Wastewater Management
"This page is Intentionally Left Blank"
Wastewater Management
Klein Gomes
Oxford Book Company Jaipur , India
ISBN: 978·93·80179·04-9
First Edition 2009
Oxford Book Company 267, 10-B-Scheme, Opp. Narayan Niwas, Gopalpura By Pass Road, Jaipur-302018 Phone: 0141-2594705, Fax: 0141-2597527 e-mail:
[email protected] website: www.oxfordbookcompany.com
© Reserved
Typeset by: Shivangi Computers 267, lO-B-Scheme, Opp. Narayan Niwas, Gopalpura By Pass Road, Jaipur-302018
Printed at: Rajdhani Printers, Delhi
All Rights are Reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, scanning or otherwise, without the prior written permission of the copyright owner. Responsibility for the facts stated, opinions expressed, conclusions reached and plagiarism, if any, in this volume is entirely that of the Author, according to whom the matter encompassed in this book has been originally created/edited and resemblance with .any such publication may be incidental. The Publisher bears no responsibility forthem, whatsoever.
Preface Wastewater Management provides state-of-the-art information on the application of innovative technologies for wastewater treatment with an emphasis on the scientific principles for pollutant or pathogen removal. Microbial granules have practical importance in anaerobic and aerobic biological wastewater treatment. Advantages of granules are retention of biomass in reactor, diversity of microorganisms, complex structure, and resistance to unfavorable conditions. Microbial granules can be used to treat municipal and industrial wastewater for removal of organic matter, xenobiotics, nutrients, and heavy metals. The book covers almost all aspects of formation and use of microbial granules in wastewater management. The data on aerobic microbial granulation are related mostly to laboratory systems due to few pilot systems in the world using aerobic microbial granules. However, by the analogy with anaerobic granulation, which is now used worldwide, it is possible to predict wide applications of aerobic granulation. This book will help researchers and engineers develop new biotechnologies of wastewater treatment based on aerobic granulation
Klein Gomes
1 .
"This page is Intentionally Left Blank"
Contents Preface
v
I. Wastewater Treatment
1
2. Ecological Principles of Wastewater Treatment
25
3. Ecology of Activated Sludge
58
4. The Relevant Aspects of Biology
73
5. The Ecology of Bacteria Beds
114
6. Ecological Operation of Bacteria Beds
150
7. Groundwater Contamination
163
8. Industrial Wastewater Treatment
175
9. Aquifer Recharge with Wastewater
197
10. Irrigation with Wastewater
208
1I. Agricultural Use of Sewage Sludge
231
12. Wastewater Use in Aquaculture
237
13. Sewerage
245
14. National Fresh Water Recreation Benefits and Water-pollution Control
15. Financing Wastewater Index
Managemen~
270 283 299
"This page is Intentionally Left Blank"
Chapter 1
Wastewater Treatment The principal objective of wastewater treatment is generally to allow human and tindustrial effluents to be disposed of without danger to human health or unacceptable damage to the natural environment. Irrigation with wastewater is both disposal and utilization and indeed is an effective form of wastewater disposal (as in slow-rate land treatment). However, some degree of treatment must normally be provided to raw municipal wastewater before it can be used for agricultural or landscape irrigation or for aquaculture. The quality of treated effluent used in agriculture has a great influence on the operation and performance of the wastewater-soil-plant or aquaculture system. In the case of irrigation, the required quality of effluent will depend on the crop or crops to be irrigated, the soil conditions and the system of effluent distribution adopted. , Through crop restriction and selection of irrigation systems, which minimize health risk, the degree of pre-application wastewater treatment can be reduced. A similar approach is not feasible in aquaculture systems and more reliance will have to be placed on control through wastewater treatment. The most appropriate wastewater treatment to be applied before effluent use in agriculture is that which will produce an effluent meeting the recommended microbiological and chemical quality guidelines both at low cost and with minimal operational and maintenance requirements. Adopting as Iowa level of treatment as possible is especially desirable in developing countries, not only from the point of view of cost but also in acknowledgement of the difficulty of operating complex systems reliably. In many locations it will be better to design the reuse system to accept a low-grade of effluent rather than to rely ort advanced treatment processes producing a reclaimed effluent which continuously meets a stringent quality standard. Nevertheless, there are locations where a higher-grade effluent will be necessary and it is essential that information on the performance of a wide range of wastewater treatment technology should be available. The
2
Wastewater Treatment
design of wastewater treatment plants is usually based on the need to reduce organic and suspended solids loads to limit pollution of the environment. Pathogen removal has very rarely been considered an objective but, for reuse of effluents in agriculture, this must now be o~ primary concern and processes should be selected and designed accordingly. Treatment to remove wastewater constituents that may be toxic or harmful to crops, aquatic plants (macrophytes) and fish is technically possible but is not normally economically feasible. Unfortunately, few performance data on wastewater treatment plants in developing countries are available and even then they do not normally include effluent quality parameters of importance in agricultural use. The short-term variations in wastewater flows observed at municipal wastewater treatment plants follow a diurnal pattern. Flow is typically low during the early morning hours, when water consumption is lowest and when the base flow consists of infiltrationinflow and small quantities of sanitary wastewater. A first peak of flow generally occurs in the late morning, when wastewater from the peak morning water use reaches the treatment plant, and a second peak flow usually occurs in the evening. The relative magnitude of the peaks and the times at which they occur vary from country to country and with the size of the community and the length of the sewers. Small communities with small sewer systems have a much higher ratio of peak flow to average flow than do large communities. Although the magnitude of peaks is attenuated as wastewater passes through a treatment plant, the daily variations in flow from a municipal treatment plant make it impracticable, in most cases, to irrigate with effluent directly from the treatment plant. Some form of flow equalization or short-term storage of treated effluent is necessary to provide a relatively constant supply of reclaimed water for efficient irrigation, although additional benefits result from storage. CONVENTIONAL WASTEWATER TREATMENT PROCESSES
Conventional wastewater treatment consists of a combination of physical, chemical, and biological processes and operations to remove solids, organic matter and, sometimes, nutrients from wastewater. General terms used to describe different degrees of treatment, in order of increasing treatment level, are preliminary, primary, secondary, and tertiary and/or advanced wastewater treatment. In some countries, disinfection to remove pathogens sometimes follows the last treatment step. A generalized wastewater treatment diagram is shown in Figure 1.
3
Wastewater Treatment Preliminary
Pnmary
Advanced
Secondary
Effl~
I DIsinfectionJ.Screening Comminution Grift Removal
Effluent ..
Effl.nt
rl
lLOW Rate Processes stabilization ponds aerated lagoons
Disinfect on
'.--_[§ ID~ls~InEfe~ct~lo~nE~~~ Nitrogen Removal nltrificallon-denitrificatlon selective ion exchange break pOint chlorination gas stnPP,ng overland flow
High Rate Processes activated sludge trickling filters rotating biocontractors
r+ Sludge Processing
I
I
Non biological thickening conditioning dewatering filter centrifuge InCineration
Biological thickening digestion dewatering filter certrifuge drying beds
Phosphorus Removal chemical precipitation
I
It--~~
suspended Solids Removal ~ chemical coagulation filtration
H
UOrganics and Metals RemovalL..... Carbon adsorption
n
I'"
U
Dissolved Solids Removal reverse osmisis electrodialysIs distillation
~
Disposal
"'
FIg:l. Generahzed Flow DIagram for MUnICIpal Wastewater Treatment Preliminary Treatment
The objective of preliminary treatment is the removal of coarse solids and other large materials often found in raw wastewater. Removal of these materials is necessary to enhance the operation and maintenance of subsequent treatment units. Preliminary treatment operations typically include coarse screening, grit removal and, in some cases, comminution of large objects. In grit chambers, the velocity of the water through the chamber is maintained sufficiently high, or air is used, so as to prevent the settling of most organic solids. Grit removal is not included as a preliminary treatment step in most small wastewater treatment plants. Comminutors are sometimes adopted to supplement coarse screening and serve to reduce the size of large particles so that they will be removed in the form of a sludge in subsequent treatment processes. Flow measurement devices, often standing-wave flumes, are always included at the preliminary treatment stage. The objective of primary treatment is the removal of settleable organic and inorganic solids by sedimentation, and the removal of materials that will float (scum) by skimming. Approximately 25 to 50% of the incoming biochemical oxygen demand (BOD 5), 50 to 70% of the total suspended solids (55), and 65% of the oil and grease are removed during primary treatment. Some organic nitrogen, organic phosphorus, and heavy metals associated with solids are also removed during primary sedimentation
4
Wastewater Treatment
but colloidal and dissolved constituents are not affected. The effluent from primary sedimentation units is referred to as primary effluent. In many industrialized countries, primary treatment is the minimum level of preapplication treatment required for wastewater irrigation. It may be considered sufficient treatment if the wastewater is used to irrigate crops that are not consumed by humans or to irrigate orchards, vineyards, and some processed food crops. However, to prevent potential nuisance conditions in storage or flowequalizing reservoirs, some form of secondary treatment is normally required in these countries, even in the case of non-food crop irrigation. It may be possible to use at least a portion of primary effluent for irrigation if off-line storage is provided. Primary sedimentation tanks or clarifiers may be round or rectangular basins, typically 3 to 5 m deep, with hydraulic retention time between 2 and 3 hours. Settled solids (primary sludge) are normally removed from the bottom of tanks by sludge rakes that scrape the sludge to a central well from which it is pumped to sludge processing units. Scum is swept across the tank surface by water jets or mechanical means from which it is also pumped to sludge processing units. In large sewage treatment plants, primary sludge is most commonly processed biologically by anaerobic digestion. In the digestion process, anaerobic and facultative bacteria metabolize the organic material in sludge, thereby reducing the volume requiring ultimate disposal, making the sludge stable (nonputrescible) and improving its dewatering characteristics. Digestion is carried out in covered tanks (anaerobic digesters), typically 7 to 14 m deep. The residence time in a digester may vary from a minimum of about 10 days for high-rate digesters (well-mixed and heated) to 60 days or more in standard-rate digesters. Gas containing about 60 to 65% methane is produced during digestion and can be recovered as an energy source. In small sewage treatment plants, sludge is processed in a variety of ways including: aerobic digestion, storage in sludge lagoons, direct application to sludge drying beds, in-process storage (as in stabilization ponds), and land application. Example: Biological treatment biochemistry Secondary Treatment
The objective of secondary treatment is the further treatment of the effluent from primary treatment to remove the residual organics and suspended solids. In most cases, secondary treatment follows primary treatment and involves the removal of biodegradable dissolved and colloidal organic matter using aerobic biological treatment processes.
Wastewater Treatment
5 Arueroblc Ileatment C0n\'\."r~I"nS Comrit"ll( Org,lnK I\'laltc': ICarl:0hydlates ProtelnS, LIPldsl
1 1
Hyri, ,·IV
.n,:yme~J
.~~I-
-+.
_ _ _ _ hlmplt'f Orgaml.· Comp,)tll1d ... -:-_ _ _ _ l~ugars,
Anuno AGds,
PeptH.:ie~
I
~ldl"'gent"SlS
A ... [Fer:n~ntatlVt'
Volalile Fattv AC'Jds [rrop'0ndle, Illliyrdte, ,-Ie
Rill tt"rJ/IJ
I
!
~
A(ct0genesls [Hydrogf>n l'roducmg: A..cf·te>gf'l1J('
.. H,lCh'TLll
--------------+..
ACt"t:lk
\'1 E'thf'lnCll~,"'nsl<;;;
~1('thanog~;nsls
t",h;thar1c1gcmcj [tlactcnal
[Hydwgen l1s1ng Mcthanogentcl [Bactenal
lAceto("jC1..;;t}C
Aerl'ble 'I'reatment L' 0li,'erS10ns. Comrle). Organl(" r..L1ttef ICar~~")hydral."'';;, Protf>IY'';;, I Jr1rl..;1
~I mplcr
I t
Organic ComFc'Llnds
Oxygen
S1 nthe",
HvcilOlv>I' [I;xtrd-celluldr En" llw'l
I
A
AeroI'lL MICl00rgdntSmS
En"'bY
End rro~iu~ ts [Carhlm fJh)Xld(', VV',ltC'f Ammor.l.l,
I'Ioh 1Flasni [t\ew Cellsl
t'k
1
Fig: 2.
Aerobic biological treatment is performed in the presence of oxygen by aerobic microorganisms (principally bacteria) that metabolize the organic matter in the wastewater, thereby producing more microorganisms and inorganic end-products (principally CO 2, NH 3, and H 20). Several aerobic biological processes are used for secondary treatment differing primarily in the manner in which oxygen is supplied to the microorganisms and in the rate at which organisms metabolize the organic matter. High-rate biological processes are characterized by relatively small reactor volumes and high concentrations of microorganisms compared with low rate processes. Consequently, the growth rate of new organisms is much greater in high-rate systems because of the well controlled environment. The microorganisms must be separated from the treated wastewater by sedimentation to produce clarified secondary effluent. The
6
Wastewater Treatment
sedimentation tanks used in secondary treatment, often referred to as secondary clarifiers, operate in the same basic manner as the primary clarifiers described previously. The biological solids removed during secondary sedimentation, called secondary or biological sludge, are normally combined with primary sludge for sludge processing. Common high-rate processes include the activated sludge processes, trickling filters or biofilters, oxidation ditches, and rotating biological contactors (RBC). A combination of two of these processes in series (e.g., biofilter followed by activated sludge) is sometimes used to treat municipal wastewater containing a high concentration of organic material from industrial sources. Activated Sludge
In the activated sludge process, the dispersed-growth reactor is an aeration tank or basin containing a suspension of the wastewater and microorganisms, the mixed liquor. The contents of the aeration tank are mixed vigorously by aeration devices which also supply oxygen to the ulOlogical suspension. Aeration devices commonly used include submerged diffusers that release compressed air and mechanical surface aerators that introduce air by agitating the liquid surface. Hydraulic retention time in the aeration tanks usually ranges from 3 to 8 hours but can be higher with high BODs wastewaters. Following the aeration step, the microorganisms are separated from the liquid by sedimentation and the clarified liquid is secondary effluent. A portion of the biological sludge is recycled to the aeration basin to maintain a high mixed-liquor suspended solids (MLSS) level. The remainder is removed from the process and sent to sludge processing to maintain a relatively constant concentration of microorganisms in the system. Several variations of the basic activated sludge process, such as extended aeration and oxidation ditches, are in common use, but the principles are similar. Trickling Filters
A trickling filter or biofilter consists of a basin or tower filled with support media such as stones, plastic shapes, or wooden slats. Wastewater is applied intermittently, or sometimes continuously, over the media. Microorganisms become attached to the media and form a biological layer or fixed film. Organic matter in the wastewater diffuses into the film, where it is metabolized. Oxygen is normally supplied to the film by the natural flow of air either up or down through the media, depending on the relative temperatures of the wastewater and ambient air. Forced air can also be supplied by blowers but this is rarely necessary.
Wastewater Treatment
7
The thickness of the biofilm increases as new organisms grow. Periodically, portions of the film slough off the media. The sloughed material is separated from the liquid in a secondary clarifier and discharged to sludge processing. Clarified liquid from the secondary clarifier is the secondary effluent and a portion is often recycled to the biofilter to improve hydraulic distribution of the wastewater over the filter. Rotating Biological Contactors
Rotating biological contactors (RBCs) are fixed-film reactors similar to biofilters in that organisms are attached to support media. In the case of the RBC, the support media are slowly rotating discs that are partially submerged in flowing wastewater in the reactor. Oxygen is supplied to the attached biofilm from the air when the film is out of the water and from the liquid when submerged, since oxygen is transferred to the wastewater by surface turbulence created by the discs' rotation. Sloughed pieces of biofilm are removed in the same manner described for biofilters. High-rate biological treatment processes, in combination with primary sedimentation, typically remove 85 % of the BODs and SS originally present in the raw wastewater and some of the heavy metals. Activated sludge generally produces an effluent of slightly higher quality, in terms of these constituents, than biofilters or RBCs. When coupled with a disinfection step, these processes can provide substantial but not complete removal of bacteria and virus. However, they remove very little phosphorus, nitrogen, non-biodegradable organics, or dissolved minerals. Tertiary and/or Advanced Treatment
Tertiary and/or advanced wastewater treatment is employed when specific wastewater constitUents which cannot be removed by secondary treatment must be removed. Individual treatment processes are necessary to remove nitrogen, phosphorus, additional suspended solids, refractory organics, heavy metals and dissolved solids. Because advanced treatment usually follows high-rate secondary treatment, it is sometimes referred to as tertiary treatment. However, advanced treatment processes are sometimes combined with primary or secondary treatment (e.g., chemical addition to primary clarifiers or aeration basins to remove phosphorus) or used in place of secondary treatment (e.g., overland flow treatment of primary effluent). The Bardenpho Process adopted is shown in simplified form in Figure 3. Effluent from primary clarifiers flows to the biological reactor, which is phYSically divided into five zones by baffles and weirs. In sequence these zones are: • Anaerobic fermentation zone (characterized by very low dissolved oxygen levels and the absence of nitrates).
8
Wastewater Treatment
• • • •
Anoxic zone (low dissolved oxygen levels but nitrates present). Aerobic zone (aerated). Secondary anoxic zone. Final aeration zone.
4!'t'".lf hture 1 B.!nJenph:>
process tram
ChlontiP
Fig:3. Simplified Flow Diagram of Bardenpho-plant
The function of the first zone is to condition the group of bacteria responsible for phosphorus removal by stressing them under low oxidation-reduction conditions, which results in a release of phosphorus equilibrium in the cells of the bacteria. On subsequent exposure to an adequate supply of oxygen and phosphorus in the aerated zones, these cells rapidly accumulate phosphorus considerably in excess of their normal metabolic requirements. Phosphorus is removed from the system with the waste activated sludge. Most of the nitrogen in the influent is in the ammonia form, and this passes through the first two zones virtually unaltered. In the third aerobic zone, the sludge age is such that almost complete nitrification takes place, and the ammonia nitrogen is converted to nitrites and then to nitrates. The nitrate-rich mixed liquor is then recycled from the aerobic zone back to the first anoxic zone. Here denitrification occurs, where the recycled nitrates, in the absence of dissolved oxygen, are reduced by facultative bacteria to nitrogen gas, using the influent organic carbon compounds as
Wastewater Treatment
9
hydrogen donors. The nitrogen gas merely escapes to atmosphere.. In the second anoxic zone, those nitrates which were not recycled are reduced by the endogenous respiration of bacteria. In the final reaeration zone, dissolved oxygen levels are again raised to prevent further denitrification, which would impair settling in the secondary clarifiers to which the mixed liquor then flows. An experimentation programme on this plant demonstrated the importance of the addition of volatile fatty acids to the anaerobic fermentation zone to achieve good phosphorus removaL These essential short-chain organics (mainly acetates) are produced by the controlled fermentation of primary sludge in a gravity thickener and are released into the thickener supernatent, which can be fed to the head of the biological reactor. Without this supernatent return flow, overall phosphorus removal quickly dropped to levels found in conventional activated sludge plants. Performance data over three years have proved that, with thickener supernatent recycle, effluent quality median values of 0.5-1.38 mg/l OrthoP, 1.4-1.6 mg/l Total nitrogen and 1.4-2.0 mg/l nitrate-N are achievable. This advanced biological wastewater treatment plant cost only marginally more than a conventional activated sludge plant but nevertheless involved considerable investment. Furthermore, the complexity of the process and the skilled operation required to achieve consistent results make this approach unsuitable for developing countries. In many situations, where the risk of public exposure to the reclaimed water or residual constituents is high, the intent of the treatment is to minimize the probability of human exposure to enteric viruses and other pathogens. Effective disinfection of viruses is believed to be inhibited by suspended and colloidal solids in the water, therefore these solids must be removed by advanced treatment before the disinfection st~p. The sequence of treatment often specified in the United States is: secondary treatment followed by chemical coagulation, sedimentation, filtration, and disinfection. This level of treatment is assumed to produce an effluent free from detectable viruses. In Near East countries adopting tertiary treatment, the tendency has been to introduce pre-chlorination before rapid-gravity sand filtration and post-chlorination afterwards. A final ozonation treatment after this sequence has been considered in at least one country. Disinfection
Disinfection normally involves the injection of a chlorine solution at the head end of a chlorine contact basin. The chlorine dosage depends
10
Wastewater Treatment
upon t"he strength of the wastewater and other factors, but dosages of 5 to 15 mg!l are common. Ozone and ultra violet (uv) irradiation can also be used for disinfection but these methods of disinfection are not in common use. Chlorine contact basins are usually rectangular channels, with baffles to prevent short-circuiting, designed to provide a contact time of about 30 minutes. However, to meet advanced wastewater treatment requirements, a chlorine contact time of as long as 120 minutes is sometimes required for specific irrigation uses of reclaimed wastewater. The bactericidal effects of chlorine and other disinfectants are dependent upon pH, contact time, organic content, and effluent temperature. Effluent Storage
Although not considered a step in the treatment process, a storage facility is, in most cases, a critical link between the wastewater treatment plant and the irrigation system. Storage is needed for the following reasons: • To equalize daily variations in flow from the treatment plant and to store excess when average wastwater flow exceeds irrigation demands; includes winter storage. • To meet peak irrigation demands in excess of the average wastewater flow. • To minimize the effects of disruptions in the operations of the treatment plant and irrigation system. Storage is used to provide insurance against the possibility of unsuitable reclaimed wastewater entering the irrigation system and to provide additional time to resolve temporary water quality problems. Reliability of Conventional and Advanced Wastewater Treatment
Wastewater reclamation and reuse systems should contain both design and operational requirements necessary to ensure reliability of treatment. Reliability features such as alarm systems, standby power supplies, treatment process duplications, emergency storage or disposal of inadequately treated wastewater, monitoring devices, and automatic controllers are important. From a public health standpoint, provisions for adequate and reliabile disinfection are the most essential features of the advanced wastewater treatment process. Where disinfection is required, several reliability features must be incorporated into the system to ensure uninterrupted chlorine feed.
Wastewater Treatment
11
NATURAL BIOLOGICAL TREATMENT SYSTEMS
Natural low-rate biological treatment systems are available for the treatment of organic wastewaters such as municipal sewage and tend to be lower in cost and less sophisticated in operation and maintenance. Although such processes tend to be land intensive by comparison with the conventional high-rate biological processes already described, they are often more effective in removing pathogens and do so reliably and continuously if properly designed and not overloaded. Among the natural biological treatment systems available, stabilization ponds and land treatment have been used widely around the world and a considerable record of experience and design practice has been documented. The nutrient film technique is a fairly recent development of the hydroponic plant growth system with application in the treatment and use of wastewater. Wastewater Stabilization Ponds
A recent World Bank Report came out strongly in favour of stabilization ponds as the most suitable wastewater treatment system for effluent use in agriculture. A comparison of the advantages and disadvantages of ponds with those of high-rate biological wastewater treatment processes show that stabilization ponds are the preferred wastewater treatment process in developing countries, where land is often available at reasonable opportunity cost and skilled labour is in short supply. Key: FC = Faecal coli forms; SS = Suspended slids; G = Good; F = Fair; P = Poor. Wastewater stabilization pond systems are designed to achieve different forms of treatment in up to three stages in series, depending on the organic strength of the input waste and the effluent quality objectives. For ease of maintenance and flexibility of operation, at least two trains of ponds in parallel are incorporated in any design. Strong wastewaters, with BODs concentration in excess of about 300 mg/I, will frequently be introduced into first-stage anaerobic ponds, which achieve a high volumetric rate of removal. Weaker wastes or, where anaerobic ponds are environmentally unacceptable, even stronger wastes (say up to 1000 mg/l BODs) may be discharged directly into primary facultative ponds. Effluent from first-
12
Wastewater Treatment
stage anaerobic ponds will overflow into secondary facultative ponds which comprise the second-stage of biological treatment. Following primary or secondary facultative ponds, if further pathogen reduction is necessary maturation ponds will be introduced to provide tertiary treatment.
---G:J~~----I .....
~~~~-~-~ (lphonai
r----I \1
r--
'-----
r----' M r--
'-----
-c : ~"-_-~~-JFig:4. Stabilization Pond Configurations AN = anaerobic pond; F = facultative pond; M = maturation pond
Anaerobic Ponds Anaerobic ponds are very cost effective for the removal of BOD, when it is present in high concentration. Normally, a Single, anaerobic pond in each treatment train is sufficient if the strength of the influent wastewater, LI is less than 1,000 mg/l BODS" For high strength industrial wastes, up to three anaerobic ponds in series might be justifiable but the retention time tan' in any of these ponds should not be less than 1 day.
Wastewater Treatment
13
Anaerobic conditions in first-stage stabilization ponds are created by maintaining a high volumetric organic loading, certainly greater than 100g BOD 5/m 3 d. Volumetric loading, lv' is given by:
A-= Li Q V where: Li = Influent BOD 5, mg/l, Q = Influent flow rate, m 3/d, and V = Pond volume, rn3 or, since V/Q = tan' the retention time:
Av
=~ tan
Very high loadings, up to 1,000g BOD 5/m 3 d, achieve efficient utilization of anaerobic pond volume but, with wastewater containing sulphate concentrations in excess of 100 mg/l, the production of H 2S is likely to cause odour problems. In the case of typical municipal sewage, it is generally accepted that a maximum anaerobic pond loading of 400g BOD 5/m 3 d will prevent odour nuisance. Table:l. BOD Removals in Anaerobic Ponds Loaded at 250 G BOD s/M 3d Retention tan days BODs removal %3 1
~
2.5
60
5
m
Anaerobic ponds normally have a depth between 2 m and 5 m and function as open septic tanks with gas release to the atmosphere. The biochemical reactions which take place in anaerobic ponds are the same as those occurring in anaerobic digesters, with a first phase of acidogenesis and a second slower-rate of methanogenesis. Ambient temperatures in hot-climate countries are conducive to these anaerobic reactions and expected BOD 5 removals for different retention times in treating sewage have been given by Mara as shown in Table 1. More recently, Gambrill et al. have suggested conservative removals of BOD5 in anaerobic ponds as 40% below lOoC, at a design loading, lv' of 100 g/m 3 d, and 60% above 20°e, at a design loading of 300 g/m 3 d, with linear interpolation for operating temperature between 10 and 20°e. Higher removal rates are possible with industrial wastes, particularly those containing significant quantities of organic settleable solids. Of course, other environmental conditions in the ponds, particularly pH, must be suitable for the anaerobic microorganisms bringing about the breakdown of BOD. In certain instances, anaerobic ponds become covered with a thick scum layer, which is thought to be beneficial but not essential, and may
Wastewater Treatment
14
give rise to increased fly breeding. Solids in the raw wastewater, as well as biomass produced, will settle out in first-stage anaerobic ponds and it is common to remove sludge when it has reached half depth in the pond. This usually occurs after two years of operation at design flow in the Case of municipal sewage treatment. Facultative Ponds
The effluent from anaerobic ponds will require some form of aerobic treatment before discharge or use and facultative ponds will often be more appropriate than conventional forms of secondary biological treatment for application in developing countries. Primary facultative ponds will be designed for the treatment of weaker wastes and in sensitive locations where anaerobic pond odours would be unacceptable. Solids in the influent to a facultative pond and excess biomass produced in the pond will settle out forming a sludge layer at the bottom. The benthic layer will be anaerobic and, as a result of anaerobic breakdown of organics, will release soluble organic products to the water column above. Organic matter dissolved or suspended in the water column will be metabolized by heterotrophic bacteria, with the uptake of oxygen, as in convential aerobic biological wastewater treatment processes. However, unlike in convential processes, the dissolved oxygen utilized by the bacteria in facultative ponds is replaced through photosynthetic oxygen production by micro algae, rather than by aeration equipment. Especially intreating municipal sewage in hot climates, the environment in facultative ponds is ideal for the proliferation of microalgae. High temperature and ample sunlight create conditions which encourage algae to utilize the carbon dioxide (C02) released by bacteria in breaking down the organic components of the wastewater and take up nutrients (mainly nitrogen and phosphorus) contained in the wastewater. This symbiotic relationship contributes to the overall removal of BOD in facultative ponds, described diagrammatically by Marais as in Figure 5. Light Effluent BOD '-_oAAlgae Soluble solids
Sludge Layer
Fig:5. Energy Flows in Facultative Stabilization Ponds
15
Wastewater Treatment
To maintain the balance necessary to allow this symbiosis to persist, the organic loading on a facultative pond must be strictly limited. Even under satisfactory operating conditions, the dissolved oxygen concentration (DO) in a facultative pond will vary diurnally as well as over the depth. Maximum DO will occur at the surface of the pond and will usually reach supersaturation in tropical regions at the time of maximum radiation intensity. From that time until sunrise, DO will decline and may well disappear completely for a short period. For a typical facultative pond depth, D f, of 1.5 m the water column will be predominantly aerobic at the time of peak radiation and predominantly anaerobic at sunrise. The pH of the pond contents will also vary diurnally as algae utilize CO2 throughout daylight hours and respire, along with bacteria and other organisms, releasing CO2 during the night. 9
::r: 0.8 10
7
6 am
12 n
6 pm Time of day
12 m
6 am
Fig:6. Diurnal Variation of Dissolved Oxygen and pH in Facultative Pond, pH; Dissolved Oxygen Wind is considered important to the satisfactory operation of facultative ponds by mixing the contents and helping to prevent shortcircuiting. Intimate mixing of organic substrate and the degrading organisms is important in any biological reactor but in facultative ponds wind mixing is considered essential to prevent thermal stratification causing anaerobiosis and failure. Facultative ponds should be orientated with the longest dimension in the direction of the prevailing wind. Although completely-mixed reactor theory with the assumption of first-order kinetics for BOD removal can be adopted for facultative pond design (Marais and Shaw, 1961), such a fundamental approach is rarely adopted in practice. Instead, an empirical procedure based on operational experience is more common. The most widely adopted design method currently being applied wherever local experience is limited is that introduced by McGarry and Pescod.
16
Wastewater Treatment
A regression analysis of operating data on ponds around the world relating maximum surface organic loading, in lb/acre d, to the mean ambient air temperature, in of, of the coldest month resulted in the following equation (now converted to metric units): A.s(rnax) = 60.3 (1.099f where: As = surface or arael organic loading, kg BODs/ha.d T = mean ambient air temperature of coldest month, °C Subsequently, Arthur (1983) modified this formula and suggested that best agreement with available operating data, including a factor of safety of about 1.5, is represented by the relationship: As = 20 T - 60 This surface (or areal) BODs loading can be translated into a middepth facultative pond area requirement (Af in m 2 ) using the formula: _10LiQ Af - - A. s
Thus:
A - LiQ 1- 2T-6 and the mean hydraulic retention time in the facultative pond (tfin days) is given by:
AIDI Q
tl = - - · The removal of BODs in facultative ponds (lr in kg/ha d) is related to BODs loading and usually averages 70-80% of Is. Retention time in a properly designed facultative pond will normally be 20-40 days and, w!th a depth of about 1.5 m, the area required will be signficantly greater than for an anaerobic pond. The effluent from a facultative pond treating municipal sewage in the tropics will normally have a BODs between 50 and 70 mg/l as a result of the suspended algae. On discharge to a surface water, this effluent will not cause problems downstream if the dilution is of the order of 8:1 and any live algae in the effluent might well be beneficial as a result of photosynthetic oxygen production during daylight hours. Efficiently operating facultative ponds treating wastewater will contain a mixed population of flora but flagellate algal genera such as Chlamydomonas, Euglena, Phacus and Pyrobotrys will predominate. Nonmotile forms such as Chlorella, Scenedesmus and various diatom species will be present in low concentrations unless the pond is underloaded. Algal stratification often occurs in facultative ponds, particularly in the absence of wind-induced mixing, as motile forms respond to changes in light intensity and move in a band up and down the water column.
Wastewater Treatment
17
The relative numbers of different genera and their dominance in a facultative pond vary from season to season throughout the year but species diversity generally decreases with increase in loading. Sometimes, mobile purple sulphur bacteria appear when facultative ponds are overloaded and sulphide concentration increases, with the danger of odour production. High ammonia concentrations also bring on the same problem and are toxic to algae, especially above pH 8.0. Maintenance of properly designed facultative ponds will be limited to the removal of scum mats, which tend to accumulate in downwind corners, and the cutting of grass on embankments. To ensure efficient operation, facultative ponds should be regularly monitored but, even where.this is not possible, they have the reputation of being relatively trouble-free. Maturation Ponds
The effluent from facultative ponds treating municipal sewage or equivalent input wastewater will normally contain at least 50 mg/l BODs and if an efQuent with lower BODs concentration is required it will be necessary to use maturation ponds. For sewage treatment, two maturation ponds in series, each with a retention time of 7 days, have been found necessary to produce a final effluent with BODs < 25 mg/l when the facultative pond effluent had a BODs < 75 mg/l. A more important function of maturation ponds, however, is the removal of excreted pathogens to achieve an effluent quality which is suitable for its downstream reuse. Although the longer retention in anaerobic and facultative pond systems will make them more efficient than conventional wastewater treatment processes in removing pathogens, the effluent from a facultative pond treating municipal sewage will generally require further treatment in maturation ponds to reach effluent standards imposed for reuse in unrestricted irrigation. Faecal coliform bacteria are commonly used as indicators of excreted pathogens and maturation ponds can be designed to achieve a given reduction of faecal coliforms (FC). Protozoan cysts and helminth ova are removed by sedimentation in stabilization ponds and a series of ponds with overall retention of 20 days or more will produce an effluent totally free of cysts and ova. Reduction of faecal coliform bacteria in any stabilization pond (anaerobic, facultative and maturation) is generally taken as following first-order kinetics:
where: Ne = Number of faecal coliforms/lOO ml of effluent.
18
Wastewater Treatment
Ni = Number of faecal coliforms/100 ml of influent. Kb = First-order rate constant for FC removal, d-I. t = Retention time in any pond, d. For n ponds in series, Equation becomes:
N
= e
Ni (1 + Kbtan )(1 + Kbt f ) ... (1 + kbtmo)
where: tmn = Retention time in the nth maturation pond. The value of Kb is extremely sensitive to temperature and was shown by Marais to be given by: Kb(7) = 26(1.l9)r-2c where: Kb(7) = value of Kb at PC A suitable design value of Ni in the case of municipal sewage treatment is 1 x 108 faecal coliforms/IOO ml, which is slightly higher than average practical levels. The value of Ne should be obtained by substituting the appropriate levels of variables in equation assuming a retention time of 7 days in each of two maturation lAgoons (for sewage). If the calculated value of Ne does not meet the reuse effluent standard, the number of maturatibn ponds should be increased, say to three or more each with retention time 5 days, and Ne recalculated. A more systematic approach is now available whereby the optimum design for maturation ponds can be obtained using a simple computer programme. Polprasert et al. have published an approach to the assessment of bacterial die-off which attempts to take into account the complex physical characteristics of ponds and biochemical reactions taking place in them. A multiple-regression equation involving parameters such as retention time, organic loading, algal concentration and ultra-violet light exposure has been suggested. The Wehner and Wilhelm non-ideal flow equation, including the pond dispersion number, was adopted to predict bacterial survival, in preference to the first order rate equation. Maturation ponds will be aerobic throughout the water column during daylight hours and the pH will rise above 9.0. The algal population of many species of non-flagellate unicellular and colonial forms will be distributed over the full depth of a maturation pond. Large numbers of filamentous algae, particularly blue-greens, will emerge under very low BOD loading conditions. Very low concentrations of algae in a maturation pond will indicate excessive algal predation by zooplankton, such as Daphnia sp, and this will have a deleterious effect on pathogen die-off, which is linked to algal activity.
19
Wastewater Treatment
Saqqar (1988), in his analysis of the performance of the Al Samra stabilization ponds in Amman, Jordan, has shown that the coliform and faecal coliform die-off coefficients varied with retention time, water temperature, organic loading, total BODs concentration, pH and pond depth. Total coliform die-off was less than the rate of faecal coliform dieoff, except during the cold season. For the series of ten ponds, including at least the first five totally anaerobic, the faecal coliform die-off coefficient, k, for the temperature range 12-15°C increased through the pond sequence from 0.11 per day in the first anaerobic pond to 0.68 per day in the final two ponds, which operated as facultative ponds.
Overland Treatment of Wastewater Apart from the use of effluent for irrigation of crops, termed 'slow rate' land treatment in the US Environmental Protection Agency's Process Design Manual for Land Treatment of Municipal Wastewaters, and 'rapid infiltration' or 'infiltration percolation' of effluent discussed as soil-aquifer treatment in a later section of this document, the EPA manual deals with 'overland flow' as a wastewater treatment method. In overland flow treatment, effluent is distributed over gently sloping grassland on fairly impermeable soils. Ideally, the wastewater moves evenly down th~ slope to collecting, ditches at the bottom edge of the area and water-tolerant grasses are an essential component of the system. This form of land treatment requires alternating applications of effluent (usually treated) and resting of the land, to allow soil reaction and grass cutting. The total area utilized is normally broken up into small plots to allow this form of intermittent operation and yet achieve continuous treatment of the flow of wastewater. Although this type of land treatment has been widely adopted in Australia, New Zealand and the UK for tertiary upgrading of secondary effluents, it has been used for the treatment of primary effluent in Werribee, Australia and is being considered for the treatment of raw sewage in Karachi, Pakistan. Table:2. Site Characteristics and Design Features for Overland Flow Treatment of Wastewater Grade Field area required (ha) Soil permeability impermeable barriers) Annual application rate (m) Typical weekly application rate (cm)
Finished slope 2-8% 6.55-44 Slow (days, silts and soils with 20-Mar Jun-40
Basic site characteristics and design features for overland flow treatment have been suggested by EPA as shown in Table 2. It was pointed out that steeper land slopes might be feasible at reduced hydraulic
20
Wastewater Treatment
loadings. The ranges given for field area required and application rates cover the wastewater quality from raw sewage to secondary effluent, with higher application rates and lower land area requirements being associated with higher levels of preapplication treatment. Although soil permeability is not critical with this form of land treatment, the impact on groundwater should not be overlooked in the case of highly permeable soils. The application rate for wastewaters will depend principally on the type of soil, the quality of wastewater effluent and the physical and biochemical activity in the near-surface environment. Rational design procedures, based on the kinetics of BOD removal, have been developed for overland flow systems by Middlebrooks et al. Slope lengths from 3060 m are common in the US for overland flow systems. The cover crop is an important component of the overland flow system since it prevents soil erosion, provides nutrient uptake and serves as a fixed-film medium for biological treatment. Crops best suited to overland flow treatment are grasses with a long growing season, high moisture tolerance and extensive root formation. Reed canary grass has a very high nutrient uptake capacity and yields a good quality hay; other suitable grasses include rye grass and tall fescue. Suspended and colloidal organic materials in the wastewater are removed by sedimentation and filtration through surface grass and organic layers. Removal of total nitrogen and ammonia is inversely related to application rate, slope length and soil temperature. Phosphorus and trace elements removal is by sorption on soil clay colloids and precipitation as insoluble complexes of calcium, iron and aluminium. ·Overland flow systems also remove pathogens from sewage effluent at levels comparable with conventional secondary treatment systems, without chlorination. A monitoring programme should always be incorporated into the design of overland flow projects both for wastewater and effluent quality and for application rates. Macrophyte Treatment
Maturation ponds which incorporate floating, submerged or emergent aquatic plant species are termed macrophyte ponds and these have been used in recent years for upgrading effluents from stabilization ponds. Macrophytes take up large amounts of inorganic nutrients (especially Nand P) and heavy metals (such as Cd, Cu, Hg and Zn) as a consequnce of the growth requirements and decrease the concentration of algal cells through light shading by the leaf canopy and, possibly, adherence to gelatinous biomass which grows on the roots. Floating macrophyte systems utilizing water hyacinth and receiving primary sewage effluent in Florida have achieved secondary treatment
Wastewater Treatment
21
effluent quality with a 6 day hydraulic retention time, water depth of 60 cm and hydraulic loading 1860 m 3jha d. The same authors suggested that similar results had also been observed for artificial wetlands using emergent macrophytes. In Europe, the land area considered to be necessary for treatment of preliminary-treated sewage is estimated at 25 m 2 per population equivalent to achieve a secondary effluent quality. Floating Aquatic Macrophyte Systems
Floating macrophyte species, with their large root systems, are very efficient at nutrient stripping. Although several genera have been used in pilot schemes, including Salvinia, Spirodella, Lemna and Eichornia, Eichornia crassipes (water hyacinth) has been studied in much greater detail. In tropical regions, water hyacinth doubles in mass about every 6 days and a macrophyte pond can produce more than 250 kg/ha d (dry weight). Nitrogen and phosphorus reductions up to 80% and 50% have been achieved. In Tamil Nadu, India, studies have indicated that the coontail, Ceratophyllum demersum, a submerged macrophyte, is very efficient at removing ammonia (97%) and phosphorus (96%) from raw sewage and also removes 95% of the BOD 5 . It has a lower growth rate than Eichornia crassipes, which allows less frequent harvesting. In such macrophyte pond systems, apart from any physical removal processes which might occur (especially sedimentation) the aquatic vascular plants serve as living substrates for microbial activity, which removes BOD and nitrogen, and achieves reductions in phosphorus, heavy metals and some organics through plant uptake. The basic function of the macrophytes in the latter mechanism is to assimilate, concentrate and store contaminants on a short-term basis. Subsequent harvest of the plant biomass results in permanent removal of stored contaminents from the pond treatment system. The nutrient assimilation capacity of aquatic macrophytes is directly related to growth rate, standing crop and tissue composition. The potential rate of pollutant storage by an aquatic plant is limited by the growth rate and standing crop of biomass per unit area. Water hyacinth, for example, was found to reach a standing crop level of 30 tonnes (dry weight)/ha in Florida, resulting in a maximum storage of 900 kg N/ha and 180 kg P/ha. Fly and mosquito breeding is a problem in floating macrophyte ponds but this can be partially alleviated by introducing larvae-eating fish species such as Gambusia and Peocelia into the ponds. It should be recognized that pathogen die-off is poor in macrophyte ponds as a result of light shading and the lower dissolved oxygen and pH compared with algal maturation ponds. In their favour, macrophyte ponds can serve a useful purpose in
22
Wastewater Treatment
stripping pond effluents of nutrients and algae and at the same time produce a harvestable biomass. Floating macrophytes are fairly easily collected by floating harvesters. The harvested plants might be fed to cattle, used as a green manure in agriculture, composted aerobically to produce a fertilizer and soil conditioner, or can be converted into biogas in an anaerobic digester, in which case the residual sludge can then be applied as a fertilizer and soil conditioner. Maximum removal by water hyacinth was 5,850 kg N/ha year, compared with 1,200 kg N/ha year by duckweed.
Emergent Macrophyte Treatment Systems In recent years, natural and artificial wetlands and marshes have been used to treat raw sewage and partially-treated effluents. Natural wetlands are usually unmanaged, whereas artificial systems are specially designed to maximize performance by providing the optimum conditions for emergent macrophyte growth. The key features of such reed bed treatment systems are: • Rhizomes of the reeds grow vertically and horizontally in the soil or gravel bed, opening up 'hydraulic pathways'. • Wastewater BOD and nitrogen are removed by bacterial activity; aerobic treatment takes place in the rhizosphere, with anoxic and anaerobic treatment taking place in the surrounding soil. • Oxygen passes from the atmosphere to the rhizosphere via the leaves and stems of the reeds through the hollow rhizomes and out through the roots. • Suspended solids in the sewage are aerobically composted in the above-ground layer of vegetation formed from dead leaves and stems. • Nutrients and heavy metals are removed by plant uptake. The growth rate and pollutant assimilative capacity of emergent macrophytes such as Phragmites communis and Scirpus lacstris are limited by the culture system, wastewater loading rate, plant density, climate and management factors. Table:3. Growth and Nutrient (N & P) Contents of Selected Macrophytes Biomass Tissue Standing Growth Composition Crop Rates N P _g kg-l_ t (dw) ha- 1 t ha-1 y,-l FLOA TING MACROPHYTES: Eichhornia crasspipes (water hyacinth) 20.0-24.0 60-110 10-40 1.4-12.0 Pistia stratiotes (water lettuce) 6.0-10.5 50-80 12-40 1.5-11.5 Hydrocotyle spp. (pennywort) 7.0-11.0 30-60 15-45 2.0-12.5 Alternanthera spp. (alligator weed) 18.0 78 15-35 2.0-9.0
23
Wastewater Treatment Lemna spp. (duckweed) Salvinia spp. EMERGENT MACROPHYTES: Typha (cattail) Juncus (rush) Scirpus (bulrush) Phragmites (reed) Eleocharis (spike rush) Saururus cernuus (lizardis tail)
1.3 2.4-3.2
6-26 9-45
25
4.0-15.0 1.8-9.0
4.3-22.5 22.0
8-61 53
6.0-35.0 8.8 4.5-22.5
10-60 26
5-24 15 8-27 18-21 9-18 15-25
0.5-4.0 2.0 1.0·3.0 2.0-3.0 1.0-3.0 1.0-5.0
Growth rates for emergent macrophytes are also provided in Table 3 as well as nutrient contents. High tissue N concentrations have been found in plants cultured in nutrient enriched (wastewater) systems and in plants analyzed in the early stages of growth. Maximum storage of nutrients by emergent macrophytes was found to be in the range 200-1560 kg N/ha and 40-375 kg P/ha in Florida. More than 50 per cent of the nutrients were stored in below-ground portions of the plants, tissues difficult to harvest to achieve effective nutrient removal. However, because emergent macrophytes have more supportive tissue than floating macrophytes, they might have greater potential for storing the nutrients over a longer period. Consequently, frequent harvesting might not be so necessary to achieve maximum nutrient removal although harvesting above-ground biomass once a year should improve overall nutrient removal efficiency. Nutrient Film Technique
The nutrient film technique (NFT) is a modification of the hydroponic plant growth system in which plants are grown directly on an impermeable surface to which a thin film of wastewater is continuously applied. Root production on the impermeable surface is high and the large surface area traps and accumulates matter. Plant top-growth provides nutrient uptake, shade for protection against algal growth and water removal in the form of transpiration, while the large mass of selfgenerating root systems and accumulated material serve as living filters. Jewell et al. have hypothesized the following mechanisms, taking place in three plant sections: • Roughing or preliminary treatment by plant species with large root systems capable of surviving and growing in a grossly polluted condition. Large sludge accumulations, anaerobic conditions and trace metal precipitation and entrapment characterize this mechanism and a large portion of wastewater BOD and suspended solids would thereby be removed. • Nutrient conversion and recovery due to high biomass production.
Wastewater Treatment
24
•
Wastewater polishing during nutrient-limited plant production, depending on the required effluent quality. A three year pilot-scale study by Jewell et al. proved this to be a viable alternative for sewage treatment. Reed canary grass was used as the main test species and resulted in the production of better than secondary effluent quality at an application rate of 10 cm/d of settled domestic sewage and synthetic wastewater. The highest loading rates achieved were equivalent to treating the sewage generated by a population of 10,000 on an area of 2 ha. Plants other than reed canary grass were also tested and those that flourished best in the NFT system were: cattails, bulrush, strawflowers, Japanese millet, roses, Napier grass, marigolds, wheat and phragmites.
Puitied Water
Capillery Ped
Fig:7. Nutrient Film Technique Variation of Hydroponic Plant Production Systems
Chapter 2
Ecological Principles of Wastewater Treatment The biological oxidation of an organic waste-sewage or industrial waste-either in bacteria beds or by activated sludge is basically the use of the organic matter as food by heterotrophic microorganisms, mostly bacteria or fungi. Part of the waste is used as the fuel in respiration to produce the energy for life processes and is broken down to carbon dioxide, water and the usual end-products. A second portion, however, is synthesized into protoplasm in the form of new cells-the "film" of bacteria beds and the microbial floes of activated sludge. This microbial mass must be removed from the water before the effluent is discharged to the receiving stream.
Sewage or Industrial (Waste)
Micro-Organisms (Film)
End Products CO2 H 2 0 NHa S02 P02
Fig:l. Synthesis and Energy Production in Biological Oxidation of Organic Matter
The subsequent treatment and disposal of this sludge is outside the scope of our subject but the amount and nature of the sludge formed in the biological oxidation plant is of practical importance in this respect. American workers have reported that approximately half of the waste is resynthesized into microbial mass and half is respired. This proportion, however, varies with different wastes and, as previously mentioned, with different microorganisms involved. In practice the amount of sludge produced will also vary with the method of operation depending on the degree of endogenous respiration involved.
26
Ecological Principles of Wastewater Treatment
THE ROLE OF BIOLOGICAL OXIDATION IN WASTEWATER TREATMENT
Biological oxidation plants are best suited to treat organic wastes in solution or in non-settleable form. Inorganic compounds such as cyanides, sulphides, thiocyanates and thiosulphates, capable of being oxidized by autotrophic bacteria, may also be treated. The use of the adsorptive properties of the film or flocs to remove settleable matter, or matter not biologically oxidizable, should be strictly limited. Particulate matter is better removed by settlement, possibly preceded by a process of floccula tion. Matter in solution which is incapable of biological oxidation, such as metallic salts, may prove injurious to the microorganisms in the plant. Furthermore, although metals may initially be removed from the liquor by adsorption or even synthesized in the microbial growth, they cannot be broken down like organic matter. They may leave the plant as humus or as excess sludge, in which case they may cause difficulty in the treatment and subsequent disposal of the sludge. Another fraction, however, may return into solution as when the film in a bacteria bed is removed by grazing fauna or when film and activated sludge undergo auto-oxidation under conditions of starvation. Stone found that with different metals different percentages (60-100) of the applied metal concentration appeared in the effluent. Of this amount different proportions were present in the humus and in solution; with copper, for example, approximately one third was in solution, whilst with nickel the amount was three quarters. Thus a considerable proportion of the applied concentration may appear in the final effluent after the settlement of the humus and result in a toxic effluent. The lethal concentration of the substances to stream life is probably much lower than that for microorganisms and grazers in the biological oxidation plants. Even though the concentration of metals in the waste does not seriously interfere with the biological oxidation, considerable reduction in toxicity may be needed to ensure a non-toxic effluent. It is possible that in treatment plants the metals become complexed with protein molecules, thereby reducing the toxicity. Metals may be present in wastes other than those from metal processing plants. Malt-distillery waste waters, for example, have been reported as containing 66.0 ppm Zn, 6.0 ppm Cu, 4.5 ppm Fe and 2.0 ppm Pb. The metals were probably derived from the vessels and pipelines used in the process. Radioactive wastes, like metals, cannot be destroyed in biological oxidation plants. Although radioactivity decays at a rate characteristic of
Ecological Principles of Wastewater Treatment
27
the material, this rate, measured in terms of its half-life, cannot be altered by treatment methods. Belcher reported that certain radioactive substances were removed in biological oxidation plants; the radioactivity is transferred to the sludge, however, and the problem of disposal remains. The most widely used method for treating waste waters of low to medium levels of activity is by chemical precipitation, thus transferring the radioactivity to a solid phase, which of course still has to be disposed off. In the latter half of the last century the association of such diseases as cholera and typhoid fevers with the polluted conditions of the rivers was probably a major factor in drawing attention to the need for adequate sewage treatment, even though the causative organisms were then unknown. The removal of pathogens is not now, however, generally considered a primary function of sewage treatment in this country. In America and in Russia much more attention is given to the bacterial standards of effluents. Although some plants effect a high degree of Bact. coli removal, some newer processes which are more efficient in the oxidation of organic matter are less efficient in reducing the number of Bact. coli. The BOD of an effluent is no true indication of its bacterial content.
THE ASSESSMENT OF PLANT EFFICIENCY The purpose of wastewater treatment is to prevent the pollution of the receiving water. Considering pollution as "an act which changes the natural qualities of the water in a river or stream" the ultimate assessment of the effectiveness of a plant is the extent to which the effluent affects the nature of the receiving water. What constitutes the "natural qualities" of water is the subject of some controversy because of the different interests involved. In this country, fishery interests in preserving the conditions of our streams suitable for fish life, to some extent safeguard the interests of others whose specific requirements may, however, differ. Aesthetic considerations, for example, should largely be satisfied. Public health requirements, however, are concerned both with the chemical content of the water and with the pathogenic organisms present. Although by satisfying the fishery requirements the chemical aspects are to a large degree safeguarded, the pathogenic content of the water is not usually considered important. Conversely a chlorinated water, satisfactory from a public health aspect, may be quite unsuitable for fisheries. In this country these two needs are satisfied by preserving the streams in as near natural condition as possible and chlorination is carried out after extraction for drinking purposes. The discharge of an effluent may affect the natural quality of a water in several ways. These have been discussed at length
28
Ecological Principles of Wastewater Treatment
elsewhere, and will only be briefly summarized here in discussing methods used to assess them. Organic Pollution
Biologically oxidizable organic matter discharged to a stream is oxidized by microorganisms as in the treatment plant. In the process the oxygen dissolved in the stream water is utilized and when the rate of removal is greater than the rate of re-aeration, resulting from surface aeration and the photosynthetic activity of plant life, the oxygen is depleted. Because of the time lag and flow of the stream, this deoxygenation is greatest some distance below the effluent and is followed by a recovery when the respiratory activity is successively reduced because of the progressive reduction in organic matter present. This phenomenon is known as the "oxygen sag" and the process as "selfpurification". Synthesis, associated with the biological oxidation of the introduced organic matter, results in an increase in saprobic organisms-bacteria, fungi and protozoa-some of which form macroscopic growths attached to the stream bed; these growths are commonly known as "sewage fungus". Both the depletion of oxygen and the development of sewage fungus adversely affect the quality of the stream. The potential oxygen demand of an effluent can be assessed directly by measuring the rate of oxygen uptake of a given volume of the effluent contained in a sealed vessel under carefully controlled conditions. Such manometric tests can be carried out over long periods and give not only the ultimate oxygen demand of an effluent but also a curve showing the course of oxidation. Techniques involving this principle, reviewed by Jenkins, are unfortunately not practicable as routine tests for large numbers of samples. Manometric methods have, however, been suggested for BOD determinations to eliminate errors due to dilution. The biochemical oxygen demand (BOD) has been generally accepted as a simple measure of the potential deoxygenating effect of the biologically oxidizable matter present in an effluent. It represents the oxygen uptake over the initial five-day period of the total respiratory curve. As a comparative test of the oxygen demand of effluents it is only strictly valid if the ratio of the BOD to the ultimate oxygen demand is constant, i.e., if the respiratory curves are truly exponential. With sewage effluents it has been shown that their oxidation takes place in at least two definite stages representing the oxidation of carbonaceous and nitrogenous matter; sometimes two stages of nitrogenous oxidation are distinguishable-the first representing the oxidation of ammonia to nitrite and the second of nitrite to nitrate. These
Ecological Principles of Wastewater Treatment
29
different stages result in definite steps in the oxidation curve and thus the ratio of BOD to ultimate oxygen demand remaining will vary at different degrees of oxidation. It can be seen that it is possible for the BOD over a five-day period to be greater when the oxidation is more advanced; compare, for example, the uptake between 5 and 10 days with that between 15 and 20 days when active nitrification was occurring. The implicit faith by some in the BOD test is exemplified by the fallacious argument that nitrification is undesirable in sewage treatment plants because it tends to increase the BOD value! The same effect probably occurs with some effluents from plants treating mixed industrial organic wastes. Thus, although the BOD is a simple and convenient test, care is needed in interpreting the results. Similarly, as pointed out by Wilson, the efficiencies of treatment plants cannot be measured in terms of percentage B.O.D. removal unless the ratio of BOD to the ultimate oxygen demand was the same for influent and effluent. In contrast to these direct biological tests for assessing the oxygen demand, chemical tests are also used. By determining the oxidizable substances present in the effluent and knowing the course of oxidation, the theoretical ultimate oxygen demand can be calculated. Gameson and Wheatland calculated the ultimate oxygen demand for sewage effluents on this basis, accounting for the complete oxidation of the organic carbon, ". organic nitrogen, ammonia and nitrite by the equation UOD = 2.67 (organic carbon) + 4.57 (ammoniacal N + organic N) + 1.14 (nitrous N) The results they obtained were in good agreement with those obtained by direct manometric methods. The above equation, however, assumes that the carbonaceous matter concerned has a respiratory quotient of unity, i.e., one volume of oxygen is utilized to produce one volume of carbon dioxide, one molecule of oxygen being required for each molecule of carbon. With sewage and sewage effluents this was apparently valid but other carbonaceous matter present in industrial wastes may have a different respiratory quotient and the figure of 2.67°C is not, therefore, necessarily generally applicable. Furthermore, although with the sewage effluents used, nearly all the organic matter was biologically oxidizable, with industrial wastes such as gas liquor, some may resist biological oxidation and therefore will not contribute to the oxygen demand, although it may represent a toxic fraction and therefore be of importance in assessing the overall polluting effect. The determination of organic carbon is a lengthy process and for routine work the amount of oxygen absorbed from acid potassium
30
Ecological Principles of Wastewater Treatment
permanganate or from boiling acid potassium dichromate is used as a measure of the oxygen demand. These two tests, the "Permanganate Value" (4 hr OA) and the "Dichromate Value" (Chemical Oxygen Demand) measure the chemically oxidizable matter present. These values do not represent the full oxygen requirements for the oxidation of all the biologically oxidizable organic matter. On the other hand they may include biologically stable organic matter. The permanganate test is well, established in sewage practice and pollution control in this country, probably because of its simplicity and the speed with which it can be carried out. It is, however, essentially an empirical measure and although of undoubted value in the operational control of individual plants continuously treating the same waste, its value as a comparative test is limited except for plants treating similar wastes such as domestic sewage. The relationship between the permanganate value, the BOD and the ultimate oxygen demand varies considerably with wastes having different organic substances present. The dichromate value has been advocated in the "Recommended Methods for the Analysis of Trade Effluents" for industrial wastes because of its reproducibility and applicability to a wide variety of wastes. It has been suggested that it is more truly proportionate to the calculated ultimate oxygen demand than is the permanganate value. For routine plant control one or more of these oxygen demand tests are necessary and where practicable these should be supplemented from time to time by determining the ultimate oxygen demand either by calculation or by manometric means. However accurately the theoretical oxygen demand of an effluent can be estimated, the prediction of the actual deoxygenating effect of its discharge into a receiving stream is complicated by several factors. Dilution, rate of re-aeration, rate of oxidation, temperature and the biological condition of the stream are inter-related factors which must be taken into consideration. The extent to which the ultimate oxygen demand of an effluent needs to be satisfied by the stream depends on the time of retention of the oxidizable matter in the stream in relation to its rate of oxidation. The pertinent retention time is not necessarily that of the water, for flocculated solids may settle on the stream bed and soluble organic matter may enter food chains and be synthesized into living organisms, thus further complicating the oxygen balance. In lakes and estuaries with long retention times the full oxygen demand will probably have to be met. In certain circumstances, however, such as the pollution of a small tributary stream which at a short distance below the discharge, is greatly diluted on entering a major river, the rate
Ecological Principles of Wastewater Treatment
31
of oxidation is probably more important in assessing the potential deoxygenation of the tributary stream itself. In such cases the BOD, which by involving a time factor (5 days), is to this extent a rate, is probably a more appropriate measure than the ultimate oxygen demand. The actual deoxygenation resulting from a discharge can, of course, be measured by a series of dissolved oxygen determinations. Because of diurnal and seasonal fluctuations, large numbers of determinations are required, preferably a continuous record. A recording instrument for such determinations has been developed by the W.P.R. Laboratories. Dissolved oxygen determinations, however, only measure one aspect of organic pollution-deoxygenation; the effects on the stream bed, where most of the stream organisms exist, are more difficult to measure analytically. Animal and plant communities living in the beds of streams exist in a dynamic state of balance which is very sensitive to changes in the environment. Organic pollution, by changing the environment, results in a change in the nature of the stream community to an extent depending upon the degree of pollution. Such effects have been studied by many workers and their results have been reviewed by Klein and Hynes. Based on such findings the degree and extent of organic pollution can readily be assessed by the biological examination of the stream bed. Toxic and Physical Pollution
Some effluents affect the natural qualities of the water by addition of toxic or poisonous substances. Where specific substances are known to be involved these can be assessed analytically. In many cases, however, a whole range of substances may be present in the waste and then routine determinations could prove a formidable task. In such cases the biological assay of the toxicity of the effluent, using fish or invertebrate stream fauna as test animals, is sometimes practised. As with organic pollution, difficulties arise in using the results of such tests to predict the effects of discharges on streams. Even when full analysis is practicable, because of interactions involving synergistic and antagonistic effects and other factors such as pH and dissolved oxygen, it is difficult to predict the overall effect on a stream. Although bio-assay measures more directly the toxicity of an effluent to the animals used in the test, the effect on the complex stream community and environment in which organisms naturally exist is agair:t difficult to forecast. In contrast to organic pollution, which by encouraging the activity of certain species causes a change in the balance of populations, toxic pollution tends to suppress activity generally. Because different species are tolerant of different concentrations of a
32
Ecological Principles of Wastewater Treatment
given poison, it may result in a change in the relative balance of populations and in some cases the more tolerant species may increase in number due to the reduced interspecific competition. The general effect, however, is to reduce the numbers of individuals and the number of species present. A discharge, although neither toxic nor deoxygenating, may affect the natural quality of the water by changing its physical properties. Discharges which appreciably raise the temperature, increase the turbidity or impart colouration are examples of physical pollution. Inert matter which settles on the stream bed may change appreciably the environment of the bottom dwelling organisms. Such deposits can seriously affect the spawning grounds of some fish, for example. Physical pollution usually has a marked effect on the stream organisms, especially when it affects the physical nature of the stream bed. In such cases it results in a change in the nature of the community; for example, the deposition of solids on a naturally stony bed causes the replacement of a stone-loving fauna by one typical of silted conditions. Thus both toxic and physical pollution can readily be detected by biological examination of the receiving stream. Apart from the above mentioned types of pollution the discharge of substances may alter the chemical nature of the water in other ways. Some readily oxidizable inorganic substances, such as sulphites in paper wastes, bring about simple de-oxygenation. Other wastes may alter the degree of hardness of the water or its salinity and thereby affect its natural qualities. Most effluents affect the receiving stream in more than one of the above mentioned ways. The type of tests chosen for operational control of a plant should be determined by the nature of the possible pollution and the nature of the receiving stream. In most cases such tests can with advantage be supplemented by biological examination of the receiving stream. THE CHOICE OF PROCESS-ACTIVATED SLUDGE OR BAOTERIA BEDS
Many factors (and possibly some prejudices) will determine the choice of plant, but here we shall briefly consider the ecological considerations involved in a choice between the two most common biological oxidation processes used in this country-activated sludge and bacteria beds. The theoretical considerations have already been discussed. The sludges resulting from the two processes represent different energy levels, the activated sludge being of a higher energy order than humus sludge. Thus, from a point of view of energy conservation, activated sludge is superior. The "sludge" developed from certain wastes could possibly
Ecological Principles of Wastewater Treatment
33
be exploited as animal feeding stuff; vitamin B12 is extracted from act:vated sludge. In sewage practice activated sludge is, however, more difficult to dispose of than humus sludge. Activated sludge is generally more sensitive to fluctuations in flow and load and to toxic discharges than are bacteria beds. On the other hand, if a bacteria bed breaks down, due to overloading or toxic discharges, the period needed for the recovery of both microorganisms and grazing fauna is longer than for the re-establishment of the simpler community of activated sludge. Some wastes, such as milk wastes, are not amenable to treatment by activated sludge because they tend to give rise to filamentous sludges which do not readily settle. Other wastes which are biologically oxidizable but which prevent the establishment of an efficient grazing fauna, or which cause solids to be deposited in the bed which are not readily removed, are best treated by activated sludge. Because of the longer period of maturation needed for bacteria beds, seasonal wastes and those which fluctuate appreciably in volume seasonally are best treated by activated sludge when possible. Since low temperatures have a less serious effect on activated sludge than on bacteria beds, the former are to be preferred for localities subject to severe winter conditions. The more complex community of a bacteria bed, having organisms on several trophic levels, is more stable than that of activated sludge. Because of this, an activated sludge plant is more sensitive to chance changes in conditions, but for the same reason is more responsive to intentional changes imposed by operational control. Thus, with bacteria beds, less frequent adjustment to operating conditions is required. Activated sludge, however, is more amenable to frequent adjustments. The choice of plant will, therefore, be affected by the availability of experienced plant operators. The possibility of a greater degree of variability in bacteria bed operation, such as variable frequency dosing has, however, reduced this difference in the degree of plant control required. Other recognized comparisons between the two processes, such as operational costs, the inevitable linking of fly nuisance with bacteria beds, and the acceptance of non-nitrified effluents from activated sludge, all need to be re-assessed in light of developments in both processes. DETERMINING A SUITABLE ENVIRONMENT FOR THE REQUISITE ORGANISMS
Although physical adsorption may account for the initial removal of
34
Ecological Principles of Wastewater Treatment
organic matter from the wastewater, the breakdown of the organic matter is the result of biological activity. The rate at which this occurs under optimum conditions depends on the readiness with which the microorganisms are able to utilize the waste, this in turn is determined by the intrinsic properties of the waste and the respective microorganisms. The theoretical rate at which a waste is oxidized biologically is known as the reaction constant (K) which is a measure of the oxidizability of the waste. In bacteria beds, according to Velz, "the rate of extraction of organic matter per interval depth of a biological bed is proportional to the remaining concentration of organic matter measured in terms of its removability". Expressed mathematically-
LD = IO- KD L where L = Total removable BOD in feed LD = Remaining removable BOD at depth D K = The reaction constant. In practice, this maximum rate may be further limited by different environmental conditions imposed by the waste or by the plant. Such conditions may be studied in isolation in the laboratory but, as mentioned in our study of autecology, care must be taken in the application of the results. For example, the interaction of factors and their effect on the balance of populations must be considered. In this respect, results are probably more applicable to the simpler community of activated sludge than to bacteria beds. Nevertheless such studies provide results which, if intelligently applied, can give a useful guide as to the range of tolerance, if not indicating the optimum conditions for most efficient purification. Environmental Conditions Imposed
by the Waste
Nutrition For those concerned with wastewater treatment it is an encouraging thought that all naturally occurring organic substances and many synthetic ones are attacked by at least one species of microorganism and that organisms exist which are potentially capable of utilizing organic compounds not yet synthesized. The presence of specific substances in the waste, is therefore a most important factor in the environment. Some organisms are able to utilize a wide range of organic substances as their primary food source but others appear to be specific in their requirements. In the latter case it is necessary to ensure a continuous supply of the specific waste, or if this is not practicable, to retain a stock culture of the organisms. All microorganisms require certain basic nutrient elements-c, N, P, and 5, together with certain trace elements such as K, Ca, Zn, Mg, Fe, Mn,
Ecological Principles of Wastewater Treatment
35
Cu and Co. Furthermore the C, N, and P are required in roughly balanced amounts. In physiology the proportion of C to N is known as the C/N ratio; in wastewater treatment practice it is usual to measure the C as BOD and the proportion is then expressed as BOD:N or BOD:P. The optimum ratio for different organisms varies somewhat but as a basis for experiment a ratio of BOD:N:P of 100:6:1.5 could be used. The nitrogen is considered only fully available when present as NH3 and the phosphorus as soluble P04' although other forms of nitrogen which can be converted into NH4 may thus become available. Domestic sewage usually provides a nutritionally balanced food with the necessary trace elements and vitamins for bacterial activity. The presence of a large proportion of trade wastes, however, may upset the balance. Some wastes such as those from milk processing and plastic manufacture may increase the carbon-nitrogen ratio and encourage troublesome filamentous growths. Pre-treatment of sewages containing trade wastes, before treatment in biological oxidation plants, may possibly upset the balance; a sewage with a high iron content, if subjected to a process of flocculation, could appreciably reduce the phosphate content for example. Industrial wastes may be deficient in nitrogen, e.g., cider wastes, cotton-kiering wastes; or nitrogen and phosphorus, e.g., citrus waste, . brewery waste and paper wastes. Such deficiencies can be made good by the addition of the requisite amount of ammonium salt and phosphate to the influent. When practicable, domestic sewage could be used as a source of nitrogen and phosphorus although Wilson found that by supplying the phosphorus, deficient in a chemical manufacturing waste, by the addition of sewage, troublesome slime growths accumulated on the medium in the bacteria beds. As a result, agricultural grade triple superphosphate was added to provide a concentration of phosphate in excess of that found necessary by laboratory tests-O.4 ppm P. The actual amount of nitrogen and phosphorus required depends to some extent on the . method of plant operation. In a plant where the organisms are subjected to endogenous respiration. the autolysis may release nitrogen and phosphorus for further synthesis-the carbon being lost as carbon dioxide. In treating wastes deficient in nutrients, the recycling of effluent containing a higher proportion of such nutrients may be one advantage of recirculation. Toxicity
The presence of toxic substances in a waste may impair biological activity for reasons previously discussed. The structure of the bacteria bed film or sludge floc affords the embedded bacteria some protection against toxic flushes, but above certain concentrations they seriously affect the
36
Ecological Principles of Wastewater Treatment
activity of the cells. In bacteria beds, toxic matter may be adsorbed on the film in the upper layers of the bed, thus reducing the concentration to which the films lower in the bed is subjected. No such protection is afforded in the activated sludge system where any part or all of the sludge may be subjected to high concentrations of toxic matter. This may be the reason why bacteria beds are usually considered better able to withstand toxic discharges. It is not possible to state the concentrations at which different toxic substances seriously affect the efficiency of biological oxidation plants. For one thing, the toxicity depends upon such other factors as pH, dissolved oxygen, temperature and the presence of other toxic products. The toxicity of metallic ions in wastes admixed with sewage may be reduced if they are complexed with the proteins present in sewage. A further difficulty is the variation in sensitivity of the different organisms. Because of these complications, reference to the literatures gives a wide range of tolerable toxic concentrations. The heavy metals are usually considered toxic at above 1-3 ppm, but it has been reported by Ross and Sheppard that the efficiency of bacteria responsible for the oxidation of phenolic wastes from oil refineries was not affected by concentrations of copper up to 100 ppm. Under steady low concentrations of toxic substances it is possible that a more resistant micro-flora will become established, although this may not be as efficient as the original one. In this respect occasional strong toxic discharges are more serious than a steady weak discharge. As previously mentioned, however, metals are not amenable to biological oxidation and to ensure a non-toxic effluent their concentration in the feed should be reduced to a minimum. Toxic non-metallic wastes such as phenols, thiosulphates, thiocyanates, cyanides, sulphides, formaldehydes, etc. although inhibiting the biological oxidation of other substances, are themselves subject to attack by specific organisms. Cyanides, for example, although interfering with biological treatment at concentrations between 1 and 2 ppm HCN, can be oxidized to ammonia and oxidized nitrogen in bacteria beds at concentrations of 60 ppm HCN without the addition of sewage. Ware and Painter isolated an organism, an actinomycete, which proved capable of breaking down cyanides and which readily colonized bacteria beds. Where practicable such wastes are best treated separately. Since they are oxidizable to relatively non-toxic compounds they can be accepted in controlled concentrations for treatment at sewage works although this would necessitate a larger oxidation plant, both because of their slower rate of oxidation and their suppressing effect on the biological oxidation of the more readily oxidizable matter.
