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. (C) 2017 Elsevier Ltd. All rights reserved.