Feasibility of High Strength Waste Co-Digestion for the San Francisco Public Utilities Commission

The San Francisco Public Utilities Commission is interested in evaluating the economic feasibility of co-digesting various high strength waste material, mainly grease trap waste and food waste at their anticipated new anaerobic digestion facility expected to be operational at the Southeast Water Pollution Control Plant in 2022. High strength waste material increases the production of biogas when co-digested with municipal sludge, resulting in an additional renewable energy source that can be beneficially used. However, processing high strength waste at the new Southeast Plant’s digestion facility results in increased digestion capacity, increased biogas recovery system capacity, increased dewatering requirements, and increased biosolids for hauling and beneficial use.The quantities and characteristics of yellow grease, grease trap waste and food waste available to the SFPUC were first estimated. Characterization of the waste was based on the commission’s own work and available literature. The baseline anaerobic digestion process chosen for this evaluation was a temperature-phased anaerobic digestion (TPAD) process sized to process the projected Southeast Plant’s 2045 biosolids loads. It was also assumed that the high strength waste would be pre-processed offsite and arrive at the Southeast Plant in a slurry form ready for anaerobic digestion.Various options for co-digesting the available high strength waste were considered and the impact on anaerobic digestion and biogas generation was quantified. Economic analysis compared the baseline TPAD digestion process with the considered options in terms of increased project cost, increased operation and maintenance costs, and increased electricity generation potential using co-generation technology.The results of the various co-digestion options considered in this study showed no economic benefit from co-digesting the available high strength waste. Strategy of co-digestion should be based on using the extra digestion capacity that will be available at the new facility’s startup, and gradually curtailing high strength waste addition as the digestion facility reaches its design capacity in 2045.