What is the largest wastewater treatment plant in Oregon?

Willow Lake WWTP in Salem, treating an average of 24.62 MGD under NPDES permit ORL026409. Durham Waste Handling in Tigard is second at 18.4 MGD.

How much wastewater do Oregon’s largest plants treat?

The 50 largest plants treat 109 MGD combined, 96% of the flow reported by all 125 publicly owned plants in Oregon.

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 Oregon

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

Ranked by average flow · 109 MGD combined · 96% of the state's reported flow

Willow Lake WWTP in Salem is the largest publicly owned wastewater treatment plant in Oregon by reported average flow, treating about 24.62 million gallons per day. The 50 plants below treat 109 MGD between them, 96% of the flow reported by all 125 plants in the state, and serve about 1,160,961 people. Every plant links to its profile with permit, technology and compliance data.

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

The 50 largest publicly owned wastewater treatment plants in Oregon 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	Willow Lake WWTP	Salem	24.62	284,938	ORL026409	Dec 2030	Clarifiers, Carrousels / oxidation ditches, Biogas silos	2
2	Durham Waste Handling	Tigard	18.4	53,093	ORL028118	Dec 2030	Clarifiers, Uncovered lagoons, Biogas silos	6
3	Medford Rwrf	Central Point	14.5	159,933	OR0026263	Jul 2026	Clarifiers, Uncovered lagoons, Biogas silos	1
4	Gresham WWTP	Portland	8.99	126,805	OR0026131	Aug 2026	Clarifiers, Biogas silos	0
5	Albany-Millersburg WRF	Albany	4.59	52,976	OR0028801	Jan 2031	Clarifiers, Biogas silos	2
6	Grants Pass STP	Grants Pass	4.48	49,734	OR0028843	Sep 2014	Clarifiers, Uncovered lagoons, Biogas silos	16
7	Tryon Creek WWTP	Lake Oswego	3.51	86,722	OR0026891	Apr 2029	Clarifiers, Biogas silos	2
8	Mcminnville WWTP	Mcminnville	2.73	34,615	OR0034002	Dec 2008	Clarifiers, Carrousels / oxidation ditches, Biogas silos	2
9	Astoria STP	Astoria	1.75	10,404	OR0027561	Oct 2016	Aerated lagoons, Uncovered lagoons	14
10	Oak Lodge Water Reclamation Facility	Milwaukie	1.74	35,370	OR0026140	Mar 2027	Clarifiers, Uncovered lagoons	3
11	Ashland WWTP	Ashland	1.71	21,788	OR0026255	Jan 2027	Clarifiers, Carrousels / oxidation ditches, Biogas silos	15
12	Lebanon STP	Lebanon	1.63	17,335	OR0020818	Oct 2030	Clarifiers	54
13	Hermiston WWTP	Hermiston	1.56	18,775	OR0020761	Jan 2028	Clarifiers, Uncovered lagoons, Biogas silos	0
14	Lincoln City STP	Lincoln City	1.44	14,102	OR0020478	Mar 2031	Uncovered lagoons	2
15	Dallas WWTP	Rickreall	1.42	16,555	OR0020737	Oct 2030	Clarifiers, Aerated lagoons, Uncovered lagoons, Carrousels / oxidation ditches	13
16	Seaside WWTP	Seaside	1.2	6,875	OR0020401	Jul 2029	Clarifiers, Aerated lagoons, Carrousels / oxidation ditches	27
17	Hood River STP	Hood River	1.13	8,717	OR0020788	May 2031	Clarifiers, Uncovered lagoons, Biogas silos	46
