Struvite Production from Dairy Processing Waste

Abstract

Food security depends on sustainable phosphorus (P) fertilisers, which at present are mostly supplied from a finite rock phosphate source. Phosphate (PO43−) and ammonium (NH4+) in dairy processing wastewater can be recovered as struvite (Mg + NH4+ + PO43− 6H20), a nutrient rich mineral for fertiliser application. The objectives of this study were to (1) quantify the effects of, pH, temperature and Mg: PO43− dosing rates on nutrient (PO43− and NH4+) removal and struvite precipitation from post anaerobic digested dairy processing wastewater, and (2) co-blend different dairy processing wastewaters to improve the reactant stoichiometry of NH4+ and PO43− for optimal struvite recovery and NH4+ removal. Phosphate removal (>90%) and struvite production (>60%) was achieved across a range of synthesis conditions, and was significantly impacted by pH as determined by response surface modelling. A combination of disproportionate molar ratios of PO43− and NH4+, presence of calcium and the apparent mineralisation of organic N, resulted in co-precipitation of hydroxyapatite and elevated levels of residual aqueous NH4+. In the second phase of this study, struvite was successfully precipitated and NH4+ removal was improved (~17%) however, higher concentrations of calcium in the wastewater blends resulted in greater hydroxyapatite co-precipitation (up to 30%). While struvite was the desired product in this study the formation of multiple heterogenous P-rich products (struvite and hydroxyapatite) has the potential to improve P recovery from dairy processing wastewaters and produce a fertiliser blend with amenity and value in agricultural systems.