**1. Introduction**

Increasing water scarcity has become a global issue. Freshwater supply is limited and has been remarkably affected by the degradation of water quality in natural water bodies, while the demand for freshwater has continued to increase. Besides water consumption minimization by improving water use efficiency, conventional water treatment and desalination are employed to reclaim the polluted water and freshwater to increase the supply. Especially in water-scarce regions, where the water source is mainly from precipitation, the water supply has been unreliable due to the influence of global climate change [1]. Water desalination has been widely applied in the world. A report from the Water Desalination Report by the International Desalination Association presented the current installed capacities of the world desalination by countries [2]. Figure 1 shows the water desalination capacities

of the world by countries from 2010 to 2016 [2]. In the last six years, the world total water desalination capacity, including brackish water and seawater desalination, increased steadily with an annual rate of about 9%. A large proportion came from the Gulf region, and not surprisingly, the Kingdom of Saudi Arabia (KSA) and the United Arab Emirates (UAE), which take a proportion of 15% and 11% of the world's total desalination capacities in 2016. Next is the USA, which takes 10% of the world total installed desalination capacity. China has the fourth largest water desalination capacity, with a share of 5% of the world total installed capacity.

**Figure 1.** Water desalination capacities of the world and selected countries from 2010 to 2016 (derived from [2]). KSA: Kingdom of Saudi Arabia; UAE: United Arab Emirates.

Water desalination is an energy intensive approach for freshwater production [3], and the rapid increase of installed capacity has resulted in increasing resource (mainly energy) consumption and environmental impacts. Based on the water desalination capacities and the energy consumption factor provided by [4], the energy consumption of the world overall water desalination is estimated and as shown in Figure 2.

The environmental impact of water desalination has been focused on theoretical and scenario analyses [5]. Cornejo et al. [6] found that reverse osmosis (RO) technologies have lower GHG emissions than thermal desalination technologies. The estimated GHG emissions footprint of seawater RO desalination (0.4–6.7 kg CO2eq/m3) is generally larger than brackish water RO desalination (0.4–2.5 kg CO2eq/m3) and water reuse systems (0.1–2.4 kg CO2eq/m3). Shrestha et al. [7] determined that the associated CO2 emissions for seawater desalination (0.25 Mt/y) are 47.5 % higher than that for water conveyance (0.17 Mt/y). The GHG footprint values vary due to the variability of location, technologies, life cycle stages, parameters considered, etc.

For producing freshwater, seawater desalination has been strongly implemented in the Gulf region and is emerging in East Asia, where are facing serious water stress issues. China has the fourth largest capacity of seawater desalination in the world, and water desalination is still an emerging industry. The major driving force is increasing water shortage. China is becoming one of the countries with a severe water shortage especially in the most developed northeast region [8]. For example, in 2016, the average water resource per capita was 2,355 m<sup>3</sup> [9], which is about 40% of the world average value [10]. For the capital city of China, Beijing, the amount is 162 m<sup>3</sup> [11], which is less than 3% of the world average value. The population and urban land in these water stress areas are still increasing [8], which indicates an increasing demand for freshwater. One fact is that about 71% of the Earth's surface is covered by water, and the oceans hold about 96.5% of all Earth's water [12]. Consequently, the water shortage is a shortage of clean freshwater, which will lead to an increase in economic cost and resource consumption, as well as potential environmental impacts.

Facing the water shortage issue, China is making a major effort with increasing the water use efficiency and eliminating water waste. On the other hand, for the regions with severe water shortage, there are mainly two possibilities to increase the amount of available freshwater. One solution is water transfer projects, including the South–North Water Transfer Project and the Water Transfer from Yellow River to Qingdao Project [13]. These projects are carried out by constructing water channels to transfer freshwater from water-rich areas to water-scarce regions, mainly Beijing, Hebei, Henan, and Shandong. Another action is the promotion of seawater desalination techniques and projects. From 2006 to 2016, the installed seawater desalination has increased from 20 × 10<sup>6</sup> m3/y to 390 × 10<sup>6</sup> m3/y [14]. Until 2016, there have been 15 newly released standards by the governmen<sup>t</sup> to facilitate the promotion and managemen<sup>t</sup> of water desalination projects [14]. Seawater desalination has been considered as one of the most promising techniques due to the abundance of seawater and the improving operating efficiency [15]. With an increased capacity, the potential of resource consumption and environmental impacts are also concerned.

