modern hydrology and sustainable water development (gupta/modern hydrology and sustainable water...

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10 Urban hydrology

Urbanization, which has of late become an impor-tant environmental issue, involves excessive physi-cal growth of urban areas, in rural or natural setting,as a result of population migration to and aroundexisting urban areas, mostly for generating liveli-hood. Effects of urbanization include changes inthe density of population and consequent pressureon civic administration to provide various ameni-ties. While the exact definition and population sizeof urban areas vary in different countries, urbaniza-tion involves growth of cities. The United Nationshas defined urbanization as the migration of peoplefrom rural to urban areas. People move to cities toseek or further economic opportunities. In rural ar-eas on small agricultural farms it is often difficult toimprove one’s living standard beyond basic suste-nance. Agricultural economy is largely dependenton unpredictable environmental conditions and intimes of drought, flood, or epidemics, very survivalof affected population becomes difficult. Further-more, as a result of industrialization, agriculturaloperations have progressively become more mech-anized, rendering many manual labourers jobless.Many of the basic services as well as other special-ized services are not available in rural areas. Also,there are more job opportunities and a variety ofjobs available in urban areas. Health care is anothermajor concern in rural areas. People, especially theelderly, are often forced to move to cities wherebetter medical facilities exist. Other factors includemore recreational facilities (restaurants, movie the-atres, theme parks, etc.) and a better quality of

education, offered by universities as well as spe-cialized institutions. These have come about dueto changes in the life style from a pre-industrial so-ciety to an industrial one, and more recently dueto the effect of globalization and the free marketeconomy. In addition, many new commercial en-terprises have created new job opportunities in thecities.

For the first time in the history of human civi-lization, almost half of the global population wasresiding in cities by the end of 2008. In cities of thedeveloped world, urbanization traditionally mani-fests as a concentration of human activities and set-tlements around the downtown area, the so-calledin-migration. Recent developments, such as inner-city redevelopment schemes, imply that new im-migrants to cities do not necessarily settle in thecentre of the city. In some developed regions, how-ever, the reverse process has also occurred, withcities losing populations to rural areas. This is par-ticularly true for affluent families and has becomepossible because of improved communication net-work and also due to factors such as rising crime inpoor urban environments. When a residential areaexpands outwards and new areas of populationconcentration form outside the downtown areas,a networked poly-centric concentration emerges.Los Angeles in the United States and Delhi in Indiaare two examples of this type of urbanization. Acommon problem often associated with urbaniza-tion is settlement of rural migrants in shanty townsunder conditions of extreme poverty, lacking even

Modern Hydrology and Sustainable Water Development S. K. Gupta

© 2011 S. K. Gupta. ISBN: 978-1-405-17124-3

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298 MODERN HYDROLOGY AND SUSTAINABLE WATER DEVELOPMENT

Home

WatertreatmentStation

Uptake

Distribution

Riv

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Water Storage

RedistributionStation

Wastewatertreatment

plant

Rainfall Rainfall

Gro

un

dw

ater

Su

pp

ly

Sewer systemWastewaterWater

losses

Waterlosses

Infestedwater

Infestedwater

StormwaterStormwater

Water supply

Treatedwater

Drainage

Drainage

Sidewalk

Waterlosses

Flooding from surroundingcatchment areas

Receivin

g R

iver

Riv

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lood

ing

Fig. 10.1 Schematic representation of water formation, circulation, and distribution in an urban area. Redrawn fromhttp://echo.epfl.ch/VICAIRE/mod 1b/ chapt 10/ main.htm.

basic amenities such as a water supply and sani-tation. Urbanization in India and other developingcountries is taking place at a faster rate than inthe rest of the world. According to the UN report‘State of the World Population 2007’, about 41% ofIndia’s population will be living in urban areas by2030, compared to about 28% now. According tothis report, over 90% of slum-dwellers live in de-veloping countries, with China and India togetheraccounting for 37% of them.

Urbanization reduces the cumulative volume ofwater storage in soils and vegetation, increases thefraction of rainfall that becomes surface runoff dur-ing storms, creates structures that accelerate themovement of runoff to streams, reduces evapora-tion, and increases the runoff yield of the water-shed. Protecting water resources and water qualityof streams in newly urbanized basins necessitatescomprehensive implementation of structural aswell as non-structural mitigation measures that arenot limited to just retention of natural vegetation.

The obvious definition of urban hydrology isthe study of hydrologic processes occurring withinthe urban environment, where a substantial part

of the area consists of nearly impervious surfacesand artificial land relief as a result of urban devel-opments. However, further consideration of forma-tion, circulation, and distribution of water in an ur-ban area (Fig. 10.1) clearly reveals the inadequacyof this simplistic concept.

In urban areas, natural drainage systems are mod-ified and supplemented by a sewerage network.The effects of flooding are mitigated by creatingengineering structures, such as dams or storageponds. In the initial stages of urban development,septic tanks are employed for disposal of domesticwaste waters. As the urban area grows, seweragesystems are installed to divert sewage to treatmentplants and the treated effluent is returned to lo-cal water courses or, in coastal locations, to theocean. In the initial stages of development, watersupplies are drawn from local surface and ground-water sources to minimize the cost. However, withincrease in population and consequently the rise indemand for water, additional supplies can only beobtained from remote locations. Both waste waterdisposal and water supply, therefore, extend the in-fluence of the urban area well beyond its immediate

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geographical boundaries. Most urban areas andtheir surroundings are not self-sufficient in terms ofwater availability and they can develop only withinthe framework of a well-organized water supplyand flood prevention/mitigation systems. The big-ger their size, the greater is their vulnerability andthe need for integrated water management, there-fore, becomes imperative.

The process of urbanization may thus be seento create three major hydrologic problems: (i) pro-vision of adequate water resources for urban ar-eas, both in terms of quantity as well as qual-ity; (ii) prevention of flooding within urban areas;and (iii) disposal of waterborne wastes from ur-ban areas without impairing the quality of localwater courses.

Of these three problems, water supply formspart of the wider subject of water resources de-velopment. Often sources of water for cities andheavily industrialized areas are from rivers and/orground water that are generally not found in ade-quate quantities within urban areas and often wa-ter has to be imported from the adjoining areas tomeet the demand. Although such water sources arenot included within the scope of urban hydrology,their role cannot be overlooked when dealing withurban hydrology. The other two urban hydrologicproblems, that is, prevention of flooding within ur-ban areas and disposal of waterborne wastes fromurban areas, are more specific to individual urbanareas and some common links are considered in thefollowing.

10.1 Water balance in urban areas

A major source of formation and circulation of nat-ural water in an urban area is the local precipitationand storm water generated either locally or in thesurrounding rural and suburban areas and flowinginto the urban area. An additional source is floodwater in rivers flowing through the area and stormwater runoff that collects in lakes or ponds that ex-ist inside the urban area or in its close proximity.Storm water generated from impervious surfaces(to which the snowmelt is added where applica-ble) is drained through pipes and canals that con-stitute the urban sewerage network. Part of the

precipitation falling on pervious areas (surround-ing rural areas, urban yards, parks, stadiums, andsports fields) is absorbed by the soil and eventuallyrecharges the groundwater and the remaining parteither joins the sewerage network via drains or isevaporated.

Rivers and groundwater (especially in deepaquifers) are sources of water supply for meetingdemands of domestic as well as industrial consump-tion and also for energy generation. Water is con-veyed from the uptake points through canals andpipes to the water treatment plant for purificationto meet the required quality standards for variousend uses. The required quantity of surface water ismet by reservoir regulation systems in the frame-work of integrated water engineering structures.In water supply from groundwater, the capacitymay be enhanced by artificial recharging of waterdiverted from rivers, provided that the river waterquality is good and does not cause groundwaterpollution. From the treatment plant, water is con-veyed to domestic and industrial users as well asfor public uses (road washing, recreational parksirrigation, etc.).

The components that constitute the output termin the water balance equation for an urban areaare the water: (i) carried through the seweragenetwork; and (ii) lost through evapotranspiration.Sewerage systems usually mix the waste water withstorm water but in some cases there are two sepa-rate systems for conveying the waste water and thestorm water. Schematic representation of the wa-ter balance of both natural water as well as wastewater generated in a typical urban area is given inFig. 10.2.

10.1.1 Influence of urbanization on theformation and circulation of wateroriginating from rainfall andsnowmelt

Modification of thermal and radiation balance in theatmosphere due to absorption of the long-wave ra-diation emitted by the ground is likely to result inan increase in the intensity of rainfall spells. Thishappens because the urban land cover is darkerthan the open spaces in rural areas. Furthermore,aerosols and trace gases generated by polluting

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Fig. 10.2 Schematic showing water balance ofnatural and generated water supply and wastewater. Redrawn fromhttp://echo.epfl.ch/VICAIRE/mod 1b/chapt 10/ main.htm.

sources in the urban air also contribute to modi-fication of the local micro-climate that may affectthe rainfall over the area, particularly the num-ber and severity of extreme events. Finally, the‘cupola effect’ of the city leads to the increaseof the convective motion of the air, which re-sults in more intense cumulonimbus-type of cloudformation causing higher rainfall in urban zones ascompared to nearby rural areas.

