The 50 largest wastewater treatment plants in Massachusetts
Ranked by average flow · 392 MGD combined · 100% of the state's reported flow
Deer Island Wastewater Treatment Facility in Winthrop is the largest publicly owned wastewater treatment plant in Massachusetts by reported average flow, treating about 233.7 million gallons per day. The 50 plants below treat 392 MGD between them, 100% of the flow reported by all 66 plants in the state, and serve about 4,137,942 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 | Deer Island Wastewater Treatment Facility | Winthrop | 233.7 | 2,317,098 | MA0103284 | Jul 2005 | Biogas silos, Biogas balloons | 3 |
| 2 | Upper Blackstone Wastewater Treatment Plant | Millbury | 24.1 | 257,969 | MAR053209 | Feb 2026 | Clarifiers | 15 |
| 3 | Greater Lawrence Wastewater Treatment Facility | North Andover | 21.7 | 412,411 | MAL100447 | Dec 2030 | Clarifiers, Biogas silos | 10 |
| 4 | South Essex Wastewater Treatment Plant | Salem | 17.57 | 376,847 | MAL100501 | Dec 2030 | — | 33 |
| 5 | New Bedford Wastewater Treatment Facility | New Bedford | 16.7 | 103,942 | MA0100781 | Nov 2013 | — | 14 |
| 6 | Brockton Advanced Water Reclamation Fac | Brockton | 9.9 | 120,494 | MAL101010 | Dec 2030 | Clarifiers, Biogas silos, Biogas balloons | 10 |
| 7 | Pittsfield Wastewater Treatment Facility | Pittsfield | 7 | 48,150 | MA0101681 | Oct 2026 | Clarifiers, Carrousels / oxidation ditches, Biogas silos, Biogas balloons | 84 |
| 8 | Haverhill Wastewater Treatment Facility | Haverhill | 6.58 | 61,329 | MAL101621 | Dec 2030 | — | 4 |
| 9 | Taunton Wastewater Treatment Facility | Taunton | 5.36 | 39,644 | MA0100897 | Jul 2020 | Clarifiers, Biogas balloons | 52 |
| 10 | Holyoke Water Pollution Control Facility | Holyoke | 5.02 | 34,070 | MAL101630 | Dec 2030 | Clarifiers | 4 |
| 11 | Westborough Wastewater Treatment Plant | Westborough | 4.17 | 51,131 | MA0100412 | Apr 2027 | Clarifiers, Carrousels / oxidation ditches, Biogas balloons | 2 |
| 12 | Northampton Wastewater Treatment Plant | Northampton | 3.84 | 26,565 | MA0101818 | Nov 2028 | Clarifiers, Biogas balloons | 40 |
| 13 | Amherst Wastewater Treatment Plant | Hadley | 3.33 | 35,110 | MA0100218 | Jan 2029 | Clarifiers | 42 |
| 14 | Billerica Wastewater Treatment Facility | Billerica | 2.88 | 35,443 | MA0101711 | Feb 2029 | Clarifiers | 52 |
| 15 | Westfield Wastewater Treatment Plant | Westfield | 2.4 | 2,559 | MA0101800 | Nov 2028 | Clarifiers | 0 |
| 16 | Gardner W W T F | East Templeton | 2.28 | — | MAG590013 | Oct 2027 | Clarifiers, Biogas silos | 75 |
| 17 | Hoosic Wastwater Treament Plant | Williamstown | 2.04 | 22,403 | MA0100510 | Dec 2028 | Clarifiers | 2 |
| 18 | Marlborough DPW Water Division East Plt | Marlborough | 2.04 | — | MA0100498 | Mar 2027 | Clarifiers | 13 |
| 19 | Rockland Wastewater Plant | Rockland | 2 | — | MAG590038 | Oct 2027 | Clarifiers, Biogas silos | 32 |
| 20 | Erving Wastewater Treatment Plant #2 | Erving | 1.72 | 956 | MA0101052 | Nov 2026 | Clarifiers, Aerated lagoons, Biogas balloons | 36 |
| 21 | Newburyport Wastewater Treatment Facility | Newburyport | 1.45 | 17,891 | MAG590035 | Oct 2027 | Clarifiers, Biogas silos, Biogas balloons | 0 |
| 22 | Greenfield Water Pollution Control Facility | Greenfield | 1.44 | 13,193 | MAG590014 | Oct 2027 | Clarifiers | 31 |
