This work deals with the study of technologies aimed to upgrade the existing waste water treatment plants, paying attention to high process efficiencies and low costs. We verified the possibility to reach high N removal efficiencies in extended aeration activated sludge plants which are not equipped with specific denitrification steps. The experimented process is based on particular conditions, kept in the biological reactor, which allow simultaneous nitrification and denitrification, even without alternating (in time or in space) of anoxic and aerobic phases: aeration system is controlled by means of dissolved oxygen and redox potential measurements. Our research was carried out, for about three years, on a laboratory plant (V = 40 L) fed with synthetic waste water, a pilot scale plant (V = 1,400 L) fed with waste water coming from a real plant, and two full-scale plants (1,500-2,000 p.e.). The main advantages of the studied process are the following: high total N removal efficiencies (up to 90%) without the realization of new specific basins for the denitrification process; decreasing of operating costs (savings in Electric Energy consumption) due to the low oxygen concentration required in the biological reactor.