Performance of a semi-pilot tubular microbial electrolysis cell (MEC) under several hydraulic retention times and applied voltages L. Gil-Carrera, A. Escapa, B. Carracedo, A. Morán, X. Gómez ⇑ Chemical and Environmental-Bioprocess Engineering Department, Natural Resources Institute (IRENA), University of Leon, Avda. de Portugal 41, Leon 24009, Spain highlights  A tubular MEC meets legal requirements for COD removal (domestic wastewater).  HRTs below 4 h required the use of a second MEC module acting as a polishing step.  Net energy consumption was in a range between 0.2 and 0.9 Wh g- COD 1 . graphical abstract article info Article history: Received 20 May 2013 Received in revised form 4 July 2013 Accepted 7 July 2013 Available online 11 July 2013 Keywords: Microbial electrolysis cell (MEC) Hydrogen production Domestic wastewater abstract 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 chem- ical oxygen demand (COD) removal efficiency of 80%, as well as an energy consumption of 0.3– 1.1 Wh g-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. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Microbial electrolysis cells (MECs) are electrochemical devices that employ electrogenic bacteria to produce hydrogen by oxidiz- ing organic matter (using the anode as an electron acceptor) through the hydrogen evolution reaction at the cathode of the electrochemical reactor. This process requires a small amount of electrical energy compared to hydrogen production through water electrolysis and also has the additional advantage of simulta- neously treating biowastes or waste waters (Logan et al., 2006; Gómez et al., 2011), offering a renewable and potentially autono- mous energy technology for wastewater treatment (Logan et al., 2006; Logan, 2004; Rozendal and Buisman, 2005; Rozendal et al., 2006). MECs are a relatively new technology, and therefore few scale- up experiences have been reported so far. In a previous work it was demonstrated a successful scale-up of the combined wastewater treatment and H 2 production process from a 50 mL laboratory MEC to a 10 L MEC (Gil-Carrera et al., 2013a). Yet, other studies conducted at a larger scale (120–1000 L), revealed that even though it is a promising technology for urban and industrial waste- water treatment, several difficulties still need to be overcame (Cusick et al., 2011; Heidrich et al., 2012). The scaling-up of tubular MECs (all the above referred scaling- up experiences were carried-out on flat designs) has also proven to be feasible, achieving 85% COD removal using domestic waste- water as substrate in a semi-pilot 4-L reactor with relatively low 0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.07.020 ⇑ Corresponding author. Tel.: +34 987 29 1845; fax: +34 987 29 1839. E-mail address: xagomb@unileon.es (X. Gómez). Bioresource Technology 146 (2013) 63–69 Contents lists available at SciVerse ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech