What We Do
Combined Sewer System Overview
- Warning Light System
- What is a Combined Sewer?
- Where are Combined Sewer Overflow (CSO) Outfalls?
- When Do CSOs Occur?
Warning Light System
In accordance with the 3-Party Consent Decree (PDF) [Civ. Action No. 1:02-02511(TFH)], DC WASA has installed two CSO Event Indicator Lights to notify river users of CSO events. The Potomac River light is located on the north shore, near the mouth of Rock Creek. The Anacostia River light is located on the north shore, east of the South Capitol Street Bridge, in front of DC WASA's Main Pumping Station. A red light will be illuminated during a CSO occurrence and a yellow light will be illuminated for 24 hours after CSO has stopped. The Anacostia River CSO Event Indicator light will be operated by a signal from CSO outfall #007 (13th Street & Ridge Place, SE) and the Potomac River CSO Indicators light will be operated by a signal from CSO outfall #021 (Northeast of Roosevelt Bridge, NW).
topWhat Is A Combined Sewer?
Pumping station
DC WASA 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:
Separate Sanitary and Stormwater Sewer Systems
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:
Combined Sewer Systems
The combined sewer area is illustrated below:
Combined Sewer Area Map
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:
Typical Regulator: Dry Weather Normal Operation
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:
Typical Regulator: Wet Weather Normal Operation
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:
Typical outfall
Where are CSO Outfalls?
Maps showing the location of each CSO Outfall are shown below:
CSO Outfalls and CSS Drainage Areas
Anacostia CSO Outfalls and CSS Drainage Areas
Potomac & Rock Creek CSO Outfalls and CSS Drainage Areas
DC WASA has also posted signs at each CSO outfall. Look for these signs near outfalls:
CSO Outfall Sign
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.
Download a summary, based on computer modeling, of overflow events in an average year with typical rainfall (PDF 58 kb) . The document summarizes, for each CSO:
- Number of overflows,
- Total overflow volume,
- Estimated minimum rainfall volume necessary to cause an overflow,
- Average duration of overflow.