Management of water pollution

CHAPTER 9 MANAGEMENT OF WATER POLLUTION 9.1 Water Pollution Management Approaches 9.1.1 Outline of Existing Models 9.1.2 Wastewater Disposal Options and Receiver Water Quality Problem

CHAPTER

9

MANAGEMENT OF WATER POLLUTION

9.1 Water Pollution Management Approaches 9.1.1 Outline of Existing Models

9.1.2 Wastewater Disposal Options and Receiver Water Quality Problems

9.1.2.1 Disposal Into Lakes 1.2.2 Disposal Into Rivers 1.2.3 Disposal Into Coastal Waters 1.2.4 Disposal on Land

.1.4.5 Formulation of Control Strategies

9.2 Water Quality Models

9.2.1 Introduction 9.2.2 Models for Water Quality in Lakes

9.2.2.3 Fate of Non-Conservative and Phosphorus Pollutants

9 1 Description of the Model

9.2.3 Models for Water Quality in Rivers

9.2.3.1 Critical BOD,, Loading

9.2.3.1.1 Description of the Model 9.2.3.1.2Example

9.2.3.2 Conservative, Non-Conservative and Microbial Pollutants

9.2.3.2.1 Description of the Model 9.2.3.2.2Example

9.2.4 Models for the Quality of Coastal Waters

9.2.4.1 Dilution Through Marine Outfalls

9.2.4.1,1 Description of the Model 9.2.4.1.2 Example

9.2.4.2 Eutrophication in Closed Bays or Harbour Basins

9.2.4.2.1 Description of the Model

9.2.4.2.2Example

9.3 Bibliography

Water Pollution Management Approaches

9.1.1 Outline of Existing Models

The analysis of existing water pollution problems and the formulation of

control strategies through the systems approach described in Section 7.3

is facilitated in practice by the use of appropriate tools In this book

a number of such tools are offered:

The source inventory model presented in Section 4.2.2, allows the as-

sessment of waste load releases, and defines the data requirements from field surveys This model is thus a valuable tool in water pollution inventory studies, not only for computing the effluent loads, but also for providing guidance on the data to be collected

during the field survey work, as well as for organizing and

presenting such data in a concise manner (see also Sections 4.2.3

and 4.2.4) The above constitute key elements in the analysis of

the existing water pollution problems

In addition, the model in Section 4.2.2 provides a fairly compre-

hensive list of the alternative controls for each activity, list-

ing the established control technologies, and allows fast quan-

tification of the impact of alternative contro] measures on

discharged Toads The above provide valuable guidance on the for-

mulation of rational strategies for any given urban or industrial area, as they make it possible to quantify the ensuing overall

pollutant load reductions

Selected models for the quality of lake, river and coastal waters are

given in Section 9.2.2, 9.2.3 and 9.2.4 respectively These models

were selected so as to address the critical pollution problems

generated in the above receivers from the discharge of domestic and industrial effluents, as well as for their ease of use and re- duced data requirements Moreover, the inputs to these models are

streamlined and compatible with the outputs from the inventory

models presented in Section 4.2 and introduced above

As mentioned in Section 7.3, the basic function of such dispersion

models is to translate the effluent loads, as predicted from the

source inventory models under current or strategy conditions, into pollutant concentrations The latter constitute the criteria of effectiveness of any strategy, as they can be compared with the WHO surface water quality guidelines listed in Table J.2-1, or

with the applicable National or regional water quality standards,

such as these listed in Tables I.2-3 to I.2-8 The observation of such guidelines or standards constitutes, as we have seen, the typical objective of water pollution management studies

To ensure compatibility of the model predictions with the water

quality standards when dealing with river or coastal discharges, seasonal variations in flow rates must be considered, for example due to tourist influx, and care must be taken in the selection of

the critical river flow rate or of the critical current velocity and direction for coastal waters

The selection of the critical river flow rates is usually based on

seasonal low-flow rates, but the latter have a statistical distribution due to variations from year to year The design

of assimilation capacities is often based on low flows with recurrence intervals of 5 to 10 years or more Obviously, the longer the recurrence interval the stricter the treatment requirements

The selection of the critical current velocity and direction is

affected by the prevailing dilution mechanism and the defi- nition of a water quality standard Indeed, if the initial dilution is the dominant mechanism, as is normally the case with properly constructed outfall and diffuser systems, the Tower the current velocity the worse the situation If the applicable standard refers to average concentration or to a concentration level which is not to be exceeded a certain percentage (for example 80 %) of the time, as middepth current velocity towards the critical direction one could select the average velocity, or the lower 80 percentile, of

the current velocities respectively

Wastewater Disposal Options and Receiver Water Quality Prob- Tems

9.1.2.1 Disposal Into Lakes

Lakes are the natural collectors of water runoff from large drainage

areas, and often of the direct or indirect discharges from industrial

and domestic sources Lakes are sensitive receivers and eutrophication

is their most common and serious problem

The term eutrophication is a generic one describing in a rather qualita-

tive form the complex state of the lake ecosystem Based on OECD (Ladner

and Wahlgren, 1988), eutrophication is defined as the enrichment of nu-

trients that causes a stimulation of a Jarge number of symptomatic

changes, among which are the augmentation of the production of algae and

macrophytes, the deterioration of the water quality and other changes,

which are negative and interfere with the usage of the affected waters

The eutrophic state: It is characterized by the growth of algae and

other aquatic plants, and depends on the availability of a number

of essential elements Among them, the element existing in the

weakest concentrations in relation to the limiting need for the

Plant growth is the limiting nutrient, and this in the majority of cases is phosphorus The concept of the limiting nutrient may be somewhat simplistic, but it has been found useful for defining the eutrophic state and for designing effective remedial measures

Section 4.2 enables the user to assess the rates of pollutant or- ganic, nutrient and toxic loads entering the lake from the various domestic, industrial and diffuse sources The user needs to be

aware however, that the loads from the latter category, carried through the water runoff, are usually significant, but exhibit a

very wide variation from place to place For this reason, monitor- ing of the water runoff is highly recommended whenever possible

Section 9.2.2.1 provides a lake eutrophication model allowing the

user to calculate the critical phosphorus loading, -based on which the lake eutrophication state can be assessed and, if necessary, remedial action can be formulated

metals, pesticides and other inorganic and organic substances:

They may also constitute a problem if present in significant con- centrations The geological character of the drainage area, the land uses, including the agriculture, the deforestation, and the presence of industrial and domestic effluent sources are some

important parameters that affect influx rates

The liquid waste loads and controls model of Section 4.2.2 pro- vides information about the influx of heavy metals and toxic sub- stances entering the Jake from industrial sources The loads of

relevant substances that may enter through rain run-off need how- aver to be determined through monitoring, in cases where such sources are believed to be important contributors

Section 9.2.2.2 provides a model allowing the user to compute the steady state concentration of conservative substances The latter should be stable, not settle, and not participate appreciably in the chemical and/or biological reactions that take place over the

rather long residence times involved, Examples of such substances

are some stable anions (C1-, $0,~, etc), cations (Nat, Ca‘, Mg**, etc), or non-degradable chlorinated organic substances Heavy met- als tend to participate in biological reactions and a portion of their input loads is absorbed by the biomass and settles on the

bottom Consequently, predictions of heavy metal concentrations

through the use of the conservative substance model can be viewed

as an indication of their maximum possible-, rather than of their actual-, level in the water

