Why mixing storm runoff and sanitary sewage in the same pipe causes a real, named pollution event — and why a separate system structurally can't have that same failure.
Two sewer collection strategies exist in nearly every city, often within the same city at once. A combined sewer carries sanitary wastewater and stormwater runoff together, in the same pipe, to the same treatment plant. A separate sewer keeps them in two independent networks entirely. The difference sounds like bookkeeping — until a heavy storm hits. Then it becomes the difference between a system that quietly handles the rain and a system that is forced, by its own design, to dump raw sewage into a river.