As already mentioned, due to the large fractionof land surface being increasingly covered with im-pervious materials (concrete, asphalt, stone-pavedroads, roofs, etc.), the infiltration of rain water intothe soil is reduced. This leads to an increase of thesurface runoff as well as the peak discharge rate inurban areas. There is also a substantial increase inthe velocity of the surface runoff as well as of thewastewater flow through the sewerage system, ascompared to the velocity of overland flow in naturalcatchments or the flow in river channels. Thus theduration of flood wave propagation is shorter in ur-ban areas compared to that in rural basins, leadingto higher peak discharge in urban catchments.

The runoff generation process is impeded mainlydue to water accumulation held in detention stor-ages in surface depressions and also due to infiltra-tion in pervious/semi-pervious zones of the urbanarea. The element that has a dominant effect onrunoff is the infiltration that may be computed by

use of the well-known empirical equations of infil-tration of the Horton, Philip, or Green-Ampt type(see Section 2.5.3.3). Another method for calcula-tion of infiltration is by using runoff coefficientsthat express the ratio between the effective (orrunoff equivalent) rainfall and the input rainfall, de-pending on the type of land cover and land usein the area. In practical situations, the runoff co-efficient is taken to be constant during the entireduration of a rainstorm. The runoff coefficients fordifferent types of land cover are given in Table 10.1and as functions of land use in Table 10.2.

Table 10.1 Runoff coefficients vis a vis the nature of landcover. Adapted from http://echo.epfl.ch/VICAIRE/mod 1b/chapt 10/main.htm.

RunoffLand cover type coefficient

Dense pavement, asphalt or concrete 0.70–0.95Ordinary pavement or brick 0.70–0.85Roofs of buildings 0.75–0.95Lawn on sandy terrain with slope <2% 0.05–0.10Lawn on sandy terrain with slope >7% 0.15–0.17Lawn on average and hard texture terrainwith slope <2%

0.13–0.17

Lawn on average and hard texture terrainwith slope >7%

0.25–0.35

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Table 10.2 Runoff coefficient as function of land use in an urban area. Adapted fromhttp://echo.epfl.ch/VICAIRE/mod 1b/chapt 10/main.htm.

Use in the urban area Runoff coefficient

Commercial{

DowntownOther urban areas

Residential

⎧⎨⎩

Buildings with yardsDwellingsSuburban area

0.70–0.950.50–0.700.30–0.350.50–0.700.25–0.40

Industrial zones (average) 0.50–0.80Heavily industrialized zones 0.60–0.90Parks 0.10–0.25Sport fields or arenas,recreational areas

0.20–0.35

Unused terrain 0.10–0.30

In the case when runoff is computed, startingwith the rainfall averaged over the entire urbanarea encompassing all types of land-cover zones ordifferent land uses, the runoff coefficient is con-sidered as an average value α over the entire areaF:

α = 1

F

n∑i=1

αi . fi (10.1)

where a given value of fi represents an area coveredby a specific type of urban land use or land cover,with its respective runoff coefficient αi.

Effective rainfall, heff, is computed as:

heff = α . Pn (10.2)

where the net rainfall Pn = P – INT ; P being the av-erage rainfall over the entire urban area and INT (inmm) the amount of rainfall interception. The rain-fall interception in an urban area plays a significantrole only in the case of dense parks and trees alongthe streets. The equation given by Horton (1919)for accounting for the interception is:

INT = a + b. Pn (10.3)

The parameters of the Horton equation for differ-ent land cover and trees are given in Table 10.3.

Furthermore, the effective rainfall is transformedinto a runoff hydrograph by the use of an integra-tion procedure. One of the commonly used integra-tion procedures that is applied in urban areas is the‘unit hydrograph method’ (see Section 2.7.1.3).

10.1.2 Hydraulic effects of changes instream channels and floodplains

Development along stream channels and flood-plains can modify the water-carrying capacity ofa stream channel and can increase the height ofthe water surface (known as stage) for a given dis-charge passing through the channel. In particular,structures that encroach upon the floodplain, suchas bridges, can increase upstream flooding by nar-rowing the width of the channel, which increasesthe resistance of the channel to flow. As a result,water flows at a higher stage as it flows past theobstruction, creating a backwater that inundates alarger area of the flood plain upstream.

Table 10.3 Parameters of the Horton equation (Eqn 10.3)for estimation of interception from a tree coveredlandscape.

Land cover a [mm] b n

Orchards 1.016 0.18 1.0Ash trees 0.508 0.18 1.0Beech trees 1.016 0.18 1.0Willow trees 1.27 0.18 1.0Oak trees 1.016 0.18 1.0Maple trees 0.508 0.40 1.0Resinous trees (pine-tree, fir-tree, etc.)

1.27 0.2 0.5

Source: (VICAIRE, 2003).

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Sediment and debris carried by flood waters de-posited in the riverbed can further constrict a chan-nel and increase flooding in the floodplain of theriver. This hazard is greatest upstream of culverts,bridges, or other locations in the river channelwhere debris transported during high floods col-lects. For example, small stream channels can befilled with sediment or become choked with de-bris because of undersized culverts. This creates aclosed basin with no outlet for the passage of floodwaters. Although river channels can be trained torapidly convey flood water and debris downstream,local benefits of this approach must be balancedagainst the possibility of increased flooding down-stream.

Erosion caused by urban streams is yet anotherconsequence of urbanization. Frequent floodingin urban streams increases both channel bed- andbank-erosion. Where channels are straightened andvegetation is removed from the catchment area,stream flow velocities increase, resulting in highersediment loads carried by the stream. In many ur-ban areas, stream-bank erosion, caused by higherstream flow velocities coupled with removal of veg-etation from the banks, represents a serious threatto roads, bridges, and other structures that is diffi-cult to control, even by strengthening the streambanks.

10.1.2.1 Reducing flood hazard in urban areas

There are many approaches for reducing floodhazard in urban basins under development. Areasthat are identified as flood-prone can be used forconstruction of parks and playgrounds, as thesecan withstand occasional flooding. Buildings andbridges can be elevated, protected with floodwallsand levees, or designed to withstand temporary in-undation. Drainage systems can be augmented toincrease their capacity for detaining and conveyinghigh stream flows, for example, by using rooftopsand parking lots to store water. Techniques thatpromote infiltration and storage of water in thesoil profile, such as infiltration trenches, perme-able pavements, soil amendments, and reducingthe proportion of impermeable surfaces can alsobe incorporated into new as well as existing resi-dential and commercial complexes to reduce storm

runoff from these areas. As an example, wet-seasonrunoff from a neighbourhood in Seattle, Washing-ton, was reduced by 98% by reducing the widthof the street and incorporating vegetated swalesand native plants in the street right-of-way, that is,an area dedicated to public streets, sidewalks, andutilities (Konrad 2003).

10.2 Disposal of waterborne wastes

In addition to natural precipitation and the runoffgenerated, urban hydrology is mostly concernedwith disposal of waterborne waste generatedwithin urban and peri-urban areas, which in mostcases gets mixed with the storm water. The mainvehicle for wastewater collection and disposal isthe sewerage system. Two types of sewerage sys-tems in use are: (i) mix type, where the waste wateris mixed with storm water; and (ii) separate type,where there are two separate pathways of convey-ing generated waste and storm waters towards thereceiving river or any other water body.

Historically municipalities, consulting engineers,and individuals had the option of choosing be-tween centralized or decentralized wastewatermanagement and chose from a variety of collectionand disposal technologies to implement their man-agement strategy. The final choice was based ona combination of cost/benefit consideration, urbandevelopment patterns, its scientific basis, tradition,socio-cultural attitudes of the concerned popula-tions, prevailing public opinion on sanitation, po-litical environment of the region, and several otherfactors. Two main types of wastewater manage-ment strategies can be identified: (i) centralized,where all waste water generated is collected andconveyed to a central location for treatment and/ordisposal; and (ii) decentralized, where the wastewater is treated to the primary level or disposed offon-site or near the source itself.

10.2.1 Municipal waste water

Municipal waste water is a combination of liq-uid or water-borne wastes originating in the sani-tary systems of households, commercial/industrialunits, and institutions, in addition to any ground-water, surface water, and storm water that may be

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generated in the area and discharged into thesewerage system (Fig. 10.1 and Fig. 10.2). Un-treated waste water generally contains high levelsof organic material, variety of pathogenic micro-organisms, as well as nutrients and toxic com-pounds. It thus entails environmental and healthhazards and consequently must immediately beconveyed away from its source locations andtreated appropriately before its final disposal. Theultimate goal of waste water management is pro-tection of the environment in a manner com-mensurate with public health and socio-economicconcerns.

