San Francisco Sequencing Batch Reactor Temperature Phased Anaerobic Digestion: Efficient Adaptation to Class A Biosolids

San Francisco Public Utilities Commission (SFPUC), in collaboration with CH2M HILL, evaluated and designed the conversion of its existing high-rate mesophilic anaerobic digestion process at OSP to a SBR TPAD process in order to produce Class A biosolids, per USEPA Title 40 Code of Federal Regulations Part 503 (40 CFR 503) Alternative 1 - Time and Temperature. Approximately 30 percent of San Francisco's biosolids are land applied in neighboring Solano County during the dry season (April through October). Recently, Solano County revised Chapter 25 of its County Code biosolids regulation to requires that a portion of a utility's biosolids be treated to Class A standards, in order to allow the utility to continue to land apply Class B biosolids in the county after October 2012. Pursuant to these pending requirements, SFPUC determined the best near-term approach to meet Class A requirements during the dry season requires a solution that avoids environmental permitting delays and minimizes capital expenditures, while expediting construction. SFPUC staff developed a temperature-phased digestion approach to convert the mesophilic anaerobic digestion system to SBR TPAD (Jolis, 2009) without constructing additional digester volume.SBR TPAD is phased anaerobic digestion. Phase 1 will consist of three sequentially fed, thermophilic phase digesters (TPD) (Digesters 2, 3, and 4) at 131°F/55°C in fill/hold/draw mode with the hold digester allowing a minimum of 24 hours of reaction time at 131°F without additional digester feed, to provide sufficient destruction of pathogens as required by 40 CFR 503. Phase 2 will consist of one mesophilic phase digester (MPD) (Digester 1 is designated and Digester 2 is designated back-up) at 100°F/38°C in flow-through mode receiving digested solids from the TPD draw, which is transferred to biosolids dewatering.