Sterilization by flocculants in drinking water treatment

In this study, the relationship between coagulation-flocculation and sterilization was studied using two conventional flocculants, polyaluminum chloride (PAC) and poly dimethyl diallyl ammonium chloride (PDMDAAC). Simulated wastewater containing kaolin and Escherichia coli was prepared. The bacterial removal rates of PAC and PDMDAAC under the same dosage are 46.3% and 19.5%, respectively. Which is far less than that of combined flocculants (94% bacterial removal rate). The synergistic effect of two flocculants was proved by contour method. β-Galactosidase activity, extracellular polysaccharides release, bacterial respiration activity and the activity of triphenyl tetrazolium chloride (TTC) dehydrogenase were investigated to clarify the sterilization kinetics and mechanisms. Two flocculants play the different roles in the treatment. PAC plays role of coagulation, charge neutralization, but PDMDAAC with quaternary ammonium groups is responsible for flocculation to form big flocs, which further enhances its contact with E. coli and contributes to sterilization. Sterilization by flocculants is divided into two processes, rapidly adsorbing and slowly killing, since the respiratory activity of E. coli in flocs gradually decreases with increasing contact time. The increase of flocculant dosage overcomes the hindrance of the secretion of extracellular polysaccharide polymers originating from E. coli to coagulation and sterilization. Sterilization of flocculant should be fully considered in drinking water treatment.