, the system departs from the equilibrium, and the chemical potentials of electrons and holesare no longer equalas they are under equilibrium, nonirradiated conditions. As a result, the Fermi level splits into two quasi-Fermi levels, one for the electron and one for the hole. The chemicalpotentials become functions of the nonequilibrium concentrations of electrons and holes. These concentrations are dependent on the absorbed ,light intensities. These differences are useful in redox reactions involving the electroactive species in the medium. The transfer of mobile charge carriers between the semiconductor andthe electrolyte is an important step in the photocatalytic degradation of toxic species. When anelectroactive species is present in the electrolyte solution, the charge transfer can take place directly across the semiconductor-solution interface. This will create a space charge layer at the interface, and the valence and conducting bands will be bent. This will affect the effectiveness of the redox processes possible in the presence of the illuminated semiconductorparticles. If the majority carriers are depleted from a colloidal semiconductorin solution and the particles are too small to develop a space charge layer, the electric potential difference resulting from the transfer of a charge from the semiconductor to the solution must drop in the Helmholtz layer (neglecting diffusion layer contributions). As a consequence, the positions of the band edges of the semiconductor particle will shift. For example,if after photoexcitation of a colloidal n-typeparticle, holes are transferred rapidly to an acceptor in solution while electrons remain in the particle, a negative shift in the conduction band edge at the surface will take place. In the case of colloidal semiconductors, the band bending is small, and charge separation occurs via diffusion. The absorption of light leads to the generation of electron-hole pairs in
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369
Photocatalytic Degradation
ox
RED Electrolyte
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Figure 4 A schematic presentationof photocatalyzed oxidation atan illuminated, n-type semiconductor. (From Ref. 18.)
thereby generating a hole in the valence band. Afterthese charges migrate to the surface of the particles, the reaction of the photogenerated electron with a reducible absorbed species and/or the reaction of the hole with an oxidizable species can take place. Figure 4 is a simple representation of the photocatalyzed reactionat the surface of an n-type semiconductorparticle [18]. These principles have found extensive application in the field of light-induced generation of fuels and that of organic transformations, but only recently in the field of environmental chemistry and pollution control. In fact, with the exception of a few reports concerning some contaminants, photoeffects at semiconductor interfaces have been largely ignoredby environmental chemists [7].
B. Applications The observation that a number of solute species present in the solution were photocatalyzed at the illuminated semiconductor surface evolved fromthe photodecomposition studies of water with solar radiation. Bard and coworkers [26-301 have shown that many hazardous species, including someof the EPA priority pollutants, could be degraded into innocuous speciesat the illuminated titania semiconductor surface. Their work formed the nucleus for the concept of photocatalytic degradation of hazardous wastes using semiconductor particles. Following this lead, our laboratory demonstratedthat ferricyanide species could be completely mineralized in the presence of illuminated colloidaltitania semiconductor sol [8]. Table 2 shows someof these results. Other investigations have shown the photocatalytic oxidation of a number of organic species at an illuminated semiconductor surface. Ollis and coworkers [7,9] summarized the available results. The use of chlorine as a disinfectant in urban water supplies is not without its problems. The halomethanes that are formed during the chlorine treatmentbecomehazardousspecies. Therefore, a number of investigations were carried out to study the fate of these halomethanes in the presence of near-ultraviolet-irradiated TiO,. The results were very encouraging.
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Table 2 Photocatalytic Degradation of Ferricyanide in Presence of Illuminated Titania Sol"
Fraction of Time (hr)
Fe(CNp- converted
0 1.5 3.0 6.0 9.0
0.0
-46%
1009bb
78% 93%
'Initial conc. of ferricyanide = I M , pH = 10. bIIluminated to midday sunlight in a fused silica vessel. All the others were exposed to a 4-W mercury lamp. Source: Taken from Bhakta et al. [8].
ing. Pruden and Ollis [32,34] clearly demonstrated that the trichloromethane (chloroform) is completely oxidizedto chloride and carbon dioxide. Theyalso established that the degradation reaction was indeed photocatalyzedat the surface of the illuminated Ti02 particles. In a series also a of papers, Ollis and coworkers [31-371 have shown that not only the chloroform but numberof other low molecular-weightchloro-organiccompounds (e.g., CH2CI2,CHCl,, CH2ClCH2Cl, CH2C1COOH, andCHC1,COOH) were photocatalytically completely mineralized at the surface of UV-illuminated titania semiconductor slurries. In all these systems, they did not observe the formation ofany intermediate species. From their results, itwas concluded that the photocatalytic degradation oflow molecular weight chloro-organics was complete if the carbon atom bondedto the halogen also had a hydrogen atom attachedto it. On the other hand, they notedthat the halocarbons containing halide-saturated carbon (e.g., CCl,, CC1,COOH) did not degrade into carbon dioxide. Trichloroethylene and perchloroethyleneare two chloroalkenesthat have widespread use, and hence the wastes containing these compounds are considered hazardous. The photocatalyzed destruction of these at the surface of illuminated TiO, slurries has been studied. Both have been shown to mineralize completely into carbon dioxide and chloride [34,35]. In the presence of dissolved oxygen, it was shown that the degradation of perchloroethylene goes through an intermediate of 1 ,I-dichloroacetaldehyde.However, the intermediate species was also observed to degrade under the experimental conditions. Pruden andOllis [34] established that the photodegradation of trichloroethylene in the presence of UV-irradiated anatase slurries was a photocatalytic process and not photodecomposition. The degradation products were shown to be chloride (measured witha chloride ion electrode) and carbon dioxide (precipitating barium carbonate from barium hydroxide solution by bubbling the effluent gases through it). For a reaction time of about 120 min. they reporteda conversion factorof about 97-100%. Ollis [36] expressed the reaction rate in terms of the equation l/rate = l/k
+ l/kK X
I/c
(2)
where k and K are the parameters deduced experimentally and c is the concentration of the reactant. Ollis [36] has listed the values of k and K for several of the chlorocarbon photocatalysis reactions from whichthe photocatalytic degradation rates could be evaluated. Some of these data are presented in Table 3. A very wide rangeof aromatic hydrocarbons and their haloderivatives find their way into the environment throughthe use of these compounds as preservatives (woods, paints, drilling muds, photographic emulsions, hides and leathers, textiles), antimicrobials (industrial cooling
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371
Table 3 Rate Parameters for the Photocatalytic Degradation of Halocarbonsa Reactant
K-Value
C1H2C-COOH C1,HC-COOH CH,CI, CHCI, CCI,
CH2Br2 CHBr,
ClH,C-CH,CI CI,C=CCCIH CI,C=cCI,
Br2HC-CH2Br Br,HC-CH,
k-Parameter 5.5 8.5 1.6 4.4 0.18 4.1 6.2 1.1 830.0 6.8 2.2 3.9
“k = ppm/(min. g catalyst); K = ppm Source: Ollis [9].
0.002 0.003 0.02 0.003 0.005 0.02 0.01 0.01 0.01 0.02 0.02 0.02
- 1.
waters, pulp and paper mill operations, etc.), herbicides, insecticides, and disinfectants and other health care products. Most of them are hazardous. A great dealof attention has been paid to removing these species from the wastes containing these compounds. Several investigators have examined the application of photocatalytic degradation at the surface of semiconductors with varying degrees of success. Direct photodecomposition of monochlorobenzene at h. = 254 nmby Tissot et al. [40] resulted in the formation of phenol, which is also a toxic compound. Ollis et al. [35] studied the reaction of chlorobenzenein the presence of illuminated Ti0, slurry with light ofh > 300 nm and foundthat the primary photocatalytic productswere chlorophenols. The chlorophenols subsequently dechlorinated and formed benzoquinones. The use of anatase (another form of TiO,) of a much higher specific surface area had resulted in complete mineralization of chlorobenzene. Oliver et al. [41] showed that 1,4dichlorobenzenewas also rapidly mineralized in the presence of UV-irradiated 1% TiO, slurries. The results of several investigations [7,42-441 have clearly shown that photocatalytic destruction of several chloroaromatic hazardous compounds is almost complete. The haloaromatics that have been shown todegrade in the presence of illuminated semiconductor slurries are Cchlorophenol, 3,4-chlorophenol, 2,4,5-trichlorophenol, pentachlorophenol, chlorobenzene, 1,4-dichlorobenzene, trichlorobenzene, 2,3,5-trichlorophenoxyacetic acid, 4,4’-dichlorodiphenyltrichloroethane,3,3’-dichlorobiphenyl,and 2,7-dichlorodibenzo-p-dioxin.With many of these compounds, the conversionrates of the intermediates is sufficiently fast that no intermediate species are observed via gas chromatography/mass spectrometryof the liquid-phase samples. Almost all of the studies have usedTiO, slurries. Ollis et al. [7]summarize the data and present the half-life values for the photocatalytic degradation of some of the haloaromatics. These values for half-lives are generally for the initial reaction rates and vary from a low of 10 min to a high of 90 min. Therefore, one can conclude that the complete photocatalytic degradation of the species at concentrations of about 6-45 ppm will be onthe order of 1-6 hr. There are exceptions. For example, 2,7-dichlorodibenzo-p-dioxin,with an initial concentration of about 18 ppm, required 90 hr for its complete disappearance (7,42). Thus, the photocatalytic destruction of these toxic species present in wastewaters shows great promise as a treatment process.
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In addition, there have been some studieson the photodegradation of common surfactants in the presence of Ti02 slurries [43,45,46]. At least one member of each of the three types of surfactants (anionic, cationic, nonionic) has been shown to exhibit complete photocatalyzed conversion. Total mineralization into CO2, though, has not been demonstrated in all cases. And, for all three typesof surfactants, degradation of the initial reactant is found to relatively rapidly destroy the surface activity of the surfactants. The majority of the studies have used semiconductorslurries asthe photocatalytic media. As the photochemical reactions take place primarily at or near the surface of the semiconductor particle, the use of colloidal sols of semiconductors will definitely enhance the conversion rates. The complexed cyanide species, which are very resistant to the chlorine oxidation process, can be easily degraded at the surface of colloidal TiO, sol. Even solar radiation was shown to photocatalyze the degradation process [8]. A review of the literature has shown that many of the hazardous pollutantsin waste streams can in principle be completely mineralized into innocuous species using semiconductor sols. However, no commercial treatment planthas been built and none is in operation that is based on this principle. Recently, Ollis and Turchi [37] discussed the engineering design aspects of such photoreactors for waste treatment processes.
IV. PHOTOCATALYTIC DEGRADATION OF TOXIC SPECIES IN MICELLAWMICROEMULSION MEDIA Micelles and microemulsions havealso been observed to act as photocatalysts for the decomposition of some organic pollutants. Micellar and microemulsion media are particularly attractive for the photocatalyticdegradation of nearly insoluble organic pollutants such as PCBs and insecticides. The ability of these microheterogeneous media to solubilize the organics is an added advantage in these processes. The photochemical degradation of such organic species catalyzed by the surfactant-generated micelles or microemulsion medium has been observed. Thus, micellar mediamay catalyze photochemical decompositionof a few very important pollutants on the EPA priority list. The remainder of this section will review the available data on microheterogeneous micellar media.
A. Micellar and Microemulsion Media Surfactants are the most common surface-activeagents, and their use is varied and vast in industrial as well as consumer markets. Surfactantsare amphiphilic molecules witha hydrophobic tail attachedto hydrophilic head group. In aqueoussolutions, above a critical concentration (called the “critical micellar concentration,” cmc), a surfactant dynamicallyassociates to form large molecular aggregatesof colloidal dimensions termed micelles. Abovethe cmc, there exists a dynamic equilibrium betweenthe monomers and the micelles. Each micelleis composed of a certain number of surfactants molecules(the aggregation number) that dictates thegeneral size and geometryof the particular micellar system. Thestructure of a normal micellar system is such that the hydrophobic tails are all directed away from the aqueous phase and toward the center of the micelle, forming the core. The hydophilic head groupsare directed toward andin contact with the aqueous solution phase, thus formingthe polar surface. The nature of the hydrophilic moietydetermines whether the micellar systemcan be classified as anionic, cationic, zwitterionic, or nonionic. Extensive compilations of the preparation, properties, and critical micellar parametersof these classes of surfactants have been published, andone should consult them for further details [17]. The charge distribution at the surface of the micelle playsa dominant role in the behavior of the micellar system.A two-dimensional schematic representationof a spherical, ionic micellar
Degradation 373
Photocatalytic
system is shown in Figure5 [47]. The structure is one in which the hydrophilic head groupsare directed toward and in contact with the aqueous phase and the hydrophobic tails are directed away from the water phase, forminga central nonpolar core. In the Stern layer, the drop in the electric potential is very sharp, while in the Guoy-Chapman layer it is rather gradual. Solubilizing power is the most useful and practically important property of micellar systems. Solubilization is a dynamic equilibrium process and depends on the temperature, surfactant concentration, the nature of the solute, and the type of micellar system. Thereare several possible sites for solubilization in a micellar system, and the site occupied by the solubilizate depends onthe nature of both the soluteand the micelle. In a normal micelle, a nonpolar solute may be located nearthe center of the hydrophobic core. An amphipathic solute may be oriented in the micelle so that the hydrophilic moiety is either near to or far from the Stem layer. Ionic solutes may be adsorbed on the polar micellar surface. Another important feature of micellar systems is their ability to serve as a novel reaction medium in which the rates, equilibrium position, products, and even stereochemistry may be affected. They caneither inhibit or accelerate the rates of chemical reactions as well as shift the
mHydrophoblcTall, 0
Head GWP,
f CounterIon
Figure 5 A two-dimensional representation of an ionic spherical micelle.
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position of the reaction equilibrium depending on the nature of the reaction and the type of micellar system. These effects are the consequence of solubilization and are attributed to the balance of the hydrophobic and electrostatic interactions occurring between the micellar system and the reactants. Microemulsions are similar to micellar systems. A simplistic description of them is that microemulsions are swollen micelles. A common microemulsion is obtained by proper processing of water, surfactant, another hydrocarbon, and a cosurfactant. One significant difference between the two is the average size. While ordinary micelles are about 2-3 nm in size, oil20-40 in-water microemulsions have an average size in the range of nm. As a consequence, the solubilizing capacity of microemulsions is far greater than that of micelles. Their structural characteristics are very similar to those of micelles. In summary, micellesand microemulsions affect the redox equilibrium concentrations and solubilize very many sparingly soluble organic pollutants. This makes them attractive photocatalysts in the photochemical degradation of toxic species that are not easily brought sointo lution in aqueous media. Like the semiconductor sols, micellar and microemulsion systemsare generally optically transparent.
B. Photocatalytic Studies in MicelladMicroemulsion Media Though extensive investigations of micellar and microemulsion systems have been reported, photochemical degradation studiesof toxic pollutantsare limited. The presence of the micellar or microemulsion medium alters the photochemical reactionsof a species from its behavior in of the photochemical processes in micellar a homogeneous medium. A comprehensive account and microemulsion media is given in the book edited by Kalyanasundarum [48]. Though micellar media offer several unique advantages for the photochemical decomposition of nearly water insoluble organic toxic species (insecticides, pesticides, pentachloropheor photochemicaldegradationstudiesofhazardouswastesin nols,etc.),photocatalytic micellar media are very limited. Shukla et al. [49] recently reported the resultsof their investigation of photodegradation of pentachlorophenol in sodium didecylsulfate (SDS) and cetyl trimethylammonium bromide (CTAB) micelles and microemulsions. The micellar solutions consisted of 0.1 M solutions of either SDS or CTAB. The microemulsions were prepared by (4.2 wt % mineral oil mixing the surfactants, mineral oil, and pentanol in appropriate amounts 26.2% pentanol 9.2% SDS water; 4.0% mineraloil + 16.0% pentanol + 28.00% CTAB water). An alkaline solution of pentachlorophenol (PCP;0.5 n M ) was mixed with the micellar solutionor microemulsion, and the resulting mixture was illuminated withUV a light source. The concentration of the PCP in the solution system was monitored spectrometrically. The details are presented elsewhere [49]. The results are presented in the Table 4. The photodegradation rates are relatively slow comparedto the rates shown in Table4 for the photocatalytic degradationat the surfaceof the semiconductor sols. But the results indicate the potential of these media where the toxic substances are water-insoluble. Pellizzetti et al. [46] reported complete mineralization of nonylphenol ethoxylated surfactants.
+
+
+
+
V. CONCLUSIONS Photocatalytic degradation of toxicspecies in waste streamsis the best alternative processing method for better management of hazardous industrial wastes. The results of many investigaof hazardous species (intions have demonstrated the feasibility of mineralization of a number on the EPA prioritypollutantslist)intoenvironmentallybenign cludingseveralspecies products using microheterogeneous media, especially the semiconductor slurries. Thus, a sci-
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Table 4 Photodegradation of PCP in Micellar Media (hr)
dPercent Medium
DS
0.1 M
72 micelle 0.1 M CTAB 23 CTAB microemulsion 4 microemulsion* CTAB ~~
56 12 35
~~~
"100-W UV light source. All the other results were obtained using a 4-W
UV lamp.
Source: Shukla [50].
entific basis for the photocatalytic degradation ofmanytoxic species inhazardouswaste streams has been established. Having established feasibility, development of these treatment processes can now be addressed. The solubilization properties of micellar systems for nearly insoluble organic toxic species offersa distance advantage for further studies.
1. Conner, J. R., Chemical Fixation and Solidification of Hazardous Wastes, Van Nostrand Reinhold,
NewYork,1990. Feb. 22-28,1992,New 2. ProceedingsofHMCSouth1992ConferencesponsoredbyHMCRI, Orleans. 3. Proceedings of the National Research and Development Conference on Control of Hazardous Materials, Feb. 20-21, 1991, Anaheim, Calif. 4. Benested, C., Hagen, I., Jebens, A., Oehme, M., and Ramdahl, T., Waste Manage. Res., 8, 193 (1990). 5. Cesareo, D., de Domenico A., Marchini, S., Passerini, L., and Tosato. M. L., in Homogeneous and Heterogeneous Photocatalysis (E.Pelizettia and N. Serpone, eds.), D. Reidel, Amsterdam, 1986, pp. 593-627. 6. Ku, Y., and Ho, S . C., Environ. Prog., 9, 218 (1990). 7. Ollis, D. F., Pelizzetti, E., and Serpone, N.,in Photocatalysis: Fundamentals andApp1ication.s(N. Serpone and E. Pelizzetti, eds.), Wiley, New York, 1989, Chapter 18, pp. 603-637. 8. Bhakta, D., Shukla, S. S., Chandrasekharaiah, M. S., and Margrave, J. L., Environ. Sci. Technol., 26, 625 (1992). 9. Ollis, D. F.,J. Catal., 97, 564 (1986). Catalysis, Academic, New York, 10. Gratzel. M. (ed.), Energy Resources Through Photochemistry and 1983. (ed.), Photoelectrochemistry, Photocatalysis and Photoreactors, D. Reidel, 11. Schiavello,M. Dordrecht, The Netherlands, 1985. 12. Photocatalysis: Fundamentals and Applications,Serpone, N., and Pelizzetti E. (eds.), Wiley, New York,1989. Photochemistry and Photophysics, Vols. I-IV, CRC Press, Boca Raton, Ha., 13. Rabek,J.F. (4.). 1991 . 14. Schiavello, M,, and Sclafani, A., in Photocatalysis: Fundamentals and Applications (N. Serpone and E. Pelizzetti, eds.), Wiley, New York, 1989, pp. 159-173. 15. Kisch, H., in Photocatalysis: Fundamentalsand Applications (N. Serpone andE. Pelizzetti, &S.), Wiley.NewYork,1989,pp.1-8. 16. Fendler, J. H., Chem. Rev., 87, 877 (1987). 17. Hinze, W. L., in Solution Chemistry of Surfacmnts, Vol. 1 (K. L. Mittatl, d.),Plenum, New York, 1979, pp. 79-127. 18. Gratzel, M., Heterogeneous Photochemical Electroni'Fansfr, CRC Press, Boca Raton, Ha., 1989. 19. Kalyanaundarum, K., Gratzel. M., and Pelizzetti, E., Coord. Chem. Rev., 69, 57 (1986). 20. Henglein, A., Pure Appl. Chem., 56, 1215 (1984).
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35. 36. 37. 38. 39. 40. 41. 42.
St. Johno. M. R., Furgala, A. J., and Sommells, A. F., J. Phys. Chem., 87, 801 (1983). Tangnay,J.F.,Suib, S. L., and Coughlin, R. W.,J. Catal., 117, 335 (1989). Maser,, J., and Gratzel, M., Helv. Chim. Acra, 65, 1436 (1982). Duonghong, D., Borgarello, E., and Gratzel, M., J. Am. Chem. Soc., 103,4685 (1981). Fox, M. A.,Acc. Chem. Res., 16, 314 (1983). Bard, A. J., J. Phys. Chem., 86, 172 (1982). Frank, S. W., and Bard, A. J., J. Am. Chem. Soc., 99, 303 (1977). Frank, S. W., and Bard, A. J., J. Phys. Chem., 81, 1848 (1977). Krautler, B., and Bard, A. J., J. Am. Chem. Soc.,100, 2239, 4317, 5985 (1978). Ward, M. D., White, J. R., and Bard, A. J., J. Am. Chem. Soc., 105, 27 (1983). Child, L. l?, and Ollis, D. F., J. Caml., 66, 393 (1980). Pruden,A. L., and D. F. Ollis, Environ. Sci. Technol., 17, 628 (1983). Hsiao, C. Y., Lee, C. L., and Ollis, D. F.,J. Caral., 82, (1983). Pruden, A. L., and Ollis. D. F., J. Caral., 82, 404 (1983). Ollis, D. F., Hsiao, C. H., Budiman, L., and Lee, C., J. Catalysis, 88, 89 (1984). Ollis, D. F., Environ. Sci. Technol., 19, 480 (1985). Ollis, D. F., and Turchi, C., Environ. Prog., 9, 229 (1990). Finklea, H. 0..J. Chem. Educ., 6 0 , 325 (1983). Turner, J. A., J. Chem. Educ., 6 0 , 327 (1983). Tissot, A., Boule, l?, and Lemaire, J., Chemosphere. 12, 859 (1983). Oliver, B. J., Cosgrove, E. G . . and Carey, J. H., Environ. Sci. Technol., 13, 1075 (1979). Barbeni,M.,Pramauro, E., Pelizzetti, E., Borgarello, E., Serpone, N., andJamieson,M.A.,
43.
Hidako,H.,Kuboto,H.,Gratzel.M.,Serpone,
N., andPelizzetti, E., Nouv. J. Chim., 9, 67
44. Hidako, H.,Kuboto, H., Gratzel, M., Pelizzetti,
E., and Serpone, N., J. Phorochem., 35, 219
21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33.
34.
45.
Chemosphere, 15, 1913(1988). (1985).
(1986).
Hidaka, H., Fujita, Y., Ihaa, K., Yamada, S., Suzuki, S . K., Serpone, N., and Pelizzetti, E,,J.
Jpn. Oil. Chem. Soc. (Yukagaku),36, 836 (1987).
46. Pelizzetti, E., Miero, C., Maurino, B., Sciafani, A., Hldaka, H., and Serpone, N.,Environ. Sci. Technol., 23, 1380 (1989).
47. 48.
Maheshwari, D.K., M.S. Thesis, Lamar University, Beaumont, Tex., 1990. Kalyanasundarum, K. (ed.), Phorochemisrry in Microheterogeneous Media,Academic, New York,
49.
Shukla, A., M.S. Thesis, Lamar University, Beaumont, Tex.,
1988.
1990.
17
Photocatalytic Oxidation of Organic Contaminants
Allen l? Davis University of Maryland College Park, Maryland
1.
INTRODUCTION
Contamination from organic compounds represents a significant problem bothin water supplies and in industrial andother hazardous wastewaters. Studies conducted by the U.S.Environmental Protection Agency (EPA) between1977 and 1982 found that 21% of the approximately IO00 drinking water systems sampled contained detectable levels of one or more volatileor synthetic organic compounds [l]. Many of these substances are suspected of being carcinogenic, and their occurrence in potable water causes serious concern. Additionally, organic compounds make up the majority of substances onthe EPA priority pollutants list and includechlorinated solvents, petroleum products, phenols, and pesticides. The application of a single process to treat contaminated waters is not possible owing to variations in water characteristics and the physical and chemicalproperties of the organic contaminants. The effectiveness of remediation processes depends upon such factors as organic volatility and solubility, pH, presence ofother compounds, required water quality, and volume treated. Consequently, treatment of organic-contaminated waters must be specifically tailored to the type and degree of contamination present. Currently several processesare being used for the decontamination of waters tainted with volatile or synthetic organic compounds. A comparison of organic removal methods was summarized by Clark et al. [2]. A list of some of the various processes and an indication of the effectiveness of the treatment for some selected compounds are presented in Table 1. A processcommonlyused for the removal of volatileorganiccontaminants is airstripping, whereby air and the contaminated water are mixed,usuallyin a countercurrent fashion. In this process, the volatile organic compounds are transferred from the aqueous to the gas (air) phase. Consequently, the pollutants must be sufficiently volatile for successful treatment. The efficiency of the process is controlled by mass transfer rates between the two phases as well as the air/water equilibrium partitioning, quantified by the Henry’s law constant. Due to stringent air pollution standards, most air-stripping processes currently require
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Table 1 Summary of the Performance for Organic Decontamination Technologies Removal efficiency" Granular Ozone Packed activated oxidation tower aeration ppm) (2-6
bon compounds Organic
vocs Alkanes Carbon tetrachloride 1,2-Dichloroethane 1,1,1 -Trichloroethane 1.2-Dichloropropane Ethylene dibromide Dibromochloropropane Alkenes Vinyl chloride 1, I-Dichloroethylene cis-l ,2-Dichloroethylene trum-l ,2-Dichloroethylene Trichloroethylene Aromatics Benzene Toluene Xylenes Ethylbenzene Chlorobenzene o-Dichlorobenzene p-Dichlorobenzene Styrene Pesticides
Pentachlorophenol 2,4-D Alachlor Aldicarb Carbofuran Lindane Toxaphene Heptachlor Chlordane 2,4,5-TP Methoxychlor
++ ++ ++ ++ ++ ++
+ ++ ++ ++ ++ ++++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ NA ++ ++ ++ ++ ++ ++ NA ++
Other
Acrylamide Epichlorohydrin PCBs
+
NA NA
++
'++ = good, = fair; - = poor, NA = data not available. Source: Clark et al. [2]
++ ++ ++ ++ ++ + ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ -
-
++ -
++ ++ -
NA NA
-
++
-
-
-
++ ++ ++ ++ +
++ ++ ++ + + + ++ ++ + ++ NA ++ -
NA
+ +
NA
NA
-
NA
minants PC0 of Organic
379
subsequent treatment of the off-gas. Vapor-phase carbon adsorption or a catalytic oxidation process is commonly used. Activated carbon adsorption is the other conventionally used remediation process. The efficiency of adsorption depends upon the physicochemical characteristics of the target organics as well as the water chemistry, e.g., pH. Complications can result from various aspects of competitive adsorption, either from multiple contaminants or with naturally occurring dissolved be exorbitant, organic matter.In addition, the costsof disposal/regeneration of spent carbon can and used carbon may be classified as a hazardous waste. Other treatment processes for organic contamination that may be employed under various conditions include biological oxidation, chemical oxidation, and membrane processes. Concerns of secondary contamination from air-stripping off-gas and spent carbon and the difficulties encountered in treating many recalcitrant substances have evoked interest in advanced oxidation processes (AOPs). Such processes involve supplementation of traditional oxidants with additional stimuli such as high temperature or UV light to create highly reactive radical species to oxidize difficult-to-treat substances suchas saturated organic molecules and pesticides. Examples of AOP include ozone-H202-UV combination systems, wet air oxidation, and photocatalytic oxidation. The increasing concern over water contamination and the need for ultimate decontaminationas opposed to phase transfer create an impetus and interest in examining these novel treatment operations. Recently, there has been a considerable amount of research and development examining the ( K O ) forthedecontaminationof phenomenonofsemiconductorphotocatalyticoxidation tainted waters. Most of the work with PC0 has been oriented toward low organic concentrations (parts per billion or low parts per million) where oxidation processes can be competitive with other organic treatment technologies. Compounds such as phenols, benzenes, and chlorinated solvents, as well as PCBs (polychlorinated biphenyls), pesticides, and dioxins,are oxidized to simple, environmentally acceptable products such as carbon dioxide and chloride. The majority of these investigations have been based on oxidizing organic solvents that are PC0 process for treating solvent-contaminated present in contaminated waters; a full-scale groundwater has shown promising preliminary results[3].
A. OrganicOxidation The oxidationoforganiccompounds,althoughthermodynamicallyfavorable, is normally kinetically limited and thus very slow. Processes that use strong oxidizing agentsare energyintensiveandusuallyhaveprohibitivecostsforwastetreatment.Someofthetraditional compounds used for oxidation are oxygen, chlorine, ozone, permanganate, chlorine dioxide, and hydrogen peroxide. Novel processes, such as the supplementation of radicalinitiating ultraviolet light, have been found to increase the reactivity of some of these common oxidizing agents. Oxidation is defined as the loss of electrons by a substance; concurrently, the oxidizing agent is reduced by the electrons gained. Organic chemical oxidation typically occurs via addition or substitution. 1. Addition is the incorporation of the oxidant into the chemical structure of the organic compound. An exampleofoxidativeadditionischlorine or ozoneaddingacrossthe double bond of an olefin. Compounds with a high electron density (such as those with double or triple bonds or an aromatic ring) are susceptible to addition of electrophilic oxidants. Furthermore, the unsaturation of the organic molecule easily allows oxidant addition.Thisdifferenceinreactivitybetweensaturatedandunsaturatedorganics is
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noted in Table 1 by the promising use of ozone in oxidizing alkenes butits limited effectiveness in reaction with alkanes. 2. Substitution is the replacement of a reduced atom or group in the organic substanceby all or a portion of the oxidizer. For example, chlorophenol is formedby substitution of a hydrogen in phenol upon reaction with chlorine. Similarly, hydroxyl radical attack ona phenolic hydrogen atom will result in the production of a dihydroxybenzene. Recently there has been a renewal of interest inoxidative processes as a means of treating hazardous organic materials. Oxidation hasthe potential to transform organic contaminantsto environmentally acceptable forms such as carbon dioxide andchloride. Thus concern oversecondary waste production, such as solid or gas-phase residuals, is relieved. The challenges with chemical oxidation are to ensure complete oxidation of hazardous substances to compounds that do not exhibit harmful or toxic characteristics and to enhance reaction conditions so that this transformation is accomplished in an economically feasible manner.
B. PhotocatalyticActivation As a result of solid-state quantum effects, semiconductor materials possess two allowable elec-
tron energy bands. The lower energy region is the valence band; electrons in this energy band are binding electrons and are somewhat restricted in movement. The higher region is the conduction band. These electrons, to a first approximation, are free to move throughout the solid and produce conductivity similar to that of metals. Between these two regions is a forbidden zone, or bandgap. Photoexcitation in a semiconductor occurs as the absorption of radiation of energy equal to, or greater than, the bandgap energyexcites an electron (e-) into the conduction bandof the solid. There is, correspondingly, an electron vacancy or hole (h’) that remains in the valence band (Figure 1).
4
hv
Semiconductor ”* e-
-0.3
Potential
(V)
2.1
+ h+
Ti4 Conduction Band
Reduced Product Oxidized Reactant
Recombination Reduced Reactant
Ti02Valence Band
Oxidized Roduct
Figure 1 Creation of electron-hole pairs in illuminated semiconductors, and subsequentphotocatalytic redox reactions.
P C 0 of Organic Contaminants
381
Table 2 Some Common Semiconductors, Their Bandgap Energy (pH 0), and Corresponding Excitation Wavelength
Semiconductor TiO, ZnO
ZnS CdS Fe203 WO3
Bandgap Wavelength (eV)
(nm)
3.0-3.2 3.2 3.7 2.4 2.3 2.8
Source: Maruska and Ghosh [4]; Sakata and Kawai
413-388 388 335 516 539 443
[5]
These holes, having an affinity for electrons, are very strong oxidizing agents. The number of electron-hole pairs is dependent on the intensity of the incident light and the material's electronic characteristics that prevent them from recombining and releasing the absorbed energy. The electron is free to move throughout the solid in the nearly unoccupied conduction band. Similarly, the hole can migrate by a valence band electron filling the vacancy, leaving behind another hole in the previous position. The bandgap energy and corresponding wavelength required for excitation for some common semiconductorsare given in Table 2. The semiconductor potentials for the valence band and conduction band are significantly different. This difference avoids rapid recombinationof the e"h+ pairs. The band potentials are a function of pH and decrease by 0.059 V per pH unit increase as predicted by the Nernst equation [6,7]. The holes in the semiconductor solidare attracted to the oxide/sulfide surface, where they oxidize an adsorbed water molecule or hydroxide ion. h+ h+
+ HzO(ads) + OH. + H+ + OH-(ads) + OH.
Hydroxyl radicals are very reactive neutral species with an unpaired electron. They ieact rapidly and nonselectively in the oxidation of organic compounds and are the common oxidizers in AOP and high-pH ozone systems. Reaction (2) is likely to occurfor two reasons. Oneis that large quantities of OH- and H20 groups are available as adsorbates, and the chances of holes reacting withthese groups on the semiconductor surface are high. The second reason is that for several semiconductors the oxidation potentials of these reactions are above (more negative than) the potential forthe valance band over the entire pH range. Reaction (2a) is favored at low pH and reaction (2b) at higher pH values [7].
II. ORGANICPHOTOCATALYTICOXIDATION Titanium dioxide (Ti0,) is the semiconductor material with the most promise for the photocatalytic treatment of hazardous wastewaters.TiO, is an active photocatalyst and is extremely stable, i.e., it does not dissolve or corrode under photoexcitation. Barbeni et al. [8] reported the photodegradation of several chlorinated benzenes and phenols, 3,3'-dichlorobiphenyl, and2,7-dichlorodibenzo-p-dioxinusing illuminatedTiO,. A comparison among several semiconductors indicated that TiO, produced a faster rate than ZnO, CdS, or WO,. The photocatalyticoxidation (PCO) of oxalic acid was also much more efficient using TiO, than other semiconductors such as ZnO, WO,, and F%O, [9], as was the K O of
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polychlorinated dioxins andPCBs [lo]. Oxidation of dioxin is much slower than that of simple compounds, apparently because of the large size of the molecule. Consequently, PC0 processes entail the addition of TiO, particles or powder to contaminated waters with concurrent exposure to UV radiation. An advantage of photocatalytic oxidation over ozone-H202-UV processes is the higher wavelength (lower energy) requirement of photocatalysis. TiO, photoactivation requireslight at wavelengths less than about 380nm (Table 2), whereas ozone-H2O2-UV systems operate at approximately 254 nm. Thus PC0 provides the possibility of savings in both capital (lamps) and operating (power) costs.
A. Titanium Dioxide Surface Reactions Titanium dioxide is rapidly hydrated in aqueous solution due to the partially uncoordinated states of its surface titanium and oxygen atoms. The amphoteric nature of the hydrated TiO, surface results in pH-dependent equilibrium between protonated and deprotonated hydrous surface species [l l],
I I I-0-H,, I I - T i -OH,' I
I - T i -OH,*
-H' "-)
<"+H'
(a)
I I I-0-H
I - T i -OH
l I
I - T i -OH
(b)
-H'
"->
("-
+H'
1 I m-0l I - T i -0' I
I - T i -0'
(c)
where Irepresents thebulk solid. The pH at which the overall TiO, surfaceis neutrally charged is termed the zero point of charge or pH,. Reported values of the pHzpcfor TiO, range from 3.5 to 6.7 depending on purity and crystal structure [12-141. TiO, can be viewed as a solid diprotic acid having two acidity constants. p&, represents the equilibrium between (a) and (b) of reactions (3). Similarly, the equilibrium between 3(b) and 3(c)is denoted bypK,. The theoretical maximum surface coverageof TiO2 with surface hydroxo groups is 5-15 OH groups per square nanometer [1 l]. As discussed previously, thermodynamically permissible traps for valence band holes in aqueous TiO, systems include adsorbed water molecules or hydroxide ions, which react with the holes to produce hydroxyl radicals [reaction (2)]. Hydroxyl radicals have been detectedby [15,16] and are widely electron spin resonance(ESR) in illuminated aqueous TiO, suspensions believed to be the most important oxidizingspecies in these systems. Oxidizable species such as hydrocarbons (RH)or other organic contaminantsin the solution subsequently react with the hydroxyl radicals [171. -H.
RH
+ OH. "* ROH
Continued radical attack will result in the complete oxidation of the organic compound via pathways common to all hydroxyl radical oxidations. It has also been proposed [l81 that adsorbed organic species may react with valence band holes at the TiO, surface, resultingin the formation of an organic radical. Despitethe favorable thermodynamics of such a reaction, however, there is strong evidence that H,O and OH-, and not organic compounds, are the principal hole traps in TiO, photocatalytic systems. For example, Cunningham -and Srijaranai [l91 found that substitution of deuterium (D) in C-H and 0-Hbonds in isopropanol and cyclobutanol did not affect the rate of oxidation of these com-
aminants PC0 of Organic
383
pounds by TiO, photocatalysis. However, they found that use of 4 0 instead of H20 as the solvent in the same system significantly decreased the initial oxidation rate of the two unlabeled alcohols, suggesting that the rate-determining step in TiO, photocatalytic oxidation involves H 2 0 and OH- and not organic species. The electron migrates to the TiO, surface, where it reduces surface Ti(1V) ions.
+
Ti(IV)sufi e-
”*
(5)
Ti(III)sud
Subsequently, oxygenor another oxidizing agent reacts with the reduced metal center, trapping conduction band electrons and producing the superoxide radical anion (0,”) [20-221. OZ(ads) + Ti(II&,,fi + Oy-(ads)
+ Ti(IV)sufi
(6)
This frees the flow of electrons (and therefore that of the holes) and inhibits the e--h+ recombination process. Hydrogen peroxide (H202) is consequently formed through various radical reactions [20,22,23].
Cleavage of H20pmay also result in the formation of OH. radicals through several reactions [6,20,22,24]. H202 + Ti(III)sufi-+ Ti(IV)sufi+ OH. + OH-(Fenton-type) (Haber-Weiss) H202 9 . - ”* OH. + OH- + 0 2 H2025 2 OH. (photodissociation)
+
However, several studies have suggested that these routes of OH- formation for organic oxidation are secondary to reactions (2a) and (2b) [20,24,25]. Within the last few years, interest in PC0 has exploded since a wide variety of organics can be both nonselectively and completely oxidized using this process. A representative (but not exhaustive)listing of the organic pollutants successfully treated by photocatalytic oxidation is given in Table 3.
B. Oxidation of Organic Compounds The complete degradation of chloroform (CHCl,) to carbon dioxide, water, and chloride ions has been demonstratedby Pruden and Ollis [271 using TiO,. A decrease in chloroform, along with formation of carbon dioxide and chloride, was found according to the stoichiometry CHC13 + H20
+ 0.5 0 2 ”* CO, + 3 HCl
(9)
Continued work by the same researchers has examined the mineralization of several simple chlorocarbon solvents andacids [26,28,31,32]; complete degradation of these reactants to inorganic products was observed. However, the reaction to these products is not direct and involves several steps. Mineralization of two organophosphorus insecticides to CO,, Cl-, P O : , and H+ was accomplished at rates three to four times slower than the disappearance of the parent compounds, indicating the formation and subsequent destruction of intermediate substances [ 4 6 ] . Similarly, CO2
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Table 3 organic Contaminants That Have Been Successfully Treated by Photocatalytic Oxidation Using TiOz Compounds
Reference
Solvents
Chloroform. dichloromethane Trichloroethylene Benzene, perchloroethylene Methyl vinyl ketone Perchloroethylene, dichloroethane, mono-, di-, and triacetic acid Trichloroethylene, trichloromethane
Hsiao et al.. [26] Pruden and Ollis [27] hvden and Ollis [28] Turchi and Ollis [29] Muneer et al. [30] Ollis et al. [31] Ahmead and Ollis [32]
Phenols
Phenol Pentachlorophenol Fluorinated phenols 4Chlorophenol Chlorinated phenols 2-Chlorophenol, 3-chlorophenol 2,4-Dichlorophenol Trichlorophenoxyacetic acid, 2,4,5-trichlorophenol 0 - , m-, and p-Cresols Pesticides Atrazine, simazine, prometryn Atrazine, simazine, propazine Dimethyl-2,2-dichlorovinylphosphate PCBs and Dioxins PCBs and dioxins Phenols, benzene, biphenyl, dioxin 3 ,4-Dichlorobiphenyl
Okamoto et al. [24] Augugliaro et al. [33] Tseng and Huang [34] Barbeni et al. [35] Minero et al. [36] Barbeni et al. [37] AI-Ekabi and Serpone [211 Tseng and Huang [38] D’Oliveira et al. [39] Ku and Hsieh [40] Barbeni et al. [41] AI-Ekabi et al. [421 Terzian et al. [43] Pelizzetti et al. [M] Pelizzetti et al. [45] Harada et al. [46] Pelizzetti et al. [lo] Barbeni et al. [8] Tunesi and Anderson [47]
Other organic compounds
Benzoic and salicylic acids, naphthalene Formic acid Acetic acid Oxalic acid Chloral hydrate Azo dyes Sodium dodecylbenzene sulfonate Nonylphenol ethoxylated surfactants Methylene blue, salicylic acid 12 Nitrogen-containing organic compounds 4 Ring nitrogenous compounds 21 Organic compounds 22 Solvents 11 Organic compounds 23 Organic compounds
Matthews [48] Bideau et al. [49] Bideau et al. [50] Bideau et al. [S11 Herrmann et al. [9] Tanaka et al. [52] Hustert and Zepp [53] Hidaka et al. [54] Pelizzetti et al. [ S ] Matthews [S61 Low et al. 1571 Low et al. [58] Matthews [S91 Matthews [60] Matthews [61] Sabin et al. [62]
385
aminants PC0 of Organic
evolution was much slower than the photocatalytic disappearance of methylene blue [56] and methyl vinyl ketone [30]. Matthews [59] noted that 21 different substances, including some aromatic hydrocarbons, are degraded completely to COz. The general reaction presented is C,H,X+702+6CO2+HX+2H20
(10)
First-order Pc0 rate constants for 23 organic compounds were evaluated by Sabin et al. [62]. Their data show several trends; for example, alkanes are more difficult to treat than alkenes or aromatics, and chlorine-substituted compounds have lower reaction rates. Both of these trends are typical for organic oxidation processes.Somehighly fluorinated compounds were very resistantto degradation. Meanwhile, Matthews [61] noted that Pc0 rate constants for 10 different compounds (organic acids, phenols, alcohols, and nitrobenzene) were quite similar.
C. Kinetics The initial photocatalytic reaction rate follows a Langmuir-Hinshelwood type of relationship with respect to initial organic concentration [9,26-29,31,33,39,42,48,49,59,6,63-651: r, = rate = 1
NCO KC,,
+
where C, is the initial concentration of the target organic compound. The constants k and K represent collections of oxidation, recombination, and possiblyadsorption terms. This expression describes a first-order reaction at low substrate concentrations, with a transition to a rate independent of organic concentration at high substrate loadings (Figure 2).The rate expression can be linearized by reciprocation, resulting in
thus describing a linear plot for llr, versus K,, (Figure 2, curve B). Secondary organic substances, formed via the oxidation process, complete with the original substrate for the oxidant (hydroxyl radicals), so later reaction rates may be slower than the initial rate. The effects of several operational and solution parameters on the PC0 reaction rate have been evaluated. In numerous cases, effects of these parameters are compared using rate constants (koh) derived from first-order plotsthat are linear over a short range. Others have made comparisons among various reaction conditions using initial rates (r,), where a linear decrease in concentration is measured over a brief time period. A Langmuir-Hinshelwooddependenceonoxygen concentration has beendetermined [33,63]: Rate =
k' K1[OJ 1 K1[02]
+
Oxygen is necessary to complete the oxidation reaction by reacting with the photoproduced electrons [surficial Ti(III)] to maintain electroneutrality via reaction (6). The incident light intensity controls electrodhole production and thusthe hydroxyl radical formation rate. For example, a direct linear relationship between light intensity and PC0 rate has been found for rn-cresol [43]. However, a transition from first-order to 1/2-order is found at high light intensities [63,66]. This is apparently due to an increase in hydroxyl radical recombination,
386
Davis
c
c
K
0.2
1
2 C rnrnoll" o/
Figure 2 (A)Initial 3-chlorophenol concentration versus initial Pc0 rate. (B) VC, versus W., (Reprinted with permission from D'Oliveira et al. [39]. Copyright 1990 American Chemical Society.)
yielding a recombination rate proportional to [OH.]'". This consequently producesan overall organic oxidation rate proportional to the square root of light intensity [67]. Quantum yield is a measure of the efficiency of light utilization and is defined as the ratio of the number of photons entering into a photochemical reactionto the number of photons a p plied. The former parameter is usually evaluated by the conversion of the substrate, the latter by measurement of the incident light characteristics. The degradation of 3,4-dichlorobiphenyl over illuminated Ti02 was examined by lbnesi and Anderson [47]. ' b o light intensities were evaluated; increasesin light intensity result in a faster removal rate but a smaller quantum yield, due to inefficient light utilization and increased radical recombination, as discussed above. Intermediate products isolated include linto ear andbranchedhydrocarbonsandsomephenols.Quantum yields rangedfrom 7.4 X l .6 X IOF3. PC0 quantum yields of 0.06 for acetic acid [51] and 0.022 for salicylic acid [48] have also been reported. Since key steps to TiO, photocatalytic oxidation occur at the TiO, surface [i.e., reactions (2) and (6)], thereaction rate would be expected to increase linearly with availablecatalyst. At dilute TiO, concentrations, such a relationship is observed. However, above certain concentrations, the rate of oxidation does not increase [34,36,63], as shown in Figure 3, and may decrease with further increasing TiO, concentration. For example, Matthews [20] found that the rate of salicylate formation in the oxidation of sodium benzoate increased with the quantity of titanium dioxide upto 2 g/L but decreased slightly at higher loading. Augugliaro et al. [33] observed the same phenomenon with a maximum phenol PC0 rate at 1 g/L TiO,. Matthews
PC0 of Organic Contaminants
1 4-f luorophenol
387
I
Figure 3 Dependence of kobs onphotocatalystconcentrationinthephotocatalyticoxidation of 4fluorophenol. The inset shows a linear reciprocation of a Langmuir-Hinshelwood expression. (Reprinted with permission from Minero et al. [36]. Copyright 1991 American Chemical Society.)
[61], analyzing 4-chlorophenol degradation using Equation (lla), found that k decreased while
K increased as TiO, loading varied from 0.2 to 2 @L. These results are explained by the fact that above certain Ti02 concentrations, there is a stoichiometric TiO, loading that is sufficient to use all available photons emitted at a given intensity. Increasing the TiO, concentration above this level becomes inconsequential because all available light is being utilized. In addition, other factors that contribute to the rate independence on TiO, concentration include the reactor configuration, reflection, and solution opacity, which may prohibit some of the light from activating available photocatalyst. The effect of pH on PC0 reaction kinetics is still very much unresolved. Some typeof pH dependence, although it is usually slight, is noted with almost every organic substrate. However, it has not been possible to draw any general conclusions with respect to pH about photocatalytic oxidationkinetics.Insome cases, there are discrepanciesabout the pHof the maximum rate for the same organic compound. For example, the maximum rate for phenol PC0 has been reported at pH 3 by Augugliaro et al. [33], at pH 4-5 by Okamoto et al. 1241, and a pH 5-9 by Tseng and Huang [341. The PC0 rates of some nonionic compounds, such as solvents, are affected by solution pH. Matthews [65] found thatrates of CO2 production from benzene, nitrobenzene, and chloroform [as quantified by the product kK using Equation (1l)] were faster at pH 4.5 than at pH 3.0. However, dioxin Pc0 was found to be more rapid under basic conditions [lo]. The effectof pH on the PC0 of dissociating organic acids is complicated by the speciation changes of these compounds. In most cases, anionic organics are more reactive than the protonated molecularspecies, which is common for electrophilicoxidation. Oxidation of the pentachlorophenolate ion at high pH proceeds faster than that of molecular pentachlorophenolat pH 3 [35]. Similar results have been found for m-cresol [43] and 4-chlorophenol [37]. Palmisano et al. [68] studied the effect ofpH on the initial reaction rates of phenol and 2-, and
388
Davis
3-, and 4-nitrophenols.The highest photoactivity was foundtheinalkaline region. 3-Nitrophenol photocatalytic oxidation was less pH-dependent than that of the 2- and 4-isomers. The PC0 rate of oxalic acid was maximumat pH 2.3 and decreasedat higher pH, apparently due to the change in speciation from HC,O4- to C 2 0 2 - 191; however, acetic acid photocatalytic oxidation is maximum at pH 3 [5 l]. The photocatalyticoxidation of three commercialazo dyes and a model compound, 4-hydroxyazobenzene,was examined by Hustert and &pp [53]. Orange G was oxidized much faster at pH 12 than at pH 7 , apparently due to the deprotonation of this weak organic acid dye. Mechanisms other than substrate speciation that may be responsible for pH dependencies include surface hydrolysisof the TiO, (with subsequentchanges in the electrical double layer), pH dependencies of any adsorption of products or reactants (O,, H20, OH-, or organic), and pH dependencies of the reaction rates of specific organic substrates.For example, Kormannet al. 1661 correlated photocatalytic oxidation rates as a function of pH with calculated surface speciation for both trichloroacetate and chloroethylammoniumion adsorption onto TiO,. Titanium dioxide has two common crystal structures, anatase and rutile. Several studies have noted that rutile is not an active photocatalyst [24,33]. However, two of the three rutile catalysts examined by Auguliam et al. [33] have specific surface areas significantly lower than that of the anatase sample; thus a direct comparison may not be completely valid. Nevertheless, Sclafani et al. [69] noted an inactiverutile that has a specific surface arealarger (20 m2/g) than that of an active anatase (14 m2/g). An active rutile photocatalyst was found by Davis et al. [70]. Apparently the reactivity of rutile TiO, dependson the method of Ti02 synthesis [69]. The bandgap energy for rutile is 3.0 eV, as compared to 3.2 for anatase. Thus the oxidationreduction potentials are slightly less for the rutile phase and, thermodynamically, some reactions may not be favored with rutile. Sclafani et al. [71] attributed PC0 activity differences between anatase and rutile TiO, to the thermodynamics andkinetics of the reduction reactions. Although crystal structure plays a role in the photocatalytic reactivityof Ti02, there are other controlling parameters that must be considered when evaluating TiO2 from different sources. In theory, a higher TiO, specific surface area will increase its photocatalytic activity due to increased area for adsorptionof H,O and OH- and the corresponding subsequentgeneration of OH. radicals [reaction (2)]. Supporting this assumption, Matthews [59] observed that the degradation rate of 4-chlorophenol was much lower with an equal concentration of La Porte TiO, (specific surface area = 9 m2/g) than with Degussa TiO,(50 m2/g), presumably because of its much lower surface area. On the other hand, this theory has not been found to be consistent in severalother studies. Investigations by Cuendet and Gratzel [72] on pyruvate photocatalytic reactions found similar rates for two TiO, samples with different specific surface areas (145 m2/g versus 50 m2/g). Tanaka et al. [73] studiedthe degradation of trichloroethylene (TCE), methylchlorideacid, and phenol on 12 commercially available TiOz samples. Thedegradation rates varied among TiO, types, but there was no correlation with the specific surface area. The rates were found to be dependent onthe crystallite size of the anatase form presentin the TiO, sample; the larger the crystallite size, the faster the reaction rate. The two TiO, samples (with 100% anatase) in which the fastest rates were observed had specific surface areas of 17.3 m2/g(Fujititan TP-2) and 9.5 m2/g (Aldrich). Sclafani et al. [69] examined several commercial and synthetic types of TiO, possessing a wide range of physical and chemical properties, such as differing crystal structu~ and .~ specific surface areas; correspondingly,a wide range of PC0 rates was found. There was no photocatalyst propertythat produced a correlation with respect to reaction rates. Particle size may affect PC0 rates through an effect on the degree of electron/hole trapping. As particle size increases, the distance that the electron-hole pair must diffuse through the solid before reacting at the TiO, surface increases. This concurrently increases the proba-
PC0 of Organic Contaminants Sampling Port
Longwave
.. . ,..
..
.
..
1
389
:
Temperature Controlled Reactor
W Lamp
\\
TiO, suspension Magnetic stirrer
L
Figure 4 Qpical recirculating batch reactor used in photocatalytic oxidation kinetic studies.
bility that recombinationwill occur. Consequently, any decreased rate of PC0 for larger TiO, particles may be due to the greater degree of electron-hole recombination. Davis et al. [70] found that the photocatalytic activity of a specific TiO, catalyst depends onthe synthesis (manufacturer) of the TiO,, the crystal structure, and anypretreatment of the solid. No correlation was found between particle size of the TiO, and initial PC0 rate of toluene. Several investigatorshave reported onthe detrimental effect of chloride ions onPC0 rates [26,31,33,38,66]. The presence of chloride significantly decreases organic oxidation rates, possibly by scavenging an active radical species [74]:
Inhibition of the PC0 process by chloride is a serious concern becausechloride is formed during the mineralizationof chlorocarbon compounds, many of which are found in contaminated waters. Abdullah et al. [74] and Tseng and Huang [381 examined the effect of several other anions on the PC0 rate of organic substrates. Phosphatesand sulfates decreased reactionrates, apparently by adsorbing ontothe TiO,, resulting in the deactivation of some active sites; NO3and C104- had no effect on PC0 reaction rates. Photocatalytic oxidationrates are decreased in the presence of bicarbonate ions. The presence of 500 mglL bicarbonate decreased TCErate constants by a factor of 2-5 in bench-scale studies [75]. Bicarbonate ionsare well known as radical scavengers through work investigating ozonation kinetics. Apparently the HC0,- acts in the same manner during K O , scavenging hydroxyl radicals, thus preventing reaction with the target substrate. Inmany instances PC0 kinetics have been investigated using recirculating systems in which a mixed reactor feeds an isolated plug-flow photoreactor, similar to that in Figure 4. In this manner, sampling and suspension chemistry monitoring and adjustmentsare performed in the mixed reactor, away from the photoreactor. Studies have shown that PC0 first-order rate constants (kobs)are a function of flow rate (FR)through these reactors andare described by the equation [56]
k* FR 1 ~ F R
+
At small flow rates, an increase in flow results in a higher reaction rate constant; the rate becomes independent of flow at higher flow rates. Similar results were found by Al-Ekabi
390
Davis
and Serpone [21] in an examination of flow through a coiled glass tube withan internal coating of TiO,. Minero et al. [36] noted that the PC0 rates of five different fluorophenols and difluorophenols were very similar. Analogously, the rates of phenol, the three chlorophenol isomers, five different dichlorophenols, and 2,4,6-trichlorophenol were all within about 30% [38]. Both of these investigationsillustrate the nonselectivityof PC0 systems. However, in contrast, Tenian et al. [43] found that the PC0 rate of m-cresol was about 2 112 times slower thanthat of either 0- or p-cresol under identical conditions. Competitive interactions among 4-chlorophenol, 2,4-dichlorophenol, and 2,4,5-trichlorophenol were examined by Al-Ekabi et al. [42]. The PC0 rates of all three compounds were slowed in a ternary mixture. However, the sum of the three individual rates was equal to that for PC0 of a single compound, indicating thata fixed amount of oxidant was being produced by the illuminated TiO, system. is not possible due Absolute comparisonof organic PC0 rates among various investigators to variations in substrate and catalyst loading, light intensity, and reactor configurations. Some have based rates on target organic disappearance, whileothers have monitored the production of CO, or Cl-. Batch and flow reactors using suspendedas well as immobilized photocatalyst have been examined. Discussionson the effect of reactor dynamics onPC0 rates are presented by Davis and Hao [76] and " c h i and Wolfrum [77].
D. Enhancement of ReactionRates In order to utilize PC0 as a viable treatment process, the oxidation should be rapid and efficient. Severalmethodologies havebeen examined, affecting boththeTiO,solidand the makeup of the aqueous suspension, to enhance the effectiveness of K O . An enhanced PC0 of organophosphorus insecticide by the addition of hydrogen peroxide was reportedby Harada et al. [46]. Tanaka et al. [52,78] noted that low concentrationsof H,02 enhanced the photocatalytic oxidation of TCE and chloral hydrate,but rate inhibition occurred at higher H202 additions. H202 can photolytically form OH. via reaction (8c) or it can act as the electron scavenger as presented in reaction (8a). In either case, the hydroxyl radical concentration in the suspension shouldbe increased andthe corresponding oxidation reaction promoted. Nevertheless, peroxide canalso promote hydroxyl radical recombination reactions [67]:
Overall:
H202
+ 2 OH*
4
2H20
+02
( W
Several other reports of both enhancement and inhibition by hydrogen peroxide havebeen published [22]. PC0 rates of several organic contaminantswere increased by the addition of inorganic oxand chlorate idizing agentsto the suspension [22]. Peroxydisulfate(S2OS2-),periodate (IO4-), (ClO,-) enhanced oxidation rates by both scavenging electrons [analogous to reaction (6)] and direct reaction with the substrate. The photoactivityof the Ti02 solid can be modified by subjecting it to various physical and chemical treatments. Abrahamset al. [1051 found an increased photocatalytic reductionrate by heating theTiO, to 350°C, which was attributed to an improvementin the TiO, crystallinity by annealing. Heller et al. [79] found that heating TiO, in the presence of oxygen or boiling it in acid increasedthe reactivity; however, they used TiO,that originally had a passive surface coating, and any enhancement should primarily be induced by the removal of this coating. Heat
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treatments in air up to 500°C had a slight effect on toluene PC0 rates, but this process also decreased the TiO, specific surface area [83]; overnight washingin strong acidor base did not affect reactivity. The heatindannealing process eliminates imperfections and defects fromthe TiO, solid state. These imperfectionscan act assites in which electron-hole recombination can easily take place. As a result, the heating step increases photocatalytic reactivity. Mechanical stressing of titanium dioxide particles by ball milling resulted in a decrease in photoactivity, corresponding to the milling duration [79];this effectwas attributed to a disruption in the particle crystallinity. Anpo et al. [80] foundenhancedphotoreactivityingas-phaseorganictransformations when TiO, was coprecipitated with a fraction of A1,0,. Also, using laser flash photolysis, it has been determinedthat iron-doped (0.5 wt %) TiO, is more photoreactive thanthe pure photocatalyst [81].It was suggested thatthe iron additionalters the photochemical behaviorof the TiO,, allowing a longer e-/h+ lifetime andcurtailing recombination. However, dopings greater than 1% apparently created an Fe2Ti05phase that decreases overall photoactivity. Another recent study found that Ag+ ions (1%) impregnated into TiO, enhance chloride and CO2 production from the photocatalytic degradation of chloroform and urea, respectively [82]. Davis and Espitallier [83]have completed a comprehensive investigation on the effect of TiO,dopingwithmetals.Variousmetalshavebeenexamined,including V(V), Mn(VII), Mn(IV), Mn(II), Fe(III), Al(III), Cu(II),Ni(II), and Ag(1); these metals were chosen to cover a range of valences. Doping was accomplished usinga commercial TiO, mixed with a solution of the metal, which was calcined at 500°C for 6 hr. Specific surface areas were quantified in all cases. Initial PC0 rates of toluene were used to quantify doping effects. In several cases specific reaction rates (rates per unit surface area) were increased by the metal doping, but the calcining step concurrently decreased the specific surface area, resulting in a decrease in the overall reaction rate. Only doping with Ag(1) resulted in a significant enhancement in the PC0 rate; initial rates were enhanced by a factor of up to 3. Unfortunately, the reactivity of the silver-enhanced photocatalyst slightly decreases over the course of several experimental runs, apparently due to the leaching of Ag+ from the TiO,, as also reported by Kondo and Jardim [82]. While the cost of silver doping is certainly high, practical use of this technology may be feasible because a significant rate enhancement results.If loss of leachable silver fromthe TiO, can be averted andan efficient TiO, encapsulation processor capture processfor the suspended TiO, particles is developed, silver losses fromthe treatment system canbe prevented and this metal doping may be economically beneficial. Another procedure that has received considerable attention for the enhancement of PC0 rates involves the supplemental addition of dissolved metals to theTiO, suspension. Addition of dissolved transition metals has been observed to increase the rate of TiO, photocatalytic the absence ofanymetal ions oxidation by a factor of between 1 and 5 overthatin [6,10,24,50,51,60,84-871. For example, Pelizzetti et al. [lo]found that addition of Ag+ and Fe3+ions to an illuminatedTiO,reactionsystemincreased the degradation rate of 2chlorodibenzo-p-dioxin. Sclafani et al. [87]and Wei et al. [86] found that added Fe3+ over a range of 0 to 5 X lO-,/M increased the oxidation rate of phenol.Similarly, the initial PC0 rates of phenol [24]and formic acid [50] were increased by the addition of dissolved Cu2+. The copper co-anion slightly affected the reaction rate. In most cases the enhancement was less at higher metal concentrations, and an optimum concentration of dissolved metals was noted. The observed rate increase resulting from the metal addition has been attributedto several mechanistic phenomena.First, enhancement dueto electron trapping by the metal at the semiconductor surface is possible [10,24,501:
Davis
392
M"+
+
e-
+
M(""')+
( 17)
where M"+ represents Ag+, Cu2+, or Fe3+. This reaction prevents electron-hole recombination, correspondingly producingan increased rate of formation of OH- radicals through reaction (2). Likewise, the detrimental effectsof high metal concentrationshave been attributed to the reverse of reaction (17), i.e., oxidation of reduced metals by photogenerated holes, proceeding in competition with reaction (2) [6,84],
M("-')+ + h+
M"+
( 18)
or by hydroxyl radicals. In order for reactions (17) and (18) to efficiently transpire, they must be preceded by adsorption of the metal onto the TiO, surface. A second mechanismto describe the enhancement assumesthat metal cations increase the oxidation rate by participation in Fenton reactions to catalytically produce OH- radicals in an alternative pathway to reactions (2a) and (2b) [20,24,87],
M("-')++ H202
+ H+* M"' + OH. + H20
( 19)
and/or to oxidize organic species or their radical intermediates [20]. The experimental results of Butler and Davis [88] indicate that dissolved Cu(II), Fe(III), and Mn(I1) have both concentration- and pH-dependenteffects on the rate of toluene and chlorobenzene oxidation in TiO, photocatalytic systems. The initial reaction rate under the optimum conditions [lo" M Cu(I1) at pH 31is double that without metals. There was negligible adsorption of Cu(I1) onto the TiO, surface at the low metal concentrations and low pH values at which the optimum rate was observed, indicating that metals enhance the removal rate through a homogeneous reaction pathwayrather than through a TiO, surface reaction.A mechanism involvinga reactive complex betweenthe dissolved metal, the organicsubstrate, and an , " was suggested. oxygen-containing species such as H,02 or 0 In the presence of dissolved Cu(II), the PC0 kinetics can be described by a modified Langmuir-Hinshelwood rate of the form (Figure 5 )
m
Rate = kk'KC
where kt represents the enhancement of the organic oxidationrate by a metal-catalyzed homogeneous reaction [88]. Using Equation (20), the values of k and K do not change appreciably from thosein the absence of dissolved Cu(I1). Inthe presence of dissolved Fe(III), an additional term is required to account for the absorption of UV light by the iron. At high metal ion concentrations, the detrimental effect on the reactionrate is likely due to absorbance of UV illumination at high concentrations, in competition with absorption by the photocatalyst [87,88].
E. MechanismsandIntermediates As a result of the current interest and investigative activity into various aspects of the PC0 process, some consistent mechanisms of oxidation are beginning to emerge, although many conflicting ideas are still ubiquitous. As noted, PC0 demonstrates a Langmuir-Hinshelwood relationship with respect to organic substrate concentration [Equation (ll)]. It has been assumed that this dependence was directly due to a Langmuir isotherm dependence of the substrate adsorption. For example, Matthews [56] investigated both the PC0 and dark adsorption of methylene blueonto Ti02. Both were described using a Langmuir isotherm expression, suggesting that adsorption of the substrate is a critical step in the oxidation. Several descriptions , and Cl- have of organic adsorption and competition from reactants and products such as 0
393
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30 6
20
"
'E .-
E
E IO
0 0.0
0.5
1.o Initial TolueneConcentration
1.5
2.0
(mM)
Figure 5 Enhancement of PC0 rates in the presence of dissolved copper (lo-' as a function of substrate concentration. (From Butler and Davis [88].)
M)and iron (
M)
been published. For example, Al-Ekabi and Serpone [21] studied the photocatalytic oxidation of chlorinated phenols. They attempted to establish the role of organic adsorption in the PC0 process, although possible competitive effects of oxygen are ignored. Kormann et al. [66] suggested that under properpH conditions, when the organic substrate is favorably adsorbed, the reactions take place at the surface of the catalyst; a second-order kinetic expression was presented based on adsorbedsubstrate and oxygen concentration, both of which were quantified using Langmuir adsorption expressions. Conversely, CIR-FTIR examination of a photocatalyst suspension revealed no chemical bonding of 3,4-dichlorobiphenyl to the TiOz [47]. Currently, the most widely accepted PC0 mechanism employs hydroxyl radicals as intermediates in the oxidationprocess.Adsorption of the substrate is notrequired to produce Langmuir-Hinshelwood kinetics if electron-hole and radical recombination reactionsare considered in the overall photocatalytic mechanism [7,89]. Evidence has consistently accrued implicating OH. radicalsthe in photocatalytic oxidation of organics. Studyingthe role of OH. radicals in the oxidation of salicylic acid, Matthews [20] found that the rate of degradation of the organic acid decreasedin proportion to the increase in OH- radical scavenger concentration. ESR spectroscopy has confirmed an abundance of OHradicals produced upon the photoexcitation of TiO, [15,16]. As discussed earlier, Cunningham and Srijaranai [l91 noted a photocatalytic kinetic isotope effect usingD20 instead of H20, but not when an isotopic exchange was made on the organic substrate. The OH. radicalsthat are produced at the catalyst surface may attack organic substrate that is also adsorbedon the catalyst surface or theymay desorbandthenreactwith the organic in the bulk solution [7,43]. Recently Lawlesset al. [90] conducted studiesto identify the role and importance of organic degradation by free versus Ti02 surface-bound OH. radicals.
394
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Externally produced hydroxyl radicals were quickly adsorbed by TiO, particles, producing a TiO, surface that had properties similar to those of the surface under PC0 conditions. This suggests that surface-boundOH-radicals are formed during PCO. Their results also imply that the organic oxidation occurs via these adsorbed OH. species. Minero et al. [91] noted that decafluorobiphenyl (DFBP) that is strongly adsorbed onto A1,03 is not photocatalytically oxidized when mixed with illuminated TiO,. However, degradation of adsorbed DFBP does occur when hydroxyl radicals are supplied by H202in the presence of UV light. This evidencealso suggests thatthe organic substrate must be in close contact with the Ti02 surface for degradation to occur and that the OH. radicalsthat are formed do not travel far into the bulk solution. Minero et al. [36] compared the diffusion rate of hydroxyl radicals in aqueous solution with experimentally determined organic oxidation rates. Based on the assumption that these rates are equal, it was shown thatthe OH. radicals could not diffusefar from the photocatalyst into the bulk solution beforereacting, even at very low concentrations of the organicsubstrate. Thus photocatalytic degradation processes occur either on or very near (within a few monolayers) the particle surface. Others have proposed that OH. radicals formed by oxidation of surface water or hydroxo groups eventually enter the solution to oxidize organics [16,92]. The formation of aqueous H202 [through reactions (7b) and (7d)l and hydroxylated organic intermediates during photocatalysis provided the basis for their conclusion. Peterson et al. [67] conducted a series of experiments using TiO, immobilized on a conducting carbonpaste to study some of the reactions suggested to occurin a photoelectrochemical slurry cell. If hydroxyl radicals were formed only at the TiO, surface, they would act as recombination centers and form H202, producing an anodic photoresponse in their system. However, if the OH- radicals were escaping into the bulk solution, a cathodic photoresponse would be expected. A cathodic response was confirmed in the cell, leading to the conclusion that OH.radicals do escape into the bulk solution. Turchi and Ollis [7] have presented possible reaction mechanisms basedon four types of interactions between photocatalytically formed OH. and the organic reactants: 1. The OH. radical species and the organic substrate are both adsorbed onthe catalyst surface. The OH. radical migrates to the bulk solution (free OH- species) and reacts with the organic substrate in solution. 3. The adsorbed OH- radical reacts with the bulk solution organic substrate. 4. The free OH.radical reacts with the surface-adsorbed organic substrate.
2.
Considering rates of recombination between holes and electrons, illumination intensity, and additional information on catalyst physical properties, a mechanistic rate expression was derived for the PC0 process. The resulting expression was in the form of Equation (11). The rate parameter k was found to be constant for all four examined mechanisms and predictedto be a function of the catalyst properties and reaction conditions;it was found to be essentially independent of the organic substrate involved in the photocatalytic oxidation process. Experimental results have confirmed this prediction of k being reactant-independent [7]. Turchiand Ollis proposed that, for the mechanisms inwhich the organic substrate is present in the bulk solution, the parameter K is a function of the second-order reaction rate constant for reaction of the OH- radical and the dissolved organicsubstrate. Conversely, if the organic substrate is adsorbed on the catalyst surface, K is also proportional to the equilibrium adsorption constant of the organic substrate. Mechanisms of degradation by photocatalysis have been presented for several organic substrates. Compounds such as hydroquinone, pyrocatechol, 1,2,4benzenetriol, pyragallol, and
minants PC0 of Organic
395
2-hydroxy-l ,Cbenzoquinone have been isolatedas intermediates for the photocatalytic oxidationofphenolusingTiO,[24].Suchhydroxylatedcompounds are suggestive of hydroxyl radical attack. Catechol and quinone were also detected in the PC0 of phenol by Augugliaro et al. [33]; hydroquinone and a phenol dimer were observed by Tseng and Huang [34]. A mechanism involving hydroxyl radicals from illuminated TiO, 2,4,5-trichlorophenoxyacetic for acid and 2,4,5-trichlorophenol was postulated by Barbeni et al. [41]. Chlorinated quinones, multihydroxylated chlorobenzenes, and other aldehyde and ketone chloroaromatics were isolated. Final products were CO2 and HCl. Methyl catechol, methyl resorcinol, and methyl hydroquinone, all hydroxylated cresols, were isolated during the PC0 of o-, m- and p-cresols by Minero [43]. A detailed examination of the PC0 of fluorinated phenols was presented fluoride appearance et al. [36].Stoichiometricproduction of CO, andF-wasfound; CO2 and nearly coincides with the disappearance of the was much more rapid than that of parent compounds, suggesting that defluorination occurs early in the degradation process. Intermediate products that were identified include mono- and dihydroxylated derivatives of the parent compounds. A plausible mechanism for phenol oxidationto catechol is that of reaction (21) [24]. Similar mechanisms canbe invoked for the formation of hydroquinone from phenol and for reactions of chlorinated and fluorinated phenols in which substituted catechol and hydroquinone compounds have been detected. Continued hydroxyl radical attack should follow mechanisms similar to those presented where OH. radicals are formed from sources other than photocatalysis, i.e., high-pH ozone decomposition or radiolysis.
A detailed mechanism for the degradationof triazine herbicides has been published[45]. The primary degradation pathway is through alkyl chain oxidation. The final product for the herbicide PC0 is cyanuric acid,
which is stable and not subject to further degradation [441. At this point, cyanuric acid is one of only a few organic compounds that are photocatalytically unreactive. Several publications have investigated the PC0 of other nitrogen-containing heterocyclic organic compounds. Low et al. [S81 monitored the concentrations of ammonium, nitrate, and carbon dioxide from several of these compounds. The appearance of ammonia was very rapid, indicating quick cleavage of the ring to release the nitrogen. Rates of NO,- production suggested that its formation was entirely due to the oxidation of previously synthesized ammonium. Ammonium was also detected in the PC0 of nitrophenols[57],indicating a reduction of the substrate nitro group. As with all organic oxidation processes, there is concern over the formation of intermediate compounds that may be more toxic than the parent material. Unknown intermediate compounds from the partial PC0 of pentachlorophenol and 2,4-dichlorophenol demonstrated an increase in toxicity to an activated sludge system over that of the parent compound [93].
396
Davis
Nevertheless, long-term PC0 of these compounds, as well as PC0 of methyl vinyl ketone, decreased toxicity to activated sludge. In summary, the mechanistic work withPC0 has demonstrated the formations of hydroxylated intermediates, indicative of hydroxyl radical oxidative pathways. Less understood are the formation and interaction of hydroxyl radicals and organic substrate, either at the photocatalyst surface or at some small distance into the solution. More studies are needed for complete comprehension of these mechanisms.
111.
LARGE-SCALEAPPLICATIONS
Pilot- and full-scale studies using solar illumination and reactor volumes of 100-300 gal have been investigatedby the National Renewable Energy Laboratory (NREL) Golden, in Colorado. PachecoandHolms [94] describe engineering-scale experiments for the detoxification of solvent-contaminated groundwaters.' h 0 reactors were used: a parabolic concentrating trough (7 ft by120 ft with a 51-fold concentration efficiency) and a falling film reactor. PC0 of a model compound, salicylic acid, was examined. The light intensity ranged from 1032 to 484 W/m2 for clear to cloudy days, respectively. Resultsfor the two reactors are presented in Figure 6, where the exposure time is given as the total time multiplied by the ratio of light-exposed water volumeto total water volume.Thus less than3 min exposure to illumination was required for effective removal of this compound. The temperature of the suspension rose from7 to 53°C as a result of the applied concentrated solar illumination, which may have enhanced the reaction rate. A field experiment using concentrating trough reactors is described by Mehos et al. [95] and Tbrchi and Mehos [3]. W Osets of parabolic reactors, 120 ft long, were used for treating deionized water spiked with TCEand, subsequently, contaminated groundwaterat Livermore, California. A TiO, loading of 0.8-0.9 g/L was used. A flow rate of 4 gpm allowed a 10-min exposure time and heated the water to 140°F. Results from the treatment of the contaminated
4 350 ?
U .-
3
15
0 .-
3
Falling Film:
220 w-w/I
10
d
5
0 0
1
2
3
Exposure Time (min)
Figure 6 Salicylic acid destruction using full-scale photocatalytic reactorwith solar illumination. (Reprinted with permission from Pacheco and Holm [94]. Copyright 1990 American Chemical Society.)
aminants PC0 of Organic
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groundwater showed a decrease in TCE from 107 ppb to 10 ppb. However, by reducing the water pH to 5.6 to decrease the effect of bicarbonate ions, the effluent TCE concentration was reduced to < O S ppb. Recent work at NREL [3,95] found that, on a basis normalizedby the photoreactor surface area, PC0 employing nonconcentrating reactorsis more efficient than concentrating (i.e., parabolic) reactors. Thisis attributed to (1) the high light reflection losses in large-factor concentrating reactors; (2) the importance of capturing diffuse UV radiation, which is not collected in concentrating reactors; and (3) the decrease in PC0 efficiency due to radical recombination processes at high light intensities, as discussed in Section 1I.C. "hrchi etal. [96] presenta detailed analysisfor the treatment of TCE-contaminated groundwater usinga full-scale photocatalytic reactor. Modelsof solar irradiation are used in conjunctionwith calculations of weather conditions to estimate available solar flux for the PC0 reaction. Incases where bicarbonateconcentrations are high, pre- and post-treatmentsteps consisting of, respectively, a lowering and neutralizingof pH must be employed in the system for efficient PCO. A strainer and filter areused to remove any particulate matter that may inhibit light transmission through the contaminated water. For a 100,OOO-gpd PC0 treatment system, land use for solar reactors ranges from 0.7 to 1.5 acres [96]. Solar sensors are used to adjust flow rates through the reactor system; higher flow rates are used during periods of high solar intensity. The photocatalytic reactor would operate at its maximum rate during midday, slow downas solar intensity decreases during the evening, and shut off completely overnight. Surge tankswould be used to store the water both before and after the phototreatment. This would allow continuous operation of groundwater pumping wells as well as the pre- and post-treatment processes. PC0 reactor schemes evaluated include parabolic reflector flow reactors as well as sequencing batch reactors and a continuous serpentine reactor, each approximately 1 m deep. A detailed cost analysis usingPC0 reactors for a 100,OOO-gpd contaminated groundwater cleanup facility in Livermore, California yields costs of $5.00-$6.00 per 1OOO gal treated. In comparison, activated carbon treatment wasestimated at $6.20/1OOO gal and UV/H,02 oxidation as $4.40/1OOO gal, thus indicating that PC0 can economically compete with these other common groundwater remediation processesin an area that has a favorable solar flux. A major concern in full-scale PC0 processes is the separation of the photocatalyst from the water stream after the completion of the reaction. Most commercially availableTiO, particles are on the order of 1-10 pm in diameter, making efficient removal difficult. An active research area incorporates the fixation of TiO, particles on various supports to allow easy separation. For example, the PC0 of several organic dyes using Ti0,-coated sand has been demonstrated [97]. However, rates are apparently limited by mass transfer of the substrate to the immobilized photocatalyst.
IV. OTHERENVIRONMENTALAPPLICATIONS The success of organic PC0 processes has led to investigations into the application of photocatalysis to manage other environmental concerns. The catalytic redox properties of illuminated Ti0, have recently been exploitedfor the treatment of inorganic contaminants. Pollema et al. [98] present results from a detailed examination of the PC0 of cyanide. LangmuirHinshelwood kinetics with respect to CN- concentration were found, and varying TiO, concentrations had negligible effects on the rate. Cyanate accumulated from the initial cyanide oxidation; eventually the cyanate was converted to nitrite and finally to nitrate. Tennakone et al. [99] also demonstrated that nitrite can be photocatalytically oxidized to nitrate. Cyanide removal from a copper cyanide solution viaPC0 was investigated using TiO, and ZnO [loo];
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TiO, was more efficient. The presence of copper enhanced the removal of CN- over that of a simple KCN solution. Bhakta et al. [loll found that ferricyanide can be efficiently degraded using PCO. Another application of photocatalysis involves the use of reduction mechanism to treat metals. For examples, hexavalent chromium can be reduced to the trivalent species. Also, dissolved metals can be completely reduced to the metallic form. Both Hg(I1) and CH,Hg(II) were reduced to metallic mercury from 100-ppm solutions using TiO, [102]. Gold(II1) is reduced to the metal by anatase titanium dioxide [103]: 3 e-
+ Au(II1)-+
Au(s)
(22)
In both cases, oxygen production from water oxidation may be the hole scavenger; however, adding methanol or ethanol increases the rate. Adsorption onto the TiO, and aqueous complexation of the metal ion control the reaction rate.
V. SUMMARY Photocatalytic oxidation has unlimited potential as a watedwastewater treatment process. PCO has its greatest potential for use in areas of high solar irradiation where sunlight can provide the TiO, excitation. A recent economic analysis suggests that PCO is competitive with common organic treatment systems in areas of high solar flux, with current costs estimated at $5-$6 per 1000 gal treated [96]. Also the oxidation typically proceeds to CO,, water, and inorganic compounds (Cl-, F-, PO,,-, NO,-, NH4+). The reaction is rapid and efficient. Therefore the accumulation of possible toxic intermediates is avoided. Oxidation provides chemical treatment of organic contaminants as opposed to phase separation processes, which transfer the pollutant to a different waste stream. Some of the present concerns of PCO include the separation of the TiO, from the effluent after the reaction and possible long oxidation times for large molecules or under nonoptimum conditions. As with all oxidation processes, a major wastewater application may involve pretreatment with PCO, followed by a traditional biological treatment. The partial photocatalytic oxidation would convert difficult-to-treat compounds to substances that are much more amenable to biodegradation. Photocatalytic oxidation is a promising waste treatment technology and has the potential to exert a significant impact in the field. Recently the use of PCO has been extended to the treatment of organic-contaminated soils [ 1041.
ACKNOWLEDGMENTS Support for some of this work came from the National Science Foundation, grant number BCS90 10434. Appreciation goes to Elizabeth Butler, Sonal Sanghavi, Phil Espitallier, and Muhammad Shariq Vohra. I also thank Oliver J. Hao and C. F? Huang for reviewing this chapter.
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PCO of Organic Contaminants 4. 5.
6.
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24. 25.
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Maruska, H. I?, and Ghosh, A. K., Photocatalytic decomposition of water at semiconductor electrodes, Solar Energy, 20, 443-458 (1978). Sakata, T., and Kawai, T., Photosynthesis and photocatalysis with semiconductor powders, in Energy Resources through Photochemistry and Catalysis ( M . Gratzel, ed.), Academic, New York, 332-358, 1983. Fujihira, M., Satoh, Y., and Osa, T., Heterogeneous photocatalytic reactions on semiconductor materials. 111. Effect of pH and Cu2+ ions on the photo-Fenton type reaction, Bull. Chem. SOC.Jpn., 55, 666-67 1 ( 1982). Turchi, C. S., and Ollis, D. F., Photocatalytic degradation of organic water contaminants: mechanisms involving hydroxyl radical attack, J. Catal. 122, 178-192 (1990). Barbeni, M., Parmauro, E., Pelizzetti, E., Borgarello, E., Serpone, N., and Jamieson, M. J., Photochemical degradation of chlorinated dioxins, biphenyls, phenols and benzene on semiconductor dispersion, Chemosphere, 15, 1913-1916 (1986). Herrmann, J.-M., Mozzanega, M.-N., and Pichat, I?, Oxidation of oxalic acid in aqueous suspensions of semiconductors illuminated with UV or visible light, J. Photochem., 22, 333-343 (1983). Pelizzetti, E., Borgarello, M., Borgarello, E., and Serpone, N., Photocatalytic degradation of polychlorinated dioxins and polychlorinated biphenyls in aqueous suspensions of semiconductors irradiated with simulated solar light, Chemosphere, 17, 499-510 (1988). Boehm, H. I?, Acidic and basic properties of hydroxylated metal oxide surface, Disc. Faraday SOC., 52, 264-275 (1971). Parks, G., The isoelectric points of solid oxides, solid hydroxides, and aqueous hydroxo complex systems, Chem. Rev., 65, 177-198 (1965). Ahmed, S. M., and Maksimov, D., Studies of the double layer on cassiterite and rutile, J. Colloid ZnterJace Sci., 29, 97-104 (1969). Davis, A. I?, Butler, E. C., Sanghavi, S. I?, and Shach, S. I?, Physicochernical Factors Affecting Organic Photocatalytic Oxidation Kinetics, NSF Prog. Rep. BCS-9010434, 1991. Jaeger, C. C., and Bard, A. J., Spin trapping and electron spin resonance detection of radical intermediates in the photodecomposition of water at TiO, particulate systems, J. Phys. Chem., 83, 3 146-3 152 (1979). Ceresa, E. M., Burlamacchi, L., and Visca, M., An ESR study on the photoreactivity of TiO, pigments, J. Muter. Sci., 18, 289-294 (1983). Izumi, I., Dunn, W. W., Wilbourn, K. O., Fan, F.-R., and Bard, A. J., Heterogeneous photocatalytic oxidation of hydrocarbons on platinized TiO, powders, J. Phys. Chem., 84, 3207-3210 (1980). Hashimoto, K., Kawai, T., and Sakata, T., Photocatalytic reactions of hydrocarbons and fossil fuels with water. Hydrogen production and oxidation, J. Phys. Chem., 88, 4083-4088 (1984). Cunningham, J., and Srijaranai, S., Isotope-effect evidence for hydroxyl radical involvement in alcohol photo-oxidation sensitized by TiO, in aqueous suspension, J. Photochem. Photobiol. A: Chem., 43, 329-335 (1988). Matthews, R. W., Hydroxylation reactions induced by near-ultraviolet photolysis of aqueous titanium dioxide suspensions, J. Chem. SOC., Faraday Trans. 1 , 80, 457-47 1 (1984). Al-Ekabi, H., and Serpone, N., Kinetic studies in heterogeneous photocatalysis. 1. Photocatalytic degradation of chlorinated phenols in aerated aqueous solutions over TiO, supported on a glass matrix, J. Phys. Chem., 92, 5726-5731 (1988). Pelizzetti, E., Carlin, V., Minero, C., and Gratzel, M., Enhancement of the rate of photocatalytic degradation on TiO, of 2-chlorophenol, 2,7-dichlorodibenzodioxin and atrazine by inorganic oxidizing species, New J. Chem., 15, 351-359 (1991). Izumi, I., Fan, F. F., and Bard, A. J., Heterogeneous photocatalytic decomposition of benzoic acid and adipic acid on platinized TiO, powders. The photo Kolbe decarboxylative route to the breakdown of the benzene ring and to the production of butane, J. Phys. Chem., 85, 218-223 (1981). Okamoto, K., Yamamoto, Y., Tanaka, H., Tanaka, M., and Itaya, A., Heterogeneous photocatalytic decomposition of phenol over TiO, powder, Bull. Chem. SOC. Jpn., 58, 2015-2022 (1985). Kormann, C., Bahnemann, D. W., and Hoffmann, M. R., Preparation and characterization of quantum size titanium dioxide, J. Phys. Chem., 92, 5196-5201 (1988).
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45. Pelizzetti, E., Minero, C., Carlin,V., Vincenti, M., Pramauro, E., and Dolci, M., Identification of photocatalytic degradation pathways of 2-Cl-S-triazine herbicides and detection of their decomposition intermediates, Chemosphere. 24, 891-910 (1992). 46. Harada, K., Hisanaga, T.. and Tanaka, K., Photocatalytic degradation of organophosphorous insecticides in aqueous semiconductor suspensions, Water Res., 24, 1415-1418 (1990). 47. 'hnesi. S., and Anderson, M. A., Photocatalysis of 3,4-DCB in TiO, aqueous suspensions: effects of temperature and light intensity, Chemosphere, 16. 1447-1456 (1987). 48. Matthews, R. W., Photooxidation of organic impuritiesin water using thin films of titanium dioxide, J. Phys. Chem., 91, 3328-3333 (1987). 49. Bideau, M., Claudel, B., and Otterbein, M.. Photocatalysis of formic acid oxidation by oxygen in an aqueous medium, J. Photochem., 14, 291-302 (1980). 50. Bideau, M., Claudel, B., Faure, L., and Rachimoellah, M., Photooxidation of formic acid by oxygen in the presence of titanium dioxide and dissolved copper ions: oxygen transfer and reaction kinetics, Chem Eng. Commun., 93, 167-179 (1990). 51. Bideau, M., Claudel, B., Faure, L., and Kazounan. H., The photooxidation of acetic acid by oxygen in the presence of titanium dioxide and dissolved copper ions, J. Photochem. Photobiol. A: Chem., 61, 269-280 (1991). 52. Tanaka, K., Hisanaga, T., and Harada, K., Efficient photocatalytic degradation of chloral hydrate in aqueous semiconductor suspensions, J. Photochem. Photobiol. A: Chem., 48, 155-159 (1989). 53. Hustert, K., and Zepp, R. G . , Photocatalytic degradation of selected azo dyes,Chemosphere, 24, 335-342 (1992). 54. Hidaka, H., Kubota, H., Gratzel, M.. Pelizzetti, E., and Serpone. N., Photodegradation of surfactants. 11. Degradation of sodium dodecylbenzene sulphonate catalyzed by titanium dioxide particles, J. Photochem., 35, 219-230 (1986). 55. Pelizzetti, E., Minero, C., Maurino, V., Sclafani, A., Hidaka, H., and Serpone, N., Photocatalytic degradation of nonylphenol ethoxylated surfactants, Environ. Sci. Technol., 23, 1380-1385 (1989). 56. Matthews, R. W., Photocatalytic oxidation and adsorption of methylene blue on thin films of nearultraviolet-illuminated TiO,, J. Chem. Soc., Faraday Trans. I , 85, 1291-1302 (1989). 57. Low, G . K.-C., McEvoy, S. R., and Matthews, R. W., Formation of nitrate and ammonium ions in titanium dioxide mediated photocatalytic degradation of organic compounds containing nitrogen atoms, Environ. Sci. Technol., 25, 460-467 (1991). 58. Low, G . K.-C., McEvoy, S. R., and Matthews, R. W., Formation of ammonium and nitrate ions from photocatalytic oxidation of ring nitrogenous compounds over titanium dioxide, Chemosphere, 19, 1611-1621(1989). 59. Matthews, R. W., Photo-oxidation of organic material in aqueous suspensions of titanium dioxide, Water Res., 20, 569-578 (1986). 60. Matthews, R. W., Kinetics of photocatalytic oxidation of organic solutes over titanium dioxide,J. Catal. 111, 264-272 (1988). 61. Matthews, R. W., Purification of water with near-U.V. illuminated suspensions of titanium dioxide, Water Res., 24, 653-660 (1990). 62. Sabin, F., lbrk, T., and Vogler, A., Photo-oxidation of organic compoundsin the presence of titanium dioxide: determination of the efficiency, J. Photochem. Photobiol. A: Chem., 63, 99-106 (1992). 63. Okamoto, K., Yamamoto, Y.,Tanaka, H., and Itaya, A., Kinetics of hetergeneous photocatalytic Bull.Chem. Soc. Jpn., 58, 2023-2028 decompositionofphenoloverAnataseTiO,powder, (1985). 64. Matthews, R. W., Photocatalytic oxidation of chlorobenzene in aqueous suspensions of titanium dioxide, J. Catal., 97, 565-568 (1986). of 65. Matthews,R. W,, Carbondioxideformationfromorganicsolutesinaqueoussuspensions ultraviolet-irradiated TiO,. Effect of solute concentration, Aust. J. Chem., 40, 667-675 (1987). 66. Kormann, C., Bahnemann, D. W., and Hoffmann, M. R., Photolysis of chloroform and other organic molecules in aqueous TiO, suspensions, Environ. Sci. Technol., 25, 494-500 (1991).
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l8 Biodegradation of Organic Pollutants in Soil
Paul D.Kuhlmeier Consulting Environmental Engineer Boise, Idaho
1.
OVERVIEW
Soil that is accidentally contaminatedby releases of hazardous chemicalsmay be classified as hazardous waste under state or federal environmental statutes.When the amounts of contaminated soil are sufficiently large, themost often used methods of disposal, incineration or land disposal, can become prohibitively expensive. In addition, soil pollutants act as a secondary source for groundwater contamination. The U.S. Environmental Protection Agency has recently implemented a strategy to address the complex problems associatedwith soil pollution through the application of innovative technologies [l]. One of the most frequently studied alternatives of late for the treatment of several classes of organics, notably petroleum hydrocarbons, is bioremediation. Long maligned as a technology that simply uses pollutants to attack other pollutants, the method was revived by Lee Thomas, former EPA administrator, and its use has spread to 130 contaminated sites throughout the country. Out of these 130 sites, 85 are targeted to treatsoils, 55 groundwater, and to a much lesser extent sediments, sludge, and surface water (Figure 1). Of the two classical methods for bioremediation, ex situ and in situ, fully 75% are using the ex situ method via some form of bioreactor, landfarming, static pile, or aerated lagoon. The vast majority of sites undergoing bioremediation use aerobicas opposed to anaerobic methods[l]. Although considered a new “innovative” technology by the EPA, bioremediationis anything but new. The mechanisms from which bioremediation research and application were deis routinely extracted rived are deeply rootedin biological wastewater treatment. Groundwater and treated in on-site bioreactors that behave much like activated sludge systems, with three unique differences: groundwater influentis more dilute than wastewater, groundwater influent can containa single or select few substrate types, and if bioremediation is properly performed, substrate levels continuously decline. A similar analogy can be drawn between soil treatment and fixed-film wastewater reactors. Kinetic equations for activated sludge and trickling filter reactors have been applied since at least the 1930s. R. L. Raymond was the first researcher to 405
406
Kuhlmeier
"
8070 60-
50-
403020 10 -
08
2 l
SOIL
GROUND WATER
SEDIMENTS
SWDGE
SURFACE WATER
Figure 1 'Qpes of media treated by bioremediator. (After U.S. EPA [l].)
patent the applicationof subsurface microorganisms to degradea variety of groundwater contaminants as early as the 1970s [2-41. The process of degrading hazardous organicsin shallow soils to be discussed here usually involves stimulating the indigenous subsurface microflora to enhancethe decomposition rate. Although engineered microorganisms with specialized metabolic capabilities have been developed andapplied, indigenous organism stimulation is being used at three outof every foursites [l]. Irrespective of the microorganism selection, the goal of biodegradation is to convert organic wastes into biomass and harmless by-products of microbial metabolism such as HzO, COz, CH4, and inorganic salts. To investigate the abilities of this remedial alternative, one must understand the waste characteristics and microbial ecology unique to each site, the limitations of the method, and the mechanisms by which its effectiveness can be optimized.
II. STRUCTUREANDBIODEGRADABILITY A. Structure Relationships
e
There is no clear and consistent way by which to predict biodegradationof a specific compound in soil absent specific site-related experiments. Thereare rules of thumb, however, that can be applied when assessing the potential for a compound to be significantly degraded. These general theories are rooted in chemical class or type, solubility, and chemical structure. The following rules of thumb can generally be applied within classes of chemicals: Water-soluble compoundsare usually degraded faster than less soluble compounds. Branching from the main chain decreases the biodegradation rate. Hydroxylandcarboxyl functional groups onbenzeneringsusuallyincreasebiodegradation rates. Halogen, nitro, and sulfonate functional groups on benzene rings usually decrease biodegradation rates.
407
Biodegradation of Organic Pollutants in Soil
Table 1 Degradability by OrganicClass Contaminant groups
Biodegradability
lfeatment range (%) ~
Halogenated volatiles Halogenated semivolatiles Nonhalogenated volatiles Nonhalogenated semivolatiles PCBs
Pesticides Dioxinslfurans Organic cyanides Organic corrosives
Effective Effective Potential effectiveness Effective Potential effectivenesdeffective
Effective Not effective
Potential effectiveness not effective
~~
50- 100 75-100 20-95 20-85
35-100 <30 10-70 <30
As the number of halogen atoms, e.g., chlorine, within the molecule increases the biodegradation rate decreases. Biodegradation rates are very slow for condensedor fused aromatic and cycloparaffinic molecules with four or more rings. The number and positions of substituents on an organic molecule will affect loss rates. Alcohols and acids are more susceptible to biological decomposition than corresponding alkanes and ketones. Substituents on aromatic rings at the paraposition appear to be more readily available to bacteria than those at the ortho or meta positions. Unsaturated aliphatics degrade more readily than similar saturated hydrocarbons. The availability of various enzymes has been suggested as the cause for this observation. n-Alkanes, n-alkylaromatics, and aromatic hydrocarbons in the C d , range are biodegradable, but in most environments volatilization proceeds faster than biodegradation. 1. A synopsis of compound groups and their relative biodegradability is shown on Table
B. General Pathways for Degradation All microorganisms need a source of energy for maintenance of cell viability and growth. The be classified according to the manner in which energy is obtained varies, and bacteria can source of their energy requirement. Phototrophs such as cyanobacteria use light directly in a photosynthetic process. Chemotrophs oxidize organicor inorganic compounds. The most common method of gaining energy is through oxidation reactions, which are normally coupled to the formation of ATP and other high-energy molecules. Many different kinds of substrates can be oxidized, but eventually the substratesare modified to metabolites that can enter one of only a few pathways for carbon dissimilation. These pathways can be divided intotwo categories: fermentation pathways in which organic compounds serve as both the electron donor and electron acceptor, and respiratory pathways in which oxygen or an inorganic compound or ion serves as the terminal electron acceptor. Aerobes utilize a respiratory pathway knownas the tricarboxylic acid cycle. The intermediates in the cycle are precursors to important cell macromolecules and may be used to fulfill other needs. Other metabolic reactions act to replace the intermediates in order to maintain functioning of the cycle. A wide varietyof microbial species can degrade organic pollutants; there is a rather limited series of sequences or pathways by which any particular class of organic may degrade [5,6]. Noteworthy study in this area was performed by Dagley [7],who critically reviewed several common soluble organic wastes, coming to the conclusion that direct insertion of oxygen is
408
Kuhlmeier
required very early in the catabolic process. Either of two types of enzymes is associated with this reaction, a monooxygenase or dioxygenase. The reaction is RH
+ 0 2 + [H-]
+ ROH
+ H20
(1)
where R denotes an organic carbon chain.This reaction is usually dependenton nicotinamide adenine dinucleotide (NADH).The second is a coenzyme-independent dioxygenase: RH2
+ 0 2 + R(OH)2
(2)
Some evidence suggests that these reactions, although associated with a large A H , may proceed without being coupled with adenine triphosphate (ATP)or NADH production [g]. Enzymes are proteins and are the most well-known catalysts for biochemical processes. They serve to increase the rate of a reaction, often causing a reaction to occur under physiological conditions that otherwise could occur only under extremes ofpH, temperature, or concentration. Degradation pathways tendto converge owinglow substrate specificity of enzymes early in the decomposition pathway. This observation holds generallytrue for a wide variety of soluble organics, with notable exceptions being classes of compounds subject to cometabolism such as chlorinated ethenes or very stable branched organics such as PCBs. Genetic research has also produced an explanation for pathway convergence. Studies [9] focusing on pathways for two specific plasmids found that degradative pathwaysfor aromatics are plasmid encoded and that the organisms appearto be similar because theyare coded by the same plasmids. From a practical viewpoint, knowledgeof the intricacies of enzyme coding andits effect on degradability is in its infancy, and applications in the field may not be commonly understood and utilized until well into the next century.
C. Degradability of Petroleum Hydrocarbons To date, over 70% of biodegradation studies and applications have focused on petroleumhydrocarbons. This situation is perhapsan artifact of the vast number of gas stations and petroleum refining and storage locations that have released substantial amountsof petroleum over decades of use, and the fact that compoundsof this class are among the most readily degraded. The biodegradability and biodegradation pathways of petroleum hydrocarbons have been exhaustively reviewed [lo-121. More specific reviews based on compound class have also been performed for alkanes [13,14], cycloalkanes [15], and aromatics [16-191. For unweathered petroleum, the primary catalysts for biological attack are oxygenases, whichrequire the presence of free oxygen. Monooxygenase attack results in alcohols from alkanesby typically attacking at theterminal position. The resulting alcohol is oxidized further to an aldehyde and finally to a fatty acid by beta-oxidation. Methyl branching decreases the rate of biodegradation by interfering with the beta-oxidation [20].This problem is usually overcomeby the introductionof a secondary substrate such as methanol, which appears to promote diterminal attack or other bypass mechanisms on the aldehydes. A general rule can be derived here: n-alkanes will degrade on the order of twice as rapidly as isoalkanes. Cycloalkanes are degraded more slowly than the isoakanes. They are transformed by a poorly understood oxidase system to cyclic alcohols, which are in turn dehydrogenated to ketones. A separate and unique monooxygenase then lactonizesthe ring, which is slowly opened by a lactone hydrolase. Itis salient to note that these two oxygenasesystems rarely occurin the same microorganisms,which supports the use of heterogeneousindigenouspopulations or heterogeneousinoculainjection (i.e., sewage sludge, manure) as opposed to engineered pure cultures.
Biodegradation of Organic Pollutants in Soil
409
Perhaps seeing the widest range of study are the aromatics. Procaryotic microorganisms convert aromatic hydrocarbons, through an initial dioxygenase attack, to trans-dihydrodials that are further oxidizedto dihydroxy products, catechol in thecase of benzene [21]. Catechol is the key intermediate in aromatic biodegradation.The ring is openedby either ortho or meta cleavage, resulting in muconic acidor 2-hydroxymuconic semialdehyde. Following the tricarboxylic acid cycle, both products are further reduced. Polycyclic hydrocarbons are degraded one ring at a time. It has been demonstrated that compounds containing four or more rings, such as pyrene and chrysene, cannot be used directly as substrates by microorganisms [22], however, they have been successfully degraded through cometabolic transformations [23,24]. Observations by the author in remediating railyard contamination suggest that 2 weeks of acclimatization time mustbe added for effecting long-term polycyclic aromatic degradationper ring from three to six, beyond which no relationship or degradability could be inferred.
