NOVELTY - Preparation of methane-rich biogas comprises: anaerobic fermentation (1) of biomass and/or other substrates to raw fermentation gas (5), where the process liquid is enriched with oxygen and at least a part of the oxygen comes from an oxygen-enriched, methane-rich fermentation gas (7); biochemical desulfurization (2) of the produced raw fermentation gas; and photo-biological carbon dioxide reduction (3) of the desulfurized fermentation gas, where oxygen-enriched, methane-rich fermentation gas and algal biomass (14) are produced.    USE - The invention deals with the preparation of methane-rich biogas.    ADVANTAGE - The process uses efficient and energetic material fermentation gas. The methane-rich fermentation gas has high-quality source of energy.    DETAILED DESCRIPTION - Preparation of methane-rich biogas comprises: anaerobic fermentation (1) of biomass and/or other substrates to raw fermentation gas, where the process liquid is enriched with oxygen and at least a part of the oxygen comes from an oxygen-enriched, methane-rich fermentation gas (7) that is produced in a photo bioreactor; biochemical desulfurization (2) of the produced raw fermentation gas, results in a separable methane-rich fermentation gas (8) and desulfurized fermentation gas (6);and photo-biological carbon dioxide reduction (3) of the desulfurized fermentation gas, where oxygen-enriched, methane-rich fermentation gas and algal biomass (14) are produced, after which the oxygen-enriched, methane-rich fermentation gas is led to the process of biochemical desulfurization.    An INDEPENDENT CLAIM is included for a plant for the production of methane-rich fermentation gas comprising at least a biogas container, a biochemical desulfurization container and a photo-bioreactor container    DESCRIPTION OF DRAWING(S) - The figure shows the flows chart of the process:    Anaerobic fermentation; (1)    Biochemical desulfurization; (2)    Photo bioreactor; (3)    Biomass decomposition; (4)    Raw fermentation gas; (5)    Desulfurized fermentation gas; (6)    Oxygen enriched methane rich fermentation gas; (7)    Methane rich fermentation gas; (8)    Substrate; (9)    Algal biomass. (14)