This work elaborated an analysis of urban water metabolic system. The system’s flows and processes were modeled and accounted on the basis of the ecosystem cumulative energy availability, also known as emergy. In detail, both the urban domestic water supplying process metabolism model and accounting framework were defined. Then, the whole process of the supplying of domestic water was analyzed, considering Beijing (China) as a case study. In particular, the existing water sources were included: surface water, underground water, water of the South-to-North Water Transfer Project; potential desalinated water from Tianjin. The results showed that, for the supply of 1 m3 of tap water, the total emergy input from the above-mentioned four sources are 3.22Eþ۱۲, ۳٫۳۴Eþ۱۲, ۴٫۵۵Eþ۱۲, and 12.55Eþ۱۲ sej. These values reflect the different energy costs of the existing supply systems, that are related to water transportation, treatment and distribution, Moreover, the emergy cost of desalinated water is about 4 times higher than the one of surface water. Conversely, the value of South-to-North Water Transfer Project is not much higher than that of surface water. Finally, the higher costs are related to the water treatment phase. Consequently, some policy recommendations and future research directions are identified for improving the sustainability for Beijing domestic water supply.

Introduction

Finite water resources, increasing demands and aging water infrastructures are some of the greatest challenges for China and many other regions of the world. The rapid increase in water demand and the reduction of the fresh water supply, resulting in water shortages, are now a serious problem in many countries (Wang et al., 2016). The United Nations Educational, Scientific, and Cultural Organization (UNESCO) predicts that global water demand will increase by 44% in 2050, with residential water growing nearly 1.5-fold (UNESCO, 2014). Without a constant supply of water, human society cannot smoothly and continuously develop (Chen et al., 2016). Urban areas are especially vulnerable to these problems, due to their higher population density. This is why, nowadays, an accurate planning for a sustainable use of water resources is of paramount importance. With this respect, complex water issues cannot be solved applying a chambered water management approach, especially at the urban scale. For example, Hu et al. (2013) showed that the majority of the present water consumption in Beijing was due to family use, i.e., domestic water. Before the operation of the South-North Water Transfer Project (SNWTP) in 2014, Beijing’s water mainly came from the local surface water and groundwater. However, this limited supply couldn’t meet the growing demands. As an “alternative source” of traditional surface water and groundwater, SNWTP greatly alleviated the pressures on the Beijing water supply. In a water-connected world, sustainable solutions would require a system-based approach. In particular, water services (traditionally: wastewater, stormwater, and drinking water) should be integrated with the effort of maximizing the recovery of resources (i.e.: energy, nutrients, materials, and, obviously, water). The United States Environmental Protection Agency (US EPA) Safe and Sustainable Water Resources (SSWR) research program represented an example of a holistic approach to water resources management.