Modeling the Plantwide Implications of Struvite Loss from Sidestream Precipitation Reactors

The combination of enhanced biological phosphorus removal (EBPR) with sidestream struvite precipitation is a synergistic treatment approach for P removal and recovery. However, periodic disruption events in crystallization reactors can cause fine struvite particle washout followed by P solubilization, leading to decreases in EBPR effluent quality, yet modeling tools do not exist to quantify plantwide impacts of struvite loss. To understand the impacts of struvite loss and dissolution on EBPR, this work first quantifies the dissolution rate of field-harvested struvite using the surface area-dependent shrinking object model and presents a novel particle population balance dissolution modeling tool for integration with a plantwide process simulator (SUMO). Analysis of time series P concentration data from dissolution experiments yields a rate constant of 1.14 mm min-1. Simulations of intra-plant phosphorus dynamics indicate that when struvite reactors operate with a high capture efficiency, the majority of influent orthophosphate can be captured through sidestream precipitation. In the event of struvite loss, large particles would not fully dissolve and are removed through sedimentation during primary clarification without significant disruption of EBPR. In contrast, the loss of 200 μm-sized struvite particles can lead to a difference in effluent phosphate of up to 0.8 mg P/L. Our results suggest that surface area-dependent models are essential to quantify the impacts of struvite loss and that reactor design should center on particle retention, rather than conversion of soluble P to struvite.