Nutrient recovery from manure digestate by using waste magnesite powder and bone meal as sustainable substitutes for struvite precipitation

BACKGROUND Struvite (MgNH 4 PO 4 .6H 2 O) precipitation is a sustainable approach that can offer simultaneous removal/recovery of nutrients from biogas plant effluents. However, most biogas plant effluents contain a higher molar concentration of nitrogen (N) than magnesium (Mg) and phosphorus (P). Therefore, the external addition of Mg 2+ and phosphate (PO 4 3‐ ) salts is needed to maximize the recovery. In this study, Mg‐rich waste magnesite powder and P‐rich bone meal were used as sustainable additives. The Box–Behnken design was applied to determine optimum conditions of process parameters (pH, Mg:N and P:N molar ratios) to maximize ammonium‐nitrogen (NH 4 ‐N), PO 4 ‐P and Mg 2+ recoveries. RESULTS NH 4 ‐N, PO 4 ‐P and Mg 2+ recoveries were in the ranges 78.5%–96.9%, 69.2%–96.3% and 75.1%–99.9%, respectively. The actual and predicted values showed significant consistency, indicating that the model is satisfactory. Under optimum conditions (pH = 9.0, Mg:N = 2.2, P:N = 1.8), 97.8 ± 0.1% NH 4 ‐N, 96.6 ± 0.31% PO 4 ‐P and 84.4 ± 0.9% Mg 2+ recovery were attained. The X‐ray diffraction results confirmed the sole presence of struvite crystals. Scanning electron microscopy images showed irregular prismatic orthorhombic crystals and amorphous material depositions on precipitates of ~25 μm crystal size. The product was 49% struvite with 15.2% P content. The heavy metal content was lower than regulatory limits. CONCLUSION Even though waste material and industrial by‐products have been used as additives in this process, high NH 4 ‐N and PO 4 ‐P recoveries were recorded under optimum conditions. These results are promising and illustrate a powerful example of industrial symbiosis, and the precipitate containing struvite can serve as a valuable product.