Separate sewerage systems
It is not really necessary to treat rain water and sewage works served by combined sewerage systems have to be bigger than necessary to cope with rain water. So back in the 1960s developments were built with separate sewerage systems with one sewer for the foul sewage and another, bigger one for rain water - called a surface water sewer.
The foul sewers go to the sewage treatment works where the sewage is treated. The surface water sewers go to a river or stream so that the rain water can be returned to the environment. The system works well so long as people don't put foul sewage into a surface water sewer.
Diagram of separate sewers
