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SFPUC staff favors thermal‑hydrolysis for Southeast biosolids project after alternatives review
Summary
After reviewing four state‑of‑the‑art options, staff recommended moving forward with a thermal hydrolysis pretreatment (THP/CAMBI) for the Southeast Plant biosolids replacement project, citing lower operating costs, reduced digester volume and higher energy reuse potential; the Alternatives Analysis Report will continue and environmental review will follow.
The San Francisco Public Utilities Commission heard an informational update on the Southeast Plant Biosolids Replacement Facility and the alternatives analysis that will shape the project’s technology selection.
Emilio Cruz, assistant general manager for infrastructure, walked the commission through the project objectives — meet regulatory requirements, convert biosolids from Class B to Class A, achieve 100% beneficial reuse, reuse biogas, minimize neighborhood impacts, and support community benefits during construction. Staff reviewed a long list of technologies and narrowed options to four state‑of‑the‑art alternatives: (1) mesophilic anaerobic digestion with mechanical drying, (2) mesophilic digestion plus composting, (3) temperature‑phased anaerobic digestion, and (4) thermal hydrolysis pretreatment (THP/CAMBI) followed by mesophilic digestion.
Cruz said the analysis showed construction costs across alternatives were roughly within 10% of one another, but operating costs, energy consumption and greenhouse gas emissions were distinguishing factors. He summarized staff’s findings: mechanical drying operated at high energy and labor costs; composting required large land footprints or long‑term contracts; temperature‑phased digestion required more digester volume; and THP offered significant reductions in digester volume and produced a Class A product while yielding energy benefits through increased biogas production. "Alternative 4 was clearly the winner in looking at all these alternatives and all of these selection criteria," Cruz said.
Technical benefits and risks: staff reported THP pretreatment can reduce required digester volume by about 30% and decrease final biosolids mass by 10–15%, improving manageability and reuse options. The technology is established in Europe and has recent U.S. deployments in Washington, D.C., and Hampton Roads; staff visited plants in Europe and reviewed operator experience. Cruz acknowledged operational and maintenance considerations and said the vendor has iterated on design and reliability.
Costs and schedule: Cruz noted a general anticipated construction value in the neighborhood of $800 million for the overall project site (construction‑only figure), with a broader program level cost estimate in the conversation near $1 billion for full alternatives. Staff is completing a detailed Alternatives Analysis Report and plans to return to the commission in December to recommend a preferred alternative and, if approved, move into environmental review and conceptual engineering with the environmental phase (CER) expected to take up to 24 months.
Next steps: staff will complete deeper analyses of alternatives 3 and 4, confirm reuse markets and energy projections, undertake environmental analysis, and return with a recommendation to select a preferred alternative and proceed to the CER and conceptual engineering phases.
