Can Iron Redox-Mediated Phosphorus Immobilization Replace Enhanced Biological Phosphorus Removal in Wastewater Treatment Processes?

The incorporation of in situ crystallization of vivianite (Fe₃(PO₄)₂·8H₂O) in an enhanced biological phosphorus removal (EBPR) process using an Fe-retrofitted University of Cape Town wastewater treatment process coupled with a membrane bioreactor is shown to be a highly effective strategy to improve the efficiency and stability of phosphorus removal during wastewater treatment over a 400-day trial period. Iron-mediated phosphorus removal (IPR)─driven by iron redox cycling and phase transformations in anaerobic–aerobic chambers─enhances polyphosphate synthesis/hydrolysis activity by polyphosphate-accumulating organisms (PAOs) with the synergy of IPR and EBPR in adsorbing phosphorus, increasing the efficiency of phosphorus removal to 90% during long-term operation. An essential decrease in the contribution of EBPR to final phosphorus removal as a result of the introduction of IPR reduced the requirement for a carbon source either from influent or via supplementation and also reduced the need for discharge of a phosphorus-rich sludge, thereby substantially decreasing the cost of phosphorus removal and the associated carbon emissions. In addition, phosphorus immobilization via the IPR pathway enables the recovery of this element in the form of separable vivianite granules, which aligns with the evolving role of WWTPs as resource recovery facilities.