Lagoon systems: wastewater treatment plants in Montana
73 plants · 67% of the state · 30 MGD reported by 50 plants
Klarifi identifies 73 publicly owned wastewater treatment plants with lagoon systems in Montana, 67% of the 109 plants in the state (30 MGD reported by 50 plants). The largest is Town Of Lavina WWTF in Lavina at 18.51 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 Montana
| # | Facility | City | Avg flow (MGD) | NPDES permit | Expires | Violations (3y) |
|---|---|---|---|---|---|---|
| 1 | Town Of Lavina WWTF | Lavina | 18.51 | MTG580013 | Dec 2027 | 1 |
| 2 | Lewistown WWTP | Lewistown | 1.82 | MT0020044 | Aug 2017 | 32 |
| 3 | Deer Lodge WWTP | Deer Lodge | 1.13 | MT0022616 | Feb 2018 | 3 |
| 4 | Wolf Point, City Of | Wolf Point | 1.08 | MT0030571 | Jun 2023 | 13 |
| 5 | Town Of Belt - WWTP | Belt | 0.82 | MT0021571 | Jun 2022 | 92 |
| 6 | Stevensville Waste Water Treatment Plant | Stevensville | 0.47 | MT0022713 | May 2024 | 5 |
| 7 | Choteau Sewage Treatment Ponds | Choteau | 0.47 | MT0020052 | Aug 2026 | 3 |
| 8 | Anaconda-Deer Lodge County Wastewater Holding Ponds And Ip Beds Facility | Anaconda | 0.44 | MTX000231 | May 2024 | 0 |
| 9 | Ennis WWTP | Ennis | 0.44 | MT0030732 | Apr 2019 | 8 |
| 10 | Three Forks WWTF | Three Forks | 0.44 | MT0020401 | Apr 2027 | 5 |
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 Montana
Lagoon systems in other states
Lagoon systems in Montana: 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 · Inflow and infiltration · Sludge dewatering · all terms