National Pollutant Discharge Elimination System (NPDES)
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Combined Sewer Overflows (CSOs)

Welcome to EPA’s combined sewer overflow (CSO) program website!

A combined sewer system collects rainwater runoff, domestic sewage, and industrial wastewater into one pipe. Normally, it can transport all of the wastewater to a treatment plant. Sometimes the amount of runoff exceeds the capacity of the system. When that happens, untreated stormwater and wastewater flows into nearby waterbodies. These events, called combined sewer overflows, are subject to the National Pollutant Discharge Elimination System (NPDES) permitting program.

Follow the links below to learn more about EPA’s CSO program.

  • Basics

    Combined sewer overflow pipes with water discharge

    Find out what CSOs are.

    Learn

  • Solutions

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    Read about how communities mitigate CSO impacts.

    Explore

  • Training

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    Watch trainings that explain CSO regulations and find technical assistance.

    Dive in

  • Program Progress

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    Learn how communities have made progress towards meeting water quality requirements.

    Discover

  • Policy

    Person thumbing through documents

    Get to know the foundational concepts and requirements that drive the CSO program.

    Read more

  • Resources

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    Browse resources on a variety of CSO-related topics.

    See more
Contact Us to ask a question, provide feedback, or report a problem.
Last updated on April 28, 2023