Section 9.2.2.2 also provides a model for non-conservative poTTiu-

tants, such as biocides, as well as a model allowing the user to

compute the time required for the lake to shift from one steady state to another after a step change in the loads of conservative, non-conservative and phosphorus pollutants

Suspended Solids: They are present in domestic and industrial effluents,

and, beside the organic and nutrient loads they carry, exert addi- tional harmful effects in the lake environment Indeed, while in suspension, they reduce the penetration of sunlight and affect

fish and filtering species sensitive to the blocking of branchiae,

and when settled, they clog the spawning grounds inhibiting the reproduction of fish Moreover, the accumulated sludge blankets cause asphyxiation to the benthic environment, and constitute sig-

nificant depositories of nutrients, part of which is recycled into

the lake water The loads of suspended solids in domestic and in- dustrial effluents can be computed through the use of the liquid waste loads and controls model of Section 4.2.2

Streams entering the lake transport particulate matter, clays, silts and

mineral particles, which by settling in the Jake, gradually fill it and

transform it into a swamp and finally into a terrestrial system This irreversible process is usually slow, lasting for thousands of years, but high erosion rates due to deforestation can speed up this process significantly and this can be an additional parameter to consider

9.1.2.2 Disposal Into Rivers

Rivers, like lakes, are the natural collectors of rain runoff from large

drainage areas and often of the direct or indirect discharges from in-

dustrial and domestic sources

Rivers are less sensitive receivers compared to lakes, as stratification

does not exist, waters are aerated through their perpetual motion, and

eutrophication in the real ecological sense does not normally occur due

to the limited residence time of the waters Yet, throughout history,

rivers have been the principal suppliers of fresh water to cities,

agriculture, and industry, as well as receivers for their discharges

Thus, while the maintenance of Satisfactory

water quality is a necessity, river pollution has often been a serious problem

Low dissolved oxygen and/or high bacteria concentrations are the most

frequent critical pollution problems for rivers Naturally, the prevail-

ing problem depends in each case on the nature of the sources, and the

characteristics of the stream In relation to the latter, microbial pol-

lution tends to be dominant in rivers with highly turbulent flow and low

water temperatures, as their assimilative capacity for BOD is high, but

the bacteria die off rates and resident times are Tow On the contrary,

low dissolved oxygen tends to be the dominant problem in slow moving

rivers, especially when the water temperature is high, as the oxygen up-

take rates are low, but the microbial die off rates and the resident

times are high

The Dissalved Oxygen (D.0.): The presence of D.O in river water is

essential for the survival of fish and aquatic life, and provides

a refreshing taste in the drinking water Thus its maintenance above a critical level constitutes one of the major objectives of

control strategies Relevant 0.0 standards applicable in certain

countries are listed in Table 1.2-7, while Table 9.2.3.1.1-3 pro- vides minimum D.O values for the survival of certain kinds of

fish

Organic materials entering the river consume oxygen as they decom- pose As the initial consumption exceeds the oxygen uptake from the atmosphere, the D.0 level goes through a minimum before it bounces back (see a typical oxygen sag curve in Figure 9.2.3-1c),

Microbial poljution: This is an other critical parameter, often re-

stricting the beneficial use of river water The abundant and easily identified coliform group of bacteria is widely used as an indicator of the possible presence of enteric pathogens and viruses and their number is often used as a measure of the micro- bial pollution level Relevant standards depend on the water use

The guidelines recommended by WHO for drinking water and the stan-

dards applicable in certain countries for river water used for public supply are listed in Tables [.2-1 and 1.2-7 respectively

Bacteria in the river water are gradually extinguished through

natural die off and their concentration is progressively reduced,

Figure 9.2.3-1d

Nutrients (mainly nitrogen and phosphorus): Their presence affects the

productivity of river waters and the water treatment for down-

stream users Moreover, nitrification may place an additional oxy- gen demand and reduce the D.0O levels It should be noted however,

that the sensitivity of rivers to nutrients is significantly Tower than that of lakes

Chemical pollution: It downgrades the quality of water for beneficial

uses, it reduces the species diversity, while toxic substances such as heavy metals accumulate in the food chain and can affect human health as they reach man through various paths The allow- able concentrations of various pollutants depend on the water use

WHO recommends limits for several substances, Table 9.2.2-1, while relevant standards are set by individual countries

9.1.2.3 Disposal Into Coastal Waters

The marine environment has a significant capacity to assimilate the

waste loads discharged The sea constitutes thus the receiver of choice

compared to surface waters Disposal at sea however, is not without pollution problems if wastes are not properly treated and if the outfall system is not properly designed to provide the necessary dilution

Microbial pollution: This is the critical parameter, in relation to

which the outfall systems are usually designed As in the case of surface waters, the abundant and easily identified coliforms are widely used as indicators of the possible presence of enteric pathogens and viruses

Relevant standards depend on whether the coastal waters are desig-

nated as suitable for bathing or for shellfish culture Bathing in polluted waters can cause infections, while eating of pathogen- contaminated shellfish can cause infectious hepatitis, typhoid and

cholera There are hardly any standards in relation to fishing, and this is rather strange when one considers the magnitude and significance of this activity

against microbial pollution

Suspended Solids (5.S.): Their presence exerts harmful effects on the

marine environment Indeed, while in suspension, they reduce the

penetration of sunlight, become carriers of toxic substances and

pathogenic microorganisms over great distances, and affect fish and filtering species sensitive to the blocking of branchiae When they settle, they clog the spawning grounds inhibiting the

reproduction of fish, and they form sludge blankets causing as-

phyxiation to the benthic environment

BOD and Nutrients: When properly discharged in areas with Open current

Toxic

circulation, little harm is normally caused to the environment, as

the assimilative capacity of the sea is considerable In fact, their discharge in oligotrophic waters may even be beneficial, up

to a certain point, as it tends to increase productivity

However, waste discharged in enclosed bays and harbours with fairly confined waters, may cause serious eutrophication problems

when the critical capacity of the waters is exceeded

Eutrophication problems may also be created in larger water bodies when large discharges are involved and the water recirculation and renewal patterns are not favourable

metals and chemical substances: Their presence can damage the primary and the secondary productivity and disturb higher Tinks in the food chain Persistent substances such as heavy metals can ac- cumulate in marine organisms, particularly in shellfish and affect the health of consumers

Few standards are provided for the concentration of toxic and per- sistent chemicals in the marine environment since the avoidance of

"local hot spots" is not the critical problem from the control

point of view The long term accumulation of such substances over large water bodies becomes the dominant environmental issue and its prevention necessitates stringent Measures, through which lo- cal problems are relieved as well In line with the above, toxic

and persistent chemicals have been classified by UNEP (see for ex- ample UNEP, 1982) in “black” and “grey” lists, Table 1.3-1, the

objective being to eventually eliminate the discharge of the for-

mer and to drastically restrict the discharge of the latter These

objectives, even if partially achievable in the beginning, clearly

define the desirable end target

9.1.2.4 Disposal on Land

Land disposal could be an attractive option in cases where the water is

relatively scarce, irrigable land is close to the treatment plant, and the wastewater is free from toxic substances

The land application methods in use include the slow rate, the overland flow and the rapid infiltration We will focus on the former as it is by

far the most widely used approach and the one that uses the wastewater and the nutrients it contains as a resource rather than as a disposal problem

