Activated sludge and clarifiers: wastewater treatment plants in California
118 plants · 61% of the state · 725 MGD reported by 73 plants
Klarifi identifies 118 publicly owned wastewater treatment plants with activated sludge and clarifiers in California, 61% of the 192 plants in the state (725 MGD reported by 73 plants). The largest is Hyperion Treatment Plant in Playa Del Rey at 222.56 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 California
| # | Facility | City | Avg flow (MGD) | NPDES permit | Expires | Violations (3y) |
|---|---|---|---|---|---|---|
| 1 | Hyperion Treatment Plant | Playa Del Rey | 222.56 | CA0109991 | Apr 2028 | 42 |
| 2 | Orange County Sanitation District Plant 1 | Fountain Valley | 99.43 | CA0110604 | Jul 2026 | 0 |
| 3 | Ccsf- Southeast Water Pollution Control Plant | San Francisco | 72.75 | CA0037664 | Sep 2018 | 10 |
| 4 | Stockton Rwcf | Stockton | 28.94 | CA0079138 | Jul 2030 | 61 |
| 5 | South Bay International WWTP | San Diego | 28.5 | CA0109045 | Jun 2026 | 2645 |
| 6 | Riverside City, Water Quality Control | Riverside | 27.97 | CA0105350 | Oct 2018 | 2 |
| 7 | Encina Water Pollution Control Facility | Carlsbad | 23.63 | CAL107395 | Dec 2030 | 11 |
| 8 | Regionwide Water Recycling System - Temescal Creek Discharge | Lake Elsinore | 19.76 | CA8000188 | Sep 2020 | 34 |
| 9 | Monterey Regional WWTP | Marina | 12.63 | CA0048551 | Feb 2030 | 5 |
| 10 | Oceanside Ocean Outfall | Oceanside | 10.99 | CA0107433 | Mar 2025 | 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 California
Activated sludge and clarifiers in other states
Activated sludge and clarifiers in California: 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 · NPDES permit · POTW · all terms