NOVELTY - Controlled anaerobic digestion process for biomass, comprises a hydrolysis phase (5) for the biomass and two or more fermentation phases (7-9) in separate vessels, with specific measurement and control of temperature, degree of mixing, pH value and pressure.    USE - A controlled anaerobic digestion process for biomass, producing gas and sludge.    ADVANTAGE - A wide variety of industrial and domestic waste feedstocks may be treated in this plant. Conditions are optimized at each stage for e.g. hydrolysis of cellulose, sugar and amino acids, formation of organic acids (e.g. acetic) and alcohols (e.g. ethanol) and biological methane formation by their breakdown. Durations of each stage are optimized. A higher yield of biogas is achieved. Solid and liquid products can be recycled into other processes.    DETAILED DESCRIPTION - Controlled anaerobic digestion process for biomass, comprises a hydrolysis phase (5) for the biomass and two or more fermentation phases (7-9) in separate vessels, with specific measurement and control of temperature, degree of mixing, pH value and pressure. In the hydrolysis phase the biomass is treated at 40-100 degrees C. Substrate preheating (4) employs a heat exchanger (4a) and/or steam injection (V). The hydrolysis phase follows a cooling phase (6). An acid enzyme-containing sludge (Fe) is recycled from the end of the process, into the hydrolysis and/or fermentation phases. A fermentation with methane-generating bacteria is included. Specific conditions are set up and controlled during each stage of fermentation. Sludge from the last fermenter (9) is passed to a hydrocyclone (10) or centrifuge, thickening it. Gas and residual fluid (A) are separated here. Heat from cooling (6) is used for preheating, heating the other process phases (4-9), or in another connected process. Gas is drawn off under control from each digester. A further variant based on the foregoing principles is described.    An INDEPENDENT CLAIM is included for the corresponding plant.    DESCRIPTION OF DRAWING(S) - A schematic block flow diagram of the process is presented.    preheating (4)    heat exchanger (4a)    hydrolysis phase (5)    cooling phase (6)    fermentation phases (7-9)    hydrocyclone (10)