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This study examines the feasibility of oil sands process water (OSPW) treatment in a gravitational flow-through membraneless microbial electrolysis cell (MEC) using crushed metallurgical coke as an electrode material. Synthetic OSPW was composed of the water soluble fraction (WSF) of diesel, naphthenic acids (NAs), and metals with a total chemical oxygen demand (COD) concentration of 200 mg L − 1 and total NA concentration of 9.4 mg L − 1. OSPW treatment under bioelectrochemical conditions (1.4 V applied voltage) was compared with a control system operated at 0 V. Furthermore, in an additional set of experiments, microaerobic conditions were applied to both MEC and control setups to facilitate NA biodegradation. Highest NA and COD removal rates were observed in the MEC with microaeration in electrode compartments, where total NA concentration decreased to 4.2 mg L − 1 at a hydraulic retention time of 4 days. Furthermore, microtox and microalgae toxicological assays showed substantial toxicity decrease in this MEC.