Water Supply & S

8/16/2019 Water Supply & S

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. +omestic water demand. ndustrial water demand

. nstitutional and commercial water demand. -ire demand. Water required to compensate losses

. +omestic demand his includes the water required in private buildings fordrin"ing, coo"ing, bathing, gardening, sanitary purposes, etc. he amount ofdomestic water consumption per person shall vary according to the livingstandard of consumers. he amount required ranges from /0 '12+ fordeveloping countries li"e 3thiopia to over 450 '12+ for developed countries.he total domestic demand generally accounts to 50 to 607 of the total waterconsumption.

. ndustrial water demand his includes water furnished to industries. he waterrequirement for this purpose will depend upon the number, nature, and si$e ofindustries.

able *.* Water demands of certain important industries

!. No Name of industry andproduct

8nit of production orraw material used

Approximate quantity of waterrequired per unit of production x*04 liters

* Automobile ehicle 90

/ +istillery (alcohol) :ilo liter *//*;0

4 -ertili$er onne 50/00

9 'eather onne 90

5 1aper onne /00900

6 !pecial quality paper onne 900*000

; !traw board onne ;5*00< 1etroleum refinery onne */

= !teel onne /00/50

*0 !ugar onne */

** textile onne ?4* is amount of water required

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/. -reeman formula 1 is population in thousands

+= *0*0

**46 P

Q

4. American nsurance association

( P PQ 0*.0*964; −=

. Water required to compensate losses this includes the quantity of water due towastage, losses, etc. !ome water is lost due to lea"ages at the &oints and valves.!ome loss may be ta"e place due to illegal and unauthori$ed connection.1rovision for these losses should be provided while designing water supplysystems. -or a well managed water wor"s this quantity should not exceed /07 ofthe total water supply. #ut in unmet red system it may reach up to 507

After assessing and estimating all demands, it is necessary to find out the total yearly waterdemand and the required flow rates. t is also necessary to analyses the variations in theserates of flows while planning and designing a water supply system. he following quantities

are therefore generally assesses and recorded. otal annual volume of water required

Annual average rate of flow in l@day

Annual average rate of draft in 'pcd

-luctuations in flows expressed in terms of percentage ratios of maximum or

minimum yearly, monthly, daily, and hourly

here are certain factors which control the water consumption rate. -or ma"ing a reliableassessment of water requirement they should be identified and considered.he following are some of the factors that affect the rates of water consumption. hese are

• he si$e of the community

• 2limatic condition

• !tandard of living

• ndustries

• >uality of water

• !ystem of sanitation

• 1ressure in the distribution system

• 8se of meters

• !ystem of supply

• 2ost of water

Section I.2 Estimating per capita demand

3stimating the per capita demand is required for finding out the total water requirement of atown or city or village. he per capita demand is estimated by finding out water needed foreach demand category and summing up all these demands. #oth the domestic and nondomestic needs are expressed with relation to population. Water demand of different headsmay be summed up, and the figure may be divided by the population of the concerned area,

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which will give equivalent per capita water demand. he following example may illustratethe situation.

+omestic need60 lpcd nstitutional demand *5 lpcd ndustrial demand*5 lcpd

-ire demand*0 lpcd Water unaccounted for /0lcpd

Total */0 lpcd

he per capita values used above are only for illustration purpose. he common procedure ofdividing total use by total population to derive a per capita consumption should be appliedonly with great care, since

*. the entire population may not be served by the municipal system/. there may be large industrial users which will not change with population, and4. he characteristics as well as the si$e of the population may be changing.

Section I.3 Variation in Demand and !eir Effect in t!e Design ofVarious "omponents of a Water Supply Sc!eme

he annual average demand is not sufficient, although very useful for the design of variouscomponents of a water supply scheme. here are wide variations in the use of water indifferent seasons, in different months of the year, in different days of the month, and indifferent hours of the day. hese normal variations in the demand should be assessed and"nown in order to design rising mains, service reservoirs, distribution systems, pumpingstations etc.

ourly demand rates are considered in the design of distribution systems, whereas dailyvariation is useful for designing rising main, pumping and treatment units. !easonal variationis considered for estimating the capacity of impounding reservoir.

here is no clearly defined relationship between average and pea" flow which is applicablein all communities. -or this reason each community should be carefully studied to determinevariations in rate with time and location. 1umping records, that is, the flows measured at thepumping station or water source, are extremely important in evaluating variations in demand.

n the absence of data it may be necessary to estimate the maximum rates. he maximum

daily consumption is li"ely to be *


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n this equation 1 is the percentage of the annual average rate and t is the length of theperiods in days. he formula predicts the maximum daily rate to be *


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designed based on the provision of economical conveyance at average daily flow atthe end of the design period with suitable velocities under all anticipated flowconditions.

4) he filter and other units at the treatment plant may be designed for maximum dailydraft. #ut most treatment units will be designed on the basis of average daily flow at

the end of the design period, since overloads do not result in ma&or losses of treatmentefficiency.9) he 1ump lifting the water is designed for the maximum daily draft plus some

additional reserve for brea"ing down and repairs.5) he distribution system should be designed for the maximum hourly draft of the

maximum day6) he service reservoir is designed to ta"e care of the hourly fluctuation, fire demands,

emergency reserve, and the provision required when pumps have to pump the entiredayGs water demand in fewer hours than /9 hours.

Section I.# Design periods and population forecasting

1.$.1 Design %eriods

A water supply scheme includes huge and costly structures which can not be replaced orincreased in their capacities easily and conveniently. he number of years for which aprovision is made in designing the capacities of the various components of the water supplyscheme is "nown as design period. t should be neither too long nor too short. he followingare the factors governing the economic design period of the components of a water supplysystem.

• 8seful life of component

• 3asy and difficulty that is li"ely to be faced in expansion

• Amount and availability of additional investment li"ely to be incurred for

additional provision• he rate of interest

• Anticipated rate of population growth

nn order to design parts of a water supply system, the flow at the end of the design periodmust be estimated. he +esign periods recommended for designing the various componentsof a water supply pro&ect are listed below.

Item Design period in years

!torage by dam 50

nfiltration wor"s 301umping

• 1ump house

• 3lectric motors and pumps

3015

Water treatment units 151ipes and fittings 30Service reservoir 15

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1.$.2 %opulation forecasting

!ince population is always a relevant factor in estimating future water use, it is necessary topredict what the future population will be. he date in the future the pro&ection is madedepends on the particular component of the system which is being designed. 3lements of the

system which are relatively easy to expand tend to have shorter design lives% hencepopulation pro&ections periods may range from as little as 5 to as long many as 50 years.horough "nowledge of the community and external factors which may affect its growth arevery important in population estimation. Denerally population growth of a town or a city isaffected by the following factors

#irths, t increases population

+eaths, it decreases population

Bigration, it increase@ decrease population

he present population of a town or city can be best determined by conducting an officialenumeration, called census. owever, the possible future population of the town@ city can beestimated by use of the following methods.

