Removal of fluoride ion from fertilizer industry wastewater to recover ammonium and phosphate using a five-compartment electrodialysis system

Wastewater from the fertilizer industry contains high concentrations of ammonium and phosphate ions, up to 3,395 mg.L^-1 and 12,725 mg.L^-1, respectively. These ions can be recovered as struvite precipitates. However, 1,158 mg.L^-1 of fluoride ions contained in the wastewater may influence the struvite formation. Therefore, experiments using a laboratory-scale of five-compartment electrodialysis batch reactor were performed to remove fluoride ion and subsequently recover ammonium and phosphate ions. Electrical currents of 0.05, 0.1, and 0.15 A were applied constantly to the electrodes. The initial pH of wastewater was adjusted to 5, 7, and 9. The highest removal of fluoride ion was obtained at 0.1 A, i.e. decreased to 376 mg.L^-1 and 486 mg.L^-1 at pH 5 and pH 7, respectively. At the same conditions, the final ammonium concentrations at pH 5 and 7 were 2,127 mg and 2,185 mg, respectively. Similarly, 7,878 mg and 10,669 mg phosphate ions were recovered at pH 5 and pH 7, respectively. The energy consumed for fluoride removal at pH 5 was higher than pH 7, i.e. 4 Wh.g^-1 F^- vs. 7 Wh.g^-1 F^-. Nevertheless, the suspended solid formation at pH 5 was higher than pH 7, suggesting that pH operational at 7 is preferable for application with less possibility of scaling formation.