Regulation of suspended solid properties for enhanced struvite-based nutrient recovery from food waste digestate: crystallization pathway control and nucleation mechanism.

Struvite crystallization represents a promising strategy for nutrient recovery from food waste digestate (FWD), yet the influence of suspended solids (SS), which are abundant in real digestate matrices, on crystallization performance remains insufficiently understood. This study elucidates how SS properties regulate struvite formation by integrating fractional centrifugation, in-situ crystallization monitoring, and multi-scale characterization. Size-fractionated SS exhibited divergent mechanistic roles. Large SS particles (Dv (50) = 48.8 μm) acted primarily as impurity carriers, concentrating more than 70% of metal species and electronegative organic matter, thereby promoting impurity co-precipitation, surface contamination and reduced product purity. In contrast, smaller SS particles served as heterogeneous nucleation interfaces with lower impurity burdens, facilitating crystal growth while preserving higher purity. Progressive SS removal decreased NH3-N removal from 97.1% to 92.2%, but increased NH3-N conversion from 91.3% to 96.6% and struvite purity from 75.5% to 89.8%. In-situ monitoring further revealed a transition among SS-mediated agglomeration, heterogeneous nucleation, and explosive homogeneous nucleation. Excessive SS removal shifted crystallization towards rapid supersaturation consumption and the formation of finer crystals, whereas targeted removal of large SS particles improved purity while maintaining favorable crystal dimensions. These findings establish SS as a mechanistic regulator rather than a passive interference factor in digestate crystallization, providing a conceptual basis for particle-size-selective pretreatment and advancing the mechanistic understanding of nutrient recovery from complex organic waste streams.