Wastewater Treatment Plants in Delaware
9 facilities · 88 MGD combined flow
Klarifi tracks 9 publicly owned wastewater treatment plants in Delaware with a combined average flow of 88 MGD, serving about 579,868 residents. 5 plants treat more than 1 MGD and 1 more than 10 MGD; 4 use lagoon systems and 0 have digesters or biogas infrastructure. The largest is Wilmington Wastewater Treatment Plant- 12th St & Hay Rd, Wilmington in Wilmington at 78.67 MGD. 1 NPDES permits expire this year and 0 next year.
Every plant below links to a profile with average and design flow, treatment technology, the NPDES permit and its expiry, a three-year compliance snapshot and the capital projects planned in its city.
9 plant locations in Delaware; dot size follows average flow.
NPDES permits and renewals in Delaware
Every plant discharges under an NPDES permit that is renewed every five years, usually with tighter limits. The renewal year is the year a plant budgets for upgrades.
Largest wastewater treatment plants in Delaware
| Facility | City | Avg flow (MGD) | Design (MGD) | NPDES permit | Expires | Technology | Violations (3y) |
|---|---|---|---|---|---|---|---|
| Wilmington Wastewater Treatment Plant- 12th St & Hay Rd, Wilmington | Wilmington | 78.67 | — | DEL020320 | Dec 2030 | Clarifiers, Uncovered lagoons | 5 |
| South Coastal STP | Frankford | 4.6 | — | DEL050008 | Dec 2030 | Clarifiers, Aerated lagoons | 5 |
| Seaford Wastewater Treatment Plant | Seaford | 1.57 | — | DE0020265 | Oct 2020 | Clarifiers | 0 |
| Selbyville Wastewater Treatment Facility | Selbyville | 1.26 | — | DE0020010 | Apr 2028 | Clarifiers, Uncovered lagoons, Carrousels / oxidation ditches | 1 |
| Howard Seymour Water Reclamation Plant | Lewes | 1.05 | — | DE0021512 | Oct 2023 | Clarifiers | 1 |
| Delaware City Wastewater Treatment Plant | Delaware City | 0.37 | — | DEL021555 | Dec 2030 | — | 2 |
| Laurel Water Department | Laurel | 0.34 | — | DE0020125 | Oct 2028 | Uncovered lagoons | 62 |
| Milton Wastewater Treatment Plant | Milton | 0.23 | — | DE0021491 | Apr 2016 | Clarifiers | 11 |
| Port Penn Wastewater Treatment Plant | Port Penn | 0.03 | — | DE0021539 | Oct 2026 | — | 1 |
Delaware plants by treatment technology
All 9 plants in Delaware
| Facility | City | Flow (MGD) |
|---|---|---|
| Wilmington Wastewater Treatment Plant- 12th St & Hay Rd, Wilmington | Wilmington | 78.67 |
| South Coastal STP | Frankford | 4.6 |
| Seaford Wastewater Treatment Plant | Seaford | 1.57 |
| Selbyville Wastewater Treatment Facility | Selbyville | 1.26 |
| Howard Seymour Water Reclamation Plant | Lewes | 1.05 |
| Delaware City Wastewater Treatment Plant | Delaware City | 0.37 |
| Laurel Water Department | Laurel | 0.34 |
| Milton Wastewater Treatment Plant | Milton | 0.23 |
| Port Penn Wastewater Treatment Plant | Port Penn | 0.03 |
Wastewater treatment 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 September 11, 2026.