1. Introduction
Water resources and energy are resources that are essential for maintaining a productive human society [
1,
2,
3,
4,
5]. Since water resources and energy are interdependent and closely related through processes such as material exchange, energy flow and information interaction, it is recognized that integrated management of water resources and energy is required [
6,
7,
8,
9,
10,
11]. The five stages of the water life cycle, namely extraction, purification, transportation, utilization, treatment and discharge, all consume energy. The amount of energy consumed by each stage of the life cycle varies due to their varying economic, technological and environmental conditions, which is an important component of the theory of the water–energy nexus: if there is a deficit of energy required for any stage of the water life cycle, the amount of water supply or consumption will in turn be affected [
12,
13,
14,
15]. Given the current context of global population growth, dwindling resources and deterioration of the ecological environment, the exploration of the quantitative relationships between water resources and energy resources is of great significance. An improved understanding of these relationships can help identify an effective method of improving energy efficiency of water resources to achieve sustainable development [
16,
17,
18,
19,
20,
21,
22].
Although water resources and energy systems are closely related, the processes involved remain incompletely understood due to the high diversity and complexity of water resources types, flow processes and consumption sectors. Therefore, past studies on the relationships between water and energy have mainly used a simple index method to estimate unit electricity consumption or fuel consumption of water resources use [
23], which may not be accurate. A variety of methods have been applied to study the energy demand of different water resource types. Spang et al. [
24] proposed the Energy Intensity (EI) model in a study which used monitoring data to calculate the electricity consumed by the water supply service system of a municipal area in California during the processes of water extraction, transmission and utilization. The National Technical University of Athens developed the Urban Water Optioneering Tool (UWOT) [
25] based on a genetic algorithm, which was used to calculate the energy demand of urban water supply equipment and evaluate measures to reduce drinking water demand and the impact of the urban heat island effect. Baki et al. [
25] used the UWOT model to calculate the energy consumption of the Athens urban water supply system and discussed the potential for the use of this tool in the strategic planning for urban sustainable development and for efficient management of water resources and energy. The Water-Energy Sustainability Tool Web (WESTWeb) was developed by the University of California based on the life cycle theory. The tool has since been used to calculate the energy consumption and air pollutant emissions of a water supply systems in California [
26] and Nova Scotia [
27] in the United States, where it was found that the production of 1 million liters of water consumes ~5.4 GJ of energy.
While it is evident that many past studies focused on the energy consumption of water resources use, the majority were not comprehensive or focused only on a single type of water resource or on a single stage of the water resources life cycle. For example, Spang [
24] and Baki [
25] in a study of the energy consumption of urban water systems did not consider that of water supply and water use within farmland irrigation. In addition, while Stokes [
26] analyzed the energy consumption of water resources extraction, purification and transportation in California, the energy consumption of water utilization was ignored.
The present study proposed a model to comprehensively calculate the energy demand of water resources use for a specific region based on the life cycle methodology. Energy consumption of each stage of the water resources life cycle was analyzed by focusing on the physical flow of water, and equations for quantifying the energy consumption of each stage were given. Considering the availability of data, taking the city of Ordos in Northwest China as an example, the energy consumption of water resources was calculated for the period 2013 to 2017, and the trends in energy consumption over time were analyzed and summarized, thereby providing a reference for improving the energy efficiency of water resources and furthering understanding of the water-energy nexus.
2. Data and Methods
2.1. Study Area
The city of Ordos is located in the southwest region of the Inner Mongolia Autonomous Region, China, covers an area of 87,000 km2 and has a permanent population of 2.0687 million people. Ordos is an important economic center and a key energy development area at a national level. The per capita gross domestic product (GDP) of Ordos in 2017 was 235,200 yuan, the highest in China. Ordos has a typical temperate continental arid and windy climate with low precipitation that is concentrated in summer. At present, the total natural water resources in Ordos is estimated to be 2.96 × 108 m3, of which surface water and groundwater resources are 1.31 × 108 m3 and 2.10 × 108 m3, respectively (the repeated calculation amount of surface water and groundwater is 0.45 × 108 m3).
2.2. Data
The available data extending from 2013 to 2017 were used to calculate the energy consumption of the entire life cycle process of water resources in Ordos. The data were mainly sourced from the Ordos Water Resources Bulletin (2013–2017) [
28], China Urban Water Supply Statistical Yearbook [
29], China Urban Drainage Statistical Yearbook [
30] and existing research results.
