Rainwater Harvesting

Rainwater Harvesting in urban areas is the process of collecting, filtering and using of rainwater, which falls on the rooftop (terrace or tile roof) and in the portico of the house. Rainwater harvesting can be adopted in three methods. These are; recharging of borewells near the house, recharging of ground water sources and collection of rainwater for reuse.

Borewell recharging

Recharging of borewells should be done to prevent them from drying up and improve their water table. It is usually done by the following method. First, a pit should be dug in the region surrounding the casing pipe and cement rings should be installed in it. The size of the pit should be one meter in diameter and 10 feet in depth. At the bottom of the pit, filter holes should be made and a casing pipe with steel mesh should be fixed tightly to the bore well pipe. This casing pipe will function as a filter. Cement rings matching the borewell’s diameter should be installed till the base of the pit. The pit should be filled with boulders from the base up to a height of two feet. A second layer of two feet height filled with jelly stones of 40 mm should follow this. The third layer of one-foot height should be filled with jelly stones of 20 mm size. The fourth layer should be filled with charcoal. This exercise should be repeated till there is 3 feet of space left from form the ground level. A nylon curtain should be spread on the layers and the remaining space of the pit should be filled with sand until one foot from the ground level. A pipe should be fixed to collect the rainwater from the roof and this pipe should be connected to the pit. A pipe should be fixed to prevent the excess water from running away to roadside drainage.

By following the above method, we can recharge our borewells and keep them alive always. Those having open wells too can adopt this system. By directing the filtered rainwater to the open wells, their water tables could be improved.

Ground water recharging

Ground water recharging in urban areas is done by collecting the rainwater from the rooftops and the portico of the house and by making it easily absorbed within the veranda space

The method: An absorption pit should be constructed in the sloped area of the house veranda. The size of the absorption pit should be a minimum of 5 feet in width and a maximum of 10 feet in depth. Matching cement rings should be installed till the base of the pit. The pit should be filled with boulders (or big jelly stones) and a nylon curtain should be laid at the top. The remaining part of the pit should be filled with sand till one foot of the ground level. The pipe, which is installed for collecting the rainwater from the rooftop and the veranda of the house, should be linked to this absorption pit. Another pipe should be deployed for allowing the excess water to run off into the storm water drainage. Thus, by effectively preventing the rainwater from running off into the drainage and by making it absorbed into the ground, ground water sources can be recharged. All Bangaloreans, by adopting the ground water recharging system, can save the future generations the problem of water shortages.

Reuse of rainwater

Bangalore has an annual average rainfall of about 1,000 mm. It has been estimated that 2,23,000 litres of water can be collected annually from the rooftop of a 40’x60’ house with 1,000 mm rainfall. Assuming that, on an average, a Bangalorean spends 135 litres of water, a family of four would need about 2 lakh litres of water annually. So, water needed by an average family living in a 40’x60’ house could be collected from the rainwater falling on the rooftop.

Rainwater Harvesting Method

Water falling on a flat rooftop should be made to run through a pipe connected to the roof and prevented from running off to the drainage on the roadside. This water should be filtered and stored in tanks constructed for the purpose of storing rainwater. In case of houses with sloped rooftops (R.C.C., Tiles and Asbestos), a gutter should be attached to the roof and the water collected in it should be made to go through a pipe attached to the end of the gutter. The water, which runs down from the pipe, should be filtered and stored in the rainwater harvest tanks. A good quality filter is needed for purifying the water. This is because, although the rainwater is basically pure, the water flowing from the rooftop usually will contain waste material in it.

Sand Filter

There are two types of filters and sand filter is one of them. This filter can be built in the house itself. Before storing the water in a water tank, sump or drum, the water should be purified by making it flow through several layers of filtering.
-One layer of boulders
-Over that, one layer of big jelly stones
-Over that, one layer of small jelly stones
-Over that, one layer of charcoal
-Over that, one layer of big jelly stones
-Over that, one layer of small jelly stones
-Over that, a layer of nylon curtain
-Over that, a layer of sand

Like this, a filter will have 8 layers. The rainwater will get purified when it flows through a filter prepared in this order. The purified water should be stored in a sump, tank or drum. The contents of the various layers of the filter should be changed once every five years. The water is filtered through the charcoal layer since the rainwater may sometimes contain poisons waste materials like bird excreta etc.

No water connection if there is no rainwater harvesting system
Keeping in mind the interest of the future generation, BWSSB has made a rule, which makes it mandatory for every house or building constructed in future to have a rainwater harvesting system in order to get drinking water connection. This action of the board has been appreciated in a main article published by the The Times of India newspaper, which is Bangalore’s leading English daily.
There should be no connection between underwater drainage and rooftop water

If the rainwater that falls on the rooftop is linked to the underwater drainage, there will be disorder in the underwater drainage system. If possible, the rooftop rainwater should be stored through the rainwater system. Otherwise, the rooftop rainwater should be linked to the storm water drainage system and should never be linked to the underground drainage system. First, a three feet slab should be taken out from the storm water drainage in front of the house. Thereafter, a trench of 3 to 5 feet should be dug and boulders should be spread on it and the boulders should be covered with a layer of sand. Through this system, water flowing though the storm water drain is absorbed into the ground and the ground water will be recharged.

Always use MDPE Pipe

When getting water connection from the main water pipe passing though the road, a bore should be drilled and the pipe should be attached without damaging the main pipe. Since the pipe connecting the house passes through the road, you should always use MDPE pipe to prevent the leakage. This pipe should be connected by shoving it through a G.I pipe. This will help in the prevention of leakages and in identification. This method has been made mandatory by BWSSB.

Mr.A.R.Shiva Kumar

Programme manager, Indo-Norwegian Environment (INEP),
Karnataka State Council for Science & Technology (KSCST),
Indian Institute of Science Campus,
Bangalore – 560 012
Ph/Fax. : 2346 1221
Office : 2334 8848
EMail : inep1@vsnl.net
Website : http://www.inep-karnataka.org

Residence :

#44, 3rd Mail, Basaeshwara Layout, Vijayanagar – 560 040.
Ph. : 2339 8655. Mobile : 98452 12314
You can also get more information on Rain Water Harvesting in Urban context on INEP Website, and Centre for Science & Environment (CSE)’s website mentioned below -
http://www.rainwaterharvesting.org/Urban/Howtoharvest.htm

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