water pollution

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Water PollutionA. JamesUniversity of Newcastle upon Tyne

I. IntroductionII. Fresh WatersIII. Marine WatersIV. GroundwatersV. Abstracted Waters

VI. WastewatersVII. Water Pollution Control

GLOSSARY

Alkalinity Buffering capacity resisting a lowering of pH.Biochemical oxygen demand Oxygen required for bac-

teria to oxidize organic matter in solution or insuspension.

Biomass Concentration of living organisms (e.g., algaeor bacteria).

Chemical oxygen demand Amount of oxidizing agent(usually potassium dichromate) required to oxidize or-ganic matter in solution or in suspension.

Deoxygenation Decrease in dissolved oxygen concentra-tion caused by the bacterial decomposition of organicmatter.

Dielectric constant Measure of the ability of a substanceto resist the passage of a charged particle.

Epilimnion Upper (warmer) layer of a thermally strati-fied lake.

Eutrophication Increase in algal productivity result-ing from increase in levels of nitrogen and/orphosphorus.

Heterotrophic Relying on organic substances as a sourceof energy.

Hypolimnion Lower (cooler) layer of a thermally strati-fied lake.

Lentic Very slow moving or stationary aquatic envi-ronment that allows the dominance of planktonicorganisms.

Lotic Moving aquatic environments that allow the dom-inance of benthic or attached organisms.

Mortification units Estimate of the extent of likely mor-tality in an aquatic population based on integrating con-centration over time.

Poikilothermic Living at the temperature of surround-ings.

Stratification Division of an aquatic environment intolayers due to density differences.

Trophic Classification based on the type of food consum-ed (e.g., herbivores, carnivores, primary producers).

Waste stabilization pond Low-cost device for treatingwastewaters that relies on algal production of oxygenthrough photosynthesis.

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700 Water Pollution

WATER POLLUTION is any alteration in water qualitythat may be damaging to humans or aquatic organismsor that may interfere with uses or potential uses of thewater. The pollution may come from natural causes such aslandslip or leaf fall, but is more often due to the dischargeof domestic, industrial, or agricultural wastes.

The consideration of water quality may be dividedinto (1) natural waters including fresh waters (rivers,lakes, and underground waters) and saline waters (estu-aries and seas), (2) wastewaters (domestic and industrialwastewaters and sludges), and (3) abstracted waters (do-mestic, agricultural, and industrial waters).

I. INTRODUCTION

Outside of a laboratory pure water does not exist, and itwould be uneconomic to attempt to produce it. The qualityof water is therefore judged in relation to the purpose forwhich it is to be used, as shown in Table I.

The relationship between water quality and human ac-tivities is extremely complicated. Water is used exten-sively for domestic, industrial, and agricultural purposesand after use is usually returned in a degraded state torivers, lakes, estuaries, or oceans. Wastes also find theirway by leaching into groundwater or by direct dischargeinto the estuaries or the sea. Industrial and agriculturaloperations such as smelting or crop spraying introducematerials into the aquatic environment either directly orvia the atmosphere. All stages of the hydrological cy-cle are therefore subject to human activity. Not all ofthe activity causes a deleterious change in quality. Intreatment of water and wastewaters the quality is de-liberately improved, and improvements may result inci-dentally from other activities such as the construction ofreservoirs.

TABLE I Relationship between Water Usage and Water Quality

Water use Quality requirement

Navigation Free from large masses of floating debris (e.g., vegetation), which may foul propellorsPower generation As above, to avoid damage to intake structure, plus inert suspended solids limitation to prevent erosion of turbinesRecreation Limits on organic content to prevent anaerobic conditions arising; on turbidity to give light penetration for viewing;

on suspended solids to avoid unsightly deposits; on oil, grease, or other floating matter, which renders the surfaceunsightly; on nutrients to prevent undesirable growth developing; on fecal contamination to prevent risk of spreadingintestinal and cutaneous disease

Fishing Limits on organic content to prevent deoxygenation; on turbidity to permit plant growth and photosynthesis; on suspendedsolids to avoid damage to the benthic community, especially near breeding grounds; on oil, grease, and other floatingmatter for affecting recreation capacity; on toxic matter, which may affect members of the aquatic community in the shortor long term; on nutrients to prevent undesirable growths that may cause marked diurnal fluctuations in DO or by alteringthe nature of the substrate change the flora and fauna

Abstractions for Limits on fecal contamination to avoid spreading intestinal diseases; on toxic materials to prevent chemical poisoning;domestic supply on substances affecting palatability such as taste, odor, salinity, color, turbidity, solids; on hardness

The control of water pollution is therefore essential ifwater resources are to be properly managed and damageto wildlife, recreation, crops, and human populations isto be avoided. Some legislative and administrative frame-work is required, which implies the setting of standards orguidelines for quality and some framework of monitoringto ensure that the standards are being achieved.

There are difficulties in producing surveilance pro-grams. These are partly conceptual since no completelysatisfactory parameters can be found to measure the es-thetic qualities of water such as taste and odor. There arealso difficulties in choosing sampling sites, sampling fre-quency, and so forth. Nevertheless, monitoring is an es-sential part of any water pollution control program.

In discussing water quality it is important to distinguishbetween abstracted waters and natural waters. The qualityof abstracted waters, particularly where they are intendedfor domestic supply, is more rigorously monitored and isgenerally controlled by treatment. The quality of naturalwaters can only be controlled by limiting discharges andis therefore subject to much wider variation.

II. FRESH WATERS

A. Edophic FactorsLife in water is significantly different from that in terres-trial habitats. For aquatic organisms life is dominated byproblems of obtaining sufficient oxygen and light or nu-trients. Also in moving water there is the possibility ofbeing washed away or in still water the chance of beingburied under silt. Balanced against these difficulties areadvantages such as greater thermal stability and the readyavailability of water and dissolved salts.

The physical and chemical properties of water are re-markable and have considerable significance in the study


Water Pollution 701

of aquatic ecology. Some knowledge of these peculiar-ities is a prerequisite for understanding water pollution.Although the chemical composition of water is conven-tionally expressed as H2O, it behaves as (H2O)n and in factconsists of continually changing branched chains of im-perfect oxygen tetrahedra linked by hydrogen bridges. Atlow temperatures, near freezing, the tetrahedral arrange-ment is marked; as the temperature increases, there is lessstructural association so that the possibility of close ran-dom packing increases at the same time that increasingthermal agitation makes for a looser arrangement. The in-teraction of these two processes results in a contraction onmelting and a further contraction up to 4C. Beyond thistemperature the thermal agitation becomes the dominanteffect and water expands. The most important result ofthis phenomenon is in lakes and seas, where it is respon-sible for thermal stratification (see Section I.B), althoughan attenuated stratification also can be observed in sometropical rivers. Biologically it is also important in ensur-ing the continuity of life beneath the ice in streams andlakes.

The presence of hydrogen bridges has other effects onthe physical properties of water (e.g., elevated boilingpoint and freezing point, high specific heat, and latentheat of sublimation and boiling), which ensure that wateris liquid at most environmental temperatures and that it isextremely thermally stable.

The structure of water also affects its chemicalproperties, particularly in producing a high dielectricconstant that makes it capable of dissolving almost anysubstance that can form an ionic solution. By contrast,substances that cannot ionize, such as oxygen and manyorganic compounds, are only sparingly soluble. The prop-erty of being such a good solvent has important biologi-cal consequences because as a result most natural waterscontain enough inorganic material to support the growthof plant life.

Ecology is concerned with the relationship betweenorganisms and their environment. The aquatic environ-ment may be divided into a number of chemical andphysical aspects, which are briefly discussed in thissection.

1. Dissolved GasesThe concentration of dissolved gases depends upon theoccurrence in the atmosphere, the solubility, and the ratesof production and consumption within the aquatic habitat.The main dissolved gas of biological significance is oxy-gen, but carbon dioxide is also important. Oxygen has avery limited solubility in water, as shown in Fig. 1, andthis fact limits both activity and abundance in many aquaticenvironments.

