An apparatus for the production of biomethane from organic substances comprises a digester having a vat with a floor and a centrally located vertical axis. The vat is formed by a fixed foundation and vertical side walls. A central support pillar has a vertical axis coincident with the vertical axis of the vat and is movably joined to the foundation so that the pillar can rotate about the axis. The vat has an upper portion fixed to the central pillar and rotatably supported on the side walls. A cylindrical primary compartment is located inside the vat concentrically around the central pillar. A fixed vertical partition joins the floor and radially extends between the primary compartment and the side walls of the vat. A movable partition radially extends between the primary compartment and a side wall and is capable of moving about the central axis with the upper portion and central pillar. When the movable partition moves, all substances in the vat outside the primary compartment are displaced. A recirculating pump transfers all of the substances in the vat outside the primary compartment from one side of the movable partition to the opposite side as the partition moves. A feed pump transfers substances in the bottom of the primary compartment to the top of the compartment or to the portion of the vat outside the primary compartment. Biogas is collected in a tank fixed to the upper portion of the digester and is subsequently removed from the digester.