Phosphorus recovery as ferrous phosphate (vivianite) from wastewater produced in manufacture of thin film transistor-liquid crystal displays (TFT-LCD) by a fluidized bed crystallizer (FBC)

Abstract

In this investigation, fluidized bed crystallization (FBC) is utilized to treat phosphorus wastewater that is produced by the manufacture of thin film transistor-liquid crystal displays (TFT-LCD). TFT-LCD wastewater contains 500 ± 10 ppm phosphorus. The pH and molar ratio of Fe/P for removing phosphorus was initially examined by performing a jar-test. The parameters of the FBC – effluent pH_e, Fe/P ratio and the upflow velocity (m h⁻¹) – were tested to recover phosphorus from wastewater as ferrous phosphate pellets, characterized using an X-ray diffractometer (XRD) and scanning electron microscopy (SEM), and silica sand was used as the seed material. The experimental results revealed that the control of effluent pH_e was an essential parameter in maximizing the phosphorous removal (PR%) and crystallization ratio (CR%). At pH_e 5–6, the supersaturation of phosphate precipitation by conditioning the molar ratio of Fe/P to 1.5 and the upflow rate was adjusted within the range of 30.56–68.76 m h⁻¹ in the metastable zone at a cross-section loading of 0.72 kg per P per h per m², leading to a phosphorus removal (PR) of 95% and a crystallization ratio (CR) of 63%. Under optimal hydraulic conditions, the treatment of real wastewater in a FBC process was viable by converting the pollutant into crystals with a high-purity phase of vivianite (Fe₃(PO₄)₂·8H₂O).