Scenario analysis of energy and water trade-offs in the expansion of a dual water system

Using treated wastewater effluent (reclaimed water) for beneficial purposes can be a sustainable practice that reduces demand on potable networks. However, implementing reclaimed water networks can have unintended effects, specifically unintended increases in energy consumption. This case study employs multiperiod scenario analysis to examine energy consumption associated with the potable and reclaimed water systems for the Town of Cary, North Carolina. Using hydraulic planning models of both systems provided by the design engineers, the conveyance and additional treatment energy is tabulated. This method considers uncertainty in reclaimed water demand by varying the expected demand for each build out of the reclaimed water network. Differential electricity consumption is calculated as the difference between the electricity consumed to deliver reclaimed water through a secondary network compared to the electricity consumed to deliver the same volume through the potable water network. Demand uncertainty, in conjunction with spatial growth, is found to have large impacts on differential electricity consumption. Because of the high quality of wastewater effluent, no additional energy is required for treatment, causing the reclaimed water network to consume less energy than the business-as-usual scenario, where the demands are supplied via the potable network. The differential electricity consumption decreases with network expansion because the reclaimed water system becomes less energy efficient per unit volume with increasing flow rate, while the potable water system energy efficiency remains fairly constant. Understanding the trade-offs between water and energy when planning reclaimed water networks is important for sustainable resource management within the built environment.