What are Combined Sewer Overflows (CSO) and other definitions « The City of Lakewood, Ohio -
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What are Combined Sewer Overflows (CSO) and other definitions


What are Combined Sewers?

Combined sewer systems are sewers that are designed to collect rainwater runoff, domestic sewage, and industrial wastewater in the same pipe.  Most of the time, combined sewer systems transport all of their wastewater to a sewage treatment plant, where it is treated and then discharged to a water body.  During periods of heavy rainfall or snowmelt, however, the wastewater volume in a combined sewer system can exceed the capacity of the sewer system or treatment plant. For this reason, combined sewer systems are designed to overflow occasionally and discharge excess wastewater directly to nearby streams, rivers, or other water bodies.

These overflows, called combined sewer overflows (CSOs), contain not only stormwater but also untreated human and industrial waste, toxic materials, and debris.  They are a significant water pollution concern for the approximately 772 cities in the U.S. that have combined sewer systems (EPA, 2014).

With the passage of the Clean Water Act in 1972, such overflows must be controlled in order to make public waters fishable and swimmable.

US EPA and Ohio EPA are requiring that Lakewood remove millions of gallons of combined sewer overflow that happens every year.

Acronyms and Definitions