Lagoon systems: US wastewater treatment plants
5,488 plants in 48 states · 4,639 MGD reported by 3,776 plants · 13 planning upgrades
Klarifi identifies 5,488 publicly owned wastewater treatment plants with lagoon systems in the United States (4,639 MGD reported by 3,776 plants). The largest is Metropolitan Water Reclamation in Chicago, IL at 281.73 MGD. Each state below links to its own list with every plant.
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.
Lagoon systems by state
625 plants
471 plants
350 plants
339 plants
327 plants
284 plants
235 plants
196 plants
196 plants
190 plants
172 plants
163 plants
142 plants
142 plants
140 plants
109 plants
107 plants
99 plants
90 plants
78 plants
76 plants
76 plants
75 plants
73 plants
68 plants
67 plants
64 plants
63 plants
50 plants
47 plants
46 plants
44 plants
44 plants
32 plants
30 plants
29 plants
28 plants
26 plants
25 plants
12 plants
10 plants
10 plants
9 plants
9 plants
9 plants
5 plants
3 plants
3 plants
The largest lagoons in the US
With permit expiry dates: the largest lagoon systems in the US →
Other treatment technologies
Lagoon systems: 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 · Tertiary treatment · Significant industrial user · all terms