Understanding anaerobic decolourisation of textile dye wastewater: mechanism and kinetics

Worldwide, some 280,000 tons of textile dyes are discharged per annum. Degr adation of the predominant highly soluble reactive dyes is poor within acti vated sludge plants, and instead requires anaerobic pretreatment for reduct ive cleavage of the chromogenic azo bond to precede aerobic degradation. Th e mechanism for the anaerobic reductive cleavage, which results in decolour isation, is not well understood. Further, thermophilic anaerobic pre-treatm ent has not been reported, although the wastewater is produced at high temp erature (50-80 degrees C). This project therefore aimed to compare decolour isation kinetics under mesophilic (35 degrees C) and thermophilic (55 degre es C) conditions and elucidate biotic and abiotic factors in the decolouris ation of the dye, Reactive Red 235. Preliminary experiments indicated that acclimation of the up-flow anaerobic sludge blanket (UASB) biomass was unnecessary and there was no dye toxicit y at expected wastewater concentrations (0.05-0.1g/L). Decolourisation of R eactive Red 235 was studied under batch conditions with intact and autoclav ed biomass, and also using the pre-reduced supernatant from a spent culture (0.2 mu m filtered). Based on HPLC identification of the dye and reaction products, first-order kinetics was observed and rate constants of -0.0096/m in (thermophilic) and -0.0034/min (mesophilic) were estimated for intact, v iable biomass. Abiotic decolourisation was 60-80% lower for the autoclaved samples, and under 12-35% lower for the filtered supernatant Hence it may b e concluded that active anaerobic cells give the most efficient and complet e decolourisation, especially under thermophilic conditions. Nonetheless, a biotic reduction does occur and has implications for the design of a novel uncoupled reactor system.