@Mor - I'm not sure if any cities bother to put in a sanitation plant to deal with storm water. They should, maybe, but unfortunately it's the kind of thing that only affects them peripherally, so they usually don't bother.
Sanitation plants are generally only used to deal with sewerage which simply cannot be ignored in a large city, or with processing chemicals from factories.
I personally think that in a few years, when clean, potable water becomes less available since we are using it up at a rapid rate, people will install more sanitation plants simply because they will want to be able to use storm water as drinking water.
And it wouldn't be a bad thing either. I actually think it's a shame at the moment that more households don't make use of rainfall, rather than relying entirely on city water lines.