Flocs generated by various coagulation mechanisms exhibit different size, strength and structure. The properties and fractal dimensions of flocs formed under three common coagulation mechanisms, i.e. charge neutralization, sweep and bridging, were investigated at various hydraulic conditions. The results showed that the floc size decreased with the increasing average velocity gradient G and the stable floc size exponent γ was of the following hierarchy: charge neutralization (0.6107) N sweep (0.5618) N bridging (0.3674). Furthermore, fractal dimensions of flocs were the highest when formed by sweep and the lowest when generated by bridging flocculation. The mass fractal dimensions measured by light scattering were between 2.0 and 3.0 and the floc strength was between 0.01 and 0.58 N m − 2 . An intrinsic unity of the relationship among floc size, fractal dimensions, floc strength under the three coagulation mechanisms was demonstrated.

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It is possible to produce stable preparations of partially neutralized aluminum chloride solutions in which cationic aluminum polyelectrolytes predominate, but attempts to produce similar stable preparations containing cationic iron(III) polyelectrolytes were unsuccessful. One method to improve the effectiveness of inorganic metal coagulants in water treatment is to partially neutralize concentrated metal salt solutions prior to their addition to the raw water and thus preform polymeric metal species, the actual coagulants. In this way coagulant chemistry can be controlled and proper solution conditions for the formation of desired coagulant species can be maintained. This article compares polymeric preparations of aluminum and ferric chloride salts with respect to preparation parameters and the extent and stability of polymerization.

a b s t r a c t a r t i c l e i n f oAlthough the synthetic organic polymers have been widely applied as flocculant aids to improve solid-liquid separation efficiency, it is not very clear how the charge type and molecular weight of the polymers influence the characteristics of flocs. In this paper, floc characteristics, such as floc size, density, structure, strength and reversibility were investigated in details in terms of their response to the polyacrylamide (PAM) with different charge types and molecular weights. Laser light scattering, image analysis and settling technology were employed simultaneously. The results showed that floc size induced by anionic PAM with medium molecular weight (A130) and ultra high molecular weight (A115) was 419 µm and 438 µm respectively, correspondingly, floc size induced by cationic PAM with medium molecular weight (C448) and very high molecular weight (C498) was 731 µm and 796 µm. Meanwhile, floc fractal dimension was inversely proportional to polymer molecular weight and the values were between 1.91 and 2.40. In addition, floc density was affected by molecular weight of anionic PAM more dramatically than that of cationic PAM. For a given floc size, the density increased with molecular weight. Furthermore, flocs formed by cationic PAM showed much greater shear-resistant ability and reversibility than anionic PAM. Nevertheless, the poor reversibility of the flocs formed by high molecular weight was also observed. An explanation for the resultant floc characteristics was offered in terms of flocculation mechanisms.

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