Elective RWH

8/11/2019 Elective RWH

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In the present scenario management and distribution of water has become centralized.People depend on government system, which has resulted in disruption of community

participation in water management and collapse of traditional water harvesting system.

As the water crisis continues to become severe, there is a dire need of reform in watermanagement system and revival of traditional systems. Scientific and technological studies

need to be carried out to assess present status so as to suggest suitable mitigative measures for

the revival to traditional system/wisdom. Revival process should necessarily be backed by

people's initiative and active public participation.

Living creatures of the universe are made of five basic elements, viz., Earth, Water, Fire, Air

and Sky, Obviously, water is one of the most important elements and no creature can survive

without it. Despite having a great regard for water, we seem to have failed to address this

sector seriously. Human being could not save and conserve water and it sources, probably

because of its availability in abundance. But this irresponsible attitude resulted in

deterioration of water bodies with respect to quantity and quality both. Now, situation hasarrived when even a single drop of water matters. However. " better late than never", we have

not realized the seriousness of this issue and initiated efforts to overcome those problems.

System of collection rainwater and conserving for future needs has traditionally been

practiced in India. The traditional systems were time-tested wisdom of not only appropriate

technology of Rainwater Harvesting, but also water management systems, where

conservation of water was the prime concern. Traditional water harvesting systems were

Bawaries, step wells, jhiries, lakes, tanks etc. These were the water storage bodies todomestic and irrigation demands. People were themselves responsible for maintenance to

water sources and optimal use of water that could fulfill their needs.

What is Rainwater harvesting?

The term rainwater harvesting is being frequently used these days, however, the concept of

water harvesting is not new for India. Water harvesting techniques had been evolved and

developed centuries ago.

Ground water resource gets naturally recharged through percolation. But due to

indiscriminate development and rapid urbainzation, exposed surface for soil has been reduced

drastically with resultant reduction in percolation of rainwater, thereby depleting ground

water resource. Rainwater harvesting is the process of augmenting the natural filtration of

rainwater in to the underground formation by some artificial methods. "Conscious collection

and storage of rainwater to cater to demands of water, for drinking, domestic purpose &

irrigation is termed as Rainwater Harvesting."


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Why harvest rainwater ?

This is perhaps one of the most frequently asked question, as to why one should harvest

rainwater. There are many reasons but following are some of the important ones.

To arrest ground water decline and augment ground water table

To beneficiate water quality in aquifersTo conserve surface water runoff during monsoon

To reduce soil erosion

To inculcate a culture of water conservation

How to harvest rainwater:

Broadly there are two ways of harvesting rainwater:

(i) Surface runoff harvesting

(ii) Roof top rainwater harvesting

Surface runoff harvesting:In urban area rainwater flows away as surface runoff. This runoff could be caught and used

for recharging aquifers by adopting appropriate methods.

Roof top rainwater harvesting (RTRWH):

It is a system of catching rainwater where it falls. In rooftop harvesting, the roof becomes the

catchments, and the rainwater is collected from the roof of the house/building. It can either be

stored in a tank or diverted to artificial recharge system. This method is less expensive and

very effective and if implemented properly helps in augmenting the ground water level of the

area.

Components of the roof top rainwater harvesting system

The illustrative design of the basic components of roof top rainwater harvesting system is

given in the following typical schematic diagram/

The system mainly constitutes of following sub components:

Catchment

Transportation

First flush

FilterThe surface that receives rainfall directly is the catchment of rainwater harvesting system. It

may be terrace, courtyard, or paved or unpaved open ground. The terrace may be flat

RCC/stone roof or sloping roof. Therefore the catchment is the area, which actually

contributes rainwater to the harvesting system.

Transportation

Rainwater from rooftop should be carried through down take water pipes or drains

to storage/harvesting system. Water pipes should be UV resistant (ISI HDPE/PVC pipes) of

required capacity. Water from sloping roofs could be caught through gutters and down take

pipe. At terraces, mouth of the each drain should have wire mesh to restrict floating material.


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First Flush

First flush is a device used to flush off the water received in first shower. The first shower of

rains needs to be flushed-off to avoid contaminating storable/rechargeable water by the

probable contaminants of the atmosphere and the catchment roof. It will also help in cleaning

of silt and other material deposited on roof during dry seasons Provisions of first rainseparator should be made at outlet of each drainpipe.

