NOVELTY - The process comprises: (a) providing a biogas reactor for producing biogas, where the biogas reactor comprises upper reactor portion (2) and lower reactor portion (3); (b) filling the biogas reactor with an aqueous fermentation liquid; (c) filling organic solids in the lower reactor portion for methanation; (d) partially filling a sorbent in the lower reactor portion; (e) allowing the organic solids to stay for a time period of 3-30 days in the liquid; (f) allowing the sorbent to stay for a time period of 2-54 weeks in the liquid; and (g) circulating the liquid through the biogas reactor.    USE - The process is useful for producing biogas (claimed).    ADVANTAGE - The process is capable of economically producing the biogas in a simple and efficient manner.    DETAILED DESCRIPTION - The process comprises: (a) providing a biogas reactor for producing biogas, where the biogas reactor comprises an upper reactor portion (2) and a lower reactor portion (3); (b) filling the biogas reactor with an aqueous fermentation liquid; (c) filling organic solids in the lower reactor portion for methanation; (d) partially filling a sorbent in the lower reactor portion; (e) allowing the organic solids to stay for a time period of 3-30 days in the fermentation liquid; (f) allowing the sorbent to stay for a time period of 2-54 weeks in the fermentation liquid; (g) circulating the fermentation liquid through the biogas reactor; (h) removing the biogas from the upper reactor portion; (i) removing a resulting solid fermentation residue (6) from the upper reactor portion; (j) removing the loaded sorbent from the lower reactor portion; (k) repeating the reaction steps (c)-(j); and (l) carrying out steps (b)-(k) under the absence of oxygen. The step (h) comprises circulating a partial stream of the removed biogas through the biogas reactor passing from the upper reactor portion to the lower reactor portion, where the circulated partial stream is in contact with the sorbent. The sorbent is an organic sorbent and/or an inorganic sorbent comprising zeolites and biochar. The process further comprises heating the fermentation liquid at a temperature of 30-60 degrees C. Enzymes, microorganisms comprising methanogenic bacteria and/or trace elements are added to the fermentation liquid in the step (a). The organic solids in the step (e) are allowed to stay below the liquid surface in the fermentation liquid. The solid fermentation residue in the step (i) and/or the sorbent in the step (j) are removed by a conveyor. The upper and lower reactor portions are separated by a fluid-permeable unit. An INDEPENDENT CLAIM is included for a biogas reactor for performing the production of biogas.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a plant for solid-state fermentation in the presence of sorbent with biogas reactor and a mixing container.    Upper reactor portion (2)    Lower reactor portion (3)    Biomass (5)    Fermentation residue (6)    Gas pipe. (7)