Size and structure of Chlorella zofingiensis/FeCl₃ flocs in a shear flow

Abstract

Flocculation is a promising method to overcome the economic hurdle to separation of algae from its growth medium in large scale operations. However, understanding of the floc structure and the effects of shear on the floc structure are crucial to the large scale implementation of this technique. The floc structure is important because it determines, in large part, the density and settling behavior of the algae. Freshwater algae floc size distributions and fractal dimensions are presented as a function of applied shear rate in a Couette cell using ferric chloride as a flocculant. Comparisons are made with measurements made for a polystyrene microparticle model system taken here as well as reported literature results. The algae floc size distributions are found to be self-preserving with respect to shear rate, consistent with literature data for polystyrene. Three fractal dimensions are calculated which quantitatively characterize the complexity of the floc structure. Low shear rates result in large, relatively dense packed flocs which elongate and fracture as the shear rate is increased. The results presented here provide crucial information for economically implementing flocculation as a large scale algae harvesting strategy.

Synergy of flocculation and flotation for microalgae harvesting using aluminium electrolysis

“…The floc structure has great influence on flocculation kinetics Wyatt et al, 2013). The compact flocs are resistant to breakage and beneficial to the solid-liquid separation.…”

Flocculation on a chip: a novel screening approach to determine floc growth rates and select flocculating agents

“…Shear stress can affect adsorption and collision rates but can also lead to floc breakage. 1,3,11,13 Under turbulent mixing, floc growth and floc breakage are typically in a dynamic equilibrium, and their individual rates are thus difficult to discern. A variety of analytical methods to size and count flocs have been established.…”

“…1,2 During flocculation, colloidal particles are selectively aggregated with the aid of a flocculating agent and then subsequently settle under the influence of gravity. 3,4 In industrial bioprocesses, this selective removal of key impurities is applied to remove cells and cellular debris from the fermentation medium during primary recovery, i.e.…”

Harvesting, Thickening and Dewatering Processes

“…Secondly, the counter cations such as sulphates remain in the medium and alter the medium composition when recycled. Nevertheless, flocculation using metal salts is useful as a model system to understand the fundamental mechanisms of flocculation of microalgae [47], [48].…”