Azo dye decolourisation by anaerobic granular sludge

The decolourisation of 20 selected azo dyes by granular sludge from an upwa rd-flow anaerobic sludge bed (UASB) reactor was assayed. Complete reduction was found for all azo dyes tested, generally yielding colourless products. The reactions followed first-order kinetics and reaction rates varied grea tly between dyes: half-life times ranged from 1 to about 100 h. The slowest reaction rates were found for reactive dyes with a triazine reactive group . There was no correlation between a dye's half-life time and its molecular weight, indicating that cell penetration was probably not an important fac tor. Since granular sludge contains sulphide, eight dyes were also monitore d for direct chemical decolourisation by sulphide. All these dyes were redu ced chemically albeit at slower rates than in the presence of sludge at com parable sulphide levels. Increasing sulphide concentrations, even when pres ent in huge excess, stimulated the azo reduction rate. The results indicate that granular sludge can decolourise a broad spectrum of azo dye structure s due to non-specific extracellular reactions. Reducing agents (e.g., sulph ide) in sludge play an important role. The presence of anaerobic biomass is probably beneficial for maintaining the pools of these reduced compounds. (C) 2001 Elsevier Science Ltd. All rights reserved.