Seawater desalination in China: an overview

China is facing increasing pressure on fresh water supplies (Yi et al. 2011). Its per capita water resource quantity was 2,354.9 m³ in 2016, approximately a quarter of the world's average level, and therefore it has been recognized as one of the 13 lowest water-availability countries throughout the world (Bai et al. 2007). Furthermore, the majority of the water resources is concentrated in southern China, leaving the northern and western regions to experience drought (Yi et al. 2011; Lyu et al. 2016). Some rivers, lakes and underground aquifers have dried up due to the overdraft. Some surface waters have been polluted so that they are no longer suitable for human use (Lyu et al. 2016). With population growth, accelerated urbanization and global climate change, shortages of water resources are becoming a key factor restricting economic development in China.

To address the serious disparity between water resource supply and demand, China has taken many measures to conserve and augment the limited water resources to meet the growing demands (Yi et al. 2011; Lyu et al. 2016). These measures include implementing a very stringent water management system that specifies water efficiency objectives (Wang et al. 2015), adjusting the inner structures of primary, second and tertiary industries for a series of water-saving targets (Chao et al. 2006), and implementing trans-basin water diversion projects across the country (such as the South-to-North Water Diversion Project) (Zhao et al. 2017).

In addition to regular water resources, China strives to develop alternative water resources to tackle water conflict, including wastewater recycling, household-level rainwater harvesting and seawater desalination (Zheng et al. 2014). Unlike long distance water transfer, which is uncertain and costly to develop, wastewater recycling has been considered a reliable and less costly water resource, but there are some concerns regarding environmental and health risks for wastewater recycling (Lyu et al. 2016). Household-level rainwater collection can provide only limited additional water resources because of the complex climate characteristics in China (Gu et al. 2015). With the rapid development of seawater desalination technology, desalinated seawater is playing a more important role in addressing the water shortage issue in China (Zhang et al. 2005; Zheng et al. 2014).

As an overview, 43% of China's population lives in its 11 coastal provinces with a 1,800 km coastline occupying 13.7% of China's territory (by the end of 2010) (Chinese Statistical Yearbook 2010), and the average water resources per capita in these 11 coastal provinces are only 1,915 m³ (in 2010), much less than the national level (Bulletin of Water Resources in China 2010). Furthermore, urbanization, economic development and industry are much more advanced in coastal areas compared with those in Chinese inland areas, leading to strong water resources pressure in coastal regions. From the Chinese Statistical Yearbook (2010), the Gross Domestic Product (GDP) was 24.6 trillion yuan in 2010 in coastal provinces, accounting for 61.3% of the national GDP. However, the total water resources in coastal provinces only comprise 28.6% of the total national water resources. The total water use in coastal areas is 248 trillion m³, accounting for approximately 41.2% of the total national water use. In coastal areas, 28.1% of the water resources have been used, while the national average water use efficiency is only 19.5% (Bulletin of Water Resources in China 2010). The supply and demand imbalance of water resources in coastal provinces is worsening with increasing levels of urbanization, economic development and population growth (Zheng et al. 2014). This promotes the authority to greatly advance the development of seawater desalination as a vital solution in relieving the increasing imbalance between water supply and demand within coastal provinces.

On the global scale, more than 150 countries and regions around the world had engaged in the seawater desalination industry by the end of 2015. The total desalination capacity reached approximately 51.2 million m³ per a day. It is estimated that over 75 million people worldwide obtain fresh water by desalinating seawater or brackish water. Some countries have a large desalination capacity, including the United States, the United Arab Emirates, Morocco, Tunisia and Egypt. Especially, in the Middle East, seawater desalination is a vital and dependable source of fresh water in countries such as Saudi Arabia, the United Arab Emirates, and Kuwait (Charcosset 2009; El Saliby et al. 2009). Saudi Arabia is the world's largest user of seawater desalination, and it has a capacity of 5.25 million m³/d, accounting for approximately 25.9% of the world's total seawater desalination. As the second largest user of desalination in the world, United Arab Emirates has built more than 30 sets of desalination plants, and the annual production capacity of these plants has reached 300 million m³. For Israel, 70% of potable water was being produced by desalination by 2015.

The three commonly used technologies worldwide for seawater desalination include multi-phase flash (MSF), multi-effect distillation (MED) and reverse osmosis (RO). MSF was adopted for water generation in the 1950s, and has advanced significantly since the 1980s. This technology was employed for 26% of total seawater desalination capacity worldwide (Ghaffour et al. 2013). MED was the primary technique for desalinating seawater before the 1960s, and this technology has been applied in more large desalination plants (Feng & Xie 2010). RO was applied in seawater desalination in the 1950s. With the advancement of high performance membranes, and installation of energy recovery devices, the cost and energy consumption for this technology has decreased significantly during past decades (Schiermeie 2008; Elimelech & Phillip 2011).

