Components of a Rainwater Harvesting System
All rainwater-harvesting systems comprise six basic components irrespective of the size of the system.
- Catchment area/roof: The surface upon which the rain falls; the roof has to be appropriately sloped preferably towards the Direction of storage and recharge.
- Gutters and downspouts: The transport channels from catchment surface to storage; these have to be designed depending on Site, rainfall characteristics and roof characteristics.
- Leaf screens and roof washers: The systems that remove contaminants and debris; a first rain separator has to be put in place to divert and manage the first 2.5 mm of rain.
- Cisterns or storage tanks: Sumps, tanks etc. where collected rainwater is safely stored or recharging the groundwater through open wells, bore wells or percolation pits etc.;
- Conveying: The delivery system for the treated rainwater, either by gravity or pump;
- Water treatment: Filters to remove solids and organic material and equipment, and additives to settle, filter, and disinfect.
Briefly the system involves collecting water that falls on the roof of a house made of zinc, asbestos or other material during rain storms, and conveying it by an aluminium, PVC, wood, plastic or any other local material including bamboo drain or collector to a nearby covered storage unit or cistern. Rainwater yield varies with the size and texture of the catchment area. A smoother, cleaner and more impervious roofing material contributes to better water quality and greater quantity. Each component is briefly described below.
The catchment area of a water harvesting system is the surface, which receives rainfall directly and contributes the 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. Temporary structures like sloping sheds can also act as catchments. In house compounds and threshing floors are surfaced with clay / cow dung plaster and used effectively as rainwater catchments. Rainwater harvested from catchment surfaces along the ground, because of the increased risk of contamination, should only be used for non-potable uses such as lawn watering. For in house uses, rooftop harvested rainwater is safer for drinking purposes than the runoff harvested water.
Catchment Area: Some Features
- The nature of the catchment distinguishes rainwater collection from other kind of harvesting.
- Four types of catchment areas have been considered namely; roof, rainwater platforms, watershed management and hill slopes.
- Catchments used to collect rainwater are frequently artificial or else ground surfaces, which have been specifically prepared and demarcated.
- Rainwater may be collected from any kind of roof â€“ tiles, metal, palm leaf, grass thatch.
- Lead flashing roof or roof painted with lead-based paint or asbestos roof is generally regarded as unsuitable.
- A well-thatched roof has been said not to be presenting much hazard to the collected water. These have been covered
Catchment Efficiency Depends upon Nature of Roofing & Plats area
|Roof Catchments||Tiles 0.8 – 0.9,Corrugated metal sheets 0.7 – 0.9, Thatched 0.2|
|Ground Surface Coverings||Concrete 0.6 – 0.9, Brick pavement 0.5 – 0.6|
|Untreated Ground Catchments||Soil on slopes less than 10 per cent 0.0 – 0.3, Rocky natural catchments 0.2 – 0.5, Green area 0.05 – 0.10|
With plastic sheets in some areas in Manipur (NE India).
Catchment area consisting of rooftop area / the plot area or the complex area from where the rainwater runoff is proposed to be collected has to be maintained so as to ensure that the resultant rainwater runoff is not contaminated. At times paints, grease, oil etc. are often left on the roof or in the courtyards. These can result in contamination of the rainwater runoff. Therefore, the households have to ensure that they keep the catchment area clean at all times especially during the rainfall season.
The size of a roof catchment area is the building’s footprint under the roof. The catchment surface is limited to the area of roof which is guttered. To calculate the size of the catchment area, multiply the length times the width of the gutter area.
