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

BACKGROUND

Struvite (MgNH₄PO₄.6H₂O) 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 Mg²⁺ and phosphate (PO₄^3-) 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 (NH₄-N), PO₄-P and Mg²⁺recoveries.

RESULTS

NH₄-N, PO₄-P and Mg²⁺ 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% NH₄-N, 96.6 ± 0.31% PO₄-P and 84.4 ± 0.9% Mg²⁺ 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 NH₄-N and PO₄-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. © 2024 The Authors. Journal of Chemical Technology and Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry (SCI).