Optimization of the Operating Conditions of a Methane-Producing Microbial Electrolysis Cell

A methane-producing microbial electrolysis cell (MEC) is a novel and highly versatile technology which enables generating methane from carbon dioxide reduction, while simultaneously treating a waste stream. A key feature of methane-producing MECs is the use of (mixed) microbial cultures as cheap and self-regenerating electrocatalysts of both the anodic and the cathodic reactions. In this study, we have analyzed the performance of a lab-scale methane-producing MEC continuously fed with acetate (as model substrate) under a range of operating condition. The main parameters which were explored include the applied organic load to the anode compartment (up to 2.1 kgCOD/m3 d), electrodic potentials, pH control with CO2 bubbling to the cathode, and temperature. For most conditions the performance of the system was assessed in terms of substrate removal efficiency, current and methane generation, biomass production, and energy yield. Nitrogen mass balances were also performed in order to preliminary assess the potential for ammonia recovery. The results of this study indicate that methane-producing MECs can be used to treat low-strength wastewater (< 1 KgCOD/ m3, i.e., unsuitable for anaerobic digestion), at ambient temperature, with a resulting sludge production which is substantially lower than that of conventional activated sludge plants operated at similar organic loads. Furthermore, the technology could also be applied to refine both the liquid effluent (containing diluted organics and ammonium) and the biogas (containing 25-45 vol% of carbon dioxide) deriving from a conventional anaerobic digestion process.