Frequently asked questions
Why do we need rainwater harvesting?
What is surface runoff harvesting?
What are the advantages of collecting rainwater in the sub-surface aquifers?
How much rooftop area is required? What size of storage capacity is needed?
What specifications are required for storage?
What can the harvested rainwater be used for?
What percent of our usage requirements can rain water-harvesting address?
Is it mandatory for homeowners to harvest rainwater?
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Why do we need rainwater 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 during rains
4. To augment the ground water storage and to control the decline of ground water levels
5. To reduce ground water pollution6. To improve the quality of existing ground water
7. To reduce the soil erosion
8. To supplement domestic water requirement during summers, and during droughts etc.
What is surface runoff harvesting?
The rainwater runoff can be collected from ground surfaces and directed into the existing low yield bore wells or to recharge the sub-surface aquifers.
What are the advantages of collecting rainwater in the sub-surface aquifers?
• No land is required for storage
• Ground water is not exposed to harmful pollution or evaporation
• It reduces top soil erosion
The roof acts like a collector of rainfall. The rain water can be run into storage tanks from the drain pipes and be used after filtration. Rainwater does not get contaminated if stored and sealed in a sunlight and air free sump or tank. This water will remain contamination free and the storage tank needs only an annual maintenance.
How much rooftop area is required? What size of storage capacity is needed?
1000 sq. ft. area of roof and a 6000 litre sump is sufficient.
What specifications are required for storage?
Storage in Tanks
• A storage tank should not be located close to a source of contamination, such as a septic
tank etc.
• A storage tank must be located on a lower level than the roof to ensure that it fills
completely.
• A rainwater system must include installation of an overflow pipe, which empties into a non-
flooding area. Excess water may also be used for recharging the aquifer through dug well or
abandoned hand pump or tube-well etc.
• A speed breaker plate must be provided below inlet pipe in the filter so as not to disturb the
filtering material.
• Storage tanks should be accessible for cleaning.
• Introduce guppy fish to remove mosquito larvae.
• The inlet into the Storage tank should be screened in such way that these can be cleaned
regularly.
• Water may be disinfected regularly before using for drinking purpose by chlorination or
boiling etc.
The cost of harvesting rainwater is variable, it could cost anywhere between Rs.2, 000 to Rs. 20, 000. For a 40'-60' site, the approximate costing is Rs. 3, 500 to Rs.4, 000.
a 40'-60' site, the approximate costing is Rs. 3, 500 to Rs.4, 000.
The key factors to keep in mind are:
• Cost of a Rainwater harvesting system designed as an integrated component of a new
construction project is generally low.
• Designing a system onto an existing building is costlier because many of the shared costs
(roof and gutters) can be designed to optimise system.
• In general, maximising storage capacity and minimising water use through conservation
and reuse are important rules to keep in mind.
Basic Costs Include:
• Filters: Rs.1, 300
• Storage tanks of terro-cement- Rs.2 to Rs.3.50 per litre
• 100 mm diameter pipes - Rs.15 to Rs.30 per running foot.
What can the harvested rainwater be used for?
All non-potable uses can safely be provided by filtered rainwater. If hand pumped from the sub-surface aquifer, it can be treated and even be used for drinking. It is always advisable to get the water checked in a laboratory before using it for drinking.
What percent of our usage requirements can rain water-harvesting address?
Current statistics on average daily water consumption in Bangalore indicate 100 to 140 litres/person. The average rainfall in Bangalore is 970mm and will allow a harvest of approximately 80, 000 litres per year.
For a family of 5, the collected rainwater should meet approximately 30% of the annual requirement.
The Formula to be used is:
Total quantity of water that can be collected (cu.m.) = Roof Top Area (Sq.m.) x Average Monsoon Rainfall (m) x 0.8 (amount of rainfall that can be harvested).
Is it mandatory for homeowners to harvest rainwater?
For all new construction, under the state law, harvesting rainwater is now mandatory.