TECHNOLOGY FOCUS - ENVIRONMENT - Preferred Process: The aeration required in the first process step is obtained in a closed container independent from the processing tank, and in that a continuous recirculation of sewage or effluent is obtained between the processing tank and the closed container, which recirculation distributes the flow of aerated sewage or effluent in the whole volume of the processing tank. The distance D from the bottom of the tank is defined by the function D is logax (where a is (x4+x3)/k, x is the effective height of the tank and k is an empirical constant depending on the type of effluent or sewage and on the type of treatment or purification used). For the treatment of sewage it is possible to keep in the first step dissolved oxygen (DO) of less than 0.1 mg/l, and redox potential (RedOX) of -200 m/0 mv because of the high concentration of dissolved oxygen and positive redox inside the oxygenizers, and the second step is carried out by conditions in which DO is 0 mg/1 and RedOX is -400/300 mV, thus causing a reduction of 85% of nitrogen and of 80% of organic substance. The oxygenizers are installed with their discharge axis inclined of 8-90 degrees with respect to the horizontal. Preferred Plant: The plant comprises a) closed aeration container, associated to and outside or inside the processing tank, in which the oxygenizers is located and sewage or effluent is aerated during the first process step; and either b) the pump taking the sewage or effluent from the processing tank and supplying it under high pressure into an oxygenation network and transferring it by oxygenizers into the closed aeration container from which the sewage or effluent thus aerated is supplied back into the bottom of the processing tank where a stirrer carries out the stirring of the second process step; or b1) the pump taking the sewage or effluent from the aeration container and supplying it back under high pressure into an oxygenation network and transferring it by oxygenizers into container, and a second pump taking from the bottom of the processing tank, the sewage or effluent to be aerated and sending it to the closed aeration container, the effluent or sewage thus treated being supplied back into the bottom of the processing tank where a stirrer carries out the stirring of the second process step. The plant comprises forced blowing unit that are able to send pressurized air into the oxygenizers, thus increasing the flow rate of air sucked up by Venturi effect. The processing tank is equipped with at least one inner clarifier in which settles the sludge produced and to be extracted or recirculated; or is equipped with microfiltration or ultrafiltration membranes for filtering the sludge produced and to be extracted or recirculated.    TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Pipe: The average pressure in the pipe network is 4 bars in processes for the treatment of sewage. The average pressure in the pipe network is at least 7 bars in processes for the treatment of effluents. EXAMPLE - No suitable example given.