The effect of salt on the performance and characteristics of a combined anaerobic–aerobic biological process for the treatment of synthetic wastewaters containing Reactive Black 5

The findings of the present work showed that variation in NaCl concentration, in the range 0–100 g/l, has a significant effect on the performance and characteristics of a combined anaerobic–aerobic process used for treatment of a synthetic RB5 containing wastewater. The effect of increasing NaCl concentration on anaerobic decolourisation efficiency was time dependent and two different types of relationships were observed for the initial and the later part of the process. Anaerobic decolourisation data from runs carried out under low salinity (NaCl concentration ⩽ 20 g/l) was successfully modelled using the second order kinetic rate equation, whereas at high salinity (NaCl concentration = 100 g/l) the first order kinetic model gave a better fit of the experimental data. Increase in salinity resulted in increase in Extracellular Polymeric Substances concentration throughout the anaerobic process and Soluble Microbial Products during the early stages of the process. UV–Vis spectral and FTIR analysis showed no effect of salt on the mechanism of anaerobic azo dye reduction whereas it lead to retardation or inhibition of the autooxidation of one of anaerobic RB5 reduction metabolites during the aerobic stage. The latter has implications regarding the use of the combined anaerobic–aerobic process for treatment of azo dye containing wastewaters.

► NaCl had a time dependent effect on anaerobic decolourisation efficiency.
► First order model fitted the anaerobic decolourisation data at high salinity.
► Anaerobic decolourisation at low salinity was modelled by second order rate equation.
► Increased salinity enhanced the production of SMP and EPS by anaerobic sludge.
► Increasing NaCl concentrations retarded the autooxidation of RB5 metabolite.