Abstract: Three different types of anaerobic sludge (granular, thickened digestate and anaerobic sewage) were evaluated as seed inoculum sources for the high rate anaerobic digestion of pig slurry in UASB reactors. Granular sludge performance was optimal, allowing a high efficiency process yielding a volumetric methane production rate of 4.1 L CH4 L-1 d-1 at 1.5 days HRT (0.248 L CH4 g-1 COD) at an organic loading rate of 16.4 g COD L-1 d-1. The thickened digestate sludge experimented flotation problems, thus resulting inappropriate for the UASB process. The anaerobic sewage sludge reactor experimented biomass wash-out, but allowed high process efficiency operation at 3 days HRT, yielding a volumetric methane production rate of 1.7 L CH4 L-1 d-1(0.236 L CH4 g-1 COD) at an organic loading rate of 7.2 g COD L-1 d-1. To guarantee the success of the UASB process, the settleable solids of the slurry must be previously removed.

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