DC Collection: Combined Sewers and the Clean Rivers Tunnels
A Mixed System
Most newer American cities were built with fully separate sewer systems -- one pipe network for sanitary sewage, a completely different one for stormwater. Washington, DC's older core predates that practice. Significant parts of the District, especially older downtown neighborhoods, are served by combined sewers, which carry both stormwater runoff and sanitary sewage in the same pipe. Newer parts of the city use separate sanitary sewers. A DC Collection operator needs to understand both systems and, critically, the different way each behaves during storms.
The Combined Sewer Overflow (CSO) Problem
Combined sewers work fine in dry weather -- all flow goes to Blue Plains for treatment. But during heavy rain, the combined volume of stormwater plus sewage can exceed what the pipes and the treatment