A combined sewer overflow (CSO) is a discharge of raw sewage mixed with stormwater into local waterways during a wet weather event, such as a rainstorm. Overflows occur when there is too much of this mixture for the sewer system or treatment plants to handle. To relieve pressure in the system and minimize backups into homes and businesses, excess sewage flows into local waterways.
A combined sewer is a one-pipe sewer that is designed to convey both stormwater and sanitary sewage. During dry weather, sanitary sewage alone is conveyed to a treatment facility. During wet weather, a mixture of stormwater and sanitary sewage is conveyed. Combined sewers can reach full capacity and begin to overflow to the Des Moines River.
CSOs contain raw sewage, which can be the source of disease-causing organisms. In addition, the pollutants in CSOs can adversely affect fish and other aquatic life and can create aesthetic problems, such as odors and sewage waste debris.
A CSO contains raw sewage and pollutants that include human bacteria and viruses, chemicals, oils, animal wastes and other contaminants that all have the potential to cause health concerns and illness. A CSO allows millions of gallons of untreated sewage and stormwater to enter the waterways in a typical year. Concerns associated with these discharges include:
The extent of the health concerns from a CSO discharge depend on the amount of water dilution from the size of the receiving stream, the amount of precipitation that causes the overflow, and if the overflow occurs during peak sewage periods such as the morning or evening.
A separate sanitary system is a collection of pipes located under streets and easements that are designed solely to transport sewage away from the sanitary fixtures inside homes, businesses, and industry and convey it to the wastewater treatment plant. This system protects public health by treating human and industrial wastes to reduce pollutant concentrations so they can be safely discharged. Cities that have these systems must also have a separate sewer system to handle stormwater.
A storm sewer system is a collection of inlets and pipes, located under streets and easements, designed to transport rainwater and snowmelt away from streets, homes and businesses and convey it to various receiving waters (such as streams and rivers). Storm sewers are usually much larger than sanitary sewer system pipes because peak stormwater flows from typical rain events greatly exceed sanitary flows. Water discharged through separate storm sewers generally receives no treatment.
There are currently about 770 U.S. communities subject to CSO regulations. Most of these communities are located in the Northwest, Great Lakes, and Northeast portions of the country.