Phosphate Uptake in Co-precipitation Systems Targeting Low Phosphate Concentrations

Achieving very low residual phosphorus (P) concentrations in wastewater treatment plant effluents is becoming increasingly important. The use of chemical P removal with iron(III) salts to achieve these low concentrations has been shown to be characterized by rapid equilibrium precipitation of hydrous ferric oxides occurring simultaneously with co-precipitation of phosphate, followed by a slower kinetic removal as a result of chemisorption. The effects of pH, dosing, mixing and aging conditions, as well as water chemistry, dictates the extent to which these mechanisms will perform. Model development has increased the ability to predict residual phosphate (PO43−) concentrations; however, existing models do not include all of the mechanisms responsible for P removal to ultra low levels. Currently, there is a lack of information on P removal mechanisms in transient processes and dynamics typical of wastewater treatment. In particular, there is limited information that describes the effect of solids aging and the rates of adsorption and desorption of soluble P. This paper presents preliminary data of a multiphase project aimed at resolving issues related to PO43−uptake in co-precipitation systems when extremely low PO43−concentrations (i.e.