FIGURE 1 Solubility of oxygen in water.

2. Dissolved SaltsSince water is such a good solvent, it is inevitable that mostnatural waters and all wastewaters contain a variety of in-organic substances that are derived from the atmosphere,soil, rocks, and wastes. The ionic composition is impor-tant in determining growth rates of algae, invertebrates,and fish and in determining the waters suitability for do-mestic and industrial consumption (see Section V.C). Astotal dissolved salts they fix the osmotic pressure, andaquatic environments may be divided into fresh water andseawater.

3. OrganicsOrganic matter enters aquatic environments by a variety ofroutes from terrestrial debris and the discharge of domesticand industrial wastes as well as from the waste productsof aquatic organisms and the decay of their bodies afterdeath. The level of dissolved and suspended organics playsa crucial role in determining whether an environment isdominated by primary producers or decomposers. Manyorganic substances are significant in other respects sincethey are often toxic or carcinogenic to humans or aquaticorganisms.

4. Solar RadiationSolar radiation is of crucial importance in providing the en-ergy source for photosynthesis. Much of the solar radiationthat falls on aquatic environments is lost by reflection(up to 50%). The subsequent penetration depends upon theincident light intensity, color, and turbidity of the water, as


702 Water Pollution

FIGURE 2 Light penetration in water.

shown in Fig. 2. The rate of reduction of light intensity (theextinction coefficient) is critical in determining the depthat which photosynthesis balances losses due to respiration(called the compensation depth). In clear mountain lakesand oceanic waters the compensation depth may be up to50 m, whereas in waste stabilization ponds it may be aslittle as 0.2 m.

5. TemperatureWater temperature is very important to aquatic organismssince almost all of them are poikilothermic. The tempera-ture requirements of different organisms vary, but the formof the response is similar and may be divided into zonesof cold stress, optimum, and heat stress above which isthe thermal death point. There is rarely any problem ofcold death since natural waters or wastes do not often fallbelow zero, but freezing can cause severe rates of mor-tality. Within the feasible range for an organism, increasein temperature causes an increase in metabolic rate. Theincrease is usually 22.5 times for a 10C rise, whichcan cause problems if the extra food and oxygen are notavailable.

On the basis of temperature patterns it is possible to dis-tinguish three types of habitat: Cold and thermally stablehabitats occur in the range 1C to 6C, such as the head-waters of a river, the hypolimnion of a temperate lake, andthe deep waters in oceans. Warm and thermally stable en-vironments occur in tropical rivers and lakes and surfacewaters in tropical oceans with temperatures varying be-tween 20C and 35C. The third type of environment hastemperature fluctuations large enough to be biologicallysignificant (>10C) and includes surface waters of lakesand seas and rivers in temperate regions.

6. Water MovementsWater movements are extremely important to aquatic or-ganisms since with the exception of fish and aquaticmammals, aquatic organisms tend to be carried along bythe movements. Freshwater aquatic habitats are dividedinto lentic and lotic, and the latter category can be sub-divided on the rate of movement into different types ofbed (silt, sand, gravel, boulders, and rock). In streams wa-ter movement is not uniform laterally or vertically. Thishas important consequences in providing different typesof habitat and also in causing dispersion.

In lakes and seas movement of water takes the form ofcurrents and waves. (See Section III.A for a discussion.)

B. Biotic FactorsAlthough the size and nature of an aquatic communityare to a large extent determined by chemical and physicalfactors, their influence is considerably modified by theeffect of biotic factors. The principal biotic factors arereviewed briefly in this section.

1. Trophic RelationshipsThe trophic structure of an aquatic community is illus-trated diagrammatically in Fig. 3. Organisms on the sametrophic level compete for food; those on successive levelshave a preypredator relationship. Competition for food orsunlight or space is often the principal factor determining

FIGURE 3 Trophic structure of an aquatic community.


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the degree of ecological success, with predation as an im-portant secondary influence. Other types of trophic re-lationships are also significant, in particular the role ofdecomposers that use an alternative energy source andprovide the basis for an alternative food chain and the roleof parasitism in creating hostparasite relationships thatcan also affect ecological success.

2. Energy and Nutrient FlowThe relationships with the energy sources enable organ-isms to be classified into various categories (producers;primary, secondary, or tertiary consumers; and primaryor secondary transformers). As energy flows through theecosystem, it is dissipated and only a limited amount ofrecycling takes place through dead remains and wasteproducts contributing to the detritus. The balance in anecosystem between the producerconsumer food chainand the transformer chain depends upon the relative en-ergy inputs and is easily disturbed by pollution. (SeeSection II.C.)

The flow of nutrients in an aquatic ecosystem is muchmore conservative, often with a large proportion of el-ements such as nitrogen and phosphorus being recycledthrough the transformers back to the producers. Nutrientstorage is a particular feature of lake ecosystems and maycause the long-term persistence of disturbances producedby nutrient enrichment. (See Section II.C.)

3. Population DynamicsFor individual species, factors such as growth rate, fecun-dity, and death rate play an important role in determiningabundance. Organisms have different mortality and fecun-dity patterns with age, and for some particular stages inthe life cycle these patterns may be critical.

C. Freshwater PollutionThe basic principles of aquatic ecology outlined in Sec-tion II apply equally to polluted sites. The following sec-tion discusses the ways in which pollution affects aquaticenvironments. In discussing these changes it is usefulto classify them into the following categories: organic,toxic, solids, heat, inorganic nutrients, and oil. However,it should be appreciated that the ways in which these dif-ferent forms of pollution affect a community also dependupon the nature of the habitat. In lotic waters the commu-nity is dominated by benthic organisms and it thereforemore sensitive to the deposition of solids and organic en-richment. Lentic waters are dominated by plankton andare therefore more responsive to increase in inorganicnutrients.

A further complication arises from the interaction be-tween different types of pollution. Although it is possibleto find examples of waters affected by only one type ofpollution, it is much more common to find environmentssubject to several types.

1. Organic PollutionPollution by organic matter is a complex phenomenonsince it involves both stimulation of the community by or-ganic enrichment and stress to the community through re-duction in dissolved oxygen and often alteration of the bedthrough organic deposits. The chemical changes and theirbiological consequences are represented in Fig. 4. Theprimary effect of organic enrichment is to stimulate thegrowth of heterotrophic bacteria. Some of these, especiallythe filamentous forms, grow in association with fungi andstalked ciliate protozoa to form an attached growth calledsewage fungus. This often produces a characteristic andvisible zone immediately downstream of significant dis-charges of organic wastes, particularly where the the dis-charge contains settleable organic solids. The result of thisintense microbial activity is to deplete the DO more rapidly

FIGURE 4 Effect of an organic effluent on a river. [Adapted fromHynes, H. N. B. (1964). The Biology of Polluted Waters, LiverpoolUniv. Press, Liverpool, U.K.]


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TABLE II Minimum Tolerance Levels for DissolvedOxygen

Minimum level ofOrganisms dissolved oxygen

Salmon, trout, mayflies, stoneflies 67 mg/literPerch, caddisflies, crayfish 56 mg/literRoach, tench, shrimps 35 mg/literCarp, eels, midges 13 mg/liter

than it can be replaced by reaeration, leading to an oxygensag. The extent of the sag governs the changes in the inver-tebrate and fish populations as shown in Table II and is theprincipal cause of biological changes downstream of anorganic discharge. The oxygen balance in these circum-stances is of great interest to regulatory authorities andvarious attempts have been made to simulate the impactusing mathematical models. (See Section VII.C.)

