Overcoming mass transfer limitations in biological methane removal: a "dry" biofilm approach

Biological methane (CH₄) removal from dilute air streams (200-5,000 ppm) is fundamentally constrained by slow mass transfer, primarily due to CH₄'s low solubility and diffusivity in water. Conventional biofilters, the current state of technology, suffer from long start-up periods, progressive pore clogging, and high transport energy requirements. This study investigates a "dry" biofilm concept designed to overcome these bottlenecks by minimizing the aqueous boundary layer while maintaining microbial activity via capillary-mediated nutrient delivery. Using concentrated biomass of the methanotroph Methylomicrobium buryatense 5GB1C, three generations of membrane-supported reactor configurations were evaluated. The third-generation (G3) design utilized a cellulose-bead capillary support to maintain a physical gap between the membrane and the liquid surface, enabling continuous drainage of metabolic water. Experimental results demonstrated that the "dry" G3 configuration achieved CH₄ removal rate of 148.9 mg·m^-2·hr^-1 at 4000 ppm, representing a 397% improvement over the first-generation floating mesh configuration. At 500 ppm, G3 design achieved a CH₄ removal rate of 18.6 mg·m^-2·hr^-1, corresponding to an over six-fold improvement over one of the highest reported values. Furthermore, the system enabled immediate start-up post-inoculation and maintained an optimal microenvironment pH (8.8-9.0) even as the bulk medium acidified. These results establish that replacing liquid-phase diffusion with drastically faster gas-phase transport provides a high-efficiency framework for mitigating low-concentration CH₄ emissions. With the added benefits of minimal pressure drop and easy biomass harvesting via scraping, this dry biofilm approach offers a scalable and sustainable alternative for atmospheric methane mitigation.