King County Wastewater Treatment continues working on ways to reduce combined-sewer overflows (CSO) into local waterways. One of its next projects is a million-gallon storage tank in southeast West Seattle, near the 1st Avenue South Bridge, to protect the Duwamish River. And now it’s proposing another, much-bigger storage tank, also along the Duwamish’s west shore in West Seattle further north. It’s part of this announcement today:

King County has proposed a solution to control overflows from five combined sewer overflow (CSO) outfalls at the mouth of the Duwamish River. The proposed solution to improve water quality in the Duwamish River and Elliott Bay includes a new wet weather treatment station in SODO and a new storage tank in West Seattle. It also includes a new outfall for cleaned water, supporting facilities, and new pipes to connect flows along the way.

Now that we have a proposed solution, we will prepare an engineering report for review by the Washington State Department of Ecology. We will also start the design phase and environmental review.

This update circulated today had a few more details – including a construction timeline that’s still at least four years away. We asked KCWTD spokesperson Akiko Oda about the size and location proposed for the new megatank; she replied that it would be “Approximately 7.5 million gallons of combined stormwater and wastewater,” on a site at 4034 West Marginal Way SW [map], owned by King County, though Oda added, “We still need to secure right-of-way easements for the conveyance pipes. Details for the tank capacity and easements will be finalized in the engineering report to the Washington State Department of Ecology, which is anticipated for March 2026.” Ways to find out more: Online here, and/or in person at a drop-in info session at Delridge Community Center (4501 Delridge Way SW), 10 am-noon Thursday, June 12.