Rainwater may be collected from any kind of roof. Tiled or metal roofs are easier to use, and may give clean water, but it is perfectly feasible to use roofs made of palm leaf or grass thatch. The only common type of roof which is definitely unsuitable, especially to collect water for drinking, is a roof with lead flashings, or painted with a lead-based paint. It is suggested that roofs made of asbestos sheeting should also not be used if fibres are getting detached from damaged areas. Many areas well-constructed houses have corrugated iron roofs which are used for collecting rainwater. Roof areas are large, often exceeding 100 m2, though guttering may be installed on only half the area. Caution is generally warranted regarding thatched roofs, which are reported to be sources of contamination, but there seems to be no evidence that water from a well-thatched roof presents any significantly great hazard to consumers than water from other roofs. Greater precaution may be advisable to ensure that debris from the roof does not enter the tank, and the water should usually be boiled before drinking. The most important consideration, however, is that if a project is to help low-income groups, there may be no choice but to tackle the problem of collecting water from thatch or palm-leaf roofs, either by using a low level collecting tank or by devising means of attaching gutters. If a new construction project is planned with a slanting roof, metal roofing is the preferred material because of its smooth surface and durability. Other material options such as clay tile or slate are also appropriate for rainwater intended to be used as potable water. These surfaces can be treated with a special nontoxic paint coating to discourage bacterial growth on an otherwise porous surface. Because composite asphalt, asbestos, chemically treated wood shingles and some painted roofs could leach toxic materials into the rainwater as it touches the roof surface, they are recommended only for non-potable water uses.
Most of the existing stormwater conveyance systems are designed to drain out the rainwater that falls in the catchment area into the nearest storm water drain or the sewerage system. These connections should be redirected to the recharge location so that the rainwater runoff can now be directed into the recharge structure. In already built up structure it requires certain modifications to the existing drainage system but in ongoing construction it can be easily re-designed at almost no extra cost. The choice of the material and the design are as per the discretion of the individual owners and, like any other drainage system, can be constructed utilizing a variety of materials.
Conduits are the pipelines or drains that carry rainwater from the catchment or rooftop to the harvesting system. Conduits may be of any material like Poly Vinyl Chloride (PVC), asbestos or Galvanized Iron (GI), materials that are commonly available. The diameter of pipe required for draining out rainwater based on rainfall intensity (average rate of rainfall in mm per hour) and roof surface area as shown in above Channels have to be 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 could be made using:
- Locally available material such as plain galvanised iron sheet (20 to 22 gauge), folded to the required shapes.
- Semi-circular gutters of PVC material which can be readily prepared by cutting the pipes into two 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 percent oversize. Gutters need to be supported so that they do not sag or fall off when loaded with water. The way in which gutters are fixed depends on the construction of the house; it is possible to fix iron or timber brackets into the walls, but for houses having wider eaves, some method of attachment to the rafters is necessary. These are the components which catch the rain from the roof catchment surface and transport it to the cistern. Standard shapes and sizes are easily obtained and maintained, although custom fabricated profiles are also possible to maximize the total amount of harvested rainfall. Gutters and downspouts must be properly sized, sloped, and installed in order to maximize the quantity of harvested rain.
Conveyance systems are required to transfer the rainwater collected on catchment surfaces (e.g. rooftops) to the storage tanks. This is usually accomplished by making connections to one or more down-pipes connected to collection devices (e.g. rooftop gutters). The pipes used for conveying rainwater, wherever possible, should be made of plastic, PVC or other inert substance, as the pH of rainwater can be low (acidic) and may cause corrosion and mobilization of metals in metal pipes.
When selecting a conveyance system, consideration should be given to the fact that when it first starts to rain, dirt and debris from catchment surfaces and collection devices will be washed into the conveyance systems (e.g. downpipes). Relatively clean water will only be available sometime later in the storm. The first part of each rainfall should be diverted from the storage tank. There are several possible options for selectively collecting clean water for the storage tanks. The common method is a sediment trap, which uses a tipping bucket to prevent the entry of debris from the catchment surface into the tank. Installing a first flush (or foul flush) device is also useful to divert the initial batch of rainwater away from the tank. Gutters and downpipes need to be periodically inspected and carefully cleaned. A good time to inspect gutters and downpipes is while it is raining, so that leaks can be easily detected. Regular cleaning is necessary to avoid contamination
To keep leaves and other debris from entering the system, the gutters should have a continuous leaf screen, made of 1/4-inch wire mesh in a metal frame, installed along their entire length, and a screen or wire basket at the head of the downspout. Gutter hangers are generally placed every 3 feet. The outside face of the gutter should be lower than the inside face to encourage drainage away from the building wall. Where possible, the gutters should be placed about 1/4 inch below the slope line so that debris can clear without knocking down the gutter. To prevent leaves and debris from entering the system, mesh filters should be provided at the mouth of the drain pipe (see figure). Further, a first flush (foul flush) device section should be provided in the conduit before it connects to the storage container. If the stored water is to be used for drinking purposes, a sand filter should also be provided.
A first flush (foul flush) device is a valve that ensures that runoff from the first spell of rain is flushed out and does not enter the system. This needs to be done since the first spell of rain carries a relatively larger amount of pollutants from the air and catchment surface. Roof washing, or the collection and disposal of the first flush of water from a roof, is of particular concern if the collected rainwater is to be used for human consumption, since the first flush picks up most of the dirt, debris, and contaminants, such as bird droppings that have collected on the roof and in the gutters during dry periods. The most simple of these systems consists of a standpipe and a gutter downspout located ahead of the downspout from the gutter to the cistern. The pipe is usually 6 or 8 inch PVC which has a valve and clean out at the bottom. Most of these types of roof washers extend from the gutter to the ground where they are supported. The gutter downspout and top of the pipe are fitted and sealed so water will not flow out of the top. Once the pipe has filled, the rest of the water flows to the downspout connected to the cistern. These systems should be designed so that at least 50 litres of water are diverted for every 1000 square feet of collection area. Rather than wasting the water, the first flush can be used for non-potable uses such as for lawn or garden irrigation.
Storage tanks for collecting rainwater may be located either above or below the ground. They may be constructed as part of the building, or may be built as a separate unit located some distance away from the building. The design considerations vary according to the type of tank and other factors. Various types of rainwater storage facilities are found in practice. Storage tanks should be constructed of inert material. Reinforced concrete, fiberglass, polyethylene, and stainless steel are also suitable materials. Ferro-cement tanks and jars made of mortar or earthen materials are commonly used. As an alternative, interconnected tanks made of pottery or polyethylene are also found suitable. The polyethylene tanks are compact but have a large storage capacity (1,000 to 2,000 litres). They are easy to clean and have many openings which can be fitted with connecting pipes. Bamboo reinforced tanks are less successful because the bamboo may become infested with termites, bacteria and fungus. Precautions are required to prevent the entry of contaminants into storage tanks.
There are various options available for the construction of these tanks with respect to the shape, size and the material of construction. Shapes may be cylindrical, rectangular and square.
Suppose the system has to be designed for meeting drinking water requirement of a 5-member family living in a building with a rooftop area of 112 sq.m. Average annual rainfall in the region is 600 mm. Daily drinking water requirement per person (drinking and cooking) is 10 litres. We shall first calculate the maximum amount of rainfall that can be harvested from the rooftop Following details are available:
Area of the catchment (A) = 112 sq.m.
Average annual rainfall (R) = 970 mm (0.97 m)
Runoff coefficient (C) = 0.9
Annual water harvesting potential from 112 sq.m. roof
= A x R x C
= 112 x 0.97 x 0.9
= 97.7 cu.m. (97,700 litres)
The tank capacity has to be designed for the dry period, i.e., the period between the two consecutive rainy seasons. With the rainy season extending over four months, the dry season is of 245 days. Particular care must be taken to ensure that potable water is not contaminated by the collected rainwater. Drinking water requirement for the family (dry season) = 245 x 5 x 10 = 12,250 litres. As a safety factor, the tank should be built 20 per cent larger than required, i.e., 14,700 litres. This tank can meet the basic drinking water requirement of a 5-member family for the dry period.The most commonly used material of construction is Reinforced Cement Concrete (RCC), ferrocement, masonry, plastic (polyethylene) or metal (galvanised iron).Materials needed for the construction of some widely used types of rainwater tank Particular care must be taken to ensure that potable water is not contaminated by the collected rainwater
It should be remembered that water only flows downhill unless you pump it. The old adage that, gravity flow works only if the tank is higher than the kitchen sink, accurately portrays the physics at work. The water pressure for a gravity system depends on the difference in elevation between the storage tank and the faucet. Water gains one pound per square inch of pressure for every 2.31 feet of rise or lift. Many plumbing fixtures and appliances require 20 psi for proper operation, while standard municipal water supply pressures are typically in the 40-psi to 60 psi range. To achieve comparable pressure, a cistern would have to be 92.4 feet (2.31 feet X 40 psi = 92.4 feet) above the home’s highest plumbing fixture. That explains why pumps are frequently used, much in the way they are used to extract well water. Pumps prefer to push water, not pull it. To approximate the water pressure one would get from a municipal system, pressure tanks are often installed with the pump. Pressure tanks have a pressure switch with adjustable settings between 5 and 65 psi. For example, to keep the in house pressure at about 35 psi, the switch should be set to turn off the pump when the pressure reaches 40 psi and turn it on again when the pressure drops down to 30 psi.
