South Magnolia CSO Control Project

King County to upgrade wastewater facilities in South Magnolia to protect Puget Sound

As part of its mission to protect public health and the environment, King County is responsible for conveying and treating wastewater collected by 34 local city and sewer agencies, including the City of Seattle. Many of the county's sewers and pump stations are more than 40 years old and need to be upgraded to ensure reliable operation. Some facilities must also be expanded to provide additional capacity or modified to meet more stringent regulations.

Reducing Combined Sewer Overflows (CSOs) at South Magnolia

During heavy rains when flows exceed the capacity of the sewer system, the system can overflow into Puget Sound at South Magnolia. When this happens, about 90 percent of the combined volume of the overflow is storm water and the rest is diluted sewage. These events are called combined sewer overflows, or CSOs. From 2000-2007, the South Magnolia CSO discharged on average 18 times each year, a total of 30 million gallons per year.

CSOs help to avoid sewer backups into homes and businesses and onto streets, but CSOs can be a public health and environmental concern. King County's goal is to reduce the number of CSOs each year, with a long-term goal of less than one untreated discharge per location per year to meet state regulations.

To protect Puget Sound, the county is in the process of identifying CSO control options for North Beach, West Seattle, and South Magnolia. These locations are top priority, because they are where people are most likely to come in contact with water during recreational activities such as swimming.

Although the sewage in CSOs is greatly diluted, CSOs may be harmful to public health and aquatic life because of bacteria and viruses found in sewage.

The map shows the area draining to South Magnolia and nearby facilities. At South Magnolia, CSO control options may include:

• Storage of peak flows during large storms. Once the storm passes, the existing county facilities would slowly convey flows to the West Point Treatment Plant. This option would involve constructing a large new storage tank or pipe in South Magnolia.
• Reducing flows of storm water and ground water into the wastewater collection system. The county will work with the City of Seattle to see if there are ways to reduce flows into the city or county system.
• Treatment of peak flows during large storms. This option would involve constructing a local CSO treatment facility in south Magnolia that would operate only a few times a year.

These options can be used alone or in combination, depending on local conditions at South Magnolia.

Schedule

• Evaluate alternatives for reducing CSOs: 2007-2008
• Predesign and environmental review: 2009-mid-2010
• Final design & permitting: 2010-2011

• Construction: 2012-2013