Filter

There is always some skepticism regarding Roof Top Rainwater Harvesting since doubts are

raised that rainwater may contaminate groundwater. There is remote possibility of this fear

coming true if proper filter mechanism is not adopted. Secondly all care must be taken to see

that underground sewer drains are not punctured and no leakage is taking place in close

vicinity. Filters are used fro treatment of water to effectively remove turbidity, colour and

microorganisms. After first flushing of rainfall, water should pass through filters. There are

different types of filters in practice, but basic function is to purify water.

Sand Gravel FilterThese are commonly usedfilters, constructed by brick masonry and filleted

by pebbles, gravel, and sand as shown in the

figure. Each layer should be separated by wire

mesh.

Charcoal Filter

Charcoal filter can be made in-situ or in a drum.

Pebbles, gravel, sand and charcoal as shown in

the figure should fill the drum or chamber. Each

layer should be separated by wire mesh. Thin layer of charcoal is used to absorb odor if any.


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PVC- Pipe filter

This filter can be made by PVC pipe

of 1 to 1.20 m length; Diameter of

pipe depends on the area of roof. Six

inches dia. pipe is enough for a 1500

Sq. Ft. roof and 8 inches dia. pipe

should be used for roofs more then

1500 Sq. Ft. Pipe is divided into

three compartments by wire mesh.

Each component should be filledwith gravel and sand alternatively as

shown in the figure. A layer of

charcoal could also be inserted between two layers. Both ends of filter should have reduce of

required size to connect inlet and outlet. This filter could be placed horizontally or vertically

in the system.

Sponge Filter

It is a simple filter made from PVC drum having a

layer of sponge in the middle of drum. It is the easiest

and cheapest form filter, suitable for residential units.


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Methods of Roof Top Rainwater Harvesting

Storage of Direct use

In this method rain water collected from the roof of the building is diverted to a storage tank.The storage tank has to be designed according to the water requirements, rainfall and

catchment availability. Each drainpipe should have mesh filter at mouth and first flush device

followed by filtration system before connecting to the storage tank. It is advisable that each

tank should have excess water over flow system.

Excess water could be diverted to recharge system. Water from storage tank can be used for

secondary purposes such as washing and gardening etc. This is the most cost effective way of

rainwater harvesting. The main advantage of collecting and using the rainwater during rainy

season is not only to save water from conventional sources, but also to save energy incurred

on transportation and distribution of water at the doorstep. This also conserve groundwater, if

it is being extracted to meet the demand when rains are on.

Recharging ground water aquifers

Ground water aquifers can be recharged by various kinds of structures to ensure percolation

of rainwater in the ground instead of draining away from the surface. Commonly used

recharging methods are:-

a) Recharging of bore wells

b) Recharging of dug wells.

c) Recharge pits

d) Recharge Trenches

e) Soak ways or Recharge Shafts

f) Percolation Tanks

Recharging of bore wells

Rainwater collected from rooftop of

the building is diverted through

drainpipes to settlement or filtration

tank. After settlement filtered water is

diverted to bore wells to recharge deep

aquifers. Abandoned bore wells can

also be used for recharge.

Optimum capacity of settlement

tank/filtration tank can be designed on

the basis of area of catchement,

intensity of rainfall and recharge rate

as discussed in design parameters. While recharging, entry of floating matter and silt should

be restricted because it may clog the recharge structure. "first one or two shower should be

flushed out through rain separator to avoid contamination. This is very important, and allcare should be taken to ensure that this has been done."


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Recharge Pits

Recharge pits are small pits of any shape

rectangular, square or circular, contracted

with brick or stone masonry wall with

weep hole at regular intervals. to of pitcan be covered with perforated covers.

Bottom of pit should be filled with filter

media.

The capacity of the pit can be designed

on the basis of catchment area, rainfall

intensity and recharge rate of soil.

Usually the dimensions of the pit may be

of 1 to 2 m width and 2 to 3 m deep

depending on the depth of pervious strata.

These pits are suitable for recharging of

shallow aquifers, and small houses.

