Removal of phosphate and fluoride from wastewater by a hybrid precipitation–microfiltration process

The removal of phosphate and fluoride from thin-film transistor liquid crystal display (TFT-LCD) wastewater by a hybrid precipitation–microfiltration (MF) process was studied. Calcium salt was used to form precipitates, followed by crossflow MF for solid–liquid separation. The results showed that excess calcium could induce effective removal of phosphate and fluoride at pH 8.5 and 10.5. The dominant solids were hydroxyapatite (Ca5(PO4)3OH, HAP), amorphous calcium phosphate (ACP), fluorapatite (Ca5(PO4)3F, FAP), and calcium fluoride (CaF2). Precipitation conditions affected the MF, and more significant fouling was found at pH 8.5 than pH 10.5. Permeate analysis showed increased removal of phosphate and fluoride in MF, and effective removal of turbidity. The initial flux and steady state flux increased with increasing filtration pressure and crossflow velocity. The main fouling resistance was cake resistance. This study demonstrated that the hybrid precipitation–MF process could effectively remove phosphate and fluoride from wastewater, and produce filtrate and precipitates for potential recovery and reuse.

. Wet chemical analysis of solid as affected by molar ratio and pH (normalized ion ratio in precipitates [PO4] = 1).Figure optionsDownload as PowerPoint slideResearch highlights▶ A hybrid process was efficient to remove phosphate and fluoride from wastewater. ▶ Optimum pH and molar ratio, ([Ca+2]: [PO43−]: [F−]), were determined. ▶ Both precipitates and filtrate could potentially be reused.