Focusing on China's seawater desalination, this study presents a systematic review of the development of seawater desalination in China. In this study, the history of seawater desalination is introduced in the section ‘History of seawater desalination’, and the present condition of the utilization of desalinated seawater is presented in detail in the section ‘Present condition of utilization of seawater desalination’. Some policies, regulations and technological standards governing seawater desalination are listed in the ‘Policies, regulations and technological standards governing seawater desalination’ section of this paper. The ‘Existing problems and suggestions regarding seawater desalination’ section summarizes existing problems and provides some suggestions regarding the current development of seawater desalination. Two representative examples, seawater desalination in Tianjin (a typical northern water-deficient metropolis) and Zhoushan (a typical southern island city with a sufficient amount of precipitation but still facing a serious shortage of fresh water resources), are presented in the ‘Case studies’ section as representative case studies. Finally, some simple concluding remarks are offered in the ‘Concluding remarks’ section.

China has carried out research on seawater desalination for approximately 60 years (Tan et al. 2007), and there has been significant progress in the development of seawater desalination during past decades (Zheng et al. 2014). The history of the development of seawater desalination can be simply divided into three phases, as discussed in this study.

1958–1990 was the phase of laboratory experiments and technological research. In 1958, the National Oceanic Bureau in China started research regarding electro-dialysis (ED) technology (Gao 2003). The Department of the Shipping Industry studied the instruments for pressure distillation and waste heat flash desalination. The reverse osmosis (RO) technology was investigated in the Shandong Institute of the Ocean in Shandong Province. Seawater desalination technologies, such as ED, RO, distillation (multi-stage flash distillation, MFD; pressure distillation; low-temperature multi-effect distillation, LT-MED; multi-effect distillation, MED) were extensively studied across the country from 1967 to 1969. In 1977, standpipe MED equipment was successfully developed in Dalian, Liaoning Province, with a capacity of 10–12 m³ per day. An ED seawater desalination station was established on Yongxing Island of the Xisha Islands in 1981, with a capacity of 200 m³/d (Zhu et al. 2014). An MFD seawater desalination project was set up at the Tianjin Dagang Electric Power Plant in 1990, and the project scale was 6 × 10³ m³/d (Zhu et al. 2014).

1991–2005 was the phase of industrialization of seawater desalination, including research regarding the technology and equipment for seawater desalination, the construction of seawater desalination demonstration projects, and studies regarding the strategy, policy and outline of the development of seawater desalination. The first seawater RO desalination station was constructed on Shengshan Island of Zhoushan, Zhejiang Province in 1997, with the project scale of 500 m³/d, satisfying the requirement for drinking water of local residents (Ding 1999; Tan et al. 2000). A seawater desalination plant with a capacity of 10³ m³/d was completed on Dachangshan Island of Dalian in 1999, and this plant employed RO technology (Ruan et al. 2002). In 2003, a seawater desalination system with a capacity of 5 × 10³ m³/d was established in Rongcheng, Shandong Province, and this system employed RO technology. Furthermore, a seawater desalination project with the capacity of 3 × 10³m³/d was completed at the Huangdao Power Plant of Qingdao, Shandong Province, and LT-MED technology was adopted in this project (Tan et al. 2004; Duan & Tan 2012).

2006–present is the phase of industry development of seawater desalination, including the establishment of the seawater desalination industry system (technology development, equipment manufacturing, project construction, maintenance, technology service, and raw material production) and the establishment of associated policies, standards and management systems, and seawater desalination demonstration projects. In 2005, a large-scale seawater desalination system with a capacity of 1.1 × 10⁴ m³/d was completed at the Datang Wangtan Power Plant in Hebei Province (Zhang 2011). In 2006, an RO seawater desalination plant with a capacity of 1.4 × 10³ m³/h was finished at the Yuhuan Power Plant in Zhejiang Province, and desalinated seawater was used as boiler make-up water (Yu et al. 2008; Zhu et al. 2014). In 2009, the first desalination plant with a capacity of over 10⁵ m³/d was completed in Tianjin (Zhu et al. 2014). Since 2005, the development of seawater desalination projects has increased significantly in China (Zheng et al. 2014).

The membrane industry has been well developed in China during the last six decades. Comprehensive applications of microfiltration, ultrafiltration and RO technologies in seawater desalination has advanced greatly. In 2010, the Chinese government released ‘Decision of the State Council on Accelerating the Fostering and Development of Strategic Emerging Industries’, and high performance membrane materials were listed as a strategic emerging industry in the decision. This greatly promoted the development and advancement of membrane technology, the membrane industry and the membrane application market. In the ‘National Twelfth Five-Year Science and Technology Development Plan’ released in 2011, the specific requirement for membrane materials has been outlined, which included ‘primarily develop membranes for water treatment, gas separation and speciality separation. Promote applications of membrane technologies in water treatment, iron and steel manufacturing, petrochemical industry and environment protections’. These indicate that membrane technologies, especially RO technology, are playing an important role in promoting the development of the seawater desalination industry in China (Zheng et al. 2014).

