Freshwater accounts for only 2.5% of the earth’s total water resources, whereas seawater accounts for 97.5% [1
]. Seawater desalination is an important measure for alleviating the water shortage crisis. It is a realistic choice to resolve the water shortage problem by developing technologies for desalination and comprehensive utilization in coastal areas [2
]. The study of seawater utilization has three key areas. One of these areas is desalination itself, which means using desalination methods to produce Freshwater [3
]. The second area is the direct use of seawater, which refers to employing seawater instead of Freshwater as industrial cooling water, domestic water [5
]. The third area is the comprehensive utilization of seawater chemical resources; namely, extracting chemical elements from seawater, and deep processing [6
By the end of 2015, more than 150 countries and regions around the world had instituted desalination [7
]. The total desalination capacity is currently approximately 8.89 × 107
]. The top five countries in terms of desalination capacity are KSA (Kingdom of Saudi Arabia), UAE (The United Arab Emirates), USA, China, Israel [9
]. In particular, in the Middle East, seawater desalination is a vital and dependable source of Freshwater in countries such as Saudi Arabia, the United Arab Emirates, and Kuwait [10
]. Furthermore, it is likely that desalination will continue to grow in popularity in the Middle East [11
]. Overall, it is estimated that over 2 × 108
people worldwide obtain Freshwater by desalinating seawater or brackish water [12
]. Saudi Arabia is the world’s largest user of desalination and it has a capacity of 5.25 × 106
/day, which accounts for approximately 25.9% of the world’s total seawater desalination [13
]. Saudi Arabia has the world’s largest MSF (multi-stage flash distillation)/RO (reverse osmosis) desalination plant, the “Ras Al Khair factory” [14
]; the world’s largest MED (low-temperature multi-effect distillation) desalination plant, the “Marafiq Jubail IWPP project” [16
]; the world’s largest single set of MED desalination devices, the “Shoaiba II project” [17
]; the world’s largest MSF seawater desalination plant, the “Shoaiba III factory”; and the world’s largest solar desalination plant. As the second largest user of desalination in the world, the United Arab Emirates has built more than 30 sets of desalination plants with an annual production capacity as high as 3 × 108
]. Eighty per cent of Israel’s potable water is produced by desalination as of 2017 [15
]. The Tathagata Centre in Fukuoka is Japan’s biggest water desalination facility, and the desalinated water it produces accounts for approximately 1/12 of the country’s total water supply [18
]. In addition, all capital cities in Australia have desalination plants; however, they are not being regularly used due to the end of a drought. The exception is Perth where the desalination plants are regularly used [19
The Freshwater resources per capita reflect a shortage in China [20
]. The coastal areas in China are economically developed, however, these areas have large populations [20
]. This leads to a low water resource amount per capita in these areas, limiting the development of social and economic conditions [21
]. By the end of 2014, the 11 coastal provinces of China (including autonomous regions and directly controlled municipalities, the geographic locations of the areas could be found in Figure 1
a) accounted for 30.2% of the country’s Freshwater resources, fed approximately 43.4% of the population, and generated 54.7% of the GDP [22
]. The recent distribution of regional water resources, population and GDP in 2017 is shown in Figure 1
b–d. The contradiction between domestic, agricultural, and industrial water consumption and water in the environment is obvious. Excessive exploitation of groundwater still exists, which leads to problems, such as declines in water levels, surface subsidence, and deterioration of the ecological environment. The safety of the water supply cannot be guaranteed in an emergency due to the single available water resource. Therefore, the positive development and utilization of seawater can relieve water shortages in the coastal areas of China and can also improve the water environment and the security and reliability of the water supply. Moreover, islands can be exploited to develop the marine economy and strategy [23
The overall scale of built desalination projects has grown continuously in China. According to the China Desalination Yearbook 2016–2017 release by the Desalination Branch of China Water Enterprises Confederation, by the end of 2017, 136 desalination projects had been completed and water production had been scaled up to 1.19 × 106
]. At the same time, in nuclear and thermal power plants and in the petrochemical industries, seawater cooling technology had been used widely in the coastal areas [25
]. The annual amount of seawater used as cooling water had reached 1.1257 × 1011
]. The desalination technology adopted in China (a combination of imported technology and homemade technology) is becoming mature due to nearly 50 years of research, development and demonstration projects [27
]. This work has laid a good foundation for large-scale applications and has placed China at the forefront of advanced seawater desalination technologies, along with the USA, France, Japan, and Israel [10
]. Liu et al. [29
] compared Multiple Effect Distillation (MED) and Electrodialysis method (ED) and found that MED is competitive in the production of boiler make-up water for coal-fired power plants if the cost of steam generation is ignored, and Reverse Osmosis (RO) is favoured for providing municipal water for public use. Generally, the factor that limits seawater desalination applications is its relatively high cost [30
]. The discharge of concentrated seawater damages seashore environments [32
]. Nie and Tao [33
] simulated the impact of a seawater desalination system with a hydraulic and water quality model and found that the area that reflects increased salinity after a 10-day continuous discharge is four times the one that results from a 3-day continuous discharge. In addition, the waste heat discharged by the seawater desalination process could increase seawater temperature and result in harmful algal blooms [34
]. Chae et al. studied the mass reproduction of plankton caused by cooling seawater discharge, which in turn caused blockages and caused the power generation capacity of power plants to decrease [35
]. There was also a study to show that the environmental issues related to seawater intake have an adverse effect on the plankton population [36
]. For example, Jones and Campbell [36
] showed that direct intake of surface seawater is hampered by impingement and entrainment of planktonic organisms that require additional filtration and pretreatment.
