The 44 largest wastewater treatment plants in New Hampshire
Ranked by average flow · 39 MGD combined · 100% of the state's reported flow
Nashua WWTP in Nashua is the largest publicly owned wastewater treatment plant in New Hampshire by reported average flow, treating about 9.2 million gallons per day. The 44 plants below treat 39 MGD between them, 100% of the flow reported by all 46 plants in the state, and serve about 494,815 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 | Nashua WWTP | Nashua | 9.2 | 99,918 | NH0100170 | May 2020 | Clarifiers, Biogas silos, Biogas balloons | 87 |
| 2 | Wrbp Franklin Wastewater Treatment Plant | Franklin | 3.53 | 78,331 | NH0100960 | Jun 2029 | Clarifiers, Biogas silos, Biogas balloons | 22 |
| 3 | Concord Wastewater Treatment Facility | Concord | 3.2 | 43,137 | NH0100901 | Aug 2027 | Clarifiers, Aerated lagoons | 9 |
| 4 | Pierce Island Wastewater Treatment Plant | Portsmouth | 2.38 | 22,900 | NH0100234 | Nov 2028 | Clarifiers, Biogas balloons | 47 |
| 5 | Rochester Wastewater Treatment Facility | Rochester | 2.35 | 17,500 | NH0100668 | Jun 2028 | Clarifiers, Aerated lagoons, Uncovered lagoons | 3 |
| 6 | Leavitt E. Magrath Wastewater Treatment Facility | Hampton | 2.08 | 17,009 | NH0100625 | Feb 2026 | Clarifiers | 27 |
| 7 | Huckleberry Hill Wastewater Treatment Facility | Dover | 1.91 | 31,345 | NHG58A003 | Jan 2026 | Clarifiers | 30 |
| 8 | Merrimack WWTF | Merrimack | 1.37 | 15,850 | NHR053094 | Feb 2026 | Clarifiers, Biogas silos | 9 |
| 9 | Keene Wastewater Treatment Facility | Keene | 1.26 | 25,380 | NH0100790 | Nov 2026 | Clarifiers | 8 |
| 10 | Claremont Wastewater Treatment Facility | Claremont | 1.2 | 13,595 | NH0101257 | Sep 2021 | Clarifiers, Biogas silos | 0 |
| 11 | Derry, Town Of, Wastewater Treatment Plant | Derry | 1.17 | 12,300 | NH0100056 | Feb 2016 | Aerated lagoons, Uncovered lagoons | 17 |
| 12 | Berlin Pollution Control Facility | Berlin | 1.12 | 9,425 | NH0100013 | Jun 2029 | Biogas balloons | 5 |
| 13 | Exeter Wastewater Treatment Facility | Exeter | 1.1 | 12,736 | NHG58A004 | Jan 2026 | Clarifiers, Aerated lagoons | 18 |
| 14 | Hanover Wastewater Treatment Facility | Hanover | 0.99 | — | NH0100099 | Jan 2021 | Clarifiers, Biogas silos, Biogas balloons | 45 |
| 15 | Lebanon Wastewater Treatment Facility | Lebanon | 0.89 | 14,400 | NHR053176 | Feb 2026 | Clarifiers, Biogas balloons | 2 |
| 16 | Durham Wastewater Treatment Facility | Durham | 0.83 | 14,590 | NHG58A005 | Jan 2026 | — | 8 |
| 17 | Milford Wastewater Treatment Facility | Milford | 0.78 | 12,051 | NH0100471 | Oct 2025 | Clarifiers, Biogas balloons | 191 |
| 18 | Hooksett Wastewater Treatment Facility | Hooksett | 0.68 | 6,000 | NH0100129 | Sep 2018 | Clarifiers, Uncovered lagoons | 1 |
| 19 | Suncook Wastewater Treatment Facility | Allenstown | 0.45 | 9,217 | NH0101390 | Jan 2027 | Clarifiers | 9 |
| 20 | Newmarket, Town Of - WPCF | Newmarket | 0.35 | — | NHG580013 | Nov 2026 | Clarifiers, Biogas silos, Biogas balloons | 17 |
