Coagulation efficiency and removal mechanism for composite coagulant polyaluminium chloride/polydimethyldiallylammonium chloride in treating lightly micro-polluted raw water of Yangtze River in autumn

Abstract

The lightly micro-polluted raw water of Yangtze River (YRW) in autumn was treated via enhanced coagulation by composite coagulants composed of polyaluminium chloride (PAC) and polydimethyldiallylammonium chloride (PDMDAAC), named PAC/PDMDAAC. Coagulation mechanism and removal efficiency were investigated by assessing the water quality parameters of the resulting supernatant, i.e. turbidity, COD_Mn, and NH₃-N, at the same dosage and supernatant turbidity (SDST) point as using PAC only, with controlled residual turbidity of 1.0–1.50 NTU to mimic drinking water production plant supernatant condition when using lightly micro-polluted water as raw water source . In addition, the zeta potential, floc morphology, and size analysis under the condition of SDST using PAC and composite coagulants PAC/PDMDAAC for getting insight about removal mechanism were done. The results showed that, firstly, most of the composite coagulants PAC/PDMDAAC with intrinsic viscosity [η] = 0.65, 1.60, and 2.6 dL/g; and mass ratio PAC:PDMDAAC of 5:1, 10:1, and 20:1 (m:m)) could meet the requirements of controlled supernatant turbidity between 1.0 and 1.5 NTU to mimic drinking water production plant condition using YRW (Nanjing section) that comply with the new national drinking water standards. Secondly, the seven kinds of composite coagulants PAC/PDMDAAC can maintained the advantage of enhanced coagulation removal efficiency within the SDST point as using PAC only. The COD_Mn and ammonia nitrogen removal rates using composite coagulants at SDST points were 0–6.19 %; 0–15.62%, respectively higher than using PAC only. Finally, this study deepened and expanded the existing research knowledge about composite coagulant PAC/PDMDAAC and offered the maximum limitations in removing the water quality parameters via enhanced coagulation treatment of lightly micro-polluted surface raw water in order to meet new national drinking water standards.