Ecological Principles of Wastewater Treatment
37
The partial oxidation of an industrial waste containing mixed compounds before discharging to the sewage works, although greatly reducing the oxygen demand and thereby reducing the load on the sewage works, may contribute a residual waste difficult to oxidize and possibly inhibitory to the biological oxidation of the sewage. Unless sufficiently diluted with sewage effluent it could also impart toxicity to the efflue'1t. With wastes containing a mixture of such toxic compounds the oxidation of one may be inhibited by the presence of others. In the treatment of gas liquor, for example, the biological oxidation of the thiocyanate was found to be suppressed by the presence of phenols, and in practice did not occur until the phenol oxidation was almost complete. Apart from the toxic effect of the phenol, interspecific competition between the heterotrophic phenoloxidizing bacteria and the autotrophic thiocyanate-oxidizing organism may be involved, as is thought possible in the carbonaceous. oxidation and nitrification of sewage. To overcome this inhibitory effect, gas liquor is being treated experimentally in a two stage process -the first bed for the oxidation of phenols and the second for thiocyanate oxidation. . pH
Because enzymes are sensitive to pH changes it is not surprising that biological activity is affected by changes in pH. Different organisms have different pH optima and ranges of tolerance, acid conditions generally favouring fungi. Generally a pH between 6.0 and 8.0 is desirable for the biological oxidation of most wastes. Sewage is usually well buffered and its biological oxidation results in only a slight change in pH. With some industrial wastes, however, the change in pH as the waste is oxidized may be appreciable and create conditions unfavounble for further oxidation. The film or activated sludge floc may themselves act as a buffer against some degree of fluctuation in pH but are not able to withstand large changes. When a waste is oxidizable by different organisms, these may require different pH ranges. The bacterial oxidation of phenol is best effected between pH 6.5 and 7.8 but it is also successfully attacked by a fungus between pH 2.0 and 2.5. Environmental Conditions of the Plant
Biological oxidation plants are required to provide an environment in which the organisms and the waste are maintained in intimate contact in the presence of oxygen. In the activated sludge system this is effected by creating turbulent conditions in the aeration tanks by mechanical
38
Ecological Principles of Wastewater Treatment
means, which at the same time introduces oxygen into the water from the atmosphere. In the bacteria bed similar requirements are satisfied by allowing the wastewater to flow over a static film of microorganisms, the oxygen being supplied by absorption from the air passing through the bed. Factors influencing the degree to which these conditions are provided will be discussed in the respective sections on activated sludge and bacteria beds. The waste itself, of course, creates environmental conditions within the plant other than those already mentioned Very strong organic wastes such as cotton-kiering liquor, food processing wastewaters and distillery wastes, having a high oxygen demand, result in conditions of oxygen depletion in the immediate environment of the organisms. No matter how efficient is the aeration within the plant, the rate of oxygen uptake would exceed the rate at which it could be transferred from the air through the liquid to the organism. Such liquors should, therefore, be diluted with returned effluent or by other means to produce a liquor having a BOD not exceeding 350 ppm, to ensure the prevention of anaerobic conditions within the plant. Wastes having a very high oxygen demand, a BOD exceeding 1,000 ppm, are probably best pre-treated by anaerobic processes followed by the biological oxidation of the resultant weaker liquor. It has been found that to induce satisfactory digestion of soluble organic wastes it is necessary to introduce a solid matrix. Pettet and others reported that this can be satisfactorily achieved by adding a massive inoculum of actively digesting sewage sludge which, by gradually increasing the rate at which the waste is added, can be developed into an active anaerobic sludge. This is retained in the system by settling the effluent from the digester and returning the settled sludge as in the aerobic activated sludge process. In other countries such anaerobic processes have been successfully applied to a variety of strong wastes including those from yeast production and fermentation processes, meat packing, and chewing-gum manufacture in America, and yeast wastes in Denmark and Sweden. Of the wastes investigated in pilot plants by the W.P.R. Laboratories in England, slaughterhouse wastes would appear to be the most amenable to treatment by anaerobic processes, meat wastes have also been successfully treated anaerobically in pilot plants in New Zealand by Hicks. Temperature is an important factor determining the rate of biological activity. Each species has a temperature range over which life is possible. Within this range activity increases with increase in temperature to an optimum above which the thermal denaturing of the enzymes occurs at a rapidly increasing rate, resulting in decreased activity and ultimate death of vegetative cells. Some microorganisms, however, produce highly
39
Ecological Principles of Wastewater Treatment
resistant spores capable of surviving temperatures outside the range of tolerance for the vegetative cell. Organisms active in aerobic treatment plants are usually mesophilic, having optima between 20°C and 40°C, depending on the species. Within the physiological temperature range, biological activity generally changes by a factor of 2 for each lOoC change in temperature. Manometric studies have shown that this factor, known as the QI0 value, for activated sludge was between 2.0 and 2.06, between temperatures of 0 and 25°C. In view of this significant effect of temperature on biological activity it may be expected that plant efficiency would be similarly affected. In fact, changes in temperature over a wide range of temperatures have little direct effect on the efficiency of treatment plants. There is, for example, little seasonal fluctuation in efficiency of activated sludge plants with changes in temperature, although increased temperatures could be expected to increase the efficiency both by increasing the oxygen transfer rate as well as directly increasing biological activity. Viehl and Meissner considered that the sludge organisms were able to adapt themselves to counteract the effect of low temperatures. Certainly, active sludges can be developed over a wide range of temperatures. Gehm investigated the treatment of waste waters from the pulp and paper industry which had temperatures of between 50 and 53°C; he found that such wastes could be satisfactorily treated at feed temperatures of 52°C by the activated sludge process, the efficiency being much the same as that between 30 and 40°C. Air
&
"---------',--""""',.""
~ 50 Cii ~40
Domesllc -Industrial
' __ • __ •••• --
~30~-----------------------1 Bed
50
40 30
June
2004 Sept
Oct
2005 March
Fig:2. Comparison of Seasonal Temperature Fluctuations in a Bacteria Bed Treating Domestic Sewage with One Treating Industrial Sewage Together with Corresponding Air and Waste Temperatures
40
Ecological Principles of Wastewater Treatment
Although sludges can be developed to operate at different temperatures, their efficiency is reduced if they are subjected to violent changes in temperature. In sewage plants such violent changes in temperature do not occur; in plants treating specific industrial wastes, however, they could occur due to sudden discharge of a hot waste. Although the temperature near the surface of a bacteria bed is related to the temperature of the waste and the air temperature, the temperature within the bed is more closely related to that of the waste. The temperature of the waste is, therefore, an important factor influencing the temperature of bacteria beds. Sewages containing industrial wastes are generally warmer during the winter than purely domestic sewages and thus beds treating sewages containing such wastes are not subjected to as low temperatures as are beds treating domestic sewage. The effect of the waste on the bed temperature depends to some extent on the volumetric loading as well as its temperature. The higher volumetric loadings common with filtration methods used for industrial sewages would, therefore, tend to further enhance this temperature difference. In bacteria beds marked seasonal fluctuations in efficiency may occur, but these are due to the indirect effect of temperature on the film-fauna balance, as discussed later. In experimental laboratory beds operated at different temperatures over a range 5°C-30°C, although large differences in temperature at different depths within each bed made strict comparisons difficult, it was apparent that there was little difference in the efficiency between lOoC and 30°C; at 7°C, however, there was a marked decrease in efficiency. Grazing fauna were not introduced into the beds and it was found that in the beds of similar efficiency-those operating berween lOoC and 30°C-the amount of accumulated film was the same, whereas in the beds operating at 7°C, less film had been accumulated. Thus this direct effect of temperature was only evident below lO°C. Ware found that the biological breakdown of cyanide in laboratory experimental beds was little affected by temperature cha!1ges in the range 10°-35°C, but that at temperatures outside this range, performance deteriorated rapidly. There was, however, a possibility that an organism other than the actinomyces, with which the bed had been inoculated, was active at the higher temperatures when recognizable actinomycetes were virtually absent. The lack of the expected relationship between temperature and plant efficiency may to some extent be due to adaptation or changes in the microbial flora with changing temperature, but the absence of response to short term temperature changes is more likely to be explained in terms of limiting factors.
Ecological Principles of Wastewater Treatment
41
The original theory, whereby it was considered that of the many factors influencing a biological process, only one was operative at any time and that this being so, the rate of the process could not be affected by changing the other, non-limiting, factors, is not now fully accepted. Nevertheless, by changing a factor which is not at the time limiting, the resultant effect on the process is only a fraction of that expected theoretically. On the other hand, changing a limiting factor causes the expected change in the rate of the process until such times as another factor becomes limiting. In biological oxidation plants the ultimate factor limiting the rate of oxidation is the intrinsic rate of generation, under optimum conditions, of the microorganisms concerned. Of the many factors theoretically capable of influencing this rate, the ones most commonly limiting, under normal operating conditions, are probably the rates at which food or oxygen are transferred from the liquid to the cells. With weak wastes such as domestic sewage the transfer rate of the respiratory substrate-the waste-is probably limiting but with strong industrial wastes it is probably the oxygen transfer rate that is limiting, at least in the early stages of oxidation. With the exception of the surface layers of some exposed bacteria beds in winter, temperature is generally not a directly limiting factor and rises in temperature do not, therefore, bring about the increase in rate of oxidation which would be expected from the experimental QlO value. Temperature, however, affects the transfer rates, but according to Wilson, with oxygen or nutrient availability controlling, a lOoC rise in temperature would increase the transfer rate and, thereby, the rate of biological oxidation, by no more than 20-30 per cent. He suggested that the response of a plant to temperature changes provides a rough indication of the operative limiting factors, the greater the temperature effect the less the control by oxygen and nutrient availability. As an example he quotes data from the Monsanto plant at Ruabon showing that the QlO value for a bacteria bed was 1.4 when the BOD of the feed was 322 but was only 1.08 when the feed BOD was halved. A knowledge of which factor is limiting in any plant is, of course, important in plant operation. The principle of limiting factors may also explain the discrepancies in operational experiences which result in the advocacy of different and sometimes conflicting practices in treatment methods. The change in a factor, which in one plant produced marked improvements in effiCiency, may give disappointing results in an apparently similar plant but where another factor is limiting.
42
Ecological Principles of Wastewater Treatment
The Application of Synecology in Controlling Populations
Although autecology enables us to define the conditions of a plant necessary for efficient oxidation, the operation of a plant involves the controlled activity of different populations and the transfer of materials and energy between the waste and the different populations; as such it involves us in synecology. Of the several populations present in plants, the saprobic microorganisms, including the autotrophic bacteria, are the organisms primarily concerned with the breakdown of organic wastes and it is, therefore, the control of their activity that is of primary importance. At first sight it would appear that maximum efficiency would be achieved by maintaining the organisms in a constant state of maximum growth, Le., in the log phase. In the absence of controlling factors, however, this phase cannot be indefinitely maintained. Of the several theoretical factors terminating the log phase, the impedence to the transfer of oxygen and nutrients to the increasing number of cells in the microbial mass-the activated sludge floc or the bacteria bed film-is probably the most important in biological oxidation plants. The accumulation of toxic metabolic by-products may also be a contributory factor. For efficient operation, the organisms must remain physiologically capable of log growth and, therefore, the control of the microbial popUlation is essential. Organisms in this physiological state may, however, be prevented from achieving maximum log growth by such factors as temperature, pH, dissolved oxygen and nutrient concentration. For maximum efficiency, of these factors, nutrient concentration should be the limiting factor, this, of course, being determined by the waste being treated. In practice, providing that the autecological conditions, previously discussed, are satisfied, the synecological considerations involve the establishment and maintainence of a balance between the loading imposed by the waste, the microbial population and the oxygen concentration. ECOLOGICAL OPERATION OF ACTIVATED SLUDGE PLANTS
The intimate contact of the waste with an optimum quantity of active sludge organisms in the presence of adequate oxygen supply for a requisite period of time followed by the efficient separation of the organisms and purified liquor, are pre-requisites of the process. The time of contact for a given flow is predetermined by the capacity of the aeration tanks which should be related to the strength, volume and treatability of the waste. The design, however, should provide for variability in the degree of aeration and in the amount of sludge carried in the system. Factors determining the optimum levels of these two variables in relation fo the
Ecological Principles oj Wastewater Treatment
43
loading will now be discussed in the light of the reported results of several laboratory investigations and the operational experiences of several workers-mostly in America. Aeration
Before atmospheric oxygen becomes available to partake in the biological oxidation of the waste, it must be transferred to the seat of the activity-the respiratory enzymes within the cell of the organism. Four stages must be considered in this transfer: • The transfer of atmospheric oxygen into solution in the wastewater. • The transfer of the oxygen dissolved in the wastewater to the surface of the respiring cell. • The diffusion through the cell wall and cell membranes into the cell itself. • Finally, the absorption by the appropriate respiratory enzyme. The last two steps in the transfer are governed by the properties of the cells themselves, although they may be affected by the nature of the surrounding wastewater. Work by Winzler supports the view that the limiting factor at low dissolved oxygen concentrations was the rate at which oxygen was absorbed by the enzymes and not the rate of diffusion of oxygen into the cell. In practice, however, it is necessary to maintain the dissolved oxygen concentration at the surface of the cell at a sufficiently high level to ensure the saturation of the oxygen-bearing enzymes. In plant operation this concerns us with the first two stages of oxygen transfer. Of these, the initial transfer rate of the atmospheric oxygen into solution, in the different methods of aeration, has received much attention. The dissolved oxygen concentration at the surface of the cells in the floc may, however, be quite different from that in the body of the water. To ensure adequate transfer to the cells in the floc it is necessary to ensure the movement of £loc relative to wastewater by creating turbulent conditions within the tank, as opposed to the general circulation of the liquid in which the floes could be carried in the same pocket of liquid. Pasveer calculated that the oxygen concentration at the boundary of the floc in the case of slight turbulence was only a small percentage of that existing in the case of strong turbulence. Gaden, applying the results of work on the effects of 'agitation and aeration in fermenta.tion processes, stresses the importance of agitation in the activated sludge process. He considered that air bubbles had distinct limitations as agitators. Ciliate protozoa probably assist in the aeration of the floc by producing microcurrents.
44
Ecological Principles of Wastewater Treatment
The oxygen concentration at the surface of the individual cells within the floc will also depend upon the size and texture of the floc itself. Wuhrmann has calculated that in a liquor having a dissolved oxygen concentration of 1.5-2.5 ppm the flocs should not exceed 400-500 I in diameter, if the innermost cells are to receive an adequate supply of oxygen. The structure of the floc is also probably of importance in this respect; microscopic examination of sludges reveals that some flocs have a dense appearance, whilst others are of a more open structure. Filamentous growths, although affecting the settling qualities of the sludge, present a loose, open floc with high oxidative powers. Most microorganisms in biological oxidation plants are microaerophilic, i.e., require only a low dissolved oxygen concentrations (less than 1 ppm.) for optimum activity. Many workers have reported that the respiratory rate of activated sludge is independent of the dissolved oxygen concentration above about 1-2 ppm, providing that turbulence is adequate. The rate of aeration should supply oxygen to balance the uptake by the floc in order to maintain the necessary concentration throughout the aeration tank. This demand will depend upon the rate of uptake of substrate (the waste) by the floc, this rate largely being determined by the concentration of the waste. Domestic sewage is a relatively weak nutrient solution and normally the rate of oxygen uptake will exceed the uptake of the nutrient and is not, therefore, limiting. With strong industrial wastes, however, the nutrient uptake may exceed that of oxygen which would then be limiting. The degree of aeration should, therefore, be related to the BOD loading of the plant rather than to the volumetric loading. The greatest oxygen demand occurs at the inlet end of the aeration tank, where the waste and the deoxygenated sludge enter. As purification proceeds and the oxygen demand is reduced, lower rates of aeration are possible. In such modified methods as "tapered" aeration, the degree of aeration is highest at the inlet end of the tank and is reduced with declining oxygen demand along the tank. The initial high oxygen demand is also prevented in methods in which complete mixing is practised, and to some extent in step aeration, where the waste is introduced into the aeration tank at different distances along the tank. It would appear that when oxygen is the limiting factor it is often the second stage-the transfer of oxygen dissolved in the waste to the surface of the cells-that is limiting, due to insufficient agitation. Increased purification rates claimed for different methods of aeration, such as brush aeration and coarse bubble aeration, may be due to the increased agitation,
Ecological Principles of Wastewater Treatment
45
rather than to the introduction of larger amounts of oxygen. Several workers have, however, reported increased rates of purification by increasing the rates of aeration. The two latter workers considered that this enhanced purification was due to a change in the species composition of the sludge, which became biologically adapted to the higher dissolved oxygen concentration. The more aerobic organisms which replace the micro-aerophilic species presumably have a higher rate of metabolism. Both workers reported changes in the microscopical appearance of the sludges. The process of aeration, besides providing the necessary oxygen and turbulence necessary to ensure the intimate contact of waste, oxygen and organisms, may also assist in the scrubbing out of the toxic end products of metabolism, which would otherwise accumulate in the floc. Most workers agree that the period of deoxygenation resulting from conditions in the settlement tanks may have a deleterious effect on the sludge and should, therefore, be reduced to a minimum. The establishment of a high dissolved oxygen concentration in the effluent from the aeration tanks may reduce the extent of deoxygenation in the settlement tanks, but this is probably better achieved by ensuring that the sludge load is well oxidized and therefore exerts only a small oxygen demand in the settlement tanks. Wuhrmann found, as a result of manometric tests, that the rate of disappearance of a soluble substrate was paralleled by its oxidation in respiration, contrary to the general concept of initial adsorption and storage. Later work showed that washed activated sludge, after periods of anaerobiosis for 1-4 hours, showed no difference in respiratory rates when aerobic conditions were again established. He concluded that the periodic anaerobiosis in the activated sludge cycle was probably less detrimental than is generally assumed. With soluble substances, which are oxidized as they are removed from the wastewater, this may be so. However, with wastes such as sewage, in which much of the load is in the form of suspended or colloidal solids which are first adsorbed and subsequently oxidized, the presence of unoxidized adsorbed solids in the floc under conditions of anaerobiosis and quiescence in the settlement tanks, could be detrimental to the condition of the sludge. Under such quiescent conditions there could be little or no removal of the toxic end products of anaerobic decomposition from the floc, the accumulation of which could impair the efficiency of the sludge. Until further evidence is produced on a wider range of wastes it would seem wise to accept the view that prolonged deoxygenation. of
46
Ecological Principles of Wastewater Treatment
the sludge is detrimental and to provide for the rapid return of the sludge to the aeration tanks. This involves the provision of adequate pipe-work and pumping capacity; the rate of return will then be determined by the rate of settlement of the sludge in the settling tanks. This factor is determined by the characteristics of the sludge itself, which will be discussed in the next section. By using two-stage settlement it should be possible to settle out most of the sludge in the first tank with relatively low retention time; to provide for the sludge return, and to complete the settlement of the remaining sludge in large secondary tanks, the settled sludge from which could be passed to waste. Sludge Characteristics
The ecological unit of the sludge is the floc and it is the structure and biological condition of the individual flocs that determine the general efficiency of the sludge. To be efficient the floc must satisfy two quite distinct conditions; it must first remove the waste from the water and secondly it must itself be capable of being readily separated from the purified liquor by settlement. These two characteristics are not necessarily linked; a young actively growing floc, capable of rapid removal of waste, may have poor settling properties; on the other hand, a good settling sludge may have reduced powers of oxidation. Different theories have been put forward,to explain the mechanism of the removal of organic matter from wastewaters by activated sludge. Whilst it is not the purpose of the present work to discuss such theories, it is necessary to formulate a theory as a working hypothesis for the correlation of experimental evidence and operational experience. One must, however, be willing to change or modify a theory to accommodate any new and valid evidence or experience, one's sense of judgement being consoled by the thought that in science there is no such thing as the ultimate truth! The floc may be considered as being formed initially as the result of combined biological activity and physical forces when wastewater containing the necessary inoculum of micro-organisms is aerated. The types of microorganisms which develop depend upon the nature of the waste as previously described. Bacteria rapidly multiply and at first are freely dispersed in the liquid but later agglutinate to form the primordium of th~ floc. McKinney considered that the flocculation of bacteria occurred only when their energy level was reduced sufficiently to prevent them from overcoming the physical forces, such as the Van der Waals forces of
Ecological Principles of Wastewater Treatment
47
attraction, causing them to adhere. Thus flocculation was more likely to occur in reduced nutrient concentrations. He considered that this power of flocculation was common to all bacteria and was not confined to a few zooglea forming species. Nevertheless this tendency to flocculate probably varies with different species and the difficulty experienced in developing a floc in some plants treating specific trade wastes, may be due to the dominance of a species peculiar to the waste. Such difficulties have been experienced in the treatment of phenolic wastes; although aeration resulted in a marked reduction in the phenolic content, the dispersed bacteria imparted turbidity and a high BOD to the effluent. The difficulty was overcome by discharging the effluent together with the suspended bacteria into lagoons where clarification took place, presumably due to flocculation of the bacteria under conditions of low nutrient concentrations. Renn stressed the importance of physical forces associated with interfaces-both air-water and solid-water-where, in the case of organic matter in low concentrations as in wastewater, the organic matter tends to be concentrated. These forces, he suggests, are operative on the dispersed bacteria themselves and result in the interfacial trapping of the bacteria at the air-liquid surface of the bubbles and their consequent flocculation. The bacteria in the resultant floc become embedded in a common gelatinous matrix which forms a further solidliquid interface at which the organic waste is then concentrated. This waste is then used by the bacteria for respiration and synthesis, the resultant multiplication increasing the size of the floc. The relative rates at which the waste is removed and oxidized by the floc is still a matter of controversy. Many workers consider that the organic matter is first rapidly adsorbed by the floc and that the accumulated waste is subsequently oxidized as aeration continues. This view is supported by the observation that the initial rate or removal of organic matter was many times greater than could be accounted for by the respiratory rates. It was considered that the rate of removal of soluble organic matter such as glucose was according to the simple adsorption equation. In later work, however, it was found that no glucose could be removed from sludge which had previously rem<1Ved considerable quantities of it. It was assumed that it had been ·absorbed by the floc. Wuhrmann found that the substrate respiration of the sludge ceased immediately the glucose substrate was exhausted and concluded, therefore, that no glucose had been adsorbed on the floc surface. Porges et al. found that the rate of purification of dairy wastes was twelve times the rate of oxidation. As a result of chromatographic analysis they
48
Ecological Principles of Wastewater Treatment
concluded that some of the carbohydrate waste was stored in the cells as an insoluble glycogen-like substance which was subsequently oxidized. It would appear that adsorption and absorption both result in the initial removal of waste. With wastes having suspended and colloidal organic matter, their adsorption on the floc surface accounts for the initial rapid removal of waste. Some of the adsorbed matter i-; then readily absorbed, while other more complex molecules are broken down by the action of enzymes secreted by the ceil, before being absorbed. A further fraction may be incapable of being absorbed and remains in the floc as relatively inert matter. In some industrial wastes most of the organic matter is in solution; in such cases it is possible that the waste is absorbed immediately it has been adsorbed, in which case no accumulation of the waste takes place on the floc surface. The relative rates of absorption and oxidation of the waste will probably depend upon the concentration of the waste; with a strong waste, organic matter may be stored and be available for subsequent oxidation, with a weak waste or one which is diluted as in the complete mixing systems, oxidation may proceed simultaneously with the absorption of the waste. The extent to which the waste is accumulated either on the surface of the floc or as storage products within the cell, depends upon the relative rates of adsorption, absorption and oxidation in relation to the concentration of the waste. The floc may thus increase in size by multiplication of the bacteria within the floc, and by the addition of living and non-living matter from the liquid. During development, the floc becomes colonized by bacteria-feeding organisms such as ciliate protozoa, nematode worms and rotifers. Thus a mature floc can be regarded as a microcosm, the populations of which exist in a dynamic state of balance sensitive to the environmental changes including the waste itself. Some ciliate protozoa such as Paramoecium, Colpidium and Lionotus swim freely in the interstitial liquid and cannot be considered as associated with the floc. Others, mostly belonging to the Hypotricha, such as Aspidisca and Euplotes, have cilia modified as cirri for creeping over surfaces and are distinctly associated with the floc as, of course, are the attached ciliates Vorticella, Opercularia and Epistylis which are anchored to the floc by their stalks. Microscopical examination suggests that the free swimming forms prey on the bacteria dispersed in the liquid, whereas the creeping forms, which can be seen browsing amongst the flocs, are actively controlling the bacterial population of the floc as do the nematode worms and rotifers.
Ecological Principles of Wastewater Treatment
49
The attached ciliates, however, although closely associated with the floc physically, appear to feed upon the dispersed bacteria. They can be seen to bring about the flocculation of bacteria and other dispersed solids by ingestion and the subsequent ejection of larger particles as demonstrated by Sugden and Lloyd. As the floc grows and ages, a higher proportion of it consists of dead cells and accumulated inert solids. Although the former may still be capable of enzymatic secretion and the whole floc capable of adsorption, oxidation is only possible by the active living cells and thus there is a decline in the general activity of the floc as it ages. Also as the floc increases in size, the diffusion of nutrients and oxygen to the individual cells, and that of waste products out of the floc, becomes more difficult. Thus, as with cultures, each floc may be considered as passing through different phases of growth, reaching maturity and then declining. The pressure of predatory populations delays the decline phase of the floc. In passing through these phases the floc changes in structure and in activity, both important features in operation. Several workers have reported the gradual reduction in substrate removal rate with increasing sludge age. The acceptance of this concept is implicit in the American use of "sludge age" in the operation of activated sludge plants. This may be defined as the average total time of detention of the flocs in the system. At a constant sludge concentration in the system, this value will be inversely proportional to the rate of increase of the sludge as determined by the nutrients available per floc, or expressed in more practical terms, the load per unit amount of sludge. It is 'usually calculated as: 51u d ge (days ) =
Ib dry weight of activated sludge in the system Ib dry weight of suspended solids entering the system per day
This value, although not the average age of the flocs, is proportional to it and is a useful guide to the efficiency of a sludge. The above definition supposes that the increase in sludge is proportional to the suspended solids load; whilst this may be a practical approximation with such wastes as domestic sewage, with other wastes having a higher proportion of the load in solution, the BOD loading would be a more accurate basis for calculation, as suggested by Haseltine. . A consideration of the mechanism of waste removal by activated sludge also shows the necessity of maintaining an ecological balance between food and microbial population-Le., between load and the amount of sludge-in the plant. Although the organic waste is rapidly removed by the floc, the subsequent oxidation and synthesis of the
50
Ecological Principles of Wastewater Treatment
transferred waste may take place over a longer period. For efficient operation, although oxidation of the waste may lag behind the removal, it must keep pace with it. Experience has shown that if the rate of removal exceeds that of oxidation, the sludge becomes difficult to settle and "bulking" results; in addition its powers of removing more waste become impaired. Work by the Water Pollution Research Laboratory, showed that by repeatedly aerating an activated sludge with sewage for half an hour and replacing the supernatant liquor, after settlement, with more sewage, the clarifying power of the sludge decreased rapidly with successive batches. When aerated for periods of an hour or more there was no loss of clarifying power. A further increase in the difference between the rate of removal and metabolism of the waste, however, results in a sludge having good settling properties although its powers of waste removal are still impaired. Waste load (lb BOD or 55 per day Amount of sludge in the system
This is probably the condition in the high rate activated sludge systems where rapid removal rates rather than high quality effluents are called for. In a conventional system, however, the floc should not be loaded with waste greatly in excess of that which it can oxidize in the aeration period provided. At a given flow this aeration period is largely predetermined by the tank capacity, but the sludge loading can be adjusted by changing the concentration of sludge in the system by controlled wastage over the excess sludge weir. It will be seen that these two ecological considerations expressed in practice as "sludge age" and" sludge loading" are determined by the same factors and are mathematically the reciprocals of each other. Thus the general usage of "sludge age" as an operational guide in plant operation is ecologically sound. Having accepted the principle of "sludge age" as a practical operational variable it becomes necessary to establish the optimum sludge age. In doing so it is necessary to consider both the activity of the floc and its settling properties. For optimum activity the flocs should be in the log phase of growth. A very young floc, however, although in the log phase, may be poorly flocculated and result in difficulties in separation from the purified liquor. On the other hand an aged floc, many of the bacteria in which may be in the endogenous phase, is less active. The optimum age will, therefore, lie somewhere between these extremes. The settleability of sludges is measured by the "sludge index"; two such indexes are in common use.
Ecological Principles of Wastewater Treatment
51
The sludge volume index (the Mohlman Index) is defined as the volume of sludge occupied by 19 (dry weight) of the sludge after 30 min quiescent settlement in a measuring cylinder. It is calculated thus: Settled Volume of Sludge percent after 30 min Slu d ge V0 1ume In d ex Suspended Solids percent For example, with a mixed liquor concentrate of 2,000 ppm solids, the sludge in which occupies 20 per cent of the volume after 30 min settlement, this index would be:
~=100
0.2 The same settling properties may also be expressed as the percentage of solids in the settled volume of sludge; this is known as the sludge density index (the Donaldson Index) and is given by: Percent, suspended solid mixed liquor 0 . I d Slu d ge DensIty n ex= xl 0 Percent, Volume Settled after 30 min For the example taken above this would be: 100x 0.2 =1 20 It will be noticed that:
100 Donaldson Index = - - - - - Mohlman Index Reported American operational experiences show that there is a functional relationship between the sludge index and sludge age or sludge loading. In a conventional plant 3-4 days would appear to be the optimum age, whilst in high rate processes the age should be 0.2 days in hot weather and 0.4 days in cold. Within both these ranges, the sludge indexes should be satisfactory, Le., the sludge volume index < 100. Between these ranges, however (0.5 days-3 days), a high volume index may result. With a sludge age above 5-6 days, although most of the sludge comprises small dense floes with a low index, some is present in the form of small floes, known a~ pin-point floes, which do not readily settle. Torpey and Chasick, on the basis of experience at several New York plants, found that the optimum age was 3-4 days for most conventional plants. With high rate processes a range of 0.2-0.5 day was found to be most satisfactory. Other workers have expressed the same relationship in terms of sludge loading. Haseltine concluded that the maximum BOD load should not exceed 0.5 1b per day per 1 lb of mixed liquor solids and suggests that in p~actice an average figure of 0.3-0.4 lb for larger plants and 0.2-0.3 lb for smaller plants having less attention.
52
Ecological Principles of Wastewater Treatment
Logan and Budd quoted corresponding figures of 0.22-0.3Slb as being necessary to ensure a sludge volume index of less than 100 in their pilot plant: above and below this range the sludge volume index increased. They also found that the optimum range decreased with increasing temperature and, therefore, more critical operational control was required at higher temperatures. They also produced evidence suggesting that there was a lag of 4-6 days between a specific loading and the resultant effect on the sludge index. Orford et al found, using laboratory scale plants, that a loading of 0.17 lb BOD per lb of volatile solids per day was optimum, above and below this figure the sludge volume index increased. In high rate activated sludge processes where use is made of the increased sludge density at low sludge ages (0.2-0.4days) Haseltine recommends that the corresponding BOD loading should not be less than 1 lb per day per 1 lb of sludge, to minimize the chance of bulking. Such high rate processes do not always produce a clarified effluent and Gould and others state that they may not be equally effective with all wastes especially those having a high proportion of waste in solution. As important as sludge loading is in relation to the settleability of the sludge, other factors may also cause bulking. Filamentous growths which are associated with a bulking condition, and are probably responsible for it, are encouraged by such environmental factors as acidity, low oxygen concentrations or nutrient unbalance. Again, sludge may rise in the settlement tanks due to the evolution of nitrogen gas liberated by the reduction of nitrates under anaerobic conditions. Having determined the range within which it is desirable to maintain the sludge age and sludge loading, it is now necessary to consider how these can be satisfactorily achieved in practice. For a given BOD load entering a conventional plant of fixed capacity, the sludge age can only be controlled by varying the aeration solids concentration. The maximum concentration of sludge in the aeration tanks of a conventional plant is limited by the speed at which it can be separated in the settling tanks and returned to the aeration tanks. As mentioned in the previous section on aeration, undue retention of the sludge in the settling tanks should normally be avoided. This retention time is determined by the amount of sludge entering the tanks, its settleability as measured by the sludge index, and the capacity of the pumps and hydraulics of the return sludge pipe work. For a given retention time the concentration of the returned sludge is proportional to the sludge index at a constant rate of pumping. A longer retention time will, other things being equal, result in a higher concentration of returned sludge. Thus the relationship between the sludge index and the concentration
Ecological Principles of Wastewater Treatment
53
of the returned sludge provides a useful guide to the retention time of the sludge in the settling tanks. Bloodgood considered that the percent concentration of returned sludge should never exceed the sludge density index if undue retention of the sludge were to be avoided. Thus for a sludge density index of 0.5 the return sludge concentration should not exceed 5,000 ppm. Later, the same worker reported that the percent concentration of returned sludge could be permitted to be 1.1 times the sludge density index, a concentration of 5,500 ppm being permissible in the example quoted above. In design therefore, not only should provision be made to accommodate the amount of sludge necessary to provide a satisfactory sludge age, but also for the expeditious removal and return of the sludge from the settling tanks. Such a system should be designed to operate under conditions of daily peak loadings and not daily average loadings, since most plants have to cope with daily fluctuations in strength and volume resulting not only in variations in the sludge index, but with the distribution of the sludge throughout the system. A plant producing a high quality effluent for 22 hours each day may daily produce an unsatisfactory effluent for the other two hours following peak flows, due to the discharge of sludge in the effluent. The permissible loading of a conventional plant is limited by the amount of sludge it can successfully carry in the aeration tanks without causing undue retention of the sludge in the settling tanks. Although the provision of a generous sludge return does not affect the average waste aeration period it inevitably increases the flow through the settling tanks. Haseltine, however, considered that, providing that the sludge is withdrawn from a point immediately below the tank inlet, this should not necessitate an increase in tank capacity. Many of the modified methods of operation are aimed at overcoming the above limitations of the settlement stage. By arranging for progressive mixing of the waste with the returned sludge throughout the aeration tank, it is possible to carry the necessary amount of sludge in the aeration tanks and yet ensure that the concentration entering the settling tanks is not excessive. In "step aeration" where the waste is introduced at stages along the aeration channel, a higher concentration of sludge is held under aeration at the inlet end than at the outlet end, thus reducing the concentration passing to the settling tanks. The higher the proportion of the load applied away from the inlet end, the greater this effect, the extreme case being where no waste is added to the first sections of the aeration tank; this corresponds to the practice of "sludge re-aeration" or "sludge reconditioning". The flexibility of step aeration, however, whereby the
54
Ecological Principles of Wastewater Treatment
proportion of waste applied at each stage can be varied to meet fluctuations in the load, is superior to the fixed capacity sludge reconditioning tanks. The practice of re-aeration, with reduced periods of 'mixed liquor retention times, is possible because of the fairly rapid uptake of the waste by the sludge and its continued metabolism in the re-aeration tanks. On this principle the time of retention of the waste liquor in the aeration tanks is not critical, providing that it is sufficient to permit the complete transfer of the waste to the sludge. A period of two hours should normally be adequate. Thus plant design based on the need to provide for the accommodation and recycling of the requisite microbial population, needed to balance the waste load to ensure optimum sludge conditions, is ecologically more sound than the more conventional design criteria involving the provision of tank capacity to ensure a given aeration contact time of the mixed liquor. Other systems have been developed to ensure a more uniform loading of the floes during their retention in the aeration tank. A high rate of sludge return itself tends to provide more uniform loading. In the Logan process some aeration tank effluent is returned to the inlet end of the tanks together with the returned sludge. The flow of returned mixed liquor is approximately 3-4 times that of the waste. Complete uniformity of loading is probably achieved in the complete mixing systems. Conditions in a conventional aeration tank, as opposed to an aeration channel, probably more nearly approach complete mixing of waste and sludge than is generally realized. An unconventional plant, described by Busch and Kalinske, was designed to meet the several ecological requirements discussed above. The processes of aeration and settlement are carried out in the one tank. The completely mixed waste and sludge are subjected to conditions of intense agitation in a central zone whilst quiescent settlement takes place in an upper outer zone, separated from the aeration zone by a hood. The sludge settling in the quiescent zone is rapidly carried back beneath the hood by the recirculating mixed liquor. In plant design also, provision should be made for 'the acceptance and treatment of the excess sludge in varying amounts according to the operational requirements of the aeration unit. The situation should never arise where the sludge concentration in the aeration unit cannot be reduced because of lack of capacity in the sludge treatment plant. Because of the sensitivity of activated sludge to fluctuations in load, all efforts should be made to control the discharge of strong liquors from the digestion plant to avoid increasing the normal fluctuations in load. In some cases separate treatment of such liquors could be practised with
Ecological Principles of Wastewater Treatment
55
advantage as in the Kraus process. In the foregoing discussion the efficiency of the activated sludge system has been considered in terms of carbonaceous oxidation. Activated sludge is capable of nitrification under suitable conditions of operation. Unfortunately it would appear that loading conditions needed for nitrification are not the optimum conditions for maintaining a satisfactory sludge. Nitrification is usually associated with a lightly loaded sludge, i.e., one having a high sludge age. Autotrophic nitrifying organisms have a relatively slow rate of multiplication and could hardly be expected to become established in a floc of low sludge age. Also, in competition with the more rapidly growing heterotrophic organisms, they would be less likely to succeed in a heavily loaded floc. Such underloaded sludges in which oxidation exceeds waste uptake do not produce clear effluents. Although the flocs are dense and settle well, large numbers of pin-point flocs do not settle and are discharged with the effluent. MICROCROPICAL EXAMINATION AS AN OPERATIONAL AID
In practice the autecological conditions within the plant can be checked by routine tests to determine the temperature, pH, dissolved oxygen concentration, etc. The synecological balance between load and microbial population can be maintained by adjustments based on frequent determinations of the flow and strength of the waste feed and the concentration of the sludge in the system. Sludge index determinations will act as a check on the resultant settleability. The return sludge concentration in relation to the sludge density index should prove a useful guide to the sludge retention time in the settling tanks, as described above. If the necessary capacity and operational flexibility has been provided for in the plant design, the operator using the results of these tests, should be able to maintain the sludge in a healthy efficient condition. Direct microscopical examination of the sludge will reveal to what extent he has succeeded. Frequent regular observations will indicate trends in sludge conditions and can be used as a supplementary guide in plant operation. It cannot be overstressed that the nature of the sludge and its component populations will depend upon the nature of the waste being treated. Even under otherwise identical operating conditions, plants treating different wastes will have sludges of different appearance and containing different organisms. Conversely plants treating the same waste under slightly different operating conditions may form sludges of markedly different natures. It follows therefore, that only by experience gained by continued acquaintanceship with the sludge in his plant and
56
Ecological Principles of Wastewater Treatment
by the application of general principles can an operator make full use of microscopical observations. Some criteria which have been found useful in assessing the condition of sludges treating sewage may be given as a guide as to what features to look for in routine observations. Such observations are best made on the sludge removed from the outlet end of the aeration tanks and with the minimum of delay after removal. The size, shape, structure of the individual flocs and the relative sizes of the different flocs are important features. Large tassel-like flocs although having fair settling qualities have been associated with a less efficient sludge than smaller flocs of more open texture. The flocs should be well-defined with no dispersed solids in the interstitial liquor. Examination of the floc margin under the high power of the microscope will reveal the degree of flocculation of the bacteria. In some cases the bacteria will be seen to be firmly embedded in the floc which then has a sharply defined margin. Under other conditions the bacteria are only loosely associated at the surface of the floc giving it a diffuse margin. Bacteria in such flocs are in a young active state and are capable of rapid oxidatio'1 of the waste. A slight increase in load, however, causes them to disperse into the interstitial liquid thus producing a turbid inferior effluent, even though the waste may have been completely removed. Filamentous growths such as Sphaerotilus and fungi, whether the causative organisms or not, are often associated with chronic bulking of sludges. Microscopical examination will reveal the appearance and development of such growths before serious bulking occurs and thus enable remedial action to be taken. Independent of any role they may play in the process, protozoa have been proved by experience to be of great value as indicator organisms. It is, however, dangerous to apply "rule of thumb" methods when using any such indicators, but with a knowledge of the dominant species present in his plant, an operator, working on the general principles of the " succession of dominant groups with increased purification, should find routine microscopical examination of the sludge of great value. Although Reynoldson found that there was a direct relationship between the numbers of vorticellids in the sludge and the quality of the effluent at Huddersfield, attempts to correlate the protozoa numbers with effluent quality at Birmingham showed that although such a relationship was established over periods of several weeks, no such correlation existed throughout the year; it was concluded that the nature of the protozoa community was more indicative than were specific numbers. As recently pointed out by McKinney and Gram, no simple
Ecological Principles of Wastewater Treatment
57
quantitative relationship could be expected; for example, a deCline in the free-swimming ciliates may indicate either decreased or increased efficiency. A stUdy of the whole community, however, would tell whether this decline was associated with an incr~ase in the flagellates or attached ciliates and thus would indicate the trend in efficiency. Experience at Coleshill (Birmingham) has shown that some protozoa have a greater indicator value than others and some exceptions to the generally accepted pattern have been observed. Although most species of Vorticella, an attached ciliate, occur in an efficient sludge together with Opercularia, Aspidisca and Lionotus, one species, V. microstoma, is more common at times when the effluent is inferior; it is also the dominant ciliate-usually the only one-in the partial activated sludge plants at other Birmingham works, its incidence there being more associated with the toxicity of the industrial sewages. Paramoecium caudatum, usually associated with a less efficient sludge, has at times been present in large numbers when the effluent was good, but its numbers fluctuate violently. A species of Arcelia, a rhizopod (a group quoted as indicative of inefficient sludges), has usually been found associated with the high-quality nitrified effluents. In quoting these exceptions, it is not intended to detract from the usefulness of protozoa as indicators, but merely to warn against over-rigid application of the general principle. The type of protozoan community is determined by the interaction of several factors such as the dissolved oxygen concentration, the concentration of organic matter and the bacterial flora and thus reflects the general conditions within the plant and not merely sludge efficiency. It should also be pointed out that the protozoan population is affected by the state of purification of the waste and, as in the case of a light or bulking sludge, the effluent may contain microbial masses, the presence of which results in inferior-quality effluents, this not being reflected by the protozoa. Of the other higher microorganisms occurring' in sludges, rotifers usually indicate better conditions than do the nematode worms.
Chapter 3
Ecology of Activated Sludge The environment of activated sludge can be regarded as an aquatic one. It is, however, unlike any natural aquatic habitat and although it has been colonized by numerous microorganisms, the constant agitation and recirculation of the sludge, make it inhospitable for aquatic macrofauna, which are rarely present. Bacteria, fungi, protozoa and the smaller metazoa such as rotifers and nematode worms are commonly found in activated sludges, though all may not be present in anyone sludge. Because of their need of light, algae, although they are introduced into the sludge with the sewage, rarely become established. Factors determining the dominant organisms in any sludge will be discussed later; first let us outline in :TIore detail the more frequently occurring organisms in each of the above groups. BACTERIA
Bacteria can be regarded as the basis of the activated-sludge floc, both structurally and functionally, and are universally present in the traditional activated sludge. Johnson was probably the first to report on the microorganisms in activated sludge and stated that "zooglea, assisted by other minute organisms chiefly of animal origin (protozoa) may be responsible for the rapid purification thus effected". Russel and Bartow isolated thirteen varieties of non-nitrifying bacteria from activated sludge, most of which belonged to the B. subtilis group of aerQbic spore-formers. The nitrifying bacteria Nitrosomonas and Nitrobacter were also isolated. These workers also demonstrated the importance of the non-nitrifying bacteria which they isolated in the purification of sewage. Buswell and Long as a result of microscopical examinations concluded that the sludge was composed of zoogleal masses intermixed with filamentous bacteria. Butterfield first isolated a zooglea-forming bacterium from activated sludge. When aerated in sterile sewage a pure culture of this organism produced floes similar to activated sludge and was found to be capable of removing a high percentage of the oxidizable material. This organism, identified as a variety of "Zooglea ramigera", was considered
Ecology of Activated Sludge
59
to be of importance in the process of purification; this was later confirmed by Butterfield and others. In an attempt to dete~mine whether one species of zoogleal organism was present or whether there were several species prevalent, Heukelekian and Littman examined zoogleal bacteria from 15 different sludges and concluded that they were sufficiently alike to be classed either as one species or as one genus and were also indistinguishable from the zoogleal bacterium, Zooglea ramigera, described by Butterfield. These bacteria were Gram-negative, non-sporing, motile, capsulated rods. When aerated in sterile sewage they rapidly oxidized carbohydrates and produced ammonia from gelatin, casein and peptone, producing wellorganized floes; however, no nitrification took place. Other bacteria isolated by Heukelekian and schulhoff were claimed to effect considerable clarification without producing a marked reduction in the oxygen consumption of the effluent when aerated with sewage. Generally, however, it became accepted that Z. ramigera, because of its ability to form flocs and to stabilize nutrient substrates, was the primary organism in activated sludge. Later work demonstrated that other organisms isolated from activated sludge were capable of floc formation when aerated in a suitable nutrient substrate. It has been suggested that all bacteria have, under certain environmental ~onditions, this ability to flocculate, this being determined by their relative surface-charges and energy-levels. Once the floc has started to form some bacteria die and lyse, the insoluble polysaccharides remain in the floc and entrap the less active bacteria. Wooldridge and Stand fast concluded that only a small proportidn of the bacteria in sludge was living. Bacteria, however, although rendered incapable of active growth, are still able to carry out some chemical activity by unimpaired enzyme systems. Bacteria entering the activated-sludge plant with the sewage are from two main sources, firstly those originally present in the water or in the infiltration water, and secondly, intestinal bacteria introduced with the faeces. Earlier workers found that the intestinal bacteria, particularly the Bact. coli and Bact. aerogenes group and the aerobic sporefbrming bacteriapredominated, and since many of these were found to be proteolytic, they concluded that the intestinal group of organisms played an important part in the purification of sewage. Allen, however, using an homogenizer to disintegrate the flocs, thereby isolating the bacteria within the floc and separating them from the -smaller number of bacteria in the interstitial liquid, found that the intestinal bacteria were present in negligible numbers. By this method the counts were increased from ten- to ·one
60
Ecology of Activated Sludge
hundred-fold counts of 2,200 million/ml being recorded. The majority of strains isolated were Gram-negative rods with no action on carbohydrates, though many had decidedly proteolytic characteristics. The majority were members of the genera Achromobacterium, Chromobacterium (Flavobacterium) and Pseudomonas. He concluded that, because of their temperature relations and general characters and the fact that Taylor had found that the majority of bacteria from lakes and streams were Gram-negative rods as opposed to the dominant Grampositive types in the soil, the bacteria which predominate in activated sludge are largely derived from water, intestinal forms being unimportant. The temperature of an activated-sludge plant would be expected to suit the aquatic types rather than the intestinal forms. Because of the different reported lists of species it is difficult to generalize on the dominant bacteria in activated sludge. Allen's list and that of the American workers, which included intestinal and non-intestinal forms have only Flavobacterium in common.
:~
',)1
C
"
"
'.
'
....
.
Fig:1. Some Microorganisms of Activated Sludge and Bacteria Beds: (A) Zooglea ramigera, (8) Sphaerotilus natans, (C) Phormidium sp., (D) Stigeoclonium, (E) Nematode Worm, (F) Rotifer The generally accepted Zooglea ramigera was not included in Allen's
list and it may be that it is not a true species but a growth form of various species. Allen's isolation technique by homogenization would be expected to reveal the dominant species, although on the other hand, the American workers demonstrated that their species were capable of floc formation.
Ecology of Activated Sludge
61
Apart from the zooglea-forming bacteria mentioned above, filamentous bacteria are also found. Because of their association with the condition known as bulking when the sludge becomes difficult to settle, much attention has been paid to these growths. Unfortunately the identification of the organisms is in some cases open to question. However, Sphaerotilus natans is probably the most common filamentous bacterium in activated sludge. Lackey and Wattie isolated fourteen strains of Sphaerotilus from different sources and in culture these behaved so similarly that they concluded they were all Sphaerotilus natans Kutzing and that this was capable of variation according to environment. This view is supported by the more recent report by Pringsheim that the different filamentous forms previously known as Sphaerotilus, Leptothrix and Cladothrix are in fact different growth forms of one identical organism. In view of this it is probable that Sphaerotilus natans is the most common filamentous bacterial form in activated sludge. Other forms however, besides those now known to be Sphaerotilus, have been found. Lackey and Wattie in their work on Sphaerotilus frequently isolated a similar form which they tentatively identified as Bacillus mycoides and which had different cultural characteristics. Ruchhoft and Watkins isolated filamentous bacteria from activated sludge. They described it as consisting of disc-shaped cells 2-31 each in diameter and 2-41 long, lying within a "barely perceptible sheath in straight unbranched chains 1000-5,0001 long. It differed from Sphaerotilus in showing no branching and did not produce conidia. Smit also concluded that other filamentous growths found in bulking activated sludge did not agree with the description of Sphaerotilus. Besides these unidentified forms, Crenothrix and Beggiatoa have also been reported. FUNGI
Although common in bacteria beds, fungi are relatively rare in activated sludge; at least, reference to the literature reveals few reports of their presence. Smit in studies on filomentous bacterial growths in activated sludge also found a fungus identified as a species of the genus Geotrichoides, which he named G. paludosus. He considered however that this was not the cause of the bulking of the sludge. When present, fungi may dominate the sludge under abnormal circumstances. In work on the oxidation of lactose by activated sludge the dominant organism of the sludge was found by Tomlinson to be the fungus Pullularia pullulans. In the same investigations Tomlinson also isolated species of the following fungi from the activated sludge: Phoma, Oospora and Sporotrichum. The
Ecology of Acti~ated Sludge
62
activated sludge in a pre-treatment plant at Yardley, Birmingham, is frequently dominated by growths of Oospora (Geotrichum) sometimes to the exclusion qt the bacterial floc.
PROTOZOA The presence of protozoa in activated sludge has always aroused much interest and conjecture as to their role in the process. Johnson was, again, probably first to report their presence; since then numerous workers have listed different genera from plants operating under different conditions: Richards and Sawyer, Buswell and Long, Kolkwi~z, Ardern and Lockett, Agersborg and Hatfield, Taylor and Barker. METAZOA Of the higher forms of life rotifers and nematode worms are occasionally found and at times they may become so abundant as to be considered a factor in the ecological system. In few plants, however, can they be regarded as permanent members of the community. Other higher forms are of even rarer occurrence, Cyclops, the worm Aelosoma and chironornid larvae of the Thummi group being reported from isolated plants. A
B
r.: ':
tt)K
Fig:2. Some Protozoa Common in Activated Sludge and Bacteria Beds: (A) Vorticella sp., (B) Vorticella microstoma, (C) Paramoecium candatum, (D) Bodo candatus, (E) Opercularia sp., (F) Lionotus fasciola, (G) Ampbileptus sp., (H) Amoeba limax, (I) Arcella vulgaris (surface Sand Side Views), G) Colpidium colpoda, (K) Aspidisca polystyla.
63
Ecology of Activated Sludge INTERRElATIONSHIPS IN THE SLUDGE COMMUNITY AND THEIR ENVIRONMENT
In waste treatment involving the breakdown of organic matter the saprobic forms are the primary feeders and primary agents of purification, although, as we shall see, holozoic animals also play an important secondary role. In the activated sludge and in the film of bacteria beds heterotrophic bacteria, saprophytic fungi and saprobic protozoa are the primary feeders occupying the basic trophic level. Holozoic protozoa occupy successively higher levels, the apex possibly being represented by the nematodes and rotifers. Of the three classes of protozoa represented in the activated sludge the rhizopoda engulf food particles within the pseudopodia by which they move. Soluble foods may also be absorbed, however, so they must be considered as both holozoic and saprophytic. Table:1. Some Species of Protozoa Commonly Recorded in Activated Sludge Rhizipoda Move and ingest food
Flagellata Move by flagella
by
pseudopodia Amoebasp. Amoeba adinophora Arceila vu!garis Actinophrys sp. Vahlkampfia limax V. guttula
Bodo caudatus Cercobodo longicauda Monas sp. Oikomonas termo Euglena sp. Cercomonas sp. Pleuromonas jaculans Anthophysa vegetans Peranema sp.
Ciliophora move by cilia (hair-likeprocesses) FreeCrawling Stalked swimming on sludge floc Paramoecium caudatum Paramoecium sp. Colpidium copoda Amphileptus sp.
Chilodon sp.
Aspidisca sp. Euplotes sp. Oxytricha fallax Stylonychia sp. Lionotus fasciola
Achineta sp. Podophrya fixa Vorticella sp. OpeTcuiaria sp. Epistylis plicatilis Carchesium sp.
Trichoda pura Loxophyllum sp.
The flagellates which move by one or more whip-like flagella may either be autotrophic (the pigment-bearing phytoflagellates, e.g., Euglena), saprobic, e.g., Cercobodo, or holozoic, e.g., Oikomonas. The ciliophora, which move by fine hair-like processes (cilia), are represented in activated sludge
64
Ecology of Activated Sludge
mostly by holozoic species although, again, some of these may also be capable of saprozoic nutrition. Factors determining the population of any species are the intrinsic rate of increase, the availability of food in competition with other species on the same trophic level and the predatory effect of larger organisms. Apart from these "biotic" factors the environment of an organism is also affected by physical and chemical factors. In activated sludge the availability of oxygen, the pH, temperature, inhibitory agents, either toxic or antibiotic, and the physical turbulence of the process are probably the chief factors to be considered. Many different species are introduced with the sewage into the activated sludge; many find the environment unsuitable and either die out or fail to increase; others, suited to the environment, persist. Of those on the same trophic level, competing for the same food, one becomes dominant, depending upon the relative rates of increase. According to "Gause's theorem" this situation should lead to the elimination of the other competing species, but this does not happen in activated sludge probably because the changing conditions, as the sludge passes through the system, successively favour different species, and the constant introduction of a mixed flora maintains the competition for food. The food of the primary feeders, the saprobic types, is the organic matter in the sewage. Thus many bacteria and fungi and the saprobic protozoa are in direct competition for this basic food supply. Others with different organic food requirements such as the autotrophic bacteria requiring less-complex nitrogen sources, are not in competition with these for food although in the same environment; they may, however, be competing for oxygen if that is limiting. The holozoic forms are predatory on the saprobic forms or on other holozoic forms and are therefore in the secondary or higher trophic levels. Competition may exist between these if they are dependent upon a common prey. There is also the relationship between predator and prey to be considered. The popUlation of the predator species is determined9 by the numbers of prey, an increase in the popUlation of the prey is followed by an increase in the predator population which results in a decrease in the prey and in turn in a decrease in predator; thus fluctuating populations occur. This system is influenced by the extent to which the prey can seek refuge from the predator; in activated sludge the floc is probably of ecological significance in this respect. Holozoic forms by their predatory nature actively control the population in lower trophic levels, but the control effected by the saprobic organisms as prey, is a passive one.
Ecology of Activated Sludge
65
Activated sludge may, then, be regarded as a complex ecological system, the organisms of which exist at different trophic levels, in each of which competition for common foods exists and between which there is a series of predator-prey relationships. Thus different populations exist, some dependent on, and some independent of, each other. Superimposed on these biotic forces are the physical and chemical factors of the environment mentioned before. These factors operate on the balanced community and their effects are not readily measurable in terms of their isolated effects on the individual members of the community as determined in pure-culture work; their differential effect on the different organisms is sometimes more important than their direct effect. In any such system the dominant organism of those in competition at any trophic level for a common food will be that which, under the conditions prevailing, is able to multiply most rapidly on the available food; this is largely determined by the relative size of the organisms and by their metabolic rate. It has, however, been suggested that in assessing the importance of species population in a community, biomass is more useful than numbers and that the sum metabolic activity of the population is of even greater value. In a study of the role of organisms in the process of sewage purification, the sum of the metabolic activities of the different populations would be of greater value than would numbers or biomass, but unfortunately most of the ecological work up to date on sewage plants has been on populations. On the basis of these general principles we shall now examine the findings of different workers on activated-sludge ecology. FACTORS DETERMINING THE CHARACTER
AND DOMINANT ORGANISMS OF A SLUDGE Although algae, bacteria, fungi and protozoa are introduced into the activated sludge, in the majority of cases investigated, bacteria become dominant as primary feeders on the organic waste, different holozoic protozoa occupying the secondary trophic level with possibly rotifers and nematode worms at higher levels in the food chain. Algae, because of their need of light, are rarely present; fungi may predominate as primary feeders under abnormal circumstances to be discussed later. Firstly the bacterial and protozoan populations will be considered. The dominant bacteria of the sludge must satisfy two conditions: they must be able to utilize the organic waste and also be capable of readily forming flocs to facilitate separation from the effluent, and thereby to ensure their retention in the system. The American workers demonstrated
66
Ecology of Activated Sludge
these capabilities for the species that they isolated from sludges treating sewage. The oxidation of strong non-toxic organic wastes by non-flocculent growths, involving the aeration of a soil suspension of the organisms with the waste, has been demonstrated by Heukelekian. No separation of the organisms by settlement is attempted and the effluents although greatly reduced in strength are turbid. This process is being developed as a pretreatment process for organic wastes, but for the purpose of this book is not included as an activated-sludge process. For the successive stages in the complete oxidation and mineralization of complex organic wastes in sewage a number of different bacteria, not in direct competition, would be present. From reports it would appear that one or few heterotrophic bacteria are involved in the initial stages of sewage purification, the autotrophic bacteria Nitrosomonas and Nitrobacter completing the process. These later stages have not been closely studied in the USA, probably owing to lack of interest in that country in taking purification to the stage of nitrification. Unlike the bacteria bed, where these successive stages are carried out by the respective organisms developed at different levels in the bed, all the bacteria in the activated-sludge system usually occupy the same physical niche, i.e., the floc, and, although not in direct competition for food, probably compete for oxygen. Nitrifying organisms are the most sensitive to inadequate aeration and it may be that the greater difficulty in obtaining a nitrified effluent with activated sludge than with the bacteria bed is due to this ecological difference in the two systems. The dominant bacteria will be determined largely by the nature of the waste being treated. Engelbrecht and McKinney found, by developing sludges on a range of organic compounds, that sludges developed on structurally related compounds have similar morphological appearances and produce similar biochemical changes, whilst those developed from compounds morphologically different were structurally different. The pentose sugars, xylose and arabinose produce similar dense flocs, but a floc dominated by large filamentous types is developed on the hexose sugars, glucose and fructose. Because of their assocation with bulking sludge, much attention has been paid to factors encouraging the filamentous bacteria such as Sphaerotilus. Reports of conditions under which Sphaerotilus will develop are somewhat at variance, especially in relation to its oxygen requirements. That certain compounds especially carbohydrates, encourage its developement is well-known; in streams its growth is stimulated by trade-effluent discharges from the manufacture of beet sugar, paper, rayon, glue and flour, as well as textile bleach, coke by-products, dairy wastes and spent sulphite liquors. Available carbohydrates are not of frequent occurrence in sewages and the frequency of Sphaerotilus growths would
Ecology of Activated Sludge
67
suggest that other nutrients or other causes are responsible. Apart from this nutritional effect the degree of availability of oxygen may also be important. Several workers have associated the presence of Sphaerotilus with inadequate aeration and although some workers consider it an obligate aerobe like the zoogleal bacteria, others describe it as being able to withstand a considerable degree of deoxygenation. Ingold concluded that it was a facultative anaerobe and that it grew more rapidly as the oxygen was depleted. In polluted streams, although this organism is rare in anaerobic conditions, profuse growths are found in water low in oxygen as well as in well-aerated, organically enriched waters. It would appear therefore that although Sphaerotilus may grow better at higher oxygen concentrations as reported, some strains at least are able to withstand fairly low oxygen concentrations- probably more so than the competing zoogleal forms-in which case the growth of filamentous forms growing in competition with zoogleal bacteria would be encouraged by lower oxygen concentrations. The relation between these filamentous growths and bulking is discussed later. The nature of the bacterial flora of primary feeders is then determined chiefly by the nature of the food, i.e., the organic waste, and secondly by the conditions within the plant, chiefly the degree of aeration. Allen found that a succession of dominant bacteria occurred during the development of an efficient sludge, the predominant flora changing from a nonproteolytic to a proteolytic one. This he explained by suggesting that by aerating sewage, in which the carbohydrate and protein content is relatively small, bacteria suited to such dilute fluids would first develop producing the floc; as this builds up, sufficient protein becomes available in the floc to encourage the proteolytic forms; carbohydrates, however, are still sparse and hence the absence of the saccharolytic species. Similar successions have been observed for the protozoa and these may also be explained for the most part, although not entirely, by nutritional changes. The general succession in which a fauna dominated by rhizopods and flagellates is replaced first by the free-swimming ciliates and later by the attached peritrichous forms as the sludge becomes more efficient. It must be stressed, however, that this is a general picture and that there are several exceptions, some species of Amoeba and Arcella in the rhizopoda for example, are more often associated with a more efficient sludge. Availability of requisite food, oxygen requirements, relative energy requirements and habit (i.e., whether free-swimming, crawlers, or attached) are all factors to be considered in accounting for such successions. Unfortunately our knowledge of such factors, especially nutritional, is far from complete. Initially, however, in the sewage, from which the
Ecology of Activated Sludge
68
sludge is developed, both soluble and suspended particulate organic matter is present; the primary feeders, heterotrophic bacteria and saprobic protozoa, mostly rhizopods and flagellates, will compete for food. As the bacteria increase in numbers secondary feeders become established. The holozoic flagellates will appear first because of their lower energy requirements. Later these are replaced by the holozoic ciliates with a more efficient feeding mechanism. Within the ciliates themselves there is a succession of species; the attached peritrichous forms having a lower energy level, replace a dominance of free-swimming forms as their common food becomes limited with increased purification. -Heterotrophic
Dispersed
+
Autotrophic-
-t---
Floqculated
rJJ
rJJ
'E
.!!!
~ 'u
.~
.~
en c:
E rJJ Ql Qliij Ql=
CJ)
Lto
2
'u
en
Q)
"0 Q)
~
J::
0
«
e
0
ro
:::::
Increasing Purification Efficiency
Fig:3. Hypothetical Curves Showing Successions of Dominant Protozoa in Relation to the Degree of Purification of Organic Waste and Bacterial Population
Ecology of Activated Sludge
69
Because of food preferences different species of holozoic ciliates would be expected to occur in association with the succession of domi nant bacteria. Gray found that the ciliate fauna of a Cambridgeshire chalk stream was determined by the bacterial flora, Paramoecium and Colpoda being associated with the abundance of Gram-negative rods. Further stages of purification, not usually achieved in the plant, would result in the development of autotrophic algae, such as diatoms, and these in turn would support the larger species of holozoic ciliates. This succession, although primarily determined by nutritional requirements, is also affected by the degree of tolerance to oxygen deficiency, the saprobic species generally being more tolerant. The degree of flocculation of the bacteria may also be important. In early stages of sludge development the bacteria are dispersed in the liquid and this encourages the truly free-swimming forms; as the bacteria become flocculated however, the attached forms and those which browse on the flocs become dominant. In some sludges a third trophic level is represented by rotifers and nematodes which feed on the holozoic protozoa, although some possibly may feed on bacteria and other primary feeders. These are usually associated with higher degrees of purification. The occasional invasion of the sludge by chironomid larvae must be considered as the introduction of such a trophic level, although in practice it is reported that they destroy the effectiveness of the sludge floc. Because of the recirculation of the sludge such successions do not occur in the activated-sludge process, but the protozoan fauna can be considered as being determined by the average stage of purification in the plant, being affected both by the strength of the incoming sewage and by the quality of the effluent. The presence of certain protozoa in an efficient sludge does not necessarily prove that they play an important role in the purification process; they may merely reflect the satisfactory conditions prevailing. The relative roles of bacteria and protozoa in the process have been variously assessed. Butterfield and Wattie found that efficient purification could be achieved by protozoa-free zooglea; on the other hand, Pillai and Subrahmanyan reported that pure cultures of the ciliate Epistylis were capable of effective purification and considered bacteria of secondary importance. To play the primary role of purification, however, the protozoa concerned must act as primary feeders in the ecological system. Although, as outlined, some saprobic protozoa, especially the rhizopods and flagellates, do compete with bacteria at this level, the bacteria are nearly always dominant and in an efficient sludge such protozoa are rare. It has been suggested that holozoic ciliates normally
Ecology of Activated Sludge
70
feeding on bacteria may also be capable of saprophytic nutrition. Even if this is so it is doubtful whether such facultative saprophytes could compete with the obligate saprophytic bacteria and it is therefore reasonable to conclude that the primary agents of purification are the bacteria. The relative importance of the protozoa, however, even if a secondary role, is difficult to establish. Several workers have demonstrated that different protozoa are capable of agglutinating bacteria: • Epistylis and to less extent Vorticella. • • •
Balantiophorus minutas. Oikomonas termo. Paramoecium caudatum.
Sugden and Lloyd also demonstrated the ability of the ciliate Carchesium to clarify turbid waters. The extent to which this capacity is effective in the activated-sludge process is difficult to assess. Jenkins considered that although flocculation was important in purification, it was not dependent upon protozoa. By suppressing the protozoa in an activated sludge several workers attempted to assess their importance. In most cases the reduction in protozoa coincided with a more turbid effluent of high BOD but these results may have been brought about by the effect of the suppressant on the bacterial floc. The resultant improvement in efficiency of a bacterial sludge after adding protozoa is, however, more positive evidence. Butterfield added the ciliate Colpidium to pure cultures of Zooglea ramigera and this resulted in a more efficient system. More recently McKinney and Gram in experiments designed to demonstrate competition and -I~redator-prey relationships in activated sludge, found that although pure culture~ of bacteria formed typical flocs in nutrient solutions, some active bacteria remained free, producing turbidity and contributing to the BOD of the effluent. When holophytic flagellates were added to such cultures they rapidly died off in competition with the bacteria although they were able to hve on the nutrient alone. On adding the holozoic ciliates Tetrqhymena and Glaucoma scintillans, however, these rapidly increased in numbers feeding on the free bacteria; because of the refuge of the floc, however, the bacteria were not eliminated and a balance was established. The resultant effluent was less turbid and the BOD was reduced. It would thus appear that protozoa can play an important role in the production of highly clarified effluents. Apart from enhancing purification by flocculation holozoic protozoa may also act as population stimulators. In culture work it has been shown that bacterial activity may be increased by the predatory activity of protozoa: Cutler and Bal showed that rate of nitrogen fixation by the
Ecology of Activated Sludge
71
bacteria Azotobacter was increased by the presence of protozoa; Meiklejohn showed a similar effect by the ciliate Colpidium on the breakdown of proteins to ammonia. By their predatory activity it was 'assumed that they maintained the bacterial culture in the active log-phase of growth. Recently, however, McKinney distinguished between the log-phase, in which the total metabolic activity and the synthesis of microbial material increased rapidly, and the following phase of declining growth, leading to a phase in which oxidation of the waste is continued, to produce energy for life of the organisms, but in which no synthesis ot microbial material takes place. At first sight ~t would appear that the more active log-phase should be maintained within the plant, but although oxidation is more rapid, little flocculation of the bacteria occurs and a higher proportion of the waste is converted into sludge which requires further treatment. In the later stage, although purification is slower, excellent flocculation occurs and no sludge accumulates, but because all the waste is oxidized and none synthesized as microbial material, more oxygen is reqUired and the time of retention, therefore, has to be increased. To what extent these different phases of growth found in cultures can be applied to an activatedsludge plant, is open to question; McKinney, however, considered that most plants operate between the declining growth and the later 'endogenous' phase. We now turn to the fungi and their occurrence in activated sludge plant. The saprophytic fungi are in competition with the other primary feeders, chiefly the bacteria, and since with domestic sewage the conditions appear to favour bacteria, the fungi do not become established in the sludge. Under exceptional circumstances, however, usually associated with the treatment of trade wastes, fungi may become dominant. Some factors in industrial wastes causing this have been enumerated by McKinney as: • Low oxygen caused by excessive organic loading or underaeration and resulting in acid conditions from incomplete oxidation. • Low pH which usually favours fungal growth. • Low nitrogen, fungi requiring less nitrogen per unit mass of protoplasm than bacteria. , At the Yardley Works the occurrence of ospora is associated with acid flushes of trade wastes. In laboratory experiments at Birmingham on the treatment of phenolic wastes Harkness found that a non-filamentous sludge was developed, but an accidental introduction of an acid sample of the waste resulted in filamentous growths of ospora developing within two days.
