Enhanced phosphorus reclamation from secondary effluent through Donnan dialysis and vivianite crystallization

Phosphorus (P) is both a pollutant and a finite resource. Its excess discharge into water bodies accelerates eutrophication, disrupting ecosystems and endangering public health. Meanwhile, the depletion of global P reserves highlights the need for recovery from alternative sources like wastewater. Though secondary effluents contain only trace P levels, these are sufficient to disturb aquatic balance. Therefore, polishing of secondary effluent is essential for water reuse and nutrient reclamation. This study presents a novel two-step method for P recovery from secondary-treated wastewater. First, Donnan dialysis concentrates P, making extraction more efficient. Then, chemical precipitation converts it into vivianite (Fe3(PO4)2·8H2O), a valuable iron-phosphate mineral. Process parameters – pH, Fe/P ratio, alkalinity, and dissolved oxygen (DO) – were optimized using synthetic wastewater (20 mg P/L), while response surface methodology (RSM) was applied to real wastewater. Visual MINTEQ 3.1 simulated precipitate formation. The Fe/P ratio was the most influential factor, followed by pH and DO. Optimal conditions (pH 8.5, Fe/P 2.5, DO 1 mg/L) achieved 98.47% P removal. X-ray diffraction confirmed vivianite formation, scanning electron microscopy showed its platy rosette structure, and energy-dispersive X-ray analysis indicated 72.66–86.66% purity. A life cycle assessment identified key environmental impacts, including terrestrial ecotoxicity, global warming, and resource depletion.