A Closer Look at Combined Sewer Overflows

When it is dry, sewage and wastewater from our homes, businesses and industrial buildings combine in system of pipes and sent to the wastewater treatment plant. After water is cleaned, it is released to the river.


When it rains, stormwater (blue water in illustration) is collected from streets, rooftops, parking lots and other hard surfaces and sent to the sewer system where it combines with sewage (brown water in illustration) from homes, businesses and industrial buildings. When water volume is high due to intense rainfall or rainfall over several days, excess flows to tunnels and resevoirs where it can be stored and later pumped to wastewater treatment plant for processing.

However, during intense rain events or long lasting rain events, the volume of stormwater and wastewater can exceed the tunnel and resevoir capacity. When this happens there is a combined sewer overflow (CSO) and sewage is sent into the river. According to MWRD, as little as 0.3 inches of rain can trigger a CSO, depending on location and intensity of rainfall.

Green infrastructure reduces the amount of stormwater sent into the sewer system. Roof gardens, rain gardens (using native plants), and disconnecting downspouts all reduce the water load to our sewer system.

Water conservation at home also plays an important role in reducing CSOs to the river. You can help keep our river clean by participating in Overflow Action Days. Click here it sign up for e-alerts that will let you know when water conservation is especially important and to check out other ways you can help reduce combined sewer overflows.