Recovery of ammonium ion as struvite from flue gas scrubbing wastewater

Direct disposal of flue gas scrubbing-derived waste water with a high level (9000–10,000 mg/L) of ammonium ion (NH₄ ⁺) into aquatic systems has contributed to environmental depreciation. Here, we report a feasibility study on NH₄ ⁺ recovery and conversion to struvite (NH₄MgPO₄·6H₂O), which is a slow-release fertilizer. Such conversion also aids in compliance with the discharge limits for nitrogen-based compounds. Lab-scale experiments were performed to determine the optimum pH and molar ratio (Mg²⁺:NH₄ ⁺:PO₄ ³⁻) for struvite formation. A chemical equilibrium model (Visual Minteq) was also employed to corroborate the experimental results. The optimum pH for struvite precipitation was found to be pH 9 with a molar ratio of Mg²⁺:NH₄ ⁺:PO₄³⁻ = 1:1:1. At this pH, more than 93, 92.3 and 100% of the NH₄ ⁺, Mg²⁺ and PO₄ ³⁻, respectively, were removed from the scrubbing waste water Visual Minteq simulation also demonstrated optimum struvite formation at pH 9–10. Both X-ray diffraction (XRD) and scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) analysis revealed that the synthesized struvite was comparable to that of a commercial struvite. Thus, our findings confirmed the possibility of synthesizing struvite from de-NOx scrubbing wastewater utilizing the residual ammonium ions (NH₄ ⁺).