I cleaned my brushes in a bucket of water and soap and now wonder if I should pour it into the toilet or down the street drain (which doesn’t seem right). What do you suggest?
Never pour the waste water down a street drain. The water won't receive any treatment before entering the local watershed. The best alternative is to pour the waste water down the toilet. This way the paint water will enter your local city waste water treatment plant.
The situation is different with a septic system. Dirty paint water is bad for a septic, both the tank and leach field. The only thing to do is pour the water on the land away from the well and septic system.
I'm not aware of any way to pre-treat this water.