I'm involved in local housing development and neighbourhood plans. And this question bothers me.

How do we deal with, and keep separate:-
- Run off from fields and agricultural land 
- Run off from roads
- Run off from roofs
- Grey water from household washing and appliances (showers, baths, clothes washing, dish washing)
- Foul water from sewage systems

Because each one needs it's own dedicated processing. A few SUDS and a big sewage pipe is not enough.

https://www.theguardian.com/environment/2023/oct/05/potentially-toxic-road-runoff-outfalls-polluting-england-rivers