72
Ecology of Activated Sludge
Of ecological interest is the recent report of the effect of a predatory fungus, Zoophagusinsidians, which invaded several laboratory activatedsludge units. The decreased efficiency of the sludge in removing nitriles was attributed to the predatory activity of the fungus in limiting the numbers of the rotifers which, it was considered, were the principal predators controlling the bacterial population. Besides probably causing bulking it has been shown that the 'economic coefficients of cell synthesis', determined experimentally as the ratio of dry weight of growth to the corresponding weight of glucose destroyed, was much lower for two zoogleal bacteria than for four species of fungi examined. Thus at the same efficiency more sludge would be produced when fungi became the dominant primary feeder of the sludge. BULKING OF ACTIVATED SLUDGE
The condition known as bulking occurs when the sludge becomes difficult to settle and this usually results in an inferior effluent due to the amount of sludge which it contains. This may result from a number of causes, but is usually associated with the development of filamentous growths of bacteria such as Sphaerotilus or fungi. Most workers consider that such filamentous organisms are the causative organisms. Under laboratory conditions a pure bacterial ,- culture sludge could not be induced to bulk even when conditions favoured bulking. Ruchhoft and Kachmar however cons.idered that even whefi Sphaerotilus was present at times of bulking it was not the primary cause although its presence accentuated the condition. Bulking may occur in the absence of bacterial filamentous growths; Pillai and Subrahmanyan reported that it could be brought about by the death, and subsequent bacterial attack on colonies, of Epistylis (an attached ciliate) when aeration was inadequate. At Birmingham the fungus Geotrichum is considered to be the responsible organism for bulking in a partial activated-sludge plant treating industrial sewage. It should be noted, however, that a bulking sludge although difficult to settle may by quite efficient in purifying the waste, probably because of the open nature of the floc. Whatever the mechanism of bulking, the result can be described as a biophysical response to an upset of the ecological balance. Oxygen, food supply and toxicity are probably the chief factors which cause such upsets to occur. It is necessary to distinguish between acute bulking, brought on suddenly by toxic discharges, and chronic bulking which results from adverse conditions within the plant.
Chapter 4
Th~ Relevant Aspects of Biology To the non-biologist, the naming and classification of organisms is the most difficult aspect of biology and to many it is probably a deterrent to further study of the subject. A universal system of scientific nomenclature is essential for the interchange of information on organisms and the specific identification of organisms is desirable. Nevertheless the identification of species in many groups is a task for the specialist on that group of organisms and is often outside the scope of the general biologist, let alone the chemist or engineer! With experience, however, it is possible to become acquainted with the more commonly occurring species in plants, the numbers of species being limited by the specialized nature of the habitat. The inability to name the species present in a plant should not, however, prevent one understanding the factors influencing their different activities in the process of treatment. It is usual to refer to organisms by two names: firstly, the generic name denoting the genus and secondly, the specific name by which it is distinguished from other species of the genus. In script it is conventional to underline generic and specific names and in print they appear in italics. Thus the common bacteria bed fly is Psychoda alternata and its less frequently occurring relative, Psychoda severini. In cases where specific identification is not possible it is usual to refer to the generic name only-Psychoda sp. or Psychoda spp. if,referring to more than one species. Different species cannot interbreed to produce fertile offspring but may differ only in some small detail of structure. Such closely related species are grouped together into a genus, and similar genera into families and then through orders, classes, phyla into kingdoms, each grouping thus successively containing a greater diversity of organisms. The classification of organisms is based on structure ana. is in no way ecological. Thus species of the same genus may occupy entirely different ecological niches, which they share with species from widely different
74
The Relevant Aspects of Biology
genera. It would appear that in evolution the different species of a genus have radiated to fill different available niches. The many forms of life inhabiting the earth today are considered to have been evolved from common stock and represent the more successful lines of evolution, many of the ancestral stock and less successful lines having died out. Although this selective elimination has left us with fairly well defined groups of organisms, making classification possible, the grouping is necessarily arbitrary and it is not surprising, therefore, that different classifications are found. Although not conventional, it is for our purpose convenient, to recognize three kingdoms of living organisms, plant, animal and a third kingdom which includes the fungi and bacteria. Structurally and functionally the basic unit of most organisms is the cell which is usually of microscopic size. This term was first applied to plant tissues where the cells are separated by a cellulose wall which is not found in animal tissues. Cells may be defined as unit masses of protoplasm contained within a limiting membrane and, in the case of plants, within a more rigid cell wall. Protoplasm may be regarded as a colloidal solution of proteins, lipoids and other substances which together possess the properties of life. The protoplasm of a cell is differentiated into cytoplasm, the outermost portion of which forms the limiting membrane-the cytoplasmic membrane, and a more specialized body-the nucleuswhich governs the activity of the cytoplasm. The cytoplasm may also contain other granules and vacuoles or cavities containing food reserves. The cell membranes are not only of importance structurally in determining the shape of the cell but are functionally important in that they act as selective permeable membranes which, as we shall see, are important in maintaining the water content of cells and in the selective absorption of suitable food material. The nucleus contains a specific number of thread-like structures-the chromosomes-the number and shape of which are characteristic of the species, a feature which can be used in specific identification. On the chromosomes are carried the genes which may be regarded as the hereditary blue-prints for the species. Growth or multiplication occurs by the division of one cell into two followed by the increase in size of each daughter cell. This division is preceded by a complicated division of the nucleus in which each chromosome splits lengthways, one half of each passing into the resultant daughter nuclei, thus ensuring a similar genetic constitution in the two daughter cells. The chromosomes, which are only evident at times of cell division, occur in pairs of similar shape and size, one of each
75
The Relevant Aspects of Biology
pair being contributed by each parent. In the production of sex cells (gametes), the chromosome numbers are halved, one from each pair passing to each daughter nucleus in the two resultant cells. Some Typical Cell animai cell
Fig:1. Examples of Plant and Animal Cells.
Thus, when the gametes from the two parents fuse (fertilization), the normal chromosome number is again re-established in the resultant cell (the zygote). Furthermore the segregation of the paired chromosomes in the parents and the chance regrouping, provides the possibility of infinite variation in the character of the offspring within the general plan of the species. In the case of multicellular organisms the fertilized egg, by a succession of divisions, gives rise to a new individual, the cells becoming differentially modified for their several functions. PLANT KINGDOM
Phylum Thallophyta This is the most primitive of the four phyla into which the plant kingdom is divided. Plants of this phylum show no differentiation into root, stem and leaves as do higher forms, their vegetative structure is known as a thallus. Botanists include both algae and fungi in this phylum, but as mentioned above we shall consider the fungi along with bacteria as a third kingdom separated from both animals and plants.
The Relevant Aspects of Biology
76 Algae
Algae are a diverse group of primitive plants which contain pigments, the possession of which, in common with other plants as opposed to animals, enables them to synthesize their own food material from carbon dioxide and water in the presence of light. The pigment may be diffused throughout the cytoplasm but more often is concentrated in special bodies-the chloroplasts. The shapes of the chloroplasts are sometimes a diagnostic feature of the species. Some algae, e.g., Chlamydomonas are one-celled and of microscopic size; they are free swimming and when present in sufficient numbers impart a green colour to the water. Other algae may, in their life cycles, form similar one-celled stages although one order (Conjugales) which includes the filamentous forms such as Spirogyra and the beautiful bilaterally symmetrical Desmids, is characterized by the absence of motile reproductive cells. Elaboration of the single celled type has resulted in colonial forms of definite numbers in rows e.g., Scenedesmus, plates, e.g., Pediastrum or spheres e.g., Gonium. A more common line of evolution, however, has given rows of cells of indefinite length producing filamentous growths either unbranched e.g. Ulothrix or branched e.g., Stigeoclonium. Further elaboration has led to flat or tubular ribbons (e.g., Monostroma, Enteromorpha) similar to the marine algae-the seaweeds. The classifiaction of the algae is based largely on the type of pigment they contain. Three classes are represented in biological treatment plants. Class Cyanophyceae (Myxophyceae) (blue-green algae) is a well defined class, the members differing markedly from other algae in possessing a blue-green pigmentation, their production of mucilage, often in the form of sheaths, and the absence of well defined reproductive cells. Some-the Oscillatoriaceae-exhibit gliding or oscillating movements; of these Phormidium is of common occurrence on the surface of bacteria beds, where it may form thick olive-green sheets of growth preventing the free passage of the liquid into the bed. These sheets are formed from the coalescence of the adjacent sheaths of the filaments.
Green Algae
Red Algae
Brown Algae
Fig:2. Some Different Forms of Algae.
The Relevant Aspects of Biology
77
Class Chlorophyceae
Here, the members contain the pigment chlorophyll identical with that found in higher plants. These are the most common fresh-water class of algae and are represented in waste treatment plants by unicellular forms which may be free swimming in the liquid or form ill-defined colonies on surfaces Thicker growths are produced on the surface of bacteria beds by fila. mentous forms such as Ulothrix or Stigeoclonium. The foliaceous Monostroma may form concentric rings of growth between the jet lines on circular beds with rotating distributors. Class Bacillariaceae
These organisms are unicellular although sometimes aggregated into colonies. They are characterized by possessing cell walls of silica - known as frustules-which are composed of two distinct halves, the one half fitting over the other in the manner of lid and box. Diatoms are usually brown and the frustules are beautifully sculptured; the shape and sculpturing of the frustules are the basis of classification.
Phylum Bryophyta This phylum includes the mosses and liverworts, both moisture loving groups. Although primitive, they show an advance on the algae in the greater elaboration of the thallus and the possession of characteristic sex organs; reproduction, however, as in the algae is by means of spores as opposed to the seeds of higher plants. The liverworts form flat lobed sheet-like growths in moist habitats such as on the surface of bacteria beds e.g., Marchantia. The better known mosses which usually have well defined leaves arranged spirally on a stem are also a common feature of the surface growths of some beds. The other two phyla of the plant kingdom, the Pteridophyta-the ferns and the Spermaphyta -the seed bearing plants having cones or flowers, need not concern us here. BACTERIA AND FUNGI KINGDOM
These are best considered as a separate kingdom, no true position for them being found in either the plant or animal kingdoms. They are essential for the continuation of all life in maintaining the recycling of such essential materials as nitrogen and carbon. In this respect they are the most important group in a study of wastewater treatment which may be regarded as a part of these natural cycles. It is unfortunate that the popular conception of bacteria is associated with disease. Although some are responsible for certain diseases, most
The Relevant Aspects of Biology
78
are beneficial in bringing about the decomposition of organic matter upon which the fertility of the soil depends. Fungi, on the other hand, popularly known by the fructifications of certain species, such as toadstools, growing in the decaying vegetation in woodlands, are more associated with the processes of decay. Bacteria-(Schizomycetes) (Fission Fungi)
The classification of the bacteria is based on the morphology of the cell although their biochemical activities are often used in their identification. Several classifications have been proposed. Certain bacteria are readily stained by the para-rosaniline dyes such as methyl-violet or gentian violet and can only be decolourized with considerable difficulty. Other species, however, do not readily take these stains and are readily decolourized. Using this distinction, a Dane by the name of Gram divided bacteria roughly into two groups: Gram-positive-in which the dye is retained against solvent. Gram-negative-in which the dye is not readily taken up and is easily lysed out by solvent. In identification of bacteria this is a most important feature.
,
(]
(a)
Ell
G::) (b)
.an J111H 11 II I I.LLLO'uU a; p;;u;qlI1 (d) 1....-....1-_'---'----''--...' '50~ (O.05m m)
Fig:3. Forms of Bacteria.
(a) Coccus (b) Bacillus
(c) Spirillum (d) Filament
The Relevant Aspects of Biology
79
The most common of these are the Bacterium coli (Bact. coli) group (B. according to bacteriological terminology applying to Bacillus). Bact. coli is also known as Escherichia coli (E. coli). Other important morphological features are their size and shape; three main forms of increasing size can be recognized-coccus (spherical), bacillus (rod shaped) and spirillum (spiral). Although individual bacteria are minute (microscopic) in size, being measured in microns (i = 0.001 mm.), some form colonies of visible (macroscopic) size, e.g., the gelatinous or zoogleal colonies of Zooglea ramigera and the filaments of Sphaerotilus which may produce white plumose growths in flowing waters. Multiplication of bacteria is by binary fission-a simple division of one cell into two. Some are capable of producing highly resistant spores which enable them to survive adverse conditions-a further diagnostic feature. A distinction should be made between those bacteria in the treatment plant which play an active role in the breakdown of organic matter and which are the basis of the "film" in bacteria beds and the "floc" in activated sludge and those which enter the wastewater from the intestines of man and other animals. These are nonsporing Gram-negative rods which are normal inhabitants of the digestive tract of man and many other animals and are discharged with the faeces. Although not dangerous themselves, their presence in water does indicate faecal contamination and the possible presence of other disease producing bacteria such as Salmonella typhi (Bacterium typhosa), Salmonella paratyphi, the causitive organisms of typhoid fevers, other species of Salmonella which produce food poisoning and Shigella, one of the organisms responsible for dysentery. These disease producing organisms are also discharged from patients or carriers, but except at times of epidemic, are less frequent and more difficult to isolate. Fungi
Like the bacteria they have simple cells devoid of chlorophyll but these are typically larger than those of bacteria and, with the exception of the yeasts, they occur in branched filaments, either septate or nonseptate. The filaments are known as "hyphae" and the interwoven mass of hyphae as a "mycelium", "mycology" being the study of the fungi. "Mushrooms" and "toadstools" are only the aerial fructifications formed by specialization of the mycelium of a group of fungi more advanced than those associated with treatment plants. Reproduction is typically by spores produced either sexually or asexually, the nature and arrangement of the spores and the associated elaboration of the mycelium is used as a basis for classification. Unfortunately the life cycle~ of some fungi are incomplete or not fully
80
The Relevant Aspects oj Biology
known and these are grouped together as the "fungi imperfecti"i most species of importance in treatment plants belong to this group although some belong to the Phycomycetes characterized by the non-septate hyphae. The term "sewage fungus" which appears frequently in the literature needs some explanation. It usually refers to macroscopic growths of a collection of micro-organisms, few of which are fungi, found in organically enriched waters. These organisms include the bacteria Sphaerotilus, "Zooglea ramigera", Beggiatoa (the sulphur bacterium), and a microscopic animal (protozoan) which forms macroscopic colonies-Carchesium. True fungi such as Leptomitus lacteus and Fusarium aqueductum may be found associated with such growths but in this country are not usually the dominant organisms and are not as common as they are reported to be on the continent and in America. Occasionally true fungi similar to those found in bacteria beds are the dominant organisms in the sewage fungus community. Profuse growths of such fungi cover the bed of a river polluted with an industrial sewage containing "metallic wastes and wastes from plastic works containing phenolic compounds and formaldehyde. ANIMAL KINGDOM
The animal kingdom may be divided into the animals without backbones (Invertebrates) and those with (Vertebrates). The former contains groups of animals of widely different organization which, therefore, rank as phyla, whilst the Vertebrates, although including such apparently diverse forms as fish, birds and man himself, have in fact a similar basic structural plan and are, therefore, all included in one phylum-Chordata. Although birds and sometimes rats, among the Vertebrates, frequent treatment plants, the most important animals in wastewater t.reatment are found in the several phyla of the Invertebrates. Phylum Protozoa
These are microscopic animals whose body is not made up of several cells but is differentiated into different organelles which carry out the many functions of the many-celled organs of higher forms. Previously they were defined as unicellular, but are now considered noncellular. As might be expected when we consider the three kingdoms as radiating from a common primitive form of life, some members of the lower phyla in each kingdom show marked similarities and it is, therefore, difficult to separate certain lower groups of protozoa, fungi and algae. The great diver'sity in the morphology, mode of locomotion and method of obtaining food, found in the protozoa, permits them to be classified into fairly well defined classes, three of which need concern us.
The Relevant Aspects of Biology
81
Class Rhizopoda have bodies of irregular shape which constantly changes because of the protrusion of lobe or finger-like processes known as pseudopodia. By this means they stream over the substrate and also engulf bacteria and other food material within their protoplasm. Such food matter, trapped in pockets-vacuoles-is subjected to digestive enzymes and the nutrient matter then passes into the protoplasm proper. The non-digestible portion is rejected by the protoplasm simply 'flowing from around it. Under adverse conditions the cell may assume a spherical shape and secrete an outer protective coat to form a cyst. In this condition it can withstand extremes of temperature and desiccation and may be dispersed aerially. Amoeba is a typical example of the class. Some- Testacea-are protectively enclosed within a shell of characteristic structure, e.g., Difflugia and Arcella. Class Flagellata (c) Stylonychia (a) Amoeba
(I) Tetrahymena
Fig:4. Some Members of the Phylum Protozoa
These have bodies of more definite shape and they move by whiplike processes known as flagella. Some, the Phytomastigina, resemble the flagellated algae in having pigments by which they are capable of photosynthesis, e.g., Euglena. Their abundance in organically enriched waters suggests that they are also capable of living saprophytically on dead organic matter. Other flagellates, the Zoomastigina, usually of smaller size, have no pigment and are dependent upon complex organic
82
The Relevant Aspects of Biology
matter for food, e.g., Bodo. Although most flagellates are free-swimming, some colonial forms exist in which several individuals are attached to a common stalk, e.g., Anthophysa. Class Ciliophora-contains species which are, on the whole, larger than those in other classes. They are characterized by the possession of numerous hair-like processes-cilia-which may be used in locomotion and feeding. Most species possess cilia throughout their life and these are grouped together in the subclass Ciliata. Some, the Suctoria, although possessing cilia in the larval stages, lose them on reaching the adult condition which is characterized by the possession of tentacles, e.g., Acineta. Sub-class Ciliata-subdivided into orders on the distribution and arrangement of the cilia. By rhythmic coordinated movements of the cilia not only is movement effected but water currents are produced containing bacteria and other food particles which are directed down a groove or gullet. Order Holotricha-cilia of equal size evenly distributed over the whole of the body surface. e.g., Paramoeciunl. Order Heterotricha-small cilia covering the whole of the body, but members also possess stronger large cilia usually arranged in spiral bands around the body, e.g. Stentor. Order Hypotricha -flattened organism, with a ventral gullet. The cilia are mostly confined to the ventral surface and are highly specialized as thick cirri which do not move rhythmically but act as stilt-like legs by which they creep over the substrate. The number and arrangement of these cirri are important features in specific identification, e.g., Stylonychia. Order Peritricha have the cilia restricted to a peristomal spiral. The individuals are bell-shaped and sessile, being attached by a stalk which is usually contractile e.g., Vorticella. Many form colonies, the individuals being borne at the end of the multi-branched stalks, e.g., Carchesium. In contrast with the non-cellular structure of the protozoa, the other animals have bodies composed of many cells which are differently modified for various functions; all such animals are referred to as "Metazoa" . Phylum Platyhelminthes
These are small flat unsegmented worms of simple structure having only one opening into the food canal which is usually much branched. This well defined group of worms is divided into three classes, one of which (Turbellaria) have members which are free living and are to be found in fresh-water streams e.g., Plana ria spp. Polycelis spp. and
83
The Relevant Aspects of Biology
Dendrocoelum lacteu11l. The other two classes are parasitic and show varying degrees of modification to this mode of life, the Trematoda (flukes) being less modified than the Cestoda (tapeworms).
I--_ _ _..J. 5.Omm
Fig:5. Some Invertebrate Animals of the Phyla
Platyhelminthes Asthropoda (a) Polyceiis nigra (c) Cyclops (b) Dendrocoelum lacteum (d) Asellus aquaticus (e) Gammarus pulex The latter are elongate ribbon-like worms parasitic in the gut of vertebrates, including man. They appear segmented but a study of their structure and development shows that they consist of a small head or scolex which is provided with suckers and hooks whereby the parasite is firmly attached to the gut wall of the host. Behind the head, segments known as proglottides are proliferated; as these pass backwards they enlarge and when mature, each is seen to contain a full set of sex organs. Thus each proglottis is comparable with one flat worm, the whole tapeworm being considered as a chain of individuals of varying degrees of maturity. The beef tapeworm Taenia saginata-a parasite of man-may reach a length of twenty feet! Apart from structural modifications for the parasitic mode of life, parasites, to ensure the reinfection of a suitable host, produce large numbers of eggs and may have complicated life histories. The tapeworm T. saginata may be taken as an example. After fertilization, the eggs develop into an embryo having six hooks known as a hexacanth, whilst still within the proglottis.
84
The Relevant Aspects of Biology
''''_,"''''~" '"'~ ~
by small freshwater fish Procercoid larva released from crustacean, develops mto Plerocercold l a r v a .
~~ Procercoid larvae
A
.,.
A v
Predator fish eats infected small fish
it(
A= Diagnostic Stage
•
/"
~~~
A= Infective Stage
Human ingests raw of undercooked infected fish
f
hatChfro~,
Coracidia eggs and are ingested by crustaceans •
•
•
'. ... ,-----
"" 0
Eggs embryonate in water
A
Unembryonated egge passed in fecess
AProglottids release V immature eggs
Fig:6. The life-cycle of the Beef Tapeworm
The ripe proglottides containing thousands of these embryos, each still enclosed in the egg membranes, are discharged from the human host after becoming detached from the end of the tapeworm. No further development takes place until they are eaten by a suitable secondary host-cattle in this case. Within the gut of cattle, the egg membranes disintegrate under the successive action of gastric and intestinal juices to liberate the embryos, which then make their way via the blood stream to the muscles where each develops into a cysticercus or bladder worm. Further development only takes place if the infected meat is eaten raw or insufficiently cooked to kill the cysticerci; in this event, the bladder is digested and the scolex, previously developed as an invagination of the bladder, becomes attached to the intestine wall and commences to proliferate proglottides, thus completing the cycle. It has been shown that viable tapeworm eggs are capable of passing through most treatment processes even including sand filters and micro-strainers. Silverman and Griffiths have produced evidence to suggest that the increase in bovine cysticercosis caused by the bladder worm stage of T. saginata in cattle, since the Second World War, may be due to overloaded treatment plants. The mild nature of the infections suggests a wide dissemination of eggs such as would be possible in a sewage effluent. Only a few infected
The Relevant Aspects of Biology
..
85
people in a drainage area are needed to supply eggs sufficient to infect a large number of cattle.
Phylum Nematoda These are unsegmented worms having spindle or thread-like bodies pointed at both ends. The phylum includes parasitic and free-living forms. The parasitic forms have received much attention including the eelworms, parasitic on crops, and the larger ones parasitic in man. They are capable of prolific egg production; these pass out with the faeces of the host and their possible dissemination in sewage effluents and sludge must be considered. The free-living forms inhabiting soils and fresh water have attracted less attention, although their importance in maintaining soil fertility has been demonstrated by Overgaard and their presence in fresh water and wastewater treatment plants is probably equally significant. The free living forms are usually small, the average size being 1 mm. Their movements are characteristic; unlike the true worms which move by alternate stretching and contraction, they progress in S-shaped curves of the body, which remains of constant diameter. Although readily recognized as a grqup, the different species are not readily distinguished. Phylum Rotifera These are minute unsegmented animals characterized by the presence of a ciliated structure anteriorly-the corona-which is used in feeding and locomotion. In many, this takes the form of two flat discs. Most species live in fresh water and are able to withstand being dried up by forming cysts in which form they are probably dispersed by the wind. Besides normal sexual reproduction, the eggs are capable of developing without being fertilized, a phenomenon known as parthenogenesis. In some, parthenogenetic and sexual reproduction alternate and coincide with the seasons; fertilized eggs which develop thick shells are produced in the autumn and these remain dormant throughout the winter and develop the following spring. In one family, which includes the common genus Rotifer, no males have ever been found, and in others they are minute and degenerate.
Phylum Annelida Soft bodied worms showing segmentation externally as a series of rings, the body wall being covered by a thin cuticle which is not hardened as in insects. The two classes occurring in fresh waters are readily distinguished -the Oligochaetae which are characterized by the presence of bristles (chaetae) on most segments and include the common earthworm
86
The Relevant Aspects of Biology
and related species, and the Hirudinea (Leeches) which have no chaetae but possess two suckers, one anterior and a more cbvious one posterior. They are also more flattened than the worms. Most are ectoparasites sucking the blood of other animals but some eat small aquatic animals. Class Oligochaeta is divided into eight families mainly on the basis of the number and shape of the chaetae which are arranged in four bundlestwo on the dorsal (upper) surface and two ventrally (lower). Four of these families contain members which may occur in wastewater treatment plants. The Lumbricidae are readily recognized by their larger size and their similarity to the common earthworm which is a terrestrial member of this family. The common brandling Eisenia foetida and Lumbricus rubellus are present in many bacteria bed,s treating domestic sewage. Members of the other families are much thinner. The Tubificidae construct tubes from mud from which their posterior portions protrude and wave around; many possess red blood which can be seen through the body wall. They are usually more than 3 cm long and may be as long as 20 cm. They usually have more than two bristles per bundle, the ventral ones being curved and having cleft tips. The Enchytraeidae (pot worms) are smaller (less than 3·5 cm) and are creamy white in colour. As with the Tubificidae there are usually more than two bristles per bundle but these are simple pointed. In this order Lumbricillus (Pachydrilus) lineatus is a very common member of the bacteria bed fauna. In another family, the Aelosomatidae, the organisms are only a few mm long and are characterized by highly coloured bodies contained in their integument and the presence of numerous long hair-like bristles. Young worms are budded off posteriorly and a chain of individuals may sometimes be found. A species of, Aelosoma having pink bodies in the integument, may be so abundant in activated sludge that they impart a pink colouration to the surface of the settled sludge. Under adverse conditions they are able to form resistant cysts and thus are able to withstand periods of desiccation. Phylum Arthropoda
Members of this successful phylum are characterized by having a segmented body possessing a hard protective outer cuticle, each segment typically having a pair of jointed appendages. These appendages are differently modified in the different groups to perform such functions as locomotion, respiration, food capture and mastication and sensory perception. Because of the hard external skeleton, growth occurs by a series of moults, or ecdyses, by which the outer skin is shed and the new cuticle, formed beneath, only hardening after the creature has expanded. The
The Relevant Aspects of Biology
87
discarded casts are quite thin since much of the material in the cuticle is re-absorbed for use in the new cuticle. As a group they are the most successful invertebrates and are probably man's most serious competitors on earth. Of the six classes into which the phylum is divided, representatives from four may be found in waste treatment plants, of the other two, one forms an interesting evolutionary link with the Annelida and the other (Trilobita) contains only fossils. Although the species found in treatment plants are confined to a small number of the many orders in the Arthropoda, a fairly full classification is given, as they are of importance in stream ecology, a matter which should also concern the wastewater engineer. Class Crustacea
These are nearly all aquatic organisms and are best known as marine forms such as crabs, lobsters and shrimps. The few species associated with treatment plants, however, belong to the more primitive orders and are also much smaller, Cyclops being less than 2 mm long, another, Asellus aquaticus (water hog louse). Class Insecta
Consists of arthropods in which the body of the adult is divided into three sections-head, thorax and abdomen. Typically, there are two pairs of wings arising from the thorax and three pairs of walking legs also attached to the thorax. The class is divided into two sub-classes-one small group of primitive wingless insects-the Apterygota, which includes the silverfish and springtails and a much larger group-the Pterygota-which possess wings or have lost them secondarily in evolution. The Apterygota have a simple life cycle, the eggs hatching into forms resembling the adults, but in the Pterygota the life cycle is more complicated. In the most complicated type, the egg hatches into a larva, the maggot of a fly, or caterpillar of a butterfly. This stage is the active feeding and growing stage of the insect. After a specific number of moults the insect enters what appears to be a resting stage-the pupa or chrysalis, which differs considerably in form from the larva. Within the protective thickened cuticle, however, a reorganization is taking place, for after a period the adult insect-the imago-emerges from the pupal case. These remarkable changes in appearance of the insect at the different stages are known as metamorphoses. These orders of insects having this complete life cycle-eggs, larva, pupa, imago-are grouped together in the Endopterygota-wings developing internally. Other orders of the Pterygota, however, have a shortened cycle-the
88
The Relebant Aspects of Biology
pupal stage being absent and the larval stage showing some resemblance to the adult and known as nymphs; these are grouped together in the Exopterygota-wings developing externally. The order Diptera (the true flies) have only one pair of wings, the second pair having been modified as club shaped structures-the halteres-used as balancing organs. They have a full life cycle and the larvae never possess true jointed legs on the thorax although they may have thick fleshy false legs-prolegs-on thorax or abdomen. Class Arachnida
Class Arachnida - (scorpions, spiders and mites) - these are readily distinguished from the insects in having four pairs of walking legs 3 and they are not divided into head, thorax and abdomen. In the spiders the body is divided into two by a narrow waist; the mites have rounded bodies with no division. Sometimes linked with the Arachnida -although of doubtful affinity, are the "bear animalcules" -the Tardigrada, which are sometimes found in the film from bacteria beds. They are microscopic in size and have unsegmented bodies carrying four pairs of unjointed stumpy legs. Phylum Mo"usca
This Phylum consists of soft-b0died animals showing no sign of segmentation in the adult. The body is usually protected with a shell into which the whole animal can withdraw. Two classes are represented in fresh water; the Gastropoda, including the snails and limpets, have a shell in one piece, and the Lamellibranchiata including mussels and cockles, in which the shell is bi-valved. PHYSIOLOGY
Although it is desirable to be able to identify and classify an organism, it is the functioning and activity of the organism that is of importance in determining its role in wastewater treatment. By what means does it obtain its life energy and what are the end products of its activity? Physiology is . the branch of biology which includes this aspect of the subject.The biochemical processes involved in the functioning and activity of organisms are known collectively as metabolism. Those involved in the breakdown of complex matter to simpler products are known as Katabolism and the reverse process of synthesis of complex matter from simpler substances as anabolism. Chemical reactions involved in metabolism proceed at much greater rates than could be expected for the reactions involved and the necessary limitations on extreme temperature and pressure, imposed by the living cells. Such
The Relevant Aspects of Biology
89
reaction rates are made possible by the presence of numerous catalysts, i.e., substances which affect the rate of a chemical reaction but which themselves remain unchanged at the end of the reaction. Such organic catalysts which are proteinaceous compounds produced by the living cell are known as enzymes. As catalysts they only affect the rate at which the reaction proceeds and do not influence the course or the equilibrium of the reaction. The importance of enzymes in cell metabolism and, therefore, in wastewater treatment in which the waste is used as a metabolite, warrants some consideration being given to their properties and factors influencing their activity. Importance of Enzymes in Metabolism Specificity
Enzymes are remarkably specific in their activities, a given enzyme being capable of catalysing one, or at most few, reactions. Thus numerous enzymes are required to catalyse the wide range of metabolic processes occurring within the cell. Temperature
The effect of temperature on enzymatic activity is the resultant of two opposing factors. Like most chemical reactions the reaction velocity increases with rise in temperature but the higher the temperature the less stable the enzymes become. Thus, above a certain temperature, called the optimum temperature, the thermal inactivation of the enzymes, due probably to the thermal denaturing of the proteins, more than offsets the resultant increase in the reaction velocity and results in a rapid decrease in enzymatic activity. The effect of thermal inactivation is also a function of time and, as pointed out by Baldwin, in determining the optimum temperature, the time factor must be taken into account. In tests of short duration during which the enzymes do not need to be long-lived, much higher "optimum" temperatures may be recorded than in tests of longer duration in which the effects of enzyme inactivation become evident. The latter tests are, of course, more applicable in wastewater treatment where sustained microbial activity is required.
pH A given enzyme is only active over a fairly narrow range of pH within which activity increases to a maximum at a definite pH-the optimumand then declines again. Changes in enzyme activity around the optimum are probably due to the resultant changes in the ionic state of the enzyme
90
The Relevant Aspects of Biology
and these changes are usually reversible. The marked reductions in activity towards the limits of the pH range, however, are the result of the deactivation of the enzymes due to the irreversible denaturing of the proteins. Unlike the effects of temperature, the stability of the enzyme is usually greatest about the optimum pH. Each enzyme ,thus has its characteristic optimum pH, although this may vary somewhat with the nature of the substrate due to the indirect effect of pH on the degree of dissociation of the compound concerned.
Protein Precipitants Enzymes are active in the colloidal state and substances which bring about their precipitation by forming insoluble salts with them, inhibit their activity. Thus substances such as the heavy metals and alkaloid reagents, which form insoluble salts with proteins, are powerful inhibitors of enzyme activity. Other forces which bring about protein denaturation such as violent mechanical agitation or ultra-violet radiation, also cause the inactivation of enzymes.
Concentration Although enzymes are effective at very low concentrations, their concentration in relation to the concentration of substrate may be a limiting factor affecting the rate of a reaction. To understand this effect it is necessary to appreciate the mechanism by which enzymes are considered to act. It is now thought that the enzyme (E) and the substrate (S) react together to form an unstable complex (ES). In this form the substrate molecule is considered to be more reactive and is said to have, been "activated" by the enzyme. The substrate enzyme complex is then broken down producing the end products (P and Q), the enzyme (E) again being liberated. E + 5 '~ E5 '~ E + P + Q The rate of the reaction is dependent upon the molecular concentration of the unstable complex (E5) which is in turn dependent upon both the concentration of the substrate (5) and that of the enzyme (E). With low concentrations of substrate, as in most waste waters, increase in the concentration of the substrate results in an increase in reaction velocity until a value is reached, the saturation point of the enzyme when enzyme concentration becomes limiting.
Activators and Co-enzymes Many enzymes are incapable of catalytic activity unless certain other substances are present. Baldwin distinguishes between two groups of such accessory substances. Those needed to enable the enzyme to activate the substrate and which are, therefore, an essential part of the activating system, are known as activators. Metallic ions such as Mn, Mo, Zn and
The Relevant Aspects of Biology
91
Co required as trace elements by most organisms are thought to act as such activators in enzyme systems. Even though the substrate may be successfully activated, the subsequent reaction may not proceed unless other accessory substances are present. This second group of accessory substances, which take part in the subsequent reaction, are known as co-enzymes. These are nonproteinaceous, thermostable substances, examples of which will be met with in the subsequent outline of respiratory processes. Classification
Although enzymes are produced inside the cell, some are secreted by the cell to act on an external substrate. These are known as extra-cellular enzymes in contrast to those which catalyse reactions within the cellthe intracellular enzymes. They are also classified on the type of reaction they catalyse. Hydrolases, for example, are enzymes concerned with the hydrolysis of organic matter, an important process in digestion, whereby food is rendered available as a respiratory substrate for the cell. Most extracellular enzymes fall into this group. Another important group are the dehydrogenases which catalyse the dehydrogenation of organic substrates in the process of respiration. Respiration
Basically, the energy needed for life processes is obtained by the oxidation of an oxidizable substance (the combustion of a fuel) by the process of respiration. The oxidation of one substance necessarily results in the reduction of another and a biological oxidation-reduction reaction may be regarded as the transfer of hydrogen atoms (or of electrons) from the substance oxidized (the H-donor) to the one reduced (the H-acceptor) AH + B '~ A + BH 2 (H-donor) (H-acceptor). The initial hydrogenation is catalyzed by enzymes of the dehydrogenase group and the transference of the hydrogen takes place via one or more intermediate carriers-coenzymes-which form what is termed a respiratory pathway. One possible respiratory pathway, involving several such co-enzymes. The initial hydrogen acceptor in the katabolic oxidation of the organic matter (AH) is diphospho-pyridine-nucleotide (DPN), sometimes known as co-enzyme 1. From this the H is transferred to a second co-enzyme, flavinadeninedinucleotide (FAD). The final steps in the path involve what is known as the cytochrome system. Cytochrome is an iron-containing enzyme and it is thought that the oxidation/reduction reaction concerned involves the transfer of one electron and the change from the ferric state (Fe+++) to the ferrous state
The Relevant Aspects of Biology
92
(Fe++). Cytochrome-Fe+++ + e ' ~ Cytochrome-Fe++ When the cytochrome accepts one electron a hydrogen ion (H+) is released Cytochrome-Fe+++ + H '~ Cytochrome-Fe++ + HI- The reduced cytochrome is finally oxidized by atmospheric oxygen by lOSing an electron, the released hydrogen ion (H+) being removed in the process. Along such pathways the energy yielded by the oxidation of the substrate is released at different stages as the hydrogen is "stepped down". This released energy is not, however, directly available for the organism's activities but has to be trapped and stored as chemical energy for subsequent use. DEHYDROGENASE
AHX'
DPN
A
CYTOCHROME OXIDASE
CYI-Fe"~1(20
t - - - - 2H
PNH
Cyt-Fe"
Ar
~--..J.<
O2
Synthesis Mechanical
Osmoregulation
Fig:7. A Respiratory Pathway Involving Several Coenzymes Involved in the Dehydrogenation of an Organic Substrate AH2 and the Transference of the Energy Produced to the Energy Consuming Activities of the Cell
This is achieved by a substance which participates in both the exothermic (energy producing) reactions of respiration and the endothermic (energy consuming) reactions involved in the many activities of the organism, thus coupling them together. The release of energy along the respiratory pathway is associated with the production of compounds having high-energy-phosphate bonds which convert the low-energy coenzyme, adenosine-diphosphate (ADP), to the high-energy co-enzyme, adenosine-triphosphate (ATP) thus storing the energy, the process being known as oxidative phosphorylation. The ATP then participates in the endothermic reactions of the cells associated with the several activities of the organism, releasing the stored energy in being broken down into ADP and the inorganic phosphate again. In aerobic organisms-ones living in the presence of atmospheric oxygen-oxygen acts as the ultimate hydrogen acceptor: AH2 + V20 2 ~ A + H 20, e.g., C 6H 12 0 6 + 602' ~ +6H 20 (glucose) (oxygen) (carbon) (water)
The Relevant Aspects of Biology
93
dioxide). In anaerobic organisms-those able to live in the absence of atmospheric oxygen-the ultimate hydrogen acceptor is either an organic byproduct of the breakdown process which is thereby reduced: AH2 B' -7 A + BH 2, e.g., C6 H12 6' -7 2C0 2 + 2CH3 CH 2 OH (glucose) (carbon (ethyl alcohol) dioxide) or a reducible substance in the substrate such as nitrate, bringing about de-nitrification: 4AH2 + HN03' -7 4A + NH3 + 3H20 Thus aerobic and anaerobic respiration differ essentially only in the nature of the ultimate hydrogen acceptor, although the anaerobic process does not bring about complete oxidation of the organic matter, and liberates less energy. The respiratory enzymes which bring about these different reactions are sensitive to many poisons such as cyanide, carbon monoxide and hydrogen sulphide and such substances act as respiratory poisons by inactivating a link in the chain of respiratory enzymes. Apart from providing the fuel for respiration, an organism requires basic materials for the synthesis of new protoplasm, both to replace worn out protoplasm and to produce additional protoplasm in growth. The energy needed for this synthesis is usually provided by respiration through the ADP-ATP system. The synthesis of protoplasm is reversible in as much as in the absence of external food, the cell utilizes the protoplasm as respiratory substrate to provide the energy necessary to maintain life. This form of respiration, where the cell's protoplasm is used as a respiratory substrate, is known as Endogenous Respiration. There is some doubt as to whether this endogenous respiration goes on in the presence of external food, when the resultant reduction in protoplasm would be masked by the much greater synthesis of protoplasm by the energy released from the respiration of external food, or whether protoplasm is only used under starvation conditions. The wide use of the term "endogenous" in connection with microbial cultures will be discussed later. It is essential to realise that energy production by respiration is a function of each individual living cell of the organism. Different bodies within the cell have been associated with different metabolic activities. There is evidence, for example, that certain cellular bodies, known as mitochondria, are the principal site of oxidative phosphorylation. The necessary reactants, the substrate and oxygen, must therefore first enter the cell. The method by which this is achieved and the nature and source of the primary energy producing compounds can be used to divide organisms into different nutritional groups. This grouping, as we shall see, is more important in determining interrelationships between populations than is the systematic classification. All organisms may be divided into two groups on the basis of their basic food requirements. Autotrophic forms do not use organic compounds as
°
The Relevant Aspects of Biology
94
primary sources of energy; they are able to synthesize organic compounds from carbon dioxide using either light energy (Photosynthesis) or energy from inorganic chemical reactions (Chemosynthesis). Heterotrophic organisms are incapable of such synthesis and require organic compounds as their primary source of energy, and are, therefore, dependent upon autotrophic organisms either directly or indirectly for such food. The most important autotrophic organisms are plants which, using their pigments, are able to store light energy from the sun as chemical energy in organic compounds. The process, in which carbon dioxide is the sole source of carbon, and oxygen is produced as a byproduct, is known as photosynthesis. 60 6H 0 Light Energy C H 0 6CO 2 + 2 (Carbon dioxide) (Water)
)
6 12 11 + 2 (Glucose) (Oxygen)
Besides piants, some pigment-bearing bacteria-the photosynthetic bacteria -are also able to utilize light energy for synthesis but instead of oxygen being produced an inorganic oxidation takes place. 3H2S + 6H2 0+6C0 2 LightEnergy )C6 H 120 6 + 3H2 S0 4 (Hydrogen Sulphide)
(Water)
(Glucose)
(Sulphuric acid)
Other autotrophic bacteria utilize the energy of inorganic chemical reactions, the process being known as chemosynthesis. Nitrosomonas: NH3 + 11/202 ~ HN0 2 + Energy ammonia
Nitrous acid
Nitrobacter: NO 2+ !02 ~ N0 3 + Energy Nitrite 2 Nitrate In the case of Beggiatoa, the sulphur is deposited as granules within the cells and when sulphide is no longer available it is then oxidized to sulphate liberating more energy: H 2S+ ~02 ~
S
(Sulphur)
+ H 20 + Energy
S + 1V2 02 + H 20 ~ H 2S04 + Energy Autotrophic organisms take in their inorganic nutrients in solution. In the higher plants the CO2 from the atmosphere enters the leaves through openings (stomata) and then passes into the cells in solution. The other basic nutrients such as mineral salts, including nitrogen as nitrate, are absorbed by the root system and pass to the individual cells by means of a vascular system. In the lower plants such as the algae, and in the autotrophic bacteria where the individual cells are bathed in a solution of nutrients, these enter the cells directly in solution. In heterotrophic forms the nature of the organic food and the method by which it enters the cells differ considerably. In the metazoa, particulate and soluble food material is taken into an alimentary (food) canal in which the utilizable organic
The Relevant Aspects of Biology
95
matter is rendered soluble by extra-cellular enzymes before being absorbed by the cells of the gut wall and then transported via the blood system to other cells of the animal. The material remaining unabsorbed by the digestive processes is passed out of the animal as faeces. Some protozoa use a similar method; Paramoecium ingests particulate food (bacteria), contained in a drop of water, through a gullet into the protoplasm where it can be seen as a vacuole into which digestive enzymes are secreted and the utilizable food is absorbed into the protoplasm, the unabsorbed material being ejected from the organism. The heterotrophic bacteria and fungi take in their organic food in solution. In the case of colloidal and particulate matter suspended in water, as in some waste waters, this is first adsorbed on to the outer surface of the bacterial or fungal cell. Enzymes are then secreted through the cell wall which render the utilizable organic matter capable of being absorbed into the cell. The structure of the cell membranes of microorganisms thus play an important role in the initial adsorption and the subsequent selective absorption of the substrate. Structurally the main pellicle enclosing the cytoplasm, nuclear material and other cytoplasmic contents, is the cell wall. This is predominantly polysaccharide of low chemical reactivity and determines the shape of the cell. Often, to the outside of the cell wall is a slime layer of varying thickness forming a capsule. Functionally, however, in controlling the interchange of materials between the cell and the outside, another membrane is involved. This lies within the cell wall and surrounds the cytoplasm and is known as the cytoplasmic membrane. It is a thin, although tough membrane, predominantly lipo-protein, and unlike the permeable cell wall and capsule, it is semi-permeable, selectively absorbing certain molecules. LIPID MEMBRANE
Fig:8. The Enveloping Membranes of a Typical Bacterial Cell
96
The Relevant Aspects of Biology
More important ecologically than the method of food intake is the nature of the food. It is necessary to distinguish between those heterotrophic forms which obtain their organic matter by preying on other living organisms either plant or animal, thereby affecting the population of the latter, and those which obtain their organic matter in the form of dead or decaying matter. In the present work the former group -mostly animals-·which utilize living matter, will be termed holozoic, and the latter, those utilizing dead and decaying matter will be termed saprobic. Saprobic forms may be further divided into those, mostly animals, which ingest particulate decaying matter-saprozoic, and those which, like bacteria and fungi, absorb the organic matter in soluble form-saprophytic. The reader is warned that these terms have been used rather loosely in the literature; holozoic for example is often used synonymously with heterotrophic, the term holophytic being the corresponding synonym for autotrophic. Saprozoic and saprophytic are also used synonymously. For our purpose, however, rather than introduce new nutritional terminology into a field already thick with it, we shall use the existing terms in the limited sense defined above. The oxygen needed in respiration must also enter the cell. In the case of aquatic micro-organisms it enters in solution from the surrounding water in which it is dissolved. In the metazoa the oxygen must be transported to the individual cells from the exterior. In some, the oxygen is taken in through all of the moist body surface, e.g., worms, in others, special areas of the surface are modified for gaseous exchange such as gills and lungs. From these respiratory surfaces the oxygen is transported in the blood system-usually chemically combined with a respiratory pigment such as haemoglobin-to the tissues, where the oxygen is given up to the respiring cells via the fluids in which they are bathed. Insects, most of which are aerial or terrestrial and need to conserve the water within their bodies, would be at a disadvantage if they had moist areas exposed for respiratory exchange. • They have in fact evolved an interconnecting system of tubestrachea-through which the gaseous oxygen penetrates the tissues of the body. The trachea open to the outside through spiracles, the frequency of the opening and closing of which controls the loss of water. Some aquatic metazoa also obtain oxygen from that dissolved in the water in which they live, a few are dependent on atmospheric oxygen and need to live associated with the surface of the water (mosquito larvae and pupae) or visit it from time to time (diving beetles). In plants, the oxygen produced by photosynthesis may supply their respiratory needs and often a surplus of oxygen is given off. In darkness,
The Relevant Aspects of Biology
97
however, plants need to utilize oxygen from without and in an aquatic environment this may result in diurnal fluctuations in the amount of dissolved oxygen. Excretion
In respiration, as in all forms of combustion, waste products are produced and these have to be removed from the sphere of activity in the cell where their accumulation- would prove harmful. The mechanism of removal of these waste products, known as excretion, differs widely. The two chief excretory products are those resulting from the oxidation of carbohydrates (carbon dioxide, water) and those from proteins (ammonia, uric acid, urea). In plant and animals the CO 2 passes out via the reverse path to that by which the oxygen is taken in. In plants this path is also taken by the water. Animals, however, are characterized by also producing nitrogenous waste materials. These are usually excreted in solution, together with water and salts, through special excretory organs corresponding to the kidneys of vertebrates. The waste products in the metazoa are transported from the cells to these organs of excretion via the blood system. In insects, because of their need to conserve water, although the nitrogenous waste is secreted into the excretory tubule as a soluble urate, this is converted into crystals of uric acid which is relatively insoluble, . before passing into the hind gut to be discharged as a solid with the faeces; the water and inorganic base (potassium and sodium bicarbonate) remain within the insect. Apart from excretory functions, these organs are also responsible for the maintenance of the salinity concentration of the body fluids necessary for the proper functioning of the cells. When solutions of different concentrations are separated by a semi-permeable membrane, i.e., one which is more permeable to water than to the solute, water will pass from the stronger solution to the weaker by a process known as osmosis; the stronger solution is said to have a higher osmotic pressure. Cell membranes act as semi-permeable membranes and if the cell content has a higher osmotic pressure than the outside medium, water will tend to pass into the cell. If, on the other hand, the external medium has a higher osmotic pressure, water will be extracted from the cell. Within plant cells having rigid cellulose walls, a turgor pressure is developed which limits the entry of water. When such cells are placed in stronger solutions water passes out of the cell and the cytoplasm within the cytoplasmic membrane contracts away from the cell wall-a phenomenon known as plasmolysis: when replaced in a weaker solution of lower osmotic pressure than the
98
The Relevant Aspects of Biology
cell contents, water is again taken in and the cytoplasm swells to fill the cell which again becomes turgid. Bacteria are reported to act in a similar way and are, therefore, able to withstand fairly wide changes in the osmotic pressure of their environment. The animal cell, however, having no such rigid wall, would burst if the internal osmotic pressure was much greater than that of the external medium, were it not for the presence of a mechanism to remove the excess water from the cell. Lower forms of marine invertebrate animals have internal body fluids which are of similar concentration to the sea water and are able to adjust it to the small changes in salinity of the sea. No osmo-regulatory mechanism is, therefore, needed and the organisms are said to be "stenohaline", their body fluids being in osmotic equilibrium with the surrounding medium. In fresh water animals the osmotic pressure of the body fluids is higher than that of the medium and thus they tend to absorb water from their surroundings. In such animals a method of osmo-regulation is necessary to excrete the excess water, this process usually being performed by the excretory organ. In protozoa for example the contractile vacuoles are probably more important as osmotic regulators than in nitrogenous excretion; they are usually only found in fresh water species. Fresh water animals are therefore osmotically independent of their surrounding medium, but to be so, they have to expend energy in the process of osmo-regulation. ECOLOGY
Although physiology gives us an understanding of the functional processes of organisms and the way in which organic wastes may be broken down, in treatment plants, organisms do not exist in the waste as pure cultures but as communities of varying degrees of complexity. Thus a knowledge is needed not only of the requirements of the individual organisms but of the inter-relationships between populations and factors controlling the growth and activity of populations. Ecology IS that branch of biological study which deals with these different aspects. Historically, ecology was at first solely concerned with the collecting and recording of organisms and sometimes the linking of these with the observed environmental factors. It could then have been defined as "scientific natural history". Plant ecology and animal ecology have developed as separate subjects in this country. Although we shall need to concern ourselves with communities containing animals and plants (including bacteria and fungi), it is certain principles evolved from studies in animal ecology, that are more important for our immediate purpose. The publication of Elton Animal Ecology in
The Relevant Aspects of Biology
99
1927 focused attention on the community rather than the individual as a unit for study. Since then it has been put on a quantitative basis and has led to the present day "population dynamics". A further development has been the study of the transference of materials and energy between populations-called "functional synecology" or "productivity ecology". The environmental factors affecting an organism and limiting its distribution, and in some cases its population, may be considered under three ill-defined headings: • Physical. • Chemical. • Biotic. The physical factors include the physical nature of the habitat, aquatic, terrestrial or subaerial, etc., and such climatic factors as temperature, light and humidity. The movement of the medium, such as wind speeds and the velocity of water currents are also important physical factors in the environment of some species. Chemical factors would appear to be more important in the aquatic and subterrestrial habitats where such factors as pH, calcium and oxygen content, salinity and the presence of toxic substances may all affect different species differentially. Biotic factors are those involving the inter-relationship with other organisms. These include the predator-prey relati~nship, both between animal and plant and between animal and animal. In such relationship!?, the population of the prey may be affected by the activity of the predator, and conversely, the abundance of the prey may determine the population of the predator species it can support. Competition for a limited common food supply, either between individuals of the same species (intraspecific competition), or between different species (interspecific competition), is another important biotic factor. In dense communities similar competition may exist for oxygen and for actual living space. The availability of food other than prey may also be considered a biotic factor. Non-living organic matter is thus i1 biotic factor in the environment of saprobic organisms although it also contributes to the chemical environment. In addition to creating biotic pressures, the activity of a population may affect the chemical and physical environment; indeed wastewater treatment may be regarded as resulting from the activity of organisms on one component of their environment-the waste being treated. The resultant change in the environment may be unfavourable to the population bringing it about and more favourable to another species, which thereby replaces it. The repetition of this process causes a succession of dominant species, a phenomenon known as an "ecological succession".
100
The Relevant Aspect? of Biology
Evidence of such a succession can be readily witnessed by allowing sewage to stand in an open beaker in a window. Successions may occur in relation to time or distance. As sewage is purified in percolating downward through a bacteria bed different species become dominant. A similar succession of dominant species occurs along the length of an organically polluted stream as self-purification proceeds. Ecological successions may also result from environmental changes brought about by causes other than biotic ones. The gradual erosion or silting of a stretch of stream bed, for example, will result in a succession of dominant species. The earlier type of ecology-autecology-involving the correlation of the distribution of one species with environmental factors and the associated experimental work, has enabled certain physical and chemical tolerance limits within which the species exists to be determined. In some cases the optimum of such conditions, the preferendum, has also been determined. The laboratory aspect of this work is, however, of limited practical value. In its natural habitat the organism is subjected simultaneously to several factors, the joint effect of which is not necessarily additive because of the interaction of the factors involved. Consider, for example, the interaction of two toxic substances associated with sewage effluents, ammonia and carbon dioxide. The toxicity of ammonia is greater when it is present in the un-ionized form (NH3 ) than when as the ion NH 4 +. The degree of dissociation is greater at lower pH values and thus the presence of carbon dioxide which reduces the pH reduces the toxicity of ammonia. It is now recognized that an ecological niche cannot be defined simply by the combination of a few physical and chemical factors. A fresh-water biologist with all his knowledge of the physico-chemical requirements of a species of fish is less likely to locate them in a given stream than is an experienced fisherman with his intuitive knowledge of their haunts and habits. In wastewater treatment, laboratory tests have been of use in determining the wider limits of the conditions under which certain organisms might be expected to thrive. The optimum plant conditions for any species are, however, more difficult to predict. In practice, we a.re more concerned with populations than with individual requirements. In considering factors influencing population activity and size we have to consider those factors listed under "biotic" above and become involved in "synecology". The interdependence of populations is best appreciated by studying a food cycle which represents the general food links between the major populations present in an aquatic environment. Within such a food cycle are several food chains in which successive links are represented by a series of different species which exist in a predator-prey relationship with neighbouring links.
101
The Relevant Aspects of Biology
The number of links in such chains are usually few: Diatom ~ Rotifer ~ Mayfly' ~ Fish The successive links of food chains are represented by fewer individuals, expressed by Elton as a pyramid of numbers. Considering the pyramid as representing the different nutritional groups within a community, each horizontal section is known as a "trophic level". Thus organisms occupying the same trophic level are competing for a common food supply-a horizontal relationship. Those on successive levels are linked by a predator-prey relationship a vertical relationship. These inter-relationships between populations result in a dynamic state of balance being established. Thus although theoretically all populations tend to increase at an intrinsic rate, they do, in fact, remain roughly constant, or fluctuate about a constant mean, for long periods. Larger Carnivores (Pike)
Algae (diatoms)
Fig:9. Populations in a Stream Community Represented as an Eltonian Pyramid of Numbers
Processes by which populations are controlled have been divided into two types according to whether their controlling effect varies with the population density. Those, such as adverse climatic conditions and toxins, which result in a percentage kill independent of the size of the population, are said to operate as density-independent processes. Others, such as disease and food shortage, are more effective against dense populations than low ones and are termed density-dependent. Generally the environmental factors classified as physical and chemical previously, are usually denSity-independent whereas the biotic factors operate in a density-dependent manner. Although densityindependent processes are capable of eliminating a population, the regulation of a population can only be effected by factors sensitive to the changes in the denSity of the population, i.e., density-aependent ones. The relative importance of density-dependent and density-
102
The Relevant Aspects of Biology
independent processes in controlling populations is a matter of much controversy which is outside the scope of this work. The different views, strongly held, may in part be due to the different communities and environments on which the observations have been made. In this respect my own views, based on macrofauna population studies on bacteria beds, should be pertinent to our subject; they may be summarized as follows: • The physical and chemical factors of bacteria beds create a specialized habitat which few species have been able to colonize. • Density-dependent factors, especially competition for the available food -the film of microorganisms feeding on the organic waste-are most important in limiting populations at the high densities established by the few species which have successfully colonized the habitat. • The outcome of these density-dependent processes on the competing populations is, however, largely determined by physical factors such as the downward rate of flow of the liquid and the temperature. • The effect of a physical factor operating through densitydependent biotic processes is greatly different in degree and sometimes the reverse of that to be expected if the physical factor acted directly on the population free from biotic pressure. • Slight changes in the physical environment may result in a change in the dominance of the competing species. Thus, seasonal fluctuations in temperature induce a succession of dominant species throughout the year, which in itself prevents the unlimited growth of anyone population. • Thus the macrofauna populations in bacteria beds are controlled by density-dependent biotic processes which are themselves monitored by physical factors. The operation of wastewater treatment plants may be regarded as the controlling of the popUlations and activities of the different organisms present. Some knowledge of population dynamics is, therefore, desirable. Especially relevant are the growth curves of cultures of microorganisms. A classical growth curve, as described by Monod. The lag phase may be considered as a period of acclimatization which is followed by a phase of rapidly increasing growth rate leading to the log phase in which the maximum growth rate is achieved. This is terminated when, for reasons discussed later, the rate of cell division declines (the declining growth rate) until it is equalled by the rate of death of cells (the stationary phase) and eventually is exceeded by the latter resulting in a negative growth rate. The log phase of growth is terminated by adverse conditions such as depletion of nutrient requisites,
103
The Relevant Aspects of Biology
accumulation of toxic by-products of metabolism or physical overcrowding, created by the activity of the micro-organisms themselves. I I I I
6
I'" I.r=
10l
z-
5
'iii
c (I) u ~ CD
U
10. 15
~ '"0. m u (I)
4
t1l
.s;:
Cl
3
.£
Ii)
(I)
1(1)
(I)
U
«
't1l"
.r=
0.
C;
g
iii ~ (I) c 0
0.
x
:J (I)
(I)
'"0.t1l '" .r= 0. Ol .s;:
c
,Q
>-
CiS u
c
iii 0
iii ~ CD
cr: U5
0
c
(I)
Cl 0
U
c ~ (I)
£3l u Ol Ql.s;:
00.
w
Cl
.£
2
+2
2
[!!
.r= ~
e
+1
0
0
-1 -2
Time-
Fig:l0. Characteristic Growth Curves of Cultures of Microorganisms
Under starvation conditions the cells of the culture may undergo endogenous respiration using their own protoplasm as respiratory substrate. Within the colony, however, the viable cells may also utilize the breakdown products of the protoplasm of other dead cells; this is also considered as endogenous respiration and the phase of growth is now usually referred to as the endogenous phase. If organisms in the log phase of growth are subjected to limiting nutrient concentrations-and a single nutrient factor may be limitingthe growth rate is reduced. This indeed is one of the causes terminating the exponential (log) phase of growth, but the organisms remain for some time capable of exponential growth when food is no longer limiting. At high concentrations food is not limiting. Below a certain concentration (B), however, at which food becomes limiting, there is a successive reduction in the growth rate with decrease in nutrient concentration until the rate is zero at concentration (C) which is above 0 food concentration.
The Relevant Aspects of Biology
104 +5 +4 +3
R. ------
+2 (])
co
a: +1 .c
~
e
0
(9
-1 -2 -3
ID.
B.
90
80
70
60
50
C.
40
30
20
10
0
Concentration of Waste
Fig:ll. Relationship between Growth Rate of Microorganism and Nutrient Concentration
Below this concentration (C) auto-oxidation (endogenous respiration) of the cells take place producing a negative growth rate. The concentrations at which these effects become operative differ with different organisms. In waste-water treatment plants it would appear that fungi require higher concentrations of nutrient than do bacteria and, therefore, with decreasing nutrient concentration it is possible that fungi pass into the declining growth phase and negative growth phase before bacteria. TRANSFER OF MATERIALS AND ENERGY
So far we have confined our discussion of the inter-relationships of populations to numbers. In studying the functional relationship, such as the transfer of materials and energy between populations, the amount of living matter-biomass-must be taken into account. Reconsidering the general food cycle in these terms, we see that the low energy inorganic matter is synthesized into high energy organic matter by the process of photosynthesis and protein synthesis by plants using light energy. This high energy material is used as food by a succession of organisms in a food chain, i.e., by organisms on successive trophic levels, either as predators or agents of decomposition. Each population, however, takes a 'cut' of the utilizable material it receives, this fraction is broken down in respiration to release energy for the life processes, the remainder is resynthesized as the organism's protoplasm (biomass), to become available as food to the next link in the food chain or to the next trophic level. The end-products of the material broken down in respiration then
105
The Relevant Aspects of Biology
become available for resynthesis by plants, either directly in the case of C, Hand 0 as CO2 and H 2 0 or indirectly in the case of protein respiration, via the nitrogen and phosphorus cycles. There is thus a reduction in calorific equivalent in successive trophic levels. The proportion of the material (and energy) received by a population that is re-synthesized as biomass for use by populations in the next trophic level, varies with different organisms. In studies on the productivity of lakes in connection with fish production, the efficiency of a population is taken as the percentage of the calorific equivalent of food taken in that is made available to the next trophic level. This definition, however, reflects the interest of the final link in the food chain-man! Other organisms in the chain are not, however, primarily concerned with the production of biomass for use as food by their predators, but with their own life processes. A more objective definition of efficiency would be that percentage of the food energy which is used in the organism's life processes. Which of these concepts of efficiency is more applicable to populations in wastewater treatment plants depends upon the attitude one takes to the problem of waste treatment. In the past the problem has been regarded, by most workers involved in the practice of waste treatment, as one of disposal without causing pollution of the receiving water. The production of sludge from the mechanical settlement and biological oxidation of the waste is regarded as a secondary process. In some cases the resultant sludge may, after digestion, be used, e.g., as a fertilizer. Often, however, its disposal creates a major problem, especially at sites away from the coast where disposal at sea is not practicable. The production of sludge, especially the secondary" sludges resulting from synthesis in biological oxidation, is not, therefore, generally desired. Accepting this concept of the aims of wastewater treatment, the· breakdown of as much as possible of the organic waste is preferable to its re-synthesis as biomass (sludge). On this concept, the efficiency of a population would be measured by the percentage of the material taken in by the population that is oxidized in the respiratory processes of the organisms. An alternative concept of the role of wastewater treatment lays stress on the conservation not only of the water but also of the valuable components of the waste. Hynes, discussing what he describes as the appalling waste involved in the disposal of organic wastes, particularly sewage, calculated that each day the population of Great Britain wastes approximately 500 tonnes of saline nitrogen and 30 tonnes of phosphate phosphorus in sewage effluents alone. Although this may increase the fertility of the rivers, sometimes /I
106
The Relevallt Aspects of Biology
adversely affecting man's interests as when thick growths of blanket weed (Cladophora) occur, much of it passes to the ocean and is lost by the land and man. Of the total amount of phosphorus in the sewage arriving at a works, approximately a half passes away in solution in the effluent. It has been estimated that about three quarters of the nitrogen is lost in this way. The nitrogen and phosphorus retained in the sludge is partly from the primary settled sewage solids and partly from the secondary sludges formed in the biological oxidation plant. In digestion, however, there is a further loss as the organic nitrogen is broken down to soluble ammonium salts: these are usually returned to the oxidation plant in the sludge liquors, the ammonium concentration in which may be 700 ppm (amm. N). Thus the larger the proportion of the soluble organic waste that is synthesized as biomass (sludge), the less the loss of fertility. On this basis the efficiency of populations should be based on the percentage of the material received that is synthesized as biomass. This concept has also been expressed as the "economic coefficient of cell synthesis" which may be considered as the ratio of dry weight of growth (biomass) to the corresponding weight of nutrient utilized. It has been shown that this value was lower for zoogleal bacteria than for four species of fungi common in sewage treatment plants. Thus in oxidizing the same amount of sewage, more sludge would be produced when fungi were the dominant microbial organisms than when bacteria were. In practice, however, the maximum synthesis of microbial mass in activated sludge plants and bacteria beds is not always compatible with efficient plant operation, as will be discussed later. In algal oxidation ponds where the breakdown products of the bacterial decomposition of the waste are re-synthesized by photosynthetic activity into algal cells, the policy of encouraging maximum synthesis could be applied. This method of treatment, however, depends on the presence of adequate periods of sunlight throughout the year and it is doubtful whether, under climatic conditions prevailing in the United Kingdom, algal ponds could prove a satisfactory alternative to bacteria beds and activated sludge. In high rate oxidation ponds in California, the algae have been harvested by alum flocculation or continuous centrifuging, and have been proved to be of value as a high-protein animal feed supplement. He considered it was not necessary to separate the algae from the bacterial . cells, as both were high-energy compounds suitable for feeding stuff. Attempts have been made to reclaim the nutrient salts from effluents of conventional plants by culturing rapidly growing crops in them. In South Africa experimental work has shown t!,at the water-hyacinth
The Relevant Aspects of Biology
107
(Eichhornia crassipes) could be used for such purposes. but again the lack of sunlight and the severity of winters in the United Kingdom make its use here impracticable. In countries where the nutrient matter in effluents can be recovered by the growth of plants, including algae, it is also possible to encourage further links in the food chain, such as invertebrate animals, on which fish can be successfully cultured. Ecologically the trapping of nutrients in effluents by plants is probably limited in this country by the light energy available. Hynes has suggested that fertility might be recoverable in the form not of algae but sewage fungus, the development of which is independent of light. The direct reuse of effluents for irrigation purposes in America has been summarized by McGauhey. An alternative answer to this problem of conservation is illustrated in the direct application of certain industrial wastes to crops, e.g., gas liquor wastes, cannery wastes. Such methods are similar in principle to the outmoded "sewage farms". In all such methods of conservation the public health aspect should, of course, be considered but this should not be confused with public prejudice against anything associated with sewage, which is common to the western civilization. In the direct application of waste to the land, care should also be taken to prevent the pollution of water courses via the land drains. Hynes stresses the need for some radical re-thinking of effluent disposal problems to develop practical methods of preventing this loss of fertility. However much one may agree with this view, the acceptance of any method of recovery, however technically feasible, is not in the hands of the chemist and biologist but will be decided by the accountant and economist. As long as "economy" is based on the arbitrary values of £. s. d, rather than on such intrinsic values as soil fertility, then it is feared that the declining fertility of the earth's soilS will continue to be "poured down the drain" to be lost in the oceans. THE METABOLIC ACTIVITY
The rate at which organic matter is broken down is determined by the metabolic activity of a population, apart from its efficiency as described above. The metabolic rate, usually measured in terms of the rate of oxygen consumption per unit of biomass, differs considerably with different species. Generally this respiratory rate is more closely related to the surface area of the organism rather than to its weight, thus smaller organisms generally have a higher metabolic rate; e.g., the rate for the soil inhabiting Nematode worms is some ten times that of the larger earthworm. In
108
The Relevant Aspects of Biology
assessing the relative importance of different species populations in a given community, it is not their numbers, nor their biomass, but their total metabolic activity that is important. A concept of the activity of a community in terms of energy flow has been put forward by MacFadyen. Although he himself warns us that the application of this concept to productivity studies had not yet been generally accepted, certain features would appear applicable to communities in wastewater treatment plants. He points out that in food cycles, although materials recirculate, energy does not, the energy trapped in the plant during photosynthesis being liberated at different trophic levels by the heterotrophic organisms. The amount of energy entering a community depends upon the rate at which the materials are circulated. This depends not only on the photosynthetic activity of the plants but equally upon the rate at which energy is released by the heterotrophic organisms, thus providing nutrients for the plants. An active community is thus one in which there is a rapid liberation of energy. The relative importance of different populations within a community is then measured by their contribution to the energy flow of the community. A community which has a rich variety of species, able to exploit different energy sources, is a more productive community than one having few unspecialized species in which there may be "stagnant pools" of energy instead of a rapid flow. Although in wastewater treatment plants we are not primarily interested in productivity, the rate at which organic matter is broken down is of economic importance and this concept of energy flow could be applied to the community of organisms in the treatment plants. APPLICATION OF BIOLOGICAL PRINCIPLES TO WASTEWATER TREATMENT
Of the three aspects of biology outlined in this introduction, both taxonomy and physiology are necessary for a full appreciation of ecology. In applying biological principles to wastewater treatment, involving the controlled activities of populations of organisms, the ecological approach is probably the most rewarding and will be developed in subsequent sections. The different aspects of ecology are all reflected in the several studies of the organisms found in sewage-treatment plants. Some resulted in mere systematic lists of organisms associated with different stages of the process and which, although useful as a basis for other workers, are of no practical
The Relevant Aspects of Biology
109
value to the operator. Other observers have correlated the presence of organisms with different conditions within the plant and such organisms have then been used as "indicator organisms": For example, early in the history of activated sludge, different associations of protozoa were associated with different levels of sludge efficiency and microscopical examinations of the sludge to determine these associations were found to be of value in plant control. Probably because of the practical nature of the subject, early investigators enquired into the role of the different organisms in the purification process and thus the "synecological" aspect of the subject was introduced at an early stage. An outline of the historical development of our knowledge of the ecology of wastewater treatment may be of interest. HISTORICAL DEVELOPMENTS IN WASTEWATER ECOLOGY
The discovery that microorganisms were associated with water and putrefying liquids was made possible by the invention in 1675 of the compound microscope by Leuwenhoek. It was not until 1743, however, that Baker indicated the scavenging activities of protozoa and bacteria, and in 1839 Schwann and Schultze demonstrated that microorganisms were the true agents of decomposition. Following his work, in the middle of the 19th century, on fermentation, Pasteur established the importance of microorganisms in the process and distinguished between aerobic and anaerobic organisms for the first time. On the subject of organic wastes he declared: "Dead matter which ferments and putrefies is not obedient, at any rate inclusively, to forces of a nature purely physical or chemical. It is life which rules over the work of death and the dissolution of animal and vegetable matter. This constant return to the atmospheric air and to the mineral kingdom of the constituents which vegetables and animals have borrowed from them is an act related to the development and multiplication of organized beings." The setting up of the River Pollution Commissions of 1865 and 1868, as a result of the foul conditions of the rivers of the country, first initiated a scientific study into the problems of sewage treatment. At that time, the recognized method of disposal was by land treatment, but there appears to have been no appreciation of the role of microorganisms in the process in spite of the findings of earlier workers mentioned above. As a result of his investigations on the purification of sewage, Sir Edward Frankland in the second report of the Commission reported that the process was not merely mechanical filtration but involved chemical oxidation; the role of micro-organisms was apparently not appreciated. He did appreciate, however, the need for adequate aeration and as a
110
The Relevant Aspects of Biology
result of his experiments he did devise the "intermittent filtration" method using land. It was truly claimed: "that Frankland's conclusions, based as they were on excellent experiments, have formed the foundation for all further progress relating to sewage purification up·to the present time." Unfortunately, however, his mechanical-chemical concept of the process has persisted, although in a subconscious form, and still influences the design and operation of biological treatment plants; the term sewage "filter" reflects this attitude to the process! Later work, however, by Schloesing and Muntz showed that nitrification was brought about by microorganisms and they concluded that these were essential to the purification process. Warington showed that nitrification proceeded in two stages, each stage being the result of the activity of separate organisms. These were later isolated as Nitrosomonas and Nitrobacter by Winogradsky. For a more detailed history the reader is referred to Stanbridge. The role of larger animals in the stabilization of organic matter was also reported by Dr. Sorby to the Royal Commission on the Metropolitan Sewage Discharge in 1883. He associated the disappearance of sedimentary faecal matter in the river Thames with the presence of certain crustacea and worms. Thus before the advent of the bacteria bed, which was evolved from soil and sand filters as a result of experiments at the Lawrence Experiment Station, Massachusetts, the role of both the microorganisms and the macrofauna in sewage purification had been established. The results of the Lawrence experiments stimulated investigations of the principles involved in the bacteria bed method of treatment. In these, although the role of the microorganisms was appreciated by most, and studies were carried out on the bacterial flora, the activity of the larger organisms, previously associated with purification, was at first overlooked, their presence, when observed, being regarded as incidental, as indeed it still is by some workers. Johnson, in reviewing the early works, quoted Dunbar as first drawing . attention to the function of higher animals and plants and states that later Hofer, in 1907, investigated their activities more closely. Dibdin in 1904 also observed the presence of large numbers of active insects and Annelid worms in his slate beds, and Fowler also reported on the importance of the higher forms of life in sewage purification. About the same time Harrison in his evidence to the Royal Commission on Sewage Disposa suggested that the seasonal discharge of solids from beds was probably due to the activity of the macrofauna.
