Discharges from a sewerage system that carries both sanitary sewage and stormwater runoff. Normally, combined sewers carry all wastewater to a treatment facility. During storm events, however, stormwater volume may be so great as to cause overflows. When this happens, untreated mixtures of stormwater and sanitary sewage may flow into receiving waters.
Sewer systems that combine both sewage and stormwater flow into one pipe are called combined sewer systems. Many cities found this system beneficial because, during a powerful rainstorm, water would not back up into people's basements, but would be taken directly to the outlet, usually a river or lake. Unfortunately, CSOs dump toxic pollutants, such as E. coli, into the water and create a public health threat. See also: What are CSOs