BACKGROUND To accelerate direct interspecies electron transfer (DIET), magnetite is supplemented in anaerobic activated sludge (AS). However, it is usually reduced by dissimilatory iron reducing bacteria (DIRB). Therefore, an AS system supplemented with submicron magnetite particles (Fe3O4/AS) in an up-flow microaerobic sludge reactor was established, aiming to remove tribromophenol (TBP) efficiently and alleviate the magnetite dissolution. RESULTS Under microaerobic condition (DO, 0.6 mg L-1), the efficiencies of removal of COD, TBP, and methane production in Fe3O4/AS system were, respectively, 14.3%, 16.7%, and 27.2% higher than those of the AS system. Correspondingly, the dehydrogenase (DHA) and Coenzyme F-420 (CoF420) activities were separately 1.38-fold and 1.41-fold enhanced. The enhanced biodegradation was via potential DIET, as visualized by microbes and magnetite particles, Geobacter enrichment and c-type cytochromes (c-Cyts) increasement. Notably, magnetite majorly maintained an intact structure. In effluents, the Fe(II) concentrations were maintained at less than 10 mg L-1 under microaerobic conditions, far less than the amount under anaerobic conditions, which might be due to the in situ redox cycle of Fe(III)-reduction and Fe(II)-reoxidation. CONCLUSION Significant enhancements in removal efficiencies of TBP and COD and methanogenesis were demonstrated in a Fe3O4/AS system, indicating intrinsically faster electron transfer via electrical conduction. Putative oxidation of dissolved Fe(II) in situ occurred in microaerobic environments, which could protect magnetite in the system without extra magnetite supplementation. (c) 2018 Society of Chemical Industry