Microbial Nitrate-Dependent Oxidation of Ferrous Iron in Activated Sludge

A biological reduction of nitrate and nitrite was found to take place in activated sludge concomitantly with the oxidation of ferrous iron to ferric iron. This process was shown to be predominantly biological and present in different types of activated sludge treatment plants with variable rates. The highest activity was found in plants with biological nitrogen and phosphorus removal. The highest Fe(II)-dependent nitrate removal rate was found to be 0.31 mmol NO 3 - (g VSS) - 1 h - 1, which corresponded to 68% of the maximum dissimilatory nitrate reduction rate in the presence of lactate. The Fe(II)-dependent nitrate removal rate was strongly pH-dependent with a maximal rate at pH 8 of almost four times the rate at pH 6. The main product of Fe(II)-dependent nitrate removal was most probably dinitrogen, as no accumulation of ammonia, nitrous oxide, or nitrite could be observed. The process may be of significance in the activated sludge treatment plant with regard to nitrate removal and with regard to the reoxidation of Fe(II) to Fe(III), which influences the chemical phosphorus removal and the flocculation properties of the sludge.