The VSS to suspended solids (SS) ratio and the sludge methanogenic activity of the granular samples in the bioflocculant-spiked reactor were also higher than those of the other three reactors. When compared with the control reactor, flocculant-spiked reactors performed better at most of the organic loading rates as flocculant addition resulted in a considerably higher degree of retention of biomass and lower solids washout from UASB reactors.