In this paper we operated an upflow anaerobic sludge blanket reactor (UASBR) continuously at 35 degrees C in order to observe the effects of varying the hydraulic retention time (HRT) from 3.5 to 34.5 d and varying the organic loading rate (OLR) from 1.5 to 46 kg COD m(-3) d(-1). The pH of the digester improved, which we varied from 1.5 to 14.5 g L-1 CaO-CKD, a range pH 7.5. A high COD degradation rate of 97% and mixed liquor suspended solids (MLVSS) of 99,000 mg L-1 were achieved at an HRT of 24.5 d. The maximum methane yield was 0.346 l CH4/g CODremoved. A CO2 reduction of 87% was obtained at an OLR of 26.5 (r = 0.99). The optimum conditions for digestion of the palm oil mill effluent were determined by studying the bio-kinetics of granulation. The growth yield (Y-G) was 1.45 g VSS/g CODremoved day; the specific biomass decay (b) was 0.056; the specific biomass growth rate (mu(max)) was 0.988 d(-1); the saturation constant (K-s) was 460; and the critical retention time (Theta(c)) was 2.464 d(-1). With a feed flow rate (Q(F)) of 1.65 l/d, the upflow velocity (V-up) was 0.6 m/h, and for a Q(F) of 2.45 l/d, V-up was 0.75 m/h. (C) 2014 Elsevier B.V. All rights reserved.