Biosolids Treatment and Management (Environmental Science and Pollution Control, 18)

BO l SOD lS TREATMENT MANAGEMENT Processes for Beneficial Use

edited by

Mark J. Girovich Wheelabrator Clean Water Systems lnc. Annapolis, Maryland

Marcel Dekker, Inc.

New York. Basel Hong Kong

Library of Congress Cataloging-in-Publication Data Biosolids treatment and management : processes for beneficial use / edited by Mark J. Girovich. p. cm. - (Environmental science and pollution control series ; 18)

Includes bibliographical references and index (p. ). ISBN 0-8247-9706-X (alk. paper) 1. Sewage sludge-Management. I. Girovich, Mark J. 11. Series: Environmental science and pollution control ; 18. TD767.B55 1996 628.3'644~20

95-5 1804 CIP

The publisher offers discounts on this book when ordered in bulk quantities. For more information, write to Special Sales/Professional Marketing at the address below. This book is printed on acid-free paper. Copyright

0 1996 by MARCEL DEKKER, INC. All Rights Reserved.

Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming, and recording, or by any information storage and retrieval system, without permission in writing from the publisher. MARCEL DEKKER, INC. 270 Madison Avenue, New York, New York 10016 Current printing (last digit): 10 9 8 7 6 5 4 3 2 1

PRINTED IN THE TJNITED STATES OF AMERICA

Preface Treated municipal wastewater solids (biosolids) represent a sipficant and valuable resource which can be recycled for various beneficial uses. The United States Environmental Protection Agency regulations (40 CFR Part 503 "Standardsfor the Use of Disposal of Sewage Sludge") promulgated on February 19, 1993, have provided new momentum and defined regulatory conditions for implementing beneficial uses of biosolids. Treatment and management of biosolids is one of the most challenging problems in the wastewater treatment industry. It is complicated by a variety of treatment and end-use options. New federal and state regulations conerning environmental safety and public health have imposed sigdcantly more complex requirementsfor bimlids treatment,use and disposal. Selectingfeasible options that conform to regulations, while minimizing cost, has become very challenging. With Ocean disposal no longer allowed, landfills filling up and incineration already and often socially unacceptable,biosolids recycling through beneficial use e-ve is gaining in popularity. Such recycling has been practiced by many communitiesfor years in the form of land application of stabilized biosolids. The more advanced treatment options of digestion, composting,heat drymg and alkaline stabilization have been developed in recent years to provide marketable products which, because of the additional treatment, are usually subject to fewer regulatory controls. This book was conceived late in 1993 after the new environmental, safety and public health regulations concerning municipal sludge (biosolids) treatment, management and disposal were introduced. It is written by a group of authors with many years of practice in the field and reflects their unique experience. The text emphasizestheuse of biosolids, reflecting the authors' strong belief that biosolids are a valuable resource and should be beneficially employed in the amtext of envinmnental, safety and public health regulations. By providing valuable technical and economic this book should prove to be an invaluable resource for . . tors,data, engineers, consllltants,students and practitioners in the field municipal who must know how to evaluate and select the best municipal and industrial wastewater solids (bimlids) treatment, management and disposal options. This book contains descriptions of the processing equipment, economics and regulatory and environmentalprotection issues related to biosolids management and use. I would like to expms my gratitude to all the people who have been helpful in the creation of this book. Special thanks are extended to Sue Gregory, Jamie Kaiser, andKathleen Wooldridge (Wheelabrator Clean Water Systems Inc., Bio Gro Division) for their assistance in preparing the manuscript. Mark J. Girovich

iii

This Page Intentionally Left Blank

Contents iii xi

Preface About the Contributors Chapter 1

Biosolidr Characterization,Treatment and Use: An Overview Mark J. Girovich

I.

1

Biosolids Generation and Beneficial Use

II. Biosolids Characterization A. B. C. D.

Composition and Beneficial Properties Microbiology of Biosolids Odors and Other Nuisances Other Characteristics

III. Biosolids Treatment for Beneficial Use: An Overview Beneficial Uses B. Requirementsfor Beneficial Use C. Treatment Processes: An Overview References A.

Chapter 2

Federal Regulatory Requirements Jane B. Forste

I.

47

Historical Backpound and Risk Assessment

A. B. C. D.

Inimxluction Basis for the 503 Regulations DataGathering Risk Assessment Methodology

II. Final Part 503 Regulations A. B. C.

Introduction Exposure Assessment Pathways Final Part 503 Standards

III. Pathogen and Vector Attraction Reduction References

V

Contents

vi Chapter 3

Conditioning and Dewatering Robert J. Kukenberger

I.

Sntroduction

II.

Conditioning A. Organic Polyelectrolytes B. Polymer Feed and Control Systems C. Inorganic Chemical Conditioning D. Thermal Conditioning

131

III. Dewatering A.

Process Description

B. Thickening C. Mechanical Dewatering D. Passive Dewatering IV. Odor Control V.

Chapter 4

Casestudies References

Digestion Kenneth J. Snow

I.

Biosolicls Digestion A. Introduction B. Process Fundamentals C. Equipment Review D. Economics of Digestion

n.

casestudies References

Chapter 5

Cornposting Lewis M.Naylor

I.

Introduction A. Growth ofCornposting in the United States

II. Goals of cornposting A.

B. C.

ChemicalQuality Biological Quality Customer Requirements

165

193

vii

Contents

HI. Process Fundamentals A. B. C.

Microbial Community Environmental Conditions Nutritional Considerations

IV. Solids and the Cornposting Process A. Types of Solids B. Particlesize V.

Process Energetics A. The Biological Fire B. Heat and Temperature C. Temperature Control D. Aeration

VI. Preparing a Blended Feedstock A. B. C. D.

Dry Solids and Porosity Chemical Composition Ingredient Selection Developing a Blended Feedstock Recipe

W. OdorRemoval A. OriginsofOdors B. Odor Control Technologies C. Biofilter Fundamentals and Operations D. Biofilter Challenges WI. Pre- and Post-Processing

IX. Marketing A.

X. Chapter 6

Marketing Issues

OutlookandSUmmq References

Heat Drying and Other Thermal Processes Mark J. Girovich I.

BiosolidsDqmg A. Heat Drying and Production of Fertilizer B. Partially Dried Biosolids

II. Heat Drymg Processes

271

viii

Contents

III. DryerDesigos A. B.

DirectDryers IndirectDryers

IV. Major Dryer Parametem A. Evaporation Capacity B. Energy and Dqmg Air Requirements

V.

HeatDrylngSystems A. System Components B. Handling and Treatment of Drying and Heating MeCllUftl C. EnvLonmental Control and Regulatory Issues D. Ekonomics of Heat Drying

VI. Production of Fertilizer: Case Studies A. Milwaukee Biosolids Dqmg and Pelletizing Plant B. C.

New York City Biosolids FertilizerFacility Baltimore City Fertilizer Facility

W. Other Thermal Processes A. C a r v e r - M e l d (C-G) Process B. Wet Oxidation (Zimpro Process) References Chapter 7

Alkaline Stabilization Mark J. Girovich

I.

Introduction

II. Alkaline Stabilization A. B. C.

Pre-Lime and Post-Lime Stabilization Process Fundamentals Alkaline Materials

III. Proprietary Alkaline StabilizationProcesses A. BIO*FIX Process B. N-ViroSoil Process C. RDP En-Vessel Pasteurization D. ChemfixProcess E. Other Alkaline StabilizationProcesses IV. Economics of Alkaline Stabilization References

343

ix

contents

Chapter 8

Land Application Jane B. Forste I.

389

Introduction A. Historical Background

II. Beneficial Properhes of Biosolids A. €3.

C.

Nitrogen Considerations Effects of Organic Matter from Biosolids on Soil Properties Effects of Other Biosolids Constituents

III. Site Selection, Design and Management A. B. C. D.

E.

Site Selection Nitrogen-Based Agronomic Rates Design for Non-Agricultural Sites Pathogen Considerations in Land Application Projects Agronomic Considerations

IV. Methods and Equipment A. Transportation V.

Economics of Land Application

VI. Monitoring and Recordkeeping A. B. C. D. E.

GeneralRequirements of 40 CFR 503.12 Pathogen Reduction Vector Attraction Reduction Management Practices Monitoring

W. Public Outreach A. Communication Channels References

Index

449

This Page Intentionally Left Blank

About the Contributors

Mark J. Girovieh, Ph.D., book editor and author of Chapters 1, 6, and 7 has years of experience in mechanical and environmentalengineering and project j management and is currently Director of Engineering j for the Bio Gro Division of Wheelabrator Clean Water ' Systems Inc. Dr. Girovich earned a Ph.D. in mechanical engineering from Moscow Polytechnic Institute (Russia), an M.S. in mechanical engineering ,~ Kharkov from Polytechnic Institute (Ukraine) and a B.S. in appliedphysics fromKharkov State University. , . ' '.., He is amemberof the AmericanSocietyofMechanical L i j Engineers (ASME). The author numerous of papers and several patents, he has specialized in biosolids heat drying and alkaline stabilization over the past 10 years. ""

8

1

Jane B. Forste, author of Chapters 2 and 8, is currently Vice President of Technical Servicesfor the Bio Gro Division of Wheelabrator Clean Water Systems Inc. Ms. Forste holds B.S. and MS. degrees in agronomy from the University of Vermont. She has over 20 years of experience in the field and has frequently published and presented papers at professional conferences. She is a Certified Professional Agronomistand Registered Environmental Professional. Ms.Forste serves on committees for the Water Environment Federation (WEF), American Society for Testing and Materials (AS"), Association of MetropolitanSewerage Agencies (AMSA) and the National Association of ProfessionalEnvironmental Communicators(NAPEC). Ms. Forste develops technical input to federal and state regulatory agencies and has workedextensivelywith civic and political organizations. Robert J. Kukenberger, P.E., author of Chapter 3, is currently Executive Vice & Lee, Inc. and is responsible for the firm's water and President of Blasland, Bouck wastewater engineering programs nationwide. Mr. Kukenberger received aB.S. in Civil Engineering from Tri State Universityanand M.S. from the University of Rhode Island. The author of numerous technical publications,Mr. Kukenberger has over 8 years of experience in the field biosolids treatment and management.

xii

About the Contributors

Lewis M. Naylor, Ph.D., author of Chapter 5, is Environmental Scientist for Black and Veatch. Dr. Naylor received his Ph.D. in Civil Engineering from Iowa State University, M.S. degrees in Chemistry from the University of Northern Iowa and Environmental Engineering from the University of Iowa, and B.S. degree in Chemistry from Bluffon College (Ohio). He has published numerous papers on composting, recycling and beneficial use of biosolids. Dr. Naylor is a member of the American Chemical Society (ACS), American Water Works Association (AWWA), and Water Environment Federation (WEF). For more than two decades, he has assisted industries and communities in developing beneficial use and recycling projects. Dr. Naylor has been associated with Cornell University’s Department of Agricultural and Biological Engineering and Wheelabrator Clean Water Systems Inc., IPS Division. Kenneth J. Snow, P.E., D.E.E., author of Chapter 4, is a professional engineer with over 20 years of experience with industrial and municipal wastewater treatment design, construction and operation. He is currently President of Corporate EnvironmentalEngineeringLnc. in Worcester,Massachusetts. Mr. Snow holds a B.S. degree from Lowell Technological Institute and an M.S. degree fiom Northeastern University.

Biosolids Characterization, Treatment and Use: An Overview Mark J. Girovich

Wheelabrator Clean Water Systems Inc. Annapolis, Maryland

1 BIOSOLIDS GENERATIONAND BENEFICIAL USE Municipal wastewater treatment produces two products: clean water and water slurries which are usually referred to as sludges. While clean water is dlsposed of directly into the environment, it is not feasible, environmentallyor economically, to do so with the solids generated by wastewater treatment processes. They must be treated prior to disposal or beneficial use to comply with public health, safety, environmental and economic considerations. Municipal wastewater treatment solids contain si@cant amounts of organic matter as well as inorganic elements which represent a valuable resource. These components of the solids, &er appropriate treatment, can be beneficially used (recycled) as a fertilizer, soil amendment or other beneficial use products. Energy contained in the solids can be recovered. Ash resulting fiom the solids incineration can also be used beneficially. Recognizing potential value of wastewater solids,the term biosolids is used in thisbook to mean a product of the wastewater solids treatment that can be beneficially used. In 1989,5.4milliondry metric tons of municipal solids were produced per year by approximately 12,750 publicly owned treatment works (POTWs) [l]. Current (1995) municipal solids production is approximately 8.0 million dry metric tons per year and is expected to increase substantially by the year 2000 due to population growth, improvements in POTW operation and stricter treatment standards. 1

Girovich

2

Treatment and disposal of municipal solids in the USA is a growing business estimated at several billion dollars mually. This business, similar to the municipal wastewater treatment industry, is financed by taxation (federal grants, state taxes, sewer fees, etc.) Quantities and quality of biosolids produced by the POTWs vary widely and depend upon the origin of wastewater, type of treatment and plant operational practices. Biosolids in the U.S. have beenmanaged as follows (1992, dry metric tons):

rmm 1-1 DISPOSALOF MUNICIPAL SOLIDS IN THE USA [13

I

Co-dispo~alin MSW landfills

1,818,700

34.0%

1,785,300

33.3%

Incineration

864,700

16.1%

Surface dimsal

553,700

10.3%

ocean disposal*

335,500

6.3%

5,357,900

100%

Land application

TOTAL

* Ended in June 1992 In the land applicaiim category, the following beneficial use options are growing in popularity (design capacity, dry metric tons per year, 1995 estimates): *

Heat Drylng and Pelletizing

375,000

*

Alkaline Stabilization

650,000

- Composting

550,Ooo

The European Union (Ev> (12 countries) generated over 6.5 million dry metric tonsof municipal solids in 1992. By theyear 2000, the amount will increase up to 9.0 million dm@ear. The EU directives of 1986 and 1991 generally promote beneficial use of sewage sludge solids [2]. The future of beneficial use in the EU will depend on the legislation regarding standards for beneficial use and incineration as well as the acceptance of beneficial use versus incineration by the population. At present, the beneficial use in the EU varies from 10 to 60 percent (Table 1-2). Japan generates over 1.4 million dm@ear with approximately60% incinerated. Accurate data on solids disposal in developing countries of Eastern Europe, Asia,

Biosolids Characterization,Treatment and Use

3

Latin America and Africa are not available at present. Detailed discussion on biosolids disposal worldwide is provided in References [2] and [111.

The US Environmental Protection Agency (EPA) actively promotes beneficial use of municipal solids because it decreases dependence on chemical fertilizers and provides signtficant economic advantages. Over 20 years of research have been devoted to the use of biosolids on agriculture and similar beneficial applications. Beneficial use includes: *

*

application to agricultural and nonagricultural lands alone or as a supplement to chemical fertilizers application in silviculture to increase forest productivity

4

*

* *

Girovich

use in home lawns and gardens useongolfcomes application to reclaim and revegetate disturbed sites such as surface-mined areas use as a daily, maintenance and final cover for municipal solid waste (MSW) landfills.

Land application is essentially the placement of appropriatelytreated biosolids in or on the soil in a manner that utilizes their fertilizing and soil conditioning properties. It includes agricultural, forest and site reclamation applications, and a number of biosolids-derived products, such as dlgested, dried, chemically or heat hreated biosolids, and compost. Biosolids-derived products are distributed in various forms, such as in bulk, packaged, further processed, enriched, sold to public, etc. They are applied to agricultural and nonagricultural lands, soil reclamation and revegetation sites, forests, lawns, gardens, golfcourses, parks, and so forth. Surface disposal includes disposal in monofills and dedicated sites. @Iisposal on dedicated sites is a beneficial use method of applying biosolids at greater than agnmomic rates at sites speczdy set aside for this purpose to restore disturbed soils (e.g., strip mines).] Incineration is generally regarded as a nonbeneficial disposal method of solids management unless heat recovery for steam and electricity generation is included in the process. One or more levels of treatment (i.e., primary, s e c o n w and tertiary) are used to clean wastewater. Each level of treatment provides both greater wastewater cleanup and greater amounts of municipal solids.

*

-

primar?r solids are removed by gravity settling at the beginning of the wastewater treatment process. They usually contain 3.0 to 7.0 percent total solids (%TS), 60 to 80 percent of which is organic matter (dry basis). The primary solids are generated at the rate of approximately 2,500 to 3,500 liters (660to 925 gallons) per each million liters of wastewater treated. Secondarv solids are generated by biological treatment processes called secondary treatment (e.g., activated sludge systems, trickling filters and other attached growth systems which utilize microbes to remove organic substances from wastewater). They usually contain fiom 0.5 to 2.O%TS. The organic content of the secondary solids ranges from 50 to 60%. About 15,000 to 20,000 liters (3,965 to 5,285 gallons) of these solids are generated per each million liters of wastewater treated. Advanced (tertiary) solids are generated by processes such as chemical precipitation and filtration. The solids content varies fi-om 0.2 to 1.5%TSwith the organic content of the solids in the 35% to 50% range. About 10,000 liters of solids (2,642 gallons) are generated additionalIy per each million liters of wastewater treated.

Biosoiids Characterization, Treatment and Use

5

-ABLE 1-3 SOLIDS GENERATION r i i Item

Primary

Unit

Amount generated

liters

2,5003,500

15,000-

10,000

Percenttotal solids

%

3.0-7.0

0.5-2.0

0.2-1.5

Dry biosolids

metric tons per

0.1-0.15

-0.2-0.31 0.02-0.15

I

tons per million

0.42-0.55 0.8-1.2 ~

1

0.08-0.6

I

Table 1-3 provides for quick, approximate estimates of the total biosolids generated as a function of a POTW wastewater influent flow. For example, a 30 milliongallon per day P O W (30 mgd) with primary and secondary treatment (typical for a large number of the POTWs in the USA) will generate 30 mgd x (.485+ 1.O) = 44.5 dry tuns of biosolids per day (dtd) approximately. (0.485and 1.O are average biosolids generation for primary and secondary treatment, tons per million gallons respectively.)

IL BIOSOLIDS CHARACTERIZATION Characterization of biosolids by their source (primary, secondary, etc.) provides only limited information about their properhes. There are numerous other physical, chernical and microbiological parameters which are important for biosolids' treatment and management.

k Composition and Beneficial Properties 1. Solids Concentration

Solids concentrations are measuredand expresseL either as mg (milligrams per liter) or as percent (%) of solids. In all cases in this book it is assumed that: 10,000mgA = 1% total solids (TS). Note that percentage of total solids is weighdweight ratio while solids concentration(mgA) is weightholumeratio. The equation above is valid only with the assumptionthat specific gavity of biosolids is equal to that of water (1 .O) which, in

6

Girovich

many wastewater solids,especially thoseof industrial origin, is not true. The standard procedurefor determhhg solids concentration employs drying a measured volume of

biosolids to a cunstant weight at 103 - 105 C. The solids concentration is the weight of dry solids divided by the volume of the sample expressed in mgA. In order to determine percent of total solids (%TS) as a weightlweight ratio, the identical procedure is applied with a measured weight of the biosolids sample. Total volatile solids f%TVS) are determined by igniting the dry solids at 550" + 50°C in a furnace with excess of oxygen. The residue is referred to as non-volatile or as fixed solids (ash) and the loss of weight on ignition determines total volatile solids, Both %TS and %TVS are widely used in the biosolids treatment and management practices as measures of dry matter (or moisture) and organic (combustible) matter in the biosolids. Total sumended solids (TSS) refers to the nonfilterable residue retained after fdtration of a sample of the liquid biosolids and then dried at 103 105O C to remove water. The concentration of TSS is the weight of dry solids divided by the volume of the sample usually expressed in mgA. Determination of volatile suspended solids is identical to that of total volatile solids using loss on ignition methods. Total solids are the sum of the dissolved and suspended solids. [3] Water in biosolids is usually categorized as follows: O

O

O

*

.. -

-

Free water is not attached to the biosolids particles and it can be separated by gravitational settling. Floc water is trapped within the flock and can be removed only by mechanical forces, which are usually much greater than gravitational force. Carillarv water adheres to individual particles and can be also separated by mechanical forces. Intracellular and chemicallv bound water is part of cell material and is biologically and chemically bound to the biosolids' organic and inorganic matter.

Approximate amounts of energy required to remove water h m biosolids by various methods are as follows (per cubic meter of water) [4]:

*

Gravity (thickening): 10' KW to achieve 2-6%TS Mechanical dewatering: 1-10 KW to achieve 1530% TS Thermal drying: lo3KW to achieve 85-95% TS

In other words, water removal requires approximatelyone million times more energy to achieve 95% TS than to achieve 2-6%TS in the biosolids.

Biosoli& Characterization,Treatment and Use

7

2. Chemical Composition Chemical c a n p i t i o n of municipal solids varies greatly depending upon their origin and methods of treatment. Biosolidscuntain organic matter, macro and micronutrients and water important for plant growth. Sixteen (16) elements out of ninety (90) found in plants are known to be essential for plant growth and most of these elements are present in biosolids. The elements are carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, sulfur, calcium, magnesium, iron, boron,manganese,copper, zinc, molybdenum and chlorine. Except for boron, animals require all of these elements and, in addition, sodium, iodine, selenium and cobalt. Some of these elements, however, can be detrimental to human, plant or animal life if they are present above certain limits (e.g., copper, Zinc, molybdenum, and chlorine) [6]. Certain metals and synthetic organics have been proven to be detrimental and even toxic to human, animal and plant life at certain levels and, as such, are regulated by respectivestatutes. Extensive studies conducted in the 1980's by U.S. EPA [7] and subsequent analysis of the potential pollutants' concentration, fate, toxicity and detrimental effects on humans and environment resulted in new federal regulations which limit concentrations of ten "heavy" metals (arsenic, cadmium, chromium, copper, lead, mercury, molybdenum, nickel, selenium and zinc) in biosolids applied to land or disposed of by various means. Synthetic organics (organic pollutants) are not regulated at present (1 995) unless they are at the levels that make biosolids hazardous and subject of other federal regulations (e.g., RCRA, TOSCA). Major solids characteristics are provided in Table 1-4 [S]. Nutrients present in biosolids are absorbed by plants as water soluble ions, mostly throughthe mts. Dissolved mineral matter in biosolids (and in soil) is present as cations @ Ca",I+ Me, ,K ', Na' and low levels of Fe", Mn*+,Cu", A13', Zn") and anions (HCO, COP, HSO;, SO:, Cl-,F-, HPO;, &PO;). Table 1-5 illustrates the nutrients' ionic forms generally present in biosolids and available for plants. Note that carlxm is absorbed by plants mostly through leaves as COz;hydrogen fiom water as E, HOH, and oxygen through leaves as 0,, OH- and CO,. 3 . Macronutrients

The elements generally recognized as essential macronutrientsfor plants are carbon, hydrogen, nitrogen, phosphorus, potassium, calcium, magnesium and sulfur. Nitrogen, phosphorus and potassium are the most likely to be lacking and are commonly added to soil as fertilizers. Although biosolids contain relatively low levels of macro and micronutrients when applied to soil at recommended rates they can supply all the needed nitrogen, phosphorus as well as calcium, magnesium and many of the essential micronutrients.

Girovich

8

Nitrogen, phosphorus and potassium (usually referred to as N-P-K) play significant roles in biosolids beneficial use. E 1-4 MUNICIPAL. SOLIDS CE MCTERISTICS :5]

Secondary

Primary

Item

Total dry solids (TS), %

3.0 7.0

-

I 0.5 - 2.0

Volatile solids (% of TS)

60 - 80

150-60

Nitrogen (N, % of TS)

1.5 - 4.0

I 2.4 - 5.0

Phosphorus (P,O,, % of TS)

0.8 - 2.8

0.5 - 0.7

Potash K O , % of TS) ~

Energy content (BTUAb, dry basis)

10,000 - 12,500

PH

5.0 - 8.0

I

8,000 - 10,000 6.5 - 8.0

Alkalinity (m@ as CaCO,)

a. Arsenic b. Cadmium c. chromim d. Copper e. Lead

f Mercury g. h. i. j.

Molybdenum Nickel Selenium Zinc k. Iron 1. Cobalt

m. Tin n. Manganese

Range 1.1 -230 1- 3,410 10 - 99,000 84 - 17.000 13 - 26,OOO 0.6 - 56 0.1 - 214 2 - 5,300 1.7 17.2 101 - 49,000 1,OOO- 154,000 11.3 - 2,490 2.6 - 329 32 - 9,870

-

Median 10 10 500 800 500 6 4 80 5 1,700 17,000 30 14 260

Biosolids Characterization, Treatment and Use

9

Nitrogen is the most critical part in the plant growth. Poor plant yields are most often due to a deficiency of nitrogen. It is a constituent of plant proteins, chlorophyll and other plant substances. Biosolids nitrogen exists as organic and inorganic compounds. Organic nitrogen is usually the predominant form of nitrogen in soils (90 percent) and in biosolids and it is not available to plants. It must be bacterially converted to ammonium (NH,+) and eventually oxidized to nitrate (NO;) to become biologically available.

copper

cu

cu2+

Zinc

zn

Zn2+

Molybdenum

Mo

MOO; (molybdate)

Boron

B

H,Bo3, W O i rB(OH)i

Nitrogen is a unique plant nutrient because, unlike the other elements, plants can absorb it in either cation (NH,+) or anion (NO;) form. Nitrogen forms not absorbed by plants volatilize or oxidize and are lost to atmosphere as N, or N20. Volatilization of ammofiium ion dependsupon pH. The higher the pH, the more nitrogen is released as gaseous ammonia(NH3). The nitrogen requirements of different plants range fiom 50 to 350 kg per hectare (45-3 12 pounds per acre). [ 11 Heavy application of nitrogen-containing bimlids or chemical fertilizermay result in unused nitrate migrating into surface or underground water with adverse health and environmental effects.

10

Girovich

-

The nitrogen content of primary biwlids is in the range of 2% 4%; in secondaryand anaerobicallydigested biosolids it is in the 2% - 6% range (dry basis) [7]. Nitrogen in biosolids is usuallydetermined as organic nitrogen (Org-N), soluble ammonia nitrogen (NH,-N), soluble nitrate-nitrogen (NC) -N) and total (Kjeldahl) nitrogen (TKN). Numerous studies have been conducted on the rate of organic nitrogen amversion into the biologicallyavailable forms (NH,+, NO,?, called mineralization. One method of determinkg nitrogen mineralization and availability is based on assumptionthat 15% of organic nitrogen becomes available to plants during the first year and 6% of the remaining, or residual organic nitrogen, is released during the second, 4% during the third, and 2% during the fourth growing season after application. Based on these assumptions, the available nitrogen per ton of dry biosolids and application rate can be calculated [8]. Nitrogen is usually the limiting factor in biosolids land application, unless they have been stabilizedby an alkaline material (he) or contain excessive amounts of heavy metals. In this case, calcium or heavy metal@)becomes the limiting component. A detailed nitrogen discussion is provided in Chapter 8. Nitrogen is available fiom chemical fertilizer in a relatively concentrated form, typically 8 to 40percent The chemical fertilizer's nitrogen is valued, however, at $2 to $4 pea percent point while the same for organic nitrogen in biosolids is $12 to $17 per percent (1994). Phosphorus (P) is the second most critical plant nutrient. The nucleus of each plant cell containsphosphaus. Cell division and growth are dependentupon adequate supply of phosphorus. The most common forms of phosphorus are organic phosphorus and various forms of orthophosphates (H$0 ;, HPO - , PO: -) and polyphosphates,such as Na@O,),, Na,P,O,,, Na,P,07. Phosphorus is not readily available in most unfertilized soils and is derived primarily fi-omphosphates released by organic matter decomposition. Organic phosphorus is decomposed by bacterial actioninto orthophosphatePO;. Polyphosphates also decompose (hydrolyze) in water into orthophosphates. Sigdicant portion of phosphorus compounds are water soluble. Stable orthophosphate (PO,) is predominate form which is absorbed by plants. Primary wastewater solids contain relatively small amounts of phosphorus. Secondary biosolids contain greater amounts of phosphorus which is generally removed from wastewater by biological means. Amounts of phosphorus in biosolids depends on phosphorus concentrationin the influent and type of phosphorus removal used by wastewater treatment plants. Conventional primary and waste activated processes remove only 20-30% of an influent phosphorus and, therefore, biosolids resulting fiom these processes contain a small amount of phosphorus (0.1 to 2%).

Biosolids Characterization, Treatment and Use

11

This amount is Sutticient for plant growth when biosolids are applied at the niirogen requirement rate. Excessive amounts of phosphorus can eventually be built up in the soil and result in leaching. The phosphorus requirementsof different plants range from 0 to 95 kilograms per hectare (0 to 85 pounds per acre). Phosphorus is not removed fiom wastewater by aerobic or anaerobic digestion. Only chemical precipitationusing aluminum and ironcoagulants or lime is effectivein phosphorus removal. Biosolids resulting h m these processes may contain greater amounts of phosphorus. The following reactions convert phosphorus b m wastewater into the forms found in biosolids.

1. Chemical coagulation by alum, Al,(SO,),, and ferric chloride, FeCl,, respectively: Al,(SO,), 14.3 KO + 2 PO: = 2 AlPO, + 3 SO: + 14.3 H20 FeC1, +PO;- = FePO, + 3 C1' 2. Phosphorus removal by addition of slaked lime: Ca(HCO,), + Ca(OH), = 2 CaCO, + 2 &O (removal of water hardness) 5 Ca2++ 4 OW + 3 HPO,2- = Ca, (OH)(PO,), + 3 KO Calcium ion in the second reaction forms precipitates containing orthophosphate. Potassium (K) is needed by plants for various functions, including maintaining cell permeability, increasing resistance of plants to certain diseases, and aiding in translocation of carbohydrates. Potassium in most soils is found in more than sufficient amounts, however, it is not bioavailable. As a result, use of potassium fertilizers is required. About 150 pounds per acre of potassium is needed for plant growth. Biosolids usually Contain small amount of potassium (0.02 to 2.5% dry basis). 4. Other Inorganic Nutrients Calcium (Ca) is rarely deficient in plants. It is needed for cell division, it makes cells more selective in their absorption and it is a constituent of a cell wall. Most soils (except sandy and strongly acid ones) contain suilicient calcium supply. Calcium is supplied to the plants by soluble calcium ions fiom calcium containing minerals such as CaCO, Addition of CaO, Ca(OH), or CaCO, to soils is done to correct the pH of strongly acid soils to improve nutrient availability rather than to supply calcium for plant growth. Biosolids contain calcium in small amounts unless they are the result of processes where lime is used (e.g., lime stabilization). Mamesium (Mg) is important in chlorophyll formation. There is one atom of magnesium in each chlorophyll molecule. There would be no green plants without

12

Girovich

magnesium. Most of the plant magnesium, however, is found in plant sap and the cytoplasm of cells. Magnesium is taken up by the plant roots as cation M e . Magnesium bioavailability is affected by other ions such as potassium, calcium and nitrogen. Magnesium requirements of different plants range fiom 9 to 36 pounds per acre. Biosolids usually contain small amounts of magnesium (0.3% - 2%,dry basis). sulfirr ranks in importance with nitrogen and phosphorus as an essential nutrient in the formation of plant proteins; it is also required for the synthesis of certain vitamins in plants. Sulfkantaining organic compounds are present in biosolids and bioavailable sulfate (SO;2) is produced as a result of microbial decomposition. Sulfur requirements of different plants range fiom 10 to 40 pounds per acre. Biosolids contain fiom 0.6% to 1.5% of sulfur (dry basis). 5. OrganicMatter

Organic matter, dead or alive, is largely carbon (approximately 58% by weight), with lesser amounts of hydrogen, oxygen and other elements such as nitrogen, sulfur and phosphorus. Organic matter in biosolids contains proteins, carbohydrates,fats--compounds composed of long chains of molecules with molecular weights ranging from several hundred to several million. Organic matter is a nutrient source for plants and microorganisms, in sods it improves water infiltration, aeration and aggregation of soil particles. Microorganisms (bacteria, protozoa, fungi, and others) decompose biosolids' organic matter and use some decomposition products (carbon, nitrogen and other elements) for reproduction and, as a result, change the biosolids organic matter and release certain products of decomposition (e.g., carbon dioxide, methane, volatile organics including odor pollutants, ammonia, nitrogen, water) into the environment. Bacteria, actinomyceteSand fungi are the most active decomposersof organic matter, but some algae, protam, rotifers, nematodes and others participate in organic matter dewnposition These organisms also interact in a complex manner at various stages of wastewater and biosolids treatment processes. Most soils Containrelatively snall amount of organic matter (1 -5% in the top 10 inches of soil) but it largely determines soil productivity. The biosolids organic matter can have a profound effect on the soil physical propxt~essuch as soil fertility, humus formation, bulk density, aggregation,porosity, and water retention. A decrease in bulk density, for example, provides for a better environment for plant root growth. The high organic carbon content of biosolids provides an immediate energy source for soil microbes. The nitrogen in biosolids is in a slowly available organic form which provides for a reliable nitrogen supply. Increasedaggregation results in better tilth and less potential for erosion and reduction

Biosolids Characterization, Treatment and Use

13

of runoff. Water retention and increased hydraulic conductivity provide necessary water for plants, especially during drought. Biologically active organic components of biosolids include polysaccharides, such as cellulose, fats, resins, organic nitrogen, sulfur and phosphorus compounds, etc.; they contribute to the formation of soil humus, a water-insoluble material that biodegrades very slowly and is the product of bacterial decomposition of plant material. 6. Micronutrients

Micronutrients such as iron, zinc, copper,manganese, boron, molybdenum (for Nhtion), sodium,vanadium, and chlorine, are needed by plants in small quantitiesbut they are quite important as catalysts in numerous biological processes. The role of chlorine, except for its part in root growth, is not well known. Excessive quantities of some of the micronutrients can render biosolidshazardous to human health, plant and animal life. Soil and biosolids pH influences micronutrient availability. All metals except molybdenum are more bioavailable at low pH (acidic environment). In near neutral and alkaline environments,metals form insoluble oxides or hydroxides and become nonbioavailable. Also, toxicity caused by excess level of micronutrientsmay O C C U T . A detailed discussion concerningmicronutrients is provided in Chapter 8.

7 . Pollutants Biosolids usually contain organic and inorganic components which can adversely affect plant and animal life as well as human health if present at excessive levels. Inorganic pollutants include ten "heavy" or trace metals presently regulated by the U.S. EPA arsenic, cadmium,chromium,copper, lead, mercury, molybdenum, nickel, selenium and zinc. Ranges and mediancOncentrafioIlS of heavy metals in biosolids are provided in Table 1 4 . Arsenic, a toxic metalloid, has been the chemical villain of more than a few murder plots. The combustionof coal introduces large quantities of arsenic into the a w t S o m e f m l y used pesticides contain highly toxic arsenic compounds as well as some mine tailings. The U.S. EPA has classified arsenic as a human carcinogen. Arsenic content in biosolids varies widely (Table 1-4). Cadmium comesh r nmetal plating and mining wastes. Cadmium is chemically similar to zinc (both are divalent cations) and replaces zinc causing acute cadmium poisoning (ludney damage, high blood pressure and destruction of red blood cells). According to the U.S. EPA, cadmium is a probable human carcinogen. Cadmium content in biosolidsvaries fiom few to over 3,500 mg/kg, dry basis. Chromium comes fkom metal plating and mine tailing. It is an essential trace element. Hexavalent chromium is classified by the U.S. EPA as a human carcinogen,

14

Girovich

Cmuer unnes fiom industrial discharges, mining and mineral leaching; it is an essential trace element, not very toxic to animals, toxic to plants and algae at moderate levels. Copper is not classified as a carcinogen. Lead comes from a number of industrial and mining sources, leaded gasoline, plumbing, lead bearing minerals, etc. It causes kidney, reproductive system, liver, brain and central nervous system disfunctions. Lead is classified by the U.S. EPA as a probable human carcinogen. Mercury generates the most concern of the heavy metal pollutants. It comes from minerals, coal combustion, pesticides and fungicides, batteries, and pharmaceutical products. Toxicological effects include neurological damage, birth defects, depression, etc. Mercury tends to accumulate in seafood due to formation of soluble organic mercury compounds. The U.S. EPA has not classified the carcinogenicity of mercury because of inadequate evidence. Molybdenum comes fiom natural sources and industrial discharges; possibly toxic to animals, essential for plants. The US. EPA has not classified the carcinogenicityof molybdenum because of inadequate evidence. Nickel sources are mineralsand industrial discharges. Nickel deficiency has not been demonstrated to be essential for the proper growth and development of plants or animals. However, excessive levels of this element produce toxic effects in plants, animals and humans. The U S . EPA has classified nickel as a probable human carcinogen. Selenium sources are minerals, coal and industrial discharges. It is essential at low levek, toxic at higher levels. The U.S. EPA has not classified the Carcinogenicity of selenium because of inadequate evidence. comeshindushial waste, metal plating and plumbing. It is an essential element in many metalland aids wound healing; it is toxic to plants at higher levels. Zinc occurs in biosolids in relatively large quantities and could limit land application. Zinc requirements vary considerably from crop to crop (e.g., alfalfa requirements are 0.5 pounds per acre, approximately). Organic Dollutants, among others, include hazardous organic substances regulated under RCRA or TOSCA standards. Moredetailedstudyofbiosolidspollutantscanbefoundin[9], [lo], [ll], [13], and [14].

B. Microbiology of Biorolida Biosolids contain diverse life forms. These microscopic living organisms have both beneficial and detrimental roles in the biosolids treatment and use practices. Microorganisms in biosolids can be categorized into bacteria, including actinomycetes, Viruses, helminths (parasitic worms), protozoa, rotifers and fungi. A limited number of these organisms are pathogenic (i.e., they may cause various human and animal diseases). One of the major objectives of biosolids treatment is pathogen

Biosolids Characterization, Treatment and Use

15

elimination or reduction to acceptable levels. Current regulations do not address protozoa, rotifem and fungi as organisms subject to pathogen requirements because of the lack of analytical methods and because EPA concluded that these organisms are unlikely to survivewastewater and biosolids treatment processes and, thus, should not cause an adverse effect in biosolids use [I]. Bacteria are the smallest living organisms. Most bacteria reproduce by division into two identical cells. Bacteria are very diacult to class@. They are composed of water (80%) and dry matter (20%), about 90% of which is organic. Bacterial dry matter contains carbon (48%), nitrogen (7- 12%),phosphorus (1 .O-3.0%), potassium (1.0 - 4.5%), sulfur (0.2-1.0%)and trace elements, such as magnesium, sodium, calcium, iron, copper, manganese, and molybdenum. Bacterial organic matter contains proteins (50-60%), carbohydrates (6-15%), lipids and organic acids [12]. Actinomvcetes are a large group of organisms which look like elongated cells or flaments. Same actinomycetes have characteristics common to fungi. Actinomycetes are commonin biosolids and soils. They are saprophytes and decompose a wide range of organic compounds such as difficult to decompose long chain hydrocarbons, complex aromatic compounds, pesticides and dead microbial biomass. They also decompose readily degradable compounds such as amino acids, sugars and organic acids. They usually grow more slowly than most other bacteria. Some actinomycetes are aerobes, some are anaerobes,and they thrive at a pH of 6.0 to 8.5. Actinomycetes are o h found in activated biosolids and scum developed over the aeration tanks and secondary clarifiers. The most fiequently reported genera in activated sludge processes are Arthrobacter, Corynebacterium, Mycobacterium, Nocardia and Rhodococcus [1 21. Coliform bacteria, especially fecal coliforms,are natural, normally harmless microscopic inhabitants of the intestines of all warm-blooded animals including humans. Coliform bacteria coexist in fecal material with pathogenic or diseaseCausing organisms such as Certain bacteria, viruses and protozoa. Coliform bacteria are also found in soil and on vegetable matter. They are highly concentrated in wastewater and biosolids and generally sparse or not present in other habitats. The presence of coliform bacteria in biosolids and water is considered an indication of Contamination. Typical average coliform bacteria populations in biosolids and other materials are (organisms per gram, dry weight) [l 11:

-

* 8

-

Unstabilized biosolids: Aerobically digested biosolids: Humanfeces: Disinfected effluent: Wastewater:

l,ooo,000,000 30,000-6,OOO,000 50,000,000,000 100

8,ooo,000

One of the coliform bacteria, Escberichiu coli (E. coli), is a natural inhabitant only in the intestines of warm-blooded animals and, therefore, its presence indicates

16

Glrovich

fecal contamination and possible presence of pathogens. Fecal coliforms are the principal indicator organisms (along with Salmonella sp. bacteria) for evaluatingthe microbiologicalcontamination of biosolids. Fecal stre~tomcciinclude the entexwmxs group and several species associated with nonhuman warm-blooded animal wastes. Enterococci are enteric bacteria fiom humans and their presence indicates contamination of human origin. Fecal coliform tofecal streptococcusratio can help idenw sources of contamination. Ratios greater than 4.4 indicate fecal pollution of human origin; ratios less than 0.7 indicate fecal pollution b m nonhuman sources. Salmonella SD. routinely found in unstabilized biosolids, compost handling facilities and 1andfYls cause various diseases such as acute gastroenteritis (food poisoning), typhoid fever, and salmonellosis. While indicators very useful in assessing microbiological contamination, no indicatorspecies is perfect. Coliforms, for example, die off very quickly in water (half life is about 15 hours and only few coliforms survive more than 3 days); however, coliforms may live significantly longer in biosolids. Viruses are acellular particles which carry genetic reproductive information but are incapable of living outside a host cell. They are extremely small (0.01-0.25 micmmter cr micron, pn)and extremely host-cell specific. More than 100 different viruses may be present in untreated biosolids. The major concerns with viruses are theirpotential for disease transmission and conditions necessary for their destruction in biosolids. Diseases associated with specSc pathogenic bacteria and viruses and are summarizedin Table 1-6. Protozoa ("firstanimals") are very small (5 to lo00 pm in size), single celled animals comprising a diverse group. Protozoa need water; they are present in all aerobic wastewater treatment plants. Their role is important in the activated solids processes (up to 50,OOO o r g a n i d d or about 5% of the dry weight of suspended solids in the mixed liquor); they are present in trickling filters; rotating biological contactors (RBC); oxidation ponds, and natural and manmade wetlands. Protozoa are parasitic (live on or in other life forms) or fi-ee living. Many are parasites of animals. Protozoa are of four different nutritional types: autotrophs (plant-like forms capable of absorbing sunlight and using carbon dioxide to produce organic compounds); saprobic organisms (animal-like forms that do not contain chlqhyll IWT require light but rely on the organic soluble compounds); phagotrophs (forms thatfeed on bacteria), and carnivorousprotozoa which feed on other protozoa. Some protozoa require oxygen and some do not use it and are often unable to grow in its presence. Optimum pH for protozoa survival is in the 6.0 to 8.0 range. At pH below 5.0 and higher than 8.0 their population is adversely affected. Light is important for autotrophs. Many protozoa compete with one another for bacteria as a food supply, while others compete with bacteria and other saprobic protozoa for soluble organics.

Biosolids Characterization,Treatment and Use

17

Fmtcma play a sigmficantrole in bacterial removal from wastewater including pathogenic bacteria that cause diseases such as diphtheria, cholera, typhus and streptococcal infiions, as well as removal of fecal bacteria such as Escherichia coli. Protozoa are helpful in flocculating suspended particulate matter and bacteria thus aiding both clarification of the effluent and formation of biosolids. Such protozoa as Entamoeba histoktica and Giardia lamblia are common in biosolids, especially in warm climates. Their cysts are quite resistant to disinfection. Rotifers ("wheel bearers") are the simplest and smallest of the macroinvertebrates found in wastewater and biosolids. They are fiee swimming organisms and range in size from 40 to 500 um (microns) and have an average life span of 6 to 45 days. Rotifers perform many beneficial roles in stabilizing the organic wastes of lagoons and fixed-filmand activated biosolids processes. In lagoons rotifers feed on phytoplankton or algae. In the activated sludge process, rotifers consume large quantities of bacteria and enhanceflcc f d o n . Generally in aerobic processes the rotifers' large consumption of bacteria and solids contributes to BOD reduction. Helminths (parasitic intestid worms and flukes) and Nematodes (roundworms) are tire living, microscopic (0.5-3.0millimeters long and 0.02 to 0.05 mm wide) and nacn>scopic organisms (Rrcaris lumbricoides (roundwonn), sometimes reaching 2040cm in the intestine) whch include various worms such as hookworms, pinworms, whipworms, eelworms, roundworms, and many others. They are present in aerobic processes with abundance of oxygen and microbial food (e.g., trickling filters, activated sludge processes). Some nematodes survive temperatures as high as 117OF (47°C)and are most active at pH between 3.5 and 9.0. Ascaris lumbricoides are a serious potential danger to humans from biosolids. Most helminths and nenato$eeggs and cysts tend to accumulate in primary biosolids. Table 1-6 lists major pathogenic h e h t h s and nematodes and potential diseases. Over 80,OOO fungal species have been identified, most of which decompose organic matter. They can be broadly categorized as yeasts or molds. Fungi consist of tubular, filamentous branches 10 to 50 um in diameter. They reproduce by forming spores which are quite hardy. About 50 fungal species can cause human infections afFecting primarily lungs, skin, hair and nails. Fungi are less dependent on moisture than bacteria and can grow on dry biwlids absorbing moisture fiom atmosphere. They can withstand broad range of pH and temperature. Fungi are important industrial microbes. Yeasts are used in production of alcohols and to synthesize antibiotics, organic acids and enzymes. Most wastewater treatment processes remove pathogens from the wastewater and trader them into biosolids. While wastewater is being cleaned, biosolids that are generated by the treatment processes contain a number of pathogenic organisms. Primary biosolids have large numbers of alive and dead protozoa cysts and helminth and nematode eggs. Secondarybiosolids contain sigdicant numbers of bacteria and viruses. For example, Salmonella are removed at 90% to 99% efficiencies fiom the water and end up in the biosolids.

m.

18

Girovich

orbL

Mode of Trnnsmlssion comment24

Dlsepee

Bacteria and Actinomywtca

1.1 Colifonn species 1.2 Mbrio cholera 1.3 Salmonella species Chmmon in biosolids

I

1.4 Salmonella @phi

Typhoidfever

I$:

in biosolids

1.5 Shinella

Shigellosis (bacillary djwnbry)

