DESIGN OF CONE-SHAPED FLUIDIZED BED STRUVITE CRYSTALLIZERS FOR PHOSPHORUS REMOVAL FROM WASTEWATER

Abstract

Precipitation of struvite in a cone-shaped fluidized bed crystallizer can efficiently remove excess phosphorus from wastewater. To enable design of such crystallizers, the mathematical relationships between the crystallizer performance and the design parameters are required. Such relationships, expressed in several design equations, were derived using material balances and a rate equation. The rate equation contains a constant whose value must be known in order to use the equation. A rate equation, including constant, has previously been reported for struvite precipitation. However, it is not suitable for deriving useful design equations because it treats neither the struvite constituent ion concentrations nor the reaction surface area separately, variables with which designers must deal explicitly. In the present work, the previously reported rate equation is revised into one that is more useful because it treats these variables specifically. The new rate equation is then used in deriving design equations for three alternative models, each based on different sets of assumptions about how the liquid and solid move in the crystallizer. The concentration profiles in a laboratory crystallizer were compared against profiles predicted by the design equations for each of the models at various values for the reaction constant. Predictions by one of the models, the PLMB (plug flow of liquid, mixed bed), fit the observed profiles with a rate constant of 10 to 15 dm h-1. It is recommended that design of similar crystallizers use the PLMB model and a rate constant in this range.