Lagoon systems: wastewater treatment plants in Idaho
64 plants · 68% of the state · 73 MGD reported by 47 plants
Klarifi identifies 64 publicly owned wastewater treatment plants with lagoon systems in Idaho, 68% of the 94 plants in the state (73 MGD reported by 47 plants). The largest is West Boise WWTP in Garden City at 50.91 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 Idaho
| # | Facility | City | Avg flow (MGD) | NPDES permit | Expires | Violations (3y) |
|---|---|---|---|---|---|---|
| 1 | West Boise WWTP | Garden City | 50.91 | ID0023981 | Jun 2027 | 10 |
| 2 | Pocatello City WWTP | Pocatello | 6.38 | ID0021784 | Aug 2017 | 8 |
| 3 | St Maries WWTP | Saint Maries | 2.25 | ID0022799 | Oct 2025 | 36 |
| 4 | Page WWTP | Smelterville | 2.04 | ID0021300 | Sep 2018 | 1 |
| 5 | Emmett WWTP | Emmett | 1.68 | ID0020311 | Jan 2007 | 2 |
| 6 | Star WWTP | Star | 1.52 | ID0023591 | Apr 2020 | 0 |
| 7 | Salmon Sewage Lagoon | Salmon | 1.09 | ID0020001 | May 2026 | 2 |
| 8 | New Meadows WWTP | New Meadows | 0.92 | ID0023159 | Jul 2018 | 13 |
| 9 | Montpelier WWTF | Montpelier | 0.49 | ID0025585 | Feb 2025 | 5 |
| 10 | Kootenai-Ponderay WWTP | Sandpoint | 0.38 | ID0021229 | Aug 2023 | 2 |
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 Idaho
Lagoon systems in other states
Lagoon systems 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 October 26, 2026.
Wastewater terms: WWTP · POTW · Biosolids · Equalization tank · Centrate · all terms