a) Arithmetic increase methodb) ncremental increase methodc) Deometric increase methodd) +ecrease rate of growth methode) !imple graphical methodf) 2omparative graphical methodg) Baster plan or $oning methodh) 'ogistic curve method

a) Aritmetic increase metod! " his is the simplest method of populationforecast. n this method, the increase in population from decade to decade is

assumed constant. Bathematically it can be expressed as

K dt

dP=

Where d1@dt is the rate of change of population and : is a constant. : is determinedgraphically or from populations in successive censuses as

:? Hp@Hthe arithmetic average of the population increase for the past 4 or 9 decades is usedas the design growth rate. he population in the future is then estimated from

Kt P p ot +=

Where 1t is the population at some time in the future, 1o is the present population,and t is the period of the pro&ection. t gives relatively lower result and suitable forold and saturated cities.

b) #eometric increase metod! n this method, it is assumed that the percentageincrease in population from decade to decade is constant and the increase iscompounded over the existing population every decade. t can be expressed as

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oo pr

p p*00

* += **/*00

pr

p p +=

+=

*00**

r p p o

/

/*00

*

+= r

p p o

//4 *00 p

r

p p +=

4

4 *00*

+=

r

p p o

t can be generali$ed asn

on

r p p

+=

*00*

Where n is number of decade, 1n is future population, and r is growth rate (7)t gives higher value and suitable for new@ young industrial cities. he percentage growth rate( r ) can be estimated by computing the average growth rates of several "nown decades of thepast as

pulationoriginalpo

populationincreaseinr =

:nowing r*, r/, I,rn for each decade, the average value can be found either by arithmetic orgeometric average method.

nn

n

rrrverageGeometrica

n

rrrcaverage Arithemeti

...CC

....

/*

/*

=

+++=

c) Incremental increase metod! " t is based on the assumption that the decadegrowth rate is progressively increasing or decreasing depending upon whether theaverage of the incremental increases in the past data is positive or negative. he

population for the future decade is wor"ed out by adding the mean arithmetic say(x) to the last "nown population as in the arithmetic increase method and to this isadded the average of the incremental increase say (y), once for the first decadeand twice for the second decade and so on. hus the method assumes that thegrowth rate in the first, second, a third etc. decades is (xJy), (xJ/y), (xJ4y) etcrespectively. Bathematically it can be written as

x P P o +=* =++= y x P P /*/ y x P P 4/4 ++=

y x p

y x y x p

o

o

4/

/

++=

++++=? y x po 4/ ++ JxJ4y

ynn

nx Po/

)*(

+++= ?poJ4xJJ6y

ynn

nx po

/

)*( +++=

his can be generali$ed as

1n ynn

nx po

/

)*( +++=


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d) Decrease rate of gro$t metod! " his method is applicable only in cases,where the rate of growth shows a downward trend. n this method the averagedecrease in the percentage increase is wor"ed out and is then subtracted from thelatest percentage increase for each successive decade.

3xample * he population of a town for five decades from *=60to /000 is given below asshown in able * 3stimate the population that will be expected after one, two, and threedecades beyond the last decade by using

*. Arithmetic increase method/. Deometric increase method4. incremental increase method9. decrease rate of method

able * 1opulation amount from *=60 to /000

Kear *=60 *=;0 *=


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increasemethod

?5/500 ?5


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he various source of water available on the earth can be classified into surface andsubsurface sources. he selection of a source of supply will be based on water availability,adequacy, quality, cost of development and operation, and the expected life of the pro&ect tobe served. n general, all alternative sources of supply should be evaluated to the extentnecessary to provide a valid assessment of their value for a specific installation. A

combination of surface and ground water, while not generally employed, may beadvantageous under some circumstances and should receive consideration

2.1 Surface Water Sources

2.1.1 &eneral Water that does not infiltrate to the ground is called surface water. Fr!urface sources are those sources of water in which the water flows over the surface of theearth, and is thus directly available for water supplies. t includes streams, la"es, ponds, andimpounding reservoirs. !urface water is available without ma&or digging or use of extensivemachinery and some times can be delivered to users without pumping. he quality of wateravailable is easy to determine by simple measurements. owever, the development of surface

water sources is not simple and careful planning is necessary. !urface water is sub&ect to runoff and human and animal contact and may be contaminated with feces or other wastes.!urface waters are generally characteri$ed by their variability in both quantity and quality.he investigations of surface water sources will cover the following items, as well as others,as circumstances warrant.

a. opographic maps showing pertinent drainage areas.b. ydrologic data, as required for pro&ect evaluations, e .g. rainfall, runoff,

evaporation, assessment of ground water resources and their potential as the solesource or supplementary source of supply.

c. !anitary survey findings.d. nta"e location.

e. Water quality data at or near proposed inta"e site.f. -easibility of developing supply without reservoir construction.g. Eeservoir location if reservoir is required.h. 1lans for other reservoirs on watershed.i. 1ertinent geological data that may affect dam foundation or ability of reservoir to

hold water. &. 'ocations for pump stations, supply lines, treatment plant.". 3nergy requirements for proposed system.l. Water laws, rules and regulations, procedure for obtaining right to use water, impact

of proposed use on rights of other users.m. +isposition of sludge from water treatment plant.

2.1.2 'et!ods of De(eloping Surface water) * !urface waters are available inrivers, ponds, impounding reservoirs, and la"es. #ecause most surface waters are feed bysurface runoff, treatment is necessary. Fther requirements of surface water sources are theprovision of appropriate inta"e and adequate storage facilities called impounding reservoir.mpounding reservoir is used commonly in rivers and streams because they have variableyield.

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A& Impounding 'eservoir hey are also called storage reservoirs and formed byconstruction a dam or di"e across a flowing stream. he storage reservoirs will store waterduring high rates of flow and will supply it during high rates of demand. !o the decision forthe construction of dam is based on

*. When the rate of flow in the stream is always greater than the rate of demand,there is no need of storage reservoir./. When the average annual flow is lower than the average annual demand, then

also dam should not be constructed as the deficit of the dry season can not becompensated by the surplus of wet season.

4. When the average flow is higher than the average demand, but in dry season thedischarge is lower than the water demand, then only it is advisable to construct adam and have storage reservoir.

he capacity of the storage or impounded reservoir may be calculated byA) Draphical (Bass 2urve) method and,#) Analytical Bethod

A& (ass )urve (etod his involves the following procedure.

• -irst mass curves for the runoff and the cumulative draft are plotted.

• hen tangents to the runoff curve are plotted parallel to the draft line.

• he maximum ordinate inside the loops is found out. hat gives the required

capacity.

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2.1.3 Inta+es for "ollecting Surface Waters) * nta"es are masonry or E2structures built at the surface sources for the purpose of withdrawing water safely from thesource over a predetermined range of pool levels and then to discharge this water into thewithdrawal conduit.

An inta"e structure constructed at the entrance of the conduit and thereby helping inprotecting the conduit from being damaged or clogged by ice, debris, trash, etc. vary from asimple concrete bloc" supporting the end of the conduit pipe to huge concrete towers housinginta"e gates, screens, and pumps e.t.c.

he following considerations shall be made in selecting the site of an inta"e.

• As far as possible, the site should be near the treatment plant

• he inta"e must be located in the purer $one of the source

• t should not be located at the downstream side of the point of waste water disposal.

