• Wet Weather Overflows
• Sewer Backups
• Water Quality

Problem | Solution | Projects | How to Help | FAQ | Community Benefits | Contact Us

Wet Weather Overflows

Our community experiences two different types of overflows: combined sewer overflows (CSOs) and sanitary sewer overflows (SSOs).

Combined Sewer Overflows

The sewers built in the older portions of our community, like the City of Cincinnati, carry both sewage and storm water in the same pipe. Known as combined sewers, they comprise about 40% of our current sewer system and date back to nearly 180 years old in parts.

During heavy rains, combined sewers are often filled beyond their capacity. To relieve pressure on the sewer line and prevent widespread flooding and sewage backups into buildings, combined sewers were designed to overflow directly into local streams, creeks, 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 present regulations of the federal Clean Water Act.

By the mid 20th century, combined sewers were largely discontinued in favor of separated sanitary sewer and storm water lines.

Animation courtesy of St. Louis MSD's Clean Rivers Healthy Communities Program

Sanitary Sewer Overflows

In newer areas of Cincinnati and suburban "bedroom communities" that surround the city, sanitary sewage and storm water are handled in separate sewer lines.

During heavy rains, however, storm water can enter sanitary sewer lines through manholes, defective sewer pipes, and illicit connections (e.g., downspouts connected directly to the sanitary sewer). If the sanitary sewer line is filled beyond capacity, it will overflow through sanitary sewer overflow (SSO) relief structures (constructed as part of manholes) or through manhole lids into local waterways, adjacent yards, and streets. SSOs are considered a greater danger to public health than a CSO, and therefore are not permitted under the Clean Water Act.

Overflow Facts

• MSD collects and treats about 184 million gallons of sewage a day at seven major wastewater treatment plants and three smaller facilities, equal to about 67 billion gallons a year (2008 data).
• The MSD service area receives about 41 inches of rain annually, equal to about 180 billion gallons of rain. Out of that total, about 21 billion gallons of rain enters the combined sewer system. A much smaller amount enters the sanitary sewer system through infiltration into the pipes and direct illicit connections (e.g., downspouts connected directly to the sanitary sewer).
• During heavy rains, the combined and sanitary sewer systems can be overloaded with excessive flow. In an average year, about 14 billion gallons a year of untreated storm water and sewage in combined sewers can overflow from CSO relief structures into local waterways, and about 100 million gallons a year of untreated storm water and sewage in sanitary sewers can overflow through SSO relief structures (constructed as part of manholes) or manhole lids into local waterways, adjacent yards, and streets.
• Hamilton County is among the top five locations in the nation for urban CSOs. Overflows occur as many as 105 times a year at some CSOs.
• There are 212 designed and permitted CSO relief structures in Hamilton County that are monitored for overflows. MSD is in the process of installing real-time electronic monitors at the CSOs.
• There are about 76 designated SSO relief structures (constructed in manholes) in Hamilton County that are monitored for overflows.
• Affected waterways include the Ohio River, Little Miami, Mill Creek, Muddy Creek, Rapid Run and other smaller neighborhood streams and tributaries.