The Effect of Mixing Rate on Struvite Recovery from the Fertilizer Industry

Abstract

Fertilizer wastewater contains a high concentration of ammonium and phosphate. One method of reducing the ammonium and phosphate contents is to recover them in the form of magnesium ammonium phosphate (MAP) or struvite (MgNH4PO4.6H2O). The objective of this experiment was to obtain the optimum mixing rate, pH, and molar ratio for struvite precipitation. A mixture of wastewater containing [Mg2+]:[NH4+]:[PO43-] in molar ratios of 1:1:1, 1.5:1:1, and 2:1:1 was added to a 0.5-L beaker glass. Samples were then stirred under G.t values of 0.5 106, 106, and 1.5 106 for 60 minutes and left for 30 minutes for the sedimentation process. pH was set to 7.5, 8, and 8.5. Subsequently, the experimental results were compared with thermodynamic modelling using PHREEQC v3.0. The results showed that the optimum mixing rate was 158 rpm, which is equal to a G.t of 106; the optimum pH was 8.5 0.2; and the optimum molar ratio of [Mg2+]:[NH4+]:[PO43-] was 1:1:1. The removal percentage was 86.14% for ammonium and 98.98% for phosphate. Experimental results displayed a pattern similar to that predicted by the model. Additionally, the morphology of struvite shown by SEM-EDX and XRD analysis also demonstrated that struvite was formed in the precipitate.