The Crystallization of Struvite and its Analog (K-Struvite) from Waste Streams for Nutrient Recycling

The precipitation of struvite (MgNH4PO4·6H2O) and its analogue, K-struvite (MgKPO4·6H2O), from waste streams is widely recognized as a promising strategy for nutrient recovery owing to their elemental compositions and fertilizing properties. This chapter is intended to provide a critical overview of the essential issues in the crystallization process of struvite-type compounds and to outline the current challenges and further prospects for the broad application of this technique. Based on the study of previous experimental practices on different scales, the key factors that affect the process of struvite (and K-struvite) crystallization are described and summarized. In particular, the synthesis of K-struvite via diverse routes and sources is elucidated. In addition, quantitative X-ray diffraction technology is confirmed to be a reliable and convenient method with which to quantify the content of struvite in precipitates obtained in various conditions. The fertilizer quality of struvite-type products is also discussed by examining the problem of heavy metal contamination during crystal formation. Finally, the limitations and barriers that hinder the further development of struvite-based materials are briefly noted, and potential improvements are indicated to establish a more reliable and efficient nutrient-recovery technology for recycling phosphorus, nitrogen, and potassium from waste streams