The Relevant Aspects of Biology
111
Work of a more truly ecological nature was carried out by Johnson when he studied the factors influencing the distribution of different species of macrofauna in bacteria beds at Wakefield. Parkinson and Bell demonstrated the useful activity of the springtail Achorutes subviaticus in controlling film accumulation. Later, the environment of the bacteria bed, supporting a reduced number of species, attracted the attention of more academic workers as a habitat on which to study problems of the dynamics of populations. Notable among such workers were Dr. Lloyd and his succession of students at Leeds University who carried out a series of studies on insect and protozoan populations on sewage works in the West Riding; these are summarized by Lloyd. Nuisance arising from flies leaving the bacteria beds probably attracted attention to the macrofauna before their beneficial activities were fully realized. Later, this nuisance resulted in many investigations into methods of control using different chemicals. Some such investigations were ecological in that they attempted to relate the populations to environmental conditions but most, although of practical value in assessing the effectiveness of different control measures, did not ad vance the knowledge of bacteria bed ecology. Another practical problem-the choking of the beds with accumulated film-has led to a series of ecological investigations both in this country. At Birmingham a study on the effect of insecticide treatment on the bacteria bed community and film accumulation has led to the two practical difficulties being studied as one ecological problem. In the ecology of activated sludge both academic interest and practical problems have initiated ecological investigations. Ecological successions of different species of protozoa in the development of a sludge and the relative roles of protozoa and bacteria in the process have been studied by many workers. The development of filamentous growths associated with 'bulking' has also resulted in investigations being carried out. The outcome of such ecological investigations usually results in the formulation of hypotheses which necessitate laboratory work for their proof or otherwise and much work being carried out at present by the Water Pollution Research Laboratory and others will, it is hoped, assist in the solution of outstanding ecological problems. ACTIVATED-SLUDGE AND BACTERIA-BED ENVIRONMENTS
The treatment of wastewaters by both methods involves similar biochemical processes the two differ in many respects. Firstly, whereas the activated sludge is truly an aquatic environment that of the bacteria
112
The Relevant Aspects of Biology
bed has been likened to the wrack zone of the sea shore. Also, whereas the fauna of activated sludge is normally restricted to microorganisms, the bacteria bed also supports higher forms of life such as oligochaete worms and insects. Associated with this difference in the fauna is the difference in the degree of control the operator has over the two biological systems. In the activated sludge process the amount of microbial mass is controlled by the withdrawal of excess sludge from the system; in the bacteria bed excess film is removed chiefly by biological agencies. A fundamental difference in the ecology of the two systems is that in the bacteria bed a succession of communities becomes established at different levels of the bed and associated with the corresponding different degrees of purification; whereas in the activated-sludge process the same community within the floc is at one time associated with the untreated waste and at the other extreme with the purified effluent. It is of interest to note that many of the advantages and disadvantages of the two systems are attributable to these differences; fly nuisance and the choking of the beds being connected with film control by biological agencies; bulking of the sludge, with resultant settlement difficulties, and the difficulties associated with disposal of the surplus activated sludge are probably connected with its removal from the system as microbial flocs rather than as the faeces of the grazing fauna. Again, the relative sensitivity of activated sludge to toxic discharges may be associated with circulation of the sludge, whereas, in the more robust 5 bacteria beds such discharges are first taken by the surface growths. A highly nitrified effluent is more often associated with an efficient bacteria bed than with activated sludge and this may also be connected with the differences in the relative ecological systems. Nitrifying organisms are more effective in isolation from the other processes of purification, e.g. in the cleaner depths of the bacteria bed or in the cleaner secondary bed of the two-stage process. It may be therefore, that in the multipurpose floc of the activated sludge their activity is restricted. Also the length of life of a floc in the system is largely determined by the growth rate of the heterotrophic bacteria and may be such that the Hoc is discharged before the more slowly growing autotrophic nitrifying organisms become effectively established. In nature, a succession of different ecological conditions and associated communities concerned with the breakdown of organic matter in waters has been described. A high concentration of complex decomposable organic matter is associated with the absence, or very low concentration, of oxygen - Polysaprobic condition.
The Relevant Aspects of Biology
113 Excess sludge
Nematode + Rotifera I-I
/--+--H'
I I
I
I
I I
/- - -f------:l---t-{ I
/
r-I
I
Bacteria Heterotrophic
+ Saprobic
Protozoa
I
i
-el0>1 ~
I __
Dead Organic Solids /
~~
E~
__________
~r-;
Soluble Organic Waste
I I
Degraded Organic Matter
Mineral Salts
Fig:2. Diagrammatic Representation of the Main Food-links in th~ Purification of Organic Wastes by Activated Sludge and by Bacteria Beds
As purification proceeds and'the organic matter is broken down first to aminoacids (a-mesosaprobic) and then to salts, the oxygen concentration increases until oxidation and mineralization is completed (oligosaprobic). Both the activated-sludge and the bacteria-bed environment differ from these systems in that, because of the aeration, high concentration of complex organic matter may be associated with fairly high oxygen concentrations. Obviously large numbers of organisms gain access to both activated sludge plants and bacteria beds; many find the environment inhospitable and never become established; others may persist but are not very successful, whilst the few species which find the habitat suitable, increase greatly and with reduced competition may become more abundant than in their natural habitat. In the following discussion on the ecologies of the two processes only those organisms are considered which have a significant role in the community, i.e., those organisms which if removed from the community would appreciably upset its balance of populations.
Chapter 5
The Ecology of Bacteria Beds The artificial environment of a sewage bacteria bed has been successfully invaded by truly aquatic microorganisms and some moistureloving higher organisms, and it is convenient to distinguish between the microorganisms or 'film' and the higher forms of life, the grazers, although as will be appreciated, some organisms of the film are also grazers in the strictest sense. ORGANISMS OF THE FILM Bacteria
Bacteria active in bacteria beds have attracted little attention; according to Wattie, bacteria found in filter slimes are zoogleal bacteria of one group closely related to those in activated sludge. Besides zoogleal bacteria, filamentous forms such as Sphaerotilus and Beggiatoa are also present, but since they have not been associated with nuisance such as bulking in activated sludge, their occurrence has not been studied in the same detail. The nature of the bacteria flora will be dete..rmined by the nature of the waste. Happold and Key showed that after the application of gas liquor to a bed the bacterial flora underwent considerable modification, bacteria capable of utilizing certain components of gas liquor becoming established. Specific bacteria capable of oxidizing phenols, and thiocyanates, thiosulphates and cyanides have been isolated. Fourteen well-defined pure organisms were isolated by Harrison from the Monsanto plant treating the trade efHuent containing a number of organic compounds; some were found to be specific over the range of compounds tested, whilst others attacked several compounds .. One would expect different bacteria associated with different stages of breakdown to be established at different levels of the bed, the heterotrophic forms being nearer the surface and the autotrophic ones nearer the base of the bed. Although samples of waste taken at different depths suggest that this
The Ecology of Bacteria Beds
115
is so, there appears to have been little work done on the actual distribution: of organisms. Barritt measured the nitrite produced by inoculating sterile solutions containing ammonium salts with effluent from different levels of his sectional bed. He concluded from his results that nitrifying organisms occur in all sections of the bed, but on the basis of the higher nitrite yield by the inoculum from the lower sections, it could be considered that they were more common in the lower portions of the bed. Using samples of the film removed from different levels in connection with film-accumulation studies, Harkness compared their relative nitrifying capacities and found that although nitrate production was highest in the lower samples, nitrite production was more evenly distributed throughout the different depths, indicating a concentration of nitrate-producing bacteria near the base of the bed, with the nitriteproducers distributed throughout the depth. It should be mentioned that the bed from which the sample was removed was operating on alternating double filtration. Fungi
Fungi are of more common occurrence in bacteria beds than in activated sludge. This may be because of the more suitable physical environment or may be because of the constant supply of complex organic matter at or near the surface of the bed; in an efficient activated sludge they would be subjected to starvation conditions for considerable periods. As saprophytes and primary feeders they are in direct competition with the heterotrophic bacteria and it would appear that the nature and stage of purification of the waste determines which is dominant at the different levels of the bed. With domestic sewage, bacteria usually predominate, but with the introduction of trade wastes, fungi may become dominant especially in the upper part of the bed. Numerous fungi have been isolated from beds but probably the following are the more important ecologically, although other species may be dominant locally under certain conditions: Fusarium aqueductum, Oospora (Geotrichum), Sepedonium sp., Ascoidea rubescens, Subbaromyces splendens, Sporotrichum sp., Penicillium sp. It is of interest to note that the commonly occurring truly aquatic Leptomitus lacteus does not generally appear to have colonized the bacteria-bed environment. At one works, although it was found in the feed channels in profuse growths and when detached, blocked the distributor arms, it did not establish itself within the beds. It would appear that the physical environment of the bed determines the species which can inhabit it. Those with tenacious holdfasts such as Fusarium and Geotrichum are the first to
116
The Ecology of Bacteria Beds
colonize the stones and form a basis for the subsequent establishment of such form, as Sepedonium and Ascoidea. Painter considered that the different growth-rates and nitrogen requirements of Fusarium and Sepedoniunz also contributed to this succession.
Fig:1. Some Bacteria-bed Fungi Showing Characteristic Spores (A) Fusarium aqueductum, (B) Geotrichum sp., (C) Sepedonium sp., (D) Ascoidea rubescens
Tomlinson found that although Fusarium and Geotrichum were able to withstand the direct discharge of sewage, Fusarium was dominant on the surface and Geotrichum below; factors other than the structural modifications mentioned above were obviously involved. As a result of experiments in which growths were developed in light and dark he found that Fusarium was able to compete successfully with the algae Stigeoclonium and Chlorella in the presence of light, while Geotrichum was unable to do so. In the absence of algae (in the dark) Geotrichum competed successfully with Fusarium and became the dominant fungus. Thus the dominance of Fusarium on the surface of the bed was due to its ability to compete successfully with the algae. He considered that algae might thus limit the growths of fungi on the surface of beds, Geotrichunz producing thicker mats than Fusarium. On the surface of the beds at Birmingham, served by fixed spray jets, a seasonal succession of dominant species has been observed, the bacterial zoogleal growth of the summer giving way to Fusarium which later became
The Ecology of Bacteria Beds
117
overgrown by Geotrichum and later, following incipient ponding, by thick growths of Sepedonium. The foam-like growths of Ascoidea are usually found within the bed. On beds served with travelling distributors, under the jets of which the sewage impinged on the surface with considerable force, only Fusarium and Geotrichum became established under jets but as these growths impeded the flow and the sewage spread laterally to the areas between the jets, Sepedonium and Ascoidea became established in these zones. Factors influencing the amount of fungal growths in beds will be discussed in detail later. Algae
Although algal growths, being restricted to the surface of beds, are not of primary importance in the direct purification processes, their luxuriant growths on the surface of some beds are probably of ecological importance and they may result in the choking of the bed. The following ate the more commonly occurring forms considered ecologically important on bacteria beds: Cyanophyceae
Phormidium
Chlorophyceae
Ulothrix sp. Stigeoclonium sp. Monostroma.
Associated with these are found unicellular types including diatoms, and, locally, mosses and liveworts may form luxuriant growths. Protozoa
The protozoan fauna of bacteria beds is richer than that of activated sludge. This is probably due to the reduced interspecific competition because of the stratification possible in the beds. Such stratification has been demonstrated by several workers, the species associated with a lessefficient activated sludge being found nearer the surface of the bed and those of a more efficient sludge, nearer the base of the bed, associated with the more purified state of the sewage. It was also found that the vertical distribution of these species at different works was affected by the strength of the sewage; seasonal changes, due to the sloughing of the film and possibly to temperature changes, also occur. Liebmann found horizontal as well as vertical zoning of microorganisms in bacteria beds. He also found that the vertical stratification was affected by the loading, the polysaprobic and mesosaprobic forms extending deeper into the bed the higher the load. The common species found in beds are probably the same as those listed
118
The Ecology of Bacteria Beds
for activated sludge. Carchesium, which is abundant nearer the bottom of the beds, is of more common occurrence. FACTORS DETERMINING THE ACCUMULATION OF FILM
The inter-relationship and activity of thes~ different members of the film are similar to those outlined for activated sludge, although modified to the extent that stratification is possible. By their combined activity the organic waste is removed or oxidized and as a result the microbial mass increases. Unless means were available for the removal of the excess film the beds would eventually choke. In discussing the factors influencing film accumulation those affecting both the rate of growth of the film and its reduction must be considered. Within the bed, temperature, food and aeration are probably of primary importance in determining the rate of growth of the film. Increases in temperature up to 20°C result in increased rates of growth for most organisms of the film. Both the nature and strength of the sewage are important in considering the food supply of the film. The nature of the organic waste will determine the dominant organisms of the film. As has already been mentioned, different organisms increase their mass by different amounts whilst oxidizing the same amount of organic matter; zoogleal bacteria, for example, increase less than several common sewage fungi in breaking down a corresponding weight of glucose. The strength of the liquid fed to the bed affects the rate of growth of the film more than does the loading (strength and volume).
Fig:2. Some Macrofauna of Bacteria Beds
The Ecology of Bacteria Beds
119
Being aerobic organisms, the rate of growth of organisms of the film will be restricted if the aeration of the bed is inadequate, as may occur when film accumulation has taken place or at times when the air currents through the bed are for other reasons reduced. Agencies operative in the removal of film have been described by different workers. British workers consider that the activity of the grazing fauna is of primary importance in the control of film, although Tomlinson has also reported the significance of bacterial attack on the hyphae of starving fungi. American workers, on the other hand, consider that physical scouring by the liquid and microbiological activity of the film are most important, the activity of the macrofauna being considered incidental or at most having a minor role. Usinger and Kellen, however, considered that the larvae of Psychoda were effective in film removal, thereby improving the efficiency of the bed. It is also possible that with the different filmsbacterial, fungal and surface algal growths-and also at different degrees of film accumulation, the relative importance of the different contributory factors varies. Some of the causes suggested by American workers can only be opertative if the film is dominated by fungus and anaerobic decomposition of the film is more likely to be important only when considerable film accumulation has taken place. A further possible source of misunderstanding is that it has been assumed that the factor causing the seasonal unloading is necessarily the same as that which is reponsible for the continuous removal of film which takes place throughout the year. Observations at Minworth showed that although the grazing activity of Anisopus fenestralis was important in controlling fungal film growths in the winter, the seasonal fluctuations in film were not accounted for by the differential effects of temperature on the grazing activity and on the film growth. The Vertical Distribution of Film
The percentage distribution of film within a bed depends largely upon the volumetric loading and the instantaneous rate of application. Tomlinson found that the film was more evenly distributed in beds operated on alternating double filtration than in single-filtration beds. Increasing the instantaneous rate of application by decreasing the frequency of dosing produced still greater evenness of distribution. GRAZING FAUNA
Ecologically the holozoic protozoa feeding on other microorganisms should be included in this group, but they are more usually considered as part of the film, as are the small metazoa such as nematode worms and
120
The Ecology of Bacteria Beds
rotifers. Their importance in controlling the film, however, should not be overlooked because they are less obvious. In experimental beds in which film accumulation was being studied in the absence of grazing fauna, discharge of humus solids was attributed to the large numbers of nematode worms which had accidentally become established. Of the larger members different species are dominant at different works. Locally other species may dominate: the snail Physa integra and other snails are reported from some beds in America in such quantities that they were a serious nuisance by blocking pipelines, by abrasion of sludge-pump pistons and by becoming lodged in the mercury seals of distributors, thereby stopping them. In this country Lymnaea pereger is common in the high-rate recirculation beds at Harrogate and its presence there is attributable to the operating conditions. Lloyd has shown that the grazing fauna of bacteria beds is derived from that of the mud-flats and, in a later paper, these two environnments are discussed to explain why so few of the members of the mud-flat community have been able to colonize the bacteria bed. Although toxic trade wastes limit the numbers of species, the chemical nature of sewage is not considered of primary importance in restricting the fauna. The physical environment and nature of the food are probably the most important factors limiting the macrofauna of bacteria beds. Of the sixteen species of Psychoda in the Leeds district, all except one of which are so much alike that they require microscopical examination to identify them, only three are found in bacteria beds. Table:l. Macro Fauna of Bacteria Beds • Oligochaeta (Worms) Lumbricillus (Pachydrilus) lineatus Enchytraeus albidus Lumbricus rubellus Eisenia foetida Dendrobaena subrubicunda. • Insects Collernbola (Spring-tails) (a) Achorutes subviaticus (Hypogastrura viatica). (b) Tomocerus minor (c) Foisomia sp. Coleoptera (a) Cercyan ustulatus Diptera (Two-winged flies) (a) Psychada alternala (b) Psych ada severini (c) Psychoda cinerea
The Ecology of Bacteria Beds
•
121
(d) Hydrobaenus (Spaniotoma) minima (e) Hydrobaenus (Spaniotoma) perennis (f) Metriocnemus hygropetricus (longitarslls) (g) Metriocnemus hirticollis (h) Anisopus fenestralis (i) Paracollinella (Leptocera) fontinalis (j) Scatella silacea (k) Spaziphora hydromyzina. Arachnida (Spiders and mites)
Lesserlia dentichelis Porrhomma thorellii Erigone artica var. maritima Plalyseius tenuipes. Their liking for moisture and their ability to breed in confined spaces were considered important factors enabling them to colonize the beds. The unsuitability of the bed for pupation was also considered important in preventing its colonization by many insects. Although described as 'saprophytic' feeders, most grazers should be regarded as holozoic, feeding on the living ilm; they are rarely found amongst the truly saprozoic forms in sewage sludge. The species which have been able to colonize the bacteria bed find the environment favourable for rapid multiplication. Seasonal temperature fluctuations are less marked in bacteria beds than in other more natural environments and insects which are capable of producing several generations a year do, in fact, produce more generations per year in bacteria beds than under natural conditions. Because of the natural selection by the environment there are fewer competing species for the available food, but at times when this is limiting severe interspecific and intraspecific competition takes place; under such conditions some-especially the chironomid flies-become predatory. Factors Determining the Nature of the Grazing Fauna Population
Different beds support different grazing faunas and in determining the nature of the fauna several factors are probably involved. Beds treating normal domestic sewage usually have a full complement of grazing fauna, but a strong sewage tends to limit the fauna; Psychoda alternata and Spaziphora hydromyzina then become dominant. This may be due to the high oxygen-demand of the sewage or to the nature of the film it produces. Toxic trade wastes restrict the fauna tofew species; different species probably react differently to specific toxic discharges, but generally
122
The Ecology of Bacteria Beds
Psychoda alternata, Anisopus fenestralis, Achorutes subviaJicus, Lumbricillus lineatus and Spaniotoma minima are found in beds treating industrial sewages at Birmingham. At Huddersfield Reynoldson found the more resistant worm Enchytraeus albidus replaced L. lineatus. This further restriction in fauna to a few surviving species enables high populations to become established, with the consequent risk of fly nuisance. Just as physical factors were important in restricting the numbers of species colonizing beds, they are also most important in determining the nature of the fauna in different beds. The nature and size of the medium in the bed has been found to affect the relative composition of the fauna. At one works having beds of different-sized media it was found the larger medium favoured Anisopus fenestralis and the smaller medium Psychoda alternata; in the very fine medium (lh in.) of one bed Lumbricus rubellus was dominant. Terry attributed this to a thigmotaxic effect, whereby the organism tends to maintain the maximum contact between its body surface and the surrounding medium. Comparing four different media under identical operating conditions it was found that in the bed having 2 lh in. roundgravel medium Lumbricillus lineatus was the dominant grazer whereas fly larvae were dominant in smaller media. It is possible that the form of the medium is also of importance: clinker or slag having many pits or depressions would provide a more hospitable niche than smooth gravel, for example. A further physical factor probably associated with the medium size is the downward rate of flow of the sewage over the stones; this obviously varies with the different methods of applying sewage to the beds, e.g., different rates of application and periodicity of dosing. At Minworth (Birmingham), beds receiving sewage as a gentle continuous spray supported a succession of dominant species including Achorutes subviaticus, Psychoda alternata, Anisopus fenestralis and Lumbricillus lineatus. Comparison with other beds treating the same sewage, but with increased rates of downward velocity from different operating conditions, showed that as the rate increased, Achoruies subviaticus, Psychoda alternata and Anisopus were successively eliminated leaving lumbricillid worms as the only effective grazing fauna in the alternating double filtration beds on which the distributors revolved once in 30 min, giving a high instantaneous dose and resultant high downward rate of flow. In beds served by distributors with spaced jets and no splash plates the downward flow varies horizontally across anyone bed, being almost dry between the jet-lines in some beds. Although some.lateral spread does
The Ecology of Bacteria Beds
123
occur as the sewage passes into the bed, this is not as great as had been thought when the bed is clean, and the upper zone of the bed, where the largest proportion of the grazing population are found, often provides two niches, one below the jet, where the downward flow may be great, and the other where there is hardly any flow. These conditions produce horizontal stratification of the fauna, lumbricillid worms and some fly larvae being found grazing on the film in the sub-jet zone, and Achorutes and other fly larvae (Psychoda) being more frequent in the interjet zone, which also provides's suitable conditions for pupation of the larvae from the sub-jet zone. Because of the lack of flow humus tends to accumulate in the inter-jet zone and true saprobic types such .as the beetle Ceryon ustulatus and Spaziphora hydromyzina occur here. The instantaneous rate of application also affects the vertical distribution of the macrofauna. High instantaneous rates limit some species such as Psychoda and Anisopus larvae and Achorutes in the surface layers, but after the initial flush is spent in the upper layers of the bed, the reduced scouring action permits fly larvae to become established within the bed. The relative distribution of Anisopus larvae in the upper 30" of different beds receiving sewage at different instantaneous rates because of different frequencies of dosing. Lumbricillid worms, being strongly prehensile because of their setae, are able to withstand considerable flushing action and their vertical distribution is less affected by the flow, in fact the reduction in fly larvae by high instantaneous rates may result in an increased percentage of worms nearer the surface because of reduced interspecific competition. Both the effect of medium size and downward rate of flow are affected by the degree of film accumulation. Even under the jets on large round medium where Psychoda larvae are unable to exist normally the development of thick fungal growths provides a suitable niche in which they become established. The amount of film accumulation may also determine the type of fauna: Psychoda, for example, is usually associated with thick film whilst Spaniotoma and Metriocnemus prefer a clean bed. It is also possible that the nature of the film, i.e., whether bacterial, algal or fungal, may have a selective effect on the fauna because of food preferences. Different operational practices may also affect the fauna. Lloyd attributed the absence of Metriocnemus in the Barnsley beds to the practice of resting for fortnightly intervals; because Metriocnemus is associated with the uppermost part of the bed and requires a constantly wetted surface it was thus adversely affected. The abundance of any species is largely determined by temperature,
124
The Ecology of Bacteria Beds
food supply and the abundance of competitors and predators. In beds treating domestic sewage, the number of competing species controls the abundance of anyone species, but when the fauna is restricted by trade wastes, etc., reduced interspecific competition results in larger populations of the surviving species. Lloyd reported that the abundance of Psychoda alternata was successively greater in the beds at Leeds, Barnsley and Huddersfield, the fauna being successively poorer at the three localities. The seasonal incidence of any species is influenced by the number of other competing species present in the bed; the invasion of the bed by a new species may appreciably change the incidence of the species already present. Reynoldson found that the summer depletion of the fungal film at Huddersfield by Psychoda was the chief factor determining the incidence of the worm Enchytraeus albidus. The invasion of the Barston beds by Anisopus fenestralis was shown to have restricted the incidence of Psychoda to August, September and October, whereas previously it was common between April and November. At higher temperatures, up to the optimum, insects complete their life cycles more rapidly and, other things being equal, the population will tend to increase more rapidly. Because different species have different thermal requirements there is a succession of dominant species throughout the year in beds with a mixed fauna. Lloyd also showed that periods of maximum abundance of sewage flies are not brought about by a gradual increase in numbers, but by a series of alternating peaks and depressions, and the subsequent decline is in like manner; this type of incidence is induced by sudden changes in temperature. In beds with restricted fauna the food supply in the form of the film is probably the most important limiting factor in contrOlling populations. At Minworth, where Anisopus fenestralis is the dominant grazer for most of the year, its relative seasonal abundance and incidence can be accounted for by the amount of film in the different beds. It was found experimentally that shortage of food not only resulted in a higher mortality of the larvae due to intraspecific competition, but the larval phase was lengthened and the resultant flies were smaller. Excessive film accumulation, although providing ample food, may create conditions unsuitable for the grazing fauna, the population of which is thereby suppressed. The value of the grazing fauna in controlling film has already been discussed; it is possible, however, that they playa more direct role in the process: In small-scale experimental beds both Parkinson and Bell and Reynoldson reported increased nitrification follOWing inoculation with Achorutes and Lumbricillus lineatus respectively. Dyson and Lloyd likened the grazing activity of Achorutes to the
The Ecology of Bacteria Beds
125
predatory activity of the holozoic ciliates whose activity maintained the bacteria in an active physiological state. Their grazing, however, would be expected to remove bacteria and protozoa indiscriminately as film and according to the "law of disturbed averages" by Volterra if two species are uniformly destroyed in proportion to their abundance, the mean numbers of prey increase and of predators decrease. On this basis the grazing activity of the macrofauna would tend to favour the bacterial population. The introduction of Lumbricillus lineatus to similar experimental beds at Birmingham, which were, however, already clean by virtue of recirculation, produced no increase in nitrification, so it may well be that the increased nitrification reported by previous workers, reflected improved bed conditions. Because insects excrete nitrogen in the form of uric acid or ammonium compounds, readily capable of oxidation, their activity would be expected to assist in the biochemical breakdown of the film. Unfortunately, although beneficial in the process of purification, the abundance of some members of the grazing fauna gives rise to considerable nuisance on and around sewage works. Psychoda alternata, Anisopus fenestralis and less frequently chironomid flies have been reported as causing a nuisance when numerous; although non-biting, because of their close association with sewage, their presence in the home must be considered a potential menace to health. Culicoides nubeculosus, a troublesome blood-sucking midge, is present on some works. An operator has not only to run his plant to achieve efficient purification, but must do so without causing nuisance in the vicinity; the prevention of fly nuisance is his duty. FACTORS INFLUENCING THE FILM-GRAZING FAUNA BALANCE
The factors affecting the nature and abundance of the two sections of the bacteria bed community have been discussed. The different populations, however, exist in a dynamic state of balance. The interrelationship of these population is involved in the breakdown of organic matter in a bacteria bed. The effect of chemical, physical and biotic factors on this balance of populations will now be considered. Chemical Factors
The nature of the organic waste being treated largely determines the nature of the film. This may in turn affect the grazing fauna, which may also be affected directly by the waste especially if toxic. With some wastes, although they are amenable to biological oxidation in bacteria beds, the resultant film is not suitable food for grazing fauna, with a result that the
126
The Ecology of Bacteria Beds
beds eventually become choked. At Stoke-on-Trent, experimental beds, successfully treating gas liquor, broke down due to the coating' of the medium with a dark-brown resinous matter, probably derived from the higher tar acids present in the liquor. Apart from the collembolan Folsomia sp. a few of which were found near the base of the bed, the bed was devoid of grazers and tests carried out showed that the film was not taken by several common grazers. Because of the differential toxicity to the film and fauna, some wastes, although permitting film growth-usually fungal-suppress the grazing fauna with the result that tl1e bed chokes. Less toxic substances restrict the fauna to a few species and the reduced interspecific competition may lead to fly nuisance. These chemical factors are imposed by the waste to be treated and apart from pretreatment and trade-effluent control in the case of sewage, the only practical method· of varying it is by dilution by such means as recirculation. Intentional additions of chemicals to the sewage are sometimes practised. When these are made to correct the waste either nutritionally in the case of some trade wastes, or to adjust the pH, they may be considered as modifications to the environment to encourage the suitable organisms for purification. The spasmodic application of chemicals, such as the liming and salting of beds to remove slime and the applications of insecticides in an attempt to alter the biological balance, although necessary palliative measures, are ecologically unsound. Hawkes showed that, by suppressing the Anisopus fenestralis population early in the year by insecticide treatment, the film remained available as food for later generations, which as a result were larger than in the untreated beds where the population was controlled by the limited food supply, the film having been removed by the earlier generation. Although successive applications of insecticide changed the fauna to one dominated "by Achorutes this could only be maintained by expensive insecticide treatments and when these were suspended the normal fauna was rapidly re-established. Physical Factors
The size and nature of the medium used in a bed have probably more effect on'the grazing fauna than on the film. With large medium excessive film accumulation can be better accommodated, wit!{ the result that the efficiency is less affected. However, under clean conditions beds with smaller medium are more efficient. It should be the policy, however, to prevent excessive accumulation of film and not to design to accommodate it, and therefore small medium is the ideal. Intentional additions of chemicals to the sewage are sometimes practised. When these are made
The Ecology of Bacteria Beds
127
to correct the waste either nutritionally in the case of some trade wastes, or to adjust the pH, they may be considered as modifications to the environment to encourage the suitable organisms for purification. The spasmodic application of chemicals, such as the liming and salting of beds to remove slime and the applications of insecticides in an attempt to alter the biological balance, although necessary palliative measures, are ecologically unsound. Hawkes showed that, by suppressing the Anisopus fenestralis population early in the year by insecticide treatment, the film remained available as food for later generations, which as a result were larger than in the untreated beds where the population was controlled by the limited food supply, the film having been removed by the earlier generation. Although successive applications of insecticide changed the fauna to one dominated by Achorutes this could only be maintained by expensive insecticide treatments and when these were suspended the normal fauna was rapidly re-established. The downward rate of flow of the waste through the bed can be varied at equivalent overall dosage rates by recirculation, double filtration, the frequency at which doses of the waste are applied to the bed, and the type of nozzle through which it is discharged. Although this factor affects both film and fauna, the effect on the film is largely restricted to the surface and below the surface the fauna is affected to a greater extent than the film. It has been claimed that film accumulation can be prevented by the flushing action of the sewage and the success of alternating double filtration and low-frequency dosing, Le., heavy doses at infrequent intervals, has been attributed to this physical scouring because of the high instantaneous dosage rates. Although these forces may be operative in removing humus solids previously detached by other means such as grazing, fungal growths are able to withstand very great scouring action as is shown by their presence on the impeller blades of rotary pumps revolving at approximately 1,000 rpm. Although immediately below the jet the fungal growths may be limited to Fusarium and Geotrichum, below the surface layer the downward flow of the liquid, although in some cases great enough to affect the grazing fauna, is not considered to have any significant effect on the film within the bed. Thus, because of the differential effect of the rate of downward flow, the film accumulation may be greater at higher instantaneous rates of dosing. Testing six types of distributor nozzle through which the sewage was discharged to the bed at different forces and differently distributed it was found that because of the suppression of the grazing fauna where the flow was great the film accumulation was greatest. Where horizontal
128
The Ecology of Bacteria Beds
stratification occurred due to jet spacing the two niches thus provided permitted Anisopus larvae and Achorutes to exist together. Where the distribution was even, these two grazers were in direct competition, Achorutes only becoming abundant when the A. fenestralis population was reduced in the summer. The ,continued grazing activity of the two populations is considered desirable and on this basis, jets spaced at intervals, were considered preferable to even distribution. Temperature also has a complex differential effect on the filmfauna balance. The rate of increase and activity of both film and fauna is generally greater at higher temperatures within the range usually experienced in beds. Therefore the higher general metabolic activity within the bed results in enhanced purification at higher temperatures. Low temperatures suppress the grazing activity more severely than the growth of film and this differential effect has been claimed to account for the winter accumulation of film. With a purely bacterial film this is probably the case. The relationship between fungal film and grazing fauna; although successive generations of fly larvae modulated the film incidence, they were not the cause of it. Whatever the cause, however, the winter accumulation of film and reduced efficiency are associated with lower temperatures and in many cases these are a limiting factor in the winter operation of beds. Siting of the beds and the temperature of the waste both affect the bed temperature. If the aeration of a bed is brought about by the differences in temperature throughout and above the bed, then because of the greater seasonal fluctuations in air temperature than in bed temperature, the aeration will vary throughout the year, usually being greater in winter than summer. This increased ventilation may result in increased fungal growths in the winter which then reduce the aeration. Biotic Factors
The most important biotic factor affecting the balance of populations is the basic food supply entering the community, i.e., the waste. The strength of the waste determines the growth rate of the film which in tum determines the grazing fauna population. The growth rate can, however, be controlled by controlling the feeding as opposed to the iood itself. Although the film population and fauna population mutually affect each other, it is probabl~ true to say that the amount of film is a more important factor determining the fauna population than the converse. In more natural environments, factors controlling animal populations are still not fully understood, but in bacteria beds the food supply is the most important factor. The soundest way to control a fly population breeding in the bacteria beds, therefore, is by controlling their food supply, i.e., limiting
The Ecology of Bacteria Beds
129
the accumulation of film. The biological control of insects by the introduction of competitors or predators, operating as it does on a densitydependent factor, appears attractive for application in the enclosed environment of a bacteria bed. Although this method met with initial success in other fields, especially when applied,to confined environments with an equable climate, such as tropical islands, it would appear that its applications are now becoming exhausted. The fly Spaziphora hydromyzina is predatory on Anisopus fenestralis but, presumably because of its temperature requirements, it occurs later in the summer than the peak Anisopus population; it in turn is also highly parasitized by an ichneumon fly Phygadeuon cylindraceus. As with activated sludge, a pre-requisite of the process is the intimate contact of the waste with an active microbial population in the presence of oxygen. In bacteria beds, however, this is achieved in a different manner by allowing the waste to flow over the microorganisms exposed as a zoogleal film on the bed medium, the oxygen mostly being provided by the air currents through the bed. The biophysical and biochemical processes bringing about the waste removal and its oxidation are probably similar to those operative in activated sludge. GRAZING FAUNA
1 I I
~-------
I
---I
Holozoic Protozoa
+
I
I~ I I I I I
I
r~
I I
Saprobic Micro-organisms Film
I
+ Dead Organic Solids
r
'If
I I
l-I
~
L
I ,I
1
I
"
J
Autotrophic Bacteria
: :
I!
Humus
I
I I I I
1
~
L{
-+=:- J
Organic Matter In
I
Solution and Suspension
Qegraded Organic Matter In Sol.
j
ti I
:
I
f·· .. • ....•• .. ··~· .. ••••••• .. ••• .... ··:
Influent
1 Effluent
Mineral
t
Salts
I
SyntheSIS
LBy Bio-oxidation I
By Death and Waste it Products
Fig:13. Diagrammatic Representation of the Main Paths of Material Transfer in the Purification of Organic Wastes in Bacteria Beds
130
The Ecology of Bacteria Beds
ACTIVITY iN BACTERIA BEDS
Bacteria-bed film has been credited with phenomenally high instantaneous rates of oxidation in keeping with the rapid removal of BOD during the relatively short retention time of the liquor in the bed. The relative instantaneous rates of removal and oxidation will probably depend upon the nature of the waste. With wastes having the organic matter in solution and in such concentrations that they are limiting the growth rate of the microorganisms, the organic matter could be expected. to be utilized as it is removed. With stronger soluble wastes some may be stored as reserves such as glycogen within the cells for subsequent oxidation. With wastes having organic matter as suspended or colloidal solids, these readily flocculate on the surface of the film and may only be absorbed slowly, prior to oxidation. The period of contact, the liquid retention time, should be sufficient to ensure the transfer of the waste to the film by adsorption or absorption for subsequent oxidation. To avoid the progressive accumulation of unoxidized waste in the bed, the overall rate of oxidation of the waste must equal the rate of removal of the waste. In other words, the amount of waste, the oxidation of which is completed per day, should equal the amount of waste removed per day. This does not imply that the time needed for oxidation is the same as that required for the initial removal of the waste. There is no functional relationship between the liquid contact time-the time required for the removal of the waste-and the instantaneous rate of oxidation of the removed waste. As an analogy we may consider the routine in a laboratory regularly receiving 100 samples for BOD determinations each day. Although the time needed to perform the test on 100 samples is 5 days, the overall rate of obtaining results is the same as that of the reception of sampies-IOO per day. The 100 samples received per day represent the waste removed in unit time in the bed, the 5 day incubation period of.the test represents, conveniently enough, the period between removal of the waste and the completion of its oxidation. The 100 results represent the amount of waste, the oxidation of which is completed each abovementioned unit of time. The 500 samples in the incubator represent the amount of waste stored or under oxidation in the bed. The analogy may be safely carried a step further. If the number of samples were increased to 120 per day, then, after a 5 day period, the number of results-or tests completed, would be 120 per day provided that two conditions were fulfilled. Firstly the capacity of the incubator
The Ecology of Bacteria Beds
131
must be adequate to accommodate the increased number of samples being incubated-600, and secondly, the laboratory staff, by increasing its activity or numbers, must be capable of dealing with the extra samples. Similarly, in a bacteria bed a 20 per cent increase in the load of removed waste could be dealt with provided that the film was able to accommodate the resultant increased storage matter without impairing its efficiency and secondly the microorganisms were able to increase their activity or population to deal with the increased food supply. These considerations point to two factors limiting the loading of bacteria beds; the extent to which they can accommodate the unoxidized removed waste without impairing the efficiency of the film, and the size of the microbial population that the bed can effectively support. In addition, of course, the effect of the increased load on the initial removal must also be considered. The instantaneous rate of oxidation is a function of the concentration of the substrate, thus the high rates of oxidation associated with the bacteria-bed film may be due to the concentration of the waste at the surface of the film due to the Physical forces associated with interfaces. The amount of unoxidized organic matter in a bed will depend upon the speed of oxidation. If the speed is halved the amount of unoxidized matter is doubled but providing that the film can accommodate this increase and the microbial population deal with it, the total amount oxidized per day will be the same, for although the instantaneous rate of oxidation is halved the amount available for oxidation is doubled. Thus a new balance is established. Below 50 0 P although the initial removal rate in -bacteria beds is probably not affected, the instantaneous rate of oxidation probably is. In beds treating wastes having a high proportion of suspended or colloidal matter, this may result in an excessive accumulation of unoxidized flocculated solids in the film. This resultant increase in film accumulation at low temperatures should be distinguished from that due to the increase in microbial mass, for reasons already discussed. As with activated sludge it is necessary to maintain an active population of heterotrophic microorganisms and in a bacteria bed this can only be achieved by preventing excessive overgrowth of the film. There is normally a seasonal decline in the efficiency of beds during the winter, in contrast with the activated sludge process. We have seen that there is usually more film present in the beds during the winter than in the summer. The extent to which the decreased efficiency is due to lower temperatures or increased film is difficult to assess. The seasonal fluctuations in film accumulation, temperature and
132
The Ecology of Bacteria Beds
efficiency of two beds, one in which the usual accumulation of film occurred in the winter and the other in which it was suppressed by controlled frequency of dosing. It will be seen that the efficiency of the bed in which there was no winter accumulation of film showed little seasonal fluctuation compared with that in the bed in which winter film accumulation occurred. Although not eliminating temperatUre as a factor, these results do confirm that film accumulation itself is an important factor. Whilst most workers agree on the desirability of preventing excessive accumulations of film, there would appear to be some difference of opinion as to the optimum amount needed. Some consider that as thick a growth as possible, compatible with the prevention of ponding and ensuring the adequate aeration of the bed, is desirable. Lloyd considered that were it not for the increased metabolic activity of the microorganisms at higher temperatures in the summer, their reduced numbers, due to the dominance of the grazing fauna, could result in decreased purification. Experience with beds at different works, however, has led me to believe that a very thin film indeed, perceptible only to the touch as a slime on the stones, is all that is necessary for efficient purification. This view is supported by experimental evidence. Using inclined rotating tubes it was found that the maximum efficiency of removal of organic carbon was reached when a film of only 0.12 mm average thickness was formed. Wuhrmann calculated that the thickness of film in which all the bacteria could be expected to receive adequate oxygen supply, as being 100-200 U + 03BC (0.1-0.2 mm). Thus although the amount of film which can be accommodated in a unit volume of bed, without causing ponding or impeding aeration, is limited by the void capacity between the media, the amount of active film is limited by the total surface area of the stones on which the thin film can be supported. Although only a thin bacterial film is considered necessary for efficient purification, the thickness at which its activity per unit area is impaired is more difficult to establish in practice. As a rough guide, considering medium of nominal size, the volatile solids should probably not exceed 50 g dry weight per cubic foot of bed, possibly less if nitrification is desired. When the growth is dominated by certain fungi, such as Ascoidea which forms foam-like masses filling the interstices, larger amounts-up to 200 g dry weight volatile matter per cubic foot-may be maintained in a healthy aerobic condition. Although such growths render the bed highly efficient, unless they are controlled they eventually result in ponding with
The Ecology of Bacteria Beds
133
marked decrease in efficiency. Such growths have been experienced within beds operating on ADF. Other fungi such as Fusarium and Geotrichum are, like bacteria, more closely associated with the surface of the stones. Consideration must also be given to the vertical distribution throughout the depth of the bed. In conventional beds, because of the reduction in concentration of the waste during purification as it passes downwards, most growth occurs at or near the surface. Although at times when such surface growths are sufficient to reduce the efficiency of the affected uppermost part of the bed, the film within the bed may still be in the active log phase. The surface growths, however, may cause ponding which interferes with the distribution of the waste and the aeration of the bed, thus affecting the efficiency of the whole bed. Furthermore, because of the reduction 'lin efficiency of the upper portion of the bed, the film below receives a higher concentration of the waste, thereby increasing the growth rate of the organisms. Thus a downward growth of the film takes place, the efficient portion of the bed being successively reduced and the nitrification zone lowered or even eliminated. During the unloading of the film the reverse probably occurs, the lower accumulation of solids being discharged first. Bacteria beds must not only be operated efficiently but also without causing nuisance. Flies, such as Psychoda spp. and Anisopus fenestralis, have caused serious nuisance in the vicinity of bacteria beds. Synthetic insecticides are being used successfully to control these flies but such a practice, although under many conditions necessary as a palliative measure, is ecologically unsound. The occasional application of insecticide results in a reduction in the insect population causing an upset in the ecological balance which may result in a rapid increase in film which is then available to support an increased fly population necessitating further applications of insecticide. It is possible by successive applications of insecticide to change the fauna of a bed from one dominated by flies to one having less troublesome organisms such as Achorutes or Lumbricillid worms as the dominant members. Such a balance can, however, only be maintained by expensive insecticide treatments and when these are suspended the normal fauna is rapidly re-established. A further objection to the use of insecticides on bacteria beds is the danger of discharging a toxic effluent. In more natural environments the factors controlling animal populations are still not fully understood, but in bacteria beds food supply is a most important factor. The soundest way to control a fly population breeding in bacteria beds, therefore, is by controlling their food supply,
134
The Ecology of Bacteria Beds
i.e., limiting the accumulation of film. Thus, for optimum efficiency and prevention of nuisance, the limitation of film accumulation is desirable. The control of the microbial population, necessary for efficient operation and prevention of nuisance, cannot be directly effected by such mechanical operations as adjusting the excess sludge weir as in the activated sludge plant. It can only be achieved indirectly through biological agencies by the ecological control of the competing populations. Factors influencing this balance of populations have previously been discussed and it now remains to be considered by which of the factors the desirable level of popUlation can best be maintained. PRACTICAL METHODS OF MAINTAINING THE BALANCE OF LOW LEVEl
The rate of film accumulation may be considered as a function of the rate of growth and rate of removal. (Ra) = (Rg) - (~) where Ra = Rate of accumulation Rg = Rate of growth Rr = Rate of removal To maintain Ra at zero, i.e., to prevent accumulation, two theoretical alternatives are possible. One method would be to operate the bed to encourage maximum growth-i.e., R is a maximum-for the waste being treated, and maintain conditions so t~at Rr = Rg. The alternative would be to limit the growth rate Rg by operational means so that Rg equals Rr under the operating conditions imposed. The two alternatives are thus basically different and are, as we shall see, in some ways incompatible. Of the two possible approaches, the former, whereby maximum growth rate and activity of the film are encouraged, but the accumulation of film is maintained at a low level by a dominant grazing fauna, is preferable. The resultant low level of the film accumulation would cause severe competition for the limited food among the grazing fauna. In insect popUlations the percentage number of larvae which successfully pupate, before emerging as adults, decreases with decreasing food supply when the latter is limiting. Thus under conditions of limiting food supply, much of the available food is wasted to the fly population by the death at different stages of development of larvae which have used some of the available food. This phenomenon, known ecologically as a "scramble", is attractive as a method of utilizing the benefits of an insect population without incurring the nuisance of a large adult population.
The Ecology of Bacteria Beds
135
It is a self-regulating mechanism in that the fewer eggs produced by
the reduced adult fly population would result in a reduced larval population with less severe competition and a resultant lower death rate. Unfortunately, it is only in beds treating a weak waste with resultant low growth rate -(Rg) that conditions suitable to support this balance can be maintained throughout the year. In other beds, although at summer temperatures such conditions may prevail, the lower winter temperatures result in the accumulation of film for reasons previously discussed. If the rate of accumulation is slight, it may be that the resultant accumulation throughout the winter is not sufficient to affect the efficiency of the bed before the spring unloading takes place. In more heavily loaded beds, or those subjected to lower temperatures, the greater accumulation of film results in decreased efficiency. In both, the unloading in the spring, usually brought about by an increased grazing population, uncontrolled by the available food, is associated with a potential fly nuisance. Also, by permitting this winter accumulation of film, the increased rate of discharge of humus sludge during the resultant spring unloading, imposes an increased load on the digestion plant, often when it is least able to cope with it. These effects of low winter temperatures could be overcome by heating the beds either directly or by heating the feed liquor. Fuel for this could be provided by the gas from the digestion of sludge. Alternatively in the vicinity of generating stations, cooling water could be used. In many beds, because of the nature and strength of the waste and the economic necessity of operating at higher rates, the grazing fauna is not able to dominate the film, especially in the winter, and in such cases it is necessary to adopt the second alternative and control the growth rate (Rg). Of the advantages claimed for the modified methods of filtration commonly practised, such as recirculation, alternating double filtration, and low frequency dosing, the control of film accumulation is common to all. The method by which this is achieved, however, has been variously described; some consider that the flushing action resulting from the higher rate of instantaneous application is the cause, others consider that nutritional control is the operative factor. In the light of results reported above, it is considered that flushing action can have little effect in removing attached film from within the bed at the usual rates of application practised in this country. An understanding of the controlling mechanism is not only of academic interest but is essential for its successful application and for the diagnosis of the cause of failure should this occur. We shall,
136
The Ecology of Bacteria Beds
therefore, now consider these different methods of "filtration" as film controlling processes, although in doing so their other advantages should not be overlooked. Recirculation
Ecologically the most important features of recirculation, whereby the waste is diluted with returned effluent before being applied to the bed, are the reduction in strength of feed and the increase in the hydraulic load. These two factors influence the growth rate of the film and its vertical distribution throughout the depth of the bed. Applying the growth rate/ concentration curve discussed earlier, a waste having a concentration C1 gives an arbitrary growth rate Rl on the surface of the bed operating as a single filter. By introducing recirculation and thereby reducing the concentration of the feed to C2 the resultant growth rate is reduced to R2 . Consider now the growth rate within the bed, say at a depth of 2 ft. With single filtration suppose the initial waste concentration C 1 has been reduced to C3 giving a reduced growth rate R3 . With recirculation because of the higher hydraulic loading the reduction in concentration in the upper 2 ft of bed is less than with single filtration and the resultant concentration C 4 may well be higher than the concentration C3 at the corresponding depth in the bed operating by single filtration, as indeed was found to be the case by Lumb and Eastwood. As a result, the growth rate of the film at a depth of 2 ft within a recirculating bed is greater than in a conventional bed. These results would account for the reduction in amounts of surface film and the more even vertical distribution of thE! film throughout the depth of the bed with recirculation. A greater dept!l of the bed is thus used for carb~maceous oxidation but the zone of nitrification is restricted. To effect this reduction in film growth at the surface it is essential that the dilution of the feed is sufficient to result in a concentration which falls on the slope of the growth rate curve, i.e., a concentration at which nutrient is limiting. It could be that with a strong waste Cs' and a low dilution ratio, the resultant diluted feed C6 would have the same growth rate-RS'6 at the surface of the bed, the concentration of the feed C6 not being growth controlling. Furthermore, because of the resultant greater hydraulic loading it is possible that the accumulation of film will also occur to a greater depth than with single filtration. To control film growth nutritionally in beds treating fluctuating flows, the dilution ratio should ideally be constant, i.e., the recirculation rate varied directly with the flow of waste. Lumb and Eastwood compared different applications of recirculation to beds treating fluctuating sewage
The Ecology of Bacteria Beds
137
flows. On the bed in which the recirculation rate was varied directly with the sewage flow to maintain a constant dilution ratio, there was little ponding. On a second bed, however, in which the recirculation rate varied inversely with the sewage flow, to maintain a constant flow to the bed, severe ponding occurred in the winter period. The operation of the former ideal method of recirculation involves plant of larger capacity and variable pumping capacity with the necessary control mechanism. A third method investigated by Lumb and Eastwood, whereby a constant recirculation rate calculated to give an average dilution ratio equal to the ideal system was maintained irrespective of the sewage flow, proved only slightly inferior to the ideaL Plant capacity is not as great as in the ideal system and pumping plant and control gear is the simplest of the three methods. The degree of dilution of the feed depends not only upon the dilution ratio but also on the quality of the diluting effluent. . Thus, if for any reason the efficiency of the bed falls off, normal recirculation will be less effective in controlling film, and further deterioration in conditions may result. By diluting the feed with purified effluent from a more efficient bed or activated sludge plant it is possible that further deterioration would be prevented. Besides this effect of the growth rate (Rg) recirculation may also affect the removal rate (Rr ) by changing the numbers and species of grazing fauna. The increased hydraulic rate of application tends to reduce the numbers of fly larvae such as Psychoda and the Springtails such as Achorutes. The more prehensile Lumbricillid worms persist and with the reduced interspecific competition they may increase in numbers. Such a change in fauna has been reported by Lumb and Eastwood and by Hawkes. There were indications that the Lumbricillid worms were less active in removing film than the Psychoda popUlation they had replaced. Alternating Double Filtration
The process of ADF, in which waste is treated on a pair of beds in series, each bed alternately becoming primary and secondary in successive periods, was first conceived by O'Shaughnessy as a means of controlling excessive film growth. He had observed that excessive accumulations on beds could be removed by applying the effluent from a bioflocculation plant. Thus, from the beginning it was acknowledged that the control of film was an important feature of ADF. Tomlinson later demonstrated that this was due to a starvation effect during the secondary stage of the process during which the film decreased in amount. This is also explicable in terms of the growth curve. During the
138
The Ecology of Bacteria Beds
primary stage the growth rate (R1 ) at the surface is determined by the concentration C1 of the waste, and is the same as that on the surface of a conventional bed treating the same waste. During the secondary stage the film is subjected to the settled primary bed effluent of concentration C2 giving a negative growth rate R2 . Thus the resultant growth rate Rl +R2 2 is less than R1, the rate at the surface of a conventional bed. Because of the higher hydraulic loading the film is distributed more evenly throughout the depth of the bed as in recirculation. The total amount of film, however, is less than in single beds treating the same waste since with ADF all the film is subjected to the negative growth phase in the secondary stage, whereas in a conventional bed the bulk of the film in the .upper layers of the bed is normally never subjected to starvation conditions and the resultant negative growth rate. Thus, this principle of ADF is based on a fundamentally sound theoretical basis. The process has been successfully applied to the treatment of sewage at Reading and Derby and has proved of particular value in treating wastes, such as those from milk processing, which tend to give rise to fungal growths. Experience at Birmingham, where the successful development of the process by the Water Pollution Research Laboratory led to it being adopted for large scale operation, has shown that to be successful the process must produce a primary effluent which will ensure that in the secondary stage the film is in the endogenous phase to produce a negative growth rate (R 2). Even when this is achieved the resultant growth rate is positive and to prevent film accumulation this has to be countered by an equal removal rate (Rr ). In beds operating at conventional hydraulic loadings this is mostly provided by the activity of the grazing fauna. At times when this is reduced, as when temperatures are low, it is essential that the resultant growth rate is kept low by a negative growth rate in the secondary stage. If for any reason an inferior primary effluent is produced either because of reduced efficiency or higher loadings, the carbonaceous oxidation is continued in the secondary stage and the final effluent is at first little affected apart from the reduced nitrate content since nitrification is not similarly increased in the secondary stage. However, the higher nutrient concentration results in a positive growth rate in the secondary stage and an increase in the degree of film accumulation. This could result in a further decrease in efficiency and a further deterioration in the primary effluent. Thus conditions progressively
The Ecology of Bacteria Beds
139
deteriorate until the process eventually breaks down in the complete ponding of the beds. It was found that this adverse sequence of events could be prevented by applying a further growth-controlling factor-the frequency at which successive doses of sewage were applied to the bed. FREQUENCY OF DOSING
At the same overall rate of treatment the waste may be applied as a continuous spray or as a succession of doses. The frequency at which these doses are applied may also vary, the lower the frequency the higher the instantaneous rate of application. This frequency of dosing is a most important ecological factor in the operation of bacteria beds. Early in the history of bacteria beds it was reported in evidence to the Royal Commis~ion on Sewage Disposal that intermittent dosing of beds limited the accumulation of film. It is of interest to note that one of the first bacteria beds to be operated at the Lawrence Experiment Station, Massachusetts, in 1890 was dosed intermittently at 20 min intervals. At Birmingham early in the century, one of the experimental beds was equipped with a one-armed distributor mechanically driven to rotate once every 7 min by means of a 2 hp oil engine. Lumb and Barnes at Halifax found that by s~owing down the speed of rotation of a four-armed distributor, the condition and performance of the bed was improved. Tomlinson and Hall investigated the effect of frequency of dosing on beds operating on ADF They concluded that for four-armed distributors the optimum rate of rotation under their conditions of experiment was between 15 and 30 min per rev. During the later period of these investigations comparisons of the seasonal fluctuations in film accumulation and fauna in low and high frequency dosed beds were made. In the beds served with rapidly revolving self-propelled distributors there was, each winter, an excessive accumulation of film; in similar beds treating the same sewage at the same rate but having the distributors mechanically rotated at speeds between 30 and 55 min per rev, the amount of film was uniformly low. There was also a marked effect on the grazing fauna; the fly populations, both Anisopus and Psychoda, were almost eliminated in the low frequency dosed beds. This was partly due to the reduced food supply in the form of film and also to the greater downward rate of flow of the sewage suppressing the larvae populations within the bed. The more strongly prehensile Lumbricillid worms were better able to withstand the stronger flow of sewage and, in the absence of competition from the fly larvae, they be<;ame more abundant in the low frequency dosed beds.
140
The Ecology of Bacteria Beds
Thus, by reducing the frequency of dosing, ecological conditions were created which not only maintained a low degree of film accumulation necessary for efficient purification, but also prevented fly nuisance by changing the fauna from one dominated by flies to one dominated by worms. For successful application of controlled frequency dosing it is essential to appreciate the principles underlying the process. Unfortunately, as in many developments of biological filtration where practice has led theory, the practical success has been explained by various theories. The resultant high instantaneous dosage rate is generally considered to be an important feature of the process. In view of the evidence reported earlier in this work, however, the scouring effect of this high instantaneous rate cannot be considered to be an important factor in controlling film, especially within the bed, although it does affect the fauna, the fly larvae being replaced by the more strongly prehensile Lumbricillid worms. An alternative theory suggests that because of the infrequent application of waste, the film is subjected to reduced nutrient concentration, if not starvation conditions, for some of the period between doses. To investigate this nutritional effect, samples were taken at minute intervals from side arm sampling channels collecting the effluents at 1 ft, 3 ft and 5 ft depths of the bed to measure the change in concentration between closes at 15 min intervals as measured by the OA. Similar samples from the same bed being dosed at I1h min intervals were taken on the next day. The results of both tests expressed as ppm OA remaining per 100 ppm. Only slight fluctuations in the strength of liquid occurred at anyone depth with the rapidly revolving distributor, when the distributor revolved slowly there was a marked reduction in strength over the 15 min period between the doses, this being most marked at the 1 ft depth. Applying these results to the conventional growth rate curve it is seen that at the 1 ft level, for more than two-thirds of the time the growth rate with low frequency dosing is considerably less than with high frequency dosing and may well pass into the endogenous phase towards the end of the period between doses. Within the bed at a depth of 3 ft for half the time the growth rate would be higher with high frequency dosing, although at 5 ft the growth rate would be higher with low frequency dosing. This would account for the more even distribution of film throughout the depth of the bed with low frequency dosing. The results illustrate that with low frequency dosing the average concentration of nutrients available to the film in the upper layers is reduced, thus reducing the growth rate. Therefore the control of film in
The Ecology of Bacteria Beds
141
low frequency dosed beds could be accounted for by nutritional control as in recirculation and ADF. Suggestions by Stanbridge that low frequency dosing may have advantages other than those attributable to the control of film have been investigated. To investigate this possibility, use was made of a circular bed which had a distributor fitted with variable speed drive. On several daily occasions the distributor was revolved at different speeds between 1 rev in 3 min and 1 rev in 60 min, the speed on each occasion being chosen at random; the efficiency, as assessed by the percentage OA and BOD removal and the amount of oxidized N in the effluents, was determined. On the same occasion the efficiency of a similar bed receiving the same sewage through distributors revolving at a fixed rate was measured. The differences between the efficiency of the two beds on each occasion plotted against the rate of revolution of the distributor demonstrates any effect of frequency of dosing, apart from those resulting from film control, since during the short period of the tests the amount of film could be regarded as constant. There was, as expected, over the greater part of the range, between 15 min and 60 min, a reduction in efficiency resulting from decreasing the frequency of dosing. There was, however, evidence that the optimum rate of revolution, for a clean bed, was between 10 and 15 min for a four-armed distributor with staggered jets. At speeds of revolution greater than these, there appeared to be a lowering of efficiency. It is of interest to note that nitrification was only slightly reduced by low frequency dosing. Results quoted by Levine and Byrom also showed that the efficiency as measured by the percentage BOD and OA removal respectively, was somewhat higher when dosed at 5-7 min and 8 min intervals respectively than at 2-2 min. It does not follow from the results that the optimum speed of rotation was 10-15 min per rev; to control the film at limits which do not adversely effect the efficiency, it may be necessary to revolve the distributor at speeds slower than once every 15 min, the resultant loss in efficiency more than being made good by the gain in efficiency resulting from the prevention of the film accumulation. The results nevertheless do indicate some advantages other than those attributable to film control, at least over the range 1-15 min per rev. Some such possible benefits of low frequency dosing have been discussed by Stanbridge. He suggested that although the time of retention of some of the liquid is less with low frequency dosing this may be more than offset by the longer retention time of the liquid held interstitially between doses. Work reported by the Water Pollution Research Laboratory has shown that the mean retention time may be increased by decreasing the frequency of dosing. Below a certain frequency of dosing, however, the retention time
142
The Ecology of Bacteria Beds
is, of course, reduced. As mentioned previously, however, liquid retention time itself may not be so important providing it is sufficiently long to enable the waste to be transferred to the film. The oxidation of the removed waste can proceed during the period between doses. In attempting to determine the times of contact at different rates of revolution it was observed that within a minute of the passage of the distributor arm over the section of the bed a flush of effluent was discharged from the bottom of the bed. The peak concentration of the chloride appeared in the effluent within the first minute but was still being discharged in decreasing amounts three hours later. The reduction in chloride concentration in the effluent took place not at a steady rate but in a series of regularly timed steps; a drop in concentration coincided with the surges of effluent which immediately followed the passage of a distributor arm over the surface. It would appear that when sewage is applied to beds intermittently it passes into the bed as a surge mixing with the interstitial liquid; some of the mixed liquid is displaced in the form of a flush of effluent; the remainder is held interstitially until the next dose, only a small quantity draining away during this period. The amount held interstitially within the bed is, therefore, important in determining the efficiency of a bed intermittently fed, because whilst the liquid was held in the bed, purification would be proceeding. It may well be that such a bed would be more efficient if constructed of medium smaller than the conventional medium used today; the low film concentration associated with the process would make this possible. Low frequency dosed beds are thus quite different hydraulically and in the course of purification from conventional beds. In the latter the waste is considered to be oxidized as it percolates downward over the medium, this course of oxid~tion at different depths being expressed mathematically by Velz's equation. In low frequency dosed beds the course of purification must be considered more in relation to time than depth. Certainly the concept implicit in the terms "percolating" or "trickling" filters is not applicable to low frequency dosed beds, which are better considered as surge filled aerobic contact beds. In view of the rapid mixing of feed and interstitial liquid they are in this respect analogous with the modern complete mixing activated sludge processes. Danckwerts, comparing the piston and complete mixing types of continuous flows through processing units generally, considered that because of the greater spread of the residence time with complete mixing and assuming the rate of the reaction falls off with the extent of the reaction, complete mixing types of reactor were less efficient than the piston flow type. With autocatalytic reactions (such as biological
The Ecology of Bacteria Beds
143
oxidation), however, it was considered that, because of the different rateof-reaction curve, the completely mixed reactor may be superior. In applying these views to bacteria beds and activated sludge plants, however, it should be borne in mind that the lag phase is probably absent. A further advantage of low frequency dosing, it was suggested, is that the higher instantaneous rate of dosage results in a greater volume of the bed medium being wetted and thereby the effective capacity of the bed is increased. To investigate this suggestion, the lateral spread of sewage applied to the surface of a clean bed as jets 12 in. apart was assessed by collecting and measuring the sewage in twelve 2 in. wide adjacent trays placed parallel (tangential) to the travel of the jets so as to collect the liquid from an area 1 ft x 2 ft. Measurements were made immediately above the surface of the bed and at three depths within the bed and at different speeds of revolution of the distributor arm. Most of the lateral spread occurred as the jet impinged on the bed surface; there was, however, no indication that with slower speeds of revolution the lateral spread was greater. Although other advantageous effects of low frequency dosing may exist, it is considered that the control of excessive film accumulation, with the resultant increased efficiency, is probably the most important effect. An appreciation of the nutritional method of film control operative in low frequency dosing and the resultant beneficial effects on bed efficiency, makes possible the reasonable application of the principle. No increase in efficiency could be expected of a bed in which film accumulation never occurred, by slowing down the rate of revolution of the distributor to speeds slower than 10 min per rev, for example, slower rates of revolution would probably result in decreased efficiency due to decreased retention times. For a similar reason, because film accumulation is usually associated with winter conditions, the frequency of dosing necessary to suppress winter accumulations could be lower than the optimum required for summer conditions. On this basis, variable frequency of dosing is desirable. Whether the extra cost of providing variable speed drives is justifiable will depend upon the degree of deterioration in efficiency during the summer due to the low frequency dosing. At Ewell, for example, Stanbridge considered that the decrease in efficiency in the summer months which resulted from low frequency dosing was not sufficient to justify changing the speed of rotation, which during the winter gave marked improvements in efficiency. A bed already ponding cannot be immediately cured by merely slowing down the speed of rotation of the distributor; the periods of low waste concentration between doses, necessary for nutrient control, would
144
The Ecology of Bacteria Beds
not occur under conditions of continuous ponding. If the success of low frequency dosing is due to nutrient control, then it can only be effective in controlling a film of living organisms, fungal or bacterial, and not against an accumulation of non-living solids. The possible accumulation of flocculated solids in beds treating wastes with high suspended and colloidal matter during the colder periods when oxidation is retarded, has previously been discussed. The overall effect of low frequency dosing on such beds is not fully known. One could expect that apart from nutritional control of the organisms, the resultant higher instantaneous rate of application would result in a deeper penetration of the flocculation zone to within the bed where temperature would be higher. It is also possible that the higher instantaneous rates of application could flush away some of the flocculated solids even though the living film resists such action. Similarly, if solids applied with the waste or those formed chemically or physically within the beds accumulate, they cannot be controlled by nutritional methods and low frequency dosing could only succeed by virtue of the high instantaneous rates of application as in the case of solids flocculating within the bed. Practical Methods of Controlling the Frequency of Dosing
In determining the frequency of dosing-best expressed as the time in minutes between successive doses-consideration should be given to unit small areas representative of one stone on which the microorganisms live. Thus on a circular bed served by a four armed distributor, the jets of which are not less than 1 ft apart and are staggered on all four arms, the frequency of dosing at the surface is the time of one complete revolution of the distributor. Even at a depth of 1 ft the degree of lateral spread results in the frequency of dosing at this depth, being half the time of revolution of the distributor. It is only within the depths of the bed where the lateral spread of the waste ensures that the flow from one jet is evenly distributed over at least 1 ft that the frequency of dosing can be considered as on~ quarter of the time of revolution. Troublesome growths usually occur in the upper layers of the bed and in this respect circular beds having distributors with staggered jets are capable of creating conditions of low frequency dosing in the upper layers, to prevent film accumulation and a higher dosing frequency within the bed. Rectangular beds, having the conventional reciprocating distributors, have the same frequency of dosing at all depths. Also, when dosing in both directions, the frequency of dosing differs at different positions along the length of travel. By having distributors with two arms, the jets in which
The Ecology of Bacteria Beds
145
are staggered, and which dose alternately in opposite directions, the conditions of the circular beds could be approached. The jet spacing should be sufficient to ensure that in the upper layers of the bed each unit small area receives waste mostly from one arm only. This can be ensured by having half the width of a bed dosed in one direction and the other half dosed in the opposite direction but the frequency of dosing is then the same at all depths. Low frequency dosing can be applied to conventional single filtration or to supplement the growth controlling effects of recirculation or ADF. The optimum frequency of dosing will depend upon the different operating conditions and the growth promoting properties of the waste. As mentioned previously the optimum frequency may vary seasonally, thus making some variability in dosing frequency desirable. In such two stage processes as ADF it is possible that the optimum frequency of dosing of the primary and secondary stages differs; a lower frequency being necessary to control film growth in the primary stage. The frequent alteration would be made practicable by providing variable frequency of dosing. Generally by providing for a dosing frequency range of 10 to 30 min in the upper layers of the bed, most conditions of operation should be covered. The conventional reaction-jet propelled distributors on circular beds rotate at rates varying with the flow, but usually faster than 1 rev in 3 min. This rate of rotation can be decreased somewhat by turning some of the jets to discharge in the opposite direction to the driving jets and thus act as a brake. The extent to which this method can be carried is limited by the need to ensure rotation of the distributor at low flows and under windy conditions. Generally it is not possible to satisfactorily reduce the speed of rotation to rates slower than 1 rev in 10 min by this method or by other methods involving a non-differential braking device. • Rotating distributors, propelled by a water-powered wheel travelling around a circumferential track or rail, usually have a slower rate of rotation and are more amenable to some adaptation for lower frequency dosing, by changing the travelling wheel for one of smaller diameter, for example. The application of the water wheel system has been successfully used at Coventry in a braking device which is attached to the conventional reaction-jet distributors. To obtain extreme low frequency dosing (1 rev in 20-30 min) the most satisfactory method is probably to provide some form of motor-drive. This could either be attached to the distributor arm and travel round the track as at Ewell, or be stationary and drive through geared wheels on the central distributor column. Such systems could readily include provision for
146
The Ecology of Bacteria Beds
variable speed drives. To ensure the rotation of the distributors and evenness of distribution under conditions of low flows experienced at some works, the distributors are fed through syphon-operated dosing chambers. It has been suggested that these provide some degree of low frequency dosing, but in practice this does not always appear to be effective, probably because for most of the time the period of application is too long in relation to the intervening rest periods. Most investigations on the effects of frequency of dosing have been carried out on circular beds and difficulties have been experienced in applying the results to rectangular beds with reciprocating distributors. The frequency of dosing is affected both by the speed of travel and the distance of travel. Unlike circular beds where the extent of travel-360° - is fixed, it is possible to design rectangular beds so that at speeds of travel which can be satisfactorily maintained by the driving power provided, the requisite frequency of dosing is achieved. In fact many of the water-wheel propelled distributors on rectangular beds complete one journey in about 20 min, thus achieving low frequency dosing. With such distribution, however, no provision is available for varying the frequency of dosing and unlike the rotating distributors with staggered jets, they do not provide a higher frequency of dosing within the bed than at the surface. With motor-driven distributors, by using a longer distance of traveIr although economy is effected by the use of fewer machines required, each machine must be capable of distributing a greater flow and the higher speed of travel necessary, results in greater wheel wear per machine. Application of low Frequency Dosing to Double Filtration
According to Howland, the contact time in beds is directly proportional to the depth and inversely proportional to the surface hydraulic loading D tu-QO.66 Thus theoretically, in two-stage filtration of whatever form, where, compared with a similarly loaded single bed, both D and Q can be considered as doubled, the resultant time of contact (t) is increased. As discussed previously the importance of time of contact, or liquid retention time, probably depends upon the nature of the waste; the removal of colloidal and readily physically adsorbed matter being less affected than that of wastes in true solution. Probably more important than the increased retention time with two-
The Ecology of Bacteria Beds
147
stage filtration, is the provision of two distinct environments where organisms responsible for carbonaceous oxidation and nitrifying organisms can exist separately. Without becoming involved in the controversy whether nitrification occurs in the presence of organic matter, experience with bacteria beds leads one to believe that the autotrophic nitrifying organisms-at least nitrate producing organisms-cannot successfully compete with the more vigorously growing heterotrophic organisms in the presence of available organic nutrient. Thus nitrification is suppressed in the presence of active carbonaceous oxidation, as in the upper layers of a conventional bed where the growth of the heterotrophic bacteria and fungi may produce considerable development of film. The nitrifying organisms are thus restricted to the depths of the bed where the reduced nutrient concentration limits the development of the heterotrophic organisms. Thus nitrification in the presence of organic matter could be restricted, not by the organic matter itself, but by the suppression of the necessary bacterial flora in competition for oxygen and living space with heterotrophic organisms. This view is supported by the fact than in ADF the amount of nitrification is much the same in the primary stage, where the organic concentration of the waste is higher, as in the secondary stage. On this basis it was considered that two-stage filtration without alternation would result in a higher degree of nitrification. Previous investigations comparing double filtration with other methods of filtration showed that although under favourable conditions double filtration gave good results, the tendency of the primary bed to clog during the winter made it a less satisfactory process that recirculation or ADF. The success of low frequency dosing in controlling film accumulation suggested that film accumulation in the primary bed of a double filtration plant could be prevented by low frequency dosing. This possibility is being investigated and results so far published of a period including two winters, show that excessive accumulations of film in the primary bed have been prevented. The 4 hr OA and BOD figures for the final effluent were similar to those from similar beds operating on ADF at the same overall rate-150 gyd. As expected the nitrification was somewhat greater with double filtration, the difference being more marked at higher hydraulic loadings. Comparisons of different methods of filtration by Peach at Cheltenham and Tidswell at Burton have also shown the superiority of double filtration in nitrification. If low frequency dosing of the primary bed results in double filtration being as efficient as ADF it could be that it
148
The Ecology of Bacteria Beds
is preferable since it would then be possible to differentiate in design and operation between the two beds. For example the secondary bed could have smaller medium and be dosed at a higher frequency than the primary bed. It wou.ld also be possible to have primary and secondary beds of different sizes to enable primary and secondary stages to be carried out at different dosage rates. HIGH RATE FILTRATION
This term is usually applied to several methods of filtration in which the liquor is applied to the bed at high average hydraulic loadings-SOD to several thousand gyd. In some cases, as in America, this involves the recirculation of the feed to provide a high dilution ratio. As such it is essentially a growth controlling process. The application of a strong undiluted waste at high hydraulic loadings creates different ecological conditions. The result of such a practice will probably depend on the nature of the waste and the type of growth it produces under the operating conditions prevailing. In the wastes ha ving a high colloidal or suspended organic content, the rate of flocculation probably exceeds that of oxidation and as a result much of the film is composed of flocculated solids. At the high hydraulic rates, the downward rate of flow could remove such unoxidized solids, the bed then acting as a bioflocculation plant, although according to Heukelekian it is still capable of biological oxidation. If the waste supports a bacterial, as opposed to a fungal growth, it is probable that the high hydraulic rates could bring about periodic sloughing of the film as described by Cooke and Hirsch and Reid and Assenzo. Such sloughing would only occur after considerable accumulation of the film had occurred. When the film is dominated by fungus the resultant tougher growth is able to develop to a greater thickness before being sloughed away by the downward flow of liquid. Fusarium, which has tenacious holdfasts, may form the base of such growths on which other orpanisms become secondarily established. It is when those secondary growths create unfavourable conditions below that the original anchoring growths of Fusarium die, and the whole of the film then sloughs off. In these methods of filtration which rely on the physical scouring of the film, the amount of film present is usually greater than that considered desirable for most satisfactory purification. However, such beds are capable of more than 80 per cent removal of BOD and where a high quality nitrified effluent is not essential, they form a useful economical method of reducing the polluting load of a discharge. They could also be used as
The Ecology of Bacteria Beds
149
pretreatment units before activated sludge plants or bacteria beds. Although it is not necessary to control film at the low level desirable in conventional beds, the amount present should not impede the free passage of waste or the ventilation of the bed. If ponding results, the downward flow of liquid is impeded and the scouring action reduced. Eventually the film decomposes under the anaerobic conditions so created to form a watery sludge. At such times the efficiency of the bed is greatly reduced and thus the occurrence of such conditions should be prevented. The thickness of film which can be supported without causing such conditions depends upon the size of the voids which is in tum a function of the medium size. Larger voids would also facilitate the passage of the sloughed film through the bed. The solids discharged would probably be composed of larger particles than the humus resulting from the grazing activity of the macrofauna in conventional beds. Investigations on high rate filtration showed that whereas beds constructed of smooth gravel 1V2 in. to 3 in. in size did not pond seriously when operated at 600 gyd, beds constructed of smaller medium ponded continuously during the winter at different rates ranging from 200-1000 gyd.