The land disposal of wastewaters results in water conservation through

reuse, thus saving valuable fresh water for other uses, reduces or elim-

inates the pollution of surface waters, especially during the low-flow

summer period, and can increase the crop yield, due to the presence, not only of nitrogen and phosphorus nutrients, but also of other fertilizing substances and "growth factors" Along with the above advantages, poten-

tial land pollution problems and health hazards exist, and should be carefully considered:

Pathogen transmission: The pathogenic bacteria and viruses can survive

in moist soils and on crops irrigated with wastewater from several

days to a few months, depending on the soil conditions and on the protection provided by the crops themselves During these periods

i1Iness can be transmitted through a variety of pathways:

Eating raw vegetables and salad crops, or even animals or products from animals grassed on sewage-irrigated pastures, has been held

responsible for several diseases, including outbreaks of cholera

and typhoid in the past Hence, the recommendation to avoid grow- ing vegetables and fruits that may be eaten raw

Sewage farm workers may also be affected by handling wastewater and crops or by walking barefoot in sewage farms Hookworm and other enteric infections are the most common

Aerosols produced during irrigation, especially through the use of

sprinklers, convey viruses and pathogenic bacteria over a fair distance, one kilometer or more, contaminating neighbouring crops and infecting people through inhalation

Table 1.4-1 provides relevant guidelines proposed by WHO These

depend on the type of irrigated crop and on the groups of people

affected

Organic loads: When applied excessively they can cause anaerobic

conditions in the soil, odour nuisance, and reduction of crop

yield

teaching and bacteria degradation are two mechanisms by which or- ganics are removed from the ground, the latter being dominant in

most cases For the degradation pracess to proceed under aerobic

conditions, a sufficient residence time in the soil is required,

and this places limits on the organic rates which can be applied

Thus, along with the soil watering Timits, BOD, application limits apply as well, and the most limiting of the two controls the land area requirements,

Dissolved Solids (TDS): Excessive loads and/or extremely low rain- fall leads to high TDS accumulation in the ground, which can cause

substantial reductions in the crop yield

Under normal conditions the presence of TDS in irrigation causes

ho problems as they are readily transported to leachates or to the

groundwater table

Metals: These are immobilized through ion exchange mechanisms and

are retained in the ground Human health can be affected by plants picking up heavy metals through the roots or directly by the fo- liage and passing them to the food chain Reduced crop yields may also result due to the phytotoxicity of the accumulated heavy met- als Heavy metal concentration standards have been promulgated by various authorities and the length of time for wastewater-irri- gated soils to reach the recommended heavy metal Toading limits

have been computed (see for example Papadopoulos at al, 1991)

The general conclusion from the relevant studies is that the ca- pacity of the soil in relation to heavy metal discharges is very limited, and as a result, wastewaters containing heavy metals, other than in trace amounts, should not be considered suitable for land irrigation

9.1.3 Analysis of Available Contro? Options

For the control of liquid wastes, as for the control of air emissions, a

variety of possibilities exist The use of treatment technologies to

reduce the strength of the wastes is a readily sought option However,

other measures, notably institutional measures and controls at the

source, may offer far more attractive options and profoundly affect

subsequent strategy formulation and for this reason they should not be

overlooked,

9.1.3.1 Institutional Measures

As institutional measures are those that set the stage for the control

of wastewaters at a Tevel higher than that of the control at the source

and the treatment installations These measures include the type and

design of the wastewater collection system, the combined or separate

treatment of industrial and domestic effluents, and the selection of the

wastewater receiver(s) As such they are important for they set bounds

for the attainable overall control efficiency, they exert a significant

impact on the treatment plant efficiency-, cost-, and reliability-

requirements, and they control, to a large extent, the magnitude of the

effluents’ impact on the environment Hence, they need to be considered

carefully in the context of every contro] strategy

The type and the design of the collection system sets a number of impor- tant parameters that profoundly affect the control possibilities and the

ensuing management strategy:

A separate rather than combined collection of wastewaters/rain

run-off from urban areas, relaxes the design requirements for the wastewater treatment facilities, avoids the acute stormwater over- flow pollution problems (see for example Smith and Eilers, 1978),

and sets the stage for urban water run-off controls, should this

become necessary

A collection system with multiple effluent discharge outlets or with frequent overflows makes the controls expensive and limits

the attainable overall system efficiency

A properly designed system serving industries with compatible ef-

fluents, so as to enable combined treatment with domestic efflu- ents, can be highly advantageous as discussed below

The selection of combined instead of separate treatment of industrial and domestic effluents may offer the following distinct advantages, pro-

vided that plants with compatible effluents, size and discharge patterns

are involved:

Lower costs due to centralised large-scale treatment;

Increased reliability for the treatment of the industrial wastes;

Better handling and disposal of the sludges produced, provided that no toxics are involved;

Greatly reduced monitoring and enforcement requirements

An industry site plan is deigned to control water pollution by directing industrial activity to areas where the existing receivers exhibit reduced sensitivity and/or by offering improved possibilities for treatment

The latter is particularly important in cases where pollution problems originate from small industries In such cases it 1s often impractical

to implement successful control programmes, because the small size of plants makes it uneconomical while the Jarge number of plants makes it unenforceable The solution in this case is to direct certain types of new plants and to progressively attract some of the existing plants into organized industrial zones or parks offering combined wastewater pretreatment and/or treatment (Economopoulos, 1985) More specifically:

In urban areas, numerous small plants with toxic wastes, such as elec-

troplating shops, tanneries, or acid battery manufacturing indus- tries, discharge their wastes into the sewerage system The smaller of these plants cannot economically pretreat their wastes,

Approaches for Consideration in Formulating Environmental Control Strategies

yet they are often responsible for the bulk of the toxic substances in the municipal wastes

In relation to this problem one could consider the creation of in- dustrial parks in strategic locations within the city, which can accommodate small plants with compatible toxic effluents These, through proper incentives, could attract all new plants, and as

the market and infrastructure develop, most of the existing plants

as well, The satisfactory pretreatment of the combined industrial

wastes becomes this way less expensive and easily enforceable,

In rural areas, unplanned scattering of industry can lead te widespread

pollution problems, which are very difficult to control Creation

of industrial zones with good infrastructure to accommodate all

new industries in an orderly way, and to progressively attract some of the existing ones, could provide more effective control through combined wastewater pretreatment and central

treatment facilities

Formulation of a rational policy for the relocation of certain types of

existing plants is a complex issue for it involves not only relocation

and waste treatment cost assessments, but also consideration of possible

simultaneous technology modernization, marketing and transportation of

products, transferring of personnel, etc In view of the above and of

the potential financial impacts, a good relocation policy should be a

long-term one and developed as part of a comprehensive planning policy

The former can help to relieve current problems, while the latter is a

prerequisite to an effective national water pollution control

The selection of the receiver, in cases where multiple choices exist,

determines to a large extent treatment efficiency and

reliability requirements, and also, which particular receiver will be affected and

Whether the wastewater will be reused or not Proper receiver selection

may involve the by-passing of a sensitive lake or an enclosed bay, or

the development of a plan for the rational distribution of the effluents

among multiple receivers (e.g on land and river disposal)

