Iron and phosphorus speciation in Fe-conditioned membrane bioreactor activated sludge Hao Wu a , Atsushi Ikeda-Ohno a,b,c , Yuan Wang a,d , T. David Waite a,* a Water Research Centre, School of Civil & Environmental Engineering, The University of New South Wales, Sydney 2052, Australia b Institute for Environmental Research, Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia c Institute of Resource Ecology, Helmholtz-Zentrum Dresden-Rossendorf, Bautzner Landstrasse 400, 01328 Dresden, Germany d UNESCO Centre for Membrane Science & Technology, School of Chemical Engineering, The University of New South Wales, Sydney 2052, Australia article info Article history: Received 12 July 2014 Received in revised form 19 December 2014 Accepted 9 February 2015 Available online 5 March 2015 Keywords: Membrane bioreactor Iron Phosphorus X-ray absorption spectroscopy abstract Iron dosing of membrane bioreactors (MBRs) is widely used as a means of meeting effluent phosphorus targets but there is limited understanding of the nature of iron and phosphorus-containing solids that are formed within the bioreactor (an important issue in view of the increasing interest in recovering phosphorus from wastewaters). Of particular challenge is the complexity of the MBR system and the variety of reactions that can occur on addition of iron salts to a membrane bioreactor. In this study, the performances of bench scale MBRs with dosing of either ferrous or ferric salts were monitored for a period of four months. The distributions of Fe and P-species in the Fe-conditioned sludges were determined using X-ray absorption spectroscopy (XAS) at the Fe K-edge and the P K-edge. Regardless of whether iron was dosed to the anoxic or aerobic chambers and regardless of whether ferrous (Fe(II)) or ferric (Fe(III)) iron was dosed, iron present in the minerals in the conditioned sludges was consistently in the þIII oxidation state. Fitting of the Fe K-edge EXAFS spectra revealed that an Fe(III)-phosphate species was the main Fe species present in all cases with the remaining fraction dominated by lepidocrocite (g-FeOOH) in the Fe(II)- dosed case and ferrihydrite (am-FeOOH) in the Fe(III)-dosed case. Approximately half the phosphorus in the activated sludge samples was present as a distinct Fe-PO 4 mineral (such as strengite or an amorphous ferric hydroxyl phosphate analogue of strengite) and half as phosphorus adsorbed to an iron oxyhydroxide mineral phase indicating that both co- precipitation and adsorption of phosphorus by iron contribute to removal of phosphorus from the MBR supernatant. © 2015 Elsevier Ltd. All rights reserved. * Corresponding author. E-mail address: d.waite@unsw.edu.au (T.D. Waite). Available online at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/watres water research 76 (2015) 213 e226 http://dx.doi.org/10.1016/j.watres.2015.02.020 0043-1354/© 2015 Elsevier Ltd. All rights reserved.