Designing an Elevated Water Storage Tank for Developing Countries

8/9/2019 Designing an Elevated Water Storage Tank for Developing Countries

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IWater for theIf \ 1

Designing an Elevated Storage TankTechnical Note No. RWS. 5.0.3

Elevated storage tanks are used todeliver water either through largedistribution systems or through stand-pipes located at or near the source orat other communal watering points.Elevated storage tanks are used whereground storage tanks cannot be builtdue to lack of sufficient natural ele-vation and where standpipes are servedfrom a well with a windmill or otherpowered pumps. Elevated tanks canserve either a large community or asmall group of families. Elevatedtanks do not have as large a capacityas ground storage due to the need for atower structure to support the tank.

This technical note discusses thedesign of elevated storage tanks andoffers suggestions for choosing theappropriate tank design and construc-tion materials. Read the technicalnote carefully and adapt thesuggestions to local conditions toensure that the storage meets users'needs.

The design process should result inthe following three items which shouldbe given to the construction super-visor.

1. A map of the area showing thelocation of the storage tank in rela-tion to the water source and the com-munity. Include Important landmarks,elevations, if known, and distances onthe map. Figure 1 gives an example ofthe type of map which should be pro-vided.

2. A list of all labor, materialsand tools needed. The list will helpmake sure that adequate quantities ofmaterials are available to preventconstruction delays. See Table 1 for asample materials list for an elevatedferrocement storage tank.

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Useful Definition

HEAD - Difference in water level be-tween the inflow and outflow ends of awater system.

SECTION 1ump house Storage r- Elev. 90 _

lev. IOOm Elev. 140m

10 Gate valve

0 Marker

q Dwelling0 Standpost

Figure 1 . Design Map of Storage and Dlstributlon System

Table 1. Sample Materials List for .Elevated Ferrocement Storage Tanks


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3. A plan of the storage reservoirwith dimensions shown as in Figure 2.The plan shows a side, front and topview.

r

-1.7m - Overflow 1.5m Reinforced concrete

TOP VIEW COVER

Concrete slab

CROSS-SECTION

:igure 2. Design of Elevated Storage Tank (Masonry Constructiol

Choice of Storage Tank

The choice of the best type of ele-vated tank to use depends on the typeof system being installed, thematerials available for constructionand the availability of skilled labor.Elevated storage tanks are useful forproviding water to standpipes as shownin Figure 3 and to large distributionsystems are shown in Figure 4.

Elevated tanks which serve stand-pipes generally have small capacitiesand are supported by small towers. Thetowers are usually made from brick ormasonry and have a height of l-3m.Steel and wood can be used althoughsteel is expensive and wood is lessdurable than other materials. The baseof the tower should be at least 1.2times wider than the top. Since notmuch head is needed to serve stand-pipes, the height of the tower does nothave to be great. Shorter towersrequire less material which reducescost. Determine the size of the towerdesired and the design which uses thefewest bricks. Figure 5 shows onedesign. Approximately 50 bricks areneeded for each m2 of construction. Ifpossible, use wide bricks to form awall lOO-150mm thick. A design usingfour walls that come together at a

below groundsurface

Figure 3. Example of Elevated Storage Fed by WindmiiiL

a. TANK NEAR WATER SUPPLY

b. TANK NEAR DISTRIBUTION POINT

Figure 4. Constructing Storage for Large Population\ /

center point is useful and effective.The base should be well designed andconstructed to ensure adequatestrength to support the water tank.Before designing an elevated tank, con-sult an engineer or someone withexperience in designing water tankbases.

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/Wire weld or rebar

300mm mortar

Figure 5. Ferrocement Tank

To choose the type of tank, firstdetermine the capacity needed. Thecapacity can be found by multiplyingthe number of users by the per capitaconsumption per day. When water is

distributed from a standpipe, anaverage estimate of water use is 15liters per capita per day, lpcd, asshown in Table 2. If animals will bewatered from the public supply, theirrate of consumption should be includedas shown in Table 3.

f

Y

Table 2. Estimated Water Consumption

At distance lOOOmAt distance 500-1000m

Ylllage wel (250m)

Standpipe (250m)

Yard cOnneCtlo OPsingle tap

House cOnneCtlo(multiple taps)

70

I IO-15I 10-20

I 20-60

I 50-120COPare)

Table 3. Population Growth Factors

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For example, if several standplpeswith one reservoir serve a small com-munity of 200 people and no animals andwater consumption is 15 lpcd, the totaldaily consumption is 3000 liters.Assume a peak demand of 20 percent oftotal daily demand for storage pur-poses and design the storage tank for3000 liters t 20 percent x 3000 litersor 3000 liters t 1.2 x 3000 liters =3600 liters. The tank should have aminimum capacity of 3600 liters.Assuming some growth in population andan increase in demand, the total capa-city should be at least 4000 andpossibly even 5000 liters. Table 3provides population growth factors fordifferent design periods.

