A microbial reverse-electrodialysis electrolysis cell (MREC) was used to produce hydrogen gas from fermentation wastewater without the need for additional electrical energy. Increasing the number of cell pairs in the reverse electrodialysis stack from 5 to 10 doubled the maximum current produced from 60 A/m(3) to 120 A/m(3) using acetate. However, more rapid COD removal required a decrease in the anolyte hydraulic retention time (HRT) from 24 to 12 h to stabilize anode potentials. Hydrogen production using a fermentation wastewater (10 cell pairs, HRT = 8 h) reached 0.9 +/- 0.1 L H-2/L-reactor/d (1.1 +/- 0.1 L H-2/g-COD), with 58 +/- 5% COD removal and a coulombic efficiency of 74 +/- 5%. These results demonstrated that consistent rates of hydrogen gas production could be achieved using an MREC if effluent anolyte COD concentrations are sufficient to produce stable anode potentials. (C) 2015 Elsevier Ltd. All rights reserved.