Nucleation and growth kinetics of struvite crystallization

• Struvite growth rate on synthetic wastewater and digestate was comparable.
• Crystal polymorph of wastewater derived struvite was similar to synthetic.
• Product showed narrow size distribution and favorable separation characteristics.
• Primary induction time reduced significantly above a supersaturation ratio of 1.7.
• Secondary nucleation showed 2nd order dependence on supersaturation.

Struvite crystallization technology is being widely applied in full-scale due to a surge in nutrient demand and phosphate price increases. Past investigations on struvite crystallization focused on process efficiency and thermodynamics, and less on kinetics, while mainly using fluidized bed type crystallizer. In this work, nucleation and growth kinetic data were measured using stirred vessel. The primary and secondary nucleation was measured in synthetic wastewater, and crystal growth in digested supernatant. The measured kinetic data was correlated with solution supersaturation. The dependence of growth rate on relative supersaturation in the digested was higher compared to synthetic wastewater. The crystal polymorph in synthetic wastewater and real wastewater was comparable. Products from the growth studies showed narrow size distribution and favorable separation characteristics. The secondary nucleation rate showed second order dependence on relative supersaturation. The nucleation induction time decreased with an increase in supersaturation ratio with a clear distinction between homogenous and heterogenous primary nucleation.

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