Optimizing phosphorus precipitation from acidic sewage sludge ash leachate: Use of Mg-rich mining by-products for enhanced nutrient recovery

Phosphorus recovery from Sewage Sludge Ashes (SSA) by wet chemical extraction followed by selective precipitation has gained great attention in recent years, attempting to reduce the anthropic pressure on natural reserves. This study investigates the selective precipitation process at lab- and small pilot-scales by means of two conventional and one innovative precipitating agents, the latter derived from a low-grade magnesium oxide mining by-product (LG-MgO named PC8), assessing the role of the most relevant operating parameters. Lab-scale experiments were performed on leachates obtained from bottom and fly ashes, in which several operating conditions were tested, differing in the type of precipitating agent, target pH and nutrient molar ratio. Based on experimental results, small pilot-scale experiments were conducted with Ca(OH)₂ and PC8 at pH 7. Effective phosphorus precipitation was obtained at lab-scale at pH equal to 4 for high Al/P molar ratio, while SSA leachate with low Al/P molar ratio promoted improved phosphorus precipitation (>90%) only at pH higher than 8 with PC8. Small pilot-scale findings confirmed the effectiveness of PC8 in increasing simultaneously the pH and the nutrient content of the solid precipitate. The comprehensive assessment of the samples denoted compliance with the European Regulation (EU 2019/1009), which allows the formulation of different fertilizers with agronomic relevance. This is the first time that experiments from small pilot-scale tests in the field of phosphorus recovery from SSA were investigated using an innovative precipitant providing key information for the process scale-up.