Bioreactors use methanotrophs, bacteria and archaea that oxidize methane, to break down methane in a localized system. Air must be moved through the localized reactor, either through active methods such as fans, or passive methods such as solar updraft chimneys that use convective heat transfer to move air.
Bioreactors are promising for breaking down methane at its emission sources where its concentration is elevated. For bioreactors to be a feasible atmospheric methane removal approach there would have to be a dramatic decrease in operating cost alongside breakthroughs in methanotroph cultivation at 2 ppm methane. The required airflow to achieve megaton scale atmospheric methane removal would be far beyond planned carbon dioxide direct air capture capacity in the upcoming few decades.
Bioreactors operating at atmospheric methane concentrations have not yet been demonstrated. Lab-grown methanotrophs have been shown to effectively remove methane at 500 ppm and could therefore potentially destroy emissions from some point-sources. However, significant improvements in the affinity for methane would be required to allow bioreactors to operate at 2 ppm methane. The overall climate impacts (due to the potential production of nitrous oxide) and costs of this approach have not yet been publicly evaluated.
Cost and climate impacts are also driven by the energy requirements to move air through the reactor. This could be negligible for passive air flow, but must be considered for active methods. Energy costs of active air flow methods could be lowered by co-locating the system with existing air movement systems, such as cooling towers, heat exchangers, or direct air capture facilities.
The potential scale bioreactors could reach is potentially limited by the energy requirements of air movement, resource limitations of raw materials, and methanotroph growing speeds and sustainable scales. Given the low atmospheric concentration of methane, any flow through system would have to process massive amounts of air to oxidize a benchmark annual scale of 10 million metric tons of methane (830 Mt CO2e using GWP20). For example, if you add methane breakdown reactors to every carbon dioxide direct air capture system that is projected to be installed by 2030 (~60 Mt CO₂/yr or around 5 billion cubic feet per minute), only around 0.1 Mt/yr of methane could be removed.
Even if bioreactors become feasible, it will be extremely challenging and resource intensive to reach meaningful scale, as the amount of methane addressed will be directly related to the number and scale of the reactors built. Therefore we estimate an approach of this sort would take decades to scale after feasibility was established.
In a closed reactor system, byproducts could be measured in situ and potentially be selectively removed, reducing the uncertainty as to what gasses would be produced and emitted. This drastically lowers the health and environmental risks compared to open system interventions.
Preliminary research suggests that apart from oxidizing methane or other gases, bioreactors could produce nitrous oxide as a byproduct. Because bioreactors convert part of the oxidized methane into biomass, there are also end-of-life considerations for the disposal of millions of metric tons of methanotrophs if this approach is scaled.