9.1.3.2 Controls at the source

The reduction of the volume and/or strength of wastes generated by a source represents, in many cases an approach which is not only environmentally responsible, but is also economically sound Moreover,

the relevant solutions tend to be by far the simplest and the fastest to

implement

Domestic Sources: A number of substances in consumer products inevitably find their way into the municipal sewage Reduction or elimination of the compounds that are harmful to the environment, can be achieved

either through treatment of the wastewater or through the replacement of products that generate them The latter presents intriguing advantages

that have hardly been exploited so far One of the best known measures

in this category, is the replacement of phosphatic detergents used in the study area by non phosphatic ones This is in fact a very effective and easily enforceable way of reducing the phosphorus loading from domestic effluents UNEP is currently preparing guidelines on this

important subject - control of pollutants at source

Industrial operations and services: Processes designed from the start to minimize the generation of waste volumes and loads can be the most ef- fective in this regard However, significant benefits can be obtained

from existing plants as well, through operating and/or process retrofit

changes, as these can considerably reduce the ensuing treatment costs

Some particular possibilities and opportunities for waste volume and/or strength reduction are discussed below:

1 Selection of Low Waste Processes is often a critical parameter for the waste volume and loads produced For example:

In Chlor-Alkali plants selection of the diaphragm cell process in-

stead of the mercury process reduces the waste production and eliminates the harmful discharges of mercury

In petroleum refineries the choice of water-cooled condensers over

barometric ones goes a long way to reduce the oil loads and the volume of the wastes Moreover, the choice of air-cooled con- densers effectively eliminates all relevant wastes

In electroplating plants, replacement of the cyanide baths with acid baths eliminates cyanide ions from the effluents In addi- tion, sand blasting instead of acid pickling of metals eliminates the acid wastes that also contain heavy metals

Selection of the most appropriate process is always possible when designing new plants, but it may be economically attractive with ex- isting plants as well Measures such as the replacement of the baro- metric condensers with water cooled ones in petroleum and in lub oi]

refineries, or the replacement of the cyanide with acid baths and the metal pickling with sand blasting in electroplating shops, are

easy to implement in existing installations

waste volume of about 80 % and of the BODs and SS of up to 40 %

Similar magnitudes are obtained in petroleum refineries

Counterflow washing can reduce the waste volumes up to 2.5 times

when 3 baths are used in series This technique is suitable for

electroplating and for acid pickling installations

Proper design and especially careful operation of the plant pro- cesses can minimize water consumption and loss of raw materials and products, with clear environmental and economic benefits

3 By-product recovery from wastes drastically reduces waste loads, but

also generates revenue from the products recovered (0vercash, 1986)

For example:

The recovery of starch through the use of hydrocyclones from potato pealing and washing plant wastes, reduces BOD, loads by 50

%, and from potato processing plants by 40 % Moreover, the

revenue generated from the starch produced balances to a large ex- tent the costs of recovery

The recovery of proteins from yeast-production wastes reduces BODs loads by 97 % In comparison, secondary treatment yields lower removal efficiencies at a higher cost

Chromium recovery from tannery wastes reduces the chromium loads

by 60 % and the recovered product is directly recyclable in the

process

4 Separation of very strong wastes considerably enhances the potential

for reducing polluting loads and/or treatment costs For example:

Separate collection of used lub oils by service stations yields a valuable recyclable product while eliminating the extremely

harmful oi] and heavy metal discharges

Collection of the spent electroplating baths reduces heavy metal

discharges from electroplating operations by about 30 % and this

measure is applicable even in small shops, for which wastewater

treatment may not be economically feasible

Separation of the strong from the weak wastes offers the possibil- ity of achieving high treatment efficiencies at low cost This is especially true in cases where the weak wastes can be directly discharged without prior treatment, as it is much more economic to treat small volumes of high-strength wastes than high volumes of diluted wastes

9.1.3.3 Treatment Technologies

Wastewater treatment technology aims to reduce the strength of wastes

making them suitable for reuse (e.g land application) or for safe

disposal into the receivers

The applicable technologies and the combination of processes depend very

much on the nature of the wastes and their discharge mode, as wel] as on the desirable system efficiency in relation to critical pollutants The

liquid waste inventories and controls model of Section 4.2.2 lists, for each kind of source, commonly employed treatment methodologies and

allows the assessment of their efficiencies

Treatment efficiency requirements constitute a key parameter for the

selection of the plant configuration which best serves the needs of a particular application This however, is not the only parameter: other important ones also need to be considered Among these the cost of construction and operation, and the operational reliability, both of which depend on local conditions, are perhaps the most important, as they inevitably interfere with the choice of treatment processes in every new installation

Municipal, predominantly domestic, wastes represent the most common type

of effluent, and for this reason their treatment is fairly well stan- dardized and of particular importance in water pollution management

Table 4.2.2-2 provides some qualitative information about the expected performance efficiencies of certain commonly used primary, secondary and

tertiary treatment processes Furthermore, Tables 9.1.3.3-1 and 9.1.3.3-

2 provide qualitative guidance regarding the relevant cost and perfor-

mance reliability of secondary (biological) treatment processes

The information in Tables 4.2.2-2, 9.1.3.3-1 and 9.1.3.3-2, covers the three predominant parameters that, as mentioned previously, affect the

configuration of every treatment plant Naturally, the eventual configu- ration is normally the product of a detailed analysis, however a good idea can be obtained at the strategy formulation stages through consideration of the listed information along with the local conditions

and nature of problems

Waste Stabilization Ponds

High Area, m@/capita: 2-4 (warm) & 4-12 (temperate)

Facultative Aerated Lagoons

Area, m@/capita: 0.15-0.45 (warm) & 0.45-1.00 (temperate)

Extended Aeration Systems

Area, m@/capita: 0.25-0.35 (warm) & 0.35-0,65 (temperate)

ý Conventional Activated Sludge

Area, m@/capita: 0.16-0.20 (warm) & 0.20-0.40 (temperate)

Low Trickling Filters

Area, m*/capita: 0.16-0.20 (warm) & 0.20-0.40 (temperate) Cost of Construction (Excluding Land Cost):

Extended Aeration Systems High Equipment; „ Aerators, recycle pumps, sludge serapers

Economy of scale : Considerable

Conventional Activated Sludge

Equipment : Aerators, recycle pumps, scrapers, thickeners,digesters,driers

Economy of scale’: Small

Trickling Filters

Equipment : Effluent distributors Economy of scale’: Small

Facultative Aerated Lagoons

Equipment ; Aerators only

v Economy of scale’: Very small

Waste Stabilization Ponds

Low Equipment: Nil

Economy of scale”: Very small

Qverating Cost:

Conventional Activated Sludge

High Operation: Skilled operators / tight contro]

Sludge Handling: Drying beds or Mechanical dewatering

Power Requirements: 12-17 kWh/persan/year

Extended Aeration Systems

Operation: Skitled operators Sludge Handling: DigestiontDrying beds/Filters Power Requirements: 13-20 kWh/person/year

Trickling Filters

Operation: Relatively simple Studge Handling: Digest ton+Drying beds/Filters Power Requirements: Low

Facultative Aerated Laqoons

Operation: Very simple Sludge Handling: Manua] desludging every 5-10 years Power Requirements: 12-15 kWh/person/year

v Waste Stabilization Ponds

Operation: Simplest (Part time operator for smaller units) Low Sludge Handling: Manual destudging every 5-10 years

