Treatment for the water from paint spraying booth

Well, water is so cheap , it makes sense to change it whenever it is 'appropriate'.

H&S issues with the water will be related to the presence of bacteria in it and the possibility of converting these into aerosols with the gas issuing from the spray nozzle. So use town main water rather than sewage (!).

If the water is untreated there could be bacterial growth in the water that may present a health hazard. The bacteria may be come airborne. This could cause health issues. There are ways to treat the water to reduce the growth of bacteria.

I think that your main problem is how are you going to render the water you are replacing safe to place into the sewage system. It is going to be filtering out a lot of the solids from the painting process, and depending on what kind of paint you are using this could be a problem. I would look into this with the city waste water engineers so you don't get a visit from the EPA doing a backtrack to find the nasties you may be putting into the system. If they get involved, you may as well find a lawyer to keep you out of jail and close up shop.

You shouldn't have a lot of problem with bacteria, as the water is continuously flowing in the system. After all that is how the water filter works, there should be a trap of some sort to keep the solids from clogging up the pump. As long as that is kept clear, just add water as needed.

And where I am at, water isn't cheap!