Breakage and re-growth of flocs: effect of additional doses of coagulant species |
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Authors: | Yu Wenzheng Gregory John Campos Luiza C |
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Affiliation: | a Department of Civil, Environmental and Geomatic Engineering, University College London, Gower Street, London WC1E 6BT, UK b State Key Laboratory of Urban Water Resource and Environment (SKLUWRE), School of Municipal & Environmental Engineering, Harbin Institute of Technology, No 73 Huanghe Road, Nangang District, Harbin 150090, China |
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Abstract: | 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. |
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Keywords: | Floc breakage Floc re-growth Kaolin Alum PACl |
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