On 25 September 2026, the California State Water Resources Control Board published an updated account of transboundary wastewater flows crossing the U.S.–Mexico border into the San Diego region, confirming that the South Bay International Wastewater Treatment Plant (SBIWTP) is now discharging blended effluent at a higher authorised rate under an amended permit (California State Water Resources Control Board (.gov)). The board's webpage reports that, on 8 April 2026, the San Diego Water Board adopted Order No. R9-2026-0005, which amended the NPDES Permit for the SBIWTP and authorised a 10 MGD expansion of treatment capacity (California State Water Resources Control Board (.gov)).
KEY TAKEAWAYS
- On 8 April 2026, the San Diego Water Board adopted Order No. R9-2026-0005, amending the SBIWTP's NPDES Permit to authorise a 10 MGD expansion (California State Water Resources Control Board (.gov)).
- Since September 2025, USIBWC has been treating up to 35 MGD of influent, blending up to 10 MGD of advanced primary effluent with 25 MGD of secondary effluent before ocean discharge (California State Water Resources Control Board (.gov)).
- The amendment requires a Treatment Optimization Report to the San Diego Water Board by 1 December 2026, evaluating ways to reduce pollutants in the blended effluent (California State Water Resources Control Board (.gov)).
- Weekly monitoring of fecal coliform and E. coli at Dairy Mart Bridge is now mandated under the amended Monitoring and Reporting Program (California State Water Resources Control Board (.gov)).
- In September 2024, EPA transferred USMCA funds to USIBWC for the SBIWTP rehabilitation and expansion, part of a $300 million USMCA commitment for Tijuana River watershed infrastructure (U.S. Environmental Protection Agency (.gov)).
WHAT HAPPENED
From 25 MGD to 35 MGD under an amended permit
According to the State Water Board's 25 September 2026 update, the SBIWTP had been designed for 25 MGD of secondary treatment, with the existing 25 MGD of secondary effluent blended with up to 10 MGD of advanced primary effluent before discharge to the Pacific Ocean via the South Bay Ocean Outfall (California State Water Resources Control Board (.gov)). The webpage states that the chemically-enhanced primary treatment stage uses coagulants to increase solids removal, and that in September 2025 USIBWC began treating up to 35 MGD of influent through advanced primary treatment followed by 25 MGD of secondary treatment using existing activated sludge processes (California State Water Resources Control Board (.gov)).
On 27 August 2025 the San Diego Water Board adopted Cease and Desist Order No. R9-2025-0139, which set interim effluent limits for the 10 MGD expansion and required USIBWC to meet its final permitted effluent limits by 30 June 2026 (California State Water Resources Control Board (.gov)). The 8 April 2026 adoption of Order No. R9-2026-0005 then folded that expansion into the standing permit, with the amended permit due to remain in force until the San Diego Water Board reissues the NPDES Permit in late 2027 (California State Water Resources Control Board (.gov)).
What the amendment requires, and what comes next
The State Water Board lists three specific changes bundled into the 8 April 2026 amendment: updated performance goals and influent action levels to reflect the higher flow being treated, the 1 December 2026 Treatment Optimization Report, and weekly fecal coliform and E. coli monitoring at Dairy Mart Bridge (California State Water Resources Control Board (.gov)). The underlying driver, according to the same page, is chronic transboundary flow from Mexico into the Tijuana River, with pollutants from urban runoff and sewage reaching the Tijuana River Estuary and coastal waters (California State Water Resources Control Board (.gov)).
EPA's USMCA Tijuana River Watershed page, published 15 June 2026, places the SBIWTP within a broader $300 million USMCA infrastructure programme; in September 2024 EPA formally transferred those USMCA funds to USIBWC for the SBIWTP rehabilitation and expansion project (U.S. Environmental Protection Agency (.gov)). The State Water Board adds that USIBWC awarded a contract for the rehabilitation and expansion in August 2024, expects construction through mid-2031, and that the project will double treatment capacity from 25 MGD to 50 MGD (California State Water Resources Control Board (.gov)).