Polluted water

1.6 Bacillus Anrhracis

Anthrax

Diseascofanimals,rartin humans

~~~~~

1.7 Brucella

I

I

~

Infectedmilkormeat Found in biosolids

Brucellosis

1.8 Mycyobacterium tllhlOSis

Tubennrlosis

Found in biosolids

1.9 Leptospira

Leptospkis

cordaminaiadfoodanddrink Found in biosolids

inrerohaemorrhagiae

..

1.10 Yersinia entercolitica

Gdmenkh

contaminatedfoodanddrink

1.11 Esherichia coli

Gastroenten'ti8

contaminatedwaterandfood Common in biosolids

1.12 CbsMdium rerani

Tetanus

Woundcontact Found in biosolids

1.13 Nocardia spp.

Lung disease (nocardiosis)

Inhalation and contact with

(usually nonpathoe~c)

skin Found in biosolids

Biosolids Characterization, Treatment and Use

19

TABLE 1-6 (COW.)

I 1.14 Actinomycetes israelii

Actinomycosis (meninsitiS. mdocarditis,genitalinfdons)

Mode of TrPRmtlsston Commenb

Inhalation and contact with

skin Found in biosolids

1.15 Camphlobacterspp.

Acute enteritis

contaminatedfoodanddrink Found in biosolids

2.1 Polio virus

Poliomyelitis

F d in biosolids Polio vaccine eliminates disease

2.2

v i

HeDatitis A

Found in biosolids

2.3

Cowadcievirua, echovirus

Mild infdons, meningitis,diarrhea Inhalation,water ininf~heartdisease, Found in biosolids conjunctivitis

2.4

2.5

Adenovirus,reovirus Rotavirus,calicivirus

3.1 Entamoeba histolitica

Respkatoryinfections,influenza, cold&bronchitis. diarrhea

Inhalation,water

Viralgssh’oenteritis

Inhalation,water

Amoebic dysentery

In untreated biosolids used aa

Found in biosolids

a fertilizer,resistant to disinfeaim 3.2 Giardia lamblia

Giardicrsis

Cysts are not destroyed by

disinfection; found in biosolids ~~

Found in biosolids

3.3 Criptospridium 3.4 Balantidiurn coli

Dysentcly

Found in biosolids

4.1 Ascaris lumbricoides; ascaris suum

Ascariasis (large inlestinal

Ingestion ofeggs in food or

roundworm)

drink

Awominal pain, digestive

F d in biosolids wet and dry;mostcommonof

dkhhms, fever, chest pain

hellninth

20

Girovich

TABLE 1-6 (COW.) I

I

I

Disease

duodenale, Necator americanus

Pinworm (enterobiasis)

Ingestion of eggs Easily curable with drugs

Whipworm (irichuriasis)

Ingestion of eggs Easily curable with drugs Found in biosolids

vennicularis 4.4

Trichuris trichiura

Abdominalpain,diarrhea 14.5

4.6

Taenfasaginato Cat, dog,beef, pork

IAbdominal pain, disturbances Worm infections in humans

~~

4.7 Variouskernatodes

I

~~

5.2 Candida albicans ~~

~~

IFound in biosolids

I

Ingestion of eggs

Intestid, lung and liver flukes

Ingestion of eggs Found in biosolids

Aspergillosk, lung infection

Inhalation of spores Found in biosolids and compost,mostcommonand serious offungal infections

(flukes)

5.1 Aspergillusficmigatus

ModeofTrnnsmission Comments

Ingestionof eggs Found in biosolids

4.2 Ancylostoma

4.3 Enterobius

I

I

I

Candidiasis (iection of lungs, skin, Inhalation of spores intestidtract)

Lung infection

Inhalation of spores Fungus grows on biosolids in warm and moist conditions

5.4 Blastomyces dennatitides

Blastomycosis(lung infection)

Inhalation of spates

5.5 cryptococcus neoformans

Cryptoauwxis (lung infection)

Inhalation of spores

5.6 Sporothrix schenkii

Sporotrichosis

Brokenskincontact

5.3

Coccidioides immitis

and Histo-plasma capsulatum

When biosolids are applied to land, humans and animals can be exposed to pathogens directly by coming into contact or indirectly by Consuming water or food Contaminated by pathogens. Insects, birds, rodents and people involved in biosolids processing can also contributeto the exposure.

Biosolids Characterization, Treatment and Use

21

Some pathogenic organisms have limited survival time in the environment Table 1-7 provides the survival times for major pathogens in soil and on plants. Pathogens can be destroyed by various treatments, such as heat (high temperature) ‘generated by physical, chemical or biological processes. Sufficient temperatures maintained for sufficiently long t i e periods can reduce bacteria, viruses, protozoan cysts and helminth ova to below detectable levels (helminth ova are the most resistant to heat treatment). Chemical processing with disinfectants (e.g., chlorine, ozone, lime, etc.) can also reduce bacteria, viruses and vector attraction. High pH conditions, for example, may completely destroy viruses and bacteria but have little or no effect on helminth eggs. Gamma and high energy electron beam radiation treatment depends on dosage with viiuses being most resistant. Reduction of pathogenic organisms can be measured using microbiological analysis directly or by monitoring non-pathogenic organisms-indicators.

TABLE 1-7 PATHOGEN SURVIVAL TIME IN THE ENVIRONMENT

Existing regulations require monitoring for representative pathogens and nonpathogenic indicator organisms (fecal colifom bacteria). Direct monitoring is required for the three common types of pathogens: bacteria, viruses and viable helminth ova. [ I ] There are test procedures available to determine viability of helminth ova and certain types of enterovirus species. No test procedure is available for bacteria. When direct monitoring of bacteria is important, Salnionellu sp. are used. They are good indicators of reduction of other bacterial pathogens. Microorganism density is defined by cui-rent regulations as nuitrber of mio-oorganisnw per zinir mass of rota1 solids (dy weight) [IS]. For liquid biosolids, density is determined as number of microorganisms per 100 milliliters. The microorganisms in biosolids are associated with the solid phase. When biosolids are dewatered, the number of microorganisms per unit volume changes significantly whereas the number per unit mass of solids remains almost constant. Units and methods used to count microorganisms vaiy (e.g., viable ova are

22

Girovich

observed and individually counted under a microscope). Viruses are counted in plaque-forming units (PFU). Each PFU is an infection zone were a single virus infcctcd a layer of animal cells. For bacteria the count is in colony-foiming units (CFU) or most probable number (MI"). CFU is a count of colonies on an agar plate and it is not necessarily a count of individual organisms. MPN is a statistical estimate of numbers in an original sample. The sample is diluted at least once into tubes containing nutrient medium; there are several duplicates at each dilution. The original bacterial density in the sample is estimated bascd on the numbcr of tubes that show growth. Under existing regulations, the pathogen density limits are expressed as numbers of PFUs, CFUs, or MI" per four (4.0) grams total solids. (This approach was developed because most ofthe tests start with 100 mL of biosolids which typically contain 4 grams of solids.) Densities of total and fecal colifoim, however, are given on a "per gram" basis. [3], [ 151

C. Odors and Other Nuisances Biosolids treatment and managcmcnt practices are greatly influenced by odor and other pollutant and nuisance factors such as dusts (particulate matter), gaseous and liquid discharges, and vectors (insects, birds, rodents, etc.). Biosolids are mherently odorous and if handled improperly can create offensive odors and other pollutants and nuisances which may cost a lot of time and money to con-ect. Control of odors is one of the most difficult problems in biosolids treatment and management. It is especially true if biosolids are beneficially used. Dealing with odors and nuisances is often not a technical issue but rather a matter of public acceptance, public education and human perception. Regulatory requirements concerning odors are often poorly defined leaving much room for interpretation and arbitrary action. Odor is an increasingly sensitive subject, and it is one of the first (and frequently most important) issues concerning the public. 1 . Odor co"l~'olilld~9

Although the exact origin, chemical composition and fate of many odors from biosolids are difficult to classify and not well studied, it is also true that the overwhelming majority of odor compounds are by-products of decomposition and decay of organic matter. Therefore, the most effective control measures are based on preventing or managing this decomposition cycle. Raw organic material brought by the influent settles into primary solids largely without microbiological decomposition. Primary solids are untreated solids from residential and industrial sources which makes them potentially the most odorous. Secondaiy solids are primarily products of aerobic microbiological activity.

Biosolids Characterization,Treatment and Use

23

Microorganisms decompose and metabolize organic matter, decrease the amounts of raw organics and increase organic matter attributable to live and dead microorganisms. Microbiological decomposition of raw organic materials results in the use of some elements (carbon, nitrogen and others) by microbes, formation of new and modified organic compounds and the release of products such as carbon dioxide, water, hydrogen sulfide, ammonia, methane and considerable amounts of other partially decomposed organic compounds. A sigmficant number of these organic compounds are strong odor pollutants. Fatty acids (e.g. acetic acid, a.k.a. vinegar), ammonia and amines (organic derivatives of ammonia), aromatic organics based on the benzene ring with nitrogen atom (e.g. indole and skatole), hydrogen sulfide, organic sulfides (e.g. mercaptans) and numerous other organic compounds have been identified as malodorous pollutants. Classifidon of these odor pollutants is difficult due to their low concentration, complexity of molecular sh-ucture, often short life span in the air, variety of sources and conditions, etc. However, two large groups can be identified: (1) sulfiucontaining organic compounds and (2) nitrogen-containingcompounds. Mercaptans (general formula C&SH) comprise a large group of strong odor pollutants in the sulfur containing group along with various organic sulfides ( C W ) and hydrogen sulfide N S ) . In the nitrogen containing group, various complex amines (CJ-IJW) are strong odor pollutants along with ammonia (NH3)and some others containing N, NH, radicals. With few exceptions (notably ammonia and some amines), their threshold odor concentration is very low (fiaction of ppb). Table 1-8 lists these odor pollutants, their chemical formula and their recognition threshold. 2. Odor Measurement Instrument-based odor detection has been advancingrapidly, able now to detect odors in parts per billion concentrationfor a sigdicant number of compounds. Instrumental odor detection preferred by the engineering Community has a limited use due to the complexity and variety of odors,considerable amount of time and money required and, more importantly, the availability of the human nose, still the best receptor that senses odors better (in some cases three orders of magnitude better) than any instrument has yet been able to do. Therefore, the human nose is still the most common means for detecting and measuring odors. The human nose provides a subjective odor evaluation and a number of organoleptic methods (i.e., using the human olfactory system) have been developed to improve reliability and to quant.@ odor compounds. Most common is the "odor panel technique" (ASTM Method E-679). The odor panel technique involves a panel of four to sixteen members who are exposed to odor samplesdiluted with odor-fiee air. The number of dilutions required to achieve a f%y percent positive response fiom the members is called the threshold

24

Glrovich

odor concentration (TOC) which is considered to be the minimum concentration detectable by the average person. If, for example, nine volumes of diluting air added to cme volume of odor sample generate a SffY percent positive response, this pseudoconcentrationis reported as ten dilutions to TOC. In this case, the above sample is &lined as having an "odor concentration" of ten odor units (100~). An odorous cOmpOund diluted to its TOC has a concentration of 1.Oou. The stronger the odor, the him is its odor Unit number. Several Merent nomenclatures are used to determine the number of required dilutions: a) the odor unit (ou); b) the effective dose at 50% positive panel response (ED&; c) dilutions to threshold OR);d) dilution ratio 2, etc. AU of theseam essentially the same. The odor unit and ED, are most common units in odor panel technique. The TOC is the minimum concentrationof an odorant that will arouse a sensation. A numbex of diffaent TOCs have been determined. The most common is odor recognition threshold (Table 1-8), i.e. the odorant concentrationat which 50% of the odor panel detected the odor.[161 odw cuncentratimin excess Of 50U are easily recogrued by most people and, regardless of odor type, represent the threshold level of complaint (distraction threshold) [l 11. At lOou complaints are assured. An ambient air odor concentration of 2ou maximum is often adopted at the plant boundary to avoid public complaint. A number of inherent difficulties are associated with organoleptic odor measurement. Collection and storage of samples is often an unreliable and expensive process; comparing results of Merent methods is quite confusing. A detailed discussion of odor science, measurements and odor treatment is provided in [ll], [16], [17], [19], [20]. Complete elimination of odors may be the goal; however, the best way to avoid complaints is through odor prevention, management and control to acceptably low levels. Portable Instrumental Methods: Currently there are several portable instrumental methods that can measure air concentrations of a number of gaseous compwnds(ammonia, hydrogen sulfide, etc.) quite accurately even to very low concentrations. For example, to measure H.$, there are two types of portable instruments: a gel-cell detector which detects &S in the 0- 100 ppm range and a more sensitive gold film conductivity type able to detect as little as one ppb (e.g., marketed by Arizona Instrument Co.). Draeaer Detector Tubes: Draeger detector tubes measure one odor compound at a time. A sealed graduated tube contains a special packing which changes color proportionately to the odor compound concentration. Ammonia, hydmgensulfide,dimethyl &ide,and some other odors can bedetected by

Biosolids Characterization, Treatment and Use

25

ABLE 1-8 ODOR POLLUTANTS IN BIOSOLIDS [ 16],[ 181, [ 191 Approxininte

Odor Pollutant

Forniuln

Threshold

HZS

3-5

Rotten eggs

Ally1 Mercaptan

CH,*CH-CH,*SM

0.05

Strong garlic

k n y l Mercaptan

CH,*(CH,),CH,*SII

0.3

Putrid, unpleasant

Hydrogen Sulfide Mercaptans:

Beilzyl Mercaptan

I

C,HcCH,*SH

I

0.2

I

Unpleasant

Crotyl Mercaptaii

CH,*CH.CH*CH,*SH

0.03

Skunk-like

Ethyl Mercaptan

C,H,SIi

0.2

Decayed cabbage

Methyl Mercaptan

CH,*SIi

1.o

Decayed cabbage

Propyl Mercaptaii

CH,*CH,*CH,*SIi

0.07

Unpleasant

Tert-Butyl Mercaptan

(CH,), C-SII

0.08

Skunk-like

~

Organic Sulfides: Diinethyl Sulfide

(CH,)zS

2.5

Decayed vegetables

Diplienyl Sulfide

(C3,)zS

c0.05

Unpleasant

Dimethyl Disulfide

(CH,hS,

c0.05

Unpleasant

Others: Thiomesol ~

CH,.C,H;SH

0.1

~

Thiophenol Sulfur Dioxide

Anunonia

Skunk-like ~

I

C,H,SH

0.06

Garlic-like

SO1

9.0

Pungent, unpleasant

NH,

3,000-15,000

Sharp, pungent

knines: Butylaiiiine

C,H,.CH,CH,.NH,

10

knnioniacal

Dibutylaniiiie

(C,H&*N*H

16

Fishy

Girovich

26

Approximate

Odor PoUutnnt

Formula

Odor Recognition Threshold

ChnrPetertstlC Odor

@PW Ethyl&

GWHi

800

Methylamine

CH,NH,

200

Putrid, fishy

(GH,XN

80

Ammoniacal

Triethylamine

Ammdacal

others:

NH,*(CH,),*NH,

c1.0

Decayingflesh

WJYH

<1.0

Fecal

pyridine

C6HP

3-5

Irritating

Skatnle

w

1-2

Fecal

Acetaldehyde

CH,CHO

4.0

Pungent, h i t y

Chlorophenol

CI*C,H,O

0.2

Medicinal

Cadaverine Indole

color proportionately to the odor compound concentration. Ammonia, hydrogen sulfide, dimethyl sulfide, and some other odors can be detected by Draeger tubes. Certain compounds present in the air during testing may cause interference and affect the accuracy of the odor measurement by Draeger detector tubes. Portable Gas Chrmnatomhs Portablegas chromatographs are able to analyze many different compounds. Flame ionization BID) and flame photometric detection (FPD) are used to detect hydrocarbon, nitrogen-containing(by FID) and s u h r a t a i n i n g organics (by FPD). Detection hmits are usually in the 10200 ppb range [I 13. Mass Smctrometrv/Gas ChromatomaDhv: Mass SpectrometrylGas Chromatography (MSIGS) is based on matching a unique mass spectrum of a sample cOmpOund with the spectra of different known compounds stored in the library. It is very accurate and sensitive, although a time-consuming and quite expensive methodology.

Biosolids Characterization, Treatment and Use

27

3. Odor Coiilrol Numerous odor control methods are employed in biosolids treatment and disposal. Generally, two fundamental approaches are used: a.

b.

Source control: Elimination, minimization and containing odors at the source or point of use by selecting an appropriate treatment, equipment or handling process; and Removal of odors generated by a process by treating the process exhaust prior to its release into the atmosphere.

Source controls may include the addition of chemicals to suppress generation of odors (e.g., pH adjustment by lime, injection of odor suppressing chemicals, etc.), aeration, and enclosing of the process and handling equipment.. Odor removal and reduction techniques include absorption (e.g., scrubbing by water solutions), adsorption (e.g., activated carbon systems), theimal destruction (incineration, afterburning), atmospheric dilution, masking, etc. A more detailed discussion on specific odor conti-ols is provided in Chapters 3,4,5,6 and 7.

D. Other Characteristics Other biosolids characteristics which have considerable effect on selection and perfoimance of a treatment process and type of equipment are: density particle size, granularity abrasiveness angles of repose, fall and slide flowability and transportability (rheological properties) con-osivity adhesion compressibility hygroscopicity (for dried biosolids) presence of foreign matter, grit and fibrous material 1. TraiisporIability

Liquid biosolids, whch are physically quite similar to water, can be pumped relatively easily using cenmfugal, plunger, progressive cavity, diaplu-ap, piston or other types of pumps. Liquid biosolids (up to approsimately 6% TS) are usually Newtonian fluids, i.e., pressure loss is yopoitional to the velocity and viscosity under laminar flow

28

Girovich

conditions. The flow becomes turbulent at certain critical velocity (usually 1.2-2.0 d s e c . or 4-6 Wsec.). The pressure drop in pumping liquid biosolids may be up to 25 percent greater than for water. In turbulent regimes, the losses may be two to four times the losses of water. The transition from laminar to turbulent flow regime depends on the Reynolds number which is inversely proportional to the fluid viscosity. The precise Reynolds number at which the change of regimes occurs is generally uncertain for biosolids. Thickened (and especially dewatered) biosolids are non-Newtonian fluids which become plastic as dryness increases. This means, among other things, that the pressure losses are not proportional to flow, so the viscosity is not a constant. Special procedures are required to deteimine losses for these biosolids under laminar and turbulent regimes. Prior to treatment, liquid biosolids are usually ground. Grinding is essential when pumping with progressive cavity pumps, prior to dewatering, heat treatment, or other processes. Newer designs of grinders are significantly more reliable and durable than types used in the past. In certain instances in which quality of the end product is very important (e.g.. production of pelletized fertilizer), biosolids degritting and foreign matter removal is skongly advisable. For liquid biosolids, degritting and foreign matter removal can be achieved by using cyclone degritters which employ a centrifugal force to remove heavier grit and foreign particles or by using a microscreen separation. In the case of dried biosolids, the foreign matter is usually removed by screening. fickened and especially dewatered biosolids are significantly more difficult to transport, meter and store and carehl attention should be paid to the selection of pumps, conveyance and storage amangements. Piping design is also an important factor. Biosolids piping is rarely less than 150mm (6 inches) in diameter. Deposits quite often develop inside pipes, resulting in an increase of the losses and, unless cleaning measures are implemented, ultimately in failure. Dewatered biosolids have rather ill-defined characteristics in terms of their flow properties, or rheology. Direct measurement of biosolids' rheological properties (viscosity, Reynolds number, shear stress) is time-consuming, not veiy informative and, in most cases, does not provide usehl information. Many biosolids properties are extremely dfiicult to predict due to numerous variables involved. For example, it has been observed that dewatered biosolids (in the 35 to 45% TS range) exhibit unique properties which are often referred to as a "glue-like'' or "plastic" phase. In that condition, biosolids tend to foim a sticky inass, often shaped as large balls. This gluelike material is veiy difficult to handle, and this condition should be recognized and anticipated when designing biosolids treatment or conveyance. Mechanical conveyors (predominantly screw and belt type) and, more recently, shaftless screw conveyors and sFecial positive displacement, single and double piston

Biosolids Characterization, Treatment and Use

29

pumps are used to transport dewatered biosolids. Enclosed conveyors and pumps in particular have the important advantage of containing odors.

2. Srorabilitj Storage of biosolids is often required to accommodate variations in the rate of biosolids production and to allow accumulation during periods when subsequent processing facilities are not operational (weekends, downtime, etc.). Biosolids storage is also important to ensure uniform feed to various treatment processes such as dewatering, drying, incineration, chemical stabilization, etc. Short-term liquid biosolids storage may be accomplished in settling and thickening tanks. Long-term liquid biosolids storage can be provided in aerobic and anaerobic digesters, specially designed separate storage tanks or in-ground lagoons. Tanks usually have storage capacity ranging from several hours to a few days while lagoons may provide up to several years of liquid biosolids storage. If storage is for longer periods, biosolids will deteriorate (e.g., turn septic) unless kept under aerobic conditions. Special prewvative chemicals, such as chlorine, lime, sodium hydroxide and hydrogen peroxide, have been employed to prevent deterioration but these have met with only limited success. Entirely different problems arise when dewatered biosolids are stored in silos (tanks), lagoons and stockpiles. The most challenging problem in this situation is to select a type of conveyance and a device to remove biosolids from the storage silos without material bridging and ratholing, uneven discharge, etc. Usually screw feeders and various types of "live bottom" storage bins are used with varying degrees of success.

3. Fuel Value Biosolids contain organic matcrial and thus have a fuel value that can potentially be realized. Biosolids heat content is approximately 5,500 kcalkg ( 10,000 BTU per pound) of dry volatile solids or 2,500 to 3,000 kcalkg (4,500 to 5,500 BTUAb.) of total d ~ matter. y By way of comparison, coal has a fuel value of 7,750 kcalkg (1 4,000 BTUhb.).

4. Fertilizer Value Commercial chemical fertilizers commonly contain nitrogen, phosphorus and potassium (N-P-K) simultaneously or separately. The N-P-K value of very common fertilizer is 8-8-8 (i.e., 8% nitrogen, 8% phosphorus [as P,O,] and 8% potassium [as &O]). Biosolids on average (on dry weight basis) contain 4% nitrogen, 0.1YOto 1 .O% phosphorus and an even smaller amount of potassium (an N-P-K 4-1-0).

30

Girovich

Organic nitrogen in biosolids is valued commercially higher (typically at $12 to $17 per percent of nitrogen) than chemical fertilizer's nitrogen ($2 to $4 per percentage point). Biosolids can be land applied as a fertilizer substitute, either directly or as an addtive (filler) in the chemical fertilizer mix or in combination with other beneficial products, such as lime, other alkaline materials and sludges, incineration ash, etc. Certain physical requirements (such as density, moisture, amount of dust, particle size, spreadability, uniformity, durability, minimum odors, etc.) must be addressed in order to use biosolids as a fertilizer or fertilizer additive. Despite considerable amounts of knowledge available for biosolids processing and management, a frustrating and expensive situation may occur if biosolids pi-opei-tiesare not fully evaluated on a case-by-case basis. A smoothly iunning pilot plant, when expanded into a full-scale production process, may unexpectedly fail. It is even more rislq to assume that a full-scale operation can succeed on the basis of preliminary information or laboratory studies. "ExFeiienceis the best teacher" is especially tiue in the sometimes unpredictable world of biosolids processing and disposal with its many variables and potentially costly surprises.

IU. BIOSOLLDS TREATMENT FOR BENEFICIAL USE: AN OVERVIEW A. Beneficial Uses

The organic matter and nutrient content of biosolids make them a valuable resource to use as a fertilizer and soil conditioner for agricultural crops; to improve marginal and reclaim disturbed lands; to increase forest productivity and to stabilize and revegetate forest land devastated by fires or other natural disasters. Biosolids products are veiy effective in revegetating areas destroyed by mining and dredge spoil areas. Fertilizing highway median strips and use as a landfill daily, maintenance and final cover are additional mcthods of beneficially using biosolids. While Section 405(e) of the Clean Water Act (CWA) reseivcs the choice of use and disposal practices to local communities, the U.S. EPA actively suppoi-ts the beneficial use of biosolids. When quality of biosolids with respect to pollutants is a limiting factor for beneficial use, POTWs can implement or increase local pretreatment efforts to control discharge limits for nondoniestic wastewater discharges in accordance with National Pretreatment Standards. Over 2,000 POTWs in the United States established these programs (1 992). The following beneficial use methods are practiced in the U.S.A. [ I ] :

Biosolids Characterization, Treatment and Use

31

1. Application on Agricultural Land Approximately 1.17 million dry metric tons, or 21.6% of total biosolids generated in the U.S.A. (5.4 million dry metric tons in 1989), were used to improve soils for agricultural crops and pastures [l]. Liquid biosolids are usually surface applied or injected into the surface layer of the soil. Dewatered and dried biosolids are typically applied to the land surface by equipment similar to that used for applying chemical fermizers,limestone,and animal manures and then incorporated by plowing or discing (except on pastures). The amount of biosolids applied to agricultural land should conform to agronomic rates designed to 1) provide the amount of nitrogen needed by the crop, 2) minimize the amount of nitrogen leached to the groundwater, and 3) minimizeintroduction of pollutants (heavy metals, synthetic organic compounds, etc.) into soil. 2. Application on Non-Agricultural Land

Non-agricultural land application in 1989 included forest land (32.3 thousand dry metric tons, or 0.6%), public contact sites (166.1 thousand tons or 3.1%) such as parks, golf courses, etc., and reclamation sites (65.8 thousand tons or 1.2%). Disturbed land resulting from both surface and underground mining can be swce&Uy restoredby proper application of biosolids. Approximately 1.48 million hectares were disturbed in the USA between 1930 and 1971 and only 40% of this has ever been reclaimed. Disturbed land reclamation in Pennsylvania, Ohio, Kentucky, Illinois, West Virginia, Missouri, Alabama and Florida proved to be an excellent oppcntun@for beneficial use of biosolids. Liquid, dewatered and dried biosolids can be applied at si@icantly larger amounts on non-agricultural and disturbed land (up to 180metric tonsper hectare) on a one-time basis. Instead of a nationwide nitrogen standard, application rates are established by permits (usually state-issued) on a caseby-case basis. 3. Land Application--Sale o r Give-Away Approximately 12% ofbiosolids generated in the U.S.A. were sold or given away for use on home gardens, landscaping and similar applications (1989). Presently (1 9 9 3 , biosolids in this category are either composted (500 thousand dry metric tons per year), heat dried and pelletized (350 thousand dry metric tons per year) or alkaline stabilized. Compost and pellets, in particular, are easy to transport and store for considerable periods of time. They are distributed, marketed and used as a bulk ferilizer in agriculture, on lawns, golf courses,ornamentals and home gardens. These biosolids products may also be distributed in various size bags. Significant increase in the heat dried and pelletized biosolids production occurred in the late 1980's and early 1990's when a number of municipalities

Girovich

32

renovated (Milwaukee, Wisconsin and Houston, Texas) or started (Boston, Massachusetts; New York, New York; Tampa and Largo, Florida; Cobb and Clayton Counties, Georgia; Baltimore and Hagerstown, Maryland, etc.) new heat drying and pelletizing facilities. Biosolids pellets produced by these facilities are marketed as a bulk organic fertilizer, fertilizer amendment or as a bagged retail product nationwide. Production of heat dried biosolids pellets has gained in popularity and is projected to increase substantially in the late 1990's to approximately 6% to 7% of total U.S. biosolids generation. Production of compost and alkaline stabilized biosolids in the USA has also increased substantially in the 1990's. 4. Biosolids as a Larid'll

Cover Material

Another beneficial application of biosolids is as an alternative daily, maintenance and final cover material (ACM) for landfills. A municipal solid waste (MSW) landfill typically uses approximately a six-inch compacted layer to cover the disposed MSW daily and as maintenance and final cover. The 503 Regulations do not establish standards for biosolids used as a landfill cover in municipal solid waste landfills (MSWF). Biosolids used as a cover in MSWLF must be suitable for that purpose (40 CFR Part 258). Requirements such as dryness (e.g., passing a paint filter test), granularity, spreadability, compactibility, peimeability, toxicity, odor, pathogen and vector attraction control, ability to support vegetation (if used as a final cover) must be complied with for this application. Biosolids chemically stabilized by alkaline materials such as lime, cement kiln dust, fly ash, etc. and usually mixed with earthen materials are used for landfill cover. Biosolids used as a landfill cover provide the following benefits:

. *

-

reduction of vectors and odors control of blowing litter reduction of the chance and spread offires reduction of the potential for surface and groundwater pollution enhancement of aesthetics cost savings

Most state regulations do not require that soil be used as daily cover material at MSW landfills. Regulations generally require that the daily cover should use "compacted layer of at least six (6) inches of suitable material" (e.g., Title 35, Subtitle G, Part 807, Illinois Pollution Control Board Regulations). Biosolids are also used as a final cover material usually mixed with top soil. In order to establish a vegetative cover on a closed landfill, noimally about 0.3 to 0.9 meters (one to three feet) of biosolids and top soil mixture (1 :1) is applied.

Biosolids Characterization, Treatment and Use

33

5. Disposal 011Dedicated Sites Approximately 260 thousand dry metric tons per year were applied to these sites in the U.S.A.(1 989). Th~smethod is defined as a disposal of biosolids (which may not meet standards for land application) with minimal attempt to use them beneficially at sites specifically set aside for this purpose. Dedicated sites may be owned, operated and managed either by a POTW or a private operator. Dedicated sites may be unproductive, disturbed, barren by mining or similar activities with no or minimal organic content, erodible and acidic. Such sites range fi-om a few to 10,000 acres and may include a vegetative cover crop. Measures should be taken on these sites to prevent runofi or leaching of the pollutants to waters. 6 . Biosolids as an Eiiergv Sozirre Excacting energy from biosolids can also be considered as a beneficial use. Energy can be rccovered fiom biosolids in the foim of steadelectricity using biosolids-toenergy processes such as incineration with energy recoveiy, gasification, pyrolysis, and conversion into fuel. 7 . Inciiieration Ash Utilizatiori

About 470 thousand tons per year of biosolids incinerator ash (bioash) is produced fiom 170 municipal biosolids incineration facilities in the USA. Bioash can be used as a supplement to other beneficial uses of biosolids or as a raw material in production of building and road cover products. For example, in Osaka, Japan, bioash is used as the only raw material in manufacture of decorative paving bricks. 8. Biosolids in Biological Renrediation Microorganisms in biosolids can be beneficially used in several ways: *

. .

Biosolids disposal in MSW landfills under appropriate conditions enhances decomposition of municipal solid waste and increases production of methane. Biosolids mixing with hydrocarbon-contamina~ed soils aids in establishing an appropriate microbial population to destroy hydrocarbon pollutants and to remediate sites. Biosolids addition to the soil structure to establish vegetation can improve the success of reclamation of disturbed and contaminated sites.

34

Girovich

9. Nan-Beneflcial Uses Non-beneficial uses of biosolids in the USA (1 989) include biosolids disposal into monotills (0.15 millionmetric ton of less than 3.0%), incineration (0.86 million metric tonsor 16.1%),codisposal in the MSW landfills (1.82 million metric tons or 33.9%) and surface clsposd (0.55 millionmetxic tonsor 10.3%)[13. The disposal of biosolids in MSW landfills is regulated under 40 CFR Part 258. In order to be co-disposed with MSW, biosolids must: (1) be nonhazardous and (2) pass the Paint Filter Liquid Test. If biosolids (sludges) are determined to be hazardous as defined by the Toxicity Characteristic Leachate Procedure (TCLP), they must be disposed of in compliance with the hazardous waste regulations (40 CFR Part 261-268). Also, if biosolids contain more than 50 ppm of PCBs, they must be disposed of in compliance with toxic waste regulations (40 CFR Part 761). Ocean dumping was banned by an Act of Congress after 1991. The selection of a technically and economically optimal method of converting wastewater solids into a beneficial use product is a complex process involving evaluation of potential markets, economics, technical and environmental protection issues, public response, regulatory and site specific factors. Figure 1-1 summarizes biosolids generation, treatment and beneficial use options.

B. Requirementsfor Beneficial Use Prior to beneficial use, municipal solids should be treated to satisfy the following general requirements relating to human health and environmentalprotection:

1. 2. 3. 4.

Pathoven duction: elimination or reduction to an acceptable level of potential diseasecausingorganisms in biosolids (bacteria, viruses, protozoa, helminths). Vector attraction reduction: elimination or reduction to an acceptable level of attraction to vectors (e.g., rats, insects,flies). Pollutant h t s : pollutants should not exceed lirmts established by federal, state and local regulations. These regulations apply to the trace metals content of biosolids, as well as emissions f?om processing facilities. Auulication rates and other specific reauirements (such as monitoring, remrdkeeping, public access, etc.) for biosolids used on agricultural and nonagricultural land, sold to the general public, and used as a landfill cover material must also be met.

1. Pathogen Reduction

Biosolids which are beneficially used should satisfy pathogen reduction requirements. Current US. EPA Regulations (Subpart D of 40 CFR Part 503) establish two classes

Primary Solids

-

Secondary Solids

-

2,500 3,500 L' 15,000 20,000 L' 3 7% T.S. 0.5 2.0% T.S.

-

-

10,000 L' 0.2 - 1.5% T.S

BENEFICIAL USES AND PRODUCTS 1. Land Application 2. l L a d m b m 0 0 0 0 0 0 0 0

Digested Biosolids Dried & Pelletized Biosolids Chemically Stabilized Biosolids Compost Heat Dried Biosolids Air Dried Biosolids Thermally or Chemically Treated Liquid Biosolids Irradiated Biosolids

FIG.1-1

0

Stabilized Biosolids

3. 0

Iacinerahon m e r p v Rccov~y: SteamElectrictiy

4.

Conversion to Feel

5.

Q&m Building 8t Road Material Component

.

.

.

Tertiary Solids

DISPOSAL

Monofilling MSW Co-Landfdlin Incineration without Energy Recovery

w * Per million liters of wastewater treated

BlOSOLlDS GENERATION, TREATMENT AND USAGE.

D 3 Q

36

Girovich

of pathogen reduction: Class A and Class B. Class A biosolids are essentially pathogen h e ; Class B biosolids have reduced levels of pathogens, and are subject to use restrictions to prevent immediate and direct public contact [I]. Biosolids that are sold or given away in bags or containers for land application, applied in bulk to a lawn or home garden must be Class A product.

a. Class A Pathogen Reduction Requirements The implicit objective of all Class A requirements is to reduce pathogen densities to below detectable limits which are: Salriioriella sp:

-

Enteric viruses: Viable helminth ova:

Less than 3 MPN per 4 grams of total solids (’I’S), diy weight basis Less than 1 PFU per 4 grams of TS, diy weight basis Less than 1 per 4 grams of TS, dry weight basis

The above referenced requirements define microbiological “Cleanliness” of Class A biosolids. Six alternatives are established for Class A. Any of these alternatives maybe met for biosolids to be Class A with respect to pathogens. These alternative requirements are described in detail in Chapter 2. b. Class B Pathogen Reduction Reouirements Class B requirements can be met by any of the t h e e (3) altematives. The implicit objective of all three altematives is to ensure that pathogenic bacteria and enteric vlluses are adequately reduced in density (as demonstrated by fecal coliforni density) in Class B biosolids to less than 2 MPN or CFU per gram of total solids on diy weight basis. Viable helminth ova are not necessarily reduced in Class B biosolids. Additionally, site restrictions are applied which restrict crop harvesting, animal grazing and public access for a ceitain period of time until environmental factors have further reduced pathogens. See Chapter 2 for a detailed discussion. Class B requirements apply to bulk biosolids that are applied to agricultural land, a forest, a public contact site, a reclamation site or a suiface disposal site, unless, in the latter case, biosolids are covered at the end of each operating day. c. Domestic Sentage Pathogen Reduction [503.32(c)l Class A or Class B biosolids requirements apply to domestic septage used on all types of land. No pathogen-related requirements apply to domestic septage placed on a suiface disposal site. Pathogen reduction in domestic septage applied to agricultural land, a forest or a reclamation site can be met by means of one of two options:

Biosolids Characterization, Treatment and Use

-

37

all class B site restrictionsunder 503.32(b)(S) must be met, or the pH of the septage must be raised to no less than 12 by alkali addition and maintained at this level for 30 minutes without adding more alkali,and the site restrictions on crop harvesting must be met.

2. VectorAttraction Reduction A significant route for pathogen transport is vector transmission. Vectors are living organisms capable of transmitting a pathogen from one organism to another mechanically (simply transportingthe pathogen), or biologically (by having a role in the paghogen's life cycle). Insects,birds, flies, rodents are the most likely vectors. Current U.S. EPA Regulationsestablish 12 options for vector attraction reduction. Compliance With the vector attraction reduction requirements must be demonstrated separately from compliancewith the pathogen reduction requirements. Vector attraction reduction options are discussed in Chapter 2.

3. Pollutant Limits Metal Concentrations) Current regulations establish numerid limits (ceiling concentrations) for trace metals

that must be met in order to land apply biosolids. If these limits are not met, biosolids must be managed under the surface disposal category. Ifthe biosolids @ly meets the ceiling limits, additional numerical limits (e.g., cumulative or annual loading ratas) are also provided for the metals. These numerical limits are discussed in detail in Chapter 2. Biosolids thatmeet "exceptional quality" limits with respect to the trace metals, in addition to Class A pathogen and vector attraction reduction requirements, are referred to as "Exceptional Quality" (EQ) biosolids. The EQ biosolids have fewer recordkeeping and monitoring requirements, which is particularly beneficial for distribution and management programs. Biosolids meeting these pollutant concentrations also do not have to meet annual or cumulative loading limits. "Exceptional Quality" metal concentrations are shown in Table 1-9 (Table 3 of $503.13). The requirements for use of biosolids on non-agricultural lands are the same as thosefor use on agriculturallands. The U.S. EPA requires that biosolids be applied to forest land or a public contact site at a rate that is equal to, or less than, an agronomic rate (i.e., a rate that protects the groundwater and minimizes the amount of nitrogen thatpasses below the root zone of the crop or vegetation grown on the site by matching the amount of nitrogen taka up the the crop). This management practice also applies to a reclamation site, unless otherwise specified by the permitting authority. In this case,the permitting authority may allow larger amounts of biosolids

38

Girovich

to be applied in order to provide nutrients and organic matter to revegetate disturbed sites. Ifthe biwlids are sold or given away in a bag or other container (less than one metric ton) and the metal concentrationsexceed the amount in Table 3 of 5503.13, a label must be attached to ensure that the annual pollutant loading limits (Table 4 of 5503.13) are not exceeded when the product is used.

I I

Pollutant'

Pollutant Concentration2 (mg/kg)

Arsenic

41

Cadmium

39

chromium copper

I I

1,200 1,500

Lead

300

Mercury

17

Nickel

420

Selenium

36

zinc

2,800

1

I

Both bulk and bagged products must be treated by a Class A pathogen reduction process and one of the vector attraction reduction options to be applied in bulk to a lawn or home garden or sold or given away in a bag or other container. Detailed discussion of the technical basis and requirements of the 503 Regulations is provided in Chapter 2. C. Treatment Processes: An Overview

Municipal solids are processed to achieve one or more of the following goals:

39

Biosolids Characterization, Treatment and Use

*

--

volume (weight) reduction pathogen and vector attraction reduction removal of pollutants and odors energy generation or recovery production of beneficial use products

1. Volume Reduction

Volume (weight) reduction is achieved by various types of mechanical water-solids separation techniques (dewatering), drying and incineration. Figure 1-2 illustrates volume (weight) reduction achieved by some commonly employed processes. Reduction of volume (weight) by water-solids separation (dewatering) provides considerable economic advantage by reducing costs of storage and transportation. A sigdcant number of treatment processes use dewatered biosolidsonly. High dryness dewatering is importantfor heat drymg and incineration because it reduces dramatically the amount of energy required.

1 OO?!

Bulking or Alkaline Additives OrganicMatter InorganicMatter

80%

60%

40%

20%

o?! Liq.Bimlids Dew. Bimlids

Compost

FIG. 1-2 VOLUME REDUCTION

Ahlinc Stab.

Drying

Inckatim

40

GIrovfch

2. Pathogen and VectorAttraction Reduction

As described previousl in thisChapter, a treatment is required to eliminate or reduce to acceptable levels the pathogenic organisms in biosolids and the vector attraction properties prior to beneficial use or disposal. These goals can be achieved by physical, chemical and biological means as Table 1- 10 illustrates. 3, Control of Odors and Pollutants

Reducing or eliminating pollutants from biosolids, air emission and odor control represent one of the most diacult tasks in biosolids treatment. Dilution (addition of certain materials such as lime) reduces pollutant concentation in biosolids. Special lreatments are used to reduce amount of pollutants in liquid, solid and gaseous discharges. Reduction of odors is achieved by various means, such as chemical treatment (addition of oxidizing agents such as potassium permanganate, lime, etc.) scrubbing, absorption, adsorption, masking, and incineration.Detailed discussion on specific air emission and odor control is provided in other chapters. 4. Environmental Impact

Bimlids treatment impacts upon environmental, safety, and public health conditions hcludmg (butnot limitedto) air pollution, odors,liquid sidestreams,noise, siting, and public acceptance. Certain treatment processes generate more gaseous, liquid, particulate matter and other emissions than others (e.g., incineration) and, therefore, have a greater potential social, environmental and health impact. Processes with greater impact usually require higher capital (and often higher operating) costs and may experience difficulty in achieving public and regulatory support and approval. It is impossible to quant@ social, environmental and public health impact of differentprocesses,but Table 1- 10 attempts to provide a qualitative evaluation of such potential impacts.

5 . Economic Impact The bimlids treatment and beneficial use market in the USA is estimated to be in the $1.5 to 2.0 billion range annually (1995). About 9.0 million dry metric tons are to be treated,berdicially used or disposed. Costs of treating and beneficial use of biosolids varies widely h m as low as $90 for land application of stabilized biosolids to as high as $400 per dry ton for heat drymg, including operational costs and cost of capital (Fig. 1-3). Potential revenue sources are: 1) sale of final products, and 2) sale of energy. Sale of bimlids derived products such as pellets and compost generally have higher revenue potential than other end products. Sales of digester gas, production of

TABLE 1-10 SUMMARY Treatment

Dewatering

Air Drying (1O0C/2-3 mos.)

Volume Reduction

-1

signifcant

Ti& I

Mesophilic Aerobic Digestion Anaerobic Digestion

I I

Potential for Energy Recovery

Pathogen Reduction No

C1assB

I

I

No

No

F i

Reduction of Pollutants

1”.

Revenue Potential

Product

I

Dewatered Biosolids

~

Medium to High LOW

I1

Medium

No

VeryLOW

very LOW

Medium

Medium

No

LOW

LOW

LOW

No

LOW

Medium

Medium

LOW

Medium

No

LOW

I

Medium No

Class B No

I

Significant’

I

No

I

Liquid, ClassB

Aerobic

Mesophilic

Small

Heat Treatment of Liquid Biosolids

No

ClassA

1

Medium

1

No

1

Liquid, Class A

!$urnto

I

Medium to High

TABLE 1-10 (CONT.) Treatment Pmcess Pasteurization

Volume Reduction No

Pathogen Reduction Class A

Potentialfor Energy Recovery

No

I 1

Reductionof Pollutants

I 1

No

I

~~

Final

Product

Dewatered, ClassA

I

Capital cost

O&M cost

Low

Medium

No Low

I

1

Low

Linie/Alkaline

Small Increase

Class B

No

Yes’

Alkaline Stabilized

Low

Medium

Irradiation

No

Class A

No

No

Dewatered Biosolids, Class A

No Data

No Data

No

No Data

Heat Drying for Autogenous Incineration or Land Application (40-80% TS)

Significant

Class A

Significant

No

High

Medium to High

No

Low to

Heat Drying & Pelletizing (90% TS)

High

Class A

Significant

High

Medium to High

No

Incineration

Highest

NIA

High

High to Very High

High

No

Wet Air Oxidation

No

Class A

Medium

Medium

No

~

I

Potential

No

1 I

NIA

Possible

I

I

Dried Biosolids. Fuel

I 2;”. I Fertilizer

I

I

Liquid

Medium

~~

to High

Medium

I I

I

Medium

Medium Medium

TABLE 1-10 (CON".)

1

Treatment Process

Volume Reduction

Reduction

Energy

Redudion of Pollutants

Capital Product Impact

Multi-effea Evaporation (Carver Greenfield Process)

I

(Biosolids to Oil) Melting

1. 2.

Very High

Possible

VeryHigh

I

NIA

I

I

Nf A Low

Medium

NIA

Building Materials

I

High High

Produclioii of niethaiie gas. Low to high decrease in pollutant concentralion (on dry basis) in thejnal produci due to addition ofprocessing material. N/A - Not Applicable

44

Girovich

FIG.

steam and electricity are potential sources of revenue fiom anaerobic digestion and incineration. Certain processes provide potential for energy recovery and resultant reduction in energy cost (e.g., heat treatment of liquid biosolids, heat drymg, incineration,multi-effect evaporation). Selecting an optimum treatment process for biosolids is a complicatedprocess which should take into account technical, economic and site specific issues. Detailed discussion on econOmics of various treatment and beneficial use methods is provided in other chapters.

Biosolids Characterization, Treatment and Use

45

REFERENCES 1.

2. 3. 4.

5. 6. 7. 8. 9. 10.

11.

12. 13. 14. 15.

16. 17.

18. 19. 20.

U.S. EPA, Subpait D, 40 CFR Part 503. "Standards for the Use or Disposal of Sewage Sludge", Federal Regisfer, Vol. 58, No. 32, February 19, 1993. J.E. Hall, F. Dalimer, Eur*opeatiSewage Sludge Sutvey, April 1994. StandardMethds for the Examination of Water and Wastewater. 17th Edition, Washington, D.C. Anierican Public Health Associafioti. 1980 M.J. Girovich, Simultaneous Sludge Diying and Pelletizing, Water Engineering atidManagenienf, March 1990. Metcalf & Eddy, Inc. Wastewater Engineering. McGraw-Hill Publishbig Co. 1991. R.L. Donahue, R.W. Miller & J.C. Shickluna. Soils: An Introduction to Soils and Plant Growth, Prenfice Hall, Itic. 1977. Municipal Sewage Sludge Management, Vol. 4, Water Qiialip Mariagenietit Library, Teclinortric Pirbliskitig Co. 1 992. Beneficial Use of Waste Solids, hlat~iialof Pvacfice FD-15. WPCF. 1988. S.E. Manahan. Environmental Chemistiy. 5 t h Edition. Lewis Pub/islier*s,]tic. 1991. Design of Municipal Wastewater Treatment Plants, Volume 11, Chapters 13-20, WEF Manual of Practice No. 8, ASCE Manual and Report on Engineering Practice No. 76. Book Press, Inc., Veimont. 1991. C . Lue-Hing, D.R. Z e n , & R. Kuchenrither, Municipal Sewage Sludge Management: Processing, Utilization and Disposal, Vol 4, Wafer Quality Matiagenietit Libraty, Tecktioniic Publishing Co. 1992. Wastewater Biology: The Microlife. Water Polliifion Control Federation, 1990. U.S. EPA, Fate of Priority Pollutants in POTWs. EPA 44011-82-303, Washington, DC. 1982. U.S. EPA National Sewage Sludge Suwey, Washington, DC, 1988. U.S. EPA, Control of Pathogens and Vector Attraction in Sewage Sludge: Environmental Regulations and Technology, EPA 625/R-92/0 13, December 1992. R.T. Haug, The Practical I-Iandbook of Compost Engineering: Principles and Practice, Lewis Publisliers, 1993. J.Y. Huang, G.E. Wilson & T.W. Schroepher. Evaluation of Sludge Odor Control Alternatives, Journal of Etivirotittieri~alEtigineetitig, Div. ASCE 6, 1978. Threshold Limit Values for Chemical Substances in Workroom Air, Nafional Sa&y News, 1976. C. Wilber & C. Munay, Odor Source Evaluation, BioCycle, Vol. 31, No. 3, March 1990. Odor Control for Wastewater Facilities, illanual of Practice No. 22, Washington, DC, WPCF, 1979.

This Page Intentionally Left Blank

Fed era1 Reg u 1atory Requirements Jane B. Forste Wheelabrator Clean Wafer Systems Inc. Annapolis, Maryland

I. HISTORICAL BACKGROUND AND RISK ASSESSMENT A. Introduction

The Clean Water Act (CWA), enacted in 1972, was adopted to "restore and maintain the chemical, physical and biological integrity of the Nation's water". To that end, the Act prohibits discharging pollutants into navigable waters except in compliance with the statute. The CWA led to the establishment of the National Pollutant Discharge Elimination System (NPDES), a pennitting system which establishes specific discharge limits for each pcnnitted wastewater treatment facility, including industrial, commercial and public sourccs of wastewatcr. In addition to regulating those sources discharging emucnt dircctly into the Nation's waters, the CWA rcquircs the U.S. EPA to promulgate nationally applicable pretreahncnt standxds to reshict dischxge of pollutants into publicly owned treatment works (POTWs). These indirect industrial dischargers, as well as direct dischargcrs, are subject to generally similar standards regarding levels of treatment. In addition, POTWs are required to implement local limits applicable to the specific industrial dischargers within their system to meet local requirements. Thus, direct dischaip-s must comply with NPDES permit emuent limitations and indirect dischargers must comply with pretreatment standards. Prctreatment standards must also be set for pollutants which cannot be treated readily by POTWs or which would inteifere with POTW opcrations [l]. 47

48

Forste

The pretreatment program implemented by EPA through these limits is the primary means for ensuring that the wastewater solids resulting from treatment at POTWs mean applicable quality standards for ultimate use or disposal. The wastewater from industry, domestic sources from private residences and runoff from various other sources, must all be treated at POTWs before they can be discharged to a receiving water. This treatment results in an emuent to be discharged and the solids known as biosolids. Because implementation of the CWA has rcsulted in greater levels of treatment and pollutant removal from wastewater, the quantities of solids resulting from treatment increased steadily during the 1970's. Appropriate management of these increasing amounts of solids has become more critical as the efforts to remove pollutants from wastewater have become more effective. In the United States, the quantity of solids generated from municipal wastewater treatment has almost doubled since the enactment of the Clean Water Act in 1972. EPA estimates that municipalities cuirently generate more than 6.5 million diy metric tons of wastewater solids per year[2]. An important means for managing these municipal wastewater solids is through beneficial use and recycling. The nutrients and other properties found in thcse othcr solids make them usehl as a fertilizer and soil amendment. At the same time, and in some situations, the use of these solids presents environmental conceins because of potential for contamination by haiiifil pollutants. Because coiitaminatcd or improperly managed wastewater solids can result in pollutants in those solids reentering the environnicnt and possibly contaminating various media through a variety of exposure routes, it is important to regulate the management of these materials. Failure to do so could have adverse effects on surface and/or groundwater, wetlands and possibly on human health as well. Concern for air quality requires proper controls over incineration of municipal wastewater solids. The interrelationship among these media means that a coordinated, comprchensive approach which encourages the beneficial use of biosolids and avoids creating environmental loopholes will assure that solving problems in one medium will not create problems for another. The 1972 CWA addressed biosolids use and disposal in only a veiy limited circumstance: when use or disposal posed a threat to navigablc waters (i.e., the Act prohibited the disposal if it would result in any pollutant entering navigablc waters unless done in accordance with a peinut issued by EPA). In 1977, Congress amended Section 405 to add Section 405(d) requiring EPA to develop regulations with guidelines for the use and disposal of what was then called "sewage sludge." These guidelines would identify uses, including disposal; spec@ factors to be accounted for in determining the methods and practices for each identified use, and identify concentrations of pollutants that would interfere with each use. The 1979 Interim Final Rule (257 Rule) was issued under the authority of the Solid Waste Disposal Act as amended by the Resource Conservation and Recovery Act (RCRA) of 1976 as wcll as Scction 405(d) of CWA[3]. This 257 Rule was an

Regula tory Requirements

49

interim final regulation whosc ciiteiia partially fulfilled the requirement of Section 405 of the CWA to provide guidelines for disposal and utilization of biosolids. Because of its issuance as a solid waste regulation, the 257 Rule resulted in a trend by many states to regulate biosolids under their solid waste programs. In fact, the federal rulemaking stated that facilities which did not meet the criteria of the 257 Rule would be classified as "open dumps." As specified by Section 405(d) of the CWA, the criteria contained in the 1979 regulation were designed "to avoid a reasonable probability of advcrse effcts on health or the environment from disposal of sludge on land." They were less comprchcnsive than later regulations and addressed only the placement of biosolids on the land. All such practices were required to address environmentally sensitive areas and diseases (pathogen control) in addition to protecting food chain crops from the effects of cadmium and PCBs. The evaluation of lead, pcsticides and pcrsistcnt organics was deferred to future rulemaking because information available to the agency at the time was believed to be inadequate to support specific standards. The 257 Rule The 257 Rule allowed for the beneficial utilization of biosolids as an agricultural soil conditioner or fertilizer through land application by providing criteria that peimittcd thcse activities in such a way as to prevent restricting the flow of the base flood, reducing the teniporaiy watcr storage capacity of the floodplain, or resulting of washout of the solid wastc so as to pose a hazard to human life, wildlifc or land or water rcsourccs. Facilities and practices were also required not to: "cause or contribute to the taking of any endangered or threatened species of plants, fish or wildlife . . . or result in the dcstruction or adverse modification of the critical habitat of endangered or threatencd species"; cause discharge of pollutants into waters of the U.S. in violation of the National Pollutant Discharge Elimination System (NPDES) under Section 402 of the CWA; contaminate underground drinking water sources beyond the boundary of the solid waste site except as proved by a state solid waste management plan. Many of the foregoing requirements were developed from the perspective of a "facility" site (e.g., a landfill) which bears little or no relation to the bencficial use practices employed for land application. This solid wastc-based approach has resulted in somc misinterpretation and misunderstanding of the requircments and the applicability of the 1979 regulation. The 257 Rule is also far less comprehensive than the 503 Regulations, particularly with respect to the pollutants regulated. Cadmium and PCBs were the only pollutants regulated by the 257 Rule. Cadmium was selected as the metal being of greatest concern because of its ability to accumulate in vegetative tissues of crops to levels which could potentially cause adverse effects on human health. To that end, the 1979 regulation was designed to protect land use for the production of food chain crops by imposing both annual and cumulative limits on the amounts of cadmium which could be applied to soils. In addition, the regulation required that the pH of the

50

Forste

solid wastekoil mixture be adjusted to 6.5 or greater at the time of each application of any biosolids containing concentrations of 2 m g k g cadmium or more. Cadmium Limits A phased-in annual cadmium application rate was developed to provide an opportunityfor POTWs to impose pretreatment requirements and reduce the levels of the metal in their influent and, therefore, in the solids generated by these facilities. The maximum cumulative application of cadmium was based on two factors: background soil pH and soil cation exchange capacity. Higher cumulative application rates were allowed for sites with higher cation exchange capacities that also had background soil pHs equal to or greater than 6.5. These higher cumulative application rates were allowed for soils with background pH of less than 6.5 if: (1) only animal feed crops were grown, ( 2 ) pH was adjusted to 6.5 or greater each time an application occurred; (3) a facility operating plan was developed to demonstrate to whom the animal feed would be distributed to preclude ingestion by humans; and (4)fbture property owners were notified that high cadmium application rates had been used on the site and that food chain crops should therefore not be grown. The net effect of these requirements was to establish controls based on the best available scientific estimates of cadmium bioavailability at the time. As will be discussed later in this chapter, many of the assumptions and the data used to develop these limits were unnecessarily conseivative and/or based on an incomplete understanding of the mechanisms of cadmium uptake and accumulation in crops. Nevertheless, this Rule (along with recommendations for cumulative loading limits developed by the U.S. Department of Agriculture) provided the federal basis for land application programs in the United States for considerably more than a decade after becoming effectivc October 15, 1979. PCB Limits Under the 257 Rule, solid waste containing concentrations of PCBs equal to or greater than 10 m g k g had to be incorporated into the soil when applied to land used for producing animal feed, including pasture crops for animals raised for milk. Such incorporation was not required if it was assured that the PCB content was less than 0.2 m g k g (actual weight) in animal feed or less than 1.5 mgkg (fat basis) in milk. Pathogen and Vector Attraction Reduction Pathogen reduction and vector attraction reduction requirements of the 1979 rulemakingwere also included in an appendix to the 503 Rule (with some additional monitoring requirements imposed for the Processes to Further Reduce Pathogens (PFRP) options). The processes to reduce pathogens in the 257 Rule were technology-based with no requirements for microbiological monitoring, as have been imposed in the 503 Regulations discussed later in this chapter.

Regulatory Requirements

51

In addition to cadmium, PCBs, pathogen and vector requirements, the 257 Rule contained provisions based on requirements for a solid waste facility, i.e. prohibiting open burning, generation of explosive gases, fires, bird hazards and access. Even as it was published, EPA and the scientific community identified gaps and limitations in the 257 Rule and began efforts to develop a more comprehensive riskbased rule encompassingmore of the pollutants of concern than those contained in the 257 Rule, which remained an "interim final" rather than "final" rulemaking. During the next decade, various study groups and task groups within EPA were formed to develop a final rule as mandated under the CWA. It was not until 1989, however, that such a rule was proposed to meet the mandate of Section 405(d) of the CWA. Changes in the Act enacted in 1987 for the first time set foi-th a comprehensive program for reducing the potential environmental risks and maximizing the beneficial use ofbiosolids. At that time, the Act established a time table for the development of use and disposal guidelines. Under amcnded Section 405(d), EPA was charged to first identify, based on available infoimation, toxic pollutants which may be present in [biosolids] in concentrations which may affect public health and the environment. Then, for each identified use or disposal method, EPA was required to promulgate regulations that specify acceptable management practices and numerical limitations for biosolids that contain these pollutants. These regulations must be "adequate to protect human health and the environment from any reasonably anticipated adverse affect of each pollutant" (Section 405(d)(2)@)). The statute also required EPA to promulgate regulations in two stages and periodically review these regulations for the purpose of identifLing additional pollutants for regulation. Since Congress provided little guidance for the Agency in carrying out the broad mandate to protect public health and the environment, EPA needed to address a number of issues including: scope of the regulation, what pollutants should be covered, what pathways of exposure should be evaluated, what individuals or groups of individuals, plants or animals were most likely to'be affected, as well as what type of modeling and risk assessment would be addressed in the final Rule. The Agency also needed to determine at what concentrations a pollutant can adversely affect human health and the environment, and what constitutes an adverse effect. The question of an acceptable level of risk had to be addressed in meeting the "adequate protection" mandate of the Act, taking into account background pollutant levels and the uncertainties inherent in all risk analyses. The Agency conducted an extensive infoimation gathering and analytical program beginning in 1984 to support the development of the final regulation. M e r the 1987 amendments to the CWA were adopted, the Agency's efforts increased and resulted in the development of important documentation which formed the basis for the 503 Regulations.

Forste

52

B. Basis for the 503 Regulations With the promulgation of the 503 Regulations on February 19, 1993, EPA met its longstanding obligation under the CWA to establish standards for the use and disposal of "sewage sludge." This undertaking represented an unprecedented effort on the part of the Agency to assess the potential for pollutants in wastewater solids to affect public health and the environment through various routes of exposure. This enormously complex effort required the Agency to address issues that affect many other EPA regulatory programs. For example, the risk evaluated in the 503 Regulations for biosolids applied to land required EPA to evaluate human exposure through inhalation, direct ingestion of soil fertilized with biosolids and through consumption of crops grown on such soil, as well as many others. The agency assesscd potential risk of contamination of drinking water or surface water sources when biosolids are managed on the land, and evaluated potential direct effects on crops, cattle, aquatic species and wildlife. In addition, EPA evaluated the effect of emissions from biosolids incinerators on human health. Thus, the development of the 503 Regulations had an impact on Agency activities under the Clean Air Act (CAA),the Resourcc Conservation and Recovery Act (RCRA), thc Toxic Substances Control Act (TSCA), and the Comprehensive Environmental Response Compensation and Liability Act (CERCLA), as well as the Safe Drinking Water Act (SDWA). The coordination of this rulemaking with standards of other Agency programs presented a significant challenge to the development of the 503 Regulations. The challenge was particularly great with respect to non-human health effects, given the more limited infoimation available in that area and the less well-developed methods available for cvaluating such effects as comparcd to human health effects. In promulgating the 503 Regulations, EPA expressed confidence that they do in fact adequately protect public health and the environment from all reasonably anticipated adverse affects, as required by CWA Section 405(d) based on the following: EPA evaluated the regulations for aggregate national health impact and found, based on very conservative assumptions that probably overstate exposure, that there are virtually no effects when biosolids are disposed of on land or used as a soil conditioner or fertilizer in compliance with the Rule. Even when biosolids are incinerated, the potentially greater human population exposed to emissions is still subject to extremely small risk. Beneficial use of biosolids is not new in the United States. In the development of the 503 Regulations, EPA and the Technical Peer Review scientists reviewed available scientific and technical literature and found no evidence that the use of significant or widespread adverse effects. biosolids is causing

Regulatory Requirements

53

The promulgation of the 503 Regulations represents the Agency's first step (Round I> of this rulemalung. The CWA statutory requirements mandate that the Agency develop regulations in two steps and revise such regulations periodically if additional information suggests such a need. EPA has expressed the view that the Regulations as promulgated in 1993 are adequately protective because most of the effects that the Regulations are designed to prevent are chronic, not acute. Even in the unlikely event that new information would dictate reconsideration of some of the determinations of the SO3 Regulations, no adverse short-term human health consequences would be experienced since the standards already protect against chronic effects and thus are well below acute effect levels. The Agency is also committed to investigate many of the assumptions used in deteimining the regulatory levels contained in the SO3 to insure consistency with broad protection of public health and the environment. Under Section 40S(d) of the CWA, EPA identified, based on available information, pollutants which may be present in biosolids in concentrations which might affect public health andor the environment. Then, for each identified use or disposal method, EPA specified acceptable numerical limitations and management practices for biosolids that contain these pollutants. In the absence of specific gudance fiom Congress in carrying out the broad mandate to protect public health and the environment from reasonably anticipated adverse affects of each pollutant, the Agency was required to address and clarify a number of significant issues. Remlatorv Issues To determine standards that would adequate protect public health and thc environment, EPA had to detemiine the following: Scope of the Remulation Since different types of biosolids are generated and are used or disposed in different ways, the question of which types to regulate and which use/disposal methods to regulate needed to be clearly defined. Pollutants Covered The basis for the Agency's selection of pollutants for regulation had to be defined. Exposure Pathways Air, water and soil as media of transport of pollutants had to evaluated in establishing risk-based limits for pollutants. Tarpet Organisms Individuals or groups of individuals, plants or animals which are most likely to be affected by the pollutants in biosolids had to be identified. Models The Agency had to simulate the movement of the pollutants in biosolids into and through various environmental media to the target organisms.

54

0

0

0

. 0

Forste

Tvpes of Risks Potential human health and environmental effects from the use or disposal of biosolids (e.g., eating foods grown on soil to which biosolids have been applied) had to be identified. Effect Levels The "response"to the presence of a pollutant by plant, animal and human systems had to be evaluated to determine at what point variation constitutes an adverse effect. Acceptable Level of Risk The Agency had to decide what level of risk adequately protects human health and the environment; this was unclear in the congressional mandate and varies within other EPA programs. Backmound Pollutant Levels The Agcncy had to evaluatc the impact of sources of pollutants other than biosolids (e.g., lead from gasoline). Uncertainties The Agcncy had to measure and account for the unavoidable uncertainties in its analyses by using appropriately conservative assumptions and margins of safety. Types of Effects Evaluated EPA had to determine whether to evaluate health and environment effects on the most exposed target organisms and/or based on the incidence of adverse effects on the total population associated with use or disposal. Pollutant Limits The Agency had to determine whether to establish a single pollutant limit for all use or disposal practices or separate pollutant limits for each of these. Form of the Pollutant Limits Pollutant limits could be expressed as limitations on concentrations in biosolids, on pollutant loading rates, pollutant emission rates, or various other limitations. Regulatorv Responsibility The Agency had to determine who would be responsible for meeting the requirements in the Rule (e.g., end user, treatment works, service contractor). Impacts A cost benefits analysis of the Rule had to be developed in order to determine its effects.

The final 503 Regulations contained a number of important, fundamental assumptions based on the Agency's interpretation of the requirements of Section 405(d) of the CWA and the various regulatory issues described above. These assumptions include the following:

Regulatory Requirements

55

1. Control Quality By cstablishing limits on biosolids quality, the 503 Regulations create incentives for treatment works to generate less contaminated biosolids. Those facilities that do not meet the quality conditionsunder the 503 must improve the influent quality (e.g., more rigorous pretreatment), provide better treatment of the solids (e.g., improved pathogen reduction) or select another use or disposal method.

2. Reduce Waste and Use BeneJicially EPA's 1984 Beneficial Use Policy and the 1991 Interagency Policy on Beneficial Use strongly supports beneficial use practices for biosolids--the term the Agency notes is used to distinguish solids which can be beneficially recycled[4][5]. The Agency believes that the improved productivity of land using the soil conditioning properties and nutrient content of biosolids has human health and environmental advantages beyond those that are directly associated with applying this material to the land. The Agcncy sees secondary or related bcnefits of using biosolids resulting from a reduction in adverse human health effects of incineration, a decreased dependcnce on chemical fertilizers and a reduction in he1 or energy costs that may be associated with incineration. In finalizing the Rule, the Agency considered and placed emphasis on approaches that supported its policy of beneficial use. 3. Mininiize Risks to Iiidividuals and the General Population The Agency evaluated the effects of each pollutant on a highly exposed individual, plant or animal (HEI) and on populations at higher risk. It also examined regulatory options that would base the Rule on: aggregate incident analyses only (the effect on the whole population); the ME1 analyses only; and a combination of these two approaches. The final 503 Regulations use an HE1 analysis supported by aggregate risk assessment on higher risk populations or special sub-populations (e.g., children) to insure protection of public health and the environment. 4. Proniulgate Reasonable Standards In establishing standards under 405d of the CWA, the Agency evaluated long-term pollutant exposure and circumstances leading to: 1) increased toxicity and potency of a pollutant in the environment, and 2) increased intensity of an adverse effect that the pollutant might have on human health or the environment. While this approach is used throughout the Rule, it does not include every conceivable combination of adverse conditions; however, based on the conservative assumptions built into EPA's modelling effort, the Agency expccts that few, if any, individuals will reccive greater doses of a pollutant than the doses used to establish the standards. Therefore, EPA has

56

Forste

determined that the 503 Rule meets the statutory directive contained in 405d. EPA concludes that adequate protection does not require the adoption of standards for exposure conditions that are unlikely and from which effects are not significant or widespread. 5. Proniulgate an Implenientable Rule The flexibility associated with site-specific analyses was weighed by the Agency against the simplicity of national numerical limits and a self-implementing rule. Regulations that allow exceptions for every conceivable contingency would be cumbersome and difficult to implement. Therefore, the limited exceptions in 503 to national pollutant limits are restricted to circumstances in which site specific conditions may make a significant difference in the pollutant limits without compromising public health and environmental protection. In such cases where sitespecific conditions are appropriate, recalculated numerical pollutant limits for that site can be imposed using EPA approved models by providing supporting information and recalculated numerical limits to the permitting authority for approval. The Agency places primary responsibility on the treatment works for insuring that biosolids are managed as required. Because the 503 Regulations are designed to be selfimplementing, they spell out the requirements which apply to persons using or disposing of this material. 6. Provide f o r Future Expansion Since the 503 Regulations are necessarily limited by the adequacy of idolmation available at the time, they contain standards for pollutants and use or disposal practices for which sufficient information exists. EPA may expand and refine these standards in the future. The National Sewage Sludge Survey (NSSS) was conducted by the agency to fill some ofthe information gaps. Among other things, it gathered additional infotmation on the pollutants in biosolids, as well as how they are used and disposed[6]. Those pollutants and methods of use or disposal not covered by the February 19, 1993 Final Rule will be evaluated for coverage under subsequent phases of503 rulemaking and as adequate data are developed. The 503 rulemaking was unique not only with respect to the complexity of the task at hand, but the extensive review and input which was received by the Agency on the technical standards. Experts from both inside and outside EPA reviewed the scientific literature and provided additional data and scientific and technical input which enabled the Agency to expand and refine the standards during the time following the comment period and before promulgation of the final standard. Reviewers included the EPA Science Advisoly Board, the Cooperative State Research Service, the Regional Research Technical Committee (the W- 170 Committee),

Regulatory Requirements

57

representatives of academia and other scienticltechcal entities with specific experience in the areas covered by the Rule. A complete listing of public comments and documents used in developing the final Part 503 can be found in [7]. The phased-in approach for this rulemakmg specified under Section 405d of the CWA provides for review of the Regulations at least every two years to identify additionalpollutants and promulgating regulations for such pollutants. The EPA will use data from the NSSS to identify additional pollutants in the second round of rulemaking. The 503 Regulations establish standards for final use or disposal when biosolids are applied to agricultural and non-agricultural land (including products sold or given away), placed in or on surface disposal sites or incinerated. The standards do not apply to processing of biosolids before their ultimate use or disposal. In addition, the 503 Regulations do not specify operating methods or requirements for biosolids entering or leaving any particular treatment process. They also do not establish standards for biosolids disposed with municipal solid waste in Municipal Solid Waste Landfills (MSWLF) or used as a cover material at MSWLF sites. The joint authority of Sections 4004 and 4010 of RCRA and Section 405d of the CWA contain the requirements adopted by the Agency for MSWLFs that apply to biosolids placed in such landfills. Disposal of biosolids in MSWLFs is regulated under 40 CFR Part 258 [8]. Treatment works which use an MSWLF to disposal of their biosolids must insure that the material is non-hazardous and passes the Paint Filter Liquid Test. By meeting these requirements, the treatment works will be in compliance with Section 405(e) of the CWA. The standards also are not applicable to biosolids co-incinerated with large amounts of solid wastes [9]. However, the standards established in the 503 Regulations do apply to biosolids incinerated with incidental amounts of solid waste used as an au>cilimyhe1 (i.e., 30% or less solid waste by weight). Biosolids generated by all publicly and privately owned treatment works treating domestic sewage and municipal wastewater are subject to the provisions of the 503 Regulations. They do not apply to domestic sewage that is treated along with industrial water by privately owned industrial facilities. Since EPA does have the authority under Section 405d of the CWA to regulate industrial sludges with a domestic sewage component, it plans to consider regulating these materials in the future Part 503 rulemaking, In the meantime, until the agency develops such regulations, those sludges (as well as nonhazardous industrial sludges without a domestic sewage component) will be regulated under 40 CFR Part 257. The 503 Regulations do not establish disposal standards for wastewater solids that are determined to be hazardous under procedures contained in Appendix I1 of 40 CFR Part 261. Such hazardous materials must be disposed of in compliance with the Regulations in 40 CFR Parts 261-268, and compliance with these requirements will constitute compliance for purposes of Section 405 of the CWA. Also, wastewater solids containing 50 ppm or more of PCBs are excluded from the 503 Regulations and

58

Forste

must be disposed of according to the requirements established in 40 CFR Part 76 1. Therefore, since EPA has not established specitic standards for the disposal of PCB contaminated wastewater solids, a disposer complying with 40 CFR Part 76 1 will be in compliance with Section 405d of the CWA. The Ocean dumping ban act of 1988 prohibits any person from dumping wastewater solids into ocean waters after December 31, 1991. A phased-in implementation under the Marine Protection Research and Sanctuaries Act (MPRSA) resulted in the cessation of such disposal at the end of June 1992. C. Data Gathering As required by Section 405(d), EPA relied on available information to develop the 503 Regulations. The primary source of this information regarding the occurrence and concentration ofpollutants in biosolids came from data on 40 pollutants from POTWs in 40 cities (40 City Study). [ 101 This database was selected as the primary source of information on pollutant concentrations because it provided the most comprehensive and best documented nationwide database available at the time. 40 Citv Study However, EPA recognized scveral deficiencies in using the "40 City Study" data. The key deficiency was the fact that this study did not generally provide data on final processed biosolids. In addition, POTWs selected for the 40 City Study were not selected following statistical methods which would provide unbiased national estimates of pollutant concentrations in biosolids. EPA also recognized that biosolids quality had changed since 1978 due to the initiation of many pretreatment programs, development of new industrial facilities discharging to the POTWs, and changes in wastewater treatment processes. The Agency therefore concluded that pollutant concentrations from the 40-city study did not accurately reflect the current quality of biosolids. In addition, advancements in analytical methodology since the 40-city study provide more accurate pollutant analysis. Other data sources on biosolids quality also suffered from deficiencies which made them inappropriate for establishing regulations. National Sewage Sludge Survev (NSSS) To develop more appropriate data to replace or at least supplement the data used to develop Part 503, EPA conducted the National Sewage Sludge Survey to obtain a current and reliable database. This information will also be used to develop a list of pollutants fi-om which the Agency will select additional pollutants for further analysis and potentially for regulation under Section 405(d) of the CWA. The data collection effort for the NSSS began in August 1988 and was completed in September 1989. EPA collected and analyzed biosolids samples at 180 POTWs and analyzed them for more than 400 pollutants. Using detailed questionnaires, the

RegulatoryRequirements

59

survey also collected information on the use and disposal practices from 475 public treatment plants with at least secondaxy treatment. The NSSS results provided EPA with information necessary to establish numerical pollutant limits in the final Part 503 Regulations to encourage the beneficial use of biosolids and provide a greater degree of public health and environmental protection than was contained in the February 6, 1989 proposal. To establish numerical limits, data from the NSSS regarding pollutant concentrations were required to estimate the level of risk which current quality and current use or disposal practices pose to the environment. The survey information enabled the Agency to fUrther evaluate its regulatory approach and "cap" pollutants at the 99th percentile concentration in instances where the Agency believes that the strictly risk based numerical limitations do not provide an adequate margin of safety to protect public health and the environment. EPA also used the results of the NSSS to attempt to assess potential shifts among various use or disposal practices as a result of the final regulations, since effect of a rule is important in determining how rapidly it should be implemented. For example, a slight impact from a particular numerical limitation may make immediate implementation of the regulations appropriate; however, if wide shifts in current methods of use or disposal are anticipated from the numerical limits, POTWs may need assistance in developing more stringent pretreatment limits or in adopting alteinative use or disposal practices. EPA will also study the analpcal results of NSSS to identify a preliminary list of pollutants for second round rulemaking, based to some degree on pollutants that have elevated concentrations in biosolids. Final decision on which pollutants to regulate in the second round will depend on availabilityof data regarding a pollutant's toxicity and environmental fate, effect and transport properties. EPA will follow a similar process in identifying these pollutants as they did for the 1989 rulemaking. Participants in the NSSS were selected from 11,407 POTWs in the U.S., including Puerto Rico and the District of Columbia. These facilities have been identified in the EPA 1986 Needs Survey as having at least secondaiy wastewater treatment (primary clarification followed by biological treatment and secondary clarification). The effort consisted of both a questionnaire and an analytical survey. The POTWs included in the two samples were selected according to stratified probability design. All POTWs in the analytical survey were selected from among those that had already been selected to receive the questionnaire. The questionnairewas designed to allow results to be analyzed separately by flow rate group and by biosolids use and disposal practice. Four hundred seventy-nine POTWs provided idormation concerning service area, operating parameters, general biosolids use and disposal practices, pretreatment programs, wastewater and biosolids testing frequencies and financial information. The POTWs also provided use and disposal practice specific information and indicated which practice(s) would be likely alternatives to their current ones.

60

Forste

A total of 208 POTWs fiom four flow rate categories were selected for sampling and analysis. EPA contract personnel collected the samples which were analyzed for a total of 412 analytes. These analytes include every organic, pesticide, dibenzohran, dioxin and PCB for which EPA has gas chromatography and mass spectrometry ( G C M S ) standards. The remaining pollutants are inorganic. Pollutants were also selected based on: 1) the CWA Section 307(a) priority pollutants, 2) toxic compounds highhghted in the domestic sewage study, and 3) Resource Conservation and Recovery Act (RCRA PL94-580) Appendix VIII pollutants. Sampling preservation and analytical protocols were developed specifically for the NSSS. Analytical methods 1624 and 1625 were adapted from methods to deal with the biosolids matrix for volatile and semi-volatile organics respectively. Pesticides and PCBs and dibenzohans and dioxins were analyzed using methods 16 18 and 1613, respectively. Metals and other inorganics and classicals were analyzed by standard EPA methods. The analytical methods were either developed, chosen or specifically adapted for the biosolids matrix to give the most reliable, accurate and precise measurement of the 4 12 analytes undertaken. Incinerator Field Studies A series of field studies was initiated by EPA in 1987 to support the 503 incinerator rulemaking effort. These on-site tests were designed to obtain information about: 1) percentage of hexavalent chromium in the total chromium in the exit gas, 2) percentage of nickel subsulfide in the total nickel in the exit gas, 3) total hydrocarbon (THC) emissions data, and 4) organic compounds in the exit gas. Information was collected at 10 biosolids incinerators, eight of which were multiple hearth and one was a fluidized bed; these incinerators had various combinations of air pollution control devises, including wet scrubbers and wet electrostatic precipitators. For the final Rule, EPA developed allowable pollutant concentrations for metals in biosolids based on risk-specific concentrations. The risk-specific concentration for chromium depends on the percentage of hexavalent chromium in the total chromium in the exit gas. EPA determined, based on tests at several incinerators, that the conversion to hexavalent chromium will vary with the type of biosolids incinerator and air pollutant controls. Based on those results, EPA derived different risk-specific concentration values based on four combinations of biosolids incinerators and air pollution control technologies (see Table 10 of the final Part 503 Regulations). The nickel speciation tests revealed that nickel subsulfide is not emitted from biosolids incinerators above the detection level for the analytical methods used in the tests. EPA therefore decided to be protective and base the standard risk-specific concentration for nickel on the higher of the two detection limit values for nickel subsulfide. The risk specific concentration for nickel in Table 9 of the final Rule is based on the occurrence of ten percent nickel subsulfide in total nickel emitted from a biosolids incinerator.

Regulatory Requirements

61

EPA used data from total hydrocarbon concentration in exit gas along with the aggregate risk analysis as the basis for the THC operational standards in the final part 503 regulations. This standard is technology-based since it is based on perfoimance data from biosolids incinerators. Information on total organic pollutants and THC in the exit gas provided the basis for THC being used as a surrogate for measuring organic compounds in the exit gas. Tests show that there is a significant correlation between THC and organic compounds and THC is easier and less expensive to monitor than are total organics. THC can also be measured on a continuous basis which enhances operating and management practices, Knowing which organic pollutants are in the exit gas (or potentially in the exit gas) allowed EPA to develop an ambient risk-specific concentration for the organic compounds. This value was then used to estimate the risk level for the technology-based THC limits which is an exit gas concentration. Details on the biosolids incinerator field studies can be found in the technical support document for incineration. Domestic Septage Study To characterize domestic septage, EPA initiated a 1981 sampling and analysis study because data on organic pollutants and domestic septage were not available. As part of h s study, nine samples of domestic septage were collected and analyzed for over 400 pollutants. Analytical results from the study were used to calculate the annual application rate equation for domestic septage for the final rule (based on total Kjeldahl nitrogen and ammonia concentrations), and to justify the domestic septage annual application rate. Details on the domestic septage study are found in the technical support documents for land application.

D. Risk Assessment Methodology 1. Intioducrion

In developing the 503 Regulations, the U.S. EPA had to set up and justify numerical limits for certain pollutants. EPA developed the numerical limits using the exposure assessment models as described in this Chapter, and in the final 503 Regulations also established an operational standard for total hydrocarbons emitted by biosolids incinerators. The models incorporated well established measures of human health or environmental protection as the design endpoint. Thus EPA based its environmental assessment on human health or environmental criteria already published or promulgated by the Agency, on human health criteria developed by the Agency, or on plant or animal toxicity values published in scientific literature. In its exposure assessment, if the Agency had not published or promulgated criteria for specific pollutants, EPA evaluated non-cancer human health risks from pollutant exposure using reference doses. EPA evaluated cancer risk using cancer potency factors listed in the Agency's Integrated Risk Information System (IRIS). In

62

Forste

all cases, the Agency used cancer potency values corresponding to an incremental, carcinogenic risk level of 1 x to evaluate the risk from pollutants found in biosolids (a 1 x lo4cancer risk implies that one additional cancer case will occur in a population of 10,000 exposed at that level for 70 years). The final limits for pollutants ensure that the use and disposal of biosolids does not result in ambient concentrationsof the regulated pollutants that exceed an incremental carcinogenic risk level of 1 x For biosolids disposed of in or on surface disposal sites (including monofills) or incinerated, the treatment facility may submit modelling and data analysis for certain physical parameters relating to the site. The permitting authority will review and approve the facility’s site specific modelling and data analyses used to recalculate numerical limits used in EPA’s approved exposure assessment methods. The recalculated numerical luruts will therefore be based on EPA approved models and the same health and environmental criteria as the national numerical limits and, therefore, the recalculated limits will also adequately protect human health and the environment. The 503 Regulations establish requirements for pathogenic organisms or pathogenic indicator organisms such as fecal coliforms in biosolids applied to the land. It also includes requirements for destroying or reducing the characteristics of biosolids that might attract birds, insects, rats and other animals (so-called “vectors”). Based on “vector”exposure to the pathogenic organisms potentially present in biosolids and potential spread of disease from these disease vectors to humans, the Rule requires measures for reducing the attraction of vectors to biosolids. These measures include reduction or destruction of the odor-causing properties of biosolids that lure insects and animals by a variety of means. Management practices and general requirements to protect human health and prevent environmental abuse supplement the numerical pollutant limits contained in the 503. In addition, small quantities of biosolids sold or given away in a bag or other container may require labelling with instructions on proper use of the product if such a product does not meet the most stringent requirements of the 503 with respect to pollutant concentrations. Monitoring, recordkeeping and reporting requirements are also contained in 503 at specified frequencies depending on the quantity of biosolids used or disposed by a treatment facility. The pollutants for which a treatment facility must monitor their biosolids depend on the use or disposal method employed, and recordkceping and reporting requirements are also specific to each use or disposal method. The 503 Regulations cover nearly 35,000 treatment facilities, including priniaiy treatment POTWs, secondary and advanced POTWs, privately owned treatment works, federally owned treatment works and domestic septage haulers. Based on the NSSS, EPA estimates that the 503 will affect approximately 6,300 of the 12,750 secondary, advanced and primary POTWs that use one or more of the disposal practices included in the 503. These 6,300 facilities generator or treat approximately 60% of the biosolids produced in the US. Of the remaining POTWs, EPA estimates

Regulatory Requirements

63

that 2,700 dispose of their biosolids (34% of the total generated) in MSWLFs that are regulated under 40 CFR Part 258. The remaining 3,750 POTWs use other disposal practices not covered in the 503 or the MSWLF rule. As noted above, in some cases compliance with the requirements of the other practices also constitutes compliance with 405(d) of the CWA. EPA’s aggregate risk assessment estimated that the pre-503 use and disposal practices contributed from less than one up to five cancer cases total for the entire US population. The lifetime cancer risk to a highly exposed individual fi-om existing practice ranged from 6 to 10-4for land application and surface disposal and from 6 x lo4 to 7 x 10” for incineration. Other health effects associated with usc and disposal prior to 503 wcre evaluated by the Agency as primarily relating to lead exposure. The baseline risks for the 503 are extremely low as compared to aggregate risks identificd by the Agency in other environmental rule making efforts. The aggregate risk assessment estimates are discussed later in this chapter. EPA’s regulatory impact analysis estimates that approximately 130 of the 6,300 aEected POTWs may produce biosolids which do not meet the numerical limits of the 503 Regulations (however, some POTWs may come into compliance by using site specific data to calculate new numerical limits and imposing stricter pretreatment requirements on industrial discharges). T e c h c a l support documents, aggregate human health risk analyses, regulatoiy impact analyses and the preamble to the 503 discuss the factors that EPA considered, the data and comments that were evaluated, and the determinations that it made in developing the Regulations. This information is containcd in 15 parts:

Part I contains generation, volume and constituents of biosolids and ways in which communities commonly use or dispose of their biosolids, the bcnefits of using biosolids and the risks associated with their disposal Part I1 lists existing federal and state requirements for use and disposal and their relationship to 503 standards. Part I11 describes the Agency‘smethods of selecting pollutants and developing the final Rule through a series of exposure assessment models. Part IV briefly describes the February 6, 1989 proposed rule. Part V describes the data collection and evaluation of the NSSS. Parts VI and VII discuss alteinativc regulatory approaches and commcnts thc Agency considered. Part VIII discusses proposed exposure assessment methods and public comments considered by the Agency in developing exposure assessment methodology. Part IX describes criteria used to select pollutants for regulation in the final Part 503 Rule. Part X describes the methods used to analyze the aggregate human health effects on highly exposed individuals and the nation from existing practice.

64

a

0

0

Forste

Part XI describes, in several subparts, the requirements that apply to the use and disposal of biosolids. Part XI also contains the requirements for septage use and disposal, pathogen and vector attraction requirements and monitoring, recordkeeping and reporting requirements. Part XI1 discusses implementation of the Rule through federal and state permit programs and the self-implementing nature of the 503 Regulation. A separate rulemaking by the Agency promulgated state program management requirements and changes in the NPDES permitting requirements. [ 101 Part XI11 describes benefits, costs and regulatory impact of the Rule along with discussion of the data limitations and assumptions. Part XIV provides information on obtaining copies of the 503 Rule, the technical support documents, risk assessment and regulatoq impact analysis. Part XV describes changes in 40 CFR Parts 257 and 403 (i.e., revisions to part 403 and removal from coverage in Part 257 of biosolids use and disposal methods which x e now subject to the standards established in 40 CFR Pait 503. Part XV lists the subjects in 40 CFR Parts 257,403 and 503.

2 . Selection oJPollutants for Regulation

In 1984 EPA began to assess which pollutants likely to be found in biosolids should be examined closely as candidates for numerical limits. A list of approximately 200 pollutants in biosolids that if disposed of improperly could cause adverse human health or environment al€ects was developed. The list was then revised by adding or deleting pollutants using the potential risks to human health and the environment when a particular pollutant was applied to the land, placed in a landfill or incinerated as the test for inclusion or exclusion on the revised list. EPA also requested that the most likely routes which a pollutant could travel to reach target organisms (human, plant or wild or domestic animals) be identified. This process resulted in a recommendation by the experts that the agency gather additional environmental information on approximately 50 pollutants listed in Table 2-1. In 1984 and 1985, EPA gathered data on the toxicity, persistence, transport and environmental fate of the 50 pollutants being evaluated. The Agency also developed preliminay information on the relative frequency of concentration of these pollutants in biosolids by analyzing the product of 43-45 POTWs (depending on the pollutant) in 40 cities. This 40-city study provided data on concentration of 40 pollutants (1 2 metals, 6 base neutral organic compounds, 6 volatile organic compounds, 9 pesticides, and 7 PCBs). Using this preliminay infoimation, EPA assessed the likelihood that each pollutant would adversely affect human health or the environment. For this effort, EPA used screening models and calculations to predict the concentration of a pollutant that would occur in surface or groundwater, soil, air or food, then compare that prdcted concentration with an Agency human health criterion, such as a drinking

65

Regulatory Requirements

TABLE 2-1 POLLUTANTS SELECTED FOR ENVIRONMENTAL PROFILES/I-IAZARDS INDICES Landfill

Pollutants

Incineration

X

AldridDieldrin ~

1 Arsenic

I

x

Benzene

X

X

X

X

X

Benzo(a)anthracene

x

Benzo(a)pyrene I

X

I

X

Cadmium

X

X

X

X

X X

Carbon tetrachloride

I Chlordane*

X

X

Chlorinated dibenzodioxins

X

Chlorinated dibenzofurans

X

Chloroform

X

Chromium*

X

X

X

~

Cobalt

X

X

Copper*

X

X

Cyanide*

X

X

DDTDDDDDE

X

X

I -x

2,4-Dichlorophenoxy-acetic acid* Dimethylnitrosamine*

X

X

X

Forste

66

TABLE 2-1 (CONT.)

Nickel*

X

X

X

PCBs

X

X

X

Pentachlorophenol*

X

X X

Phenathrene

X

Phenol' Selenium*

X

X

X

Tetrachloroethylene* Toxaphene

X

Trichloroethylene

X

X

X X

67

Regulatory Requirements

TABLE 2-1 (CONT.) X

Trichlorophenol Tricresyl phosphate

X

X

Vinyl chloride Zinc*

X

X

X

* Pollutant evaluated found not to interfere with one or more use/disposal option(s). water standard, to determine whether the pollutant could be expected to have an adverse effect on human heallh. For this initial screening, EPA assumed conditions that would maximize the pollutant exposure of an individual animal or plant, as well as the worst possible pollutant related effects. Based on concentration, toxicity, persistence and other factors, EPA scored each pollutant and ranked them for more rigorous analysis, excluding two categories of pollutants for M e r evaluation. Excluded were those which, when compared to a simple index, presented no risk to human health or the environment at the highest concentration found in the 40-city study or other databases, or, secondly, lacked human health criteria or sufficient data to be evaluated at the time. [ 1 11 Table 2-1 notes the pollutants EPA did not analyze hrther because of their demonstrated lack of negative eflect on human health or the environment. These pollutants are also included in the list of pollutants for which eligible POTWs, by complyingwith the requirements in Part 503, may under 40 CFR Part 403, apply for authorization to grant removal credits to their industrial dischargers, shown in Table 2-2. TheEPA also proposed to include septage from septic tanks in the definition of "scwage sludge" and thus within the scope of the proposed requirements. EPA did not propose separate standards for septage from septic tanks but rather regulated septage under the various use or disposal practices contained in the proposed Rule. 3. Proposed Standards - 1989 The pollutant limits, management practices and other requirements were specific to the use or disposal method employed. The use or disposal methods included in the proposal were 1) application to agricultural or non-agricultural land, 2) distribution and marketing (referred to in the final SO3 Regulations as sale or giveaway of biosolids), 3) disposal in monofills, 4) disposal in on surface disposal sites, and 5) incineration.

Forste

a. Land Amhcation The original proposed standards for spreading of liquid, dewatered, dried or composted biosolids on or just below the surface of agricultural and non-agricultural land contained difZaent numerical pollutant limits for agricultural and non-agricultural lands. The limits proposed for agricultural land were based on an EPA modeled assessment of potential risk to public health and the environment through 14 pathways of exposure. Numerical limits, when applied to agricultural land, were expressed as cumulative loading limits of 10 metals and annuals pollutant loadings of 12 organic pollutants. A cumulative loading rate for each metal defined the limit of how much of a given metal in biosolids could be added to the soil. This load could be applied all at once or over a period of years from repeated applications. No fbrther application would be allowed once the cumulative loading was reached. Also, the proposed Regulations provided an annual limitation on the quantity of the 12 organic pollutants that could be applied to land. The proposed Regulations required owners and operators of treatmcnt works to keep records on the amount of organic and inorganic pollutants applied to each land application site in order to insure that cumulative and annual loading rates would not be exceeded. In addition, before biosolids could be applied to the land by anyone other than the treatment works, the treatment works would have to entcr into an agreement with the distributor or applier to provide that they must comply with the standards. For non-agricultural land, EPA developed pollutant ceilings for the concentrations in biosolids of these 22 organic and inorganic pollutants. These standards were based on the assumption that pollutants in biosolids applied to non-agricultural land would not reach individuals through the food chain. The ceiling concentration (above which biosolids could not be land applied) were based on the 98th perccntilc values for pollutant concentrations in biosolids based on data from a 198 1-1982 study. b. Distribution and Marketing Different requirements were originally proposed for biosolids to be distributed and marketed (designated in the Final Rule as sold or given away) for use as a fcrtilizer and soil conditioner. EPA proposed to limit the quantity of biosolids (or product derived therefiom) of a given concentration that could be applied to land in one year. The major difference between proposed land application requirements and proposed distribution and marketing requirements was in the numerical limits for somc of thc organic pollutants and some metals. For both instances, it was assumed that biosolids would be used in the production of crops intended for human consumption. For agricultural land, numerical limits were based on crops intended for direct human consumption or fed to animals intended for direct human consumption, whichevcr was the more stringent. For the organic pollutants which can accumulate through the food

Regulatory Requirements

69

chain, the numerical limit was based on crops fed to animals intended for human consumption. In contrast, the distribution and marketing scenario centered on a fruit and vegetable home garden, not a garden in which feed for animals is raised. Therefore, the numerical limits for pollutants in distribution and marketing were somewhat higher than those for agricultural land application. c. Monofills EPA's proposed requirements applying to landfills receiving only biosolids contained numerical limits on the concentration of 16 pollutants that could not be exceeded for monofill disposal. These limits were derived from a modelled exposure pathway analysis and varied depending upon the type of groundwater under the monofill. The proposal also provided for site specific limits for monofills in defined circumstances. d. Surface Disposal In addition to disposal in monofills, EPA developed standards for another widely practiced land disposal method which EPA called "surface disposal," typically piles of material placed on the land and defined as areas of land where such material is placed for a year or longer. EPA concluded that such sites are generally small and in rural areas and therefore did not expose individuals to significant concentrations of pollutants. The Agency therefore proposed pollutant concentration limits for surface disposalbased on the 98th percentile value derived from the data on biosolids quality. Using a 98th percentile data cap would essentially freeze surface disposed biosolids at the level of quality represented by the database. Based on low aggregate effects of existing use and disposal practices, EPA then concluded that this approach would adequately protect public health and the environment. In the proposed Regulations, EPA did however commit to revisiting for the final Regulations the issue of whether to distinguish between different use and disposal methods and to develop exposure assessment models to evaluate potential risks from surface disposal units for the final rulemaking. e. Pathopen and Vector Attraction Reduction Requirements Since the solids resulting from wastewater treatment typically include bacteria, viruses, protozoa and helminth ova which can cause diseases (usually enteric diseases through direct human contact with the organism or through the ingestion of an affected animal), the proposal included requirements for the control of pathogens in biosolids. It also required measures for reducing contact of the disease "vectors" with such pathogens. The pathogen and vector attraction reduction requirements were required for biosolids applied to agricultural and non-agricultural land, distributed and marketed or disposed of on a monofill or surface disposal site.

70

Forste

Three levels of pathogen reduction were contained in the proposed Regulations with applicable restrictions on public access, crops and animal grazing which varied with the particular level of treatment selected. Two sets of numerical limits were included and their applicability depended on whether biosolids were to be used in the production of crops intended for human consumption or for animals raised for human consumption. The level of pathogen reduction Mered in the proposed Regulations for biosolids intended for land application and those intended for distribution and marketing. The highest level of pathogen reduction was required for distribution and marketing to the general public. On the other hand, land application allowed alternative pathogen reduction standards as long as they were combined with imposed public access and animal grazing controls and restricted to growing and harvesting of crops to conform to the standards of the pathogen reduction method selected. In developing the pathogen reduction requirements, EPA assumed that except for the applier, there would be little public contact with the biosolids or with the land to which they were applied and that public access would be restricted for a period of time. The premise undcrlying the distribution and marketing requirements was that biosolids would be used in a home garden where there would be immediate and continuous human contact with the biosolids or with the land to which it was applied and that under such circumstances no restricted access could be assumed. f. Incineration Requirements The proposed 503 Regulations contained requirements for biosolids that are incinerated in an incinerator firing only biosolids. Such incinerators were required to comply with the National Emissions Standards for Hazardous Air Pollutants (NESHAPS) for mercury and beryllium. For lead, arsenic, cadmium, chromium and nickel, the proposed Regulations set limits on the concentration of these mctals in the biosolids that would be incinerated based on two factors: 1) the control cficiency of the incinerator and 2) the dispersion factor (i.e., the relationship between ground level concentrations and pollutant emissions). Limits were designed to result in ground level concentrations for a given pollutant that would fall below the value required to protect human health at a cancer risk level of 10.'. For lead, the standard National Ambient Air Quality Standard for lead; for h s calculation, biosolids incinerators were assigned 25% of the air shed loading for lead. The 1989 proposed rulemaking also contained a limit for maximum allowable total hydrocarbon concentration in biosolids. Like the metal limits, this limitation would vary with dispersion factors and control efficiency. It was also designed to prevent ground level concentrations of total hydrocarbons above a level associated with a cancer risk of lo', To determine this risk-specific concentration for total hydrocarbons, EPA used a number of assumptions about which organic pollutants

Regulatory Requirements

71

comprised the total hydrocarbon mixture and the levels at which these organics were present. g. Monitoring. Recordkeepinp and Reporting The proposed Regulations required owners and operators of treatment works to sample and analyze their biosolids and keep certain records. The specific pollutants for which monitoring was required was determined by the method of biosolids use or disposal. Frequency of monitoring would vary with the design capacity of the treatment works. Treatment works were also required to monitor biosolids for compliance with pathogen reduction requirements when they were to be used or disposed of other than by incineration. For incineration, the proposal required owners or operators to monitor continuously for stack hydrocarbon concentrations, feed rate, combustion temperature and oxygen content of the exit gas. Another provision of the proposed 503 Regulations required an agreement between the treatment works and the distributor or land applier which would contain the mformation needed for proposed reporting requirements. EPA also proposed that treatment works applymg biosolids to agricultural land keep records for the life of the treatment works to insure that no cumulative pollutant loading rate would be exceeded for a paiticular parcel of agricultural land to which biosolids were applied. Similar reporting requirements were proposed for non-agricultural lands, with the cxception that treatment works did not have to keep track of annual and cumulative pollutant loading rates and therefore, need only retain records for five years. For surface disposal, five years' retention of the analytical data on pollutant concentrations and pathogen reduction were required and ten years of such recordkeeping for monofills. Under the proposal, incinerator records were also required to be kept for five years. 4. Risk Assessnient

EPA's risk assessment processes and tools have been developed to identily the potential for adverse aRects associated with a pollutant in order to deteimine what, if any, measures are needed to protect public health and the environment. In developing the 503 Regulations, EPA evaluated such risk from individual pollutants present in biosolids. This risk assessment can be broken down into four stages:

5

* *

hazard identification dose-response evaluation exposure evaluation characterization of risks

72

Forste

Hazard Identification The first element in risk assessment--hazard identification--determines the nature of the effects that may be experienced by an exposed human or ecosystem from an identified pollutant. Thus, hazard identification helps to determine whether a pollutant poses a hazard and whether sufficient information exists to perform a quantitative risk assessment. By gathering and evaluating all relevant data that help determine whether a pollutant poses a specific hazard and quantitatively evaluating those data on the bases of the type of effect produced, the conditions of exposure and the metabolic processes that govern pollutant behavior within the organism, potential hazards can be identified. Hazard identification may also characterize the behavior of a pollutant in the environment (or within an organism) as well interactions within the environment or within an organism. Hazard identification therefore helps to determine whether it is scientifically appropriate to infer that observed effects under one set of conditions (e.g., in experimental animals) are likely to occur in other settings (e.g.,in human beings) and whether a quantitative risk assessment can be developed from available data. The first step in hazard identification is to gather mformation on the toxic properties of pollutants through animal studies and controlled epidemiological investigations of exposed human populations. Animal toxicity studies are based on assumptions that effects in human beings can be inferred from effects in animals. Animals bioassays include acute exposure tests, subchronic tests, and chronic tests. Acute exposure to high doses for short periods of time (usually 24 hours or less) is most commonly measured as m e d m lethal dose (Ld&-(the dose level that is lethal to 50 percent of the test animals). Ld,is also used for aquatic toxicity tests (i.e,, concentrations of the test substance in water that will result in 50 percent mortality in test species). Substanceswith a low LD, are more acutely toxic than those with higher values; for example, sodium cyanide has an M,, of 6.4 m g k g while that of sodium chloride is 3000 mgkg. Subchronic tests utilize repeated exposure of test animals for periods ranging from 5 to 90 days depending on the animals by exposure routes which comespond to human exposures. Such tests are used to determine the no observed adverse effect level (NOAEL), the lowest observed adverse effect level (LOAEL) and the maximum tolerated dose (MTD). This latter is the largest dose a test animal can receive for most of its lifetime without demonstrating adverse effects (not including cancer). For chronic effects, test animals receive h l y doses of the test agent for approximately two to three years at doses lower than those used in acute and subchronic studies. The number of animals for these tests is also larger since the tests are trying to determine effects that will be observed in only a small percentage of the animals tests. Health effects can also be evaluated using epidemiology, the study of patterns of disease in human populations and the factors the influence those patterns. Well-conducted epidemiological studies are generally viewed by scientists as the

Regulatory Requirements

73

most valuable information from which to draw inferences about human health risks. Unlike the other approaches described above, epidemiological methods evaluate the effects of hazardous substances directly on human beings. They help to identify human health hazards without requiring knowledge of the causative factor and they complement information gained from animal studies. Epidemiological studies compare the health status of a group of people who have been exposed to a suspected causal agent with that of a comparable nonexposed group. In case control studies, individuals with a specific disease are identified (cases) and compared with individuals not having the disease (control) in order to attempt to find common exposures. Cohort studies begin with a group of people (a cohort) considered fi-eeof the specific disease; the health status of the cohort known to have a common exposure is examined over time to determine whether specific conditions or causes of death occur more frequently than might be expected from other causes. The next step in hazard identification is to combine the data to ascertain the degree of hazard associated with each pollutant. EPA generally uses different approaches to assess hazard associated with carcinogenic versus noncarcinogenic effects. For non-carcinogenic health effects (e.g., mutagenic effects, systemic toxicity), EPA's hazard identificatiodweight of evidence determination has not been formalized and is based on qualitative assessment. For carcinogenic risk assessment, EPA groups all human and animal data reviewed into the following categories of degree of evidence of carcinogenicity: sufficient limited (e.g., in animals increased incidents of benign tumors only) inadequate no data available 0 no evidence of carcinogenicity Human and animal evidence of carcinogenicity in these categories is combined into the following classifications: Group A - human carcinogen Group B - probable human carcinogen B 1 - higher degree of evidence B2 - lower degree of evidence Group C - possible human carcinogen Group D - not classifiable as to human carcinogenicity Group E - evidence of non-carcinogenicity The following factors are evaluated by judging the relevance of the data for a particular pollutant: quality of data resolving power of the studies relevance of route and timing of exposure appropriateness of dose selection

-

-

74

Forste

replication of effects number of species examined availability of human epidemiologic study data Hazard identification enables researchers to characterize the body of scientific data in such a way that the following questions can be answered: 1) Is a pollutant a hazard? 2) Is a quantitative assessment appropriate? Conducting such quantitative assessments involves dose-response evaluations and exposure evaluation. Dose ResDonse Evaluation Estimating what dose of a chemical produces a given response for a particular pollutant is the second step in the risk assessment methodology. Evaluating such data characterizesthe connection between exposure to a pollutant and the extent of injury or disease. Most dose-response relationships are based on animals studies, since even good epidemiological studies rarely will have good information on exposure. In the current context, "threshold" characteristics of a pollutant refer to exposure levels below which no adverse health effects are assumed to occur. For effects involving genetic alterations (including carcinogenicity and mutagenicity) EPA postulates that effects may take place at vary low doses and therefore these compounds are modeled with no thresholds. For most other biological effects, the usual assumption is that threshold levels exist. For non-threshold effects, the key assumption is that the dose response cuve for each pollutant exlubitiig these effects in the human population achieves zero risk only at zero dose. Mathematical modeling extrapolates response data fiom doses in the obseived (experimental) range to response estimates in the low dose ranges. EPA's cancer assessment guidelines recommend the use of multistage model which yields estimates of risk that are conservative, representing a plausible upper limit of risk. Systemic toxicants or other compounds which exhibit non-carcinogenic and non-mutagenic health effects are assumed to exhibit threshold effects. Dose response evaluations involve calculating what is known as the reference dose (oral exposure) or reference concentration (inhalation exposure) are RfD and RfC respectively. RfDs and RfCs are estimates of a daily exposure to the human population that is likely to be without appreciable risks of a negative effect during a lifetime. The RfD and RfC valucs are calculated from data establishing a no observed effect level (NOEL), no obmved adverse effect level (NOAEL), lowest observed effect level (LOEL), or lowest observed adverse effect level (LOAEL). These values are stated in mgkg per day and values are derived from laboratoiy animal and human epidemiology data. Uncertainty factors are applied to the IUD and RfC values depending upon the level of confidence EPA has in the data used to derive them. Uncertainty factors vary with the nature and quality of the data from which the NOAEL or LOAEL is derived and range from 10 to 10,000.

Regulatory Requirements

75

These uncertainty factors are used to extrapolate from acute to chronic effects, to accounf for differences in species sensitivity or variation in sensitivity in human populations and, if appropriate, to extrapolate from an LOAEL to a NOAEL. If information is available for only one route of exposure, this infomation is used to extrapolate to other routes in the absence of I s and RfCs for those routes. Once an RfD or R E is derived, the next step in risk assessment is to estimate exposure. Exposure Evaluation EPA relies on two methods to determine pollutant concentration: 1) direct monitoring of pollutant levels and 2) using mathematical models to predict pollutant concentrations. Atter environmental pollutant concentrations are determined by one of the above methods, EPA then determines the severity of the exposure by evaluating data on the nature and size of the population exposed, exposure route (i.e., oral, inhalation, dermal), the extent of exposure (concentration x time) and the circumstances of exposure. Collecting monitoring data provides the most accurate information about pollutant concentrations. Such monitoring includes personal and ambient (or site and location) monitoring. To reduce the variability of exposure assessments to individual persons, the technique of sampling air and water and then combining that information with the amount of time spent in various microenvironments (i.e., home, car or office) provides an estimate of exposure. Personal monitoring may also include sampling of human body fluids (biological monitoring or biomonitoring). Biological markers (biomarkers) can be classified as markers of exposure of effect and susceptibility. An example of a biomarker of exposure is lead concentration in blood. A biomarker of effect measures some biochemical, physiologic or other alteration within the organism that points to impaired health. Biomonitoring may also refer to the regular sampling of animals, plants or microorganisms in an ecosystem. Ambient monitoring (or site or location monitoring) involves collecting samples from the air, water, soil or sediment at predetermined locations and analyzing them to determine environmental concentrations at those locations. Exposures can be further evaluated by modeling the fate and transport of the pollutants. Measurements are a direct and preferred source of information for exposure analysis, but such measurements are expensive and are oAen limited geographically. Such data can be best used to calibrate mathematical models that simulate the movement of pollutants into and through the environment with mathematical equations or algorithms that can be more widely applied. Mathematical models must account for both physical and chemical properties related to fate and transport and must document mathematical properties, spacial properties and time properties. Of the hundreds of models for fate, transport and dispersion available for all media, there are five general types: atmospheric,

76

-.

%

Forste surfacewater, groundwater, and unsaturatedzone, multi-mediamodels and food chain models. These types of models are primarily applicable to pollutants associated with dust and other particles.

PoDulation Analvsis Population analysis describes the size, characteristicsand habits of potentially exposed human and non-human populations. Census and other survey data may be usem in iden* and describing the populations exposed to a pollutant. Integratedexposme a d y s s calculatesexpowe levels and describes the exposed population and uses this information to quantify the contact of an exposed population to each pollutant under investigationvia all routes of exposure and all pathways h m the sources to the exposed individuals. In addition,uncertainty must be described and quantified to the extent possible to achieve a valid exposure analysis. EpA's policy with respect to risk characterizationis designed to convey the extent of the Agency's confidence in risk estimates. It is also important that risk assessment information be clearly presented and separate fiom any risk management considerations. Evaluating how exposure assessments were conducted provides one method for i d e n e g the uncertainties in risks. For example, in the human health risk assessment for the 503 Regulations, the technical support documents define several exposure pathways for the three biosolids management practices. EPA used point estimates for each exposure pathway and did not consider variability of the parameters describingexposure among individuals. Based on the amount of data and the lack of accepted risk assessment methodologies in certain areas, it is difllcult to judge whether the point estimates in the human health risks assessment and assumptionsmade in the ecological effects assessment are likely to underestimateor overestimate actual risks. Some aspects of the risk analysis probably contain coI1sefv(Ltiveor protective assumptions while other factors may bias results in the opposite direction. Also,some assumptions are based on EPA policy and reflect risk management choices; some of which are conservativewhile others are less so. Human Health Assessment Based on staradard Agency practice, human-health dose-response assessmentsfor the 503 Regulations are based on RfDs for non-carcinogensand cancer potency factors for carcinogens. Both of these measures are generally considered conservative, thatis, they predict a greater impact on human health than is likely to actually occur. The referencedose is defied as an estimate of a daily exposure to the human population (including sensitive subgroups) that is likely to be without appreciablerisks of deleterious affects during a lifetime. It is based on themost sensitive adverse dect found by toxicologicaltesting and then applying

Regulatory Requirements

77

a series of unmtainly factors so that higher exposures may also not present any appmiablerisk For example, it assumesthat humans may be an order of magnitude more Sensitivethanthe animals tested. But in fact humans may also be less sensitive. This Bssesgnenf method also assumes,except as noted, that the risk assessmentsrelied an far theseregulationsare based on exposureslasting a lifetime, whereas in fact they may be much shorter. Similarly, cancer risks are calculated in the risk assessment guidelines as "plausibleupper bounds" to the actual risk. Conservativeassumptions used in these calculations, such as use of the most sensitiveanimal data in bioassays, linear extrapolaticnto low doses, species-to-species conversionbased on surface area and use of an upper confidence limit for the dose response slope make it unlikely that cancer risk would be greater than what is calculated; in fact, it could be orders of magnitude less or even zero. The application of uncertainty factors in the risk assessment process provides a conservative level of protection even in the face of limited data on the factors described above.

* Home Garden Scenario A primarily focus of concern in developingthe 503 Regulations was the human food chain pathway, that is the production of crops by gardeners and farmers. The population assessed for this pathway were people who used biosolids to produce crops for their own consumption. EPA made specific assumption about a number of variables addressinghuman behavior and properties of biosolids as describedbelow.

*

Plant Uptake of Metals The risk assessment estimated metal concentrations in plants using a linear uptake line whose slope was based on the assumption that metal uptake was proportional to cumulative applicationrates of biosolids. The uncertainty as to whether this is appropriate arises fiom data on plant concentration versus application rate which suggests a nonlinear relationship. EPA's assessment of linearity is therefore conwative because application rates allowed under 503 are in g d well in excess of test plot application rates and data ffom long-term (>20 years) studies support the conclusion that actual metal concentrations in plants plateau at these higher application rates (seeFigure 2-1). Anadditidty in the plant uptake calculation arises fiom the uses of gemnet& means for all slopes calculated fiom individual studies. Assuming a log normal distribution,the geometric mean provides an estimate of the median (50th percentile) slope. Such a value is useful in estimating uptake for "typical biosolids." Individual studies used by EPA to calculate plant uptake used bimlids with higher metals Concentrations than the "typicalbiosolids" on the

Metal level in biosolids-amended soil FIG. 2-1 ASSUMED VS. ACTUAL UPTAKE OF METALS RESULTING FROM INC R EASlNG CU MULATIVE LOAD1NGS

Regulatory Requirements

79

market today; therefore, these biosolids with higher metal concentrations most likely produced higher plant concentrations than would be likely using current values. A further uncertainty results from the way the geometric mean calculations were performed. A value of 0.001 was used as the uptake slope from studies which showed no significant increase in metal uptake by the crops. Since the geometric mean calculation is very sensitive to the inclusion of low values and it appears that 0.001 is smaller than the upperbound on uptake that would be obtained from "no significant increase in metal uptake" studies, the use of this default slope of 0.001 may therefore underestimate the typical slope for crop uptake. Dietary Consumption Pollutant limits for the 503 Regulations were calculated based on population average food consumption estimates. Survey data from short-term food consumption reports of large surveyed populations provided the basis for these estimates--an accepted basis for estimating population average food consumption rates. Such estimates do not reflect the higher food consumption rates per unit of body weight of young children compared with adults. Another limitation arises from the fact that it is possible that individuals who raise a particular crop may have a highcr consumption rate than do individuals who only buy such a crop. On the other hand, homc gardens do not produce a specific crop year round, which may offset this bias. EPA estimated the amount of food coming from biosolids treated land using USDA survey data on average percentage food consumption from home-grown crops. EPA estimated that large garden plots were required to produce the amount of home grown crops consumed although EPA believes that a relatively small percentage of gardens are that large. Also, because of seasonal factors it may be difficult for most gardeners to produce the quantities of leaf) vegctables that are assumed; leafy vegetables are an important factor in the risk assessment since they tend to have high mctal uptake slopes. Plant Toxicity and Uptake The plant toxicity (pliytotoxicity) assessment was based on the relationships between biosolids application and tissue residue, between tissue residues and growth reduction, and between growth reduction and yield reduction. The relationship between growth reduction and yield reduction is particularly uncertain and will vary with chemicals, crop species and toxic endpoints. Some crops (e.g., beans) and endpoints (e.g., reproduction) may be more sensitive to the effects of biosolids, although other crops (e.g., sudan grass) and endpoints (e.g., mortality) may be less sensitive. There are limited data about non-

80

Forste

cultivated forest species and perennials and these may differ in their response to biosolids-borne contaminants. Metal phytotoxicity depends particularly on soil pH and the degree of binding to the biosolids matrix. Since most metals are more available in acidic soil, EPA used the assumption of a soil pH of 5.5 as representing a reasonable worst case condition for agricultural soils. Based upon results from several field studies, EPA believes that metals are bound to the biosolids matrix and remain relatively unavailable biologically. 0.

Wildlife In the absence of standard methodology at EPA for assessing iisks to wildlife, there are uncertainties about how biosolids application affects terrestrial wildlife and soil biota. Available data and methodologies only describe direct toxicity to a few species. With uncertainties as to how to extrapolate this information to other birds, mammals, amphibians and soil invertebrates whose relative sensitivity to the compounds of concern is unknown. The ecotoxicological analysis focused on cadmium and lead which had the most data available. EPA used a simple linear model of bioaccumulation or bioconcentration from soil to earthworms to shrews and did not model the more complex effects of biosolids contaminants on the terrestrial food chain. In the absence of standard methodologies, EPA did not consider how amendment of forest soils or edges of agncultural fields with biosolids might change the composition of species in the plant community, either through nutrient enhancement or phytotoxicity, and any subsequent ramifications such changes might have throughout the food web. Uncertainty about the impact of biosolids on soil biota exist because the criteria are based solely on a N O E L for the earthworm (Eiseniafoetidu) which may not be the most sensitive or appropiiate species for evaluating many of the chemicals. Possible additional factors influencing soil flora and fauna include adding nutrients to the soil and possible increased exposure to organisms that feed in the litter layer due to the organic matter in biosolids. While these considerations need to be explored further, it should be noted that the same lack of data and uncei-hinties exist with respcct to the current agricultural practices on sites used to produce food and feed crops throughout the US. Section 405 of the CWA requires EPA to develop standards for biosolids use or disposal, to protcct from reasonably anticipated adverse affects, and to promulgate these standards in two stages and to revise the standards periodically. EPA concluded that the standards adopted on February 19, 1993 are adequately protective based on its assessment of available data. To verify the conclusions about the adequacy of these standards, EPA has committed to developing a comprehensive,environmental evaluation and monitoring study. The results will provide a database for the Round 2 biosolids standards and will also help the Agency in its efforts to develop a comprehensive, ecological risk assessment

Regulatory Requirements

81

methodology and to conect any uncertainties in future Part 503 rulemakings. The final plan, including study design, will be available for public comment at the time that Round 2 regulation is proposed. Risk Management Auuroach Using risk characterizationinformation, EPA can determine if a "SigIllficant"or "unreasonable"risk exjsts, what controls are needed and how to communicatethis risk to the public and regulated community. The mere identification of risk is not necessarily enough to just@ action. Also,non-risk factors, such as availability and effectivenessof controls, whether alternativesexist and any benefits lost or gained as a result of umtmls must be evaluated by the Agency to reach a decision. Under each exposure scenario, EPA identifies a range of control strategies and regulatory requirements that usually reduce exposure so that the risk or identified effect is put back into balance with the benefits. Using this information provided in therisk management step, EPA can then select the appropriate control strategy and means for communicating it. For the 503 regulations, EPA's approach was to establish management practices and numerical limits (standards) to safeguard public health and the environment by examining use or disposal practices and the probability that individuals would be exposed to pollutants Itom these practices. The Agency identified the types of risks involved (e.g., drinking water with pollutant levels exceeding the MCLs for drinking water) and examined the possibility of special populations at greater risks (e.g., small children playing in gardens where biosolids have been applied), and examined whether individuals voluntarily incurred the risks. Finally, EPA used exposure assessment models to project the effecton an individual receiving maximum dose throughout an average life span of 70 years. Aggregate effects analyses were employed to project the incidents of adverse health effects on the population as a whole (e.g., the number of people exposed to lead at levels producing adverse health effects). For its regulatoy approach in the proposed Regulations, EPA primarily focused on two types of risks: that to a most exposed individual (MEI) receiving a maximum dose and risk to the population as a whole (aggregate risk). The Agency examined both the individual and aggregate effect of each alternative to balance the uncertainties in the analyses. Available data resulted in the greater emphasis being placed on public health than on environmentaleffects, although environmentaleffects were considered in the determinationof what constituted adequate protection as long as they could be identified even qualitatively. Individual vs. aggregate risk results in divided opinion. Some argue that no individual should be at high risk and thatconsidering the number of people at risk leads to acceptance of higher individual risk when two people are exposed; using maximumindividual risk alone h u t s the effectivenessof the Regulations by not indicating how many people may be affected since they only relate to carcinogenic

82

Forste

risk to the MET Aggregate risk can also be seen as an appropriate measure of total public health impact and therefore a good indicator of whether the goal of adequately protecting public health has been achieved. A combination of approaches was selected by EPA for the purposes of the proposed 503; this was revised for the hal Regulations based on current Agency policy, public comment and scientific peer review. For the proposed 503 Regulations, using the ME1 approach, the Agency identified a most exposed individual plant or animal that remained for an extended period of time at or adjacent to the site where the maximum exposure occurs. EPA used models of 14 exposure pathways to determine the concentration of biosolids-borne pollutants that may be utilized or disposed of in each use and disposal practice without exceeding human health or environmental criterions. These criteria were taken from those already published or promulgated by the Agency, fiom human health criteria developed by the Agency or from plant and animal toxicity values published in scientific literature. For example, to protect sources of drinking water, pollutant limits were developed which would ensure that the Agency’s maximum contaminant levels were not violated. For surface water protection, Water Quality Criteria were used. For carcinogens, the risk specific doses corresponding to an incremental carcinogenic risk level of 1 x 10” were used for all use and disposal practices except when biosolids were distributed and marketed. For distribution and marketing, numerical limits were established so as not to exceed an incremental carcinogenic risk level of 1 x lo6. For all pathways, the human MEIs assumed to be the most sensitive individuals were continuously exposed over a 70 year lifetime. Endpoints for the ecologicals MEIs were conservatively developed using the most sensitive species with steady state duration and concentration of exposure over a critical life period. For all use or disposal practices, carcinogenic risk targets were applied pollutant by pollutant, except for the organic pollutants in the emissions of biosolids incinerators. In this case, EPA set a limit on total hydrocarbon emissions, rather than on each individual pollutant as described previously. For the proposed 503 Regulations, the Agency’s approach to situations where individuals were unlikely to be exposed to biosolids pollutants (i.e., forests, reclaimed lands, and others) a proposed 98th-percentile pollutant concentration was developed based on the assumption that such practices would have negligible impacts on human diets. These pollutant concentration limits also applied to surface disposal since the Agency believed such sites are generally small, located away from population centers and presented little likelihood of exposure.

Regulatory Requirements

83

5 . Conments/Review oJProposed Rule EPA solicited comments on a wide range of issues, including the hndamental principles of the 503 Regulations, the carcinogenic risk levels, other human health and environmentalcriteria, changes resulting from other Agency actions, the models used, die ME1 and aggregate risk analyses, the cost benefits estimates of the Rule and data deficiencies. The EPA committed to seek and support scientific peer review of the technical bases of the rulemaking during the public comment period (1 8 3 days). The major peer review groups with which EPA worked during the public comment period were: Land Practices Peer Review Committee -- This specially convened group of biosolids experts reviewed in depth the land application, distribution and marketing, monofilling and suiface disposal provisions. The group was comprised of nationally known experts on biosolids use and disposal, including members of the U.S. Department of Agriculture W-170 Committee along with representatives from a broad diversity of views. The Peer Review Committee's final report was officially submitted to EPA on July 24, 1989. [ 121 EPA Science Advisory Board -- The SAE3 reviewed the technical bases of the incineration portion of the incineration portion of the regulation. As various SAB committees have reviewed similar EPA incineration regulations in the past, most notably for municipal solid waste combustion and hazardous waste incineration. Their final report of August 7, 1989 was also submitted to EPA. [ 131 During the comment period, EPA received more than 5,500 pages of comments from 656 commenters during the 183 day public comment period. Comments were submitted from municipalities, industries, federal and state agencies, professional associations, academia and the general public. These comments were provided in addition to the foimal peer review described above. These public and scientific review groups provided EPA a comprehensive rangc of opinions, comments, and recommendations. Many commenters criticized the Agency's risk assessment as being overconservative for some use and disposal practices and underconseivative for others. Other significant areas of criticism included the risk levels used by EPA ( i c , which risk levels are most appropriate); data selection and the parameters used in the analysis of exposure assessment and thc impacts that the proposed nile would have on beneficial use of biosolids. Following the public comment period, EPA provided public notice of the availability of National Sewage Sludge Sluvey data along with information and data from the Sewage Sludge Incinerator Study and the Domestic Septage Study. The notice described some of the rcsults of the survey and changes the Agency considered making to the proposed 503 Regulations as a result of these studies. The notice also requested comments on a

a4

Forste

number of changes to the use and disposal standards that were being considered for the 503 Regulations in light of the comments submitted earlier the peer review and new idormation developed since the February 8, 1989 proposal. During the 60-day public comment period for the NSSS notice, the Agency received more than 1000 pages of comments from 153 commenters. Many of these comments supported the changes identified in the notice as potential revisions to Part 503. 11. FINAL PART 503 REGULATIONS

A. Introduction

Thc Agency's proposed regulatory approaches received extensive peer review and public comment, focusing especially on the ME1 exposure scenario used and the use of the 98th percentile technique. The Agency agreed with many of the comments provided by the public and the scientific peer review committees and noted that there is no clear guidance in Section 405, which contains only limited discussion of how to establish pollutant limits and concluded that its statutory duty was to protect against reasonable risks to exposed populations and not to risk associated with highly unlikely or unusual circumstances. Therefore, the Agency decided to evaluate the risk to a highly exposed individual (I-IEI) instead of the ME1 for the final 503 risk assessment. This approach more realistically protects the health of individuals or populations which are at higher risks than the population as a whole. EPA retained a 70-year exposure for the HE1 in the final risk assessmcnt as a conservative assumption in the context of a highly mobile society. Furthermore, this 70-year exposure duration represents a steady state assumption that is consistent with the measure of carcinogenic risk (i.e., the probability of contracting cancer based upon a lXetime--70 year-exposure). Tlus esposure assessment assumption, which is in part a policy judgment by EPA, was considered by the Agency to be preferable to the less conservative alternatives suggested and, in the Agency's opinion, represents an appropriate response to their obligation to protect public health. Furthermore, EPA believes that retaining the 70-year assumption insures that the population of highly esposed individuals will remain extremely small. In preparing the proposed 503 Regulations, EPA used what it believed were "reasonableworst-case assumptions." Each of these has a margin of safety associated with it, depending on the accuracy ofdata and information supporting it. For example, if EPA had insufficient data from biosolids/field studies 011 metals uptake in crops, data from biosolids/pot studies or saldpot studies were used in the original ME1 risk assessment analysis. The margin of safety associated with the data from saltlpot studies is much geater than the margin of safety associated with data from biosolids/pot studies and far greater than the margin of safety associated with data from biosolids/field studies.

Regulatory Requirements

85

The proposed Regulations were developed using each of the above types of data described above and in addition, selected data using only studies which showed an effect, thus eliminating valid field studies in favor of pot studies or salt studies which showed an effect which did not occur at similar levels in the field. Individual researchers and members of the peer review committee emphasized that data from pot and metal salt experiments should not be extrapolated to results found in the field. Thus the peer review concluded that the approach used to select data for individual contaminants and pathways in the proposed rule was scientifically unsound and needed to be revised. The peer review committee also criticized E P A s identification of an ME1 in the proposed rulemaking. As noted by the committee: "The exposures implied by the MEIs are grossly exaggerated and it is impossible to know the probability that such an ME1 exists. Because the assumptions underlying selection of the various MEIs differ, the extent of conservatism embodied in the various ME1 exposure models also differs. Further, insufficient information is available to compare the relative degree of conservatism of one ME1 with another. Consequently, it can be misleading to compare risks derived from one ME1 with another because different premises were used within and among disposal options. The reliance on "worst-case" scenarios pervades these [proposed 5031 documents and needs to be reconsidered. The ME1 is so rigidly defined that it completely overshadows other components of the risk assessment pathways and even to the extent that it makes the issue of 'inappropriate' technical material almost irrelevant."[12, p. xi] The problem occurred when a series of parameters and assumptions, each having large margins of safety, were used in the same exposure pathway assessment. This results in an unrealistically conservative analysis which the Agency acknowledged could conceivably over-regulate a useldisposal practice. Information provided by the NSSS, the incinerator study, the scientific peer review committees and the public was incorporated into the aggregate risk assessment for the final Rule and showed minimal risk from current biosolids management methods (pre-Part 503 or base line risk). EPA agreed with the public and the scientific peer review committees that the 98th percentile approach is inconsistent with the ME1 approach and that numerical limitations derived ftom the 98th percentile approach do not insure protection of public health and the environment because they lack a formal pathway risk assessment. In developing the proposed 503 standards, the Agency had relied on this approach because it did not have reliable exposure assessment models nor input data and information to conduct the pathway risk assessment for certain practices. The Agency also believed that this 98th percentile approach was supported by the aggregate risk assessment which showed low exposure

86

Forste

and minimal human health impacts on the population as a whole from the use and disposal practices which would be governed by this approach. Following the proposal, the Agency workcd with experts within and outside EPA to devclop and refine modeling techniques and supporting data to conduct a formal pathway risk asscssmcnt for these practices. Public comments and scientific peer review provided EPA with better data and lnfoimation so that numerical limitations for all biosolids use and disposal practices could be derived using risk assessment. For the final Regulations, EPA selected an approach based on a risk to highly exposed individuals (HEIs) and consideration for higher risk populations (aggregate risk assessment), not an unrealistic worst-case ME1 approach. EPA believes this approach is consistent with the congressional intent to establish standards adequate to protect public health and the environment from reasonably anticipated adverse affccts of each pollutant. In the final Regulations, EPA evaluated the risk to highly exposed individuals and populations from pollutants found in biosolids using differcnt exposure assessment pathways. In evaluating the standards for the final Rule, the Agency established criteria based not only on cancer risk but on a series of other health and environmental effects, including the overall incidence of othcr serious health effects within thc esposed population as a whole (including average exposed and highly exposed individuals and within special subpopulations such as children). EPA also evaluated effects on plants and animals and considered policy assumptions, estimation of uncertainties and margin of safety, weight of the scientific evidence for human health and environmental effects, other quantified or unquantified health and environmental effects and other impacts associated with use and disposal of biosolids before selecting the final standards. The Agency also determined that in order to insure adequate protection of public hcalth and the environment, they needed to add safety factors to the numerical criteria derived from the exposure assessments. This dccision also served a sccond critical objcctivc in the rulcmaking, that is, to promotc the use of biosolids for their beneficial properties. EPA believes that an inportant component to promote beneficial usc of biosolids involves building public confidence that using biosolids to @-OW food that the public eats is safe, and that adding a margin of safety to the model-derived criteria should help to encourage this practice. In addition, adding safety factors to the model-derived numerical criteria enabled EPA to overcome some of the unquantifiable variables in the real world movement of pollutants fi-ombiosolids to environmental end points. EPA therefore made a number of assumptions to reduce the complexity of actual experience. The Agency believcd that these exposure assessments generated numerical criteria consistent with the stated goals of the rulemaking. Also, through its exposure assessments, EPA derived numerical limitations from metals that represented the total quantity that could be added to the soil. So long as that total quantity (loading) for the metal is not exceeded, the eqosure assessment models predict that there will be no haim to the HEI. Sincc

Regulatory Requirements

a7

the model does not concern itselfwith whether the total quantity is received in a single application or over time, adopting a purely cumulative loading approach would mean that biosolids with extremely high metals concentration could be applied to the land so long as the cumulative load is not exceeded. Such an approach, which is strictly risk based, could therefore allow for degradation of current biosolids quality. EPA's aggregate risk assessment shows only small effects associated with current use and disposal--that is, biosolids used at pre-503 pollutant concentration levels presented a low risk and therefore such levels already had an inherent level of protection. The models do not look exclusively at data from the most heavily contaminated biosolids. Therefore, in order to ensure continued protection of public health and the environment EPA determined that existing quality of biosolids applied to land should be "protected" and not allowed to deteriorate above current concentration levels. EPA has stated that implicit in its numerical pollutant limits for land application is the assumption that biosolids with low concentrations of pollutants are safe and to downgrade the quality of biosolids reduces the protective levels inherent in such limits. EPA concluded that its uncertainty about the protectiveness of the numerical criteria derived from the exposure assessment models for land application is increased by adding margins of safety to the numerical criteria. Accordingly, the Agency placed a "ceiling" on the concentration of pollutants in biosolids that may be applied to land at the 99th perccntile pollutant concentration from the NSSS survey. This ceiling concentration is the &r of the 99th percentile pollutant concentration or risk based pollutant limitation and acts as a trigger, dictating when biosolids quality is no longer suitable for beneficial use, regardless of how it is applied to the land. One important purpose of the ceiling limits is to direct the "cleanest biosolids" into beneficial usc practices. The agency has also "capped" the numerical pollutant limits for land application at the 99th percentile pollutant concentration found in the NSSS. If that concentration is lower than the risk-based numerical pollutant limit, this cap dctermines when biosolids quality is suitable for beneficial use under the alternative pollutant limit concept or must be applied using cumulative pollutant loading rates as discussed below. EPA made these risk policy decisions (i.e., the capping and ceiling) to provide an additional margin of safety to protect public health and the environment beyond the risk-based standards developed for the final rule while maintaining quality to encourage utilization consistent with the Agency's beneficial use policy. A complete description of the exposure assessment methodology and risk management issues for the proposcd 503 rulemaking is found at 54 Federal Register 5764 - 5791. The following section describes the exposure assessment pathways modeled in the final Part 503 Rule and the basis for the decisions made in developing the approach for each use and disposal practice. A detailed discussion of the exposure assessment methodology (i.e., models, pathways, parameter values, assumptions and others) and the risk management decisions used by EPA to develop the final Part 503 numerical critcria are contained in the Technical Support Documents.

88

Forste

B. Exposure Assessment Pathways EPA evaluated 14 pathways of potential exposure to pollutants in biosolids of the final Part 503 Regulations. The Regulations distinguish between biosolids applied to the land for a beneficial purpose and biosolids disposed of on the land. EPA evaluated potential exposure when biosolids are used as a fertilizer or soil conditioner in one of two ways: agricultural and non-agricultural land application. Agricultural land application includes use to produce food or feed crops commercially by agricultural producers on pasture and rangeland and also by a home gardener (formerly described as distribution and marketing in the proposed rule; in the final rule characterized as biosolids "sold or given away in a bag or container"). Nonagricultural land includes forest, reclamation and public contact sites. The descriptive term "surface disposal"in the final rule applies to biosolids disposed on land either in piles or in biosolids-only landfills which are also referred to as monofills. For surface disposal, EPA evaluated two pathways of exposure. Incineration was evaluated by a single pathway of exposure--inhalation. The final exposure assessment pathways evaluated in the final Part 503 Regulations are: Land Amhxtion (Beneficial Use) Biosolids-soil-plant-human(Pathway 1) Biosolids-soil-plant-home gardener (Pathway 2) Biosolids-soil-child (Pathway 3)* Biosolids-soil-plant-animal-human (Pathway 4) Biosolids-soil-animal-human(Pathway 5) Biosolids-soil-plant-animaltoxicity (Pathway 6) Biosolids-soil-animal toxicity (Pathway 7) Biosolids-soil-plant toxicity (Pathway 8)** Biosolids-soil-soil biota toxicity (Pathway 9) Biosolids-soil-soil biota-predator of soil biota toxicity (Pathway 10) Biosolids-soil-airborne dust-human (Pathway 11) Biosolids-soil-surface water-contaminatedwater-fish toxicity-human toxicity (Pathway 12) Biosolids-soil-air-human (Pathway 1 3) Biosolids-soil-groundwater-human (Pathway 14) 9

Surface Disposal Biosolids-soil-air-human (Pathway 13) Biosolids-soil-groundwater-human(Pathway 14) Incineration Biosolids-incineration particulate-air-human (Pathway 13)

Regulatory Requirements

89

* Most limiting pathway (regulatory limit) for arsenic, cadmium, lead, mercury and selenium. ** Most limiting pathway (regulatory limit) for copper, lead, nickel and zinc.

For situations where EPA determined that these exposure assessment pathways for a particular use or disposal option did not yield adequately protective results, additional management practices were imposed under the Regulations to prevent environmental abuses and to protect public health. All pathways, except 3 , s and 7, assume the mixing of biosolids with 15 cm (i.c., six inch plow layer) of the suiface soil, either by incorporation or by natural weatheringprocesses. Using this assumption, which entails the affected surface layer having a mass of two million kghectare, pollutant concentrations in soil (per unit mass of soil) could be converted to cumulative pollutant loading rates for metals (mass of pollutant/hectare of land, In addition, annual pollutant loading rates for organics (mass of pollutant per hectare of land per 365-days) could also be converted to pollutant concentrations in soil. EPA first determined the pollutant concentration in soil that would be allowed (i.e., the maximum pollutant concentration in soil that when taken up by a plant and consumed by a target organisms does not produce undue risk for a particular pathway). Having made that determination, the model then can be used to determine the allowable pollutant loading rate in two different ways. Metals are determined based on a cumulative pollutant loading rate. That is, the total quantity of metals consistent with no undue risk. This number is derived from the allowable pollutant concentration in the soil multiplied by the mass of soil in the top 15 cm of a hectare of land. 2. The annual pollutant loading rate for organic pollutants (kg/ha/365 days) takes into account the rate of pollutant loss or decay. A first order decay of organic pollutants is assumed by the model--that is, that the quantity lost per year is directly proportional to the quantity present. Assuming continued annual applications, pollutant concentrations for organics gradually approach a plateau at which the quantity lost each year equals the quantity applied. Therefore, the annual pollutant loading rate is dcteimined such that the concentration levels off at that allowable soil concentration when biosolids are applied for n long period of time.

I.

For human exposure pathways, maximum pollutant intake allowed was based on several different EPA health effects criteria: reference dose (RtD), recommended daily allowance (RDA), or concentration (RE) for noncarcinogens, a risk specific

90

Forste

dose for carcinogens based on a risk level for all use and disposal practices. A daily dietaiy intake derived from a drinking water standard or a drinking water standard (MCL). The only exception to this approach is Pathway 11 (inhalation) which limits the pollutant concentration in the soil to an amount which will not exceed the National Institute of Occupational Safety and Health N O S H ) work place air quality criteria ifsigmficant quantities of soil become airborne. This pathway was not a limiting pathway for any pollutant in the final Regulations Brief summaries of each of the pathways analyzed for the 503 Regulations are as follows:

Pathway 1 - Evaluates liuniati exposure to crops grown with biosolids. This pathway is designed to protect consumers who eat produce grown in a soil to which biosolids have been applied. The environmental endpoint is an HE1 (highly exposed individual) assumed to live where a relatively high percentage of the available cropland receives biosolids. It is assumed that the HE1 eats a mis of crops from land where biosolids were applied and from land where this did not occur. For Pathway 1,2.5 percent of a consumer's intake of grains, vegetables, potatoes, legumes and garden fruits is assumed to come from biosolids-amended soil. Pathway 1 assumes uptake of biosolids pollutants through plant roots, not through direct adherence to crop surfaces as crops are assumed to be washed before consumption. This pathway assumes agricultural use in commercial enterprises where crops for human consumption are grown. Pathway 1 also includes the exposure of a person in a non-agricultural setting who ingests wild berries and mushrooms grown in biosolids amended soils. Exposure is based on the uptake of a pollutant by each type of beny or mushroom, a daily consumption of wild bemes and mushrooms, and a fraction of diaerent wild berries and mushrooms grown in biosolids amended soil. Thc HE1 for the non-agricultural Pathway 1 is an individual living where biosolids are applied to a forest, a public contact site or reclamation site. The dose for this pathway is the RfD for an inorganic pollutant; organics were not evaluated for this pathway because they do not concentrate in wild berries and mushrooms. Pathway 2 - Evaluates the situation it1 which biosolids are added to tlie soil in a home garden. Pathway 2 differs from Pathway 1 primarily in the fraction of food g-oups produced on biosolids amended soil. For this pathway, as much as 60 percent of the EIEI's diet of certain food groups is grown in the home garden where biosolids are used as a fertilizer. Pathway 2's home gardener produces and consumes potatoes, l e a vegetables, legume vegetables, root vegetables and garden fi-uits; grains, cereals and peanuts are not included in these crops because home gardeners do not usually consume such crops when they grow them on biosolidsamended soils.

Regulatory Requirements

91

Pathway 3 -Assesses the hazard to a child ingesting undiluted biosolids. This HE1 is the child who ingests biosolids from storage piles or from the soil surface. Pathway 3 assumes that the biosolids are not diluted with soil when exposure occurs. The ingestion rate of 0.2 grams (dry weight) per day for five years was based on the 1989 EPA soil ingestion directive suggesting this value for higher risk chldren. Pathway 3 uses the integrated uptake biokenetic model (IUBK) rather than extrapolating from cattle data as had originally been proposed. The IUBK model used a lead blood level not to exceed 10 micrograms per deciliter, a 30 percent absorption value and a 95th-percentile population distribution. Using these values in the IUBK model, results in an allowable biosolids concentration of 500 parts per million (ppm). The lead pollutant limit calculated by the peer review committee resulted from the observation that body burdens (absorption) of animals fed up to 10 percent of their diet as biosolids did not change until the lead concentration in the biosolids exceeded 300 ppm. EPA therefore decided to select the more conservative numerical limit for the final rule to minimize lead exposure to children and set the allowable lead concentration at 300 ppm for Pathway 3. Pathway 4 - Evaluates huinan exposure from the consuinption of anitnal products. The HE1 for this pathway consumes the tissue of foraging animals that have in turn consumed feed crops or vegetation grown on biosolids amended soils. Pathway 4 depends on plant uptake of a contaminant being proportional to soil concentrationsof the contaminant with uptake occurring through the roots to the edible portion or by volatilization from soil to above ground plant parts. In the non-agricultural setting for Pathway 4, an individual consumes meat or products from wild animals that consume plants grown in biosolids amended soils (meat obtained from hunting herbivorous wild animals). For both agricultural and non-agricultural Pathway 4 HEIs, a background pollutant intake is also assumed. Pathway 5 - Evaluates the consumption of anitnal tissue which has been containitiated by direct ingestion of biosolids by the animal. The HE1 consumes the tissue of foraging animals that have incidentally ingested biosolids. As with Pathway 4, the HE1 is assumed to consume daily quantities of various animal-tissue food groups and is also assumed to be exposed to a background intake of each pollutant. Pathway 6 - Establishes level of pollutants in biosolids to protect aiiinials ingesting plants grown on biosolids amended soils. This pathway assumes the livestock diet is 100 percent forage grown on biosolids amended soil and that the animal is exposed to a background pollutant

92

Forste

intake. For Pathway 6, when a sensitive species has been identified for a specific pollutant, that species is used in the exposure assessment (e.g., livestock, domestic grazing animals, birds and rodents). Pathway 7 - Designed to protect the highly sensitivelhighly exposed herbivorous livestock animal which incidentally consumes biosolids adhering to forage crops and/or biosolids on the soil surface. Pathway 7 assumes a 1.5 percent biosolids in the livestock diet and a background pollutant intake for the animal, as well as the most sensitive species for which data are available for each pollutant. Pathway 8 - Evaluates the risk of plant toxiciyfiom pollutants i n biosolids. EPA determined an allowable pollutant concentration in the soil that would be associated with a low probability (I x lo‘) of a 50 percent reduction in young plant growth (not necessarily a mature plant yield reduction). Since phytotoxicity resulting from metals is sensitive to changes in pH, plant species and the degree of binding capacity in the biosolids matrix, EPA elected to “cap“ at the 99th percentile pollutant concentration from the NSSS for some metals. Pathway 9 - Designed to assist in establishingpollutant loading litnits to protect highly exposeahighly sensitive soil biota. Since only limited field data exist which indicate levels at which inorganic pollutants become toxic to soil biota, the database available for earthworms which were raised in biosolids were used to set the criteria for this pathway. These criteria are based on a No Observed Adverse Effect Level (NOAEL) for the earthworm Eisenia foetida. Pathway 10 - Designed to assist in detemiining pollutant loading liriiits to protect highly sensitive/highly exposed soil biota predators (i.e., sensitive wildlye that consuiiies soil biota that have beenfeeding on biosolids-amended soil). A literature review identified what the Agency determined is a pollutant intake level protective of sensitive species in general. Chronic exposure assumes the sensitive species diet to consist of 33 percent of such soil biota.

-

Pathway 11 Evaluates liuiiian exposure to biosolids pollutants through inhalation. A tractor driver tilling the field is the HE1 for Pathway 1 1 which evaluates the impact of suspended particles of dewatered biosolids. Pathway 1 1 assumes the incorporation of biosolids to a depth of 15 cm and a distance from the driver to the soil surface of 1 m with no more than 10 mglcubic meter (mglm’) of total dust.

Regulatory Requirements

93

Pathway 12 - Designed to protect surface waters for benejkial use in order to protect both human health and aquatic lfe. The runoff of pollutants from soil on w h c h biosolids have been applied is calculated so that the soil pollutant concentrations would not result in exceeding a Water Quality Criterion for a pollutant if the soil enters a relatively small stream. Rate of soil loss was based on the Universal Soil Loss Equation and a sediment delivery ratio. Water Quality Criteria designed to protect human health assume exposure through consumption of drinking water and resident fish and are also designed to protect aquatic life. Pathway 13 (Land Application) - Evaluates the exposure of a fann fami4 inhaling vapors of volatile pollutants that niay be in biosolids applied to the land. Six pollutants were included in this pathway: benzo(a)pyrene, dis(2ethylhexyl)phthalate, chlordane, DDT, dimethylnitrosamine, polychlorinated biphenyls. These pollutants were selected from EPA's hazard indices screen because they are semi-volatile. Organic pollutants which are highly volatile were not evaluated for Pathway 13 because such compounds would volatilize in the wastewater treatment processes and were therefore not considered to be of concern. Similarly, non-volatile metals were not evaluated in the vapor pathway. The vapor pathway assumes that the total amount of the pollutant spread each year would vaporize during that year. The allowable annual pollutant loading rate is thus equal to the amount that may be allowed to enter the atmosphere per unit area per unit time without exceeding the allowable pollutant concentration in the air. This concentration corresponds to the R E , risk specific dose or an acceptable daily dose derived from an MCL. A plume model was used to determine the resultant pollutant concentration in the air with a never changing wind direction so that the HE1 always remains in the center line of the plume. Pathway 13 (Surface Disposal) - Evaluates the exposure of an individual inhaling vapors of any volatile pollutants for a 70-year period. The HE1 is assumed to live at the downwind edge of the site and to inhale air, at 20 cubic meters per day, for 70 years, contaminated with volatile organic compounds from a surface disposal site. Volatilization rates were calculated with equations that consider constituent parameters. Allowable lifetime exposure (at a risk level of lo4) is the basis for back-calculating the allowable loss rate to the vapor pathway. This value, divided by the fraction that vaporizes, provides the allowable pollutant concentration at the site.

94

Forste Pathway 13 (Incineration) - Evaluated exposure of an individual inhaling particulates and gasesfi.oni an incinerator 24 hours per day, 365 days per year f o r 70years. This HE1 is located when the highest annual ground concentration of incinerator emissions occurs. This pathway evaluated five metals (arsenic, cadmium, chromium, lead and nickel) and approximately 70 organic compounds which are represented by a surrogate measure of total hydrocarbons. Allowable metal concentrations are calculated by determining the removal efficiency across the incinerator and air pollution control devise site-specifically and considering the feed rate to the system. Pathway 14 (LandApplication) - Evaluated exposure of individuals obtaining drinking water fioni groundwater directly below a j e l d where biosolids were applied. The allowable pollutant rate was determined from the MCL that must be met at the groundwater interface with no allowance for dilution, the decay rate of a pollutant and other factors that affect either the time period for decay or the dispersion of the peak conccntration. Pathway 14 (Surface Disposal) - Evaluated exposure of individuals who obtained their drinking water from wells located 150 iiielers or less downgradient, using eitherMCLs or iisk level of I x IQ'for an HEI consunling two liters of water per day over a 70year lifetime.

Numerical limits were derived for both covered and uncovered suiface disposal sites, considering pollutant losses through three processes: volatilization, on-site degradation, and leaching. Public comments and the scientific peer review of 503 resulted in changes in many of the assumptions and data originally proposed for the exposure assessment methodology. These changes resulted in more realistic exposure scenarios re1at'ive to the use of biosolids which are land applied for agricultural purposes. A discussion of the major comments, responses and actions taken by EPA are contained in the final Part 503 Regulations and the Techrucal Support Document for Land Application. [ 141 These sources provide detailed information on the Agency's response to the significant comments on the exposure pathways and the final action in the 503 Rule resulting from the Agency's evaluation of such comments. Domestic SeDtane Many of the 130 commenters who provided input to the Agency on the proposed regulations for septage, opposed the 1989 proposed limits as overly stringent, burdensome and with little or no environmental benefit. As a result of these comments, the Agency agreed that the regulation of septage in a manner similar to

Regulatory Requirements

95

biosolids would have been difficult to implement and therefore unlikely to achieve the Agency's public health and environmental statutory objectives. Based on this assessment, EPA developed an alternative regulatory strategy for septage which replaced the 503 pollutant monitoring and cumulative pollutant loading limits used for biosolids with a single hydraulic loading rate (30,000 gallons per acre per year). Thc revised approach also requires short-term lime stabilization to control pathogcn and vector attraction or site use and access restrictions in the absence of pH control. EPA also simplified many of the monitoring, recordkeeping and reporting requirements for domestic septage. The h a l rule on domestic septage contains limits on hydraulic loading based on an average septage analysis to provide an estimated crop nitrogen rate. Short-teii alkaline stabilization for septage may be achieved by raising the pH to 12 or greater for 30 minutes. If alkaline stabilization is not possible and domestic septage is applied to agricultural land, crops whose edible portions may contact the surrace soil and root crops grown in the soil are prohibited for 14 and 38 months rcspectively following septage application. In addition, for sites where potential for public exposure is high (e.g., parks and recreational areas) public access must be restricted for 12 months following application of domestic septage; whcre such potential esposure is low, public access must be restricted for 30 days. Similarly, animals must not be allowed to graze or feed crops harvested for 30 days. [ 151 C. Final Part 503 Standards

The standards contained in the final Part 503 Regulations consist of general requirements, pollutant limits, management practices, operational standards and requirements for frequency of monitoring, recordkeeping and reporting. In the final regulation, EPA uses the term "land application" in a restrictive sense to delineate clearly between different regulatory requirements. Since biosolids are not only disposed on land, but in many cases also used to condition the soil or provide nutrients, the Part 503 uses the phrase land application only when referring to biosolids used for their beneficial properties. When biosolids are disposed of by placing them on the land, Part 503 rcfers to this practice as "surface disposal." Requirements for biosolids applied to the land differ for bulk material or that soldgiven away in a bag or other container. The latter is used to distinguish situations in which biosolids are typically applied in small amounts in a single application (e.g., home gardens) from those in which biosolids may be applied in large quantities ovcr wide areas (e.g., agricultural and reclamation use) or bulk material. Many of the requirements in the regulation apply to the "person who prepares" biosolids--referring to the person or entity that effectively controls the quality of the biosolids or the material derived therefrom that is ultimately either used or disposed. For esamplc, in situations where a treatment works generates biosolids that are blended with othcr

Forste

96

substences, the person blending is the one who prepares the biosolids since he controls the quality of the material that is ultimately used or disposed. The final rule requires that any person who generates biosolids or derives a material from biosolids, any person who applies biosolids to the land, any ownedoperator of a surface disposal site, and any person who fires biosolids in an incherator must maintain certain records. It also establishes reporting requirements for Class I biosolids management facilities, publicly owned treatment works (POTWs) with a design flow rate 2 1 million gallons per day, and POTWs that Serve 10,000 people or more. The "surface dqmd"subpart (C) in thefinal regulations combines the originally proposed subparts on monofills and surface disposal because differences between theae two did not merit separate treatment, since in either case the material is placed on the land for final disposal and may present essentially similar potential threats to public health and the environment. EPA also moved the subpart on removal credits fiom the final Part 503 Regulations to the general pretreatment regulations (40 CFR Part 403) since the Agency believed tha! these lists of pollutants eligible for removal credit more logically belonged with the pretreatment requirements.

TABLE2-2

POLLUTANTS IN PART 503 ELIGIBLE FOR A REMOVAL CREDIT Uae or Diapoepl Prnctke

Unlined'

AldrinlDie1cb-h(Total)

2.7

-

Benzane

16'

140

3400

-

-oopyrene

15

100'

looc

-

looo

100

100'

l00C

-

Arsenic

-

Linedb 100'

-

~

cadmium

-

Chmmium

-

-

100'

-

copper

-

46'

looC

1400

1.2

2000

2000

-

Bis(2-ethylhexyl)ate

DDD, DDE, DDT (Total)

I

97

Regulatory Requirements

TABLE 2-2 (COW.)

Molybdenum

-

Nickel

-

-

100s

-

2.1

0.088

0.088

-

30

-

-

82

82

-

Mermry

N-N-

'

ylamine

Pentachlorophenol Phenol

I

100'

lOOC

40

40

I

Polychlorinated biphenyls

4.6

<50

<50

-

Selenium

-

4.8

4.8

4.8

Toxaphene

10

26'

26'

Trichloroethylene

1O C

9500

100

-

-

4500

4500

4500

zinc

-

-

Key: LA land application, SD surface disposal I - incineration 'Sewage sludge unit without a liner and leachate collection system Sewage sludge unit with a liner and leachate collectionsystem

' Value expresd in grams per kilogram - dry weight basis.

Table 2-2 identifies pollutants by use or dqosal practice and a concentrationfor each pollutant which was determined by the EPA not to pose an unreasonable risk to public health and the environment if the concentration for those pollutants in the bimlids are below the concentrationsgiven.[161 Pollutants were placed on this list for one of two reasons. First, available data at the time the original list of pollutants

98

Forste

of concern for Part 503 was developed indicated that the existing concentrations of pollutants in biosolids did not exceed those concentrations contained on the list in Table 2-2. EPA also determined that at those concentrations the pollutants do not pose a threat to public health and the environment at the highest levels detected. Secondly, the pollutant was placed on the GI1 list shown in Table 2-2 if, after determining a risk level, EPA decided not to regulate this pollutant in the final Part 503 Regulations. The concentration specified for those pollutants to be eligible for removal credits are the concentrations developed during the risk assessment. These pollutants are designated with an asterisk on the list in Table 2-2. Based upon either low concentration or low risk as described above, a removal credit is available for those pollutants with respect to the use/disposal of biosolids if the concentration of the pollutant in the biosolids is less than or equal to the concentration shown in Table 2-2 and if the treatment works complies the applicable requirements in 40 CFR 403.7. A POTW applying for removal credits must provide proof that the pollutant concentrations in its biosolids do not exceed the pollutant concentrations shown in Table 2-2. If subsequent monitoring reveals pollutant concentrations above those levels or any more stringent level in the biosolids permit, the POTW is no longer eligible for removal credit authority for that pollutant. Removal credit eligibility is h t e d to those pollutants regulated specifically in Part 503 and to pollutants that the Agency determines do not threaten public health and the environment as specified concentrations. The general provisions of Part SO3 establish standards for biosolids applied to the land or disposed by surface disposal or incineration. The requirements for reducing organisms in biosolids that cause disease (pathogens) are contained in Subpart D of Part 503. Either Class A or Class B pathogen reduction requirements must be met when biosolids are applied to the land or placed on a surface disposal site. In addition, the regulations require reduction of vector attraction, that is, control of those characteristics of biosolids that attract disease-spreading agents (e.g., flies or rats) when applied to the land or placed on a surface disposal site. There are no pathogen or vector atlraction reduction requirements for biosolids fired in an incinerator which achieves such reduction during the incineration process. For land application, the pollutant limits and management practices protect public health and the environment from reasonably anticipated adverse affects of arsenic, cadmum, chromium, copper, lead, mercuiy, molybdenum, nickel, selenium and zinc in the biosolids. EPAevaluated the risks associated with organic pollutants and other inorganic pollutants in biosolids and concluded that numerical pollutant limits for these are not required for the reasons described in this chapter. For surface disposal, the pollutant limits and management practices are established to protect public health and the environment from the reasonably anticipated adverse affects of arsenic, chromium and nickel in the biosolids. For incineration, these limits and practices are based on arsenic, beryllium, cadmium,

Regulatory Requirements

99

chromium, lead, mercury and nickel in the biosolids. In addition, EPA established an operational standard for biosolids incinerators that limit the total hydrocarbons (THC) in the exit gas from a biosolids incinerator stack. The recordkeeping requirements of the 503 Regulations specify who must develop and retain information, what information must be developed and the length of time such information must be kept. Section 405(f) of the CWA provides that permits issued to a publicly owned treatment works or any treatment works treating domestic sewage shall include conditions to implement the Part 503 Regulations unless such are included in permits issued under other federal or approved state programs. Thus,Part 503 requirements may be implemented through a CWA permit, a Subtitle C Solid Waste Disposal Act Permit, a Part C Safe Drinking Water Act Permit, a Marine Protection Research and Sanctuaiy Act Permit, a Clean Air Act Permit, a permit under an approved state program or an EPA-issued "Sludge Only" permit. However, it should be noted that the requirements in the Part 503 must be met even in the absence of a permit, i.e., the Part 503 is self-implementing. Thus a responsible person must become aware of the Part 503 standards, comply with them, perform appropriate monitoring and recordkeeping and, if applicable, report information to the permitting authority even when a permit is not issued. These standards are also directly enforceable against any person who uses or disposes of biosolids through any of the practices addressed in the final regulations. An enforcement action can be taken against a person who does not meet those requirements even in the absence of a peimit. Relevant dates for the biosolids program are shown in Table 2-3. Pollutants Selected for Regulation Numerical limits in the 503 are imposed for 10 metals rather than the 28 inorganic and organic pollutants contained in the proposed regulation. EPA's decision to select the final 10 metals for regulation was based on analysis of information provided on the proposed Rule and the data from the National Sewage Sludge Survey. The Clean Water Act language requires EPA to develop standards on the basis of available information on pollutant's toxicity, persistence, concentration, mobility or potential for exposure. These criteria led to the original proposal of 28 pollutants, however, the Agency deteimined for the final rule that some of these should not be regulated, either because they are not present in biosolids, or, if present, the potential for exposure and therefoi-ehealth or environmental risk is small. EPA concluded that the concentrations of a pollutant do not pose a public health or environmental risk if: the pollutant is banned or restricted or no longer manufactured or used in manufacturing a product 2. based on data from the NSSS the pollutant was not found in biosolids at significant frequencies of detection 1.

100

Forste

ABLE 2-3 RELEVANT DATES FOR THE BIOSOLIDS PROGRAM Publication of Part 503 at 58 FR 9248

February 19,1993

~~~

Publication of amendments to Sewage Sludge Permit Program regulations at 58 FR 9404

February 19, 1993

Effective date of Part 503

March 22,1993

Requirements for monitoring and recordkeeping under Part 503 become effective (except for THC)

July 20, 1993

Compliance date for Part 503 requirements other than monitoring, recordkeeping and reporting (where construction is not required)

February 19,1994

Rcquireinents for reporting under Part 503 become effective (except for THC)

February 19,1994

Limited permit application information due from sludge-only facilities (not needing site-specific limits)

February 19,1994

Date for closure of active sewage sludge units 1) located within GO meters of a fault that have displacement in Holocene time (unless authorized by the permitting authority); 2) located in a wetland (unless authorized under an NPDES permit); or 3) located in an unstable area

March 22, 1994

Compliance date for Part 503 requirements other than monitoring, recordkeeping and reporting (where construction is required)

February 19,1994

Date when active sewage sludge unit owners/operators must submit closure plans

180 days prior to the date the unit closes

Permit application information duc from facilities with NPDES pcrniits (not needing site-specific limits)

At the time of the next NPDES permit renewal

Permit application information due from facilities who commence operation after 2/19/93

180 days prior to the date proposed for commencing operation

Regulatory Requirements

3.

