The rainwater harvesting structures must be built for seepage of rainwater alone, and not the household drainage water, warn experts.
Many apartment-dwellers or associations are wont to re-direct the used water after car washing or cellar cleaning into the pits dug up for rainwater, which is counter productive.
“The rainwater must flow directly from the roof into the RWH structures. Used water from the complexes would have oil and muck, which could block the seepage,” says J. Satyanarayana, a retired hydro-geologist and ground water expert with the Hyderabad Metropolitan Water Supply and Sewerage Board (HMWSSB).
Ideally, for a 100 square metre roof area, the pit should be two metres in length, 1.5 metres wide, and two metres deep. As the roof area grows, so should the structure, proportionately. However, as the city has paucity of space, structures of any size are welcome.
The bottom of the pit should be filled up to 50% with coarse metal (gravel) of 40 mm thickness, over which 20 mm thick metal should be laid up to 25%. Then a brick wall should be constructed around the pit, before filling half of the remaining pit with coarse sand and leaving the rest open. A six cubic meter pit would cost up to ₹20,000.
The water board has installed about 7,000 to 8,000 RWH structures around the city, which underwent maintenance during summer.
Four hundred more structures were grounded by the citizens and NGOs through a one-day campaign that was held recently. “The upkeep of the structures involves preventing littering and urination in them. Before monsoons, the top sand may be replaced,” Mr. Satyanarayana says.
According to the building rules, it is mandatory to have a RWH structure if the plot area is above 200 square metres. Drinking water connection would be given only after an occupancy certificate is obtained from the GHMC, for which the RWH structure is a precondition. Without occupancy certificate, three times the usual water charges would have to be paid.