The other result of the microbial activity is the conver-sion of organic nitrogen and phosphorus into their cor-responding inorganic forms. Provided that other condi-tions (light intensity, dissolved oxygen and stream bed)are suitable, production of inorganic nutrients will lead toincreased growths of algae, especially attached forms suchas Cladophora and Vaucheria. Further downstream thealgal biomass decreases with reduced nutrients. Rootedmacrophytes and mosses are generally eliminated or muchreduced by organic discharges due mainly to being smoth-ered by solids or sewage fungus.

Organic enrichment and oxygen depletion stimulate thegrowth of protozoa and detritusfeeding metazoa at the ex-pense of other invertebrates, especially if there is solidssettlement. Where the stream becomes anaerobic, only theair-breathing animals survive. In situations with low DO(12 mg/liter) and a sludge blanket, the fauna is domi-nated by tubificid worms. At slightly higher DO levels(23 mg/liter), chironomid larvae compete more success-fully, and at even higher levels (35 mg/liter) other or-ganisms such as Asellus and Gammarus succeed togetherwith leeches, mollusks, and other fly larvae. This is fol-lowed by a return to the normal clean water fauna as theeffects of organic enrichment die away.

In the zone affected by the organic enrichment there isan increase in the total biomass of the benthic fauna due tothe increased food supply. Those organisms that can toler-ate the conditions may be present in vast numbers, partlydue the increased food and partly due to the reduced com-petition and predation. Where the concentration of organicmatter from an effluent is low (BOD < 4 mg/liter), the ef-fects of the organoc enrichment are observable withoutthe attendant deoxygenation and deposition of solids. Insuch cases there is a slight shift in the species composition

of the benthic fauna in favour of detritus feeders togetherwith an increase in the overall biomass.

Where the effects of pollution are severe, fish popu-lations can be totally eliminated. Less severe pollutionwill enable the more tolerant fish to survive, and sincethe food supply is ample will lead to higher numbers.For the more sensitive fish to survive the DO should notfall below 67 mg/liter as indicated in Table II. The stan-dards set for controlling organic pollution are discussed inSection VII.A.

Certain categories of organic compounds may causesignificant changes in the stream community at levels wellbelow those causing deoxygenation. Some of these maybe toxic (see Section II.A.2), but others act in more subtleways. For example, substances that mimic the sex hor-mones have been found to reduce or eliminate fish popu-lations by inducing sterility.

2. Toxic Pollution

Apart from ammonia, domestic wastes do not contain anymaterial that causes poisoning of aquatic organisms, butthere are a vast number of toxic substances in industrialwastewaters. Toxic substances may be in solution and en-ter organisms such as algae, macrophytes, and inverte-brates by diffusion through external surfaces or via the in-ternal surfaces of fish, where the gills are the main portalof entry. The other mode of entry is through the food whenthe toxic substance is released during digestion. For somepoisons there is a mechanism of excretion and/or break-down so that a balance between uptake and elimination ispossible, but for other toxins such mechanisms are inef-fective and accumulation takes place. Toxins are generallydivided into acute and chronic. Acute toxins exert their ac-tion over a short period, usually less than a week, whereaschronic poisons continue to act over a period of months oreven years. Some poisons have a particular affinity for cer-tain organs such as the liver, but others accumulate moregenerally. Because of these physiological complication, itis difficult to generalize about toxic action, but Table IIIindicates some general classes of poisons.

Since it is not possible to predict a toxins effects from aconsideration of its chemical composition, it is necessaryto establish toxic effects experimentally. The test proce-dure is a form of bioassay in which test animals are ex-posed to a range of concentrations of toxin for periods upto 10 days. There are many practical difficulties in stan-dardizing the procedure (such as choice of test animal), butthe main difficulty is interpretation. As shown in Fig. 5,the test result can be used to estimate the safe concentra-tion by plotting the median survival times in each tankagainst concentration and from the asymptote determin-ing the concentration below which survival is independent


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TABLE III Mode of Action of Some Common Aquatic Toxins

Class of poison Mode of action Examples

Heavy metals Precipitation of proteins in gill secretions and anemia Lead, mercuryRespiratory depressants Inactivation of enzyme systems related to respiration Cyanide, sulfideInorganic acids and alkalis Corrosive poisons that attack soft tissues Sulfuric acid, sodium hydroxidePhenolic substances Corrosive poisons that also attack central nervous system Phenol, acridineSynthetic detergents Cause respiratory distress by attacking gill surface ABS, lissapolInsecticides and herbicides Disturb the action of the central nervous system DDT, malathionAmmonia Upsets water balance by increasing permeability

of the toxin (i.e., the safe concentration). Alternatively,the results can be used to calculate the concentration thatpoisons 50% after all the toxic action has been exerted(incipient TLm). This is a useful basis for comparing therelative toxicities of different poisons and can be used forestimating the safe concentrations by using the equation

safe concentration = incipient TLm application factorbut the application factor is difficult to establish.

For chronic toxins all toxicity is not exerted within the10-day period, and this type of test is therefore inappropri-ate, although a 96-hr test is sometimes used in conjunctionwith an application factor of 0.01 to estimate the safe con-centration. Long-term growth tests are a better basis forsetting safe standards for chronic toxins.

In polluted waters mixtures of poisons are commonlyencountered, and it is therefore useful to be able to deter-mine the toxicity of mixtures. It is usually assumed thatthe toxicity is additive and that the combined toxicity isthe sum of the individual toxic effects expressed in toxicunits. These are calculated as

actual concentration

toxic units = threshold concentrationincipient TLm

This approach has been shown to give good agreement inover 80% of cases and near agreement in a further 10%.

FIGURE 5 Interpretation of the results of an acute toxicity test.

In the remainder chemical reactions among toxins tend toreduce the toxicity.

Fluctuating concentrations also present a problem incalculating toxic effects. Various approaches have beentried, of which the most useful is the idea of mortifica-tion units. These are calculated by multiplying the time ofexposure by the concentration of the toxin. Mortificationunits have been shown to be a good basis for comparingexposures.

Two other aspects of the biological response add furthercomplications to toxic pollution. The first is acclimatiza-tion. This is a general property of all types of aquaticorganisms and describes their increased resistance to apoison on second or subsequent exposures. Acquired tol-erance to toxins is even more important in field situations,where it develops in populations via natural selection.

The second aspect is limited to fish and some largeinvertebrates. These more mobile species are sometimescapable of detecting poisons and may be able to avoidthem by moving into tributaries or backwaters. The abilityto detect poisons seems to be limited to a relatively smallrange of compounds, mainly corrosive substances.

From the preceding discussion it is apparent that toxicsubstances can cause severe pollution and may damageaquatic environments in many subtle ways that are difficultto quantify. Assessment of toxic pollution and standardsare discussed later.


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3. Solids PollutionParticulates are discharged into water bodies through var-ious processes, including erosion of soil and rock throughland runoff, leaf fall, discharge of sewage, discharge ofindustrial wastes, and discharge of urban drainage. Theeffect of these particles on aquatic life depends primarilyon whether they remain in suspension and to a lesser extenton whether they are organic or inorganic. Where solids re-main in suspension, they reduce the photosynthetic activ-ity by increasing the turbidity, and at high concentrations(>300400 mg/liter) they can have a direct abrasive actionon soft tissues such as the gills of fish. But for concentra-tions of suspended solids of less than 100 mg/liter, there isno evidence of any damage to aquatic communities unlessthe solids are readily degradable organics and cause otherproblems through deoxygenation.

Where solids settle they can cause severe damage toaquatic communities in very small amounts. This is espe-cially the case when the deposition takes place in the head-waters of rivers used for breeding grounds by salmonidfish. But many invertebrates are also badly affected whenthe interstices of eroding gravel are blocked by fine solids,and a layer of only 10 to 20 mm is required to completelyeliminate some species by increased stream drift.

The nature of the depositing solids does affect the ex-tent of the changes since organic solids are particularlydamaging. They often lead to totally anaerobic condi-tions that eliminate all but air-breathing invertebrates andbacteria.

