Investigation into struvite solubility, growth and dissolution kinetics in the context of phosphorus recovery from wastewater

The present research was conducted to investigate the mechanisms controlling formation, dissolution and decomposition of the mineral, struvite (MgNH₄PO₄•6H₂O) in the context of phosphorus recovery from wastewater. Solubility, thermodynamics, kinetics and thermal decomposition of struvite were studied in laboratory and wastewater treatment environments to gain knowledge to optimize the phosphorus recovery process from wastewater through struvite crystallization. The thermodynamic solubility products (Ksp) of struvite were determined by extrapolating measured solubility product values to zero ionic strength, with -log Ksp of 13.36(±0.07) at 25⁰C, using an appropriate activity coefficient model. A representative temperature compensation factor (a = 0.0198⁰C-¹) has been derived for electrical conductivity (EC) correction, and a relationship between ionic strength (I) and EC has been developed for anaerobic digester supernatant/centrate samples from five different wastewater treatment plants in western Canada. The metastable region, where nucleation is negligible, for struvite precipitation was explored in this study. This region was used in a kinetics study to suppress nucleation of struvite during growth experiments in a bench-scale fluidized bed reactor (FBR). A linear growth rate model has been tested and proposed, which was found to be effective for struvite growth determination in FBRs. The dissolution processes of struvite were investigated in a batch reactor system using two different theoretical models. The experimental values of struvite dissolution were found to fit well with both models. In a mixed flow-through reactor system, the dissolution rates for struvite pellets were found to increase with the hydrogen ion concentration in the acidic pH, while the rate of dissolution in the alkaline pH was found to increase due to hydroxyl-promoted dissolution. The thermal decomposition study of struvite showed that the simultaneous loss of both ammonia and water molecules from the struvite structure occurred gradually as a function of temperature, rather than as a distinct step. A pilot-scale struvite recovery FBR developed at The University of British Columbia (UBC) was operated, using the knowledge gained from the thermodynamics and kinetics experiments. The pilot-scale FBR was found to be effective in recovering phosphate from anaerobic digester centrate in the form of a nearly pure struvite.Applied Science, Faculty ofCivil Engineering, Department ofGraduat