Reductive Decolourisation of Sulphonated Mono and Diazo Dyes in One- and Two-Stage Anaerobic Systems

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 (R_A), a settler and a methanogenic UASB reactor (R_M). 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 (R_A) 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, R_A was responsible for up to 38 % of the total colour removal.