Effect of different redox mediators during thermophilic azo dye reduction by anaerobic granular sludge and comparative study between mesophilic (30 °C) and thermophilic (55 °C) treatments for decolourisation of textile wastewaters

The impact of different redox mediators on colour removal of azo dye model compounds and textile wastewater by thermophilic anaerobic granular sludge (55 °C) was investigated in batch assays. Additionally, a comparative study between mesophilic (30 °C) and thermophilic (55 °C) colour removal was performed with textile wastewater, either in the presence or absence of a redox mediator. The present work clearly evidences the advantage of colour removal at 55 °C compared with 30 °C when dealing with azo coloured wastewaters. The impact of the redox mediators anthraquinone-2,6-disulfonate (AQDS), anthraquinone-2-sulfonate (AQS) and riboflavin was evident with all dyes, increasing decolourisation rates up to 8-fold compared with the mediator-free incubations. The generation of the hydroquinone form AH2QDS, i.e. the reduced form of AQDS, was extremely accelerated at 55 °C compared with 30 °C. Furthermore, no lag-phase was observed at 55 °C. Based on the present results we postulate that the production/transfer of reducing equivalents was the process rate-limiting step, which was accelerated by the temperature increase. It is conclusively stated that 55 °C is a more effective temperature for azo dye reduction than 30 °C, which on the one hand can be attributed to the faster production/transfer of reducing equivalents, but also to the decrease in activation energy requirements.