101

the risk assessment for the pollutant shows no reasonably anticipated adverse affects at the 99th percentile concentration found in biosolids as per the NSSS.

The NSSS was specifically designed to resolve data deficiencies which EPA acknowledged existed in the information used to develop the proposed rule. Based on the results of the survey and the status with respect to the production of a pollutant, the above three criteria provide a reasonable certainty that numerical pollutant limits are unnecessary. For example, if the pollutant is banned from production, it is highly unlikely that it will be present in biosolids and there is no consequent need to establish numerical limits. For a number of the banned and no longer manufactured pesticides that EPA had proposed to regulate, examination of the NSSS data confirmed that these pesticides were not present in today's biosolids. Fourteen pollutants meet the criteria detailed above and are no longer regulated in the Part 503 Rule: aldriddieldnn benzene benzo(a)pyrene bis(2-ethylhexy1)phthalate chlordane DDT (and its derivatives DDD and DDE) dimethyl nitrosamine heptachlor hexachlorobenzene hexachlorobutadiene lindane polychlorinated biphenyls toxaphene trichloroethylene Details on the analysis used in deciding not to regulate are contained in Appendix A of the Technical Support Document for Land Application. m e g a t e Risk Assessment EPA's aggregate risk assessment evaluated the "baseline" public health impacts of biosolids use and disposal without the 503 Regulations, assessed the impacts after implementation and established the difference between these two estimates as the public health "benefit"of the final rule. For the baseline estimates, EPA used a sample of plants from the NSSS and used additional information on the plants from the questionnaire portion of the NSSS to develop a profile of use and disposal practices across the country. The Agency also

102

Forste

used the data to assign pollutant concentrations under baseline conditions to treatment plants in different categories. Data describing cancer potency or other effects were used to develop measures of individual risk. The results provided an aggregate determination of the likely number of individuals experiencing each effect expected per year in the affected populations as a result of biosolids. Details on the aggregate risk assessment are contained in the Technical Support Document, "Human Health k s k Assessment for the Use and Disposal of Sewage Sludge: Benefits of Regulation"[l7]. For land application, EPA used average values for the pollution concentrations from facilities practicing land application as deteimined by the NSSS. The mathematical models contained in 503 were used to predict the transport of pollutants to groundwater and ambient air for the baseline assessments. Additional modelling was used to predict the uptake of pollutants from treated soil to agricultural crops and animal tissues and to estimate contamination of surface water. Using average population densities, the Agency established current human exposure to and risk from each pollutant in biosolids through each relevant exposure pathway. The final step in the assessment of current risks was to extrapolate results to the national level based on the estimated number of facilities practicing land application and the quantity of biosolids applied. The baseline assessment showcd a pre-503 risk of less than one possible cancer case, 1000 possible cases of individuals who would exceed a threshold lead concentration and 500 possible individuals experiencing lead related effects. In no CRSC did the average exposure to the other inorganic pollutants for which limits are included in the land application exceed the reference dose for the pollutant during the baseline assessment. The aggregate risk assessment & implementation of the final part 503 land application regulations are: <1 possible cancer case,
Regulatory Requirements

103

domestic sewage or any device or system used in the storage, treatment, recycling and reclamation of municipal or domestic sewage, including land dedicated to the disposal of biosolids that are located within the confines of the facility with a design flow of 1.O MGD or more or required to have an approved pretreatment program under 40 CFR Part 503, must control stormwater. Specifically not included are farm lands, domestic gardens or land used for a beneficial use of biosolids which are not physically located within the facility. For this category, EPA viewed fac es such as large treatment works that may experience spills and bubbleovers (e.g., on-site composting and chemical storage) as suitable candidates for stormwater permits. Such activities are considered to be more akin to industrial activity in scope and size and hence should be required to obtain an NPDES permit for stormwater discharges. The 503 Regulations do not establish requirements for the specific treatment of biosolids except for properties (other than chemical composition) that may pose a threat to public health and the environment. Therefore, requirements to reduce pathogens and vector attraction are provided while processes used to prepare biosolids for final use or disposal (e.g., composting) are not subject to the 503 standards. The pathogen requirements of Part 503 are not risk-based but are pei-fomiance standards based on the demonstrated ability of treatment processes to reduce pathogens. Such requirements include raising the temperature of the biosolids and maintaining that temperature for a specific period of time. While EPA encourages the beneficial use of biosolids through such practices as land application, the specific use or disposal practice selected by a municipality or authority responsible for biosolids remains a local determination and responsibility. Therefore, the Part 503 does not establish requirements for the selection of a use or disposal practice. The Part 503 Regulations do not establish requirements for use or disposal of biosolids from an industrial facility which treats industrial wastewater because those materials are not biosolids which are generated during the treatment of donicslic sewage in a treatment works. If industrial wastewater solids are disposed on the land, the requirements of 40 CFR Pait 257 must be met. The 1987 Water Quality Act included under Section 405(d) industrial manufacturing and private processing es that treat domestic sewage combined with industrial wastewater. During the development of the Part 503, insufficient time was available to develop standards for use or disposal ofwastewater solids generated at such facilities. In addition, EPA did not have sufficient information on the number of such facilities, the amount of solids generated at them and the practices through which the solids are used or disposed in order to evaluate the impact of the Part 503 numerical standards. The Agency also questioned whether the models and data used to develop the numerical limits in 503 are appropriate for industrial wastewater solids which also have a domestic scwage component. For these reasons, the Part 503 does not establish requirements for the use or disposal of biosolids generated at an industrial facility during the treatment of industrial wastewater combined with domestic sewage. Such materials may be

104

Forste

considered in future revisions to the Part 503. It should be noted however that the Part 503 Regulations do apply to biosolids generated at an industrial facility during the treatment of only domestic sewage, that is, without combining the domestic sewage with industrial wastewater. Thus all industrial wastewater treatment facilities that treat domestic sewage, whether generated on site or off site, are considered Treatment Works Treating Domestic Sewage and may be required to apply for a permit under 40 CFR 122.21. The Part 503 Regulations do not establish requirements for wastewater solids determined to be hazardous; these materials must be used or disposed under the ,applicable requirements of 40 CFR Parts 260 through 268. Biosolids having a concentration of polychlorinated biphenols 3 50 mgkg of total solids (dry weight) must be used or disposed in accordance with the requirements of 40 CFR Part 76 I , not the Part 503 requirements. Ash from incinerated biosolids also are not subject to the Part 503 but must be used or disposed in accordance with the appropriate requirement (e.g.,40 CFR Part 257 when disposed on the land). Wastewater grit and screenings are not subject to the Part 503 Regulations as they have completely different characteristics than biosolids and must be disposed in accordance with appropriaterequirements (e.g., 40 CFR Part 257 when disposed on the land). Solids generated from the processes which produce drinking water also are not subject to Part 503 since they are not generated during the treatment of domestic sewage in a treatment works. Since the characteristics of domestic septage and the characteristics ofcommercial septage (e.g., grease from a grease trap at a restaurant) and industrial septage (e.g., from a septic tank or similar treatment works that receives industrial wastewater) are different, the Part 503 requirements for domestic septage do not apply to commercial or industrial septage and these are excluded from the Part 503 Regulations. Requirements for a Person Who Premres Biosolids When biosolids are prepared to be applied to the land, placed in a surface disposal site or incinerated, the person who performs such preparation must meet the applicable requirements in the Part 503. This preparer could be the person who generates biosolids during the treatment of domestic wastewater or a person who derives a material derived fiom biosolids. The latter would include, for example, a person that blends biosolids with some other material or a private contractor who receives biosolids from a treatment works and then blends the biosolids with some other material (e.g., a bulking agent). When biosolids are part of any material, the person "derived" the material fiom biosolids and similarly any time the quality of the biosolids is changed, a "material" is derived from those biosolids. Although not explicitly defincd in the 503 Regulations, biosolids which meet a Class A pathogen reduction requirement, an appropriate vector attraction reduction requirement and contain metals concentrations below those in 503.13(b)(3) (pollutant concentration limits) are not subject to any further regulatory requirements of 503

Regulatory Requirements

105

beyond monitoring and reporting to establish conformance with the three requirements specified above. Thus, a person who blends such biosolids is not subject to any further regulatoly requirements, including monitoring and reporting. This distinction is especially important for products (e.g., heat-dried biosolids) which are often sold to a dealer who then blends the biosolids with some other material for sale to the consumer. Pollutant Limits Based on numerous comments on both the 1989 proposed and the 1990 NSSS, EPA concurred with the view that protection of public health and the environment allowed for the development of standards for biosolids that receives minimal regulation as a result of meeting certain quality requirements. Such materials may either meet Class A pathogen reduction requirements and be subject to no fi.uther federal regulation, or meet Class B pathogen reduction requirements and be required to conform to the general requirements and management practices for land application. The numerical s t a n h d s for biosolids not subject to cumulative pollutant loading limits result from the loading rates for inorganic pollutants (i.e. trace or "heavy" metals) through the various exposure assessment pathways described above. EI'A derived the pollutant concentrations in such materials from calculations using the already established cumulative pollutant loadings and applying certain conservative assumptions to back calculate to a pollutant concentration. Maximum metal concentrationsfor such biosolids are shown in Table 2-4 (Table 3 of 6 503.13). Since pollutant concentrations are based on the cumulative pollutant loading rates and include the same conservative safety factors, they provide the same degree of protection to human health and the environment as is provided by the cumulative pollutant loading rates. For the final rule, the cumulative pollutant loading rate was converted to an annual pollutant loading rate by assuming that the entire load for the pollutant was applied in one year, which EPA believes is a conservative assumption and unlikely to occur. Annual pollutant loading rates were then converted to pollutant concentrations using the following equation:

C'

Where:

APLR AWSAR

xo.001

.......................

C = pollutant concentration in m g k g (dry weight) APLR = annual pollutant loading rate in kg/hectare/365 day period (diy weight) AWSAR = annual whole sludge application rate in metric tonshectare/ 365 day period (dry weight) 0.00 1 = conversion factor

106

Forste

EPA assumed that the AWSAR used in this equation is 10 dry metric tonshectare/year for 100 consecutive years. EPA believes that a pollutant concentration derived fiom h s equation is conservative because it is unlikely that any one site will receive 10 metric tons of biosoliddhectarehjear for 100 consecutive years. It is also unlikely that nutrient-based (agronomic) application rates for agricultural land will require such annual application rates for 100 consecutive years.

TABLE 2-4 POLLUTANT CONCENTRATION (F'C) BIOSOLIDS*

%Biosolidsmeeting these pollutant concentration lintits are not required to be tracked with respect to cuinirlative loadings on sites where they are applied to land. If biosolids meet the pollutant concentrations calculated using the above equation, the more stringent Class A pathogen reduction requirements and one of eight vector attraction requirements in the final 503 Rule, the general requirements and management practices for land application do not apply if such biosolids are applied to the land. However, minimum frequency for monitoring, recordkeeping and reporting however do apply since the EPA needs to have information to determine that

Regulatory Requirements

107

the 503 quality requirements are being met. This reduction of general requirements and management practices applies both to bulk biosolids applied to the land and to biwlids sold or given away in bag or similar container. It also applies to a material derived ffom biosolids if that material is derived fiom a biosolids that does not meet the three q d t y requirements. Land AvDlication The 503 Regulations recognize two broad categories of biosolids applied to the land and establish requirementsfor each. For both categories,biosolids must meet metal ceiling concentrations, otherwisethey cannot be applied to the land. The first broad categq is bulk biosolids applied to the land, that is, biosolids that are not sold or given away in a bag or other container. These biosolids must meet either pollutant concentration limits or the amount of the pollutant applied to the land in the bulk biosolids must not exceed a cumulative pollutant loading rate. The ceiling limits, cumulative pollutant loading rates and annual pollutant loading rates are shown in Table 2-5 (Metal Concentrations and Allowable Loadmg Rates). In addition, pathogen and vector attraction reduction requirements must be met for bulk biosolids applied to the land and general requirements and manageme$ practices may have to be met depending on the degree of treatment for pathogen reduction of the bulk biosolids. For the second broad categq--biilida sold or given away in a bag or other container for application to the land--oneof two pollutant limits also must be met. The biosolids must meet the pollutant concentrations in Table 2-4 for bulk biosolids or the amount of a pollutant applied to the land annually must not be exceeded in an annualpollutant loading rate (Table 2-5). Annual pollutant loading rates are used to calculate the application rate specified on a label on the bag or other container. This rate cannot be exceeded and is based on EPA's estimation of 20 annual applications for biwlids combined with the cumulative pollutant loading limits (the CPLR + 20). The annual rate is for the whole biosolids, not just for a single pollutant, although one such pollutant will govern the annual rate. In addition, these biosolids must meet the highest quality (Class A) pathogen reduction requirements and a vector attraction reduction requirement. Depending on quality, such biosolids may also be subject to ' general requirements and a management practice. The general requiranentSand management practices for biosolids are contained in Figures 2-2 and 2-3. The land application requirements apply to any person who prepares biwlids that are applied to the land, any person who applies biosolids to the land, to the biosolids applied to the land, and to the land on which biosolids are applied.

Pollutant Ceiling Concentretion* Pollutant

(ww (Pc)

Cumulative Pollutant Loading Rates (CpLR)

Annual Pollutant Loading Rates WLR)

@@la)

Wac)

(kglhalyr)

(Ib/ar/yr)

Nickel

420

420

375

21

19

Selenium

100

100

89

5

4.5

7,500

2,800

2,500

140

125

Zinc ~

~

'Asper§ 503.13, Table I , 2 and 4.

' Wastewaterbiosolicis or biosolids-derivedproductsthat are to be land-applied, used for reclamation purposes, or distributed and marketed must meet these criteria at a minimum. Molybdenum limits (other than ceiling concenb-ations)have been remmedfiom the 503 Regulationspending re-evaluation of scientific data and re-proposal.

'

109

Regulatory Requirements

For Bioaolidn Meeting 503.13@)3,50332(n) and one of 50333@)(1) through @)@) None (unless set by EPA or State permittmg authority on a CBsebyCBsebasis for bulk biosolids to p t e c t public health and the environment).

For PC and CPLR B l m U Theprepma. must not@ and provide informationnecessBty to comply with the Part 503 land applicationrequirementstotheperson who applies bulk biosolids to the land The prepare who provides biosolids to awther persoa who further prepares the biosolids for applicationto the land must provide this person with notification and informationnecessary to comply with the Part SO3 land application reqUirlXllentS.

Theprepara must provide written notification of the total nitrogenCOncenMion (as N on a dryweight basis) in bulk biosolids to the applier of the biosolids to agricultural land, forests, public oontact sites, or reclamation sites. The upplier of biosolids must obtain infomtion 7to comply with the Part SO3 land applicatronrequirementS, apply biosolids to the land in BEcordance with the Part 503 land application requirements, and provide notice and necess~lyinformationto the owner a leaseholder of the land on ~ c biosolids h are applied. out of state use

Thepreparermustprovidewittennotification (pior to the initial applicationof the bulk biosolids by the applier) to the permitling authority in the State where biosolids are pFoposedto be land applied when bulk biosolids aro gonerated inoae state andtranaferredto ~ 0 t State h ~for applicationto the land The notificationmust include: the location (either street address or latitude ' and longitude) of each land applicationsite;

+ ~+ + 0

theapproltimatetimeperiodthebulk biosolids will be applied to the site; the name, addaess,telephone number, and National Pollutant Discharge Elimination System (NPDES) number for both the preparer and the applier of the bulk biosolids; and a&iitionalinformationorpermitsinboth Statea,.ifrequired by the permitting -ty.

Additional Requirement. for CPLR Biosolidr The applier must not@ the permittingauthority in the State where bulk biosolids are to be applied prior to the initial applicationof the biosolids. This is a onetime notice requirementfor each land applicationsite each time there is a new applier. The notice must include: the location (either street address or latitude and longitude)of the land applicationsite; and 0 the name, addreaq telephone number, and NPDES permit number ( ifapproPriate)of the person who will apply the bulk biosolids. The applier must obtain records (if available) h m the previous applier, landowner, or pennittingauthoritythatindicatatheamountof each CPLR pollutant in biosolids thathave been applied to the site since July 20, 1993. In addition: when these records are available,the upplier mustusethisinfmationtodeterminethe additionalamount of each pollutant that can be applied to the site in Bcoordatlcewith the CPLRS in Table 2-5; the upplier must keep the previous records and also record the additionalamount of each pollutanthe or she b applying to the dte; and when records ofpast known CPLR applicationssince July 20,1993,arenot available,biosolids subjectto CPLRs cannot be applied to that site. Ifbiosolids meeting CPLRs have not been applied to the site in excess of the l i t since July 20,1993,the CPLR limit for each pollutant in Table 2-5 will determine the maximum amount of each pollutant that can be applied in biosolids

+

+

+

E

+ +

all applicablemanagement practices are followed, and the applier keeps a record of the amount of each pollutant in biosolids applied to any given site. The applier must not apply additional biosolid under the cumulativepollutant loading c4mcept tc a site whem any of the CPLRs have beenreached

* The preparer is either the person who g p the biosolids or the person who derivesa matend &ombiosolids.

FIG. 2-2 PART 503 LAND APPLICATION GENERAL REQUIREMENTS, Source: U.S. €PA. A Plain English Guide to The €PA Part 503 Biosolids Rule. EPA/8324?-93/003. September 1994

110

Forst e

For Biosolids Meeting 503.13(b)3,503.32(a) and one of 503.33(b)(l) through (b)(8) None (unless established by EPA or the State perniitting authority on a ease-by-case basis for bulk biosolids to protect public health and the environment). For PC (Pollutant Concentration) and CPLR (Cumulative Pollutant Loading Rate) Biosolids These types of biosolids cannot be applied to flooded, frozen, or snow-covered agricultural land, forests, public contact sites, or reclamation sites in such a way that tlic biosolids enter a wetland or other waters of the United States (as defined in 40 CFR Part 122.2, which generally includes tidal waters, interstate and intrastate watcrs, tributaries, tlie territorial sea, and wetlands adjaccnt to tlicse waters), cxccpt as providcd in a periiiit issued pursuant to Section 402 (NPDES perinit) or Scction 404 (Dredge and Fill Pcrniit) of the Clean Water Act, as anieiidcd. These types of biosolids cannot bc applicd to agricultural land, forests, or reclaination sitcs that arc 10 nicters or less froin U S . waters, unless otherwise specificd by tlie permitting authority. If applied to agricultural lands, forests, or public contact sites, these typcs of biosolids must be applied at a rate that is equal to or less than the agronomic rate for nitrogen for the crop to be grown. Biosolids applied to reclamation sites may exceed the agronomic rate for nitrogen as specified by tlie permitting authority. These types of biosolids must not harm or contribute to the harm of a threatened or ciidangered species or rcsult in the dcstruction or adverse modification of the spccics' critical habitat when applied to the lant. Thrcatencd or endangered species and their critical habitats are listed in Section 4 of the Endangered Species Act. Critical habitat is defined as any placc where a Ilircatcncd or ciidangered species lives and grows during any stage of its life cycle. Aiiy dircct or indirect action (or the rcsult of any dircct or indirect action) in a critical habitat that diminishes the likelihood of survival and recovery of a listed species is considered destructionor adverse modification of a critical habitat. For APLR (Annual Pollutant Loading Rate) Biosolids A label must be affixed to the bag or other container, or an infonnation sheet must be provided to tlie person who receives APLR biosolids in other containers. At a minimum,

the label or information shcct must contain tlie following information: the name and address of the person who prcpared the biosolids for sale or giveaway in a bag or othcr contaiiicr; a statement that prohibits application of the biosolids to the land except in accordance with tlie iiistructions on the label or information sheet; an AWSAR (see Figure 2-6) for tlic biosolids that do not cause tlic APLRs to be exceeded; and + the nitrogen content.

+ + +

FIG. 2-3 PART 503 LAND APPLICATION MANAGEMENT PRACTICE REQUIREMENTS Source: US.EPA. A Plairi English Guide to The EPA Part 503 BiosolidsRule. EPA/832/R-93/003.Septenr ber I994

Regula tory Requirements

111

For land application, there are instances in which not all the requirements must be met to comply with the standards: If the biosolids or material derived therefi-om meets certain pollutant limits (see Table 2-4) and operational standards for pathogen and vector attraction reduction as well as fiequency of monitoring, recordkeeping and reporting for land application, the general requirements and management practices do not apply if these three quality requirements are met. For a bulk material derived from biosolids, the land application subpart requirements do not apply when the derived material meets the pollutant concentrations in 503.13(b)(3), the Class A pathogen reduction requirements in 503.32(a) and one of the vector attraction reduction requirements in 503.33(b)( 1) through 503.33(b)(8). In this case, the biosolids used to produce the bulk matcrial already meets the three quality requirements.

In the fust instances cited above, an EPA Regional Administrator or a state dircctor in a state with an approved management program may require that any or all of the general requirements or management practices be met on a case-by-case basis, even when the biosolids or bulk material derived therefrom meets the three quality requirements, if found that these are needed to protect public health and the environment from any reasonably anticipated adverse effect of a pollutant in the biosolids. The final Part 503 Regulations do not authorize the EPA Regional Administrator or the state dircctor to impose the general requirements and management practices on a bulk material derived frJm a biosolids that already meets the three quality requirements. Because the 503 requirements do not apply to biosolids used to make that material once it has met the identified quality requirements, no records have to be kept on who receives those biosolids or what happens to them after the thrce quality requirements are met. For biosolids sold or given away in a bag or other container, the general requirements and management practices do not apply when the biosolids or material derived therefrom meet the three criteria dcscribed above. However, monitoring, recordkeeping and reporting requirements do apply to such biosolids derived material. Ceiling concentrations are included in the Part 503 to address EPA's concein about the potential impact of a "dirty" biosolids on public health and the environment. They are the less stringent of two values: the cumulative pollutant loading rate from the land application exposure assessment and assumed 100 year site life at an assumed annual application rate of 10 metric tonshectare the 99th percentile concentration fiom the National Sewage Sludge Suvey, whichever is less stringent. EPA concluded that when pollutant concentrations in biosolids do not exceed the ceiling concentrations, the potential for short term impacts on the environment from land application are greatly reduced. Two pollutant limits for bulk biosolids applied to the land are contained in the Part 503. The first limit consists of cumulative pollutant loading rates in 503.13(b)(2) (see

112

Forste

Table 2-5). The cumulative loading rate is the total amount of an inorganic pollutant that can be applied to an area of land. Cumulative pollutant loading rates must be met when bulk biosolids that do not meet the pollutant concentrations contained in 503.13@)(3). To comply with this requirement, the amount of each pollutant in the bulk biosolids must be known, records must be kept of the amount of each pollutant applied to each site. When the cumulative pollutant loading limit for any of the pollutants in Table 2-5 is reached for a site, no more bulk biosolids may be applied to that site. The ceiling concentrations and cumulative pollutant loading rates must be met when bulk biosolids are applied to agricultural land, forests, a public contact site or a reclamation site. Equation (2-1) can be used to estimate site life for bulk biosolids with a particular quality and for a certain Annual Whole Sludge Application Rate (AWSAR) when the cumulative pollutant loading rates are met. When either the quality of the bulk biosolids or the AWSAR changes, the site life for the land also changes. In order to use this equation, the concentrations of the regulated metals must be determined on a dry weight basis and the AWSAR determined-- usually based on a selected agronomic rate. The Annual Pollutant Loading Rate (APLR) may be calculated as follows: APLR = C x AWSAR x 0.001

....................

(2-2)

Where:

APLR =

Annual Pollutant Loading Rate for an inorganic pollutant in kilograms of pollutant per hectare per 365 day period C= measured pollutant concentration in milligrams of pollutant per kilogram of total solids AWSAR = Annual Whole Sludge Application Rate in metric tons per hectare per 365 day period 0.001 = a conversion factor The years an inorganic pollutant can be applied to the land is calculated by dividing the cumulative pollutant loading rate (CPLR) by the APLR calculated in accordance with equation (2-1). The lowest number of years calculated in this manner is the period that this bulk biosolids can be applied to the land without causing any of the cumulative pollutant loading rates to be exceeded, This calculation is used only to estimate the number of years biosolids can be applied to a site; when the pollutant concentrations change or when the AWSAR changes, this estimate will also change.

113

Regulatory Requirements

EXAMPLE: Given: Step 1 - Pollutant concentrations in bulk biosolids (dry weight basis): oncentration

-

Step 2 - Assume thc annual whole sludge application rate for the bulk sewage sludge is 10 MThd36.5 day period base on agronomic requirements. Step 3 - Calculate the annual pollutant loading rates for the pollutants using equation (2-2):

~~

Zinc

1201

12.00

114

Forste

CPLR (kgW

Pollutant

APLR

Years (CPLWAPLR)

OCghdyr)

Arsenic

41

0.10

410

Cadmium

39

0.07

557

chromium

3000

8.5

353

Copper

1500

7.4

202

300

3.50

85

Mercuiy

17

0.05

340

Molybdenum

18

0.09

200

420

0.42

1000

Lead

Nickel Selenium Zinc

I I

I 2800 I 100

I 12.0 I

0.05

I 233 I

2000

Step 5 - Determine the lowest number of years calculated in Step 4

For this example, the lowest number of years is 85 for lead. Bulk sewage sludge with the inorganic pollutant concentrations given in Step 1 of this procedure can be applied to the land at an AWSAR of 10 M T h a for 85 years. After that period, the cumulative pollutant loading rate for lead is exceeded. To develop the pollutant concentrations in Tabre 2-5, the cumulative loading rates were converted to annual pollutant loading rates and the calculated annual pollutant loading rates and an assumed AWSAR were used by EPA in Equation (2-1) to calculate a pollutant concentration (C = APLR f AWSAR x 0.001). The pollutant concentrations calculated using the equation (2- 1) were compared to the 99th percentile concentration values for these pollutants from the NSSS. Thus the pollutant concentration limits in Table 2-4 are either the concentration calculated using this equation or the 99th percentile concentration, whichever is more stringent. To convert the cumulative pollutant loading rates to annual pollutant loading rates, EPA assumed that the life of the sitc where biosolids are applied is 100 years and that the AWSAR is 10 metric tons/hectare/year based on an estimated agronomic rate.

Regulatory Requirements

115

Because the pollutant concentrations shown in Table 2-4 are based on conservative (100 year) site lives and typical AWSARs, the Agency concluded that records do not have to be kept of the amount of each inorganic pollutant in biosolids applied to a site. The pollutant concentrations are monthly average concentrations which may not be exceeded in bulk biosolids in order to be exempt from the recordkeeping requirement. EPA's use of the 99th percentile "cap" has been the subject of litigation. A unanimous three-judge panel of the U.S. Court of Appeals for the D.C. Circuit on November 15,1994 remanded to EPA the limits or caps on chromium and selenium. The Court rejected EPA's reliance on the National Sewage Sludge Survey for establishing the parameters by determining what limits could be met by 99% of the POTSs responding to the survey, stating that the 99th percentile caps are not related to risk. The 3,000 mgkg maximum loading for chromim and the 100 m a g cap for selenium were rejected because EPA had failed to meet the statutory burden of assessing the risk to public health and the environment. EPA will have to redetermine those caps as mandated by the Court. The Court also found EPA had failed to establish a rational relationship between the assumed application of all the covered pollutants, including those that are risk-based, in liquid biosolids and the actual usage of heat-dried biosolids. This part of the regulation was also remanded to EPA either to just@ its assumptions or to provide more tailored caps. Since no risk-based chromium limit can be established based on available field data (which show no adverse effects from chromium) it is likely that chromium will be deleted fi-om the 503 Regulations. It is also expected that the Agency will reevaluate and impose risk-based limits on selenium. For biosolids sold or given away in a bag or other container which do not meet the pollutant limits but are below the ceiling concentrations, annual pollutant loading rates must be met as shown in Table 2-5. These annual pollutant loading rates assume a cumulative pollutant loading rate to be reached in 20 years. That is, an assumed site life of 20 years. EPA concluded that 20 years is a conservative assumption because most biosolids sold or given away in a bag or other container will be applied to a lawn, a home garden or public contact site which EPA believes are unlikely to receive biosolids for longer than 20 years, particularly 20 consecutive years. These annual pollutant loading rates for "bag or other container" biosolids are also being challenged and may be deleted from Part 503 if that challenge is upheld. Domestic Septage A separate pollutant limit is contained in the 503 Regulations for domestic septage applied to agricultural land, forest or a reclamation site. This annual application rate is based on an estimate of the amount of nitrogen needed by the crop grown on the application site by the following equation:

116

Forste

.....................

N ' ANC x AAR x 8.34

.(2-3)

Where:

N= ANC = AAR = 8.34 =

pounds of nitrogen needed by the crop per acre per 365 day period available nitrogen concentration in milligrams per liter annual application rate in million gallons per acre per year a conversion factor

Solving for the annual application rate as follows: N

AAR'

................................

(2-4)

ANC x 8.34

The 503 Regulations require that the allowable annual application rate be calculated using the following equation, which is equation (2-4) with a value substituted for ANC x 8.34:

AAR'

N ................................ 0.0026

(2-5)

In tlus equation, 0.0026 is the product of an available nitrogen concentration and the conversion factor 8.34/1 million. This concentration was calculated using values for Total Kjeldahl Nitrogen (TKN) and ammonia-N in domestic septage from nine samples collected in Madison, Wisconsin. Tk available nitrogen concentrations were calculated assuming that: 5 0.

*a 00

00

domestic septage is applied to the site every year domestic septage is injected below the land surface none of the ammonia-N in the domestic septage is lost through volatilization most of the organic nitrogen (TKN minus ammonia) in domestic septage becomes available over a three year period (50,20 and 10 percent in years 1 , 2 and 3 respectively) "steady state" conditions with respect to available nitrogen are achieved in the third year after application of domestic septage

Regulatory Requirements

117

EPA allowed the annual application rate for domestic septage to be used only when it is applied to agricultural land, forests or a reclamation site because site restrictions are imposed on such sites. EPA assumed that the applier has control over the application site. Because of the difficulty of imposing site restrictions on a public contact site, lawn or home garden application of domestic septage to such site an annual application rate is prohibited under 503.

III. PATHOGEN AND VECTOR ATTRACTION REDUCTION Biosolids applied to agricultural land, forest, public contact sites or reclamation sites must meet either Class A or Class B pathogen reduction requirements. Class A biosolids are essentially pathogen free; Class B biosolids have reduced levels of pathogens, and are subject to use restrictions to prevent immediate and direct public contact.

Class A Pathopen Reduction Requirements The implicit objective of all Class A requirements is to reduce pathogen densities to below detectable limits which are: *a

00.

Salmonella sp. : Enteric viruses: Viable helminth ova:

Less than 3 MI" per 4 grams of total solids (TS), dry weight basis. Less than 1 PFU per 4 grams of TS, dry weight basis. Less than 1 per 4 grams of TS, dry weight basis.

The above-referenced requirements define microbiological "cleanliness" of Class A biosolids. Six alternatives are established for Class A. Any of these alternatives may be met for biosolids to be Class A with respect to pathogens. These alteinative requirements are described below and summarized in Table 2-6. Alternative 1: Thermallv Treated Biosolids [503.32(a)(3)] For pathogen reduction, microbiological requirements time-temperature requirements must be met. Four time-temperature regimes are specified based on percent of total solids and operating parameters. Alternative 2: Biosolids Treated bv Alkaline Materials [503.32(a)(4)] Elevating and maintaining a certain pH level for certain time (pH-time requirements) and air drying to certain level of dryness. No microbiological limits are established.

118

Forste

'ABLE 2-6 CLASS A PATHOGEN REDUCTION ALTERNATIVES

Process Requirements

Either F. colifomi or

Uilknown Processes

Either F. colifonn or

PFKP Processes

Either F. colifomi or Salmonella'

Process equivalent to PFRP

Notes:

Yes2

yes3

No

No

No

No

Salmonelia'

Either F. colifonn or ~aliiioiie~a'

No option to substitute operational monitoring for microbiological monitoring.

Seven PFRP processes listed in /Appendix B, Part 503.

Process equivalent to PFRP.

'

-the density of fecal colifonn must be less than 1,000 MPN per gram of total solids (dry weidit basis) oLthe density of Salmonella sp. bacteria must be less than 3 MPN per 4 grams of total solids (dry weight basis). hi addition to requiretnaitsin 1. above, the density of enteric viruses must be less than 1 PFU per 4 grams oftotal solids (dry weight basis). In addition to rquiremcnts in I. and 2. above, the density of viable helminth ova must be less than 1 per 4 grams of total solids (dry weight basis).

Alternative 3: Biosolids Treated in Other Processes [503.32(a)(5)] This alternative requires comprehensive monitoring of bacteria, enteric viruses and viable helminth ova. If a process has been proven to meet microbiological limits, monitoring operating paranicters can be used instead of microbiological monitoring. Alternative 4: Biosolids Treated in Unknown Processes [503.32(a)(6)] This Alternative is similar to Alternative 3, but with no option to substitute operational monitoring for microbiological monitoring.

Regulatory Requirements

119

Alternative 5: Use of PFRP [503.32(a)(7)] Biosolids must be treated in one of the seven (7) specified Processes to Further Reduce Pathogens (PFRP) listed in Appendix €3 of the 503 Regulations (see Table 2-7) & comply with microbiological limits. Alternative 6: Use of a Process Eauivalent to PFRP [503.32(a)(8)] Any process determined by the EPA's Pathogen Equivalency Committee (PEC) to be equivalent to a PFRP compliance with microbiological limits. Several processes have already (1994) been recommended as equivalent to Class A. In addition, it is required that: ( I ) Pathogen reduction be accomplished before or at the same time as vector attraction reduction except for alkaline treatment and drying. (2) Certain microbiological limits be monitored to detect regrowth at the time of use or disposal, or when biosolids are prepared for sale/give-away [503,32(a)(3)(811. Class B Pathogen Reduction Requirements

Class B requirements can be met by any of the three alternatives shown below. The implicit objective of all three altematives is to ensure that pathogenic bacteria and enteric v i ~ are w adequately reduced in density (as demonstrated by fecal colifoim density in Class B biosolids of less than 2 million MPN or CFU per gram of total solids on dry weight basis). Viable h e h t h ova are not necessarily reduced in Class B biosolids. Additionally, site restrictions are applied which restrict crop harvesting, animal grazing and public access for a certain period of time until environmental factors have further reduced pathogens. The Class B requirements apply to bulk biosolids that are applied to agricultural land, a forest, a public contact site, a reclamation site or a surface disposal site, unless, in the latter case, biosolids are covered at the end of each operating day. Alternative 1 : Monitoring of Fecal Colifonn [503.32@)(2)] This alternative requires that seven samples of treated biosolids be collected at the time of use or disposal and the geometric mean fecal colifom density of these samples be less than 2 million CFU or ME" per gram of total solids (dry weight basis). Alternative 2: Use of PSRP [503.32@)(3)] Biosolids must be treated in one of the five (5) Processes to Significantly Reduce Pathogens (PSRP) listed in Appendix B of the 503 Regulations (see Table 2-7). No microbiological monitoring is required.

Forste

120

Alternative 3: Use of Processes Eauivalent to PSRP [503.32@)(4)] Any process detennined by the EPA's Pathogen Equivalency Committee (PEC) to be equivalent to PSRP. Several processes have already (1 994) been recommended as equivalent. TABLE 2-7 CLASS A (PFRPI AND CLASS B (PSRP) OPTIONS [2] Processes to Further Reduce Pathogens (PFRPs)

Processes to Signiflcnntly Reduce Pathogens (PSRPs)

1. Composting Using either the within-vessel coniposting method or the static aerated pile coniposting method, the temperature of the biosolids is maintained at SSEC (131EF) or higher for 3 day;.

1. Aerobic Digestion Biosolids are agitated with air or oxygen to maintain aerobic conditions for a specific mean cell residence time (i.e., solids retention time) at a specific temperature. Values for the mean cell residence time and temperature shall be between 40 days at 20EC (68EF)and 60 days at l5EC (59EF).

Using the windrow composting method, the temperature of the biosolids is maintained at 55EC (131EF) or higher for I5 days or longer. During the period when the compost is maintained at 55EC (131EF) or higher, there shall be a minimum of five turnings of the windrow.

2. HeatDryIng

2. AlrDrying

Biosolids are dried by direct or indirect contact withhot gases to reduce the moisture content of the biosolids to 10% or lower. Either the temperature of the biosolids particles exceeds 80EC (176EF) or the wet bulb teniperature of the gas in contact with the biosolids as the biosolids leave the dryer exceeds 80EC (1 76EF).

Biosolids are dried on sand beds or on paved or unpaved basins. The biosolids dry for a minimum of 3 months. During 2 of the 3 months, the ambient average daily temperature is above OEC (32EF).

3. Heat Treatment Liquid biosolids are heated to a temperature of 180EC (356EF) or higher for 30 minutes.

3. Anaerobic Digestion Biosolids are treated in the absence of air for a specific niean cell residence time (i.e., solids retention time) at a specific temperature. Values for the mean cell residence time and temperature shall be between 15 days at 35EC to SSEC (131EF) and 60 days at 20EC (68EF).

Regulatory Requirements

121

TABLE 2-7 (CONT.) 4. Therniophllic Aerobic Digestion Liquid biosolids are agitated with air or oxygen

to maintainaerobic conditions and the mean cell residence time (i.e.., the solids retention time) of the biosolids is 10 days at 55EC (131EF) to 60EC (140EF).

5. Beta Ray Irradiation

Biosolids are irradiated with beta rays from an electron accelerator at dosages of at least 1.0 megarad at rmm temperature (ca. 2OEC [68EF]).

4. Cornposting Using either the within-vessel, static aerated pile, or windrow composting methods, the temperature of the biosolids is r a i d to 40EC (104EF) or higher and remains at 40EC (104EF) or higher for 5 days. For 4 hours during the 5-day period, the temperature in the compost pile exceeds 55EC (13 1EF).

5. Linie Stabllizatioii S d c i e n t lime is added to the biosolids to raise the pH ofthe biosolids to 12 afler 2 hours of contact.

6. G m i a Ray Irradlntlon Biosolids are irradiated with ganuna rays from certain isotopes, such as Cobalt 60 and Cesium 137, at dosages of at least 1.0 megarad at room temperature (ca. 20EC [68EF]).

7. Pasteurization The tempaature ofthe biosolids is maintained at 70EC (15PEF) or higher for 30 minutes or longer.

Site restrictions with respect to types of crops, animal grazing, turf growing and public access are also required for Class B biosolids [503.32(b)(5)], as shown in Figure 2-4. Class A pathogen reduction requirement for bulk biosolids applied to a lawn or home garden is based on the infeasibility of imposing site restrictions on such areas. Rather than imposing site restrictions which would be needed if the bulk biosolids met only Class B pathogen requirements, EPA has also imposed the Class A requirements for biosolids sold or given away in a bag or other container for land application.

122

Forste ~~

Food crops with harvested parts that touch the biosolids/soil mixture and are totally above the land surface shall not be harvested for 14 months after application of biosolids. Food crops with harvested parts below the surface of the land shall not be harvested for 20 months after application of biosolids when the biosolids remain on the land surface for four months or longer prior to incorporation into the soil. Food crops with harvested parts below the surface of the land shall not be harvested for 38 months after application of biosolids when the biosolids remain on the land surface for less than four months prior to incorporation into the soil. Food crops, feed crops and fiber crops shall not be harvested for 30 days after application of biosolids. Animals shall not be allowed to graze on the land for 30 days after application of biosolids. Turf grown on land where biosolids are applied shall not be harvested for one year after application of the biosolids when the harvested twf is placed on either land with a high potential for public exposure or a lawn, unless otherwise specified by the permitting authority. Public access to land with a high potential for public exposure shall be restricted for one year after application of biosolids. Public access to land with a low potential for public exposure shall be restricted for 30 days after application of biosolids. FIG. 2-4

CLASS B/SEPTAGE SITE RESTRICTIONS [2]

Domestic Septage Pathogen Reduction r503.32(c)] Class A or Class B biosolids requirements apply to domestic septage used on all types of land. No pathogen-related requirements apply to domestic septage placed on a surface disposal site. Pathogen reduction in domestic septage applied to agncultural land, a forest or a reclamation site can be met in one of two options:

Regulatory Requirements

c

123

all Class B site restrictions under 503.32(b)(5) must be met; or the pH of the septage must be raised to no less than 12 by alkali addition and maintained at this level for 30 minutes without adding more alkali, the site restrictions on crop harvesting must be met.

The same site restrictions that apply to Class B biosolids applied to the land are also imposed when domestic septage is applied, unless a pH requirement for domestic septage is met along with site restrictions concerning the harvesting of crops. These restrictions prohibit harvesting of crops, grazing of animals and public access to the site for a period of time. Restrictions on crop harvesting are part of this second alternative because EPA does not believe adequate pathogen reduction is achieved by pH adjustment to allow crops to be harvested immediately after applying domestic septage. Vector Attraction Reduction A sigrdicant route for pathogen transport is vector transmission. Vectors are living organisms capable of transmitting a pathogen from one organism to another mechanically (simply transporting the pathogen), or biologically (by having a role in the pathogen's life cycle). Insects, birds, flies, rodents are the most likely vectors. Current USEPA Regulations establish twelve options for vector attraction reduction (VAR). Compliance with the vector attraction reduction requirements must be demonstrated separately from compliance with the pathogen reduction requirements. Vector attraction reduction options are summarized in Table 2-8. When bulk biosolids are applied to agicultural land, forests, public contact sites or reclamation sites, one of ten VAR requirements must also be met. This vector attraction requirement is in addition to the pathogen requirements and both must be met. One of the first eight VAR options must be met when bulk biosolids are applied to a lawn or home garden. Injection below the land surface and incorporation into the soil cannot be used to achieve VAR in these cases since implementation would be dflicult ifnot impossible. Similarly, one of the first eight VAR requirements must be met for biosolids sold or given away in a bag or other container for the reasons just cited. For domestic septage, the pH elevation to 12 or higher for 30 minutes, injection below the surface of the land or incorporation into the soil after application will all meet the VAR requirements of 503. Management Practices When bulk biosolids are applied to agricultural land, forests, a public contact site or a reclamation site, the management practices set forth in Figures 2-2 and 2-3 are imposed. These management practices do not apply for bulk biosolids applied to a

124

Forste

lawn or home garden based on EPA's determination that large amounts of bulk biosolids will not be applied to lawns or home gardens for several applications and the pollutant limits already insure a high degree of protection. In addition, the management practices in many cases have no relevance in the lawn or home garden setting. Monitoring Monitoring frequency requirements in the 503 Regulations apply to both pollutant concentrations in biosolids and for compliance with pathogen reduction and vector attraction reduction requirements. Frequency of monitoring requirements also are included in the subparts on surface disposal and incineration in order to make the Part 503 self-implementing.

TABLE2-8 VEC' )R ATTRACTION REDUCl IN OPTIONS [3] What is Required?

Most Appropriate For:

Option 1 503.33 @XI)

At least 38% reduction in volatile solids during biosolids treatment

Biosolids processed by: Anaerobic biological treatment Aerobic biological treatinelit Chemical oxidation

Option 2 503.33 (bX2)

Less than 17% additional volatile solids loss during bench-scale anaerobic batch digestion for 40 additional days at 30EC to 37EC (86EF to 99EF)

Only for anaerobically digested biosolids that cannot meet the requirements of Option 1

Option 3 503.33 (bx3 1

Less than 15% additional volatile solids reduction during bench-scale aerobic batch digestion for 30 additional days at 20EC (68EF)

Only for aerobically digested biosolids with 2% or less solids that cannot meet the requirements of Option 1 e.g., biosolids treated in extended aeration plants

Option 4 503.33 (bX4)

SOUR* at 2OEC (68EF) is 1.5 nig oxygenlhrlg total solids

Biosolids froni aerobic processes (should not be used for composted biosolids)

Option 5 503.33 (bX5)

Aerobic treatinent of the biosolids for at least 14 days at over 40EC (104EF) with an average temperature of over 45EC (1 13EF)

Composted biosolids (Options 3 and 4 are likely to be easier to meet for biosolids froni other aerobic processes)

Requirement

--

125

Regulatory Requirements

TABLE 2-8

(cow.)

Option6 503.33 @X6)

AUcali-treatedbiosolids (alkalies include lime,fly as4 kilUdUStandWo0ddl)

Addition of sufficient alkali to raisethepHtoatleast12at 25°C(770F)andmaht&a pH 2 12 for2 hours and apH 2 11.5 for22 more hours

Biosolids treated by an aerobic or anaerobic process (i.e., biosolids that do not contain unstabilized solids generatedinprimary

option7 503.33

@X7)

wastewatertreatment)

Option 8

503.33

Biosolids that Oontain unstabilii solids generated

Percent solids 290% prior to mixhgwith dher malerials

inprimaryWastewater mdment (e.g., any heatdried

@X8)

Option9 503.33 ex91

Biosolids are injected into soil so that no significant amount of biosolids are present on the landsurface 1 hour after hj4m except Class A biosolids which must be injectedwithin8hoursafter

Biosolids applied to the land or placed on a surface disposal

option 10

Biosolids inoapotatedintothe soil withiu 6 hours after application to land or placementonaBurfacedisposal site, except Class A biosolids which must be applied to or placedonthelandsurfa

Biosolids applied to the land

503.33@X10)

site. Domesticseptage applied to agricultural land, a forest, or a reclamation site, or placedona surface disposal site

sitc. D o m e s t i c w e applialto agriarltural land, forest, or a reclamation site, or placedona d i d i s p o d Site

within8hoursafterthe DathORCXI radudOn DlWXSS

option 11 503.33 @X11)

Biosolids placed on a surface disposal sitemustbe m v d with soil or other material at the end of each opffatingday

option 12

pH of domestic septage must be d t o 2 12 at 25°C ( 7 7 3 by alkali addition and . . mmtmedat 212 for30 minutea without adding mom alkali Oxygen Uptake Rate

503.3 @XI21

SOUR:S@c

Biosolids a domestic septage placed on a surface disposal site

Domestic septage appliedto

reC~amation site or p l d on a surface dispapal site I

126

Forste

The frequency of monitoring is based on the amount of biosolids used or disposed annually. For biosolids sold or given away in a bag or other container, the land application monitoring frequency is based on the amount of biosolids received by the preparer. The frequency of monitoring specified for the lowest range (i.e., > 0 but < 290 metric tons per year) indicates that when biosolids are not used or disposed during a 365 day period no monitoring for the requirements of Part 503 is required. Biosolids must be monitored only when an amount is used or disposed in a 365 day period (Table 2-9).

I

TABLE 2-9 MONITORING FRE 2UENCY Amount of Biosolids* (Metric Tons pcr 365 Day Period - Dry Wcight Basis)

Estimated Size of Facility (MGD)***

Frequcncy

>O to ~ 2 9 (320)** 0

B0.9 to 4 . 5

,290 to <1,500 (1,653)** ~~

31,500 to <15,000 (16,530)**

I 215,000 (>16,530)**

~

24.5 to <45

L45

once per quarter

I once per 60 days I I oncepermonth I

*

Either the aitioutit of bulk biosolids applied to flie latid or the amourit of biosolids received by a persoti who prepares biosolids that is sold or given away in a bag or other containerfor application to the latid. ** Values in parentheses are dry tons. *** These valrres are not listed in the 503 Regulatiotis but are rule-ofthurrrb values based oti one diy loti of biosolids beiiig produced for eveiy iirillioti galloiis per day &fGD) of wastewaterprocessed.

Under the 503 Regulations, the peimitting authority can reduce the fiequency of monitoring for pollutant concentrations and pathogen requirements after two years of monitoring at the fiequency specified in Part 503. However, monitoring fiequency must be at least once per year when biosolids are applied to the land. Reducing the fiequency of monitoring for pathogen densities addresses enteric viruses and viable helminth ova in biosolids. As part of those requirements, the biosolids must be analyzed for viruses and ova each time the biosolids are monitored. After those two organisms are found in the influent to the pathogen treatment process and after tlie

Regulatory Requirements

127

required reduction for those organisms is demomtrated through the treatment process, the biosolids do not have to be monitored for enteric viruses and viable helminth ova when values for the process operating parameters are consistent with the documented values for those parameters. The potential reduction in frequency of monitoring applies only to pathogen density reduction requirements for enteric viruSes and viable h e h t h ova and cannot be reduced for the other pathogen density requirements. Every container of domestic septage applied to agricultural land, forests or reclamation sites must be monitored to establish compliance with the pH adjustment requirement when that requirement is met. Each container (e.g., each tanldtiuck load) must be monitored because there is no way to ensure that the pH requirement will be met if only a certain number of containers are monitored. Recordkeering The final Part 503 Regulations contain recordkeeping requirements for bulk biosolids and bulk material or biosolids sold or given away in a bag or other container regardless of whether such biosolids meet the requirements which would exempt them from the cumulative pollutant loading limits contained in Table 2-5. Recordkeeping requirements also apply to domestic septage applicd to agricultural land, forests or reclamation sites. Recordkeeping requirements specify the information that must be developed, the person who must develop and retain the infoimation and the period h a t this information must be retained. The infoimation that must be developed depcnds on which pollutant limits are met and which pathogen and vector attraction reduction requirements are met; the infomation is needed to show that the requirements i n Subpart B are met. For biosolids that do not meet the pollutant concentrations in 503.13(b)(3), the more stringent Class A pathogen requirements and one of the vcctor attractionrequucments in 503.33(b)(I ) through (8), the person who prepares the biosolids must develop certain information (e.g., pollutant concentrations in the biosolids) and the applier must develop other information (e.g., the record of the amount of each pollutant applied to the land). These persons may be the treatnicnt works, an agent of the treatment works, a private contractor or some other person For example, when the bulk biosolids meet the pollutant concentrations and the less stringent Class B pathogen requirements, the person who applies the biosolids to thc land must develop infoimation concerning how the restrictions on the application site are met and keep those records. Depending on which pollutant limits are met, infoimation must usually be retained for five years. However, when cumulative pollutant loading rates are met, records for certain information have to be retained indcfinitely in order to know how much of an inorganic pollutant has bccn applied tcj the land.

128

Forste

Future EPA Rewlatorv Actions on Part 503 Program On April 3, 1995, EPA's Office of Water published a preliminaxy report on regulatory streamlining measures within the Agency's water program regulations. Targeting the Part 503 biosolids management program, EPA plans to revise the requirements to delete chromium pollutant limits, increase the ceiling limits for pollutants in areas with high background levels of the pollutant, and eliminate the use of the 99th percentile to set pollutant concentrations. Revisions may also include new pollutant limitsfor molybdenum, revised requirements for incinerator emissions, and revisions to reporting and recordkeeping requirements. EPA's Dioxin Reassessment is currently under review, and the agenda items include: an EPA overview of the Agency's proposed dioxin research program an overview of EPA's regulatory programs that may be impacted by the Dioxin Reassessment updates from EPA and others on the Draft Dioxin Reassessment public comment responses and the Dioxin Call-in fiuther discussions of known and unknown sources EPA's Science Advisory Board (SAB) will review the Agency's dioxin risk assessment. Two separate charges--an exposure document charge consisting of 20 questions and a health document charge consisting of 23--will be addressed. The issue of toxicity equivalence factors (TEFs) is likely among the most controversial issues to be examined. TEFs are used to calculate a toxic potency, measured in terms of the potency of 2,3,7,8-tetrachlorodibenzo-p-dioxin(TCDD), for various dioxinlike chemicals. Data h i s h e d to EPA concluded that POTWs are nominal sources of dioxin releases to the environment. EPA will propose and promulgate additional standards for the use or disposal of biosolids (Round 11) following a review of contaminants from a list of 31 pollutants listed in a Consent Decree dated May 25, 1993. The Agency will then develop regulations that will establish requirements for biosolids under the following conditions: when applied to the land for a beneficial purpose, when disposed on land by placing on surface disposal sites, and incinerated. The court-ordered schedule calls for proposed Round I1 regulations by December 1999 and final promulgation by December 2001. EPA may significantly reduce the number of pollutants for M e r regulatory action and focus on only those with a human health impact potential. EPA is scheduled to propose claracations to ease implementation of the Part 503 rules and to address inconsistenciesin the regulations in 1995. According to an EPA internal memo a Federal Register notice will propose the Agency: (1) clarify the land application rules to indicate that pollutant ceiling concentration limits pertnin to any land-applied biosolids; (2) delete the requirement to record the time of day

Regulatory Requirements

129

biosolids or septage is applied to land; (3) spec@ biosolids pH must be reported at the standard temperature of 25 degrees Celsius; (4) require seven samples of Class A sludges by analyzed for fecal colifom, (5) allow permitting authorities to determine equivalency for options to reduce vector attractions reduction options other than those specified in Part 503; (6) change site-specific pollutant limits for biosolids incinerators fi-om daily conmtxations to an average monthly concentration; (7) allow permitting authorities to delete site-specific limits for incineration if the pollutant are extremely high; and (8) modify the certification statement used throughout Part 503 to allow biosolids authorities to certitjl that compliance information was collected, whether or not requirements were met.

REFERENCES 1. 33 U.S.C.A. 1251 et. seq. 2. U.S. Environmental Protection Agency. Standards for the Use or Disposal of Sewage Sludge. 40 CFR Parts 257,403 and 503. February 19,1993. 3. U.S. Environmental Protection Agency. 40 CFR Part 257, Criteria for Classification of Solid Waste Disposal Facilities and Practices. Federal Register 44( 179):53438-53464. 1979. 4. US. Environmental Protection Agency. Municipal Sludge Management Policy. Federal Register. June 12, 1984. 5. U S . Environmental Protection Agency. Interagency Policy on Beneficial Use of Municipal Sewage Sludge on Federal Land. Federal Register. July 18, 199 1. 6 . National Sewage Sludge Survey. 55FR472 10. November 1990. 7. U S . Environmental Protection Agency. Response to Public Comments on the Proposed Part 503 Sewage Sludge Use and Disposal Regulations and the National Sewage Sludge Notice of Availability. Washington, D.C. 1992. 8. 56FR50978. October 9, 1991. 9. 56FR5507. February 1 1, 1991. 10 US. Environmental Protection Agency. Fate of Priority Pollutants in PubliclyOwned Treatment Works. Vol. I. Efluent Guidelines Division, Washington, D.C. EPA 44011-82-303. 1982. 11 U.S. Environmental Protection Agency. Ground-Water Protection Strategy. Washington, D.C. 1984. 12. U.S. Department of Agriculture. Peer Review - Standards for the Disposal of Sewage Sludge US. Environmental Protection Agency Proposed Rule 40 CFR Parts 157 and 503. 1989. 13. U.S. Environmental Protection Agency. Review of Proposed Sewage Sludge Incineration Rules (40 CFR Part 257 and 503). EPA-SAB-EEC-89-035. 1989.

130

Forste

14. U.S. Environmental Protection Agency. Technical Support Document for Land

Application of Sewage Sludge. EPA 822R-93-001a. November 1992. 15. U.S. Environmental Protection Agency. Domestic Septage Regulatory Guidance: A Guide to the EPA 503 Rule. EPA 832-B-92-005. September 1993. 16. U S . Environmental Protection Agency. Standards for the Use or Disposal of Sewage Sludge, Appendix G.11. February 19, 1993. 17. US. Environmental Protection Agency. Regulatory Impact Analysis of the Part 503 Sewage Sludge Regulation. EPA 821-R-93-006. 1993.

3 Conditioning and Dewatering Robert J. Kukenberger

Blasland, Bouck 8 Lee, Inc. Syracuse, New Yo&

L INTRODUCTION Conditioning and dewatering is an integral and often necessary process associated with reuse or disposal of wastewater solids. Biosolids conditioning, the first step in the process, prepares liquid biosolids by changing chemical and/or physical properties to allow separationof the solid and liquid fractions. Dewatering, the separation of solid and liquid portions of biosolids slurries, is generally accomplished by mechanical means,although passive systems, such as drying beds and natural fieeze/thaw lagoons, are used where weather permits. Thickening can be the first phase of dewatering, or can function as a stand-alone process to remove a portion of the liquid fiaction, creating a thick slurry. Conditioning and dewatering prepares the biosolids for subsequent processes, such as dqmg, incineration, composting, alkaline stabilizationor for beneficial use as a soil amendment and fertdizer, or for disposal in landfills. The volume and weight of liquid or slurry biosolids can be reduced by as much as 20-fold, although typical processes will achieve a 10-fold reduction in volume and weight. Handling characteristicsare also improved, as dewatered biosolids can be handled as a solid or semi-solid rather than as a liquid slurry. This chapter focuses primarily on chemical conditioning with organic polyelectrolytes (polymers) and mechanical dewatering. Inorganic chemical conditioning (e.g., lime and ferric chloride) is also covered; however, the use of inorganic chemicals is becoming less common as polymer technology improves. 131

132

Kukenberger

High-pressure thermal treatment obviates the need for chemical conditioning. Thermal conditioning at atmospheric pressure in conjunction with polymers is a new technology and holds promise in plants that have low-cost heat or waste heat available for heating biosolids slunies prior to dewatering. Mechanical dewatering is generally favored over passive dewatering (drying beds and fi-eezekhylagoons) as a primary means of dewatering, except in parts of the world that have climates favorable to such technologies or for small treatment plants. Mechanical dewatering machmes are continuously being developed and improved, and new and improved equipment combined with improved conditioning chemicals has resulted in production of diyer biosolids cakes. New technologies are also being developed, requiring planners, designers, and operators to stay current with the latest conditioning and dewatering technologies. Dewatering processes discussed in this chapter include: Belt filter presses Centrifuges Pressure filters (plate and frame presses) Vacuum filters Screw presses Rotaiy presses Diying beds and fi-eezelthaw lagoons Bag dewatering Biosolids thickening is an important process that can be accomplished independently from dewatering or as an integral part of the dewatering process. Independent thickening processes include:

0

Gravity belt thickening Centrifuges Flotation thickening Gravity thickening

11. CONDITIONING Dewatering of most biological solids is not practical without conditioning by either chemical or physical means, including:

-

Organic and inorganic chemical addition Theimal treatment (with and without increased pressure) Freezelthaw Addition of bulking material

and

Conditioning

133

Conditioning represents a signifcant cost component of biosolidsandor reuse disposal; therefore, particular attention needs to be given to this process during planning, design, and operations.

A. Organic Polyelectrolytes The use of polymers in the wastewater industry began in the1960s [l]. Biosolids dewatering equipment technology has developed substantially since the introduction of polymers, leading to the extensive useof polymers for conditioning. Polymers are manmade, long-chain molecules which function in several ways coagulate and flocculate solids particles. The most important bonding mechanisms associated with polymer conditioning are: Electrostatic attraction vanderWaalsattraction Chemical bonding Electrostaticattraction is similartomagneticattractionandreliesonthe attraction of positively and negatively charged particles. Van der Waals attractionis a weak attraction caused by polarization. van der Waals attraction can be explained qualitatively by considering that even neutral atoms and molecules have systems of oscillating charges that attract each other. Chemical bonding is an attraction of highly polar molecules. Chemical forces extend only over very short distances [2].

+ W

Daa1abmz.d

Coagulation

Catlonio

rmlabnm

Flocculation

Figure 3-1 POLYMER COAGULATION AND FLOCCULATION

Kukenb erger

134

Polymers used for conditioning of biosolids can be categorized by charge, molecular weight, and foim. Polymers are available as cationic (positive), anionic (negative), or nonionic (neutral). Cationic polymers are most often used for conditioning of wastewater biosolids; however, anionic and nonionic polymers have applications for some biosolids either independently or in conjunction with cationic polymers. For example, very old biosolids can lose their charge and may require a combination of anionic and cationic polymers for eEective conditioning [3]. Figure 3- 1 depicts negatively charged biosolids particles conditioned with cationic polymers. Polymers for wastewater treatment and biosolids conditioning are usually purchased in dty, liquid, or emulsion form.

TABLE 3-1

MAJOR POLYMER MANUFACTURERS

Allied Colloids, Inc. 2301 Wilroy Road P.O. Box 820 Suffolk, VA 23439-0820

Calgon Water Management Division P.O. Box 1346 Pittsburgh, PA 15230-1346

Callaway Clieniical Division of Exxon Chemical C o P.0.Box 2335 Columbus, GA 3 1993 (800) 241-7543

Cytec Industries 5 Garret Mountain Plaza W. Patterson, NJ (201) 357-3100

Diatec Enviroimiental 1245 Paramount Drive Butavia, IL 60510 (800) 541-8041

Nalco Chemical Co. One Nalco Center Naperville, IL 60566 (708) 305-1000

Rhoiie-Poulenc, Inc. (Fomierly Polypure, Allied Signal) One Catehall Drive Parsippany, NJ 07054 (201) 232-2900

Secodyne, Inc. P.O. Box 10717 Detroit, M1 48210 (800) 824-0933

Stoc~lausen,lnc. 2408 Doyle Street Greensboro, NC 27406 (919) 333-3561

1. ClassiJicatioii of Polyiireis Cationic polymers ai-e foimed by joining aciylarnide monomers together or by modification of acrylamide monomers. Manufacture of various polymer types and

Conditioning and Dewatering

135

specific polymers are beyond the scope of this book; however, the reader is directed to the "Guidance Manual for Polymer Selection in Wastewater Treatment Plants" for specific polymer manufacturing and development information [4]. Major polymer manufacturers in the USA are shown in Table 3-1. 2. Foriiis of Po!ytner.s

Drv Polymers Dry polymers are manufactured as powder, granules, beads, or flakes. Dry polymers have a high active content, typically 90-95% [5]. Dry polymers have proven to be very effective for biosolids condilioning; however, handling of diy polymers requires more manual labor than handling of liquids and emulsions. Emulsions Emulsions are dispersions of polymer particles in hydrocarbon oil. Emulsions are high molecular weight polymcrs with high solids. Active polymer levels of emulsions usually range from 25 to 50%. Emulsions are pourable, clear to white milky liquids with viscosities ranging fi-om 300 to 5,000 centipoise (cps) [5]. Emulsions are stored in tanks and transported with pumps; therefore, many operators prefer emulsions over dry polymers. Mannichs Mannich polymers have high molecular weight and very high viscosity, typically rangng kom 45,000 to 60,000 cps. As a cornpaison, room temperature molasses has a viscosity of 24,000 cps, and water has a viscosity of 1 cps. Mannich polymers have an active solids level of 2-10%. 'The pH of Mannich polymers can be as high as 12. Mannich polymers have a I-elativelyshort shelf life, which is a factor in purchase, delivery, and storage. Maiuiichs are manufactured using formaldehyde, which has raised concerns regarding the health and safety of workers using mannichs and the environmental impacts associated with beneficial reuse of mannich-polymer-treated biosolids [5]. Liquid Polvmers There is a wide range of liquid polymer types available. Liquid polymers have low to medium molecular weight, viscosity ranging fiom 1 to 6,000 cps, and active solids ranging from 10 to 60%. The pIH of liquid polymers is generally neutral to acidic. Shelf life can be from two months to one year depending on the manufacturer [5]. 3 . Polynier. Seleclioti

Design of new or upgraded facilities should include the capability of handling both dry and liquid or emulsion polymers to give the operator flexibility. Selection of the most effective polymer for each application is based on the following criteria:

136

Kukenberger

-

Biosolids characteristics Performance objectives Type of dewatering process employed Downstream impacts of filtrate or centrate Cost

Jar testing is used for screening to determine which polymers have the most potential to be effective in full-scale dewatering applications. Polymer testing is usually conducted as an annual event; therefore, the operator should attempt to obtain biosolids samples for testing that represent, as closely as possible, conditions for the upcoming year. If a side-by-side comparison of various polymers is being made, the tests should be done simultaneously to be sure the biosolids are of similar characteiistics. Ifcost is to be a major selection factor, preliminaiy cost estimates can be made prior to jar testing based on dosage and cost estimates from the polymer vendors. Cost is typically expressed as cost per unit weight of biosolids treated (dollars per dry ton of biosolids). Polymer vendors and manufacturers will generally provide jar testing to screen various polymers and will assist the operator on full-scale field testing. Biosolids should be characterized to determine solids concentration, inorganic (ash) content, biological solids content, and biosolids chemishy. Concentrations of solids can change due to changes in the wastewater and biosolids treatment practices. Lower solids concentrations will generally require more polymers (on a pounds per dry ton basis). Expected changes in solids concentrations should be assessed, and appi-opiate adjustments made, during jar testing and field testing of conditioning and dewatering processes. Biosolids with low inorganic (high ash) content usually have cationic charge neutxalization requirements. Fresh or digestcd biosolids generally have an ash content of 1 5 5 0 % and eshibit cationic charge neutralization demands. Higher ash contents (>50%) may be encountered from lime-stabilized or chemically treated biosolids. Extremely old biosolids, such as lagoon-stored biosolids, may also have high inorganic content. Biosolids with greater than 50% inorganic content may exhibit either an anionic or nonionic charge demand. Bench testing with various polymers should be done to verify the ionic charge demand, and subsequently the most appropriate polymer. In some cases, a combination of anionic, nonionic, and cationic polymers will be more effective than any single form of polymer due to the charge neutralization requirements. The percentage of solids resulting fi-om biological treatment processes can significantly impact conditioning. Solids produced by biological (secondary) treatment processes are more difficult to dewater than raw pl-imaiy or chemical solids [ 5 ] . Unconditioned biological solids have poor characteristics for mechanical dewatering. The trend in larger wastewater treatment fac es is to condition secondary solids separately from raw or primaiy solids. It is generally less costly to

Conditioning and Dewatering

137

treat these solids separately; however, the cost and complexity of two separate systems may offset the advantages, particularly in smaller facilities. Extended storage of biosolids usually increases conditioning requirements. Only well stabilized biosolids should be stored prior to conditioning and dewatering to prevent anaerobic conditions. Biosolids should be processed within a few hours of production; however, aeration of raw biosolids for up to 4 or 5 days can improve dewatering characteristics. Dewaterability deteriorates significantly if biosolids are stored for longer than 5 days. Anaerobically digested biosolids generally have higher polymer dosage requirements than raw or unprocessed primary biosolids, and are generally more difficult to dewater. Polymers with moderate cationic charge and moderate to high molecular weight are usually most effective on anaerobically digested biosolids. Polymer dosage requirements for aerobically digested biosolids are similar to dosage requirements for anaerobically digested biosolids, however, more highly charged polymers are typically required.

B. Polymer Feed and Control Systems Polymer feed systems are selected based on the form of polymer to be used. In larger wastewater eeatment plants, designers will often select multiple feed systems to allow the operator to select the most appropriate and cost-effective polymer form.

1 , Polyirier Feed Systerri Requisenrents The key requirements of a polymer feed system are: Proper polymer storage Complete dissolution to prevent polymer waste Lack of undissolved particles to prevent clogging of equipment and pipes Proper mixing Proper equipment sizing Optimum dosage - underfeeding does not accomplish results and overfeeding can upset treatment and waste polymers Polymer feed systems should allow for dosage variations of 20 to 1 to give the operator adequate flexibility. Multiple polymer application points into the biosolids stream should be provided. It is necessaiy to provide diy, temperature-controlled storage areas to prolong the life of the polymer. Make-up water should have a pH of 6-8, be fkee of suspended solids, have a specific conductivity of < I ,200 micromho per centimeter (pmhohm), total dissolved solids of 4 0 0 mg/L, and temperature of 10"50°C (50" -120°F). Residual chlorine in the make-up water should be less than 0.5 mg/L [ 11. Typical polymer feed systems are depicted in Figure 3-2. Polymer feed

138

Kukenberger

systems can be assembled from components or purchased as skid-mounted package systems. Compact polymer blending systems have been shown to be successhl in many applications. Polymer blending systems can eliminate the need for mixing and aging tanks, which saves considerable space and operator attention [6].

Dry Feeder

Llquld or

Emulrlon Storage TMk Feed Pumps

Figure 3-2 SCHEMATIC

Transln Pumps

UetMblg Pump.

OF A TYPICAL POLYMER FEED SYSTEM

2. Polynier COWO~ Systeim

The control of polymer feed for biosolids conditioning affects the thickening and dewatering perfoimance, quality offiltrate or centrate, and the cost of chemicals. With polymer conditioning, “more is not better” because overdose as well as underdose can have deleterious effects on dewatering efficiency. Manual control is typically accomplished by measuring cake concentration, filtratelcentrate quality, and, in belt filter press installations, by visual obseivation of the conditioned biosolids on the gravity deck. Several technologies have recently become available for automatic or semiautomatic control of polymer feed, including streaming current detectors, sludge flocculation controllers, and reflective (infrared) scanning of gravity drained biosolids. The streaming current detector measures colloidal charge (streaming current) in the filtrate or centrate fi-om the biosolids dewatering process. Research has shown that a near zero charge indicates optimum dewaterability and that the centratekltrate exhibits nearly the same charge as conditional biosolids [9]. Therefore, streaming current measurements of filtrate or centrate can be used to control polymer feed. Biosolids flocculation controllers measure the rheological characteristics of the biosolids before and after conditioning. Relationships have been developed between polymer addition and rheology, which can be used to optimize polymer addition and

Conditioning and Dewatering

139

dewaterability. Flocculation controllers control polymer feed by alternately measuring raw and conditioned biosolids rheological characteristics to optimize polymer feed. Flocculation controllers appear to have difliculties in controlling polymer dose over a wide range of biosolids conditions [ 7 ] . Automatic scanning of biosolids reflective characteristics on the gravity drainage section of belt filter presses can be accomplished with infrared sensors [9]. Infrared sensors are placed along the length of the gravity deck to evaluate drainage effectiveness. Signals from the sensors can be used to control polymer feed, belt speed, and biosolids feed rate. The principle is essentially the same as visual observationby the operator, except that the sensing and control is continuous and can potentially save up to 20-30% of the polymer dose [S], [9]. 3. Polynier Consuiiiptioii Organic polymer consumption depends on a wide range of variables. Belt filter presses (BFPs) typically use polymers to condition biosolids to achieve separation of the water from the biosolids sluiiy. Use of a wide range of dosages, ranging from 1 to 25 g/kg dry solids (2-50 lbdton), has been practiced, although typical dosage requirements for BFPs are between 3 to 6 g k g dry solids [5]. Belt filter presses rely on significant removal of water in the gravity drainage section, which is highly dependent on good conditioning. Underconditioning causes poor dewatering in the gravity section, and overconditioning causes blinding of the filter press cloth (media). Organic polymers are also used with solid bowl centrifuges for wastewater biosolids dewatering. The recently developed "high solids centrifuges" rely heavily on good polymer conditioning. Typical dosages range from 2-4 g k g (4-8 lbs/ton) for traditional solid bowl centrifuges and up to 10 g/kg (20 Ibs/ton) for "high solids" centrifuges [5]. In the past, organic polymers have not been used extensively for plate and frame (pressure) filter presses. Poor and inconsistent dewatering and media blinding have been historical problems associated with polymers used with plate and frame filters. As polymer technology continues to improve, however, conditioning for plate and frame filtration has become more effective. Also, the use of precoat agents, such as diatomaceous earth or fly ash, prevents cloth blinding. As a result, polymer conditioning prior to plate and frame filtration is becoming more popular. Historically, gravity drying bed dewatering was accomplished without conditioning. Over the past several years, however, improved polymer technology has led to conditioning prior to applying biosolids to drying beds, which has improved dying bed performance. Vacuum-assisted diying beds rely on polymer conditioning for adequate dewatering, but care must be taken to prevent floc break-up, which typically occurs with pumping or transporting over distances. Floc break-up can be prevented by conditioning near the discharge to the diying beds. Typical dosage for vacuum-assisted diying beds is 15 to 25 g k g (30-50 lbs/ton) [I].

140

Kukenberger

Application

Typical Polymer Dosage gram dry polymer/kilogram dry nolidn

Vacuum Assisted

C. Inorganic Chemical Conditioning

Inorganic chemicals used for biosolids conditioning prior to dewatering typically include iron salts (fenic chloride, ferrous chloride, and ferrous sulfate) and lime. Lime (CaO and Ca(0H)J and ferric chloride (FeCl,) are the most widely used inorganic chemicals for biosolids conditioning. Use of inorganic chemicals has decreased in recent years because of improved polymers. As compared to polymers, inorganic chemicals may be more effective with certain biosolids, particularly when employing filter presses and vacuum filters, however,the use of inorganic chemicalsmay: require a higher dosage rate; increase weight and volume; reduce the heat value of the biosolids; and add metals to the final product.

Conditioning and Dewatering

141

1. Ferric Chloride

Fenic chloride is available commercially in liquid foim as a 30-35% solution. In cold climates, ferric chloride may be shipped at lower concentrations to avoid crystallization. Commercial ferric chloride is an orange-brown corrosive and acidic solution. Fenic chloride is used as a conditioner to improve biosolids dewaterability by destabilizing negatively charged particles. Flocculation with ferric chloride requires a pH above 6; therefore, lime is usually added to offset the pH reduction caused by ferric chloride. Further, rapid mixing should be provided to cause particle collision and promote flocculation. All materials handling equipment for ferric chloride must be coirosion resistant (i.e,, non-metallic). Pumps, valves, piping, tanks, etc., are provided in plastic-, fiberglass-, or i-ubber-lined steel. FeiTic chloride can cause severe burns to eyes and skin, and fiunes can cause uiteinal buns if inhaled. Protective gear and specific safety precautions must be adhered to when handling ferric chloride.

2 . Lime Lime is commercially available as quicklime (CaO) and hydrated lime (Ca(OH),). Quicklime is manufactured by burning crushed limestone in high temperature kilns. During the buning process, carbon dioxide is driven off, leaving calcium oxide (CaO). Quicklime is usually sold in pebble form for ease in handling. To use quicklime in wastewater processes, it is sluii-ied with water to form calcium hydroxide (Ca(OH),) prior to use. Hydrated lime is a powdered foim of Ca(OH),. Commercially available hydrated lime is prefeired for small and inteimediate applications because it does not require slaking. For larger applications (greater than 2 tons per day), quicklime is usually more economical, even though slaking is required. Lime is used as a biosolids conditioning agent to raise the pH, reduce odors, and add bulk via formation of calcium carbonate and calcium hydroxide complexes. Lime dust is an irritant if inhaled and can cause buins to skin and eyes due to heat generated when CaO comes in contact with water.

3 . Applicalioii of Litire and Ferric Cliloride Lime and feiric chloride are often used together to condition biosolids for dewatering on filter presses and vacuum filters. With filter presses, the dewatering process is accomplished in batches, therefore, biosolids are conditioned in a batch (or feed) tank prior to filling the filter press. Vacuum filtcrs are operated as a continuous process; therefore, lime and feiiic chloride are added just upstream of the dewatering machine.

Kukenb erger

142

Lime and ferric chloride are not typically used for belt filter presses or centrifuges, as organic polymers are better suited for these dewatering machines. F a n c chloride reacts with biosolids by neutralizing negatively charged particles and forming fmic hydroxide complexes. Because alkalinity is utilized in the reaction, pH is lowered. Lime is used to raise the pH, provide additional alkalinity, and thereby allow the reactions involving feiiic chloride to be more eEcient. Lime also forms calcium carbonate and calcium hydroxide complexes, which provides a matrix in the biosolids, improving dewaterability . Feiiic chloride is typically dosed at 2- 10% of solids on a dry weight basis. Lime is usually dosed at 0-40% on a diy weight basis. 4. Other Inorganic Cheniicols and Additives

Ferrous sulfate (Fe(S0.J) and ferrous chloride (FeC4 ) (waste pickle liquor - a byproduct of the steel industiy) may be used in lieu of ferric chloride. Evaluation of alternatives to feiric chloride should be based on performance, cost, and availability. Also, waste pickle liquor may contain by-products of the steel manufacturing processes, which could cause quality problems associated with reuse or disposal of biosolids. Ash, pulverized coal, saw dust, and diatomaceous earth have also been used as additives for filter presses and vacuum filter dewatering. These additives have limited use and depend on biosolids characteristics, type of dewatering equipment, and reuse/disposal options. These additives are noimally used in conjunction with organic or inorganic chemicals and serve as a filter aid or "precoat" to allow release of the cake fiom the filter cloth. The addition of diatomaceous earth, fly ash, and cement kiln dust wese found to be successful when using polymers with plate and frame filter presses

PI. 5. Selection Criteria The decision to use inorganic chcmicals, particularly lime and fen-ic chloride, should be based on the following criteria: 00

-0 0. 0.

0-

Perfoiinance of conditioning chemicals, as detelmined by testing or fullscale operations Type of dewatering equipment proposed or in use Life cycle costs Materials handling and storage requirements End use or disposal options for the dewatered biosolids

Conditioning and Dewatering

143

D. Thermal Conditioning 1 . High-Pressure Tliermal Conditioning

Thermal conditioning of biosolids at elevated pressures breaks down (lyses) the cell wall of the microorganisms in biological solids, which allows the release of bound water. High-pressure theimal conditioning alters the physical properties of biosolids, which results in a drier cake a s compared to that produced by chemical conditioning. High-pressure thermal conditioning is described in Chapter 6. High-pressure themially conditioned biosolids will thicken by gravity to a range of 6 to 15% solids and can be mechanically dewatered to a range of 35 to 75% solids [ 101. Vacuum filters are typically used for dewatering biosolids that have been conditioned by high-pressure thermal treatment.

2. Tliervially Enhanced Dewatering Mixed primary and biological solids dewatering can be enhanced by preheating the solids to 60°C (140°F) at atmospheric pressure prior to dewatering. Cake solids concentration may be inci-eased by up to 6%, and polymer usage may be decreased by as much as 25% with this process [ I I]. This process may be cost effective when waste heat is available fiom biosolids incinerators or fiom other sources. Digested or thickened biosolids are heated in heat exchangers prior to polymer addition and dewatering. Centrate can be returned to the head of the plant or used as a heat source in the biosolids heat exchanger. The potential for odors is greater if heated biosolids or centrate/filtrate is exposed to the atmosphere prior to cooling; therefore, odor conwool facilities need to be designed and sized to address this potential. Figure 3-3 depicts a thermally enhanced mechanical dewatering system.

180 ' C Water

PoIynmr Centrate

Polymer

I

Dewatared

Spiral Heat Exchanoer

BlOSOlld8

Dewaterho Yachhe

Figure 3-3 SCHEMATIC OF THERMALLY ENHANCED DEWATERING SYSTEM

Kukenberger

144 111. DEWATERING

A. Process Description

Biosolids are produced as a by-product in wastewater treatment facilities as a liquid or sluny, generally from less than 1% solids concentration up to 10% solids concentration. Further handling and processing of biosolids is enhanced by thickening and/or dewatering. Most thickening devices can achieve a solids concentration of 5lo%, and dewatering devices can achieve 20-50% solids concentration. Factors affecting the thickened and dewalered solids Concentration are: Characteristics of biosolids Type of conditioning employed Type of thickening or dewatering device Thickening processes include gravity and flotation thickening, centnfUge thickening, gravity belt thickeners and sotaiy drum thickeners. Dewatering processes include mechanical devices and passive systems (e.g., drying beds). Mechanical devices discussed herein include belt filter presses, cenhlfuges, pressure filters, vacuum filters, screw presses, and rotary presses. Passive systems include conventional and vacuum assisted drying beds, freezekhaw lagoons, and bag dewatering systems.

B. Thickening Biosolids thickening reduces the volume of material to be handled in subsequent processing steps. Thickening prior to digestion reduces the size of digesters, reduces energy requirements for heating anaerobic digesters, and increases digestion efficiency. Thickening reduces storage and hauling requirements when biosolids ase recycled in liquid fosm. The Madison Wisconsin Metropolitan Sewerage District (MMSD) thickens digested biosolids with gravity belt thickeners (GBTs) prior to liquid land application. MMSD achieves an average thickened solids concentration of over 6% with GBTs on a feed solids concentration of 2-3% [ 121. Thickening prior to dewateiing may be beneficial with regard to sizing of dewatering equipment, higher thsoughput, and higher dewatered solids concentration. Most mechanical dewatering systems provide thickening integral to the dewatering process. 1. Gravity Thickening

In gravity thickening, biosolids are concentrated by gravity settling and compaction. Gravity thickeners are typically used for piimaiy biosolids, lime sludges, and

Conditioning and Dewatering

145

combined primary and secondaiy (waste activated) biosolids. Gravity thickeners are usually circular tanks with steep cone-shaped bottoms to promote bottom withdrawal of thickened biosolids. Gravity thickeners can achieve undeiflow solids concentrations of 5- 10% on primaiy biosolids and 4-6%on combined primary and waste activated biosolids [ 131. Gravity thickeners are sized based on solids mass loading per unit area. Overflow rates are also an important criteria. Too high of an overflow rate can cause solids carry-over, and too low of an overflow rate may cause septic conditions. Dilution water, such as plant effluent, is typically added to maintain sufficient overflow rates. Typical design mass solids loading and oveiflow rate is presented in Water Environment Federation Manual of Practice No. 8 [ 131.

2. Dissolved Air. Flolariori (OAF) Thicketiitig Dissolved air flotation thickening is typically used for secondary waste-activated biosolids, which do not settle as well as primary biosolids. DAF thickeners rely on the addition of air in a pressurized system. Microscopic air bubbles attach to the solids particle to promote flotation. DAF thickeners are either circular or rectangular with surface skimmers and bottom solids removal mechanisms. Design criteria include solids loading rate, hydraulic loading rate, and air to solids ratio [ 131. DAF thickeners can produce a solids concentration of3-5% without conditioning and 3 . 5 4 % with the addition of polymers [ 131. 3. Ceiilvi~ugalTI1icketI it iy Centnhgal thickening is commonly applied to waste-activated biosolids. Solids bowl c e n h ~ g e for s thickening are essentially the same as those used for dewatering. The major differences between thickening centrifuges and dewatering centrifuges are the location of the discharge ports and the configuration of the scroll (conveyor). Typical thickened solids concentrations with solid bowl centrifuges range from 3-10% [13]. Solid bowl centrifuges are discussed in more detail in Section C.2. 4. Graviv Bell Tliicketiers (GBT) and Roiary Drvttr Tliicketiers

Gravity belt tluckenas and rotaiy drum thickeners are similar to the first stage of belt filter presses. Polymer conditioning is essential with municipal biosolids, as these units rely on liquidsolid separation and drainage of the free liquid through a porous belt or wedge-wire drum. Gravity belt thickeners typically achieve 4-10% solids concentration [ 131.

-

146

Kukenberger

C. Mechanical Dewatering 1. Belt Filter. Presses

Belt filter presses (BFPs) are mechanical dewatering machines that utilize two or more porous belts to dewater biosolids. There are three stages of dewatering on a BFP: *. 0. 0.

Gravity drainage Compression Shear

The BFP is a continuous process and relies heavily on polymer conditioning. Polymer conditioning agglomerates suspended solids into flocs for initial separation of the liquid fi-om the solids. Conditioning also builds a structural matnx in the biosolids that can withstand gradual increase in pressure and shearing action. The gravity drainage section of a BFP is a porous belt (or rotaiy dlum) that allows fiee liquid to drain through the belt pores (or drum slots) leaving behind a layer of partially dewatered (or thickened) biosolids. Gravity drainage is a necessary first step of dewatering to allow the compression zone to hnction without losing biosolids out the edges of the belts. The compression section of the BFP contains two belts which sandwich the biosolids. As the two belts travel over decreasing diameter rollers under pressure, compression and shear is created. Liquid is removed to create a biosolids cake of typically 16-30% solids concentration. Dewatered biosolids are discharged with the assistance of a "doctor" blade, and the belts are washed with a water spray. Washwater can be potable or plant effluent water; however, if plant water is used, it must be fiee of suspended solids to prevent clogging of spray nozzles. Dhcharge TO Pressure Zone

\t

Condiuonlng Lone Eiosolkls Feed

Gravlly Drainage Zone

Figure 3-4 BELT FILTER PRESS

Conditioning and Dewatering

147

Automatic belt tracking systems are provided by means of a roller tensioning system. The rollers are driven by variable speed motors or hydraulic drive to allow infinite variation of belt speed. Belts are typically spliced for easy removal and replacement by the operator. Figure 3-4 depicts a typical belt filter press mechanical layout. Performance of the belt filter press is controlled by biosolids feed rate, polymer selection, polymer dosage, point of feed, and belt speed. An experienced operator can visually observe dewatering characteristics on the first stage (gravity) section of the belt filter press to determine whether the biosolids are properly conditioned. In addition, solids concentration of the dewatered biosolids and solids capture (quality of the filtrate) are monitored to determine o\ierall pelfoimance.

Centrifuges for thickoning and dewatering of biosolids are typically solid bowl machines. Solid bowl centrifuges are operated as a continuous process. These machmes rely on centrifugal force that separates the solid and liquid fractions. Solids are forced away fiom rotating axes of centrifuges, due to the differences in densities of the solid and liquid phases, and the liquid moves toward the center of the machine. Both inorganic chemicals and organic chemicals (polymers) have been used successfully with centrifuges. Due to advances in polymer technology and centrifuge machine design, polymers are now used for most centrifuge dewatering systems on municipal wastewater biosolids. Modem centrifuges for dew atering biosolids consist of a solid bowl, usually tapered on one end, a scroll conveyer, a case to cover the bowl and conveyor, a heavy cast iron base, a main drive, and a back drive. The main drive rotates the bowl, and the back dnve assembly controls the conveyor speed. The range of differential speeds between the bowl and conveyor vary with different manufacturers. There are two configurations of solid bowl machines: co-cuirent and counter-current. In the cocurrent design, the liquid and solid phases travel in the same direction, and liquid is removed by an internal skimming device or ports located in the bowl. In the countercurrent design, liquid travels in the opposite direction from solids, and liquid overflows weir plates. Figure 3-5 shows the main components of a counter-current solid bowl centrifuge. Over the last 5 to 10 years there have been significant improvements in centrifuge design. These new machines, referred to as "high solids" or "compression" centrifuges, provide improved performance and less maintenance than older centrifuges. Recent improvements include:

. 0.

Higher operating speeds Less differentid in speed between conveyer and bowl, resulting in less wear

148

Kukenberger

Higher solids retention and more compression Better materials of constiuction The current generation of cenh-tfbges can achieve solids concentrations from 25-

35%, 510% higher than earlier generation centrifuges and standard BFPs [ 141. The operator controls the centrihge perfoimance by vruying feed rate, type of conditioning chemicals, dosage rate, and the bowl/conveyor speed differential. With some manufacturers, the pool depth in the centrihge can be varied, thereby affecting dewatering peifomance. The operator monitors solids concentration and solids recovery (centrate quality) as primaiy perfoimance parameters.

Damtarlno Barnoh

Figure 3-5 SOLID BOWL COUNTER-CURRENT CENTRIFUGE

3 . Pressure Filters Pressure filters (commonly refemed to as plate and frame presses) used for biosolids and industrial sludge dewateiing are recessed plate presses, either fixed volume or vaiable volume (diaphragm) machines. The teim "plate and frame press" is derived from the food industry, which developed the forerunners of today's pressure filters. Pressure filters are operated as batch processes and require storage and batch tanks to contain feed biosolids in enough volume to fill the press. In large systems, multiple presses can be used to reduce storage volume. Modein filter presses can produce dewatered biosolids with a solids content in escess of 40% [ 131. Recessed plate pressure filters are horizontally aligned to allow discharge of dewatered cake by gravity into a receiving bui or conveyor (see Figure 3-6). Recessed plates are placed on a fixed fi-ame which has a closure mechanism that presses the plates together and holds them in position during the filling and dewatering cycle.

