Critical conditions of struvite growth and recovery using MgO in pilot scale crystallization plant

The struvite crystallization process can recover struvite crystals as a valuable slow-release fertilizer from the side stream of wastewater treatment plants (WWTPs). The purpose of this study is to demonstrate the crystal growth characteristics and determine the appropriate recovery criteria for a struvite crystallization pilot plant. A pilot plant (8.6 m³/d) was designed with a feeding system of MgO (magnesium oxide), a pH controller, and a hydrocyclone for recovering struvite; the plant was operated for 42 hours at a pH range of 8.25-8.5. The removal efficiencies for PO₄-P and NH₄-N were 82.5-90.7% and 13.4-22.9%, respectively. The struvite recovered from the hydrocyclone was sifted using standard sieves and analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The dry weight fraction of the precipitate in the 300-600 μm range increased gradually from 7% to 74% in 18 hours. The XRD analysis revealed that the crystalline structure of the precipitate in the 150-600 μm range indicates struvite without any peaks of MgO, Mg(OH)₂, and MgCO₃. This indicates that the critical conditions for recovering struvite from the side-stream of WWTPs are an operation period of 18 hours and a crystal size greater than 300 μm.