Kinetics Effects on Preferential Struvite Accumulation in Wastewater

Abstract

The kinetics of struvite (MgNH4PO4⋅6H2O) formation was studied to determine the cause of preferential accumulation at certain locations within the anaerobic digestion and postdigestion processes in wastewater treatment. Mechanisms controlling nucleation were also studied in laboratory induction time experiments. A field study of struvite accumulation on postdigestion process appurtenances was conducted to evaluate mechanisms affecting crystal growth rates. Struvite nucleation was found to be reaction-controlled, with nucleation strongly dependent on the struvite supersaturation level. Increased mixing intensity at constant supersaturation had a minor effect on induction times. The crystal growth rate was found to be transport-limited, with the growth rate a function of mixing energy input. Evaluation of the effect of pH change resulting from CO2 evolution in high turbulence areas showed that it has a minor influence on the crystal growth rate. Preferential struvite accumulation was found to be primarily controlled by growth kinetics, which was strongly dependent on mixing energy. Preferential accumulation of struvite on selected construction materials was also studied, and experimental evidence suggested that surface roughness exerted the greatest control over preferential struvite accumulation on various materials under similar mixing and supersaturation conditions.