Iron and phosphorus speciation in Fe-conditioned membrane bioreactor activated sludge

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). In this study, bench scale MBRs were dosed with either ferrous or ferric salts and the Fe and P-containing mineral distributions in the Fe-conditioned sludges determined using X-ray absorption spectroscopy (XAS). 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 +III oxidation state. Significant proportions of an Fe(III) mineral exhibiting very similar EXAFS spectra to strengite were present in all cases with the remaining fraction dominated by lepidocrocite in the Fe(II)-dosed case and ferrihydrite in the Fe(III)-dosed case. Approximately half the phosphorus in the activated sludge samples was present as a distinct Fe-PO4 mineral (such as strengite or an amorphous ferric hydroxyl phosphate analog) and half as phosphorus adsorbed to an iron oxyhydroxide mineral phase indicating that both co-precipitation and adsorption of phosphorus with iron contribute to removal of phosphorus from the MBR supernatant.