Combined Sewer Overflow Technology Fact Sheet Alternative Disinfection Methods

Combined sewer overflows (CSOs) occur when flows exceed the hydraulic capacity of either the wastewater treatment plant (WWTP) or the collection system that transports the combined flow of storm water and sanitary sewage to the WWTP. When an overflow occurs, the excess flows tend to be discharged into a receiving body of water. CSOs typically discharge a variable mixture of raw sewage, industrial/commercial wastewater, polluted runoff, and scoured materials that build up in the collection system during dry weather periods. These discharges contain a variety of pollutants that may adversely impact the receiving water body, including pathogenic microorganisms, viruses, cysts, and chemical and floatable materials. Health risks associated with bacteria-laden water may result through dermal contact with the discharge, or through ingestion of contaminated water or shellfish.