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

King Street Center
201 S. Jackson St., Suite 505
Seattle, WA 98104-3855
Phone: 206-684-1280
Fax: 206-684-1741
Telecommunication device for the deaf (TTY): 711

Get Directions to our office location in Seattle, Washington.

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 average normal flow is 90,000 gallons a day and the average wet weather flow is 180,000 gallons a day. The maximum plant flow capacity is 1.4 million gallons a day.

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 into 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.

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