The majority of studies have focused on either the advancement of the desalination process or specific case plants. Sores et al. [16] proposed and tested a novel supercharger which can be applied to a seawater desalination RO system and found that the efficiency of the tidal supercharger is currently lower than 20%, although the efficiency increased from 12% to 14%, with the seawater flow rate increasing from 290 m3/h to 440 m3/h. The application of renewable energies is also discussed in the current studies. For instance, Zuo et al. [17] proposed a model of a wind supercharged solar chimney power plant combined with seawater desalination and claimed that the utilization of solar energy can be raised by 70% with integration. Li et al. developed a high-efficiency membrane for seawater desalination using solar energy [18], and the results showed a 90% efficiency of converting solar energy. Desalination plants in China have seldom been discussed. Liu et al. [19] carried out the systems process analysis of the freshwater cost of seawater desalination, with a case study of a 25,000 m3/d seawater desalination plant in Huanghua Port, Hebei Province, China. The study determined that the freshwater consumption of the plant is 4.5 × 10<sup>5</sup> m3/y, which is 5% of the annual freshwater production (9.2 × 10<sup>6</sup> m3/y). The World Resources Institute [15] investigated the carbon footprint of different scenarios in Qingdao. It showed that in 2020 with a water desalination capacity of 400 × 10<sup>3</sup> m3/d, the carbon footprint of the water desalination will be 541.31 kt CO2eq/y (with a cost of 8 CNY/m3), which is 1.81 times more expensive than water supplies from surface and groundwater (with a cost of 1.17 CNY/m3). Other studies also reported that water desalination with various techniques has other atmospheric emissions, e.g. dust, NOx, and SOx [20].

Most of the literature have focused on either the advancement of the desalination process or the specific case plants. The overall picture of the development and environmental performance, as well as the cost of seawater desalination in China, has not been thoroughly discussed. There is an urgen<sup>t</sup> need to analyze the current development of the seawater desalination in China and to benchmark the energy consumption, emissions, as well as the cost. This can provide an overall picture of the environmental and economic performance, and facilitate energy consumption minimization, GHG

reduction, and efficiency improvement in seawater desalination implementations. The aim of this paper is to provide fundamental remarks for the further studies of the water-energy nexus of seawater desalination. In order to do this, the paper first overviewed the current development and processes of seawater desalination projects in China. Then, the energy demand, GHG emissions, as well as the unit product cost of the seawater desalination plants were estimated. Based on the calculation, the future development and promising directions of water desalination studies are discussed.

#### **2. Seawater Desalination in China**

With the promotion and development of water desalination projects and more advanced technology, the total capacity of seawater desalination plants increased from 20 × 10<sup>6</sup> m3/y to 390 × 10<sup>6</sup> m3/y from 2006 to 2016, which can be seen from Figure 3 [14]. Due to the introduction of relevant policies and standards by the government, seawater desalination has been one of the promising solutions to the water shortage in China.

**Figure 3.** Total seawater desalination plant capacities in China from 2006 to 2016 (Based on [14]).

According to the State Oceanic Administration of China [14], there have been 131 seawater desalination plants/projects up until 2016 in mainly coastal cities in China, and the total capacity has reached to 1.19 × 10<sup>6</sup> m3/d. The plants with different capacities are evenly distributed.

As shown in Table 1, there are 36 plants/projects with a capacity larger than 10 km3/d, with a total capacity of 10<sup>6</sup> m3/d. 38 plants have a capacity of 10<sup>3</sup> m3/d to 10<sup>4</sup> m3/d, with a total capacity of 1.2 × 10<sup>5</sup> m3/d. Another 57 smaller plants/projects have a total capacity of 1.1 × 10<sup>4</sup> m3/d. The largest water desalination plant in China is in Tianjin, with a total capacity of 2 × 10<sup>5</sup> m3/d.

