NOVELTY - The device comprises an input opening provided for inputting organic waste to be treated and located at one of ends of a methanisation tank (1), an output opening provided for outputting the treated residue or digestate by a waterbed effect located at an opposite end of the tank, an evacuation opening provided for releasing biogas towards an upper part of the tank, and a mixing unit immersed in the waste in the course of digestion in an internal part of the tank in four successive treatment areas (Z1-Z4) along a displacement direction of the waste.    USE - The device is useful for producing biogas from an organic material waste containing a liquid fraction and solid fragments (claimed), where the organic material waste includes animal manure, culture residues, organic-waste of food industries or culture waste intended for cattle feed, wastewater, muds of purification stations, kitchen waste, household refuse and agricultural waste, and the device is used by farmers and stock breeders and is useful for small industries of food sector.    ADVANTAGE - The device includes the mixing unit that allows a different mixing of the waste in the course of digestion by the treatment areas so as to guarantee that waste is constantly subjected to the appropriate mixing with the degree of digestion and to sufficiently contact the waste in the course of digestion with microorganisms for enhancing biological degradation of the waste in the course of digestion thus improving yield of the biogas, and is compact and modular so as to avoid the need to replace an existing unit.    DETAILED DESCRIPTION - The device comprises an input opening provided for inputting organic waste to be treated and located at one of ends of a methanisation tank (1), an output opening provided for outputting the treated residue or digestate by a waterbed effect located at an opposite end of the tank, an evacuation opening provided for releasing biogas towards an upper part of the tank, and a unit for mixing the waste in the course of digestion in an internal part of the tank during a longitudinal displacement of the waste between the input opening and the output opening. The mixing unit is immersed in the waste in the course of digestion in the internal part of the tank in four successive treatment areas (Z1-Z4) along a displacement direction of the waste, where the first treatment area is equipped with pushing units for homogenization and fractionation of waste, the second treatment area is equipped with a unit of slow mixing and packaging of waste without homogeneous mixing of the waste, the third treatment area or zone of rest for allowing decantation of solid fragments of the waste that remains in suspension in a liquid fraction or gathers at a bottom of the tank when a density goes up on a surface, and a fourth treatment area is equipped with units for aspiration of the liquid fraction of the waste and lengthening of residence time of the solid fragments in the tank. The pushing units include a double reversed coil helix (7) movable in rotation around a first shaft (4) parallel with an axis of the tank and actuated at slow speed by a motor reducer (5), where a profile of the double helix has a sharp rectangular section with sharp angle. The double helix is equipped with side squares positioned perpendicularly on the section, where branches of the double helix are asymmetrical. The unit for slow mixing and packaging of waste include a paddle wheel movable in rotation around the first shaft and equipped with a series of longitudinal blades with a flat profile directed according to an axial angle of 25-30 degrees . The units for aspiration of the liquid fraction and lengthening of the residence time of the solid fragments in the tank include: two radial blade propellers that includes a concave profile in the displacement direction in the course of digestion in the two radial blade propellers, and are mounted in rotation around a second shaft parallel with the axis of the tank and pulled by the motor reducer at variable speed, where the second radial blade propeller has greater dimension than the first radial blade propeller; and a truncated cone that is coaxially mounted at the second shaft in order to surround the two radial blade propellers and widens in a direction of the output opening of the tank to surround the output opening, while releasing a free space at a right of the cone. The input opening is equipped with a feed hopper of waste to be treated emerging tangentially in the tank. The output opening is positioned opposite to the input opening and equipped with a outlet piping in a swan neck. The tank is equipped with a draining opening at a lower part of the tank.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic longitudinal sectional view of a methanisation tank.    Methanisation tank (1)    First shaft (4)    Motor reducer (5)    Double reversed coil helix (7)    Treatment areas. (Z1-Z4)