In this study a demand forecasting Optimal Life-cycle Energy Design (OLED) model is developed. The model enables the determination of life-cycle of existing Water Distribution Systems (WDSs) as well as optimal design of new WDSs. Harmony Search is used as the optimization algorithm with objective function minimizing energy expenditure per year. A WDS analysis program EPANET 2.0 is linked the optimal model for solving hydraulic constraints. Demand forecasting is also possible in the model to consider demand changes in the future. Results show that designing WDS with the developed model can minimize the environmental impact compared to some other conventional design methods