Every water drop is precious. With deficiency in the rains this monsoon, ground water table in Bangalore is probably at all time low in certain places. I think if you own the place and if you have the space, everybody should go for RWH, whether BWSSB mandates it or not. It is every denizen’s responsibility, IMO. (Have heard weird reasons – the plot size is only 2390 and not 2400; may not be alive to see ROI)
If you have no idea where to start, BWSSB has some decent info on their website. Check this Guidelines by BWSSB, pretty good! If you want to see it in action, visit the Sri M. Visvesvaraya Rain Water Harvesting Theme park in Jayanagar 5th Block.
Depending on the terrace area, the contractor or the plumber (who is trained in RWH) will tell you how big a storage tank and how big a pit you need, for ground water charging. Typically, the water collected from the roof top is first sent to a filter. If you have storage tank, the water will be stored there and then the excess water will be sent to the recharge pit. My terrace is about 1000 sq ft. We have a storage tank of 2000L capacity and the recharge pit holds about 5000L.
If you have a borewell, you can recharge that too.
If you are ok using the rainwater, you could redirect the water to the sump. Living in the polluted city, we weren’t comfortable using the rain water directly; many households have done that though.
Wish I had different plumbing for the bathrooms and could use this water for flushing toilets. Right now it is used only for cleaning purposes. I don’t even have a garden. Hopefully I’ll find more usage for the stored water in the future.
I will be happy to answer any queries you may have. BTW, not recommending my plumber because he turned a 1 week project to a 3 month project !