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3505 N. Dries Lane   •   309.494.8800

CSO Overview

What are CSOs?

Located in the oldest parts of Peoria, 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. This happens between 20 and 30 times a year. That’s called a Combined Sewer Overflow, or CSO.


A Little History

Peoria built its first sewers in the late 1800s to carry runoff away from homes, businesses and streets. When indoor plumbing arrived, property owners hooked in their sewage lines. By 1931, these combined sewers were connected to the Greater Peoria Sanitary District treatment plant, but were still able to overflow when sewage levels got too high. (Without this escape valve, raw sewage would back up into basements and streets. New neighborhoods avoid this problem by separating sewers for stormwater and sewage.)

Overflow Locations

The dots on the map show where sewers may overflow during wet weather. Due to improvements completed in the 1990s, some of these overflow pipes are inactive. During a typical rainfall, CSOs occur at an average of five locations.


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 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.

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Consent Decree Information:
City of Peoria CSO-Fact Sheet-12-9-2020

Project Summary & Preliminary Environmental Impacts Determination:
PEID Document

Green Infrastructure

Peoria is proposing a 100% green solution to this problem. This innovative idea means that we could create jobs and beatify the city.

Where We’ve Been, Where We’re Going

1920s to 1930s

Greater Peoria Sanitary & Sewage District forms, and a large “interceptor” sewer is built along the riverfront.


Federal Clean Water Act passed, regulating sources of water pollution.


Peoria implements approximately $10M of CSO improvements, reducing average annual CSO volume from 840M gallons to 160M gallons.


Peoria ordered to plan for further reduction in CSO impacts.


Public Works engages local leaders to serve on the OneWater Committee, a group tasked with evaluating and recommending stormwater solutions.


Peoria passes the stormwater utility, a dedicated funding stream for wet weather solutions


Stormwater utility launches citywide approach.


Peoria launches citywide approach to green infrastructure to proactively manage stormwater where it falls.


Peoria enters into settlement agreement with United States Environmental Protection Agency (EPA) and Greater Peoria Sanitary District (GPSD) to implement a Green-Infrastructure-based program to reduce CSOs.

2022 to 2039

City implements CSO improvements under agreed program.