In this study, the potential of a pilot hollow-fiber membrane bioreactor for the conversion of H-2 and CO2 to CH4 was evaluated. The system transformed 95% of H-2 and CO2 fed at a maximum loading rate of 40.2 m(H2)(3)/m(R)(3)d and produced 0.22 m(3) of CH4 per m(3) of H-2 fed at thermophilic conditions. H-2 mass transfer to the liquid phase was identified as the limiting step for the conversion, and kLa values of 430 h(-1) were reached in the bioreactor by sparging gas through the membrane module. A simulation showed that the bioreactor could upgrade biogas at a rate of 25 m(3)/m(R)(3)d, increasing the CH4 concentration from 60 to 95% v. This proof-of-concept study verified that gas sparging through a membrane module can efficiently transfer H-2 from gas to liquid phase and that the conversion of H-2 and CO2 to biomethane is feasible on a pilot scale at noteworthy load rates. (C) 2015 Elsevier Ltd. All rights reserved.