The 46 largest wastewater treatment plants in New Hampshire
Ranked by average flow · 49 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 9.77 million gallons per day. The 46 plants below treat 49 MGD between them, 100% of the flow reported by all 47 plants in the state, and serve about 498,193 people. Every plant links to its profile with permit and compliance data. Treatment technology is shown with a free Klarifi account.
| # | Facility | City | Avg flow (MGD) | Population | NPDES permit | Expires | Violations (3y) |
|---|---|---|---|---|---|---|---|
| 1 | Nashua WWTP | Nashua | 9.77 | 99,918 | NH0100170 | May 2020 | 78 |
| 2 | Wrbp Franklin Wastewater Treatment Plant | Franklin | 5.19 | 78,331 | NH0100960 | Jun 2029 | 26 |
| 3 | Concord Wastewater Treatment Facility | Concord | 3.5 | 43,137 | NH0100901 | Aug 2027 | 8 |
| 4 | Pierce Island Wastewater Treatment Plant | Portsmouth | 3.28 | 22,900 | NH0100234 | Nov 2028 | 47 |
| 5 | Rochester Wastewater Treatment Facility | Rochester | 2.88 | 17,500 | NH0100668 | Jun 2028 | 3 |
| 6 | Leavitt E. Magrath Wastewater Treatment Facility | Hampton | 2.24 | 17,009 | NH0100625 | Feb 2026 | 29 |
| 7 | Huckleberry Hill Wastewater Treatment Facility | Dover | 2.18 | 31,345 | NHG58A003 | Jan 2026 | 30 |
| 8 | Keene Wastewater Treatment Facility | Keene | 1.87 | 25,380 | NH0100790 | Nov 2026 | 8 |
| 9 | Berlin Pollution Control Facility | Berlin | 1.75 | 9,425 | NH0100013 | Jun 2029 | 5 |
| 10 | Derry, Town Of, Wastewater Treatment Plant | Derry | 1.46 | 12,300 | NH0100056 | Feb 2016 | 16 |
| 11 | Exeter Wastewater Treatment Facility | Exeter | 1.45 | 12,736 | NHG58A004 | Jan 2026 | 18 |
| 12 | Claremont Wastewater Treatment Facility | Claremont | 1.39 | 13,595 | NH0101257 | Sep 2021 | 0 |
| 13 | Merrimack WWTF | Merrimack | 1.29 | 15,850 | NHR053094 | Feb 2026 | 10 |
| 14 | Lebanon Wastewater Treatment Facility | Lebanon | 1.15 | 14,400 | NHR053176 | Feb 2026 | 2 |
| 15 | Milford Wastewater Treatment Facility | Milford | 1.14 | 12,051 | NH0100471 | Oct 2025 | 172 |
| 16 | Hanover Wastewater Treatment Facility | Hanover | 1.11 | — | NH0100099 | Jan 2021 | 46 |
| 17 | Hooksett Wastewater Treatment Facility | Hooksett | 0.82 | 6,000 | NH0100129 | Sep 2018 | 1 |
| 18 | Durham Wastewater Treatment Facility | Durham | 0.72 | 14,590 | NHG58A005 | Jan 2026 | 8 |
| 19 | Lancaster Wastewater Treatment Facility | Lancaster | 0.68 | 3,350 | NH0100145 | Sep 2025 | 69 |
| 20 | Littleton Wastewater Treatment Plant | Littleton | 0.55 | 3,899 | NH0100153 | Oct 2025 | 19 |
| 21 | Suncook Wastewater Treatment Facility | Allenstown | 0.53 | 9,217 | NH0101390 | Jan 2027 | 8 |
| 22 | Newmarket, Town Of - WPCF | Newmarket | 0.44 | — | NHG580013 | Nov 2026 | 17 |
| 23 | Ashland Wastewater Treatment Facility | Ashland | 0.35 | 2,100 | NH0100005 | Jan 2027 | 5 |