Where the population is spread out,more than one elevated tank with stand-pipes in several locations can beconstructed. The capacities of thesetanks can be smaller. This type ofelevated tank can receive water from awell via a windmill, a simple gravityflow system if the terrain permits, ora hydraulic ram. If pumps powered byelectricity or petrol are used, largerstorage tanks are recommended.

One of the cheapest and easiestmaterials to use for a small capacitytank is steel. Pressed steel platescan be bolted together on top of atower with little or no skilled labor.Tank size will be influence by theamount of water that needs to be storedand the size of pressed steel sheetsavailable. A tank using pressed sheets1.6m x 1.6m x 1.6m will provide astorage capacity of just over 4000liters.

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If steel tanks are impossible toobtain or are very costly, a storagetank can be constructed using brickmasonry or ferrocement. Elevatedferrocement tanks are best when theyare small diameter circular tanks withrelatively small capacities. A cir-cular ferrocement tank similar to thecistern described in "Designing aHousehold Cistern," RWS.5.D.1, can beconstructed on a raised tower. Asmall, 4-6m3, capacity ferrocement tankcan be built on the tank tower withoutusing formwork. An initial circle ofbricks forming the outside diameter ofthe tank should be placed so that thefloor diameter is 1.5-2m. A weld andchicken wire mesh cylinder 3m highshould be assembled as shown in Figure5. The tank itself should be only 2mhigh; the extra wire mesh is used forroofing. Table 1 is a sample list ofmaterials for this kind of tank.

Once the mesh is assemble, mortarmixed in a proportion of 1 part cementto 3 parts sand is applied in a coat300mm thick. Inlet, outlet andoverflow pipes should be installed.The inlet should enter the top of thetank while the screened outlet shouldbe at the bottom.

Brick masonry tanks can beconstructed upon the tower itself.Special care should be taken to ensurethat the tank is water-tight by coatingthe inside with a layer of mortar.Construction follows the basic stepsfor masonry tanks. For small waterstorage tanks, a circular tank designas shown in Figure 5 is recommended.The mortar between the bricks or rocksshould be mixed in the ratio of 1 partcement to 4 parts sand. If sand is notof good quality, then a ratio of 1:3may be used. To strengthen the mix,lime can be added to the cement in aproportion of 1 part lime to 5 partscement. Lime is especially useful whenmortar is being mixed for brick-laying.The mortar used to line the insidewalls of the tank does not requirelime. Two coats o f mortar each 150mmthick should be applied to the insideof the tank. To give the tank andtower a good finish a 200mm layer ofmortar should be applied to the outsideof the tank and tower if resources per-mit.

An inlet pipe should be attached tothe tank so that it enters near thetop. See Figure 2. A screened outletleads from the bottom of the tank tothe standpipe connections. The screenstrains out sediment that collect onthe tank bottom. An overflow should beinstalled near the very top of the

tank.large Capacity Tanks

Large capacity elevated tanks formore populated communities aregenerally made of steel or reinforcedconcrete. Figure 4 shows a typicalreinforced tank while Figure 6 is asteel tank. Steel tanks raised onsteel platforms are generally paid forby national water authorities orgovernments. They are very expensiveand generally must be imported. Thechoice to use this type of tank dependson whether they are manufactured In thecountry or can be acquired at anappropriate price.

lgure 6. Large Capacity Steel Tank

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Reinforced concrete tanks are morecommonly used than steel tanks. Theirconstruction requires cement and othermaterials which may be availablenationally or else must be imported.If a tank of standard design is manu-factured, the forms used in buildingthe first tank can be reused inconstructing later tanks and willresult in lower construction costs.Furthermore, local employment oppor-tunities will be created and the localeconomy will be helped by increasedincomes. An elevated reinforcedconcrete tank should not be constructedwithout an engineer. Tanks must bewell designed to work efficiently and agreat deal of expertise is needed inconstruction. An engineer should be atthe construction site to supervise allconstruction and no work should be donewithout expert advice.

The decision to use an elevated tankis a result of the need to provide suf-ficient head in the system. The waterlevel in the tank must be higher thanthe highest tap in the system so thattap has enough discharge pressure. Toprovide 14m of pressure in the system,the water in storage should be morethan 14m above the height of thehighest tap. Consult an engineer whendetermining minimum and maximum heightsfor the storage reservoir.

Capacity. The capacity of thestorage tank is important to the effi-cient operation of a water supplysystem. It should be large enough tostore sufficient water to meet bothaverage and peak daily demands. Whendesigning a storage tank, keep in mindthat demand for water varies during theyear. In the hotter months, people usemore water than in cooler months and oncertain religious or cultural occasionswater use may increase.

The first step in determiningstorage capacity is calculating thedemand for water in the community.Follow the steps below in estimatingdemand.