Conditioning and Dewatering

149

For a fixed volume press, the biosolids are pumped into the recesses over a period of several hours to continually increase pressure and force the liquid through the filter cloth. JAW pressure units achieve a pressure of up to 100 pounds per square inch (psi) (690 kilopascal [@a]), and high pressure units achieve a pressure of up to 225 psi (1550 Wa). The maxirnurn pressure is maintained until the filtrate essentially stops, signifying the end of the dewatering cycle. The variable volume press has a diaphragm behind the cloth media. Biosolids are pumped into the press until the recessed chambers are filled. Pumps are then shut off, and air or water is pumped into the diaphragm to create pressure in the recessed chamber. With variable volume presses, higher pressures can be achieved, cycle times are reduced, and generally more consistent dewatering results can be achieved. However, diaphragm presses are more costly than fixed volume presses and require more auxiliary equipment. Biosolids to be dewatered in pressure filters are typically conditioned with lime and feinc chloride. Recently, some municipal biosolids dewatering installations have had success with polymers alone; however, cloth blinding and difficulty with cake release have been generally observed with polymer-conditioned biosolids. However, the small decrease in performance with polymers may be offset by lower chemical costs, reduced ammonia odors, and lower metals content in dewatered biosolids. Operator control of performance is limited to conditioning; therefore, chemical selection and dosages are critical control parameters. rFilter Plates

Figure 3-6 PRESSURE FILTER

150

Kukenberger

4. Jfacu14111 Filteis

Vacuum filters have been in existence since the late 1800s and have been used in the United States for dewatering municipal sludge since the 1920s. The use of vacuum filters for biosolids dewatering has sharply decreased in recent years due to developments of more efficient dewatering equipment. Vacuum filters are still used with high-pressure, thennal-conditioned biosolids, since high solids concentrations (30-50%) can be achieved. Vacuum filters are also used for dewatering lime sludge from lime softening water treatment plants, particularly in the southeastern United States. The most common vacuum filter consists of a large, horizontally mounted rotating drum,whch is covered by a porous cloth or metal coils (see Figure 3-7). The bottom portion of the drum is submerged in a vat of biosolids. As the drum rotates, biosolids are picked up on the porous medium under a vacuum. The divm is divided into sections, and vacuum is applied by a rotary valve. The filter operates in three zones: Cake foi-niation Cake dewatering Cake discharge Vacuum filters can be successhlly used with organic (polyelectrolyte) conditioning, inorganic (lime and fen-ic chloride) conditioning, and thermal conditioning. Operating variables include drum speed, liquid level in the vat, vacuum level, and conditioning agent dosages.

Cake

Feed

Figure 3-7 VACUUM FILTER

151

Conditioning and Dewatering

5. Rotaiy Press The rotary press is a recent development in biosolids dewatering. Biosolids are pumped into a peripheral channel that has walls made up of rotating filter elements. As the mechanism rotates, compression is created and liquid is forced through the filter elements. Cake is formed in the interior channel and extruded. The rotary press is depicted in Figure 3-8 [ 161. The rotary press is a continuous dewatering process; therefore, ancillary equipment would be similar to that required for belt filter presses and centrifuges. Polymer conditioning is a necessaiy step for proper dewatering via rotary press. The machine requires relatively small floor space and is available in several sizes and in single- or multiple-channel arrangements. Static Mechanlcal

SECTION A-A

Figure 3-8 ROTARY PRESS 6. Screw Presses

Screw presses are manufactured as low-pressure, inexpensive machines for smaller installations when high solids concentrations are not required, and as large, highpressure machines that produce high solids concentrations. The low-pressure unit is typically mounted vertically, either trailer mounted or as a permanent installation. Typical solids concentrations on municipal biosolids from low-pressure screw presses are 8- 15%. Horizontally mounted, high-pressure screw presses are typically used for dewatering fibrous materials such as pulp and paper sludge [17]. High-pressure screw presses are not widely used in the wastewater industry.

152

Kukenberger

D. Passive Dewatering 1. Conventional Dtying Beds D y n g beds for municipal biosolids dewatering have been designed and constructed with many variations, including: 0

* 0

Open sand beds Open paved beds with and without drainage channels Covered sand and paved beds Vacuum-assisted drying beds Freeze/thaw lagoons and diying beds

A typical covered, paved drying bed is depicted in Figure 3-9

Ins onall

Open For

Ven tlletlon

SandJ Trench

LPerforated Drain To’ Head Of Plant

LLiner

Figure 3-9 CROSS SECTION OF COVERED/PAVED DRYING BEDS Diymg beds require larger land area than mechanical dewatering, are generally more labor intensive, and are subject to weather conditions. Because they are typically open to the atmosphere, odor control is difficult, and diying beds used in highly developed areas may result in odor complaints. However, drying beds remain in use for many municipal biosolids dewatering operations. Diying beds are used in small communities across the United States and at some large plants in areas where the weather promotes rapid diying. In recent years, polymer conditioning has been used successhlly with diying beds to promote liquidsolids separation and faster dewatering. Mechanical mixing of the biosolids following npplication to the bed is also being used, especially on larger installations. During the first several hours of

Conditioning and Dewatering

153

drying, the primary mechanism of liquid removal is by gravity drainage. Sand beds and paved beds with drainage channels are equipped with an underdrain system. Filtrate is typically directed to the head of the treatment plant for treatment with the plant's influent. After initial gravity drainage is essentially complete, evaporation becomes the primary dewatering mechanism. Final solids concentration depends on the weather conditions and the length of time the biosolids remain on the bed. Final solids concentration from drying beds can be as high as 80%; however, dust can become a problem if the solids become too dry.

2. Vacuirin-AssistedDtyiiig Beds Vacuum-assisted drying beds are constructed of a wedge-wire support system which allows fieewater to be removed much more rapidly than by gravity drainage. Liquid biosolids are pumped over the slotted surface, and vacuum is applied to the underside of the support system to enhance gravity drainage. If consbucted outside, the biosolids are allowed to continue dewatering by evaporation after free water is removed by vacuum. Biosolids are typically conditioned with polymers prior to placement on vacuum-assisted drying beds, allowing rapid removal of the liquid fraction from the solids. 3 , FreezeRiraw Lagoons

Freezehaw lagoons and diying beds have been shown to be effective in dewatering biosolids and alum sludge generated in water treatment plants [ 181. The freezing and thawing process separates the liquid and solid fraction, and it is believed that freezing breaks all walls, allowing removal of bound water after thawing occurs. Mechanical freezelthaw devices have been developed; however, energy requirements have prevented development of h s technology; therefore, freezelthaw systems are generally limited to climates that can support natural freezehhaw cycles. 4. Bag Dewatering

Small treatment plants with wastewater flow rates of less than 200,000 gallons per day (gpd) can dewater biosolids cost effectively with a bag dewatering system. The bag system is a gravity dewatering system with no moving parts. Polymer-conditioned biosolids are pumped into a gravity drainage plenum, which provides initial dewatering and distribution to the bag chambers. Woven polypropylene bags are filled with biosolids and allowed to drain by gravity. Filtrate is collected in a pan placed underneath the bags and returned to the head of the plant. Cycle time is 4 to 6 hours - 2 hours for filling and 2 to 4 hours for drainage. A standard 22-gallon bag yields approximately 20 pounds of d y solids per bag at about 10-1 2% dry solids [ 191,

154

Kukenberger

Bags can be stacked outdoors for further diying by evaporation to as high as 60% solids concentration. Figure 3-10 depicts a bag dewatering system.

,-

Levd Probe

Emergency Overllow pipe

Figure 3-10 BAG DEWATERING SYSTEM

IV. ODOR CONTROL Odor-producing compounds released from municipal wastewater biosolids include inorganic gases, hydrogen sulfide and ammonia, and organic gases, such as indoles, skatoles, mercaptans, and nitrogen-bearing organics. A list of odor-producing compounds in biosolids is included in Table 1-8. Hydrogen sulfide (H,S) is the most prevalent odorous gas associated with domestic wastewater. Hydrogen sulfide is produced during anaerobic decomposition of the organic matter in wastewater; therefore, keeping wastewater and biosolids fiom becoming septic prevents production of hydrogen sulfide. Organic vapors, particularly mercaptans, are often produced during conditioning and dewatering. Mercaptans have an extremely low odor threshold, that is, they are detectable by the human nose at very low concentrations. Odor thresholds for mercaptans range from 2.9 x I0-j to 1 . 1 x The odor threshold for hydrogen sultide is 4.7 s 10-4[20]. Mercaptans can create odor problems at concentrations as low as 20 times less than hydrogen sulfide. Odor control can generally be classified in three categories: 1) prevention of the formation of odorous compounds, 2) treatment of odorous gases prior to release to the atmosphere, and 3) odor modification and masking. Prevention of the formation of odorous compounds is accomplished by chemical addition to the liquid stream, maintaining aerobic conditions and good housekeeping. Chemicals added to liquid biosolids to reduce odors include hydrogen peroxide (H,O,) and potassium permanganate (KMn04) [20]. These chemicals are strong oxidizing agents that react with H,S to form non-odorous compounds.

Conditioning and Dewatering

155

Masking of odors is accomplished by replacing or overwhelming an objectionable odor with a more pleasant one. This approach of adding "perfume" does not solve the problem and should not be considered as a permanent odor control alternative. Collection and treatment of odorous gases from biosolids processes is generally required at treatment plants located in populated areas. The production of H,S, mercaptans, and ammonia is unavoidable, and the low odor threshold associated with many odorous compounds leads to odor problems and complaints from nearby residents. Collection of odorous gases requires enclosure of biosolids handling and treatment units, or containment within a building. To reduce air flow rates and odor treatment unit sizing, and for worker comfoi-t, it is beneficial to contain odorous gases when possible. Containment can be accomplished by enclosing handling and treatment units and utilizing systems that are self-contained. For example, odor containment and control was an important selection criteria for dewatering and conveying equipment at the Yonkers Joint Treatment Plant (YJTP), Westchester County, New York. Centrifuges were favored over open dewatering units, such as BFPs and filter presses, and dewatered biosolids pumping units were selected in lieu of open conveyors. The biosolids dewatering system is entirely enclosed in buildings, including truck loading bays. Figure 3-1 1 is a schematic of the odor containment and control system at the YJTP. Odor treatment systems for biosolids dewatering and handling include: Wet scrubbers (pack tower and mist type) Activated carbon adsorption Biofilters

Figure 3-11 BlOSOLlDS DEWATERING ODOR CONTROL SCHEMATIC. YONKERS JOINT TREATMENT PLANT, WESTCHESTER COUNTY N.Y.

156

Kukenberger

r

Alr DlstrlbutlonZone

Alr Discharge Pipe (Vpical)

Polyethylene Liner Clay Cut Off Wall

J

LDrsln pipa

Figure 3-12 CROSS SECTION OF BlOFlLTER

Wet scrubbers use a chemical solution to remove odors from the odorous air stream. Selection of chemicals for wet scrubbers should be based on the odorous compounds to be removed. The most common scrubbing solutions are sodium hypochlorite and potassium permanganate with water. Hydrogen peroxide, ozone, and proprietary chemical solutions are also employed. Adjustment to pH may be beneficial with some chemicals; therefore, acid and alkaline solutions may be provided with wet scrubbers. Activated carbon adsorption can be employed as the primary odor treatment system, or as a polishing step. When used as a polishing step, a bypass may be provided so that the carbon is only used when necessary, thereby extending the life of the carbon bed. Carbon adsorbs a wide range of odorous compounds; however, due to the non-selectivity of carbon, the capacity of the carbon can be prematurely exhausted due to the adsorption of non-odorous hydrocarbons [20]. Biofilters use bacteria naturally present in soil and compost to biodegrade odorous compounds. Biofilten can be custom designed (in ground) filters, as depicted in Figure 3-12, or manufactured units [2 I]. Custom designed biofilters should meet the following criteria: Collect leachate by sloping the bed floor and using a liner and collection system with n drainage pipe fitted with a valve or water trap to prevent leakage of untreated air through this pipe Prevent leakage around the perimeter of the bed and along the air supply pipe by estending the active media past the edge of the air distribution zone and using a flange or other device around the air supply pipe as it enters the biofilter Specify corrosion resistant air supply duct work and piping Evaluate the method and frequency of media replacement

Conditioning and Dewatering

-

157

Provide a water supply for surface sprinklers and an inlet air humidifier Use geotextile fabric between the active media and the air distribution zone to prevent migration of fines Provide condensation drainage holes, as necessaiy, in the air supply piping Avoid surface compaction by keeping equipment and vehicles off the bed during construction and operation Maintain perfoimance through regular operator attention to bed conditions and maintenance

Combustion oxidizes odorous compounds with direct flame oxidation or catalytic oxidation. In direct flame oxidation, the odorous air is mixed with combustion air at temperatures of 480 to 815°C (900-1 500°F) [EPA]. In catalytic oxidation, a catalyst causes oxidation to occur at lower temperatures and in the absence of a direct flame. The odorous air is preheated to 3 15 to 480°C (600 to 900°F). Catalytic combustion is not well understood, but generally occurs in three steps: adsorption on the active surface, oxidation, and desorption of the reactant products. Catalytic combustion is only recommended when the concentration of odorous compounds is greater than 1,000 ppm. Catalytic combustion is more widely used for removal of organics from industrial processes [EPA]. Other odor treatment systems include adsorption on activated alumina impregnated with potassium peimanganate and woodkhips with iron oxide, and oxidation with ozone. These systems can be manufactured package systems or custom designed systems for specific applications. With less common systems, pilot testing should be performed to assure that the odorous compounds can be removed and to develop design and sizing criteria.

V. CASE STUDIES Two case studies are presented in this chapter:

1. 2.

Westchester County, New York High Solids Centrifuge Dewatering Facility Yonkers Joint Treatment Plant Onondaga County, New York Belt Filter Press Dewatering Facility - Syracuse Metropolitan Treatment Plant

The Yonkers Joint Treatment Plant (YJTP) dewatering facility in Westchester County is a 60 d ~ y ton per day system constructed in 199 1 to replace ocean disposal. This facility processes biosolids produced by the 120 mgd Y JTP and several smaller County treatment plants. Figure 3- 13 is a schematic of the biosolids processing system at YJTP. Primary solids are gravity thickened, and secondary solids are hckened by flotation. Both solids streams are anaerobically digested and blended in

l!% Prlmuy Bloeollde

Sonde /

1

Cmtrlfugam

Gravity

Secondary Bloaollda

Storm* Lomdlng Hoppore

Figure 3-13 BlOSOLlDS TREATMENTSCHEMATIC YONKERSJOINTTREATMENTPLANT WESTCHESTERCOUNTY N.Y.

3-14

ONEOF SIX HIGH-SOLIDSCENTRIFUGES,YONKERS JOINT TREATMENT PLANT, WESTCHESTER CO.,NEW YORK

159

Conditioning and Dewatering

Lime ind Kln Dlul

Belt Filter Stormgo

Anieroblc Digesters

Gravity Thickeners

Ylxeri Pump. Digested Blosollds Pump8

Thkksned Elosolids PurnDs

DrVho and

Figure 3-15 BlOSOLlDS TREATMENT SCHEMATIC SYRACUSE METROPOLITAN TREATMENT PLANT ONONDAGA COUNTY N.Y.

TABLE 3-3

SYRACUSE METROPOLITANTREATMENT PLANT DEWATERING FACILITY 0 :RATING PARAMETERS [6] Cationic Polymer Six Belt Filter Presses 2.37 (1994 Average)

Feed Solids Concentration

40 Waste Activated Secondary Solids (% of Total)

60

I

Emulsion (1690 Secodyne) 35.5 lbdton Dry Solids, as delivered 35% active Dewatered Solids Concentration conditioning Cost

($/Gton)

20.8 (1994 Average) $29 (1 995) ~~

~

Solids Concentration

0.35%

BOD

640 mg/L

Nitrogen

316mglL

Quantity

1.5 mgd

~~

~

II

162

f

Conditioning and Dewatering

163

storage tanks. Stored biosolids are pumped to a battery of four Sharples DS 706 solid bowl, continuous feed, scroll-type high solids centrifuges, shown in Figure 3- 14. Dewatered biosolids drop by gravity into a battery of four Schwing KSP-25V twin cylinder, hydraulically driven, reciprocating dewatered solids piston pumps. The d e w a t a d solids pumps discharge to storage hoppers. Storage hoppers discharge by gravity to a truck loading bay. Dewatered biosolids are currently trucked to a landfill stabilization facility, followed by beneficial use. The long-term plan will incorporate trucking to an N-Vir& facility for additional treatment and ultimate beneficial reuse. Table 3-3 lists the key parameters of the Yonkers Dewatenng Facility. The average feed rate to each press is 85 gallons per minute at 2% solids concentration. The BFPs are fed with Robbins & Myers I60 HSI Moyno progressive cavity pumps (Figure 3-17) from a holding tank. Dewatered biosolids are conveyed by a serpentine conveyor to an N-Viro@ stabilization process (Figure 3-18). Stabilized biosolids are applied to agricultural land surrounding the Syracuse, New York area.

REFERENCES

1. 2. 3.

4.

5. 6.

7.

8.

Sludge Conditioning,Manual of Practice No. FD- 14, Water Pollution Control Federation, 1988. W. Strumm and J. J. Morgan, Aquatic Cheniistw, John Wiley & Sons, Inc., 1970, pp. 447,503-507. J. J. Cunnkigham, President, Cunningham Environmental Support, Inc., personal communication with R. 1. Kukenberger, Blasland, Bouck & Lee, Inc., April 1995. S. K. Dentel, M. M. Abu-Orf, and N. J. Griskowitz, Guidaance Manual for Polynier Selectiori in Wastewater Treattiient Platits, Project 9 I -1SP-5, Water Environment Research Foundation, 1993. Operationatidhfaitiretiance of Sludge Dewatevitig System, Manual of Practice No. OM-8, Water Pollution Control Federation, 1987. J. J. Saraceni, Onondaga County Department of Drainage and Sanitation, Memorandum to Blasland, Bouck & Lee,Inc., Subject: metro sludge dewatering facility operating data, and personal communication with R. J. Kukenberger, April 1995. S. K. Dentel and M. M. Abu-Orf, WEW: Full-scale evaluation of available sludge conditioning control technologies, #AC943805, presented at the Water Environment Federation 67th Annual Conference & Exposition, Chicago, Illinois, October 14- 19, 1994. Andritz PolyScan, Automated control system for sludge dewatering operations, manufacturer's literature, Andritz Ruthner, Inc., Arlington, Tesas, 1994.

164 9.

10.

11. 12.

13.

14.

15.

16.

17. 18. 19. 20. 21. 22.

Kukenberger S. Benitez, A. Rodnguez, and A. Suarez, Optimization technique for sewage sludge conditioning with polymer and skeleton builders, Water Resources, 28:10, pp. 2067-2073 (1994). Characteristics of thermally treated sewage sludge, Technical Bulletin 2302-T, Zimpro Inc., Rothschild, Wisconsin, December 1978. Alfa Laval Thermal, Spiral heat exchangers save up to 25% on sludge disposal costs, manufacturer's literature, Alfa Laval Thermal Inc., Richmond, Virginia. Blasland & Bouck Engineers, P.C., 201 Facilities Plan Anienclrnent, Madison Metropolitan Sewerage District, Volume I1 - Technical Memoranda, February 1991. Design ofhlunicipal Wastewater Treatment Plants, Volume 11: Chapters 13-20, Manual of Practice No. 8, Water Environment Federation and Manual and Report on Engineering Practice No. 76, American Society of Civil Engineers, 1992, pp. 1 148-1250. E. A. Retter and R. Schilp, Solid-bowl centrifuges for wastewater sludge treatment, presented at the Filtech Europa 93 Conference, Karlsruhe, Germany, October 19, 1993, reprinted in Filtvafionci! Separalion, June 1994. A. Davarinejad and N. Voutchkov, Comparison of high solids centrifuge performancefor digested and undigested solids dewatering, Proceedings of WEF Specialty Cotlference, June 1994, Chapter 7, pp. 47-56. The Rotary Press, A new solution to all your dewatering problems, manufacturer's literature, Les Industries Fournier Inc., Black Lake, Quebec, Canada. Andritz, A Dewatering Profile, manufacturer's literature, Andritz Ruthner, Inc., Arlington, Texas. C. S. Martel, Fundamentals of sludge dewatering in freezing beds, Water-Science Technology, 28: 1, p. 29 (1 993). L. Craig and B. Sheker, In the bag: a small flows option, Operations Forum, March 1995, pp. 22-25. Odor and Corrosion Control in Sanitary Sewerage System and Treatment Plants, Design Manual, 625/1-85/018, USEPA, October 1985. R. W. Frachetti and R. J. Kukenberger, Biofilters for odor control, Operations Forum, 1 1 :4 (1 994). T. Lauro, Deputy Director, Wastewater Treatment, Westchester County Department of Environmental Facilities, personal communication with R. J. Kukenberger, Blasland, Bouck & Lee, Inc., April 1995.

Digestion Kenneth J. Snow Corporate Environmental Engineering Inc. Worcester, Massachusetts

I. BIOSOLIDS DIGESTION

A. Introduction

Digestion typically refers to biological digestion of biosolids, either aerobic or anaerobic. Digestion of solids generated from wastewater treatment processes has been employed as a means of stabilization, volume reduction and pathogen reduction for many years. The primary objectives of stabilizing biosolids in the recent past was dependent on the regulations which governed the method of ultimate disposal such as IandfX, ocean dumping, incineration, or land application. Of these disposal options, land application has typically been considered more of a beneficial use than the other optionsbut the old regulations, 40 CFR Part 257, did not provide a clear definition of beneficial use. Options for the method of disposal are decreasing. Landfills are filling up and permitting of new landfills has become very expensive. Several states have banned landfilling of sludges and Congress has banned ocean dumping. In addition, implementation of advanced treatment of wastewater over recent years has generated even more biosolids. As a result, the current objectives for many municipalities, for digestion and other methods of biosolids treatment, is to accomplish beneficial use as defined by the 40 CFR 503 regulations, published February 1993.

165

166

Snow

Biosolids generated fi-om municipal wastewater treatment plants typically originate fiom primary clarification and " s e c o n w biological processes such as trickling filters, rotating biological contactors, and activated sludge. Grease, skimmjng and grit are usually disposed of separately and are not processed with the other biosolids. In addition, many municipal wastewater treatment plants accept residential septage via tanker trucks. Septage is introduced at a variety of locations in the treatment process train such as influent headworks, gravity thickeners, or digesters. Chemical addition to the wastewater process and methods of tertiary treatment contributes to the volume and varying characteristics of biosolids. Alum addition for phosphorus removal, polymer addition for e n h a n d solids separation and activated carbon addition for removal of a variety of substances are examples. The biosolids are often thickened prior to digestion. Digester configurations have been fairly standard over the past fifty years. Anaerobic digesters are typically two-stage concrete cylindrical covered tanks with conical bottoms. The biosolids are normally pumped into the first stage tank,where the contents are mixed and most of the digestion occurs. The displaced solids overflow into the second stage where the solids are thickened via settling and are then removed for disposal. The supernatant is generally directed back to the wastewater process for furthertreatment Aerobic digesters are generally open top tanks equipped with diffused or surface mechanical aerators. The shape of the tank is usually dictated by the selected method of mixing andor aeration. They are often equipped with some means of decanting to provide some degree of thickening. Again, the supernatant is usually directed back to the wastewater treatment process. Fairly recent improvements (in this country) to digestion processes are generally directed toward increasing efficiency, reducing maintenance, and meeting the new 503 Regulation pathogen and vector attraction reduction criteria. The improvements include: Egg-shaped anaerobic digesters which have improved efficiency and sigtufhntly reduced maintenance primarily by modifying the reactor codiguration. Autothermal Thermophlic Aerobic Digestion (ATAD) which is more efficient than conventional aerobic digestion and increases the degree of pathogen destruction. Pre-stage ATAD, which is used upstream of anaerobic digesters to accomplish pathogen destruction to meet Class A 503 regulation criteria, is typically retrofitted into existing plants employing anaerobic hgesters.

In addition, a pasteurization step can be added to the above digestion processes which can further enhance pathogen reduction and, in some cases, produce a higher class bimlids end product. Pasteurization is not considered biological digestion and discussion will be limited on this process.

167

Digestion

For new digester installations,process selection depends on ultimate use (Class) of the biosolids (dependent on marketability), volume of biosolids to be processed and economics. When reb-ofitting the digestion process of an existing treatment plant, the existing equipment, structures and process configuration must be taken into consideration to take economic advantage of previous investments.

B. Process Fundamentals 1. Aerobic Digestion Bacteria growth in a batch reactor can be represented by Figure 4-1. The aerobic digestion of biosolids originating from biological processes such as activated sludge is primarily endogenous, i.e., microorganisms consuming their own protoplasm. This is represented by the "death" phase portion of the growth curve. When biosolids from primary settling are introduced, the organic material is first oxidized resulting in the growth of new cells represented by the "growth" phase of Figure 4-1. The microorganisms are oxidized to water, carbon dioxide, and ammonia in accordance with: C,H,NO,(Cells)+SO, = 5C0,+2H20+NH,.

I

Stationary phase A

I

Time FIG. 4-1 BACTERIA GROWTH CURVE.

Snow

a. Mesophilic Aerobic Digestion

In conventional (mesophilic) aerobic digestion, the ammonia is further oxidized to nitrate. A maximum of about 75 percent of the microorganism cell can be oxidized as the remainder of the cell is inert material or organics which cannot be biologically degraded. The operation of a conventional aerobic digester is fairly simple provided equipment design and layout is not a hindrance. The operator needs to maintain sufficientdissolved oxygen (typically no less than 1 - 2 mgA throughout the reactor), mixing, and residence time. In addition, decanting or drawing off the supernatant after 30 to 60 minutes of settling will provide a thicker end product which will reduce power andor chemical requirements for subsequent treatment such as dewatering. Scum removal, foam control and odors or treatment of off gas may also need to be addressed depending on equipment configuration. Dissolved oxygen can be monitored using a portable meter with a probe which can be lowered into the reactor. Adjusting blower or surface aerator speed can be used to minimize power consumption. Adequate mixing can be penodically checked by simply probing the tank bottom to feel for solids which have been deposited or use of a velocity meter which can be lowered into the reactor. A velocity of 2 to 3 feet per second is usually adequate. Often the aeration equipment is also used to provide mixing. Depending on reactor configuration, either mixing or dissolved oxygen concentration will govern equipment operation. Laboratory tests such as the percent of volatile reduction, specific oxygen uptake rate, and biological testing, may be used to determine the required residence time. The actual monitoring and testing requirements will be based on compliance with the 503 regulations and the ultimate disposal method. Decanting, if the process is so equipped, to thicken the biosolids is usually conducted prior to andor after the digestion period. Decanting is usually used when a batch process scheme is employed. The method of decanting is dictated by equipment configuration. Thickening following digestion, as a separate process, is also common. b. Theimophilic Aerobic Dipestion Autothermal thermophilic aerobic digestion (ATAD) is aerobic digestion under thermophlic conditions (4OoCto SOOC). The term autothermal is used to describe this technology because no outside heat source is required. The organic decomposition duing digestion releases heat and maintains the thermophilic operating temperatures (typically 55'C), provided the reactor has adequate insulation. The ATAD process has several benefits over conventional aerobic digestion [I]:

Digestion

169

hgh biosolids treatment rate. Decreased reactor volume. More effective pathogen reduction. More effective vector attraction reduction. Higher volatile solidsreduction resulting in reduced downstream equipment sizes. The ATAD process can effectively produce biosolids that exceed EPA Class A pathogen reduction criteria. It can also consistently meet the specific oxygen uptake rate and volatile solids reduction U.S. EPA 503 vector attraction reduction requirements. Thermophilic Aerobic Digestion is a Process to Further Reduce Pathogens (PFRP), as defined in the U.S. EPA 503 regulations. The ATAD process can meet the PFRP definition [ 11. ATAD digestion systems are typically two-stage processes with mixing, aeration and foam control equipment. Single stage systems can provide similar volatile solids destruction but cannot destroy pathogens to the same degree. Figure 4-2 illustrates a typical ATAD system configuration. Pre- and post- thickening is usually employed. The Fuchs system (Germany) is the most common system and design and operational parameters discussed below are generally based on this process configuration. Typical design parameters are listed in Table 4-1 [2].

EXHAUST AIR

BlOSOLlDS

t

1

1

/

TO THICKENER/ DEWATERING

THICKENER/STORACE

ATAD RUCTORS

SOUDS STORACE

FIG. 4-2 TYPICAL ATAD CONFIGURATION.

Key design and process requirements include biosolids feed at 4% to 6% solids with a minimum of 2.5% volatile suspended solids. Biosolids at less than 4% solids may not allow sufficient heat generation to achieve thermophilic conditions. Biosolids at greater than 6% may prevent adequate mixing or oxygen transfer. Heat

170

Snow

p h t i o n is typically between 14,200 and 14,600kjkg 0, and oxygen requirements are 1.42 kg O@g volatile suspended solids. The thermophilic temperature range inhibits nitrificationand the associated oxygen demand. Temperatures should be kept below 65OC to prevent resolublizationof organics [11.

Parameter

Feed Total Solids Feed Volatile Solids Detention Time Temperature

Range

4%-6% 2.5% min.

5 6 days 35-50°C Reactor 1 50-65°C Reactor 2

Air Input/m’ Reactor Volume

4 m’h

The reactors typically are conical with flat bottoms.If grit is not removed in an upstream process, it may accumulate in the reactor and it may be necessaq to provide a conical bottom or other means to facilitate cleaning or prevent deposition. The method of operation of the ATAD system dictates performance, especially with respect to pathogen destruction, and establishes sizing criteria for biosolids feed pumps and gravity flow lines. Biosolids are fed in batch once per day. The solids feed volume, typically ane third of a single reactor volume, should be delivered in less than 1 hour.This allows about 23 hours per day of undisturbed digestion needed to attain high pathogen destruction. The sequence of batch feeding is as follows: s r r

Stop aerationhmag and transfer digested solids fiom the second reactor to the storage/thickenertank. Transfer solids fiom the first to the second reactor. Transfer feed solids into first reactor and start aeratiodrnixing.

The above sequence prevents parually treated biosolids fiom reinoculating the final stabilized product. The ATAD process generates foam during digestion. It has been speculated that foam provides some insulating value, improves oxygen utilization, and enhances

Digestion

171

biological activity. Cuyent designs provide for foam control as opposed to elimination. The foam bubbles are reduced in size, using foam cutters, which provides a denser foam. A freeboard of 0.5 to 1.0 m in the reactors should be allowed for foam. Thickening of the digested biosolids is generally desirable prior to transport for ultimate reuse. It should be noted that the sludge must be cooled (possibly in a storage tank) prior to using gravity thickening since thermal convection currents result in poor performance. Normally, some storage of the digested biosolids will be needed to accommodate removal and disposal fi-equencies. Uncovered tanks equipped with mixers but with no aeration will not result in objectionable odors if mixing occurs daily for at least an hour and ifthe biosolids are fully stabilized. Reactivation of pathogens or reinfection has not been a problem using the above equipment and procedures. Experience has shown that some odors are generated from the ATAD process and may, depending on receptors, require treatment. Odors will be more objectionable should the process be overloaded resulting in incomplete stabilization or if storage tanks are not mixed daily as previously mentioned. In general, aerobic digestion can, depending on design and equipment selection, have the following advantages over anaerobic: Impact to the wastewater treatment process is reduced due to lower strength @OD) supernatant recycle. Resulting biosolids dewater better. Less potential for odors and safety hazard due to explosive gases. Disadvantages: Does not produce methane which is a useful by-product. Supplying oxygen generally results in higher power cost.

2. Anaerobic Digestion Anaerobic digestion can be described as a multistage process where microorganisms break down various types of organics into simpler organics which are further converted by other types of microorganisms into even simpler organic compounds and CO,, KO,C Q , and Q S . In the first stage, the biosolids, which are made up of complex organics, lipids, l i p n s , proteins, and cellulose, are broken down into organic acids, alcohols, ammonia and carbon dioxide. The first stage products are broken down into hydrogen, carbon dioxide, and low molecular weight organic acids during the second stage. The microorganisms responsible for this conversion are referred to as “acid formers.” During the third stage, acetate, carbon dioxide and hydrogen are converted to methane and carbon dioxide and water. Two types of “methane-forming” bacteria are responsible for this third stage; the first converts carbon dioxide and

172

Snow

hydrogen to methane (CH,) while the second type converts acetate to methane and carbon dioxide. Refer to Figure 4-3 for a schematic description. The thrd stage is generally considered to be rate limiting since methane-forming bacteria grow more slowly than the acid formers. In addition, the methane formers are also more sensitive to pH. The pH must range between 6 and 8, otherwise, un-ionized volatile acids (below pH 6 ) or un-ionized ammonia (above pH 8) is toxic to methane formers. The operation of a typical two stage anaerobic digester is somewhat more complicated than an aerobic digester. A number of parameters must be monitored:

COMPLEX ORGANICS LIPIDS

LIGNINS PROTElNS CELLULOSE

ORGANlC ACIDS ALCOHOLS AMMONIA CARBON DIOXIDE

1 ~

HYDROGEN CARBON DIOXIDE ORGANIC ACIDS ACETATE

I CH,COOH acetate

HO

FIG. 4-3 BlOSOLlDS CONVERSION IN ANAEROBIC DIGESTION.

Digestion

-

173

Alkalinity PH Temperature Volatile solids loading Gas production Volatile acid concentration

The methane-foiming bacteria are very sensitive to pH and will only h v e

between pH 6.5 and pH 8.5. Therefore, operational monitoring focuses on providing the appropriate environment to keep these bacteria viable. Overloading with feed biosolids is a typical problem with anaerobic digesters. When the feed rate of volatile solids in the incoming sludge increases, the acid forming bacteria grow rapidly and produce more acids than the methane forming bacteria can digest. As a result, the excess acids lower the pH to the point where the methane formers can not function and the process stops. To help ensure the pH will remain in the proper range, alkalinity is also monitored. Generally, a range of between 2,000 to 3,500 mg/l CaCO, is acceptable. In conjunction with pH and alkalinity, volatile acid concentration should be monitored. Typically, the range of volatile acids is from 50 to 300 mg/l for well digested sludge. The optimum value of the above parameters which provide for the most efficient digestion will vary with different types of sludges. Therefore, the operator must monitor and track the results to determine the optimum range for each parameter. C. Equipment Review 1. Aerobic Digestion Equipment used for aerobic digestion of biosolids must be able to provide thorough mixing and sufficient dissolved oxygen. Tank geomehy, aeratodmixer location, and suspended solids concentration are factors which affect oxygen transfer and mixing efficiencies. Scum removal, foam control, decanting of supernatant and off gas control must also be considered. Conventional Aerobic Digestion Conventional aerobic digestion equipment designs have included a variety of tank geometries. It is usually dependent on the type of mixing and aeration equipment selected. Conventional aerobic digesters are often open top rectangular concrete tanks, sometimes with sloped bottoms to facilitate solids removal and cleaning, into which biosolids are pumped. The liquid level in the digester varies as biosolids are introduced or removed.

174

Snow

Mixing and aeration equipment can be designed as independent devices to accomplish mixing or aeration, as a single piece of equipment to accomplish both mixing and aeration or various combinations of both. All mixing and aeration equipment both mix and aerate but may be designed with a primary function as one or the other. There are many types of aeration/ mixing devices available. Equipment designed primarily for mixing would include Turbine or impeller type mounted on a vertical shaft driven by a surface mounted motor. Pumps which draw liquid from one part of the reactor and discharge to another part in a manner to promote circulation. Equipment designed primarily for aeration in conventional aerobic digesters include mechanical surface aerators and diffused aerators. For diffised aeration, an external blower delivers air to a diffiser located near the bottom of the digester. Positive displacement blowers are often used when the liquid level in the digester is designed to rise and fall. Diffusers are categorized by the orifice size which detemiines the size of bubble produced; fine bubble or course bubble. Course bubble d i h s e r s are nomially used since the high concentrations of solids tends to clog the fine bubble orifices more rapidly. Diffisers can be mounted on a "swing" arm pipe header to allow lifting out of the digester or can be mounted to the digester floor. As air is blown through the diffiser, small bubbles form providing a large aidliquid interface to promote oxygen diffusion and transfer. The action of the air rising also induces mixing. Mixing can be enhanced by strategic location of diffusers to develop circulation currents. The blowers can have variable speed dnves or two speed motors to optimize energy consumption. Mechanical surface aerators are usually constructed of a partially submerged impeller driven by a vertical shaft connected to an electric motor. The degree of impeller submergence is usually critical so the liquid level in the digester must be controlled or fixed when the aerator is rigidly mounted. As an option, the surface aerator can be mounted on floats allowing it to rise and fall with the liquid level. As the impeller rotates, the liquid is "sprayed" radially outward. The turbulence and formation of small liquid droplets cause a large airfliquid interface which promotes oxygen transfer. Depth of submergence, often controlled by adjustable weirs on the digester outlet, and variable or dual speed motors allows control of power usage. Equipment designed to provide both mixing and aeration include: Static aerator Mechanical aspirator Venturi aspirator

Digestion

175

Static aerators are rigidly mounted, usually in a grid pattern, across the floor of the digester. Static aerators are usually cylindrical tube-shaped, vertically mounted and air is introduced into the bottom to the tube. A series of baffles and the current induced by the rising air draws the liquid into the base of the tube, mixes it, and discharges from the top. U x i n g and aeration are controlled by adjusting air flow to the static mixers. Mechanical aspirator type mixeiherators are manufactured in a variety of configurations and are selected bascd on digester geometry, location within the digester and the size and shape of the effective mixing zone. Generally, the equipment consists of an impeller mounted on a hollow shalt which rotates at high speed. The rotation ofthe impeller creates low pressure which draws in (aspirates) air. The venturi type mixerherator employs a pump which directs the liquid through a venturi device. Air is drawn in to localized low pressure, and the discharge stream is directed such that the force of the stream promotes mixing. The discharge can be submerged or discharge above the liquid level, depending on the application. Conventional aerobic digesters typically use plant water sprayed through a series of nozzles across the top of the liquid. The spray breaks up the foam but has the disadvantage of adding "dilution" water to the biosolids. Decanting is typically conducted by shutting off the mixing and aeration equipment and allowing the solids to settle. The relatively clear supernatant is then drawn oE. Telescopic valves, submersible pumps that can be raised and lowered into the liquid, and multiple draw off lines at various elevations along the tank wall are some of the more common configurations used to decant and thus thicken biosolids before or after digestion. In some cases the digesters are covered and the off gas is directed to some type of treatment such as intake to blowers providing air to aeration basins, scrubbers, or activated carbon canisters. Off gas treatment is usually to control odors. Autothermal Thermonhilic Aerobic Digestion ATAD digesters are relatively new and equipment types in use are limited. The ATAD reactor installations are usually insulated cylindrical aboveground steel tanks which are fully enclosed. Steel is cumently less expensive than concrete but retrofitting existing concrete tanks may be desirable. The efficiency of aeration and mixing equipment is of particular concein in the thermophilic process in order to minimize the heat loss from the reactor through air exhaust. The Fuchs ATAD system, which is one of the more common, uses the aspirator type mixer/aerator described above. The CBI Walker, Inc. system employs the ventun type mixedaerator with exteinal liquid recirculation. The biosolids are pumped from the top third of the reactor through a venturi where air is drawn in and discharged back into the reactor near the bottom. This system also has the ability to

176

Snow

recirculate air from within the reactor head space to the venturi aspirator which can be used to slow down the biological oxidation should temperatures become too great. Foam is generated in ATAD reactors. As previously discussed, foam control, not elimination, is desirable. Foam cutters are typically set at a specific height above the liquid level. An impeller, which can be mounted horizontally or vertically, rotates and the blades “cut”the foam, reducing the bubble size, making it more dense and less voluminous. Various foam control, mixing and aeration equipment are available from several manufacturers, some of which are proprietary. Babcock, CBI Walker, Fuchs, Limus, and Thieme are some of the manufacturers. Odor control equipment to treat the off gas from the reactors will, in many situations, be needed. Depending on state regulations, air peimits may be required. 2. Anaerobic Digestion Anaerobic digestion must take place in reactors which are sealed from the atmosphere. Covered cylindrical concrete tanks have been the most frequently designed. More recently, egg-shaped digesters are being designed and installed and offer improved efficiency and reduced maintenance. The digester equipment must provide for mixing, scum and grit removal, foam control, off gas handling and heating. Conventional cvlindrical dipesters employ a variety of covers to seal the contents from the atmosphere which maintains anaerobic conditions, collects useful gas, minimizes odors,provides insulation, and prevents explosive mixtures of air and gas from forming. Covers are either floating or fixed. Floating covers offer several advantages; prevention of the formation of excess pressure or vacuum or drawing of air into the digester during transfer of digester contents and, in some instances, minimizes scum/foam accumulation because the cover rests directly on the liquid surface. There are a variety of cover types available. Conventional anaerobic digesters are equipped with a variety of mixing devices including gas recirculation and mechanical mixing methods [3]. The major gas recirculation mixing systems include: sequentially discharged lances floor mounted diffusers draft tubes bubble guns Each requires some type of compressor. Gas recirculation usually promotes foaming problems. Mechanical mixing typically consists of a motor, drive shaft and propeller combination or a recirculating pump. Propeller mixers can be mounted at various locations throughout the digester but most are used with draft tubes to direct flow. The recirculating pump mixer withdraws biosolids from one or more locations,

Digestion

177

sometimes through a draft tube, and discharges through nozzles at the digester wall. The nozzles direct the flow to induce a circulation pattern. Convention digesteis are not typically equipped with grit, scum or foam removal devices. Mixing is usually relied upon to prevent accumulation. Many digesters experience problems as a result and must be cleaned every 3 to 8 years [3]. Anaerobic digesters generate methane which can be used as fuel to provide heat to maintain proper operating temperatures of the digester contents andor building heat. External hot water heat exchangers are frequently used to heat the digester contents. The gas is commonly used to fire hot water boilers or as a primary fuel for reciprocating internal combustion engines. Gas collection equipment includes flame traps, pressure relief valves, check valves, waste gas burners, accumulators and compressors. k - s h a D e d digesters (ESD) have eliminated the shaip transition along the digester wall and reduced the liquid surface area at the top. The ESD offers a number of advantages over conventional digester configurations which are primarily a result of the modified shape and combination of mixing methods: Mixing, provided by gas injection, propeller or pump in combination with a central draft tube, is very effective in concert with the shape of the EDS. The mixing energy is lower which results in operating cost reduction. Combinations of these mixing methods can be employed. The thorough mixing improves digestion efficiency by better substrate/microorganism contact and elimination of "dead" zones. Grit accumulation is eliminated by the shape of the ESD and thorough mix systems. Eliminating the cleaning of grit, the lost volume due to accumulated grit, and the associated cost and down time improves efficiency and economics. Scum/foam accumulation can be effectively controlled as a result of the limited liquid suface and operation of the central draft tube mixer in the down mode. In addition, a scum chute at the top of the digester allows removal of accumulated floatables if needed. Eficiency and economics are improved for the same reasons as stated for grit accumulation. Generally, the footprint is smaller and there is usable space below the digester. Figure 4-4 illustrates a ESD in schematic form which is based on the CBI Walker, Inc. design. The mixing system shown utilizes a jet pump with a.draft tube assembly. The valves and heat exchanger are not shown for simplicity. The jet pump can discharge into either the top or the bottom of the draft tube. This promotes blending of either floating or settled materials with the remainder of the digester contents. The jet pump directs the motive fluid into one end of the draft tube which draws in additional liquid. This mixes liquid which is taken from different locations within the digester, resulting in a more homogeneous mixture. This pump system eliminates moving parts from within the digester. In place of the jet pump, a

178

Snow

W

DIGESTER

1

I

COLLECTION

SCUM FUNNEL

\

I

-D DIGESTER

VESSEL

\ DIGESTED BlOSOLlDS

1

\

1

FEED

BlOSOLlDS

FIG. 4-4 EGG SHAPED DIGESTER.

M TUBE

179

Digestion

mechanical mixer (propeller) can be located near the top portion of the draft tube. This mixer also mixes the contents fiom top to bottom. Compressed gas is not used in either configuration which minimizes foaming problems.

D. Economics of Digestion One of the primary factors in selecting a digestion process is the ultimate disposal or use of the biosolids, i.e., production of Class A or Class B end products. This must be determined first so the appropriate methods of digestion can be compared on an economic basis. Conventional aerobic digestimcannot reliably meet Class B pathogen and vector attraction reduction criteria when digesting primary and secondafybiosolids at typical retention times [7]. In some cases, achieving Class A or B end products is not requued, yet it still may be economical to do so as illustratedby the study conducted by the Grand Chute-MenashaWest Sewerage Commission [5]. In 1993, the Grand Chute-MenashaWest Sewerage Commission conducted a cost comparison between conventional anaerobic digestion with gas mixing and ATAD. The upstream wastewater process included primary cl&ers followed by activated sludge at an average design flow of 5.2 MGD. The schematic arrangement fop the two digestion process is shown in Figures 4-5 and 4-6 located in the following section The cost for the digestion process, excluding thickening, dewatering, and odor control, are as shown below.

CAPITAL AnnualO&M Power Labor Parts & Supplies Digester Heat TOTAL 0&M

CONVENTIONAL ANAEROBIC

ATAD

$2,455,000

$1,270,000

$ $ $

5,000 52,500 15,000

$

15,000

$

87,500

$ $

60,Ooo

35,000 15,000 $ $ 110,Ooo $

To meet Class A standards, a comparison between ATAD and ATAD followed by anaerobic ESD can be made. The cost has been found to be within 10% for facilities treating five to eight MGD. The ATAD followed by anaerobicESD will be more economical at larger facilities.

180

Snow

For Class B product, the cost to digest biosolids from wastewater facilities treating between three to five million gallons per day (MGD) is about the same for ATAD and anaerobic ESD. Digestion costs at smaller facilities will generally be lower using the ATAD process. This is primarily due to capital cost of some equipment, such as gas monitoring and safety, in anaerobic systems which change very little with changes in digester size. Life cycle costs are very important when conducting a comparison between processes. Labor cost associated with operation are about the same for all three processes. However, energy costs are about half the cost for systems which utilize anaerobic digestion. The reduced energy cost combined with the energy production potential makes the anaerobic digestion process more attractive for the larger facilities. Utilization of the ATAD system with anaerobic digestion hrther enhances the energy producing characteristics of the anaerobic system. Capital cost comparison between methods of digestion presented below will be limited to the more advanced processes, i.e., autothermal thermophilic aerobic digestion (ATAD), anaerobic digestion utilizing Egg-Shaped Digesters (ESD), and aerobic thermophilic followed by anaerobic ESD. Biosolids characteristics and volume generated from a typical municipal secondary treatment facility will be a common basis of comparison. The capital cost information on the following pages has been provided by CBI Walker for their AutoThermm (ATAD), ESD, and AeroThennTM(ATAD followed by ESD) systems [S]. In developing the three different designs and associated capital cost, a new facility is assumed. Each of the costs are based on the same design information and assume a site in the Midwest with open shop construction forces for the field erection. The cost estimates provided for the three designs were developed by comparing information from recent (1995) projects with an accuracy of about 10% to 15%. Other factors which have an impact on the cost estimates are the site location, time of year in the field, final design of the system and its components, system layout and the existing facility. Regardless of the system preference, one should consider life cycle costs when malung a final selection of a system. The ATAD ( AutoThermTM)system is the least capital cost for the design example. However, as an aerobic digestion process, the ATAD system is an energy consuming process. The ESD system is an anaerobic process and therefore is an energy producing process. While the ATADESD (AeroTheimm) system includes an aerobic process, it is utilized with anaerobic digestion to produce a two stage digestion process. This results in a net energy production similar to the anaerobic digestion process.

Digestion

181

ATAD DESIGN AND CAPITAL COST MIDWEST WWTP APRIL 1995 PROCESS DESIGN INFORMATION

A.

PLANT DESIGN SUMMARY: Plant Design Flow Rate -

2.0 MGD (Current)

4.5MGD (Future)

B.

Sludge Design Flow Rate -

8,100 GPD (Current) 18,300 GPD (Future)

Design Loading -

3400 #/Day Solids Loading (Current) 7600 #/Day Solids Loading (Future) 5% Feed (Assumed Design) 75% Volatile Solids

Present Facility -

Activated Sludge Facility 45% Primary 55% Secondary No Existing Sludge Treatment System

ATAD DESIGN SUMMARY: Reactor Design Reactor Detention Air Injection Mxing Requirements Heating Requirements Energy Requirements Sludge Storage Detention Time Energy Requirements -

3 @ 39,600 Gals. Capacity, Each 6.5 Days @ Future Design Flow Aspiration Through Venturi Effect External Pump Recirculation External Heat Not Required 150 k W l 0 0 0 Gals. (Raw Feed Sludge) 1 Vessel @ 370,000 Gals. 20 Days @ Future Design Flow 5 k W l 0 0 0 Gals. (Raw Sludge Feed)

182

Snow

ATAD CAPITAL COST ESTIMATE MIDWEST WWTP APRIL 1995

1.

System Effective Footprint

FT2

50' x 120' 2.

Site Work:

3.

Control Building Excavatiodl3ackfYl Concrete Foundation

Tankage:

4.

$ 475,000.00

$ 880,000.00

Vessels Insulation System Stair Tower Access Walkway & Platforms

MechanicalElectrical Process Equipment (Installed):

-

$1,150,000.00

Mixing System Air Injection Pumps & Valves Process Piping Instrumentation & Control Process Engineering Start-up Service

50,000.00

5.

Vent Air Odor Control System (Installed)

$

6.

Miscellaneous

$ 245,000.00

TOTAL COSTS

$2,800,000.00

183

Digestion

ESD DESIGN AND CAPITAL COST MIDWEST WWTP APRIL 1995

PROCESS DESIGN INFORMATION

A.

B.

PLANT DESIGN SUMMARY: Plant Design Flow Rate -

2.0 MGD (Cuirent) 4.5 MGD (Future)

Sludge Design Flow Rate -

8,100 GPD (Current) 18,300 GPD (Future)

Design Loading -

3400 #ma:<Solids Loading (Current) 7600 #R?ySolids Loading (Future) 5% Feed (Assumed Design) 75% Volatile Solids

Present Facility -

Activated Sludge Facility 45% Primary 55% Secondary No Existing Sludge Treatment System

ESD DESIGN SUMMARY: Digester Design Digester Detention Mixing Design Heating Requirements Energy Requirements Gas Production Methane Content Gasholder Sludge Storage Detention Time Energy Requirements -

1 @ 4 10,000 Gals. Capacity Vessel 22 Days @ Future Design Flow External Pump Recirculation 432,OO BTU/HR 15 LWH/lOOO Gals. (Raw Sludge Feed) 15 FT’/#VS Detroyed 60% Methane 500 FT3 1 Vessel @ 370,000 Gals. 20 Days @ Future Design Flow 5 k W l 0 0 0 Gals. (Raw Feed Sludge)

Snow

ESD CAPlTAL COST ESTIMATE MIDWESTWWTP APRIL 1995 1.

System Effective Footprint

4950

FTZ

45'X110' 2.

Site Work:

$

580,000.00

ControlBuilding Excavation5acldill Concrete Foundation 3.

0

4.

$1,540,000.00

Tankage: Vessels Insulationsystem StairTower Access Walkway & Platforms

MechanicaVElectrical Process Equipment (Installed):

$ 720,000.00

Mixing system Heating System pumps & valves Process Piping Gas Safely Equipment Instrumentation & Control Process Engineering start-up service 5.

Gas Storage (Installed) 500 FT' Capacity

$ 40,000.00

6.

Miscellaneous

$

TOTAL COSTS

280,000.00

$3,160,000.00

185

Digestion

ATAD/ESD DESIGN AND CAPITAL COST MIDWEST WWTP APRIL 1995 PROCESS DESIGN INFORMATION

A.

PLANT DESIGN SUMMARY: 2.0 MGD (Current) Plant Design Flow Rate 4.5 MGD (Future) 8,100 GPD (Current) Sludge Design Flow Rate 18,300 GPD (Future) 3400 #/Day Solids Loading (Current) Design Loading 7600 #/Day Solids Loading (Future) 5% Feed (Assumed Design) 75% Volatile Solids Activated Sludge Facility Present Facility 45% Primary 55% Secondary No Existing Sludge Treatment System

B.

ATAD DESIGN SUMMARY: 1 @ 19,800 Gals. Capacity Reactor Design 26 Hours @ Future Design Flow Reactor Detention Aspiration Through Venturi Effect Air Injection External Pump Recirculation Mixing Requirements Heating Requirements 360,000 BTU/HR Energy Requirements 20 k W 1 0 0 0 Gals. (Raw Sludge Feed)

C.

ESD DESIGN SUMMARY: Digester Design 1 @ 278,000 Gals. Capacity Vessel Digester Detention 15 Days @ Future Design Flow Mixing Design External Pump Recirculation Heating Requirements Included in the ATAD Design Summary Energy Requirements 10 k W l 0 0 0 Gals. (Raw Sludge Feed) Gas Production 15 FT31#VSDetroyed 65% Methane Methane Content Gasholder 500 FT’ Sludge Storage 1 Vessel @ 370,000 Gals. Detention Time 20 Days @ Future Design Flow Energy Requirements 5 k W 1 0 0 0 Gals. (Raw Feed Sludge)

Snow

186 ATAD/ESD CAPITAL COST ESTIMATE MIDWEST WWTP APRIL 1995

1.

System Effective Footprint

6ooo

FT'

5UX120' Site Work: Control Building 4 ExcavationBacMill Concrete Foundation

$ 530,000.00

3.

Tankage: ATAD Vessels Anaerobic DigesterIStorage Vessels Insulation System Stair Tower Access Walkway & Platforms

$1,460,000.00

4.

MechanicalElectrical Process Equipment (Installed): Mixing System 4 Air Injection System 4 Heating System Pumps & Valves 4 Process Piping Gas Safety Equipment Instrumentation & Control 4 Process Engineering Start-up Service

2.

4

4

$ 950,000.00

4

4

5.

Vent Air Odor Control System

$ 20,000.00

6.

Gas Storage (Installed) 500 FT3 Capacity

$ 40,000.00

7.

Miscellaneous

$ 300,000.00

TOTAL COSTS

%3,300,000.00

187

Digestion When developing the life cycle costs, the following should be addressed: 1)

Manpower Both systems will require about the same manpower for operation of the process.

2)

Energy

3)

Solids The volatile solids reduction for an aerobic digestion system Reduction has been characterized to be within a 40%-50% range. Anaerobic digestion is typically expected to be within a 45%-55% range.

The ATAD (AutoTherm) system utilizes about 150 k W 1 0 0 0 gals. of raw sludge feed per day. The ESD system utilizes about 15 k W 1 0 0 0 gals. of raw sludge feed per day. The ATADESD system utilizes about 20 k W 1 0 0 0 gals. of raw sludge per day. Both the ESD and the ATADESD system will produce about 15 fi’ of gasAb volatile solids destroyed.

Many times there will be other factors to consider which may be more or less tangible for a given project. Some things worth considering include the type of wastewater facility and location. Northern climates with low volatile waste sludge may require additional heat for the ATAD system. This will drive up the capital cost and make the process less attractive for a given project. Warm climates may also be more favorable to the ATAD system due to little need for heat in the winter and no desire to use cogen as a means for reducing electrical costs.

11.

CASE STUDIES

Several studies have been conducted which compare conventional anaerobic digestion to the more recent aerobic and anaerobic thermophilic processes. More specifically, the ATAD and thermophilic anaerobic processes were evaluated during two studies summarized below [5][6]. 1. ATAD vs. Convenlional Anaerobic Digestion

The Grand Chute Menasha West Wastewater Treatment Facility, Wisconsin, a medium sized facility with projected flows of 5.2 MGD for the year 2010, was constructed in 1983 and was in need of expansion and upgrade. Specific biosolids process for pathogen and volatile solids reduction were the focus of concern and could not be met using the existing gravity thickenindaerobic digestion process. The planned liquid process train includes fine screens (0.635 cm opening), vortex type grit removal, primary clarification, activated sludge, ferric chloride addition to the secondary process for phosphorous removal and UV disinfection. The

ia8

Snow

projected biosolids from the primary clarifiers and waste activated sludge process assume 50% suspended solids removal as the primary clarifier, 0.6 kgkg BOD removed in the activated sludge process and a 30 mg/l dosage of fenic chloride.

Primary Solids

Secondary Solids

Ferric Chloride

Total

Annual Avg. (kdday)

1,954

1,549

1,077

4,580

Max. Month (kg/day1

2,344

1,796

1,322

5,462

PRIMARY & THICKENED WASTE ACT. SLUDGE

L

METHANE (MIXING

To OWATERING

SUPERNATANT

CAS)

+

FIG. 4-5 PROCESS FLOW DIAGRAM, ANAEROBIC DIGESTION.

189

Digestion

DIGEST€ rn iuir

I

1

/

SLUDGE WITHDRAWAL

AERATION & MIXING

Fig.4-6 PROCESS FLOW DIAGRAM, AUTO-THERMAL AEROBIC DIGESTION.

ATAD

Parameter

Present worth cost

23% less

Safety

No explosive gases

--

Mechanical Simplicity

Anaerobic

-GeneratesMethane

Has floating covers, and gas handlinglsafety equipment

Odors

None if well operated .

Has odors

Tank Size

6 day hydraulic

25 day hydraulic retention time

retention time Quallfy for PFRP

I Yes

I No

The process flow schematicswhich are the basis for the comparison between anaerobic digestion and ATAD are shown in Figures 4-5 and 4-6.

I

190

Snow

In this study, the ATAD process for solids treatment was selected over conventional anaerobic digestion. The ATAD process was evaluated to be less expensive for both capital and operation and maintenance, safer, mechanically simpler, less odorous, smaller with respect to tank sizes and able to qualify for PFRP. Table 4-4 presents the advantages and disadvantages. 2 . Thennophilic Anaerobic vs. Conventional Anaerobic Digestion The Greater Vancouver Regional District conducted full-scale thermophilic anaerobic digestion testing to determine if a Class A sludge, based on pathogen density criteria could be achieved. This experiment was conducted at the Lion's Gate Wastewater Treatment Plant which is a 20.5 MGD average diy weather flow with primary treatment only. The primtxy sludge feed into the digesters during the experiment ranged between 130 and 160 m3/day at an estimated 5% solids. Two of the existing four digesters, operated in series in contmuous flow mode, were used for digestion. These two digesters provide a retention time of 20 days each. The remaining two digesters were used for storage of digested sludge prior to dewatering. The experiment intended to operate only the first digester at the thermophilic temperature of 55" C at a retention time of 20 days. This far exceeded the Part 503 process requirement for Class A combination of time and temperature:

D = 50,070,000/10 0 ~ 1 4 L where: D = Retention Time in Days t = Temperature in Degrees C Using the above formula, only 1 .O days retention time is required at 55°C. No provisions were made to cool the biosolids between the first and second digester. Consequently, the temperature of the second digester rose to 47°C which resulted in poor pei-foimance. To correct this problem, the temperature of the second digester was raised to 55°C. The process then stabilized and concentrations of fecal colifoim in the second stage digester were sustained below Class A levels of 1,000 MPN per gram total solids for several months. The fecal coliform concentrations in the first stage digester were not below Class A levels. Several important conclusions andor observations can be identified based on this study:

Digestion

0.

*.

0.

0.

191

Single stage continuous feed thermophilic (55°C) anaerobic digestion of primary sludge with a retention time of 20 days was unable to meet Class A pathogen levels. Two stage continuous feed thermophilic (55°C) anaerobic digestion of primary sludge with a retention time of 20 days in each stage is able to meet Class A pathogen levels. Modification of existing mesophilic continuous feed digesters to achieve thennophilic operation may be economically attractive depending on marketing options for biosolids. Si@icant odors from subsequent dewatering process are likely. Odor control should be planned.

REFERENCES 1.

2.

3. 4.

5.

6.

7.

8.

M. Poeckes, D. Oerke, M. Maxwell, S. Rogowski, and I-I. Kelly, Evaluation of the Autotheimal Thermophilic Aerobic Digestion (ATAD) Biosolids Stabilization Process to Meet the New EPA 503 Requirements, Proceedings of the WEF Specially Coilference, Washington D.C., June, 1994. U. S. EPA, Autotheimal Thermophilic Aerobic Digestion of Municipal Wastewater Sludge: Environmental Regulations and Technology, EPA 62511090/007, September 1990. Anaerobic Sludge Digestion, IYEFMmiral of Practice No. 16. G. Balog, K. L. Nelson, and M. A. Patel, Back River Egg-Shaped Digesters: Technology-Transfer Adapts German Approach to Wastewater Solids Digestion, Proceedings of the WEF Specially Conj&wice, Washington D.C., June, 1994. T.E. Vik & J.R. G k , Evaluation of the Cost Effectiveness of the Auto Thermal Aerobic Digestion Process, Proceedings of WEF 66111 ilriti~alCorlferetice, 1993. G. Volpe, B. Rabinowitz, C. Peddie & S. Krugel, Class A (High Grade) Sludge Process Design for the Greater Vancouver Kegional District Annacis Island Wastewater Treatment Plant, Proceedings of IVEF 66th Atitiiral Corference, 1993. G. Shmp, J. Sandino and R. ShamsKhoizani, Peifoimance of Aerobic Digestion in Meeting the 503 Requirements for Pathogen and Vector Attraction Reduction, Proceedirigs of die WEF Specialty Cotlferetice, Washington D.C., June, 1994. J. Currie, P.E., CBI Walker, Inc., Aurora, Illinois, Personal conversation (Unpublished).

This Page Intentionally Left Blank

Cornposting Lewis M. Naylor Black and Veatch Gaithersburg, Maryland Too often scientists r e b on their sophisticated instrumentation and computers and Linus Pauling not on their unique abiliv to reason.

L INTRODUCTION Biological decompontionis as ancient as the existence of organic matter on the earth. With division of the firstcell and germination of the first seed, amino acids making up proteins and glucose links in cellulosic chains initiated the first step toward chemical and biological breakdown, returning to the earth nutrients and energy for other life forms. This natural process of cleansing the surface of the earth enabled life as we h o w it to exist today. It is the first step in the process of composting. Without the natural decomposition of dead organic materials, recycling of the nutrients and the biochemical energy in the carbon contained in organic matter would be slowed dramatically. While periodic fires could rapidly release such energy and nutrients, organic matter production vastly exceeds the scale of the land area for decomposltimof organic matter involved in fires. Without this natural decomposition process, dead organic matter would accumulate at the rate of about 15 feet of depth each millennium. Human beings have walked the earth for a million years or so,and plant life has existed much longer. Thus,just since humans began to walk the surface of the earth, dead organic matter in the absence of decomposition would have accumulated to a depth of nearly 3 miles. Cornposting is a natural process of aerobic, thermophilic microbiological de@m of organic wastes into a stabilized, useful product that is free of odors and pathogens, w ill not attract rodents and insects, and can be used beneficially for horticultural and agricultural purposes. During this process the waste is stabilized biologically. This means that the readily biodegradable components of a mixture

193

194

Naylor

of organic materials being composted are stable or resistant to change biologically. Wastes are broken down sufficiently that further decomposition is very slow and does not cause problems during use of the material. Such problems could include odors, attraction of flies, or regrowth of pathogens. Virtually all carbonaceous, biodegradable materials are compostable under suitable environmental conditions. Such conditions are essentially those favorable for microbial growth: appropriate moisture content, an aerobic environment, and biologically available carbon for energy and nitrogen for growth and reproduction of the microbial population. The blended feedstock to be composted should contain about 40% dry solids (60% moisture), a balanced nutrient (nitrogen) and energy (biodegradable carbon) supply. The aerobic process should supply periodic mixing to improve the contact between food and microorganisms and increase the rate of stabilization. A. Growth of Composting in the United States

Beginning in the 1950’s and continuing through the 1960’s enthusiasm exceeded the reality of operations and the demand for mediocre quality compost in the United States. The chief problems were unrealistic economic expectations, poor compost quality, and process failure. Facilities started up with a notion that the value of the finished compost would pay for operations and capital cost of the system. Alas, neither the demand nor the price of the compost met expectations and nearly all facilities from that era failed, or at least hesitated severely. This rough start placed a gloomy shadow on composting as a viable, economic alternative to landfilling and combustion. However, in the last 10 years or so the industry has matured and composting is perceived as a legitimate, engineered technology with wide scale opportunities for recycling organic residuals. We can get a notion of the growth of the composting industry, chronicled annually since 1985 by BioCycle, Journal of Composting and Recycling. In December of each year BioCycle publishes a summary of composting activities in the United States. Data show that in the decade since 1985, the number of compost facilities in all stages of development has grown from 173 to 3 18 [7][8]. Of these facilities, the number in construction, desigdpermitting, planning/consideration, pilot operation, and those shut down has remained relatively constant. The remarkable growth has been in the number of operational facilities, their number having grown from 79 to 20 1. Of the operational facilities in the United States, the largest growth in terms of number of facilities is the aerated static pile, growing from 49 to 99. In terms of the percentage increase, the in-vessel facilities have grown by a factor of 15. Invessel type systems can be divided into vertical silo type reactors, agitated bed systems, tunnel reactors, and rotary drum units. In 1985, there were 3 operating invessel composting systems, all of the silo type, with an additional 11 silos or

Composfing

195

agitated bed facilities in some phase of construction or start-up. By 1994,45 invessel systems were operating or in construction. Of these were 26 agitated bed facilities, 15 silo type reactors, 3 rotary drums, and 5 tunnel reactors. Compost quality and marketing strategies have likewise improved markedly. The horticultural industry has shown a profound recognition of the value of compost as a soil amendment. Increasingly compost is specified by landscapers and architects. University research has more fully defined the value of compost to impart disease resistance for container crops, field plantings, and turf grass. Market surveys for compost suggest two quite different markets: a high volume market (Figure 5-1) and a high dollar market (Figure 5-2) [ 151. The high volume market, estimated at nearly 900 million cu. yd. by The Composting Council, is based almost entirely on marketing compost to agricultural crops. In contrast, the high dollar value is based on marketing a high quality product supported by technical staff. Customers use directly or resell the compost in bulk, bags, or with a horticultural product such as turf grass sod or a shrub. Obviously, both markets are important. The market targeted for a specific composting program will dictate to a large extent the range of ingredients selected for the compostable blended feedstock and the post-processing required. 11. GOALS OF COMPOSTING

The overall goal of composting is to produce a high quality compost using a technology that is protective of the environment. When the focus is on the production of a quality product, the facility will remain open, the process will succeed and the compost will be readily marketable. This goal is so important that it precedes all others as the foundation of a successful project. Basically, there are three requirements of quality compost. A. Chemical Quality

Compost produced must meet the proscribed standards of the environmental regulatory community. The primary standards are those of state and federal regulators. For compost containing biosolids, these would include standards promulgated by the U.S. EnvironmentalProtection Agency as part of the U.S. EPA Part 503 Rule, discussed at length in Chapter 2 [24]. Those standards which were developed through a risk-based approach assure that the compost would be safe enough to eat, though few would look forward to such an opportunity. The standard for lead content of biosolids products, for example, is based on a child consuming minute quantities of biosolids daily for several years. Thus, achieving the regulated chemical standards assures not only the manufacturer of compliance, but also the ultimate compost user of a virtually risk-free product.

Naylor

196

Landfdl cover

Reclamation Sod production

Silviculture

Agriculture

Fig. 5-1 The potential high volume market for composted materials. [15]

lo

1

xv;

Landscapen

Topsoil

Bagsed

Container

Field

Fig. 5-2 The potcntial high value market for compost. [ 151

Composfing

197

B. Biological Quality While the presence of certain metals is of concern in compost used without restriction by the general public, the presence of pathogens may be of more practical significance. Toxic effects of most metals become apparent only after many years of exposure through ingestion. In contrast, the presence of biological pathogens such as bacteria, viruses, parasites and protozoa can be immediate and uncomfortable. Achieving conditions which assure complete pathogen destruction during composting is essential to meeting not only the regulatory requirements, but also to protecting the most sensitive of compost users. An infective dose of pathogenic organisms over time is generally required for healthy persons, but certain sensitive individuals may respond differently. As an example, a mother and son bring home several bags of a composted biosolids product. The mother begins to work the compost into her flower bed, while the son using his toy dump truck faithfully delivers the compost to each appointed plant. All the while he is sucking on a piece of hard candy, which he allows absent mindedly to fall out of his mouth into the toy truck load of compost. The young lad glances surreptitiously at his mother and quickly picks up the piece of candy and pops it back into his mouth. Compost manufacturers need to consider this young lad as the person they are protecting as they monitor the conditions required to meet highest standard of pathogen reduction. C. Customer Requirements

The final standard the compost manufacturer must meet is the one established by the customer. For the most part, such standards are not reviewed by regulatory bodies, but they are equally important to marketing the finished compost. Customers have requirements that relate both to their perception of what compost ought to be, such as color, texture, and aroma, as well as how the compost performs in a growing medium, e.g. pH, salinity, and nutrient levels. The compost must meet such requirements, which will vary enormously between customers, or the compost will not be readily marketable. Thus, the three goals of compost quality are to meet chemical quality, biological quality, and client requirements. When these goals are achieved, the composting process will be effective and the compost will be readily marketable.

In. PROCESS FUNDAMENTALS Composting is a living process and proper care and feeding of the active microbial community is essential to good composting.

198

Naylor

A. Microbial Community

The microbial community does not consist of a single type or even a single group of organisms. Rather, many different types of bacteria and fungi play important roles in the decomposition of the organic matter. The conditions produced by one population of organisms establish the character of the food and the working environment of a subsequent group. The formation of compost is a natural, but extremely complex biochemical process in which cellulosic materials, proteins, fats, and carbohydrates are decomposed and transformed into a humus-like material. The vast and immensely diverse population of fungi, protozoa, and bacteria initiating this process grow, reproduce, and die as environmental conditions in the composting materials change. Cell contents and cell walls of these organisms are decomposed by successive generations of microbes, contributing to the humus content of finished compost, along with the non-degradable or slowly degradable fraction of the input feed-stock. This is the final stage in the production of the finished compost, a living fertilizer and a marketable product. Complex carbohydrates and proteins are degraded into simple sugars and amino acids by bacteria and fungi. Temperatures rise into the thermophilic range in the moist composting environment, cooking and softening organic matter particles and enabling physical break-down of the material. With physical degradation, new surfaces are exposed expanding the available food and supporting a larger microbial population. As the most easily degradable materials decompose, woody materials remain in predominance. These materials which consist of layers of cellulose bound by glue-like lignin are degraded by fungi. As the intense biological activity diminishes, temperatures gradually decrease and the composting materials begin to dry, forming conditions suitable for increasingly complex groups of fungi. While the composting process never grinds completely to a halt, the process is so slow that physical and chemical changes in the materials become noticeable only over months. The compost at this stage is ready to use, and with plant growth the recycling of nutrients and energy continues. Four general physiological groups of microorganisms are active within the temperature range found commonly in well-managed composting processes. As illustrated in Figure 5-3, these groups are psychrophiles, mesophiles, facultative thermophiles, and thermophiles. Psychrophiles grow or tolerate in temperatures found in a northern climate, -5 to 35°C. Mesophiles thrive on temperatures found in the tropics, about 15 to 45°C. Facultative thermophiles tolerate temperatures from room temperature, 25"C, to that of scalding water, 60°C. Finally, thermophilic organisms tolerate very high temperatures ranging from 45 to 75 "C, well into the range at which food can be cooked and milk pasteurized. In general, growth rate increases with temperature, as illustrated in Figure 5-4. However, the relationship is not linear within any one physiological group over its entire

Cornposting

199

temperature tolerance range. Each group can survive throughout its temperature tolerance range, but thrives within its preferred range. 80 70

60

'z

-$

50 40

E

Thermophiles

0)

s

0.

30

I-

Facultative thermophiles

20 10

Mesophiles

0 -1 0

Psychrophiles Physiological groups

FIG. 5-3 TEMPERATURE RANGE FOR GROWTH O F SEVERAL PHYSIOLOGICAL GROUPS OF ORGANISMS [Zl].

While growth rate of individual organisms increases with temperature, fewer individuals of the population thrive. As noted in Figure 5-5, the total number of cells produced by a physiological group diminishes as temperature increases. Within the mixed culture of the compost heap, all physicological groups are present. As each group's activity diminishes because of temperature sensitivity, the next most heat tolerant takes over the decomposition labor. At some point, however, even the thermophiles struggle with the heat and this heat stress begins to reduce the rate of the composting process. The overall objective of temperature control is to maintain a temperature that accommodates the maximum number of organisms with the greatest average biological activity. Similar to other living organisms, the composting microbial population has certain requirements for growth and reproduction. Metabolic activities of the organisms reflect environmental conditions as they vary from optimum to tolerable to intolerable.

200

Naylor

Regremion line: Rate, Ill,hr = 0.0438 + 0.016 x T.C

0.4

-2x

0

5

10

15

-

20

25

Temperature, .C

FIG. 5-4 INCREASE IN GROWTH RATE OF A PSYCHROPHILIC BAClLLUS SP. WITH TEMPERATURE [21].

psychrophllla Baclllus sp.

Cornposting

201

B. Environmental Conditions Conditions which a compost facility operator must pay close attention to are temperature, oxygen levels, and moisture of the cornposting materials. The optimum temperatures for composting are in the range of 35 to 65°C. Microorganisms that efficiently drive the composting process will be most active within this temperature range. As noted previously, mesophilic and thermophilic bacteria and fungi predominate at various stages within the composting process. The growth dynamics of celluloyticbacteria and fungi (those microbes which break down cellulose) during composting of organic waste is shown Figure 5-6. As the composting and curing process continues, more and more microbial activity must be directed at decomposition of the lignin and cellulose in the relatively nonbiodegradable woody products. Since fungi tend to be more proficient at lignin decomposition their population continues to increase, whereas the bacterial numbers decline.

V I

0

.

I

I

10

20

I

I

I

30

40

50

~~

Time, days FIG. 5-6 CHANGE IN THE NUMBERS OF CELLULOYTIC BACTERIAL AND FUNGI DURING THE COMPOSTING AND CURING PROCESS[6].

202

Naylor

Sufficient oxygen must be present in the composting mixture to sustain the needs of the aerobic bacterial population. Five to 10 percent oxygen will generally suffice for aerobes. However, measurement of oxygen concentrations within the bulk compost pile can give misleading results. Oxygen levels within the center of large, non-porous clumps may drop to zero even while reading 10% oxygen within the bulk pile. Simple forced air or suction aeration of a compost pile can be inadequate when the composting material tends to stick or clump together. Physical turning and abrasion to break up the clumps is generally beneficial to assure good oxygen penetration throughout the entire composting mass. Moisture levels in the composting materials must be maintained at about 40 to 50% to support the physiological systems of the compost microbes. Cell walls must be permeable to the flow of soluble nutrients into the cell by osmosis. Enzyme systems must be able to function properly. While complete desiccation can cause gradual decline and complete cessation of biological activity, reduction of moisture content in the compost will reduce the composting efficiency and restrict the active organisms to those more tolerant of low moisture. As with the case for temperature, excessively low or high moisture contents of a composting mixture can impair the composting process by forming conditions that favor the growth of organisms less efficient in the degradation process. For example, if the mix is excessively wet, poor oxygen penetration is likely because of voids being plugged with water. Formation of very low oxygen conditions can enhance the survival of acid-forming bacteria that can decrease the pH of the composting materials. The acids formed, termed volatile acids because they readily vaporize, possess an odor some find unpleasant. Such is the case for the formation of vinegar (acetic acid) and other volatile longer-chain,more odorous acids formed in a pile of fermenting apples.

C. Nutritional Considerations All living organisms have fundamental nutrient needs: carbon (C), nitrogen (N), phosphorus (P), sulfur ( S ) , trace nutrients, and minor amounts of vitamins. Water discussed above is likewise a nutrient solvent and carrier as well as an environmental requirement. The basic raw materials or feed-stock used and metabolized during composting are composed of cellulosic materials (polysaccharides), proteins (sources of nitrogen and sulfur), and sugars, fats and carbohydrates (energy sources). Polysaccharides are essentially polymers or long chains of glucose sub-units with the formula (-C6H,205-). Proteins are composed of various amino acids, also linked to each other in long chains. A common chemical form of an amino acid is (R-CH(NH,)-CO-NH-). Some of the amino acids also contain sulfur. These materials make up the diet of the composting organisms which must be balanced nutritionally for optimum composting.

Cornposting

203

Appropriate and balanced amounts of such nutrients are generally present in a heterogeneous composting mix containing material such as food waste, biosolids, source separated organic waste, leaves and yard waste. However, the ratio of certain nutrients, notably carbon and nitrogen, can be out of balance for certain types of mixes. Carbonaceous and nitrogenous compounds are important to living organisms as a source of energy (biodegradable carbonaceous compounds) and protein (biodegradable nitrogenous compounds) for growth and reproduction. During composting, these microbes metabolize 15 to 30 parts of carbon for each part of nitrogen. This is true for organisms as large as a dairy cow to those as small as the decomposer bacterial cell. While balancing a feed ratio for a Holstein may need to be more precise, the blended feedstock for composting facility must be adjusted for the relative amounts of carbon and nitrogen. Thus, a compost facility operator must consider himself to be a microbial nutritionist as well as manager of environmental conditions within the cornposting materials. The compost process manager should have access to a variety of materials to balance the nutrient and moisture content of the blended feed stock. In general, the ratio of biodegradable carbon to biodegradable nitrogen (the C:N ratio) in a blended compost feedstock is suggested to be about 20 to 40 for good composting. Organic wastes such as biosolids tend to be moist and nitrogen rich, relative to their content of carbon, with C:N ratios of typically 5 to 10. Materials such as sawdust used as amendments, in contrast, tend to be dry and carbon rich. Sawdust has a typical carbon to nitrogen ratio of about 200 or more. Since the blended feedstock should have a C:N ratio of about 20 to 40, mixing these two materials together balances the moisture content as well as the carbon to nitrogen ratio. The relatively wide, acceptable C:N ratio range is indicative of two things: 1, the enormous dietary flexibility within the heterogeneous microbial community (How many of us weigh out our protein, fiber, and energy components each day?), and 2, the dearth of research to enable understanding of the biodegradability of various feedstocks and the consequences of improving the precision of the optimum C:N range. Carbon is combined with organic matter, and can be estimated from the organic matter content because of the relative uniformity of organic matter composition. Organic matter is the principal component of the volatile solids of a material. An ingredient in a compostable mix consists of a liquid fraction, i.e. water, and a solid fraction or dry matter, as depicted in Figure 5-7. Most analyses express the composition of compostable materials as a percentage of dry matter. A proportion of the dry matter consists of minerals with the remainder making up organic matter which is combustible. This combustible fraction is considered to be volatile since it can be burned off, and is named volatile solids. The mineral residue, or ash, is often referred to as futed solids. Since carbon is a portion of the organic matter, percent carbon in a material can be estimated indirectly from its

204 organic mattercontent. A varietyofratioshavebeenused to estimate the proportion of carbon in organic matter.The values for percentage carbon insoil organic mattervary h m approximately to 58% estimate commonly used is about 55.6% of organic matter is carbon, based on data nearly ahalf century old Evaluating the nitrogen content of materialsis generally straightforward and accurate. Most feeds and forage laboratories, as well as environmental testing laboratories have provisions to test materials for total kjeldahl nitrogen ammonia, nitrate, and nitrite. The total kjeldahl nitrogen content includes organic nitrogen, that bond with organic matter, and ammonia. In general, the TKN does not include all of the nitrate and nitrite in a sample, and these anions must be analyzed separately, usuallyin the liquid phasefiom leaching a sample. However, in terms of composting process and feed stock control, nitrate and nitrite content is not very important. TKN concentrations in individual components range from 5,000 to 60,000 mg/Kg, whereas the concentration nitratehitrite combined is generally less than100 mg/Kg.

FIG. S-7 COMPOSITION OF THE SOLIDS IN A COMPOSTABLE

composting

205

The operator can evaluate the C:N ratio of his compostingmaterials reasonably well hthe weight and percentage composition and once the blended feedstock has begun to compost. Unforhmately, at this point adjustingthe mix is nearly impossible. As mixtumlow in carbon, e.g. C:N < 20,begin to compost, the compostingprocess is likely to be accompanied by the loss of excess nitrogen as ammonia because of the proportionally large amount of nitrogen. Conversely, mixtures deficient in nitrogen, e.g. C:N > 40, tend to compost slowly, and have poor heat generation. From an operator’s perspective, some ammonia aroma should be evident during composting and the mix should heat up well. However, if very high concentrationsof ammonia are released, causing eye irritation or breathing difliculty, then the operator should know that the C:N ratio is too low and adjust the proportion of biodegradable carbonaceous materials in subsequent batches. Experiencx is a pow& teacher of compost process managers.

N. SOLIDS AND THE COMPOSTINGPROCESS C c n n w g as a non-biological process is essentially a materials handling exercise. This includes managing moisture, particle size, biodegradability, and porosity. These properties control to a large extent the efficiency of gaseous exchange within the composting materials, heat output and temperature achieved. A. Types of Solids

Materials to be composted consist of dry matter and water. The dry matter, or dry solids @S), consists of organic matter and minerals. The organic matter is combustible and can be burned off or volatilized, leaving the mineral fraction or ash, Figure 5-7. The ash or fixed solids (FS) consist of minerals including calcium (Ca), magnesium (Mg), sodium (Na), iron (Fe), manganese (Mu), and trace metal compounds. The anion moiety of the metal compounds include carbonates (CO?), bicarbonates (HCO;), sulfates (SO:-1, phosphates (PO:- ), nitrates (NO; ), and other auions. The carbonates and bicarbonates help buffer the acidity and alkalinity of the cOmpOShng materialsby reacting reversibly with the carbon dioxide produced during the composting process. The combustible fraction or volatile solids (VS) is correlated highly with the organic matter cunteslf and reliably indicates the organic matter content of a material. A portion of the volatile solids is biodegradable (BVS). This ffaction can be further broken down into thosematerials that are readily biodegraded, e.g. biosolids and food waste, and those that are slowly biodegraded such as woody materials. For many operators, little clarity exists regarding biodegradability of

206

Naylor

carbonaceous materials and fibrous materials. In fact, even at the research level there is much to be learned and taught regarding biodegradability and factors influencing biodegradability of organic wastes. As a consequence, operators use their best judgment until corrected by experience. In general we can judge several high nitrogen, fairly biodegradablematerials, Table 5-1, to include biosolids, spring and early summer grass clippings, and certain fresh animal manures without bedding. Highly biodegradable, carbonaceous materials include shredded leaves, fruit and vegetable waste, and apple pomace. Two practical approaches to assessing the biodegradabilityof organic materials are to consider their digestibility by animals and humans and to recall their disappearance in garden compost piles or when worked into the soil and broken down by soil microorganismsand macroinvertebrates. Respirometry has been used to experimentally judge biodegradability. In this experimental method, a substance is allowed to biodegrade under known conditions of temperature and pressure, and either oxygen absorbed or carbon dioxide evolved is measured during a specific time period. The oxygen absorbed or carbon dioxide evolved is a function of the conversion of bound carbon to carbon dioxide.

B. Particle Size Particle size is one of the most important physical properties of the composting materials. Both the physical size and distribution of the particle sizes are important to porosity of a compostablemix. The more uniform in size the materials in a mix, the greater the porosity of the mix. Conversely, as the heterogeneity of particle sizes widens, the porosity decreases. Small particles fill the voids between larger particles, Figure 5-8, restricting the flow of water and gaseous exchange. Most composting technologies now include mixing of the composting mass because developing a uniform particle size is difficult to achieve under even the best of conditions for most composting facilities. Grinders and shredders, unless followed by screens to grade the product, produce a very heterogeneous material with coarse particles and chips to fine dust and grit. Regular mixing of a blended feedstock with a broad particle size range ameliorates some of the negative effects of low porosity by exposing the entire pile of material to the atmosphere. 1. Porosity

Reduced porosity can have a number of undesirable effects during the composting process. To maintain an aerobic composting environment, thorough exchange of oxygen and carbon dioxide must take place. As the porosity increases, exchange becomes more efficient. Heating and cooling are likewise important control parameters. In active compost piles, heat generated can produce temperatures too high for efficient composting. Air flowing through the composting materials

207

Cornposting

Uniform particle size

Heterogeneous particle sizes FIG. 5-8

UNIFORM PARTICLE SIZE PRODUCES GOOD POROSITY WITH LOTS OF VOIDS FOR EXCHANGE OF GASES AND WATER. HETEROGENEOUS PARTICLES FILL IN VOIDS, REDUCING POROSITY

Naylor TABLE51

DRY SOLIDS AND C:N RATIO IN BIODEGRADABLE ORGANIC MATERIALS DS, % C:N ratio Biodegradability. Drganic Material Resources

I

I

I

Apple pomace [20]

20 - 35

48

H

>75%

Fruit waste

10 - 25

43

H

775%

Vegetable waste

10 - 25

28

H

> 80%

Mixed food waste & paper

25 - 35

14

H

55 %

54

22

H

75 %

80 - 95

177

L

20 %

95

924

L

20%

Corrugated paper

90 - 95

427

L

20

Waxed milk cartons

90 - 95

560

L

Fruit and vegetable materials (a%. 1 Mixed paper Newsprint

w 20 w

209

Cornposting

DS, %

Organic Material Resources

C:N ratio Biodegradability'

90 - 95

300

L

20 %

Junk mail

95

226

L

20 %

Paper and paper products (avg.1

95

324

L

20 %

Paper food cartons

IBacteria

IC5H702N

I

Fungi

C 1OH 1706N

4.29 8.57

Wood

C295H4200186N

253.0

Grass

C23H380 17N

19.7

Garbage

C 16H2708N

13.7

Food wastes

C 18H26010N

15.4

Mixed paper

C266H4340210N

228.0

Yard wastes

C27H380 16N

23.1

Biosolids

ClOH1903N

8.57

IRefuse 1

IRefuse 2

IC64H104037N

IC99H148059N

I

I I

I Biodegradability, expressed as % of volatile solids, estimated. I

54.9 84.9

I

I

I

210

Naylor

will absorb moisture and heat energy, cooling and drying the material. If the air flow is restricted, cooling may be non-uniform leaving hot spots in the composting materials which could also become deficient in oxygen. Such hot, poorly aerated pockets are ripe for odor production when the compost is prepared for use. 2. Biodegradability

Particle size can influence gross biodegradability of a material. Composting is a surface phenomenon since micro-organismsare active only on surfaces. They have no way to access, for example, the interior of a wood chip. As the overall surface area of a given mass of waste increases, the quantity of food available for the microbes will increase. Hence, the overall rate of conversion of biodegradable material into biologically stable compost will increase. While the biodegradability of a material at the molecular level may not vary appreciably with particle size because the rate of biochemical transformations and enzymatic reactions at a specific temperature are fKed, a smaller particle size does expand the surface area for the microbial attack. An expanded surface area enables the support of a larger number of organisms each of which contributesto the overall biodegradation of the material. As an example, shredded leaves have a larger surface area and break down more rapidly than unshredded leaves because a greater surface area is exposed for degradation. Conceptually, we may think of the composting materials as spheres. It can be shown that the surface ( S ) to volume (V) ratio is equivalent to 3/r, where r is the radius of the sphere. As r decreases the S to V ratio increases. For example, a sphere with a radius of one cm. has a volume of about 4.2 cu cm and a surface area of about 12.6 sq cm. The surface to volume ratio will be 3 : 1. If this small sphere is divided into four equal spheres containing the same total volume as the initial sphere (4.2 cu cm) ,each will have a radius of about 0.63 cm, a total volume of 4.2 cu cm, but the total surface area will be 20 sq cm. The resulting surface to volume ratio increasesto 4.7. Now if that one cm. sphere is ground into a 100 tiny spheres, the surface area to volume ratio will be about 14, with the total surface area exposed for biological degradation also 14 times as great. This example also illustrates the importance of regular agitation which physically degrades lumps that may be present in the raw materials, reducing their size and increasing their total surface. Increasing the surface area cannot increase the metabolic rate of the microorganisms in terms of unit weight of material metabolized per organism per unit of time. The basic biochemistry does not change. However, the greater surface area exposed does increase the total amount of food available to the organisms so that greater numbers can be supported. Thus, the practical impact and advantage of reduced particle size is that the rate of composting in terms of tons per hour will be increased.

Composfing

211

V. PROCESS ENERGETICS

Living organisms convert food into energy for growth and synthesis of new cells. This process is not 100% efficient. As a consequence waste energy is produced. Within the composting process the waste energy appears as sensible heat and is the basis for warming the composting materials.

A. The Biological Fire The source of the heat energy within the composting process is the metabolic oxidation of organic matter into carbon dioxide and water. The heat generated by the intense biological activity of the organisms is the driving force of the compost processes. Conceptually,the composting process could be considered a bioLogical $re. Biodegradable volatile solids are the fuel for this biological fire. Oxygen is consumed, and metabolic water, carbon dioxide, heat energy, and compost are produced. The compost produced represents the materials that do not biodegrade within the time frame of the managed compost process.

Microbes Fuel (biodegradables) + 0, -> Wafer + CO, + Energy + Compost (slowly biodegradables) . . . . . . . . . . . . . . . (5-1) The heat energy may be lost by ventilation or radiation to the atmosphere, or it may be conserved within the composting materials. Whether lost or conserved, the heat energy warms the environment and control over the segment of the environment warmed is the basis for achieving temperature increases within the composting mass. B.

Heat and Temperature

Heat energy is the capacity to do work, such as warm the composting mass or evaporate water. Temperature is a sensory measurement. On a personal level, hot and cold are relative and judged differently from one person to the next. To increase our objectivity and eliminate personal bias, temperature is measured against a standard using a thermometer or other recording instrument. By way of illustration, temperature is comparable to the speed of an object. Following the same analogy, heat is similar to the momentum of the object: the speed times the weight of the object. A bird in flight and a moving truck may have the same speed, but the momentum of the two objects and their relative effects when they bump into a window of a house is enormously different. Thus, temperature is not a good basis on which to judge the heat energy possessed by an object. Heat energy is a function of not only the temperature of

212

Naylor

an object, but also its mass. For example, a bathtub full of warm water contains more heat energy than a cup of hot tea even though the temperature of the tea is much higher. Heat energy is quantified by: Heat energy = mass of an object x heat capacity x temperature . . . . . . . . . . . . . . . (5-2) In this equation, temperature is measured in degrees Celsius or Fahrenheit. Heat energy is measured in joules, calories or British Thermal Units. (The irony is that Americans are the last major country to continue to use the British Thermal Unit or BTU on a regular basis.) Heat capacity, or specific heat, of an object is the capability of that object to absorb or release heat as related to its temperature change. Specific heat is measured in units of calories per gram per degree Celsius. Water has a specific heat of one calorie per gram per degree Celsius, or one BTU per pound per degree Fahrenheit. That means that one calorie of heat energy will increase the temperature of one gram of water by one degree Celsius. Conversely, one gram of water cooling by one degree Celsius will release one calorie of heat energy. Not all materials have the same heat capacity. While water has a heat capacity of 1.00, wood has a heat capacity of about 0.4, and air 0.25. The dry matter of compost probably has a heat capacity of about 0.4. Based on the heat capacities of these three materials, a given amount of heat produced during the composting process will increase the temperature of the same weight of air the most, of the compost dry matter the next greatest, and of water the least. One calorie of heat would increase the temperature of one gram of air by four degrees Celsius:

Temperature change, A " C =

T,h 0 C =

heat energy, cal wt. of object, g x heat capacity, ca1lgl"C

1 cal = 1 g x 0.25 cal1gl"C

. . . . . . . . . . . . . (5-3)

As the proportion of water in the cornposting materials increases, i.e., the wetness of the compost increases, more and more energy will need to be generated to increase the temperature of the composting materials because of the greater

Cornposting

213

capacity of water to absorb heat energy. For a given amount of heat generated, a smaller temperature increase will be observed. As will be noted later in this Chapter, this has important implications for the compost process manager since he is interested in achieving specific minimum temperatures during the composting process. C.

Temperature Control

Temperatures in the thermophilic range are beneficial to the composting process. They increase the rate of conversion of biodegradable organic matter into biologically stable compost. Such temperatures assure rapid stabilization of the organic matter, and destruction of plant and animal pathogens and weed seeds. However, the temperature of the composting materials must be managed carefully to assure optimum environmental conditions for the microbial population. 1.

Feedstock Adjustments

It is evident that for a given weight of material, a larger amount of heat produced will result in a greater temperature rise. One question the compost process manager must consider is how can the character and composition of that weight of material be managed to result in the greatest temperature increase. Conditions influencing temperature change of composting materials include: a. b. c. d.

the heat production capability of the material, the ambient temperature surrounding the composting materials, the moisture content of the composting materials, and the heat losses from the composting materials to the external environment.

Heat production of the composting materials is, as we learned earlier, a function primarily of the biodegradability, and secondarily, of the particle size of the composting materials. Any change the process manager can make to improve the biodegradability of a blended compost feedstock, or of the composting materials, will enhance the heat output. For a specific material, a smaller particle size will generally improve the rate of conversion (composting rate) of the bulk material to carbon dioxide, water, and heat because of the larger surface area exposed for microbial attack. Changing the biodegradable character of the input ingredientsto the feedstock can also enhance heat output. Compost mixes rich in highly carbonaceous, slowly biodegradable sawdust tend to reduce heat output. In contrast, increasing the proportion of a highly biodegradable material such as food waste will increase the heat output. Using the illustration of diet, most of us know what contributes to our

rABLE 5-2

CHEMICAL PROPERTIES OF COMMON FOODS [25]

s3

'D

g. TABLE 5-2 (CONT.)

1

Element

Bibb Lettuce

Fried Liver

2.0% Milk

Salad Oil

Frozen Peas

Boiled Potato

Raw Spinach

White Sugar

I

(Q

g/100 g dry weight of food, or YOdry basis

Y

cn

216

Naylor

caloric intake and potential weight gain most. Certainly it is not high fiber vegetables such as celery or lettuce, Table 5-2. Rather it includes sugars, meats, fats and oils. These have a high digestibility and fats and oils have a high caloric density. This is not to imply that the process manager should add disproportionate amounts of fats and oils to the blended feedstock. It does suggest that one can use common sense in developing a blend of materials of diverse nature so as to enhance the overall biodegradability and heat output of a compostable mix. Temperature increases within a composting mass are also a function how the heat produced is absorbed by the individual components of the feedstock, and conserved or lost from the composting mass. Heat energy produced by the biological fire of the composting process is absorbed by the materials that make up the blended feedstock. Since temperatures within a well-managed composting mass will be in the range of 40 to 60"C, everything mixed into the pile will have to be brought to desired composting temperature, i.e. 55 to 60°C. This includes not just the solid ingredients, but also all of the air that is added to the mix to maintain aerobic conditions. During hot weather when ambient temperatures approach 30°C. However, as ambient temperatures drop, additional energy will be absorbed just to warm up the composting mass to the operating temperature. For cold weather composting, particularly in the northern climates where temperatures can fall to - 10 to -2O"C, considerably more care and thought must be given to blending the compost ingredients to maximize biodegradability and minimize extraneous heat losses. Thus, under these cold weather conditions, air used to ventilate the compost may have to be warmed by 50 to 80°C. The input ingredients will need to be warmed by 30 to 50°C. If the energy output from the composting process is unable to match these heat demands, temperatures mandated for pathogen destruction will not be achieved. The blend of materials is also important from the perspective of the proportion of water in the composting materials. As the proportion of water in the mix increases, the potential temperature rise will decrease because of the greater heat capacity of water relative to the compost dry matter. With a larger proportion of water, the ratio between the heat producing fraction (the biodegradable material) and the heat absorbing fraction (the water) decreases. As a result, during cold weather the compost process manager should develop mixes that are drier and more biodegradable than would be needed during warm weather.

2.

Facility Design Considerations

Heat loss and conservation is also related to the size of the composting mass and to the compost technology. In contrast to the desirability of a larger surface to volume ratio to enhance biodegradability,best heat conservation is obtained with the lowest surface to volume ratio. For example, in a small compost pile a portion

Composfing

217

of the heat energy will be radiated to the atmosphere and lost, reducing potential temperature increases in the composting materials. Such would be observed in many backyard compost piles which tend to be fairly small and not well-insulated fiom the outside temperature. Larger compost piles a have smaller, heat radiating surface area per unit of volume than do small piles. In a well-managed composting facility, heat energy will be conserved within larger compost piles. Compost within an enclosed composting container or vessel is engineered to minimize heat loss. Pile depth is managed and container walls can be insulated to optimize temperature increases. D.