2.3. Methods
The concept of a life cycle was originally conceived within the biological sciences and referred to the developmental stages an organism experiences over its entire lifespan, including birth, development, maturity, aging and death. Later, the concept of a life cycle was introduced into social and economic sciences to describe the life cycle process of resources and products. Within this context, a life cycle can be understood as the sum of all stages of an object’s lifespan while being exploited for human benefit.
It should be noted that the water resources life cycle in the context of the present study has a different meaning to the natural water cycle (hydrological cycle). The water cycle refers to a continuous natural process by which water is circulated throughout the earth and the atmosphere through evaporation, condensation and precipitation. The natural water cycle is specifically manifested in different natural forms of water such as surface water, groundwater, soil water and atmospheric water, and these forms remain in an equilibrium and are not greatly changed by the influence of external factors. The life cycle of water resources in the context of the present study includes all aspects of human exploitation of water, and include water extraction, storage, purification, transportation, utilization, consumption, discharge back to nature and sewage reuse. The sum of these aspects of human exploitation can be regarded as a water resource life cycle from ‘‘birth to death”. The life cycle of water resources can also be called the “urban water cycle”. In comparison with the natural water cycle, the life cycle of urban water resources is a complex system incorporating societal, economic and environmental aspects, which include all interactions with energy during exploitation for human benefit. The life cycle of water resources starts from the extraction of water from a compartment of the natural water cycle (such as surface or groundwater) and ends with discharge or evaporation of water back into the natural water cycle or through recycling through advanced treatment [
31,
32,
33,
34,
35,
36,
37].
Based on this concept of a life cycle methodology and the physical flow process of water resources, the entire life cycle of water resources can be categorized into five stages: (1) extraction, (2) purification, (3) transportation, (4) utilization and (5) sewage treatment. The present study calculated the energy consumption of each stage in the life cycle of water resources from 2013 to 2017, using the city of Ordos as an example. It should be specially pointed out here that the energy consumption in the process of urban water cycle we studied refers to the energy directly consumed in the process of urban water resources recycling, such as electricity for pumping, heating and so on, but does not include some indirect energy consumption in the process of urban water cycle, such as various chemicals used in the process of wastewater treatment.
2.3.1. Energy Consumption of the Water Extraction Stage
The different types of exploitable water resources include surface water, groundwater, rainwater, desalinated seawater and reclaimed water. The energy consumed during the reclaiming of water was calculated during the sewage treatment stage. The energy consumed during rainwater harvesting and desalination mainly relates to the power consumed by relevant equipment. Since there was no rainwater harvesting or desalination in Ordos over the period 2013–2017, these two categories of water extraction could be ignored in the present analysis.
2.3.2. Energy Consumed during the Water Purification and Transportation Stages
The water purification process involves a series of operations implemented within a water treatment plant, including sedimentation filtration, sterilization and disinfection, and consumption of energy during water purification is mainly due to electric energy consumed by the operation of an electric mixer and sedimentation tank. The water transportation stage involves the conveyance of purified water to end users through the water reticulation system, with energy consumed mainly related to the electric energy required for the operation of a water tower, clean water tank, pump station and other equipment used in the water reticulation system. Since it is difficult to calculate the specific energy consumption of each process during the purification and transportation stages, energy consumption was calculated by multiplying volumes of water purification and transportation by the corresponding unit power consumptions. The data required for the calculation were sourced from the China Urban Water Supply Statistical Yearbook (2013–2017) [
29].
2.3.3. Energy Consumed during the Water Utilization Stage
The main uses of water resources can be broadly divided into four categories: (1) domestic water, (2) industrial water, (3) agricultural water and (4) ecological water. Among the categories, agricultural water mainly relates to water used for irrigation and rural livestock watering. The energy consumed during the extraction of surface water and groundwater was calculated during the water resource extraction process. Ecological water refers to the water resources used to maintain the normal development and relative stability of various ecological systems within a specific time and space, such as water for greening, water for rivers and lakes, etc. Since ecological water does not directly benefit the domestic and economic sectors and the energy consumed is negligible, the current study ignored the energy consumption of ecological water.
Energy Consumed during the Process of Domestic Water Use
Domestic water includes household water and public water. Energy consumed during the use of household water mainly includes heating energy consumed for domestic drinking water and bathing as well as mechanical energy consumed by domestic appliances such as washing machines and other equipment. Public domestic water includes drinking water, cleaning water and other basic domestic water and infrastructure water, and the energy consumed during the use of public water mainly relates to the conversion of electric energy into thermal energy or mechanical energy. Jiangshan [
40] found that the unit energy consumption of domestic water and public domestic water in Ordos is 12.62 kWh·m
−3 and 11.6 kWh·m
−3, respectively.
