Relative importance of hydrolyzed Al(III) species (Ala, Alb, and Alc) during coagulation with polyaluminum chloride: A case study with the typical micro-polluted source waters

The relative importance of three different Al species, Ala (monomeric species, instantaneous reacted species), Alb (medium polymer species, reacted less than 120 min), and Alc (colloidal or solid species, no reaction), defined by timed complexation reaction rate measured by using ferron reagent in polyaluminum chloride (PACl) was investigated in terms of DOC (dissolved organic carbon), UV254, and turbidity removal efficiencies. Micro-polluted, typical North China, source waters were used to conduct the experiments. The results show that DOC removal is correlated well to the content of Alb. Removal of UV254 is determined by the content of Alb and Alc, particularly Alc. Turbidity removal is primarily related to the content of Alc; however, Alb could destabilize particles efficiently, and the flocs formed by Alb are not as large as those formed by Alc, which affected the settling efficiency. Unlike the preformed Alb, the in situ formed Alb could remove turbidity more efficiently since Alc is the dominant final species formed during coagulation. Ala shows a strong ability to react with some unsatisfied coordinate bonds of organic matter to facilitate particle and DOC removal. The distinct coagulation feature of Ala, Alb, and Alc can be applied to develop tailor-made PACl (with the correct distribution of Al species) to match the characteristics of raw water for optimized coagulation.

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