The 50 largest wastewater treatment plants in Washington
Ranked by average flow · 331 MGD combined · 96% of the state's reported flow
Metro West Point in Seattle is the largest publicly owned wastewater treatment plant in Washington by reported average flow, treating about 106.93 million gallons per day. The 50 plants below treat 331 MGD between them, 96% of the flow reported by all 150 plants in the state, and serve about 3,428,837 people. Every plant links to its profile with permit and compliance data. Treatment technology is shown with a free Klarifi account.
| # | Facility | City | Avg flow (MGD) | Population | NPDES permit | Expires | Violations (3y) |
|---|---|---|---|---|---|---|---|
| 1 | Metro West Point | Seattle | 106.93 | 1,342,009 | WAL029181 | Dec 2030 | 275 |
| 2 | King County South WWTP | Renton | 74.4 | 791,766 | WAR002511 | Dec 2029 | 3 |
| 3 | Chambers Creek WWTP | University Place | 22.49 | 245,871 | WAR000727 | Dec 2029 | 0 |
| 4 | Three Rivers Regional WWTP | Longview | 17.43 | 45,361 | WA0037788 | Feb 2026 | 0 |
| 5 | Post Point Plant | Bellingham | 15.62 | 103,199 | WAR005561 | Dec 2029 | 1 |
| 6 | Lott Wastewater Treatment Plant | Olympia | 12.08 | 111,878 | WAG994545 | Dec 2026 | 0 |
| 7 | Yakima Regional Wastewater Facility | Yakima | 8.67 | 120,355 | WA0024023 | Mar 2029 | 2 |
| 8 | Richland City Wastewater Lab | Richland | 5.91 | 56,193 | WA0020419 | Sep 2021 | 0 |
| 9 | Puyallup WWTP | Puyallup | 5.2 | 41,000 | WAR313277 | Dec 2030 | 0 |
| 10 | Lakota WWTP | Federal Way | 4.96 | 69,957 | WA0022624 | Sep 2023 | 4 |
| 11 | Lynnwood City Wastewater Treatment Plan | Edmonds | 3.96 | 41,049 | WAG994526 | Dec 2026 | 14 |
| 12 | Miller Creek WWTP And Compost Facility | Seattle | 3.6 | 28,342 | WA0022764 | Sep 2023 | 0 |
| 13 | Ellensburg WWTP | Ellensburg | 3.59 | 20,891 | WA0024341 | Dec 2023 | 0 |
| 14 | Salmon Creek WWTP | Seattle | 3.03 | 31,051 | WAG994551 | Dec 2026 | 0 |
| 15 | Wenatchee Wastewater Treatment Plant | Wenatchee | 2.86 | 36,733 | WA0023949 | Aug 2015 | 10 |
| 16 | Port Angeles WWTP | Port Angeles | 2.72 | 21,841 | WA0023973 | Jan 2021 | 0 |
| 17 | Pullman City Muni SW | Pullman | 2.46 | 34,135 | WA0044652 | May 2019 | 1 |
| 18 | Picnic Point Wastewater Treatment Facility | Edmonds | 2.45 | 12,595 | WAG994579 | Dec 2026 | 0 |
| 19 | Ferndale STP | Ferndale | 2.26 | 13,198 | WAR308713 | Dec 2025 | 1 |
| 20 | Port Orchard WWTP | Port Orchard | 1.98 | 13,095 | WAG994523 | Dec 2026 | 4 |
| 21 | Enumclaw STP | Enumclaw | 1.85 | 12,500 | WA0020575 | Jun 2030 | 0 |
| 22 | Monroe STP | Monroe | 1.79 | 19,838 | WA0020486 | Nov 2023 | 0 |
| 23 | Port Of Sunnyside Industrial WWTF | Sunnyside | 1.64 | — | WA0052426 | Aug 2019 | 79 |
| 24 | Burlington WWTP | Burlington | 1.56 | 14,629 | WA0020150 | Jan 2023 | 0 |
| 25 | Lynden STP | Lynden | 1.56 | 13,805 | WA0022578 | Oct 2022 | 6 |
| 26 | Sunnyside STP | Sunnyside | 1.34 | 18,327 | WA0020991 | Jan 2020 | 1 |
| 27 | Hoquiam STP | Hoquiam | 1.19 | 9,902 | WA0020915 | Oct 2030 | 0 |
