Coagulation efficiency of polyaluminum chloride for natural organic matter removal from low specific UV absorbance surface water and the subsequent effects on chlorine decay

Abstract

The effect of polyaluminum chloride (PAC) dosage on the coagulation performance and the floc characteristic with respect to the treatment of surface water with low specific UV absorbance (SUVA) value was investigated in this paper. The subsequent effect on chlorine decay was studied by a first decay model and AQUASIM modeling software. The results showed that natural organic matter (NOM) removal increased with the increasing dosage of PAC, while the turbidity removal decreased as the zeta potential further increased to the positive side. At low PAC dosages, charge neutralization was the main mechanism for NOM removal. For the dosage of PAC increased, entrapment, adsorption and complexation played important roles in removing NOM. The growth rate of the flocs was raised with the PAC dosage. Flocs formed at a PAC dosage of 3 mg/L were larger than those at high dosages (12 mg/L and 20 mg/L) and became more compact during the slow stir process. D f values of flocs with 12 mg/L and 20 mg/L of PAC exhibited a slight decrease at the end of slow stir stage. Floc breakage behavior showed that flocs formed at different dosages of PAC gave different floc strength and the strength factors were in the order: 3 mg/L < 12 mg/L < 20 mg/L. While the floc regrowth factors at different dosages of PAC were in the order: 3 mg/L > 20 mg/L > 12 mg/L. Coagulation treatment with 20 mg/L of PAC resulted in lower chlorine decay rate, but did not lower the total reacting compounds in the effluent when compared with coagulation with the PAC dosage of 12 mg/L. Furthermore, minimal amount of disinfection by-products (DBPs) would be possibly produced after treatment with 12 mg/L of PAC due to the minimal reacting compounds present in the water in this investigation.