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Seattle Public Utilities and King County are building underground stormwater and sewer storage.
Sewage and stormwater from many older parts of the city, including Ballard, Fremont, Wallingford, and Queen Anne, all funnel into one set of pipes. In dry weather conditions, all sewage flows to King County’s West Point Wastewater Treatment Plant in Magnolia. During wet weather conditions, polluted runoff can exceed the pipes’ capacity and send a mixture of stormwater (90%) and sewage (10%) to outfalls that flow into the nearest water body. These combined sewer overflows, or CSOs, contain contaminants that can make people sick and harm fish, wildlife, and the environment.
Seattle Public Utilities and King County are working together to build an underground storage tunnel to reduce the amount of polluted water that spills into the Lake Washington Ship Canal from Ballard, Fremont, Wallingford, and Queen Anne. The 2.7-mile, approximately 18’ diameter tunnel will capture and temporarily hold polluted stormwater and sewage that overflows during heavy rains. When the storm passes, flows will be sent to King County’s treatment plant to be treated and discharged.
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This project will keep approximately 70 million gallons of polluted stormwater and sewage out of the Lake Washington Ship Canal, Salmon Bay, and Lake Union each year. The project will limit CSOs to no more than one per outfall per year on average, in compliance with state and federal laws.