This work aims to evaluate the effect of different velocity gradient and rapid mixing time on floc properties of picophytoplankton, using polyaluminum chloride (PACl) and polysilicato-iron (PSI) as coagulant. The growth, breakage, and re-growth of flocs were followed by a particle size analyzer in order to investigate the underlying mechanisms. Three different zones were found to be depending on G value in coagulation process. At low G value (G < 200 s-1), the aggregation dominated over breakup. At mean Gvalues (G = 250 s-1 for Synechococcus sp.-PSI flocs and G = 546 s-1 for kaolin-PSI flocs; G = 200 s-1 for Synechococcus sp.-PACl flocs and G = 390 s-1 for kaolin-PACl flocs) flocs formation wAS maximized, producing the largest flocs. For large G values (G > 250 s-1for Synechococcus sp.-PSI flocs and G > 546 s-1 for kaolin-PSI flocs; G > 200 Synechococcus sp.-PACl flocs and G > 390 s-1 for kaolin-PACl flocs), breakup dominated over floc formation. Broken flocs did not fully re-grow after breakage; however, the re-growth rate of Synechococcus sp. flocs was larger than that of kaolin flocs, probably as a result of the surface properties of Synechococcus sp. The rapid mixing time of 60 and 90 s led to maximum floc formation Synechococcus sp. and kaolin systems, respectively.
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