Method of thermal hydrolysis of sludges implementing a group of thermal hydrolysis reactors characterized in that it comprises successions of cycles, each of these successions of cycles being dedicated to one of said thermal hydrolysis reactors , each cycle comprising: - a step a) for conveying a batch of non-preheated sludges to be treated into a thermal hydrolysis reactor, said step for conveying comprising the continuous passage of the sludges of said batch of sludges into a dynamic mixer comprising an essentially cylindrical chamber having a small volume of a few liters and receiving blades mounted on a rotation shaft driven by a rotor rotating at a speed of over 500 rpm, said dynamic mixer being provided with an inlet of recovery steam into which recovery steam is injected, the residence time of the sludges in said essentially cylindrical chamber being only a few seconds; - a step b) for injecting live steam into said thermal hydrolysis reactor containing said batch of sludges so as to increase the temperature and the pressure prevailing in this reactor; - a step c) of thermal hydrolysis of said batch of sludges in said thermal hydrolysis reactor; - a step d) for emptying the content of said batch of hydrolyzed sludges of said thermal hydrolysis reactor towards a recovery vessel and for concomitantly de-pressurizing said reactor prompting the emission of recovery steam from said recovery vessel ; the cycle starting points of the successions of cycles being staggered in time so that the steps a) of a succession of cycles are concomitant with the steps d) of another succession of cycles, the recovery steam emitted during the steps d) of a succession of cycles constituting the recovery steam injected during the steps a) of another succession of cycles. - a step a) for conveying a batch of non-preheated sludges to be treated into a thermal hydrolysis reactor, said step for conveying comprising the continuous passage of the sludges of said batch of sludges into a dynamic mixer comprising an essentially cylindrical chamber having a small volume of a few liters and receiving blades mounted on a rotation shaft driven by a rotor rotating at a speed of over 500 rpm, said dynamic mixer being provided with an inlet of recovery steam into which recovery steam is injected, the residence time of the sludges in said essentially cylindrical chamber being only a few seconds; - a step b) for injecting live steam into said thermal hydrolysis reactor containing said batch of sludges so as to increase the temperature and the pressure prevailing in this reactor; - a step c) of thermal hydrolysis of said batch of sludges in said thermal hydrolysis reactor; - a step d) for emptying the content of said batch of hydrolyzed sludges of said thermal hydrolysis reactor towards a recovery vessel and for concomitantly de-pressurizing said reactor prompting the emission of recovery steam from said recovery vessel ; the cycle starting points of the successions of cycles being staggered in time so that the steps a) of a succession of cycles are concomitant with the steps d) of another succession of cycles, the recovery steam emitted during the steps d) of a succession of cycles constituting the recovery steam injected during the steps a) of another succession of cycles. <EOS>