| 21 | Colebrook Wastewater Treatment Plant | Colebrook | 0.3 | 1,062 | NHG580315 | Nov 2026 | — | 33 |
| 22 | Littleton Wastewater Treatment Plant | Littleton | 0.27 | 3,899 | NH0100153 | Oct 2025 | Carrousels / oxidation ditches, Biogas silos | 17 |
| 23 | Lancaster Wastewater Treatment Facility | Lancaster | 0.23 | 3,350 | NH0100145 | Sep 2025 | Aerated lagoons, Uncovered lagoons | 37 |
| 24 | Sunapee, Town Of-WWTP | Sunapee | 0.22 | — | NHG580017 | Nov 2026 | Carrousels / oxidation ditches, Biogas balloons | 27 |
| 25 | Town Of Bristol WWTF | Bristol | 0.21 | 3,072 | NHG580021 | Nov 2026 | Carrousels / oxidation ditches | 0 |
| 26 | Penacook Wastewater Treatment Plant | Concord | 0.18 | 9,100 | NH0100331 | Jul 2025 | Aerated lagoons, Uncovered lagoons | 5 |
| 27 | Woodsville Wastewater Treatment Facility | Haverhill | 0.18 | 814 | NHG580978 | Nov 2026 | — | 26 |
| 28 | Waterville Valley Wastewater Treatment Plant | Waterville Valley | 0.14 | 1,140 | NH0100781 | Mar 2027 | Aerated lagoons | 6 |
| 29 | Epping Wasterwater Treatment Facility | Epping | 0.13 | 1,800 | NHG580012 | Nov 2026 | Uncovered lagoons | 122 |
| 30 | Henniker Wastewater Treatment Facility | Henniker | 0.11 | 1,850 | NHG580018 | Nov 2026 | Biogas balloons | 75 |
| 31 | Winchester Wastewater Treatment Plant | Winchester | 0.11 | 2,200 | NHG580404 | Nov 2026 | — | 1 |
| 32 | Newington Wastewater Treatment Plant | Portsmouth (newington) | 0.1 | 105 | NHG581141 | Nov 2026 | — | 82 |
| 33 | Whitefield Wastewater Treatment Facility | Whitefield | 0.09 | 431 | NHG580002 | Nov 2026 | Uncovered lagoons | 21 |
| 34 | Woodstock WWTF | Woodstock | 0.08 | 952 | NHG580020 | Nov 2026 | Clarifiers, Carrousels / oxidation ditches | 29 |
| 35 | Hinsdale Wastewater Treatment Plant | Hinsdale | 0.07 | 3,204 | NHG580016 | Nov 2026 | Carrousels / oxidation ditches, Biogas silos | 6 |
| 36 | W Swanzey WWTF | Swanzey | 0.05 | — | NHG580015 | Nov 2026 | Aerated lagoons | 208 |
| 37 | Charlestown Wastewater Treatment Plant | Charlestown | 0.03 | 3,025 | NH0100765 | Sep 2025 | Uncovered lagoons | 21 |
| 38 | Town Of Hopkinton WWTP | Hopkington | 0.03 | 628 | NHG580579 | Nov 2026 | — | 48 |
| 39 | Northumberland Village Water Pollution Control Facility | Groveton | 0.02 | 334 | NHG581206 | Nov 2026 | Aerated lagoons, Uncovered lagoons | 22 |
| 40 | Stratford Village System | North Stratford | 0.02 | 146 | NHG580536 | Nov 2026 | — | 8 |
| 41 | Lisbon Wastewater Treatment Facility | Lisbon | 0.02 | 1,669 | NHG580421 | Nov 2026 | Aerated lagoons | 25 |
| 42 | Cheshire County Maplewood Nursing Home | Westmoreland | 0.01 | 314 | NHG580391 | Nov 2026 | — | 14 |
| 43 | Wallis Sands State Park | Rye | 0.0021 | — | NHG580014 | Nov 2026 | — | 43 |
| 44 | Lancaster Grange Wastewater Treatment Plant | Lancaster | 0.0011 | 36 | NHG581249 | Nov 2026 | — | 3 |
Largest plants in New Hampshire: 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.