In this paper, the mechanism of NOM removal by coagulation is investigated using coagulant polyaluminium chloride (PACl), compared with AlCl3. The kinetics of Al ion hydrolysis, interaction of hydrolyzed species of Al and NOM, and hydraulic power condition in coagulation process are investigated by programmable jar test, photometric dispersion analyzer and ferron assaying. After the coagulant dosing, Al ion would hydrolyze very quickly, and then monomeric Al and small polymeric Al would further hydrolyze to form larger polymer and precipitate, based on pH condition, while the further hydrolyzed process is relative slow. Although complexation between NOM and Al appears to be several orders of magnitude slower than hydrolysis of monomeric forms of Al, it would be faster than further polymerization and precipitation of Al during aging. However the further polymerization of fresh Al would benefit to Al-NOM complex aggregation and settlement. Therefore, PACl, with stabile preformed polymer shows significantly different performance in NOM removal compared with AlCl3. Although the hydraulic power condition plays significant role in collision frequency and efficiency of particles removal, the process of the aggregation to form larger floc to settle down is relative slower than interaction of Al and NOM, and it is not very correlative to the performance of NOM removal if only it can provide enough mixture.

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