The South Bay International Wastewater Treatment Plant, just north of the border in San Ysidro, treats sewage piped over from Tijuana and flows caught by the U.S. canyon collectors. It's being rebuilt and doubled in size.
Why it matters
The plant has been too small and in poor repair for years. In an August 2023 storm, flows reached more than three times its design capacity, and it had to bypass part of its treatment. When pipes fail in Tijuana, excess sewage lands here first, so a bigger, sturdier plant is the main U.S.-side protection against dry-weather spills.
Where it stands (as of October 2026)
- Interim expansion, done. Capacity rose from 25 to 35 million gallons a day on August 28, 2025, in about 100 days.
- Full expansion, underway. The rebuild is designed for 50 million gallons a day of average flow, with capacity for peaks up to 75. The design-build contractor is PCL Construction, with Stantec. USIBWC is doing early work, including site preparation and ordering equipment with long lead times.
- First major construction contract. In September 2026, USIBWC awarded the first major construction phase, for the expanded primary treatment works, within budget. EPA says construction should start soon after, and the full design is due for approval in October 2026.
- Timeline. In 2025, EPA said expansion to 50 MGD or more was being considered for completion by 2027. Earlier plans allowed up to five years of construction. A firm completion date hasn't been published.
Funding
The project has been reported as fully funded. In October 2024, USIBWC estimated the full cost at $600 million, and said the estimate would be refined as design progressed; no newer figure has been published. About $300 million came from the 2020 U.S.–Mexico–Canada trade agreement, which EPA transferred to USIBWC for this project in September 2024. The rest is funded jointly by EPA and USIBWC.
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
Design approval in October 2026, the start of construction, a published completion date, the total cost, and whether annual budgets keep pace.
Related
The map: South Bay plant. Project tracker.
Sources
- U.S. EPA, Tijuana River watershed public updates, quarterly updates from November 2025 to September 2026
- Wastewater Digest, South Bay plant completes 10 MGD expansion, 2025
- Project Coronado, South Bay plant launches expansion, October 2024
- U.S. EPA and USIBWC, press release on South Bay plant repairs, 2023
- San Diego Regional Water Board, executive officer report, September 2024
- Coronado Times, IBWC provides update on the fully funded treatment project, February 2025
- USIBWC, partners celebrate launch of the South Bay plant rehabilitation and expansion, October 2024, for the 50/75 MGD design, the contractor and the $600 million estimate
- U.S. EPA, USMCA Tijuana River watershed, for the $300 million and its 2024 transfer to USIBWC