111.
BIOKINETICS
Once the physiochemical site conditions have been assessed and the biofeasibility of degrading a target pollutanthas been determined through soil respirometry or other similar studies, a conceptual model for estimating field reaction rates can be constructed. Several theoretical models have been proposed for simulating bioremediationof contaminated soil and groundwater [26281. These models consider contaminant movement from the bulk liquid phase to adjacent particle surfaces. Baveye and Valocchi [29] considered several models critically in light of their applicability to soil remediation, ultimately proposingnew a model roughly based on the theory of attached growth on fine media. Kuhlmeier[30] found that empirically derived data applied to slight modifications of classical Monod relationships [31] provide the best-fit results across a wide spectrum of nonchlorinated organics.
A. Biomass-SubstrateRelationships The Monod relationships for describing the growth and decay of microorganisms and consumption of organic substrate takes the form
and
where X is the microbial concentration (mg/L), pmis the maximum specific utilization rate (g/g), S, is the amountof limiting substrate(mg/L), Uday), Y is the microbial yield coefficient kd is the microbial decayrate (Uday), and K, is the substrate half-saturation constant (mg/L). Growth takes the form of a hyperbolic functionof a limiting nutrient, usually nitrogenor phosphorus. The interreactivity of oxygen availability and biomass density often renders specific nutrient stress difficult to pinpoint, however. If bench-scale studies allow for sensitivity analysis, Equation (1) can be expandedto a multiple-compound analysis in the form of
where Ciis the concentration of the ith limiting nutrient.
410
Kuhlmeier
If Ci greatly exceeds Ki, the term Ci/ (Ki + Ci) goes to 1.0 and as a result has no effect on microbial growth. A practical limiting value for a multiple of Kiis approximately 3 for short-chain aliphatics using landfarming, to as much as 10 in situations where inhibitory compounds suchas chlorinated solventsare commingled inthe soil to be treated. Concentrations of trichloroethylene as low as 50 pg/L can inhibit activity in a 0.2 mgd slurry bioreactor. Conversely, when Ci < Ki, the growth rate will be roughly proportional to the concentration of nutrient i. A critical element to field monitoring of minimum nutrient loading (Cmin)can be derived from this analysis. Cmincan be written as
Recognize that Equation (4) represents a one-substrate scenario and that in practice this condition is rare. Thus, organic degradation is generally not limited to the implied boundary condition. Using the example of petroleum degradation, growth rates tend to decline substantially at pollutant concentrations below 100 mgkg soil. To offset these losses, secondary soluble substrate additions in the form of dilute primary alcohols can be added to the nutrientwater injection solution. This technique is particularly effective in landfarms. To develop a more accurate model for expressing microbial kinetics, decay must also be considered. The rate of biomass growth in the bulk liquid phase can then be described as
where COis the concentration of oxygen in the liquid phase. Here the first term on the righthand side of the equation represents growth, and the second term represents decay. In soil, the immobile microcolonies can havea significant effect on removalrates; in fact, in many cases it can exceed the effect of mobile colonies, particularly in fine-grained soils (permeability less than c d s ) whereinsitutreatment is beingconducted.Exchange rates are not controlled by transport within the bulk pore network because the microcolonies are found attached at the interface between the liquid and solid phases. To account for this phenomenon, local adsorption-desorption equilibrium shouldbe included in an overall massrate expression. The simplest form is based on a linear relationship that can be expressed as qb = Kdx
(8)
where q6 is the effective concentration of biomass in the solid phase (g/g) and Kd is the partition coefficient of the biomass (g/g). The rate of biomass growth in the form of microcolonies at the solid-liquid interface can be expressed as
For simplicity, it is assumed that the concentrations of substrate and oxygen extracted by the microcolonies are equal to those in the bulk liquid phase, which, in reality, is rarely true. The rate of substrate degradation in the bulk phase can be estimated by
Similarly, the rate of substrate degradation by the microcolonies at the immobile interface is
Biodegradation Pollutants of Organic
in Soil
411
To calculate the oxygen consumption required to keep population growthat a minimum, the yield coefficient for oxygen(Yo) is substituted for Y in Equation (10).
B. Inhibitory Wastes Discussion of kinetics to this point has assumed thatone or more soluble substrates are availof inhibitory or toxic chemicals,howable to the resident microbial population. The existence ever, more adequately reflects reality. Inhibitory waste is any waste that, when introduced, can effect a reduced respirationrate or a reduced growth rate.A compound's inhibitory actionmay be membrane mediated or cytoplasmically mediated. Membrane mediation has been linked to lipophilicity, particularly at low concentrations [32]. Research in this area has advanced rapidly within the wastewater treatment industry. Extensions of this research to hazardous waste bioremediation is now being investigated. Existing theory in the wastewater regime suggests that operational difficulties associated with inhibitory wastes arise todue process dynamics.A similar paradigm canbe demonstrated in soil remediation; landfarming systemsare more effective than in situ systems. The primary reason is the ability to control operating dynamics to a greater extent with the ex situ method. Substrate-inhibited growthmay be described by a variation of Monod theoryas expressed by the Haldane equation [33]
where p is the actual specific growth rate and KI is the inhibition constant. Quantification of 1.1and KI is typically accomplished through respirometric studies. It is interesting to notethatevenwhentreated as anempiricalequationdescribingsubstrateinhibitedgrowth,theHaldaneequationsuggeststhatperformanceenhancementscan be achieved through stringent process control.
W.
PRACTICALLIMITATIONS
Given the complexity of the bioremediation process, it is often as critical to know the limitations of the site-specific scenarioas it is toknow how to apply the technology. Bioremediation is not unlike a golf swing; to play well, good mechanicsare everything. Yet, recognizing flaws and adjusting them seems to be an insurmountable task. Environmental factors may stunt or preclude biodegradation even in the presence of engineered organisms. Biodegradationat residual concentrations of the target substrate frequently is a function of the presenceof toxicants [34]. During acclimatization and log growth phase, insufficient oxygen and nutrient are the below 200 mgkg dominant constraintsto growth rate. As a target substrate is reduced to levels soil, substrate concentrationand substrate inaccessibility may reduce the microbial population to maintenance levels.If the applicationis being attempted in situ, the transport of oxygen and nutrients to the contaminated zone and the extractability of the affected pore water present yet another set of constraints. The challenge of effectively introducing nutrients and oxygen to the subsurface can be daunting. Bioslime buildup adjacent to injection wells or drains is a common problem. Subsurface heterogeneitiesand sorption of nutrients lead to preferential flow pathways for oxygen
412
Kuhlmeier
and nutrient transport, leaving pocketsof the contaminated zone virtually untouched. Field experience acquired from responses to tanker and railroadcar spills indicates that a practical limit to the size of an in situ treatment system ranges from3 acres in fine-grained soils (sandy silt) to perhaps 20 acres in porous sands and gravels with native organic contents greater than 3%. Bioremediation in situ for contaminated soil over5 acres or contaminated groundwater greater than 20 acres in size is not recommended as a sole treatment but may be included in a combination system with such technologies as soil venting, in situ air sparging, or hydraulic containment with above-grade treatment. Recall that in Section I1 it was noted that sufficient oxygen was requiredto commence the catabolism process. Alexander [34] found that operating a system in an oxygen-deficient environment inhibited the metabolism of simple carbohydrates. Aromaticdegradation may cease at oxygen concentrations less than 50% of optimum [35]. Lee and Ward [36] confirmed these findings by observing a large decay rate for naphthalene, dibenzofuran, and phenanthrene in oxygenated groundwater in comparison to oxygen-depleted water. Aerobic degradation is not the only pathway for aromatic degradation, however. Mineralization of xylenes in river alluvium under denitrifyingconditions has been reported by Kuhn et al. [37]. Benzene, toluene, and alkylbenzenes detectedin landfill leachate were mineralized by methanotropic bacteria in a study by Wilson and Rees [38]. Anaerobic decomposition has not been the subject of significant research efforts to date and currently is not being proposed at any of the 80 soil remediation sites under the purview of the EPA [l]. Macronutrient levels should be maintained in excess of optimum whenever practical. In order of importance to growth rate sensitivity, the major macronutrients are nitrogen, phosphorus, potassium, sodium, and calcium. The effect of declining substrate concentration on biodegradationhas been documented in soil and groundwater[39,40]. Kuhlmeier [39] postulated that a threshold may exist for degrading petroleum hydrocarbonsin landfarms at 30-70 mg/kg. Functional thresholdsfor numerous other contaminants such as acetone, bromodichloromethane, and phenolshave been raised by others [41-431. Tbming once again to experience acquired fromthe wastewater industry, another limiting factor can be the amount of inoculum introduced. The theoryis that the low cell densitythat is introduced intosoil may not replicate enough to promote effective bioremediation. The effects of predators (protozoa) on the growth of a strain of Pseudomonas bacteria in a sandy loam injected with 50 mg/L methanol was tested. At a cell density of 520 cells/mL, the bacteria declined when protozoa were not inhibited. However, when the protozoa were inhibitedby cycloheximide and nystatin, the inoculated bacterium multiplied after a 30-h lag period to over 1.7 X lo4 cells/mL (Figure 2). Other factors such as soil permeability,native organic content, sorption, pH, and temperature also play a role in limiting biological degradation rates. Soils exhibiting a permeability greater than 1 x cmls and a total organiccontent of 3% are moreamenable to biorestoration. Free product in the subsurface should be removed prior to commencement of biofeasibility studies. Sorption has the tendency to concentrate nutrients and canalso render the target substrate unavailable to the microbial population. Solubilization techniques such as the addition of surfactants can aid in minimizing this problem. Soil pH plays a key role in sorption phenomena of ionizable compounds and affects enzyme activity. Several strains of microorganisms known to degrade aromatic hydrocarbons are inhibited at pH values less than 6 [44],and activity approaches zero at soil pH below 5.5. In some cases it is preferable to adjust soil pH where possible to slightly basic. Hambrick et al. 1451 observed that mineralization of naphthalene in sediment was faster at pH 8 than at pH 5.
Biodegradation of Organic Pollutants in Soil
m 0
d.
l0,Ooo lOm0 7500 7 w
413
j -
m40
80
80
Figure 2 Effects of predator organismsoninoculumgrowth mg / L methanol).
100
(soil amendedwith 50 mglL and 100
A comparison was not madeat the less acidic conditions ofpH 6 or 6.5, so true optimization could not be determined. Temperature has a profound effect on microbial activity, particularly in landfarm applications. Several researchers have reported direct relationships between degradation and temperature[39,46,47].Fungitend to be moretemperature-tolerantthanbacteria.Ageneral benchmark for temperature thresholds is 55°F for inhibition and 42°F for cell die-off.
V. APPLICATIONS IN PRACTICE A. Oxygen Hydrogen peroxide is often usedto increase the amountof oxygen availableto microbes when applying in situ techniques. Hydrogen peroxide may use be beneficial in landfarming situations as well. Although it is an excellent alternative oxygen source, hydrogen peroxide is toxic at concentrated levels. Lee et al. [48] suggested that 200 ppm can be toxic to microorganisms. Their work, unfortunately, was limited to published literature. Introduction of stepwise increases of hydrogen peroxide is possible up to at least 2 %.If added immediately after wetting a landfarm where hydrophobic compoundsare of interest, concentrations of 4% have proved successful in enhancing biodegradation rates. The combination of surfactants and hydrogen peroxide injection in the waterstream is also showing promise in degrading hydrocarbons adsorbed to the clay fraction in a fine-grained soil matrix. Hydrogen peroxide, if introduced into the subsurface, should be added after nutrient addition to aid in controlling biofouling of injection points.
B. Nutrients Certain nutrients are required by all cells. Carbon is the most important, and those cells that are referred to as heterotrophs. Autotrophs canfix carbon from obtain it from organic substrates
414
Kuhlmeier
carbon dioxide.A few specialized groups can use other substrates; methylotrophs,for example, can oxidize methane at aerobic-anaerobic interfaces. Other essential nutrients include phosphorus, usually derived from phosphates, and nitrogen, usually obtained from nitrate or ammonia. These three elementsare the most common nutrientsthat limit growth. Other necessary growth factors include sulfur, magnesium, potassium, calcium, and other metallic elements. While some bacteria synthesize all their required vitamins and growth factors, other bacteria must obtain some from the environment. Water is also a specific requirement in cellular metabolism. The bacterial cell is about 80% water, and water is both the solvent and a specific cofactor in many biochemical reactions. Nitrogen and phosphorus are the nutrients that typically limit microbial growth and metabolism. Although other macroand micronutrients are essential, soil and groundwater contain sufficient levels of these to sustain unlimited microbial growth. Therefore, the quantitative analysis of nutrients other than nitrogen and phosphorus is not typically necessary. Industrial wastewater is a possible exception where other nutrientsmay become limiting. Ammonia is the preferred nitrogen sourcefor bacteria. Soils, groundwater, or process water are tested for residual levelsof ammoniacal nitrogen. Sincethe total amount of organic carbon is usually determinedat some point during the site assessment, the proper nitrogen addition rate can be calculated based on the size of the site and the estimated total mass of organic material that needs to be biodegraded. The residual level of orthophosphate in a sample is also determined. Based on the total organic carbon content of the sample, estimates can be made regarding the amount of phosphorus needed to bioremediate a site or waste stream. Based on the concentration of calcium and magnesium in aqueous systems, phosphate in excess of that needed to remediate the site may be required to prevent precipitate formation. The availability of nutrients and the ability to move nutrients througha matrix such as soil are important considerations when evaluating a site for in situ treatment. The aqueous phase interaction of phosphorus, calcium,and magnesium should be considered. Soil itself poses another potential problem. Ions tend to interact with soil particles byionic interactions. The strength and magnitude of suchinteractions can influence themovement of nutrients through an in situ treatmentarea. In order to determine thecorrect nutrient additionrate, the ability of the so soil to adsorb ammonia and phosphorus should be known. Adsorption immobilizes nutrients that their movement through the soil is seriously impeded. This effect can usually be counteracted by adjusting the nutrient addition rate to account for adsorption. Although often not considered inorganic nutrients, trace levels of many metals, e.g., calcium, magnesium, manganese, copper, cobalt, zinc, iron, and molybdenum, are essential for microbial growth and metabolism. However, high concentrations of the heavy metals, especially mercury, lead, and cadmium, can be toxic to microorganisms. Interestingly, areas that have been contaminated with heavy metals often have microbial populations that have become resistant to heavy metals. Therefore,the presence of heavy metals at a site or in a waste stream does not preclude bioremediation. The presence of high concentrations of calcium and magnesium in variably saturated soil may seriously interfere with the physical process of transporting nutrients and oxygen through a subsurface system. Both calcium and magnesiumwill combine with phosphate to form an insoluble precipitate. Precipitation of calcium and magnesium phosphates can clog geological formations during in situ soil and groundwater treatments, thus preventing proper circulation of nutrients and oxygen throughthe treatment area. In order to avoid practically irreversible clogging, the calcium and magnesium concentrations must be determined when the in situ flushing technique is under consideration. The amountof phosphorus added for in situ treatment can usuallybe adjusted to prevent precipitate formation even in situations where high concentrationsof calcium or magnesium are present.
Soil Biodegradation in Pollutants of Organic
415
Divalent cation chelators appear at first to be useful amendments for binding calcium and magnesium. This makes the ions unavailable for interaction with phosphorus. Citric acid and EDTA (ethylenediaminetetracetic acid) are both effective chelators for calcium and magnesium ions. However, both compounds are organic, and both are easily biodegraded. The usual result of adding citric acid or EDTA is that these compounds serve as alternative and preferred substrates for the bacteria. Insteadof biodegrading the compounds of interest, the bacteria degrade the chelator. Therefore, the benefits and effects of these chelators shouldbe thoroughly defined prior to application.
C.SurfactantEnhancement A persistent and often frustrating constraint on biodegradation effectiveness is the adsorption of target substrates onto fine-grained soils and within the immobile liquid phase[49,50]. Because of the persistence and sorption of hydrophobic chemicals, it is probable that sorption acts as a limiting mechanism for biodegradation. Solubilization of chemicals through the application of surfactants has received increased attention [5 l]. The effect of various anionic and nonionic surfactants on the solubilization of petroleum hydrocarbons including anthracene, been demonstrated in soil-water suspensions [52]. Theory on the phenanthrene, and pyrene has subject to date implies that if a surfactant solubilizesthe sorbed compound, the molecule will then become readily available in the bulk fluid phase for microbial attack [53]. A similar notion also promote immobile-phase extraction advanced by Kuhlmeier [54] found that surfactants can of trace chlorinatedaliphatics, rendering them more amenable for vapor-phase extraction. Beneficial uses of these productshave also been reported in soil-washing and pump-and-treat technologies [55,56]. What makesa surfactant molecule effectiveis its amphophilic nature, as it has two distinct structure moieties, one polar andthe other nonpolar. The polar moietyof the molecule has an affinity for water and many other polar substances. Conversely, the nonpolar moietyis hydrophobic. A surfactant molecule can dissolvein water as a monomer, adsorb to a solid surface, or be incorporated with other surfactant molecules as part of a micelle. Critical micelle concentration (CMC)is the surfactant concentrationat which monomers begin to assemble incolloidal aggregates[57]. At surfactant concentrationsgreater than the CMC, additional surfactant is incorporated intothe bulk solution through micelleformation. CMC values for a number of surfactant solutions have been compiled by Mukerjee and Mysels [58]. Solubilization of petroleum hydrocarbons and other hydrophobic substances starts at the CMC and followsa linear function of surfactant concentration overa wide range of concentrations [57]. Evaluation of a surfactant for its ability to solubilize a given chemicalmay be accomplished using the molar solubilization ratio (MSR). The MSR is defined as the number of moles of organic compound solubilized per mole of surfactant added to solution [59]. The increase in solubilizate concentration per unit increase in micellar surfactant concentrationis equivalent to the MSR. It is represented by the slope of a curve plotting solubilizate concentration against surfactant concentration. The MSR for aromatic hydrocarbons can be calculated as
where S, is the total apparent solubilityof a chemical in micellar solution (moYL); Sapp,the apparent solubility of a chemical at the CMC (mol/L); and CS"*, the surfactant concentration at which S, is evaluated (mol/L). Liu et al. [52] presented an alternative solution thatstill relies on batch column test design theory. This theory attempts to partition the organic compound between micelles and monomeric solution with a mole fraction micelle phase/aqueous phase partition coefficient. The
Kuhlmeier
416
Table 2 AromaticSolubilityApplyingSurfactants Aromatic solubility (mollL) Compound
No.Without rings surfactant CMC
Naphthalene Phenanthrene 8 Pyrene
(X
3
3X I X
4
X
2
10-4 10-4
IO”
l0-q
2.5-5.0 0.1-0.4 0.001-0.005
MSR
log K m
0.2-0.5 0.1-0.2 0.01-0.08
4.2-5.8 5.4-6.5 6.0-7.0
micelle phase/aqueous phasepartition coefficient, K,, is the ratio of the mole fraction of the compound in the micellar phase, X,, to the mole fraction of the compound in the aqueous phase, X,. Using batch tests, K, can be calculated as
and
X,
= MSW(1
+ MSR)
(15)
The mole fraction of aromatic compound in the aqueous phasein dilute solutions(<0.5%) can be approximated by
x, = S,, v,
(16)
where V, is the molar volume of water (Umol). K , can then be expressed as
Experimental observations have been reported for only a sparingly few compounds. Results will also be specific to surfactant type. Nonionic surfactantsare preferred over anionicor cationic surfactants becauseof differences in surfactant toxicity, biodegradability, and change. Polyoxyethylene (POE) compounds comprise over 75% of nonionic surfactants produced in America. From this majorclass, two octylphenol-POE andnonylphenol-FQE types are recomC,,, (CMC), MSR, andK , are given in Table 2 for two- to mended. Representative ranges for four-ring aromatic hydrocarbons.
VI. CASE HISTORIES Results fromfour independently performed petroleum landfarm sites are compared in an effort to quantify the significance of several limiting growth parameters previously discussed andto determine a threshold concentration below which biological activity ceases. At lower concentrations, it was expected that the microbial mix would change from predominantly eutrophic species to oligotrophic species.The oligotroph is able to grow at low substrate levels, whereas the eutrophic species multiplies at high concentrations. Consider that a single organism must use up a certain amount of organic macronutrientsand substrate to provide enough energy to maintain the cell. Therefore, whenthe nutrient concentration is quite high, diffusion will provide molecules to the cell surface at a rate that is rapid enough to meet the energy needs for maintenance and growth. As substrate declines, growth ceases, and, finally, at a critical substrate concentration, even maintenance cannot be satisfied.
Biodegradation Pollutants of Organic
417
in Soil
A. Selection of Test Systems In this reviewwe consider exclusively land treatment systems. Land treatment, or landfarming, is a method by which contaminated soilsor sludges are excavated and placed in a lined treatment cell. This technique has several distinct advantages over in situ treatment. As previously mentioned, it allows for better control of the system by controlling the depth of soil and the surface area exposed. In turn,one is able to more accurately control soil temperature, nutrient concentrations, moisture content, and oxygen availability. Placing a liner with a leachate collection system beneath the soil column prevents any additional contamination potential and provides for recovery and recyclingof water and nutrient. An added benefit of landfarming is found in the ease of sampling and hence cleanup verification. Zones of slow degradation can be readily identified and targeted for enhancement. This method allows for system optimization in a manner in which other techniques do not. Drawbacks to landfarming are primarily those of economics. Excavation and construction of the treatment cellmay be costly, but the cost of off-site excavation and disposal exceeds that of landfarming by a factor of 2 or more. Available space to operate is the other significant constraint. Soil depthsare maintained between6 and 10 in. to accommodate complete tillingby a disk or chisel plow. Therefore, the total surface area required mayexceed 2 acres for a medium-size tank farm remediation project. The four landfarms chosen for this review represent varying climatic conditions and initial chemical concentrations. In addition, discrete parameter sensitivity analyses were performed at one or more of the treatment sitesso that data could be presented both independently and collectively. Each of the four locations (Ohio, California, Michigan, and Texas) contained petroleum hydrocarbon-contaminated soils.
B. Discussion
A synopsis of significant sitedata is presented in Table3. The maximum initial concentration encountered was approximately 5100 mg/kg, and the lowest initial concentration was 3940 mgkg. The minimum concentration achievedat any site was74 mg/kg of total petroleumhydrocarbons (TPH). Values less than 100 mg/kg represented less than 10% of the total number 50 of observations, thereby suggesting that a threshold for effective biodegradation is between and 125 mg/kg, accounting for measurement accuracy and precision. Biodegradation flux or loss rates were calculated for the four sites collectively and partitioned by concentration incrementsas shown in Table4. Loss rate declines generally fit a first3000 mg/kg TPHdegrading orderdecayfunction,withacclimatedsoilscontainingover substrate at a mean rate of 95 mg (kg day). Dispersivity of data at concentrations above 500 mg/kg was low, typically less than10%of the mean.As substrate declines and competition
-
Table 3 LandfarmingPetroleum-Contaminated SoilsSummary Statistics for Four Sites ~~
Initial conc. TPH (mgkg) No. Avg Location Max. 1 2 3 4
TPH
Ohio California Michigan Texas
5 100 4650 2150 3940
Final conc. TPH (mgkg)
.
Min. Avg. Max.
3800 2611 1875 18 17
194 96 147 133
= total petroleum hydrocarbons. 'Activity ceased at point where cleanup criteria were exceeded.
Duration Plot (weeks)
158 87 112 106
107 8 74 93 89
14-23' 10-42 8 16-38 7 18
size Depth (acres) (in.) 2 1.4 1.5 0.5
12
Kuhlrneier
418
Table 4 Petroleum Loss Rates ~
~
~~
Loss rate (mg/kg)
Concentration*C,, (mgkg)
Max.
Avg
~4OOo 1000-4OOO 500-1000 250-500 100-250 <100
157 71 57 23 8.0
95 53 38
1.3
.
9.5 5.4 0.8
Min. 74
40 26 4.0 1.9 0
'Lowest observed value, 74 mg/kg.
sites disappear, pockets of viable and inviable activity form, and dispersivity increases to as much as 80% of the mean value at concentrations less than 200 mglkg. The effects of the several individual parameters contributing to this phenomenon are discussed in turn below. Specific observations were made with respect to threshold values and inhibition points for four parameters-oxygen, biological population enhancement,soil moisture, and temperature. As noted previously, these factors do not constitute all of the potential constraints on biodegradation of petroleum in soils; however, they representcritical elements that can often be controlled in a applications environment. 1. OxygenLimitations The primary purpose of tilling in a landfarm is to optimize the contact of soil microbes with atmospheric oxygen. In soils, aeration depends on the total amount of air-filled pore space. Elimination of air-filled porosity through excess wateringor compaction reduces oxygen transfer between the microbial colonies and the substrate. Large amountsof biodegradable organics in the top layer of soilwill deplete oxygen reservesin the soil and slow down oxygen diffusion to the deeper layers. Oxygen levels were measured at selected points in each of the four tests at l-week intervals. More frequent oxygen monitoring was performed in the first 2-3 weeks for purposes of evaluating tilling and moisture requirements.At three s i t e s d h i o , California, and Michigantilling biweekly was optimal. At the Texas site, where temperatures were significantly higher, oxygen transferwas more efficient and frequent; weeklytilling produced markedly higher degradation rates. This observation is somewhat contrary to that expected. The maximum oxygen content measured in soil water was 6.5 mg/L, which is approximately 80% of saturation. The minimum amount of oxygen measured,0.9 mg/L, was at 6 in. below the surface in a soil that had not been tilled for 4 weeks. Microbial growth was foundto be inhibited at concentrations below 2.5 mg/L, and total cell loss was detected at oxygen concentrations below 1.4 mg/L, which were found in plots with very low soil moisture.
2. MicrobialDensityEnhancement One way to enhance biodegradation of organic compounds is to inoculate the soil or groundwater (as appropriate) with microorganisms known to metabolize the chemical readily. This concept has been roundly debated for its performance by various researchers and commercial vendors of inoculum [m]. Both successes and failures have been noted in the literature. It has been my experience that commercial bacteria cannot be relied upon as a sole source as their populations tend to die off quickly, thus requiring numerous reapplications. In the natural en-
Biodegradation of Organic Pollutants in Soil
419
'h.. .... ."..
1oC
Control (Tilled) .".""".
",; TI
\
\
% TPH Remaining 50
\
\
""*
".". -......
40302010 -
0
1
2
3
4
5
6
7
8
Q
1 0 1 1 1 2 1 3 1 4 1 5
Time (Weeks)
Figure 3 Effects of a proprietary inoculum to degrade TPH compared to a horse manure as a bacteria additive over time.
vironment the hybrid species face competition, predation, or parasitism. Any of these interactions could account for the inability of introduced microbes to survive [61]. The potential of hydrocarbon mixtures to be eliminated at a higher rate from soil when hydrocarbon-degrading bacteria are added to the soil is also a much disputed matter. A general criticism of the seeding approach is that the use of an allochthonous microbial population may not be necessary or effective in most cases. Also, most isolates implicated in petroleum hydrocarbon biodegradationare gramnegative, non-spore-forming bacteria. These are difficultto store in large quantities in a manner that preserves their viability. Bioaugmentation was usedat the landfarm in Ohio.The ability of a proprietary inoculum to degrade TPH was compared to horse manureas a bacteria additive and the simple addition of sufficient macronutrients (fertilizer). Resultsof this evaluation are given in Figure 3. Inoculum was applied on a biweekly basis as population density levels were found difficultto maintain. Manure was found to lastat least 4 weeks before a reapplication was needed. While the commercial additiveneeded substantially less acclimatization time in the soil-4 days as compared to 3 weeks for the manure-its effectiveness diminished rapidly after the first month of by the testing. After 15 weeks of activity the horse manure subplot performed the best, followed commercial additive subplot and then the subplot with only macronutrient additions. Although the performance difference between horse manure and commercial organisms is statistically insignificant, the test indicated that continuous use of commercial bacteria was not beneficial. It was determined, however, that initial injection of commercial bacteria decreased acclimatization time, and ongoing injection of 5-15% of such bacteria with fertilization probably is valuable and economically sound. 3. MoistureEffects
Moisture is essential to the growth of soil microorganisms. Water provides the mechanism for the exchange of reactants and food absorption through the cell walls. Excessive amounts of moisture can be disruptive. Commensurately, when soil moisture drops to near the point of
Kuhlmeier
420 o lo
-
9080-
70 -
96 TPH
m 85+% Saturation
60-
B4 5 % Saturation
Remaining 50 -
l1
40-
M
lo 0’
At 4 Weeks
At 8 Weeks
At 12 Weeks
Figure 4 Effects of moisture on petroleum degradation, plot 3.
specific retention, usually corresponding to 10% by volume or less, metabolic activity ceases. Bossert and Bartha [62] suggested that moisture contents be maintained in the range of 2080%. Biodegradation studies performedat the site in Michigan sought to define a more accurate or narrow range where moisture content is optimum. Moisture data from three different subplots are shown on Figure 4. All other parameters such as temperature, soil type, and nutrient loadings were kept as constant as possible; only moisture content was deliberately varied. Values of 15%, 50%, and 85% of saturation were chosen for the test. These levels were checked once a day for a total test period of 12 weeks. Losses from the 15% moisture subplot were minor over the entire duration of the test. TPH loss that was observed is believed to be due almost exclusively to volatilization, not biodegradation. It was thus confirmed that moisture contents below 20% are threshold limiting to biodegradation. Biodegradation was observed in the 85% subplot but was only about one-third the loss observed in the 50% moisture subplot. This evidence suggests that indeed inhibition 80% above is present but is not a threshold condition.It is further offered, basedon the performance of this test, that optimum moisture levels are probably in the range of 4040% of saturation. Additional study in this area is warranted. TemperatureEffects Temperature affects the respiration rates of microorganisms, and, through controlling respiration, temperature affects metabolism. It also affects the solubility of the host hydrocarbons. Temperature is directly related to the amount of evaporation and volatilization from the soil, which in turn impacts microbial populations. Mesophilic organisms in general perform bestat about 95°F. Huddleston and Cresswell [63] reported petroleum degradationat temperatures as found that inhibition became easilydetectable at low as 30T.Kuhlmeier and Sunderland [M] temperatures below 55T. Temperature was closely monitored at each of the four test locations. Mean TPHloss rates were calculated across a wide range of temperatures. Results of this analysis are presented in Figure 5 . Beginning and ending TPH concentrations for a given period in which the average
4.
.
Biodegradation of Organic Pollutantsin Soil
421
90
70
%Loss 50TPH
403020-
lo] 0
40
50
60 70 Tmperahrre (4)
80
Figure 5 Effects of temperature on microbial activity, plots 1-4.
daily temperature was in a designated temperature bracket (i.e.,OS, OS, etc.) were summed, and the mean loss was derived from data in all tests. There was virtually no biodegradation associated with observations at temperatures under 50°F. Losses shown in Figure 5 represent volatilization losses and measurement inconsistencies. There was a significant difference between degradation rates below 80°F from those above this reference value. Interestingly, biologically attributable losses above 95°F did not prove to be significantly greater than thoseat 85". Although less than 2%of the total observations were at temperatures above 100"F, there was a noticeable difference from the balance of the data points. When ambient temperatures exceeded 100°F the mean percent TPH loss was observed to be 92%. The slight dropoff from the80s-90s bracket is postulatedto be attributable to high initial volatilization losses but subsequent cell mortality due to heat stress, rapid moisture loss, or nutrient constraints or possibly some combination of these factors.
VI. CONCLUSIONS A reviewof data collected at four independent petroleum landfarming operations has suggested that biodegradation of contaminated soils may be kinetically limited at concentrations below 50 mg/kg. Loss rates decline dramatically when remaining substrate dips below 150 mg/kg. Oxygen levels become criticalat 2 mg/L asmeasured in soil moisture. Ideally, soil moisture should be maintained in the 4 0 4 0 % of saturation range. Ambient temperature hits a threshold limit for degradation at approximately 50°F and reduces microbes to cell maintenance levels once temperatures drop to 45". The addition of commercial microorganisms was not found to be highly beneficial. Natural additives such as sewage sludge or horse manure do enhance microbial activity by introducing large quantities (and diverse types) of microbes. Macronutrients, although not detailed specificallyinthediscussion, are importantforcellactivity.Acarbordnitrogen ratio of 160: 1 as suggested by the American Petroleum Institute appears to be overly conservative.
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Carbodnitrogen ratiosof 6 0 : 1 to 160: 1 were used at the various sites, with little evidence that the lower concentration of fertilizer was inhibiting growth.
1. U.S. Environmental Protection Agency (EPA), Bioremediation in the Field, EPA/540/N-92/002, Office of Solid Waste, 1992. 2. Raymond, R. L., U.S.Patent 3,846,290 (1974). 3. Raymond, R. L., Jamison, V. W., and Hudson, J. O., Beneficial Stimulation of Bacterial Activity in Ground Waters Containing Petroleum Products, API Publ. 4427, Am. Petroleum Inst., Washington,D.C.,1975. 4. Raymond, R. L., et al.. U.S. Patent 4,588,506 (1986). 5 . Sinton, G. L., Fan, L. T.,Erickson, L. E., and Lee, S. M., Biodegradation of 2,4-D and related xenobiotic compounds, Enzyme Microb. Technol., 8 365 (1986). 6. Stanier, R. Y., Adelberg, E. A., and Ingraham, J., The Microbial World, 4th ed., Prentice-Hall, Toronto,1976. 7. Dagley, S., Microbial degradation of stable chemical structures: general featurs of metabolic pathProc. Conf. San Francisco, ways, in Degradation of Synthetic Organic Molecules in the Biosphere, p. 1. Calif., 1971, Nat. Acad. Sci., Washington, D.C., 1972, 8. Clarke, P. H., and Omston, L. N., Metabolic pathways and regulations. I and 11, in Genetics and Biochemistry of Pseudomonas (P. H. Clarke and M. H. Richmond, eds.), Wiley, New York, 1975, p.191. in Microbial Degradationof Organic 9. Britten, L., Microbial degradation of aliphatic hydrocarbons, Compounds (D. T. Gibson, ed.), Marcel Dekker, New York, 1984. 10. Chapman, I? J., Degradation mechanisms,in Proc. Workshop, Microbial Degradationof Pollutants in Marine Environments (A. W., Bourquin, and P. H. Pritchard, eds.), EPA-66019-79-012, Environmental Research Laboratory, Gulf Breeze, FI., 1979, pp. 28-66. 11. Higgins, I. J., and Gilbert, P. D., The bidegradation of hydrocarbons, in The Oil Industry and MicrobialEcosystems (K.W.A. ChaterandH.J.Somerville,eds.),Heyden,London,1978, pp. 80-1 17. 12. Jensen, B., Arvin, E., and Gunderson, A. T., The degradation of aromatic hyudrocarbons with ba teria from oil contaminated aquifers,Proc. NWWAIAPI Con$ on Petroleurn Hydrocarbons and Organic Chemicals in Ground Water-Prevention, Detection, and Restoration, Nat. Water Well Assoc. Worthington, Ohio, 1986, p. 421. 13. Ratledge, C., Degradation of aliphatic hydrocarbons, in Developments in Biodegradation of Hy1-46. drocarbons, Vol. l (J. R. Watkinson, ed.), Applied Science Pub., London, 1978, pp. 14. Yaniga, P. M.,andMulry, J., Accelerated aquifer restoration: in situ applied techniques for enProc. NWWAIAPI hanced free .product recoverylabsorbed hydrocarbon reduction via bioreclamation, DetecCon$ on Petroleum Hydrocarbons and Organic Chemicals in Ground Water-Prevention, tion, and Restoration, Nat. Water Well Assoc., Worthington, Ohio, 1985, p. 421. 15. Perry, J. J., Microbial metabolism of cyclic hydrocarbons and related compounds, Crit. Rev. Microbiol., 5 387-412 (1977). 16. Dagley, S., Newperspectives in aromatic catabolism, in Microbial Degradation of Xenobiotics
(T. Leisinger,R.Hutter,A.M.Cook,andJ.Nuesch,eds.),Academic,NewYork,1981,pp.
181-187. Degradationof in 17. Fewson, C. A., Biodegradation of aromatics with industrial relevance, Microbial Xenobiotics (T. Leisinger,R.Hutter,A. M. Cook, andJ.Neusch,eds.),Academic,NewYork, 1981,pp.141-152. 18. Gibson, D. T., Biodegradation of aromatic petroleum hydrocarbons, in Fate and Efects of Petroleum Hydrocarbons in Marine Ecosystems and Organisms (D. Wolfe, ed.), Pergamon, New York, 1977, pp. 36-46. 19. Wilson, J. T., and Ward, C. H.,Opportunities for,bioreclamation of aquifers contaminated with petroleum hydrocarbons, J. Ind. Microbiol., 27, 109 (1987).
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20. Rrnik, M. l?,Microbial oxidation of methyl branched alkanes,Crit. Rev. Microbiol., 5, 413-422 (1977). 21. Rittman, B. E., and Kobayashi, H., Microbial separation for trace-organic removal, in Detoxification of Hazardous Waste (J. H. Exner, ed.), Ann Arbor Science, Ann Arbor, Mich., 1982, p. 323. Microbiology in 22. Cerniglia, C. E., Microbial transformations of aromatic hydrocarbons.Petroleum (R. M. Atlas, d . ) , Macmillan, New York, 1984, pp. 99-152. 23. Bauer, J. E., and Capone, D. C., Degradation and mineralization of the polycyclic aromatic hydrocarbons anthracene and naphthalenein intertidal marine sediments,Appl. Environ. Microbiol., SO, 81-90 (1985). 24. Herbes, S . E., and Schwall,L. R., Microbial transformations of polycyclic aromatic hydrocarbons, Appl. Environ. Microbiol., 35, 306-316 (1978). 25. Bouwer, E. J., and McCarty, l? L., Modeling of trace organics biotransformations in the subsurface, Ground Water, 22, 433 (1984). 26. Kosson, D. S., Agnihotri, G . C., and Ahlert, R. C., Modeling and simulation of a soil-based microbial treatment process, ./. Hazardous Mater., 14, 191 (1987). 27. Windowson, M. A., Molz, J. F., and Benefield, L. D., A numerical transport model for oxygenWater and nitrate-based respiration linked to substrate and nutrient availability in porous media, Rsources Res., 24, 1553 (1988). 28. Molz, F. J., Windowson, M. A., and Benefield, L. D., Simulation of microbial growth dynamics WaterResources Res., 22, 1207 coupledwithnutrientandoxygentransportinporousmedia, (1986). 29. Baveye, l?,and Valocchi, A., An evaluation of mathematical models of the transport of biologically reacting solutes in saturated soils and aquifers, Water Resources Res., 25, 1413 (1987). 30. Kuhlmeier, l? D., Biodegradation of acetone in soil,Proc. Purdue Univ. Industrial WasteConf., 43, 71-79 (1988). 31. Monod, J., Recherches sur la croissance des cultures bacteriennes,Hermann, Paris, 1942. 32. Beltrame, l?, Beltrame, l? L., Carniti, l?,Guardione, D., and Lanzetta, C., Inhibiting action of chlorophenols on biodegradationof phenols and its correlation with structural propertiesof inhibitors, Biotechnol. Bioeng., 31, 821 (1988). 33. Haldane, J. B. S., Enzymes, Longmans, London, 1930. 34. Alexander, M., Environmental and microbiological problems arising from recalcitrant molecules, Microb. Ecol., 2, 17 (1975). 35. Kuhlmeier, l? D., and Sunderland, G.L.,Biotransformation of petroleum hydrocarbons in deep unsaturated sediments, Proc. Petroleum Hydrocarbons and Organic Chemicals in -Groundwater, NWWA,Nov.15,1986,Houston,Tex.pp.86-98. 36. Lee, M. D., and Ward, C. H., Microbial ecology of a hazardous waste disposal site: enhancement of biodegradation, Proc. 2nd Int. Conf. on Groundwater Res. Qualify Res. (N. N. Durhamand A. E. Redelfs, eds.), Oklahoma State Univ. Printing Services, Stillwater, Okla., 1985, p. 25. 37. Kuhn, E. F!, Colberg, l? J., Schnoor, J. L., Warner, 0 . . Zehnder.A. J. B., and Schwanenbach, R. F!, Microbial transformation of substituted benzenes during infiltration of river water to ground water: laboratory column studies, Environ. Sci. Technol., 19, 961 (1985). 38. Wilson, B. H., and Rees, J. F., Biotransformation of gasoline hydmarbons in methanogenic aquifermaterial, Proc. M A J A P I Conf. onPetroleum HydrocarbonsandOrganicChemicalsin GroundWater-Prevention, Detection, and Restoration, Nat. Water Well Assoc., Worthington, Ohio,1986.p.128. Proc. 39. Kuhlmeier, l? D., Threhold limited kinetics of aromatic hydrocarbons in shallow soil systems, Environmental Remediation '91, U.S. DOE, Richland, Wash., 1991, pp. 651-657. 40. Alexander, M., Biodegradation of organic chemicals, Environ. Sci. Technol., 18, 106 (1985). 41. Wilson,J.T.,McNabb, J. F., Balkwill, D. L., and Ghiorse, W. C., Enumeration and characterization of bacteria indigenous to a shallow water-table aquifer, Ground Water, 21, 134 (1983). Proc. 42. Jhaveri, V., and Mazzacca, A. J., Bio-reclamation of ground and groundwater. A case history, 4th Natl. Conf. on Management of Uncontrolled Hazardous Waste Sites, Washington, D.C., October 1983, p. 242.
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Suflita, J. M., and Miller, G. D., Microbial metabolism of chlorophenolic compounds in ground water aquifers, Environ. Toxicol. Chem., 4, 751 (1985). 44. Alexander, M., Soil Microbiology, Wiley, New York, 1977, p. 467. 45. Hambrick, G. A., 111, DeLaune, R. D., and Patrick, W. H., Jr., Effect of estuarine sediment pH and oxidation-reduction potential on microbial hydrocarbon degradation, Appl. Environ. Micro-
43.
bid., 40, 365 (1980).
46. Bartholomew, G.' W., and Pfaender, F. K., Influence of spatial and temporal variation on organic Appl. Environ. Microbiol., 45, 103 pollutantbiodegradationratesinanestuarine environment, (1983). 47. Atlas, R. M., Effects of temperature and crude oil composition on petroleum biodegradation,Appl. Microbiol., 30, 396 (1975). 48. Lee, M. D., Wilson, J. T., and Ward, C. H.,In situ restoration techniques for aquifers contaminated with hazardous wastes, J. Hazardous Mater., 14, 71 (1987). 49. Means, J., Wood, S., Hassett, J., and Banwart, W., Sorption of polynuclear aromatic hydrocarbons by sediments and soils. Environ. Sei. Tech., 14 (12), 1525-1529 (1980). 50. Hassett, J. J., andBanwart,W. L., in ReacrionsandMovement of Organic Chemicals in Soils (B. L. Sawhney and K. Brown, eds.), Am. Soc. Agronomy, Madison, Wisc., 1989, pp. 31-44. L., Surfactant Screening of diesel contaminated 51. Peters, R. W., Montemagno, C. D., and Shem, soil, Haz. Waste and Haz. Matls., 9 (2), 113 (1992). 52. Liu, Z., Laha, S., and Luthy, R., Surfactant solubilization of polycyclic aromatic hydrocarbons in soillwater suspensions, Water Sci. Technol., 475, 23 (1991). 53. Rosenberg, E., CRC Crit. Rev. Biotechnol., 3 , 109-132 (1986). 54. Kuhlmeier, F? D., Vapor phase trichloroethylene recovery from hydrogen peroxide stimulated soils, Proc. Nut. R&D Con$onthe Control of Hazardous Materials, HMCRI, San Francisco, Calif., 1992, pp. 232-236. 55. Nash, J. H., and Traver, R. I?,Field evaluation of in-situ washings of contaminated soils with waterlsurfactants, Proc. 12th Annual Res. Symp., EPAl600/9-86/022, Cincinnati, Ohio, 1985. 56. McDermott, J. B., 'Itvo strategies forPCB soil remediation: biodegradation and surfactant extraction, AIChE Meeting, New Orleans, La, Spring 1988. 57. Rosen, M. J., Surfactants and Interfacial Phenomena,2nd ed., Wdey, New York, 1989. 58. Mukerjee, I?, and Mysels, K. J., CriticalMicelle Concentrations of Aqueous Surfactant Systems, NSRDS-NBS 36, U.S. Depart. of Commerce, Washington, D.C., 1971. 59. Attwood, D., and Florence, A. T.,Surfacant Systems: Their Chemistry, Pharmacy and Biology, Chapman and Hall, New York, 1983.
m. Goldstein, R. M., Mallory, L. M., and Alexander, M., Reasons for possible failure of innoculation to enhance biodegradation, Appl. Environ. Microbiol., 50, 977 (1985). 61. Walter, M. V., Barbour, J., McDowell, M., and Seidler, R. J., A method to evaluate survival of genetically engineered bacteria in soil extracts, Curr. Microbiol., 15, 193-197 (1987). of hydrocarbons during oily sludge disposal in 62. Bossert, I., Kachel, W. M.. and Bartha, R., Fate soil, Appl. Environ. Microbiol.. 47, 763-767 (1984). 63. Huddleston, R. L., and Cresswell, L. W., Environmental and nutritional constraints of microbial hydrocarbon utilization in the soil,Proc. 1975 Eng. FoundationCon,: The Role of Microorganisms in the Recovery of Oil, NSFIRANN, Washington, D.C., 1976, pp. 71-72. 64.
Kuhlmeier, I? D.,and Sunderland, G. A., Biodegradation of petroleum hydrocarbons in deepunsaturated sediments, Proc. Petroleum Fate in Unsaturated Environments, IAHS Symp. Ser., 1984.
19 Siallon: The Microencapsulation of Hydrocarbons Within a Silica Cell
Tom McDowell Siallon Corporation Laguna Niguel, California
1.
INTRODUCTION
The magnitude of the effort required for remediation of hazardous waste sites throughout the country is staggering. The number of underground storage tanks that are registered with the U.S. Environmental Protection Agency (EPA) alone is estimatedat between 1.8 million and 2 million, of which25% are assumed to be leaking [l]. The costs associated with cleanupof our hazardous waste problems are even more appalling. The remediation of underground storage tanks (USTs) alone is estimated to cost $67 billion, assuming that current regulatory policies do not get any more stringent. The cost of cleaning up all hazardous waste sites could reach $1.7 trillion by the year2020 [2]. Numbersof this magnitude will make the remediation of hazardous wastesone of the most pressing and vital matters for the next several decades. Cost/benefit analysis will become the driving force in the environmental community; no longer will webe able to pay exorbitant prices for soil remediation, nor will we have to. The responsible party (RP) isnow more educated, more cost-conscious, more aware of the available options, and much more receptive to new and innovative methods. Such willbe the focus of the next generation, a move away from the “dig and haul” mentalityto treatment and eliminationof the problem. One of the low-cost soil and sludge remediation methods that has been widely used for many years is the solidification or’stabilization of heavy metals. These techniquesare generally based on the addition of a cementitious materialto the contaminated soil and the formation of a solid monolith. Due to the multitude of reactions that take place both within the cementitious material and between theheavy metals and the cement,this process can work well, with a few drawbacks. The most significant downsideof well-solidified heavy metal-contaminated soil is the large volume increase requiredto provide an adequate degree of nonleachability. This volume increase can be as little as 15% or as much as 150% of the original contaminated soil volume [3,4]. The advantages ofsolidificatiodstabilization,however, most often far outweigh these disadvantages. High volume throughput (100 yd3/hr is common) and low cost makethis an attractive process for heavy metal solidification. 425
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It is these distinct advantages that have led many people to consider and actuallyuse this type of process for the remediation of hydrocarboncontaminated soils. These attempts have met with varying degrees of success, from outright failure [5] to some measure of solidification that is usually contaminant-dependent [6]. These inconsistent results haveled EPA to state that “immobilization of organics is uncertain” [7,8] and that the most stringent tests for total hydrocarbons should be used in assessing the results of any organic solidification/ stabilization process. The main reason for poor success is the fact that all of the conventional solidification, . stabilization, or fixation processes use some type of cementitiousor pozzolanic material as the mainstay of the process. Materials such as portland cement, fly ash, kiln dust, and lime (the four major cementitiousor pozzolanic materials used) all undergo hydration reactions as part of the curing process. It is well known within the concrete industrythat the setting of concrete can be retarded by adding a small amount of diesel fuel prior to pouring the concrete. The hydrophobic diesel fuel absorbsonto the crystal faces of the pozzolanics and effectively blocks the infusion of water and subsequent hydration and thus the curing of the cement. This dichotomy of trying to solidify something that inherently prevents solidification has given solidification of organics its “uncertain” reputation and more recently has ledto vendors looking at additives such as carbon and clays to improve the retention of hydrocarbons within a solidified matrix [9]. The Siallon processfor the microencapsulation of organics and hydrocarbonsis a unique approach that does not use any cementitious or pozzolanic materials. It is a simple two-step chemical reaction that results in the hydrocarbon being encapsulated within a micrometer-sized particle of silica [10,11]. As there are no large amounts of pozzolanics used, there is no large volume increase with treated soil and no attenuated cure time, and results are always determined using total concentration analysis.
II. THE SIALLON PROCESS A. Overview The chemistry of the patented [l21 Siallon process is simple in that it is a two-stage reaction involving two water-based products. The first step uses a water-based emulsifier that first desorbs the hydrocarbon from the soil and then emulsifies it within a water phase. The second stage is the addition of the Siallon reactivesilicate solution tothe mixture of soil and emulsified hydrocarbon. The silicate undergoes an acid-base reaction with the emulsifier micelle and is neutralized, forming a micrometer-sized particle of silica around the droplet of hydrocarbon. As the essential reaction, the neutralization of the silica, is a simple acid-base reaction, it is virtually instantaneous.The speed of this reaction leads to very high volume throughput during soil remediation; depending upon site-specific conditions, throughput ranges from 25 to 200 tonshr. The only limitation on the process is that the hydrocarbon or organic contaminant has to be emulsifiable. This means thata wide variety of contaminants such as gasoline, dieselfuel, jet fuel, motor oil, crude oil, greases and lube oils, coal tars, PCBs, chlorinated solvents, and many others can all be successfully treated withina soil or sludge matrix. Similarly,the physical nature of the host material plays little part in the final remediation results. All types of soils, from sand to clay, sludges, and tars, respond well to treatment, with the only modifications being to the actual processing or mixing equipment utilized. Pug mills, ribbon blenders, Bomags, and modified soil composting equipment have been usedfor the application and mixing of the Siallon reagents.
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Applications of this process range from remediation of gasoline-contaminated soil from a UST location to in situ remediation of PCB-contaminated soil at a transformer manufacturing plant. Encapsulation efficiencies for benzene, ethylbenzene, toluene, and xylenes (BETX) 99.97% and total petroleum hydrocarbons (TPH) in the gasoline-contaminated soil ranged from to 100%. Encapsulation efficiencies for the in situ PCB remediation ranged from 99.9991% to 99.9999%.
B. Siallon Chemistry As mentioned previously, the Siallon process of microencapsulation is a simple two-step procedure. The first step, the emulsification of the hydrocarbon, is the heart of the process. Surfactant chemistry shows that when a hydrophobic materialis emulsified it forms a micelle[l31 within the aqueous phase. This micelle is composed of a droplet of hydrocarbon surrounded by surfactant or emulsifier. The reason for this is the molecular structure of the surfactant. Molecularly all surfactants, emulsifiers, wetting agents,and detergents have a common structure, a hydrophobicor oil-loving “tail” that is oil-soluble and a terminal hydrophilic or water-loving portion or “head” that is water-soluble. The hydrophobic tail will attach itself to the hydrocarbon particle, with the hydrophilic head conferring water solubility on the entire molecule. As emulsion proceeds, moreand more surfactant molecules attachto the hydrocarbon particle in the same orientation. Such extremely close packing of the surfactant molecules occurs that the concentration of surfactant around the micellesin a given solution is much higher than the concentration within the bulkof the solution. The final result is a spherical form of hydrocarbon surrounded by close-packedsurfactantmoleculeswithinthewaterphase.Theactual amount of surfactant requiredto first produce desorption from soil and subsequently emulsify the hydrocarbon is extremely small in relation to the mass of soil. Consider that the clothes washing done in the average home is with an aqueous solution containing less than O.OOO1 mol% surfactant for removal of body oils and grease[141. This very high rateof activity allows the Siallon process to remediate contaminated soil with relatively small amounts of additives and no volume increase between the untreated soil and the treated soil. The Siallon emulsifier differs from conventional surfactants or other emulsifiers in thatit contains an acidic moiety as part of the hydrophilic end or head of the molecule. The acidic moiety will orient itself along with the hydrophilic end toward the aqueous phase, in essence producing a hydrocarbon microdroplet surrounded by and attached to the oil-loving portion, surrounded in turnby the water-loving portion and thenby the acidic moiety. In essence there are a series of spheres or shells around the hydrocarbon, ending with a shell that has acidic reactive sites on its surface. The average diameter of these shellsor micelles is less than2 pm. The small size of the micelles results in a very stable emulsion with or no littletendency to split or revert. All individual hydrocarbon compounds are susceptible to emulsification, and each compound responds to a very specific ratio between the size of the hydrophobic and hydrophilic portions of the emulsifier molecule. A product such as crude oil is composed of hundreds of individual compounds, each emulsifying best at a specific hydrophobic/hydrophilic ratio. To overcome this problem and produce the smallest possible micelle and most stableposemulsion sible for the various hydrocarbon products requires a range of emulsifiers, each covering a range of hydrophobichydrophilic ratios. Each of the Siallon emulsifiers is a water-based, nontoxic product having a pH in the 4.2-5.5 range. The second step of the Siallon process is the addition of a water-based silicate that is slightly alkaline with pH a of 9.5. The silicate undergoes an acid-base reaction with the acidic sites on the emulsifier, which neutralizes the silicate. This reaction forms silica, water, and a
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trace amount of salt. As the acidic sites are formed in a sphere around the emulsified micelle ofhydrocarbon, the silica in turn forms a cell around the micelle. The final product is a micrometer-sized particle of silica containing bothemulsifier and the drop of hydrocarbon. Extensive characterization of the morphology and chemical nature of the silica cell was undertaken. Chemical analysis of the bulk material by both X-ray spectrometry and energydispersive X-ray analysis (EDXA)showed that the material formed was more than 98% SiO,, with a trace of salts. These results were consistent with the neutralizationof silicates in which the reaction products are normally silica, water, and simple salts.
C. Siallon Cell Morphology The evaluation of the physical characteristics of the Siallon silica cellwas aimed at answering two questions: How durable is the cell? and How is the hydrocarbon held within the interior? In most cases at least two different analytical or instrumental techniques were employed for each step in order to confirm the answers by separate methods. Samples of motor oil that had been encapsulated withthe Siallon process were usedas the basis sample formost of the evaluations of morphology. The first item consideredwas the exactcrystalline structure of the silica cell. The generally accepted structures of silica are crystalline forms of quartz, cristobalite, and tridymite, of which only the quartz is thermodynamicallystable [U].Amorphous silica canbe formed by the very slow cooling of molten silica, resulting in a random array of three-dimensional units or chains of SiO,. When the Siallon silica cells are examined with a microscope under planepolarized light, the cells appear to be highly colored. Under plane-polarized light, crystalline materials transmit the light without distortion and appear as bright or white areas. It was obvious that the Siallon cells were amorphous silica.This was reaffirmed by the use of selected area electron diffraction, where allof the samples show an amorphous haze, with the exception of the trace of a salt formed as a by-product of the reaction that was confumed to be sylvite, a common form of NaCl. The next step was evaluation of the surface characteristics of single silica cells. Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed. The SEM micrographs show the surface to be fairly regularin appearance, nonporous, and with a solid monolithic character. The TEM micrographs show that the silica cell is essentially a solid particle rather than a hollow sphere. This left the question of how the hydrocarbon was held within the silica cell. It was obvious fromthe SEM work that there was encapsulation of the hydrocarbon rather than adsorption, as the surface was nonporous, and also obvious that it was not a zeolite typeof open-lattice structure thatwould lead toabsorption. Thenext step then became the cutting of thin sections of single silica cells using a diamond knife in an ultramicrotome. The sections were first evaluated with SEM at 15,OOOX, 30,OOOX, and 60,OOOX magnification and shown to be a random mix of solid matter and void space. They were then analyzed by EDXA, which showed that the solid matter on the interior of the cell was essentially pure SiO,. To further evaluate the configuration of the interior, the SEM micrographsof the thin sections were computer-enhanced; the results show a three-dimensional, mazelike, random honeycomb structure very much like a closed-cell urethane foam. The results of the chemical and morphologicalinvestigations can be summarized as follows: The Siallon reaction productis essentially pure silica-this provides a material with negligible water solubility, extreme hardness and durability, and resistance to acids and alkali. The silica is a stable amorphous form and as such is not prone to fracture or cleavage along crystal planes.
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The surface is solid and nonporous and will thus preventor retard leaching out of the encapsulated hydrocarbon. The interior of the micrometer-sized silica cell is a mazelike configuration of almost infinite tortuosity within which the hydrocarbon is firmly embedded.
111.
LEACHABILITYSTUDIES
1
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. . , , . * , * A . : ’
wsw P*’
A. Waste Leaching Tests-An Overview In general a leaching test involves contacting the waste material with a liquid, the leachant, to determine how much Contaminant is released or dissolved into the liquid. There are two basic types of leach tests, the extraction tests suchas EPA’s Toxicity Characteristics Leaching Procedure (TCLP), inwhich the leachate is not renewed during the test, and dynamic testsassuch the column leach tests used in this study, in which the leachant is periodically replaced or renewed. Extraction tests can be further categorized according to whether the sample is agitated or nonagitated and whether only one extraction is run or multiple or sequential extractionsare used. Dynamic tests can be further categorized according to how the waste and leachant are brought into contact. Serial tests involve the use of a granular or crushed sample that is leached by periodic leachant renewal. Flow-through and flow-around tests are dynamic tests in which the sample are typically used for a vais a porousor solid configuration, respectively. Flow-through tests riety of soil samples. Procedures such ‘as column or lysimeter studies are typical of flowthrough tests. With any leach test there areat least five variables that canbe manipulated to ensure that the test represents either a real-life situation, a worst-case scenario, or equilibrium conditions. Sample preparation can involve simple grinding, aging, curing, compaction, or surface washbe varied from simple distilled waterto ing of monolithic samples. Leachant composition can actual site leachate or to a variety of acidic or alkaline synthetic solutions, each designed to provide specific contaminant-release information. The method of contact of the waste and the leach solution canbe agitated or nonagitated, open to the atmosphereor a closed system.The amount of leachate used relative to the amount of sample defines the liquidkolid ratio for the procedure. Thisratio should be low enough to allow analysis of the contaminant and high enough that solubility of the contaminant is not a limiting factor. The fifth variable is the amount of time a sampleand the leach solution are in contact. For extraction tests using ground samples, contact time is usually24-48 hr, however, for the same test for monolithic samples, the time requiredto reach equilibrium conditions couldbe weeks or months. In dynamic tests such as column extraction tests, contact time is simply a function of the flow rate of the leach solution and the amountof leach solution passed through the column. The data generated from a properly designed leaching protocol can be used to predict the mechanism by whichaparticularcontaminantisreleasedfromawaste(e.g.,diffusioncontrolled), to predict relationships between time and contaminant release, or to simulate reallife conditions and the resultant flux or release of contaminant per unit area.
B.Column
LeachTest-Description
When hydrocarbon-contaminated soil that has beenmated by the Siallon process is analyzed, the appropriate total concentration analysis is used. These methods will generally involve solvent extraction followed by gas chromatographic analysis, which gives a result that is indicative of the sample as it stands or at a fixed point in time. To assess the effects of both time and groundwater intrusion on Siallon-treated soils an extensive leachability study was undertaken.
McDoweLL
430
The studyinvolvedmorethan 80 samplesandmorethan 600 separate analyses ofboth gasoline- and diesel-contaminated sandy soil, silty sand soil, and clayey soil. The soil samples used forthis study were prepared in the laboratory by spiking the appropriate soil type with approximately 10,OOO mgkg of gasoline or diesel fuel. The spiking was done in a side-rolling, airtight mixer with theappropriate amount of gasoline or diesel sprayed into a mixing chamber, containing approximately70 lbof soil, over a 6-hr period, and then the soil was homogenized by continued mixing for a further 6 hr. The samples were treated by adding the Siallon reagents while mixingin a laboratory-scale pug mill capable of processing 12 lb of soil per minute. Each treatment required35-40 Ib of contaminated soil, and the treatments were each done in triplicate for each contaminant (gasoline anddiesel) and for each of the three types of soils [16,17]. The samples were then analyzed for TPH using EPA Method’ 8015 as modified by the California Departmentof Health Services (DHS). Thisparticular variation of Method 8015 used methylenechloride and sonication to extract the hydrocarbons from the soil, followed by gas chromatographic analysis of the extract. Table 1 shows the results of the treatment of gasoline-contaminated sandy soil anddieselcontaminated sandy soil. The average difference between the treated and untreated samples equates to a 99.9% encapsulation efficiency.To ensure that the leaching study results were not all “non-detect” results, the treated samples with the highest residualTPH-in other words, the samples with the lowest encapsulation efficiency-were used for the balance of the study. These worst-case resultsof the treated samplesam listed in Table2, showing the initial soil analysis andthe treated sample analysis.All analyses were run with EPA Method 8015 as modified by California DHS. The 12 samples from Table2 were used for both phases of the leaching study. at ambient pressure on both comPhase 1 of the study involved column leachability studies pacted and uncompacted samples. Phase2 consisted of column leachability studies similar to those in Phase 1 but in a closed system at pressures of 500-1800 pounds per square inch (psi). The leaching solution used for all samples was 0.1 M sodium acetate at pH 4.9, and analysis of the leachate was with EPA Method 8015 as modified by California DHS. Each of the 12 samples was run through the Phase 1 and Phase 2 procedures both in an “as is” state and after being compacted to 100% compaction.
C.ColumnDesign For the Phase 1 ambient pressure study, graduated glass chromatography columns,5 cm in diameter and 70 cm in length and equipped witha stopcock, were used. The columns were first
Table 1 Comparison of TreatedandUntreated SamplesSandy Soil ~~~~~
~
ID.
Contaminant Sample Sample Gasoline Gasoline Gasoline Gasoline Diesel Diesel Diesel Diesel
Control Run 1 Run 2 Run 3 Control Run 1 Run 2 Run 3
S- 16-6 S- 16-1
S-16-2 S- 16-3 S- 14-6 S-14-1 S- 14-2 S- 14-3
TPH = total petroleum hydrocarbons.
~
Initial untreated Siallon-treated TPH (mg/kg) TPH (mg/kg)
7400 c0.05
5.4 0.9
lo400 6.6 12.2 13.4
t.D.
Siallon:
431
Table 2 Summary of Relevant 'Ihxtability Results for Gasoline- and Diesel-Contaminated Soils Initial untreated Sample S-16-6 S-16-2 S- 16-12 S-16-8 S-16-18 S-16-13 S-14-6 S- 14-3 S-14-10 S-14-8 S- 14- l6 S-14-13
Soil type
Gasoline Gasoline Gasoline
Gasoline Gasoline Gasoline Diesel Diesel Diesel Diesel Diesel Diesel
"W-I
Sandy
7400
Sandy Silty sand Silty sand
8350
Clay Clay
Sandy Sandy Silty sand Silty sand Clay
Clay
Siallon-treated
pH 5.4 34.1
9150 78.4 lo400 13.4 10700 78.2 9200 412.5
charged with3 cm of glass wool followed by 30 g of clean sand and another 3 cm of glass wool. The soil, compactedor uncompacted, was then added in three lifts totaling 250 The g. soil was rod to remove air voids and channels. Each column then gently tamped with a 1 in. aluminum was equipped with a gravity feed systemto provide a constant volumeof 600 mL of leachant above the top of the soil column. As the columns were allowed to run, the leachate was collected in 250-mL volumes, equivalent to one pore volume of the soil. A total of six pore volumes was collected from each column, the diesel collected at ambient temperature and the gasoline collected into VOC containers in ice baths set at -5°C. Each of the Siallon-treated samples was runin triplicate with one run of the untreated control sample, requiring four columns for each sample run, a totalof 24 columns for the compacted samples and 24 columns for the uncompacted samples. 2 in. The Phase 2 high pressure study required construction of a stainless steel column, by 12 in. stainless steel tube equipped with threaded endsand sealed with caps. The interior was fitted with a 3/8-in. (179-mm) porous glass frit and No. 4 Whatman filter paper. The column was equipped with two pressure gauges, one within the soil area to measure the soil pressure and one above the soil leachant interfaceto measure the solvent pressure. Inlet and exit valves were installed in the sealed caps. The same mass of soil as in the Phase l study, 250 g, was added to the open column, followedby 500 mL of leachant. The chamber was then sealed and hydraulic pressure was applied. The leachate was collected in 200-mL increments, equal to one pore volume of the soil, for a total of six pore volumes.
D. Leachability Results There are three key analyses from this study, total concentration analyses by EPA Method 8015 as modified by California DHS of the untreated and treated samples and the cumulative total of the amount of TPH released over the six pore volumes in the column leaching. As shown in Table 3, the Siallon encapsulationof the diesel-contaminated soils isso confining or tight that even the aggressive solvent extraction of Method 8015 Modified cannot extract the contaminant in the treated samples.However, as is also shownin Table 3, the total amount of contaminant leached out of the treated samples is much less than is shown as available for leaching or as unencapsulated by the TCA method. This held for all of the samples, with the total amount leached ranging from only10.2% to 67.6% of the available TPHas determined by TCA. Obviously, when the TCA type of analysis is used on Siallon-treated samples, it will detect not
432
McDowell
Table 3 Comparison of TCA and Total Leachate Release for Sandy Soil Samples EPA Method 8015 Modified
After Contaminant
Before treatment mg.
Diesel
2600
Total mg leached
After treatment (mg)
Before treatment (mg)
treatment (mg)
3.35
2605
1.9
only the unencapsulated hydrocarbon but also the partially or poorly encapsulated hydrocarbon. In essence, the TCA methods overstate the impact on the environment. As was mentioned previously, all of the samples were subjected to the column leach proof cedure in triplicate, resulting in more than75 samples being analyzed for each pore volume leachate. A portion of the mass of data generated is summarizedand averaged for the ambient pressure study in Table 4. As can be seen in Table 4, regardless of soil type, contaminant type, compacted or uncompacted, the decrease in TPH leached over the six pore volumes follows the same rapid decline for both treated and untreated samples. When the cumulative releaseof TPH is plotted pore volume hasa time and flowrate factor), the against the number of pore volumes (and each
Table 4 ColumnLeachateStudyResults-AmbientPressure Total petroleum hydrocarbon (m&) Pore volume
Soil type Sandy Sandy Sandy Sandy Sandy silt Sandy silt Sandy silt Sandy silt Clay Clay Clay Clay Sandy Sandy Sandy Sandy Sandy silt Sandy silt Sandy silt Sandy silt Clay Clay Clay Clay
Contaminant Sample Compaction condition 1 Gasoline Gasoline Gasoline Gasoline Gasoline Gasoline Gasoline Gasoline Gasoline Gasoline Gasoline Gasoline Diesel Diesel Diesel Diesel Diesel Diesel Diesel Diesel Diesel Diesel Diesel Diesel
Untreated Treated Untreated Treated Untreated Treated Untreated Treated Untreated Treated Untreated Treated Untreated Treated Untreated Treated Untreated Treated Untreated Treated Untreated Treated Untreated Treated
2 Uncompacted Uncompacted Compacted Compacted Uncompacted Uncompacted Compacted Compacted Uncompacted Uncompacted Compacted Compacted Uncompacted Uncompacted Compacted Compacted Uncompacted Uncompacted Compacted Compacted Uncompacted Uncompacted Compacted Compacted
3
4
5
ND 5 ND ND 360 110 40 10 0.13 ND ND ND 900 650 120 80 1.3 0.4 ND ND 1040 450 150 60 0.65 0.2 ND M) 1100 950 600 400 3.6 1.9 0.65 5.3 950 700 650 300 4.8 3.4 2.2 1 3100 1150 420 100 1.3 0.3 ND ND 4400 2300 600 300 80 3.2 0.6 0.1 ND ND 4200 1800 750 120 50 27.8 8.9 4.3 1.3 0.5 3600 1400 600 100 80 1.5 0.6 ND 17.3 6.3 3350 1150 420 60 5 33.4 19 6.1 148.6 68 2400 850 450 100 20 73.3 27.1 9.2 4 1.3
2350 1.77 1450 1.25 2100 7.1 1650 2.6 1250 9.9 875 7.9 5600 6.2
&Q0
60
0.37 ND
6 ND ND 5
ND 20 ND 20 ND 250 0.2 120 0.4 50 ND 20 ND 10
0.2 20 ND 1
2.1 5 0.6
433
Siallon: Hydrocarbon Encapsulation
INCREASINGPOREVOLUME
Figure 1 Cumulative releaseof TPH gasoline vs. increasing pore volume.(.) Sandy treated; untreated; (+) silty sand treated; (0) silty sand untreated; (A) clay treated; (A) clay untreated.
(D)
sandy
resultant curves shown as Figure 1 have identical shapes for both treated and untreated samples. The releaseof TPH fromthe untreated samplesis due to simple washoff or desorption from the soil surface. There obviously has been no treatment of these samples, since diffusion and advection do not play a part in contaminant release. As the Siallon-treated samples show identically shaped release versus pore volume curves, the method of contaminant removalmust be very similar to that in the untreated samples-a simple washoff of the unencapsulated TPH. The differences lie in the amounts of TPH released. One of the uses of leach tests is to determine the actual kinetics of contaminant leaching from within treated wastes. There are several mechanisms for leaching contaminants from encapsulated or solidified materials. At the particle-leachate interface, both dissolution and desorption can occur. The contaminant can diffise out of the encapsulated or solidified matrix. Dissolution of the encapsulant or solidified material will leadto steadily increasing cumulative release results. Chemical reaction can occur between the leachant used and either the encapsulation or solidifying material or between the leachant and the contaminant. According tothe Godbee and Joy [l81 model, a linear relationship between the cumulative fraction of TPH leached and the square root of time for the leaching is a consequence of diffusion control. When the results from Table 4 are plotted this way, the resulting curve shows a rapid rise followed by a straight line of zero slope. Since the slope is equivalent to D,' (the apparent diffision coefficient), it holds that followingthe rapid washoff of the unencapsulated TPH, there is no detectable diffusion from within the Siallon silica cell. This was to be expected because the Siallon silicacell is resistant to the extraction effectsof even an aggressive solvent suchas methylene chloride. This resistanceto extraction meansthat the solvent cannot penetrate the silica cell to extract the hydrocarbon; similarly,the hydrocarbon cannot diffuse out of the cell in any leaching protocol.This is clearly shown in Figure 2 and is especially evident in the clay soil sample, where thereis a rapid washoff of the unencapsulated TPH followedby a straight line with zero slope, indicating no diffusion from within the Siallon silica cell.
434
McDowell
O
L 019
*
I
13.4
--
*
z
*
23.2
26.8
m 30
32.9
-
Square R o o t Time
Figure 2 Cumulativefractionleached vs. square mot of time-siallon-treated diesel-contaminated soil. (M) Sandy; (A) silty sand; (4)clay.
E. LeachabilityConclusions This intensive study of the leachability of hydrocarbons encapsulated with the Siallon process is only a small part of the ongoing researchinto the durability, integrity, and life span of processed materials.This study did, however, provide a number of significant conclusionsas to the behavior of the Siallon silicacell. 1. The Siallon process was effective in encapsulating both diesel fuel and gasoline in sandy
soil, sandy silt soil, and clayey soil. 2. The Siallon silica cell is not affected by compaction or the tamping pressure of compac-
tion.
3. The Siallon silica cell is not affected by pressures up to 1500 psi. 4. Hydrocarbons that are encapsulated within the silica cell will not diffuse out of the cell or
leach into the environment. 5 . The limiting criterion for the long-term effectiveness of Siallon-encapsulated material is not soil type, contaminant type, or the volume of leachant; rather it is the efficacy of the initial Siallon treatment. One of the main results of these conclusionsis that the Siallon process can easily be monitored for effectiveness by simple total concentration analysis methods such as EPA Method 8015 as modified by California DHS. Along with this conclusion isthe fact that when Siallon treatment is used at a contaminated site and is required to meet state cleanup levels, the actual effect of the treatment on the environment will be even less than that expected from the set levels.
IV. SIALLON APPLICATIONS AND MIXING EQUIPMENT The Siallon microencapsulation process is applicable to the treatment of a wide varietyof waste materials. The hydrocarbonsthat have been successfully treated and foundto be amenable to treatment by microencapsulation include crude oil, gasoline, coal tars, creosote, lubricants, diesel fuel, jet fuel, PCBs, and refinery wastes. The differencesbetween many types of wastes are related not so much to the type of contaminant as to the physical form of the waste matrix. The treatments of pure crude oil, crude
Siallon:
435
oil-contaminated soil, and crude oil sludgeare all chemically similar; the differences lie in the physical form of the material to be treated and in how it is handled during treatment. The most important aspect of any Siallon treatment is that adequate mixing of the host material must be obtained during the application of the reagents. For materials differing in physical characteristics, it is obvious that different mixing equipmentor methods are required. For remediation of normal contaminated soil,pug a mill has been found to meet all of the requirements for efficient application of the Siallon process. Apug mill is simply a continuous mixer with twin side-by-side helical mixing blades. The pug mill is used in many industries for a variety of mixing and blending operations. The concrete industry and the mining industry are large users of pug mills. For the processing of Siallon-treated soil, the large throughput of a pug mill combined with the instantaneous reaction of the Siallon reagents makes the pug mill an ideal tool for remediation of small to large quantities of soil. Pug mills for soil remediation range in size from15 tonshr up to 200 tonshr. At these rates the Siallon process becomes one of the most cost-effective remediation methods available. Processing contaminated soil through pug a mill is a relatively simple operation.The soil is added to the pug mill hopper via a conveyor or auger system. As the soil enters the hopper, the Siallon emulsifier is appliedby spray. Approximately halfway down the pug mill, the Siallon reactive silicate is added. The soil exits the pug mill after a 90-sec residence time and is ready for final disposition. The pug mill is adjustable for mixing speed and residence time as well as flow rate and pressure of the spray-applied reagents. This allows a pug mill to easily be configured to treat many different contaminants, soil types, levels of contaminant, and various site conditions with the Siallon process. For sludges or heavy clays of high moisture content, a ribbon blender is the most appropriate mixing equipment. Ribbon blenders are used in a wide variety of industries for mixing and blending everything from powdersto pastes. The food, chemical, detergent, pharmaceutical, and paint industries all make use of this versatile mixer. These mixers come in watertight configurations for mixing pastesor fluids, which makes them ideal for use as a mixing system for sludges, tars, or other high water content materials. The use of ribbon blenders for soil or sludge remediation with the Siallon process is a batch-type production. The mixer is charged with known a quantity of sludge and is then turned on. After 30 sec of mixing to ensure homogeneity of the mixture, the Siallon emulsifier is automatically sprayed into themixer. This is mixed in for 2 min, after which the Siallon reactive silicate is automatically added. Afurther 2 min of mixing ensures that total reaction has taken place, and the treated soil is dumped through a bottom valveto a conveyor. 1 to 30 yd3 capacity,so processing 100 tonshr Ribbon blenders come in sizes ranging from of sludge is a simple matter. The watertight nature of the blender ensures that everything that comes out of the system is treated and encapsulated. QNQCprocedures as all of the processing The useof these blenders provides for excellent parameters can be easily monitoredor adjusted. For soil remediation on specific sites where the contaminant has little or no volatile component, there is a large amount of debrisor cobble in the soil, and there is sufficient space to lay out the contaminated soil in windrows, a tractor-driven soil-mixing machine has found application. The Dirt Witch is similarto equipment used for composting soil in its overall design. The differences are in the mixing blades, the spray system, and the control system. Theequip ment used for the application of Siallon reagents is modified to provide intimate soil mixing rather than the simple blending accomplished with cornposting machines. The mixing blades are L-shaped to provide maximum turbulence in the soil. Reagent is added from a bulk storage tank controlledby pressure settings and flow rate to provide the proper spray pattern and force
436
McDowell
of impingement on the soil particles. A tachometer is used to ensure constant mixing speeds, and a low-speed speedometeris used to ensure a constant mixing timeon any given soil block. There are several advantages to the use of this equipment for ex situ remediation. Only debris larger than 10 in. has to be removed from the soil. Processing rates are upwards of 100 tonslhr, depending uponsite conditions. The equipmentis simple and inexpensive, working off the power take-off of a standard tractor. To remediate soil with the Dirt Witch, the soil is laid out in windrows on a plastic liner. The windrows are generally 6 ft high and 8 ft wide, the lengthof the windrow being governed by the site. The Dirt Witch moves through the windrow, mixing and adding the reagents in a sequential manner. In other words, it will makea complete pass mixingand applying the Siallon emulsifier and then a second complete pass mixing and applying the Siallon reactive silicate. For on-surface or shallowdepth soil remediation, the use of a Bomag or similar type of mixing equipmenthas beem found to be the most practical. These mixersare generally used for soil or sludge solidification, and the only modifications necessary are the addition of the reagent flow control and spray system. Their mixing depthis usually limited to 18 in., and their speed in applying Siallon materials is 1 mph on an 8-ft-wide path. This makestheir use one of the most cost-effective and efficient methods of remediating large sites where the contamination is all within the top foot of surface.
V. SITE REMEDIATION OF GASOLINE-CONTAMINATED SOIL The actual remediation of a site is always the true test of whether or not a process has applicability within the environmental industry, and whetheror not it will find acceptability among the regulatory agencies and clients.’The following documents the remediation of gasolineimpacted soilat a UST site in Los Angeles, California.As with the majority of service stations, speed is of the essence when it comes to soil remediation and tank replacement. If a service station is shut down for tank replacement and the associated soil remediation, is not it pumping gas, and in the case of some of the large stations in the Los Angeles area this can representthe loss of sales of tens of thousands of gallons. With speed ofremediation in mind, the client chose the Siallon process for remediation of the soil at this particular site.
A.SiteCharacteristics The site is in Los Angeles and is located about 1.5 mi east of the Pacific Ocean. It lies at an y p e ,with fair permeability. elevation only slightly above sea level. The soil was of a silty sandt Approximately 700 yd3 of contaminated soil had been excavated and stockpiled on site and covered with plastic during sampling, analysis, permitting, and work plan preparation. The soil contamination at this site resulted from spillage as well as leaking underground storage tanks (USTs) and their associate piping. It had an average TPH of 6700 ppm and an average BTXE contamination of 320 ppm. The “hot spots” ranged as high as 67,000 ppm TPH and 2300 ppm xylenes. All the soil had to be remediated as part of the work plan. Outside the “hot spots,” the TPH levels ranged from nondetectable up to 5200 ppm, with an average TPH of 1920 ppm. The BTXE components range from nondetectableto 190 ppm xylenes, with an average BTXE of 79 ppm.
B. RegulatoryAffairs Siallon Corporation provided a pug mill, fully permitted as a transportable treatment unit (“U) capable of processing 20 tons of soil per hour. Siallon Corporation sampled the heated
Siallon:
437
material and had an on-site mobile, state-certified laboratory perform EPA 8015 as modified by California DHS and EPA 8020 analyses on all appropriate samples. The soil was to be remediated on-site with the TTU-permitted pug mill. At present the operation of a TTU for remediation is allowed under Permitby Rule (PBR), which requires all appropriate regulatory agenciesto be notified prior to each site remediation. As the movement of contaminated soilprior to remediation may lead to some air release of volatile hydrocarbons, approval by South Coast Air Quality Management District (SCAQMD) to operate was also required through their Rule 1166 permitting procedure. The regulatory agencies that were required to be notified were SCAQMD, Los Angeles Fire Department, Los Angeles County Department of Public Works, California Department of Health Services, and the Regional Water Quality Control Board.
C. AnalyticalandSampling Methods All analyses were performed by a state-certified laboratory in accordance with the following methods:
Analyte
Total petroleum hydrocarbons Total recoverable petroleum hydrocarbons (TRPH) Benzene, toluene, ethylbenzene. and total xylenes (BETX) Organic lead
Analytical procedure EPA Method 8015, Modified EPA Method 418.1 EPA Method 8020 California DHS-LUFT Method
Sampling of the Siallon-treated material was done with the aid of an automatic samplerat the exit to the pug mill. The sampler retrieveda sample every6 min and placed it in a sealable container for cornpositing prior to analysis. The regulatory requirements for analysis required one sample for every 75 yd3 of soil processed.
D. SiallonTreatment Siallon Corporation provided a fully permitted pug mill capable of processing 20 tons of soil per hour, two experienced operator/technicians, and the labor and equipment required to load the contaminated soil into the pug mill and stockpile the clean material on-site. Siallon Corporation also regularly sampled the treated material and had a certified laboratory perform EPA 8015 and 8020 analyses on all appropriate samples. The pug mill used for the encapsulation of the gasoline-contaminated soil entails several preparatory steps: setupof the unit, prescreening of the soil, and pretreatmentof the soil. In this case the pretreatment step required that the contaminated soil be sprayed with a dilute (1 part Siallon emulsifier to 10 parts water) Siallon emulsifier solutionto minimize the possibility of any volatile emissions duringthe soil prescreeningstep. The treatment unitwas transported to the site via a 45-ft trailer. The soil that had been excavated previously was analyzed for TPH by the client’s consultant, thus allowing forthe calculation of the appropriate dose rates for the Siallon reagents. The choice of dose rate was sufficient to encapsulatethe hydrocarbons at the highest contamination levels. The actual dose rate used was 0.007% by weight of soil of both the Siallon emulsifier and the Siallon reactive silicate.This would equate to 14 lb of emulsifier and 14 lb of reactive silicate per ton of soil.
438
McDavell
The influent soilwas continuously monitoredfor volatization of hydrocarbons with a portable organic vapor analyzer(OVA), because exceeding 50 ppm of volatile organics in the air would have required a major interruption of the remediation. There wasno major interruption in the remediation of the soil stockpile. VOC emissions, the contamAfter being wet withthe dilute emulsifier solution to control inated soilwas loaded into a feed hopper that was equipped witha shaker screen.The soil was then automatically conveyedto the back portion of the twin-screw pug mill for mixing. There the emulsifier blend was sprayed on the soil in thefirst portion of the mixing chamber. The soil was mixed and moved downthe pug mill and approximately halfway along the Siallon reactive silicate solution was spray-applied. The treated soil was then discharged to a stockpile until analysis was completed. After effective Siallon remediation, the treated soil was transported for use as day cover at a designated Class I11 landfill as the client had used a mixture of sand and pea gravel to backfill the excavated tank cavity.
E. Remediation Results The client was concerned about the time requiredfor remediation, and thesite was cleaned in less than 7 working days.A summary of results is presented as Table 5 . The summary clearly shows the efficacy of the Siallon process in remediation of contaminated soils. An encapsulation efficiency of greater than 99.9% and essentially nondetect results for all contaminants was two orders of magnitude lower than the required regulatory cleanup levels of 100 mg/kg as TPH gasoline. Clearly, a significant amount of contaminated soil had been promptly and effectively remediated with the Siallon microencapsulation process, yielding a soil that easily met the regulatory target. Following the Siallon soil treatment, the regulatory agencies allowed the soil to be used in a Class I11 landfill and the facility to be delisted. The environment had been protected quickly and completely, with no expectation of any further impact from the contaminated soil.
VI. CONCLUSIONS This chapter has shown how the Siallon process for the microencapsulation of hydrocarbons differs from conventionalsolidification or stabilization processes. Its simple two-step chemical process avoidsmany of the pitfalls inherent inother processes. It has clearly shown itself to be another tool in the remediation of contaminated soil, and as such its advantage of speed and efficiency can be teamed with other remediation methods to provide the client with the best possible alternatives at the lowest costs. The Siallon process can be used to remediate the source area of the highest contamination ona site where biodegradation is being used for remediation of the plume area. At a site where vapor extractionis operating in a plume area, the
Table 5 Gasoline-ContaminatedSoil-RemediationResultsSummary Highest before (mag)
Contaminant treatment ~~~~
TPH gasoline Benzene Toluene Ethylbenzene Xylenes
~~
~
Avg before treatment (mgkg)
Avg after treatment (mg/kg)
Encapsulation percentage
~
67O ,OO 120 420 350 2,300
6670 12 48 36
220
CO.01 0.04 <0.01
0.19
>99.99 >99.9 99.9 >99.9 99.9
Siallon: Hydrocarbon Encapsulation
439
Siallon process can quickly clean up the heavy source contamination to prevent any further migration into the plume or groundwater. The Siallon microencapsulation process is designed to reduceor eliminate the mobility and toxicity of the contaminant and thereby lessen or eliminate the contaminant's impact on the environment andhuman health. This reduction in toxicity and mobility is a function of the efficiency of the Siallon treatment, the rate of leaching of any untreated or unencapsulated material, and the lifespanof the silica cell.Whenthesefactors are optimized, aninformed decision as to the safety and reliability of the encapsulation can be made.
REFERENCES 1. The Jennings Group, Underground Storage Tank Markets, 1991-1995, Chatham, N.J., 1991. Univ. Tennessee Waste Manage2. M. Russel et al.,Hazardous Waste Remediation: The Task Ahead, ment Research and Education Institute, December 1991. Solidfied Waste, US.Environmental 3. U.S. EPA, Guide to the Disposal of Chemically Stabilized and Protection Agency Publ. SW-872, Washington, D.C., September 1980. 4. Solidtech Inc. SolidificationlStabilization Process-Applications Analysis Report, U.S. EPA Publ. EPA/54OIA5-891005, Risk Reduction Engineering Laboratory. 5. Quicklime PCB treatment, Hazardous Waste Consultant, SeptemberlOctober 1991. 6. Caldwell, R. J., and Cote, F? L., Investigation of solidificationfor the immobilization of trace organic contaminants, Hazardous Waste Hazardous Mat., 7, 3 (1990). 7. Office of Solid Waste and Emergency Response, Immobilization as Treatment, U.S.EPAPubl. 9380.3-07FS, 1991. 8. Funderbruk, R., EPA still doubtjkl about organic immobilization, Hazmat World, February 1991. 9. StabilizationlSolidification of CERCLA and RCRA Wastes, U.S. EPA Publ. EPA/625/6-89/022, May1989. 10. McDowell, T. K.,Microencapsulationof hydrocarbons in soil using reactive silicate technology, in Hydrocarbon Contaminated Soils and Groundwater, Vol. 2 (Calabrese and Kostecki,eds.), Lewis, Chelsea, Mich., 1991. 11. Siallon Technical Manual, Siallon Corp., Laguna Niguel, Calif., 1992. K.,Oil treatment method, U.S. Patent 5,076,938 (December 12. Noonan,W.R.,andMcDowell,T. 1991). 13. Shwartz, A. M., and Perry, J. M., Surface Active Agents, 3rd ed., Interscience-Wiley, New York, 1949. 14. Considine, D. M., ChemicalandProcessTechnologyEncyclopedia, McGraw-Hill,NewYork, 1974. 15. Cotton, F. S., and Wilkinson, G., Advanced Inorganic Chemistry, Interscience, New York, 1962. 16. Gasoline Fuel Contaminated Soils, Siallon Corp. Rep. B102A. September 1991. 17. Diesel Fuel Contaminated Soils, Siallon Corp. Rep. BlOlB, August 1991. 18. Godbee, H.W., and Joy, D. S., Assessment of theLoss of Radioactive Isotopes from Waste Solids to the Environment, Part 1: Background and Theory, Oak Ridge National Laboratory, Oak Ridge, Tenn., Publ. No. T"4333, 1974.
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20
Remediation of Heavy Metal Contaminated Solids Using Polysilicates George J. Trezek Greenfield Environmental Carlsbad, California and Universiq of California at Berkeley Berkeley, California
1.
INTRODUCTION
The polysilicate technologyfor the remediation of heavy metals in solidor semisolid matrices is a commercial process capable of cost-effectively treating materials at rates on the order of 100 tonsihr. This process is known in the industry as the STS technology. The process evolved through a series of laboratory bench-scale studies, pilot field tests, and the construction of commercial-scale mobile systems. The development of the process began inearnest in early 1985 as a project to ascertain the technical and economic feasibilityof treating the heavy metals containedin automobile shredder residue. The California Departmentof Health Services (DHS) requiredthat this material be managed as a hazardous waste because it failed to pass the California wet extraction test(CAM test) for certain metals such as lead, cadmium, and zinc. After the successful development of a treatment protocol on the laboratory scale, a pilot system was installed as part of the shredding plant process line. A full-scale permanent on-line treatment system was installed toward the end of 1985. Following the initial success with auto shredder residue,the treatment was applied to mitigating heavy metals in other types of materials. These have included heavy metal-contaminated soil, bag house dust, electric arc furnace dust, incinerator ash, filter press cake, foundry sand, wastewaster treatment sludges, and sludges from a variety of other manufacturing operations. Thus far, the majority of experience with the technology has been through its application of remediating heavy metals contained in soil-like substrates such as soil, clay, sand, sludges, and residues as well as mixtures of these materials. Depending upon the particular circumstance or site, thequantities of treated material range from several thousandto several hundred thousand tons. The nature of the metals also varied fromone predominant constituent to a material containing five or more elevated metal concentrations. Consideration will be givento the nature of the technology, the development of treatment protocols, the delivery of the technology for site remediation includingcase studies, and a de441
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scription of the governing parametersthat encompasses hydration andcuring, compactability, final particle size distribution, and long-term effects.
11.
NATURE OF THE TECHNOLOGY
The plysilicatetreatment technologyis a chemical treatment that uses commercially available soluble silicate solutions and various cementitious materials such as cement, lime, pozzalime, and fly ash [1,2]. Relatively small amountsof polysilicates andcementitious materials are used to change the chemical characteristics of heavy metals containedin solid or semisolid matrices. Consequently, this chemical treatmentdiffers significantly fromthe so-called solidificationhtabilization technologies, requiring as much as a 100% addition of reagents. Three principal steps are involved in delivering the treatment: (1) thorough wetting of the material containing the heavy metals with a silicate-water blend, (2) the addition of appropriate cementitious materials, and (3) curing of the mixed material into a friable form suitable for backfilling.The following discussion deals with the chemistry associated with these steps [3]. Typically, the common metallic compounds found in materials requiring treatment are a mixture of free metallic ions and other metallic ions resulting from metal chlorides, sulfates, carbonates, etc., and metallic oxides and hydroxides as shown below. Free Metallic Ions
+ OH+ a e- + (6G - S@)
2 H20 r"-H3O+
M'"*M'
Other Metallic Ions
+ a ClMb(S04&C- b Ma+ + c S042Mb(C03)ce- b M" + c C q 2 MCl,
F+
M"+
Metallic Oxides/Hydroxides
In field applications, the free metallic ions are usually present in small quantities. The actual concentrations of these ions and of the other metallic ions resulting fromsalts, oxides, and bases are determined by the equilibrium Gibbsfree energy of the constituents of the mixture. These concentrations are also influenced by various kinetic factors relatedto the presence of catalysts and particle size distributions. On the other hand, theconcentrationsof the metallic oxides and hydroxides are also typically related to the material generation source and influenced by historical temperature, chemical medium, moisturecontent, and age. These metallic substances can be contained in a wide variety of substrates such as various soils, clays, soilclay mixtures, amorphous silica containing a broad spectrum of grain sizes, sludges generated as a by-product of industrial processes, and combinations of these materials.The following lead and zinc reactions are typical of metallic constituents requiring treatment:
H20
+ 2 e- + ZnO C - ZnO2 + H2C- Zn(0H)z
Remediation Contaminated Solids Metal of Heavy
443
As previously mentioned, the first element in the treatment involves wetting with the polysilicate-water blend that is created priorto its introduction into the mixing process. The most effective treatments have used potassium silicates, which are a family of chemicals with a wide range of physical and chemical properties. They are clear, highly viscous liquids having a pH in the range of11.3-1 1.7. The viscosity is affected by the Si0,/K20 ratio, concentration, and temperature. For example, the lower theratio and the more alkaline at a given solids content, the lower the viscosity. When the silicate is mixed with water it quickly forms a solution whose viscosity approaches that of water. Also, relatively small temperature increases, on the order of 10°C, can cause a fivefold decrease in viscosity. represented by the following equations. The formation of the liquid silicate polymerbecan Water
Liquid Silicate
As shown, the liquid silicates depolymerize when mixed with water, revealing their active negatively charged oxygen sites. Further, the silicon backbones themselves go into smaller, ionically charged clusters. The following shows the nature of the reactionthat occurs between the active metallic elements, divalent in this case, in the material and the liquid silicate polymer.'
2x K+
+ 2x OH- + 2u KOH
' This balance considers the active metals. not the silicates, to
be the limiting reactants.
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OH
H0 Here, ionically active metallic compounds react with the liquid silicates to form stable metal silicate chelates. The formation of these metal silicates is kinetically relatedto a number of factors suchas themetal ionizationstate, particle size,reaction temperature, andactivation energy. Further, when a combination of heavy metalsare present in a material requiring treatment, reaction priority is given to the more electropositive heavy metal. Thermodynamically, the reaction progresses toward a minimum potential energy equilibrium state. After the material has been thoroughly wettedby the polysilicate-water blend, a cementitious material is introduced into the process. The metal polysilicate chelates, and other metallic compounds are incorporatedinto a crystalline cementitious matrix. The following example illustrates the hydration of cement and lime, which are two commonly used cementitious materials. Portland Cement (In cement chemistrynotation2)
+ 6H + 3C.2S.3H + 3(CH) + 4H + 3C.2S.3H + CH + 1OH + 2(CH) + 6C-A.F.12H + 12H + CH + 3C.ACH.12H + 10H + CS.2H + 3C.A.CS-12H
2C3.S 2C2.S 4C-A-F 3C-A 3C-A Lime
+
2CaO + 2H20 + CaOeH20 + Ca(OH)2 27.5 kBN(lb-mo1) Ca(OH)2+ C a 2 + + 2 OHa Ca2+ a OH- + Ma+ + M(OH), a Ca2' CaCO3 + 78 kBtu/(lb.mol) + CaO CO, (gas)
+
+
+
445
Remediation Contaminated Solids Metal of Heavy
During the hydration of portland cement, active ionic bonds begin to transform into covalent bonds of greater stability. As the metal polysilicate chelates are incorporated into the cementitious structure, the size and stability of the usual 7-10-silicon backbone crystalline structure is increased. In addition, the metal polysilicates, along withactive metal oxides and hydroxides, tend to chelate the cementitious silicate chains by providing the necessary electronic bonding clouds. Again, the relatively weak ionic bonds are transformed into strong covalent bonds linking metal to oxide or hydroxide and the oxide or hydroxide to the silicon backbone. Typically, lime is used in combination with cement. Whenisitapplied separatelyto multimetallic materials, highly soluble metal hydroxidechelates can form. However, in combination with cement, it can provide the heat necessary to activate and accelerate metal oxidation and cement hydration reactions, thereby yieldinga stronger structure. The resulting configuration is a three-dimensional, low potentialenergy crystalline cementitiousmatrix that has orderly passivated metallic elements integrated into itsstructure. An example of the transformation of this structure from the ionic to the covalent state is shown below.
I
I
I
I
I
I
I
0
0
0
0
0
0
0
0
0
0
0
0
0
0
I
I
I
I
I
I
I
Si-0-Si- 0-Si-0-Si-0-Si-0-Si-0-Si
I
I
o/c" I
'0
I
I
OH ' M
l
I
Si-O~i-O-~i-~-~i-O--Si~-Si-O-si
I
0
111.
I
0
I
'0
I
0
OH). 0
I I
0
I l
OH ' M '0
I
I
0
I I
OH 0
I
I
0
DEVELOPMENT OF TREATMENT PROTOCOLS
A series of bench-scale treatability studies are commonly used to establish aninitial treatment protocol. Typically,a series of samples taken from representative areas in the field, particularly those having the highest concentrations, are evaluated in the laboratory and then subjectedto a range of treatment protocols.Thus, the treatability studies providethe initial data that validate the applicability of the process for the associated metals. These testsalso give an indication of the effectiveness of the reagents, that is, the polysilicate blend, and the quantity and composition of the cementitious materials. After a basic protocol is developed and optimized, the treatability studies provide an initial estimate of the treatment costs. The treatment protocolsused in the application of the STS technology are sensitive to the substrate, and the overall physical nature, type,and combination of metals present, the material
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characteristics of the material. These featuresare organized into a database (Table 1) containing the following principal information: Laboratory identification for internal tracking Generator information Waste classification Physical characteristics Treatment reagents Extraction information Pre- and posttreatment soluble and total metalconcentrations The database is designed to include other nonsilicate reagents thatmay be applicable tothe use of the STS technology in conjunction withother procedures suchas metal extraction, transformation of metal states, and the combined treatment of hydrocarbon- and metal-contaminated materials. Provisions are also made for the evaluation of both the STLC and TCLP extraction methods. (The STLC procedure is the so-called California test based on sodium citrate as the leaching agent.) Further, information on metals other than those regulated is included because of their importancein the overall treatment dynamics terms in of metal-metal interactions and matrix bonding stability. The results of approximately 800 treatability and commercial treatment evaluations are currently stored in the database. In addition to providingthe information necessary to quantify the chemical nature and validity of the treatment, the database offers valuable insights intothe development of protocols for new materials requiring treatment. Examples of the efficacy of the treatment are given in Table2 for five metal groups. With of the same six metalsthe exceptionof group 4, these groups contain various concentrations arsenic, copper, lead, nickel, cadmium, and zinc. In each case, values are given for the total metal concentration (TTLC) and the pre- and posttreatment soluble metal concentration as measured by either the STLC or TCLP procedure. The important features are summarized as follows. Metal system 1. Here copper is the predominant metal as given by both the total and soluble concentrations. The treatment resulted in a primary reduction in copper, a secondary reduction in lead, and minor ductions in the other metals. Metal system 2. Based on total concentrations, arsenic and copper are the primary metals; however,only the soluble concentration of arsenic exceeds regulatory limits. Here the treatment resulted in a primary reduction in arsenic, secondary reductions in copper and zinc, and essentially no detectable reductions in the other metals. Metal system 3. This system is characterized by elevated levels ofcopper, lead, and zinc. Arsenic is essentially not involved in this metal system. Here the treatment significantly lowered the levels of all the metals. MetaI system 4. The metals of principal concern in this system are copper, lead, cadmium, andzinc.In this metalgroup,nickelremainedrelativelyunchangedwithtreatment, whereas the other metals experienced significant reductions. Metal system 5. This system exhibits elevated total concentrations of arsenic, copper, and lead. Based only on totals, this would be classed as a three-metal system. However, this group also exhibits elevated soluble levelsof nickel and cadmium,so it behaves as a fivemetal system. The treatment significantly reduced the soluble levels of arsenic, copper, and cadmium. The soluble levels of zinc and nickel were reduced to a lesser extent, whereas the level of lead remained essentially unchanged. The reactions in these five metal groups are complex and often not exactly predictable. Competing ions suchas calcium, magnesium, iron, and, to a lesser extent, sodium, potassium,
Remediation Contaminated Solids Metal of Heavy
447
and cationic charged substances like clays, often occupy active sites within the developing cement-silicate matrix preferentially over the contaminant ion and can decrease the stability of certain precipitants prior to the poyolanic reaction. Equilibrium between the metals present, their oxidation states, and the competition exhibited during induced reactions are complicated and are typically determinedby experimental processes. In this regard, the use the of database allows treatment options to be predetermined, thereby permitting actual treatment protocols to be expeditiously established.