Wastewater quality may be defined by its physi-cal, chemical, and biological characteristics. Phys-ical parameters include colour, odour, tempera-ture, and turbidity. Included in this category areinsoluble substances such as solids, oil, and grease.

Solids may be further subdivided into suspendedand dissolved solids, as well as organic (volatile)and inorganic mineral fractions. Chemical parame-ters associated with the organic content of wastewater include biochemical oxygen demand (BOD),chemical oxygen demand (COD), total organic car-bon (TOC), and total oxygen demand (TOD). In-organic chemical parameters include salinity, hard-ness, pH, acidity, and alkalinity, as well as ionizedmetals such as iron and manganese, and anionicspecies such as chlorides, sulphates, sulphides,nitrates, and phosphates. Bacteriological parame-ters include coliforms, faecal coliforms, specificpathogens, and viruses. Both types of constituentsand their concentrations vary with time and localconditions. Typical ranges of concentration of var-ious constituents in untreated domestic waste wa-ter are shown in Table 10.4. Sewage waters are

Table 10.4 Typical composition of untreated domestic waste water. Adapted from Metcalf and Eddy (1991).

Concentration

Contaminants Unit Weak Medium Strong

Total solids (TS) mg l−1 350 720 1200Total dissolved solids (TDS) mg l−1 250 500 850

Fixed mg l−1 145 300 525Volatile mg l−1 105 200 325

Suspended solids mg l−1 100 220 350Fixed mg l−1 20 55 75Volatile mg l−1 80 165 275

Settleable solids mg l−1 5 10 20BOD5, 20◦C mg l−1 110 220 400TOC mg l−1 80 160 290COD mg l−1 250 500 1000Nitrogen (as total N) mg l−1 20 40 85

Organic mg l−1 8 15 35Free ammonia mg l−1 12 125 50Nitrites mg l−1 0 0 0Nitrates mg l−1 0 0 0

Phosphorus (as total P) mg l−1 4 8 15Organic mg l−1 1 3 5Inorganic mg l−1 3 5 10

Chlorides mg l−1 30 50 100Sulphate mg l−1 20 30 50Alkalinity (as CaCO3) mg l−1 50 100 200Grease mg l−1 50 100 150Total coliforms No.

per/100 ml10−6–10−7 10−7–10−8 10−7–10−9

Volatile organic compounds µg l−1 <100 100–400 >400

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Table 10.5 Important contaminants in terms of their potential effects on receiving waters and their treatment concerns.Adapted from Metcalf and Eddy (1991).

Contaminant Reason for importance

Suspended solids (SS) Can lead to development of sludge deposits and anaerobicconditions when untreated waste water is discharged into anaquatic environment

Biodegradable organics These are made up mainly of proteins, carbohydrates, andfats. They are commonly measured in terms of BOD andCOD. If discharged into inland rivers, streams, or lakes, theirbiological stabilization can deplete natural oxygen levels andcause septic conditions that are detrimental to aquatic species

Pathogenic organisms These can cause infectious diseases

Priority pollutants, including organic and inorganiccompounds

These may be highly toxic, carcinogenic, mutagenic, orteratogenic

Refractory organics including surfactants, phenols andagricultural pesticides

These tend to interfere with conventional wastewatertreatment

Heavy metals These are usually contributed by commercial and industrialactivities and must be removed for reuse of waste water asthey tend to bio-concentrate in the food chain

Dissolved inorganic constituents such as calcium, sodiumand sulphate

May be undesirable for many wastewater reuse applications athigh levels of concentration

classified as strong, medium, or weak, dependingon the level of their contaminant concentration.

The effects of discharging untreated waste waterinto the environment are manifold and depend onthe type and concentration of various pollutants.Important contaminants in terms of their potentialeffects on receiving waters and treatment concernsare outlined in Table 10.5.

10.2.2 Wastewater treatment technologies

Physical, chemical, and biological methods are usedto remove contaminants from waste water. In orderto achieve different levels of contaminant removal,individual wastewater treatment procedures arecombined into a variety of systems, classified asprimary, secondary, and tertiary treatments (Fig.10.3). More rigorous treatment of waste water in-cludes removal of specific contaminants as well asremoval and control of nutrients. Natural systemsare also used for treatment of waste water in land-based applications. Sludge resulting from wastewa-ter treatment operations is treated by various meth-

ods in order to reduce its water and organic contentand make it suitable for final disposal and/or reuse.This section briefly describes various conventionaland advanced technologies that are in current useand explains how these are applied for effectivetreatment of municipal waste water.

Preliminary treatment prepares influent wastewater for further treatment by reducing or eliminat-ing unfavourable wastewater characteristics suchas the presence of large solids and rags, abra-sive grit, odours, and sometimes unacceptably highhydraulic or organic loading. Preliminary treat-ment processes consist of physical unit operations,namely screening and comminution for removalof debris and rags, grit removal for elimination ofcoarse suspended matter, and flotation for removalof oil and grease. Other preliminary treatment oper-ations include flow equalization, septage handling,and odour control methods.

Primary treatment involves partial removal ofsuspended solids and organic matter from wastewater by means of physical methods such as screen-ing and sedimentation. Pre-aeration or mechanical

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Fig. 10.3 Flow diagram showing various treatment levels in conventional wastewater treatment plants. Preliminarytreatment operations include screening and comminution, grit removal, and flow equalization, and to prepare the influentwastewater for further treatment. Redrawn from (UNESCWA, 2003).

flocculation with chemical additives can be used toenhance the effect of primary treatment, which is aprecursor to the secondary treatment. The effluentfrom primary treatment contains a good deal of or-ganic matter and is characterized by relatively highBOD.

The purpose of secondary treatment is removalof soluble and colloidal organics and suspendedsolids that are not taken care of in the primarytreatment. This is done typically through biologicalprocesses, namely treatment by activated sludge,fixed-film reactors, or lagoon systems and sedimen-tation.

Tertiary treatment is a step beyond the conven-tional secondary level treatment in removing signif-icant amounts of nitrogen, phosphorus, heavy met-als, biodegradable organics, bacteria, and viruses.In addition to biological nutrient removal pro-cesses, unit operations frequently used for thispurpose include chemical coagulation, floccula-tion, and sedimentation, followed by filtrationand passing the sewage through an activated car-bon bed. Less commonly used processes includeion exchange and reverse osmosis for removal

of specific ion(s) or for reduction of dissolvedsolids.

As mentioned above and shown in Fig. 10.3,wastewater treatment involves a combination ofseveral physical, chemical, and biological pro-cesses. Various unit operations included in eachcategory are listed in Fig. 10.4.

10.2.2.1 Physical unit operations

10.2.2.1.1 Screening

One of the oldest treatment methods, such asscreening of waste water, removes gross pollu-tants from the waste stream to protect downstreamtreatment plant equipment from damage, avoid in-terference with plant operations, and prevent un-desirable floating material from entering the pri-mary settling tanks. Screening devices may consistof parallel bars, rods or wires, grating, wire mesh,or perforated plates, to intercept large floating orsuspended material. The openings may be of anyshape, but are generally circular or rectangular. Thematerial retained in the waste water, after manual

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Fig. 10.4 Various unit operations and processesin a conventional wastewater treatment system.Redrawn from (UNESCWA, 2003).

or mechanical cleaning of bar racks and screens,is referred to as ‘screenings’ and is disposed off ei-ther by burial or by incineration or returned intothe waste flow after grinding (Metcalf and Eddy1991).

10.2.2.1.2 Comminution

Comminution is a process involving pulverizationof large floating material in the wastewater flowwhere handling of the screenings is impractical,generally between the grit chamber and the pri-mary settling tanks. For this, comminutors witheither rotating or oscillating cutters are installed.Rotating-cutter comminutors either employ a sep-arate stationary screen alongside the cutters or acombined screen and cutter rotating together. Adifferent type of comminutor, known as a barmin-utor, involves a combination of a bar screen androtating cutters (Liu and Liptak 2000). Comminu-tion reduces odours, fly menace, and unsightliness.

10.2.2.1.3 Flow equalization

Flow equalization is a technique employed to im-prove the effectiveness of secondary and advancedwastewater treatment processes by levelling outoperating parameters such as flow, pollutant lev-els, and temperature over a period of time. Fluctu-ations in flow are damped out until a near-constant

flow rate is achieved, minimizing the downstreameffects of these parameters.

10.2.2.1.4 Sedimentation

Sedimentation is a widely used unit operation inwastewater treatment and involves gravitationalsettling of heavy particles suspended in a mixture.This process is used for removal of grit and particu-late matter in the primary settling basins, biologicalflocs in the activated sludge settling basins, andchemical agglomerates when a chemical coagula-tion process is used.

