TECHNOLOGY FOCUS - MECHANICAL ENGINEERING - Preferred Plant: In the plant for the treatment of biological sludges, the reactor (303) that performs the process of alkaline protein lysis at medium-low temperature is set at atmospheric pressure, comprises feed means of lime milk or caustic soda to obtain a suspension with pH higher than 12; heating means to take the temperature to 40-90 degrees C; mixing means for the suspension, and has a volume arranged for ensuring a hydraulic retention time of the incoming sludges of 2-10 hours depending on the temperature used and on the degree of solubilization of the organic fraction to be obtained. The forced filtration device (304) comprises a centrifuge. The plant comprises a thermophilic aerobic fluidized bed reactor (501), arranged for treating the protein lysate broth and for completing the purification cycle of the organic load contained in the aqueous phase forming the basic protein lysate; a device (502), arranged for performing stripping and subsequent acid absorption of the Ammonia contained in the treated waters delivered from the reactor (501) produced starting from the organic Nitrogen contained in the sludges, transforming it into ammonium sulfate; an anaerobic digester (404) arranged for treating at least the protein lysate broth and for transforming the organic carbon into biogas, and arranged for supplying the residual organic load contained in the liquid phase delivered from the digester to the thermophilic aerobic reactor (501); a biogas purification device (601); a gas holder (602) for storage; a boiler (603) for producing steam; a plant (604) for lowering combustion gas emissions; a condensing turbine (605); a high temperature hot water circuit (606) and a low temperature hot water circuit (607); a volume (305) for the basic protein lysate susceptible to be re-used both in animal nutrition, as a supplement, and in agriculture; a volume (701) for containing the ammonium sulfate destined both for agriculture and for industry; a volume (702) for containing the lime milk deriving from purging of a circuit for recovery of lime inside the plant. The plant as well comprises a calcination kiln (801) of the cake of solids separated by centrifugation of the basic protein lysate; a volume (802) for containing quicklime polluted by inert substances and heavy metals obtained in the kiln (801); a reactor (808) for slaking the quicklime with treated recycled water and producing calcium hydroxide, with purging of a part to avoid accumulation of impurities and destined to be reused externally; a lime milk production device (804); and an active sludge biological treatment (901) of the output waters from the thermophilic aerobic reactor (501), after stripping the ammonia in the device (502); and a volume (902) for accumulation of the purified waters reusable both for preparing/slaking the lime and for initial dispersion of the biological sludges received to the plant in solid physical state.