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Combined Sewer Overflows

CSO Overview

The sewers built in the older portions of our community carry both sewage and storm water in the same pipe. Known as combined sewers, they comprise about 40% of our current sewer system, some of which are over 180 years old. During heavy rains, combined sewers are often filled beyond their capacity. To relieve pressure on the sewer line and prevent street flooding and sewage backups into buildings, combined sewers were designed to overflow directly into local streams and rivers through outfall structures known as combined sewer overflows or CSOs. At the time they were built, CSOs were an acceptable way of handling excess flows, but their environmental impacts are now controlled under the federal Clean Water Act.

The discharge of untreated storm and wastewater from a combined sewer into the environment typically occurs when combined sewers fill up with too much water for the system to handle, most often during heavy rains, and the excess water is released into a stream or river. Power failures and pipe blockages also can cause CSOs. In a combined sewer system, stormwater is the major culprit in combined sewer overflows (CSOs). Solutions known as sustainable or green infrastructure, reduce CSOs by reducing the amount of stormwater runoff entering the combined sewer system. They can range from rain gardens and pervious pavement to stormwater detention basins and stream restoration (daylighting).

Cincinnati CSOs

The range of typical water quality characteristics for Cincinnati CSOs are shown in the table below.

Value

TSS

(mg/L)

CBOD5

(mg/L)

E. Coli

(ct/100 mL)

Min

60

10

105

Average

154.6

39.2

106

Max

470

91

107

 To see CSO site specific configurations, select from the representative CSOs below.