Evaluation of the effect of temperature changes on chlorine mass deterioration in the water supply network using EPANET (II) qualitative-hydraulic simulation system

Environmental and health organizations believe that water entering the water distribution network should always have a certain amount of free chlorine remaining (at least 0.6 mg/L according to standard 1053). The founding philosophy of this standard is based more on the possibility of re-contamination in the water network. Experience in the operation of urban water distribution networks shows that various factors can lead to microbial growth and contamination in treated water. These parameters include the presence of biofi lms in parts of pipes, the entry of external contaminants (microorganisms active in the soil) due to the phenomenon of leakage, the entry of air pollution in open tanks and the transfer of pathogens intentionally or Unintentionally pointed out. However, factors such as the reaction of chlorine with the wall of the distribution system pipe or the reaction with the matrix of materials in the water lead to a decrease in the concentration of free chlorine remaining throughout the network. Meanwhile, chlorine mass decay is a function of the materials in the water and the water temperature in the network. Therefore, this study intends to evaluate the effect of temperature changes on the residual chlorine concentration of the network using qualitative-hydraulic modeling and simulations. Studies have shown that in different time series, the remaining free chlorine changes up to 32% in different months of the year.


Introduction
One of the missions of the water resources and water supply management system is to maintain the physical, chemical, and microbial quality of water in the entire water supply network. Raw water (surface water) enters the municipal water treatment plants after treatment and disinfection are disinfected and enter the water supply network. In the secondary or fi nal disinfection process, various oxidizing compounds and substances such as chlorine gas, chlorine dioxide, sodium hypochlorite (bleach), ozonation, or irradiation systems such as UV are used [1]. But chlorine and its compounds are of special importance due to the residual effect on the water supply network [2]. After chlorine injection, part of it reacts with organic substances and compounds in the water matrix, which is called compound chlorine. The rest of the chlorine is left free in the water (free chlorine) to oxidize in the event of microbial or chemical re-contamination [3]. However, in the real conditions and dimensions of the network, the concentration of free chlorine remaining changes and decreases due to the two phenomena of wall decay and mass decay [4][5][6]. Chlorine wall degradation is due to the reaction of chlorine to the wall of the water distribution network pipe, which depends entirely on the material and age of the water pipe. This is while the mass decay of water is a function of the matrix of materials in water and its temperature. Alyan Koopayi and Haghighi [7] conducted a study on the effect of temperature on chlorine decay coeffi cients in the water distribution network. In this research, EPANET dynamic qualitative model has been used. The results of this research showed that the reaction coeffi cients in the fl uid mass follow the fi rst degree, whose values in summer and winter are -0.53d -1 and -0.207d -1 , respectively. Next, the researchers found that the rate of chlorine degradation with the pipe wall was zero. The coeffi cients obtained for summer and winter were 3-and 2-mg/m 3 /day, respectively. Ameri Safi abadi, et al. [8] in a review study to determine the reaction rate and kinetic coeffi cients of chlorine decay in water distribution systems. It should be noted that in this study, the main approach was based on the principles of network operation. Ahmadi Barghani, et al. [9] also investigated the rate of chlorine deterioration in the leached water supply network (a network that has been cleared of biofi lms by disinfection). The mentioned research has been done in the branch network of Darab city. The results of this study showed that after rinsing the grid, the rate of chlorine degradation decreases; So that the reaction constant coeffi cient (k) before the washing operation was about 0.00056 lit/min and after this operation, it reached 0.00031 lit/min. In another study, Yazdanpour, et al. [10] used artifi cial neural networks to model residual chlorine in urban water distribution systems. In this study, using two models of linear regression and an artifi cial neural network, a multilayer perceptron was used to predict the concentration of chlorine in the water supply network. Research has shown that both methods are successful in this regard, But the artifi cial neural network method is more effi cient. This study also intends to investigate the effect of temperature and the consequent decay of chlorine in the Plinze Braille water supply network using the EPANET (II) qualitativehydraulic model.

The network under study
The network studied in this research is a Plinze Braille network with 36 nodes, a tank and a pumping station, which is described in Figure 1. It is worth mentioning that the maximum height code in this network is related to the reservoir with a height of 70.5 meters and the minimum height is related to node No. (1) with 15.01 meters.
In this part of the study, for each node, the assignment of operations (Demand Assigning) has been done according to Figure 2. The studied network has pressure fl uctuations overtime during the consumption pattern, which is shown in Figure 3.
Finally, it should be noted that in the process of qualitativehydraulic simulation of this network, the values of wall decay coeffi cients and Hazen-Williams are fi xed and equal to 1 and 100, respectively. The consumption pattern of the mentioned network is also shown in Figure 4.

Qualitative network simulation
As mentioned earlier, the mass degradation coeffi cient depends on the temperature and the water matrix. In this study, the mass degradation coeffi cient based on the proposal of the American Water Works Association (AWWA) is 1.35d -1 at a temperature of 20°C. Equation (1) is also used to convert the values of this coeffi cient at different temperatures. The temperature conditions considered in this study are calculated   It should also be noted that to determine the amount of mass deterioration, node (2) was considered as a control node. 20 20 For qualitative simulation in EPANET (II) medium [11], chemical reaction for wall and wall deterioration of fi rst degree was selected.

Chlorine injection system
In this network, injection is done inside the tank, and the injection pattern is in accordance with the fl ow pattern

Results and discussion
As stated earlier, with temperature changes, the amount of chlorine remaining in the nodes of the water supply network changes. As seen in Figure 7, around 10 to 12 o'clock, the consumption pattern, and around the 22nd to 24th hours, major changes are made from this pattern. The maximum of these tolerances in the fi rst interval (HR= [10][11][12]) between the coldest and warmest network temperature is equal to 32%. This Identifying changes in network water quality and controlling the minimum and maximum values of the remaining free chlorine standard is of great importance. As shown in Figure  9, the maximum percentage difference between the residual chlorine concentration (between the hottest and coldest temperatures of the year) with 48% is related to nodes 10, 28, 33, 34, 35, and 36.

Conclusion
One of the main tasks of the quality management system of water supply networks is to control the error of chlorine consumption within the standard limits. Therefore, qualitativehydraulic simulation programs such as EPANET (II) help to predict and evaluate the amount of residual chlorine in the network before designing or laboratory sampling. Chlorine decomposes as an oxidizing chemical by reacting with the pipe wall of the distribution line or the material inside the matrix of water entering the network. Wall decay is a function of pipe