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Anaerobic Phased Solids Digestion of Mixed Wastes
Author: Robert Beggs, Perry Schafer, Adam Ross, Dave Konwinski (Onsite Power Systems CA); Ruihong Zhang (U.C. Davis)
Date: 10/07
WEFTEC Conference October 2007

The anaerobic phased solids (APS) digestion process is a two-phase semi-batch feed thermophilic digestion process that is particularly well suited to high solids content wastes. The APS process couples multiple batch fed hydrolysis digestion tanks with a methanogen-friendly biogasification tank using intermittent circulation of free liquid between the tanks. Lab scale testing has been followed by a commercial demonstration system completed in late 2006. Lab scale test results were promising in that good volatile solids (VS) destruction and continuous overall gas production were observed despite the batch feeding of the hydrolysis tanks. Digestion of a 50%-50% mixture (on a VS basis) of green waste and food waste produced 0.32 liter (L) methane per g of VS and an 82% VS reduction. The commercial demonstration APS system at the University of California, Davis (UC Davis) is being used to evaluate green waste, food wastes, and waste mixtures for materials handling considerations, process optimization, and gas production. Materials handling equipment adapted from various industrial sources has generally worked well during initial runs of the commercial demonstration system. Initial gas production results have been in line with expectations from lab test results. Improvements are being made to the heating and mixing systems. Additional tests are planned for co-digestion of other mixed wastes, including wastewater sludge and food processing wastes.