Wastewater Treatment Plants in Washington
125 facilities · 70 MGD combined flow
Klarifi tracks 125 publicly owned wastewater treatment plants in Washington with a combined average flow of 70 MGD, serving about 3,630,475 residents. 8 plants treat more than 1 MGD and 2 more than 10 MGD; 52 use lagoon systems and 21 have digesters or biogas infrastructure. The largest is Riverside Park Water Reclamation Facility in Spokane at 31.72 MGD. 14 NPDES permits expire this year and 2 next year.
Every plant below links to a profile with average and design flow, treatment technology, the NPDES permit and its expiry, a three-year compliance snapshot and the capital projects planned in its city.
125 plant locations in Washington; dot size follows average flow.
NPDES permits and renewals in Washington
Every plant discharges under an NPDES permit that is renewed every five years, usually with tighter limits. The renewal year is the year a plant budgets for upgrades.
Largest wastewater treatment plants in Washington
| Facility | City | Avg flow (MGD) | Design (MGD) | NPDES permit | Expires | Technology | Violations (3y) |
|---|---|---|---|---|---|---|---|
| Riverside Park Water Reclamation Facility | Spokane | 31.72 | — | WA0024473 | Aug 2027 | Clarifiers, Biogas silos | 4 |
| Chambers Creek WWTP | University Place | 17.07 | — | WAR000727 | Dec 2029 | Clarifiers, Biogas silos | 0 |
| Richland City Wastewater Lab | Richland | 6.31 | 22.8 | WA0020419 | Sep 2021 | Clarifiers, Biogas silos | 0 |
| Camas STP | Camas | 2.62 | — | WA0020249 | Jul 2030 | Clarifiers, Uncovered lagoons, Biogas silos | 19 |
| Pullman City Muni SW | Pullman | 2.13 | 21.5 | WA0044652 | May 2019 | — | 1 |
| Sunnyside STP | Sunnyside | 1.31 | 22.4 | WA0020991 | Jan 2020 | Clarifiers, Uncovered lagoons, Biogas silos | 1 |
| Othello STP | Othello | 1.27 | 10 | WA0022357 | Sep 2025 | Uncovered lagoons | 85 |
| Port Of Sunnyside Industrial WWTF | Sunnyside | 1.17 | 8.1 | WA0052426 | Aug 2019 | Aerated lagoons, Covered lagoons | 79 |
| Clarkston WWTP | Clarkston | 0.71 | 15.6 | WA0021113 | Apr 2021 | Clarifiers | 0 |
| Toppenish Wastewater Treatment Plant | Toppenish | 0.67 | 1.67 | WA0026123 | Dec 2030 | Clarifiers, Biogas silos | 29 |
| Colville WWTP | Colville | 0.62 | 73.1 | WA0022616 | Jul 2020 | Aerated lagoons, Uncovered lagoons | 0 |
| Omak Wastewater Treatment Plant | Omak | 0.53 | 3.78 | WA0020940 | Dec 2020 | Clarifiers, Carrousels / oxidation ditches | 0 |
| Cashmere STP Facility | Cashmere | 0.39 | 2.03 | WA0023183 | May 2025 | Clarifiers, Uncovered lagoons | 30 |
| Medical Lake WWTP | Medical Lake | 0.27 | 24.6 | WA0021148 | Jul 2019 | Clarifiers, Carrousels / oxidation ditches | 13 |
| Montesano Sewage Treatment Plant | Montesano | 0.24 | — | WA0024660 | Nov 2029 | Clarifiers, Aerated lagoons, Uncovered lagoons | 0 |
| Benton City STP | Benton City | 0.22 | — | WA0051349 | Aug 2019 | Aerated lagoons, Uncovered lagoons | 0 |
| Dayton STP | Dayton | 0.19 | — | WA0020729 | Sep 2016 | Clarifiers | 2 |
| Chewelah WWTP | Chewelah | 0.19 | — | WA0023604 | Nov 2016 | Clarifiers, Carrousels / oxidation ditches | 1 |
| Brewster STP | Brewster | 0.18 | 1.22 | WA0021008 | Jan 2021 | Clarifiers, Carrousels / oxidation ditches | 0 |
| Tulalip Wastewater Treatment Plant | Marysville | 0.17 | — | WA0024805 | Oct 2014 | Clarifiers, Carrousels / oxidation ditches | 1 |
| Coulee Dam WWTP | Coulee Dam | 0.16 | 0.5 | WA0020281 | Nov 2022 | — | 0 |
| Suquamish WWTP | Suquamish | 0.16 | 0.4 | WA0023256 | May 2013 | — | 0 |
| Davenport WWTP | Davenport | 0.15 | — | WA0045578 | Jul 2019 | — | 3 |
| Okanogan WWTP | Okanogan | 0.15 | 3.78 | WA0022365 | Sep 2030 | Clarifiers | 1 |
| Winlock Wastewater Treatment Plant | Winlock | 0.15 | — | WA0021199 | Feb 2028 | Clarifiers | 0 |
Washington plants by treatment technology
All 125 plants in Washington
Wastewater treatment 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 September 11, 2026.