Raw biogas obtained from a sewage plant was successfully purified by a single-step method to a quality compatible with compressed natural gas (CNG) standards. For this purpose, thin-film composite membranes with polyamide skin layer were evaluated at varying temperatures, pressures, feed and sweep flow rates. The wetting of the polyamide skin layer was analyzed under different experimental conditions. Optimization of the purification process resulted in a better separation than that in previous studies. The achieved CH4 and H2S levels are conform to the required standards for commercialization in the Czech Republic. A unique feature of the presented approach, distinguishing the water-swollen thin-film composite membranes from polymeric membranes under dry conditions, is that the condensing water absorbs a significant amount of the minor impurities of biogas, such as H2S.