• here should be sufficient space for further expansion.

• he site should permit greater withdrawal of water.

• t should be easily accessible during floods

• n meandering rivers, it should not be located on curves or at least on sharp curves. f

they have to be located on curves, it is better to locate them on concave ban"s.

+epending on the nature of sources, the following types of inta"es are generally used.. Eiver inta"es

. Eeservoir inta"es

*4

Eequiredcapacity ofstorage

+raft curve

!tart ofdry period


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'iver inta*es are towers usually circular in crosssection and constructed on the ban" of theriver. he inta"e tower itself serve as a sump well at the lower part and the upper half act aspump house and also as control room. -or letting water into the sump well, a number ofopenings or ports are provided at different levels. -or small water supply scheme the water is

avoided and only pipe is "ept in the water with suitable protection.

'eservoir inta*es are similar to river inta"es but they are constructed inside thereservoir@la"e@pond i.e. in deep water near the upstream toe of the dam. he inta"e isprovided with inlet ports at different levels. he inlet ports at different levels may be operatedfrom the gate house with the fluctuation of water levels. Access to the inta"e tower foroperation of gates, pumps and other purposes is made by means of foot bridges.

2.2 Ground Water Sources

2*2*1 &eneral) * Dround water is water contained in beds of roc", gravel, and sand,

termed Laquifers,L beneath the land surface. he principal source of ground water is rainfall,and aquifers are replenished, or recharged, by seepage of rainfall into the ground. AnaquiferMs recharge area may be close to or distant from a well location. Deology controls theabundance of ground water. n general, wells drilled into dense roc"s, such as granite, do notyield large quantities of ground water. Fn the other hand, wells that penetrate unconsolidatedformations of loose sand and gravel will often yield large quantities of water.

An aquifer may be defined as a formation that contains sufficient saturated permeablematerial to yield significant quantities of water to wells and springs. his implies an ability tostore and transmit water% unconsolidated sands and gravels are a typical example. Bany typesof geologic formations can serve as aquifers. he "ey requirement is their ability to store

water in the roc" pores. Aquifers are generally really extensive and may be overlain orunderlain by a confining bed, which may be defined as a relatively impermeable materialstratigraphically ad&acent to one or more aquifers.

Water enters an aquifer from natural or artificial recharge% it flows out under the action ofgravity or extracted by wells. Aquifers may be classified as unconfined or confineddepending on the presence or absence of a water table% while a lea"y aquifer represents acombination of the two types. he following figures show the schematic representation ofaquifer types.

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Figure! +.1 !chematic cross section illustrating unconfined and confined aquifers.

a. Availa,ility! Dround water is found in most parts of the world and can be a reliablesource of drin"ing water. !ources of groundwater are usually free from disease causingbacteria. here is usually less seasonal variation in ground water quantities than in surfacewater. Within a given area, there may be considerable variation from place to place in theyield of aquifers. #efore an appraisal of the cost of a ground water supply system can beprepared, information is needed regarding well location, depth, productivity, and spacing.!ometimes reliable information can be obtained from a study of existing wells in the samevicinity. arious investigation techniques (!urface and !ubsurface), including a testdrillingprogram in some important and large scale pro&ects, are often required to determine welllocation and probable productivity. A test drilling program will provide reliable informationabout the following matters% location of wells, depth of wells, well spacing, well design,probable productivity of completed wells, probable water quality, materials of wellconstruction, and pumping equipment. 3xisting wells, drilling contractors familiar with thearea and related regional or district offices can serve as sources of information regarding well

location, depth, and capacity.,. -uality! Denerally ground has less suspend impurities and high dissolved minerals andmetals as compared to surface waters. Bany ground waters have high levels of hardness andmay require softening. ron and manganese are also troublesome constituents of manyground waters% if concentrations in excess of * .0 mg@l iron and@or .*5 mg@l manganese areencountered, treatment will be a requirement for domestic uses. Fther common constituentsof ground water are chloride and sulfate. 2hloride is ob&ectionable if present in excess of

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about 900 mg@l and, for sulfate, 500 mg@l. Dround waters contain varying amounts ofdissolved carbon dioxide. Binor concentrations are not harmful% excessive amounts can bereadily removed by aeration processes. ydrogen sulfide is a noxious constituent of someground waters. t has an extremely unpleasant odor, is corrosive, and causes disinfectiondifficulties. reatment, usually by some form of aeration, is required for its removal. n spite

of the relatively high concentration of dissolved impurities in groundwater, it is the first andmost often choice of source of water supply for domestic supply with no or minimumtreatment. Bost urban and rural areas in 3thiopia are getting their supply from groundwatere.g. #ahirdar.

c. conomy! " Where water requirements are moderate, ground water may prove economicalif the supply can be obtained from a few highcapacity wells. owever, if the yield ofavailable aquifers is limited, a number of lowcapacity wells will be required, resulting ingreatly increased capital costs for well construction. !ystem operating and maintenance costswill also be considerably increased. 8nder such conditions, the economy of a ground waterversus surface water supply needs to be carefully examined. he study should include an

appraisal of operating and maintenance costs as well as capital costs. No absolute rules canbe given for choosing between ground and surface water sources. n general, wells requiringminimum or no treatment or only disinfection will be the preferred supply method whencompared with surface sources. 3ach situation must be examined on its merits with dueconsideration to treatment as well as installation and pumping costs.

2.2.2 Important considerations in locating well site) #oth well location andconstruction are of ma&or importance in protecting the quality of water derived from a well.

a. Sanitary survey! " 1rior to a decision as to well or well field location, a thoroughsanitary survey of the area should be underta"en. !uch effort will usually provide a good

picture of the pollution problems in the area and their possible impact on the ground water aswell as an assessment of the probable quality of the water that will be obtained from newwells in the area. !uch a survey will involve interviewing personnel from state and localhealth departments and other state agencies as well as -ederal agencies having "nowledge ofwater quality and water use in the area. Also, data related to the following should be obtainedand analy$ed .'ocations and characteristics of sewage and industrial waste disposal.

• 'ocations of sewers, septic tan"s, and cesspools.

• 2hemical and bacteriological quality of ground water, especially the quality of water

from existing wells.

• ndustrial and municipal landfills and dumps.

• +irection and rate of travel of usable ground water.

,. /ell location he well or wells should be located on the highest ground practicable,certainly on ground higher than nearby potential sources of pollution. he well casing shouldbe carried at least */ inches above the elevation of the ground surface at the site and thesurface near the site should be built up, by fill if necessary, so that surface drainage will beaway from the well in all directions. Where flooding is a problem, special well design will be

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necessary to insure protection of wells and pumping equipment from contamination anddamage during flood periods and to facilitate operation during a flood.

c. (inimum distance from pollution sources Binimum distances from "nown potentialsources of pollution should be carefully considered in deciding upon well location.

