Rainwater harvesting Fact Sheet - Water Aid

8/9/2019 Rainwater harvesting Fact Sheet - Water Aid

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This document is

one of a series oftechnical briefsproduced byWaterAid.

Rainwater harvesting

Technical brief

WaterAids missionis to overcomepoverty by enabling

the worlds poorestpeople to gainaccess to safe water,sanitation andhygiene education.

Introduction

Where there is no surface water, or wheregroundwater is deep or inaccessible due to hardground conditions, or where it is too salty, acidic orotherwise unpleasant or unt to drink, another sourcemust be sought. In areas which have regular rainfall themost appropriate alternative is the collection of rainwater,called rainwater harvesting.

Falling rain can provide some of the cleanest naturallyoccurring water that is available anywhere. This isnot surprising, as it is a result of a natural

distillation process that is at risk only fromairborne particles and from man-madepollution caused by the smoke and

ash of res and industrial processes,particularly those which burn fossil fuels.

Most modern technologies for obtainingdrinking water are related to theexploitation of surface water from rivers,streams and lakes, and groundwater fromwells and boreholes. However, these sources

account for only 40% of total precipitation.

It is evident, therefore, that there is considerable scope for the collectionof rainwater when it falls, before huge losses occur due to evaporationand transpiration and before it becomes contaminated by natural meansor man-made activities.

The term rainwater harvesting is usually taken to mean the immediatecollection of rainwater running off surfaces upon which it has fallendirectly. This denition excludes run-off from land watersheds intostreams, rivers, lakes, etc. WaterAid is concerned primarily with theprovision of clean drinking water; therefore the rainwater harvestingprojects which it supports are mainly those where rainwater is collectedfrom roofs, and only to a lesser extent where it is collected from smallground, or rock, catchments.


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Advantages of rainwater harvesting

Relatively cheap materials can be used for construction ofcontainers and collecting surfaces

Construction methods are relatively straightforward

Low maintenance costs and requirements Collected rainwater can be consumed without treatment

providing a clean collecting surface has been used

Provides a supply of safe water close to homes, schools orclinics, encourages increased consumption, reduces thetime women and children spend collecting water, reducesback strain or injuries from carrying heavy water containers

Disadvantages of rainwater harvesting

Supplies can be contaminated by bird/animal droppings on

catchment surfaces and guttering structures unless they arecleaned/ushed before use

Poorly constructed water jars/containers can suffer fromalgal growth and invasion by insects, lizards and rodents.They can act as a breeding ground for disease vectors ifthey are not properly maintained

How it works

Roof catchments

Rainwater can be collected from mostforms of roof. Tiled roofs, or roofssheeted with corrugated mild steel

etc are preferable, since they are theeasiest to use and give the cleanestwater. Thatched or palm leafedsurfaces are also feasible; althoughthey are difcult to clean and can oftentaint the run-off. Asbestos sheetingor lead-painted surfaces should beavoided.

The rainwater is collected in gutteringplaced around the eaves of the

building. Low cost guttering can bemade up from 22 gauge galvanisedmild steel sheeting, bent to form aV and suspended by galvanisedwire stitched through the thatch orsheeting, as shown in the followingdiagram:




WaterAid/Caroline Irby

The guttering drains to a down-pipewhich discharges into a storage tank.The down-pipe should be made toswivel so that the collection of the

rst run-off can be run to waste (therst foul ush), thus preventingaccumulated bird droppings, leaves,twigs and other vegetable matter,as well as dust and debris, fromentering the storage tank. Sometimesa collecting box with a mesh strainer(and sometimes with additional ltermedia) is used to prevent the ingress ofpotential pollutants.

Alternatively, a foul ush box, whichcan be drained separately, may betted between the down-pipe and thestorage tank. The run-off from a roof isdirectly proportional to the quantity ofrainfall and the plan area of the roof. Forevery one millimetre of rain a squaremetre of roof area will yield one litre ofwater, less evaporation, spillage lossesand wind effects. The guttering anddownpipes should be sized so as to becapable of carrying peak volume of runoff; in the tropics this can occur duringhigh intensity storms of short duration.

