DC Collection: Advanced CSO Control & the Clean Rivers Tunnel Program
This guide extends the Beginner-level overview of DC's combined/separate sewer system into the engineering and regulatory depth expected of Grade 3 and Grade 4 operators, who are more likely to be involved in system-level CSO management rather than routine maintenance alone.
Combined vs. Separate Sewers in DC
DC's collection system is a mix: older parts of the District (primarily downtown and adjacent areas) still use combined sewers, carrying both sanitary sewage and stormwater in a single pipe, while newer areas use separate sanitary sewers. This is unusual for a mid-size modern city, most of which were built entirely with separate systems. During heavy rain, combined sewer capacity can be exceeded, causing combined sewer overflows (CSOs) -- a mix of untreated sewage and stormwater discharging dir