As part of its mission to protect public health and the environment, King County is responsible for conveying and treating wastewater collected by 34 local city and sewer agencies in the King County region, including the City of Seattle (external link).
Four county pump stations (53rd Avenue, 63rd Avenue, Murray Avenue and Barton Street) located along the Puget Sound shoreline send wastewater flows from the local sewer system through the West Seattle Tunnel to the West Seattle Pump Station. From there, flows are sent to the West Point Treatment Plant, located next to Discovery Park, Seattle. During storm events, some excess flow is treated at the Alki Combined Sewer Overflow (CSO) Treatment Plant.
In the mid-1990s, the West Seattle Pump Station and tunnel were constructed, and the 63rd Avenue Pump Station and Alki CSO Treatment Plant were upgraded to transfer flows to West Point for treatment.