Iron reduction in activated sludge measured with different extraction techniques

Pools of Fe(II) and Fe(tot) in activated sludge from a nutrient removal plant were determined after extraction with HCl, oxalate or Ferrozine. Of a total Fe content in fresh sludge of 1.12 mmol per g dry sludge (62.6 mg/g TS), HCl and oxalate extracted 80.7 and 90.5%, respectively. More than 70% was as Fe(III). Fe(III) reduction was immediately initiated when sludge was stored anaerobically. The levels of Fe(II) observed during the anaerobic storage period depended on the method used. With HCl and oxalate extraction techniques Fe(II) continuously increased during two days of storage, whereas Ferrozine extractable Fe(II) accumulated more slowly over the first 24 h of anaerobic storage and did not increase beyond that time. Almost all produced Fe(II) remained associated to the floc matrix, so soluble Fe2+ was very low. In short-term (10 h) and long-term (2 days) anaerobic incubations Fe(II) accumulation rates were in the range 6.0–29.4 μmol per g organic matter per hour at 20°C, where the highest rates were obtained in short-term experiments. Fe(III) reduction was of minor significance as electron acceptor for turnover of organic substrate compared to oxygen or nitrate, but could be of importance compared to other anaerobic processes. The reduction was of significance for possible release of phosphate into bulk water in an anaerobic bio-P tank and by anaerobic storage of sludge before dewatering.