Figures 1 and 2 show the number growth and capacity growth of seawater desalination plants in China during 1981–2015. It is clear that the development of seawater desalination plants in China has entered a rapid growth phase since 2006. By the end of 2015, there were 139 seawater desalination plants put into operation in China, with a total production capacity of 1 × 10⁶ m³/d (Utilization report of seawater in China 2015). Table 1 lists the 10 largest seawater desalination plants present in China. Furthermore, by the end of 2015, 31 seawater desalination plants with capacities of over 10⁴ m³/d were constructed throughout the country with a total capacity of 8.9 × 10⁵ m³/d. Approximately 7.2% (10) and 19.4% (27) of the total seawater desalination plants have a capacity from 5,000 to 10,000 m³/d and from 1,000 to 5,000 m³/d, respectively, producing approximately 1.2 × 10⁵ m³/d in total. There were 71 seawater desalination plants with capacities below 1,000 m³/d across China, yielding a new water resource quantity of 1.6 × 10⁴ m³/d (Utilization report of seawater in China 2015).

Figure 3 shows the distribution of different utilization sectors of desalinated seawater in 2015 (Utilization report of seawater in China 2015). Of the total capacity (1 × 10⁶ m³/d) of seawater desalination plants in 2015, 6.5 × 10⁵ m³/d was adopted by industry, accounting for 63.6%, while 35.7% of desalinated seawater has been used for domestic living of residents, with a capacity of 3.7 × 10⁵ m³/d. The remaining 0.8% has been adopted in other sectors, such as shipping, harbour service and military affairs.

In industrial applications, the power generation industry accounts for the greatest portion, 35.8%; the petrochemical and iron and steel industry comprise 12.4% and 9.8%, respectively; and approximately 2.9% and 2.6% have been utilized in the paper and chemical industries, respectively. In summary, the power generation industry, domestic life of residents, petrochemical industry and iron and steel manufacturing are the four largest consumers of desalinated seawater in China.

In the power generation industry, desalination plants are used to produce boiler make-up water. This is because local authorities in coastal regions are greatly encouraging seawater desalination plants to adopt desalination technology to provide water (Zhu et al. 2012). With the policy incentives and further development of desalination technology, seawater desalination could provide the majority of boiler make-up water for power plants in the future (Man et al. 2014).

At present, the water price in China is relatively high due to the high costs associated with the production, distribution, maintenance and management of domestic living water and strong requirements for its water quality. With the advancement of seawater desalination technologies and the construction and distribution of large-scale seawater desalination plants, the cost of the desalinated seawater will decrease (Zhang et al. 2005; Zheng et al. 2014). Seawater desalination plants would produce a large amount of water for the domestic living needs of residents within coastal regions.

Desalination plants can reuse the heat and steam generated in the petrochemical and iron and steel industries. Therefore, seawater desalination plants can not only produce reliable water resources but can also reuse heat and steam, reducing the associated costs (Zhou 2009; Shi et al. 2012). It is possible that more petrochemical and iron and steel industries could adopt seawater desalination technology in order to provide more water resources.

Figures 4 and 5 show the geographic distributions of the number and the capacity of seawater desalination plants along the Chinese coastline by the end of 2015 (Utilization report of seawater in China 2015). There are many seawater desalination projects with a wide geographic distribution, ranging from the eastern sea of Liaodong Island (to the north) to the South China Sea (to the south). In terms of number, most of the desalination plants were located in four provinces: Zhejiang, Shandong, Liaoning and Hainan. From the perspective of capacity, the majority of the desalination plants were concentrated in the six provinces of Tianjin (an autonomous city), Zhejiang, Hebei, Shandong, Guangdong and Liaoning. Forty-five seawater desalination plants were put into operation in Zhejiang, with a total capacity of 2 × 10⁵m³/d. Only nine desalination plants were in Tianjin, but the total capacity reached 3.2 × 10⁵m³/d, showing there were some large-scale plants in Tianjin. There were 30 desalination plants in Shandong, with a total capacity of 1.6 × 10⁵m³/d, whereas only nine seawater desalination plants were constructed in Hebei, but they had a capacity of 1.7 × 10⁵m³/d. There were 16 and eight desalination plants in Liaoning and Guangdong Provinces, yielding 6.8 × 10⁴ and 8.2 × 10⁴m³/d of water, respectively. The other 22 seawater desalination plants were set up in Fujian, Jiangsu and Hainan Provinces, but they produce a smaller total capacity of 2.8 × 10⁴m³/d. The desalination plants in northern China focused on large-scale projects for industrial needs, whereas the desalination plants in southern China focused on island projects for domestic living objectives (Zhu et al. 2012).

