Enhanced struvite (MgNH₄PO₄·6H₂O) granulation and separation from synthetic wastewater using fluidized-bed crystallization (FBC) technology

Struvite (MgNH₄PO₄·6H₂O) could be recovered from wastewater containing phosphorus and showed high potential as an alternative source of phosphorus fertilizer. A comparison of batch experiment and continuous crystallization processes was carried out using batch reactor and fluidized bed reactor (FBR) methods, respectively. The continuous crystallization processes for phosphorus granulation were conducted both in a seeded (FBC) and unseeded (FBHC) system. Various seed materials such as dolomite-(CaMg(CO₃)₂), CaHPO₄, and SiO₂ were applied as seed material in FBC. In the batch reactor, the operating parameters, including pH, [Mg]₀/[P]₀ molar ratio, and initial phosphorus concentration were first optimized. The effects of the coexisting ions (SO₄²⁻, Cl⁻, NO₃⁻, Na⁺, K⁺, Ca²⁺) and organic pollutants (Ethylenediaminetetraacetic acid-EDTA, and Citric acid) on phosphorus-containing wastewater treatment were also examined. The hydraulic parameters of FBR (Fluidized-bed reactor), including effluent pH_e, and cross-sectional surface loading (L, kg m⁻² h⁻¹) were investigated to recover phosphorus and ammonium as struvite pellets. Under optimum conditions (pH_e 8.8, [Mg]₀/[P]₀ = 1.3/1, L = 2.4 kg-P m⁻² h⁻¹), FBC using dolomite as seed resulted in higher phosphorus removal efficiency than the FBHC system with total removal (TR) = 93.0 % and a crystallization ratio (CR) = 85.0 %. As confirmed by the XRD pattern, the solid products consist of magnesium ammonium phosphate (MgNH₄PO₄·6H₂O) from both FBC and FBHC processes. The high crystallization ratio (CR = 85 %) and the recovery of crystal pellets (>1 mm) in the FBC process reduce the production of sludge compared to traditional chemical precipitation and FBHC processes.