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.
Find out what CSOs are.
Read about how communities mitigate CSO impacts.
Watch trainings that explain CSO regulations and find technical assistance.
Learn how communities have made progress towards meeting water quality requirements.
Get to know the foundational concepts and requirements that drive the CSO program.
Browse resources on a variety of CSO-related topics.