A novel approach describing struvite crystal aggregation and granulation in the fluidized bed for phosphorus recovery from swine wastewater

Fluidized bed is commonly used for struvite (MgNH4PO4·6H2O) recovery from wastewater. However, effective methods are lacking to describe the properties of the millimeter-scale struvite pellets in the fluidized bed, with the consequence of not predicting and optimizing the granulation process effectively. In this work, swine wastewater was used as the medium for struvite recovery by using fluidized bed reactor. A method combining with image processing and composition analysis was adopted to describe the aggregation and granulation stages of struvite crystal evolution. Based on this method, the properties of struvite granules, including granule type, particle size distribution, number density, mass concentration and crushing strength, were evaluated. Accordingly, the growth rates of the granules, including population growth, mass increase and particle size enlargement, were also determined. It was observed that struvite particles in the fluidized bed evolved into four stages, including aggregation, aggregate compaction, clusteragglomerating and coating growth. Up-flow rates of 30-80 mm/s in the fluidized bed sustained 600-876 g/L granular solids. Different from the conventional gas-solid granulation models, these stages could occur simultaneously or subsequently. Results revealed that the coating-growth granules were formed with compact aggregates or cluster-agglomerating granules as the nuclei. A novel model for struvite granulation was developed.