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Decolourisation of the azo dye model compound, Congo Red (CR), and real textile wastewater, was assessed in one- and two-stage anaerobic treatment systems (R₁ and R₂, respectively). High colour removals were achieved in both treatment systems even when a very high CR concentration (1.2 mM) was applied. However, R₂ presented a slightly better stability, in which the acidogenic reactor (R(2,A)) played a major role on dye reduction, as compared to the methanogenic reactor (R(2,M)), evidencing the role of fermentative microorganisms. The minimum electron donor concentration required to sustain dye reduction was much higher than the stoichiometric amount. Additionally, a decrease on the hydraulic retention time (from 24 to 12 h) did not significantly affect decolourisation, indicating that electron transfer was not a concern. Finally, experiments with real textile wastewater showed low decolourisation efficiencies in both systems, most likely due to the presence of dyes not susceptible to reductive decolourisation under these experimental conditions.

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Azo dye reduction at 55 degrees C by thermophilic anaerobic granular sludge was investigated distinguishing between the biotic and abiotic mechanisms. The impact of the redox mediator anthraquinone-2,6-disulfonate (AQDS) on colour removal and co-substrate oxidation was also investigated. Metabolic activities of the thermophilic inoculum induced a fast azo dye reduction and indicated a biotic predominance in the process. The addition of co-substrate enhanced the decolourisation rates 1.7-fold compared with the bottles free of co-substrate. Addition of AQDS together with co-substrate enhanced the k value 1.5-fold, compared with the incubation containing co-substrate in the absence of AQDS. During a comparative study between sludge samples incubated under mesophilic (30 degrees C) and thermophilic (55 degrees C) conditions, the decolourisation rate at 55 degrees C reached values up to sixfold higher than at 30 degrees C. Biological treatment at 55 degrees C showed a fast initial generation of reducing compounds via co-substrate oxidation, with AQDS increasing the azo dye reduction rate in all the incubations tested. Nevertheless, high concentrations of AQDS showed severe inhibition of thermophilic acetate and propionate oxidation and methane production rates. These promising results indicate that there may be good prospects for thermophilic anaerobic treatment of other reductive transformations such as reduction of nitroaromatics and dehalogenation.

The effect of temperature, hydraulic retention time (HRT) and the redox mediator anthraquinone-2,6-disulfonate (AQDS), on electron transfer and subsequent color removal from textile wastewater was assessed in mesophilic and thermophilic anaerobic bioreactors. The results clearly show that compared with mesophilic anaerobic treatment, thermophilic treatment at 55 degrees C is an effective approach for increasing the electron transfer capacity in bioreactors, and thus improving the decolorization rates. Furthermore, similar color removals were found at 55 degrees C between the AQDS-free and AQDS-supplemented reactors, whereas a significant difference (up to 3.6-fold) on decolorization rates occurred at 30 degrees C. For instance, at an HRT of 2.5 h and in the absence of AQDS, the color removal was 5.3-fold higher at 55 degrees C compared with 30 degrees C. The impact of a mix of mediators with different redox potentials on the decolorization rate was investigated with both industrial textile wastewater and the azo dye Reactive Red 2 (RR2). Color removal of RR2 in the presence of anthraquinone-2-sulfonate (AQS) (standard redox potential E(0)' of -225 mV) was 3.8-fold and 2.3-fold higher at 30 degrees C and 55 degrees C, respectively, than the values found in the absence of AQS. Furthermore, when the mediators 1,4-benzoquinone (BQ) (E(0)' of +280 mV), and AQS were incubated together, there was no improvement on the decolorization rates compared with the bottles solely supplemented with AQS. Results imply that the use of mixed redox mediators with positive and negative E(0)' under anaerobic conditions is not an efficient approach to improve color removal in textile wastewaters.