Figure 6 shows the distributions of the number and the capacity of seawater desalination plants with different desalination technologies by the end of 2015 (Utilization report of seawater in China 2015). Most desalination plants have employed RO and/or MED technologies. For the number of desalination plants, 120 seawater desalination plants have adopted RO technology, and 17 desalination plants have employed MED technology to desalinate seawater. There was only one of each type of desalination plant that used MSF or ED technology. In terms of the capacity, the capacity of desalination plants with RO technology is 6.6 × 10⁵ m³/d, accounting for approximately 63.9% of the total capacity, whereas the desalination plants using MED technology have a total capacity of 3.6 × 10⁵ m³/d, even though there are only 17 plants, showing that MED technology is generally used for large-scale desalination plants. The capacities of seawater desalination plants using other technologies such as MSF and ED are 6,000 and 200 m³/d, respectively, accounting for only 0.6% and 0.02% of the total capacity.

The major drawback of RO technology is that the semipermeable membrane in the RO system is very sensitive to oxidants, organisms, algae, bacteria, and other pollutants contained in the seawater, as well as the pH of the seawater. Hence it is necessary to carry out rigid pre-treatment towards seawater. Scale and fouling easily happen on the surface of the semipermeable membrane, consequently leading to a declining ratio of desalinization and unstable water quality. Therefore it is necessary to wash and replace the semipermeable membrane in a timely manner (Zheng et al. 2016).

For MED technology, because the heat transfer coefficient for phase transition increases with rising temperature, a low top-brine temperature will hinder the improvement of heat efficiency. Scale deposits easily form on the outer wall of the heat exchange tube of MED systems. Hence it is important for a timely cleanout of the heat exchange tube to dislodge the scale, in order to maintain the effective operation of the desalination system (Zheng et al. 2016).

The major drawback of MSF desalination technology lies in the fact that structural materials are prone to corrode, and seawater is easily leaked into the condenser pipe when corrosion failure happens. Additionally, the MSF system needs an amount of seawater to circulate, leading to a large power consumption for the pump (Zheng et al. 2016). For ED technology, ions will gather on the surface of the electrode and the ion exchange membrane, consequently leading to the formation of scale deposits. Additionally, it should be noted that there are some new methods (such as microbial desalination cells) for water desalination in the literature (e.g. Cao et al. 2009; Chen et al. 2011).

At present, some policies and regulations have been published in nine provinces of the total of 11 coastal provinces. These are listed in Table 2.

At the country scale, seawater utilization had been incorporated as an important issue into some formal files by the end of 2015, including ‘Suggestions on the Thirteenth Five-Year Planning of Economy and Social Development Formulated by the Central Committee of the Communist Party of China’, ‘Suggestions on Speeding Up the Construction of Ecological Culture issued by the Central Committee of the Communist Party of China and the State Council’, and ‘Water Pollution Prevention and Control Plan’ issued by the State Council.

By the end of 2016, China had published 115 technological standards to govern the development and utilization of seawater desalination, including 23 national standards, 91 professional standards, and one provincial standard. These 23 national standards are listed in chronological order in Table 3.

The publishing of these polices, regulations and technological standards has played a critical role in the rapid development of the seawater desalination industry in China during the past decades, significantly relieving the serious situation of the imbalance between water supply and water demand within coastal provinces.

At present, the Water Law of China does not incorporate the utilization of seawater into the category of water resources and does not specify the responsibility and obligation regarding seawater use, leading to a lack of legal security for the promotion of seawater utilization-related policies (Chen et al. 2015). There is also insufficient policy support for the utilization of desalinated seawater at present. The capital investment for industry development could not be completely guaranteed, and seawater desalination is not also incorporated into the subsidy range provided by the government (Zhang et al. 2005). The policy related to the investment and water price subsidy remains incomplete. At present, a rational market mechanism for the development and utilization of water resources including seawater desalination is not formed.

The cost of seawater desalination is comprised of the investment, operation and maintenance, and energy consumption costs. The operation and maintenance cost includes maintenance, chemicals, membrane exchange, management and labour. With the advancement of seawater desalination technology and the increase in capacity, the cost for seawater desalination has decreased, concentrating at 5–8 yuan/m³ in China, but when compared with tap water, this cost is still high. This may lead to a decrease in competitive benefits of desalinated seawater compared with other alternative water resources (Liu et al. 2010; Zheng et al. 2014; Chen et al. 2015).

The capacities of seawater desalination plants are different depending on their utilization objectives. At present, desalination plants for domestic living of island residents operate with different capacities according to the water needs of residents, while those desalination plants for industrial use have capacities adjusted according to the industrial water need. However, for some desalination plants for water use in both domestic living and industry, their actual capacities are low because there is a constraint for putting desalinated seawater into municipal pipelines (Wang et al. 2013; Chen et al. 2015). For example, it has been reported that there is some idle capacity in desalination plants in Tianjin, China (Wang et al. 2011).

