BACKGROUND: High-rate anaerobic reactors represent suitable technologies to achieve efficient decolorisation of azo dyes. However, several anaerobic bioreactors have shown poor performance or even collapsed during decolorisation of azo dyes owing to the inhibitory effects of these contaminants. In this study the inhibitory effects of the azo dye Reactive Orange 14 (RO14) on different syntrophic groups in methanogenic sludge, as well as the catalytic impact of riboflavin, were assessed in batch incubations. RESULTS: Low rates of decolorisation of RO14 by the anaerobic sludge were observed, which decreased with increasing concentration of RO14 (up to 4.8 mmol L-1), when incubations proceeded in the absence of riboflavin. Concomitantly, the methanogenic activity of the anaerobic sludge diminished by up to 65% compared with controls not exposed to RO14. Methylotrophic methanogens were the group most affected by RO14, followed by acetoclastic methanogens, while formate-utilising methanogens emerged as the group most tolerant to the inhibitory effects of RO14. Acidogenic micro-organisms were also inhibited by RO14 during decolorisation assays. Addition of riboflavin as a redox mediator increased the decolorisation rate of RO14 up to 53-fold compared with controls lacking this catalyst, However, the methanogenic activities did not vary significantly regardless of the presence or absence of riboflavin, suggesting that methanogens were not responsible for the enhanced decolorisation observed. CONCLUSION: Redox mediators could prevent the inhibitory effects of recalcitrant azo dyes in anaerobic consortia by decreasing the time of exposure of azo-reducing micro-organisms to the pollutants. (C) 2008 Society of Chemical Industry