Is it illegal to dispose the treated water from STP to the main sewage line?

Hi, We have a sewage treatment plant (STP) in our apartment complex in Whitefield, Bangalore. The treated water from the STP is reused in toilets and gardening but we are facing the issue of excess water getting generated from the STP and there is no storage for the excess treated water. Initially, we thought of discharging the excess water to the main drain outside of our apartment complex, but came to know that it is not allowed and there are huge penalties if BWSSB/KSPCB gets to know about it. So I would like to know from the experts whether is it really illegal to discharge treated water to the main sewage lines. Is there a regulation that we can refer to? And if it is really illegal, then I would like to know the alternate methods of disposing the excess treated water. Thanks, Arinjay