Potential of Struvite Recovery from Compost Leachate Using Multistage Processes

Leachates produced from composting activities often contain high levels of ammonium and phosphate. If left untreated, these compounds can pollute water bodies, but at the same time, they also represent nutrients that could be recovered and reused. This study explored the possibility of transforming composting leachate into struvite, a slow-release fertilizer, by applying sequential recovery processes. Initial characterization of the leachate showed high concentrations of phosphate (1179.8 mg.L-1) and ammonium (312.5 mg.L-1), while magnesium was almost absent (0.017 mg.L-1), confirming the need for external magnesium addition. A series of experiments and PHREEQC simulations were carried out to examine the influence of magnesium sources (MgCl₂ and MgO), pH adjustment, molar ratios, and mixing conditions. The results showed that MgCl2 consistently enhanced nutrient removal compared with MgO, with the best outcomes occurring at around pH 9 for ammonium and pH 10 for phosphate. Increasing the magnesium dosing ratio to 5:1:1 further improved precipitation, while controlled mixing (fast followed by slow) supported crystal formation. The presence of struvite crystals confirmed the effectiveness of the process. These findings suggest that composting leachate, which is often considered problematic waste, can be converted into a valuable product while reducing environmental risks.