IV.DELIVERY
OF THE TECHNOLOGY
For site remediation applications, the technology is delivered by means of a mobile system that can be operated under a TTU (Transportable Treatment Unit) permit. The equipment is versatile and self-contained, requires a relatively small space, and can be made operational on a site within two to three working days. The principal elementsof the treatment system, shown schematically in Figure 1, consist of the feeder, magnet, screen, pug mill mixing plant, and polysilicate blending unit. The actual are opequipment used in the latter two elements is shown in Figure 2. These unit operations erated as an in-line continuous system with a throughput of 100-125 tonshr. 1) follows. Soil requiring treatment is taken from A brief description of the system (Figure stockpiled material with a front-end loader and fed into a variable-speed feeder equipped with a set of grizzly bars to remove large tramp material. In some cases, to improve material handling and eliminate certain objects that could fall through the grizzlies (which could cause tears in the feeder belt or jams in the feeder, resulting in excessive equipment maintenance downtime), stockpiled material is prescreened with a short residence time mobile screen. After the material exits the feeder, it passes under a cross belt magnet to remove any ferrous material that may be present. (This fraction is common in scrap yard remediations.) The material then enters a triple-deck screen, where the large oversize fraction consisting of pieces of concrete, asphalt, wood, etc., is removed on the top deck. After the middle fraction (i.e., smaller rocks, stones, nonferrous metals, etc.) is removed, the remaining material is the undersize soil that is suitably conditioned for treatment. The feedrate of the material entering the feed hopper on the treatment unit is recorded by a certified belt scale. The polysilicate additives and mixing process are configured into two mobile treatment units. The mixing unit (Figure 2) consists of two feed hoppers, a twin-screw pug mill, a ceof storing apmentitious material storage silo, and a discharge conveyor. The silo, capable proximately 50 tons of material, is hydraulically elevated after the unit arrives on the site. Although a diesel engine generator system is mounted on the mixing trailer to provide a selfcontained source of power, the demands of the ancillary equipment may require the use of a separate mobile generator on the site. The polysilicate delivery system is contained in a separate trailer. A 2OOO-gal buffer tank allows the polysilicate-containing water blend to be delivered to a spray nozzle system at the point where the soil enters the pug The mill.polysilicates are added to the buffer tankby means of calibrated metering pumps connected to four 250-gal tanks. Polysilicate in 55-gal drums can be directly pumped into these tanks from outside the trailer. Typically, water enters the buffer tank directly from an on-site hydrant. Appropriate instrumentation allows for the measurement of the flow rates of all water and chemicals entering and exiting the chemical delivery system. Thus, along with the measurement of the soil throughput, a complete mass balance can be performed on the system. During operation, material requiring treatment enters the rear of the twin-screw pug mill. The diluted polysilicate blend is sprayed onto the feed soil. Intensive wetting of the soil with the polysilicates occurs in the first half-portion of the mixer. The cementitious material is
Table 1 Elements of the Metals Treatment Database Laboratory I.D.
Sample # Lab I.D.
Generator information
Generator Address
Site Location
Waste classification
Waste type Waste code Character Treatment date
Physical characteristics
% Moisture.
Density Comprehensive strength Size < 1/4 in. Size > '/4 in. Size > % in. Size > % in. Treated size Waste m t e d (g)
'Zteatment reagents
Extraction information
Silicate 1 (g) Silicate 2 (g) Cement (g) Cement type Lime (g) Lime type Fly ash (g) Fly ash type Kiln dust (g) Kiln dust type Polysulfide (g) Polysulfide type Phosphate (g)
Ext. date (totals) Ext. date (TCLP) Ext. date (STLC)
Pre- and post-treatment concns.
Ag SIZC Ag TCLP Ag TTLC A1 STUJ
Al TCLP
E Ni STLC Ni TCLP Ni TTLC PbsTLc Pb TCLP P b m
AlTTLC AssLC As TCLP A s m
Sb SIZC Sb TCLP Sb TTLC
Ba STLC Ba TCLP Ba TTLC Be STLC
Se STLC Se TCLP Se TTLC T1 STLC
Y
5%
Phosphate type Acid (g) Acid type Base (g) Base type Oxidant (g) Oxidant type Reductant (g) Reductant type Other 1 (g) Other 1 type Other 2 (g) Other 2 type
Be TCLP Be TIu3 Cd sIu3 Cd TCLP Cd TTLC c o STLC Co TCLP c o TTLC
cam
= =
Cr(II1) TCLP Cr(1II) TTLC cr(w Cr(VI) TCLP c r ( w TTLC
cu STu=
Cu TCLP c u TIu3 FTTZC
T1 TCLP TI TTLC Vslzc V TCLP V TTLC zn STLC Zn TCLP ZnTTLC ca STLC Ca TCLP
B
5 3
%
2 Q
4 % r8
E.
c1 STLC
2
C1 TCLP
CI TTLC F e r n Fe TCLP FeTTLc
F TCLP
Hg Hg !m-.c Hg TCLP Mo sTu= Mo TCLP Mo TTLC
Na STLC Na TCLP Na TTLC Final dry wt. pH solid DHextract
=
r8
cam
Mg STLC Mg TCLP Mg
FSlZC
F
3
3
Table 2 Total Metal and he-and Posttreatment Concentrations for Metal Gmum 1-5 ~
Extraction procedure
Character
Units
Arsenic
~~~
Copper
Lead
Nickel
Cadmium
Zinc
1.24 1S 2
0.01 0.01
70
0.88
0.01
Metal system 1
Totals 3050 STLC
STLC Totals 3050
TCLP #1 TCLP #2
Initial initial Treated
mg/kg m a
lnitial Initial Treated
mgkg mg/L mg/L
Totals 3050 STLC
Initial Initial
STLC
Treated
Totals 3050
snc
Initial Initial TEated
Totals 3050 TCLP #2 TCLP #2
Initial Initial Treated
STLC
16,300
2
m a
0.005 589 11.23 0.001 Metal system 2 3217 140 0.09
I359 1.11 0.12 Metal system 3
0.001
mg/L mglL mag mg/L mg/L
mglkg m gn
mpn
3917
o.OoO1
383 O.OOO1 0.88 Metal system 4 N/A NIA NIA
350 19.01 7.28 Metal system 5
2,759 158.3 0.068
1,627
34.7 0.075
2 0.27 0.02
21 0.06 0.05
770
72 0.075
537 31.99 0.62
1,550
0.33 0.28
29 0.1 0.1 30
2 0.01 0.01
40
2.89 0.01
0.28 0.005
92 2.56 1.64
4.95 0.46 0.005
470
41.25 20.91
48 1.54 0.005
0.01 0.01
398 2.2 0.003 5400
915 21 -0.5 1,012 87.04 0.84
325 15.7 0.534
Remediation of Heavy Metal Contaminated Solids
?
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8
.p!
B
5
B
451
1 I
Fa
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452
Figure 2 Equipment used in the delivery of the STS technology.
introduced at the midpoint of the mixer. The feed rate canbe adjusted by controlling the variable-speed drive on the silo rotary vane feeder.The residence timein the mixer is controlled by the blade angles. For soil, a 22" blade angle is used in the first half of the mixer to enhance retention (i.e., increase the contact timebetween the silicates and the material). In the section after the cementitious material is added, the blade angles are set at approximately 45" to enhance mixing and removal of the treated material from the chamber. As the treated material exits the pug mill unit, a radial stacking conveyor pilesthe material. The process is complete after the material has cured in the stockpiles. The treated stockpiles typicallyare turned with a front-end loader on a daily basis for several days. A Bomag unit can also be used to cure treated material. Here, the material is arranged in 3-44? lifts. After the material has partially set, the Bomag unit traverses the lift, creating suitably sized material. The final aspect of the treatment includes the sampling and subsequent analytical evaluation of the cured material. This involves obtaining representative samples from treated stockpiles and subjectingthe material to an extraction test. If the EPA TCLP test is used, the treated material (crystalline cementitious matrix) is extracted witha sodium acetate-acetic acid buffer solution. In this process, metallic elements can be leached from the matrix to form metal acetates. As shown below, the final extraction product is an equilibrium mixture of metal polysilicates, metal acetates, acetic acid, metal hydroxides, metal oxides, etc. a CH3COO-
+ b H+ + c Na+ + d H20 + e M(OH), + fCa(OH)2
+ g metalpolysilicates + - -
4
+
h (CH3COO),M + i CH3COONa + j CH3COOH k H20 n NaOH o ca3(C~H&)~ p metal polysilicates
+
+
+
+ l M(OH), + m Ca(OH)2
Remediation Contaminated Solids Metal of Heavy
453
test, a stable matrix Thus, in order for a treatment to be successful with regard to the TCLP must be created that resists the attackof the aggressive leachingfluid, i.e., one in which only negligible quantities of soluble metallic elementsare present. Upon completion of the analytical data, the treatedfriable material can then be backfilled on the site with conventionalearthmoving equipment.
V. CASESTUDIES Two examples of applying the technology to actual site remediations are given here. These particular case studies were selected becauseof their diversityin terms of both site conditions and types of metals. The first case study involves a heavy metal-contaminated site in the Port of Los Angeles where there had been an extensive metal salvaging operation dealing with a variety of operations that included ship breaking. Asa result of these activities, the soil was contaminated with lead, zinc, cadmium, nickel,and copper. The treatmentoperations were carried out on a clay pad constructed on site. The second case study is an example of “treatment in tank” that requiredthe construction of an RCRA tank before treatmentoperations were begun. In this case, arsenic was the principal metal of concern, and its remediation required a modification of the processing system to include an additionalsilo for the delivery of a second cementitious material.
A. Case Study I This case study deals with the remediation of a 23.5-acre site located in the Terminal Island District of the Portof Los Angeles [4]. The initialsite characterizationanalytical data indicated that approximately 18-24 in. of top soil material, or about 60,OOO tons of soil, would require treatment. In actuality, 106,700 tons of soil was treated in the overall project. A clean areawas prepared onsite for the mobile equipment treatment operations. The contaminated layer was removed in a 300 ft by 300 ft area, stockpiled in an adjacent location on the site, and replaced or backfilled with clean decomposed granite soil. This area provided a working pad for the equipment and the curing of treated material. The material requiring treatment was not typical soil. Because of the prior metal salvaging activities, the material containeda variety of ferrous and nonferrousmetals, rocks and stones, pieces of wood and asphalt, and other miscellaneous items. The size distribution of these materials spanned several orders of magnitude ranging from less than l in. to several feet. Occasionally, various parts of ships (i.e., riveted and welded beams, parts of anchors, sections of mechanical equipment, etc.) were uncovered in the excavation of the site and found their way into the stockpile for treatment. Consequently, the heterogeneous natureof the material dictated the type of preprocessing unit operations prior to mitigating the heavy metals. Although the project was permitted to operate from 6:00 A.M. to 6:00 P.M. five days a week, the South Coast Air Quality District imposed the added restriction that all operations including operating rolling stock cease by 5 0 0 P.M. Consequently, the effective daily treatment window was approximately 10 hr or less, depending upon downtime. A period of at least 1 hr was required for cleanup, maintenance, moving piles, etc. at the end of each shift. Thus, in order to meet the project schedule,a nominal 1OOO tons of material per day had to be processed within these time constraints. Material requiring treatmentwas arranged in 1000-ton stockpiles, 30 by 150 by 8 ft high, on the site adjacent to the clean soil equipment zone. Samplesof this material for laboratory analysis of heavy metals were taken as the piles were generated. Thesedata were used to s u p plement the original site characterization data and provide guidance in establishing the daily
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treatment protocols.The sampling protocol also involved the collection of samples of untreated and treated material at 15-min intervals during operation. These samples formeda daily composite, which was split for independent certified laboratory analysis. After receiptof the laboratory report and acceptance by the Port inspectors, the material was backfilled on the site. Additional samples of the in-place material were also taken. 1. TreatmentLevels Testing of the contaminated soil for all 17 metals revealed that only five had elevated levels requiring treatment. The range of these metals in terms of both the soluble (STLC) and total (TTLC) concentrations is summarized as follows: (1) lead, STLC 11-121 mg/L; TTLC 271500 mg/kg; (2) zinc, STLC 14-320 mg/L; TTLC 242-3130 mgkg; (3) cadmium, STLC 0.11.9 mg/L; TTLC 2-12 mgkg; (4) nickel, STLC 0.2-7 mg/L; TTLC 30-600 m a g ; and (5) copper, STLC2-96 mgL; TTLC 70-2610 mgkg. The soluble concentrationswere determined by the CAM wet extraction method, which involves milling to pass a No. 10 standard sieve followed by 48 hr of extraction in a sodium citrate solution. The relationship between the total and soluble concentrations summarized inFigure 3 shows the respective ranges for each metal. In effect, the treatment process must deal with metals whose concentrations cover a range of four orders of magnitude. It should be noted that the values of lead shown in Figure 3 are plotted as Pb/100 to aid in pattern recognition. Thus, the concentrationsof lead are in the same general band as copper and the lower range of zinc. 2. TreatmentResults The actual treatmentactivities began on October9, 1989 and terminated on April 12, 1990. The treatment of the initial 60,OOO tons of material was completed by the contracted scheduledate of January 15, 1990. The project period was then extended to treat the additional 47,000 tons of soil. Approximately 16,000 tons of nonhazardous oversize material was removed in the screening operation. With the exception of the ferrous metals, this fractionwas disposed of in a Class 111 landfill. The quantities of polysilicates and cementitious material were adjusted to coincide with the concentrations of metals in the in-feed material. Because of a combination of logistical, economic, andtreatability considerations, cement wasused as thecementitiousmaterial. Throughout the course of the treatment, the additionof cement ranged from 10.14 to 11.08%. Even with the wide range of STLCconcentrations of the various metals,the use of polysilicates varied over a relatively narrow band, which ranged from 0.513 to 0.59 gaton of soil.
B. Case Study II This study involves the remediation of 40,OOO tons of heavy metals-contaminated soil at the Thompson-Isaacson site in 'hkwila, Washington. In this soil, arsenic was the principal metal of concern. Theother metals thatwere evaluated were barium, chromium, copper, nickel,lead, and zinc. The need for the remediation arose asa result of the planned developmentof the site, which entailed the construction of an industrial structure necessitating substantial excavations for foundations and pedestrian tunnels[5]. Elevated levelsof arsenic are believed to have occurredas a result of the dredging,filling, and straighteningof the Duwamish channel beginning aroundthe turn of the century. Presently, one of the site boundaries is the Duwamish Waterway, which once flowed through the approximate center of the site before the channel was straightened and redirected. Various site investigations [6,7] indicated thatthe arsenic contaminationmay have resulted froma variety of fill materials generated by smelting and other ore processing operations.
Remediation of Heavy Metal Contaminated Solids
0
0
-
0
0 0
9 0
0 0
0
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A number of factors entered intothe decision to use on-site treatment forthe remediation of the heavy metals. Basically, thisalternative allowed the site owner to maintain control in that the treated material would remain on-site, thereby reducing the reliance on off-site hazardous waste landfill disposal and its associated long-term liabilities. Further, the useof on-site treatment at high processing rates (100 tons/hr) eliminates the excessive time associated with trucking large quantities of material considerable distances to Class I disposal sites. Finally, in terms of construction, since the characteristics of the backfilled STS-treated materialare suitable for the placement of buildings, the need and costs associated with imported fill materials can be significantly reduced or eliminated. The conditions for theuse of on-site treatment were consistent with the treatment in tank by generator regulations. Consequently,an RCRA tank was designed and constructed withthe appropriatemembraneliner,asphaltconcretepavement,catchbasins, etc. Basically,this RCRA tank was designed to not only contain the soil during treatment, but also to collect all contact and noncontact liquids,which could be subsequently transferred toappropriate storage tanks. A secondary containment system was also installed for the purpose of monitoring any leakage ftom this system. Normally, full-scale or commercial processing would follow the treatability study. However, in thisproject, the Washington State Department of Ecology, as part of the approval of the Site Remediation Action Plan[g], requested a 4000-5000-ton pilot field test prior to full-scale remediation for the purposeof verifying the treatability data and the effectiveness of the treatment under actualfield conditions. It was necessary to first construct the previously mentioned RCRA tank before the test could begin. The mobile treatment system was then erected in the completed tank. This system consistedof the feed hopper, mixing unit, chemical delivery system, and associated feed and discharge conveyors. In this project, the curing and subsequent stockpiling of the treated material occurred in the tank. Approximately 4500.tons of soil was treated at commercial processing rates of 80-100 tonskr during the 5-day pilottest; Material requiring treatmentwas excavated to the water table (about 12 ft below the surface) using a standard backhoe machine. Prior to entering the treatment unit, the excavated soil was screened to a 314 in. particle size through a two-step process using grizzly bars and a trommel screen. Stockpilesof the screened material were sampled and evaluatedfor heavy metals concentrations before treatment. Composite samples of the material were also evaluated before and after treatment. Control of the treatment protocol was accomplished through a mass balance, which entails continuous measurement of the rate at which material and reagentsenter the treatment unit. The necessary instrumentation included a certified belt scale on the material in-feed conveyor, calibrated and computer-controlled rotary feeders for dry reagents, and in-line flow meters and calibrated delivery pumps for liquids. 36,000 tons of material After the successful completion of the pilot test program, the remaining was processed and subsequently backfilled into the excavation. 1. TreatmentLevels In this particular soil, as in Case I, a comparison of the total (TTLC) and soluble concentrations (TCLP) for arsenic, copper, and zinc (Figure 4) illustrates that the concentrationsof heavy metals can vary over several orders of magnitude throughout the site. Arsenic is the principal metal of concern; copper and zinc exhibitedthe next highest concentrations but did not exceed TCLP limits. The data shown in Figure 4 were obtained during the pilot test program. During the production phase, the evaluation of total concentration levelswas not included in the routine data collection. An analysis of the data in Figure 4 indicates that the degreeof solubility is a function of total concentration. In the case of arsenic, the soluble concentrations were about five times
Remediation Contaminated SolidsMetal of Heavy
457
Figure 4 Relationship between soluble and total concentration levels in untreated soil (Case Study 11).
greater at the highest TTLC levels. In other words, the TCLP levels range between 200 and lo00 times less than the TTLC levels as they decrease from lO,o00 to 100. Copper exhibits similar behavioreven though the ranges in TTLC and TCPL levels only span an order of magnitude. On the other hand, the levels of zinc are clustered such thatthe TCLP levelis about 100 times less than the TTLC concentration. Itis interesting to comparethis soil with that of Case I, which also exhibited concentration levelsof the aggregate or mixture of metals that spanned several orders of magnitude (Figure 3). However, unlike the arsenic in this soil, the metals in the Case I soil were grouped or clustered in relatively narrow ranges withinthe overall distribution. Also, another notable exceptionwas zinc, which was not clustered. In these particular arrangements (Figure 3), the average soluble concentrations (determined by the STLC procedure) were about 20 times less than the total concentration. Extraction procedures will affect the behavior of soluble concentrations in a mixture of metals. In terms of the above comparison, this would involve differences inthe leaching characteristics between the previously discussed TCLP procedure and the STLC extraction, which uses a sodium citrate buffer with a tenfold dilution factor. 2.TreatmentResults The treatment protocol used two cementitious materials, cement and lime, in the ratios of 20:3 or 15:5, with polysilicate additionsof approximately 0.5 gaton. An example of the ability of the process to reduce the soluble concentration is given for arsenic treatment (Figure 5) during the production phase.It is important to notethat in this case the treatment standard for arsenic was set at 1 ppmby the site owner instead of the regulated level of 5 ppm. The analytical evaluation protocol was established at 0.2ppm, so values belowthis level were not determined.
458
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Number of Samples
figure 5 Comparison of untreated and treated TCLP levels of arsenic (Case Study 11).
In this project, a daily production goalof 750 tons of treated material was established as being reasonable for the site conditions, which were constrained by the ability to deliver material to the process (i.e., excavation, screening, sampling,etc.), the cycle time for curing and stockpiling in the tank, and the subsequent backfilling. The ability of the system to meet the processing goals is illustrated in Figure 6 in terms of daily tonnage and daily average tonnage over the production phase of the project. At times it was not possible to meet the daily production schedules, the reasons being (1) a mechanical failure in the lime delivery system on days 3 and 4, (2) the unavailability of feed material on day 3 1, and (3) the reduced quantityof feed material at the end of the major portion of the project during days 37 and 38. An additional feature of this project concerned the contaminated water collected on-site in the process. The RCRA tank allowed liquids to be collected from the processing areas that included both the screening and treatment activities. Approximately 12,000 gal of water per day was generated froma combination of spraying for dust controland rain. An on-site systemwas implemented that allowed this water to be incorporated into the treatment process. The water from the collection system was introduced into the first of a series of four Baker tanks. This provided suitable capacity and time for the settling of suspended solids. The liquid from the fourth tank was then returned to the polysilicate-water blending tank in the chemical mixing portion of the treatment system. The concentration of arsenic in the liquid entering this tank, as determined by EPA methods SW-846,3010, and 6010 was on theorder of 4 mg/L. Blending this liquid with the process water at a ratio of 1:20 did not impact the treatment protocol. In addition, the project benefited from a significant savings in off-site disposal costs.
VI. ANCILLARY TREATMENT PERFORMANCE PARAMETERS In addition to the task of reducing metal solubility,other important featuresof this technology are considered. These include (1) the nature of hydration and curing; (2) the compactability of the treated material as related to on-site backfilling; (3) the effect of pre- and posttreatment particle sizes, which influence metal concentrations in dispersed material; and (4) the long-termviabilityof the treatment as measuredthroughmultipleextractionprocedures.
459
Remediation of Heavy Metal Contaminated Solids
10
15
20
Days Running
25
30
35
Figure 6 h e s s i n g system throughput performance (Case Study II).
A. HydrationandCuring As previously mentioned, curing the treated material is the final step in the process. In terms of the field application of the technology, this basically amounts to liberating the moisture in the treated material, i.e., allowing it to dry. However, this process has other fundamental and serious impacts on the efficacy of the technology in terms of(1) the proper hydration of cementitious materials, which is relatedto the transformation of ionic bonds to covalent bonds, and (2) the use of sampling procedures that ensure that the curing process has been completed before the material is subjected to TCLP or other extraction protocols. The following discussion considers the interrelationship between hydration, curing, and sampling, which were extensively evaluated for the soil treated in Case Study 11. The treatment protocol developed for the remediation of the arsenic-contaminated soilof Case Study I1 used cementnime ratios of either 20:3 or 15:5 on the basis of weight. The effect of these mixtures on the hydration characteristics of the treated material is illustrated in Figure 7 for a bench-scale test using an initial 500-g sample of soil having a moisture content 12%. of After the addition of reagents and water in the amounts of 85 and 100 mL, respectively, the mixture weight was about 700 g for both cement-lime combinations. Here water essentially behaves as a reaction catalyst. Based on prevailing accepted stoichiometric data, the amountof water bound to cement and lime is on the order of 13% and 30% of their weight, respectively. This would indicate that approximately 17-18 g of water would be retained in the matrix. In
460
Trezek
-
Time hours
Figure 7 Hydration characteristics of cement-lime mixtures.
other words, the dilution factor due to water would be approximately 3.5%. As expected, the mixture with the higher percentage of lime begins to initially lose its free water at a faster rate (Figure 7) due to the increased heat of hydration. After thisinitial period, a similar linear rate of water loss occurs in both mixtures over a 72-h period. During this time, the samples at 8-hrintervals were subjected to a 70°F, 65% relative humidity environment and were broken to expose new surface. Between 72 and 75 hr into the test period, heat lamps were used to accelerate the water loss. The samples were allowed to equilibrate to room conditions for 1 hr before final weighing. The data obtained fromthe above exerciseillustrate that (1) the majority of the free water used as a catalyst will be liberated and(2) the retentionof water in the matrix appearsto be less than that derived from conventional stoichiometry. However, more important in terms of operations, using artificial means such as heat lamps to accelerate the loss of free water does not alter the fundamentals of the treatment. In certain cases, a procedure of this nature, referred to as a lab cure, is used to expedite the turnaround time of obtaining analytical laboratory TCLP data. Operationally, this can allow naturally cured stockpiles to be backfilled at the time the analytical data are available. A comparison of the lab and field cure TCLP arsenic data (Figure 8 ) illustrates that the two methods yield the same results with proper field curing. It shouldbe noted that in Figure 8 , arsenic data are given for both the pilot test and the production phase where the detection limits were 0.01 and 0.2 mg/L, respectively.The acceptable TCLP levelof arsenic in the treated material was set at 1.0 mg/L. Basically, the elevated levels in the field data shown in Figure8 indicate that the material was prematurely sampled.Sufficient free water was still present; that is, the process shown in Figure 7 was not completed. The tendency of the treated material to form monoliths is another ramification of improper field curing. Stockpiled material must be turned, usually on a daily basis, to (1) enhance the rate of curing through accelerated heat and mass transfer to the environment and(2) mechanically overcome the tendency of cementitious mixtures to fuse or solidify into a continuous substance, which would negate the desirable feature of a friable matrix structure.
Remediation of Heavy Metal Contaminated Solids
461
Number of Samples
Figure 8 Comparison of laboratory and field curing on TCLP arsenic concentrations.
B. Compactability An important feature of the process is the ability to backfill or recompact the treated material into the excavation. Standard tests of treated material using ASTM Method D-l557 yield a to achieve this compaction of about 97%. The following procedure is typically used in the field compaction. Stockpiled cured material is loaded into a dump truck and moved to the excavation. Here the dumped pile is spread with a bulldozer into an 8-12-in. lift. The moisture content is adjusted according to the ASTM test data to be consistent with optimum compaction. A drum vibrating compactor is then usedto complete the placementof the lift. The volume expansion due to treatment is estimated to be on the order of 20%. Operationally, after the removal of the oversize fraction, the backfilled treated material occupies the volumeof the excavation. Several field observations indicate that the features of the backfilled treated material are suitable for construction. For example, backhoe digging tests on material in place for 28 days showed that it remained friable with sufficient structural stability for trenching; that is, sharp vertical cuts couldbe made. The equipment operators indicated that the backfilled material was similar to digging hardpan.
C. Effect of Particle Size An additional featureof the STS polysilicate technology is its ability to indirectly mitigate total metal concentrations through increased posttreatment particle The size.basic mechanism stems from the fact that the emissionrate for total respiration particulate matter(E,,) has significant E,, is illustrated through the work of Cowherd et al. [9] particle size sensitivity. This effect on and summarized in the California Site Mitigation Decision Tree F’rocess [lo]. The basic calculational methodology follows. 1.ParticleSizeDistribution The f m t step in the procedure involves the determination of the characteristic particle size. This is typically accomplishedby subjecting representative samples of pre- and posttreated ma-
462
c
Trezek
Size mm Figure 9 Size distribution of pre- andposttreated soil.
terial to a sieve analysis andarranging the results in terms of percent passing vs. size (Figure 9). It is interesting to note that the characteristics of the treated material (straight line on a semilog plot) are representative of a Rosin Rammler distribution, which will have a characteristic particle size (X,) corresponding to 63.2% passing. Consequently, for illustrative purposes, the value of X, for the particular treated material shown in Figure 9 would be given by a size of about 6.4 mm. The comparable size for the untreated material can thenbe taken as 0.8 mm. Thus, as illustrated in Figure 9, the agglomerative featuresof creating the metasilicate and cementitious matrix during the treatment process hasthe effect of shifting the size distribution of the treated materialto larger sizes. The increase inthe characteristic size is usually about an order of magnitude. Here this increase is a factor of 8. 2.ThresholdFriction Velocity As illustrated in the work of Cowherd [9], the threshold friction velocity (Uf) is logarithmically related to the so-called aggregate size distribution mode. Basically, this parameter is intended to be representative of the wind speed at ground level corresponding to the occurrence of soil erosion. In termsof the characteristic particle size, the valueof Uffor the 0.8-mm and 6.4-mm particles are 60 c d s e c and 140 cdsec, respectively. Thus, an eightfold increase in particle size slightly more than doubles the threshold friction velocity. 3. Roughness Height Roughness height is used to convert the value of U, at ground level to the wind speed at a typical 7-m-high weatherstation. Values of roughness height(Z,) range from0.1 cm for natural snow to IO00 cm for high-rise buildings. Similarly,plowed fields and grasslands are given by Z, values of 1.O cm and 2.0-4.0 cm, respectively. A Z, value of 1 cm is typically used for soil being remediated.
Remediation Contaminated Solids Metal of Heavy
463
4.Threshold WindVelocity The threshold wind velocity (Ur),i.e., the wind speed necessary to initiate soil erosion as determined by 7-m-high weather station data, is given in terms of the value of Ufaccording to a relation developed by Cowherd,
Ut = U, (13.1 - 2.5 In G) The corresponding values of U,for the 0.8- and 6.4-mm particles are 7.86 and 18.34 m/ sec, respectively.
5. Respirable Particulate EmissionRate The following relation developed by Cowherd was used to evaluatethe emission rate of respirable particle matter from erodible surfaces: El0
= 0.036 (1 - V ) ( I Y / U , )F(x) ~
where E,, is the emission rate for total respirable particular matter [g/(m 2.hr)]; V , the fraction of exposed contaminated area thatis vegetated (V = 0 for bare soil); and U,the mean annual wind speed (m/sec). The parameter F(x) is given by the relation
F(x) = 0.18(82
+ 12x)/8*
where x = 0.886
(UJU)
Assuming a nominal 10 mph wind speed (4.47 m/sec), the value of E,, for the 0.8-mm particle size is 5.15 X low3g/(m 2*hr).The corresponding value for the 6.4-mm particle is 7.3 X lo-* g/(m 2+r). Thus, an eightfold increasein particle size reduces the value of E,, by in threshold friction velocity due to increased 7 X lo4. Alternatively, relatively small increases particle size result in large decreases in respirable emission rates. The E,, behavior with wind speedfor 0.8-mm and 6.4-mm particles is illustrated in Figure 10. Each particle exhibits a characteristic steep rise in concentration at elevated wind speed. Basically, the larger particles can sustain higher wind speeds for a given concentration. For g/(m2-hr)at 10 mph. A similar example, the 0.8-mm particle has an E , , value of 5.15 X value of E,, for the 6.4-mm particle would be reached at a 23 mph wind speed. 6. DownwindConcentrations The effect of particle size on concentrations of specific constituents reachinga downwind receptor can be obtained from therelation
X = Q/n ayazU where X is the concentration of the constituent in ambient air (pg/m); Q is the emission rate (pg/sec); ayand a, are the standard deviationof horizontal and vertical dispersion, respectively (m);and U is wind speed (m/sec). The following assumptions are made to illustrate the effect of particle size on downwind concentrations for a working area of IO00 m' and a length of 100 m to the receptor. Using the data of lbrner in the Workbook of Atmospheric Dispersion Estimates[1l] and ClassC stability, the values of ayand a, are 13 and 7 m, respectively, fora 100-m distance. Thus, for a 10 mph ng/m3for the 0.8 and 6.4wind speed, the valuesof X become 1.12 X IO3 and 1S 8 X mm particles, respectively. Thus, an estimate of the downwind concentration of any specific
46.4
Trezek
A. 3
0
U C
1.OOE-(
_
c
6.4 mm PARTICLE
X I- o.amm PARTICLE _
i 38
1.00E-07
E10 (gms/sqm.hr)
, l .OOE-
1H 1 .OOE-0'
1.00E+00
Figure 10 Influence of wind speed on concentration. constituent can be calculated by multiplying the downwind concentration X by the mass fraction of that constituent. The results of this calculation are summarized in Table3 for the total 2 for metal system5 . Basically, these results follow the E,, metal concentrations given in Table effect such that the downwind concentrations become negligible for the larger particle size.
VII. MULTIPLEEXTRACTIONPROCEDURE The standard extraction tests (TCLPor California STLC) provide a one-time equilibrium solubility valueof the heavy metal compounds in the extraction fluid do butnot give an indication of the time-dependent stability of the residual matrix.The EPA multiple extraction procedure, designed for evaluating the leachabilityof materials exposedto so-called acid rain conditions, was used to test the stability of the polysilicate cementitious treatment protocol.
Table 3 Effect of Particle Size on Constituent Downwind Total Metal Concentrations for Metal System 5" Compound Arsenic Copper Lead Nickel Cadmium 0.364 Zinc ~
Concn, Mass fraction X
0.174 0.076
2.759 2.57 1.627 12.45 S5 0.47 0.048 0.325
~~
'Mean wind speed = IO mph.
Concn, 0.8 mm particle (ng/m3)
6.4 mm particle(ng/m3 X 10"~)
3.09 1 .S2 1.74 0.526 0.054
4.36
465
Remediation Contaminated Solids Metal of Heavy
STLC
Ar4 Arl Ar3 Ar2
Ar5 Ar7 Ar6
STLC - Acid Rain Extractions
Ar8
Ar9
Arlo
Figure 11 Effect of multiple extractions on soluble metal concentrations.
The evaluation consistedof an initial STLC extraction (as described in CCR title 26, Ch22a solution of 66261 Appendix 2) followed by 10 consecutive acid rain extractions (using 60/40 sulfurichitric acid with apH of 3 as described inSW846 Volume C, Ch. 6,Method 1320).The predominant metals in the sample being treated were copper, lead, and zinc with initial STLC concentrations of 22, 110, and 106 ppm, respectively. Thus, in this sample the summed value be 238 ppm. Following treatment, the summed for the STLC soluble metal concentration would 11.7 ppm. The results of the subsequent acid rain extractions for STLC concentration value was both the treated and untreated materials are shown in Figure 11. Basically, the stability of the treated material is illustratedby the fact that the equilibrium leachable value was reached following the second acid rain extraction and remained unchanged. On the other hand, the metalliccompoundsintheuntreatedmaterialcontinuedtoionize andleachfollowingeach additional extraction step.
VIII. CONCLUDING REMARKS The STS polysilicate technology provides an effective and relatively low cost method of remediating soils contaminated with heavy metals. In this system, the metals become partof a covalent bonded matrix created through the action of the silicates and cementitious materials. of severalorders of magnitude are Reductioninheavymetalsolubleconcentrationlevels achievable. In terms of site remediation, the processing equipment can be configured in the form of a mobile system capable of operatingat high throughput rates. When properly cured, 97% compaction. An orderthetreatedmaterial is friableandcan be backfilledwitha of-magnitude increase in the mean particle size of mated the material significantly reduces the
466
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downwind total metal concentrationsof potentially airborne material. Finally, multiple extraction procedures validate the long-term survival of treated material in an acid rain environment.
ACKNOWLEDGMENT I wish to express my appreciation to Ms. Nancy Wright for her patience and competent assis-
tance in preparing the manuscript.
1.
2. 3. 4. 5.
6. 7. 8. 9. 10.
11.
PhysicallChernical Processes,In'Ifezek, G . J., Polysilicate heavy metals mitigation technology, in novative Hazardous Waste Treatment Technol. Ser., Vol. 2, Technomic, Lancaster, Penna., 1987, pp.155-163. Trezek, G . J., Polysilicate treatment of heavy metals in soil, Fifth Annual Hazardous Materials Management Conf./West, November 1989. Trezek, G . J., Raphael, G . , Wilber, J. S., and VanPelt, R. E., Remediation of arsenic contaminated soil using polysilicates, HMCRI R&D '92 Conf., San Francisco, February 1992. Trezek, G . J., Heavy metal contaminated soil remediation at high throughput, Proc. Superfund '90, Washington, D.C., November 1990, pp 673-676. Landau Associates, Inc., Edmonds, Wash., Thompson-Isaacson Site Soil Stabilization Pilot Test Summary Report, Prepared for The Boeing Company, July 1991. Wicks, F! H., Bothell. Wash., Proposed Remedial Action at Isaacson Corporation Property, Seattle, Washington, Prepared for Isaacson Corporation, 1984. PropDames & Moore, Seattle, Wash., Report of Evaluation of Site Contamination, Isaacson Steel erty, Prepared for the Boeing Company, 1983. LandauAssociates,Inc.,Edmonds,Wash.,andParametrix, Inc.,Bellevue,Wash.,ThompsonIsaacson Site Soil Remedial Action Plan, Prepared for The Boeing Company, 1990. Cowherd, C. M., Muleski, G . E., Englehart, F! J., and Gillette. D. A. Rapid Assessment of Exposure to Particulate Emissions from Surface Contamination Sites, Midwest Research Institute, Kansas City, MO., 1984. California Department of Health Services, Toxic Substances Control Division,The California Site Mitigation Decision Tree Manual, Sacramento, Calif., 1986. 'lbmer. D. B., Workbook ofArtnospheric Dispersion Estimates, HEW, Washington, D.C., 1970.
21
Fluidized Bed Combustion for Waste Minimization: Emissions and Ash Related Issues
E. J. Anthony and F. Preto CANMET Ottawa, Canada
1. INTRODUCTION Fluidized bed combustion (FBC) is a versatile technology that can be used in burning a wide variety of fuels. Fuels may range from conventional premium fuels (coal, oil, and natural gas) to municipal solid wastes (MSW), refusederived fuels (RDF), coal rejects (up to 70% ash or more than 50% moisture), biomass and biomass wastes, sludges, pitches, and tars [l-51. Another advantage of FBC is that, due to lower combustion temperatures, NO, levels are considerably lower than for conventional combustion systems. Furthermore, SO2emissions can be reduced by using limestone as the bed material to capture sulfur (as CaSO,)[6-81. Paradoxically,despitethelowercombustiontemperatures(typically800-950°Ccompared to 1200-1400°C for conventional combustion), FBC also produces very low levels of polyaromatic hydrocarbons (PAH) and other volatile organic compounds(VOC), due to a number of factors. First, the residence times of gases and solids in the combustion zone are relatively long compared to conventional combustion; second, FBC systems provide excellent, bed turbulentmixing;finally, it appearsthat at FBCcombustiontemperatures,limestone particles act as a catalyst to destroy organics [9-131. In a study on both FBC and pulverized fuel boilers burning coal, it was concluded that “in general, the levels for the limited polychlorinated dibenzofurans and dioxins that have been detected in fly ashes from these power stations are several orders of magnitude lower than levels reported in municipal incinerator ashes” [Ill. The use of sorbents as bed material results in the generation of ashes with unique properties. The management, i.e., disposal or utilization, of these ashes must be taken into account as part of the overall waste minimization scheme. For high sulfur fuels as such coal, petroleum coke, and some pitches and tars, the ash produced has high calcium oxide content and be can associated with exothermic reactions, dimensional instabilityin waste disposal sites, and high pH leachate (typically pH 11-12). However, the alkaline nature of these ashes does confer the benefit that leaching of heavy metals from such residues in minimal [14,15]. 467
468
Anthony and Preto
CANMET (the research division of Energy, Mines and Resources Canada, the federal government department responsible for energy-related activities in Canada) has been investigating fluidized bed combustion for coal, industrial wastes, and biomass since 1975. Research using bench-scale and pilot-scale reactors throughout this period has shown that fluidized bed combustion appears to be an ideal technology for efficiently burning a wide variety of fuels and wastes while also achieving verylow emissions of pollutants. The only significant difficulties have been related to feeding nonconventional fuels using pilot-scale equipment. These difficulties are resolvable in full-scale units.
II. FLUIDIZEDBEDCOMBUSTIONPRIMER
A fluidized bed consist ofbed a of particles suspended above a grid (e.g., perforated plate) by a fluid (air) moving upward through holes in the grid. The velocity of the fluid must be sufis too ficient to suspend the particlesso that they move freely in the bed region. If the velocity low, the particles are immobile in a “fixed bed”; at velocities greater than the particle’s terminal velocity, the particles are blown completely out of the bed, i.e., elutriated. In the fluidized bed regime, particles remain in the bed (hence good residence time) yet move about freely (hence good mixing). These characteristics, i.e., good mixing and long residence time, are the primary reasons for the numerous benefitsof using fluidized beds. Fluidized bed combustion (FBC) is the technique whereby a fluidized bed of particles is used as a medium for the combustion process. The bed particles themselves are not usually consumed in the combustion process but may play important roles in emissions control, for example, limestone particles are used to capture sulfur dioxide (forming calcium sulfate, i.e., anhydrite). The fuel particles typically comprise a very small fraction of the bed, e.g., less 1% for burning coal. Good particle mixing results in nearly uniform bed temperatures. This together with high thermalinertia due to the considerable mass of particles in thebed allows for virtually isothermal reaction at the optimum operating temperature. This confers upon FBC a number of advantages over conventional combustion systems. Some of FBC’s advantages ca be summarized as follows.
Fuel particles are rapidly heated to stable combustion temperatures. Almost any fuel can be handled, including nonhomogeneous fuels withlow volatility or high moisture content. are minimal; washing, drying, and pulverization are genThe requirements for fuel preparation erally unnecessary. Judicious selection of bed material can result in the in-bed capture of gaseous pollutant sp Low combustion temperatures (typically800-950°C) ensure that the formation of nitrogen oxides from air nitrogen is negligible.
1. A blower suppliesair to the A schematic diagramof the FBC process is shown in Figure windbox, which then discharges it through the distributor plate into the bed. The particle bed is traversed by heat exchanger tubes to provide for heat extraction from the bed. Above the particle bed a “freeboard” region is providedto allow particles ejected fromthe bed to settle back and to allow gas-phase reactionsto proceed to greater completion.The cyclone separator removes all but the finest particles carried by the exhaust gases. The cyclone product can be either recycled to the bed for further reaction or removed from the system. The gases leaving the cyclone are passed through a fabric filter “baghouse” to remove fly ash. The type of FBC unit shown in Figure1 is generally referred toas a bubbling bed because bed through whichair in excessof that required for fluidization passesas there exists a distinct “bubbles.” Superfkial fluidizing velocities are typically 1-4 &sec. If the fluidizing velocity
. 469
Fluidized Bed Combustion
L
I
I
1
CYCLONE
+
STACK
I
BAGHOUSE
f
COMBUSTOR
OVERBED FUEL & S *O R B E N T
+
* *
"
COOLIN1 -. I3 WATER -.
UNDERBED FUEL & *BENT
AS RECYCL -
-
WINDBOX
?
V
CiYCLONE P'RODUCT
AIR
UNDERBED OVERBED BAGHOUSE PRODUCT PRODUCT PRODUCT
Figure 1 Schematic diagram of FBC process.
is increased substantially, i.e., to 4-8 d s e c , the bed particles become entrained and are carried out of the combustor. These particles can, however, be captured by a "hot" cyclone and returned to the bed. A hot particle circulation loop is thus established that allows particle reactions to continue throughout the complete cycle. This type of system offers some advantages for capture of pollutant species and combustionof low-reactivity fuelsdue to the considerably increased residence time of fine particles in the bed. The actionof this systemis referred to as circulating fluidized bed combustion (CFBC). This section is included only as a basic introduction toFBC.The literature on FBC is quite voluminous, a recommended source beingthe American Society of Mechanical Engineers' series of conferences, themost recent of which is the 12th International Conference onFBC held in San Diego in May 1993.
Anthony and Preto
470
111.
FBC FOR WASTE MINIMIZATION
Due to low organic emissions levels, FBC is a promising technology for disposal of organic material-laden wastes. Donlee Technologies (USA) has developed a CFBC boilerto incinerate infectious hospital wastes by cofiring with coal [16]. Based on pilot-scale testing, they report that PAH, dioxin, and furan emissions were typically well below those achieved with other typesofhospitalwasteincinerators. Furthermore, suppression ofHClofup to 50% was achieved. Organic emissions metboth the Pennsylvania Department of Environmental Resources and proposed California restrictions. FBC-based technology meeting all emission limits for PAH, dioxins, and furans has also been offered by Ogden Environmental Services (USA) for the successful destruction of a number of hazardous wastes, including PCB-contaminated soil [17,18]. FBC is being examined in Italy for the combustion of organic material-laden industrial, agricultural, and domestic wastes. Published results[l91 indicate emission levelsof dioxins and furans much lower than proposed European legislated limits.The potential of FBC technology as an inexpensive method for disposal of hazardous wastes is being examined in India by Sandoz (India) and Thermax [20,21]. Thermax also reports that there are now 200 FBC installations operating in India burninga wide rangeof fuels fromlow grade coals to agrowastes such as rice hulls [21]. In India, FBC is overtaking rotary kilnsas the preferred technology for the incineration of solid and liquid wastes [21]. The use of FBC (primarily bubbling bed technology) for incineration is strongly established in Japan [22,23]. FBC is routinely used to burn MSW, plastics, and tires among other materials (one supplier alone, EBARA Corp., has 40 FBC incinerators operating [23]). It has been reported that there are currently 113 FBC incinerators operating in Japan [24]. In Europe there are over 50 FBC incineration plants [25]. Studies have also shown that FBC can burn RDF and industrial waste plastics either directly or by cofiring with coal [26,27]. A particularly interesting design concept, the fast internally circulating bed [25], used FBC technology to achieve “100%” elimination of all pollutants. The principal disadvantageof the concept was that the payback periodin the Austrian contextwas inherently long, i.e., 15 years for a 10 MW(th) unit. In North America, the use of bubbling fluidized bed combustion is increasingly finding more acceptanceas a means for the production of energy from the combustion of used tires and municipal wastes [28]. In the UnitedStates, circulating fluidized bed combustion is being used for disposal of hazardous wastes [17,18]. In Canada, a 10 MW(e) revolving fluidized bed unit has recentlybeen built to cleanly burn residues fromtar ponds near Sydney,Nova Scotia [29]. Although the main thrust of this project is to clean up 700,000 tons of coke oven residues,the energy production is not unimportant. This unit is due to become fully operational in 1993. Cofiring involvesthe burning of a waste “fuel” in conjunction with a premium fuel(coal, gas, or oil). Cofiring is used to supplement a waste fuel that may not be able to sustain combustion on its own or as a topping fuel if insufficient waste fuel is available. Cofiring, also known as “smart-burn,” does not prevent recycling programs,as any reduction inthe quantity of waste (particularly MSW) can be made up with the premium fuel. Cofiring thus appears to offer a palatable solution to the considerable municipal waste disposal problems now being experienced across North America. Traditional resistance to incinerators is likely to be reduced since as little or asmuch MSW canbe burned or recycled as desired, based on local economic or legislative requirements. Recently, the idea of cofiring coal and MSW or RDF has begun to find favorin the United States [30]. This is especially true in California, where the use of such wastes allows positive returns (extension of the lifetime of disposal sites; reduction in waste site emissions of green-
471
Fluidized Table 1 BubblingFBCCombustion of Biomass paper husks waste basis Corn pellets Rice grass Alfa Dry
cobs
59.8 83.1 15.2 0.2 1.6 HHV (MJkg) 18. l Particle size 40-50 mm diam, 150mm-long cylinders Feed rate 40-80 (kg/hr) Fluidization 0.9-1.2.4 velocity (dsec) >98% Combustion efficiency 100-400 ppm CO % Volatiles % Fixed C % Sulfur % Ash
80.9 14.6
-
13.4
-
4.4 22.2 18.8 14.1 3-cm fragments 5 10-mmhusks
-
Wood pellets
-
-
-
-
l-cm pieces
2-3-cm pieces
70
80-90
-
-
61
20-70
2 1.4
0.4-2.2
1.3
96% >96%
>97%
>90%
1.9%
>200(up ppm
0.1-0.3%
0.6-1.0%
to 0.5 %)
60-150 NO, @Pm) 100-180
so, (PPm)
100-200
>450
155
-
50-150
-
60-80
-
house gases and other toxic products; and generation of energy in the form of heat, process stream, and/or electricity) while still meeting the strict state environmental guidelines[31]. Elliot [31] quotes a figure of 3.6-4.1 cents/kWh for cofiring MSW compared with approximately 5.5 cents/kWh for a new coal-burning plant.Furthermore, these figures do not include a credit for the extension of the lifetime of the waste disposal facility or reduced liability costs for storage of potentially hazardous materials such as used tires. The economics of cofiring are therefore very favorable, findings that have been reinforced by a recent detailed literature survey on application of FBC to MSW [32]. In Canada, a number of companies are evaluating this technology and CANMET is also actively supporting FBC technology for biomass combustion [33,34].
A. Firing of Waste Biomass CANMET has tested a number of waste biomass fuels in pilot-scale bubbling fluidized bed combustors (40cm X 4 cm units with a total height of 5 m) [35]. The fuels that have been burned are paper waste pellets, alfa grass, wood chips, corncobs, and rice husks. Proximate analyses of the fuels, where available, operating conditions, combustion performance, and emission levels are given in Table 1. The only serious problem associated with burning these fuels was feeding them into the bed. This was largely due to the pilot scale of the equipment; the alfa grass and wood chips had a tendency to “bridge” in the fuel hopper; the cyclone was overwhelmed by ash during the alfa grass trials. Both the alga grass and corncob exhibited significant particle carryover. Freeboard combustionwas found to be significantfor all biomassfuels, particularly if the bed temperature was low. For instance, for the paper pellet tests, when the bed temperatures
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were between 600 and 800”C, the freeboard temperatures were typically in the range of 10001100°C. However, if the bed temperature was increased to 700-95OoC, the freeboard temperatures dropped to 650-950°C. In the case of this particular feedstock there were also some problems associated with defluidization of the bed. This was theorized as being due to agglomeration, possibly caused by the chlorine contentof the fuel. Scanning electron microscopic examination of the bed material showed high levels of chlorine in the limestone particles. Rice husks proved to be an ideal fuel for pilot-scale trails as their small uniformsize and low compressibility allowed them to be easily screw-fed into the bed. Their combustion characteristics were excellent (Table 1). The principal finding of this work was that the primary challenge for FBC utilization of waste biomass fuels is that of feeding the fuel. Once this can be done successfully, complete combustion and low emissions of both SO, and NO, can be achieved. Reduction in CO emissions and particle carryover require only a longer residence time, i.e., a substantial freeboard. Combustion of wood waste (sawdust and hog fuel) has beencarried out at the University of British Columbia using a 150-mm internal diameter circulating fluidized bed reactor [33]. NO, emissions were found to be low (
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B. Cofiring of Pulp Mill Sludges The Canadian pulp and paper industry produces on the order of 2200 tonnes/day of dry sludge (>50%) from wastewater treatment operations. These materials typically contain high moisture and have low heating values. Approximately half of this material is incinerated, and the remainder is used as landfill. Increasing concerns about heavy metal contamination may make the landfill option less attractive in the future. A study has been directed by CANMET, using a 150-mm-diameter CFBC unit, to inves[34]. Eight trails were pertigate the cofiring of these sludges with high sulfur bituminous coal formed with coaYsludge mass ratios varying between 3:1 and 1 :3. Sludge moisture ranged form 50 to 60%. One test, burning only sludge, quickly demonstrated that the sludge alone could not sustain combustion. In general, combustion efficiencies of 93-98% were achieved (based on carbon conversion). Emissions were low; NO, varied from 100 to 200 ppm, and CO levels ranged from 30 to 700 ppm. The only significant problem was, once again, feeding due to the fibrous nature of the sludges. To echo an earlier conclusion: If feed problems can be resolved, there are no barriers to using FBC technologyfor minimization of pulp mill sludges. The economics of sludge combustion would probably be significantly improved by better dewatering techniques. In a joint project, CANMET and Air Products and Chemicals, Inc. (USA) have used a 40 cm X 40 cm FBC pilot plant [35] to study the combustion performance of deinking sludges cofired with propane. Again,once feed problemswere overcome, all of the combustion results were successful. Due to low sulfur content anda high degree of sulfur capture inherentin the sludge, SO, concentrations remained below 5 ppm. NO, and CO concentrations were easily maintained below 150 ppm and 100 ppm, respectively. HCl levels were typically less than 15 ppm. At low temperatures (<850°C), N,O concentrations of approximately 10 ppm were recorded, dropping to a few ppm at higher temperatures. A complete set of organic analyses showed that PAH, dioxins, andfurans were much lowerthan Canadian guidelinesfor municipal incinerators. The ash leachate characteristicsas measured by the Toxicity Characteristic Leaching Protocol (TCLP) were also well within limits.
W.
GASEOUSEMISSIONS
A. Nitrogen-BasedSpecies As indicated earlier, fluidized bed combustors operate at temperatures of 800-95OoC, several hundred degrees below temperatures for suspension or pulverized fuel fired boilers. At these lower temperatures, nitrogen inthe air is not fixed, and thereforethe majority of NO, (emitted primarily as NO rather than NO,) originates from fuel-bound nitrogen. Conversion of fuel nitrogen to NO, is typically in the 10-25% range. The bulk of the fuel nitrogenis either directly converted to N, or reduced to N, by reaction of NO, with the char present in the bed [42,43]. As a general rule, higher nitrogen content of the fuel leads to higher NO, emissions [42-441. The volatile content of a fuel can also affect NO, emissions, with NO, increasing with increasing volatile content. The following relationship has been found to be hold for numerous fuels [44,45]: Nitrogen conversion
a
Vol(%) X N(%)
where nitrogen conversion can be either the percentage of fuel nitrogen convertedto NO, or the actual NO, emission referenced toa constant oxygen level. Vol(%) is the percentage of volatiles in the fuel, and N(%) is the percentage of nitrogen in the fuel.
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As expected, higher temperatures leadto higher NO, emissions, with increases as high as 260 ppm being reported [42-441. Nevertheless, bubbling FBC systems can normally operate with NO, concentrations lower than 250 ppm. Circulating FBC systems can usually operate with NO, concentrations below 150 ppm due to an inherent staging capability in these units. Biomass fuels typically have significantly less fuel nitrogen (<0.3%) than coal (typically
in the 0.8-1.5% range). NO, emissions are therefore not usually a concern for these types of fuels. In cases whereNO, emissions are significant, NH, injection has been shown to bea very successful technique for achieving extremely low NO, levels [46,47]. Leckner and Karlsson[48] studied the effect of cofiring wood chips and sawdust with coal in an 8-MW(th) CFBC boiler. They concludedthat the conversion of fuel-bound nitrogen was higher for wood (about 20%) than for coal (
B. Halogen Species The principalCl-containing species producedin fluidized bed combustion of chlorinecontaining wastes is HC1 [57]. It has been frequently claimed that FBC systems with limestone additionhave the potential to capture halidesin general andchlorine in particular via the mechanism
Fluidized Bed Combustion CaO
+ 2 HX e CaX2 + H20
475
(1)
where X is a halogen such as F, Cl, or Br. However, work on CaCl,, CaBr, and PVC addition to both bubbling and circulating fluidized beds has shown that despite such claims, chlorine and bromine are not effectively captured by limestone sorbents [57,58]. Except for CaF,, the calcium halides tend to decompose under typical FBC conditions (800-950°C) and capture is negligible above 850°C. Experiments with additives containing Cl, Br, and I, either as the alkali metalsalts or in the form of volatile halogen compounds (halons), have shown that the presence of halogens has a pronounced effect on the CO concentration in the flue gases [59,60]. Massive addition of chlorine-containing feedstocksto a fluidized bed combustor indicates that the conversion of CO to CO, can be adversely affected [59-611. It has been suggested that these effects are due to suppression (to equilibrium levels)of radical populations in the FBC environment, thus inhibiting reaction of CO with the OH radical [57, 59, 60, 621: CO+OH=COz+H
(2)
Other undesirable effects have also been noted, in particular an increase in NO, formation. Anthony et al. [62] have speculated that this is due to modification of the limestone sorbent surface. Capture of chlorine in the baghouse is possible and has been reported by Itaya et al. [63], have who injected Ca(OH), into a bag filter to help control HCl emissions. Weinell et al. [M] also shown that the optimum temperatures for HCl capture with slaked lime [Ca(OH),] and CaC03 is in the range of 500-600°C or below 150°C with Ca(OH), in the presence of H20 (which they ascribe to the presence of a liquid product phase). Such capturein an FBC boiler or incinerator is thus somewhat limited unless conditions can be properly optimized. In fact, significant chlorine capture by limestone sorbents in an FBC environment is not even a particularly desirable phenomenon. CaCl, melts at about 782°C. Therefore, large excesses of such materials could lead to agglomeration of the bed material. Experience with CANMET and Queen’s University bubbling fluidized bed pilot plants has shown significant corrosion of metal internals, which can be traced to high levels of addition of PVC and CaC1,. Equally, the presence of high levelsof CaC1, in electrostatic precipitators (ESPs)or baghouses can be expectedto lead to problems. CaCl, is a potentially corrosive and deliquescent solid that can be expected to cause corrosion in an ESP and caking and “blinding” of baghouse filters. Although fluidized bed combustors have been showntobe able to burnmixed plastic wastes, PCB-contaminated soil, and other chlorine-contaminated wastes [l ,17,18,26,27], it seems likely that the use of some additional control technology suchas scrubbers and perhaps combustion modificationof the FBC system itself (by additionof a premium fuel such as natural gas) is required for firing wastes containing very high levelsof chlorine.
C. Organic Species In general, due to “low”-temperature operations, fluidized bed combustionhasnotbeen thought of as appropriate for the destruction of hazardous organic wastes. Infact, FBC systems types of wastes. As mentioned earlier, can perform extremely wellas incinerators for almost all FBC can perform welldue to very good mixing, long residencetimes (up to hours for solids in the combustor), and potential catalytic effects of bed particles. The proper combination of these features allows FBCto be very effective in destroying organic species [12,13]. There are probably more elusive mechanismsat work in the destruction of organic species by FBC. Classical “low”-temperature combustion such as occurs in “cool flames” involves the formation of oxygenated species such as alcohols, aldehydes, and organic acids.At higher
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temperatures (>725”C), H, 0, and OH radicals become responsible for chain branching, and this leads to a combustion process dominated by free radical processes [65]. High concentrations of H and OH radicals canbe expected to destroy organics such asPAH provided that the residence time and mixing (turbulence) are sufficient. The importanceof radical processes for incineration was reexamined by Lyon [66] via the oxidation kineticsof CHJCl, c6H6, and C6H,Cl (as representative of fuels thatare “difficult” to incinerate). The conclusion was that besides the familiar lower flammabilitylimits there is a second and “unrecognized” threshold for effective combustion “i.e., if a reaction is to occur with the high rate a superequilibrium free radical concentration can provide, then the initial concentration of the fuel must be sufficiently high to generate that superequilibrium concentration” [66]. Furthermore, improvements in toxic organic material destructionby a factor of up to 10,OOO achieved by the additionof a “clean” fuel were also reported. Similar results have been reported by Banaszak, Miller, and coworkers, who studied improvements in the destruction efficiencyof organic species inan electrically heated gas stream by the addition of a small “pilot” flame [67,68]. In these studies a temperature of approximately 1100 K was necessary for the chain-branching reaction H+02+OH+O
(3)
to be faster than the reaction H+O2M%H02+M which otherwise served as a sink for the free radicals. Radicals multiplied rapidly at the location where the gas streams mixed (i.e., waste gas and “pilot” flame), thus allowing attack of the organic “wastes” [69]. It is well known that in flame combustion there exists a significant excess of radicals (above the thermodynamically predicted concentrations) in the flame reaction zone [70]. It has been demonstrated that radical concentrationsin FBC systems are also likely to be present at above-equilibrium concentrations[57,59,60,62]. Thus, the low level oforganic emissions seen from FBC systems is not particularly surprising, and from the point of view of emissions of unburned hydrocarbons, FBC should be regarded as a high-temperature combustion technology Another particularly important class of pollutants comprises dioxins and furans. Griffin [71,72] speculated that the “Deacon reaction” is a source of elemental chlorine and hence dioxins inthe combustion environment. He further suggested that the presenceof sulfur at S/Cl ratios greater than 10 can completely suppress dioxin formationby removing the chlorine via the reaction
-
Cl2 + SO2 + H20 4 SO3 + 2 HCl
(5)
If this effect can be confirmed, then this might provide a new niche for high sulfur fuels for cofiring with industrial wastes-a novel method for dioxin and furan control.
V. ASH MANAGEMENT Fluidized bed combustion ashes can be classified into two distinct categories depending on whether the fuel sulfur content is high enough to require limestone (or dolomite) addition (to reduce SO2). If limestone addition is not required, then the property of the ashes is strongly dependent on the natureof the waste being combusted and the material added to maintain particle bed inventory (e.g., sand). In that case very little of a general nature canbe said about the ashes other than that FBC can reducethe massholume ratio of the waste being burnedby one
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or two orders of magnitude. Even if the ashesare hazardous and must be disposed of in hazardous waste disposal sites, the quantity of material is considerably reduced. FBC therefore offers not only a net saving in terms of overall waste disposal, but also potential for the recovery of energy as steam or power. One of the few exceptions to this is combustion of coal washery rejects, which can contain upto 70% or more of ash and for which any gains in net reduction of materials are minimal. The potential energy from waste gain is still significant enough to make thistype of project economically feasible.
A. SulfurCapture For coals of moderateor high sulfur content, petroleum coke, and any high sulfur waste such as pitch and tars from upgrading processes (all suitable FBC fuels [4,45]), sorbent (limestone or dolomite) must be added to reduce SO, emissions. The properties of the ashesare strongly affected by the presence of limestone. In an atmospheric fluidized bed combustor, limestone calcines and then reacts via the reactions CaCO3 e CaO
+ C02
(6)
and CaO
+ SO;! + 3:1 0 2 e Cas04
(7)
Both of these reactions are reversible. However, the calcination reaction (6) normally goes to near completion andat FBC temperatures canbe considered to be relatively rapid in comparison to the sulfation reaction (7) [7,8,73]. Typically, when the ashes from FBC are examined they are found to contain less than9%CaC03, and it is possible thatat least some of the carbonate may be due to recarbonization. On thermodynamic grounds, the sulfation reaction (7) should go to completion; however, pore plugging normally prevents conversion (i.e., sorbent utilization) from going beyond 50% (molar basis) [7,8]. Pore plugging occurs during sulfate formation because the molar volume of the CaS04 product is greater than that of the original bed combustor at CaCO,. Canada’sfirstindustrial-scale[22MW(e)]circulatingfluidized Chatham, New Brunswick is able to achieve sorbent utilization of up to 51% [74]. The best reported performance for a large bubblingbed facility is the Tennessee Valley Authority’s 20MW(e) unit with sorbent utilization up of to 45% [75]. In comparison,flue gas desulfurization technologies suchas wet anddry scrubbing exhibit calcium utilization efficiencies on the order of 85-92% [50]. As a result of below stoichiometric sorbent utilization, FBC systems produce a relatively large amount of waste solids. For example, Minto or coal Syncrude petroleum coke (both up to 8% sulfur) can produce 0.65 kg ofash for every kilogram of fuel fed, assuming a sorbent utilization of 4596, 5% CaC03 in the final ash, and 90% SO2 removal. The significant ash production quantities make it important that the ashbe well characterized and that disposal costs be as low as possible, or alternatively that suitable applications be found for the ash.
B. Ash Disposal Successful disposal of high-calcium FBC ashes is currently practiced throughout the world, are over resulting in an increasing body of experience in this domain. In the United States there 150 operating FBC units producing4.5 GW(e) [47] for which adequate ash management practices exist. Good overviews of ash disposal techniques are availablein the literature [76-781. In Canada, Environment Canada, the federal government agency charged with developing environmental standards, has developed interim guidelines for dealing with CFBC ashes [79].
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The disposal of FBC ash is complicated by the fact that such ashes show a tendency to react with moisture in the atmosphere, leadingto expansion anddeterioration of the integrity of the disposal site. Canadian experience witha number of test cells set up to study optimum hydration and compaction showed that the permeability of an optimally compacted test cell deteric d s e c within 2 yearsofplacement orated from cndsec (equivalent to a clayliner) to [80]. Work is proceeding to use a new patented hydration process (CERCHAR, France) to selectivelyconverttheCaO component of Ca(OH),withouthydratingCaSO, or forming aluminosulfates like ettringite or aluminosilicates [81,821. This process controls subsequent expansion and improvesthe behavior of the ashes for both disposal and utilization. Another technique for improvingthe behavior of ash is pelletization [83], which also offers the potential of producing a commercial grade aggregate fromthe ash. Canadian and Polish researchers (CANMET, Committee on Atlantic Coal, Environment Canada, Canadian Electrical Association, Cracow University of Technology, Academy of Mining and Metallurgy, Cracow, Poland) are jointly investigating ash disposal by mine backfill. This technique produces ash water dense slurries (AWDS) that can be pumped over a kilometer into abandoned mines.This technology is being used extensively in Poland for disposal of pulverized coal ashes. Such slurries have been shown to be able to set to a low strength material with permeabilities of about cmlsec [84]. If successfully applied to FBC ashes, this technique will become a useful adjunct to existing disposal technologies.
C. Ash Utilization As the scarcity and costsof landfill increase, utilizationof FBC ashes becomes moredesirable and even necessary. Given that disposal costs for FBC residues approach $20/ton, there is a major economic incentive to turn theashes into a salable product rather than a liability. In principle, there are many potential uses for the high calcium residues from FBC [85,86]: agriculturaluse,lime substitute in acidicwaste neutralization, wastestabilizationagentinlime/ pozzolan systems, asphaltic concrete, synthetic aggregate, low-strength backfill applications (with and without portland cement), and roller-compacted concrete. Agricultural use of the ash as a lime substitute represents an obvious method of taking advantage of the high CaO content and is already being applied [87]. Agricultural useis, however, limited by a number of factors. Obtaining the necessary license may be a lengthy and costly undertaking. The lime component may be only a third of the ashes by weight, and the ashes must therefore be used at twice the agricultural lime rate. Transportation costs for this lower value product are significant, and in practice it is seldom economic to transport these ashes beyond 150 km. Acid neutralization is a very promising if restricted use for FBC-derived residues. Petroleum coke, which often hasa very high sulfur content (leading to the use of considerable quantities of limestone for sulfur capture) and a low fuel ash component, is particularly suited to such applications. This, in fact, is the method of disposal employed for two coke-fired [l00 MW(e)] CFBC units operated by NISCO in Lake Charles, Louisiana [88]. Ultimately, construction material applications may represent the most promising methods for dealing with FBC-derived ashes. Unfortunately, some of the most technically promising applications such as asphaltic concretes use only small amounts of FBC-derived materials, and the economic benefits of such useare at best marginal [76,85]. FBC-derived residues have been to produce materials withcementitiousproperties, and shown to beable to react with pozzolans there are already a number of demonstrations of the use of such residues: low-strength backfill applications, roadbase materials, mortars, masonry blocks, etc. [86]. Nevertheless, before
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such uses can become widespread, considerable effort will be required to better understand the cementitious reactions further, to reduce setting times, to improve the freeze-thaw behavior, and also to resolve problems causedby destructive expansion often seenin FBC-derived products. Furthermore, the variability in such residues is likely to delay their widespread use. Finally, obtaining market acceptance for such products remains a major obstacle that must be overcome before their use can become commonplace.
VI. SORBENTUTILIZATIONISSUES An alternative to attemptingto use large quantities of ashes fromfluidized bed combustion of high sulfur fuels is to improve the efficiency of the sulfur capture process. FBC sorbent utilization for sulfur capture is relatively low and can range from30 to 45% [7,8]. Given that both sorbent transportation costs and disposal costare often on the order of $20/ton, major savings can be realized if near-stoichiometric utilizations canbe achieved. For example, consider the 165-MW(e) CFBC unit being constructed in Point Aconi, Nova Scotia, which will burn a high sulfur coal. This unit is expected to generate up to 1000 tonnes of ash per day [89]. Nearstoichiometric utilization of the limestone could produce savingsof up to $20,000 a day. Research on reactivation or enhancement of sorbent performance has a long history. Argonne National Laboratories have used both halide salts and sodium carbonate addition [90]to enhance sorbent performance anda direct hydration technique using steamin a fluidized bed hydrator to reactivate spent sorbent for reinjection and reuse [91,92]. The first process works by surface modification of the limestone in the combustor allowing moderate sintering of the sorbent pores so that they become available for further reaction with SO,. In the hydration process, the spent sorbent is reactedwithwater,whichbreaks the sulfate shell that forms around spent sorbent particles, thus allowing unreacted limestone to be used on subsequent reinjection intothe combustor. In principle, hydration can be repeated several times until nearstoichiometric utilization of the sorbent is achieved. These methods have been duplicated by other researchers with similar success[60,93], however, the use of additives is not promising because of the corrosion and fouling problems likely to be experienced if such salts were introduced into a boiler. CANMET has also developed a technique for reactivation that involves fine grinding and pelletization of limestone to produce a sorbent with enhanced porosity[7,8]. Improvements in sulfur capture of up to a factor of 5 have been obtained. This work was aimed at bubbling fluidized beds; the advent of circulating fluidized beds has raised concerns about the physical strength of such “artificial” sorbents in the more vigorous CFBC environment. European research intothe use of synthetic reusable sorbents [94] hasbeen somewhat successful; however, the approach does not appear to be economically viable. Currently, itwould appear that thereare two promising approaches to improve sorbent performance: an improved hydration technology(e.g., CERCHAR process), which delivers a dry product with complete hydrationof the CaO componentof the ash, and injectionof spent sorbent in a slurry form either into the combustor or into the back end of the boiler. These processes are different from the hydration techniquesin that they rely on reactions with the fuel ash componentsto form high-surface-area silicate and aluminosilicate products,which are then capable of secondary reaction withSOz. The most noticeable of these processes is perhaps the proprietary (ABB-CE) ADVACATE process [95]. These types of processes are of limited values for use with petroleum cokeor pitches or tars that have very low ashes, in that these fuels do not provide the required silica products in the fuel ash. For these types of fuels, hydration process may well offer the best hope for ash reactivation.
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A Canadian study [96] on CFBC bottomash from Elmtree limestone found that the degree of hydration of the spend ashes was directly proportional to the degree of reutilization of the sorbent. The results were correlated by Z Utilization = 0.23
X
(% degree of hydration)
+ 38
(8)
This suggests that a process that achieves effectively complete hydration (e.g., CERCHAR process [82]) may be particularly suitable for reactivation. The highest CaO contents are typically foundin bed ashes; therefore,a promising strategy may be to take the bed ashes, hydrate them for reactivation, and reinject them. This would also ensure that only one ash stream is produced for disposal.
VII. CONCLUSIONS Research on bench-scale and pilot-scale reactors throughout the last two decades has shown that fluidized bed combustion appearsto be an ideal technology for efficiently burninga wide variety of fuels and wastes whilealso achieving very low emissions of pollutants. The only significant difficulties havebeenrelated to feedingnonconventionalfuelsusingpilot-scale equipment. These difficulties are resolvable in full-scale units. A number of large-scale [>20 MW(e)] unitshave recently begun operating, successfully burninga wide variety of industrial and agricultural wastes. FBC cofiring is increasingly being adopted to burn a waste fuel that may not be able to sustain combustion onits own or as a topping fuelif insufficient waste fuelis available. Cofiring does not discourage recycling programs,as any reduction in the quantity of waste (particfuel. Cofiring thus appears to offer a palatable ularly MSW) can be made up with the premium solution to the considerable municipal waste disposal problems now being experienced across North America. In terms of emissions, the application of FBC for waste minimizationis characterized by the following statements: NO, levels are much lower thanfor conventional systems, due primarily to lower combustion temperatures. SO, levels can be reduced by using limestone or dolomite as bed material for in situ sulfur capture. FBC’s turbulent mixing and increased residence times result in excellent organic compound destruction, and FBCshould therefore be regarded as a high-temperature combustiontechnology. N,O concentrationsfor fluidized bed combustion of biomass fuels are typically below 10 ppm. Although fluidized bed combustors have been shown to be able to burn mixed plastic wastes, PCB-contaminated soil, and other chlorine-contaminated wastes, it seems likely that the use of some additionalcontrol technology is required forfiring wastes containing very high levels of chlorine. must be addressed in anyconsideration of comFluidized bed combustion ash management bustion of waste fuels. In some cases ashes canbe used as agricultural or construction material additives, thus further minimizing the amount of waste that must be discarded. Even if the ashes cannot be used, FBC offersa considerable reduction in the mass of material over thatof candidate for combining energy recovery the waste fuel. FBC’s fuel versatility makes it an ideal and waste minimization.
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REFERENCES V. V., Preliminary Report on Combustion Trials of a 1. Anthony, E. J., Desai, D. L., and Razbin. Pelletized Paper Product in a Pilot-Scale Fluidized Bed Combustor, ERL Division Rep. ERPIERL 84-02(Int), August 1984. 2. Gendreau, R. J., and Raymond, D. L., An assessment of waste fuel burning in operating circulating fluidized bed boilers, AIChE Natl. Meeting, New Orleans, La., April 6-10, 1986. 3. Preto, F., Anthony, E. J., Desai, D. L., and Friedrich, F. D., Combustion of rice hulls in a pilotscale FBC, Ninth Int. Conf. FBC Combustion, Boston, Mass., May 3-7, 1987. 4. Zhang, J. Q., Anthony, E. J., Desai, D. L., and Friedrich, F. D., Fluidized Bed Combustion of Heavy Liquid Fuels, ERL Division Rep. ERL 87-22 (CF), March 1988. 5. Daw, C. S., Chandran, R. R., Duqum, J. N., Perna, M. A., and Petril, E. M., FBC engineering correlations for estimating the combustion efficiency of a range of fuels, Tenth Int. Conf. on FBC, San Francisco, Calif., April 30-May 3, 1989. 6. Whaley, H., Anthony, E. J., and Lee, G. K., Control of acid rain precursors, in Emissionsfrom Combustion Processes (R. Clement and R. Kagel, eds.), Lewis, Chelsea, Mich., 1990, Chapter 21. 7. Hamer, C. A., Methods to Improve Limestone Utilization in Fluidized Beds, MSL Division Rep. 84-77(IR), July 1984. 8. Hamer, C. A., Evaluation of SO2 Sorbent Utilization in Fluidized Beds, CANMET Rep. 86-9E, December 1986. 9. Chess, E. K., Later, D. W., Wilson, B. W., Harris, W. K., and Remensen, J. F., Chemical and Toxicological Characterization of Organic Constituents in Fluidized-Bed and Pulverized Coal Combustion: A Topical Report, DOE Rep. DE-AC06-76W 1830, April 1984. 10. Cianciarelli, D., Characterization of Semi-volatile Organic Emissions from the Chatham 20 MW Circulating Fluidized Bed Demonstration Unit, Environment Canada draft report, January 1989. 11. Zenon,Characterization of OrganicContaminantsinAshSamplesfromPulverizedCoal-Fired Power Generation Stations, Environment Canada Rep. EPS 3/PG/9, February 1987. 12. Vural, H., Walsh, F!, Samfim, A. F., and Beer, J., Destruction of tar during oxidation and nonoxCombust. Sci. Technol., 63, 229-246 idativepyrolysisofbituminouscoalinafluidizedbed, (1989). 13. Ellig, D. L., Lai, C. K., Mead, D. W., Longwell, J. I?, and Peters, W. A., Pyrolysis of volatile Ind. Eng. Chem. ProcessDearomatic hydrocarbons and n-heptane over calcium oxide and quartz, sign Dev., 24, 1080-1087 (1985). 14. Anthony, E. J., Doiron, C. C., Kissel, R. K., and Ross, G. G., Properties and Disposal Characteristics of Solid Wastes from Circulating Fluidized Bed Combustionof High Sulphur Coal, ERL Division Rep. ERPIERL 88-67(J), 1988. 15. Anthony,E.J.,Ross,G.G.,Berry, E. E., Hemings,R.T.,Kissel,R.K.,andDoiron,C.C., Characterization of Solid Wastes from Circulating Fluidized Bed Combustion, ERL Division Rep. ERL 89-07(0PJ), 1989. 16. Colthard, E. J., Korenberg, J., and Oswald, K., Co-fiiing coal and hospital wastes in a circulating fluidized bed boiler, Eleventh Int. Conf. on FBC, Montreal, Quebec, April 21-24, 1991. 17. Yip, H. H., and Diot, H. R., Circulatingbed incineration, Ogden Environmental Services, San Diego, Hazardous Materials and Management Conf. and Exhibition of Canada, 1987. 18. Diot, H. R., and Young, D. T., Fluidized bed PCB incineration in Alaska,Remediution, 1(2), 199209 (1991). 19. Brunetti, N., De Cecco, C., and Zuccotti, S., Fluidized-bed incineration of solid wastes and sludges: a viable technology for energy and the environment, Int. Symp. Environ. Contamination in Central and Eastern Europe, Budapest '92, Budapest, Hungary, Oct. 12-16, 1992. 20. Sethumadhaven, R., Vasudeven, R., Sahasrabudhe, D. K., and Biswas, Incineration of industrial toxic wastes:an efficient method of waste disposal, Eleventh Int. Conf. on FBC, Montreal, Quebec, April 21-24, 1991. 21. Sen, F! K., and Joshi, N. D., Fluidized bed combustion-Indian Experience, Inst. Energy's Fifth Int. Fluidized Combustion Conf., London, UK, 10-11, 1991.
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22. Minoura, T., Sakamoto, Y.,andToyama, S., Simulation of fluidized bed refuse of incineration, Tenth Int. Conf. on FBC Combustion, San Francisco, Calif., April 30-May 3, 1989. 23. EBARA. State-of-the-Art Incineration Overcomes Ash Problems, pamphlet EBARA Corp., 1990. 24. Howe, W. C., and McGowin, C. R., Fluidized bed combustion of alternate fuels: pilot and commercial plant experience, Eleventh Int. Conf. on FBC, Montreal, Quebec, April 21-24, 1991. 25. Ganster, G., Waste incineration based on the FICB process, IEA-AFBC Tech. Meeting, Palo Alto, April 30, 1988. 26. Kiorboe, L., Fluidized bed combustion of industrial waste plastic, IEA-AFBC Tech. Meeting, Tokyo, Japan, Oct. 20. 1987. bed in-bed feeding device for solid wastes, IEA-AFBC 27. Kiorboe, L., Development of a fluidized Tech. Meeting, Palo-Alto, Calif., April 30, 1988. 28. Pope, K. M., Tires and Municipal Wastes to Energy in a Fluidized Bed Combustion System, EPI pamphlet,1990. 29.Boraston, G . W., Revolvingfluidizedbedtechnologyforthetreatmentofhazardousmaterials, CANMET Conf. on Energy and the Environment, Toronto, Mar. 26-27, 1991. 30. McCarthy, F? D., and Colville, E. E., Repowering of the Tacoma steam plant fluidized bed combustor fired on RDF, wood, and coal, Eleventh Int. Conf. on FBC, Montreal, Quebec, April 21-24, 1991. 31. Elliot, T., Combine recycling, “smart-burn” programs to control waste disposal, Power, October 1990. 32. Legros, R., and Rognon, S., Valorisation des Wchets-Revue dans le Domaine de la Combustion des Wchets Solides en Lit Fluidise, Ecole Polytechnique report to CANMET, ProjectNo. C.D.T. P1609. March1992. 33. Grace, J. R., and Lim, C.J., Circulating Fluidized Bed Combustion of Coal, Woodwaste and Pitch, Final report prepared for Energy, Mines and Resources Canada, contrast 24ST.23440-6-9007, December 1987. 34. Salib, F!, Evaluation of Circulating Fluidized Bed Combustion of Pulp Mill Sludges, Final report to CANMET, Energy, Mines and Resources, contract No. 23216-8-9056/01-SZ, October 1991. 35. Anthony, E. J., Desai, D. L., Friedrich, F. D., and Smith, D., Description of the Mark I1 Atmospheric Fluidized Bed Combustor at the Combustion and Carbonization Research Laboratory, ERL Div. Rep. ERP/ERL 86-45(TR), August 1986. 36. Gibbs, D. R.. Babcock & Wilcox, Fluid Bed Boiler List, June 1992. 37. Belin, F., James, D. E., Walker, D. J., and Warrick, R. J.. Waste wood combustion in circulating fluidized bed boilers, Second Int. Conf. on Circulating Fluidized Beds, Compeigne, France, Mar. 14-18,1988. 38. Dryden, R., Clean coal combustion in California, Institute of Energy’s Fifth Inter. Fluidized Combustion Conf., London. UK, 10-11,1991. 39. Brooks, M., Ahlstrom Pyropower, Pyropower Pyroflow Reference List, June 1992. 40. Hansen, J. L., Agricultural waste fired fluid bed combustor, Delano, California, Eleventh Int. Conf. on FBC, Montreal, Quebec, Apr. 21-24, 1991. 41. Astrom, L., and Vayda, S . . The combustion of sludges and other low grade waste fuels, Inst. Energy’s Fifth Int. Fluidized Combustion Conference, London. UK, 10-11,1991. 42. Zhang, J. Q., and Jones, W. E., Evaluation of SO, and NO, Emissions in Fluidized Bed Combustion,FinalreportforCANMET,EnergyMinesandResources,undercontract007SQ23440-8-9213, August 1990. SO, andNO, 43. Zhang, J. Q., and Jones, W. E., AnExtensionoftheStudyonFactorsAffecting Emissions in Fluidized Bed Combustion, Final report for CANMET, Energy, Mines and Resources, under contract 007SQ-23440-8-9213, December 1990. 44. Brereton, C., Grace, J. R., Lim, C. J., Zhu, J., Legros, R., Muir, J. R., Zhao, J., Senior, R. C., Luckos, A., Inumura, N.. Zhang, J., and Hwang. I., Environmental Aspects, Control and Scaleup of Circulating Fluidized Bed Combustion for Applicationin Western Canada, Final report, prepared for Energy, Mines and Resources Canada, under contract 55SS 23440-8-9243,December 1991.
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45. Grace, J. R., Brereton, C. M. H., Lim, C. J., L.egros. R., Zhao, J., Senior, R. C., Wu, R. L., Muir, J. R., and Engman, R., Circulating Fluidized Bed Combustion of Western Canadian Fuels, 5288.23440FinalreportpreparedforEnergy,MinesandResources,Canada,undercontract 7-9136,August 1989. 46. Kurjatko, I., and Placer,F., Inferential Smith predictor for fluidized bed boiler control, Eleventh Int. Conf. on FBC, Montreal, Quebec, Apr. 21-24, 1991. 47. Friedman, M. A., Divilio, R. J., and Brown, R. A., Environmental performance of atmospheric FBC’s, Application of Fluidized-Bed Combustion for Power Generation Utility Conf., sponsored by EPRI, Cambridge, Mass., Sept. 23-25, 1992. 48. Leckner, B., and Karlsson, M., Emissions from Combustion of Wood in a Circulating Fluidized Bed Boiler, Chalmers Univ. Technol., Sweden, Rep. A 92-200,May 1992. 49. Hulgaard, T., Nitrous oxide from combustion, Doctoral Thesis, Technical Univ. Denmark, May 1991. 50. Ram, K., Environment Canada, Private communication, November 1992. 51. Preto,F.,andAnthony,E.J.,Assessment of Nitrous Oxide Formation in Power Plant Plumes, PhaseICANMETReportofJointEnvironmentCanadaKANMETresearchprogram,October 1991. 52. Gustavsson, L.,and Leckner, B., N,O reduction with gas injection in circulating fluidized bed boilers, Eleventh Int. Conf. on FBC, Montreal, Quebec, Apr. 21-24, 1991. ., O emissions from CFB boilers: experi53. Hiltunen, M., Kilpinen, F!, Hupa, M., and Lee, Y. YN, mental results and chemical interpretation, Eleventh Int. Conf. on FBC, Montreal, Quebec, Apr. 21-24. 1991. 54. Brown, R., and Muzzio, L., N,O emissions from fluidizedbed combustion, Eleventh Int. Conf. on FBC, Montreal, Quebec, Apr. 21-24, 1991. O., Theinfluenceofsomeoxideandsulphate 55. Miettinen,H.,Stromberg,D.,andLindquist, 21-24, surfaceson N,O decomposition,EleventhInt.Conf.onFBC,Montreal,Quebec,Apr. 1991. 56. Leckner, B., Karlsson. M., Mjornell, M., and Hagman, U., Emissions from 1a65 MW, circulating fluidized bed boiler, J. Inst. Energy, 65(464), 122-130 (1992). 57. Liang, D. T., Anthony, E. J., Loewen, B. K., and Yates, D. J., Halogen capture by limestone during fluidized bed combustion,Proc. EleventhInt. Conf Fluidized Bed Combustion,Montreal, Quebec, 2, 917-922 (1991). 58. Becker, H. A., Code, R. K., Gogolek, F! E., and Poirier, D. J., A Study of Fluidized Dynamics in Bubbling Fluidized Bed Combustion, part of the Federal Panel on Energy R&D (PERD) Tech. Rep. QFBC.TR91.2, 1991, prepared for CANMET. 59. Bulewicz, E. M., Janicka, E., and Kandefer, S., Halogen inhibition of CO oxidation during the combustion of coal in a fluidized bed, Proc. Tenth Int. Conf. on Fluidized Bed Combustion. San Francisco, 1989,pp. 163-168. of NaClonflue-gasdesulphurisationby 60. Bulewicz,E.M.,andJanicka,E.,Catalyticeffect limestone-based sorbents during the FB combustion of coal, J. Inst. Energy, Sept., 124-130 (1990). 61. Bloomer, J. J., and Miller, D., Characteristicsof methyl chloride/propane combustion in a fluidized 21-24, 1991. bed combustor, Eleventh Int. Conf. FBC, Montreal, Quebec, Apr. 62. Anthony, E. J., Bulewicz, E. M., and Preto, F., The effect of halogens on FBC systems, Welfth Int. Conf. FBC, San Diego, May 9-13, 1992. 63. Itaya, M., Yamahata, Y., Hasebe, Y., Harada, H., and Mikawa, Y.,Removal of hydrogen chloride gas from MSW fluidizedbed incinerator using bag filter, Int. Conf. Municipal Waste Combustion, Hollywood, Ha., 1989. 64. Weinell, C. E., Jensen, I? I., DamJohansen, K., and Livbjerg, H., Hydrogen chloride reaction with lime and limestone: kinetics and sorption capacity,Ind. Eng. Chem. Res., 31,164-171 (1992). Fossil Fuel Combustion-A Source Book 65. Haynes, B. S., Soot and hydrocarbons in combustion, in (W. Bartok and A. F. Sarofim. 4s.). Wiley, New York, 1991. 66. Lyon, R. K.,The existence of a second threshold for combustion and its implication to incineration, Proc. Twenty-third Symp. (hr.) Combustion, Orltans, France, 1990, pp. 902-908.
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67. Banaszak, T.,Miller, R., and Zembrzuski, M,,The influence of flame-generated free radicals on the thermal oxidation of waste gases, JAPC, 37, 1434-1438 (1987). 68. Banaszak, T., Miller, R., Rybak, W., and Zieba, A., Thermal-catalytic incineration of waste gases, First Int. Conf. Combustion Technologies for a Clean Environment, Vilamoura (Algarve), Portugal, Sept.3-6,1991. 69.Bulewicz,E. M., Cracow Univ. Technology, Poland, Private communication, November 1992. 70. Gaydon and Wolfhard, Flames: Their Structure and Radiution, Chapman and Hall, 1979. 71. Griffin, R. D., A new theory of dioxin formationin municipal solid waste combustion, 78th Annual Meeting of the Air Pollution Control Assoc., Detroit, MichJ, June 16-21, 1985. Chemo72. Griffin, R. D., A new theory of dioxin formation in municipal solid waste combustion, sphere, 15, 1987-1990 (1986). 73. Couturier, M. F., Sulphur Dioxide Removal in Fluidized Bed Combustors, Tech. Rep. QFBC.TR.86.1 to Energy, Mines and Resources, Canada, 1986. 74. W. H. Richards et al., ChathamCFB test bum of Nova Scotia coal and limestone, Tenth Int. Conf. Fluidized Bed Combustion, San Francisco, Calif., April 30-May 3, 1989, p. 887. 75. Bass, J. W., 111, Experience at 20 MW, Shawnee Power Plant, AFBC Development Project Tech. Papers, TVA/OP/ED and T-87/24, 1987. 76. Smith, I., ManagementofAFBCResidues, I E A CoalRes.Rep.IEACW21,February1990. L. E., FBC waste characterisation, utilization and disposal-recent 77. Berry, E. E., and Holcombe. experience, 1991 CANMET CFBC Ash Management Seminar (E. J. Anthony and F. Preto, eds.), Halifax, Nova Scotia, July 2-3, 1991. G., and Anthony, E. J., Experience with the management of CFBC resi78. Kissel, R. K., Ross,G. dues, 1989 Joint Power Generation Conf., Dallas, Tex., Oct. 22-26, 1989. 79. EPS, Interim Recommended Practises for the Management of Solid Residues h m the Circulating Fluidized Bed Combustion, Environment Canada Rep. EPS 1/PG/4, May 1992. 80. Georgiou, D. N., Kissel, R. K., and Ross, G. G., Geotechnical characteristics and landfilling of CFBC residues, Eleventh Int. Conf. on FBC, Montreal, Quebec, Apr. 21-24, 1991. Proceedings of the 1991 CANMET CFBC Ash Man81. Blondin, J., The CERCHAR hydration process, agement Seminar (E. J. Anthony and F. Preto, ed~.),1991. 82. Blondin, J., Anthony, E. J., and de Iribarne, A. F?,A new approach to hydration of FBC residues, Welfth Int. Conf. FBC, San Diego, May 9-13, 1993. 83. Bland, A. E,, Kissel, R. K., and Ross, G. G., Enhanced FBC waste management using pelletization, Eleventh Int. Conf. FBC, Montreal, Quebec, Apr. 21-24, 1991. 8 4 . Bulewicz, E. M., Zoldani, E., and Dudek, K., Site Evaluation of the Ash Water Dense Suspension Process, report to CANMET under contract 23440-1-9061/01-SQ, September 1991. 85. Berry, E. E., Anthony, E. J., and Kalmanovitch, D. F?,The uses and morphology of atmospheric fluidized bed combustion wastes from Canada's first industrial AFBC boilersJ. Energy Resources Techno!., 109(3). 148-154 (1987). 86. Bland, A. E., Kissel, R. K., and Ross, G. G., Utilization of CFBC ashes in roller compacted concrete applications, Eleventh Int. Conf. FBC, Montreal, Quebec, Apr. 21-24. 1991. Proceedings of the 87. Lewnard, J. J., Agricultural applications of CFBC ashes from western coals, 1991 CANMET CFBC Ash Management Seminar (E. J. Anthony and F. heto, eds.), 1991. 88.Zierold,D.,NISCOcogenerationfacility,ApplicationofFluidized-BedCombustionforPowet Generation Utility Conf., sponsored by EPRI, Cambridge, Mass., Sept. 23-25, 1992. 89.Gridley, N., andCousins,D.,NovaScotiaPowerCorporationPointAconigeneratingstation: CFBC residue management, Proceedings of the 1991 CANMET CFBC Ash Management Seminar (E. J. Anthony and F. Preto, eds.), 1991. 90. Johnson, I., Lenc, J. F., Shearer, J. A., Smith, G. W.,Swift,W.M.,Teats,F. G., "her, C. B., and Jonke, A. A, Support Studies in Fluidized-Bed Combustion, Argonne Natl. Lab. Quart. Rep., ANL/CEN/FE-79-8, April-June 1979. 91. Johnson, I., Myles, K. M., Fee, D. C., Lenc, J. F.,Moulton, D. S., Shearer, J. A., Smith, G . W., 'hmer, C. B., and Wilson,W. I., Exploratory and Basic Fluidized-Bed Combustion Studies, Semiann. Rep., ANUCENIFE-81-2, October 1980-March 1981.
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92. Shearer,J.A.,Smith, G. W., Moulton,D. S., Smyk,E.B.,Myles, K. M., Swift, W.M.,and Johnson, I., Hydration processfor reactivating spent limestone and dolomite sorbents for reuse in 9fluidized-bed coal combustion, Sixth Int. Conf. Fluidized Bed Combustion, Atlanta, Ga., Apr.
11,1980.
93. Razbin, V.V.,Anthony,E.J.,Desai,D.L.,andFriedrich.
F. D., FluidizedBedCombustionof High-Sulphur Maritime Coal, ERL Div. Rep. 85-44(0PJ), December 1984. 94. Wolff, E. H. P, Regenerative sulfur capture in fluidized bed combustionof coal: a fixed bed sorption study, Doctoral Thesis, Delft Univ., 1991. 95. Sedman, C. B., Maxwell, M. A., Jozewicz, W., and Chang, J. C. S., Commercial development of theADVACATEprocess for flue gas desulfurization, 25th Intersoc. Eng. Conf. Environmental Control, Reno, Aug. 16, 1990. %. Marquis, D. L., Couturier, M. F., and Steward, F. R., A Thermogravimetric Study on the Reactivation of Spent CFJ3 Limestone by Hydration, Report to Energy, Mines and Resources, CANMET, August1991.
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Part I11
WASTEWATER TREATMENT
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22
An Overview of Physical, Biological, and Chemical Processes for Wastewater Treatment Kanti L. Shah Ohio Northern University Ada, Ohio
1.
WATERPOLLUTANTS
Pollutants can originate from either a point source or a dispersed source. A point source is a channel, pipe, or any other confined source such as a pipe discharging wastewater treatment plant effluent or untreated wastewater intoa stream. A dispersed sourceis an unconfined area from which pollutantsenter a body of water. For example, surface runoff from agricultural and urban areas carrying such pollutantsas silt,fertilizers, animal wastes, pesticides, and oil drips do not enter at one particular point. These materials can enter a body of water as it flows through the area. Also, acidic runoff from mining areas is a dispersed pollutant. Water pollutants from both point and dispersed sources can be classified into groups of materials, based mainly on their environmental or health effects. The following list indicates common types of pollutants of concern: Pathogenic organisms Oxygen-demanding materials Plant nutrients Suspended solids and sediments Toxic chemicals and metals Radioactive substances Oil Thermal (heat) pollution Municipal and industrial wastewaters and runoff from farms and other open areas (streets, parks, lawns, etc.) are sources of the first five types of pollutants.
A. Pathogens Diseasecausing microorganisms are excreted in the feces of infected humansand animals and may be carried into the waters receiving the wastewater. 489
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B. Oxygen-Demanding Matter Oxygen is used in the aerobic stabilization of organic waste matter. Anything that can be oxidized in water with the consumption of dissolved molecular oxygen is termed an oxygendemanding, or aerobic substance. This materialis usually biodegradable organic matter, buta few inorganic compounds also oxidize in this manner. Oxygen is needed for bacterial oxidation of pollutants during biodegradation. This consumption of dissolved oxygen may exceed the capacity of surface waters to replenish the dissolved oxygen by natural means and pose a threat to higher forms of aquatic life that need oxygen to live. Thecritical level of dissolved oxygen (DO)' varies among species. Some game fish may require more than 3.5 mg DOIL.
C. PlantNutrients Nitrogen and phosphorus are two nutrients that are of concern in water pollution. Both contribute to the growth of algae and the eutrophication of lakes. Nitrogen occurs in many forms, e.g., organic nitrogen, ammonia, nitrate, and nitrite nitrogen. In wastewater or contaminated water, the main source of nitrogen and its complex compounds is human and animal feces and fertilizers. Excessive nitrates in drinking water can pose health problems. Phosphorus is also an essential nutrient that contributes to the growth of algae and other plant life in lakes. The sources of phosphorus in water include agricultural runoff containing fertilizers, animal wastes, and common detergents.
D. SuspendedSolidsandSediments Organic or inorganic particles that are carried by wastewater intoa receiving waterare termed suspended solids (SS). Colloidal particles in suspension cause the turbidity observed in many surface waters. Organic suspended solids also exert an oxygen demand. Inorganic suspended solids are dischargedby some industries, but their main source is soil erosion due to logging, stripmining, and construction activities.
E. ToxicChemicalsand Metals A wide variety of toxic organic and inorganic substancesare found in water in very small or trace amounts. Some of these substances are from natural sources, but many come from industrial and agricultural activities. Many of these contaminants can be dangerous to public health. Some heavy metals that are toxic are cadmium, chromium, lead, silver, and mercury. Arsenic, barium, and selenium are also poisonous inorganic elements. All these chemicals and metals must be monitored in drinking water.
E RadioactiveSubstances Radioactivity is widespread in surface water because of fallout from testing of nuclear weapons and wastewaters from nuclear power plants, hospitals using radioactive substances for treatments, university labs, and industrial users. Radioactivityin drinking water can be due to natural or artificial radionuclides. In a few water supplies, radioactivity can reachconcentrations that pose a risk of cancer. 'See Glossary for definition.
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G. Oil Oil pollution can enter the surface water and groundwater from various sources such as oil spills on the groundor surface waterand industrial activity. The majority of oil and oily wastes generated are not classified as hazardous. Nonhazardous oils and oily wastes are amenable to recovery.
H. Thermal Pollution Heat is considered to be a pollutant because of the adverse effect it canhave on oxygen levels and aquatic life in rivers and lakes. The cooling water used in industries and power plants carries away waste heat as it passes through the condensers in the plant. Cooling water temperature may increase up to 15°C after it serves to condense the steam. The discharge of warm water into surface waters is called thermal pollution.
II. WASTEMANAGEMENT Waste management is an all-encompassing term. It consists of several processes: the elimination or reduction of waste, the recycling or reuse of waste material, and the destruction or treatment of waste to render itharmless through chemical,physical,and/orbiological processes. Waste reduction or minimization can be incorporated into all aspects of the production process, recyclingor reuse occursafter a product hasbeen used anddiscarded, and waste treatment is essentially an addition to end-of-process, i.e., “end-of-pipe,” pollution control. Sections III-V discuss the state-of-the-art technology of wastewater treatment and disposal.
111.
PURPOSES OF WASTEWATER TREATMENT
Wastewater is collected in combined and separate sewers from industries, commercial establishments, and householdsand transported to the treatment plant. The plant effluentis usually disposed of by being discharged into rivers, lakes, or estuaries. See Figure 1. Wastewater typically contains many different substances, both suspended and dissolved, inorganic and organic. The total amount of organic material is related tothe “strength” of the wastewater. This is measuredby the biochemical oxygen demand (BOD).Another measurerelated to “strength” and “quality” of wastewater is the total amountof suspended solids (TSS). Another group of impurities of major significancein wastewater are plant nutrients, specifically compounds of nitrogen (N) and phosphorus (P). Oil and grease are also common pollutants. The amount of pathogens (disease-causing microorganisms) in wastewater is usually proportional to the concentration of fecal coliform bacteria. BOD,TSS, N, P, and coliform so many concentration are important parametersof water quality. Actually, wastewater contains different substances that itis impractical to identify each substanceor microorganism. The approximate composition of an average domestic wastewater is givenin Table 1. Wastewater is treatedto meet the minimum level of effluent quality setby the state or the U.S. Environmental Protection Agency (EPA). The purposes of wastewater treatment are to remove most of the suspended and dissolved organic and inorganic material and destroy pathogenic microorganisms. In many cases it is also necessary to remove plant nutrients-nitrogen and phosphorus. Some pollutants, both inorganic and organic, are resistant to conventional treatment processes. Such contaminants are called “refractory” contaminants. Phosphate, for example, is present inverylow concentrations in natural waters but is increasedinwastewatersfrom
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Combined wastewater
Domestic wastewater
Industrial wastes (usuallypretreated)
t
Disposal Wastewater Treatment Plant
I
Watersupplies Figure 1 Location of a municipal wastewater treatment plant.
Table 1 ApproximateComposition of an Average Domestic Wastewater (rng/L) ~
Component sedimentation sedimentation Before After Biologically treated Total solids Total volatile solids
Suspended solids 30 Volatile suspended solids BOD Ammonia nitrogen as N 30 Total nitrogen as N Soluble phosphorusas P Total phosphorus asP Source: Adopted with
800 440 240 180 200 15 35 7 10
I
permission from Viessman and Hammer (1993).
680
340 120 100 130
15 7 9
530 220 20 30 24 26
7 8
Physical, Processes Chemical Biological, and
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domestic sources due to the use of synthetic detergents and runoff from fertilized fields. Organic nitrogenous compounds decompose to ammonia and oxidize to nitrates during waste treatment. Certain high molecular weight materials (e.g., dyes and surfactants) are not biodegraded. Also, agricultural land runoff includes pesticides such as chlorinated hydrocarbons. Excessive concentrations of nutrients often lead to large growths of algae, which in turn become oxygen-demanding materials when they die and settle to the bottom. Conventional waste treatment is only 3 0 4 0 % effective in removing these nutrients.Advanced waste treatment methods (tertiary treatment) are required for greater nitrogen reduction and phosphorus removal.
