Understanding anaerobic decolourisation of textile dye wastewater: mechanism and kinetics

Abstract

Worldwide, some 280,000 tons of textile dyes are discharged per annum. Degradation of the predominant highly soluble reactive dyes is poor within activated sludge plants, and instead requires anaerobic pretreatment for reductive cleavage of the chromogenic azo bond to precede aerobic degradation. The mechanism for the anaerobic reductive cleavage, which results in decolourisation, is not well understood. Further, thermophilic anaerobic pre-treatment has not been reported, although the wastewater is produced at high temperature (50–80°C). This project therefore aimed to compare decolourisation kinetics under mesophilic (35°C) and thermophilic (55°C) conditions and elucidate biotic and abiotic factors in the decolourisation 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 toxicity at expected wastewater concentrations (0.05–0.1g/L). Decolourisation of Reactive Red 235 was studied under batch conditions with intact and autoclaved biomass, and also using the pre-reduced supernatant from a spent culture (0.2 μm filtered). Based on HPLC identification of the dye and reaction products, first-order kinetics was observed and rate constants of –0.0096/min (thermophilic) and –0.0034/min (mesophilic) were estimated for intact, viable biomass. Abiotic decolourisation was 60–80% lower for the autoclaved samples, and under 12–35% lower for the filtered supernatant. Hence it may be concluded that active anaerobic cells give the most efficient and complete decolourisation, especially under thermophilic conditions. Nonetheless, abiotic reduction does occur and has implications for the design of a novel uncoupled reactor system.