Breakage and re-growth of flocs formed by charge neutralization using alum and polyDADMAC

The formation, breakage and re-growth of flocs were investigated using alum and polyDADMAC to explore the reversibility of floc breakage. There is a significant reversibility of the breakage process, i.e. the broken flocs can re-grow to the size before breakage, when charge neutralization dominates the coagulation mechanism. However, for higher alum dosage, the break-up process displayed a distinct irreversibility. When coagulated in charge neutralization, the re-growth process of alum was nearly the same as that of polyDADMAC. The average size, coagulation rate and fractal dimension of flocs before and after breakage were nearly the same, including alum and polyDADMAC. While at higher alum dosage, the average size, coagulation rate and fractal dimension of flocs after breakage were much lower than that before breakage. Most important is that the number of small flocs after breakage and re-growth was much less than before breakage when charge neutralization dominated the coagulation mechanism. On the contrary, at higher alum dosage, the small flocs, after breakage and re-growth, increased. The fractal dimension of flocs with alum increased as coagulation time increased until a limiting floc size was reached, while for higher alum dosage, it decreased, whether before or after breakage. The determining parameter for floc re-growth is probably not the fractal dimension, but rather the chemical characteristics of the flocs surface. (C) 2010 Elsevier Ltd. All rights reserved.