W. TREATMENTPROCESSES Conventional wastewater treatment consists of preliminary, primary, and secondary m t m e n t processes. Primary treatment consists of pumping, screening, and sedimentation, which includes grit removaland primary settling. Screening, grit removal,and primary settling remove large and heavy solids and floatable materials. In secondary treatment, biological aeration is used to metabolize and flocculate colloidal and dissolved organics. Tertiarytreatment(advanced wastewater treatment), e.g., chemicalcoagulation,isan additional step applied only after the wastewater has undergone conventional primary and secondary treatment processes. Tertiary treatment is used to remove contaminants from wastewater that are usually not taken out by conventional techniques, for example, phosphorusand nitrogen. Figure 2 is a flow diagram showing the stages of wastewater treatment.
A. PreliminaryTreatment The first level of wastewater treatment includes the physical processes of screening and sedimentation, which remove large floating objects and settleable solids. These account for about 60% of the total suspended solids (TSS) and approximately 35%of the biological oxygen demand (BOD). Raw wastewater first undergoes coarse screening. Bar screens made of long narrow metal as large fish and other bulky objects bars spaced l in. (25 mm) apart retain floating debris such that could clog pipesor damage mechanical equipment. In large wastewater treatment plants, the bar screens are cleaned by an automatic mechanical device. A mechanical cutting device called a comminutor is installed after the coarse screens in some treatment plants. A comminutor consists of a slotted cylinder with a moving cutter blade that shreds and chops solids that pass through the bar screen. The shredded material is then removed from the wastewater in the primary settling tank (Figure 2). In small sewage treatment plants, a manually cleaned bar screen is generally installed in a channel nextto the comminutor to serve as a bypass during comminutor repair.
B. Primary Treatment Wastewater contains suspended matter that has a higher specific gravity than the liquid and is in a relatively quiescent state, the particles will settle out because of gravity. Grit chambers remove grit, including gravel, sand, and heavy particulate matter such as corn kernels, bone chips, coffee grounds, and broken glass. All of this accounts for only a portion of the inert suspended solids in raw sewage. After grit removal, the wastewater still contains suspended solids, which are then removed by sedimentation.
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Shah. Raw
wastewater
r
r Grit Chamber
Basin
Tertiary Treatment
Biological Treatment
Advanced Waste Treatment
+ I
*
ReceivineBody
Figure 2 Stages of wastewatertreatment. 1. Grit Removal For design purposes, grit is defined as 0.2-mm particles with a specific gravity of 2.65 and a settling velocity of 0.075 fps (0.0225 &sec). Grit removal in municipal waste treatment protects mechanical equipment and pumps from abnormal abrasive wear, prevents pipe clogging, and reduces accumulationsin the settling tanks. ' b o types of grit removal units that are used in wastewater treatment plants are the horizontal flow type and the aerated type.
Physical, Biological, and Chemical Processes
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Figure 3 Grit collector assembly with mechanical scraper.
Horizontal flow type. In sewers, the flow velocity is generally not less than 2.00 fps (0.6 dsec), which is “self-cleaning velocity.” Most of the gritty material settles out of the flow by gravity if the velocity is reduced to 1 fps (0.3 &sec). Therefore, to prevent scouring of the settled grit and other material, the horizontal velocity of the grit removal unit is controlled at approximately 1 fps (0.3 dsec). Figure 3 diagrams a chain-and-bucket grit collector. Aerated type. Aerated-type grit-removal units may be square or rectangular hopper-bottom tanks with the inlet and theeffluent weir on opposite sides of the tank.The shearing action of the air bubbles strips the inert grit of the organic material that adheres to its surface. These unitsare often designedto keep the organic solids in suspension whilethe grit settles out. Design of an aerated hopper-bottom grit tank is based on a detention time of approximately 1 min at peak hourly flow. Settled matter is removed from.the hopper bottom by gravity flow, pumping, screw conveyor,or bucket lift. The grit removed from thegrit chambers is high in organic content. In most plants, the grit is washed before disposal in the landfill. Washing can be done by a counterflow grit washer, which functions like a screw conveyor. The waste organics are returned to the plant influent. Another methodis to use a centrifugal pump to lift the slurry from the grit pit to a centrifugal cyclone, which separates the grit from the organic material and discharges it toa classifier for washing. The wash water with organics is returned to the wastewater. Primary settling tanks are used to remove the readily settleable solids preceding further treatment. With chemical treatment and flocculation, the primary settling tank (clarifier) can remove a significant amount of colloidal particles. Intermediate and final settling tanks are
496
lr
-
Q-
Inlet Zone
Shah Baffle
Effluent Weir
U
Outlet
Settling Zone
Zone
-- ---------SludgeZone
(4
Sludge Zone
(b) Figure 4 (a) Horizontal settling tank. (b) Upflow settling tank.
used to remove the settleable solids following biological treatment prucesses. These settling tanks can be either circular or rectangular and are designed to operate on a continuous flowthrough basis. Figure 4 is a sketch showing the four zones-inlet, settling, outlet, and sludge storage.The inlet zone distributes the flow evenly across the cross section of the settling zone. It consists of inlet pipes andbaffles. Wastewater enters at the center behind a stilling baffle and travels down and outwardtoward the effluentweir. The inletline usually terminates near the surface, butthe wastewater must travel down behind the baffle before it enters the actual settling zone. The baffle reduces the velocity and forces downward motionof the solids. Following the inlet zoneis the settling zone. A good inlet designis important for efficient removal of suspended solids in the settling zone. The flow velocity decreases in the settling zone, and suspended solids settle out and accumulate in the sludge zone. Theconfiguration and depth of the sludge storage zone depends upon the frequency of cleaning, method of cleaning, and estimates of sludge production. Mechanically cleaned tanks have bottom scrapers. The sludge is continuously scrapedto the hopper,from which it is pumped out.The tank bottom has a 1% slope in the direction of the sludge removal point. The sludge hopper is designed with sides at a vertical-to-horizontal slope of between 1.2: 1 and 2: 1. The function of the outlet zone is toremove the settled water from the basin without carrying particles with the effluent. The outlet zone should prevent short-circuitingand loss and destruction of floc. A proper outlet design can prevent scouring and thus loss of floc.
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In the design of a sedimentation tank, the settling velocity (V,) of the particles to be removed should be greater thanthe overflow rate (Vo).The determinationof the particle-settling velocity is different for differenttypes of particles. Settling properties of particles are characterized into the following classes. Type I sedimentation is characterized by particles that settle in a discrete manner at a constant velocity, suchas sand andgrit material. Particles do not come into contact with each other. I settling occurs in grit chambers. Type IZ sedimentation is characterized by particles that flocculate while settling. Since flocculation constantly changes the size of particles, the velocityof the particles is also changing. These types of particles are present in the settling tanks following tricklingfiltration. In Qpe IZZ sedimentation, the particles tend to settle as a mass, leading to a separation of a clear zone anda sludge zone. Zone settling occurs in the tank following an activated sludge unit and sludge thickeners.
C.SecondaryTreatment(Biological) Physical treatment of raw municipal wastewater by sedimentation removesmost of those pollutants that will either float or settle out by gravity, which accounts for only approximately 35% of the BOD. The major purpose of secondary treatment is to remove nonsettleable (colloidal and dissolved) solids in the wastewater. “Secondary treatment” is generally considered to mean at least 85% efficiency in reducing BOD and now represents the minimum degree of treatment required by law in most cases. Some of the plant nutrients are also removed. The removal of organics and nutrients helpsto protect the receiving watercourse. Secondary treatment processes are almost always biological systems. Biological treatment systems are living systems that rely on mixed biological cultures to break down waste organics and remove organic matter from the solution. A biological waste treatment system provides an artificial and controlled environment suitable for the growth of microorganisms that can stabilize the organic pollutants in the wastewater before it is discharged intothe surface waters. These living microorganisms, including bacteria and protozoa, consume the organic pollutantsas food. They metabolize the biodegradable organics, converting them into carbon dioxide, water, and energy. The primary use of energy is for synthesis. The maximum rate of synthesis occurs simultaneously with the maximum rate of energy yield (maximum rate of metabolism). Aerobic metabolism requires oxygen for the processes of metabolism and synthesis. In anaerobic metabolism, the metabolism and synthesis take place in of organic matter results from an incomplete the absence of oxygen.A low energy yield per unit reaction. When the supply of biologically available energyis exhausted, the processes of metabolism and synthesis cease. To keep the microorganisms productive in their taskof wastewater treatment, they require an ample supply of oxygen, suitable temperatures and ph, a nontoxic environment, and other favorable conditions. The design and operation of the secondary treatmentis based upon these factors. Several types of biologicaltreatmentsystems are stabilizationpond,oxidation ditch, biotowers, trickling filter, and activated sludge. The latter two are the most common. 1. TricklingFilters A trickling filter consists of a bed of coarse material, such as stones, slats, or plastic media, over which wastewater is applied. A widely used design is a bed of stones approximately5-7 ft (1 S-2.1 m) deep; it is usually circular and may be as large as 200 ft (60 m) in diameter. Figure 5 shows a trickling filter, and Figure 6 is a flow diagram of a trickling filter plant.
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498
Figure 5 A tricklingfilter.
Trickling filters are usually preceded by primary treatment to remove large and settleable solids. The wastewater is typically distributed over the surface of the rocks by a rotating arm of the trickling that has nozzles along its length and sprays wastewater evenly over the surface filter. The distributor arm is mounted on a center column in the trickling filter. It is driven around by the reaction force of the wastewater. The underdrain system serves to collect and carry the wastewater from the bottom of the bed and also permits air to circulate through the bed. The primary effluent is sprayed on a bed of crushed rock or other media coated with biological films. The biological slime layer consists of bacteria, protozoans, and fungi. Sludge Q
+
Q..
Q + QR
Wet well
Figure 6 How diagram of a trickling filter plant.
499
Physical, Processes Chemical Biological, and
worms, rotifers, filter-fly larvae, and other higher animals also grow in this environment. As the wastewater flows over the microbial film, the soluble organics are metabolized and the colloidal organicsare absorbed ontothe surface, thus removing organic substances from the wastewater. Air circulating through the void spaces in the bedof stones provides the oxygen for stabilization of the organics by the microorganisms. The rocks in the trickling filter are usually approximately 3 in. (75 mm) in size to provide a large surface area for the biological growths, and the large voids allow air circulation. Sometimes materials such as modules of corrugated plastic or redwood are used as the medium instead of rocks. As the microorganismsgrow and multiply, the slime layer thickens. Dueto its weight and the flushing action of flowing wastewater, the slime is washed off the rock surfaces. The trickling filter effluent is collected in the underdrainsystem, from where itflows to a sedimentation tank called a secondary or final clarifier. To maintain a uniform flow rate through the trickling filter and to keep the distributor arm rotating during periods of low wastewater flow,a portion of the effluent is pumped back to the trickling filter inlet. Recirculation alsoserves to improve the treatment efficiencyof the trickling filter; it allows a certain portion of the wastewater to make a second pass throughthe film of microbes on the rocks. Thereare many recirculation patterns and configurations of trickling filter plants, and these may be direct or indirect. Recircularion. The amount of recirculation can vary. It is characterized by a recirculation ratio, which is the ratio of recycled flow to raw wastewater flow, R =
QR/Q
where R is the recirculation ratio (dimensionless), QR is the recirculated flow rate (fi3/sec or m3/sec), and Q is the wastewater flow rate (ft3/sec or m3/sec). The recirculation ratio, R , is generally in the range of 0-3.0. Hydraulic Load and BOD Load. The rate at which the wastewater flow isapplied to the trickling filter surface is the hydraulic load. The rate at which organic material is applied to the trickling filter is called the organic or BOD load. The hydraulic load depends on the recirculated flow QR;the total flow through the trickling filter is equal to Q QR.
+
Hydraulic load =
Q + QR AS
where A , is the trickling filter surface areain square meters or square feet. Hydraulic load can be expressed in cubic feet per square foot per day, cubic meters per square meter per day, or millions of gallons per acre per day. The rate at which organic material is applied to the trickling filter is expressed as BOD load. It doesnot include the BOD of the material in the recirculated flow.The formula for BOD load can be expressed as
or Organic load
= 8340 X
Q X BOD
v
Ib/(lo00 @day)
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I
Figure 7 An activated sludge unit.
where BOD is the biological oxygen demand of the primary effluent in milligrams per liter or parts per million and V is the volume of the trickling filterbed in cubic meters or cubic feet.
Eflciency. The BOD reduction efficiency of a trickling filter unit depends on organic load, recirculation ratio, and temperature. Generally, the efficiency increases with increasing recirculation and temperature and decreasing organic load. 2.
ActivatedSludgeTreatment The activated sludgeprocess is a biological wastewater treatment technique in which a mixture of wastewater and microorganisms is agitated and aerated. Wastewater is fed continuously int an aerated tank, where the microorganisms metabolize and biologically flocculate the organics using oxygen provided in the compressed air. Figure 7 shows an activated sludge unit.
%Fits + oxygen + CO1 + Hz0 + Energy Aeration and mixingare achieved by continuously injecting compressed air into themixture through porous diffusers located at the bottom of the tank (Figure 8a). Sometimes mechanical devices such as propeller-type mixers located at the liquid surface are used (Figure 8b). The propeller blades mix air with the wastewater and keep the contents of the tank in suspension. The aerobic microorganisms in the tank grow and multiply, forming an active suspension of biological solids called activated sludge. The mixture of the activated sludge and the waste-
501
Physical, Biological, and Chemical Processes Mechanical Aerator
I k/-
7
Air suppl’ Line
Figure 8 An activated sludge unit achieves aeration and mixing (a) with diffused air Or (b) with a mechanical aerator.
water in the aeration tank is called mixed liquor. In most cases the aeration period is 6-9 hr. The biological solids are subsequently separated by gravity from the mixed liquor under quiescent conditions in the final clarifier. The clarified water near the surface (the supernatant) is discharged from the final clarifier overweir. a The settled sludge is pumped out from a sludge hopper at the tank bottom. A portion of the sludge is returned to the aeration so tank that active and acclimatized microorganisms can absorband metabolize organics more efficiently. Since the organisms grow and multiply greatly, it is not possible to pump all the sludge to the aeration tank. Therefore, excess sludge is diverted to the sludge handling unit for treatment and disposal. General loading and operational parameters for the activated sludge processes used thein are listed in Table2. The wide rangeof aeration periodsand treatment of municipal wastewater BOD loadings used in activated sludge processes differentiate the processes from each other. Also, the aeration tank‘s size and shape influence the process. An important indicator used in the design and operation of activated sludge systems is the food/microorganism ( F M ) ratio. The ‘ 6 f ~ o dor ” the BOD in the influent wastewater (without regard to return sludge) is expressed in pounds or kilograms per day of liquid volume in the aeration tank. The concentration of the suspended solids, which mainly consistof active microorganisms, is called themixed liquor suspended solids(MLSS). FIM can be computed from the formula Table 2 Loadings and Efficiencies of Activated Sludge Systems
Loading (Ib BOD/day Aeration Efficiency Process per High rate
Conventional Extended aeration
of
period (hr)
BOD removal (%)
1-2
4
80-85
0.2-0.5 0.05-0.2
6
Ib MLSS)
30
85-90 90-95
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Waste sludge Return sludge
" FF
Sewage Influent
Final
Effluent
Waste sludge Return sludge Screen k GritChamber
-r
Figure 9 Activatedsludgeplant;(a)conventional
I
I
(b) stepaeration.
BOD X Q
"F
= MLSS X V
where BOD is the applied 5-day BOD, in mg/L or ppm; Q is the wastewater flow rate, in mUday or mg/day; M U S is mixed liquor suspendedsolids, mg/L, and V is the volume of the aeration tank, mL. Types of Activated Sludge Processes. Several types of systems have been developed. Many of these serve to increase plant capacity or reduce the tank volume requirement. 1.
In the conventional activated-sludgeprocess, the aeration basin is a long rectangular tank with air diffusers alongone side of the tank bottom to provide aeration and mixing. Settled raw wastewater and returning activated sludge enter the headof the tank and flow down in a spiral flow pattern. The air supply is tapered to provide a greater amount of diffused air near the head where the rate of biological metabolism and consequently oxygen demand are the greatest (Figure 9).
503
Physical, Biological, and Chemical Processes Screen & GritChamber
-
c-
Sewage Influent
Return sludge A Pump
Screen & GritChamber
Aeration
Sewage Influent
Effluent
Waste sludge
Return sludge
1
f Influent
U W Effluent
(c)
Figure 10 Activatedsludgesystems;(a)contactstabilizationplant,(b)completelymixedactivated sludge plant, and (c) extended aeration plant.
2.
The step-aeration activated-sludgeprocess is a modification of the conventional process. It provides multiple feed points of the primary effluent into the aeration tank, unlike in the conventional process where flow enters only at the head end. Distributing the influent load along the tank produces a more uniform oxygen demand (see Figure10). 3. Extended aeration systems are generally small packaged plants used for treating low wastewater flow rates from hotels, schools, suburban residential development, and other iso-
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lated sources.In this process the screened wastewater flows directly into theaeration tank without any primary settling, and the detention time is 24 hr or greater. The system operates on a low fodmicroorganism ratio( F M ) . (See Table2.) The low FIM ratio and the “extended” period allow for the stabilization of most of the organics. Excess sludge is generally not wasted continuously, only periodically. Figure 1% is a flow diagram of an extended aeration system. 4. Table 2 indicates that the high-rate (completely mixed activated sludge)process operates with a high BOD load per unit volume of aeration tank and a short aeration period. Many such units use a combination of compressed-air aeration and mechanical mixing. Also, in large, completely mixed aeration basins a mechanical impeller is placed above an uptake tube for deep mixing along with surface aeration. (See Figure lob.) 5. The contact stabilization activated-sludgeprocess provides for reaeration of the returned activated sludge from the final clarifier, allowing the use of a smaller aeration tank and therefore relatively shorter aeration periods. After a short contact time, the mixed liquor enters a clarifier and the activated sludge settles out; the clarified liquid flows over effluent weirs. The settled sludge is pumped into another aerated tank for reaeration. This type of process is shown in Figure loa. 6. Air is approximately 2 1% oxygen. The major componentsof a pure oxygen aeration system are an oxygen generator,a covered aeration tank, a final clarifier, and recirculation pumps. Primary effluent, return activated sludge, and oxygen are introduced into the first compartment of a multistage covered tank. Mechanical agitators mix the oxygen with the wastewater in the tank. High-purity-oxygen activated sludge has several advantages. High efficiency is possible at increased BOD loads and reduced aeration periods. Waste sludge production is also less. Operation and Control of Activated SludgeProcesses. Operation of an activated-sludge treatment plantis regulated by the quantity of air supplied, the rate of activated sludgerecirculation, and the amount of excess sludge withdrawn. The settleability of the mixed liquor in the final clarifier governs the rate of activated sludge recirculation. Poorlyflocculated particles and filamentous growths that do not separate by gravity in the final clarifier contribute to the BOD and suspended solids in the effluent. Excessive carryover of floc is called sludge bulking. This condition can be controlled by appropriate adjustments in the mixed liquor suspended solids concentration andfodmicroorganism ratio. Also, sludge bulkingmay be caused by excessive agitation or insufficient aeration. The settleability of mixed liquor is defined by the sludge volume index (SVI), which is equal to the volume occupied by l g of settled sludge and is expressed in milliliters per gram (mug). To determine SVI, a sample of mixed liquor fromthe aeration tank is allowed to settle for 30 min in a l-L graduated glass cylinder.The volume of settled sludgeis read fromthe scale on the cylinder, and the MLSS is also measured. The SVI is then computed as SVI = v x lo00 MLSS where V is the volume of settled sludge (mVL), and M U S is the mixed liquor suspendedsolids (mg/L).
-
3. RotatingBiologicalContractors
A rotating biological contractor (RBC) isa secondary treatment devicethat consists of a series of large plastic disks mounted on a horizontal shaft. The lightweight disks are approximately 10 ft (3 m) in diameter and are spaced approximately 1.5 in. (40mm) apart on the shaft. The disks are usually made of corrugated plastic sheets bonded together.
Physical, Biological, and Chemical Processes
505
Table 3 High "bo-stage rate ~~
BOD loading [lb/(lOOO ft3-day)] Hydraulic loading (gPdfi*) 0.16-0.48
25-45
45-70
0.16-0.48
1.0 lb/(1000 @.day) = 16 g/(m3.day). 1.0 X Id gal/(acwday) = 0.935 m3/(m2.day).
The disks are approximately 40% submerged in primaryeffluent. As the shaft rotates, the disks are alternately in contact with air and with the wastewater. The resultis similar to that of the trickling filter system. When disks are rotated out of the tank, air enters the spaces while the liquid trickles out over the biological film on the disks. The microbes that form the film absorb the organic material in the wastewater. These microbial solids grow on the medium. Excess microbial solids (slime) break away from the medium due to their weight and hydraulic forces and are carried out in the process effluent for gravity separation in the final clarifier. Stabilization or OxidationPonds For suburban or rural areas with seasonal industries such as fruit and vegetable canning facilities and where land is relatively cheap, wastewater lagoonsmay be used for secondary treatment. These lagoons are also called stabilizationor oxidation ponds. A stabilization pond is a flat-bottomed pond enclosed by an earth dike. It canbe round, square, or rectangular of length approximately three times the width. The operating liquid depths are 2-6 ft (0.6-1.83 m) and 3 ft (0.91 m) dike freeboard. A minimum of 2 ft (0.6 m) liquid depth is needed to prevent the growth of weeds. Liquiddepths of 6 ft (1.83 m) or greater may give out odors because of anaerobic decomposition near the pond bottom. Where the required lagoon area is greater than 6 acres, it is good engineeringto have multiple cells that can be operated individually,in series, or in parallel. If the soil is pervious, the pond bottom and dikes should be sealed with clay or other sealantto prevent groundwater pollution. Dikes and surroundingsare seeded with grass, graded to prevent runoff water from entering the pond, and fenced. A majority of lagoons are facultative ponds; i.e., both aerobic and anaerobic biochemical reactions take place. Raw wastewater enters the pond without primary treatment. Organic solids that settle in the bottom decompose anaerobically, producing organicacids, methane, and hydrogen sulfide. Near the mid-depth of the pond, most of the organic matter is decomposed by facultative bacteria, bacteria that can grow in either an aerobic or anaerobic environment. In the presence of oxygen, aerobic decomposition occurs mostly in the top half of the liquid. A major portion of the oxygen added to the wastewater in the pond is due to the mixing of air at the pond surface by wind action. A minor portionof the oxygen is from photosynthesis as algae present in the pond use energy from sunlight. The algae grow and multiply by consuming carbon dioxide and other inorganiccompoundsreleased by bacteria. Bacteria and protozoa in turn use oxygen released by algae and from surfaceaeration to decompose the organic matter in the wastewater. The overall process in a stabilization pond is the sum of the reactions of the bacteria, protozoans, and algae. BOD loadings on a stabilization pond are generally in the range of 20-40 lb of BOD per acre per day or 0.46-0.92 lb of BOD per lo00 ft2 [2.2-4.4 g/(m2.day)]. Higher loadingsare possible in areas with warmer climates. Although the algae playa role in the purification process in a lagoon, they can causea problem when theyare carried out of the pond in the effluent flow by increasing the levels of total suspended solids.
4.
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Despite many drawbacks, such as difficulty with total suspended solids removal efficiency and potential groundwater pollution, wastewater lagoons are being used where land is relatively cheap. The low construction costs and ease of operation and maintenance are distinct advantages. 5. SecondaryEffluentChlorination The final step in the secondary wastewater treatment process is usually disinfection by chlorination. The main purpose is to destroy pathogens in the effluent before discharging it into a body of water used for swimming or water supply downstream. The chlorine demand of secso that a combined chlorine ondary effluent is usually high. Chlorine dosage should be adjusted residual is approximately 0.5 mg/L in the secondary effluent.
D. Tertiary or Advanced Treatment Secondary treatment followedby disinfection can kill nearly all pathogens and remove8 5 4 0 % of the BOD and TSS in raw wastewater. However, only very little of some pollutants, suchas nitrogen, phosphorus, andheavy metals, canbe removed by biological treatment. Nitrogen and phosphorus are important plant nutrients. If they are discharged into a lake, algal blooms and eutrophication (accelerated aging) may occur. Also, nitrogen in the secondary wastewater effluent may be present mostly as compounds of ammonia. These exert an oxygen demand the in receiving water as theyare converted to nitrates. This process is known as nitrification. Heavy metals in the effluent may be toxic or carcinogenic, a function of concentration and type of metal. In cases where pollutants are not removed to a sufficient extent by secondary treatment, additional or advanced treatmentsteps must betaken. Thisis also called tertiary treatment. The cost of construction, operation, and maintenance for this type of treatment is very high.
1. Filtration By using a filtration process it is possible to remove most of the residual suspended solids, BOD, and bacteria fromthe secondary effluent. This is accomplished by multimedia filters that consist of low-density coal for the large grain sizes, medium-density sand for intermediate sizes, and a high-density medium for the smallest filter grains. Qpically, filtration can reduce activated sludge effluent suspended solids from 25 mg/L to approximately 10 m@. However, coagulationandsedimentationfollowed by filtration canremovesuspendedsolidsalmost entirely. 2. CarbonAdsorption Many soluble organic materials that are resistant to biological breakdown will persist in the effluent even after secondary treatment, coagulation, sedimentation, andfiltration. A reliable and practical method for removing persistent organics is to adsorb them onto activated carbon, which is usually packed in a cylinder through which the effluent is passed. Organic materials accumulate at the particle interfaces. Adsorption is a surface phenomenon, and thegreater the surface area of the carbon, the greater is the capacity to remove organics fromthe liquid. The large surface areais provided by a large number of pores. After the adsorption capacityof the carbon has been exhausted, it can be restored by heating the carbon at a high temperature to burn or drive off the adsorbed organics. 3. Microstraining
A microstraining process can also be used to remove suspended solids. The microstrainers (also called microscreens) are made of woven steel wire or special cloth fabric mounted on a re-
Physical, Biological, Processes and Chemical
507
volving drum. The rotating drum is partially submerged in the wastewater after secondary treatment. The effluent flows into the drum and then throughthe microscreen, where thesolids are captured. 4.
Phosphorus Removal
As previously discussed, phosphorus is one of the key plant nutrients that contribute to eutrophication of lakes. Untreated wastewater contains approximately 10 mgL of phosphorus from household detergentsas well as from sanitary wastes. This phosphorusis primarily in the form of organic phosphorus and phosphate components. Only 20%of the phosphorus is removed in secondary treatment. (See Table 1.) The tertiary treatment process used to move phosphorus from the secondary effluent is typically chemical precipitation of phosphorus with alum, ferric chloride, or lime. The precipitation reactions are shown below. 2 HP0:- + 2 AIP044 2 H+ 3 SO:Alum: A12(S04), Ferric chloride: FeCl, + + FeP04i + H+ 3 ClLime: 5 Ca(OH), + 3 HPOZ- ”* Ca,(P04)30Hi + 3 H20 6 OHThe pH is very important for these reactions. The effective range of PH for alum andferric chloride is 5.5-7.0. The precipitation of phosphorus requires a reaction basin and a settling tank to remove the precipitate. However, ferric chloride and alum may be added directly to the aeration tank in the activated sludge system. In this case, the aeration tank serves as a reaction chamber and the precipitate is removed in the secondary clarifier. This is not done withthe lime because the highpHneeded to form a precipitate is not tolerated by microorganisms in thebioaeration tank.
+
+
PO:-
+
5 . NitrogenRemoval Nitrogen can exist in wastewaterin the form of organic nitrogen, ammonia,or nitrates. Nitrogen in any soluble form is a nutrient in the receiving body and can cause algal blooms. In addition, nitrogen in the form of ammoniaexerts an oxygen demand and canbe toxic to fish. For these reasons, it is sometimes necessary to remove the nitrogen from the wastewater effluent before discharge. Removal of nitrogen can be accomplished either biologically or chemically. The biologicalprocess is called nitrificatioddenitrification. Thechemicalprocess is calledammonia stripping. NifrificationlDenirrificafion. This process consists of two basic steps.First, the secondary effluent is further aerated by maintaining a cell detention time of 15 days or more. In this first step (nitrification), the ammonia nitrogenis converted to nitrate nitrogen, producinga nitrified effluent. This step is expressed in terms of the chemical reaction
NHQ
+ 202”*NO; + H20 + 2 H +
A second biological treatmentstep, denitrification, is necessaryto actually remove thenitrogen from the wastewater. If the nitrogen level is not of concern for the receiving body of water, the wastewater can be discharged after settling. If nitrogen is of concern, the nitrification is followed by denitrification. The denitrifying bacteria use the carbon from the organic matter and the oxygen from the nitrates in their metabolic processes. This step is expressed in chemical equation form as 2 NO;
+ organic matter -+
NZ?
+ C02 + H20
The organic matter may be a synthetic material such as methanol.
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Ammonia Stripping. Ammonia stripping is a physicochemical process. Nitrogen is removed from wastewater in two steps. First, the pH of the wastewater is raised to convert the ammoniumion into ammonia. Second, the ammonia is stripped by passing large volumes of air through the wastewater. The ammonia stripping reaction is NH;
+ OH- + NH3T + H20
The hydroxide is usually supplied by adding lime. The combination of ammonia stripping with phosphorus removal using lime a coagulant as is cost-effective andefficient, since lime canalso serve to raise the pH of the wastewater. Low temperatures can cause problems suchas reduced stripping.
V. WASTEWATER SLUDGES In the processof removing pollutants from wastewater during treatment, sludge accumulates in the settling tanks and mustbe withdrawn, treated, and disposed of. The amount of solids generated in a wastewater treatment system isa function of the degree of treatment provided and the amounts of chemicals added. The handling, treatment, and disposal of wastewater sludge can be a complex and costly operation. The sludge is composedof materials that have settled out from the raw wastewater and protoplasmof organisms. Land disposal of sludge or sludge incineration is usually preceded by some type of treatment to render it easier and less offensive to handle and reduce its volume. A discussion of sludge characteristics and qualities and several common treatment methods follows.
A. Characteristics of Sludges Sludge initially forms at the bottom of a clarifier orsettling tank. The concentrationof organic matter in wastewater is approximately 0.02%, while that in a typical raw waste sludge is a p proximately 4%. This sludge is odorous and putrescible andmust be further processed and reduced in volume for final disposal. The concentration of solids has a significant effect on the total volume occupied by the liquid sludge.The total sludgevolume is inversely proportionalto the solids concentration. For example, if the waste is thickened from 2% to 4% solids, the volume is reduced by one-half. During this concentration processthe water content is reduced from98% to 96%. Primary sludge typically has a solids concentration of 6-8%. The solids are 70-80% organic. Primary or raw sludge is highly odoriferous, greasy, and gray. Secondary sludge consists of microorganisms andinert materials that have been accumulated as wastes from the secondary treatment processes. Trickling filter sludge contains 2-4% solids, while activated sludge waste is typically 0.5-2.0% solids with an organic fraction Of 80-90%. The characteristics of tertiary sludges depend on the nature of the process. For example, these sludges may contain largequalities of chemical precipitates because chemicalsare added during phosphorus removal. This chemical sludge is combined with the biological sludge.
B. SludgeTreatment Sludge is treated prior to ultimate disposal for two reasons: volume reduction andstabilization of organics. A reduced sludge volume minimizes pumping and storage requirements and lowers sludge handling costs. Stabilized sludge has a less offensive odor, and health hazard and nuisance conditions are minimized.
Physical, Processes Chemical Biological, and
509
Common methods to accomplish the two objectives include thickening, biological digestion, dewatering, and incineration. When incineration is used, there is no need to stabilize the sludge.
1. Thickening It isalmost impracticalto treat thin sludges witha very high water content and very little solids concentration. Thickening serves to increase solids concentration.It is accomplished in oneof two ways: the solids are floated to the top of the liquid (flotation) or allowed to settle to the bottom. The later process is called gravity thickening. The objective is to remove as much water as possible before final dewatering or digestion of the sludge. 2. Stabilization The stabilizationof sludge is accomplished by a sludge digestion processin which organic solids are biochemically decomposed. The organics are converted into simpler and more stable it putrescible andeasier to dewater, products. Digestion reducesodors of the sludge, makes less and destroys pathogens. Sludge may be digested under anaerobic or aerobic conditions. Anaerobic Digestion. The anaerobic digestion of sludge occurs in two stages. The first stage is commonly referred to as acid fermentation. In this stage organic material is converted to organic acids, alcohols, andnew bacterial cells.In the second stage the end products the of f i t stage are converted togases, mainly methane and carbon dioxide. This stageis called methane fermentation. The two stages are simultaneous. The primary acids produced during acid fermentation are propionic and acetic acids, which are precursors for methane formation. The methane gas may be burned to provide power for other plant pmesses and equipment as well as for heating the plant buildings. two covered circular In modern anaerobic systems, the digestion process takes place in tanks. These tanks are typically about 80 ft (25 m) in diameter and approximately 50 ft (15 m) deep. The sludge in the first tank is heated to a temperature of approximately 90°F (35"C), and the contents are thoroughly mixed. The digestion process is completed inthe first tank within 10-15 days of detention time. The sludge thenflows to the second tank for settling and storage. The digester supernatant (relatively clean liquid above the digested sludge) is pumped to the inlet of the treatment plant. Digested sludgeis removed from the bottom ofthe second tank for further processing (e.g., dewatering) and final disposal. Aerobic Digestion. In aerobic sludge digestion, the sludge is aerated in an open tank similar to the activated sludgeaeration tank. The sludgeis aerated fora long period (approximately 30 days or more), during which most organics are stabilized and the amount of sludge is reduced. The BOD loading on the digestion tank is kept very low. Eventually the microorganisms consume theirown cellular mass. Thisis called endogenous decay. The aerobic digester is followed by a settling tank unlessthe liquid is to be disposed of by land. The supernatant(effluent) from the clarifier is recycled back to the head end of the plant. Not all of the sludge decomposes. Some of it eventually has to be removed from the tank for disposal. Because the fraction of the volatile matter is reduced, the specific gravity of the digested sludge solids will be higher than that of the original sludge. Thus, the sludge settles to a more compact mass,and the clarifier concentration can reachto a maximum of 3%. It is a thin sludge that is difficult to thicken and dewater. Conditioning 3. Conditioning of sludge facilitatesthe separation of the liquid and solids. Chemical conditioning involves additionof coagulants such as lime, ferric chloride, or organic polymers. Sludge particles carry a small electrostatic charge that causes'particles to repel each other. The coagu-
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lants, however, neutralize the effect of particle charges so that particles can collide and form larger and heavier particles to facilitate separation from the water. These conditioning chemicals (coagulants) are mixed into the sludge just before dewatering. Heat Treatment. Another methodof conditioning the sludge is to heat it at high temperatures (175-230°C) and pressures (1000-2000 kPa). Under these conditions, water that is bound up with organic solids is released. Thus, dewatering of the sludge becomes easier. Dewatering. The process of removing water from the sludge is called sludge dewatering or drying even though the dewatered sludge may still be 70-75% water. The dewatered sludgeis no longer a liquid and can be handled easily. Sludgeis usually dewateredprior to incineration. There are several methodsby which sludge canbe dewatered. The most popular methodof dewatering in the past hasbeen the use of sludge dryingbeds. The beds usually consistof 6-9 in. (150-225 mm) of coarse sand supported on a graded gravel bed. The digested sludge is spread on an open bed of sand, where drying takes place by a combination of evaporation and drainage. A piping system with open joints placed in the gravel or crushed stone bed collects the water that drains from the sludge. Rational designis difficult because several variables affect the dryingrate. These include climate (temperature,rainfall, humidity, and sludge velocity), sludgecharacteristics (degree of stabilization, grease content, solids concentration), and frequency of sludge application. Solids loadings average about 20lb/(ft2-yr)in northern states and about 40 lb/(ft2.yr) in southern climates. Drying time ranges from a few weeks to several weeks depending on the drainability of the sludge and weather conditions. Sludge is appliedto the sand bed to a depth of 12 in. (300 “cake,” as it iscalled, has a solids contentof about 30-40%. mm). A typical dewatered sludge It is removed from the sand with a pitchfork or front-end loader. Sand drying beds are the oldest, most common type of beds. Since a relatively large amount of land area is required to construct a bed, this method of sludge drying is most common in rural or suburban areas. When there is not enough land area available at the plant site or weather is a problem, a mechanical system may be used to dewater the sludge. A mechanical system needs less space and offers more operational control than the sand drying beds. ”bo of the most common types of mechanical systems for sludge dewatering are the vacuum filter and the centrifuge. A vacuum filter consists of a drum covered witha special filtering fabric, which rotates as it is partially submerged in a vat of conditioned sludge.A vacuum or suctionis applied inside the drum to extract water, leaving the solids on the filter medium. A thin layer of dewatered sludge, called filter cake, sticks to the fabric. As the drum completes its rotational cycle, a blade scrapesthe filter cake intoa hopper as the drummoves. It is often necessary toadd chemicals (ferric chloride, lime, or polymers) to the sludgeto improve the drainability. Thisis called sludge conditioning. In the centrifuge dewatering system, the sludge is pumped into a cylinder that rotates at are also injected. Thesolids are removed to the wall high speed. Sludge conditioning chemicals of the rotating cylinder by centrifugal force, and the liquid is pumped to the inlet end of the plant for treatment. 4. Incineration The dewatered sludge canbe incinerated to convert itinto inert ash. Incineration is used when suitable land is not available. The exhaust gases from the incinerator must be treated to meet air quality standards. Thereare two types of incinerators: the multiple-hearth furnace and the fluidized bed incinerator. In the multiple-hearth furnace, dewatered sludge passes through a series of hearths, where it is dried and heated to the ignition stage. Fuel burners provide the heat for start-up; afterwards sludge serves as a fuel to keep incineration going. In the fluidized bed incinerator, air
Physical, Processes Biological, Chemical and
SI I
flowing upwardis mixed with the sludge andis forced through a bed of hot sand. The passage of air causes the sand bed to expand (This is the fluidized bed). The sludge is burned as it passes through the hot sand. The ash, including partially burned particulates, makes up the exhaust gases and is removed by the air pollution control equipment.
C. Sludge Disposal The dewatered or incinerated sludge may be applied to plots of land for the purpose of recovering nutrients or reclaiming despoiledland. In contrast to other land disposal techniques, this type of land spreading is land-use-intensive. Applicationrates are controlled by the soil characteristics and intended crops. Sludge landfill is the planned burial of wastewater solids, which may include screenings, grit, processed sludge, and ash, at a suitable site. A trench is excavated, and solids are put in it and covered with a layer of soil that does not permit water to percolate. Wastewater sludge may also be used for cornposting or for cofiring with municipal solid waste.
GLOSSARY Biochemical oxygen demand (BOD). The amount of oxygen required by microorganisms to decompose organic wastein water, usually during a 5-day incubation at 20°C (68°F). BOD is a measure of the amount of organic pollution. It is usually givenin mg/L or ppm. It can also be expressed in pounds of total oxygen required by wastewater or sludge. Dissolved oxygen. or ppm.
The amount of oxygenpresentin the water,usuallyexpressedasmg/L
Biodegradable. Readilybrokendown or decomposed into simpler substances by biological action of microbes. Eutrophication. The natural aging of a lake due to high concentration sive plant growth, and accumulation of bottom sediments.
of nutrients, exces-
Total solids. The amount of residue on evaporation, including organic matter and dissolved salts; usually expressed in mg/L or in pounds per unit of liquid. Suspended solids. Solids carried in water or sewage that would be retained on a glass fiber filter in a standard laboratory test. The units are mg/L or pounds per unit of liquid.
RECOMMENDED READING ASCE (1982). Wastewater Treatment PlantDesign, ASCE Manual on Engineering Practice No. 36. Cheremisinoff, F! N., and Young, R. A. (1976). Pollution Engineering Practice Handbook, Ann Arbor Science, Ann Arbor, Mich. Davis, M. L., and Cornwell, D. A. (1989). Introduction to Environmenrul Engineering, PWS Publishers. Viessman,W., Jr. andHammer, M.J. (1993). Water Supply andPollution Control. 5th d..Harper Collins.
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23 Freeze Concentration: Its Application in Hazardous Wastewater Treatment
Ray Ruemekorf NIRO, Inc. Columbia, Maryland
1.
INTRODUCTION
Melt crystallization is a standard chemical industries purification process for chemical products such as naphthalene, para-xylene, dichlorobenzene,acrylic acid, monochloroacetic acid, bisphenol A, and others [l]. It is seen as an extremely selective separation process that can reduce contaminant levelsto the ppm range. This provides an effective means to purify some products. Freeze concentration (FC) is usually associated with the process when the crystallizing componentis water. In this chapter we focus onthe crystallization of water and limitour discussion toFC of aqueous solutions. The purpose of isFC to provide a means toseparate pure water froma solution. This will provide either high quality water and reduced volumes of waste or, in the case of food liquids, a high quality concentrate with only water removed from the original feed product. NIRO (formerly Grenco) process technology has developed anFC system used commercially in the food processing industry (the NFC process). TheNFC process is described, and the results from pilot plant tests ona caustic wastewater are presented along with economicsfor a typical treatment case. Freeze concentration has been applied in the food industry [2,3] for a variety of products including fruit and citrus juices, coffee and tea extracts, beer and wine, vinegar, and dairy products. It featuresmany product quality advantages over the more conventional concentration processes of evaporation and membranefiltration. Freeze concentration can produceconcentrates up to 45-50% total solids PS),not as high a concentration as evaporators but significantly higher thanthat achieved with membranes.The process operates at freezing temperatures. This has obvious benefits for the quality of food products but also reduces corrosion, which can occurat the elevated temperaturesused in evaporation. The crystallization method is extremely selective in that only water is included in the crystal. Coupled with an efficient separation unit, FC can remove pure water from the product, eliminating the need for further processing. Evaporation canbe limited by the relative volatility 513
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Table 1 Characteristics of the Niro Freeze Concentration Process Crystallization of water using ripening process Pure spherical ice crystals. No solids incorporated in the crystal itself. Ice crystal separation using a wash column Effective separation; losses usuallyin the ppm or ppb range. No recycle of washwater. Efficient separation,reliable operation through mechanically forced bed transport. Low processing temperatures Always below the freezing point of water. Operation with heat-sensitive materials. Reduction of corrosion. Closed, pressurized, flooded system No gas-liquid interface; reduces oxidation problems and eliminates loss of volatile materials. Self-stabilizing process Simple control system. Relatively insensitive to changesin feed characteristics. Low-speed rotating equipment Low maintenance costs. Modular design Flexibility-multistage systems can be split for dual product operation (or online maintenance). Easily expandable as capacity demands increase. Clean energy source Electrically powered, which can provide for a positive environmental impact due to controlled centralized production. Local utility companies may provide economic incentives for installation of electric equipment. of the constituents, some of whichmay be carried over with the water vapor and require another treatmentlrecovery step. The advantages for hazardous waste disposal lie in the purityof the discharged water.The NFC process uses separate nucleation and growth zones, which produce pure spherical ice be discharged crystals that can be easily separated and discharged as pure water. This water can directly or reused in the process, reducing water requirements. The system operates continuously and does not require shutdown for periodic cleaning of the components. It is a closed system, which eliminates vapor lossesand oxidation through contact withair. It is a relatively 100% turndown ratio from stable operation with a simple control process. It provides for a design capacity, is relatively insensitive to fluctuations in feed rate and composition, and is modular in design, improving expansion capabilities. These process characteristics are summarized in Table 1. The FC process is dependent ontwo main characteristics of the concentrated product.The viscosity of the concentrate at its freezing point determines the maximum concentration obtainable by FC. Figure 1 shows the viscosity curves for caustic wastewater, beer,and milk for comparison. The maximum concentration is reached when the viscosityof the liquid prevents the growth of ice crystals. The separation then becomes too difficult to maintain the purity of the discharged water. Fromthe standpoint of economics, the maximum concentration may be reached long before the product reaches this viscosity. The crystal growth rate decreases as viscosity increases, and the system requires longer residence times (and thus larger equipment) to reach a separable crystal size, The freezing point curves for the same productsas in Figure 1 are shown in Figure 2. The freezing point depression due to the solute concentration can be so great that the lower temperature limit for the
515
Freeze Concentration in WastewaterTreatment 90 80 -
Viscosity 70 (mm2/s)
60 -
50 -
40 -
30 -
20 10-
o
o
5 15 10
25
20
35
30
40
Concentration
Figure 1 The relationship of viscosity to concentration for a caustic wastewater solution (% total solids), skim milk (% total solids), and beer (‘Plato).
refrigerant may be reached. This can be overcome by using specialty refrigerants and multistage refrigeration systems, but these are usually too expensive to be feasible. Qpically the NFC system can operate with to up40-50% total dissolved solids with a product freezing point of around - 10 to - 15°C and viscosities of 150-500 cSt. Evaporation is a well-developed unit operation andbecah used in most instances requiring concentration. Even though water requires more energy to evaporate than to crystallize(2325 kJkg heat of vaporization compared to334 kJkg heat of fusion for pure water), commercial evaporators with energy recovery systems use approximately the same amount of energy as FC systems. The evaporator condensate from a wastewater treatment system is usually not suitable for direct discharge becauseof carryover of volatile organics. The condensate stream thenrequires further treatment before discharge or reuse. Many wastewaters contain chemicals that are quite corrosive at the elevated temperatures in the evaporators and therefore require expensive materials for equipment construction [4] or limit its lifetime. Precipitates usually build up on the heat transfer surface, leading to additional cleaning cycles, lost production time, and production of an additional waste stream thatmust be treated.
-
0 -2
Milk
-10 -12 -14-16-
-18 -20 -22 -24 -26
Caustic Water Waste ‘ 5
10
15
20
25
30
35
40
Concentration
Figure 2 The relationship between equilibrium temperature and concentration for a caustic wastewater solution (%TS), skim milk (%TS), and beer (“Plato).
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The treatmentof wastewater streams with membranes depends greatly on the type on contaminant found in the wastewater. Membrane systems are effectively used in the treatment of oily wastewaters[5]. The costs are reasonable, and their operation is routine. As with all membrane systems, pretreatment is necessary to prevent damage and reduce fouling of the membrane. Low molecular weightcompoundsfoundinmanyreactor effluents require tighter membranes. Membrane use in these situations is generally more expensive due to the larger surface area necessary and requires additional treatment steps before final discharge of the water. Regular cleaning cycles add to the cost of operation due to lost production time and additional treatment systems for the cleaning water supply.
II. DISCUSSION In its most basic form, FC has been employed since as far back as the Middle Ages. Beer or wine in wooden barrels would be exposed to the cold winter nights, causing water to freeze along the barrel walls and leaving a concentrated product in the middle of the ice shell. In more recent times, a few companies have tried to commercialize FC processes. The main problem with most of these attempts was the difficulty in separating the ice from the concentrate. Filter presses, centrifuges, and various types of wash columns have been used with limited success [3]. Unacceptable losses of product with the ice, low production capacities, and high equipment cost were normally associated with FC processes during this time. Professor H. A. C. Thijssen, working at the Technical University of Eindhoven, was the first to develop a practical solutionfor this problem.His solution consistsof two parts. An efficient crystallization process provides ice crystals of sufficient size and purity to allow proper separation, and an efficient wash column provides for complete separation of the ice and concentrate, using the melted ice to wash the incoming crystals without recycling the wash water. Thijssen’s original work provided the basis for the development of the present NFC process. The following sections describe the general NFCprocessandequipmentusedinthe wastewater project. Commercial applicationsin the food industry use the same basic process. The NFC can be separated into the twoparts noted in Thijssen’s work, thecrystallization section and the separation section 161.
A. The Crystallization Section The crystallization process is built up of three components as shown schematically in Figure 3, starting with a set of scraped surface heat exchangers (SSHEs) that remove the necessary heat from the system to provide for icecrystal formation. The SSHEsare connected to the recrystallizer by a recirculation stream through a scraped filter in the recrystallizer. This provides a continuous crystal free stream over theSSHEs where small ice crystals (nuclei) are formed and pumped into the recrystallizer. Of these small crystals, some survive and grow while the majority melt, absorbing energy from the surroundings and causing water to crystallize on the surviving ice crystals. This growth process is known as ripening and can be described by the Gibbs-Thomson formula depicted in Figure 4 for an aqueous sugar solution. Small crystals have a slightly lower equilibrium temperature, the temperature at which they neither grow nor melt, than larger crystals, When small and large crystals are mixed in an adiabatic vessel, the temperature of the bulk solution will reach a value somewhere between the equilibrium temperaturesof the large and smallcrystals [ 7 ] . This puts the larger crystals in an environment below their equilibrium temperatureso they will not melt, whereas the smallcrystals are in an environment above their equilibrium temperature so they tend to melt, which removes energy from thebulk solution that canbe exactly balanced by the recrystallization of
517
Freeze Concentration in Wastewater Treatment
Heat
1
I The small ice
crystalsform in the S H E
When enoughi c e crystals have formed, a sluny stream is divertedfrom the recrystallizer to the washcolumn. The iceis separated from the concentrate and discharges after being melted.
The ice crystalstravel in the product flow to the rec~ystalliier,where the ripeningprocess begins. Most of the small cryatals melt,but a few survive and start growing.
Feed
A aystal-freeliquid stream of product is
continuously recirculated over the SSHE prwidinga constant supply of small crystals to fuel the ripening process.
New product from the feed tank replaces the discharged water so that thesolute concentration in the system increases to the desired production value when we can withdraw part of the recirculation streamas product.
Figure 3 Schematic buildup of an externally cooled crystallizer with separation section. (a) The small ice crystals form in theSSHE. (b) The ice crystals travel in the product flow to the recrystallizer, where the ripening process begins. Most of the small crystals melt, but a few survive and start growing.A(c) crystal-free liquid stream of product is continuously recirculated over the SSHE, providing a constant supply of small crystals to fuel the ripening process. (d) When enough ice crystals have formed, a slurry stream is diverted from the recrystallizer to the wash column. The ice is separated from the concentrate and discharges after being melted. (e) New product from the feed tank replaces the discharged water so that the solute concentration in the system increases to the desired production value when we withdraw part of the recirculation stream as product.
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Diameter (microns) 100
000
Drivingforce for growth
melt Driving force for
1 I: Figure 4 Schematic representation of ripening for ice in a 30% sucrose solution. water on the larger crystals. Therefore, the small crystals melt and disappear while the larger crystals grow larger. By providing a continuous supply of small crystals to an existing slurry of larger crystals we can force the growth of crystals large enough to be cleanly separated from the concentrate. This recrystallization carried out in a mixed vessel provides for very pure spherical crystals. The round crystal shape is clearly an advantage in the following s e p aration section.
B. TheSeparationSection The separation unit shown in Figure5 can be described as a packed bed pressurized washcolumn with mechanically forced bed transport. The piston type consists of a cylinder, a moving piston with a porous head, and a rotating scraper for ice removal. The slurry from the recrystallizer enters at the bottom of the wash column througha central inletin the porous head. The liquid concentrate can leave through the head while the ice crystals remain to form a packed bed of crystals. The round crystals pack, leaving a certain void space that remains filled with liquid concentrate. The moving piston forcesthe ice bed into the scraper at the top of the cylinder. Iceis scraped off and enters the melting circuit, where itis circulated through and melted in a small heat exchanger. Controlling the pressurein the melting circuit allows the possibility to force the melted water back throughthe ice bed, which washesthe incoming crystals. The area where wash water meets concentrate in the ice bed is called thewash front andis generally very well defined.This stable displacement also depends on therecrystallizationprocess shown in Figure 6 . The water (freezing at OOC) meets crystals coming in with the slurry at the equilibrium temperature in the recrystallizer, say -8°C. This water willrecrystallize onto the surface of the existing crystals and force theconcentrate away from the crystal surface and towardthe porous piston head. The wash water, now frozen, is carried toward the top of the column along with the washed ice bed to be melted and discharged. Washingcarried out in this manner prevents
Freeze Concentrationin Wastewater Treatment
519
Melter Water discharge
Fill with slurry
(4
Compression and washing
4
Figure 5 Packed bed wash column (piston type). (a) The column is filled with slurry. (b) The piston compresses the slurry. The upper portionof ice bed is removedby a rotating ice scraper. The crystals are melted by circulation of ice and water through the melter. Washing the crystal bed is completed by increasing the pressure in the melting circuit.
mixing, or recycling, of the wash water with concentrate, whichprovidesfor separation.
an efficient
C. The Process When the system is first started, the unit is filled with product without ice. The product is first cooled to form a slurry of approximately 30% ice. Then the wash column is started, and water is removed from the system. This discharged water is replaced with fresh feed containing the solute(s) being concentrated. As water is removed from the system in this manner the solute concentration of the liquid in the system will increase. When the desired solute concentration, or the maximum allowed by product characteristics, is reached, concentrate is removed from the filtrate line of the recrystallizer. The solids removal rate (product output) is controlled to maintain a constant solute concentration in the final stage. In practice this is quite simple. A system will be set to produce (and remove) a fixed amount of ice, and the product flow can be adjusted accordingto the equilibrium temperature in the recrystallizer. A product has a certain freezing point at each solute concentration. While operating as a two-phase mixture in the recrystallizer the system has a given equilibrium temperature for that solution. This means that as the solute concentration of the liquid phase increases, its equilibrium temperaturewill decrease. This signal canbe used to control the flow of product from the system. Therefore, for a given solution, the ice production and product temperature (concentration) are set so that the system will react automatically to changes in feed concentration. Higher feed concentrations under the previous conditions will result in higher production rates and vice versa. This is the basic form for a single-stage FC system.
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Washwater
Washwater moves downward dueto the pressure differencebetween the top and bottom of the ice bed.
p,
-8 "C
1
P,
' p,-
i
L
Concentrate
Water recrystallizesonto the surfaceof the incoming ice crystals in the form of dendrites. This increases the flow resistance of the washed partof the bed. Since the washed part now has a greater flow resistance water will flow along path "A" to maintain a horizontal washfront. The water that has recrystallized is now carried back to the topof the icebed and discharged. This provides of the for an efficient washing action without recycle wash liquid.
Figure 6 Wash water flow in a packed bed wash column. Wash water moves downward due to the pressure difference between the top and bottom of the ice bed (P,> P*).Water recrystallizes onto the surface of the incomingice crystals in theform of dendrites. This increases theflow resistance of the washed part of the bed. Since the washed part now has a greaterflow resistance, water will flow along pathA to maintain a horizontal wash front. The water that has recrystallized is now carried back to the top of the ice bed and discharged. This provides for an efficient washing action without recycling the wash liquid.
The single-stage systemmust operate at the highest viscosity and lowest equilibrium temperature for a particular product. This implies that the ice crystals must grow and the wash column must remove these crystals under the least favorable conditions [3]. The high product viscosity reduces thecrystal growth rate, increases pumping and mixing energy requirements, reduces heat transfer coefficients, and limitsthe wash column capacity. The refrigeration system must remove all the heat at the lowest temperature, increasing its size and reducing efficiency. These have the effect of reducing the overall capacity and total efficiency of the process. Multistage systems withup to six separate crystallization sections and only oneseparation section (which may consist of a number of wash columns) overcome theselimitations by dividing the ice production over a range of product concentrations. Figure 7 shows an example illustrating the increased capacity provided by a multistage system. In Figure 7 you see a typical single-stage system with a water removal capacity of 250 kg/hr (550 Ib/hr) and in comparisona three-stage system witha water removal capacityof 1500 kg/hr (3300 lbhr). This is twice the sum of the individual units. Each stage is a complete crysare pumped to the tallization and recrystallizationsection. The ice crystals formed in one stage next. This ice is replaced with concentrate from the previous stage, resulting in a counter current flow of ice and soluble solids. Multistaging increases the capacity of a freeze concentration system by separating the ice production section from the separation section. This allows the wash column(s) tooperate with a liquid much lower in concentration and viscosity. Multistaging reduces energy consumptionby dividing the ice production over the stages so that more ice is made at higher freezing temperatures and lower viscosities, which improves the energy ef(SSHEs) and reduces the overall energy needed ficiency of the scraped surface heat exchangers for pumping and mixing. The average crystal growth rate is increased by reducing the residence time needed per stage. The crystals produced in stage 1 are transported to stage 2, where they
ncentration Freeze
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grow larger, andso on. These crystals are also exposed to the more favorable growth conditions of lowersolute concentrationand lower viscosity. Multistage systems also provide for a wider capacity range since the componentsare modular and can be addedas greater capacity is needed.
111. TEST DESCRIPTION The purpose of the test run was to demonstrate the operation of the NFC process on a wastewater and determine the concentration limit forthis particular product. The test was completed using Niro's FC-W6 Chemie pilot plant. The pilot plant is a complete skid-mounted singlestage FC system as describedin Figure 3. It has a volume of 150 L, a piston-type wash column, and 0.5 m*of surface area in two SSHEs. This provides for 10-20 kgihr ice production and removal for most products. The test solutionwas prepared to simulate a typical caustic wastewater contaminated with organics. It contained approximately 1.5%acrylic acid and 3% sodium hydroxide, anda silicon dioxide suspension was used to simulate suspended solids at -1100 ppm. The freezing point depression for this mixture was approximately 1°C for each %TS in solute concentration. The maximum temperature difference for stable wash column operation is normally around IO'C, so a product concentration of at least 10% TS was expected in a single-stage unit. The wash column usually will not performsatisfactorily with a slurry supply of much lower than- lO'C, but this product could probably be concentrated to a much higher degree on a multistage system. If the system can operate with some losses, due to the reduced efficiency of the wash column, then the maximum concentration for the crystallization section can be determined. Accepting losses and performing a two-step test like this allows us to determine the operating conditions for a multistage system on a single-stage pilot plant. A twofold concentration of the caustic solution produced water with only 24 ppm total dissolved solids with 2 ppm from total organic carbon.The second part of the test showed that the crystallization section could operate at over a 3.5-fold concentration with expected losses in the water. Composite samplesof the feed, water, and concentrates taken duringthe run were analyzed for total dissolved solids (TDS), total organic carbon (TOC), total suspended solids (TSS), and alkalinity. The results [8] are given in Table 2. This indicates that a commercial system couldbe designed with twoor more stages to produce a caustic wastewater concentrate for reuse inthe process. of around 22-23% total dissolvedsolids while maintaining water suitable Although specific cases must be analyzed individually to determine their economic feasibility, the following example illustrates the cost benefits for FC in one particular system.
Table 2 Results of Analysis of Composite Samples from Pilot Plant Tests using Niro's Chemie W6 Freeze Concentration Unit WaterConcentrate
Feed ~~~~
Part I (wash column limit) Total organic carbon Total suspended solids Total dissolved solids Alkalinity Part I1 (crystallizer limit) Total organic carbon Total suspended solids Total dissolved solids Alkalinity
7,860 124 61,400
40.600
17,650 24 1 137 ,000 91,900 36,000 389 225 ,000 174,000
2 <2 24 13 6,900 154 45,300 33,000
523
Freeze Concentration in Wastewater Treatment Table 3 Results of Economic Analysis for Example Wastewater Disposal Costs (U.S.$/year) Option'
Capital ~~
Energy
Total
~
1. Incineration 2. Evaporation
Biotreatment
395,000 Incineration ,000
720,000 420,000 360,000 35,000 70,000 95 300,000
3. Freeze concentration
Incineration
Ob
440,000 70,000 370,000 300,000395,000 95 ,000
l , 140,000 360,000 105,000 860,000 835,000
"See text. bzem energy assumes additional heat recovery equipment inuse.
Example Case Study For this example we assume that a processor has a waste stream contaminated with various organics havingproperties similar to those of the solution tested onthe pilot plant. The stream is 2400 kg/hr wastewater with 10% TSS. Three options are determined: Option 1. Incinerate the complete stream without pretreatment. Option 2. Concentrate the stream threefold withevaporation, and incinerate the concentrate. Employ additional biotreatmentof the condensate before reuse. Option 3. Concentrate the stream threefold with FC and incinerate the concentrate. Reuse water in process without additional treatment. Table 3 gives a summary of the economics of the three options [4]. It is clear that straight incineration (option 1) is the most expensive and also has the highest energy consumption. Evaporatiodincineration (option 2) offers lower energy costs when a heat recovery system is incorporated but requires the additional biotreatment process. Freeze concentratiodincineration (option 3) provides the least expensive processing. Note that the zero energy costs for evaporation depend the on installation of a heat recovery system for the incinerator,which can double the capital investment for the evaporator. This additional cost was not included in the calculation. W.
CONCLUSION
Freeze concentration is presently a viable alternative when concentration by evaporation or membrane filtration is not possible. Present applications for freeze concentration benefit entirely fromthe improved qualityof the final concentrate due to the low processing temperatures and selective separation of water. Through continuing improvementsin process design and reduced equipment costs, the areas of application that can profit from the use of this unit operation are expanding. Freeze concentration has demonstrated its potential in the treatment of hazardous wastewater.
ACKNOWLEDGMENTS The incinerator data for the economic analysis andsupport for the pilot plant test program were provided by T-Thermal, Inc., Conshohocken, Pennsylvania.
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REFERENCES 1. Wynn, N. P, Separate organics by melt crystallization, Chem. Eng. Prog., 88(3), 52 (1992). 2. Desphande, S. S., Cheryan, M., Sathe, S. K., and Salunke, D.K., Freeze concentration of fruit juices, CRC Crit. Rev. Food Sci. Nutr., 20(3), 173 (1984). 3. Van Pelt, W. H.J. M., and Swinkels, W. J., Recent developments in freeze concentration, in Food Engineering and Process Developments (M.LeMaguer and P. Jelens, eds.), Elsevier Applied Sciences, London, 1986. 4. Jansen, H.A., and Klomp, R., Grenco Freeze Concentration Technology, Tech. Rep., Grenco Pro-
cess Technology, 1991.
5. Cheryan, M., Ultrafiltration Handbook, Technomic, Lancaster, Penna., 1986.
6. Niro Process Technology bv, Current Large Scale Commercial Application of Freeze Concentration in the Food Industry, Tech. Publ., 1992. 7. Huige, N. N. J., Nucleation and growthof ice crystalsfrom water and sugar solutions in continuous stirred tank crystallizers, Ph.D. Thesis, Eindhoven Univ. Technol., Eindhoven, The Netherlands, 1972. 8. Kom, S., T-Thermal, Inc., personal communication, 1992.
24
Organoclay Sorbents for Selective Removal of Organics from Water and Wastewater Steven K. Dentel, Ahmad I.Jamrah, and Michael G.Stapleton University of Delaware Newark, Delaware
1.
REGULATORYBACKGROUND
Cost-effective removal, or even reconcentration, of specific organic contaminants from aqueous systems is of increasing technological priority in light of increasingly stringent environmental regulations.Thisis the case forwater,wastewater,andcontaminatedgroundwater 1986 Amendtreatment. In the United States, EPA regulations developed in implementing the ments to the Safe Drinking Water Act require substantial improvement in water treatment technologies. Certain organic substances must be removed to extremely low levels. Maximum contaminant levels (MCLs) havebeen established for eight volatile organic chemicals (VOCs) [l] and for33 contaminants, including22 synthetic organic chemicals (SOCs) [2]; a wide range of disinfectant by-products (DBPs) are also be be regulated [l]. The MCL for benzene, for example, is 5pg/L with an MCL goal (MCLG) of 0 pg/L. Current technologies available for attaining these low concentrations remain expensive. In wastewater treatment, the Clean WaterAct of 1977’s list of 129 priority pollutants necessitated consideration of treatment process efficiency in terms of specific removal of targeted chemical compounds. These are relevant both for publicly owned treatment works (POTWs) and for facilities dischargingto POTWs. A desirable removal process in such cases may also be one capable of specific uptake of one organic contaminant, or group of contaminants, in the presence of other less critical but more highly concentrated organic components. Industrial wastewater generatorsmust also be concerned with the presence of hazardous constituents in a waste, which may determine whether the waste is hazardousby definition. A process capable of concentrating selected waste components from a complex organic matrix may allow recovery and reuse as well. Thus a process capable of selective uptake of organic contaminants from watermay have a variety of uses. A sorbent material that, unlike activated carbon, could exclude innocuous components from the sorption zone would provide such performance. 525
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II. POTENTIALAPPLICATION OF ORGANOCLAYS One emerging technologyof possible use in these areas is the useof surfactant-modified smectite clays, also termed organoclays, as a sorbent material. The organoclay is created by insertion of anorganic cationic surfactant into the interlamellar spacings of the clay. The exchangeable metal cations are replaced, to a varying degree, by the surfactant molecules [351, which act as pillars to hold the aluminosilicate sheetsapart at an increased distance[ 6 ] .The hydrophobic portions of the surfactant are oriented into. this interlamellar spacing, creating an organophilic sorption zone. Hydrophobic organic compounds will be sorbed from water into this organic phase, which actsas a solubilizing medium similar to a bulk solvent phase suchas octanol or hexane [ 6 ] . The sorption characteristics of the organoclay that is created are directly related to the organic cation that has been placed onto the clay surfaces. Essentially, the sorption zone is of adjustable dimensions, and hydrophobicityis determined by surfactant selection. For example, if the surfactant possessesat least one aliphatic chain, the chainlength may have the following effects: Short chains tend to act as independent agents on the surface and may exclude aqueous components from the sorption zone as a function of size and shape. Longer chains have the ability to overlap onthe surface and form a uniform organic layer. This layer serves to remove water sorbed on the mineral surface and change the surface to a more hydrophobic character. These traits are also a function of the clay properties, since the configurationof the surfactant is determined by such properties as the cation-exchange capacityof the clay (and thereby the distance between exchange sites). Thepotential to produce a selective sorbentthrough the addition of specific organic cations on the surface is one of the features that makes organoclays attractive. The sorptive performance of some organoclays, usinga variety of clay and surfactant combinations,is summarized in this chapter. Some organoclay characteristics were established 20 years ago as such materials were evaluated for their rheological properties; these materials are used as thickeners in paints, lubricants, and evenlipstick. However, it is more recent research that provides the possibility of economic applications in environmental engineering areas. According to small-scale studies to date, properly tailored organoclays may selectively sorb and remove certain organic contaminants from water. By contrast, activated carbon, though exhibiting a high overall adsorption capacity, is relatively nonselective, and substantial adsorptive interference is observed in the complex chemical matrix of a surface water. By maximizing selectivity toward the uptake of the organic contaminant in the presence of innocuous, but competing, natural organic matter, the advantages of organoclays may become significant. The reported use of organoclay materials in chromatographic applications implies reversibility of this sorption process (at least from a vapor phase). Thus, kinetic aspects of these applications of organoclays must also be considered. For example, desorptionof the surfactant pillars must be slow within the time frame of the treatment process if the application is to be feasible for production of drinking water. At the same time, uptake of organic contaminants must be relatively rapid. Kinetic data for the sorption process may be necessary for proper process evaluation and design and should be obtainedin a realistic multicomponent matrix similar to the water to be treated.
Organoclay Sorbentsfor Removal of Organics
527
111. AREAS OF APPLICATION Immobilization or separation of contaminants contained in polluted waters is an objective of increasing importance in a variety of environmental settings. Recent research has suggested in clear terms that chemically modified clay minerals represent a new and promising class of adsorbent materials for accomplishing this. Specific areas of application exist in potable water treatment [7,8], industrial wastewater treatment [9,10], and treatment or confinement of contaminated groundwater [11-13]. Specific removal of hydrophobic organic pollutants, such as benzene-related compounds, certain pesticides, and precursorsof carcinogenic trihalomethanes, is of key interest in all such applications. In the areaof potable watet treatment, removal of such specific micropollutants as these is often desired inthe presence of much higher concentrations of other innocuous organics. Due to its high overall adsorption capacity, activated carbon is generally the adsorbent of choice when adsorption is employed as a treatment process. Unfortunately, activated carbon is quite nonselective, thus expending muchof its capacity in removal of incidental constituents. Thus, the presence of natural organic matter (NOM) hasbeen shown to severely impair carbon’s effectiveness in removing such organics from solution as trichlorophenol, trichlorobenzene, and trichloroethylene [14-201. The competition and coverage effects of theNOM are exacerbated with time [20]. Thus, in many water treatment applications, activated carbon adsorption isotherms may significantly overestimate its capacity due to background NOM competition. In this respect, any adsorbent providing more preferential uptake would offer obvious treatment and economic advantages. Evidence suggests that surfactant-modified clays may provide this selectivity. Furthermore, production from inexpensive base materials and the possibility of chemical rather than thermal adsorbent regenerationadd to the potential benefits. Membrane processes are also being advancedas promising water treatment technologies, particularly in France and Japan. Although these offer numerous advantages over conventional water treatment processes, they appear to do poorly in removinglow molecular weight hydrophobic organic compounds such as atrazine. Initial research in France indicated [8] that the addition of a sufficiently selective organoclay sorbent preceding the membrane would capture such compounds in small enough mass that head loss to the membrane would not suffer, and the sorbent could be periodically removed from the membrane by backwashing. It should be added that such an organoclay material could also be added to conventional treatment schemes in the rapid mix stage and removed in sedimentation and filtrationjust as powdered activated carbon is sometimes now employed. The use of organoclays in wastewater treatment has also been suggested. Cadena [9,21] studied the use of an organoclay in removal of benzene-related hazardous wastes from aqueous solution. Srinivasan and Folger [37] investigated application of organoclay sorbents as “molecular forceps” to remove toxic organics in a waste stream. Elution into an organic solvent would then allow reuse or thermal destruction of the removed organic and reuse of the organoclay. Boyd et al. [5] first indicated that claysor soils heated with organic cations mightbe used to improve attenuationof contaminant migration in groundwaters. Eitherin situ applicationof the organic cationto a soilor clay or its additionto bentonite slurry wallsor clay landfill liners were citedas possibilities. Subsequently, Soundararajan et al. [121 suggested the useof organoclays in solidificatiodstabilizationof organic wastes. Their approach was to combine the organoclay, as a minor constituent, with a cement, fly ash, or slag powder binder to enhancethe binder’s ability to retain the organics. Burris and Antworth [l31 also suggested the use of organic modifiers for in situ modification of aquifer material to enhance retardationof organic
528
et
Dentel
al.
chemicals and establish a stationary zone of surfactant-modified aquifer material without significant decrease in the hydraulic conductivity.
IV. TECHNOLOGY BACKGROUND Organoclays are typically smectite slays treated with an alkylamine or quaternarized cationic surfactant [22]. Adsorptive properties of the clay are modified due to insertion of the surfactant molecules into interlamella, providing an extensive volumethat apparently serves as a hydrophobic “phase” into which hydrophobic contaminants can then be partitioned [3]. Characteristics of this phase can be altered by appropriate matching of clay and surfactant, enabling in theorythecustomized design of such sorbents.Variables that may be consideredinclude charge density of the clay [11,231 and properties of the surfactant such as alkyl chain length [24], number and basicity of positive charges [10,24], structural aspects (number of alkyl branches, presence of aromatic moieties), and polarity index of the alkyl groups [12]. Subsequent treatment of the organoclay with other inorganic materials can improve uptake as well (Figure 1). Wyoming montmorillonite andhectorite are the baseclays that have been usedin research studies [e.g., 3,25-281 and also used as base materials in some commercially available organoclays (e.g., Bentones B27, B34, B38, produced by NL Industries for use as viscosity modof positivecharge due to isomorphic ifiers). These are smectites thatpossessdeficits substitution of cations in the octahedral sheets.Substitution in this sheet distancesthe origin of charge from the interlayer and thus delocalizes the charge imbalance. Because there are no negatively charged sites per se in the sheets adjoiningthe interlayer, compensatorycations located in the interlayer do not serve as firm anchors between adjacent layers. The consequent disorder may enhance access to adsorption or exchange sites. On the other hand, it is possible that desorbability of inserted surfactant may also be facilitated. Montmorillonite, with a dioctahedral structure, differs from hectorite (which is trioctahedral) in allowing greater distortions in the sheet structures due to the unfilled cation sites. This is demonstrated by the substantial
Ce mg/l
Figure 1 Results of adsorption of 2,4,5-trichlorophenol. onto calcium montmorillonite (Ca-M), montmorillonite modified withdioctadecyldimethylammonium(dd-M), and montmorillonite modified with dioctadecyldimethylammonium and then treated with CaCI, (Ca.dd-M). Solid/liquid mass ratio 0.0005; equilibration time 15 hr. temperature 35°C. analyses by UV absorption. (From Demougeot [25] and Dentel et al. [26].)
J
Sorbents Organoclay
for Removal of Organics
529
dimensional differences betweenAi3+ and the substituting Mg2+. Isomorphous substitution in hectorite instead replacesLi+ for Mg2+ because these have similar dimensions, andlittle distortion results. The more planar interlayer geometry may provide more uniform adsorption selectivity. Charge density or cation-exchange capacity (CEC)of the smectite can have a significant effect onproperties of the resulting organoclays. If charge densityor the CEC is high, the spacing of intercalated surfactantsis increased and theorientation is more likelyto be parallel to the silica sheets [23]. However, more recent findings [l11 show superior removals of benzene and toluene using a smectite with a lower CEC (Wmeqll00 g)than with a higher CEC (120 meq/ 100 g). Evidently, the greater amount of free volume provided between pillars is the critical factor, allowing placement of benzene and toluene parallel to the silica sheets in a more favorable configuration. A range of cationic surfactants have been used in previous studies with organoclays.Early studies employed primary amines such as propylamine and octadecylamine [e.g., 291. These were readily intercalatedinto interlayers, but the apparent conformationis with the alkyl chain parallel to the basal surfaces [30]. The extent and control overd(OO1) spacing is thus limited. McBride et al. [3] suggested that it is the lowbasal spacing and thus close packing of nalkylammonium ions thatrestricts the interlamellar volume availablefor adsorption. A further limitation is the pH dependency of the cationicity of these surfactants. Wolfe et al. [4,31] reported comparatively poor sorptive removal using 11 different organics with such materials The study also used dodecyldiammonium (dodecylammoniumandpropylammonium). [H3N+(CH2),2N+H3]with the assumption that fewer of the divalent ionswould be required to neutralize the anionic charge on the anionic silicate sheets, leaving morefree interlamellarvolume [22]. Whilethis was apparently thecase in adsorption fromvapor, removals from aqueous suspensionswerenotimproved.Plausibly,watermoleculesinthe interlayer succeededin shrinking the cavities regardless of the presence of any of these primary amines. It should be noted that somedata presented by Wolfe et al. [4,31] are not consistent with more recent findings, possibly due to volatilization difficulties or the use of procedures in which organic solubility limits were exceeded. Quaternary ammoniumionshave also beenemployed as claymodifiers. A somewhat greater basal spacing may provide a looser packing in this case. Furthermore, a significant change in the properties of the modifiedclay may create a morefavorablesorbent.The change includes the surface properties, which become organophilic as the mineral’s surface area covered by the organic exchange cations increases [1l]; this leads to an increase in the organic carbon content of the clay as well. The porosity of the clay will also be altered. Since the porosity is related to the distance between the sheets of the clay andis dependent onthe size and natureof the organic exchange cations [26], modified clays can be designed to have surface accessibility to adsorbates of varioussizes.Tetraalkylammoniumanionssuch as tetramethylammonium (TMA+), tetrapropylammonium, and others with a variety of alkyl and aromatic groups have been used, as exemplified by Table 1, from Boyd et al. [32]. Boyd et al. found that the larger, more hydrophobic pillars led to the best removals of pentachlorophenol (PCP) from solution (Figure 2). The large? spacing appeared to enable partitioning of the sorbate into an organic “phase.” (Thus the more general termsorption, rather than adsorption, appears to be more appropriate
’
‘Low adsorption levels reported by Wolfe have been questioned by Cadena [g] due to the possibility of volatilization losses, emphasizing the need for zero-headspace procedures.
%e enlarged d(100) spacings after insertionof the larger surfactants are considerable (e.g.. 10-14 DODMA+ (281.
A increases with
Dentel et al.
530
Table 1 Structure of Quaternarized Cationic Surfactants Used in the Research of Boyd et al.
[32]
Ion
Dioctadecyldimethylammonium
DODMA+
CH3
I
CH3 - H+ - ( C H Z )~ ~CH3
I
(cH2) 17 - CH3
Hexadecyltrimethylammonium
HDTMA+
Hexadecylpyridinium
HDPY
Trimethylphenylammonium
TMPA+
Tetramethylammonium
TMA+
CMercaptopyridinium
4"P+
+
SH
b H
Ammonium
m
a
+
H
I I H
H-N+-H ~
Source: Boyd et al. [32]. Reprinted with permission.
in describing uptake into organoclays.) Consistent with this hypothesis was the lack of competitive effect in PCP uptake when up to 0.5 mmol of 3,4,5-trichlorophenol was also present. by smectite modBoyd et al. [32] suggested that the uptake of hydrophobic organic compounds ified with HDTMA was due to solute partitioning into a highly nonpolar organic phase formed by the hydrophobic HDTMA ions. The partitioning behavior of HDTMA-smectite was supported by (1) the highly linear isotherms forthe adsorption of the organic compounds,(2) the dependence of the sorption coefficient on the solubility of the organic compounds,(3) the general agreement between the organic matter normalized partition coefficient of the organoclay and the corresponding octanollwater partition coefficient of the organic compounds, and (4) of the modified clay. the dependenceof the adsorption capacity on the organic carbon content Smith and Jaff6 [33] used thermodynamic arguments-low or negative heats of sorption as
Organoclay Sorbentsfor Removal of Organics
S31
0.12
t
9 0.10
P
e
E
oy 0.08 W
m
g 0.06 pt
0
U
Q
g 0.04 0.02
PCP EOUIUBAIUM CONCENTRATION (mmole1100 ml) Figure 2 Results of Boyd et al. [32] for adsorption of pentachlorophenol onto smectite modified with organics listed in Table1 and onto activated carbon.Temperature 20°C. [From Clays and Clay Minerals, 36 (2), with permission.] evidenced by isotherms at different temperatures-to support partitioningas the uptake mechanism of tetrachloromethane using decyltrimethylammonium bentonite. If a decyltrimethyldiammonium bentonite organoclay was used,however, the predominant mechanism appeared to be adsorption rather than partitioning. Consistent with the workof Wolfe et al. [4] is a lesser d spacing due to attachment of the diammonium structure to the same mineral surface. The diammonium clay exhibited competitive adsorption in the presence of benzene with the tetrachloromethane,whilethe decyltrimethyldiammonium bentonitesorbedequalamounts of tetrachloromethane whether the benzenewas present or not. Such sorbent materials thus show promise in separation of more hydrophobic classes of contaminants from wateras a class. One the other hand,Lee et al. [l13 reported that the use of smaller TMA+ ions allowed quite selective sorption, removing benzene while excluding toluene because of its somewhat larger dimensions. Cadena [9] showed the extent of organic u p >> toluene > o-xylene, although each was take by a TMA bentonite to be in the order benzene removed to a much greater extent thanby an unmodified bentonite. This is consistent with the findings of McBride et al. [3], who observed high selectivity of “A+-exchanged montmorillonites exposed to single-ring aromatic hydrocarbonsand was greatly favored by small planar molecules suchas phenol and benzene. Although Cadena surmised a partitioning mechanism, the consensus of other investigatorsis that adsorption predominates with the smaller cationic modifiers. Lee et al. [6] suggested “adsorption” as the mechanism of uptake for nonionic organic compoundsby TMA smectite. Their conclusion was based on the following: (1) A curvilinear isotherm was observed for sorption of benzene by TMA smectite, while a linear isotherm was observed byHDTMA smectite; (2) more soluble compounds such as benzene showed
532
Dentel et al.
strong affinity for TMA smectite, despite the fact that with a partitioning mechanism the higher the water solubility,the lower the partitioning coefficient; and(3) the uptake of benzene from aqueous solution was higher for TMA smectite than for HDTMA smectite, although TMA smectite has a much lower organic carbon contentthan that of HDTMA smectite. Smith that those with alkyl et al. [34] in comparing the behavior of 10 different organoclays, indicated carbon content lessthan 3 functioned by adsorptive uptake; those with 12 carbons or more exhibited removal by partitioning (no carbon contents of 3-11 were characterized). These studies have made clear the potential for optimizing organoclay configurationfor a as those in Figure 2 indicate that given use or in idealized experimental systems. Results such properly synthesized organoclays approach within an order of magnitude the performance of activated carbon [36] as an adsorbent in a “clean” system. Evidence to date suggests that the modified clays may perform equally as well as carbon in a complex system where selective adsorptive properties are advantageous. Cost advantagesmay also be significant in comparing organoclays to carbons, due to energy costs in production and regeneration of the latter. Such comparisons may well extend to use in the more complex chemical environment of water treatment. Recent work[7] suggests the practicability of using these materials in actual surface waters; however, analytical difficulties may be encounteredin determining removals for separation from a comtargeted componentsif measures are not taken for proper extraction and plex water or wastewater matrix.
V. FEASIBILITYSTUDIES Specific application of tailored organoclay sorbents must accordinglybe evaluated on a sitespecific basis. Organoclay media to be evaluatedmust be chosen with appropriate understanding of clay and modifierproperties and desired interactions with boththe target organic(s)and the other organic matter to be excluded. Experimental procedures begin with equilibrium isotherm data, generated by standard batch-test methods using analytical methods appropriate for the particular organic and anticipated concentration range.Closed, zero-headspace containers are to be used, particularly if treatment of volatile organic compounds is considered. Proper attention must be given to pH and ionic strengtheffects on both adsorption and analytical methods. A number of past studies have ignored such influences, particularly that of pH in the case of ionizable chlorophenolics. Isotherms must also be produced in a system with background components representative of those to be encountered in full-scale treatment. In water or wastewater applications where the types and concentrations of background organics, surface-active substances, and/or colloidal material may vary diurnally or seasonally, the effectsof these should be examined by appropriate use of actual or simulatedexperimental matrices, Temperature(s) of anticipated operation are of particular importance if an adsorptive rather than partitioning uptake is anticipated. In reactor design, the kinetic aspects of sorption and release should be evaluatedby use of variable equilibration times in batch studies or through use of stirred flow reactor experiments for more rapid reactions. Standard proceduresfor predicting scaled-up performancefor sorption processes may then be employed in projecting feasibility and costs. However, recent studies [20,35] have demonstrated flaws in these procedures when activated carbon is employed as an absorbent. Difficulties lie in proper simulation of competitive effects and apparent catalytic activity at the carbon surface, neither of which is probable in the use of organoclays. Nonetheless, a comparison of activated carbon with organoclay for full-scale use requires attention to accurate be scale-up predictionsfor both alternatives. In addition, the regeneration costs for each should included in any comprehensive economic evaluation.
Sorbents Organoclay
for Removal of Organics
533
Results of such feasibility studies will indicate to engineering practitioners whether organoclays are feasible alternatives to technologies suchas activated carbonand, if so, under what circumstances larger, field-scale studies of the organoclay sorbents may be warranted. Potential applications on this basis are likely in water, wastewater, or contaminated groundwater treatment.
VI. SUMMARY Current research is characterizing the actual mechanisms responsible for selective adsorption of water contaminants on modified clays. In an engineering sense, a more fundamental understanding of how the modifications determine selectivity will enable the virtual design of adsorbents which are more specific and thus more economical for a given application. Potential of such materials include (1) use as additives to the conventional watertreatmentuses coagulation-flocculation-sedimentation sequence;(2) use in a granular form in a column-type adsorber process; and(3) use in conjunction with advanced membrane processes. Applications in related areas also exist, such as the creation, possibly in situ, of clay barriers to prevent transport of organic solvents from contaminated groundwaters and the removal of specific priority pollutants from industrial wastewaters.A selective adsorption process will be beneficial in all of these applications and would be likely to significantly reduce costsin comparison to present, less selective removal technologies such as activated carbon adsorption.
ACKNOWLEDGMENTS Funding support is gratefully acknowledged from the U.S.Geological Surveyand, for S.K.D., from Lyonnaisedes Eaux while on a research sabbaticalat the Institut National Polytechnique de Lorraine. J. Y. Bottero provided invaluable assistance at INPL.
REFERENCES 1.
Pontius, F. W., Complying with the new drinking water quality regulations,
J. A W A , 82(2), 32
(1990).
Pontius, F. W., Federal drinking water regulation update, J. A W A , 85(2). 42-51. 1993. McBride, M. B., Pinnavaia, T. J., and Mortland, M. M., Adsorption of aromatic molecules by clays in aqueous suspension, Adv. Environ. Sci. Technol., 8, 145-154, 1977. 4. Wolfe, T. A., Demirel, T., and Baumann, E. R., Interaction of aliphatic amines with montmorillonite to enhance adsorption of organic pollutants, Clay Clay Min., 33, 301 (1985). 5. Boyd, S. A., Lee, J. -F., and Mortland, M. M., Attenuating organic contaminant mobility by soil modification, Nature, 333, 345-347 (1988). J. F., Mortland, M. M., Chiou, C. T., and Boyd, S. A., Shape selective adsorption ofaro6. matic molecules form water by tetramethylammonium-smectite,J. Chem. Soc. Faraday Tmns. I , 2. 3.
Lee.
85, 2953-2962 (1989).
Jamrah,A. I., andDentel, S. K., Selectiveremovaloforganics by surfactant-modified clays, h o c . Joint CSCE-ASCE Nut. Conf Envtl. Engg., Montreal, 1073-1080, July 12-14 (1990). sur des zeolithes et des 8. Jucker, C., Isothermes d’adsorption de carbone organique total et d’atrazine argiles greffees. -Rapport de Stage, Institut National Polytechnique de Lorraine,1990. 9. Cadena, F., Use of tailored bentonite for selective removal of organic pollutants,J. Environ. Eng.,
7.
115, 756-767 (1989). 10. Srinivasan, K. R., and Fogler, H.S., Use of inorgano-organoclays in the removal of prioritypollutants from industrial wastewaters, Clay Clay Min., 38, 277-293 (1990). 11. Lee, J. F., Mortland, M. M., Chiou, C. T., Kile. D. E., and Boyd, S. A., Adsorption of benzene,
tetramethylammonium-smectites havingdifferentchargedensities, Clays Clay Min., 38(2), 113 (1990).
toluene,andxylenebytwo
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12. Soundararajan, R., Barth, E. F., and Gibbons, J. J., Using an organophilic clay to chemically stabilize waste containing organic compounds, Hazardous Mar. Control, 3 (l), 42 (1990). 13. Burris. D. R., and Antworth, C. I?,In situ modification of an aquifer material by a cationic surJ. Contam. Hydrology,10,325-337 (1992). factant to enhance retardation of organic contaminants, 14. Robeck, G . G . ,Dostal, K. A., Cohen, J. M., and Kreissl, J. F., Effectiveness of water treatment processes in pesticide removal, J. AWWA, 57 2 (1965). 15. Lalezary, S., Pirbazari, M., andMcGuire, M. J., Evaluating activated carbon for removing low concentrations of taste and odor producing organics, J. AWWA, 78,11-16 (1986). 16. Milner, R. J., Baker,D.B.,Speth,T.F.,andFronk,C. A., Treatment of seasonal pesticides in surface waters, J. AWWA, 81(l), 43-52 (1989). 17. Summers, R. S., Haist, B., Koehler. J., Ritz, J., Zimmer,G . , and Sontheimer, H., The influence of background organic matter on GAC adsorption, J. AWWA. 81(5),66 (1989). 18. Najm, I. N.. Snoeyink, V. L., Suidan, M. T., Lee, C. H., and Richard, Y., Effect of particle size and background natural organics on the adsorption efficiency of PAC,J. AWWA, 82(1), 65 (1990). 19. Speth, T.F.,EvaluatingcapacitiesofGACpreloadedwithnaturalwater, J. Environ. Eng. Div.ASCE, 117(l), 66 (1991). 20. Carter, M. C., Weber, W. J.. and Olmstead. K. I?,Effects of background dissolved organic matter on TCE adsorption by GAC, J. AWWA. 8,81-91 (1992). 21. Cadena, F., and Jeffers,S. W., Use of tailored clays for selective adsorption of hazardous pollutants, Proc. 42nd Indust. Waste Conf., Purdue Univ., West Lafayette, Ind., 113-119. 22. Barrer, R. M., Zeolites and Clay Minerals as Sorbents and Molecular Sieves, Academic, London, 1978. 23. Weiss, A., Mica-type layer silicates with alkylarnmonium ions, Clays Clay Min., 10, 191 (1961). 24. Barrer, R. M., andMillington, A. D.,Sorptionandintercrystallineporosity in organo-clays, J. Colloid Interface Sci., 25, 359 (1967). et 25. Demougeot,H.,Adsorptiondemoleculespolluantesdel’eau:phenol,2,4,5-trichlorophenol acide tannique sur des argiles modifiees, DEA Rep. Institut National Polytechnique de Lorraine, 1990. 26. Dentel, S. K., Bottero, J. Y., Demougeot, H., Duguet, J. E,and Anselme. C., Adsorption of tannic acid, phenol, and 2,4,5-trichlorophenol on organoclays, Water Res., (1993), submitted. its adsorption capacity for 27. Wolfe, T. A., Addition of aliphatic amines to montmorillonite to improve organic pollutants in aqueous solution, Ph.D. dissertation, Iowa State Univ., Ames, Ia (1981). 28. Hommeril, F., Utilisation d’argiles greffks en vue de 1’6limination de micropolluants organiques, Rapport de Stage, Institut National Polytechnique de Lorraine, 1986. 29. Jordan, J. W., Organophilic bentonites. I. Swelling in organic liquids, J. Phys. Colloid Chem., 53, 294-306 (1949). 30. Theng, B. K. G . , The Chemistry of Cluy-Organic Reactions, Wiley, New York, 1974. 31. Wo1fe.T. A., Demirel, T., and Baumann, E. R., Adsorption of organic pollutants on montmorillonite treated with amines, J. Water Pollution Control Fed., 58,68 (1986). 32. Boyd, S. A.,Shaobai, S., Lee,J.-F.,andMortland, M. M., Pentachlorophenolsorptionby organo-clays, Clays Clay Min., 36(2) 125 (1988). 33. Smith, J. A. and Jaffk,I? R., Comparison of tetrachloromethane sorption to an alkylammonium-clay and alkyldiammonium-clay, Environ. Sci. Technol.,25, 2054-2058 (1991). 34. Smith, J. A., Jaff6, I? R., and Chiou, C. T., Effect of ten quaternary ammonium cations on tetrachloromethane sorption to clay from water, Environ. Sci. Technol.,24, 1167-1 172, 1990. 35. Vidic, R. D., Sorial, G . A., Papadimas, S. I?,Suidan, M. T., and Speth, T. F., Effect of molecular oxygen on the scaleup of GAC adsorbers, J. AWWA, 8,98-105 (1992). 36. Dobbs, R. A., and Cohen, J. M., Carbon Adsorption Isotherms for Toxic Organics, EPA 609/880-023, MERL, Office of Research and Development, U.S. Environmental Protection Agency, Cincinnati, Ohio, 1980. 37. Srinivasan, K. R., and Fogler, H. S., Use of modified clays for the removal and disposal of chlorinated dioxins and other priority pollutants from industrial wastewaters, Chemosphere, 18,333342,1989.
25
Removal of Chromate, Cyanide, and Heavy Metals from Wastewater
Klaus Schwitzgebel and David M. Manis EET Austin, Texas
1.
INTRODUCTION
The Clean Water Act of 1983 imposed national limitations on the concentrationof metals allowed to be discharged into the waters of the United States. States and municipalities have the authority to impose even more stringent pretreatment standards. In order to comply with the regulations, technologies were developed and continue to be developed that reduce the concentrations of metals in wastewater effluentsto acceptable levels. The technologies currently available to reach the treatment goals have been reviewedin several EPA documents [l-61. In September 1988, the State of California Department of Health Services published a study of the technologies availableto treat aqueous waste. The California study made recommendations regarding the adoption of wastewater pretreatment standards prior to discharge. Performance data of treatment plants are included in this report [7]. The treatments include oxidation of cyanide complexes, reduction of chromates, precipitation of metals as hydroxides and sulfides, coprecipitation with iron, and ion exchange. The basic chemistries for cyanide oxidation, chromate reduction, and the precipitation of metals as metal hydroxides and metal sulfides are discussed in Section 11. Wastewater treatment gets complex if cyanide, chromate, and a mixture of heavy metals are generated in a plant. A novel process [8] to treat such a complex matrix from a custom plating shop is described in Section VI. Cyanide-containing streams are separated from the general rinse and destroyed by hypochlorite in the cyanide reactor. The effluent from the cyanide reactoris fed together with the general rinse into the main reactor. Here chromate (Cr"') is reduced by ferrous sulfate, with Cr"' and Fe"' being the reaction products. Heavy metals present are coprecipitated with Fe(OH),. Chromate reduction and heavy metal precipitation can be performed in one vessel at pH 9.5-10. This approach greatly reduces the number of clarifiers and reactors needed for treatment. Performance data for this treatment scheme are provided. 535
Schwitzgebel and Manis
536
II. CHEMISTRY OF AQUEOUS TREATMENT FOR CONTROL O f CYANIDE, CHROMATE,AND HEAVY METALS The removal of heavy metals as hydroxides or sulfides from aqueoussolutions requires the oxidation of cyanide and the reduction of chromate before the precipitation step. Cyanides are oxidized above pH 11 using elemental chlorine (Cl,) or hypochlorite (C10-) solutions. Rapid decomposition of the intermediate, cyanogen chloride (CNCl), occursat this high pH with cyanate (CNO-) as the end product. This compound can be further oxidized to N2 and CO2 ina second chlorination step at pH 7.5. Very concentrated cyanide solutions, such as cyanidebased heat treatment melts used in the metal industry or spent cyanide-based electroplating baths, are best destroyed electrolytically. Chromates are normally removed in a two-step process: reduction of chromate at pH 2-3 using sulfite and precipitation of chromium hydroxide at pH 8.5. The reduction and precipitation can be reduced to one step in the alkaline region using a ferrous compound. Precipitation of metals as hydroxides encountersdifficulties in a multielement matrix, especially if amphoteric elements are present. Coprecipitation with iron widens the pH region suitable for metal precipitation. Sulfides show lower solubilities than hydroxides, but dosage and odor problems are the main obstacles to the wide use of this precipitation reagent.
A.CyanideOxidation Cyanide (CN-)-based plating baths exhibit the unique property of producing fine crystalline, shiny metal coatings. They are therefore widely in use despite the high toxicity of cyanide and the greater control effort necessary in pretreatment. Cyanide destruction is commonly achieved through oxidation inthe alkaline region using chlorine gas or sodium hypochlorite. Peroxide oxidation and treatment with ozoneare reported to be feasible. Electrolytic oxidation has advantages in treating highly concentrated cyanide solutions stemming from heat treatment melts and spent cyanide baths [9]. 1. AlkalineChlorination Oxidation of cyanide (CN-) to cyanate (CNO-) in the alkaline region is achieved either through chlorination with chlorine gas or through treatment with hypochlorite. The highly toxic cyanogen chloride (CNCl) is formed as an intermediate. The overall chemical reactionsare
2 Cl2
+ 2 NaCN
2 CNCl
4
+ 4 NaOH
+(1) 2 NaCl 2NaCNO + 2 NaCl + 2 H20
2 CNCl
4
(2)
The cyanogen chloride in reaction (1) is formed instantly, andthe formation rate is independent of pH. The hydrolysisreaction, reaction (2), however, is pH- and temperature-dependent. Entwistle [9] reports that at 15°C and pH 11the cyanogen chloride hydrolyzes to about 0.1% of its original value within10 min. The CNCl decompositionrate slows appreciablyat lower pH values. The cyanide oxidation is therefore performed at pH values of 11or greater to avoid buildup of cyanogen chloride and the escape of toxic CNCl gas. The useof gaseous chlorine (Cl,) necessitates the installation of a chlorinator. This is avoided by adding thechlorine in the form of sodium hypochlorite (NaC10). Sodium hypochlorite is commercially available as an aqueous solution with15% available chlorine. The overall reactions using hypochlorite are
+ 2 NaCN + 2 H20 + 2 CNCl + 4 NaOH (3) 2 CNCl + 4 NaOH + 2 NaCNO + 2 NaCl + 2 H20
2 NaOCl
(4)
Removal of Chromate, Cyanide, and Heavy Metals
537
Table 1 DissociationConstants of MetalCyanideComplexes at 25°C Equilibrium expression
Complex Cd(CN):Zn(CN):Ag(CN); Fe(CN):Cu(CN)ZNi(CN)zFe(CN)Z-
1 I 1 1
x 10-6 X 10-17
x 10-2' X
I X
10-25
1 X 10-30
1 X
10-31
Cd(CN):- G Cd" + 4 CN+ 4 CN-. Zn(CN)zZn2+ Ag(CN); e Ag+ + 2 CNFe(CN);- S Fe2+ + 6 CNCu(CN)z- e Cu2+ + 4 CNNi(CN):- S Ni2+ + 4 CNFe(CN)Z- S Fe3+ + 6 CN-
Source: From Refs. 10 and 11.
The cyanate (CNO-) formedin reactions (2) and (4) is much less toxic than cyanideitself. It decomposes slowly in water to form ammonia and carbonic acid: HCNO
+ 2 H20
-
NH3
+ H2CO3
(5)
Local regulations may allow direct dischargeof cyanate (CNO-). If cyanate (CNO-) discharge is prohibited, its decomposition into CO, and N, can be accomplished ina second chlorination step using chlorine [reaction (6)] or hypochlorite [reaction (7)l.
+ 6 NaOH + 2NaCNO + 6 NaCl + N2 + 2 NaHCO3 + 2 H20 3 NaOCl + H20 + 2 NaCNO 3 NaCl + NZ + 2 NaHC03 (7)
3 Cl2
(6)
These reactions take place at a pH 7.2-7.7. At higher pH values, other oxidation products such as nitrates are also formed. This causes additional chlorine consumption. In the presence of heavy metals, e.g., zinc, cadmium,or copper, cyanideis complexed and the initial cyanide-chlorine reaction rate is slowed down. This necessitates reaction times of 30-40 min in the cyanide reactor. Nickel cyanide complexes require 24 h for destruction, while Fe cyanides are only oxidized to the ferricyanide complex (Fe(CN)d3-. For this reason it is essential tostrictly separate cyanide-containingstreams from nickel-andiron-containing rinses. Further destruction of ferricyanide is reported to be feasible with hydrogen peroxide and ultraviolet light irradiation' [ 5 ] . The dissociation constantsof the heavy metal cyanide complexes givean indication of the difficulty of cyanide destruction throughchlorination. Table 1 lists these constants for themetals commonly encountered in the plating industry [10,1l]. 2. Electrolytic Oxidation Special problemsarise during treatmentof highly concentrated cyanidesolutions such as spent electroplating and heat treatment baths. Heat treatment baths typically contain BaCI, (26%), NaCN (m%),Na2C03 (13%), NaCl (21%), and graphite. Cyanide destruction can be achievedin a process described by Entwistle [9]. The process equipment consists of five elements: spray dissolution chamber, settling chamber,electmlytic cell, crystallizer, and scrubber. The destruction of heat treatmentsalts is possible in a closed-loop operation. The salts are dissolved in a spray dissolution chamber, and insolubles are removed through settling. The electrolytic cell is constructed of a mild steel tank, graphite anodes, and mild steel cathodes. A dc voltageof 2.8-3. l V is applied. The current reaches several thousand amperes depending on the cell size. Chloride must be presentin sufficient quantityto generate hypochlorite within the cell.
Schwitzgebel and Manis
538
Products generated in the electrolytic cell are sodium carbonate and bicarbonate, sodium formate, sodium oxalate, and ammoniumcarbonate. Sodium cyanateis present as a minor component (0.5%). Some ammonia escapes with the off-gas and is controlled by wet scrubbing. of crysThe waterin the feed stream canbe balanced through evaporation losses and water tallization, which is mainly removed with the sodium carbonate decahydrate. Barium forms insoluble BaCO,. Heavy metals like Cd, Cu, and Zn can be recovered a metal deposits on the cathodes. The process efficiency depends on the NaCN concentration. Direct current power consumption ranges from 0.004 to 0.019 k W g CN- for cyanide concentrations of 100 g/L and 5 g/L, respectively. At lower cyanide concentrations the efficiency drops sharply (Figure 1). Iron and nickel cyanides are reported to be destroyed in the process [9].
