Seawater as Magnesium Source for Struvite Recovery from Wastewater

Abstract

Phosphorus is essential for living organisms and is widely used in various industries. Phosphorus naturally exists as phosphates (PO43-) and is mainly exploited from phosphate ores, a mineral that is predicted to be exhausted in the next century. On the other hand, extra phosphorus released into water bodies can cause eutrophication issues. Hence, phosphorus recovery from agricultural wastewater is one option to tackle future phosphorus shortage and eutrophication issues. This project investigated the feasibility of saline water (seawater and its concentrate) as a magnesium source for struvite recovery from wastewater. The research focused on: the effect of ionic strength and the presence of acetic acid in feed water on struvite recovery, the development of a soft sensor to predict the phosphorus removal and struvite purity, the application of struvite and K-struvite for immobilising heavy metals in contaminated soil, and a case study on the use of nanofiltration retentate of Takapuna seawater as a magnesium source for phosphorus recovery. This study indicated the feasibility of using saline water as a magnesium source for phosphorus recovery from wastewater. The experiment results confirmed that the phosphorus removal process was not sensitive to the ionic strength and the presence of acetic acid, indicating that the process has reasonable robust control. A soft sensor has been successfully developed using shape factors of struvite, ionic strength, and saturation index to predict phosphorus removal and struvite purity, allowing real-time monitoring of struvite quality. Struvite and K-struvite have the potential for the immobilisation of certain heavy metals from contaminated soil. Struvite produced with magnesium source from nanofiltration retentate of Takapuna seawater demonstrated similar characteristics as the one produced with pure magnesium chloride reagent. The findings of this study provide insight into the application of alternative magnesium sources for phosphorus recovery as struvite and other potential applications of struvite for remediation of contaminated soil.