This paper deals with the development of technologies aimed to upgrade existing waste water treatment plants, paying attention to high process efficiencies and low costs. We established conditions for good N removal efficiencies in extended aeration activated sludge plants which are not equipped with specific denitrification steps. The experimental process is based on establishing conditions in the biological reactor which allow simultaneous nitrification and denitrification without alternating (in time or in space) anoxic and aerobic phases; the aeration system is controlled by means of dissolved oxygen and/or redox potential measurements. The research was carried out on two real plants (design size: 2,500 p.e. and 440,000 p.e. respectively). The main advantages of this process (even if some aspects are still under investigation) are: total N removal efficiencies similar to a pre-denitrification process, without the need for an anoxic basin and decrease of operating costs (savings in electric energy consumption in particular) due to the low oxygen concentration required in the biological reactor.

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