Factors affecting bio-decolorization of azo dyes and COD removal in anoxic–aerobic REACT operated sequencing batch reactor

•Longer anoxic-REACT period resulted in higher dye decolorization efficiency.•Higher decolorization rate observed for Acid Red 14 (naphthol type) azo dye.•Higher COD removal rate found for Acid Yellow 9 (non-naphthol type) azo dye.•Dye molecular structures affected performance of mixed bacterial culture in SBR.•Better performance of SBR operated without co-substrate in color and COD removal.

The main objective of this study was to investigate the effects of (i) time ratios of operating periods; (ii) azo dye molecular structure and (iii) presence/absence of co-substrate on the performances of the anoxic–aerobic REACT operated sequencing batch reactors (SBRs). The results show that among the anoxic REACT periods of 6, 10 and 16 h studied, the highest bio-decolorization efficiency of AO7 was achieved with the longest anoxic REACT period while the highest COD removal rate was attained for the shortest anoxic REACT period. The molecular structures of the investigated azo dyes, namely Acid Orange 7 (AO7), Acid Orange 10 (AO10), Acid Red 14 (AR14) and Acid Yellow 9 (AY9), were found to affect the performances of the SBRs in terms of the bio-decolorization and COD removal efficiencies. A higher bio-decolorization rate was observed for naphthol-type azo dyes whereas the highest COD removal rate was achieved for non-naphthol type azo dye. Comparison of AO7 bio-decolorization and COD removal in the SBRs operated with and without co-substrate showed better performance achieved by the latter, thus suggesting the possibility of using this approach for the treatment of industrial wastewater contaminated with spent dye.