Rainwater Harvesting Techniques
There are two main techniques of rain water harvesting:
(1) Storage of rainwater on surface
The structures used include underground tanks, ponds, check dams, weirs etc. which receive direct rainfall (footpaths, roads and rooftops). However, the ideal catchment is the rooftop as a large volume of runoff is generated from it and also contamination of water is less on roof-tops as compared to other two options.
(a) Rooftop RWH. Either flat or inclined roofs are suitable for RWH. Heavy rains yield sizeable volume of water while drizzles don’t. The roof area determines the quantum of water that could be collected, which is generally 20 litre/Sq m or 2 litre/Sq feet. Down-ward water pipes made out of PVC, HDPE, cement pipes or half cut horizontal plastic pipes (for inclined roofs) are fixed to the rooftop and connected to a storage sump or PVC tank with specially designed filter, mesh etc. The stored water can be used for 4 secondary purposes and, if treated through boiling, filtering and other purification methods, it can be used for drinking. To avoid contamination, precaution should be taken to protect stored water by covering it properly so that either sunlight or dust does not enter the sump/tank. It is also important to manually maintain the storage system by cleaning of filters, removing of leaves and twigs and other wastes from the mesh etc, for obtaining good results.
(b) Rooftop Rain water for recharging of open wells and bore wells Rainwater collected from rooftops may be filtered using suitable filters to recharge groundwater in an existing open well or bore well. The filtered rainwater may be directly let into an open well through a pipe from any side of the well. Care should be taken that the water pipe is slightly projected into the well and a bend at the end of the pipe guides the water flow downwards. This avoids splashing of water causing damage to the well-wall. Water level in the well could fluctuate according to the intensity of the rains as well as the rate at which ground-water is recharged by the collected rainwater. It is suggested not to allow the filtered rainwater into a bore-well, functioning or defunct, because the filter may not hold fine silt or dust from the rooftops. If silt or dust passes through the filter, the micro pores or aquifers in the bore can get blocked and cause permanent damage to the bore-well. . Therefore it is advised to build a specially designed infiltration gallery next to the bore well to inject the filtered rainwater into the bore well.
(2) Groundwater Recharge Technique
The percolation of excess rain water through an infiltration system to the subsurface is called ‘Artificial Ground water Recharge’. In the floor area, water recharge would be about 1lt/Sq m, whereas in the pavement area i.e. through pavers with gaps, it would be about 10 lt/Sq m. Another estimate indicates that when the breadth of a footpath is 3m, 3,00,000 liters of rainwater flows for 1 mile; more will be the flow of rainwater if more is the breadth of the footpath (BBMP brochure). 5 Plate 4: Ground water recharge pit, Theme Park, RWH in Bangalore Bangalore’s urbanization process has been alarmingly fast, making it a challenge for the government to build the much needed infrastructure facilities. Bangalore, the capital city of Karnataka, happens to be India’s sixth most populous city and also the fifth most populous urban agglomeration. Bangalore, with its strategic location as well as congenial climate, attracts people from all over the country. Of the many challenges that urban Bangalore faces, water scarcity remains one of the critical issues. Availability, Accessibility, Equity and Quality have been the serious challenges, while environmental concerns like ground water depletion and contamination are equally challenging. Bangalore draws water from river Cauvery, about 100 km away incurring huge financial costs. Water flows against gravitational force from Cauvery, and is pumped at various stages before reaching Bangalore. Thus, Cauvery travels a distance of 100km to a height of 500mts against gravity using 71mw of electricity. A huge amount of Rs.25 crore is being paid as power charges every month, but Bangalore still faces an acute deficit of water supply against the total demand. Hence, BWSSB has drilled a large number of bore wells for alternative water supply. In addition, citizens also drill private bore wells in order to ensure reliable supply of water. This has resulted not only in the rapid increase of the number of bore wells throughout Bangalore, but also overexploitation of ground water. It is estimated that currently the number of bore-wells in Bangalore is around 2 lakhs. Several defunct wells and failed (dry) bore-wells can be seen throughout the length and breadth of Bangalore. Another issue that adds to groundwater depletion