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National Pollutant Discharge Elimination System (NPDES)

Combined Sewer Overflows (CSO)

Most older cities in the United States, including Park Ridge, were built with combined sewer systems. This means the sewer system was designed to collect rainwater runoff, domestic sewage and industrial wastewater all in a single pipe and transport all of the wastewater to a sewage treatment plant where it is treated and then discharged to a water body. During periods of heavy rainfall or snowmelt, the runoff volume can exceed the capacity of the sewer system or treatment plant, resulting in the discharge of excess wastewater directly to nearby streams, rivers or other water bodies.

The long term plan for the region to reduce and/or eliminate direct discharges of wastewater is called the Tunnel and Reservoir Plan (TARP), sometimes referred to as the Deep Tunnel. The TARP consists of a series of tunnels and reservoirs to temporarily store excess runoff during rain events so it can be treated before discharge into the waterways.

Metropolitan Water Reclamation District
The Metropolitan Water reclamation District (MWRD) maintains a network of flow monitoring equipment throughout the Chicago area to detect CSOs, including the seven City of Park Ridge outfalls along the Des Plaines River. In addition, the MWRD has created a public notification plan that includes signage at the outfalls, a web page to inform the public of CSO occurrences, and an email address book of interested parties. Up-to-date information is available at the Metropolitan Water Reclamation District's website.

Additional Information

Combined Sewer Overflow Location Map

Combined Sewer Overflow Operational and Maintenance Plan   

Combined Sewer Overflow Pollution Prevention Plan

Combined Sewer Overflow Public Notification Program

A full explanation of the National Pollutant Discharge Elimination System (NPDES) CSO program and information about combined sewer overflows can be found at the United States Environmental Protection Agency’s website.

Information about the Tunnel and Reservoir Action Plan (TARP) is also available on the Metropolitan Water Reclamation District's website.

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