To improve the coagulation performances, new composite coagulants were used to treat different water samples. The results indicate that Ca2+ has no significant effects on the removal efficiency for turbidity in the kaolin system. The residual aluminum decreased from 0.15 mg·L-1 to 0.10 mg·L-1 (AlCl3 was used as coagulant and the coagulant dosage was 0.10 mmol·L-1). The presence of Ca2+ led to the decrease of the amount of negative charges in the HA system and the residual aluminum decreased due to the decrease of the complexation between the HA molecules and Al-based coagulants. When the raw water contained BSA molecules and the coagulant dosage was 0.16 mmol·L-1, the flocs formed by PACl with Ca2+ were larger (~50 μm) than the flocs generated by PACl and the settleability also improved. Under alkaline conditions (pH=8.5), the DOC concentration decreased after coagulation process by~0.2-0.6 mg·L-1 and the residual aluminum decreased by~0.4-0.7 mg·L-1 using composite coagulants. Under acidic conditions (pH=5.5), the concentrations of DOC and residual aluminum did not significantly differ.
Keywords: Ca2+; coagulation; composite coagulant; floc characteristics; residual aluminum.