Effect of coagulation and applied breakage shear on the regrowth of kaolin flocs

A computer-controlled digital camera was used to examine the formation, breakage, and regrowth of flocs formed by kaolin or the mixture of kaolin and humic acid with aluminum sulfate (alum) at different intensities of applied shear, as well as for continuous optical monitoring (Photometric Dispersion Analyzer). The computer-controlled digital camera could record the flocs size and morphology. There was full regrowth of flocs in kaolin suspension when their zeta potential was close to zero, regardless of intensity of the applied shear, which indicated a significant reversibility of the floc break-up process. However, such a process displayed a distinct irreversibility at higher alum dosage when the flocs were positively charged as the intensity of applied shear increased, though there was full reversibility of floc breakage at low applied breakage shear. Addition of humic acid weakened the reversibility of broken flocs in all cases, especially at high breakage shear. Two-dimensional fractal dimension (D₂) of regrown flocs in kaolin suspension coagulated under condition of charge neutralization was not influenced by the applied shear except at high applied shear (400 rpm), whereas at a higher alum dosage the D₂ decreased as the applied shear increased. The D₂ was lower in the presence of humic acid. Further, the D₂ of regrown flocs with charge neutralization was higher than that with higher alum dosage, whether the model water contained humic acid or not. Without humic acid in water, the residual turbidity and particle number after breakage and regrowth were both decreased as the applied shear increased up to 250 rpm. Higher breakage shear, such as 400 rpm, gives higher residual turbidity (or residual particle number). This work gives new information and a better understanding of the regrowth of broken flocs, a subject which is of considerable practical importance in water and wastewater treatment.