Oxygen control and improved denitrification efficiency by dosing ferrous ions in the anoxic reactor

Small concentrations of dissolved oxygen (DO) in the range 0.2–0.4 mg L−1 normally are present in biological pre-denitrification reactors. This situation causes adverse effects on denitrification rate and, consequently, on the process efficiency. The results presented show the possibility to control the DO in the anoxic reactor by dosing ferrous Fe(II) ions. The experiments were carried out on both batch samples and a pilot plant and proved that oxidation of Fe(II) to Fe(III) is very efficient in the DO control. Moreover, Fe(III) reacts with phosphorus which recipitates as ferric orthophosphate. A dose of 6 mgFe2+ L−1 decreased the mean DO concentration from 0.45 to 0.28 mg L−1; as a consequence, the denitrification efficiency (ηDEN) increased from about 65–77%. ηDEN reached up to 89% with 9 mgFe2+ L−1 (50% over the stoichiometric for phosphorus removal) thanks to an average DO concentration of 0.08 mgO2 L−1 in the denitrification stage. The results also highlighted the strong influence of DO (and consequently the dosage of Fe2+) on the specific denitrification rate suggesting to maintain DO concentration in the pre-denitrification reactors lower than 0.2 mg L−1 in order to achieve high operation efficiencies.