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Designing a GroundTechnicalNotelevel Storage TankNo. RWS. 5.D.2Under suitablecircumstances,groundlevelstoragetanks may be used todeliverwater to users by gravityflow.Storagetanks are a very importantpartof a water system because they ensurethat adequatequantitiesof water areavailableto meet demand.Storagetanks also help in preservingwaterquality.FUseful DefinitionsALGAE - Tiny greenfloatingin surfacepart of pond scum.HEAD - Differencetween the inflowwater system.plantswater;usuallyfoundmay formin water levelbeand outflowends of aThis technicalnote discussesthedesign of ground levelstoragetanksand offerssuggestionsfor nd selectingappropriateconstructionmaterials.Read the technicalnote carefullyand attempttoadapt the suggestionsto the localenvironmentto ensure successfuldesignof the storagetank.The design processshould resultinthe followingthree items which shouldbe given to the constructionsupervisor.1. A map of the area showing thelocationof the storagetank in relation to the water source and the community.Includeimportantlandmarks,elevations,if known, and distancesonthe map. Figure1 gives an example ofthe type of map which should be provided.2. A listof all labor,materialsand tools needed similarto Table 1.The listwillhelp make sure that adequate quantitiesof materialsareavailableto preventconstructiondelays.3. A plan of the storagetank withdimensionsshown as in Figure2. Theplan shows a side,top and end view.Distribution System Elev: 620 1- -.-.i.-.-Flgure 1. Location MapSite Selection and Tank locationThe most Importantconsiderationinthe choice of a site for a storagetankThe heightof wateris the elevation.stored,measured from the bottom of thetank, must produce sufficienthead orpressureto enable water to flowThethrougha pipelineto the users.heightneeded is determinedby theheightof the taps in the system andthe amount of pressuredesiredfor thedistributionsystem.

r4fITable 1. Sample Materials pliesPortlandcementClean sand and gravelClean waterReinforcingrodPipe for inlets,overflowanddrain(PVC or steel)Screeningand wire mesh for pipesTarformsBoards and lumberLubricatingoilRocks,ifmasonryShovels,picks and othertoolsMeasuringtapePlumb lineHammerSawBucketsCarpenterssquareLevelblnor machineMixingPipe nkdlgglngTotal\fCoattankFloatvalve(if needed)Cut-offvalvesLock for manhole coverIron for steps In tankLumber and rafters,If woodenCover Str"Ct"PeShinglesPlywoodRope or stringNails,t1e wireToolsEstimatedEstimatedCost /Manhole and coverSide View--- - ----------L rE.cnI”1OverflowManhole andcoverWidth -- -End View--mm----\L-- ------Top View. .I--Figure 2. Storage Tank Design/2

A generalrule to followis thatsmall water systems should have atleast14m of pressure.This means thatthe bottom of the storagetank must beat least14m higherthan the highestA generalrule is that the minitap.mum water levelin the storagetankshould be 20-4Om above the area served.Figure3 shows a profileof a system.Note that the elevationof the highesttap is 210m and that the system isbuiltfor a minimum of 14m pressure.The ground storageis on a hillat anelevationof 230m which providessufficientpressureto reach the highesttap in the community.If no locationof suitableheightis available,anelevatedstoragetank may be needed.For informationof the design of elevated storagetanks see "DesigninganElevatedStorage Tank," RWS.5.D.3.\ Figure 3. Distribution/In order to save money, try tolocatethe storagetank as close aspossibleto the water source and thepopulationbeing served.If possible,put it between the source and the populationto limitthe need for long linesof pipe.The locationof storageshown in Figure3 is distantfrom theofpump. This has the advantagedrawing water from the pump and tankdrum peak demand periods.For completeinformationabout the design of watertransmissionlines,the choice of pipe,and head losses due to friction,see"Designinga TransmissionMain,RWS.4.D.3 and System with Ground StorageTable 2. Population/’Growth Factors1. Determinethe populationof theUse census data or initiatecommunity.a survey to obtainpopulationfigures.Check past recordsto determinetherate of populationgrowth over theyears.If funds permit,the storagetank should be designedto last fortwenty-fiveyears.Therefore,use theestimatedpopulationfor 25 years inthe futureto determinedemand forwater.Use the growth factorsin Table2 when estimatingfuturepopulation.If money is short,the storagetank canbe sized to serve only the currentpopulationand the size Increasedlateron, if necessary.Tank CapacityThe capacityof the storagetank isimportantfor the efficientoperationof a water supply system.The tankshould be large enough to store sufficientwater to meet both average andpeak dailydemands.When designingastoragetank keep in mind that demandfor water variesduringthe year.Inthe hottermonths, people use morewater than in coolermonths and on certain religiousor culturaloccasionswater use may increase.For example,the presentpopulationof a community Is approximately1300and it has been growingat a rate of 3percentper year.To determinethepopulationin 25 years multiply1300 bythe populationgrowth factor1.81 foundin the row marked 20 and the 3 percentcolumn in Table 2.The firststep in determiningstoragecapacityis calculatingthedemand for water in the community.Follow the steps below in estimatingdemand.Population 1300 x 1.81Population 2350 or approximately2400.The reservoirshould be designedpopulationof 2400 people.3fora

