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PUC pushes thermal hydrolysis pretreatment for Southeast digester rebuild after European site visits
Summary
After a five-day international site visit, SFPUC staff and Supervisor Cohen recommended including a thermal hydrolysis pretreatment (THP/CAMBI) in the Southeast Treatment Plant digester rebuild, citing odor containment, reduced digester volume, and Class A biosolids production; the Commission heard questions about food waste logistics, costs, and next steps for CER and detailed design.
Supervisor Cohen and SFPUC staff presented the findings of a multi-site international visit and recommended moving forward with the thermal hydrolysis process (THP) as a core pretreatment for the Southeast Treatment Plant digester rebuild. Cohen said the trip gave him confidence that the technology can reduce odors and produce a Class A biosolids product, and offered to sponsor and advocate for the project.
Why it matters: The digester rebuild is a major component of the Sewer System Improvement Program (SSIP). Staff told commissioners that THP can reduce solids volumes and the number/size of digesters required, contain odors within the plant, and produce a marketable Class A product. Staff presented site visits to plants in Dublin (Ringsend), Crowley, Oxford, the CAMBI fabrication facility, and the Davie Hume plant — with the largest CAMBI facility producing ~250 dry tons/day and supplying Class A biosolids for agricultural use.
Details and numbers: Kathy Howe, presenting the PUC findings, said the THP equipment investment is "about $25 to $35,000,000" within an overall construction program of over $1,000,000,000 for the full digester rebuild project. Staff said THP pretreatment reduces pathogen loads (pasteurization), lowers solids volume going to digesters, and can be prefabricated on skids for constrained sites.
Commission questions and concerns: Commissioners asked about residual odors at intake/headworks and whether food-waste co-digestion would be practical in San Francisco. Staff and Supervisor Cohen said THP treats the solids stream that most drives digester odors and that the pretreatment "goes a long way" toward odor reduction, but that headworks and food-waste intake remain odor-control challenges. Staff noted logistical constraints: collection and sorting of food waste (plastic contamination) and available space at Southeast limit some options.
Vendor selection and experience: Staff compared CAMBI and Veolia THP offerings and said CAMBI has a longer track record in the plants they visited; Veolia has had operational challenges at some sites. Presenters described CAMBI units shipped on prefabricated skids and committed to providing start-up and operator training.
Next steps: The project team intends to complete the Notice of Preparation / CER work and advance to detailed design with THP included as the preferred pretreatment. Commissioners and members of the public asked for continued outreach, transparency on alternatives and costs, and clear reporting back to the Commission during CEQA and procurement milestones.
Quotes: "This is the biggest decision that we're making," Supervisor Cohen said of the digester rebuild, adding the site visits made him a fierce advocate for modernizing the Southeast plant. Kathy Howe summarized the technical case: "THP provides for pretreatment of solids before the material goes into the digesters" and produces a Class A product staff said the Commission could "take pride in."
What’s next: Staff signaled they are moving toward completing the CER, advancing detailed design with THP, and returning to the Commission with procurement and CEQA milestones.