10.2.2.1.5 Flotation

Flotation is a unit operation employed to removesolid or liquid particles from the liquid phase byintroducing a gas, usually air bubbles. The gas bub-bles either adhere to the liquid or are trapped in theparticulate structure of the suspended solids, rais-ing the buoyant force of the combined particle–gasbubbles. Particles that have a higher density thanthe liquid can thus be made to rise and float onthe surface of the liquid. In wastewater treatment,flotation is used mainly to remove suspended mat-ter and to concentrate biological sludge. The mainadvantage of flotation over sedimentation is thatvery small or light particles can be removed moreor less completely and in a shorter time. Onceparticles have been raised to the surface by

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flotation, they can be skimmed out. Flotation, ascurrently practised in municipal wastewater treat-ment, uses air exclusively as the floating agent. Fur-thermore, various chemical additives can be intro-duced to accelerate the removal process (Metcalfand Eddy 1991).

10.2.2.1.6 Granular medium filtration

Granular medium filtration of effluents as partof wastewater treatment processes is a relativelyrecent practice but has come to be widely usedfor supplemental removal of suspended solids fromwastewater effluents of biological and chemicaltreatment processes, in addition to the removal ofchemically precipitated phosphorus. The completefiltration process comprises two phases: filtration,and cleaning or backwashing. The waste water tobe filtered is passed through a filter bed consist-ing of granular material (sand, anthracite, and/orgarnet), with or without adding any chemicals.Within the filter bed, suspended solids containedin the waste water are removed by means of a com-plex process involving one or more removal mech-anisms such as straining, interception, impaction,sedimentation, flocculation, and adsorption. Thephenomena that occur during the filtration phaseare basically the same for all types of filters usedin wastewater filtration. The cleaning/backwashingphase, however, differs depending on whether thefiltration process is continuous or semi-continuous.In semi-continuous filtration, filtering and clean-ing operations occur sequentially, whereas incontinuous filtration, filtering and cleaning op-erations occur simultaneously (Metcalf and Eddy1991).

10.2.2.2 Chemical unit processes

Chemical processes employed in wastewater treat-ment are always used in conjunction with physicalunit operations and biological processes. In gen-eral, chemical unit processes have an inherent dis-advantage compared to physical operations in thatthey are additive processes. This means that there isusually a net increase in the dissolved constituentsof waste water. This can become a significant factorif the waste water is to be reused.

10.2.2.2.1 Chemical coagulation

Chemical coagulation of raw waste water be-fore sedimentation promotes flocculation of finelydivided solids into more readily settleable flocs,thereby enhancing removal efficiency of sus-pended solids, BOD5, and phosphorus, as com-pared to the basic process of sedimentation with-out coagulation. The degree of clarification ofwaste water achieved depends on the amount ofchemicals used and the precision with which theprocess is controlled (Metcalf and Eddy 1991).Selection of the coagulant is based on consid-erations of its performance, reliability, and cost.Some commonly used chemical coagulants inwastewater treatment include alum (Al2(SO4)3

14.3 H2O), ferric chloride (FeCl3 6H2O), ferric sul-phate (Fe2(SO4)3), ferrous sulphate (FeSO4 7H2O),and lime (Ca(OH)2). Sometimes organic poly-electrolytes are also used as flocculation agents. Re-moval of suspended solids through chemical treat-ment involves a series of three unit operations:rapid mixing, flocculation, and settling. First, thechemical is added and completely dispersed in thewaste water by rapid mixing for 20–30 secondsin a basin employing a turbine mixer. Coagulatedparticles are then brought together by flocculationby mechanically inducing velocity gradients withinthe liquid. Flocculation takes 15–30 minutes in abasin containing turbine or paddle-type mixers.The advantages of coagulation include greater re-moval efficiency, feasibility of using higher over-flow rates, and consistent performance. On theother hand, coagulation results in a large mass ofprimary sludge that is often difficult to thicken anddewater. It also entails higher operational costs anddemands care on the part of the plant operator.

10.2.2.2.2 Adsorption

Adsorption is the process of collecting dissolvedsubstances in a solution on a suitable interface (seeSection 6.1.10). In wastewater treatment, adsorp-tion with activated carbon – a solid interface –usually follows normal biological treatment and isaimed at removing a part of the remaining dissolvedorganic matter. Particulate matter present in thewaste water may also be removed in this process.Activated carbon is produced by heating charcoal

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to a high temperature and then activating it by ex-posure to an oxidizing gas at high temperatures(600–1200◦C). The gas develops a porous structurein the charcoal and thus creates a large internal sur-face area. The activated charcoal can then be sepa-rated into various sizes with different adsorption ca-pacities. The two most common types of activatedcarbon are granular activated carbon (GAC) with agrain size of more than 0.1 mm, and powdered acti-vated carbon (PAC) with a grain size of less than 200mesh (opening 0.0686 mm). A fixed-bed columnis often used to bring the waste water in contactwith GAC. The waste water is applied at the top ofthe column and withdrawn from the bottom withcarbon held in its place. Backwashing and surfacewashing are applied to limit build-up of head loss.Expanded-bed and moving-bed carbon contactorshave also been developed to overcome the prob-lem of head loss build-up. In the expanded-bed sys-tem, the influent is introduced at the bottom of thecolumn and is allowed to expand. In the moving-bed system, the spent activated carbon is contin-uously replaced with fresh carbon. Spent granu-lar carbon can be regenerated by removal of theadsorbed organic matter from its surface throughoxidation in a furnace. The capacity of the regener-ated carbon is slightly inferior to that of the virgincarbon. Wastewater treatment using PAC involvesaddition of the powder directly to the biologicallytreated effluent or after the physico-chemical treat-ment process. PAC is usually added to waste waterin a contacting basin for a certain length of time. Itis then allowed to settle to the bottom of the tankand removed. Removal of the powdered carbonmay be facilitated by addition of poly-electrolytecoagulants or filtration through granular-mediumfilters. A major problem with the use of powderedactivated carbon is that the methodology of its re-generation is not well defined.

10.2.2.2.3 Disinfection

Disinfection refers to the selective destruction ofdisease-causing micro-organisms. This process isof importance in wastewater treatment owing tothe nature of waste water, which harbours a num-ber of human enteric organisms that are associatedwith various waterborne diseases. Commonly used

means of disinfection include: (i) physical agentssuch as heat and light; (ii) mechanical means suchas screening, sedimentation, filtration, etc.; (iii) ra-diation, mainly gamma rays; (iv) chemical agentsincluding chlorine and its compounds, bromine,iodine, ozone, phenol and phenolic compounds,alcohols, heavy metals, dyes, soaps and syntheticdetergents, quaternary ammonium compounds, hy-drogen peroxide, and various alkalis and acids.The commonly used chemical disinfectants are ox-idizing chemicals, and of these, chlorine is themost widely used (Qasim 1999). Disinfectants actthrough one or more of a number of mechanismsincluding damaging the cell wall, altering cell per-meability as well as the colloidal nature of the pro-toplasm, and inhibiting enzyme activity of micro-organisms. In applying disinfecting agents, severalfactors need to be considered, that is, contact time,concentration and type of chemical agent, intensityand nature of physical agent, wastewater tempera-ture, number of micro-organisms present, and na-ture of the suspending liquid (Qasim 1999).

10.2.2.2.4 Dechlorination

Dechlorination is removal of free and total com-bined chlorine residues from chlorinated wastewa-ter effluent before its reuse or discharge to receiv-ing waters. Chlorine compounds react with manyorganic compounds in the effluent to produce un-desired toxic compounds that cause long-term ad-verse impacts on the water environment and po-tentially toxic effects on aquatic micro-organisms.Dechlorination may be brought about by use of ac-tivated carbon or by addition of a reducing agentsuch as sulphur dioxide (SO2), sodium sulphite(Na2SO3), or sodium metabisulphite (Na2S2O5). Itis important to note that dechlorination does not re-move toxic by-products that are already producedduring the process of chlorination (Qasim 1999).

In addition to the chemical processes describedabove, various other methods are occasionallyapplied in wastewater treatment and disposal(ESCWA 2003).

10.2.2.3 Biological unit processes

Biological unit processes are employed to convertthe finely dispersed as well as dissolved organic

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matter in waste water into flocculent settleableorganic and inorganic solids. In these processes,micro-organisms, particularly bacteria, convert thecolloidal and dissolved carbonaceous organic mat-ter into various gases and into cell tissues that are re-moved in sedimentation tanks. Biological processesare used generally in conjunction with physical andchemical processes, the main objective being to re-duce the organic content (measured as BOD, TOC,or COD) and nutrient content (notably nitrogen andphosphorus) of waste water. Five major biologicalprocesses used in wastewater treatment may beclassified as: (a) aerobic processes; (b) anoxic pro-cesses; (c) anaerobic processes; (d) combined pro-cesses; and (e) pond processes. These processesare further subdivided, depending on whether thetreatment takes place in a suspended-growth sys-tem, an attached-growth system, or a combinationof both. Some of the commonly used biological sys-tems are trickling filters, activated sludge process,aerated lagoons, rotating biological contactors, andstabilization ponds.

