King County's plan for Mouth of Duwamish Wet Weather Facilities

King County, Washington sent this bulletin at 05/14/2025 09:25 AM PDT

May 2025

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King County’s plan for Mouth of Duwamish Wet Weather Facilities

One step closer to improving water quality in the Duwamish River and Elliott Bay

King County is taking on our largest combined sewer overflow (CSO) project to date to reduce overflows from five outfalls at the mouth of the Duwamish River. To address this challenge, we are proposing to build facilities, which include a new wet weather treatment station in SODO and a new storage tank in West Seattle (Industrial District West). The Mouth of Duwamish Wet Weather Facilities is our final step in controlling CSOs in the Duwamish River, preventing approximately 430 million gallons of polluted water from entering Elliott Bay and the Duwamish River per year on average. This major investment will support a healthier waterway for people, fish, and orcas. Read on to learn more about the proposed solution, how we got here, what is coming next, and how to share your questions.

Identifying solutions for CSOs in King County

In older parts of our wastewater system, including areas in Seattle, rainwater flows into the same pipes as wastewater. This is called a combined sewer system. CSO outfalls are relief points designed to prevent sewer backups in homes, businesses, and streets during heavy rain. When heavy rains fill the pipes, CSO outfalls release sewage and stormwater into local water bodies. Today, King County is modernizing its wastewater treatment system so that it can improve water quality while withstanding larger storms and other impacts from climate change. We conduct long-term planning to identify possible CSO control options for each outfall that we need to control. We must fulfill legal requirements with the U.S. Environmental Protection Agency and the Washington State Department of Ecology. A controlled CSO can overflow no more than once per year on a long-term average.

Proposed facilities

The proposed solution includes five types of facilities. Together, these facilities will control CSOs from the five project outfalls.

Wet weather treatment station: During periods of heavy rain, stormwater and wastewater that currently overflow into the Duwamish River will be directed to the wet weather treatment station. There, the combined stormwater and wastewater will be cleaned before it is discharged into the Duwamish River.

Storage tank: The storage tank will help control CSOs by holding combined stormwater and wastewater that would otherwise overflow into the Duwamish River during large rainstorms. Once heavy rain subsides, the combined stormwater and wastewater held in the storage tank will flow back into pipes and on to West Point Treatment Plant for cleaning.

Outfall for clean water: After leaving the new wet weather treatment station, clean water will be released into the Duwamish River from a new outfall between SODO and Harbor Island.

New pipes: We will install new pipes to connect the new wet weather treatment station and storage tank to the existing system serving homes and businesses.

Supporting facilities: New and upgraded above-and-below-ground facilities will connect the wet weather treatment station and storage tank to the existing system and direct flows on rainy days. We will share more information about the location of these facilities, once available.

Proposed Mouth of Duwamish Wet Weather Facilities, study area, and outfalls.

The best solution

We studied many different options combining proposed facilities in different locations. Each option included different connections to the existing system and the Duwamish River or Elliott Bay. For each option, our team considered community and social impact, cost, environmental impacts, operations and maintenance, and schedule. We identified the proposed solution as the best option.

The proposed solution sites the wet weather treatment station and storage tank locations near existing sewer pipes and close to the Duwamish River. This means that the proposed solution requires less construction for new pipes compared to other options. Less construction for new pipes means less time needed to build, lower costs, and fewer impacts on local roads.

What comes next?

Now that we have a proposed solution, we will prepare an engineering report for review by the Washington State Department of Ecology and move into environmental review. We will also start the design phase. We will have more to share in the coming months about these processes and how you can get involved.

Timeline for the planning, design, and construction of Mouth of Duwamish Wet Weather Facilities

Want more information?

Use the links below to learn more about our proposed solution.

Read the proposed facilities fact sheet

Explore the proposed solution frequently asked questions

Our commitment to you

We want to hear your feedback, questions, and concerns. Connect with our team in one of the following ways:

Attend one of our upcoming drop-in sessions
- Home Depot Drop-In: Tuesday, June 10, from 10 a.m. to noon at 2701 Utah Ave. S.
- Delridge Community Center Drop-In: Thursday, June 12, from 10 a.m. to noon at 4501 Delridge Way S.W.
Invite us to a meeting hosted by your group or organization
Visit kingcounty.gov/MDWetWeatherFacilities to learn the latest updates about the project, access all project materials, and sign up for email updates
Contact Bibiana Ocheke-Ameh at dwetweafacilities@kingcounty.gov or (206) 477-5604 to have a one-on-one conversation by phone, video chat, email, or in person

If you have suggestions for other ways we can work with you and your neighbors, please let us know!

Interpretation and translation services are available to you at no cost. If you need them please contact Bibiana Ocheke-Ameh, dwetweafacilities@kingcounty.gov, (206) 477-5604

Los servicios de interpretación y traducción están disponibles para usted sin costo alguno. Si los necesita, comuníquese con Bibiana Ocheke-Ameh, dwetweafacilities@kingcounty.gov, (206) 477-5604

សេវាបកប្រែនិងបកប្រែអាចរកបានសម្រាប់អ្នកគ្មានការចំណាយ។ បើត្រូវការគេសូមទំនាក់ទំនងមកកាន់ Bibiana Ocheke-Ameh, dwetweafacilities@kingcounty.gov, (206) 477-5604

免費為您提供口譯和筆譯服務。如果您需要這些服務，請聯絡 Bibiana Ocheke-Ameh, dwetweafacilities@kingcounty.gov, (206) 477-5604

免费为您提供口译和笔译服务。如果您需要这些服务，请联络 Bibiana Ocheke-Ameh, dwetweafacilities@kingcounty.gov, (206) 477-5604

Dịch vụ thông dịch và biên dịch được cung cấp miễn phí cho quý vị. Nếu quý vị cần những dịch vụ này, xin vui lòng liên hệ với Bibiana Ocheke-Ameh, dwetweafacilities@kingcounty.gov, (206) 477-5604

Alternative Formats Available: 206-477-5371 or TTY Relay: 711

Update on RainWise in your area

New neighborhoods are now eligible for RainWise! Eligible properties can get a cistern or rain garden installed through the RainWise program and then receive a rebate to pay for up to 100% of the cost!

Find out if you are eligible

Shape the future of clean water

Your voice matters as we plan the future of clean water for our Puget Sound region. King County’s Vision for Clean Water sets our clean water goals for our regional wastewater system for the next 75 years. Help shape our region’s vision by taking a 15-minute survey at publicinput.com/VisionforCleanWater.

About King County Wastewater Treatment Division

The King County Wastewater Treatment Division protects public health and the environment by collecting and cleaning wastewater while recovering valuable resources for a thriving Puget Sound region. We provide services to 18 cities, 15 local sewer districts, the Muckleshoot Indian Tribe, and nearly 2 million people across a 424-square-mile area in King, Snohomish, and Pierce counties.

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