December 9, 2025

GC/CM finalists named for $1B SoDo wet weather treatment station

Photo via King County [enlarge]

King County built a similar facility in 2022, the Georgetown Wet Weather Treatment Station at Fourth Avenue South and South Michigan Street.

King County's Wastewater Treatment Division named its shortlist last week for a heavy civil general contractor/construction manager to build a wet weather treatment station in SoDo as part of its Mouth of Duwamish Combined Sewer Overflow treatment effort.

A notice ran in the DJC Dec. 3 that Hoffman Construction, Kiewit Infrastructure West and MWH Constructors of Broomfield, Colorado, are finalists.

The facility will be able to treat about 240 million gallons of combined stormwater and wastewater per day during heavy rains before discharging into a new outfall in the east waterway at the mouth of the Duwamish River.

Preconstruction services for the facility are estimated at $10 million and are expected to last just under 2.5 years, with a nearly eight-year GC/CM MACC estimated at over $1 billion. Preconstruction is expected to start in 2026, and construction is slated to start in 2028-2029.

The project is part of a larger effort to meet current water quality requirements by treating combined sewer overflows (CSOs) prior to discharging them. The Mouth of Duwamish project addresses five CSOs built many decades ago to serve as overflow points during heavy rain, discharging that water without treatment.

Map via King County [enlarge]

Proposed Mouth of Duwamish facilities, study area and outfalls, including the wet weather treatment station at First Avenue South and South Spokane Street in SoDo.

A 2013 consent decree with the state Department of Ecology, the U.S. Environmental Protection Agency, and the U.S. Department of Justice requires the county and the city of Seattle to add storage and treatment facilities to reduce untreated discharges.

The agreement's targets were adjusted last year to bring the projects in line with current climate change estimates that require larger and more resilient facilities to clean all of the extra water predicted to enter the system in the coming years, and included extending deadlines for the increased work. Work must now be completed by 2037.

The larger Mouth of Duwamish project includes a 7.5-million-gallon underground storage tank near the intersection of Southwest Dakota Street and West Marginal Way Southwest in West Seattle, a new underwater outfall between SoDo and Harbor Island, and new pipes and support facilities.

The wet weather treatment station will be built on about 8 acres of land near the intersection of First Avenue South and South Spokane Street in SoDo. During periods of heavy rain, stormwater and wastewater that currently overflow into the Duwamish from four of the five outfalls will be directed to the station.

The facility will be able to treat up to about 240 million gallons of combined stormwater and wastewater per day — nearly four times as much as the Georgetown Wet Weather Treatment Station the county built in 2022 — before discharging through a new outfall into the east waterway between SoDo and Harbor Island.

Work includes demolishing existing structures and foundations, building a below-grade 4-million-gallon equalization basin, shoring and excavation work, and soil management (including contaminated soils).

The GC/CM will construct piping, screening and pumps; a vendor-supplied ballasted sedimentation technology for solids removal, and a chlorine disinfection system using UV light disinfection; solids storage; and instrumentation and controls. The GC/CM will also make site surface improvements.

The site is expected to include deep liquifiable soils with potential for seismically induced settlement, necessitating deep foundations for the facility.

The county is using the heavy civil GC/CM method for the infrastructure-heavy project that involves complex scheduling and coordination, so the contractor can be involved during design, and because the project requires a complex and technical work environment.

The GC/CM will also need to coordinate with contractors working on the other projects to tie the piping into the new facility.

Following feedback from people who live and work in the area, the county said the facility will be designed to reflect the history and importance of the Duwamish River, and work will also include site improvements including planting trees and fixing sidewalks.

In all, the county said the Mouth of Duwamish projects will prevent 430 million gallons of untreated water from entering Elliott Bay and the Duwamish River each year.

In separate CSO treatment news, the county announced in Monday's DJC its intent to award a roughly $50 million contract to Ceccanti of Tacoma for a 1.25-million-gallon underground storage tank in South Park. The West Seattle West Duwamish CSO control facility is expected to start construction in early 2026 and continue through 2028.

Related Stories:

• County seeks contractor to build $56M West Seattle wet weather water storage  07-30-2025
  
• Weigh in on future county CSO project  03-19-2025