What is the largest wastewater treatment plant in Delaware?

Wilmington Wastewater Treatment Plant- 12th St & Hay Rd, Wilmington in Wilmington, treating an average of 78.67 MGD under NPDES permit DEL020320. South Coastal STP in Frankford is second at 4.6 MGD.

How much wastewater do Delaware’s largest plants treat?

The 9 largest plants treat 88 MGD combined, 100% of the flow reported by all 9 publicly owned plants in Delaware.

How is plant size measured?

By reported average daily flow in million gallons per day (MGD) from EPA discharge monitoring reports, the standard measure of a plant’s size. Design capacity, the flow a plant is permitted to treat, is usually higher and is shown on each profile.

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The 9 largest wastewater treatment plants in Delaware

Name: Largest wastewater treatment plants in Delaware
Creator: Klarifi
License: https://klarifi.io/terms-of-service

Ranked by average flow · 88 MGD combined · 100% of the state's reported flow

Wilmington Wastewater Treatment Plant- 12th St & Hay Rd, Wilmington in Wilmington is the largest publicly owned wastewater treatment plant in Delaware by reported average flow, treating about 78.67 million gallons per day. The 9 plants below treat 88 MGD between them, 100% of the flow reported by all 9 plants in the state, and serve about 579,868 people. Every plant links to its profile with permit, technology and compliance data.

All plants in Delaware →The 100 largest US plants →Delaware NPDES permits →

The 9 largest publicly owned wastewater treatment plants in Delaware by average flow (MGD), with population served, NPDES permit, expiry, treatment technology and three-year violation count.
#	Facility	City	Avg flow (MGD)	Population	NPDES permit	Expires	Technology	Violations (3y)
1	Wilmington Wastewater Treatment Plant- 12th St & Hay Rd, Wilmington	Wilmington	78.67	530,079	DEL020320	Dec 2030	Clarifiers, Uncovered lagoons	5
2	South Coastal STP	Frankford	4.6	21,810	DEL050008	Dec 2030	Clarifiers, Aerated lagoons	5
3	Seaford Wastewater Treatment Plant	Seaford	1.57	9,572	DE0020265	Oct 2020	Clarifiers	0
4	Selbyville Wastewater Treatment Facility	Selbyville	1.26	2,459	DE0020010	Apr 2028	Clarifiers, Uncovered lagoons, Carrousels / oxidation ditches	1
5	Howard Seymour Water Reclamation Plant	Lewes	1.05	8,269	DE0021512	Oct 2023	Clarifiers	1
6	Delaware City Wastewater Treatment Plant	Delaware City	0.37	2,890	DEL021555	Dec 2030	—	2
7	Laurel Water Department	Laurel	0.34	4,044	DE0020125	Oct 2028	Uncovered lagoons	62
8	Milton Wastewater Treatment Plant	Milton	0.23	—	DE0021491	Apr 2016	Clarifiers	11
9	Port Penn Wastewater Treatment Plant	Port Penn	0.03	745	DE0021539	Oct 2026	—	1

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 September 11, 2026.

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