Combined Sewer System

What Is A Combined Sewer?

Separate Sanitary and Stormwater Sewer Systems
Separate Sanitary and Stormwater Sewer Systems

DC Water operates a wastewater collection system comprised of "separate" and "combined" sewers. 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 of Columbia is served by separate sewer systems. Separate systems are illustrated here.

Combined Sewer Systems
Combined Sewer Systems

The remaining one-third of the District of Columbia is served by a combined sewer system (CSS), which was developed before 1900. A combined sewer system conveys both sanitary sewage and storm water in one piping system and is illustrated below:

The combined sewer area is illustrated here.

Typical Regulator: Dry Weather Normal Operation
Typical Regulator: Dry Weather Normal Operation

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 here.

Typical Regulator: Wet Weather Normal Operation
Typical Regulator: Wet 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 into 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 here.

Typical outfall
Typical outfall

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 permit from the United States Environmental Protection Agency. A typical CSO outfall is shown here.

Where are CSO Outfalls?

Maps showing the location of each CSO Outfall are shown below:

Combined Sewer Area Map
Combined Sewer Area Map
CSO Outfalls and CSS Drainage Areas
CSO Outfalls and CSS Drainage Areas
Anacostia CSO Outfalls and CSS Drainage Areas
Anacostia CSO Outfalls and CSS Drainage Areas
Potomac & Rock Creek CSO Outfalls and CSS Drainage Areas
Potomac & Rock Creek CSO Outfalls and CSS Drainage Areas

When do CSOs Occur?

CSO outfall Warning sign
CSO Outfall Warning Sign

CSOs should only occur during wet weather. Whether an overflow occurs and the magnitude of the overflow depends on many factors including rainfall volume, rainfall intensity, and recent rainfall in the area. CSOs typically overflow more in wet years than dry years and more intense rains also make it more likely that CSOs will occur.

DC Water has also posted signs at each CSO outfall. Look for these signs near outfalls.

Avoid Contact with Combined Sewer Overflows

DC Water is committed to improving the quality of water within the District of Columbia. Discharges may be dangerous to the public, due to the high flow of water that may exit these sewer outfalls. Potentially harmful substances may also be present in these discharges. The public is advised to stay away from any sewer pipe discharge.

For small rainfalls, the effects of CSOs typically last less than 24 hours. For larger rainfalls, greater than 1" of rain, the effects of CSO on water quality can last up to three days. Please note that the District of Columbia Department of Health has imposed a ban on swimming in all rivers and creeks in the District of Columbia and any contact with rivers and creeks immediately downstream of any sewer pipe discharge should be avoided.

Warning Light System

DC Water 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 Water'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).

What can you find in this section?

  • CSS Contacts
    Find contact information and additional resources for issues related to the Combined Sewer System (CSS) and Municipal Separate Storm Sewer System (MS4).
  • Preventing Sewer Overflows
    Learn what you can do to help local water quality.
  • CSO Predictions
    CSOs should only occur during wet weather. This page explains more and presents predictions on their occurance.
  • CSS Reports
    Monthly and quarterly reports on program status and consent decree compliance.

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