Due to rapid urbanization, infiltration of rainwater into the subsoil has decreased drastically and recharging of groundwater has diminished. This scenario requires an alternative source to bridge the gap between demand and supply. Rainwater, which is easily available and is the purest form of water, is an immediate source to augment the existing water supply by “catching water wherever it falls” says, A.R. Shivakumar, Principal Investigator – RWH, Karnataka State Council for Science and Technology, Indian Institute of Science, Bangalore.
Buildings are usually constructed with hard material rooftops like reinforced cement concrete (RCC), tiles, asbestos, galvanized iron or zinc sheets which allow rainwater runoff and loss due to evaporation or is let off into storm water drains which eventually goes away from the city. Alternatively, rooftop rain water can be put to good use by storing water on either roof itself, at ground level or below the ground, by using storage devices like masonry tanks, ferro cement tanks, plastic or metal containers.
Flat roofs normally have water proofing on the surface as a surface finish. This top surface is provided with a slope towards down water pipes. For efficient collection and effective usage, slope on the roof need to be given towards the storage device placed for rainwater harvesting. This will minimize pipe length to the storage system. Sloping of roofs on the other hand, lead the water to the lower edge of the roof. For rainwater harvesting, a gutter made out of sheet metal or PVC has to be installed at the lower edge to collect and channel water to the down water pipes.
Down water pipes made out of PVC, HDPE or cement pipes can be used for transporting rainwater collected from roofs to the filtration system before storing. The size of the down take pipe varies depending on the roof area, which is connected to the down pipe. The rainwater is then stored for direct use in tanks above ground or underground sumps / overhead tanks and used directly for flushing, gardening, washing etc. Rainwater is also channelled to recharge ground through recharge pits, dug wells, bore wells, soak pits and recharge trenches.
Rainwater Harvesting potential
Rainwater harvesting potential in urban areas is huge. For instance, in a residential site of 40ft x 60ft (an area of 2400sqft / 223sqm.), with an annual rainfall of about 1000 mm or 39.4 inches (Bangalore receives around 1000 mm of rainfall annually), about 2, 23,000 litre of rainwater can be harvested depending upon the potential rainfall, catchment area available and collection efficiency.
Residence of A R Shivakumar, “Sourabha” is located in south western part of Bangalore. Plot area is 2400sqft. (40ft X 60ft). House constructed during 1995 depends only on rainwater for all its needs. There is no municipal or BWSSB water connection to the house. Clean and safe water is available 24 hours 365 days a year and the water is periodically tested to verify drinking quality.
Bangalore has an advantage of having nearly 70 rainy days spread throughout the year. It receives around 1000mm of rainfall in a year which translates in to around 2, 23,000 litre per year cumulatively in a plot of 60ft X 40ft Rainwater falling inside the plot is channelled to
1. Roof top rainwater harvesting
2. Roof top rainwater channelled to recharge ground water
3. Percolation in garden area to recharge ground water.
Majority of the rainwater falling on the roof (85%) is allowed to flow in to a tank of 4500 litre capacity built on the ground floor roof. PopUp filter along with a stabilization tank specially designed filters the roof water before storing. This water is used during the rainy season. The over flow of this tank is allowed to rundown through rainwater pipe on the wall to an underground sump-1 of 25,000 litre capacity built under the portico at the entrance of the house. In the event of these two tanks getting full, the rainwater is diverted to percolate into the ground through a system of percolation tanks / infiltration gallery (4 recycled plastic drums interconnected and buried underground with their bottom cut open) to recharge ground water.
The remaining portion of the roof water (15%) is allowed to channel through rainwater pipe on the wall and a PopUp filter installed at the ground level filters suspended and floating material. Relatively cleaner water after filtration flows to an underground sump-2 of 10,000 litre capacity built inside the car park (garage). Sump-1 and sump-2 together with 35,000 litre capacity are interconnected and the stored rainwater water is used during the non rainy days when the roof top tank water is not available.
In addition, rainwater falling in the open area or the garden area around the house is allowed to percolate in to the ground to recharge ground water. In this manner not a drop of rainwater falling in the plot is allowed to flow out.
‘Sourabha’ house has inbuilt simple mechanisms to effectively harness all the rainwater falling in the plot and channel the same for different purposes. The family of four needs around 15,000 litre of water per month. Out of which 400 litre of water per day is for daily needs excluding toilet flushing and part of gardening. The need of toilet flushing is met by recirculation of used water from laundry washing machine. Used water from washing machine is stored in an underground tank and is pumped up to a tank on the roof for toilet flushing automatically. Wash water from kitchen sink is diverted for gardening. During rare periods of more than 100 days of continues no rains, water drawn from a shallow tube well, which gets recharged from rainwater, meets the requirement. Ground water table in and around the plot before the rainwater harvesting was as deep as 200ft. Within one year of ground water recharge, water is available at around 40ft.
In Sourabha the amount of water used is far less than the amount of rainwater harvested and ground water recharged. Consequent to this, a social cause to recharge the ever-dwindling ground water is also attempted. There is no recurring cost involved except for the electricity used for pumping up water. The electricity consumption is around 80kWh (units) per month, which includes water pumping also. Low electricity consumption in the house is due to several energy conservation methods and renewable energy techniques adopted in the house.