عنوان مقاله [English]
Chlorine disinfection (chlorination) is one of the common methods in the water purification process. Chlorine is used as the most common disinfectant in water supply systems due to its residual preservation, effective performance as well as economic advantages.
Chlorine decay occurs as it moves along a pipe or settles in storage reservoirs due to reaction with inorganic and organic matter in the volume of water (bulk decay) and biofilms and materials on the wall of the pipe (wall decay). Due to the lack of chlorine concentration, the risk of microbial contamination increased. On the other hand, increasing chlorine concentration during the treatment process can cause pipe corrosion, taste and odor problems and disinfection by-products formation. In the field of quality assessment of drinking water distribution and conveying lines, the study of decay and chlorine reactions is of particular importance. The reaction of chlorine and organic matter is one of the problems that many water supply lines are facing.
In this article, the residual chlorine concentration in the Isfahan water supply line from Baba Sheikhali water treatment plant to Naeem is simulated by applying the first-order model, parallel first order model as well as second order single-reactor model in the EPANET-MSX software. The line’s total length is 259 Km and supplies drinking water for more than 4 million consumers. The nominal discharge of the line is 11.7 m3/s. EPANET is a widely used software for modeling the hydraulic and water quality of drinking water in distribution systems. While the water quality component in the original version of EPANET is limited to tracking the transport and value of just a single chemical species, the MSX extension provides qualitative simulation with a multi-species approach. The newly developed multi-species extension (EPANET MSX) brought enhanced capabilities for the simulation of chlorine residuals in water supply systems that allow the modeling of chemical reactions with any level of complexity. First, the hydraulics simulation of the model was performed and the results were compared with the measured pressures along the line. The model is calibrated and validated with the available pressure data. Then the quality simulation is applied. To determine the bulk coefficient, the bottle tests were performed at 6, 13 and 18 ° C. In order to extend the results of the experiment to different temperatures, the temperature coefficient is calculated based on Arrhenius law. Two methods of integrated and bisection are applied for numerical modeling in spring, summer and winter. In the first method, chlorine simulation was performed taking into account the whole Isfahan water conveyance line as one section (integrated) and determining a bulk decay and wall decay coefficient for the entire conveyance line. In the second method, the line was divided into two sections (bisection) and decay coefficients of chlorine for each section separately were dedicated.
The model’s results were compared with the measured data at different nodes at the conveyance line. The results indicated that by separating the line into two parts and applying independent coefficients and decay parameters to each part, the RMSE value has been reduced for the measured and simulated chlorine concentrations, from 0.09 to 0.03 mg/L in summer and from 0.064 to 0.025 mg/L in winter. Comparison of the root-mean-square error (RMSE) value in different models between the measured data shows that if in simulation, the line is divided into two or more parts and the different coefficient is considered for each, the results are considerably improved and models show no significant advantages. The results indicated that the chlorine concentration is high at the beginning and lower than the minimum at the end of the conveyance line. In order to comply with the standard range, re-chlorination was investigated through two approaches. In the first approach, in order to meet the required minimum concentration at the end of the conveyance line, re-chlorination was performed at an intermediate node. The chlorine injection remained constant at the same rate as it is. In the second approach, the injection rate was reduced at the beginning of the line and three nodes were selected for re-chlorination. A comparison of the results showed that chlorine injection along the line results in maintaining the minimum chlorine level in the whole line and reducing the concentration of chlorine in the leading regions. Moreover, by application of the new method, the consumption of chlorine is reduced up to 50%.