At present, there are only small-scale enterprises that could carry out equipment manufacturing for seawater desalination and associated projects, and a weak foundation for manufacturing hinders the industrialization of seawater desalination technology (Zhou 2009; Duan & Tan 2012; Zhu et al. 2012). Furthermore, the construction of seawater desalination plants is in an isolated and scattered state (Chen et al. 2015). The large-scale companies that have strong competitive power around the world in the field of equipment manufacturing for seawater desalination have not appeared in China until the present. International desalination companies will still dominate the seawater desalination market for the foreseeable future (Zheng et al. 2014). The market in China still needs activation.

At present, there are a few key seawater desalination technologies that have self-help intellectual property rights, and there is a relatively weak ability for equipment manufacturing in China (Duan & Tan 2012; Zhu et al. 2012). Most seawater desalination plants, especially with capacities of over 10⁴ m³/d, have employed imported technologies from international desalination companies (Chen et al. 2015). For example, for some key materials and equipment in RO desalination instruments, such as energy recovery equipment, the seawater membrane unit, high-pressure pumps and some chemical materials, China should still import them from international desalination companies due to the immature technology and a long period needed for their final applications in China (Ruan 2008; Chen et al. 2015). Additionally, some technologies in which desalination technology, nuclear energy, wind energy and solar energy are coupled together are in the preliminary development phase, and a long time is needed for their final applications in China.

In the process of seawater desalination, a large amount of concentrated seawater (brine) will be produced. At present, there are two kinds of comprehensive utilization methods for brine. The first kind is direct discharge of brine: discharge brine into ocean, surface water and sewage treatment systems directly through the pipeline, or lead it into an evaporation tank or bore. This method does not need any treatment measures, and can only be applicable for small-scale discharge of brine (Yu et al. 2017; Dai et al. 2018). The direct discharge of brine will damage the coastal water environment (Yu et al. 2005; Zhang et al. 2010). Additionally, the waste heat discharged from the seawater desalination process could increase the seawater temperature of regional waters, resulting in rapid reproduction and highly dense phytoplankton, and sometimes a phenomenon termed ‘harmful algal blooms’ occurs in some coastal waters (Ma 2008; Nie & Tao 2008). The increasing seawater temperature lowers the concentration of dissolved oxygen in seawater, influences the metabolism of organisms, and finally causes deterioration of the living environment of organisms (Zhou 2009). For example, seawater desalination facilities were identified as the potential reason for harmful algal blooms in the sea area of 1.6 × 10³ km² around Huanghua, Hebei Province (Ma 2008).

The second method is resource reuse of brine: salt manufacturing, and extraction of chemical raw materials (Yu et al. 2017; Dai et al. 2018). Circumfluence and distillation are two common methods adopted for small-scale brine treatment in China. There are some developed technologies for the comprehensive utilization of brine, including multi-effect distillation of brine, softening of calcium and magnesium of brine, salt-making method of brine, and bromine and potassium extraction from brine (Dai et al. 2018). Zero discharge technology is an ideal way of brine treatment. It not only protects the water environment and ecological balance of the inner sea, but also recycles brine resources and provides resource guarantee for economic development. This technology adopts the evaporator, brine concentrator and crystallizer to complete the separation of chemical products (such as calcium chloride, calcium carbonate and sodium carbonate) from water. There is potential for zero-discharge technology to be applied to large-scale brine treatment in China (Yu et al. 2017).

The successful implementation of seawater desalination projects heavily depends on public involvement and a positive attitude towards desalinated seawater (Fielding et al. 2015; Hurlimann & Dolnicar 2016). At present, the public lack strong knowledge of the role that seawater desalination plays in the integrated water resources supply system, and people are more or less concerned about environmental and health risks related to the utilization of desalinated seawater (Yu et al. 2005; Zhang et al. 2010).

With the abovementioned existing problems in mind, we propose the following suggested measures.

It is necessary to set up a complete legal system related to seawater desalination according to the strategic requirements and actual demand for sustainable utilization of water resources, promoting the utilization of seawater desalination in a gradually standardized and legal manner (Chen et al. 2015). The law related to seawater desalination should specify the responsibility and obligations for individuals and enterprises using desalinated seawater, especially specifying the legal responsibility to those individuals and enterprises who can adopt the desalinated seawater but still do not use it.

It is of great importance to decrease the effective cost of seawater desalination by coupling it with some production segments, such as solar electrical energy generators, wind electrical energy generators, heat systems, seawater desalination, and salt chemical production, forming a comprehensive utilization system of resources and expanding the industry chain of seawater desalination. Meanwhile, it is also of vital importance to strengthen the competitive power of the seawater desalination industry by combining it with regional water resources demand and by producing salted and bottled water in China (Liu et al. 2010; Zheng et al. 2014).

