Role of the organic loading rate and the electrodes' potential control strategy on the performance of a micro pilot tubular microbial electrolysis cell for biogas upgrading

An innovative biogas upgrading process consists in the utilization of a microbial electrolysis cell (MEC) in which a biocathode performs the bioelectromethanogenesis reaction reducing the CO2 into CH4 while an additional CO2 removal mechanism consists in the CO2 sorption as HCO3-, due to alkalinity generation in the catholyte. Here, a two chamber 12-liter tubular MEC has been developed to upgrade biogas by using bioelectrochemical organic matter oxidation at the anode to partially sustain the energy demand of the process. In the tubular MEC, the electroactive microorganisms' selection was obtained by polarizing the anode chamber at + 0.2 V vs. SHE (Standard Hydrogen Electrode). Under this condition, three values of the applied organic loading rate (OLR) have been investigated. Once the best OLR was selected at 2.55 gCOD/Ld, the potentiostatic control of the tubular MEC was switched from the anode to the cathode. As reported in a previous experiment, the potentiostatic control shift resulted in a sharp decrease of the process' energy consumption thanks to minimization of the anodic overpotential. Moreover, three different runs were conducted with the cathodic potential controlled at -1.3 V; -1.8 V; -2.3 V vs. SHE to investigate the performances of the CO2 abatement. The lowest energy consumption for CO2 removal was obtained during the -1.3 V vs SHE condition with a consumption of 0.5 kWh/ Nm3 of removed CO2. Those results indicate that the potentiostatic control switch from the anode to the cathode permits to minimize the energy consumption of a micro pilot MEC having a tubular configuration.