Frequently Asked Questions


Q: What is a combined sewer overflow CSO?
A CSO is a discharge from a combined sewer system directly into a waterway. A combined sewer system is designed to collect a mixture of rainfall runoff, domestic and industrial wastewater in the same pipe for conveyance to a wastewater treatment plant. A CSO may occur during heavy rainfalls when the inflow of combined wastewater exceeds the capacity of the combined sewer system and the wastewater treatment plant. The CSO outfalls act as relief points for the excess flow in the sewers, thereby reducing the frequency and severity of sewer backups and flooding.

Q: What are the impacts of CSOs?
Although CSOs may contain highly diluted sewage that could include bacteria which may cause illness, they may also cause temporary water quality degradation. Regardless of CSO activity, any standing water may contain disease-containing bacteria at any time. Therefore, swimming, canoeing or other activities where immersion in water is possible should be avoided and is not recommended during and immediately following rainfall.

Q: Why does the Joliet area have CSOs?
Joliet and other older suburbs typically have a combined sewer system in which both sanitary waste and storm water are conveyed in the same pipe. Suburbs built since 1950 have separate sanitary and storm sewer systems.

Q: What is being done to reduce the occurrence of CSOs?
The City has incorporated numerous programs to address CSOs which include the elimination of storm drains that were tied into the sanitary sewer. Building dedicated storm sewer systems. The footing tile separation program which replaces footing tiles that were connected to the sewer lines. Repairing and replacing old sanitary sewers that are getting a lot of inflow from groundwater sources.
How can the Public reduce CSOs? During wet weather events and periods of high flow thereafter, every gallon of wastewater and storm water kept out of the sewer system is a gallon that will not add to a CSO discharge. Examples of ways to reduce the wastewater load include avoiding any unnecessary water usage which may include clothes washing, showering, dishwashing and toilet flushing. Residents and businesses can install rain barrels to collect rainwater runoff from roofs. This water could be used for garden and lawn watering and similar uses thereby reducing the impact of wet weather events and providing water for non-potable uses.