Improved decolorization and mineralization of azo dye in an integrated system of anaerobic bioelectrochemical modules and aerobic moving bed biofilm reactor

• Stacked BES-MBBR was developed for efficient decolorization and mineralization. • BES modules showed improved pretreatment performance in AO7 decolorization. • MBBR played an important role in further products removal and COD removal. • Mechanism of azo dye degradation in the integrated system was investigated. In this study, a stacked integrated system with anaerobic bioelectrochemical system (BES) and aerobic moving bed biofilm reactor (MBBR) was developed to improve the decolorization and mineralization of azo dye. This stacked BES-MBBR exhibited better performance with acid orange (AO7) decolorization of 96.4 ± 0.6% and chemical oxygen demand (COD) removal of 87.7 ± 4.4%. Contribution of each module in the BES and MBBR stages indicated that BES modules enhanced the pretreatment process in AO7 decolorization, and MBBR played an important role in further removal of COD. The mechanism analysis indicated that the azo bond was cleaved with reductive decolorization at biocathode in the anaerobic BES stages, and then the intermediate products can be further oxidized with COD removal in the aerobic MBBR stage. This work demonstrated that the integrated system with stacked anaerobic BES and aerobic MBBR could provide a promising way for the pretreatment and post-treatment of refractory wastewater.