Vivianite precipitation in high recovery reverse osmosis desalination of anaerobic wastewater effluent

Wastewater reuse is essential to water supply and conservation efforts. Anaerobic membrane bioreactors (AnMBR) are emerging as an efficient treatment, enabling high effluent quality and low energy consumption. Subsequent reverse osmosis (RO) treatment can achieve potable water quality. Although mineral fouling in RO has been extensively studied, research on iron-based minerals, particularly under anaerobic conditions, remains notably scarce. Specifically, the potential fouling caused by Vivianite (Fe3(PO4)2·8H2O) – a mineral prone to precipitate and clog pipes of anaerobic wastewater streams – during high recovery RO applied to anaerobic effluents (e.g., following AnMBR) had not been systematically studied. This study used geochemical modeling and filtration experiments to test the potential mineral fouling of ferrous ion-bearing minerals in RO of synthetic and real AnMBR effluents. The geochemical model calculated mineral precipitation potential at pH 5–10 and an 85–95 % recovery ratio. Vivianite and amorphous calcium phosphate (ACP) were the dominant precipitants. Filtration experiments at varying feed Fe 2+ concentrations (0–10 mg/L) and pH (6.0–7.1) revealed the significant effect of these parameters on mineral fouling type and extent, agreeing with the geochemical model. Vivianite precipitated at all pH values, significantly reducing (>90 %) at pH 6 compared to pH 6.75 & 7.1. Higher Fe 2+ feed concentrations were correlated with a higher extent of vivianite mineral fouling. ACP was observed only at pH 7.1 since it competed with vivianite over P utilization. This research presents the first systematic investigation of vivianite mineral fouling in RO processes, informing process design of anaerobic effluent water reuse processes. • Ferrous iron is largely unremoved by anaerobic membrane bioreactors. • Fe(II) concentration and pH affect mineral fouling in anaerobic effluent RO. • Vivanite fouling was prevalent in high-recovery RO At high Fe(II) (>2 mg/L, pH ≥ 6.75). • Vivianite and amorphous calcium phosphate compete for P utilization At pH ≥ 6.75. • Geochemical equilibrium modeling successfully predicted type of mineral fouling in anaerobic effluent RO.