Enhanced removal of sulfamethoxazole by an anaerobic/aerobic SBR with an oxidation-reduction cycle of magnetite

This study aims to evaluate an advanced activated sludge process that integrates activated sludge process with biological Fenton reaction to enhance antibiotic treatment in wastewater. The target antibiotic used in this study was sulfamethoxazole (SMX). An anaerobic/aerobic sequencing batch reactor (SBR) containing magnetite was proposed. Anaerobic and aerobic sludge was exposed to alternating anaerobic and aerobic conditions for treating synthetic wastewater containing 1 mg/L SMX and 405 mg/L chemical oxygen demand (COD). Stable and enhanced SMX removal was observed in the SBRs with magnetite, which achieved more than 86% removal, 40% higher than that without magnetite. Magnetite dosage at 1 and 3 g/L did not significantly affect SMX removal. Fluorescence observations indicated the formation of hydroxyl radicals (·OHs) in the matrix of magnetite-sludge aggregates. This elucidated the primary mechanism of the significant SMX removal. It was shown that Fe (III) in the magnetite was reduced to Fe (II) and then oxidized to Fe (III) under anaerobic and aerobic conditions respectively, creating continuous oxidation-reduction of magnetite. Moreover, hydrogen peroxide (H2O2) was microbially generated under aerobic conditions and drove the biological Fenton reaction. Approximately 90% of COD was removed under steady-state conditions in the SBRs containing magnetite. The higher COD removal in SBRs with magnetite was due to microbial Fe (III) reduction.