Coagulation Properties of Biodegradable Chitosan-CTA Coagulants in Turbidity Removal

In this present study, strongly cationic coagulants were prepared by modifying chitosan with 3-chloro-2-hydroxypropyl trimethyl ammonium (CTA). The coagulation properties of this cationic chitosan (chitosan-CTA) were systematically examined in laboratory and pilot scale studies. At the laboratory scale, the effects of coagulant dosage, temperature, initial turbidity of untreated water, pH, and salinity were investigated by a jar test using kaolin suspension as synthetic water. Compared with several conventional coagulants, chitosan-CTA showed a lower dosage requirement of approximately $0.05\mathrm{mg}/\mathrm{L}$, but higher turbidity removal efficiency of approximately 93.3%. Moreover, raw water from the Zhenjiang part of the Yangtze River in China was used as test water for further investigation on a pilot scale. The influences of dosage, sedimentation time, and mechanical mixing rate on the coagulation properties were studied by an orthogonal test and the optimal experimental conditions were obtained. The dosage of coagulants was $1.1\mathrm{mg}/\mathrm{L}$, sedimentation time was 20 min, and the mechanical mixing rate of the first triple coagulative sedimentation tank was 200 rpm in this case. The effect of the substitution degree of CTA on the coagulation properties was also investigated at the laboratory and pilot scales. Above all, the coagulation experiments indicated that the cationic chitosan-based coagulants showed good coagulation performance in turbidity removal.