Minimum distance between sewage and drinking water lines in Kharaghpur, West Bengal

Dear Sir,
1) We have a municipality drinking water connection at our residence. We have our house in Kharagpur, West Bengal.
2) The water supply line was laid first from one side of the street road.
3) Now it is being planned by a neighbour to lay a waste water drainage line over the existing water supply line.
I would like to know if this is allowed. We are obviously against this proposal and would like to know the suggested/PWD norms. Please let us know the minimum distance between sewage/wastewater line and potable water line for residential areas. Any PWD resources on this topic and the general norms would be appreciated too.
It is an urgent matter, please give us your valuable advice.

Chandrani Biswas