The denitrification process reduces nitrates in the nitric oxide, nitrous oxide and nitrogen gas, through the action of facultative heterotrophs bacteria, under anoxic conditions. These bacteria oxidize the organic substance, that is used as an energy source, taking advantage of the oxygen of the nitrate as electron acceptor.

Biological denitrification, therefore, involves several next stages which, starting from nitrates, lead to the nitrogen gas as shown below:

During this process, 3,57 gr of alkalinity as CaCO3 are produced per each gram of reduced N- NO3-.

If in the water to be treated there is an insufficient amount of nitrogen in ammoniacal form N-NH4+, a small portion of N-NO3- (equal to 5% of BOD removed) is used in assimilative biological processes to satisfy the necessity of nitrogen for the synthesis of new cells.

Biological denitrification is an integral part of the process of biological nitrogen removal, which therefore includes the steps of nitrification and denitrification. The most diffuse scheme of plant, to perform these treatments, is to provide a basin upstream of the oxidation, with the function of pre-denitrification, and to perform the Nitrification step directly in the aerated tanks, but with machineries for the oxygen transfer suitably enhanced. For the process, it is necessary that the nitrates produced in the aerobic tank are then recirculated to the anoxic tank, sizing the recirculation so that the amount of nitric nitrogen, pumped to the anoxic reactor is suitably greater than that which must be removed.

This technology is an integral part of the treatment process of a Biological plant.