10.2.2.3.1 Activated sludge process

The activated sludge process is an aerobic,continuous-flow system containing a mass of

activated micro-organisms that are capable of sta-bilizing organic matter. The process consists of de-livering clarified waste water after primary settle-ment into an aeration basin where it is mixed withan active mass of micro-organisms, mainly bacteriaand protozoa, which aerobically degrade organicmatter into carbon dioxide, water, new cells, andother end products. The bacteria involved in acti-vated sludge systems are primarily Gram-negativespecies, including carbon oxidizers, nitrogen ox-idizers, floc formers as well as non-floc formers,aerobes, and facultative anaerobes. The protozoa,for their part, include flagellates, amoebas, and cil-iates. An aerobic environment is maintained in thebasin by means of diffused or mechanical aeration,which also serves to keep the contents of the reac-tor (or mixed liquor) well mixed. After a specificretention time, the mixed liquor passes into thesecondary clarifier where the sludge is allowed tosettle and a clarified effluent is produced for dis-posal. The process recycles a portion of the settledsludge back to the aeration basin to maintain therequired activated sludge concentration (Fig. 10.5).In this process, a portion of the settled sludge is de-liberately discarded to maintain the required solidsretention time (SRT) for effective removal of or-ganics. Control of the activated-sludge process is

Primarysedimentation

Gritchamber

Barracks

Influent

Effluent Chlorinecontact

chamber

Settlingtank

Aerationtank

Return sludge

Screenings Grit Sludge

Wastesludge

Cl orNaOCl

2

Wastesludge

Fig. 10.5 Typical flow diagram of an activated-sludge process. Redrawn from (UNESCWA, 2003).

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important to maintain a high treatment perfor-mance level under a wide range of operating con-ditions. Important factors in process control are:(a) maintenance of dissolved oxygen levels in theaeration tanks; (b) regulation of the amount ofreturned activated sludge; and (c) control of thewaste activated sludge. The main operational prob-lem encountered in such a system is sludge bulk-ing, which can be caused by absence of phospho-rus, nitrogen, and trace elements and large fluctu-ations in pH, temperature, and dissolved oxygen(DO). Bulky sludge has poor settleability and com-patibility due to the excessive growth of filamen-tous micro-organisms. This problem can be con-trolled by chlorination of the returned sludge (Liuand Liptak 2000; Metcalf and Eddy 1991).

10.2.2.3.2 Aerated lagoons

An aerated lagoon is a basin of depth between1 and 4 m, in which waste water is treated ei-ther on a flow-through basis or by recycling ofsolids. The microbiology involved in this processis similar to that of the activated-sludge process.However, differences arise because the large sur-face area of a lagoon may cause more temperature-dependent effects than are ordinarily encounteredin conventional activated-sludge processes. Wastewater is oxygenated by surface exposure, turbineaeration, or diffused aeration. The turbulence cre-ated by aeration is used to maintain contents ofthe basin in the state of suspension. Depending onthe retention time, aerated lagoon effluents containapproximately one-third to one-half the incomingBOD value in the form of cellular mass. Most of thesolids must be removed in a settling basin beforethe effluent is finally discharged (Fig. 10.6).

10.2.2.3.3 Trickling filters

A trickling filter is the most commonly encoun-tered aerobic attached-growth biological treatmentprocess used for removal of organic matter fromwaste water. It consists of a bed of highly per-meable medium, which attaches micro-organisms,forming a biological slime layer through whichwaste water is made to percolate. The filter mediumusually consists of rock or plastic packing material.The organic material present in the waste wateris degraded by adsorption onto the biological slimelayer. In the outer portion of this layer, waste wateris degraded by aerobic micro-organisms. As micro-organisms grow, the thickness of the slime layerincreases and the oxygen is depleted before it haspenetrated the full depth of the slime layer. Ananaerobic environment is thus established near thesurface of the filter medium. As the slime layer in-creases in thickness, the organic matter is degradedbefore it reaches the micro-organisms present nearthe surface of the medium. Deprived of their ex-ternal organic source of nourishment, the micro-organisms die and are washed off by the flowingliquid. A new slime layer grows in its place. Thisphenomenon is referred to as ‘sloughing’ (Liu andLiptak 2000; Metcalf and Eddy 1991).

After passing through the filter, the treated liq-uid is collected in an under-drain system, togetherwith any biological solids that have detached fromthe medium (Fig. 10.7). The collected liquid thenpasses to a settling tank where solids are sep-arated from the treated waste water. A portionof the liquid collected in the under-drain systemor the settled effluent is recycled to dilute thestrength of the incoming waste water and to main-tain the biological slime layer in a moist condition(Fig. 10.8).

BarRacks

Activatedlagoon

Settlingtank

Chlorinecontact

chamber

Influent Effluent

ScreeningsReturn sludge

SludgeCl orNaOCl

2

Fig. 10.6 Typical flow diagram of an aerated lagoon. Redrawn from (UNESCWA, 2003).

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Under drain

DistributorFilter material

Filter floor

Fig. 10.7 Cutaway view of a trickling filter. Redrawn from (UNESCWA, 2003).

10.2.2.3.4 Rotating biological contractors

A rotating biological contractor (RBC) is anattached-growth biological process that consists ofone or more basins in which large closely spacedcircular disks mounted on horizontal shafts rotateslowly through waste water (Fig. 10.9). The disks,made of high density polystyrene or polyvinyl chlo-ride (PVC), are partially submerged in the wastewater so that a bacterial slime layer forms on theirwetted surfaces. As the disks rotate, the bacteriapresent in the waste water are exposed alternatelyto waste water from which they adsorb organicmatter and to air from which they absorb oxygen.Their rotary movement also allows excess bacte-

ria to be removed from surfaces of the disks, thusmaintaining a suspension of sloughed biologicalsolids. A final clarifier is needed to remove sloughedsolids. Organic matter is degraded by means ofmechanisms similar to those operating in tricklingfilters. Partially submerged RBCs are used for car-bonaceous BOD removal, combined carbon oxida-tion, and nitrification of secondary effluents. Com-pletely submerged RBCs are used for denitrifica-tion (Metcalf and Eddy 1991). A typical arrange-ment of RBCs is shown in Fig. 10.9. In general,RBC systems are divided into a series of indepen-dent stages or compartments by means of baffles ina single basin or separate basins arranged in stages.Compartmentalization creates a plug flow pattern,


Gritchamber

Barracks

Influent


chamber

Settlingtank

Tricklingfilters

Return effluent


Wastesludge

Cl orNaOCl

2

Wastesludge

Fig. 10.8 Typical flow diagram of a trickling filter treatment unit. Redrawn from (UNESCWA, 2003).

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Gritchamber

Barracks

Influent



chamber

Settlingtank

RBC Units

Sludge

Cl orNaOCl

2

Drive motor

Baffle

Fig. 10.9 Typical flow diagram of a RBC treatment unit. Redrawn from (UNESCWA, 2003).

increasing the overall removal efficiency of micro-organisms. It also promotes a variety of conditionswhere different organisms can flourish to varyingdegrees. As waste water flows through the variouscompartments, each subsequent stage receives in-fluent with a lower organic content than the previ-ous stage; the system thus enhances removal of theorganics.

10.2.2.3.5 Stabilization ponds

A stabilization pond is a relatively shallow bodyof waste water contained in an earthen basin, us-ing a completely mixed biological process with-out solids being returned. Mixing may be eithernatural (caused by wind, heat, or fermentation) orinduced (due to mechanical or diffused aeration).Stabilization ponds are usually classified on the ba-sis of the nature of the biological activity that takesplace in them – as aerobic, anaerobic, or a com-bination of aerobic-anaerobic processes. Aerobicponds are used primarily for treatment of solubleorganic wastes and effluents from wastewater treat-ment plants. Aerobic-anaerobic (facultative) pondsare the most common type and are being usedto treat domestic waste water and a wide varietyof industrial wastes. Anaerobic ponds are particu-larly effective in bringing about rapid stabilization

of high concentrations of organic wastes. Aerobicand facultative ponds are biologically complex. Thebacterial population oxidizes organic matter pro-ducing ammonia, carbon dioxide, sulphates, water,and other end products that are subsequently usedby algae during sunlight hours to produce oxygen.Bacteria use this oxygen and also the oxygen pro-vided by air flowing over the pond to break downthe remaining organic matter. Wastewater reten-tion time ranges between 30 and 120 days. This isa treatment process that is commonly employed inrural areas, because of its low construction and op-erating costs. Fig. 10.10 presents a typical flow dia-gram of stabilization ponds (Liu and Liptak 2000).

