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What are Combined Sewers?
Combined sewers carry raw sewage, industrial waste and storm water in a single pipe. In dry weather and light rain, all the water goes to one of the District's three wastewater treatment plants.
During a large rain storm, run off can cause a dramatic increase of water flowing through the combined sewers. When this happens, control devices may allow some of the flow - a combination of storm water and sewage - to overflow into area waterways to prevent urban flooding. This event is called a Combined Sewer Overflow (CSO).
Click here to view an illustration of a CSO.
What problems do CSOs cause?
Because CSOs are a mixture of storm water and sewage, pollutants are discharged to area waterways during overflow events. Floating material and debris are a highly visible problem that CSOs can cause. A more significant problem is the bacteria present in CSOs. High bacteria counts in area waterways may pose a health risk to people (particularly children, the elderly and those with chronic illnesses) involved in recreational activities such as swimming.
Public health officials recommend that people not go into water that receives flow from CSOs for two days after a heavy rain.
For more information, click here to view information from the Center for Disease Control.
For additional information about Beach Water Quality Advisories, visit the Ohio Department of Health's website.
How much rain does it take for a CSO discharge to occur?
It varies by CSO location, depending on how the system has been designed and built. Some locations will have CSO discharges during a moderate storm, while other locations will only overflow during the worst storms.
Click here to see a summary of the predicted frequency of CSOs in the District's service area.
How long is the receiving water affected by a CSO after a rain event?
Natural water bodies, such as streams and lakes, may receive contamination from a wide range of sources. Additionally, the characteristics of these waters and storm events vary greatly. As a result, it is impossible to accurately predict how long pollutants will be present at elevated levels. As a general rule, however, public health officials advise against recreational contact with urban area water bodies for two days after a rain event.
What is being done to control CSOs?
Under the Clean Water Act, the District is required to plan, design and construct the combined sewer overflow control program. The District's Combined Sewer Overflow Control Program is a $1.6 billion effort that consists of 103 additional miles of tunnels and connecting sewers under 65 total construction projects, which will dramatically reduce the frequency and volume of combined sewer overflows.
Click here to see a map of CSO project locations.
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