Phosphorus and nitrogen recovery from wastewater in the form of struvite: Alternative magnesium sources

Recovery of phosphorus and nitrogen from municipal and other types of wastewater may be achieved through the direct crystallization of struvite (magnesium ammonium phosphate hexahydrate, Mg·NH4·PO4·6H2O). Wastewater deficiency in magnesium makes it necessary for the external addition of magnesium to make up for the supersaturation needed. In the present work we have tested artificial seawater and saturated with respect to calcined magnesia (MgO) aqueous solutions as magnesium sources, as both sources are low cost and readily available. Both the spontaneous precipitation and the seeded crystal growth have been investigated at constant supersaturation (pH 9.0- 10.0, 25°C). Seawater accelerated the precipitation of struvite and narrowed the stability domain of the supersaturated solutions. Saturated magnesia solutions were shown to broaden the stability domain of the struvite supersaturated solutions. Magnesia suspensions were also tested with respect to their ability to induce crystallization of struvite. Measurement of the kinetics of crystal growth by potentiometric methods maintaining solution supersaturation, and AFM observations showed that the solid particles in magnesia saturated suspensions, provide favorable substrate for the overgrowth of struvite in combination with the appropriate supersaturation conditions.