The current study reports on biological biogas upgrading by means of hydrogen addition to obtain biomethane. A mesophilic (37 degrees C) 0.058 m(3) trickle-bed reactor with an immobilized hydrogenotrophic enrichment culture was operated for a period of 8 months using a substrate mix of molecular hydrogen (H-2) and biogas (36-42% CO2). Complete CO2 conversion (> 96%) was achieved up to a H-2 loading rate of 6.5 m(n)(3) H-2/m(reactor)(3) vol. x d, corresponding to 2.3 h gas retention time. The optimum H-2/CO2 ratio was determined to be between 3.67 and 4.15. CH4 concentrations above 96% were achieved with less than 0.1% residual H-2. This gas quality complies even with tightest standards for grid injection without the need for additional CO2 removal. If less rigid standards must be fulfilled H-2 loading rates can be almost doubled (10.95 versus 6.5 m(n)(3), H-2/m(reactor)(3) vol, x d) making the process even more attractive. At this H-2 loading the achieved methane productivity was 2.52 m(n)(3) CH4/m(reactor)(3) vol. x d. In terms of biogas this corresponds to. an upgrading capacity of 6.9 m(n)(3) biogas/m(reactor)(3) vol. x d. The conducted experiments demonstrate that biological methanation in an external reactor is well feasible for biogas upgrading under the prerequisite that an adequate H-2 source is available. (C) 2016 Elsevier Ltd. All rights reserved.