Yeah, I'm from the US.
We used to have combined storm/sanitary sewers, and some older areas still do. Boston is notorious for this as an example. The intent is to have separate storm and sanitary lines because as soon as you mix the drainage in with the poop, it's considered poop. Also, where you have combined systems, you occasionally get untreated overflows to waterways. So it's generally frowned upon these days, and cities spend millions on projects to separate them.
Stormwater is a real pain in the butt because there are all these quality and quantity criteria you have to meet.
What sort of plant do you work at? Typical primary clarifier->nitrification->denitrification->secondary clarifier->chlorinattion type of setup or something fancy?
Considering the massive problems with runoff (and people just being stupid and dumping their used car oil down the storm drain), it might not be a bad idea to treat both as contaminated anyway.
Stormwater, from any developed and/or impervious area, is considered to be contaminated and treated for both quality and quantity and treated accordingly.
You ever see one of those big drainage basins next to a shopping center? That removes suspended particles and nutrients, and attenuates peak flow and temperature. You know, so the river it eventually flows into doesn't overflow, turn green, and kill the fish.
They make underground vaults with various filter media to get rid of floatables like oils and chemicals.
(Stormwater is my biggest area of expertise, I could go on all day.)