Power Requirements: © kWh/person/year

* Sensitivity of Unit Cost (construction cost per capita) to the Size of Population Served

Table 9.1.3.3-2 Comparison of the Operating Reliability of Biologi-

cal Treatment Processes

Adaptation to Organic and Toxic Shock Loads:

Facultative Aerated Lagoons Trickling Filters

Extended Aeration Systems

Low Conventional Activated Sludge

| Waste Stabilization Ponds

Suitability for Intermittent Operation:

Facultative Aerated Lagoons Trickling Filters, Biodiscs

Extended Aeration Systems Low Conventional Activated Sludge

I Waste Stabilization Ponds

Independence from Operators’ Training:

Facultative Aerated Lagoons Trickling Filters, Biodiscs

Extended Aeration Systems

Low Conventional Activated Sludge

I Waste Stabilization Ponds

9.1.4 Synthesis of Rational Pollution Control Strategies

For the formulation of rational pollution control strategies one has to

carefully consider the particular sensitivities associated with each type of waste receiver These impose definite priorities, distinctly

characteristic of the receiver type, in the control of particular

pollutants and define, to a large extent, the severity of the wastewater treatment requirements Thus, the choice of the particular receiver sets

a rather rigid framework for the ensuing management strategy The following Sections, 9.1.4.1 to 9.1.4.4, focus on receiver sensitivities and analyse the associated management framework

In Section 9.1.3 we have seen that through the available control? options

the user may be able to select wastewater receivers, and that, once the choices are made, several alternative control options are at his disposal for addressing their sensitivities and for mitigating the ad- verse consequences of the effluent discharge into them

Based on the above it would appear that for the formulation of a ratio- nal strategy one needs to thoroughly review the particular sensitivities

9-18 Approaches for Consideration in Formulating Environmental Control Strategies

associated with each receiver along with the available control measures

(institutional, controls at the source and treatment technologies) and

to formulate a strategy that carefully balances the entire picture This

subject is discussed in Section 9.1.4.5,

9.1.4.] Management of Lake Pollution Problems

For the control of take pollution there are three basic approaches: One

is to reduce the incoming polluting loads, the other is to alter the hy-

draulic state of the Take, so as to increase the system tolerance to the

incoming loads without adverse effects, and the last one is to minimize

the adverse effects by manipulating the produced biomass, so as to hin-

der the recycling of the nutrients in the water body

Reduction of the incoming pollution loads: This constitutes the most di-

rect approach and includes some of the most frequently employed control

measures The choice of the particular set of measures depends on the

nature, the location and the relative significance of each contributing

source Some of the measures, such as the industrial and/or domestic

wastewater treatment systems are well established, others however, espe-

cially those dealing with the control of rain runoff need special atten-

tion as the volumes may be large and the relevant design experience may

not be as extensive (U.S EPA, 1991) Some of the most important

measures in the present load reduction category are analysed below:

Diversion of the incoming nutrient-rich effluents: This, whenever possi-

ble, is clearly the most effective way to eliminate the influx of

pollutants, especially from industrial and domestic sources The diverted effluents could be discharged downstream of the lake, to

a river, an estuary or to the sea, where the sensitivity of the receiving waters is reduced They could be used also for irriga- tion in cases where the land use, the climate, the nature of the wastewater and other conditions permit

Control at the source: This offers important and advantages and is often

the essential measure in load reduction:

Encouraging the use of non-phosphatic detergents in communities, the effluent of which find their way into the lake, is a rela- tively simple and fast to implement measure, yielding significant reduction of the phosphorus loads from domestic effluents at Tow

or negligible cast

Control of the nutrient loads in the water run off from the drainage basin can be effective, especially in arid or semi arid

areas where soil arosion is significant Selective forestation,

especially in steep slopes and along the river banks, appropriate soil use in agricultural areas, restrictions in the use of fertil- izers, or controls in the cattle pasture within the drainage basin are some of the relevant measures

Control through the use of appropriate technologies: This kind of

control is frequently practiced and can, in many situations, significantly reduce the total organic, nutrient and toxic Toad inputs into the Jake

Control of industrial and/or domestic effluents is often neces-

sary, and typical control options available for each kind of

source are provided in the liquid waste inventories and controls model of Section 4.2.2 The need for increased reduction of nutri- ents in the case of discharges into lakes, dictates to a large extend the kind of treatment techniques to be used, shifting the

technologies, which are particularly effective in this regard

Control of nutrient-rich rain runoffs through pre-reservoirs serv- ing as bioreactors, may be effective for removing a portion of the nutrients, as well as of silt Alternatively, control through

physical and/or chemical methods can be used for the removal of

phosphorus

Modification of the hydraulic situation: This approach is not always

feasible, but offers important possibilities in certain cases Addition

of external water of good quality, if available in large volumes from

nearby surface or underground sources, can increase the critical phos-

phorus loading, Equation (9.2.2.1-1) and Figure (9.2.2.1-1), with obvious effects on the eutrophic condition of the Take The anticipated positive effects can be further enhanced if the removed water is si-

phoned from the nutrient-rich hypolimnion and especially if this opera- tion is undertaken selectively, e.g at the end of the stratification

Removal of the produced biomass: This, through the periodic harvesting

of aquatic plants and fish, eliminates the nutrients fixed in them and prevents their recycling Macrophytes grow from the bottom in shallow lakes, or may float on the surface The mechanical removal

of nutrients may adversely affect the wild life living in it, while the collection of the macrophytes may increase the algal production in the take

Dredging of sediments: This may be an expensive proposition as a nu-

trient removal measure However, selective dredging around wastewater outfalls, in areas with thick deposits of accumulated sludge, may speed up the lake recovery process after appropriate control action has been taken

9.1.4.2 Management of River Pollution Problems

Control of river pollution is mainly achieved through the appropriate

treatment of domestic and/or industrial wastes Through such controls

significant reductions are possible in the loads of important pollutants

such as BOD, bacteria, suspended solids and toxic substances

Dissolved Oxygen (D.0.): This can be maintained above the desirable

level by controlling the organic loads discharged, as the decompo-

sition of the latter depletes the oxygen supplies of the water

Domestic and industrial sources are often the major contributors

to the organic loads discharged into rivers

The effluent loads and control model of Section 4.2.2 allows as- sessment of the organic Toads generated by each major source and the efficiency of the alternative control options The water qual-

ity model shown in Section 9.2.3.1 yields the critical (maximum

allowable) BOD load, which can be discharged into a river so as to maintain the D.O above the desirable level Application of the this model allows the user to assess whether a problem is likely

to exist in a given situation and/or to compute the required BOD removal efficiency from the treatment system(s)

Microbial pollution: Microorganisms, some of which are pathogenic, along

with viruses and helminths (parasitic worms) are abundant in do- mestic wastewaters, but also in the effluents from certain indus- trial operations (e.g animal farms, slaughterhouses, tanneries)

These can be controlled, up to a certain point, through wastewater treatment installations

Raw domestic wastewater is often assumed to contain about 108 col- iforms per 100 ml and suspended or attached growth biological treatment processes remove roughly 90% of them Chlorination, if applied, may be effective in reducing the total coliforms, but many pathogenic bacteria and viruses are little affected and the coliform count loses its value as an indicator As the allowable coliform presence for various river water uses is of the order of

