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 CO 2 to CH 4 . The objective of this study was to evaluate gas (N 2 , CO 2 , CH 4 , and H 2 ) 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 N 2 , a gas commonly used for flushing anoxic systems, out of the anode headspace ranged from 3.7 to 6.2 L/d‐atm‐m 2 , normalized to the proton exchange membrane (PEM) surface area and net driving pressure (NDP). CO 2 was transported from the cathode to the anode headspace at rates from 3.7 to 5.4 L/d‐atm‐m 2 . The flux of H 2 from cathode to anode headspace was 48% greater when the system had a biocathode (AAn‐BCa) than when H 2 was produced at an abiotic cathode (BAn‐ACa), even though the abiotic cathode headspace had 75% more H 2 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 CH 4 production in the biocathode was 27% greater on a molar basis than the initial CO 2 supplied to the biocathode headspace, indicating conversion of CO 2 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.