What is the largest wastewater treatment plant in Montana?

City Of Billings Wastewater Treatment Plant in Billings, treating an average of 20.31 MGD under NPDES permit MTR000459. City Of Missoula WWTF No Exposure in Missoula is second at 7.52 MGD.

How much wastewater do Montana’s largest plants treat?

The 50 largest plants treat 60 MGD combined, 99% of the flow reported by all 102 publicly owned plants in Montana.

How is plant size measured?

By reported average daily flow in million gallons per day (MGD) from EPA discharge monitoring reports, the standard measure of a plant’s size. Design capacity, the flow a plant is permitted to treat, is usually higher and is shown on each profile.

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The 50 largest wastewater treatment plants in Montana

Name: Largest wastewater treatment plants in Montana
Creator: Klarifi
License: https://klarifi.io/terms-of-service

Ranked by average flow · 60 MGD combined · 99% of the state's reported flow

City Of Billings Wastewater Treatment Plant in Billings is the largest publicly owned wastewater treatment plant in Montana by reported average flow, treating about 20.31 million gallons per day. The 50 plants below treat 60 MGD between them, 99% of the flow reported by all 102 plants in the state, and serve about 387,331 people. Every plant links to its profile with permit, technology and compliance data.

All plants in Montana →The 100 largest US plants →Montana NPDES permits →

The 50 largest publicly owned wastewater treatment plants in Montana by average flow (MGD), with population served, NPDES permit, expiry, treatment technology and three-year violation count.
#	Facility	City	Avg flow (MGD)	Population	NPDES permit	Expires	Technology	Violations (3y)
1	City Of Billings Wastewater Treatment Plant	Billings	20.31	117,116	MTR000459	Jan 2028	Clarifiers, Uncovered lagoons, Biogas silos	7
2	City Of Missoula WWTF No Exposure	Missoula	7.52	47,463	MT0022594	Apr 2020	Clarifiers, Biogas silos	7
3	City Of Bozeman WWTP	Bozeman	7.35	38,264	MT0022608	May 2017	Clarifiers, Biogas silos	0
4	City Of Kalispell WWTP	Kalispell	3.21	30,040	MT0021938	Jul 2020	Clarifiers, Biogas silos	11
5	City Of Helena WWTP	Helena	3.14	32,091	MT0022641	Sep 2017	Clarifiers, Biogas silos	37
6	Lewistown WWTP	Lewistown	2.38	7,675	MT0020044	Aug 2017	Clarifiers, Uncovered lagoons, Carrousels / oxidation ditches, Biogas silos	32
7	Havre WWTF	Havre	1.64	9,762	MT0022535	Aug 2026	Clarifiers	0
8	Miles City WWTP	Miles City	1.11	9,074	MT0020001	Sep 2023	Clarifiers, Carrousels / oxidation ditches	3
9	City Of Glendive Water Resource & Recovery Facility	Glendive	0.84	5,185	MT0021628	May 2027	—	3
10	City Of Hamilton Wastewater Treatment Plant	Hamilton	0.79	4,784	MT0020028	Dec 2026	Carrousels / oxidation ditches	0
11	Deer Lodge WWTP	Deer Lodge	0.72	2,900	MT0022616	Feb 2018	Clarifiers, Uncovered lagoons	3
12	Water Reclamation Facility	Livingston	0.69	8,061	MT0020435	Oct 2021	Aerated lagoons, Biogas balloons	0
13	City Of Glasgow Parks Maintenance Bldg	Glasgow	0.66	3,826	MT0021211	Nov 2026	—	21
14	Anaconda-Deer Lodge County Wastewater Holding Ponds And Ip Beds Facility	Anaconda	0.66	—	MTX000231	May 2024	Uncovered lagoons	0
15	Town Of Poplar WTP	Poplar	0.59	3,289	MT0030597	Jun 2025	Clarifiers, Carrousels / oxidation ditches	176
16	Red Lodge WWTF	Red Lodge	0.57	2,472	MT0020478	Apr 2027	Aerated lagoons	3