Soak away or Recharge Shafts

Soak away or recharge shafts are

provided where upper layer of soil

is alluvial or less pervious. These

are bored hole of 30 cm dia. up to10 to 15 m deep, depending on

depth of pervious layer. Bore

should be lined with

slotted/perforated PVC/MS pipe to

prevent collapse of the vertical

sides. At the top of soak away

required size sump is constructed to

retain runoff before the filters

through soak away. Sump should be

filled with filter media.

Recharging of dug wells

Dug well can be used as recharge

structure. Rainwater from the rooftop is

diverted to dug wells after passing it

through filtration bed. Cleaning and

desalting of dug well should be done

regularly to enhance the recharge rate.

The filtration method suggested for bore

well recharging could be used.


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Recharge Trenches

Recharge trench in provided

where upper impervious layer

of soil is shallow. It is a trench

excavated on the ground andrefilled with porous media like

pebbles, boulder or brickbats. it

is usually made for harvesting

the surface runoff. Bore wells

can also be provided inside the

trench as recharge shafts to

enhance percolation. The length

of the trench is decided as per

the amount of runoff expected.

This method is suitable for

small houses, playgrounds,

parks and roadside drains. The

recharge trench can be of size

0.50 to 1.0 m wide and 1.0 to

1.5 m deep.

Percolation tanks

Percolation tanks are artificially created surface water bodies, submerging a land area with

adequate permeability to facilitate sufficient percolation to recharge the ground water. These

can be built in big campuses where land is available and topography is suitable.

Surface run-off and roof top water can be diverted to this tank. Water accumulating in the

tank percolates in the solid to augment the ground water. The stored water can be used

directly for gardening and raw use. Percolation tanks should be built in gardens, open spaces

and roadside green belts of urban area.

Do's and Donts

Harvested rainwater is used for direct usage or for recharging aquifers. It is most important to

ensure that the rainwater caught is free from pollutants. Following precautionary measures

should be taken while harvesting rainwater:-

Roof or terraces uses for harvesting should be clean, free from dust, algal plants etc.

Roof should not be painted since most paints contain toxic substances and may peel

off.

Do not store chemicals, rusting iron, manure or detergent on the roof.

Nesting of birds on the roof should be prevented.

Terraces should not be used for toilets either by human beings or by pets.

Provide gratings at mouth of each drainpipe on terraces to trap leaves debris and

floating materials.

Provision of first rain separator should be made to flush off first rains. Do not use polluted water to rechargeground water.


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Ground water should only be recharged by rainwater.

Before recharging, suitable arrangements of filtering should be provided.

Filter media should be cleaned before every monsoon season.

During rainy season, the whole system (roof catchment, pipes, screens, first flush,filters, tanks) should be checked before and after each rain and preferably cleaned

after every dry period exceeding a month. At the end of the dry season and just before the first shower of rain is anticipated, the

storage tank should be scrubbed and flushed off all sediments and debris.


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COMPONENTS OF A RAINWATER HARVESTINGSYSTEM

A rainwater harvesting system comprises components ovarious stages - transporting rainwater through pipes ordrains, filtration, and storage in tanks for reuse or recharge.The common components of a rainwater harvesting system

involved in these stages are illustrated here.

1. Catchments:The catchment of a water harvestingsystem is the surface which directly receives the rainfall andprovides water to the system. It can be a paved area like a

terrace or courtyard of a building, or an unpaved area like a lawn or open ground. A roof made oreinforced cement concrete (RCC), galvanised iron or corrugated sheets can also be used for waterharvesting.

2. Coarse meshat the roof to prevent the passageof debris

3. Gutters:Channels all around the edge of a sloping roof to

collect and transport rainwater to the storage tank.Gutters can be semi-circular or rectangular and couldbe made using:

Locally available material such as plaingalvanised iron sheet (20 to 22 gauge),folded to required shapes.

Semi-circular gutters of PVC material can be

readily prepared by cutting those pipes intotwo equal semi-circular channels.

Bamboo or betel trunks cut vertically in half.

The size of the gutter should be according to the flow during the highest intensity rain. It is

advisable to make them 10 to 15 per cent oversize.

Gutters need to be supported so they do not sag or fall off when loaded with water. The wayin which gutters are fixed depends on the construction of the house; it is possible to fix iron ortimber brackets into the walls, but for houses having wider eaves, some method of attachment tothe rafters is necessary.

