Hydrogen produced from periodic excess of electrical energy may be added to biogas reactors where it is converted to CH
4
that can be utilized in the existing energy grid. The major challenge with this technology is gas-to-liquid mass transfer limitation. The microbial conversions in reactors designed for hydrogenotrophic methanogenesis were studied with microsensors for H
2
, pH, and CO
2
. The H
2
consumption potential was dependent on the CO
2
concentration, but could partially recover after CO
2
depletion. Reactors with 3-dimensional biofilm carrier material and a large gas headspace allowed for a methanogenic biofilm in direct contact with the gas phase. A high density of Methanoculleus sp. in the biofilm mediated a high rate of CH
4
production, and it was calculated that a reactor filled with 75% carrier material could mediate a biogas upgrading from 50 to 95% CH
4
within 24 h when an equivalent amount of H
2
was added.
