Structure and porosity of alum coagulation flocs

It is generally assumed that flocs grow according to the following model: primary particles form compact flocculi which assemble themselves into microflocs. Microflocs bind together to form floc aggregates. The main difference between flocculi, microflocs and floc aggregates is their structure. Porosity can be used to indicate differences in this structure. The objective of this study was to establish whether alum flocs disperse into floc components that represent different structures, as indicated by different porosities. Alum coagulation flocs were broken up with increasing amounts of mixing energy. The size and porosity of the studied particles were determined on microtome sections of the resin-stabilized samples. The results, as expected, indicated that the size and porosity of alum flocs decreased with the increase of applied mixed energy. Fractal analysis of pores revealed that alum flocs contained at least three distinctive populations of pores: small, medium and large. The three pore populations indicate three levels of aggregation, i.e. flocculi, microflocs and floc aggregates. The results of this study confirm the previous theoretical and experimental investigations that argued for the multilevel floc structure.