18	Canby WWTP	Canby	0.99	16,800	OR0020214	Nov 2027	Clarifiers, Uncovered lagoons, Biogas silos	0
19	Monmouth STP	Monmouth	0.79	9,940	OR0020613	Dec 2030	Aerated lagoons, Uncovered lagoons	3
20	Warrenton STP	Warrenton	0.76	6,013	OR0020877	Nov 2025	Uncovered lagoons	0
21	Umatilla STP	Umatilla	0.72	6,574	OR0022306	Dec 2027	Clarifiers, Carrousels / oxidation ditches	0
22	Sweet Home STP	Sweet Home	0.7	9,718	OR0020346	Mar 2010	Clarifiers, Biogas balloons	27
23	Lakeview STP	Lakeview	0.68	—	OR0041386	Jul 2031	Uncovered lagoons	143
24	Scappoose STP	Scappoose	0.67	7,610	OR0022420	Oct 2014	Clarifiers, Uncovered lagoons	1
25	Florence STP	Florence	0.64	7,588	OR0020745	Apr 2019	Clarifiers, Biogas silos	0
26	Sutherlin WWTP	Oakland	0.59	8,787	OR0020842	Nov 2029	—	3
27	Junction City STP	Junction City	0.55	8,977	OR0026565	Oct 2028	Aerated lagoons	167
28	Independence STP	Independence	0.47	11,355	OR0020443	Aug 2027	Uncovered lagoons	10
29	Myrtle Point STP	Myrtle Point	0.46	2,535	OR0020435	May 2029	—	57
30	Estacada STP	Estacada	0.44	4,035	OR0020575	Sep 2015	Clarifiers	41
31	Philomath WWTP	Corvallis	0.43	5,431	OR0032441	Jun 2017	Uncovered lagoons	11
32	Coquille City Shop And Waste Water Plant	Coquille	0.43	3,920	OR0026247	Jul 2029	—	56
33	Creswell STP	Creswell	0.35	5,585	OR0027545	Sep 2027	Aerated lagoons, Uncovered lagoons	42
34	Depoe Bay STP	Depoe Bay	0.35	4,943	OR0026107	Jan 2029	—	0
35	Odell STP	Hood River	0.33	1,827	OR0022471	Aug 2029	Clarifiers, Aerated lagoons	7
36	Cannon Beach WWTF	Cannon Beach	0.32	1,718	OR0020222	Jul 2030	Aerated lagoons, Uncovered lagoons	3
37	Toledo STP	Toledo	0.32	3,709	OR0020869	Nov 2010	Clarifiers, Aerated lagoons, Biogas silos, Cooling towers	31
38	Nehalem Bay STP	Nehalem	0.3	3,218	OR0023370	Jun 2028	Aerated lagoons, Uncovered lagoons	0
39	Aumsville STP	Aumsville	0.3	4,000	OR0022721	Aug 2026	Aerated lagoons, Uncovered lagoons	5
40	Shady Cove STP	Shady Cove	0.29	3,051	OR0030660	Mar 2016	Clarifiers	1
41	Lafayette STP	Lafayette	0.27	4,338	OR0022551	Mar 2031	Uncovered lagoons	8
42	Bandon STP	Bandon	0.26	3,451	OR0020206	Aug 2029	Clarifiers	39
43	Enterprise STP	Enterprise	0.24	—	OR0020567	Mar 2025	—	1
44	Oakridge STP	Westfir	0.22	3,479	OR0022314	Apr 2012	Uncovered lagoons	26
45	Carlton STP	Carlton	0.2	2,319	OR0020541	Jun 2015	Aerated lagoons, Uncovered lagoons	66
46	Warm Springs WWTP	Warm Springs	0.2	3,161	OR0032638	Jun 2026	Uncovered lagoons	261
47	Rockaway Beach STP	Rockaway Beach	0.19	1,390	OR0023302	Oct 2027	Clarifiers, Aerated lagoons, Uncovered lagoons	0
48	Cave Junction Wastewater Treatment Facility	Cave Junction	0.19	1,975	OR0028339	May 2027	Clarifiers, Uncovered lagoons	8
49	Bay City STP	Bay City	0.17	1,462	OR0022578	Sep 2029	Uncovered lagoons	0
50	Hubbard STP	Hubbard	0.17	3,315	OR0020591	Aug 2027	Clarifiers	10

Largest plants in Oregon: 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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