NOVELTY - Purifying polluted waters in a processing tank (1) with continuous supply and discharge involves blowing sewage or effluent and air with conventional oxygenizers (5) at a pressure of at least 2 bars in the upper pipe network and at a distance D from the bottom of the tank varying from about half the tank height to about the liquid surface in the tank; and stirring the sewage or effluent without blowing air by at least one stirrer (10) that is able to break the biological foam previously formed.    USE - For purifying polluted waters in a processing tank; for purification of sewages or effluents (claimed); for reducing the nitrogen content and the bad smell in highly polluted waters, more specifically of livestock sewage, digestates from biogas.    ADVANTAGE - The process can take place in one processing tank with continuous feed and discharge, and using lower energy consumption. The high pressure in the pipe network creates within each oxygenizer such a high specific air-liquid exchange surface to enable the transfer into the liquid of the greater air flow rate supplied by the forced aeration system. The high pressure in the upper pipe network causes in the oxygenizers the effluent or sewage to be pulverized and mixed with air, so as to support a highly effective and easily managed biological process. The level at which the exit orifices of the oxygenizers lie, which level is always high with respect to the height of the tank and optionally near the surface of the liquid in the tank, as well as the high pressure in the upper network ensure an extremely high effectiveness of the nitrification step. The method allows implementing more than one type of treatment of polluted effluents. The method allows a reduction of nitrogen and organic substance of 75-80%. Oxygen supplied to these plants is exactly the same amount consumed for reducing nitrogen and a part of organic substance. Thus the whole organic charge is not consumed and the discharge obtained from the plant still has a certain fertilizing power. The plant is highly flexible and the process is very stable and effective, thus it is suitable for a large number of highly polluting effluents/sewages. The process is always self-adjusting at a temperature of 30-38 degrees C and pH of 7-7.8. The redox potential and oxygen concentration depend on the type of treatment, more or less forced, which is to be carried out, according to users'requirements. Eventually, the forced aeration unit associated to the plant enables a reduction of processing costs in all types of plants and treatments, with the same sewage or effluent to be treated. Long stainless steel pipe networks are not necessary inside the processing tank, which means lower costs for the plant. Since the container is separated from the processing tank, all maintenance operations are much simpler and faster. Moreover, it is no longer necessary to empty the processing tank for maintenance in the upper network and in the oxygenizers immersed in the tank. The system is much more flexible for oxygenation management, since there is no longer a strict connection between oxygenation effectiveness (which depends on the level of liquid above the oxygenation network) and retention time (which depends on the volume, and thus on the level, of liquid contained in the tank).    DETAILED DESCRIPTION - Purifying polluted waters in a processing tank (1) with continuous supply and discharge involves a nitrification step, in which blowing sewage or effluent and air with conventional oxygenizers (5) at a pressure of at least 2 bars in the upper pipe network and at a distance D from the bottom of the tank varying from about half the tank height to about the liquid surface in the tank; and stirring the sewage or effluent treated in the first step without blowing air by using at least one stirrer (10) that is able to break the biological foam previously formed, where the sewage or effluent and air are blown with conventional oxygenizers (5) installed along a pipe network (7) and equipped with unit for sucking up atmospheric air to an effect only conventionally (Venturi effect), and alternating times and step duration is selected as a function of nitrogen load being input. An INDEPENDENT CLAIM is included for a plant for purifying polluted water comprising a processing tank receiving from outside the polluted sewage or effluent; at least one pump (2) sucking up sewage or effluent from the bottom of the tank and sending it into a pipe network including several oxygenizers immersed in the sewage or effluent; and a filter (4) being placed between the pump and a pipe sucking up the sewage or effluent from the bottom of the tank, where the pump produces in the pipe network a pressure of at least 2 bars and the discharge openings of the oxygenizers are located at a distance D from the bottom of the tank varying from about half the effective height of the tank to the surface of the sewage or effluent in the tank.    DESCRIPTION OF DRAWING(S) - The figure shows a first side view of the plant.    Tank (1)    Pump (2)    Duct (3)    Self-cleaning filter (4)    Oxygenizer (5)    Upper pipe network (7)    Stirrer (10)