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* '''How large should the sump be?''' Even a small storage system can generate large collection provided we use the rainwater every time it rains. In Bangalore for example, with it’s evenly spread rains, a 500 litre sump tank can generate 25,000 litres of rainwater in a year. An ideal size is 2000 litres of sump storage for 100 square metre of roof area. This is as per the building bye law for Bangalore. 6000 litres of storage for 100 square metre roof area should give you 70,000 litres of water in a well distributed rainfall year<ref>[http://www.archiestudio.com/userfiles/Harvesting%20rainwater%20in%20underground%20sump%20tanks.pdf HARVESTING RAIN IN UNDERGROUND SUMP TANKS]. S.Vishwanath</ref>.

* A first rain separator is crucial to the collection of clean rainwater in sump tanks<ref>[http://www.archiestudio.com/userfiles/Harvesting%20rainwater%20in%20underground%20sump%20tanks.pdf HARVESTING RAIN IN UNDERGROUND SUMP TANKS]. S.Vishwanath</ref>.