[milanhbs]

Your driveway drains to a nearby body of water, most likely, while your bathroom trains into sewage which is treated.

[hermitcrab]

It makes more sense if your run-off and sewage are treated separately - assuming the car washes get theirs treated.

My understanding is that the UK has a combined system where rainwater and waste go into the same system and is all treated the same. More because of history than because anyone now thinks that is a good idea. Maybe someone who knows more about this could confirm?

[hermitcrab]

I'm in the UK and our privatised water companies seem to mostly just pump stuff into the nearest river or coastline, untreated. Treating the sewage or building infrastructure would eat into their dividends and bonuses. Trebles all round!