Effects of low temperature on floc fractal dimensions and shape factors during alum coagulation

Over 2,000 flocs were involved in the calculations of floc fractal dimensions and floc shape factors to examine the effects of low temperature on floc structures. The 3-D fractal dimension of flocs was found to decrease from 2.48 at 22°C to 2.24 at 2°C, indicating flocs are more compact at the higher temperature. At 22°C, floc fractal dimensions were not significantly affected by flocculation time. However, at the low temperature, 2°C, fractal dimensions were found to increase with flocculation time, indicating that flocs generated at low temperature need more time to be well formed. On the other hand, low temperature did not exert significant influence on floc shape factors. Statistical analyses show that the examined shape factors of the flocs, roundness R and smoothness S, followed probability rules. Around 80% of flocs have an R value ranging from 0.5 to 0.8, and about 75% of flocs have an S value in the same range. The mechanism of floc growth during coagulation of kaolinite suspensions is reaction-limited cluster-cluster aggregation (RLCA).