Combined Sewer Overflow

Published: March 30, 2017 | Last updated: July 5, 2023

What Does Combined Sewer Overflow Mean?

Combined sewer overflow (CSO) is the discharge of a combined sewer system into an unprotected stream, river or other body of water. Intended to prevent damage to combined sewer lines, the overflow relieves the sewer system’s overburden.


Trenchlesspedia Explains Combined Sewer Overflow

CSOs happen when the pressure inside a combined sewer exceeds system capacity. The excess pressure on the sewer system can result from heavy rain, snowmelt or the limited ability of a treatment plant to process incoming flows. CSOs are the result of a combined sewer design feature that relieves system pressure by allowing the excess to overflow from the combined sewer into nearby bodies of water without treatment. According to the EPA, CSOs “…contain not only storm water but also untreated human and industrial waste, toxic materials, and debris.”

Similar to sanitary sewer overflows, the EPA further states that about 772 cities in the United States have combined sewer systems with this feature and that CSOs are a “major water pollution concern.”


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