Acidic effluent discharged after the hydrogen fermentation of sugarcane juice was used to produce methane by batch fermentation and UASB reactor. Significant parameters affecting methane production including substrate to biomass (S/X) ratio, nickel (Ni) concentration, and cobalt (Co) concentration were optimized by response surface methodology with central composite design in batch mode. A maximum methane yield (MY) of 305.4 mL CH4/g-volatile solid (VS)(substrate) ((sub)-added) was achieved at an S/X ratio of 0.83 g-VSsub/g-VSinoculum, a Ni concentration of 0.53 mg/L, and a Co concentration of 0.06 mg/L Continuous methane production was conducted at various hydraulic retention times (HRT) using the optimum conditions obtained from the batch experiments. The optimum HRT of 4 days in a UASB reactor resulted in a maximum methane production rate (MPR) and MY of 1.27 +/- 0.05 L-CH4/L-culture day and 348 +/- 13 mL-CH4/g-COD, respectively. Total energy generated was 219.23 kJ/L-substrate or 8.77 kJ/g-COD, and COD removal efficiency was 75.60%. (C) 2015 Elsevier Ltd. All rights reserved.