Effect of Al speciation on residual turbidity and Al minimization by coagulation with single and dual dosing

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.