17	City Of Shelby Wastewater Lagoons	Shelby	0.53	3,728	MT0031488	Feb 2025	—	20
18	City Of Polson WWTP	Polson	0.5	5,148	MT0020559	Aug 2024	—	0
19	City Of Belgrade- Belgrade Wastewater Treatment Plant	Belgrade	0.5	10,460	MTX000116	Sep 2023	Aerated lagoons, Uncovered lagoons, Carrousels / oxidation ditches, Biogas silos	30
20	Northern Cheyenne Utility Commission Wastewater Treatment Plant	Lame Deer	0.47	2,522	MT0029360	Jun 2029	—	737
21	Columbia Falls WWTP	Columbia Falls	0.43	3,643	MT0020036	May 2030	—	0
22	Manhattan WWTF	Manhattan	0.32	3,366	MT0021857	Aug 2015	—	20
23	Bigfork County Water And Sewer	Bigfork	0.32	4,683	MT0020397	Apr 2023	—	1
24	Three Forks WWTF	Three Forks	0.31	2,049	MT0020401	Apr 2027	Covered lagoons, Uncovered lagoons	5
25	City Of Laurel, Water Treatment Plant	Laurel	0.29	—	MT0030261	Oct 2027	—	0
26	Great Falls WTP	Great Falls	0.27	—	MT0000442	May 2029	—	0
27	Choteau Sewage Treatment Ponds	Choteau	0.25	1,952	MT0020052	Aug 2026	Aerated lagoons, Carrousels / oxidation ditches, Biogas silos	3
28	Libby WWTP	Libby	0.23	2,879	MTR000752	Jan 2028	Carrousels / oxidation ditches, Biogas balloons	34
29	Stevensville Waste Water Treatment Plant	Stevensville	0.21	1,983	MT0022713	May 2024	Uncovered lagoons, Biogas balloons	5
30	Ronan City WWTF	Ronan	0.2	1,696	MT0021474	Jun 2027	Aerated lagoons, Uncovered lagoons	523
31	Forsyth WWTP	Forsyth	0.19	1,948	MT0021288	Jan 2026	—	0
32	City Of Townsend Wastewater Treatment Plant	Townsend	0.19	—	MTG581020	Dec 2022	Aerated lagoons, Covered lagoons	0
33	City Of Bozeman - Lyman Creek Reservoir	Bozeman	0.19	—	MT0031631	Nov 2029	—	0
34	East Helena WWTF	East Helena	0.18	1,944	MT0022560	Nov 2024	Aerated lagoons, Uncovered lagoons	50
35	City Of Cut Bank WWTF	Cut Bank	0.17	3,394	MT0020141	Feb 2017	Clarifiers, Uncovered lagoons	42
36	City Of Conrad WWTF	Conrad	0.16	2,318	MT0020079	Nov 2023	Uncovered lagoons	13
37	River Rock Subdivision	Belgrade	0.15	3,399	MTX000147	Sep 2022	Aerated lagoons	0
38	Town Of Columbus - Waste Water Treatment Facility	Columbus	0.14	1,857	MT0031801	Oct 2028	Aerated lagoons	24
39	Ennis WWTP	Ennis	0.13	1,371	MT0030732	Apr 2019	Aerated lagoons, Uncovered lagoons	8
40	Gardiner Wastewater Treatment Facility	Gardiner	0.13	884	MT0022705	Sep 2027	Aerated lagoons, Uncovered lagoons	119
41	Bozeman Water Treatment Plant	Bozeman	0.13	—	MTX000224	Mar 2027	—	4
42	Montana State Hospital Sewage Treatment Lagoons	Warm Springs	0.12	—	MTG581004	Dec 2022	Covered lagoons	4
43	City Of Chinook WTP	Chinook	0.11	—	MT0030473	Mar 2024	—	0
44	Hot Springs, Town Of	Hot Springs	0.1	557	MT0020591	Jun 2023	Aerated lagoons	31
45	City Of Troy Wastewater Treatment Facility	Troy	0.1	—	MTG581038	Dec 2022	Aerated lagoons, Uncovered lagoons	30
46	Park City County Wastewater Treatment Facility	Park City	0.1	1,187	MTG581007	Dec 2022	Aerated lagoons	15
47	Town Of Plains WWTP	Plains	0.1	—	MT0030465	Apr 2027	Aerated lagoons, Uncovered lagoons	10
48	Town Of Eureka WWTP	Eureka	0.09	—	MTG581032	Dec 2022	Aerated lagoons, Uncovered lagoons	0
49	City Of Boulder WWTF	Boulder	0.09	1,297	MT0023078	Nov 2025	Uncovered lagoons	68
50	Town Of Philipsburg WWTP	Philipsburg	0.07	1,064	MT0031500	Sep 2020	Aerated lagoons, Uncovered lagoons	8

Largest plants 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.

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