The storage tanks

The capacity of the storage tank isbased upon several design criteria:rainfall patterns and volume, theduration of the dry period and, ofcourse, the estimate of demand.

Sometimes sophisticated calculationsare involved, but these tend not totake into account human behaviourand the willingness to use water if itis available and not to conserve it forfuture use, in the hope that the dryspell will soon be over.

The following simple calculation canbe used to approximate the potentialsupply of rainwater from a collectingsurface. This can help to determine thecapacity of storage tanks:

S = R x A x Cr

S = Mean rainwater supply in m3

R = Mean annual rainfall in mm/year

A = Surface area of catchment in m2

Cr = Run-off coefcient

The run-off coefcient accounts forlosses due to splashing, evaporation,leakage and overow and is normallytaken to be 0.8.




The provision of the storage tank isthe most costly element of a rainwaterharvesting project, usually about 90%of the total cost. Storage can rangefrom small containers made for otherpurposes, for example oil drums, food

cans etc, but used as domestic storage,up to large tanks of 150 cubic metresor more at ground level, or sometimesbeneath it. These tanks are made ofconcrete or ferrocement and are usedas storage for schools, clinics or otherinstitutions with large areas of roof.

Domestic storage tanks

Tanks for household use can be madecheaply in a variety of ways. Basket

tanks are baskets made of bamboo,originally intended for carrying orstoring maize, which have beenplastered internally and externally, intwo stages, with sand/cement mortar.Storage of up to two cubic metrescan be provided by such baskets.Corrugated galvanised mild steelsheeting, bent and welded or boltedinto a circular plan, and often coated

with sand/cement mortar, can providesimilar storage capacity, but at agreater cost.

Eight year old Falidalls her jerry can atone of the two new4000 litre capacityrainwater harvestingtanks at her school

in Kitayita, Uganda.Previously, water hadto be collected froman unprotected sourcehalf a kilometre away.These were the rstjars constructed inthe community andthe exercise was usedas a workshop to trainlocal masons in jarconstruction.


Tanks of larger capacity can be made offerrocement, which substitutes chickenwire for the bamboo reinforcement ofthe basket tank. These are cheaper toconstruct than tanks made of masonry,blockwork, reinforced concrete etc,

and do not require the renderingwith waterproof cement mortar thatmasonry and blockwork often need.

Ferrocement tanks

Above ground level, tanks areconstructed with a plain or reinforcedconcrete base, cylindrical walls offerrocement and a roof of ferrocement,or sometimes mild steel sheeting.The construction of ferrocement walls

is carried out by rst assembling acylindrical mesh of chicken wire and/or fence wire reinforcement, withor without the aid of formwork. Onto this, a cement-rich mortar of 3:1sand:cement is applied by troweland built up in layers of about 15millimetres to a nished thicknessof between 30 to 100 millimetres,depending on wall height and tank

diameter. Thicker walls may have twolayers of mesh. The mesh helps tocontrol local cracking and the higherwalls may call for the provision of small




Four wraps at bottom




diameter vertical steel reinforcing barsfor bending resistance. Sometimesbarbed fence wire is wound spirally upthe wall to assist with resistance to ringtension and stress distribution.

Effective curing of the mortar betweenthe trowelling of each layer is very

important and affects the durabilityof the material and its resistance tocracking. Mortar should be still greenwhen the next layer is placed. Thismeans that the time gap betweenlayers should be between 12 and 24hours. The nished material shouldthen be cured continuously for up to10 days under damp hessian, or othersheeting. A ferrocement tank is easyto repair and, if the mortar has been

properly applied and cured, shouldprovide long service as a water-retaining structure at a fraction of thecost of a reinforced concrete structure.

WaterAid/Chris Leake

A rainwater harvesting tank at a school in Guirhora Kello, Burkina Faso.

Rock catchments

Just as the roofs of buildings canbe exploited for the collection ofrainwater, so can rock outcrops beused as collecting surfaces. Indeed,if access to the catchment areaby animals, children etc, can be

prevented, a protected catchment cancollect water of high quality, as longas its surfaces are well ushed andcleaned before storage takes place.