Energy Consumed during the Process of Industrial Water Use
Industrial water consumption can be divided into three broad categories, namely, (1) thermal power generation, (2) general industrial production and (3) domestic water consumption of employees working at a factory. The energy consumed through thermal power generation includes that for the heating of water in a boiler and for circulating cooling water. The energy consumed through general industrial production is mainly related to the circulating cooling system. Domestic water consumption by factory employees usually accounts for 5–10% of total industrial water consumption, was taken as 5% in the current study [
40] and was calculated in a similar way to that for public domestic water consumption.
The equations used to calculate the energy consumed during industrial water consumption were as follows:
In Equations (6)–(9),
,
and
are the energy amounts consumed through the use of water for thermal power generation, general industrial production and domestic consumption by factory employees, respectively (kWh);
is the weight of boiler heating water (kg); Δ
t is the heating temperature (generally, water at 25 °C is heated to 400 °C; therefore, Δ
t = 375 °C);
c is the specific heat capacity of water (c = 4.2 kJ·kg
−1·°C
−1);
is the water consumption efficiency of boiler heating (
β4 = 75%) [
39];
D is thermal power generation (kWh);
α is the power consumption rate of the water pump in the circulating cooling system (
α = 1.55%) [
42];
is the quantity of water recycled in general industry (m
3);
is the power consumption of circulating 1 m
3 water (
= 6.4 kWh·m
−3 [
40];
is the operating efficiency of the circulating cooling system (
β5 = 50%) [
40]; and
is the total amount of industrial water (m
3).
2.3.4. Energy Consumed during Sewage Treatment
Wastewater treatment involves the collection and treatment of wastewater. Sewage treatment plants (STPs) can be broadly categorized into ordinary STPs and STPs that reclaim water. Wastewater includes domestic sewage, rainwater in contact with streets or highways and water contaminated by industry. There are various forms of wastewater treatment technology, including physical treatment, chemical treatment and biological treatment. The energy consumption intensity of each wastewater treatment technology is different. The sum of power consumption of all sewage treatment plants is taken as the total energy consumption of sewage treatment stage. The calculation of energy consumed during the sewage treatment process is similar to that for the purification process, and energy consumed is calculated by multiplying the quantity of sewage treated or water reclaimed by the corresponding unit power consumption. The data required for this analysis were obtained from the China Urban Drainage Statistical Yearbook (2013–2017) [
30].
4. Conclusions
The present study proposed a method to calculate energy consumption during the entire water resources life cycle, which addressed certain shortcomings of existing methods that focus on only one or several stages of the water resources life cycle. Year-by-year changes in energy consumed by each stage of the water resources life cycle were analyzed with the aim of reducing energy consumption through the use of water resources and of improving the efficiency of energy use. This method could be used to analyze and evaluate the historical, current and future trends in the energy consumption of water resources, can further enrich the basic theory of the water-energy nexus and can provide a basis for formulating water and energy planning strategies.
Year-by-year total energy consumption of the entire life cycle of water and the energy consumption of the per unit water over the period 2013–2017 in Ordos showed a decline, with a generally consistent downward trend. Energy consumed during the water utilization stage accounted for the largest proportion of total energy consumed, accounting for ~95% every year. Energy consumed by the extraction of water was the second highest and increased from 1.94% to 3.12%. The energy consumption values of other stages accounted for less than 1% of total energy consumed. It was found that energy consumed during the process of boiler water heating in the water consumption stage was the largest among any of the studied stages and that total energy consumption over the entire water resources life cycle can be reduced by ~0.8% by improving the efficiency of boiler water heating by 1%. Therefore, improving the efficiency of boiler water heating water can be an effective approach to improving energy consumption efficiency and reducing energy consumption over the entire water resources life cycle.
Potential exists for extending the present study by incorporating a comparison of the results of this paper that used the water resources life cycle methodology with results of studies which employed other methods. The focus of future research can be taken into account the impact of environmental factors. For example, life cycle methodology can be used to calculate the emissions of environmental pollutants in each stage of the life cycle of urban water resources, and probability estimation methodology can be used to study how to save the energy consumption of water resources as much as possible under the premise of ensuring the environmental sustainable development.