It is necessary to adopt a ‘market-oriented operation and enterprise management’ pattern for the development of the seawater desalination industry. It is also important to set up a pluralistic, multi-channel and multilevel investment assurance system for the development and utilization of seawater desalination (Zheng et al. 2014). It is necessary to construct a special fund for seawater desalination to support some demonstration projects for seawater desalination (Utilization report of seawater in China 2015). A favourable tax policy and an economic subsidy for the management of seawater desalination facilities should be supported (Liu et al. 2010).

Governments should encourage special studies on the key technologies of the seawater desalination process and comprehensive utilization process, import and absorb advanced desalination technologies and engineering experiences from overseas, and finally speed up the localization of key seawater desalination equipment (Duan & Tan 2012; Zhu et al. 2012).

It is critical to stress the industrial layout and large-scale development of seawater desalination. Some industrial matching should be considered before constructing new desalination projects. Pipeline development should be planned before planning the desalination industry, and it is necessary to incorporate desalinated seawater into an integrated water-resources allocation system within coastal water-deficient regions (Chen et al. 2015).

Applications of combined water and power and some desalination technologies are commonly used to mitigate environment risks. They could decrease the cost of steam generation by using the low-grade superheated steam and reduce the energy consumption by using cooling water. Some studies have indicated that it is possible to combine the process of seawater desalination and salt production (Feng & Xie 2010). Concentrated seawater utilization and zero-discharge technology are warranted to facilitate the sustainable development of the seawater desalination industry (Ruan 2008).

It is important to improve public awareness and receptivity towards desalinated seawater. Measures should be adopted to encourage the public to engage in the comprehensive management of water resources and to decrease their concern regarding the environmental and health risks, especially when the desalinated seawater enters the municipal pipelines (Fielding et al. 2015; Hurlimann & Dolnicar 2016).

Tianjin is a large, rapidly growing urban centre in northern China. The annual water use per capita in Tianjin is 153 m³, which is substantially less than the national average of 455 m³ (Tianjin Water Resources Bulletin 2015). This is because there are a very limited surface and ground water resources in Tianjin due to the semi-arid climate, with long-term annual precipitation of 550 mm (Tianjin Water Resources Bulletin 2015). Tianjin has a thriving economy and a growing population with a substantial and growing water need. To solve the serious water shortage issue in Tianjin, water diversion and alternative water resources have been developed to supply water to Tianjin (Gu et al. 2015). Several large-scale water diversion projects are operating (Tianjin Water Resources Bulletin 2015), including the South-to-North Water Diversion, the East Route Diversion and the Luan River Diversion. Of the alternative water resources, seawater desalination plays a major role.

By the end of 2015, the total capacity of seawater desalination plants reached 3.3 × 10⁵m³/d in Tianjin (Utilization report of seawater in China 2015): the Dagangxinquan seawater desalination plant with a capacity of 10⁵ m³/d, the seawater desalination plant of Dagang electric power supplies a capacity of 0.6 × 10⁴ m³/d, the seawater desalination plant of Taida new water source provides a capacity of 2 × 10⁴ m³/d and the seawater desalination plant of Beijiang electric power provides the most capacity at 2 × 10⁵ m³/d. The seawater desalination plant of Beijiang electric power transported and distributed the desalinated seawater to the tap water plant by constructing a special water supply pipeline. The Dagangxinquan seawater desalination plant supplied a new water source for large-scale ethylene projects by also constructing a special water transport pipeline. The seawater desalination plant of Dagang electric power provided the water source for boiler make-up water. In 2013, all of the seawater desalination plants provided new water resources of 3.7 × 10⁷ m³, of which 2.8 × 10⁷ m³ was for industrial needs and the remaining 0.9 × 10⁷ m³ was for domestic living of residents. In 2014, the total water resources provided by all of the desalination plants was 3.1 × 10⁷ m³, including 2.6 × 10⁷ m³ for industrial development and the remaining 0.5 × 10⁷ m³ for domestic living.

The following are brief introductions to four large-scale seawater desalination plants (Utilization report of seawater in China 2015).

The Dagangxinquan seawater desalination plant is located in Dagang Chemical Industrial Park in Tianjin, and it is a financial investment of the Kaifa Group of Singapore. The first phase capacity of this plant was 10⁵ m³/d since it went into operation in 2010. This project used a double membrane system as the technology for water production, and seawater pretreatment of this project adopted a hyper-filtration membrane system. The plant employed reverse osmosis and an energy recycling system to improve the desalination capacity and water-recycling rate.

