BACKGROUND Anodic oxidation, a promising approach for the removal of refractory organic pollutants in wastewater through advanced oxidation, still faces the challenge of high energy consumption. This study coupled an O-2-reducing biocathode, a cathode thermodynamically more favorable than conventional abiotic ones, with an MnOx/Ti electrocatalytic porous anode, to produce a novel biocathode-electrocatalytic reactor (BECR). RESULTS After inoculation, the BECR was successfully started up to generate current at a voltage of 0.7 V, which was lower than the startup voltages of electrocatalytic reactors with abiotic cathodes. The cathode potential of the BECR was increased by approximately 300 mV. Abundant electrochemically active microorganisms were found on the BECR cathode with Pseudomonas and Acinetobacter as the dominant genera reducing O-2 to H2O. Furthermore, hydroxyl radical (center dot OH) generation was detected in the anode chamber of the BECR after startup at 0.7 V and contributed to its current production. This phenomenon was not observed for electrocatalytic reactors. The mechanism of the BECR in reducing the applied voltage to generate center dot OH was well elucidated using the thermodynamic theory of Gibbs free energy. CONCLUSIONS The coupling of the O-2-reducing biocathode improved the thermodynamics of the BECR cathode toward a high redox potential and consequently reduced its applied voltage to generate center dot OH, thereby rendering it as potentially energy saving and suitable for use in wastewater treatment. (c) 2019 Society of Chemical Industry