The invention relates to a method for ultra-dehydrating thickened or pasty biomass products, in particular sludge from wastewater treatment plants, according to which the products, in particular products having a dryness of 4% to 25%, are subjected to the following steps: hydrothermal carbonization treatment, including pressurization (1, 2) and thermal conditioning (30) for a duration in a closed reactor (4); followed by dehydration of the products by a ram press (12), obtaining a dryness of more than 50%; the temperature of the product upstream from the ram press dehydration being regulated by cooling between 40° C. and 90° C., advantageously to around 70° C., in order to optimize the filterability in the piston press; the steps of the method being performed in a confined space making it possible to prevent the release of smells into the atmosphere.