This study demonstrates microbial electrolysis cell (MEC) scale-up from a 50 mL to a 10 L cell. Initially, a 50 mL membraneless MEC with a gas diffusion cathode was operated on synthetic wastewater at different organic loads. It was concluded that process scale-up might be best accomplished using a "reactor-in-series" concept. Consequently, 855 mL and 10 L MECs were built and operated. By optimizing the hydraulic retention time (HRT) of the 855 mL MEC and individually controlling the applied voltages of three anodic compartments with a real-time optimization algorithm, a COD removal of 5.7 g L(R)(-1)d(-1) and a hydrogen production of 1.0-2.6 L L(R)(-1)d(-1) was achieved. Furthermore, a two MECs in series 10 L setup was constructed and operated on municipal wastewater. This test showed a COD removal rate of 0.5 g L(R)(-1)d(-1), a removal efficiency of 60-76%, and an energy consumption of 0.9 Wh per g of COD removed. Crown Copyright (C) 2012 Published by Elsevier Ltd. All rights reserved.