Climate change is driven by anthropogenic greenhouse gas (GHG) emissions [1]. Most attention has focused on carbon dioxide (CO₂), while methane (CH₄) has gotten less attention despite having a warming potential 28 times greater than CO₂ [2]. Current efforts to remove GHGs have focused on removal at the source of production even though major GHG emissions are not point- source, which makes these dilute emissions hard to control. Therefore, there is a need to develop direct air capture (DAC) technologies to extract CH₄ from the atmosphere. While large scale DAC technologies exist to remove CO₂ [3] no effective solution exists to remove atmospheric CH₄. The low CH₄ concentrations in air present a major challenge towards CH₄ removal since low concentration slows reaction kinetics. However, methanotrophic microbes have evolved the capacity to consume CH₄ at high rates and produce the 28x less climate active CO₂ by catalyzing the following reaction (CH₄ + 2O₂ → CO₂ + 2H₂O).

While DAC of CO₂ has seen success [3], the removal of CH₄ is challenging because the differences in atmospheric concentrations (450 ppm CO₂; 2 ppm CH₄) make CO₂ removal more favorable [2, 4]. Effective reactor systems with high biomass concentrations and efficient energy use are needed to ensure a process remains net carbon negative. The water profession spent decades optimizing reactor systems to remove high volumes of dilute pollutants [5], which can offer inspiration for bio-based CH₄ removal. Biofilters were found to be highly ineffective systems, while biofilm reactors achieve higher process rates, with less space, and reduce energy input. Hollow fiber membrane (HFM) reactors force air directly through biofilms, hence avoiding mass transfer limitations associated with gas-liquid-biofilm mass transfer [6] yet biofilm growth can be a lengthy process. New DAC technologies should incorporate pre-established methanotrophic biofilms to remove large fluxes of CH₄ per surface area.

Alarmingly, CH₄ concentrations have been quickly increasing over the past century. Since CH₄ is a powerful GHG, implementation of DAC technologies would achieve a significant and rapid reduction of atmospheric CH₄, which in turn would decrease the cumulative GHG warming potential and tropospheric ozone production from CH₄ [7]. High affinity methanotrophs offer great potential to lower atmospheric CH₄ concentrations but their full potential cannot be utilized at scale without a) obtaining high biomass concentrations and b) effective bioreactor units. We propose a compact reactor unit with capacity to treat large volumetric loads of air to oxidize CH₄ by coating methanogens in a densely packed hydrogel layer onto a hollow fiber membrane reactor unit.