Oxidation ditches: US wastewater treatment plants
1,508 plants in 48 states · 2,454 MGD reported by 1,398 plants
Klarifi identifies 1,508 publicly owned wastewater treatment plants with oxidation ditches in the United States (2,454 MGD reported by 1,398 plants). The largest is Northeast WPCP in Philadelphia, PA at 153.22 MGD. Each state below links to its own list with every plant.
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.
Oxidation ditches by state
261 plants
112 plants
95 plants
64 plants
64 plants
63 plants
58 plants
54 plants
46 plants
40 plants
37 plants
37 plants
36 plants
36 plants
33 plants
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28 plants
25 plants
25 plants
21 plants
21 plants
21 plants
17 plants
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12 plants
9 plants
8 plants
8 plants
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7 plants
6 plants
6 plants
5 plants
5 plants
3 plants
3 plants
2 plants
2 plants
1 plants
1 plants
1 plants
The largest oxidation ditch plants in the US
Other treatment technologies
Oxidation ditches: 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 · Lift station · Inflow and infiltration · all terms