Eecommended minimum distances for well sites, under favorable geological conditions,from commonly encountered potential sources of pollution are as shown in able /*. t isemphasi$ed that these are minimum distances which can serve as rough guides to goodpractice when geological conditions are favorable. 2onditions are considered favorable whenthe earth materials between the well location and the pollution source have the filteringability of fine sand. Where the terrain consists of coarse gravel, limestone, or disintegratedroc" near the surface, the distance guides given above are insufficient, and greater distanceswill be required to provide safety. #ecause of the wide geological variations that may beencountered, it is impossible to specify the distance needed under all circumstances.2onsultation with local authorities will aid in establishing safe distances consistent with theterrain.

able /.* Binimum +istances from 1ollution !ources.

(inimumSource oriontal Distance

#uilding !ewer *5.0 meter!eptic an" *5.0 mete+isposal -ield 40.0 mete!eepage 1it 40.0 mete+ry Well *5.0 mete2esspool 95.0 mete

2.2.3 'et!ods of De(eloping Sources of &round water) * he most commonlyused methods for the development of ground water are

a) Water well constructionb) !pring +evelopment

Fther methods li"e infiltration wells and galleries could also be used at some appropriateplaces.

a, Water supply wells A well is a hydraulic structure that permits the withdrawalof water from the interstices of a water bearing formation. A water supply well can be

considered to consist of two basic components (*) a conduit section that houses pumpingequipment and provides piping for upward flow of water to the ground surface, (/) the inta"esection equipped to promote free entry of water from the aquifer into the well. n roc"formations, the conduit portion is usually cased from the surface to the top of the aquifer. heremainder, or inta"e portion, of the well is uncased. n sandgravel aquifers, the conduitsection is cased and the inta"e portion consists of a screen or a screen plus a gravel pac". hefunction of the screen and gravel pac" is to prevent fine aquifer material, such as sand, from

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entering the well while permitting the inflow of water. hose responsible for planning anddesigning water supply wells should recogni$e the following principles

• Bethods of well location

• Bethods of well design and construction

• Well completion

• Well development

Bany methods exist for constructing wells. !election of a particular method depends on thepurpose of the well, the quantity of water required, depth to groundwater, geologic conditionsand economic factors. Denerally well can be classified based on their depth and method ofconstruction as

!hallow wells and,

+eep wells

Attention to proper design will ensure efficient and long lived wells. After a well has beendrilled, it should be completed, developed for optimum yield, and tested

Well design and construction Wells are constructed as dug wells, driven wells, &etted wells, bored wells, drilled wells, or collector wells. here is no single correct methodof well construction. he choice depends on si$e, depth, formations encountered, andexperience of local well contractors. +etailed guidance on water supply well design andconstruction is contained in the boo" Dround water ydrologyO written by +evied. :. odd.t contain detailed information on drillingmethods, types of wells, well casings and screens,testing for yield and drawdown, grouting and sealing, disinfection, samples and records,protection of water quality, and sealing of abandoned wells . !ummary of some of theimportant points are presented below.

A& Sallo$ /ells

!hallow wells are generally less than *5m in depth. hey are constructed by digging, boring,driving, or &etting.

I. Dug /ells

• +epth range up to /0metres or more

• +iameters are usually fromm* to 5 meters

• 3xcavated by hand

• 2oncrete, #ric", or stone lining be employed

• Bost wells yield less than 500m4@day

• Dravel is bac"filled around the curb and bottom of the well to control sandentry and possible caving.

• he series limitation of dug wells is pollution by surface water.

II. 2ored $ells

• hey are constructed with hand operated or power driven earth augers

• and #ored wells seldom exceed /0cm in diameter and*5meres in depth

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• 1ower driven augers will bore holes up to *meter in diameter and 40meters in

depth

• Augers wor" best in formations that do not cave

III. Driven $ellst consists of a series of connected lengths of pipe driven by repeated impacts into the

ground to below the water table.

• he lower end of the well consists of a screened cylindrical section that allow

entrance of water into the well

• +iameter ranges from 4 to *0cm and depth usually lass than *5meters

• Water table must be near the ground surface (with in 4 to 5 meters below the

ground surface)

• Kields range from *00 to /50m4@day

• hey can be constructed in short time at minimum cost.

Fig +.+ dug $ell $it a roc* cur, concrete seal and and pump.IV. 4etted $ells

• Are constructed by a cutting action of downwarddirected stream of water

• +iameter range from 4 to *0cm and depth up to *5m

• Are best adopted to unconsolidated formation

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• !implification can be obtained by using a self &etting well point.

Article III. Deep wells

Bost deep and high capacity wells are constructed by drilling. A large number of methods

e.g. cable tools, hydraulic rotary or reverse rotary methods are in use for boring or drillingdeep wells.

I. )a,le tool metod t drills by lifting and dropping the tools alternatively in the drilled hole to brea"

up the formation. he lifting and dropping of drilling bit and bailer can be accomplished by hand or

percussion rigs n soft formation it is possible to bore *0 to /5cm diameter well up to a depth of

=0 to */0 meters by hand boring set he average Pperformance of hand boring set is *.5 to 4.0meters per < hours

1ercussion rig used normally range from light type (weight of tools /50 to ;50"g)which can drill *5 to /5cm hole up to a depth of =0 to */0 meters in soft andmedium hard formation, to medium type (weight of tools *000"g ) which can drillholes of /0 cm to a depth of /00meters in soft, hard or boulder formation. heaverage drilling rate may range between *.5 and 9.5 meters per


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!prings are developed by constructing a spring box around the spring outlet. A small area isdug out around the spring and lined with gravel. A concrete box with a removable cover isplaced over the spring to collect and store the water. he cover prevents outsidecontamination and should be heavy enough not to be displaced by people. A tap and an over

flow should be provided to prevent bac"up in the aquifer. -ig /.4 !how the details of springbox.

3. Distri-ution systems

3.1. General

Water, whether it is drawn from surface or groundwater supplies, must be conveyed to thecommunity and distributed to the users. 2onveyance from the source to the point of treatmentmay be provided by aqueducts, pipelines, or open channels, but once the water has beentreated it is distributed in pressuri$ed closed conduits. o deliver water to individualconsumers with appropriate quality, quantity, and pressure in a community requires an

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extensive system of pipes, distribution (service) reservoirs, pumps, and relatedappurtenances. he term distribution system is used to describe collectively the facilities usedto supply water from its source to the point of usage. t consist a transmission line and pipenetwor" for distribution of water.

3.2. Transmission Line

ransmission main, is the pipe line or conduit that carries water from the inta"e to the servicereservoir or treatment plant. t is some times called as supply main or rising main. 2onduitsused for transmission of water may be divided into two types based on the condition andcharacteristics of flow.

*. Dravity conduit/. 1ressure conduit

*) #ravity conduits are those in which the water flows under the mere action ofgravity. n such cases the water is at the atmospheric pressure. he hydraulic gradient line is

parallel to the bed of the conduit. hey can be in the form of canals, flumes, or aqueducts andusually used to transport water from the source to the treatment plant.

/) %ressure )onduits are closed conduits and the water flows under pressure aboveatmospheric pressure. hey have the following advantages

a. hey are very economical hence they can be laid at the shorter routes.b. he flowing water has no chance or very less chances of getting polluted.

Thus pressure pipes are invariably and universally used for water supply.1ressure pipes are used in the following situations.

