Factors influencing urease driven struvite precipitation

• An increase in ionic strength resulted in a decrease in ureolytic activity.
• A low sludge concentration resulted in larger struvite crystals.
• Large Mg2+ dosages did not significantly affect the phosphate removal efficiency.
• Calcium interfered with struvite precipitation in wastewater containing < 35 mg.L-1 PO4-P.

The concern about phosphorus depletion is driving more efficient phosphorus use and recycling. In this context the precipitation of phosphate as struvite from wastewaters has recently gained considerable traction. This study investigates the influence of different parameters on the ureolytic phosphate precipitation (pH increase by means of urease activity). Wastewater with a high phosphate concentration (>50 mg L−1 PO4–P) and low volatile suspended solids (⩽3.0 g L−1 VSS) resulted in a high phosphate removal efficiency (87%) and struvite crystals that could be separated from the sludge. An increase in ionic strength resulted in a decrease in ureolytic activity. An increase in Mg2+:PO43- molar ratio increases the degree of supersaturation with respect to struvite formation, which enhances the phosphate removal. Above a Mg2+:PO43- molar ratio of 3, additional Mg2+ dosage did not significantly affect the phosphate removal efficiency. Wastewater containing low phosphate concentrations (<50 mg L−1 PO4–P), needed disproportionally large chemical additions and are thus not economically rewarding. For those wastewaters the presence of calcium interfered with the struvite precipitation rendering the precipitation of calcium phosphate a better option.