Comment time for next phase of 1.25-million-gallon overflow-storage tank plan near Duwamish River

Another major combined-sewer-overflow storage tank is planned for our area, this time on the east edge of West Seattle, near the 1st Avenue South Bridge. This is the West Duwamish Combined Sewer Overflow (CSO) Control Project, and its aim is to comply with orders to reduce the number of overflows into the Duwamish River – specifically one outfall on SW Michigan Street that overflows an average of 4.6 times a year and one at Terminal 115 that overflows an average of 1.7 times a year. The new facility near 2nd SW/SW Michigan will be on 60,600 square feet of T-115, which is owned by the Port of Seattle, centered on a 1.25 million gallon underground tank – 140′ x 110′, 26′ deep – and related pipes. Here’s the King County Wastewater Treatment Division map:

For comparison, the storage capacity is 25 percent more than the big tank built across from Lowman Beach five-plus years ago, now known as the Murray Wet Weather Facility. The West Duwamish project will also include an above-ground 5,300-square-feet “facility building and outdoor odor control area” plus landscaping including a “stormwater bioretention facility.” The project also includes associated facilities such as a “diversion structure” near West Marginal Way and Highland Park Way SW.

Construction is not expected to start until 2025, but another key comment period is open now. Through February 14th, comments are being taken on the environmental checklist for the project – you can see it here. You can comment via email at You’ll get a chance for a briefing and Q&A about the project at an upcoming HPAC meeting – watch for word of that.

6 Replies to "Comment time for next phase of 1.25-million-gallon overflow-storage tank plan near Duwamish River"

  • Mj February 3, 2022 (5:15 pm)

    Was a budget identified?

    • Don Brubeck February 4, 2022 (12:06 pm)

      King County does not make the cost easy for ordinary taxpayers to find. But they do list an upcoming construction contract estimate of $32,000,000 for the West Duwamish CSO project.  That does not include land acquisition or engineering design or project management.

      • WSB February 4, 2022 (12:26 pm)

        Thanks, I looked through several of the documents for the $ before publishing this but finally had to move on; am following up with KCWTD, though.

  • Jethro Marx February 3, 2022 (7:33 pm)

    We’re a rainy city; make the infrastructure big enough for future calamities and also pleasant. The Murray CSO is awesome.

  • Steve Richmond February 4, 2022 (9:17 am)

    The cheapest stormwater infrastructure are evergreen trees, shrubs, and groundcovers in forests to intercept winter rains. And in yards as much as practical, instead of lawns.  For more info:

    • WSB February 4, 2022 (10:23 am)

      The county’s documentation says green stormwater infrastructure was explored (as they’ve done in other areas) but wouldn’t do enough for this area.

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