The main problem in assessing the effect of the dis-charge of solids into water bodies is in predicting theirhydraulic behavior, particularly in streams where condi-tions can change from deposition to erosion with changingriver flow. Setting standards for solids is therefore partic-ularly difficult.

4. Heated DischargesIncreasing the temperature of aquatic environments candamage the community in various ways, notably by

1. Lowering the solubility of dissolved oxygen2. Decreasing the availability of oxygen by stimulating

bacterial activity (the increase in activity is approximately10% per degree Celsius)

3. Increasing the requirements for both food and oxy-gen of aquatic organisms (the increase in metabolic ratevaries for different organisms but can be up to 250% per10C increase)

4. Disturbing the reproduction pattern, particularly offish, so that males or females are mature at the wrong timeor the eggs hatch too soon

FIGURE 6 Acclimatization to heat by an aquatic organism.

5. Causing physiological distress (for most aquatic or-ganisms there is a very small range between optimum andmaximum temperatures)

Heated discharges arise from a variety of industrial op-erations such as smelting and coke production, but theymainly come from thermal power stations. Because of thefluctuating nature of the demand for electricity, there areoften marked seasonal and diurnal fluctuations in the quan-tities of heat discharged. This creates a similar problem tothat caused by fluctuating concentrations of toxins (seeSection II.C) and is usually handled in a similar mannerby the use of mortification units.

Acclimatization to heat is also similar to acclimatizationto toxins. This is illustrated in Fig. 6.

5. EutrophicationThe concentration of inorganic forms of nitrogen andphosphorus (and sometimes silicon) often restricts theamount of algal growth, especially in lentic waters thatare particularly suitable for planktonic organisms. Am-moniacal nitrogen is toxic to some aquatic organisms (seeSection II.C), and concentrations of nitric nitrogen above10 mg/liter are associated with increased incidence ofmethemaglobinemia (see Section V.C), but the main ef-fect is eutrophication.

The general effect of inorganic nutrient release in rivershas already been discussed in relation to organic dis-charges. Where inorganic materials are discharged di-rectly into streams, the same effects appear but may bein a more severe form. Large growths of Cladophora(and other filamentous algae) may make the river un-sightly and in association with rooted vegetation mayalter the nature of the bed and cause siltation. But the


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TABLE IV Approximate Requirement of Nitrogen andPhosphorus in Relation to Algal Growth (All Levels ing/liter)

Nutrient Plankton Benthic

Minimum level of nitrogen 10 500Saturation level of nitrogen 1000 5000Minimum level of phosphorus 1 100Saturation level of phosphorus 100 1500

chief concern of nutrient addition to streams is wherethe water is subsequently impounded. Nutrient enrich-ment in lakes and reservoirs takes various forms, depend-ing on whether the water body is used for recreation orwater supply. Recreational problems are caused by algalgrowth causing diurnal fluctuations in pH, alkalinity, anddissolved oxygen and through causing increased turbid-ity and in extreme cases producing floating mats of al-gae. The problems caused in abstracted waters are mainlydue to increased levels of turbidity plus other problemsof taste and odor. These are discussed in more detail inSection V.C.

In considering the significance of nutrient dischargesit is important to have a grasp of the levels at whichnitrogen and phosphorus affect algal growth. These aresummarized in Table IV as average levels for minimumand maximum growth. Individual species obviously varyconsiderably in their requirements. For example, blue-green algae are capable of utilizing dissolved nitrogengas and are therefore independent of ammoniacal and ni-tric forms. But the figures do show the higher nitrogenrequirement compared with phosphorus and the muchhigher nutrient requirements of the benthic algae. Nu-trient enrichment of streams and lakes is due to contri-butions from many sources. These are summarized inTable V.

The pattern of nitrogen and phosphorus levels differsgeographically. In Europe and the United States, phos-phorus is the main limiting nutrient in freshwater systems

TABLE V Sources of Nitrogen and Phosphorus Discharges

Nutrient load

Source Nitrogen Phosphorus

Domestic waste 9 g/person day 2 g/person dayIndustrial wastes 0 up to fertilizer Usually low except

waste detergent-rich wastesUrban runoff 26 kg/ha yr 12.5 kg/ha yrArable runoff 1020 kg/ha yr 0.51 kg/ha yrPasture runoff 610 kg/ha yr 0.10.5 kg/ha yr

FIGURE 7 Correlation between winter nutrient levels and algalconcentrations in summer. [Reprinted with permission from Lund,J. W. G. (1971). Eutrophication. In The Scientific Management ofAnimal and Plant Communities for Conservation, Symp. Br. Ecol.Soc. (E. Duffy and A. S. Watts, eds.), Blackwell, London.]

and its main source is detergents. Point sources contributeup to 80% of the total phosphorus budget. In tropical areasof Africa, South America, and Asia, soils are deficient innitrogen and this is the main limiting nutrient in most wa-ter bodies. This is in marked contrast to countries such asthe United Kingdom, where lower rainfall combined withextensive use of nitrogenous fertilizers lead to significantrunoff of nitrogen from farmland, which is responsible forover 60% of the total nitrogen budget in rivers.

Since the consequences of eutrophication are moreserious, greater attention has been paid to the connec-tion between nutrient loads and the levels of algae. Therelationship between nutrient inflow to a reservoir and thenutrient concentration in the reservoir water is somewhatcomplex. In general terms the degree to which the nutri-ent content of the reservoir will approach that of inflowsis affected by retention time, number of algae present, andtheir rate of growth. In temperate reservoirs with seasonalgrowth, there is some correlation between winter nutri-ent levels and the algal concentrations in the followingsummer, as shown in Fig. 7.

A more generalized approach is to assess the potentialfor eutrophication in terms of the nutrient loading, as illus-trated in Table VI. This has been shown to apply to many

TABLE VI Permissible Loadings of Nutrients forLakes and Reservoirs

Permissible Dangerousloading loading

Meandepth N P N P

5 1.0 0.07 2.0 0.1310 1.5 0.10 3.0 0.2050 4.0 0.25 8.0 0.50

100 6.0 0.40 12.0 0.80


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European lakes and has since been found to be applicablein other parts of the world.

6. Minewater PollutionSubsections 1 to 5 have characterized different types ofpollution, but in the case of minewater the pollution arisesfrom a combination of effects caused by the presence ofiron, manganese, aluminum, acidity, and/or alkalinity.

Anaerobic conditions underground may allow iron andmanganese to pass into solution in their soluble diva-lent states. Also pH reduction due to high concentrationsof sulfate and similar ions may allow aluminum to dis-solve. Once these materials enter surface waters iron andmanganese may be precipitated because of oxidation,blanketing the bed. Also, the iron, manganese, and alu-minum in solution may cause toxic effects.

The problems caused by minewaters are exacerbated bythe large and varying flows that make treatment arrange-ments difficult and expensive. Additionally, the minewa-ters continue to flow indefinitely, long after the mines havebeen abandoned.

7. Oil PollutionOil pollution in fresh water is generally as aesthetic ratherthan a biological problem. Small spillages of oil can cre-ate visible surface films over a wide area, and this is acommon source of complaint to water pollution controlorganizations.

Where large quantities of oil are discharged, oxygendepletion and smothering action may damage the aquaticcommunity. But the effects are short-lived and the com-munity usually recovers rapidly.

The effects of oil pollution are more frequently encoun-tered in the marine environment (see Section III.B.4).

FIGURE 8 Diversity in an unpolluted estuary.

III. MARINE WATERS

A. Marine EcologyAlthough the sea is an aquatic environment that sharesmany of the characteristics of freshwater environments,there are some important differences.

1. Osmotic PressureThe salt concentration in seawater (3437 g/liter) is toohigh for the majority of organisms adapted to fresh water.Very few organisms can tolerate both fresh-water and ma-rine conditions, although there are important exceptions,such as salmon and eels. The abrupt biological transitioncaused by increased salinity can be observed in estuaries,as shown in Fig. 8.