Chapter 6
Ecological Operation of Bacteria Beds Even in methods of filtration involving nutritional control of the film, the growth rate (Rg) will be positive and has to be countered by a removal rate (Rr ), Unless tnis is achieved, ponding will eventually result and the efficiency of the growth-limiting processes will be further reduced, bringing about a rapid deterioration in conditions. Also the higher instantaneous rates of application, usually involved in such processes, result in more serious ponding if the process breaks down. In most beds operating at conventional rates, the chief film removal factor is provided by the activity of the grazing fauna. Even high rate beds which normally rely on physical control of film may at times depend upon the activity of grazing fauna for their continued efficiency. In the investigations mentioned above it was found necessary to allow an Anisopus and Psychoda population to become established before the beds could be successfully operated. The establishment and maintenance of an active grazing fauna is thus essential for the successful operation of bacteria beds. The operational requirements necessary to ensure suitable ecological conditions for such a population are a useful guide to bacteria bed operation. MATURATION
With many wastes, including domestic sewage, the microorganisms responsible for their breakdown readily become established in the plant, being introduced with the wastewater. With specific organic wastes the establishment of the necessary flora may take some time. In such cases the process of maturation can be speeded by inoculating the bed either from a stored culture of the organisms concerned or from a plant treating the same waste. A satisfactory practical method of inoculating, in the latter case, would be to apply fresh unsettled effluent containing the humus from an appropriate bed, or the effluent from an aeration tank containing activated sludge known to be capable of oxidizing the waste. Since the lag phase of growth of micro-organisms is longer at lower temperatures, an active
Ecological Operation of Bacteria Beds
151
population of organisms is more rapidly established in the warmer months. A bacteria bed is often considered matured once nitrification becomes established. Ecologically, however, a bed is only mature when a satisfactory balance between film and grazing fauna has been established. Because microorganisms are more ubiquitous and more widely dispersed in water and air and because of their rapid rate of multiplication, not only is their chance of being introduced into the bed greater than that of grazing fauna, but their initial rate of colonization is more rapid. Initially then, there is no effective removal rate (Rr) and until such a force b'ecomes established the growth rate (Rg) of the film should obviously be limited to prevent excessive accumulation of film and ponding. At first little purification of the waste is effected and as a result the concentration of waste at different depths of the bed is much the same and thus the growth rate (R g ) of the film is similar throughout the depth of the bed. As the developing film in the upper part of the bed removes more organic matter, the concentration available to the film in the depths of the bed reduces the growth rate. The extent to which this control of film within the bed is effected depends on the temperature and hydraulic rate of application, the controlling effect being greater at higher temperatures and low rates of application. It is thus essential that at first the load to the bed should be severely restricted and that it should only be increased in stages to match the increasing purification capacity of the developing film. The design flow should not be applied until an active grazing fauna population has become established. It is possible that before such a condition is reached, satisfactory purification, even with nitrification, may be obtained. However, the positive growth rate of the film will eventually bring about a reduction in efficiency unless a grazing population becomes established. Taylor, in reporting his experiences in maturing beds at Bradford, noted that although during the first few months there was a gradual improvement in effiCiency, this was usually followed by a marked deterioration for one or two weeks before the beds fully matured. The time needed for the establishment of the necessary grazing fauna is determined by several factors. Organisms may enter the beds by chance, as eggs introduced by the wastewater, or by aerial invasion by adult insects. In the case of beds brought into operation in the vicinity of existing beds, there is every chance of a rapid aerial dispersal to the new beds by flies during periods of suitable climatic conditions. Also beds constructed on the site of land irrigation areas could similarly be inoculated. Although
152
Ecological Operation of Bacteria Beds
in time most beds would be invaded by macro-fauna, the process can often be speeded up by the intentional introduction of organisms. The introduction of grazing fauna into beds was widely practised some years ago. Achorutes were not only despatched all over this country, but were also exported to places as far afield as Stockholm, Chicago, Singapore and Pretoria. They became popularly known as "Bell's Bugs" after the worker who publicized their beneficial activities in bacteria beds. The organisms introduced should, of course, be a species suited 'to the environment imposed by the waste and the operational methods practised. For example, with low frequency dosed beds, the introduction of Lumbricillid worms would be more suitable than introducing Achorutes. Different practical methods are available for effecting their introduction. Where practicable, medium from a suitable bed containing an abundance of different stages of grazing fauna should be carefully transferred into pockets in the surface of the new bed. On a large scale it is more practicable to apply fresh humus sludge from an appropriate bed via the feed. Although the intentional introduction of fauna may be necessary to speed up maturation, it is more important to maintain conditions to provide a suitable environment in which the desirable organisms can become established. By increasing too rapidly the rates of application before an effective grazing fauna is established, excessive film growth may occur throughout the depth of the bed and the resultant ponding would prevent its successful colonization by the introduced fauna, and its eventual maturation would thus be considerably delayed. The necessity to severely restrict the loading during the initial stages of maturation can be satisfied in different ways. Probably the most satisfactory way is to operate the beds continuously at very low rates of application. With distributors designed for high flows, such as those on beds operating on double filtration and recirculation systems, it may not be possible to satisfactorily distribute the waste at low flows required. In such cases, beds should be operated at as low flows as possible for part of each day followed by a period of rest. In hot weather this rest period should be during the night when the temperatures are lower, and in cold weather during the day. In maturing beds designed to operate by the nutritional control of film, although initially the film growth can be encouraged by the conventional operation of the beds, the later stages of maturation should be carried out whilst operating on the appropriate system. This would not only limit the rate of film accumulation but ensure the establishment of the species of grazing fauna characteristic of the system.
Ecological Operation of Bacteria Beds
153
Small Scale Experimental Beds
For experimental purposes and to assess the treatability of wastes, small beds usually consisting of cylinders 6 ft in depth and up to 1 ft in diameter, have been widely used. Within a few weeks it is possible to .assess the degree to which the waste is removable. So much can also be assessed by the use of rotary tubes. The suitability of bacteria beds as a method of treatment, however, can only be determined after longer periods when the bed has been fully matured and a film-grazing fauna balance has been established. With such beds it is usually necessary to introduce the grazing fauna. Care should be exercised in assessing the rates at which the waste should be treated on large scale beds. The size of medium in such beds should, ideally, be such that its diameter is not greater than one eighth of that of the bed. Account should also be taken of the temperatures at which the beds are operated in relation to those which may be expected in the large scale beds. A further difficulty is in obtaining an effluent sample representative of the final humus tank effluent on the large scale plant. Although satisfactory small humus tanks can be designed for use with such beds, their performance may not be repeated on the large scale plant. The use of paper-filtered samples ignores the polluting load of an effluent attributable to the suspended solids present. Even for comparative investigations, although paper-filtered samples make comparisons easier, the removal of suspended solids eliminates a variable since the settleability of humus solids produced from beds in different conditions may differ. Routine Operation of Beds
Once a bed has been matured, the essence of efficient operation is the maintenance of the balance of populations. To maintain this ecological balance in popUlations, continuity of operation is essential. The effect of a period of disuse of a bed depends upon its duration, the original condition of the bed and the climatic conditions prevailing. For some time after the application of the waste to the bed is stopped, the interstitial liquid gradually drains out of the bed for a period depending upon the waterholding capacity of the film. As the surface layers dry out the grazing fauna retreat into the depths of the bed and soon large numbers of grazing organisms will be observed leaving the bed. The last drainings from the bed are accompanied by vast numbers of the motile stages of insects and worms, migrating from the inactive bed. The bed is thus seriously depleted of active grazing fauna and presumably the young worms and larvae which hatch from the remaining
154
Ecological Operation of Bacteria Beds
eggs find conditions unsuitable for development in the drying bed. The pupal stages give rise to flies which, if climatic conditions are favourable, leave such beds in vast numbers. Under such conditions spiders sometimes dominate the scene and whole beds are covered, and the distributors festooned, with their webs. If a bed, having suffered this depletion of grazing fauna, is put back to operate at normal rates of application, serious consequences will probably result. Such beds, unlike newly constructed ones, are already well inoculated with the necessary microorganisms, and the film, although possibly having suffered as a result of the period of inactivity, rapidly recovers when the application of the waste is resumed. In the absence of grazing fauna the film may rapidly accumulate and result in ponding which then delays the establishment of a grazing fauna. Such a series of events can result in a bed having a reduced efficiency for long periods, in some cases for over a year. If the operation of a bed has to be interrupted, then the period of inactivity should be restricted to the minimum and if practicable the bed should be operated for periods whenever possible, even though this may not be convenient. By operating beds during the nights only, it has been possible to carry out major modification over a period of a week or more, without affecting their conditions or efficiency. Under conditions of extreme temperatures, a period of inactivity of longer than one day may have adverse effects; when the weather is mild and wet, longer periods are permissible. When prolonged periods of inactivity are unavoidable, it is then essential to carefully re-mature the bed in a similar manner to the initial maturation process. Under favourable conditions, usually in the spring, and when flow circumstances permit, the periodic resting of a bed, for periods not exceeding 24 hours, can have beneficial results on a bed in which film accumulation has occurred. Besides the period of starvation imposed on the film, the grazing activity of the macro-fauna is encouraged by the absence of the physical flow of liquid which normally limits their activity. By such a practice the spring unloading, when this is necessary, can be speeded up. Fly Control
Although ecologically fly populations are best controlled by nutritional control, i.e., by limiting the degree of film accumulation, it may on occasions be necessary to prevent serious nuisance by direct action. Where practicable this may be achieved by flooding the bed and provided
Ecological Operation of Bacteria Beds
155
that this is of short duration, the film and probably the worm population will not be seriously affected. A more common practice is the application of insecticides. Although these are probably more selective, worms and Collembola not being affected at the concentrations necessary to kill the flies, the bed effluent may be rendered toxic to life in the receiving stream. For this reason only a small portion of the bed area should be treated at any time. It is usually most practicable to apply the insecticide to the bed as an emulsion or water dispersible powder in the sewage feed. Benzene hexachloride (BHC) at the rate of 1.3 lb of the active gamma isomer per acre of bed has proved successful against Psychoda and Anisopus. 'the effectiveness of such treatments is limited by the degree to which the insecticide is evenly distributed to the bed. Flies which emerge from the beds often congregate in sheltered positions in the vicinity of beds, where they can be attacked by spraying insecticide. This method of attack has least effect on the ecology of the bed and should not, if carefully carried out, result in a toxic effluent. Theoretically the successive applications of insecticides to a fly population in such an isolated habitat might be expected to result in resistant strains being evolved. So far, however, no such case has been substantiated. Even when insecticide treatments have to be resorted to, they should be restricted to a minimum. A knowledge of the habits of the fly enables insecticides to be used only when nuisance is likely to occur. Fluctuations in the aerial density of Anisopus above beds, for example, have been shown to be closely related to climatic conditions. There is also evidence that, although Anisopus may be abundant above the beds throughout the summer, they rarely enter near by dwellings as they do in the spring, and applications of insecticide are, therefore, not necessary after the end of June. By a continuous succession of insecticide applications it is possible to replace a fly population by one of worms or Collembola which does not give rise to nuisance and which, once the balance of populations has been re-adjusted, is equally efficient in controlling the film. Once the insecticide treatment ceases, however, the original fauna is rapidly reestablished and the continuous use of insecticides, needed to maintain the modified fauna, increases the possibility of a toxic effluent. Such a change in the fauna is best maintained by controlling the physical environment, as achieved by low frequency dosing. In this process, not only is the fauna popUlation limited by the nutritional control of its food-the film, but the resultant higher instantaneous rate of application is considered to favour the more prehensile worms. The degree
156
Ecological Operation of Bacteria Beds
to which this is effective will probably be affected by such factors as the type of distributor jet, jet spacing, type of medium and the hydraulic loading. In the work previously reported, where low frequency dosing resulted in such a marked change in the fauna, the beds were operating on ADF and thus the instantaneous rate of application would be double that of a bed operating on single filtration. Design Considerations
Although a plant has to be designed before it is operated, operational requirements should be the basis of design. The operator should not find himself severely restricted in the degree of control by over-rigid design based on arbitrary criteria. For this reason, the brief discussion on the ecological factors which should be considered in design of beds has been left to follow the operational considerations. Site
Because of the adverse effect of low temperatures, beds should be sited in sheltered positions. Enclosed beds should prove advantageous in this respect. Shelter provided by a screen of trees can, however, cause a nuisance at leaf-fall. Temperatures within the bed are largely determined by the temperature of the waste and in this connection the proximity of the plant to the source of waste may be important, reductions in sewage temperatures may occur in long trunk sewers, for example. Because of the possibility of fly nuisance beds should be at some distance from dwellings, etc, especially those to the leeward in relation to the prevailing winds. Hydraulic considerations are paramount in the design of most plants and the choice of site is obviously important in this respect. In some situations careful siting can enable double filtration to be practised without additional pumping. Medium
The function of the medium is to provide the necessary surface on which the active biological film can be supported and at the same time permit the free flow of waste and air to the film. The size, shape and surface nature of the medium are important features determining the degree to which these functions are carried out and hence affect the efficiency of a bed. Theoretically it appears wasteful in plant capacity that, to provide the necessary surface area, more than half the volume of a bed constructed of conventional medium is occupied by inert matter-the medium. In America several attempts have been made to construct suitable media in
Ecological Operation of Bacteria Beds
157
units of uniform size, e.g. "Aero-blocks", "Raschig rings", and "Straight's blocks". Goldthorpe designed the "Huddersfield Tile". The more recent developments of plastics have permitted the prefabrication of corrugated plastic surfaces as bed medium, trade registered as "Dowpac". Although with this medium the surface area per unit volume of bed is no greater than that of stone media, only 6 per cent of the bed volume is occupied by the material. A development of this type of medium to increase the surface area should further increase its efficiency. The extent to which the grazing fauna could become established on such medium is not known and although bacterial film may be removed hydraulically the possibility of clogging by profuse growths of fungus should not be overlooked. Until a satisfactory and economic alternative is found, the conventional medium will continue to be used in most cases. Since the type of medium used may affect the ultimate efficiency of the plant, careful consideration should be given to its choice. It has been shown that the liquid retention time of liquids flowing over spheres varies inversely with the two-thirds power of the diameter of the spheres. Thus, medium size could be expected to affect liquid retention time. More important, however, is the effect of medium size in determining the amount of film which a given volume of bed can support. The optimum size of medium is determined by two opposing factors; the smaller the medium, the greater the surface area, but the more restricted the interstitial spaces in which the film can accumulate and through which the waste and air can flow. The optimum size of medium is thus the smallest on which the maximum degree of film accumulation, resulting from the conditions of operation, can be accommodated without interfering with the ventilation of the bed or the even dil'tribution of the waste. Since the degree of film accumulation depends upon several factors, including the nature of the waste,and method of operation, the optimum size of medium is affected by such factors. It is thus unlikely that there can be a single standard medium for all beds, considering the wide range of conditions under which they are operated. Since, at least with conventional beds, the degree of film accumulation is greater in the upper layers, on this basis smaller medium could be used in the depths of the bed. However, the amount of humus in the liquor increases as it passes downward and thus very small medium would be unsuitable. With low frequency dosed beds in which the load is more evenly distributed the size of medium throughout the bed should be the same, providing the lower-most medium is sufficiently large to prevent it passing through the openings in the under-draining.
158
Ecological Operation of Bacteria Beds
Where the degree of film accumulation can be limited, the use of smaller medium is possible in such beds. The possible use of smaller medium in the secondary bed of double filtration has already been mentioned. The base of the bed should have sufficient slope to prevent the accumulation of humus solids under the tiles which could impede the ventilation of the bed. The presence of "fines" of appreciably smaller size than that considered suitable, even in relatively low proportions by weight, is undesirable. Medium of mixed sizes tends to become consolidated and the interstitial spaces between the suitable medium become restricted by the smaller components. For the same reason the media used should not be susceptible to disintegration and should be handled carefully when filling the beds. As important as size, is the shape of individual units of medium. Ideally, they should be almost spherical. Excessively long or flat particles tend to pack and restrict the interstitial spaces. The nature of the surface of the particles is another factor to be considered. Bacterial slimes and fungal growths can develop on smooth surfaces, such as washed gravel, although the initial lag phase may be somewhat longer. It is probable, however, that a rough surface such as cracked granite, provides a better holdfast for certain grazing fauna. The honeycomb-like structure of slag or clinker is ecologically superior to other standard types of medium. It presents a larger surface area than other media of the same size, even though all of the surface may not be wetted by the waste. Furthermore, it provides an ideal niche for the grazing fauna. One has only to examine a matured piece of slag medium and see the worms, larvae and blue Springtails housed in their ready-made caves and burrows and grazing on the film growing on their doorsteps, to appreciate the hospitable environment it provides. Economic considerations will undoubtedly be a major factor in the choice of medium. It may be more economical, for example, to use suitable locally available material, even though this may necessitate having slightly larger beds than would be required if a more costly superior form of medium were used. This consideration, however, only applies to the nature of the medium and not to its grading. To use cheap media of indifferent grading containing "fines", or media that will disintegrate to produce "fines" is certainly false economy. Such media will most probably give rise to trouble under extreme operating conditions which could only be remedied at considerable expense and inconvenience.
Ecological Operation of Bacteria Beds
159
DEPTH Little consideration has been given to the question of bed depth, and by tradition, most beds in England are approximately 6 ft deep. The depth of the bed, however, is ecologically important in as much as, at a constant loading (gyd), it affects the surface rate of application, and thereby the downward rate of flow of the waste. In conventional beds with high frequency dosing, the increase in surface loading with increase in bed depth could cause excessive accumulations of solids near the surface of deep beds which would reduce their efficiency. With low frequency dosing, however, the more even distribution of the film throughout the depth of the bed should make possible the use of beds considerably deeper than 6 ft. Where this is possible it would reduce the land area required and the size of base and underdraining. The numbers of distributors would be reduced but their capacity would need to be greater. The retaining walls, however, become increasingly more expensive with increase in height above ground level; this being more so with rectangular beds than with circular ones. Where hydraulic requirements demand, it will be necessary to provide pumping capacity together with an adequate standby pump. In two stage filtration or recirculation, where pumping is only required for the primary or returned effluent, it is often possible to continue to apply the waste by gravity to the beds when pump failure occurs. This would not be possible in th~ case of a deep (high) bed where pumping was normally required. Theoretically on the basis of the formula: D
tu QO.66 where D = Depth and Q = Surface loading, by increasing the depth and proportionately increasing the surface loading to maintain the same gyd. loading, the liquid retention time is increased.
Shape Although the old method of fixed spray distribution allowed full use to be made of any shape of available area, modern methods of distribution usually determine the bed shape as rectangular or circular. Other shapes demand complication in the design of distributors. Rectangular beds are usually only found on the larger works although some large works have circular ones. Ecological differences occur because of the different distribution methods imposed by the different shapes. The rectangular beds, although
160
Ecological Operation of Bacteria Beds
more readily ensuring an even distribution of waste across the width of the bed, are not so readily adapted for low frequency dosing, as previously discussed. Ventilation
Although ventilation is essential for the successful operation of beds, the method by which it is effected is not fully understood. It is probable that several forces are at play in effecting ventilation, and the extent to which each contributes varies under varying operating conditions. In most beds, at most times, the temperature differences between the interior of the bed and the air above the bed results in convection currents which effect ventilation. It has been calculated that a difference of 1°F is sufficient to give adequate ventilation to a bed treating sewage at normal rates. The importance of maintaining a thin film so as not to impede the ventilation currents and to ensure that all the organisms in the film receive an adequate supply of oxygen has already been discussed. Structurally, provision should be made for the ventilation of the bed from below. With circular beds having underdrains discharging into a circumferential open channel, these should provide adequate ventilation, providing that they are not surcharged. The use of open stone walling for such beds is probably not essential and it certainly provides an attractive shelter for flies. Where the underdrains discharge into one small common effluent channel within the bed, the provision of ventilation shafts on the periphery of the bed would seem to be a wise measure. With rectangular beds having the underdrains discharging independently to a longitudinal channel to the outside of the bed, or to a channel in a well ventilated duct within the bed, the underdrains should themselves provide adequate ventilation providing that they are kept open. From an aeration point of view beds cannot be over ventilated, although in the winter this could lead to an undesirable reduction in bed temperatures. Since in most beds ventilation is probably not a limiting factor, the provision of forced ventilation has not always proved beneficial. It could possibly increase the efficiency of enclosed beds and those treating strong wastes. Distribution of Waste
As we have seen, the method of distributing the waste can markedly affect the ecology of the bed and thereby its efficiency. Distributor design
Ecological Operation of Bacteria Beds
161
is an integral part 0f bed design and should be considered in rebtion to the proposed method of operation. Difficulties arise in providing for even distribution of the waste over a wide daily range of flows experienced at some works. At very low flows recirculation of effluent is recommended. On the whole, if even distribution over the whole flow range is unattainable, the distributor should be designe.d to ensure even distribution at average flows and above, rather than at very low flows. This being so, the uneven distribution at low flows can be regarded as producing a greater degree of underloading in some places, but the uneven distribution at high flows can only result in a greater over-loading in some places-a much more serious effect. The view that a dribbling jet is likely to produce more growth than a normal jet is based on the erroneous mechanistic concept of film control. Apart from the possibility of freezing in winter, and provided that the distributor is in motion, dribbling during periods of low flow should have no ad verse effect. The provision of motordrives for distributors ensures movement at times of low flow besides providing for controlled frequency dosing and makes unnecessary the provision of syphon dosing chambers. With normal jets the spacing between them should be such that at average flows the distance between jets lines made by all the distributor arms serving one area of the bed are not greater than 6 in. On circular beds, to ensure even distribution along the radius, some distributors have jets of the same size spaced at increasing distances towards the centre of the bed. Even distribution, however, is probably better achieved by having the jets equally spaced and having a progressive reduction in the diameter of the jets towards the centre of the bed. In beds where nutritional methods of film control are practised, the use of more widely spaced jets with splash plates, to en;;ure even distributions, should be equally satisfactory. By spraying or cascading the waste from the distributors, the increased aeration should be beneficial but the amount of oxygen so dissolved can only satisfy a fraction of the total oxygen demand. Fixed spray-jets, because they provide a continuous rain of waste on the surface of beds, encourage excessive growths of film and most have now been replaced by travelling distributors giving a lower frequency of dosing. They could, however, be used en the secondary bed of a double filtration unit, where the low nutrient concentration should not result in excessive growths of film. Such a method of application would probably enhance nitrification within the bed.
162
Ecological Operation of Bacteria Beds
Because of the need to ensure continuity of operation, distributors independently driven are superior to those rope-hauled in groups. Even when use is made of the provision for uncoupling a faulty distributor for attention, when this is possible, the area served by it receives no sewage, whereas it is sometimes possible to permit an independently driven distributor to cover the extra area by extending the length of travel. Ancillary Plant
As mentioned previously, beds should not be used to remove settleable solids and it is usually necessary to provide adequate settlement of the waste before it is applied to the beds. It is equally important to remove the humus solids discharged from the bed before the effluent passes to the receiving stream. The conventional method is to settle these solids in humus tanks; deep tanks of small cross sectional area which provide a fairly even rate of upward flow are probably the best type. The humus sludge should not be retained for any length of time. If nitrification is occurring in the bed, the denitrification, which would rapidly occur in the anaerobic conditions in the tank, could liberate nitrogen gas which, in rising, would carry the sludge to the surface. The sludge should be removed at least once or twice each day if not continuously. The monthly washing down of humus tanks, where practicable, should also prove beneficial. Humus from beds differs considerably, depending on the waste being treated and the condition of the bed, some having better settling qualities. However efficient the humus tanks, some finely divided solids are, in practice, always present in the effluent and these form a considerable portion of the oxygen demand of many effluents. Their removal can be effected by sand filtration or microstrainers. Where land is available the irrigation of the effluent over a grass plot is a simple and effective method of removing most of the solids and effecting clarification when necessary. Such grass plots should be used for the removal of the finely divided solids which do not settle in humus tanks, and not to replace humus tanks or supplement grossly inefficient tanks, unless the frequent removal of the accumulated solids from the plots is anticipated.
Chapter 7
Groundwater Contamination Visibility in the national parks primarily involves a preservation issue instead of one of amelioration. Historically, Americans have placed a high value on good visibility, that is, the ability to see distant objects clearly. This appreciation of atmospheric visual clarity is evidenced in the country's early literature and art, including the journals of Lewis and Clark as well as the masterpieces of the great American landscape artists of the nineteenth century. Today that yearning is demonstrated not only by the millions who flock each year to our western parks, but also in the high prices brought by those artists works of a century ago and by the interest in Ansel Adams's simple, yet dramatically clear, black-and-white photographs of Yosemite and other wonders. Over the past one hundred years, Congress has, acted to preserve many of the nation's natural wonders. It did so by creating and by continually expanding the national parks, wilderness areas, monuments, recreation areas, and wild and scenic rivers. Since the 1950s, there has been an increasing concern that this. beauty is threatened by industrial development and population growth. Pollution from coal-fired power plants became a special concern in 1963 with the advent of the first unit of the Four Corners Power Plant ~ near- Farmington, New Mexico. It produced a plume that could be seen clearly for many miles, reducing the clarity of the visual experience in areas of northwestern New Mexico, southeastern Utah, southwestern Colorado, and northeastern Arizona. By the later 1960s and the early 1970s, smog began to appear in Yosemite Valley, California, on warm summer days. Battles erupted over the visibility effects of proposed coal-fired power plants on the Kaiparowits Plateau and near Capitol Reef National Park, both in southern Utah. The increased publicity and concern resulted in magazine and newspaper articles decrying the loss of visual clarity, particularly in the western United States, and precipitated political pressures in Congress
164
Groundwater Contamination
for legislative steps to protect visibility. Those pressures culminated in the August 1977 adoption by Congress of the nation's first specific visibility protection requirements for national parks and national wilderness areas. One of the large issues raised by these developments is whether the value of visibility protection outweighs the cost, including both air pollution control equipment and the regulatory system. The study reported in this section was designed to improve our ability to measure the benefits of visibility and to provide some preliminary estimates of the value of that visibility in several major national parks and for the region in which they are located. Visibility is the ability to see both colour and detail over long distances. Human perception of visual air quality is associated with the apparent colour contrast of distant visual targets. As contrast is reduced, a scene "washes out" both in terms of colour and in one's ability to see distant detail. What, then, is the nature of the preservation value of visibility? That value has at least two possible components. First, a scenic resource such as the Grand Canyon attracts large numbers of recreationists. The quality of the experience of these individuals depends in great part on air quality, in that scenic vistas are an integral part of the Grand Canyon "experience." Accordingly, the air quality at the Grand Canyon is valuable to recreationists. We might call this economic value, or willingness to pay by users for air quality at the Grand Canyon, user value. Thus, recreationists in the national parklands of the Southwest should be willing to pay some amount to preserve air quality for each day of their own use if their recreational experience is improved or maintained by good air quality. The second component of preservation value we have termed existence value. Individuals and households who may never visit the Grand Canyon may still value visibility there simply because they wish to preserve a national treasure. Visitors also may wish to know that the Grand Canyon retains relatively pristine air quality even on days when they are not visiting the park. Concern over preserving the Grand Canyon may be just as intense in New York or Chicago as it is in nearby states and communities. Thus, preservation value has two additive components, user value and existence value. During the summer of 1980, more than 600 people in Denver, Los Angeles, Albuquerque, and Chicago were shown five sets of photographs depicting both clear conditions and regional haze. Each set consisted of five photographs of a national park vista with different visual air quality of a general nature, that is, generally increased haziness. The vistas shown
165
Groundwater Contamination
were from Grand Canyon, Mesa Verde, aIld Zion. Summer was chosen for the survey because it is the season of peak visitation. These photographs were placed on display boards as full-frame, 8x10-inch textured prints, arranged from left to right in ascending order of visual air quality, with each vista in a separate row. The participants were asked how much they would be willing to pay for visibility as shown in the five sets of photographs, from worst to best. They were also asked how much they would be willing to pay to prevent a plume from being seen in a pristine area. Two photographs were used in this connection, one with and the other without a plume. The photographs were taken from Grand Canyon National Park at the Hopi fire tower observation point and toward Trumbull Mountain. They were both taken at 9:00 am, so the lighting on the canyon wall and other features is the same. Both photographs have the same light, high cirrus cloud layer in the sky. The plume is a narrow gray band crossing the entire vista in the sky, except where it appears in front of the top of Trumbull Mountain.
IWORSTI
D D D EJ D D D EJ DESERT VIEW 9 AM
IWORSTI
TRUMBULL MT. 9 AM
IWORSTI
DDDB TRUMBULL MT 3 PM
Fig:l. Grand Canyon Photograph Board The source was not industrial or municipal pollution, but a controlled burn in the area around the Grand Canyon. However, the effect was comparable to what a large industrial source might produce. The bidding game based on these photographs reveals the household's willingness to pay for preserving or improving the degree of visibility in specific locations of the national park area des~ribed earlier. The bids offered by respondents in the aggregate preservation-value section of the survey encompass both pure existence value and users' valuations of preserving visibility. Since the results did not permit a completely clean distinction between the two types of bids, further discussion will concentrate on the
166
Groundwater Contamination
preservation-value section of the survey. The benefits derived from the interview results can be extrapolated to populations larger than that in the sample (the sample was chosen in as random a manner as practical) by applying statistical techniques to the results of the survey. The amount of bids offered by interviewees to preserve or improve visibility is related to such factors as income, education, and other personal characteristics. These relationships can be quantified using regression analysis. After this is done, it is possible to estimate the value of benefits to residents of the whole Southwest region as well as the entire nation. This is done by substituting the average values for these characteristics for each state into the relationship established by the regression analysis and calculating what the average value of the bid of a person in that state would be. This value can then be multiplied by the population of the state as a whole to get a total bid. When the analysis is performed for the southwestern United States, the following values are obtained. The figures show the annual willingness to pay for preserving present average conditions, as contrasted with the next worse condition depicted by the pictures, and for preventing plume blight. To estimate the aggregate national benefits from preserving visibility, a similar analysis is done for the entire United States, but additional survey data from Chicago are included, and the following values are obtained. Total (million $) Annual Benefits for Grand Canyon 3,370 5,760 The Grand Canyon region 2,040 The plume These figures, even though their accuracy is highly uncertain, imply that very large existence values characterize the areas in question. However, some recent and highly preliminary experiments with surveys imply that these figures may be much too high. Several other observations on the outcomes of the analysis of the actual interview results are worth mentioning here. First, in the conventional view of the demand for environmental quality, there is a smooth tradeoff between higher successive levels of environmental quality and economic benefits, with successive units commanding less incremental willingness to pay. The survey respondents, however, placed a much higher value on a small initial diminution in visual clarity than on comparable subsequent decreases. Second, and again somewhat contrary to expectations, neither past nor prospective
Groundwater Contamination
167
visits to the Grand Canyon region were shown to be important determinants of preservation value. On the average, those who had never seen the Grand Canyon valued it as highly as those who had. Third, and again unexpectedly, a household's distance from the region had no significant relationship to the size of its bids. When corrected for income and other differences, people in Chicago bid fully as high as those closer by. However, preliminary further investigation suggests that this result may not be very robust, being sensitive, for example, to the sequence in which people are asked about their valuation of various public goods. Because the Grand Canyon is the dominant feature in a region with many visitor attractions, one must be especially cautious in extending these preliminary findings to other recreational attractions. It seems likely that there are only a very few natural phenomena in the United States about which Americans have such strong feelings. Obvious candidates for-this short list would be Old Faithful (in Yellowstone National Park), Niagara Falls, and perhaps a few others. The main conclusion of this visibility study is that the magnitude of the annual benefits, when aggregated across households, is impressive. While these are necessarily rather crude extrapolations, the survey results suggest that Americans place great value on the preservation of air quality in the Grand Canyon region, and that this valuation is not necessarily localized in the Southwest. Further, the survey results suggest that pure existence value may overwhelm a substantial user value for the national parks in the region. ACID RAIN
Acid rain and other mechanisms for the dispersion and deposition of acid formed from sulfur and nitrogen emitted from various sources are complex and ill-understood phenomena. In addition, methods for estimating the economic losses resulting from damages or economic benefits of prevention of acid rain are not well developed, nor was it possible within the scope of the project described here to make much progress in developing them. Consequently, since the estimates of benefits made for controlling acid rain are very crude and of no particular interest in terms of methods development, the discussion here will be very brief. It is included primarily because of current intense interest in the phenomenon, and because the analysis that was done provides some guidance concerning directions for future research. Acid depOSition, among all the areas covered in this volume, is perhaps the one most crying out for additional methods
168
Groundwater Contamination
development and improved estimates. Let us turn first to its possible effects on agriculture and forestry. Increases in soil acidity can negatively affect the yields of certain field crops, but this can be offset by modestly increasing liming operations which already occur for acid-sensitive crops. Therefore, the benefits of controlling acid rain for this purpose would be small. It is known that direct damage to the plant can occur from acid deposits on leaves, flowers, and fruits, but there is virtually no basis for estimating the amount of such an effect. Forest growth can be affected in a similar fashion in that there can be both indirect effects, through the soil, as well as direct effects. As in the case of certain field crops, there may even be short-run favourable effects, for example, when the acid dissolves plant nutrients and makes them more available to the trees. Nonetheless, in the longer term this could result in reduced soil fertility and slower tree growth. If some strong assumptions are made, an estimate can be made of damages resulting from retarded growth. If one assumes-and there is some evidence pointing in this direction from Swedish studies-that acid rain would reduce timber growth in Minnesota and east of the Mississippi (the area of the country thought to be most affected by acid) by 5 per cent annually, the reductions in yield would decrease the worth of timber production about $600 million per year. Assuming that other services of forests, such as watershed protection, fishing, and hunting, also were to be reduced by 5 per cent, and based on crude estimates by others of the possible overall value of these services, the total damage including timber and other services might come to about $1.75 billion. This is a substantial sum, but not very large relative to the costs of controlling acid deposition. There might also be effects on human health, say, by the acid dissolving and mobilizing heavy metals so that larger concentrations would get into drinking water or the human food chain. The present state of knowledge does not permit even very crude estimates to be made of this possibility. Higher acidity in municipal and industrial water systems might also result in increased corrosion in piping, appliances, and cooling systems. But the diminishing acidity in such systems, by the use of lime, is a routine operation and can be accomplished at small cost. The big danger in watercourses appears to be those features of the aquatic ecosystem itself which mankind values. Acid conditions in a watercourse tend to destroy the small plants and animals (plankton), that are the initial links in the
Groundwater Contamination
169
fish food chain, and this has a negative effect on fish population. But the primary way in which fish populations are destroyed is different. As noted earlier, acid in water bodies tends to mobilize toxic metals and increase their concentration in the water. The reproductive capacity of many species of animals, including fish, is adversely affected by the presence of excessive amounts of toxic metals. Thus, for a time, as fish numbers decline, the ones that remain increase in size as competition for food declines, but then rather abruptly there are none left. This, of course, destroys commercial and recreational fisheries_ The value of fish taken by commercial freshwater fishing in the United States is not very large, so the loss there, at least as measured by present market prices, would not be very great. On the other hand, the value of fresh water recreational fisheries is relatively enormous. Let us make the extreme assumption that all such fisheries in Minnesota and other areas east of the Mississippi would totally disappear. If we then take estimates of willingness to pay for fishing from other studies, it appears that the loss could, at an outside limit, be on the order of $10 billion per year in 1979 prices. Additional losses would be caused by the decline of terrestrial and aquatic animals (other than fish) who are partly or wholly dependent on the aquatic food chain-certain species of water fowl, for example. The other area where our study suggests really major damages might occur is deleterious effects on materials. As noted previously, acid corrodes metals, eats away limestone, is harmful to paints and other coatings and finishes, and damages cloth. Given the huge number of such items which exist and are exposed to the atmosphere, it is not very surprising that benefits from protecting them might be large. Again, in Sweden, where the problems of acid rain first received widespread attention (because of prevailing winds, Sweden gets inputs of sulfur and nitrogen compounds from the Ruhr, the Rotterdam petrochemical complex, and Great Britain), a study has been made of per capita damages of corrosion and soiling. If one makes the assumption, once again very gross, that this same estimate can be applied to all persons dwelling in Minnesota and east of the Mississippi, one gets an annual benefit from avoiding acid rain of about $14 billion. Putting together the various dollar estimates, the benefits of preventing acid rain in 1978 dollars amounts to nearly $26 billion-a hefty amount indeed. But as stated, many extreme assumptions were made in generating these numbers, and they are no doubt too high by far. An educated guess by the research team was that the actual figure is probably not more than $5 billion for a condition that is characterized by severe effects on the entire eastern United States.
170
Groundwater Contamination
Of course this number cannot be taken very seriously, because even if it were correct, in all other respects it neglects the large adjustments in demand and supply which would accompany the types of changes contemplated. An approach much more like that described for the agriculture study in the South Coast Basin would be appropriate in a more sophisticated study. Where are the greatest potential benefits of protection from acid rain likely to lie? These are, in rough order, materials damage, aquatic ecosystems effects, and effects on agriculture and forestry. These categories of damages certainly merit further study. But our progress in economic research on these questions is highly dependent on improved dose-response relationships. The relationship between the intrusion of acid into a watercourse and its ultimate effect appears to be extraordinarily complex. For example, what sudden changes could occur after a previously uneventful period, and what difficulties would be encountered in reversing them after they have occurred? Indeed, could they be reversed completely? Ecologists place great value on diversity of species as an indicator of a healthy, stable ecological system. Acidification of streams is known to reduce diversity. But it is not well understood how this ultimately affects characteristics of the stream that we value. This problem seems ripe for joint work between economists and ecologists. GROUNDWATER CONTAMINATION
While the extent of groundwater contamination is not accurately known, it is thought to be widespread and is the focus of much public apprehension. Contaminants in groundwater range across an enormous list of chemical substances, and usually no thorough checks for contamination are made until there is reason to suspect a problem. Even at extremely low concentrations, many toxic chemicals pose serious, irreversible, health risks. In many of the cases investigated, well water has been found to contain concentrations above, and often several orders of magnitude higher than, those commonly encountered in raw or .treated drinking water drawn from contaminated surface sources. Thus, while water from most wells no doubt is safe, the widespread nature of the contamination and its potential seriousness merit the public attention the problem is getting. The intent of the case study in this chapter is to develop methods for estimating benefits from preventing contamination of groundwater-based drinking-water supplies. This, so far as we know, is the first study to attempt to quantify such benefits. As in the studies discussed in other
Groundwater Contamination
171
chapters, the quantitative results reported here must be regarded as largely experimental, but the numbers turn out to be impressively large. For any chemical source, the extent of groundwater contamination is determined by the characteristics of the underground storage mediumcalled an aquifer. Groundwater in shallow, alluvial aquifers, typically moves less than a foot per day. That flow is governed by recharge and discharge rates from the aquifer, and by the aquifer's permeability. Contaminants are transported by diffusion together with the slow underground flow of groundwater. In that oxygen-poor environment, chemical or physical processes of contaminant degradation proceed very slowly. Thus the contaminant plume may move great distances, with hardly a change in toxicity levels, and may therefore reach drinkingwater wells. Among the principal sOurces of groundwater contamination are wastedisposallandfills and impoundments, accidental pills of chemical substances, and abandoned oil and gas wells. Most groundwater contamination can be traced to chemicals leaching into the aquifer from poorly constructed and managed industrial or municipal landfills, surface impoundments, or outright illegal dumps. Contamination from such sources has often been in process for years, and sometimes for decades. To date, most groundwater contamination incidents have been discovered only after a drinking-water source has been affected. By the time suspected aquifer contamination is verified in samples drawn from drinking-water wells, the problem may be irreversible. Stricter regulation of the disposal of potential contaminants in other environmental media, particularly air and surface waters, and the consequent rising cost of such disposal, is likely to increase the flow of wastes to land disposal and aggravate the threat to groundwater. Benefit analysis of controlling groundwater contamination requires, as usual, quantification of several link between sources and receptors. One must know the location and strength of actual or potential sources of contamination, and must be able to model the spread of the contaminant plume in the aquifer. One must know the numbers of persons exposed to contaminated groundwater and the extent and timing of their exposures. One must know the" dose-response relationship", the nature' and extent of health effects on the population at risk. , And finally, one needs a way of converting health effects into monetary, or dollar, values. We are far from being able to quantify these link between sources and receptors with precision. In each case, there is a need for substantially improved methods and data. Actually, while it is referred to as a landfill, this is a rather euphemistic term-dump would
172
Groundwater Contamination
be a better word - but we shall stick with the conventional usage. Price applied to the New Jersey Department of Environmental Protection for a license to conduct a sanitary landfill operation. The application listed the materials that Price intended to accept at the landfill, and specifically excluded "Chemicals". He was issued a certificate authorizing operation of a solid waste disposal facility. Authorities inspected the landfill, citing Price for accepting chemical wastes and formally advising him of the violation. Nonetheless, Price continued accepting significant quantities of chemical wastes. After that date, no chemical wastes were disposed of at the landfill, although it continued in operation. Price terminated the landfill operation and covered the site with fill material. The site has not been used since then. Price accepted approximately 9 million gallons of toxic and flammable chemical and liquid wastes, either in drums or directly into the ground. Among others, these included glycolic, nitric, and sulfuric acid, caustics and spent caustic wastes, cesspool waste, chemical resins and other waste chemicals, chloroform, and cleaning solvents. Price's Landfill is situated over the Cohansey aquifer, the principal source of Atlantic City's water supply, and the separation between landfill and aquifer is a relatively permeable layer. Waste from the landfill is free to leach into the aquifer; the direction of flow in the aquifer is eastward, toward Atlantic City's wells. Chemicals in the leachate therefore can be carried into the private and public water-supply wells, and people can be exposed to these chemicals in drinking water. Test wells drilled near the landfill by the EPA show that groundwater in the aquifer is contaminated and that the plume of contamination indeed is moving toward Atlantic City's wells. But estimation of actual or potential human exposures requires either considerable information on, or heroic assumptions about, the mechanism by which toxics are transported from the source of contamination to the water-supply wells. This is the second linkage mentioned earlier. Shortly it will be clear why discussion of this linkage logically precedes the first quantification of the source of the contamInation. Efforts to understand and model the source to receptor linkages, called groundwater solute transport, are relatively recent. While there has been considerable earlier work on salinity transport, study of the more difficult cases of chemically reactive toxic groundwater contaminants is less advanced. Improvements in our ability to model these phenomena must be a prime objective for future research. For purposes of analyzing the Price's landfill situation, the researchers
Groundwater Contamination
173
chose and estimated numerically a technique called the Wilson-Miller solute-transport model. This relatively simple model was chosen because of time and funding limitations for the research. The model chosen does appear to fit the Price's landfill situation relatively well and has been judged adequate for conducting this experiment. Future research should determine whether more complex models yield substantially different results. But to apply any solute transport model, it is first necessary to have so-called source-term information, that is, the amounts of material entering groundwater and their distribution over time. Much of the activity at Price's landfill was illegal. It therefore seems unlikely, to say the least, that careful records of what went into the pit were kept. Indeed, for a large number of chemical substances there is no information at all about the amounts that have been dumped there. Where such records exist, or if leaching rates are known or can be calculated, deliveries of pollutants to the aquifer can be estimated directly. In the situation exemplified by Price's landfill, which is typical of much groundwater contamination, there is only one way to estimate the quantity. Since we have information on what is already present in test wells drilled by the EPA, the solute-transport model can be run "backwards", so to speak, and used to infer what amount there had to be to produce the existing groundwater concentrations. This is why, logically, the discussion of the transport model precedes discussion of the source term. The reader should be cautioned that this estimate, while necessary, is based on many assumptions and involves great uncertainty. Just to give one example, the procedure assumes that releases occur at a constant rate. This may not be true for some pollutants, and "slugs" may be released which cause transients of pollution in much higher concentrations than would be predicted by the model. But given the computed source term, the model can be run "forward" to compute concentrations at any well drawing on the aquifer-the production wells of Atlantic City, for example-and for any time after some contaminant enters the aquifer. Those concentrations, and the times at which they are projected to occur, were computed for the wells from which Atlantic City's Municipal Water Authority pumps its water. Assuming that no mitigating action is taken, this provides the link that specifies the exposure of the population to contamination from Price's landfill. To take the next step, one must have dose-response informationthat is, the actual health risk stemming from the contamination. To make
174
Groundwater Contamination
this link, information published by the EPA was used. There is a section of the Clean Water Act that requires the EPA to estimate excess cancer risks for 129 chemicals called priority pollutants. Many of these priority pollutants are leaching from Price's landfill into the Cohansey aquifer. Using this information, the probability of excess mortality from cancer was estimated for the population of Atlantic City. While this procedure is the best available based on existing information, the reader should be aware that, for this purpose, the risk factors provided by the EPA are both incomplete and very uncertain. For example, there are many pollutants that have been identified in groundwater that are not on the EPA list, and extrapolations from animal toxicity tests to human risks are quite uncertain. Additionally, it is assumed that each chemical risk is independent of each other chemical risk so that risks can simply be added up across chemical categories. It is well known that a synergistic effect can occur, making the combined toxicity of two chemicals greater than the sum of the effects of each one taken independently. Again, with all these cautions in mind, we turn to the next, and final, step, the monetary evaluation of damages. The values ranged from $100,000 to $1 million per death. These were then multiplied by the mortality numbers calculated in the risk analysis to obtain the total benefit from averting the damage which would otherwise emanate from Price's landfill. The total benefit ranges from $180 million to $1.8 billion. Those are large amounts, and one must be clear about what they mean. Say that, at a site similar to Price's landfill, there is a comparable release of contaminants into a similar aquifer, and that the release goes unnoticed for two decades. Then there will be human exposures through drinking water, and incremental mortality risks faced by the exposed population over their remaining lifetimes. Valuing this incremental mortality risk produced the figures cited above. At a site at which groundwater contamination has already occurred, those figures represent the damages that might be avoided by measures taken to prevent future exposures, either by restricting access to, or by cleansing, the aquifer. Needless to say, those figures are impressively large. But such limited information as there is indicates that the costs of cleansing aquifers are always large and the cost of obtaining an alternate water supply may be large. This analysis, shaky as the numbers necessarily are, suggests that where groundwater contamination is affecting drinking water supplies, prevention is probably the best cure.
Chapter 8
Industrial Wastewater Treatment Industrial wastewater treatment covers the mechanisms and processes used to treat waters that have been contaminated in some way by anthropogenic industrial or commercial activities prior to its release into the environment or its re-use. Most industries produce some wet waste although recent trends in the developed world have been to minimize such production or recycle such waste within the production process. However, many industries remain dependent on processes that produce wastewaters. SOURCES OF INDUSTRIAL WASTEWATER Iron and Steel Industry
The production of iron from its ores involves powerful reduction reactions in blast furnaces. Cooling waters are inevitably contaminated with products especially ammonia and cyanide. Production of coke from coal in coking plants also requires water cooling and the use of water in by-products separation. Contamination of waste streams includes gasification products such as benzene, naphthalene, anthracene, cyanide, ammonia, phenols, cresols together with a range of more complex organic compounds known collectively as polycyclic aromatic hydrocarbons (PAH). The conversion of iron or steel into sheet, wire or rods requires hot and cold mechanical transformation stages frequently employing water as a lubricant and coolant. Contaminants include hydraulic oils, tallow and particulate solids. Final treatment of iron and steel products before onward sale into manufacturing includes pickling in strong mineral acid to remove rust and prepare the surface for tin or chromium plating or for other surface treatments such as galvanisation or painting. The two acids commonly used are hydrochloric acid and sulfuric acid. Wastewaters include acidic rinse waters together with waste acid. Although many plants operate acid recovery plants, (particularly those using Hydrochloric acid), where the
176
Industrial Wastewater Treatment
mineral acid is boiled away from the iron salts, there remains a large volume of highly acid ferrous sulfate or ferrous chloride to be disposed of. Many steel industry wastewaters are contaminated by hydraulic oil also known as soluble oil. Mines and Quarries
The principal wastewaters associated with mines and quarries are slurries of rock particles in water. These arise from rainfall washing exposed surfaces and haul roads and also from rock washing and grading processes. Volumes of water can be very high, especially rainfall related arisings on large sites. Some specialist separation operations such as coal washing to separate coal from native rock using density gradients can produce wastewater contaminated by fine particulate haematite and surfactants. Oils and hydraulic oils are also common contaminants. Wastewater from metal mines and ore recovery plants are inevitably contaminated by the minerals present in the native rock formations. Following crushing and extraction of the desirable materials, undesirable materials may become contaminated in the wastewater. For metal mines, this can include unwanted metals such as zinc and other materials such as arsenic. Extraction of high value metals such as gold and silver may generate slimes containing very fine particles in where physical removal of contaminants becomes particularly difficult. Food Industry
Wastewater generated from agricultural and food operations have distinctive characteristics that set it apart from common municipal wastewater managed by public or private wastewater treatment plants throughout the world: it is biodegradable and nontoxic, but that has high concentrations of biochemical oxygen demand (BOD) and suspended solids (55). The constituents of food and agriculture wastewater are often complex to predict due to the differences in BOD and pH in effluents from vegetable, fruit, and meat products and due to the seasonal nature of food processing and post harvesting. Processing of food from raw materials requires large volumes of high grade water. Vegetable washing generates waters with high loads of particulate matter and some dissolved organics. It may also contain surfactants. Animal slaughter and processing produces very strong organic waste from body fluids, such as blood, and gut contents. This wastewater is frequently contaminated by significant levels of antibiotics and growth
Industrial Wastewater Treatment
177
hormones from the animals and by a variety of pesticides used to control external parasites. Insecticide residues in fleeces is a particular problem in treating waters generated in wool processing. Processing food for sale produces wastes generated from cooking which are often rich in plant organic material and may also contain salt flavorings, coloring material and acids or alkali. Very significant quantities of oil or fats may also be present. Complex Organic Chemicals Industry
A range of industries manufacture or use complex organic chemicals. These include pesticides, pharmaceuticals, paints and dyes, petrochemicals, detergents, plastics, etc. Wastewaters can be contaminated by feed-stock materials, by-products, product material in soluble or particulate form, washing and cleaning agents, solvents and added value products such as plasticisers. Water Treatment
Water treatment for the production of drinking water is dealt with elsewhere. Many industries have a need to treat water to obtain very high quality water for demanding purposes. Water treatment produces organic and mineral sludges from filtration and sedimentation. Ion exchange using natural or synthetic resins removes calcium, magnesium and carbonate ions from water, replacing them with hydrogen and hydroxyl ions. Regeneration of ion exchange columns with strong acids and alkalis produces a wastewater rich in hardness ions which are readily precipitated out, especially when in admixture with other wastewaters. TREATMENT OF INDUSTRIAL WASTEWATER
The different types of contamination of wastewater require a variety of strategies to remove the contamination. Solids Removal
Most solids can be removed using simple sedimentation techniques with the solids recovered as slurry or sludge. Very fine solids and solids with densities close to the density of water pose special problems. In such case filtration or ultra-filtration may be required. Alternatively, flocculation may be used using alum salts or the addition of polyelectrol ytes Oils and Grease Removal
Many oils can be recovt!red from open water surfaces by skimming
178
Industrial Wastewater Treatment
devices. However, hydraulic oils and the majority of oils that have degraded to any extent will also have a soluble or emulsified component that will require further treatment to eliminate. Dissolving or emulSifying oil using surfactants or solvents usually exacerbates the problem rather than solving it, producing wastewater that is more difficult to treat.
0-)
Skimmed Oil To Sump
Top View
-~ I
I
I
Side View 1 Trash Trap(inciined rods) 2 Oil retention baff les 3 Flow distributors (vert ical rods) 4 Oi l layer 5 Slotted Pipe Skimmer 6 Adjustable overflow we ir 7 Slugge Sump 8 C ham and flight Scraper
Fig:1. A Typical API Oil-water Separator Used in Many Industries
The wastewaters from large-scale industries such as oil refineries, petrochemical plants, chemical plants, and natural gas processing plants commonly contain gross amounts of oil and suspended solids. Those industries use a device known as an API oil-water separator which is deSigned to separate the oil and suspended solids from their wastewater effluents. The name is derived from the fact that such separators are designed according to standards published by the American Petroleum Institute (API). The API separator is a gravity separation device designed by using Stokes Law to define the rise velocity of oil droplets based on their density and size. The design is based on the specific gravity difference between the oil and the wastewater because that difference is much smaller than the specific gravity difference between the suspended solids and water. The suspended solids settles to the bottom of the separator as a sediment layer, the oil rises to top of the separator and the cleansed wastewater is
Industrial Wastewater Treatment
179
the middle layer between the oil layer and the solids. Typically, the oil layer is skimmed off and subsequently re-processed or disposed of, and the bottom sediment layer is removed by a chain and flight scraper (or similar device) and a sludge pump. The water layer is sent to further treatment consisting usually of a dissolved air flotation (DAF) unit for additional removal of any residual oil and then to some type of biological treatment unit for removal of undesirable dissolved chemical compounds. Inlet
Adjustable weir
, I
Adjustable weir outlet
I
Fig:2. A Typical Parallel Plate Separator
Parallel plate separators are similar to API separators but they include tilted parallel plate assemblies (also known as parallel packs). The parallel plates provide more surface for suspended oil droplets to coalesce into larger globules. Such separators still depend upon the specific gravity between the suspended oil and the water. However, the parallel plates enhance the degree of oil-water separation. The result is that a parallel plate separator requires significantly less space than a conventional API separator to achieve the same degree of separation. Removal of Biodegradable Organics
Biodegradable organic material of plant or animal origin is usually possible to treat using extended conventional wastewater treatment processes such as activated sludge or trickling filter. Problems can arise if the wastewater is excessively diluted with washing water or is highly concentrated such as neat blood or milk. The presence of cleaning agents, disinfectants, pesticides, or antibiotics can have detrimental impacts on treatment processes. Activated Sludge Process
Activated sludge is a biochemical process for treating sewage and industrial wastewater that uses air (or oxygen) and microorganisms to
180
Industrial Wastewater Treatment
biologically oxidize organic pollutants, producing a waste sludge (or floc) containing the oxidized material. Treated Water
Air
Aeration Tank
Recycle Sludge
To Sludge Treatment
Fig:3. A Generalized, Schematic Diagram of an Activated Sludge Process
In general, an activated sludge process includes: • An aeration tank where air (or oxygen) is injected and thoroughly mixed into the wastewater. • A settling tank (usually referred to as a "clarifier" or "settler") to allow the waste sludge to settle. Part of the waste sludge is recycled to the aeration tank and the remaining waste sludge is removed for further treatment and ultimate disposal. Trickling Filter Process Waste Water
Microbial
Treated Water
Fig:4. A Schematic Cross-section of the Contact Face of the Bed Media in a Tricklfng Filter
A trickling filter consists of a bed of rocks, gravel, slag, peat moss, or plastic media over which wastewater flows downward and contacts a layer (or film) of microbial slime covering the bed media. Aerobic conditioI'ts
Industrial Wastewater Treatment
181
are maintained by forced air flowing through the bed or by natural convection of air. Rotating Influent Distributer
l
Treated Water
Sludge
Fig:5. A Typical Complete Trickling Filter System
The process involves adsorption of organic compounds in the wastewater by the microbial slime layer, diffusion of air into the slime layer to provide the oxygen required for the biochemical oxidation of the organic compounds. The end products include carbon dioxide gas, water and other products of the oxidation. As the slime layer thickens, it becomes difficult for the air to penetrate the layer and an inner anaerobic layer is formed. The fundamental components of a complete trickling filter system are: • A bed of filter medium upon which a layer of microbial slime is promoted and developed. • An enclosure or a container which houses the bed of filter medium. • A system for distributing the flow of wastewater over the filter medium. • A system for removing and disposing of any sludge from the treated effluent. The treatment of sewage or other wastewater with trickling filters is among the oldest and most well characterized treatment technologies. A trickling filter is also often called a trickle filter, trickling biofilter, biofilter, biological filter or biological trickling filter. Treatment of Other. Organics
Synthetic organic materials including solvents, paints, pharmaceuticals, pesticides, coking products and so forth can be very difficult
182
Industrial Wastewater Treatment
to treat. Treatment methods are often specific to the material being treated. Methods include advanced oxidation processing, distillation, adsorption, vitrification, incineration, chemical immobilization or landfill disposal. Some materials such as some detergents may be capable of biological degradation and in such cases, a modified form of wastewater treatment can be used. Treatment of Acids and Alkalis
Acids and alkalis can usually be neutralised under controlled conditions. Neutralization frequently produces a precipitate that will require treatment as a solid residue that may also be toxic. In some cases, gasses may be evolved requiring treatment for the gas stream. Some other forms of treatment are usually required following neutralization. Waste streams rich in hardness ions as from de-ionisation processes can readily loose the hardness ions in a buildup of precipitated calcium and magnesium salts. This precipitation process can cause severe furring of pipes and can, in extreme cases, cause the blockage of disposal pipes. A 1 metre diameter industrial marine discharge pipe serving a major chemicals complex was blocked by such salts in the 1970s. Treatment is by concentration of deionization waste waters and disposal to landfill or by careful pH management of the released wastewater. TREATMENT OF TOXIC MATERIALS
Toxic materials including many organic materials, metals (such as zinc, silver, cadmium, thallium, etc., acidst alkalis, non-metallic elements (such as arsenic or selenium) are generally resistant to biological processes unless very dilute. Metals can often be precipitated out by changing the pH or by treatment with other chemicals. Many, however, are resistant to treatment or mitigation and may require concentration followed by land filling or recycling. Disolved organics can be incinerated within the wastewater by advanced oxidation processes: SEWAGE TREATMENT
Sewage treatment, or domestic wastewater treatment, is the process of removing contaminants from wastewater, both runoff (effluents) and domestic. It includes physical, chemical and biological processes to remove physical, chemical and biological contaminants. Its objective is to produce a waste stream (or treated effluent) and a solid waste or sludge suitable for discharge or reuse back into the environment. This material is often inadvertently contaminated with many toxic organic and inorganic compounds.
Industrial Wastewater Treatme1lt
183
Sewage is created by residences, institutions, hospitals and commercial and industrial establishments. It can be treated close to where it is created (in septic tanks, biofilters or aerobic treatment systems), or collected and transported via a network of pipes and pump stations to a municipal treatment plant. Sewage collection and treatment is typically subject to local, state and federal regulations and standards. Industrial sources of wastewater often require specialized treatment processes. The sewage treatment involves three stages, called primary, secondary and tertiary treatment. First, the solids are separated from the wastewater stream. Then dissolved biological matter is progressively converted into a solid mass by using indigenous, water-borne microorganisms. Finally, the biological solids are neutralized then disposed of or reused, and the treated water may be disinfected chemically or physically (for example, by lagoons and micro-filtration). The final effluent can be discharged into a stream, river, bay, lagoon or wetland, or it can be used for the irrigation of a golf course, green way or park. If it is sufficiently clean, it can also be used for groundwater recharge. Description
Raw influent (sewage) includes household waste liquid from toilets, baths, showers, kitchens, sinks, and so forth that is disposed of via sewers. In many areas, sewage also includes liquid waste from industry and commerce. The draining of household waste into greywater and blackwater is becoming more common in the developed world, with greywater being permitted to be used for watering plants or recycled for flushing toilets. A lot of sewage also includes some surface water from roofs or hardstanding areas. Municipal wastewater therefore includes residential, commercial, and industrial liquid waste discharges, and may include stormwater runoff. Sewage systems capable of handling stormwater are known as combined systems or combined sewers. Such systems are usually avoided since they complicate and thereby reduce the efficiency of sewage treatment plants owing to their seasonality. The variability in flow also leads to often larger than necessary, and subsequently more expensive, treatment facilities. In addition, heavy storms that contribute more flows than the treatment plant can handle may overwhelm the sewage treatment system, causing a spill or overflow (called a combined sewer overflow, or CSO, in the United States). It is preferable to have a separate storm drain system for storm water in areas that are developed with sewer systems.
184
Industrial Wastewater Treatment
As rainfall runs over the surface of roofs and the ground, it may pick up various contaminants including soil particles and other sediment, heavy metals, organic compounds, animal waste, and oil and grease. Some jurisdictions require stormwater to receive some level of treatment before being discharged directly into waterways. Examples of treatment processes used for stormwater include sedimentation basins, wetlands, buried concrete vaults with various kinds of filters, and vortex separators (to remove coarse solids). The site where the raw wastewater is processed before it is discharged back to the environment is called a wastewater treatment plant (WWTP). The order and types of mechanical, chemical and biological systems that comprise the wastewater treatment plant are typically the same for most developed countries: • Mechanical treatment. • Influx (Influent). • Removal of large objects. • Removal of sand and grit. • Pre-precipitation. • Biological treatment. • Oxidation bed (oxidizing bed) or aeration system. • Post precipitation. • Chemical treatment (this step is usually combined with settling and other processes to remove solids, such as filtration. Primary treatment removes the materials that can be easily collected from the raw wastewater and disposed of. The typical materials that are removed during primary treatment include fats, oils, and greases (also referred to as FOG), sand, gravels and rocks (also referred to as grit), larger settleable solids and floating materials (such as rags and flushed feminine hygiene products). This step is done entirely with machinery. Removal of Large Objects from Influent Sewage
In primary treatment, the influent sewage water is strained to remove all large objects that are deposited in the sewer system, such as rags, sticks, tampons, cans, fruit, etc. This is most commonly done with a manual or automated mechanically raked screen. The raking action of a mechanical bar screen is typically paced according to the accumulation on the bar screens and/or flow rate. The bar screen is used because large solids can damage or clog the equipment used later in the sewage treatment plant. The solids are collected in a dumpster and later disposed in a landfill. Primary treatment also typically includes a sand or grit channel or chamber where the velocity of the incoming wastewater is carefully
Industrial Wastewater Treatment
185
controlled to allow sand grit and stones to settle, while keeping the majority of the suspended organic material in the water column. This equipment is called a detritor or sand catcher. Sand, grit, and stones need to be removed early in the process to avoid damage to pumps and other equipment in the remaining treatment stages. Sometimes there is a sand washer (grit classifier) followed by a conveyor that transports the sand to a container for disposal. The contents from the sand catcher may be fed into the incinerator in a sludge processing plant, but in many cases, the sand and grit is sent to a landfill. Sedimentation
Many plants have a sedimentation stage where the sewage is allowed to pass slowly through large tanks, commonly called "primary clarifiers" or "primary sedimentation tanks". The tanks are large enough that sludge can settle and floating material such as grease and oils can rise to the surface and be skimmed off. The main purpose of the primary clarification stage is to produce both a generally homogeneous liquid capable of being treated biologically and a sludge that can be separately treated or processed. Primary settling tanks are usually equipped with mechanically driven scrapers that continually drive the collected sludge towards a hopper in the base of the tank from where it can be pumped to further sludge treatment stages. Secondary Treatment
Secondary treatment is designed to substantially degrade the biological content of the sewage such as are derived from human waste, food waste, soaps and detergent. The majority of municipal and industrial plants treat the settled sewage liquor using aerobic biological processes. For this to be effective, the biota requires both oxygen and a substrate on which to live. There are number of ways in which this is done. In all these methods, the bacteria and protozoa consume biodegradable soluble organic contaminants (e.g., sugars, fats, organic short-chain carbon molecules, etc.) and bind much of the less soluble fractions into floc. Secondary treatment systems are classified as fixed film or suspended growth. Fixed-film treatment process including trickling filter and rotating biological contactors where the biomass grows on media and the sewage passes over its surface. In suspended growth systems-such as activated sludge-the biomass is well mixed with the sewage and can be operated in a smaller space than fixed-film systems that treat the same amount of water. However, fixed-film systems are more able to cope with drastic changes in the amount of biological material and can provide higher
186
Industrial Wastewater Treatment
removal rates for organic material and suspended solids than suspended growth systems. Roughing filters are intended to treat particularly strong or variable organic loads, typically industrial, to allow them to then be treated by conventional secondary treatment processes. Characteristics include typically tall, circular filters filled with open synthetic filter media to which wastewater is applied at a relatively high rate. They are designed to allow high hydraulic loading and a high flowthrough of air. On larger installations, air is forced through the media using blowers. The resultant wastewater is usually within the normal range for conventional treatment processes. Activated Sludge
In general, activated sludge plants encompass a variety of mechanisms and processes that use dissolved oxygen to promote the growth of biological floc that substantially removes organic material. The process traps particulate material and can, under ideal conditions, convert ammonia to nitrite and nitrate and ultimately to nitrogen gas. Surface Aerated Basins Electric Motor With propeller and Slinger-ring on vertic,,1 motor shaft
Bernt
Basin
Most biological oxidation processes for treating industrial wastewaters have in common the use of oxygen (or air) and microbial action. Surface-aerated basins achieve 80 to 90% removal of Biochemical Oxygen Demand with retention times of 1 to 10 days. In an aerated basin system, the aerators provide two functions: they transfer air into the basins required by the biological oxidation reactions, and they provide the mixing reqUired for dispersing the air and for contacting the reactants (that is, oxygen, wastewater and microbes). Typically, the floating surface aerators are rated to deliver the amount of air equivalent to 1.8 to 2.7 kg. Biological oxidation processes are sensitive to temperature and, between O°C and 40°C, the rate of biological reactions increase with temperature. Most surface aerated vessels operate at between 4°C and 32°C.
