Oxidation ditches: wastewater treatment plants in Washington
36 plants · 24% of the state · 15 MGD reported by 35 plants
Klarifi identifies 36 publicly owned wastewater treatment plants with oxidation ditches in Washington, 24% of the 150 plants in the state (15 MGD reported by 35 plants). The largest is Lynden STP in Lynden at 1.56 MGD.
Oxidation ditches are ring-shaped activated sludge reactors with long retention times, common at plants between 0.5 and 10 MGD. They are recognisable from the air by their looped channels and rely on surface aerators or brush rotors, which are frequent replacement and energy-efficiency projects.
The largest oxidation ditch plants in Washington
| # | Facility | City | Avg flow (MGD) | NPDES permit | Expires | Violations (3y) |
|---|---|---|---|---|---|---|
| 1 | Lynden STP | Lynden | 1.56 | WA0022578 | Oct 2022 | 6 |
| 2 | Hoquiam STP | Hoquiam | 1.19 | WA0020915 | Oct 2030 | 0 |
| 3 | Washougal STP | Washougal | 1.09 | WA0037427 | Nov 2016 | 0 |
| 4 | Sedro Woolley City Of | Sedro-Woolley | 0.96 | WA0023752 | Dec 2023 | 1 |
| 5 | Grandview Wastewater Treatment Plant | Grandview | 0.93 | WA0052205 | Dec 2024 | 195 |
| 6 | Port Townsend WWTP | Port Townsend | 0.83 | WAG994538 | Dec 2026 | 0 |
| 7 | Stanwood STP | Stanwood | 0.67 | WAG994534 | Dec 2026 | 0 |
| 8 | Sequim Water Reclamation Facility | Sequim | 0.64 | WAG994556 | Dec 2026 | 7 |
| 9 | Sultan WWTP | Sultan | 0.59 | WA0023302 | Mar 2023 | 0 |
| 10 | Naches STP | Naches | 0.54 | WA0022586 | Sep 2019 | 40 |
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
Oxidation ditches in other states
Oxidation ditches 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: Oxidation ditch · Activated sludge · Biological nutrient removal · Inflow and infiltration · Sludge dewatering · all terms