Aeration

Air circulation through the composting materials is an important process control feature. Aeration cools the vigorously composting material, removes excessive moisture to produce a product that is readily screened and transported, and exchanges oxygen and carbon dioxide to maintain aerobic conditions within the composting materials. 1.

Temperature Control

Air is circulated through the composting material to maintain the optimum temperature environment for the microbial composting community. While sustaining temperatures of 55 to 70°C for several days is needed for assured pathogen control, such high temperatures are not necessarily advantageous to the overall composting process efficiency. Extended high temperatures can destroy beneficial organisms that are essential to the cornposting processes. Air can be circulated through the composting materials naturally, by mechanical turning, or by using mechanical forced air or suction. As the air flows through the composting materials, heat is absorbed by the cooler air and carried from the composting materials decreasing the temperature. Best temperature control is effected using a temperature feedback mechanism controlling a mechanical aeration system. In this approach the temperature of the composting materials is sensed electronically. The observed temperature is compared with a set-point using a microprocessor,and a blower is turned on if the observed temperature exceeds the set-point. The sensor monitors the compost temperature periodically and when the temperature drops below the set-point, the blower is turned off. The cycling of blowers off and on can produce a very uniform temperature within the vicinity of the temperature sensor. However, if the porosity of the composting material is poor, air flow through the mix and, hence, temperature uniformity may be non-uniform. Non-uniform air flow can result in hot spots that are not cooled, anaerobic conditions, or overly cool locations. Regular agitation or turning along with mechanical aeration will improve the

218

Naylor

homogeneity of the material and keep the compost mix open and porous to good air movement. 2.

Moisture Removal

Aeration not only sweeps heat from the composting materials, but also removes water vapor. Removal of water vapor has two effects. First, the removal of the water vapor begins the compost drying process. Second, the evaporation of water and removal of the vapor cools the compost. The heat that develops during composting warms the composting materials, but it also evaporates water. This water is derived from two sources: 1, moisture in the feed-stock (biosolids, wood chips, sawdust, etc.) and 2, metabolic water from microbial oxidation of the feed-stock carbon or volatile solids. The incoming feed-stock generally contains about 40 to 45% dry matter, or about 55 to 60% moisture. During composting, aeration sweeps out a portion of this moisture which has vaporized, leaving the drier product with only 40 to 50% moisture. This amounts to removal of a substantial quantity of water in vapor form. For example, for each 100 tons of raw material composted daily, its weight after composting will have diminished to about 67 tons, assuming no changes in the weight of the dry matter. Thus, 33 tons of water will be evaporated and removed from the composting materials each day. On an annual basis, this 33 tons surges to nearly 12,000 tons of water, equivalent to approximately 3,000,000 gallons of liquid water. Ignoring the management of this water vapor, and potentially corrosive liquid water condensate, has caused untold pain and suffering among compost process managers and engineers. The second source of water removed by aeration is metabolic water which is derived from the composting materials. If we consider, as an example, that the fuel for the biological fire is the cellulose monomer glucose, we may estimate the amount of water that will be produced by the metabolic processes of the composting organisms by degradation of volatile solids. From the following equation, and the associated formula weights of the materials:

C,H,,O, 180

+ 6 O2 --> 192

6 C02 + 6 H 2 0 + Energy . . . . . . . . (5-4) 264 108

From this equation, we can show that for every 180 pounds of cellulose or glucose monomer (volatile solids) lost, approximately 108 pounds of water is produced. Scientistshave suggested that about 15 to 25% of the dry weight of the composting materials is lost or biologically burned during the composting process. Each day the volatile solids loss from a 100 wet ton load at 40% solids could amount to 4.7 tons, with the consequent release of about 3.3 tons of metabolic

Cornposting

219

water. On an annual basis, the formation of this water amounts to about 1,200 tons, or approximately 292,000 gallons. A portion of this water will also be evaporated by the aeration process. While aeration cools the composting materials as the air absorbs heat, evaporation of water is the major cooling factor. As noted earlier, air has a heat capacity of 0.25 caVg/"C, as contrasted with water which is 1.00 caVg/"C. Cool air moving through the hot compost will increase in temperature by 4°C for every calorie of heat energy absorbed. However, evaporation of water absorbs substantially larger quantities of heat energy. To evaporate one gram of water at 100°C will require 540 calories of energy. At a temperature of 70", about 560 cal/g is required to evaporate a gram of water, over ten times more than the 50 calories that would be required, for example, to increase the temperature from 20°C to 70°C. As is evident, evaporation of water is the greatest heat absorbingkooling force within the composting material. While adequate aeration is required to control temperature and remove water vapor, over-aeration of the composting materials can prove detrimental to the composting process. Over-aeration can remove so much heat and moisture that the composting materials cool excessively,reducing the metabolic rate of the microbes and, as a consequence, the composting rate. Over-aeration can also dry the composting materials excessively so that metabolic activities of the microbes is reduced, or in some cases, stopped altogether due to lack of moisture in the materials. Thus, careful control of aeration, and process monitoring is important to good composting. 3 . Oxygenation The third hnction of aeration is to maintain adequate oxygen and, hence, an aerobic environment in the composting materials. Compost aeration (the process air) is provided through the aeration piping located typically under the composting beds. The composting materials are also aerated during agitation and turning, exposing each particle of the composting materials to the atmosphere. Maintaining an aerobic environment for the compost organisms assures that these organisms operate at peak efficiency and prevents most odors associated with anaerobic conditions from forming.

VI. PREPARING A BLENDED FEEDSTOCK

As the compost process manager considers ingredient options for a blended feedstock, the physical and chemical characteristicsof each potential material must be known. There must also be some notion of how each material will influence the overall character of the compostable mix. Of the most important characteristics,

Naylor

220

percent dry solids (%DS) is probably the number one consideration, followed closely by the carbon to nitrogen ratio (C:N). Then would follow biodegradability, porosity, and finally a host of characteristics related more to marketing a wellcomposted material than operating the compost process successfully. Since materials used to adjust such properties amend the physical and chemical character of the blended feedstock, they are often called amendments. Thus, the process manager must know his ingredients,how they impact the composting process, and

ABLE 5-3 AMENDMENT SELECTION Selection of an amendment to modify and enhance physical and chemical properties of blended feedstock for optimum composting is a key consideration in the production of high quality, readily marketable compost. Factors to consider during selection of an amendment include: 1. The compost customer's requirements 2. Dryness of the amendment 3. Organic matter content 4. Bulkdensity Porosity and particle size distribution 5. pH, nitrogen concentration and other chemical considerations 6. 7. Color and texture

Guidelines for Optimum Performance Parameters Dry matter, YO

I

Minimum 50

I

Maximum 95

I

Typical 55 to 75

Organic matter, YO

50

95

60 to 80

Bulk density, lb/cu ft

8

30

15 to 30

PH

4

7.5 (except ash)

6 to 7

0.01

2

0.5 to 1.0

Mulch

Topdressing

General Use

112 to 2

1/16 to 1/4

114 to 1

dark brown

brown to black

brown to black

coarse

fine

medium

Nitrogen, %

Particle size, in Color Texture

Cornposting

221

how they will ultimately influence the marketability of the finished compost. Guidelines for selection of amendments are suggested in Table 5-3. A second, though no less important basis for developing a blended feedstock is that the compost process manager must be guided by the initial purpose for developing and operating the compost facility. Operational objectives for a composting facility range from being a solution to a waste management problem to manufacturing compost for an identified compost market. Such objectives may set fairly narrow constraints on the ingredient options by the process manager. For example, many compost facilities are developed to help solve a problem with biosolids, leaves, and yard wastes. For such facilities, the goal is to develop a blend that 1) composts all of the targeted wastes within a timely fashion, and 2) produces a marketable product. So long as the identified blend of wastes produces a readily compostable mixture the process manager’s daily activities are straightforward. However, when that blend of materials no longer produces a readily compostable mix under the prevailing conditions, then the operator will need to seek out and evaluate other ingredient options. A. Dry Solids and Porosity

Obtaining appropriate dry solids content of a blended feedstock with sufficient porosity is important biologically to give the composting microbes adequate moisture, and physically to enable gaseous exchange. Important physical properties of individual materials that influence dry matter content and porosity of a blended feedstock include dry matter content, moisture absorptivity, particle size, distribution of particle sizes, and bulk density. For the most part, a good rule is to develop a blended feedstock containing 40 to 50% dry solids. Agitated bed composting facilities will be able to handle materials at the lower end of the range while vertical silo-type systems and windrows will need to be at the higher end. Assuring adequate porosity is the main reason for the dry solids adjustment. Excessive porosity is not normally a problem so long as the correct moisture content and good biodegradability exists in the feedstock. Porosity is assured when voids between bulk particles such as wood chips are filled with air and not with water or small particles. Windrows rely on porosity within the blended feedstock to enable gaseous exchange, whereas porosity in agitated beds is derived in part from regular turning. 1, Increasing Dry Solids

Ingredient options for increasing the dry solids content are listed in Table 5-4, along with some comments on their use. The materials include both organic and mineral products, not all of which will be appropriate at each composting location. For example, use of shredded pallets as a source of dry matter and bulkiness may

222

Naylor

be economical in an area with a pallet manufacturing or recycling facility. In the mid-west where agriculture predominates,use of various agriculturalwastes or byproducts may be more economical or reliable. Wood fred power plants not uncommon in the northeast may make wood ash an important dry matter component in a blended feedstock in that area. Each locale must evaluate materials discarded within a reasonable hauling distance and consider opportunities to work such materials into a blended feedstock as beneficial ingredients to the process or the marketing and use of the finished compost. If the purpose of an amendment is to increase the dry matter content of a blended feedstock, usually the drier the material, the greater its value. As will be noted later, particle size and moisture absorptivity are also important because of their effect on porosity and absorption of free water.

Material

Comments on the Product

can be dusty, avoid nails in all woody shredded materials, very dry

Wood chips, shavings, woody construction waste

I

Shredded pallets

I

Kiln dried sawdust & green sawdust Shredded paper

I

dryness will vary, coarseness will improve porosity, some limitation on availability, leaves & grass are very biodegradable

Shredded brush & leaves, bark, plant fibers such as shredded hay

I

very dry, course finished product, coarseness will improve porosity

I

very dry & dusty, smaller quantities, moisture will vary widely, competition for farm use shred finely, very dry, moderately biodegradable ~~

dry, paper covering is an organic matter source, gypsum may help hold ammonia, presence of gypsum in compost a plus Wood ash

very fine material, may reduce porosity, very dry & dusty, carbon particles may absorb odors

Coal fly ash

not a food grade material, will require regulatory review, some restrictions on use, must pass TCLP test

I

223

Cornposting

TABLE 5-4 (CONT.)

I

~

Material

~

-1

Comments on the Product

Oat hulls, rick hulls

hulls are very stiff, low biodegradability & porosity

Grain milling & cereal waste

corn, wheat, oats fines, very dry & biodegradable

Wet grains

wet, very biodegradable

Poultry broiler bedding

nitrogen source, can be dry & coarse, good porosity

High sugar waste

very biodegradable, good energy, high C:N

I

-

Organic Wastes and Slurries Dewatered biosolids

waste activated, anaerobically & aerobically digested, air dried material, raw primary, dewatered septage, low C:N, biodegradable

Dewatered water treatment plant sludge

low volatile solids, may be wet, low biodegradability, presence of Al, Ca, or Fe

Dewatered milk wastes, processing wastes, brewery wastes, beverage filter cake residues

potentially very odorous, may contain diatomaceous earth, high biodegradability, good energy source

Paper mill fibrous residuals & biosolids

biodegradability varies, variable organic matter content, low nitrogen, check molybdenum

Coffee & tea wastes, fruit processing wastes

highly biodegradable, acidic, low nitrogen content

Meat packing wastes, hatchery wastes, fish processing wastes, egg breaking wastes

high nitrogen, highly biodegradable, high odor potential

From time to time, porosity needs may be less important than increasing the

dry solids content of a mix. In such cases, additional options are available. Several very dry materials that happen to be fairly fine are available and sometimes may be obtained at little or no cost. These would include sawdust, either kiln-dry or green, wood ash, ground dry wall (gypsum), and perhaps coal fly ash, if allowed by regulation. Some materials that may be available locally in selected areas include ground corn cobs, dust from grain storages, animal bedding such as wood shavings from horse stables, and shredded papers. Each of these has specific benefits, but also handling problems. For most of the very fine materials such as kiln dry sawdust, dustiness is a problem the operator must be prepared to contend

Naylor

with. For a few, such as grain dust or animal bedding, rapid biodegradability or odors may need to be considered. However, these problems are not insurmountable and the process manager should consider broadly all ingredient options available as tools to manage the composting process. 2 . Porosity

In general, bulky, woody materials, one-inch and smaller in diameter, make good amendmentsto increase dry solids content as well as porosity. Such materials are also called bulking agents for obvious reasons: they increase the bulkiness or porosity of a mix. Post-process screening can be used to remove and reuse the larger pieces, enabling the operator to get multiple-duty from the chips. This can be particularly important if the chips must be purchased, or their procurement cost, e.g. chipping, grading, and transport, is higher than their cost of simple disposal. However, in some cases the porosity lending materials were considered in the original design to be included in the fmished product as a means of their disposition and beneficial use. In such instances, the process manager must give additional thought on how the materials will impact the marketability of the fmished product. 3 . Particle Size

The particle size distribution of a dry amendment will influence the porosity of a mix. A uniform particle size will result in many voids between the particles, just like a stack-up pile of oranges. However, if there is a wide range of particle sizes, the smaller particles will plug the voids between the larger particles. While the mix may be drier, the porosity may not be appreciably improved. See Figure 5-8.

4. Bulk Density Bulk density of a material can be important in terms of the finished weight and transportation economics of the marketable compost, transportation cost of the amendment, ease of turning the composting mixture. Materials can vary from extremely light for Styrofoam beads sometimes added to the finished product for marketing purposes to relatively dense in the case of wet wood chips.

5 . Moisture Absorptivity Moisture absorptivity of a dry amendment may be an important consideration if one or more of the other ingredients contains some fiee water. Free water is water that will flow from a material by gravity. For example, a dropped, fresh, ripe watermelon which contains 90% water will manifest free water or juice, whereas a crushed potato that contains 80% water, most of which is inside cell walls (bound

Composting

225

water) has almost none. If the objective of adding a dry amendment is to absorb the free water within a mix, then the absorptivity of that material is important. Shredded rubber tires have from time to time been used as an amendment. Such material would surely provide porosity, but would have no impact on free water. In contrast, shredded paper would absorb a great deal of water, but would not increase porosity appreciably.

6 . Stickiness Stickiness is a troublesome property than can be adjusted by increasing the dry solids content of a mix. This problem tends to assert itself with fluid wastes that have been dewatered using polymers. Under such conditions the overly moist mix will stick to equipment such as conveyors and loaders, increasing the weight load on the equipment and bearings. In this case a drier mix tends to be less sticky, turns easier, and keeps the mix reasonably porous to air. A good test of whether a mix is too wet or too sticky is to make a ball about the size of an apple with the mix. If the ball barely holds together, the dry solids content is about right. If the ball sticks together firmly, even if dropped on the composting floor, more dry matter is needed. 7 . Adding Moisture Generally we consider options for increasing the dry solids content of a compostable mix, but there are circumstances in which a reduction in dry solids, i.e., more moisture, is warranted. For example, biosolids are often used as a cocomposting ingredient with leaves or other dry organic residues. Where leaves and yard trimmings are the main focus, dewatered or even fluid biosolids could be considered the amendment. The water in the biosolids is an essential component in the blended feedstock. One could consider that in this case, the biosolids are added as a source of moisture and nitrogen. Dry solids adjustment may involve either increasing or decreasing the solids content of a blended feedstock, depending on the ingredients. However, the compost process manager must focus on producing quality compost and all that is implied in that objective. Initially the emphasis is on developing a compostable mix and managing the process successfully, but ultimately the customer’s requirements are primary. It is clear that these two requirements are intertwined and both the process and the customer must be satisfied.

B. Chemical Composition Chemical composition is of interest in a blended feedstock in order to judge the carbon to nitrogen ratio of the mix and to assure that the mix meets regulatory

Naylor standards and compost user requirements. 1.

Carbon to Nitrogen Ratio

The main purpose of adjusting the carbon to nitrogen (C:N)ratio is to assure the compost microorganismsa balanced diet. As discussed earlier, the carbon provides the energy and the nitrogen furnishes the protein for growth and reproduction. Materials that are rich in carbon include woody materials, paper, sugar, and certain types of wood ash. However, these materials do not all have the same proportion of carbon that would be available to a microbe during the time frame of a typical two to four week composting period. A common notion is that the amendment is added to adjust the C:N ratio as well as the dry solids content of a blended feedstock. While this is not an incorrect strategy, it is certainly not an adequate picture of the true situation. The dry solids adjustment can be made with all sorts of dry materials that may or may not have any impact whatsoever on the C:N ratio. Obviously, an amendment such as kiln dry sawdust will adjust the dry solids content of a mix, and will have some effect on increasing the C:N ratio since the sawdust is very fine, with a high surface to volume ratio. In contrast, wood chips that are one-inch diameter by a quarter inch thick will have almost no impact on the C:N ratio. Microbes have relatively little access to the bulk of the carbonaceouscomponent buried within the interior of the chip. Likewise having no effect on C:N are materials such as wood ash with chunks of carbon which are virtually non-biodegradable and ground up gypsum (calcium sulfate) board which is very dry,but contains no carbon at all. Sometimes non-woody materials make the best carbon sources. These may or may not be dry. Examples include sugary wastes, fruit wastes, and shredded leaves which have high C:Nratios and are very biodegradable. Thus, as the operator evaluates the overall blend, consideration must be given to the biodegradabilityof the materials as well as their C:N ratio to effectivelyjudge their ability to adjust the C:N of the blended feedstock. 2 . Regulated Chemicals

All ingredients incorporated into the blended feedstock contain all ofthe chemicals regulated by environmental agencies, although for many their concentrations are of no significance. There is a notion on the part of some that certain materials are clean whereas others represent potential risk as a consequence of the presence of such chemicals. In reality, presence equals neither toxicity nor hazard. All materials represent a continua of chemical compositions and concentrations. Beneficial use of a material is possible because the composition is known, the appropriate rate of use to achieve some beneficial end is understood, and a use rate is identified that makes negligible any potentially harmful effects due of the

227

Cornposting

presence of chemicals. For example, application rates of chemical fertilizers are established to assure nitrogen is not lost to ground water. The proportion of biosolids compost blended into a container mix is adjusted to avoid salinity problems. Nonetheless, strict attention must be paid to the chemical composition of individual components of the blended feedstock. In general, it is wise to have a chemical analysis of a material before incorporating it into the compostable mix. Once a material is mixed into the feedstock, separation is virtually impossible. If a material is known to contain concentrations of a regulated chemical in excess, blending with other materials so as to produce a finished product that meets the standard is acceptable under EPA Part 503, but not necessarily under all state environmental regulations [24]. In some cases, blending into a mix a material containing excessive concentrations of a regulated chemical will cause the entire mix to enter the realm of questionable material. It is best to know the regulatory situation and to know the composition of the materials used in the blended feedstock prior to mixing.

3 . Non-regulated Chemicals While the regulatory community has a set of chemicals that it is concerned with, this set does not overlap very much with chemicals of concern to the users of the finished compost. Where such chemicals do overlap the concern tends to be sufficiency rather than excess. Users are interested primarily in macro-nutrients and trace minerals, pH, organic matter, salinity, along with physical characteristics such as porosity, water holding capacity, and bulk density. The macro-nutrients consist of nitrogen, phosphorus, and potassium. These nutrients are of interest in the blended feedstock for several reasons. Nitrogen in the materials will influence the C:N ratio, and have some effect on the nitrogen content of the fmished product. Phosphorus and potassium content will influence the finished product quality. Phosphorus is often listed as soluble phosphoric acid on description sheets of the nutrient content of materials with a guaranteed or approximate fertilizer value. What is meant is not that the material contains phosphoric acid, but that the water soluble phosphorus concentration is expressed as phosphoric acid, usually as P,O,. This formula gives a P205concentration about 2.29 times higher than the concentration listed as P. The reason is that the formula contains oxygen. A similar situation exists for potassium which is usually listed as soluble potash, K,O. In this case the value as K 2 0 is about 1.21 times higher than for K. The following two formulas show how the proportion of K (eq. 5-5) and of P (eq. 5-6) in the respective oxide forms is calculated: 2xK-K,O

2x39 -- 1 (39x2+16) 1.21

. . . . . . . . . . . . . (5-5)

228

Naylor

--2xP P,O,

2x31 - 1 (2x31 +5x16) 2.29

--

. . . . . . . . . . . . . . . (5-6)

Other elements that may be present at appreciable concentrations include calcium, magnesium, sodium, aluminum, and iron. Calcium, magnesium, potassium, and sodium (Ca, Mg, Na, and K) contribute to the salinity of a material, and correlate closely with pH. Salinity and pH are of interest to the compost process manager since excess salinity can impact the finished product quality and pH can influence nitrogen loss as ammonia. Aluminum and iron (A1 and Fe) can react with soluble phosphorus to form an insoluble compound, and reduce plant available phosphorus in the finished product. Calcium, magnesium, potassium, and sodium are alkaline earth elements that will contribute to the salinity of a material and ultimately to the finished product since leaching losses of salts will be negligible. Excessive salinity in the finished product will reduce the range of uses to those which are less salt sensitive, e.g. mulch around trees and shrubs or field plantings in humid regions. Salinity tolerant plants include most grasses, cotton, and the beet family. Of the four elements, sodium is the least essential to most plants. Thus, the operator needs to review analyses of individual ingredients for excessive salinity or, as a general indicator, sodium. Concentrationsof sodium high enough to be a problem obviously depend on the proportion of a particular ingredient in the blended feedstock. However, as the concentration of sodium rises above one percent in a material, the manager needs to consider potential impacts on the fmished product. At high enough concentrations lime or other alkaline materials can contribute to the initial of the mix. High pH materials are commonly derived from lime treatment of a material such as biosolids or potable water treatment residuals. pH is generally only of transient consequenceto the composting process because of the buffering action of carbon dioxide, a weak acid, and ammonia, a weak base. Ammonia is released as a natural consequenceof the composting process. A high pH in a blended feedstock will tend to encourage volatilization of nitrogen as ammonia during aeration or turning of the composting materials. At pHs greater than 8.0 to 8.5, loss of the volatile ammonia can be uncomfortable to operators in confined composting spaces, whereas at pHs of 7.5 or lower, ammonia loss may not be particularly noticeable. neutral NH,+ C-> non-volatile

alkaline NH, + W . ............................ volatile

(5-7)

TABLE 5-5

COMPOSITION OF FOOD PROCESSING RESIDUALS [171

quor). 3. Dewaiere N

5:

NayIor

230

This chemical reaction is reversible and ammonia, along with carbon dioxide, (eq. 5-8) buffer the pH of the composting materials. Ammonia will react with volatile acids and other acidic compounds, such as acetic acid, formed during composting, neutralizing the acids.

acetic acid ammonia ammonium acetate HC2H301+ NH3 &> NU,+ + C2HsOf.. . . . . . . . . . . . . . . . ( 5 - 8 ) volatile non-volatile Since the reaction tends to reduce the content of the volatile, odorous components, this reaction may reduce or at least change the odors evident at a composting facility. However, balancing the pH of the composting materials so as to neutralize all odors by this strategy is not practical. Within the composting process, a pH increase is generally temporary. The enormous quantity of carbon dioxide, a weak acid, produced during the composting process buffers changes in pH. The buffering reaction is reversible, but tends to depress the pH of the composting materials by reaction of carbon dioxide with alkalinity:

C02+OH-c->HC03- o C O , ” + H +

. . . . . . . . . . . . . . . . . . . (5-9)

As a consequence of the buffering action of ammonia and carbon dioxide, well made compost tends to have a pH in the range of 7.5, even though the initial pH of the blended feedstock may vary from pH 5 to pH 10. Aluminum and iron are frequently added to slurries to aid in dewatering, or in some cases to water or wastewater to react with soluble phosphate (orthophosphate). Aluminum forms a very insoluble precipitate which can be settled out with biosolids or other sludge to produce a very low phosphorus effluent. When unreacted aluminum is present in a waste it will react with phosphorus from other components in the composting feedstock, and can reduce the water soluble or plant available proportion of the phosphorus in the finished product. This will not be evident, however, from the analysis of the total phosphorus (strong acid soluble) present in the finished product but will be shown in certain tests for plant available phosphorus. Aluminum is not an essential plant nutrient, but is phytotoxic at low pH. Iron has a similar effect on available phosphorus, although not so strong as aluminum, plus the advantage that the iron is an essential nutrient. Reaction of soluble aluminum with soluble phosphorus in materials to be composted is not usually desirable. One approach to judge the extent to which the aluminum has reacted with the phosphorus is to compare the percentages of each in a material. The formula for aluminum phosphate is AlP04. The molar ratio of

Composfing

231

A1:P in the formula is 1:1. Since the atomic weight of A1 is 27 and that of P is 32, an A1:P percentage ratio of 27:32 could theoretically precipitate all the phosphorus. However, soluble aluminum can react with other cations. As a practical matter a molar ratio of I .4:1 was observed experimentally to precipitate only 75% of the soluble phosphorus from wastewater [23]. Thus, it can be shown that 1.2% A1 in a material will precipitate 1.O% phosphorus. From the perspective of the compost process manager reviewing an analysis report, we can say that the percentage phosphorus in excess of the percentage aluminum will approximate the unreacted and potentially plant available phosphorus. So if a waste contains 5% A1 and 2% P, then the manager could expect the waste to react with other sources of P in other materials in a blended feedstock. If the %P exceeds the %Al, then most of the A1 will have reacted and should not be a problem due to reactivity with soluble phosphorus.

C.

Ingredient Selection

1.

Chemical Properties of Common Materials

One of the problems compost facility managers face is lack of ready information on the composition of common materials. Balancing ingredients from materials received at a composting facility is always a challenge because analyses of such items are not always available prior to receipt or in some cases at the time of delivery. Dry solids can be tested rapidly using a microwave drier, but other elements are not so straightforward. Data are presented for various foods and food processing residuals (Tables 5-2 and 5-5 ), wood processing by-products and fiber residuals (Table 5-6), and various types of animal wastes (Table 5-7). Analyses furnish data that would help answer questions related to dry solids content, C:N ratio, concentrations of regulated or non-regulated elements. While not as good as an analysis of the actual material, these data are useful as a guide for preliminary evaluation and selection of amendments and other ingredients in a blended feedstock. 2.

Food Processing Wastes

Source separated organic wastes and various residuals fiom food processing (Table 5-8) are ideal materials to blend with biosolids in a blended feedstock. The wastes derived from the processing of fruits and vegetables are highly biodegradable, lower in C:N than biosolids, and as a result can supply a readily biodegradable source of carbon. Pallets, corrugated boxes, and slip sheets can be shredded and used as a source of dry matter in the blend. In a highly controlled composting facility, a compostable blend of certain meat packing wastes can be specially formulated. Not all wastes fiom the food processing facility are appropriate. To

Jute Fiber

Sawdust KilnDry

Primary Sludge

Wood Ash

Pulping Liquor

Wood Bark

White Tissue

Colored Tissue

Chip Board

Totalnitrogen

1.1

0.11

0.29

0.12

0.18

0.99

0.28

0.3

3.17

Ammonia

---

0

0.01

0

0

0

0

0

3.17

organic nitrogen

-__

0.1 1

0.28

0.12

0.18

0.99

0.28

0.3

0

Phosphorus

0.13

0.02

0.01

0.33

0.22

0.07

0

0

0.02

Potassium

0.34

0.11

0.02

1.ti6

0.28

0.11

0.03

0.04

0.05

Calcium

5.03

0.1

0.15

12.8

0.18

1.12

0.16

0.16

0.11

Magnesium

0.36

0.02

0.02

0.81

0.04

0.08

0.02

0.03

0.13

TABLE 5-7 COMPOSITIONOF SOME ANIMAL WASTES r7i

Magnesium

0.44

1.09

0.75

,043

3.19

0.80

0.21

0.2 I

.030

Sodium

0.05

0.59

0.69

0.08

0.1 1

0.08

0.2 I

0.21

0.30

Iron

0.34

0.06

0.18

0.10

.046

1.94

0.15

0.07

0.05

Aluminum

0.15

0.04

___

0.06

0.24

---

0.21

0.02

0.05

Cornposting

235

be avoided are salty residues, metal and glass contamination, plastic strapping and sheeting, and in general slurries from treatment of cooling water or boiler blowdown.

D.

Developing a Blended Feedstock Recipe

Cornposting is an art and a science. To learn the art takes a good deal of experience, whereas working with blended feedstock recipes based on science can furnish even the novice with the correct starting point. For the experienced, the scientific approach can at least help the process manager know why the mix did or did not compost well. This section discusses the science of developing a good mix. The process manager is a chef, and for the preparation of a good product he must know his ingredients and understand how these ingredients interact during the composting process, their effect on the finished product, the process biology, and the specific composting technology used. Understanding the art will come with time. 1.

Facility Design Considerations

Each composting facility is sized based on a specified quantity of biosolids at a specified minimum per cent dry solids, and a minimum per cent dry solids amendment. A typical composting period is three to five weeks during weather conditions when composting temperature levels can be maintained and reasonable drying of the composting materials can be expected. Under these conditions, marketable finished compost containing 50 to 60% solids would be projected. Impact of Drv Solids. As properties of the input materials vary from these expected values, capacity of a facility and the dryness of the finished product will also depart fiom projected values. As an example, the capacity of a facility will be diminished as the biosolids dry solids content drops below 20%. With a wetter biosolids material, a greater weight and volume of amendment at, for example, 55% solids must be added to bring the compostable mixture to 40% solids. Since a larger proportion of a facility is filled with amendment, the capacity of the equipment and the facility to handle biosolids is diminished. Likewise, as the amendment dry solids dips below 55%, a greater volume will be required to h i s h dry matter as an amendment for the 20% biosolids to produce the desired dry solids content in the input mix. Conversely, as the solids content of biosolids or amendment increases, larger quantities of biosolids or smaller quantities of amendment can be used. Thus, the dry solids contents of the raw materials to be composted have great importance in the capacity of a composting facility as well as the quantity and cost of procurement of amendment. Nutrient Considerations. A second factor is that nutrient deficiencies can impair the compost process efficiency. Such deficiencies can also limit drying of

TABLE 5-8 COMPOSTABLE WASTE FROM THE FOOD PROCESSING INDUSTRY Raw vegetables and fruit spoiled, bruised, spilled shole or partial items, e.g. apples, corn cobs, mushroom stems and butts, skins, peel and cores, pea and bean waste, cherries and grapes Process residues peeling waste, cooked product, sugar solutions or other concentrated high BOD liquid or semi-solid wastes, grains or whole fiuitlvegetables, spoiled processed product, pie fill, edible fats and oils, sugar beet pulp Juice Operations juice pressing residuals (pomace) with or without press aids such as shredded paper or rice hulls, filter aid with retained materials, apple or grape pomace, juice and beverage filtration residues from wineries, breweries, fruit juice operations, fermentation residues fiom wine tanks, spent grains from breweries Waste treatment waste screenings, wastewater treatment primary (settleable) and secondary (biological) residuals or sludges Solid waste shredded pallets, cardboard boxes and slip sheets, labels Bread. cakes. and cereals whole bread, spilled flour or grain dust fiom silos, chocolate chips, cereal processing residues Fish and chicken processing and packing (depending on locale and regulations) whole or partial birds, hatchery wastes, fish waste, blood if absorbed into sawdust or other dry material, paunch manure, high strength egg breaking wastes and shells Avoid: high salt residues (pickling operations), dilute liquid wastes, cooling water and boiler blow-down water, metal glass or hydrocarbon compound contaminated wastes, waste containing non-compostable solid waste such as large seeds or pits. Note: Shredding and screening with recycle of overs will be required for most food processing waste.

Cornposting

237

the finished product. Composting is a microbiological process in which nutrition of the organisms must be considered. Under most circumstances, the waste materials to be composted contain adequate nitrogen and energy. Biosolids material is very moist and contains plenty of nitrogen. Amendment tends to be much drier, and contains much less nitrogen. Balancing these two materials not only adjusts moisture in the input mix, but also shifts the overall nitrogen balance of the mix. The ratio of nitrogen to carbon (protein to energy) as discussed earlier, is an important consideration in the composting process, especially as the input mix becomes very rich in biosolids or very rich in amendment. For example, an input mix will tend to be rich in amendment or yard waste if the biosolids material is very wet. As the amendment increases in moisture, even larger proportions must be added, M e r widening the carbon to nitrogen ratio, or C:N ratio. At some point the composting process will become nitrogen limited. An acceptable upper limit for the C:N ratio is 40. As the C:N ratio becomes larger and larger, the microorganisms begin to become nitrogen limited. In such a situation, while we are not in trouble nutritionally, the composting process begins to slow causing a hrther retardation in the rate of energy or heat production. With less energy produced, the drying capability will be diminished resulting in a wetter finished compost. Energv Considerations. Finally, not only does the biosolids composting capacity diminish as amendment solids content decreases, but the dryness of the finished product may also decrease. The microbially available energy content of most amendment materials tends to be lower than that of the biosolids. Volatile solids derived from biosolids is about 30 to 50% biodegradable, whereas for an amendment such as sawdust the volatile solids are only about 15 to 25% biodegradable. Thus, on a dry weight basis, energy available microbially from sawdust may be comparable to that from biosolids even though the biosolids may contain only 50% VS as compared with sawdust which may contain 90% VS. As the amendment dry solids content decreases, proportionately more is needed to produce an input mixture with the desired solids content, for example 40% in this case. As the proportion of sawdust in the input mixture increases, the available energy decreases. With less energy available to warm the compost and evaporate water, more moisture remains in the compost. Hence, the finished product will tend to be wetter (lower dry solids content) when the amendment is wetter. 2.

The Two Ingredient Mix

The simplest compostable mix will consist of two ingredients: biosolids and a dry amendment. For example, biosolids and sawdust, biosolids and shredded yard waste, or biosolids and wood chips are compostable mixes. Literally hundreds of such mixes are prepared daily in all sorts of composting facilities throughout the

Naylor

238

world. The single objective of the recipe is generally to develop a mix with the correct dry solids content, about 40 to 50% dry solids, depending on the composting technology. As it turns out, such mixes have a C:N ratio of about 20 to 40, which is appropriate. The mixes usually have adequate porosity. And the mixes usually compost well. Thus, for ordinary day-to-day composting balancing the dry solids content is a good approach, and certainly a good first step, to developing a compostable mix. Obtaining the correct dry solids content at the time the blended feedstock is prepared is important to successful composting, not to mention low frustration levels by the staff. Once the initial mix has been prepared, wetting is inconvenient, if not very difficult, and drying a too wet mix is nearly impossible without completely starting over and adding more dry material. Weight basis mix ratios. The proportion of biosolids and amendment in a mix

CALCULATION OF WET WEIGHT OF AN AMENDMENT TO ADJUST DRY SOLIDS CONTENT OF A COMPOSTABLE BLENDED FEEDSTOCK drv weight waste + drv weight amendment = %DS mixture or %DS, wet weight

TABLE 5-9

waste + wet weight amendment wet weight waste x YoDS, + wet weight amendment waste + wet weight amendment

x

%DS,

= %DS,

wet weight

wet weight waste x YoDS, + wet weight amendment x %DS, = %DS, (wet weight waste + wet weight amendment) = %DS, x wet weight waste + %DS, x wet weight amendment wet weight amendment x YoDS, - wet weight amendment x YoDS, =wet weight waste x YoDS, - wet weight waste x %DS,v wet weight amendment (%DS, - %DS,) = wet weight waste (%DS, - YoDS,) &DSADS,) (DS, - %DS,)

=

wet weipht waste wet weight amendment

wet weight amendment = wet weight waste WODS,-O/DS,~) (%DS, - YoDS,) Where:

w = waste, a = amendment, m = blended feedstock mixture, YoDS = percent dry solids

Cornposting

239

are a function of the dry solids content each of these two ingredients and the target dry solids content of the blended feedstock. The derivation of mathematical equation, shown below, for calculation ofthe amendment quantities (weight basis) is given in Table 5-9.

Net weight amendment =

wet weight waste (%DS,-%DS,)

. . . .(5-10)

(YoD S,-YOD S,)

Where: w = waste, a = amendment, and m = blended feedstock mixture; %DS = percent total dry solids of the indicated material as subscript Using this equation, the amendment weight varies the greatest when in the range of 50 to 60% as shown in Figure 5-9. For example, the quantity of amendment required, weight basis, at 50% DS to achieve 40% DS in the input mix is double that required at 60%. But the amount of amendment required at 60% is double that required at 80%. Similar differences are noted in the effect of the dry solids in the biosolids on the amendment requirements. Thus, dry solids content

c

3'0i

Bio6olids. % dry solids

+12% DS +18%

DS

-24%

DS

~

3

0 DS %

U Ic

.-

0.5

Weight ratio6 50

55

60

65

70

75

Amendment, %DS

130

85

90

Flg. 6-9 INFLUENCE OF DRY SOLIDS CONTENT OF BIOSOLIDS AND AMENDMENT ON THE WEIGHT RATIOS OF THE TWO INGREDIENTS TO ACHIEVE 40% DS IN THE BLENDED FEEDSTOCK.

Naylor

240

of the available amendments and of the biosolids are very important considerations as the compost process manager evaluates the supply of dry materials available to be used in a biosolids-amendment input mix. This ratio is important to transportation costs, truck traffic, finished product handling and post-processing, quantity of finished compost to be marketed, and capacity of storage facilities. It is also an important consideration in evaluating the quantity of a dry waste material such as yard waste that can be absorbed by the composting facility. Volume basis mix ratios. Mixing materials on a weight basis provides the most accurate, reproducible, and generally most successful approach. However, materials are mixed from time-to-time on a volumetric basis, especially once the bulk density of the material is well-known fiom practice. To develop a mix based on the volume of the ingredients their bulk densities must be known. Bulk densities were calculated from weight of a cubic foot of material for various samples of sawdust, a common amendment, and the data are plotted in Figure 5-10. As is clear from the graph, considerable variation exists in the bulk density of sawdust samples, even samples with the same dry solids content. Sawdust is a relatively homogeneous material as contrasted with, for example, yard wastes, leaves, and even wood chips. The latter materials are non-uniform, and pack tightly or loosely depending on the particle size, moisture, and stiffness (green vs. dry). Developing mixes based on volume ratios for such non-homogeneous materials is not very accurate, but experience operators have had reasonable

Fig. 6-10 BULK DENSITY OF SAWDUST AS A FUNCTION OF DRY

SOLIDS CONTENT.

241

Cornposting

success. Nonetheless, mix ratios based on weights of ingredients are always more accurate than those based on volumes. As the dry solids of a homogeneous amendment such as sawdust decreases, i.e. the material becomes wetter, the bulk density increases (Figure 5-10). Water has filled openings in the material that were previously filled with air. From Figure 5-9, a decrease in the dry solids of an amendment will increase the quantity required to achieve a desired dry solids content in the blended feedstock. This requirement for a larger weight of an amendment when combined with an increase in bulk density as amendment moisture increases would seem to be selfcompensating. But these two factors do not quite compensate for each other volumetrically, as shown in Figure 5- 1 1. Roughly half as much amendment by volume is required at 65% DS as at 50% DS. It should be noted that the mix ratios shown in Figure 5-9 (weight basis) and Figure 5- 1 1 (volume basis) are specific to the conditions represented on the graphs. Those conditions are 40% DS in the blended feedstock, amendment bulk density as represented by the curve in Figure 5-10, and a biosolids bulk density of 59 lb/cu ft. Volumetric ratios are particularly sensitive to variations in bulk density of the

1

2 5

Biosolids, % dry solids

Volume ratios

+12%

DS

18% DS -+-24% DS -0- 30% DS

--A-

.. C al E

'

0

.-0

U

m K

v

U

A v

0

Biosolids bulk density = 59 lblcu ft

c

01 50

I

55

I

I

60

I

I

65

70

Amendment, % DS

I

I

75

80

Fig. 5-11 INFLUENCE OF DRY SOLIDS CONTENT OF BlOSOLlDS AND AMENDMENT ON T H E VOLUME RATIOS OF T H E T W O INGREDIENTS TO ACHIEVE 40% DS IN THE BLENDED FEEDSTOCK.

1

85

Naylor

242

amendment. If the conditions change from these assumptions, the curves become less representative and should be redrawn for better accuracy.

3.

Multiple Ingredient Mixes

Compostable feedstocks have become more complex as compost process managers become increasingly knowledgeable and a wider diversity of ingredients become available. Public works directors and government officials are becoming increasingly aware of the ability of compost facilities to handle the source separated organic wastes portion of the solid waste stream. These wastes include grocery store wastes; certain non-recyclable paper products; food wastes from homes, restaurants, fast food establishments, hospitals and schools; and a wide variety of organic wastes and residues from industries. Developing a recipe for such a complex mixture can be a challenge because of the diverse physical and chemical properties of the various ingredients. However, all potential ingredients with the exception of mineral residues such as wood ash are biodegradable within the composting process just as they would be in a natural setting such as a woods or field. Blending such materials does not guarantee rapid composting, but the process manager can expect that the materials will eventually break down. The manager’s objective is to optimize the blend of composting materials and the composting conditionsto allow the fungi and bacteria to function most efficiently. The first objective the process manager must address is to adjust is the dry solids content of the blended feedstock. As with the two ingredient mixture, a feedstock with appropriate dry solids content will generally have adequate porosity and a reasonable C:N ratio. The simplest approach is to convert the diverse ingredients to a two component mixture by combining the relatively moist materials into one group, and then choosing one dry material or a group of dry materials as the amendment. Then the recipe for the feedstock mixture thus divided can be developed using the formula for the two component mix. A blended feedstock recipe calculation example is shown in Table 5- 1Oa. A blank template is provided in Table 5-lob. The calculations directed in Table 5-10 can be readily transferred to a computerized spreadsheetto make the recipe calculator interactive. The example in Table 5- 1Oa suggests the availability at a composting facility of biosolids, paper, yard trimmings, food waste, dry IC&I (industrial, commercial, and institutional wastes), and a dry amendment that is used to balance the solids content of the mixture of the other ingredients. The wet (as received) weights of the waste materials shown are arbitrary for this example. In practice, the materials and their weights will be those actually available or received at the composting facility. No weight or mass units are shown in the example since it is the weight ratio of the various materials that is key. While yard trimmings are shown here as a waste,

243

Cornposting

the material could also be used as an amendment, leaving a blank for the yard trimmings box. In other words, the example shows a weight of 10 for the yard trimmings, and a weight of 14 for amendment. We couldjust as easily use a weight of 24 for the yard trimmings. 'ABLE5-10a MULTIPLE INGREDIENTSCOMPOST RECIPE CALCULATOR Dry SolIda,

WredienQ

Wet Weight

Dry Weight

%

fill in 96DS and w t w i g h t Biosolids

paper YardTrimming Food Waste Dry IcBtI

multiply

to get

divide

55% 32%

divide dry d u e t wt x 100toget%DS

Total wastes Blendedfeedstock

add wetwts

add dry wts

<< Fill in desired Yo dry solids in feedstock %dry solids inamendment

Amendment

amendmaRYoDS x wetwt= dry weight

wet weight wet weight (~alc.below) =

How to eplcnlntethe weight of amendment to nchleve the dealred % dry r o ~ d aha the blended feedstock: 1. Add up dry weights of in@enta. Add up weights of ingredients. Keep all Units the same, for example, all weights in tons, Mg, kilograms or Ibs. 2. Divide total weight by the tdal wet weight to give percent dry solids of the compositemix 3. Calculate amdment guantity to achieve the desired percent solids in the blended feedstock Amendment wt =

-

(wet wt wastes x feedstodc DS %I 100 drv wt wastes) (amendment DS YO feedstock DS %)I 100

-

wetweieht x

-

feedstockDS.%+

lamend DS. YO feedstock DS. YO) +

100

=

value 100

=

12.11

a-b

244

Naylor

Depencllng on the actual materials, their weights, and their dry solids content, it is possible for the calculated amendment weight to be negative value. When this is the case,the dry solids content of the mixture of the wastes will be higher than that desired in the blended feedstock In such cases, a material wetter than the blended feedstock will be used to balance the dry solids content. For example,

TABLE 5-lob MULTIF'LE INGREDIENTS COMPOST RECIPE CALCULATOE Ingredients Dry Solids, Yo 'ill in % DS and wet weight

Dry Weight

Wet Weight

100

divide dry wt/wt wt X

add wetwts

loOtoget%DS

add dry wts

0

rdal wastes

<< Fill in desired %dry solids in feedstock << Fill in known% dry solids in amendment

Blendedfeedstock Amendment

wet weight

wet weight (~alc.below) =

1

ammdment%DSxwetwt= dry weight

1

0

How to cpleulnte the weight of amendment to achieve the dedred % dry solids in the blended 1.

2.

3.

Add up dry weigMs of ingredients. Add up weights of ingredients. Keep all units the same, for example, all weights in tons, Mg, kilograms or Ibs. Divide total dry weight by the total wet weight to give percent dry solids of the composite mix Calculate amendment quantity to achieve the d e s i percent solids in the blended feedstock

-wt=

-

(wetwtwastes X feedstock DS %I 100 drvwtwastes) (amendment DS YO feedstock DS YO)I 100

-

wetweight

X

n1 -

+

TI=

dryweightwastes

=

Composfing

245

food waste or biosolids could become the amendment if a wet material is needed to lower the percent dry solids of the waste materials.

4. Carbon to Nitrogen Adjustments The C:N ratio is important in developing a compostable mix in order to assure proper nutrition for the microbiological decomposers involved in the composting process. The C:N ratio is calculated by dividing the % carbon (%C) by the % total nitrogen (%N). As noted previously, the % carbon (%C) can be estimated fiom the % volatile solids (%VS). Research has indicated a very general relationship between these two parameters fiom the study of soils [22]. This study suggests that the % volatile solids divided by 1.8 approximates the % carbon in a sample. Although this is only an estimate, it does provide guidance in developing compostable mixtures.

TABLE 5-11 CALCULATION OF C:N RATIOS

Total Mix$

1.2%

t % Carbon = % VS + 1.8 % Blended Feedstock *

43%

78%

tt

C:N=%C+%N

Dry weight. These weights may be transferred from the data in Tab. 5-10a.

35

Naylor

246

In general, it is best to develop a blended feedstock recipe initially by adjusting the dry solids content to the desired level because moisture content is so critical to good composting. Then the C:N ratio can be checked to be sure it is within the range of 20 to 40. If the ratio is much outside this range, subsequent batches can be adjusted. Adjusting material that is already prepared is usually not necessary. Unless the biosolids used in the initial mix is very dry (e.g. > 30% DS), a C:N of about 15 to 18 is the lowest that would ordinarily be obtained with a biosolids -wood chips or sawdust mixture provided the dry solids content of the mix is correct. Mixes with this much nitrogen will be expected to generate a good bit of ammonia when they are turned or aerated. In a static pile situation, the situation can be corrected if the piled material can be leveled, more carbonaceous material layered on top with a front end loader, and then the pile reformed. For vessel type systems, adjustment is generally not practical. A procedure to calculate the C:N ratio of a blended feedstock is presented in Table 5-1 1. Weight of dry solids for each of the waste materials, the amendment, and the input mix can be transferred from Table 5-10a and used as input values where appropriate in Table 5-1 1. If spreadsheets are used for the calculations, these can be linked so that the data is automatically transferred and C:N calculation is immediate. This enables working interactively with the quantities of the materials to adjust not only the dry solids content, but also the C:N ratio. This is not to say, of course, that a compost process manager must have a computer to do good composting. A hand calculator can work just as well, but will take somewhat longer. If a deficiency of carbon or nitrogen is evident to the process manager, corrective steps in the next mixture should be undertaken. A large number of materials with their dry solids content, C:N, and estimated biodegradability are listed in Table 5-1. Review of these materials may be helpful to the process manager in locating ingredients that can correct the available carbon or available nitrogen in the blended feedstock. 5.

Energy Balance

As discussed earlier, the energy output as heat during composting is the driving force of the process. Energy generated is used to increase temperatures into the pathogen killing range, to increase the microbial rate of decomposition, to elevate the temperature of the feedstock and air, and to evaporate water. When a mix has inadequate energy, one of these uses suffers, usually moisture evaporation. Occasionally it is useful to be able to estimate the energy content of various components in a compostable mix and the blended feedstock. This is especially helpful if an input mix contains a high proportion of inert (non-biodegradables such as sand or ash) or slowly biodegradable materials (lots of paper or wood chips).

Composting

247

Energy can be estimated from the percent biodegradabilityof the volatile solids and the amount of energy obtained from degradation of those solids. Oxidation of volatile solids may be estimated from the energy output during combustion of a material. For organic matter the common value used is 10,000 BTU's per pound. While this value may not be exactly correct for each ingredient, it will furnish an approximation of the relative heat output from a mixture of ingredients. A protocol for calculatingthe estimated energy from ingredients and for the blended feedstock is presented in Table 5-12. This energy is considered to be released during the three to seven week composting period plus curing time. Weights and solids data for this calculation can be obtained from Tables 5-10 and 5- 1 1. Estimates for biodegradability are shown in Table 5- 1. Tables 5- 10 to 12 developed as computer spreadsheets can be linked to automatically update data transfers and make the blended feedstock recipe (materials balance), C:N ratio, and energy output interactive. Data calculated from Table 5- 12 also enables estimation of the dry weight and wet weight of the finished product. Examining the thermodynamicsof the composting process can lead to a better understanding of energy output during the composting process, how that energy is used, and the inter-relationship of biodegradability, volatile solids lost, metabolic water formed, and the weight of the finished compost. To produce a dry, high quality product, the composting process must generate enough energy to not only heat the compost to pathogen-killing temperatures, but also to evaporate moisture in the composting materials. This energy is produced by the microbial oxidation of biodegradablevolatile solids, the biological fire. This discussion assumes an energy yield of 10,000 BTU per pound of volatile solids destroyed, and the energy to remove the moisture (latent heat of evaporation) is considered to be about 1,000 BTU per pound of water evaporated at 140°F. For the purposes of this example, the biosolids dry matter is considered to contain 60%volatile solids, of which 35% is considered to biodegrade within the active composting period. While the amendment contains a greater concentration of volatile solids, 90%, this material is known to be less biodegradable because of the lignin in the wood. The amendment is considered to be 25% biodegradable assuming it to be primarily ground up wood mixed with some leaves and/or grass clippings. The example data in Table 5-13 illustrates a case in which 30 wet tons of biosolids are composted with 40 wet tons of amendment. The blended feedstock contains 70 wet tons or 28 dry tons at 40% DS. Of the dry material in the feedstock, 3.60 tons of VS is derived from biosolids and 19.8 tons from amendment, for a total of 23.4 tons. During the cornposting process, 1.26 tons of biosolids VS plus 4.95 tons of amendment VS for a total of 6.21 tons VS are biologically burned releasing about 124.2 million BTU of energy, eq. 5-1 1 .

248

Naylor

TABLE 5-12

ESTIMATION OF ENERGY CONTENT

Example Material

DS,%

Wetwt.

Drywt.

VS,%

Weight

b

axb

C

axbxc

Weight Water b-a x b

Biosolids

30.0

6.0

60%

3.60

24.0

Paper

5.0

4.5

60%

2.70

0.5

5.5

80%

4.40

4.5

3.2

95%

3.04

6.8

90%

1.35

90%

6.93

a

10.0 Food Waste

10.0

Dry IC&I

2.0

Amendment

14.0

Totals Total Mix

Biosolids

I

1 ::: 1 1 1 :I 1

71.0

28.4

-I

22.0

f+e x 100%

f

1.26

12,600

I

40% a+b x 100%

d

e

6.0

3.60

35%

Yard Trimmings Food Waste Dry IC&I

Amendment Totals k

I

32%

I +k

x

I

42.6

78%

Paper

Total Mix

vs

%

I

100%

Y BTU's = Ib VS lost x 10,000. Example assumes weights are in Ibs. Multiple by 2000 for tons, or other factor to convert to other units. t Biodegradability of volatile solids. tt Ib VS lost = Ibs VS x % biodegradability

I

4.7

249

Composffng

.........................

6.21 tons VS lost x 2,000 lbshon x 10,000 BTUAb VS lost = 124.2 M BTU

.(511)

Assuming the 28 tons of dry solids and 42 tons of moisture increase in temperature by 70°F fiom 70°F to 140°F during composting, approximately 6.86 millionBTU's of energy will be absorbed by the campostingmaterials (eq. 5- 12), plus an additional amount for the air moving through the materials and warmed. In this example, we will ignore the quantity of heat absorbed by the air. Further details on energy balances and composting thermodynamics are discussed in exhilarating detail in the text by Haug [9]. The amount of heat required to warm water and solids is, respectively, 1.OOBTUAb-F" for water and 0.25 BTUAb-F" for solids. (28 tons solids x 2,000 lbhon x 0.25 BTUAb-F + 42 tons waterx 2,000 lbhon x 1.00 BTUAb 4) x 70 OF = 6.86 M BTU ..........................

(5-12)

Most of the energy will be absorbed by evaporation of water. The blended feedstock mix is 40% DS initially, and the finished compost is considered to be 55% DS. From Table 5-13, the 70 wet tons of feedstock contains 42 tons of water and 28 tons of dry matter. During the composting process 6.21 tons of volatile solids are biologically b u m 4 leaving only 21.8 tons of dry matter. If this remaining material, the finished compost, is 55% dry solids, the wet weight will be: 21.8 dry tons + 0.55 dry tons per wet tons = 39.6 wet tons ..........................

(5- 13)

39.6 wet tons - 21.8 dry tons = 17.8 tons water in the finished compost 42.0 tons water in feedstock - 17.8 tons water in compost = 24.2 tons water lost However, the biodegradation of the volatile solids generates water as well as heat,ammonia, and carbon dioxide, eq. 5-14. This water is a product of the metabolic activities of the microbes and is called metabolic water. The quantity of metabolic water can be calculated assuming that the biodegradable volatile solids have the empirical formula C,,,H,,O,N, formula weight of 201, which is oxidized by 12.5 moles of O2producing 10moles of CO, 1 mole of NH and 8 moles of 5 0 ,formula weight 18 [9].

Naylor

250

TABLE 5-13 ESTIMATION OF ENERGY CONTENT -EXAMPLE Example Material

DS, %

Wet,wt.

Drywt.

a

b

axb

a+b

d

e

x 100%

Weight DS h

x

Weight

vs i

Loss? %ofVS j

vs

C

axbxc

Weight Water b-axb

fie

f

g

BTU outputg

Final wt.

VS, %

Weight

100%

Wt. VS 1osttt ixj

DS h-ixj

Food Waste

Amendment

I + k x 100% .Y BTU's = Ib VS lost x 10,000 BTU per Ib. Data shown here assume weight units are in tons.

The calculation is: M BTU's = (tons Vs lost x 2000 Ib/ton x 10,000BTUllb) + 1,000,000 t Biodegradability of volatile solids. tt Ib VS lost = Ibs VS x % biodegradability

251

Composting

6.2 1 tons C,,H,,O3N + 12.5 201

0 2

-->

4.4 tons 10 CO,+ NH3 + 8 HZO . . . . . . . . . . . . . . . . (5-14) 8 x 1 8 = 144

Oxidation of 6.21 tons of biodegradable volatile solids as calculated in Eq. 514, produces (6.21 x 144 -+ 201) x 2,000 = 4.4 tons of water. This weight must be also added to the total evaporated. Adding the weight of the metabolic water (4.4 tons) to the amount calculated earlier (24.2 tons) we get a total of 28.6 tons of water lost. Evaporation of this water at 140°F will require 57.2 million BTU's for evaporation, over 8 times more energy than for warming the composting materials. 28.6 ton x 2000 Ib/ton x 7,000 BTU/lb water lost = 57.2 M BTU .............................

(5-15)

Adding the 57.2 M BTU for evaporation of water to the 6.86 M BTU to warm the compost gives a total energy absorption of 64.1 M BTU. This leaves 60.1 million BTU in excess that will be absorbed by the air used to cool the compost or lost to the environment. VII. ODOR REMOVAL

Odor ranks number one of all the pollutants that may get the public's immediate attention. Complicating the odor problem is the highly subjective nature of odor. What one person finds acceptable, another many find extremely objectionable. Odors are produced on the farm, during rendering and food processing activities, and by a variety of waste management activities, including composting. Agricultural areas have organized into political agricultural districts with legal right-to-farm laws supporting normal farm operations including the spreading of manure with any associated odors. Thus, odors are not specific to biosolids or composting, but are common in all areas of life from the home to the farm. The key to being a good neighbor, whatever the odor source, is to keep the odors separate from the noses. For a composting facility to be successful and continue operations, this objective must not just be a goal, but an accomplishment. In recent years the science and technology of odor control has greatly improved. Odor control can be provided using a number of strategies including the following: * Control of composting process c Enclosure of composting areas 'I. Treatment of odorous pollutants c Locating composting site at safe distance from receptors

Naylor

252

A high percentage of composting facilities use biofiltration but a number of very large facilities do use chemical scrubbers. No reports of commercialuse of thermal oxidizers can be found in the literature researched. This section reviews causes of odors and their removal by biofiltration, an increasingly used odor control technology.

A.

Origins of Odors

Odors are produced during incomplete microbial oxidation of organic materials, principally carbohydrates and proteins. Carbohydrates are chemicals that consist of carbon, hydrogen, and oxygen in the general form - (C,H,,O,) -. Most common of the carbohydrates are cellulose and sugars found in virtually all natural products. In the absence of adequate aeration, carbohydrates break down into alcohols, esters, aldehydes, and organic acids. Each of these natural compounds has a characteristic aroma from sweet to sour to putrid. Proteins are compounds consisting of carbon, hydrogen, nitrogen, oxygen, and sulfur. Odors produced from degradation of proteins in the absence of adequate oxygen consist of ammonia, amines, mercaptans, and others. Some of the nitrogen and sulfur containing odors are very intense, tend to be more objectionablethan carbohydratebased odors, and may be evident at atmospheric concentrations in the ppm and ppb range. Composting biosolids aerobically will generate some odors as a consequence of normal biological activity. Intensity and character of such odors will be a function of the C:N ratio of the blended feedstock, the quantity of material composted, odor dilution potential, operational factors such as agitation, temperature of the composting process, and the odor potential of raw materials. While odors from such sources are not readily controlled, other potential sources can be minimized. Prevention is the best initial step for control of odors. Maintenance of aerobic composting conditions will help to a large extent in minimizing formation of odors. But this cannot eliminate all odors. Sources of preventable odors are spills, biodegradable amendment storage piles, and even clean up activities. Noncomposted biosolids tracked onto roads, parking areas, or even off site are troublesome due to both odors and appearance. Good housekeeping in the composting area is essential to eliminate unwanted debris or piles of fresh materials from accumulating. Any spill or even clean up activity leading to evaporation of volatile organic compounds will contribute to odors. A bucket on a tractor is an ideal way not only to clean up spills, but to also generate an odor-producing thin layer of biosolids on a hard surface. But biosolids are not the only sources of odors. Other sources would include wet leaves and especially grass clippings used in the composting process. Stockpiles of such materials may turn anaerobic and release odors during turning.

Cornposting

253

Once the preventable odors are eliminated,the focus is on minimizing contact of the residual odors with sensitive noses. This is accomplished through a combination of facility isolation, odor control technologies, and natural dilution with air. Facility isolation must be a consideration during the initial planning process, and subsequent changes are seldom an option. Thus, odor destruction and dilution of any residual odors are the basis of being a good neighbor.

B.

Odor Control Technologies

Attempts at odor control have included masking agents (covering up the odor with another, more acceptable odor); oxidation of the odorous material with ozone, potassium permanganate, or chlorine containing compounds in commercially available wet scrubbers; adsorption using activated carbon filters; thermal oxidation using afterburners; and absorptiodbiological oxidation using biofilters. All of these technologies can be successful in removing odors. However the chief benefit of the biofilter is the simplicity of this technology matching the simplicity of composting. Although biofiltration is accepted to varying degrees within the compost science and engineering community, biofilter use is growing. Major benefits of biofiltration relate to its simplicity. Odor removal is reasonably effective even under poor management, and very effective under average to best management. A broad spectrum of chemicals and concentrations can be removed. Biofilter media materials are readily available through local landscaping supply yards. Piping and blowers to distribute air to the biofilter are readily available at many equipment supply houses. Virtually no chemicals are required for successful operation. Moisture flowing through the biofilter contains mostly soluble salts (nitrates, carbonates, and sulfates). This liquid can be captured in a sump and may be returned to the composting materials as a source of moisture. The biofilter is resilient to varying environmental conditions such as heat and cold, snow and rain, although much less so to drought. Relatively simple and infrequent maintenance is required for satisfactory odor removal. While there are a number of benefits, the long term biofilter efficiency is dependent on the maintenance of the physical, chemical, and biological condition of the biofilter media.

Nay/or C. Biofilter Fundamentals and Operations A properly operating biofilter is, at its most fundamental level, a forced air, unmixed, very high C:N ratio compost pile. In most cases heat build-up is negligible, though fieezing is uncommon in northern climates. Components of the media must furnish porosity for good air flow and low head loss through the media, physical structure to support the media and minimize collapse during the life of the biofilter, an active biological population, and moisture for support of this biological community. To function efficiently, the biofilter must be maintained to sustain both the physical structure and the biological population. The success of a well-maintainedbiofilter to control odors is a function of the efficiency of two processes: sorption and regeneration. 1.

Sorption Processes

The biofilter's odor removal capability is due initially to interaction with particle surfaces, the active sites. This takes place by adsorption onto particles of the media, absorption or dissolution into water, chemisorption, catalytic contact on particle surfaces, and ion exchange on particle surfaces. Odorous compounds in an air stream flow through the biofilter media matrix and become attached through adsorption or absorption onto particle surfaces, removing the odors from the air stream. Since removal occurs onto surfaces, the surface area of the media is fairly important. Thus, odor removal efficiency is a function the strength of the physicaVchemica1 sorption processes and the surface area of the sorptive particles in the biofilter. For example, hydrogen sulfide can be precipitated onto particles as sulfides of iron or other metals. Hydrogen sulfide is first dissociated into HS- and H'and then precipitated as the metal sulfide. In the case of ammonia, the ammonia is dissolved in water, dissociating into andOH with sorption of the ammonium ion, and neutralization of the hydroxyl ion by soil acidity. Volatile, long-chain, lipophilic organic molecules are sorbed most effectively by moist organic matter. However, the sorption capacity of the particles is limited, and regeneration of the medium's sorption capacity must be achieved by chemical and microbial oxidation of the odorous compounds. When the biofilter is in long-term use, the sorption-regeneration processes reach steady-state conditions.

m'

2. Regeneration Processes

Regeneration of the sorbed chemical is moderated by microbial biodegradation, thermal processes, and chemical reactions. Thermal and chemical removal by biofilters play a small role in odor removal because of their high energy requirements. In contrast, microbial processes, which have low energy

Composting

255

requirements, are the dominant mechanisms in biofilters. Biofilter media possess an abundance of diverse soil bacteria and other organisms which participate in these processes. Because these organisms are dependent on the water in the soil for their biological processes, biodegradation and oxidation of sorbed odors is most efficient in media with adequate moisture. As an example, hydrogen sulfide, and H S are oxidized by thiobacillus bacteria to hydrogen ions and sulfate ions, odorless chemical entities. In the case of ammonia, the NH4+is oxidized to nitrite by nitrosomonas bacteria, and then to nitrate by nitrobacter bacteria. Volatile organics such as the odorous butyric acid are oxidized to carbon dioxide and water by a succession of bacterial groups.

3 . Steady-state Operation The capacity of the biofilter media to remove odorous chemicals depends on the simultaneous operation of both the sorption processes and the regeneration processes. As a result, the biofilter's odor removal potential can be exceeded by overloading the system through excessive air flow rates, so that sorption rates are lower than the rate at which the chemicals pass through the filter. Once all the sorption sites are occupied, odor removal is rapidly diminished. A second limiting factor is the microbial regeneration rate of the sorbed chemical which must equal or exceed the sorption rate. Toxic chemicals can interfere with microbial processes until a microbial population develops which can metabolize the toxic chemical. Soil acidity may reduce the removal efficiency due to an inhospitable environment for soil bacteria. Soil acidity may also increase removal by reacting with ammonia to form the non-volatile ammonium ion. In most cases of biofilter failure, however, the limiting factor is simply overloading of the filter rather than impairment of microbiological processes because of the great diversity and numbers of soil bacteria. 4.

Design Suggestions

Construction of biofilters is straightforward with proper planning and adequate attention to design and construction. Key factors in biofilter design are media composition, air retention time, head loss through the media, and pollutant loading rate. Biofilter media generally consist of a blend of materials to achieve the desired properties of structural longevity,high C:N ratio, porosity, biological activity, and absorptivity. Individual components of the biofilter sorbing media have included well composted leaves or yard trimmings, wood chips, bark, peat, sand, soil, cured biosolids compost, fibrous peat, volcanic, and ash. The choice of media tends to be somewhat site specific. After placing, the media the should have a porosity in the range of 32 to 55%, possess 40 to 60 % moisture, and the pH should be in the

256

Naylor

range of 5.5 to 8.0. Odorous gases must reside in the filter long enough to achieve maximum sorption. While residence time is influenced by the structure, permeability, and porosity of the filter media, the volume of the bed has been treated for design purposes as an unobstructed volume. Thus, residence time is determined by the volume of the filter divided by the specific loading rate of the waste gas [l]. Air flow rates have varied from 0.5 cubic feet per minute per square foot of biofilter surface area (cfm/ft2)tomore than 30 cfndff, with a range of 1 to 6 cfm/fi . At least 30 seconds is suggested for gases with low odor concentrations. Retention times of 45 to 60 seconds are commonly used for biofilters in the northeastern United States [ 161. To achieve these contact times at high airflow volumes using a biofilter media of typically three to four feet in depth, the biofilter footprint can be quite large. With this media depth, a good rule-of-thumb is an air flow rate of about 3 cfm/ft2. Surface area requirements can be a significant disadvantage for locations where land is at a premium. Several equipment suppliers are marketing enclosed biofiltration units of varying capacities. Most include some type of air moistening capability and are ready to be attached to a point source of odor. D. Biofilter Challenges

While there are a good many positive aspects of biofiltration, there are also many challenges that must be taken care of during design, construction and operation. Biofilter media are considered to possess uniform porosity and uniform moisture. These properties are important to uniform air flow and efficient odor removal, but are seldom achieved throughout the filter.

1.

Construction

Problems during placement include compaction, vertical and/or horizontal interfaces between consecutive placements, non-uniform mixing of the component materials, and non-uniform composition of materials. During construction, media must be placed without compaction using a back hoe or a crane. After placement the media should be leveled by hand raking. Traffic must be avoided on the media during or after placement. Compaction decreases porosity, resulting in nonuniform air flow and poor odor removal. 2.

Operation

Short circuiting of odorous air through the biofilter reduces odor absorption efficiency. Short circuits can be caused by tap roots of plants allowed to grow in the media, by dry zones which possess more porosity than adjacent moist areas, and

Cornposting

257

macro-pores caused by earthworms, rodents, or water flow. Leaks can be caused around the edge of the biofilter by improperly placed or sealed liners. Although there is some concern with overly wet media, drying tends to be a more common problem. Air flowing to the biofilter is seldom saturated with moisture, and thus has the potential to remove moisture from the saturated atmosphere within the media voids. Removal of water from the voids in the biofilter increases porosity. With higher porosity comes preferentially higher air flow and the capacity to firher dry the media. Thus, once a dry channel forms in the media, the drying problem always gets worse. Rewetting of the media should be initiated as soon as a media dryness is discovered. Biofilter media can be rewetted by moistening the air flowing to the biofilter, using soaker hoses below the surface, and providing surface irrigation. Rain and especially snow are very effective in maintaining media moisture levels. Where extensive drying has occurred, the dry media may need to be excavated, rewetted in bulk, and then replaced. Turning off airflow to the section of the biofilter being wetted can improve penetration of the water. Many biofilters are now designed with synthetic liners enabling water flowing through the media to be directed to a sump. Water from the sump may contain nitrate, carbonate, and sulfate salts and should be tested prior to returning to the biofilter surface or addition to composting materials. The biofilter media tends to degrade physically with time. Even though the media possesses a very high C:N ratio, odors from biosolids composting operation generally contain varying amounts of ammonia. As the ammonia is absorbed by the media, microbes can use this nitrogen source for degradation of the carbonaceous media. The consequences of this physical breakdown are shrinkage, cracks, and structural collapse. Consequences include direct leaks, shorter retention times, increasing pressure drop, and poor odor removal. The life of the media will be affected by its biodegradability, the nature and loading rate of the gases flowing to it, and the maintenance it has received. A three to seven year media life is common.

3.

Monitoring Media Condition

On a weekly basis, record the pressure drop through the media, check for dry spots on the surface, remoisten as necessary, and remove weeds and grass from the surface. Increases in static head pressure indicate plugging or media collapse, while decreases may indicate leaks. Variation from week to week is normal when induced by rainfall or other temporary changes. What is important are pressure changes that persist, or trends over time. Graphing the data is often helpful to get a visual presentation of changes in head pressure within the biofilter. Each month make an extensive survey of the media surface and correct any problems. These include surface porosity, moisture in the media, rodent holes that could develop

258

Naylor

into short-circuits, and any odor leaks. On an annual basis, chemically test samples of the biofilter media to check pH, and concentrations of nitrogen compounds. Screen the media at one inch, and measure the C:N ratio on the finer materials. Excavate and examine the media from surface to base to assess its physical condition. 4.

Monitoring Air Qualiy

Samples of dischargedltreatedair can be checked fiom if required using an inverted m e 1 or plastic bucket dischargingto a plastic sampling bag. Samples should be selected randomly at points on a grid superimposed on the media surface. Collection of a number of samples is recommended because of non-uniform air flow due to non-homogeneityof the media. Monitor air flow rates and temperature at each collection point and at a monitoring point leading to the biofilter to evaluate airflow rates and mass loading accurately. Untreated and treated air samples can be analyzed for specific chemicals of interest or by odor panelists. The success of the biofilter is derived from a combination of physical, chemical, and biological processes that occur in the media. Odors are sorbed by the organic matter particles, dissolved in moisture, and broken down and destroyed by microorganisms. The process is self-sustainingand no supplemental chemicals are needed. Consequently, little maintenance is required and any liquids generated are often recyclable.

VIII.

PRE- AND POST-PROCESSING

A successful composting facility will need a receiving and processing train to prepare the ingredients for the composting process and the finished compost for the customer. This includes a receiving area, shredding or particle size reduction capability, moving the finished compost to a screening area, and a curing or storage area. he-processing prepares the raw materials for the composting process and the blending together of ingredients to be used to develop the finished compost. The raw materials will affect the quality of the finished compost and its marketing so the presence of any contaminants must be evaluated at this stage. The receiving area should enable receipt of wet materials such as dewatered biosolids and dry materials such as wood chips, sawdust, leaves, or yard trimmings. Provisions must be made to store dry materials before and after shredding. Sawdust and wood chips can be readily stored under cover to keep them dry; the purpose of the dry materials is to use their dryness to adjust the moisture content of the biosolids. Leaves and yard trimmings can be stored outside before grinding and after grinding, if necessary. These piles are essentially static compost piles and

Cornposting

259

will generate heat that tends to keep the materials fairly dry on the inside. Because the piles do generate heat, periodically monitoring their temperature is essential to avoid overheating, charring, or worse. If heat builds up beyond 60 to 70"C, the piles should be turned and aerated to release the heat. The dry materials will be processed to uniform particle size by shredding using a tub grinder, a slow speed auger shredder, or other type of grinding or shredding technology. The particle size should be guided by the composting technology selected and the requirements of the customer for the finished compost. Once the dry materials to be used as amendments are received and ground, these materials will need to be stored until blending into the composting feedstock. The purpose of post-processing is to prepare the finished compost for marketing. Preparation will include conveyance of the finished compost from the active composting area to the curing, screening, and preparation areas. Trommel screens and belt shredders are frequently used. Shredding can precede or follow curing. In some cases, double screening is preferable, especially for the horticultural market where price opportunities and product quality required enable somewhat more time to be spent on product quality control. The compost will need to be cured and stored for 30 days to up to 6 months or more. The length of curing and storage is a function of the composting process, regulatory requirements, marketing strategy, and customer requirements. One benefit of screening before curing is that the quantity of material stored is smaller, reducing the size of the curing area. On the other hand, the presence of coarser materials in the curing compost tends to keep the static pile more open, enabling some exchange of air and heat release. Compost can be cured and stored outside. Heat generated by this product tends to dry the piles on the inside. The piles will become wetted on the surface to a depth of 6 to 12 inches. The site should be laid out to avoid surface water run-on to the storage area. In most cases, site runoff control is required. Surface water from the area can be collected and used as a moisture source in the process in many cases. IX. MARKETING

Recycling composted organic wastes through soil-plant systems provides an environmentallybeneficial, ultimate disposition of such materials. Such a program is possible only if the product meets the quality compost standards: chemically acceptable to regulatory agencies, biologically free of pathogens and weed seeds, and meets the needs and expectations of prospective customers. The composting process transforms biosolids and other organic residues into a material with a physically appealing character, in contrast with the general acceptability of raw materials. Finished compost contains carbon, oxygen, hydrogen, nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, and a

260

Naylor

diverse group of trace elements such as copper, zinc, iron, and boron. These elements as well as the bulk provided by the organic matter in compost can improve the humus content of soil and contribute greatly to the chemical and physical properties of a fertile soil. However, once the finished compost is prepared, the necessary next step is distribution and marketing of the product to the consumer. Composted biosolids are marketed to a large extent for the establishment and maintenance of turf-grass, garden and horticultural use, production of nursery crops, and as a source of organic matter in bedding plant mixes. This practice saves customers money while yielding excellent horticultural and floricultural results. These markets depend on readily available sources of consistent quality and supply of organic matter for production of high quality plants. Compost derived from an effective composting process guided by a quality assurance program is a good competitor in such markets.

A. Marketing Issues Issues that can influence marketing fall into two general areas: (1) consistent composition and assured delivery, (2) quality concerns: physical, chemical, and biological. 1.

Consistent Composition & Assured Delivery

On-time delivery of the required quantity of a soil amendment with a known quality is essential. While sizable budgets are allocated for the use of growing media and soil amendments, they may be small relative to the cost of an entire project. Composts may be blended with other ingredients such as lime, soil acidifiers, sand, etc. to achieve desired properties in the finished product. These blends are used to produce platings that account for the entire year’s sales. If the quality of the compost changes during the blending program, the finished soil product may be unsuitable for the planned use. Plant production schedules, fertilization programs, harvest schedules and marketing plans are heavily dependent on the quality of the growing media and its consistent performance. Growers, landscapers, top soil dealers, golf course superintendents, and greenhouse managers cannot afford to have a project delayed nor the quality of their work diminished because of unreliable delivery schedules or compost of a quality different from what they were expecting. Delays cost money and can hold up work that must follow in sequence. Inability of a compost supplier to meet both the delivery quantity and schedule can mean lost business by a customer. While high quality compost can often deliver better results at a lower cost, the seller of the displaced material will still be competing for the same business. To successfullymarket compost in a competitive environment, the producerhupplier

Composting

26 1

must achieve assured delivery of the quality and quantity of the material ordered at a competitive price. 2. Quality Concerns

Physical, chemical, and increasingly, biological properties of compost are of major importance in determining compost use and value. Physical properties influence porosity to air and water; chemical characteristics control nutrient availability, while biological properties can influence plant disease control. Phvsical DroDerties. Physical properties of interest to users would include traditional characteristics such as porosity (70 to 90%), bulk density (0.1 to 0.6), water holding capacity (100 to 200%), and organic matter content (60 to 85%) [5][10]. But color and particle size can also be important considerations. Customers tend to compare the color and texture of the new product with whatever they are currently using. Education by the salesperson,actual use by the customer, and analysis by a reliable soils laboratory can improve acceptability of a new product. In general, compost must match physical properties of currently used products, or the salespersonmust be prepared to effectively demonstrate equivalent or better performance. Although woody particles are acceptable and even desirable in compost, the presence of non-organic materials should be evaluated carefully. Unacceptable contaminants would include pieces of sheet plastic larger than about 'I8 inch, metal objects, and sharp pieces of glass (but probably not rounded pieces the size of a pebble or smaller). Not all non-organic materials are unacceptable, however. For example, shredded dry wall (gypsum board) has been included in compost, and may be acceptable chemically and physically. The gypsum particles add sulfur and calcium, and have a physical appearance similar to perlite, a common ingredient in soil mixes. The principal value of compost, though, is its ability to supply organic matter. Physical characteristics of compost provide important soil conditioning benefits [3]. These benefits include enhanced aggregation (the ability of the soil to form discrete particles or aggregates), increased soil aeration (helps keep the soil biologically healthy), lowered bulk density (lightens the soil which eases weeding and improves root development),and increased water infiltration and water holding capacity (maintains the proper moisture balance in the soil for plant growth). Compost added to soil improves spring warming and enables earlier working of the soil and planting of crops. Compost added to sandy soils increases the water holding capacity and the amount of water available to plants. This reduces the need for irrigation and minimizes the loss of soluble nutrients such as nitrates. In heavy textured clay soils the added organic matter increases the soil's permeability to water and air. Such improvements enhance the plant root environment and thus, aid root development and overall plant growth.

262

Naylor

Chemical characteristics. While the major use of compost is related to its organic matter content, the macro- and micro-nutrient content are also important. The nitrogen (N), phosphorus (P), potassium (K) and calcium (Ca) composition of composts will vary depending on the source of the wastes. However, compost dry matter typically contains about 0.5 to 3.0 % total nitrogen, 0.5 to 3.0 % P,05, 0.2 to 2.0 % K20, 0.5 to 5 % calcium, and 0.1 to 3 % iron. Concentrations of these nutrients substantially beyond this range, suggesting higher nutrient content, can be produced by adjusting with sources of fertilizer concentrates such as guano, bone meal, blood meal, or rock phosphate. In addition, most composts contain variable amounts of other macro-nutrients such as magnesium and sulfur, and micro-nutrients such as boron, copper, manganese, molybdenum, and zinc. Total analysis of several biosolids based composts and the amendment used are listed in Table 5-14. Analyses include macro-nutrients and several micronutrients of horticultural interest, pH (5050 mix in water), total solids (dry matter content), volatile solids (organic matter content), fixed solids (ash content), carbon to nitrogen (C:N) ratio, and N-P,O,-K,O fertilizer value (on a dry solids basis). Growers are also interested in the concentrations of nutrients that plants can actually access during the growing season. Available or exchangeablenutrients are the concentration of nutrients present in a material in a form that a plant root can absorb, sometimes within a specific time period. For this test, sample processing is less vigorous since the laboratory is trying to simulate chemically the same extracting power as fluids emitted by root hairs or the potential for the nutrient to dissolve in soil water. In this case prior to actual chemical analysis, the sample is typically prepared by extraction with a weak acid solution such as acetic acidammonium acetate buffer. Results are commonly expressed as ppm or mg/l in the extracting solution or the sample dry matter, or sometimes as lbs. per acre. In some cases the laboratory reports extractable nutrient concentrations for the sample and also the optimum or typical range of such concentrations for commercial container mixes. This strategy is useful to a potential customer because it enables the economic and cultural comparison with a familiar growing media. The total and available nutrient analysis present a fairly accurate estimate of the nutrients in compost in both the long term (total) and the short term (available). The exception may be nitrogen availability. Forms of nitrogen present in most composts can be divided into inorganic nitrogen and organic nitrogen. Inorganic nitrogen forms include principally nitrate and ammonia. These forms are sometimes described as mineral forms because they can be found in nature or can be purchased commercially as a mineral such as sodium nitrate. Since these two nitrogen forms are soluble and can be extracted by water or extracting solutions, available nitrogen estimated by extraction will report both nitrate and ammonia. However, organic nitrogen is chemically bound to organic matter and the organic matter must be biodegraded or mineralized to release the nitrogen. Estimating the

Cornposting

263

amount of nitrogen that may become available as the organic nitrogen mineralizes (biodegrades into the mineral or inorganic form) is not described by either test. While there are no tests to determine precisely the proportion of this organic nitrogen that may become available from a specific compost during a growing season, the results of many laboratory studies of nitrogen release from composts suggest that about 10 to 15% of the organic nitrogen will be released during the growing season. The residual nitrogen is retained in the soil and continues to provide nitrogen for plant growth for several years. Compost is a living fertilizer. Because some the nutrients in the compost are combined with organic matter, these nutrients, especially nitrogen, are released as this organic matter slowly breaks down through biological activity of living microorganisms. This biological activity is moderated by the same environmental conditions that influence plant growth: moisture and temperature. Thus, as with many organic fertilizers, the plant nutrients tend to be released gradually, and in response to the conditions that also stimulate plant growth. Such a release pattern tends to result in more efficient use of the nutrients. Because of this slow release nature of compost nitrogen, a second, but equally important, benefit is that nutrient loss through leaching is greatly reduced or eliminated. Specific parameters. While environmental regulators show a great deal of interest in metal concentrations in compost, potential customers, i.e. growers, have much simpler analytical interests once the compost has passed the regulatory standard. Thus, a quality assurance program should focus on the grower’s interests. Analyses of interest are typically pH, soluble salts, organic matter content, C:N ratio, and odor. The pH of a growing media strongly influences the solubility of minerals in the root zone of the plant and the hospitably of the root zone to plant pathogens. As the growing media becomes more acidic or lower pH, the solubility of aluminum, iron, manganese, zinc, copper, and macro-nutrients such as calcium, potassium, magnesium, and phosphorus increases [4]. For ericaceous plants such as azaleas, gardenias, rhododendron, or laurel a growing media pH of 4.5 - 5.5 is necessary. On the other hand, non-ericaceous plants prefer a media pH of 5.5 to 6.8. Turfgrass recommendations suggest a soil pH of 6 to 7, but can use higher pH. Most composts have a pH of 6.8 to 7.5, although some composts can be lower (Table 5-14). Thus, testing the pH of compost or other material intended for use is important so that the pH can be adjusted, if necessary, for the intended crop. Concentrations of soluble salts tends to be higher in compost than that present in common media such as peat, bark, soil, or sand. For example, biosolids can be rich in calcium, potassium, magnesium and sodium. These elements contribute to the soluble salts or salinity of a compost. While such minerals are essential for plant growth, excess can damage plant roots and inhibit germination, especially in container crops. The carbon to nitrogen ratio of a compost, usually shown as C:N, is important

TABLE 5-14 COMPOSITION OF BIOSOLIDS COMPOSTS

3b. Fairfield, Conn. (leaves); 4. Dartmouth, Mass. (brush); 5. Guilderland, N.Y. (brush); 6. Holyoke, Mass. (sawdust); 7. Lockport, N.Y. (pallets); 8. Plymouth, N.H. (sawdust, woodash); 9. State College, Penn. (sawdust); 10. Rochester, N.H. (woodash, sawdust, brush). IPS Cornposting Facilities * Total concentration after strong acid digestion ** Volatile solids = organic matter content

266

Naylor

because it indicates generally whether a compost will compete with plants for nitrogen. The C:N ratio should be in the range of 15 to 25, i.e. the carbon content should be 15 to 25 times greater than the nitrogen content. Extra soluble nitrogen can be added to a growing media to prevent nitrogen deficiencies, provided the problem is known ahead of time. Alternatively, if the problem is discovered after planting, a soluble nitrogen source can be added during watering. Odor is a very personal sensation and one that can influence compost sales. Compost produced fiom wastes tend to have some odor associatedwith them. The odor often reflects the material being composted. For example, some ammonia may be noted in compost produced fiom manures. Biosolids-based composts tend to have a musty aroma. While such odors may be of some concern initially, they usually dissipate once the material is mixed with the other ingredients or incorporated into field soils. Odor in composts applied to the soil surface as a surface mulch or top-dressing dissipates after watering or exposure to sun, air, and rain. The public and compost customers often have questions regarding the biological condition of compost, particularly biosolids derived compost. These questions center on pathogens. Compostingtime and temperature very effectively destroy pathogens: bacteria, viruses, protozoans, and helminths. State and federal regulation of these conditions have been in effect for many years, and experience indicates that where these regulations have been followed, pathogens are effectively eliminated fiom compostedbiosolids. However, human pathogens may not be the problem grower customers are most interested in. Weed seeds in compost are devastating to a landscaper who has invested hundreds of hours in developing a beautiful garden only to discover after the first rain a blanket of weeds. The compost facility will effectively destroy weed seeds if the time and temperature requirements are met. However, that is only the first step. The next step is protecting the compost fiom being infected with wind-blown weed seeds during storage or curing outside. Protection can be achieved by covering the material, or pushing aside the surface layer into a pile allowing it to reheat and destroy the weed seeds. Weed control around the storage site can also be effective in isolating the compost fiom weeds. A property of compost that has long been suspected by organic gardeners, and is now being investigatedand proven scientificallyis the ability of some composts to control plant pathogens. Homick, et al. reported that as early as 1974, composts were thought to control some root rots and plant pathogens, and that in 1983 research suggested that plant nematodes would also be controlled [12]. Kuter, et al. found that specific h g a l populations colonizing compost were responsible for suppression of Rhizoctonia damping-off [14]. Nelson has continued this work focusing on pythium blight and dollar spot control on turf and has found that bacterial populations can also play an important role [ 181[ 191. Much of the current understanding of the widespread mechanisms of disease suppressing effects of

Cornposting

267

compost has been reviewed by Hoitink, Boehm, and Hadar [ 1 11. 3.

Quality Compost

Achievement of quality compost is not only indicative of a well-operating composting facility, but essentialto successfulcompost marketing. Marketing such products within a competitive economic environment requires a sound quality assurance program focused on delivering a material with consistent physical, chemical and biological properties. While meeting environmental regulations is required by law for biosolids derived composts, customers have an additional set of chemical parameters of interest. These include organic matter content, total and available nutrients, pH, carbon to nitrogen ratio, salinity, and odor. Compost customers rely on the supplier to have the capability to deliver not only a consistent product, but to deliver that product in the required quantity and on schedule as promised. Recycling composted biosolids as a soil amendment is not only environmentally responsible, but sound from a plant production and economic perspective as well.

X. OUTLOOK AND SUMMARY In the past decade composting has developed from a fledgling art into an engineered technology. The science of the composting process is increasingly well understood. Mechanical composting technologies are improving in reliability and ease of operation. Process control has become computerized to enhance the environmental conditions for the compost microbial community and improve process efficiency. Best of all, the quality assurance of the finished product is increasingly being guided by customer requirements and process knowledge, improving the marketability of the compost. While substantial strides have been made, there is opportunity for improvement. Continuing efforts must be directed toward economical and effective odor management strategies. Because of the potential growth within the industry, effort is needed to expand even further opportunities for beneficial use of composted biosolids. Public acceptance of biosolids products varies from place to place. While we have supportive regulations guiding biosolids use backed by a sound technical understanding of risks and benefits, the societal aspects need W e r work. Perhaps it is time to request the assistance of the social scientists in moving forward on this front. Technically the priorities for guiding biosolids composting are to know the qualities of the biosolids and the amendments used in developing the blended feedstock, and to know the expectationsof the market. Compare the quality of the compost produced with that likely to be expected by the compost market. Once

268

Naylor

the inputs and the markets are known then decisions can be made regarding selection of the most appropriate composting technology whether in-vessel or windrow type. Selection of pre- or post-processingtechnologies will be guided by the characteristics of the ingredients, the quality of the fmished compost, and the composting technology. In short, with a sound understanding of the art and the science of composting a quality product can be produced. A quality product is the foundation of readily marketable compost.

REFERENCES

1.

E. Allen, H.U. Hartenstein, Biofiltration: An Odor Control Technology for a Wastewater Treatment Facility, Environmental Engineering Sciences Dept., University of Florida, Gainesville, FL. 1986. 2. L. Allison, Organic Carbon, p. 1367 In Methods of Soil Analysis, C. A. Black, Ed., American Society of Agronomy, Madison, WI, 1965. 3. G. Arkin, H. Taylor, Modifying the Root Environment to Reduce Crop Stress, ASAE Monograph No. 4, American Society of Agricultural Engineers, St. Joseph, MI. 1981. 4. N. Brady, The Nature and Properties of Soils, Macmillan Publishing Co., New York, 1984. 5. G. Bugbee, C. Frink, Quality of Potting Soils, Bulletin 812, The Connecticut Agricultural Experiment Station, New Haven, CT, 1983. 6. M. de Bertoldi, F. Zucconi, Microbiology of the Transformation of Urban Solid Waste in Compost and its Use in Agriculture, Ingeneria Ambientale 9 (3): 209-216, 1980. 7. N. Goldstein, D. Riggle, R. Steuteville, Biosolids Cornposting Strengthens its Base, BioCycle 35 (12): 8-57, 1994. 8. N. Goldstein, Sewage Sludge CompostingFacilities on the Rise, BioCycle 26 (8): 19-24, 1985. 9. T. Haug, The Practical Handbook of Compost Engineering, Lewis Publishers, Boca Raton, Florida, p. 249, 1993. 10. C. Henry, M. Knoop; K. Cutler-Talbott, Technical Information on the Use of Organic Materials as a Soil Amendment:Literature Review, University of Washington College of Forest Resources, Seattle, WA, 1990. 11 . H. Hoitink, M. Boehm, Y. Hadar, Mechanisms of Suppression of Soilborne Plant Pathogens in Compost-amended Substrates, p. 601. In Science and Engineering of Compost, Eds. H. Hoitink and H. Keener, Renaissance Publications, Worthington, OH, 1993. 12. S. Hornick, et al., Utilization of Sewage Sludge Compost as a Soil

Cornposting

269

Conditioner and Fertilizer for Plant Growth, Agriculture Information Bulletin Number 464, U.S. Department of Agriculture, Agricultural Research Service, Beltsville, MD, 1984. 13. Incineration Application of Combustion, Wheelabrator Technology, Inc., Hampton, NH. Internal Document, J. Sciabba, April 1, 1994. 14. G. Kuter, et al., Fungal Populations in Container Media Amended with Composted Hardwood Bark Suppressive and Conducive to Rhizoctonia Damping-off, Phytopathology 73, 1983. 15. R. Monk, Digging in the Dirt, Unearthing Potential, World Wastes 37 (4): CS2-CS14, 1994. 16. L. Naylor, P. Gormsen, Odor Control by Biofiltration in the Northeast: Inventory and Operations, Presented at the New England Water Pollution Control Association, Boston, MA, January 1992. 17. L. Naylor, W. Doty, Food Processing Residuals: Management Strategies for Beneficial Use. pp. 115 - 125 in Proc. Con$ Waste Management and Water Conservation in the Food Processing Industry, P.D. Robillard and H.A. Elliott, Eds, The Pennsylvania State University, University Park, PA, 1989. 18. E. Nelson, C. Craft, A Miniaturized and Rapid Bioassay for the Selection of Soil Bacteria Suppressive to Pithium Blight of Turfgrasses, Phytopathology 82,206-2 10, 1992. 19. E. Nelson, C. Craft, Suppression of Dollar Spot on Creeping Bentgrass and Annual bluegrass turf with compost-amended topdressings. Plant Disease 76, 954,1992. 20. Northeast DHIA Forage Laboratory, 1994 Tables of Feed Composition, Northeast DHIA, Ithaca, NY. 1994. 21. M. Pelczar, Jr., R.D. Reid, Microbiology, McGraw-Hill Book Co., New York. p. 115, 1972. 22. Second interim report of the Inter-departmental Committee on Utilization of Organic Wastes, New Zealand Eng., November-December, 1951. 23. G. Tchobanoglous, F. Burton, Wastewater Engineering: Treatment, Disposal, and Reuse, McGraw-Hill Publishing Co., New York, p. 744, 1991. 24. U.S. EPA, Subpart D, 40 CFR Part 503. Standards for the Use of Disposal of Sewage Sludge, Vol. 58, No. 32, February 19, 1993. 25. B. Watt, A. Merrill, Composition of Foods, Agricultural Handbook No. 8, U.S. Department of Agriculture, Washington, D.C., 1975.

This Page Intentionally Left Blank

Heat Drying and Other Thermal Processes Mark J. Girovich Wheelabrator Clean Water Systems Inc. Annapolis, Maryland

Heat (thermal) treatment in biosolids processing has been used for several purposes, such as a) promoting separation of solids from liquids by releasing bound (cell) water to improve dewaterability; b) meeting pathogen and vector attraction reduction requirements (i.e., sterilization and disinfection), c) enhancing wasteactivated biosolids digestibility; d) incineration; and e) drying and production of biosolids-derived fertilizer. To meet current U.S. EPA 503 Regulations pathogen and vector attraction reduction requirements, the biosolids heat treatment processes should meet certain time-temperature requirements depending largely on the solids content (Subparagraphs C & D of §503.32(3)(ii), [l]). These requirements are typically applied for direct thermal treatment of liquid biosolids (1. 7% TS), aerobic thermophilic stabilization (ATS) processes and pasteurization. This chapter’s focus, however, is on biosolids heat drying for production of beneficial use products.

I. BIOSOLIDS DRYING

Production of dried beneficial use products from municipal solids offers a viable management solution for many municipalities. Open air solar drying has been practiced for many years using fiee, nonpolluting, renewable and abundant energy

271

272

Girovich

source. However, using a solar radiation is not an applicable method for the overwhelming majority of municipalities in the U.S. where more reliable sources of energy must be used. Air (solar) drying is only feasible at favorable climate conditions (significant percent of sunshine, rate of precipitation, air temperature, wind velocity, etc.). Biosolids are dried to a level of dryness which depends upon largely uncontrollable circumstances and, therefore, the establishment of a standard operating procedure is impossible. For air drying, biosolids are typically placed on an open surface (lagoons, sand drying beds, etc.) and left there until they have dried to a desired solids content. Biosolids should be stabilized prior to air drying to prevent odors and vector attraction. Air drying can be enhanced by a vacuum or mechanical agitation, with the latter possible only if biosolids initial dryness is at least 12-15% TS. Another solar drying method tested in Florida is to dry biosolids by air heated in solar panels; however, this technology has not developed beyond a pilot plant. Air drying is a low cost alternative but it requires large parcels of land and cooperating weather. It has been practiced primarily in the Southern United States, however, some installations have been as far north as Chicago, Illinois. Air dried biosolids are Class B products with respect to the pathogen and vector attraction reduction requirements and as such they can be land applied on the agricultural sites specifically permitted for this purpose. A. Heat Drying and Production of Fertilizer

The rapidly growing biosolids treatment option for beneficial use is heat drying. Heat drying and production of biosolids-derived fertilizer has been in use in the U.S. since 1925 when the City of Milwaukee, Wisconsin produced Milorganite, a fertilizer made from municipal solids. Biosolids heat drying in general provides the following major advantages: eliminates pathogens and sterilizes the end product, fully satisfying the U.S. EPA "Standards for the Use or Disposal of Sewage Sludge" (40 CFR Part 503) reduces the volume and weight of the biosolids considerably preserves and recycles valuable organic and inorganic nutrients found in biosolids converts biosolids into a valuable end product with revenue potential (fertilizer, soil amendment or fuel) improves transportability, storability and social acceptance of biosolids derived products

Thermal Treatment

273

Heat drying can generate the following products: a) b) c)

dried biosolids pellets with no less than 90% TS for marketing as a fertilizer; partially dried biosolids for land application and landfilling (typically 40-60% TS); and dried biosolids for incineration, i.e. fuel production (40-90% TS).

In order to successfully market biosolids fertilizer, the following major criteria should be met: 1. Moisture Content

Pellet moisture content should not exceed ten percent (10%) since a higher moisture content encourages microbiological regrowth and product overheating. The finished product with minimum dryness of 90% is a Class A product and as such it can be used as fertilizer with no site restrictions provided its heavy metal and synthetic organic compounds content is in compliance with the limits established by federal and local regulations.

