Monitoring flocs produced by water treatment coagulants

Pre-polymerized inorganic coagulants, such as polyaluminum chloride (PAC), are claimed to have several advantages over conventional aluminum and iron salts. However, their mode of action is not fully understood and progress in this area has been made largely by empirical approaches. In this study we have followed the dynamics of flocculation of model clay/humic acid suspensions following the addition of different coagulants, including aluminum sulfate, ferric sulfate and two commercial PAC products. Experiments were conducted using controlled stirring rates in a modified jar test procedure, in which the formation, break-up and re-formation of flocs could be monitored continuously by a flow-through optical technique. The results show significant differences in the behavior of the coagulants, notably in the rate of flee formation and flee strength. In all cases, only very limited re-growth of flocs occurred after breakage. Further experiments along these lines should help to elucidate the mode of action of different forms of coagulant and to aid in their selection for specific applications.