Chlorine Decay Variation And Influential Factors In Water Distribution Network

Water distribution system (WDS) is a main part of urban water cycle. Research of water quality variations of WDS and how to enhance water supply security has become the focus of water industry in the 21st century. No matter in the process of water treatment or water distribution, disinfection plays an important role. The most widely used potable water disinfectant in the world is aqueous chlorine because of its relatively low cost, high disinfection efficiency and ability to provide a residual concentration in the network to protect against (further) bacterial growth. Therefore, it is significant to investigate the forecast and modeling of chlorine decay. During years of operation in water supply pipeline, anomalous annularity mixture will grow up along pipe wall, which is called‘growth ring'. It is the mix of sediment, erosion, dirty and biofilm inside pipeline. When water flows along pipeline wall, it is not the wall which directly contact with water, but“growth wing”. Therefore, it is the main reason why water quality will change in WDS. In this research, in order to ensure water quality of WDS, chlorine decay rule was discussed through dynamic and static experiments. The measured chlorine decay data was analysed throuh exponentially fitted method. Results show that chlorine decay has positive correlation with factors such as: temperature, TOC, oxygen consume, THMs and ammonia nitrogen. Initial chlorine concentration and pipe diameter has negative correlation with chlorine decay coefficient. Furthermore, chlorine decay rate after re-chlorination was distinctly less than that before re-chlorination. Chlorine decay in cast iron pipes was the most between stainless steel, PVC and cast iron pipes. For common cast iron pipes, in bigger diameter pipe, the effect of flow velocity on decay coefficient is less and that of pH value is more. Chlorine decay is mainly occurred in the reaction with pipe wall. pH value is the main factor that effects chlorine decay in cast iron pipes. The close relationship between hydraulic model and water quality model was analysed. On the basis of hydraulic model, chlorine decay model was built on the premise of overall analysis of chlorine residual status. Aiming at the localization of current chlorine decay models, a practical and very reliable semi-empirical numerical chlorine decay model was proposed, which will be defined as the variable rate coefficient (VRC) model. Based on analysis of influence factors of chlorine decay, factors such as temperature, chlorine dosage and re-chlorination was mainly considered during the establishment of VRC model. The proposed model requires the calibration of 4 coefficients: X0,?0, kmin and?. The model was based on second order bimolecular decay kinetics, and a differential equation was defined to describe the change in the average rate coefficient. Chlorine decay process was simulated by the combination of the two factors. The model coefficients?0?kmin and?are calibrated using the Shuffled Complex Evolution (SCE) algorithm for a range of given X0. The optimal values for the initial and booster chlorine concentrations for each data set are determined by using a local search method. The VRC model, the first order chlorine decay model and a second order reactive species model by Boccelli et al. are calibrated and compared using experimental data from 6 water sources. In this research, SCE algorithm was first used in the research and application of WDS. The results show that calibration results of SCE method is better and VRC model shows very good agreement with observation and outperforms the other models in all cases.