Effect of the anode operating conditions on the performance of a continuous-flow methane-producing microbial electrolysis cell

Microbial electrolysis cells (MECs) have emerged as a highly versatile technology which enables coupling wastewater treatment to the generation of chemicals and energy carriers (such as H2 and CH4) [1,2]. In a microbial electrolysis cell, “electro-active” microorganisms use a solid-state anode as terminal electron acceptor for the oxidation of organic waste substrates to carbon dioxide, while simultaneously releasing protons to the solution. Electrons flow from the anode to the cathode through an external circuit while protons diffuse to the cathode through a proton-exchange membrane separating the two electrodic compartments. At the cathode, in the presence of a suitable (bio)catalyst, the electrons combine with a soluble electron acceptor, generating a target product. In most cases, MECs require the potential generated from substrate oxidation at the anode to be boosted with an external power supply in order to make the cathodic reaction thermodynamically feasible. Even though the need for external power decreases the net energy balance with respect to the traditional Anaerobic Digestion (AD) process, the MEC approach brings some specific advantages, such as the possibility to deal with diluted wastewaters and to operate the process even at ambient temperature. Furthermore, the possibility to integrate the AD process with bioelectrochemical systems has also been proposed and it seems to be particularly interesting when AD is combined with a methane-producing MEC.