Before making a decision about what type of water treatment methods to use, water should be got tested by an approved laboratory and determine whether the water could be used for potable or nonpotable uses. The types of treatment discussed are filtration, disinfection, and buffering for pH control. Dirt, rust, scale, silt and other suspended particles, bird and rodent feces, airborne bacteria and cysts will inadvertently find their way into the cistern or storage tank even when design features such as roof washers, screens and tight-fitting lids are properly installed. Water can be unsatisfactory without being unsafe; therefore, filtration and some form of disinfection is the minimum recommended treatment if the water is to be used for human consumption (drinking, brushing teeth, or cooking). The types of treatment units most commonly used by rainwater systems are filters that remove sediment, in consort with either ultraviolet light or chemical disinfection.
A filter is an important part of the inflow structure of a RWH System. Once screens and roof washers remove large debris, other filters are available which help improve rainwater quality. Keep in mind that most filters available in the market are designed to treat municipal water or well water. Therefore, filter selection requires careful consideration. Screening, sedimentation, and pre-filtering occur between catchment and storage or within the tank. A cartridge sediment filter, which traps and removes particles of five microns or larger is the most common filter used for rainwater harvesting. Sediment filters used in series, referred to as multi-cartridge or inline filters, sieve the particles from increasing to decreasing size. These sediment filters are often used as a pre-filters for other treatment techniques such as ultraviolet light or reverse osmosis filters which can become clogged with large particles. Unless something is added to rainwater, there is no need to filter out something that is not present. When a disinfectant such as chlorine is added to rainwater, an activated carbon filter at the tap may be used to remove the chlorine prior to use. It should be remembered that activated carbon filters are subject to becoming sites of bacterial growth. Chemical disinfectants such as chlorine or iodine must be added to the water prior to the activated carbon filter. If ultraviolet light or ozone is used for disinfection, the system should be placed after the activated carbon filter. Many water treatment standards require some type of disinfection after filtration with activated carbon. Ultraviolet light disinfection is often the method of choice.
Types of Filtration Systems -Types of Filtration Systems
Gravity Based FilterThis consists of construction of an underground / above ground filtration chamber consisting of layers of fine sand / coarse sand and gravel. The ideal depths from below are 60 cm thick coarse gravel layer, 40 cm coarse sand and 40 cm fine sand. Alternatively only fine sand can also be used along with the gravel layer. Further deepening of the filter media shall not result in an appreciable increase in the rate of recharge and the rate of filtration is proportional to the surface area of the filter media. A unit sq.m. surface area of such a filter shall facilitate approx. 60 litres./hr of filtration of rainwater runoff. In order to determine the optimum size of the surface area just divide the total design recharge potential with this figure. A system of coarse and fine screen is essential to be put up before the rainwater runoff is allowed to flow into the filtration pit. A simple charcoal can be made in a drum or an earthen pot. The filter is made of gravel, sand and charcoal, all of which are easily available.
All rainwater-harvesting systems comprise six basic components irrespective of the size of the system.
Sand filters are commonly available, easy and inexpensive to construct. These filters can be employed for treatment of water to effectively remove turbidity (suspended particles like silt and clay), colour and microorganisms. In a simple sand filter that can be constructed domestically, the top layer comprises of coarse sand followed by a 5-10 mm layer of gravel followed by another 5-25 cm layer of gravel and boulders. These filters are manufactured commercially on a wide scale. Most of the water purifiers available in the market are of this type.
