Coagulation characteristics of titanium (Ti) salt coagulant compared with aluminum (Al) and iron (Fe) salts

In this study, the performance of titanium tetrachloride (TiCl4) coagulation and flocculation is compared with commonly used coagulants such as aluminum sulfate (Al-2(SO4)(3)), polyaluminum chloride (PACl), iron chloride (FeCl3), and polyferric sulfate (PFS) in terms of water quality parameters and floc properties. TiCl4 flocculation achieved higher removal of UV254 (98%), dissolved organic carbon (DOC) (84%) and turbidity (93%) than other conventional coagulants. Charge neutralization and physical entrapment of colloids within coagulant precipitates and adsorption, seemed to play a significant role during TiCl4 flocculation, while the main mechanism for conventional coagulants was bridge-aggregation and adsorption. The aggregated flocs after TiCl4 flocculation showed the fastest growth rate compared to the other coagulants, with the largest floc size (801 mu m) occurring within 8 min. The floc strength factor of PACl, Al-2(SO4)(3), PFS, FeCl3 and TiCl4 was 34, 30, 29, 26 and 29, respectively, while the floc recovery factor of the TiCl4 coagulant was the lowest. Based on the results of the above study, it is concluded that the TiCl4 flocculation can reduce the hydraulic retention time of slow and rapid mixing, however, careful handling of sludge is required due to the low recoverability of the aggregated floc. (C) 2010 Elsevier B.V. All rights reserved.