| 28 | Birch Bay Water And Sewer District WWTP | Blaine | 1.17 | 8,413 | WAG994542 | Dec 2026 | 0 |
| 29 | Cle Elum WWTP | Cle Elum | 1.13 | 5,996 | WA0021938 | Sep 2024 | 0 |
| 30 | Washougal STP | Washougal | 1.09 | 16,266 | WA0037427 | Nov 2016 | 0 |
| 31 | Willapa Regional WWTF | Raymond | 1.01 | — | WA0041041 | Feb 2030 | 0 |
| 32 | Sedro Woolley City Of | Sedro-Woolley | 0.96 | 8,268 | WA0023752 | Dec 2023 | 1 |
| 33 | Grandview Wastewater Treatment Plant | Grandview | 0.93 | 12,500 | WA0052205 | Dec 2024 | 195 |
| 34 | Orting STP | Orting | 0.91 | 7,750 | WA0020303 | Jan 2017 | 4 |
| 35 | Othello STP | Othello | 0.89 | 8,494 | WA0022357 | Sep 2025 | 86 |
| 36 | College Place WWTP | Walla Walla | 0.87 | 9,775 | WA0020656 | Feb 2019 | 23 |
| 37 | Port Townsend WWTP | Port Townsend | 0.83 | 10,508 | WAG994538 | Dec 2026 | 0 |
| 38 | Chelan WWTP | Chelan | 0.75 | 4,915 | WA0020605 | Jan 2022 | 0 |
| 39 | Prosser City WWTP | Prosser | 0.74 | 6,616 | WAR303447 | Dec 2020 | 9 |
| 40 | Clarkston WWTP | Clarkston | 0.69 | 11,600 | WA0021113 | Apr 2021 | 0 |
| 41 | Stanwood STP | Stanwood | 0.67 | 6,233 | WAG994534 | Dec 2026 | 0 |
| 42 | Toppenish Wastewater Treatment Plant | Toppenish | 0.66 | 7,885 | WA0026123 | Dec 2030 | 29 |
| 43 | Sequim Water Reclamation Facility | Sequim | 0.64 | 10,170 | WAG994556 | Dec 2026 | 7 |
| 44 | Bainbridge Island WWTP | Bainbridge Island | 0.63 | 2,730 | WAG994558 | Dec 2026 | 1 |
| 45 | Sultan WWTP | Sultan | 0.59 | 5,329 | WA0023302 | Mar 2023 | 0 |
| 46 | Naches STP | Naches | 0.54 | 830 | WA0022586 | Sep 2019 | 40 |
| 47 | Winlock Wastewater Treatment Plant | Winlock | 0.54 | 1,534 | WA0021199 | Feb 2028 | 0 |
| 48 | Omak Wastewater Treatment Plant | Omak | 0.54 | 5,412 | WA0020940 | Dec 2020 | 0 |
| 49 | Kalama Wastewater Treatment Plant | Kalama | 0.49 | 2,707 | WA0020320 | Nov 2014 | 0 |
| 50 | Lighthouse Point Water Reclamation Facility | Blaine | 0.42 | 5,386 | WAG994581 | Dec 2026 | 0 |
Treatment technology for each plant is shown with a free Klarifi account. Create one →
Largest plants in Washington: questions
Sources and methodology
Flow, permit and violation figures come from the EPA's ICIS-NPDES discharge monitoring records. Population served comes from the Clean Watersheds Needs Survey 2022. Treatment structures are identified by Klarifi's review of aerial imagery, and capital improvement projects are extracted from published municipal budget documents. Violations are counted over the last three years of monitoring reports.
Data refreshed October 26, 2026.
Wastewater terms: WWTP · POTW · MGD · NPDES permit · Biological nutrient removal · Tertiary treatment · all terms
Cite this page
Journalists, researchers and educators are welcome to quote these figures. Please credit Klarifi as the source and link to this page.
Klarifi. “Largest wastewater treatment plants in Washington (2026).” Updated October 26, 2026. https://klarifi.io/facilities/washington/largestNeed a custom cut of the data for a state, a county or a list of plants? Email info@klarifi.io.