What is a combined sewer system and how does it affect water quality?
A. A combined sewer system (CSS) carries both sewage and stormwater runoff. Normally, its entire flow goes to a waste treatment plant, but during a heavy storm, the stormwater volume may be so great as to cause overflows. CSSs are designed to allow overflows (a Combined Sewer Overflow or CSO) under wet weather conditions when system capacity is exceeded. Storm water flows are greater than the sanitary sewage flow; therefore small rainfall events can cause the system to exceed its capacity and overflow. When this happens, untreated mixtures of stormwater and sewage may flow into surface waters. Control of this unsanitary water pollution is a national environmental priority.