The water supply target of the Dagangxinquan seawater desalination plant is some large-scale ethylene enterprises in the petrochemical industry of Tianjin. The water supply has reached 6–7 × 10⁴m³/d since operation in 2010. The water supply cost is not high due to the short supply distance between the desalination plant and the enterprises and the relatively low requirement for water quality of the supplied water in the petrochemical industry. Due to the low cost, the price of desalination could be kept initially at 6.7 yuan/m³, and adjusted to 7.6 yuan/m³, similar to the price of tap water in Tianjin. With the increasing petrochemical projects and the increasing water demands, the capacity of the Dagangxinquan seawater desalination plant will be expanded greatly in future.

The Dagang electric plant has four thermal generator sets, and each set has a volume of 300 MW. Desalinated seawater comprises 90% of boiler make-up water. Seawater desalination in this plant belongs to a dual-purpose power and water plant; that is, it utilizes the waste heat from the electric plant to produce the desalinated seawater. The employed technology is MSF, with a capacity of 6 × 10³ m³/d.

This plant is located in an economically developed region of Tianjin. It imported the technology from Vaire Corporation of France and employed LT-MED technology with a capacity of 2 × 10⁴m³/d. The majority of the desalinated seawater produced from this plant was utilized as boiler make-up water for Taida No. 5 thermal source plant during the heating period, and the remainder is used for the water requirement of the enterprises in an economically developed region of Tianjin.

This plant is located in Yingcheng Town of Hangu in Tianjin, occupying an area of 2.2 × 10⁶m². The plant plans to construct four coal-fired generator sets with a capacity of 10⁶W and seawater desalination equipment with a capacity of 5 × 10⁵m³/d. The first stage of the project has constructed two coal-fired generator sets with a capacity of 10⁶W and seawater desalination equipment with a capacity of 2 × 10⁵m³/d. This plant has employed LT-MED desalination technology and uses the waste heat to desalinate seawater.

Beijiang electric power achieved a capacity of 10⁵m³/d in 2010 and 2 × 10⁵m³/d in 2014. The desalinated seawater has been transported directly into the Hangu Water Plant through a specific water transport pipeline with a discharge of 6–8 × 10³ m³/d since October 21, 2010. The desalinated seawater was first mixed with tap water before entering the forebay of the water plant and was then treated using the treatment technology for tap water, before finally entering the municipal pipe network in Hangu. The water quality of desalinated seawater, before entering the forebay and after mixing, was determined to ensure it satisfied the standard for tap water in China. This was the first time a municipal pipe network was utilized to transport desalinated seawater in China. In 2015, the seawater desalination plant of Beijiang electric power produced 2 × 10⁴m³/d of desalinated seawater for self-use in the electric power plant and an additional 2–3 × 10⁴m³/d of desalinated seawater for the Taida Water Plant and Hangu Water Plant; this water was transported into the municipal pipeline after being mixed with regular water and then treated according to the treatment technology for tap water.

Zhoushan is located in the north-eastern part of Zhejiang Province, south of the passage of Yangtze River to the East China Sea, and it is an island group comprised of 1,390 small islands. The annual average precipitation in Zhoushan is 1,209 mm, and the per capital water resource quantity in Zhoushan is 601 m³ (Bulletin of Water Resources in Zhejiang Province 2015). However, the islands within Zhoushan are very scattered, and the majority are mountainous or have hilly lands, leading to difficulty in producing large runoff and effectively collecting the limited fresh water resources (Wang et al. 2013). Meanwhile, the islands in Zhoushan have complex topography (exhibiting a radial pattern and incomplete development conditions of surface water systems), which makes it much more difficult to carry out water diversion projects in Zhoushan (Wang et al. 2013).

The advancement of urbanization and improvement of people's livelihoods lead to an increasing need for fresh water. Furthermore, the water pollution issue due to economic growth has become increasingly serious, and some large steelworks, dockyards and electric power plants have been constructed in Zhoushan since 2006 (Zhejiang Statistical Yearbook 2015). These factors have caused a serious shortage of fresh water resources in Zhoushan. To relieve this serious shortage of fresh water resources and ensure the sustainable development of the economy in Zhoushan, it has become urgent for the Zhoushan Authority to strive to develop seawater desalination.

Zhoushan is a region in which the earliest seawater desalination was carried out across China. The first RO seawater desalination project, with a capacity of 5 × 10⁵ m³/d, was constructed and put into operation in Shengshan Town, Shengsi County in 1997 (Chen et al. 2015). Then, seawater desalination was applied widely around Zhoushan, including in Gaoting Town, Qushan Town, Changtu Town, Dongji Town, Liuheng Town, Yangshan Town and Gouqi Town. By the end of 2015, there were 21 seawater desalination projects in operation in Zhuoshan, and the total capacity reached 1.2 × 10⁵ m³/d (Utilization report of seawater in China 2015). Table 4 lists all of the seawater desalination projects present in Zhoushan. As seen in the second column of this table, the majority of the seawater desalination plants are developed for municipal needs, while the remainder are for nearshore industrial development, port logistics and marine tourism.

