Coagulation and sludge recovery using titanium tetrachloride as coagulant for real water treatment: A comparison against traditional aluminum and iron salts

• TiCl4 was an effective coagulant for real water treatment.
• TiCl4 flocs exhibited higher growth rate and larger floc size than Al and Fe salts.
• TiCl4 and FeCl3 flocs showed comparable compactness, higher than those by Al2(SO4)3.
• Using TiCl4 as pre-coagulant resulted in the most severe membrane fouling.
• The TiCl4 coagulated sludge can be recycled to produce anatase TiO2.

Coagulation/flocculation performance of titanium tetrachloride (TiCl4), ferric chloride (FeCl3) and aluminum sulfate (Al2(SO4)3) was comparatively investigated for real water treatment. Comparisons were made under different coagulant dose and initial solution pH conditions and their performances measured in terms of UV254 (absorbance at 254 nm) and DOC (dissolved organic carbon) removal and residual turbidity. Characteristics of aggregated flocs during the coagulation/flocculation process by the three coagulants were studied using a laser diffraction particle sizing device. The performances of the three coagulants were also assessed in terms of the membrane fouling potential of the ultrafiltration (UF) membrane or during coagulation–ultrafiltration (C–UF) process using a stirred and dead-end batch UF unit. Additionally, the TiCl4 flocculated sludge was recovered to produce titanium dioxide (TiO2) under thermal treatment. The results indicate that the TiCl4 showed superior coagulation performance compared to that of FeCl3 and Al2(SO4)3, with the optimum removal of UV254 and DOC of 54.9% and 55.1%, respectively. The aggregated flocs formed by TiCl4 showed the highest growth rate with the largest size compared to those by FeCl3 and Al2(SO4)3, but with the weakest floc strength and the worst re-growth ability. The TiCl4 and FeCl3 yielded the flocs with comparable degree of compaction, higher than that by Al2(SO4)3. Additionally, the investigation of membrane fouling demonstrated that the severity of flux decline followed the order of Al2(SO4)3 < FeCl3 < TiCl4. TiCl4 coagulated sludge was also characterized by X-ray diffraction, Thermal analysis and scanning electron microscope.

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