Lagoon systems: US wastewater treatment plants
5,827 plants in 48 states · 4,269 MGD reported by 3,282 plants · 12 planning upgrades
Klarifi identifies 5,827 publicly owned wastewater treatment plants with lagoon systems in the United States (4,269 MGD reported by 3,282 plants). The largest is Back River Wastewater Treatment Plant in Baltimore, MD at 114.1 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
632 plants
481 plants
373 plants
341 plants
290 plants
288 plants
260 plants
238 plants
197 plants
195 plants
189 plants
182 plants
161 plants
150 plants
149 plants
147 plants
110 plants
107 plants
102 plants
92 plants
82 plants
82 plants
77 plants
74 plants
74 plants
74 plants
69 plants
64 plants
61 plants
52 plants
51 plants
43 plants
43 plants
42 plants
41 plants
35 plants
31 plants
30 plants
25 plants
20 plants
15 plants
12 plants
11 plants
10 plants
10 plants
8 plants
4 plants
3 plants
The largest lagoons 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 September 11, 2026.