Al(OH)2+, Al(OH)2+, Al2(OH)24+, and Al3(OH)45+,etc, easily neutralize the exterior negative charges of particles and organic matters, and further destabilized the colloids. However, the positive charges were not enough for full charge neutralization, yielding the flocs with negative charges. When the initial pH is between 6 and 9, the polymeric hydrolyzates with high positive charges and large surface area are formed [38, 39]. The colloids are easily adsorbed, neutralized and co-precipitated by the hydrolyzates, producing the flocs with positive charges. For all the three coagulants, the slight decrease in UV254 and/or DOC removal at high pH values could be attributed to the mutual repulsion between particles with same charges. 3.3 Floc characterization To understand the relative merits of different coagulants, the properties of the flocs formed by TiCl4, FeCl3 and Al2(SO4)3 were investigated under optimum coagulant dose and solution pH conditions. Fig. 3 presents the dynamic variation of floc size with time during coagulation process. The floc size quickly increased with time, reaching a steady stage during floc growth period, which was immediately reduced following an increase in shear. The flocs were broken to different degrees, and then began to regrow as the high shear was reduced again. The floc characteristics were then investigated in detail in terms of growth rate, size (d1, d2, and d3), strength, recoverability and fractal dimension. 1 summarizes the selected floc parameters under optimum coagulant dose and solution pH conditions. Compared to FeCl3 and Al2(SO4)3, TiCl4 coagulant exhibited the highest floc growth rate at around 429.8 µm/min, which was approximately 3.5 times that by FeCl3 and 6.5 times that by Al2(SO4)3. The resultant flocs were with the largest size irrespective of the growth, breakage and regrowth process. Fig. 3 shows a sharp increase in size of the flocs by TiCl4 coagulation during the first 2.5 min, achieving the largest floc size of around 950
Al(OH)2+, Al(OH)2+, Al2(OH)24+, and Al3(OH)45+,etc, easily neutralize the exterior negative charges of particles and organic matters, and further destabilized the colloids. However, the positive charges were not enough for full charge neutralization, yielding the flocs with negative charges. When the initial pH is between 6 and 9, the polymeric hydrolyzates with high positive charges and large surface area are formed [38, 39]. The colloids are easily adsorbed, neutralized and co-precipitated by the hydrolyzates, producing the flocs with positive charges. For all the three coagulants, the slight decrease in UV254 and/or DOC removal at high pH values could be attributed to the mutual repulsion between particles with same charges. 3.3 Floc characterization To understand the relative merits of different coagulants, the properties of the flocs formed by TiCl4, FeCl3 and Al2(SO4)3 were investigated under optimum coagulant dose and solution pH conditions. Fig. 3 presents the dynamic variation of floc size with time during coagulation process. The floc size quickly increased with time, reaching a steady stage during floc growth period, which was immediately reduced following an increase in shear. The flocs were broken to different degrees, and then began to regrow as the high shear was reduced again. The floc characteristics were then investigated in detail in terms of growth rate, size (d1, d2, and d3), strength, recoverability and fractal dimension. 1 summarizes the selected floc parameters under optimum coagulant dose and solution pH conditions. Compared to FeCl3 and Al2(SO4)3, TiCl4 coagulant exhibited the highest floc growth rate at around 429.8 µm/min, which was approximately 3.5 times that by FeCl3 and 6.5 times that by Al2(SO4)3. The resultant flocs were with the largest size irrespective of the growth, breakage and regrowth process. Fig. 3 shows a sharp increase in size of the flocs by TiCl4 coagulation during the first 2.5 min, achieving the largest floc size of around 950
Coagulation%20and%20sludge%20recovery%20using%20titanium%20tetrachloride%20as%20coagulant%20for%20real.docx