The high turbidity in surface water may make it difficult for water treatment plants to supply drinking water. Chitosan, a natural linear cationic polymer, and aluminum chloride, a metal salt, and the mixture of the two coagulants were used to treat highly turbid raw water in this study according to the residual turbidity, sludge volume and residual aluminum concentration. The residual turbidity was less than 50 NTU but the sludge/water volume ratio was over 150 mL/L after aluminum salt coagulation (135 mg/L as Al), which could stop the sedimentation process. The amount of sludge produced after chitosan coagulation (5 mg/L) was only about 1/5 of that for aluminum coagulation for the similar turbidity removal. Chitosan coagulation, however, still has two problems that need to be solved. First, the residual turbidity of treated water is still too high for sand filtration. Second, the colloid particles may restabilize if chitosan is overdosed. Adding a comparative low dosage of aluminum salt (13.5 mg/L as Al) with chitosan can successfully solve both of the problems. The sludge volume ratio only increased slightly and the residual turbidity was less than 10 NTU. Moreover, the restabilization of colloids did not occur. The residual aluminum concentration, which could lead to Alzheimer's disease, can also be reduced significantly after addition of chitosan. (C) 2012 Elsevier B.V. All rights reserved.