[Effect of pH on precipitate composition during phosphorus recovery as struvite from swine wastewater]

Phosphorus recovery as struvite from swine wastewater was carried out. Fourier transform infrared spectroscopy (FTIR), Xray diffraction (XRD) and mass balance analysis were utilized to analyze the species of precipitated minerals under different pH conditions. Results showed that increasing pH from 8.0 to 9.0 resulted in the increase of phosphorus removal efficiency from 85% to 94%. A relatively stable phosphorus removal at 94% was observed at pH 9.0-11.0, whereas a drastic decline to 70% was detected when pH increased to 12.0. The minerals precipitated in the deposits were struvite (MgNH4PO4 x 6H2O), K-struvite (MgKPO4 x 6H2O), amorphous calcium phosphate [Ca3 (PO4 )2 (x) xH2O, ACP] and Mg (OH)2. Struvite was the dominant species in the precipitate at pH 8.0-9.0. Enhancing pH from 9.0 to 10.0 resulted in struvite decline and gave rise to K-struvite and ACP steadily. With regard to highly alkaline conditions at pH above 10, drastic decrease of struvite and rapid increase of ACP and Mg(OH)2 were observed. Maximum concentration of K-struvite was obtained at pH 11.0, following a sharp decline to pH 12.0. Controlling pH between 8.0 and 9.0 could inhibit other minerals formation and obtain the highly pure struvite crystal product.