What is the largest wastewater treatment plant in Maryland?

Back River Wastewater Treatment Plant in Baltimore, treating an average of 114.1 MGD under NPDES permit MDL021555. Patapsco Wastewater Treatment Plant in Baltimore is second at 49.01 MGD.

How much wastewater do Maryland’s largest plants treat?

The 50 largest plants treat 279 MGD combined, 99% of the flow reported by all 118 publicly owned plants in Maryland.

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 50 largest wastewater treatment plants in Maryland

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

Ranked by average flow · 279 MGD combined · 99% of the state's reported flow

Back River Wastewater Treatment Plant in Baltimore is the largest publicly owned wastewater treatment plant in Maryland by reported average flow, treating about 114.1 million gallons per day. The 50 plants below treat 279 MGD between them, 99% of the flow reported by all 118 plants in the state, and serve about 1,735,703 people. Every plant links to its profile with permit, technology and compliance data.

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

The 50 largest publicly owned wastewater treatment plants in Maryland 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	Back River Wastewater Treatment Plant	Baltimore	114.1	432,114	MDL021555	Dec 2030	Clarifiers, Uncovered lagoons, Biogas silos, Biogas balloons	32
2	Patapsco Wastewater Treatment Plant	Baltimore	49.01	293,367	MDL021601	Dec 2030	Clarifiers, Biogas silos, Biogas balloons	22
3	Piscataway Swge Plnt	Accokeek	19.79	160,827	MDL021539	Dec 2030	Clarifiers, Biogas silos, Biogas balloons	25
4	Little Patuxent Water Reclamation Plant	Savage	16.68	125,350	MD0055174	May 2031	Clarifiers, Biogas balloons	8
5	Sod Run Wastewater Treatment Plant	Perryman	10.81	122,618	MDL056545	Dec 2030	Clarifiers, Uncovered lagoons, Biogas silos, Biogas balloons	0
6	Annapolis WRF	Annapolis	8.48	47,859	MD0021814	Jan 2030	Clarifiers, Uncovered lagoons, Biogas silos	6
7	Ballenger-Mckinney WWTP	Frederick	6.92	69,525	MD0021822	Dec 2030	Clarifiers, Biogas balloons	1
8	Parkway Wastewater Treatment Plant	Laurel	6.35	56,093	MDL021725	Dec 2030	Clarifiers, Biogas silos	2
9	Ocean City Wastewater Treatment Plant	Ocean City	5.88	12,500	MDL020044	Dec 2030	Clarifiers, Biogas silos, Biogas balloons	2
10	City Of Frederick Wastewater Treatment Plant-2012	Frederick	5.4	72,481	MD0021610	May 2031	Clarifiers, Aerated lagoons, Carrousels / oxidation ditches, Biogas silos	23
11	City Of Salisbury Wastewater Treatment Plant	Salisbury	5.05	24,205	MDL021571	Dec 2030	Clarifiers, Biogas silos	83
12	Broadneck Water Reclamation Facility	Annapolis	3.68	—	MDL021644	Dec 2030	Clarifiers, Carrousels / oxidation ditches	106
13	Westminster WWTP	Westminster	3.41	38,000	MD0021831	Jul 2031	Clarifiers	9
14	Easton WWTP	Easton	2.02	16,143	MDL020273	Dec 2030	Clarifiers, Uncovered lagoons, Carrousels / oxidation ditches	2
15	Freedom District WWTP	Sykesville	1.97	24,913	MD0021512	Oct 2029	Clarifiers, Carrousels / oxidation ditches	32
16	Conococheague WWTP	Williamsport	1.84	12,180	MD0063509	May 2031	Clarifiers, Carrousels / oxidation ditches	0
17	Maryland City Water Reclamation Facility	Laurel	1.53	20,309	MDL062596	Dec 2030	Clarifiers	27
