Combined sewers cause serious water pollution problems during combined sewer overflow (CSO) events. This occurs when combined sewage and surface runoff flows exceed the capacity of the wastewater treatment plant or the maximum flow rate of the system which transmits the combined sources. The overflow is often discharged to a receiving body of water that combines a variable mixture of untreated human and industrial waste, polluted runoff, debris and scoured materials that build up in the collection system during dry weather periods. Discharge contains a variety of contaminants such as chemicals, pathogenic microorganisms, viruses and cysts that impair water quality and impact human health and wildlife. Combined sewer overflows are the leading source of water pollution in the United States, with approximately 772 communities being susceptible as of 20171.
Combined sewer overflow events occur intermittiently and vary in flow rate, this makes it difficult to add and regulate a chemical disinfectant. Additionally, many CSO outlets are located in areas that are not readily accessible, leading to the requirement of an automatic system.
1Ref: The 2017 Infrastructure report by the American Society of Civil Engineers (ASCE)
