Combined Sewer Overflow

Combined sewers collect and convey sanitary wastewater (AKA sewage) and stormwater (AKA runoff). Under normal conditions, they carry both to the wastewater plant for treatment. But sometimes after a heavy rain or snow melt, stormwater overwhelms the system and untreated sewage is released into the Illinois River. That's called a Combined Sewer Overflow, or CSO. Learn more about Combined Sewer Overflows here.

Stay in the Know About CSOs

During Combined Sewer Overflows (CSOs), untreated sewage is released into the Illinois River. CSOs occur throughout the year and on average, Peoria experiences between 20 and 30 overflow events per year at 16 locations. CSOs cause elevated bacterial levels in the river exceeding the levels considered safe for contact.

Stay current on CSOs by signing up for CSO notifications and visiting this page.

The Federal Mandate

The Clean Water Act makes it unlawful to discharge pollutants from sewer systems into U.S. waters without a permit from the National Pollutant Discharge Elimination System (NPDES). Peoria has long maintained this permit. Now, the Environmental Protection Agency (EPA) has ordered us to develop a long-term plan to get our CSOs as close to zero as possible. We're committed to meeting our federal responsibilities. We'd rather keep our dollars here to improve the river and our city's infrastructure than send fines to Springfield and Washington, D.C.

Consent Decree


Watch the March 10, 2021 Zoom Meeting.


Preliminary Environmental Impacts Determination (PEID)