South Magnolia CSO Control Project

King County to begin the alternatives selection process for CSO Control in the South Magnolia basin

To protect Puget Sound and public health, King County is identifying CSO control options for Barton, Murray, North Beach, and South Magnolia. These locations are top priority because people are most likely to come in contact with water during recreational activities such as swimming.

South Magnolia CSO basin area

King County has identified several approaches to CSO control for these projects. These include the following:

• Conveyance/treatment --Sending flows from the basin to a regional treatment plant.
• Storage --Retaining flows during storms to prevent CSOs. Following the storm event, flows are sent for secondary treatment following the event.
• On-site treatment --Providing primary treatment for flows exceeding system capacity during large storms.
• Demand management (stormwater flow reduction) --Implementing one or more methods of limiting stormwater flow into the sewer system.
• Combination of approaches--These options can be used alone or in combination, depending on local conditions at North Beach.

Beginning fall 2009, King County’s CSO Beach Project team will begin work to evaluate alternatives, approaches combined with potential sites, for CSO Control projects in all four basins.

Why are CSOs a problem 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. CSOs help to protect sewer facilities and avoid sewer backups into homes and businesses and onto streets during storms. The South Magnolia CSO discharged on average 19 times each year, a total of 31 million gallons per year.

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. The county is in the process of identifying CSO control approaches for South Magnolia, as well as for three other locations on Puget Sound: Barton and Murray (in West Seattle) and South Magnolia. These locations are top priority because people are most likely to come in contact with water during recreational activities such as swimming.

Schedule

• Evaluate alternatives for CSO control: 2009-2010
• Alternative selection and environmental review: December, 2010
• Final design & permitting: 2011-2013
• Construction: Begins late 2013