The effect of age and material on the deterioration of chlorine separation in the water supply network using the qualitative-hydraulic model EPANET (II)

The free chlorine remaining in the water supply networks guarantees the quality and health standards of drinking water. However, it should be noted that this factor does not exist in equal parts in all parts of the network and has fl uctuations. In some parts of the network, the amount of chlorine is too much and produces taste and smell, and in other parts of the network, the amount is less than the allowable limit, leading to an increased risk of epidemics of microbial diseases. The phenomenon of free chlorine reduction in water supply networks is referred to as chlorine decay. Several factors and reactions such as the presence of soluble and suspended organic matter, iron, manganese, and ammonia mineral compounds, as well as various microorganisms and algae compounds, lead to changes and reduction of free chlorine concentration in the water supply network. Chlorine degradation in two general cases, mass degradation, and wall degradation lead to a decrease in free chlorine concentration along with the water supply network. Mass decay is a function of the temperature and matrix of materials in the fl uid, while the deterioration of the wall depends on the age and material of the pipe. Qualls and Johnson [1] and Dominguez, et al. [2] investigated the reaction between chlorine and organic compounds in water. In this study, fl uoric acid was used for the experimental simulation of organic matter in water and groundwater, respectively. The results of the study by Qualls and Johnson [1] showed that the reaction lasts only fi ve minutes, which reacts quickly and then slowly in the fi rst minute. Hence, for the fi rst time, the rates of rapid and slow decay for the chlorine reaction were Abstract


Introduction
The free chlorine remaining in the water supply networks guarantees the quality and health standards of drinking water.
However, it should be noted that this factor does not exist in equal parts in all parts of the network and has fl uctuations.
In some parts of the network, the amount of chlorine is too much and produces taste and smell, and in other parts of the network, the amount is less than the allowable limit, leading to an increased risk of epidemics of microbial diseases. The phenomenon of free chlorine reduction in water supply networks is referred to as chlorine decay. Several factors and reactions such as the presence of soluble and suspended organic matter, iron, manganese, and ammonia mineral compounds, as well as various microorganisms and algae compounds, lead to changes and reduction of free chlorine concentration in the water supply network. Chlorine degradation in two general cases, mass degradation, and wall degradation lead to a decrease in free chlorine concentration along with the water supply network. Mass decay is a function of the temperature and matrix of materials in the fl uid, while the deterioration of the wall depends on the age and material of the pipe. Qualls and Johnson [1] and Dominguez, et al. [2] investigated the reaction between chlorine and organic compounds in water. In this study, fl uoric acid was used for the experimental simulation of organic matter in water and groundwater, respectively. The

The network understudy
The network required for the study in this study, the Brains-Polynes network with 36 nodes, a mixture, and a pumping station is described in Figure 1. It is worth mentioning that the maximum height code in this network is related to the mixture with a height of 70.5 meters and the minimum height is related to node number (1) with 15.01 meters.
In this part of the study, for each node, Demand Assignment operations have been performed 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 value of mass degradation coeffi cients is constant and is considered equal to zero, respectively. The consumption pattern of the mentioned network is also shown in Figure 4.

Qualitative-hydraulic simulation
The reaction between chlorine and the pipe wall of the distribution and water transmission network, which leads to a decrease in chlorine concentration, refers to wall decay. As can be seen in relation to number (1), there is a direct relationship between application level (surface to volume ratio), mass transfer coeffi cient (KW), and chlorine wall decay rate.
Many researchers [12,13] have emphasized that the degree of wall decay reaction is primary.
In relation to number (2) It is worth mentioning that the mentioned qualitativehydraulic simulations will be performed in the environment of the EPANET (II) model. To observe the changes in the concentration of free chlorine remaining in the time series, node (2) was selected as the control node. Also, the qualitative behavior of all nodes at peak consumption was examined.

Age and material of pipes
In this part of the study, the effect of age and material of distribution network pipes on chlorine wall decay will be evaluated. In the fi rst step, the analysis of the network with PVC and cast iron materials will be evaluated, the characteristics of their wall degradation coeffi cient are stated in Table 1.
In this section, the effect of different ages of cast iron pipes in the water supply network will be examined; The details of their coeffi cients are presented in Table 2.

Results and discussion
As shown in Figure 5, PVC pipes have less defi ciency than cast iron pipes. Statistical analysis shows that in the time series, you can have the maximum consumption pattern of this difference between cast iron and PVC in the control node (node number (2)) equal to 14.7%.
The values of the difference in the concentration of free chlorine remaining in the various nodes of the Brains-Polynes network (at peak consumption) are shown in Figure 6. The maximum difference values between cast iron and PVC are related to node number (22) with a value of 17.82%.
Then, the effect of age of cast iron pipes on the decay of chlorine wall was evaluated, which is described in Figure  7. Examination of the time series of consumption pattern in the control node showed that the maximum difference in free chlorine concentration between the newest and oldest cast iron pipe is 19.5%.    Also, the results showed the difference in the concentration of free chlorine remaining in different nodes (at peak consumption); In the case of maximum node number (22), there is a 34.71% distance between the newest and the oldest pipe. All these differences are presented as the concentration gap under the infl uence of tube age in Figure 8.
Comparison of the percentage difference between the age and material of the pipe shows that the age of the pipe has a greater effect on the decay of chlorine wall than its material. Some studies have recommended a specifi c type of pipe material. For example, in Learbuch, et al. [14] research, PVC has shown better performance than other pipe materials in the water distribution network. These studies have also been performed about other materials of pipes; for example, copper and composite plastic in a pilot-scale water distribution network has been examined, and chlorine has decreased the microbial numbers successfully in the water of composite plastic pipes [15,16]. When comparing to previous studies, the infl uence of pipe material has been tested on the occurrence and persistence of the target microorganisms that comparing iron, cement, and PVC pipe materials in different ages were fed either chlorinated water, and the infl uence of pipe material and age of pipes have become apparent to low disinfectant residual [5,17,[18][19][20][21][22][23][24][25].
All statistical evaluations are done by descriptive models.

Conclusion
The free chlorine remaining in the water supply networks ensures the microbial and to some extent chemical quality of the water. Therefore, predicting and investigating the factors affecting chlorine degradation is of special importance.
Chlorine mass decay occurs within the water matrix and is a function of temperature and constituents in the water. This is while the deterioration of the wall is a function of the age and