18	The City Of Bowie (ssos)	Bowie	1.4	25,000	MDL021628	Dec 2030	Clarifiers, Carrousels / oxidation ditches	8
19	Northeast River Advanced WWTP	Charlestown	1.25	27,330	MDL052027	Dec 2030	Clarifiers, Uncovered lagoons, Carrousels / oxidation ditches	38
20	Town Of La Plata	La Plata	1.04	21,326	MDL020524	Dec 2030	Clarifiers, Uncovered lagoons	111
21	Celanese WWTP	Cumberland	0.8	2,122	MDL063878	Dec 2030	Clarifiers	0
22	George'S Creek WWTP	Westernport	0.77	6,035	MD0060071	May 2026	Clarifiers, Aerated lagoons, Uncovered lagoons, Carrousels / oxidation ditches	0
23	Crisfield WWTP	Crisfield	0.74	4,487	MDL020001	Dec 2030	Clarifiers	45
24	Wssc Damascus WRRF	Gaithersburg	0.72	5,246	MD0020982	Dec 2028	Clarifiers	3
25	Mount Airy WWTP	Mount Airy	0.69	9,701	MD0022527	Dec 2025	Clarifiers	142
26	Perryville Wastewater Treatment Plant	Perryville	0.64	4,478	MDL020613	Dec 2030	Aerated lagoons, Uncovered lagoons	57
27	Chestertown WWTP	Chestertown	0.63	6,200	MD0020010	Sep 2026	Clarifiers, Uncovered lagoons	5
28	Delmar WWTP	Delmar	0.54	7,624	MD0020532	Jul 2031	Uncovered lagoons	6
29	Piney Orchard WWTP	Odenton	0.52	8,500	MDL059145	Dec 2030	Clarifiers, Carrousels / oxidation ditches	6
30	Pocomoke City Wastewater Treatment Facility	Pocomoke City	0.52	4,905	MD0022551	May 2028	Aerated lagoons, Covered lagoons, Uncovered lagoons, Carrousels / oxidation ditches	0
31	Taneytown Wastewater Treatment Plant	Taneytown	0.51	6,746	MD0020672	May 2031	—	3
32	Oakland WWTP	Oakland	0.5	2,058	MD0020648	Mar 2024	Aerated lagoons	1
33	Hampstead WWTP	Hampstead	0.48	4,207	MD0022446	Aug 2030	Aerated lagoons, Uncovered lagoons, Carrousels / oxidation ditches	25
34	Manchester WWTP	Manchester	0.43	5,118	MD0022578	Feb 2027	—	70
35	Worton-Butlertown WWTP	Worton	0.42	1,160	MD0060585	Oct 2029	—	5
36	Fruitland WWTP	Fruitland	0.41	4,127	MD0052990	Feb 2029	Uncovered lagoons	36
37	Denton WWTP	Denton	0.4	5,490	MD0020494	Jul 2025	Clarifiers, Aerated lagoons, Uncovered lagoons	0
38	Poolesville WWTP	Poolesville	0.34	5,500	MD0023001	Nov 2027	Uncovered lagoons	6
39	Indian Head WWTP	Indian Head	0.3	3,625	MD0020052	Feb 2030	Clarifiers	1
40	St. Michael'S WWTP	Saint Michaels	0.29	4,445	MD0023604	Nov 2030	Uncovered lagoons, Carrousels / oxidation ditches, Biogas balloons	0
41	Town Of Emmitsburg	Emmitsburg	0.27	2,900	MD0020257	Feb 2030	Clarifiers, Aerated lagoons	10
42	Snow Hill WWTP	Snow Hill	0.26	2,879	MD0022764	Dec 2028	Clarifiers	61
43	Boonsboro WWTP	Boonsboro	0.26	7,998	MD0020231	Dec 2030	Aerated lagoons	1
44	Rock Hall WWTP	Rock Hall	0.22	2,961	MD0020303	Jul 2029	Clarifiers, Uncovered lagoons, Carrousels / oxidation ditches	7
45	Federalsburg WWTP	Federalsburg	0.21	2,794	MD0020249	Jan 2029	Clarifiers	392
46	Rising Sun Town Of WWTP	Rising Sun	0.2	3,168	MD0020265	Apr 2028	Clarifiers, Uncovered lagoons, Carrousels / oxidation ditches	22
47	Smithsburg WWTP	Smithsburg	0.19	3,250	MD0024317	Mar 2031	Uncovered lagoons	14
48	Middletown - East WWTP	Middletown	0.19	2,964	MD0067628	Dec 2021	—	3
49	Grantsville WWTP	Grantsville	0.16	1,253	MD0020761	Jul 2025	—	142
50	Middletown WWTP	Middletown	0.15	1,622	MD0024406	Jul 2026	Aerated lagoons	0

Largest plants in Maryland: 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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