Combined Sewer Overflow Treatment

Abstract Combined sewer overflows (denoted hereafter as CSOs) occur when flows exceed the hydraulic capacity of either the wastewater treatment plant (denoted as WWTP) or the collection system that transports the combined flow of storm water and sanitary sewage to the WWTP. The principal components of a combined sewer system include (1) the contributing drainage area (catchment) and wastewater sources, (2) the combined sewer pipe network and interceptor(s), (3) the regulator and diversion structures, and (4) the CSO outlets. When an overflow occurs, the excess flows tend to be discharged into the neighboring receiving body of surface 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. These discharges contain a variety of pollutants that may adversely impact the receiving waterbody, including pathogenic microorganisms, viruses, cysts, and chemical and floatable materials.