Optimum Conditions for Enhancing Chitosan-Assisted Coagulation in Drinking Water Treatment

Coagulant aids are useful chemicals that enhance the efficiency of coagulation sedimentation treatment. For this purpose, it is necessary to choose safe chemicals to avoid various risks to the health of those who use the treated water. The use of chitosan, an abundant natural polysaccharide, as a coagulant aid is significant not only for safe water treatment but also for the effective utilization of unused natural water resources, which are mostly wasted. We experimentally determined the optimal treatment conditions for using chitosan as a coagulant aid in water treatment. The most efficient use was identified as adding chitosan at the stage of rapid stirring after the addition of coagulant accelerated initial dispersion. When used with the main coagulant polyaluminum chloride (PACl), the optimal concentration of chitosan was 0.8 mg L−1, as estimated using the ζ potential showing isoelectricity at the optimal chitosan concentration. Determining the chitosan concentration using the minimum ζ potential was also valid for estimating the optimum concentration of chitosan, which is an extension of the method used at much higher turbidity, as seen in wastewater. Thus, the ζ potential-based prediction of the optimum chitosan concentration was effective even when the effect of sweep coagulation, which is normally induced at higher turbidity, was negligible. The superiority of using the coagulant PACl in combination with chitosan as the coagulant aid was demonstrated by comparing the in situ-observed coagulation process to cases with other coagulants and coagulant aids using direct time-series observation of the coagulation process. The use of chitosan with PACl was found to make the flocs easier to remove because it resulted in the largest mass fraction of the resultant floc sedimentation on the bottom of the vessel. In this study, using the PACl coagulant in combination with chitosan as the coagulant aid was found to be as viable as using the current popular combination of aluminum sulfate and polyacrylamide. Replacing polyacrylamide with chitosan contributed to reducing the potential risk to the health of those to use the treated water.