Skip to main content
close

KingCounty.gov is an official government website. Here's how you knowexpand_moreexpand_less

account_balance

Official government websites use .gov

Website addresses ending in .gov belong to official government organizations in the United States.

lock

Secure .gov websites use HTTPS

A lock lock or https:// means you've safely connected to the .gov website. Only share sensitive information on official, secure websites.

King County Logo

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

An aerial view of the Elliott West Wet Weather Treatment Facility.

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.

For project updates, join our email list.

Project schedule

The project schedule spans from 2023 to 2032. Design 2023-2027, Construction 2028-2032, and Community Outreach 2024-2032,

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.

Project location

expand_less