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 roughly 240,000,000 gallons per day," Hummel said. He described a new outfall that would discharge treated flow to the east waterway of the Duwamish River.

For West Seattle, staff recommended a roughly 7,000,000-gallon storage facility and piping to the Chelan Avenue regulator along with regulator modifications. Hummel said the project would be coordinated with East Waterway cleanup and Superfund-area remediation because the outfall pipeline will be in a cleanup area.

Hummel explained the recommended approach after evaluating wet-weather treatment, storage, sewer separation and green stormwater infrastructure (GSI). He said wet-weather treatment combined with some storage was the least-cost solution and that GSI or full sewer separation would not address the large volumes at issue. The evaluation used tiered criteria with a primary emphasis on community equity/social justice, "good neighbor" considerations, environmental impacts, operations and maintenance, schedule and cost. Hummel said the team engaged community organizations and NGOs to inform siting and design decisions.

Crystal Fleet, capital portfolio planning and analysis manager for King County Wastewater, presented how CSO forecast assumptions have shifted over time and how schedule changes move expenditures into near-term forecast windows. Fleet said WTD moved to a 20-year forecast and that changes in regulatory assumptions, project sizing (including results of alternatives analyses) and updated methodologies for forecasting capital spending materially affect near-term rate forecasts.

Schedule and next steps described by staff: Hummel said the consent-decree milestones include completion of an engineering report by December 2026, bidding completed by 2029 and construction finished by 2034; King County plans to submit its engineering report to Ecology in August 2025 and advance 30% design in first quarter 2026. Fleet and Hummel said cost certainty will improve as the project moves from a Class 5 estimate toward a Class 3 estimate in 2026, after 30% design and geotechnical/survey work and with contractor input.

No formal committee vote on the project was taken; staff presented the recommendation for committee information and asked for committee input and continued coordination. Hummel emphasized the program is schedule-driven and that modest shifts in timing can materially change near-term rate forecasts because large capital expenditures fall into the ten-year forecast window as delivery dates move forward.

Ending: Staff will document the alternatives and recommendations in the engineering report submitted to Ecology, continue environmental review and outreach to property owners, and proceed with design and site investigations so the project can reach greater cost certainty in 2026.