Bioelectrochemical conversion of carbon dioxide to methane for biogas upgrading
Dykstra, Christine M.
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Biogas produced by anaerobic digestion contains a mixture of carbon dioxide (CO2), methane (CH4) and other trace gases. To increase the energy content (i.e., CH4) of biogas, current methods of biogas upgrading separate or sequester CO2. Instead, bioelectrochemical systems (BESs) may be utilized to directly convert biogas CO2 to CH4. However, fundamental questions remain to be answered before BES biogas upgrading technology can be scaled up. Therefore, the overall objective of this research was to develop and test a bioelectrochemical system (BES) designed to convert carbon dioxide into methane for the purpose of upgrading the energy content of anaerobic digester biogas, with the specific objectives to: i) analyze and describe the processes taking place in an electromethanogenic BES under well-defined conditions; ii) investigate the microbial communities in an electromethanogenic BES under varied conditions; iii) test BES performance under applied, non-ideal conditions (e.g., hydrogen sulfide contamination of biogas feed, actual digester biogas feed); and iv) develop and test a zero-valent iron (ZVI) amended biocathode to improve CH4 production. To achieve these objectives, BESs were developed with acetate-fed bioanodes and CO2-fed biocathodes, which were maintained at -0.8V vs. SHE. By comparing abiotic systems with biological systems, this research demonstrated that the transport of carbon and gases through the proton exchange membrane of a methanogenic BES can influence the extent of biocathode CH4 production. The microbial community of a methanogenic biocathode was examined by comparing a biocathode inoculated with a mixed methanogenic (MM) culture and a biocathode inoculated with an enriched hydrogenotrophic methanogenic (EHM) culture, developed from the MM culture following pre-enrichment with H2 and CO2. Inoculum pre-enrichment resulted in a 3.8-fold larger CH4 production rate, although the archaeal communities in both biocathodes converged primarily on a single phylotype (Methanobrevibacter arboriphilus). However, the bacterial community of the EHM-biocathode bacterial community was enriched in Proteobacteria, exoelectrogens and putative producers of electron shuttle mediators relative to the MM-biocathode, indicating an important role for Bacteria in biocathode methanogenesis. A biocathode produced 277% more CH4 following amendment with 1 g/L ZVI and CH4 production by the ZVI-amended biocathodes remained elevated throughout subsequent feeding cycles with no new ZVI addition. Changes in the bacterial community following ZVI exposure and the formation of a redox-active precipitate in the ZVI-amended biocathode were two possible contributors to the increased CH4 production. The addition of up to 3% v/v H2S initial concentration to the CO2 in the cathode headspace resulted in up to a two-fold increase in CH4 production due to the transport of H2S to the anode compartment and subsequent donation of electrons to the anode. However, CH4 production declined above 3% H2S, indicating an inhibitory effect. A microbial community analysis of four BESs with biocathodes exposed to different conditions (control, H2S-amended, ZVI-amended, and H2S- and ZVI-amended) indicated the effect of biofilm, ZVI and H2S on both anode and cathode communities. Additionally, this research evaluated biocathode performance when fed gas from a stock anaerobic digester, and assessed the performance of a catholyte recycling system to guide the future designs of biogas upgrading systems.