• o transport water from the surface sources to treatment plants

• o transport water from wells to service reservoirs

• o transport water from treatment plants to service reservoir• n the distribution networ".

he hydraulic gradient should be such as to generate velocities which are neither so small asto require large si$e diameter pipe, nor so large as to cause excessive loss of pressure head.he velocity should be nonsilting and nonscouring. he flow velocities are normally "eptbetween 0.= to *.5m@s.

t is a normal practice to design the pipes in such a way that the available pressure headbetween the source and supply points is &ust lost in overcoming the frictional resistanceoffered to the flow by the pipe interior.he head loss caused by the pipe friction can be found by using either of the following

formulae.*) Darcy"/eis,ac Formula +arcy, Weisbach, and others proposed that, on the basis

of experiment that the energy loss resulting from friction varies as

=

g

V

d

Lf H L

/

/

//


23/55

in which f is a friction factor, ' is the pipe length, d its internal diameter, and /@/g thevelocity head. f depends on the Eeynolds number NE and the relative roughness e@d, where eis the height of surface roughness on the wall of the pipe and depends on the pipe material./) aen"/illiams euation!" n addition to the +arcyWeisbach equation, a numbr ofmore or less empirical equations which are more easily silved have been developed for use in

pipe flow problems. he a$en QWilliamsGs equation is one of such a relation and has theform59.064.0


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Q ! =6=.0= o Q ! //.*=

Where +? economic diameter in meter> ? +ischarge in m4@s

! i $ e

3 c o n o m i c + i a m e t e r

2

o

s

t

1 u m p i n g c o s t

1 i p e c o s t

- i g 4 . * 3 c o n o m i c s i $ e o f a r is i n g m a i n

3.3. Distriution !etwor"

Water collected from the selected sources through the provision of appropriate inta"es, madesafe by various treatment processes, and transported to the service reservoir by supply mainhas to be supplied to the consumer via the distribution networ". A distribution networ" mayconsist of

• 1ipe lines for carrying the water

• arious types of alves for flow regulation and networ" maintenance

and proper functioning.• Beters for measuring discharge

• !ervice reservoir for storing water

• 1umps for lifting water

• -ire hydrants

he distribution system may supply water to the public either continuously for /9 hours ofthe day, or it may supply intermittently during certain fixed hours of the day.

he following are some of the requirements of good distribution system. t should be capable of supplying water for all required purposes with

sufficient pressure. t should be simple and easy to operate and repair.

t should be safe against pollution.

/9


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3.3.1 Distri-ution eser(oir/Ser(ice eser(oir

+istribution reservoirs also called service reservoirs are the storage reservoirs made of steel,E2 or masonry, which store water for supplying it during emergency such as fire, and also tohelp in absorbing the hourly fluctuations in the normal water demand.

Bain functions served by the distribution reservoirs are

• o equali$e supply and demand

• o furnish water for emergency

• o reduce si$e of treatment plants and number of wells

• o reduce pumping capacity

• o maintain uniform pumping rate

• o reduce si$e of transmission lines e.t.c.

n the design of water supply schemes, provisions should be made for adequate capacitydistribution reservoir.

2apacity of distribution reservoir depends upon a number of factors. hese include*. ariation between demands/. the reserve required for fire protection4. he stand by pumping capacity9. nterconnection in the distribution mains

2apacity of the reservoir is usually between *@9th to *@4rd i.e. 6 to < hours of maximum daydemand in case of large community or city water supply and R or */ hours of the maximumday demand for small community or rural water supply.

3.3.2. 0ayout and Design of a Distri-ution etwor+s

he networ"s of conduits that convey water to the point of use from the service reservoir@

supply main are "nown as the distribution networ"s. !treet plan, topography and location ofsupply wor" and distribution storage establish the type of distribution system and characterof flow through it. he following are the types of distribution systems

A. +ead end system#. Drid iron system2. Eing system+. Eadial system

Any one of these either singly or in combination, can be used for a particular place,depending upon the local conditions and orientation of roads.

A& Dead end system in the dead end system which is also sometimes called tree

system% there is one main supply pipe, from which a number of sub main pipesoriginates. 3ach sub main then divides into several branch pipes, called laterals.

his system is suitable for localities which expand irregularly and where the water pipes haveto be laid at random due to absence of any planned road networ"s. he system has thefollowing advantages and disadvantages.Advantages

t is cheap and simple to expand and extend

/5


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3asy to design

'esser number of cutoff valves are required

+isadvantage !tagnation of water and accumulation of sediments at dead ends

Water supply has to be cutoff to large area during repair +uring emergency such as firefighting the discharge is limited.

2& #rid Iron system in this system the mains, sub mains, and branches are allinterconnected with each other. he system is more suitable for all planned towns and cities.

/6

Bain 1ipe


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he system has the following advantages and disadvantages.

Advantage• Water reaches at different places through more than one routes

• n case of repair very small area is affected

• Water remains in continuous circulation

• Bore water can be diverted during emergency to the affected area

+isadvantage

• he system requires more length of pipes and large number of valves

• ts construction is costlier

• he design difficult

2) 'ing system in this system, a closed ring of the main pipe, either circular orrectangular, is formed around the area to be served.

he system has the same advantage and disadvantage with the grid iron system. !ome timesthe system is used as a looped feeder placed centrally around a high demand area along withthe grid iron system. n such a case, it enhances the capacity of the grid iron system and willimprove the pressure at various points.

/;

Bain i e


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+) 'adial system if a city or town is having a system of radial roads emerging fromdifferent centers, the pipe line can be best laid in a radial method by placing thedistribution reservoir at these centers. his system gives quic" service with outmuch loss of head.

3)

+esign calculations are also simple. owever, cost of distribution reservoir may be more.

Design of Distri,ution 7et$or*

he design of a distribution system involves the determination of si$e of pipes that ensuresavailability of water at the end points of the pipe with the minimum allowable pressure at thetime of maximum demand.

he design of a water distribution system for a new area can be outlined as follows.*. Fbtain a detailed map of the area to be served on which topographic contours and the

locations of present and future streets and lots are identified./. #ased on the topography, select possible locations for distribution reservoirs. f the

area to be served is large, it may be divided into several sub areas to be served withseparate distribution systems.

4. 3stimate the water demand for each area.9. 3stimate pipe si$es on the basis of water demand and local code requirements.

/


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5. 'ay out a s"eleton system of supply mains leading from the distribution reservoir orother source of supply.

6. Accomplish hydraulic analysis of the distribution system.;. Ad&ust pipe si$es to reduce pressure irregularities in the distribution system.


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heir si$e is compact and require very limited space

he discharge obtained is steady

hey can be used to pump water containing silt, sand etc.

hey are quite durable and safe against high pressures

Disad(antage) Absence of self priming arrangement

-or high heads the efficiency is low

heir ordinary suction head is limited

#.2 (ead) %ower) and *fficienc$ of %ump

1ressure and discharge are inversely related in pump design so pumps which produce highpressure have a relatively small discharge and pumps which produce a large discharge arecapable of relatively low pressures.

he capacity (flow rate) of a pump is the volume of fluid pumped per unit of time whichusually is measured in cubic meters per second (m4@s). he term head refers to the elevationof a free surface of water above or below a reference datum.

he total dynamic head () is the head against which a pump has to wor" when water isbeing pumped. t consists of

. !tatic suction head (hs) which is the difference between the suctionliquid level and the centerline of the pump impeller. f the suction liquid level isbelow the centerline of the pump impeller, it is a static suction lift.

