The effect of second coagulant dose on the regrowth of flocs formed by charge neutralization and sweep coagulation using titanium tetrachloride (TiCl4).

Characteristics of flocs formed by charge neutralization and sweep coagulation using titanium tetrachloride (TiCl(4)) were investigated with humic acid-kaolin suspension by continuous optical monitoring. This paper focused on the regrowth ability of broken flocs after addition of second TiCl(4) dose. Variation of floc size and the fractal dimension of flocs versus second TiCl(4) dose after regrowth were investigated. Second TiCl(4) dose was added during the floc breakage period, and addition time of second TiCl(4) dose was also investigated. The results showed that, when coagulated by charge neutralization at pH 6, an appropriate second TiCl(4) dose improved regrowth ability of broken flocs at low initial TiCl(4) doses. While for high initial TiCl(4) doses, second TiCl(4) dose lowered floc re-growth ability. When coagulated by sweep coagulation at pH 10, second TiCl(4) dose made regrown flocs larger than those without second TiCl(4) dose. Floc structure analysis showed that it was determined by not only the fractal dimension of flocs, but also the chemical characteristics of floc surface. Addition time of second TiCl(4) dose had a great effect on floc regrowth ability, suggesting that the broken flocs had better regrowth when second TiCl(4) dose was added at the end of the breakage period.