Biofiltration of methane using hybrid mixtures of biochar, lava rock and compost

Using hybrid packing materials in biofiltration systems takes advantage of both the inorganic and organic properties offered by the medium including structural stability and a source of available nutrients, respectively. In this study, hybrid mixtures of compost with either lava rock or biochar in four different mixture ratios were compared against 100% compost in a methane biofilter with active aeration at two ports along the height of the biofilter. Biochar outperformed lava rock as a packing material by providing the added benefit of participating in sorption reactions with CH₄. This study provides evidence that a 7:1 volumetric mixture of biochar and compost can successfully remove up to 877 g CH₄/m³·d with empty-bed residence times of 82.8 min. Low-affinity methanotrophs were responsible for the CH₄ removal in these systems (K_M(app) ranging from 5.7 to 42.7 µM CH₄). Sequencing of 16S rRNA gene amplicons indicated that Gammaproteobacteria methanotrophs, especially members of the genus Methylobacter, were responsible for most of the CH₄ removal. However, as the compost medium was replaced with more inert medium, there was a decline in CH₄ removal efficiency coinciding with an increased dominance of Alphaproteobacteria methanotrophs like Methylocystis and Methylocella. As a biologically-active material, compost served as the sole source of nutrients and inoculum for the biofilters which greatly simplified the operation of the system. Higher elimination capacities may be possible with higher compost content such as a 1:1 ratio of either biochar or lava rock, while maintaining the empty-bed residence time at 82.8 min.