It will break down, but in the mean time it's a hazard to animals and a poison. Even the city prefers you not dump it in the drains as it messes with the sewerage treatment plant.
I drain what I can into a five gallon drum and take it to the cities vehicle maintenace facility. Figure what little remains will be dilluted enough with running water that it's ok to put down the sewer. I never drain into the street.
That or I use the auto hobby shop on base, where they have a flushing station.