2 . Nutrient Content Pellet fertilizer value is based primarily on nitrogen content. Nitrogen is the primary plant nutrient found in biosolids, typically in the range of 4% - 6% on a dry weight basis. As a "stand alone" fertilizer product, pellets should contain a minimum of 5% total nitrogen consistently. If used as a fertilizer ingredient to be blended with other fertilizer materials, pellets should contain at least 3.0%total nitrogen. Phosphorus (as P,O,) and water soluble potassium (as y 0) are also important nutrients which are usually present in biosolids at low concentrations. Heat drying generally does not diminish the fertilizer or soil amending qualities of biosolids as other treatment processes can. However, certain loss (approximately 20% - 60%) of water soluble ammoniacal nitrogen (M-h+) has been observed during heat drying, especially if the incoming biosolids pH is above neutral. Fortunately, the biosolids ammoniacal nitrogen is typically in the 0.25% - 0.75% (dry weight) range only. Organic nitrogen, the predominant form of nitrogen in biosolids, on the other hand remains practically unchanged. Organic nitrogen has the advantage of a slow release rate over time and it is much less susceptible to run-off loss after rainfall. Micronutrients,for example iron, may also enhance the product value. The pellets can also be enriched by adding nutrients (nitrogen, magnesium, iron, etc.) as needed prior to marketing.

274

Girovich

3. Physical Quality The fertilizer industry demands biosolids fertilizers be consistent with other fertilizer materials they utilize. The U.S. Fertilizer Institute's (Washington, D.C.) Bulk Blend Quality Control Manual (1987) addresses some properties for the fertilizer's amendments (fillers). These properties include particle size, bulk density, uniformity, chemical compatibility, pH, critical humidity and visual appearance. Physical quality criteria include: Particle Size Range - For most agricultural applications, pellets should fall in the size range of 1 to 4 millimeters. For example, granular commercial fertilizer particle size specificationused by the U.S. Agency for International Development (USAID) establishes: 0% wt over 4.75 mm (US #4 mesh) 90% wt minimum in the 1.OO to 3.35 mm (US #6 to #18 mesh) 2% wt maximum below 0.6 mm (US #30 mesh)

Some agricultural and other non-agricultural fertilizer outlets may require different pellet sizing. Pelletized biosolids with narrow particle size distribution and minimum dust command higher prices. Bulk Density - The bulk density of most fertilizer constituents ranges from 40 to 80 pounds per cubic foot. Typically, biosolids fertilizer bulk density values fall at the lower end of this range or below. DurabiliR - The pellets should be able to withstand the normal rigors of transportation, handling and mixing without producing excessive levels of dust. 4. Pathogen and Vector Attraction Reduction

The pellets must comply with the Class A standards for pathogen and vector attraction reduction requirements contained in the 503 Regulations. In order to do so, the biosolids heat drying should meet either the direct microbiological limits (see Chapter 2) or the following process-based requirements: biosolids are dried in direct or indirect contact with hot gasses to reduce the moisture content to 10% or lower (290% TS) either the temperature of the biosolids particles exceeds 80°C (176°F) or the wet bulb temperature of the gas in contact with biosolids as biosolids leave the dryer exceeds 80°C (176°F). Heat drying is an effective pathogen and vector attraction reduction process which produces Class A biosolids. If the biosolids also meet applicable quality

Thermal Treatment

275

standards with respect to trace metals and synthetic organics, they may be sold without restriction for use on all crops. The pellets must also comply with federal, state, and local standards governing marketing and distribution of biosolids as a fertilizer. Depending upon the desired outlet, other qualities will determine pellet marketing success. A very important characteristic is odor, which is largely dependent upon the type of biosolids processed (e.g., raw, primary, secondary, digested, etc.). Heat drying with pelletization generates so far the most acceptable, safe, often revenue-producing product. Pellet fertilizer qualities which can be assigned a value, depending upon the application, include:

5.

Slow-release organic nitrogen content Micronutrient content Organic matter content Blended fertilizer material conditioning properties Reduced likelihood of plant nutrient contamination of groundwater compared to many chemical fertilizers

Markets

Biosolids fertilizers meeting regulatory standards can be used for a range of fertilizer applications from the most basic such as agricultural fertilizer filler to the more sophisticated such as the feeding of sensitive turf and ornamental plants. As such, the application opportunities for biosolids exist throughout the U.S. and beyond. Presently ,there is a substantial demand for heat-dried biosolids fertilizers in the US. The Florida fertilizer industry consumes biosolids-derived products for agricultural applications including citrus, other fruit, and vegetable fertilization. Fertilizer blenders can replace currently used lime and clay by biosolids pellets as a filler material. Non-agricultural applications include golf courses, fairways, greens and tees; residential and commercial property lawn care; and landscape plant nursery feeding. Florida is currently the distribution end point for a number of in- and out-of-state pellet producers. Number of producers and amount of pellets are expected to increase in upcoming years which may potentially flood the Florida market. Major new markets outside the primary market area of Florida should certainly be developed to accommodate the increasing production. Biosolids product marketing typically involves displacement of existing product rather than creation of a product to satisfy an unmet demand; and, therefore, effective marketing depends on a demonstration that the product's benefits exceed those of competing products [2].

276

Girovich

Typically, biwlids fertilizer prices in Florida range from $1 5 to over $100 per ton at the point of use and depend largely on nitrogen content (up to $15.00 per percent point of nitrogen for a good quality product). Products with established reputation and guaranteed nutrient composition, such as Milorganite, command signilicanehigher prices in the fertilizer market ($90 - $120 per ton). Future price dynamics will depend upon supply-demand relationship with a possibility of si@cant price decrease due to flooding of the prime Florida market. New markets must be developed, especiallynear production plants to Sustain reasonable price. The production of a fertilizer from the municipal biosolids offers a viable management solution for many municipalities with sufficient biosolids generation, especially those located in densely populated areas. This option is stimulatedthrough s i m c a n t improvement in biosolids quality throughout the U.S. and it grows quite rapidly. At present, large biosolids heat drymg facilities operate in New York, New York; Milwaukee, Wisconsin, Boston, Massachusetts; Tampa and Largo, Florida, Baltimore, Maryland; Houston, Texas, and other locations with total annual design capacity of over 350,000 dry tons. A list of heat drymg and pelletizing facilities producing commercial fertilizer in the U.S. is provided in Table 6-1. By the end of the 1990's, heat dried bimlids production for fertilizer application in the U.S. is projected to increase from the current 0.3 million tons approximatelyto in excess of 0.5 million tons per year with the majority of this amount being utilized in the South. This production level will still comprise only a small portion of the total U.S. chemical fertilizer consumption (approximately50 million tons annually).

B. Partially Dried Biosolids Bimlids can be dned partially, i.e. to less than 90% TS, (typically in the 40% - 70% TS range), in order to reduce volume prior to land application or landfilling. Partially dried biosolids are used also for subsequent autogeneous incineration. Landfilling of partially dried biosolids is typically practiced when they contain trace metals and other pollutants in excess of the limits established by the 503 Regulations and could not be used as a fertilizer. Other requirements of landfilling should also be met. Partial drymg of biosolids makes economic sense mainly because it reduces the volume and cost of disposal. Heat +g is alsoutilized to reduce or eliminate the amount of external energy required for biosolids incineration (i.e., to reach an autogenous

277

Thermal Treatment

TABLE 6-1

BIOSOLIDS COMMERCIAL HEAT DRYING PELLETIZING FACl ITIES IN THE USA (1994)

Location

Wilwaukee, Wisconsin

Zlayton County, Largo, Florida

Capacity, dtd (Evaporation Capacity, 200.0 (12 x 4.1)

32.0 (2 x 2.0) 36.0 (2.5)

Pollution Control

1925

Direct rotary drum drying, cyclones & electrostatic precipitators

(renovated in 1994) 1980

Direct rotary drum drying, wood chip combustion

1976

Direct rotary drum drying, after-burners

(renovated in ~

1993)

Cobb County, Georgia

32.0 (2 x 2.0)

Direct rotary drum drying, chemical scrubbers

1980

Tampa, Florida

64.0 (2 x 4.0)

Direct rotary drum drying, after-burners

1987

Direct rotary drum drying, chemical scrubbers

1990

Hagerstown, Maryland

14.0 (1.5)

AND

Boston, Massachusetts

130.0 (4 x 4.5)

Direct rotary drum drying, after-burners

1991

New York, New York

300.0 (6 x 5.0)

Direct rotary drum drying, after-burners

1993

Houston, Texas

90.0 (9 x 1.0)

Flash drying

1976 (7) 1987 (2)

Baltimore, Maryland

1 10.0 (3 x 4.5)

Indirect multitray dryers (3)

1994

278

Girovich

Pollution Control

*

Design capacity available in 1995.

incineration). The dried biosolids contain a substantial amount of energy (5,000 9,000 BTUAb, dry weight) and can be used as a fuel. Two methods of bimlids drylng are used for autogenous or partially autogenous incineration: 1.

The entire amount of biosolids is dried to an autogenous level (typically 40% TS) and then fed into an incinerator.

Thermal Treatment

2.

279

A portion of biosolids is dried to no less than 90% TS and mixed with wet biosolids feed to arrive at an autogenous level prior to feeding an incinerator. A portion of the dried biosolids can also be diverted for marketing as a fertilizer.

Biosolids drying for autogenous incineration is most popular in Europe and Japan where cost of energy is high and the biosolids use as a fertilizer is limited. A substantial number of incineration systems there employ indirect horizontal dryers to dry biosolids to autogenous level prior to incineration. The second method can be used for a dual purpose system, i.e. for simultaneous production of pellet fertilizer andor incineration during inclement weather or market downturns. (See Figure 6-16)

11. HEAT DRYING PROCESSES

Diversity of materials to be dried and relatively high capital cost of drying systems resulted in the evolution of many dryer and drying process designs. Drying is essentially the process of thermally removing volatile substances (above all, moisture in case of biosolids) to yield a dry product. All dryers are evaporators, i.e., they convert water to vapor which expands the volume about 1500 times thus expelling the vapors. Moisture in biosolids may be either free (unbound) or biologically and chemically bound. During the first stage of drying, the drying (evaporation) rate is constant. The biosolids particle surface contains free moisture which evaporates at constant rate. At the end of the constant rate period moisture has to be transported from the inside of the particle to the surface by capillary and diffusion forces and the drying rate begins to fall. As the dry surface develops on the particles and as the free moisture content is lowered by drying, the rate of internal movement of this moisture decreases and the drying rate falls continuously. The bound moisture evaporation proceeds even more slowly for it requires breaking the biological cell structure or chemical structure of compounds having bound water in their composition. As a result, the drying rate falls even more rapidly. This is of course the mechanism of all solids drying processes but it is considerably complicated by many other factors, such as mechanical agitation, mixing, etc. In the direct rotary drum dryers, for example, biosolids cascade through the dryers with the cascading interrupted by the "resting in flights" periods. In indirect drying, the solids are agitated and mixed by paddles, discs, stationary bars, etc. These mechanical motions are design-specific: and, therefore, the process is extremely difficult to describe in mathematical equations of heat and mass transfer.

280

Girovich

WATER VAPOR CASCADING OR SUSPENDED PARTICLES

DRYING MEDIUM

DIRECT

INSULATED SHELL

(CONVECTIVE) DRYING

WATER VAPOR

BlOSOLIDS PARTICLES

INDIRECT (CONDUCTZVE) DRYING

Fig.6-1 DIRECT VS. INDIRECT DRYING

Thermal Treatment

281

Extensive work has been done in developing theories of drying. A systematic comprehensive review and classification of drying processes and dryers is beyond the scope of this book and the reader is referred to specialized sources [3], [4],[5], [61. The most common classification of drying is based on methods of heat transfer, namely: 1) conduction or indirect drying; 2) convection or direct drying; 3) radiant drying and 4) dielectric (microwave) drying. Freeze drying can be classified as a special case of conduction drying. Dryers can also be classified based on the type of drying hardware (rotary drum, fluidized bed, spray, tray, conveyor, etc.), mode of operation (continuous, batch), residence time, etc. Both indirect and direct drying processes are used in biosolids treatment (Figure 6-1): 1. Indirect (conductive) drying uses a heat medium such as steam, thermal oil or hot gas to heat a metal surface over which biosolids are transported. Conduction transfers heat from the hot metal surface to the cooler particles and then between the particles. It is the principal heat transfer mode for indirect dryers. Indirect drying is a solid-liquid-vapor system and the process is nonadiabatic, i.e., heat is added from outside to the dryer. In indirect drying, solids and water can be heated above the water's boiling point. Indirect drying is more efficient and uniform when agitation brings cooler, wet particles into contact with the heating surface. Heat transfer fluids in the indirect drying may be of either the condensing type (e.g. steam) or the liquid type (e.g., hot water, oils). Indirect drying has several distinctive features: 1) cross contamination is avoided since the product does not contact the heating medium; 2) since a very small amount of air is introduced, air handling, cleaning and odor control is considerably less expensive; and 3) fire and explosion potential in the dryers is reduced significantly. Examples of indirect dryers are horizontal "en masse" (hollow disc, paddle, screw, etc.), vertical multi-tray, rotary, mechanically fluidized bed dryers, etc.

2. Direct (convection)drying heats the biosolids by contacting them directly with hot gas (air). Direct drying uses relatively large amounts of hot air (usually mixed with combustion products). Convection transfers heat between adjacent volumes of hot air and particles, within porous particles and outside of all particles. It is the principal heat transfer mode for direct drying. Direct drying is a solid-liquidvapor-gas system and the process is adiabatic, i.e., no heat is added or taken from the dryer, assuming perfect insulation [4]. In direct drying, water and solids are not heated much above the wet-bulb temperature which is well below the water's boiling point. The higher water vapor pressure drives water vapor into the gas which has a low vapor pressure. Drying stops when these two pressures are equal.

Girovich

282

Direct dryers may use hot air, combustion products, inert gas, and superheated vapor as the heating medium. Examples of direct dryers are rotary drum,fluidized bed, flash, spray and conveyor dryers. Direct rotary drum dryers are the workhorses of biosolids drying; they are the least expensive of all dryers and have high unit capacity. However, all direct dryers have a common disadvantage: the amount of heating medium (usually hot air mixed with products of combustion) is large which results in energy loss in the exhaust and requires expensive air handling and poilution control equipment to clean the gas prior to its discharge to the atmosphere. This serious disadvantage can be neutralized to a certain degree by using drying air recirculation as described in the following sections. Both drying processes are complex. Drying involves a number of physical and chemical processes such as heat and mass transfer, mixing, combustion, conveying, size separation, psychrometry,evaporation, etc. So far, however, there is very little help and transfer from the theoretical studies to the drying practice. Drying is still more art than science and more so for biosolids drying. 111. DRYER DESIGNS

Numerous dryer designs have evolved for drying of various products including biosolids. Table 6-2 depicts dryer design availability and use for biosoiids drying in the USA (1995).

Dryer Design

Direct

Indirect

1. Rotary drum, single pass 2. Rotary drum, triple pass 3. Horizontal, "en masse" 4. Multi-tray 5. Flash 6. Fluid bed 7. Conveyor 8. Spray

A

A NA A A NA A A NA

A = commercially available NA = commercially not available

A NA A A A A A

Installations in the USA 14 (direct) 19 (direct) 8 3 (indirect) 9 (direct) 1 (direct) 0 0

Thermal Treatment

283

The following dryer designs are currently used in the U.S.A. for commercial production of biosolids-derived fertilizer (Table 6-2):

rn

Rotary drum, single and triple pass, direct drying (9 operational installations, over 30 dryers) Horizontal "en masse", indirect drying (6 operational installations, 8 dryers, including partial drying for autogenous incineration) Flash dryer, direct type (4 operational installations, 9 dryers) Multi-tray, indirect drying (one operational plant, 3 dryers)

There are also a few pilot installations in the U.S.A. using radiation heated dryers and directly heated fluidized bed dryers. Dielectric heating has not yet developed beyond the experimental phase. A. Direct Dryers

1. Rotary Drum Dtyers

In the rotary drum dryers, drying air supplies the heat and carries off the evaporated moisture. Volume of drying air is the most important factor which determines the dryer and the drying system cost. The best drying energy efficiency is achieved at the highest air inlet and the lowest air outlet temperatures. These temperatures are limited by various factors (fire hazard, type of fuel, design of the heat generator, sensitivity of the end product, water condensation at the downstream equipment, etc.). Direct rotary drum dryers typically use cocurrent air flow which allows high (900°F) for biosolids. Exit temperature varies inlet temperature, up to within 85"(180"-200°F). In cocurrent flow with high inlet temperature the moisture does not protect the small particles from over-heating and potential fire; however, the drying is fastest in this case. Most drying takes place as the solids cascade off lifting flights (or get fluidized) and the air passes around them. The dryer shell also heats the solids. In rotary dryers, solids are moved through the shell by the combined effect of air flow, gravity, internal flight design, speed of rotation and the shell slope. Baffles and dam rings are also used to retard the forward motion of the solids and to increase their residence time. Evaporation of free water typically occurs in minutes. The smaller particles dry faster and are carried off more readily and often are overdried. Their removal and return to the process may be troublesome for it requires special air-solids separation and transport techniques. The end product physical characteristicsimportant for biosolids beneficial use are moisture content, particle size distribution, density and amount of dust. Due to rapid evaporation in cocurrent rotary dryers, a particle's density often decreases

Girovich

284

and particle sue distribution widens. Significant amounts of fine particles (dust) are produced by abrasion and overheating. Typical rotary drum speeds are between 5 and 25 rpm depending upon the drum's diameter. The percent of the shell loading is typically 10 to 20 percent and the solids retention time is 10 to 25 minutes. Drying air retention time is only several seconds. Single pass and triple pass direct rotary dryers are used to dry biosolids. At low capacities and low gas temperatures, direct drum dryers lose their cost effectiveness. The biosolids fed into the dryer must be a post glue-like phase, granular (i.e., 50 - 60% TS) material, therefore, dry material recycling and backmixing is required to avoid sticky biosolids inside the dryers and to generate a pelletized product. Dry,fine material recycling and backmixing in the upstream mechanical mixer is extremely important because it forms wet pellets (50-65%TS) in the mixer which are subsequently dried in the dryer resulting in high quality pelletized product with 90-95%TS. Two types of direct rotary drum dryers are used in biosolids drying: single and triple pass. The triple pass dryer (Figure 6-2, courtesy Baker-Rullman, Inc.) technical data is provided in Table 6-3. Fourteen single pass and nineteen triple pass rotary drum dryers are currently (1995) used in the USA for biosolids drying and pelletizing. TABLE 6-3

1. 2. 3.

TRIPLE PASS ROTARY DRUM DRYER (courtesy of Baker-Rullman, Inc.)

Biosolidsfeed at 20% TS; product at 2 90% TS; dry recycle; 24 hour day. Dry air inlet = 480°C (900"F), outlet = 85°C (180°F). MBH = million BTU/hr; ACFM = actual cubicfeet per minute; dmtd = dry metric ton per day.

MHAUST TO APC

COM AIR

-----

Fig.6-2

TRIPLE: PASS ROTARY DRUM D R n R (Courtesy of Baker Rullman Co.)

l ) B U M R , 2)FURh?ACE, 9)OUTER INSULATED SHELL, 4)INTEMEDU TE DRUU, 5)L"ER DRUM, S)SEALS, ?)A.lR-SGLIDS SEPARATOR, 8)DRNB BASE. 9)FUGHTS. 1O)IDLER BASE

286

Girovich

Numerous other single and triple pass dryers are used in Canada, Europe and Japan for biosolids drying. The rotary drum dryer is the most widely used design with the lowest capital cost. Disadvantages of direct rotary dryers, especially ones without drying air recirculation (so called open-cycle system), include: heat losses in the dryer's exhaust; use of expensive systems to clean large amounts of air from particulate and gaseous pollutants including odors often with application of costly afterburners; potential for dust fire and explosion, Flash Drver (FD) is a direct type, cocurrent unit that is essentially a long vertical tube with no moving parts (similar to pneumatic conveyance). The FDs have short solids residence time (0.5-3.5 seconds) and high unit evaporation rate. Good mass and heat transfer rates are obtained due to high relative velocity between the feed and drying air. Typically, @/wet biosolids mixture is fed in a cage mill where hot gas is also introduced. Drying occurs in the mill, ductwork and partially in the cyclones. Dried material is separated from drying air in the cyclones. Vapors are burned in the heat generator ( h a c e ) . The FD do not produce high quality pellets. The FDs are used in biosolids drying in the USA by the City of Houston, Texas (nine flash dryers by C-E RaymondCombustion Engineering Co.). Houactinite, a biosolids derived product produced in Houston is marketed primarily in Florida. Disadvantages are: complex operation and potential for fire. Fluid bed h e r s fFBD) are also direct, cocurrent units. The FBD involves the suspension of solid particles in upwardly moving hot air, which is introduced through a distribution plate. The FBD are usually of direct type, however, in recent years indirect FBD have been introduced. The biosolids feed must be granular and backrnixing and feed grinding before drying is typically required; particles should not be so heavy that they drop out. Retention time is very short (few seconds) because air velocity must be kept above a certain minimum. The FBD do not produce a high quality pelletized product; however, the end product is marketable. Numerous fluid bed dryer designs are available (stationary, vibrated fluid bed, continuous fluid bed, mechanically agitated, etc.) However, only a few experimental FBDs for biosolids fertilizer production are in operation in the USA. Disadvantages include: poorly granulated product; potential for dust fires/explosions and complex operation. Convevor Drvers use steel belt, screw or vibrating trough to carry the biosolids through a directly or indirectly heated chamber. Depending upon the design, hot air may or may not pass through the bed of biosolids on the conveyor. Indirect conveyor dryer may use radiation as a means of heating.

Thermal Treatment

287

B. Indirect Dryers Indirect dryers also come in a variety of designs. The most popular design is horizontal, "en-masse" (i.e., moving, mixing and drying biosolids in a plug or semiplug flow fashion) type. In this design, biosolids are simultaneously heated, dried and transported horizontally from the feed to the discharge end. Other types of indirect dryers include multi-tray, fluidized bed, rotary drum and conveyor designs. Indirect "en masse" dryers come in a variety of shapes and forms. Some units have single rotors while others may have two or more. Flights may be fully circular (disc), helical (heated screws), blade-like (paddles) or circular segments. Flight cross sections may be tapered, parallel, or wedge shaped. The stator (housing) is horizontal and it may be cylindrical, U-shaped or ox-bow in section. The indirect dryers use very little or no drying air and as a result they are generally more energy efficient than direct ones. The energy required to operate dryers is delivered by an external energy source, typically steam boiler or thermal oil heater. The heat is transferred from hot metal surface to biosolids. A significant number of proprietary horizontal "en massel' designs have been developed by various manufacturers (Stord, Bepex, BUSS, Blaw-Knox, Tsukishima Kikai, Nara Machines, Komline-Sanderson, and others). Indirect disc dryer (Figure 6-3, courtesy of Stord, Inc.) data is provided in Table 6-4. In the disc (flight) dryer, the external surface of the discs (flights) provides 85 to 100% of the total heat transfer surface of the dryer. The heat transfer medium, usually steam, flows into the discs (flights), and is discharged as condensate after transferring its latent energy to the product. Other thermal mediums, such as hot water or hot oil, may be used as well. The filly circular, hollow discs of the Stord dryer (Figure 6-3) provide a smooth heat transfer surface, which reduces fouling, product accumulation in "dead" spots and wear. The rotor rotates at a constant speed [7]. The discs are fabricated of carbon or stainless steel, or a combination of both. The stator is filled with biosolids so that the rotor is submerged and the heating surface is almost fully utilized. Agitator (scraper) bars are mounted fiom the stator, ending just above the rotor shaft at the base of the discs. Large paddles are mounted on the tips of the discs and each disc has a combination of neutral and adjustable paddles. A portion of the upper part of the stator extends into a plenum to form a vapor dome. The vapor exits from the dome near the product inlet where the evaporation rate is the greatest. Wet biosolids enter the dryer near the top of the unit on one end and the finished product exits the dryer at the opposite end. The dryer is operated as a 'lclosedl' unit. No air is deliberately allowed to enter and the exhaust is nearly pure

h)

00 00

EXHAUST

/

Fig.6-3 INDIRECT DISC DRYER (Courtesy of Stord, Inc.) 1)EOLLO W STATOR (HOUSIUC), 2)HOLLOW ROTOR. 3)PRODUCT DISCBdRCB. 4)CONDENSAT.E OUTLET, S)rrouoW DISCS, 8 W E R BARS, 7)VBpoR DOME, 8 BXaaUST OUTLET, 0)DRIVR. i0)SU PORTS, 1l)DISCS PADDLES, i2)KATBAIdt INLET, 13)STYU.M INLET

P

I

Thermal Treatment

289

TABLE 6- 4 DISC DRYER TECHNICAL DATA Parameter

;I 3

Heating Surface Evaporation Capacity

I Weight

I

Horsepower Number of discs Specific Thermal Rate (STR)

I

7

1. 2.

3.

Unit

1

Range

450 - 3,500

I tons, metric kLca1

12 - 75

I

I

30 - 200

650 - 670(') I

kg H2O

I

Specific Evaporation Rate (SEW

kg H,O m2 hr.

Feed: 22% - 30% TS; product: 25% - 95% TS; no recycle, constant rate heat transfer. Product at 95% TS. SER increases to 16.0 ifproduct is at 35% TS. Lower values of STR and SER are for higher product dryness. 100% surface utilization. SER - Specific evaporation rate, STR - Specific Thermal Rate, see Section IV of this Chapter.

vapor. It is intended that biosolids co-rotate with the rotor. However, simultaneous co-rotation is prevented by the agitator bars. The disc tip paddles aid the progression of the product in both rotational and longitudinal directions. The biosolids travel through the dryer in a plug flow fashion, and they must pass through the annulus formed between the inner wall of the stator and the tips of the discs. Heat is transferred from the heat transfer surface to the biosolids by contact and by internal distribution of the heat in the product. Disc dryers manufactured by Stord, Inc. are used in the USA primarily for production of partially dried biosolids for landfilling, land application or subsequent incineration. In order to produce biosolids pellets, mechanical pelletizing equipment is added to the dryer. Twelve Stord disc dryers are currently used to dry biosolids in the USA. Five of them produce dried biosolids for subsequent incineration (Buffalo, NY; Pittsburgh, PA; Los Angeles, CA).

F’ig.6-4

SOLID PADDLE DRYER (Courtesy of Tsukishima Kikai Co.)

1 ) 4 I R T E W INLET, 2)STEAM INLET, 3)EXlUUST OUTLET, 4)SOLID PADDLES, 5)CONDENUTE OUTLET, 6 HOLLOW ROTOR, 7)MTERI.U OUTLET. 8)HOLLOW STATOA. @)SUPPORTS, 1 OLRIVE

0

Thermal Treatment TABLE 6-5

291

NARA PADDLE DRYER TECHNICAL DATA (Courtesy of Komline Sanderson) Parameter Unit

Evaporation Capacity

kg/hr

35 - 1,500

3

Weight

tons, metric

1-50

4

Horsepower

kw

2-95

5

Specific Thermal Rate (STR)

kcal kg H2O

Specific Evaporation Rate (SER)

m2 8 hr.

1,600 - 3,000

I

40 - 70

I

45 - 95

650 - 680(') 10 - 16")

Lower values are for higher product dryness.

Indirect two or four shaft Nara Daddle drvers manufactured by KomlineSanderson, Inc. have hollow steam (or oil) heated rotors and hollow heated wedge shaped, self-cleaning blades. The omega shaped stator (trough) is also heated. Typical rotor speed is 10 - 30 rpm. Major Nara paddle dryer technical data are provided in Table 6-5. Other types of indirect horizontal dryers, solid paddle and inclined flight are shown in Figures 6-4 and 6-5 respectively (courtesy of Tsukishima Kikai Co.). Solid paddle dryer (Figure 6-4) has multiple solid (not heated) paddles installed onto the hollow steam heated rotor contained in the hollow steam heated stator (housing). Heat is conducted through stator and rotor to the biosolids. Rotor moves at relatively high circumferential speed (5 - 15 m/sec). The paddles perform multiple actions: agitate, scrap, transfer and granulate (particle size: 2 15 mm). Disadvantages are: short retention time (1 - 5 min.); high product moisture content (40% - 6O%TS); relatively high electric power demand. Currently no paddle dryers of this type are in use in the USA. Paddle dryers are used mainly in Japan to dry biosolids prior to incineration. Specific evaporation rate (SER) is in the 9 to 18 (kg H20/m28 hr.) range. Lower value of the SER is for higher product dryness. Steam heated inclined flight dryer (Figure 6-5) originates from the jacketed dual-shaft screw conveyors. Both rotor and stator conduct heat. Biosolids are transported and mixed by solid (not heated) flights in a plug-flow fashion eliminating the need for paddles or scraper bars. The rotor speed is relatively high

N (0

N

F'ig.6-5

INCLINED FLIGHT DRYER (Courtesy of Tsukishima Kikai Co.) 1)AlilTERUL INLET. 2)EXlUUST OUTLET, S)PADDLES, 4)INCUNED FLIGHTS, 5)VdpoR DOME, 6)DAM ADJUSTER, 7)CONDBNSATE OUTUT, 8)MBTERUL OUTLET, 9)HOLLO W ROTOR, 1O)HOLLO W STATOR, 1 l)SUPFORTS, I2)DRWE. f 3)STEAhf I N U T

Thermal Treatment

293

(30 - 45 rpm) which ensures good mixing and heat transfer. Typical product dryness is in 45 - 60% TS; SER is 10 - 20 kg H,0/m2 -hr. Granulation is poor (5 - 40 mm, with lumps). These dryers are also used in Japan for drying prior to incineration or for volume reduction. Advantages of the en masse horizontal dryers include: compact, horizontal arrangement; effective utilization of heating surface; relatively high energy efficiencyand low cost of environmentalcontrol. Disadvantagesare: end product is poorly granulated, dusty with lumps. Production of high quality pellets is not feasible without additional processing ( e g , mechanical compaction, milling, screening, etc.). Indirect multi-tray drver (Figure 6-6, courtesy of Wheelabrator Clean Water Systems, Inc.) is a vertical multi-tray dryer to which biosolids are fed via the top inlet and moved by rotating arms equipped with adjustable scrapers from one hollow, oil heated tray to another in a zigzag motion until they exit at the bottom as a dried, pelletized product with dryness in excess of 90% TS. The rotating arms are installed on a vertical shaft rotated by a variable speed drive. The dryer's doughnut-shaped trays are heated by recirculating thermal oil. The dryer's scrapers move and tumble biosolids over the tray's heated metal surface in small windrows and thin layers (30-60 mm) constantly exposing new surfaces for heating and thus improving heat transfer. Only top surface of the tray is in contact with biosolids. Multi-tray dryers are typically fed by a biosolids mixture with 55-75% TS to avoid glue-like plastic phase inside the dryer and, therefore, they require dried material recycle and backmixing. The biosolids mixture is prepared upstream of the dryer by mixing dry fine recycled material (2 90% TS) with wet biosolids (20-30% TS). In the upstream mixer, dry particles, which form a nucleus of hture pellets, are coated with wet biosolids-forming pellets with a dry core and thin wet coat. In the dryer, only the outside thin (0.1-0.2 mm), wet layer has to be dried. In the drying systems based on the multi-tray dryer this process is repeated several times until the pellets grow to a predetermined size and then removed for beneficial use. As a result of this "pearling"process, water evaporation is more effective and heat transfer rates are higher than in the "en masse" dryers. Pellets produced by this process tend to have higher bulk density and less dust than other dryers. Indirect multi-tray dryers are licensed from Seghers Engineering Company (Belgium) and marketed under a trade-name PelletechO by Wheelabrator Clean Water Systems, Inc., Bio Gro Division [ll], 11141. Major multi-tray PelletechB technical data are provided in Table 6-6. Advantages of the multitray dryer are: low cost of environmental control; high heat transfer rates; low power consumption; high quality of pellets with narrow particle size distribution and minimum dust; small footprint due to vertical arrangement.

294

Girovich

Fig.6-6 INDIRECT MuLmRAY (PELLeTECH@) DRYER (Courtesy of Wheelabrator Clean Water System, Inc.) 1)INSLVATED SEE& 2 E O U Y TRdYS, 3 ROTATING ARMS WITa SCRAPERS. 4 ) T d R W L OIL INLET .dlNIFOLD, 6)S€L4FT, 6 ) D i U W . 7)OUTLET OIL MANIFOLD, 8)UPPER BBdRINcr @)LOWER BZWUNG 1O)PRODUCT LNUT AND 0 U m T VXLVZS, 1I)DEFLECTOR. 12)TRdY'S SKIRTS, IS)ClXAN-UP SCRMER, 14)SUPPORTS

Thermal Treatment

295

Disadvantages are: significant vertical space required; relatively high weight and cost; only half of the heating surface is fully utilized.

TABLE 6-6

I. 2. 3.

MULTI-TRAY PELLETECHO DRYER TECHNICAL DATA (courtesy of WCWS)

Only top tray surface is in contact with biosolids. Feed @ 65% TS; pellets @ 2 90% TS; dry recycle; oil @ Lower values are for higherfeed dryness.

(500°F).

IV. MAJOR DRYER PARAMETERS A. Evaporation Capacity

Biosolids quantity is expressed in either dry or wet weights. Dry and wet weights are related as follows:

D

=Wed

D = dry weight W = wet weight d = biosolids dryness, TOTS

............................

(6-1)

296

Girovich

20

25 30 FEED DRYNESS, X TS

F'ig.6-7 EVAPORATION CAPACITY

AND ENERGY

DEMAND VS. FEED DRYNESS A dryer's throughput, whether it is direct or indirect, can be measured by its evaporation capacity and by its dry weight capacity. Traditionally in the U.S. throughput is expressed in dry tonnage (e.g., dry ton per day); however, it is often more useful to express production in terms of evaporation capacity (e.g., tons of water evaporated per unit of time). The relationship between the dryer's evaporation capacity (E) and dry weight capacity (D) is:

E =D

(k i) -

. . . . . . . . . . . . . . . . . . (6-2)

E = evaporation capacity, i.e., weight of water evaporated per unit of time; D = dry weight capacity (throughput), i.e., weight of biosolids dried to 100% TS per unit of time; d , do = biosolids input and output dryness (%TS).

Thermal Treatment

297

For example, if the biosolids input dryness is 22% TS, the end product dryness is 92% TS, and dry biosolids throughput required is 1.O dry tonhour, then the evaporation rate needed is E = 1.0(1/.22 - U.92) = 3.46 tons of water. Evaporation rate more than any other process parameter determines the size and, therefore, cost of the dryer and drying system. Biosolids feed dryness is the most significant process parameter which determines the dryer energy requirements. Figure 6-8 illustrates that at the biosolids feed dryness of 30% TS the evaporation rate and amount of energy required to dry one ton (2,000 lbs.) of dry biosolids is almost two times less than at the feed dryness of 18% TS. Assuming the cost of energy at $4.50 per million BTU, the feed dryness increase from 18% TS to 30% TS will save approximately $34 per dry ton of biosolids. Figure 6-8 also illustrates that the higher feed dryness increases the drying system's evaporation rate or, in other words, the same system can process more biosolids if the feed has higher solids content. For indirect dryers, it is also useful to express the dryer's evaporation capacity as amount of water evaporated per unit of heating surface per unit of time, i.e., by using Specific Evaporation Rate (SER). The SER indicates how efficient the indirect dryer heating surface is utilized.

. . . . . . . . . . . . . . . . (6-3)

Where: SER = specific evaporation rate, kg/m**hror BTU/ft2*hr E = total dryer evaporation capacity, kg (lbs) of water evaporated per hour S = indirect dryer heating surface, mz (ft2). Heat transfer in the indirect dryers occurs typically in several zones: heating, constant rate drying and falling rate drying. For each zone, the SER will be different. The SER can be obtained from a dryer's manufacturer. The SER allows one to quickly estimate the amount of heating surface for a required evaporation capacity. B. Energy and Drying Air Requirements

All dryers, direct or indirect, require an external source of heat energy. Usually dryers are fired on natural gas or oil. Digester gas or other sources of heat energy can also be utilized.

298

Girovich

A dryer can be characterized by the amount of heat energy required to evaporate a weight unit of water expressed as kilocalories per kilogram or BTU per pound of water evaporated (kcaykg or BTU/lb). Total amount of heat energy required by a dryer is: QT=

Q l + Q2+

Q,+ Q4

. . . . . . . . . . . . . . (6-4)

energy required to heat dry substance in the feed; Q2= energy required to heat water in the feed up to an evaporation temperature; Q3= energy required to evaporate water and to heat water vapor; Q4= dryer's heat losses (for well insulated dryers it is usually less than 1% of total energy required). Q1=

Q, is the largest energy component (approximately 970 BTU per pound of water, or 540 kcalkg at standard conditions). The heat losses also occur outside the dryers (in conveyance, feed and product handling systems, air pollution control, afterburners, etc.) and, as a result, the overall drying system's energy demand is considerablyhigher than that of the dryer proper, usually by 10% to 20%depending on the drying system design and energy recovery provisions. It is useful to characterize dryers by their specific thermal rate (STR), i.e., amount of heat energy required to evaporate a unit of water. The greater the STR the less efficient is the dryer.

STR = Specific Thermal Rate, kcaykg (BTU/lb) of water evaporated; QT= total amount of energy required, kcal (BTU) per hour; E = evaporation capacity, kg (lbs) of water evaporated. The average STR for modem dryers is typically in the 640 - 780 kcal per kg of water range (1,150 - 1,400 BTU/lb) depending on inlet and outlet temperatures of drying and heating mediums, dryer's throughput, design, etc. As with the SERs, the STRs are different for different heating zones. The STRs are provided and must be guaranteed by a dryer's manufacturer. The overall drying system's STR is higher than that of the dryer proper. Mass and energy balance calculations are performed to determine the dryer's heat

Thermal Treatment

299

requirements, heating surface, heating medium flow, evaporation rate, and other dryer parameters. The following input data are typically required: 0

biosolids feed rate, moisture content and inlet temperature product moisture content and temperature drying air conditions (for direct dryers) heating medium type, temperature and heat transfer rates (for indirect dryers)

Major calculating needs are associated with determination of heat needed for evaporation and, for direct dryers, amount of drying air. Calculations alone do not complete a design. Experience, manufacturer's data and tests are essential components of dryer and drying process design. a. Indirect Dryers Heat transfer in the indirect dryers occurs typically in three zones: heating, constant rate and falling rate drying. In the first zone, wet biosolids are heated by hot metal surface of the dryer and the heat transfer coefficient is initially high due to high temperature differential and abundance of free water. As biosolids temperature rises, the heat transfer begins to fall. In the second zone, the entire mass is heated almost to a boiling point (100°C) and evaporation of ffee water is essentially completed at the end of the zone. This zone is characterized by an approximately constant heat transfer coefficient. In the third zone, water bound in the biosolids is evaporated, and as the biosolids dryness increases it becomes more and more difficult to evaporate the remaining water. The heat transfer coefficient falls sharply. Figure 6-8 illustrates an indirect drying process in the horizontal steam heated dryer. Moisture content of the feed and heat transfer rates are plotted against dryer's length [7]. Typically, about 15 percent of dryer length (heating surface) is used to reach the boiling point, 25 percent of the length is at the constant and 60 percent is at the falling heat transfer rates. For each zone, amount of heat energy transferred from the heating surface to the product is defined as follows (heat losses are not included):

Q = heat transferred, kcaVhr or BTU/hr; S = heating surface, m2(ft2); T, = log mean temperature difference, "C (OF); U = heat transfer coefficient (different for different zones); kcaVm2*"C-hror BTU/ftz*"F*hr

300

Girovich

. . . . . .. . . .

05-71

ti, to= biosolids temperatures, inlet and outlet, "C (OF); Ti, To= heating medium temperatures, inlet and outlet (steam, oil); "C ( O F )

Heat transfer coefficients, U,are typically in the 40 - 80 kcaVm2*"C=hror 8 - 16 BTU/ft2-"Phr range with higher values for the constant rate zone. Indirect dryer calculations are based on mass and energy balance using heat transfer coefficients obtained experimentally for various drying zones. These calculations are quite straightforward and center around determining evaporation capacity, heat energy and amount of heating medium flow needed, taking into account feed and product moisture and temperature, heat medium type (steam, oil) and temperatures, heat sources and losses, etc. Thermal Fluid vs. Steam Thermal fluid heating systems provide certain advantages as compared to steam. Thermal fluid heating systems are used typically for process temperatures beyond the range of steam. It is often overlooked that thermal fluid systems are economical compared to steam heating, even at temperatures where steam can be used easily. As the steam temperatures increase, so does pressure. For a steam temperature of 187"C, the steam pressure is 10.54 kg/cm2 (150 psig). This increases to 30 kg/cm2(425 psig) for steam temperatures of 230°C. Such high pressure steam systems are expensive to install and operate. This is the main reason for selecting thermal fluid versus steam. Heat transfer fluids (thermal oil) are manufactured from refmed petroleum, synthetically formulated hydrocarbons or siloxanes (silicone). They are able to provide high temperature at very low vapor pressures. Thermal oil systems, if used properly, offer safety, low maintenance and extended operating lifetimes. For process temperatures of about and above, thermal fluid heating is superior to steam heating in almost all respects. Thermal fluid heating uses only sensible heat since it is in a liquid state. Hence, there are no "condensate losses." This is one of the reasons why the thermal fluid heating system has better efficiency than steam. Thermal fluids have relatively high boiling points, depending upon their application. The operating pressures of thermal fluid systems are from 5 to 15 kg/cm2.

m

U L

Or4

&:

I

E

1.600

s

n

3 F

1,200

I

I

I

-

I

40

vi UJ

B

E 30

-

40

8 V

aoo -

t;

60

20

0

E

400

5 10

-

3 8

a n

I-

v

Y& ' n

z !l

E

8

r

-1----1---- - - --I----

2.

9

2

_--

I

2,000

80

I

v

8

3 1

0

I

I 20

1

FIG.6-8

I

1

I

40 1 60 a0 PERCENT OF DISTANCE THROUGH DRYER FALUNG RATE HEATING CONSTANT RATE ZONE ZONE ZONE

___t

100%

L

INDIRECT DRYER HEAT TRANSFER COEFFICIENT U, STR AND SER.

302

Girovich

The selection of thermal fluids is essentially dependent on the maximum temperature required by the process. Each thermal fluid has a maximum temperature recommended by the manufacturer. The life of the fluid is dependent on the maximum film temperature and not necessarily the bulk temperatures. When the fluid is being heated, the film of fluid adjacent to the tubewall and exposed to the flame is at a higher temperature than the average temperature of the fluid at that point. For a well-designed heater, the film temperature would be about 30°C higher than the bulk temperature. The heart of the thermal fluid system is a heater where the fluid is heated. The hot fluid is pumped to the process to give up its heat and then returned to the heater. The fluid expands considerably during heating so an expansion tank is required to prevent system overpressure. The thermal fluid heater is generally a forced circulation unit. Sufficient velocities, about 3 to 5 m/s, are maintained in the coil to prevent overheating of the fluid. The licensed boiler engineer is not required to operate thermal oil heater. The thermal fluid system does have disadvantages: Petroleum-based thermal fluids are flammable, and some of them are toxic, so precautions have to be taken. Accidents have happened, but very few in chemical plants with good maintenance. Care must be taken in using thermal fluid systems because any leakage may have serious consequences. Temperature control is more complex than in a condensing system (like steam) though temperature control of 2 to can be achieved. The heat transfer coefficients on the thermal fluid side are lower than steam condensing coefficients. But the higher temperature of thermal fluid increases the overall heat transfer rate. In practice, the heat transfer rate is always equal to or more than that of steam. When the process needs a cooling cycle, design of heat exchanger for cooling the thermal fluid is quite complex. Measures must be taken to prevent oil leakage, to avoid water in the system and maintain the system's tightness and cleanliness. Only high temperature closedcell insulation should be used in leakage-prone areas; the system's equipment (pipes, pumps, expansion joints, strainers, gaskets, packing seals, flanges, etc.) must be in accordance with pertinent standards and best engineering practice. The amount of thermal oil or steam flow for indirect drying can be determined using formulae (6-8), (6-9) and (6-10) below. Formulae (6-9) and (610) are used for noncondensing heating medium (thermal oil, hot water) and formula (6-8) for the condensing medium (steam).

303

Thermal Treatment

. . . . . . . . . . . . . . . (6-8)

..............

(6-9)

Formula (6-9) can be written as follows:

F =

QT

. . . . . . . . . . . . . (6-10)

CD*d*(TiT0)*60

Where: m = mass flow of heating medium, kg/hr (lbhr) Q T = total amount of energy required, kcaVhr (BTUhr) C, = specific heat, kcaVkg."C or BTU/lb*"F Ti, To= heating medium temperature, inlet and outlet, "C ( O F ) hi, h, = heating medium enthalpies, inlet and outlet, kcaVkg (BTU/lb) d = density of thermal oil at operating temperature, Ib/gal F =thermal oil flow, gpm For example, to evaporate four (4.0) tons of water per hour in an indirect dryer with the STR = 1,260 BTU/lb, the amount of heat energy required is: QT= STR x 4.0 x 2,000 = 1,260 x 4.0 x 2,000 = 10 million BTU/hr, approx. (STR, Specific Thermal Rate, can be obtained kom the dryer's manufacturer data). Using formula (6-lo), (thermal oil with C, = 0.65, d = 6.2 lb/gal; Ti = 450°F; To= 410°F) flow of thermal oil through the dryer: F = 10 x lo6 : 0.65 : 6.2: (450 - 410) : 60 = 1,034 gpm

b. Direct Drvers Calculations for direct dryers with hot air as a drying medium are quite different from those for indirect dryers. They are also based on mass and energy balance but involve the psychrometric relations of moist air.

304

Girovich

In the direct dryers, as soon as wet biosolids particles come into contact with drying air, evaporation begins from the saturated vapor film which is quickly established at the particle's surface. The temperature of the biosolids particle surface approximates to the wet-bulb temperature of the drying air. At fust, there is sufficient moisture within the particle to replenish that lost at the surface. Unbound moisture transport from within the particle maintains saturated surface conditions and, as long as this is possible, evaporation proceeds at a constant rate. After a dried shell forms at the particle's surface, evaporation is dependent largely upon the moisture diffusion through the dried surface and, as a result, evaporation rates begin to fall. The addition of heat to a wet particle is insufficient by itself to dry a particle. Removal of moisture in the direct drying depends upon humidity (moisture content) of the drying air. To maintain high drying rates, cool humid air around a particle must be moved away and replaced with hot, low humid air. Humidity (amount of water vapor or absolute humidity) defines the moisture content of the air and it depends upon the air temperature and is independent of air pressure. Absolute humidity is expressed as weight of water vapor per unit weight of dry air (kgkg DA). Absolute humidities for air-water mixtures (i.e., drying air) are read directly from psychrometric charts (enthalpy/temperature--humidity). Relative humidity is water vapor content expressed relative to water content at saturation at the temperature of the mixture. The mixture relative humidity at saturation is designated 100%. Saturation temperature (dew point) is the temperature to which wet air must be cooled at constant pressure to cause condensation. The dry bulb temperature of air-water mixture can be recorded by an ordinary thermometer. Wet bulb temperature is recorded by a thermometer whose surface is saturated with liquid water and exposed to an air-water vapor mixture. The wet bulb temperature is lower than dry bulb temperature. The wet bulb temperature corresponds to the temperature at which the air would be saturated without any change in its heat content. For the saturated mixture wet and dry bulb and dew point temperatures are the same. Air-water mixtures are unique in that for all practical purposes, wet bulb and adiabatic saturation temperatures can be considered identical and wet bulb and adiabatic cooling lines coincide on psychrometric charts. Psychrometric charts allow determination of important direct drying process parameters such as evaporation rates, energy demand and efficiency, drying air parameters, wet bulb temperature, particle temperature, etc. Psychrometric charts are often used to illustrate or calculate direct drying operations. Several forms of psychrometric charts are used. Mollier charts plot moisture against enthalpy (in metric units) and are widely used in Europe and elsewhere outside the US. (In the US, Mollier diagram refers to an enthalpy-entropy chart). The most common US psychrometric chart plots temperature and enthalpy on the ordinate (Y-axis) against humidity in kgkg of dry

Thermal Treatment

305

air (DA) on the abscissa (X-axis). Psychrometric diagrams along with computerized calculations are used by direct and indirect dryer manufacturers. These calculations are design specific and the reader is referred to specialized sources and manufacturers' data [3], [4], [5]. Typically, dryer calculations are based on a computer program developed by a dryer manufacturer and vary from one type of dryer to another. Calculations, however, cannot complete a design. Empirical data, tests, and rules of thumb are required to transform data into a successful drying system design. However, a simplified method of determining two major direct dryer parameters, drying air flow and total heat requirements which are essential for sizing and estimating the price of the drying system is described below [4]. The nomograph in Figure 6-9 depicts dryer inlet (heavier curved lines) and outlet temperatures (lighter curved lines). Drying air flow in cubic feet per minute per pound of evaporated water (cfm/lb) is plotted on the abscissa against specific thermal rate (STR in BTU per pound of water evaporated) plotted on the ordinate. The nomograph is based on typical conditions: open cycle drying (once through drying air flow); atmospheric pressure; air supply temperature 60°F and 0.007 pounds of moisture per pound of dry air; feed at 60°F and 50% TS; heat loss -0.5%.

With natural gas or indirect heat the error in the airflow rate is less than 0.5%. To correct the heat load, add 4% for natural gas heating and subtract 7.5% for indirect heating. Both values are roughly 9% higher at 80% TS and 9% lower at 20% TS in the feed [3]. As an example, an 800°F inlet and 200°F outlet temperature lines' intersection has coordinates of 2.75 and 1500 approximately (Figure 6-9). Thus, the evaporation rate of 8,000 l b s h (4 to*), an airflow of 2.75 x 8,000 = 22,000 cfm (at the dryer outlet) and a total heat energy demand of 1,500 x 8,000 = 12 million B T U h are required.

V. HEAT DRYING SYSTEMS (€IDS) A. System Components

Dryers comprise an important,but not the only part of a heat drying system (HDS). Each HDS contains the following major components: (1)

(2) (3) (4) (5)

Dryer Heat Generator (Furnace) Feed Preparation and Handling Dried Product Preparation and Handling Air Pollution and Odor Control

U

E

0

Drying Air Flow/Evaporation, CFM per poundhour of evaporated water

Fig. 6-9 DRYING AIR FLOW AND ENERGY DEMAND IN DIRECT DRYERS

Thermal Treatment

307

1. Heat Generator (Furnace) All HDS require an external source of energy to heat the drying air (direct drying) or heating medium (indirect drying) such as thermal oil or steam. Typically, natural gas is used by a majority of the existing HDS; however, digester gas and waste heat from other sources (e.g., incinerator flue gas, steam, gas turbine exhaust, etc) is utilized also. For direct HDS, the heat generator (furnace) fired on natural gas is usually a cylindrical refractory lined chamber with a natural gas or dual fuel burner installed at the front end. Combustion air fan, burner controls and necessary ductwork are also part of the heat generator. The burner combustion products reach high temperature of 1700-1800°C (3 100-3250°F). For direct drying this temperature is reduced to an acceptable level of 37W50"C (700-850°F) prior to introduction in the dryer by addition of ambient air or air vented from other parts of the HDS, other equipment, housing building, etc. In indirect drying, high temperature combustion products produce steam in a boiler or heat thermal oil, water or air indirectly. 2. Feed Preparation and Handling Each HDS requires that the feed (mechanically dewatered biosolids or mixture of DWB and dry recycled product) be prepared prior to entering the HDS. Commonly DWB is either conveyed into the dryer without mixing with dry recycle or DWB/dry recycle mixture is prepared in a mixer (e.g., rotary drum, multi-tray, fluid bed, flash dryers, etc.). Formation of wet pellets occurs in the mixer and selection of the mixer's design is very important. Single shaft, high speed pin mixers or dual shaft relatively slow speed ( 2 W O RPM) paddle mixers (pug mills) are typically used. Measures must be taken to reduce wear of pins or paddles using abrasion resistant alloys. Wet pellets should be preserved from deterioration as much as possible prior to their introduction into the dryer. 3. Dried Product Preparation and Handling In the modem HDS which produce pelletized biosolids, dry product (290%TS) exiting the dryer typically undergoes the following treatment: (1) screening to separate fine, oversized and on-spec ( 1 4 mm) size fractions; (2) recycling the fine fraction to mixer; (3) reducing (crushing) the oversized fraction and recycling the fine material to mixer;

308

Girovich

(4) cooling the "on-spec", commercial size pellets prior to storage to no less than 30°C (90°F) to avoid problems in storage; (5) storing the commercial pellet fertilizer in silos provided with overheating and fire and explosion prevention devices.

In the HDS using heat drying with subsequent mechanical pelletizing additional treatment of dry product is required (i.e., compaction, screening, milling, etc.). 4. Air Pollution and Odor Control

Control of particulate matter (PM) and gaseous pollutants including malodorous compounds has played an extremely important role in the evolution of the HDS. With air emission and odor control regulations becoming more stringent on federal and state levels and ever growing public concern about environmental impact of any industrial process the utmost attention should be given to the design of the HDS air pollution and odor control equipment. Historically, removal of particulate matter (dust) was the prime concern. Initially simple PM removal devices such as cyclones have been used. Later, odor control was introduced with application of "chemical" scrubbers which use oxidants (sulfuric acid, caustic, sodium hypochloride, etc.) to control ammonia, hydrogen sulfide and malodorous organics. Significant improvement in controlling organics emission, including odors, has become possible with application of external thermal afterburners. An afterburner is a separate combustion chamber through which the exhaust gas passes after particulate removal devices and wet scrubbers. Combustion within the afterburner raises the temperature of exhaust above the auto-ignition temperature of most hydrocarbon compounds thus destroying them. The best destruction of organic compounds is achieved at a temperature above (1400°F) with sufficient turbulence and residence time (temperature-time-turbulence or "Three T" conditions). Operation above 760 "C may not be desirable because it increases NO, emissions, especially if biosolids have an elevated pH (7.0-7.5). Introduction of indirect, semi-closed and closed cycle direct HDS provides for another viable option of a considerable decrease in the gas flow and improvement in many aspects of air emission and odor control. Currently, all modem HDS are able to comply with existing air emission standards. However, there is a tendency to apply higher standards locally depending on local air quality situation. Significantly more stringent air emission and odor control limits could be imposed in the nonattainment areas under National Ambient Air Quality Standards (NAAQS). Serious analysis of applicable environmental regulations must be done prior to fmal design and selection of the HDS equipment.

Thermal Treatment

309

B. Handling and Treatment of Drying and Heating Medium Like the dryers, the HDS can be classified based on the methods of heat transfer (direct, indirect, radiation, etc.) as described in previous sections. The HDS can also be classified in accordance with methods of handling and treating their drying and heating mediums. 1. Direct HDS

Three major groups of direct HDS can be identified depending on the drying medium flow arrangements: a) open, b) semi-closed and c) closed cycles (Figure 6- 10). ODen cvcle HDS (Figure 6-10, a) bum fossil fuel (natural gas being most common) in a furnace. Combustion products are diluted by ambient air or by air vented from other parts of the HDS, from the housing building, etc. Hot mixture of combustion products and dilution air (hot drying medium) is then ducted directly into a dryer (rotary drum,fluid bed, spray or any direct type) at appropriate temperature and volume. The drying medium exits the dryer laden with water vapor, particulate and gaseous matter, including odor pollutants. After removing particulate matter (PM) in a dry air-solids separator (usually cyclones) the drying medium is cooled to condense the water vapor. The liquid condensate is then returned to sewer. Condensation of water vapor is typically accompanied by a wet scrubbing which, if designed properly, is also capable of removing some fine PM and scrubbing some gaseous pollutants, such as ammonia, and to a much lesser degree, hydrogen sulfide, mercaptants, etc. from the drying medium. However, the above mentioned treatment of the drying medium is often insufficient because it does not completely remove volatile organic compounds (VOCs) including odors and fine submicron particles from the gas stream to the levels presently required. New stringent air pollution and odor control regulations require that PM and VOCs be reduced below the levels normally achieved by the dry PM removal devices and wet scrubbers, especially with respect to malodorous organic compounds, nitrogen oxides, sulfur oxides, etc. As a result, the drying medium needs to be treated either by "chemical" scrubbers which should be designed to oxidize malodorous compounds or by high temperature "afterburners"to thermally destroy malodorous compounds and burn fine dust. In order to comply with stringent VOC removal requirements, the open cycle HDS should be equipped with an afterburner. In this case, no drying air is released into the atmosphere without complete treatment, including efficient PM removal, water vapor condensing, wet scrubbing and high temperature thermal treatment. Open cycle HDS (also called once-through air flow) are relatively simple, however they have the following disadvantages:

310

Girovich

FUEL

Q

a) DIRECT OPEN CYCLE

-

R.T.O. @(Am-@+

BURNER)

3

FURNACE

b) SEMI CLOSED CYCLE

-

Mi%%

.,15%

m I

FURHPCE

c) CLOSED CYCLE WITH INDIRECT HEATING

FURNACE

Fig.6-10 DRYING AIR FLOW ARRANGEMENTS 1)FUEL. 2)HOT DRYING AIR, 3 ) D R m R EXHAUST, 4)DRYER EXEAUST W T H PARTICUUTE M T T R R REMOVED, 5)CLiUNED AND COOLED DRYING AIR, 6)DRYING AIR RhTYCLE. 7)DILUTION AIR, 8)COMBUSTION AIR, 3)DRY PRODUCT, 1O)COOLING WATER, I l)CONDENSATE, 12)COMBUSTION PRODUCTS

Thermal Treatment

311

high volume of drymg medium increases capital and operational costs additional energy required by afterburners heat loss in exhaust potential for fire in the dryer due to direct contact of hot combustion products with dry biosolids Open cycle HDS, also known in the USA as an ESP process since the 1970s, are now marketed by Wheelabrator Clean Water Systems, Inc. (WCWS). The ESP process utilizes either "chemical" scrubbers or afterburners and it is protected by a number of USA patents [8], [9]. Afterburners with heat recovery, called RTO or Regenerative Thermal Oxidizers, which reduce energy consumption sigruiicantly, are employed by the modem ESP process. A principal flow diagram for a direct open cycle HDS based on the ESP technology with the RTO and triple pass dryer is shown in Figure 6-1 1. Similar open cycle HDS without the afterburner @TO) is used in production of Milorganite by the City of Milwaukee. Currently, there are eight (8) operational open cycle HDS in the USA (1 995). In order to reduce disadvantages of the open cycle HDS, the drylng air recirculation is used. The drymg air recirculation can be accomplishedby: Semi-closed cvcle: Partial recycle of drying air with direct heating, i.e. recirculating part of the drymg air flow after condensingwater vapor. Closed cvcle: Recycle of drymg air with indirect heating and separating drymg air fiom the heating medium. Figures 6-lob and Figure 6-1Oc depict these two arrangements. Both methods increase energy efficiency and reduce amount of air emissions although to Werent degrees. The semi-closed HDS using partial recycle of drymg air with direct heating burns fossil fuel in a furnace (Figure 6-10.b). Combustion products diluted by ambient air or by air vented fiom other parts of the HDS are ducted directly into the dryer similarlyto the open cycle HDS. The dqmg air exiting the dryer is treated in the air-solidsseparation devices (typically cyclonehaghousecombination)to remove PM and then the drymg air is cooled in a condenser to remove water vapor. After the coladenserthe nonmdensable portion of drying medium (air with some moisture) is divided into two gas streams: 1) 15%-25% of the total flow is released into the atmosphere without further treatment, and 2) 75%-85% of the flow is recycled back to the furnace. The recycled portion of the drying air serves as a dilution air and its particulate and gaseous pollutants (malodorousorganics, dust, etc.) are thermally destroyed by high temperatureof the furnace. Thus the heat generator (furnace) is used also as an "afterburner." However, certain portion (up to 25%) of dqmg air is not thermally treated in the furnace. Advantages of the semi-closed system are: 1) higher energy efficiency as compared with the open cycle system with the afterburner, 2) si@cant reduction

Fig.6-11 DIRECT OPEN CYCLE HDS WITH TRIPLE PASS ROTARY DRYER 1)LIQUID BIOSOLLDS, 2)DEWATRRED BIOSOWDS (D WB), 3)FEED MXTURB, 4)DRIED PRODUCT, S)COMRCL4L G W E PBLGBTS (FBRTI.LJZER), 6)DRY PRODUCT RECYCLE, 7)DRyBR E . U S T , 8)CLEANED AND COOLED DRITNG RLR 9)COMBUSTION AND DILUTION AlB, lO)FUEL, 1l)EOT D R m G AIR, 12)COOUNG WATER, lS,kONDBNsdTE

Thermal Treatment

313

of stack gas volume, 3) reduction of oxygen content in the drying air which reduces potential for fires (but does not totally eliminate them). Disadvantages are: 1) relatively low PM removal efficiency, especially for fine particles below 10 microns (PM,,); 2) some gaseous pollutants including malodorous organics and fme PM escape thermal treatment and are released into the atmosphere. The semi-closed HDS is marketed in the USA by Andritz Co. and some others. Currently (1995), there are two semi-closed system under construction in the USA (Figure 6- 12). Further improvements in the drying air handling and treatment are provided by the closed cycle HDS (Figure 6-10 c). In closed cvcle HDS fuel is also burned in the furnace but the combustion products heat the drying air indirectly via a heat exchanger so the combustion products and drying air are separated and as a result, the combustion products do not come in contact with biosolids and are not contaminated. The drying air indirectly heated in the heat exchanger is ducted into the dryer. After exiting the dryer, the drying air is treated in an air-solids separator (typically cyclones and a baghouse) with high PM removal efficiency, including PM,, removal. After the PM removal, the drying air is divided into two gas streams: 1) 15%-25% of the total flow is constantly drained to a condenser and then, after cooling and substantially removing water vapor it is preheated and recycled back to the furnace; and 2) 75%85% of the drying air flow is recycled back to the drying also after being reheated in the heat exchanger. Air vented from certain process equipment (conveyors, screens, etc.) is cleaned in a separate baghouse to remove PM and ducted to the furnace for thermal treatment. No drying air is released into the atmosphere without complete treatment, including efficient PM removal, water vapor condensing, wet scrubbing and high temperature thermal treatment. Advantages of the closed cycle HDS are: 1) increased energy efficiency as compared with other direct HDS, 2) separation of combustion products from biosolids and drying air and reduction of oxygen content in the drying air, which considerably reduces fire potential, 3) significant improvement in air emission and odor control, 4) decrease in the stack gas volume released to the atmosphere as compared with other direct HDS. Disadvantages are: 1) more complicated equipment and controls with the increased cost as compared with other direct HDS, 2) still uses more air than indirect HDS. The closed cycle HDS are marketed in the USA by Wheelabrator Clean Water Systems Inc. under a license agreement with Swiss Combi, Inc. (Switzerland). The process is protected by the US patent [ 101. Currently (1995), there is one closed cycle HDS under construction in the USA; over a dozen closed cycle installations are operational in Europe and Canada.

U

[ MIXER I

I RECYCLE

90-95m

Fig.6-12

SEMI-CLOSED HDS WITH SINGLE PASS ROTARY DRYER (Courtesy of Andritz, Co.) BIOSOLlDS (U), 2)DWATERED BIOSOUDS (DlVB), S)FEBD MDTURE, 4 ) D m D PRODUCT8 6 ) G O ~ R C L A LCRdDB P E U T S %)DRY PRODUCT RECYCLE, 7)aoT D R m G dlR. 8)DRyBR EXE4UST. 8)COMBUSTlON AMD DILUTION AlR, IO)FUEL, COOLED DRZWC ALR, 1.2)CUOKUC WATER, IS)COhDENSAlT, 14)DRWt'C AIR RBCYCU

Fig.6-13 DIRECT CLOSED CYCLE HDS WITH N I R E C T HEATING (Courtesy of WCWS/Swiss Combi) 1)LIQUID BIOSOLIDS (D). 2)DEWATERED BIOSOLIDS (D WB), 9)FEED MlXl'UR& 4)DRLED PRODUCT, 6)COMIvBRCU GRADB PELLETS (FERTILIZER), 6)DRY PRODUCT RECYCLE, 7)DRYXR E m U S T , 8)RECIRcuWITINc DRnNG AIR, 9)CLEANED DRYING AIR, lO)FUEL, 1l)COMBUSTION PRODUCTS, 12)COMBUSTION AIR, 1 S)COOLING M TER, 14)CONDENU TE

316

Girovich

2. Indirect HDS The indirect HDS use very little air and only to facilitate removal of water vapor from the dryer. The air used in the indirect HDS is not the heating medium. It is called "sweep" air and it plays insignificant role in the overall energy balance. Typically the %weep'' air comprises from 10% to 30% (by volume) of the total indirect dryer exhaust with the remainder being water vapor with small amounts of PM and gaseous pollutants including malodorous compounds. As an example, the amount of sweep air required for an indirect drying with evaporation capacity of 4.0 metric tonhour is only 3 0 4 0 cubic meterdmin (1200 -1500 cubic feet/min approximately), while the amount of drying air required for the same size direct rotary drum dryer is in the 700 -900 cubic meter/min. range. As a result, handling and treatment of the sweep air is significantly less expensive than in the direct HDS. However, even this small amount of the "sweep" air unless properly treated can cause odor problems. Similarly to the direct HDS the sweep air handling and treatment can be accomplished in open and closed cycles. In the closed cycle arrangement, which is used by almost all indirect HDS, the dryer exhaust is first cooled to condense water vapor and then scrubbed to remove PM and gaseous pollutants. A relatively clean noncondensibleportion (mainly air with some VOC, submicron PM,,, organic malodorous compounds, etc.) is treated typically by a condenser and wet venturi-scrubber and then it is ducted to the high temperature zone of the HDS furnace (or boiler) for thermal destruction and deodoration (Figure 6-14 and 6-15). The furnace serves as a built-in afterburner. No dryer exhaust is released into the atmosphere without complete treatment, including efficientPM removal, water vapor condensing,wet scrubbing and high temperature thermal treatment. Advantages of the indirect closed cycle HDS are: 1) high energy efficiency due to heat recovery as compared with direct HDS; 2) separation of heating medium (steam, thermal oil) from biosolids and reduction of oxygen content in the dryer exhaust, which considerably reduces fire/explosion risks; 3) significant improvement in air emission and odor control; 4) substantial decrease in the stack gas volume released to the atmosphere as compared with direct HDS. Disadvantages are: 1) higher cost of indirect dryer, and 2) indirect HDS with horizontal en-masse dryer produce low quality product (inconsistency of particle size, low durability, etc.) The indirect HDS based on horizontal en masse (Figure 6-1 5) dryer typically require additional mechanical pelletizing and classification equipment to produce an acceptable pelletized product (fertilizer). The indirect HDS with multitray PelletechB dryer are marketed in the USA by the Bio Gro Division of WCWS under a license agreement with Seghers Engineering Co. (Belgium).

Fig.6-15 INDIRECT CLOSED CYCLE HDS WITH MECHANICAL PELUTIZING (Courtesy of Stord, i n c . ) LIQUID BIOSOLIDS LB , 2 DEWATERED BIOSOLIDS DWB), FEED, 4 D R m D d7RODUCT, 5 COMMERl d C d D E PELLETS FERTILIZ6R , 6 D 1Y PRODJCTAIR# RECYCLE, ) D R m R E&UST, 8)CLEMED AND COOLE6 SWEEP Ad#!&OMBUSTION 9

1

4

d

10 FUEL, 1 1 STEAM# 12 CONDENSATE RETURN, 1S)COOLING WATER, I4)CONDENSATE. lt&OMBUS ION PRODU TS

Thermal Treatment

319

The process is protected by the U.S. patent [ll]. Currently (1995), there are three operational systems of this type in the USA (City of Baltimore, Figure 6-14). The multitray indirect dryer can be used in conjunction with an incinerator in a dual purpose heat drymg-incineration system to produce pellets for beneficial use andor incinerate biosolids (Figure 6- 16). Such an HDS will have the advantage of producing a beneficial use product (pellets) in Bccordance with seasonal market demand and incinerating biosolids when demand is down while maintaining full processing capacity. Currently, there is no dual use HDS in the USA, one similar system operates in Belgium [141.

C. Environmental Control and Regulatory Issues

Early biosolids HDS operatedin the 1970s in Europe and the USA had only primitive partidate matter (PM) control devices such as mechanical cyclones with low pressure drop (6 - 8" WC). With gradual introduction of en~onmentalregulations throughout the industrialized nations of Europe and the USA, the biosolids HDS enviramnental control has been evolving toward the use of more efficient and costlier air anissianand odor control technologies to comply with ever tightening regulations. New limitshave been established for particulate and gaseous pollutants including total suspended partidate (TSP), PM,, (particulate matter smaller than 10 ym), SO,, NO,, CO, chlorides, total hydrocarbons(THC), volatile organic compounds (VOC), toxic and hazardous air pollutants (TAP, HAP)and metals (lead, mercury, beryllium). The U.S. EPA has established National Ambient Air Quality Standards (NAAQS) for six "criteria pollutants" (SO,, NO , PM, CO, ozone and lead). Until early 1990, most air quality regulations focused on control of the criteria pollutants. The 1990 Clean Air Act (CAA) requires also that major VOC sources in w t areas be quantiiied and reduced. Nonattainment areas are geographic regions where ambient air concentration of one or more of the criteria pollutants exceeds the established limit. The US.EPA is also scheduled to propose new standards for control of HAP from the POTW in late 1994. Subpart E of the 503 Regulations specifically regulates the operation and emissions fiom the biosolids incinerators. The regulatory air permitting process for a HDS can be complex especially in the states that regulate toxic (hazardous) air pollutants as well as criteria pollutants. It is almost certain that these pollutants as well as odors will be of public concern during the HDS permitting. Public relations and possible opposition may be in fact time-consumingand difficult process. In addition to air quality approval the HDS will be required to obtain other permits and approvals in order to construct and operate the system. These will include biosolids utilization permits, water and sewerage construction permits,

W h)

0

HDS WITH PEUTECN@DRYER

AND INCINERATOR

1)UQUID BIOSOLIDS, 2)DBWAlTRED BIOSOUDS, 3)FEED To DRYER (60-70XTS), 4)FRED TO FLUIDIZED BED (40-4%TS), 6)DRY PRODUCT, 6)FBRlTLIZER PBLLETS, 7)DRY FINE RBCYCJX, 8)DRlTR BXTUUST, 9)FB EXHAUST, 1O)FLUIDIZING AIR, I l)RBClRCUL4TINC T E E W OIL, 12)FUEL, 1S)COOLlNG WATER, 14)CONDENS!4TB AND SCRUBBER BLOW-DOm, f6)ASH TO LANDFILL

Thermal Treatment

321

requirements (stormwater, soil and erosion control management, wetlands impact, impacts in critical areas, etc.), approvals to market and distribute the biosolids derived product, etc. Usually air quality permits are the most difficult and time consuming to obtain. 1. Criteria Pollutants

Under federal regulations, new sources must provide estimates of emissions of criteria pollutants. Allowable emission rates of each of the criteria pollutants and required control technologies depend somewhat on air quality conditions in the particular area where the new source is to be located. In general, under federal regulations, emissions from sources in areas that have been determined to have relatively good air quality ("attainment areas") are regulated under Prevention of Significant Deterioration (PSD) regulations. Sources in areas with relatively poor air quality (nonattainment areas) are regulated under nonattainment review regulations. The HDS usually will not be subject to Prevention of Significant Deterioration (PSD) regulations, since it is not large enough to be considered to be a "major stationary source." A HDS itself would have to emit more than 250 tons per year of any regulated pollutant to be considered a major source for PSD purposes. However, some states have developed specific regulations in nonattainment areas such as, for example, VOC and NOx for the ozone nonattainment areas. In Maryland, for example, these regulations, known as the New Source Impacting a Nonattainment Area (NSMA) regulations, apply to new sources or modifications to existing sources that emit 50 tons per year, 1,000 pounds per day, or 100 pounds per hour of VOCs, whichever is most restrictive. Direct and indirect HDS seem likely to be able to meet these limitations unless they are of very high capacity. There has been a revision in federal particulate matter regulations that is of interest in terms of the HDS permitting. In 1987, the US EPA changed its ambient standard for particulate matter from a limit on total suspended particulate matter (TSP) to a limit on particulate matter less than 10 microns in diameter (PM,,). The standard was revised to account for the fact that smaller particles pose more of a public health threat than larger particles because they are inhalable. Most states are in the process of redefining attainment and nonattainment areas for PM,,.

2 . Toxic (Hazardous) Air Pollutants (TAPSand HAPS) In many states now a HDS will also be subject to review of TAP (HAP) emissions. For example, under Maryland TAP regulations, the HDS construction permitting will require that the applicant for a new source: - quantify TAP emissions from the proposed project,

322

-

Girovich

demonstrate use of Best Available Control Technology for Toxics (T-BACT), and - perform an Ambient Impact Analysis for the TAP emissions. The Maryland TAP regulations exempt "fuel burning equipment" which is defmed as "any boiler or furnace which has the primary function of producing hot air, hot water, or steam through indirect heat transfer from the burning of fuel." One interpretation of these regulations would exempt an indirect HDS from requirements of TAPS. However, this furnace may also serve as an afterburner. As such, it is the sole emission point from the system. It is likely, therefore, that the indirect HDS furnace may be subject to the TAP regulations. The direct HDS will be subject to the TAP regulations in Maryland in any case. The TAP regulations cover emissions of any substance emitted to the atmosphere not already regulated under federal criteria pollutant or National Emissions Standards for Hazardous Air Pollutants (NESHAPs) regulations. This means essentially,that any substance emitted by the sources will be regulated under the TAP program. Metals and organics emitted from the HDS will be subject to TAP review. The second requirement of the TAP regulations in Maryland compels a new source to demonstrate that it will employ the most stringent emissions control technology available for toxic pollutant emissions, referred to as T-BACT. The TBACT demonstrationmust be made for each regulated TAP emitted by the source. Maryland Department of Environment (MDE) uses a "top down'' approach for reviewing various control technologies proposed by a new source. This means that a source must first consider installing the most stringent control in use by any other similar type of source, unless it can be shown that that particular control is not feasible in terms of economics or energy use. The source must then use the second most stringent control, unless it too is deemed to be too expensive, and so on until a control strategy is found. T-BACT reviews are performed by MDE on a case-bycase basis. New sources must employ T-BACT, according to the regulations, regardless of whether the source demonstrates that its emissions are within allowable levels. In order to fulfill the third requirement of the TAP regulations, the Ambient Impact Analysis, the source must successfully demonstrate to MDE that TAP emissions will not unreasonably endanger the public health. This analysis can be performed in a number of ways, but generally involves estimating TAP emissions, modeling how emissions will affect ambient concentrations of that pollutant, and comparing ambient concentrationswith toxicological screening levels to assess the public health impact. The modeling can be accomplishedby several methods, from simple, conservative (overestimating)hand calculator techniques to more complex computer air quality dispersion modeling. The analysis must review impacts to the public of emissions of both carcinogenic and noncarcinogenic substances. If emissions of any TAP do not pass this initial screening, the source must perform

Thermal Treatment

323

more complex modeling and risk assessment techniques to prove that although emission levels produce impact above screening levels, the projected off-site ambient concentrations will not unreasonably endanger the public health. Pollutants regulated under the federal NESHAPs program (or pollutants for which the EPA is proposing to regulate under the NESHAPs program) are exempt from the T-BACT and Ambient Impact Analysis portions of the TAP regulations. EPA has currently promulgated NESHAPs governing emissions of eight pollutants from a specific set of sources. One NESHAP governs mercury emissions from facilities that incinerate or dry wastewater treatment plant solids (40 CFR 61SO). A biosolids dryer is defined as "a device used to reduce the moisture content of sludge by heating to temperatures above 65°C (- 150°F) directly with combustion gases" (40 CFR 61.50(m)). Thus the EPA recognizes the direct drying technology as a potential source of hazardous mercury emissions and would regulate it under NESHAPs. The indirect drying system, on the other hand, does not appear to be covered by NESHAPs, and would be regulated under the Maryland TAP program only. NESHAPs limit mercury emissions from HDS to 3,200 grams of mercury per 24 hours (or approximately 7 pounds per 24 hours, or 1,820 lbs/yr (assuming operations at 5 daydweek, 52 weekslyear)). The NESHAPs also require applicable sources to test mercury emissions within 90 days of start of operations. By contrast, allowable mercury emissions under TAPS would be a function of facility design and may be less than or greater than the NESHAPs limit. 3. Odor Emissions

Under air quality regulations practiced by many states, sources are prohibited from discharging odors "beyond the property line in such a manner than a nuisance or air pollutant is created." There are no quantitative limits placed on odor emissions as there are on other air emissions; however, sources are, by regulation, prohibited from generating any emissions that can be detected outside the property boundary. In general, regulators are reluctant to specify odor concentrations or BACT for odor control. These parameters will be evaluated on a case-by-case basis taking into consideration the economics and effectiveness of the control technologies. Some states (Virginia, Pennsylvania) require afterburner technology on the biosolids HDS if biosolids have a high VOC content. Indirect and closed cycle direct HDS have a lower overall potential for emitting pollutants, primarily because their air flows are considerably lower than that of direct open cycle HDS. The reduced air flow allows the incorporation of higher performance emission controls and BACT into the HDS at a low cost. Even with the same control efficiencies, pollutant loadings from the open cycle direct HDS would be still higher due to the order of magnitude difference in the gas volume discharged.

324

Girovich

Despite the differences in emission rates and pollution control efficiencies, practically all modern HDS could obtain permits and approvals in accordance with existing regulations. However, should air emission regulations tighten, the indirect HDS would have less problems to comply. 4. Air Pollution and Odor Control Equipment

a. Particulate Matter Removal For the PM removal, modern HDS use mechanical cyclones, fabric filters, wet venturi-scrubbers and afterburners. In the direct open cycle HDS (Figure 6-1 1) mechanical cyclones are used to remove biosolids pellets and coarse PM from the dryer's exhaust. Mechanical cyclones are generally inefficient for the removal of PM,,. The PMloremoval is largely accomplished by a wet venturi followed by an impingement or packed bed scrubber and the afterburner. The venturi agglomerates fine PM which enhances PM removal in the downstream wet scrubber. The wet scrubber also serves as a condenser and a gaseous pollutant removing device. Venturi-wet scrubber is well suited as a condenser and as a coarse PM removing device but it is not effective for removal of water insoluble organic and inorganic species and submicron PM. Venturi-wet scrubber removal efficiency (RE) for hydrocarbons is generally 5% to 30% only and for the submicron PM is usually less than 60% unless high pressure drop venturi is employed. In the closed cycle HDS with indirect heating (Figure 6-13), a combination cyclone-fabric filter has been used to effectively remove coarse and fine PM. Use of fabric filters should be carefully analyzed to avoid water condensation and blinding of the filter bags. In the indirect HDS, the PM removal is effectively accomplished by a wet venturi-scrubber followed by a built-in afterburner due to small air flow and ability to use high pressure drop (Figure 6-14). Usage of a fabric filter in this case is not recommended because of the small air flow and high humidity of the gas. b. Gaseous Pollutants and Odor Removal The modem HDS with well designed furnaces fired on natural gas do not emit carbon monoxide, sulfur oxides and chlorides (as HCI) above limits established for this gaseous criteria pollutants and no control devices are usually employed to remove them. A totally different situation is observed with respect to total hydrocarbons, ammonia, nitrogen oxides, hydrogen sulfide, organic sulfides, amines, mercaptans and some other volatile organic compounds.

Thermal Treatment

325

Regulations now establish strict stack limits for total hydrocarbons (THC) or VOC and nitrogen oxides. The BACT for the THC (VOC), organic sulfides, mercaptans, etc. removal is afterburning or thermal oxidation. However, some HDS especially at the low capacity end may still use "chemical scrubber" to remove NH,, H,S, THC (VOC), mercaptans and other organic, often malodorous compounds. In this case, removal of ammonia and H,S is typically accomplished by wet scrubbers using acid (H,SO,) or caustic (NaOH) respectively. Removal of organic species is achieved by wet scrubbers using strong chemical oxidizers such as sodium hypochloride (NaOC1) and hydrogen peroxide. Sufficient gas residence time and the amount of scrubbing medium and chemical oxidizing agents should be incorporated into the chemical scrubber design to ensure that organic species will be oxidized and deodorized. Removal efficiencies of the chemical scrubbers vary significantly for different organic species. Separate or built-in afterburners (RTO) operating at 700-800°C with 0.5-2 seconds of retention time provide 90-95% removal efficiency for all organic species. (Figures 6-1 1,6-12,6-13 and 6-14) Operating the afterburners at higher temperature is not recommended due to increase in NOx emission. Potential source of NOx emission is the release of gaseous ammonia from biosolids with an elevated pH during drying and subsequent partial conversion of ammonia into NOx in the afterburners at high temperatures. To reduce or eliminate this phenomenon, ammonia must be substantially removed from the gas stream in the wet scrubber prior to its entering the afterburner. The wet scrubber should be designed to perform as an ammonia and PM removal device with sufficient liquid to gas ratio and pressure drop and injection of sulfuric acid to convert ammonia into ammonia sulfate salt which is removed with the scrubber blowdown. D. Economics of Heat Drying

Selection of a heat drying process should be based on thorough technical, environmental and economic evaluation. Some evaluation criteria such as capital, operational and maintenance costs, utilities, equipment, etc. are relatively easy to quantify. Others such as future changes in biosolids quantity and quality and environmental laws, market trends, etc. are dificult or impossible to attach a dollar value. To initiate a meaningful evaluation process, the following major parameters should be analyzed and incorporated in the design concept: 1) required throughput in dry tonnage and water evaporation capacity; 2) mode of operation; 3) biosolids quality (dryness, chemical composition, compliance with the established limits of pollutants, etc.); 4) product storage capacity; 5) methods of product disposaUmarketing;6) applicable environmental regulations concerning particulate, gaseous and liquid emissions and odor control; 7) site specific conditions such as

326

Girovich

availability and cost of all utilities, including process water, wastewater discharge, fuel, electricity, etc.; traffic patterns and access roads, proximity of residential areas, zoning, etc.; and 8) public acceptance or opposition to the project. 1. Operating and Maintenance Cost

Operating and maintenance (0 & M) costs of an HDS operation includes the following major components: a. Labor Modem, well-designed HDS are automated and usually run 3 shifts, 24 hours a day, 6-7 days a week. Typically, a single dryer HDS employs two operators per shift. Supervisory, maintenance and clerical personnel depend on the overall facility design, mode of operation, etc.

Fuel (usually natural gas) comprises the major 0 & M expense (between 20% and 25%). Natural gas cost varies from $3.0 to $4.5 per million BTU. If the overall STR for HDS is known (or better, guaranteed by the system's supplier), the fuel cost per hour of operation can be calculated as follows:

C, = STR

E

k

. . . . . . . . . . . . . . . . . . . . . . . . . (6-11)

To calculate the fuel cost per dry ton of biosolids processed, the following formula is applied:

C, = fuel cost, $ k r C, = fuel cost, $/dry weight unit STR = specific thermal rate, kcalkg (BTUAb) of evaporated water E = system's evaporation capacity, kg/hr (lbhr) of evaporated water k = cost of fuel, $ per unit of energy di, do= biosolids and product dryness respectively

Thermal Treatment

327

For example, using formula (6-12) for a natural gas fired HDS with the STR = 1,500 BTU/lb of evaporated water; incoming biosolids dryness of 22% TS and

product dryness of 92% TS; k = $4.5 per million BTU (2,000 lbs = 1.O dry ton):

C, = 1,500 x 2,000 (1/.22 - 1/.92) x $4.5

x

lo6 = $46.7 per dry ton

The fuel cost (per unit of dry weight) depends greatly on the incoming biosolids dryness (Figure 6-8 and Figure 6- 18) so that at the dryness of 30% TS the fuel cost drops to $30.3 per dry ton. c. Electricity A single train HDS with evaporation capacities between 1.O and 4.0 metric ton per hour requires electrical power in the 200 - 300 kw range depending upon the HDS type. Electricity consumption per dry ton will depend on the HDS capacity and its utilization, mode of operation, etc. Typically, 180 - 320 kilowatt-hours per dry ton of biosolids processed is required, or $12.6 - $22.5 per dry ton approximately (at $0.07/kwh).

d. Reuair. Maintenance and Maintenance Reserve Expenses associated with equipment repair, replacement, maintenance and up-keep are site specific and usually between 2.0 and 2.5% of the HDS capital cost (as installed).

e. Water and Wastewater The HDS require substantial amount of water for purposes of exhaust gas cooling, condensing, scrubbing, washing, etc. Potable water is also required for personnel. f. These include overhead expenses such as supervision, insurance, phone, security, clerical, etc. g.

;

The biosolids fertilizer produced by a HDS can be sold to various consumers. Expenses associated with this activity may include transportation, promotion and marketing, etc. Revenue from the biosolids fertilizer sales help to reduce the cost of production.

Girovich

328

z

405

0

I-

>D L 360 c2

2 3

315

-t

cn

8 270 Z zz

6 225 3 a

0 lr

a

180

. .

2

I-

135

CT

w

L 0 a 1

90

0

8m 45 5,000( 15)

10,000(30) 20,000( 60) PRODUCTION CAPACITY, DRY TONS PER YEAR (PER DAY)

+

Fig.6- 17 BIOSOLIDS FERTILIZER PRODUCTION COST

Thermal Treatment

329

Figure 6-17 illustrates 0 & M cost component's dependence on the HDS production capacity (economy of scale). Profit, cost of land, insurance, taxes and similar project specific expenses are excluded. No fertilizer sale revenue are included. Typically the biosolids production cost (or tipping fee) components are as follows: Labor - 15 -25% Fuel 20 - 30% Electricity - 10% Maintenance and repair - 15 - 25% Overhead and administrative expenses - 15 - 25% Miscellaneous - 5%

-

2. Method to Improve the HDS Performance In order to improve performance, increase productivity and reduce operational cost of the HDS, both technical and nontechnical methods can be employed. Increase drvness of the feed. Higher dryness dramatically improves throughput and decreases fuel consumption (per dry ton). For example, increasing the incoming biosolids dryness from 18% TS to 30% TS reduces the cost of energy (per dry ton) by almost two times and increases dry throughput accordingly. Increase the AT, Use higher inlet and lower outlet temperatures for drying air (direct dryers) and heating medium (indirect dryers). Watch for fire danger at the inlet and water condensation at the outlet of the dryers with high inlet and low outlet temperatures. Recvcle drvinv air. Recycle improves thermal efficiency, decreases potential for fire and reduces environmental impact. Automate Furnace Operation bv usinv reliable and accurate firine controls. Recover heat enerev by combustion air preheating, biosolids preheating, air recirculation, etc. Among nontechnical methods the following are beneficial: Combine dewatering and heat drvinp in one plant. Such combination always saves labor and simplifies biosolids handling. Buv natural eas on a mot market instead buying from a utility. Often better gas prices can be enjoyed in this case. If you buy from the utility, consider socalled "intermptable" service which is less expensive. However, in this case either an alternative fuel system should be installed or the HDS gas supply might be cut during certain periods of the year. Install high efficiencv furnaces (boilers) and heat exchangers. Improve aualitv of Delletized fertilizel; (eliminate odor and dust; improve nutrient content, etc.)

330

Girovich

Establish aggressive marketin? program backed up by quality control to maximize your revenue from the fertilizer sale. Use "economv of scale," i.e., the higher the HDS capacity the better is its economics (Figure 6-17). VI. PRODUCTION OF FERTILIZER: CASE STUDIES A. Milwaukee Biosolids Drying and Pelletizing Plant

For nearly 70 years, biosolids generated at the City of Milwaukee's Jones Island WWTP were dewatered and dried in direct rotary drum dryers to produce Milorganite, a popular biosolids fertilizer product with 6% nitrogen, 1% phosphorus and 4% iron. In 1994 the facility was retrofitted to dewater, dry and pelletize 200 tons (182,000 kg) of biosolids per day [ 131. Liquid biosolids from different sources are blended to maintain consistent composition and conditioned by ferric chloride to maintain a guaranteed 4% iron and to facilitate coagulation; by cationic polymer to assist in flocculation, and by hydrogen peroxide to control odor. Twenty-four (24) two meter wide belt filter presses (BFP) dewater biosolids to 16-1 8% TS. Each BFP is capable of dewatering up to 10 tons (9,000 kg) per day of liquid biosolids ranging from 4% to 5% TS. The drying and pelletizing plant uses an open cycle HDS and employs twelve 8 ft. by 50 ft. direct rotary drum dryers, each with evaporation capacity of 9,000 Ibs. (4,100 kg) per hour. Assuming 16% TS in the biosolids feed, it amounts to a total production capacity of a minimum 10.5 dry t o n s h . Heat sources for the drying include waste heat from the turbine exhaust and natural gas firing. Turbine exhaust at 900°F (482°C) from two natural gas fired 15 MW turbines which provide electrical power for the entire WWTP is conveyed to the dryers. The turbine exhaust is augmented by firing natural gas. The dryer's exhaust is first cleaned in cyclones to remove coarse PM from the gas stream and then further treated by an impingement quench chamber and a wet electrostatic precipitator (WESP) prior to its release to the atmosphere via 350 ft (107 m) stack. The WESPs are permitted to release less than 0.005 grainshcf of particulate matter. The greater Milwaukee area is a nonattainment area for sulhr dioxide under the National Ambient Air Quality Standards, Consequently,the air permit contains stringent pollutant emission limitations. The regulated pollutants include PM, sulfur dioxide, nitrogen oxides, carbon monoxide, hydrocarbon and volatile organic compounds, cadmium, lead and mercury. Testing is required for these pollutants plus nickel, chromium, chloride as HCI, and opacity.

Thermal Treatment

331

Dried product handling system separates oversized, fine and on-spec dried products. The oversized material is reduced in size and recycled as required. Dry fine biosolids are separated, recycled and mixed with wet biosolids feed in the mixers. The on-spec pellets are cooled to 32°C (90°F) to prevent heat generation during storage and fire potential. After cooling, Milorganite is pneumatically transported to and stored for long periods (up to three months or 18,000 tons) in 14 silos. Due to seasonal changes in nitrogen content, Milorganite with substandard nitrogen level is stored separately for later blending with high nitrogen content product to maintain the guaranteed 6% nitrogen. Due to relatively large generation of fines (over 40 tons per day), a special fine material processing system is included. The fines are mixed with hot water and extruded to form pellets. The pellets are cooled by air, reduced in size and returned to the dried product handling system. Fine material collected from the cyclones (so-called chaff) is treated by an outside contractor which converts it into a product similar to standard Milorganite. B. New York City Biosolids Fertilizer Facility

The world's largest direct heat drying and pelletizing facility was put into operation in 1993 in the City of New York (Bronx). The facility uses an open cycle direct HDS (ESP process) (Figure 6-1 1) with regenerative thermal oxidizer (RTO). The New York facility is capable of processing up to 270 dry metric tons a day (300 dtd) and it employs six (6) independenttriple pass rotary dryer process trains. The facility, privatized by Wheelabrator Clean Water Systems Inc., processes dewatered biosolids from New York City's dewatering plants (Figure 6-18, photo courtesy of WCWS). Trucks dump dewatered biosolids (DWB) into large holding pits in an enclosed tipping area. A series of screw conveyors located at the bottom of the pits move the DWB to conveyor belts. The belts move the DWB into hoppers which feed the material into mixers at a controlled rate. In the mixers, the DWB are mixed with recycled dry product to produce a wet granular feed for the triple pass rotary dryers. The DWB are dried fiom approximately25 - 30% TS to 95% TS in the rotary dryer. Upon leaving the dryer, dry pellets are separated by size. Oversized pellets are reduced to fines before being mixed with the undersized pellets in a recycle bin. The fine pellets are fed into the pin mixer to begin the process again. "On spec", market size pellets ( 1 4 mm) are pneumatically conveyed after cooling to 30°C to storage silos before being loaded into closed-hopper rail cars and shipped for beneficial reuse. The entire process is controlled by computers in a centrally located control room.

332

Thermal Treatment

333

Drying air is treated by dry cyclones, recirculating wet venturi-scrubbers with sulfuric acid injection designed to remove PM and ammonia and the RTOs (afterburners). The RTO operates at approximately 1560°F (with 95% thermal energy recovery) which destroys organic compounds present in the drying air prior to its discharge to the atmosphere. The RTO destruction and removal efficiency with respect to the VOCs is no less than 97%. The facility was constructed at the total cost of approximately $120 million (1992). The facility achieved high level of energy efficiency (less than 1500 BTU/pound of water evaporated including the RTO) and very low stack emission levels in full compliance with stringent environmental permits. The facility is permitted to emit no more than 0.008 grains of PM per dry standard cubic feet (corrected to 1% COJ; no more than 16 ppm of NO,.. 8.8 ppm of CO; 12 ppm of total non-methane hydrocarbons (TNMHC). The biosolids pellet fertilizer is marketed in Florida and some other states.

C. Baltimore City Biosolids Fertilizer Facility The world's first and the largest indirect heat drying and pelletizing facility was put into operation in 1994 in the City of Baltimore, Maryland. The facility is located on a 1.5 acre site at the 466 acre Back River WWTP. The 180 mgd plant serves 1.3 million people living in Baltimore City and Baltimore County. The facility uses a closed-cycle indirect HDS (Figure 6- 14) based on the Bio Gro-Seghers patented process [ 113. The process was developedjointly by Seghers Engineering Company (Belgium) and the Bio Gro Division of Wheelabrator Clean Water Systems Inc. The facility is designed to process 55 dtd (230 wtd) with 100% redundancy (total capacity 110 dtd). The facility normally receives liquid biosolids and its three centrifuges dewater them from 2 4 % TS up to 26% TS. The dewatered biosolids are conveyed to the HDS. Biosolids dewatered by the City's centrifuges can also be received and processed. The facility is privatized by WCWS. The City pays approximately $96 per wet ton of biosolids processed which includes cost of financing. Three (3) independent Bio Gro-PelletechO multitray dryer process trains are used to simultaneously dry and pelletize the biosolids (Figure 6-19, photo courtesy of WCWS). Each dryer (Fig. 6-6) is capable of evaporating not less than 4.0 metric tons of water per hour (35 dry tons per day approximately). The PelletechO dryer uses thermal oil heated to 260°C (500°F) in the natural gas fwed thermal oil heater. Thermal oil (heating medium) circulates through the dryer hollow trays. A small amount of sweep air is used in the dryer. The dryer's exhaust is first cooled by liquid biosolids to recover energy and to improve the dewaterability (Figure 6-14). The liquid biosolids are preheated up to 140°F (60°C) which improves the overall system thermal efficiency by approximately 3 4 % .

Thermal Treatment

335

Improvement in dewaterability due to the liquid biosolids preheating is manifested in higher solids concentration of the centrifuged biosolids. According to tests, for every 10°F increase in temperature, the solids content increases by approximately 0.1% at the same polymer dosage. The dryer's exhaust is further cooled by the process water (plant effluent) and treated in a condensor and a venturi-wet scrubber to remove water vapor, PM and gaseous pollutants. Cleaned non-condensible gas (air) still containing some VOCs including malodorous compounds is ducted to an indirect preheater and into a high temperature zone of the thermal oil heater for thermal destruction and deodorization. The heater serves as an afterburner. The condensate (along with centrate from the centrifuges) is sent back to the host WWTP. The pelletized product exiting the dryer is conveyed to the dry product preparation and handling system where it is screened and separated into oversized, fine and commercial grade size fractions. The oversized fraction is then reduced in size and recycled back to the mixer along with the fine fi-action. The commercial grade size fraction (usually between 1 and 3 mm) is indirectly cooled by water to approximately 30°C (90°F) and stored in the product silos for marketing and distribution. A liquid nitrogen system is used to prevent overheating in the product silos. All system components such as dryers, screens, crushers, conveyors, etc. are enclosed and operate at slightly negative pressure to eliminate odor and dust escape. A microprocessor-basedcontrol system automates and monitors the entire process. The facility total cost, including dewatering, was approximately $30 million (1992). The facility has achieved high energy efficiency and very low stack emissions. The facility is permitted to emit no more than 0.03 gr/dscf. VII. OTHER THERMAL PROCESSES

In addition to drying and incineration, several other heat treatment processes are used in the biosolids management. Some of them are employed to thermally condition liquid biosolids prior to dewatering [ Z h p r o process, Envirotech (formerly Porteous) process, Nichols (formerly Dorr Oliver Farrer System), Zurn, etc.)]. Multi-effect evaporation process (Carver-Greenfield System) is also employed as biosolids thermal processing prior to dewatering. Numerous innovative thermal processes have been developed in the last decade, the majority of which have not yet been proven in a full-scale municipal plant project (e.g. gasification, oil from sludge (OFS) processes; bricks and tiles from sludge, etc.). A description of some of these innovative processes is provided in [ 151.