. !tatic discharge head (hd

) which is the difference in elevation betweenthe discharge liquid level and the centerline of the impeller.. -riction head (hf) which is the head of water that must be supplied to

overcome the frictional head loss caused by the flow of fluid through the pipe system.he frictional head loss in the suction and discharge piping system may be computedwith the a$enWilliams and +arcyWeisbach equations.

he total dynamic head on a pump can be determined by considering the static suction head(hs), the discharge head (hd), and the frictional head (hf). he expression for the determiningthe total dynamic is

? hs J hd J hf

he energy (#ernoulliGs) equation can also be applied to determine the total dynamic head onthe pump. he energy equation written between the suction and the discharge no$$le of thepump is

?

++−++ s

ssd

dd " V p

" g

V P

/=/

//

γ γ

4/


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he power required to operate a pump is directly proportional to the flow rate, dischargepressure head, and the specific gravity of the fluid, and inversely proportional to the pumpefficiency. his power must be supplied by a motor to the pump drive shaft so the pumpimpeller can impart the power to the water at the relevant pump efficiency.

1ump performance is measured in terms of the capacity that a pump can discharge against agiven head and at given efficiency. 1ump efficiency T, the ratio of the useful power output tothe power input, is given by

i

o

p

P=η

1ump efficiency usually range from 60 to ? capacity (flow rate or discharge) m4@s ? otal dynamic head (m)T ? 1ump efficiency

$. %ipes ittings and 4oints

+.1 %ipes

A pipe is a circular closed conduit through which the water may flow either under gravity orunder pressure. hey are mostly used for the transportation and distribution of water. 1ipecan be divided into different type depending upon the type of material they are made. heseare

• Iron pipes ron pipes are extremely durable and may be expected to have a

service life in excess of *00 years. t is, however, sub&ect to corrosion, whichmay produces a phenomena called tuberculation, in which scales of rust coat

the inside of the pipe, reducing its diameter and increasing its relativeroughness. ron pipes can be cast ron or ductile ron pipes. +uctile ron isnowadays used mostly, since for a given strength it is lighter and is lessbrittle. 2ast ron pipes have the following advantages.

o hey have moderate cost

o heir &oining is easier

o hey have long life

o !ervice connection can be easily made in them.

44


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he have on the other hand the following disadvantageso hey are sub&ect to tuberculation

o hey are heavier

o hey are li"ely to brea" during transportation

o hey can not be used for pressures greater than ;"g@cm/.

• Steel pipes !teel pipes may be used for water lines,

particularly in circumstances where diameters are large and pressures arehigh. !teel has economic advantages in such circumstances, since it isstronger and thus lighter for give strength. !teel pipe has relatively thin wallthat ma"es it li"ely to be structurally damaged by corrosion than iron pipes.

• %lastic %ipes !ince plastic pipe is far easier to handle and

install and generally cheaper than other pipes such as iron they are widelyused both in domestic plumbing and in water distribution systems. heyhave the following advantage and disadvantages

o Advantages

hey are free form corrosion

hey are cheaper

heir &oining, bending, and installation is easy

hey are highly resistant to acidic water

hey are smooth and possess low hydraulic resistance

hey are good electric insulators

hey are durable and unaffected by age, sunlight or

weather

o +isadvantages hey have low resistance to heat

• )oncrete pipes and As,estos cement pipes

!election of pipe depends on !trength

ydraulic properties

Eesistance to 2orrosion

+urability and period of life

3conomy

andling and &oining

+.2 ,ales and Appurtenances

A variety of valves and speciali$ed appurtenances are used in water distribution systems.hese may include

49


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• Gate Valves: - hese are the most commonly used valves for onoff service.

hey are located at regular intervals through out the distribution systems andat &unctions. !o that brea"s in the systems can be readily isolated. Bost gatevalves will operate properly only when installed in a vertical position.

• Check Valves: hese valves permit water to flow in only one direction and

are commonly used to prevent reversal of flow when pumps are shut off.2hec" valves installed at the end of a suction line are called foot valves. heseprevent draining of the suction line and loss of prime when the pump is shutdown. 2hec" valves are also installed on the discharge side of pumps toreduce hammer forces on the pump mechanisms.

• Pressure regulating Valves hey automatically reduce the pressure on the

down stream side to any desired level. hey function by using the upstreampressure to throttle the flow through an opening. he throttling valve willclose (or open) until the down stream pressure reaches the preset value.

• Blow off Valves At low points in the system, blow off valves are provided to

drain the pipe line and permit removal of sediment.

• Vacuu and !ir relief valves igh points should be provided with vacuumand airrelief valves to admit air when the line is being emptied and to releaseair which is initially in the line or which accumulates during use. Admittingair is particularly important with thin walled pipe (such as steel pipe) whichmay buc"le under compressive load. igh points in the line should be "eptbelow the hydraulic grade line, since negative pressure at such locations willlead to accumulation of gases which eventually may bloc" the flow.

ion III.1 'onstruction and maintenance of water distriution s$stem

1ipe lines are normally installed along the streets. here are several steps in installing pipe.hese are

i. renching and stringingii. #edding

iii. 1ressure testingiv. #ac" fillingv. +isinfecting

Trencing and Stringing as a general rule do not excavate a trench too far a head oflaying the pipe. Avoiding long stretches of open trench will minimi$e the ris" of flooding andcaving and will reduce ha$ards that might cause accidents. renches should be no wider thannecessary to permit wor"men easy access to install the pipe and required fittings. 2learanceof about *50mm on either side is normally adequate. 1ipes must be buried deep enough toprevent it from surface activity, vehicle loads, and high temperature. Normally a minimumdepth cover of 60cm will provide adequate protection. rench excavation can be done byhand labour or motori$ed equipment. t is a common practice to deliver pipe directly alongthe route of the trench and unload it along the trench line. his called stringing and savesdouble handling of the pipe.

45


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2edding n roc" formation for the purpose of providing uniform support along theunderside of the pipe trench bottoms should be leveled and bedded with selected materials.

%ressure testing After a pipe line has been installed, it should be tested for lea"s andpressure resistance. his will ensure that pipes and &oints are free from lea"ages and also

sound enough to with stand the maximum pressure li"ely to be developed under the wor"ingconditions. his can be done by filling the section to be tested with water under pressure.

2ac*filling 1roper bac"filling is essential in order to protect the pipe, prevent erosion, andavoid too much settlement of the filled trench. +ry soil that is free from roc"s and organicmaterials should be used. f the excavated material is not free of roc"s, then selectedmaterials should be used.

Disinfecting te System When installation has been completed, the entire system shouldbe flushed with clean water to remove dirt and sediment and then disinfected. his can bedone by filling the system with water containing at least 50mg@liter of chlorine and leaving it

in the main for /9hours.

