Struvite precipitation with MgO as a precursor: Application to wastewater treatment

Phosphorous (P) is an essential element and plays a key role for life on Earth. It stiffens the bone structure, is a major component of DNA and is involved in the metabolism of living cells. One of the most widely known metabolic processes is photosynthesis, which, amongst many other things, fuels the growth of plants. Modern agriculture uses fertilizers to supply plants with additional phosphorous in order to support photosynthesis and maximize yields. Still today, the primary source for fertilizers is phosphate rock, the supply of which is finite and estimated to be depleted within in the next century. In recent years, several techniques were developed to recycle phosphorous and prevent future scarcity of this vital element. This study examines a method to recover P from nutrient-rich wastewater streams by crystallizing struvite (MgNH4PO4·6H2O). A low-grade magnesium oxide, a by-product of the magnesite industry, is added to the effluent to precipitate struvite. Precipitation was investigated in terms of the amount of magnesia added, and its influence on crystallization kinetics and the resulting particle morphologies and sizes. The amount of magnesia determines the supersaturation of the solution by fixing the pH and the chemical potential of magnesium ions in solution, thereby inducing crystallization. Here, the use of an MgO suspension couples the struvite precipitation to a prior MgO dissolution step. This step is controlled by surface reaction and it is assumed that the suspended particles facilitate heterogeneous nucleation. Apparently, the amount of magnesia solids has a strong influence and precipitation mainly occurs around the MgO particles.