336

Girovich

A. Carver-Greenfield (C-G) Process

The C-G process uses a solvent (oil) to disperse a wet material (e.g., liquid biosolids) and evaporate water out of the suspended oiVsolids emulsion. The C-G technology started in the mid 1940's and the frst commercial C-G plant was built in Philadelphia to dry rendering plant waste. In 1964 a C-G plant was constructed for the Hershey Corporation sludges. Numerous C-G plants are operational worldwide treating food, dairy, pharmaceutical, oily and other sludges [12]. In 1984, the U.S. EPA evaluated the C-G process and declared it as "innovative and alternative," thereby permitting municipalities to obtain federal grant subsidies for C-G process plants under the Clean Water Act of 1977. As a result, four municipalities were contracted to build C-G process plants in the U.S.: the City of Los Angeles (Hyperios WWTP), the County Sanitation District of Los Angeles, Mercer County Utility Authority (NJ) and Ocean County Utilities Authority (NJ). All USA C-G biosolids processing facilities have rather troubled history. The City of Los Angeles built and started up the first light solvent C-G plant for municipal sewage sludge in 1987. Dried biosolids generated by the C-G process are incinerated in the fluid bed furnace producing 11 MW of electricity. However, the Los Angeles C-G plant experienced serious operating problems. Following a number of changes to the C-G plant, the City of Los Angeles has been operating one train at a time at a rate of about 50 dry tons per day [1995]. The City plans to make further improvements to the plant and to operate two trains for a combined capacity of 160 dry tons per day some time in the future. Other changes are also being implemented (e.g. installation of Stord indirect steam dryers as a back-up). The Ocean County Utilities Authority (OCUA) build their C-G plant in 1990 at the cost of approximately $23 million. This plant was designed at about the same time as the other U.S. municipal plants, but differed from the others in two important ways. First, a mechanical vapor recompression system was used instead of a four-effect evaporation system. Second, the feed contained only 7 percent dry solids, compared with 20% TS at the other plants. As a result, three times as much water had to be evaporated (per dry ton of solids) compared with the other plants. The OCUA operated the plant at up to 30 dry tons per day (design rate-50 dtd) and sold the dried and mechanically pelletized biosolids in Florida. The plant experienced severe downtime problems and cost of treatment was very high (approximately $600/dt). Consequently, in the fall of 1992, the OCUA shut the plant down. The other two U.S. C-G plants, owned by the County of Los Angeles and Mercer County, New Jersey, have been built but have not yet been started up (1 994).

Fig.6-20 SIMPLIFIED CARVER-GREENFIELD PROCESS FLOW DIAGRAM t)UQlIID BIOSOLIDS. 2)BIOSOLIDS-OIL SLURRY, 3)FLUIDIZING OIL, 4)WATER FREE SLURRY, 5 PROCESS S T E M , 6)RGCOYERED OIL, 7)OIL AND CONDENSATE, 8)LOW OIL SOLIDS, 9IkENl' GAS, f0)OIL VXPOR AND S T E M

338

Girovich

The C-G process is a multiple-effect evaporation system that utilizes a fluidizing oil to overcome the problems of pumpability and heat transfer, which limit the efficiency and cost-effectivenessof other evaporation systems. The C-G process uses multi-stage (multi-effect) evaporation. The process starts by mixing biosolids slurry with a light carrier oil in a fluidizing tank at a ratio of between five and ten parts oil to one part biosolids. It is then fed to the fust stage of a multiple-effect evaporation system. Hot vapor from the preceding evaporator, or effect, is used to drive off water from the sludge in the following effect. The pressure inside the evaporators is reduced by a vacuum system to allow vaporization to occur at a lower-than-normal temperature. The last effect - where the final quantities of moisture must be removed - consumes steam. Temperatures decrease from the last effect to the first while the vacuum is increased by means of a condenser. As the biosolids pass from one effect to the next, its viscosity increases, making transport more difficult. The carrier oil prevents plugging that would result from viscosity effects through the system. After virtually all water is evaporated, a centrifuge is used to remove approximately half of the oil from the solids-oil emulsion from the second (last) effect. The resultant biosolids cake is then transported into a steam-heated deoiler where most of the remaining oil is evaporated. Carrier oil retention in the solids leaving the deoiler varies from 0.2 to 0.5 percent. The C-G process is significantly more efficient thermally than conventional drying systems because it combines the energy efficiency of multi-effect or mechanical vapor recompression (MVR) evaporation with the use of a light fluidizing carrier oil. Modem HDS typically have the energy demand (STR) in the 1,500-1,600 BTU range per pound of water evaporated. However, in a C-G process with MVR, total STR can be reduced to less than 800 BTU per pound of water evaporated (theoretically to 300-400 BTU). All of the solvent is recovered and reused within the process. Extracted compounds (oils and some fine biosolids) are recovered and burned separately to produce steam. Since the biosolids are raised to above 300°F for almost an hour, any pathogens or other microorganisms present are destroyed. The product is Class A biosolids with regard to pathogen reduction; it typically contains less than 2% of water. However, the product is very dusty and as a result, it requires additional mechanical pelletizing using extrusion to produce cylindrical particles which are then separated into commercial, fine and oversized fractions. The fme fraction and the oversized fraction after crumbling is recycled to the extrusion step. Commercial grade pellets were sold in Florida. After the C-G plant shut-down in 1992, the OCUA plans to construct a 50 dtd heat drying-pelletizing system. The new plant will also include a dewatering facility.

Thermal Treatment

339

B. Wet Oxidation (Zimpro Process) Wet oxidation is an aqueous-phase oxidation process brought about when an organic andor oxidizable inorganic-containing liquid is mixed with gaseous oxygen (usually air) at temperatures of 150 to 325°C (300 to 617°F). Pressures of 2,000 to 20,500 kPa (300 to 3,000 psi) are maintained to promote reaction. Wet oxidation was initially developed in the early 1960s for recovery of pulping chemicals from waste liquids produced by the paper mills. In this application, the upper range of temperature/pressure was used. For biosolids conditioning, the lower range of temperatures (175 to 205 "C) has been used. Wet oxidation is "combustion without fire" in a sense that most organic compounds can be "burned" (decomposed) at the above temperatures/pressures. Wet oxidation reaction is not as sensitive to pressure as it is to temperature. Wet oxidation of biosolids is available from Zimpro, Inc. utilizing an aboveground vertical column reactor. The Zimpro process is the most widely used wet oxidation technology in the biosolids treatment mainly as a thermal conditioning prior to mechanical dewatering (Figure 6-21). Typically, a 400 psig steam is injected into the reactor where liquid biosolids are maintained at 300-350°F for a period of 15 to 30 minutes. The reactor is kept at approximately 100 psig. The heat exchangers are used to recover some of the heat exiting with the processed biosolids. A by-product of thermal conditioning is strong odor which requires subsequent treatment such as incineration. Thermal conditioning by Zimpro process breaks down cell matter in the sludge and generates significantly more dewaterable biosolids with no chemical polymers added. Biosolids treated by Zimpro process can be mechanically dewatered up to 4 0 4 0 % TS. They are a Class A product with respect to pathogen reduction requirements. Zimpro conditioned biosolids also have an increased heating value (10,000 - 15,000 BTU/lb of volatile solids) which makes them more suitable for incineration. However, Zimpro systems operated in the USA have experienced severe problems which led, in many instances, to the system's shutdown. (e.g. Zimpro systems in Kalamazoo, Michigan, 1990; Green Bay, Wisconsin, 1995; Troy, Ohio, 1993, etc.) Some Zimpro systems are reasonably successful (e.g. Southerly Plant, Cleveland, Ohio; Passaic Valley, New Jersey; Mississango, Ontario; Minneapolis, Minnesota; etc.) Major problems include complexity of the system, high cost of maintenance, corrosion and erosion, low reliability, and the requirement for highly skilled operating personnel. Another variation of wet oxidation process is the aqueous phase oxidation (APO) process marketed by VerTech Treatment Systems. The unique feature of the VerTech technology is the processing of biosolids using high purity oxygen in

Girovich

340

a closed loop below-ground oxidation vessel (reactor) extending to a depth of 4,000 ft. (1,200 m) approximately which results in high pressure at the bottom of the reactor without the need for high pressure pumps. In the mid-1980s a VerTech APO demonstration facility with 25 dmtd capacity was operated in Longmount, Colorado. The first commercial below-ground, 64 dmtd APO system was built in the Netherlands. Since its start-up in early 1993, the APO system experienced operational problems and underwent a number of changes and improvements. The process had not reached its design potential at the time this book was written.

\z

HEAT EXCHANGER

t' 6)

lf

BOllER

* DEWATERING-@+ I

F'ig.6-21 SIMPWED Z W R O PROCESS FLOW DIAGRAM 1)LIQU.D BIOSOLIDS, 2)PREHEATBD WQUID BIOSOLIDS, 3)THERMULY CONDITIONED BIOSOLIDS, 4)TREATED AND TEIChTNBD BIOSOLIDS, 6)CENTRATE. 6 PROCBSS STEAM, 7)BOILXR FEEDWATBR, 0)OFF-GAS 0 TREATMENT, SJDECBNT TO TRIUTMENT, I O)COMPMSSED AIR, I 1)DE WATERED BIOSOLIDS

i

Thermal Treatment

341

REFERENCES 1. 2. 3. 4. 5. 6. 7.

8. 9. 10. 11. 12.

13. 14. 15.

U.S. Environmental Protection Agency. Standards for the Use or Disposal of Sewage Sludge. 40 CFRPart 503.February 19,1993. Grace, N., Gonzales, A. Biosolids Marketing--End Users Constraints, Florida Water Resources Journal, April 1994. Mujumdar, A.S. Handbook of Industrial Drying. Marcel Dekker, Inc. New York. 1987. Cook, E.M. and DuMont, M.D. Process Drying Practice. McGraw Hill, Znc. 1991. Masters, K. Spray Drying Handbook. John Wiley & Sons. New York. 1979. Keey, R.B. Introduction to Industrial Drying Operations. Pergamon Press, 1978. Lackemacher, P, Low, B.J. Sludge Dehydration to Meet PFRP via Indirect Heated, Disc Type Processor. Proceedings of the 85th Annual Meeting and Exhibition, Kansas City, Missouri, June 1992. The US Patent # 4,956,926, September 18, 1990. The US Patent # 4,953,478, September 4, 1990. The US Patent # 5,271,162, December 21, 1993. The US Patent # 5,069,801, December 3, 1991. Holcombe, T.C., Sukkel, J. Conversion of Biosolids to Fertilizer with the Carver-Greenfield Process. 204th American Chemical Society National Meeting, Washington D.C. August 1992. Guthrie, M., Scrivner, A., Kutz, D., McCarthy, P.C. Sludge Drying Technology for the Future. CH2M Hill Report. 1993. Girovich, M.J. Simultaneous Sludge Drying and Pelletizing. Water Engineering and Management. March 1990. Outwater, A.B. Reuse of Sludge and Minor Wastewater Residuals, Lewis Publishers, 1994.

This Page Intentionally Left Blank

Alkaline Stabilization Mark J. Girovich Wbeelabrator Clean Water Systems Inc. Annapolis, Maryland

I. INTRODUCTION Use of various chemicals in wastewater slurries processing (chemical treatment) has been known for many years and in many applications. Chemicals are used to improve dewaterability, for odor control, pH modification, pasteurization, disinfection, stabilization, as oxidizing agents in odor control equipment, etc. Chemical reactions take place in many wastewater treatment processes. Chemicals commonly used in wastewater slurries treatment are listed in Table 7-1. Lime and chlorine are the primary chemicals that have been extensively researched and used. Chlorine, a strong oxidant, has been used to inactivate or destroy microorganisms in drinking and wastewater treatment. However, use of chloriie is quite expensive and safety concerns are substantial. Lime or lime containing materials have been widely used to raise pH to inactivate or destroy pathogenic microorganisms. Lime is less effective than chlorine, but it is safer, cheaper and easier to use. Common use of lime in wastewater solids treatment is to 1) stabilize biosolids and, 2) condition wastewater slurries to improve dewaterabilityand suppress odors. This Chapter discusses alkaline treatment of wastewater slurries to produce a beneficial use product. In this application lime and lime containing materials are the principle treating chemicals used in numerous waste water treatment plants in the USA and abroad. In addition to lime other chemicals such as cement and lime kiln dusts (CKD & LKD), certain types of fly ash obtained fiom burning wood and fossil fuels, by-products of stack gas desulfurization, drinking water treatment sludges and other materials have been used to stabilize wastewater sludges and to produce beneficial use products.

343

Girovich

344

The early use of lime is lost in the antiquity of civilization. The fvst (unrecorded) use of lime to treat human wastes was perhaps in pit latrines to reduce odors in Roman camps. Diseased animal bodies were buried in quicklime to reduce the danger of infection.

TABLE 7-1 CHEMICALS USED IN BIOSOLIDS TREATMENT Chemical Formula CaO Major Applications Quick Lime

2.

I

Hydrated Lime Dolomitic

4.

Potassium Permarganite

5.

Ferric Chloride

6.

pH modification, odor control, pasteurization, disinfection, stabilization, conditioning to improve dewaterability

I

1

Aluminum Sulfate

8.

Ozone

9.

I Chlorine

CaO-MgO

I

I

Odor and pH control Odor control (strong oxidizer)

FeCl,

I Ferric Sulfate I

7.

Ca(oH)2

pH modification, stabilization, conditioning

Fe2(S04), AL,(SO,)- 18.3 H2O

I

Odor control, conditioning to improve dewaterability

I Conditioning, coagulation Conditioning, coagulation

O,(gas)

Disinfection

Cl,(gas)

I Disinfection

10.

Sodium Hypochloride

NaOCl

Oxidizing agent in odor control

11.

Sulfuric & Phosphoric acids

H2S04

pH modification in odor control

12.

Polymers & Flocculants

H2P04 Complex organic compounds

Conditioning to improve dewaterability

Alkaline Stabilization

345

Chemical treatment of wastewater sludges was first tried in Paris around 1740, however little progress occurred until about 1860. The use of lime in sewage treatment was practiced in England from the 1890s. In fact, the problem of deodorizing and disinfecting sewage had already been studied seriously. British patent of 1871 describes the production of a powder (made from lime ashes, Portland cement or similar materials) for use in privies or water closets. It describes how this powder, or materials like lime or kiln dust, could be combined with night soil or manures to create a product for reuse "in the service of agriculture" [ 11. By 1910, several U.S. cities were using lime for precipitation of solids from wastewater, and by 1915 lime was being actively promoted. A major problem at that time was improvement of the effluent quality, and liming proved to be an economical and effective method. Before biological treatment was invented approximately 70 years ago, lime was frequently used to treat raw sewage. Drying lime treated sludge on sand beds reduced odor problems, and subsequently, the dried biosolids were used in agriculture. The pre-liming of sludge before vacuum filtration (often in conjunction with ferric chloride) also became an accepted method of solids removal. Studies of liming conducted between 1942 and 1967 were mostly concentrated on the effects of pH, temperature and time on bacteria survival in water and sludge. An important study of lime stabilization was published by Farrell ef al. in 1974 [2]. Major studies of lime stabilization were conducted between 1975 and 1989 [3], [4], P I ? [61. Alkaline stabilization research and development was also underway in Europe, primarily in Norway and Sweden [7]. In 1967 Swedish patent was awarded for the fust advanced lime stabilization process. In 1978, a similar process patent was granted in Germany. The distinction of these patents was their lower temperatures, typically 58 "C to effect pasteurization. Other European alkaline stabilization processes and patents have since been developed [8]. In Sweden (Gothenburg - Rya WWTP), sea dumping of sludge was replaced in the early 70s by the lime stabilization process producing the end product for agricultural purpose. The method added hydrated lime to liquid sludge for flocculation, and quicklimeto the dewatered sludge to produce heat for sterilization and enhanced drying. The resultant product was bagged and marketed for agricultural purposes. The Gothenburg-Rya was the first relatively large plant (630,000 population in 197 1) which selected quicklime stabilization instead of aerobic or anaerobic digestion. Even in the 18th century, farmers knew the agronomic benefits of using night soils and burnt limestone (quicklime)to enhance crop yields. In 1974, the first use of burned lime to improve soil productivity was recorded in Pennsylvania.

346

Girovich

Edmund Ruffin, called the "Father of Soil Chemistry in the U.S.," presented a paper in 1818 stating that southern soils, through single cropping and other bad farming practices, had become too acidic to retain fertilizer. With application of burned lime, soil acidity was reduced and enhanced yields of corn and wheat were experienced. Farmers in southeastern Maryland and in New York regularly followed the practice of liming with burned lime (CaO). Prior to the 1900's, mechanical equipment did not exist to grind limestone (CaCO,) into the agricultural limestone we know today. Consequently,the limestone was burned, which resulted in a soft, porous material that could be used directly on soils. Pulverized limestone became more available around 1909. Exaggerated claims from the producers led to considerable conhsion concerning agronomic benefits of different forms of lime. Pulverized limestone, due to its low cost and broad availability, became the preferred form of lime used in agriculture. However, numerous studies conducted since 1912 revealed that "the value of any form of basic lime used in soil treatments is dependent upon the calcium and magnesium oxide content. If applied to the soil on the basis of equal lime oxides of similar fineness, the various forms of basic lime compounds have the same crop producing value. There are no significant differences between the nitrogen and organic matter content of the untreated soil, the limestone, and the burned lime treatments. There is, however, a significant difference in favor of burned lime when used with manure" [9]. The term "stabilized" or "stable" biosolids has been defined in different ways in the literature and criteria for assessing biosolids stabilization have not been universally accepted. Pathogen contents, volatile solids content, and odor intensity (putrescibility) have often been used as an indicator of "stability". Anaerobic and aerobic digestion, composting and alkaline (lime) treatment are stabilization processes commonly used in practice. The U.S. EPA 40CFR Part 503 Regulations promulgated in February, 1993 established quantitative criteria for biosolids "stabilization"by setting up standards for pathogen and vector attraction reduction. An alkaline stabilization process can meet the U.S. EPA Part 503 Class A (PFRP) pathogen reduction requirements by using either pasteurization (Appendix B) or pH, time, temperature and dryness standards as specified in paragraph 503.32 (a)(4). For example, N-Viro Soil process meets the Paragraph 503.32 (a)(4) requirements. The BIO*FIX and RDP EnVessel Pasteurization processes meet pasteurization standards. (The temperature of the sewage sludge is maintained at 70°C (158°F) or higher for 30 minutes or longer.") To meet Class B (PSRP) requirements an alkaline (lime) stabilizationprocess should add "sufficient lime... to the sewage sludge to raise the pH of the sewage sludge to 12 after 2 hours of contact". Additionally, vector attraction reduction requirements are imposed on both Class A and B alkaline stabilization processes.

Alkaline Stabilization

347

The evolution of alkaline stabilizationtechnology, with process modifications including various alternative alkaline materials and advancements in processing equipment, has allowed the production of end-products that can be used in many ways. Major beneficial use options are: * Organic fertilizer or soil amendment; . Agricultural liming agent; . Structural fill material; Landfill daily and final cover; and Erosion controVslope stabilization. The selection for a management option for alkaline stabilized biosolids depends on many economic and marketplace factors. However, the treated product must fit the requirements for the end-use. Alkaline stabilization can be an effective treatment process for biosolids with various characteristics. Depending on the enduse, the chemically-treated sludge must be: (1) stabilized; (2)non-hazardous; (3) non-infectious; (4) reasonably odorless; and ( 5 ) physically and chemically suitable. Although alkaline stabilization has been used for many years in both the United States and in Europe, its usage to produce a microbiologically "clean" product has been limited. Although viruses and bacteria are partially inactivated by raising the pH to 12 for two hours (Class B or PSRP treatment), additional treatment is required to inactivate certain pathogens and reach Class A (PFRP) level. There are numerous studies documenting the inactivation of viruses and bacteria and regrowth potential through alkaline stabilization [2][4][5]. The conventional alkaline stabilization of wastewater solids through the addition of lime has been a commonly used practice for many years. According to the U.S. EPA Needs Survey (1989) over 250 POTWs use lime to stabilize wastewater solids. Lime stabilization is most popular in small and medium size plants. New forms of alkaline stabilizationother than conventional lime treatment have been developed and used. These technologies add alkaline materials such as cement kiln dust (CKD),lime kiln dust (LKD), Portland cement, scrubber ash, fly ash, etc. and involve special equipment or processing steps. Adjustments to the conventional alkaline stabilization process allows the production of a product with different physical and chemical characteristics, or a "designer product," depending on the intended end-use as well as compliance with more stringent pathogen, vector attraction and odor reduction requirements. By varying process parameters such as the type and dosage of the alkaline material, mixing configurations, heating, windrowing, or drying advanced alkaline stabilizationprovides diversification and flexibility, which municipalities so often

Girovich

348

need when managing their biosolids. Generally two types of lime stabilizationprocesses are utilized with respect to their location in the overall wastewater residuals treatment: 1) 2)

Lime stabilizationbefore dewatering, often called pre-lime stabilization, and Stabilization after dewatering called post-lime stabilization.

Proprietary advanced alkaline post-liming technologies available now in the U.S. include: BIO*FIX process, N-Viro's Advanced Alkaline Stabilization with Subsequent Accelerated Drying Process (ASSAAD), RDP's En-Vessel Pasteurization, Chemfix process, and Leopold Willotech process. With the enforcement of the U.S. EPA 503 Regulations and as the end-use opportunities grow, the number of enhanced alkaline stabilization processes and facilities will continue to increase.

11. ALKALINE STABILIZATION A. Pre-Lime and Post-Lime Stabilization 1.

Pre-Lime Stabilization

Generating a beneficial use material from liquid biosolids by alkaline (lime) stabilization has been common practice for many wastewater treatment plants for many years, usually by small capacity plants (less than 5 mgd). Liquid (not mechanically dewatered) wastewater residuals can be treated by quick or hydrated lime to reach the pH 2 12 and then applied (injected or sprayed) to soil. In certain circumstances this practice is quite an economical way of recycling municipal biosolids. Pre-lime stabilization typically produces liquid product in compliance with Class B standards. Average lime dosages (lime ratios) for the Class B liquid biosolids are shown in Table 7-2. TABLE 7-2 PEE-LIME STABILIZATION LIME RATIOS r 101 YOTS Average Lime Ratio * Type of Biosolids 3-6

0.12

12.7

Waste Activated

1 - 1.5

0.30

12.6

Anaerobic

6-7

0.19

12.4

Primary

I * pounds

pH

Alkaline Stabilization

349

With the passage of the 503 Regulations new pathogen and vector attraction reduction requirements should be met for pre-lime stabilization (Table 7-3). A number of additional restrictions apply (public access, monitoring and record keeping, type of crops, etc.). The pH of the pre-lime stabilized biosolids reduces gradually depending upon the lime ratios. At high dosages (lime ratio of 0.3) a pH of 12 can be maintained for days and possibly weeks; but it drops quickly (hours or few days) at lime ratios below .O. 15. Other alkaline materials have been tried for pre-lime stabilization (e.g. fly ash), however there is no sufficient data concerning environmental and health compliance or economics of the alternative materials. Pathogen and vector attraction reduction requirements for Class B alkaline stabilized biosolids are shown in Table 7-3. TABLE 7-3 REGULATORY REQUIREMENTS FOR CLASS B (PSRP) Standards Requirements

I

I. Pathogen Reduction

Raise pH to 12 after 2 hours of contact (Appendix B) or Comply with Fecal Coliform Density standards [503.32(b)(2)] Either: 1. At least 38% reduction in volatile organic solids [503.33(b)(l)]; or 2. Raise pH to at least 12 (at 25°C) and maintained pH t 12 for 2 hours and pH 2 11.5 for 22 more hours [503.33(b)(9)]; or 3. Inject into soil so that no significant amount is present on the land surface 1 hour after injection [503.33(b)(9)]; or 4. Incorporate into soil within 6 hours after application [503,33(b)(lO)l.

2.

Post-Lime Stabilization

As a final processing step before beneficial use or disposal, alkaline treatment of dewatered biosolids has gained popularity in the USA in the late 80's to early 90's due to the following major factors: a) promulgation of the 503 Regulations; b) process simplicity; c) low capital cost; d) relatively quick permitting and construction. Alkaline stabilization became especially attractive to a municipality under a court mandate to cease ocean dumping or other unacceptable existing

350

Girovich

practice. (Middlesex Co., NJ; Bergen Co., NJ; Howard Co., MD; South Essex Sewerage District, MA; Toledo, OH; etc.) Significant development work and regulatory attention was initially focused on alkaline treatment of industrial rather than municipal sludges with special attention given to the disposal of industrial and nuclear wastes. Earlier developments and related patents were made by Chemfut, Roediger Pittsburgh Inc., N-Viro, RDP Company, Dravo Lime Co., Envirotech Corp., Hazcon Inc., F. B. Leopold Co. lnc., Soliditech, Inc., Lodige, Rhenuslager, and Albert Klein KG (Germany). The purpose of the post-lime stabilizationis to reduce (Class B) or to eliminate (Class A) pathogenic organisms, to comply with vector attraction reduction requirements, minimize odors and nuisance conditions and, last but not least, to produce a marketable beneficial use product with minimum cost. Majority of the post-lime stabilized biosolids are beneficially land applied or used as a substitution for landfill cover material. Post-lime stabilizationtypically produces the following beneficial use products: a) Class B (PSRP) material to use as a soil fertilizer for specifically permitted sites; b) Class A (PFRP) material to use as an agricultural liming agent; c) Landfill cover material in compliance with physical properties required by landfill operation; it can be either Class A or B material. Several studies have demonstrated that biosolids lime-treated to a pH of 12.0 achieve significant pathogen reduction. In fact, it was shown that post-lime stabilization was as good or better in terms of fecal coliform and fecal streptococcus pathogen reduction (but excluding helminth ova), than the liquid sludge pre-lime stabilization,mesophilic aerobic digestion, anaerobic digestion and mesophilic composting [2][5][10][11]. Exothermic reaction of quicklime (CaO) can yield Class A (PFRP) biosolids provided sufficient lime is added to achieve pasteurization (70°C for no less than 30 minutes). External heat can also be utilized to reach pasteurization level and to reduce CaO consumption. It is convenient to express lime dosage in tons of CaO per dry ton of solids (lime ratio). Theoretical lime dosages (lime ratios) required to achieve pH of no less than 12 and a temperature of no less than 70°C are highly dependent on solids content and vary from 0.3 to 2.5 (Figure 7-1). However, in practice higher dosages of lime or addition of a bulking material are often needed to meet specific requirements of the end use such as dryness, granularity, spreadability, compactability, nutrient content, etc.

Alkaline Stabilization

351

Major advantages of pre- and post-lime stabilization are: . Meeting pathogen and vector reduction requirements of the 503 Regulations; such compliance can be achieved without biological treatment (digestion) which is often economically beneficial especially for small POTWs. . Reduction of odors.

A

1

0 Dosage

to Achieve Class A (Pasteurization) Only

v)

3,OOt-

3.0 0 CA

i2

2.5

n

0

2,SO t

v)

P

d

-

0

2.0

g=., 2800 tc 0) 04

.-*0

2 1.5 li .C1

d

1.O

P

-El 1,50[-

.% 1-

a 0

"0 l,OO[-

Class B Stabilization

a"E

2

0.5

-& 500-

0

10

20

30

40

50

60'

Fig. 7-1 QUICKLIME DOSAGE FOR CLASS A AND CLASS B BIO'FIX ALKALINE TREATMENT

352

Girovich

Disadvantages are: . Limited markets for alkaline stabilized biosolids especially for the pre-lime stabilization of liquid biosolids; * Substantial storage is required due to seasonal and market variations. . Re-lime stabilization (i.e. alkaline stabilization of liquid biosolids) can meet only Class B (PSRP) requirements. Process Fundamentals

B.

A number of chemical reactions take place during alkaline (lime) stabilization process. The principal reaction is that of calcium oxide (quicklime) with water, producing hydrated lime and heat: CaO + H20= Ca(OH), + Heat . . . . . . . . . . . . . . . . . (7.1) 15,300 calories/gram*mol(at 27,500 BTU/lbamol 491 BTU/lb of lime Hydration of one pound of quicklime ties 0.32 pounds of free water available in biosolids to produce 1.32 pounds of dry hydrated lime approximately. Hydrated lime is sparingly soluble in water and disassociatesto produce an equilibrium pH of 11.3 to 12.53 as follows [12]: CaO, grams/ liter

0.064

0.122

0.164

0.271

0.462

0.710

1.027

1.160

PH

11.27

11.54

11.66

11.89

12.10

12.31

12.47

12.53

(25 "C)

At 1.16 gramdliter calcium hydroxide solution is saturated at a pH of 12.53. Solubility of CaO decreases as the temperature increases and pH is correspondingly lower at higher temperatures. A pH should always be measured at 1.

Heat Generation

The most significant exothermic (heat releasing) reaction is that of quicklime hydration. 491 BTUs per each pound of lime are generated within 3 to 20 minutes during the quicklime hydration process.

Alkaline Stabilization

353

Exothermic reactions between lime and silica, aluminum and ferric oxides also occur when these oxides are present: CaO + Fe,O, + Al,03 + H,O = Ca(OH),~A1203~Fe,03~nH,0 Ca(OH), + H,O + A1,03 = CaOmAl,O,mCa(OH),mnH,O Ca(OH), + S O z = CaO*SiOpnH,O . . . . . . . . . . . . . . . . (7.2) These reactions produce pozzolanic (cementitious) products (complex hydrates). C02produced by sludge decomposition and CO, in the atmosphere react with quicklime: CaO + CO, = CaCO, + Heat . . . . . . . . . . . . . . . . . . . (7.3) a. 43,300 cal./grammmol (at 25°C) a. 78,000 BTU/lbmmol a. 1,393 BTU/lb of CaO The reaction between quicklime and carbon dioxide is highly exothermic, however it is of an order of magnitude slower than that of quicklime and water due to low CO, concentration in the biosolids and the atmosphere. The atmospheric CO, reaction with dry quicklime leads to gradual lime deterioration in storage. 2. Alkaline and Other Reactions The term pH expresses the hydrogen ion activity, i.e. intensity of the acid or alkaline condition of a solution and it is defined as follows: p H = - log [H'] . . . . . . . . . . . . . . . . . . . . . . . .(7.4) Pure water disassociatesto small degree yielding hydrogen ions (H') equal to

l o 7 moldliter; thus pure water has a pH of 7. It is also neutral since lo-' mole/liter of hydroxide ion (OH-) is produced simultaneously,(H,O = H' + OH). Addition of an alkali such as Ca(OH), reduces the number of free hydrogen ions causing an increase in pH, because additional OH- ions unite with K ions. The pH scale is acid from 0 to 7 and basic (alkaline) from 7 to 14. The pH adjustment by lime is a two step process: first the CaO reacts with water in biosolids to produce calcium hydroxide [Ca(OH),] and then the calcium hydroxide raises the pH of biosolids. The hydration of calcium oxide is a quick process (approximately 1 to 1.5 minutes) provided water is available. Availability of free water in biosolids is an important factor: generally, the wetter the dewatered biosolids, the quicker the reaction will be. If biosolids are dry (L 30% TS) then more retention time is required for lime

354

Girovich

hydration and temperature to rise. Other factors are the lime reactivity and particle size. Generally the smaller the particles the better lime utilization can be achieved. Alkaline materials used in the biosolids stabilization may also contain other base forming compounds such as calcite (CaCO,, limestone), 5 0 , dolomite (CaMg(CO,),), N&CQ (in cement kiln dust), silicates (in fly ash) and metal oxides. Reactions of these compounds with water in biosolids effect the pH adjustment and heat release. The chemistry of the lime stabilization of biosolids has not been completely understood and described in detail. The following reactions between lime and organic and inorganic ions in biosolids possibly occur [ 1 11: Ca2' + 2HC0,- + CaO = 2CaC0, + H,O ..................... Q 2POi3 + 6H' + 3Ca0 = Ca, (PO,), + 3H20 . . . . . . . . . . . . . . . . . . 06) Organic Acids (RCOOH) + CaO t+ RCOOHCaOH . . . . . . . . . . v.3 Fats + CaO 4 Fatty Acids ............................... V.8)

Ammonia

Fig. 7-2 EFFECT OF PH ON SPECIATION OF AMMONIA [113

355

Alkaline Stabilization

As a result of these reactions, the amount of available organic matter and phosphorus in the end product decreases. If insufficient lime is added, the pH may decrease also due to the above referenced reactions. Theoretically, the amount of lime required to raise the pH to a given value can be calculated, however, these calculations are often unreliable. 3.

Odors

Odors in wastewater solids are largely a result of organic matter decomposition. Odors are generally comprised of nitrogen and sulfur containing organic and inorganic compounds and certain volatile hydrocarbons. The nitrogen compounds contained in biosolids include soluble ammonium (NH,'), organic nitrogen, nitrates and nitrites. Total amount of nitrogen is quantified by the Total Kjeldahl Nitrogen (TKN) analysis. Ammonium ions in alkaline conditions are converted into ammonia gas: NH,'+OH--NH,+H,O . . . . . . . . . . . . . . . . . .(7.9)

PH Fig. 7-3 EFFECT OF PH ON SPECIATION OF HYDROGEN SULFIDE [ I 13

356

Girovich

The higher the pH, the more gaseous ammonia (NH,) is released from the alkaline stabilized biosolids. Figure 7-2 illustrates the effect of pH on ammonia release at various pH levels. At the pH of 11, all ammonium ions are converted to ammonia gas which if not controlled, can cause considerable odor problems. On the other hand, high pH levels essentially eliminate odors due to hydrogen sulfide and possibly other sulphur-containingodor pollutants such as mercaptans and organic sulfides. Figure 7-3 illustrates that as pH increases, the gaseous hydrogen sulfide (H,S) decreases while water soluble H S and S- increase. At pH of 9, gaseous H,S release is essentially at 0% [l I]. This phenomenon explains why alkaline stabilized biosolids exhibit predominantly ammonia-based odors while other severe odor pollutants are generally eliminated or inhibited [13]. High pH levels typically reached in alkaline stabilization inactivate or destroy microorganisms involved in decomposition of biosolids, thus contributing to odor suppression. However, it was observed that odors due to nitrogen-containing organic compounds which are products of organic matter decomposition continue to persist even at the high pH. These include amines, and more specifically trimethylamine (TMA) and possibly cadaverine, indole, etc. (Chapter 1, Table 1-8). 4.

Acidic Soil Neutralization

Alkaline stabilized biosolids are used as an agricultural lime (CaCO,) substitute to modify the soil pH to a near neutral level to improve soil productivity. When lime-stabilized biosolids are added to an acid soil (pH of 4.5 - 5 3 , the initial acid neutralization reaction in the soil is: Ca(OH), + 2H+(soil)= Ca2+(soil) + 2H20 ............... (7.10) Acidic soil neutralization using calcium carbonate limestone is as follows: CaCO, +2H+(soil) = Ca2+(soil) + CO, + H,O . . . . . . . . . . . . (7.1 1) In both reactions, two hydrogen ions in the soil are exchanged for one calcium ion thus reducing the hydrogen ion concentration and raising the soil pH [ 151. Hydrated lime is 100 times more soluable than lime and as a result, the 7.10 reaction is significantly quicker than the 7.1 1. Hydrated lime is generally more effective in reducing soil acidity than limestone [9]. The acid neutralizing capacity of a liming agent is determined by acid titration to a pH of 7. This is then expressed as calcium carbonate equivalence (CCE) which is the acid neutralizing capacity of the material relative to that of pure CaCO,. A CCE value of 50% means that it would take two times more of the

Alkaline Stabilization

357

material to satisfy a soil liming requirement of pure CaCO,. Special laboratory methods are employed to determine the CCE [ 151. The amount and type of alkaline material determines the residual alkalinity of the end product and its forms (Ca(OH), or CaCO,). Typically, Ca(OH), has an equilibrium pH in water of about 12.5 and that of CaCO, is approximately 8.2. Usually, Ca(OH), determines the end product pH. 5.

Immobilization of Heavy Metals

High pH level of alkaline stabilized biosolids causes conversion of water soluble metal ions (except for molybdenum and selenium) into water insoluble metal hydroxides: Me' + 20H- Me(OH)* . . . . . . . . . . . . . . . . . . (7.12) (Me indicates a metal)

-.

Metal hydroxides precipitate from the soil solution and become largely unavailable for the plant's uptake. Metal mobility is greatly reduced as well. Metal immobilization is a complex process which also involves absorption to mineral surfaces, precipitation as carbonates, complexation by organic matter, etc. C. Alkaline Materials

1. Lime Lime in its various forms, as quicklime and hydrated lime, is the principal, lowest cost alkali. It is the second (after sulfuric acid) greatest basic chemical in tonnage produced. It is manufactured by approximately 80 plants in 29 of 50 US states. Over 18.7 million tons of lime are consumed annually (1993) in the United States and over 137 million tons worldwide. Lime is the greatest tonnage chemical consumed in water treatment, in sewage and industrial waste treatment. In recent years lime has become a major chemical for desulfurization of stack gases at power plants and industrial boilers. Lime (calcium oxide or burned lime) is produced by burning limestone (CaCO,). Burned lime was manufactured from limestone as early as 1661 in Providence, R.I. Lime is a general term but by strict definition it only embraces burned forms of lime: quicklime, hydrated lime, and hydraulic lime. The two forms of particular interest to biosolids treatment, however, are quicklime and to a lesser degree hydrated lime. Limestone (calcium carbonate) is occasionally but erroneously referred to as "lime".

358

Girovich

High Calcium

Dolomitic

CaO

CaO and MgO

Specific Gravity

3.2 - 3.4

3.25 - 3.45

Bulk Density (Pebble Lime), Ib./cu.ft.

55 - 60

55 - 60

0.19

0.2 1

50 - 55

50 - 55

High Calcium

Dolomitic

Primary Constituents

Ca(OH)*

Ca(OH),+MgO

Specific Gravity

2.3 - 2.4

2.7 - 2.9

25 - 35

30 - 40

0.29

0.29

70

70

High Calcium

Dolomitic

CaCO,

CaCO, and

2.65 - 2.75

2.75 - 2.90

87 - 95

87 - 95

Primary Constituents

Specific Heat at 100"F.,Btdlb. Angle of Repose (approx. average for pebble)

Bulk Density, lb./cu.ft. Specific Heat at 100"F., Btdlb. Angle of Repose

Primary Constituents Specific Gravity Bulk Density (3/4-in. Stone), lb./cu.ft. Specific Heat at 100"F., Btu/lb.

Alkaline Stabilization

359

Limestone, calcium oxide and hydrated lime have been used extensively in agriculture to neutralize acidic soils. When pulverized limestone became a competitor on the agricultural lime market about 1909, a competition arose between the producers of various types of lime products. Exaggerated claims led to considerable confusion concerning agronomic values of the different forms of lime. Numerous studies performed since 1912 proved that there is no principle difference between different forms of lime with respect to their agronomic value. If applied to the soil on the basis of equal lime oxides of similar fineness, the various forms of basic lime compounds have the same crop producing effect [9]. Various types of lime are used for acid neutralization in waste treatment processes. Basic data on the properties of the common types of lime products are provided in Table 7-4. Quicklime's ability to raise the temperature over time (reactivity) and percent of available CaO in commercial quicklime are important factors for an alkaline stabilization process involving temperature-time. Particle size distribution is another factor effecting lime usage. These factors should be specified when purchasing quicklime. a.

Quicklime

Quicklime (burned lime) is the product resulting from the calcination of limestone and to a lesser extent shell in rotary or vertical kilns at temperatures of 2000" 2400" F. It consists primarily of the oxides of calcium and magnesium. On the basis of their chemical analyses, quicklimes may be divided into three classes: 1 . High calcium quicklime - containing principally calcium oxide (CaO) and less than 5 percent magnesium oxide (MgO). 2. Magnesium quicklime - containing 5 to 35 percent magnesium oxide. 3. Dolomitic quicklime - containing 35 to 40 percent magnesium oxide. Magnesium quicklime is relatively rare in the United States and is only available in a few localities. Quicklime is available in a number of more or less standard sizes, as follows: 1. Lump lime - the product with a maximum size of eight inches in diameter down to two to three inches, produced in vertical kilns. 2. Crushed or pebble lime - the most common form which ranges in size from about 2 to '/4 inches, produced in most kiln types. 3. Granular lime - the product that has a particulate size range of 100% passing a # S sieve and 100% retained on a #SO sieve (a dustless product).

360

Girovich

Ground lime - the product resulting from grinding the larger sized material and screening off the fine size. A typical size is substantiallyall passing a #8 sieve and 40 to 60% passing a #lo0 sieve. 5. Pulverized lime - the product resulting from a more intense grinding than is used to produce ground lime. A typical size is substantially all passing a #20 sieve and 85 to 95% passing a #lo0 sieve. 6. Pelletized lime - the product made by compressing quicklime fines into about one-inch sized pellets or briquettes.

4.

TABLE 7-5 SCREEN SIZES (ASTM)

Major screen sizes are provided in Table 7-5. Quicklime is shipped in either bulk carloads or tanker trucks or in 80-pound multiwall paper bags. Lump, crushed, pebble, or pelletized lime are rarely handled

Alkaline Stabilization

361

in bags and are almost universally shipped in bulk. The finer sizes of quicklime, ground, granular, and pulverized, are readily handled in either bulk or bags. Quicklime storage must be designed to avoid any accidental contact with water. Lime Reactivity Availability of active quicklime and its reactivity level are important factors for alkaline stabilization processes. Reactivity of CaO varies in limes from different suppliers. The quicklime used in alkaline stabilization process, especially when heat of its exothermic reaction is important (i.e. pasteurization), should be of the quality known as "quick slaking" and shall slake easily, readily disintegrating into a suspension, while producing a required temperature rise of 40°C (72°F) in predetermined time. Three major types of CaO are defmed as follows (ANSVAWWA B202-93 "Standard for Quicklime and Hydrated Lime"): 1) A high reactive lime will show a temperature rise of 40°C (72°F) in 3 minutes or less and the reaction will be complete within 10 minutes. 2) A medium reactive lime will show a temperature rise of 40°C (72°F) in 3-6 minutes and the reaction will be complete within 10-20 minutes. 3) A low reactive lime will require more than 6 minutes to raise temperature to 40°C (72°F) and more than 20 minutes to complete the reaction. Quicklime will deteriorate in storage at a much more rapid rate than hydrated lime. Under good storage conditions, with multiwall moisture proofed bags, quicklime may be held as long as six months, but in general should not be stored over three months. b.

Hydrated Lime

Hydrated lime is: "A dry powder obtained by treating quicklime with water enough to satisfy its chemical affinity for water under the conditions of its hydration." (ASTM C5 1-81) The chemical composition of hydrated lime generally reflects the composition of the quicklime from which it is derived. A high calcium quicklime will produce a high calcium hydrated lime containing 72 to 74 percent calcium oxide and 23 to 24 percent water in chemical combination with the 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 only

362

Girovich

a small portion of the magnesium oxide hydrates (about 5 to 20%). The composition of a normal dolomitic hydrate will be 46 to 48 percent calcium oxide, 33 to 34 percent magnesium oxide, and 15 to 17 percent water in chemical combination with the calcium oxide. (With some soft-burned dolomitic quicklimes, 20 to 50% of the MgO will hydrate.) A "special" or pressure hydrated dolomitic lime has almost all (more than 92%) of the oxides hydrated. Hydrated lime is air-classified to produce the fineness necessary to meet the requirements of the user. The normal grades of hydrate used for chemical purposes will have 75 to 95 percent or more passing a #200 sieve, while for special uses the hydrate may be classified as fine as 99.5 percent passing #325 sieve. Hydrated lime is packed in paper bags weighing 50 pounds net; it is also shipped in bulk. 2.

Other Materials

a.

Kiln Dusts

Kiln dust is a by-product of the production of cement (CKD) or lime (LKD). Cement kiln dust is readily available throughout the USA as over 12 million tons are generated annually. Kiln dust is collected by a variety of systems such as mechanical cyclones, bag filters and dry electrostatic precipitators (ESP). Approximately 64% of all cement kiln dust is now recycled into the manufacturingprocess. About 7% of CKD produced (approximately900,000 tons) was sold for off-site use, most of it as a waste stabilizing additive or liming agent (1992). Utilization of kiln dust available at low cost (or no cost) for biosolids alkaline stabilization provides an alternative for the more expensive quicklime. Typical chemical composition of CKD is depicted in Table 7-6. In order to be efficient in the biosolids stabilization kiln dust should contain a sufficient amount of active CaO (25% - 40%) and its heavy metal content must be strictly controlled. The CKD and LKD chemical composition varies widely depending on the type of raw material and fuel involved, variations in the manufacturing process, etc. Some cement manufacturersuse waste or even hazardous materials to supplement fossil fuel so quality control is important when using kiln dusts €or biosolids stabilization. On February 7, 1995 the U.S. EPA determined that "additional control of CKD is warranted in order to protect the public from human health risks and to prevent environmental damage resulting from . . . disposal of this waste." The U.S. EPA will develop a set of standards under Subtitle C of RCRA for the use and disposal of CKD. Until these standards are published, CKD will continue to be exempt from hazardous waste status. The cement manufacturing plants

Alkaline Stabilization

363

which use hazardous waste as a fuel will be required to test their CKD to prove that it remains unaffected. TABLE 7-6 TYPICAL CEMENT KILN DUST COMPOSITION Component 'YORange Component mgkg, dry basis ~

CaO

25 -40

~

Arsenic

1 - 80

Mercury

1-5

Si02

14- 15

Cadmium

1-45

A1203

5-6

Nickel

3 -60

Fe203

11-2

I Copper

I 500 - 1,000

MgO

1-2

Lead

5 - 2,620

so3

6-7

Chromium

5 - 100

KZO

4.0 - 4.5

Zinc

500 - 1,000

I

No 200 mesh

I

Total Solids

1 100

I

I Silver

YO Strontium

b.

Flv and FGD Ash

Millions of tons of fly ash and flue gas desulfurization(FGD) ash are produced as a result of intensive use of coal for electrical power generation. These ashes contain some essential plant nutrients. Numerous studies have been conducted to evaluate the effects of ash application on properties of soil and its use in biosolids treatment. Composition of ash varies considerably. In general, fly and FGD ashes are rich in Ca and Mg and therefore can be used as a liming agent either alone or in alkaline treatment of biosolids. Application of fly and FGD ash mixed with biosolids increases soil moisture-holding capacity. Its effect on soil microbial activity if applied at low rates is negligible.

364

Girovich

Ash trace metal content should be carehlly analyzed to avoid potential adverse effects on crops. A typical chemical analysis of calcium and magnesium based scrubber ash is provided in Table 7-7.

Calcium Ash #

Components

YOby Weight

Magnesium Ash, YOby Weight

9

K,O

0.6 - 1.0

0.4 - 0.45

10

Na,O

0.15 - 0.25

0.0 - 0.1

11

pHunits

12.4

9.65

12

Density, IbkF

60 - 80

53 - 60

Calcium- and magnesium-based FGD ash can be used in biosolids alkaline stabilization. Magnesium-based FGD ash contains significant amounts of magnesium sulfite (MgSO,) which reacts exothermically with water providing additional heat. Presence of magnesium and sulhr, important plant nutrients, enhances agronomic value of the stabilizedbiosolids treated with magnesium based scrubber ash. Calcium based FGD ash which contains sizable amounts of active CaO can be used as a source of heat and as a bulking agent. FGD ash heavy metal content should be strictly monitored and controlled.

Alkaline Stabilization

365

scrubber ash. Calcium based scrubber ash which contains sizable amounts of active CaO can be used as a source of heat and as a bulking agent. Scrubber ash heavy metal content should be strictly monitored and controlled. 3.

Transportationand Storage of Alkaline Materials

The principal form of lime and other dry alkaline materials transportation is pneumatic trucking. Pneumatic (blower) trucks are available with compartmented tanks varying from 700 to 1300 cu. ft. capacity (the latter delivers up to 20 tons of hydrate and 24 tons of pebble lime). Air for blowing is provided by a blower mounted on the trailer. The largest size of pebble lime that can be pumped efficiently from blower trucks is 1 '/s in., although 1 in. top size is preferred. Pebble lime may be blown as much as 100 ft. vertically and 150 ft. on a combined vertical and horizontal run. Hydrated lime can be blown readily up to 300 ft. combined vertical and horizontal run. Normally a blower truck can be unloaded in about 60 minutes. Airslide trucks are also used for lime delivery, particularly for the finer lime products (hydrate and pulverized quicklime). In these trucks the lime is fluidized by low pressure air and conveyed in a slightly inclined trough to the discharge end of the truck. A separate conveyor system then transfers the lime to storage. A separate fan mounted on the truck provides the air for fluidizing. Covered rail hopper cars haul up to 100 tons of quicklime or 50 tons of hydrated lime. The lime is discharged to an undertrack hopper, then taken by screw conveyor and bucket elevator to plant storage, or pneumatically unloaded. The newest type of rail car for lime is the pneumatic, either the pressure differential or the airslide car, comparable to the blower and airslide trucks, respectively.

TABLE 7-8 SCREW CONVEYOR DATA [I21 Screw Size Normal RPM (inches)

Tons Quicklime (per hour)

6

50

2 - 2.5

9

50

7-8

12

50

15 -20

16

50

45 - 50

366

Girovich

containers are removed and stored at the plant until use. In addition to pneumatic conveyance there are two other means of transporting bulk alkaline materials to storage: mechanical and vacuum. With mechanical handling the lime and similar materials are generally discharged into a hopper, then transferred by conveyor to a bucket elevator, then elevated to storage. Pan and drag conveyors are preferred for larger sized quicklime, and for long runs. Belt conveyors are not recommended for transferring lime because of dusting. Data on screw conveyor and elevator capacity which can be used for estimating purposes are given in Tables 7-8 and 7-9; however for final design purposes, the equipment manufacturer should be consulted. Lime and other alkaline materials are not corrosive and carbon steel or, less often concrete bins and silos are used for storage. The silos must be watertight and airtight. Basic data for a popular 12 ft. diameter silo are provided in Table 7-10 and Figure 7-4.

TABLE 7-9 ELEVATOR DATA [ 121

Quantity and size of storage silos is dictated by such factors as daily consumption, number of alkaline materials used, type of delivery, fbture needs, etc. The total storage capacity should be at least 50% greater than the minimum truck or rail delivery in order to guarantee adequate lime supply. In case of steady daily demand it may be prudent to have even greater reserve capacity to insure at least a week of supply. Selection of a storage silo involves numerous considerations such as silo shape, transportability, height to diameter ratio, type and slope at cone bottom, use of vibrators or air pads for improved flowability, preventing material bridging and ratholing, type of a discharge device, dust

Alkaline Stabilization

367

collection, etc. The flow of lime varies from good for pebble and granulated quicklime to erratic for pulverized quicklime, hydrated lime and fine materials. Quicklime tends to absorb moisture readily, forming an adherent soft cake which can cause bridging in storage. Hydrated lime tends to form rat holes, craters, or chimneys due to its fluffy texture and also possibly to electrostatic charges. Because of the inherent problems with lime's flowability, several methods of design have been developed to facilitate unloading. These include special design considerations in silo construction, the use of external vibrators and air pads, as well as internal anti-packing and anti-bridging devices and live bin bottoms. The interior silo walls should not be painted, and should be kept as smooth as possible, fiee from projections like bolt heads, welding ridges, offset joints, etc. which could restrict material flow. In some cases plastic materials have been used to coat the hopper bottoms to reduce fiiction and improve flowability. Height to diameter ratio of 2.5 - 4.0 is most desirable. Hopper silo bottoms should have a minimum slope of 60" fiom the horizontal; for hydrate lime a greater slope angle is desirable. An electromagnetic vibrator, attached to the outside of the silo, is the most popular device to improve flowability. It is more suitable for quicklime than hydrate, because vibration tends to pack the hydrate. With pebble quicklime the vibrator can be operated continuously during discharge. Best results are achieved with vibrators attached to a cone which is connected to the silo via a flexible joint (vibrating bin activators). Vibrators should only be operated while the hopper is open to flow to prevent packing.

TABLE 7-10 STORAGE SILO DATA* [12]

*I2 ft. diameter silo with 60" cone. Several internal devices have also been developed to prevent bridging. One such internal device is a double-ended cone which is mounted near the base of the conical hopper. The cone decreases the downward pressure of material above the

Girovich

368

discharge opening, thus helping to prevent packing and minimize rat-holing. There are other designs to prevent bridging.

D (12')

X

X-

/

-0 --Q ToFeeder

-

Fig. 7-4 TYPICAL STORAGE SILO 1. Support Structure; 2. Activated Suspended Cone; 3. Pneumatic Feed Pipe;4. Fabric Filter; 5 . Safety Valve; 6. High & Low Level Indicators; 7. Vibrator

Alkaline Stabilization

369

Air jets and pulsating air pads are also used on silos to facilitate flow of hydrated lime. Normally, air activation is not recommended for quicklime since any moisture in the air would cause air slaking. Each silo should be vented and equipped with a dust collection device. In earlier designs a small cyclone collector may suffice, however at present the more efficient bag-type collector is required to clean air expelled during truck unloading. Normally 1 sq. ft. of cloth area per 2 cu. ft. of air is recommended, indicating 375 sq. ft. of cloth area for a large truck rotary blower of 750 cu. ft. capacity. Additional cloth area of 100-300 sq. ft. may be justified, however, to accommodatethe final clean-out period. An air permit is required for each storage silo equipped with a dust collector. The lime feeders are usually located beneath the silos, with the lime simply flowing by gravity directly to discharge conveyor. With multiple silo storage, either mechanical or pneumatic conveyors are used to transfer lime to the feeder hoppers. Pneumatic transfer systems comprise pressure (high and low), vacuum (low, medium, and high) and combination. The most popular pneumatic system used for lime transfer is the low pressure system.

111. PROPRIETARY ALKALINE STABILIZATION PROCESSES

A. BIO*FIX Process

BIO*FIX is a patented alkaline stabilization process marketed by the Bio Gro Division of Wheelabrator Clean Water Systems Inc. The process uses quick lime (CaO) to treat biosolids prior to beneficial use and to meet pathogen and vector attraction reduction requirements established by the US EPA 503 Regulations as well as other requirements associated with the end product use. The BIO*FIX end products are marketed as a fertilizer, agricultural lime substitute, soil conditioner or used as daily and final cover material at landfills [13]. Quicklime (and other types of materials) is mixed with biosolids at appropriate ratios to produce a Class A (PFRP) or Class B (PSRP) biosolids-derived product. The BIO*FIX process achieves Class A standards for pathogen reduction by achieving pasteurization (70" C for 30 minutes). The exothermic reaction of quicklime with water present in the biosolids produces rapid increase in temperature. This temperature increase combined with the elevated pH results in destruction of pathogens and elimination of vector attraction properties. Class B standards for pathogen reduction are achieved by adding sufficient lime to raise the pH of the biosolids to 12 after 2 hours of contact.

370

Girovich

Vector attraction reduction requirements for the BIO*FIX Class A product are met either by compliance with the 503.33(b)(6) (pH2 12 for more than 2 hours and pH2 11.5 for 22 hours) or by compliance with the 503.33(b)(l)(at least 38% reduction in volatile solids). Vector attraction requirements for the BIO*FIX Class B product are met by compliance with the 503.33(b)(6) or 503(b)(10). (Chapter 2, Table 2-8) The major process difference between production of Class A and Class B products is related to the different lime ratios used during the treatment process. The lime ratio is expressed as tons of quicklime per ton of dry biosolids used during treatment. A higher lime ratio is required to produce a Class A product than to produce a Class B product. BIO*FIX processing offers the following advantages: production of a diversity of beneficial use products using the same equipment . efficient quality control of air emission and odors immobilization of heavy metals and reduction of their concentrations . automatic process control small footprint * low capital cost 1

Disadvantages are: . increase in product weight/volume (15 - 30% by weight as compared with incoming dewatered biosolids) . relatively high cost of lime when producing Class A product BIO*FIX Class A product remains stable over months and generally does not cause odor problems or attract vectors even after extended periods of storage. This stability results fiom residual alkalinity and significantly reduced levels of volatile solids [14]. Trace (heavy) metals present in sludge become insoluble at the high pH levels attained by the BIO*FIX process. The alkaline nature of the end product immobilizes and prevents trace metals plant uptake or movement to groundwater. The addition of clean alkaline material dilutes the trace metal concentration of the end product by 2 to 2.5 times which is important for biosolids with marginal heavy metal concentrations [ 131. Studies have confirmed that quicklime is a cost-effective additive in production of a marketable Class A and especially Class B products as compared to other alkaline materials such as cement and lime kiln dust, fly ash, etc. [ 16][171. BIO*FIX Class A product is are a soil-like, practically odorless material containing organic matter, calcium and micronutrients. The product is reasonably dry (50-60%TS), light grey in color, has a crumbly texture and is easily

Lime & Alkaline Additive Storage

I I I

I

I

Process Water

to Sewer

p

Ventilation Point

---- APC Duct

I

Marketing & Distribution

Fig. 7-5 BIO*FIX@ ALKALINE STABILIZATION PROCESS

Girovich

372

spreadable by common means. Major BIO*FIX product data are provided in Table 7-1 1. BIO*FIX products with sufficient dryness and spreadability are successfully used as a landfill cover material substitute [ 18][19]. As an agricultural liming agent, BIO*FIX Class A product has been used with very few site restrictions. BIO*FIX Class B product is typically utilized on

TABLE 7-11 MAJOR BIO*FIX PRODUCT DATA Item Class A ~~

~~

I 5 - 15

~

Class B 22 - 40

50 - 60

1. Total Solids, % 2. Total Volatile Solids, %

~~

I 50-60 2-5

4. Phosphorus, P, % Potassium, K, YO

As in biosolids feed

As in biosolids feed, diluted by the addition of lime

I 5. Calcium (%, dry basis)

30 - 40

0.5 - 1.0

6 . Trace Metals mg/kg, dry

Concentrations are reduced by a factor of 2 - 2.5. Metals are immobilized,

As in feed

~~~

~~~

~

7. pH

11.5 for several months

8. CaCO, Equivalent, (CCE), %

70 - 95

9. Bulk Density, lb/cf

52 - 58

10. Physical characteristics

soil-like, near odorless, crumbly material with very good spreadability and storability

