Staff recommends 240-million-gallon-per-day wet-weather treatment station at mouth of Duwamish; engineering report due this summer

On May 7, 2025 King County Wastewater staff described a recommended system to control combined sewer overflows (CSOs) in the Maltha/Duwamish area, proposing conveyance, storage and a 240,000,000 gallons-per-day wet-weather treatment station and announcing plans to submit the engineering report to the Washington State Department of Ecology this summer.

Stan Hummel, CSO delivery unit manager for King County Wastewater, told the Regional Water Quality Committee he and his team evaluated more than 60 planning concepts and narrowed options to a Best Apparent System Alternative (BASA). Hummel said the BASA would divert flows near the Lander Street and Hanford Street regulators, route large flows south, and convey them to a wet-weather treatment facility just north of the Spokane Street Viaduct and slightly west of First Avenue. "The capacity needs when we model the system and the peak flows that need to be treated are…