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Wastewater Treatment

King County, Washington

For questions about the Wastewater Treatment Division website, please send an e-mail message or contact us at:

King Street Center
201 S. Jackson St., Suite 500
Seattle, WA 98104-3855
Phone: 206-477-5371
Fax: 206-684-1741
Telecommunication device for the deaf (TTY): 711

Get Directions to our office location in Seattle, Washington.

Staff Contacts

History of Puget Sound Beach CSO Control Projects

Project Planning Phase (2007-2010)

South Magnolia CSO Control Project

March 2012 Update
Design on the South Magnolia CSO Control project began in 2011.
>> For current information on this project, view HERE.

Learn more about the decision process and recommended proposal completed in 2010.

Why are CSOs a problem at South Magnolia?

Like many cities around the country, the older parts of King County's wastewater system carry both wastewater and stormwater to the treatment plant. When heavy rains fill the pipes, excess stormwater and sewage flow directly into Puget Sound at South Magnolia. These events, called combined sewer overflows, or CSOs, help to avoid sewer backups into homes and businesses and onto streets during storms, but they are a public health and environmental concern. When a CSO happens, about 90 percent of the overflow is stormwater and the rest is dilute sewage.

In 2008, King County reported that South Magnolia CSO facility has 19 overflows per year on average that discharge a total of 31 million gallons into Puget Sound off of South Magnolia.

King County's goal is to reduce the number of CSOs each year, with a long-term goal of no more than one untreated discharge per location per year on a long-term average to meet state regulations.

Proposed project

King County recommends a project for combined sewer overflow (CSO) control in South Magnolia

King County proposes to design and build an underground diversion structure and tank to store approximately 1.8 million gallons of peak flows during large storm events. A diversion structure at 32nd Avenue West will transfer flows through a gravity sewer line in the right-of-way to an underground storage tank located in the Smith Cove Park/Port of Seattle West Yard area. After storms have passed, in-tank pumps and force main will send flows back to the wastewater treatment system for conveyance to West Point Treatment Plant in Discovery Park.


Learn more about the project schedule.

New project web page

For more information