A continuous flow microbial reverse-electrodialysis electrolysis cell (MREC) was operated under non-buffered substrate with various flow rates of catholyte effluent into anode chamber to investigate the effects on the hydrogen gas production. Adding the catholyte effluent to the anolyte influent resulted in increased salt concentration in the anolyte influent. The increasing anolyte influent salt concentration to 0.23 M resulted in improved hydrogen gas production, Coulombic recovery, yield, and hydrogen production rate to 25 +/- 1.4 mL, 83 +/- 5%, 1.49 +/- 0.15 mol-H-2/mole-COD, 0.91 +/- 0.03 m(3)-H-2/m(3)-V-an/day, respectively. These improvements were attributed to the neutral pH rather than increase in anolyte conductivity as there was no significant improvement in the reactor performance when the NaCl was directly added to the reactor. These results show that addition of catholyte effluent into the anode chamber improved the MREC performance. (C) 2017 Elsevier Ltd. All rights reserved.