Magnesium supplementation for improved struvite recovery from dairy lagoon wastewater

Abstract Recovery of phosphate (PO4-P) as struvite (MgNH4PO4·6H2O) from dairy lagoon wastewater (DW) limits contamination of surface water arising from application of DW to crop fields. Magnesium (Mg) addition to DW during PO4-P recovery is used to promote struvite precipitation and suppress formation of undesired PO4-P mineral products. A pilot-scale aerated fluidized-bed reactor was used to treat DW for PO4-P recovery. The effect of Mg concentration (0–20 mM) and reaction pH (9–10) on mineralogy, thermal stability and micronutrient content of collected solids was evaluated. Elemental analysis, scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDX), X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy confirmed that the major PO4-P and NH4-N mineral recovered from DW was struvite (20–30% w/w). Simultaneous thermal analysis with evolved gas analysis (STA-EGA) showed that ammonium (NH4-N) in recovered struvite had two thermal maximums of decomposition into ammonia gas (NH3(g)): low temperature (LT-NH3) at 100–109 °C, and high temperature (HT-NH3) at 295–318 °C. Increasing Mg concentration did not improve PO4-P recovery as struvite and reduced the fraction of HT-NH3 in solids from 50% to 14%. Higher Mg concentrations also decreased the sorption of micronutrients such as zinc and iron with recovered solids. These observations indicate a potential benefit to application of lower Mg concentrations during PO4-P recovery that will increase the content of micronutrients and reduce thermal losses of NH4-N in the produced struvite fertilizer.