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Elliott West Wet Weather Treatment Station upgrade
King County is planning to upgrade the Elliott West Wet Weather Treatment Station (EWWTS). The upgrade will ensure compliance with stricter environmental regulations, meet the demands of a growing population, and handle a changing climate.
Project description
The Elliott West Wet Weather Treatment Station (EWWTS) is a combined sewer overflow (CSO) facility that was first completed in 2005. Since then, it has stored and treated sewage and stormwater during periods of heavy rain. EWWTS now needs to be upgraded to meet stricter environmental standards and to adapt to changing climate.
This project will ensure that the upgraded facility meets its permit requirements. This is a major investment to ensure King County continues to protect public health and the environment for years to come.
More information about the upgrade plans will be available this summer and fall. To stay updated, subscribe to our email list.
Project update
June 20, 2025
State Environmental Policy Act (SEPA) Determination of Non-Significance (DNS) issued for upcoming King County Sewer Upgrade Project
King County is planning to upgrade the Elliott West Wet Weather Treatment Station (EWWTS), located at 601 Elliott Ave W. Like many cities around the country, older parts of sewer system use the same set of pipes to carry both sewage and stormwater. When the system is overwhelmed, it is designed to overflow into waterways. This is called a Combined Sewer Overflow (CSO). EWWTS is a CSO facility that provides partial treatment to sewage and stormwater during large storms to reduce untreated overflows of combined wastewater into Elliott Bay.
EWWTS needs an upgrade to ensure compliance with stricter environmental regulations, meet the demands of a growing population, and handle a changing climate. Upgrades will include expansion of the existing facility to the property to the south and decommissioning the existing dechlorination system at the Denny Way Regulator Station (3155 Alaskan Way). If it is determined that additional improvements are necessary to ensure water quality compliance, the Elliott West CSO Outfall may also be modified.
Environmental Review
King County must perform an environmental review as required by the State Environmental Policy Act (SEPA). The SEPA review process involves evaluating how the project will affect the environment.
King County issued a SEPA Determination of Non-significance (DNS) for this project on June 20, 2025. This determination means the County’s SEPA review found that the project will not have significant adverse impacts to the environment. The determination will be reviewed by the Washington State Department of Ecology.
To view the project’s SEPA Environmental Checklist, visit: https://kingcounty.gov/wtdenvreview.
For more information about environmental review, please contact Jessica Conquest at: (206) 263-3721 or jconquest@kingcounty.gov.
Previous project updates
- Just over a week left 'til our online briefing on May 29! May 21, 2025
- Have you heard about the upgrade coming to the Elliott West Wet Weather Treatment Station? May 2, 2025
- Maintenance work near Elliott West Facility February 13, February 12, 2025
- Updates on the Elliott West Wet Weather Treatment Station upgrade, June 2024
- Survey for Elliott West Wet Weather Treatment Upgrade project closes Sunday, Sep 24 at midnight! September 22, 2023
- Elliott West Wet Weather Treatment Upgrade project website is now LIVE! September 5, 2023
- New Project Alert! The Elliott West Wet Weather Treatment Station is getting an upgrade and we need your input! August 25, 2023
Project schedule
Our team is in the early stages of designing the needed upgrades to EWWTS. Project timelines may change due to a number of factors. We will provide updates as our work progresses and any schedule changes are needed.