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CSO Illustration
Map of CSO Locations
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What is a CSO? [PDF]

 CSO Education

Above: Combined sewers carry stormwater and sewage. When it rains, debris (like plastic bottles and waste from street curbs) floats into combined sewers. Heavy rains can cause combined sewers to fill very quickly, and to prevent flooding in homes or treatment facilities, they can discharge to the environment, causing floating debris shown here to enter local waterways.


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 rain storm, water flowing over hard surfaces rushes quickly into sewers. This flow is known as runoff, and can cause a dramatic increase of water flowing into and through the combined sewers. When this happens, control devices (known as regulators) may allow some of the flow (a combination of stormwater and sewage) to overflow into area waterways to prevent urban flooding and damage to wastewater treatment facilities. This event is called a combined sewer overflow or CSO.

What problems do CSOs cause?

Because CSOs are a mixture of stormwater 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. During beach season, the Sewer District posts the advisory status for Edgewater Beach on its homepage thanks to a service called Nowcasting.

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 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, known as Project Clean Lake, is a $3 billion 25-year effort which will dramatically reduce the frequency and volume of combined sewer overflows.