What is a Combined Sewer Overflow?

Combined sewer systems (CSS) were state-of-the-art solutions for the disease-ridden, flood-prone urban areas of the late 1800s and early 1900s when they were built, because they were able to remove sewage and stormwater quickly. But when it rains, the stormwater can overwhelm the capacity of the combined sewer system. The systems have release valves of sorts — outfalls where the mix of sewage and stormwater spills out and into waterways. When this occurs, it is called a combined sewer overflow. These types of systems are an expensive and complicated problem for communities around the country.



These CSOs pollute rivers and bays during rain events. Combined sewer overflows additionally can cause sewer backups into basements and streets, threatening human health, and they have a significant environmental impact, causing closure of beaches and shellfish beds and impairing fish and other aquatic life and their habitats.

A Video Primer on CSOs

Watch what happens when it rains…

H2 Oh No! from the Center for Urban Pedagogy on Vimeo.



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