10.2.2.3.6 Completely mixed anaerobicdigestion

Anaerobic digestion involves biological conver-sion of organic and inorganic matter in the absenceof molecular oxygen to a variety of end-products,including methane and carbon dioxide. A host ofanaerobic organisms work together to degrade or-ganic sludge and waste in three steps comprising:(i) hydrolysis of high molecular mass compounds;(ii) acidogenesis; and (iii) methanogenesis. The pro-cess takes place in an airtight reactor. Sludge is in-troduced either continuously or intermittently and

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Fig. 10.10 Typical flow diagram of stabilization ponds. Redrawn from (UNESCWA, 2003).

retained in the reactor for varying periods of time.After withdrawal from the reactor, whether con-tinuous or intermittent, the stabilized sludge haslower organic and pathogen contents and is non-putrescent. The two widely used types of anaero-bic digesters are standard-rate and high-rate. In thehigh-rate digestion process, contents of the digesterare heated, mixed completely, and retained typi-cally for a period of 15 days or less. In the standard-rate digestion process, contents of the digester areusually not subjected to either heating or mixingand are retained for a period ranging from 30–60days. A combination of these two basic processesis known as the two-stage process, which is used toseparate the digested solids from the supernatantliquor. However, additional digestion and gas pro-duction may occur (Metcalf and Eddy 1991). Anaer-obic digesters are commonly used for the treatmentof sludge and waste water with high organic con-tent. Disadvantages of a system of this kind, as com-pared to aerobic treatment, stem directly from theslow growth rate of methanogenic bacteria. A slowgrowth rate necessitates a relatively long retentiontime in the digester for adequate waste stabiliza-tion to occur. An advantage of this type of systemis production of methane gas, which can be used asa fuel if produced in sufficient quantities. Further-more, the system produces a well-stabilized sludge,which can be safely disposed off in a sanitary land-fill after drying or dewatering. On the other hand,the fact that a high temperature is required for ade-quate treatment is a major drawback of this system.

10.2.2.3.7 Biological nutrient removal

Nitrogen and phosphorus are the two main nu-trients of concern in waste water, as these may

accelerate eutrophication of lakes and reservoirsand stimulate the growth of algae and rootedaquatic plants in shallow streams. A significant con-centration of nitrogen may have other undesirableeffects, such as depletion of dissolved oxygen inreceiving waters, causing toxicity to aquatic life,adverse impact on chlorine disinfection efficiency,creation of public health hazard, and waste waterthat is less suitable for reuse. Nitrogen and phos-phorus can be removed by physical, chemical, andbiological methods. Biological removal of these nu-trients is described below.

10.2.2.3.8 Nitrification-denitrification

Nitrification is the first step in the removal of ni-trogen using a biological process. It is the workingof two bacterial genera: (i) Nitrosomonas, whichoxidizes ammonia to the intermediate product ni-trite; and (ii) Nitrobacter, which converts nitriteto nitrate. Nitrifying bacteria are sensitive organ-isms and are extremely susceptible to a wide va-riety of inhibitors such as high concentrations ofammonia and nitrous acid, low DO levels (<1mg l−1), pH outside the optimal range (7.5–8.6),and so on. Nitrification can be achieved throughboth suspended growth and attached growth pro-cesses. In suspended growth processes, nitrifica-tion is brought about either in the same reactorthat is used for carbonaceous BOD removal or ina separate suspended-growth reactor following theconventional activated sludge treatment process.Ammonia is oxidized to nitrate either by air or high-purity oxygen. Similarly, nitrification in an attachedgrowth system may be brought about either in thesame attached growth reactor that is used for car-bonaceous BOD removal or in a separate reactor.

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Trickling filters, rotating biological contactors, andpacked towers can be used as nitrifying sys-tems.

Denitrification involves removal of nitrogen inthe form of nitrate by conversion to nitrogen gas un-der anoxic conditions. In denitrifying systems, DOis a critical parameter. Its presence suppresses theenzymatic action needed for denitrification. Theoptimal pH is in the range 7–8. Denitrification canbe achieved through both suspended and attachedgrowth processes. Suspended growth denitrifica-tion takes place in a plug flow type of activatedsludge system. An external carbon source is usuallynecessary for micro-organism cell synthesis, sincethe nitrified effluent is low in carbonaceous mat-ter. Some denitrification systems use the incomingwaste water itself for this purpose. A nitrogen gasstripped reactor should precede the denitrificationclarifier, because nitrogen gas hinders settling ofthe mixed liquor. Attached growth denitrificationtakes place in a column reactor containing stonesor one of a number of available synthetic mediaupon which bacteria grow. Periodic backwashingand an external carbon source are necessary in asystem of this kind.

10.2.2.3.9 Phosphorus removal

Phosphorus appears in water as orthophosphate(PO4

−3), polyphosphate (P2O7), and organicallybound phosphorus. Microbes utilize phosphorusfor their cell synthesis and also for their metabolicprocesses. As a result, 10 to 30% of all influentphosphorus is removed during secondary biologi-cal treatment. More phosphorus can be removedif one of the several specially developed biologi-cal phosphorus removal processes is used. Theseprocesses are based on exposing microbes in anactivated-sludge system to alternating anaerobicand aerobic conditions. This causes stress on themicro-organisms, as a result of which their uptakeof phosphorus exceeds the normal levels.

10.2.3 Natural treatment systems

Natural systems for wastewater treatment are de-signed to take advantage of physical, chemical, and

biological processes that occur in the natural envi-ronment when water, soil, plants, micro-organisms,and the atmosphere interact with each other(Metcalf and Eddy 1991). Natural treatment systemsinclude land treatment, floating aquatic plants,and constructed wetlands. All natural treatmentsystems are preceded by some form of mechanicalpre-treatment for removal of bulk solids. Wheresufficient land suitable for the purpose is available,these systems can often be the most cost-effectiveoption in terms of both construction and oper-ation. They are generally well suited for smallcommunities and rural areas (Reed et al. 1988).

10.2.3.1 Land treatment

Land treatment involves controlled application ofwaste water to the land at rates compatible with thenatural physical, chemical, and biological processesthat occur in the soil at a given location. Threemain types of land treatment systems in use are:(i) slow rate (SR); (ii) overflow (OF); and (iii) rapidinfiltration (RI) systems.

10.2.3.1.1 Slow rate systems

Slow rate (SR) land application systems are impor-tant amongst the various land treatment systems formunicipal and industrial waste water. This tech-nology incorporates wastewater treatment, waterreuse, utilization of nutrients for growing crops,and wastewater disposal. It involves application ofwaste water to vegetated land by means of varioustechniques, including sprinklers or surface tech-niques such as graded border and furrow irrigation.Water is applied intermittently (every 4–10 days) tomaintain aerobic conditions in the soil profile. Theapplied water is lost by evapotranspiration as wellas by percolation vertically downward and also lat-erally through the soil profile. Any surface runoff iscollected and reapplied to the system. Treatmentoccurs as the waste water percolates through thesoil profile (Table 10.6). In most cases, the perco-lated water either joins the underlying groundwateror it may be intercepted by natural surface waters,or recovered by means of sub-surface drains or byrecovery wells (Reed et al. 1988).

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Table 10.6 Mechanisms of wastewater constituent removalby slow rate (SR) land treatment systems.

Parameter Removal mechanism

BOD Soil adsorption and bacterial oxidation

SS Filtration through the soil

Nitrogen Crop uptake, denitrification, ammoniavolatilization, soil storage

Phosphorus Chemical immobilization (precipitationand adsorption), plant uptake

Metals Soil adsorption, precipitation, ionexchange, complexation

Pathogens Soil filtration, adsorption, desiccation,radiation, predation, exposure to otheradverse environmental factors

Trace organics Photodecomposition, volatilization,sorption, degradation

Source: (UNESCWA, 2003).

SR systems can be classified into two types, Type1 and Type 2, based on the design objectives. Type1 systems are designed primarily for wastewatertreatment itself rather than for crop production.Accordingly, in systems of this kind, the maximumpossible amount of water is applied per unit landarea. Type 2 SR systems, in contrast, are designedmainly with a view to reuse water for crop produc-tion, and consequently the amount of water appliedin a system of this kind is just enough to satisfy theirrigation requirements of the crop being grown. Ofall natural treatment systems, SR systems have thehighest potential for land treatment of waste water.

10.2.3.1.2 Rapid infiltration systems

Rapid infiltration (RI) is the most intensive of allland treatment methods. Relatively high hydraulicand organic loadings are applied intermittently toshallow infiltration or spreading basins (Fig. 10.11).The RI process uses the soil matrix itself for phys-ical, chemical, and biological treatment. Physicalstraining and filtration occur at the soil surface andalso within the soil matrix. Chemical precipitation,ion exchange, and adsorption occur as water per-colates through the soil. Biological oxidation, as-similation, and reduction occur within top one ortwo metres of the soil. Vegetation growth is not

allowed in systems of this kind. The RI system isdesigned to meet multiple performance objectives,including the following (Metcalf and Eddy 1991;Sanks and Asano 1976):

1. Recharging of streams by interception ofgroundwater;

2. Recovery of water by wells or sub-surface drainswith subsequent reuse or discharge;

3. Groundwater recharge;4. Temporary storage of renovated water in local

aquifers.