Industrial Wastewater Treatment
187
Fluidized Bed Reactors
The carbon absorption following biological treatment is particularly effective in reducing both the BOD and COD to low levels. A fluidized bed reactor is a combination of the most comrrlon stirred tank packed bed, continuous flow reactors. It is very important to chemical engineering because of its excellent heat and mass transfer characteristics. In a fluidized bed reactor, the substrate is passed upward through the immobilized enzyme bed at a high velocity to lift the particles. However, the velocity must not be so high that the enzymes are swept away from the reactor entirely. This causes low mixing; these type of reactors are highly suitable for the exothermic reactions. It is most often applied in immobilized enzyme catalysis. Filter Beds (Oxidizing Beds)
In older plants and plants receiving more variable loads, trickling filter beds are used where the settled sewage liquor is spread onto the surface of a deep bed made up of coke (carbonised coal), limestone chips or specially fabricated plastic media. Such media must have high surface areas to support the biofilms that form. The liquor is distributed through perforated rotating arms radiating from a central pivot. The distributed liquor trickles through this bed and is collected in drains at the base. These drains also provide a source of air which percolates up through the bed, keeping it aerobic. Biological films of bacteria, protozoa and fungi form on the media's surfaces and eat or otherwise reduce the organic content. This biofilm is grazed by insect larvae and worms which help maintain an optimal thickness. Overloading of beds increases the thickness of the film leading to clogging of the filter media and ponding on the surface. Biological Aerated Filters
Biological Aerated (or Anoxic) Filter (BAF) or Biofilters comb.ine filtration with biological carbon reduction, nitrification or denitrification. BAF usually includes a reactor filled with a filter media. The media is either in suspension or supported by a gravel layer at the foot of the filter. The dual purpose of this media is to support highly active biomass that is attached to it and to filter suspended solids. Carbon reduction and ammonia conversion occurs in aerobic mode and sometime achieved in a single reactor while nitrate conversion occurs in anoxic mode. BAF is operated either in upflow or downflow configuration depending on design specified by manufacturer.
188
Industrial Wastewater Treatnze1lt
Membrane Bioreactors
Membrane bioreactors (MBR) combines activated sludge treatment with a membrane liquid-solid separation process. The membrane component uses low pressure microtiltration or ultra filtration membranes and eliminates the need for clarification and tertiary filtration. The membranes are typically immersed in the aeration tank (however, some applications utilize a separate membrane tank). One of the key benefits of a membrane bioreactor system is that it effectively overcomes the limitations associated with poor settling of sludge in conventional activated sludge (CAS) processes. The technology permits bioreactor operation with considerably higher mixed liquor suspended solids (MLSS) concentration than CAS systems, which are limited by sludge settling. The process is typically operated at MLSS in the range of 8,000-12,000 mg/l, while CAS are operated in the range of 2,000-3,000 mg/I. The elevated biomass concentration in the membrane bioreactor process allows for very effective removal of both soluble and particulate biodegradable materials at higher loading rates. Thus increased Sludge Retention Times (SRTs)-usually exceeding 15 days-ensure complete nitrification even in extremely cold weather. The cost of building and operating a MBR is usually higher than conventional wastewater treatment, however, as the technology has become increasingly popular and has gained wider acceptance throughout the industry, the life-cycle costs have been steadily decreasing. As well, in developed urban areas where the footprint of the treatment plant is considered a limiting tactor MBR facilities can be considered a desirable option. Secondary Sedimentation
The final step in the secondary treatment stage is to settle out the biological floc or filter material and produce sewage water containing very low levels of organic material and suspended matter. Rotating Biological Contactors
Schematic diagram of a typical rotating biological contactor (RBC) in given in figure 7. The treated effluent clarifier/settler is not included in the diagram. Rotating biological contactors (RBCs) are mechanical secondary treatment systems, which are robust and capable of withstanding surges in organic load. RBCs were first installed in Germany in 1960 and have since been developed and refined into a reliable operating unit. The rotating disks support the growth of bacteria and microorganisms present in the sewage, which breakdown and stabilise organic pollutants.
Industrial Wastewater Treatmellt Media discs or panels
One media pack ,--A--.
'"
Shaft ~
189
Media disc or panel /fo==n===~ Motor
A r,5
ISlde Vlewl
IFront Viewl Optional air distributor pipe
Fig:7. Schematic Diagram of a Typical Rotating Biological Contactor (RBC)
To be successful, microorganisms need both oxygen to live and food to grow. Oxygen is obtained from the atmosphere as the disks rotate. As the microorganisms grow, they build up on the media until they are sloughed off due to shear forces provided by the rotating discs in the sewage. Effluent from the RBC is then passed through final clarifiers where the microorganisms in suspension settle as a sludge. The sludge is withdrawn from the clarifier for further treatment. Tertiary Treatment
Tertiary treatment provides a final stage to raise the effluent quality before it is discharged to the receiving environment (sea, river, lake, ground, etc.). More than one tertiary treatment process may be used at any treatment plant. If disinfection is practiced, it is always the final process. It is also called "effluent polishing". Filtration
Sand filtration removes much of the residual suspended matter. Filtration over activated carbon removes residual toxins. Lagooning
Lagooning provides settlement and further biological improvement through storage in large man-made ponds or lagoons. These lagoons are highly aerobic and colonization by native macrophytes, especially reeds, is often encouraged. Small filter feeding invertebrates such as Daphnia and species of Rotifera greatly assist in treatment by removing fine particulates. Constructed Wetlands
Constructed wetlands include engineered reedbeds and a range of similar methodologies, all of which provide a high degree of aerobic
190
Industrial Wastewater Treatment
biological improvement and can often be used instead of secondary treatment for small communities. One example is a small reedbed used to clean the drainage from the elephants' enclosure at Chester Zoo in England. Nutrient Removal
Wastewater may contain high levels of the nutrients nitrogen and phosphorus. Excessive release to the environment can lead to a build up of nutrients, called eutrophication, which can in turn encourage the overgrowth of weeds, algae, and cyanobacteria (blue-green algae). This may cause an algal bloom, a rapid growth in the population of algae. The algae numbers are unsustainable and eventually most of them die. The decomposition of the algae by bacteria uses up so much of oxygen in the water that most or all of the animals die, which creates more organic matter for the bacteria to decompose. In addition to causing deoxygenation, some algal species produce toxins that contaminate drinking water supplies. Different treatment processes are required to remove nitrogen and phosphorus. Nitrogen Removal
The removal of nitrogen is effected through the biological oxidation of nitrogen from ammonia (nitrification) to nitrate, followed by denitrification, the reduction of nitrate to nitrogen gas. Nitrogen gas is released to the atmosphere and thus removed from the water. Nitrification itself is a two-step aerobic process, each step facilitated by a different type of bacteria. The oxidation of ammonia (NH3) to nitrite (N02 ) is most often facilitated by Nitrosomonas spp. (nitroso referring to the formation of a nitroso functional group). Nitrite oxidation to nitrate, (N0 3), though traditionally believed to be facilitated by Nitrobacter spp. (nitro referring the formation of a nitro functional group), is now known to be facilitated in the environment almost exclusively by Nitrospira spp. Denitrification requires anoxic conditions to encourage the appropriate biological communities to form. It is facilitated by a wide diversity of bacteria. Sand filters, lagooning and reed beds can all be used to reduce nitrogen, but the activated sludge process (if designed well) can do the job the most easily. Since denitrification is the reduction of nitrate to dinitrogen gas, an electron donor is needed. This can be, depending on the wastewater, organic matter (from faeces), sulfide, or an added donor like methanol. Sometimes the conversion of toxic ammonia to nitrate alone is referred to as tertiary treatment.
Industrial Wastewater Treatment
191
Phosphorus Removal
Phosphorus can be removed biologically in a process called enhanced biological phosphorus removal. In this process, specific bacteria, called polyphosphate accumulating organisms, are selectively enriched and accumulate large quantities of phosphorus within their cells (up to 20% of their mass). When the biomass enriched in these bacteria is separated from the treated water, these biosolids have a high fertilizer value. Phosphorus removal can also be achieved by chemical precipitation, usually with salts of iron (e.g., ferric chloride) or aluminum (e.g. alum). The resulting chemical sludge is difficult to handle and the added chemicals can be expensive. Despite this, chemical phosphorus removal requires significantly smaller equipment footprint than biological removal, is easier to operate and can be more reliable in areas that have wastewater compositions that make biological phosphorus removal difficult. Disinfection
The purpose of disinfection in the treatment of wastewater is to substantially r.educe the number of microorganisms in the water to be discharged back into the environment. The effectiveness of ·disinfection depends on the quality of the water being treated (e.g., cloudiness, pH, etc.), the type of disinfection being used, the disinfectant dosage (concentration and time), and other environmental variables. Cloudy water will be treated less successfully since solid matter can shield organisms, especially from ultrflviolet light or if contact times are low. Generally, short contact times, low doses and high flows all militate against effective disinfection. Common methods of disinfection include ozone, chlorine, or ultraviolet light. Chloramine, which is used for drinking water, is not used in wastewater treatment because of its persistence. Chlorination remains the most common form of wastewater disinfection in North America due to its low cost and long-term history of effectiveness. One disadvantage is that chlorination of residual organic material can generate chlorinated-organic compounds that may be carcinogenic or harmful to the environment. Residual chlorine or chloramines may also be capable of chlorinating organic material in the natural aquatic environment. Further, because residual chlorine is toxic to aquatic species, the treated effluent must also be chemically dechlorinated, adding to the complexity and cost of treatment. Ultra-violet (UV) light can be used instead of chlorine, iodine, or other chemicals. Because no chemicals are used, the treated water has no adverse effect on organisms that later consume it, as may be the case with other
192
Industrial Wastewater Treatment
methods. UV radiation causes damage to the genetic structure of bacteria, viruses, and other pathogens, making them incapable of reproduction . ....",The key disadvantages of UV disinfection are the need for frequent lamp maintenance and replacement and the need for a highly treated effluent to ensure that the target microorganisms are not shielded from the UV radiation (i.e., any solids present in the treated effluent may protect microorganisms from the UV light). In the United Kingdom, light is becoming the most common means of disinfection because of the concerns about the impacts of chlorine in chlorinating residual organics in the wastewater and in chlorinating organics in the receiving water. Edmonton, Alberta, Canada also. uses UV light for its water treatment. Ozone (03) is generated by passing oxygen O 2 through a high voltage potential resulting in a third oxygen atom becoming attached and forming 0 3 , Ozone is very unstable and reactive and oxidizes most organic material it comes in contact with, thereby destroying many pathogenic microorganisms. Ozone is considered to be safer than chlorine because, unlike chlorine which has to be stored on site (highly poisonous in the event of an accidental release), ozone is generated onsite as needed. Ozonation also produces fewer disinfection by-products than chlorination. A disadvantage of ozone disinfection is the high cost of the ozone generation equipment and the requirements for special operators. Package Plants and Batch Reactors
In order to use less space, treat difficult waste, deal with intermittent flow or achieve higher environmental standards, a number of designs of hybrid treatment plants have been produced. Such plants often combine all or at least two stages of the three main treatment stages into one combined stage. In the UK, where a large number of sewage treatment plants serve small populations, package plants are a viable alternative to building discrete structures for each process stage. One type of system that combines secondary treatment and settlement is the sequencing batch reactor (SBR). Typically, activated sludge is mixed with raw incoming sewage and mixed and aerated. The resultant mixture is then allowed to settle producing a high quality effluent. The settled sludge is run off and re-aerated before a proportion is returned to the head of the works. SBR plants are now being deployed in many parts of the world including North Liberty, Iowa, and Llanasa, North Wales. The disadvantage of such processes is that precise control of timing, mixing and aeration is required. This precision is usually achieved by computer controls linked to many sensors in the plant. Such a complex, fragile system is unsuited to places where such controls may be unreliable,
Industrial Wastewater Treatment
193
or poorly maintained, or where the power supply may be intermittent. Package plants may be referred to as high charged or low charged. This refers to the way the biological load is processed. In high charged systems, the biological stage is presented with a high organic load and the combined floc and organic material is then oxygenated for a few hours before being charged again with a new load. In the low charged system the biological stage contains a low organic load and is combined with £loculate for a relatively long time. Sludge Treatment and Disposal
The sludges accumulated in a wastewater treatment process must be treated and disposed of in a safe and effective manner. The purpose of digestion is to reduce the amount of organic matter and the number of disease-causing microorganisms present in the solids. The most common treatment options include anaerobic digestion, aerobic digestion, and composting. The choice of a wastewater solid treatment method depends on the amount of solids generated and other site-specific conditions. However, in general, composting is most often applied to smaller-scale applications followed by aerobic digestion and then lastly anaerobic digestion for the larger-scale municipal applications.
Anaerobic Digestion Anaerobic digestion is a bacterial process that is carried out in the absence of oxygen. The process can either be thermophilic digestion, in which sludge is fermented in tanks at a temperature of 55°C, or mesophilic, at a temperature of around 36°C. Though allowing shorter retention time (and thus smaller tanks), thermophilic digestion is more expensive in terms of energy consumption for heating the sludge. One major feature of anaerobic digestion is the production of biogas, which can be used in generators for electricity production and/or in boilers for heating purposes.
Aerobic Digestion Aerobic digestion is a bacterial process occurring in the, presence of oxygen. Under aerobic conditions, bacteria rapidly consume organic matter and convert it into carbon dioxide. The operating costs are characteristically much greater for aerobic digestion because of the energy costs needed to add oxygen to the process.
Composting Composting is also an aerobic process that involves mixing the sludge
194
Industrial Wastewater Treatment
with sources of carbon such as sawdust, straw or wood chips. In the presence of oxygen, bacteria digest both the wastewater solids and the added carbon source and, in doing so, produce a large amount of heat.
Thermal Oepolymerization Thermal depolymerization uses hydrous pyrolysis to convert reduced complex organics to oil. Sludge Disposal
When a liquid sludge is produced, further treatment may be required to make it suitable for final disposal. Typically, sludges are thickened (dewatered) to reduce the volumes transported off-site for disposal. There is no process which completely eliminates the need to dispose of biosolids. There is, however, an additional step some cities are taking to superheat the wastewater sludge and convert it into small pelletized granules that are high in nitrogen and other organic materials. This product is then sold to local farmers and turf farms as a soil amendment or fertilizer, reducing the amount of space required to dispose of sludge in landfills. Treatment in the Receiving Environment
Many processes in a wastewater treatment plant are designed to mimic the natural treatment processes that occur in the environment, whether that environment is a natural water body or the ground. If not overloaded, bacteria in the environment will consume organic contaminants, although this will reduce the levels of oxygen in the water and may significantly change the overall ecology of the receiving water. Native bacterial populations feed on the organic contaminants, and the numbers of disease-causing microorganisms are reduced by natural environmental conditions such as predation exposure to ultra-violet radiation, for example. Consequently, in cases where the receiving environment provides a high level of dilution, a high degree of wastewater treatment may not be required. However, recent evidence has demonstrated that very low levels of certain contaminants in wastewater, including hormones (from animal husbandry and residue from human hormonal contraception methods) and synthetic materials such as phthalates that mimic hormones in their action, can have an unpredictable adverse impact on the natural biota and potentially on humans if the water is re-used for drinking water. In the US and EU, uncontrolled discharges of wastewater to the environment are not permitted under law, and strict water quality requirements are to
Industrial Wastewater Treatment
195
be met. A significant threat in the coming decades will be the increasing uncontrolled discharges of wastewater within rapidly developing countries. Sewage Treatment in Developing Countries
There are few reliable figures on the share of the wastewater collected in sewers that is being treated in the world. In many developing countries the bulk of domestic and industrial wastewater is discharged without any treatment or after primary treatment only. In Latin America about 15% of collected wastewater passes through treatment plants (with varying levels of actual treatment). In Venezuela, a below average country in South America with respect to wastewater treatment, 97 per cent of the country's sewage is discharged raw into the environment. In a relatively developed Middle Eastern country such as Iran, Tehran's majority of population has totally untreated sewage injected to the city's groundwater. Israel has also aggressively pursued the use of treated sewer water for irrigation. In 2008, agriculture in Israel consumed 500 million cubic metres of potable water and an equal amount of treated sewer water. The country plans to provide a further 200 million cubic metres of recycled sewer water and build more desalination plants to supply even more water. Water utilities in developing countries are chronically underfunded because of low water tariffs, the inexistence of sanitation tariffs in many cases, low billing efficiency (Le., many users that are billed do not pay) and poor operational efficiency (i.e., there are overly high levels of staff, there are high physical losses, and many users have illegal connections and are thus not being billed). In addition, wastewater treatment typically is the process within the utility that receives the least attention, partly because enforcement of environmental standards is poor. As a result of all these factors, operation and maintenance of many wastewater treatment plants is poor. This is evidenced by the frequent breakdown of equipment, shutdown of electrically operated equipment due to power outages or to reduce costs, and sedimentation due to lack of sludge removal. Developing countries as diverse as Egypt, Algeria, China or Colombia have invested substantial sums in wastewater treatment without achieving a significant impact in terms of environmental improvement. Even if wastewater treatment plants are properly operating, it can be argued that the environmental impact is limited in cases where the assimilative
196
Industrial Wastewater Treatment
capacity of the receiving waters (ocean with strong currents or large rivers) is high, as it is often the case. Benefits of Wastewater Treatment Compared to Benefits of Sewage Collection in Developing Countries
Waterborne diseases that are prevalent in developing countries, such as typhus and cholera, are caused primarily by poor hygiene practices and the absence of improved household sanitation facilities. The public health impact of the discharge of untreated wastewater is comparatively much lower. Hygiene promotion, on-site sanitation and low-cost sanitation thus are likely to have a much greater impact on public health than wastewater treatment.
Chatper 9
Aquifer Recharge with Wastewater SOIL-AQUIFER TREATMENT
Where soil and groundwater conditions are favourable for artificial recharge of groundwater through infiltration basins, a high degree of upgrading can be achieved by allowing partially-treated sewage effluent to infiltrate into the soil and move down to the groundwater. The unsaturated or "vadose" zone then acts as a natural filter and can remove essentially all suspended solids, biodegradable materials, bacteria, viruses, and other microorganisms. Significant reductions in nitrogen, phosphorus, and heavy metals concentrations can also be achieved. After the sewage, treated in passage through the vadose zone, has reached the groundwater it is usually allowed to flow some distance through the aquifer before it is collected. This additional movement through the aquifer can produce further purification (removal of microorganisms, precipitation of phosphates, adsorption of synthetic 6rganics, etc.) of the sewage. Since the soil and aquifer are used as natural treatment, systems such as those in Figure are called soil-aquifer treatment systems or SAT systems. Soil-aquifer treatment is, essentially, a low-technology, advanced wastewater treatment system. It also has an aesthetic advantage over conventionally treated sewage in that water recovered from a SAT system is not only clear and odour-free but it comes from a well, drain, or via natural drainage to a stream or low area, rather than from a sewer or sewage treatment plant. Thus, the water has lost its connotation of sewage and the public see it water more as coming out of the ground (groundwater) than as sewage effluent. This could be an important factor in the public acceptance of sewage reuse schemes. SAT SYSTEM LAYOUTS
The simplest being where the sewage effluent is applied to infiltration basins on high ground from where it moves down to the groundwater
Aquifer Recharge with Wastewater
198
and eventually drains naturally through an aquifer into a lower area. This lower area can be a natural depression or seepage area, a stream or lake, or a surface drain. SAT systems as in Figure 1 also serve to reduce the pollution of surface waters. Instead of discharging wastewater directly into streams or lakes, it is applied to infiltration basins at a higher elevation so that it receives soil-aquifer treatment before entering the stream or lake. The system shown in Figure is similar to that shown in A but the treated sewage water, after SAT, is collected by underground, agricultural-type drains. Systems A and B have the advantage that the entire SAT process is accomplished without pumping. Where the groundwater is too deep to collect the renovated sewage water by gravity, pumped wells must be used and there are two basic layouts.
; & III ~,-- --_ .~/I) -.......: '", .
A
B
';},S"
;;:;s,"'JJffffiflyr;;n,."", " .. ,;"
.; ) ...... ---..... ./ ~----/
--. --.
2 ; ;;;: ;;;; J"
1>;,ml"ffrMni,;;m;;,;r;;;fA
--
- ~\ \ \.""OBS
C
D
Welt
J ' ""
...... _
--
-- -
-
"... ••
--
,Impeaveabl
! ~ "11} 2}J/ .. ;1",;1)'777'1,1);' , ,Tn 7; 111,
---
fft.J7h}'" ,7'> ",,.. " ').','l.» ... .,
71~ I
r"') i, ,.'!I;,1
Fig:1. Schematic of Soil-aquifer Treatment Systems
In one, the infiltration basins are arranged in two parallel strips and the wells are located on the line midway between the two strips. In the
Aquifer Recharge with Wastewater
199
other, the infiltration basins are located close together in a cluster and the wells are on a circle around this cluster. The system of Figure C can be designed and managed so that the wells pump essentially all renovated sewage water and no native groundwater from the aquifer outside the SAT system. Systems as in Figure are more likely to deliver a mixture of renovated sewage water and native groundwater. Systems C and D can be used both for seasonal underground storage of sewage water, allowing the groundwater mound to rise during periods of low irrigation water demand (winter), and for pumping the groundwater mound down in periods of high irrigation water demands (summer). The type of SAT system shown in Figure would be suitable for small systems where there are only a few basins around a centrally located well. While SAT systems give considerable water quality improvement to the sewage effluent, the quality of the resulting renovated water is not often as good as that of the native groundwater. Thus, SAT systems should normally be designed and managed to prevent encroachment of sewage water into the aquifer outside the portion of the aquifer used for soilaquifer treatment. For systems A and B in Figure, this could be achieved by ensuring that all the renovated water is intercepted by the surface or subsurface drain, which would result from excavating or installing the drain deeply enough to make sure that groundwater on the other side of the drain also moves toward the drain. For system C in Figure, movement of renovated sewage water to the aquifer outside the SAT system can be prevented by managing infiltration and pumping rates so that the groundwater table below the outer boundaries of the infiltration strips never rises higher than the groundwater table outside the SAT system. This requires groundwaterlevel monitoring in a few observation wells installed at the outer edges of the infiltration strips. In the case of system D in Figure, movement of renovated sewage water into the aquifer outside the circle of wells can be prevented by pumping the wells at sufficient rate so that there is movement of native groundwater outside the SAT system toward the wells. Sewage water should travel sufficient distance through the soil and aquifer, and residence times in the SAT system should be long enough, to produce renovated water of the desired quality. While 100 m underground travel and one month underground retention time have been suggested as rule-of-thumb values, the actually required values depend on the quality of sewage effluent infiltrating into the ground, the soil types in
Aquifer Recharge with Wastewater
200
the vadose zone and aquifer, the depth to groundwater, and the desired quality of the renovated water. Most of the quality improvement of sewage effluent moving through an SAT system occurs in the top 1m of soil. However, longer travel is desirable because it gives more complete
removal of microorganiSOPOlisaatment.
® WELL
DD
Fig:2. Schematic Representation of Four Small Infiltration Basins with Well in Centre for Pumping Renovated Sewage Water from Aquifer
SOIL REQUIREMENTS Infiltration basins for SAT systems should be located in soils that are permeable enough to give high infiltration rates. This requirement is important where sewage flows are relatively large, where excessive basin areas should be avoided (due to land cost) and where evaporation losses from the basins should be minimized. The soils, however, should also be fine enough to provide good filtration and quality improvement of the effluent as it passes through. Thus, the best surface soils for SAT systems are in the fine sand, loamy sand, and sandy loam range. Materials deeper in the vadose zone should be granular and preferably coarser than the surface soils. Soil profiles consisting of coarse-textured material on top and finer-textured material deeper down should be avoided because of the danger that fine suspended material in the sewage will move through the coarse upper material and accumulate on the deeper, finer material. This could cause clogging of the soil profile at some depth, where removal of the clogging material would be very difficult. Vadose zones should not contain clay layers or other soils that could restrict the downward movement of water and form perched groundwater mounds. Aquifers should be sufficiently deep and transmissive to prevent excessive rises of the groundwater table (mounding) due to infiltration. Groundwater tables should be at least 1 m below the bottom of the infiltration basins during flooding. Above all, soH and aquifer materials should be granular. Fractured-rock aquifers should be protected by a soil mantle of adequate texture and thickness (at least a few metres). Shallow soils underlain by fractured rock are not suitable for SAT systems.
Aquifer Recharge with Wastewater
201
OPERATIONS HYDRAULIC CAPACITY AND EVAPORATION
Infiltration basins in SAT systems are intermittently flooded to provide regular drying periods, for restoration of infiltration rates and for aeration of the soil. Flooding schedules typically vary from 8 hours dry-16 hours flooding to 2 weeks dry-2 weeks flooding. Therefore, SAT systems should have a number of basins so that some basins can be flooded while others are drying. Annual infiltration amounts or "hydraulic loading rates" typically vary from 15 m/year to 100 m/year, depending on soil, climate, quality of sewage effluent, and frequency of basin cleaning. Thus, assuming a sewage production of 100 I/person day, a city of 100,000 people, and a hydraulic loading rate of 50 m/year, an SAT system for the entire sewage flow would require about 7.3 ha of infiltration basins. This shows that SAT systems do not necessarily require very large land areas, provided, of course, that the soils are permeable enough and the sewage is of such a quality (low suspended solids content) so as to allow high hydraulic loading rates to be maintained. Evaporation losses from free water surfaces in dry, warm areas typically range between 1 and 2 m/year. Since the soil of infiltration basins will be mostly wet during drying, evaporation from intermittently flooded basins will be almost the same as that under continuous flooding conditions. Assuming an SAT system with a hydraulic loading rate of 50 m/year and evaporation losses of 1.5 m/year, evaporation losses would be 3% of all the sewage applied which would cause a 3 % increase in the concentration of dissolved salts in the sewage water. BASIN MANAGEMENT
Bare soil is often the best condition for the bottom of infiltration basins in SAT systems. Occasional weeds are no problem but too many weeds can hamper the soil drying process, which delays recovery of infiltration rates. Dense weeds can also aggravate mosquito and other insect problems. Low water depths (about 20 cm) may be preferable to large water depths (about 1 m) because the turnover rate of sewage applied to shallow basins is faster than for deep basins of the same infiltration rate, thus giving suspended algae less time to develop in shallow basins. Suspended algae can produce low infiltration rates because they are filtered out on the basin bottom, where they clog the soil. Also, algae, being photosynthetic, remove dissolved carbon dioxide from the water, which increases the pH of the water. At high algal concentrations, this can cause the pH to rise to 9 or 10 which, in turn, causes precipitation of calcium carbonate. This cements the soil surface and results in further soil
202
Aqu~fer
Recharge with Wastewater
clogging and reduction of infiltration rates. Because suspended algae and soil clogging problems are reduced, shallow basins generally yield higher hydraulic loading rates than deep basins. During flooding, organic and other suspended solids in the sewage effluent accumulate on the bottom of the basins, producing a clogging layer which causes infiltration rates to decline. Drying of the basins causes the clogging layer to dry, crack, and form curled-up flakes; the organic material also decomposes. These processes restore the hydraulic capacity so that when the basins are flooded again, infiltration rates are close to the original, high levels. However, as flooding continues, infiltration rates decrease again until they become so low that another drying period is necessary. Depending on how much material accumulates on the bottom of infiltration basins, periodic removal of this material is necessary. Removing the material by raking or scraping is much better than mixing it with the soil with, for example, a disk harrow. The latter practice will lead to gradual accumulation of clogging materials in the top 10 or 20 cm of the soil, eventually necessitating complete removal of this layer, which could be expensive. For clean secondary sewage effluent with suspended solids concentration of 10 to 20 mg/l, flooding and drying periods can be as long as 2 weeks each, and cleaning of basin bottoms may be necessary only once a year or once every 2 years. Primary effluent, with much higher suspended solids concentration, will require a schedule which might be 2 days flooding-8 days drying, and basin bottoms might be expected to require cleaning at the end of almost every drying period. The best schedule of flooding, drying, and cleaning of basins in a given system must be evaluated by on-site experimentation. PRETREATMENT
The main constituent that must be removed from raw sewage before it is applied to an SAT system is suspended solids. Reductions in BOD and bacteria are also desirable, but less essential. In the USA, there are several hundred SAT systems and, prior to land application, the sewage typically receives conventional primary and secondary treatment because that is the treatment normally prescribed before anything can be done with the effluent. Secondary treatment removes mostly biodegradable material, as expressed by the BOD, but bacteria in the soil can also degrade organic material and reduce the BOD of the sewage to essentially zero. Thus, where pretreatment is followed by SAT, primary treatment would normally be sufficient. The primary effluent would have a higher
Aquifer Recharge with Wastewater
203
BOD and suspended solids content than secondary effluent and this would result in somewhat lower hydraulic loading rates for the SAT system and would require more frequent basin cleaning. However, elimination of the secondary step in conventional pretreatment of the effluent would result in very significant cost savings for the overall system.
EFFECTS As mentioned previously, the main constitutents that must be removed from sewage effluent before it can be used for unrestricted irrigation are pathogenic organisms. Nitrogen concentration might also have to be reduced and suspended solids and biodegradable materials should perhaps be removed to protect the irrigation system or for aesthetic reasons. If the renovated water is to be used for recreational lakes or discharged into surface water, phosphorus should also be removed to prevent algal growth in the receiving water. The following sections describe how these constituents are removed or reduced in SAT systems.
Suspended Solids After appropriate pretreatment, the suspended solids in sewage effluent are usually relatively fine and in organic form (sewage sludge, bacteria, floes, algal cells, etc.). These solids accumulate on the soil in the infiltration basins, requiring regular drying for infiltration recovery and periodic removal from the soil by raking or scraping. For loamy sands and sandy loams, few suspended solids will penetrate into the soil and then, usually, only for a short distance. In dune sands and other coarser soils, fine and colloidal suspended solids (including algal cells) can penetrate much greater distances. Except for medium and coarse uniform sands, soils are very effective filters, and suspended solids will be essentially completely removed from the sewage effluent after about 1m of percolation through the vadose zone. Additional details regarding suspended solids removal and clogging are given in Bouwer and Bouwer and Chaney.
Organic Compounds Most organic compounds of human, animal or plant origin in sewage effluent are rapidly decomposed in the soil. Under aerobic conditions (intermittent flooding), breakdown is generally faster and more complete (to carbon dioxide, minerals and water) than under anaerobic conditions. The latter prevail in the soil profile during continuous or long-term flooding. Stable, non-toxic organic compounds such as humic and fulvic
204
Aquifer Recharge with Wastewater
acids can be formed as products of reactions between proteins and carbohydrates (cellulose or lignin). The BODs of sewage varies from several hundred to about 1000 mg/l for raw sewage, and from about 10 to 20 mg/l for good quality secondary effluent. SAT systems can handle high BOD-loadings, probably hundreds of kg/ha day, and BOD levels are generally reduced to essentially zero after a few metres (often less) of percolation through soil. However, the final product water from SAT systems still contains some organic carbon, usually a few mg/l. This is probably mostly due to humic and fulvic acids but also to synthetic organic compounds in the sewage effluent that do not break down in the underground environment. Halogenated hydrocarbons tend to be more resistant to biodegradation than non-halogenated hydrocarbons. Synthetic organic compounds in the renovated water from SAT systems are generally present at very low concentrations, usually at the ppb (micrograms/I) level, and are not considered a problem when the water is used for irrigation. If it were to be used for drinking, however, additional treatment of the water by, for example, carbon filtration and reverse osmosis, would be necessary to remove the organic compounds. Additional details regarding BOD removal in SAT systems are given in Bouwer and Bouwer and Chaney. Bacteria and Viruses
Pathogenic organisms in sewage effluent include salmonella, shigella, mycobacterium, and Vibrio comma. Specific tests for these bacteria are not routinely carried out but, instead, the numbers of faecal coliform bacteria are normally determined. Escherichia coli are indicator organisms that are widely used to detect faecal contamination of water and the assumption is that if faecal coliform bacteria are present in a sample, then human pathogenic bacteria could also exist. It is also inferred that if faecal coliform bacteria are no longer present, pathogenic bacteria are also absent. Viruses in sewage effluent include entero- and adeno-viruses. Hepatitis viruses are of special concern. Viruses in renovated water from SAT systems are tested for by passing large volumes (1000 to 2000/1) through positivelycharged filters to trap the viruses. Subsequently the viruses are determined in the laboratory as plaque-forming units (PFUs), which usually represent clusters of viruses. Specific viruses are tested for serologically. Other pathogens in sewage effluent include protozoa and helminth parasites, which are discussed elsewhere. Soil is an effective filter to remove microorganisms from sewage effluent (except, of course, coarse soils such as sands and gravels, or fractured rock). Bacteria are physically strained from the water, whereas
Aquifer Recharge with Wastewater
205
the much smaller viruses are usually adsorbed. This adsorption is favoured by a low pH, a high salt concentration in the sewage, and high relative concentrations of calcium and magnesium over monovalent cations such as sodium and potassium. Human bacteria and viruses immobilized in the soil do not reproduce, and eventually die. Most bacteria and viruses die in a few weeks to a few months, but much longer survival times have also been reported. Many studies indicate essentially complete faecal coliform removal after percolation of 1 to a few metres through the soil. However, much longer distances of underground travel of microorganisms have also been reported. Usually, these long distances are associated with macropores, as may be found in gravelly or other coarse materials, structured or cracked clay soils, fractured rock, cavernous limestones, etc. The best protection against breakthrough of pathogenic microorganisms in the renovated sewage water from SAT systems is to reduce bacterial levels in the sewage effluent before infiltration, to avoid coarse textured materials in the SAT systems, and to allow long underground travel distances and retention times. Additional information on this subject is provided in Bouwer, Bouwer and Chaney and Gerba and Goyal. Nitrogen
Nitrogen levels in sewage can range from 20 to more than 100 mg/l, depending on in-house water use and diet of the local people and on the treatment of the sewage effluent prior to SAT. Nitrogen is primarily present as organic, ammonium, and nitrate nitrogen. The relative amounts of these nitrogen forms depend on the form of treatment prior to SAT. For secondary effluent, much of the nitrogen will often be in the ammonium form but some processes are designed to achieve nitrification and the effluent will then contain primarily nitrate-nitrogen. Raw sewage has considerable organic nitrogen. The desirable form and concentration of nitrogen in the renovated sewage water from an SAT system depends on the nitrogen and water requirements of the crops to be irrigated, the need for preventing nitrate pollution of groundwater in the irrigated area due to excess nitrogen application to the crops, and on other possible uses of the water (including fish ponds, for which low concentrations of ammonium are required). Control of the form and concentration of the nitrogen in renovated water from an SAT system is possible by properly selecting hydraulic loading rates and flooding and drying periods for the infiltration basins.
206
Aquifer Recharge with Wastewater
For example, if the nitrogen in the sewage effluent is mostly in the ammonium form, short flooding periods and frequent drying of the infiltration basins (for example, 2 days flooding-5 days drying) will cause essentially complete nitrification of the ammonium in the soil, due to frequent aeration of the soil profile and resulting aerobic conditions. Thus, almost all the nitrogen in the renovated water from the SAT system will then be in the nitrate form and at concentrations about equal to the total nitrogen concentration in the sewage effluent applied to the basin. Long flooding and drying periods (for example, 1 month flooding1 month drying) would eventually lead to complete breakthrough of ammonium in the renovated water because of anaerobic conditions in the soil and absence of nitrification. If flooding and drying periods are of intermediate length (for example, 1 to 2 weeks flooding-l to 2 weeks drying), there will be a succession of aerobic and anaerobic conditions in the upper part of the soil profile, which stimulates nitrification and denitrification. The latter is an anaerobic bacterial process that reduces nitrate to free nitrogen gas and oxides of nitrogen that return to the atmosphere. With this process, about 75% of the nitrogen in sewage has been removed in an SAT system in Arizona, USA, with almost all of the remaining nitrogen in the renovated water occurring in the nitrate form. Denitrification requires the presence of nitrate and organic carbon (an energy source for denitrifying bacteria) under anaerobic conditions. About 1 mg/l of organic carbon is required for each mg of nitrate nitrogen to be denitrified. If the nitrogen in the sewage is already mostly in the nitrate form and the water quite stabilized, organic carbon (as primary effluent, for example) may have to be added to the sewage effluent to achieve sufficient denitrification when the system goes anaerobic. Local experimentation is usually required to find the optimum schedule for flooding and drying, hydraulic loading, and organic carbon addition for stimulating denitrification. More information can be found in Bouwer and Bouwer and Chaney. Phosphorus
Sewage effluent can contain 5 to 50 mg/l phosphorus, depending on diet and water use of the local population. During pretreatment of the sewage, and in passage through the soil of the SAT system, organic phosphorus is biologically converted to phosphate. In calcareous soils and at alkaline pH, phosphate precipitates with calcium to form calcium phosphate. In acid soils, phosphate reacts with iron and aluminium oxides in the soil to form insoluble compounds. Sometimes, phosphate is initially
Aquifer Recharge with Wastewater
207
immobilized by adsorption to the soil and then slowly reverts to insoluble forms, allowing more adsorption of mobile phosphate, etc. In clean sands with about neutral pH, phosphate can be relatively mobile. Further information is given in Bouwer and Bouwer and Chaney. Trace Elements and Salts
Sewage effluent contains a wide spectrum of other chemicals at low concentrations. These include heavy metals, fluorine, and boron. Unless these elements were already present in large concentrations in the drinking water or added to the sewage in significant amounts by industrial discharges, their concentrations in sewage are usually below the maximum limits for irrigation water. Metals are significantly retained in most soils but a high pH favours immobilization. Fluoride can form calcium fluoride, which has a very low solubility, in the soil and is also adsorbed by various soil components, especially hydrous aluminium oxides. Boron is mobile in sands and gravels but can be adsorbed on clay. Thus, SAT systems can significantly reduce the concentrations of trace elements in sewage effluent. Total salt concentrations in sewage effluent can be several hundred mg/l higher than in drinking water. Since SAT systems generally have sandy soils, hydraulic loading rates will be much higher than evaporation losses (for example, 50 m/yr vs 1.5 m/yr). Hence, the salt concentration in the renovated water from SAT systems will be about the same as (or slightly higher than) that of the sewage effluent. If clay or organic matter is present in the soil, there will be cation adsorption and ion exchange when the SAT system is first put into operation. However, eventually, the ionic composition of the renovated sewage water will be essentially the same as that of the sewage effluent going into the ground. SAT systems do not remove salts from sewage.
Chapter 10
I rrigation with Wastewater CONDITIONS FOR SUCCESSFUL IRRIGATION
Irrigation may be defined as the application of water to soil for the purpose of supplying the moisture essential for plant growth. Irrigation plays a vital role in increasing crop yields and stabilizing production. In arid and semi-arid regions, irrigation is essential for economically viable agriculture, while in semi-humid and humid areas, it is often required on a supplementary basis. At the farm level, the following basic conditions should be met to make irrigated farming a success: • The required amount of water should be applied. • The water should be of acceptable quality. • Water application should be properly scheduled. • Appropriate irrigation methods should be used. • Salt accumulation in the root zone should be prevented by means of leaching. • The rise of water table should be controlled by means of appropriate drainage. • Plant nutrients should be managed in an optimal way. The above requirements are equally applicable when the source of irrigation water is treated wastewater. Nutrients in municipal wastewater and treated effluents are a particular advantage of these sources over conventional irrigation water sources and supplemental fertilizers are sometimes not necessary. However, additional environmental and health requirements must be taken into account when treated wastewater is the source of irrigation water. Amount of Water to be Applied
It is well known that more than 99 per cent of the water absorbed by
plants is lost by transpiration and evaporation from the plant surface. Thus, for all practical purposes, the water requirement of crops is equal to the evapotranspiration requirement, ETc. Crop evapotranspiration is mainly determined by climatic factors and hence can be estimated with
209
Irrigation with Wastewater
reasonable accuracy using meteorological data. A computer programme, called CROPWAT, is available in FAO to determine the water requirements of crops from climatic data. Table presents the water requirements of some selected crops, reported by Doorenbos and Kassam. It should be kept in mind that the actual amount of irrigation water to be applied will have to be adjusted for effective rainfall, leaching requirement, application losses and other factors.
Quality of Water to be Applied In addition, farm practices, such as the type of crop to be grown, irrigation method, and agronomic practices, will determine to a great extent the quality suitability of irrigation water. Table:l. Water Requirements, Sensitivity to Water Supply and Water Utilization Efficiency of Some Selected Crops Crop
Water Requirements (mmlgrowing period)
Alfalfa
800-1600
Banana
1200-2200
Bean
300-500
Cabbage
380-500
Citrus
900-1200
Cotton
700-1300
Groundnut
500-700
Maize
500-800
Potato
500-700
race
350-700
Safflower
600-1200
Sorghum
450-650
Wheat
450-650
Sensitivity to Water Supply (klJ)
Water Utilization Efficiency for Harvested Yield, Ey, kglm3 (% moisture)
low to medium-high (0.7-1.1) high (1.2-1.35)
1.5-2.0 hay (10-15%) plant crop: 2.5-4 ratoon: 3.5-6 fruit (70%) lush: 1.5-2.0 (80-90%) medium-high dry: 0.3-0.6 (10%) (1.15) 12-20 medium-low head (90-95%) (0.95) 2-5 low to medium-high fruit (85%, lime: 70%) (0.8-1.1 ) 0.4-0.6 medium-low seed cotton (10%) (0.85) 0.6-0.8 low unshelled dry nut (15%) (0.7) 0.8-1.6 high grain (10-13%) (1.25) 4-7 medium-high fresh tuber (70-75%) (1.1) 0.7-1.1 high paddy (15-20%) 0.2-0.5 low seed (8-10%) (0.8) 0.6-1.0 medium-low grain (12-15%) (0.9) 0.8-1.0 medium high grain (12-15%) (spring: 1.15; winter: 1.0)
Some of the impQrtant farm practices aimed at optimizing crop production when treated sewage effluent is used as irrigation water.
210
Irrigation with Wastewater
Scheduling of Irrigation
To obtain maximum yields, water should be applied to crops before the soil moisture potential reaches a level at which the evapotranspiration rate is likely to be reduced below its potential. The relationship of actual and maximum yields to actual and potential evapotranspiration is illustrated in the following equation:
ETa) (1- YYam )=1cy(1- ETm where: Ya = Actual harvested yield Ym = Maximum harvested yield Icy = Yield response factor = Actual evapotranspiration ETm= Maximum evapotranspiration Several methods are available to determine optimum irrigation scheduling. The factors that determine irrigation scheduling are: available water holding capacity of the soils, depth of root zone, evapotranspiration rate, amount of water to be applied per irrigation, irrigation method and drainage conditions.
ETa
Irrigation Methods
Many different methods are used by farmers to irrigate crops. They range from watering individual plants from a can of water to highly automated irrigation by a centre pivot system. However, from the point of wetting the soil, these methods can be grouped under five headings, namely: • Flood irrigation: Water is applied over the entire field to infiltrate into the soil (e.g., wild flooding, contour flooding, borders, basins, etc.). • Furrow irrigation: Water is applied between ridges (e.g., level and graded furrows, contour furrows, corrugations, etc.). Water reaches the ridge, where the plant roots are concentrated, by capillary action. • Sprinkler irrigation: Water is applied in the form of a spray and reaches the soil very much like rain (e.g., portable and solid set sprinklers, travelling sprin~lers, spray guns, centre-pivot systems, etc.). The rate of application is adjusted so that it does not create ponding of water on the surface. • Sub-irrigation: Water is applied beneath the root zone in such a manner that it wets the root zone by capillary rise (e.g., subsurface irrigation canals, buried pipes, etc.). Deep surface canals or buried pipes are used for this purpose.
211
Irrigation with Wastewater
•
Localized irngation: Water is applied around each plant or a group of plants so as to wet locally and the root zone only (e.g., drip irrigation, bubblers, micro-sprinklers, etc.). The application rate is adjusted to meet evapotranspiration needs so that percolation losses are minimized.
Leaching
Under irrigated agriculture, a certain amount of excess irrigation water is required to percolate through the root zone so as to remove the salts which have accumulated as a result of evapotranspiration from the original irrigation water. This process of displacing the salts from the root zone is called leaching and that portion of the irrigation water which mobilizes the excess of salts is called the leaching fraction, LF. Leaching Fraction(LF}
depth of water leached below the root zone depth of water applied at the surface
= -~------------
Salinity control by effective leaching of the root zone becomes more important as irrigation water becomes more saline. Drainage
Drainage is defined as the removal of excess water from the soil surface and below so as to permit optimum growth of plants. Removal of excess surface water is termed surface drainage while the removal of excess water from beneath the soil surface is termed sub-surface drainage. The importance of drainage for successful irrigated agriculture has been well demonstrated. It is particularly important in semi-arid and arid areas to prevent secondary salinization. In these areas, the water table will rise with irrigation when the natural internal drainage of the soil is not adequate. When the water table is within a few metres of the soil surface, capillary rise of saline groundwater will transport salts to the soil surface. At the surface, water evaporates, leaving the salts behind. If this process is not arrested, salt accumulation will continue, resulting in salinization of the soil. In such cases, sub-surface drainage can control the rise of the water table and hence prevent salinization. STRATEGIES FOR MANAGING TREATED WASTEWATER ON THE FARM
Success in using treated wastewater for crop production will largely depend on adopting appropriate strategies aimed at optimizing crop yields
212
Irrigation with Wastewater
and quality, maintaining soil productivity and safeguarding the environment. Several alternatives are available and a combination of these alternatives will offer an optimum solution for a given set of conditions. The user should have prior information on effluent supply and its quality, to ensure the formulation and adoption of an appropriate on-farm management strategy. Basically, the components of an on-farm strategy in using treated wastewater will consist of a combination of: • Crop selection. • Selection of irrigation method. • Adoption of appropriate management practices. Furthermore, when the farmer has additional sources of water supply, such as a limited amount of normal irrigation water, he will then have an option to use both the effluent and the conventional source of water in two ways, namely: • By blending conventional water with treated effluent. • Using the two sources in rotation. These are discussed briefly in the following sections. CROP SElECTION To Overcome Salinity Hazards
Not all plants respond to salinity in a similar manner; some crops can produce acceptable yields at much higher soil salinity than others. This is because some crops are better able to make the needed osmotic adjustments, enabling them to extract more water from a saline soil. The ability of a crop to adjust to salinity is extremely useful. In areas where a build-up of soil salinity cannot be controlled at an acceptable concentration for the crop being grown, an alternative crop can be selected that is both more tolerant of the expected soil salinity and able to produce economic yields. Table:2. Information Required on Effluent Supply and Quality Informatioll Effluent supply
Decision on Irrigation Management
The total amount of effluent that would be made available during the crop growing season. Effluent available throughout the year.
Total area that could be irrigated.
Storage facility during non crop growing period either at the farm or near wastewater treatment plant, and possible use for aquaculture.
Irrigation with Wastewater The rate of delivery of effluent either as m 3 per day or litres per second. Type of delivery: continuous or intermittent, or on demand. Mode of supply: supply at farm gate or effluent available in a storage reservoir to be pumped by the fa~mer.
213
Area that could be irrigated at any given time, layout of fields and facilities and system of irrigation. Layout of fields and facilities, irrigation system, and irrigation scheduling. The need to install pumps and pipes to transport effluent and irrigation system.
E/flueut quality Total salt concentration and/or electrical conductivity of the effluent. Concentrations of cations, such as Cart, Mg++ and Na+. Concentration of toxic ions, such as heavy metals, Boron and Cl-. Concentration of trace elements (particularly those which are suspected of being phyto-toxic). Concentration of nutrients, particularly nitrate-No Level of suspended sediments.
Levels of intestinal nematodes and faecal coliforms.
Selection of crops, irrigation method, leaching and other management practices. To assess sodium hazard and undertake appropriate measures. To assess toxicities that are likely to be caused by these elements and take appropriate measures. To assess trace toxicities and take appropriate measures. To adjust fertilizer levels, avoid overfertilization and select crop. To select appropriate irrigation system and measures to prevent clogging problems. To select appropriate crops and irrigation systems.
There is an 8-10 fold range in the salt tolerance of agricultural crops. This wide range in tolerance allows for greater use of moderately saline water, much of which was previously thought to be unusable. It also greatly expands the acceptable range of water salinity (Ee w ) considered suitable for irrigation. The relative salt tolerance of most agricultural crops is known well enough to give general salt tolerance guidelines. Figure 1 presents the relationship between relative crop yield and irrigation water salinity with regard to the four crop salinity classes. The following general conclusions can be drawn from these data: • Full yield potential should be achievable with nearly all crops when using a water with salinity less than 0.7 dS/m, • When using irrigation water of slight to moderate salinity (i.e., 0.7-3.0 dS/m), full yield poh::ntial is still possible but care must
214
Irrigation with Wastewater be taken to achieve the required leaching fraction in order. to maintain soil salinity within the tolerance of the crops. Treated sewage effluent will normally fall within this group,
o
5
10
i
iii'
i
Ii'
i
15
20
;
15 ,
i
20 i
,
25
'\
ECw
35 ECe
~~TT~TT~TT~~~~~~~~rrrrrn
EC,;Electrrcal Conductivity of the Solunatlon Water (dS/m)
80 ECw;Electrrcal Conductivity of the Irrigation Water
;;!1 u Qj
>=
60
EC,=1.5 ECw
c.
e
u
QJ
>
~
Unsuitable
40
for Crops
81 20 00
35 ECe
, 0
I
I 5
J
i
I
i
I
I
"
15
10
20
dS/m Fig:l. Divisions for Relative Salt Tolerance Ratings of Agricultural Crops • For higher salinity water (more than 3.0 dS/m) and sensitive crops, increasing leaching to satisfy a leaching requirement greater than 0.25 to 0.30 might not be practicable because of the excessive amount of water required. In such a case, consideration must be given to changing to a more tolerant crop that will require less leaching, to control salts within crop tolerance levels. As water salinity (Eew) increases within the slight to moderate range, production of more sensitive crops may be restricted due to the inability to achieve the high leaching fraction needed, especially when grown on heavier, more clayey soil types. • If the salinity of the applied water exceeds 3.0 dS/m, the water might still be usable but its use may need to be restricted to more permeable soils and more salt-tolerant crops, where high leaching fractions are more easily achieved. This is being practised on a large scale in the Arabian Gulf States, where drip irrigation systems are widely used. If the exact cropping patterns or rotations are not known for a new area, the leaching requirement must be based on the least tolerant of the
Irrigation with Wastewater
21;:;
crops adapted to the area. In those instances, where soil salinity cannot be maintained within acceptable limits of preferred sensitive crops, changing to more tolerant crops will raise the area's production potential. If there is any doubt about the effect of wastewater salinity on crop production, a pilot study should be undertaken to demonstrate the feasibility of irrigation and the outlook for economic success. To Overcome Toxicity Hazards
A toxicity problem is different from a salinity problem in that it occurs within the plant itself and is not caused by water shortage. Toxicity normally results when certain ions are taken up by plants with the soil water and a~cumulate in the leaves duriNg water transpiration to such an extent that the plant is damaged. The degree of damage depends upon time, concentration of toxic material, crop sensitivity and crop water use and, if damage is severe enough, crop yield is reduced. Common toxic ions in irrigation water are chloride, sodium, and boron, all of which will be contained in sewage. Damage can be caused by each individually or in combination. Not all crops are equally sensitive to these toxic ions. However, toxicity symptoms can appear in almost any crop if concentrations of toxic materials are sufficiently high. Toxicity often accompanies or complicates a salinity or infiltration problem, although it may appear even when salinity is not a problem. The toxic ions of sodium and chloride can also be absorbed directly into the plant through the leaves when moistened during sprinkler irrigation. This typically occurs during periods of high temperature and low humidity. Leaf absorption speeds up the rate of accumulation of a toxic ion and may be a primary source of the toxicity. However, urban wastewater may contain heavy metals at concentrations which will give rise to elevated levels in the soil and cause undesirable accumulations in plant tissue and crop growth reductions. Heavy metals are readily fixed and accumulate in soils with repeated irrigation by such wastewaters and may either render them nonproductive or the product unusable. Surveys of wastewater use have shown that more than 85% of the applied heavy metals are likely to accumulate in the soil, most at the surface. The levels at which heavy metals accumulation in the soil is likely to have a deleterious effect on crops. Any wastewater use project should include monitoring of soil and plants for toxic materials. SElECTION OF IRRIGATION METHODS
Under normal conditions, the type of irrigation method selected will
216
Irrigation with Wastewater
depend on water supply conditions, climate, soil, crops to be grown, cost of irrigation method and the ability of the farmer to manage the system. However, when using wastewater as the source of irrigation other factors, such as contamination of plants and harvested product, farm workers, and the environment, and salinity and toxicity hazards, will need to be considered. There is considerable scope for reducing the undesirable effects of wastewater use in irrigation through selection of appropriate irrigation methods. The choice of irrigation method in using wastewater is governed by the following technical factors: • The choice of crops. • The wetting of foliage, fruits and aerial parts. • The distribution of water, salts and contaminants in the soil. • The ease with which high soil water potential could be maintained. • The efficiency of application. • The potential to contaminate farm workers and the environment. A border (and basin or any flood irrigation) system involves complete coverage of the soil surface with treated effluent and is normally not an efficient method of irrigation. This system will also contaminate vegetable crops growing near the ground and root crops and will expose farm workers to the effluent more than any other method. Thus, from both the health and water conservation points of view, border irrigation with wastewater is not satisfactory. Furrow irrigation, on the other hand, does not wet the entire soil surface. This method can reduce crop contamination, since plants ar.e grown on the ridges, but complete health protection cannot be guaranteed. Contamination of farm workers is potentially medium to high, depending on automation. If the effluent is transported through pipes and delivered into individual furrows by means of gated pipes, risk to irrigation workers will be minimum. The efficiency of surface irrigation methods in general, borders, basins, and furrows, is not greatly affected by water quality, although the health risk inherent in these systems is most certainly of concern. Some problems might arise if the effluent contains large quantities of suspended solids and these settle out and restrict flow in transporting channels, gates, pipes and appurtenances. The use of primary treated sewage will overcome many of such problems. To avoid surface ponding of stagnant efflue~t, land levelling should be carried out carefully and appropriate land gradients should be provided.
Irrigation with Wastewater
217
Table:3. Evaluation of Commoll Irrigation Methods in Relation to the Use of Treated Wastewater Parameters of Evaluation
Furrow Irrigation
Border Irrigation
Sprinkler Irrigation
Drip Irrigation
No foliar Some bottom Severe leaf damage can injury occurs leaves may be affected but the occur resulting under this damage is not in significant method of so serious as to yield loss irrigation reduce yield Salts move ver- Salt moveSalt move2 Salt accumuSalts tend to ment is radial lation in the root accumulate in tically downwards ment is downwards along the zone with repea- the ridge which and are not likely to accuand root zone direction of ted applications could harm the is not likely to water movemulate in the crop accumulate ment. A salt root zone wedge is forsalts med between drip points 3 Ability to Plants may be Plants may be Not possible Possible to maintain high subject to stress subject. to to maintain maintain high water stress soil water between high soil wa- soil water pobetween potential irrigations ter potential tential throughout the groirrigations throughout the growing wing season and minimize season the effect of salinity Fair to medium. Fair to medium. Poor to fair. Excellent to 4 Suitability to handle brackish With good ma- Good irrigation Most crops good. Almost all crops can wastewater nagement and and drainage suffer from without signidrainage accep- practices can leaf damage be grown with ficant yield table yields are produce accep and yield is very little reduction in table levels of low loss possible yield yield
1 Foliar wetting and consequent leaf damage resulting in poor yield
No foliar injury as the crop is planted on the ridge
Sprinkler, or spray, irrigation methods are generally more efficient in terms of water use since greater uniformity of application can be achieved. However, these overhead irrigation methods may contaminate ground crops, fruit trees and farm workers. In addition, pathogens contained in aerosolized effluent may be transported downwind and create a health risk to nearby residents. Generally, mechanized or automated systems have relatively high capital costs and low labour costs
218
Irrigation with Wastewater
compared with manually-moved sprinkler systems. Rough land levelling is necessary for sprinkler systems, to prevent excessive head losses and achieve uniformity of wetting. Sprinkler systems are more affected by water quality than surface irrigation systems, primarily as a result of the clogging of orifices in sprinkler heads, potential leaf burns and phytotoxicity when water is saline and contains excessive toxic elements, and sediment accumulation in pipes, valves and distribution systems. Secondary wastewater treatment has generally been found to produce an effluent suitable for distribution through sprinklers, provided that the effluent is not too saline. Further precautionary measures, such as treatment with granular filters or microstrainers and enlargement of nozzle orifice diameters to not less than 5 mm, are often adopted. Localized irrigation, particularly when the soil surface is covered with plastic sheeting or other mulch, uses effluent more efficiently, can often produce higher crop yields and certainly provides the greatest degree of health protection for farm workers and consumers. Trickle and drip irrigation systems are expensive, however, and require a high quality of effluent to prevent clogging of the emitters through which water is slowly released into the soil. Table presents water quality requirements to prevent clogging in localized irrigation systems. Solids in the effluent or biological growth at the emitters will create problems but gravel filtration of secondary treated effluent and regular flushing of lines have been found to be effective in preventing such problems in Cyprus. Bubbler irrigation, a technique developed for the localized irrigation of tree crops avoids the need for small emitter orifices but careful setting is required for its successful application. Table:4. Water Quality and Clogging Potential in Drip Irrigation Systems Potential Problem Physical Suspended Solids Chemical pH Dissolved Solids Manganese Iron Hydrogen Sulphide Biological Bacterial populations
Units None mg/l
mg/l mg/l mg/l mg/l maximum number/ml
Degree of Restriction on Use Slight to Moderate Severe
< 50
50-100
> 100
< 7.0 < 500 < 0.1 < 0.1 < 0.5
7.0 - 8.0 500-2000 0.1 - 1.5 0.1 - 1.5 0.5 - 2.0
> 8.0 > 2000 > 1.5 > 1.5 > 2.0
< 10000
10000 - 50 000
> 50000
Irrigation with Wastewater
219
When compared with other systems, the main advantages of trickle irrigation seem to be: • Increased crop growth and yield achieved by optimizing the water, nutrients and air regimes in the root zone. • High irrigation efficiency - no canopy interception, wind drift or conveyance losses and minimal drainage losses. • Minimal contact between farm workers and effluent. • Low energy requirements - the trickle system requires a water pressure of only 100-300 k Pa (1-3 bar). • Low labour requirements - the trickle system can easily be automated, even to allow combined irrigation and fertilization (sometimes terms fertigation). Apart from the high capital costs of trickle irrigation systems, another limiting factor in their use is that they are only suited to the irrigation of row crops. Relocation of subsurface systems can be prohibitively expensive. Clearly, the decision on irrigation system selection will be mainly a financial one but it is to be hoped that the health risks associated with the different methods will be taken into account. The method of effluent application is one of the health control measures possible, along with crop selection, wastewater treatment and human exposure control. Each measure will interact with the others and thus a decision on irrigation system selection will have an influence on wastewater treatment requirements, human exposure control and crop selection (for example, row crops are dictated by trickle irrigation). At the same time the irrigation techniques feasible will depend on crop selection and the choice of irrigation system might be limited if wastewater treatment has already been decided before effluent use is considered. FiElD MANAGEMENT PRACTICES IN WASTEWATER IRRIGATION
Management of water, soil, crop and operational procedures, including precautions to protect farm workers, play an important role in the successful use of sewage effluent for irrigation. Water Management
Most treated wastewaters are not very saline, salinity levels usually ranging between 500 and 200 mg/l (Ee w = 0.7 to 3.0 dS/m). However, there may be instances where the salinity concentration exceeds the 2,000 mg/l level. In any cas~, appropriate water management practices will have to be followed to prevent salinization, irrespective of whether the salt content
220
Irrigation with Wastewater
in the wastewater is high or low. It is interesting to note that even the application of a non-saline wastewater, such as one containing 200 to 500 mg/l, when applied at a rate of 20,000 m 3 per hectare, a fairly typical irrigation rate, will add between 2 and 5 tonnes of salt annually to the soil. If this is not flushed out of the root zone by leaching and removed from the soil by effective drainage, salinity problems can build up rapidly. Leaching and drainage are thus two important water management practices to avoid salinization of soils. Leaching
The concept of leaching has already been discussed. The question that arises is how much water should be used for leaching, Le., what is the leaching requirement? To estimate the leaching requirement, both the salinity of the irrigation water (EC w ) and the crop tolerance to soil salinity (ECe) must be known. The necessary leaching requirement (LR) can be estimated from Figure 2 for general crop rotations reported by Ayers and Westcot. A more exact estimate of the leaching requirement for a particular crop can be obtained using the following equation: LR=
EC w 5(EC e -EC w )
where: LR = minimum leaching requirement needed to control salts within the tolerance (ECe ) of the crop with ordinary surface methods of irrigation. EC w = salinity of the applied irrigation water in dS/m. ECe = average soil salinity tolerated by the crop as measured on a soil saturation extract. It is recommended that the ECe value that can be expected to result in at least a 90% or greater yield be used in the calculation. Figure was developed using ECe values for the 90% yield potential. For water in the moderate to high salinity range (>1.5 dS/m), it might be better to use the ECe value for maximum yield potential (100%) since salinity control is critical in obtaining good yields. Where water is scarce and expensive, leaching practices should be designed to maximize crop production per unit volume of water applied, to meet both the consumptive use and leaching requirements. Depending on the salinity status, leaching can be carried out at each irrigation, each alternative irrigation or less frequently, such as· seasonally or at even longer intervals, as necessary to keep the salinity in the soil below the threshold above which yield might be affected to an unacceptable level.
221
Irrigation with Wastewater Unsuitable __ ___
10
Tolerant crops
8
E
'III ""0
,......
-----(1$ 6
e
~ c:
Moderately '" tolerant crops ~ 4 o
Vl
Assumed crop water use pattern Moderately sensitive crops
Sensitive crops
1
2
4
6
8
10
Salinity of applied water (ECw) in ds/m
Fig:2. Relationship between Applied Water Salinity and Soil Water Salinity at Different Leaching Fractions
With good quality irrigation water, the irrigation application level will almost always apply sufficient extra water to accomplish leaching. With high salinity irrigation water, meeting the leaching requirement is difficult and requires large amounts of water. Rainfall must be considered in estimating the leaching requirement and in choosing the leaching method. The following practices are suggested for increasing the efficiency of leaching and reducing the amount of water needed: • Leach during cool seasons instead of during warm periods, to increase the efficiency and ease of leaching, since the total annual crop water demand (ET, mm/year) losses are lower. • Use more salt-tolerant crops which require a lower leaching requirement (LR) and thus have a lower water demand. • Use tillage to slow overland water flow and reduce the number of surface cracks which bypass flow through large pores and decrease leaching efficiency. • Use sprinkler irrigation at an application rate below the soil infiltration rate as this favours unsaturated flow, which is significantly more efficient for leaching than saturated flow. More irrigation time but less water is required than for continuous ponding.
222
Irrigation with Wastewater
•
• •
•
•
Use alternate ponding and drying instead of continuous ponding as this is more efficient for leaching and uses less water, although the time required to leach is greater. This may have drawbacks in areas having a high water table, which allows secondary salinization between pondings. Where possible, schedule leachings at periods of low crop water use or postpone teachings until after the cropping season. Avoid fallow periods, particularly during hot summers, when rapid secondary soil salinization from high water tables can occur. If infiltration rates are low, consider pre-planting irrigations or off-season leaching to avoid excessive water applications during the crop season. Use one irrigation before the start of the rainy season if total rainfall is normally expected to be insufficient for a complete leaching. Rainfall is often the most efficient leaching method because it provides high quality water at relatively low rates of application.
Drainage
Salinity problems in many irrigation projects in arid and semi-arid areas are associated with the presence of a shallow water table. The role of drainage in this context is to lower the water table to a desirable level, at which it does not contribute to the transport of salts to the root zone and the soil surface by capillarity. What is important is to maintain a downward movement of water through soils. van Schilfgaard reported that drainage criteria are frequently expressed in terms of critical water table depths; although this is a useful concept, prevention of salinization depends on the establishment, averaged over a period of time, of a downward flux of water. Another important element of the total drainage system is its ability to transport the desired amount of drained water out of the irrigation scheme and dispose of it safely. Such disposal can pose a serious problem, particularly when the source of irrigation water is treated wastewater, depending on the composition of the drainage effluent. Timing of Irrigation
The timing of irrigation, including irrigation frequency, pre-planting irrigation and irrigation prior to a winter rainy season, can reduce the salinity hazard and avoid water stress between irrigations. Some of these practices are readily applicable to wastewater irrigation. In terms of meeting the water needs of crops, increasing the frequency
Irrigation with Wastewater
223
of irrigation will be desirable as it eliminates water stress between irrigations. However, from the point of view of overall water management, this may not always produce the desired results. For example, with border, basin and other flood irrigation methods, frequent irrigations may result in an unacceptable increase in the quantity of water applied, decrease in water use efficiency and larger amounts of water to be drained. However, with sprinklers and localized irrigation methods, frequent applications with smaller amounts may not result in decrease in water use efficiency and, indeed, could help to overcome the salinity problem associated with saline irrigation water. Pre-planting irrigation is practised in many irrigation schemes for two rea,sons, namely: • To leach salts from the soil surface which may have accumulated during the previous cropping period and to provide a salt-free environment to germinating seeds (it should be noted that for most crops, the seed germination and seedling stages are most sensitive to salinity). • To provide adequate moisture to germinating seeds and young seedlings. A common practice among growers of lettuce, tomatoes and other vegetable crops is to pre-irrigate the field before planting, since irrigation soon after planting could create local water stagnation and wet spots that are not desirable. Treated wastewater is a good source for pre-irrigation as it is normally not saline and the health hazards are practically nil.
Blending of Wastewater with other Water Supplies One of the options that may be available to farmers is the blending of treated sewage with conventional sources of water, canal water or groundwater, jf multiple sources are available. It is possible that a farmer may have saline groundwater and, if he has non-saline treated wastewater, could blend the two sources to obtain a blended water of acceptable salinity level. Further, by blending, the microbial quality of the resulting mixture could be superior to that of the unblended wastewater.
Alternating Treated Wastewater with other Water Sources Another strategy is to use the treated wastewater alternately with the canal water or groundwater, instead of blending. From the point of view of salinity control, alternate applications of the two sources will be superior to blending. However, an alternating application strategy will require duel conveyance systems and availability of the effluent dictated by the alternate schedule of application.
224
Irrigation with Wastewater
land and Soil Management Several land and soil management practices can be adopted at the field level to overcome salinity, sodicity, toxicity and health hazards that might be associated with the use of treated wastewater. Land Development
During the early stages of on-farm land development, steps can be taken to minimize potential hazards that may result from the use of wastewater. These will have to be well planned, designed and executed since they are expensive and, often, one time operations. Their goal is to improve permanently existing land and soil conditions in order to make irrigation with wastewater easier. Typical activities include levelling of land to a given grade, establishing adequate drainage (both open and sub-surface systems), deep ploughing and leaching to reduce soil salinity. Land Grading
Land grading is important to achieve good uniformity of application from surface irrigation methods and acceptable irrigation efficiencies in general. If the wastewater is saline, it is very important that the irrigated land is appropriately graded. Salts accumulate in the high spots which have too little water infiltration and leaching, while in the low spots water accumulates, causing waterlogging and soil crusting. Land grading is well accepted as an important farm practice in irrigated agriculture. Several methods are available to grade land to a desired slope. The slope required will vary with the irrigation system, length of run of water flow, soil type, and the design of the field. Recently, laser techniques have been applied to level land precisely so as to obtain high irrigation efficiencies and prevent saliniza ti on. Deep Cultivation
In certain areas, the soil is stratified, and such soils are difficult to irrigate. Layers of clay, sand or hard pan in stratified soils frequently impede or prevent free movement of water through and beyond the root zone. This will not only lead to saturation of the root zone but also to accumulation of salts in the root zone. Irrigation efficiency as well as water movement in the soil can be greatly ~nhanced by sub-soiling and chiselling of the land. The effects of sub-soiling and chiselling remain for about 1 to 5 years
225
Irrigation with Wastewater
but, if long term effects are required, the land should be deep and slip ploughed. Deep or slip ploughing is costly and usually requires the growing of annual crops soon after to allow the settling of the land. Following a couple of grain crops, grading will be required to re-establish a proper grade to the land. Crop Management and Cultural Practices
Several cultural and crop management practices that are valid under saline water use will be valid under wastewater use. These practices are aimed at preventing damage to crops caused by salt accumulation surrounding the p10nts and in the root zone and adjusting fertilizer and agrochemical applications to suit the quality of the wastewater and the crop. Placement of Seed
In most crops, seed germination is more seriously affected by soil salinity than other stages of development of a crop. The effects are pronounced in furrow-irrigated crops, where the water is fairly to highly saline. This is because water moves upwards by capillarity in the ridges, carrying salts with it.
