What is the largest wastewater treatment plant in Illinois?

MWRDGC Terrence J O'Brien Water Reclamation Plant in Skokie, treating an average of 170 MGD under NPDES permit IL0028088. Sanitary District Of Decatur in Decatur is second at 26.53 MGD.

How much wastewater do Illinois’s largest plants treat?

The 50 largest plants treat 402 MGD combined, 80% of the flow reported by all 641 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 · 402 MGD combined · 80% of the state's reported flow

MWRDGC Terrence J O'Brien Water Reclamation Plant in Skokie is the largest publicly owned wastewater treatment plant in Illinois by reported average flow, treating about 170 million gallons per day. The 50 plants below treat 402 MGD between them, 80% of the flow reported by all 641 plants in the state, and serve about 2,989,019 people. Every plant links to its profile with permit, technology and compliance data.

All plants in Illinois →The 100 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, treatment technology and three-year violation count.
#	Facility	City	Avg flow (MGD)	Population	NPDES permit	Expires	Technology	Violations (3y)
1	MWRDGC Terrence J O'Brien Water Reclamation Plant	Skokie	170	946,333	IL0028088	Jul 2022	Clarifiers, Biogas balloons	18
2	Sanitary District Of Decatur	Decatur	26.53	78,304	ILL028321	Dec 2030	Clarifiers, Biogas silos, Biogas balloons	1
3	Springbrook Water Reclamation Center	Naperville	17.26	162,500	ILL034061	Dec 2030	Clarifiers, Biogas silos, Biogas balloons	0
4	Bloomington Normal WRD -West STP	Bloomington	10.71	57,849	ILL027731	Dec 2030	Clarifiers, Aerated lagoons, Uncovered lagoons, Biogas silos, Biogas balloons	0
5	Fox River Water Reclamation Dist	Elgin	10	107,193	ILL028657	Dec 2030	Clarifiers, Biogas silos	51
6	Flagg Creek Water Reclamation	Burr Ridge	8.31	61,953	ILL022586	Dec 2030	—	5
7	Elmhurst Stp, City Of	Elmhurst	7.92	46,532	ILL028746	Dec 2030	Clarifiers, Biogas silos	8
8	Lake County Public Works Department	Buffalo Grove	7.89	72,507	ILL022055	Dec 2030	Clarifiers	1
9	Spring Creek STP	Springfield	7.59	106,720	ILL021989	Dec 2030	Clarifiers, Uncovered lagoons, Biogas silos	28
10	Glenbard Wastewater Authority	Glen Ellyn	7.04	70,797	ILL021547	Dec 2030	Clarifiers, Uncovered lagoons, Biogas silos, Biogas balloons	27
11	Joliet West Stp, City Of	Joliet	6.67	75,393	ILL033553	Dec 2030	Clarifiers, Biogas silos, Biogas balloons	0
12	Downers Grove Sanitary District	Downers Grove	6.39	48,727	ILL028380	Dec 2030	Clarifiers, Biogas silos	5
13	Urbana & Champaign Sd	Urbana	6.2	116,900	ILL031500	Dec 2030	Clarifiers, Aerated lagoons, Biogas silos, Biogas balloons	2
14	Woodridge-Greene Valley WWTP	Woodridge	6.1	56,482	ILL031844	Dec 2030	Clarifiers, Biogas silos	2
15	Alton Waste Water Treatment Plant	Alton	5.38	—	ILL027464	Dec 2030	Clarifiers	39
16	Danville Sanitary District STP	Danville	5	83,005	ILL020788	Dec 2030	Uncovered lagoons	11
17	Romeoville Wastewater Treatment Facility	Romeoville	4.81	39,430	ILL048526	Dec 2030	Biogas balloons	0
18	Wheaton Sanitary District WWTF	Wheaton	4.5	52,451	ILL031739	Dec 2030	Clarifiers, Uncovered lagoons, Biogas silos	7
19	Plainfield North Stp, Village Of	Plainfield	4.34	44,542	ILL074373	Dec 2030	Biogas balloons	0
20	Sterling WWTP	Sterling	4.31	14,764	IL0060569	Sep 2024	Aerated lagoons	5