As both unsaturated soil zone and the underlyingphreatic aquifer are involved in this treatment pro-cess, rapid infiltration systems are also known assoil aquifer treatment (SAT) systems.

10.2.3.1.3 Overland flow systems

Overland flow (OF) is a treatment process in whichwaste water is treated as it flows down a network ofvegetated sloping terraces. Waste water is appliedintermittently to the top part of each terrace, whichflows down the terrace to a runoff collection chan-nel at the bottom of the slope (Fig. 10.12). Applica-tion techniques include high-pressure sprinklers,low-pressure sprays, or surface methods such asgated pipes. OF systems are normally used with rel-atively impermeable surface soils, since in contrastto SR and RI systems, natural infiltration throughsoil is limited. The effluent waste water undergoesa variety of physical, chemical, and biological treat-ment mechanisms while it flows as surface runoff.Overland flow systems can be designed for sec-ondary treatment, advanced secondary treatment,or nutrient removal, depending on user require-ments (Reed et al. 1988; Sanks and Asano 1976).

10.2.3.2 Constructed wetlands

Wetlands are inundated land areas with waterdepths typically shallower than 0.6 m that sup-port growth of emergent plants such as cattail, bul-rush, reeds, and sedges. The vegetation providessurfaces for attachment of bacteria, aids filtrationand adsorption of wastewater constituents, trans-fers oxygen into the water column, and controls

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Fig. 10.11 Schematic diagram showing rapid infiltration treatment system. Redrawn from (UNESCWA, 2003).

growth of algae by restricting penetration of sun-light. Two types of constructed wetlands have beendeveloped for wastewater treatment, namely: (i)free water surface (FWS) systems; and (ii) subsur-face flow systems (SFS) (Reed et al. 1988; Sanksand Asano 1976).

10.2.3.2.1 Free water surface systems

Free water surface (FWS) systems consist of paral-lel shallow basins with depth ranging from 0.1–0.6m or channels with relatively impermeable bottomsoil or subsurface barrier and emergent vegetation(Fig. 10.13a). As a rule, pre-clarified waste wateris applied continuously and undergoes quality im-provement as it flows through the stems and rootsof the emergent vegetation.

10.2.3.2.2 Subsurface flow systems

Subsurface flow systems (SFSs) consist of beds orchannels filled with gravel, sand, or other perme-able media planted with emergent vegetation (Fig.10.13b). Waste water is treated as it flows horizon-tally through the media-plant filter. Systems of thiskind are designed for secondary or higher levels oftreatment.

10.2.3.3 Floating aquatic plants

This system is similar to the FWS system, exceptthat the plants used are of the floating type, suchas water hyacinths and duckweeds. Water depths,ranging from 0.5–1.8 m, are greater than in the

Fig. 10.12 Schematic showing cross-section across an overland flow system for wastewater treatment. Redrawn from(UNESCWA, 2003).

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URBAN HYDROLOGY 317

Fig. 10.13 Schematic of two types of constructed wetlands for wastewater treatment: (a) Free water surface (FWS) system;(b) subsurface flow system. The essential difference between the two is the level of water surface. Redrawn from Qasim(1999).

case of wetland systems. The floating plants shieldthe water from sunlight and reduce the growthof algae. Systems of this kind are effective in re-ducing BOD, nitrogen, metals, and trace organicsand also in removing algae from lagoons and stabi-lizing pond effluents. Supplementary aeration hasbeen used with floating plant systems to increasetreatment capacity and to maintain the aerobicconditions necessary for biological control ofmosquitoes (Metcalf and Eddy 1991).

10.2.4 Sludge treatment and disposal

Sewage sludge consists of organic and inorganicsolids present in the raw waste water and removedin the primary clarifier, in addition to organic solidsgenerated in secondary/biological treatment andremoved in the secondary clarifier or in a sepa-rate thickening process. The generated sludge isusually in the form of liquid or semi-solid, con-taining 0.25 to 12% solids by weight, dependingon the treatment method used. Sludge handling,treatment, and disposal are complex operations,owing to the presence of offensive constituents,depending upon the source of waste water and thetreatment processes employed (Metcalf and Eddy1991). Sludge is treated by means of a variety ofprocesses that can be used in various combina-tions. Various unit sludge treatment operations andprocesses currently in use include thickening, con-ditioning, dewatering (primarily to remove mois-ture from sludge), digestion, composting, incinera-tion, wet-air oxidation, and vertical tube reactors totreat or stabilize the organic matter present in thesludge.

10.3 New approaches and technologies forsustainable urbanization

The focus on urban hydrology discussed thus farhas been on understanding the hydrologic impactsof urbanization and the conventional approachesto providing water supply, treatment, and disposalsystems for storm and waste water. These ap-proaches forming the basic framework of urbandevelopment and planning have made cities theworld over more liveable with regard to the publichealth and recreational aspects and will no doubtcontinue to be relevant for a long time to come. Buta price has to be paid – in terms of jeopardizing theenvironmental sustainability and consequently thewell-being of future generations. Even as cities be-come more populous and their number increases,impacts of urbanization spread to surrounding re-gions as a result of which water supplies and sanita-tion services, including wastewater treatment andsafe disposal, get stressed to meet demands of ever-increasing populations and correspondingly the fi-nancial resources. New approaches are, therefore,being formulated and practised in some areas, withthe objective of ensuring environmentally soundand sustainable cities.

Broadly, the objectives of sustainable urban de-velopment are: (i) operational sustainability forpublic health, flood protection/mitigation, andstructural integrity; (ii) environmental sustainabil-ity including pollution mitigation, efficient useof natural resources including reuse of wastes,and preservation of the natural hydrologic cycle;and (iii) economic sustainability, including cost-effective technology and cost recovery.

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10.3.1 The need

As discussed above, concentrated human settle-ments with their propensity to create hard, imper-meable surfaces for building houses and roads, andthe need for water intake and outflow in a varietyof forms, are not in harmony with the natural hy-drologic cycle. The adverse effects of creating im-pervious surface cover in urbanized watersheds, re-ducing the groundwater recharge and consequentreduction in the base flow of the stream/river flow-ing through the area, are well documented (Cian-frani et al. 2007). Sewerage and water supply sys-tems serving dense settlements can further inter-fere with groundwater and surface water hydrol-ogy. Urban settlements also create the ‘heat islandeffect’, reduce evapotranspiration, and modify lo-cal microclimates (Ward and Trimble 2003).

The historic shift from population settlementbased on resource exploitation to one basedon economy driven, transportation, and amenity-based settlement patterns poses even greater chal-lenges for achieving sustainable water supplies andwater management, as it puts increasingly largerpopulation and land-use transformation in areasthat previously served as ‘water banks’ for meetingthe requirement of population residing in the area.

The major driving force for change is essentiallypopulation growth coupled with rising standardsof living globally, a combination that has resultedin resource over-exploitation, including water. Thecurrent world population is about 6 billion, whichis expected to grow to about 9 billion by the year2050. When the population was much smaller (e.g.<2 billion) and when per capita use of resourceswas also much smaller, the traditional pattern of re-source consumption, namely, ‘take, make, waste’was sustainable. However, what is now neededis to recycle and reuse all types of resources (in-cluding water) and also increase the use of re-newable resources. Water stress currently affectsonly a modest fraction of the human population,but it is expected to affect 45% of the popula-tion by 2025 (Daigger 2007; WRI 1966). This situa-tion may be further exacerbated by global climatechange, which may alter water supply and stor-age patterns in ways that would make the existingwater-management infrastructure ineffective.

Recycling technologies can significantly reducenet water abstraction from the environment butmany of these technologies require an increasein the use of other resources, especially energy.In our resource-constrained world, increasing theconsumption of any resource, even for necessaryfunctions such as water management, must be care-fully considered.

Another aspect of water stress caused by ur-ban water-management systems is the increase inthe amount of nutrients, especially phosphorus, inthe aquatic environment (Wilsenach et al. 2003).Mined as phosphate rock, phosphorus is used formanufacturing fertilizers that are used widely toincrease crop production for human consump-tion. Phosphorus (and other nutrients) then passesthrough human metabolism and ends up in thewastewater discharge. When these effluents are dis-charged into the aquatic environment, the excessnutrients can cause eutrophication of surface wa-ter bodies. At the current rate of consumption, thesupply of phosphate, an essential nutrient with noknown replacement, is expected to be exhaustedin about 100 years. Thus, there are at least two ur-gent reasons for us to recover phosphate from thewastewater stream.

Two other factors must be taken into consid-eration. First, although water supply is uniformlyprovided in the developed world, approximately 1billion people in the developing world do not haveaccess to safe drinking water, and more than 2.5billion do not have access to adequate sanitation.Clearly to meet global needs, a more efficient urbanwater management system is needed.

