Molasses wastewater contains high levels of organic compounds, cations, and anions, causing operational problems for anaerobic biological treatment. To establish a high organic loading treatment system for industrial molasses wastewater, this study designed a combined system comprising an acidification tank, a thermophilic multi-stage (MS)-upflow anaerobic sludge blanket (UASB) reactor, mesophilic UASB reactor, and down-flow hanging sponge reactor. The average total chemical oxygen demand (COD) and biochemical oxygen demand removal rates were 85% +/- 3% and 95% +/- 2%, respectively, at an organic loading rate of 42 kgCOD(cr) m(-3) d(-1) in the MS-UASB reactor. By installation of the acidification tank, the MS- UASB reactor achieved low H-2-partial pressure. The abundance of syntrophs such as fatty acid-degrading bacteria increased in the MS- UASB and 2nd-UASB reactors. Thus, the acidification tank contributed to maintaining a favorable environment for syntrophic associations. This study provides new information regarding microbial community composition in a molasses wastewater treatment system. (C) 2015 Published by Elsevier Ltd.