In this study, a three-stage upflow anaerobic sludge blanket (UASB) system was developed and tested for hydrogen (H-2) and methane (CH4) production from cassava wastewater with an emphasis on CH4 production. The experiment was carried out at mesophilic temperature (37 degrees C) at different chemical oxygen demand (COD) loading rates from 5 to 18 kg/m(3)d (based on total liquid holding volume) with a recycle ratio of the final effluent to both the first and second bioreactors at a constant 1: 1 flow rate ratio of feed: final effluent. The first bioreactor was maintained pH at 5.5 while those of the other two bioreactors were not controlled. At an optimum COD loading rate of 15 kg/m(3)d, the system provided the highest COD removal level (92.5%) and the highest H-2 and CH4 yields of 0.43 mL H-2/g COD applied, and 328 mL CH4/g COD applied, respectively. The very high productivity of CH4 with the very low H-2 productivity resulted from the recycled methanogen sludge from the third bioreactor to the first and second bioreactors. The process performance of the three-stage UASB system in terms of optimum COD loading rate and total energy yield was much higher than those of single and two-stage anaerobic processes. (C) 2018 Elsevier Ltd. All rights reserved.