Reductive decolourisation of sulphonated mono and diazo dyes in one- and two-stage anaerobic systems

Abstract

This work assessed the application of one- and two-stage mesophilic anaerobic systems to colour removal of sulphonated mono and diazo dyes with ethanol as electron donor. The dyes Congo Red (CR), Reactive Black 5 (RB5) and Reactive Red 2 (RR2) were selected as model compounds and tested separately in seven different periods. The one-stage system (R1) consisted of a single up-flow anaerobic sludge blanket (UASB) reactor, whereas the two-stage system (R2) consisted of an acidogenic UASB reactor (RA), a settler and a methanogenic UASB reactor (RM). For CR and RB5, no remarkable difference was observed between the colour removal performance of both anaerobic systems R1 and R2. The experiments with RR2 revealed that R2 was more efficient on colour removal than R1, showing efficiencies almost 2-fold (period VI) and 2.5-fold (period VII) higher than those found by R1. Additionally, R2 showed a higher stability, giving a good prospect for application to textile wastewaters. Finally, the acidogenic reactor (RA) had an important role in the overall decolourisation achieved by R2 during the experiments with CR and RB5 (>78 %), whereas for RR2, a more recalcitrant dye, RA was responsible for up to 38 % of the total colour removal.