Activated sludge and clarifiers: wastewater treatment plants in Delaware
6 plants · 67% of the state · 87 MGD combined
Klarifi identifies 6 publicly owned wastewater treatment plants with activated sludge and clarifiers in Delaware, 67% of the 9 plants in the state (87 MGD combined). The largest is Wilmington Wastewater Treatment Plant- 12th St & Hay Rd, Wilmington in Wilmington at 78.67 MGD.
Activated sludge is the standard mechanical process for cities: aeration basins where bacteria consume organic matter, followed by clarifiers that settle the biomass out. These plants are identified by their circular or rectangular clarifiers and by the suspended-growth process code, and they are the main market for blowers, diffusers, clarifier mechanisms and instrumentation.
The largest activated sludge plants in Delaware
| # | Facility | City | Avg flow (MGD) | NPDES permit | Expires | Violations (3y) |
|---|---|---|---|---|---|---|
| 1 | Wilmington Wastewater Treatment Plant- 12th St & Hay Rd, Wilmington | Wilmington | 78.67 | DEL020320 | Dec 2030 | 5 |
| 2 | South Coastal STP | Frankford | 4.6 | DEL050008 | Dec 2030 | 5 |
| 3 | Seaford Wastewater Treatment Plant | Seaford | 1.57 | DE0020265 | Oct 2020 | 0 |
| 4 | Selbyville Wastewater Treatment Facility | Selbyville | 1.26 | DE0020010 | Apr 2028 | 1 |
| 5 | Howard Seymour Water Reclamation Plant | Lewes | 1.05 | DE0021512 | Oct 2023 | 1 |
| 6 | Milton Wastewater Treatment Plant | Milton | 0.23 | DE0021491 | Apr 2016 | 11 |
Contacts and project signals for every plant on this list
Klarifi adds verified decision-maker contacts, permit limits, discharge monitoring data and the capital projects planned in each city, filterable by technology, size and compliance record.
Activated sludge and clarifiers in other states
Activated sludge and clarifiers 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.