I 1. Application

Liming Agent, Landfill Cover

~

~~~~

11 - 12 for 24 hours

,

5 - 10 5 8 : 60

moist, dark brown; good spreadability

Soil amendment & fertilizer

Alkaline Stabilization

373

agricultural lands, land reclamation projects and similar sites specifically permitted for this purpose. At present (1995) there are over a dozen operational BIO*FIX installations in the U.S.A. processing over 70,000 dry tons of biosolids annually, the majority of which are owned and operated by the Bio Gro Division of Wheelabrator Clean Water Systems Inc. The largest BIO*FIX facility producing Class A material is located at the Bergen County Utility Authority (BCUA) WWTP in New Jersey. The BCUA BIO*FIX facility currently processes 40 dry tons per day of dewatered biosolids (at 20 - 24% T.S.) and generates approximately 235 tons per day of BIO*FIX Class A material for beneficial use, largely as a landfill cover material at several MSW landfills. The facility design capacity is 105 dry tons per day. 1.

Process Description

A typical BIO*FIX process flow diagram is provided in Figure 7-5. Dewatered biosolids are transported into a receiving hopper by conveyors or pumps. At the bottom of the hopper a variable speed screw feeder delivers biosolids to a process mixer. The process mixer is of a dual shaft, plow, high speed (60 - 100 rpm) type. Quicklime and other materials, as required for a specific end use, are delivered by pneumatic trucks and stored on-site in silos. These materials are typically fed to the mixer via a variable speed airlock valve and screw conveyor. The end product discharged from the mixer is conveyed via screw conveyor(s) directly into vehicles and then to application sites or stored for krther marketing and distribution. The conveyors, mixer and loading area are enclosed and maintained under slightly negative pressure to eliminate the possibility of fugitive odors. The exhaust air from this equipment is typically treated by a wet multistage scrubber prior to being released into the atmosphere. An induced draft fan provides sufficient flow to ventilate all process equipment and the loading area and, after treatment, to release clean, odorless air into the atmosphere via stack. A microprocessor-basedautomatic programmable logic system controls the entire process. The above referenced system is capable of producing either Class A or Class B products by changing the alkaline materials' feed and speed of conveyance to the mixer. The system operates both in manual and automated modes. In the later mode, the feed is automatically adjusted in relation to the mixer's exhaust temperature (Class A). Fluctuations in the incoming biosolids flow and moisture change the temperature of the mixer's exhaust. The lime feed is adjusted automatically in response to this temperature change to maintain stable process temperature and

Girovich

374

to ensure consistent quality of the end product and its compliance with regulatory standards. The control system monitors and records alkaline material and biosolids feeds, temperature of the end product, status of all system components and historic trends. All process records are stored electronically and are available for review and inspection [ 13][20]. Typical BIO*FIX facility data utilizing a medium capacity mixer (2.0 dry tonshour) are provided in Table 7-12.

6

Process Water

gPm

50

7

Building Size (L x W x H)

Ft. (SF)

7 0 ' ~ 2 0 ' ~ 1 7(1,400 ' SF)

30

2. Environmental Control The BIO*FIX Class A process is characterized by a rapid increase in pH and temperature. As a result, the ammonium ions (NH,') normally present in the biosolids are forced out of solution to become gaseous ammonia (NH,). The amount of NH,released from the process depends on the ammonia nitrogen content of the biosolids, pH and temperature achieved in the process. It is possible that

Alkaline Stabilization

375

some organic nitrogen is also mineralized and is partially converted into gaseous ammonia. During the process liberation of ammonia occurs primarily in the enclosed process equipment. This provides BIO*FIX operations with an opportunity to ensure that odors are contained, minimized and controlled before they become a problem. Other odor pollutants such as hydrogen sulfide (H,S), organic sulfides and mercaptans are largely suppressed due to the high pH environment. The primary odor concern during Class A production is ammonia. Typical uncontrolled ammonia release is in the 8 to 25 pounds per dry ton range (before scrubbing) depending on the type of biosolids. Ammonia is contained and is treated by the wet multistage packed bed scrubber to a removal efficiency of no less than 99% with typical ammonia stack concentration in 5 - 10 ppm range. (Human detection concentration is typically 3 - 15 ppm.) Odors due to m i n e s (nitrogen containing organic compounds) can also occur. Some volatile organic compounds (VOCs) are generated at the process temperature involved (80DC). These VOCs, if present in the incoming biosolids should be controlled by oxidizing chemical scrubbers. Metals are not expected to volatilize at the process temperatures maintained during BIO*FIX processing [ 131. To remove ammonia and particulate matter (PM) two types of wet scrubbing systems are used: 1) once-through and 2) recirculating scrubbing medium (water) flows. If an inexpensive source of water is available the first type of scrubbing system is used. If water is unavailable or expensive, a recirculating scrubber system is recommended to minimize water consumption and waste water discharge. In this case, sulfuric (phosphoric) acid is typically used to facilitate ammonia removal by controlling pH in the scrubbing medium.

B. N-Viro Soil Process Post-lime stabilization using lime and kiln dusts called the N-Viro Soil Process was developed and patented in the late 1980's by N-Viro Energy Systems, Inc. (Toledo, Ohio). It is a combination of relatively low temperature, high pH and drying ("curing") steps to achieve compliance with the U.S. EPA Class A (PFRP) requirements. The final product is reasonably dry,practically odorless, granular material. Beneficial uses for the N-Viro products are: liming agent, landfill cover material and top soil supplement. There are three alternatives of the N-Viro Soil process [21]:

.

Alternative 1: The dewatered biosolids, lime and kiln dust mixture is dried while the pH remains above 12.0 for at least seven (7) days. The product must be held for at least thirty (30) days and until solids content is at least 65% by

376

-

Girovich weight. Ambient air temperatures during the first seven (7) days of processing must be above 5°C. Alternative 1 is presently not popular due to the significant space and time required for processing. Alternative 2: The dewatered biosolids, lime and kiln dust mixture is heated while the pH exceeds 12.0 using exothermic reactions fiom the quicklime and alkaline materials in the kiln dust. The material must be stored in such a way (e.g., in a bin) so as to maintain uniform minimum temperature of 52°C for at least twelve (12) hours. The heat pulse (raising the temperature to 52°C) is a function of the CaO content. Variation in the CaO content of the kiln dust(s) will require adjustment to prevent under- or over-heating of the product. Following the heat pulse, the product is air dried (while pH remains above 12.0 for at least three (3) days by windrowing until the solids content is > 50% by weight. This alternative has no ambient air temperature requirements. Alternative 3: Same as Alternative 2, but instead of windrow drying, a thermal drying is offered as an alternative (1 995).

At present (1995) there are over 30 operational N-Viro Soil installations; a majority of them use the Alternative 2 process also known as the Advanced Alkaline Stabilization with Subsequent Accelerated Drying (AASSAD). The NViro Soil Alternative 2 is an EPA approved Class A (PFRP) process [§503.32(a)(4)]. A typical process flow diagram for the N-Viro Soil AASSAD Alternative 2 process is shown in Figure 7-6. Raw primary, activated or digested biosolids with 18 - 40% TS content are mixed with lime and kiln dusts (and other materials as qualified by N-Viro Energy Systems, Ltd). in a single shaft mixer. The mixer is enclosed to prevent release of dust and to allow exhaust air containing ammonia fumes to be treated. Kiln dust and CaO dose rates depend highly on the incoming biosolids dryness and CaO content in the kiln dust. Quicklime (CaO) is added when the active lime content of the kiln dust is insufficient to raise the temperature to the required level. Kiln dusts are by-products of the production of cement or lime. Cement kiln dust (CKD) and high calcium lime kiln dust (LKD) are used by the N-Viro process. Analysis of kiln dusts to determine available reactive lime (CaO) content is critical for the process. Strict quality control of kiln dusts should be instituted to control trace metals which are normally present in the kiln dust (cadmium, lead, mercury, copper, zinc, boron, etc.) [21]. On February 7,1995, the U.S. EPA ruled that a set of standards for the use and disposal of CKD will be developed "to protect the public and to prevent environmental damage resulting fiom this waste." Until new standards are published, CKD is not classified as a hazardous waste.

(2

for L 12 hours)

@H

7

Air Drying 12; L 72 hours; 50% T.S.)

L L

Emission Point

Fig. 7-6 N-VIRO SOIL PROCESS FLOW DIAGRAM

Distribution

Girovich Alkaline materials used in the N-Viro process are stored in silos and can be blended to produce the end product. Alkaline materials are normally conveyed from the silo(s) to the mixer pneumatically or by a screw conveyor(s). The conveyors are to be enclosed to prevent release of dust and odors. The blended material must be "cured" for at least 12 hours in heat pulse containers, storage bins or enclosed piles while the temperature is maintained above 52°C and below 62°C (pasteurization). After the heat pulse the material is transported and discharged into long piles forming windrows. The material is aerated and intermittently windrowed for 3 - 7 days and is complete when the solids content of the material is above 50% TS, while the pH remains above 12. Further windrowing may be desirable to reduce volumes of material to bring the solids content to 60 - 65% TS. Drying is performed on an asphalt or concrete pad, outdoors or inside a building, typically using a SCARAB turner or similar windrowing equipment. The product is easily handled and can be stockpiled. As in other alkaline stabilizationprocesses, odor control is extremely important. Large space and relatively long time required for curing, windrowing and air drying of the N-Viro Soil allow for more odor pollutants to be released. Long term odor control is maintained by degradation and stabilization of organic materials by the remaining heterotrophic microorganisms. The mesophilic (relatively low) process temperatures and other stresses are claimed to be enough to kill the pathogens without destroying the more ubiquitous heterotrophic microorganisms which continue to degrade organic matter making the N-Viro Soil more stable with time. Serious odor problems have been observed during the N-Viro Soil windrowing and air drying due to the release of ammonia and certain mines such as trimethylamine (TMA) [21]. To control the odors the heat pulse bins and curing (windrowing) facility should be enclosed and serviced by an odor control device (scrubber). NViro Soil product properties are depicted in Table 7-13 [21]. The N-Viro Soil process is typically licensed by N-Viro Energy Systems, Ltd. to a private fm or a municipality to utilize the process. Royalty and management fees related to the CKD quality control are usually charged in addition. Advantages of the N-Viro Soil process are: . Good quality, stable product . Immobilization of heavy metals Relatively low operational cost Disadvantages are: . Increase in product weightholume 50% - 70%by weight as compared with incoming dewatered biosolids. . Large space required for windrowinglair drying

379

TABLE 7-13 N-VIRO SOIL PROPERTIES I 11 ITEM

I

1. 2.

50 - 60

Total Solids, % by weight

9.3

Total Volatile Solids (organic matter), % by weight

~~

DATA

~~

, 3.

Total Nitrogen, % dry basis

1 - 1.5

4.

Phosphorus, P, % b y weight Potassium, K, % b y weight

0.39 1.o

I 5. 8.

I 9.

I

Ca, % by weight

CaCO, Equivalent, % CKD Content, % b y weight

50 - 80 35

Dependent upon biosolids and admixture content 11 - 12

11. pH ~

. .

I II

+ 20.0

10. Trace Metals, m a g , dry basis

12. Buk density, Ib/cf

I

I

50 - 62

13. Physical Characteristics

Soil-like, near odorless, granular material with very good spreadability and stability

14. Application

Liming Agent Landfill Cover

Odor control is expensive (for heat bins,windrowing/curing building) Temperature control is manual and frequent alkaline feed adjustments are required. Relatively high capital cost.

The largest (190 dry tons per day) N-Viro installation in the U S A . is located at the 147 mgd Middlesex County, New Jersey WWTP. The facility uses Alternative 2 AASSAD process to generate N-Viro Soil for beneficial use primarily

380

Girovich

as a landfill cover material. The facility capital cost was $16.8 million (1992) including $8.6 million for air pollution control. C. RDP En-Vessel Pasteurization En-Vessel Pasteurization process is marketed by the RDP Company (Plymouth Meeting, PA). The patented process consists of two steps: 1) mixing quicklime with dewatered biosolids and 2) heating the mixture with supplemental external (usually electrical) heat. External heat is used to supplement the quicklime-water exothermic reaction heat to reduce the lime dosage. A typical process flow diagram is shown in Figure 7-7. The end product meets the requirements of Class A biosolids. The system consists of a dewatered biosolids feeder, dual shaft ThermoBlender mixer, lime storage bin with variable speed lime feeder, and a pasteurizationvessel. The ThermoBlender mixes lime with the biosolids and heats the mixture to approximately 70°C. The biosolids feeder, and the mixer, are electrically heated and along with the pasteurization vessel are insulated. The heated mixture is held in the pasteurization vessel for no less than 30 minutes at no less than 70°C to meet pasteurization requirements. En Vessel Pasteurizationhas eight (1994) operational installations in the USA [23]. D. Chemfix Process The Chemfix Process uses lime, Portland cement and proprietary pozzolanic compounds based on soluble sodium silicate, This technology was patented in the 1970's for use in treatment of industrial and municipal wastes. Chemfix-stabilized biosolids under a trade name NATUFUTE were marketed from several plants in the 1980's. However, the first Chemfuc installationsexperienced serious problems (odors, low solids content, etc.). In the early 1990's an improved process was introduced (ChemPost System) with the end product called ChemPost [24][25]. Through the use of lime and specially designed chemical reagents (Chemset reagents) ChemPost System heats and raises the pH to comply with the Class A (PFRP) requirements. It produces alkaline stabilized biosolids within three to six hours. A process flow diagram of the new ChemPost System is shown in Figure 7-8. Dewatered biosolids are fed via a variable speed feeder into a first mixer where a proprietary dry reagent(s) is added from a silo(s). A proprietary liquid reagent from a storage tank is also added. The first mixer discharges into a second mixer where the dry and the liquid reagents are added as well. The mixture is then conveyed to a storage vessel-conveyor (Thermoveyor) where it is held for some

Electrical

T Emission Point

Fig. 7-7 RDP EN-VESSEL PASTEURIZATION PROCESS

Girovich

382

time. After the Thermoveyor, the mixture is crumbled in a particle size-reduction unit and discharged into a Degasser. In the Degasser removal of ammonia, water vapor and other gases takes place. Temperature of the ChemPost product can reach 90°C. In two to three hours the end product dryness is claimed to reach 55 6OYoTS and 60 - 65YoTS in 24 hours. The end product exhibits residual alkalinity (PHbetween 11 and 12) for several months. Due to a relatively high capital cost, special reagents required and relative complexity the ChemPost System is limited to at least 25 - 35 wet ton per day capacity. The O&M cost is reported as 150 250 per dry ton. Currently (1995), there are no commercial ChemPost installations in the USA.

E. Other Alkaline Stabilization Processes The ROEMIX Process marketed by Roediger Pittsburgh, Inc. in the USA comes from Germany where it was first applied in 1976. The first US installation was built in 1984. The ROEMIX process is basically mixing lime with dewatered biosolids using the Roediger paddle mixer. In Germany M E N U S Co. markets mixing systems and a line of additives called RHENIPAL since 1981 using lime and brown coal ash. The GFS Company markets a stabilization process called LUTAFORTE N using fly ash from utility boilers and incinerators. In England, the SELOSAFE process was developed for solidification of biosolids and other wastes using fly ash from utility boilers and incinerators. A biosolids stabilization-solidification process called "SOLIROC" is marketed in Belgium. There is insufficient information concerning the compliance of the above referenced processes with the requirements of the USEPA 503 Regulations with respect to pathogen and vector attraction reduction requirements.

IV. ECONOMICS OF ALKALINE STABILIZATION Alkaline stabilization offers a viable treatment and management option for beneficial use of biosolids especially when the end product@)can be economically utilized. Market availability for the beneficial use or, at least, inexpensive disposal of the end product(s) in compliance with pertinent regulations is crucial for economic success. Major operational costs associated with the alkaline stabilization are:

ro APC A

t

U

Thennoveyor (Reactor)

Degasser

Mixer 2

To Beneficial Use

T Emission Point Conveyor

Fig. 7-8 CHEMFIX (CHEMPOSTT')

PROCESS

384

Girovich 1. alkaline additive@); 2 . transportation of the end product from the facility to beneficial use site(s); 3. beneficial use (land application, landfill cover, etc.); 4. transportation of the biosolids to the facility; 5. labor; and 6. utilities and maintenance.

Typically alkaline additives, transportation and beneficial use comprise 70 80% of total operating expenses. Additionally capital cost amortization (cost of financing) should be considered during evaluation of the available alkaline stabilization processes. Alkaline Additives Cost Cost of quicklime is typically in the $55 - 90 per ton range depending on cost of transportation to a site. Cost of various by-product alkaline additives such as CKD, LKD, FGD and fly ash, etc. depends primarily upon transportation cost and it is in the $10 - 30 per ton range (1995). End-Product Transportation and Beneficial Use Costs Cost of transportation to a beneficial use site (a farm, reclamation site, landfill, etc.) and land application of the alkaline stabilized biosolids comprise a significant portion of the total cost (30 - 50%). These expenses vary widely from site to site. Rarely the alkaline stabilized biosolids generate sale revenue. Table 7-14 illustrates comparative costs of alkaline additives and the end product transportation costs per dry ton of incoming biosolids (at 20% TS) for the two proprietary processes (BIO*FIX and N-Viro Soil). Cost of lime, kiln dust, and the end product transportation cost to a beneficial use site are assumed at $70, $26, and $12 per ton respectively. As can be seen from Table 7- 14, cost of alkaline additives and the end product transportation to a beneficial use or disposal site are the two major operating expenses for BIO*FLX and N-VIRO Soil processes. Labor, utilities, maintenance, etc. are approximately equal for all alkaline stabilization technologies. As the dryness of incoming biosolids increases, the amount and cost of alkaline additives decreases sharply (e.g. Fig. 7-1) and, consequently, the cost of transportation to a beneficial use site goes down. Use of external heat (e.g. RDP En Vessel Pasteurization) helps to reduce the cost of alkaline additives, especially for wet biosolids (12 - 20%TS). With the inclusion of labor, utilities, maintenance, beneficial use/disposal cost, etc., the cost of alkaline treatment is typically in the $190 - $220 per dry ton range for a medium-sized alkaline stabilization installation (15 - 30 dtd), exclusive of capital amortization (1995). Alkaline stabilization is a viable and economic

Alkaline Stabilization

TABLE7-14

385

COMPARATIVE OPERATING DATA AND COST OF

Item

N-Viro Soil

BIO*FIX (Class A)

11. End Product

Dryness, % T.S.

I

55-56

I

56 - 58

Weight, toddry ton

5.35

6.70

Weight Increase: %

33 - 34

65

Girovich

386

alternative for treatment and production of beneficial use products fiom wastewater sludges. The costs of an alkaline stabilizationprocess should be evaluated with respect to the total costs of the option over its useful life, using the present worth, equivalent annual cost, tip fee, or a similar method. In addition, the costs of a privatized option, if that is the preferred procurement method, should be compared to alternative management options that involve ownership and operation by the municipality. The privatized facility is designed, operated, constructed, krnished with all the necessary equipment, and eventually operated as a commercial enterprise by a private firm. Flexibility of the alkaline stabilizationprocess and the use of existing facilities are also important factors when evaluating potential sludge management options. Alkaline stabilizationprocesses are not labor- or equipment-intensive,and they can be a cost-effective option, depending on the product market and end-use program. Site preparation is generally minimal. An additional advantage of these processes is the ability to start up operations in a short time period. Because of their low capital cost, flexibility, and ease of start up, alkaline stabilization technologies can be effective biosolids management options. Some f m s have mobile equipment which can be used for backup or emergency situations or demonstration programs to explore the end product marketability and encourage public approval.

REFERENCES 1.

2. 3. 4. 5.

6.

W. H. Hughan, Improvements in the Utilization of Night Soil, Ashes, and Cements or Limy Matter for the Production of Manure and Other Useful Purposes, British Patent #3060, Nov. 1871. J. B. Farrell, J. E. Smith Jr., S. W. Hathaway, R. B. Dean, Lime Stabilization of Primary Sludges, Journal WPCF 46, Jan. 1974. C.A. Counts, A. J. Shuckrow, Lime Stabilized Sludge: Its Stability and Effect on Agricultural Land EPA-670/2-75-0 12, Battelle Memorial Institute, 1975. R. M. Otoski, Lime Stabilization and Ultimate Disposal of Municipal Wastewater Sludges, U.S. EPA-600/52-8 1-076, 1981. U.S. EPA, Environmental Regulations and Technology Control of Pathogens in Municipal Wastewater Sludges, EPA/625/10 -89/096, Sept. 1989. A. Westphal, G. L. Christensen, Lime Stabilization: Effectiveness of Two Process Modifications;Journal WPCF, 1983.

Alkaline Stabilization

387

B. Paulsrud, A.S. Eikum, Lime Stabilization of Sewage Sludges, Water Resources (Great Britain), 1975. 8. L. A. Stone, J.F. Dausman, R. S. Reimers, The Historical Development of Alkaline Stabilization, Proceedings of the Water Environment Federation Conference, Vol.1, July 1992. 9. T. W. White, The Use of Burned Lime Products in Soil Improvement; Pit and Quarty, May 1947. 10. C. Lue-Hing, D.R. Zenz, R. Kuchenrither, Municipal Sewage Sludge Management: Processing, Utilization & Disposal, Vol. 4, Water Quality Management Library, 1993. 1 1. Design of Municipal Wastewater Treatment Plants, Vol. 11, WEF Manual of Practice No.8, Chapter 18, 1992. 12. Lime: Handling, Application, and Storage in Treatment Processes, Bulletin 213, National Lime Association, Arlington, VA. 13. T.R. Weisinger, M.J. Girovich, Evaluation of Chemical Stabilzation Process, Remediation, Winter 1994-5. 14. G. L. Christensen, Storage Characteristics of BIO*FLX Biosolids, Villanova University, May 1992. 15. S.E. Manahan, Environmental Chemistry, Lewis Publishers, 5th Edition, 1991. 16. K. R. Tsang, J. A. Bauer ,Evaluation of Five Alkaline Stabilization Processes, Proceedings of the A WWNWPCA Conference,Nov. 1992. 17. G. W. Foess, D. Fredericks, F. Coulter, Evaluating Biosolids Stabilization Technologies, Water Environment h Technology, April 1994. 18. M. A. Barlaz, R.D. Rhew, Use of Lime Treated Wastewater Sludge-Soil Mixtures for Daih Cover in Solid Waste Landfills, University of North Carolina, Water Resources Institute, March 1993. 19. Art$cial Soil Demonstration Project, Final Report, Black & Veatch Co. for Charlotte-Mecklenburg Utility Department, November 1993. 20. M. J. Girovich, BIO*FIX Chemical Stabilization, Florida Water Resources Journal, April, 1994. 21. J. C. Burnham, N. Hatfield, G. F. Bennett, Use of Kiln Dust With Quicklime for Effective Municipal Sludge Pasteurization and Stabilization with the NViro Soil Process. 22. M. Ponte, V. Santamarina, K. Aiello, Control of Odors, VOCs, and Particulates at the Middlesex County (NJ) Utilities Authority Chemical Stabilization Facility, Proceedings of the WEF Special& Conference, June 1994. 23. P. G. Christy, Lime Pasteurization, An Extended Evaluation, Proceedings of the WPCF Conference, August 199 1.

7.

388

Girovich

24. P. N. Baldwin, W. A. Brown, R. S . Reimers, T. G. Akers, M.D. Little, The

Enhanced Chemfur Process: Achieving Improvements in the Resulting Naturite End Product, Proceedings of the WPCF Conference, August 1991. 25. United States Patent No. 5,246,596, September 4, 1992.

Land Application Jane B. Forste Wheelabrator Clean Water Systems Inc. Annapolis, Maryland

I.

INTRODUCTION

The treatment of wastewater fiom modem urban society produces a rich resource: biosolids. Some eight million dry metric tons of biosolids are produced annually in the United States in 1995, a significant growth from the five million tons generated in 1990 [ 11. Of the technologies currently available to use this natural resource, the most direct and most commonly employed is land application-the term used to describe the application of biosolids to land for purposes of agricultural production, production of other non-agricultural crops (e.g., forest application) or use as a soil amendment/fertilizerto reclaim areas which have been disturbed by mining or other activities. In light of the significant benefits which biosolids can provide for the above uses, it remains somewhat surprising that such uses frequently encounter apprehension and even strong opposition fiom the general public. The reasons for such attitudes have not been fully identified but are becoming recognized as a significant impediment to the development of land application programs for biosolids. With the improvement in terminology which "biosolids" represents, the opportunity exists for better communicating the true nature of the product of modem wastewater treatment. For better understanding to result, biosolids must be dealt with in arenas other than the conference rooms of sanitary engineers and contractors who design and build bigger and better treatment plants. These

390

Forste

technical and operational managers of biosolids have reached a greater understanding in the last decade. However, the challenge to communicate effectively with the various publics who express concern has yet to be adequately addressed. This Chapter will discuss the beneficial properties of biosolids, the methods by which appropriate land application of this material can be developed, the relevant environmental and health issues and agricultural and crop production considerations, all of which must be addressed for land application to succeed. A.

Historical Background

Even before the development of modem wastewater treatment facilities, the solids from household sewage were recognized as a potential resource. As far back as the Roman Empire, human wastes were used as fertilizers, and the practice continued for centuries throughout the world in Europe, North America and the Orient. These practices were vastly different from the regulated, managed methods developed in the last several decades for land application of biosolids. Perhaps the most important distinction centers on the obvious health hazards associated with using raw wastes to fertilize farmlands. The pathogens contained therein represent a very real concern unless the foods are boiled during their preparation to destroy the organisms which can cause amoebic dysentery, salmonella poisoning or other diseases. Unfortunately, the potential for illness which results from the indiscriminate use of raw sewage may be misperceived by many people as a potential danger to them if they eat any foods from crops grown in soils fertilized with processed biosolids. This misconception must be addressed head-on and dispelled in order to change public attitudes towards land application. In the United States throughout the 19th and into the 20th Century, fanners and scavengers collected human wastes from urban cesspools to use as a fertilizer on a wide variety of crops. They were often sold to neighboring farmers who used them as a fertilizer or, if the city had a farm, they were used there. The 1880 census contained a compilation by one of the leading sanitarians in the U.S. which reported that 103 of the 222 cities listed reported that their wastes were applied to the land to grow crops. The New England and middle Atlantic states followed this practice more widely than did southern and western areas of the country [2]. Several maj0rU.S. cities, including New York, Baltimore, Cleveland, Philadelphia and Washington, D.C., marketed a fertilizer material derived from cesspooVprivy wastes. Regulations focused mostly on allowing the emptying of the cesspools and privy vaults only at night and prohibiting the use of this night soil on farms within the gathering ground of the city's water supply. The City of Baltimore, which was without a system of municipal sewers until 1912, continued to fertilize garden crops with urban night soil until the sewers were constructed. Farmers in Maryland and Virginia purchased over 12 million gallons of such waste from Baltimore annually, using it to grow crops such as cabbage, kale, spinach, potatoes and

Land Application

391

tomatoes which were then sold in the Baltimore market. Baltimore was the only major city in the nation that continued this recycling of urban wastes on neighboring farms into the 20th century. Three factors led to the disappearance of such agricultural use of urban wastes: increasing urban population, technology changes, and increasing concern for public health. By the mid- 19th century, the method of having farmers and scavengers empty urban cesspools became increasingly inefficient with rampant urban growth. The deficiencies and imperfections in the system were exacerbated in the cities' densely packed slums which were often neglected by the scavengers. Water-carrying systems for removing human wastes were urged by health officials and sanitarians as a means of better addressing the problem. During the same time period, a number of the largest U.S. cities installed piped-in water supplies as did a number of smaller cities. Running water was a necessity because of the pollution of local water sources and the need for large quantities of water for fire fighting. This greatly increased water consumption was too great a volume to be handled by the existing cesspools and street gutters and urban residents soon demanded sewers to handle household and human wastes. Finally, and perhaps most importantly, the substitution of sewers for the old system was necessary to address the concern for cholera and yellow fever epidemics that plagued American cities throughout much of the 19th Century. By the 1SSO's, most large and many small American cities were building or planning to build sewerage systems. The advent of this water carriage system led most cities to dump their sewage in streams and rivers, thus creating serious pollution problems, particularly for the downstream cities which drew their water supply from rivers into which upstream cities discharged. The development of centralized wastewater treatment plants began in the early 20th Century and was accelerated significantly by the increasing concern about water pollution in the mid-20th Century, culminating in the Clean Water Act passed by the U S . Congress in 1972. Until the 1960's, most local wastewater treatment plants employed only primary treatment. The federal funding programs which resulted from the Clean Water Act provided virtually all U S . cities with the opportunity to provide secondary and, in many cases, advanced wastewater treatment. With these increased levels of treatment, the amount of solids generated also increased to reach its present estimated level of over eight million metric tons. As noted in Chapter 2, the US.EPA developed policies for encouraging the beneficial use of wastewater solids and continues to prefer, wherever possible, well managed beneficial uses to disposal practices [3]. The key to implementing such beneficial uses is to develop application systems which take advantage of the beneficial properties of biosolids while ensuring that environmental and health considerations are also addressed.

Forste 11. BENEFICIAL PROPERTIES OF BIOSOLIDS Land application of biosolids results in improved soil properties, primarily resulting from two components contained in the biosolids: plant nutrients and organic matter. Both of these components represent a significant resource for agricultural and other uses of biosolids. \ The benefits of adding minerals to soils to improve plant growth has been recognized for centuries. However, it was only in the 19th Century that this information was compiled by Justus von Liebig so that the mineral nutrition of plants had a scientific basis. As a result, by the end of the 19th Century large amounts of potash, super phosphate and eventually inorganic nitrogen were used in agricultural and horticultural production. However, Liebig’s observations were not based on precise experiments,which led to numerous studies being conducted at the end of the 19th Century. These investigations led to the recognition that neither the presence nor the concentration of a particular mineral element in a plant provides evidence that this element is essential, since plants can selectively uptake essential elements as well as those which are not necessary for growth and which may, in fact, be toxic. The technique of selective omission of particular mineral elements led to a better characterizationof the essentiality of plant elements. The term essential mineral element is based on the following three criteria: (1) a given plant must be unable to complete its life cycle in the absence of the element, (2) the element’s function cannot be replaced by another element, and (3) the element must be directly involved in plant metabolism (either as a component of an essential plant constituent or as an acquired component for a distinct metabolic process). Generalizations are still somewhat difficult in discussing the essentiality of mineral elements for plant growth. However, for higher plants, 13 mineral elements have been well-established as essential elements. The essential macronutrients are: nitrogen, phosphorus, sulfur, potassium, magnesium and calcium; micronutrient essential elements are: iron, manganese, zinc, copper, boron, molybdenum, and chlorine. In addition, the “beneficial” elements which have been found to compensate for the toxic effects of other elements or which replace some of the less specific functions of mineral nutrients are: sodium, silicon and cobalt [4]. All of these elements represent potentially beneficial components fiom biosolids used in a soiVcrop system. Most commonly, biosolids are applied to soils at rates based on the primary fertility element required by farmers for crop production (i.e., nitrogen). A. Nitrogen Considerations

The nitrogen content of biosolids varies considerably, particularly among different sources which may have received different wastewater treatment processing. This

Land Application

393

variability underlies the need for frequent biosolids analyses to develop a technically sound land application program. Such regular analyses provide a database from which nitrogen available for crops (see below) can be determined more accurately and reliably. The U.S. EPA 503 requirement that biosolids be land applied at "an agronomic rate" means that such a rate must be calculated for specific crops grown on various soils in different parts of the country in order to minimize the amount of nitrogen from the biosolids that could potentially leach below the root zone of the crop or vegetation to groundwater. For most biosolids land applied under the requirements of the 503 regulations, nitrogen will be the limiting constituent on an annual basis. Therefore, the application rate must be calculated giving careful considerationto the kinds and amounts of nitrogen in the biosolids. Typical ranges for these forms ofN in anaerobicallydigested biosolids are shown in Table 8-1 [ 5 ] .

TABLE 8-1

I

I

COMPOSITION OF BIOSOLIDS

Component Organic nitrogen

REPRESENTATIVE ANAEROBIC

I I

Range* 1% - 5%

Ammonium nitrogen

1% - 3%

Total phosphorus

1.5% - 5%

Total potassium

0.2% - 0.8%

I I

Organic nitrogen is estimated as total nitrogen (TKN) minus NO,-N and NH4N. Both of these latter forms of N are inorganic and therefore more readily available for the various processes of transformation in the soil N cycle (see Figure 8-1). Organic nitrogen must be mineralized to these inorganic forms through a biological decomposition process in the soil in order to become more available to plants. These processes occur in various combinations depending upon environmental conditions. For example, nitrification converts NH,-N to NO,-N by means of an aerobic process. Conversely, the anaerobic denitrification process converts NO,-N to gaseous N forms (N2 or N,O), and occurs to a significant degree in water-saturated soils. Immobilization occurs when soil microorganisms use inorganic N in such a way that it is no longer available for plant uptake. Nitrogen may also be lost through volatilization of NH,. The calculations which are

Gaseous

Gaseous

NH3

N,O, N, De-

Minera ization

Organic Nitrogen A

Nitrification

NO,Soil adsorption

Leaching

FIG. 8-1 Biosolids N Transformation In Soils.

F a

3

Land Application

395

generally used to characterize these N transformations in land application are shown below in the discussion of application rates. The unavailability of organic nitrogen to plants and the transformation to forms available for plant uptake provides the basis for land application programs. Since organic nitrogen must be converted to inorganic forms before plants can absorb it, the rate of nitrogen mineralization is extremely important. As noted above, the microbial conversion of organic nitrogen to mineral nitrogen is affected by various soil factors such as moisture, temperature, aeration and pH. Many studies have been conducted on the rates of nitrogen mineralization [ 6 ] . This research has demonstrated that the nitrogen mineralization rates for biosolids vary significantly with the type of treatment to which the biosolids have been subjected (e.g., anaerobic vs. aerobic digestion). Table 8-2 shows the ranges of factors which have been estimated for nitrogen mineralization of various types of biosolids and are commonly used to calculate plant-available nitrogen (PAN) [7].

Time after Application (Years)

Unstabilized Primary and Waste Activated % of No

Aerobically Digested and Lime-Stabilized % of No

Anaerobically Digested % of No

Composted Yo of N.

0- 1

40

30

20

10

1-2

20

15

10

5

2-3

10

8

5

3

Mineralization factors are employed by state regulatory agencies and land application practitioners to develop appropriate calculations for nitrogen-based (agronomic) application rates. It should be noted that while there is some variability in these estimates, the amount of nitrogen which is absorbed by plants also varies considerably with differing weather, moisture, pH and other soil conditions. Therefore, for purposes of agricultural production, a reasonable database which contains analytical information on the forms of nitrogen contained in a particular biosolids source, coupled with a knowledge of the processes used to treat the biosolids, provides a means to reasonably estimate how much nitrogen that biosolids source will supply.

396

Forste

The appropriate rate for biosolids application based on nitrogen may be determined using the recommendations of a state cooperative extension service at land grant universities, state departments of agriculture or other agricultural advisory groups. The objective is to provide sufficient nitrogen to obtain the maximum crop yield possible based on existing yield capabilities for the local soil, weather and other conditions which govern agricultural yields. Since land applied biosolids generally represent a no- or low-cost nitrogen source, economic yield need not be the limiting factor as is generally the case with chemical fertilizer inputs. An application rate should be designed to provide maximum yields commensurate with the goal of minimizing nitrogen leaching in order to provide the greatest benefit to the farmers who participate in land application programs. Biosolids used in reclamation projects are of value for both the slow-release nitrogen provided by this organic source and the soil enhancement properties of biosolids, specifically organic matter, which is generally lacking on such sites. To achieve these benefits, application rates considerably higher than those designed for agricultural crop production are common when biosolids are used for reclamation purposes. Generally, this application is a one-time rate in order to rejuvenate the disturbed soil and provide a basis for a stable ecosystem. Application rates for agronomic purposes generally range from 5 to 20 dry metric tons per hectare applied repetitively (every 1-5 years) while initial reclamation rates typically range fiom 50 to 150 dry metric tons per hectare. Silviculturalapplication rates usually fall somewhere between agronomic and reclamation and are applied only once or very infrequently for these slow growing and relatively long life-cycle crops.

B. Effects of Organic Matter from Biosolids on Soil Properties Applying organic matter in the form of biosolids can exert significant influence on soils' physical, chemical and biological properties. In general, organic matter (the non-living, heterogeneous mixture of organic components from the microbial and chemical transformations of organic debris) have long been recognized as contributing greatly to soils' productive capacity [8]. When incorporated into the soil, organic matter can affect its structure (porosity, aggregation and bulk density) as well as altering the content and transmission of water, air and heat and soil strength. Nutrients such as nitrogen are mineralized during decomposition of organic matter as described above. This results in an increase in carbon, nitrogen and cation exchange capacity following biosolids additions. Changes in other soil chemical properties, such as pH, electrical conductivity and redox potential, also occur with the addition of organic matter. Since this organic matter represents new energy sources for organisms, changes in biological populations will also occur, in turn influencing synthesis and decomposition of humic substances produced by

Land Application

397

those microbial populations, nutrient availability and interactions with soil inorganic constituents. The profound and complex interactions of soil organic matter are by no means filly understood, although it is commonly recognized that organic matter imparts to soils a desirable physical condition as well as modifying various chemical and biological relationships. Investigations into the bulk density (BD), the weight of oven dry soil per unit volume, provide an indicator of a soil's physical condition. Bulk density is related to the porosity, texture, hydraulic conductivity, aggregation and compaction properties of soils. In general, the desirable effect of lowered BD values occurs to a greater degree as the rate of biosolids application increases, probably due both to the dilution effect from adding less dense organic matter to the more dense mineral matter and also to increased soil aggregation or binding together of individual soil particles. A well structured soil with a relatively high level of strongly bound aggregates has greater erosion resistance and improved aidwater relationships, and increasing aggregation generally provides increasing hydraulic conductivity, infiltration rate, air diffusivity, surface drainage and ease of root penetration. Porosity, the total soil volume occupied by pore space, is inversely proportional to bulk density and is another index of soil physical properties. Numerous studies have demonstrated increased porosity in various types of soils from the addition of several different types of biosolids. Similarly, saturated hydraulic conductivity as a measure of the rate of water movement through soils is increased significantly by the addition of biosolids. Biosolids also increase soil water retention at both field capacity and wilting point, an important factor in crop production where irrigation is practiced andor drought conditions exist. Unlike fresh animal or plant residues incorporated into the soil, most biosolids have been through a biological treatment where partial decomposition has occurred. Thus, biosolids decomposition rates in soils may be slower than most fresh organic residues, resulting in longer-lasting effects on soil organic matter levels and changes in the composition of the organic fraction of soils. Abundant research has demonstrated the close relationship between organic matter and soil organisms, but few data are available to determine the specific effects of biosolids organic matter on soil organisms. The effects on flora and fauna often cannot be separated from the primary effects on soil physical and chemical properties. The bacteria, actinomycetes,fungi, alga6 and soil micro- and macro-fauna contribute significantly to biosolids recycling by decomposing the organic compounds, eliminatingpathogens, influencingthe solubility and mobility of inorganic ions in soil and through involvement in the N, P and S cycles. Existing data on soil microflora in biosolids amended soils have generally shown increased microbial populations in agricultural soils. While better information is needed to establish the population shifts which occur from biosolids additions to soils, it should be noted that the addition of an energy source as well as a diverse

Forste

398

microbial population from biosolids may broaden the spectrum of soil microbial activity and increase the potential to decompose organic contaminants added to soils. The microfauna present in soils (e.g., earthworms, mites, nematodes) can alter the magnitude and pathways of biosolids organic matter decomposition by their feeding activities. The relatively scarce information on these effects indicates that earthworms may accelerate decomposition and stabilization of biosolids in the soil as well as mechanically mix and change the distribution of organic and mineral matter. At the same time, labile constituents of biosolids are converted to earthworm biomass and respiration, thus reducing odors and pathogens. Addition of biosolids to soil decreases root penetration resistance (as related to the physical properties described above) and results in improved root:shoot ratios as compared to inorganic fertilizer additions. Increased yields from biosolids application have been well documented in numerous research studies; however, it has been suggested that the most interesting effects of biosolids was not the direct short-term fertilizer effects, but the longer term effect of soil organic matter (i.e., an increase in soil productivity which cannot be explained by the mineral nutrients alone). These increases are probably attributable to increased soil moisture resulting from higher organic matter levels as well as from the slow release of N and P from biosolids. Substantial amounts of N, P and K as well as micronutrients and organic matter can be recycled through harvested drops and provide benefits in both yield and quality.

C.

Effects of Other Biosolids Constituents

When biosolids are considered in the context of a recycling rather than a disposal mode, the environmental problems associated with trace elements become much less critical. After applying biosolids, chemical and physical reactions begin to affect elemental availability. The accumulation, volatilization, translocation and removal of these elements will largely be controlled by the soil and crop management practices on land application sites. By understanding the mechanisms involved and evaluating the data on the chemical, physical and microbiological properties of the biosolids, as well as application site characteristics, land application systems can be developed which optimize the recycling of these elements. Biosolids additions to soils increase the total soil N content (as discussed above) due to the addition of both inorganic and organic nitrogen sources from biosolids. Transformations of nitrogen from biosolids encompass the whole spectrum of the soil N cycle. The ability of biosolids to supply N for crop growth decreases as the biosolids decompose over time.

Land Application

399

Alterations in soil pH from the application of biosolids depend on various soil properties, including texture, buffering capacity and length of time after the last biosolids application. Biosolids which do not have lime added as part of the treatment process generally tend to cause a decrease in pH from the production of organic acids during biosolids decomposition and/or by NH,-N nitrification. This change tends to stabilize over time. If plants are sown directly into soil amended with fresh biosolids, seed germination can be inhibited either from high levels of ammonia or salts which may be present in biosolids. These effects are generally short-lived and dependent on the rate of application. Most of the evidence for the inhibitory effects, however, is based on laboratory or greenhouse experiments which have been shown not to occur in field studies. The large quantities of salts present in biosolids may increase the soil solution electrical conductivity (EC). In humid climates, the addition of salts via biosolids application is of less significance than are such additions in arid climates where salts tend to accumulate in the surface horizon of the soils (see details under Agronomic Considerations as follows). The amount of phosphorus applied by biosolids can vary by two orders of magnitude (see Table 8-l), and the amounts of P associated with the organic and the inorganic biosolids fractions also varies. Biosolids application provides substantial amounts of P for crop growth and increases P concentration in the soil. Since soils fuc substantial amounts of P, its vertical movement is often limited even when large quantities of biosolids-containing P are supplied. However, increased organic matter may decrease the soil's ability to sorb P, thus allowing for some movement down through the soil profile. P solubility is influenced by soil pH, becoming less soluble at values less than 5.5 or greater than 7.5. The rate of P movement through biosolids-amended soils depends upon many variables, including texture; iron, aluminum and calcium content; and redox potential. Much applied P can remain in a form unavailable for plant uptake. These factors suggest that the major consideration with respect to P additions to soils from land application are related to the potential for surface water pollution when any soils high in P are subject to sediment erosion. Additional research into the forms and relative bioavailability of P in both biosolids-amended soils and sediments enriched with P from biosolids application will help to resolve some of the issues regarding organic P sources and water quality concerns. The amounts of metals contained in biosolids have been found to vary greatly, as shown in Chapter 1, Table 1-4. However, the requirements which must be met for metals content in biosolids applied to land as regulated by 40 CFR Part 503 ensures that these metals are not present at levels which would cause agronomic, environmental or human health problems as discussed in detail in Chapter 2.

400

Forste

111. SITE SELECTION, DESIGN AND MANAGEMENT A. Site Selection For land application of biosolids, yield of vegetation or crop rather than disposal of the material, is the primary objective. Probably the most useful single source of information for initially determining site suitability for land application is provided by soil surveys. These are generally available for the soils used most intensively (i.e., in agricultural production). They provide detailed soil maps on a photographic background, a general soil map, soil description by series and mapping unit, data on drainage and agronomic properties of soils and interpretive tables. Soil surveys are prepared by the Soil Conservation Service in cooperation with agricultural experiment stations and local government units. Experienced agricultural professionals (e.g., soil scientists, agronomists) use soil surveys extensively to identify potential sites that meet regulatory and agronomic requirements for land application of biosolids. A significant advantage of the agronomic application of biosolids is the minimal need for detailed site investigations. Since the application is a low analysis fertilizer constrained by nitrogen requirements of the crop and with appropriate quality standards or cumulative limits imposed with respect to metals, a combination of soil survey maps and site visits generally will provide enough information on the site characteristics to identify appropriate areas for land application. Routine soil tests which are conducted to develop recommendations for crop-specific fertilizer application provide the basis for nitrogen-limited application rates. Soil characteristics which lend themselves to agricultural production also provide the necessary characteristicsfor using biosolids in a land application program. Most states have guidelines or regulations for separation distances between the areas receiving biosolids and adjacent site features such as developments, dwellings, surface water, wells, roads and rights-of-way. The primary reason for these setbacks is a potential for surface runoff of liquid biosolids. Immediate incorporation or injection into the soil generally requires reduced setback distances. Some representative setback distances are shown in Table 8-3. These setback requirementsshould be taken into account when evaluating site suitability-they may represent constraints which render a site unusable from a practical standpoint; in any case, they alter the net acreage available for biosolids application. When potential sites are identified, the soil survey reports provide a basis for preliminary selection; field inspections and investigations are necessary to c o n f m these selections. Quadrangle maps published by the U.S. Geological Survey may also be useful during preliminary planning and screening to estimate slope, topography, depressions or wet areas, rock outcrops, drainage patterns and water

Land Application

401

table elevations. Because of their scale, neither of these two types of maps can be relied upon for evaluating small parcels and do not eliminate the need for field investigation of potential sites.

TABLE 8-3

RANGES OF SETBACKS (BUFFER ZONES) TYPICALLY REOUIRED FOR LAND APPLICATION

Feet

1 Public road I House

I

I

(Meters)

0-50

I

(0 - 15)

20 - 500

I

(6 - 152)

Well

100 - 500

(30 - 152)

Surface water

25 - 300

(8 - 91)

Property line

none listed - 100

(none listed - 30)

10 - 200

(3 - 61)

Intermittent stream

Some important criteria to be evaluated in eliminating potentially unsuitable areas for land application include: 5 5 5

+

+

*

steep areas with sharp relief undesirable soil conditions (shallow) environmentally sensitive areas (e.g., intermittent streams, ponds) rocky, non-arable land wetlands and marshes areas bordered by surface water bodies without appropriate setback areas

The location of sites will vary depending upon the location of the treatment plant, the amounts of biosolids to be land applied and the fanning practices within a given radius of the treatment facility. Obviously the closer the land is to the treatment plant, the more cost effective the program will be. Soil characteristics which affect operations (e.g., drainage, texture) will also have a significant impact on site suitability seasonally. After identifying potential sites and discussing the potential for land application with the landowner/farm operator, a more detailed site survey should

Forsfe

402

be conducted. A drive or walk through of the proposed land application areas will verify or provide additional information on: topography drainage distance to surface water distance to water supply wells availability of access roads cropping cycles or existing vegetation Table 8-4 contains a sample form for a field survey. The extent of field investigation necessary will vary depending upon specific regulatory requirements imposed by the state, as well as the option(s) being considered (e.g., agricultural, reclamation) and the completeness of the information on site suitability,topography and other factors obtained from sources such as the SCS and USGS. Site specific information should generally include: property ownership information physical dimensions of the site overall boundaries and portion usable for land application current and planned future land use surfacelgroundwaterconsiderations - location and depth of wells - location of surface water (seasonal and permanent) - history of any flooding and drainage problems - groundwater seasonal fluctuations for agricultural crops: - cropping patterns - average yields - tillage practices - irrigation practices - use of the crop grown (food, feed, fiber or other) vehicular access within the site for forest land: - ageoftrees - species - commercial or recreational forest - fertilizer practices - irrigation practices - vehicular access within the site

-

Land Application

TABLE 8-4

403

SAMPLE FORM FOR PRELIMINARY FIELD SITE SURVEY*

A.

PROPERTY LOCATION

PROPERTY OWNERRAWER

B.

TOPOGRAPHY Relief (sharp, flat, etc.) Slope Estimate Drainage Patterns - Open/Closed - Drainage Class** - Any Underdrains

C.

DISTANCE FROM SITE BOUNDARY TO: Surface Water Water Supply Well

D.

ESTIMATE OF SITE DIMENSIONS Area Natural Boundaries Fences

E.

AVAILABLE ACCESS Road Types Other

F.

EXISTING VEGETATION/CROPS AND COMMONLY USED CROP ROTATIONS

G.

SOIL Texture Variability

--

* Adaptedfiom Reference 7. ** Soil Conservation Service Drainage Classes:

very poorly drained, poorly drained, somewhat poorly drained, moderately well drained, well drained, somewhat excessively drained, excessively drained.

404

Forste

-. -

for reclamation: existing vegetation, if any - cause of site disturbance (e.g., strip mining or coal) - previous attempts at reclamation, if any need for grading - existence of approved reclamation plan, if any, for site

-

Reviewing this type of information will further refine the site selection process. When sites are determined by the above procedures to be suitable, soil test data should be obtained for each field. For agricultural purposes, soil testing should include pH, cation exchange capacity, nutrient status and, as appropriate, background trace metals analysis (i.e., metals regulated by federal and state requirements as well as important micronutrient trace elements). Soil physical properties such as profile depth and texture may be helpful but are generally assumed suitable for agronomic purposes if the site is currently being used in agricultural production. For forests or reclamation sites, more extensive investigationof groundwater characteristicsand underlying geologic features may also be appropriate. These physical properties are much less critical for sites used for agricultural application at agronomic rates. Final site selection is often a decision based on availability of the most suitable sites, particularly for small communities. For large scale projects, it is imperativethat many sites be selected and be permitted by state agencies to provide a variety of sites to which biosolids can be applied throughout most, if not all, of the year. The basic design approach for agricultural land application is to optimize crop yields on privately owned land which has been permitted for land application. One advantage of the agronomic option is the minimal site investigation required. The application rates typically employed for agriculture range from 5 to 20 dry metric tons per hectare. These rates are designed to match the crop uptake for nitrogen and provide assurance that excessive nitrogen will not be made available for movement to groundwater. Use of appropriate application techniques and runoff control measures for different slopes will also minimize any potential for contamination of surface waters. Conservationtechniques (e.g., strip cropping, terraces, grassed waterways) and reduced tillage (e.g., chisel plowing, no-till planting) are agricultural practices designed to prevent soil erosion which also prevent surface runoff from soils treated with biosolids. Vegetative cover or crop residue is also effective in reducing runoff, even from steeply sloping soils. Incorporation into the soil is another means of reducing the potential for loss of biosolids constituents. Selecting the appropriate application method in conjunction with currently recommended practices for controlling soil erosion will essentially eliminate potential contamination of surface water or adjacent land by biosolids.

Land Application

405

The philosophy inherent in federal and state regulations is to design biosolids land application systems that are based on sound agronomic principles so that using biosolids poses a lesser or no greater threat to groundwater resources than current agricultural practices. Nitrate leaching can occur if excessive amounts of N in any form are applied to soils. Therefore, controlling the nitrogen rate provides a minimum potential for nitrate leaching on land application sites. Downward movement of metals, phosphorus and synthetic organic compounds are not usually encountered since these biosolids constituents are relatively immobile in soils. Essentially all of the applied metals, pathogens, phosphorus and organics remain in the upper 50 to 30 centimeters of soil (i.e., the depth of tillage). Soil sampling and analysis is needed to determine amounts of supplemental fertilizer, to evaluate soil pH and, in many cases, to estimate potential crop yields. This information is used to calculate annual biosolids application rates. Soil sampling procedures and information are available from university or private soil testing laboratories and each field which is farmed as a unit should be sampled separately. Limitations on total (cumulative)amounts of trace metals that can be applied under Federal (503) regulations are imposed only for biosolids which exceed the metal concentrations shown in Chapter 2, Table 2-4 (Pollutant Concentration Biosolids). However, some states which have not yet modified their regulations to conform to the Federal standards may also impose cumulative metal limits on sites regardless of concentration of these metals in the biosolids. B.

Nitrogen-Based Agronomic Rates

Nitrogen is a primary plant nutrient that is required in available forms in relatively large amounts by growing crops. It is therefore often the most limiting plant nutrient in soils for obtaining optimum crop yields. The soil furnishes nitrogen by fertilizer addition, decompositionof soil organic matter, crop residues and manure and soil bacteria fmation of nitrogen. Nitrogen deficiency is usually recognizable first as a yellow-green or pale green coloration, usually more visible on the lower leaves of the plant. Nitrogen deficiency can also result in slow growth and stunting of the crop. Nitrogen compounds in older plant tissue break down and the nitrogen moves to younger tissue. Thus nitrogen deficiency occurs first in the tips of older leaves, and as it becomes acute, effects the entire plant. Plants take up nitrogen in either the nitrate or the ammonium form, however, most uptake is probably in the nitrate form since the ammonia rapidly converts to nitrate in warm, well-aerated soils. Due to its positive electrical charge, ammoniaN is adsorbed on the negatively charged clay particles in soil. The amount of ammonium which a soil can retain is largely dependent on the amount and kind of clay in the soil (cation exchange capacity). Other positively charged ions of calcium, magnesium, potassium, hydrogen and aluminum are also held by some of

406

Forste

these negative soil sorption sites in a complex system which is one of the major differences between soils and other plant rooting media. Nitrogen transformations depend on photosynthetic energy which gives rise to a soil carbon:nitrogen ratio typically around 1O:l on a mass basis. When N uptake by plants or the addition of organic matter low in nitrogen changes this ratio, the microbial actions work to return the ratio to its normal amount in that soil. The soil solution concentrations of ammonium in well aerated soils are normally less than 1 ppm. The amount of exchangeable ammonium is nearly always small in comparison to other exchangeable cations. Compared to NO,, ammonium ions are much less mobile and thus much less subject to leaching to the crop root zone. Ammonia volatilization occurs fairly readily particularly from the surface of alkaline soils while denitrification is much less likely to occur. Because the nitrogen forms and uptake mechanisms occur in oxidationheduction reactions which occur in the aqueous component of the soil system, redox reactions govern the nitrogen chemistry of agricultural soils. While the transformations of nitrogen are usually described in terms of the microbes which carry out the various oxidation and reduction steps, these microbes and the enzymes involved in the reactions are only catalysts for carrying out the essential redox reaction that is determined by electron availability. The oxidation reduction range in soil systems is the range of water stability, which in turn is pH dependent [9]. Nitrogen has many oxidation states which can be stable within the stability range (Eh) of water. By comparing electrode potentials of possible redox combinations, the stable oxidation states can be determined. As one example, reduction of nitrate to nitrite (NO,- ) occurs at Eho = 0.95V and reduction to N, at EhO = 1.25V as long as the redox reaction is reversible. Given that reversibility, adding a reducing agent to a nitrate solution will reduce all of the nitrates to N, before the electrode potential is low enough to reduce nitrate to nitrite. Therefore, nitrite ions are unstable and will spontaneously decompose to N,. While this does occur, it is a slow decomposition due to the irreversibility of nitrogen redox reactions. Given the many possible redox reactions for nitrogen, the stable oxidation states within the stability range for water are nitrate, N,, and ammonia. The relationships are dominated by N, since nitrate is stable only under oxidizing conditions and at pH greater than 3, and ammonia is stable only under reducing conditions. The organic forms of nitrogen (e.g., amino acids) are generally more stable to oxidation than is ammonia. Presumably proteins are more stable than amino acids, as confmed by the more common presence of proteins in nature. Most of the nitrogen in the world exists as N,, some as amino nitrogen in reduced carbon compounds in living organisms and dead organic matter. Only a very small fraction occurs as nitrates. The general irreversibility of nitrogen reactions prevents the formation of nitrates which, if it were to occur, would consume virtually all atmosphericoxygen and would result in surface waters which

Land Application

407

would be dilute HNO,-a reaction which now occurs only to a very limited extent when lightning provides the necessary activation energy to overcome the general irreversibility of nitrogen reactions. Unstable compounds such as nitrite and nitrous oxide (N,O) would not be produced if soil microbial actions were only based on optimal use of chemical energy. Warming temperatures can bring about temporary nitrite accumulation in soils because the specific organisms responsible for reducing nitrite are less responsive to increased temperature than are those reducing nitrate to nitrite. Fertilizing with a nitrate source or adding water to a dry soil will also stimulate N,O production so that up to 50% of the nitrogen may be lost by denitrification. Both these microbial conversions are temporary and related to rapid microbial activity following a sudden environmental change. Potential problems associated with N additions to soils from biosolids relate primarily to the potential for high concentrations of nitrate in crops or drainage water, however, there is no evidence that N in biosolids is any more of a hazard than equivalent amounts of nitrifiable or plant available N supplied by inorganic sources. To address this issue, the mineralization factors and other N transformations which are applicable to a particular site or method of application must be considered in developing a land application program. Nitrogen management for biosolids application has the goal of calculating the amount of N in the biosolids that will be available to the subsequent crop (potentially available N or PAN). PAN is generally calculated assuming 100% availability of NO,-N and NH,-N if the biosolids are injected or incorporated into the soil. For surface application, a 50 percent availability is generally calculated for NH4-N to allow for volatilization losses. Mineralizable N prediction is somewhat more difficult and variable. Incubation procedures which estimate mineralized N as a percentage of organic N have been developed, as has use of a chemical extractant to be used as a substitute for these incubation procedures. None of these have gained widespread acceptance,probably because the potentially mineralizable nitrogen will vary with soil type, temperature and soil moisture content as well as the type of biosolids and the method of processing. Values commonly used for first year mineralization are: 40% for biosolids fiom a waste activated or primary process, 30% for lime stabilized, or aerobically digested, 20% for anaerobically digested and 10% for composted biosolids. Using this information, the concentration of PAN can be estimated as follows:

PAN = NNO,+ XNNH, + YN,,,

...................

Where: X is fraction of NH,-N that does not volatilize Y is fraction of NoRGthat is expected to mineralize based on the representative values shown above

408

Forste

Site Management As a land application program is developed, the permitting process must be initiated for the amount of land (the land base) required to sustain the project. Depending upon the state in which the permit is obtained, the timeframe for obtaining permits ranges from six months to two years. A continuing permitting effort will provide a suitable land base to conform to regional agricultural operations and specific agricultural practices. As permits are obtained they must be evaluated for their immediate suitability and included in the overall operational management of the project. Before applying biosolids to a given field, it is important to perform an evaluation of the available land base permitted for a particular generator’s biosolids. Application must be scheduled to coincide with each farmer’s agricultural practices, the time of the year that the application will occur, the type of crop that the farmer plans to plant, the physical characteristics of the biosolids to be applied, and many other permit and regulatory requirements that affect operations. It is advisable to develop a checklist for each state or area in which operations are proposed that includes all the regulatory requirements that must be performed at each specific site. Pre-application checklists should verify and identify such requirements as: that the site is properly permitted that the required time interval has elapsed since the last biosolids application that loading limits will not be exceeded by the proposed application that ownership of the land has not changed since the last application that any seasonal water (e.g., water table) restrictions are not in effect that flagging of the field to identify boundaries, restricted areas, setbacks, and other requirements has been performed that any field changes and adjustments to the net acreage available for application have been made that the farmer and landowner consent forms have been completed and signed that the calculation of the application rate of the field is based on the permitted net acreage of the crop to be grown, the appropriate application rate and that a calculation of the number of trucks needed to apply the calculated amount of biosolids has been established A pre-operating checklist should be completed by both the technical staff and any field operator involved in the direct application of biosolids. Such a procedure ensures that all operational considerations that must be employed during field application are addressed. A method of establishing truck reporting based on scales; if available, or daily reports should also be developed. It is advisable to summarize all activities for a two week period for most ongoing projects in order to keep a running check on the accuracy of the tracking

Land Application

409

system. A computerized monitoring information system for large-scale projects may be used to generate the reports required by the permitting agency and to provide data to participating farmers on the nutrients and metal loadings (if applicable) for each field. Farmer reports are not a regulatory requirement but provide a valuable service to farmers who agree to participate in land application programs and whose land is essential to the success of the project. With the development of complex state and federal requirements for land application, standard operating procedures should be developed and maintained for each project. Technical staff should be thoroughly trained in these SOPS which also should be updated regularly as needed. C.

Design for Non-Agricultural Sites

While most regulatory and technical experience in land reclamation relate to such efforts on land which has been surface mined for coal or sand and gravel, the principles involved may be modified to apply to other types of marginal or disturbed sites. Roadway and other construction, depositing of dredge materials and fly ash, as well as forestry related activities can result in large areas which are frequently difficult to reclaim conventionally with inorganic fertilizers and other soil amendments. The potential for biosolids use on such sites is significant, and combining the use of such sites with agricultural land application can also provide sites during periods of the year when agricultural or some other crop production land alone would not be available. Thus reclamation, while conducted using somewhat different methods and application rates than those for agriculture, is a significant opportunity for the beneficial use of biosolids in land application. Application on construction sites and roadways, as well as areas drastically disturbed (e.g., copper mines, oil shale mine areas, zinc and lead smelters and landfills), provides an opportunity for using biosolids to stabilize and revegetate eroded, unproductive soils and man-made disturbed land. The environmental benefits achieved by providing a medium for the re-creation of a soihegetation system cannot be measured in terms of dollar value alone. Areas which are now a biological desert due to high metal contamination (e.g., zinc, lead and copper) have been successfully reclaimed using biosolids when virtually no other reclamation effort could succeed on such a site [lo]. Since the 1977 Surface Mining Control and Reclamation Act established regulations for the revegetation of currently mined land, reclamation efforts have been focused on the requirement to establish a diverse, effective and permanent vegetative cover of the same type of plants native to the area where the mining activity occurred. The stringent requirements of the Act are difficult to meet using conventional reclamation techniques and the use of biosolids represents a very effective option for meeting the regulatory requirements.

410

Forste

Extensive research has been carried out in the United States on the practice of reclaiming disturbed land with biosolids. This research, along with large-scale projects, have demonstratedthat biosolids provide an excellent soil amendment and chemical fertilizer substitute for reclamation purposes. Many state regulations and guidelines include provisions for application rates higher than those established for agronomic crops in order to establish growth on these infertile sites. Such applications may not exceed cumulative loading limits under 503 (and most state) regulations and still qualify as land application. The use of biosolids for reclamation provides the most dramatic evidence of the benefits of using biosolids to enrich soils and enhance plant growth. The organic matter content which is valuable in agriculture is critically important in disturbed sites where topsoil is inadequate or on sites where topsoil does not exist. While organic matter is the most important single factor in the improvement of soil physical properties which are achieved using biosolids, the fertilizer elements and neutralizing compounds that improve soil fertility and pH are also of great value for reclamation purposes. The increasing number of successful reclamation projects using biosolids across the U.S. is a further demonstration of the value of biosolids as a recyclable resource rather than a waste product. The use of biosolids for reclamation purposes has been most prevalent in the Midwest, Northeast and mid-Atlantic areas of the U.S. These projects resulted in excellent growth of forage and cover crops on a variety of research, demonstration and full-scale operations. Many sites are seeded with a mixture of grasses (to provide quick vegetative cover) and legumes (which eventually predominate as permanent vegetative cover). The application of biosolids has also helped in the establishment of tree seedlings on reclaimed areas, by improving growth rates, particularly for the hardwood species when they are seeded simultaneouslywith the grass legume mixtures. The potential for disturbedmined sites to receive biosolids is enormous, both on sites which have been abandoned and are not subject to the reclamation law requirements (“orphaned lands”) and on active mine sites which are required to reclaim areas as the mining operation proceeds. Since reclamation sites which are operated as land application projects are subject to the same regulatory requirements for trace elements as are agricultural sites, the application of biosolids at reclamation rates will not adversely affect the ability of reclaimed areas to support agricultural production or any other land use which is appropriate to the site. The research results from such sites have paralleled the results found for agricultural crop production and have demonstrated clearly that the usability of the site has been significantly improved by the application of biosolids. Most research results have shown that trace element concentrations in biosolids-reclaimed mined areas are within the range considered normal for such metals in unpolluted and unamended soils.

Land Application

41 1

The immediate goal of reclamation is to establish a vegetative cover to prevent soil erosion; in the longer term, a stable soil ecosystem is the goal. Since such areas lack microbial activity and organic matter, the normal soil microbial processes and N-cycling are also a measure of the degree to which these reclaimed areas resemble undisturbed soil. Application of biosolids has uniformly resulted in increased microbial populations and activity--bacteria, fungi, and actinomyces. The use of biosolids as reclamation material has been shown to eliminate the initial lag period of several years that is characteristic of conventionally reclaimed areas during which microbial activity and plant growth are minimal. For such conventionally reclaimed areas to eventually acquire “soil” characteristics, intensive reclamation and management techniques, along with annual fertilizer additions, are usually necessary. By contrast, normal soil populations and processes in the surface soil of biosolids reclamation sites can be achieved within two years and deteriorate over time. The beneficial effects of biosolids application on the whole ecosystem of reclamation sites have been demonstrated in a variety of settings. With the exception of a brief and temporary elevation of N03-N in soil percolate water due to the higher-than-agronomic application rate, no other negative effects on water quality have been demonstrated. The temporary elevation of nitrates represents a minimum ecological impact which is clearly offset by the positive environmental benefits achieved by reclaiming these sites with biosolids. A number of wildlife studies conducted on reclamation sites have found no adverse effects on the health of domestic or wild animals and buds living on such sites. Research and practice has clearly shown that municipal biosolids, if applied properly under present regulations and guidelines, can be successfully used to revegetate mined or other disturbed lands in an environmentally safe manner. Revegetation of a variety of otherwise devastated sites has been demonstrated in many studies using various types and application rates of biosolids. All of the results validate the present regulatory framework which protects the environment, as well as animal and human health, when using biosolids to reclaim drastically disturbed land. Design for Forest Land Utilization In general, the chemical, biological and physical interactions of biosolids and soil in forest applications are similar to those in agricultural operations. Trees have been shown to respond positively to nutrient additions, particularly when forest soils are low in nitrogen and the surface litter layers have comparatively high N storage (immobilization)capacity. Some advantages of forest application include the greater flexibility in scheduling because forests are perennial (which may also translate to lowered storage requirement) and the extensive acreages of forest land in many regions which provide large areas for application.

412

Forste

The type of biosolids application to particular forested areas will depend to a large degree on the growth stage of the forest. When the stand is developing high nutrient content tissues (early development), nutrient accumulation will be significantly higher than that occurring in near-mature or climax forests which maintain biomass rather than synthesizing new growth. Applying biosolids on forest land may also pose special operational challenges. Forest land is frequently rugged, steeply sloped and relatively inaccessible. In addition, the tree stand itself may present obstacles to biosolids distribution methods. Such factors must be taken into account in developing forest land application for biosolids. Forest land application can be conducted on commercial timber and fiber production lands, federal and state forests as well as privately owned forest areas. Biosolids may also be used in nurseries, green belt management and Christmas tree production as appropriate in specific situations. The three common scenarios for forest application include: (1) recently cleared land that has not yet been planted, (2) young plantations, and (3) established stands. Each of these must be considered separately when designing a forest land application program. Physical features which should be considered include: Proximitv to ~ u b l i caccess (e.P.. recreational areas. dwellinps. public roads and hikine trails) The application sites should be as removed from these public access areas as practical. Many states will also impose the minimum setbacks or buffer zones which have been developed for agricultural biosolids application. Proximitv to surface waters The application site should be located and managed to avoid contamination of surface waters, including setbacks (which most states require). For steep slopes and /or relatively impervious soil, greater runoff is likely to occur and increased setback distances should be considered. Proximitv to drinking water supplies Where forest application is to occur in water supply sensitive sites, provisions should be made for biosolids quality control, minimization of biosolids movement and possibly monitoring of surface and/or groundwater quality in the area. Distance to groundwater Guidelines suggest that forest application sites have an average groundwater table distance of one meter (0.7 meters minimum) below the soil surface.

Land Application

413

This recommendation is to prevent seasonal surface flooding and boggy conditions which might cause biosolids migration. Forest soils are often infertile by agronomic standards--deficientin organic nutrients and low in available nitrogen. Added organic N from biosolids may make up this deficit without adverse impacts on groundwater. Tree species differ significantly in their uptake of available nitrogen; there is also a large difference in N uptake by seedlings, rapidly growing young trees and mature trees as noted previously. The average annual N uptake of hlly established and vigorously growing forest ecosystems varies from 100 to 400 kilograms per hectare per year depending upon species, age and other factors. These estimates include the overstory and understory vegetation or gross N uptake by the forest ecosystem. For specific projects it is recommended that state or regional forestry management or research agencies be contacted in designing forest utilization projects. While information on forest uses of biosolids is not as extensive as that relating to agricultural production, there has been a growing interest in such application in the last decade, particularly in the Pacific Northwest. Research conducted at the University of Washington, College of Forest Resources, coupled with a number of operating projects, has provided information and relevant experience for implementing forest application projects. Technical data are available from these efforts in a variety of silvicultural settings [ 1 11. Application of biosolids to recently cleared forest sites has the advantage of providing better access for application equipment and the possible option of incorporating the biosolids into the soil if the site is sufficiently cleared. It may also be easier to control public access to a cleared forest site (which is generally less attractive than wooded areas for forest recreational activities). There may also be an option to select species which are superior in growth and survival characteristics on biosolids amended sites. Some disadvantages of applying biosolids to recently cleared areas include the potential for salt or ammonia damage to new seedlings for some species (this can be overcome by allowing the biosolids to age for six months or more). Seedlings also have a low nitrogen uptake rate and biosolids application may be inappropriate if there is an underlying potable aquifer which could potentially be impacted by excess N. Extensive weed control is also necessary to prevent competition with tree seedlings for up to three to four years and browsing by deer and other pest species may also require control measures since animals may selectively feed on biosolids-amended sites due to their higher food value. Applying biosolids to young forest plantations (over two years old) provides a greater tolerance of fresh biosolids application, less concern for weed control than with cleared sites, more rapid nitrogen uptake, reasonably good access and rapid growth response from many tree species. However, biosolids application by spraying over the canopy may be restricted to certain times of the year, some weed

Forste

414

control still will probably be necessary and plant nitrogen uptake is less than that of a well established forest cover.

D.

Pathogen Considerations in Land Application Projects

Returning organic wastes such as biosolids and animal manures to the land is part of a natural cycle in which the soil decomposes these residues, thus recycling and decontaminating them of any disease agents they may contain. Any system of biosolids management which is to succeed must neither avoid nor abuse this cycle. Land application systems must strike an acceptable balance between the benefits derived fiom using this material and the potential risks to human and animal health. Each disease agent has a life cycle that usually includes a saprophytic or nonhost stage during which transition fiom one host to another occm. This is also the stage in which the greatest destruction of pathogens occurs and survival away fi-om the host is so rare that millions of pathogen propagules usually are required for a successful infection. Survivability of pathogens in the outside environment (particularly soil, air and water) varies greatly with only a few of the hundreds of disease agents having high enough survival rates in soil and water that they need to be addressed in a biosolids recycling program [ 121. Land application programs and restrictionsfor biosolids (which have not been treated to Class A pathogen reduction levels, thus eliminating the concern) focus on making sure that pathogens do not escape the destructive action of the soil or survive it long enough to complete the cycle back to man. For land application, this recycling to hosts would be most likely to occur by direct contamination of food crops, as exemplified by the use of raw sewage in truck crop production practiced in some parts of the world. This route back to the host is the most direct and also the easiest to block as has been done in the site restrictions imposed by federal and state requirements as described in Chapter 2. Exposure potential from viable pathogens diminishes over time as these organisms are destroyed by heat, sunlight, drying and competing microbes in the environment. Protecting public health and animals fi-ombiosolids-bornepathogens is accomplished by: reducing pathogens in the biosolids through treatment or natural die-off reducing pathogen transport by reducing the materials attractiveness to disease vectors (e.g., insects, birds, rodents) limiting human and animal contact with biosolids through site restrictions which allow natural die-off of pathogens. Regulatory approaches represent a combination of these approaches [ 131. The Part 503 Rule specifies treatment technologies that are deemed sufficient to reduce pathogens in biosolids to levels which will protect public health and

Land Application

475

animals. Other technologies that produce a comparable reduction may be allowed if their efficacy is demonstrated through microbiological monitoring. The site restrictions (described in Chapter 2) provide for M e r pathogen reduction through environmental factors which occur when biosolids are beneficially used. Many soil and environmental processes provide destruction of pathogenic agents when biosolids are spread on the soil surface or incorporated. Viruses do not multiply outside of living hosts, neither do most pathogenic bacteria and parasites. On the soil surface, high temperature, ultraviolet radiation and desiccation are all lethal to pathogens if exposure and intensity are sufficient. When incorporated into the soil, the natural soil microflora which have adapted to metabolize almost any organic substance will gradually attack dormant pathogens. There are also believed to be many antibiotic, antagonistic or predatory activities occurring in soil against "alien" organisms such as human pathogens. Ultimately these forces, plus the alternation of freezing and thawing, flooding and drying, aerobic and anaerobic conditions will further reduce pathogens. In addition, once a disease agent is incorporated into the soil a few inches, aerosol dispersal or surface runoff cannot occur; if soil cultivation brings them to the surface, drying and solar radiation will rapidly destroy these organisms. A review of the literature on disease aspects of using municipal biosolids on land in the United States finds virtually no evidence linking human disease to the presence of pathogens in biosolids. The "hazard" or ''risk" which is hypotheticallyposed by the presence of pathogens has never been shown epidemiologically. One epidemiological study which evaluated health effects from land applied biosolids (both clinical and subclinical manifestations of infection) found no differences between families living on sites where biosolids were applied as compared to control farms [14]. The potential for surface runoff of pathogens into surface water supplies is also addressed by the management practices (e.g., setback distances, slope restrictions) which are part of every regulatory program for Class B biosolids. It is safe to conclude that the land application of biosolids from a generally healthy population collected in sanitary sewage systems and treated or stabilized before use does not represent a significant public health risk. While it is obvious that human disease can be spread by uncontrolled use of raw sewage on food crops, especially fiom populations with a high incidence of intestinal parasites and enteric diseases, this problem is perpetuated by the poor sanitation, ignorance and lack of effective waste handling systems which such a practice represents. This is not a practice carried out in the United States and is certainly not relevant to the land application of treated biosolids described in this Chapter. Epidemiological evidence to date has shown that the segments of the US. population living near treated plants or on farms where biosolids have been applied are just as healthy as the rest of the population, as are workers in treatment plants or those engaged in hauling and applying biosolids to land. While further epidemiologicalstudies may be helpful in reassuring the general public with respect to the health aspects of land

416

Forste

application, common experience strongly shows a lack of any significantly greater incidence of disease associated with sewage treatment or biosolids use. It would therefore be very surprising if more detailed studies which are extremely expensive, difficult to conduct properly and take many years to complete would reveal a serious problem. Under these circumstances, it seems justifiable to continue to use the prudent management practices which have been developed and codified in federal and state regulations for the beneficial use of biosolids and which have been found to be compatible with high public health standards.

E.

Agronomic Considerations

Microelement Considerations Because nearly all biosolids contain levels of microelements higher than those naturally present in soils, agronomistshave studied the short- and long-term effects of land applying these microelements through biosolids recycling. Some microelements(Zn, Cu, B, Fe) may serve to correct plant deficiencies. Excessive use of high levels of biosolids with elevated concentrations of Zn, Cu, Ni or B might lead to phytotoxicity effects and crop yield reductions. As noted in Chapter 2, the non-nutrient microelements (Cd, Pb, Hg) have also been addressed by research and research-based regulatory constraints. These limits have been described in the discussion on risk assessment contained in Chapter 2. Since biosolids, like most organic by-product materials, simultaneously add several microelements, as well as phosphorus, and may influence soil pH, the plant uptake of biosolids-bornemicroelements is complex. In general, elements such as Mn, Zn, B, Mo, and Se can move readily to plant tops when applied to soil. Others are intermediate (Ni, Co, Cu), and still others seldom reach plant tops (Cr, Pb, Hg). In addition, other soil and plant factors strongly influence the movement of microelements to edible plant parts. Soil pH is predominant among soil factors which affect microelement uptake because it controls the degree to which these elements react with the soil. In general, metal uptake decreases linearly with increased soil pH; an exception is Mo which becomes more plant available in neutral to alkaline soils. Other factors which affect metal diffusion include the concentration in the soil matrix, soil texture, clay and organic matter contents, other nutrients, soil bulk density and soil moisture content. Growing roots can also cause local pH and bulk density changes which influence microelement uptake. A number of factors present in biosolids tend to reduce uptake, particularly as compared to metal salts and the results of pot studies. These factors, while not completely understood (particularly with respect to their interactions with each other) have clearly demonstrated that field studies of microelement uptake in plants provide the only legitimate basis for evaluating uptake rates from biosolids. Interactions which influence microelement chemistry in biosolids-amended soils include: temporary

Land Application

417

change in pH from mineralization of organic-N; alkalinity contained in or added to biosolids; relative insolubility of the forms of metals initially present in biosolids; increased soil organic matter and cation exchange capacity from biosolids; addition of phosphorus which strongly influences metal interactions; increased available soil Fe; the addition of a “balanced” mix of microelements;and generally higher fertility of biosolids-amended soils. These factors can be managed judiciously by excluding biosolids contaminated with excessive amounts of microelements and limiting their maximum total applicationsto levels safe to crops and the food chain (as per 503 regulations) and by managing biosolids-amended soils according to appropriate plant and soil tests. Soluble Salt Considerations When applying biosolids to crop land, the basic principles of salinity management apply and factors must be considered to alleviate any potential salinity hazards associated with biosolids application. The two most common hazards are excess total salts and high sodium levels. Excess total salts cause reduced germination and growth while high sodium levels cause dispersion of soil particles, poor soil structure and reduced infiltration rates. Because of these differing effects, the nature of the salts in biosolids should be determined in evaluating application rates. Soluble salt content can be determined by analyzing for each constituent, or more simply by measuring the electrical conductivity (EC) and applying a factor which calculates total soluble salt content. Measuring EC usually is accurate enough to determine the effects of a biosolids application rate. A simple procedure is to add 10 grams of biosolids to a one liter volumetric flask filled with distilled water and measure the EC of the decantate. The EC provides an estimate of salt content assuming a factor of 700 ppm for one mmho/cm of EC. More detailed information for estimating salt effects in soils such as changes in exchangeable sodium (Na) are used to calculate sodium adsorption ratios (SAR):

SAR =

Na

+

...................

The ions are in milliequivalentsper liter. This ratio is used to express the relative activity of sodium in exchange reactions with the soil [ 151. Typically in biosolids the four cations which constitute about one half the total salt content are Na, K, Ca and Mg. Given the balance of these cations, the primary salinity hazard of utilizing biosolids on soils is generally

478

Forste

total salts rather than sodium alone. Compared to animal wastes, biosolids are usually low in soluble salts because many of them have been removed with the effluent from the treatment plant. However, the principles developed to reduce the effects of soluble salts from animal waste apply equally to biosolids. Part of the salts in biosolids is not present initially but is released through mineralizationwherein the conversion of organic N to inorganic forms causes a rise in salinity. Conductivity level found in soils will also vary with soil texture. A fine textured soil containing 0.1 percent salt (approximately 2200 kgha in 15 cm of soil) would have a saturation extract EC value of 3mmhos/cm. This amount would be added with 30 dry metric tonshectare of a material containing 7.5 percent salt. The same amount of salt in a sandy soil would result in an EC value of over 6 mmhoskm. Thus, soil texture is an important factor in determining whether a biosolids application is likely to have an adverse affect on soil salinity. By the same token, coarse textured soils allow more rapid leaching of salts than do the finer textured ones, a consideration which should be examined if annual applications of biosolids are contemplated. The example discussed above is based on a maximum EC of 4 mmhos/cm in the root zone which allows essentially all crops to be grown without salt management practices. If higher EC values are allowed, crop selection and salt management practices should be imposed; however, many crops can be grown satisfactorily at levels up to eight mmhodcm as shown in Table 8-5.

Salinity EC

Crop Yields

0-2

mostly negligible effects

2-4

very sensitive crops may be affected

4-8

many crops are affected

8 - 16

salt tolerant crops only not affected

above 16

a few very tolerant crops not affected

The most important factor in preventing harmful effects of salts from biosolids or other organic sources is water management. To prevent excessive increases in EC in the soil solution, the amount of water moving through the soil

Land Application

419

must be increased as added salts are increased through biosolids applications. Under non-irrigated conditions, approximately 40 dry metric tonshectare of biosolids could normally be added annually without excessive salinity buildup when 10 cm of water move through the soil profile annually (a rate which is generally exceeded in much of the Eastern U.S.). By contrast, little or no leaching occurs in arid areas necessitating careful attention to soil salinity levels and appropriately low application rates unless irrigation is practiced. In any case, care must be taken to insure that salts do not accumulate. Since irrigation water is also a source of salt addition to soils it should be taken into account in managing salinity. pH Considerations Micronutrient availability in soils is highly dependent on soil pH. Therefore, the addition of biosolids which have been amended or treated with lime is a consideration which needs to be addressed in planning a land application program. In particular, the availability of manganese is most sensitive to elevated pH which results from the application of lime treated biosolids. Manganese availability decreases as soil pH increases and is the micronutrient which is the most sensitive to elevated pH. In acid soils, manganese becomes soluble and available to plants and can even cause toxicity at extremely low soil pHs. Manganese deficiencies can occur at pHs as low as 6.3, which makes it the primary consideration with respect to elevating pH by using lime-treated biosolids. Manganese deficiency commonly occurs in well-drained alkaline or neutral soils, such as peat and muck soils, and is in direct proportion to the high leaching potential and lower amount of manganese naturally occurring in these types of soils. Manganese deficiencies are also frequently observed in poorly drained soils with low redox potential. This deficiency is caused by the solubility of manganese under anaerobic soil conditions which leads to manganese leaching from the soil, thus the potential for manganese deficiency can be inferred from the natural soil drainage class since those developed under poor drainage conditions will likely contain less manganese than those developed under good drainage. As one example, the coastal plain soils of Maryland and Virginia which commonly experience seasonal high water tables are also subject to manganese deficiencies at moderately elevated soil pHs. Because it is a recognized agricultural phenomenon, manganese deficiency has been examined in considerable detail and recommendations have been established for both soil and foliar applications to correct such deficiencies. If land application of a lime-amended biosolids source is conducted in an area where manganese deficiencies have been experienced, land appliers should consult agricultural extension sources and include methods for determining when such deficiencies do occur and provide a means for addressing this issue.

420

Forste

IV. METHODS AND EQUIPMENT

Land application of biosolids involves a variety of methoL, and equipment. Land application generally entails the following:

0.

Transportation from a treatmentlstorage facility to application sites Storage, as needed Application to agricultural, reclamation or other sites

A.

Transportation

0.

*.

Biosolids transportation may employ trucks, rail cars, barges, pipelines, and combinations thereof. Liquid biosolids can be transported by barges, railroad, tank trucks, and pumping/pipelines, with tank trucks being the most common choice. Tank trucks are used to transport liquid biosolids either to an application site or to an intermediate point such as storage lagoon, railroad or barge loading area. Many tank truck configurations are available (e.g., tank trucks range in size from 500 to 6,500 gallons; tank trucks may be modified for field applications;tank wagons may be pulled by a farm tractor for short hauls). Semi-solid (dewatered) biosolids are transported by rail cars, specialized highway trucks (dump trucks), tractor-trailers, and roll-on containers. Truck capacities vary from 10 to 25 tons (8 to 30 cubic yards). A truck cover (hard top or tarped) is typically required to prevent odors and spills. Other specialized features include leak-prooftailgates with seals and wide anti-splash shields. A hard top tractor trailer is shown in Figure 8-2. Many factors influence the type, size, and number of vehicles needed for biosolids transportation (quantity of biosolids, distance, road conditions, seasonal variability of land application, equipment utilization considerations, etc). Generally, truck transportation of dewatered biosolids is significantly less expensive than transportation of an equivalent quantity of liquid biosolids (by as much as 30% in some cases). Detailed information on the selection of means of biosolids transportation is provided in Refs. [16] and [17]. Many municipalities use private contractors for biosolids transportation and land application; private contract versus use of publicly-owned equipment and employees is generally determined by economics. Pipeline transport of liquid biosolids has limited application due to high capital cost which can be justified only for high volume. The lack of flexibility of pipelines limits the selection of application sites for land application. Rail and barge transport is employed for high volume and long distance hauling, and rail transport is generally limited to dewatered biosolids.

Land Application

.,,. -

..~. "~.. . . ,

Forste Biosolids Armlicators Biosolids application is accomplished by either surface or subsurface methods. In both cases, the biosolids eventually become incorporated into the soil by mechanical means or naturally over time. Biosolids are applied either in liquid or in dewatered form. Surface application on tilled land is usually followed by mechanical incorporation of biosolids in the soil. Standard agricultural discing or other tillage equipment can be utilized (e.g., discs or disc harrows). Discing is inappropriate when biosolids are applied to existing pasture or hay fields, crops which would be damaged by this operation. Most existing land application programs in the U.S. are on privately owned land. Such operations require flexible arrangements and a variety of equipment to conform with local farming practices. Equipment used varies from a simple tank truck with surface application nozzles (liquid biosolids) or a box spreader drawn by a tractor (dewatered biosolids) to more sophisticated and high-volume machinery. In the latter category, specialized self-propelled biosolids applicators for liquid and dewatered biosolids are available from many manufacturers (Figures 83, 8-4, 8-5 & Table 8-5). These machines feature high flotation tires which minimize soil compaction and can apply liquid biosolids on the surface of the soil or subsurface inject them up to 17 inches. The machines can be equipped with a tank and a pump for liquid biosolids or a spreader box for dewatered biosolids applications with a capacity of up to 17,000 gallons per hour of liquid or 75 wet tons per hour of dewatered biosolids. The techniques available for subsurfaceapplication of liquid biosolids include direct injection beneath the soil surface or injection ahead of plow blades. Another alternative is spreading of liquid biosolids on land and incorporating them into the soil by plowing or discing. In subsurface injection, odors and vectors are effectively controlled, pollution of surface waters through runoff and loss of ammonia nitrogen through volatilization are minimized. Subsurface injection of liquid biosolids also has the advantages of minimal impact on the soil and crops (Figure 8-4), however, it is often more expensive overall due to higher volume and associated handling cost. Spray irrigation is used to disperse liquid biosolids on agricultural land or a forest site. A typical spray application system consists of a rotary sprayer (rain gun), storage lagoon (tankage), pumps and piping. The system can be stationary (Figure 8-6) or moving. Storage Biosolids storage is often required to compensate for changes in production or application rates, equipment breakdown, seasonal demand (i.e., cropping cycles)

Application

423

424

Forste

426

Forste

Tank Capacity (gal)

Spreader Box Capacity,

Application Rate

Model

HP

TerraGator 1664

25 0

TerraGator 004

24 5

3,100

12.0

15,000

50

x135 373x124 x132

TerraGator 2505

25 0

4,000

15.0

17,000

70

I 426x147

AgGator 1004

19 5

1,600

7.2

9,600

Ag-

19

2,200

10.0

12,000

2,000

c.y.

gaVhr

10.0

12,000

I

~~

I

x136

and weather conditions. Storage can be provided at either the treatment plant, land application site, or at other locations. Both short- and long-term (up to several years) storage of biosolids is practiced. Liquid biosolids can be stored in digesters (short-term), dedicated storage tanks, and lagoons (impoundments). Dewatered biosolids can be stored in specializedhoppers (short term), stockpiles, and lagoons (long term). The storage capacity required is greatly influenced by site-specific factors and climate considerations. U.S. EPA data indicate that storage capacity requirements range from 30 days in hot, dry climate areas to 200 days in cold, wet areas. A simple method of calculating storage capacity is to estimate the maximum number of days biosolids generation will require storage, factoring in climate and scheduling. A more sophisticated method is to prepare a mass flow diagram of

Land Application

427

cumulative biosolids production and projected biosolids land application volume and determine storage requirements [18]. A majority of states regulate the design and operation of impoundments (lagoons) and storage tanks, and require a permit to operate storage facilities. Typically, a biosolids lagoon is required to have an impermeable (plastic or clay) liner, some freeboard, prevention of surface runoff, and a groundwater monitoring system. Above-ground storage tanks may be required to have a secondary containment system at least equal to the largest above-ground tank volume. Lagoons are the least expensive storage option. Several types of lagoons are used for liquid biosolids including anaerobic, aerobic, and facultative lagoons.

Af'PUCATlON

BOOS

LIQUID BIOSOUDS DELIVERY

SPRAY GUN

FIG. 8-6 SPRAY APPLICATION OF LIQUID BIOSOLIDS

Anaerobic lagoons are used for storage and sometimes for pre-treatment of liquid biosolids prior to land application. The biological processes which take place in the lagoons are similar to those in an anaerobic digester (reduction of organic matter and gravity settling), except there is no mixing or heating equipment and the processes are significantly slower and influenced by weather. In a longterm anaerobic lagoon, significant amounts of organic nitrogen will be converted to ammonia and escape to the atmosphere. Organic matter reduction will also be significant if the biosolids have not already been treated. Serious odor problems are often associated with anaerobic lagoons. Aerobic lagoons are either naturally or mechanically aerated lagoons. Oxygen is provided to control odors and support aerobic microbiological activity. In naturally aerated lagoons, a relationship is developed between anaerobic or

Forste

428

facultative bacteria degrading organic matter in the bottom of the lagoon and oxygen production by algae near the surface. The oxygen demand must not exceed the algae ability to supply the necessary oxygen. Otherwise, the lagoon will become anaerobic and offensive odors may result. Mechanically aerated lagoons do not depend on algae oxygen [ 191. Liquid and dewatered biosolids removal from storage is accomplished by a variety of means ranging from stationary and floating pumps for liquid biosolids to front-end loaders, bulldozers, grapple cranes and trucks for dewatered biosolids removal from storage lagoons and pads. The cost of this removal may be significant and should be carefully analyzed when planning a land application program.

V.

ECONOMICS OF LAND APPLICATION

Land application cost depends upon numerous factors. The major cost components are: 0.

-0 -0 0.

Transportation Storage Site preparation, biosolids application, and incorporation in the soil Permitting, engineering, monitoring, and recordkeeping

Cost of transportation from the point of fmal biosolids treatment (e.g., digestion, chemical or heat treatment, dewatering) to the application sites depends largely on hauling distance, amount of biosolids transported and type of vehicles. Typically, cost of transportationranges from $2 to $8 per ton. The cost of vehicles varies from $120,000 for a specializedtractor trailer to approximately $150,000 for a three-axle dump truck (1 995). Cost of storage in lagoons, tanks, digesters, or other facilities includes costs of additional construction, transportation, maintenance and labor. Cost of application and incorporation in the soil depends upon biosolids volume, availability of permitted land, labor, energy, maintenance, cost of capital, etc. Typically, application and incorporation cost ranges from $5 to $10 per wet ton for dewatered biosolids or $0.04 to $0.15 per gallon of liquid biosolids. Site preparation for land application includes topographic, soil, water and drainage mapping, access roads, site fencing and security. In general, for agricultural land application programs on existing farm land, extensive site preparation and modification are not typically required. In case of biosolids applications to disturbed land, dedicated land sites and similar programs with high application rates, site grading, surface water control measures, access roads, security, and fencing may be required.

Land Application

429

LarPe-Scale Land Apdication Program As an example, a large volume (up to 1,500 wet ton per day of dewatered biosolids) agricultural land application program in the Mid-Atlantic region of the United States is described below (courtesy of the Bio Gro Division of Wheelabrator Clean Water Systems Inc.).

Dewatered biosolids are transported by tractor trailers to the agriculturalsites permitted within a 60-1 00 mile radiusfrom the WWTP. A crew comprised of one supervisor, three t o j v e operators, and one agricultural technical specialist is required to run the program. During favorable weather seasons (March - November), the dewatered biosolids are reloaded from the transportation vehicles (tractor trailers with 20-24 wet ton capacity) into biosolids applicators (self-propelled or spreader boxes &awn by a farm tractor) and applied (spread)on thefarm land which previously had beenpermitted andpreparedfor such activity (Figure 8-7). Up to 45 acres per day are needed to accommodate the above referenced volume (based on a typical application rate of 7 dry todacre). During inclement weather (December - March), dewatered biosolids are stored in lagoons. Up to 30,000 wet tons of storage may be required. The following shows 1995 costs (excluding delivery) for equipment (or comparable) requiredfor a large-scaleproject:

ml?lGam2 1. Front-end loader (John Deere, 644E/4wd) 1 2. Disc (John Deere, Model 555) I

3. 4. 5.

6.

$140.0 $ 15.0 Farm Tractor (John Deere 49G0/4wd) 2 $ 90.0 Biosolids Applicator (Terra-Gator 2505) 1 $180.0 Spreader Box (tractor drawn) I $ 29.0 Pick-up Truck -I $ 20.0 Total $564.0

Tractor-trailers/trucks ($120,000 to $150,000 each) to deliver dewatered biosolids to thejeld are not included in the above costs. Medium Scale Land Application Program (up to 100 wet tons Per day)

430

Forste .,,.,. _,,

.‘..,

Land Application

431

Medium-scale land application programs are performed similarly to the large volume program described above, however, they may require significantly less permitted land (2-3 acres per day) and can be accomplished by afield crew of two or three operators. Thefollowing shows 1995 costs (excluding de1ivery)for equipment (or comparable) requiredfor a mediumscale project:

QlYLL2&2!m 1. Front-end loader (John Deere, 644U4wd) I 2. Disk (John Deere, Model 555) I

3. Farm Tractor (John Deere 485014wd) 4. Spreader Box (tractor drawn) Total

I

1

$140.0 $ 15.0 $ 90.0

w $2 74.0

Storage requirements are also required for medium-scale projects; although for proportionately smaller volumes. VI. MONITORING AND RECORDKEEPING Any successful land application program requires that a system be implemented to comply with specific permit conditions as well as all state and federal regulations. Such an approach involves: providing the appropriate permitted land base for application activities and monitoring of field applications; developing procedures and mechanisms to ensure that operations will conform to permit conditions and regulations field-by-field; and collecting, recording and reporting all required information to demonstrate conformance with permit conditions, regulations and biosolids quality requirements. Adequate staffing must be provided in order to implement measures that will fulfill the above objectives. The actual steps performed to conform to permit requirements and relevant regulations are described below. With the initiation of a new project, a code system which will individually identify the various sites dedicated to a specific project should be established. At the same time, information on the quantity and quality of biosolids, methods for determining loading rates, analytical requirements and frequency and specific

Forste management practices in a particular state or other geographic area should also be compiled. This infomation is needed to initiate the permitting process for the land base needed for the project. Depending upon the state in which permits are to be obtained, a time fiame h m six months to two years may be required to obtain site permits. For large projects in particular, the submittal of permit applications for a land base suitable for beneficial use is a continuing activity, even though a specific site may not receive an application of biosolids for up to several years. This continuing permitting of a suitable land base ensures that enough land will be available for the project to operate subject to the seasonal constraints and the specific farm management and cropping practices which determine individual farming activities. Dunng the site selection process, the signature of consent for both the individual who will be farming the land application site and the landowner should be obtained. It is advisable for these agreements to spell out as far as possible the various responsibilitieswhich the applia has and those which must be observed by the farmer and/or landowner &er the application. In some cases, farmers or landownersmay request thatthq be provided with an InderndlcationAgreement protecting them for potential legal liability associated with the improper land application of biosolids. Figure 8-8Contains for projects in which a contractor performs the biosolids application under a permit issued by the permitting authority. The rmrdkeeping and reporting requirements of the 503 Rule are mandated throughoutthe United States;other individualpermitting requirements may vary &om state to state. Federal and state regulations and guidance documents should be consulted to develop a system to comply fully with all applicablerequirements. For land applicationunder 503,the number and stringency of requirements depends upon the quality of thebiosolids and the circumstancesunder which they are being applied [20]. Land appliers are those who apply biosolids as a soil conditioner andor to fertilize crops or vegetation grown in the soil. Under 503, this term includes those applying large quantities of bulk biosolids to agricultural land as well as those applying smaller quantities which may be distributed in bags to a lawn or home garden. The definition of “land applier” is therefore very broad and not all land appliem are required to comply with the same provisions of the Part 503 Regulations as discussed in Chapter 2. The requirements for biosolids @ty are usually the responsibility of the person who prepares the biosolids, not the land applier. However, such quality plays a si@icant role in determining land application requirements and is relevant to the discussion of the compliance requirements for land appliers. Biosolids meeting the most stringent limits for pollutant conwtrations, pathogen and vector attraction reductionare considered comparable to commercial fertilizer products and therefore am not subject to any additionalrequirementsfor complianceunder 503. Additional

Land Application

433

In&mnrw&n

Agreement

Contractor agrees to indemnify, defend and hold harmless (Landowner) fi-om md against any and all claims, suits,actions, demands, losses, costs, liabilities, and expenses (includingremediation costs and reasonable attorneysfees) to the extent such losses result h m :1) Contractor’s or Generator’s violation of applicable laws or regulations in effect at the time of biosolids application; or 2) the negligence or willful misconduct of Contractor in the delivery and epplicationof biosolids to the undersigned Landowner’sproperty. In the event this i n d d c a t i o n is enforced against Contractorfor a violation of law by a Generator, Landowner agrees to assign and subrogate to Contractor its claim against Generator. This indemnificationshall survive termination of this Agreement until the expiration of any applicable statutes of limitations. Landowner shall promptly not@ contractor in the event of a third party claim and Contractor shall have the right to provide and oversee the &fense of such claim, and enter into any settlementof such claim at its discretion. Landowner agrees to l l l y cooperate with Contractor in the defense against any third party claim. Landowner

Contractor

Date:

Date:

IG. 8-8 INDEIvlNIFICATION AGREEMENT.

quimnents are imposed on biosolids which do not meet one or more of the preceding requirements to ensure the same level of protection for human health and the environment is met in all cases. If the applier changes the quality of the biosolids prior to application, such a change may also influence the number of requirements with which he must comply. Monitoring data which establishes and certifies biosolids quality is provided by the person who prepares the biosolids (often the generator) who is then responsible for meeting the preparation requirements before the biosolids can be land applied. If the land applier then alters the quality fi-om that which was received h m the preparer, the land applier becomes a preparer and assumes responsibility for monitoring and CerGfYing biosolids quality with respect to pollutant limits, level of pathogen reduction and level of vector attraction reduction. If the biosolids have met the three criteria referred to above and are then mixed with other non-503 regulated substances (e.g.,

Forste

434

fertilizer materials, bulking agents), the person performing that mixing operation is not required to re-evaluate the product’s final quality. In other instances, however, where biosolids quality is considered to have been changed, for example when bulk biosolids from several sources which do not meet the above three criteria are mixed prior to land application or when a source of biosolids which does meet these three requirements is mixed with a source which does not, the resulting quality of the mixtures must be determined in order to correctly land apply. Detailed information regarding these requirements is contained in Reference P11.

A.

General Requirements of 40 CFR 503.12

Land application of bulk biosolids subject to this section must include the: 0.

0.

0.

transfer of sufficient information (Notice and Necessary Information) among the preparer, land applier, landowner and permitting authority tracking of cumulative pollutant loading limits from biosolids that do not meet the concentrations contained in 503.13,Table 3 movement of biosolids across state lines

If the land applier is not the same person who prepared the biosolids, the preparer is responsible for providing documentation to the land applier on the quality of the biosolids before they may be applied to land. The suggested format providing this Notice and Necessary Information (NANI) is contained in Figure 89. Before land application, the land applier will need to obtain the following, independently or from the preparer:

*.

0.

pollutant concentrations nitrogen concentrations class of pathogen reduction level achieved vector attraction reduction option achieved, if any

=.

0.

The land applier must also provide the landowner/farmer with any information needed to comply with land application requirements (e.g., site restrictions). It is also advisable to provide the landowner/farmerwith the biosolids quality information including nitrogen content and with appropriate documentation that biosolids have been applied in accordance with all relevant Part 503 requirements.

Land Application

435

As a practical matter, the permitting process for a land base will entail interaction with the landownedfarmer in obtaining information typically required by state regulatory agencies (e.g., maps, cropping information, soil test results). For bulk biosolids subject to cumulative pollutant loading rates (CPLR), the land applier must notify the permitting authority in the state where the biosolids will be applied of his intent to apply to a particular site before the initial land application. Such notification must include the location of each land application site and the name, address, telephone number and a National Pollutant Discharge Elimination System (NPDES) permit number (if applicable) of the land applier. In addition, before beginning land application on a site, a land applier must consult the permitting authority to determine whether past applications of biosolids subject to cumulative pollutant loading rates (CPLRs) have been made after July 20, 1993. If no biosolids were applied after that date, the land applier must then begin keeping records of cumulative pollutant loadings for each of the ten regulated metals. Multiple land appliers on a single site must make their application data available to one another and coordinate the tracking of cumulative loadings to ensure that CPLRs are not exceeded. If biosolids subject to CPLRs were applied after July 20, 1993,the land applier is responsible for finding out the amount of each regulated metal that was applied and subtracting those loadings from the allowable CPLR for each pollutant. If the levels of pollutants were not documented in a previous application and the applier is unable to take these past loadings into account, no biosolids that are subject to CPLRs can be applied to the site. When a land applier determines that bulk biosolids subject to CPLRs is to be land applied in a state other than that where it was generated, the land applier should notify the preparer, who is then responsible for notifying the permitting authority in the state where biosolids are to be applied before initial application to any site. The land applier should contact the preparer to conform that this notification has been submitted before beginning application. In the case of interstate land application of biosolids subject to CPLRs, the land applier as well as the preparer must send prior written notice to the permitting authority in the state where the application will occur. The Part 503 Regulations provide four sets of pollutant limits: ceiling concentrations, pollutant concentration limits, cumulative pollutant loadings and annual pollutant loading rates. These limits are contained in 40 CFR Part 503.13, Tables 1-4as described in Chapter 2. The ceiling concentrations establish the maximum concentrations of each metal that biosolids can contain and still be land applied. Each sample analyzed must meet the ceiling concentration limits (i.e., they are maximum, never to be exceeded values, not averages). Biosolids not meeting any or all of these specified thresholds cannot be land applied. After determiningthat biosolids meet the ceiling concentrations, it must then be determined which of the three remaining sets of pollutant limits applies to a particular biosolids source.

436

Forste

This/om1 ir 10 ossisi complinnce with the bulk biosolids notflation requirements 503.12@. I/rhe biosolids mccr :he Crccprionnl Quoliry requirements. howcur. then the norijicniion requirementsdo nor apply.

*Biosoli& may not be land applied if any pollutant mcen~ations in any sample ucttdli these values.

B.

-

Pahopen Reduction (40 CFR 503.32) Pleaw indicate the level achieved ClmB

ClnssA

C.

--

Vector AtWaclion Reduction (40 CFR 503.33) Please indicate the option performed

0Option I

0

option2

0

option3

Option4

Option5

0

Option6

0

Option7

Option8

A. Nnme and Oflicial Tille (weorprint)

(1 C.

signnture

~

B. Arm Codc and Telephone Number D. Dau Signed

FIG. 8-9 SAMPLE FORMAT FOR PROVIDING NOTICE AND NECESSARY INFORMATION.

Land Application

437

A

If the pollutant lcvcls m the biosolids do not meet the pollutant concentration limits in Table 3 of 40 CFR Part 503, then the land applier should provlde the landowner with the followmg dormaUon

1

Locauon of land applicauon site

2

Number of hectues where the bulk biosolids w m applied

3

Date and ( M e bulk biosolids werc applied

4

Amount of bulk biosolids applied m mctnc tons, dry wnght

5

Record the mount of each metal and mtrogcn applied and appropnate units (I e ,kilograms per hectare, pounds per acre)

B

Clnss B pathogen reduction alternative was used (see Part I), then the followmg site reslncuons must be met Please check the boxes for the site restnchons mel, d any Food crops that may touch the biosoliddsoil ~XtUrecannot be harvested before the end of the follouing waiting pencd A 0 1 If harvested parts nre tolally above the land wmt to harvest for 14 months after the application of biosolids 0 2 Ifharvested parts arc below the land surface and the biosolids mnam on top of the soil for 4 months or longer before the field WBS plowed. wait lo harvest for 20 months after the uuhal applicallon of biosolids 0 3 Ifharvested parts are below the land surface, and the biosolids werc incorporated into the soil wthin 4 months of being applied. wait to harvest for 38 montha after the mihd applicauon B 0 Food crops that do not touch the biosoliddsoil mixwe. feed crops. and fiber crops C D M O ~be harvested for 30 days nfier biosolids applicauon C 0 Anlmals cannot be g r d on the land for 30 days after applicauon of the biosolids D 0 Ifharvested turfis used for a lawn or other purpose where there is a htgh potential for public e\poswe, then Ihc turf cannot be harvested for 1 year after the applicauon of the biosolids to the land E 0 Public access to land with a high potential for public expsure (e g ,parts. playgrounds. golf couises) will be restncted for 1 y e a after the applicauon of the biosolids F 0 Public access lo land w~tha low potential for public upsure (e g ,pnvate property. remote or reslncted public lands) will be renncted for 30 days after the applicauon of the biosolids

C

If the pnparer did not perform any of the vector atunction reduction Opuons 1-8 (seePan I). then either Option 9 or 10 must be

If8

performed by the land applier Please indicate foption 9 or 10 was performed Check appropnate bo\

0 Option 9 -- Subsurface Injection D

0 Opuon 10 -- Incorporated @lowed) Into the Soil

0 NIA

CERTIFICATION

I m@. under pennlly of law. that this document and all atlachments were prepared under my direction or supenwion in accordance with a system designed to assure that qualliled pmonnel properly gather and evaluate the information submitted Based on my inquiry of the p m o n or pmons who manage the syslcm or these p m o n s directly responsible for gathenng the information, the informnuon submitted is, to the best of my knowledge and belief, b e . accurate. and complete 1 nm nware that there are slpllilcant penalties for submilung false mfoormauon. including the possibility of fine and impnsonment for hnowing violations A Nnme and Oflicial Title (ypr orpnnr)

B Area Code and Telephone Number

Forste

438

Biosolids containing pollutant concentrationsbelow those contained in Table 3 of 503.13 impose no requirements relative to those pollutants on the land applier. These pollutant concentration limits are monthly average values in mgkg, dry weight basis. Cumulative pollutant loadings (Table 2 of 503.13) establish the maximum amount (mass) of each regulated pollutant that can be applied to a site (kgha) during the life of the site. The greatest number of recordkeeping and reporting requirements for the land applier pertains to biosolids of this quality. It should be noted that in addition to the 503 requirements, state regulations may also mandate recordkeeping and reporting for biosolids which are not subject to these requirements under 503. The land applier must maintain records of the amounts of each pollutant applied to the site in the biosolids including any applied in previous applications occurring after July 20, 1993. For biosolids that meet ceiling concentrations (Table 1 of 503.13) but not pollutant concentration (Table 3 of 503.13) limits, Annual Pollutant Loading Rates (APLRs) establish the maximum amount (mass) of pollutants in biosolids that can be applied to a site during a 365 day period only when these biosolids are sold or given away in a bag or other container for application to the land (Table 4 of 503.13). These AF'LRs are imposed for biosolids which might commonly be used by home owners and for whom it would be impractical to track cumulative pollutant loadings. Note that such biosolids must also meet the highest quality requirements for pathogen reduction (Class A) and a vector attraction reduction in order to be sold or given away.

B.

Pathogen Reduction

The preparer of biosolids is responsible for monitoring and certifying pathogen reduction. A land applier may, however, at any time choose to verify this information independently. Land appliers who apply biosolids certified by the preparer as Class A have no requirement relative to pathogens. If biosolids are Class B treated by a process, site restrictions must be imposed to allow time for natural processes to further reduce pathogen levels. Site restrictions for Class B address: (1) public access to the site and (2) crop harvest and grazing of animals at the site. Public access must be restricted for at least 30 days on all land application sites that receive Class B biosolids; this is usually accomplished by selecting sites on farmland in rural areas, remote lands or fenced areas. If the site is used fiequently by the public or the potential for public contact is high, such access must be restricted for one year after Class B biosolids are applied; such land would include parks, playgrounds and golf courses. Besides public access, other site restrictions may also apply depending on the use of the site. If food crops are grown, certain waiting periods must be observed

Land Application

439

prior to harvest and waiting periods must be observed on sites where feed and fiber crops, as well as turf, are grown and where animals are grazed. The following summarizes the site restrictions associatedwith application of Class B biosolids in order to achieve the same level of protection to public health and the environment as is provided by Class A biosolids treatment: public access to land with a high potential for public exposure is restricted for one year after biosolids application public access to land with a low potential for public exposure is restricted for 30 days after biosolids application food crops, feed crops or fiber crops are not harvested for 30 days after biosolids application food crops with harvested parts that touch the biosolids/soil mixture and are totally above the land surface (e.g., melons, cucumbers) are not harvested for 14 months after application of biosolids food crops with harvested parts below the surface of the land (e.g., root crops such as potatoes, carrots, radishes) are not harvested for 20 months after application when the biosolids are not incorporated into the soil or remain on the soil surface for four or more months prior to incorporation into the soil. food crops with harvested parts below the surface are not harvested for 38 months if the biosolids are incorporated into the soil within four months after biosolids application animals are not grazed on a site for 30 days after biosolids application turf shall not be harvested for one year after biosolids application if it is placed on land with high potential for public exposure or on a lawn unless otherwise specified by the permitting authority These site restrictions to control public access or crop harvest and grazing animals must be implemented by either the land applier or the landowner/fanner. At a minimum, the land applier must provide the landowner/farmerwith a list of these restrictions and inform him that they must be met for each site where Class B biosolids are applied. If the land applier will implement the restrictions, he must certify that they have been met and maintain this certification in his records for a five year period. If it is agreed that the landowner/farmer will implement the appropriate restrictions, the iand applier must provide him with a list of the restrictions and certify that he was appropriately informed. C.

Vector Attraction Reduction

The ten vector attraction reduction options specified in 503 are treatment options (1-8) and barrier options (9 and 10). Treatment options are undertaken by the biosolids preparer, and in such a case the land applier has no requirements relative

440

Forste

to vector attraction reduction once he has been notified by the preparer that this requirement has been met. If such a treatment option is not performed by the preparer, the land applier must implement and certify compliance with one of the barrier options of injection below the soil surface or immediate incorporation into the soil following application to the surface. D.

Management Practices

Management practices applicable to the land application of biosolids address the following:

9

threatened or endangered species flooded, frozen or snow-covered land distance to waters of the United States agronomic rates

For biosolids sold or given away in a bag or other container, the only management practice required is the provision of a label or information sheet indicating the appropriate application rate for the quality of biosolids. The applier is then required to read and correctly follow these instructions. Threatened or Endangered SDecies Bulk biosolids subject to the management practices of 503.14 may not be applied to the land if an adverse effect on threatened or endangered species or their designated critical habitat is likely to occur. An “adverse effect” includes any direct or indirect action that reduces the likelihood that a threatened or endangered species will survive or recover from an impact. The critical habitat is any location where a threatened or endangered species may live or grow during its life cycle. Threatened or endangered species are listed in 50 CFR 17.11 and 17.12, published by the U.S. Department of Interior, Fish and Wildlife Service (FWS). The normal tillage, cropping and grazing practices, mining, forestry and other activities that involve turning the soil and impacting vegetation are not likely to cause any increase in negative impact on endangered species and may be beneficial because of the enhanced nutrient status and soil building which the application of biosolids imparts. Therefore, the application of biosolids to land would not normally be considered to have an adverse impact on threatened or endangered species or their habitat. If there is some specific reason to believe otherwise, a land applier should evaluate whether any threatened or endangered species or habitats at the site could potentially suffer a negative impact.

Land Application

441

Flooded. Frozen or Snow-Covered Land While bulk biosolids may be applied to flooded, fiozen or snow-covered lands, this practice must not result in the biosolids entering wetlands or other waters of the United States unless specifically authorized by a permit under Sections 402 or 404 of the Clean Water Act. By insuring that proper runoff prevention andor control measures exist to prevent biosolids from entering the waters of the U.S., the land applier is allowed to apply biosolids to flooded, frozen or snow-covered areas. Many state programs include such measures as slope restrictions, buffer zones, tillage requirements, crop residue or other means of insuring that biosolids will not migrate to wetlands or waters of the U.S. The land applier should be familiar with the details of such requirements and, in some cases, season restrictions which relate to this requirement. E

S

Bulk biosolids may not be applied to agricultural land, forest or reclamation site within 10 meters (approximately 33 feet) of any waters of the US. unless otherwise specified by the permitting authority. This restriction applies to intermittent flowing streams, as well as creeks, rivers, wetlands or lakes. The permitting authority (U.S. EPA or a delegated state) may give approval for application within 10 meters of waters of the U.S. for site-specific conditions or to enhance the local environment. Revegetating a stream bank suffering from severe erosion is an example of the situation wherein the permitting authority could reduce or eliminate this requirement. Additional management practices are sometimes required by states to minimize runoff and ponding of biosolids. These management practices would be specific to a particular situation and land appliers should familiarize themselves with state and regional requirements which relate to this issue. Avronomic Rate Part 503 requires biosolids application at a rate equal to or less than the agronomic rate for a particular site. As discussed in more detail earlier in this chapter, the agronomic rate is the dry weight application designed to provide the amount of nitrogen needed by the crop or vegetation while minimizing the amount of nitrogen that passes below the root zone to groundwater. For reclamation sites the permitting authority may specifically authorize application of biosolids at a rate above the agronomic. Since such an application usually occurs only once to improve soil physical properties and supply sufficient nitrogen, organic matter and other nutrients to establish vegetation, land appliers should obtain approval for such an application rate from the permitting authority. After the site has been reclaimed, any future applications of biosolids should be limited to the agronomic rate discussed earlier in this chapter.

442

Forste

E. Monitoring The preparer of biosolids must monitor their quality. It is the responsibility of the land applier to obtain information fiom the preparer or through independent verification by additional testing on the pollutant concentrations in bulk biosolids. EPA approved procedures contained in 503 must be used to test biosolids. The following references provide detailed guidance on the collection and analysis of biosolids samples: 9

9

POTW Sewage Sludge Sampling and Analysis Guidance Document (EPA, 1989, and updates) Standard Methods for the Examination of Water and Wastewater, 18th Edition (APHA, 1992) Test Method for Evaluating Solid Waste, Physical/Chemical Methods, EPA Publication SW-846 (EPA, 1986) Environmental Regulations and Technology: Control of Pathogens and Vector Attraction in Sewage Shdge (EPA, 1992)

The preparer may, by contractual arrangement, designate the land applier as the party responsible for sampling andor testing the preparer’s biosolids. In that case, the land applier will also need to keep records documenting those results and provide them to the preparer. All preparers must keep records on biosolids quality regardless of what that quality is. Land appliers are only required to keep records for biosolids quality if they change its original quality and, therefore, meet EPA’s defmition of a preparer. Land appliers of bulk biosolids which are subject to the management practices of 503 (i.e., do not meet any one, two or three of the quality parameters for Class A pathogen reduction, vector attraction reduction or Table 3 metal concentrations) must document the implementation of applicable management practices. If the biosolids do not meet Table 3 metal limits, the land applier is also required to track the cumulative pollutant loadings. If the bulk biosolids has been treated to a Class B level of pathogen reduction, the land applier must also keep records on the implementation of the site restrictions required for this material. If the biosolids has not already met a vector attraction reduction requirement, the land applier must also keep records documenting the implementation of vector attraction reduction Option 9 or 10. Land appliers must sign Certification Statements relating to compliance with site restrictions, vector attraction reduction andor management practices whenever such requirements are applicable to the land applier. These records must be maintained and be readily available to inspectors for a five year period. If the biosolids do not meet the 503 pollutant limits of Table 3, the land applier is

Land Application

443

required to maintain records documenting the cumulative pollutant loadings for every application site indefinitely. The reporting requirements of 40 CFR 503.18 are described in Chapter 2, Regulatory Requirements. These apply to major municipal NPDES permittees and Class I biosolids management facilities. “Majors” are POTWs with a design flow rate 2 1 MGD, and those with a service population of 10,000 people or more. Class I biosolids facilities are generally POTWs that must have an approved pretreatment program. An EPA Regional Administrator also has discretion to designate other treatment works as Class I biosolids management facilities. This designation can also be applied to a land applier by the EPA Regional Administrator. A land applier who is a preparer must report compliance information if the land applier is a major municipal permittee or a Class I biosolids management facility. Preparers should consult the manual entitled Preparing Sewage Sludgefor Land Application or Surface Disposal--A Guide for Preparers of Sewage Sludge on the Monitoring, Recordkeeping and Reporting Requirements of the Federal Standardsfor the Use or Disposal of Sewage Sludge, 40 CFR Part 503 (EPA 1993) for further guidance on the information which a preparer must provide. Since transferring of land application information fiom the land applier to the preparer is not required by P a 503, a written agreement between the two parties should be developed to ensure that this flow of information and exchange of information, along with any other agreement necessary to meet the requirements of Part 503 by both parties, is clearly defmed and legally implemented. The permitting authority has.discretionary ability to take enforcement action against either a land applier or preparer or both if a violation of land application requirements occurs. Such enforcement actions will depend on the nature and circumstances surrounding any violation.

VII. PUBLIC OUTREACH

Beneficial use of biosolids has often been hampered by public concern and opposition which in turn results in institutional barriers to beneficial use practices. The attitudes which impose constraints on a community’s use of biosolids may be an even more important barrier than are formal federal, state or local regulations governing biosolids [22]. Concerns about health risk, nuisances and environmental quality are often coupled with political and attitudinal constraints which are not based on either science or regulation. Public outreach efforts for biosolids projects must be based on the recognition that scientific data and information about risk assessment do not necessarily result in public acceptance for projects. The perceived risk is just as real to the individual concerned as is scientificallyderived risk assessment and the perception of risk has

444

Forste

repeatedly been shown to vary significantly from the actual or scientific risk involved. However, perceptions about many subjects can and do change in time and with accurate, credible communication efforts. In the case of biosolids, altering the risk perception surrounding beneficial uses should heavily rely on the extensive, detailed scientific data which has been compiled during the last several decades and has been codified and documented in the comprehensive 503 rulemaking. Efforts to communicate with concerned individuals, groups and institutions should focus on the quality standards for safe, beneficial use which are based on protective assumptions about the impact of biosolids on the environment, crops, animals and humans. The very common and often natural human apprehension of the unknown or little known represents part of the reason for a negative perception. Since the average citizen knows little or nothing about wastewater treatment, industrial pretreatment programs, or the biological and chemical composition of biosolids, a public outreach effort must include communication about these factors which protect the integrity of the practice of beneficial use. In developing a dialogue with communities where biosolids will be processed or used, it is vital to establish the link between a national commitment to clean water and the necessity to manage the solids which result from wastewater treatment. A clear distinction between a waste material (even a hazardous waste in some cases) and the inevitable production of biosolids from the treatment of wastewater is a fundamental first step positioning beneficial use of biosolids as a positive environmental practice. Detailed discussions of research fmding on attitudes, effective messages and elements required for an effective communicationsplan form the basis of the Water Environment Federation’s ongoing effort to achieve public acceptance for beneficial uses of biosolids [23]. Outreach materials and an ongoing effort to achieve public acceptance by WEF members incorporate the messages of environmental benefit, agricultural benefit and protection of health and safety as embodied in existing practices and regulatory frameworks. Achieving public acceptance includes both information and outreach efforts to the general public, and, even more importantly, the development of local acceptance. It is highly unlikely that a beneficial use project or program can succeed without addressing potential risk of local negative opinion at the outset. Both facilities which process and treat biosolids and operating projects such as agricultural land application require different approaches to achieving local acceptance. In both cases, open communication and responsiveness are essential to develop alliances and support. For land application programs, the farming community is the obvious and first choice in the public acceptance effort. Agricultural organizations, fanners, and extension agents can provide a strong local support which makes programs successfbl. Their alliance is also very helpful in developing the understanding and

Land Application

445

therefore the approval of local officials. The value of biosolids as a significant resource to agriculture must be recognized and articulatedby the farmers or others who use them for the practice to remain viable. Appropriate field management which involves coordination with individual farming practices is essential for the agricultural component of biosolids beneficial use. Land appliers should pay carehl attention to the timing, method of application and housekeeping details during operations to ensure continued cooperation by meeting farmers’ needs as well as the needs of generators. In this regard, a highly motivated technical staff with experience in such disciplines as crop science, soil science, natural resources and environmental science can provide the liaison with individual farmers. Their role in the permitting and monitoring requirements for land application as well as their responsivenessto farmers’ needs will provide the necessary link between the biosolids generator, the field operation and the local communities. The following activities also maintain and expand agricultural support for land application programs: *. *.

.. 0.

-. *.

arranging field operational demonstrations sponsoring local programs (e.g., agricultural appreciation days) providing biosolids nutrient information to agricultural extension agents funding research and extension publications presenting information, organizationalmeetings for the farmer community providing farmers with the results of soil testing and field application reports

Providing farmers with actual cost savings from biosolids as compared to current fertilizer prices enables them to both appreciate the value of a biosolids program and integrate this low- or no-cost input into their farm economic plan. To expand the base of local support for biosolids in an agricultural setting, it is important to identify and meet with key individuals in the local communities:

-. 0. 0.

0.

-.

0.

administrators and managers elected officials state legislative representative environmental organizations community associations civic groups

During these meetings, it is important to stress the use of biosolids as a resource, not a waste material. When local communities recognize that all biosolids, including those which they generate, result from the treatment of

Forste

446

wastewater, they can understand that both wastewater treatment and biosolids recycling benefit the environment and society in general. A.

Communication Channels

Public acceptance efforts must include an ongoing method for maintaining the flow of information to keep communication lines open among all interested parties. Concerns about health, odor, groundwater contamination, decline in property values and other legitimate issues can be anticipated and defused in a wellmanaged public acceptance program. Some or all of the following outreach methods may be included in a public information program: media contact--news releases and informational materials describing land application in general and project specifics written and audiovisual materials--detailed land application manuals, specific project information,brochures, question-and-answerpamphlets, news reprints and slide presentations public meetings--public information, regulatory, and special interest groups tours--in the field and in conjunction with the POTWs treatment of wastewater to discharge clean effluent regulatory liaison--to provide regulators with technical and operational information beyond the minimum required for the permitting process; regulators may also be included in non-regulatory meetings, field days, tours and other community outreach programs The effort to gain and maintain public acceptance is an integral part of a beneficial use land application project throughout its lifetime. Needs and concerns differ from project to project, but community acceptance can never be taken for granted. It is the strongest underpinning for any successful environmental project being operated today. The beneficial use of biosolids is no exception. To ensure ongoing and up-to-date scientific information is available for outreach efforts, methods for compiling data and exchanging information on beneficial use issues are a critical need. Agricultural research in related areas should be evaluated for its relevance to the issues surrounding land application and the general agricultural community should be made aware of the research information and practices of biosolids beneficial use programs. Efforts to gain new insights and data should be accompaniedby a commitment to provide these results to the agricultural community and to the interested segments of the general public.

Land Application

447

REFERENCES 1. National Sewage Sludge Survey. 55FR47210. November 1990. 2 . Goldstein, J. Sensible Sludge. Rodale Press. 1977. 3. Lue-Hing, C., et al. (eds.). Municipal Sewage Sludge Management: Processing, Utilization and Disposal. Water Quality Management Library, Volume 4. Technomic Publishing Co., Inc. 1992. 4. Marschner, H. Mineral Nutrition of Higher Plants. Academic Press. 1986. 5. Knezek, B.D., et al. (eds.). Application of Sludges and Wastewater on Agricultural Land: A Planning and Education Guide. US.Environmental Protection Agency, Oflce of Water Program Operations. March 1978. 6. Task Force on Beneficial Use of Waste Solids. Beneficial Use of Waste Solids. Water Enviroment Federation. 1989. 7. U.S. Environmental Protection Agency. Process Design Manual €or Land Application of Municipal Sludge. EPA-625/1-83-016. October 1983. 8. Chen, Y. and Y. Avnimelech (eds.). The Role of Organic Matter in Modern Agriculture. Martinus Nijhoff Publishers. 1986. 9. Bohn, H., et al. Soil Chemistry. John Wiley & Sons. 1979. 10. Sopper, W. E. Municipal Sludge Use in Land Reclamation. Lewis Publishers. 1993. 1 1 . Henry, C. and R. Harrison (eds.). Literature Reviews on Environmental Effects of Sludge Management. University of Washington, College of Forest Resources. July 1991. 12. Elliott, L. F. and F. J. Stevenson (eds.). Soils for Management of Organic Wastes and Waste Waters. Chapter 22. Soil Science Society of America, American Society of Agronomy, Crop Science Society of America. 1977. 13. Page, A. L., et al. Utilization of Municipal Wastewaters and Sludge on Land,. University of Calqornia. 1983. 14. Ohio Farm Bureau Development Corp. Demonstration of Acceptable Systems for Land Disposal of Sewage Sludge. US. Environmental Protection Agency. May 1985. 15. Elliott, L. F. and F. J. Stevenson (eds.). Soils for Management of Organic Wastes and Waste Waters. Chapter 9. Soil Science Society of America, American Society of Agronomy, Crop Science Society of America. 1977. 16. U.S. Environmental Protection Agency. Process Design Manual for Sludge Treatment and Disposal. EPA-625/1-79-01I . September 1979. 17. U.S. Environmental Protection Agency. Transport of Sewage Sludge. EPA600/2-77-216. December 1977. 18. U.S. Environmental Protection Agency. Process Design Manual for Land Application of Municipal Sludge. EPA-625/1-83-016. October 1983. 19. Miner, J. R. and T. E. Hazen. Transportation and Application of Organic Wastes to Land. Oregon State University. 1977.

448

Forste

20. U.S. Environmental Protection Agency. Land Application of Sewage Sludge: A Guide for Land Appliers on theRequirements of the Federal Standardsfor the Use or Disposal of Sewage Sludge, 40 CFR Part 503. EPAA31-B-93-0026. December 1994. 21. U.S. Environmental Protection Agency. Preparing Sewage Sludge for Laud Application or Surface Disposal-A Guide for Preparers on the Monitoring, R d e e p i n g and Repcntq requirernentS of the Federal Standardsfor the Use or Disposal of Sewage Sludge Management, 40 CFFt Part 503. EPA 8313-93002a. 1993. 22. Forster, D. Lynn, et al. Institutional Constraintson Residuals Use in Utilization, Treatment and Disposal of Waste on Land. 1986 23. Forste, J. B. and P. S. Macho. Public Acceptance of Biosolids-What’s in a Name? PmxdhgshThe Management of Water and Wastewater Solidsfor the 21st h b n y : A Global Perspective. Water Environment Fedemtion, 1994.

Index AAR (annual application rate), 116 Actinomycetes, 15 Advanced solids, 4 Aerobic digestion, 167 Afterburners, 311,325 Alkaline materials, 357-369 Alkaline stabilization, 2,343-386 BIO*FJX process, 346,369-375 Chemfix process, 380 economics, 382-286 end products, 372,279 N - V i PWSS, 375-379 odors, 355 post-lime stabilization, 349 prelime stabilization, 348 process fundamentals,352,373 RDP process, 346,380 Amines, 25 AmmoNa, 25,355,391-393 Ammonium, 355,391-393 Anaerobic digestion, 171 ANC (available nitrogen concentration), 116 APLR (annual pollutant loading rate), 105,112 Arsenic, 13 AWSAR (annual whole sludge application rate), 105,112 Bacteria, 15 Baltimore Heat Drying Facility, 319 Belt mter press, 146 w e n County Alkaline Stabilization Plant, 373 Biofilters, 256-260 BIO*FD(, 369-375 BioGro,316 Biosolids alkaline stabilization, 343-386 applicators, 420-424 beneficial uses, 30-34 caloric content, 29 comparative cost of treatment, 44 characterization, 8

piosolids] composition, 57 composting, 193-269 conditioning, 132,140-143 dewatering, 144-163 digestion, 165-191 drying, 2,27 1-340 energy content, 29 exceptional quality, 38 fuel value, 29,278 fertilizer value, 29,272 generation, 1 land application, 4,31,33,389446 macronutrients, 7,414 microbiology, 14 micronutrients, 13 non-beneficial uses, 34 organic matter, 12,396-398 pathogens, 18-22,34,414,438 p~llutants,37,64,96,97-99 production of fertilizer, 2,272,330 storability,29 transportability,27 transportation, 420-422 treatment for beneficial use, 34, 38-43 Cadmium, 13,50 Calcium, 11 Calcium carbonate, 346 Calcium hydroxide, 353 Carver-Greenfieldprocess, 336 Cement kiln dust, 347,362,363 Centrifuges, 147 CFU (colony forming unit), 22 Chemfix process, 380 Chromium, 13 Clean Air Act (CAA), 52,319 Clean Water Act (CWA), 47 Coliforms, 15 Compost amendments, 220-226 biological quality, 197 chemical quality, 195

449

450

tCompo4 composition, 226-232 Composting, 2,193 energy balance, 212,248 facility design, 217 growth in the U.S.,194 marketing, 261 odors, 253-256 process fundamentals, 197-205, 21 1-217 Conditioning, 132 chemical, 132-137,140-142 thermal, 143 copper, 13 Criteria Pollutants, 319,321

Daily cover, 32 Dedicated sites, 33 Dedicated sites disposal, 433 Dewatering, 146 bag, 153 belt filterp a , 146 wtrifuge, 147 drying beds, 152 fkezdthaw lagoons, 153 odor control, 154-157 pressure filters, 148 rotary press, 151 screw press, 151 vacuum filters, 150 Digestion, 165 aerobic, 167 anaerobic, 171 ATAD (autothermal thermofdic aerobic), 168 economics, 179-187 equipment, 173-179 mesophilic, 168 process fundamentals, 167 Digesters ATAD,175 conventional aerobic, 173 egg-shaped, 177 Fuchs ATAD, 169 Dryers conveyor, 282,286

Index IDryerSl dmt,281,282 disc, 287 drum, 282-284 requirements, 297-300 en massc, 287 evaporation capacity, 295 flash, 286 fluid bed, 286 indirect, 281,282,287 multitray, 293-295 paddle, 291 SER (specific evaporation rate), 297 STR (specific thermal rate), 298 triple pass, 282,284 Drying gygfems, 27 1,277-279,305 Andritz, 313 air pollution, 308 Bio Gro-Seghers, 316,333 closed cycle, 311,3 16 economics, 325-329 environmentalcontrol equipment, 324 ESP process, 311 feed preparation, 307 heat generation, 307 heating medium, 300 material handling, 307 mixers, 284,307 odor control, 308,323 open cycle, 309 regulatory issues, 319 semi-closedcycle, 311 SwissCombi, 313 Wheelabrator Clean Water SystemsInc.,311,313,316 European community, biosolids generation, 2 Fecal coliforms, 15 Fecal strcptocccci, 16 FGD ash, 363 Final cover, 32 Fly ash, 363 Fungi, 17

lndex Gas chromatography, 26 Gravity thickeningJ44 Heat drying, see w i n g systems Heavy metals, 7,8,13,14,37,105, 106,114 Helminths, 17 Highly exposed individual (HEl), 84, 86 Humus, 13 Hydrogen sulfide, 25,355 Hyperion Carver-Greenfield plant, 336 Incineration, 4 Incineration ash utilization, 33 Indicator organisms,l 5 Japan, biosolids generation, 3 Kjeldahl nitrogen, 10,391 Lagoons, 29,425 aerobic, 425 anaerobic, 425 Land application, 31,38946 agronomic rates, 405,441 buffer zones, 401 economics, 428431 management practices, 440 monitoring and recordkeeping,431434,442 non-agricultural,409 site selection, 400 Landfill cover, 32 Lcad,13 Lime, 141,344,357-359 Lime kiln dust, 347 Lime stabilization,see Alkaline stabilization, 343 Limestone, 346 Liquid biosolids generation, 5 lime stabilization of, 348,349 spray systems, 427 storage, 422 subsurface injection, 422

451 Macronutrients, 7 Magnesium, 1 1 Manganese, 13 Mass spectrometry, 26 MEI (most exposed individual), 81,84,

85 Mercaptans, 23,25 Mmury, 13 Micronutrients, 13 Microorganism density, 21 Microorganisms, 4 Milwaukee drying facility, 330 Molybdenum, 13 MPN (most probable number), 22 NAAQS (National Ambient Air Quality Standards), 308,319 NESHAP (National Emission Standards for Hazardous Pollutants), 322 New York heat drying facility, 331 Nickel, 13 Nitrate nitrogen, 10,391 Nitrogen, 9,390 NPDES (National Pollutant Discharge Elimination System), 47 NSSS (National Sewage Sludge Survey), 56,58 N-Viro process, 375-379 Ocean County Carver Greenfield plant,

336 Odors,22,323 compounds, 22,25-26 control, 27 detector tubes, 24 measurement, 23,24 panel, 23 portable instruments, 24,26 Oil from sludge (OFS)process, 335 Organic matter, biosolids, 12,394 Organic nitrogen, 10,406408

Part 503 Regulations, 47-130 Pathogens Class A reduction, 36,117-119,439

hdex [pathogens] Class B reduction, 36,119-120,438 potential diseases, 18 survival times, 21 PCBs (polychlorinated biphenyls), 34 POW (publicly owned treatment works), 47 Pellets, 273 density, 274 nutrient content, 273 particle size,274 PFRP (process to Further Reduce Pathogens), 50,120,350 PFU plaque Forming Unit), 22 pH, 353,419 Phosphorus, 10,397 Polymers, 133 consumption, 139 dry, 135 emulsions, 135 feed and control systems,137,138 inorganic, 140 liquid, 135 mannichs, 135 manufacturers, U.S, 134 organic polyelectrolytes, 133 Potassium, 11 Primary solids, 4 Production of fertilizer, 2,272,330 Protozm, 16 PSRP (Process to SiBnificantly Reduce Pathogens), 50,120,350

Salmonella, 16,17 Scrubbers,311,316,324 Scrubber ash, 363 Secondary solids, 4 Selenium, 13 Septage,dornestic,36,94,115,122 Solids, 4 concentration,4 total (TS), 5 total suspended (TSS), 6 total volatile (TVS),6 Soils acidic, 356 electrical conductivity, 397,416 nitrogen, 390 organic matter, 394 salinity, 417 Sulfur, 12 Surface disposal, 4 SyracuseMetropolitanDewatering Facility, 157-159

RDP Envessel pasteurization,346,380 Regulations cadmium limits, 50 distributionand marketing, 68 Final Part 503,84 hazard identification, 72 incineration,70 land application, 68,107 monitoring, recordkeepingand reporting, 71,126 monofills, 69 pathogen and vector attraction reduction, 50,69,120,123

TCLP (Toxicity Characteristic Leachate Procedure), 34 T-BACT (Best Available Control Technologyfor Toxics), 322 Thickening, 144 centrifugal, 145 DAF,dissolved air flotation, 145 gravity belt, 144 rotary drum, 145 Treatment processes, overview, 30 - 34 Toxic (hazardous) air pollutants (TAPS, HAPS), 319,321 TKN (Total Kjeldahl nitrogen), 10,391

Begulations] PCB limits, 50 pollutant limits, 37,105-115 pollutant selection,64-67,96-101 Recordkeeping, 127 Risk assessment analysis, 6163,71, 81,101 agregate risk, 63 exposure pathways, 75-80,88-94 Rotifen, 17

Index Yonkers Joint Treatment Plant, 157 Vector attraction reduction,37,123 125,439 Viruses, 16 Volatile Organic Compounds(VOC), 309,319,321 Volume reduction, 39

453 Wastewater treatment plants, see POW Wet oxidation, 339 Wheelabrator Clean Water SystemsInc.,311,313,316 Zimpro process, 339 Zinc, 13