Factors Affecting the Crystal Size of Struvite‑K Formed in Synthetic Urine Using a Stirred Reactor

The crystallization of struvite-K (MgKPO4·6H2O) can simultaneously recover phosphate and potassium from source-separated urine. However, its crystal size has been rarely reported despite significantly affecting the crystallizer performance. Experimental results show that the average size of struvite-K was 15–63 μm in ammonium-depleted synthetic urine under the examined conditions with a stirred reactor. A low supersaturation ratio (Sr) or the same Sr induced by low pH and high Mg:P ratio resulted in large crystal size. Large crystals also formed at high agitation speed, small feed volume, or feeding close to the stirring impeller. The size distribution of struvite-K crystals was predicted using the coupling resolution of the population balance equation to on-site turbidity measurement. The estimated kinetic parameters quantitatively described the nucleation and crystal growth of struvite-K under different conditions of supersaturation, feeding, and mixing. This work proposes approaches to obtain large crystals while reducing fines of struvite-K