What is the largest wastewater treatment plant in Illinois?

Metropolitan Water Reclamation in Chicago, treating an average of 281.73 MGD under NPDES permit ILL028061. MWRDGC Terrence J O'Brien Water Reclamation Plant in Skokie is second at 205.79 MGD.

How much wastewater do Illinois’s largest plants treat?

The 50 largest plants treat 835 MGD combined, 83% of the flow reported by all 664 publicly owned plants in Illinois.

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 Illinois

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

Ranked by average flow · 835 MGD combined · 83% of the state's reported flow

Metropolitan Water Reclamation in Chicago is the largest publicly owned wastewater treatment plant in Illinois by reported average flow, treating about 281.73 million gallons per day. The 50 plants below treat 835 MGD between them, 83% of the flow reported by all 664 plants in the state, and serve about 4,368,071 people. Every plant links to its profile with permit and compliance data. Treatment technology is shown with a free Klarifi account.

All plants in Illinois →The 50 largest US plants →Illinois NPDES permits →

The 50 largest publicly owned wastewater treatment plants in Illinois by average flow (MGD), with population served, NPDES permit, expiry and three-year violation count.
#	Facility	City	Avg flow (MGD)	Population	NPDES permit	Expires	Violations (3y)
1	Metropolitan Water Reclamation	Chicago	281.73	1,346,237	ILL028061	Dec 2030	10
2	MWRDGC Terrence J O'Brien Water Reclamation Plant	Skokie	205.79	946,333	IL0028088	Jul 2022	16
3	Sanitary District Of Decatur	Decatur	34.42	78,304	ILL028321	Dec 2030	1
4	Springbrook Water Reclamation Center	Naperville	20.15	162,500	ILL034061	Dec 2030	0
5	Greater Peoria Sanitary And Sewage District	Peoria	19.22	126,196	ILL021288	Dec 2030	12
6	Spring Creek STP	Springfield	16.41	106,720	ILL021989	Dec 2030	29
7	Bloomington Normal WRD -West STP	Bloomington	16.33	57,849	ILL027731	Dec 2030	0
8	Fox River Water Reclamation Dist	Elgin	13.07	107,193	ILL028657	Dec 2030	50
9	Glenbard Wastewater Authority	Glen Ellyn	11.79	70,797	ILL021547	Dec 2030	27
10	Collinsville Wastewater Treatment Plant	Collinsville	11.53	33,769	ILL028215	Dec 2030	5
11	Flagg Creek Water Reclamation	Burr Ridge	11.38	61,953	ILL022586	Dec 2030	6
12	Downers Grove Sanitary District	Downers Grove	10.66	48,727	ILL028380	Dec 2030	6
13	Elmhurst Stp, City Of	Elmhurst	10.25	46,532	ILL028746	Dec 2030	8
14	Lake County Public Works Department	Buffalo Grove	9.09	72,507	ILL022055	Dec 2030	0
15	Woodridge-Greene Valley WWTP	Woodridge	8.92	56,482	ILL031844	Dec 2030	7
16	Joliet West Stp, City Of	Joliet	8.9	75,393	ILL033553	Dec 2030	0
17	Danville Sanitary District STP	Danville	8.45	83,005	ILL020788	Dec 2030	9
18	Alton Waste Water Treatment Plant	Alton	7.8	—	ILL027464	Dec 2030	40
19	Urbana & Champaign Sd	Urbana	7.76	116,900	ILL031500	Dec 2030	2
20	Galesburg Sanitary District STP	Galesburg	7.4	30,052	ILL023141	Dec 2030	6
21	Wheaton Sanitary District WWTF	Wheaton	7.27	52,451	ILL031739	Dec 2030	7
22	Romeoville Wastewater Treatment Facility	Romeoville	6.43	39,430	ILL048526	Dec 2030	0
23	Plainfield North Stp, Village Of	Plainfield	6.07	44,542	ILL074373	Dec 2030	0
24	Highland Water Reclamation	Highland	5.55	9,919	IL0029173	Mar 2031	28
25	Jacksonville STP	Jacksonville	5.44	19,446	ILL021661	Dec 2030	1
26	Frankfort, Village Of	Frankfort	4.97	19,411	ILL072192	Dec 2030	7
27	St. Charles Wastewater Treatment Facility	Saint Charles	4.44	32,709	ILL022705	Dec 2030	2
28	North Slope Treatment Plant	Moline	4.25	15,605	ILL029947	Dec 2030	1
29	East Moline Wastewater	East Moline	4.16	32,163	IL0028550	Mar 2031	2
30	Bloomington-Normal WRD SE WWTP	Bloomington	4.04	74,683	ILL073504	Dec 2030	0
31	City Of Monmouth	Monmouth	3.88	8,886	ILL036218	Dec 2030	19
32	Fox River Water Reclamation Di	Elgin	3.79	22,420	ILL035891	Dec 2030	0
33	Village Of Bolingbrook STP #3	Bolingbrook	3.67	26,649	ILL069744	Dec 2030	17
34	Moline South Treatment Plant	Moline	3.64	25,460	ILL029939	Dec 2030	4
35	City Of East Peoria Wastwater Treatment Plant	East Peoria	3.53	19,416	ILL028576	Dec 2030	6
36	Kewanee STP	Kewanee	3.5	12,174	IL0029343	Apr 2031	37
37	Sycamore STP - North Plant	Sycamore	3.26	18,317	ILL031291	Dec 2030	0
38	Addison Stw North 3	Addison	3.21	23,173	ILL033812	Dec 2030	0
39	Edwardsville Stp, City Of	Edwardsville	3.11	25,189	ILL026310	Dec 2030	2
40	Pekin STP #1	Pekin	3.05	33,338	ILL034495	Dec 2030	6
41	Algonquin Stw, Village Of	Algonquin	2.89	30,730	ILL023329	Dec 2030	13
42	Metropolitan Water Reclamation District	Lemont	2.85	17,338	ILL028070	Dec 2030	2
43	Morton STP 2	Morton	2.7	10,225	ILL030015	Dec 2030	222
44	Streator STP	Streator	2.7	12,993	ILL022004	Dec 2030	51
45	Rochelle City Of	Rochelle	2.68	9,799	ILL030741	Dec 2030	0
46	Geneseo Stp, City Of	Geneseo	2.68	6,505	ILL021814	Dec 2030	3
47	Marion SE Stp, City Of	Marion	2.63	17,620	ILL029734	Dec 2030	1
48	O Fallon STP	O Fallon	2.57	30,999	IL0021636	Mar 2031	0
49	Village Of Mokena Stw	Mokena	2.56	20,050	ILL024201	Dec 2030	2
50	Lake In The Hills Sd	Lake In The Hills	2.56	28,982	ILL021733	Dec 2030	2

Treatment technology for each plant is shown with a free Klarifi account. Create one →

Largest plants in Illinois: 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 October 26, 2026.

Wastewater terms: WWTP · POTW · MGD · NPDES permit · Centrate · Ozonation · all terms

Cite this page

Journalists, researchers and educators are welcome to quote these figures. Please credit Klarifi as the source and link to this page.

Klarifi. “Largest wastewater treatment plants in Illinois (2026).” Updated October 26, 2026. https://klarifi.io/facilities/illinois/largest

Need a custom cut of the data for a state, a county or a list of plants? Email info@klarifi.io.

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