Is K-Struvite Precipitation a Plausible Nutrient Recovery Method from Potassium-Containing Wastes?—A Review

The definition of technologies capable of removing and recovering nutrients from polluting effluents is a key environmental challenge. Through these technologies, it would be possible to protect aquatic systems and prevent the consumption of natural resources for the production of commercial fertilizers. In this regard, the application of the precipitation processes of struvite-type compounds is an attractive approach. Indeed, these processes are potentially able to remove nutrients from many effluents and produce a precipitate reusable as a slow-release fertilizer. The scientific community has largely focused on the precipitation of magnesium ammonium phosphate (MgNH₄PO₄·6H₂O, MAP), while the recovery of the analogous magnesium potassium phosphate (MgKPO₄·6H₂O, MPP) has received extensive attention in the last decade. Research on this topic is continuously progressing to improve the precipitation process in different aspects (working conditions, reaction units, interference elimination, etc.). Until now, there has been no paper that comprehensively reviewed the applicability of MPP precipitation for the removal and recovery of nutrients from aqueous waste. To fill this gap, the present paper aimed to provide an exhaustive analysis of the literature reports on MPP processes to help researchers understand the theoretical and applicative aspects, the main problems, and the need for further research. In this regard, the applications in the treatment of various aqueous wastes were considered. The theoretical concepts, the main process parameters, and the effects of inhibiting substances and impurities are presented. Moreover, the development of reactor configurations and their working conditions are evaluated. Finally, the potential use of MPP as a fertilizer and some economic evaluations are reported. On the basis of the conducted analysis, it emerged that the recovery of MPP was mainly affected by the pH, dose, and nature of reagents, as well as the presence of competitive ions. The optimal pH values were reported to be between 9 and 11. Reagent overdoses with respect to the theoretical values improved the process and the use of pure reagents guaranteed superior performance. The stirred-tank reactors and fluidized bed reactors were the most used units with high process yields. The applicability of MPP in agronomic practices appears to be a suitable option.