Effect of advanced water treatment on behaviour of residual chlorine and development of chlorine control method in distribution systems

The behaviour of residual chlorine concentration in drinking water treated by the mid-chlorination or the advanced water treatment (AWT) was investigated. The AWT removes dissolved organic matter more than the mid-chlorination, thus the rate of residual chlorine reduction is decreased. The analysis of chlorine concentration data measured by automated continuous monitoring devices revealed that the introduction of AWT reduced the maximum and mean chlorine concentration by 0.33 mg L⁻¹ and 0.21 mg L⁻¹ compared to the mid-chlorination respectively, while the minimum concentration was maintained at around 0.3 mg L⁻¹ in the whole city. The overall chlorine reduction rate coefficient in the AWT was reduced to approximately half of that in the mid-chlorination. Volatilization of chlorine from a water surface could be expressed by the zero-order rate of reaction from laboratory experiments. The rate of volatilization increases with increasing water temperature and shallower water depth. The residual chlorine prediction models were developed based on the results of data monitoring and laboratory experiments. The accuracy of simulated values is good and almost all of the predicted concentrations are in the range of±0.1 mg L⁻¹ of measured concentrations. The models are very effective to control residual chlorine concentration at predetermined target value.