A lab-scale upflow anaerobic sludge blanket (UASB) has been run for 250 days to investigate the influence of influent COD/SO42- ratios on the biodegradation behavior of starch wastewater and process performance. Stepwise decreasing COD/SO42- ratio enhanced sulfidogenesis, complicating starch degradation routes and improving process stability. The reactor exhibited satisfactory performance at a wide COD/SO42- range >= 2, attaining stable biogas production of 1.15-1.17 L L-1 d(-1) with efficient simultaneous removal of total COD (73.5-80.3%) and sulfate (82.6 +/- 6.4%). Adding sulfate favored sulfidogenesis process and diversified microbial community, invoking hydrolysis-acidification of starch and propionate degradation and subsequent acetoclastic methanogenesis; whereas excessively enhanced sulfidogenesis (COD/SO42- ratios <2) would suppress methanogenesis through electrons competition and sulfide inhibition, deteriorating methane conversion. This research in-depth elucidated the role of sulfidogenesis in bioenergy recovery and sulfate removal, advancing the applications of UASB technology in water industry from basic science. (C) 2016 Elsevier Ltd. All rights reserved.