NOVELTY - The method comprises: combining a mesophilic or thermophilic anaerobic digestion step (20) and a biological solubilization anaerobic treatment step (30) of a stream of sludge (1) to be treated; dehydrating (10) the sludge to be treated in upstream of the anaerobic digestion step; and mixing the dehydrated sludge with a recirculated fraction of sludge that is more liquid originating from recycling of the anaerobic digestion step and/or from the anaerobic treatment step, and/or centrates originating from a final dehydration step (50) of the treated sludge.    USE - The method is useful for reducing the production of sludge by municipal or industrial wastewater purification plants (claimed).    ADVANTAGE - The method is capable of reducing the production of sludge by the municipal or industrial wastewater purification plants in an energy-effective manner.    DETAILED DESCRIPTION - The method comprises: combining a mesophilic or thermophilic anaerobic digestion step (20) and a biological solubilization anaerobic treatment step (30) of a stream of sludge (1) to be treated; dehydrating (10) the sludge to be treated in upstream of the anaerobic digestion step; and mixing the dehydrated sludge with a recirculated fraction of sludge that is more liquid originating from recycling of the anaerobic digestion step and/or from the anaerobic treatment step, and/or centrates originating from a final dehydration step (50) of the treated sludge. A recirculation rate is chosen such that the mixture has a dryness suitable for digestion. The mixture is being directed towards the anaerobic digestion. The dryness of sludge at the first dehydration is 20-30% of dry material. The recirculation rate is chosen such that the dryness of the mixture is lower than 6 mass% of dry material. The anaerobic treatment step is located downstream from the anaerobic digestion step and placed on a recirculation of the anaerobic digestion step. The final dehydration step is carried out by centrifugation. The method further comprises: measuring nitrates in an input of the anaerobic digestion step and measuring ammonium and pH in output of the anaerobic digestion and the recirculation rate from the measurements; shredding/desanding the sludge to decrease silting of digester and the abrasion of the components in the agglomeration of a circuit; and transferring heat between sludge leaving the anaerobic treatment step and sludge leaving the anaerobic digestion step. A part of the biogas produced during of the anaerobic digestion is used as energy source to heat or maintain a temperature of a reactor in which the anaerobic treatment step is carried out. The heat transfer is of indirect type of sludge/water and water/sludge, where the water is used as coolant for the exchange between two sludges. A dilution step is provided on a loop of recirculation connected to the exit of the anaerobic digestion step to allow to control of operating conditions of the anaerobic digestion. Flows of return coming from the anaerobic digestion step or the final dehydration are controlled according to the operating conditions of the anaerobic digestion and the production of biogas and according to the quality of the solid residue. An INDEPENDENT CLAIM is included for an installation for reducing the production of sludge by municipal or industrial wastewater purification plants.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a method for reducing the production of sludge by municipal or industrial wastewater purification plants (Drawing includes non-English language text).    Sludge (1)    Dehydrating step (10)    Mesophilic or thermophilic anaerobic digestion step (20)    Biological solubilization anaerobic treatment step (30)    Final dehydration step. (50)