Baintenance of distribution systems includes occasional cleaning, servicing of valves andhydrants, lea" surveys, repairs, and disinfection of repaired section.

%art wo) Sewerage

Article I,. Source and t$pes of sewae

"portant definitions

Se$age! " !ewage is the liquid waste conveyed by a sewer and may include domestic andindustrial discharges as well as storm sewage, infiltration, and inflow. #oestic is that whichoriginates in the sanitary conveyances of dwelling, commercial and institutional facilities. "ndustrial waste includes the liquid discharges from industrial processes. A combination ofdomestic and industrial sewage is called sanitary sewage. $tor sewage is flow derived fromrainfall events and deliberately introduced into sewers intended for its conveyance.

"nfiltration is water which enters the sewers from the ground through lea"s. "nflow is waterwhich enters the sewers from the surface, during rainfall events through defects in thesystem, or through connections to roof or basement drains.

! sewer is pipe or conduit, generally closed, but normally not flowing full, which carriessewage. A common sewer serves all abutting properties A sanitary sewer carries sanitarysewage and is designed to exclude storm sewage, infiltration, and inflow. A storm sewercarries storm sewage and any other wastes which may be discharged into the streets or onto

46


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the surface of the ground. A combined sewer carries both domestic and storm sewage. Asystem composed of combined sewers is called a combined system. While one whichsegregates the storm water is called a separate system.

Se$erage system! t is the system of sewers including all appurtenances required for

sewage collection, treatment, and disposal. A sewage system will consist of mainly• )ollection $or*s

• Treatment $or*s

• Disposal $or*s

he sewerage system can be*. !eparate system it collects transport and disposes safely either sanitary sewage

or storm water./. 2ombined system which collects and dispose safely both sanitary sewage and

storm waters.4. 1artially separate is that type of sewerage system which collects sanitary

sewage and a portion of storm waters specially those from roof gutters andstreet drains.

The ai of providing either of this syste or a cobination of the in a city is to create good sanitary environental syste that results fro the safe collection% treatent% and disposal of sewage.

Section I,.1 Introduction

3very community produces both liquid and solid wastes. he liquid portion, wastewater orsewage is essentially the water supply of the community after it has been used for a variety of

purposes.

What will happen if untreated waste water is allowed to accumulateV

• here will be environmental pollution

• 8ntreated waste water usually contains numerous pathogenic or disease

causing microorganisms

• he decomposition of the organic materials it contains produce large

quantities of gases which have ob&ectionable smell

• t also contains nutrients which can stimulate the growth of aquatic plants and

may contain toxic compounds.

-or the above reasons the immediate and nuisance free removal of wastewater from itssource of generation followed by treatment and disposal is not only desirable but alsonecessary.

he following table discusses the ma&or elements of wastewater management system andassociated engineering tas".

9(7T 7#I7'I7# TAS:

4;


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Article V. ype and Quantity of Sewage

/.1. *stimatin t&e 0uantit$ of arious wastewater

As pointed out in chapter 6 wastewater (sewage) collected from a community may includethe following.

*. +omestic wastewater./. ndustrial wastewater4. !torm wastewater9. nfiltration@inflow.

1& Domestic /aste$ater Source and Flo$ rates he principal sources of domesticwastewater in a community are the residential areas, commercial districts, institutional, andrecreational facilities. -or areas already served with sewers, wastewater flow rates arecommonly determined from existing records or by direct field measurements. -or new

developments, wastewater flow rates are derived from an analysis of population data andcorresponding pro&ected unit rates of water consumption of rXfrom estimates of percapitawastewater flow rates from similar communities.'esidential areas -or many residential areas, wastewater flowrates are commonlydetermined on the basis of population density and the average per capita contribution ofwastewater. Where possible, these rates should be based on actual flow data from selectedsimilar residential areas. n many cases design flows are fixed by water and wastewateragencies. f these data are not available an estimate of ;0 to


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nstitutional -acilities Actual records of similar institutions are the best source of flow datafor design purpose. When records are not available, the flow from institutional facilities canbe estimated with the data given below.

!ource 8nit -low '@unit@day

Eange ypicalospital Bedical bed 500=50 650

3mployee /060 90

ospital Bental bed 400500 900

3mployee /060 90

1rison nmate 400600 950

3mployee /060 90

!chool day with cafeteria, gymnasium, andshower

student 60**5


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nfiltration water entering a sewer system, including sewer service connection, from theground through such means as defective pipes, pipe &oints or manhole walls.

nflow this consists of water discharged into sewer pipes from sources such as foundationdrains, roof leaders, yard and area drains, other clean water discharges from commercial and

industrial establishments etc.

he rate and quantity of infiltration depends on the length of sewers, the area served, the soiland topographic conditions and to a certain extent the population density (which affects thenumber and total length of house connections).

nfiltration and inflow should be reduced as much as possible. Ftherwise there will beadditional cost of collection and treatment of @ water.

nfiltration rates in old systems have been measured to be from 45 to **5m 4@ ("m.day) per(mm) of diameter. !pecification for new sewer pro&ects now limit infiltration to 95'@

("m.day) per mm of diameter. !ince sewers deteriorate with age, estimates of infiltration,even for new systems, should be reasonably generous.

3xample ;.* 2alculate the infiltration and compare this quantity to the average dailydomestic wastewater flow for the following data.

!ewered population ?/90,000

Average domestic contribution ?*;0'pcd

nfiltration rate ? 95

l@"m.day per mm of diameter

he sanitary sewer system consists of

*00 mm dia of building sewers of 5


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%averageflo

peakflo% Peakfactor =

he following empirical formulae may be also used for estimating maximum rate ofdomestic sewage flow from small areas.

p

p

Q

Q Peakfactor

+

+==

9

*<

min

max#y armon

5min

max 5

pQ

Q Peakfactor == #y #abbit

Where B? pea" factors (the ratio of pea" rate to average)1 ? 1opulation served in thousands.

'ow flow in sewers is observed during nights. he effect of this low flow is maximum in thelaterals and minimum in main and trun" sewers. he minimum flow rate is critical in fixingthe flattest possible slope of a sewer so that the velocity is sufficient to prevent deposition ofgrit, silt, and accumulation of grease in the sewer wall.

elocities in sewers are selected with the goal of "eeping the solids in the sewage insuspension or at least in traction. !anitary sewers should be si$ed to provide minimum selfcleansing velocity of 0.6m@s which is adequate to "eep grit in traction when flowing full.

!torm sewer velocities are normally higher than those in sanitary sewers because of therelatively coarse solids which they must convey. elocities flowing full are generally "eptbetween 0.;5 to /.5m@sec. he maximum velocity is limited to reduce the potential forabrasive damage to the maximum the sewer.

!ome regulatory agencies specify minimum slopes for sewers of various diameters. heseslopes are those which are calculated to give the minimum self cleansing velocity when thesewers are flowing full.

