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What are Combined Sewers and Combined Sewer Overflows (CSOs)?


Three types of sewers exist in the City of Akron: separate storm sewers, separate sanitary sewers and combined sewers. Separate storm sewers collect rain water that falls on roads, parking lots and roof tops and sends the rainwater directly to streams, rivers or lakes. Separate sanitary sewers are typically smaller in diameter and collect domestic sewage and industrial waste from houses, schools, stores, factories and other buildings. The sanitary sewers transport the waste to larger combined interceptor sewers which sends the waste directly to the Akron Waste Water Treatment Plant. The third type, Combined sewers, carries domestic sewage, industrial waste and rainwater. Combined sewers are larger in diameter and make up the main sewage network of the Akron Sewer System. When it is not raining, all the domestic sewage and industrial wastewater goes through combined sewers and transports the waste directly to the Akron Waste Water Treatment Plant. When it does rain significantly and the amount of collected rain plus the domestic sewage and industrial wastewater exceed the capacity of combined sewers, the combined sewers overflow into the Cuyahoga River, Little Cuyahoga River, Ohio Canal and Camp Brook. Such an event is called a Combined Sewer Overflow, frequently abbreviated to CSO. All interceptors are combined sewers. Thus, all separate sanitary sewers must travel through a combined sewer interceptor to be treated at the Akron Waste Water Treatment Plant.

Fully Separate Sanitary and Storm Sewer System

Combined Sewer System

The majority of overflow from a CSO is rain water, although the overflow does include a small percentage of the domestic sewage and industrial waste. Different types of contaminants can be present in the overflow including pathogens, oxygen-demanding pollutants, suspended solids, nutrients, toxics and floatable matter.

Approximately 772 cities in the US, including the City of Akron, have combined sewers.

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