NOVELTY - The method comprises collecting sludge or water to be heated in a basin or pipe according to a liquid layer (15); injecting hot gas or vapor loaded with water vapor from the equipment of the plant into the liquid layer, below the upper level so as to form bubbles and to heat the water or the sludge not only through the transfer of sensible heat from the gases and by transfer of the latent heat of condensation of water vapor. The water or sludge is collected in a bubbling tank. The liquid layer has a depth of 30 cm. The sludge to be heated has suspended solid concentration of 2-6%.    USE - The method is useful for reheating of thickened sludge or water of wastewater sewage treatment plant (claimed).    ADVANTAGE - The method is capable of improving energy balance of the wastewater treatment plant.    DETAILED DESCRIPTION - The method comprises collecting sludge or water to be heated in a basin or pipe according to a liquid layer (15); injecting hot gas or vapor loaded with water vapor from the equipment of the plant into the liquid layer, below the upper level so as to form bubbles and to heat the water or the sludge not only through the transfer of sensible heat from the gases and by transfer of the latent heat of condensation of water vapor. The water or sludge is collected in a bubbling tank. The liquid layer has a depth of 30 cm. The sludge to be heated has suspended solid concentration of 2-6%, which is directed in the bubbling tank. The sludge is preheated upstream of a digester (6) at 40-55 degrees C to cause reduction of energy consumption for heating of the thickened sludge in the digester. The gas injected into the bubbling tank is formed by vapor outlet of a thermal dryer (10), and/or biogas combustion gases from a boiler (8) providing steam, or a thermal fluid or hot water, and/or combustion gas from a biogas combustion engine (13), and/or combustion gases from the combustion of raw sludge. The injection of hot gas or vapor in the effluent is at a depth of 500-600mm, below the upper level of the liquid layer. The injection takes place from a duct fixed on a vertical wall of the tank. An INDEPENDENT CLAIM is included for a wastewater treatment plant.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a wastewater sewage treatment plant.    Digester (6)    Boiler (8)    Thermal dryer (10)    Biogas combustion engine (13)    Liquid layer. (15)