Combined Sewer Outfall Sewer Shed

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.

DC Water运营着一套由分流制下水道（separate sewers）与合流制下水道（combined sewers）组成的污水收集系统。本辖区约三分之二的区域采用分流制下水道系统，剩余三分之一则采用合流制下水道系统。合流制下水道系统可在同一管网内同时输送生活污水（sanitary sewage）与雨水（storm water）。本辖区的合流制下水道系统始建于1900年之前。分流制系统则包含两套独立管网：一套用于输送生活污水（即来自住宅与商业场所的污水），另一套用于输送雨水。在正常干燥天气条件下，通过称为调蓄闸或拦水坝的设施，合流制下水道内收集的生活污水会被输送至蓝平原（Blue Plains）处理设施，通常不会有污水溢流至本辖区的河流与溪流中。在降雨时段，合流制下水道的处理容量可能会超出上限，此时调蓄闸会将过量混合水流（即雨水与生活污水的混合物）直接排放至本辖区的河流与溪流中，此类过量水流被称为合流制下水道溢流（combined sewer overflow，简称CSO）。排放此类过量水流是防止住宅、地下室、商业场所及街道发生内涝的必要措施。在多数中等强度降雨事件中，合流制下水道溢流会通过CSO排放口直接排入阿纳卡斯蒂亚河（Anacostia River）、罗克溪（Rock Creek）、波托马克河（Potomac River）及其支流水域。