In order to investigate the effect of total hardness on coagulation efficiencies of Al species, different Al coagulants - conventional Al salt (AlCl3), Polyaluminum chloride (PAC-1) and the purified polyaluminum chloride (PAC-2) - were used to treat humic acid (HA) and kaoline clay in this study. The coagulation efficiency and coagulation kinetics were investigated by treating synthetic water with different total hardness. The results suggest that total hardness can influence the coagulation efficiencies of aluminum salts. The coagulation efficiency of AlCl3, PAC-1 and PAC-2 increased when total hardness increased. The investigations on coagulation kinetics showed that total hardness influences the flocs aggregation in the coagulation process of aluminum salts. Higher total hardness results in longer reaction time in all cases. The maximum floc size of AlCl3, PAC-1 and PAC-2 occurs when total hardness reaches 8,4, and 1 mmol L-1, respectively. (C) 2009 Elsevier B.V. All rights reserved.