/2. Once the populationis known, thedemand for water can be calculated.Demand can be estimatedby consideringthe type of distributionsystem used.Table 3 shows estimatedwater consumption rates for differenttypes ffectingdemand isthe use of water for purposesotherthan householddrinkingand cleaning.If the community has hotelsandrestaurantsor if animals willbewatered from the publicsystem consumption figureswould reflectthese uses.Table 4 shows estimatedwater use forvariousinstitutionsand for animals.Use these figureswhen designingthecapacityof the reservoir.Table 3. Estimated Water ConsumptionType of WaterAt distanceAt distanceVillagePeak day 1.2liters/dayAveragewater (250m)(250m)Yard connections1ng1e tapornouse 0;;I:;12IO-151510-204020-607050-120(or more)\//\Table 4. Water Use rpersonBus Horsesliters/dayx 120000wellStandpipe3. Once the totaldailydemand isdetermined,peak demand should be considered.Peak demand is the highestrate of demand duringthe day.Usuallypeak demand occurs duringthe morningwhen people get up to begin the day andin the earlyevening afterwork iscompleted.Peak demand is estimatedbyadding 20-40 percentto average dailydemand.Multiplythe average dailydemand by 1.2 or 1.4.For example,day 120000SupplyCommunity water point(i.e.,well, spring,public standpipe)The totaldailydemand for water canbe calculatedusing WorksheetA. Thecalculationsare done for a populationof 2400 people in a town that has asmall hospitalwith twenty beds, onehotelfor 75 people and two schools.Alarge chickenfarm with 5000 chickensalso uses water from the publicsystem.It is estimatedthat 40 percentof thepopulationwillbe served by multipletaps,35 percentby singletaps in theyard and 20 percentby standpipe.Fivepercentwillhave no service.AverageLand MulesPoultry*Chickens 1440000.15-0.25*Ifat allpossible,usepublicsupplyfor livestockshouldbe avoided.A generalrule to followis that thecapacityof the storagetank should be20-40 percentof the peak day waterdemand.With a peak dailydemand of144000 liters,the capacityof the tankshould be at least30m3: 144000 litersx.2 30000 liters.At the 40 percentvalue,the tank would be 58m3: 144000litersx .4 58000 liters.In thisof between 40case, a reservoir\ofperheadwaterfromand poultrya/50m3 would be needed to meet peakA possibledesign size woulddemand.For this capacitybe 5.5m x 4m x 2.3m.the water heightin the tanktank,should be at least2m and some spaceshould be availablebetween the highwater leveland top of the tank.4

The storagetanks can be builteitherabove or partiallyor completelybelow ground.Undergroundstructuresprovideadded supportfor the walls.If soilconditionspermitand elevationis sufficient,a storagetank partiallyor totallyundergroundis recommended.Where such a tank cannot be Installed,at leastbuildthe form work In theground for support.If a steeltank ispurchased,the tank willbe placeddirectlyon a concreteslab on the surface of the ground.Steel tanks provide good storagebut may not befeasib-lein many places due to theircost.Tank DesignGround levelstoragetanks aregenerallymade from reinforcedconcrete,masonry or brickdependingonthe materialsavailableIn the area andthe skillsof the localpeople.Steeltanks may be purchased.Reinforcedconcreteis used in manyareas.Its advantagesare that it provides a very sturdywatertightstructure that willlast many years,and ituses less concretethan mass concretestructureswhich reduces constructioncosts.A disadvantageof using a reinforcedstructureis that steel,lumberfor forms and skilledlabor and supervisionare needed to buildthe tank.Neitherthe materialsnor the expertisemay be affordableor available;if not,an alternativestructuredesign shouldbe chosen.All tanks must have covers.In somecases,reinforcedconcreteis used butforms are expensiveand constructiondifficult.Cast-in-placeroofingmaynot be possible.A concretecover canbe builtby castingseveralsections;;z fY&ingthem togetheron the top ofSee Figure5. The advantageof castingin sectionsis that thesmallersectionscan be cast at groundleveland liftedinto place.If large buildingstone isavailable,the tank can be builtofmasonry.When buildingwith masonry,no forms are necessaryand constructionis generallyeasier.Masonry tanks arenot watertight,however.For bestresults,make thin masonry wallsandfillIn between them with concreteasshown in Figure4./CROSS-SECTION/Cover sectionsSupporting wall for- coverof large tankCover\END VIEW/coversectionConcrete mortar on1oncrete foundation and floorForms full 01concrete, troweledsmooth‘Form half fullof concrete,reinforcingbars in placeWooden formsFigure 4. Example of Mass Concrete Wall ConstructionFigure 5. Casting Cover Sections5