What is the largest wastewater treatment plant in Washington?

Riverside Park Water Reclamation Facility in Spokane, treating an average of 31.72 MGD under NPDES permit WA0024473. Chambers Creek WWTP in University Place is second at 17.07 MGD.

How much wastewater do Washington’s largest plants treat?

The 50 largest plants treat 70 MGD combined, 100% of the flow reported by all 125 publicly owned plants in Washington.

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 Washington

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

Ranked by average flow · 70 MGD combined · 100% of the state's reported flow

Riverside Park Water Reclamation Facility in Spokane is the largest publicly owned wastewater treatment plant in Washington by reported average flow, treating about 31.72 million gallons per day. The 50 plants below treat 70 MGD between them, 100% of the flow reported by all 125 plants in the state, and serve about 660,080 people. Every plant links to its profile with permit, technology and compliance data.

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

The 50 largest publicly owned wastewater treatment plants in Washington 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	Riverside Park Water Reclamation Facility	Spokane	31.72	185,960	WA0024473	Aug 2027	Clarifiers, Biogas silos	4
2	Chambers Creek WWTP	University Place	17.07	245,871	WAR000727	Dec 2029	Clarifiers, Biogas silos	0
3	Richland City Wastewater Lab	Richland	6.31	56,193	WA0020419	Sep 2021	Clarifiers, Biogas silos	0
4	Camas STP	Camas	2.62	22,459	WA0020249	Jul 2030	Clarifiers, Uncovered lagoons, Biogas silos	19
5	Pullman City Muni SW	Pullman	2.13	34,135	WA0044652	May 2019	—	1
6	Sunnyside STP	Sunnyside	1.31	18,327	WA0020991	Jan 2020	Clarifiers, Uncovered lagoons, Biogas silos	1
7	Othello STP	Othello	1.27	8,494	WA0022357	Sep 2025	Uncovered lagoons	85
8	Port Of Sunnyside Industrial WWTF	Sunnyside	1.17	—	WA0052426	Aug 2019	Aerated lagoons, Covered lagoons	79
9	Clarkston WWTP	Clarkston	0.71	11,600	WA0021113	Apr 2021	Clarifiers	0
10	Toppenish Wastewater Treatment Plant	Toppenish	0.67	7,885	WA0026123	Dec 2030	Clarifiers, Biogas silos	29
11	Colville WWTP	Colville	0.62	5,369	WA0022616	Jul 2020	Aerated lagoons, Uncovered lagoons	0
12	Omak Wastewater Treatment Plant	Omak	0.53	5,412	WA0020940	Dec 2020	Clarifiers, Carrousels / oxidation ditches	0
13	Cashmere STP Facility	Cashmere	0.39	3,000	WA0023183	May 2025	Clarifiers, Uncovered lagoons	30
14	Medical Lake WWTP	Medical Lake	0.27	5,620	WA0021148	Jul 2019	Clarifiers, Carrousels / oxidation ditches	13
15	Montesano Sewage Treatment Plant	Montesano	0.24	3,700	WA0024660	Nov 2029	Clarifiers, Aerated lagoons, Uncovered lagoons	0
16	Benton City STP	Benton City	0.22	3,600	WA0051349	Aug 2019	Aerated lagoons, Uncovered lagoons	0
17	Dayton STP	Dayton	0.19	2,825	WA0020729	Sep 2016	Clarifiers	2
18	Chewelah WWTP	Chewelah	0.19	3,297	WA0023604	Nov 2016	Clarifiers, Carrousels / oxidation ditches	1
19	Brewster STP	Brewster	0.18	2,589	WA0021008	Jan 2021	Clarifiers, Carrousels / oxidation ditches	0
20	Tulalip Wastewater Treatment Plant	Marysville	0.17	—	WA0024805	Oct 2014	Clarifiers, Carrousels / oxidation ditches	1
21	Coulee Dam WWTP	Coulee Dam	0.16	2,020	WA0020281	Nov 2022	—	0
22	Suquamish WWTP	Suquamish	0.16	—	WA0023256	May 2013	—	0
23	Davenport WWTP	Davenport	0.15	1,980	WA0045578	Jul 2019	—	3
24	Okanogan WWTP	Okanogan	0.15	2,959	WA0022365	Sep 2030	Clarifiers	1
25	Winlock Wastewater Treatment Plant	Winlock	0.15	1,534	WA0021199	Feb 2028	Clarifiers	0
26	Ilwaco Wastewater Treatment Plant	Ilwaco	0.15	9,303	WA0023159	Aug 2030	—	4
27	Tonasket POTW	Tonasket	0.1	1,173	WA0052337	Apr 2031	Aerated lagoons, Uncovered lagoons	7
28	Pomeroy Sewage Treatment Plant	Pomeroy	0.09	1,631	WA0021164	Oct 2023	Clarifiers	0
29	Pacific Beach STP	Pacific Beach	0.09	2,003	WA0037095	Aug 2030	Clarifiers, Uncovered lagoons, Carrousels / oxidation ditches	37
30	Asotin STP	Clarkston	0.08	1,437	WA0020818	Aug 2023	Clarifiers, Uncovered lagoons, Carrousels / oxidation ditches	1
31	Lummi Kwina Road WWTP	Bellingham	0.07	—	WA0026727	Dec 2027	—	7
32	Toutle STP	Toutle	0.06	690	WA0037770	Jun 2030	Clarifiers, Carrousels / oxidation ditches	7
33	Rock Island WWTP	Rock Island	0.06	1,320	WA0501487	Feb 2031	—	0
34	Yakama Legends Casino Hotel	Toppenish	0.06	—	WA0026743	Oct 2018	—	0
35	North Bonneville STP	North Bonneville	0.06	1,105	WA0023388	Jun 2029	—	0
36	Winthrop Wastewater Treatment Plant	Winthrop	0.06	469	WA0020885	Jul 2020	Uncovered lagoons	40
37	Tekoa STP	Tekoa	0.05	939	WA0023141	May 2019	Clarifiers, Biogas silos	35
38	Peshastin Wastewater Treatment Plant	Peshastin	0.05	1,163	WA0052175	May 2024	—	5
39	Wa Doc Cedar Creek Correctional Ctr	Littlerock	0.04	—	WA0037737	May 2029	—	0
40	Palouse Wastewater Treatment Plant	Palouse	0.04	1,162	WA0044806	Mar 2020	—	21
41	Olympic Corrections Center	Forks	0.04	—	WA0038938	Apr 2030	—	2
42	Harrah WWTP	Harrah	0.03	—	WA0022705	May 2030	Aerated lagoons	2
43	Crescent Bar Inc	Quincy	0.03	325	WA0991013	Jan 2031	—	0
44	Reardan Town STP	Reardan	0.03	696	WA0045306	Jun 2015	—	49
45	Onalaska Wastewater Treatment Plant	Chehalis	0.03	356	WA0024546	Aug 2030	—	0
46	Vantage STP	Ellensburg	0.03	302	WA0050474	Feb 2030	—	2
47	Lyle POTW	Lyle	0.03	783	WA0050482	Oct 2020	—	1
48	Metaline STP	Metaline	0.02	183	WA0020699	Dec 2023	Uncovered lagoons	10
49	Moclips River Estates WWTP	Moclips	0.01	—	WA0026603	Apr 2031	—	0
50	Wishram Wastewater Treatment Plant	Wishram	0.01	211	WA0051292	Aug 2019	Aerated lagoons	0

Largest plants in Washington: 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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