Lagoon systems: wastewater treatment plants in California
99 plants · 52% of the state · 204 MGD reported by 67 plants
Klarifi identifies 99 publicly owned wastewater treatment plants with lagoon systems in California, 52% of the 192 plants in the state (204 MGD reported by 67 plants). The largest is Riverside City, Water Quality Control in Riverside at 27.97 MGD.
Lagoon systems treat wastewater in large earthen ponds over weeks of retention time, with or without mechanical aeration. They serve thousands of small towns because they are cheap to run, but they struggle with ammonia and nutrient limits, fill with sludge over decades and are the plants most often converted or upgraded when a permit is renewed.
The largest lagoons in California
| # | Facility | City | Avg flow (MGD) | NPDES permit | Expires | Violations (3y) |
|---|---|---|---|---|---|---|
| 1 | Riverside City, Water Quality Control | Riverside | 27.97 | CA0105350 | Oct 2018 | 2 |
| 2 | Colton/san Bernardino Stp, Rix | Colton | 20.79 | CA8000304 | Jul 2018 | 0 |
| 3 | Regionwide Water Recycling System - Temescal Creek Discharge | Lake Elsinore | 19.76 | CA8000188 | Sep 2020 | 34 |
| 4 | Monterey Regional WWTP | Marina | 12.63 | CA0048551 | Feb 2030 | 5 |
| 5 | Victor Valley Wastewater Reclamation Authority | Victorville | 8.44 | CA0102822 | May 2025 | 7 |
| 6 | Corona WWTP 1 | Corona | 7.32 | CA8000383 | Jun 2025 | 9 |
| 7 | Simi Vly Cnty Sanitation | Simi Valley | 7.12 | CA0055221 | Jul 2031 | 36 |
| 8 | Manteca Wwqcf | Manteca | 6.91 | CA0081558 | Jul 2031 | 7 |
| 9 | Lompoc Wastewater Plant | Lompoc | 6.37 | CA0048127 | Apr 2027 | 11 |
| 10 | San Luis Obispo WWTP | San Luis Obispo | 5.6 | CAL049224 | Dec 2030 | 23 |
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
Lagoon systems in other states
Lagoon systems 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: WWTP · POTW · Biosolids · Activated sludge · Oxidation ditch · all terms