High-resolution 16S biogas upgrading communities: contrasting in situ and ex situ setups

As biogas from anaerobic digestion becomes an increasingly attractive biofuel, the need to improve the quality of biogas has come to the fore. Biological upgrading focuses on adding enough hydrogen to an anaerobic biogas reactor to allow methanation of the remaining carbon dioxide by methanogenic Archaea (in situ upgrading). Alternatively, biogas and hydrogen can be mixed in the absence of feedstock, in a reactor operated exclusively to facilitate methanogenesis (ex situ upgrading). This novel technology can encounter inhibition at high loading rates of hydrogen: however, in contrast to anaerobic digestion, the dynamics of this thermophilic functional microbial community are sparsely characterised. High-resolution 16S rDNA community profiles from four anaerobic biogas upgrading reactors were constructed to determine how feedstock, hydrogen, and CO₂ influence biomethanation. Presence/absence of a feedstock led to large differences between in situ and ex situ communities, determining the dominant methanogen genera, and encouraging distinct populations of hydrolysing and fermenting Firmicutes. Although high hydrogen flow rates (∼37 L/day) caused a collapse in methanogenic Methanothermobacter populations in situ, ex situ hydrogen rates greatly exceeded these levels (∼400 L/day) without collapse of Methanobacterium, despite some observed instability and proliferation of likely homoacetogens. Subsequent reduction of hydrogen rates ex situ (259 L/day) appeared to create a niche for hydrogen production, indicated by increased abundance of various syntrophic fermenters known to supply biogenic hydrogen. In either upgrading setup, instability due to increased hydrogen levels manifested as a disruption of fermenting and hydrolysing populations prior to disruption of methanogens.