B. ChromateReduction Hexavalent chromium is toxic, and most chromates are water-soluble. OnlyBa2', Pb2+, Hg , and Ag+ form insoluble chromates. Pollution with chromatesis especially troublesome, since chromates are not adsorbed by soil and thus reach the groundwater table rapidly. Most chemical control strategies are based on the reduction of hexavalent chromium (Cr") to trivalent chromium(Cr"'). Trivalent chromium is much less toxic than chromates and forms a hydroxide with a solubility minimum inthe pH range 8.0-9.0. Common reducing agents used are sulfur dioxide (SO,) and its salts, ferrous salts, and iron sulfide. +
1.
ChromateReductionwithMetabisulfite
The most widely used reducing agent in SO2-based processes is metabisulfite (Na2S205).It crystallizes from solutions containing NaOH and SOz in a mole ratio of 1 :1. In aqueous solutions it forms an equilibrium with the disulfite ion: S2052disulfite
+ H20 e
2HS03
metabisulfite
Aqueous solutions of sodium metabisulfite react acidicly. The solubility in water is 54 @l00 mL at 20°C [ 121. Metabisulfite reduces chromates in acidic solutions: 4 H2CT04
+ 3 Na2S205 + 6 H2SO4Cr2(SO4)3
+ 6 NaHS04 + 7 H20
(9)
The sulfite (S4+) is oxidized to sulfate (S6'). The reaction consumes acid and is strongly temperature- and pH-dependent. The reaction rate increases with decreasing pH. The sulfur dioxide equilibrium pressure also increases with decreasing pH, causingSOz gases to escape that are toxic and cause corrosion. Thereforea pH range of 2-3 is generally maintained in the reduction reactor as a compromise between SO2 backpressure and reaction rate. 2.
Chromate Reduction with Fe" in Acidic Medium
Chromate reductionis feasible with ferrous salts in acidic solutions.The redox potentialof the Fe"'/Fe" chain is pH-independent in the acidic region, while the oxidation potential of chromate is strongly dependent on the hydrogen ion concentration. Chromate reduction using ferrous ions in the acid region is performed at pH values 1 3 [13]. The overall reaction,
2 H2CT04 + 6 FeS04 + 6 HzS04 + Cr2(SO4)3 + Fe2(SO4)3+ 8 H20
( 10)
consumes 3 mol of Fe" per mol of Crv' reduced to Cr"'. Fe" is oxidized to Fe"' in the reac1.5 mol tion. This stoichiometry is less favorable than using SO2-based reducing agents where
539
Removal of Chromate, Cyanide, and Heavy Metals 100
80
60
40
2c
c 0.00
0.01
0.02
0.03
[kWh/g CN] Figure 1 Electrolytic decompositionof cyanide. Effect of cyanide concentration (glL NaCN) on process efficiency (kwhlg CN- destroyed). (Data from Ref. 9.)
Schwitzgebel and Manis
540
of SOz is oxidized for every molof Crv' being reduced. An additional disadvantage is the introduction of iron into the solution, which increases the sludge production in the chromium precipitation step. These disadvantages must be weighed against the advantages offeredby the iron process. Chromate reduction by iron is odorless. No toxic and corrosive gases escape from the reactor as in SO2-based processes. In addition, heavy metals are precipitated to concentrations one to two orders of magnitude lower than is achievable by hydroxide precipitation of these metals in the absence of iron. These advantagesare compounded if the chromate reductionis performed in the alkaline region. Chromate reduction and metal precipitation can be achieved in one reactor at one pH under alkaline conditions. 3. ChromateReductionwithFe"inAlkalineMedium Iron (11) andiron (111) precipitate in the neutralandalkalineregion as hydroxides.The hydroxide Fe"' hydroxide is by 24 orders of magnitude less soluble than the Fe" [Ks,,(Fe(OH)3) = 10-37.4; KSp(Fe(OH)J = 10"3.8]. The Nernst equation describes the redox potential of the Fe"'/Fe" cham:
E = 281
- 59
X pH
(m V)
( 12)
The Fe"' and Fe" activities are expressed as functionsof pH using the solubility productsof the hydroxides, the ion product of water (K,,,), and the definition of pH. 2
Ksp(Fe(OH)2)=
URII U OH-
K,(Fe(OH)3) =
U ~ I U
pH = -IOg
( 13)
20"-
(14)
(17)
aHt
The potential described by Equation (12) is more negative in the entire alkaline rangethan the oxidation potentialof the Crvl/Cr"' chain. This makes the reductionof chromate feasible in the pH range of 7-1 1. Fe" can be added in the formof ferrous sulfate, which is a by-product from acid pickling in steel mills. The Fe" can also be produced electrochemically by anodic dissolution of steel anodes [7]. 4. ChromateReductionwith Iron Sulfide Chromate can be reduced by the addition of FeS at pH 7. Fe" is oxidized to Fe'", and the sulfide ion is oxidized to elemental sulfur [4]. H2CI04
+ FeS + 2 H20 + Cr(OH13 +(OH)3 Fe
+S
The reaction time needed for the completion of reaction (18) is several hours [14].
(18)
Removal of Chromate, Cyanide, and Heavy Metals
541
Table 2 Theoretical pH for Metal Precipitation to 0.1 ppm
4.9 3.4 1.6 'Kspvalues from Ref. 15.
C. Metal Hydroxide Precipitation After the oxidation of cyanide and reduction of chromate, the metals can be precipitated as hydroxides. A prerequisite isthe absence of strong complexing agents suchas ethylenediamine tetraacetic acid (EDTA). The degreeof metal precipitation is dependent onpH. At a given pH, different metals exhibitdifferent solubilities. The amount of metal in solution as a function of pH can be calculated theoretically from the solubility product (Ks,,)of the hydroxide. For divalent metals this takes the following form:
Kw
COH- =
G
CM:+
This correlation relates the equilibrium metal concentration with the hydrogen ion concentration C, +.Ion pairing is neglected, and the activity coefficients are assumed to be unity in this derivation. Table 2 lists the solubility products for the most common metal hydroxides W]. The theoretical pH values necessary to reach an equilibrium metal concentration in solutions of 0.1 ppm are calculated from Equation (23).
Schwitzgebel and Manis
542
In practice, the equilibrium concentration of 0.1 ppm is not reached at the pH values inof freshly precipitated hydroxide dicated in Table 2. This is due to ion pairing and the tendency solutions to stay supersaturated. The metal solubility can be reduced by a pH increase above the values given in Table 2. This increases the driving forcefor hydroxide precipitation. However, this approach has limits, especially when dealing with hydroxides that resolubilize in excess base. Such amphoteric behavior is typical for beryllium, lead, zinc, tin, chromium, aluminum, and antimony. The hydroxides of these metals redissolve in excess base and form [Be(OH),]'-, [Pb(OH),I'-, [Zn(OH),]-, [Sn(OH),]-, [Cr(0H),l3-, [AI(OH),I-, and [Sb(OH),I- complexes. Ammonium salts, if present, also can hinder hydroxide precipitation due to the formation of soluble ammonium complexes: [Ag(NH,),] +,[Fe'1(NH,)6]2+,[Ni(NH3)6I2+, [cd(NH,),]'+, [Mn(NH3)612',[Zn(NH3)412't [CU(NH~)~I*+. The previously mentioned tendency of freshly precipitated hydroxide solutions to stay supersaturated becomes obvious if the results of Table 2 are compared with the experimental data shown in Figure 2. The metal concentration in solutions for single-metal precipitation experiments are one to two orders of magnitude higher than would be expected from theoretical calculations. Also shown in Figure2 are the residual metal concentrations in a mixture of 20 ppm each of CrV1,Pb, Cu, Cd, Zn, and Ni. CrV1was reduced with Fe", and the hydroxides precipitated at different pH values. Due to coprecipitation with Fe(OH), there is considerable decrease in residual metal concentrations compared to that measured for individually precipitated metals. This coprecipitation is the reason for the good performance of iron-based processes observed in the industry, especiallyfor the removal of a number of elements in the presence of chromate.
D. Metal SulfidePrecipitation The solubility productsof metal sulfides are several orders of magnitude lower than thoseof the corresponding hydroxides. This should make it feasible to precipitate the heavy metals from solutions containing complexing agents that prevent metal removal by hydroxide precipitation. Also, the pH range for the precipitation of metals as sulfides is much broader. Table 3 lists the solubility products for metal sulfides of the type MeS, Me$, and Me2S3 [15]. Metal sulfide precipitation occurs if the solubility product is smaller than the product of metal concentration times sulfide ion concentration: Table 3 Solubility Products (Ksp)for Metal Sulfides of Type MeS, Me& and Me$, MeS 10-21.6 10-21.7
10-27.0 10-27.9
MnS FeS ZnS a-Cos p-cos a-NiS p-NiS SnS CdS PbS
cus
HgS PtS
KSP
10- 15.2
10-21.3 10-26.7 10-20.5 10-26.0
10-28.6 10-40.2 10-51.2 10-72. I
Source: Data from Ref. 15.
Me$
6,
KSP
10- 19.9
TI$ Hg2S
10-47.0
10-85.0 Ag2S
10-52.2
Bi2S3
10-28.4 10-58.5 10-71.8
c02s3
10- 124.0
Sb2S3
Removal of Chromate, Cyanide, and Heavy Metals
543
100
P
5
.l
.Ol
Imp
l
l 0 0
S
e
S ’
Figure 2 Metal removal efficiency by hydroxide precipitation and Fe”’ coprecipitation. (---) Solubilities of metals precipitated with NaOH from single-metal solutions as a functionof pH. (0) Solubilities of metals precipitated with ferrous sulfate-NaOH from a solution containing 20 ppm each of Cr”’, Pb, Cu, Cd, Zn, and Ni as a function of pH.
544
Manis
Ksp S
Schwitzgebel and
CS2-
Sulfide (S2-) is the dissociation product of H2S. The solubility of H2S in water is 0.1 moYL. H2S dissociates in two stages: H2S HS-
+ P
"*
HSS2-
+ H" + H+
K1
= 10-6.9m01/l
KZ = 10"2.9 mol/!
(25)
(26)
We find for the overall reaction: H2S + 2H+
+ S*-
K 3 -- 10"9.8 (mo1/1)2
(27)
From Equation (27). we calculate that at pH 7 a hydrogen sulfide solution of 0.1 mol/L contains only 10" mollL of free S2- ions. The most solublesulfide (MnS) listed in Table 3 would moYL = 5.5 mg/Lunder shownanequilibrium concentration of [Mn] = K,,,/lO" = these precipitation conditions. In practice, processes are designed to operate with lower sulfide concentrations in order to minimize the H2S odor in the working environment. Theodor detection level of hydrogen sulfide is 0.1-1 ppm. Because H2S is toxic, the workplace H,S limit is set by the Occupational Safety and Health Administration (OSHA) at IO ppm. Sulfide can be added in soluble form as Na2S or NaHS. The problem with this approach is the correct sulfide dosage. Encouraging results using a sulfide-specific electrode were reported [4]. The Sulfex process [4] uses freshly precipitated FeS as the source for sulfide and operates in the neutral pH range. FeS dissolves to its equilibrium concentration. The sulfide ions liberated in this fashion precipitate all sulfides with a solubility product smaller than that of FeS. From Table 3 it is seen that only manganese would not be precipitated as MnS. The dissolved iron precipitates as Fe(OH),. The FeS dosage necessary in this approach is several times the theoretical amount. The amount of sludge produced is therefore large in comparison to the amount of heavy metals removed. Chromate, if present, is reduced and precipitates as Cr(OH),.
111.
ORP and pH MEASUREMENT
To apply the chemistry discussedin Section I1 to an automated process, it iscritical to control the pH and the oxidation reduction potential(ORP). The adjustment of the pH and the ORP is achieved with electrodes in combination with millivolt controllers and chemical feed pumps. The electrode system for pH determination consists of a glass electrode and a reference electrode. The mostcommonreference electrode is the saturated calomel electrode [Hgl Hg,Cl,/KCl(sat)] with a constant potential of 241.2 mV at mom temperature. Similarly, the ORP is determined with a platinudcalomel electrode chain. The electrodes are interfaced with a pH/ORP meter. The pH electrode system is calibrated using buffersof known pH. A pH 7 solution is used to set the intercept at the pH meter, while buffers at pH 10 or 4 are used to set the slope. If no commercially available solutionsare at hand, the buffers can be prepared from base standards W]. Buffer systems for other pH values are given in the Handbook of Chemistry and Physics W]. The ORP measuring system canbe calibrated using a buffer solution saturated with quinhydrone. Quinhydrone decomposes in aqueous solution intoan equimolecular mixtureof hydroquinone and quinone, which in turn impose a fixed potential on the platinum electrode. The
Removal of Chromate, Metals Cyanide, Heavyand
545
potential of the quinhydrone is pH-dependent. If a calomel electrode is used as a reference electrode, the following potential difference results:
AE (Wcalomel in saturated quinhydrone) = 699.7 (mV) - 241.2 (mV) - 59.2 (mV) X pH
(28)
For a buffer of pH 4.00 saturated with quinhydrone, a potential difference of 222 mV results. The potential of a saturated silver electrode is 45.5 mV less than the potential of a saturatedcalomel electrode. Thepotential of a WAglAgClchainin saturated quinhydroneis therefore AE(WAglAgC1 in saturated quinhydrone) = 699.7 (mV) - 195.7(mV) - 59.2 (mV) X pH
(29)
For quinhydrone dissolved in a buffer of pH 4.00, a potential difference of 267 mV results. The ORP measuring devices (ORP controllers) normally requirean intercept and a slope for calibration. The intercept is set by shorting the electrode input terminals (AE = 0). The slope is set usingthe known potentialof the Pt/Ag/AgCl or Pt/Hg/Hg,CI2 chain in pH 4 buffer saturated with quinhydrone. Quinhydrone is very insoluble in water; only 50-100 mg need be added to 250-500 mL of the buffer solution. The pH and ORP curves necessary for process control are determined during bench-scale testing in the laboratory. Figure 3 gives the ORP curve for a sodium cyanide-based solution titrated with sodium hypochlorite solution. In the example shown, the equivalence point is reached at 100-200 mV. For the purposes of process control, the amount of NaCIO) added for cyanide oxidation is therefore sufficient if the potential in the reactor is in the 250-350 mV range. Figure 4 shows the ORP curve for a 100 ppm Cr"' solution titrated with FeSO, reagent at pH 9.5. The equivalence point is reached at -200 mV. The amount of iron (11) added is sufficient for chromate reduction if the potential is slightly more negative than -200 mV. Chemical addition control usingelectrodes is feasible for both batch and continuous systems once the ORP curve is established. The oxidation reduction potential is often dependent on pH. In this case, simultaneous control of the pH is necessary. ORP curves for chromate reduction with sulfite are established in this manner. These curves are strongly pH-dependent; therefore close pH control is necessary. The pH range of 2-3 is normally chosen, becausethe SO2 backpressure gets too high below pH 2 and the reaction rate is not fast enough above pH 3.
IV. DOSAGE OF CHEMICALS USING PROPORTIONAL FEED PUMPS The slopes of the ORP curves shown in Figures 3 and 4 are steepest at the equivalence point. This creates the potential of overdosing the amount of reactant, which is alleviated by the use of a proportional addition system.For this purpose a millivolt-to-frequency converter is added to the ORP controller.This converter is interfaced with the chemical feed pump and controls the pump stroke frequency linearly betweenlow and high setpoints as indicated in Figure 5. In this fashion, the chemical feed rate increases linearly with an increase in the ORP deviation from the ORP set point. The feed rate (pump stroke frequency) slows downas the ORP approaches the desired ORP set point. The dosage of the chemicals with proportional feed pumps is especially useful ifthe pollutant concentrations vary in the feed streams.The pH adjustments are achieved in the same way. In addition, ORP and pH controllers can activate relays at low and high alarm set points. In this fashion, visual or acoustical alarms can be activated and feed pumps can be shut off.
546
Electrode Potential [mVI
Schwitzgebel and Manis
joO1 t
4001
EquivalencePoint
""""""""""""""""""""""""-.
I
0
2
4
146
8 12
1
-
1
10
HypochloriteSolution
[ml]
Figure 3 Oxidation-reduction potential (ORP) of a KCN solution. 100 ppm of CN- titrated with hypochlorite reagent. Electrodes: Pt/saturated calomel.
V. PROCESS PERFORMANCE IN INDUSTRY The State of California Departmentof Health Services collected performancedata on treatment of aqueous wastes. The findings were published in 1988 [7].Table 4 summarizes a portion of the performance data established in this study for Cd, Cr, Cu, Pb, Ni, and Zn. Unfortunately, the pH operating ranges are not well defined in the report.
VI. AUTOMATED PROCESS FOR REMOVAL OF CYANIDE, CHROMATE, AND METAL MIXTURES In this section, the chemistry andcontrol principles described inthe previous sections will be used for effluent pretreatmentin a custom plating shop.Very complex situations were encoun-
547
Removal of Chromate, Cyanide, and Heavy Metals
Potential [mVI
'"1
EquivalencePoint
Iron SulfateTitrant [m11
Figure 4 Oxidation-reductionpotential (OW) of a K,CrO, solutiontitrated with FeSO,reagent. Electrodes: Wsaturated calomel.
tered here, since these shops perform a multitude of plating, cleaning, and stripping operations. A control system had to be installed to meet effluent guidelines that were site-specific.
A. Problem Definition Table 5 shows the bath types and chemicals used in the custom plating shop. The effluent are used intermittently. The following ranges were esconcentrations vary because some baths tablished through effluent monitoring: Cr"', 1-4 ppm; Ni, 7.5-107 ppm; Cu, 1.5-9 ppm; Zn, 1.4-19 ppm; CN-, 1.9-18 ppm. In addition, small amounts of Cd, Pb, Mn, and Se were found as well as the Ag and Sb from to be present. The amounts of Au dragged out from the gold bath the silver bath were small since the precious metal baths are used sparingly. The streams listed in Table 5 can be grouped into four categories:
548
Schwitzgebel and Manis
Pump Stroke Freqjsnq 100%
'
1
0
-100
I
0
1
High Alarm
Low Alarm
I
100
*0°
t
t
Electrode Potential [mvl
300 400 Low Set High Set
Figure 5 Chemicaldosage by proportionaladditionusing an O W controller and amillivoltThe set points for hypochlorite to-frequency converter in combination with a proportional feed pump. addition to KCN solution are shown. Cyanide streams containing Cu, Zn, Ni Chromate streams containingCr"' Metal streams containing Zn, Ni, Se Other: containing Pb, Sn, Fe, Cu, Zn, Ni After elimination of the cyanide-based Ni-stripping operation, CuandZn are the primary heavy metals in the cyanide stream. Cyanide can be destroyed at pH 2 11. Cu(OH)*and Zn(OH), require a precipitation pH range of 10-1 1. The conventional procedure for Cr"' removal using sulfite operates at pH 2-3 in the re)~ reactor. duction tank and at pH 8.5 in the C I ~ O Hprecipitation The last two streams containPb, Zn, Sn, Cu, and Ni, with optimum hydroxide precipitation pH ranging from 8.5 (Pb) to 11 (Ni). An additionalcomplication arises when chlorine is used to treatcyanide-containing streams. Excess chlorine used in the cyanide oxidationstep must be destroyed before discharge. The pH of the discharge stream must lie within a pH range of 6-10. Conventional pretreatment technology for treating copper cyanide, chromium (VI), and 6. In the situation nickel wastewater streams requires the system arrangement shown in Figure at hand, a fourth treatment line mayhavetobe installed to treat the lead-, zinc-, and tincontaining streams,which require a lower pH environment than nickel forquantitative precipitation of metal hydroxides. The expense of a continuous systemwould be excessive dueto the multiple reactors, clarifiers, and controls. Batch treatment would be complex and very laborintensive. A great simplification can be achieved if the treatment scheme shownin Figure 7 is implemented. The cyanides are destroyed in reactor 1. The effluent of the cyanide reactor isfed together with the chromate and generalrinse into reactor 2, which operates at pH 9-10. Here chromate and excess chlorine from reactor 1 are reduced with Fe", and all metals are coprecipitated with Fe"' generated in the reduction of Cr"' and hypochlorite.The effluent from reactor 2 can then be further processed in the conventional way.
Removal of Chromate,Metals Cyanide, Heavyand
S49
Table 4 Process Performance Data for the Control of Cd, Cr, Cu, Pb, Ni, and Zn Metal
Average treatment (mg/L)
Process Hydroxide precipitation
NaOH mfg. battery Cadmium Lead
0.055
0.08 0.067 0.02 2.63 0.58 0.11 0.14
battery NaOH Lead mfg. Zinc battery mfg. Lime precip. Lime finishing Metal
gMetal Copper
Commercial precip. Lime TSD
eryLead Lead components Electr. NaOH
H ponents Electr.Nickel
Lime D ommercial Zinc
0.52 1.8
Lime Commercial TSD mfg. Battery
0.5 1.05
Hydroxide precipitationin the presence ofof
Cadmium finishing Metal Metal finishing Lead battery mfg. components Electr. Copper
Electr. components Metal finishing Commercial TSD
Commercial TSD Lead Lead battery mfg.
components Electr. Nickel Electr. components Metal finishing Commercial TSD Battery mfg.
ng Metal Zinc Commercial TSD
FelAlum
FeCI, + base FeSO, + base Ferrite coprecip. NaOH + FeSO, NaOH + FeSO, FeSO, + hydroxide NaOH + FeCI, NaOH + FeCI, Ferrite coprecip. NaOH + FeSO, NaOH + FeSO, FeSO, + hydroxide NaOH + FeCI, Ferrite coprecip. FeSO, + hydroxide NaOH + AI coprec.
0.07 0.579 0.008 0.17 0.59 0.505
1.2 0.07 0.01
0.165 0.63 0.018 1.1 0.2 0.005 4.1
Surfide precipitation
Cadmium Copper smelter Commercial TSD Army report Commercial Copper TSD
EPA test run
Commercial TSD Lead Copper smelter EPA test run Commercial TSD Nickel EPA test run
ng Metal Zinc Foundry
N%S + lime Lime + sulfide Na,S + FeSO, coprec. Lime + sulfide Sulfide + ion exchange Lime + sulfide Na,S + lime FeS + ion exchange Lime + sulfide Sulfide precip. + ion exchange FeS + ion exchange Sulfide precip.
0.01 0.5
0.06 0.15 2.3 0.01
0.05;0.2 0.5
0.34 13.4 0.5
0.88
Process
azardous edients (gallday) Dragouttype
550 Manis
and
Schwitzgebel
Table 4 Continued Chromium treatment
Average treatment (mg/L) Plant Metal finishing Metal finishing Metal finishing Commercial TSD Commercial TSD Lead battery Lead battery
Reduct. precip. Reduct. precip. FeSO, + hydroxide NaHSO, reduct. 0.12 NaHSO, reduct. FeS precip. Na,S precip.
2.14 2.36 0.151 0.1 0.63 0.014
0.04
0.005
0.005
Source: Data from Ref. 7.
Table 5 Bath Types in Custom Plating Shop, Estimated Dragout and Bath Composition
Bath Aluminum cleaner
114
Steel cleaner
1l2
Mild acid Die cast boil Nickel strike Copper solution
112 118 118 114
Chrome solution Nitric acid Zincate Nickel solution
1 114 114 3.0
Gold solution
1110
Brass solution Silver strike Silver solution
114 1/10 114
Antiquing
114
Antiquing Cleaner
114 114
stripping Paint stripping Silver stripping Nickel Nickel stripping Hydrochloric acid
112 114 112 112 112
Sodium tetraborate pentahydrate, zinc sulfate, sodium nitrate, tetrasodium pyrophosphate hySodiumcarbonate,tetrasodiumpyrophosphate,sodium droxide, ethtoxylated amphoteric sodium salt, sodium metasilicate Sodium bisulfate, sodium fluoride, sodium chloride Not given Nickel sulfate solution Copper cyanide, potassium hydroxide, sodium cyanide, zinc cyanide, sodium hydroxide, thallium carbonate Chromic acid, magnesium silica fluoride, sulfuric acid. Nitric acid Sodium hydroxide, zinc oxide Nickel carbonate, nickel chloride, nickel sulfate, sulfuric acid, boric acid, saccharin (Besniflec 812) Potassium cyanoaurate, cobalt complex, nickel complex, potassium citrate Copper cyanide, zinc oxide, sodium cyanide Sodium cyanide, potassium cyanide, silver cyanide Potassiumcyanide,sodiumcyanide,silvercyanide,potassium antimonyl tartrate Selenous cupric acid, sulfate, phosphoric sulfate, zinc acid, ammonium molybdate Cupric sulfate, selenous acid nitric acid, Sodium hydroxide, sodium metasilicate pentahydrate, sodium carbonate, tetrasodium pyrophosphate Sodium hydroxide, phenyl diethanolamine, sodium metasilicate cyanide Sodium acid Sulfuric Sodium cyanide, sodium hydroxide Hydrochloric acid 50%
551
Removal of Chromate, Cyanide, and Heavy Metals corrm
CYANIDE
e
OXlDAllON
And PREclPrrAnoN
EXCESS
p11 11.5
SWWE HOLDINQ
CHROMAT6
NICE
e
PRECIPITATION
pH 11.2
Figure 6 Conventionalpretreatmentsystemarrangement cyanide, chromate, and nickel.
for treatingwastewatercontainingcopper
CHROMATE AND GENERAL RINSE
7
CYANIDE-
CYANIDE DESTRUCTION pH 11.5
CHROMATE REDUCTION AND METAL PRECIPITATION pH 9.5 to 10
-+
ClARlFlER
"I-IARGE
Figure 7 'bo-step system arrangement for treatment of cyanide, chromate, and heavy metals. mate is reduced by Fe". and all metals are coprecipitated with Fe"' at pH
9.5-10 in one reactor.
chro-
552
Schwitzgebel and Manis
Table 6 SyntheticStreamComposition (mg/L) Noncyanide Inorganic (pH ~~
Copper Nickel Zinc Silver Chromium Cyanide Selenium Cadmium Lead Manganese
Cyanide 11.7). stream 1 ~~~
(pH 2.7). stream 2
~~~~
90 8.6 14 22
-
250 0.02 0.22
Gold
-
-
-
1.4
99 5.9
-
67
-
0.087 0.18 0.05
-
0.87
B. Process Implementation The cyanide streams contained nickel stemming from the nickel stripping operation. Since Ni(CN)42- cannot be destroyed throughchlorination, this bath was substituted by a bath con7 had taining noncyanide chemicals. The feasibility of the process arrangement shown in Figure to be tested in the laboratory. Flow rates and stream compositionswere needed. Theflow rates were estimated from the individual rinse water streams to be approximately 5 gal/min for the cyanide-containing streams and 15-20 galhin for the remaining streams (general rinse). The cyanide-based baths were operated intermittently whereas the chromium and nickel lines were in continuous use. The concentrations expected in the cyanide and general rinse streams after stream segregation were estimated from the bath dragout rates. A dilution of 1:250 to 1 :loo0 is expected in the rinse tanks.With these data, a synthetic cyanideand synthetic general rinse effluent was composited from bathconcentrates, which were mixed in the sameratios as the estimated bath dragout rates. The resulting composite was finally diluted 1:250 to obtain simulated cyanide and general rinse effluent streams. These synthetic effluents contained all the chemicals, additives, and brighteners used in the process in their approximate ratios and concentrations and were used in bench-scale testing to prove system feasibility. Analysesof these composites are shown in Table 6 . The cyanide was treated for cyanide destruction using hypochlorite at pH values greater of the redox potential. than 11. The hypochlorite addition was monitored through measurement The cyanide concentration decreased from 250 ppm to <0.02 ppm in this step. Following cyanide destruction, the treated cyanide streamwas combined with the general rinse streamsin a ratio of 1:4 to simulate the reactor 2 influent. This solution was treated with FeS04 reagent at pH 10. The metal concentrations before and after the bench-scale treatment are summarized in Table 7. The discharge limitsare also shown for comparison. The treatment strategy results in an effluent quality well below the discharge standards. The data obtained in the bench-scale study were used to build a fully automated systemas shown in Figure 8. It incorporates the following features. 1. The cyanide stream is collected in a holding tank and is uniformly fed to the cyanide de-
struction reactor.
553
Removal of Chromate, Cyanide, and Heavy Metals Table 7 TreatmentEfficiency and DischargeStandards (mglL)
Tteated effluent Inorganics Reactor
influent
(pH 10)
18 82 7.4 4.1 55 0.18 <0.02
Arsenic
-
0.20 0.12 0.02 0.03 C0.05 <0.008 <0.02
Cadmium Lead
0.07 0.15 0.04 0.16
<0.009
Copper Nickel Zinc Silver
Chromium Selenium Cyanide
Manganese Gold
-
Discharge standards (pH5.5-10.5) 2.02 2.33 l .45 0.24 1.67 0.02 0.64 0.70 0.07 0.42 1. 0 0 NA
~~
NA = not available.
2. This reactor is well stirred. The pH and the ORP are continuously monitoredby electrodes. NaOH and hypochlorite are automatically fed into the reactor to maintain preset pH and ORP values determined duringthe bench-scale phase of the program. 3. The effluent of the cyanide reactor is fed by gravity feed into the main reactor. 4. The daily rinse is collected in a holding tank and is uniformly fed into the main reactor. This vessel is well stirred. The pH and the redox potentials are monitored by electrodes. FeSO, reagent and sodium hydroxide are automatically metered into the reactorto maintain preset ORP and pH values. This allows for chromium reduction andheavy metal precipitation in one reactor and at one pH: Chromium reduction: Cr6'
+ 3 Fe2'
+ C?+
+ 3 Fe3+
Metals precipitation:
+ 3 OH- + Cr(OH)3 Fe3+ + 3 OH- + Fe(OH)3 CU" + 2 OH- + CU(OH)~
C?'
+ 2 OH- + Ni(OH)2 Cd2+ + 2 OH- "* Cd(OH)2 Pb2+ + 2 OH- + Pb(OH)2 Zn2+ + 2 OH- -+ Zn(OH),
Ni2+
5 . Precipitated metal hydroxides usually form fine particles, which settle slowly. Addition of a polymer achieves agglomeration of the precipitate, greatly improving the solid-liquid separation in a gravity clarifier. 6. The main reactor effluent solution is transferred to a lamella clarifier with built-in flocculator. The flocculator provides gentle, shear-free mixing for about 3 min of retention
E1echodes &
Feed Tank Sulfate
Level Conmllers
Control Panel
Filter Press
I
Cyanide Collection
1I
9r;( I II Rinse collectioo
,II 7
I
I
I Sludge Tank Overflow Return
1
Figure 8 Flow schematic of a two-step system for cyanide oxidation, chromate reduction, and metals precipitation.
Hydroxide Sludge
-.
osite
555
Removal of Chromate, Cyanide, and Metals Heavy Table 8 SystemPerformance,Values in mglL
24-hr
samples
As co.01 co.01 co.01 Ba C0.7-C0.7 C0.7 Cd co.01 co.01 co.01 Cr C0.06 C0.06 C0.06
-
-
0.40 0.03 C0.06 C0.06
Pb Mn
co.04
c0.02 c0.02 c0.02
Hg
co.04
Ni Se
<0.008
Ag
c0.02
Zn Fe CN COD
monitoring compliance
8/27/90 8/30/90 8/31/90 9/1/90 10/9/90 12/3/90 2/25/91 4/2/91
Parameter 8/26/90
cu
Grab startup period
0.06 0.4
0.30
-
0.03
-50.06
CO.04
co.04
co.04
co.04
CO.008
CO.008
c0.02
CO.01 0.2
c0.02 cO.01 0.3
0.04
C0.02
-
-
-
-
0.12 -
-
0.04
-
-
-
-
-
-
C0.005 0.02
co.01 co.01
c0.02
0.17 co.10 co.10 co.10
co.02
c0.005 C0.005 C0.005 <0.02 c0.02 c0.02 co.01 co.01 c0.02 c0.02 c0.02 0.12 0.07 0.05 0.190
c0.02
co.001 co.001 0.200.14 0.13
0.005
C0.005
c0.02
0.02
-
-
0.10 0.14 0.11
-
0.06
-
43
19
c0.02 <0.001
c0.02 co.001
CO.10
CO.005 C0.005 c0.02 c0.02
-
0.08
28
c0.02 16
time, allowing the particlesto grow. The clarifier overflow is discharged to a holding tank. A final filtration step can be added if needed. 7. The sludge collected in the clarifier is discharged from the bottom via a diaphragm pump be adjusted by a timer located on the for which the frequency and duration of operation can central control panel. Sludge from the clarifier is collected in the sludge holding tank, from which it is pumped to the filter press. A level indicator will alert operators if the sludge exceeds a preset level. 8 . Sludge that has been separated and prethickened by the clarifier is further thickened to 25-35% solids using a filter press equipped with gasketed, recessed polypropylene filter plates and a high pressure feed pump. Thickened sludge is collected in a self-dumping sludge hopper or in 55-gal drums. a which 9. A final pH adjustment is not necessary, since the system is operatedpHat 9.5-10, is well within the discharge limits of 5.5-10.5.
C. System Performance The system described was started up during July and August of 1990. Table 8 summarizes the system performance during thestartup period as well as the monitoring results established using 24-hr composites.
SYMBOLS akw a,+" CH
+
CM:+ cOH-
Fe3+ ion Activity theof Activity ofFe2+ ion the Concentration of the hydrogen ion Concentration hydroxyl the of ion Concentration theof Me2+ ion
H+ OH-
Schwitzgebel and Manis
Concentration of the sulfide ion S'' Normal potential Potential Faraday constant Dissociation constant Solubility product Solubility product of Me(OH), Ion product of water Metal Oxidation reduction potential Gas constant Absolute temperature, "K
REFERENCES 1. EPA, Environmental Regulations and Technology, The Electroplating Industry, EPA 625/10-80-001 (1980). 2. EPA, Control and Treatment Technology for the Metal Finishing Industry, In-Plant Changes, EPA 62518-82-008 (1982). 3. EPA, Environmental Pollution Control Alternatives: Economics of Wastewater Treatment, Alternatives for the Electroplating Industry, EPA 625/5-79-016 (1979). 4. EPA, Control and Treatment Technology for the Metal Finishing Industry, Sulfide Precipitation, EPA 625/8-80-003 (1980). 5. EPA, Control and Treatment for the Metal Finishing Industry, Ion Exchange, EPA 625/8-81-007. 6. EPA, Environmental Pollution Control Alternatives: Centralized Waste Treatment Alternatives for the Electroplating Industry, EPA 6235-81-017 (1981). B., Staff Report on Proposed Treatment Standards for Metal-Containing Aqueous Wastes, Fi7. nal Draft, State of California, Department of Health Services, Sacramento, Sept. 8, 1988. 8. Schwitzgebel, K., Integrated process for cyanide and heavy metal removal from plating process waste streams, U.S. Patent 5,106,508 (1992). 9. Entwistle, J. E., The electrolytic processingof cyanide wastes,Efluent Water TreatmentJ . , March 1976. 10. Brown,T. L., andLeMay, H. E., Chemistry: TheCentralScience, Prentice-Hall,Englewood Cliffs, N.J., 1977, p. 51 1. 11. Blackmer, G. L.,Analytical Chemical Methods, Texas Tech Univ. Press, 1980, p. 245. 12. Handbook ofchemistry and Physics, Weast. R. C., (ed.). 59th e d . , CRC Press, 1978-79. 13. U.S. EPA, Treatability Manual,Vol. 2, Technologiesfor ControllRemovalof Pollutants, EPA-600-, 2-82-001c, U.S. Environmental Protection Agency, Washington, D.C., 1981. 14. Grosse. D., A review of alternative treatment processes for metal-bearing hazardous waste streams, JAPCA, 36(5), 603-614 (1986). 15. Jander, G., and Blasius, E., Lehrbuch der analytischen and prc7parativen anorganischen Chemie, S. Hirzel Verlag. Stuttgart, 1988.
Lee.
26
Neutralization Tactics for Acidic Industrial Wastewater Christopher A. Hazen and James I. Myers Miles Inc. New Martinsville, West Virginia
1.
INTRODUCTION
This chapter was written to provide insight into the background and tactics surrounding the neutralization of acidic industrial wastewater. Several major topicsare covered, including pH measurement, lime and caustic use,and design considerations when using either of these two as a chemicals alone or in a two-stage neutralization technique. This chapter is meant only be obtained in the refguide to neutralization tactics; more detailed technical information can erences listed at the end of the chapter.
II. pH MEASUREMENT pH is a unit of measure that describes the degree oforacidity alkalinity of a solution.The origin of the terminology comes from the letterp , meaning power, and the letter H symbolizing hydrogen [l]. Formally defined, pH is the negative logarithm of the hydrogen ion activity: pH = -loglo[H+] Since pH is a logarithmic function, a change of one pH unit represents a tenfold change in hydrogen ion concentration. Figure 1 includes a table correlating pH to the hydrogen ion and hydroxyl ion concentrations, [H+] and [OH-]. The control and measurement of pH are necessary in a variety of chemical processes. In acidic industrial wastewaters it is necessary to maintain pH within the range6-8 to effectively meet state and/or federal permit conditions. A rough indication of pH can be obtained using pH papers or indicators, which change color as the pH level varies. These indicators have limited accuracy and can be difficult to interpret correctly in colored or murky samples. 557
Hazen and Myers Acid pH Range
0
Neutral “ -
” _
” ” _
-> 7
Base
> 14
” ” ” ” ” “
pH = -Log of Concentration of H+
pH of 1 = -Log(. 1) pH 1.0000E-14 1 2 3 4 5
6 7 8 9 10 11
12 13
14
H+Conc. 0.1 0.01 0.001 o.Ooo1 o.oooo1 0.000001
1.0000E-13 1.0000E-12 1.0000E-l1 1.0000E-10 0.00000000
0.0000001
0.00000001
0.00000001 0.00000000 1.0000E-10 1.0000E-11 1.MxHIE-12 1.oooOE-13 1.oooOE- 14
0.0000001 0.000001 o.oooo1 o.Ooo1
OH- Conc.
0.001
0.01 0.1
Figure 1 Understanding pH. More accurate pH measurements are obtained with a pH measuring system. The basic pH measuring system consists of three parts: a glass electrode, a reference electrode, and a high input impedance pH meter. The glass electrode is a hydrogen-ion-sensitive glass bolb with a millivolt output that varies with the changes in the relative hydrogen ionconcentrationinside and outsidethe bulb. The millivolt output is developed as a result of differences betweenthe hydrogen ion activity in the sample and that in the standard solution contained within the electrode. This potential, measured relative to the potential of the reference electrode, gives a voltage that is expressed as pH [2]. As stated before, the glasselectrode alone is not sufficient to measure the potentialof the liquid system. A reference electrode is needed to complete the measuring circuit. The reference electrode provides a stable potential by surrounding an internal elementwith a known solution. The reference electrode has three principal parts:an internal element, a filling solution, and a permeable junction through which the filling solution escapes from the electrode. The liquid junction can take several forms, but its principal function is to allow small quantities of the reference electrode’s filling solution to slowly leak or migrate intothe sample being measured. Reference electrodes are available in a variety of types to accommodate many different types of samples. The third componentof the pH measuring systemis the pH meter. The glasselectrode bulb has a high resistance acrossit of about 100 megohms. This fact prevents the use ofan ordinary voltmeter for reading out the electrode potentials. A meter that has a high input impedance or low bias current is required [l]. It is important, when trying to maintain proper functionof the pH measuring system, that the electrode and the samplebe in intimate contact and thatthere be no surface contamination to provide a barrier between the ions in solution and the surface of the glass [2].
Tactics Neutralization Wastewater for Acidic
559
Devices usedto measure pH are generally rugged and built to withstand very adverse conditions. Several electrode systems and highly reliable electronic circuits havebeen developed for pH measurement and control. Among these devices and techniques are specific-ion electrodes, coulometric analysis, and polargraphic analysis [3]. The accuracy of a pH measurement depends to a large extent on bias, span, and nonlinearity errors from a varietyof sources. Even under the best industrial conditions, pH electrode potential measurements are only accurate to within k0.017 pH. At pH 13 this is an error of approximately 20.13%. Common sourcesof error in a pHmeasuring systemare broken bulbs, fill contamination, bulb abrasion, bulb dehydration, and coating of the bulb [4]. Periodic maintenance of glass electrodes is normally required to keep the glass membrane free of pluggage and prevent coating with the alkali. To obtain accurate pH measurements, a regular schedule must be establishedto clean the pH probes and calibrate the devices.
111.
LIME: TYPES, USES, COMPOSITION, AND TREATMENT
Acidicindustrialwastewatercan be neutralizedwithslakedlime[Ca(OH),],causticsoda it is the (NaOH), or soda ash (Na2C03). Since slaked lime is less expensive than other bases, most commonly used chemical for acidic neutralization [5]. W Otypes of commercial lime with their associated hydrates are high calcium quicklime, otherwise known as calcium oxide (CaO), and dolomitic quicklime (CaO MgO). and The composition of these will depend on the source of the parent limestone and the method of lime production, particularly the type of fuel used to convert the limestone to quicklime [6]. A high calcium quicklime will produce a high calcium hydrated lime containing 72-74% calcium oxide and 23-24% water in chemical combination with calcium oxide. A dolomitic quicklime will produce a dolomitic hydrate. Under normal hydrating conditions the calcium oxide fraction of the dolomitic quicklime completely hydrates, but generally only a small portion of the magnesium oxide hydrates. The compositionof a normal dolomitic hydrate will be 46-48% calcium oxide, 33-34% magnesium oxide, and 15-17% water in chemical combination with the calcium oxide. npical compositions of commercially available lime are given in Tables 1 and 2. Note the differences between the compositionsof high calcium hydrated lime and pebble quicklime. Slaked lime, commonly known as hydrated lime, is obtained by adding enough water to quicklime to satisfy its affinity for water. Wettingor dissolving tanks are usually designed for 5 min of detention time with 0.5 lb/gal (0.06 kg/L) of water or 6% slurry at the highest feed 250 lb/hr (113.3 rate. Hydrated lime is often used where the maximum feed rates are less than kg/hr) [7]. A general reaction of hydrated lime with acidic wastewater is Ca (OH)2
+ acid S salt + H20
A common system for the storage, transfer, and reaction of hydrated lime is diagramed in Figure 2. Hydrated lime is loaded into a storage silo and metered out using a lime screw/auger, rotary valve, or other metering device. The lime is then transferred by either a lime auger or pneumatic line to themix tank. The mix tank or other device should provide the retention time necessary for complete reaction of the lime. Neutralization is not instantaneous, and contact 5 min, depending on the wastewater composition. time mustbe provided, usually not to exceed It should also be noted that it is very difficult to hold pH values between 6 and 8, since slight changes in lime feed causes wide swings in pH as neutrality is approached [8]. In one industrial wastewater treatment center the lime storage was silo used for limestone, and a pneumatic transfer system was used to transfer the lime from the silo to a mix tank. When
CaO
Si
Hazen and Myers
560
Table 1 High-CalciumHydratedLimeAnalysis results testanalysis Chemical
Total calcium oxide, CaO Available calcium oxide, CaO Calcium hydroxide, Ca(OH), Magnesium oxide, MgO Silicon dioxide, SiO, Total sulfur, S Iron oxide, Fe,O, Aluminum oxide, AI,O, Free moisture Physical analysis test results % Passing 100 mesh screen % Passing 200 mesh screen % Passing 325 mesh screen Bulk density Loose Settled
72.5 70.6 95.8 l .02 1.5 0.01 0.33 0.37 0.97 99.5 98.8 98.7 27 PFC
40 PFC
Table 2 PebbleQuicklimeChemicalAnalysisTest Results (%) Total, Available calcium oxide, CaO MgOMagnesium oxide, Silicon, AI,O,oxide, Aluminum oxide, Iron Fe,O, sulfur, Total S Zinc,
98.8 93.7 1.9 1.36 0.64 0.34 0.035 0.013
r l Ume Sllo
U
Figure 2 Schematic diagram of lime transfer and use.
Tactics Neutralization Wastewater for Acidic
561
the change was made to hydrated limeit quickly became apparent that the “dusty” nature of the hydrated lime made it very difficultto handle with the off-gas collection system. At the exit from mix tank was awet scrubber, meantto scrub out any chlorine that may have been released during the neutralization process andto knock out any dust particles entrained in the air. This scrubber was overwhelmed by the magnitudeof dust particles and could not effectively remove them from the air. The change was then made to a lime auger, which eliminated the majority of the “dusting” problems while still providing effective transfer. Also particular to this system is themix tank. This mix tank is operated as a slurry tank for the hydrated lime. Lime introduced to this tankis combined with water exiting the scrubber, and the mixture is gravity fed to a process trench. The process trench and subsequent basins noted thatat this provide adequate retention time for complete reaction of the lime. Itbeshould time the neutralization requirements are not completely satisfied by hydrated lime. A two-stage neutralization system is used. This type of system uses limeto raise the pH initially and finish the neutralization process using caustic soda (NaOH). This process is detailedin Section V.
W.
CAUSTIC SODA: TYPES, USES, COMPOSITION, AND TREATMENT
Causticsoda,otherwise known as sodiumhydroxide(NaOH),isanothercommonwastewater neutralizing agent. Its most popular form is a solution containing 50% sodium hydroxide by weight.
A.ManufacturingMethods Several well-known processes by which caustic soda is produced are those employing a diaphragm cell, bipolar diaphragm electrolyzers, mercury cell, or membrane cell. In the diaphragm cell process, water is added to sodium chloride with direct current to produce chlorine, hydrogen, and sodium hydroxide. The overall reaction is 2 NaCl
+ 2 H20 + direct current + Cl2 + H2 + 2 NaOH
An asbestos diaphragm is used to separate the anodeand cathode; this serves to keep the caustic soda and hydrogen separated from the analyte as well as to control the flow of electrolyte to the cathode. This situation allows for the most efficient formation of caustic soda and chlorine. Bipolar diaphragm electrolysis is similar to the diaphragm cell process but employs a number of bipolar diaphragm cells in a series circuit of electrolyzers. This design permits current to flow internally within an electrolyzer from one cell to another. Major benefits from this type of design are a higher current efficiency, lower power consumption, and decreasedfloor space requirements. The mercury cell process involves two stages or two cells. In the first stage, sodium and chlorine are releasedby the electrolysisof brine. The sodium dissolves in the mercury cathode, forming an amalgam, while the chlorine is drawn off. In the second stage this amalgam reacts with water to form sodium hydroxide and release hydrogen. The main reactions are
+ H& + directcurrent 2 Na + H& + Cl2 2nd stage: Na (Hg), + 2H20 + direct current + 2 NaOH + H2 + 2 (Hg), 1ststage: 2 NaCl
This type of reaction can occur in several different types of designs, most often in a horizontal or vertical arrangement.
18
562
Hazen and Myers
Table 3 PhysicalDatafor100%SodiumHydroxide(NaOH) Property Molecular weight Boiling point at 760 mm Hg Melting point Specific gravity at 20°C Solubility in Water at 0°C Solubility in water at 100°C Heat of formation at 25°C Latent heat of fusion at 320°C
40.01 1390°C 318.4"C 2.13 42 g/100 mL 347 @l00 mL 101.96 kg callmol 42.5 callg
Holtzclaw [101 ~ e r r y[31 P ~ W[31 Perry 131 Perry P
I
perry [31 Perry [3] h g e [l11
And finally, the membrane cell process is very similar to the diaphragm cell process except that it involves the use of an ion-exchange membrane in place of the asbestos diaphragmused in a diaphragm cell, and nickel instead of steel for the construction of the cathode compartment. This system is used with both monopolar and dipolar arrangements and can produce a higher strength, higher purity caustic [9].
B. Selection Factors thatenter into the selection of the most economical grade and formof caustic soda for a particular application, in addition to purchase price, are the cost of transportation, unloading, handling within the plant, preparing caustic soda for use, and the investment in equipment for unloading, storage, handling, and preparation of the solution [9]. Tables 3 and 4 list the general properties of 100% sodium hydroxide and a 50% solution of sodium hydroxide. Other factorsare the safety and handlingaspects of caustic soda. Caustic sodais poisonous and can be dangerous if handled improperly. To avoid accidental spills, all pumps, valves, and lines shouldbe checked regularly for leaks. Operators should be properly instructedin the precautions needed for the safe handling .of this chemical. Emergency eyewashes and showers to protect personnel fromthe should be provided close to the caustic soda storage and feed area harmful effects of accidental spills [7]. Table 4 Physical Data for 50% Sodium Hydroxide, 50% Water Property Molecular weight NaOH H*O Boiling point at 760 mm Hg Melting point Specific gravity at 20°C Solubility in water at 0°C Solubility in water at 100°C Heat of formation at 25°C Average molecular wt. Viscosity at 40°C Viscosity at 70°C Specific heat of 120°F Solution
40 [IO]
Holtzclaw
288°F 12-15°C 1S253 42 g1100 mL 347 gll00 mL 101.96 kg caVmol 29 22.1 CP 7.1 CP 0.775 Btu (Ib-"F)
[IO] PmPI MSDS [l31 P11 err^ [31 Perry [3] Perry [3] Calculated P m 191 pm [91 Calculated
563
Tactics Neutralization Wastewater for Acidic
Caution must also be exercisedin selecting materials of construction for the caustic soda storage and handling equipment. Iron and steel are the usual choice for equipment handling 50% solutions of caustic sodabelow 140°F. The most popular choice for storage tank construction is mild steel because of its lower cost and adequate performance under most conditions. Certain metals, e.g., aluminum, magnesium, zinc, and tin, are attacked by caustic soda. In addition, silica-containing materials suchas glass, brick, and tiles are broken down by caustic liquor. The action is slow and willat first only contaminate the caustic with silica, but failure of the material will eventually follow [9].
V. DUAL NEUTRALIZATION: A TWO-STEP APPROACH TO NEUTRALIZING WASTEWATER A. Background Because of the downturn of the economy during 1991, the Environmental Control Department (ECD) at Miles Inc. began looking for ways to reduce variable costs within its department [12]. Utilizing a breakdown of departmental costs, the .variable expenses attributed to the neutralization of wastewater were identified as an area of major concern (Figure 3). This wastewater, which has an average pH less than 2, must be treated to meet discharge standards as set by the West VirginiaDepartmentofNaturalResources(WVDNR). The WVDNR issues permits that regulate wastewater discharges and require constant monitoring of the plant effluent pH. Adjusting the wastewater to pH 7 would be similar to raising the pH of vinegar (acidic) to that of tap water (neutral). Consider the historyof wastewater neutralizationat this site. In the 1970s and early 1980s, lime was used as the primary neutralizing agent for wastewater. However, lime loses its efficiency as a neutralizing agentas the pH approaches 7 , and any excessor unreacted lime settles out during clarification. (See Figure 4). Dueto the organic compounds in the wastewater, the resultant sludge is classified as a hazardous wasteand must be disposed of accordingto RCRA hazardous waste regulations. In 1986 the switch was made to caustic sodaas the principal neutralizing agent in an effort of to reduce sludge generation rates. This system was very effective in limiting the amount
1
Ash blspoeal
l
Caustlc
FigUte 3 General cost breakdown. Caustic soda used to neutralizethewastewater is seen to represent the greatest cost factor.
564
Hazen and Myers
Lime EfficiencylSludge Generation 50%
10096 0
I
I
I
I
I
I
1
1
2
3
4
5
6
7
8
PH Figure 4 Lime efficiency/sludge generationversus pH.
wastewater sludge produced, but in 1990 caustic soda experienced a 52% increase in price, which dramatically increased the variable cost of neutralization.
B. Approach The solution selected to lower neutralization costs while minimizing sludge generation rates was to employ a dual neutralization process. This process uses less expensive materials to adjust the wastewaterpH from less than 2 to approximately 3.5 and then finishes the neutralization to pH 7 by using the higher priced caustic. This means that approximately95% of ECD’s neutralization requirements can be met usingthe lower priced lime. (Figure 5 presents a general pH diagram that displays the change in pH achieved with hydroxideaddition, and Figure 6 plots the cost of neutralizing agent versus pH achieved.) The process was tested initially in the laboratory, using several neutralizing materials. It was found that lime couldbe used successfully to achieve lowerpH values with highefficiency. In implementing this process, limewas metered viaa rotary valve to a screw conveyor that conveys the lime into a mixing tank. In the mixingtank, water is addedto create a lime slurry, which is then directed to the process trench and combined with wastewater. This operation occurs just upstream of our neutralization basin, wherecaustic is then used to finish the neutralization process. (See Figure 7.) Implementation of this process is being conducted on a staged basis to evaluate its effect on the rest of our treatment system. To date, no adverse effects have been noted.
C. Results The dual neutralization process has been active since September 24, 1991. During this time caustic use has decreasedby 37% and the variable cost associated with neutralizing wastewater has decreased significantly. There have been nodeleterious effects to the incineration system, and the percentage of acid neutralized with lime is being increased.
565
Neutralization Tacticsfor Acidic Wastewater
PH 10
~
~~~
Hydroxide Addition Figure 5 Hydroxideaddition versus pH.
cost 0 -................................................................................................................................................................................................
............................................... ..""""~.."""'~""~~""
n """"a." _. ..................
-"--".y" &,..W " YYUU"UUl.r.fl.'....... ......................................... ................. -.".... L
8
1.4 1.6
I
I
1.45 1.55
1.5
.......... - - &"........... ""-L's's':'5. ...............*. ................ """"": :&. ...........................
.........................................................................................
---.:&.L
8
--""".
I
I
I
I
I
1.65
1.7
1.75
PH Caustic
Ume
......Both 0......
Figure 6 Cost of neutralizing Agent vs. pH. (0)Caustic soda; (A) lime; (0)both.
1.8
566
Hazen and Myers Wastewater Flow -W
Neutralization Basin
-0 caustic
M Mixing Tank
W
Figure 7 Limeneutralizationprocessdiagram.
REFERENCES 1 . Wescott, C. C., pH Measurements, Academic. New York, 1978. pp. 1-12. 2. OMEGA. The pH and Conductivity Handbook, OMEGA Engineering, Inc., Stamford, Cl', 1992,
pp. A-3, A-4,A-8.
3. Perry, R. H., Perry's ChemicalEngineers' Handbook, 6th ed., McGraw-Hill, New York, 1984, pp. 3-20, 3-153, 22-50. 4. McMillen. G . K.,pH Control, Instrument Society of America, Charlotte, N.C., 1985, pp. 54-68. 5. Wentz, C. A., Hazardous Waste Management, McGraw-Hill, New York, 1989, p. 146. 6. Gutschick. K. A., Lime for Environmental Uses, ASTM Spec. Tech. Publ. 931, ASTM, Philadelphia, 1987, pp. 54-68. 7. Culp, G . L., and Williams, R. B., Handbook of Public Water Systems. Van Nostrand Reinhold, NewYork, 1986, pp. 263-273. 8. ASTM Committee D-19, Manual on Industrial Water and Industrial Waste Water, 2nd ed., Spec. Tech. Publ. 148-E, American Society for Testing and Materials, Philadelphia, 1960. p. 72. 9. PPG, NaOH: Caustic Soda. Tech. brochure. PPG Industries, Inc., Pittsburgh, Penna, 1986, pp. 6-68. 10. Holtzclaw, H. F., Jr., Robinson, W. R., and Nebergall, W. H., General Chemistry, 7th ed., D.C. Heath,Lexington,Mass., 1984, p. 105. 1 1 . Lange, N. A., Lunge's Handbook of Chemistry, Tenth Edition, McGraw-Hill, New York, 1967, p. 1150. 12. Hazen, C. A., Dual Neutralization: a two step approach to neutralizing wastewater, Hazardous and Industrial Wastes: Proceedingsof the Twenty-Fourth Mid-Atlantic Industrial Conference, edited by Brian E. Reed and William A. Sack. Technomic, Lancaster, Penna., 1992, pp. 431-434.
ADDITIONAL READING Boynton, R. S. (1980). Chemistry and Technology of Lime and Limestone, 2nd ed., Wiley, New York. Mattock, G . (1961). pH Measurement and Titration, Macmillan, New York.
Part IV
MODELING FOR POLLUTION CONTROL
This Page Intentionally Left Blank
27
Introducing Uncertainty of Aquifer Parameters into an Optimization Model
Robert L. Ward Ohio Northern Universiv Ada, Ohio
1.
INTRODUCTION
'Qpically, some of the parameters of conceptual hydrologic models are calibrated using limited hydrologic information. The purposeof this chapter is to describe how uncertain knowledgeof aquifer parameters can be incorporated into an optimization program. The optimization program used as the basis for this paper has been described in detail elsewhere [l]. The mathematical description of groundwater flow requires specific assumptions in order to fit the physical events into a set of equations for which a solution exists. The expression of the physical events by mathematical equations, the estimation of the aquifer parameters, and the approximation of complex analytical expressions by their discrete analogs are important sources of error. Because theseerrors introduce uncertainty into groundwater modeling, future projections cannot be made with absolute certainty. The validity of these mathematical equations and the errors introduced by numerical methods have been discussed elsewhere. This work is only concerned with errors introduced by inaccuracies in aquifer Parameters. In the practical simulation of real dynamic systems we are immediately faced with uncertainty as to exact physical parameters. The investigator must establish tolerances within whichthe parameters of the physical system may vary without appreciably affecting the model results. In light ofthe inherent randomnessof subsurface flowand the existenceof uncertainties in be treated as a stochastic process, and aquifer parameters, the groundwater flow system should aquifer parameters should be considered random variables. To carry this argument further, groundwater management modelsshould, if possible, be able to consider the random nature of the subsurface flow system andderive management decisions accordingly. Inthischapter,thedevelopment of a multiperiod stochastic groundwater contaminant management model is illustrated using the Theis equation. The model considers explicitly the random characteristics of transmissivity and effective porosity in a confined aquifer. The stochastic management model is formulated by transforming theobjective function and constraint 569
570
Ward
equations containing random aquiferproperties into a chance-constrained expression that specifies the reliability requirements of the system performance (i.e., the user’s confidence in the system results).
II. PREVIOUS WORK There have been very few studies that have used stochastic concepts at the macroscopic scale in subsurface flow models. The work that has been done can be placed into one of the three possible categories of uncertainty that have been investigated in model solutions: (1) uncertainties caused by measurement errors in the input parameters, (2) those caused by spatial averaging of the input parameters, and (3) those associated with the inherent stochastic description of nonhomogeneous or heterogeneous porous media. The error propagation study of Sagar and Kisiel [2] falls into the first category. They investigated the influence of errors in initial head, transmissivity,and effectivity porosity on the drawdown patterns predicted by the Theis equation for pumpage from a homogeneous isotropic confined aquifer. They used uniform frequency distributions for the input parameters, noting that this is the usual Bayesian “know nothing” prior distribution. They produced plots that show the growth throughtime of the percent error in hydraulic head at various radial distances from a pumping well with various input errors. They also concludedthat a far more general and better (yet mathematically complicated) method of investigating error would be to consider the parameters as stochastic processes. McElwee and Yukler [3] looked at the sensitivity of groundwater models with respectto variations in transmissivity and effective porosity. They obtained sensitivity coefficients by finding the partial derivatives of the Theis equation with respectto each of these parameters. In general, they discovered that a 20% deviation in transmissivity or effective porosity can be handled adequately (error of less than 5% of drawdown) by the first-order formulation that is used in this chapter. The work of Bibby and Sunada[4] combines aspectsof categories 1 and 2. In their analysis they used a numerical simulation model of transient flow toa well in a confined aquifer. They used Monte Carlo simulation to investigate the effect on the solutions of normally distributed measurement errors in initial head, boundary heads, pumping rate, aquifer thickness, hydraulic conductivity, and storage coefficient. In addition they analyzed the uncertainties introduced into the solutions by choosing spatially averaged parameter values at each grid point in the nodal mesh used in the numerical method. They assumed that within each nodal block, each input parameter (hydraulic conductivity, for example) can be represented by a general linear function that fully describes the spatial trends within the block. Theuncertainties in the values of the coefficients of this general linear function (whichare related to the number of available measurements) leadto uncertainty in the spatially averaged value usedat each node in the simulation. This type of analysis leads to the normal distribution for the hydraulic conductivity values. This normal distribution identifies the approach as having more in common with the analysis of measurement errors (category 1) than with stochastically defined media (category 3) where hydraulic conductivity is usually recognized as being log-normally distributed. A paper by Freeze [5] falls into category 3. He concluded that the most realistic representation of a nonuniform homogeneous porous mediumis a stochastic set of macroscopic elements in which the two basic hydrogeologic parameters (hydraulic conductivity and porosity) within these elements are assumed to come from frequency distributions. lbng [6] developed a multiperiod stochastic groundwater management model using the Cooper-Jacob equation and the concept of unit response functions. His general conclusions were that effort should be given to better evaluate transmissivityand its variability. The effec-
571
Parameters Uncertainty of Aquifer
tive porosity in a modeling process can be treated as deterministic, and its accuracy is not critical. However, when the uncertainty of transmissivity is large, the normality assumption for random drawdown may not be appropriate. Furthermore, the assessment of statistical properties of drawdown using first-order analysis may not be appropriate. There have been some investigations regarding the appropriateness of first-order analysis applied to situations where variation of system components is large. Loaiciga and Marino [7] develop a method for estimating the elementsof parameter matrices in the governing equation of flow in a confined aquifer. The estimation techniques for the distributedparametersinverseproblemincludelinearleastsquaresandgeneralizedleast squares methods. Second, a nonlinear maximum likelihood estimation approach to the inverse are derived, problem is presented.The statistical properties of maximum likelihood estimators and a procedure to construct confidence intervals and do hypothesis testing is presented.
111.
MODELDEVELOPMENT
A. Deterministic Model Unit response functions describe relationships between state variables of an aquifer system such as drawdown and management decision variables suchas pumping. The continuous form of convolution relations between aquifer drawdown and discharge for a linear flow system can be expressed as M i= 1
where si,,is drawdown at observation point j at time t; 6ij,T"1+1 is drawdown at observation at point i during time t; qi,,is unit pumpingat point j resulting from a unit impulse of pumping well i during time t; and M is the total number of pumping wells under consideration. The represents incremental drawdownof each time-dependent drawdown response function, observation point at j at time t resulting from a unit impulse of pumping at each discharging well applied at time t = 0. When the time scale is discretized, Equation (1) can be expressed in an equivalent form as
aiJVt,
M
n
i=l
k=l
where si," is drawdown at the jth observation point at the end of the nth period; aijakis the response function for the kth period relating drawdown at the $h observation point to unit pumping at the ith discharging well; and qi.n"k+I is pumping at the ith discharging well during the kth period, k < n. In groundwater management practices, the entire planning horizon is generally divided into operational intervals. An operation policy or management decision may vary from one operational interval to another, but it generally remains the same within each operational interval. As a result, a discrete formulation of the convolution relation, Equation (2), is more practical than the continuous formulation in groundwater management. The unit response function 6 can be obtained from a distributed parameter groundwater simulation model.However, when hydrogeologic information of an aquifer systemis lacking or unavailable, some closed form of analytical solution to an idealized condition can be used to
572
Ward
derive the unit response function. Inthis paper, a stochastic groundwater management model is developed for a confined, homogeneous, and nonuniform aquifer with the following assumptions: (1) The aquifer is nonleaky and infinite in horizontal extent; (2) there is a radial flow pattern; (3) wells fully penetrate the entire thickness of aquifer; and (4) the piezometric head prior to pumping is uniform throughout the entire aquifer. Under these assumptions, the unit response function can be obtained from the well functions:
where qiJ,k= ( I / ~ ) ~ T W [ ~ U in which ~ ~ , ~W.] ] , is the well function, UiJ,k = ( ? ~ ) / ( 4 ~ r , )r; is the distance betweenthe pump well and the observation point; S is the storage coefficient; T is the aquifer transmissivity; andtk is the time at the end of the kth period. The well function for the Theis equation can be written as
In this paper,the Theis equation is used to demonstrate the development of a stochastic groundmodel will be used water management model.A groundwater contaminant plume management to demonstrate our developmentof a stochastic management model. A detailed description. of the model is presented in a previous paper by Peralta and Ward [l]. The problem is to determine the optimal pumpage and pumping pattern over a specified are no undesirable consequences.In general, undesirable conplanning horizon such that there sequences such as depletion of aquifer and land subsidence can be avoided by properly controlling aquifer drawdown. Since the response function characterizes an aquifer pumping-drawdown relationship, a functions are groundwater managementmodel can be very easily formulated once the response defined. Without considering the random natureof aquifer properties, the deterministic management model can be stated as follows. Objective: Minimize the cost of pumping to produce as nearly as possible a horizontal hydraulic gradient within a specified time frame, subject to 1. Upper and lower limits ondrawdown 2. Upperandlowerlimitsonpumping
The details of the objective function and constraints are also described in detail in Ref. 1. Values for transmissivity T and storage coefficient S are normally derived from a pump well test, and as such a test provides in situ values of aquifer parameters averaged overa large and representative aquifer volume, T and S should be treated as random variables. Consequently, the response function 6 and the drawdown constraint are random in nature because they contain random variables T and S. The objective function equation and the drawdown constraint equations are affected by drawdown. This implies that theobjective function and constraints cannotbe known with certainty. Thus, it is more appropriate and realistic to examine both probabllistically. In a stochastic environment, we want to specify limitations on allowable risk or required reliability of constraint performance.The reliability we specifyfor our constraintsis actually the confidence limits we are setting for our optimal pumpingvalues. This reliability can be determined based on the confidence of the model user in his measurement of the aquifer parameters. Our expert system (the preprocessor for user input before optimization) uses the confidence in the input and, onthe basis of Bayesian theory, producesa confidence factor (reliability pj,Jfor its
573
Uncertain0 of Aquifer Parameters
input into the optimization program. Then, if we impose a restriction on drawdown at any point j at the end of the nth period resulting from the pumping over the entire well field that cannot exceed a specified value sjmnwith a reliability pj,n, the drawdown constraints can be expressed as
Pr
{ 9
6 i jq. ik. t - k +
I
ki == ll
A probability statement of drawdown constraint(or anywhere drawdown is used suchas in the objective function in our case) like Equation ( 5 ) is not mathematically operational, andfurther modification is needed. To make Equation ( 5 ) operational, it is necessary to assess statistical properties of random terms in chance-constrained equations. There havebeen a number of field investigations and laboratory experiments assessing the probability distributionof aquifer transmissivityand hydraulic conductivity. Most findings indicate that the hydraulic conductivity has a log-normal distribution. Because the response function 6 is a nonlinear function of transmissivityand effective porosity, the probability function of 6 as well as drawdown at any observation point cannot be easily be assessed. Therefore, first-order analysis is applied to estimate statistical properties of the response function and drawdown at each observation point. First-order analysis is a useful method to estimate statistical characteristics such as the mean and variance of a function involving random functions.In first-order analysis, the function containing random variables is expandedin a Taylor series about the mean values ofrandom variables, that is,
fly) = A V )
+ f’(v) [x-x(v)]
2!
+p(v,[x-x(v)]*
+- -
n! (6)
in which&) is a function involving a random variable x,flv) is themean value of&), and x(v) is the value of the random variableat the mean. Derivations of statistical properties of drawdownat each observation point, assuming independence of tranmissivityand effective porosity, are given in the Appendix, and the results are as follows. I
t
i=l
k=l
in which E[sjJ and var[sj.,] are the mean and variance, respectively of drawdown at observaare the standard deviations of the transmissivity tion wellj at the endof the t period; sdt and sds and effective porosity, respectively, and B_,A_, are coefficients that are functions of the mean transmissivity and effective porosity.As can be seen, the mean of drawdownis a linear function of pumping, but the variance is a quadratic function of the pumping. Derivation of Equations (7) and (8) enables the development of a deterministic equivalent for Equation ( 3 , as shown in the nextsection, that is mathematically operational, and the random characteristics of the aquifer properties are explicitly incorporated in the management model.
e
574
Ward
B. Stochastic Model The total drawdown at any control point is the sum of the drawdowns created by many individual pumps. Since drawdown is a random variable, the central limit theorem applies. That theorem states that if n is large, a set of random variables has approximatelya standard normal distribution. Therefore, the total drawdown at each observation point can be assumed to have a normal distribution with a mean and variance given by Equations (7) and ( 8 ) , respectively. Under the normality assumption, the original chance-constrained Equation (5) can be expressed as
where Z is a standard normal randomvariate with mean zero andunit variance. By substituting Equation (7) into Equation (9), an equivalent expression can be written as
in which F" [ p ] is a standard normal deviate correspondingto the normal cumulative distribution functionof p. A plus sign producesthe equation stating that thereis a p probability that ~, a minus the actual drawdowns at pumping wells are less than the calculated values ~ , whereas sign produces the equation stating that there is a 1 - p probability that the actual drawdowns at observations wells are less than the calculated value. Note that the second termin Equation (10) involvesa square root of the varianceof drawdown at each observation point, which, in turn, is a quadratic function of unknown decision variables q. The determidistic equivalent of a chance-constrained equation is nonlinear. Standard linear programming codes cannot solve problems with nonlinear constraint equations. However, as suggested by lhng [ 6 ] ,quasi-linearization can be employed to linearize the nonlinear term in Equation (10). The quasi-linearizationresults in a linear approximation for the stochastic equivalent to the original deterministic constraint on drawdown: I
t
i=l
k=l
where Eij,k = 5 j . k i-F"[p]gij,k for the drawdown Constraint, Eij,k = 5 j . k - F"[P]gij,k for the objective function, and O_ is a stochastic term derived during this process. Checking the signs for the B_ and O_ coefficients reveals that the stochastic unit influence coefficient E responds the same whether showing the influence of an injection well or that of an extraction well. At injection wells, bothB_ and Q are negative values. Therefore, E is larger in absolute magnitude than the deterministic unit influence coefficient for the drawdown constraint. E is smaller than the deterministic coefficient for theobjective function. At extraction wells, both B_ and O_ are positive, producing a larger absolute value for E in the drawdown constraint and a smaller value for the objective function. To convert the original deterministic model into a stochastic model, replace the drawdown for 8iJ.k in the objective function. Clearly EijVkcan constraint with Equation (10) and use
Uncertainty of Aquifer Parameters
575
...
, -. .. ..- - DOES NOT EXCEED GROUND SURFACE
FINAL ELEVATION
95% CERTAINTY THAT FINAL
MODEL
ELEV. AT OBSERVATION WELL BYISCALCULATED 2 ELEVATION BY MODEL
” ”
X S s X CERTAINPI THAT FINAL
ELEV. AT PUMP WELL DOES A CRITICAL NOT DROP BELOY LOWER ELEV.
._
INITIAL
DIRECTION OF FLOW
Figure 1 Cross section demonstrating sample stochastic constraints on final water levels.
be considered a stochastic unit response function derived from the Theis equation. And it should be noted that the deterministic model actually represents a reliability of 0.50 (when F”[0.50] = 0). 1. ReliabilityDetermination There are drawdown terms (for observation wells) in the objective function as well as in the drawdown constraint (for pumping wells). Reliability is treated differently in the two cases. Refer to Figure 1 during the following discussion. Let’s assume a reliability level of 0.95. In a drawdown constraint one wishes to be 95% sure that the change in water level does not exceed the prespecified maximum change (i.e., does not violate predetermined bounds on the head). One uses the standard normal deviate (F”[p]) corresponding to a reliability of 0.95 for the drawdown constraint (i.e., F-It.951 = 1.M) The . procedure described previously computesa stochastic unit response coefficient for than a deterministic coefficient (which corthe 95% confidence level. The coefficient is larger responds to a 50% confidence level). Since unit pumping causesa greater change in head using the 95%probability influencecoefficient, less pumpingis feasible before drawdown constraints become tight. When considering the objective of raising water levelsto prevent contaminant movement, one wishesto be 95%confident that head changes equalor exceed calculated values. Therefore, with the objective function one uses the standard normal deviate correspondinga to reliability of 0.05. This produces stochastic influence coefficients thatare numerically smallerthan 95% of all deterministic influence coefficients. For identical pumping values, the 95% probability change in water levelsneeded to achieve a horizontal gradientis much greater than that needed using deterministic coefficients. This guarantees that pumping values calculatedby the model are equal to or greater than those required by the deterministic model to produce a horizontal gradient. However, this guarantee also causes the constraintthat final heads at downgradient observation wells are greater than the final head at the source to force the objective function value
576
Ward
to be larger than an objective function value resulting from only trying to minimize the head differences between the contaminant sourceand all observation wells. Greater pumping values may actually cause the heads at the downgradient observation wells to “overshoot” the head at the source and produce a reverse gradient. This is demonstrated in Section IV, where the objective function and reverse gradient increase as aquifer parameter uncertainty increases. The “tight” downgradient observationwell is the one whose final head is equal to the final headat the source. All other downgradient observation well headsare higher than the source head and therefore produce a larger objective function value. 2. Determination ofAquifer Parameters Estimation of transmissivity and effective porosity has received much attention in the literature in recent years and was discussed in Section 11. From Equations (7) and (8), it is seen that the mean and variance of transmissivity and effective porosityare needed in the stochastic version of the optimization model. Many methods for determining these statistics are described in the literature. Here a Bayesian approach is usedto derive the mean and variance for transmissivity and effective porosity. The Bayesian approach uses a prior (also called unconditional) probability distribution function (pdf) and a likelihood pdf to determine the mean and variance forthe aquifer parameters. This mean and variance describe the posterior or conditional pdf used within the stochastic model. The prior pdfisbasedonknowledgeoftheaquiferobtainedfrompast experience. This study suggests using aquifer material(soil type) as the basis for the prior pdf. The likelihood pdf is developed from current information (field or lab data) about the aquifer in question. In this chapter the standard deviationof transmissivity and effective porosity are varied to determine how these changes affect the objective function value. However, in a real situation, one would estimate a mean and variance for these aquifer parameters from a prior pdf and a “likelihood” pdf. The user would select a description of the soil type from a given list. Based on a range of values of transmissivity and effective porosity associated with each soil (detype rived from numerous references), a prior pdf mean (Xo) and variance (V,) are determined. This determination is made by assuming that the range of values spans three standard deviations each side of the mean (99% confidence interval). With this assumption and assuming a lognormal pdf for transmissivity and a normal pdf for effective porosity, one can compute the mean and standard deviation. If there are no field data values for the problem, the prior pdf becomes the posterior pdf. If one has field data values, the mean (X) and variance ( V ) are determined using standard equations for mean and variance of a data population. This mean and variance for the field data values define the likelihood pdf. Themean and variance for transmissivityare calculated using the natural log of all transmissivity values because these log values are known to be normally distributed. The relationship between posterior pdf,the prior pdf, andthe likelihood pdf can be expressed as
Posterior distribution a prior distribution X likelihood distribution The mathematicsof multiplying a normally distributed likelihood pdf by a normally distributed prior pdf has been previously derived [ S ] . Assuming that the natural logdata values for transmissivity and the data values for effective porosity are normally distributed, the posterior mean, E ( . ) , and posterior variance, var (.),for either parameter are calculated from
Uncertainty of Aquifer Parameters
577
and var (.) =
( v ~+-V~- * ) - ’
( 13)
The expected value, E(.), and the variance, var(.), for effective porosity are used as the posterior mean and variance. However, because natural log values are used to determine the expected value and variance for transmissivity, these values must be converted back to represent the mean and variance of the actual transmissivity values.Standard equations for the mean and variance of a population that has a log-normal pdf (when the expected value and variance of its natural log values are known) are
-
Mean = exp [ E
-
+
and Variance = [exp (var
+ 2E)] [exp (var) - 11
(15)
These two equations are used on the assumption that the entire population of values is available. Since the prior pdf uses the knowledge of a large amount of data for each soil type, this assumption is sound.
IV. APPLICATION,RESULTS, AND DISCUSSION The stochastic optimization model was applied to the same hypothetical groundwater contamination problem that was usedto analyze the deterministic version. Aquifer parameters (transmissivity and effective porosity), coefficient of variation (CV, ratio of standard deviation to mean), and required solution reliability were varied in consecutive runs. The simulation component and optimization component were run on anIBMATwith 640K bytes of RAM, a 30 MB internal hard disk a with a floppy disk drive, and math coprocessor. Physical parameters for model run Id include a transmissivity of 1255 m2/day (13,500ft?/ day) and an effective porosity of 0.3. The original hydraulic gradient was 0.54%. Maximum and minimum acceptable pumping rates, based on available equipment, are 135 and 0 Usec. This was based on the performance curve fora pump that can discharge 150Usec against 6 m on head at all injection wells wasthe ground surface of head at 80% efficiency. The upper limit (5.8 m, above the initial water table). This should prevent pressurized injection[9]. The lower limit on head at extraction wells prevented such changes in tranmissivity that would invalidate the use of superposition. In general, if the changein transmissivity is less than lo%, the aquifer can be treated as a confined aquifer system. The stochastic model was applied to the same hypothetical system described for the deterministic model in Ref. l . Results are shown in Tables l and 2 for comparison with the deterministic model (run Id). The coefficients used for this analysis were Wf = l .O and c‘ and c’’ equal to their original values. Therefore,the results shown for this analysisare for a strongly hydraulic objective function. The initial pumping (Q,) used in the iterative solution procedure of the stochastic model was the optimal pumping from the deterministic model run. It was found that two iterations brought acceptable agreement (convergence within 5%) between the “estimated” pumping values and the final optimal pumping. The weight factor in the objective function was adjusted for identical runs as described in the previous paper [l], but, as was found then, all weight factors of 1.O and greater produced the same results. Subsequent tests used a weight factor of 1.O.
578
Table 1
Ward Effect of AquiferParameterUncertaintyon
95% ReliableOpti-
mal Unsteady Pumping Strategy for Hypothetical Problem'
Run 5s
4s
3s
2sId
1s
Pumping (Vsec) Day 1 2 3 4 5 6 7 8 Avg. pumping Avg. gradient (%) Gradient SD Sum of squared head diff. (m') Obj. function 0 & M costsb ($10~)
96.1 90.1 84.9 80.2 76.9 36.9 0.0 0.0 58.1 0.08 0.058 1.24
85.8 76.4 70.4 66.3 63.2 57.3 0.0 0.0 52.4 0.079 0.043 1.08
70.2 63.4 59.3 56.4 54.2 52.5 28.7 0.0 48.1 0.085 0.057 1.30
51.4 47.1 44.7 43 .O 41.7 40.7 40.0 25.6 41.8 0.095 0.062 1.72
85.3 74.8 68.3 60.9 58.7 0.0 23.3 54.4 0.098 0.061 1.79
83.3 70.9 63.7 59.2 56.2 54.2 52.8 0.0 55.0 0.14 0.084 4.99
15.63 2.31
13.54 1.93
15.66 1.65
19.82 1.32
21.18 1.93
55.53 1.84
Transmissivity CV Run
64.0
CVporosity Effective ~~
1s 2s 3s 4s 5s
0.2
0.8
'Run Id is deterministic model. bo C% M = operations and maintenance.
In all, 10 stochastic optimizations were performed. These used a range of values for the coefficient of variation (CV) for both transmissivity and effective porosity and used two reliabilities (a constant for all wells and all time periods for each run). Figures 2 and 3 graphically depict the pumping strategies developed for the five stochastic model runs made at the 95% reliability level. Figure 2 shows the pumping trends as the uncertainty of transmissivity increases from run 1s to 3s as compared to the deterministic run (Id). Figure 3 shows the pumping trends as the uncertainty of effective porosity increases from run1s to runs 4 s and 5s. The same general pumping trendsare evident for the runs made at the 80% reliability level. To analyze the predictabilityof these results we look first at the equation forthe stochastic influence coefficientE [Equation (1l)] and refer to Figure1. From a table of standard normal deviates it is known that as reliability [p = F(z)] increases, z (which equalsF"[p]) increases. Therefore, lookingat Equation (1 1) we see thatas reliability increases,E for the objective function decreases and E for the drawdown constraint increases. In summary, an increase in the uncertainty of aquiferparametersproduces the sameresult as an increase inreliabilitysmaller E for the objective function and larger E for the drawdown constraint. As stated, for the drawdown constraints, increasing reliabilityor uncertainty of parameters produces a larger influence coefficient. This causes a greater reaction of the potentiometric
5s
Uncertainty of Aquifer Parameters
579
Table 2 Effect of Aquifer Parameter Uncertainty on 80% Reliable Optimal Unsteady Pumping Strategy for Hypothetical Problem
4s
3s
2s Id
1s
Pumping (Usec) ~
Day 1
%.1
Avg. pumping Avg. gradient (%) Gradient SD Sum of squared head diff. (m2) Obj . function 0 & M costs
94.6 86.0 90.1 78.8 84.9 73.9 80.2 70.2 76.9 36.9 21.5 0.0 0.0 0.0 0.0 58.1 53.1 0.08 0.067 0.058 0.047 1.24 .77
85.7 76.7 71.1 67.1 64.1 44.9 0.0 0.0 51.2 0.070 0.048 .85
69.8 63.2 59.2 56.4 54.3 52.7 20.1 0.0 47.O 0.076 0.050 1.04
93.2 85.1 77.6 72.4 68.7 36.2 0.0 0.0 54.1 0.076 0.049 1.01
90.6 82.0 74.7 69.3 65.6 63.O 0.0 4.9 56.3 0.097 0.060 1.70
15.63 2.31
11.03 1.89
12.80 1.62
12.89 2.04
20.36 2.06
2 3 4 5 6 7 8
($10~)
10.37 2.04
Figure 2 Daily pumping strategies for increasing uncertainty of transmissivity as compared to the deterministic strategy (95% reliability level).
580
Ward
Figure 3 Daily pumping strategies for increasing uncertaintyof effective porosity as compared to the deterministic strategy (95% reliability level).
surface to a unit of pumping. Therefore, this increase allowsfor less pumping during a unit of time because the upper bound on drawdownis reached more quickly. In thecase of a reliability is equal to or larger than 95% of all F”[p] of 0.95 we know that the F” [0.95] value (1 .a) values; thusthe E value for a reliability of 0.95 is equal to or greater than 95% ofE values for the same aquifer parameters. This confirms the stochastic constraint that in the field the upper bound on drawdown will not be exceeded 95% of the time. Tables 1 and 2 reflect the trend of increasing reliability or increasing uncertainty of parameters and the resulting decrease in allowable pumping. Why, then, does the pumping increasefor the last time period, or why are there more time periods of pumping as reliabilityor CV increases? Whilethe large coefficients are causing large head increases at the injection wells (thus restricting the amount of pumping), the small stochastic influence coefficients for the objective function cause a much smaller reaction of the potentiometric surfaceat the observation wells. Thus, lower pumping values caused by increasing the reliability or uncertainty have even a smaller effect on drawdown at the observation wells. Yet the goal is still to minimize the objective function. To do this, additional pumping periods are needed or more pumping is required during the last time period as reliability or uncertainty increases. This trend is shown in Tables 1 and 2. The objective function uses the large drawdowns at the pumping wells to calculate pumping costs, thus producing the highest costs. The objective function uses the small drawdowns at the observation wells to determine the difference in head, thus producing a large sumof head differences.Thus we are assured that the objective function valueis the largest expected for the given input andthat the results in the field will probably not exceed the calculated value. However, the constraint that downstream heads must be higher than the head at the contaminant source, because it uses the smaller E values for the observation well head calculations, actually causes the hydraulic gradient to “overshoot” horizontal. The smaller E values produced at the 0.05 reliability level forobservation well head calculations give usa 95% confidence that the heads are those calculated (using these E values) or greater, thus causing the
581
Parameters Uncertainty of Aquifer
Table 3
Summary of TrendsProducedbyStochasticAnalysis(HydraulicObjectiveFunction) Effect on uncertainty Effect on reliability
Value affected
IntransmissivityIneffectiveporosity
1. Influence coefficient used with:
Objective function DD constraint 2. Dailypumping 3. Totalpumping 4. Gradient (reverse) 5 . Objective function value
Decrease Increase Decrease Decrease Steeper and less smooth Increase
Large decr. Small decr. Large incr. Small incr. Small decr. Large decr. Large decr. Small incr. Steeper and less smooth incr. Large incr. Small
reverse gradient. Remembering that the final gradients are always reverse gradients, Tables 1 in the reverse and 2 show that as reliability or uncertainty increases, the final gradient is larger direction. The confidence in the final gradient is further complicated by the fact that the target elevation (normally the head at the contaminant source) is itself stochastic. Therefore, the actual reliability of the final gradient will be something less than the specified value, but that reliability cannot be determined with precision. Table 3 summarizes the trends that developed as uncertainty of aquifer parameters and reliability were systematically varied. Figures 4 and 5 graphically show the trends in total pumping and the resulting final gradient. Figure 4 shows the five stochastic runs using a reliability of 95% normalized to the deterministic run (Id). Figure 5 shows the five runs nor80%. As the coefficient of variation (CV) for tranmissivity malizedusingareliabilityof increases (runs Is, 2s, and 3s), the influence coefficients for the drawdown constraint increase and those of the objective function decrease. The expected result is a decrease in pumping for each time period (but larger total pumping) and an increase in the final average gradient and objective function value. Runs Is, 4s, and 5s show the results of increasing the CV for the effective porosity while holding the transmissivity CV constant. The general trend for these runs is the sameas those for runs Is, 2s, and 3s. The resulting gradient and objective function for runs4s and 5s show
0f i n a l
gradient 160
l20 100 80 60
Stochastic
Runs
Figure 4 Comparisons of final gradient and total pumping for the stochastic runs at the 95% reliability level.
582
Ward
ae
c
.4
180-
0f in81 gradient
-total
pumping
- 180
Y
io0
eo 60
Stochastic Runs figure 5 Comparisons of final gradient and total pumping for the stochastic runs at the80% reliability level.
a sharp increase from run 1s. The increased CV produces larger influence Coefficients for the drawdown constraint and smaller coefficients for the objective function just as the increased CV for transmissivity does. However, the changes in these coefficients are small compared to those produced by comparable increasesin transmissivity CV and cause only small differences in pumping between runs Is, 4s, and 5s. In comparison, the resulting gradient and objective function are much worse than those resulting Erom comparable tranmissivity changes runs in 2s and 3s. To explain this difference we look at the difference in sign between the A_ coefficients, Equation (19), which are affected by changes in transmissivity CV, and the P_ coefficients, Equation (21), which are affected by changes in effectivity porosityCV. The negative sign with the P_ coefficient indicatesit will affect the optimalstrategy in a manner oppositeto that of the A_ coefficient. As the CV of transmissivity is increased, there is a large changein pumping and a small change in gradient and objective function. For the same CV increase in effective porosity, thereis a small changein pumping anda large changein gradient and objective function. The two parameters (transmissivity andeffective porosity) cause an opposite relationship between pumping and its effect on the objective function and the constraints. Table 2 displays results of the same variation in the CV of the two parameters, computed using a reliability level of 0.80. As expected, the reduction in reliability increases the optimal pumping values and improvesthe final gradient and objective function. The smaller reliability produces smallerstochastic unit response coefficients. Resultingstrategies and water levelsare more similar to those from the deterministic madel (reliability = 0.50) than are those developed using a 0.95 reliability. Strategies for runs 4 s at the 95% reliability level and5s at 80% reliability have no pumping 8. This is a definite change in the overall pattern of the on day7 and yet require pumping on day stochastically optimal pumping strategies. However, a look at the sensitivity values for the pumping during days 7 and 8 gives an indication that it is not a major change. The sensitivity value (amountthe objective function would change witha unit increasein pumping during that day) associatedwith each pumping value for days7 and 8 for those tworuns is very small. For example, these sensitivities are in the range of 10-4-10"5 as comparedto a sensitivity of 0.71.3 for the tight pumping value in most other runs. This indicates that the pumping for day 8 could also be 0 without any significant change in the objective function. Therefore, the zero
Parameters Uncertainry of Aquifer
583
pumping for day 7 and a pumping valuefor day 8 of these runs could be zero pumping for both days 7 and 8 without a dramatic change in the overall pattern of the results. Comparisons to Tung’s [6] analysis are difficult to make because his objective function was to maximize pumping, which is not affected by the stochastic influence coefficient. The only constraint was on drawdown. In addition, the Cooper-Jacob equation (which is only appropriate for small values of the Boltzmann variable, U 5 0.02) used to derive the stochastic P_ to be equal to 0 except for the first time period. However,the unit influence coefficient shows general trends "brig speaks of concerning transmissivity applyto is analysis: (1) Pumping increases as reliability or CV decreases and (2) uncertainty of tranmissivity causes a larger change in pumping than does a comparable change in effective porosity. This study indicates that effective porosity has an effect on the drawdownat the observation wells (something Tung considers negligible) and hence has an effect on the objective function value. In addition, the daily pumping increases with decreasing effective porosity CV, but at the same time the total pumping decreases. In summary, the trends shown in this analysis are found in Table 3.
V. CONCLUSIONS
A procedure developed by Tung [6] was used to incorporate uncertainty of aquifer parameters into our model. A stochastic unit response function ( E , based on the Theis well function) was developed and usedin the same manneras the unit response functionin a deterministic model. This E value is dependent upon the uncertainty of aquifer parameters as measuredby the COefficient of variation and a specified reliability of the solution. Drawdown at observation wells (which affect the objective function and gradient constraints) must be treated differently than drawdown at the pumping wells (which affect the drawdown constraint). For example, if a reliability of 95% is specified for our solution, an E value correspondingto a reliability of 0.95 is used for the drawdown constraint because the user bq’the optimal pumpingat the wants to be 95% confident that the resulting drawdown produced or less, whereas the E value corresponding toa reliability pumping wells is the value calculated of 0.05 is used to determine drawdown at the observation wells because the user wants to be 95% confident that the actual drawdown (produced by the optimal pumping)at the observation wells equals or exceeds the calculated value. This meansthat E values corresponding to a reliability of 0.95 are used for the drawdown constraint, and those corresponding toa reliability of 0.05 are used with the objective function, Equation (10). This theory guarantees the user a 95% confidence levelfor the drawdown constraint. However, because the objective function minimizes the head differences between the observation wells (whose valuesare stochastic) and the source (whose valueis also stochastic), a joint 95% confidence level cannot be guaranteed. It would be some value slightly less than 95% and cannot readily be determined. The major differences between Tung’s analysis and this study are that (1) Tung used the Cooper-Jacob equation to derive the stochastic coefficients and (2) Tung’s objective function was to maximize pumping, and therefore the objective function did not incorporate stochastic coefficients. The study results shown in Tables 1 and 2 agree in general with the conclusions of Tung. As the reliability leveldecreases or as aquifer parameter uncertainty decreases, the pumpingfor each time period increases. As a consequence, the objective function improves. The results of changes in uncertainty of effective porosity differ from those of Tung. Tung’s derivation of the P_ coefficient (the partial derivative of drawdown with respect to effective porosity) usingthe Cooper-Jacob equationshowed it to have a value of zero for alltime periods except the first. Therefore, changesin uncertainty of effective porosity had almost no effect on
584
Ward
the optimal pumping values. This may be due to the fact that the Cooper-Jacob equation is only valid for small values of the Boltzmann variable (U 5 0.02). Our study shows that the P_ coefficient has values for all time periods. For equal changesin CV, effective porosity produces smaller changes in pumping than does transmissivity. However, the resulting final gradients produced by these small changesin pumping are much poorer than the final gradient produced by a comparable changein CV of transmissivity. These results indicate that uncertainty in effective porosityhas little effecton allowable pumpingas Tung concluded, but the final gradient is affected in an adverseway. Four general statements canbe made from the stochastic analysis of this mode: 1. Introducing stochasticity into the optimization model increases the value of the objective function. 2. Lowering the reliability level produces a model that allows more pumping (increased operations and maintenance cost) and produces an improved final gradient. 3. Changes in uncertainty of transmissivity and effective porosity both produce the same general changes in optimal daily pumping and final gradient. 4. Changes in uncertainty of transmissivity and effective porosity produce opposite effects on the total optimal pumping required.
List of Symbols' A_ stochastic coefficient producedby taking the partial derivative of drawdown with respect to transmissivity, [T/L2] &J,k the unit response function for a stimulus at well i on an observation point j at time period k; calculated using the mean values of transmissivity and effective porosity, [T/L2] 8iJ.t-k + I the incremental drawdown at a well j in time period t caused by unit volume of pumping at well i in time k , [l/L2] CV coefficient of variation g stochastictermusedtofindstochasticunitresponsefunction E(Sj,,) mean of drawdown at observation well j at endof time period t , [L] EiJ,k stochastic unit response function for stimulus at well i and response point j for time period k, [T/L2] E(T) mean of transmissivity, [L*/"] E($) mean of effectiveporosity $ effective porosity AQ) standarddeviationofdrawdown,[L] F"[p] standard normal deviation corresponding to a normal cumulative distribution function 4.g groundelevation at pump i, [L] hi,o head at pumpwell i at time 0, [L] hit head at pumpwell i at time t , [L] hzi lowerlimitonhead at pump i, [L] hui upperlimitonheadatpump i , [L] (hj,=)d head at observation well j , which is downgradient of the contamination source at the end of the modeling period TT, [L] head at ,contaminant source at end of modeling period T T , [L] I totalnumber of pumpingwells 'Dimensions given in terms of unit length [L] and time [TI.
Uncertainty of Aquifer Parameters
585
hydraulicgradientinthe x direction, [UL] hydraulicgradientinthe y direction, [UL] J totalnumber of observationwells n porosity P_ stochastic coefficient producedby taking the partial derivative of drawdown with respect to effective porosity q Darcy's velocity, [UT] lower limit on pumping at all wells, [L~/T] 4" upperlimitonpumping at allwells, [L~/T] qf pumping at allwells at time t , [L~/T] Q, initialestimateofoptimalpumpingforstochasticmodel, [L3/Tl r distancefromstimulus i to observationpoint j , [L] re effectiveradius of thepumpwell,[L] r,, radiusofthepumpwell,[L] p specified reliability S storage coefficient sdtstandarddeviationoftransmissivity sdsstandarddeviation of effectiveporosity calculated drawdown at pump i at time I , [L] TT totaltimeforoptimalpumpingstrategy,[TI T transmissivity, [L2/T] Tavg averagetransmissivitybetweenpumpwell p andobservationwell 0 , [L2/T] t timeperiodwithintime T T , [TI U Boltzmann variable v seepage velocity, [UT] var(sj,,) variance of drawdown at observation well j at the end of time period t var(T) variance of transmissivity, var(+)varianceofeffectiveporosity V varianceoffield data V, varianceofpriorprobabilitydensityfunction W, weight factor to convert the square of hydraulic head differences to dollars, [$/L2] X mean of field data X, meanof priorprobabilitydensityfunction Z standardnormalrandomvariatewithmeanzeroandunitvariance
,i i,
APPENDIX-ANALYSIS
OF UNCERTAINTY IN DRAWDOWN
Discrete formulation of drawdown given by
i=l
at observation point j at theend of the nth period is
&=l
where Gij,,"k+I is the unit response function, which can tion as
be derived from the Theis equa-
586
Ward
where
Since T (transmissivity) and0 (effective porosity)are random variables, the unit response function and drawdown are both random variables because theyare functions of random variables. To estimate statistical properties of random variables, the first-order analysis of uncertainty is employed, Taylor’s expansionof drawdown about the mean values of T and @ can be expressed as
~
i=l
k=I
m,
where 5 . J . k iscomputedusingmeanvaluesand and HOT represents the higher order terms. The time increments of k and t - k + 1 are reversed from those in (Al),but they produce the same result. First, we compute the middle term onthe right-hand side. The first-orderpartial derivative of S,., with respect to T can be obtained by the Leibnitz rule for differentiating an integral [lo, p. 181:
Performing the mathematics of the differentiation in three parts, we define
L
i=l
J
k=l
For the first time on the right-hand side of (A3),
For the second term, db f l b ( c ) , c ] a= 0
because b = constant (m)
For the third term,
where du ” ”
dT-
Therefore,
dT
- ”=” 4T2k
U
T
587
Uncertainty of Aquifer Parameters 1
e"' iu - 4nT2
f l a ( c ) ,da c b= 4 i T ky ]-=
Adding the three terms, we have
i=l
k=l
in which
Similarly, the first-order partial derivative of drawdown with respect to the effective porosity can be obtained in three parts from the Leibnitz rule. For the first term on the right-hand side of (A3),
For the second term, db A b ( c ) , c h= 0
because b = constant (m)
For the third term,
where
Therefore, 1
Only the third has a value, and
where
I
t
i=l
&=l
588
Ward 1 4zT+ e"'t 1 4nT+(e"''
atk = 1
"
5j.k
=
"
- e"'"')
at k > 1
The partial derivatives of drawdown with respectto transmissivity and effective porosity agree with those shown by McElwee and Yukler [3]. Ignoring the higher order terms in (A5), the expectation of drawdown canbe approximated by the equation
i=l
&=l
Which is Equation (7) of the text. Furthermore, assuming independency of T and ance of drawdown can be approximated as
-1
[z 'sdt']+ [asj.t
var(sj,,) = 8Sj.t
+, the vari-
sds'
which is expressed in the text as [Equation (8)l
where sdt and sds are the standard deviations of the transmissivity and effective porosity,respectively.
REFERENCES 1.
2. 3.
4. 5.
6.
7. 8. 9.
Peralta, R. C., and Ward, R. L., Optimal piezometric surface management for groundwater contaminant control, Paper no. 86-2513, Presented at ASAE Winter meeting, Chicago, 1986. Copies may be obtained by writing ASAE. ProSagar, B., and Kisiel. C. C., Limits of deterministic predictability of saturated flow equations, ceedings of the Second Symposium on Fundamentals of Transport Phenomenain Porous Media, Vol. 1, International Association of Hydraulic Research, Guelph, Ont., Canada, 1972, pp. 194-205. McElwee, C. D.,and Yukler, M. A., Sensitivity of groundwater models with respectto variations in transmissivity and storage, Water Resources Res., 14(3), 451-459 (1978). Bibby, R., and Sunada, D. K., Statistical error analysis of a numerical model of confined groundwater flow, Stochastic Hydraulics: Proceedings First IntemutionalSymposium on Stochastic Hydraulics (C. L. Chiu, e d . ) , 1971, pp. 591-612. Freeze, R. A., A stochastic-conceptual analysis of one dimensional groundwater flow in nonuniform homogeneous media, Water Resources Res., 11(5) 725-740 (1975). mng, Y. K., Groundwater management chance-constrained model, J. Water Resources Planning Manage., 112(l), 1-19 (1986). Loaicigia, H. A., and Marino, M. A., The inverse problem for confined aquifer flow: identification and estimation with extensions,Water ResourcesRes., 23( l), 92-104 (1987). Lindley, D. V., Bayesian Statistics: A Review, Society for Industrial and Applied Mathematics, Philadelphia,1970. Lefkoff and Gorelick, 1986. Design and cost analysis of rapid aquifer restoration systems using flow simulations and quadratic programming. Groundwater,24. No. 6, pp. 777-790.
Uncertain@ of Aquifer Parameters IO.
589
Greenberg,M.D., Foundations of Applied Mathematics, Prentice-Hall, Englewood Cliffs, N.J., 1978.
ADDITIONAL READING Bear, J. (1979). Hydraulics of Groundwater. McGraw-Hill, New York. Dagan, G. (1985). Stochastic modelingof groundwater flowby unconditional and conditional probabilities: the inverse problem,Water Resources Res., 21(1). 65-72. Freeze, R. A., and Cherry, J. A. (1979). Groundwater, Prentice-Hall, Englewood Cliffs, N.J. McWhorter, D., and Sunada, D. K. (1977). Groundwater Hydrology and Hydraulics, Water Resources Publications, Colorado. Murtagh, B. A., and Saunders, M. A. (1983). MINOS 5.0 Users Guide, Tech. Rep. SOL 83-20. Stanford Univ., Stanford, Calif. Todd, D. K. (1980). Groundwater Hydrology, 2nd ed.. Wiley, New York.
