Rainwater harvesting system

we have received a small fund to install a rainwater harvesting system in Chennai in PWD premise with PWD counterpart of the project. We have a sub-contractor who will construct a water source structure (kind of reservoir to store the water). Is this sub-contract work (for small water supply) is exempted from all Taxes. If yes what is CBDT circular for this? Thank you in advance. Best regards, Lalit