To maximize the performance of bioelectrochemical system (BES) for simultaneous sewage sludge degradation, Cr(VI)-laden wastewater treatment and electricity generation, a three-chamber bioelectrochemical reactor (BER) that used sewage sludge as the anodic fuel and Cr(VI) laden-wastewater as the catholyte was constructed, and an in-depth investigation of the electron fluxes, microbial communities and metagenomics genomes was performed to quantify the performance of the BER at different Cr(VI) concentrations. The maximum total chemical oxygen demand (TCOD) removal achieved with the BER was obtained using 75 mg/L of Cr(VI) catholyte, and transphilic neutral fractions, fulvic acid-like substances and aromatic hydrophobic acids were mainly removed. The quantification of electron fluxes revealed that more electrons were consumed by electrogenesis than methanogenesis at increased Cr(VI) concentration, and a maximum power density of 9.8 W/m(3) was obtained using 150 mg/L Cr(VI). In the sight of metagenomics, increased catholyte Cr(VI) concentration caused an increase in the abundance of exoelectrogens (Geobacter, Comamonas, Desulfovibrio, Desulfobulbus, Aeromonas, and Desulfotomaculum) and a decrease in the major methanogenic groups of gene encoding enzymes. The shift in methanogenic communities induced by the competition change was the main driver that shaped the structures of methanogenic functional genes during the operation of the BER at different Cr(VI) concentrations. The results might provide a solid theoretical basis for the simultaneous treatment of sewage sludge and Cr(VI)-laden wastewater using BERs.