Artificial recharge to groundwater is a process by which the groundwater reservoirs are augmented at a rate exceeding that obtaining under natural conditions or replenishment. Any man-made schemes or facilities that add water to an aquifer may be considered to be artificial recharge systems. To ensure that rainwater percolates into the ground instead of draining away from the surface, various kinds of recharge structures are possible. Some structures like recharge trenches and permeable pavements promote the percolation of water through soil strata at shallower depth, while others like recharge wells carry water to greater depths from where it joins the ground water.At many locations, existing structures like wells, pits and tanks can be modified to be used as recharge structures, eliminating the need to construct any new structures. A few commonly used recharging methods are explained here. Innumerable innovations and combinations of these methods are possible. Rainwater may be charged into the groundwater aquifers through any suitable structures like dug wells, bore wells, recharge trenches and recharge pits. A wide spectrum of techniques is in vogue in different countries to recharge ground water reservoirs. Similar to the variations in hydrogeological framework, the artificial recharge techniques too vary widely. The artificial recharge techniques can be broadly categorized as follows:
Direct surface techniques/spreading methods
- Flooding techniques
- Basins or percolation tanks
- Stream augmentation/channel method
- Ditch and furrow system
- Over irrigation
Direct sub-surface techniques/pit method
- Injection wells or recharge wells
- Recharge pits and shafts
- Dug well recharge
- Borehole flooding
- Natural openings, cavity fillings.
Combination of surface-cum-sub-surface techniques/well method
- Basin or percolation tanks with pit shaft or wells.
- Induced recharge method
- Aquifer modification
Artificial Recharge Structures Particularly in Urban Context
are constructed for recharging the shallow aquifers. These are generally constructed 1 to 2 m. wide and 2 to 3 m. deep. After excavation, the pits are refilled with pebbles and boulders as well as coarse sand. The excavated pit is lined with a brick/stone wall with openings (weep-holes) at regular intervals. The top area of the pit can be covered with a perforated cover. Design procedure is the same as that of a settlement tank. The size of filter material is generally taken as below: Coarse sand : 1.5 – 2 mm, Gravels : 5 – 10 mm, Boulders : 5 – 20 cm
The filter material should be filled in graded form. Boulders at the bottom, gravels in between and coarse sand at the top so that the silt content that will come with runoff will be deposited on the top of the coarse sand layer and can easily be removed. If clay layer is encountered at shallow depth, it should be punctured with auger hole and the auger hole should be refilled with fine gravel of 3 to 6 mm size.
- Recharge pits 1 to 2 m wide and 2 to 3 m deep are constructed to recharge shallow aquifers.
- After excavation, the pit is refilled with boulders and pebbles at the bottom followed by gravel and then sand at the top.
- The collected water from the rooftop is diverted to the pit through a drainpipe.
- Recharge pit can be of any shape i.e. circular, square or rectangular. If the pit is trapezoidal in shape, the side slopes should be steep enough to avoid silt deposition.
- This method is suitable for small buildings having the rooftop area up to 100 sq.m.
is a bored hole of up to 30 cm diameter drilled in the ground to a depth of 3 to 10 m. The soakaway can be drilled with a manual auger unless hard rock is found at a shallow depth. The borehole can be left unlined if a stable soil formation like clay is present. In such a case, the soakaway may be filled up with a filter media like brickbats. In unstable formations like sand, the soakaway should be lined with a PVC or MS pipe to prevent collapse of the vertical sides.
The pipe may be slotted/perforated to promote percolation through the sides. A small sump is built at the top end of the soak away where some amount of runoff can be retained before it infiltrates through the soak away. Since the sump also acts like a buffer in the system, it has to be designed on the basis of expected runoff.
The above figures show typical systems of recharging wells directly with rooftop runoff. Rainwater that is collected on the rooftop of the building is diverted by drainpipes to a settlement or filtration tank, from which it flows into the recharge well (bore well or dug well).
If a bore well is used for recharging, then the casing (outer pipe) of the bore well should preferably be a slotted or perforated pipe so that more surface area is available for the water to percolate. Developing a bore well would increase its recharging capacity (developing is the process where water or air is forced into the well under pressure to loosen the soil strata surrounding the bore to make it more permeable). If a dug well is used for recharge, the well lining should have openings (weep-holes) at regular intervals to allow seepage of water through the sides. Dug wells should be covered to prevent mosquito breeding and entry of leaves and debris. The bottom of recharge dug wells should be desilted annually to maintain the intake capacity.
