Morphological properties of flocs under turbulent break‐up and restructuring processes

Abstract

Bentonite flocculation was performed in a Taylor–Couette reactor coupled with an in situ method of image acquisition and analysis. A hydrodynamic sequencing is imposed to perform successive cycles of flocculation and breakage. Depending on the shear rate applied during the breakage step, one or two cycles are needed after the first flocculation step to recover a full reversibility on both size and shape factors. The breakup step produces flocculi that are the building blocks for the next. The re‐flocculation steps produce smaller sizes and more regular shapes than the initial growth step. The floc size is calibrated by the turbulence as the radius of gyration is close to the Kolmogorov microscale whereas the floc structure is determined by flocculi aggregates. An analysis of the change of the flocs morphology, despite of their diversity, can also be achieved thanks to some relevant moments of the distributions. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3706–3716, 2017

Persistent reshaping of cohesive sediment towards stable flocs by turbulence

“…This process resembles a toddler shape-sorting different objects into their respective slots by persistently trying different options until the correct solution is achieved. In a somewhat similar manner, under strong enough turbulent shear, large non-compact flocs break into smaller flocs, with the latter quickly reaggregating to reform a typically compact floc [50][51][52] . Turbulence seems to "try" several repeated breakage-regrowth steps until a stable configuration, which cannot be broken at that level of turbulence, is reached www.nature.com/scientificreports/ (Fig.…”

Performance assessment of pulsating floc blanket clarifiers and conventional clariflocculators in pilot‐scale models

“…Moreover, the adsorption isotherm in case of alum is “Freundlich” type. This leads to characteristic formation of multiple layers (surface coverage is characteristically high) with voluminous (less dense but large and fluffy) amorphous precipitates (Pernitsky & Edzwald, 2006; Vlieghe et al, 2017; Wu et al, 2007). Therefore, and on the basis of above analyses, one can contemplate on the possibility that the speciation of alum brought about in the high solids contact zone and the effectively increased residence time offered in the lower columns of the PFBC enhance the coagulation–flocculation efficiency and result in a better utilization of coagulant alum.…”

Mechanisms behind overshoots in mean cluster size profiles in aggregation-breakup processes

“…Identification of the mechanism that controls the overshoot through direct experimental investigation can be demanding and also may require sophisticated instrumentation [12,11,60], especially in the case of complex systems such as wastewaters [16] or microalgae [8]. However, indications from which the controlling mechanism can be deduced can be obtained from measuring the response of the mean aggregate size upon a step change in the shear rate, a technique frequently applied to study the dynamics of aggregation and breakup processes [24,23]. In the following we propose a simple protocol how a step change in the shear rate at a specific moment in time allows for distinguishing among the four mechanisms discussed in this work.…”

“…The proposed protocols rely on step changes of the stirring speed during the aggregation experiment and measure the response of the system. Step changes in the stirring speed are relatively easy to implement in lab experiments [23,24] making the proposed protocols readily available. Being able to identify the controlling mechanism that causes the overshoot is important for optimizing aggregation processes and tailoring the produced flocks to fit the requirements of the specific application.…”