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
may be due to the remaining organic matter. P-25 showed the mixture of both anatase and rutile TiO2. The TiO2 with anatase structure is generally accepted as the most active and efficient photocatalyst compared to rutile and brookite [45]. Therefore, the TiO2 obtained at 600 ℃ is expected to be an efficient photocatalyst, which coincides with Lee et al. [4] and Okour et al. [46]. Here, the 600 ℃ was selected as the most efficient temperature taking into account the energy require to recover the photocatalyst from the sludge. 3.5.2 Thermal analysis (TG/DSC) The thermogram recorded for the settled after TiCl4 coagulation is shown in Fig. 5b. A continuous weight loss distributed between 25-557.39 ℃ in the TG plot can be seen. The thermal treatment of sludge up to 153.77 ℃ led mainly to the loss of adsorbed water content (16.65%) while it resulted in the removal of organic matter (12.67%) between 153.77-493.06 ℃. Another weight loss of 0.9919% at 493.06-557.39 ℃ may be caused by the release of constitution water. The DSC curve of the settled flocs was comparatively smooth and there were no obvious peaks except at 542.72 ℃ and 806.99 ℃. The transformed temperature of the anatase phase to rutile was approximately 550 ℃ according to Liao et al. [47]. However, this transformation occurred at approximately 800 ℃ according to results of XRD analysis (Fig. 5a), corresponding well with the endothermic peak at 806.99 ℃. The peak at 542.72 ℃ was speculated to be the indication of transformation from amorphous structure to apparent anatase phase of TiO2. This is in agreement with the obvious anatase TiO2 at 600 ℃ shown in Fig. 5a. 3.5.3 SEM analysis As mentioned in section 3.5.1, the 600 ℃ was selected as the optimum temperature for
Table 1 Selected floc parameters under optimum coagulant dose and pH conditions Coagulant Floc growth rate (µm/min) d1 (µm) d2 (µm) d3 (µm) Sf (%) Rf (%) Df Before breakage breakage After regrowth TiCl4 429.8 859.7 320.6 334.0 37.3 2.5 2.44 2.52 2.52 120.8 422.9 187.5 196.3 44.3 3.8 2.49 2.55 2.56 Al2(SO4)3 65.1 130.2 103.9 108.3 79.8 16.6 2.23 2.25 2.25