Activated sludge and clarifiers: wastewater treatment plants in Illinois
164 plants · 25% of the state · 873 MGD reported by 155 plants
Klarifi identifies 164 publicly owned wastewater treatment plants with activated sludge and clarifiers in Illinois, 25% of the 664 plants in the state (873 MGD reported by 155 plants). The largest is Metropolitan Water Reclamation in Chicago at 281.73 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 | Metropolitan Water Reclamation | Chicago | 281.73 | ILL028061 | Dec 2030 | 10 |
| 2 | MWRDGC Terrence J O'Brien Water Reclamation Plant | Skokie | 205.79 | IL0028088 | Jul 2022 | 16 |
| 3 | Sanitary District Of Decatur | Decatur | 34.42 | ILL028321 | Dec 2030 | 1 |
| 4 | Springbrook Water Reclamation Center | Naperville | 20.15 | ILL034061 | Dec 2030 | 0 |
| 5 | Greater Peoria Sanitary And Sewage District | Peoria | 19.22 | ILL021288 | Dec 2030 | 12 |
| 6 | Spring Creek STP | Springfield | 16.41 | ILL021989 | Dec 2030 | 29 |
| 7 | Bloomington Normal WRD -West STP | Bloomington | 16.33 | ILL027731 | Dec 2030 | 0 |
| 8 | Fox River Water Reclamation Dist | Elgin | 13.07 | ILL028657 | Dec 2030 | 50 |
| 9 | Glenbard Wastewater Authority | Glen Ellyn | 11.79 | ILL021547 | Dec 2030 | 27 |
| 10 | Collinsville Wastewater Treatment Plant | Collinsville | 11.53 | 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 October 26, 2026.
Wastewater terms: Activated sludge · Biological nutrient removal · Equalization tank · FOG · Centrate · all terms