At present, all seawater desalination plants in Zhoushan have employed the RO desalination technology (Wang et al. 2013). Additionally, some desalination technologies are associated with solar energy technology. The Liuheng seawater desalination project, with a capacity of 10⁵ m³/d in Putuo district, Zhoushan, was chosen as a demonstration project by the National Key Technology Research and Development Program of the Ministry of Science and Technology of China during the Eleventh Five Year Plan (2006–2010).

From the perspective of engineering operations, local water authorities and tap water companies are responsible for operating most desalination projects. Desalinated seawater has been transported directly into municipal pipelines in order to solve the drinking water problems of local residents (Wang et al. 2013). Desalinated seawater was supplied for the domestic needs of local residents during dry seasons. Taking Shengsi County in Zhoushan as a typical example (Wang et al. 2013), the desalinated seawater supply accounted for approximately 80% of the total water supply. Local health and epidemic prevention departments have continually checked the water quality of desalinated seawater to ensure the safety of the municipal water supply in Zhoushan. Additionally, with the increasing capacity of seawater desalination, the cost of desalinated seawater has decreased to 5.5 yuan/m³ at present.

With the advancement of seawater desalination technology and the increase in seawater desalination capacity, seawater desalination is becoming a vital water supply source for the development of the Zhoushan region, playing an important role in ensuring the safety of water resources in Zhoushan.

China, especially its coastal provinces, is facing a serious shortage of fresh water resources and a water pollution issue, restricting further development. To address the serious imbalance between water resource supply and demand, in addition to efficiently utilizing the regular water resources, China has strived to develop alternative water resources to combat the water crisis, among which seawater desalination plays an important role. This paper reviewed the current situation of the utilization of desalinated seawater in China, including the points outlined below.

The history of seawater desalination in China was simply classified into three phases according to chronological order: the phase of laboratory experiments and technological research (1958–1990), the phase of industrialization of seawater desalination (1991–2005), and the phase of industry development of seawater desalination (2006–present).

The number and capacity of seawater desalination plants rapidly increased during 1981–2015. Most of the desalinated seawater has been utilized in industry sectors (mainly including the power generation industry, and the petrochemical and iron and steel industries) and domestic living of residents. In terms of the number of seawater desalination plants, most of the desalination plants are located in four provinces: Zhejiang, Shandong, Liaoning and Hainan. From the perspective of capacity, the majority of the desalination plants are concentrated in the six provinces of Tianjin (an autonomous city), Zhejiang, Hebei, Shandong, Guangdong and Liaoning. Most desalination plants have employed RO and/or MED technologies.

Policies, regulations and technological standards governing seawater desalination were listed in this study along with existing problems and some suggestions regarding the present status of seawater desalination have been proposed.

Two representative examples were given: seawater desalination in Tianjin (a typical northern water-deficient metropolis); and Zhoushan (a typical southern island city that has a sufficient amount of precipitation but still faces a serious shortage of fresh water resources).

On August 14th 2017, the Ministry of Water Resources of China first published an official file ‘Guide on putting alternative water resources under an integrated water resource allocation in China’ that specifies allocation requirements, allocation areas, strengthening measures, supervisionary managements and organizational guarantees using alternative water resources for all of the provincial administrative regions in China in order to speed up the development and utilization of alternative water resources. The allocation of alternative water resources (not including the directly used seawater) was planned to exceed 10¹⁰ m³ by 2020 in this file, and in particular, at least 2 × 10⁹ m³ of alternative water resources needs to be allocated in the Beijing-Tianjin-Hebei regions. The file specifies the allocation of the desalinated seawater for industrial needs: the desalinated seawater should be encouraged to allocate for industrial water use in coastal water-deficient regions; coastal regions should greatly utilize seawater as industrial cooling water. Some strengthening measures have been taken to promote the allocation of desalinated seawater in an integrated water resources system in this file, including strengthening the planning guide, strengthening the management of planned water use, accelerating the construction of alternative water resources projects, and implementing examination and motivation mechanisms. Some super-visionary management measures were also proposed in the file, including making up a complete technological standard system, strengthening the monitoring and statistical management of seawater desalination, and strengthening the supervision and control of the safety of seawater desalination. Finally, some organizational guarantee measures were also presented in the official file, such as making up a complete operating system, supporting the market to play a role, strengthening policy motivation using a positive guide, and improving the public involvement by broadly disbursing information.

With the advancement of seawater desalination technology, increasing capital investment and strong policy support, seawater desalination is developing rapidly, and desalinated seawater is expected to play a vital role in dealing with the serious shortages of water resources within the coastal regions of China.

This work was supported by the National Natural Science Foundation of China (Number: 51479003, 51509004). Considering some references cited in this paper are in Chinese, the interested readers could feel free to contact the authors for the English translated edition of those references.