Activated sludge and clarifiers: wastewater treatment plants in Illinois
160 plants · 25% of the state · 413 MGD reported by 150 plants
Klarifi identifies 160 publicly owned wastewater treatment plants with activated sludge and clarifiers in Illinois, 25% of the 641 plants in the state (413 MGD reported by 150 plants). The largest is MWRDGC Terrence J O'Brien Water Reclamation Plant in Skokie at 170 MGD.
Activated sludge is the standard mechanical process for cities: aeration basins where bacteria consume organic matter, followed by clarifiers that settle the biomass out. These plants are identified by their circular or rectangular clarifiers and by the suspended-growth process code, and they are the main market for blowers, diffusers, clarifier mechanisms and instrumentation.
The largest activated sludge plants in Illinois
| # | Facility | City | Avg flow (MGD) | NPDES permit | Expires | Violations (3y) |
|---|---|---|---|---|---|---|
| 1 | MWRDGC Terrence J O'Brien Water Reclamation Plant | Skokie | 170 | IL0028088 | Jul 2022 | 18 |
| 2 | Sanitary District Of Decatur | Decatur | 26.53 | ILL028321 | Dec 2030 | 1 |
| 3 | Springbrook Water Reclamation Center | Naperville | 17.26 | ILL034061 | Dec 2030 | 0 |
| 4 | Bloomington Normal WRD -West STP | Bloomington | 10.71 | ILL027731 | Dec 2030 | 0 |
| 5 | Fox River Water Reclamation Dist | Elgin | 10 | ILL028657 | Dec 2030 | 51 |
| 6 | Elmhurst Stp, City Of | Elmhurst | 7.92 | ILL028746 | Dec 2030 | 8 |
| 7 | Lake County Public Works Department | Buffalo Grove | 7.89 | ILL022055 | Dec 2030 | 1 |
| 8 | Spring Creek STP | Springfield | 7.59 | ILL021989 | Dec 2030 | 28 |
| 9 | Glenbard Wastewater Authority | Glen Ellyn | 7.04 | ILL021547 | Dec 2030 | 27 |
| 10 | Joliet West Stp, City Of | Joliet | 6.67 | ILL033553 | Dec 2030 | 0 |
| 11 | Downers Grove Sanitary District | Downers Grove | 6.39 | ILL028380 | Dec 2030 | 5 |
| 12 | Urbana & Champaign Sd | Urbana | 6.2 | ILL031500 | Dec 2030 | 2 |
| 13 | Woodridge-Greene Valley WWTP | Woodridge | 6.1 | ILL031844 | Dec 2030 | 2 |
| 14 | Alton Waste Water Treatment Plant | Alton | 5.38 | ILL027464 | Dec 2030 | 39 |
| 15 | Wheaton Sanitary District WWTF | Wheaton | 4.5 | ILL031739 | Dec 2030 | 7 |
| 16 | Scwrd Sugar Creek WWTP | Springfield | 3.95 | ILL021971 | Dec 2030 | 9 |
| 17 | East Moline Wastewater | East Moline | 3.74 | IL0028550 | Mar 2031 | 2 |
| 18 | North Slope Treatment Plant | Moline | 3.43 | ILL029947 | Dec 2030 | 1 |
| 19 | Frankfort, Village Of | Frankfort | 3.32 | ILL072192 | Dec 2030 | 7 |
| 20 | Galesburg Sanitary District STP | Galesburg | 3.22 | ILL023141 | Dec 2030 | 6 |
| 21 | Fox River Water Reclamation Di | Elgin | 3.21 | ILL035891 | Dec 2030 | 0 |
| 22 | Belleville Wastewater Treatment Facility | Belleville | 3 | ILL021873 | Dec 2030 | 2 |
| 23 | Moline South Treatment Plant | Moline | 2.72 | ILL029939 | Dec 2030 | 3 |
| 24 | City Of Monmouth | Monmouth | 2.64 | ILL036218 | Dec 2030 | 20 |
| 25 | Collinsville Wastewater Treatment Plant | Collinsville | 2.59 | ILL028215 | Dec 2030 | 5 |
Contacts and project signals for every plant on this list
Klarifi adds verified decision-maker contacts, permit limits, discharge monitoring data and the capital projects planned in each city, filterable by technology, size and compliance record.
Other technologies in Illinois
Activated sludge and clarifiers in other states
Activated sludge and clarifiers 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.