The effect of SiO₂ on phosphate and ammonium recovery from fertilizer wastewater

Phosphate and ammonium recovery from wastewater could produce magnesium ammonium phosphate (MAP). Several previous studies have found that precursor ions have an impact on MAP formation, such as the presence of calcium, sulfate, fluoride, and other ions. The wastewater from the petrochemical industry that mainly produces fertilizer contains a high concentration of ammonium, phosphate, and silica. This study aims to determine the effect of silica on MAP precipitation. The batch system used a jar test at an optimum speed of 158 rpm for 60 min. MgCl₂ was used as a precipitation agent. Synthetic wastewater with a [Mg]/[NH₄–N]/[PO₄] and [SiO₂] ratio was predicted using the PHREEQC model. The pH variations of the sample were 8, 8.5, and 9. SiO₂ concentration varied in the range of 20–1000 mg/L. The parameters measured were pH, silica, phosphate, ammonium, and magnesium concentration, and MAP morphology. MAP precipitation has an impact on increasing the concentration of silica in wastewater. Recovery of phosphate and ammonium reduction reached 20% and 10%, respectively, with the concentration of silica of 1000 mg/L at pH 9 ± 0.2. The morphology of MAP as analyzed by SEM and XRD shows that silica has an impact on the purity of the MAP.