From energy balance to energy efficiency indicators including water losses

The collection and distribution of drinking water resources generally require large quantities of energy, that vary according to factors related to the characteristics of the served area, as well as to design and management choices. Energy intensity indicators (energy per unit of volume) are insufficient to assess the weight of different factors that affect the energy consumption and appear not suitable for the comparison of different water supply systems. The key step of this work is to define a methodology for assessing the energy efficiency of water supply systems. In particular, water losses in water distribution systems, generally assessed in relation to the quantity of high quality water dispersed in the environment, are herein considered in relation to their energy content. In addition to the evaluation of energy balance using the approach proposed by Enrique Cabrera et al. in ‘Energy audit of water networks’ (see J. Water Res. Plan. Manage.136 (6), 669–677) an overall efficiency indicator WSEE (Water Supply Energy Efficiency) is then proposed. Its decomposition finally leads to the definition of further indicators, which may help to assess how the structure of the network, leakage rate and/or pumps affect the energy efficiency of the water system. Such indicators can be used to compare different water supply systems and to identify the impact of individual interventions. The proposed energy analysis was applied to two case studies in Northern Italy.