What Is A Combined Sewer?
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The District of Columbia Water and Sewer Authority (DCWASA) operates a wastewater collection system comprised of "separate" and "combined" sewers. Since the early 1900's, sewers constructed within the District have been separate systems and no new combined sewers have been built. 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. Currently, approximately two-thirds of the District is served by separate sewer systems. Separate systems are illustrated below:
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The remaining one-third or approximately 12,478 acres of the District is served by a combined sewer system (CSS). The areas served by the CSS were primarily developed before 1900. A combined sewer system conveys both sanitary sewage and storm water in one piping system and is illustrated below:
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The combined sewer area is illustrated below:
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During normal dry weather conditions, sanitary wastes collected in the combined sewer system are diverted to the Blue Plains Advanced Wastewater Treatment Plant. The sanitary wastes are diverted at facilities called regulators. Typical regulator operation during dry weather is shown below:
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During periods of significant 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 storm water and sanitary wastes, to be discharged directly to the Anacostia River, Rock Creek, the Potomac River, or tributary waters. This excess flow is called Combined Sewer Overflow (CSO). Release of this excess flow is necessary to prevent flooding in homes, basements, businesses, and streets. Typical regulator operation during wet weather is shown below:
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Combined Sewer Overflow is discharged, during certain rain events, to the Anacostia River, Rock Creek, the Potomac River or tributary waters at CSO outfalls. There are 53 CSO outfalls listed in the existing National Pollutant Discharge Elimination System (NPDES) permit from the United States Environmental Protection Agency (EPA). A typical CSO outfall is shown below:
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| Typical Outfall CSO 040 Rock Creek |
Where are CSO Outfalls?
Maps showing the location of each CSO Outfall are shown below:
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DCWASA has also posted signs at each CSO outfall. Look for these signs near outfalls:
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When do CSOs Occur?
CSOs should only occur during wet weather. Whether an overflow occurs and its magnitude depends on many factors including rainfall volume, rainfall intensity, whether or not it has rained in previous days, and on the natural variability of rainfall. CSOs typically overflow more in wet years than dry years. More intense rains also make it more likely that CSOs will occur.
In an average year with typical rainfall, the table below summarizes the following for each CSO:
- Number of overflows
- Total overflow volume
- Estimated minimum rainfall volume necessary to cause an overflow
- Average duration of overflow
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© DC Water and Sewer Authority
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