Need advice on cost-effective treatment & reuse of rainwater, grey water in Howrah, West Bengal

I am a resident of Shibpur, Howrah in West bengal. We stay in a multi-storied flat building having 7 flats. Apart from rain water from the roof outlets, effluent may be collected from kitchen/ toilet/ washbasin exit-pipes.

1. I was wondering whether there can be a cost-effective way of treatment of rainwater and domestic water effluent?
2. Whether rain qater and other effluents need to be treated separately or in a combined form?
3. Can you help in this regard by providing any information about this. Can it be a feasible proposition?

Shyamasis Ghosh