Coupled precipitation-ultrafiltration for treatment of high fluoride-content wastewater

• Precipitation-UF effectively treats high fluoride content wastewater.
• Flux increased with increasing MWCO of membrane and crossflow velocity.
• Pre-flocculation by PAA improved filtration.
• Cake formation was the main fouling mechanism.

High fluoride-content wastewater is commonly found among various industries, such as fertilizer, aluminum, and semiconductor. Its treatment was studied by using calcium precipitation followed by ultrafiltration (UF). Experimental results indicated that fluoride could be reduced from 305 mg/L to 15 mg/L by the addition of calcium chloride (CaCl2) at molar ratio of calcium to fluoride, [Ca2+]/[F−], of 0.7. Filtrate of very low turbidity was found and fluoride was further reduced to <10 mg/L. Initial flux and steady flux increased with increasing molecular weight cut-off (MWCO) of membrane, transmembrane pressure (TMP), and crossflow velocity. Filtration was improved by pre-flocculation by polyacrylic acid (PAA). It was found that PAA with higher molecular weight (450 K) was more effective than PAA with lower molecular weight (90 K). Cake formation was the main fouling mechanism. The study demonstrated that coupled precipitation-UF is effective for treatment of high fluoride-content wastewater.

Both fouling resistance (Rb) and cake resistance (Rc) were significant firstly. However, cake resistance became the predominant resistance as filtration continued.Figure optionsDownload as PowerPoint slide