The reductive biotransformation of acid orange 7 (AO7) was explored in a lab-scale upflow anaerobic sludge blanket (UASB) reactor at low hydraulic residence times (HRT). A colour removal of 85% was achieved when the reactor was operated at a HRT of 6 hours, but decreased up to 70% when the HRT was lowered to 2 hours. Addition of the quinone model compound, anthraquinone 2,6-disulfonate (AQDS), as redox mediator, allowed for a considerably higher decolourising efficiency (>90% at all the HRT evaluated). The results indicate that the use of catalytic concentrations of AQDS (AQDS/AO7 molar ratio about 0.01) can accelerate decolourising processes achieving satisfying extent of decolourisation.

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