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

  

  Find out what CSOs are.

  Learn

• Solutions

  

  Read about how communities mitigate CSO impacts.

  Explore

• Training

  

  Watch trainings that explain CSO regulations and find technical assistance.

  Dive in

• Program Progress

  

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

  Discover

• Policy

  

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

  Read more

• Resources

  

  Browse resources on a variety of CSO-related topics.

  See more