The significance of microbial Fe(III) reduction in the activated sludge process

The significance of microbial Fe(III) reduction in activated sludge was evaluated with regard to: its importance as electron acceptor; as a producer of acetate during anaerobic conditions; for phosphate release; and for its role in the floc structure. Potential Fe(III) reduction rates were measured in 6 wastewater treatment plants with and without biological P-removal and found to be in the range of 0.9-5.4 mgFe/gVSS h. Assuming an incomplete oxidation of organic matter leading to acetate formation, Fe(III) reduction was a major acetate source, providing substrate to phosphorus-accumulating organisms (PAO) during anaerobic conditions. The observed high potential Fe(III) reduction rate might also be responsible for a significant chemical phosphate release due to reduction of Fe(III) to Fe(II) in clarifies, sludge storage tanks and anaerobic tanks in plants with biological P-removal. Investigation of the concentrations of Fe(II) in a full-scale treatment plant in anaerobic tanks, oxic/anoxic tanks and return sludge indicated that both reduction and reoxidation took place in the treatment plant. Reoxidation of Fe(II) to Fe(III) in activated sludge was shown to take place with oxygen and probably also during anoxic conditions with nitrate and nitrite as electron acceptors. The results indicate that Fe may be more involved in important processes in activated sludge than hitherto assumed, so a better understanding of Fe interactions in activated sludge is desirable.