2. Wave Action

A wave is composed of a cone of rotational movementsin which velocities may be sufficient to cause scouringof solids from the bed. In most seas the waves generatedat the surface can only resuspend solids up to a depth ofabout 50 m. Any solids that settle in deeper waters remainundisturbed, but at lower depths they will be resuspended.Wave action is also very important to littoral communities.

3. CurrentsThere are three main types of currents that are important:tidal, wind-induced, and oceanic. Tidal currents are re-sponsible for much of the advection and dispersion thattake place near coastlines. The majority of wastes enterthe sea in the zone from estuaries or outfalls, and the mix-ing and movements caused by tidal currents are therefore


Water Pollution 709

critical in determining the pattern of concentration aroundthe point of discharge.

Wind-induced currents are limited to the top 1 to 2 m.Within the top-meter velocities may be 5% to 10% of thewind velocity and cause rapid translocation of bacteria,oil, and other surface pollutants back onto a beach. Theyare of particular interest in the design of marine outfalls.

Oceanic currents are caused by convection in tropicalregions, which induce surface movements toward the poleand deep-water return currents. These are relatively slow(a few kilometers per day) but over a long period of timeare responsible for the global distribution of pollutants.

4. Dilution

The sea has an almost unlimited capacity for dilution.Provided that polluting materials are rapidly dispersed,the resulting concentrations are below toxic thresholdsand the rates of decomposition of organic matter makelittle impact on the dissolved oxygen concentration. Acorollary of the great dilution capacity is an almost infi-nite retention time, and any pollutants discharged to thesea that do not degrade will remain there almost indefi-nitely. This gives rise to the possibility of reconcentrationthrough biological agencies (food chains) or geochemi-cal processes (adsorption or sediments) or both. Recon-centration may cause persistent pollutants to exceed toxicthresholds, particularly in predators near the top of a foodchain.

5. LightThe light and temperature pattern in the sea is similar tothat in lakes except that the much greater depths dictatethat the vast bulk of the seas (i.e., below 100 m) are per-manently cold (


710 Water Pollution

microorganisms (and heavy metals) up to dangerous lev-els. Therefore, protection of shellfish grounds is required.

4. Oil PollutionMarine environments are particularly susceptible to pol-lution by oil from accidents, spillages, and deliberate dis-charges of bilge waters associated with the global transportby sea. Very small quantities of oil are needed to producea visible slick (around 0.0001-mm-thick layer). Thickerlayers can cause biological damage especially to seabirdsand littoral communities through toxic action and by coat-ing the organisms and preventing respiration, feeding, andphotosynthesis. The toxicity is inversely correlated withthe molecular weight, and as the lighter, more volatilefractions evaporate, the oil becomes less toxic (96-hr TLmgoing from around 10 mg/liter to over 1000 mg/liter). Withthe possible exception of seabirds, the effects of oil pollu-tion do not have long-term effects on marine populations.The fate of oil in the sea depends to some extent on the oilsmolecular weight. The lower-molecular-weight fractionas well as evaporating is more easily emulsified, and theresulting droplets are rapidly degraded by phytoplanktonand bacteria. The heavier fractions tend to settle and de-grade much more slowly, but biological effects even fromsevere pollution do not usually persist for more than 1 or2 years. The main reason for control measures is, there-fore, to avoid damage to tourist areas. Control is by burn-ing, accelerated sinking, or increased dispersion, althoughcontainment and recovery may occasionally be possible.

IV. GROUNDWATERS

Quality considerations obviously differ from those thatapply to surface waters since the absence of light and tosome extent dissolved oxygen prevents the developmentof any significant macroscopic community. Concerns overquality are therefore limited to two main areas:

(a) Where groundwaters are a source or potential sourceof water for drinking or for industrial or agriculturalsupplies

(b) Where groundwaters are in hydraulic contact withsurface waters and influence their quality

Water quality changes underground are extremely com-plex and varied, but the general pattern of change is asfollows. Water flowing over a permeable surface suchas soil permeates downward through a saturated stra-tum (the water table or phreatic surface). Water qualityin the unsaturated zone is generally improving as it iscut off from further contamination (except for leachates

from solid waste depositories and effluents from septictanks); passage through the soil acts as a filter for sus-pended solids, and dissolved organics are oxidized. Oncebelow the phreatic surface the water gradually becomesanaerobic; further filtration and oxidation produce a high-quality water in deep aquifers. However, depending on thenature of the rock formation, prolonged exposure can pro-duce groundwaters with high concentrations of dissolvedsolids due to ions such as sulfate, chloride, and bicarbon-ate. By contrast, shallow underground sources are gener-ally of poor quality because of solids pollution, often witha high concentration of bacteria.

Pollution of underground waters is a particularly seriousconcern because such occurrences are much less evidentthan contamination of surface sources. Many countrieshave introduced legislation to protect aquifers from haz-ards such as solid waste disposal sites, pesticide and nu-tient runoff from agriculture, and subsurface disposal ofeffluents from septic tanks.

V. ABSTRACTED WATERS

A. Sources and QualityWater may be required for a wide variety of purposes, suchas domestic or industrial or agricultural supply, or it may beused for hydropower generation or fire-fighting. In plan-ning any supply it is important to consider the potentialdemand and to balance this against the volume and reli-ability of each alternative source. Quality considerationsare obviously important but perhaps less critical becauseit is always possible to alter the quality of abstracted wa-ters. The reliable yield is the primary concern, after whichconsiderations of quality, conveyance, and storage play apart in the choice of supply source.

Water sources are generally nonsaline and usuallyare based on streams, groundwaters, or lakes, but othersources such as rainwater or sewage or seawater may beused. The best quality and the most reliable are deep un-derground waters.

B. Collection and StorageThe quality of water in any natural system is continuallychanging. This applies to water that is being collected andstored for abstraction, and so it is sensible to consider theseas part of the water treatment process.

In the case of surface waters it has long been recog-nized that protection of the catchment is essential in min-imizing the cost of obtaining the required water quality.But the emphasis on catchment protection has changedwith improved technology of treatment and increasing


Water Pollution 711

pressures on catchment areas for recreation. The main pri-ority in catchment management is concentrated on reduc-ing nutrient loads entering reservoirs so as to minimizealgal growths. The second important aspect of catchmentmanagement is concerned with prevention or reduction ofsoil erosion, since this not only reduces the storage capac-ity, but also can impair the water quality due to increasedBOD or suspended solids. The importance previously at-tached to avoiding bacterial contamination of catchmentshas been somewhat reduced. Although bacterial contami-nation is still regarded as undesirable, it is appreciated thatthe complete prohibition of recreation and agriculture areunnecessary since the technology for bacterial reductionis reliable and relatively inexpensive.

The need for storage for a surface source can have a ma-jor influence on the quality of the water. The water qualitymay improve because of sedimentation, oxidation of or-ganic matter, and die-off of bacteria and other pathogens.But the water quality may also deteriorate mainly due toalgal growth. Factors such as thermal stratification andeutrophication largely control the changes in quality dur-ing storage and need to be carefully considered during thedesign and construction of a reservoir.

Once a lake or reservoir is used for storage, algalgrowths need to be monitored and if necessary controlled.Techniques for destratification by induced circulation areavailable mainly by the use of compressed air and sub-merged jets. These same techniques help to reduce therate of algal growth and prevent deoxygenation of the hy-polimnion with the consequent problems of sulfide pro-duction and increased levels of manganese and iron. Al-ternatively, algal growths may be controlled by the useof algicides. Various compounds have been found thatsuccessfully inhibit the growth of algae, notably coppersulfate and 2:4 dinaphthoquinone (2:4D), but these areexpensive, may create secondary toxicity problems, andprovide only a temporary amelioration since the algae willgrow again the following year.

