NOVELTY - The installation comprises: a primary reaction vessel or digester (11) configured to produce methane by anaerobic fermentation of an organic waste; a secondary reaction vessel or post-digester (12); a set of pipes connecting the two vessels to each other; a pre-tank (13) in which a mixture supplied to the reactor vessels is prepared, where the pre-tank is connected to the digester vessel via a pipe, and the vessels produce methane by fermentation of organic waste; and a circulating pump (111). The digester and the post digester are sealed with a flexible envelope of plastics material.    USE - The installation is useful for anaerobically producing methane from a mixture consisting of organic matter, pure water or leachate (claimed), agricultural waste and plant waste.    ADVANTAGE - The installation is capable of economically producing the methane from the mixture with maximum efficiency, and can be removed in a simple manner so as to be reused on another site.    DETAILED DESCRIPTION - The installation comprises: a primary reaction vessel or digester (11) configured to produce methane by anaerobic fermentation of an organic waste; a secondary reaction vessel or post-digester (12); a set of pipes connecting the two vessels to each other; a pre-tank (13) in which a mixture supplied to the reactor vessels is prepared, where the pre-tank is connected to the digester vessel via a pipe, and the vessels produce methane by fermentation of organic waste or residue; a circulating pump (111); and a set of pipes connecting the two vessels to a recirculation pump and the circulation pump connected to the pre-tank. The digester and the post digester are sealed with a flexible envelope of plastics material configured to rest on a floor previously prepared to provide a mounting surface in a shape of truncated cone. A central portion of the envelope forms a trough, and a lower face of the envelope is provided with an opening at its center. The opening is connected to the recirculation pump through the pipes. Diameter of the opening in the pipes and the circulation pump is determined to allow evacuation of sand and sludge accumulated on the bottom of the digester and the post-digester. A part of the digestate product through the circulation pump is connected to the pre-tank by a pipe that is arranged to supply the sludge collected in the digester or the post-digester. The digester and the post digester are interconnected by the pipe so that the passage of the digester to the post-digester occurs according to a principle of communicating vessels when the material level of post-digester is lower than that of the digester. The pre-tank: produces a new mixture of organic material, digestate, water or leachate; comprises a mixer that mixes incoming goods introduced to form a mixture having a form of a fluid paste; and is positioned in height relative to the ground level. The diameter of the pipe connecting the pre-tank to the digester is defined so that the mixture prepared in the pre-tank is introduced into the digester by gravity. The pre-tank is cylindrical, and has a bottom of truncated cone shape in which the sludge mixed with the digestate reintroduced by recirculating pump is deposited by settling during mixing of its contents. The bottom of the pre-tank includes an orifice blocked by a valve through which the settled sludge is extracted from the pre-tank. The digester and the post digester are of low height. The recirculation pump is a high flow and low pressure pump configured to raise the mixture of sludge and digestate taken from the digester or the post-digester to the pre-tank. The recirculation pump has a short free end screw whose diameter is defined to not allow a free flow of the mixture of digestate and sludge taken from the digester or the post-digester. The installation further comprises: a unit for automatically supplying fresh organic matter to the pre-tank; a control unit configured to control and maintain a constant volume of produced methane based on solids content of the product mixture introduced in the digester, where the solids content are adjusted by a recirculation device so as to introduce more or less significant amount of digestate extracted from the digester or to extract the digestate from the post-digester; and a unit for purifying the methane produced by a bioreactor. The purifying unit comprises timber baffles arranged within the pipe collecting the methane produced by the digester and the post digester.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic perspective view of an installation for anaerobically producing methane (Drawing includes non-English language text).    Primary reaction vessel or digester (11)    Secondary reaction vessel or post-digester (12)    Pre-tank (13)    Circulating pump (111)    Extraction pump. (113)