21	Scwrd Sugar Creek WWTP	Springfield	3.95	41,092	ILL021971	Dec 2030	Clarifiers, Uncovered lagoons, Biogas balloons	9
22	East Moline Wastewater	East Moline	3.74	32,163	IL0028550	Mar 2031	Clarifiers, Biogas silos, Biogas balloons	2
23	North Slope Treatment Plant	Moline	3.43	15,605	ILL029947	Dec 2030	Clarifiers	1
24	Frankfort, Village Of	Frankfort	3.32	19,411	ILL072192	Dec 2030	Clarifiers, Uncovered lagoons, Carrousels / oxidation ditches, Biogas balloons	7
25	Galesburg Sanitary District STP	Galesburg	3.22	30,052	ILL023141	Dec 2030	Clarifiers, Aerated lagoons, Biogas silos	6
26	Fox River Water Reclamation Di	Elgin	3.21	22,420	ILL035891	Dec 2030	Clarifiers, Biogas silos	0
27	Belleville Wastewater Treatment Facility	Belleville	3	21,449	ILL021873	Dec 2030	Clarifiers, Aerated lagoons, Biogas silos	2
28	Bloomington-Normal WRD SE WWTP	Bloomington	2.94	74,683	ILL073504	Dec 2030	Uncovered lagoons	0
29	Moline South Treatment Plant	Moline	2.72	25,460	ILL029939	Dec 2030	Clarifiers, Uncovered lagoons, Biogas silos, Biogas balloons	3
30	City Of Monmouth	Monmouth	2.64	8,886	ILL036218	Dec 2030	Clarifiers, Aerated lagoons, Covered lagoons, Uncovered lagoons, Carrousels / oxidation ditches	20
31	Collinsville Wastewater Treatment Plant	Collinsville	2.59	33,769	ILL028215	Dec 2030	Clarifiers, Uncovered lagoons, Biogas balloons	5
32	Village Of Bolingbrook STP #3	Bolingbrook	2.52	26,649	ILL069744	Dec 2030	Carrousels / oxidation ditches, Biogas balloons	18
33	Pekin STP #1	Pekin	2.4	33,338	ILL034495	Dec 2030	Clarifiers, Uncovered lagoons, Biogas silos	7
34	Edwardsville Stp, City Of	Edwardsville	2.3	25,189	ILL026310	Dec 2030	Clarifiers	1
35	Algonquin Stw, Village Of	Algonquin	2.3	30,730	ILL023329	Dec 2030	Clarifiers, Biogas silos	13
36	Sycamore STP - North Plant	Sycamore	2.26	18,317	ILL031291	Dec 2030	Clarifiers	0
37	City Of East Peoria Wastwater Treatment Plant	East Peoria	2.25	19,416	ILL028576	Dec 2030	Clarifiers, Aerated lagoons, Uncovered lagoons, Biogas balloons	6
38	Addison Stw North 3	Addison	2.24	23,173	ILL033812	Dec 2030	Clarifiers, Biogas silos	0
39	Lake In The Hills Sd	Lake In The Hills	2.19	28,982	ILL021733	Dec 2030	Clarifiers, Carrousels / oxidation ditches	2
40	Jacksonville STP	Jacksonville	2.16	19,446	ILL021661	Dec 2030	Uncovered lagoons	1
41	Rochelle City Of	Rochelle	2.12	9,799	ILL030741	Dec 2030	Clarifiers, Covered lagoons, Uncovered lagoons, Biogas silos, Biogas balloons	0
42	Morton STP 2	Morton	2.11	10,225	ILL030015	Dec 2030	Uncovered lagoons, Biogas balloons	222
43	Metropolitan Water Reclamation District	Lemont	2	17,338	ILL028070	Dec 2030	Clarifiers	2
44	Yorkville-Bristol Sd STP	Yorkville	1.99	20,101	ILL036412	Dec 2030	Biogas balloons	216
45	O Fallon STP	O Fallon	1.98	30,999	IL0021636	Mar 2031	—	0
46	Marion SE Stp, City Of	Marion	1.96	17,620	ILL029734	Dec 2030	Clarifiers	1
47	Village Of Mokena Stw	Mokena	1.84	20,050	ILL024201	Dec 2030	Clarifiers	2
48	Deerfield WRF	Deerfield	1.78	19,528	ILL028347	Dec 2030	Clarifiers	11
49	Woodstock Northside Wastewater Treatment Plant	Woodstock	1.77	15,634	ILL031861	Dec 2030	Clarifiers, Uncovered lagoons	3
50	Caseyville Township East STP	O Fallon	1.75	35,113	ILL021083	Dec 2030	Clarifiers	3

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 September 11, 2026.

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