What is a Combined Sewer System?
Combined sewers were built many years ago to carry wastewater and stormwater from homes and businesses. Under dry conditions, wastewater from homes and businesses flows through combined sewers to water reclamation centers (WRCs) for treatment and eventual discharge into a water body. When it rains, stormwater flows into the same sewers, creating “combined flow.” The combined sewers convey combined flow to the WRCs for treatment. To protect the WRCs from flooding during heavy rain, regulators redirect the combined flow to a combined sewer overflow (CSO) treatment facility. These facilities treat the combined flows through screening and disinfection prior to discharge into a river or stream. During larger storms, flow can exceed the capacity of the CSO facility.
