Performance of a semi-pilot tubular microbial electrolysis cell (MEC) under several hydraulic retention times and applied voltages

•A tubular MEC meets legal requirements for COD removal (domestic wastewater).•HRTs below 4h required the use of a second MEC module acting as a polishing step.•Net energy consumption was in a range between 0.2 and 0.9Whg-COD−1. The influence of applied voltage and hydraulic retention time on the performance of a semi-pilot modular tubular wastewater-fed microbial electrolysis cell (MEC) with high scalability was investigated. A chemical oxygen demand (COD) removal efficiency of 80%, as well as an energy consumption of 0.3–1.1Whg-COD−1 removed, were achieved. Hydrogen production was limited by the reduced amounts of organic matter fed into the reactor, the poor performance of the cathode, and COD consuming by non electrogenic microorganisms. The presence of COD consuming microorganism that do not contribute to electrogenic metabolism severely affected the MEC performance.