1. Introduction
Water scarcity can cause severe socio-economic consequences from local to global scales [
1,
2]. The World Economic Forum rated water crises as one of the major global risks over the next decade [
3]. Energy and chemical industry is one of the largest water consumers; it demonstrates high water sensitivity because each stage of the entire life cycle of its productions needs water (e.g., mining or extraction, processing and conversion) [
4]. The International Energy Agency projected a rise of 60% in global water consumption for primary energy production and power generation through 2040 [
5]. In China, the energy and chemical uses have dramatically increased in last decades due to rapid economic expansion. Consequently, China released the Energy Production and Consumption Revolution Strategy (2016–2030), which set up a series of targets for 2030 including the share of non-fossil fuel in the energy mix, and the nation’s energy self-sufficiency rate [
6]. However, coal-based energy will continue to form the major part of China’s energy mix over the next decade due to the low cost and the abundance of domestic reserves [
6].
There are two major problems exist in China’s fossil energy resources endowment. First, the energy structure of China is characterized by “rich coal, meager oil, and little gas”; the proven reserves are comprised of 94% coal, 5% crude oil, and 0.6% natural gas [
7]. China now has become the world’s largest and second largest importer of crude oil and natural gas, respectively [
8]. The dependence of imported energy and chemicals poses a great threat to China’s energy and chemicals supplies. Second, the distributions of coal and water resources are severely mismatched across the country’s territory. Nearly 70% of coal production is concentrated in the northern and western provinces, that only account for 6.5% of China’s total water resources, making water a significant vulnerability in the country’s energy and chemical supplies [
9]. To cope with these problems, China gave great priority to the development of 14 large coal energy bases and four large modern coal chemical industry bases during its 12th and 13th Five-Year periods (2011–2020). To reduce its dependence on foreign petroleum, China also made great efforts to develop technology to convert abundant coal into clean fuels and value-added chemicals [
10]. However, producing coal-based fuel and chemicals in these coal-rich water-limited energy bases has been controversial due to the high water-consuming processes. Large-scale water-intensive industrial production within an industrial base potentially threat the local environment. Thus, life cycle assessments related to water scarcity for the arid industrial bases in China is of great importance to achieve environmental sustainability.
Water footprint (
WF) can be used as an indicator of environmental sustainability in water use. The
WF concept was first introduced in 2002 by [
11]; it functions as a multidimensional indicator of freshwater use (i.e., blue
WF and green
WF) and pollution status (i.e., gray
WF) [
11,
12,
13]. The Water Footprint Network (
WFN) community considers
WF as a volumetric metric and focuses on the consumptive freshwater use. Simultaneously, the Life Cycle Assessment (LCA) community converts
WF into an environmental impacted-oriented indicator by a weighting scheme called characterization, which is recommended in ISO document 14046 on
WF [
14]. Over the past decade, researchers in the two communities have given rise to a debate over so-called better
WF accounting approach [
15,
16,
17,
18,
19,
20,
21]. Nevertheless, there is no contradiction in fundamental principles of the methods proposed by two sides; information provided by volumetric
WF and impacted-oriented
WF should be complementary rather than competing [
22], and the choice of the two
WF accounting depends on the purpose of a study.
Previously, the
WFN-
WF has been adopted in studies focusing on the optimal water resources allocation and productivity of freshwater use [
10,
23,
24], whereas LCA-based
WF accounting using input-output (IO) approach has been used in assessing the potential environmental impact of products [
4,
25,
26,
27,
28]. The IO framework are extensively used to estimate the
WF of industrial sectors at global scale (e.g., [
29,
30]), national or multiregional scale (e.g., [
31,
32,
33,
34,
35,
36]), and basin scale (e.g., [
37,
38,
39,
40]), but rarely used to assess the potential impact of the production of interdependent products at local or sub-local scales, because IO tables are compiled only at national or provincial levels due to cost and resource constraints. This study attempts to fill in this research gap by introducing a methodological framework for assessing the
WF sustainability of multiple interdependent products in a system. The Mixed-Unit Input-Output (MUIO) model is adopted in the framework to account
WF of the products, and three sustainability assessment indicators are then proposed. A large modern coal chemical industry base in Northwest China is used as a case study. Technical coefficient matrix and direct water consumption vector for the products in the study area were constructed based on a database, which was built by our research team through on-site survey and investigation. Since zero liquid regulation has been enacted in China’s major arid industrial bases, the assessment was conduct at product and regional levels with a focus merely on blue
WF.
5. Conclusions
This study attempts to fill in a current research gap by introducing a methodological framework for assessing the blue WF sustainability of multiple interdependent products in a system. The Mixed-Unit Input-Output (MUIO) model is adopted in the framework to account WF of the products, and three sustainability assessment indicators are then proposed. A large modern coal chemical industry base in Northwest China, in which 19 major coal-based energy and chemical products are produced is used as a case study. Technical coefficient matrix and direct water consumption vector for the MUIO model were constructed based on first-hand data collected by on-site field research in the study area, after which WF accounting and sustainability assessment were conducted at product and regional levels. The conclusions drawn from the proposed framework, as well as from the results and discussion of the real-world case are as follows: (1) although the top-down approach is usually applied to investigate the interdependent among industry sectors in terms of water consumption, our method has generalized it to calculate the blue WF of multiple interdependent products. The validation results indicate the good performance of the model. (2) Instead of using the IO tables that are directly compiled at national or provincial levels for regional and global scale WF analysis, the proposed method usually requires on-site data collection and computations, based on which the technical coefficient matrix and direct water consumption vector of the products are constructed. (3) In the case study, the blue WF of the coal-based products differ significantly. The standard deviation of the blue WF of the products in the study area is 14.83 m3/t, to which the indirect water contributes much more than direct water. (4) Generally, lowering the indirect water use is the key to WF reduction for the downstream products whereas lowering direct water use is more important for the upstream product WF reduction. (5) Although the blue WF of all products manufactured in the study area are sustainable at both product and regional levels, further WF reducing measure should be implemented for several major products such as CTL and MTO. (6) To enhance the blue WF sustainability, the ratio of nontraditional water resources to total water use should also be further increased. (7) The LCT should be adopted to provide a comprehensive approach in support of the overall reduction of environmental impacts in water resources utilization in China’s arid coal bases. (8) It is suggested to establish standardized national or regional WF benchmarks for the major products in the water-intensive coal-to-chemical industry.