The 8 largest wastewater treatment plants in Delaware
Ranked by average flow · 10 MGD combined · 100% of the state's reported flow
South Coastal STP in Frankford is the largest publicly owned wastewater treatment plant in Delaware by reported average flow, treating about 4.37 million gallons per day. The 8 plants below treat 10 MGD between them, 100% of the flow reported by all 8 plants in the state, and serve about 49,789 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 | South Coastal STP | Frankford | 4.37 | 21,810 | DEL050008 | Dec 2030 | 5 |
| 2 | Seaford Wastewater Treatment Plant | Seaford | 1.61 | 9,572 | DE0020265 | Oct 2020 | 0 |
| 3 | Howard Seymour Water Reclamation Plant | Lewes | 1.29 | 8,269 | DE0021512 | Oct 2023 | 1 |
| 4 | Selbyville Wastewater Treatment Facility | Selbyville | 1.24 | 2,459 | DE0020010 | Apr 2028 | 1 |
| 5 | Laurel Water Department | Laurel | 0.5 | 4,044 | DE0020125 | Oct 2028 | 62 |
| 6 | Delaware City Wastewater Treatment Plant | Delaware City | 0.42 | 2,890 | DEL021555 | Dec 2030 | 2 |
| 7 | Milton Wastewater Treatment Plant | Milton | 0.23 | — | DE0021491 | Apr 2016 | 11 |
| 8 | Port Penn Wastewater Treatment Plant | Port Penn | 0.04 | 745 | DE0021539 | Oct 2026 | 1 |
Treatment technology for each plant is shown with a free Klarifi account. Create one →
Largest plants in Delaware: 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 · Biological nutrient removal · Tertiary treatment · 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. “The 8 largest wastewater treatment plants in Delaware (2026).” Updated October 26, 2026. https://klarifi.io/facilities/delaware/largestNeed a custom cut of the data for a state, a county or a list of plants? Email info@klarifi.io.