Reactive textile dye colour removal in a sequencing batch reactor

Two sequencing batch reactors (SBRs) with sequenced anaerobic/aerobic phases were used to study biological colour removal from a simulated cotton textile effluent containing an azo reactive dye. One of the reactors was daily fed with Remazol Brilliant Violet 5R dye and the other was used as control. When operating with a sludge retention time (SRT) of 15 days the total COD removal was around 80%, with 30% being removed anaerobically. After 40–50 days of acclimatization the colour removal efficiency reached a maximum, stable value of 90% from a feed dye concentration of 90 mg/l, almost all being removed during the anaerobic phase. This colour removal was attributed to microbial degradation rather than adsorption and colour removal capacity was not lost even after a seven-day absence of dye in the fed substrate. The dye-fed reactor experienced a reduction in the ORP values attained during the non-aerated phase, after acclimatization, an effect not observed in the dye-free control. Under denitrifying conditions it was observed that the decolouration levels achieved in the anaerobic phase decreased from 90% to 70% after only two cycles with a feed containing 45–60 mg NO₃/l. Reduction of the SRT value from 15 to 10 days reduced the biomass concentration from 2.0 to 1.2 g VSS/l and lowered colour removal levels from 90% to 30–50%. When the SRT value was increased back to 15 days the colour removal capacity of the system was completely recovered, suggesting that with a SRT of 10 days the adequate microbial population could not be installed in the reactors.