Water Harvesting is a subject every where discussed now a day. I worked in Minor Irrigation department for 6 (1970 - 76) years & use to design & construct earthen village ponds (Percolation Tanks). During this time I saw 
many villages where village women use to bring water from 4 miles every day on head. These villages benefited from such ponds. 

Here is a simple suggestion for saving water. This is applicable to the buildings where under ground tanks are not provided and water directly goes to OHT.

Problem.

1. Most of the buildings have an overhead tank.
2. There is an overflow pipe on the top portion of the tank.
3. Though the float cock is provided most of the time it fails to work.
Thanks to Indian manufacturers and their substandard product and design.
4. Water overflows from the terrace to the ground and drains out.

Solution.

1. Join this overflow pipe to supply line which goes to flats.

When water overflows this extra water directly goes to sintex tanks of the flats.

If anybody require a sketch or has any doubt please contact me.

Ravindra