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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: 206-684-1280
Fax: 206-684-1741
Telecommunication device for the deaf (TTY): 711

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Staff Contacts
Puget Sound shoreline next to the West Point Treatment Plant, Seattle

Vashon Treatment Plant

The process

The Vashon Plant serves the Vashon Sewer District. The plant is designed to treat an average of 180,000 gallons a day. In 2009, it treated 140,000 gallons per day. The maximum plant flow capacity is 1.4 million gallons a day. The plant is not designed to treat peak flows on an ongoing basis.

Wastewater coming into the plant undergoes a series of treatments, including the following:

  • Preliminary treatment: where trash is removed from the wastewater.
  • Biological treatment: next, the water goes to an oxidation ditch where hungry "bugs" eat up contaminants. The water then flows to a clarifier which acts like a giant bathtub and the solids fall to the bottom. The clean water is ready to move to disinfection.
  • Disinfection: (ultraviolet light) destroys most remaining pathogens, or disease-causing bacteria before the final effluent is released through an outfall pipe and diffuser into Puget Sound.
  • Solids removal: the solids are stored, settled and loaded into tanker trucks and taken to the South Treatment Plant in Renton for solids handling. They are ultimately recycled in various beneficial ways.

Related information

Vashon process diagram
Wastewater Treatment Process
How is wastewater treated at King County's Vashon Treatment Plant?
(PDF, 289KB)

The Vashon Plant discharges treated wastewater under permits issued by the Washington State Department of Ecology.

Visit the Resource Recovery site to learn how King County recovers and recycles beneficial resources from the wastewater treatment process.

How can you help improve the treatment process? Don't use your toilet as a trash can!