NOVELTY - Preparing a biogas containing methane from an organic matter amenable to anaerobic digestion comprises: feeding an anaerobic digester with the organic matter; and converting part of the organic matter into biogas, where the total inorganic carbon concentration of the aqueous liquid of the digestion medium is maintained during the anaerobic digestion above 9000, preferably 15000 mg of equivalent calcium carbonate/1 and the buffering capacity of the digestion medium is maintained above 200 mmole/l, preferably above 250 mmole/l.    USE - The sodium bicarbonate is useful to reduce the hydrogen sulfide content of the biogas (claimed).    ADVANTAGE - The process improves the anaerobic processes for biogas production, enhances biogas quality, improves the anaerobic digestion productivity of the anaerobic digesters and reduces the amount of hydrogen sulfide of the generated biogas using sodium bicarbonate.    DETAILED DESCRIPTION - Preparing a biogas containing methane from an organic matter amenable to anaerobic digestion comprises: feeding an anaerobic digester with the organic matter, where the anaerobic digester contains an aqueous medium comprising microorganisms capable of digesting the organic matter to obtain a digestion medium comprising an aqueous liquid; and converting part of the organic matter into biogas by the addition of a buffering reagent comprising sodium bicarbonate to the digestion medium, where the total inorganic carbon concentration of the aqueous liquid of the digestion medium is maintained during the anaerobic digestion above 9000, preferably 15000 mg of equivalent calcium carbonate/l and the buffering capacity of the digestion medium is maintained above 200 mmole/l, preferably above 250 mmole/l.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic illustration of a process for preparing a biogas.    Cooling fluid (12)    Partially exhausted digestion medium (11)    Raw organic matter (14)    Hydrolyzer-reactor (13)    Heating fluid. (15)