The combined sewer area makes up about 5% of MMSD's total service area. Compared to storm sewers found in seperate sewer systems, combined sewers provide a water quality benefit to Lake Michigan by capturing highly polluted stormwater runoff.

Storm sewers deliver polluted runoff to rivers and lakes every time it rains or the snow melts. Unfortunately, heavy rains can lead to combined sewer overflows that send a combination of polluted stormwater and wastewater into Lake Michigan untreated.

Combined sewer overflows occure significantly less frequently than they did in the past. However, overflows are ocassionally needed to prevent sewage from backing up into the basements of homes and businesses.