4. ConduitsConduits are pipelines or drains that carry rainwater from the catchment or rooftop area to theharvesting system. Conduits can be of any material like polyvinyl chloride (PVC) or galvanized iron(GI), materials that are commonly available.

The following table gives an idea about the diameter of pipe required for draining outrainwater based on rainfall intensity and roof area:

Sizing of rainwater pipe for roof drainage

Diameter

Of pipe(mm)

Average rate of rainfall in mm/h

50 75 100 125 150 200

50 13.4 8.9 6.6 5.3 4.4 3.3

65 24.1 16.0 12.0 9.6 8.0 6.0

Source: A water harvesting manualfor urban areas

Source: A water harvesting manual for urban areas
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75 40.8 27.0 20.4 16.3 13.6 10.2

100 85.4 57.0 42.7 34.2 28.5 21.3

125 - - 80.5 64.3 53.5 40.0

150 - - - - 83.6 62.7

mm/ h - millimeters per hour; m - meters

Source: National Building Code

5. First-flushingA first flush device is a valve that ensures that runoff from the first spell of rain is flushed out anddoes not enter the system. This needs to be done since the first spell of rain carries a relativelylarger amount of pollutants from the air and catchment surface.


6. Filter

The filter is used to remove suspended pollutants from rainwater collected over roof. A filter unit isa chamber filled with filtering media such as fibre, coarse sand and gravel layers to remove debrisand dirt from water before it enters the storage tank or recharge structure. Charcoal can be addedfor additional filtration.

(i) Charcoal water filterA simple charcoal filter can be made in a drum or an earthenpot. The filter is made of gravel, sand and charcoal, all ofwhich are easily available.

(ii) Sand filtersSand filters have commonly available sand as filter media.Sand filters are easy and inexpensive to construct. These filterscan be employed for treatment of water to effectively removeturbidity (suspended particles like silt and clay), colour and

microorganisms.

In a simple sand filter that can be constructed

domestically, the top layer comprises coarse sand followed bya 5-10 mm layer of gravel followed by another 5-25 cm layerof gravel and boulders.

(ii. a) Dewas

filtersMost residents inDewas, MadhyaPradesh, have wells in their houses. Formerly, all thatthose wells would do was extract groundwater. Butthen, the district administration of Dewas initiated agroundwater recharge scheme. The rooftop waterwas collected and allowed to pass through a filtersystem called the Dewas fillter, designed byMohan

Rao, district collecter of Dewas, and engineers of therural engineering services. The water thus filtered is

Source: A water harvesting manualfor urban areas

Source: A water harvesting manual for urbanareas
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put into the service tubewell.

The filter consists of a polyvinyl chloride (PVC) pipe 140 mm in diameter and 1.2m long.There are three chambers. The first purification chamber has pebbles varying between 2-6 mm, thesecond chamber has slightly larger pebbles, between 6 and 12 mm and the third chamber has thelargest - 12-20 mm pebbles. There is a mesh at the outflow side through which clean water flowsout after passing through the three chambers. The cost of this filter unit is Rs 600.

Filter for large rooftopsWhen rainwater is harvested in a large rooftop area,the filtering system should accommodate the excessflow. A system is designed with three concentric circularchambers in which the outer chamber is filled withsand, the middle one with coarse aggregate and theinner-most layer with pebbles.

This way the area of filtration is increased forsand, in relation to coarse aggregate andpebbles. Rainwater reaches the centre coreand is collected in the sump where it istreated with few tablets of chlorine and ismade ready for consumption. This system wasdesigned byR Jeyakumar(Source: Jeyakumar; Rain water Harvest Manual P-21)

Varun:S Vishwanath, a Bangalore water harvesting

expert, has developed a rainwater filter "VARUN". According to him, from a decently clean roof'VARUN' can handle a 50 mm per hour intensity rainfall from a 50 square metre roof area. This

means the product is relatively standardised. For new house builders we therefore can recommendthe number of downpipes they have to optimise on and the number of filters they will need.

'VARUN' is made from a 90 litre High Density Poly Ethylene(HDPE) drum. The lid is turned over and holes are puched in it. This isthe first sieve which keeps out large leaves, twigs etc. Rainwatercoming out of the lid sieve then passes through three layers of spongeand a 150 mm thick layer of coarse sand. Presence of sponge makesthe cleaning process very easy. Remove the first layer of sponge andsoak /clean it in a bucket of water (which you then don't waste but useit for plants). The sand needs no cleaning at all. The basic cost of thefilter is about Rs 2250/-

ii. b. Horizontal roughing filter and slow sandfilterThe introducton of horizontal roughing filter and slowsand filter (HRF/SSF) to treat surface water hasmade safe drinking water available in coastal pocketsof Orissa. The major components of this filter aredescribed below.

