Hydrothermal oxidation device for treatment of a material in a supercritical medium, comprising: - a main body provided with a cold interface flange at a first of its ends, sealing means between the main body and the flange, the main body, the flange and the sealing means being made from materials resistant to pressures and temperatures in supercritical media; - an internal tube placed inside the main body so as to form an annular zone along the main body, the internal tube comprising a cold first end and a hot second end, the first end of the internal tube being fixed in a sealed manner to the cold interface flange, the internal tube delimiting an internal reaction zone; - a stirring means placed in the internal reaction zone of the internal tube and actuated by a rotating shaft; - a cooling means to cool the treated material located in the internal reaction zone of the internal tube before it is evacuated from the oxidation device through a treated material outlet; - an inlet for the water and oxidant mix located at the cold first end of the main body, the water and oxidant mix passing through the annular zone from the cold first end to the hot second end of the internal tube, before penetrating into the internal reaction zone of the internal tube; - an effluents outlet located at the cold first end of the main body; - an inlet for material to be treated, located at a hot second end of the main body, this inlet opening up into the internal tube, at its second end; characterised in that the oxidation device also comprises an inlet for a diluted effluent, located in the cold interface flange and connected to a preheating coil located along the internal wall of the main body and extending from its cold first end to its hot second end, the preheating coil opening up into the reaction zone of the internal tube at its hot second end. - a main body provided with a cold interface flange at a first of its ends, sealing means between the main body and the flange, the main body, the flange and the sealing means being made from materials resistant to pressures and temperatures in supercritical media; - an internal tube placed inside the main body so as to form an annular zone along the main body, the internal tube comprising a cold first end and a hot second end, the first end of the internal tube being fixed in a sealed manner to the cold interface flange, the internal tube delimiting an internal reaction zone; - a stirring means placed in the internal reaction zone of the internal tube and actuated by a rotating shaft; - a cooling means to cool the treated material located in the internal reaction zone of the internal tube before it is evacuated from the oxidation device through a treated material outlet; - an inlet for the water and oxidant mix located at the cold first end of the main body, the water and oxidant mix passing through the annular zone from the cold first end to the hot second end of the internal tube, before penetrating into the internal reaction zone of the internal tube; - an effluents outlet located at the cold first end of the main body; - an inlet for material to be treated, located at a hot second end of the main body, this inlet opening up into the internal tube, at its second end;