Lagoon systems: wastewater treatment plants in Washington
47 plants · 31% of the state · 32 MGD reported by 44 plants
Klarifi identifies 47 publicly owned wastewater treatment plants with lagoon systems in Washington, 31% of the 150 plants in the state (32 MGD reported by 44 plants). The largest is Yakima Regional Wastewater Facility in Yakima at 8.67 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 | Yakima Regional Wastewater Facility | Yakima | 8.67 | WA0024023 | Mar 2029 | 2 |
| 2 | Ellensburg WWTP | Ellensburg | 3.59 | WA0024341 | Dec 2023 | 0 |
| 3 | Ferndale STP | Ferndale | 2.26 | WAR308713 | Dec 2025 | 1 |
| 4 | Port Of Sunnyside Industrial WWTF | Sunnyside | 1.64 | WA0052426 | Aug 2019 | 79 |
| 5 | Lynden STP | Lynden | 1.56 | WA0022578 | Oct 2022 | 6 |
| 6 | Sunnyside STP | Sunnyside | 1.34 | WA0020991 | Jan 2020 | 1 |
| 7 | Hoquiam STP | Hoquiam | 1.19 | WA0020915 | Oct 2030 | 0 |
| 8 | Birch Bay Water And Sewer District WWTP | Blaine | 1.17 | WAG994542 | Dec 2026 | 0 |
| 9 | Cle Elum WWTP | Cle Elum | 1.13 | WA0021938 | Sep 2024 | 0 |
| 10 | Washougal STP | Washougal | 1.09 | WA0037427 | Nov 2016 | 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 October 26, 2026.
Wastewater terms: WWTP · POTW · Biosolids · Significant industrial user · Filter press · all terms