Precautions should be taken to ensure that physical matter in the runoff like silt and floating debris do not enter the well since it may cause clogging of the recharge structure. It is preferred that the dug well or borewell used for recharging be shallower than the water table. This ensures that the water recharged through the well has a sufficient thickness of soil medium through which it has to pass before it joins the groundwater. Any old well, which has become defunct, can be used for recharging, since the depth of such wells is above the water level.
Quality of Water Recharged
The quality of water entering the recharging wells can be ensured by providing the various elements in the system. These are: (1) Filter mesh at entrance point of rooftop drains; (2) Settlement chamber; (3) Filter bed.
Mini Artificial Aquifer System (MAAS)
is a unique artificial recharge structure, which is ideally suitable for open areas particularly low-lying areas. This structure is also suitable for junctions of roads, street corners, parks, stadiums, play grounds, bus terminus, theatres, open area of public buildings, schools, colleges etc.
In open areas, the topsoil and clayey portion of sub-surface should be excavated and the excavated portion may be filled with locally available boulders of various sizes in ascending order from the top. The top portion may be filled with river sand. Two or three recharge shafts may be constructed at the bottom of the excavated portion. These recharge shafts of site-specific dimensions can be constructed penetrating through the layers of impermeable horizon to the potential to prevent clogging.
Recharging through recharge trenches, recharge pits and soakaways is simpler compared to recharge through wells. Fewer precautions have to be taken to maintain the quality of the rainfall runoff. For these types of structures, there is no restriction on the type of catchment from which water is to be harvested, i.e., both paved and unpaved catchments can be tapped. A recharge trench is a continuous trench excavated in the ground. These are constructed when the permeable strata is available at shallow depths.
The trench may be 0.5 to 1 m. wide, 1 to 1.5 m. deep and 10 to 20 m. long depending upon availability of water. It is back filled with filter materials like pebbles, boulders or broken bricks. In case a clay layer is encountered at shallow depth, a number of auger holes may be constructed and back filled with fine gravels. The length of the recharge trench is decided as per the amount of runoff expected. The recharge trench should be periodically cleaned of accumulated debris to maintain the intake capacity. In terms of recharge rates, recharge trenches are relatively less effective since the soil strata at depth of about 1.5 metres is generally less permeable. For recharging through recharge 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.
To collect the runoff from paved or unpaved areas draining out of a compound, recharge troughs are commonly placed at the entrance of a residential/institutional complex. These structures are similar to recharge trenches except for the fact that the excavated portion is not filled with filter materials. In order to facilitate speedy recharge, boreholes are drilled at regular intervals in this trench. 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.
In this method water is not pumped into the aquifer but allowed to percolate through a filter bed, which is comprised of sand and gravel. A modified injection well is generally a borehole, 500 mm diameter, which is drilled to the desired depth depending upon the geological conditions, preferably 2 to 3 m below the water table in the area. Inside this hole a slotted casing pipe of 200 mm diameter is inserted. The annular space between the borehole and the pipe is filled with gravel and developed with a compressor till it gives clear water. To stop the suspended solids from entering the recharge tubewell, a filter mechanism is provided at the top.
Due to severe depletion of ground water table Many open wells, bore wells and hand pumps become dry. Instead of discarding these wells, they can be converted into useful recharge wells. Roof water and run-off water can be diverted into these wells after filling them with pebbles and river sand. The wells should be fully desilted before diverting the water into them. In alluvial and hard rock areas, there are thousands of wells Which have either gone dry or whose water 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 settlement or filtration tank, from which it flows into the recharge well (bore well or dug well). If a tube well is used for recharging, then the casing (outer pipe) should preferably be a slotted or perforated pipe so that more surface area is available for the water to percolate. Developing a bore well would increase its recharging capacity (developing is the process where water or air is forced into the well under pressure to loosen the soil strata surrounding the bore to make it more permeable). If a dug well is used for recharge, the well lining should have openings (weep-holes) at regular intervals to allow seepage of water through the sides. Dug wells should be covered to prevent mosquito breeding and entry of leaves and debris. The bottom of recharge wells should be desilted annually to maintain the intake capacity. Providing the following elements in the system can ensure the quality of water entering the recharge wells:
- Filter mesh at entrance point of rooftop drains
- Settlement chamber
- Filter bed
Settlement tanks are used to remove silt and other floating impurities from rainwater. A settlement tank is like an ordinary storage container having provision for inflow (bringing water from the catchment), outflow (carrying water to the recharge well) and overflow. A settlement tank can have an unpaved bottom surface to allow standing water to percolate into the soil. Apart from removing silt from the water, the desilting chamber acts like a buffer in the system. In case of excess rainfall, the rate of recharge, especially of bore wells, may not match the rate of rainfall. In such situations, the desilting chamber holds the excess amount of water till it is soaked up by the recharge structure. Any container with adequate storage capacity can be used as a settlement tank. Generally, masonry or concrete underground tanks are preferred since they do not occupy any surface area. Old disused tanks can be modified to be used as settlement tanks. For over ground tanks, prefabricated PVC or ferrocement tanks can be used. Prefabricated tanks are easier to install compared to masonry and concrete tanks.