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28
Application of Total Quality Management (TQM) Principles to Pollution Prevention Programs Prasad S. Kodukula Woodward-Clyde Consultants Overland Park, Kansas
1.
INTRODUCTION
Total qualitymanagement ("QM) and pollution prevention are relatively new conceptsin American industry. Although TQM may be a new concept to many American companies, Japanese manufacturers have been implementing TQM principles in their manufacturing processes for several years. On the other hand, environmental regulatory emphasis the United in States is shifting from end-of-pipe controlsto pollution prevention and waste minimization. The latter approaches are not new to some American manufacturers, however. For example,3M Company has been a pioneer in minimizing waste generation in manufacturing, and saved over half a billion dollars during the last decade. Both TQM and pollution prevention represent major shifts in the philosophy underlying production and environmental management, respectively. These new concepts are beginningto replace traditional methods. They have many conceptual similarities. These similarities are briefly discussed in Section11, and Section 111 presents a case study wherein TQM principles are being applied to achieve pollution prevention and waste minimization.
II. TQM AND POLLUTIONPREVENTION According to W. Edwards Deming, an American management visionary, some of TQM principles are
the core
Top management commitment to quality Zero defects Quality through process improvement, not product inspection Continuous process improvement Measurement of success through statistical methods Team effort 591
592
Kodukula
Empowerment of workers Training Interestingly enough, successful pollution prevention programs also involve the same core principles. Unless there is strong commitment at the top management level (e.g., corporate CEO or president), neither TQM nor pollution prevention programs can be successful. The main reason is that the goals of these programs are extremely ambitious and require strong commitment. TQM has an absolute goal of zero defects, and pollution prevention strives for zero pollution. The traditional American manufacturing practice has been to inspect the product for defects, and defective productsare either thrown outor reworked. But according to TQM, quality comes through process improvement. This improvement should take place starting at the research and development stage and continuing through design and manufacturing. Similarly, pollution prevention cannotbe achieved by simply monitoring wastestream discharges for environmental compliance. Manufacturing processes and waste management practices need to be improved or even completely altered starting at the drawing board to minimize, preferably to eliminate, the creation of waste in the first place. The ambitious goalsof zero defectsand zero pollution can be achieved through continuous process improvement. Further, statistical methods can be used effectively to measure the success of achieving both of these goals. The “team” concept is important in the TQM philosophy, and pollution prevention can be effectively achieved only by team effort. This is especially true because environmental issues typically are multidisciplinary andrelate to several different aspects of manufacturing. By empowerment of the team members as well as other individuals involved, innovation canbe prothe company.Training is extremely moted thatcanpositively affect the bottomlinefor important in the environmental area, especially considering the rapid evolvement of new regulations and new technologies and practices developed to keep up with the regulations.
111. CASE STUDY The case study presented herein involvesa large chemical manufacturing plant with more than 25 individual production units.As with many other chemical manufacturers, this plantand the parent corporation as a whole, are finding itdifficult to meet the applicable environmental regulations in a cost-effective manner. Furthermore, due to the ,nature of the types of chemicals handled, this particular facility has been under rigorous scrutiny by the regulators. In this climate, the company has no other choice but to seek pollution prevention and waste minimization not only to comply with the regulations but also to minimize overall environmental costs. The corporate CEO of the subject facility has recently made a commitment to reduce waste generation at each of the company’s manufacturing plantsby 20% each year over the next 5 years. As a first step, a task forcewas formed to undertake the waste reduction programat this facility.
A. Phase I As part of Phase I of the program, one of the first objectives the task force accomplished was to develop a waste generation database for the plant. The major categories of wastes included in this inventory were Hazardous wastes shipped to a hazardous waste landfill Solid wastes shipped to an industrial landfill Other hazardous and solid wastes sent off-sitefor treatment and/or disposal
Application of TQM to Pollution Prevention Hazardous and solid waste residues sentto the on-site industrial boiler Liquid waste streams treated in the on-site wastewater treatment facility Fly ash from the power generation facility at the plant Air emissions at the plant
593
0
Among these waste categories, liquid waste streams producedby the manufacturing units were identified as top priority for further study for the following reasons.
1. From the waste generation data collected, liquid waste streams were determined to be a of these major source of pollutants. Furthermore, the costs associated with treatment streams is a major portion of the overall environmental management costs at the plant. 2. The effluent of the WWTF, where the liquid wastes are treated, exhibited aquatic toxicity and recently resulted in discharge permit violations. Further violations would trigger a formal investigation under a regulatory-controlled consent order, that could potentially cost the plant several hundreds of thousands of dollars. 3. The surface impoundments that serve as the aeration basins of the WWTF’s biological treatment system need to be replaced with above-ground tanks in the near future. This is based on a corporate decision to eliminate the use of surface impoundments in order to minimize any potential future liability. Since the capital cost associated with the tankbased system is basically a function of the organic loading in the waste streams to be treated, there is a great incentive to minimize the loading. A team was formed to address the more pressing aforementioned effluent toxicity problem. This team was headedby the director of the environmental department of the plant. Other team members consisted of aWWTF operator, two chemists, two technicians, and a technical consultant. The primary goal of the team wasto reduce the effluent toxicity. The team used a TQM approach to accomplish this objective. Since the corporation had already been involved in TQM and the team members had been exposed to the quality principles, it was easyto integrate TQM into this project. The team met twice a week over a period 2ofmonths. The initial meetings involved goal setting and program organization followedby investigation of sources of toxicity. Based on a review of the existing WWTF data and results from previous investigations, itwas suspected that there were three sourcesof toxicity:
1. A cationic polymer used in the waste treatment processto enhance separation of biological solids. 2. Unused ammonia nitrogen (ammonia fromthe WWTF influent, ammonia formedby mineralization of organic nitrogen in the influent, and ammonia addedto the WWTF influent to supplement the biological requirement of the microorganisms in the treatment system) in the biological treatment system 3. Untreated organics of the WWTF influent and possible intermediates formed during the biological treatment After the identification of “suspect” causes, laboratory tests were conducted for confirmation. Then pilot-scale testing was conducted to verify if removal of the suspect sources would result in toxicity reduction. At the end of these studies, the following remedies were suggested: 1. Replacement of the cationic polymer with another, less toxic polymer
594
Kodukula
2.
Better process control of the polymer system (including regular “jar tests” to estimate proper polymer doses, proper maintenance of polymer feeding system, etc.) 3. Monitoring of ammonia levels within the waste streams (including regular monitoring of influent and effluent nitrogen parameters, supplemental addition of nitrogen to the biological treatment system onlywhen required, proper aeration basin pH control, etc.) 4.Operatorandtechniciantraining Furthermore, it was recommended that the WWTF influent and the individual process streams constituting the influent should be better characterizedto evaluate the presenceof potentially toxic and nonbiodegradable substances. The project team was given authority to investigate and implement selected remedies in an expeditious manner, especially consideringthe enormous costs associated witha potential consent order-driven investigation. The recommended remedies were implemented immediately.As a result, no effluent toxicity permit violationshave been experienced for the last 3 years. Furthermore, the replacement of the polymer has so far resulted in an operational cost savingof $750,000. Thiswas possible because thenew polymer demand was much lower than the previous one. Also, since a consent order from the regulatory agencieswas avoided, additional savings of several hundreds of thousands of dollars were realized. As part of Phase I1 of the waste reduction program, studiesare currently under way to reduce the amount of toxic organics released intothe WWTF influent.
B. Phase II The ultimate goal of this phase of the program is to reduce the amount of toxic as well as nontoxic organics in the liquid waste streams producedby different manufacturing unitsat the plant. The specific objectives of this program are 1. To review the existing waste stream flow and TOC loading data of all the manufacturing units 2. To prioritize the individual process streams based on their TOC loading 3. To characterize the flow and chemical composition of the top five waste streams 4. To evaluate waste minimizatiodpollution prevention options at each waste stream source 5. To implement those options that are deemed most cost effective The first three tasks have already been completed. Out of 15 significant waste streams,10 were targeted for flow and characterization study. The chemical characterization included many conventional parameters,priority pollutants, and several organic chemicals thatare specific to the waste streamof concern based onthe manufacturing process sources involved. The sampling of these waste streams constituted sarhpling of 24-hr flow-weighted composite samples for 1-2 weeks. In cases where flow-based compositing was not possible, time-weighted samples were taken. Based on the flow and analytical data, chemical losses have been quantified. Specific pollution prevention and waste minimization options are currently being evaluated. The next phase of the program involves evaluation of other important solid hazardous waste streams and air emissions for exploring waste minimization opportunities.
IV. CONCLUSION The waste reduction program initiated at the subject manufacturing facility is based on TQM principles. Thereis a strong commitment fromthe CEO of the corporation with a specific goal to reduce wastesat the rate of 20% every year overthe next 5 years. This goal is currently being achieved by a team effort at this facility, which involves process changes as well as improved operations that are supported by employee training. Oneparticular team that worked ona spe-
Application of TQM Prevention to Pollution
595
cific project not only helped the facility meet its liquid effluent discharge toxicity limits also but saved more than $750,000 over the last 3 years. This team has received a corporate quality excellence award forits teamwork and successful results. The company is now involved in the next phase of pollution prevention, demonstrating its commitment to the TQM principle of “continuous improvement.” Companies with established TQM systems can easily integrate their pollution prevention and waste minimization programs into TQM. Such integration could result in reduced waste management costs, increased environmental compliance, reduced liability, and greater community acceptance.
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29
PC Software.for Optimizing Groundwater Contaminant Plume Capture and Containment Richard C. Peralta, Herminio H. Suguino, and Alaa H. Aly Utah Stare University Logan, Utah
1.
INTRODUCTION
Simulatiodoptimization ( S O ) models canbe used to speed the process of computing desirable groundwater pumping strategies for plume management. They make the process of computing optimal strategies fairly straightforward and can help minimize the labor and cost of groundwater contaminant cleanup. (S) models currently used by over 98% Differences betweenS/O models and the simulation of practitioners are discussed in Section I1 [l], followed by an overview of the two most common forms of groundwater managementS/O models, their strengthsand limitations, in Section 111. In Section IV, currently available PC-based S/O models are discussed, and the ways in US/ whichtheywouldbeappliedtorepresentativesituationsareillustrated.Includedis WELLSD,an easy-to-use deterministicmodel that requires minimaldata but will address aqui[2] and Glover and fer and stream-aquifer systems where the analytical solutions of Theis Balmer [3] are appropriate. Also included is US/REMAXB, appropriate for heterogeneous, multilayer systems.To ease use, that code accepts datain format readableby MODFLOW [4], the most widely used flow simulation model in the United States today. These twoS/O models are selected because they are the only ones we are aware of(1)that are available for use onPCs, (2) include with them the optimization algorithms necessary for solution, and (3) use superposition. As explained later, these characteristics make them especially useful for plume management by consultants and water resource managers.
II. COMPARISON BETWEEN COMMONLY USED SIMULATION MODELS AND SlMULATION/OPTIMIZATION MODELS A simulatiodoptimization (90)model contains both simulation equationsand an operations research optimization algorithm. The simulation equations permit the modelto appropriately represent aquifer response to hydraulic stimuli and boundary conditions. The optimization al597
598
Peralta et al.
gorithm permits the specified management objective to serve as the functiondriving the search for an optimal strategy. Themodel computes a pumping strategy that minimizes(or maximizes) the value of the objective function. Table 1 shows generic inputs and outputs of the generally used simulation (S) model and those of an S/O model. The normal S models compute aquifer responses to assumed (input) boundary conditions and pumping values. Using such models to develop acceptable pumping strategies can be tedious and involve much trial and error. For example, simulated system response to an assumed pumping strategy might cause unacceptable consequences. In thatcase, the user must assume another pumping strategy, reuse the model to calculate aquifer response, and recheck for acceptability of results. This process of assuming, predicting, and checking might have to be repeatedmany times. The numberof repetitions increaseswith the number of pumping locationsand control locations (places where acceptability of system responsemust be evaluated and ensured). When using an S model, as the number of possible pumping sites increases the likelihood that the user has assumedan “optimal” strategy decreases.Also, as the number of restrictions on acceptable system response to pumping increases, the ability of the user to assume an optimal strategyalso decreases. Assuminga truly optimal strategy becomes impractical or nearly impossible as problem complexity increases. There are too many different possible combinations of pumping values. Furthermore, even if the computation process is automatedin a computer program, the act of checking and ensuring strategy acceptability becomes increasingly painful as the number of control locations becomeslarge. In essence, it becomes impossibleto compute mathematically optimal strategies for complicated groundwater management problems using S models. Alternatively, S/O models directly calculatethe best pumping strategies for the specified management objectives and ensure that the resulting heads and flows lie within prespecified limits or bounds (Table 1). The upper and lower boundsreflect the rangeof values that the user considers acceptable for cell pumping rates and resulting heads. The model automatically considers the bounds while calculating optimal pumping strategies. The user might choose to use lower bounds on pumping at currently operating public supply wells. Hehhe might choose to limit pumping at the upper end of the range, depending on hardware availability or legal restrictions. The user might impose lower bounds on head, at a specific distance below current water levels or above the base of the aquifer. Upper bounds might be the ground surface or a specified distance below the ground surface. Assume, for example, a situation in which a planning agency is attempting to determine the least amount of groundwater pumpingneeded to capture a contaminant plume and the locations where it should be pumped, i.e., the spatial distribution of the withdrawals and injections. If a pumping strategy isnot implemented to achieve capture, the contaminant will reach public supply wells, resulting in litigation and undesirable costs. Table 1 ComparisonBetweenSimulationandSimulatiodOptimizationModels Model type
values
Computed Input values
Simulation (S)
boundary Some flows boundary Some flows cells SomeboundaryheadsHeadsat“variable”head Pumping boundary flows Optimal boundary flows Simulatiodoptimization (90) Some SomeboundaryheadsOptimalheadsat“variable”head Bounds pumping, on Optimal pumping heads, flows
cells
PC Software for Optimizing Plume Capture
599
An S/O model can be used to directly calculate an optimal pumping strategy for the goal of minimizing the pumping needed to capture the plume without causing unacceptable consequences. For example, assume that no injection mounds should reach the ground surface and that no drawdowns should exceed 2 m. In addition, assume that potentiometric surface gradients near the plume should be toward the plume source. The S/O model will directly calculate the minimum total pumping rate needed and will identify how much should be pumped from each pumping location. The potentiometric surface heads and gradients that will result from the optimal pumping willlie within the bounds specified initially (Table 1). In other words, future heads will not reach the ground surface, future heads will not be more than 2 m below current heads, and final gradients will be toward the contaminant source. Thus, the very first optimal pumping strategy computed by an S/O model will satisfy all specified management goals.
111. COMMON S10 MODELING APPROACHES AND LIMITATIONS Most S/O models employ either an embeddingor a response matrix approach for representing system (head) responseto pumping [5]. Embedding models contain finite-difference or finiteelement equations embedded directly as constraints. In a finite-difference embedding model, head and pumping values (or other flows) must be computed for each time step at each cell. This is a very useful approach for those situationsin which (1) pumping should be a decision andeither variable at most cells,(2) head must be constrained in a high proportion of cells, (3) a steady-state strategy should be developed or there need be very few time steps. It is not as desirable if there are relatively few pumping cells and control pointsor if many time steps are needed. Thus, embedding models have been mainly used for steady-state regional planning and for small hypothetical problems. Response matrix S/O models use linear systems theory and superposition with influence coefficients (e.g., [6]-[14] and many others). The matrix containing the influence coefficients and superposition(summation equations) is termed theresponsematrix.Responsematrix (RM) models use a two-step process. First, normal simulation (analytical or numerical) is used to calculate system response to assumed unit stimuli. Then optimization is performed by an SI0 model that includes summation equations (discretized forms of the convolution integral). Response matrix models are ideal for transient management situations. They require constraint equations for only those specific cells and time steps at which head or flow (other than To predict system response to the optimal pumping) must be restricted during the optimization. strategy at locations and times other than those constrained in S/O the model, an external simulation model is used after the optimization. Regardless of the simulation approach used,S/O models share some of the limitations of standard simulation models. Poor physical system representationor inadequate data will cause error. One cannot properly optimize management of system processes that one cannot correctly simulate. Useful simulatiodoptimization modeling presupposes that aquifer parameters are appropriate and that actual boundary conditions are represented adequately within the model. Both embedding and RM SI0 models generally assume system linearity during at least some part of their processing operation. Confined aquifers are linear, unless they become unconfined. Unconfined aquifers are nonlinear, but frequently the change in transmissivity is insignificant, and they can be treated as if they were linear. Most commonly, system nonlinearity is addressed by cycling. Cycling involves(1) assuming aquifer parameters (and computing in(2) calculating an optimal strategy,(3) recalculating sysfluence coefficients for RM models), tem parameters, (4) comparing assumedand newly calculated parameter values, and(5) either stopping or returning to step 2 and repeating the process (if the assumed parameter values are
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still inappropriate for the problem or if the optimal strategy is still changing with cycling). Frequently, three cycles are sufficient for this convergence process.Thus, although both types of models are completely applicablefor confined aquifers, some adjustmentsmust be made to accurately apply them to unconfined aquifers. Within S/O models, plume captureis generally achievedby controlling hydraulic gradients and thus controlling advective transport. Generally, nonlinear transport equations are not included. This approach permits the modeler to retain use of the characteristics of linear systems (superposition, etc.). All of the RM model applications presented below achieve capture via gradient control. Concerning data input, SI0 models require all of the data needed by simulation models, plus information on lower and upper bounds on decision variables (pumping rate, location) and state variables (head, gradient, etc.). Although the same sort of information shouldbe required when using an S model, the forced codification of these data as SI0 model input is helpful. It causes the modeler to specify strategy acceptability criteria earlierthan he/she might otherwise. Concerning model results, an S/O model might tell a user that the posed problem is infeasible. This means that the user has posed a problem for which all the constraints cannotbe satisfied simultaneously. For example, the user might have instructed the model to cause the head near an injection cell to reach at least 100 m above meansea level and simultaneously told it that the upper bound on injection in 50 m3/day. If that injection rate is inadequate to cause the required change in head, the model will declare the problem to be infeasible. The model will be unable to determine even one pumping rate that can satisfy both conditions. Of course, if there is more than one potential injection well, the same problem might be feasible. In that case, the model can compute an optimal pumpingstrategy (probably the user would have requested a strategy that minimizesthe total pumping neededto achieve that head). Fortunately, S/O model users rapidly getbeyond the stage wherein they try to develop impossible pumping strategies (force the model to achieve goals that are impossible or mutually exclusive when considering both the laws of nature and the goals of humans). Experience brings the S/O modeler great ability to address common management problems.
IV. PC-BASED S/O MODELS AND SAMPLE APPLICATIONS A. USM/ELLSD for Systems Addressable Using Analytical Solutions 1. ModelBackground USWELLSD (Utah State extractiodinjection well system for optimal groundwater management) is a deterministic version of an RM model. It uses influence coefficients based on analytical equations for potentiometric surface response to pumping and river depletion resulting from pumping. It is appropriate for systems where those analytical approaches are appropriate-presumably relatively homogeneous systems. (Of course, in the management and consulting arena, suchapproaches are commonlyapplied to heterogeneoussystems,with acceptable error.) Characteristicsof US/WELLSDare summarized in Table2. The overviewbelow is derived from the user’s manual [15]. The objective function of the optimization module in USWELLS is generally applicable and easily used for a variety of situations. The user can select either a linear or a quadratic form. The linear objective function is to minimize 2
K
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for Optimizing Plume Capture
Table 2 Characteristics of US/WELLSD and USIREMAX~ US/WELLSD Systems addressed One Layer, homogeneous
Management period One
Streadaquifer Stream stage not affected by pumping or two stress periods of equal or unequal duration Steady stateor transient Can rep. transient evolutionary era with terminal steady-state conditions.
Influence coefficients Deterministic Objective function Min
Boundsandconstraints
USIREMAX~
Multilayer heterogeneous Streadaquifer Stream stage affected by pumping One or multiple stress periods of equal duration Steady stateor transient
Based on analytical expressions by Theis and Glover and Balmer or max pumping or combination Time-varying weight for extraction and injection
Deterministic Based on finitedifference simulation (MODFLOW+STR) Min or max pumping or combination Diff. weight for each pumping location
g L Ig 5
gL
hLSh5hU
S g Igu hLIh5hU
Ah:,,
5
Ahl,2
S
Gt.2 S G1.2 IG
AhF2
U
. 2
v4.2 5 v1.2 5 vy2 = 1.0 2
l.x
x (Ext) X (Ij)
' e o
= 1.0 2
1.0
X(Ext)L5 X(Ext)5 X(Ext)' Notes: Superscripts L and U refer to lower and upper bounds; g = extraction or injection, [L3/T]; h = head.; A h , G,.2,V,.2 = head-difference, gradient, and velocity, respectively, between any two locations, [L], dimensionless, or [UT]; (Ext), H (Inj) = total extractionor injection, [L3/T]; d = stream depletion, [L3/T].
where WE,xand W,,x are the cost coefficientor weight assignedto extraction ( E ) or injection (f) rates in the x , time period, [$/(L3.T)] or dimensionless; Ej, and JkJ are extraction ( E ) or time period, [L3/T]; and J and K are number of injection (0 rate at well j (or k) in the extraction (J)or injection ( K ) wells. Potential constraints are the following. 1. Hydraulic gradient between any gradient control pair of wells at any time period must be within user-specified bounds. This can ensure that wateris moving only in the desired direction. The maximum value can differ for each gradient control pair and time period. This constraint is useful, for example, when US/WLLSDis used for groundwater contaminant plume immobilization or for any situation where hydraulic gradient control is desired. 2. Extraction or injection rate at any well must be within user-specified bounds (lower and upper limits). If the user cannot decide if a certain well should be used for extraction or injection, he can locate one of each at the same location. The model will then determine either an extraction or an injection rate, or neither, for that location.
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3. Hydraulichead at any injection, extraction, or observation wellmustbewithin
userspecified lower and upper bounds. For example, a lower bound may be used to maintain adequate saturated thickness. An upper bound may be used to prevent surface flooding or to eliminate theneed for pressurized injection. These lower and upper bounds can differ for different locations. The boundsare the same for both time periods. 4. Total import or export of water can be controlled to be within a user-specified range. The user canalso completely prevent importor export of water or both. If no importor export of water is allowed, the total optimal extraction must equal the total optimal injection. 5. Depletion from the river must be within user-specified bounds (lower and upper limits). This is applicable only if a river exists in the considered system. 6. Constraint 3 is modified such that the probability that the actual change in head at any point in the groundwater systemis not less than the change calculated by the model or is not greater than the change calculatedby the model and is at least equal to the reliability level specified by the user. (This ability is found only in an alpha-test chance-constrained version of the model, US/WELLSS, which considers the stochastic nature of hydraulic conductivity. The utilized chance constraint is more accurate than previously reported formulations.) Optionally, US/WELLSDcan use a quadratic objective function to minimize
x= I
j= 1
j= I
k= I
where Hi, is the dynamic lift, the difference between ground surface elevation and optimal potentiometric head resulting at extraction well j at the end of the xth time period, [L]; and W,, is the weight assigned to the power used for extraction in the time period, [$/L.T)]. The weighting factors can be used to emphasize differentcriteria and different timeperiods. For example, assume a problem of minimizing the total extraction usingthe linear objective function. If the second time period is chosen be to much longer than the first time period and the weights assigned to extraction and injection in the second time period are larger than those used for the first time period, then the solution will tend to minimize steady-state extractiodinjection rates, andlessattentionwillbegiven to the short-term transientrates. Through the weighting factors, US/WELLSD can also be used for maximizing pumping rates for water supply problems.
2. ApplicationandResults Here we illustrate the use of US/WELLSDto determine the optimal time-varying sequenceof extraction and injection of water in prespecified locations needed for first immobilizing and then extracting a groundwater contaminant plume.In this example, the user specifiespotential locations of extraction and injectionwellsaroundthecontaminantplume(Figure 1). US/ WELLSD then determines optimal extraction and injection rates for different time periods. To illustrate model flexibility, four potential extraction wells and five potential injection wells are considered for placement outsidethe contaminant plume during the first period. In the second time period, three extraction wells are considered for placement inside the plume (to extract contaminated water) and five potential downgradient injection wells are considered. During both periods, the resulting hydraulic gradients (between 10 pairs of head observation locations) must be toward thecenter of the plume. Alternatively, the user could choose to minimize the pumping needed to capture the plume using only internal extraction wells in one or both periods.
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0 Potentlal inlectlon Well Observation Well
1
Contaminated Plume
3
0 I
Figure 1 Hypothetical
1
6
lnltial Groundwater Gradient
I
0.1 96
1
K :75 mlday (isotropic)
.
X
study area for Example A, addressable with
USWELLSD.
Here, the quadratic objective function is used and employs greater weights for the second time period than the first period. This supports the fact that the second period is much longer than the first. In addition, neither export nor import of water is allowed-total injection must equal total extractionin each period. All the above considerationsare incorporated within the model via the input data [15]. The user also specifies lower and upper bounds on head and pumping rates. Figure 2 shows US/WELLSD output, in meters and m3/day. This contains, in addition to the input bounds(L.Bound and U.Bound), the optimal values of the decision variables (pumping), state variables (head and gradient), and marginal values. The marginal is definedas the valueby which the objective function will change if a tightly bounded variable changes one unit. If a variable’s optimal value is not equal to either its lower or upper bound, its marginal will be zero. That is, the marginal will be nonzero only if the optimal value of the variable equals one of its bounds. In this case, the marginal shows the improvement of the value of the objective function resulting from relaxing thisbound by one unit. Marginals are valid only as long as no other variable also changes in value. Thus they might be valid for only a small range of change in the bound. To illustrate, the output file (Figure 2) shows that the marginal of the optimal injection rate in the firsttimeperiod at injectionwell 3 is -45.3. Theobjectivefunctionvaluewas 334,668.1. If the upper bound on injection in the first time period is relaxed by one unit at the 901isinstead of W),one would expect the value mentioned well (that is, the new upper bound of the objective function to change by about -45.3 to 334,622.8. If this change is actually -45.4. made and the model is rerun, the resulting change in objective function value is Marginals are useful in determining how to refine an optimal strategy. They help one to decide which bounds or constraints shouldbe looked at more closelyand perhaps relaxed. They also indicate the trade-off between that bound and objective achievement. They showhow much one is giving up in terms of objective attainment to satisfy that restriction.
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604 MODEL STATUS : OPTIMAL SOLUTION FOUND VALUE OF OBJECTIVE FUNCTION 334668.1 OPTIMALEXTRACTIONRATES Well No
1 2 3 4 5 6 7 No Well 1
FIRSTTIMEPERIOD L.Bound Optimal 0.00 745.42 0.00 447.60 0.00 448.71 0.00 747.86 0.00 0.00 0.00 0.00 0.00 0.00 SECONDTIMEPERIOD Optimal L.Bound 0.00
0.00
3 4 5 6
0.00 0.00 0.00 0.00 0.00
7
0.00
0.00 0.00 0.00 426.53 883.77 428.90
2
"""""""-""""""""""""""""""""""""""""""
U.Bound 900.00 900.00 900.00 900.00 0.00 0.00 0.00
U.Bound 0.00 0.00 0.00 0.00 900.00 900.00 900.00
Marginal 0.000 0.000 0.000
0.000 0.000 0.000 0.000
Marginal 81.955 81.627 81 -605 81 -913 0.000
0.000 0.000
......................................
OPTIMALINJECTIONRATES L-BOund Optimal No Well
l
2 3 4
5 Well No 1 2 3 4 5
FIRSTTIMEPERIOO
0.00 211 -66 0.00 900.00 328.89 0.00 900.00900.00 0.00 900.00 0.00 900.00 49.04 0.000
U.Bound 900.00 900.00
SECOND T I M E PERIOO L.Bound Optimal U.Bound 0.00 0.00 900.00 0.00 900.00293.53 0.00 900.00 900.00 0.00 545.67 900.00 0.00 0.00 132.583900.00
Marginal 0.000 0.000 -45.342 < = = = eMdnedin text 0.000
Marginal 132.584 0.000 -4.3E+2 0.000
...................................... ~~~~~~"""~~""""""""~""~""""""""""""
OPTIMAL HEADS AT OBSERVATION WELLS Well No
1 2 3 4 5 6 7 8
9 10 11 12 13 14
15 16
FIRSTTIME L. Bound 30 00 30.00
-
30.00 30.00
30.00 30.00 30.00 30.00 30.00 30.00 30.00 30.00 30.00 30 00 30.00 30.00
-
PERIOO Optimal 35.69 35.54 35 -60 35.55 35.70
35 -79 35.92 35 -88 35.84 35.77 35 -65 35 -60 35.65 35.79 35 -88 35.77
U.Bound 40.00 40.00 40.00 40.00 40.00 40.00 40.00 40.00 40.00 40.00 40.00 40.00 40.00 40.00 40.00 40.00
Figure 2 US/WELLSDoutput file for Example A.
Marginal 0.000 0.000 0.000
0.000 0.000 0.000 0.000 0.000 0.000 0.000
0.000 0.000 0.000 0 000 0.000 0.000
-
PC Sofmare for Optimizing Plume Capture SECOND TIME NoWell
1 35 2 3 4 35.62 5 35 6 35.66 7 35.75
.00 0.00 40.00 40.00 40.00
a
9 10 11 35.56 12 35.54 13 14 15 35.74 16
605
PERIOD
L. Bound
30.00 30.00 30.00 30.00 30.00 30.00 30.00 30.00
Optimal
-62 35 -62 35-68 -63
35-74 35-71 -64
30.00 30.00 30.00
35.S6
30.00
"""""""""""""""""""""""""""""""""""""_""""
Marginal U.Eound
40.00
0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
U.Bound
Marginal
40.00 40.00 40.00 40.00 40.00 40.00 40.00
40.00 40.00 40.00 40.00
OPTIMAL HEADS AT EXTRACTION WELLS FIRST
TIMEPERIOO
L. Bound 30.00 30.00 30.00 30.00 30.00 30.00 30.00
Well No 1 2 3 4
5 6 7
Optimal 35-09 35.29 35.29 35.09 35.61 35-62 35.61
40.00 40.00 40.00 40.00 40.00 40.00 40.00
0.000 0.000 0.000
0.000 0.000 0.000 0.000
SECONDTIMEPERIOD
L.Bound
Well No 1 2
30.00
3
40.00
4 35.70
5 35.24. 6 7
30.00 30.00 30.00 30.00 30.00 30.00
Optimal 35.69
U;Bound
35.72 35.73
40.00 40.00 40.00 40.00
Marginal
34-90 35.25
40.00 40.00
0.000 0.000 0.000 0.000 0.000 0.000 0.000
U.Bound
Marginal
40.00
0.000 0.000 0.000
........................................................................... OPTIMAL HEADS AT INJECTION WELLS FIRSTTIMEPERIOO L.Bound
Well No
1 40.00 2 3 40.00 4 5
36.03 36.46 36.47
30.00 30.00 30.00 30.00 30.00
Optimal 35-90
35.a3
40.00
40.00
0.000 0.000
SECONDTIMEPERIOO Well No 1 2 3 4 5
Optimal L.Bound 30.00
30.00 30.00 30.00
35-65 35-88 36.33 36.08 35.67
U.Eound 40.00 40.00 40.00 40.00 40.00
Marginal 0.000 0.000 0.000 0.000 0.000
Peralta et al. OPTIMALHYDRAULICGRADIENTS FIRSTTIMEPERIOD From 1 -> 3 -> 5 -> 6 ->
7 -> 8 -> 9 -> 10 ->
To
L. Bound 0.D0000
11 12 13 14 14 15 16 16
0.00000 0.00000 0.00000
0.ooooo 0 .ooooo 0.00000 0.00000
Optimal 0.00055 0.00003 0.00019 0.00000 0.00157 0.00000 0.00087 0.00000
U. Bound
0.01D00 0.01000 0.01000 0.01000 0.01000 0.01000 0.01000 0.01000
Marginal 0.000 0.000 0.000 1.17E+7 0.000 3.26E+7 0.000 1.17E+7
SECONDTIMEPERIOD
L. Bound
From To 1 -> 1 1 3 -> 12
5 -> 6 -> 7 -> 8 -> 9 -> 10 ->
0.00000 0.00000 0.00000
13 14 14 15 16 16
0.00000 0 .ooooo
0.00000 0.00000 0.00000
Optimal 0.00082 0.00241 0.00027 0.00014 0.00115 0.00000 0.00081 0.00000
U. Bound 0.01000 0.01000 0.01000 0.01000 0.01000 0.01000 0.01000 0.01000
Marginal 0.000 0.000
0 IO00 0.000 0.000
2.88E+8 0.000 0.000
Figure 2 Continued.
B. US/REMAXBfor Heterogeneous Multilayer Systems 1 . Model Background For optimizing management of complex heterogeneous systems, one would rather useUS/ REMAXB [l61 than US/WELLSD.This is the basic version of the Utah State response matrix model. To develop influence coefficients, it uses code modified fromMODFLOW, a modular finite-difference groundwater flow simulation model [4], and STR, a related stream routing the same module [17]. The physical system data neededby US/REMAXDcanbeinputin format as is used by MODFLOW and STR. Internally, US/REMAXB also uses a portion of PLUMAN, a decision support system for optimal groundwater contaminant plume management [18], and other code. The optimization model formulation capabilities are similar to those of US/WELLSD (Table 2). For steady state, the generic objective is to minimize J
K
where Wjis the weight assigned to pumpingin cellj, dimensionless or [$.T/L3].US/REMAXB can employ constraints 1-3 of US/WELLSDfor multiple layers. Similar to the US/WELLSD constraint 4, US/REMAXB can force total extraction to exceed, equal, or be less than total injection. Again, via the sign on the weighting coefficients, one can perform maximization. One can also achieve multiobjective optimization by the weighting method. Whereas in US/ WELLSD the same weight must be applied to all extraction wells in a time step (and a different weight can be used for injection wells, but the same must be applied to all such wells in a particular time step), in US/REMAXB eachwell can employ a different weight.
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2.Application andResults Introduction. For illustration, we discuss addressing a contaminant plume in a representative study area. First, the study area is described and the results of continuing current management are predicted, using MODFLOW+STR for flow simulation and MOC [l91 for transport simS/O model is ulation. Then an approach to developing an optimal strategy is discussed, the applied, and an optimal strategy is computed. Next, the system response to implementing the optimal strategy is verified using MODFLOW+STR and M W . Finally, slight variations in themanagementgoal or situation areassumed andnew optimalstrategiesaredeveloped. Computed optimal strategies are compared. Suguino [20] first addressed this study area using PLUMAN. Some of the discussion below follows his development. Study Area Description and Situation. The area (Figure 3) measures about 4.3 km by 4.3 km. It is bounded on the north by a large saltwater body; on the south, east, and northwest by impermeable material; and on the west by a lake. A river transects the area from south to north. Aquifer parameters of this example study area were obtained from ranges reported by Todd [21]. For the unconfined upper layer (layer l), parameters are as follows. Hydraulic conductivity: 1stzone: 2nd zone: 3rd zone:
45 d d a y (coarsesand)fromlaketocontaminantspillarea(columns 57-58) 30 d d a y (medium sand) in irrigated area (columns 51-56) 450 &day (fine gravel) in contaminant spill area (columns 37-50).
Specific yield: 1st zone: 0.27 (coarse sand) 2nd zone: 0.28 (medium sand) 3rd zone: 0.25 (fine gravel)
Figure 3 Finite-difference grid for the area addressable with US/REMAXB.
1-36and
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1.167 X 1.928 X
d s e c in nonirrigated area d s e c in irrigated area
In the confined lower layer (layer 2): 0.1564 m*/sec Transmissivity: Saturated thickness: 30.0 m Storage coefficient: O.OOO1 Finite-difference modelsare to be used in this study. This requires systemdiscretization. The resulting block-centered cell grid (Figure 3) has 58 columns and 39 rows. Cell side lengths range from 3 to 400 m. Because MOC will be used for transport simulation near the plume, cells of uniform size are specified for that region. The resulting 17 row by 20 column region (subsystem) near the plume has squarecells of 15.2 m (50 ft) side length. A conservative (nonreactive) contaminantis assumed to be spilled in the top aquifer layer (layer 1) of cell (22, 18) or (ll,, 3,). (The subscript “S” after a cell row or column index indicates that the cell is in the subsystem.) This cell is treated as a continuous source during the management period. Initially, pumping for water supply occurs in two cells between the plume and the river. One well is in layer 1 of (23, 15) or (12s, 15,). The other well is in layer 2 of (18, 18) or (7,, 18,). There is immediate concern about the potential for contamination reaching the supply well in layer l . Nonoptimal System Response Determination (Step 1). Before one attempts to develop an optimal strategy, one usually demonstrates the need for such a strategy. This requires predicting system response if no optimal strategy is implemented. Frequently, simulation models are used for this action. Here, MODFLOW+STR computes the potentiometric surface that will result from assumed steady-state conditions (Figure 4). Because of the gradient, the contaminant will tend to migrate toward the supply wells. M W is used to quantify the migration resultingin the subsystem from the steady flow. Figure 5 shows the 210 ppb contour expected to result 60 days after contamination begins. Furthermore, concentrationin the cell containing the drinking well (12,, 15,) reaches 3 17 ppb 8 months after the spill. We assume that this concentration level exceeds the health advisory for human consumption and that developing a plume capture strategy is desirable. Management Goals Specification and SI0 Model Formulation for Scenario l (Step 2). The assumed goal is to minimize the steady pumping (extraction and injection) needed to capture the plume. Plume capture will presumably be achieved when hydraulicgradients, just outside the plume boundary, all point toward the plume interior. We also want the head at extraction wells not to drop too far (to avoid reducing saturated thicknessby more than about 10%)or the head at injection wells not to rise to the ground surface. These criteria identify the example problem termed Scenario 1 . The SI0 model formulation for this scenario is shown below. The model computes the pumping strategy that minimizesthe value of the objective function, subject to the stated constraints and bounds. Locations of potential injection and extraction wells to be considered by the model are shown in Figure. 5. Figure 6 identifies head difference (gradient) control cell pairs and shows the direction that will be imposed on the hydraulic gradient by any computed optimal strategy. Theseare placed to enclose the plume projected to exist by day 60.A modeler can select potential well locations on the basisof practical experience.For example, the closer the injection wells are to the head gradient control locations, the less pumping is needed to
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I H
Figure 4 Nonoptimal (unmanaged) steady-state potentiometric surface contour map for the study area of Scenario 1 (meters above MSL). Js 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 ~ 1 1 5 1 5 ~ 1 8 1 9 2 0 legend 12
l3
14
U 16
l7 18 19 20 21
8 9 ‘S
10 11 12
22
23
l3
24
14
25
K
26 27 28
16 17
16ll79
21
23
25
27
29
31
33
’
unmanaged wen [ I 1 layer 1 [ 2 ) layer 2 potential managed injection well potential managed extraction well contamlnantsource
B 210 ppb concentration contour
0
meter 30 45
K
5
35
J
Figure 5 Subsystem discretization, potential well locations for Scenario 1, and 210 ppm contour, 60 days after contamination begins.
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J*
1 2 3 4 5 ' 6 7 8 9101112l314lSl617181920
1
12 13 14
2 3 4 5
unmanaged wen ( 1 1 layer 1 ( 2 1 layer2
Is l6
6
'S
Legend:
potential managed
lnJectionwell (upper bound on head)
l7
7
#)
8 9
19
14 15 16
2o I 21 22 23 24 25 26 27
l7
28
ta 11
l2 l3
16 17
l9 2321
25 2927 d
31
33
Q potential managed extraction well (lower bound on head)
contaminant source +
gradient canstraint 0
IS 30 45 m
tS¶
35
Figure 6 Head-difference constraint locations applied within the S/O model in Scenario 1.
satisfy the head-difference constraint. Thus, the modeler might want the model to consider pumping sites near the location where heads need most to be affected. The model objective is to minimize the value of Equation (3), using weights of 1, subject to
. . . , 22
Gb 2 0.01,
for B = 1,
hp
for 2 = 1,
(5)
for 2 =
(6)
2
15.0,
h* I25.0,
. . . ,6 1, . . . , 25
(4)
where GBis the difference in head between a pair of cells, the first located farther from the plume. A positive value denotes a higher head farther from the plume, [L]; ha is the hydraulic head just outside the casingof pumping well P located in the center of a pumping cell, [L]; 6 is the index denoting pair of cells head-difference (gradient) control pair; and 6 is the index denoting pumping well at the center of cell j or k. Here j = 6 and k = 25. Note that identifyingthe location of potential extraction and injection wells for the model (Figure 5 ) does not mean that the model will choose to pump at those locations. Via the optimization process, the model might choose to pump at only a few of the potential sites. The computed strategy will require less total pumping than any other strategy possible forthe specified potential well locations and imposed bounds and constraints.Furthermore, since this isa steady-state problem, steady-state system responseto implementing the strategy computedby the model will satisfy all those bounds and constraints. This is verified in the next step. Optimal Strategy Computation and Verificationfor Scenario I (Step 3). The optimal strategy computed for Scenario 1 is shown in Table 3. Because the model is minimizing pumping only for plume containmentin layer 1, no extraction is shown for layer 2. The original unmanaged pumping does continue from original supply wellsin both layers (Figure3) but is not included in Table 3 because the model is not optimizing that pumping.
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Table 3 Pumping Results for the Sample Scenarios g(eW
b(inj.)
(g
+ b) total
(@m) (m’lsec) Layer 2nd1st Layer
Constraints Scenario
~~~
1
2 3
onconstraint Gradient same the located on heads layer, head constraint on injection and extraction well. constraint: pumping Added = total extraction of sum total sum of injection. heads Gradient constraint on same and located theon on different layers, head constraint on injection and extraction wells.
0.01338 (212.05)
-
0.03358 0.02020 (320.13) (532.18)
0.01702 (269.74)
-
0.03404 0.01702 (269.74) (539.48)
-
-
~
0.00300 0.00329 0.03786 0.04415 (47.54) (52.14) (600.03) (699.69)
Figure 7 shows the locations of wells that will pump, according to the optimal strategy. It also shows the head-difference constraints [Equation (4)] that will be tight. Tight constraints are those that are satisfied exactly. The other gradient constraints are also satisfied, but the so. These latter head-difference constraints are “loose” (there model had no difficulty in doing at the two cells coupled by an arrowin Figure is more than0.01 m difference between the heads 6 but not shown at all in Figure7). No heads are against their bounds. Therefore neither Equation (5) nor (6) is tight. It is appropriate to verify that the computed strategy accomplished its goal of plume capture. MODFLOW+STR can be used to demonstrate how quickly the optimal steady pumping
%
l 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 1819 20
12 13 14 15
16
n
18
Is
19 20 I 21
9 10
22 23 24 25 26
11 12 l3 14 15
16 ti
27
28 16 l7
19
21 23
25272931 J
33 35
I.egend:
unmanaged well ( 1 1 layer 1 c21 layer 2 activemanaged injection well actiuemanaged extraction well 1contaminant MUI ’ce
+ lightgradient constraint meter 0 63045
Ea
612
Peralta et al.
strategy will cause the desired gradients to occur. Transient simulation demonstrated that the gradient constraintswould be satisfied 30 days after implementing the optimal pumpingstrategy (Figure 8). Figures 9 and 10 show the ultimate steady-state surface resulting from strategy implementation. Clearly, a groundwater divide has been formed between the plume and the supply well. MOC is used to predict the pollutant transportthat would result from strategy implementation. No contaminant moved past the injection wells. Theoretical verificationof the optimality of the computed strategyis beyond the scope of this document. However, many texts onoperations research and linear programming assure the optimality of solutions to models havinga linear objective function and constraints'. Alternative Scenarios. Scenario 2. This scenario differs from the previous in the addition of a constraint forcing total injection to equal total extraction around plume. the Again, pumping fromthe two supply wells is not included in the total. aquifer optimal a l l gradients contam. punping constraints beg ins ach i eved I 0
-I
I
60
90
check of cont.conc. I 240
days
Figure 8 Time scale of Scenario 1.
J -
column
3
0
c l
H
Figure! 9 Subsystem potentiometric surface resulting from implementing the optimal pumping strategy for Scenario 1 (meters above MSL).
613
PC Software for Optimizing Plume Capture Ips 1.36
Well
I 2
3
4 S
0.62 "0.60 9.69 0.85
Well 6 7 8
gprn
2l.56
9.89 9.49
- -EX55
9 10
13.46
Ips gpm -449.02 -3.09 79-58 5.02 'M.67 4.46 7.88 l2496
-100.00 "158482
Figure 10 Subsystem potentiometric surface resulting after6 months of optimal pumping for Scenario 1 (meters above MSL).
Results in Table 3 show an increase in extraction and a decrease in injection. Total pump the phenomeingneeded for plume containment increased slightly (1.4%). This illustrates of the non-increasing the number or restrictiveness of constraints does not improve the value objective function. Although total pumping increased, one less extraction well is used in this strategy than in the previous (Table 4). The same number of gradient constraintsare tight, but the locations of the tight gradient constraints differ slightly. Scenario 3. This scenario demonstrates what might happen if involved managers have conflicting goals. It differs slightly fromScenario 1. In addition to controlling the plume, the agency wishes to extract more from layer 2 for water supply. Three new potential extraction wells are located in cells (19,25), (20, 25), and (21, 25), as if along a nearby road. Pumping is not permitted to change at the two initial supply wells.
Table 4 Numbers of Managed Wells that Will Pump Under the Optimal Strategies for the Tested Scenarios g(extr.1 Scenario l 2 3
Layer Layer 2nd 1st
Hinj.)
3 2
-
4
3
6 6 14
(g
+ b) total 9 8 21
614
Peralta et al.
As a result, the objective function is altered to maximize new extraction from layer 2 while still minimizing the pumping in layer 1 needed to capture the plume. This is achieved by assigning a negative sign to extraction from the supply wells, and minimizing: K
I
Since minimizing a negative number is the same as maximizing a positive number, minimizing negative extraction in layer2 means maximizing that extraction. Also added are new constraints imposed on vertical flow in cells (21, 21) and (22, 21). There, the head in the lower layer is forced to exceed that in the upper layer by 0.01 m, preventing the downward migrationof contaminant. Figure 11 shows the resulting optimal injection and extraction well locations and tight gradient constraints. The optimal pumping strategy includes seven extraction wells and 14 injection wells. Although extractionof polluted water decreases, injection increases with respect to Scenario 1 (Table 3). Extraction of water for public supply increases by 31% above the unmanaged rate. Although the gradient constraints are all satisfied by the optimal strategy, subsequent simulation demonstrated that the vertical gradient is reversed in some plume-containing cells in be careful in placing head which the gradient was unconstrained. This illustrates that one must or gradient control in appropriate locations. In practice, another optimization would be performed, using additional vertical head-difference or gradient constraints. Processing Considerations. It is useful to consider the resources required to address optimization problems. First, the total computer time needed to solve an optimization problem is of Js
1 2 3 4 5 ' 6 7 8 9101112I3MlSl6~181920 l2 1 l3 2 14 3 15 4 16 5 n 6 18 7 19 8 9
~~~d
B unmanagedWen ( 1 1 layer 1 ( 2 I layer 2
actiuemanaged inJection well actiuemanaged wtraction well
20 I
21 22 23
Is 10 l1 l2 l3 14 15
24
25 26 27 28
16 17 16 17 9.
21 23
25
27 29
31
contaminant source tight grad. wnstr. (heads on samelaywl ++ tight grad.constr. (heads ondiff. layer1 meter
c
0 15 3045
Ea
33 35
J
Figure 11 h a t i o n of optimal pumping wells andtight head-difference (gradient) constraints for Sce-
nario 3.
min)
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PC Software Capturefor Plume Optimizing
interest. Table5 illustrates the time neededto address Scenario 1. Included stages useeither the discussed simulation models or the PLUMAN code on a 386 PC running at 33 MHz and having 4 MB RAM. Time required for US/REMAXB is comparable to thatof PLUMAN, since it uses many of the same solution procedures. Clearly, the stage of computing influencecoefficients, arranging the optimization model, and calculatingan optimal strategyis the most computationally intensive.For this scenario and stage, two steps can be distinguished. Thefirst involves computing influence coefficients. The second is model organization and optimization problem solution. Here, the step of generating influence coefficients requiresby far the most time. This results because this act essentially involves one simulation of a modified MODFLOW+STR per potential pumping location. Since thereare 31potential pumpinglocations, 31 simulations are performed to develop the influence coefficientsneeded for the response matrix. Themore decision variables (potential pumping rates), the more computer time involved in this step. The step involving model formulation and calculation of the optimal strategy is fairly short. The time needed to perform the optimization is a function of the number of decision variables (potential pumping rates) and state variables (heads or gradients that must be constrained within the optimization model). The larger these numbers, the more time required. Second, the size of the optimization problem being solved is of interest. For example, the special versions of US/WELLSD and US/REMAXB that are released in shortcourses are limited in the number of nonzero values they can have in the optimization formulation. (Even optimization algorithms that are not part of water management models are commonly limited either in the number of nonzeros or in the number of rows and columns in their constraint equations.) By way of explanation, there is one row in the response matrix per heador gradient constraint equation per time step of constraint. There is one column in the matrix per decision variable. For a steady-state problem, total matrix size is the product of the number of control locations andthe number of decision variables.The matrix contains one nonzero coefficientfor each potential pumping location-head control location pair (per time step of active constraint). For the steady-state Scenario 1, there are 31 X (22 31), or 1643, nonzeros due to influence coefficients. Thereare also 31nonzeros due to the weighting coefficients (even if they are 1in value) assigned to decision variables in the objective function. Thus, the optimization model formulation for Scenario 1employs almost 1700 nonzeros. (That of Scenario A using by considering US/WELLSD includes919 nonzeros.) This number can be reduced significantly injection in only every other cell on the plume periphery rather than in each cell. For example, 12 injectionwellswere considered, thenumber of nonzeroswouldbeabout ifonly 18 X (22 18) 18, or 738. In addition to reducing problem size, this would significantly reduce computational time.
+
+
+
Table 5 Computer Time Required to PerformEach Activity for Scenario 1 Step
Time
Software used
1
MODFLOW+STR head) nonoptimal (compute
2 3
MOC nonoptimal (predict atransport solute potent surface) in PLUMAN (compute influence coefficients, formulate management model and determine optimal pumping strategy) MODFLOW+STR (compute transient head response to optimal pumping) MOC (compute head and solute transport response to optimal pumping)
4
5
5.0 35.0 150.0 1.3 8.0
Peralta et al.
616
Reducing the number of nonzeros below IO00 is important becausethat is theupper limit on problem size in the inexpensive “special” versions of US/REMAXB and US/WELLSD.If problem size increases beyond that, software price increases dramatically. The full professional versions of the software can address problemsof virtually unlimited size.
V. SUMMARY Use of simulationloptimization modelscan significantly aid managementof groundwater contamination. It can speed the design process and reduce manpower costs. It can improve the produced remediation designs and reduce remediation costs. It can easily address problems previously considered very difficult. S/O modeling methods for groundwater flow management have been well established in research literature. Now, generally applicable S I 0 models are available for use on Pcs. The discussed models, US/WELLSD and US/REMAXB, use linear systems theory, influence coefficients, and superposition. These models can addressa wide rangeof problems. Easyto use, they include all simulation and optimization algorithms neededto compute optimal strategies. US/WELLSD and US/REMAXBare perfectly applicable to linear (confined) aquifer systems and can be applied to nonlinear systems. The former is most appropriate for fairly homogeneous aquifer and stream-aquifer systems.The latter can address complex heterogeneous multilayer stream-aquifer systems. Increasing use of these PC-based S I 0 models is anticipated, especially as user-friendly options increase. Even the special versionsof these models (releasedat shortcourses),can solve important real-world problems.
REFERENCES 1. Peralta, R. C., and Willardson, L. S . , Optimizing ground water planning and management, U.S. Committee on Irrigation and DrainageNewsletter, AprilIJune 1992, Denver, Colo., pp. 61-65. 2. Clarke, D., Microcomputer Programsfor Groundwater Studies, Elsevier, New York, 1987. 3. Glover, R. E., and Balmer. G. G., River depletion resulting from pumping a well near a river, Trans. AGV, 35(3), (1954). 4. McDonald, M. G., and Harbaugh, A. W., A modular threedimensional finite-difference groundwater flow model, inTechniques of Water-Resources Investigations,U.S. Geological Survey, 1988, Chapter Al, Book 6. 5. Gorelick, S. M., A review of distributed parameter groundwater management modeling methods, Water Resources Res., 19(2), 305-319 (1983). 6. Morel-Seytoux,H. J., A simple case of conjunctive surface-ground-water management, Ground Water, 13(6) (1975). 7. Verdin, K. L., Morel-Seytoux, H. J., and Illangasekare, T. H., Users Manual for AQUISIM: FORTRAN N Programs for Discrete Kernel Generationand for Simulation of Isolated Aquifer Behavior in 2 Dimensions, HYDROWAR Program, Colorado State Univ., Fort Collins, Colo., 1981. 8. Heidari, M., Application of linear systems theory and linear programming to groundwater management in Kansas, Water Resources Bull., 18(6), 1003-1013 (1982). 9. Illangasekare,T. H.,Morel-Seytoux, H. J., and Verdin, K. L., A technique of reinitialization for efficient simulation of large aquifers using the discrete kernel approach, Water Resources Res., 20(11), 1733-1742 (1984). 10. Danskin,W. R.,and Gorelick,S. M., A policy evaluation tool: management ofa multisystem using controlled stream recharge, Water Resources Res., 21( 1l), 1731-1747 (1985). 11. Lefkoff, L. J., and Gorelick, S . M.,AQMAN. Linear and Quadratic Programming Matrh Generator Using lbo-Dimensional Ground-waterFlow Simulation for Aquifer Management Modeling,
U.S.Geological Survey Water-Resources Investigations Rep. 87-4061, 1987.
oftware PC
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12. Reichard, E. G., Hydrologic influences on the potential benefits of basinwide groundwater management, Water Resources Res., 23(1), 77-91 (1987). 13. Ward,R. L., andPeralta,R.C.,EXEIS-ExpertScreeningandOptimalExtractionllnjection Pumping Systems for Short-Term Plume Containment, Rep. ESLTR-87-57, Air Force Engineering
and Services Center, Tyndall AFB, Fla., 1990. 14. Willis, R., and Finney, B., Optimal control of nonlinear groundwater hydraulics: theoretical development and numerical experiments, Water Resources Res., 21(10), 1476-1482 (1985). 15. Aly, A. H., and Peralta, R.C., USIWELLSD, Extractionllnjection Well System for Optimal Groundwater Management: User’sManual, Biological and Irrigation Eng. Dept., Utah State Univ., Logan, Utah, 1992. 16. Peralta. R. C., Aly, A. H., Suguino, H. H.,Belaineh, G . , and Miyojim, M., USIREMAX’, Utah State Model for Optimizing Management of Stream-Aquifer Systems Using the Response Matrix LoMethod: User’s Manual, Version f.05,Biological and Irrigation Eng. Dept., Utah State Univ., gan, Utah, 1992. 17. Prudic, D. E., Documentation of a Computer Program to Simulate Stream-Aquifer Relations Using
a Modular, Finite-Difference, Ground-water Flow Model, U.S. Geological Survey, Open-File rep. 88-729, 1989. 18. Suguino, H., and Peralta, R. C., PLUMAN A Decision Support System for Optimal Groundwater Contaminant Plume Management: User’s Manual, Version 1.0, Software Eng. Div., Dept. Biological and Irrigation Eng., Utah State Univ., Logan, Utah, 1992. 19. Konikow. L. F., and Bredehoeft, J. D., Computer model of two-dimensional solute transport and dispersion in ground water, in Techniques of Water Resources Investigations,USGS, Washington, D.C., 1984, Book 7, Chapter C2. 20. Suguino, H., A decision support system for optimal groundwater contaminant plume management, Ph.D. dissertation, Dept. Agricultural and Irrigation Eng., Utah State Univ., Logan, Utah, 1992. 21. Todd, D. K., Groundwater Hydrology, Wiley.NewYork, 1980.
ADDITIONAL READING Colarullo, S. J., Heidari, M., and Maddock, T.,111 (1984). Identification of an optimal groundwater management strategy in a contaminated aquifer, Water Resources Bull., 20(5), 747-760. Gharbi, A, (1991). Optimal groundwater quantity and quality management with application to the Salt Lake Valley, Ph.D. Dissertation, Dept. Agricultural and Irrigation Eng., Utah State Univ., Logan, Utah. Peralta, R. C., Killian, F? J., Yazdanian, A., and Kumar,V. (1989). SSTAR UsersManual; Rep. IIC-8913, International Irrigation Center Utah State Univ., Logan, Utah.
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30
Horizontal Wells for Subsurface Pollution Control
George Losonsky Eastman Christensen Environmental Systems Houston, Texas
Milovan S . Beljin University of Cincinnati Cincinnati, Ohio
1.
INTRODUCTION
Horizontal wells are emergingas a new technology for solving problems in the environmental industry. Horizontal wellscreen orientation complements typical aquifer geometry, groundwater flow patterns, and common site logistics. Soils are naturally stratified, and individual aquifers or water-bearing zones are much wider than they are thick. Contaminant concentrato remedial options are often highest directly beneath buildings, landfills, and other obstacles erations, so treatment facilities are constructed tens or hundreds of feet away from the target zone of remediation. Despite the dominance of the horizontal direction in aquifer shapes and groundwater flow, the predominant tool for extracting contamination from subsurface sources is a vertical well. However, in many environmental remediation scenarios, a horizontal well offers a better matchof form and function than a vertical well. The tabular geometry ofmany aquifer zones renders horizontal wells more productive than vertical wells. The specific capacity ratio of horizontal-to-vertical wells increases with decreasing aquifer thickness. Extraction of contaminated groundwater is often more efficient with horizontal Flow wells. A horizontal well placed characteristics ofmany aquifers create elongated contaminant plumes. through the coreof a plume can recover higher concentrations of contaminants at a given flow rate than a vertical well. Hydraulic barriersare most efficiently created using horizontal wells oriented perpendicular to groundwater flow direction. Logistical advantagesof horizontal wells are obvious. Horizontal wells avoid theneed for installing wellheads inside buildingsor in the midst of complex manufacturing or process fapenetrated to extract leachateor cilities. Landfills, spoil mounds, and landfill liners needbenot other underlying contaminants. Fractures in an aquiferare commonly vertical. Because fluidor vapor recovery from fractured zones requires penetration of numerous fractures, a horizontal well oriented normally to vertical fractures is the optimal tool for pump-and-beat or soil vapor extraction systems in vertically fractured zones. By analogy, vertical wells are efficient in highly stratified soils with 619
620
Losonsky and Beljin
little vertical communication between strata, where fluid or vapor recovery from many thin layers t h u g h a single wellbore is required. Injection of groundwater is part of some remediation systems, either to create a water table mound or to reinject treated effluent from a manufacturing plant into the subsurface. Water table mounds can help to control flow of contaminants toward recovery wells or trenches, or they can serve as hydraulic barriers. Manufacturing plants can avoid high sewer discharge costs if their treated plant .effluent can be reinjected into a non-drinking water aquifer. Reinjection can cause mounding, but the mounding can be minimized by using horizontal wells.
II. DRILLINGTECHNOLOGIES Various technologies can be used for installing horizontal wells for subsurface pollution control. Such wells are typically installed in unconsolidated soils 10-200 feet deep. Selection of drilling technique depends on surface access, well placement and completion requirements, and subsurface hydrogeology. Jetting and moling are the most common techniques used to create horizontal brings in shallow, unconsolidated soils. Both are mechanically simple and require minimal labor, using small tubulars that can be handled by one person. Small trucks can carry the hardware for both techniques. Moling employs a rotating bit, compressing soil into the hole wall. This requires a soft, compressible soil. Directional control is limited andrelies chiefly on trial-and-error targeting methods. Holes are only a few inches in diameter, limiting completion options. No drilling fluid is used, and no cuttings are generated. Clayey soils may become “damaged” (i.e., their permeability may be reduced) because the cuttings are not removed from moling boreholes. Jetting employs an off-axis, high-pressure water jet to fluidize the formation. Downhole transmitters anda walk-over receiver are used for surveying thewell location and determining jet orientation. The operator can steer the system accuratelyin unconsolidated, homogeneous soils. Jetting requires that there be no interference sources between the surface receiver and downhole transmitter.The transmitter must not be located deeper than25 ft below the surface. Drilling fluid lubricates the drill string. Cuttings mixed with drilling fluid enter the soil formation along the wellbore, which may cause formation damage as with moling. Rotary drillingemploys a stabilized assembly allowing control of well path inclination but no horizontal directional capability. Thedrilling rig must be large enough to providerotating, push, and pull forces. The technique is used for oil and gas recovery and for installing utility lines under rivers. Low drilling fluid flow rates are used in utility line installation, where the fluid serves only to carry cuttings throughthe hole wall to keep it open and lubricate to the soil. High fluid flow rates are used in hydrocarbon recovery to remove cuttings from the wellbore and thereby preserve formation permeability. A large radius of curvature is required in rotary drilling to prevent failure of tubulars. The larger the tubular diameters, the larger the curve radius required to prevent failure of tubulars. Surveying toolsare similar to those used for magnetic and gravitational orientation surveys. These tools work at any depth, but only in the absence of magnetic interference. Positive displacement steerable motor technology offers vertical and horizontal directional control. The radiusof curvature is small compared to that of rotary drilling, because the drillstring rotation is minimal. Penetration rates are high because ofrapid bit rotation. Drilling fluid forms an impermeable layer on the borehole wall, preventingfluid and cuttings from entering the formation, as with rotary drilling. The impermeable layer also prevents hole collapse by maintaining positive fluid pressure within the wellbore. The impermeable layer must be removed following installation of the screened casing, to expose undamaged formation. Drilling
Horizontal Wellsfor Pollution Control
621
fluid must remove cuttings from the wellbore. Viscosity and turbulence of the drilling fluid allow it to clear the wellbore. By comparison, vertical wellbores rely solely on viscosity. Turbulence prevents cuttings from accumulating along the bottom of the wellbore. Since turbulence increases with decreasing viscosity, ideal drilling fluid balances both factors. Thixotropic gels are well suited as drilling fluids. lbrbulence is also maintained with the help of centralizers, which provide a uniform annulus around the tubulars [l]. Percussion and vibration drilling techniques use air as a drilling fluid. These techniques are particularly suitable for drilling through heterogeneous soils containing bouldersor coarse gravel or through highly fractured rock formations. The wellbore remains dry, so fluid loss is avoided. However, percussion and vibration drilling techniques currently lack well-developed steerability and directional control.
111.
HORIZONTALWELLHYDRAULICS
Recently, due to increased interest in horizontal wells for oil production, a large number of papers have been published regarding the reservoir engineering aspects of horizontal drilling and reservoir simulation[e.g., 2-41. In the groundwater industry, thefirst theoretical analysis of groundwater flowto horizontal drains (collector wells) can be traced back the to early 1960s [ 5 ] . In the last few years, there has been renewed interest in horizontal wells for subsurface remediation [6-81 and for groundwater monitoring [9]. Langseth [lo] performed a numerical analysis of horizontal well performance and comparedinstallation and operation costs of horizontal and vertical wells.
A.Steady-StateFormulas Whereas a vertical well drains a cylindrical volume, a horizontal well of length L drains an ellipsoid. The zoneof influence is elliptical, with endpoints of the well constituting the foci of , is the ellipse. The area of the drainage ellipse, A
A,
=
n Reva
(1)
where R,, is the effective drainage radius of a vertical well in the same aquifer, anda is half the major axis of the ellipse [ll]: a = In order to compare the drainage area of a horizontal well with that of a vertical well, the drainage radius of a horizontal well, R,, measured in the horizontal plane that contains the well, is defined suchthat the corresponding circular areaA, equals the elliptical drainage area A, of the well: 2 A, = A, = n Rch
(3)
Combining Equations (1H3) and solving for a , a = (1512)[ O S
+ J0.25 + (We&)4]
0.5
A formula forestimating steady-state flow to a horizontal well in a homogeneous and isotropic aquifer is given as [12,13] 2nKBAs
(5)
622
Losonsky and Beljin
where Qh is the flow rate, [L3iT]; As is the drawdown, [L]; L is the length of horizontal well, [L]; r, is the well radius, [L]; K is the hydraulic conductivity, [ m ] ; and B is the aquifer thickness, [L]. The specific capacity of a horizontal well, Jh = Q,,/&, is usually greater than that of a comparable vertical well, J , = QJAs, except in relatively thick and highly permeable aquifers. In Figure 1, the ratio of the specific capacityof a 500-ft-long horizontal well,Jh, to that of a fully penetrating vertical well, J,, is plotted as a function of the hydraulic conductivityK (gpd/ft2) for three different aquifer thicknesses(B = 5 , 20, and 100 ft). The figure reveals that the ratio of the horizontal to vertical well specific capacity (productivityratio) is the greatest for relatively thin aquifers with low hydraulic conductivity. Thus, horizontal wells are most effective in thin, low-permeability aquifers where vertical wells commonly fail to produce significant volumes of groundwater.
B. Anisotropy Effect When the vertical conductivity,K,, is different from the horizontal conductivity,Kh, the flow rate to a horizontal well can be computed as follows:
where
.
=
l0O0? Aquifer Thickness:
.001
.01
.l
1
K [gpdlft2] Figure 1 Productivityratiovs.hydraulicconductivity.
10
mmmmmm~
b=20ft
*M*UUVU
b=lOOft
100
1000
Wells
Horizontal
623
for Pollution Control
Figure 2 plots the specificratio of a 500-ft horizontal well as a functionof the conductivity contrast and the aquifer thickness. In the case of anisotropic aquifers, the specific capacity ratios of horizontal to vertical wells increase if the hydraulic conductivity ratios, K,,/K,, decrease. This increase is inversely proportional to aquifer thickness.
C. Formation Damage and Effective Well Radius During drillingof a well, drilling mud can invade the aquifer and change its permeability in the vicinity of the well and cause formation damage or skin effect. The thickness of the “skin zone” will depend not only on drilling technology but also on the permeability of the aquifer. High-permeability aquifers exhibit a larger skin zone than low-permeability aquifers. However, the reduction in permeability is smaller in a high-permeability zone than in a low-permeability zone. The additional drawdown due to the change in permeability and the turbulent flow around well losses. Because of lower flow rate per unit screen length, horizontal wells the well is called show smaller well losses due to drilling mud invasion than vertical wells. Van Everdingen [l41 defined a dimensionless “skin factor” S as s = [ ( W
- 11log(rirw)
(7)
where r, is the radius of the skin zone with permeability k, in an aquiferof permeability k, [L2]. The effective radius of a well,rk, is defined as the theoretical radius of the well required to match the observed pumpingrate and drawdown. The effective radiusof a hypothetical vertical well that pumpsat the samerate as a horizontal well canbe computed using the equation
1000 3
100
10
1
Figure 2 Productivityratiovs. anisotropy.
Aquifer thickness:
Losonsky and Beljin
624
The following formula relates the skin factor, S, and the effective radius of a well, rk:
rk = r, exp(-S)
(9)
Renard and Dupuy [l51 gave the following solution for computing flow rate, Qh, from a horizontal well with the skin factor S: Qh =
cosh”(2a/L.)
2nKBAs (B/L)log(B/23crb)
+
+
S
If the length of a horizontal well greatly exceeds aquifer thickness, L % B , or if well length is small compared to drainage radius, R,, Equation (5) reduces to the well-known Thiem’s equation,
with the effective well radius being equalto one-fourth of the horizontal welllength, rk = L/4.
D. Off-Center Horizontal Well All solutions presentedso far have assumed that the horizontal well is located at midheight in the aquifer cross section. The distance, 6, from aquifer mid-height to the horizontal well is called the eccentricity of the well. The flow rate of an eccentric horizontal well can be calculated using the formula
2nK#As Qh =
[
a+-
1%
L12
]
-k
log
[
+
]
(pB/2)2 p2a2 pBrJ2
As long as thehorizontal well eccentricityis relatively small, 6 < 2 B/4, the performance of a horizontal well is not significantly affected [16].
IV. TRANSIENT SOLUTION Gringarten et al. [l71 used source and Green’s functions to solve a problem of transient flow into a well with a single vertical fracture. For the uniform-flux fracture, i.e., a fracture with uniform fluid entry along its length, the pressure distribution is given by
where S, is dimensionless drawdown;rD is dimensionless time;,x = x/xf; yo = y/xf;xf is half fracture length, [L]; x is thedistance measured fromthe well center along the fracture, [L]; and y is the distance perpendicular to the fracture, [L]. The concept of uniform flux and infinite-conductivity fracture can be applied to a horizontal well by assuming that the horizontal well is a vertical fracture with a width equal tothe radius of the well [16]:
Horizontal Wellsfor Pollution Control
625
where LD = [ W 2 B ] m ; rwD = 2rJL; XD = YD = 2yIG zWD= z JB; and z, is the vertical distance from the aquifer bottom, [L].
Z,
= zIB;
For a long horizontal well,the summation termin Equation (14) approaches zero, and the horizontal well solution reduces to the fracture solution, Equation (13).
V. EXAMPLEAPPLICATIONS A. Case 1: Texas Gulf Coast Industrial Landfill Site Before the advent of horizontal wellbore technology, vertical wells were used to recover contaminated groundwater under landfills. In aquifers with high hydraulic conductivity, vertical wells placed along the periphery of a landfill can influence the aquifer below the landfill. If the size of the plume is small comparedto the size of the landfill, migration of contaminants from to vertical extraction wells along the periphery spreads the coreof the plume below the landfill the contamination. Low hydraulic conductivity requires placement of vertical wells within the landfill, penetrating the landfill liner. Long-term sealing of the liner around the vertical well casing cannot be guaranteed. Horizontal wells avoid these problems by placing a production screen directly in the plume without penetrating the landfill. In 1991, a horizontal well was installed beneath an RCRA facility on the Gulf Coast of 900 ft Texas to recover contaminated groundwater. The landfill occupies an area approximately by 900 ft. Its depth is 30 ft, and the slope along its sides is 27" from horizontal (Figure 3). Numerical modeling prior to installation predicted that a single horizontal well wouldbe hy-
RED-BROWN CLAY GRAY-TAN SILTY CLAY RED-BROWN CLAY FINE SAND
""""""""""""I""""""""""""""""""""""""""". GRAY CLAY
Figure 3 Texas Gulf Coast landfill-cross section with horizontal well path.
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draulically more efficient than a vertical well system [18]. The single horizontal well replaces five vertical recovery wells. Fewer pumps and a lower volume of recovered water save operation and maintenance costs of the remediation system. 1. Hydrogeology Interbedded, 20-30 ft-thick reddish brown clay and gray and tan silty or sandy clay characterize the subsurface from0 to 98 ft depth, corresponding to 26 ft above mean sea level(MSL) to 72 ft below MSL (Figure 3). The uppermost silty clay layer and the top of the underlying reddish-brown clay layer have been excavated to accommodate the landfill. Tan fine sand occurs at 100-128 ft depth from the surface,or 70-95 ft below the base of the landfill. Blue-gray and black-gray clay underlies the sand. The silty or sandy clay layers and the sand layer are saturated. The reddish-brown clays are aquicludes or aquitards. The tan sand layer is the target of the remediation effort. The hydraulic conductivity, K, of the sand is 30 ftlday, and storativity, S, is 1 X lo-', indicative of a confined aquifer. There is no significant potentiometric gradient in the absence of pumping. 2. Well Specifications Sixty feet of 65/-in. slotted stainless steel screen was installed beneath the landfill at a total vertical depthof 114 ft. This depth is 4.7 ft below mid-height in the aquifer, givingthe well an eccentricity of 4.7 ft. The wellhead protrudes at a 41 S " angle, 20 ft from the edgeof the landfill. The horizontal displacement from the wellhead to the beginning of the screen is 248 ft, reflecting a radius of curvature of 275 ft. The curved sectionof the well was cased with 10 -in. of high-density polyethylene liner to isolate soil zones during drilling and to provide structural support for the inner stainless steel casing. 3.DrillingProcedure Drilling commenced with augering of a 16-in. wellbore to a measured depth of 37 ft, which corresponds to the kickoff point forthe curved section of the wellbore. A steel conductorpipe, 14 in. in nominaldiameter, was cemented in the 16-in. wellbore. The cement was allowed to set for 12 hr. The curve drilling assembly then drilled and simultaneously caseda 13-in. borehole with an effective drilling radius of 275 ft to a measured depthof 272 ft. The curved sectionwas drilled using a 13-in.wing bit. High-densitypolyethylenecasing 10 in.thickwasthencehad set, the horizontaldrilling mented intothe curved section of the wellbore. After the cement assembly drilled an 8%-in. wellbore to a measured depth of 352 ft, with a final inclination of 90.5" from vertical. The horizontal section was drilled using an 8%-in. rock bit. Schedule 40, entire 3 16 SS ERW stainless screen casing, with6Ys-in. outer diameter, was installed along the length of the wellbore. Only the60-ft section of stainless steel casing installed withinthe target zone was slotted. Slot size was 0.02 in. 'henty-foot sections of stainless steel casing were welded together, increasing installation time. Six 1 0 4 sections of prepacked screen were installed inside the stainless steel screen in the target zone. The prepacked screen was attached directly to an electric submersible pump oriented horizontally within the unscreened stainless steel casing. Well development required 18 hr,after which clear water was recovered. Residual drilling fluids were flushed out of the well with a submersible waterjetting tool. Drilling, installation, and well development required approximately 10 days. Duringdrilling, continuous updatesof inclination and tool faceorientation were provided to ensure proper wellbore placement. TruTracker surveys were taken as needed, at least once every 20 ft, to provide actual bit locations with respect to the proposed well path direction. The horizontal wellbore remained within a 4-ft vertical target range, thus exceeding the required accuracy of k 5 ft (Figure 3). The well remained within a 20-ft-wide horizontal target zone (Figure 4). Approximately 18,000 gal of cuttings, drilling fluids, and cement returns were generated dur-
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Figure 4 Texas Gulf Coast landfill-plan view
of horizontal well path.
ing the installation of the horizontal well. Nearly half of that volume was generated during installation of the curved section of the well. 4.
WellPerformance 1-2 ft at distances of upto 300 The well produces upto 7 gpm of water, creating drawdown of ft radially from the midpoint of the well. The drawdown created by the horizontal well is consistent with results of analytical modeling shown in Figure 5 (distances are given in feet, and the contour interval is 0.1 ft). The model assumes steady-state flow in a homogeneous, isotropic aquifer. Less than 100 ft from the well, the potentiometric surface defines an elliptical
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Figure 5 Texas Gulf Coast landfill-horizontal
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trough of depression. Beyond 100 ft, potentiometric surface contours are circular. The model indicates that drawdown along the well is 2.7 ft at a pumping rate of 7 gpm. The specific capacityof the horizontal well is only 1.16 times that of a comparable vertical well with fully penetrating screen. Therefore,the primary advantageof this horizontal well is not hydraulic, but logistical. The risks involved in penetrating an existing landfill liner, this scenario, outweigh the cost whichwould be requiredfor vertical wellinstallationin difference.
B. Case 2: Industrial Chemical Plant in Southern Louisiana Contaminated clayey soils beneath industrial plantsare commonly excavated to prevent migration of contaminant5 into underlying aquifers. Low hydraulic conductivities of such soils require close spacing of vertical wells, but dense wellfields are difficult or impossible to install because of logistical obstructions such as underground utility lines and overhead steel structures. Furthermore, drilling vertical wells in the midst of active operating facilities raises concerns about worker health and safety as well as plant productivity losses. Recently, several horizontal wells have been installedas an alternative to excavation in sandy clay soils beneath petrochemical complexes alongthe Mississippi River in the industrial corridor between Baton Rouge and New Orleans, Louisiana. Two such wells were installedat a petrochemical complex located southof Baton Rouge. The wells were installed to recover dissolved ethylene dichloride and monochlorobenzene from shallow silty clay soils. 1. Hydrogeology Clay and silt dominate the subsurface to a depth of 60 ft at thesite. Eight soilstrata have been identified. The top 8-10 ft consists of dark brown to gray clay and orange, green, and brown to gray silty clay. Below that is a 10-ft-thick orange and gray to brown clayey silt, which is underlain by alternating clay and silt.The 10 ft of silt isthe target of the remediation effort. Its hydraulic conductivity is 1.7 Wday in the vicinity of the contaminant plume. Vertical permefffday. The clay ability in the clayey soils aboveandbelow the silt is approximately 3 X sediments behave likeaquitirds. The potentiometric gradient in the 10 ft of silt is 0.002 eastnortheast.
2.Well Placement The shallower of the two wells (Well A)was installed at a total vertical depth of 12 ft beneath an existing superstructure. The overheadstructures and the concrete foundation below ground level containedactive unit process equipment.The horizontal section of the well is 356 ft long, and it was installed alongthe midline of a 20-ft-wide corridor bounded by 60-ftdeep concrete and wood pilings. A vertical well was located only 5 ft away from the wellpath, demanding high placement accuracy. The deeper well (Well B) was installedat a total vertical depth of 14 ft beneath an existing concrete road, along a pipe rack and. railroad loading dock. The horizontal screenof the well is 400 ft long, and it was installed between pilingssupporting the pipe rack on one side and a freshwater drainage ditch onthe other side. 3. Well Specifications
The curved section of Well A (Figure 6) is constructed of 10%-in. -high-density polyethylene casing cementedin a curved 12!4-in. wellbore. The curve ends at a measured depth (measured along the wellbore) of 80 ft. The horizontal section is completed with 65h-in. highdensity polyethylene slotted liner (slot size is 0.02 in.) installed within an 8Ya-in. wellbore. The horizontal section ends at a measured depth of 436 ft. Blank (unslotted)highdensity polyethylene
ells
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Figure 6 Louisianachemicalplant-horizontal
well path, well A.
casing is installed inside the loyein. curved casing, rising to the wellhead. An electric submersible pump, rated at 10 gpm, was placed inside the @&in. casing at the bottom of the curved section of the well. Stainless steel wire-wrapped prepacked screen was installed inside the horizontal high-density polyethylene casing and attached the to pump to filter silt-sized formation material. Horizontal displacement between the wellhead and the beginningof the screen is approximately 70 ft. The horizontalsection remains withina 3-ft tolerance envelope, ranging in total vertical depth from 11.8 to 14.9 ft. Horizontal accuracy is within 2 ft of the planned termination point. Well B is constructed similarly to Well A (Figure 7), with total vertical depth ranging from 12.4 to 17 ft. Four undisturbed horizontal core samples were extracted during drilling-three from the shallower well and one from the deeper well. Eachcore is 5 ft long and 2 in. in diameter. Coring is accomplished by setting a hydraulic coring tool (Figure8) into the soil at the end of the
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Figure 7 Louisianachemicalplant-horizontal
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Figure 8 Hydrauliccoringtool.(Source:EastmanChristensenEnvironmentalSystems.)
wellbore with a moderate amount of pushdown force to isolate the inner barrel from the drilling fluid and contaminated soiland groundwater. Pressure is hydraulically appliedto create a constant load on the outer tubeof the coring device.An accumulator located on the drillingrig is plumbed in parallel with the drill string circulation path to maintain a constant punch force. Pressure is raised to the calculated punch release force, which breaks shear pinsand accelerates the inner tube into the formation. Pressure is maintained in the system to hold the outer tube against the formation to prevent the drilling medium from coming into contact with the sample as it is pulled back into the outer barrel. The core barrel is then retrieved to the surface for recovery of the undisturbed sample. The sample is contained within a disposable plastic liner. The core barrel requires a radiusof curvature of at least 100 ft to pass through a 5Vi-in. borehole. The total groundwater recovery of the two wells is 23 gpm, which is comparable to over 50 vertical wells in the same water-bearing zone. Vertical wellsas far as 70 ft away from the horizontal wells exhibit drawdown causedby pumping from the horizontal wells. 4.
AnalyticalModeling Results of analytical modeling of capture zones that develop in response to groundwater recovery from the two horizontal wells are shown in Figure 9. Recovery of the contaminant tank car unloading areas, process sump, and glyoxal unit, shown in plume in the vicinity of the two 400-ft-long horizontal wells placed within the waterFigure 9, canbeachievedwith bearing silt zone.The southern well would be placed 30 ft north of the glyoxal unit,as shown
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Figure 9 Louisianachemicalplant-groundwatercapturezones.
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in Figure 9. Shaded areas indicate 4-year and 8-year capture zones of the two wells. The capture zones are calculated using a groundwater code called PAT [19], which assumes a homogeneous, isotropic aquifer. The assumed aquifer parameters, input data, and the results are listed in Table 1. Results of analytical modeling of aquifer response to groundwater recovery fromthe two horizontal wells are presented in Figure 10 (distances are in feet and the contour interval is 1 ft). The shaded region represents the chlorobenzene (MCB) plume. Groundwater recovery rates are calculated using W E L L , an analytical model [20]. Aquifer response tothe two proposed horizontal recovery wellsis estimated by simulating each horizontal well by a series of closely spaced vertical wells with total pumping rate equal to that calculatedby W E L L . This method provides a good estimate of drawdown curves generated by more complex, numerical methods, and hence it is a good tool for predicting actual drawdown contours.The model used for this simulation is a modified version of THWELLS [21], which solves the Theis equation for multiple wells and accounts for unconfined aquifer conditions. The model assumes non-steady-state conditionsin a homogeneous, isotropic aquifer with infinite areal extent and with a hydraulic gradient of 0.002. Time elapsed since the onset of pumping is 100 days. The analytical models suggest thatthe two horizontal wells installed at the plant could capturethe plume during an 8-10-year period if the total pumpingrate were only 5 gpm. Actual recovery rates are over four times that amount, so closure levels of contaminant concentrations may be achieved sooner.
VI. CONCLUSIONS Horizontal wells offer significant advantages over vertical wells in environmental remediation and protection in many hydrogeological scenarios. These include thin and/or low-permeability aquifers, where many closely spaced vertical wells are replaced by a single horizontal well. Current high installation cost of a horizontal well compared to a vertical well is offset by operation and maintenance cost savings. New developments in horizontaldrilling technology will further reduce the cost of installing horizontal wells, and subsurface pollution control with horizontal wells should become as common as vertical wells. Table 1 Input data Hydraulic conductivity Storage coefficient Flow rate of horizontal well Time of pumping (capture zones) Time of pumping (drawdown contours) Aquifer thickness Horizontal well length Wellbore radius Radius of influence of horizontal well Horizontal well eccentricity Regional flow gradient Results Half the major axis of drainage ellipse Specific capacity, horizontal well Drawdown in absence of flow gradient
1.67 ft/day 0.05 2.5 gpm
4and8yr 100 days 8 ft
mft 0.7 ft 330 ft 2 ft
0.0021
361.9 ft 68.34 gpdft2 7.04 ft
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Figure 10 Louisianachemicalplant-aquiferresponsetohorizontalwells.Potentiometricsurface contours interval is 1 ft. Distance scale isin ft. Initial potentiometric surface elevation is 10 ft at origin (SW comer).
REFERENCES 1. Chin, W. C., Borehole Flow Modeling in Horizontal, Deviatedand Vertical Wells,Gulf Publ. Co.,
Houston, Tex., 1992. 2. Babu, D. K., and Odeh, A. S., Productivity of a horizontal well, Paper SPE 18298, SPE Reservoir Evaluation, November 1989, pp. 417-421. SPE 3. Giger, F. M., Horizontalwellsproductiontechniquesinheterogeneousreservoirs,Paper 13710, presented at the 1985 SPE Middle East Oil Technical Conference and Exhibition, Bahrain, Mar.11-14,1985. 4. Goode, F! A., and Kuchuck. F. J., Inflow performance of horizontal wells, SPE Reservoir Eng.. August 1991, pp. 319-323. 5. Hantush, M. S., and Papadopulos, I.S., Flow of ground waterto collector wells,J. Hydraul. Div., Proc ACSE, 1962, pp. 221-244. 6. Dickinson, W., Dickinson. R. W., Mote, F!, and Nelson, J., Horizontal radials for hazardous waste remediation, NSWMA Waste Tech '87 Conference, San Francisco, Calif., Oct. 26-27, 1987. 7. Kaback, D., Looney, B., Corey, J., Wright, L., and Stele, J., Horizontal wells for in-situ remediation of groundwater and soils, NWWA Outdoor Action Conf., Orlando,FL., May 22-25, 1989. 8. Looney, B., Kaback, D. S., and Corey, J. C., Field demonstration of environmental restoration using horizontal wells, Third Forum on Innovative Hazardous Waste Treatment Technologies, June 11-13, 1991, Dallas, Tex. 9. Karlsson, H., and Bitto,R., New horizontal wellbore system for monitor and remedial wells, Proc. Superfund '90, pp. 357-362. 10. Langseth, D. E., Hydraulic performance of horizontal wells, Superfund '90, Washington, D.C., NOV. 26-28,1990.
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11. Fritz, R. D., Horn, M. K., and Joshi, S. D., Geological Aspects of Horizontal Drilling, ASFG Course Notes 33, 1991. 12. Borisov, J. P., Oil Production Using Horizontal and Multiple Deviation Wells, Nedra,Moscow, 1964. 13. Joshi, S. D., Augmentation of well productivity with slant and horizontal wells, JPT, June 1988, pp.729-739. Trans. 14. van Everdingen, A. F., The skin effect and its influence on the productive capacity of a well, AIME, 198, (1953). 15. Renard, G . I., and Dupuy, J. M., Influence of formation damage on the flow efficiency of horizontal wells, Presented at the Formation Damage Control Symposium, Lafayette, La., Feb. 22-23, 1990. 16. Joshi, S. D., Horizontal Well Technology, Pen Well Books, 'hlsa, Okla., 1991. 17. Gringarten, A. C., Ramey, H. J., Jr., and Raghavan, R., Unsteady-state pressure distributions created by a well with a single infinite-conductivity vertical fracture,SPE J . , August 1974. pp. 347360. 18. Speake, R. C., 'Itpjan, M.. and Wang, Z. Z., Modeling the performanceof a horizontal groundwater recovery well, Proc. Outdoor Action Conf., Las Vegas. Nev., Natl. Ground Water Assoc., 1991, pp. 399-415. Model, Kassel, Stutt19. Kinzelbach, W., and Rausch, R.,PAT: Pathlines and Traveltimes Groundwater gart, Germany, 1990. 20. Beljin, M. S . , and Losonsky, G., HWELL a horizontal well model, Solving Ground WaterProblems with Models, NGWNIGWMC Conference, Dallas, Texas, 1992, pp. 45-54. 21. van der Heijde, P. K. M., THWELLS, International Ground Water Modeling Center, Holcomb Research Institute, Indianapolis, Ind., 1991.
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Part V
INDUSTRY-SPECIFIC POLLUTION CONTROL
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31
Pollution Control and Waste Minimization in Military Facilities
Merrit l? Drucker Army Management StaffCollege
Fort Belvoir, Virginia
1.
INTRODUCTION
Military facilities include a wide range of bases, installations, maintenance facilities, manufacturing plants, factories, as well as installations in foreign countries and temporary facilities established in support of combat operations. Some military facilities are similar to cities and towns; others are highly specialized facilities producing unique munitions;others resemble office complexes. Some facilities are rented. Military facilities present a variety of problems for the environmental engineer. First, military bases are generally organized to support a wartime or contingency mission. This means that environmental considerations may not be as prominent as in municipalities and private industry, although this attitude is changing rapidly. Second, because of their organization and function, the waste stream from military facilities is likely to be more complex and, in some cases, more toxic or hazardous than the waste stream from a comparable civilian facility. Finally, military facilities are operated undera different budgeting and financial system than municipalities and private industry. Environmental engineers employedby military facilities ought of the culture, or sociology, to be aware of these differences and should make themselves aware of the military facility where theyare working, the military chainof command, andthe peacetime and wartime missions of the facility. This is not merely “nice-to-know” information. It is the essential first step to designing a pollution control and waste minimization strategy for an installation.
II. MATERIALS
AND PROCESS ANALYSIS
The first step in controlling pollution and minimizing waste from a military facility is to find out how much material and what type of material is being brought into the facility. This information is not easily obtained, and few if any installations can tell for certain what all their material imputs are.However, even imprecise estimates of the amountof material brought into
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the facility can helpdetermine start points for control strategies. Often, after merely lookingat what the facility is ordering, substantial reductions or effective substitutions can be made. A centralized computer system that could identifythe total amount of material coming into the installation would be a useful management tool. Unusually large orders or inputs of toxic or hazardous materials are good initial places to look for reductions. vpically, pollution control and waste minimizationhave been thought of as logistical or engineering functions.A more effective approachwould require managers at all organizational levels to know what all of their inputs are (materials, energy, utilities) and to examine their first reducing the amount of inputs caneliminate much input stream for reductions. Focusing on waste. Often smaller quantities can be ordered, less toxic material substituted, or some items eliminated entirely. Working with suppliers can be especially helpful, as suppliers of hazardous materials are under intense pressure to reduce the hazard level of theirproducts, to reduce packaging, and to accept empty containers for reuseor recycling. The second step is to examine the various processes at the facility to find economies and efficiencies. Most industrial and materials processes at military facilities were not designed with pollution control or minimization in mind until relatively recently. All processes mustbe continuously evaluatedso they can be made to consume fewer inputs of materials, energy, and hazardous materials. Process improvement may entail improved housekeeping, better maintenance of equipment, training of employees, replacementof older equipment with newer, more efficient models, or completely new ways of performing required industrial tasks. Engineers ought to be especially active in asking employees who actually runthe processes how savings can be achieved.Often, the employee closest to the process can providethe best suggestions on ways to make the process less polluting, less energy-intensive, or less expensive. Nor should relatively inexpensive, simple solutions be overlooked. Actions such as providing plastic covers for metal drums can prevent contamination from rainwater, thus protecting products stored outside and avoiding large disposal fees. Since many military installations share similar processes andfacilities, it is often possible to obtain information fromother facilities on how to improve operations. Informationsharing and exchange are vital elements in any plan to reduce waste or minimize pollution. Processes that have been implementedat one installation can often be implemented at another with little modification. The United States Department of Defence is undertaking a department-wide effort to reduce the amount of hazardous waste generated. Specialized agencies throughoutthe be called department serve as consultantsor centers of expertise for waste reduction and should on for assistance. Environmental engineers assigned to military facilities should aggressively pursue efforts to replace toxic material with less toxic substances. It is especially importantfor military facilities to plan and budgetfor pollution control and waste minimization. These facilities do not have the flexibility of private industry, and procurement times tend to be longer. Installation and organizational plans, both long-term and short-term, as well as budgets, must include planning and funding for new equipment and facilities. The military facility’s higher headquarters must be kept fully informed as to the status of minimization efforts and the need for capital investment. The third, and final, step is to control and properly dispose of the wastes that are generated. This requires disposal systems and options, and education of the installation population. The primary type of disposal of wastes from militaryfacilities should be recycling. Almost all military facilities have recycling systems, and thereis an accounting system in place to return profits from the recycling program to the facility. However, recycling could be greatly expanded, resulting in significant cost savings as disposal fees for waste increase. All wastes should be evaluatedfor recycling before disposal, and consideration should be given to giving away waste material such as scrap lumber or metal rather than disposing of it as pure waste.
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Recycling programs on military facilities shouldbe fully integrated with the local community. The military recycling program should be made available to the local community, and those who live and work on the installation should be encouraged to participate in community recycling programs. It is essential that recycling programs be fully coordinated and integrated into programs sponsored by other military facilities and the local community, inboth the United States and in foreign countries. Current and pending legislation, primarilyat the state level, will require that certain percentages of the waste stream be recycled. It is impossible to determine if the military facility is meeting this goal unless the waste managers have accurate information on both the amount of material coming into the installation and the amount of waste being generated. This reinforces the need for automated materials tracking systems. There is a considerable amount of technical information available on pollution control and waste minimization, much of which is readily applicable to military facilities. Employees and managers at all organizational levels should be required to be on the lookout for alternative processes and methods and ways to improve existing processes. It is critically important that waste minimization and pollution controlbe seen as a primary responsibility of those who actually work with the systems and materials and not the responsibility of the environmental enbe seen as a line, and not a staff, gineer.Pollutioncontrolandwasteminimizationmust function; these requirements shouldbe part of efficiency reports and performance appraisals. While supply personnelare in a critical position, leaders and managers must specify that they want nontoxic materials whenever possible, and must specify that they prefer products made from recycled, postconsumer waste. One extremely valuable, yet sometimes overlooked, resource available to military facilities is audit capability. Most military facility commanders have professional auditors available to support them. These auditors can examine all phases of the pollution control, waste minimization, and recycling program and can assist in achieving considerable savings and efficiencies. Environmental engineers working on military facilities are encouraged to seek auditing support. The payoff can be enormous.
111.
LEGALREQUIREMENTS
In the United States, federal facilities must comply with local, state, and federal environmental, public health, and safety laws. Although there are some minor exceptions, the general thrust of environmental legislation is toward more and stricter regulation; military facilities are no exception. For the environmental professional, a detailed knowledge of the law is essential. Law and technology combine to form an interactive process.As technology improves and allows us to detect and control more pollutants, laws are amended or written to make the technological ability a requirement.New laws, on the other hand, often contain requirements that serve as a spur or stimulus to technological innovation. Legal requirements are important and must be complied with, but they are only a small part of the pollution and waste control effort at military facilities. Further, focusing only on legal requirements may mask or distract attention from the possibility for considerable waste minimization and economic savings resulting from a more systemic, global view of pollution control and waste minimization. Environmental engineers must be looking for additional ways to prevent or minimize pollution to levels well below those required by law. All U.S. military commands have legal staff support available to them. Environmental engiearly very in the neers are well advisedto bring the legal staff into all environmental engineering planning stage. Continuous coordination and consultation with the legal staff during all phases of base operations will help eliminate environmental problems before they become legal problems.
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Military commanders and federal employees can be held both civilly and criminally liable for violating environmental laws. This makes compliance with the law quite important for mil itary facilities and increases the importanceof close consultation with supporting legal staffs. Legal problems with state or federal regulatory agencies have the potential to delay projects or military missions. Military commanders are extremely concerned about the possibility of delay on military missions andwant to be kept fully informed on possible problems or delays. Engineers working on military facilities should ensure that they have open communications channels with the installation commander.
IV. MOBILIZATIONANDDEPLOYMENT A unique military environmental problem is the amount of waste generated by military mobi-
lizations and deployments. During times of national emergency or war, the population of an of 2 or 3 days. Industrial and production activities, installation can doubleor triple in the space such as maintenance and ammunition manufacturing, cango from very low rates to very high rates in a matter of days. Surging troop populations preparing for overseas deployment dema all manner of goods and services, resulting in the generation of large amounts of waste of all types in a short time period. It is difficult to minimize waste during mobilizations, since they are short-duration, resource-intensive activities. Good planning, however, can prevent pollution and recover such waste for recycling. It is important that military facilities be planned and operated in such a way that all environmental laws and practices followed in peacetime can be followed during rapid wartime expansion. In most cases, this canbe done if the installation or facility infrastructure has been properly constructed and if adequate environmental planning has been accomplished. Typical, specific problems have emerged from past mobilizations and deployments; theseare predictable problems that can be prevented by good planning. Perhaps the single biggest environmental problem during mobilizations is wastewater resulting from increased troop populations. Large flows from troop housing areas, mess halls, vehicle washing, and industrial operations can quickly overwhelm wastewater treatment sysbe designed to accommodate tems. To prevent this, military wastewater treatment systems must the maximum or peak troop population during mobilization. This may seem wasteful or too expensive; however, the costs of environmental cleanup, fines, and health problems may well exceed the initial cost of larger capacity wastewater systems. Vehicle painting operations present another problem. Due to the toxicity of military cambe painted in special booths intended to protect worker safety and ouflage paints, vehicles must prevent the release of toxic fumes to the atmosphere. While a typical facility may haveorthree four booths, a wartime emergencymay require the rapid painting of hundreds of military vehicles. Unless sufficient booths to accommodate wartime surges have been constructed, the vehicles may be painted in the open or workers not protected. Environmental planners must ensure that adequate facilitiesare constructed or that contract arrangementsare made to have the vehicles painted in appropriate facilities. A more effective long-term solution wouldbe to design a paint system that does not have toxic components. Painting and industrial coating p cesses for military equipment (ammunition, weapons, naval vessels, aircraft electronic equipment) tend to present the same types of problemsas vehicle painting, particularly when there is a surge in production. These “pollution surges” must be planned for in all phases of the engineering process. Increased populations will generate substantially more quantities of solid waste, and increased industrial operations will result in the generationof increased quantities of hazardous waste. These additional quantities can usually be disposed of by expanding existing disposal
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contracts for waste disposal and recycling. Should planning estimates reveal that likely mobilization surges will overwhelm existing capacity, arrangements shouldbe made to have “contingency” contractsprepared for immediate implementation when needed. The to keys effective waste disposal are good recycling programs, good contracting, and thoughtful planning. One potentially significant pollution problem resulting from increased military training is the pollutionof streams and waterways with soil or other material eroded from military training areas. Large military tracked vehicles, such as tanks and armored personnel carriers, can severely damage the land, making erosion possible. Careful training area management and appropriate revegetation and repair of damaged land must be planned for during mobilizations.
V. WARTIMEOPERATIONS A time of crisis or war does notmean that environmental standards are discarded. Rather, peacetime pollution control and waste minimization should continue in domestic facilities as well as in the theaterof operations. Thiswill present somedifficulty; however, intelligent prior planning and appropriate discipline in the theater of operations can prevent large amounts of environmental destruction. First, logistic support plans should clearly specify what will be done with any hazardous waste as well as solid waste generated in the war zone.Much of this material will be generated at fixed, semipermanent, or permanent facilities at rear areas of combat zones, so intense enemy interferencewith many support activities will be limited.Waste maybedisposedof through the system of the host country or retrograded back to the United States for disposal. or Wastes may be pretreated to reduce bulkor hazard. It is essential that waste not be dumped abandoned, even if that is standard practicein the country our forcesare in. For legal, political, ecological, and moral reasons, wastes shouldbe disposed of, whenever possible, in accordance with existing U.S. standards. Second, nonhazardous solid waste should,to the greatest extent possible, be made available to local recyclers. Open burning and burial should be avoided unless otherno options exist be constructed and the waste becomes a health problem.If possible, engineered landfills should and the location and closure plans made available to the appropriate national environmental agency. Excess supplies or equipment should be returned for reuse or abandoned, butnot burned or buried. Third, battlefield cleanup must be plannedfor and conducted as soon as possible after the cessation of combat. Normally, troop units can remove and dispose ofthe vast majority of battlefield debris. Specially trained and equipped units willbe required to recover explosives, unexploded projectiles, mines, and chemical weapons. Someitems, such as vehicles destroyedby depleted uranium rounds, may be returned to the United States for secure disposal.
VI. TRAINING An effective training program is essential if military facilitiesare to minimize waste and control pollution. Military facilities usually have a diverse work force of military and civilian personnel; if the facilityis in another country there will most likely be employees from the host nation. All members of the facility will need to be informed on the procedures for recycling, waste disposal, and pollution control. The best, most expensive, most technologically sophisticated pollution control or recycling system will be of little use if it is not understood by those who must use it. There are a variety of training systems available. The environmental engineer or recycling coordinatorcanprovideinstruction or information to unitenvironmentalrepresentatives
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through a variety of installation councils and meetings. The unit environmental representatives can then train the members of their organization and serve as a means of communicating new information to their organizations. Many facilities use installation newspapers or periodicals to communicate information be a very useful adjunct to a formal training quickly to all members of the installation. This can PWPam. Some trainingis quite technical and is requiredby federal law. This training mustbe conducted by qualified instructors,and careful records maintainedto ensure that reporting requirementsaremet.Militaryfacilitiesinthesamegeneralgeographicareashouldconsider consolidating this type of training to cut training expenses. All training should be very high quality, and standards kept high. Training must be repeated periodically, especially for new members of the installation team. Trainingat facilities in foreign countries must be conducted in the language of the employees. This is especially important, as many logistics functions for U.S. forces overseas are performed by foreign nationals.
