Lagoon systems: wastewater treatment plants in Washington
52 plants · 42% of the state · 8 MGD reported by 16 plants
Klarifi identifies 52 publicly owned wastewater treatment plants with lagoon systems in Washington, 42% of the 125 plants in the state (8 MGD reported by 16 plants). The largest is Camas STP in Camas at 2.62 MGD.
Lagoon systems treat wastewater in large earthen ponds over weeks of retention time, with or without mechanical aeration. They serve thousands of small towns because they are cheap to run, but they struggle with ammonia and nutrient limits, fill with sludge over decades and are the plants most often converted or upgraded when a permit is renewed.
The largest lagoons in Washington
| # | Facility | City | Avg flow (MGD) | NPDES permit | Expires | Violations (3y) |
|---|---|---|---|---|---|---|
| 1 | Camas STP | Camas | 2.62 | WA0020249 | Jul 2030 | 19 |
| 2 | Sunnyside STP | Sunnyside | 1.31 | WA0020991 | Jan 2020 | 1 |
| 3 | Othello STP | Othello | 1.27 | WA0022357 | Sep 2025 | 85 |
| 4 | Port Of Sunnyside Industrial WWTF | Sunnyside | 1.17 | WA0052426 | Aug 2019 | 79 |
| 5 | Colville WWTP | Colville | 0.62 | WA0022616 | Jul 2020 | 0 |
| 6 | Cashmere STP Facility | Cashmere | 0.39 | WA0023183 | May 2025 | 30 |
| 7 | Montesano Sewage Treatment Plant | Montesano | 0.24 | WA0024660 | Nov 2029 | 0 |
| 8 | Benton City STP | Benton City | 0.22 | WA0051349 | Aug 2019 | 0 |
| 9 | Tonasket POTW | Tonasket | 0.1 | WA0052337 | Apr 2031 | 7 |
| 10 | Pacific Beach STP | Pacific Beach | 0.09 | WA0037095 | Aug 2030 | 37 |
| 11 | Asotin STP | Clarkston | 0.08 | WA0020818 | Aug 2023 | 1 |
| 12 | Winthrop Wastewater Treatment Plant | Winthrop | 0.06 | WA0020885 | Jul 2020 | 40 |
| 13 | Harrah WWTP | Harrah | 0.03 | WA0022705 | May 2030 | 2 |
| 14 | Metaline STP | Metaline | 0.02 | WA0020699 | Dec 2023 | 10 |
| 15 | Wishram Wastewater Treatment Plant | Wishram | 0.01 | WA0051292 | Aug 2019 | 0 |
| 16 | Rockford WWTP | Rockford | 0.00029 | WA0044831 | Nov 2018 | 17 |
| 17 | Big Lake WWTP | Mount Vernon | — | WA0030597 | Feb 2026 | 0 |
| 18 | Birch Bay Water And Sewer District WWTP | Blaine | — | WAG994542 | Dec 2026 | 0 |
| 19 | Buena WWTF | Zillah | — | WA0052132 | Jan 2024 | 3 |
| 20 | Carbonado STP | Carbonado | — | WA0020834 | Nov 2022 | 25 |
| 21 | Clallam Bay Correction Center STP | Clallam Bay | — | WAG994541 | Dec 2026 | 0 |
| 22 | Cle Elum WWTP | Cle Elum | — | WA0021938 | Sep 2024 | 0 |
| 23 | Colfax Wastewater Treatment Plant | Colfax | — | WA0020613 | Oct 2015 | 4 |
| 24 | Colton STP | Colton | — | WA0023175 | May 2020 | 69 |
| 25 | Eatonville STP | Eatonville | — | WA0037231 | Apr 2014 | 0 |
Contacts and project signals for every plant on this list
Klarifi adds verified decision-maker contacts, permit limits, discharge monitoring data and the capital projects planned in each city, filterable by technology, size and compliance record.
Other technologies in Washington
Lagoon systems in other states
Lagoon systems 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.