A 3,000-litre tanker costs around ` 500 in Bangalore. We relied on two such tankers per day for our 30-flats apartment complex in Bangalore North. Then we went in for rain water harvesting. And things changed. The dependency on the tankers lessened, our ground water got recharged, and the bore well yield increased significantly. A similar story unfolded in south of Bangalore, this time with a 220-apartment complex, SLS Splendour, which used to bring in about 22 tankers with 6,000 litres water capacity daily. With rain water harvesting in place, they were able to save 3-4 tankers of water for every 45 minutes of water they captured. This is the power of rain water harvesting. It recharges the surrounding aquifers, prevents rain water flooding in the city roads due to runoff water, and what’s more, is economical in the long run.
Rain water harvesting, an old tradition
Rain water harvesting is not something new. In fact, it is as old as 3000 B.C., you could say. Rain water harvesting was a natural thing to do in villages till the 18th century. The kalyani and keres in temples of Karnataka, the surangas and madakas in Kerala, the step wells and tankas of Gujarat, the talabs found in most parts of North India; all these are just some examples of traditional rain water harvesting done in our country. While keres and talabs are lakes, madakas are depressions or low lying lands that accumulate water. Surangas are caves/ horizontal wells while tankas are underground reservoirs for storing water. Tanks, channels, embankments, canals, pits, check dams – different parts of the country followed different means to capture rain water. Yet, over time, this traditional wisdom has been lost and neglected, with cities and villages reeling under water scarcity.
Today, however, interest in rain water harvesting has been revived. The rising cost of tankers, low yield of bore wells and in some cases, legislations too have helped. In Bangalore, for example, apartments will be penalised if they are found without a rain water harvesting unit in their premises. A 25% fine will be levied on the water bill for the first three months that changes to 50% after that. Chennai has made rain water harvesting compulsory for three-storied and above buildings, while cities like Ahmedabad have made a percolation well compulsory for buildings covering an area of 1500 square metre. Some cities like Indore offer a 6% rebate on property tax for buildings with rain water harvesting systems in place.
Rain water harvesting, a system to capture and store water or recharge ground water is easy to implement if you have a catchment area, say a terrace or ground/driveway. The water thus collected via these open spaces can be put to domestic use like drinking or cooking, or can be used to further recharge the ground water.
The basic components of a rain water harvesting unit
Paved-unpaved surfaces, open grounds, lawns, rooftops and driveways can be considered for harvesting rain water. Apart from the catchment area, you will require pipes, rain water separators, filters and storage tanks. The storage tanks can be sumps, tanks, or simply rain barrels to store water, or they can be open wells, bore wells or recharge pits to recharge the ground water levels. The storage sumps and tanks can be above or below the ground.
Rooftops and driveways used for rain water harvesting: The process of installing a rain harvesting unit is simple. In case of rooftops, PVC pipes are attached from the terrace to carry rain water below to the sump. As the first burst of rainfall on the terrace is likely to be heavy on suspended particles and pollutants, there is generally a rain water separator with a gate valve to let off this water and prevent it from entering the sump. The pipes lead to a sump or a barrel after passing through filters that make the water suitable for consumption. Overflows from the sump can be directed to open wells or recharge wells through an overflow pipe.
Similarly, driveways can also be used to conserve rain water. The water collected via driveways is best directed to recharge wells as it contains large amounts of silt. Storm water drains, gutters and bumps are generally used to direct the runoff water appropriately and divert it to recharge wells to increase the water quotient of the water table.
The total water harvested from rain water harvesting units depends on catchment area and annual rainfall. It is a product of these two coupled with runoff coefficient, i.e., losses due to evaporation etc. The runoff coefficient is taken as a number between 0.8-0.95. So if Bangalore has an average annual rainfall of 972mm and if the roof top area is 100 square metre in a house, then volume harvested turns out to be approximately 83,000 litres.
Common filters used: PVC drums, stainless steel filters, Ferro cement filters are generally used for further filtration for storage tanks. The filters are attached to the down pipes. At the top of these filters there is a wire mesh to filter the suspended particles. The drums or filter containers are filled with sand, gravel and charcoal to remove particles and odour. A stainless steel filter does away with sand and gravel. And that’s one of the reasons why they are expensive compared to other types of filters. For storage tanks on the ground like rain barrels, the top of the tanks have a filtration mechanism consisting of an aluminium box with mesh and sand, gravel, jelly or sponge that filter the water impurities before it is stored.
Recharging ground water: The runoff water can also be used to recharge the ground water by constructing structures like recharge pits and recharge wells. Dry wells, trenches and tube wells can also be used for recharging aquifers. In areas where permeable rocks are present on surface, recharge pits are one of the better options. A recharge pit is a pit that is dug until you hit a weathered rock or porous soil. To filter the water entering into the pit, large stones or jellies are put at the bottom of the pit, then smaller-sized stones, gravel and pebbles in between, and finally sand at the top. This ensures that water is filtered in stages before it percolates to the ground.
Recharge wells on the other hand, are dry wells fitted with perforated rings. In such wells, the water percolates both vertically and then horizontally through holes in the rings. Over time, a recharge well too has the potential to become an open well that yields water. Rain water from the terrace and water through storm water drains can be diverted to the well through proper filtering. The inlet pipe to recharge well generally has a netlon mesh for filtration, and the well is covered with a slab or a steel grill. A 20-feet recharge well can store up to 4000 litres of water.
The cost of installation: The cost of constructing a rain water harvesting unit depends on the site. If there are pipe outlets from the terrace or there are open wells/tanks/sumps that can be reused, then the expenses come down considerably. The cost can be from anywhere between Rs 3,000 for a single-block apartment, to Rs 50,000 or even more for a bigger apartment complex. The apartment complex SLS Splendour spent about 3.3 lakhs for the harvesting unit that included piping, trenching, adding filters etc.
The biggest expense is the storage tank. In areas which boast of good rainfall, the storage requirements are low, and hence the cost of tanks too comes down. If the dry months are more, the tank capacity also increases, in which case the cost goes up. The storage capacity of a tank is determined by the dry season, number of family members and their consumption. So it is better to talk with a rain water expert in your area to get a fair idea of what components to use, the design and layout best suited for your terrain, to make your rain water harvesting unit work successfully.