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This work assessed the impact of microaeration and the redox mediator anthraquinone‐2,6‐disulfonate (AQDS) on both azo dye reduction and by‐products degradation (aromatic amines) in a mesophilic reactor. The experiment was carried out in an up‐flow anaerobic sludge blanket (UASB) reactor with 3.2 L of working volume and hydraulic retention time (HRT) of 24 h. The synthetic effluent contained 50 mg L−1 of the dye Reactive Red 2 (RR2). The reactor was initially operated under anaerobic conditions and, subsequently, was microaerated with atmospheric air (1 mL min−1) and then supplemented with the redox mediator (AQDS) at a concentration of 25 μM. The microaeration effect on dye reduction and by‐products formed was not evident. However, AQDS considerably enhanced RR2 reduction, and its regeneration is not affected by the presence of oxygen. It seems that the cleavage of the double bond between nitrogen groups is a much easier way to start the attack of the azo dye, and it is independent of the presence of oxygen. Thus, the introduction of hydroxyl groups into the aromatic ring was not playing a role in the color removal process. In terms of ecological parameters, the ecological niches were maintained, and the distribution of species increased, despite the majority being generalists, i.e., not specialized in dye degradation. Furthermore, anaerobic species are dominant over facultative species, which explains why there was no population diversity recovery after the microaeration started. [ABSTRACT FROM AUTHOR]

Abstract: 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 (1.2mM) was applied. However, R2 presented a slightly better stability, in which the acidogenic reactor (R2,A) played a major role on dye reduction, as compared to the methanogenic reactor (R2,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 12h) 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. [Copyright &y& Elsevier]