Role of C/N ratio in a pilot scale Microbial Electrolysis Cell (MEC) for biomethane production and biogas upgrading

Microbial electrolysis cells (MECs) permit to couple the oxidation of waste organic streams (e.g., wastewater, fermentate or digestate) with the reduction of carbon dioxide into products with a high market value (e.g., methane or acetic acid). MECs exploit the ability of electroactive microorganisms to use a solid electrode as final electron acceptor or donor. Here, a micro pilot tubular MEC has been set up combining the anodic oxidation of the organic matter with the bioelectromethanogenesis reaction in the cathodic chamber. Seven different synthetic feeding solutions, simulating a domestic wastewater or an acidogenic fermentate, have been used to test different C/N ratio on the performance of the MEC bioanode in the range between 25 and 0.4 (molC/molN). As a main result it was found that, under the same operating conditions (i.e., anode potential controlled at + 0.2 V vs SHE and HRT of 0.5 d), a high C/N ratio (e.g., 19 mol/mol) promotes the bioelectrochemical metabolism of the electroactive biofilm. These findings are relevant for a practical application of the technology considering the variable content of carbon and nitrogen in real feedstocks.