Characteristic analysis on temporal evolution of floc size and structure in low-shear flow

A series of flocculation tests were performed to investigate the effect of low-shear rates (G = 3–16 s−1) on flocculation of kaolin suspension by polyaluminum chloride (PACl), with the goal of understanding floc growth mechanisms. Results were reported in terms of floc average size (dp) and boundary fractal dimension (Dpf), derived from a non-intrusive optical sampling and digital image analysis technique. As expected, the rate of floc aggregation increased with increasing G, resulting in faster changes in aggregate size and structure in the initial stage of flocculation. Nevertheless, steady state was attained faster for Dpf than for dp at the same shear rates, possibly due to the self-similarity of fractal aggregates. An interesting finding was that at G = 3 s−1, an obvious plateau was observed for the average-size evolution at steady state; for shear rates of 6 and 7 s−1, the flocs exhibited some decrease after reaching the peak of size, mainly as a result of floc settling at steady state; and for G = 11–16 s−1, a decrease in floc size was possibly attributed to the irreversibility of PACl-floc breakage. The process of floc growth was described using a fractal growth model, which defined flocculation as the result of the combined processes of aggregation and restructuring. The conceptual model could effectively characterize temporal changes in floc size and structure, and found that fragmentation followed by reformation seemed to be more effective in forming larger and more compact aggregates than the restructuring process due to erosion and reformation, which may provide useful insights for the design of flocculation reactors.

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► We investigate the effect of low-shear rates on floc size and structure.
► The higher the shear the greater changes in size and structure initially.
► Steady state is attained faster for floc structure than for its size.
► We use a fractal growth model to describe the process of floc growth.
► Restructuring due to fragmentation and reformation may be more effective.