Towards a methodology for recovering K-struvite from manure

BACKGROUND: This study provides a methodology for recovering two essential macronutrients (potassium and phosphorus) from the liquid fraction of manure. This methodology paves the way for K-struvite recovery from manure by determining the suitable operational conditions (pH and temperature). RESULTS: The theoretical operational conditions were established by modelling and experimentally adjusted with manure, achieving the highest recoveries (80% of P-recovery) at pH 10 and 38 °C when using a 10% manure solution. Then, the suitable operational conditions were evaluated using 0–10–50–100% manure solutions. The lowest percentages of manure (0–10%) favoured the co-precipitation of other products besides K-struvite (magnesium phosphate and magnesium hydroxide); and at the highest percentages of manure (50–100%), all the phosphate was recovered as K-struvite, but also the co-precipitation of magnesium hydroxide due to the Mg/P molar ratio applied (Mg/P = 2). Suspended particles acted as nuclei for heterogeneous nucleation and favoured the aggregation of crystals resulting in a star-asterisk form. CONCLUSION: The viability of the process was demonstrated, this study being the first proof-of-concept of K-struvite recovery from complex waste streams.