What is a Combined Sewer Overflow (CSO)?

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CSO - Combined Sewer OverflowCombined sewer systems are sewers that are designed to collect rainwater runoff, domestic sewage, and industrial wastewater in the same pipe. Most of the time, combined sewer systems transport all of their wastewater to a sewage treatment plant, where it is treated and then discharged to a water body. During periods of heavy rainfall or snowmelt, however, the wastewater volume in a combined sewer system can exceed the capacity of the sewer system. For this reason, combined sewer systems are designed to overflow occasionally and discharge excess water directly to nearby streams, rivers, or other water bodies.

These overflows, called combined sewer overflows (CSOs), contain not only stormwater but also untreated human and industrial waste, toxic materials, and debris. They are a major water pollution concern for the approximately 772 cities in the U.S. that have combined sewer systems.

For this reason, The City of Joliet has created a public notification plan to ensure that the public receives adequate notification of CSO occurrences and CSO impacts. This plan includes signage [PDF] at the outfalls, a listing of current overflows, and an email address book for the sending of CSO notifications to interested parties. The City of Joliet shall monitor outfall events. Should the Administration consider the condition to be an emergency, then the City’s Emergency Operations Plan shall be implemented.

A full explanation of the NPDES CSO program and information about combined sewer overflows can be found at the USEPA’s website. Also see our FAQs.

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