NOVELTY - The method comprises converting an organic material as feedstock in presence of water by adding water and/or a catalyst in the absence of air when compared with standard conditions of elevated pressure and temperature by a hydrothermal conversion reaction in a coal and an aqueous liquid. The feedstock: is formed from different aqueous organic component and a dry component; and consists of cellulose and sawdust. The aqueous organic component includes aqueous biomass from organic wastes such as vegetable and/or animal constituents, seaweed and/or small- or micro-organisms, and/or bacteria.    USE - The method is useful for the hydrothermal carbonation of organic material such as biomass (claimed).    ADVANTAGE - The method allows reliable and energy-efficient hydrothermal carbonation of the organic material in rapid manner.    DETAILED DESCRIPTION - The method comprises converting an organic material as feedstock in presence of water by adding water and/or a catalyst in the absence of air when compared with standard conditions of elevated pressure and temperature by a hydrothermal conversion reaction in a coal and an aqueous liquid. The feedstock: is formed from different aqueous organic component and a dry component; and consists of cellulose and sawdust. The aqueous organic component includes aqueous biomass from organic wastes such as vegetable and/or animal constituents, seaweed and/or small- or micro-organisms and/or bacteria, and sludge. A ratio of the mass, molecular weight and/or volume fractions of the two components is predetermined, where the ratio of the components to one another is 80-20. One of the components such as sewage sludge is pretreated and/or treated prior to addition to the feedstock by mechanical, chemical and/or biological pathways such as fermenting, digesting, filtrating, mixing, drying, humidifying, crushing and/or homogenizing and by a purification plant process. The feedstock is pretreated before and/or during the hydrothermal carbonation by mechanical, chemical and/or biological pathways and by the purification plant process. The feedstock and/or the component has a maximum water content of 60% prior to the hydrothermal carbonization. An INDEPENDENT CLAIM is included for a device for the hydrothermal carbonation of organic material such as biomass.