Essential aspects of floc structure and breakage

Flocculation is an essential operation in many industries employing solid-liquid separation processes; examples include mineral processing, fines retention in paper-making, paint production, and water and wastewater treatment. To better understand and model flocculation processes, it is necessary to characterize the evolution of the physical properties of aggregates as a function of process conditions. This requires an in-depth understanding of the effects of interactions between process chemistry, hydrodynamics, and coagulation mechanisms. The emphasis of this investigation with respect to floc characteristics has been placed upon floc size, density (interparticle distance), strength, and dewaterability.; In this study, a state-of-the-art, high-speed video system was used to observe and record floc breakage occurring in the thin shear layer of a turbulent jet. The unique macrovideographic data provide details of the breakage process that were not available previously and were used to validate and quantify an improved model of the aggregates' response to hydrodynamic stress; the model consisted of three distinct components corresponding to rotation, translation, and extensional deformation. An experimental program was developed to manipulate the physicochemical conditions of flocculation processes in order to generate flocs with certain structural characteristics by employing various combinations of hydrodynamic profile and polymer dosage. Macrovideography and a novel light obscuration technique were applied to assist evaluation of the evolving aggregation processes and the resulting floc structural characteristics. It was shown that an experimental procedure using multiple polymer dosing along with episodes of intense agitation at each interval of polymer addition can produce flocs with unusual compact structures (higher densities). Experimental studies have also been performed for the kaolin-polymer floc system to explore the role of the changes in adsorbed macromolecular conformation upon the structure's mean interparticle distances. The evolution of floc size was monitored with the help of macrovideography and a novel light obscuration method. In addition, an extensive series of filtration experiments was carried out to assess the dewaterability of flocs generated by different combinations of physical and chemical conditions. It was found that efficient coagulation processes must compromise because ideal floc characteristics (dense, strong, and easy to dewater) are generally incompatible to one another; such processes must employ optimal operational conditions in order to produce flocs suitable for subsequent solid-liquid separations and/or sludge volume reduction.