R~:~:D
A /
"·y·
\E
Double Row BED
Single-Row Sloping BED
Double-Row Sloping BED
Fig:3. Schematic Representations of Salt Accumulation and Planting Methods in ridge and Furrow Irrigation
When water is either absorbed by roots or evaporated, salts are deposited in th'e ridges. Typically, the highest salt concentration occurs in the centre of the ridge, whereas the lowest concentration of salt is found along the shoulders of the ridges. An efficient means of overcoming this problem is to ensure that the
226
Irrigation with Wastewater
soil around the germinating seeds is sufficiently low in salinity. Appropriate planting methods, ridge shapes and irrigation management can significantly decrease damage to germinating seeds. Some specific practices include: • Planting on the shoulder of the ridge in the case of single row planting or on both shoulders in double row planting. • Using sloping beds with seeds planted on the sloping side, but above the water line. • Irrigating alternate rows so that the salts can be moved beyond the single seed row. PLANNING FOR WASTEWATER IRRIGATION Central Planning
Government policy on effluent use in agriculture will have a deciding effect on what control measures can be achieved through careful selection of site and crops to be irrigated with treated effluent. A decision to make treated effluent available to farmers for unrestricted irrigation or to irrigate public parks and urban green areas with effluent will remove the possibility of taking advantage of careful selection of sites, irrigation techniques and crops in limiting the health risks and minimizing environmental impacts. However, if a Government decides that effluent irrigation will only be applied in specific controlled areas, even if crop selection is not limited (that is, unrestricted irrigation is allowed within these areas), public access to the irrigated areas will be prevented. Without doubt, the greatest security against health risk and adverse environmental impact will be achieved by limiting effluent use to restricted irrigation on controlled areas to which the public has no access but even imposing restrictions on effluent irrigation by farmers, if properly enforced, can achieve a degree of control. Cobham and Johnson have suggested that the procedures involved in preparing plans for effluent irrigation schemes are similar to those used in most forms of resource planning and summarized the main physical, social and economic dimensions as in Figure 4. They also indicated that a nUJnber of key issues or tasks were likely to have a significant effect on the ultimate success of effluent irrigation, as follows: • Organizational and managerial provisions made to administer the resource, to select the effluent use plan and to implement it. • The importance attached to public health considerations and the levels of risk taken.
Water resources Components · (quality and quantity) infra Popu Iallon SewageTopography climate and consumer treatments structure
Stages Project area resource appraisal
I
demand
I
Wastewater/ sludge
Evaluation opportunities and constraints for after uses
I
Evaluation of socioeconomic environmental and reuse implications
Wastewater/ sJudge
I
Management capabilities
~nd
Quality
private land
I
I
I
Soil and land capability Public
I
Quantity Assessment of single-purpose I and multipurpose Recreation uses which match resources to opportunities
I
Land and associated items for agricultural and environmental protection, beautification, aquaculture and other uses
com~ercial
othe~
Land and applications efficiencies . and. I I Industrial Agriculture Environmental Beautification Aquaculture Ground water Cereal protection I recharge fodder d d Roads vegetables Crops an san Park perimeters j ~,________________________________~ ;f~rU~it~~~______~st~a~b~il~iz=a~t~~n~__~S~TW~~e~nv~i~ro~n~s______-'_ Cost-benefit appraisal
I i i
I}
V Identification of intangibles
Public health and social implications
Organizational and resource requirements
Control and monitoring
End products Environmental improvement of project area
Increased Optimum Advisory manuals Demonstrations Increased efficiency allocation and other aids food of water of wastewater production application between Landowners MunlclpalltlesGovernment Landowners Municipalities the chosen and farmers advisory and farmers uses office
.1 ..
II
Fig:4. Main Components of General Planning Guidelines for Wastewater Reuse
228
Irrigation with Wastewater
• • •
The choice of single-use or multiple-use strategies. The criteria adopted in evaluating alternative reuse proposals. The level of appreciation of the scope for establishing a fOfest resource. Adopting a mix of effluent use strategies is normally advantageous in respect of allowing greater flexibility, increased financial security and more efficient use of the wastewater throughout the year, whereas a singleuse strategy will give rise to seasonal surpluses of effluent for unproductive disposal. Therefore, in site and crop selection the desirability of providing areas for different crops and forestry so as to utilize the effluent at maximum efficiency over the whole yearly cycle of seasons must be kept in mind. Desirable Site Characteristics
The features which are critical in deciding the viability of a land disposal project are the location of available land and public attitudes. Land which is far distant from the sewage treatment plant will incur high costs for transporting treated effluent to site and will generally not be suitable. Hence, the availability of land for effluent irrigation should be considered when sewerage is being planned and sewage treatment plants should be strategically located in relation to suitable agricultural sites. Ideally, these sites should not be close to residential areas but even remote land might not be acceptable to the public if the social, cultural or religious attitudes are opposed to the practice of wastewater irrigation. The potential health hazards associated with effluent irrigation can make this a very sensitive issue and public concern will only be mollified by the application of strict control measures. In arid areas, the importance of agricultural use of treated effluent makes it advisable to be as systematic as possible in planning, developing and managing effluent irrigation projects and the public must be kept informed at all stages. The ideal objective in site selection is to find a suitable area where long-term application of treated effluent will be ·feasible without adverse environmental or public health impacts. It might be possible in a particular instance to identify several potential sites within reasonable distance of the sewered community and the problem will be to select the most suitable area or areas, taking all relevant factors into account. The following basic information on an area under consideration will be of value, if available: • A topographic map. • Agricultural soils surveys. • Aerial photographs. • Groundwater reports and well logs.
Irrigation with Wastewater
229
• Boring logs and soil test results. • Other soil and peizometric data. At this preliminary stage of investigation it should be possible to assess the potential impact of treated effluent application on any usable aquifer in the area(s) concerned. The first ranking of sites should take into account other factors, such as the cost and location of the land, its present use and availability, and social factors, in addition to soil and groundwater conditions. The characteristics of the soil profile underlying a particular site are very important in deciding on its suitability for effluent irrigation and the methods of application to be employed. Among the soil properties important from the point of view of wastewater application and agricultural production are: physical parameters (such as texture, grading, liquid and plastic limits, etc.), permeability, water-holding capacity, pH, salinity and chemical composition. Preliminary observation of sites, which could include shallow handauger borings and identification of vegetation, will often allow the elimination of. clearly unsatisfactory sites. After elimination of marginal sites, each site under serious consideration must be investigated by onsite borings to ascertain the soil profile, soil characteristics and location of the water table. Peizometers should be located in each borehole and these can be used for subsequent groundwater sampling. A procedure for such site assessment has been described by Hall and Thompson and, if applied, should not only allow the most suitable site among several possible to be selected but permit the impact of effluent irrigation at the chosen site to be modelled. When a site is developed, a long-term groundwater monitoring programme should be an essential feature of its management. Crop Selection Issues
Normally, in choosing crops, a farmer is influenced by economics, climate, soil and water characteristics, management skill, labour and equipment available and tradition. The degree to which the use of treated effluent influences crop selection will depend on Government policy on effluent irrigation, the goals of the user and the effluent quality. Government policy will have the objectives of minimizing the health risk and influencing the type of productivity associated with effluent irrigation. Regulations must be realistic and achievable in the context of national and local environmental conditions and traditions At the same time, planners of effluent irrigation schemes must attempt to achieve maximum productivity and water conservation through the choice of
230
Irrigation with Wastewater
crops and effluent application systems. A multiple-use strategy approach will require the evaluation of viable combinations of the cropping options possible on the land available. This will entail a considerable amount of survey and resource budgeting work, in addition to the necessary soil .and water quality assessments. The annual, monthly and daily water demands of the crops, using the most appropriate irrigation techniques, have to be determined. Domestic consumption, local production and imports of the various crops must be assessed so that the economic potential of effluent irrigation of the various crop combinations can be estimated. Finally, the crop irrigation demands must be matched with the available effluent so as to achieve optimum physical and financial utilization throughout the year. This process of assessment is reviewed by Cobham and Johnson (1988) for the case of effluent use in Kuwait, where afforestation for commercial purposes was found to offer significant potential in multiple-use effluent irrigation.
Chapter 11
Agricultural Use of Sewage Sludge CHARACTERISTICS OF SEWAGE SLUDGE
Most wastewater treatment processes produce a sludge which has to be disposed of. Conventional secondary sewage treatment plants typically generate a primary sludge in the primary sedimentation stage of treatment and a secondary, biological, sludge in final sedimentation after the biological process. The characteristics of the secondary sludge vary with the type of biological process and, often, it is mixed with primary sludge before treatment and disposal. Approximately one half of the costs of operating secondary sewage treatment plants in Europe can be associated with sludge treatment and disposal. Land application of raw or treated sewage sludge can reduce Significantly the sludge disposal cost component of sewage treatment as well as providing a large part of the nitrogen and phosphorus requirements of many crops. Very rarely do urban sewerage systems transport only domestic sewage to treatment plants; industrial effluents and storm-water runoff from roads and other paved areas are frequently discharged into sewers. Thus sewage sludge will contain, in addition to organic waste material, traces of many pollutants used in our modern society. Some of these substances can be phytotoxic and some toxic to humans and/or animals so it is necessary to control the concentrations in the soil of potentially toxic elemen~s and their rate of application to the soil. The risk to health of chemicals in sewage sludge applied to land has been reviewed by Dean and Suess. Sewage sludge also contains pathogenic bacteria, viruses and protozoa along with other parasitic helminths which can give rise to potential hazards to the health of humans, animals and plants. A WHO Report on the risk to health of microbes in sewage sludge applied to land identified salmonellae and Taenia as giving rise to greatest concern. The numbers of pathogenic and parasitic organisms in sludge can be significantly reduced before application to the land by appropriate sludge
Agricultural Use of Sewage Sludge
232
treatment and the potential health risk is further reduced by the effects of climate, soil-microorganisms and time after the sludge is applied to the soil. Nevertheless, in the case of certain crops, limitations on planting, grazing and harvesting are necessary. Apart from those components of concern, sewage sludge also contains useful concentrations of nitrogen, phosphQrus and organic matter. The availability of the phosphorus content in the year of application is about 50% and is independent of any prior sludge treatment. Nitrogen availability is more dependent on sludge treatment, untreated liquid sludge and dewatered treated sludge releasing nitrogen slowly with the benefits to crops being realised over a relatively long period. Liquid anaerobically-digested sludge has high ammonia-nitrogen content which is readily available to plants and can be of particular benefit to grassland. The organic matter in sludge can improve the water retaining capacity and structure of some soils, especially when applied in the form of dewatered sludge cake. Directive prohibits the sludge from sewage treatment plants from being used in agriculture unless specified requirements are fulfilled, including the testing of sludge and soil. Parameters subject to the provisions of the Directive include the following: • Dry matter (%). • Organic matter (% dry solids). • Copper (mg/kg dry solids). • Nickel (mg/kg dry solids).
•
pH.
• Nitrogen, total and ammoniacal (% dry solids). • Phosphorus, total (% dry solids). • Zinc (mg/kg dry solids). • Cadr~1ium (mg/kg dry solids): Lead (mg/kg dry solids). • • Mercury (mg/kg dry solids). • Chromium (mg/kg dry solids). To these parameters the UK Department of the Environment has added molybdenum, selenium, arsenic and fluoride in the recent 'Code of Practice for Agricultural Use of Sewage Sludge'. Sludge must be analyzed for the Directive parameters at least once every 6 months and every time significant changes occur in the quality of the sewage treated. The frequency of analysis for the additional four parameters may be reduced to not less than once in five years provided that their concentrations in the sludge are consistently no greater than the following reference concentrations: Mb - 3mg/kg dry solids, Se - 2mg/kg dry solids, As -2mg/kg dry solids and FI - 200mg/kg dry solids.
Agricultural Use of Sewage Sludge
233
SLUDGE TREATMENT
Except when it is to be injected or otherwise worked into the soil, sewage sludge should be subjected to biological, chemical or thermal treatment, long-term storage or other appropriate process designed to reduce its fermentability and health hazards resulting from its use before being applied in agriculture. Table 1 lists sludge treatment and handling processes which have been used in the UK to achieve these objectives. Table:l. Examples of Effective Sludge Treatment Processes Process
Descriptions
Sludge Pasteurization
Minimum of 30 minutes at 70°C or minimum of 4 hours at 55°C (or appropriate intermediate conditions), followed in all cases by primary mesophilic anaerobic digestion Mesophilic Anaerobic Mean retention period of at least 12 days primary Digestion digestion in temperature range 35°C +/- 3°C or of at least 20 days primary digestion in temperature range 25°C + / - 3°C followed in each case by a secondary stage which provides a mean retention period of at least 14 days Thermophilic Aerobic Mean retention period of at least 7 days digestion. All sludge to be subject to a minimum of 55°C for a period Digestion of at least 4 hours Composting (Windrows The compost must be maintained at 40°C for at least or Aerated Piles) 5 days and for 4 hours during this period at a minimum of 55°C within the body of the pile followed by a period of maturation adequate to ensure that the compost reaction is substantially complete Lime Stabilization of Addition of lime to raise pH to greater than 12.0 and sufficient to ensure that the pH is not less than 12 for a Liquid Sludge minimum period of 2 hours. The sludge can then be used directly Storage of untreated liquid sludge for a minimum period Liquid Storage of 3 months Dewatering and Storage Conditioning of untreated sludge with lime or other coagulants followed by dewat~ring and storage of the cake for a minimum period of 3 months if sludge has been subject to primary mesophilic anaerobic digestion, storage to be for a minimum period of 14 days
The second edition of a 'Manual of Good Practice on Soil Injection of Sewage Sludge' has been produced by the Water Research Centre in the UK and describes suitable equipment! and techniques for what is now the only method permissible within the EEC for applying untreated sludges to grassland.
234
Agricultural Use of Sewage Sludge
SLUDGE APPLICATION The concentrations of potentially toxic elements in arable soils must not exceed certain prudent limits within the normal depth of cultivation as a result of sludge application. No sludge should be applied at any site where the soil concentration of any of the parameters mentioned in Section, with the exception of molybdenum, exceed these limits. Maximum permissible concentrations of the potentially toxic elements in soil after application of sewage sludge. For zinc, copper and nickel, the maximum permissible concentrations vary with the pH of the soil because it is known that crop damage from phytotoxic elements is more likely to occur on acid soils. When sludge is applied to the surface of grassland, the concentrations of potentially toxic elements should be determined in soil samples taken to a depth of 7.5 cm. In order to minimize injestion of lead, cadmium and fluoride by livestock, the addition of these elements through sludge application to the surface should not exceed 3 times the 10 year average annual rates. EFFECTS OF SLUDGE ON SOILS AND CROPS
The natural background concentration of metals in soil is normally less available for crop uptake and hence less hazardous than metals introduced through sewage sludge applications. Research carried out in the UK has shown that the amounts of Cd, Ni, Cu, Zn and Pb applied in liquid sludge at three experimental sites could be accounted for by soil profile analyses five years after sludge applications, with the exception of Cu and Zn applied to a calcareous loam soil. These field experiments also determined the extent of transfer of metals from sludge-treated soil into the leaves and edible parts of six crops of major importance to UK agriculture and the effect of metals on yields of these crops. Although all the plots received sufficient inorganic fertilizer to meet crop requirements for nutrients, the applications of sludge had some effects on crop yields. In 60% of the cases stlldied crop yields were not significantly affected but in 26% of the cases liquid sludge application resulted in significantly increased crop yields, attributed to the beneficial effects on soil structure. Reductions in wheat grain yield, from 6-10%, were noted on the clay and calcareous loam soils treated with liquid sludge and the sandy loam and clay soils treated with bed-dried sludge. However, this yield reduction was not thought to be due to metals but the most likely explanation was lodging of the crop as a result of excessive nitrogen in soil. Increases in metal concentrations in soil due to sludge applications produced
Agricultural Use of Sewage Sludge
235
significant increases in Cd, Ni, Cu and Zn concentrations in the edible portion of most of the crops grown: wheat, potato, lettuce, red beet, cabbage and ryegrass. In most cases there was no significant increase of Pb in crop tissue in relation to Pb in soil from sludge application, suggesting that lead is relatively unavailable to crops from the soil. The availability of metals to crops was found to be lower in soil treated with bed-dried sludge cake compared with liquid sludge, the extent being dependent on the crop. Even though the Ni, Cu and Zn concentrations in soils treated with high rates of application of liquid and bed-dried sludges were close to the maximum levels set out in the EC Directive and the zinc equivalent of sludge addition exceeded the maximum permitted in UK guidelines, no phytotoxic effects of metals were evident, with one exception. This was in lettuce grown on clay soil, when Cu and Zn levels exceeded upper critical concentrations at high rates of sludge application. PLANTING, GRAZING AND HARVESTING CONSTRAINTS
To minimize the potential risk to the health of humans, animals and plants it is necessary to coordinate sludge applications in time with planting, grazing or harvesting operations. Sludge must not be applied to growing soft fruit or vegetable crops nor used where crops are grown under permanent glass or plastic structures. The EC Directive requires a mandatory 3-week no grazing period for treated sludge applied to grassland but prohibits the spreading of untreated sludge on grassland unless injected. Treated sludge can be applied to growing cereal crops without constraint but should not be applied to growing turf within 3 months of harvesting or to fruit trees within 10 months of harvesting. When treated sludge is applied before planting such crops as cereals, grass, fodder, sugar beet~ fruit trees, etc., no constraints apply but in the case of soft fruit and vegetables, the treated sludge should not be applied within 10 months of crop harvesting. In general, untreated sludge should only be cultivated or injected into the soil before planting crops but can be injected into growing grass or turf, with the constraints on minimum time to harvesting as already mentioned. ENVIRONMENTAL PROTECTION
Care should always be taken when applying sewage sludge to land to prevent any form of adverse environmental impact. The sludge must not contain non-degradable materials, such as plastics, which would make land disposal unsightly. Movement of sludge by tanker from sewage treatment plant to agricultural land can create traffic problems and give
236
Agricultural Use of Sewage Sludge
rise to noise and odour nuisance. Vehicles should be carefully selected for their !ocal suitability and routes chosen so as to minimize inconvenience to the public. Access to fields should be selected after consultation with the highway authority and special care must be taken to prevent vehicles carrying mud onto the highway. Odour control is the most important environmental dimension of sludge application to land. Enclosed tankers should be used for transporting treated sludge, which tends to be less odorous than raw sludge. Discharge points for sludge from tankers or irrigators should be as near to the ground as is practicable and the liquid sludge trajectory should be kept low so as to minimize spray drift and visual impact. Untreated sludge should be injected under the soil surface using special vehicles or tankers fitted with injection equipment. Great care is needed to prevent sludge running off onto roads or adjacent land, depending on topography, soil and weather conditions. On sloping land there is the risk of such runoff reaching water-courses and causing serious water pollution. Sludge application rates must be adjusted accordingly and, under certain circumstances, spreading might have to be discontinued. In addition to the problem of surface runoff, pollution may arise from the percolation of liquid sludge into land drains, particularly when injection techniques are used or liquid sludge is applied to dry fissured soils. In highly sensitive water pollution areas, sludge should be used only in accordance with the requirements of the pollution control authority as well as of good farming practice. Sludge storage on farms can optimize the transport and application operations but every effort must be made to ensure that storage facilities are secure.
Chapter 12
Wastewater Use in Aquaculture BIOTA IN AQUACULTURE PONDS Food Chains
The objective in fertilizing an aquaculture pond with excreta, nights oil or wastewater is to produce natural food for fish. Since several species of fish feed directly on faecal solids, use of raw sewage or fresh nights oil as influent to fish ponds should be prohibited for health reasons. Edwards has represented the complex food chains in an excreta-fed fish pond, involving ultimate decomposers or bacteria, phytoplankton, zooplankton and invertebrate detritivores. Inorganic nutrients released in the bacterial degredation of organic solids in sewage, nightsoil or excreta are taken up by phytoplankton.
Gill]
Fig:1. Food Chains in an Excreta-fed Aquaculture System
Zooplankton graze phytoplankton and small detritus particles coated with bacteria, the latter also serving as food for benthic invertebrate
238
Wastewater Use in Aquaculture
detritivores. Plankton, particularly phytoplankaton, are the major sources of natural food in a fish pond but benthic invertebrates, mainly chironomids, also serve as fish food, although they are quantitatively less important. To optimize fish production in a human waste fed pond, the majority of the fish should be filter feeders, to exploit the plankton growth. A wide range of fish species has been cultivated in aquaculture ponds receiving human waste, including common carp (Cyprinus carpio), Indian major carps (Catla catlax, Cirrhina mrigala and Labeo rohita), Chinese silver carp (Hypophthalmichthys molitrix), bighead carp (Aristichthys nobilis), grass carp (Ctenopharyngodon idella), crucian carp (Carassius auratus), Nile carp (Osteochilus hasseltii), tilapia (Oreochromis spp.), milkfish (Chanos chanos), catfish (Pangasius spp.), kissing gouramy (Helostoma temmincki), giant gourami (Osphronemus goramy), silver barb (Puntius gonionotus) and fresh water prawn (Macrobrachium lanchesterii). Fish Species
The selection reflects local culture rather than fish optimally-suited to such environments. For example, Chinese carps and Indian major carps are the major species in excreta-fed systems in China and India, respectively. In some countries, a polyculture of several fish species is used. Tilapia are generally cultured to a lesser extent than carps in excretafed systems although, technically, they are more suitable for this environment because they are better able to tolerate adverse environmental conditions than carp species. Milkfish have been found to have poorer growth and survival statistics compared with Indian major carps and Chinese carps in ponds fed with stabilization pond effluent in India. Edwards gives a thorough review of current knowledge on the various fish species which can be cultured in ponds fed with human waste. It would appear that considerable confusion still exists with regard to fish feeding on natural food. Although fish are generally divided into types according to their natural nutritional habits-those that feed on phytoplankton, or zooplankton or benthic animals-several species are known to feed on whatever partides are suspended in the water. There is also uncertainty about the types of phytoplankton fed upon by filter-feeding fish. For example, although blue-green algae are thought to be indigestible to fish, Tilapia have been shown to readily digest these algae and there is evidence that silver carp can do the same. Aquatic Plants
Aquatic macrophytes grow readily in ponds fed with human waste and their use in wastewater treatment has been discussed. Some creeping
Wastewater Use in Aquaculture
239
aquatic macrophytes are cultivated as vegetables for human consumption in aquaculture ponds and duckweeds are also cultivated, mainly for fish feed. Among the aquatic plants grown for use as vegetables are water spinach (Ipomoea aquatica), water mimosa (Neptunia oleracea), water cress (Rorippa nasturtium-aquaticum) and Chinese water chestnut (Eleocharis dulcis). The duckweeds Lemna, Spirodela and Wolffia are cultivated in some parts of Asia in shallow ponds fertilized with excreta, mainly as feed for Chinese carps but also for chickens, ducks and edible snails. TECHNICAL ASPECTS OF FISH CULTURE Environmental Factors
In a successful aquaculture system there must be both an organismic balance, to produce an optimal supply of natural food at all levels, and a chemical balance, to ensure sufficient oxygen supply for the growth of fish and their natural food organisms and to minimize the build-up of toxic metabolic products. Chemical balance is usually achieved through organismic balance in waste-fed ponds because the most important chemical transformations are biologically mediated. It is now recognized that depletion of dissolved oxygen in fertilized fish ponds is due primarily to the high rates of respiration at night of dense concentrations of phytoplankton. Romaire et al. introduced Equation to cover the factors influencing waste-fed fish pond dissolved oxygen (DO) at dawn: DOdn = DOdk±DOdrDOm-DOrDOp where: DOdn = DO concentration at dawn. DOdk = DO concentration at dusk. DO df = DO gain or loss due to diffusion. DO m = DO consumed by mud. DO f = DO consumed by fish. DO p = DO consumed by plankton. Bacterial respiration is not specifically mentioned in this equation but is included in the mud consumption of DO and in the planktonic DO consumption. In a well-managed waste-fed fish pond the DO in the morning should be only a few mg/l whereas in late afternoon the pond should be supersaturated with DO. Mud respiration probably lowers DO by less than 1 mg/l overnight and a fish population weighing 3,000 kg/ha would also lower DO by only about 1 mg/l overnight. Phytoplankton photosynthesis is the major source of oxygen during daylight hours and, during the night, the major cause
240
Wastewater Use in Aquaculture
of oxygen depletion is respiration. It has been estimated that respiration of plankton (bacterioplankton, phytoplankton and zooplankton) can lower pond DO by 8-10 mg/l overnight. By far the greatest proportion of the DO depletion overnight is caused by the respiration of the phytoplankton that develops as a result of the nutrients contained in the waste. Phytoplanktons provide feed for the largest percentage of fish farmed in Asia. They also- exhibit a positive net primary productivity on a 24hour basis and are net oxygen ccntributors to a fish pond. The objective in a waste-fed fish pond should be to maintain an algal standing crop at an optimum level for net primary productivity by balancing the production of phytoplankton biomass, in response to waste fertilization, with the grazing of phytoplankton biomass by filter-feeding fish. Fish mortality in a waste-fed pond can result from at least three possible causes. First, the depletion of oxygen due to bacterial oxygen demand caused by an increase in organic load. Second, the depletion of oxygen overnight due to the respiratory demand of too large a concentration of phytoplankton, having grown in response to an increase in inorganic nutrients, caused by an organismic imbalance. The third possible cause is high ammonia concentration in the waste feed. All three causes of fish mortality have been reported in respect of sewage-fertilized fish ponds. The sensitivity of fish to low levels of DO varies with species, life stage (eggs, larvae, adults) and life process (feeding, growth, and reproduction). A minimum constant DO concentration of 5 mg/l is considered satisfactory, although an absolute minimum ~onsistent with the presence of fish is probably less than 1 mg/l. Fish cultured in waste-fed ponds appear to be able to tolerate very low DO concentrations, for at least short periods of time, with air-breathing fish (such as walking catfish (Clarias batrachus) being the most tolerant, followed in decreasing order of tolerance by tilapia, carps, channel catfish and trout. Reducing phytoplankton biomass to maintain a reasonable DO in the early morning hours might well depress fish growth more than exposure to a few hours of low DO. A wastewater fertilized aquaculture system might occasionally require a stand-by mechanical oxygenation system for use during periods when DO would otherwise be very low. However, if the system is well managed to avoid overloading, this expense can be avoided. Unionized ammonia (NH3) is toxic to fish in the concentration range 0.2 - 2.0 mg/l. However, the tolerance of different species of fish varies, with tilapa species being least affected by high ammonia levels. Bartone et al. found that satisfactory growth and survival of tilapia was possible in fish ponds fed with tertiary effluent in Lima, Peru when the average
Wastewater Use in Aquaculture
241
total ammonia concentration was less than 2 mg Nil and the average unionized ammonia concentration was less than 0.5 mg Nil, with the latter ,only exceeding 2 mg Nil for short periods. "'l In ponds receiving large quantities of organic matter, sediments tend to accumulate and release anaerobic breakdown products, such as methane and sulphides, which can inhibit fish growth. Bottom feeding fish, such as the common carp (Cyprinus carpio), are most affected by such conditions, especially if the macrozoobenthos disappear. Fish Yields and Population Management
A wide range of yields has been reported from waste-fed aquaculture systems, for example: 2-6 tonslha yr in Indonesia, 2.7 - 9.3 tons/ha yr in China and 3.5 - 7.8 tonnes/ha yr in Taiwan. Although the majority of wastefed fish ponds stocks carps, research in Peru and Thailand has demonstrated the potential of tilapia for such systems. Management of fish ponds can have a significant effect on fish yields but the maximum attainable yield in practice is of the order of 10 - 12 tonnes/ha yr. Increase in weight of small fingerlings stocked in a pond follows a sigmoidal curve. The first phase of growth is slow, so a high stocking density can be adopted to better utilize the spatial and nutritional resources of the pond. Alternatively, this can be achieved by stocking with larger fish having a higher initial weight, following growth in nursery ponds. Fish yield is positively correlated with the size of the stocked fish at a given stocking density. In South China, til apia are stocked once a year at rates of either 30 g fish and 0.15/m2 or 1.3 g fish at 2.3 - 3.0/m2 stocking density. An increase in weight of fish in a pond leads initially to an increase in yield or production but there is subsequently a reduction in the growth rate of individual fish because of the limitation of natural food production in the system. The third phase of slow growth in Figure 2 is because the total weight of fish in the pond is approaching the carrying capa~ity. Intermediate harvesting when the rapid growth ceases, at the end of phase 2, should lead to significant increases in total yield. The high yields of tilapia reported in South China sewage-fed ponds are due to high stocking density and frequent harvesting. Clearly, the key to achieving high yields in a waste-fed pond is to determine the carrying capacity of the pond, the maximum standing stock of fish. This can be assessed by varying the waste load and determining the maximum production of natural food consistent with satisfactory water
242
Wastewater Use in Aquaculture
quality, sustainable through a fish culture cycle. Fish stocking density is related to carrying capacity according to the desired weight of individual fish at harvest, as follows: Fmal Ha.rvest
l Smgle Harvest
Tnne
Fig:2. Fish Growth Cycle
Fish stocking density Carrying capacity (kg/ha) Harvesable weight of individual fish (kg) Experience has shown that there is a limit to the fish yield attainable from a waste-fed fish pond. Higher yields can be achieved by addition of energy-rich supplementary feed, such as cereals, cereal brans or pelletedfeed. The highest yields are only achieved with a sufficiently high fish stocking density to benefit from the improvement in pond nutrition. There appears to be increased efficiency of utilization of supplementary feed by fish in ponds receiving sewage effluent. Marketable weights of fish vary with species and local market preferences but, in general, desirable sizes of the following fish range from 0.25-0.6 kg for tilapia, 0.5-1.5 kg for Indian major carps (mrigal 0.5, rohu 1.0, catla 1.5 kg) and perhaps 1-2 kg for Chinese carps. Thus, for a particular carrying capacity, Chinese carps should be stocked at an intermediate density. The length of culture cycle, or frequency of harvesting, depends on the time it takes stocked fish to reach marketable size. It should be recognized that the size of individual fish is o~ly significant if the product is to be consumed by humans. When fish are raised as high-protein feed for carnivorous fish or livestock, size is relatively unimportant. Nevertheless, it is now appreciated that sustainable yields of even high densities of small-size fish with a high specific growth rate are not significantly different from the yield of table fish for human consumption (6.2-7.8 tonnes/ha yr). (number/ha)
Wastewater Use in Aquaculture
243
Health Related Aspects of Fish Culture
Although it is good practice to limit the discharge of toxic materials to sewerage systems, inevitably some of these materials gain access and heavy metals and pesticides are frequently present in municipal sewage. This gives rise to concern about bioaccumulation when sewa~ effluent is used in aquaculture. Algae are known to accumulate various heavy metals but, with the possible exception of mercury, fish raised in sewagefed ponds have not been observed to accumulate high concentrations of these toxic substances. It would appear that the concentrations of heavy metals in the pond water may be accumulated at slower rates than new tissues develop in rapidly growing fish, such as tilapia. In the case of mercury, the position of fish in the food chain seems to be important in determining their mercury uptake, with carnivorous fish accumulating more than herbivores. .,; Fish, apparently, have the ability to regulate the heavy metal content of their tissues, except for mercury, and tend to accumulate metals in parts other than muscle tissue. There is little information on the uptake of toxics other than heavy metals but a high phenol content in the sewage fed to fish ponds in Wuhon, China caused the fish flesh to become unpalatable due to the odour of phenol. Weis et al. have reported on the effects of treated municipal wastewater on the early life stages of three species of fish and indicated that moderately toxic effluent (organic fractions) caused cardiovascular and skeletal defects, depression of heart rate and poor hatching, larval and juvenile growth rates. The health effects of aquacultural use of human wastes in respect of pathogenic organisms have been discussed. Depuration was mentioned as a means to decontaminate fish grown in waste-fed aquaculture. It is generally believed that holding fish in clean-water ponds for several weeks at the end of the growing cycle will remove residual objectionable odours and pathogens and provide fish acceptable for market. However, there is a lack of data on depu~ation practice and experimental assessment. What little evidence there is suggests that depuration of heavily contaminated fish with bacteria in muscle tissue will not oe effective. Relatively short depuration periods of one to two weeks do not appear to remove bacteria from the fish digestive tract. Considering the lack of verification of the effectiveness of depuration as a health protective measure, Edwards has not included it in his suggested strategies for safeguarding public health in aquaculture.
244
Wastewater Use in Aquaculture
Fig:3. Aquacultural Reuse Strategies with Different Types of Excreta to Safeguard Public Health
Chapter13
Sewerage Sewerage, a general term for the process of systematically collecting and removing the fouled water-supply of a community. The matter to be dealt with may conveniently be classified as made up of three parts: • Excreta, consisting of urine and faeces. • Slop-water, or the discharge from sinks, basins, baths, and the wastewater of industrial processes. • Surface water due to rainfall. Before the use of underground conduits became general, the second and third constituents were commonly allowed to sink into the neighbouring ground, or to find their way by surface channels to a watercourse or to the sea. The first constituent was conserved in middens or pits, either together with the dust, ashes, kitchen waste and solid waste generally or separately, and was carried away from time to time to be applied as manure to the land. In more modern times the pits in which excrement was collected took the form of covered tanks called cesspools, and with this modification the primitive system of conservancy, with occasional removal by carts, is still to be found in many towns. Even where the plan of removing excrement by sewers has been adopted, the kitchen waste, ashes and solid refuse is still treated by collecting it in pails or bins, whose contents are removed by carts either daily or at longer intervals, the refuse frequently being burned in destructors. It therefore forms no part of the nearly liquid sewage which the other constituents unite to form. The first constituent is from an agricultural point of view the most valuable, and from a hygienic point of view the most dangerous, element of sewage. Even healthy excreta decompose, if kept for a short time after they are produced, and give rise to noxious gases; but a more serious danger proceeds from the fact that in certain cases of sickness tl).ese products are charged with specific germs of disease. Speedy removal or destruction of excremental sewage is therefore imperative. It may be removed in an unmixed state, either in pails or tanks or (with the aid of
Sewerage
246
pneumatic pressure) ·by pipes; or it may be defaecated by mixture with dry earth or ashes; or, finally, it may be conveyed away in sewers by gravitation, after the addition of a relatively large volume of water.
Facultative Pond Raw Sewage----+-,. HRAP Influent Influent Anaerobic Fermentation Pit (section) Final Effluent to ..._ _ _ Maturation Pond
3
I
Algal Settling Pond (section)
I
Drying Bed High Rate Algal Pond (Plan)
Fig:l. Sewerage This last mode of disposal is termed the water-carriage system of sewerage. It is the plan now usually adopted in towns which have a sufficient water supply, and it is probably the mode which best meets the needs of any large community. The sewers which carry the diluted excreta serve also to take slopwater, and mayor may not be used to remove the surface water due to rainfall. The watercarriage system has the disadvantage that much of the agricultural value of sewage is lost by its dilution, while the volume of foul matter to be disposed of is greatly increased. COLLECTION Of SEWAGE
House drains, that is to say, those parts of the domestic system of drainage which extend from the soil-pipes and waste-pipes to the sewer, are generally made of glazed stoneware pipes having a diameter of 4 in., 6 in, or sometimes 9 or 12 in., according to the estimated amount of waste to be removed. In anent ordinary domestic dwellings there is rarely any occasion to use pipes of a greater diameter than 6 in., and this only for the main drain, the branches and single lines of piping being 4 in. in diameter. It is a good rule to make the pipes and other fittings, such as channels and bends, as small in diameter as possible, having due regard to efficient capacity. Such a drain is more cleanly than one too large for its purpose, in that it is more thoroughly flushed when in use, the sewage running at a much faster speed through a full pipe than through one only partially
Sewerage
247
full. For this reason a pipe having too great a capacity for the work it has to do is liable to become corroded by sediment deposited from slowly moving waste. The pipes are made in 2 ft. lengths and are formed with a socket at one end into which the straight end of the next pipe fits loosely. This is wedged in position with a little gasket and the remaining space then carefully filled with neat Portland cement. Pipes are made also with a bituminous substance in the socket and around the spigot end, and by merely pushing the one into the other the joint is made. The bitumen is curved to allow self-adjustment to any slight settlement, so that damage to the joint is avoided. A composite joint may be used having the bitumen lining reinforced with the ordinary Portland cement filling. This type is somewhat more expensive than the ordinary jointing, but it makes a powerful and effective connexion. The method of connecting two lead pipes by a " wiped solder joint" the method of connect amen bihimatn ing a lead pipe into the socket of a stoneware one, a brass sleeve piece or ferrule being used to give the necessary stiffness to the end of the lead pipe. This arrangement is frequently used, for example, at the base of a soil-pipe at its junction with the drain. The lead pipe has a brass socket attached to it to take the plain end of a stoneware pipe. This form of connexion is used between a watercloset and a lead trap. In the water-carriage system of drainage each house has its own network of drain-pipes laid under the ground, into which are taken the waste-pipes which lead from the closets, urinals, sinks, lavatory basins, and rain-water and other gulleys within and about the house. The many branches are gathered into one or more manholes, and connexion is finally made by means of a single pipe with the common public sewer. Gas from the sewer is prevented from entering the house drains by a disconnecting trap fixed in the manhole nearest the entrance to the sewer. The fundamental maxims of house sanitation are first, that there shall be complete disconnexion between the pipes within and without the house, and second, that the drainage shall be so constructed as to allow for the free admission of air in order to secure the thorough ventilation of all parts of the system and avoid the possibility of the accumulation of gas in any of the wasteor drain-pipes. The drains must be planned to conduct the waste material from the premises as quickly as possible without leakage or deposit by the way. The pipes should be laid in straight lines from point to point to true gradients of between 2 to 4 in. in io ft. Junctions with branch pipes and any bends necessary should be gathered, as far as practicable, in inspection chambers fitted with open channels instead of closed pipes.
248
Sewerage
Large Driving Face (for impact resistance)
Double Thickness Socket wall
Tapered Spigot End
Tapered Socket
Internal Shoulder (for spigot end stop)
End Plug or Grout Shoe Fitted at base of Pile
Fig:2. Iron Spigot and Socket Joint
This allows of easy inspection and testing, and provides means of access for the drain-rods in cases of blockage. Sometimes it is desired, for reasons of economy or otherwise, to avoid the use of a manhole at a change of direction in the drain. A branch pipe which may have a specially shaped junction for cleaning the pipes in both directions is taken up with a slope to the ground qr floor level and there finished with an air-tight cover which may be removed to allow the introduction of drain-rods should the pipes become blocked. Junctions of one another should be made obliquely in the direction of the floor. Stoneware pipes should be laid upon a bed of concrete not less than 6 in. thick and benched up at the sides with concrete to prevent any movement. When such pipes pass u'n der a building they should be entirely surrounded by a concrete casing at least 6 in. in thickness. No drain should lie under a building if it is possible to avoid it, for injury is very liable to occur through some slight settlement of the building, and in a position such that the smells escaping from the damaged pipe would rise up through the floor into the building this would be an especially serious matter. The expense and annoyance of having the ground opened up for the repair of defects in the pipes beneath is another strong argument against drains being placed under a house. Where this is really necessary, however, pipes of cast iron are recommended instead of the ordinary
Sewerage
249
stoneware pipes, as being stronger; being made in lengths of 6 and 9 ft, they have a great advantage over the 2 ft. long stoneware tubes, for the joints of the latter are frequently a source of weakness. The joints, fewer in number, are made with molten lead, or flanged pipes are used and the joints packed with bolted. The principle of disconnexion and outdoor pipes should be retained between the latter and the sewer, and the domestic system should be cut off from the public drain by means of a dis connecting trap. This appliance is usually placed in a small chamber or manhole, easy of access for inspection, built close to the boundary of the premises, and as near as possible to the sewer into which the house drain discharges.
Fig:3. Iron-flanged
The cap to the clearing arm has a chain attached by which it can be removed in case of flooding. The channels are benched up at the sides with cement, and the manhole is rendered on the inside with a cement lining. A fresh air inlet is taken out near the top of the chamber and is fitted with a mica flap inlet valve. The cover is of cast-iron in a cast-iron frame shaped with grooves to afford a double seal, the grooves being filled with a composition of tallow and fine sand. Where there is a danger of a backflow from the sewer due to its becoming flooded, a hinged flap should be placed at the junction of drain and sewer to prevent sewage from entering the house drain. A ball trap for this purpose may be used in place of a flap, and is more satisfactory, for the latter is liable to become corroded and work stiffly. In the balltrap appliance the flowing back of the sewage forces a copper ball to fit tightly against the drain outlet, the ball dropping out of the way of the flow directly the pressure is relaxed . The water-carriage system of drainage is undoubtedly the most nearly perfect yet devised. At the same time it is a very costly system to install
250
Sewerage
with its network of sewers, stations, and arrangements for depositing the sewage either in the sea or river, or upon the land or " sewage farm." In country districts and small towns and villages, however, excreta are often collected in small vessels Section. Manhole Lid Manhole Frame
I~~==t~~ Adjustment Rings
Branch Sewer
/'
Flow
Flow
Main Sewer
Fig:4. Manhole The dry-earth system oxidizing effect which a porous substance such as dry earth exerts by bringing any sewa'ge with which it is mixed into intimate contact with the air contained in its pores. The system is of rather limited application from the fact that it leaves other constituents of sewage to be dealt with by other means. But so far as it goes it is excellent, and where there is no general system of watercarriage sewerage an earth-closet will in careful hands give perfect satisfaction. Numerous forms of earth-closet are sold in which a suitable quantity of earth is automatically thrown into the pan at each time of use but a box filled with dry earth and a hand scoop will answer the purpose nearly as well. A plan much used in towns on the continent of Europe is to collect excrement in air-tight vaults which are emptied at intervals into a tank cart by a suction pump. Another pneumatic system adopted on the continent has the cesspools at individual houses per manently connected with a central reservoir by pipes through which the contents of the former are sucked by exhausting air from the reservoir at the central station. Newly laid drains should be carefully tested before the trenches are Testing filled ;i, to detect drains. any defectsin the pipes or joints. These
Sewerage
251
should be made good and the test again applied until the whole system is in perfect order. . Cement joints should be allowed to set for at least fortyeight hours before the test is made. There are several methods of testing. For the stoneware drains laid under the ground the water test is generally adopted. After -the lower end of the length of drain to be tested has been securely stopped the drain is filled with water from its upper end until the desired pressure is obtained. To obtain the required head of water extra lengths of pipe are sometimes taken up temporarily at the upper end of the drain or, as an alternative, both ends of the pipe may be plugged and water introduced under pressure by a force pump through a small aperture provided in the plug. The exact pressure may then be ascertained by a water pressure gauge. An escape of water through some defective portion of the drain is indicated by the subsidence of the level of the water in the upper part of the drain or by a diminution of the pressure shown by the gauge. Then the defect must be located and remedied and the drain re-tested until all weak points are eliminated. This process must be repeated in each section of the drainage system until the whole is found to be sound and tight. It is not necessary to test drains laid with ordinary socket joints made in cement with a greater pressure than is obtained with a 5 ft. or 6 ft. head of water. A foot head of water gives at its base a pressure of .433 lb per square inch, so that a head of 6 ft. would result in a pressure of just over lb per square inch. Cast-iron drain-pipes with caulked lead joints will withstand a pressure of nearly 90 lb per square inch of internal surface, but in actual practice it is sufficient if they are tested with a pressure of or say a head of 20 to 24 ft. The atmospheric or air test is sometimes applied instead of the water test. The drain is plugged, as in the latter, and air is then pumped into the pipes until the desired pressure is registered by the gauge attached to the apparatus . This pressure should be maintained without appreciable diminution for a stipulated period before the drains are passed as sound. The smoke test is generally used for testing vertical shafts such as soilpipes and ventilators to which the water test cannot be conveniently applied owing to the excessive pressure produced at the lower portion of the pipe by the head of water. It is applied by stopping the ends of the pipes and introducing smoke by a drain rocket or by a smoke-producing machine which forces volumes of thick smoke through an aperture in the stopper. The pipes and joints are then carefully inspected for any evidence of leakage. The scent test is occasionally employed for testing soil and ventilating
252
Sewerage
pipes, but the apparatus must be carefully handled to avoid the material being spilt in the building and thus misleading the operator. The test is made by introducing into the drain some substance possessing a powerful odour such as oil of peppermint, calcium carbide or other suitable material, and tracing any defect by means of the escaping odour. This is not so effective a method as the smoke test, as there is more difficulty in locating leakages. Gulleys, traps and other similar fittings should be tested by pouring in water and observing whether siphonage or unsealing occurs. This of course will not happen if the appliances are of good design and properly ventilated. A section of a drain plug or stopper. It has a band of indiarubber which expandswhen the screw is turned and presses tightly against the inside of the drain-pipe. In the centre of the plug is a capped aperture which allows for smoke testing and also allows the water gradually to escape after a test by water.
Fig:5. Drain Stopper
Existing drains which have become defective and require to be made good must be exposed, taken up and relaid with new pipes, unless advantage be taken of a method which, it is claimed, renders it possible to make them permanently watertight so as to withstand the water test under pressure, and at the same time to disinfect them al1d the surrounding sub-soil. This end is accomplished with the aid of patent machines which on being passed through the drain-pipe first remove all obstructions and' accumulations of foul matter and then thoroughly cleanse and disinfect it, saturating the outside concrete and contaminated soil adjacent to any leak with strong disinfectants. Subsequently, loaded with the best Portland cement, another machine is passed through the drain, and, by powerful evenly-distributed circular compression, forces the cement into every hole, crack or crevice in the
Sewerage
253
pipes and joints. This work leaves the inner surface of the pipes perfectly clean and smooth. After the usual time has been allowed for the cement to set the air test is applied, and the drain is claimed to be equal to, if not better than, a new drain, because the foundation is not dis turbed by the process, and the risk of settlement, which is often the ~ause of leaky drains, is remote. Every sanitary fitting should be trapped by a bend on the waste-pipe; this is generally made separately and fixed up near to the sink, closet or basin, as the case may be. The traps of small wastes such as those of sinks and lavatories should be fitted. with a brass screw cap to facilitate clearing when a stoppage occurs. Their object is to hold a quantity of water sufficient to prevent the access of foul air through the waste-pipe into the house. The depth of the water "seal" should not be less than 2 in., or it may become easily unsealed in hot weather through the evaporation of the water. Unsealing may be caused, too, by "siphonage", when a number 'of fittings are attac;hed to the same main waste without the branches being properly ven tilated just below each trap. The discharge from one fitting in this case would create a partial vacuum in the other branches and probably suck the sealing water from one or more of the traps. To obviate such an occurrence an "anti-siphonage" pipe is fixed having its upper end open to the air and provided with branches tapping such waste-pipe just below the trap. Then, with this contrivance, a discharge from any fitting, instead of causing air to be sucked in through the trap of another fitting, thereby breaking the seal and allowing foul drain air to enter the house, merely draws the necessary air through the anti-siphonage pipe, leaving the other traps with their seals intact. There are many forms of traps for use in different positions although the principle and purposes of all are identical. Two forms commonly used are known as the S and the P trap. The bell trap and the 0 trap are obsolete. To collect the rain and wastewater from areas, yards, laundry and other floors and similar positions an open trapped gulley is used. It is usually of stoneware and fitted with an open iron grating which admits the water. Many of these gulleys are made too shallow and speedily get choked if. the water they receive is charged with mud or sand. The gulleys are made with a deep container and are often fitted with-a perforated basket of galvanized iron which catches the solid matter and has a handle which allows for its easy removal when necessary. Gulleys with slipper or channel heads are required to be fitted in some districts to receive the waste from sinks. The warm wastewater from scullery and pantry sinks contains much grease, and should discharge into a trapped gulley specially constructed to prevent the passag~ of the
254
Sewerage
grease into the drain. It should be of ample size to contain sufficient cold water to solidify the fat which enters it. This forms in cakes on the top of the water and should be frequently broken up and removed. Great attention has been directed to the design of sanitary fittings, with the object of making them as nearly self-cleansing as possible. In the fixing of closets the wood casings whic.h used to be fixed around every water-closet are going steadily out of use, their place being taken by a hinged seat supported on metal brackets - an arrangement which allows every part of the appliance to be readily cleaned with a cloth. In hospitals and similar institutions a form of closet is made fitted with lugs which are built into the wall; in this way support is obtained without any assistance from the floor, which is left quite clear for sweeping. Lavatory basins and sinks are also supported on cantilevers in the same way, and the wood enclosures which were formerly often fixed around these appliances are now generally omitted. There are several distinct types of water-closets. Each type is made in many different patterns, both good and bad from a sanitary point of view, and, whatever the type decided upon, care is necessary in selecting to obtain one efficient and hygienic in shape and working. The principal kinds of closets now in use are the washdown, siphonic, valve, washout and hopper. Washdown closets are most commonly used. They are inexpensive to buy and to fix, and being made in one piece and simple in construction without any mechanical work ing parts are not liable to get out of order. When strongly made or protected by brick or concrete work they will stand very rough usage. The objection is sometimes raised with regard to washdown closets that they are noisy in action. Washdown designs have been greatly improved in this respect, and when fitted with a silent flushing cistern are not open to this objection. Siphonic closets are a type of washdown in which the contents of the pan are removed by siphonic action, an after flush arrangement providing for the resealing of the trap. They are practically silent in action and with a flush of three gallons work very satisfactorily. Where the restrictions of the water company require the usual two gallon flush the ordinary washdown pan should be used. Valve closets are considered by many authorities on sanitation to be preferable to all other types. For domestic buildings, hotels, and where not subjected to the hardest wear, they are undoubtedly of great value. They should have a three gallon flush, and on this account they cannot be used in many districts owing to the water companies' regulations stipulating that a flush of not more than two gallons may be used . The washout closet is a type that never attained much popularity as it has been found by practical experience to be unsanitary and objectionable. The
255
Sewerage
standing water is too shallow, and the receiving basin checks the force of the flush and the trap is therefore frequently imperfectly cleared. Hopper Closets are of two kinds
• Long hopper. • Short hopper. These are the forerunners of the washdown closet which the short hopper pan resembles, but instead of pan and trap being made in one piece the fitting consists of a fireclay or stoneware hopper, with straight sloping sides and central outlet jointed to a trap of lead or other material. The joint should be placed so as to be always kept under water the seal of the trap. The long hopper pan is a most objectionable type of closet which should be rigorously avoided as it easily becomes foul and is most insanitary. In most districts its use is prohibited. From Restaurant
Side View
Fig:6. Grease Trap
A water-waste preventer is a small tank fixed usually 4 or 5 ft. above a closet or urinal and connected therewith by a flushing pipe of 4 in. or greater internal diameter. This tank usually contains a siphon, and the flush is actuated by pulling a chain which admits water to the siphon; the contents are then discharged with some force down the flushing pipe into the pan of the closet, clearing out its contents and replacing the fouled water with clean. The flushing tank is automatically refilled with water by a valve fitted with a copper ball which rising on the surface of the incoming water shuts off the flow when the tank is full is a sectional drawing of one of the latest patterns and clearly shows its construction. The water-supply is shown near the top with the regulating ball valve attached. An overflow is provided and a pipe is led from this to an external outlet. The capacity of the ordinary domestic flushing cistern is two gallons, which is the maximum quantity allowed by most water companies. A three gallon flush is much better, however, and where this larger quantity is allowed should be adopted. Larger tanks for ranges of closets or urinals are often made to flush automatically when full, and for
256
Sewerage
these the rate of water supply. The by-laws of the London County Council contain very full regulations respecting the construction and fitting up of waterclosets. These may be summarized as follows: A water closet or urinal must be furnished with an adequate flushing cistern distinct from any cistern used for drinking water. The service pipe shall lead to the flushing cistern and not to any other part of the closet. The pipe connecting the cistern with the pan shall have a diameter of not less than 14 in. in any part. The apparatus for the application of water to the apparatus must provide for the effectual flushing and cleansing of the pan, and the prompt and effectual removal therefrom, and from the trap connected therewith of all solid and liquid filth. The pan or basin shall be of non-absorbent material, of such shape, capacity and construction as to contain a sufficient quantity of water and to allow all filth to fall free of the sides directly into the water. No container or similar fitting shall be fixed under the pan. There shall be fixed immediately beneath or in connexion with the pan an efficient siphon trap constructed to maintain a sufficient water seal between the pan and the drain or soil pipe. No 0 t.rap or other similar trap is to be connected with the apparatus. If more than one water-closet is connected with a soil-pipe the trap of each closet shall be ventilated into the open air at a point as high as the top of the soil-pipe, or into a soil-pipe above the highest closet. This ventilating (or anti-shiphonage) pipe shall be not less than 2 in. in diameter, and connected at a point not less than 3 and not more than 12 in. from the highest part of the trap. Baths may be made of many different materials; copper, castiron, zinc and porcelain are those most generally employed. Metal, baths have the great advantage of becoming hot with the water, while baths of porcelain, stoneware and marble, which are bad conductors of heat, impart to the user a sense of chilliness even though the water in the bath be hot. Copper baths are best; they may be finished on the inside by tinning, enamelling or nickel plating. Iron baths, usually tapering in shape, are very popular and are usually finished in enamel, but sometimes tinned good type of cast-iron bath with standing waste. A good feature of this bath lies in the fact that all parts are accessible and easily cleaned. Porcelain baths are cumbersome and take a long time to heat, but they are often used for public baths. The practice of enclosing the bath with a wood casing is fast dying out; it is insanitary in that it harbours dust and vermin. Baths are now usually elevated upon short legs, so that every part of them and of the adjacent floor and wall is accessible for cleaning. The fitting is supported
Sewerage
257
upon galvanized iron cantilever brackets which are built into the wall. Like closets, urinals have undergone much improvement in design and manufacture. The best types are of glazed ware, and have vertical curved backs and sides about 4 ft. high with a flushing rim round the top and terminating in a base discharging into an open glazed channel waste, which, in the case of a range of urinals, collects the discharge from all and conveys it into a trapped gulley at one end of the range. This is the type usually fixed in street conveniences and similar positions. Plate and iron urinals are often fixed, but there is more difficulty in keeping them dean on account of the sharp angle and the unsuitability of the material. Urinals are seldom fixed in private houses or offices, an ordinary washdown pedestal closet with hinged "tip-up" seat serving every purpose. Such seats are often fitted with balance weights to cause them to lift automatically when not in use as a closet. Unless kept very clean and well flushed with water, urinals are liable to become a nuisance. In London among other towns the system of drainage is a""combined" one, that is, the storm water and the domestic sewage and waste is all collected in one sewer. For many reasons it is more satisfactory to have the two drains quite separate. In many districts this is done, but it entails the provision of a double system of drainage for each house, one drain being provided for rainwater, the other for ground plans of the same house, a semi-detached suburban residence, one with combined drainage and the other with separate dr~ins for storm water and sewage. Where combined drainage is installed an excess of water poured into the sewers during a storm often results in back flow and the flooding of basements and cellars with where there is a separate sewer fcr the storm water, but in this case the flooding would be with comparatively harmless rain-water instead of sewage and filth. The London by-laws regulating drainage are very full and are strictly enforced. They include requirements regarding the size, form, gradient and methods of construction and repair of drains, together with regulations affecting the design and fixing of traps, fittings and other apparatus connected with sanitary arrangements. Some of the headings of the different clauses of the by-laws are subjoined: - water-closets; earth-closets; drainage of subsoil; drainage of surface water; rain-water pipes; materials, , for drains; size of drains; drain to be laid on bed of concrete 6 in. thick; if under buildings to be encased with 6 in. of concrete; drain to be benched up with concrete to half its
258
Sewerage
diameter; fall of drain; joints of drain; drain to be water-tight; thickness and weight of iron pipes; thickness of sockets and joints of stoneware pipes; drains under buildings; composition of concrete; every inlet to drain to be trapped; drain beneath wall to be protected by arch, flagstone, or iron lintel; drain connected with sewer to be trapped and means of access to trap provided; no right-angled junctions to be formed either vertical or horizontal; at least two untrapped openings to be provide<:i for ventilation, each fitted with a grating or cowl with apertures for passage of air equal in area of the pipe to which it is fitted; ventilating shafts to be at least 4 in. in diameter, and if possible all bends and angles to be avoided; ventilating shafts to be of the same material, construction and weight as soil-pipes; no unnecessary inlets to drains to be made within buildings; waste-pipes from sinks and lavatories to be of lead, iron or stoneware, trapped immediately beneath the fitting; bell traps, dip traps and D traps are prohibited waste-pipes to discharge in the open air into a properly trapped gulley; soil-pipes wherever practicable to be situate outside the building and to be of drawn lead or heavy cast-iron; if fixed internally the pipes to be of lead with wiped joints; iron pipes to have socket joints not less than 22 in. in depth and to be made with molten lead or flanged joints securely bolted with some suitable insertion; the soil-pipe not to be connected with any rain-water or waste-pipe, and no trap to be placed between the soilpipe and the drain; the soil-pipe to be circular with an internal diameter of not less than 32 in, and to be taken up above the building and its end left open as an outlet for foul air; methods of connecting a lead pipe with an iron one; connexion of stoneware and lead, connexion of iron and stoneware; ventilation of trap of water-closet with an anti-siphonage pipe of not less than 2 in. diameter and ventilated into the open air or into the soil-pipe at a point above the highest fitting on the soil-pipe; construction of slop sinks and urinals. From a general point of view the requirements of the American bylaws as to materials and methods of construction vary in a very slight degree from those in force under the London authorities. It is in the regulations affecting the execution of the work that we find a great difference, and these in New York are of a more stringent character than in any other capital. Thus no sanitary, plumbing or lighting work may be undertaken without first submitting for approval to the Department of Buildings complete and suitable drawings and particulars of the materials to be used. Such a notice is necessary even in the case of repairs and alterations to existing work. As a further guarantee of the work being satisfactory it is ordained
Sewerage
259
that no such work shall be executed except under the superintendence of a registered plumber. Every master plumber in the city of New York or others working therein as such must obtain a certificate of competency from the Examination Board and be registered afresh every year during the month of March, as without such certificate or licence no work can be undertaken; any person violating such requirements shall upon conviction be fined for each offence $250 or undergo three months' imprisonment or both, while in the case of any certificated plumber or his employes wilfully breaking, with his knowledge, any of the rules and regulations relating to drainage and plumbing, the certificate of the master is to be forfeited in addition to the aforementioned fine. CONVEYANCE OF SEWAGE
For small sewers, circular pipes of glazed stoneware or of moulded cement are used, from 6 in. to 18 in. and even in. in diameter. The pipes are made in short lengths, and are usually jointed by passing the end or spigot of one into the socket or faucet of the next. Into the space between the spigot and faucet a ring of gasket or tarred hemp should be forced, and the rest of the space filled up with cement. Other methods of jointing have already been described and illustrated. The pipes are laid with the spigot ends pointing in the direction of the flow, with a uniform gradient, and, where practicable, in straight lines. In special positions, as under the bed of a stream, castiron pipes are used for the conveyance of sewage. Where the capacity of an 18 in. circular pipe would be insufficient, built sewers are used in place of stoneware pipes. These are sometimes circular or oval, but more commonly of an egg-shaped section, the invert or lower side of the sewer being a curve of shorter radius than the arch or upper side. The advantage of this form lies in the fact that great variations in the volume of flow must be expected, and the eggsection presents for the small or dry-weather flow a narrower channel than would be presented by a circular sewer of the same total capacity. Built sewers are most commonly made of bricks, moulded to suit the curved structure of which they are to form part. Separate invert blocks of glazed earthenware, terracotta or fire-clay are often used in combination with brickwork. The bricks are laid over a templet made to the section of the sewer, and are grouted with cement. The thickness of brickwork for sewers over 3 ft. in diameter should not be less than 9 in, but for smaller sewers laid in good ground at depths not exceeding 20 ft. from the surface a thickness of 41 in. will suffice if well backed up with concrete. The
260
Sewerage
thickness of brickwork for a sewer of any size may be determined in feet by the formula: dr/lOO, where d = depth of excavation in feet and r = external radius in feet. An egg-shaped sewer, made with two thicknesses of brick, an invert block, and a concrete setting. Concrete is largely used in the construction of sewers, either in combination with brickwork or alone. For this purpose the concrete consists of from 5 to 7 parts of sand and gravel or broken stone to 1 of Portland cement. It may be used as a cradle for or as a backing to a brick ring, or as the sale material of construction by running it into position round a mould which is removed when the concrete is sufficiently set, the inner surface of the sewer being in this case coated with a thin layer of cement. A develop the construction of concrete sewers, whether laid in pipes or constructed and moulded in situ, is the use of iron or steel bars and wires embedded in the material as a reinforcement. Such conduits can be constructed of any size and designed to withstand high pressures a section of a concrete sewer having a diameter of more than 9 ft. constructed with round rod reinforcement. With regard to the method for calculating the proportions, generally speaking the thickness of the concrete shell should in no place be less than one-twelfth of the greatest internal diameter of the tube, while the steel reinforcement should be designed to resist the whole of the tensile stress. Where the safe tensile stress in the steel is 8 tonnes per sq. in. P= the pressure in pounds per sq. in, and r= the internal radius in inches; the weight of the reinforcement per sq. ft = Pr/450, while its area at each side of the pipe per longitudinal foot, when f= safe tensile stress in the reinforcement in pounds, is I 2 Pr/f. In determining the dimensions of sewers, the amount of sewage proper may be taken as equal to the water supply (generally about 30 gallons per head per diem), and to this must be added D (when the "combined" system is adopted) an allowance for the surface water due to rainfall. The latter, which is generally by far the larger constituent, is to be estimated from the maximum rate of rainfall for the district and from the area and character of the surface. In the sewerage of Berlin, for example, the maximum rainfall allowed for is $ of an inch per hour, of which one-third is supposed to enter the sewers. In any estimate of the size of sewers based on rainfall account must of course be taken of the relief provided by storm-overflows. and also of the capacity of the sewers to become simply charged with water during the short time to which very heavy showers are invariably limited.
261
Sewerage
Rainfall at the rate of 5 or 6 in. per hour has been known to occur for a few minutes, but it is unnecessary to provide (even above stormoverflows) sewers capable of discharging any such amount as this; the time taken by sewers of more moderate size to fill would of itself prevent the discharge from them from reaching a condition of steady flow; and, apart from this, the risk of damage by such an exceptional fall would not warrant so great an initial expenditure. Engineers differ widely in their estimates of the allowance to be made for the discharge of surface water, and no rule can be laid down which would be of general application. In order that sewers should be self-cleansing, the mean velocity of flow should be not less than 22 ft. per second. The gradient necessary to secure this is calculated on principles which are stated in the article Hydraulics. The velocity of flow, V, is V =G-Vim, where i is the inclination, or ratio of vertical to horizontal distance; m is the hydraulic mean depth," or the ratio of area of section of the stream to the wetted perimeter; and c is a coefficient depending on the dimensions and the roughness of the channel and the depth of the stream. A table of values of c will be found in 98 of the article referred to. This velocity multiplied by the area of the stream gives the rate of discharge. Tables to facilitate the determination of velocity and discharge in sewers of various dimensions, forms and gradients will be found in Latham's and other practical treatises. Where the contour of the ground does not admit of a sufficient gradient from the gathering ground to the place of destination, the sewage must be pumped to a higher level at one or more points in its course. To minimize this necessity, and also for other reasons, it is frequently desirable not to gather sewage from the whole area into a single main, but to collect the sewage of higher portions of the town by a separate highlevel or interception sewer. It is undoubtedly necessary to construct overflows for storm water in connexion with combined systems of sewerage. No combined sewer of such size as will make it comparatively Storm self-cleansing under normal conditions can hope to carry water off the volume of water resulting from heavy rain. It might be thought that the overflow resulting from a storm would consist of nearly pure rain-water, but this is not the case, as the pressure of storm water has the effect of scouring out from the sewers a great deal of foul matter that is deposited when the flow is small. This being the case it is obviously bad policy to take the overflow into a stream, which would thereby suffer contamination. A better plan is to direct the discharge into a dry ditch or channel where the liquid may soak into the soil and the solid particles by contact with the air may quickly 1/
262
Sewerage
become oxidized. In agricultural districts it might be possible by arrangement with farmers to run the overflow over grass-land, as it has good manurial properties. Occasionally when a sewer has to cross a stream or other obstruction it is found impossible to bridge or carry the pipe across and preserve its proper gradient. In such cases it must be carried under the obstruction by means of an inverted 5, siphon. The exact form that should be given to inverted p siphons is disputed, but it is generally agreed that they are expedients to be avoided wherever possible. The majority take roughly the form of the stream section, that is, they have two sloping pieces corresponding with the banks with a flat cross-piece under the bed of the stream. The pipes are invariably of iron and should be laid in duplicate, as they are liable to silt up in the flat length. For this reason it is usual in constructing a siphon to place permanent chains in the pipes, and these are periodically pulled backward and forward to stir up the silt. Brushes may also be attached to the chains and pulled through from end to end. At either end of the siphon pipes there are manholes into which the pipes are built. Penstock valves also should be provided at each end so that sewage can be shut out of one or both of the siphons as desired for clearing purposes. Tumbling bays being prohibited, the usual method of leading a highlevel sewer into a low-level sewer is by means of a ramp. This is constructed in connection with a manhole into which the end of the highlevel sewer is taken and finished usually with a flap valve. Some distance back along with this sewer a wide-throated junction is put in the invert of the sewer, and from this junction a ramp-pipe is taken down to the invert of the low-level sewer, so that the sewage in the upper sewer instead of having a direct fall runs down the slope of the ramp. The ramp-pipe is usually constructed of iron and is of smaller section than the high-level sewer because of the greater fall and pressure. In the low-lying parts of towns storage tanks are often constructed to receive the sewage of such districts. They are periodically emptied of their contents, which are pumped up into the main sewers through which the sewage travels to the outfall. This storing of sewage should be avoided whenever possible. It is much better to provide for raising it as it is produced either by an installation of one or more automatic lifts, such as Adams's sewage lifts, or, where a large amount of material is to be dealt with, necessitating continual pumping, by a Shone ejector worked by compressed air. Sewer gas is a term applied to the air, fouled by mixture with gases
Sewerage
263
which are formed by the decomposition of sewage, and by the organic germs which it carries in suspension, that fills the sewer in the variable space above the liquid stream. It is universally recognized that sewer gas is a medium for the conveyance of disease, and in all well-designed sewers. systems of sewerage stringent precautions are taken to keep it out of houses. It is equally certain that the dangerous character of sewer gas is reduced, if not entirely removed, by free admixture with the oxygen of fresh air. Sewers should be liberally ventilated. Only for this reason, but to prevent the air witlVp them from ever having its pressure raised (by sudden influx of water) so considerably as to force the "traps" which separate it from the atmQsphere of dwellings. The plan of ventilation now most approved is the very simple one of making openings from the sewer to the surface of the street at short distances-generally shafts built of brick and cement-and covering these with metallic gratings. Under each grating it is usual to hang a box or tray to catch any stones or dirt that may fall through from the street, but the passage of air to and from the sewer is left as free as possible.
Fig:7. Brick Sewer The openings to the street are frequently made large enough to allow a man to go down to examine or clean the sewers, and are then called "manholes". Smaller openings, large enough to allow a lamp to be lowered for purposes of inspection, are called "lampholes", and are often built up of vertical lengths of drain-pipe, 6 in. or 9 in. in diameter, and finished at the surface with a cover similar to that used for a manhole but smaller. A length of 150 ft. of pipe sewer is about the limit that can be sighted through. Lampholes are mostly used in the construction of pipe and other small sewers. To facilitate inspection and cleaning, sewers are, as far as possible, laid in straight lines of uniform gradient, with a manhole or lamphole
264
Sewerage
Flashing at each change of direction or of slope and at each junction F sewers. of mains with one another or with branches. The sewers may advantageously be stepped here and there at manholes. Sir R. Rawlinson pointed out that a difference of level between the entrance and exit pipes tends to prevent continuous flow of sewer gas towards the higher parts of the system, and makes the ventilation of each section more independent and thorough. When the gradient is slight, and the dry-weather flow very small, occasional flushing must be resorted to. Flap valves or sliding penstocks are introduced at manholes; by closing these for a short time sewage (or clean water introduced for the purpose) is dammed up behind the valve either in higher parts of the sewer or in a special flushing chamber, and is then allowed to advance with a rush. Many self-acting arrangements for flushing have been devised which act by allowing a continuous stream of comparatively small volume to accumulate in a tank that discharges itself suddenly when full. A valuable contrivance of this kind is Rogers Field's siphon flush tank. When the liquid in the tank accumulates so that it reaches the top of the annular siphon, and begins to flow over the lip, it carries with it enough air to produce a partial vacuum in the tube. The siphon then bursts into action, and a rapid discharge takes place, which continues till the waterlevel sinks to the foot of the bellshaped cover. Adams's "Monster Flusher" is constructed on similar principles and is of simple and strong design. Its flushing power is claimed to be greater than that of the ordinary siphon. By the use of this appliance, which is automatic in action, shallow sewers can be effectively flushed. DISPOSAL OF SEWAGE
The composition of domestic sewage is now fairly well known and is generally reduced for the purposes of comparison to a standard; that is to say, ordinary sewage is that due to a water-supply of about 30 gallons per head per diem. If the supply is less, and there is no leakage of subsoil water into the drainage system, the sewage will be stronger; conversely, if there is leakage, the sewage will be more dilute, but obviously, the quantity of impurities will, for any given population, be the same in amount. For all practical purposes we may say that average sewage contains two tonnes of suspended matters in each million gallons, one-half of which is mineral matter. When, however, we come to a consideration of trade waste, the question becomes difficult in the extreme, because of the great variety of trades, and the ever varying quantities added to the sewage.
