Breakage and re-growth of flocs: Effect of additional doses of coagulant species

Several polyaluminum chloride (PACl) coagulants were prepared, with different OH/Al ratios (B values), and characterized by Ferron assay. These were used in studies of floc formation, breakage and re-growth with kaolin suspensions under controlled shear conditions, using a continuous optical monitoring method. Particular attention was paid to the effect of small additional coagulant dosages, added during the floc breakage period, on the re-growth of broken flocs. The results showed that the re-growth ability was greatly dependent on the nature of the PACl species added as second coagulant. The re-growth ability of broken flocs was greatest when the second coagulant was PACl0 (i.e. AlCl3, with B = 0) and least with PACl25 (B = 2.5). In the latter case there was no effect on floc re-growth, irrespective of the initial coagulant used. PACls with intermediate B values gave some improvement in floc re-growth, but less than that with PACl0. Additional dosage of PACl0 gave re-grown flocs about the same size or even larger than those before breakage. The re-growth of broken flocs is significantly correlated with the species Ala (monomeric) and Alb (polymeric), as determined by Ferron assay. The amorphous hydroxide precipitate formed from PACl0, (mainly Ala) can greatly improve the adhesion between broken flocs and give complete re-growth. However, for PACl25, mostly composed of Alb, the nature of the precipitate is different and there is no effect on floc re-growth.

► Breakage of flocs formed by hydrolyzing coagulants not fully reversible.
► Complete re-growth of flocs can occur if small additional coagulant dosage added during breakage.
► Effect of additional dosage greatly dependent on coagulant type.
► PACl samples with high basicity are ineffective in promoting floc re-growth.
► Model proposed based on ‘active’ and ‘inactive’ regions of floc surface.