Biofilters are the state of technology (SOT) in biological CH4 removal, which have been considered promising for CH4 capture from air. Biofilters are packed bed reactors with biocatalysts forming a biofilm on the surface of the porous packing material. Although biofilters have been proven effective in removing CH4 from point sources (>1%), they are not feasible for CH4 at 200 – 5,000 ppm: the high energy requirement (due to high pressure drop) will be prohibitive for treating large volumes of air (~500-5,000 W/m3) [3]. In addition, the biomass in biofilters is not harvested, and will end up as CH4 and/or CO2 again after biomass decomposition.
To fill this gap, we have proposed a novel concept of a multi-tray “dry” biofilm reactor and developed a patent-pending reactor prototype for CH4 removal from the air at the sites with elevated CH4 concentration (200 – 5000 ppm). One key challenge of biological CH4 removal from air is the rate-limiting CH4 transport from the gas phase to the biocatalyst. Our solution addresses these challenges with the following: (1) a highly efficient (demonstrated highest CH4 consumption rate at 200 ppm) and highly intensified (biofilm) biocatalyst; (2) CH4 capture directly from the air - by eliminating the bulk liquid phase in conventional biofilm reactors, the chance for CH4 molecules to collide with the biofilm is increased by an order of compared to those in aqueous solutions.
Compared to biofilters, our technology eliminates the circulating liquid medium and the significant energy consumption associated with it. This seemingly insignificant change comes with significant advantages: CH4 concentration in air is 34 times higher than its equilibrium concentration in water [4]; the diffusivity of CH4 in air is 16,000 times higher than that in water [5]. Since the collision frequency , CH4 molecules in air have times higher chance of colliding with a biofilm than CH4 molecules in water. Also, our technology enables easy harvesting of the biomass produced from CH4 capture. If the harvested biomass is used as aquafeed, very little (if any) will end up as CH4. Since fish will be eaten by people, some of the C will stay in people for decades. In contrast, if the produced biomass is not harvested but treated as waste or fertilizer, as in biofilters, a significant percentage (up to 40%) will end up as CH4 again after biomass decomposition, which significantly diminishes the net CO2e reduction.
In this talk, we will present our preliminary experimental results, which show significant advantages over biofilters. Specifically, we will demonstrate a proof-of-concept prototype of the proposed “dry” biofilm reactor. Our preliminary results demonstrate that the proposed technology significantly reduces energy consumption compared to biofilters, while increasing CH4 elimination capacity (EC), in g CH4 removal/m3 reactor volume/hr, by 2 - 5 times. In addition, the proposed technology will not only remove CH4 from CH4-elevated air but also store captured CH4 as aquafeed - with one tonne of CH4 removed, 0.8 tonne of microbial biomass (a co-benefit) will be produced and harvested for aquafeed production.
Reference
[1] D. Shindell et al., “Simultaneously mitigating near-term climate change and improving human health and food security,” Science (1979), vol. 335, no. 6065, pp. 183–189, 2012.
[2] L. He et al., “A methanotrophic bacterium to enable methane removal for climate mitigation,” Proceedings of the National Academy of Sciences, vol. 120, no. 35, p. e2310046120, 2023.
[3] N. J. R. Kraakman, J. Rocha-Rios, and M. C. M. van Loosdrecht, “Review of mass transfer aspects for biological gas treatment,” Appl Microbiol Biotechnol, vol. 91, pp. 873–886, 2011.
[4] NIST, “Methane.” Accessed: Sep. 25, 2024. [Online]. Available: https://webbook.nist.gov/cgi/cbook.cgi?ID=C74828&Units=SI&Mask=10#Solub…
[5] EPA, “EPA On-line Tools for Site Assessment Calculation.” Accessed: Sep. 25, 2024. [Online]. Available: https://www3.epa.gov/ceampubl/learn2model/part-two/onsite/estdiffusion-…