Before the full rebuild began, the South Bay plant was given a quick increase in how much it could take, to buy room while the larger work runs.
Why it matters
The plant had been running past its capacity for years. When a pipe fails in Tijuana, the excess lands here first, so every extra million gallons a day is sewage that does not go down the river instead.
Where it stands
Done. Capacity rose from 25 to 35 million gallons a day on August 28, 2025, in about 100 days.
- 25 million gallons a day receives full secondary treatment.
- The added 10 million gallons a day receives advanced primary treatment and is blended before discharge.
That distinction matters: the extra capacity is not the same quality of treatment as the original, and the plant's discharge has been under a state cease and desist order since 2025 over permit violations.
The extra 10 million gallons a day depends on the advanced primary system working. When it needed repairs in September 2026, USIBWC asked Mexico to cut flows to the plant back to 25 million gallons a day, because it could not treat 35 and still meet its permit. Repairs were finished by September 21, and the plant was cleared to return to 35.
Who's responsible
The U.S. Section of the International Boundary and Water Commission, with EPA funding. The San Diego Regional Water Board sets its discharge permit.
What to watch
Whether the blended discharge affects permit compliance, and the full expansion to 50 million gallons a day behind it.
Related
South Bay plant expansion to 50 MGD. The map: South Bay plant.
Sources
- USIBWC, 100-day goal of increasing San Diego plant capacity, August 2025
- San Diego Regional Water Board, executive officer report, August 2025
- USIBWC, repairs to the South Bay plant's advanced primary treatment system, September 14, updated September 21, 2026