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