32 Waste Reduction Strategies for Small Businesses
Dan A. Philips Pensacola Junior College Pensacola, Florida
1.
INTRODUCTION
Some historians believe that the 1990s will be rememberedas theenvironmental decade.Even small businesses cannot remain in operation today without considering the repercussions of each of their actions on the natural environment, the regulatory environment, and the public perception of the environment. The late 1960sand early 1970sbrought aboutmany changes. Hair became longer and skirts shorter. For any number of reasons, the public consciousness was raised along with hemlines. One of the major controversiesto arise concerned the role of business in maintaining the ecology, includingthe detrimental effects that many industrial processeshave on the integrity of the environment. By exerting a great deal of political pressure, the public forced government into the position of “protector of the environment.” The government lived up to its newly established role by developing a myriad of bureaucratic agencies, committees, subcommittees, and review boards to address environmental issues. The late 1970s brought about concrete, tangible legislation on waste treatment and pollution prevention. The basic legislation was merely a small snowball balancing precariously on the edge of a ski slope. Over the years to follow, regulations grew until small businesses were facing a veritable avalanche of rules, acts, regulations, and amendments. EPA, DOT, CERCLA, SARA, RCRA, and OSHA are not charitable foundations requesting tax-deductible donations.They are regulatory agencies and regulatory acts. The cumulative results of their efforts comprise an extensive list of emission limits, exposure limits, disposal limits, etc. Failure to comply with their standards can result in tremendous fines. Onefine from any of these agencies could translate into bankruptcy for a small business. Today, small business operators may be confronted by over 10,OOO pages of regulations, and the number continues to grow. It is estimated that each new page of federal regulations costs businesses about $10 million. Unfortunately, over 90% of the money, efforts, and regulations focus on end-of-the-pipe wastes. Little emphasis is placed on the nature of the process 443
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being regulated, i.e., the beginning of the pipe. Obviously, what goes in directly affects what comes out. Most federal andstate environmental funds are spent on pollution control and remediation. $70 billion, with two-thirds National spending on pollutionand waste control is approximately of that amount being suppliedby industry. That represents a large sumof money that could be used for investments, expansions, and capital outlay if the waste stream could be reduced or eliminated [13. To make matters worse, the land available for disposal is quickly disappearing, and government regulations on land burial of waste are becoming more stringent. Once a waste is created, the creator is responsible forever. Long-term waste liability may be 200 times as costly as the initial disposal. Obviously, this can drastically affect the cost of doing business. Approximately 300 million tons of hazardous waste is buried annually in the United States. Over 7 billion tons of solid waste is generated each year by 72,000 businesses. Such estimates provide a driving forceto regulate what is happeningto these wastes. The reduction of waste has been a regulatory requirement for several years, providing a strong motivationto comply; however, this has traditionally not been sufficient to ensure full compliance with existing regulations. Businesses generally consider wastes as end products requiring disposal and not as expenses that need to be reduced. Traditionally, most businesses have preferred to use waste management techniques such as treatment, incineration, or land burial. Such techniques are applied strictly to the end of the process[2]. These old techniques pose environmental risks, require meeting costly regulatory compliance standards, and increase future liabilities. Disposal costs will continueto increase as regulationsconcerningwastebecomemorerestrictive.HalfofallU.S.landfillsarealready nearing or exceeding their designed capacity. Some government agenciesare already stressing a zero-discharge philosophy. In meeting these new regulations, companies will need to place a strong emphasis on source reduction and on-site recycling while shifting away from off-site recycling or disposal [3]. In 1976, the Resource Conservation and RecoveryAct (RCRA) established a “cradle-tograve” responsibility for the generator of a hazardous waste. Theneed for better waste man1984, agement was increased by the Hazardous and Solid Waste Amendments (HSWA) of which applied even more restrictions. The intent of these laws is to prevent environmental degradation and to protect natural resources. This philosophy is clearly stated in the Pollution Prevention Act of 1990, which reads: The Congress hereby declares it to be the national policy of the United States that, polbe prelution be prevented or reduced at the source wherever feasible; pollution that cannot vented should be recycled in an environmentally safe manner whenever feasible; pollution that cannotbe prevented or recycled should be treated in an environmentally safe manner; and disposal or release into the environment shouldbe employed only as a last result. The Comprehensive Environmental Response, Compensation and Liability Act (CERCLA,
or Superfund) was passed in1980, raising the stakes in the waste disposal game considerably. CERCLA is responsible for repairing environmental damage done because of past and present improper management of wastes. In 1986, the Superfund Amendments Reauthorization Act (SARA) was passed. SARA assigned liabilityto the responsibleparties and provided a means by which to hold the parties financially responsible.The principle of jointand several liability, upheld in the U.S. Supreme Court decisionU.S. v Stringfellow, makes all parties whose waste is contained at a Superfund site equally responsible for cleanup costs[4]. The reduction of waste has been a regulatory requirement for several years. However, there was no common description of what constituted waste reduction, there were few data
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on the extent of industrial waste reduction, and data that were collected were often measured incorrectly [5]. Under the Clinton-Gore administration, waste reduction will probably receive increased governmental emphasis. In Vice President Gore’s book Earth In The Balance, he points out that it will be increasingly important for all companies to incorporate standards of environmental responsibility into their entire operation [6]. By using materials more efficiently, industry can reduce waste. This in turn will allow greater protectionof human health and the environment while ensuring regulatory compliance. Other factors are beginning to have a considerable impacton how waste is handled. Of these, probably the most overwhelming incentive is liability. The generator of a hazardous waste isultimately and solely responsible for what happens or has happened to that waste in the past, present, and future. The imminent hazard provisions of SARA and RCRA affect waste managed on-site as well as waste shipped off-site and can close down a business as well as imposing heavy fines and penalties. EPA can assess triple damages (three times the amount of the cleanup) and assess a lien on a business property to collect [l]. There are many compelling reasonsto minimize waste; Table1 summarizes someof them. RCRA regulations require that generators of hazardous waste “havea program in place to reduce thevolume and toxicity of waste generated tothe extent that is economically practical.” A waste reduction program isan organized, comprehensive, and continual effort to systematically reduce the generation of waste. It should become part of a company’s everydayoperating policy. While the main goal is to reduce or eliminate waste, it will also bring about an improvement in production efficiency. Under the more stringent legislation being considered, waste reduction programs would include all environmental pollutants regardless of whether their disposal is permitted. This includes solid waste, wastewater, hazardous waste, andair pollutants. Also being consideredare bills restricting waste reduction options by considering only source reduction or on-site recyor any cling techniques. The strictest legislative packages would not consider off-site recycling treatment separate from the production process within the scope of waste reduction. Although these proposals have developed support fromcertain congressional quarters and are supported Table 1 WasteMinimizationIncentives ~
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Economics Landfill disposal cost increases Costly treatment technologies Savings in raw material and manufacturing costs Tax incentives for pollution prevention Regulations Certification of a waste minimization program on the hazardous waste manifest Biennial waste minimization program reporting Land disposal restrictions and bans Increasing permitting requirements for waste handling, and treatment Liability Potential reduction in generator liability for environmental problems at both on-site and off-site treatment, storage, and disposal facilities Potential reduction in liability for worker safety Public image and environmental concern Improved image in the community Concern for improving the environment
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by the Office of Technology Assessment, such restrictive legislation is not favorable to either manufacturers or the developing recycling industry. Strong efforts by generators to reduce waste generation are encouragedby the EPA in order to avoid more restrictive regulation[ 5 ] .
II. BARRIERS TO WASTE REDUCTION Although economic factors often work in favor of pollution prevention approaches, there is always some resistanceto change of any kind. Waste reduction projects can reduce operating costs and improve environmental compliance, but they frequently bring out conflicts between different groups within acompany. Economic barriers to pollution prevention include the following: Inaccurate market signals. Sometimes the immediate cost of releasing toxic substances is less than the cost of implementing a pollution prevention project. This occurs when the longrange cost of the release is not included in the calculations. Incomplete accounting. Indirect benefits, such as lower future liabilities and improved public image, are not commonly considered in a financial analysis. Fear of lowering quality. This is very common in situations where unused feed materials are recovered from a waste and recycled back into the process. If not done properly, such processes may affect quality. Workerfear ofjob loss. If employees or labor groups look upon pollution prevention as a threat to their jobs, these concerns may pose a barrier to new processes. Fear of losing market share. Surveys suggest that a significant barrier to pollution prevention is reluctance to tamper with proven processes for fear of adverse effects on product quality [7]. Attitude-related barriers must be overcome before any pollution prevention program is tried, or it is destined to fail. A prevailing attitude is to maintain the status quo and avoid the unknown. There is also a fear that a new program may not work as advertised. Without the commitment of everyone involved, a pollution prevention program is doomed to failure. The Case of the Proverbial Potato' In 1986, a coatings manufacturer in Hamburg, Germany hired a bright young process chemist for one of its factories.The young chemist was involved in preparing a high quality varnish to be used as a sealer on musical instruments. While reviewing the formula, he was surprised to see that it called for a potatoto be added after each new ingredient was mixed into the batch. Curiosity got the best of him. He asked head the chemist what physicalor chemical properties the potato addedto the batch. The head chemist abruptly told the young man that potatoeshad been added to the varnish batches for over50 years and reprimanded him for his insolence. Not easily discouraged, the young chemist began asking other chemists in the lab the about addition of potatoes. Unable to provide an answer, they too became curious about the use of potatoes in the varnish batches. One of the older employees on the staff suggested that the young man contact a retired chemist who lived in a nearby village. The old chemist had worked at the factory for many years before his retirement. He explained to the inquisitive young man that during the war years, thermometers had beem very hard to acquire. Potatoes had been used to tell when the batchhad reached the proper temperature for adding the next ingredient. When this information was conveyed to management, the use of potatoes was halted at the factory and thermometers were substituted. 'Adapted from Ref. 8.
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This story exemplifies our reluctance to change. It also demonstrates that one concerned individual, through persistence, can make a difference. Of course, a potato is not considered a hazardous or toxic substance, but it is symbolic. Often, less toxic and equally effective substitutes are available for a process, but they are not used because “this way is we’ve the always done it.” When a person does not fully understand the nature of a proposed option and its impact, a common attitude is that “it just won’t work.” Attitudinal changes are more likely to occur when people are presented with stimuli that encourage self-persuasion. Techniques of persuasion used by others may force the confused individual into a defensive posture. When this happens, creative options may be dropped before they can be evaluated. Oneway to avoid this is to use idea-generating sessions (brainstorming), encouraging participants to propose a large number of options without regardto cost, technology, or barriers. The more impractical ideas will be dropped anyway, but previously undiscovered problem-solving solutions may emerge. Many business managers are surprised to discover that waste reduction does not always necessitate large-scale investments. Significant savings have resulted from such simple, commonsense improvementsas better housekeeping, preventive maintenance, balancing inputs and outputs (inventory control), and minor process changes. Waste minimization shouldbe viewed as an investment in the company’s future. It begins with a conceptas simple as “Waste-If you don’t produceit, you won’t haveto dispose of it.” At first, this statement seems oversimplified and naive; however, upon further review, many managers have become impressed with how often this principle can be applied. Waste minimization is a relatively low-cost but effective procedure for reducing liability and increasing profits. Often, waste minimization processes do not require high technologyor expensive equipment. In many cases, very simple shop practices and procedures, combined with low-cost equipment, can greatly reduce the amount of hazardous waste generated. In addition, 42 states presently offer tax credits or exemptions for companies purchasing pollution prevention equipment [9].
111.
LEVELS OF WASTE MINIMIZATION
There are three major levels in the waste minimization hierarchy. These levels canbe viewed as steps to improving production processes and saving money. The levels, in orderof priority, are source reduction, recycling, and treatment.
A. Source Reduction Source reduction involves the minimization or elimination of wastes at their source. It often includes changesin procedures rather than in technology or machinery, makingit simple, easy, and cost-effective. Because littleor no wasteis generated, source reduction alleviates the problems associated with handling and disposing of wastes. It is therefore the most desirable option in the waste reduction hierarchy.
1. ImprovedOperatingPractices Operatingpracticesincludeprocedural,administrative,andinstitutionalmeasuresthata company can useto reduce waste. Many of these measures have been used for decades by successful businesses as efficiency improvements and sound management practices. They can ofGood operating ten be implemented with little cost and have a high return on investment. practices include Management and personnel involvement Material handling and inventory practices
Philips Loss prevention Waste segregation Complete accounting practices Production scheduling Management and personnel involvement includes employee training, incentives and bonuses, and other programsthat encourage employees to conscientiouslystrive to reduce waste. Material handling and inventory practices include programsto reduce loss of input materials due to improper handling, expired shelf life, or improper storage conditions. Loss prevention minimizes wastes by such means as avoiding leaks and spills from equipment and preventing evaporation by keeping solvent containers closed tightly. Waste segregation practices reduce the volume of hazardous wastesby preventing the mixing of hazardous and nonhazardous wastes. Complete accounting practices involve allocating waste treatment and disposal costs directly to the departments or groups that generate wasterather than chargingthese costs to general company overhead accounts. In doing so, the departments or groups that generate the waste become more awareof their treatment and disposalpractices, thus providing a financial incentive to minimize waste. By judicious production scheduling of batch runs, the frequency of equipment cleaning and its resulting waste can be reduced. This technique is particularly useful in printing and painting operations. Example: Good OperatingPractices-Management. A printing companyreducedpaper plates, and presseswere waste by 30% simply by implementing process control measures. Inks, kept at peak efficiency, reducing the need for reprinting. Example: Good Operating Practices-Material Handling and Inventory. A consumer product company in California adopted a corporate policy to minimize the generation of hazardous waste. In order to implement the policy, the company mobilized quality circles made upof employees representingareas within the plant that generated hazardous wastes. The company experienced a 75% reduction in the amount of wastes generated by instituting proper maintenance procedures suggestedby the qualitycircle teams. Since the team members were also line supervisors and operators, they made certain that procedures were followed. Process changes provide the best opportunities for reducing waste. One way to alter the process is to use different raw materials. It may be possible to choosea material that allowsfor a greater percentage of product in the end. A material that is nontoxic, evenif it does become a waste, will not pose as great a danger to the environment. Employees’ assignments canbe planned carefully in advance so they are done efficiently and thoroughly. Putting excessive pressure on workers to complete jobs quickly may mean that the job will not bedone properly thefirst time because itwas rushed. Consequently, moretime and materials will be required to correct the mistakes. Another way to reduce waste is through better housekeeping. Keeping an operation clean and orderly improves safety, delays deterioration of equipment, reduces breakdowns, and increases efficiency. It will also assist in discovering leaks and spills when they first occur. Improving inventory control has been found to be a big step in reducing waste. Records should keep track of which materials are being used and howquickly they are being used. Buying large quantities of materials just because the unit cost is lower will not save money if a majority of the material sits on the shelf anddries out or becomes unusable because of chemical decomposition.
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To avoid exceeding an expiration date, it is also useful to rotate stock periodically. The freshest material cango to the back of the shelf, with the oldest moved to the front. Ordering environmentally safe products greatly reduces disposal costs should a material be stored past its useful shelf life. Preventive maintenance will also contribute greatly to waste reduction.If a tool or piece of equipment is maintained properly, it will last longer and provide a more productive of the use process for which it was designed. are more procedural than technological. Many helpful changes that foster waste reduction Improved operating proceduresdo not have to be elaborateor expensive; they just need to get the job done. 2.
TechnologyChanges Technology changes are oriented toward industrial process and equipment modifications that will reduce waste, primarily in a production setting. Technology changes may range from minor improvements that can be implemented in a matter of days at low cost to new processes with startup costs that require large capital outlays. These changes include Alterations in the production process Changes in equipment, layout, or piping Use of automation Changes in process operating conditions, suchas flow rates, temperatures, pressures,and residence times Example: Technology Changes-Equipment, Piping, or Layout. In many dry cleaning facilities, existing equipment is retrofitted with ventilation and vapor recovery machinery. It is costeffective to invest in modern equipment that can practically eliminate emissions and human contact with solvents. Example: Technology Changes-Process. Many companies are installing equipment to recycle wastewater by separating the hazardous components from the water, which can then be reused. Example:TechnologyChanges-Process. A manufacturer of fabricated metal products foran alkaline chemical bath priorto using the wire in merly cleaned nickel and titanium wire in the product. In 1986, the company began to experiment with a mechanical abrasive system. The wire was passed through the system, which uses silk and carbide pads and pressure to brighten the metal.The system worked but required passing the wire through the unit twice for complete cleaning. In 1987, the company bought a second abrasive unit and installed it in series with the first unit. This allowed the company to completely eliminate the need for the chemical cleaning bath. 3. InputMaterialChanges
Input material changes accomplish waste reduction by reducing or eliminating hazardous mabe made to avoid terials that enter a production process. Also, changes in input materials can generation of hazardous wastes within a production process. Input material changes include material purification and material substitution. Example:InputMaterialChanges. An electronics manufacturing facility originally cleaned printed circuit boards with organic solvents. The company found that by switching from an organic-based cleaning system to a water-based system, the same operating conditions and workloads could be maintained and the aqueous system cleaned six times as effectively. Thisresulted in a lower product reject rate and eliminated theneed for disposal of a hazardous waste.
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ProductImprovements Product improvements are performed by the manufacturer of a product with the intent of reducing waste resulting from a product’s use. Product improvements include product substitution, product conservation, and changes in product composition.
4.
Example: Product Improvements-Product Substitution. In the paint manufacturing industry, water-based coatings are finding increasing application where solvent-based paintswere previously used. These products do not have the toxic or flammable characteristics that make solvent-based paints hazardous.Also, it is not necessary to clean the applicators with solvent. The use of water-based paints instead of solvent-based paintsalso greatly reduces volatile organic compound emissions to the atmosphere. In many cases, water-based productsare cheaper than those based on organic solvents.
B. Recycling 1. Extended Use andReuse
Recycling via use and/or reuse involves either returning a waste materialto the originating process as a substitute for an input material or using it in another process as an input material. Return to original process Raw material substitution for another process Example: Reuse-Return to Original Process. A printer of newspaper advertising purchased an ink recycling unit to produce black newspaper ink from its various waste inks. The unit blends different colors of waste ink together with fresh black ink and black tonerto create the new black ink. This ink is then filtered to remove flakes of dried ink. The recycled inkis used in place of fresh black ink and eliminates the need for the company to ship waste ink off-site the savings for disposal.The price of the recycling unit was paid off18inmonths based only on in fresh black ink purchases. The payback period improved to 9 months when the costs for disposing of ink as a hazardous waste were included. Reclamation Reclamation is the recovery of valuable material from a hazardous waste. Reclamation techniques differ from use and reuse techniques in that the recovered material is not used in the facility but is sold to another company. Waste exchanges provide valuable information to link companies that wish to transfer wastes and those that wish to use them [8]. Materials to be reclaimed may be processed for resource recovery or processed as a by-product.
2.
Example: Reclamation. A photoprocessing company uses an electrolytic deposition cell to recover silver from the rinsewater from film processing equipment. The silver is then sold to a small recycler. When the silver is removed from this wastewater, the wastewater can be discharged to the sewer without additional pretreatmentby the company. This unit paid for itself in less than 2 years due to the value of the recovered silver. The company also collects used film and sells it to the same recycler. The recycler bums the film andcollects the silver from the residual ash. Removing the silver fromthe ash renders the ash nonhazardous. Many environmental groups focus on recycling as the solution to solid and hazardous waste problems;however, in most recycling projects, somematerials are lost, contaminated, or leached into the environment. Evenin Japan, where industries and municipalities have the world’s most successful recycling program, all of their recycling practices combined reduce waste by only 65%. Recycling is no substitute for source reduction [l].
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C. Treatment Treatment shouldbe considered onlyas a last resort when implementing a waste reduction program. Waste treatment may involve conversion of hazardous waste to a less toxic form by chemical, biological,or physical meansor it couldbe as simple and dangerous as placement in landfills, deep-well injection, or ocean dumping [6]. Treatmentmayinvolvemoreexoticprocessesforcleaningup or usingcontaminated wastes. Many shop wastes canbe cleaned up using activated carbon filtration, biological treatment, and extraction. Some wastes can be put to good use as a fuel source for on-site heating units. Creative processes and ideas can accomplish wonders in reducing wastes. Even though new ways of thinking and reformulating processes may be required, it is less difficult to improve on waste minimization techniques thanto improve on waste treatment techniques. Some of the real incentives that waste minimization has going forit are that most of the procedures are that can be carried out are not expensive, save money over treatment costs in the long run, not time-consuming, and will help protect business interestsas well as the environment.
IV. ORGANIZING A WASTE REDUCTION PROGRAM Because a waste reduction program affects many groups within a company, a program task force should be assembled. This group must include members of any department that has a significant interest in the outcome of a waste reduction program. The formalityor informality of a waste reduction program will depend on the nature of the company. A program in a large, highly structured company will probablybe quite formal, one in a small company, less formal. Table 2 lists the typical responsibilities of a waste reduction program task force. The task force draws on the expertise of everyone in a small company. Even in a small business, several people willbe required to implement a successful waste reduction program. People with responsibility for waste treatment and disposal, production, facility maintenance, and quality control should be included on the team. At a small facility, a single person, for example, an owneror manager, may have all of these responsibilities; however, even with a small facility,at least two people shouldbe involved to get a variety of viewpoints and perspectives. Some larger companies have developed a system in which assessment teams visit different facilities within thecompany. Benefits result through sharingof ideas and experiences. Similar results can be achieved for smaller businesses by visiting other facilities with similar operations. Table 2 Responsibilities of the Waste Minimization F %m TaskForce Get commitment and a statement of policy from management.
Establish overall waste minimization program goals. Establish a waste-tracking system. Prioritize the waste streams or facility areas for assessment. Select assessment teams. Conduct (or supervise) assessments. Conduct (or monitor) technical/economic feasibility analysesof favorable options. Select and justify feasible options for implementation. Obtain funding, and establish the schedule for implementation. Monitor (and/or direct) implementation progress. Monitor performance of the option once it is operating.
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Table 3 Attributes of EffectiveGoals Acceptuble to those who will work to achieve them Flexible and adaptable to changing requirements Measurable over time Motivarioml for all employees Suitable to the overall corporate goals and mission Understandable by all employees Achievable with a practical level of effort
Planning a waste reduction strategy is similar to a big league manager getting his team ready for baseball season. It is best to Establish a game plan. Set specific long-term and short-term goals. Keep your eye on the ball. Concentrate on the worst streams first. Know your spike zone. Set realistic objectives that center on production activities. Keep score. Examine possible cost savings for various process changes. Get the best equipmentfor your players. Do not be afraid to make improvements that will pay for themselves in the long run. Remember that expansion teams do not per&orm well early in the season. Invest sufficient time and effort before evaluating results.
A. SettingGoals The first priority of a waste reduction task forceis to establish goals that are consistent with management policy. Waste reduction goals can be qualitative, for example, “a significant reduction of toxic substance emissions into the environment”; however, it is better to establish measurable, quantifiable goals, since qualitative goals canbe interpreted ambiguously. Quantifiable goals establish a clear guideas to the degree of success expected of a program. Other attributes of effective goals are listed in Table 3. Waste reduction must be reviewed periodically. As the focus of a waste reduction program are becomes more defined, the goals should reflect any changes. Waste reduction assessments not one-time projects. Periodic reevaluation of goals is necessary because changes occur in available technology, raw material supplies, environmental regulations, and economic climate.
B. Planning a Strategy The steps involved in planning and organizing a waste reduction program are summarized in Table 4. Most small businessesdo not realize how much waste they produce. Ask most managers if they use all of their raw material,or input, and. they will probably say that they do. What they actually mean is that they process allof the raw material needed, not that they use the entire amount purchased. If they feel they are getting their money’s worth, most managers will not give a second thought to what is lost or wasted. This philosophy is common in smaller businesses because they often do not have the personnel, the expertise, or the budget to spend on developing adequate inventory control. The larger the business, the more resources are available to scrutinize inventory control. Waste reduction in any size industry can only work if it is embraced by top management. Small operations often have an advantage here because the owner is usually the immediate supervisor and remains on-site for most of the workday. Through leadership and example, employees will be motivated to participate in the overall program.
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Table 4 PlanningandOrganizationActivitiesSummary Setting Up the Program
Get management commitment to Establish waste minimization as a company goal. Establish a waste minimization program to meet this goal. Give authority to the program task force to implement a program. Starting the Program Task Force
Find a “cause champion” with the following attributes: Familiar with the facility, its production processes, and its waste management operations. Familiar with and respected by the employees. Familiar with quality control requirements. Good rapport with management. Familiar with new production and waste management technology. Familiar with waste minimization principles and techniques and environmental regulations. Aggressive managerial style. Getting Company-wide Commitment
Find people who know the facility, processes, and procedures. Find people from the affected departments or groups. Incorporate the company’s waste minimization goals into departmental goals. Solicit employee cooperation and participation. Develop incentives and/or awards for managers and employees. Several large corporations, e.g., General Dynamics, Borden Chemical, and the 3M Corporation, have pioneered effective waste reduction programs with procedures thatfit equally well in smaller operations. These large industries were driven by upper management pressures, profit margins,and corporate images. Inherent in each program is a strong waste reduction plan and direct involvement by employees from all departments [ 8 ] . Small businesses may not have the capital to spend on expensive equipment or to change existing processes overnight, but theydo possess a valuable resource in their employees. Creative ideas and active participationby all employees can translate into unexpected cost savings and profits for any entrepreneur. Acting on an employee’s recommendation, General Dynamics was able to eliminate disposal costs of a caustic solution by finding a market for it. Borden Chemical greatly reduced effluent wasteby encouraging good housekeeping techniques. In the area of waste reduction, employers should never overlook the obvious and never reject another’s ideas as being too simplistic.
V. THE ASSESSMENTPHASE
A. Selecting a Team A program task force should be concerned with reducing waste for an entire facility; however, an assessment team should focus on one particular waste streamat a time if the stream can be linked to a single process.Team membership should notbe limited to environmental managers
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and engineers. Teams should include all people with direct responsibility and knowledgeof a particular waste stream or area of a plant. In addition to internal staff members, assistance from outsiders should be considered, especially during assessment and implementation phases. These outside people may be trade association representatives, consultants, or experts from a different facilityof the same company. One or more outsiders can bring in new ideas and providean objective viewpoint. An outsider is more likely to counteract bias brought about by the “sacred cow” syndrome, such as that encountered when an old process area rich in tradition undergoes an assessment. Free or inexpensive assistance may be available froma variety of souices, including universities and community colleges, management associations, state agencies, insurance companies, and retired employees [ 8 ] . Although their services are expensive, professional consultants bring a variety of experiare especially usefulto smaller ence and expertise to a waste reduction assessment. Consultants companies, which may not have in-houseexpertise in relevant waste reduction techniques and technologies. Trade associations, community colleges, and public agencies will sometimes have very knowledgeable p p l e available to provide advice forlittle or no compensation. Production operators and line employees should not be overlooked as a source of waste reduction suggestions. They possessfirsthand experience with processes, and their assistance may be useful in assessing operational changesor equipment modifications thataffect the way they perform their work. “Quality circles” have been instituted by many companies, particularly in manufacturing industries, to improve product quality and production efficiency. These quality circles (sometimes referred to as TQM, total quality management) consist of workers and supervisors sharing ideas on proposed improvements. Qualitycircles are often successful because they involve production people who are closely associated with the operations and truly appreciate being part of the decision-making process. Several companies with quality circles have used them effectively to solicit suggestions for waste reduction that have saved millions of dollars.
B. Collecting and Compiling Data In order to develop options, a detailed understanding of a plant’s wastes and operations is a must. An assessment should beginby examining information about processes,operations, and waste management practices. There are several questions that need to be answered [lo]: 1. What waste streams does the business generate? How much? 2. With which processes or operations do these waste streams originate? are not?Whatmakesthem 3. Whichwastes are classified as hazardous,andwhich 4.
5. 6.
7. 8. 9. 10.
hazardous? What are the inputmaterials used that generate the waste streamsof a particular process or plant area? How much of a particular input material enters each waste stream? How much of a raw material can be measured throughfugitive losses? How efficient is the process? Are unnecessary wastes generatedby mixing otherwise recyclable hazardous wastes with other process wastes? What types of housekeeping practices are used to limit the quantity of wastes generated? What types of process controls are used to improve process efficiency? Additional types of information for assessing wastes are listed in Table 5.
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Table 5 Facility Information for Waste Minimization Assessments Design information
Process flow diagrams Material and heat balances (both design balances and actual balances) for (1) production processes and(2) pollution control processes Operating manuals and process descriptions Equipment lists Equipment specifications and data sheets Piping and instrument diagrams Plot and elevation plans Equipment layouts and work flow diagrams Environmental information ;
Hazardous waste manifests Emission inventories Biennial hazardous waste reports Waste analysis Environmental audit reports Permits andlor permit applications Raw materiallproduction information Product composition and batch sheets Material application diagrams Material safety data sheets Product, utility, and raw material costs Operating and maintenance costs Departmental cost accountingreports Economic information
Waste treatment and disposal costs Product, utility, and raw material costs Operating and maintenance costs Departmental cost accountingreports Other information
Company environmental policy statements Standard procedures Organization charts
C. Site Inspection Once an assessment team isin place and a specific areaor waste stream has beenselected, the assessment may continue with a visit to the site. Although collected informationis critical to gaining an understanding of the processes involved, it is helpful to witness an actual operation. In many instances, a process unit is operated differently from themethod described in an operating manual. Modifications may have been made that were not recorded on flow diagrams or equipment lists. An assessment team should preparea list of needed information andan inspection agenda prior to conducting the site inspection. This can bea checklist detailing objectives, questions and issues to be resolved, andlor further information needed. An agenda and information list should be given to appropriate plant personnel beforethe visit to allow them time to assemble the requested information.With a carefully thought-out agenda andchecklist, important points are less likely to be overlooked during an inspection. the In performing a site inspection, an assessment team should observe each process from point where rawmaterials enter to the point where products and wastes leave an area. The team
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should identify suspected sources of waste. Thismay include the production process, maintenance operations, storage areas for raw materials, finished product, and work-in-progress. Even a plant’s waste treatment area may itself offer opportunities to minimize waste. An inspection often results in preliminary conclusions being drawn aboutthe causes of waste generation. Full confirmationof conclusions may require additionaldata collection, analysis, and/ or site visits. The following are some guidelines for asite inspection [lo]. 1. Prepare an agenda in advance that covers all points that still require clarification. Provide staff contacts in each area being assessed with an agenda several days before the inspection. 2. Schedule an inspection to coincide with a particular operation that is of interest (make-up chemical addition, bath sampling, bath dumping, start-up, shutdown, etc.). 3. Monitor an operation at different times during shifts, especially when waste generation is highly dependent on human involvement (e.g., painting or parts cleaning operations). 4. Interview operators, shift supervisors, and foremen in an assessed area. Do not hesitate to question more than one person. Assess operators’ and supervisors’ awareness of waste generation aspects of their operation.Note their familiarity (or lack thereof) with the impacts their operation may have on other operations. 5 . Photograph an area of interest. Photographs are valuable in the absence of plant layout be forgotten drawings. Many details canbe captured in photographs that otherwise would or inaccurately recalled at a later date. 6. Observe “housekeeping” aspects ofan operation. Check for signsof spills or leaks. Visit the maintenance shop and ask about problems in keeping equipment leak-free. Assess the overall cleanliness of the site. Pay attention to odors and fumes. 7 . Assess the organizational structure and the levelof coordination of environmental activities between various departments. 8. Assess administrative controls, such as cost accounting procedures, material purchasing procedures, and waste collection procedures.
D. Flow Diagrams and Material Balances Different materials and proceduresare used to execute each process; however, every operation maoffers an opportunityto track material flow. Figure1 provides an extremely oversimplified terial flow chart. Before a waste reduction plan can be formulated, it is crucialto determine the amount of waste that the business is already producing. How? Perform a waste audit. What is a waste or audit? It is the direct application of the law of conservation of matter taught in Physics Chemistry 101: Matter is neither created nor destroyed. In other words, nothing ever disappears. Everything thatwas here on the first day this planet was created is still with us today, although it may be in a different form. or goes “away,” it must be somewhere. So to find out where Since nothing ever disappears the “matter” is going, a waste reduction team must perform a material balance. In this process, a quantitative relationship is established between raw materials, product, and waste. Material balances allow for quantifying many losses or emissions that were not previously recorded by any bookkeeping or inventory system. In its simplest form, a material balance is representedby the equation Mass in = mass out
+ mass accumulated
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Strategies for Small Businesses AIR EMISSIONS
SPILLS
Figure 1 Simplified flow chart indicating process components to be included in a material balance study.
An independent material balance should be made for all componentsthat enter and leave a process. When chemicalreactions take place in a system, an “elemental balance” should be performed for specific chemical elements. Material balances assist in determiningconcentrationsof waste components where data are limited. Theyare particularly useful when there are points in a process where itis difficult (due to inaccessibility) or uneconomical to collect analytical data. A material balance candetermine whether or not fugitive losses are occurring. For example, the evaporation of solvent from a the tank and solvent cleaning tank canbe estimated as thedifference between solvent put into removed from the tank. effort; however, Characterizing wastestreamsby material balance can require considerable the result is a complete picture of a waste stream. This helps to establish a focus for waste reduction activities and provides a baseline for measuring performance. 1.
Sources of Material Balance Information
Material balance includes materials entering and leaving a process. The following list exemplifies potential sources of material balance information: Samples, analyses, and flow measurements of feedstocks, products, and waste streams Raw material purchase records Material inventories Emission inventories Equipment cleaning and validation procedures Batch make-up records Product specifications Design material balances Production records Operating logs Standard operating procedures and operating manuals Waste manifests
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Material balances are easier, more meaningful, and moreaccurate when done for individual units, operations, or processes. It is important to define a material balance envelope properly. An envelope should be drawn around a specific area of concern rather than a larger group of areas or an entire facility. An overall material balance fora facility can then be constructed from individual unit material balances. This highlights relationships between units and helps to point out areas for waste reduction. Several factors mustbe considered when preparing material balances in order to avoid errors that could significantly overstate or understate the waste streams. The precision of analytical data flow measurements may not allow an accurate measure of a stream. In processes with very large inlet and outletstreams, the absolute error in measurement of these quantities may be greater in magnitude than the waste stream itself. Whenever a reliable estimate of a waste stream cannot be obtained by simply subtracting the quantity of material in a product from that in a feed, the team must be creative in developing a more advanced methodof quantitative analysis. Time spanis important whenconstructing a material balance. Material balances performed over the duration of a complete production run are typically the easiest to construct and are reasonably accurate. Time duration also affects the use of raw material purchasing records and on-site inventories for calculating input material quantities. The quantities of materials purchased during a specific time period may not necessarily equal the quantity of materials used in production duringthe same time period. Purchasedmaterials can accumulatein warehouses or stockyards. Developing material balances around complex processes can bea complicated undertaking, especialy if recycle streams are present. Such tasks are generally performed by chemical engineers, often with the assistance of computerized process simulators. Material balances will often be needed to comply with SARA Title I11 reporting requirements to establish emission inventories for specific toxic chemicals. EPA's Office of Toxic Substances has prepared a guidance manual entitled Estimating Releases and Waste Treatment Eflcienciesfor the Toxic Chemicals InventoryForm (EPA 560/4-88-02). This manual contains additional information on developing material balances for listed toxic chemicals [ll]. 2. TrackingWastes By tracking wastes ona regular basis, seasonalvariations in waste flows or single-batch waste streams are distinguished fromcontinual, constant flows. Changesin waste generation cannot be meaningfully measured unless the information is collected both before and after a waste reduction option is implemented. Fortunately, it is easier to do material balances the second time, and it continues to becomeeasier as more are completed. It is relatively simple and inexpensive, with today's computer software,to use computerizeddatabase systems to track and monitor waste streams.
E. Prioritizing Waste Streams and/or Operationsto Assess Ideally, all waste streams and plant operations should be assessed. Prioritizing waste streams and/or operations to assess is necessary when available funds, personnel, or time is limited. Waste reduction assessments should concentrate on the most important waste problems first and then move to lower priority problems as time, personnel, and budget permit. Setting priorities for waste streams or facility areas requires a great deal of care and attention, since this step focuses the direction of an assessment activity. Important criteria to consider when setting these priorities include the following. Compliance with current and future regulations
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Costs of waste management (treatment and disposal) Potential environmental and safety liability Quantity of waste Hazardous properties of a waste (including toxicity, flammability, corrosivity, and reactivity) Other safety hazards to employees Potential for removing bottlenecks in production or waste treatment Available budget for waste minimization assessment program and projects Small businesses with only a few waste-generating operations should assess their entire facility. It is also beneficial to lookat an entire facility when thereare a large number of similar operations. The implementationof good operating measures such as soliciting employee suggestions, awareness-building programs, better inventory and maintenance procedures, and internal cost accounting changes can be implemented on a facility-wide basis. Many of these options do not require large capital expenditures and can be put into operation in a relatively short time. Performing the mass balance may be the most difficult part of the waste audit, but it will make the remainder of the waste reduction process easier and more effective.
VI. WASTEREDUCTIONOPTIONS Once the origins and causes of waste generationare understood, the assessment process enters the creative phase. Theobjective of this step is to generate a list of waste reduction options for further consideration. Following the collection of data and site inspections, members of the team will have begun to identify possible ways to minimize waste in an assessed area. The identification of potential options relies on both the expertise and creativity of all team members. Much knowledge comes from education and on-the-job experience; however, the use of technical literature, contacts, and other sources is always helpful. When identifying options, team members should follow the waste reduction hierarchy in which source reduction possibilitiesare explored first, followed by recycling options. With an ever-increasing emphasis being placed on zero-discharge and on-site recycling, waste treatment technology will eventually be phased out ofmany operations. Source reduction is preferred because it reduces or eliminates waste from the beginning of a process. Recycling should be considered only when all attempts at performing source reduction have been exhausted, and then on-site recycling should bethe primary method considered.
A. Methods of Generating Options The process whereby waste minimization options are generated should occurin an environment that encourages opennessand independent thinkingby members of the assessment team. While individual team members will suggest many potential options on their own, the process can be enhanced,by using some common group decision techniques. These techniques allow an assessment team to identify options that individual members might not have come up with on their own. Brainstorming sessions with team members are an effective means of developing options. It is likely that all or most of the options being presented have some merit and some drawbacks. During a brainstorming session, it is best not to dwell on details but to simply list as many options as possible.
B. Screening and Selecting Options for Further Study Several waste reduction options will be identified in a successful brainstorming session. It is then necessary to identify the options that offer the best potential for minimizing waste and
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reducing costs. Because a detailed evaluation of technical and economic feasibility is often costly and time-consuming, the proposedoptions should be screenedto identify the ones that deserve further evaluation. An initial screening will eliminate options that appear unreasonable, impractical, or inferior without a detailed and more costly feasibility study. Screening procedures can range from a simple vote by team members to sophisticated quantitative decision-making formulas. Generally, an informal evaluation utilizes an unstructured conferencesetting by which an assessment teamor program task forceselects options that appear tobe feasible. This methodis used most often by small businessesor in situations where only a few options are practical.
VII. FEASIBILITYANALYSIS Once the assessment team has agreed to evaluate selected options, a feasibility analysis is conducted to determine which options are technically and economically practical. The level of analysis required is directly proportional to the complexity of the option being considered. Simple options, such as housekeeping improvements, would not require as thorough an evaluation as equipment changes[7]. The final product of an assessment phaseis a list of feasible options for an assessed area. The assessment will have screened out impracticalor unattractive options. The next step is to determine if remaining options are technically and economically feasible. An evaluation should be conductedto determine whether a proposed option will workfor a specific application. The evaluation of an option should include facility constraints, product requirements, capital costs, and operating costs. A fiscal evaluation should consider standard measures of profitability such as payback period, return on investment, and net present value. Each organization has its own economic criteria for selecting projects; however, as in any project, the cost elementsof a waste reduction project can be broken down into long-term and short-term capital costs and operating costs. All affected groups in a facility should contribute to and review the results of a technical evaluation. Prior consultation and review with affected groups is needed to ensure the viability and acceptance of an option. If the option calls for a change in production methods or input materials, the project’s effects on final product quality must be determined. If, after technical evaluation, the project appears impossible or impractical, it should be dropped.
VIII. IMPLEMENTING WASTE REDUCTION OPTIONS The assessment team’s evaluation report should providethe basis for obtaining company funding of waste reduction projects. Projects should notbe marketed ontheir technical or environmental merits alone. An analytical, fiscally sound description of both short- and long-term benefits can help edgea proposed project past competing projects for funding. Itis also helpful to have someone from the manufacturing, purchasing, or accounting section of the facility present the proposal. This will enlarge the imageof the projectbeyond environmental improvements, to depict the proposed action as a sound business investment. The presenters of the proposal should be flexible enough to develop alternatives or modifications. They shouldalso be sufficiently committedto the projectto obtain background data and support studies and to anticipate potential problems that may be encountered during implementation. Above all, team members should keep in mindthat an idea will not sell if team members are not sold on it themselves. This is why management and production staff people should support the concept of waste reduction from the beginning of the process.
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A. ObtainingFunding Waste reduction projects generally call for improvements in process efficiency and/or reductions in operating costs of waste management. These are the points that shouldbe emphasized in the decision-making process. An assessment team made up of financial and technical personnel helps to ensure that a sponsor’s enthusiasm is balanced with objectivity. Even if a project promises a high rate of return, a small company may have difficulty acquiring funds for capital investment. In this case, the company should look to outside financing. It generally has two major sources to consider: private sector financing and government-assisted funding. If the venture is prudent and cost-effective, many lenders will gladly provide loans for pollution prevention or waste reduction projects, particularlyif they have already backed the company with other loans. Government grants and loans arealso available for pollution prevention projects. If the project involves a new or unusual approach to waste reduction, the entire proposalmay be funded by a research grant.
B. Installation Waste reduction proposals that involve operational, procedural or material changes canbe implemented as soon as funding has been approved. For projects involving equipment modificabe involvedwithplanning,design,comparisons, tions or new equipment,theteamwill procurement, and construction of thenew equipment.
C. MeasuringWasteReduction Once a waste reduction project has been implemented, it mustbe monitored to determine how effective the option actually turns outto be. Projects that do not measure up to their original performance expectations may require reworking or modification. One measureof project effectiveness is its payback time. A project should pay for itself in a reasonable time through reduced waste disposal costs and/or reduced raw materials costs. The primary goal of the project, however, is to reduce waste. This should be the major consideration when implementing a program. Accurate measurements must be takento express the extent of waste reduction. The process by which the wastes are reduced must be fully understood and compared to previous processes to determine the total impact of the new method.
D. AssessingNewProcesses It is important to avoid trading one waste problem for another. Waste reduction principles should be applied tonew projects experimentally on a small scale. Iteasier is to observe waste generation during research and development than to go back and modify a process after it has already been installed. With new emphasis on attaining zero discharge and eliminating off-site recycling, companies must strive to implement fully contained systems.
E. KeepingtheFaith A waste reduction program is an ongoing rather than a one-time effort. Once high priority streams havebeen assessed and reducedor eliminated, an assessment team should look at low priority streams. The ultimate goal of the program should be to reduce the generation of waste to the maximum extent achievable.
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To be truly effective, a philosophy of waste reduction must be developed in the organization. This meansthat waste reductionmust be an integral part of the company’s operations. The most successful waste reduction programs to date have all developed this philosophy within their companies.
REFERENCES 1. 2. 3. 4. 5.
6. 7. 8. 9. 10.
11.
Long, R. E., The problem of waste disposal, Rd. Shelf, 60, 41-48 (1989). University of Tennessee Teleconference,Promote Landfill AlternativesNow. PLAN Booklet, 1992. Council for Solid Waste Solutions, The Solid Waste Management Problem, 1989, pp. 7-10. Marburg Associates, Site Auditing: Environmental Assessments of Property, STP Specialty Technology Publishing, 1992, Appendix U.S. Office of Technology Assessment,Serious Reductionof Hazardous Waste,Booklet U.S. Congress, Washington, D.C., 1991. Gore, A., Earth In the Balance, Houghton Mifflin, Boston, 1992, pp. 342-343. Theodore, L. and Mffiuinn, Y., Pollution Prevention, Van Nostrand Reinhold, New York, 1992, p. 170. Philips, D. A., Waste Reduction in Industrial Processes, Florida Dept. of Environmental Regulation,1991,pp.1:16-24. Environmental Information, Ltd., Tax incentives for pollution abatement, Environ. Dig. State Programs, Minneapolis, Minn., May1992,pp.18-21. Freeman, H. A.,Hazardous Waste Minimization, Mffiraw-Hill, New York, 1990, pp. 77-107. HowToComply Handbook SARA Title III FloridaStateEmergencyResponseCommission, 1990, p. 7.
33 Contaminated Soils in Highway Construction
Namunu J. Meegoda New Jersey Instituteof Technology Newark, New Jersey
1.
INTRODUCTION
In 1988, there were approximately4 million milesof roads in the UnitedStates of America, of which 2.3 million miles were surfaced with asphalt or concrete. Of the surfaced roads, approximately 96%, or 2.2 million miles, had asphalt pavements[l]. More than 95% of the2 trillion vehicle miles traveled each year occur on asphalt-paved roads. In 1988, expenditures for highways amounted to over $68 billion at all levels of government [l]. During 1988, 500 million tons of hot mix asphalt (HMA) was produced and placed, and about 250 million tons of HMA was used for construction, rehabilitation, and maintenance of the highways [l]. The HMA industry directly employs 300,000 people and indirectly accounts for an additional 600,000 jobs [l]. The hot mix asphalt concrete consists of a combination of aggregates blended and uniformly mixed, coated with asphalt cement, and compacted into a dense material. The materials in HMA consist of (1) coarse aggregates with sizes ranging from 1.5 in.to U.S. sieve No. 4, (2) fine aggregate or sand with sizes passing U.S. sieveNo. 4 and retained on U.S.sieve No. 200, (3) mineral filler such as crushed stone dust or lime passing U.S. sieve No. 200, and (4) asphalt cement. A typicalHMA composition consistsof 50% coarse aggregates, 40% fine aggregate, 5% mineral filler, and 5% asphalt cement. Asphalt cement is obtained by distillation of petroleum crude. The asphalt cements obtained from refineries are classified as AC-2.5, AC-5, AC-10, AC-20, AC-30, or AC-40 based on viscosity. To obtain sufficient fluidityof asphalt cement for proper mixing and compaction, both the aggregate and the asphalt cement are heated prior to mixing; hence the product is called hot mix asphalt (HMA) concrete. qpically 5-10% waste products such as recycled asphalt pavements, tire rubber, glass, municipal solid waste (MSW)ash, roofing shingles, polythene waste,fly ash, bottom ash, ore slug, and petroleum-contaminated soils are addedto HMA without sacrificing its strength and performance [2-51. U.S. Department of Transportation data [l] suggest that an estimated 25 million tons of industrial waste can be recycled and consumed annually by the U.S. asphalt
663
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industry, by simply adding 5% waste productsto all HMA mixes. Therefore, all levels of government in the UnitedStates emphasize the inclusion of waste products in HMA. Usually separate mix designsare not performedto include such waste material. Either the amount of materi in the original mix is proportionately reduced or waste products replace the mineral filler. Petroleum-contaminated soil (PCS), one of the industrial solid waste products used in HMA, is generated from leaking underground storage tanks (USTs), including their piping systems. During the 1950s and 1960s, the construction of many gasoline stations and chemical manufacturing and processing facilities led to the installation of millions of USTs. Several million USTs in the United States contain petroleum products. Tens of thousands of these USTs, including their piping systems, are currently leaking [6].The U.S. Environmental Protection Agency estimates that there are more than400,OOO leaking USTs with petroleum hydrocarbons. Many more are expected to leak in the future. Most states vigorously encourage the removal of all tanks after 25 years of service. It is estimated that the removal of a leaking tank generates approximately 50-80 yd3 of contaminated soil. In addition, soils that surround petroleum refineries and crude oil wells are contaminated with petroleum products. Since groundwater is a source of drinking water, federal legislation seeks to safeguard our nation’s groundwater resources. Congress responded to the problem of leaking USTs by adding Subtitle I to the Resource Conservation and RecoveryAct (RCRA) in 1984. The current federaland state statutes require that leaking USTs be removed to prevent further contamination. Petroleum-contaminated soils consist of mixtures of natural sands, silts, and clays with petroleum products. A small portion of light petroleum product mixed with asphalt cement merelyproduces an asphaltcementofslightlydifferentspecification or characterization. Therefore, it is believedby many involved with the asphalt industry that a small increase in the quantity of light petroleum substanceswould not damage the HMA. This is, in fact, the basis for the theory that contaminated soils can be used in asphalt concrete paving. However, the inclusion of natural soils inHMA and the environmental impact dueto such inclusion require an in-depth study. When PCSs are added to HMA, three beneficial actionsoccur: incineration, dilution, and solidification. Part of the petroleum is used as a fuel and is burned during the production of asphalt concrete; thus a majority of the contaminantsare eliminated beneficially, which reduces fuel costs. Since only5-3096 petroleum-contaminated soilis added to virgin aggregates during the production of asphalt concrete, there is spreading and dilution. The asphalt cement actsas a binder in asphalt concrete; therefore, the remaining diluted contaminants are solidified and stabilized in the final asphalt concrete matrix. Strength or stability, durability, and workabilityare the primary factorsto be considered in hot asphalt mix designs [7]. The secondary factors are flexibility, permeability,fatigue resis[7].Since the HMA produced withPCSs is used for tance, skid resistance, and stripping action paving, it must satisfy all the engineering and environmental requirements specified by the appropriate federal, state, and local agencies. In 1990, a major laboratory and field research project fundedby the New Jersey Department of Environmental Protection and Energy (NJDEPE) was initiated to investigate the feato sibility of using PCSs in HMA. An extensive laboratory and field study was conducted evaluate the use of PCSs as an aggregate replacement in the production of hot mix asphalt (HMA) concrete. In the laboratory and field investigations, petroleum-contaminated soils were , added to HMA, and the resulting asphalt concrete mixes were tested for strength, durability, permeability, and leachability. During the field production of HMA and PCSs, emissions of volatile organic compounds(VOCs) were also monitored. In this chapter, the engineering performance of HMA with K S s is summarized and the environmental impact of the process is described in detail.
Soils Contaminated
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in Highway Construction
Table 1 DataonSixContaminatedSoilsfromNewJersey Soil 1 sand
Soil 2
Soil 3
Soil 4
Soil 5
classification Soil Well-graded Clayey Silty sand Poorly siltSilty clay graded sand
Soil 6 Poorly with silt
moisture In situ 7.3 24.1 14.3 content (a) qpe and amount 1100 ppm 1200 ppm of contamination heating heating oilheating oilgasoline oil gasoline gasoline
6600 ppm
14.4
19.6
10.1
25 ppm
1500 ppm
330 ppm
Source: Meegoda et al. [ 8 ] .
II. EXPERIMENTAL PROCEDURE
AND RESULTS
A. Engineering Performance of HMA with PCSs 1.SoilClassification Six contaminated soils providedby NJDEPE from sites identifiedas containing soils with less than 1% total petroleum hydrocarbons (TPHs) were selected for testing and for characterization. Three soils were contaminated with heating oil, and the other three were contaminated with gasoline. The degree of contamination for oil-contaminated samples was determinedby the Soxhlet oil and grease extraction method (USPHS standard method for the analysis of water and wastewater). Table 1 gives the soil classification data and lists the type and amount of contaminants in each soil. 2. Stability or Strength of HMA with PCSs All the asphalt concrete samples were designed for New Jersey surface coarse mix (NJ 1-3). A control mix was designed and tested for comparison. The mix designs to include soils were prepared for each of the six soils based on the sieve analysis data. (Please refer to Meegoda data for soilsand aggregates and the NJ 1-3 specifications.) The et al. [8] for grain size analysis optimum percentages that may be usedin the NJ 1-3 mix based on aggregate blending for the above six soils wereas follows: Soil I , 35%; soil 2, 10%; soil 3, 20%; soil 4, 15%; soil 5, 10%;andsoil 6 , 15% [8]. These percentages are muchhigherthanthecurrentpracticeof 5% PCSs. Once the maximum amountof PCS that may be added to HMA was determined, the suitability of such an addition was evaluated.The Marshall stability test was performed using ASTMD1559.BasedontheNewJerseyDepartmentofTransportation(NJDOT)requirements for Marshall strength, bulk density, air voids, voids in mineral aggregates (VMA), and flow, the optimum asphalt content for each mix was selected. The optimum asphalt contents based on Marshall test results for the control mix and for the six contaminated soils are shown in Table 2 [8]. Table 2 also shows the dry density, Marshall stability, air voids, VMA, and flow values at the optimum asphalt contents for the control as well as for HMA made with 1-3 mix are also shownin each soil type. The New Jersey specifications for high traffic volume Table 2. If an asphalt concrete meets all state specifications and if it is a workable mix, then it is accepted as a paving material. Exceptfor the control mix and HMA mixed withPCS No. 4, as shown in Table2, all the HMA mixes with PCS were acceptableas paving materials. The control mix and the HMA mix with soil No. 4 had low flow values. A higher flow value can be selected for all three mixes from Marshall test results with higher asphalt contents, but it 8006 N). If a lower Marshall strength value will result in lower stability values (lower than
Meegoda
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Table 2 Optimum Properties of Asphalt Concrete with PCSs for NJ 1-3 Mix Asphalt concrete Allowable for property NJ 1-3 Control Mix Strength (N) Flow (0.25 mm) Air voids (%) VMA (%) Density (W/m3) Optimum asphalt content (%)
>8006 >6.0 2.0-8.0 >13.0
8006 4.0 7.0 18.0 24.3 5.0
NIA 4-8
Soil 1
Soil 2
Soil 3
Soil 4
Soil 5
Soil 6
8228 11.0 7.5 17.8 24.8 4.5
8450 8.0 3.0 14.0 24.5 4.5
10229 7.5 5.7 16.8 24.1 5.0
8450 3.5 8.0 18.0 23.4 4.5
8317 6.5 4.0 14.7 24.6 4.5
10452 7.7 3.4 14.2 24.5 4.5
N/A= not available. Source: Meegoda et al. [8].
(say 6671N) is acceptable, then the control mix and the HMA made with soil No. 4 are acceptable as paving materials. Based on the test results shown in Table 2, it can be stated that the HMA with PCSs produced better asphalt concrete thanthe control. This may be due to the better blend obtained by adding natural soils. 3. Durability ofHMA with
PCS
ASTM D4867 describes the test procedure for determining the effect of moisture on asphalt concrete mixtures,a factor that is very important for the durabilityof hot mix asphalt concrete. It has a section on freeze-thaw conditioning of a mixture. However, the freeze-thaw and wetdry tests require onlyone cycle eachof freeze-thaw or wet-dry. There was no rationale for the selection of one cycleof freeze-thaw or wet-dry to evaluate the moisture damage when, in the real world, asphalt concrete pavements are subjected to several freeze-thaw and wet-dry cycles under service conditions before they are removed for resurfacing. Therefore, an experimental program was designed to evaluate the moisture damage of the control mix and one mix prepared with PCS No. 3. In this experiment HMA specimens were subjected to several cycles of freeze-thaw and wet-dry. For this experiment, 18 specimens of control mix and 18 specimens of HMA mixed with PCS No. 3 were used. The experimental test resultsof an extended freeze-thaw test are plotted in Figures 1 and 2. Figures 3 and 4 show similar data for wetdry tests. In this extended durabilitytest, the control mix and HMA with PCS No. 3 were tested for one, three, seven, and 14 cycles, with each cycle taking approximately48 hr. Upon completing 14 freeze-thaw and wet-dry cycles, data were collected and graphically displayed. It can be concluded from thesetest results that as the temperature drops to freezing temperatures for the freeze-thaw specimen, the asphalt concrete contracts and becomes brittle, creating tiny cracks on the surface of the sample, which provide entry points for water. Water inside the specimen causes moisture damage due to strippingand volume expansions during subsequentfreeze cycles. With the increasein the number of freeze-thaw cycles, the cracks get larger,letting more water into the specimen and eventually leading to the failure of the specimen. The data from cyclic freeze-thaw tests indicate that the percentage swell increased rapidly during the first cycle and then gradually reacheda maximum before the specimen failed. It is believed thatthe specimen reached its maximum percentage swell when it was totally saturated, and stripping occurred before complete failure. The tensile strength ratio also decreased rapidly during the first cycle and also began to level off to zero strengthafter 14 cycles. The behavior seemsto be similar for the control mix and the mix containing PCS No. 3, suggesting comparable durability
Contaminated Soils in Highway Construction
.-
667
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Figure 1 Extended freeze-thaw test for control mix and HMA with PCS No. 3-TSR Meegoda et al. [9].
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Figure 2 Extended freeze-thaw test for control mix and HMA with PCS No. 3-swell values. (From Meegoda et
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values for these two mixes. It also seems that there is a correlation between the percentage swell and the tensile strength ratio. The data show that most of the strength is lost during the f i i t cycle, and hence there was no need to test beyond one freeze-thaw cycle as suggested by ASTM D4867. The cyclic wet-dry test also indicated that the first cycle was the p i n t where attention must be focused. For the wet-dry test, the tensile strength ratio declined during the f i t '
668
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6 8 10 NUMBER OF CYCLES
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Figure 4 Extended wet-dry test for control mix and HMA with PCS No. 3-swell values. (From Meegoda et al. [9].)
cycle and increased thereafter. This is believed to be due to oxidation of asphalt during the drying cycle. Therefore, it was concluded that the first wet-dry cycle should yield the critical conditions. It can be concluded from these tests that freeze-thaw and wet-dry tests with one cycle indicate whethera specimen is durable, and a strength lossof more than20% indicates that the specimen will not withstand harsh weathering conditions.
Contaminated Soils in Highway Construction
669
Table 3 Durability of HMA with PCSs HMA mix
Freeze-thaw Wet-dry test test
82.3Control 89.0 with soil 1 HMA 100.0 with soil 2 HMA HMA with soil 3 87.0 HMA with soil 4 98.4 HMA with soil 5 HMA with soil 6 Source: Meegoda et
91.7 98.0 89.3 87.2 83.8 93.4 100
93.9
100
al. [9].
Table 3 shows the tensile strength ratio (TSR) values of wet-dry and freeze-thaw tests performed based on ASTM D4867 for the control mix and for the HMA made with six soils. TSR values for HMA with PCSs were not significantly different from that of the control, indicating that HMA with PCSs produced durable asphalt concrete. Some of the wet-dry and freeze-thaw test results show that the HMA withPCS are better than theHMA produced with virgin aggregates. 4.
Permeability of HMA Prepared with PCSs Hydraulic conductivity tests were performed on laboratory-compacted HMA specimens. The of a 100 mm diameter flexible-wall permeameter. A cell pressure specimens were placed on top of 50 psi (344kPa), a back-pressure of 30 psi (206 P a ) , and a desired pressure difference (mainly 1 psi or 7 kPa) were applied to the specimen. Twenty-four hours after the in-flowbecame equalto the outflow and when the hydraulic conductivity didshow not further reduction, the permeability test was stopped. The permeability test was conducted concurrently on three PCS. A bladder accumulator was connected between specimens of HMA made with the same the permeameter and the pressure panel to separate the permeant from the distilled water used in the pressure panel. This procedure eliminated the contaminationof the pressure panel. 5 shows At the end of the permeability test, the specimen height was measured. Figure typical permeability test results where the variation of hydraulic conductivity with time is shown. The average saturated hydraulic conductivities of the control mix and HMA mixes with PCSs are shown in Table 4. Table 4 shows the saturated hydraulic conductivity data for HMA with and withoutPCSs. Only one mix (with PC No. 2) showed a lower saturated hydraulic conductivity value than the control mix. However, the saturated hydraulic conductivity values of all the HMA mixes with PCSs werelessthan 2.0 X acharacteristicvaluefor low permeableclay-typesoilsand hence canbe considered acceptable. Table4 also shows the other parameters of asphalt matrix that contribute to the saturated hydraulic conductivity. It appears that the combination of air voids, asphalt content, and dlo size (aggregate size correspondingto the 10% finer fraction in the gradation curve) control the saturated hydraulic conductivity. With a higher percentage of air voids, one would expect a higher hydraulic conductivity as a larger fractionof asphalt concrete matrix will be porous, allowing fluid to flow through those voids. In geotechnical engiof with soils neering, it is an accepted fact d,, thatsize controls the hydraulic conductivitysoils, that have higher d l , sizes having higher hydraulic conductivity values. However, since all the HMA mixes tested in this research were NJ 1-3 mixes, there should not be drastic differences in d l , size as shown in Table 4; hence its influence here is minimal. The asphalt content will also play a major role when it is higher than optimum, as the excess asphalt cement, after coating all the particles, will block the interconnecting voids,
670
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; inlet 0 :outlet
......
I
‘0
Time (hrs)
Figure 5 A typical hydraulic conductivity test result. (From Meegoda
et al. [9].)
Table 4 Hydraulic Conductivities and Other Related Parameters of HMA with PCSs Saturated hydraulic HMA conductivity mix (cm/sec) (mm) mix voids Air 4.75 Control HMA with soil HMA with soil HMA with soil 4.25with soil HMA HMA with soil HMA with soil
1
2 3 4 5 6
4.7 2.3E-07 3.3E-07 1.6E-07 1.6E-06 6.9 1 .OE-06 8.3847 4.6E-07
d,, size inthe (%)
0.18 4.50 0.21 0.13 0.12 0.27 4.50 0.14 4.50 0.15
Asphalt content
(%)
5.6 6.9
7.5
4.50 5.00
7.3 6.3
Source: Meegoda et al. [9].
causing a reduction in hydraulic conductivity. However, since all the mixes were tested around the optimum asphalt content values for the corresponding mixes, its contribution to the test results are minimaltoo.Therefore,thehydraulicconductivityvaluesshouldincreasewith the percentage of air voids, and there appears to be a direct correlation between percentage of air voids with the hydraulic conductivity except for mix having PCS No. 3. We suspected measurement errors for this mix [9]. Therefore, it canbe concluded that the additionof PCSs may not change the saturated hydraulic conductivityof the asphalt concrete and the change is due only to the difference in air voids, dlo size, and asphalt content associated with the mix design [9].
B. Environmental Impact of Adding PCSs to HMA 1. Leachability Test for HMA with PCS The increase in the number of organic contaminants being detected in groundwater as well as in surface water is causing concern because of potential health risks claimed to be associ-
Contaminated Soils Construction in Highway
671
ated with human exposure to these substances. Soil contaminated with petroleum product is a potential threat to both surface water and groundwater; thuskey a issue is whether hydrocarbonwillleach out from the asphalt mixture when it is mixedwith PCSs andused to pave roads. The short-term test conducted by Eklund [IO] on the environmental effects of paving showed that after allowing a pavement to leach for7 days with distilled water, the amount of petroleum-based compounds leached out and detected was less than 2 ppm. In another series in cold of experiments by Eklund [lo] with 6% waste oil with an appreciable lead content mix asphalts (CMA), when the CMA was leached for a week in an acid rain simulation, the leachate contained less than 3 ppm of lead. Although short-term tests showed no sign of any harmful leachate generation, tests should be conducted to determine the environmental impact of long-term exposure of asphalt produced with contaminated soils on the environment. A uniform leaching test was used to estimate the quality of leachate that would be produced by the asphalt mixture. A new leachability test was designedto simulate the rate of release of contaminants when HMAs mixed with PCSs are exposed to the actual performance environments when used as pavements. The test methodology and test standards were developed in cooperation with the New Jersey Departmentof Environmental Protection and Energy (NJDEPE). The experimental protocol was evolved based on the information and experience of EPA toxicity characteristics leaching procedure (TCLP) and waste stabilizatiodsolidification program, the waste solidification program of the U.S.Army Corps of Engineers, and the nuclear waste research program at Brookhaven and Oak Ridge National Laboratories. The uniform leaching procedure gives an indicationof the amount of each organic compound that is leachable under specific experimental conditions. The structural integrity of sami.e., inthose plewaskeptinthis test, whereparticlesizereductionwasinappropriate, instances where solidification of the waste is needed to meet the best demonstrated available technology provision of environmentallaw. Grinding may not adequately represent the actual process. Particle reduction alters the physical charactermany of solidified wastesby destroying the cementitious property of these wastes in such a way as to show an unrealistically high leaching rate. This water-leachability test was based on the EPA draft on Solid Waste Leaching Procedure (SWLP). This document recommends using laboratory reagent wateras the leaching mebe chosen to mimic the environment where the dium. Chemical and physical conditions should HMA with PCS is used. The leaching medium in this test was chosenas reagent water with a pH value of 6.8 at a temperature of25°C (77°F). Since the asphalt mixturesare used as paving materials, the reagent water should be more representative of the actual environment than an acidic leaching medium. The reagent water was prepared by boiling deionized water for 15 min. Subsequently, while maintaining the water at about 90°C (194"F), nitrogen gas was bubbled through the water for 1 hr. The HMA specimen was treated as a monolithic waste (i.e., the to testing). The asphalt concrete with PCSs were compacted specimen was not pulverized prior into a specially designed stainless steel mold 2 in. X 2 in. X 2 in. before being tested. Since 2-in. cubes were needed for this test, HMA could not be compacted to the same densityas in Marshall tests. However, a loose matrix should produce a higher and conservative leaching rate. HMA specimens were prepared as for the Marshall test, where the blended aggregates were heated to 130°C(268°F). Then heated asphalt cement was added to the dry mix and wet-mixed for 1 min. Then a sufficient quantity of HMA was placed inside the steel molds and compacted with 50 tamps and the HMA specimen was allowed to cool. The compacted HMA specimens with PCSs were placed in 480-mL glass containers and sealed with Teflon septum caps. The volume of the container was at least three times that of the sample,so sufficient space was available in the container and the sample was surrounded on all sides by
672
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leaching medium. There was zero headspace to minimize the effects of volatilization. Containers were placed on an Orbit shaker table at 75 rpm for a period of 96 hr. After the end of the shaking period, the containers were opened and a 25-mL water sample was immediately drawn using a gastight syringe. The extracted solutionwas analyzed using an EPA purge-andtrap method. The Purge and Trap Method and GC Settings. The analytical system usedin this test was an ALS 2016 desorber and Tekmar 2000 purge-and-trap concentrator interfacedto a Varian 3400 gas chromatograph (GC) with a high resolution capillary column and a flame ionization detector (FID). The parameters of A L S 2016 were as follows:
Preheat pwe Dry Purge Cooldown Desorb preheat Desorb
Inject Bake
2 min 15 min. helium flow at 40 d m i n 4 min - 150°C 150°C 5 min at 180°C 3 min at 190°C 10 min at 240°C
The column used for GCFID analysis was a cross-linked 5% phenyl and 95% methyl silicone gum, 50 m long, 0.2 mm in diameter, and 0.5 pm film thickness fused silica-bonded high resolution capillary column. Flow rates for the GC were
Hydrogen Air Carrier gas Make-up gas
30 d m i n
30mUmin 1 mUmin 30 d m i n
The GC parameters were
Initial column temperature Initial column hold time Program 1 final column temperature Program 1 column rate Program 1 column hold time Program 2 final column temperature Program 2 column rate Program 2 column hold time Inject temperature Detect temperature
40°C 5 min 65°C 2"Umin 0
190°C 8"Umin 5 min 210°C 250°C
I
Contaminated Highwuy Soils in
Construction
673
Table 5 Method Specifications and Regulatory Limits for Fraction Leached (proposed NJ) in ppb Compound
MDL
Dichloromethane 1 ,ZDichloroethane l,l,l-TCE Hexane Benzene Toluene F’erchloroethene Ethylbenzene p,m-Xylene o-Xylene
0.29 0.47 0.39 0.22 0.26 0.25 0.13 0.28 0.28 0.24
f S,
CV and concentration
Regulatory level
82 f 32% 76 f 21% 73 f 28% 106 f 25% 88 f 16% 94 f 18% 108 f 24% 79 f 17% 95 f 20% 91 f 21%
12.6%112 24.8%126 2 1.4%18 18.5%18 14.3%114 16.8%114 19.7816 19.7%/6 9.6%/5 12.6%15
270 820 110
P
NIA
410 350 100 360 340 340
N/A = not available. Source: Meegoda et al. [9].
An external standard was used for the calibration. The standard consistedof a mixture of targetcompounds in methanol. Theywere dichloromethane.1 ,Zdichloroethane, 1,1,1trichloroethane, hexane, benzene, toluene, perchloroethylene, ethylbenzene, p,m-xylene, and o-xylene. Calibration and Quantification of Data. To determine the precision of the measurements, three injections were made for each concentration level, and the results were analyzedto obtain the mean and the standard deviation. Seven standard samplesof the same concentration were analyzed to obtain the coefficient of variation (CV)to determine the reproducibilityof his analytical method for each compound. The detection limit of the method (MDL) is defined as the minimum concentration of a substance that can be identified, measured, and reported with 99% confidence when the analytic concentration is greater than zero and is determined from analysisof a sample in a given matrix containing analytic compounds. In this test, reagent water was used to estimate the MDL concentration. Standards (analytic in reagent water) at concentrations equal to 1 to 5 times the estimated MDL were used to calculate the MDL. The MDL was reported in concentration units as the standard deviation (SD) of the replicates multiplied by the appropriate Student’s t-value (for a one-tailed test at 99% confidence) for the number of replicates. In this test, the number of replicates was chosen as seven, so MDL was defined as MDL = 3.143
X
SD
Each day, three or four sampleswere spiked withat least 10% of the samples and analyzed to monitor and evaluatethe experimental data quality. The percentage recovery was calculated
using the equation P = 100 (A - B)% / T where A is concentration after spiking, B is the background concentration, and Tis the known true value of the spike. After the analysis of 10 spiked samples,the average percentage recovery( P ) and the standard deviationof the percentage recovery(Sp) values were calculated and expressed as P f Sp. Table 5 shows those values. A major sourceof interference in this test was cross-contamination of samples. To prevent such cross-contamination, reagent water blankswere run twice before running each sampleto
674
Meegoda
Table 6 Leachate Concentration (ppb) from Monolithic HMA Specimens with Pc& Compound Dichloromethane 1 ,2-Dichloroethane l,l,l-TCE Hexane Benzene Toluene Perchloroethene Ethylbenzene p,m-Xylene o-Xylene
Pcs 1
Pcs 2
Pcs 3
Pcs 4
Pcs 5
PcS 6
ND ND ND 0.23 ND ND ND ND ND ND
ND ND ND ND ND ND ND ND 0.32 ND
2.% 0.88 8.01 6.46 3.11 ND 1.19 ND 0.36 0.37
ND 0.16 ND ND ND ND ND ND ND ND
ND 0.18 5.43 2.43 2.19
ND 1S 1 5.14 0.85
ND
ND ND
0.97 ND 0.33 ND
ND ND 1.69 ND
ND=none detected or less than the detection limit. Source: Meegoda et al. [9].
demonstrate that interferences from the analytical system were under control. The following procedure was used to clean the vials and glass containers: 1.
2.
3. 4. 5.
Wash all the vials and containers thoroughly in hot water using a detergentto remove the particulate matter and contaminants. Rinsethoroughlyusingtapwater. Rinsethreetimesusingdeionizedwater. Place in a vacuum oven at 105°C for 12 hr to bake all volatile compounds. Cool for 30 min, screw the lids tightly, and store in an area not subject to contamination by air or other sources.
Test Results. This method can estimate the concentrations of target chemicals up to a minimum concentration of around one-tenth of 1 part per billion (ppb). Table6 shows the testresults. Ethylbenzene did not leach out from any of the six mixes. Benzene, toluene, and xylene 8 ppb. For the HMA mix containing PCS No. concentrations in all the leachates were less than 3 with an initial petroleum concentration of 6600 ppm in the soil, the total leachate concenPCS No. 3 was tration of 10 keychemicals in leachate from solidified and stabilized HMA with less than 25 ppb. The experimental results clearly demonstrate that the HMA with PCSs solidifies and stabilizes most of the petroleum contaminants within the asphalt matrix and the very small concentrations of organic chemicals that may be leaching are from the soil particles that are not completely coated with asphalt cement. To further evaluate the long-term leaching of contaminants, three specimens of HMA and shaken for durations of made with PCS No. 3 were prepared, placed on top of the shaker, 1 day, 1 week, and 1 month. Those test results are shown in Table 7 and are graphically displayed in Figure6 . Results do not show any significant increase in contaminant concentrations with time for any of the compounds tested.
2. Analysis of Air Quality During the Production of HMA with PCS Czarnecki [4] obtained a permit from the Massachusetts Department of Environmental Quality Engineering (DEQE) to process petroleum-contaminated soils subjected to the following conditions: It should process 95% virgin aggregate and 5% PCS, and 95% of the hydrocarbons should be incinerated during the process.To demonstrate the95% destruction, a mass balance of pure chemicals was performed for the incineration system. The test used sand contaminated with a 3% concentration of a 50150 blend of xylene and toluene. Three points in the system
Contaminated Soils in Highway Construction
675
Table 7 Leachate Concentration (ppb) with TimefromMonolithic HMA Specimen with PCS No. 3
eekOne day Compound One 2.33 Dichloromethane 0.81 1 ,2-Dichloroethane 1,l.l-TCE 6.29 Hexane 6.32 Benzene 2.32 Toluene 1.11 Perchloroethene Ethylbenzene 1.65 p,m-Xylene o-Xylene 0.28
1.82 0.82 5.35 6.01 2.19 ND 1.01
ND
3.12 1.03 9.43 6.87 2.29 ND 1.12
ND
ND
ND
1.80 0.30
0.33 0.26
ND=none detected or below detection limit for each compound. Source: Meegoda et al. [9].
10
g
P v
0
"i
." .............-
"'B
"
"
..... .... "
"
"
A
x
X
i
Time (days)
30
Figure 6 Long-term leaching test results. (From Meegoda et
al. [9].)
were selected for analysis: incoming sand from the conveyer belt before the aggregate dryer, sand stripped of contaminants from the storage silo, and air samples from the stack. For three tests, Czarnecki [4] showed 99% destruction of hydrocarbons. Following are the test results from this study; the ai+ sample collection method was not documented:
Incoming sand concentration 30,000.0 ppm Sand coming out from aggregate dryer 0.9 ppm Air samples from the stack 0.2 ppm The measurement of volatile organic compounds (VOCs) has become an important aspect in understanding photochemical reactions and providing an index of hydrocarbons present in the atmosphere. Knowledge of the levels of such materials in the ambient atmosphere isalso
676
Meegoda
required in order to determine human health impacts. However, the determination of toxic organic compounds in ambient air is a complex task, primarily because of the wide variety of compounds of interest and the lack of standards and procedures for analysis. The U.S. Environmental Protection Agency (EPA) has developed several standard analytical methods forthe measurement of volatile organic compounds present in ambient air. Sample collection has been reported as the weakest link in the analytical chainfor the determination of airborne organics, which is critical to the accuracyof the results. The sampling methodsused for measurement of VOCs can be categorizedas adsorption by a solid, cryogenic trapping,or whole air collection. Adsorption by a solid. The solid adsorbents include(1) organic polymers (Tenax, XAD2), (2) inorganic materials (silica gel, florisil), and (3) carbon (activated carbon, carbon molecular sieves). In this technique VOCs are collected on a solid sorbent material while the bulk constituents (e.g., nitrogen, oxygen) are allowed to pass through the sorbent. The VOCs adsorbed are then desorbed, and the sample is injected in a gas chromatograph (GC). Stripping of the adsorbed analytes from the adsorbent is typically accomplishedby either thermal or solvent desorption. Solvent desorption, although useful in many applications, generally requires sample preconcentration before analysis.This adds to the complexity of the final method and increases the sample handling time and the possibilityof contamination. The thermal desorption of analytes has the advantages of reduced sample handling and increasedsensitivity because of the transfer of all analytes onto the chromatographic column. The main advantages of the use of a solid adsorbent for organic polymers are that (1) little water is collected in the sampling process and (2) a large volume of air can be sampled relative to other techniques such as cryogenic sampling. The analytes adsorbed onto Tenax, desorbed, and analyzed using GC/MS can determine volatile nonpolar organics (e.g., aromatic hydrocarbons, chlorinated hydrocarbons) having boiling pointsin the range of 80-200°C [1l]. The PUF/XAD-2 adsorption with GC and HPLC detection can be applied todetermine polynuclear aromatic hydrocarbons. A major disadvantage of these materials is the breakthrough of volatile compounds. Degradation products of the trapping materials are frequently found in adsorbent tubes such as Tenax. Incomplete desorption is also a problem with these methods. Inorganic adsorbents includesilica gel, alumina, florisil, and molecular sieves. Thesematerials are considerably more polar than the organic polymeric adsorbents, leading to the efficient collection of polar materials. Unfortunately, water isalso efficiently captured, leading to rapid deterioration of the adsorbents. Carbon adsorbentsare relatively nonpolar compared to the inorganic adsorbents, and hence water absorption is not a significant problem. Carbon tends to exhibit much strongeradsorption properties than organic polymeric adsorbents, allowing the efficient collection of volatile materials such as vinyl chloride. However, the strong adsorption of carbon adsorbents can be a disadvantage. The desorption of target compounds from the carbon tubes is a common problem. For example, carbon molecular sieves used in EPA Method TO-'2 bind aromatic compounds tightly, and a high temperature (=4OO"C) is required to desorb them. Finally, moisture affects the trapping and desorption efficiencyof the charcoal tubes. The adsorption on carbon molecular sieve followed by desorption and GUMS analysis can be used to determine highly volatile nonpolar organics (e.g., vinyl chloride, vinylidene chloride, benzene, toluene) having boiling points in the range of - 15°C to + 120°C. Cryogenic Trapping. The collection of atmospheric organics by condensation in a cryogenic trap is an attractive alternative to adsorption or impinger collection. The primary adthe collection of a wide range of organic materials, (2) vantages of this technique include(I) avoidance of the contamination problem associated with adsorbents other and collection media, (3) the availability of the sample for immediate analysis without further work, and (4) consis-
Soils Contaminated Construction in Highway
677
tent recoveries. But the disadvantage of this method is that it is suitable only for volatile and nonpolar organics having boiling points the in range of - 10°C to +200"C. This technique provides quantitative concentrations of identified species of lower molecular weights suchas CT C,, compounds typically observed in ambient air.However, an important limitation to this technique is the condensation of large quantities of moisture and carbon dioxide and lesser amounts of certain reactive gases. Whole Air Collection. Collection of whole air samples using stainless steel canisters, evacuated glass bulbs, or similar devices is probably the simplest sampling approach. This approach is most useful for relatively stable volatile compounds such as hydrocarbons and chlorinated hydrocarbons with boiling points below 150°C. The canister samplers have the following advantages compared to solid sorbent tubes: (1) Breakthrough does not occur with canister sampling because the actual air sample is collected; (2) no thermal desorption is required; (3) canister pressure canbe used as an indicator of correct sampler operation; (4) analysisof the canister sample can be repeated by using the remainder of the sample in the canister; and (5) the evacuated canisters can be used for sampling without power in the sampling location. SUMMA passivated canister sampling with GC was developed by USEPA to determine semivolatile and volatile organic compounds. The canister is a sampling deviceused to collect and store whole air samples. It can be pressurized, thereby increasing the volume of the collected air sample. It has been demonstrated that the SUMMA passivation process, in which a pure chrome-nickel oxide layer is coated on the inner metal surface, increases the stability and the storage life of many organic compounds. However, certain compounds pose stability problems associated with storageby the formation of an oxide coating. Selection of an alternative container material can circumvent these problems in many cases. The most difficult problem associated with this method is the quantity of moisture collected in the canister. Too much moisture can clog the cryogenic trap and the capillary interface.
GC Due to the complexity of ambient air samples, only high resolution (capillary column) techniques are acceptable for most of the above methods. The GC/MS system should be capable of programing subambient temperatures. Unit mass resolution shouldbe better than 800 amu, and GC/MS shouldbe capable of scanning the 30-440 arnu region every 0.5-1 sec. The measuring device shouldbe equipped with a data system for instrument controlas well as data acquisition, processing, and storage. Impinger bubblershave traditionally beenused for the collection of volatile organics from air. They are charged with a trapping solvent and require much more sampling time for collecting sufficient amounts of the analyte. The growing demand forgreater sensitivity and time resolution suggestedthe development of a system based on solid adsorbent sampling and thermal desorption of VOCs followed by gas chromatographic analysis with cryogenic refocusing techniques. An air sampling and analysis systemfor monitoring VOCs being emitted from the stacks of asphalt plants was designed. It was based on solid adsorbent sampling tubes with thermal desorption GC analysis using capillary column separations with cryogenic refocusing techniques. Air samples were collected using the solid adsorbent Tenax in stainless gas collection . tubes. Stripping of the adsorbed analytes from the sampling tube was accomplished by thermal desorption and followed by gas chromatographic analysis. Preparation of Tenux Cartridges. The following procedure was usedto prepare a 5/8 in. tube containing Tenax cartridges. Prior to use, the Tenax resin was subjected to a series of solvent extraction and thermal treatment steps. The operation was conducted in an area where levels of volatile organic com-
Meegoda
678
pounds (other than the extraction solvents used) are minimal. All glassware usedinTenax purification and all cartridge materials were thoroughly cleanedby rinsing with water followed by rinsing with acetone and drying in an oven at 250°C. The bulk Tenax was placed in a glass extraction thimbleand held in place with a plug of clean glass wool. The resin was then placed in the Soxhlet extraction apparatus and sequentially extracted with methanol and then withpentane for 16-24 hr for each solvent for approximately 6 cycleshr. Glass wool for cartridge preparation was cleaned in the same manner as that for Tenax. The extracted Tenax was placed immediately in an open glass dish and heated under an infrared lamp for 2 hr inside a hood. Care was exercised to avoid overheating the Tenax with the infrared lamp. The Tenax was then placed in a vacuum oven (evacuated using a water aspirator) without heating for 1 hr. Then it was purged with an inert gas (helium or nitrogen) at a rate of2-3 m u m i n to aid the removal of solvent vapors. Then the oven temperature was increased to 110°C while maintaining the flow of inert gas for 1 hr. Then oven temperature control was shut off, and the oven was allowedto cool to room temperature. Before opening the oven, it was slightly pressurized with nitrogen to prevent contamination with ambient air. The Tenax was removed from the oven and sieved through a 4 / 6 0 mesh sieve (acetone rinsed and oven-dried) into a clean glass vessel. in a clean glassjar with a Teflon-lined screw If the Tenax was to be used later, it was stored cap and placed insidea desiccator. The cartridge used for the monitoringof air was packed by placing a 0.5-1 cm long glass wool plug at the bottom of the cartridge and then filling the cartridge with Tenax up to approximately 1 cm from the top. Then a 0.5-1 cm long glasswool plug was placed on top ofthe Tenax. Thecartridges were thermally conditionedby heating for 4 hr at 270°C while purging with an inert gas (helium at 100-200 mumin). The Desorption and GC Settings. The air sampling and analysis system was based on solid adsorbent sampling tubes and thermal desorption gas chromatographic analysis. After collection of air samples, sampling tubes were put into a desorber. Then the VOCs were twice condensed in cryogenic trap to improve capillary column resolution. Before cryogenic refocusing, a purge step served to remove any oxygen that remained inthe tube. This eliminated the problem of the solid adsorbent reactingwith the oxygen when heated, and it also removed traces of water from the tube. Collected air samples in Tenax traps were first desorbed using a Tekmar Model 5010 automatic desorber connected to a Varian 3400 high resolution capillary GC with a flame ionization detector. The two systems were interfaced to automate the entire analysis. The desorption conditions are as follows.
Prepurge Desorb
Cryotrap 1 Cryotrap 2 Transfer Inject
5 min at 10 mUmin 8 min, 210°C, 10 mUmin 150°C 150°C 10 min. 210°C 0.5 rnin, 210°C
The column used for GC/FID analysis was a cross-linked 5% phenyl and 95% methyl silicone gum, 50 m long, 0.2 mm diameter, and 0.5 pm film thickness fused silica bonded high resolution capillary column. Flow rates for the GC were
Contaminated Soils in Highway Construction
679
30 mUmin 30 mUmin, 1 mUmin
Hydrogen Air
Carrier gas Make-up gas
30 mUmin
The temperature program was (1) an initial temperature of 15°C for 8 min and (2) temperatures programed up to 210°C at 4"CImin. The detector range was selected as 10. A schematic diagram of the analytical system is shown in Figure 7. Calibration and QuantificationofData. External standards were used to calculate a response factor for each compound of interest. The process involvedthe analysis of four calibration levels for each compound during a given day for the determination of the response factor (area/ picomole). The linear least squares fit of a plot of picomoles versus area was used for the determination of the response factor. If substantial nonlinearity is present in the calibration curve, a nonlinear least squares fit should be employed. This process involvesfitting the data to the equation
Y
=
A
+ BX + DX2
where Y is the peak area, X is the quantity of the component in picomoles, and the constants
A, B , and D are coefficients to be determined fromthe regression analysis. If the instrumental response is linear over the concentration rangeof components, a linear equation (D = 0 in the equation above) can be employed. The systemdetection limit for each component can be calculated from the calibration standards, where the detection limit is defined as
DL
=A
+ 3.3SD
where DL is the calculated detection limit in picomoles, A is the intercept of the regression analysis equation, and SD is the standard deviation of the samples with replicate determinations of the lowest concentration level. The standard gas used for calibration consisted of a mixture of chloromethane, hexane, chloroform, 1,1, l-trichloroethane, carbon tetrachloride, trichloroethene, toluene, trichloroethylene, benzene, mpxylene, and o-xylene. These compoundswere injected into an evacuated and clean 13-L stainless steel cylinder, and the cylinder was pressurized with zero grade helium. The preparation and analysis of the standard was done by Alphagaz, Edison, New Jersey. A 2-mL volume sampleloop was used for the GC analysis. When the sample loop wasflushed, the standardgas mixture passed throughthe sample loop. A three-port valve witha switch was used to allow helium gas to pass throughthe loop, flushing the standard gas onto a Tenax cartridge. The compoundsin the standard gas were adsorbed by Tenax and quantifiedby gas chromatography. The following ideal gas equation was used for the calculation of the component concentrations in the standard gas. n = PVC/(R/T) where P is pressure (inatm), V is the volume of the sample loop, C is the concentration of each compound expressed as a fraction, R is the universal gas constant (0.082), Tis temperature (in kelvins), and n is the amount of each standard gas injected (in moles). At 25"C, sample concentration in ppb can be expressed as Sample concentration =
area of sample X C X 24.5 X lo9 area of the xv
Meegoda
680
Automatic Desorber Tenax trap with contaminants
i
IOpsig, I mVmin and then desorborganics
Printer
t Computer
t GC Server 4 A
i
,
,..., .~,...
.,
Air, 6Opsig 30 d m i n
Hydrogen, 4Opsig 30 mVmin ' , -
.
. . . .
Nitrogen, 8Opsig 30 mumin ;make up for capillary column
...........
:
*
.....................................: *
High Resolution CapillaryG a s Chromatograph Figure 7 A schematic diagramof the analytical system usedfor the air quality analysis. (From Mesoda et al. [g].)
where C is concentration of the compound in standardgas and V is the volume of the sample in liters. If V is at a different temperature and/or pressure, then the volume of gas should be converted to that at 25" and a pressure of 1 atm. The air samples were collected by drawing air through theTenax cartridge, a 5/8 in. stainless steel tube packed with 1.5 g of 60/80 mesh Tenax using a vacuum pump. Figure 8 shows air samples from stacks. Samples were drawn at approximately 500 the schematic for extracting &min. In this experiment, 10 Tenax blank traps were spiked with known quantities of standard and then desorbed into the analytical system. The reproducibility can be expressed by the coefficients of variation(CV) of the target compounds. The CV andDL values for all the tested organic compounds are shownin Table 8.
Contaminated Soils in Highway Construction
681
Tenax Cartridge
Meter
L
x
l
+
Vacuum Pump
Valve
Reducing Union
I
.
/
l Tenax
EndCap
\
Glass Wool
Swagelok Fitting
Vent
Metal Cartridge
Figure 8 Schematic diagrams of the setup used for air quality analysis. Prom Meegoda et al. [8].) Test Datu. An air sampling and analysis system was designed to monitor volatile organic compounds emitting from stacks of asphalt plants. It was based on solid adsorbent sampling tubes with thermal desorption gas chromatographic analysis and used capillary column separations with cryogenic refocusing techniques. Air samples were collected using the solid adsorbent Tenax in stainless gas collection tubes. Stripping of the adsorbed analytes from the sampling tube was accomplishedby thermal desorption and followedby gas chromatographic analysis. During the field study, air samples were drawn for3 min from the stackin the asphalt plant PCS was prepared.The air samples were drawn through while the HMA concrete blended with a filter to remove the dust going into the Tenax cartridge. The Tenax cartridge was connected 3 min for test, and the Tenax samples to a vacuum pump. Three air samples were obtained for soils were analyzed for concentrations of target organic compounds. Test results (firsttwo day, are shownin withfourdifferentblends;secondday,onesoilwiththreedifferentblends) Table 9.
Meegoda
682
Table 8 Method Specifications and Regulatory Limits for Fraction Released (Proposed N.J.) (ppb)
(PPW
DL Compound 270 Chloromethane NIA Dichloromethane NIA Hexane 6 Chloroform 494 1,1,1 -Trichloroethane 13.8 Trichloroethylene 16.7 Benzene 988 Toluene 9.9Tetrachloroethylene 148p,m-Xylene 148o-Xylene
Regulatory level as average flux (mg/m2* day)
CV
0.54 0.13 0.03 0.44 0.22 0.05
0.07 0.06 0.03 0.05
0.06
20.5% 9.2% 11.8% 9.8% 10.5% 21.4% NIA 6.7% 8.7% 7.5% 9.5%
N/A =not available.
Areas of all peaks were added to computethe total nonmethane organic carbon (NMOC) emissions. A standard propane/heliumgas mixture was usedto obtain the GC calibration curve for concentration. Various known amounts of the mixture were injected to the GC and the peak areas were obtain to establish the calibration curve. Then the concentration corresponding to sum of all peak areas was calculated from thecalibration curve and reported as total NMOC in Table 9. Figure 9 shows a comparison of totalNMOC emission from a regular asphalt plant and that uses PCSs (day 2 test 3 in Table 9). The concentrations of the target chemicals were less than 1 ppm, and total concentrations of VOCs were lower than the NJDEPE specification for regular asphalt plants (c250 ppm). Table 9 AirQualityTestResults
Concentration in ppb
otal Cs Hexane 0-Xylene Xylene P&M Toluene No. Test 8
1
ant
2 3 4 2 2 2 plant around
Day581 Test 1 52 Day 1 Test Day 1 Test Day 1 Test Day Day Day Conc. levels 2,650 149 after stopped Background conc. Stack conc. from an ordin. HMA plant without PSCs
52 90
69
53 14,880
99155,230 11 4
2
25
2
24
16
Source: Meegoda et al., 1992 [9].
6
11 283
0
1,210
0
26,700
Soils Contaminated Highway
in
Construction
683
NMOC
100 I
80
60
40
20
n "
Normal HMA
=
PPmC
IIMA with PCSs I b Carbonlhr
Figure 9 Comparison of non-methaneorganiccarbon (NMOC)emission by aregularasphaltplant and one that uses PC%. Solid bars, ppm carbon; hatched bars, pounds of carbon per hour. (From Meegoda et al. [ g ] . )
Furtherdetailscan be fohndinthe 1991 report to NJDEPE ontheuseofpetroleumcontaminated soils in construction material production [ 121.
111.
SUMMARY AND CONCLUSIONS
Leaking underground storage tanks (USTs) are one of the primary sources of groundwater contamination in the United States. The soils contaminated by leaking USTs are treated as solid waste. The quantities of such soils are projected to increase substantially over the next few years. In this report the feasibility of using petroleum-contaminated soil (PCS) in the production of asphalt concrete is discussed. When F'CSs are used in asphalt production, three beneficial actions occur: incineration, dilution, and solidification. An in-depth laboratory study was performed to determine the feasibility of producing HMA with PCSs. For the heating oil-contaminated soil used in this study, it was possible to include upto 35% PCS based on the total weight of the aggregates in the HMA mix. This value is much higher than any reported in the literature. The impact with respect to strength and durability of asphalt concrete dueto the additionof PCS to HMA was also evaluatedby performing the Marshall stability test. The test results showed that HMA with PCS produced a much better paving material than the control. The extended durability test showed that one cycle of freeze-thaw and wet-dry was sufficient to evaluate the durability of HMA with PCS. The durability of HMA produced with PCS was found to be the same as that of the control mix,
684
Meegoda
suggesting that there are no harmful effects from the addition ofPCS. The hydraulic conductivity of HMA with allPCSs produced asphalt concrete with hydraulic conductivities less than 2.0 x A new leachability test was designed for the monolithic sample. The leachability of hydrocarbons based on the EPA purge-and-trap method showed that maximum release was less than 25 ppb after the specimens were immersedin analytical water and subjected to4 days of vigorous shaking. To further evaluate the leaching of contaminants, a long-term leaching test was performed. The test results show no significant increase in contaminant concentrations with time for all the compounds tested. An extensive field studywas conducted to demonstrate the applicability of this process. An air sampling and analysis system was designed for monitoring volatile organic compounds being emitted from asphalt plant stacks. It was based on solid absorbent sampling tubes with thermal desorptiongas chromatographic analysis using capillary column separations with cryogenic refocusing techniques. The concentrations of the target chemicals were less than1 ppm, and the concentrations ofVOCs were lower than the NJDEPE specification for regular asphalt plants (<250 ppm). be given to this On the basis of these results it is recommended that much greater attention process because it is a highly cost effective recycling technology.
1. USDOT, Selected Highway Statistics and Charts, Federal Highway Administration, U.S. Dept. of
Transportation,1988. 2. Collins, R. J., Assimilation of waste and by-products into the highway system: status report and regulatory influences, Second Interagency Symp. Soil Stabilization, New Orleans,La., November 1992, Section 7, pp.3-11. 3. Flynn, L., Recycling: will roads become “linear landfills?,” Roads Bridges, October 1992, pp. 65-70. 4. Czamecki, R., Making use of contaminated soils, Civil Eng., December 1988, pp. 72-74. 5. Czamecki, R., Hot mix asphalt technology and cleaning of contaminated soils, inPetroleum Contaminated Soils, Vol. I1 (€?Kostecki and E. J. Calabrese, ed~.),Lewis, Chelsea, Mich., 1989, pp 267-277. 6. Fairweather, V., U.S. trackless leaking tanks, Civil Eng., 6 0 , 46 (1990). 7. Asphalt Institute, The Asphalt Handbook. College Park, MD, Asphalt Institute, MS4, 1989. of petroleum contaminated 8. Meegoda, N. J., Huang, D. R., DuBose, B., and Mueller, R. T.. Use soils in asphalt concrete, in Hydrocarbon Contaminated Soils, Vol. I1 (I?T. Kostecki, E. J. Calabrese, and M. Bonazountas, eds.), Lewis, Chelsea, Mich., 1992, Chapter 31,. pp. 529-548. 9. Meegoda, N. J., Huang, D. R., DuBose, B. H., Chen, Y.,and Chuang, K. Y., Use of Petroleum of EnvironContaminated Soilsin Construction Material Production, Second Report to N.J. Dept. mental Protection and Energy, Div. Sci. Res., Submitted by the N.J. Inst. Technology, Newark, N.J. January1993. 10. Eklund, K., Incorporation of contaminated soils into bituminous concrete,in Petroleum Confamimted Soils, Vol. I (P Kostecki and E. J. Calabrese, eds.), Lewis, Chelsea, Mich., 1988, pp 191199. 11. Tsuge. S., Matsushima, Y., Watanabe, N., Shintai, A., Nishimura, K., and Hoshia, Y., New automated thermal desorption system for gas chromatographyof volatile componentsin environmental and polymeric samples, Anal. Sci. 3, 101-107 (1987). Y.,Use of Petroleum Contaminated 12. Meegoda, N. J., Huang, D. R., DuBose, B. H., and Chen, Soils in Construction Material Production, Interim Report to the N.J. Dept.of Environmental Protection, Division of Science and Research, submitted by N.J. Inst. Technology, Newark, N.J., August 1991, pp.1-80.
34
Management of Waste Compressed Gases
Dan Nickens Earth Resources Corporation Ocoee, Florida
1.
INTRODUCTION
Professionals in the field of hazardous waste management are increasingly faced with problems associated with waste compressed gases. The problems stem from the specialized hazards posed by pressurized materials. The commonuse of gases increases the likelihood of encountering this type of waste. Gases, in compressed or liquefied form, are prevalent in industrial and governmental a p plications. These materials are also commonly used in many households (e.g., propane cylin(i.e., oxygen ders for outdoor gas grills). Gasesare required in hospitals for medical purposes and anesthetic gases), and agriculture employs gases on a routine basis (i.e., anhydrous ammonia for fertilization of crops). Cutting and weldingare most commonly completed by using compressed gases. A compressed gas is defined by the American Society for Testing and Materials (ASTM) as any material or mixture having an absolute pressure exceeding 40 psi at 70°F.Regardless of the pressure at this temperature, a compound that has a pressure exceeding 104 psi absolute at 130"F, or any liquid flammablematerial having a vapor pressure exceeding40 psia at 100°F is classified as a compressed gas (ASTM Test D-323). There are two major categories of compressed materials: compressed gases and liquefied compressed gases. Liquefied compressed gases condense at normal ambient temperatures under pressures of up to 2500 psig. In addition to these major classes, gases can be generally divided into groups related by common origins, properties, or uses. These include atmospheric gases, gases produced through fractionation (argon and the rare gases), fuel gases, refrigerant gases, and poison gases. Options available for management of waste gases are more limited than for almost any greater for gas other type of waste. Constraints on safe management techniques become even cylinders that are in poorcondition.Proceduresandtechnologiesthathaverecentlybeen 685
686
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developed offer increased capabilities and important improvements over historical practices for safe and proper management of these wastes.
II.HEALTH
AND SAFETY CONSIDERATIONS
Waste compressed gas cylinders pose special hazards that are peculiar to these materials. The potential hazards include all those that are associated with other chemical wastes, and in addition there are hazards associated with the energy of compression and the mobility of the materials when released. Many compressed materials exhibit chemical hazards that must be taken into consideration in managing their disposition. These hazards include characteristics of ignitability, reactivity, corrosivity, toxicity, and combinations of these characteristics. It is extremely important to identify the contents of compressed gas cylinders to determine the nature of the risks posed by the wastes. The very nature of compressing these compounds generates physical hazards from possible rupture or other uncontrolled release. A 1 S-ft3 cylinder pressurized to 2000 psig contains an energy equivalent to 1.4 lb of TNT. If cylinder failure occurs, this energy is released in an uncontrolled and extremely dangerous manner. of asphyxiation. Any Many gases that are not otherwise hazardous can present the danger gas capable of displacing oxygen offers a potential hazard from asphyxiation. This is most common in confined environments with limited air circulation. The asphyxiation hazard posed by some gases is reduced by their good warning properties. For example, any worker entering an environment that contains high levels of ammonia will immediately recognize its presence. Other gases, however, are more insidious. Nitrogen is particularly hazardous because of its poor warning properties and its common use in enclosed areas. Other gases that tend to accumulate in low-lying areas (e.g., sulfur hexafluoride) can displace oxygen in these areas. Other characteristics of compressed gases have more direct dangers. Many gases can be extremelytoxic to humansthroughinhalation or contactwiththeskin or eyes.Some gases can be hazardous simply because of cryogenic temperatures or cooling associated with volatilization. Many toxic gasesare used in commercial applications. Examplesof these extremely toxic gases include arsine, phosphine, germane,and diborane, which are used in the electronics industry. Injuries and fatalities have resulted from accidents involving these gases. The extreme toxicity of some gases is illustrated by exposure limits establishedas low as 0.001 ppm. Toxic gases affect humans primarily through respiration. When released from pressurized cylinders, toxic gases can rapidly reach lethal concentrations in surrounding areas. Liquefied toxic gases suchas chlorine are of particular concern due to the sheer volume of material that can be stored as a liquefied compressed gas. Flammable gases that present both fire and explosion risks are some of the most common gases (e.g., propaneand acetylene). Flammable gases must be isolated from sources of or heat ignition. Leaks of flammable gases can traverse long distances to sources of ignition, resulting in explosions and fire. Reactive materials are often present in compressedgas cylinders. These include powerful oxidizers suchas fluorinated compoundsand oxygen. Oxygen can cause an explosion when the released gas contacts organic materials. Fluorine is even more reactive and can ignite metal in otherwise inert environments. Many gases usedas rocket fuels (suchas hydrazines and chlorine trifluoride) are capable of explosive reactions at very low initiation energies.