Reported figures at a glance
| Item | Reported value | Source |
|---|---|---|
| Original SBIWTP secondary treatment capacity | 25 MGD | California State Water Resources Control Board (.gov) |
| Authorised 10 MGD expansion (advanced primary stage) | 10 MGD | California State Water Resources Control Board (.gov) |
| Current operating flow under Order R9-2026-0005 | 35 MGD | California State Water Resources Control Board (.gov) |
| Rehabilitation and expansion target capacity | 50 MGD | California State Water Resources Control Board (.gov) |
| USMCA infrastructure funding committed to Tijuana River watershed | $300 million | U.S. Environmental Protection Agency (.gov) |
| Rehabilitation and expansion construction window | August 2024 award through mid-2031 | California State Water Resources Control Board (.gov) |
| Treatment Optimization Report due | 1 December 2026 | California State Water Resources Control Board (.gov) |
| Final permitted effluent compliance deadline | 30 June 2026 | California State Water Resources Control Board (.gov) |
SPECIFICATION READ
Reported plant scale. The 25 MGD baseline is the only single-number flow the sources give for the SBIWTP, and the order-of-magnitude check is straightforward: 25 MGD equals about 95,000 m³/day, while the post-expansion 35 MGD equals about 132,000 m³/day, and the 50 MGD rehabilitation target equals about 189,000 m³/day (arithmetic from the 25 MGD, 35 MGD and 50 MGD figures reported by California State Water Resources Control Board (.gov)). The sources do not report influent BOD, COD, TSS or population equivalent for the SBIWTP, so those numbers cannot be cited here; a buyer scoping a similar transboundary plant has to treat influent strength as an open input.
Treatment train and what this event bears on. The State Water Board page describes a screening-to-ocean train running from raw transboundary influent through chemically-enhanced primary treatment, followed by 25 MGD of activated-sludge secondary treatment, with the 10 MGD of primary effluent blended into the 25 MGD secondary stream and discharged to the Pacific Ocean via the South Bay Ocean Outfall (California State Water Resources Control Board (.gov)). The 8 April 2026 permit amendment bears directly on the primary-and-secondary interface: it sets the new influent action levels and performance goals for that higher combined flow, and it forces a Treatment Optimization Report that will evaluate how to reduce pollutants in the blended effluent by 1 December 2026 (California State Water Resources Control Board (.gov)).
What it means for your plant. If you operate a coastal or transboundary municipal works in the rough 95,000–190,000 m³/day band, receiving a blended stream of treated wastewater, untreated wastewater, groundwater and stormwater, and you are under permit pressure to add capacity without a full secondary-train rebuild, the SBIWTP pathway is a chemically-enhanced primary stage running ahead of an existing activated-sludge secondary train, with the primary effluent blended into the secondary flow and discharged via an existing ocean outfall (California State Water Resources Control Board (.gov)). Operators in that position should pay particular attention to the monitoring step now bolted on at Dairy Mart Bridge (weekly fecal coliform and E. coli) and to the 1 December 2026 optimization report deadline, since both shape the next permit cycle (California State Water Resources Control Board (.gov)). For plant managers comparing integration options on the primary-to-secondary interface, the reference designs on our Municipal hub and the Integrated Sewage Treatment product hub cover the equipment classes this event implicates; if your screening turns on biological-stage selection for similar flows, the SBR for Hotel Wastewater: 2026 Process Design & Buyer's Guide walkthrough sets out the kind of decision points a buyer would map onto an SBIWTP-style expansion.
FAQ
What is the SBIWTP, and who owns it?
The SBIWTP is a federally owned transboundary wastewater treatment plant operated by the United States section of the International Boundary and Water Commission on the U.S. side of the Tijuana border, and it is regulated under San Diego Water Board Order No. R9-2021-0001 as amended (California State Water Resources Control Board (.gov)).
What did Order No. R9-2026-0005 actually authorise?
It amended the SBIWTP's NPDES Permit to authorise the 10 MGD expansion, set updated performance goals and influent action levels for the higher flow, require a Treatment Optimization Report by 1 December 2026, and add weekly fecal coliform and E. coli monitoring at Dairy Mart Bridge (California State Water Resources Control Board (.gov)).
What is the headline cost figure tied to this work?
EPA reports that the U.S. government committed $300 million under USMCA for treatment works in the Tijuana River watershed, with funds transferred to USIBWC for the SBIWTP rehabilitation and expansion project in September 2024 (U.S. Environmental Protection Agency (.gov)).
How long will the larger rehabilitation and expansion take?
USIBWC awarded the rehabilitation and expansion contract in August 2024 and expects construction to run through mid-2031, doubling capacity from 25 MGD to 50 MGD while the plant stays in operation (California State Water Resources Control Board (.gov)).
