The chambers for acid (1) and regressive acid (2) fermentation of tank in ground are connected to prechamber (3) for methane fermentation. The chamber (5) for alkaline fermentation is connected to the chamber (5) for methane fermentation.    Devices (6,7) for grinding the substrate suspension are in input lines connected to heaters (8) and mixer devices (9) with strings (10) in the tank.    The output lines have microfilters (11,12) and disintegrators (13,14) in recirculation lines.    The chamber (2) for regressive acid fermentation has a settling section (15) formed by curtains (16,17) of strings connected at level B to prechamber (3).    The chamber (4) for alkaline fermentation is connected to section (18), and at level B to chamber (5) for methane fermentation containing gas-injectors (20).    The gas space (21) of chambers (1,2) is connected to prechamber (3), and the gas-space (22) of chamber (4) is connected to chamber (5) for methane fermentation.    Chamber (5) is also connected to collector (23) formed by covers (24,25).    The unit also comprises:i) mixer (26);ii) separator (27);iii) bio-cultivator (28);iv) micro-filter (29);v) disintegrator (30);vi) heat-exchanger (31); and,vii) transport (32).    Prechamber (3), chamber (5), and bio-cultivator (28) contain:i) overflow pipes (33);ii) granular packing (34) for immobilisation of microorganisms; and,iii) input gas pipes (35).    Outflow of animal and aviary origin is input to the mixer (26) for dilution with hot water from heat-exchanger (8) to a concentration of 1-10 g./l. at 36-38C, then to separator (27) for separation of floating suspension which is reduced in particle size in devices (6,7) to that comparable with microorganisms.    Formed biogas containing 95-98% of methane is directed to the collector (23) between covers (24,25).    USE -   In production of biogas from outflow from animal and aviary farms in treatment plants in agriculture, and obtaining product which can be used as protein-vitamin additive to fodder.    ADVANTAGE -   Increased output of biogas because of increased efficiency of separation of silty water; the decomposition of organic material is increased from 5-50% to 90-95%, the biogas output from 0.6-0.8 kg. per 1 kg. of organic material to 1.3-1.5 kg. per 1 kg., and the methane content of biogas from 65-70% to 95-98%; the biogas becomes calorifically equivalent to benzene.