| 24 | Sunapee, Town Of-WWTP | Sunapee | 0.3 | — | NHG580017 | Nov 2026 | 28 |
| 25 | Penacook Wastewater Treatment Plant | Concord | 0.3 | 9,100 | NH0100331 | Jul 2025 | 5 |
| 26 | Colebrook Wastewater Treatment Plant | Colebrook | 0.28 | 1,062 | NHG580315 | Nov 2026 | 33 |
| 27 | Town Of Bristol WWTF | Bristol | 0.2 | 3,072 | NHG580021 | Nov 2026 | 0 |
| 28 | Waterville Valley Wastewater Treatment Plant | Waterville Valley | 0.2 | 1,140 | NH0100781 | Mar 2027 | 6 |
| 29 | Woodsville Wastewater Treatment Facility | Haverhill | 0.18 | 814 | NHG580978 | Nov 2026 | 26 |
| 30 | Epping Wasterwater Treatment Facility | Epping | 0.18 | 1,800 | NHG580012 | Nov 2026 | 121 |
| 31 | Lisbon Wastewater Treatment Facility | Lisbon | 0.18 | 1,669 | NHG580421 | Nov 2026 | 26 |
| 32 | Charlestown Wastewater Treatment Plant | Charlestown | 0.17 | 3,025 | NH0100765 | Sep 2025 | 20 |
| 33 | Winchester Wastewater Treatment Plant | Winchester | 0.14 | 2,200 | NHG580404 | Nov 2026 | 2 |
| 34 | Henniker Wastewater Treatment Facility | Henniker | 0.13 | 1,850 | NHG580018 | Nov 2026 | 81 |
| 35 | Whitefield Wastewater Treatment Facility | Whitefield | 0.13 | 431 | NHG580002 | Nov 2026 | 22 |
| 36 | Hinsdale Wastewater Treatment Plant | Hinsdale | 0.12 | 3,204 | NHG580016 | Nov 2026 | 6 |
| 37 | Woodstock WWTF | Woodstock | 0.11 | 952 | NHG580020 | Nov 2026 | 32 |
| 38 | Newington Wastewater Treatment Plant | Portsmouth (newington) | 0.11 | 105 | NHG581141 | Nov 2026 | 83 |
| 39 | Groveton Wastewater Treatment Plant | Northumberland | 0.09 | 1,278 | NHG580226 | Nov 2026 | 15 |
| 40 | W Swanzey WWTF | Swanzey | 0.06 | — | NHG580015 | Nov 2026 | 182 |
| 41 | Town Of Hopkinton WWTP | Hopkington | 0.04 | 628 | NHG580579 | Nov 2026 | 25 |
| 42 | Northumberland Village Water Pollution Control Facility | Groveton | 0.04 | 334 | NHG581206 | Nov 2026 | 25 |
| 43 | Stratford Village System | North Stratford | 0.02 | 146 | NHG580536 | Nov 2026 | 8 |
| 44 | Cheshire County Maplewood Nursing Home | Westmoreland | 0.01 | 314 | NHG580391 | Nov 2026 | 14 |
| 45 | Lancaster Grange Wastewater Treatment Plant | Lancaster | 0.0034 | 36 | NHG581249 | Nov 2026 | 27 |
| 46 | Wallis Sands State Park | Rye | 0.0005 | — | NHG580014 | Nov 2026 | 51 |
Treatment technology for each plant is shown with a free Klarifi account. Create one →
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 October 26, 2026.
Wastewater terms: WWTP · POTW · MGD · NPDES permit · Activated sludge · Oxidation ditch · all terms
Cite this page
Journalists, researchers and educators are welcome to quote these figures. Please credit Klarifi as the source and link to this page.
Klarifi. “Largest wastewater treatment plants in New Hampshire (2026).” Updated October 26, 2026. https://klarifi.io/facilities/new-hampshire/largestNeed a custom cut of the data for a state, a county or a list of plants? Email info@klarifi.io.