Revised 5/25/18
Combined Sewer Overflows
In older cities, sewers carry both wastewater and rainwater in a single pipe system that is
known as a combined sewer system. Most of Camden City has a combined sewer system. During
heavy rainfall events, the flow in combined sewers can reach capacity and threaten to back up
into homes and businesses. To prevent this, flow relief structures known as Combined Sewer
Overflows (CSOs) are constructed, typically sending excess flow into bodies of water. CSOs have
a clear issue of pollution where untreated sewage and stormwater is sent into local waterbodies:
they pose a risk to public health, impact and restrict water uses (swimming, boating, and fishing),
and stress the aquatic environment. Bacterial contaminants typically last for 48 hours after a
heavy rainfall event. More modern separate sewer systems are installed with rain runoff
discharging to surface waters via storm sewers while all wastewater is routed in its own pipeline
to a wastewater treatment plant.
Graphic attributed to D.C. Department of Energy and Environment appearing in, “Big
Greenscaping for a Big City: Washington, D.C., mixes green techniques, gray engineering to
reduce stormwater flows.” Chesapeake Quarterly, vol. 15, no. 1, Maryland Sea Grant, April
2016, College Park, MD. http://www.chesapeakequarterly.net/V15N1/main1/
[Text adapted from Mystic River Watershed Association, “Combined Sewer Overflows” page,
https://mysticriver.org/csos]
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