Many cities have “combined” sewers. in good weather, these systems funnel sewage to treatment plants. during rainstorms street runoff is combined with sewage. combined sewers in cities can be problematic because the sewers cannot handle the volume of waste from street runoff and sewage sewers are often blocked by trash that people have thrown into them street runoff cannot flow into the sewers and be safely taken to treatment plants street runoff often causes the sewers to deteriorate