50 to 5,000 per 100 mI (Tables I.2-1 and 1.2-7), reductions of several order of magnitudes remain to be achieved through the ini- tial dilution and the natural bacteria die-off process that takes place within the river For a 99 % reduction due to natural col- iform die-off, 24 to 48 hours are usually required, and this may render large stretches of river downstream from a major discharge unsuitable for drinking water and other uses (see example in Sec- tion 9.2.3.2.2) A model enabling the user to assess the level of microbial pollution in river waters is provided in Section 9.2.3.2

Use of stabilization ponds for the treatment of domestic wastewa- ter in temperate and hot climates may be advantageous from the mi- crobial pollution control point of view, as they can be designed for very high bacteria removal efficiencies, especially during the hot summer months, where the dilution is normally minimum due to seasonal low-flow conditions

Nutrients (mainly Nitrogen and Phosphorus}: Domestic and certain indus-

trial wastes, as wel] as rain run-off from urban and agricultural

areas are major sources of the nutrients discharged into rivers

Control at the source includes encouragement of the use of non-

phosphatic detergents This measure can significantly reduce the phosphorus loading in domestic effluents and be applied easily

with no significant costs It may also include controls in the ap- plication of fertilizers in the adjacent agricultural lands

The conventional biological wastewater treatment processes have rather limited nutrient removal efficiencies The effluent loads and control model of Section 4.2.2 allows assessment of the nutri- ent loads generated by such sources and provides some information

about the efficiency of the treatment plants If high nutrient re-

moval efficiencies are desired, physicochemical or tertiary treat- ment technologies, which are particularly effective in this re- gard, should be employed

Toxic and other chemical substances: Industries are often the major

source of such substances, but water run-off from urban areas may also be a significant contributor The allowable concentrations in the river water depend on the intended water use and limiting val- ues for several substances are set by WHO (Table I.2-1), as well

as by individual countries (Tables 1.2-2 to I.2-4, and 1.2-6)

The effluent loads and control model of Section 4.2.2 provides some information on the loads generated by individual sources and

on the efficiency of the treatment installations Industrial wastewaters with toxic and chemical substances are best treated at the source, before they are mixed with other wastes and get di- 1uted

The water quality model of Section 9.2.3.2 allows the user to as- sess the concentration of conservative substances, such as metals

9.1.4.3 Management of Coastal Water Pollution Problems

Pollution management includes treatment of industrial effluents

containing toxic substances at the source, primary treatment of domestic

and industrial wastes to remove suspended solids {S.5.) and part of the

organic loads, as well as proper design of the outfall systems to provide adequate dilution Only when large and/or multiple discharges are involved, or when effluents are discharged in enclosed bays or harbours with restricted water renewal, does secondary wastewater treatment need to be considered

Microbial pollution: This is normally the critical parameter for domes-

tic waste water discharges into the sea, and as such it controls

the design of the outfall systems

Primary and secondary wastewater treatment plants have, with the

exception of stabilization ponds, rather limited efficiency in

removing coliforms Chlorination, if applied, may be effective in reducing total coliforms, but viruses and many pathogenic bacteria are much Jess susceptible and the coliform count loses its value

as an indicator Moreover, chlorination yields chlorinated by- products, such as chloramines produced from the ammonia in effluents, which are themselves toxic to marine fauna

Based on the above, one has to rely on properly designed outfall

systems to provide the necessary dilution so as to restrict microbial pollution within the acceptable limits for the intended water use Direct water discharges without outfalls tend to render

extensive coastal stretches unsuitable for bathing or shellfish cultivation (Section 9.2.4.1 and Table 9.2.4.1.1-2) The frequent

lack of such outfalls indicates that the magnitude of the problem

is not fully recognized

The water quality model of Section 9.2.4.1 allows analysis of a particular situation in order to assess whether a problem is likely to exist It also allows the preliminary design of proper outfall systems, which, in tandem with the treatment plant, ensure

that the applicable standards are met

Suspended Solids (SS): Their adverse environmental effects make the

primary treatment of wastewaters containing appreciable amounts of

SS, such as the domestic wastewaters, highly desirable Only in

exceptional cases, where strong currents prevail and the deposi-

tion of the 5.5 is prevented, may sedimentation be unnecessary

The liquid waste Toads and controls model of Section 4.2.2 yields the SS loads of domestic and industrial effluents and provides the

removal efficiency of treatment installations

The BOD and Nutrients: The assimilative capacity of the sea is

substantial, but eutrophication problems may arise in cases where

effluents from relatively large sources are involved and the

receiving area is enclosed reuslting in restricted water renewal

tertiary, treatment may be required

The liquid waste loads and controls model of Section 4.2.2 allows the computation of the BOD, nitrogen and phosphorus loads from do-

mestic and industrial effluents and indicates the efficiency of

the alternative control options

The water quality model presented in Section 9.2.4.2.1, allows the

assessment of the critical organic loading for domestic sewage discharged into confined coastal areas This model may be used as

a screening tool for identifying bays and harbours with potential eutrophication problems, as well as for conveniently assessing the required efficiency of wastewater treatment plants receiving

effluents from small or medium size cities, in cases where no

other information is available

Toxic metals and chemical substances: Their major source is often

industrial activity The liquid waste loads and controls model of

Section 4.2.2 provides information on the toxic pollutant loads generated by individual sources and on the efficiency of alterna- tive treatment installations Industrial wastewaters with toxic

and persistent substances are best treated at the source, before they are mixed with other wastes and diluted

9.1.4.4 Management of Land Disposal Problems

Prerequisites for the successful land disposal of wastewaters are the availability of irrigable land reasonably close to the treatment Plant and wastewater free from toxic substances Wastewater from domestic sources, or from certain industries such as slaughterhouses, canneries, dairies, sugar factories, breweries, alcohol distilleries, soft drink plants etc has been successfully used for land irrigation

The basic requirements for a wastewater disposal system is high reli- ability, reduction of coliforms and maintenance of acceptable irrigation rates Agronomic measures (proper irrigation methods, control of the type of crops produced, and protection of the health of sewage farm workers) are also important More specifically:

Pathogen transmission is a problem in cases where domestic sewage is

significantly reduce the associated health risks

Table I.4-1 provides relevant quidelines proposed by WHO, and lists typical suitable technologies Considering that in raw mu- nicipal sewage the number of intestinal nematopodes eqgs per lit- ter is about 800 and that the fecal coliforms are about 0.46*108 per 100 ml, the efficiency requirements for the treatment plant are high The safest approach for achieving the required standards

is through the use of stabilization ponds, which can be designed

for very high pathogen removal efficiencies, while meeting the high operational reliability requirements at the same time Pond

area requirements are as Tow as 3 m? per capita in hot climates

and as high as 12 m@ in temperate climates if adequate protection

is to be provided

Along with proper wastewater treatment, it is good practice to discourage sprinkler irrigation, which promotes the direct contact between wastewater and crops, generates aerosols that carry pathogenic microorganisms over long distances, and encourages the cultivation of of products that can be eaten raw

The health risk for sewage farm personnel can be effectively pro- tected through the use of boots and gloves and proper personal hy- giene,

The area requirements for irrigation land depend on climatic and soil

conditions, but also on two limiting parameters, soil watering and BODs

application limits The former emerges as a controlling factor in the case of treated effluents, and the latter in the case of untreated, and especially, industrial effluents

Soil watering limitations impose irrigable land area requirements which

typically range from 100 to 300 m@ per m’/day of wastewater flow In ad-

dition, alternative ways of storing or disposing of the waste, when not required for farming, must be provided Table 9.1.2.4-1 provides typical organic loading for some types of wastes, which have been found to cause

no tong term adverse effects

Table 9.1.4.4-1 Typical Organic Loading Used in Practice for Land

Application (Adapted from S.J Arceivala, Marcel Dekker, Inc.)

