Microbial Fe(II)-oxidation by nitrate in activated sludge

The oxidation of Fe(II) to Fe(III) by addition of nitrate and nitrite to activated sludge was studied to determine whether the process was biological or chemical (chemodenitrification). It was shown that the process was mainly biological, although the microorganisms involved have not yet been described. Investigations in a full scale treatment plant suggested that the process most likely took place in the anoxic (denitrification) tank. Details of the kinetics and stoichiometry have not yet been determined, but the process may be of significance for keeping Fe(III) oxidized, which is important for P-removal and for floc structure. Furthermore, in some treatment plants, the oxidation may also be of significance for nitrate removal (denitrification).