Innovative combined technique for high concentration of azo dye AR18 wastewater treatment using modified SBR and enhanced Fenton process as post treatment

•Combined modified SBR and enhanced Fenton process as post treatment was evaluated.•Alternating anaerobic–aerobic modified SBR with external feeding was investigated.•Enhanced Fenton process using H2O2 and ZVI with ultrasonic irradiation was performed.•Detection of the dye metabolites by HPLC and UV–vis analyses were conducted.•About 99% of dye, 97% of COD, 87% of phosphorus and 89% of metabolites was removed. The purpose of this research was to evaluate the performance of combined biological/advanced oxidation process (AOP) system for treatment of wastewater containing high concentration (500mg/L) of azo dye Acid Red 18 (AR18). Two alternating anaerobic–aerobic sequencing batch reactors (SBR1 and SBR2 without and with external feeding at the beginning of aeration cycle, respectively) were operated. The effluent of the SBRs was then post treated through enhanced Fenton process (using zero-valent iron combined with ultrasonic irradiation). More than 90% and 97% of COD was removed in the combined SBR/AOP system without external carbon source (CTS1) and with external feeding (CTS2), respectively. The analysis of dye and its metabolites using UV–vis and HPLC analysis also proved that 99% of the original dye was decolorized and more than 89% of its metabolites were degraded through CST2 which is significantly higher than the reported values in the literature. Besides, more than 87% of phosphorus removal efficiency was obtained in CST2 compared to only 54.5% removal efficiency in CST1. Regarding the findings of this study, the proposed combined treatment system (CTS2) can be suggested as an effective technique for treatment of high azo dye AR18 concentration wastewater.