Organic Loading

Raw domestic ãâ industrial (India) 12.5 (2.5-15)

Municipal after secondary treatment 0.2-0.5

Liquid sigested sludge 0.2-9

Potato starch plants 60

Diaries without milk processing 1.2-12.5

nization Indeed, the treatment plant operation must be highly reliable,

toxic substances should not be present, and agronomic measures, related

to irrigation systems, type of crop production, and worker health must

be maintained

In view of the above, the entire system must be designed as safely as possible and this justifies the earlier recommendation of the use of stabilization ponds as a domestic wastewater treatment method It justi- fies also the recommendation that wastewaters containing heavy metals

should not be used for land irrigation, Section 9.1.2.4

Management of Water Pollution 9-25

9.1.4.5 Formulation of Control Strategies

Systematic classification of the water receivers according to their in- tended, present and future use is required in the early stages of the pollution abatement programme, as the applicable water quality guidelines on standards, depend on it (Appendix I) Meeting these stan- dards becomes the major objective of the management plan (Section 7.3)

Selection of the wastewater receiver(s) is a key issue that has also to

be addressed in the early stages of the strategy formulation procedure,

as it defines quite rigidly the ensuing strategy framework as well as the severity of the treatment efficiency requirements

The priorities in the receiver selection, tend to be as follows:

Open coast, enclosed bay or harbour, river, Take The inherent as- sumption in setting the above priorities, or indeed any relevant

priorities, is that the applicable water quality standards, as well

as the limiting organic Joading conditions for enclosed bays, Section 9.2.4.2, can be met through ordinary control measures (see below)

Disposal on Tland may have a high priority depending on the suit- ability of the wastes (lack of toxic and persistent substances, rel- atively Tow dissolved and suspended solids, presence of phosphorus and nitrogen nutrients), on the nature of the irrigated soil, on the types of crops raised, and especially on the climatological condi- tions (in arid or semi arid regions the reused wastewater may con-

stitute a valuable asset)

The urban area sewerage system is an additional "receiver", which

needs to be avaluated for industrial wastes Smaller industries within the urban area may have to discharge into the sewerage system, but many plants, especially larger ones, are located near a

final receiver (sea, river, lake or land) and for these a choice may

be available as to whether they should be allowed to discharge into

the sewerage system or directly into the final receiver:

The combined treatment of industrial effluents along with the

domestic wastes tends to be advantageous (Section 9.1.3.1), if the former do not contain toxic or persistent substances and if

they need to be treated so as to have their organic loads re- duced (e.g prior to discharge in a river) or their nutrient loads reduced (e.g prior to discharge in a lake)

Separate treatment, or pre-treatment for industrial wastes is normally required in cases where they contain toxic substances

Indeed, the removal of such substances is more efficiently and

more economically achieved before the toxic wastes get diluted through their discharge into the sewerage system

The presence of toxic substances in the sewerage system can be pre- vented by not allowing new sources to discharge into the system as mentioned above, but still a problem may exist from the sources al- ready connected to the sewer A medium to Tong-term industry

9-26 Approaches for Consideration in Formulating Environmental Control Strategies

location plan and selective controls, on a priority basis as

described below, help to alleviate this problem The end objective

of such measures is first of all to safeguard the normal operation

of the treatment plant (relevant limiting concentrations of toxic substances in the sewage are listed in Section 1.5), to enable the

safe disposal of the produced sludge (the portion of heavy metals

and other toxic substances removed by the treatment plant are accumulated in the sludge, often creating serious sludge disposal problems) and to protect the final receiver from the portion of toxic substances that are not removed by the treatment plant (see for example Section 9.1.4.3 and Section I.3)

The design of collection systems must also be reviewed in the early

strategy formulation stages along with the selection of the final re-

ceiver(s):

Lack of a central collector from urban sewerage systems result in the existence of several discharges into the final receiver(s) This situation can be particularly serious from the effluent control

point of view, as either multiple treatment plans would be required,

or central collector(s) would have to be constructed The choice be-

tween the former, the latter or a balance between the two depends on the local conditions, but the entire situation needs to be carefully reviewed before one focuses on the treatment plant aspects

Overloaded or poorly designed sewage collection systems may be re- leasing excessive overflows of untreated sewage into the final re- ceiver This can considerably reduce the overall control efficiency

of the entire system and may create health hazards The opposite situation of overdesigned systems, in which the sewage flow is too slow, may cause septic conditions with odour problems especially in hot climates, that may affect the operation of the treatment system

Expansion of the sewerage system might have to be considered so as

to address the choice of the final receiver and the possible com-

bined treatment from neighbouring industries and/or communities For example, to divert direct discharges into a sensitive lake, a cen- tral collector might have to run along the shore collecting the wastewater from several communities and industries to a central treatment plant, and the treated effluents could be discharged into the river downstream of the lake, the sensitivity of which is re- duced

The formulation of rational strategies can be pursued once the fundamen-

tal issues of the municipal and industrial wastewater collection system

and receivers are settled This can be achieved through computation of

effluent loads, analysis of the impact of such loads on the receivers

and assessment of the treatment efficiency requirements The effluent

loads and control model in Section 4.2 allows the user to assess the ef-

fluent loads from each source and the efficiency of the alternative

treatment possibilities The models in Section 9.2 address the critical

impacts on each receiver and allow the user to conveniently assess which

treatment efficiency is required By combining the data and information

of these models the user can, in principle, derive sound pollution con-

trol strategies:

Management of Water Pollution 9-27

The particular sensitivities associated with each type of waste re- ceiver impose definite priorities, distinctly characteristic of the receiver type, in the contro) of particular pollutants, and define

to a large extent the severity of the wastewater treatment require-

ments Thus, the choice of the particular receiver sets a rather rigid framework for the ensuing management strategy and detailed ac- tion programme This subject is analysed in Sections 9.1.3.1 to 9.1.3.3 and 9.1.4.1 to 9.1.4.4, and outlined below:

For lakes the major pollution problem is usually eutrophication and the critical pollutant that controls this problem is phosphorus

Diversion of effluents into downstream waters or on land, con- trol at the source through the use of non phosphatic detergents etc, and use of physicochemical or tertiary treatment processes with high efficiency in phosphorus removal, are the principal control] measures

For rivers, the major pollution problem is the oxygen deficiency caused by the discharge of organic loads Microbial pollution can also be a critical problem in the case of domestic wastewa- ter discharges

Secondary wastewater treatment processes are often sufficient to address the organic load problem, but are marginally effective

in removing microorganisms, unless properly sized waste stabilization ponds are used

For coastal waters, microbial pollution is the critical problem when Municipal waste waters are involved Eutrophication may also be

a problem when discharging into enclosed bays or harbours

In coastal areas with unrestricted current movement and water renewal, primary effluent treatment and properly designed out-

falls are normally sufficient to address the problem In en-

closed bays or harbours secondary waste treatment may be

required so as to prevent eutrophication

For land disposal, pathogen transmission is a critical problem, uniess sufficient land area is available to meet the critical hydraulic and organic loading restrictions

