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Combined sewer overflows: What are they, why are they a problem, and what we are doing about them

 Local Sewers
Smaller local sewers (combined sewers, sanitary sewers and storm sewers) are owned and maintained by the local municipalities. Local sewers transport wastewater to the District's interceptors, which lead directly to the wastewater treatment plants.

Combined sewers

The area's earliest sewers, primarily those built within the City of Cleveland and portions of surrounding suburbs, are combined sewers. Combined sewers carry sanitary waste, industrial waste and storm water runoff in a single pipe. The District's service area includes over 75 square miles served by combined sewers, located primarily in the City of Cleveland.

Combined sewers are designed to allow normal, dry weather flow to go to the wastewater treatment plant. During a rainstorm, runoff can cause a dramatic increase of water flowing through the combined sewers to the treatment plant. When this happens, control devices may allow some of the flow to overflow into area waterways to prevent urban flooding. There are approximately 126 points in the District's service area where combined sewers can overflow into the environment during a rainstorm. These points are called Combined Sewer Overflows (CSOs).

Separate sewers

Beginning in the 1920s, developing suburbs began building separate sewer systems. Separate sanitary and storm sewer areas are comprised of two sets of pipes that transport wastewater and storm water separately and are not connected. Storm water is directed to the creeks, rivers and Lake Erie during rain events, while sanitary wastewater flows to the treatment plants. While separate sewer systems are generally preferred, combined sewers do have the advantage of providing treatment for a portion of storm water runoff that can also carry a high pollutant load.

Issues with storm water still may arise. Even with separate sewers, storm water can still flow into sanitary sewers, a process called infiltration and inflow (I/I). This may lead to pipe overflow, which may in turn cause residential sewer backups.

Environmental impact of Combined Sewer Overflows

Floating matter and debris are a highly visible problem that CSOs cause. A more significant problem, however, is the harmful bacteria carried in the overflow. High bacteria counts pose health hazards to people involved in contact recreation, such as swimming.

Reducing Combined Sewer Overflows

During the 1970s, the District pioneered the development of technology to maximize the storage of combined wastewater in existing pipes. Construction of computer-regulated gates and dams within the sewer system allows storage of some wastewater during storms. After the rainstorm, the wastewater is released for treatment. Currently, the District operates 29 automated regulator systems.

At the Westerly Wastewater Treatment Plant in Cleveland, a CSO treatment facility captures and treats combined sewer flows. This has resulted in dramatic water quality improvements at Edgewater State Park. The Southwest Interceptor and the Heights/Hilltop Interceptor transport wastewater from separate sanitary sewer areas to the treatment plants. This removes some of the load from the combined sewer system.

Local Sewers: Maintenance responsibility

The Northeast Ohio Regional Sewer District's responsibility includes construction and maintenance of intercommunity relief sewers, interceptors and wastewater treatment plants. The Cuyahoga County Sanitary Engineer has full or partial responsibility for sewer maintenance in most Cleveland suburbs. In the City of Cleveland and severalhe local Sewer Maintenance Department or Engineering Office of that city handles sewer maintenance.