| 23 | Plymouth Wastewater Treatment Facility | Plymouth | 1.31 | 12,121 | MAG590040 | Oct 2027 | Uncovered lagoons, Biogas balloons | 5 |
| 24 | Palmer Water Pollution Control Facility | Palmer (three Rivers) | 1.2 | 7,927 | MA0101168 | Nov 2028 | Clarifiers | 4 |
| 25 | Westerly Wastewater Treatment Facility | Marlborough | 1.16 | 40,085 | MA0100480 | Dec 2026 | Clarifiers | 26 |
| 26 | Middleborough WPCF | Middleborough | 1.09 | — | MAG590042 | Oct 2027 | Clarifiers | 11 |
| 27 | Hull Wastewater Treatment Plant | Hull | 1.03 | 7,478 | MAL101231 | Dec 2030 | Clarifiers, Biogas balloons | 3 |
| 28 | Hudson W W T F | Hudson | 0.78 | 19,692 | MAG590029 | Oct 2027 | Clarifiers | 78 |
| 29 | Uxbridge Wastewater Treatment Facility | Uxbridge | 0.71 | 8,412 | MAG590003 | Oct 2027 | Clarifiers | 24 |
| 30 | Concord Wastewater Treatment Facility | Concord | 0.68 | 5,492 | MAG590030 | Oct 2027 | Clarifiers, Uncovered lagoons | 5 |
| 31 | Orange Wastewater Treatment Plant | Orange | 0.61 | 3,747 | MAG590019 | Oct 2027 | Clarifiers | 508 |
| 32 | Salisbury Wastewater Treatment Facility | Salisbury | 0.59 | 6,401 | MA0102873 | Jan 2021 | Clarifiers, Aerated lagoons | 16 |
| 33 | Montague Wastewater Treatment Facility | Montague | 0.54 | 5,764 | MA0100137 | Dec 2028 | Clarifiers | 237 |
| 34 | Ipswich Wastewater Treatment Plant | Ipswich | 0.5 | 6,824 | MAG590036 | Oct 2027 | Clarifiers | 2 |
| 35 | Lee Wastewater Treatment Facility | Lee | 0.47 | 5,157 | MAG590023 | Oct 2027 | — | 0 |
| 36 | Hardwick WPC - Gilbertville | Gilbertville | 0.44 | — | MAG580016 | Nov 2026 | Clarifiers | 31 |
| 37 | Spencer Wastewater Treatment Plant | Spencer | 0.44 | 4,749 | MAG590010 | Oct 2027 | Clarifiers, Biogas balloons | 20 |
| 38 | Hadley Wastewater Treatment Plant | Hadley | 0.34 | 2,952 | MAG580013 | Nov 2026 | Clarifiers | 6 |
| 39 | Charlton W W T P | Charlton | 0.31 | — | MAG580039 | Nov 2026 | Clarifiers | 129 |
| 40 | Lenox Center Wastewater Treatment Facility | Lenox | 0.3 | 3,379 | MAG590024 | Oct 2027 | Clarifiers | 12 |
| 41 | North Brookfield Waste Water Treatment Plant | North Brookfield | 0.29 | 938 | MAG580037 | Nov 2026 | Clarifiers | 20 |
| 42 | Manchester Wastewater Treatment Facility | Manchester | 0.29 | 3,418 | MAG580043 | Nov 2026 | Clarifiers | 33 |
| 43 | Merrimac Wastewater Treatment Plant | Merrimac | 0.28 | 3,993 | MAG580030 | Nov 2026 | Clarifiers, Carrousels / oxidation ditches | 0 |
| 44 | Cohasset Wastewater Treatment Facility | Cohasset | 0.21 | 4,646 | MAG580033 | Nov 2026 | — | 66 |
| 45 | Templeton W W T P | Templeton | 0.2 | — | MAG580038 | Nov 2026 | Aerated lagoons | 19 |
| 46 | M C I Norfolk Walpole | Norfolk | 0.18 | — | MAG580036 | Nov 2026 | Clarifiers | 4 |
| 47 | Oxford Rochdale Wastewater Treatment Plant | North Oxford | 0.17 | 3,183 | MAG580026 | Nov 2026 | Clarifiers, Uncovered lagoons | 0 |
| 48 | Leo L. Dubois WPCF - Bridgewater Correction Complex | Bridgewater | 0.15 | — | MAG580040 | Nov 2026 | Clarifiers, Uncovered lagoons, Carrousels / oxidation ditches | 40 |
| 49 | Hatfield Wastewater Treatment Facility | Hatfield | 0.14 | 1,659 | MAG580015 | Nov 2026 | — | 0 |
| 50 | Sunderland Wastewater Treatment Facility | Sunderland | 0.13 | 2,720 | MAG580005 | Nov 2026 | Clarifiers | 0 |
Largest plants in Massachusetts: 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.