Effect of different redox mediators during thermophilic azo dye reduction by anaerobic granular sludge and comparative study between mesophilic (30C) and thermophilic (55C) 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<br/>The impact of different redox mediators on colour removal of azo dye model compounds and textile wastewater by thermophilic anaerobic granular sludge (55 degreesC was investigated in batch assays. Additionally, a comparative study between mesophilic (30 degreesC and thermophilic (55 degreesC 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 degreesC compared with 30 degreesC 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 AH(2)QDS, i.e. the reduced form of AQDS, was extremely accelerated at 55 degreesC compared with 30 degreesC. Furthermore, no lag-phase was observed at 55 degreesC. 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 degreesC is a more effective temperature for azo dye reduction than 30 degreesC 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. (C) 2004 Elsevier Ltd. All rights reserved