**Table 1.** Water desalination projects development in China until 2016 (Based on [14]).


Regionally, large-scale seawater desalination plants are located on the northeastern coast of China with relatively severe water scarcity issues, e.g. Tianjin, Shandong, and Hebei. Figure 4 shows the seawater desalination capacities of the plants by desalination process, as well as the estimated

water stress index of the provinces with seawater desalination plants. Tianjin, as a coastal city next to Beijing (the capital of China), has the largest total capacity of 317 × 10<sup>3</sup> m3/d, with RO and multiple-effect distillation (MED) being the main processes, multi-stage flash (MSF) and electrodialysis (ED) are less applied in China. The Tianjin Beijing seawater desalination plant, with a capacity of 200 × 10<sup>3</sup> m3/d, is the largest seawater desalination plant in China. Tianjin also has the largest installed seawater desalination plants with MED process. Shandong has the second largest seawater desalination capacity (282 × 10<sup>3</sup> m3/d). The largest water desalination plant in Shandong is located in Qingdao, an international city facing severe water shortage issues, with a capacity of 10<sup>5</sup> m3/d. The total capacity of water desalination plants in Qingdao reached 235× 10<sup>3</sup> m3/d in 2016, which takes 83% of the total capacity of Shandong province [21]. One large-scale plant in southern China is in Zhejiang Province, with a capacity of 228 × 10<sup>3</sup> m3/d. Other seawater desalination plants in southern China (e.g. Guangdong, Fujian, Jiangsu, and Hainan) are mainly small plants. Most of the plants are built after 2009, and the main processes applied are reverse osmosis (RO, with a proportion of 86%) and MED (with a proportion of 12%). Other technologies such as MSF and ED are also applied in few plants, with a total ratio of 2%.

**Figure 4.** Water desalination capacity by provinces (derived from [14]). WSI: water stress index, RO: reverse osmosis; MED: multiple-effect distillation; MSF: multi-stage flash; ED: electrodialysis.

Figure 4 also indicates that the provinces/cities with large seawater desalination plants also have a higher water stress index (WSI), which is the ratio of the annual water consumption and the available natural water resources [22]. A higher WSI indicates that consumption is closer to the available water resources, thus higher water stress. When the value is higher than 1, it means the water consumption of the region has exceeded the available freshwater resources, and the region has a high water stress level. The WSI of the selected provinces are estimated with the method of [22] based on the data from [9], and the values are shown in Figure 4. The green dash line is the reference WSI with a value of 1. For most of the selected provinces, provinces with higher water stress have higher seawater desalination capacity, which indicates the driving factors of the development of seawater desalination. Two exceptions are Zhejiang and Jiangsu. Zhejiang has the third largest seawater desalination capacity, but the water stress is rather small. Jiangsu has a very small seawater desalination capacity but with relatively high water stress.

In terms of water use, as shown in Figure 5, a major proportion (66.6%) of desalted water is used in industries, and followed by domestic use with a ratio of 33.1%, and a small fraction (0.3%) is used for other purposes, e.g. watering in parks and greenbelts. Within industry, main users of desalinated water are fossil fuel power plants (31.6% of total), steel making industries (13.1%), and petrol chemical industries (12.3% of total). The major use in industries indicates that the quality of the desalted seawater needs to meet the quality requirement for industrial use.

**Figure 5.** Distribution of desalinated water use in 2016 (Based on [14]).

Water treatment, seawater desalination, and water resource transfer are the main solutions for the freshwater shortage in China. Wastewater treatment, as the most conventional approach, has been well developed and implemented. The wastewater treatment approach can only offset part of the consumed water by removing the contaminants from the polluted water and return the treated water back to the available natural resource, but usually, the water quality of the discharged water is not as high as previous. When there is not enough available natural water resource, the cycle of supply- use-treatment-return cycle would be difficult to maintain. Water transfer is usually a huge project that has considerable economic and ecological cost, with large-scale changes to the inhabitants along the channel. Due to this reason, the existing water transfer projects in China are still under controversial discussion [23]. Seawater desalination is considered as one of the most promising approaches to produce freshwater.

Although seawater desalination is a reliable water supply and is not vulnerable to climate change, it consumes a lot of energy and it requires a lot of investment and public acceptance. The increasing capacity and wide distribution in different regions also result in the issue of increasing energy consumption, GHG emissions, as well as the economic cost. It is important to analyze and benchmark the environmental and economic performance, in order to provide insightful data for the further planning and optimization of energy use and emission reduction in seawater desalination.
