Evaluation of gas and carbon transport in a methanogenic bioelectrochemical system (BES)

Bioelectrochemical systems (BESs) may be used to upgrade anaerobic digester biogas by directly converting CO₂ to CH₄ . The objective of this study was to evaluate gas (N₂ , CO₂ , CH₄ , and H₂ ) and carbon transport within a methanogenic BES. Four BES configurations were evaluated: abiotic anode with abiotic cathode (AAn-ACa), bioanode with abiotic cathode (BAn-ACa), abiotic anode with biocathode (AAn-BCa), and bioanode with biocathode (BAn-BCa). Transport of N₂ , a gas commonly used for flushing anoxic systems, out of the anode headspace ranged from 3.7 to 6.2 L/d-atm-m² , normalized to the proton exchange membrane (PEM) surface area and net driving pressure (NDP). CO₂ was transported from the cathode to the anode headspace at rates from 3.7 to 5.4 L/d-atm-m² . The flux of H₂ from cathode to anode headspace was 48% greater when the system had a biocathode (AAn-BCa) than when H₂ was produced at an abiotic cathode (BAn-ACa), even though the abiotic cathode headspace had 75% more H₂ than the AAn-BCa biocathode at the end of 1 day. A 7-day carbon balance of a batch-fed BAn-BCa BES showed transient microbial carbon storage and a net transport of carbon from anode to cathode. After a 7-day batch incubation, the CH₄ production in the biocathode was 27% greater on a molar basis than the initial CO₂ supplied to the biocathode headspace, indicating conversion of CO₂ produced in the anode. This research expands the current understanding of methanogenic BES operation, which may be used in improving the assessment of BES performance and/or in the development of alternative BES designs and mathematical models. Biotechnol. Bioeng. 2017;114: 961-969. © 2016 Wiley Periodicals, Inc.