Combined Sewer Overflows (CSO) are a discharge of a mixture of storm water and untreated domestic wastewater that occurs when the flow capacity of a sewer system is exceeded during a rainstorm. CSOs are excess flows from a combined sewer system that are discharged into a receiving water without being treated. Combined sewer systems are found primarily in older metropolitan communities.

Newer systems have separate sewers for storm water and domestic wastewater. Storm water is collected in a separate sewer and is typically discharged directly into the receiving water. Sanitary Sewer Overflow (SSO) occurs when excessive infiltration and inflow during wet weather periods enters the sanitary sewer.

CSOs and SSOs are major sources of water pollution in many areas.