In this case the rooftop runoff is not directly led into the service tubewells to avoid chances of contamination of ground water. Instead rainwater is collected in a recharge well, which is a temporary storage tank (located near the service tubewell), with a borehole, which is shallower than the water table. This borehole has to be provided with a casing pipe to prevent the caving in of soil, if the strata are loose. A filter chamber comprised of sand, gravel and boulders is provided to arrest the impurities.
Bore wells / tube wells can be used as recharge structures for recharging the deeper aquifers and roof top rainwater is diverted to recharge well for further recharging to groundwater. The runoff water may be passed through filter media to avoid choking of recharge wells.
For recharging the shallow aquifers which are located below clayey surface at a depth of about 10 to 15 m, recharge shafts 0.5 to 3 m. diameter and 10 to 15 m. deep are constructed depending upon availability of runoff. These are back filled with boulders, gravels and coarse sand. For lesser diameter shafts, reverse / direct rotary rigs are used and larger diameter shafts may be dug manually. In the upper portion of 1 or 2 m depth, the brick masonry work is carried out for the stability of the structure.
If the aquifer is available at greater depth say 20 or 30 m, a shallow shaft of diameter 2 to 5 m and depth 5 to 6 m may be constructed depending upon availability of runoff. Inside the shaft, a recharge well of 100 to 300 mm diameter is constructed for recharging the available water to deeper aquifer. At the bottom of the shaft a filter media is provided to avoid choking of the recharge well.
For recharging the upper as well as deeper aquifers, lateral trench 1.5 to 3 m. wide and 10 to 30 m. long with one or more bore wells may be constructed depending upon availability of water. The lateral trench is backfilled with boulders, gravels and coarse sand.
After construction the trench can be covered with detachable slabs. Vehicles can move over it and children can play without fear or lawn can be grown over it after putting soil over the slabs leaving provision for periodical cleaning.
Rainwater collected from the terraces of a row of houses may be led into the nearby ponds through pipelines for recharging the groundwater aquifers. Runoff water can be diverted into this pond after proper desilting.
The ground level near the gate should be raised to retain as much water as possible inside the compound. Alternatively, a sloping gutter may be constructed across the gates and the rushing water directed towards the rainwater harvesting structure. For multistoried buildings, it is better to direct this water to a recharge well.
An enormous quantity of surface water generated during the rainy season flows through paved roads particularly concrete roads of residential colonies. This precious water can be checked and made to recharge then and there by constructing site-specific recharge structures on the road itself. All the water extracting structures of colonies, particularly bore wells and hand pumps of houses on the sides of the roads, are likely to give sustainable yield in due course.
By constructing artificial recharge structures like percolation pits and Dug Cum Bore well (DCB) on the storm water drains or on the sides, the run-off water can be effectively used for recharge purpose.
Unpaved surfaces have a greater capacity of retaining rainwater on the surface. A patch of grass would retain a large proportion of rainwater falling on it, yielding only 10-15 per cent as runoff. A considerable amount of water retained on such a surface will naturally percolate into the ground. Such surfaces contribute to the natural recharge of groundwater. If paving of ground surfaces is unavoidable, one may use pavements which retain rainwater and allow it to percolate into the ground.