Sewerage
265
Some of the principal trade wastes are from dye-works, print-works, bleach-works, chemical works, tanneries, breweries, paper-makers, woollenworks, silk-works, iron-works and many others. In some cases one only of these trade wastes finds its way to the sewers; in others, several of them may be found. In some instances, again, these trade wastes are of an alkaline nature, in others they are acid; the mixtures may be either, and of greatly varying character. Next comes the manner in which sewage is discharged at the works. The flow is variable throughout the entire 24 hours, but in the case of sewers discharging domestic sewage only, such sewage being of the standard strength, it will be a close approximation to the facts to say that about two-thirds is discharged between the hours of 7 a.m. and 7 p.m, one-half during the eight hours of maximum flow, two-fifths during the six hours of maximum flow, and about 71 % per hour during the two hours of maximum flow. These data will be sufficient for the design of the works intended for dealing with the sewage. Separate calculations must be made if there is trade refuse, or much leakage of subsoil water. In very large systems, again, the maxima are rather less because of the time occupied by the sewage in travelling to the outfall from the more remote parts of the district. In cases where one set of sewers is employed for both sewage and rainfall the sewage flow may be increased more than a hundredfold within a few minutes by heavy rainstorms. Of course the sewage disposal works can only deal with a small proportion of such flow, and the balance is discharged into some convenient water-course or other suitable place. Even when the separate system is employed, as in the case of the smaller towns, the flow may be increased ten to fifteen times by rain, because it is unusual to carry two sets of drains to the backs of the houses. In designing outfall works, therefore, all these circumstances must be carefully considered. Again, when the sewage is pumped, as is frequently the case, the size of the tanks must often be increased, because in the smaller installations the whole of the day's sewage is frequently pumped out in a few hours; this fact must also be remembered when designing filters. Nearly every town upon the coast turns its sewage into the sea. That the sea has a purifying effect is obvious. The object to be attained is its dispersion in a large volume of sea-water. As it is lighter than salt water it tends to rise after leaving the sewer; the outfall should, therefore, if practicable, terminate in deep water, so that the two liquids may become well mixed. The currents must be studied by means of floats, and in most cases
266
Sewerage
the sewage must be discharged upon the ebb tide only, and then perhaps not throughout the entire period, the object being to prevent it from being carried towards the shore. That the purification is effected mainly by means of living organisms is well established, and it has been urged by competent authorities that this system is not wasteful, since the organic matter forms the food of the lower organisms, which in turn are devoured by fish. Thus the sea is richer, if the land is the poorer, by the adoption of this cleanly method of disposal. The next step is the partial purification of the sewage by means of a chemical process. When a town lies some distance up an estuary, as for example London, Glasgow, Rochester and many others, the dilution may be insufficient to prevent a nuisance, or the suspended matters may be deposited upon the foreshore to be uncovered at low water. The first stage of purification is then employed, namely, clarification in tanks. Practice varies with regard to tank capacity, but as a general rule it should be at least equal to half a day's dry weather flow. This will enable the works manager to turn out a good effluent, even in wet weather, when the volume is much increased. With regard to the practical effect of any particular treatment, cit is now recognized that the matters in solution are scarcely touched by any chemical process that can be employed, but the removal of the suspended matter is a great gain, as has been proved in the case of London. Briefly, a good chemical process will do about one-half of the work of purification; and in many cases it is not necessary to go further. With regard to the kind of chemical to use, lime, either alone or in conjunction with aluminium sulphate or with ferrous sulphate, is most frequently employed. When the resulting sewage sludge has to be filterpressed, lime is almost essential for the primary treatment of the sewage, in order to destroy the glutinous nature of the sludge. In the case of large towns like London, Manchester and Salford, the sludge is shipped in specially designed steamers, of 600 tonnes to 1000 tonnes burden, and discharged into the sea at a distance from the coast. The London outfall works have a fleet of six steamers, which convey the sludge out to Barrow Deep, a channel in the North Sea about 10 m. east of the Nore lightship. Each vessel has four oblong tanks having a total capacity of 100 tonnes of sludge, which can be discharged in seven minutes when the valves are fully opened. The sludge is discharged about 10 ft. under the water and being agitated by the action of the ship's screws is very completely diffused. The sand and earthy matters soon subside and the organic matter is rapidly consumed by the organic life in the sea-water. A careful microscopical examination and chemical analysis failed to detect more than the merest trace of the mineral portion of the sludge, either in
Sewerage
267
dredgings from the bottom of the channels or on the surface of the sandbanks. The cost of the disposal works out at about 41d. per tonne of sludge. In the case of towns situated on rivers above the range of tidal waters, the further purification is effected either on land, or by means of artificial filters, or a combination of the two. The question of land treatment is frequently considered from the standpoint of so many persons to the acre; but the best method is to ascertain how many gallons per day an acre of land will purify. As the quality of land varies greatly, the proper volume to be applied per acre can only be ascertained after a good deal of experience. The range lies between about 3,000 gallons per acre per day in the case of poor land, to about 30,000 gallons in the same period in the case of the best. Let us assume an instance of the latter kind. The works have been designed on a basis of 1,000 persons per acre, producing 30,000 gallons of sewage per day; the land being of a highly suitable character, and the sewage having been clarified, success is assured. But, conversely, through faulty construction of the sewers, the sewage amounts, say, to 60 gallons per head; the land, unable to deal with the liquid, quickly becomes water-logged and offensive, and the works are a failure. Precisely the same remarks apply to artificial filters, which are always designed upon the basis of so many gallons per square yard of filtering material. Many failures of both land and filters have been due to the fact that the actual sewage flow was greatly in excess of the original estimates. We may say that clay soils lie at one end of the scale, and very porous sands or gravels at the other; obviously, therefore, each case must be considered on its merits. It should be remembered that when such moderate quantities as 3,000 gallons per acre per day are applied to land, there is no necessity to remove the suspended matter; broad irrigation being resorted to, the land readily assimilates the solids, and thus one source of expense may be eliminated. The artificial filters are now generally called bacteria beds; although filters have been in constant use in some cases, as for instance at Wimbledon, for a great number of years. The first filters constructed at these works were made in 1876, and were about 7,000 sq. yds. in extent. With the growth of popUlation additions have been made of at least five times that area. One of the original beds was used for crude sewage, but the mineral matter choked it completely, and experience pointed to the necessity of clarifying the sewage before filtration. Whether the treatment should be in open or in closed tanks, or whether chemicals should be added, has
268
Sewerage
been much debated; but seeing that ordinary sewage contains one ton of suspended mineral matter in each million gallons, it is clear that if this is not removed before filtration, it will be retained in the filters and ultimately choke them, as happened at Wimbledon. The common cesspool has been resuscitated and improved under the name of a septic tank. In this the disintegration of the suspended matter is brought about by anaerobic organisms, and the liquid in passing slowly through the tank absorbs most of the gases due to the breaking down of the organic matter. There is no oxidation at this stage. The liquid is next passed through artificial filters, of which there are many types. What is known as a "contact" filter was constructed, probably for the first time on a large scale, at the London (Barking) works. The object sought to be attained was that of making each cubic yard of filtering material perform the same amount of work, and the least expensive way was apparently to close the outlet, and charge the filter with liquid, allowing it to remain in contact for about two hours, and then drawing it off so that the bed could be thoroughly aerated. No doubt a better way would be to distribute the sewage in the form of a shower of liquid, and work the beds continuously, but this involves a good deal of expense for spreading appliances, and a fall is necessary in the works, which is not always obtainable. Probably the most complete installation of the kind last referred to is that at Salford. Iron pipes are led over the surface of the filters, and spraying nozzles are placed at short intervals, so that the sewage is applied in the form of a heavy shower. But whatever form the filters and appliances may assume, the final result is the same. If the beds are properly aerated, the aerobic organism establishes itself in prodigious numbers, and attacks the organic matter, breaking it down into harmless, soluble and gaseous products. It is, of course, assume~ that the filters are adequate in area, and are properly managed. With regard to the materials to be employed in making sewage filters, it is now well established that the size of the particles has a more important bearing than their composition. At the same time, it may be remarked that materials with very rough surfaces, as for instance coke breeze, are more effective than those with smooth surfaces. Doubtless the former classes afford, in the interstices, a lodging for the bacteria, and no doubt a given quantity of material with rough surfaces will harbour greater numbers than the same amount of smooth. A reference must be made to the Manchester experiments. The experts' report suggested the provision of 60 acres of filters for dealing
Sewerage
269
with the sewage of the city, which is said to average 30 million gallons per day in dry weather. But after inquiry into the merits of the proposal the officials of the Local Government Board recommended that the filters should be 92 acres in extent, and that the effluent should be finished on land. Storm water filters to take the excess after the sewage was diluted six times were also recommended, such filters being designed to pass 500 gallons per sq. yd. per diem. In this case clarified sewage was to be dealt with on filters 3 ft. 4 in. in depth, composed of clinkers broken to pass a sieve with meshes of 12 in., but retained on one with meshes. It will be observed, therefore, that the bacterial treatment of sewage has scarcely as yet emerged from the experimental stage, but it will certainly be adopted in many cases where it is impracticable to obtain good land in sufficient quantity for the purification of the sewage. With regard to the disposal of sewage-sludge in inland towns, until it has been fairly established by a long trial that bacteria will dispose of this material, the reduction of its bulk by means of filter-presses will be 'found to be the most satisfactory method of dealing with it. The practical effect is the conv~rsion of 5 tonnes of offensive mud into 1 tonne of hard cake, which may be readily handled and carted. The cost is usually about 2S. 6d. per ton of cake, and a million gallons of average sewage produce about 8 tonnes.
Chapter 14
National Fresh Water Recreation Benefits and Water-pollution Control This chapter presents two studies that endeavour to develop methods for assessing the national benefits associated with improvements in, or maintenance of, the quality of surface waters. The first focuses exclusively on freshwater fishing and builds up a national total from regional estimates. The second uses a national sample survey technique to elicit individual's valuation of some broad national water-quality goals. NATIONAL FRESHWATER RECREATION BENEFITS
Among the more important pieces of national environmental legislation created during the 1970s were the comprehensive amendments to the Federal Water Pollution Control Act. These amendments, signed into law in 1972 and further amended in 1977, in reality constituted a major piece of legislation in their own right, dramatically redirecting the nation's efforts at water-pollution control and setting out ambitious national goals, expressed both in terms of discharge controls and of resulting water quality. Criticism of the amendments and debate over their goals and requirements began during the legislative process and has continued, with more or less heat, to the present. Some critics argue that the goals are too ambitious, that is, the benefits of meeting the goals (and related requirements) are thought to be too small to justify the costs of compliance. This argument over the·.balance of benefits and costs can never be resolved entirely by research, but the RFF project described here was undertaken in the conviction that it should be possible to improve methods for estimating at least some of the benefit categories associated with waterpollution control, in this case, the benefits from recreational fishing in freshwater bodies. From the outset the intent was to design a method for estimating benefits for the nation as a whole rafher than benefits for particular sites. In this respect, it resembles the study discussed in the
National Fresh Water Recreation Benefits and Water-pollution Control
271
last part of chapter. In undertaking this project, a primary question concerned the ways in which water-quality improvement would favorably affect freshwater fishing. Two major ways were identified. First, it tends to increase the total availability of fishable fresh water bodies by reducing the incidence of conditions such as low dissolved oxygen that result~ from the bacterial degradation of organic materials and heavy sediment loads that make it difficult for fish to survive. Second, it produces changes in the types of fish that can survive in particular water bodies. Simply put, clean water means game" fish such as trout or bass, and dirty water means rough fish such as carp or buffalo. In general, fishermen prefer game fish. Therefore, pollution control tends to increase the amount of water yielding high-quality fishing relative to that yielding low-quality fishing. Given this view of the benefit-producing mechanisms, one can work toward a methodology for making national benefit estimates based on it. As explained earlier, benefit estimation for environmental improvement requires the understanding of a number of links. For this particular study, the following questions should be considered: • How will implementation of the law affect pollution discharges. by location, quantity, and pollutant type across the entire nation? • How will the pre- and postpolicy discharge levels affect ambient water quality? • Or how does ambient quality change as discharges. • change not only in terms of such familiar indicators as dissolved oxygen, but also in terms of supportable fish population types? • How will increases in total amounts of water supporting recreational fishing and shifts in the composition of that water toward more highly valued fish species affect the number of anglers and the amount of time they spend fishing? • In addition, one needs to be able to value fishing activity of various kinds-that is, for practical purposes, how many days are spent fishing for various species-rough fish versus game fish? The novelty of this study and its main contribution to methodological development lies in the ingenious way it is able to link models together to structure these linkages and how it is able to take existing and newly developed data sets to estimate them quantitatively. We turn now to a discussion of each step in the procedure. II
DISCHARGE REDUCTIONS AND LINKS TO AMBIENT QUALITY AND fiSH
Initially, one must have an understanding of the llfishability" of the
272
National Fresh Water Recreation Benefits and Water-pollution Control
nation's water prior to the implementation of the Federal Water Pollution Control Act. A data base is available from the Fish and Wildlife Service that permits estimates of fishable water by state (the state is the basic geographic unit on which this study is operated), but these data do not provide a basis for the breakdown between rough fish and game fish that is basic to the methods used in this study. For this reason, the researchers surveyed state fish-and-game officials asking them for a breakdown by species category within their own states. Using these data, they found that for the contiguous forty-eight states and the District of Columbia, there are about 30.6 million acres of fishable fresh water consisting of about 20.4 percent cold-water game fisheries, 68.4 percent warm-water game fisheries, and 11.2 per cent rough fisheries. To determine how the implementation of discharge controls would affect the current status requires a knowledge of the amount and location of discharges prior to, and following, the implementation of the 1972 amendments. Then it is necessary to estimate how this change will affect ambient conditions in water courses, and how, in turn, these will affect fishability. The first three kinds of information have been established by the use of RFF's Water Quality Network (WQN) model. This model, designed specifically to answer those questions, was run for four scenarios representing-albeit roughly in some cases-stages in the implementation of the law. In what follows, we will focus on only one of these stages-the Best Practical Control Technology Currently Available. This is for simplicity and also because the quantitative benefits still must be regarded as experimental. The Best Practical Control Technology Currently Available (BPT for short) requirement was to be achieved by all point sources (that is, discharges from confined channels, such as pipes) of wastewater discharge. This unmet goal may reflect where we currently are in our control efforts. At best, the WQN model provides a reasonable estimate of the impact of policy changes on one important aspect of ambient conditions: dissolved oxygen. However, it does not translate directly into fishability. Indeed, making that step is an undeveloped discipline, calling for heroic measures. Fortunately, a fisheries biologist, willing to use his knowledge and skill to survey the literature, developed a set of rules that appear to capture whatever consensus exists on the water-quality conditions appropriate to the survival and reproduction of various fish populations. These rules can be applied to the results of the WQN model to provide estimate~ of the acreages of different kinds of fishing availability by state, and by aggregation, for the nation as a whole. The reader may be struck
National Fresh Water Recreation Benefits and Water-pollution Control
273
by how small the increases in total fishable water are-only about 100,000 acres from a base of more than 30 million. This is because a very large proportion of US fresh waters already was fishable before implementation of the water-pollution law. However, at the same time, it is projected that the waters regarded as unfishable or capable of supporting rough fish only will decline dramatically. This does not mean a proportionate decline in rough fish populations, but rather a large increase in the water that rough fish will share with warmand cold-water game fish. The next step is to devise ways of converting the water-quality results into changes in fishermen's participation in varioas kinds of fishing. Before proceeding, however, it is pertinent to note that what has been discussed so far is not types of research and modeling that are in the usual purview of economics. But the situation here, is reflective of the fact that existing models of natural systems rarely fit the needs of the economist who would estimate the benefits of environmental improvement. Accordingly, he is often forced into disciplinary imperialism. BEHAVIORAL ECONOMIC ASPECTS OF THE STUDY
I now turn to steps in the analysis that are more clearly economic in character. In order to estimate total activity in various types of fishing, the individual fisherman's chain of decision about recreational fishing must be broken down into several logical stages. The first choice is whether to do any fishing at all. The researchers' hypothesis is that the decision of whether to fish is sensitive, among other things, to the opportunity to fish, represented by the quantity of fishable water. The object of this first stage of the research is then to quantitatively estimate how the decision to fish is influenced, in the population at large, by the availability of fishable water. Regression analYSis is the method used to determine the separate influences of availability of fishing opportunity and those other factors that might affect the decision (for example, income or sex of respondent). The indicators of existing availability of fishable water are the statelevel estimates divided by the state population to get 'a per capita measure. This is rather crude, but a more refined indicator was not available at the time. The other data needed for this stage of the research were obtained from a very large survey conducted by the US Department of the Interior, Fish and Wildlife Service. The first or screening stage of this survey was conducted by telephone interview of more than 100,000 households (300,000 individuals). Its primary intent was to determine whether individuals participated in
274
National Fresh Water Recreation Benefits and Water-pollution Control
hunting, fishing, and other recreational activities associated with ~ildlife. The survey also contained information on other pertinent variables such as age, sex, income, and other factors so that it was possible to include them in the regression analysis and control for their possible effects on participation. The dependent variable was the decision to fish or not to fish. Since the availability of fishable water was included among the independent variables, once the coefficients of the equation have been estimated, the size of the availability variable can be changed and the corresponding change in participation calculated. We have seen regression analysis results used in a similar way in other chapters, for example, in projecting the effect of air-quality improvement. So far, all the analysis permits us to do is to project fishing in general as a function of water quality. But since, as I have indicated, different types of fishing (warm-water game fishing, cold-water game fishing, and rough fishing) probably differ in value, we must also be able to project how likely a representative individual is to pick each of these types if he or she does decide to fish. For this purpose, data obtained by the Fish and Wildlife Service in the second stage of the 1975 survey was used. A questionnaire was mailed to more than 50,000 persons who had declared themselves to be hunters or fishermen in the screening stage. For this subgroup, detailed information was gathered on their participation patterns, socio-economic characteristics, and preferences. Data for the fishermen only was used in analyzing the second stage in the decision chain-namely, once a person has decided to fish how likely is he or she to participate in each of the three types of fishing given the availability of water suitable for each type? Because doing some trout fishing, for example, does not rule out doing some bass or rough fishing as well during the course of the year, the regression equa'tions for the type of fishing decision might best be characterized in "some-of" terms. Either a person did some cold-water game fishing or he or she did not. But the individual also might have done some bass fishing. In any case, whether some of a particular kind of fishing was done was hypothesized to be a function of the availability of water suitable for that kind of fishing as well as other characteristics of the participant. The final stage in the decision chain is the decision on how much time (how many days) will be spent in that activity. The same set of survey data was used in the analysis of this question, and regression analysis was the tool for connecting reported decisions on days of fishing to the independent variables, including the availability of water suitable for the fishing in question.
National Fresh Water Recreation Benefits and Water-pollution Control
275
The drift of the analysis is now clear. The steps are as follows: the amount of increase in total fishable water and fishable-type water associated with water-pollution control is given for the nation as a whole from the models of the previous sections. Given this, the results of Stage 1 are used to calculate how much fishing participation will increase in general. Then, the results of Stages 2 and 3 are used to calculate how this increase in participation will be distributed across the fishing types and how many days of increased fishing of each type will occur nationally as a result of the pollution-control policy. The final problem confronted by this research on the benefits from improved freshwater fishing opportunities is how to assign dollar-value benefits (that is, willingness to pay) to the increase in each category of fishing activity. The approach adopted estimated a demand curve for fishing days for each category and used those to calculate the average consumer's surplus per day. The travel-cost method was the technique selected. We now tum to a brief discussion of how it was applied in this study. Recall that the basic assumption of the travel-cost method is that higher costs of access, as reflected in distance from a recreational site, will have the same effect on visitation as an equivalent admission fee assuming zero distance from the site. We presented a very simple example of how this relationship can be used to develop a demand curve by assuming successively higher admissions fees and using information on access costs to estimate their effects on visitation. This establishes points on a demand curve, that is, the relationship of price to the number of visitor-days. The area under the demand curve, is the total willingness to pay of participants for the total number of visitordays to the site, say, a trout fishery. If one then divides the number of visitor-days into this number, one obtains the average willingness to pay for a day of fishing for trout. The researchers who conducted the study collected data from a large number of fishing sites around the country which permitted them, by statistical means, to make exactly such a calculation yielding average willingness to pay per visitor-day for each type of fishery. ESTIMATING NATIONAL WATER-QUALITY BENEFITS
We are now at a point where a national benefits estimate can be made. All the earlier machinations were designed to estimate how many days of increased recreational fishing of each type would correspond to the water-quality changes resulting from a reduction of wastewater discharges corresponding to the implementation of a pollution-control policy. Having these numbers in hand, it is a simple matter to multiply them by average
276
National Fresh Water Recreation Benefits and Water-pollution Control
willingness to pay for a day by fish type and get a total benefit number for freshwater fishing in the United States. When this is done, the following results are obtained for Best Practical Control Technology Currently Available. Valuation Base
Total Annual Benefits over Base (millions)
Low High
307 683
A few words of explanation are needed about the difference between the low and the high estimates. For the low estimate, travel cost is based only on out-of-pocket expenses-gasoline, restaurant food, motels, and others. This is the conventional method. The higher estimate takes account of the fact that the fisherman may also attach a cost to the time it takes to get to the site. For the higher figure, an estimate of this cost is made by attaching average wage rates to the travel time needed to reach the site. Needless to say, large uncertainties attend these numbers and, because of this, they must be regarded as largely experimental. Nevertheless, in view of the heavy costs of the national programme for water-quality improvement they may strike the reader as being quite low. There are several things to be said in this connection. First, the reader should recall that in terms of the availability of fish species the vast majority of the nation's fresh water was already fishable prior to the 1972 Amendments. Second, these estimates are partial in the sense that they consider only the fresh waters and even then they do not include values that may accrue to fishermen from the possible effects of pollution control on the aesthetic aspects of the fishing experience. At present, research is under way to extend the methodology developed in this study to effects of pollution control on marine (saltwater) recreational fisheries, and on both marine and freshwater swimming and recreational boating. METHOD FOR ESTIMATING NATIONAL WATER-QUALITY BENEFITS
The research reported in the previous section was designed to yield national recreational fishing benefits of water-quality improvement. Basically, it used subregions as units of analysis and aggregated t~m by adding up the results. Thus it can be described as a large-scale simulation falling somewhere between a particular site (or micro) study and a national survey that asks respondents directly about their willingness to pay for national programmes of pollution control. This last procedure has been called the "macro" approach. Among other potential advantages of such an approach, two are especially imp,ortant.
National Fresh Water Recreation Benefits and Water-pollution Control
277
First, a randomized national sample of persons can be interviewed which permits the use of well-established statistical procedures to extrapolate the results to the entire population. Second, one can inquire about "intrinsic" or existence benefits as well as user benefits. The second reason invites more explanation. Because the US popUlation politically supports very expensive programs of waterpollution conrol-much more costly than the benefits estimated for recreational users in the previous section-the researchers were led to believe that there must be some form or forms of benefits accruing to persons who do not actually use particular water bodies. We have termed such benefits variously as intrinsic or existence benefits. These benefits may accrue because persons value the options for possible use that are opened to them when water bodies are cleaned up. This type of value, discussed widely in the economics literature, has come to be called the option value. Other intrinsic values may accrue from a sense of national pride or rectitude associated with having clean waters. One of the main conclusions of the research reported in this chapter and in the following one is that intrinsic benefits definitely exist with respect to environmental improvements or maintenance. Moreover, and with the usual caution about accuracy of results, not only do they exist, but they are large, perhaps even larger than user benefits in some instances. Some aspects of water quality make it more appropriate than air quality for an experimental application of the macro approach. Chiefly, goals of our national policy are set out in a manner that would let most of the population understand what they mean in terms of ordinary experience. The objectives are to make all the nation's water fishable and swimmable in successive stages. Furthermore, much of the cost of these programs is to be paid from taxes levied at the national levels (taxes financing subsidies to local governments) so that respondents can be realistically asked how much in added tax burden they are willing to pay for improved water quality across the whole nation. Neither one of these situations holds with respect to air quality, so it would be much harder to pose understandable and realistic alternatives in a national clean air survey. A macro study, then, is potentially useful for doing a benefit-cost analysis for national water programs. It should be noted, however, that it is not a substitute for site-specific studies in other applications. For example, determining whether the benefits outweigh the costs of a programme for water-quality improvement in the Potomac estuary would require a site-specific study.
278
National Fresh Water Recreation Benefits and Water-pollution Control
Research Procedures One problem with national surveys is that they are quite expensive. What made it possible to conduct an experiment with the macro approach, given available resources, was that the researchers were able to add some water-quality questions onto a survey being funded by another source. After the interview for the other survey was completed, the interviewers administered a sequence of benefits questions that had been carefully pretested by researchers on the benefits project. From the respondents' perspective, the two interviews appeared as one long interview. In all, 1,576 personal interviews of a national probability sample of persons eighteen years of age and older were completed. The sample was designed and the interviews were conducted by Roper and Cantril, a national polling firm. A penalty of this add-on approach proved to be that an unfortunately large number of persons failed to complete all of the questions. In part this was bec'ause they came at the end of an already fairly lengthy survey and in part because it was not possible to undertake special training of the interviewers to administer the benefits section. Because of the likelihood of item-response bias (caused by respondents failing to answer individual items), the researchers regard their estimates as only suggestive and warn against accepting them as definitive. The main intent of the experiment was not to develop definitive estimates at this stage but to test whether a macro approach is applicable to an investigation of waterquality benefits. The low response rate presumably can be cured by an improved questionnaire and by training of the interviewers. A study is currently being planned in which both of these elements will exist. THE WATER-POLLUTION LADDER AND VALUE LEVELS
The levels of water quality for which the research team sought willingness-to-pay estimates are "boatable", fishable", and "swimmable" These levels are described in words and depicted graphically by means of a "water-quality ladder". Us~ of these categories, two of which are embodied in the law mandating the national programme for waterpollution control, permitted avoidance of the communications problems associated with describing water quality in terms of the numerous abstract technical measures of pollution (oxygen depletion, for example). Although the boatable-fishable-swimmable categories are widely understood by the public, they did require further specification to ensure that different people perceived them in a similar fashion. Boatable water was defined as an intermediate level between water which "has oil, raw
National Fresh Water Recreation Benefits and Water-pollution Control
?79
sewage and other things in it, has no plant or animal life and smells bad" on the one hand, and water which is of fishable quality on the other. As discussed earlier, fishable water covers a fairly large range of water quality. Game fish such as bass and trout cannot tolerate water in which certain rough fish such as carp and catfish flourish. In pretests, experiments were made with two levels of fishable water-one for rough fish like carp and catfish, and the other for game fish like bass-but a single definition of fishable was adopted as water "clean enough so that game fish like bass can live in it", under the assumption that the words "game fish" and "bass" had wide recognition and denoted water of the quality that Congress had in mind. Swimmable water appeared to present less difficulty for popular understan.ding since the enforcement of water-quality standards for swimming by health authorities has led to widespread awareness that swimming in polluted water can cause illness. Because willingness-to-pay questions have to describe in some detail the conditions of the "market" for the good, they are inevitably longer than the usual survey research questions. Respondents quickly become bored and restless if material is read to them without giving them frequent opportunities to express judgements or to look at visual aids. The questionnaire for this experiment was designed to be as interactive as possible by interpreting the text with questions which required the respondents to use the newly described water-quality categories. They were also handed a card depicting the water-quality ladder which was referred to constantly during the sequence of benefits questions. Willingness-to-Pay Questions and Answers
Questions about willingness to pay should seem realistic to respondents. Accordingly, they were couched in terms of annual household payments in higher prices and taxes because this is the way people do pay for water pollution control. A portion of each household's annual federal tax payment goes toward the expense of regulating water pollution and providing construction grants for sewage-treatment plants. Local sewage taxes pay for the maintenance of these plants. Those private users, such as manufacturing plants, who incur pollution-control expenses ultimately pass much or all of the cost along to consumers in higher prices. Thus, this payment method has a ring of truth to the respondents. As explained earlier "starting-point bias" can be an important problem in bidding games and surveys. That is, a high starting bid from an interviewer may elicit a higher bid from a respondent than a low
280
National Fresh Water Recreation Benefits and Water-pollution Control
starting bid. A major methodological innovation of the research reported in this chapter is the development of a device for eliminating such a bias, the "payment card~" In this technique, the respondent is given a card which contains a menu of alternative amounts of payment beginning at $0 and increasing by a fixed interval until an arbi.trarily determined large amount is reached. When the time comes to elicit the amount one is willing to pay, the respondent is asked to pick a number from the card (or any number in between) which "is the most you would be willing to pay in taxes and higher price each year" (italics in the questionnaire) for a given level of water quality. Thus, the interviewer suggests no bid at all. It turns out, however, that this presents some problems of its own. In initial pretests, it was found that the respondents had considerable difficulty in determining their willingness to pay when a card was used which only presented various dollar amounts. A number of them expressed embarrassment, confusion, or resentment at the task, and some who gave amounts indicated they were very uncertain about them. The problem lay with the lack of benchmarks for their estimates. People are not normally aware of the total amounts they pay for public goods even when that amount comes out of their taxes, nor do they know how much such goods cost. Without a way of psychologically anchoring their estimate in some manner, they were not able to arrive at meaningful estimates. They needed benchmarks of some kind which would convey sufficient information without biasing their responses. Their most appropriate benchmarks for willingness to pay for water-pollution control would appear to be the amounts they are already paying in higher prices and taxes for other non-environmental, publicly provided goods and services. Amounts were identified on the card for several such goods, and further pretests were conducted, indicating that the benchmarks made the task meaningful for most people. But the use of payment cards with benchmarks raises the possibility of introducing its own kind of bias. Are the respondents who gave amounts for water-pollution control using the benchmarks for general orientation or are they basing their amounts directly on the benchmarks themselves in some manner? In the former case, respondents would be giving unique values for water quality; in the latter case, they would be giving values for water quality relative to what they think they are paying for a particular set of other public goods. If the latter case holds and their water-quality values are sensitive to changes in the benchmark amounts, or to changes in the set of public goods identified on the payment card, their validity as estimates of consumer
National Fresh Water Recreation Benefits and Water-pollution Control
281
surplus for water quality are suspect. A test for this kind of bias was conducted in the pretest by using different versions of the payment card with the amounts paid for other publicly provided goods changed by modest amounts. No bias was found, and so the "anchored" payment card was deemed to be a suitable device for the full-scale experiment. Tests were also conducted to attempt ,to discover if any of the other sorts of bias were inherent in the questionnaire. Again, none was found. A final point should be made regarding the payment card. What people actually pay for publicly provided goods varies with their income. To correct for this, four different payment cards were developed corresponding to four income classes. At the appropriate point in the interview, the interviewer gave the respondent the payment card for his or her income category, which had been established by a prior question. As already discussed, the respondents valued three levels of water quality which were described in words and depicted on the water quality ladder. They were first asked how much they were willing to pay to maintain national water quality in the boat~ble level. Subsequent questions asked them their willingness to pay for o~erall water quality to fishable quality and swimmable quality. The average willingness-to-pay amounts given by the respondent for the two higher levels consists of the amounts they offered for the lower levels plus any additional amount they offered for the higher level. The average annual amounts per household for those respondents who answered the willingness-to-pay questions turned out to be: Water Quality Boatable
Fishable Swimmable
Total $
Mar>:inal $
152
152
194
42
225
31
The most substantial benefit is for boatable water. The respondents are willing to give about 20 per cent more for fishable water than boatable water, but only an additional 15 per cent to make the water swimmable. The data also permitted one to make a rough distinction between the type of recreation and the intrinsic values discussed earlier. Since the willingness-to-pay questions measure the overall value that respondents have for water quality, the amount given by each respondent represents the combination of recreational and intrinsic values held by that person. But it was possible to tell from the questions whether a person actually engaged in water-based recreation. It was reasoned that the values expressed by the respondents who do not engage in in-stream recreation
282
National Fresh Water Recreatioll Benefits and Water-pollution Control
should be almost purely intrinsic in nature. In calculating the average willingness-to-pay amount for the nonrecreationists alone, therefore, we get an approximation of the intrinsic value of water quality. By subtracting this amount from the total the recreationists are willing to pay, one can estimate, in a rough way, the portions of the recreationists' benefits which are attributable to recreation and intrinsic values. When this is done, it is found that intrinsic value constitutes about 45 per cent of the total value for recreationists, 100 per cent for the nonrecreationists (by assumption), and about 55 per cent for the sample as a whole. If this is a correct reflection of reality, it is a major find;ng and may have large implications for the future study of benefits from environmental improvement. It was noted earlier that, while the sample of persons interviewed was initially chosen at random, quite a few respondents failed to give usable answers. Any aggregate national benefit estimate based on these data therefore could not be put forward as accurate. Thus, we make such an estimate simply to illustrate that the results of this experiment imply very large values. There are about 80 million households in the United States. Assume that the sample results imply that to have high-quality recreational waters throughout the country there is an annual willingness to pay of $200 per household. This would imply a total willingness to pay of $16 billion. According to the earlier discussion, this would divide about equally between user and nonuser values. At first this might seem out of line with the value of well under the billion dollars that was calculated for recreational fishing. But this is not necessarily the case. Recall that that estimate is for a relatively small increase in the nation's fishable waters over the actual conditions of the early 1970s, and tRat the estimate from the national survey is the value people attach to making and maintaining the whole of the nation's fresh waters of high recreational qllality where the alternative is almost total degradation of most of the nation's water-courses. In other words, both the baselines and the routes of benefit accrual considered are different in the two studies. A somewhat closer comparison, though still not a perfect one, is between the survey's reported willingness to pay for an improvement from boatable to fishable water and the largest value found in the fishing study for essentially complete cleanup (in fishing terms) of the nation's fresh water- roughly $1 billion. The objective of this experiment was not to produce an accurate estimate of national benefits, rather it was to test the feasibility of using a macro approach to the estimation of water-quality benefits. In that, it succeeded.
Chapter 15
Financing Wastewater Management Urban sanitation is a priority issue for cities everywhere. Major deficiencies in the provision of this basic service contribute to environmental health problems and the degradation of scarce water resources. The rapid growth of cities and the accompanying concentration of population leads to increasing amounts of human wastes that need to be managed safely. The relative success in providing cities with usable water has led to greater volumes of wastewater requiring management, both domestic and industrial. As population densities in cities increase, the volumes of wastewater generated per household exceed the infiltration capacity of local soils and require greater drainage capacity and the introduction of sewer systems. Wastewaters flowing out of cities can, in turn, affect downstream water resources and threaten their sustainable use. The mix of problems and the capacity to deal with these sanitation problems varies amongst cities and countries. Confronting these problems requires an ability to face a number of challenges, including different environmental health challenges as well as financial, institutional and technical challenges. THE CHALLENGES OF URBAN SANITATION
The environmental health challenges facing the urban sanitation subsector in developing countries are of two types. First, there is the "old agenda" of providing all urban households with adequate sanitation services. Second, there is the "new agenda" of managing urban wastewater safely and protecting the quality of vital water resources for present and future populations. The relative importance of each agenda normally depends upon the level of development, although these two "agendas" coexist in most cities of the developing world, even in some of the most modem cities.
284
Financing Wastewater Management
Table:1. Economic-Environmental Typology of Urban Sanitation Problems Urban sanitation problems
Lower-Income countnes « US$ 650 per capita)
Access to basic sanitation services
Lower mlddleIncome countnes (USS 650-2,500 per capita) Low coverage, Low access for espeCially for urban urban poor, poor, mainly nonIncreasmg use sewered optIOns of sewerage
Upper middle-Income Upper-Income countnes (US$ countnes (> US$ 6,500 per capita) 2,500-6 500 per capita)
Wastewater treatment
Virtually no treatment
Few treatment IncreaSing treatment Generally hl9h ladlltles, poorly capacity, operational treatment levels, operated deficlenoes major Investments over past 30 years
Water pollution Issues
Health problems from Inadequate sanitation and raw domestiC sewage "n the streets"
Severe health problems from untreated munlopal discharge
Generally acceptable Good coverage; coverage, higher mainly sewered sewerage levels
Severe poHutJon Pnmarlly problems from poorly concerned wrth treated muniCipal and amenity value and mixed Industnal tOXIC substances discharges
BASIC SANITATION SERVICES FOR URBAN HOUSEHOLDS The provision of sanitation services, including sewerage, has not kept pace with population growth in urban areas, Despite this, the significant progress that was achieved by countries during the 1980s has resulted in a 50 per cent increase in the number of urban people with adequate sanitation facilities. These achievements, although impressive, were not sufficient because the number of people without adequate sanitation actually increased by 70 million in the same period, and as many remained unserved as were provided with service. The health consequences of the service shortfalls are enormous and fall most heavily on the urban poor. In most low-income communities, the pollutant of primary concern is human excreta. It has been reported by WHO that 3.2 million children under the age of five die each year in the developing world from diarrhoeal diseases, largely as a result of poor sanitation, contaminated drinking water and associated problems of food hygiene. Infectious and parasitic diseases linked to contaminated water are the third leading cause of productive years lost to morbidity and mortality in the developing world. Diarrhoeal death rates are typically about 60 per cent lower among children living in households with adequate water and sanitation facilities than those in households without such facilities. Looking to the future, the challenge of the next two decades dwarfs the progress made in the past decade; some 1,300 million new urban residents will require sanitation services in addition to those presently without service. In total, this is roughly six times the increase in service provided during the 1980s. Clearly, the aim of providing all urban households with adequate sanitation services still poses large financial, institutional and technical challenges.
285
Financing Wastewater Management
URBAN WASTEWATER MANAGEMENT AND POLLUTION CONTROL A "new agenda" of environmentally sustainable development has emerged forcefully, and appropriately, in recent years. One aspect of sustainable development is the quality of the water environment which is seen as a global concern about sustainable water resources. The situation in cities in developing countries is especially acute. Even in middle-income countries, sewage is rarely treated. Buenos Aires, for example, treats only 2 per cent of its sewage, a percentage that is typical for the middle-income countries of Latin America. There is also the problem of uncontrolled industrial discharges into municipal sewers, increasing organic1oads and introducing a range of chemical contaminants that can damage sewers, interrupt treatment processes, and create toxic and other hazards. Water quality is far worse in developing countries than in industrialized countries. Furthermore, while environmental quality in industrialized countries improved through the 1980s, it did not improve in middle-income countries, and even declined sharply in lower-income countries. The costs of this degradation can be seen in many ways. The vast majority of rivers in and around cities in developing countries are little more than open sewers. Not only do these degrade the aesthetic quality of life in the city, but they constitute a reservoir for cholera and other waterrelated diseases. 14
Low-income Families
Middle-income Families
High-income Families
~ ~6 .0 ~ .l!l
TI
o g. (/) (/)
(5
o
••
4
() ()
co 2
o
-S 0
Fig:1. Dissolved Oxygen Concentrations in Rivers in Developing and Developed Countries The cause of the major outbreak of cholera in Peru in 1991 could be traced to inadequate urban sanitation and water contamination that cost the Peruvian economy over US$ 150 million in 1991-92 in direct and indirect health impacts. Similarly, the otherwise inexplicable persistence of typhoid in Santiago over four decades has been attributed to the pollution of irrigation waters by untreated metropolitan discharges. Energetic emergency measures, taken as a result of the Latin American
2~6
Financing Wastewater Management
dholera outbreak in 1991, prevented the spread of cholera in Santiago and brought typhoid under control with estimated savings in direct and ~ndirect health costs in the order of US$ 77 million. , The costs of urban water pollution also create an additional burden for cities in the form of higher water supply costs. In metropolitan Lima, for example, the cost of upstream pollution has increased water treatment costs by about 30 per cent. In Shanghai, China, water intakes had to be moved upstream more than 40 km at a cost of about US$ 300 million. CONNECTION BETWEEN SANITATION SERVICES AND ENVIRONMENTAL ISSUES
To understand the connection between sanitation services and environmental issues, it is necessary to consider the sequence in which people demand water supply and sanitation services. For a family which migrates into a shanty-town, the first environmental priority is to secure an adequate water supply at reasonable cost. This is followed shortly by the need to secure a private, convenient and sanitary place for defecation. Families show a high willingness to pay for these household or private services, in part because the alternatives are so costly. Accordingly, they pressure local and national governments to provide such services, and in the early stages of economic development much external assistance goes to meeting the strong demand for these services. The very success in meeting these primary needs, however, gives rise to a second generation of demands, namely for the removal of wastewater from the household, then from the neighbourhood and then from the city. As cities succeed in meeting this demand another problem arises, namely the protection of the environment from the degrading effects of such large and concentrated pollution loads. Thus it is no surprise that the portfolio of external assistance agencies has focused heavily on the provision of water supply. For example, World Bank lending for water and sanitation over the past 30 years has only included about 15 per cent for scfnitation and sewerage, with most of this spent on sewage collection arid only a small fraction spent on treatment. In a description of the Orangi Pilot Project in Karachi, Pakistan, Hasan describes how forcefully poor people demand environmental services, once the primary demand for water supply is met, and how it is possible to respond to the challenge of these new demands. THE
Financing Wastewater Management
287
progress on wastewater management and pollution control creates major financial challenges for developing countries. Mobilising the necessary financial resources requires both recognising the need for an urban sanitation subsector and reliance on new ways of financing urban sanitation, sewerage and wastewater management. RESPONDING TO THE DEMANDS OF HOUSEHOLDS AND COMMUNITIES
In recent years there has been a remarkable consensus on marketfriendly and environment-friendly policies for managing water resources and for delivering water and sanitation services on an efficient, equitable and sustainable basis. At the heart of this consensus are three closely related guiding principles expressed at the 1992 Dublin International Conference on Water and the Environment, namely: • The Ecosystem Principle: Planners and policy makers at all levels should take a holistic approach linking social and economic management with protection of natural systems. • The Institutional Principle: Water development and management should be based on a participatory approach, involving user, planners and policy makers at all levels, with decisions taken at the lowest appropriate level. • The Instrument Principle: Water has an economic value in all its competing uses and should be recognised as an economic good. The challenge facing the urban sanitation subsector is to put these general principles into operation and to translate them into practice on the ground. The new consensus gives prime importance to a central principle of public finance, i.e., that efficiency and equity both require that private resources should be used for financing private goods and that public resources should be used only for financing public goods. Implicit in this principle is a belief that social units themselves, whether households, commercial organisations, urban communities or river basin associations, are in the best position to weigh the costs and benefits of different levels of investment. The vital issue in the application of this principle to the urban sanitation subsector is the definition of the decision unit and the definition of what is internal (private) and external (public) to that unit. For each level, the demand for sanitation services must be understood, and each social unit should pay for the direct service benefits it receives. To illustrate the application of this emerging ideal, it is necessary to consider how urban sanitation should be financed.
Financing Wastewater Management
288
SANITATION, SEWERAGE AND WASTEWATER MANAGEMENT The benefits from improved sanitation, and therefore the appropriate financing arrangements, are complex. At the lowest level, households place high value on sanitation services that provide them with a private, convenient and odour-free facility which removes excreta and wastewater from the property or confines it appropriately on-site. However, there are clearly benefits which accrue at a more aggregate level and are, therefore, "externalities" from the point of view of the household. Willingness-to-pay studies have shown consistently that households are willing to pay for the first category of service benefits, but have little or no interest in paying for external (environmental) benefits that they consider beyond their concern.
11~~~~~~~~
...
Country River basin
~~~=:::J Region
=~:r=:::JCity
~=:r~;:::== Neighbourhoo~
~!!~~~ Btock ~ Household
Fig:2. Levels of Dicision-making on Water and Sanitation
At the next level (i.e., the block) households in a particular block value services which remove excreta from the block as a whole. Moving up a level, to that of the neighbourhood, residents value services which remove excreta and wastewater from the neighbourhood, or which render these wastes innocuous through treatment. Similarly, at the level of the city, the removal and/or treatment of wastes from the city and its surroundings are valued. Cities, however, do not exist in isolation - wastes discharged from one city pollute the water supply of downstream cities and of other users. Accordingly, groups of cities (as well as farms and industries and others) in a river basin can perceive the collective benefit of environmental improvement. Finally, because the health and well-being of a nation as a whole may be affected by environmental degradation in one particular river basin, there are sometimes additional national economic, health and environmental benefits from wastewater management in that basin. The example of typhoid in Santiago illustrates the latter point. The fundamental principle of public finance is that costs should be assigned to different levels in this hierarchy according to the benefits accruing at
Financing Wastewater Management
289
the different levels. This suggests that the financing of sanitation, sewerage and wastewater treatment should be allocated approximately as follows: • Households pay the cost incurred in providing on-site facilities (bathrooms, toilets, sewerage connections). • The residents of a block collectively pay the additional cost incurred in collecting the wastes from individual homes and transporting these to the boundary of the block. • The residents of a neighbourhood collectively pay the additional cost incurred in collecting the wastes from blocks and transporting these to the boundary of the neighbourhood (or of treating the neighbourhood wastes). • The residents of a city collectively pay the additional cost incurred in collecting the wastes from blocks and transporting these to the boundary of the city (or of treating the city wastes). • The stakeholders in a river basin (cities, farmers, industries and environmentalists) collectively assess the value of different levels of water quality within a basin and decide on the level of quality they wish to pay for, and on the distribution of responsibility for paying for the necessary treatment and water quality management activities. • The nation, for the achievement of broader public health or environmental benefits, may decide to pay collectively for meeting more stringent treatment standards. Sanitation and Sewerage
Although there are complicating factors to be taken into account (including transaction costs of collection of revenues at different levels and the inter-connectedness of several of the benefits), the principles discussed above are reflected both in the way some industrialised countries finance sewerage investments and in the most innovative and appropriate forms of subsector financing observed in developing countries. In many communities in the USA, for example, households and commercial organisations pay for sewer connections, primary sewer networks are financed by a sewer levy charged to aU property owners along the streets served, and secondary sewers and major collectors and interceptors are often financed by improvement levies on all property owners in the serviced areas. Innovative sewerage financing schemes are now being observed in developing country cities. In Orangi, an informal urban settlement in Karachi, a hierarchical scheme for financing sewerage services has developed in which households pay the costs of their "on-lot" (Le., onsite) services (e.g., latrines and septic tanks), the primary sewers are paid
290
Financing Wastewater Management
for by the households along the "lane" (public passageway between rows of houses), contiguous "lanes" pool their resources to pay for neighbourhood sewers, and the city (via the Municipal Development Authority) pays for trunk sewers. The arrangements for financing condominial sewers by the urban poor in Brazil follow a remarkably similar pattern; households pay for the onsite costs, blocks pay for the block sewers (and decide what level of service they want from these), with the water company or municipality paying for the trunk sewers. Lack of access to credit may impede investment in sanitation, drainage and other essential urban environmental services, especially in small cities and towns. This problem has been overcome in some cases by creating special municipal development funds or rotating funds to finance environmental investments. For example, the World Bank has supported the creation of municipal development funds in the State of Minas Gerais, Brazil, for environmental improvements in small cities and towns, and in Mexico for qmnicipal water supply, sewerage and solid waste investments in intermediate cities. Similarly, poor urban households need mechanisms to finance sewer connections and in-home sanitary facilities. Some cities provide credit to poor households for these investments that can be paid off in instalment payments (not subsidised) over periods of three to five years. Where there are well-managed water and sewerage utilities, the instalment payments can be collected as part of the monthly water bill. In some cases, households can provide "sweat equity" (labour inputs provided by the community for self-help construction schemes) or even make partial payment in the form of construction materials. A special sanitation credit fund has been established in Honduras for poor urban households, fashioned along the lines of the well-known Grameen rural credit bank in Bangladesh. Such experiences show that the urban poor will invest in a healthier environment if they can spread the initial costs over time. Similarly, innovative schemes for providing urban households access to credit for sanitation investments have been demonstrated in Lesotho and in Burkina Faso. Wastewater Treatment
Even when the appropriate financing and institutional principles are followed, very difficult issues can still arise with respect to the financing of wastewater treatment facilities. In industrial countries, two very different models are used. In many industrialized countries, the approach
Financing Wastewater Management
291
followed has been to set universal environmental standards and then to raise the funds necessary to finance the required investments. It is becoming increasingly evident that such an approach is proving to be very expensive and not financially feasible, even in the richest countries of the world. In the UK, the target date for compliance with the water quality standards of the European Union (EU) is being reviewed as customers' bills rise astronomically to pay the huge costs involved. In the USA, US$ 56,000 million in federal construction grants were provided to local governments from 1972-89 to build mandated secondary treatment facilities, but these grants have now been eliminated (and replaced by State revolving funds for loans to municipalities) at the same time that increasingly stringent environmental standards are being proposed. Many local governments are now refusing to comply with the unfunded mandates of the Federal Government. The city of San Diego, for example, has refused to spend US$ 5,000 million on federally-mandated secondary treatment, arguing that it is more cost-effective to use long, coastal outfalls for sewage disposal. San Diego brought suit against the Federal Government and recently won its case in the federal courts. The US National Research Council has advocated a change in which costs and benefits are both taken into account in the management of sewage, with a shift to a water quality-based approach at the coastal zone, watershed or basin level. In a few countries, a different model has been developed. In these countries, river basin institutions have been put into place which: • Ensure broad participation in the setting of standards, and in making the trade-offs between cost and water quality. • Ensure that available resources are spent on those investments which yield the highest environmental return. • Use economic instruments to encourage users and polluters to reduce the adverse environmental impacts of their acti vities. These institutional arrangements are described more fully below. In river basins in Germany and France, and more recently in Brazil, river basin financing and management models are applied in order to raise resources for wastewater treatment and water quality management from users and polluters in the basin. The stakeholders, including users and polluters as well as citizens' groups, are involved in deciding the level of resources to be raised and the consequent level of environmental quality they wish to "purchase". This system has proved to be efficient, robust and flexible in meeting the financing needs of the densely industrialised Ruhr Valley for 80 years, and for the whole of France since the early 1960s.
292
FinaJtcing Wastewater Management
There is growing evidence that if such participatory agencies were developed, people would be willing to pay substantial amounts for environmental improvement, even in developing countries. In the state of Espirito Santo in Brazil, a household survey showed that families were willing to pay 1.4 times the cost of sewag~ collection systems, but 2.3 times the higher cost of a sewage collection and treatment system. In the Rio Doce Valley, an industrial basin of nearly three million people in south-east Brazil, a river basin authority is in the process of being developed. Stakeholders have indicated that they are willing to pay about US$ 1,000 million over a five-year period for environmental improvement. In the Philippines, recent surveys show that households are often prepared to make substantial payments for investments which will improve the quality of nearby lakes and rivers. For developing countries, the implications of the experience of industrialised countries are clear. Even rich countries manage to treat only a part of their sewage, e.g., only 52 per cent of sewage is treated in France and only 66 per cent in Canada. As in the USA, Japan and France, most countries have provided some form of environmental grants to municipalities in order to achieve their present levels of treatment. Given the very low initial levels in developing countries (e.g., only about 2 per cent of wastewater was treated in Latin America at the beginning of the decade) and the vital importance of improving the quality of the aquatic environment, an approach is needed that simultaneously makes the best use of available resources and provides incentives to polluters to reduce the loads they impose on surface and ground waters. An effluent tax is one form of incentive that is used in many countries, ranging from France, Germany and The Netherlands to China and Mexico. It cim be applied to any dischargers, cities or industries, with two benefits; it induces waste reduction and treatment and can provide a source of revenue for financing wastewater treatment investments. The dramatic impact of the Dutch effluent tax on industrial discharges is described by Jansen. The overall industrial effluent loads decreased by two-thirds between 1969, when an effluent tax was first applied, and 1985 (falling from 33 million to 11 million population equivalents). The experience of China in the application of an industrial effluent tax for financing industrial wastewater management improvements has been described by Suzhen. In France and Mexico, the effluent tax is applied equally to municipal and industrial effluents, thus encouraging local investment in municipal wastewater treatment plants. An effluent tax, however, should be used in combination with municipal sewer use charges in order to ensure that industries do not escape paying for their discharges
Financing Wastewater Managemel1t
293
by passing the cost on to the municipality, as well as to ensure that the municipal sewerage authority has sufficient revenues to build and to operate sewerage and treatment works. Community Participation
The aspiration of most urban households, including the urban poor, is to have access to cost-effective and affordable sanitation services via public or private utilities. Consequently, they would be willing to participate, as responsible users, by paying the appropriate service charges. In the cities of many developing countries, however, such services are not yet universally accessible and poor communities must, themselves, get involved in the planning and delivery of sanitation and sewerage options. The examples of the condominial sewer system in Brazil and the , Orangi Pilot Project indicate an important institutional approach to community participation in which a productive partnership is formed between community groups and the municipal government or the utility. Often, such a system involves public provision of the external or trunk infrastructure, which may be operated by either the public or private sector, and the community providing and managing the internal or feeder infrastructure. The link between feeder and trunk infrastructure is essential for the evacuation and disposal of human waste collected by the community, but it is too easily overlooked. Many forms of community participation are possible for the provision of sanitation and sewerage services, such as: • Information gathering on community conditions, needs and impact assessments. • Articulation of, and advocacy for, local preferences and priorities. • Consultations concerning programmes, projects and policies. • Involvement in the selection and design of interventions. • Contribution of "sweat equity" or management of project implementation. • Information dissemination. • Monitoring and evaluation of interventions. Promoting and enabling community participation can take many forms. Where political will exists, governments may promote participation and create the conditions under which communities and households, as well as NGOs and the private sector, can play their appropriate roles. The World Bank-financed PROSANEAR project in Brazil, for example, provides a framework and the resources for municipalities and utilities to experiment with innovative technical and institutional arrangements for providing sanitation services to the urban poor. When such
294
Financing Wastewater Management
government support is absent, alternative approaches have commonly been used to stimulate community involvement and to build the necessary political will. First, NGOs or community-based organizations (CBOs) often playa catalytic role in mobilizing communities and forming partnerships. In one of the largest scale examples involving an NGO, Sulabh Shauchalaya International began, in 1970, promoting the construction of pour-flush latrines in Delhi and other Indian cities, and over a period of 20 years assisted in building over 660,000 private latrines and 2,500 public toilet complexes with community participation and government support. Second, consultations and town meetings are increasingly used as a forum to discuss and agree on environmental priorities, and to propose participatory solutions. Finally, communities may engage in public protests or legal actions as a means of building a constituency of the urban poor, and applying pressure on local governments and utilities for dialogue and acti~n. The Orangi Pilot Project had its origins in the discontent of local residents with excreta and wastewater overflowing in the streets as a result of the failure of the Karachi Development Authority to provide adequate sewerage. A Role for the Private Sector
Financial resources can also be mobilised through the private sector; poor service provision by the public sector often suggests a need for increasing partnerships with the private sector. Private sector participation, however, is only one possible opportunity; it is not a panacea. In situations in which existing sanitation service delivery is either too costly or inadequate, private sector participation should be examined as a means of enhancing efficiency and lowering costs, and of expanding the resources available for service delivery. In deciding whether to involve the private sector, it is important to assess several key factors which have been summarized by the Infrastructure for Development: World Development Report, 1994. Introducing competition is the most important step in creating conditions for greater efficiency by both private and public operators; some services can be split into separate operations to help create contestable markets. The principle of accountability to the public should be maintained through transparent contractual agreements that are open to public scrutiny and should help to minimize risks to public welfare, create real competition, ensure efficiency, and promote self-financing. Paradoxically, public sector capacity may have to be strengthened in order to achieve effective private sector participation which requires public
Financing Wastewater Management
295
sector agencies with sufficient capacity to prepare bidding documents and performance indicators, assess proposed outputs and costs, administer the contracting process, and regulate contract performance. In Mexico, municipalities are granting concessions to the private sector to build and operate wastewater treatment plants, both as a means of financing investments in plants through the private sector and to overcome problems with weak local operating capacity. The Puerto Vallarta wastewater treatment plant was the first of many new plants to come on line in the past few years. An important point to remember in cases such as Puerto Vallarta is that the private sector performs the necessary function of mobilising financing for needed investments, but the investments made together with operations, maintenance and depreciation costs will all have to be recovered through. tariffs charged to domestic and industrial customers. Another innovative example is a concession to 26 industries in the Vallejo area of Mexico City to form a new enterprise, Aguas Industriales del Vallejo, to rehabilitate and expand with its own funds an old municipal wastewater treatment plant, treat up to 2001 s-1 of sewage, and sell the treated water to shareholders at 75 per cent of the public utility water tariff. STRATEGIC PLANNING AND POLICIES FOR SUSTAINABLE SANITATION SERVICES
Applying a strategic planning approach to urban sanitation problems should result in choosing the right policy instruments, agreeing priorities, selecting appropriate standards for service provision, and developing strategic investment and cost recovery programmes. The question of appropriate service standards is a particularly vexing one that, in the end, should be answered by considering user preferences and willingnesstopay. In a large city with many pockets of poverty, service standards are likely to be spatially differentiated because many households cannot afford conventional sewerage without massive government subsidies. The Kumasi Strategic Sanitation Plan provides an example of a differentiated plan matching housing types, income levels and user preference; the plan recommends that sewers be used in tenement areas, latrines in the indigenous areas, and flush toilet/septic tank systems in high income and new government areas. Willingness-to-pay surveys \vere carried out, and the results were used to help define differentiated financing options. Explicit subsidies were targeted to the city's low-income population. Municipal wastewater treatment is a particularly costly and long-term undertaking so that sound strategic planning and policies for treatment are of special importance. The recently endorsed Environmental
296
Financing Wastewater Management
Action Programme for Central and Eastern Europe (CEE), formulated with ilie assistance of the World Bank, recognises that the CEE countries will require a plan to move towards Western European standards over a period of 15-25 years as financial resources become available. Although urban sewerage levels in the CEE are generally adequate, 40 per cent of the population are not, at present, served by wastewater treatment plants. The domestic pollution load represents 60-80 per cent of the combined municipal and industrial organic waste load in many CEE cities. Furthermore, many of the existing plants are currently overloaded, poorly operated and maintained, or bypassed. The following is a checklist of policy questions posed in the CEE Action Programme to be answered before proceeding with municipal waste-water investments: • Have measures been taken to reduce domestic and industrial water consumption? • Has industrial wastewater been pre-treated? • Is it possible to reuse or recycle wastewater? • Can the proposed investment be analyzed in a river basin context? If so, have the merits of the investment been compared with the benefits from different kinds of investments in other parts of the river basin? (Note that a least-cost solution to achieve improved water quality may involve different, or no, treatment at different locations.) • Has the most cost-effective treatment option been used to achieve the desired ambient water quality? • Has there been an economic analysis to assess the b~nefits (in terms of ambient water quality) that could be achieved by phasing investments over 10 years or more? COST-EFFECTIVE TECH NOlOG I ES
Developing country cities are beginning to recognize that poor urban residents cannot afford, nor do they necessarily want or need, costly conventional sewerage. Beyond the dense urban centres, the average household cost of conventional sewerage may range from US$ 300-1,000. This is clearly too expensive for many households with annual incomes well below US$ 300. Fortunately, a broad range of cost-effective technological options are available to respond to the demands of urban consumers beyond the urban centre, with the potential to reduce costs to the order of US$ 100 per household. The lJNDP/World Bank, Water and Sanitation Programme has worked with many countries over the past decade to develop,
Financing Wastewater Management
297
demonstrate, document and replicate many of these low-cost sanitation options. The examples drawn upon throughout this chapter illustrate many of the options available to households (e.g., ventilated improved pit (VIP) latrines in Lesotho, Sulabh pour-flush latrines in India, condominial sewers in Brazil and simplified sewerage in Pakistan), as "Yell as the supporting institutional and financial systems that make possible the wide-scale application of these options. Wastewater treatment technologies also have a wide range of costs. Conventional treatment processes may cost US$ 0.25-0.50 per cubic metre. If nonconventional options can be used, it may be possible to cut these costs by at least onehalf. Promising low-cost treatment approaches, especially for small and intermediate cities,-range from natural treatment systems (such as waste stabilisation ponds, engineered wetlands systems and even ocean outfalls), to decentralized treatment systems (such as are used in Curitiba, Brazil), to new treatment processes (for example, anaerobic treatment processes such as the upflow anaerobic sludge blanket (UASB) reactors presently operating in cities in India, Colombia and Brazil). In large cities, land or other constraints may result in conventional treatment being the most cost-effective approach for achieving the desired water quality objectives, although this should always be a decision resulting from an economic analysis. Lifetime costing should always be used to compare and to choose among treatment options, because operations and maintenance constitute a major share of the costs. CONSERVATION AND REUSE OF SCARCE RESo.URCES
Cornerstone ecological principles for sustainable cities include the conservation of resources and the minimization and recycling of wastes. Translating these principles into urban policies for wastewater management should emphasise the strategic importance of water conservation and wastewater reclamation and reuse in cities. Successful conservation and reuse policies, moreover, need to achieve a balance between ecological, public health and economic and financial concerns. Pricing and demand management are important instruments for encouraging efficient domestic and industrial water-use practices and for reducing wastewater volumes and loads. Water and sewerage fees can induce urban organizations to adopt water-saving technologies, including water recycling and reuse systems, and to minimize or eliminate waste products that would otherwise end up in the effluent stream. In addition to price-based incentives, demand
298
Financing Wastewater Management
management programmes should include educational and technical components, such as water conservation campaigns, advice to consumers, and promotion, distribution or sale of water-saving devices like "six-litre" toilets which use less than half the volume of water per flush than a standard toilet. Wastewater reclamation and reuse is increasingly recognized as a water resources management and environmental protection strategy, especially in arid and semi-arid regions. The use of reclaimed urban wastewater for non-potable purposes, such as in-city landscape irrigation and industry or for peri-urban agriculture and aquaculture, offers a new and reliable resource that can be substituted for existing freshwater sources. Water pollution control efforts can make available treated effluents that can be an economical source of water supply when compared with the increasing expense of developing new sources of water. Conversely, in developing countries only recently embarking on major wastewater treatment investments, reuse has the potential to reduce the cost to municipalities of wastewater disposal. A framework for the economic and financial analysis of reuse projects has been provided by Khouri et al. in a planning guide that integrates economic, environmental and health concerns with agronomic concerns for the sound management of crops, soil and water.
Index A Acid Rain 167, 168, 169, 170 Acidogenesis 13 Activated Sludge 6,32,44,61,64,66, 76, 191, 197, 201, 203, 205, 207 Algae 14, 15, 16, 17, 18, 22, 58, 65, 69, 75,76,77,80,81,94,106,107,116, 190,201,202,238,253 Alternating Double Filtration 115, 119, 122, 127, 135, 137 Ancillary Plant 162 Animal kingdom 77, 80 Application of Low Frequency 146 Aquaculture 1, 212, 237, 238, 239, 240, 241,243,298 Aquatic plants 2,238,239 Aquifer Recharge 197 Areal 16
B Bacteria 4, 5, 7, 8, 9, 14, 15, 17, 18, 22, 25,26,27,28,32,33,34,35,36,37, 38,40,41,42,46,47,48,49,50,56, 67,68,69,70,71,72,73,74,75,76, 118, 119, 120, 121, 123, 125, 128, 147, 148, 149, 150, 151, 152, 153, 193, 194, 197, 202, 203, 204, 205, 269,271 Bacteria and viruses 204, 205 Bardenpho 7, 8 Basic sanitation services 284, 290 Basin management 201 Biofilm 7, 187
Biofilters 6, 7, 183, 187 Biogas 22, 193 Biological aerated filters 187 Biological Principles 108 Biomass 14, 20, 21, 22, 23, 65, 104, 105) 106, 107, 108, 185, 187, 188, 191, 240 Biotic factors 99, 101, 125 Bulking of Activated 72 C
Central planning 226 Ceratophyllum 21 Chemical Factors 64, 65, 99, 100, 125 Chi orella 16, 116 Collection of Sewage 246 Comminutors 3 Community participation 293, 294 Concentrations 5, 13, 16, 17, 18,23,31, 35,36,43,44,47,52,90, 103, 176, 19~201,204,205,206,20~243
Conservation and reuse 297 Contaminents 21 Conventional wastewater 2, 17, 179, 188 Conveyance of Sewage 259 Coontail21 Cost-effective technologies 296 Crop selection 219,226,228,229 D
Daphnia 18, 189 Demersum 21 Depolymerization 194
Index
300 Design Considerations 156 Dewatering 4, 233 Discharge Reductions 271 Disinfection 2,7,9, 10, 189, 191, 192 E
Ecological Operation 42, 150 Ecology 34,42, 65, 87, 98, 99, 100, 108, 109, Ill, 112, 155, 160, 194 Eichhornia 22, 107 Environmental factors 52, 98,99, 100, 101,239 Environmental Protection 19,172,235, 298
F Field management practices 219 Filamentous 18, 33, 35, 58, 61, 66, 67, 71,72,76, Ill, 114 Filter beds 187 Financing Wastewater 283, 292 Fish species 21, 238, 271, 276 Fish yields 241 Fluidized bed reactors 187 Fly Control 154 Food chains 30, 100, 101, 237 Food industry 176 Fungi 25, 28, 37, 56, 58, 61, 63, 64, 65, 71,72,78,79,80,95,104,106,115, 116, 118, 119, 132, 133, 147, 187
G Gambusia 21 Grazing fauna 26, 33, 112, 119, 120, 121, 122, 124, 125, 126, 127, 128, 132, 134, 135, 137, 138, 139, ISO, lSI, 152, 153, 154, 157, 158 Groundwater 20, 170, 171, 172, 173, 174,183, 195, 197, 198, 199,200, 205,211,223,229,292 Groundwater Contamination 163, 170, 171, 173, 174 H
Health related aspects 243
High Rate Filtration 148, 149 Hopper closets 255
I Industrial waste 12, 13, 25, 29, 30, 37, 40,41,44,48,71,107,175,177,179, 186,195,292,296 Irrigation with Wastewater I, 208, 216, • 224
L Lacstris 22 Land and soil management 224 Leaching In, 173, 174, 208, 209, 211, 213,214,220,221,222,224 M
Macrophyte 2, 20, 21, 22, 23, 189, 238, 239 Macrophyte Treatment 20, 22 Membrane bioreactors 188 Metazoa 62, 82 Methanogenesis 13 Method for Estimating 270, 276 Middlebrooks 20 Mines and quarries 176
N National Fresh Water 270 Nitrogen 3, 7, 8, 9, 14, 20, 21, 22, 28, 98, lOS, 106, 116, 125, 162, 167, 169, 186, 190, 194, 197,205,206 Non-motile 16 • Nutrient film Technique 11, 23, 24 Nutrient removal 23, 190
o Oils and grease removal 177 Organic compounds 26, 34, 66, 93, 94, 114, 175, 181, 182, 184, 191, 203, 204 Overland treatment 19 Ozonation 192
301
Tudex
p Package plants 192, 193 Phacus 16 Phosphorus 3, 7, 8, 9, 14, 21, 35, 105, 106, 190, 191, 197, 203, 206, 231, 232 Phragmites 22, 23, 24 Phylum Annelida 85 Phylum Arthropoda 86 Phylum Bryophyta 77 Phylum Mollusca 88 Phylum Nematoda 85 Phylum Platyhelminthes 82 Phylum Rotifera 85 Phylum Thallophyta 75 Phytotoxic 218, 231, 234, 235 Pistia 22 Preliminary treatment 3, 23 Protozoa 28, 43, 48, 56, 57, 58, 62, 63, 64,65,67,68,69,70,71,80,82,95, 98, 109, 111, 117, 119, 125, 185, 187,204,231 Pyrobotrys 16 R
Reaeration 9 Requisite Organisms 33 Research Procedures 278 Respiration 9, 25, 35, 45, 47, 86, 91, 92, 93,96,97,103,104,105,239,240 Responding to the demands 287 Rhizosphere 22
5 Salvinia 21,23 Saqqar 19 SAT system layouts 197 Secondary sedimentation 6, 188 Secondary treatment 4, 5, 6, 7, 9, 20, 185,186,188,190,192,202,291 Sedimentation 3, 4, 5, 6, 7, 9, 17, 20, 21, 177,184,185,188,195,231 Sewage treatment 4, 14, 17, 18, 24, 27, 29, 106, 109, 183, 184, 192, 197,
228,231,232,235 Sewerage 228, 231, 243, 246, 250, 260, 261,263, 284, 286, 287, 288, 289, 290,293,294,295,296,297 Sludge application 234, 235, 236 Sludge disposal 231 Sludge treatment 54,185,231,232,233 Soil requirements 200 solids removal 203 Spirodella 21 Strategic planning 295 Stratiotes 22 Supernatant 50 Suspended solids 2, 3, 4, 6, 7, 23, 49, 153, 176, 178, 186, 187, 188, 197, 201,202,203,216 T
Transfer of Materials 42, 104 U
Urban wastewater 215,285,283 V
Ventilation 128,149,157,158,160,247, 258,263,264
w Wastewater stabilization 11 Wastewater treatment 1, 2, 3, 7, 9, 10, 11,14,17,19,27,34,35,77,80,85, 86, 88, 89, 99, 100, 102, 105, 108, 109, 175, 176, 179, 182, 184, 188, 191, 193, 194, 195, 196, 197,212, 218, 219, 231, 238, 289, 290, 291, 292,295,296, 29 0 298 Water management 219, 220, 285, 287, 288,292,297 Werribee 19 Wetlands 21, 22, 184, 189,297 Z Zooplankton 18,237, 238, 240
