(558d) A Novel “Dry” Biofilm Reactor for Atmospheric Methane Removal

Because of CH₄’s high global warming potential and relatively short half-time in the atmosphere, CH₄ removal can contribute substantially to near-term climate mitigation. It has been reported that there are hundreds of thousands of sites where the methane in the overlying air is estimated to be in the 200-2000 ppm range, but no proven scalable technology currently exists to capture CH₄ in this concentration range.

Bacteria (aerobic methanotrophs) in the soil and water provide the second-largest natural sink for atmospheric CH₄, accounting for ~7% of the total CH₄ sink. We believe engineering innovations can significantly increase this capability.

Biological atmospheric CH₄ removal has two major advantages: (1) it occurs at ambient temperature and pressure without releasing toxic byproducts ^{(Lawton & Rosenzweig, 2016)}, providing a safe pathway with known impacts. (2) only about 50 mol % of CH₄ is converted to CO₂, while the other half is assimilated to produce microbial biomass (a co-benefit), which reduces the amount of CO₂ released back into the atmosphere.

However, the very low atmospheric CH₄ concentrations (2ppm overall, and 200 – 2000 ppm for areas with elevated CH₄ concentrations) pose significant challenges for biological CH₄ removal. Specifically, CH₄'s low concentration and its very small solubility in water results in low throughput and significant energy consumption to overcome the mass transfer resistance of the gas substrate. This is why available CH₄ bioconversion technologies all target point-source methane with high concentrations (>5%). As such, existing biological CH₄ conversion approaches (all targeting point sources with >5% concentrations) are not suitable for CH₄ removal from air (La et al., 2018; Smith et al., 2001).

Recently, the Lidstrom lab at the University of Washington showed that Methylotuvimicrobium buryatense 5GB1C can efficiently remove CH₄ at 500 ppm (He et al., 2023). However, even with this efficient biocatalyst, to remove CH₄ from air at scale, we need a bioreactor that can effectively address the above-mentioned mass transfer-related challenges. To fill this gap, we introduce a new concept of “dry” CH₄ bioconversion and a novel scalable bioreactor design that has the potential for atmospheric CH₄ removal. In the proposed bioreactor, biocatalyst forms biofilm on a substratum tray placed horizontally. “Dry” means there is no bulk liquid phase and biofilm is directly exposed to gas phase. By eliminating the bulk liquid phase, we hypothesize that the “dry” biofilm reactor will not only drastically enhance CH₄ mass transfer, but also significantly reduce energy and water use.

In this talk, we will present our bioreactor prototype to demonstrate the new concept. In addition, preliminary experimental results are presented to support our hypothesis.

He, L., Groom, J. D., Wilson, E. H., Fernandez, J., Konopka, M. C., Beck, D. A. C., & Lidstrom, M. E. (2023). A methanotrophic bacterium to enable methane removal for climate mitigation. Proceedings of the National Academy of Sciences, 120(35), e2310046120.

La, H., Hettiaratchi, J. P. A., Achari, G., & Dunfield, P. F. (2018). Biofiltration of methane. Bioresource Technology, 268, 759–772.

Lawton, T. J., & Rosenzweig, A. C. (2016). Methane-oxidizing enzymes: an upstream problem in biological gas-to-liquids conversion. Journal of the American Chemical Society, 138(30), 9327–9340.

Smith, P., Goulding, K. W., Smith, K. A., Powlson, D. S., Smith, J. U., Falloon, P., & Coleman, K. (2001). Enhancing the carbon sink in European agricultural soils: including trace gas fluxes in estimates of carbon mitigation potential. Nutrient Cycling in Agroecosystems, 60, 237–252.