Magnesium supplementation for improved struvite recovery from dairy lagoon wastewater

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.