What are Combined Sewer Overflows (CSOs)?
Combined sewer overflows, or CSOs, were built as part of sewer collection systems that were designed to carry both sewage and stormwater in the same pipe. When there is not a lot of stormwater, this mix is transported to a wastewater treatment plant where it is processed. However, after heavy rainfall or snowmelt, stormwater and sewage overload the system. Without CSOs, this mix would back up into homes, businesses, and public streets. Combined sewer systems have regulator structures that allow overloaded systems to discharge into rivers, lakes and coastal areas subjecting them to higher pollutant loads. This can compromise a water body’s uses and lead to water quality violations in the receiving waters.
