NOVELTY - Biogas tank (1) for storage of combustible biogas mixture (2), comprises a container wall, which limits the storage volume of the biogas tank and prevents the diffusion leakage of combustible components (4) of biogas mixture. At least a portion (6) of the container wall comprises a selectively permeable membrane material and a selectively permeable film material, where the membrane material or film material is diffusion-impermeable for combustible components of biogas mixture. The membrane material or film material is at least partially diffusion-permeable for carbon dioxide molecules (3).    USE - The biogas tank is useful for storage of combustible biogas mixture.    ADVANTAGE - The biogas tank achieves increase in methane content and reduction in relative carbon dioxide content, and thus it allows improved processing of biogas mixtures in cost efficient manner.    DETAILED DESCRIPTION - Biogas tank (1) for storage of combustible biogas mixture (2), comprises a container wall, which limits the storage volume of the biogas tank and prevents the diffusion leakage of combustible components (4) of biogas mixture. At least a portion (6) of the container wall comprises a selectively permeable membrane material and a selectively permeable film material, where the membrane material or film material is diffusion-impermeable for the combustible components of biogas mixture. The membrane material or film material is at least partially diffusion-permeable for the carbon dioxide molecules (3) present in biogas mixture. INDEPENDENT CLAIMS are also included for:    (1) a membrane or film material for manufacturing the biogas tank, which supports a combustible biogas mixture; and    (2) preparing the combustible biogas mixture, comprising (a) introducing the biogas mixture in the biogas tank, (b) depleting the relative carbon dioxide content in the biogas mixture through partial removal of carbon dioxide molecules from the biogas tank by diffusion of the carbon dioxide molecules through the selectively permeable membrane material or film material, and (c) removing the remaining biogas mixture based on concentration relative to methane content from the biogas tank.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic cross-sectional view of the biogas tank.    Biogas tank (1)    Biogas mixture (2)    Carbon dioxide molecules (3)    Combustible components (4)    Portion of container wall (6)