The 47 largest wastewater treatment plants in Idaho
Ranked by average flow · 94 MGD combined · 100% of the state's reported flow
West Boise WWTP in Garden City is the largest publicly owned wastewater treatment plant in Idaho by reported average flow, treating about 62 million gallons per day. The 47 plants below treat 94 MGD between them, 100% of the flow reported by all 94 plants in the state, and serve about 704,678 people. Every plant links to its profile with permit, technology and compliance data.
| # | Facility | City | Avg flow (MGD) | Population | NPDES permit | Expires | Technology | Violations (3y) |
|---|---|---|---|---|---|---|---|---|
| 1 | West Boise WWTP | Garden City | 62 | 380,979 | ID0023981 | Jun 2027 | Clarifiers, Uncovered lagoons, Biogas silos | 9 |
| 2 | Pocatello City WWTP | Pocatello | 6.79 | 72,392 | ID0021784 | Aug 2017 | Clarifiers, Uncovered lagoons, Biogas silos | 8 |
| 3 | Jerome WWTP | Jerome | 3.59 | 12,641 | ID0020168 | Jun 2015 | Clarifiers, Biogas silos | 4 |
| 4 | Lewiston WWTP | Lewiston | 3.31 | 45,845 | ID0022055 | Jan 2021 | Clarifiers, Biogas silos, Biogas balloons | 0 |
| 5 | Oxbow WWTP | Shelley | 1.93 | 39,474 | ID0020133 | May 2019 | — | 0 |
| 6 | Emmett WWTP | Emmett | 1.76 | 7,647 | ID0020311 | Jan 2007 | Uncovered lagoons | 2 |
| 7 | Star WWTP | Star | 1.58 | — | ID0023591 | Apr 2020 | Aerated lagoons | 0 |
| 8 | Page WWTP | Smelterville | 1.36 | 17,263 | ID0021300 | Sep 2018 | Clarifiers, Aerated lagoons, Uncovered lagoons | 1 |
| 9 | Kuna North WWTP | Meridian | 1.23 | 29,000 | ID0028355 | May 2014 | — | 5 |
| 10 | Rigby WWTP | Rigby | 1.2 | 4,117 | ID0020010 | Dec 2021 | Clarifiers, Carrousels / oxidation ditches | 9 |
| 11 | Salmon Sewage Lagoon | Salmon | 1.02 | 3,119 | ID0020001 | May 2026 | Aerated lagoons, Uncovered lagoons | 0 |
| 12 | Ketchum Sun Valley WWTP | Ketchum | 1.02 | 5,338 | ID0020281 | Jul 2017 | Biogas balloons | 0 |
| 13 | Payette WWTP | Payette | 1 | 8,415 | ID0020672 | Oct 2019 | Clarifiers, Carrousels / oxidation ditches | 1 |
| 14 | Soda Springs WWTP | Soda Springs | 0.86 | 3,084 | ID0020818 | Jun 2025 | — | 1 |
| 15 | Moscow STP | Moscow | 0.85 | 25,702 | ID0021491 | Jan 2031 | Clarifiers, Carrousels / oxidation ditches | 0 |
| 16 | Heyburn WWTP | Heyburn | 0.71 | 3,592 | ID0020940 | Jan 2007 | Clarifiers, Carrousels / oxidation ditches | 17 |
| 17 | American Falls WWTP | American Falls | 0.53 | 5,243 | ID0020753 | Jul 2019 | — | 2 |
| 18 | Homedale WWTP | Homedale | 0.5 | 2,881 | ID0020427 | Sep 2018 | Covered lagoons, Uncovered lagoons | 18 |
| 19 | Driggs WWTP | Driggs | 0.39 | 4,169 | ID0020141 | Dec 2015 | Aerated lagoons | 163 |
| 20 | Grangeville WWTP | Grangeville | 0.39 | 3,187 | ID0020036 | Apr 2025 | Clarifiers, Carrousels / oxidation ditches | 3 |