1) Filter channel :One square metre in cross-section and eight m in length, laid across the tankembankment, the filter channel consists of threeuniform compartments, the first packed with brokenbricks, the second with coarse sand, followed by finesand in the third compartment. The HRF usually consists of filter material like gravel and coarse

sand that successively decreases in size from 25 mm to 4 mm. The bulk of solids in the incomingwater is separated by this coarse filter media or HRF. At every outlet and inlet point of the channel,fine graded mesh is implanted to prevent entry of finer materials into the sump. The length of a
http://www.rainwaterharvesting.org/People/innovators-urban.htm#rjeyhttp://www.rainwaterharvesting.org/People/innovators-urban.htm#rjeyhttp://www.rainwaterharvesting.org/People/innovators-urban.htm#rjeyhttp://www.rainwaterharvesting.org/People/innovators-urban.htm#svishttp://www.rainwaterharvesting.org/People/innovators-urban.htm#svishttp://www.rainwaterharvesting.org/People/innovators-urban.htm#svishttp://www.rainwaterharvesting.org/People/innovators-urban.htm#rjeyhttp://www.rainwaterharvesting.org/People/innovators-urban.htm#rjey


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channel varies according to the nature of the site selected for the sump.

2) Sump:A storage provision to collect filtered water from the tank through the filter channel forstorage and collection.

While HRF acts as a physical filter and is applied to retain solid matter, SSF is primarily abiological filter, used to kill microbes in the water. Both filter types are generally stable, making fulluse of the natural purification process of harvested surface water and do not require any chemicals.

For more details:Making Water Everybody's Business

iii. Rain PCAcquaSure, a consortium of three specialist Netherlands-basedcompanies, has developed a system for the conversion orainwater to drinking water in the form of a RainwaterPurification Centre (RainPC).

RainPC is developed by scaling down the multi-stagedwater treatment method (MST), which involves screening,

flocculation sedimentation and filtration and incorporatingexisting technologies like upward flow fine filtration, absorptionand ion exchange. Coming in a small compact 26 kg unit, theRainPC offers an affordable solution by converting rainwaterinto drinking water.

RainPC is made of ultra violet resistant poly-ethylenehousing and cover, stainless steel rods and bolts, a nickel-brassvalve and an adapter for maintaining constant volume.

Xenotex-A and activated carbon catridges along with ultra membrane filtration or micro-membranefiltration modules incorporated in the RainPC has the capacity to deal with E-coli and the potential

of meeting the Dutch as well as World Health Organisations (WHO) water regulation standards. Thecomponents can also be transported individually to be assembled at the site. Three product typesare available based on their microbial contaminant removal capacity. This technology is ideally

suited for virtually any situation and is a blessing particularly for those who have little or no accessto regular safe drinking water.

The salient features of Rain PC are:

Simple straight-forward installation

Easy to operate and maintain

Needs no power and operates at low gravity pressure(0.1 bar upward).

The system is capable of providing a constant flow ofabout 40 liters of rainwater per hour, enough for a familyof five for drinking, cooking and bathing purposes.

Maintains nearly constant volume irrespective of waterpressure.

The Xenotex-A and activated carbon cartridge processesup to 20,000 liters and can be regenerated up to 10times.

Cost per 1000 litres is as low as US$ 2 to 3.

(The above information is as per the manufacturers' claims and not based on any study byCSE.)

For further information Email:[email protected]

iv. Rainwater harvester

EA Water Pvt Ltd has launched a unique Rainwater Harvester, which filters runoff water from roads,
mailto:[email protected]:[email protected]:[email protected]:[email protected]


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which generally contains oil and grease. This system has been installed in the Gymkhana club,Sector-15, Faridabad, Haryana. Rajit Malohtra, project in charge, of this company explained thatthe water harvesting system installed at the club has a sand filter, which filters silt from runoffharvested from roof, lawns and parking area. The cost of the filter is around Rs 60,000.

