Inducing in Situ Crystallization of Vivianite in a UCT-MBR System for Enhanced Removal and Possible Recovery of Phosphorus from Sewage.

Abstract

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.