The role of rapid mixing condition on picophytoplankton floc growth

Abstract

A B S T R A C TThis work aims to evaluate the effect of different velocity gradient and rapid mixing time on floc properties of picophytoplankton, using polyaluminum chloride (PACl) and polysilicatoiron (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 À1 ), the aggregation dominated over breakup. At mean G values (G = 250 s À1 for Synechococcus sp.-PSI flocs and G = 546 s À1 for kaolin-PSI flocs; G = 200 s À1 for Synechococcus sp.-PACl flocs and G = 390 s À1 for kaolin-PACl flocs) flocs formation wAS maximized, producing the largest flocs. For large G values (G > 250 s À1 for Synechococcus sp.-PSI flocs and G > 546 s À1 for kaolin-PSI flocs; G > 200 Synechococcus sp.-PACl flocs and G > 390 s À1 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.