Combined sewer systems (CSSs) are wastewater collection systems designed to carry sanitary sewage (domestic, commercial, and industrial wastewater) and surface drainage from rainfall or snowmelt in a single set of pipes. During periods of wet weather the total flow can exceed the capacity of the Water Pollution Control (sewage treatment) Plant. When this occurs, the untreated or partially treated Combined Sewer Overflows are discharged into rivers and streams.

In April, 1994, the US Environmental Protection Agency (EPA) released its final Combined Sewer Overflow (CSO) Control Policy. The requirements of the CSO Policy are to be included in enforceable documents, usually an National Pollutant Discharge Elimination System (NPDES) permit. These permits tell a city where they can discharge wastes, how much, and what standards those wastes must meet. The NPDES permit covers the Water Pollution Control Plant and any Combined Sewer Overflows (CSOs).