Sequential Anaerobic-Aerobic Biological Decolorization/Degradation of a Reactive Azo Dye Using Anaerobic and Activated Sludge Under Varying Salt Concentrations

Textile dyeing wastewaters usually contain a significant concentration of both reactive azo dyes and salts (mainly NaCl). Recently, sequential anaerobic-aerobic biological processes have been considered for the treatment of reactive azo dye containing wastewaters but little attention has been paid to the effect of salt concentrations on the color removal performance of these processes. In the present study the effect of NaCl concentration in the range 0-100 g/l on the decolorization and degradation of Reactive Black 5 – a common dye in textile dyeing – during a sequential anaerobic–aerobic bacterial process at RB5 concentrations of 100 and 200 mg/l was studied. Statistical analysis showed that – for both dye concentrations - increase in salt concentration in the range 0-60 g/l lead to a statistically significant increase in anaerobic decolorization efficiency whereas a further increases up to 100 g/l resulted in a statistically significant decrease. Kinetic analysis of the anaerobic decolorization data showed that for salt concentration in the range 0-80 g/l a second order model best describes the data whereas at 100 g/l the anaerobic decolorization data was best fitted using a first order model. UV-Vis spectral analysis indicated that aromatic amines formed as a result of anaerobic reduction of RB5 partly biodegraded in the subsequent aerobic phase. This analysis also showed that for NaCl concentrations up to 40 g/l the aromatic amines autoxidised whereas higher salt concentrations inhibited the autoxidation reaction. As a result – during the aerobic phase - at salt concentrations ≤ 40 g/l slight recoloration of the anaerobically decolorized wastewater occurred whereas at higher salt concentration there was no pronounced change in the color of the wastewater.