Phosphorus Recovery from Municipal Wastewater: Brucite from MgO Hydrothermal Treatment as Magnesium Source

Abstract

Crystallization of struvite (MgNH4PO4·6H2O, MAPH) for both N and P recovery from municipal wastewater is an attractive alternative. The low magnesium concentration in municipal wastewater calls for low-cost magnesium sources. In the present work, the precipitation of struvite from supersaturated solutions was investigated. The supersaturated solutions were prepared by the addition of either slurry of Mg(OH)2 (brucite) obtained from the hydrothermal treatment of magnesium oxide (MgO) or using the aqueous solution equilibrated with brucite after separation from the solid. In both cases, in the supersaturated solutions prepared by the addition of ammonium dihydrogen phosphate, the Mg:NH4:P ratio was 1:1:1, pH 7.0–11.0. The apparent order for the precipitation suggested that the dominant mechanism was surface diffusion according to the polynuclear model. The apparent rate constant for struvite precipitation was two orders of magnitude smaller for the slurry prepared supersaturated solution in comparison with the respective prepared from the brucite-equilibrated solution. Simulated wastewater (SWW), was supersaturated with respect to struvite, by inoculation with brucite. The apparent rate constant was two orders of magnitude higher than that corresponding to the supersaturated solutions prepared by the addition of solution equilibrated with brucite. The P recovery from SWW reached 70%.

Keywords

• municipal wastewater
• phosphorus recovery
• magnesium oxide
• struvite