Where it is impossible or uneconomical to control thegrowth of algae, the water leaving the reservoir or lake willrequire additional treatment to remove not only the algae,but also their metabolic products, which can give rise tounpleasant tastes and odors. Also, chlorination of thesemetabolic products can lead to a risk of the formation ofcarcinogens if they are not removed.

Surface storage of water in tropical climates can haveadditional problems due to the growth of aquatic macro-phytes. The difficulties caused by these large macroscopicgrowths chiefly affect fishing, navigation, and recreationaluses, but such growths also affect water treatment plantsadversely by causing blockage of intakes, disruption ofsedimentation, and other problems, as well as by increas-ing the organic debris in the water.

Storage of water underground is generally free from theproblems that beset surface storage, particularly where thegroundwater is in a confined aquifer. However, problemsof chemical contamination can arise because of leachingfrom waste tips, discharge from septic tanks, or sprayingof pesticides and herbicides. Where the aquifer is near thesurface or contains fissures, it is also possible for microor-ganisms to gain access.

Various chemical and biological processes occur in thestored water mainly because of bacteriological activity,which leads to a decrease in organic content, a decreasein dissolved oxygen, and an increase in carbon dioxide.The increase in CO2 may cause a fall in the pH and wherethe aquifer is calcareous can lead to an increase in hard-ness. Other chemical changes occur as a result of contactwith minerals in the aquifer. Often the rock strata can be-have as an ion exchange medium that alters the cationcomposition. Metal complexes may be formed owing tothe high carbonic content that brings previously insolublemetals such as uranium into solution as complex hydratedcarbonates.

The main dangers to the quality of water stored un-derground come from saline intrusion and contamina-tion from leachates. Saline intrusion can occur when thedrawdown of the water table allows seawater to enter theaquifer. This problem is usually confined to coastal ar-eas but can place severe constraint on the yield from theaquifer. Contamination from leachates is nearly alwayschemical in nature since the movement of bacteria or otherpathogens is very limited underground unless large fis-sures are present. The material that causes most difficultiesis nitrate since this is usually used as a fertilizer, is readilysoluble, and is not easily removed during storage under-ground. The other agricultural sources of pollution are pes-ticides and herbicides, particularly chlorinated organics.Other sources are deposits of domestic or industrial sludgeand solid wastes, which may yield metals or other toxins.

C. Quality RequirementsThe quality of a water is judged in relation to its use. Thisprinciple applies especially where the water quality is be-ing changed by treatment since unnecessary expendituremay be incurred. The quality requirements for domesticsupplies, for industry, and for agriculture will therefore beconsidered separately.

Domestic water supplies must satisfy three criteria:(1) they must be free from injurious chemicals, (2) theymust be free from harmful pathogens, and (3) they mustbe aesthetically satisfactory (see Tables VIIIX).

The list of potential toxins in water is virtually infi-nite since even a substance like common salt becomesharmful if present in sufficient concentration. Table X


712 Water Pollution

TABLE VII Some Inorganic and Organic Constituents ofHealth Significance

GuidelineConstituent value Remark

Inorganics (mg/liter)Antimony 0.005Arsenic 0.01 Skin cancer riskBarium 0.7Boron 0.3Cadmium 0.003Chromium 0.05Copper 2.0Fluoride 1.5 Depends on dietLead 0.01Manganese 0.5Mercury 0.001Molybdenum 0.07Nickel 0.02Nitrate (as NO3) 50Selenium 0.01

Organics (g/liter)Carbon tetrachloride 2Vinyl chloride 5Trichloroethene 70Benzene 10Acrylamide 0.5Tributyltin oxide 2

summarizes the limits as suggested by World Health Or-ganization to some common toxins. It must be appreciatedthat the guidelines given in Table X refer to the quality ofwater as supplied to the consumer. It is perfectly normalto take water that does not conform to these requirementsand then alter the quality by appropriate treatment.

The processes normally used in water treatment are acombination of flocculation, sedimentation, filtration, anddisinfection. With the exception of disinfection, they aredesigned for solid removal. Pollutants that are in suspen-

TABLE VIII Bacteriological Guidelines for Drinking Water

Type of water Organism Guideline value

All water intended for drinking Escherichia coli or thermotolerant coliforms Must not be detectable in any 100-ml sampleTreated water entering the distribution system E. coli or thermotolerant coliforms Must not be detectable in any 100-ml sample

Total coliforms Must not be detectable in any 100-ml sampleTreated water in the distribution system E. coli or thermotolerant coliforms Must not be detectable in any 100-ml sample

Total coliforms Must not be detectable in any 100-ml sample. Inthe case of large suppliers where sufficientsamples are examined, must not be detectablein 95% of samples taken throughout any12-month period.

sion are therefore removed effectively as are those that areadsorbed onto solids (e.g., metals), but pollutants that arein true solution such as nitrates are not readily removedby conventional treatment and require special techniques.

Much of the natural coloring material in surface watersis humic acid derived from decomposition of plant debris.This is present as negatively charged colloids and is effec-tively removed by the addition of a coagulant (aluminumor iron salts), which destabilizes the colloid and forms asludge blanket that helps with sediment removal. Adjust-ment of pH and alkalinity is often required to keep theseprocesses at maximum efficiency.

The requirement to be free from harmful pathogens isto avoid the risk of spreading disease, since waterborneinfections have been the cause of much morbidity andmortality. The microbiological criteria used in assessingthe quality of drinking water are summarized in Table XI.One important feature of the microbiological guide-linesis their reliance on indicator bacteria. It is not normal prac-tice in water examination to search for pathogens such asVibrio cholerae or Salmonella typhosa for the followingreasons: (1) The aim of the examination is not to establishthe presence of pathogens, but to search for evidence of fe-cal contamination. (2) The pathogen content of sewage issmall and extremely variable, whereas the indicator organ-isms give a much more reliable quantitative assessment ofthe degree of fecal contamination. (3) Techniques for theenumeration of pathogens are not as reliable and quanti-tative as those for coliforms and other indicators.

It must be appreciated that this reliance on measure-ment of total coliforms and fecal coliforms assumes thatthey are always present in sewage and absent from envi-ronments that are free from sewage. This is almost alwaystrue of fecal coliforms, but other types of coliforms aremore widely distributed and may lead to erroneous con-demnations of safe supplies. Where there are doubts aboutthe coliform results, other tests such as the enumerationof fecal streptococci or anaerobic clostridia may be usedas confirmatory tests together with sanitary surveys to dis-cover any possible sources of contamination.


Water Pollution 713

TABLE IX Some Substances and Parameters That MightGive Rise to Consumer Complaint

Levels likely to Reasons forgive complaint complaint

Physical parametersColor 15 TCU AppearanceTaste and odor Should be acceptableTemperature Should be acceptableTurbidity 6 NTU Appearance and

effective disinfectionInorganic constituents

Aluminum 0.2 mg/liter Deposition anddiscoloration

Ammonia 1.5 mg/liter Odor and tasteChloride 250 mg/liter Taste and corrosionIron 0.3 mg/liter StainingManganese 0.1 mg/liter StainingSodium 200 mg/liter TasteSulfate 250 mg/liter Taste and corrosionTotal dissolved solids 1000 mg/liter TasteZinc 3 mg/liter Appearance and taste

Organic constituentsToluene 0.0240.17 mg/liter Odor and tasteXylene 0.021.8 mg/liter Odor and tasteStyrene 0.0042.6 mg/liter Odor and tasteTrichlorobenzenes 0.0050.05 mg/liter Odor and taste

The routine examination for other types of microbialpathogens such as viruses or protozoa presents formidabledifficulties. Techniques are available for virus examina-tion, but they require sophisticated laboratory facilities.Instead, reliance is usually placed on the maintenance ofa sufficient chlorine residual.

The number of bacteria and other microorganisms isconsiderably reduced during water treatment. Processessuch as coagulation, sedimentation, and filtration cannotbe relied upon to achieve a satisfactory reduction, and thefinal stage in most water treatment plants is disinfection.