1. Determine the population of thecommunity. Use census data or initiatea survey to obtain population figures.Check past records to determine therate of population growth over the

years. If funds permit, the storagetank should be designed to last fortwenty years. Therefore, use the esti-mated population for twenty years inthe future to determine demand forwater. Use the growth factors inTable 3 when estimating future popula-tion.

For example, the present populationof a community is approximately 1300and it has been growing at a rate of 3percent per year. To determine thepopulation in twenty years, multiply1300 by the population growth factor1.8l.found in the row marked 20 and the3 percent column in Table 3.

Population = 1300 x 1.81

Population = 2350 or approximately2400.

The reservoir should be designed for apopulation of 2400 people.

2. Once the population is known, thedemand for water can be calculated.Demand can be estimated by consideringthe type of distribution system used.Table 2 shows estimated water consump-tion rates for different types ofdistribution arrangements. Anotherimportant factor affecting demand isthe use of water for purposes otherthan household drinking and cleaning.If the community has hotels andrestaurants or if animals will bewatered from the public system, con-sumption figures would reflect theseuses. Table 4 shows estimated wateruse for various institutions and foranimals. Use these figures whendesigning the capacity of the reser-voir.

The total daily demand for water canbe calculated using Worksheet A. Thecalculations are done for a populationof 2400 people in a town that has asmall hospital with twenty beds, onehotel for 75 people and two schools. Alarge chicken farm with 5000 chickensalso uses water from the public system.It is estimated that 40 percent of thepopulation will be served by multipletaps, 35 percent by single taps in theyard and 20 percent by standpipe. Fivepercent will have no service.

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Table 4. Water Use Requirements

Category

Schools

Hospital

(with laundry)

(without laundry)

Clinics

Hotels

Restaurants

Dffice

Bus Station

Livestock*Cattle

Horses and Mules

Sheep

Pigs

Poultry*

Chickens

Typical Water Use(Liters/Day)

15-30 per pupil

200-300 per bed

120-220 per bed

15-30 per patient

80-120 per guest

60-90 per seat

25-40 per person

15-20 per user

25-35 per head

20-25 per head

15-25 per head

lo-15 per head

0.15-0.25 per head

*If at all possible, use of water from apublic supply for livestock and poultry

should be avoided.

3. Once the total daily demand isdetermined, peak demand should be con-sidered. Peak demand is the highestrate of demand during the day. Usuallypeak demand occurs during the morningwhen people get up to begin the day andin the early evening after work iscompleted. Peak demand is estimated byadding 20-40 percent to average dailydemand. Multiply average daily demandby 1.2 or 1.4. For example,

Average day = 120000 liters/day

Peak day = 1.2 x 120000 = 144000liters/day.

A general rule to follow is that thecapacity of the storage tank should be20-40 percent of the peak day waterdemand. With a peak dally demand of144000 liters, the capacity of the tankshould be at least 30m3; 144000 litersx . 2 = 30000 liters. At the 40 percentvalue, the tank would be 58m3; 144000liters x .4 = 58000 liters. In thiscase, a reservoir of between 40-50m3would be needed to meet peak demand.

Structure Design. Large capacityreinforced concrete tanks should bedesigned by engineers to ensure thatthe structure is sound. A typical tankis elevated on a tower made of rein-forced concrete with a tank built as acontinuation of the supporting struc-ture. Large quantities of material are

Worksheet A. Calculating Water Demand

Population 2400

1. Multiple taps in homes of 40% of population40% x 2400 x 70 lpcd

2. Single tap in yards of 35% of population35% x 2400 x 40 lpcd

3. Standpipe for 20% of population20% x 2400 x 15 lpcd

4. Two schools with students001000 x 20 lpcd

5. Chickens (5000)5000 x 0.2 lpcd

6. Hospital with 20 beds without laundry20 x 160 lpcd

7. Hotel 75 guests75 x 75 lpcd

Total

67200

33600

720~

2000

1000

3200

5625

119825

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required for building the tank. Thecommunity should be sure that allmaterials can be obtained beforedeciding to contruct such a tank.

The below ground base, the legs ofthe tower and cross pieces should allbe reinforced using at least 8mmdiameter reinforcing rods. A largerdiameter rod may be preferable if it isavailable at a cost that is not toohigh.

The inlet and outlet pipes should beconnected as described for smallertanks. The outlet pipe should bescreened to prevent sediment fromentering the distribution system. Atight-fitting cover should be placed atthe top of the tank so the tank can beentered for cleaning. A ladder on theoutside of the tank gives access to themanhole. A small ladder or ringsshould be placed on the inside of thetank so people can enter safely.

Summary

There is little chance that a com-munity can build a large capacity ele-vated storage tank without outsidetechnical and financial assistance.Such tanks are recommended for use onlywhen these resources are easilyavailable to the community. Smallerelevated tanks, as described in theearlier section of this paper, can bebuilt with local supplies and laborwith minimal outside assistance. Thechoice of storage tank design should bemade only after careful considerationof the needs of the community and theresources available.

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Designing an Elevated Water Storage Tank for Developing Countries