Anaerobic-aerobic processes for the treatment of textile dyeing wastewater containing three commercial reactive azo dyes: Effect of number of stages and bioreactor type

[Display omitted] •Anaerobic-aerobic processes evaluated for treatment of simulated dyeing wastewater.•Two stage anaerobic–aerobic process had better COD and color removal than one stage.•Higher COD and color removal rate was observed in anaerobic MBSBBR compared to SBR.•In sequential SBR-MBR process COD and color removal were 77.1 ± 7.9% and 79.9 ± 1.5%•Nitrate in MBR increased due to partial aerobic oxidation of aromatic amines. In this study, the effect of number of stages and bioreactor type on the removal performance of a sequential anaerobic–aerobic process employing activated sludge for the treatment of a simulated textile dyeing wastewater containing three commercial reactive azo dyes was considered. Two stage processes performed better than one stage ones, both in terms of overall organic and color removal, as well as the higher contribution of anaerobic stage to the overall removal performance, thereby making them a more energy efficient option. The employment of a moving bed sequencing batch biofilm reactor, which uses both suspended and attached biomass, for the implementation of the anaerobic stage of the process, was compared with a sequencing batch reactor that only employs suspended biomass. The results showed that, although there was no meaningful difference in biomass concentration between the two bioreactors, the latter reactor had better performance in terms of chemical oxygen demand (COD) removal efficiency and rate and color removal rate. Further exploratory tests revealed a difference between the roles of suspended and attached bacterial populations, with the former yielding better color removal whilst the latter had better COD removal performance. The sequential anaerobic–aerobic process, employing an aerobic membrane bioreactor in the aerobic stage resulted in COD and color removal of 77.1 ± 7.9% and 79.9 ± 1.5%, respectively. The incomplete COD and color removal was attributed to the presence of soluble microbial products in the effluent and the autoxidation of dye reduction metabolites, respectively. Also, aerobic partial mineralization of the dye reduction metabolites, was experimentally observed.