CSO_diagram_US_EPACombined sewer overflows (CSOs) occur in combined system sewer systems, which are sewer systems that carry wastewater (sewage) from residences and businesses and stormwater in the same pipe, eventually conveying it to a wastewater treatment plant. Combined systems are older systems, and any new sewer systems installed in Ohio are required to create separate sewer pipes for wastewater and stormwater.

In general, CSOs are not a problem during dry periods because sewers have the capacity to carry both wastewater and stormwater to be treated at a local wastewater treatment plant.Essexville Wastewater Treatment Plant  During wet periods, however, excess stormwater is created when rain contacts impervious surfaces (e.g. pavement), and this stormwater then enters sewers through storm drains and downspouts; this excess stormwater overwhelms the capacity of the sewer. When this happens, the sewer has to reduce the amount of material it is carrying until to below its maximum capacity, resulting in a CSO, or discharge of waste and stormwater into a waterway.

The waste that CSOs discharge into waterways poses risks to human health and local ecology.  Raw sewage contains numerous waterborne pathogens, including salmonella and giardia, and these pathogens can sicken humans and animals that come in contact with the waterway.  Discharges also contribute to nutrient (nitrates and phosphates) loading, possibly leading to eutrophication, and increase suspended solids and reduce dissolved oxygen, resulting general water degradation.


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