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Rain water harvesting--COSTING

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  • shaji john
    Message 1 of 1 , Apr 25, 2002

      envo wrote:

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      In spite of astonishing achievment in the field of science and technology, nature remain to be a mystery for human being . though water is obtained through desalination, artificial rain by cloud seeding etc. the shortage of water even for drinking purpose is a perpetual phenomenon throughout the world,in most of the cities , the water supply sector is facing a number of problems and the income in population in urban area have resulted in exploitation of water resources to the extremes . fresh water sources are being heavily exploited to meet the demands of urban population . failure of monsoon fail to meet the ever increasing demands and ground water reserves are tapped , often to unsustainable levels. Unplanned and uncontrolled extraction of ground water would disturb the hydrological balance . hence it is necessary to take up measures to conserve and management of the renewable water sources in all possible ways. Ground water recharge by rain water harvesting (RWH) is the simple and cost effective way.

      Rooftop Rain Water Harvesting is the technique through which rain water is captured from the roof catchments and stored in reservoirs. Harvested rain water can be stored in sub-surface ground water reservoir by adopting artificial recharge techniques to meet the household needs through storage in tanks. The Main Objective of rooftop rain water harvesting is to make water available for future use. Capturing and storing rain water for use is particularly important in dryland, hilly, urban and coastal areas. In alluvial areas energy saving for 1m. rise in ground water level is around 0.40 kilo watt per hour.

      Need for Rooftop Rain Water Harvesting

      1. To meet the ever increasing demand for water

      2. To reduce the runoff which chokes storm drains

      3. To avoid flooding of roads

      4. To augment the ground water storage and control decline of water levels

      5. To reduce ground water pollution

      6. To improve the quality of ground water

      7. To reduce the soil erosion

      a.. To supplement domestic water requirement during summer, drought etc.

      Advantages of Rain Water Harvesting

      1. Provides self-sufficiency to your water supply

      2. Reduces the cost for pumping of ground water

      3. Provides high quality water, soft and low in minerals

      4. Improves the quality of ground water through dilution when recharged to ground water

      5. Reduces soil erosion in urban areas

      6. The rooftop rain water harvesting is less expensive

      7. Rainwater harvesting systems are simple which can be adopted by individuals

      8. Rooftop rain water harvesting systems are easy to construct, operate and maintain

      9. In hilly terrains, rain water harvesting is preferred

      10. In saline or coastal areas, rain water provides good quality water and when recharged to ground water, it reduces salinity and also helps in maintaining balance between the fresh-saline water interface

      11. In Islands, due to limited extent of fresh water aquifers, rain water harvesting is the most preferred source of water for domestic use

      12. In desert, where rain fall is low, rain water harvesting has been providing relief to people


      CATCHMENT - ROOF The catchment area is the surface through which rain water runoff is harvested i.e. roof. The water to be used for non-drinking purposes can be collected from any roof. Water to be used for drinking purposes should, however, not be collected from roofs with damaged asbestos sheets or from roofs covered with asphalt and lead flashings or lead based paints as lead contamination may occur in the collected water. Regardless of roofing material, generally a loss of upto 20% may takes place due to evaporation and inefficiencies in collection processes. Thus only 80% of monsoon rainfall can be harnessed through rooftop.

      Gutters: Gutters are of great significance especially in

      hilly terrains.

      Gutters collect rain water from the roof and transport it to the inflow pipe. The gutters could be of various shapes, sizes and material.

      1. The size of the gutters should be selected keeping in view the flow during highest intensity of rainfall.

      2. The gutter could be made using - Alluminium or galvanised iron sheet. - Half cut PVC rigid pipe - Half cut large diameter bamboo trunk

      3. To keep leaves and other debris away from entering the system, the gutters should be covered with 1/4" wire mesh (jali) along their entire length. The conduits should be free from lead or any other toxic material likely to contaminate water.

      Inflow Pipe:

      Inflow pipe or drainpipe is the pipe which connects the gutter to the filter and then to tank or reservoir.

      It could be made from:-

      - PVC pipe

      - Cast iron pipes

      - Cement pipes

      At the place of junction of the gutter and inflow pipe, the gutter may be enlarged to accommodate the greater influx of water at the bend.

      A wire mesh should be fixed at the mouth of the inflow pipe.

      For domestic consumption through tanks :- For direct consumption, a by-pass arrangement should be made at the point where the inflow pipe is connected to the filter. The objective is not to collect first shower water. This can be done by using a stopper and a by-pass pipe just above the filter as shown in the figure.

      By-Pass Arrangement

      At the time of first shower, gate valve -1 is opened while gate valve -2 is closed to remove the first showers. After few minutes, Gate valve - 1 is closed and gate valve - 2 is opened to pass rain water into the filter.

