Evaluation of the performance of titanium and zirconium salts as coagulants in industrial wastewater treatment: pollutant removal, sludge production, and sludge characteristics

In this study, titanium tetrachloride (TiCl4), zirconium tetrachloride (ZrCl4), and zirconium oxychloride (ZrOCl2 center dot 8H(2)O) were evaluated using jar test experiments as coagulants and compared with traditional aluminum sulfate (Al-2(SO4)(3)center dot 18H(2)O) and ferric chloride (FeCl3) for industrial wastewater treatment. The effects of the initial pH of 4-10 and initial coagulant doses of 10-100 mg/L on chemical oxygen demand (COD) and total suspended solids (TSS) removal were investigated. The performances of the five coagulants were also assessed in terms of the settled sludge volume, the sludge volume index (SVI), and removal efficiencies of metals, color, and total phosphorus (TP) under optimum conditions. In addition, the contents of the residual sludge produced for all five tested coagulants under optimum conditions were determined. The results showed that the maximum removal efficiency of COD (69.33%) was achieved using 100 mg/L TiCl4 at pH 8. The maximum removal efficiency of TSS (98.32%) was achieved using 50 mg/L Al-2(SO4)(3)center dot 18H(2)O at both pH 8 and 10. The settled sludge volume and SVI generated by TiCl4 were lower than that for the other four tested coagulants. ZrCl4, ZrOCl2 center dot 8H(2)O, FeCl3, and Al-2(SO4)(3)center dot 18H(2)O resulted in 128.13, 92.39, 72.26, and 69.66 mL/g SVI, while that using TiCl4 was 48.84 mL/g. Ti- and Zr-based coagulants achieved better removal efficiencies of TP, Zn, and Cu than FeCl3 and Al(SO4)(3)center dot 18H(2)O. The residual sludge from using Ti and Zr coagulants had a very high TP content. The results indicated that Ti- and Zr-based coagulants could be used as alternatives to traditional coagulants for industrial wastewater treatment.