It is challenging for a drinking water treatment plant to carry out effective coagulant dosing with polyaluminum chloride (PACl) for turbid-varied water. The goal of this study is to investigate effects of Al speciation on the minimization of residual turbidity and Al in coagulation-sedimentation for low and high turbid water. Two coagulant dosing approaches, including single and dual dosing with different commercial PACl and FeCl3 coagulants, were used to evaluate coagulation performance in terms of turbidity and residual Al. The results showed that single dosing by PACl-1 with high monomeric Al (48%) is effective in reducing turbidity at low turbidity, but its residual Al is higher than that by PACl-2 with high monomeric Al (54%) and polymeric Al (35%), while PACl-2 coagulation brings less turbidity reduction than PACl-1 coagulation regardless of Al minimization. Dual dosing with FeCl3 and PACl improve Al reduction but turbidity reduction improvement depends on Al/Fe dosing ratio. The most effective reduction in residual turbidity and Al is achieved at the Al/Fe ratio of 1:2 for low-turbidity water, whereas dosing ratio is insensitive to residual turbidity and Al at high turbidity. Dual dosing can achieve more effective residual turbidity and Al minimization for low turbidity water.

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