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Gases
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Corrosive gases form another classification of hazardous gases. Examples include anhydrous hydrogen chloride and hydrogen fluoride. On exposure to moist air, many gases will react to form acids. Some unstable gases are inherently hazardous due to their potential for exothermic polymerization. These include such gases as hydrogen cyanide, ethylene oxide, and acetylene. The handling of a cylinder containing unstabilized hydrogen cyanide resulted in an explosion that destroyed windows several blocks removed from the blast site in Texas. Acetylene in its free state is known to explosively decompose at pressures above 15 psig (it is also shock-sensitive in the liquid and solid states). Ordinarily, reactive mixtures of gases are not stored in a single cylinder. Manifolding of cylinders can result, however, in the creation of potential bombs. Such a cylinder containing silaneandnitrousoxidedetonatedduringhandling,killingthreeworkers inNewJersey. The resultant fii destroyed the laboratory building and forced evacuation of the surrounding neighborhood. Hazards are often associated with materials stored in compressed gas containers even though they may not be pressurized. A tragic example was an accident involving cylinders at a waste disposal facility. Residual contents of “empty” devalved cylinders exposed several unprotected workers to lethal vapors. Compressed gas cylinders are commonly used to stored a variety of hazardous nonpressurized materials. The hazards associated with this situationare illustrated by a fatal accident. A lecture bottle containing pentaborane (a liquid at ambient temperature and pressure) was removed from a sampling device by an unprotected worker. Even though there would have been no indication of residual pressure, volatilization killed the worker and a bystander and injured others in the vicinity, including rescue workers. Even though the original material may have been introduced in the gaseous phase, polymerization or other degradation reactionsmay result in the formation of liquids or solids in the cylinder. Other hazardous materials storedin cylinders include radioactive gases. Biological agents be isolated from have also reportedly been stored in cylinders. Almost any material that should the environment for some reasonmay be contained in a cylinder. All of these hazards mustbe addressed to safely manage waste compressedgas cylinders. When the contents of a cylinder are not positively identified, it must be assumed that any of these hazards may be present.
111. REGULATORYCONSIDERATIONS The management of waste compressed gas cylinders is governed by federal, state, and local regulations. Federal regulations apply to the interstate movement and filling of cylinders. Hazardous waste management and air pollution regulations may be applicable. Local buildingand fire codes also apply to cylinder use and storage. and reFederal Departmentof Transportation (DOT) regulations govern the transportation filling of compressed gas cylinders. Under these regulations (found in 49 CFR), appropriate containers must be used for eachgas or liquid. be stamped to indicate information concerning DOT regulations require that each cylinder bethe rating and manufacture of the cylinder. High pressure cylinders have DOT ratings of tween 900 and 6OOO psig. Low pressure cylinders range from 240 to 500 psig. Acetylene cylinders are included in a special classification (the container is filled with a porous material and the gas is dissolved in acetone).
688
Nickens
In order to be refilled, cylinders must meet DOT requirements forstructural integrity. Periodic hydrostating of the vessel is required. Standardshave also been developedfor corrosion. Conditions thatmay makethe cylinder unsafefor transport include extreme corrosion, bulging, or damage to the vessel or its valve. Prior to transportation, a cylinder must be carefully inspected to ensure that it is suitable for safe shipment. A leaking cylinder cannotbe legally transported and should be handled only be obtained fromthe owner of the cylinder if the by professionally trained workers. Support can owner is a major gas supplier. been misAn exemption inthe regulations allowing the shipment of laboratory samples has used to move unknown gases in lecture bottles. The exemption applies only to the quantity of material necessary for analysis. Since analysis of gases can be accomplished with extremely small volumes, the exemption does not apply to lecture bottle quantities. The Resource Conservation and Recovery Act (RCRA) specifically includes compressed gases in its definition of solid wastes. A waste gas is classified as a hazardous waste if it has a listed characteristic (ignitability, reactivity, or corrosivity) or if its components are listed as hazardous. Historically, there hasbeen a differentiation betweenthe handling of compressed gas cylinders that are owned by gas suppliers and thoseowned by a generator. If a gas supplier rents the cylinderto the user, it can be returned when it is no longer needed. The suppliers treat any residual gases as part of their manufacturing process under operating (air discharge) permits. This hasbeen used as a vehicle for avoiding having to treat residual gasesas hazardous wastes. Where title to the cylinder and its contents is transferred to the user, the user may be subject to RCRA regulations for disposition of residual waste gases. Under these regulations, a container approaching atmospheric pressure is considered tobe empty and need not be handled as a hazardous waste. Only those cylinders with residual pressure must be treated as a hazardous waste. Even though the regulations may not cover cylindersat atmospheric pressure, the contents may still be hazardous. Improper handling of these materials can result in injury or death. Waste compressed materials subject to RCRA regulations must be disposed of at a permitted facility. Appropriate disposal options include incineration and chemical treatment. Very few options presently exist in the United States for disposal of compressed gases at permitted facilities. The listof gases that can be disposed of at these facilities does not include all compressed gases. One commercial disposal facility handling compressed was gasesclosed by state regulatory offkials in 1991. The Aquatech facility in Greer, South Carolina nowisa Superfund site. A New Jersey facility specializingin waste compressed gases closed in1986 after a worker was killed. To accept a compressed gas for disposal, the contentsof the cylindermust be identified and the valve mechanism must bein operable condition. No commercial facility can legally accept gas cylinders whose contentshave not been positively identified. Because of the limited disposal options, some users have previously obtained permission for disposal by detonation of the cylinders at remote sites. For this operation, shaped charges are attached to the cylinder and detonated. This may also include ignition of fuel surrounding the cylinder to bum flammable gases. This procedure is not effective with many hazardous gases. Its use has been severely curtailed by responsible regulatory officials. The risks associated with detonation of unknown cylinders as a disposal optionhave been illustrated on many occasions. For example, several unidentified gases were detonatedat a remote site. In one case, the charges failed to completely sever the cylinder. The resulting explosion rocketed portions of the cylinder over several hundredyards, releasing a toxic gas over a broad area.
Management Compressed of Waste
Gases
689
Even if hazardous waste management regulations are inapplicable, air discharge regulations may cover management of a compressed gas. An example is sulfur dioxide. mically, state air regulations apply to emissions, or potential emissions, without regard to treatment systems. Local building and fire codes often apply to Compressed gases.These may cover the storage of cylinders. Some local regulations now include aspects of storage such as emergency containment and treatment. Codes developed by associations such as the National Fire Protection Association may be adopted by local agencies for compressed gas cylinders.
IV. IDENTIFICATION OF CYLINDER CONTENTS Where identification of a compressed gas is necessary, safety of the sampling effort becomes a critical concern. Because of the chemical characteristics of some gases, fire or explosion or other release of toxic gas may occur during sampling. When cylinders or valves have deteriorated, it is very possible to have failures resulting in uncontrolled release of the contents. Procedures and technologiesthat have historically been usedfor management of cylinders in poor condition have not always met adequate safety standards. These techniques have included uncontrolled release, valve removal, and uncontained tapping. Uncontrolled release has historically been the methodology employed for problem cylinders. This includes penetration of the cylinder shell with projectiles or explosives. Variations have provided this penetration in conjunction withfire pits. None of these provide for protection of the environment and rely upon distance for protection of personnel. One proposedvariation of this technique involves explosive detonation inside a “gastight” bomb chamber. With some gases., however, the energy released by the detonation can easily exceed the maximum rating of any chamber that can be built. Valve actuation insidea containment unit canbe accomplished ina foreign-designed overpack vessel. This procedure is ineffective where the valve is blocked, defective, damaged, or failed in a closed position. Experience has also raised concerns over failures associated with gases passing through dynamic seals of the containment unit. Major compressed gas companies with this type of equipment will not permit its use with many hazardous gasesor for unknown cylinders. In a variation of this technique, the valve may be removed rather than actuated. It is possible, however, for the valve to break off or for the cylinder orifice to be completely blocked. Either of these problems will result in a much more hazardous situation following the attempt. One often proposed technique involves cold or hot tapping of the cylinder. A similar technique is used for low pressure gas lines. Guidelines developedby the American Petroleum Institute (API) clearly demonstrate, however, that it is inappropriate and highly dangerous in situations involving high pressure cylinders with unknown contents. Guidelines for tapping are publishedinAPIPublication 2201 (third edition, October 1985). Bum-through prevention cannot be ensured unless base metal thicknessis greater than 3/16 in. Very few cylinders meetthis requirement. The procedure requiresthat the base metal be free of laminations, corrosion, and other imperfections. Further, hot tapping cannotbe done where cylinder contentsmay contain oxygen-vaporair mixtures in an explosive range; hydrogen, acids, chlorides, peroxides, or chemicals that may explosivelydecompose(includingacetylene); caustic soda or amines; or unsaturated hydrocarbons (e.g., ethylene). Unless the contents and pressure are absolutely known, this procedure cannot be used. The cold tapping proceduremay be used for known low pressure gases. It is crucial, however, that the cylinder contentsbe known with certainty. For example,a high pressureinert gas,
690
Nickens
often used with some liquefied gases, could overpressure the device. Because of the tenuous nature of the gasket seal, a suitable surface must be prepared. As with any extraordinary procedure, provision must be made for emergency containment and response. A proposed variation of this procedure is installation of a new valve in the body of the cylinder. Along with previously cited technical objections, this is hazardous because the cylinder shell may have been weakened by corrosion. A major problem with these techniques is graphically demonstrated by reactions involving strong oxidizers. Even in an inert environment, exposure of bare metal to fluorine gas will result in an ignition that destroys the cylinder shell and any associated appurtenances. sufficient Adequate records and labels documenting the origin and use of cylindersbemay for identification of cylinder content. If, however, there is a potential for mixing of different gases (e.g., by common manifolding) or other reason to suspect the accuracy of records, further efforts to obtain confirmatory identification are necessary. Prior to developing a sampling approach it is desirable to obtain as much information as possible as to possible contents. Inventory records, usage history, and personnel interviews may be useful. This typeof information may allow the investigationto be focused on probable contents. After background information has been obtained, the cylinder should be inspected. Depending upon the circumstances, personal protective equipment, including supplied air and encapsulating suits,may be required. A high level of protection is appropriate when the cylinders potentially contain hazardous materials (this mustbe assumed unless adequate information to the contrary exists) and if the cylinders are stored in an enclosed area. The inspection should be conducted in a manner that minimizes disturbance of the cylinders. Cylinders in poor condition may either fail or begin to leak if moved. Simply moving cylinders with unstable contents (e.g., unstabilized hydrogen cyanide or tetrafluorohydrazine) can cause detonationof the contents. It is especially important to avoid manipulation of the cap or valve until it has been deare in place. termined that the cylinder is stable and emergency response procedures be familiarwithcompressedgases and The personconductingtheinspectionshould be sufficientlyexperiencedtoevaluatethepotentialhazcylinders.Theinspectorshould ards associated with these containers and their contents. As with other potentially hazardbe completed without experienced, professional ousoperations,theinspection should not assistance. Labels can be used for preliminary indicationof contents. The labels will typically be applied to the body or shoulder of the cylinder. Unfortunately, these are not indestructible and be misleading whereit has may no longerbe either readableor even present. Labeling may also not been updated or contaminants have been introduced. DOT requires that the Some cylinder information is stamped into the body of the cylinder. and manufacturer by identified on the cylinder type,service pressure, serial number, test date, body of each cylinder. Although these are usually preserved, in some cases corrosion can obscure this information. DOT has developed specifications for cylinder types based on classes of gases.DOT has also adopted recommendationsof the Compressed Gas Association for valve typesto be used with most gases. An example of the information obtained from valve type is shown by identifying a cyl3AAwithaCGA 580 valve.Thesecylinderandvalvetypes are speciinder as a DOT fied for inert gases such as nitrogen. It should not, however, be assumed that this will always be the nature of the contents. It is not uncommon for cylinders to be filled outside of these guidelines.
Gases Management Compressed of Waste
691
These conventionsdo not applyto cylinders of lecture bottle size. These cylinders contain only small volumes (less than 112 L liquid volume). In recent years a color code has been adopted and used for medical gases. Color of the cylinder is typically not, however, a reliable indicator of contents. Most manufacturers have different color codes, and these may not have been historically uniform. Some “experts” offer cylinder identification based solely on visual inspection. Relying on The technical director for the Compressed Gas this type of identification is risky and negligent. Association has written that these consultants are like “snake oil salesmen.” The general appearance of the cylinder is important to developing a safe sampling plan. The overall condition of the cylinder should be noted, especially with respect to denting or corrosion. Evidence of exposureto fire may be presented by burn marks. If possible the valve should be examined without unduly disturbing the cylinder. If the possibility of radioactive gases cannot be eliminated, the initial inspection should include a scan for external radiation. Any detection of radioactivity shouldbe cause to develop appropriate protective procedures. Evaluation of information obtained during the inspection will permit preparation of a suitable sampling plan. The sampling approach is largely dependent upon the degree of certainty with which the cylinder contents can be deduced and the potential hazards associated with a release. Only cylinders in good condition shouldbe considered for sampling through the cylinder valve. Handling should be minimized in any case until the contents have been identified. Safely obtaining a suitable sample is the principal challenge in the identification process. The degree of hazard is largely dependentupon the contents of the cylinder. If there is no indication of the contents, a worst-case approach must be used. As with samplingof other types of unidentified pressurized wastes, sampling must be completed using remote techniques with an explosion-resistant barrier [see regulations promulgated by the Occupational Safety and Health Administration, 29 CFR 1910.120(j)]. Cylinder carts or forklifts with suitable attachments are useful for moving larger cylinders. During any handling the cylinder should be monitored for any temperature increases associated with polymerization of unstable gases. Unless the natureof the cylinder contents canbe determined with reasonable certaintyat this stage, remote sampling procedures are required. The mere operation of a cylinder valve has type have included gases suchas hydrogen been known to result in detonation. Incidents of this and ethylene oxide and mixtures suchas a mixture of deuterium and oxygen. Accessing the cylinder valve may pose a potential problem. It is common for the protective cap to corrode to such an extent that it cannot be unscrewed. Upon removal of the valve cap, the valve itself shouldbe carefully inspected. To be conbe in good condition with sidered for the remote valve sampling operation, valve threads must an adequate sealing surface. Discoloration, excessive valve corrosion, or damaged threads are cause for discontinuing the operation. The construction of sampling apparatus mustbe compatible with the expected gas.If the of contents are unknown, universal compatibility is required. This typically includes the use stainless steel, passivated steel,or Teflon that has been cleanedof all contaminants. The Compressed Gas Association (CGA) specifies cleaning procedures for use in oxidizing environbe capableofsafely ments(oxygen or fluorineservice).Further,allcomponentsmust containing the maximum pressure that may be exerted by the compressed gas. It is critical that the atmosphere within the sampling system be inert. Exposure of pyrobe ignited at mom temperatures) to air can result in combustion or phoric gases (gases that can explosion.
. 692
Figure 1 Valve sampling system. (Courtesy of Earth Resources Corporation,
Nickens
Ocoee, Florida.)
Obtaining a minimal sample quantity at less than atmospheric pressureis desirable. This can be accomplished by using appropriate controls. Should leakage occur during sample transport, it will be into the sample container. Provisions should be made for response to valve failures during sampling. It is not uncommon for leakage to occur following valve actuation, especially with packed valves. Treatment or containment options should be provided for this eventuality. It is also possible for a valve to fail during opening, preventing closure. Roughly 1-3% of sampled waste cylinders have experienced some typeof valve failure. For the remote operation to provide protection against potential explosive reactions that could occur during sampling, the cylinder must be isolated. This canbe accomplished by erecting barriers such as sandbags or a containment chamber. An example of a mobile sampling system is shown in Figure 1. A valve opening mechanism consisting of a pneumatically actuated wrench has been developed. The wrench contains adaptersto allow it to fit various types of valves. The control is operated from a remote location outside the protective barrier. Valve movement does not always indicate that the cylinder contents have been accessed. This must be verified by positive means. It is possible that there is internal blockage or the valve stem has broken inside the valve body. The remote valve opener should be constructedto permit maximum application of toque without overstressingthe valve. If this mechanism is incapable of actuating the valve, it cannot be safely opened. Cylinders or valves that are in poor condition requirespecial handling procedures. These procedures are also required where the valve is inaccessible or cannot be operated.
Management Compressed of Waste
Gases
693
A patented device has been developed for sampling under these circumstances-the Cylinder Recovery Vessel (CRV). This equipment permits remote release of the gas in an inert contained environment. Figure 2 shows the key components of this sampling system. The Cylinder Recovery Vessel was designed to control and contain all common compressed gases and liquids. The system provides for remote release and recontainerization of pressurized gases and liquids in a completely contained inert environment. The CRV is an ASME-rated pressure vessel.The waste cylinder can be pierced by a drilling mechanism housed within the vessel. Priorto drilling, a vacuumis obtained andan atmoare hydraulically actuated, removing sphere of inertgas is introduced. All internal components possible sources of ignition. The interior of the vessel and its associated systems are composed of passivated steel, stainless steel, or Teflon. All hydrocarbons and other reactive materials are excluded from possible contact with cylinder contents. The hydraulic fluid used for CRV components is an inert fluid. A secondary containment chamber housesthe CRV and its systems. This reinforced steel chamber issealed to contain any release from the primary system. All of the equipment in the chamber is suitable for operation in a Class I, Division I1 explosive environment. Ports are attached so that any released gases can be withdrawn and treated in the unlikely instance of a leak in the primary system. Temperature inside the chamber be cancontrolled by a heating and cooling unit. All operations are controlled from a panel located outside the trailer. In this manner personnel are isolated from the sampling operation. After contents of the cylinder are released inside the CRV, a sample can be withdrawn of the trailer. An evacuated sample cylinder attachedto through a port extending to the exterior the port is opened to obtain a small volume of the contents for analysis. On-site analytical equipment provides an identification of the contents within minutes. The CRV provides for recontainerization of both gasesand liquid cylinder contents. The cylinder contents are transferred to a new DOT-approvedcylinder for subsequent disposal. The contents can also be transferredto an appropriate treatment system. Appurtenances to the vessel include a cylinder clamping and locking mechanism and a roller mechanism for rotating liquid cylinders. Diaphragm compressors complete the purging and recontainerization process. Included in the operation are controls to verify the functioning of the equipment. The entire operation is monitoredby a remote video camera that shows the interior ofCRV. the Pressure and temperature are monitored at numerous points in the process system. These systems allow the operator to effectively control the process. When the cylinder processing has been completed, the empty target cylinder is removed from the chamber. The cylinder is then cut into halves with a power saw. Any solid residue inside the cylinder isremoved and containerized. The clean, empty remains of the cylinder is containerized and staged for disposal. Analyses of gases canbe completed with a variety of instrumentation. m i c a l techniques used include mass spectroscopy (MS), Fourier transform infrared spectroscopy (FTIR), gas chromatography (GC), and wet chemical testing. Gas chromatography is useful when the identity of the gas is known or suspected. The technique provides for selective absorption and elution in a column. m i c a l l y a standard of the test gas is required. The mass spectrometer (or residualgas analyzer) is a vacuum analyzer that will measure total pressure and partial pressure. The analyzer is typically capable of separating the ions
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Gases
695
formed in an electron impact source according to their mass-to-charge ratio. The signal collector may be either a Faraday cup or a secondary emission multiplier. The FTIR can be used for identification of unknown gases based on absorbance spectra. The infraredspectrumcontains characteristics thatpermitidentificationofthefunctional groups or “working parts” of molecules. Through the use of an interferometer, infrared wavelengths are passed through a sample simultaneously. After obtaining the sample transmittance, the spectrum is mathematically converted to absorbance. The absorbance spectrum is then compared to spectra contained in various libraries. This can be accomplished through computer programs. Wet chemical methods can also be used. ’ZLpically this will involve reaction with a variety of reagents and calorimetric materials. These are inexpensive procedures that can be used if the gas is known. With unknown gases the methodology can be extremely hazardous. Information generated in the analytical process may be insufficient to provide an identification of the cylinder contents. Inmany cases it is necessaryto have associated observations to reach a reasonable interpretationof the results. A typical problem in obtaining an analysis is related to the reactivity of the sample gas. Many gases will react during the sampling or analytical process to form other compounds. The analysis will generally show the reaction products and not the cylinder contents. An example of the problems associated with obtaining a sample of a reactivegas is illustrated by fluorine. Because of its extreme reactivity, it is likely that the sample will react with some contaminant (residual wateror air or unpassivated surfaces) during the process. The gas can similarly react within the analytical equipment, evento the point of damaging the instruments. Analysis may show reaction products such as hydrogen fluoride. Most analytical libraries of spectraare incomplete with respectto compressed gases. Further, depending upon environmental conditions, the spectramay appear to be different. Complicating the analysis are the many potential mixtures that may be present. This can lead to some ambiguity in interpretation. After tentatively identifying a compound, it is importantto relate it back to the physical characteristics observed during sampling. For example, it is easy to confuse the varietyof hydrocarbon spectra. If significant pressure was noted during sampling, hydrocarbons with lowa vapor pressure can be eliminated. The same conclusion cannot, however, be reached where there are additional components indicated (e.g., metal anhydridesmay be dissolved in an organic solvent). All of the observations made during the analytical process should be combined with experience with compressed gases and cylindersto reach a reasonable interpretation. The sampling and identification process is fraught with potential hazards for the unsusbe professionals with extensive expecting. It is crucial that those involved with the sampling perience in handling compressed gases. Unfortunately there have been examples of contractors misrepresenting their experience and capabilities. Given the ramifications and liabilities associated with an accident, careful screening of contractors is imperative.
V. EXAMPLEMANAGEMENTPROGRAMS The Los Alamos National Laboratory is a research facility operated by the University of California for the U.S. Department of Energy. Since its inception1942, in the laboratory has han1990 a facility-wide dled a variety of hazardous materials, including compressed gases. In program was undertakento manage compressed gases considered to be in excess of their users’ requirements.
696
Nickens
The objective of the program was to identify and properly manage cylinders containing compressed gases. The need for this service was originally identified by the Waste Management Group (HSE-7). This group has accumulated approximately 400 waste compressed gas cylinders at a secure storage location. The cylinders of concern were those owned by the laboratory. Cylinders thatwere leased by the laboratorycould, in many cases, be returned directly to the vendor. Historically, compressed gas users at the laboratory had purchased cylinders for their use. Under waste regulations, the laboratory became responsible for dispositionof the waste gases. Rather than adopta piecemeal approach,HSE-7 decided to attack the entirety of the problem. A contract was let for a single source outlet to collect, sample, identify, and dispose of these materials. Concurrently, notices were sent to the technical groups at the laboratory advising users of the collection program. A preliminary listing of the gases believed to be present at the storage location indicated the potential range of hazards. These gases included extremely toxic gases (suchas arsine and nickel carbonyl), reactive gases (fluorine and chlorine trifluoride), unstable gases (tetrafluorohydrazine), pyrophoric gases (silane), radioactive gases (tritium), and corrosives (hydrogen fluoride and hydrogen chloride). Most of the cylinders were in good condition. A significant number, however, were in a deteriorated condition. Some were nonstandard containers that did not meet DOT container specifications. A primary concern for proper management of the cylinders was identification of the contents. For most of the cylinders, this required sampling. Facilities were established ina remote area of the laboratory for processingthe cylinders. These facilities included a vapor containment structure, cylinder sampling equipment, and an on-site laboratory. Sampling and analysis of the gases began in August 1990. The vapor containment structure was central to the processing effort. This structure consisted of a large temporary building (approximately 30 ft by 100 ft and 15 ft in height). Cylinders were staged inside the structure for processing. Activities within the structure were monitored by several cameras. The structure included several emergency treatment systems. A liquid impinging scrubber and activated carbon scrubber/molecular sieve adsorbent canisters were provided to treat the atmosphere insidethe structure in the event of a release. The vapor containmentstructure could be sealed to capture and contain accidental releases. Incorporated intothe vapor containment structure was a valve sampling room. Inthis area, those cylinders in good condition with operable valves were remotely sampled. The valve sampling setup was similar to that shown in Figure 1. The cylinderswere placed into a containment chamber and attachedto a remotely operated valve actuation mechanism. Cylinders in poor condition or with inoperable valves were sampled using the patented Cylinder Recovery Vessel (CRV). This equipment also provided recontainerization capabilities for containers not meeting specifications. An on-site laboratory was located adjacent to the vapor containment site. The laboratory was equipped for radiological analysis and chemical identification. Chemical analysis of cylinder contents was accomplished using a combination of techniques. The primary methods were Fourier transform infrared spectroscopy and mass spectroscopy. Following identification using these techniques, a small sampleof the gas was dissolved in a compatible solvent. The solvent was placed in a liquid scintillation counter for detection of low levels of radioactive materials.
697
Management of Waste Compressed Gases
The initial task required in the management processW= inspection Of the CYlinders- The purpose of the inspection was to categorize each cylinder accordingto its condition and SUSpetted contents. This task was assignedto a cylinder inspection team consisting of a cylinder specialist, a health physics technician, andan emergency coordinator. SUPCategorization of the cylinders was based on external characteristics and information plied by the users. The four general categories used W e r e 1. Unrestricted. Standard DOT containers in good condition whose contents have’been cer-
tified by the user
2.’ Restricted.Labeledcylindersingoodcondition 3. unknown. Cylindersthat are suitablefortransportationbuthavenoindicationof 4.
contents Unstable.Cylindersinpoorcondition
or with unstablecontents
Contents of unrestricted cylinders were identified based upon certifications of theTOusers. be acceptable for certification, several criteria had to be met: 1. The user had to have personal knowledge as to its contents. 2. The purchase and usage history of the cylinder had to be documented. 3. The cylinder could not have been used in applications with other commonly manifolded
4.
gases. The cylinder must not have been connected become contaminated.
to a process in a manner in which
it could
Initially only a few cylinders were certified. As a cost-saving measure, a laboratory task team was organizedto work with the users to document cylinder contents. In the latter stages of the project .a significant perciintage of the cylinders were accepted under this classification. If the cylinder configuration and labeling was consistent with its certified contents, the cylinder was taken directlyto a storage area for direct disposition. Because of inconsistencies between labeling and analyses, however, the laboratory decided to sample a representative number of the certified cylinders. In addition to visual classification of cylinder condition, the exterior was scanned for evidence of radioactivity. The health physicist completed a radiation survey and obtained a surface “swipe” for analysis. had to Because the facilitiesat Los Alamos are spread out over a large area, the cylinders be collectedand moved to the processing facility. Fortunately, the transportation routes did not extend outside of laboratory-controlled areas. Regulatory prohibitions would have prevented transportation of unidentified gases over public roadways. Transportation was accomplished using a specially modified truck. An emergency response crew accompanied the truck throughoutthe short trip to the processing facility. More than 1500 cylinders were sampled during the courseof the one-year program. The variety of cylinder types and gases encountered posed significant challenges for both sample collection and analysis. Of those sampled, most were accessible through the cylinder valve. A total of 1316 cylinders (84% of the number sampled) were processed in this manner. Approximately 2-396 of the attempts to sample in this manner were unsuccessful owing to valve failure. The valve sampling system provided a mechanismfor sampling all standard valves [those designated by the Compressed Gas Association (CGA)]. Because of the higher potential for 110 type)were not sampled in failure, those cylinders with petcock valves (discontinued CGA also excluded because this configuration this manner. Cylinders with multiple valves were could have resultedfrom failure of the primary valve.
698
Nickens
In some cases cylinder valves were found to be leaking upon removal of the dust cap protecting the connecting threads. Because the caps were removed by workers using totally encapsulating suits with supplied air, there was no exposure to hazardous gases. Further, sampling was completed inside the vapor containment structure, which was designed to contain any release to the environment. Failure of the cylinder valve to open was a typical problem. Although the maximum recommended torque was applied, some valves would not actuate. Even when the valve handle rotated, there were cylinders whose contents could not be accessed. Because of the obvious safety concerns, an important part of the sampling operation was verifying that there was free access through the valve to the interior of each cylinder. As expected, an incredible variety of gases were identified. Analytical results proved the prudence of the sampling program. At the midpoint of the project, a survey showed more than 45% of labeled cylinders contained different or additional constituents. Ten percent increased in hazard classification based on sampling results. Several extremely hazardous cases illustrate the problem posed by inadequate labeling of cylinders. An oxygen cylinder with a standard medical post valve was found to contain radioactive tritium. Some cylinders configured for inert gases contained either poisonous or reactive gases. Many of the cylinders and valves failed to follow standards of the Compressed Gas Association. For example, the CGA 540 oxygen valve was frequently found on cylinders containing a variety of other gases. By the conclusion of the project, a total of 262 cylinders were processed through the Cylinder Recovery Vessel. Because of the relatively slow processing rate, CRV operations extended beyond valve sampling by several months. Without the capabilities of the CRV, more than 16%of the cylinders could not have been handled. Only the CRV was capable of managing the extremely powerful oxidizing gases commonly used at Los Alamos. The unique ability to manage these gases in a remote, contained process was crucial to the success of the project. Some of the cylinders were found to contain air or inert gases at atmospheric pressure. Cylinders that were not reusable were decommissioned and disposed of in a landfill. Many of the cylinders were reclaimed by the gas plant at the laboratory for reuse. Gases that were typically used by the laboratory were likewise recycled by the gas plant. These were generally common, innocuous gases such as nitrogen or argon. Originally a permitted hazardous waste incinerator was slated for use for disposal of waste gases. Developmental and operational difficulties, however, delayed the facility in accepting most of the gases except for freons and inert gases. A specialty gas manufacturer was approved for purchase and recycling of cylinders. This option was deemed viable for most commercially available gas except radioactive gases. The cylinder management program was predicated upon principles of safety and environmental protection. The variety of hazardous gases and condition of cylinders posed significant challenges to these goals. Engineering controls were instrumental in providing adequate safety for workers. As previously discussed, critical sampling operations were completed remotely. Engineering controls were also in place for preventing accidental releases. All of the sampling apparatus had some manner of secondary containment. Further, the entire operation occurred inside a vapor containment structure with emergency treatment equipment. The effectiveness of these controls was clearly demonstrated in one of the two safety incidents that occurred in the course of the project. A cylinder that may have contained residual oxygen difluoride was removed from the CRV. When monitoring systems detected an atmo-
Management of Waste Compressed Gases
699
spheric contaminant inside the structure, emergency treatment systems were activated. Exterior monitoring of the structure found no detectable release to the atmosphere. Personnel who had been working inside the structure in protective equipment were sent to the hospital as a precautionary measure, Examinations showed that there was no evidence of exposure. One other safety incident of note involved radiological analyses. During sample preparation, phosgene apparently degassed from a small vial of scintillation fluid. The laboratory was evacuated as a precautionary measure. Although there was no hazardous exposure to personnel, procedures were modified to provide for complete isolation of scintillation fluids. The management of hazardous gases at the Los Alamos National Laboratory was a major step in dealing with problem materials that had accumulated over 50 years of operation. The procedures demonstrably accomplished the goals of worker protection and environmental protection. In August 1991 the project was suspended for lack of funding. Despite its premature termination, significant progress was made toward identifying the full scope of the problem and acceptable means of managing it. Procedures for management of these materials were thoroughly tested and evaluated in the course of the project. The lessons learned and the success of the operation provide standards for handling compressed gases at other laboratory, governmental, and industrial facilities.
VI. ABANDONED WASTE SITES The techniques described in the preceding sections have been tested on several sites with a variety of gases. The project sites have included abandoned Superfund sites such as the Fike Chemical site in Nitro, West Virginia and the Chemical Control Corporation Superfund site in Elizabeth, New Jersey. The Fike Chemical site in Nitro, West Virginia was a chemical manufacturing operation that was abandoned and taken over under the Superfund program. Approximately 60 cylinders in varying conditions remained at the site during the remediation. The cylinders at the Fike Chemical site were found at various locations throughout the facility. Initial cylinder reconnaissance was completed by the EPA's technical assistance team (TAT). TAT provided Earth Resources Corporation (ERC) with the locations of the cylinders on the site. The cylinders were then inspected by ERC personnel to determine if they would be valve sampled or processed in the CRV. Cylinders were also checked for labels, markings, and leakage and to determine if they were safe for transport to the vapor containment building. Cylinder processing at the Fike site was accomplished using both the remote valve sampling device and the CRV. Cylinders that could be valve sampled were processed first. A total of 36 cylinders were sampled using the remote valve sampling technique. Cylinders that could not be sampled with the valve sampling device were processed with the CRV. Twenty-one cylinders were processed using the CRV. Of the cylinders processed in the CRV, 62% contained air or constituents of air. The remaining 38% contained one or more of the gases 1-butane, boron trifluoride, hydrogen chloride, hydrogen bromide, and hydrogen sulfide. Contents of cylinders processed in the CRV were treated on-site. Some of the cylinders that were valve sampled were returned to the manufacturer after analysis (provided the cylinder met appropriate shipping requirements). Those cylinders that were valve sampled but were of unknown origin were treated on-site. The Chemical Control Superfund site in Elizabeth, New Jersey was a former waste disposal facility that burned in 1980. Thousands of drums and many cylinders were removed dur-
700
Nickens
ing an initial remediation. Approximately 200 cylinders remained at the site, however, in severely deteriorated condition after exposure to thefire and weathering. In 1987 the cylinders were placed in protective overpacks and staged at the site. Because the cylinders at the Chemical Control site were contained in overpacks, part of the work required handling these materials outside of the CRV containment structures. TO provide for emergency containment in the event that a leak occurred during one of these p e s s e s , a temporary vapor containment structure was erected at the site. This structure also contained the principal treatment systems. The general site configuration is illustrated by Figure 3. Various treatment systems were established in conjunction with the operationto provide for the disposalof the compressed gases. These systems included molecular sieve and activated carbon adsorbents, a liquid impinging scrubber,and a flare unit. The initial task of operations required that the overpack cylinders moved be into the containment structure. Once inside the structure, the overpacks were attached to a sampling and of the overpacks were obtained using a vacuum system that was firs treatment system. Samples purged with an inert gas. Each of the overpacks was initially sampled.The purpose of this sampling was to determine whether the cylinder inside had failed during storage. The first overpack sample indicated a pressure of approximately 30 psig and an explosive mixture of hydrogen and air.The presence be repeated in approximately 70% of the overpacked of hydrogen and air was a pattern to samples. Since the cylinders contained in these overpacks were intact and contained other gases, it became clear that the hydrogen must have been generated from a different source. It is hypothesized that the hydrogen was generated through a reaction involving residual water and iron: Fe H,O + Fe0 + Hz. The presence of an explosive mixture of hydrogen and air around potentially incompatible cylinders greatly increased the risks associated with the work. In handling the overpacks, it w determined that the hydrogen could be removed through a controlled venting procedure. Excess pressure in the overpack was allowed to exhaust through theflare stack without burning. After equalization of pressure, a vacuum was created inthe overpack, and it was exhausted through the flare stack. This procedure was followedby purging with inert gas. During this process, eachof the overpacks was carefully grounded. This procedure related to the potential for building up a static charge through the passage of gas in the lines. Any sparking with the mixture in the overpack would havebeen disastrous. Of the 181 overpacks, only 10 (6%) contained gases that originated from leakage through failure of the cylinders. The gases in the overpacks included hexafluoroethane, monomethylamine, ethane, propane, isobutane, acetone, 2-methyl propanethiol, nitrous oxide, and methane. In the 26% of the overpacks that did notshow a potentially explosive mixture, there was, however, some enrichmentof hydrogen. It appears that these overpacks did not have the same atmosphere or moisture available to generate the hydrogen. Additionally, in those overpacks where leakage occurred from the original cylinders, hydrogen was not detected. Of the cylinders removed from the overpacks, approximately 10%were clearly perforated. These cylinders were sent directly to the cylinder rinse area after determination through inspection that they contained neither a solid nor a liquid. After immersion in the rinse tank, the cylinders were removed and cut intotwo pieces for subsequent disposal. Most of the cylinders that were intactat the time of processing proved to contain onlyair or water. These cylinders did not exhibit a significant pressure increase upon release into the CRV. It is possible that these cylinders failed during thefire or subsequent weathering.
+
Management of Waste CompressedGases
I
701
702
Nickens
'Ilventy-two percentof the intact cylinders containeda variety of materials. These included flammable gases, reactive gases, toxic gases, and pyrophoric liquids and solids. Flammable gases encountered at the site included cyanogen (one cylinder), ethane (two cylinders), and ethylene oxide (one cylinder). Flammable gases were readily treated through the flare stack. These gaseswere introduced to a flame in a controlled manner. Air monitoring using organic vapor detectorswas employed during the treatment process. Air was monitored at both upwind and downwind locations to detect any differences from background. One of the flammable gas cylinders processed createda particular problem for waste disposal. An acetylene cylinder that was drilled contained solid material that was determined to be an asbestos fiber. Asbestos is a common filler for cylinders of this type. The CRV was decontaminated usinga HEPA filter vacuum. The cylinder and the recovered asbestos were packagedin disposal bags for subsequent handling.Air monitoring following the procedures demonstrated thatthe vessel had been effectively decontaminated. A variety of potentially toxic gases were encountered during the operations. Among the particularly hazardous gases were cyanogen [threshold limit value (TLV) of 10 ppm], phosgene (TLV of 0. l ppm), and iron pentacarbonyl (TLV of 0.1 ppm). Several reactiveor corrosive gasesand other materials were found in the cylinders. These included anhydrous hydrogen chloride and hydrogen fluoride. Among the most difficult materials to deal with were reactive organometallic compounds. The reactive materials were treated using the liquid impinging scrubber. A caustic solution was used as the reagent in the scrubber for the acid gases (e.g., anhydrous hydrogen chloride). The Fike Chemical and Chemical Control Corporation Supefind sites illustrate the successful employment of conservative methodologies for handling difficult waste management problems. The projects were successful intesting all of the cylinder contents and disposingof the materials on-site. This was accomplished without releasing the materials to the environment or exposing workers and the general public to hazards associated with them. The gases encountered during these projects support the selection of this conservative methodology over others that had previously been proposed. In retrospect, it is evident that rejected off-site transportation and disposal techniques would have been inadequate for the materials at these sites. Transportationof the materials would have been extremely hazardous, as the cylinders were in poor conditionto begin with and itis possible that a release couldhave occurred. One of the alternatives investigated for the cylinders was detonation in a remote area. Some of the materials encountered at these sites, however, would not have been destroyed by an uncontrolled explosion. Many of the materials would have been released and would have subsequently presented hazards to workers and the environment. Detonation of the acetylene cylinder containing asbestos would have generated a major problem of airborne asbestos. Othermaterials, such as the nonvolatile organometallics,would also have been spread into the environment in an uncontrolled fashion. Nonpyrophoric gases such as hydrogen fluoride and chlorine would simply have been released in an uncontrolled fashion and would not have been affected by the detonation or consumed by fire.
VII. CONCLUSION Compressed gases have been an area of increasing concern for waste managers. Although these materials have been widely used, application of management regulations has progressed slowly in comparison with other types of waste.
Management Compressed of Waste
Gases
703
Compressed gas wastes and their containers pose significant hazards and potential liabilities. The number and typesof accidents and the closure of disposalfacilities have highlighted the necessity of a conservative managementphilosophy. Procedures and equipment have been developed to greatly increase the safety of handling waste compressed gases. Demonstrated success on major projects has increased the acceptance of these as standard practices. As more regulatory attention is focused on these wastes, the expediency of these practices will become more apparent.
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35
Pollution Control in the Dairy Industry
T.Viraraghavan University of Regina Regina, CaMda
1. INTRODUCTION The dairy industry occupies an important place in the food supply in many countries. In the United Kingdom the dairy industry provides24% of the protein content of the nutritionaldiet [l]; in some countries, e.g., New Zealand, it earns a sizable portion (approximately 20%) of the foreign exchange [2]. Major categories of dairy products are (1) fluid milk products, (2) butter, (3) cheese, (4) ice cream and frozen desserts, and(5) condensed and evaporated products. In some cases, several types of dairy products are produced at a single facility. Figure 1 shows a flowsheetof an integrated dairy process.
II. DAIRYPLANTWASTES Dairy wastesare generally dilutionsof milk or milk products resulting from product losses during handling and transport together with detergentsand sanitizers from intensive washingo p erations. The following operations contributeto the major product losses 131: 1. 2. 3. 4. 5. 6. 7.
Sludge discharges from clean-in-place (CIP)clarifiers Start-up, shutdown, and changeover in high temperature, short time Evaporatorentrainment Bottleand case washing Tank,can, piping, and other equipment washing Breakage and spillage in packaging equipment Production changeover in filling machines
(HTST) pasteurizers
Cheese and butter making processes produce whey and buttermilk, respectively. These are high strength wastes that increase the organic waste load if discharged accidentally or intentionally. 705
706
t
Ll
m
1
,c-* 4J
t
4
9. a
{
l
a 8-4 P)
X rd
a
Viraraghavan
# m
Pollution Control in the Dairy Industry
707
Table 1 Summary ofConversionFactorsRelatingDairyProduct to Milk Equivalents ~~~~
~
equivalent Product Milk (Ib)
lk
milk milk
eam
to 1 Ib product
Butter Whole cheese Evaporated Condensed Whole Cottage Non-fat dry Whey Dry whey Whey Dry Ice cream'
21.3 9.9 2.1 2.4 13.5 7.12 12.5 1.1 17.6 40.7 249.0 2.67 ~~~~
%e gallon of ice cream weighs 5.4 lb. Source: Environment Canada [3].
Table 2 Water Usage in theCanadianDairyProductIndustry Total water usage (Llkg ME)' ~~~~
Commodity
Range
Fluid milk products Butter products 0.79-5.90 Cheese products Ice cream and frozen desserts Condensed and evaporated products
Median 3.90 1.30 2.75 1.78 1.45
1.21-9.15 0.82-3.21 0.33-4.23 0.37-2.59
'ME = milk equivalent.
Source: Environment Canada [3].
Generally water useand process effluent loadings are related to the throughput in terms of milk equivalents. Table 1 provides a summary of conversion factors relating actual product to milk equivalents. Table 2 provides a summary of water use in the Canadian dairy product industry by commodity segment. The raw waste loads by commodity segment for the Canadian dairy industry are included in Table 3. The data for the American dairy industry are summarized in Table 4. Tables 3 and 4 show that waste loads in Canadian and American dairy plants are similar. The strengthand volume of the wastewater from any dairy plant will depend upon the processes employed, the volume of milk handled, the commodity produced, the condition and type of equipment, the waste reduction practices, the attitude of the management and staff, and the of amount of water used in cooling and washing[5]. The chemical oxygen demand (COD) the dairy wastewater is saidto vary from less than100 mg/L to more than 20,000 mg/L depending [6];biological oxygen demand (BOD) varies from40 mg/L to over 10,OOO on the dairy product mg/L, and wastewater flows vary from542 U1000 kg milk equivalent to 900 WlOOO kg milk equivalent [7]. Table 5 summarizes the characteristics of dairy wastewaters in the United States. Milk product losses typically range from 0.5% in large advanced plants to in excess of 2.5% in small old plants [3]. Many of the water-saving and waste control changes specific to the dairy industry include a number of steps[3], some of which are outlined below.
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Viraraghavan
Table 3 Summary of Raw Waste Loads from Canadian Dairy Products Industry ~
BOD (kg/lOOO kg ME) Range Commodity
Median
Range
~~
TSS (kg/lOOO kg ME)
Median
Fluid milk products Butter 0.53-8.78 Cheese Ice cream and frozen desserts Condensed and evaporated products
3.8 1.4 5.1 6.4 2.2
0.75-7.430.59-2.42
1.4 0.5
0.19-1.45
2.12-8.48 0.72-2.51 1.3 0.85-21.31 0.22-11.05 2.4 0.99-4.40 0.6
0.08-1.40
*ME = milk equivalent; TSS = total suspended solids. Source: Environment Canada [3].
Table 4 Summary of American Dairy and Milk Processing Plant Effluent Characteristics
Avg
Product Avg
Range
Milk Cheese Ice cream Cond.milk Butter Powder Cottage cheese Cottage cheese and milk Cottage cheese, ice cream, and milk Mixed products Overall
No. of plants 6 3 6 2 1 2 3 19 9 5 56
Waste vol. coefficientb BOD coefficienf
0.1-5.4 1.63-5.7 0.8-5.6 1.0-3.3
-
1.5-5.9 0.8-12.4 0.05-7.2 1.4-3.9 0.8-4.6 0.1-12.4
3.25 3.14 2.8 2.1 0.8 3.7 6.0
1.84 2.52 2.34 2.43
0.2-7.8 1.0-3.5 1.9-20.4 0.2-13.3
-
0.02-4.6 1.3-71.2' 0.7-8.6' 2.3-12.9 0.9-6.95 0.2-71.2
4.2 2.04 5.76 7.6 0.85 2.27
34.0 3.47 6.37 3.09 5.85
'Volume: kg wastewaterkg milk (or milk equivalent) processed. %OD kg BOD/1000 kg milk (ormilk equivalent) processed. Whey included; whey excluded from all other operations manufacturing cottage cheese. Source: Harper and Blaisdell [4].
1. 2. 3. 4. 5. 6. 7. 8.
9. 10. 11.
Complete drainage of all tanks and pipes before rinsing Recovery of low-volume, concentrated initial rinsesof tanks, cans, and equipment Automatic shutoff valves on all hoses Recovery of solid material such as fruit and cheese curd prior to discharge to a sewer system Removal of defective containers from bottling lines Installation of control systems on all equipment where overflow might occur Use of drip savers for milk car unloading in small plants Routine inspectionof all lines, valves, and pumps to eliminate leakage, especially CIP in lines Adequate temperature control coolers in and heaters to prevent freeze-on or burn-on leading to excessive product loss and water consumption during cleaning Drip shields to prevent spilled products from entering the sewer system Adequateentrainmentcontrolequipment on evaporatorsequippedwithbarometric condensers
*
709
Pollution Control in theDairy lndustry
Table 5 Characteristics of Wastewaters from Dairy Plants in United States Value" Characteristic BOD COD
ss
Total solids Nitrogen Phosphorus Carbohydrate
PH Temperature ("C)
Range
Mean -
40-48,ooO 80-95.000 24-4500 135-8500 1-180 P-2 10 250-930 4.4-9.4 18-55
2300 4500 820
2500 64 48 520 7.2 35
'All values except pH and temperature are in mg/L. Source: U.S. EPA [8,9].
12. Use of postcleaning rinses as make-up water for sanitizing and/or cleaning 13. Control or collection of leakage from damaged product containers 14. Collection and recovery of valuable by-products, especially whey An extensive accountof good managementpractices in waste control inthe dairy industry is included in a publication issued by US. EPA [8]. Milk rinsings and whey contribute to the dairy waste loads and can be recovered. Whey is a by-product of cheese-making and can be used as a supplement in livestock or poultry feed. Harper et al. [lo] discuss the xesults of a study undertaken to reduce water and waste discharges at the Kroger DairyCompany, a complex multiproductdairy plant in Indianapolis, Inof equipmentandprocesses.Before diana, throughmanagementcontrolandmodification control procedures were introduced, the plant discharged 400,000-800,000 gal of wastewater per processing day, with a total BOD load of 10,000-15,000 lb. As long as direct supervision was maintained on a regular basis, water use in the plant was reduced by one-third and the organic waste load was reducedby one-half. Process and equipment changeswere made in the plant that reduced waste discharges by 100,000 gaYday and reduced waste strength about20% at an economic savings projected at $200,000 per year.
111. DAIRYWASTEWATERTREATMENT Since dairy wastewaters are mainly composed of soluble organic materials, they respond ideally to treatment by biological methods; the conventional methods generally used are (1) activated sludge, (2) trickling filtration, (3) lagooning, (4) anaerobic digestion, and ( 5 ) irrigation or land application [l 13.
A. AerobicBiologicalTreatment Tabulated information on a wide range of activated sludge plants, both laboratory and industrial units, treating dairy wastewater is available elsewhere [8].The treatment efficiency values of about 85%.With good vary from about 25%to over 99% with an average removal efficiency operation, BOD removal efficiencies in excess of 90% are easily attainable with activated sludge systems; however, even an efficient system may not be able to reduce the BOD from
710
Viraraghavan
2000 mg/L in the dairy waste to 20-30 mg/L in the effluent, which may be required by the regulatory authorities. Westage systems are frequently required to meet such standards [5]. An extended aeration activated sludge systemtreating a dairy wastewater, under proper operating conditions, removed 99% BOD and 95% SS at an organic loading of 0.14 kg BOD/(kg mixed liquor suspended solids (MLSS) day), producing an average effluent BOD and suspended solids (SS) of 22 and 55 mg/L, respectively [12]. Guo et al. [l31 evaluated the performance of an oxidation ditch treating 140-218 m3/day of dairy wastewater with a BOD in the range of 380-1900 mg/L (average 950 mg/L) duringthe summer months. The organic loading was 0.08 kg BOD/(kg M U S day) and 0.32 kg BOD/ (m3 day). The average effluent BOD to 99% BOD and 98% SS removal. and SS were 7 and 28 mg/L, respectively, which corresponded The wastewater treatment process employed by a condensed milk production plant consisted of a combined extended aeration, trickling filter process treating 160m3/day [14]. The average BOD and SS of the wastewater were lo00 mg/L and 300 mgL, respectively. When operating well, an effluent BOD of less than 25 mg/L andSS of less than 100 mg/L were generally achieved. The wastewater treatment facilityfor a 7-day week milk processing operation (producing cheese and butter) consisted of a seven-stage RBC unit followedby aerated lagoons. The wastewater BOD and SS were 3400 mg/L and 3200 mg/L, respectively, on average [14]. The final effluent from the aerated lagoon system contained less than 25 mg/L of BOD SS, andfor overall removals in excess of 99% for both parameters. Exhaustive performance data on trickling filters and lagoon systems treating dairy wastewaters are included in a publication of the U.S. EPA [8]. Goronszy and White [l51 present data on large-scale cyclically operated activated sludge facilities (batch reactors)treating cheese whey and dairy processing wastewater. Bothfacilities included a captive selector reactor zone where biomass from the main aeration zone was mixed with incoming wastewaters according to specific initial loading criteria. Both plants produced effluents of acceptable quality. Schulte [l61 providesdetails of the design and operations of a sequencing batch reactor (SBR)treating Meadowland Creamery wastewaterat Conroe, Texas; the effluent quality consistentlymet the city’s requirementsof BOD andSS less than 200 mg/L for the discharge of dairy wastewater to its sewer system.
-
B. AnaerobicBiologicalTreatment Anaerobic processes have been employed recently for treating dairy wastewaters. A full-scale anaerobic treatment plant for treating dairy wastewater was first installed in Canada at Granby, Quebec. This plant, which treats the cheese wasteand several other processing wastes, consists of two units-a downflow fixed film reactor and an upflow sludge blanket reactor [17-201. The second Canadian anaerobic digestion system, at Millbank Cheese and Butter Ltd., is treating cheese whey only [20]. Switzenbaum [21] reported onthe use of an anaerobic expanded-bed process for treating whey waste; Vandamme and Waes [22] used an anaerobic contact process to treat dairy wastewater; and Sutton [23] demonstrated the use of a membrane anaerobic reactor system (MARS) to treat cheese whey permeate. Zaal [24] showed that the dairy waste could be efficiently treated in an upflow anaerobic sludge blanket reactor. One of the major limitations in the treatment of organic wastes by fixed-film anaerobic reactors is the long start-up period that is required before optimum performance is achieved. The reason for this is the length of time necessary for the development of a stable biofdm. Start-up time varies from 4 to 8 months [25]. For starting up an anaerobic reactorfor treating dairy wastewater, different approaches have been followedby different researchers. Varied ex-
Pollution Control in the Dairy Industry
71I
periences with respect to durations of start-up problems encountered during this period have been reported. Samson et al. [26], who started up the reactorby inoculating the sludge froma municipal wastewater treatment plant in treating the cheese waste, ran into difficulties during a 4-month start-up period mainly because of the large fluctuations in the composition of the plant effluent (pH varying from 2 to 11 and COD from lo00 to 10,OOO mg/L). Backman et al. [27] started up anaerobic filters by initially seeding the reactors and adding sodium bicarbonate into the feed influent (dairy wastewater). Both alkalinity and COD concentrations were 10oO mg/L. They reported that there were operational failures duringthe first month that were due to the increased productionof volatile acids (1215 mg/L) and consequently low pH values (5.6) found in the effluent. Because of these problems, the removal efficiency fluctuated between 12.5 and 92.0%. To some extent the problems encountered duringthe start-up operation can be minimized by carefully monitoring influent and effluent characteristics. The microorganism populations in the reactor should be stabilized until the steady state is achieved. For instance, Landine et al. [28] started up a downflow stationary fixed-film reactor treating dairy wastewater with only 20% raw waste. The strength was gradually increased to its full concentration (100%) over a period of10 weeks, during which period no operational difficulty was reported. Hall and Adams [29] and Wilson and Murphy[l81 started up their dairy waste reactorsin a manner similar to the procedure describedby Landine et al. [28]. They did not report any difficulty during the start-up period of 240 days. Various investigators treated different types of dairy wastes such as milk waste, whey waste, and synthetic milk waste of varying strengths using fixed-film reactors. The COD removal efficiencies of these filters have generally ranged from50% to 98%. These studieswere conductedwithrespect to differentorganicloading rates and differenthydraulicretention times. Rittmanet al. [30] reported that the efficiency of the filter dropped drastically from over 80% to less than 30% when the hydraulic retention time was changed to 1 day from 3 days while treating milk waste (synthetic waste anddairy wastewater) with a strength of 3900 mg/L ofC/N ratio while treating the diluted cheese whey COD. De Haast et al. [31] studied the effect waste and concluded that a value muchin excess of 20 would lead toreactor failure,due to poor buffering. Samson et al. [26] examined the effect of recirculation while treating the dairy wastewater. Nieuwenhof [32] demonstrated that dairy wastewater could be treated effectively by means of a two-stage anaerobic process. Cordoba et al. [33] were able to treat whey waste efficiently in a horizontal flow anaerobic filter. A laboratory investigation was carried out to evaluate and compare the performances of three plastic medium upflow anaerobic filters operating at 12.5"C7 21"C, and 30°C treating dairy wastewaters [34].' b o of the reactors were started at 21°C and the third at 30°C. Steadystate operation was carried out at three different temperatures: 12.5"C7 21"C, and 30°C. Each reactor was operated sequentially at different hydraulic retention times (HRTs) of 6, 4, 3, and 1 day. The difference in start-up performance (COD removal) between anaerobic filters operating at 21°C and 30°C was not substantial. At an HRT of 4 days, COD removals in the three anaerobic filters were approximately 92%, 85%, and 78% at 30"C, 21"C, and 12.5"C7 respectively.
IV. LAND APPLICATION Land application of dairy wastewater is extensively covered in a publication of the U.S. EPA [8] and by Marshall and Harper [5]. Environment Canada [35] presented details of the use of land applicationby four milk processing plants and one cheese plant. Performance data on these land treatment systems are not available, however.
Viraraghuvan
712
V. CONCLUSIONS Substantial cost savings canbe realized through good management practices suchas reducing rewater use and minimizing raw material and product loss. Such a management strategy will duce capital and operating costs for treatment systems. Dairy wastewaters contain soluble organic matter and are therefore amenableto biological treatment. Although aerobic biological treatment systems have been popular in thepast, there is a growing interest in the design and operation of alternative anaerobic systems, especially is availwith their capacityto produce biogas. Land treatment of dairy wastewaters, where land able, will be advantageous from an economic and environmental perspective.
REFERENCES 1. Hemmings, M. L., The treatment of dairy wastes, Dairy Ind. Int., 45( l l ) , 23-28 (1980). 2. New Zealand Dairy Board, Annual Report for Year Ended 3 1 May 1982, New Zealand Dairy Board, Wellington, New Zealand, 1982. 3. Environment Canada, Evaluation of Physical-Chemical Technologies for Water Reuse, Byproduct Recovery and Wastewater Tmatment in the Food Processing Industry, Rep. No. EPS 3-WP-79-3, Ottawa, Canada, 1979. 4. Harper, W. J., and Blaisdell, J. L., State of the art dairy plant wastes and waste treatment, Second Natl.Symp.on Food ProcessingWastes,U.S.EnvironmentalProtectionAgency,Washington, D.C., 1971. 5 . Marshall, K. R., and Harper, W. J., The treatmentof wastes from the dairy industry, inIndustrial Wastewater Treatment,Vol. 1 (D. Barnes, C. F. Forster, and S. E. Hrudey, eds.), Pitman, London, 1984, pp. 296-376. 6. Choi, E., and Burkhead, C. E., Anaerobic treatment of dairy wastes using fixed-film and withoutmedia reactors, Doc. 39th Industrial Waste Conference,Purdue Univ., West Lafayette, Ind.,1984, pp. 223-233. 7. Brown, G. J., Landine, R. C., Steeves, A. L., Bough, W. A., Clark-Thomas, D. L., Burk, E., and
8.
9.
10.
11. 12.
13.
14. 15. 16.
Eckert, R., Anaerobic treatment of dairy wastewater, 58th Annual Conference, Water Pollution Control Federation, Kansas City, MO., 1985. U.S. EPA, Dairy Food Plant Wastes and Waste 'lhatment Practices, Rep.12060 EGU 03/71, U.S. Environmental Protection Agency, Washington, D.C., 1971. U.S. EPA, DevelopmentDocumentfor EffluentLimitationsGuidelinesforNewSourcePerformance Standards for Dairy Products Processing-hint Source Category, U.S. Environmental Protection Agency, Washington, D.C., 1974. Harper, W. J., Delaney, R. A. M., Igbeka, I. A., Parkin, M. E. Schiffermiller, W. E., Ross, T. E., and Williams, R. A., Strategies for Water and Waste Reduction in Dairy Food Plants. Rep. No. EPA/600/S2-85/076, U.S. Environmental Protection Agency, Cincinnati, Ohio, 1985. Nemerow. N. L.,and Dasgupta, A.,Industrial and Hazardous Waste Treatment,Van Nostrand Reinhold, New York, 1991. Guo, F? H.M.,Fowlie, F? J. A., Cairns, V.W.,and Jank, B. E., Performance Evaluation of an Extended Aeration Activated Sludge b e s s Treating Dairy Wastewaters, Rep. No. EPS 4-WP79-8, Environment Canada, Ottawa, Canada, 1979. Guo, F? H. M., Fowlie, F? J. A., Cairns, V.W.,and Jank, B. E., Performance Evaluation of an Oxidation Ditch Treating Dairy Wastewater, Rep. No. EPS4-WP-79-7, Environment Canada, Ottawa, Canada, 1979. Environment Canada, Biological Treatment of Food Processing Wastewater-Design and Operations Manual, Rep. No. EPS 3-WP-79-7, Ottawa, Canada, 1979. Goronszy, M. C., and White, J., Activated sludge treatmentof high COD food processing wastes, 1988 Food Processing Waste Conference, Georgia Instituteof Technology, Atlanta, Ga., 1988. Schulte, S. R., SBR technology for dairy wastewater, 1988 Food Processing Waste Conference, Georgia Inst. Technol., Atlanta, Ga., 1988.
Pollution Control in the Dairy Industry
713
17. Maaskant, W., and Zeevalkink,J. A., First full-scale anaerobic treatment plant for dairy wastewater in the world, Proc. Eur. Symp. Anaerobic Wastewater Treatment, Noordwijkerhout, Netherlands, 1983, pp. 442-446. 18. Wilson, R. W., and Murphy, K. L., Full-scale anaerobic treatment of dairy effluent, Proc. Seminar on Anaerobic Fired Film Digestion, Pollution Control Association of Ontario, Toronto, 1986, pp. 201-220. 19. Environment Canada, Anaerobic Treatmentof Dairy Effluent, Rep. No. EPS 3/FP/1, Ottawa, Canada, 1986. 20. Environment Canada, Anaerobic Technology: A Review of Research, Development and Demon4/AN/1, Ottawa, stration Activity in the Agrifood and Pulp and Paper Industries, Rep. No. EPS Canada, 1988. 21. Switzenbaum, M. S., Anaerobic expandedbed treatment of wastewater Proc. Workshop-Anaerobic Filters: An Energy Plusfor Wastewater Treatment,Argonne, Ill., 1980, pp. 115-128. 22. Vandamme, K., and Waes, G . , Purification of dairy wastewater in a two stage treatment plant, including anaerobic pretreatment,Milchwissenschft, 35( 1 l), 663-666 (1980). 23. Sutton, I? M., Active biomass retention: the key to anaerobic process efficiency,Proc. Seminar on Anaerobic Fired-Film Digestion, Toronto, Ont.. 1986, pp. 55-64. 24. Zaal, R., Anaerobic treatment of some dairy wastewaters,Proc. Eur. Symp. Anaerobic Wastewater Treatment, Noordwijkerbout, Netherlands, 1983, pp. 464-465. 25. Henze, M., and Harremoes, l? Anaerobic treatment of wastewaters in fixed film reactors-a literature review, Water Sci. Technol., 15(8/9), 1-102 (1983). B., Dairywastewatertreatmentusing 26. Samson, R., Peters, R., Hade,C.,andvandenBerg,
27.
28.
29. 30. 31. 32. 33. 34.
industrial-scale fixed-film and upflow sludge bed anaerobic digesters: design and start-up experi1984, ence, Proc. 39thIndustrialWasteConference, PurdueUniversity,WestLafayette,Ind., pp. 235-241. Backrnan, R. C., Blanc, F. C., and O’Shaughnessy, J. C., The treatment of dairy wastewater by anaerobic upflow packed bed reactor,Proc. 40th Industrial Waste Conference,Purdue Univ., West Lafayette, Ind., 1985, pp. 361-372. Landine, R. C., Cocci, A. A., Brown, G . J., Pyke, S. R., and Steeves, A. L., Experiences with Proc. Seminar on Anaerobic Fired-FilmDigestion, anaerobic treatment using fixed film processes, Pollution Control Association of Ontario, Toronto, 1986, pp. 141-190. Hall, E. R., and Adams, G. P, Anaerobic treatment of cheese whey, Proc. Seminar on Anaerobic Fixed-Film Digestion, Pollution Control Association of Ontario, Toronto, 1986, pp. 65-80. Rittman, B. E., Strubler, C. E., and Ruzicka, T., Kinetics of anaerobic filter pretreatment of dairy Proc. Natl. Con$ Environmental Engineering, Environmental wastewater at ambient temperatures, Engineering Division, ASCE. New York, 1981, pp. 332-339. De Haast, J., Britz. T. J., Novello, J. C., and Vervwey, E. W., Anaerobic digestion of deproteinated cheese whey, J. Dairy Res. (GB),52, 457-467 (1985). Nieuwenhof, F. F. J., Anaerobic treatmentof dairy wastewater, Proc. Eur. Symp. Anaerobic Wastewater Treatment, Noordwijkerhout, Netherlands, 1983, p. 172. Cordoba, I? R., Riera, F. S., and Sineriz, F., Treatment of dairy industry wastewater with an anaerobic filter, Biotechnol. Lett., 6(11). 753-758 (1984). Viraraghavan. T., and Kikkeri, S. R., Dairy wastewater treatment using anaerobic filters, Can. Ag-
ric. Eng., 33(2) 143-149 (1991). 35. Environment Canada, Land Application of Food Processing Wastewater, Rep. No. 3-WP-78-5, EPS Ottawa, Canada, 1978.
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36 Landfill Gas Collection and Destruction Systems: Evaluating Toxic Emissions and Potential Health Risk Karnig Ohannessian, Anna Peteranecz, and Thomas Kear OP&L, Inc. San Diego, California
1.
INTRODUCTION
Concerns in the UnitedStates over limited landfill space have until recently overshadowed the potential health effects from exposure to landfill gas that is emitted into the air from municipal solid waste landfills. The constituents of landfillgas vary. Generally, landfill gas consists of 40-75% methane 50% CH4 (CH,) by volume and25-65% carbon dioxide(CO,) by volume; a safe assumption is and 50% CO,.In addition, landfill gas contains traces of non-methane organic compounds U.S. EnvironmentalProtection (NMOCs),includingaircontaminantsthataretoxic.The Agency (EPA) has based its standards and guidelines for the control of landfill gas on emissions of NMOCs. The control of landfill gas would reduce the adverse health effects NMOCs of that are toxic is or carcinogenic. Another benefitwould be the reduction of methane emissions. Methanesuspected to be a greenhouse gas. Also, landfill gas has been known to migrate under buildings near landfills, where the methane has caused explosionsand fires. Individual states often require that landfill gas control systems not only meet EPA the standards and guidelines, but also be shownto have insignificant potential health risks. A convenient tool (one that is sometimes required by regulatory agencies fornew or modified sites)for evaluating health effects is a health risk assessment (HRA). A health risk assessment is a quantitative analysis of the dispersion of airborne toxins, a to these contaminants, and the assessment determination of the potential exposure of the public of the health risks that may result because of this exposure. An excellentofsetHRA guidelines (CAPCOA) for has been published by the California Air Pollution Control Officers Association use by facilities as a way of determining the potential risk they poseto the surrounding community [l]. The EPA also has HRA guidelines that are in many ways less stringent than the CAPCOA guidelines. 715
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Ohannessian et al.
The processes, site descriptions, and assumptions found in this chapter have been taken from work that we have done for numerous landfills. The cases presented are composites of several sites, and process conditions suchas equipment design parameters, chemicals present, and the risk and ambientconcentrations of these chemicalshave been changed to ensure client confidentiality. However, the processes described are representative of the industry. The landfills are located in California, a trendsetter in environmental regulations. Therefore, the toxic emissions evaluations and risk assessments are performed to take into accountthe requirements of the California Solid Waste Management Board, the California Department of Health Services, and local air pollution control districts.
II. LANDFILLGASGENERATION The rate at which landfill gas is generated depends on a variety of factors that determine the rate at which the refuse in the landfill breaks down. Thesesite-specificfactors include moisture content, temperature, waste composition, and acidity. Gas generation occurs over long periods of time, sometimes lasting up to 50 years. The EPA has published a landfill gas emissions estimation model that is available on computer disks [2]. The model can predict gas generation rates based on site characteristics. Conservative default valuesfor estimating emission rates are provided for cases where site data are not available. Site information includes the landfilling history, refhe acceptance rate, design capacity, NMOC concentration, methane generationrate constant (based ona first-order decay equation), and methane generation potential. Individual air toxics concentrations can also be used as input. The model calculates emission rates for methane, carbondioxide, and NMOCs, as well as for individual toxic air components. Obtaining site-specific NMOC concentration9 and landfill gas generation rates requires testing that is often expensive.For this reason landfill operators base their initial emission rate estimates on the conservative default values. Emissionrate estimates are important becausethe EPA and state standards requirethe installation of costly landfillgas control equipmentfor sites exceeding established NMOC emission rates. If the default values yield high NMOC emission rates, the operator can then decide whether actual testing would be warranted. The California Air Resources Board (CARB) has documented two methods of estimating average landfill gasgeneration rates over a 70-year period. One method, developed in 1982 by the South Coast Air Quality Management District (SCAQMD), suggests that methaneis produced at a rate of 972 tondyear per million tons of in-place refuse [3]. The second method, recommended for work done tosatisfy the requirements of California’s AirToxics “Hot Spots” Program (Assembly Bill2588). suggests that methane is producedat a rate of 3000 ft3 per ton of refuse over a 70-year period [4,5]. CARB assumes that landfill gas consists of 50% by volume carbon dioxide and50% by volume methane forthe purpose of calculating emission rates; this assumption partially accounts for the variations in evolution rate observed in the landfill gas with respectto time [4]. Thus, on average, landfill gas is generated at twice the volumetric rate at which methane is generated. Both California methods assume a constant landfill gas generation rate over a 70-year period (average human lifespan). This time period doesnot necessarily reflect the actual period of landfill gas generation.The 70-year periodhas been adoptedby California as a conservative estimate of exposure based on the average human life span. The air emissions estimates, coupled with unit cancer risk factors for carcinogenic chemicals published in risk assessment guidelines, can be used to conservatively estimate the increased likelihoodthat an individualmay develop cancer over the course of a 70-year exposure period [l]. In reality, the generation rate of landfill gas will initially be much greater than the
Landfill Gas Collection and Destruction
717
Table 1 Landfill Gas EmissionRateAssumptions
Emission rate Assumption A. B.
C.
3000 ft3 of methane per ton of refuse per 70 years (CARB) 972 tons of methane per million tons of refuse per year (CARB) 370 ft3/min million tons per of refuse (BAAQMD)
(Ib landfill gadyear) 6.7 X lo6 7.3 x IO6 15.2 X lo6
estimated rate, but eventually it will taper off. Landfill gas generation reaches its peak rate during or just after landfill closure. Therefore, a decade after landfill closure,would one expect to see gas generation rates that are lower than those estimated by either of these California techniques [3,5]. For this reason, calculated health risks using these estimation techniques may [5]. These methodsare conservative enough, however, not be representative of actual exposure to satisfy the requirements of regulatory agencies. Another emission estimation technique that has been used by the Bay Area Air Quality Management District (BAAQMD) assumes that 370 ft3/min of landfill gas is evolved per million tons of refuse for 10 years [6].This is a much higher estimated rate of production over a much shorter amount of time than is used in the CARB methods. Table 1 compares the landfill gas generation rates from each of the California methods per million tons of refuse onefor year. Aside from the presented emission rate assumptions, actual measured landfill gas generation rates can be used for calculation of cancer risks, and chronic (long-term exposure) and acute (short-term exposure) hazard indices, over a 70-year exposure period. Note that these methods can be employed in a health risk assessment to calculate average acute hazard indices, as opposed to the peak acute hazard indices thatwould result during the peak landfillgas generation period within the first few years after landfill closure.
111.
LANDFILL GAS COLLECTION SYSTEMS
Landfill gas collection systems comprise verticalor horizontal gas extraction wellsor trenches that are connected to header pipes, and gas moving equipment such as blowers or compressors. This type of system is called an active collection system, because the gas is extracted from the landfill by creating a pressure gradient. Thereare also passive collection systems that channel the generated landfill gas to control devices without pumping. The EPA suggests that active vertical collection systems can best meet its emission standards and guidelines [7]. Vertical extraction wells are normally placed in areas of a landfill that are no longer accepting waste. The wells are spaced according to the vacuum generated such that gas is collected from every part of the landfill.An effective collection system shouldbe able to handle the maximum predicted landfill gas generation rate. The wellhead is connected to the header pipes,whichsometimesareburiedunderthetopsoilcover.Eachwellheadassemblyis be regulated and gas samples can equipped with valvesso that the flow through each well can be taken. The operator must guard againsttoo large a pressure gradient that would cause large amounts of air to penetrate into the landfill.An excess of air in the landfill has been linked to underground fires. Wells are often made of polyvinyl chloride (PVC) or high-density polyethylene (HDPE) of of the pipe about4 in. in diameter.The EPA requires that wells descend to a minimum75% landfill depth [7].The bottom two-thirds of the pipe is perforated with slots. The pipe is surrounded by a 2-ft bore, which is backfilled with gravelup to 1 ft above the perforations, then with bentonite, concrete, and top cover soil [7].
Ohannessian et al.
718
Blowers or compressors conveythe gas through the header piping. The gas is directed to a knockout drumto remove water, and sometimes atoscrubber to remove particulates. The gas then flows to the control equipment. Compressors causethe moisture to condense more easily because of the higher gas pressures, but they are more costly than blowers. Collection efficiencies generally range from 40 to 6095, with some systems having efficiencies as high as 90% [3]. Covering closed landfills with plastic lining material and compacted soil (e.g., clay) results in greater gas collection rates. Also, a liner system installed during landfilling to seal the bottom would control gas migration. In the toxics emission evaluation presented in Section V it is assumed that the efficiency of the landfill gas collection system to be installed at the hypothetical landfills inquestion would be 60%. Because of the difficulties in estimating the actual landfill gas generation rates and other factors that influence the emission rate of landfill gas (e.g., temperature, type of refuse, fluctuations in groundwater depth), the actual efficiencyof any landfill gas collection system may differ from the design efficiency of the system. If the design efficiency is not reached in the landfill gas collection system, the health risks associated with the landfill will be higher than those calculated, because a smaller amount of gas will reach the control devices.
IV. LANDFILL GAS TREATMENT SYSTEMS A.Energy-RecoveringTreatment
Systems
Landfill gas is usually burned. There are cases where the gas is purified by removing the caras a product gas with a heating valueof about bon dioxide, NMOCs, and water and is then sold lo00 Btu/ft3. Carbon dioxide is removedwith gels, activated carbon, molecular sieves, or now membranes [8]. In recent years this process has become too expensive, and most landfills simply bum the collected gas. Four common devices used for combustion of collected landfill gas are flares, internal combustion (IC) engines, gas turbines, and boilers. IC engines, turbines, and boilers are used to recoverenergyfrom the combustionprocess.However,whentheseenergy-recovering devices are used as the primary destruction devices, flares are also employed as a backup for periods during downtime or maintenance. If there is a market for the energy generated, then energy-recovering devices may be economically more attractive and viable alternatives than flares.
B. Flares Flares are increasingly becomingthe only control devices installed, because they have higher destruction efficiencies, can operate at wider gas flow ranges, and are easier to operate and maintain than other devices. The current trend is toward enclosed flares (EPA recommends either open or enclosed flares [7]). Enclosed flares have burners at ground level that are enclosed by a shell that acts as a stack. This simplifies the gas sampling and source testing procedure.The flares are equipped with flame arresters, temperaturesensors,and other controlinstrumentation. The pilot flame burns either natural gas or propane. Exhaust gas temperatures range from 1200 to 1700"F, with a typical value of 1400°F. The destruction efficiency is required by regulatory agencies to be greater than 98% for NMOCs; destruction efficiencies regularly reach levels above 99%. Higher temperatures resultin higher destructionefficiencies but also result in higher emission rates of nitrogen oxides (NO,) and carbon monoxide (CO). In California, the NO, emission limit is 0.06 lb per million Btu heat input, and CO emission limit is 0.30 lb per million
719
Landfill Gas Collection and Destruction
Table 2 Enthalpies of Combustion Product Gases
-mol)] EnthalpyChemical 19,138 14,832 12,092 12,805
Btu. Many regulatory agencies also have established emission limitsfor PM,,, or particulates with diameters less than or equal to 10 pm. The combustion of landfill gas in flares requires a supply of forced excess air to assure complete oxidation. The following is a quick and rough procedure for estimating the air requirements. The air requirement information can be used to size the flare. Assume that 1400 standard cubic feet per minute (scfm)of landfill gas is to be burned at 1700°F.The inlet gas is at 70°F and consistsof 50% CH, and 50% CO2. The gas has a heating value of 460 BNft3 [assume a molar volume of 379 ft3/(lb-mol)]. The combustion reactionfor 1 mol of gas is
+
+
+
0.5 C& 0.5 C02 X(02 3.76 N2) "* C02 + H20 (X - 1)02 (X) (3.76)N2
+
+
where X - 1 is the excess amount of air required. The heat released is
(460Btulft?) [379 f 9 / (lb-mol)] (1 Ib-mol) = 174,340Btu
(2)
The enthalpies of the product gases at 1700°F are shown in Table 2. The heat balance (assuming that 20% of the heat is lost by radiation) is
0.8 (174,340)= 19,138 + 14,832 + (X - 1) (12,805)+ (X) (3.76) (12,092) (3) The solution gives
X = 2.03,
X -(4)1 = 1.03
Therefore, 100% excess air is neededin becomes 0.5 C&
C02
the flare. The balancedchemical
equation
+ 0.5 C02 + (2)(02+ 3.76 NZ)"*
+ H 2 0 + 0 2 + (2)(3.76)N2
The exhaust flow rate is
(1400scfm) [l
+ 1 + 1 + (2)(3.76)] = 14,728 scfm
(6)
The excess air also serves to maintain the exhaust gasesat the desired temperature. Without the excess gas, the temperature rise would be
19,138 +
1700 - 70 = 0.0205"F/Btu 14,832 + 3.76(12,092)
and the exhaust gas temperature rise would be
(0.0205) (0.8)(174,340)
= 2860°F
(8)
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Ohannessian et al.
V. LANDFILL GASEMISSIONS A. Emissions of Non-Methane Organic Compounds At the hypothetical subjectsite presented in this chapter, landfill gas is collected and is routed to an enclosed flare. The flare oxidizes any toxic compounds present into less harmful compounds(ideally,carbondioxideandwater).Becauselandfillgas after combustionhas a significantly lower health risk than unburned gas, the choice of the landfill gas collection system efficiency has a large effect on the final estimate of the health risks in the vicinity of the landfill. Emission factors of individual chemicals inthe toxics evaluationare based on source tests for landfill gas from actual landfills. Flares are assumed to have a destruction efficiency of 99.8%for the toxic constituents based upon the same source tests. This destruction efficiency is higher than the 98% NMOC destruction efficiency required by the EPA. There is no destruction of the NMOCs being emitted fromthe landfill surface. It is also assumed that metals present in the landfill gas are not reduced by the flares. It is believed that combustionof landfill gases may actually add metals into the exhaust [lo]. Three different kindsof data are available to quantify the presence of toxic constituents in landfill gas: 1 . Emission factors from local regulatory agencies based on submitted reports 2. Source test results of inlet landfill gas and exhaust gases 3. AirSolid Waste Assessment Test (Air-SWAT) reports for compounds assessed at landfills in California
The data from these sources are shown in Table 3. Dioxins and furans will notbe formed in the flares. Recent research indicatesthat dioxins and furans are destroyed at flame temperatures exceeding 1380°F [9]. Occasionally, data from the regulatory agencies indicate that several metalsthat should not be present in the landfill gas are present in the exhaust gases. Examples are cadmium, manganese, nickel, and zinc. It is believed that these metals result from corrosion. Such effects have been documented in other equipment such as IC engines located on landfills andare believed to be caused by high levels of chlorine, in the form of chlorinated hydrocarbons, in the landfill gas. Combustion of these halocarbons resultsin decomposition into hydrogenchloride and carbon dioxide withinthe IC engines. The acid then attacks engine componentsand creates a source of metal release and contamination of the exhaust stream [lo]. Most of the compounds that were detected in the data from the regulatory agencies, the source test data, and the Air-SWAT data had different concentrations in each data set. One reason for the variations is that landfill gas, even gas from different regions of the same landfill, will vary greatly in composition.
B. Emission Rates of Exhaust Gases Since the actual landfill gas flow rate varies over time, the California method of 3000 ft3 of methane produced per ton of refuse per 70 years is used to estimate the gas flow rate. This model accountsfor human exposure to the toxic pollutants over a lifetime, which corresponds with the assumptions made in the health risk assessment methods. Assuming a landfill capacity of 20 million yd3 and a refuse density of 1200 lb/yd3, the result is 12 million tons in place at landfill closure. This amount yields an average landfillgas flow rate of 2000 scfm.
Landfill Gas Collection and Destruction
721
Table 3 Non-Methane Organic Compounds in Landfill Gas" RegulatorySubstance
Acetaldehyde Benzene Carbon Chlorobenzene Chloroform 1,l-Dichloroethane 1,1-Dichloroethylene 1 Ethylene Formaldehyde Hydrogen Hydrogen Hydrogen Methyl Methylene Perchloroethylene PAHs) hydrocarbons aromatic Polycyclic Styrene Toluene Trichloroethylene Vinyl Xylene Zinc Acrolein Chromium(VI) Lead Mercury Selenium Phenol Vinylidene chloride
ND
D
ND ND
-
ND -
-
ND
-
ND
-
-
-
D
-
D
ND ND ND
ND ND
ND ND
-
ND ND
D
-
D
-
-
'D denotes compounds that were detected. ND denotes compounds with concentrations at or below the detection limit. Blank entries (-) denote compounds that were not tested.
For design purposes it mustbe assumed that theflare can handle allof the landfill gas that is generated. If the flare operates at 1400°F and 100% excess air is used, then the exhaust flow rate is 75,260 cfm. Assuming an exhaust velocity of25 Wsec, the flare will have a diameter of 8 ft. For air dispersion modeling purposes, the flare is assumed to be 40 ft tall. If the heating value of the gas is assumed tobe 520 BNft3, then the flare will be rated at 62 million Btu/hr. California NO, and CO emission limitsof 0.06 and 0.30 lb per million Btu heat input, respectively, are used. An industry-wide emission limit of 40 lb/106 ft3 for total suspended solids, of which 50% is PM,,, is also used. To estimate emissions of toxic chemicals, it is assumed that60% of the total landfill gas generated is collected and directed to theflare, and 40% is emitted from the landfill surface. Toxics emissionrates from the flare are calculatedby multiplying the landfill gas inletrate by the chemical concentration and the destruction efficiency. For emissions from the landfill surface, the destruction efficiency is not applied. These emission rates are used in the evaluation of health effects.
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VI. MODELINGMETHODOLOGY A. AirDispersionModeling For air dispersion modeling purposes, the enclosed flare is treated as a point source (stack), and the landfill surface is treated as an area source. Commonly usedair dispersion models are the EPA’s IndustrialSourceComplex, Short Term(ISCST)andSCREENmodels[11,12]. SCREEN, which is referred to as a “screening” model, is easy to run. ISCST is a refined model that is much more difficult to run, requires many more site-specific parameters, and yields more representative values than SCREEN. an emission rate of 1 @sec, facilitating scaling of the predicted The modeling is done using relative concentrationsto compound-specific concentrations.The equation that is used forsealing of relative groundlevel concentrations, obtainedfromdispersionmodeling runs, to compound-specific concentrations based on actual emission rates is [l11 (xu/Qu)Qo
=
X,
(9)
where X J Q , is the relative or unit concentration, (pg/m3)/(g/sec);Q, is the compound-specific actual emission rate, g/sec; and X , is the compound-specific actualconcentration, pg/m3. UnlikeISCST,SCREENdoesnottakeactualmeteorological conditions intoaccount. SCREEN calculates the maximum hourly concentration using worst-case meteorological conditions. Actual concentrations would be much lower than these hypothetical concentrations.
B. PointSources Flare emissions are modeled with the SCREEN model at a 1 @sec emission rate. SCREEN does have a flare emission release type that requests specific emission parametersas input, but this release type applies to open flares. For exhausts from enclosed flares (flame at ground level), it is more appropriate to input the point source (stack) parameters. SCREEN calculates the maximum relative concentrations of pollutants from the flare. Using Equation(9), the emission rates for each chemicalin the flare exhaust are multiplied by the relative concentration to yield the maximum predicted ambientconcentrations for each substance.
C.AreaSources Area sourcesare the best representation for fugitive releases froma landfill surface. One characteristic of an area source (with sides of length L) is that estimated concentrations are inacIn addition, the EPA curate withindistances of lessthan L to the areasourceitself. recommends using area sources withside lengths of between 50 and 150 m. Therefore, if 100 m by 100 m area sources were used, 100 of these area sources would be needed to cover a square landfill surface 1 km2. vpically ISCST, which can model multiple sources in one run, by 100 sources would be used for such an application. However,the resources and time required are extensive. An estimate of the health risk associated with the fugitives can be calculated using the SCREEN model as follows. Find the nearest distance to the property boundary fromthe landfill surface. This point is the nearest receptor site. SCREEN is not direction-specific in its air dispersion modeling, so the nearest off-site receptor will usually be the point of maximum impact. If the model is run using the regulatory defaultoptions, it will automatically findthe worst-case hypothetical meteorological conditions (wind speed, stability, and mixing height) for air dispersion. Although these conditions may actually never occur at a particular site, this procedure gives the worst-
Landfill Gas Collection and Destruction
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case ground level air concentrations. In the hypothetical subject site presented here, assume that the point of maximum impact is 200 m from the midpoint of an area source with a side length of 100 m located on the edge of the landfill. Determine the maximum number of 100 m by 100 m area sources acrossthe surface of the landfill that lie along a straight line tothe receptor point. Thesearea sources lie along the plume line of the dispersed gases.In the subject landfill area of1 km2, there would be a maximum of 10 such area sources. It is assumed that all of the emissions from the area sources are impacting the chosen receptor point throughout the entire year. This is a conservative assumption that increases the calculated maximum ambientconcentration but drastically reduces the number of sources that are modeled. The maximum numberof area sources that can impact this receptor witha single wind direction throughout the year is the maximum number of area sourcesthat can be aligned in a straight line with the impact point.SCREEN assumes onewind direction. Sources thatare not lined up inthe single wind direction cannot affect the impact point. Forthis reason, not all the 100 square area sources need to be modeled. In this case (modeling 10 area sources) there is a substantial reduction fromthe number of sources thatwould have been modeled (100 area sources) using a refined algorithm such ISCST as that accounts for real meteorological conditions. The number of sources modeled can befurther reduced to one, since the modeling parameters are the same for the identical areas. The concentrations at 10 discrete distances 100 m apart are calculated, beginning at the nearest distance from an area source and ending at the farthest distance. In the subject landfill, the nearest distanceis 200 m, and thefarthest distance one area source correspondto the distances from the midpoints is 1100 m. These distances from of the given number of aligned squares(10) to the single point of impact locatedat the nearest distance downwind from the first area source. Therefore,this method is equivalent to separately modeling 10 area sources, each with a single distance.
D. Calculation of Ground Level Concentrations The predicted concentrations at each distanceare then summed to yield the cumulative impact from all of the fugitive emissions fromthat particular landfill. Since each area source is modeled as emitting 1@sec, the cumulative concentration is divided by the number of sources to yield the relative concentration. This is the unit concentration based ona 1 @sec emissionrate from the fugitive sources. Equation (9) is used with chemical emission rates from the landfill surface to calculate substance-specific concentrations. All of the emissions calculatedfor the total landfill surfaceare assumed to be emitted from the modeled area sources. Again, this is a conservative assumption that supposes that allof the emissions impact a single point. These emissions rates are multiplied by the relative concentration to yield calculated maximum concentrations. A less conservative approach would be to assume that the emission rate from each area the landfill surface, whereN is the total number ofarea source is 1/N of the total emissions from sources that lie on the landfill. SCREEN calculates maximumhourlyconcentrations.Factors are available to convert these values to annual average concentrations. Annual averageconcentrations are used for the health risk assessment calculations regarding long-term health effects. Maximum hourly concentrations are used for short-term health effects.
VII.DEVELOPINGHEALTH
RISK EVALUATIONS
There are eight primary steps necessary to quantify the health risks associated with landfiil gas emissions:
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Ohannessian et al.
1. Determination of the landfill gas generation rate 2. Determination of the efficiency of the landfill gas collection system 3. Determination of the compounds that are present in the landfill gas 4. Development of a dispersion modeling scenario and emission rates 5 . Completion of dispersionmodeling 6. Scaling of the dispersion model results for specific compounds 7. Application of unit risk values 8. Summation of the risks for the maximum exposed individual
The EPA and individual regulatory agencies have developed extensive guidance for the quantification of health impacts, from ambient concentrations of a wide rangeof air toxics [l]. California methodologiesare based on dose-response curves developed by the EPA, the California Air Resources Board, and the California Department of Health Services. Three surrogates for potential health risk are used: cancer risk, chronic hazard indices, and acute hazard indices. Cancer risk is calculated by multiplying the annual averageconcentration of a compound by a compound-specific unit risk factor. Unit risk factors are available from regulatory agencies. Cancerrisk is a conservative estimate of the likelihood that an individual whois exposed to the emissions in question for 70 continuous years will contract cancer as a result of this exposure. Chronic hazard indices are a measure of the likelihood that an individual who is exposed to the emissions in question for 70 continuous years will contract a noncancer health effectin one of several target organdsystems as a result of inhalation of emissions. Other pathwaysof exposure include ingestion of contaminated soil, water, food, and mother’s milk. These noncancer healtheffects may occur because of the toxicityof the chemicals. Chronic hazard indices are calculated by dividing the annual average concentration of a compound by a compoundspecific acceptable chronic level of exposure. That is, a chronic hazard index is the ratio of calculated long-term exposure to acceptable long-term exposure. Acute hazard indices are a measure of the likelihood of an individual suffering adverse health effects asa result of an acute exposure via emissions inhalation. Acute hazard indices are calculated by dividing the maximumhourlyaverage concentration of a compound by a compound-specific acceptable acute level of exposure. That is, an acute hazard index is the ratio of calculated short-term exposure to acceptable short-term exposure. Once all of the surrogates for the health risk are calculated, like surrogates are summed over all of the compounds present at the point of maximum concentration. The point of maximum concentration is the peak off-site concentration for the fugitives and the peak receptor from the flare.This calculation yields the maximum health risk associated with each emitting source. It is assumed thatthe sources are placed in a manner that would cause their respective peak concentration points tooverlap, a conservative assumptionthat is physically impossible. The total healthrisks from the flare are added to the health risks associated with the fugitives. This summation is a result of the assumption thatthe peak receptors from each source overlap.
VIII. RESULTS The resulting assessment displaysa total cancer risk from all of the sources of less than 10 in a million, the value that is generally consideredsignificant. This value represents the potential risk of 1 excess cancercase per 100,OOO individuals. However, the exposure must take placefor 70 continuous years in order for the excess cancer burden to be representative. Values representing chronic and acute hazard indices are found to be less than 1.O, the value that is gen-
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erally considered to be significant. A value under 1.O for either of these indices represents an acceptable level of exposure for an individual. Emissions of the EPA criteria pollutants, NO,, CO, and PM,,, are found to be lower than the rates acceptable by the regulatory agencies. Although 60% of the gas is assumed to be collected and diverted to the flare, the risk associated with flare emissions is orders of magnitude lower thanthe risk from landfill surface emissions.This is a resultof the destruction of the toxicchemicals in the flare during combustion. Therefore, higher landfill gas collection efficiencies result in lower associated health risks.
1.
2. 3.
4. 5.
6. 7. 8. 9.
10. 11. 12.
CAPCOA, Air Toxics “Hot Spots” Program, Risk Assessment Guidelines,California Air Pollution Control Officers Association. January 1991. I .I . User’sManual, PB9 -11677 18, prepared EPA, Landfill Air Emissions Estimation Model Version by Radian Corp., Research Triangle Park, N.C., April 1991. for Landfill Gas Emissions, California Air Pollution Control CAPCOA, Suggested Control Measure Officers Association, August 1990. CARB, AB 2588 Landfill Gas Emission Rate Estimation Technique (letter toAir Pollution Control Officers of various Districts), April 1990. CARB, The Landfill Testing Program: Data Analysis and Evaluation Guidelines,California Air Resources Board, August 1990. BAAQMD, Suggested Generation Rate and Unit Risk Factors for Landfill Gas Emissions (letter from Laura Harnish of CH2MHillto Steve Hill of the Bay Area Air Quality Management District), January 1990. EPA, Air Emissionsfrom Municipal Solid Waste Landfills-Background Information for Proposed Standards and Emission Guidelines, EPA-450/3-90-01l(a), Research Triangle Park, N.C., 1991. Christensen, T. H., Cossu, R., and Stegmann,R. (eds.), Sanitary Landfilling:Process, Technology and Environmental Impact, Academic, San Diego, Calif., 1989. Wong, T. S., Dioxin formation and destruction in combustion processes, 77th Annual Meeting of the Air Pollution Control Association, June 1984. Dernbach, H., Corrosion of gas engines at landfill gas utilization plants caused by chlorohydrocarbons, Recycling International, 1989. Wagner, C., Industrial Source Complex Dispersion ModelUser’s Guide, 2nd ed. rev., Vol. 1, EPA450/4-88-002a, U.S. EPA, Washington, D.C., 1987. Brode, R., Screening Proceduresfor Estimating the Air Quality Impact of Stationary Sources, EPA45014-88410, U.S. EPA, Washington, D.C., 1988.
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Index
Activated sludge treatment, 500-504 Aerobic digestion, 509, 709-710 AHP (analytic hierarchy process), 42 Air dispersion modeling, 722 Alternate point of compliance (APC), 119-142
Amalgam, 561 Ammonia stripping, 508 Anaerobic digestion, 509, 710-711 Aquifer parameters, 581 Arsenic contamination, 454 Ash, 476-478 Baseline risk assesment, 87 Bayesian approach, 576-577 Biodegradation, 276, 405 BOD (biochemical oxygen demand),491493,497,499-506,509
Calcium oxide, 559 Cancer, 724 Carbonation, 315-318, 321, 322, 327-328 Carbon dioxide, 715 Casing, 311-320, 322, 329, 330 Caustic soda, 559, 561-563 Cavern closure, 35 1-352 Cement-based techniques, 272-273
Cementitious materials, 442, 445, 447 CERCLA, 119, 644 Chelates, metal silicate, 444 Chlorination, 536 Chlorine, 475, 536 Chromate, 535, 554 reduction, 538, 540 Cleanup level, 87 Clean Water Act, 535 Confining zone, 354 Cost analysis, 20-21, 28-44, 397 Creosote, 89, 94 Cyanide, 535-537, 554 Cyanogen chloride, 536 Dairy wastes, 705 wastewater treatment,709-7 11 Decision theory, 30-44 Denitrification, 507 Department of Defense, 17 Destruction efficiency, 720 Deterministic management model, 572-573 Deterministic simulation, 105-1 18 Dewatering, 322, 329, 510 Diaphragm cell, 561 Dioxins, 384 727
728
Index
Dispersion, 461, 464 DO (dissolved oxygen), 490 Electrode calomel, 544 glass, 558 reference, 558 saturated silver, 545 Electrolytic oxidation, 537 Electrowinning, 312, 318-19, 324, 326, 328,330
Environmental Protection Agency, 17,20 Equilibrium temperature, 515 Exposure pathways, 724 Extraction prccedures, 446, 453, 464 FBC (fluidized bed combustion), 467-470
FC (freeze concentration), 513 Feasibility analysis, 660 Feed pumps, 545, 548 Filters, trickling, 497-500 Flares, 718 Flow rates, 389, 552 Fluosilicic acid, 318-319, 324-325 Fly ash, 271 Freeze-thaw, 290 GIS (geographic information systems), 32 Gross national product, 26 Groundwater, 569,577 Halogens, 474 Hazardous materials, 18, 21-22 Hazardous waste residuals, 331 Hazard severity, 19 Heat treatment, 510 Heavy metals, 271 arc furnace dust, 441 auto shredder residue, 441 groups, 446, 450 sludges, 441 soil, 441 wastewater, 535 HRA (health risk assessment), 715, 723-725
HSWA (Hazardous and Solid Waste Amendments), 644 Hydraulic load, 499
Hydrocarbons petroleum, 405 polycyclic aromatic (cPAHs), 88-94 Hydrochloric acid, 474 Hydrogen peroxide, 390 Incineration, 510 Injection interval, 354 zone, 354 Intrinsic properties, 19 Kinetics, 385 Land application, 711 Landfarming, 405 Landfill gas, 71 collection systems, 717 emissions, 720 generation, 716 treatment systems, 718 Leachability, 276, 290-291 Lead, 31 1-330 Legal requirements, 639 Life cycle, 20 Light intensity, 385 Lime, 559-561 Material balances, 656-658 Membrane cell, 561-562 systems, 516 Mercury cell, 561 Methane, 715 Military facilities, 637-642 Mishap probability ( M E ) , 19 MLSS (mixed liquor suspended solids), 501-504
Mobility index, 124 Monte Carlo simulation, 105-118 MSDS (material safety data sheets), 22 Multimedia exposure assessment model (Multimed), 88, 101-118 Multistage systems, 520 National priorities list, 31, 40 Navy substitution algorithm, 21 Neutralization, 557-566 Nitric acid, 317-318, 322 Nitrificatioddenitrification,507
729
Indf?X Nitrogen, 491-492,506-508 NMOCs, 715 NO,, 473 N,O, 474
Ripening, 516 Risk, 36-44,94 RIS (risk information system), 43 Rotating biological contractors, 504-505
Optimization, 569 Organics, 405 concentration, 385 load, 499 organic leachate model (OLM), 88,
Salt deposits, 332 SARA (Superfund Amendments Reauthorization Act), 644-645 Semiconductors, 381 Simulation, 32 Site inspection, 655-656 Sludges disposal, 511 heavy metals, 441 pulp mill, 473 residuals management, 360 wastewater, 508 Soil, 311-314, 319-322, 324-330, 405 petroleum contaminated, 280 Solar illumination, 396 Solidification, 346, 349, 442 Solution-mined repository, 332 Solution-mining, 333 Solvents, 384 Sorbent techniques, 272, 275 utilization, 479 Source reduction, 647 Specific surface area, 388 SS (suspended solids), 490, 492 Stabilization ponds, 505 Stochastic, 569, 574 Stochastic unit influence coefficient, 574 Strategic Petroleum Reserve, 333, 340 STS technology, 4 4 1 Substitution methods, 17 Superfund, 94, 311, 330 Surface reactions, 382-383 SVI (sludge volume index), 504 Synthetic general rinse effluent, 552 System performance, 555
120- 142
polymer techniques, 272, 275 ORP (oxidation-reduction potential), 546-547
Oxygen concentration, 385 Particle size distribution, 461 PCBs, 384 Permissible exposure limits (PEL), 18 Pesticides, 384 pH, 387,557-559 Phenols, 88, 94, 384, 395 Phosphorus, 491-492, 506-508 Photoexcitation, 380 PM,,, 719 Pollution prevention, 17, 591 Pollution Prevention Act of 1990, 644 Polysilicate, 441, 447 Potency factors, 89-92 Precipitation metal hydroxide, 541, 554 metal sulfide, 542 Priority measurement, 19 Problem avoidance, 34, 38, 42 Process changes, 648 Process performance data, 549 Pug mill, 447 Quinhydrone, 544 RAC (risk assessment code), 18-19 RASS (resource allocation support system), 43 RCRA (Resource Conservation and Recovery Act), 644-645 tank, 453, 456 Records of decision (RODS), 119 Recycling, 638-639, 641, 650 Relative ground level concentrations, 722 Residuals management, 360
Titanium dioxide, 381-383, 386 Toxicity aquatic, 593 and cancer, 724 effects, 22 effluent, 593 hazards, 22 TQM (total quality management), 591
730
Training, 641-642 Treatability studies, 445 Treatment activated sludge, 500-504 aerobic, 509, 709-710 anaerobic, 509, 7 10-7 1 1 caustic, 559, 561-563 chemical, 442 dairy wastes, 709-71 1 efficiency, 553 heat, 510 landfill, 7 18 levels, 454, 456 metals, 444, 541-542, 554 on-site, 453, 454 oxidative, 537 preliminary, 493 priority, 493-494, 498 secondary, 494,497, 506 studies, 445 tertiary, 494, 506
Index
Treatment units, transportable, 447, 452 TSS, 491, 493, 506 Unit response functions, 571-572 Uranium, 641 Vertical and horizontal spreading (VHS), 88, 120- 142
Viscosity, 5.15 Vitrification, 272, 275, 276 VOC (volatile organic compound), 20 Wartime operations, 641 Wash column, 518 Waste minimization, 591 reduction, 646-647 streams, 658
WIPP (waste isolation pilot plant), 340, 35 1
