Lagoon systems: wastewater treatment plants in Montana
74 plants · 73% of the state · 29 MGD reported by 38 plants
Klarifi identifies 74 publicly owned wastewater treatment plants with lagoon systems in Montana, 73% of the 102 plants in the state (29 MGD reported by 38 plants). The largest is City Of Billings Wastewater Treatment Plant in Billings at 20.31 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 | City Of Billings Wastewater Treatment Plant | Billings | 20.31 | MTR000459 | Jan 2028 | 7 |
| 2 | Lewistown WWTP | Lewistown | 2.38 | MT0020044 | Aug 2017 | 32 |
| 3 | Deer Lodge WWTP | Deer Lodge | 0.72 | MT0022616 | Feb 2018 | 3 |
| 4 | Water Reclamation Facility | Livingston | 0.69 | MT0020435 | Oct 2021 | 0 |
| 5 | Anaconda-Deer Lodge County Wastewater Holding Ponds And Ip Beds Facility | Anaconda | 0.66 | MTX000231 | May 2024 | 0 |
| 6 | Red Lodge WWTF | Red Lodge | 0.57 | MT0020478 | Apr 2027 | 3 |
| 7 | City Of Belgrade- Belgrade Wastewater Treatment Plant | Belgrade | 0.5 | MTX000116 | Sep 2023 | 30 |
| 8 | Three Forks WWTF | Three Forks | 0.31 | MT0020401 | Apr 2027 | 5 |
| 9 | Choteau Sewage Treatment Ponds | Choteau | 0.25 | MT0020052 | Aug 2026 | 3 |
| 10 | Stevensville Waste Water Treatment Plant | Stevensville | 0.21 | MT0022713 | May 2024 | 5 |
| 11 | Ronan City WWTF | Ronan | 0.2 | MT0021474 | Jun 2027 | 523 |
| 12 | City Of Townsend Wastewater Treatment Plant | Townsend | 0.19 | MTG581020 | Dec 2022 | 0 |
| 13 | East Helena WWTF | East Helena | 0.18 | MT0022560 | Nov 2024 | 50 |
| 14 | City Of Cut Bank WWTF | Cut Bank | 0.17 | MT0020141 | Feb 2017 | 42 |
| 15 | City Of Conrad WWTF | Conrad | 0.16 | MT0020079 | Nov 2023 | 13 |
| 16 | River Rock Subdivision | Belgrade | 0.15 | MTX000147 | Sep 2022 | 0 |
| 17 | Town Of Columbus - Waste Water Treatment Facility | Columbus | 0.14 | MT0031801 | Oct 2028 | 24 |
| 18 | Ennis WWTP | Ennis | 0.13 | MT0030732 | Apr 2019 | 8 |
| 19 | Gardiner Wastewater Treatment Facility | Gardiner | 0.13 | MT0022705 | Sep 2027 | 119 |
| 20 | Montana State Hospital Sewage Treatment Lagoons | Warm Springs | 0.12 | MTG581004 | Dec 2022 | 4 |
| 21 | Hot Springs, Town Of | Hot Springs | 0.1 | MT0020591 | Jun 2023 | 31 |
| 22 | City Of Troy Wastewater Treatment Facility | Troy | 0.1 | MTG581038 | Dec 2022 | 30 |
| 23 | Park City County Wastewater Treatment Facility | Park City | 0.1 | MTG581007 | Dec 2022 | 15 |
| 24 | Town Of Plains WWTP | Plains | 0.1 | MT0030465 | Apr 2027 | 10 |
| 25 | Town Of Eureka WWTP | Eureka | 0.09 | MTG581032 | Dec 2022 | 0 |
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 September 11, 2026.