Wastewater from certain industries such as canneries, dairies, sugar factories, breweries, alcohol distilleries, soft drink plants etc has been successfully used for land irrigation with- out treatment For municipal wastewaters high operational relia- bility and coliform removal efficiency requirements make the use

of waste stabilization ponds the safest and, for this reason, the recommended chaice

The discharge of toxic substances is highly undesirable in atl cases; and strict control measures are always required Land ex- hibits the highest sensitivity among the receivers due to heavy metal accumulation problems For this reason, planners should be re-

luctant to dispose of toxic industrial effluents or municipal

wastewaters containing toxic industrial effluents on land, even if

pre-treatment at the source removes the toxic substances High treatment reliability and strict controls would otherwise be required

The discharge of suspended solids into Jakes, rivers and coastal wa- ters is harmful, and for this reason primary treatment is the mini-

mum recommended process for municipal wastewaters

The formulation of detailed action programmes can proceed once the

fundamental issues of the municipal and industrial wastewater collection

system and receivers are settled, and the kind of treatment requirements

established This comprises the drafting of specific measures, which are

required for achieving the pollutant reduction objectives:

Controls at the source, whenever applicable, tend to have high

priority because of their high effectiveness, low cost and ease of implementation The same applies for industry-location plans (see Section 9.1.3.1), which, in addition to offering long-term

environmental benefits in relation to the existing plans, are invaluable in providing the foundations for sustained development

Technical measures include the (pre)treatment of the effluents dis-

charged into the sewerage system or into the final receiver, so as

to have the toxic and/or organic and/or other pollutant loads re-

duced These measures, along with the outfall systems, constitute the last line of defense and the need for them eventually becomes apparent As the effluents entering the sewerage system and/or the final receiver are normally generated from multiple sources, the dilemma of strict controls on few selected sources or relaxed controls across the board has to be addressed:

There is a limit to the number of measures, which can be

effectively promulgated at any given time, especially in the

early pollution abatement stages and this makes imperative the

simplest and the most effective among all possible alternatives for the given situation (Section 7.3)

Such prioritization is of key importance for it greatly enhances

the impact of the action programme, while simultaneously reducing the time and cost required to achieve noticeable

results

As a rule, strict prioritization leads to action programmes in- volving the larger pollution sources Indeed, this is the most effective approach as it involves large scale wastewater treatment, requires minimum enforcement because of the limited number of sources involved, and because a small number of large industries (e.g.10 % of the plants operating in a given area)

are responsible for the bulk (e.g 80%) of the total toxic or other pollution loads generated from the entire industrial sector

Management of Water Pollution 9-29

The imposition of effluent treatment on selected sources is thus in-

strumental for the initial development of highly effective and en- forceable pollution abatement programmes, even when faced with tight

organizational, technical and financial constraints Programmes of

this sort, which impose strict treatment only on selected sources, may be practical as far as their implementation ts concerned, effec- tive and highly economic from the overall plan point of view, they raise however the question as to whether selective allocation of the treatment burden is acceptable and can be justified

The above question could be of academic interest in cases where charges are levied for the discharge of wastes, in proportion to

their volume and strength Big polluters having to treat their effluents would automatically be compensated through tower

charges of this nature This approach, despite its apparent

fairness and theoretical elegance, is difficult to implement in practice for it requires well developed waste monitoring

procedures

In most cases, where effluent charges are non-existent, imposi- tion of treatment on selective sources could be acceptable if properly documented and impartial In fact, the uneven treatment requirements depending on source type and size usually become an accepted and little challenged notion justified by economic realities and environmental pressures The Best Practical

Control Technology Economically Achievable is based on this

notion and aims to maximize environmental benefits

9.2 Water Quality Models

9.2.1 Introduction

This section presents a number of water quality models which were

selected so as to address the critical pollution problems created by

effluent discharges into lakes, rivers and coastal waters All models

are easy to use and their input data requirements are fairly simple and

streamlined with the output generated by the inventory model of Section

4.2,

Use of these models can be made for the purpose of assessing the exist-

ing quality of waters, as well as for assessing wastewater treatment

requirements so as to maintain the applicable water quality standards

As such, they are invaluable in the analysis of existing water quality

problems as well as in the formulation of rational water pollution

Management strategies

For lakes, eutrophication is the major problem and phosphorus is the

controlling variable for this condition Section 9.2.2 comprises models

allowing the user to calculate the critical phosphorus loading, the

steady state concentration of conservative and non-conservative pollu-

tants, as wel] as the time required for the lake to recover after a step

change in the Toads of such substances and of phosphorus

For rivers, BOD and Dissolved Oxygen (DO) are critical and interacting

quality parameters In relation to these, a simple model is presented,

which allows the assessment of the maximum BOD Loading, which can be as-

similated by a river, without its DO level dropping below a desirable

minimum Models are also presented for assessing the concentrations of

conservative, non-conservative and microbial pollutants,

For coastal waters with relatively unrestricted current circulation,

microbial pollution emerges as the controlling pollution factor A sim-

ple model is presented allowing the user to assess the concentration of

coliforms, as well as that of other conservative and non-conservative

substances Moreover, for enclosed water bodies, such as bays and

harbours, a simple model is presented allowing the user to quickly

assess their capacity to assimilate organic materials so as to avoid

eutrophication problems

9.2.2 Models for Water Quality in Lakes

Lakes depending upon their pollution state are generally classified into

two broad categories, oligotrophic and Eutrophic:

Oligotrophic Jakes have low levels of nutrients and are characterized by the presence of oxygen in their lower strata (hypolimnion), even during the summer stagnation period They thus provide an environment where organic joads, locally produced or

discharged into the lake, are completely oxidized, while phosphate

ions, which are liberated from insoluble complexes precipitate continuously to the sediment

Eutrophic lakes are characterized by the enrichment of nutrients,

Which causes a stimulation of a large number of symptomatic

changes, among which are the augmentation of the production of algae and macrophytes, the deterioration of water quality and other undesirable changes that interfere with the usage of the affected water

A simple screening model is presented in the section that follows, which can be used to decide whether a lake is in a satisfactory oligotrophic state, and what would be the critical loading to safeguard this situa-

tion, or to bring a eutrophic lake back to such a state In addition,

simple models are presented for substances which are conservative, such

as heavy metals, or with a constant decay rate or half life, such as pesticides and other toxic materials

9.2.2.1 Lake Eutrophication

9.2.2.1.1 Description of the Model

In oligotrophic takes the concentration of phosphorus is low and tends

to exhaust first Indeed, in cases where the nitrogen to phosphorus ra-

tio is greater than 12, and this is the rule rather than the exception, the eutrophication is phosphorus-controlled, and the phosphorus input

into the lake is a key index to assess whether a problem exists or not

A particularly useful relation for the purposes of environmental management is the following one developed by Vollenweider (1976):

Le = Critical normalized phosphorus loading, mg total P/m2-yr,

above which eutrophication conditions may begin to develop

(eutrophic conditions may be expected in the Jake when the actual phosphorus load, L, equals 2 to 3 times L,)

Gg = Overflow rate, in m/yr (equal to the total annual inflow

rate, in m/yr, divided by the lake area, m2)

Z = Lake mean depth, m