Section ,.2 0uantit$ of Storm Sewae

When rain falls over the ground surface apart percolates into the ground, apart is evaporatedto the atmosphere and the remaining part overflow as flood water. he quantity of the storm

water overflowing on the ground surface and reaching the sewers or drains is called stormsewage (surface runoff) and is very large as compared with sanitary sewage.

he following factors mainly affect the quantity of storm sewage.

• Area or si$e of the catchment

• !lope and shape of the catchment

• Nature of the soil

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• 'and use and coverage

• ntensity and duration of rain fall

he quantity of storm sewage may be determined for storm water sewer design by differentmethods. his may include

ormulaehe following may be mentioned as an example.

i. -ullerGs formula/4.*4


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0;.*=6.0/;.*4

−= ' AQ

=5.0/*.;*

−= AQ' (=/.0=4.0

50 0


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1aved driveways and wal"s 0.;50.


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A

#

! e w e r * ! e w e r /

( * ( /

-or an area served by the laterals of sewers the concentration time will be computed asillustrated below. he flow from area A enters at inlet * and that from area # at inlet / hetime of concentration at / is either the time of concentration for area # or the time ofconcentration of area A plus the time of flow in sewer from * to /, which ever is greater.

he time of concentration for each sewer is determined in similar fashion, by comparing thetime of concentration for the area immediately tributary to the sewer inlet and the time ofconcentration plus time of flow for upstream tributary areas. When there is more than one

upstream tributary area, the time of concentration is the longest of those possible.

Article VI. Sewer design

.1 General

he hydraulics design of sewers, which means finding out their crosssections and gradients,is generally carried out on the same lines as that of the water supply pipes. owever, there

are two ma&or differences between the characteristics of flows in sewers and water supplypipes. hese differences are*. Water supply pipes carry relatively pure water but sewer lines carry sewage

containing main impurity. n order to prevent the clogging of sewers anddeposition of suspended materials in sewer, it is necessary to provide sewer pipeshaving si$es and gradients sufficient enough to generate self cleansing velocity.

/. Water supply pipes carry water under pressure, and hence, they may be carried upand dawn the hills and the valley% whereas, sewers carry sewage are gravityconduits (or open channels), and they must, therefore, be laid at a continuousgradient in downward direction up to the outfall point.

!ewers are generally designed as open channels except when it is required to design them asflowing under pressure, as in the case of inverted siphons and discharge lines from pumpingstations. he common open channel uniform flow empirical formulae can be used todetermine the necessary crosssections and gradients. hese may include

i. (anning6s Formula

/*

4/*s

nv =

9;


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ii. )ey6s Formula

scv =

Where v is velocity of flow in the channelE is hydraulic mean radius! is hydraulic gradient

Section ,I.1 low in %artiall$ filled sewer

2ircular sewer pipes are most widely adopted in sewerage system. he sewers are usually runin partially full except in some cases. herefore, the hydraulic performance of partially filledas well as filled section must be well understood. he concern is specially on how tomaintenance self cleansing velocities in both cases.

When sewers run full, their hydraulic properties will be as given below.

Area of crosssection9

/

! A Π== where + is diameter of sewer

Wetted perimeter ! P Π==

ydraulic mean depth9

!

p

A ===

When sewers run partially full at a depth, say d, as shown in the -igure below, the hydraulicelements can be wor"ed out as follows.

+

d

*) +epth at partial flow?d

−=−=

/cos*

//cos

//

α α ! ! !d

1roportionate depth? )/

cos*(/

* α −=

!

d

/) Area of partially filled section?aA? area of sector Q area of triangle

9


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6) +ischarge in partially filled section ? qav+ =

1roportionate discharge?Q

+

4

/

/sin460*

/sin

460.

−

−===

πα α

π α α

V v

Aa

AV av

Q+

n all the above derived equations, except [, every thing is constant, and hence by givingdifferent values to [ all the six proportionate elements can be easily calculated.

#y ta"ing proportionate depth (d@+) as reference values of other elements, the remaining canbe found quic"ly from diagram called partial flow diagram which is shown below.

=ample 1 +esign an outfall circular sanitary sewer for a town with a population of*50,000 and average per capita sewage contribution of


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0.4= 0.


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Article VII. Sewer 'aterials and Sewer Appurtenances

he pipe materials which are used to transport water may also be used to collect wastewater.t is more usual, however, to employ less expensive materials since sewers rarely are requiredto with stand any internal pressure. he most commonly used sewer materials are clay pipes and concrete pipes.

5/


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!ewer systems require a variety of appurtenances. hese include

• Banholes

• nlets

• !ewer outlets and outfalls

• 1umping stations

• nverted siphons

i) (anoles they are used as means of access for inspection repair and cleaning.

hey are placed at intervals of =0 to *50m and at point where there is change indirection, change in sewer si$e, and substantial change in grade. he design ofmanholes is fairly well standardi$ed. A typical bric" manhole, shown in -igure=./ has a cast iron frame and cover with a 500 to 600mm opining. he walls aretypically /00mm thic" for depths up to 9m and increase by *00mm for eachadditional /m of depth. he interior of bric" manholes is often plastered with1ortland cement. he bottom of the manhole is normally concrete, sloping toward

54

>.1


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an open channel which is an extension of the lowest sewer. he open channel issometimes lined with halfround or split sections of sewer pipe. he channelshould be sufficiently well defined and deep enough to prevent sewage fromspreading over the bottom of the manhole.

ii) Inlets they are structures through which storm water enters the sewers. heirdesign and location require consideration of how far water will be permitted toextend into the street under various conditions with minimum interference to bothpedestrian and vehicular traffic. hey are placed at gutters, at street intersectionsand at intervals of *50m.

iii) Se$er outlets and ?utfall storm water and treated wastewater may bedischarged to surface drainage or to bodies of water such as la"e, or ocean.Futlets to small streams are similar to the outlets of highway culverts, consistingof a simple concrete headwall and apron to prevent erosion. !ewers discharginginto large bodies of water are usually extended beyond the ban"s into fairly deep

water where dispersion and diffusion will aid in mixing the discharge with thesurrounding water.

iv) %umping stations sewage is required to be lifted up from a lower level to ahigher level at various places in a sewage system. !ewage may have to be liftedby pumps under the following circumstances.

When the area is flat, the laying of sewers at their designed gradients may

involve deeper excavation in the forward direction of flow. -or disposing of the sewage from building basement which are below the

grade of the sewer.

When the outfall sewer is lower than the level of treatment plant. o discharge treated wastewater to streams which are above the elevation

of the treatment plant. With in sewage treatment plants.

v) Inverted sipons t is a section of sewer which is dropped below the hydraulicgrade line in order to avoid an obstacle such as a railway or highway, or a stream.An inverted siphon is thus a sewer section constructed lower than the ad&acentsewer sections and it runs full under gravity with pressure greater than theatmospheric.

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nverted siphons are designed to develop relatively high self cleansing velocities(at least 0.=m@s) even during minimum discharge to prevent deposition of solids

in locations which would be very difficult or impossible to clean.

!ince sewage flow is sub&ect to large variations, a single pipe will not serveadequately in this application. nverted siphons normally include multiple pipes

Figure >.+ 2ric* manole

Figure >.3 )oncrete manole $it @unction of ,ranc se$er

Water Supply & S

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