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Treatment process at Vashon

Learn about the process we use to clean wastewater at Vashon.

Vashon is a secondary treatment facility that uses an activated sludge process. The Vashon Treatment Plant collects wastewater from pipes owned and maintained by the Vashon Sewer District. We cycle that wastewater through oxidation and clarification tanks and disinfect the water with ultraviolet light. It is then released into the Puget Sound. Any excess solids are sent to South Treatment Plant in Renton for further treatment.

Graphic displaying the Vashon Treatment Plant treatment process steps

Download the treatment process diagram (PDF) to learn more.

NPDES permits

The Washington State Department of Ecology administers National Pollutant Discharge Elimination System (NPDES)  under authority delegated to the department by the U.S. Environmental Protection Agency. These permits outline the conditions under which King County can discharge treated wastewater.

Learn more about Vashon and other King County wastewater permits.