The success of surface water treatment strongly depends on the effectiveness of coagulant performance. Aluminium sulfate (alum), the most widely used coagulant in water treatment plants in Canada, is well known for its poor performance in cold water. Polyaluminium chloride (PACl), a relatively new polymeric aluminium coagulant increasingly being used in water treatment plants, is found to have many advantages over conventional alum. However, PACl hydrolysis reaction is quite complex and its action is not fully understood. In this research, a series of bench-scale jar tests with alum and PACl was conducted. Alum and PACl coagulation flocs were analyzed for the evaluation of coagulant performances at 19°C and 5°C for the Split Lake water treatment plant.; The results of this research indicated that the settling properties of PACl flocs were superior to those of alum flocs, especially at the lower temperature. The average size of PACl flocs was relatively smaller than that of alum flocs. The density of PACl flocs could be higher than that of alum flocs. And the number of settled PACl flocs could be higher than that of settled alum flocs. The effects of temperature on alum flocs and PACl flocs were different. Alum flocs size decreased at 5°C. This is most likely due to the existence of monomeric aluminium species in alum aqueous solution. PACl flocs size did not change significantly at the 5°C. This may be due to the existence of Al137+ polymeric species in PACl aqueous solution.
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