Process for treating biodegradable materials, in particular fermentation residues, poultry manure, pig manure or cattle manure, in which the biodegradable material is hygienized in a conditioner (1, 1'), in which, during the hygienization in the conditioner (1, 1'), volatile substances, in particular ammonia, are expelled from the biodegradable material and are enriched in a gas mixture, in which the enriched gas mixture is drawn off from the conditioner (1, 1'), is fed to a purification process and the volatile substances in the purification process are washed out of the gas mixture with a washing liquid, and in which the biodegradable material, which is then low in ammonium, is removed from the conditioner (1, 1') separately from the gas mixture and is fed to a further use, wherein a negative pressure is present in the conditioner (1, 1'), and wherein the volatile substances are removed from the biodegradable material on the basis of their respective partial pressure by the negative pressure present, characterized, in that the negative pressure applied to the conditioner (1, 1') causes a stepwise pressure reduction in the conditioner (1, 1'), in that the pressure reduction in the conditioner (1, 1') at a first pressure level causes volatile substances, in particular ammonia, to be expelled from the biodegradable material on the basis of the respective partial pressure, that, after the volatile substances have been expelled, the pressure in the conditioner (1, 1') is lowered to a second, lower pressure stage, so that the boiling point in the conditioner (1, 1') shifts in such a way that water contained in the conditioner (1, 1') changes its aggregate state and water vapor is formed in the conditioner (1, 1'), 1'), that the water vapor formed in the conditioner (1, 1') is fed to a cooling unit (5, 5') as a gas mixture containing water vapor, and that the water vapor is condensed in the cooling unit (5, 5'). in which the biodegradable material is hygienized in a conditioner (1, 1'), in which, during the hygienization in the conditioner (1, 1'), volatile substances, in particular ammonia, are expelled from the biodegradable material and are enriched in a gas mixture, in which the enriched gas mixture is drawn off from the conditioner (1, 1'), is fed to a purification process and the volatile substances in the purification process are washed out of the gas mixture with a washing liquid, and in which the biodegradable material, which is then low in ammonium, is removed from the conditioner (1, 1') separately from the gas mixture and is fed to a further use, wherein a negative pressure is present in the conditioner (1, 1'), and wherein the volatile substances are removed from the biodegradable material on the basis of their respective partial pressure by the negative pressure present, characterized, in that the negative pressure applied to the conditioner (1, 1') causes a stepwise pressure reduction in the conditioner (1, 1'), in that the pressure reduction in the conditioner (1, 1') at a first pressure level causes volatile substances, in particular ammonia, to be expelled from the biodegradable material on the basis of the respective partial pressure, that, after the volatile substances have been expelled, the pressure in the conditioner (1, 1') is lowered to a second, lower pressure stage, so that the boiling point in the conditioner (1, 1') shifts in such a way that water contained in the conditioner (1, 1') changes its aggregate state and water vapor is formed in the conditioner (1, 1'), 1'), that the water vapor formed in the conditioner (1, 1') is fed to a cooling unit (5, 5') as a gas mixture containing water vapor, and that the water vapor is condensed in the cooling unit (5, 5').