TECHNOLOGY FOCUS - MECHANICAL ENGINEERING - Preferred Component: A mixer is provided in the anaerobic hydrolysis tank for continuously or discontinuously mixing the content of the tank. A first separator is connected to the first reactor output for selective separation of particles larger than a first threshold size from the digested waste and has an output for the separated large particles. The anaerobic tank is connected to the first separator output for anaerobic hydrolysis of the separated particles. A second separator is connected to the hydrolysis tank output for selective separation of particles larger than a second threshold size from the hydrolyzed waste and has an output for the separated particles that is connected to the second reactor for digestion of the hydrolyzed particles and an output for the reject water. The output for the reject water is connected to a denitrification chamber of a wastewater treatment plant or to an anoxic fermentation chamber of a biological bio-P reduction chamber of a wastewater treatment plant. A gas circuit is connected to the headspace of at least one of the first and second reactor having a heat exchanger for cooling and condensation of recycled biogas for increased evaporation of ammonia in the reactor.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Component: The organic waste is organic fertilizer, manure, semi liquid manure, livestock dung, animal remains, animal feed remains, bacterial material, household waste, industrial waste, industrial waste water, sludge, corn, grass, dry grass, fresh or dry straw, straw contained in livestock dung, straw contained in deep-bedding, fibers, and/or silage. The gas passed through the headspace is a nitrogen gas, a non-oxygen containing gas, low oxygen containing gas, and/or exhaust gas. Preferred Method: The evolved gases are removed by passing a gas through the headspace of the anaerobic hydrolysis tank. The gas passed through the headspace is biogas output from the first reactor or from the second reactor. The gas passed through the headspace is a combined biogas output from the first and the second reactor. The evolved gasses are removed from the headspace of the anaerobic hydrolysis tank by a piping to a downstream processing. The piping to a downstream processing is provided by a communication between the headspace of the anaerobic hydrolysis tank and either the biogas outlet from the first reactor or the biogas outlet from the second reactor, or by a communication between the headspace of the anaerobic hydrolysis tank and the combined biogas outlets from the first reactor and the second reactor. The digestion step is performed at 10-70degreesC, preferably 40degreesC for 1-50 (10-20) days, or 35-50degreesC for 15-30 days. The hydrolysis step is performed at 55-95degreesC, preferably 80degreesC for 0.25-60 (14-16) hours. The digestion in the second reactor is performed at 15-70degreesC, preferably 55degreesC for 1-50 (10-15) days. The gas to be passed through the headspace of the anaerobic hydrolysis tank is heated in a heat exchanger at 15-95degreesC, preferably 75-85degreesC. The gas, that has been passed through the headspace of the anaerobic hydrolysis tank, is cooled in the heat exchanger (16, 18, 20) so that condensable gases of the gases removed from the anaerobic hydrolysis tank content condense, the condensed water containing the removed gases. EXAMPLE - Experiments was performed on semi liquid manure to investigate the effect of a hydrolysis tank with a communication between the headspace of the anaerobic hydrolysis tank and a biogas outlet from a reactor compared to a conventional closed hydrolysis tank. After the first digestion of the material, experiments were performed. The total biogas products after digestion steps were 107 m3/1000 kg dry matter supplied biogas produced using hydrolysis in a hydrolysis tank with a communication with between the headspace and a biogas outlet from a reactor compared to 72m3/1000 kg dry matter supplied biogas produced using hydrolysis in a closed hydrolysis tank.