The separation of methane from raw biogas was the main purpose of this study. A polymer membrane was used in order to obtain the high energy product, which can be utilized in cogeneration systems (CHP) or as a natural gas substitute. The study showed that using a polyimide hollow fiber module for biogas purification was an efficient method (low energy consumption, small-sized devise and a simple separation module). The satisfying results of laboratory tests caused scale up the installation. Different synthetic gas mixtures were used at the lab-scale, while in the field tests, raw biogas from a Polish two-stage agricultural biogas plant was processed. The plant used the following substrates: maize silage, grass silage and blends of these substrates with different supplements. The concentration of methane in the raw gas was up to 70% volume and contained up to 250 ppm of H2S. In both cases (laboratory and field tests), the retentate after membrane treatment was characterized by high methane concentration (up to 90% volume) and was free of H2S. The applied membrane demonstrated high selectivity for separating CH4 from CO2, H2S and H2O. The permeate stream contained less than 5% volume of CH4, which ensured low losses of the desired biogas component. The influence of pressure (below 10 bars) and stage cut on the quality of the product were analyzed to develop optimal process conditions for mobile plant construction. (C) 2013 Elsevier Ltd. All rights reserved.