UBC Theses and Dissertations
Phosphorus recovery from wastewater through struvite crystallization in a fluidized bed reactor : kinetics, hydrodynamics and performance Rahaman, Md. Saifur
Struvite crystallization from wastewater, using a novel fluidized bed reactor developed at UBC, offers a significant reduction (80—90%) of soluble phosphate from waste streams and generates a product that can be reused as a slow release fertilizer. To implement this green technology at a plant scale, a reactor model that incorporates process kinetics, thermodynamics and the system hydrodynamics, is required. Therefore, the main objectives of this dissertation were to study the struvite precipitation kinetics, thermodynamics, and fluidization characteristics of the struvite crystals bed, and finally, develop a model based on this information. Both dissolution and precipitation experiments were carried out in a jar test apparatus to study the solubility and precipitation kinetics of struvite. The struvite solubility product, pKsp values were found to vary from 13.43—14.10, for different water and wastewater samples tested at 20°C. Also, a correlation was developed to estimate struvite solubility at different temperatures. In struvite precipitation experiments, the operating conditions of supersaturation, pH, Mg:P ratio, mixing and seeding conditions were varied to identify the effect of those process parameters on the precipitation kinetics. The kinetic rate constant increases with increasing both the supersaturation and Mg:P ratios. Both the mixing energy and seeding rate were found to have minimal effect on ortho-P removal. Detailed experimental and numerical investigations of the fluidization characteristics of struvite crystals were performed. The bed expansion behaviour of mono-sized struvite crystals can be represented reasonably well by the Richardson-Zaki relation and the expansion characteristics of poly-dispersed struvite crystals bed can be predicted by the ‘serial model’. The CFD simulated bed expansion behaviour of the crystals bed was found to be consistent with the experimental results. Also, CFD simulations were capable of capturing the mixing/segregation behavior of a fluidized-bed of multi-particle stnivite crystals. Finally, a mathematical model was developed by assuming a complete segregation of the bed crystals and liquid movement as plug flow in the reactor. The model predictions provided a reasonably good fit with the experimental results for both P0₄-P and NH₄-N removal. The model predicted mean size of product crystals matched reasonably well with pilot scale experimental results.
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