Performance of chitosan/PAC enhanced coagulation for low-temperature and low-turbidity Pi River source water

This work aims to investigate the effect of combined polyaluminum chloride (PAC) and chitosan (CTS) on improving coagulation performance in treating special water, such as low-temperature and low-turbidity water. Furthermore, the study aims to elucidate the mechanism of action involved in applying PAC and CTS in the removal of turbidity, organic matter, and other pollutants. A coagulant of chitosan (CTS)/polyaluminum chloride (PAC) was formulated for the treatment of low temperature and low-turbidity Pi River source water, located in the southwest of Anhui Province. PAC was used as the main coagulant, and CTS was used as the coagulant aid. The effects of the com-pounding ratio of CTS/PAC, agitation intensity during coagulation, and sedimentation time on tur-bidity reduction, decolorization, and organic matter removal were investigated through coagulation beaker tests. Finally, the electric potential after coagulation was analyzed, and the structure of CTS and PAC was characterized. Results showed that the highest removal efficiency of turbidity, chro-maticity, and ultraviolet absorbance at 254 nm (UV254) was obtained when the volume ratio of CTS/ PAC was 0.4:1, reaching 86.32%, 16.98%, and 32.79%, respectively, which was 17.9%, 3.77%, and 8.2% higher than those of PAC alone. The optimal agitation intensity was attained at a mixing speed of 300 rpm and a flocculation speed of 150 and 100 rpm. Besides, sedimentation time showed the most significant impact on turbidity. Moreover, scanning electron microscopy and zeta potential results confirmed that PAC coagulation mainly relies on electrical neutralization, while CTS coagulation is highly attributed to adsorption and bridging. The results suggest the formation of a new structure via the compounding of CTS and PAC, which further strengthened the electrical neutralization and bridging and netting capabilities, consequently improving the coagulation effect.