Factors affecting phosphorus recovery as struvite: Effects of alternative magnesium sources

Phosphorus depletion around the globe has made it a critical element for sustainable agriculture resulting in significant research into options to recover and recycle this non-renewable resource. Phosphorus recovery as struvite usually requires the dosing of supplemental commercial magnesium and can account for between 10 and 75% of the cost of struvite production. Therefore, cheaper alternatives to commercial magnesium sources such as seawater and concentrates from RO desalination plants have been sought. Here, we examine the effects of alternative magnesium sources on the phosphorus removal efficiency, particle size, rate constant, morphology and purity of struvite precipitated from synthetic solutions. It was found that the phosphorus removal reduced for struvite precipitation using seawater (14%) compared to the control (29%), but increased for struvite precipitation using bittern (37%). Precipitation using seawater also increased the calcium content from <0.1% for the control and bittern to 6.6% and reduced the average particle size from 88 μm for the control to 50 μm for seawater and to 67 μm for bittern in 1 L jar tests. Batch reactor tests indicate that both magnesium sources produced struvite, though the particle size was smaller for trials using alternative magnesium sources which may limit the efficiency of solid-liquid separation