For more details, contact

EA Water PVT Limited,

504 empire apartments,Mehrauli-Gurgaon Road,SultanpurNew Delhi-110 030Phone: 011-2680062Website:www.eawater.com

Filters available in the German MarketAccording to Wessels (1994), concerns over the possible negative health effects of rainwaterutilisation led to some opposition. The Federal Office of Health, for example, intially objected to itsuse for washing clothes, personal hygiene and even for toilet flushing, due to possible risks ofinfection and allergic reactions. Long-term investigations by the health offices in Hamburg andBremen, however, have yielded positive results with respect to the use of water for washingpurposes and have confirmed that rainwater sources do not present a health risk.

(i) Filters developed by WISYPrivate companies such as WISY, based in Kefenrod in Germany, are playing an important role inpromoting rainwater use by developing pumps and filter devices to improve water quality. WISYhas developed a simple filter system, which can be attached to a standard household downpipe.Under conditions in Germany (assuming a mean annual rainfall of 650mm/year), this can divertand filter 90 per cent of the runoff from a roof area of up to 200 square metre.

(fig a). A filter collector diverts 90 per cent of rainwater toa storage tank through a 0.17 mm stainless steel mesh filter.

(fig b). A larger vortexfine filter can cope with run-off

from roof areas of up to 500square metre.

(fig c).Afloating fine suctionfilter for ensuring that thewater pumped from the tank isextracted from the cleanest

part of the tank and is free ofparticulates has also been

developed.

For details contact:WISY (Winkler system)OT Hitzkirchen, Oberdorfstrasse 26,D-63699, Kefendrod-HitzkirchenGermany; fax:[email protected](Source: John Gould and Erik Nissen-Petersen, 1999: Rainwater CatchmenSystems for Domestic Supply - Design, Construction and Implementation,Intermediate Technology Group)

(ii) Filters developed by MALLBETON

Another company, MALLBETON, a manufacturer of concretetanks and filters, based in Germany, is marketing a tankdesign which manages any overflows (Konig, 1998). This is

(a)(b)

(c)
http://www.eawater.com/http://www.eawater.com/http://www.eawater.com/mailto:[email protected]:[email protected]:[email protected]://www.eawater.com/


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done by constructing the top half of a sub-surface tank from a porous concrete ring, which allowswater to gradually seep into the ground. While this reduces the volume of water available, it doesmake householders eligible for waivers on their rainwater drainage fees. These fees are alreadyapplied to householders and businesses in about 25 per cent of Germany. The charges that arelevied on each square metre of roof area and sealed surroundings can be substantial, such as inBonn, so waivers often provide significant savings.(Source: John Gould and Erik Nissen-Petersen, 1999: Rainwater Catchment Systems for Domestic Supply - Design, Construction

and Implementation, Intermediate Technology Group)

7. Storage facility

There are various options available for the construction ofthese tanks with respect to the shape, size and the material ofconstruction.

Shape:Cylindrical, rectangular and square.Material of construction:Reinforced cement concrete,(RCC), ferrocement, masonry, plastic (polyethylene) or metal

(galvanised iron) sheets are commonly used.Position of tank:Depending on space availability these tanks could be constructed above ground,partly underground or fully underground. Some maintenance measures like cleaning anddisinfection are required to ensure the quality of water stored in the container.

8. Recharge structures

Rainwater may be charged into the groundwater aquifers through any suitable structures likedugwells, borewells, recharge trenches and recharge pits.

Various recharge structures are possible - some which promote the percolation of waterthrough soil strata at shallower depth (e.g., recharge trenches, permeable pavements) whereasothers conduct water to greater depths from where it joins the groundwater (e.g. recharge wells).

At many locations, existing structures like wells, pits and tanks can be modified as rechargestructures, eliminating the need to construct any structures afresh. Here are a few commonly usedrecharging methods:

1. Recharging of dugwells and abandoned tubewells.In alluvial and hard rock areas, there are thousands of wells which have either gone dry or whosewater levels have declined considerably. These can be recharged directly with rooftop run-off.Rainwater that is collected on the rooftop of the building is diverted by drainpipes to a settlementor filtration tank, from which it flows into the recharge well (borewell or dugwell).

