## 1 Introduction  
One of the primary objectives for the sustainable development of today's societies is to guarantee access to drinking water for the entire population. Considering climate change and its consequences, the design of infrastructures, both current and future, must be as efficient and sustainable as possible. This means that they must be capable of meeting their intended service requirements with the lowest possible energy consumption, and at the lowest possible cost.  
In this sense, the ideal design for water distribution networks is one in which the water inside the entire system flows by gravity because it is not necessary to provide external energy so that the water reaches all supply points. This means avoiding the need to install pumping stations to increase the water head pressure, which implies significant energy and cost savings. In other words, the design of the infrastructure must be optimal in terms of construction and operation.  
The main objective of this study was to optimize the water distribution network of Cheongna International City in the Korea. For this purpose, the Harmony Search (HS) algorithm [1] was implemented to determine the optimal pipe diameter considering the required limitations on the head pressure in the nodes and the flow velocity in the pipe network. The pipe diameter was chosen to be the main element for optimization because of its high impact on water flow followed by the roughness coefficient and conduit length [2].  
In this manner, the aim of this optimization process was to reduce the pressure inside the water distribution network to avoid problems related to leakage and the quick deterioration of the elements of the system due to high pressure.  
The second part of this research was to find the critical conduits in the system by applying the same HS algorithm scheme using a different approach. This consists of using the \(C\) value of the roughness coefficient as the parameter to be optimized in the objective function, applied to the original water distribution network (WDN). Therefore, the outcomes of this approach provide the \(C\) value range that should be maintained in order to maintain the desired level of serviceability in the system. Consequently, conduits exhibiting higher values of \(C\) can be identified as the critical pipes that should be substituted in the first term as the WDN ages. Therefore, as the Cheongna International City WDN is formed by ductile iron pipes, whose \(C\) value can be assumed to decrease by a factor of ten every 10 years [3], the objective of this part of the study was to provide a useful tool for decision making in the assessment of deteriorated pipes.  
The implementation of the HS algorithm was carried out using the EPANET-MATLAB Toolkit [4], an open-source software tool that allows EPANET to interface with MATLAB. This tool allows the user to integrate the governing equations and functions of EPANET using different mathematical approaches for WDN optimization. Some of the different approaches used by other researchers include linear programming (LP) [5; 6], simulated annealing (SA) [7; 8], and genetic algorithms (GAs) [8; 9]. However, the HS algorithm was chosen for this study because of its relatively new meta-heuristic approach that encompasses convenient implementation and rapid convergence with reasonable computational cost [1; 10; 11; 12; 13].  
The structure of this paper begins with a brief review of the theoretical background needed to conduct this experiment (use of the HS algorithm and EPANET), followed by a description of the study area, discussion of the model construction, presentation and discussion of the results, and the conclusions and future work.