      Filter: Filter is to be used when the water is to be stored in tanks for direct consumption. Filtration and disinfection of water are necessary before human consumption.


      Filter can be divided into three parts:-

      - Container: It can be made from either galvanised iron sheet or ferrocement.

      - Perforated plate: A non-corroding material Steel, PVC etc. is used to make a perforated plate with 1 cm diameter holes.

      - Filtering Media: Filtering material should be composed of following layers :

      Layer 1 : Layer of sand-fine to medium

      Layer 2 : Layer of gravel (4 to 16mm)

      Layer 3 : Layer of medium pebble bed (16-50 mm)

      For Recharge to Ground Water Reservoirs :- For storage in ground water reservoir the filter in the inflow structure is not required. The water, however, should pass through a desilting pit before entering the aquifer. A desilting pit is essential to let the suspended material settle down before the water is introduced into the aquifer. Except for the recharge through handpump or tubewell, the filter should be constructed as shown in the figure below

      By-Pass Arrangement

      At the time of first shower, gate valve -1 is opened while gate valve -2 is closed to remove the first showers. After few minutes, Gate valve - 1 is closed and gate valve - 2 is opened to pass rain water into the filter.

      STORAGE 1.


      Rooftop Rain Water collected may be recharged to Ground Water reservoir through:

      Abandoned dug well

      1. Water to be recharged is guided through a pipe to the bottom of the dry/unused dug well or below the water level to avoid scouring of bottom and entrapment of air bubbles in the aquifer.

      2. Bottom of the dug well should be cleaned and all fine deposits should be removed before its use for recharge.

      3. Recharge water should be silt free.

      4. The well should be cleaned regularly.

      5. Periodic chlorination is required in order to control bacteriological contamination.

      6.This method is suitable for large buildings having the roof area of more than 1000 sq.m.

      Gravity head recharge well

      1. Borewells/tubewells can be used as recharge structures. 2. The rooftop rain water is channelised to the well and recharged under gravity flow condition. 3. Recharge water should be silt free. 4. The well can also be used for pumping. 5. The number of recharge structures can be determined in limited area around the buildings depending upon the rooftop area and aquifer characteristics. 6. This method is applied where land availability is limited and when aquifer is deep and overlain by impermeable strata. 7. The technique is most suitable in areas where ground water levels are very deep.

      Recharge shaft

      1. A recharge shaft is dug manually or drilled by the reverse/ direct rotary drilling method.

      2. Diameter of recharge shaft varies from 0.5 to 3 m depending upon the availability of water to be recharged.

      3. Recharge shaft is backfilled with boulders, gravel and coarse sand.

      4. The bottom of the shaft should end in permeable strata i.e. sand.

      5. Recharge shaft should be constructed 10 to 15 meters away from the buildings for their safety.

      6. It should be cleaned regularly by scraping the top sand layer and refilling it periodically.

      7. Recharge shaft is constructed where shallow aquifer occurs below clay layer.


      Rain water may be collected and recharged into the ground water aquifers through the following method .

      1) Rainwater harvesting through borewell method


      Roof Area = 6211 sq mtr( Case Study),

      Peak rainfall in 15 minutes= 0.025 mtr (In South Delhi)

      Runoff Coefficient = 0.85

      Runoff quantity = 132 cu mtr

      Approx Cost per well :

      Sr NO Scope of work RATE AMOUNT(Rs)

      1. Drilling , 13 inch, Calax/DTH 25 MTR depth x 750/- = 18,750/=

      2. Pea Gravel --- L.S. = 8,500/=

      3. Housing 8" MS Pipe 9 x 570/= = 5,130/=

      4. Slotted 8" MS pipe 16 x 640/= = 10,250/=

      5. Developement by compressor L.S. = 8,300/=

      6. Extra Casing L.S. = 1,500/=

      7. Pipe lowering L.S. = 2,000/=

      8. Transportation L.S. = 1,000/=

      9. Logging 01x15,000/= = 15,000/=

      10. Supervision L.S. = 15,000/=


      TOTAL 85,430/=


      1. Excavation 3 m dia and 2 mtr depth : 15 cu mtx 60/- = Rs. 1,000/=

      2. PCC :1.1Cu mtr x1700/- =Rs.1870/=

      3.Brick work in cement mortar(1:6) : 8 cu mtr x 1500/= =Rs.12,000/=

      4.Plaster(1:6) :40 cu mtr x 100/-= Rs.4000/=

      5.Filter for injection well :1 Jobx 2000/= = Rs.2000/=


      TOTAL CIVIL WORK = Rs.20,870/=


      TOTAL= Rs. 1,06,300/= per well of 25 meter depth, 13 inch bore with 8 inch MS pipe.

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