You might be surprised at what goes into storm sewers.
The plant I worked at had a "branch" of about 40 people. Anyway, because of its small size, it was legal for sewage and industrial waste to go down the storm sewer. Dont know the restrictions, but I did check with the local water polution person who said it was legal.
And, in case of overflow or nasty factory owners, you will also ses industrial waste in storm sewers.