BACKGROUND The anaerobic treatment of wastewaters is usually applied to remove the organic matter, converting it into methane. The presence of sulfate in some wastewaters produces sulphide during anaerobic biological processing, which is known as an inhibitor of biological processes. In this study, a novel anaerobic bioreactor with a fixed structured bed (ABFSB) was subjected to simultaneous sulfate reduction and organic matter removal. RESULTS Synthetic wastewater was used as organic substrate and the bioreactor performance was studied with different COD/[SO42−] ratios: 0.72, 1.7, 3.5 and 6.1. The highest COD and sulfate removal efficiencies were achieved at a COD/[SO42−] ratio of 1.7 (82 and 89%, respectively). Also, in this condition a greater overall kinetic apparent parameter (0.96 h−1) was observed. The kinetic parameters indicate that the sulfate reducing bacteria played an important role in the organic matter removal process over a range of COD/[SO42−] ratios from 6.1 to 1.7. CONCLUSIONS Concerning the organic matter removal, the best synergy between sulfate reduction bacteria and metanogenic archaea communities was achieved in COD/[SO42−] ∼1.7. The ABSFB was suitable for the simultaneous organic matter and sulfate removal, especially when the COD/[SO42−] ratio was 1.7, but demonstrating a stable and efficient process in all conditions studied. © 2013 Society of Chemical Industry