In addition, some scholars have also studied future development trends, as well as the problems and management policies associated with desalination research, in China. Zhu et al. [38
] predicted that, in 2030, the total demand for desalinated seawater will be up to 1.44 × 109
/year in the coastal areas of China. This amount corresponds to 0.52% of the coastal water demand and 0.21% of the national total water demand. The quantity of seawater utilized directly will be 1.789 × 1011
/year. Chen et al. [39
] noted that problems still exist in the process of seawater desalination. For example, the supporting policies are poor, the independent industry is developing slowly, and the management system is inefficient. They also put forward several corresponding suggestions, including putting seawater utilization into the system of water resources allocation projects, supporting it as a public water conservancy engineering project, constructing a production-learning-research platform, setting up a special fund, and carrying out demonstration projects. Deng et al. [40
] discussed three situations combining seawater desalination and energy system utilization to solve the problems of the lack of freshwater and low efficient use of low-temperature thermal energy in coastland areas.
Zhu et al. [15
] summarized the status of seawater desalination in China in 2015. However, the current utilization status of desalinated seawater keeps unclear since the seawater desalination industry is developing rapidly in China and plays a more important role in relieving the water crisis especially in coastal areas of China. More importantly, the direct utilization issues of seawater and the utilization of seawater as chemical resources in China, which are processing fast and becoming more important, were not mentioned in the work of Zhu et al. [15
]. Besides, a deep discussion regarding the restrictive factors, which hinder the development of the seawater desalination industry in China and a detailed presentation of some niche targeting and practical countermeasures seem also insufficient in Zhu et al. [15
]. Therefore, this study introduces three aspects of the comprehensive utilization of seawater in China, including desalination, the direct use of seawater, and the use of seawater as a chemical resource. Based on this analysis of the comprehensive utilization of seawater in 2017, the study also presents the concept of optimizing the utilization of seawater from a technological perspective. Moreover, a deep exploration of the factors that restrict the development of seawater desalination and the potential countermeasures are presented in this study. This study could provide some reference for other countries facing similar water shortage state.
6. Concluding Remarks
This paper introduces comprehensive seawater utilization in China in three aspects, including the desalination of seawater, the direct use of seawater, and the use of seawater as a chemical resource.
First, the history of development and the status quo of desalination in China were analysed. Since the “7th Five-Year Period” (1986–1990), China has supported research into and development of related technologies. By the end of 2017, the available desalination capacity in China had reached 1.18 × 106 m3/day. China had built 136 desalination engineering projects. These projects are mainly concentrated in Tianjin, Shandong, Zhejiang, Hebei and Guangdong Provinces. The major technologies used to perform seawater desalination in China, reverse osmosis (RO), low-temperature multi-effect distillation (MED) and multi-stage flash distillation (MSF), have further broadened the desalination market. By the end of 2017, 117 plants employ RO technology to desalinize seawater, whereas 16 plants employ the MED technique.
Next, the direct utilization of seawater was discussed. By the end of 2017, the amount of seawater used for cooling water had reached 1.34 × 1011 m3. In 2017, the annual seawater utilization in Guangdong, Zhenjiang, Fujian, Liaoning Provinces were 4.18 × 1010 m3, 3.07 × 1010 m3, 2.26 × 1010 m3 and 0.92 × 1010 m3, respectively. Furthermore, by analysing the comprehensive utilization of seawater in China, the perspective of optimizing the utilization of seawater resources was presented to achieve the “minimum quantitative” discharge.
Finally, restrictive factors and potential countermeasures of the increased use of seawater desalination are investigated. Several specific recommendations are presented, specifically improving the laws, implementing regulations and standards related to desalination, strengthening the policies that support the enterprises that use desalination, improving the localization rate of key technologies and equipment gradually and devoting additional attention to the problems associated with brine processing.
The utilization of seawater resources is in the developmental stage in China, and there are differences in the utilization of water resources in different regions. Improving the relevant support policies for desalination is necessary. The preferential policies currently introduced have not been sufficiently supportive for desalination enterprises, and there is a lack of specific support, guidance and encouragement policies such as running water and South-to-North Water Transfer. The seawater desalination price mechanism is still unreasonable. At present, a benign water price mechanism has not been established, which has caused seawater desalination to remain at a disadvantage in terms of price. The strategic positioning of seawater use is not yet clear. In the Water Law of the People’s Republic of China (revised in July 2016), seawater was not included in the conventional water resources management system, and the corresponding preferential policies could not be obtained. In future law formulation, consideration should be given to increase the utilization of seawater. Compared with Freshwater resources, seawater does not have a prominent advantage in terms of price. Along with the severe shortage of water resources, long-term work is still required to reasonably formulate and improve regulations and standards, and to determine how to incorporate seawater into a regional unified allocation of water resources. These issues require further comprehensive study.