DC Water operates a wastewater collection system comprised of "separate" and "combined" sewers. Approximately two-thirds of the District is served by a separate sewer system, while the remaining one-third is served by combined sewers. A combined sewer system conveys both sanitary sewage and storm water in one piping system. The District's combined sewer system was developed before 1900. Separate systems are comprised of two independent piping systems: one system for "sanitary" sewage (i.e., sewage from homes and businesses) and one system for storm water. During normal, dry weather conditions: Sanitary wastes collected in the combined sewer system are diverted to Blue Plains at facilities called regulators or dams. Typically, there are no overflows to the District's rivers and creeks. During periods of rainfall, the capacity of a combined sewer may be exceeded. When this occurs, regulators are designed to let the excess flow, which is a mixture of stormwater and sanitary wastes, be discharged directly into the District's rivers and creeks. This excess flow is called combined sewer overflow or CSO. Release of this excess flow is necessary to prevent flooding in homes, basements, businesses, and streets. CSO's are discharged to the Anacostia River, Rock Creek, Potomac River or tributary waters at CSO outfalls during most moderate rain events. There are a total of 48 potentially active CSO outfalls listed in the existing National Pollutant Discharge Elimination System permit from the United States Environmental Protection Agency.