The Environmental Protection Agency’s (EPA's) Combined Sewer Overflow (CSO) Control Policy is a framework for developing CSO control programs. Published on April 19, 1994 (at 59 Fed. Reg. 18688), the CSO Policy is monitored through the National Pollutant Discharge Elimination System (NPDES) Permitting Program and provides guidance to municipalities and state and federal permitting authorities on how to meet pollution control goals as stated in the Clean Water Act.
The first milestone under the CSO Policy was reached January 1, 1997 and included implementing minimum technology-based controls, also known as the "nine minimum controls." The nine minimum controls are measures that can reduce the prevalence and impacts of CSOs and that are not expected to require significant engineering studies or major construction. Service providers with combined sewer systems were also expected to develop Long-Term CSO control plans.