is the levelling up of tanks. Bangalore has an advantage of having nearly 70 rainy days spread over the year. However, Bangalore’s drainage system (infrastructure) can handle only 30mm of rainfall in one hour. So whenever there is a heavy down pour, the city faces flooding and water-logging. There were about 370 tanks in Bangalore, but their number has declined over time due to improper management and several of them have transformed into Stadiums, Parks, Bus stops and Buildings. Major lakes in the city have given way to buildings, parks, bus stands, Stadiums etc, and tar/ cement roads have gobbled up space, leaving no place for rain water run offs to seep into. The rainwater infiltration has also declined as the soil exposed to this purpose has decreased over time. Further, the storm water drains were designed about 20 years ago and their carrying capacity remains the same even though the city has expanded enormously. Although BBMP has the technical expertise to re-design/widen the drainage network, the process is evidently time consuming and involves huge costs. Frequent incidents of storm water getting mixed with sewage are another issue that needs serious attention. Besides, the sewer system of Bangalore is 80 years old and lacks the capacity to take large volume of sewage water. Even as BWSSB has a plan to make zero sewage storm water drains so that they are free of wastes that block rainwater flow. The Bangalore Water Supply and Sewerage Board (BWSSB) has taken up various steps to improve its services with water conservation as one of its focus areas. RWH would play a dual role by way of solving water crisis on the one hand and mitigating the havoc often caused by heavy rains, on the other. It is estimated that 4 crore liters of water can be collected a year if RWH is adopted on one acre of land. Therefore, if RWH is carried out in an area of 9300 acres of land, about 1500 crore liters of water could be collected a year (BBMP, Brochure). In view of its inherent advantages, RWH has been made mandatory for households in Bangalore city with a site dimension of 40 ft. x 60 ft. and above from 2009. There has been a gradual increase in the number of RWH installation after the BWSSB amendment Act became operational. Several scientists and architects have worked on the methods to be adopted in respect of RWH. Theme park, the first information and research centre on of RWH established jointly by BWSSB and KSCST at 5th block, Jayanagar, functions as a centre for creation and dissemination of awareness on RHW. Currently, per capita water demand in Bangalore works out to about 150 ltrs. to 200 ltrs. per day, while the actual supply comes to about 100 ltrs. to 125 ltrs per. day, bringing the shortage of water to about 40%
- Rainwater storage (surface tank or underground sump) and ground recharge (RCC precast ring well) of minimum 20 liters per square meter of roof area and a minimum of 10 liters per square meter of paved open space provision shell be made.
- The open well / recharge well of depth of 3 meter (minimum) and diameter of .9 meter (minimum) without filling in the well (like aggregates, jelly, sand etc.) provision shell be made.
- You may store rainwater in a tank / sump or recharge rainwater in to the ground through a well or you can use a combination of these two to achieve the stipulated capacity of ‘X’ liter. Where X = roof area in sqm x 20 + paved open area in sqm x10.
- The open well should be of minimum 3 ft diameter and 10 feet deep.
- Ground water recharge should not be attempted at places where the ground water level is very high and at places where hard rock strata exists at shallow depths.
- It is suggested to build a shallow sump or install a surface tank for storing harvested rainwater.
- Rainwater from the roof and through a filter should not be allowed directly in to a live borewell.
- It is advisable to allow the filtered rainwater in to an open well beside the borewell. In case of a failed or very low yielding borewell, you may allow a portion of overflow from your tank / sump which collects filtered rainwater from the roof. However a skilled person’s advice is required before recharging the borewell directly.
- Existing sump can be used for storing rainwater. However, the harvested rainwater from the roof may have bacterial contamination which needs to be treated before using for cooking and drinking.
- It is required to ensure adequate storage space in the sump for rainwater at any given point of time.
- It is advisable to filter rainwater from the roof before discharging in to the well. If filters are not used it is advised to discharge rainwater in to the annular space filled with aggregates around the cement rings of the well.
- It is not mandatory to use any particular type of filter. You can choose the type of rainwater filter suitable for your requirement.