| 21 | Buhl WWTP | Buhl | 0.29 | 4,558 | ID0020664 | Oct 2012 | Clarifiers, Uncovered lagoons, Carrousels / oxidation ditches | 2 |
| 22 | St Anthony Sewage Lagoon | Saint Anthony | 0.26 | 4,525 | ID0020401 | Nov 2014 | Aerated lagoons | 7 |
| 23 | Aberdeen WWTP | Aberdeen | 0.2 | 1,756 | ID0020176 | Feb 2021 | Clarifiers | 18 |
| 24 | Shoshone WWTP | Shoshone | 0.13 | 1,653 | ID0023728 | Apr 2024 | Uncovered lagoons | 22 |
| 25 | Council WWTP | Council | 0.12 | 867 | ID0020087 | Aug 2025 | Aerated lagoons, Uncovered lagoons | 3 |
| 26 | Kamiah Regional STP | Kamiah | 0.12 | 2,133 | ID0028002 | May 2028 | — | 11 |
| 27 | Kooskia WWTP | Kooskia | 0.11 | 685 | ID0021814 | Dec 2024 | Aerated lagoons | 21 |
| 28 | Parma WWTP | Parma | 0.11 | 2,096 | ID0021776 | Mar 2022 | Aerated lagoons, Uncovered lagoons | 82 |
| 29 | Grace WWTP | Grace | 0.08 | 920 | ID0023825 | Jun 2027 | Clarifiers, Carrousels / oxidation ditches | 1 |
| 30 | Wilder WWTP | Wilder | 0.08 | 1,597 | ID0020265 | May 2010 | Aerated lagoons, Uncovered lagoons | 8 |
| 31 | Horseshoe Bend WWTP | Horseshoe Bend | 0.07 | 715 | ID0021024 | Jul 2023 | Covered lagoons | 3 |
| 32 | Mullan WWTP | Mullan | 0.07 | 814 | ID0021296 | Nov 2025 | — | 30 |
| 33 | Hansen STP | Hansen | 0.07 | 1,346 | ID0022446 | Oct 2012 | Clarifiers, Uncovered lagoons, Carrousels / oxidation ditches | 7 |
| 34 | Mackay WWTP | Mackay | 0.07 | 608 | ID0023027 | May 2023 | Uncovered lagoons | 5 |
| 35 | Riggins WWTP | Riggins | 0.07 | 372 | ID0020931 | Aug 2017 | Clarifiers | 0 |
| 36 | Smelterville WWTP | Smelterville | 0.06 | 670 | ID0020117 | Sep 2018 | Clarifiers, Aerated lagoons, Uncovered lagoons | 21 |
| 37 | Dover WWTP | Dover | 0.05 | 752 | ID0027693 | Jul 2023 | — | 0 |
| 38 | Firth WWTP | Firth | 0.05 | 1,011 | ID0024988 | Mar 2018 | Uncovered lagoons | 0 |
| 39 | Pierce WWTP | Pierce | 0.05 | 501 | ID0020206 | May 2023 | — | 1 |
| 40 | Roberts WWTP | Roberts | 0.04 | 436 | ID0026913 | Apr 2009 | — | 56 |
| 41 | Troy WWTP | Troy | 0.04 | 760 | ID0023604 | Jul 2027 | Aerated lagoons, Uncovered lagoons | 26 |
| 42 | Culdesac WWTP | Culdesac | 0.03 | 413 | ID0024490 | Sep 2028 | Uncovered lagoons | 58 |
| 43 | Winchester WWTP | Winchester | 0.03 | 288 | ID0020184 | Sep 2025 | — | 75 |
| 44 | Moyie Springs, City Of - Moyie Springs WWTP | Moyie Springs | 0.03 | — | ID0030040 | Jun 2028 | — | 231 |
| 45 | Juliaetta WWTP | Juliaetta | 0.02 | 632 | ID0023761 | Jul 2023 | — | 10 |
| 46 | Ahsahka Wastewater Treatment Plant | Orofino | 0.02 | 194 | ID0025224 | Dec 2026 | — | 0 |
| 47 | Kendrick WWTP | Kendrick | 0.01 | 288 | ID0024554 | Aug 2023 | Aerated lagoons, Uncovered lagoons | 32 |
Largest plants in Idaho: 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.