The aims of disinfection are twofold: to reduce the bac-terial concentration to an acceptable level and to provideresidual protection in case of contamination in the distribu-tion system. These goals are generally achieved by dosingthe water with chlorine in the form of either chlorine orhypochlorite. The technical problems of handling lique-fied chlorine restrict its use to large treatment plants, andthere are drawbacks to using chlorine in any form since itreacts with many trace organic substances present in thewater and is liable to form carcinogenic products. Thereare, however, considerable advantages to using chlorine,notably its reaction with ammonia to form chloramines.These are relatively stable and yet bacteriostatic com-pounds that serve to provide residual protection.

Effective disinfection with chlorine requires low tur-bidity [


714 Water Pollution

TABLE XI Microbiological Guidelines for Drinking Water Quality as Suggested by WHO (1984)Guideline

Type of supply Indicator bacteria value Remark

Piped supply of treated water entering Fecal coliforms 0/100 ml Turbidity


Water Pollution 715

character. Wastewaters (with the exception of sludges)flow sufficiently easily to be collected by a surface chan-nel and underground pipe system (sewers). Sewer systemsconvey domestic and industrial wastes (foul sewers) thatmay be separate from urban drainage (storm sewers), orthe two types of sewers may be conveyed in a single sys-tem (combined sewers). The design of a sewer system is adifficult hydraulic problem because of the need to provideself-scouring velocities under varying flows in a systemthat usually operates under gravity.

Another useful distinction is between point sources (i.e.,wastes that can be economically collected together fortreatment or disposal) and diffuse sources. The latter in-clude agricultural wastes and leachates. In many parts ofthe world, because of severe economic constraints, it isnot possible to collect domestic wastes. Also in such com-munities industries may be in small and widely scatteredunits. Control of water pollution in such communities re-lies upon nonsewered devices such as septic tanks and pitlatrines.

B. CharacteristicsThe composition of wastewater needs to be assessed inrelation to the following considerations: (1) suitability fordischarge to a sewer, whether the wastewater is liable todamage the fabric of the sewer or to cause risk to health forpeople working in the sewer; (2) suitability for treatment,whether the wastewater is liable to cause difficulties in thetreatment process due to high organic strength, nutrientimbalance (e.g., C:N:P not sufficient), toxicity (e.g., heavymetals), foaming, or high suspended soils; and (3) suitabil-ity for discharge after treatment, whether the wastewateris liable to cause problems in receiving waters even af-ter treatment because of the presence of recalcitrant sub-stances or high levels of pathogenic organisms.

Domestic wastes vary somewhat in composition aroundthe world depending upon diet, especially water consump-tion. In Europe the daily production is around 0.06 to0.08 kg of BOD and 0.8 to 1.0 kg of SS, which witha daily water usage of 180 liters per person results in asewage of 300 mg/liter of BOD and 400 mg/liter of SS.The corresponding figures in the United States are about25% less because of higher water usage.

In other parts of the world the composition of sewage ismore varied, tending particularly in arid rural areas to bestronger owing to low water consumption and in tropicalareas tending to be septic.

The composition of wastes from urban areas also de-pends upon the pattern of surface water drainage. Wherecombined sewers are used, very high flows of low-strengthwastes can result, especially in tropical regions. Whereseparate sewers are used, the flow of domestic wastes is

FIGURE 9 Diurnal variations in the flow of domestic wastes.

much more even but still has a pronounced diurnal pat-tern, as shown in Fig. 9. These fluctuations are importantnot only in sewer design, but also because they imposevarying loads on the wastewater treatment plant.

Apart from organic strength and suspended solids, themost important constituents of sewage are nitrogen, phos-phorus, and intestinal bacteria. The nitrogen concentrationin raw wastes varies from 60 to 100 mg/liter, of which 20 to30 mg/liter is ammoniacal and the remainder in the form oforganic compounds, mostly urea. The per capita produc-tion of phosphorus is about 2 g/day and is much lower thanthe 9 g/day of nitrogen. The concentration of phospho-rus in raw domestic waste is correspondingly less (about3040 mg/liter), of which about half is organic and therest is orthophosphates. The concentration of nitrogenand phosphorus in sewage is more than ample for thenutritional needs of the bacteria that are used in treat-ing wastewaters. The ratio of BOD to nitrogen should benot more than 100:10, and the ratio of BOD to phospho-rus should not be more than 100:1. The ratio in domesticwaste is usually 100:20:2. The bacterial content of domes-tic wastes is extremely high, as shown in Table XIII.

The composition of industrial wastes is much more var-ied than domestic sewage, so much so that it is impossibleto characterize it briefly. Table XIV shows the composi-tion of only some of the more important types of industrialwastewaters.

TABLE XIII Bacterial Content of DomesticWastewaters

Organism Concentration range

Total coliforms 107109/100 mlFecal coliforms 107108/100 mlFecal streptococci 105106/100 mlSalmonellae 0102/100 ml


716 Water Pollution

TABLE XIV Composition of Some Common IndustrialWastewatersa

Approximate compositionType of

wastewater BOD SS N P

Brewery 8001000 600800 3040 810Cannery 5005000 2002000 100500 550Metal processing 100300b 100500 1020 210Coke production 5002000b 200400 100300 520

a All concentrations are in milligrams per liter.b Estimated from COD. Due to the toxic nature of waste, BOD is

not an appropriate parameter.

The discharge of industrial wastewaters may be morevaried than domestic sewage, especially in industries thatuse batch processes or where factories open for only cer-tain periods (e.g., 8 hr/day or 5 days/week). Fluctuations inflow may also be accompanied by fluctuations in quality.Despite the variability of industrial wastewaters, it is pos-sible to make a number of generalizations, among themthat industrial wastewaters usually contain many fewerbacteria than domestic sewage and that they often containmany more toxic substances.

The third main source of wastewater is the surfacerunoff from urban areas, which is often referred to as stormwater. The polluting material in storm water is a reflectionof wastes brought into solution or suspension during thewaters passage over the surface. The most outstandingcharacteristic of urban drainage is its extreme variability,as shown in Table XV. Part of this variability is due todifferences in the nature of the material on the surface andthe length of time of contact, but it is due mainly to thefirst flush phenomenon, as shown in Fig. 10.

Sludges are an especially difficult type of wastewa-ter to dispose of. They are intermediate in character be-tween solid and liquid wastes, with a solid content thatmay vary from 0.5% up to 10% or even 20%. Sludgesarise from many industrial processes (e.g., neutralizationof acid wastes with lime). Also biological sludges are pro-duced during the primary and secondary sedimentation inwastewater treatment.

TABLE XV Characteristics of Urban Drainage

Parameter Concentration range

Biochemical oxygen demand 30500Suspended solids 207000Total nitrogen 670Total phosphorus 0.112Heavy metals 0.310Total coliforms 102108

FIGURE 10 Relationship between flow and quality during astorm.

The important characteristics of sludges are thefollowing.

1. Solids content: Sludge handling and disposal usu-ally rely upon dewatering the sludge to around 5% to10% solids to produce a material that can be handled as asolid.

2. Organic content: The amount of degradable organicmatter determines the method of disposal and subsequentproblems such as odor production.

3. Pathogen content: The concentration of bacteria,viruses, helminth eggs, and protozoa cysts can be veryhigh in sludges from wastewater treatment and may re-quire some form of disinfection before disposal.

4. Toxic content: Persistent toxins such as heavy met-als and chlorinated organics limit the disposal of sludges,especially for agricultural purposes.

