By mimicking iron­(Fe)-based phosphorus (P) immobilization in natural environments, an Fe-retrofitted UCT-MBR involving in situ vivianite crystallization for removing and recovering P from sewage was developed, and its performance was examined in this work. We show that dosing of ferrihydrite, once biological P uptake reached its limit, enabled effective ongoing P removal; whereas conventional conditions in the anaerobic chamber of the University of Cape Town (UCT) system (i.e., a sludge retention time of hours and a completely mixed sludge phase) was insufficient for a satisfactory Fe­(III) bioreduction, with the overaccumulation of Fe­(III) as fine particles finally resulting in severe membrane fouling and collapse in P removal. The enhancement of reductive conditions in the anaerobic chamber by lowering agitation and adding biocarriers to favor Fe­(III) reduction was found to be effective in enabling ongoing P removal and recovery. The average level of effluent P was as low as 0.18 mg/L for a period of 258 d under this condition. Using chemical and spectroscopic methods, the P product was identified as primarily vivianite: Fe3(PO4)2·8H2O. The in situ crystallization of vivianite as a sink for P enabled the UCT-MBR to continuously remove and recover sewage P with no need for sludge discharge.

No abstract

No abstract

The formation of vivianite (Fe3(PO4)2·8H2O) in iron (Fe)-dosed wastewater treatment facilities has the potential to develop into an economically feasible method of phosphorus (P) recovery. In this work, a long-term steady FeIII-dosed University of Cape Town process-membrane bioreactor (UCT-MBR) system was investigated to evaluate the role of Fe transformations in immobilizing P via vivianite crystallization. The highest fraction of FeII, to total Fe (Fetot), was observed in the anaerobic chamber, revealing that a redox condition suitable for FeIII reduction was established by improving operational and configurational conditions. The supersaturation index for vivianite in the anaerobic chamber varied but averaged ∼4, which is within the metastable zone and appropriate for its crystallization. Vivianite accounted for over 50% of the Fetot in the anaerobic chamber, and its oxidation as it passed through the aerobic chambers was slow, even in the presence of high dissolved oxygen concentrations at circumneutral pH. This study has shown that the high stability and growth of vivianite crystals in oxygenated activated sludge can allow for the subsequent separation of vivianite as a P recovery product.

Vivianite (Fe3(PO4)2·8H2O) crystallization has attracted increasing attention as a promising approach for removing and recovering P from wastewaters. However, FeII is susceptible to oxygen with its oxidation inevitably influencing the crystallization of vivianite. In this study, the profile of vivianite crystallization in the presence of dissolved oxygen (DO) was investigated at pHs 5–7 in a continuous stirred-tank reactor. It is found that the influence of DO on vivianite crystallization was highly pH-related. At pH 5, the low rate of FeII oxidation at all of the investigated DO of 0–5 mg/L and the low degree of vivianite supersaturation resulted in slow crystallization with the product being highly crystalline vivianite, but the P removal efficiency was only 30–40%. The removal of P from the solution was substantially more effective (to >90%) in the DO-removed reactors at pH 6 and 7, whereas the efficiencies of P removal and especially recovery decreased by 10–20% when FeII oxidation became more severe at DO concentrations >2.5 mg/L (except at pH 6 with 2.5 mg/L DO). The elevated degree of vivianite supersaturation and enhanced rate and extent of FeII oxidation at the higher pHs led to decreases in the size and homogeneity of the products. At the same pH, amorphous ferric oxyhydroxide (AFO)the product of FeII oxidation and FeIII hydrolysisinterferes with vivianite crystallization with the induction of aggregation of crystal fines by AFO, leading to increases in the size of the obtained solids.