A signicant proportion of Gibraltarswater is obtained from sloping rockcatchments on the Rock. At the footof the slopes, collecting channelsdrain into pipes which lead to tanksexcavated inside the rock. Some

articial collection surfaces have alsobeen formed: cracks and voids in rocksurfaces have been lled in and alarge, soil covered, sloping area has




been covered in corrugated mild steelsheeting supported on short pilesdriven into the subsoil. This is a hugeexample of what may be possible on asmaller domestic or village scale.

Sometimes it proves difcult toprevent the collected water from beingpolluted. If so, it is sensible to use thiswater for purposes that do not requirea potable water supply, such as housecleaning, laundry, horticulture etc, andreserve for drinking water, cooking andpersonal hygiene the better qualitywater which has been collected from aclean roof.

Use can also be made of other forms of

ground catchment where, although thecollection coefcient can be as low as30%, useful volumes of water can becollected and used for agriculture andanimals.

References

Nissen-Petersen E (2007) Water fromroofs, Danida

Gould G, Nissen-Petersen E (1999)Rainwater catchment systems, IT

Publications, LondonPacey A, Cullis A (1986) Rainwaterharvesting: The collection ofrainfall and run-off in rural areas, ITPublications, London




Case study Construction of household rainwater harvesting jars in Uganda

WaterAid has been working to improve household water supplies in a Ugandan village where 3,000people lack access to safe water. The community is heavily affected by HIV/AIDS and there are manywidows and single occupancy households. The terrain here is hilly, often with very steep gradients,making it difcult for the old or inrm to access safe water sources.

Local masons have been trained in theconstruction of rainwater harvestingjars. These jars are made from locallyavailable materials and have a capacityof 1,500 litres which is equivalent to 75jerry-cans of water. The objective hasbeen to help the community constructon-site water supplies, close to thehome, thus removing the need for theold or inrm to travel long distances

across difcult terrain to collect water.The jars have a long design life andonce constructed can provide a stablewater source for many years. There aretwo dry seasons and two wet seasonsin this part of Uganda and the jarsaugment supply over the dry seasons,although the water inside may not lastfor the duration of a whole dry season.

Anna Maria Nanvubya and husband Zevilio NakuzabasajjaKwafu constructing a rainwater harvesting jar at their homein Kitayita village, Uganda.


Construction methodology:

Fifty bricks are used to assemble a stable platform upon which the jar isconstructed.

A one metre long copper pipe is shaped and laid in the brick base. This willchannel water from the jar to the tap tting.

A reusable wooden mould is assembled from 12 component pieces on top ofthe brick base.

The outside of the mould is smeared with mud for approximately three hours.

It is left to stand for three days after which it is plastered with normal cement.

The jar is left to dry for four days, giving time for the cement to set.

After four days the mould is removed by extracting individual pieces fromthe mouth of the jar. The layer of mud inside the jar is also removed.

The inside of the jar is then sealed using water proof cement.

Community members provide guttering for their roofs and a plastic basinwhich is perforated to act as a lter at the top of the jar.

Some jars have lockable tap chambers attached to their base to prevent

theft of water.

All jars are made in situ as they are fragile to transport. One jar costs around35 to make. This cost can be shared between ve households.

Materials requiredfor construction:

Locally madebricks for base(50)

Three bags ofsieved sand

One bag ofnormal cement

One kilogrammeof waterproofcement

A woodenmould that canbe reused tomake a numberof jars

One metre

copper pipe

One brass tap


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WaterAids mission is to overcomepoverty by enabling the worldspoorest people to gain access tosafe water, sanitation and hygieneeducation.

For more information, pleasecontact:WaterAid, 47-49 Durham Street,London SE11 5JD, UKTelephone: + 44 (0) 20 7793 4500Fax: + 44 (0) 20 7793 4545Email: [email protected]

Registered charity numbers

288701 (England and Wales)and SC039479 (Scotland)

www.wateraid.org

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Rainwater harvesting Fact Sheet - Water Aid

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