C. Treatment and DischargeIt is evident from the preceding discussion that the qualityof wastewaters renders them unsuitable, in most instances,


Water Pollution 717

FIGURE 11 The relationship between treatment and disposal.

for discharge without some form of treatment. The degreeof treatment required depends mainly on the quality ofthe wastewater, the quality requirements in the receivingwater and the extent of the dilution available. In almostall circumstances there will be a minimal treatment re-quirement to avoid obvious signs of wastewater disposal.Beyond the minimum the aim is to find a balance be-tween avoiding water pollution and causing an unreason-able economic burden on the industry or municipality. Thegeneral relationship between treatment requirements andreceiving waters is illustrated in Fig. 11. It is importantto appreciate that wastewater treatment is carried out ina stepwise manner. For example the BOD of a domes-tic wastewater is likely to drop from 200250 mg/liter asraw waste, to 130170 mg/liter after primary treatment, to1520 mg/liter after secondary treatment, to 25 mg/literafter tertiary treatment.

TABLE XVI Chemical Classification of River Water Quality in the United Kingdom

River class Quality criteria Current and potential use

1A DO > 80% saturation; BOD < 3 mg/liter; ammonia < 0.4 mg/liter; Water of high quality suitable for game fishery and with a highnontoxic to fish amenity value

1B DO > 60% saturation; BOD < 5 mg/liter; ammonia < 0.9 mg/liter; Water of less high quality but suitable for substantially thenontoxic to fish same purposes

2 DO > 10% saturation; BOD < 9 mg/liter; nontoxic to fish Water supporting coarse fishery and of moderate amenity value3 DO > 10% saturation; BOD < 17 mg/liter Fish only sporadically present but water unlikely to cause

a nuisance4 Inferior to class 3 Waters that are grossly polluted and liable to cause a nuisance

VII. WATER POLLUTION CONTROL

A. Assessment

Water pollution is a biological phenomenon and shouldtherefore be assessed biologically. There are many advan-tages in using biological assessment, notably that benthicorganisms intergrate numerous environmental and give adirect answer as to whether a particular combination issuitable. Secondly, the effects are intergrated over time,especially with the longer lived macroinvertebrates. Thepresence of mature individuals can indicate appropriateconditions over the past 6 to 12 months. Also, biologicaldata can clearly show sublethal as well as lethal effects.The former are very difficult to discover from purely chem-ical observations.

Despite these obvious advantages, most water pollutionassessment is based on chemical monitoring. This is, tosome extent, a reflection of the ease with which collectionand analysis can be carried out, but it is mainly due to thequantitative nature of the result that is obtained. This maybest be illustrated by reference to the examples of chemicaland biological classification of rivers in Tables XVI andXVII. Where a standard of 4 mg/liter is adopted for BOD,an analytical result will give a quantitative indication ofthe amount of additional treatment required. However, aresult from a biological survey classified as class III givesno real indication of the BOD removal needed to bring theriver up to class VII or VIII.

One further problem with biological assessment is thedifficulty in devising a universal system. Most system relyon changes in the community due to some or all of thefollowing factors:

(a) Diversityrange of species present(b) Spectrumtype of species present(c) Abundancenumbers of individuals present in each

species

But the changes in these parameters due to pollution aresuperimposed on the changes due to other edaphic factors,


718 Water Pollution

TABLE XVII Trent Biotic Index

Diversity of Number of other groups presentIndicator group indicator group 01 25 610 1115 16+

Plecoptera nymphs present >1 species VII VIII IX XOnly 1 species VI VII VIII IX

Ephemeroptera nymphs >1 species VI VII VIII IXpresent Only 1 species V VI VII VIII

Tricoptera larvae present >1 species V VI VII VIIIOnly 1 species IV IV V VI VII

Gammarus present All above species III IV V VI VIIpresent

Asellus present All above species II III IV V VIpresent

Tubificid worm and/or All above species absent I II III IV Chironomid larvaepresent

All above types absent Only organisms not O I IIneeding DO present

and so each system tends to work best in the area forwhich it was devised and cannot be easily applied to otherregions without modification. Special problems in assess-ment are posed in streams by suspended solids and heateddischarges. The control of suspended solids has to be ex-ercised on an individual basis for each stream situationsince the effects depend upon the local hydraulic regime.The effect of heated dischages is also site specific.

Suggested standards for these are summarized inTable XVIII.

Assessment of pollution in the marine environment ismuch more difficult than for freshwaters. The physicalenvironment is much more complex, especially the hy-draulics and the greater diversity of habitat is reflected in amuch greater range of species. Attempts at biological clas-sification of marine pollution have not been as successful.From an anthropomorphic standpoint the interest in con-

TABLE XVIII Suggested Standards for Pollutants

Pollutant Guideline

Inert solids


Water Pollution 719

FIGURE 12 Mass balance approach to simulating the fate of apollutant in a stream:

CM = ((QRCR) + (QECE))/(QR + QE)CT is some function of CM and travel time.

of recovery after the cleanup operation. For example, therecovery of stream communities may be only 1 to 5 yearsdepending on the possibilities of recolonization. However,where sediments are involved in lakes and estuaries withthe storage of nutrients or heavy metals, then the time scalemay be decades or even longer for marine environments.

For these reasons regulatory authorities make extensiveuse of mathematical modeling of water pollution to simu-late the potential impact of discharges on receiving waters.At the simplest level this may involve no more than a massbalance approach as shown in Fig. 12. But the models maybe much more sophisticated to take account of some or allof the following:

(a) Stochastic variation in the flow or discharge(b) Water quality variations across wide rivers due to the

bank-side nature of most discharges

(c) The diffuse nature of inputs of contaminants such asland runoff containing nutrients or pesticides

(d) Diurnal and other cyclical variations due to solareffects

SEE ALSO THE FOLLOWING ARTICLES

ENVIRONMENTAL TOXICOLOGY POLLUTION CONTROL POLLUTION, ENVIRONMENTAL SOIL AND GROUNDWA-TER POLLUTION TRANSPORT AND FATE OF CHEMICALSIN THE ENVIRONMENT WASTEWATER TREATMENT ANDWATER RECLAMATION WATER CONDITIONING, INDUS-TRIAL WATER RESOURCES

BIBLIOGRAPHY

Heitman, H.-G. (1990). Saline Water Processing, VCH Publishers,New York.

Hynes, H. N. B. (1964). The Biology of Polluted Waters, LiverpoolUniv. Press, Liverpool, U.K.

James, A. (1984). An Introduction to Water Quality Modelling, Wiley,London.

Lund, J. W. G. (1971). Eutrophication, In The Scientific Managementof Animal and Plant Communities for Conservation, Symp. Brit. Ecol.Soc. (E. Duffey and A. S. Watt, eds.), Blackwell, London.

Streeter, H. W., and Phelps, E. B. (1925). A study of pollution andnatural purification of the Ohio River, U.S. Public Health Ser., Bull.No. 146.

Vollenweider, R. A. (1968). Fundamentals of the Eutrophication ofLakes and Flowing Waters, Organization for Economic Cooperationand Development, Paris.

World Health Organization (1984). Guidelines for Drinking WaterQuality, 2nd ed., World Health Organization, Geneva.

Water PollutionGlossaryIntroductionFresh WatersEdophic FactorsDissolved GasesDissolved SaltsOrganicsSolar RadiationTemperatureWater Movements

Biotic FactorsTrophic RelationshipsEnergy and Nutrient FlowPopulation Dynamics

Freshwater PollutionOrganic PollutionToxic PollutionSolids PollutionHeated DischargesEutrophicationMinewater PollutionOil Pollution

Marine WatersMarine EcologyOsmotic PressureWave ActionCurrentsDilutionLightNutrients

PollutionPollution by Persistent ToxinsSurface Slick FormationMicrobiological Contamination of Swimming WatersOil Pollution

GroundwatersAbstracted WatersSources and QualityCollection and StorageQuality Requirements

WastewatersSources and CollectionCharacteristics

Water Pollution ControlAssessmentEnforcementModeling

See also the Following ArticlesReferences

water pollution

Documents

water qualitythat

qualityof water

water pollutiona

industrial waters

water pollutionwater

natural waters

consideration of water

laboratory pure water