If a tubewell is used for recharging, then the casing (outer pipe) should preferably be aslotted or perforated pipe so that more surface area is available for the water to percolate.Developing a borewell would increase its recharging capacity (developing is the process wherewater or air is forced into the well under pressure to loosen the soil strata surrounding the bore tomake it more permeable).

If a dugwell is used for recharge, the well lining should have openings (weep-holes) atregular intervals to allow seepage of water through the sides. Dugwells should be covered toprevent mosquito breeding and entry of leaves and debris. The bottom of recharge wells should bedesilted annually to maintain the intake capacity.

Providing the following elements in the system can ensure the quality of water entering therecharge wells:1. Filter mesh at entrance point of rooftop drains2. Settlement chamber3. Filter bed

A storage tank made of galvanised iron sheets
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2. Settlement tankSettlement tanks are used to remove silt and other floatingimpurities from rainwater. A settlement tank is like anordinary storage container having provisions for inflow(bringing water from the catchment), outflow (carryingwater to the recharge well) and overflow. A settlementtank can have an unpaved bottom surface to allow

standing water to percolate into the soil.

In case of excess rainfall, the rate of recharge, especially of borewells, may not match therate of rainfall. In such situations, the desilting chamber holds the excess amount of water till it issoaked up by the recharge structure. Thus, the settlement chamber acts like a buffer in thesystem.

Any container, (masonry or concrete underground tanks, old unused tanks, pre-fabricatedPVC or ferrocement tanks) with adequate capacity of storage can be used as a settlement tank.

3. Recharging of service tubewells.In this case the rooftop runoff is not directly led into the service tubewells, to avoid chances ocontamination of groundwater. Instead rainwater is collected in a recharge well, which is a

temporary storage tank (located near the service tubewell), with a borehole, which is shallowerthan the water table. This borehole has to be provided with a casing pipe to prevent the caving inof soil, if the strata is loose. A filter chamber comprising of sand, gravel and boulders is provided toarrest the impurities.

4. Recharge pitsA recharge pit is 1.5m to 3m wide and 2m to 3m deep. The excavated pit is lined with a brick/stonewall with openings (weep-holes) at regular intervals. The top area of the pit can be covered with aperforated cover. Design procedure is the same as that of a settlement tank.

5. Soakaways / Percolation pit

Percolation pits, one of the easiest and most effectivemeans of harvesting rainwater, are generally not more than60 x 60 x 60 cm pits, (designed on the basis of expectedrunoff as described for settlement tanks), filled with pebblesor brick jelly and river sand, covered with perforated concreteslabs wherever necessary.

6.Recharge trenches

A recharge trench is a continuous trench excavated in theground and refilled with porous media like pebbles, bouldersor broken bricks. A recharge trench can be 0.5 m to 1 m wideand 1 m to 1.5 m deep. The length of the recharge trench is decided as per the amount of runoffexpected. The recharge trench should be periodically cleaned of accumulated debris to maintain theintake capacity. In terms of recharge rates, recharge trenches are relatively less effective since thesoil strata at depth of about 1.5 metres is generally less permeable. For recharging throughrecharge trenches, fewer precautions have to be taken to maintain the quality of the rainfall runoff.Runoff from both paved and unpaved catchments can be tapped.

7.Recharge troughs


To collect the runoff from paved or unpaved areas draining out of a compound, rechargetroughs are commonly placed at the entrance of a residential/institutional complex.These structures

A settlement chamber

Filter materials in a soakaway


16/16

are similar to recharge trenches except for the fact that the excavated portion is not filled with filtermaterials. In order to facilitate speedy recharge, boreholes are drilled at regular intervals in thistrench. In design part, there is no need of incorporating the influence of filter materials.This structure is capable of harvesting only a limited amount of runoff because of the limitation with

regard to size.

8. Modified injection wellIn this method water is not pumped into the aquifer but allowed topercolate through a filter bed, which comprises sand and gravel. A modifiedinjection well is generally a borehole, 500 mm diameter, which is drilled tothe desired depth depending upon the geological conditions, preferably 2 to3 m below the water table in the area. Inside this hole a slotted casing pipeof 200 mm diameter is inserted. The annular space between the boreholeand the pipe is filled with gravel and developed with a compressor till itgives clear water. To stop the suspended solids from entering the rechargetubewell, a filter mechanism is provided at the top.

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Elective RWH