Mixing collected rainwater and trucked in water

The prevention valve which you have fixed earlier to prevent contamination, have you not taken the permission of water department at that time? If you have not taken the permission and they are objecting this point now, then obviously you have to follow their instruction. And in such case, it will be better for you to install the reduced pressure principle backflow preventer. It will also do the work of prevention of water contamination in your water tank.