10.3.2 Achieving sustainability goals

Urban water supply, storm water, and wastewatermanagement is at a critical juncture all over theworld. Methods must evolve in response to urbandevelopment, population growth, and diminishingnatural resources. Based on information available inrecent literature, three aspects of urban water man-agement are becoming increasingly important andwill continue to be important in the foreseeablefuture. These aspects are: (i) decentralized wastewater management (DWM); (ii) wastewater recla-mation and reuse; and (iii) increased attention to

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URBAN HYDROLOGY 319

Aquifer for saline water

Rainwater Harvesting

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Fig. 10.14 Schematic diagram of a decentralized urban water management system. Modified from Daigger (2008).

wet-weather flow (WWF) management. Currently,consideration of these three aspects in urban watermanagement planning is improving the functional-ity of wastewater systems and creating sustainablealternatives to the traditional centralized combinedor segregated sewerage systems.

However, if sustainability goals as defined aboveare to be achieved, the current ‘take, make, waste’approach to water and resource managementneeds to give way to a closed-loop approach, involv-ing a combination of decentralized and centralizedelements for recycling both liquid as well as solidwaste material. Because of their superior environ-mental performance, closed-loop systems have thepotential to meet the environmental, social, andeconomic goals of sustainability (Daigger 2007).

A closed-loop system for urban water manage-ment is schematically depicted in Fig. 10.14. Thewater supply, both domestic and commercial, issegregated into water for potable uses, such as di-rect consumption and bathing, and water for non-potable uses, such as toilet flushing, laundry, irriga-tion, and industrial uses. Overall, the requirementof potable water is quite small, though it shouldbe of the highest quality in respect of its physical,chemical, biological, and radiological properties.The amount of water needed from the environ-ment necessary for this purpose is much smaller

than the amount of domestic water that is beingcurrently provided. In fact, the demand for potablewater can be met either from local water suppliesor by importing modest quantities of potable wa-ter. By separating potable and non-potable water,the net removal of water from the environmentfor potable uses can be dramatically reduced. Thebulk of the domestic and commercial water require-ment is of non-potable water quality, which can besupplied from a variety of local sources, includ-ing recycled water and captured rainwater, supple-mented by modest import of water. As shown inFig. 10.14, storage of non-potable water is a criticalcomponent of the system. Non-potable water canbe stored either in an aquifer underlying the urbanarea (as shown) or in a surface storage facility if therequisite land area is available.

The repeated recycling of water may result in thebuild-up of dissolved solids, including salts, whichmust be managed to maintain the quality of the wa-ter for its intended use(s). Mixing with rainwateror employing reverse osmosis (RO) and other pro-cesses can dilute/remove the dissolved salts. Thewaste brine can be discharged into a saline-wateraquifer (Fig. 10.14) or disposed off in some otherway.

A question that is often asked relates to desali-nation of brackish/saline groundwater or surface

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source, or even sea water in water-scarce regions.Although this is technologically feasible, desalina-tion does not always meet the environmental crite-ria for sustainability because of its significant energyrequirements. Even though technological advancescontinue to reduce the energy requirement, it willalways be higher than for treating waste water, be-cause the content of dissolved solids in waste water(typically ∼1000 mg l−1) is much smaller than thatof sea water (35 000 mg l−1). Therefore, dependingon the need for desalination, water with the leastsalt content should be preferred.

10.3.2.1 Available tools/technologies

The main hydrologic consideration in achievingsustainability is conservation of local water re-sources to meet a variety of local needs withoutcompromising living standards to a significant de-gree. Fortunately, tools and technologies are avail-able that enable: (i) more efficient harvesting andlocal use of storm water; (ii) better water conserva-tion by reducing the consumption; (iii) reclamationand reuse of waste water for a variety of applica-tions; (iv) management and extraction of energyfrom the wastewater stream; (v) recovery of nu-trients; and (vi) separation of specific wastewatersources that are difficult to treat by the commonlyemployed methods. Several of these technologiesare presently in use, though not widely, and havedemonstrated the ability to facilitate implementa-tion of systems, such as the one shown in Fig.10.14, and for improving decentralized and cen-tralized water and resource management.

Technologies are available for managing stormwater that can be collected and used either di-rectly or treated by natural methods and in-filtrated into the groundwater for subsequentuse (see Section 10.2.3, and Chapter 6). Addi-tional technologies include construction of: (i)permeable pavements to augment soil infiltra-tion in urban landscapes; (ii) green roofs to re-duce and in some cases completely eliminatethe storm water contribution from the existingstructures; and (iii) surface depressions that areplanted as rain gardens with the objective of ab-sorbing rainwater runoff generated from imper-vious urban areas such as roof tops, driveways,

walkways, and compacted lawn areas. In the pastdecade, as understanding of these systems has im-proved, storm water harvesting and treatment havebecome much more reliable and predictable.

Water- and wastewater-treatment technologiesare crucial components of urban water manage-ment systems. Membrane technologies for remov-ing particulate matter (micro- and ultra-filtration)and dissolved substances (nano-filtration and RO)are increasingly being used. When particle-removalmembranes are coupled with biological systems,they can create membrane bioreactor (MBR) that isfast becoming an essential water-reclamation pro-cess (Judd 2006). Advanced oxidation processes in-clude combinations of ozone, ultraviolet (UV) light,and hydrogen peroxide to create the highly reac-tive hydroxyl radical (OH). In addition, activatedcarbon is still widely used for water reclamation.

10.3.2.1.1 Tools that address environmentalgoals

Some other tools and technologies do not necessar-ily reduce the overall abstraction of water but docontribute significantly to meeting environmentalgoals, such as optimal use of energy and reducednutrient dispersion. For example, as shown in Fig.10.15, water from laundries and bathrooms (typi-cally referred to as grey water) that contain fewpollutants, constitute the largest component of ur-ban waste water (Henze and Ledin 2001). Becauseof its low-pollutant content, grey water requires aminimal degree of treatment to become reusablefor non-potable purposes. Recycling this large vol-ume of waste water does not require much energyand thus needs fewer resources than recycling com-bined potable and non-potable waste water.

Organic matter in several components of thewastewater stream represents a significant sourceof energy. Most of the organic matter (quantified interms of the five-day biochemical oxygen demand,or BOD5) is contained in toilet- and kitchen- waste(typically referred to as black water). The amountof waste water generated by these components isquite small, suggesting that the black-water frac-tion can be used efficiently for energy production.Technologies for energy generation from organicmatter in black waters include thermal combustion

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URBAN HYDROLOGY 321

Fig. 10.15 Typical distribution of fourimportant constituents in domestic wastewaters of different sources. Source: Henzeand Ledin (2001).

and anaerobic treatment for producing biogas(Grady et al. 1999), which can be used in com-bined heat and power systems. The microbial fuelcell is an emerging energy-production technology(Logan et al. 2006). A microbial fuel cell is a devicethat converts chemical energy to electrical energyby catalytic reaction of micro-organisms (Allen andBennetto 1993). A typical microbial fuel cell con-sists of anode and cathode compartments separatedby a cation-specific membrane. In the anode com-partment, fuel is oxidized by micro-organisms, gen-erating electrons and protons. Electrons are trans-ferred to the cathode compartment through an ex-ternal electric circuit and protons are transferred tothe cathode compartment through the membrane.Electrons and protons are consumed in the cath-ode compartment, combining with oxygen to formwater.

Several nutrients are found in human urine (typ-ically referred to as yellow water). Developmentof urine-separating toilets and technologies fortreating urine to produce fertilizer products hy-gienically is the key to managing nutrients withminimal requirements of external resources, par-ticularly additional sources of energy. When en-ergy management and nutrient recovery are com-bined with source separation, energy can be ef-ficiently produced and extracted from the wastewater stream along with nutrient recovery. A va-riety of technologies are available for nutrient re-

covery. For example, biosolids containing nitro-gen and phosphorus produced from treatment andnutrient-recovery processes, can be applied di-rectly to agricultural lands as fertilizers, as an alter-native to phosphorus-based fertilizers. The stormwater capture and water-reclamation technologiesare most effective at the local (decentralized) level.Water-reclamation technologies result in reducedpumping requirements, because the reclaimed wa-ter is produced close to the place where it isused. In contrast, energy-management and nutrient-recovery technologies are most effective in large-scale centralized systems.

A recent survey by the British Medical Journal(BMJ 2007) found that modern water supply andsanitation has been the most significant contribu-tion to public health in the past 150 years. TheUS National Academy of Engineering listed modernwater supply and sanitation systems as one of thegreatest engineering achievements of the 20th cen-tury (Constable and Sommerville 2003). Despitethese developments, the present situation is grimand new approaches to water and sanitation sys-tems are urgently needed. We are thus faced withmany new, interesting, yet formidable challengesin water supply and sanitation management.

Fortunately, many technologies to meet thesechallenges already exist and work is continuingon refining them and integrating them into high-performance and more sustainable systems.

modern hydrology and sustainable water development (gupta/modern hydrology and sustainable water...

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