Seawater desalination in China: an overview

China, especially its coastal provinces, is facing water shortage issues, restricting its further development. To tackle the serious imbalance between water resource supply and demand, China has strived to develop alternative water resources to combat the water crisis, among which seawater desalination plays a major role. This paper reviews the current situation of utilization of desalinated seawater in China and includes: (1) a history of seawater desalination classi ﬁ ed into three phases; (2) analysis of utilization sectors, geographic distribution and employed technologies of the desalination plants; (3) summaries of the policies, regulations and technological standards governing seawater desalination; (4) proposals for existing problems and some suggested measures regarding the current condition of seawater desalination; and (5) the seawater desalination programs in Tianjin and Zhoushan are presented as two representative examples. China ’ s seawater desalination experience can provide some guidance for other countries facing similar water resource situations.


INTRODUCTION
China is facing increasing pressure on fresh water supplies (Yi et al. ). Its per capita water resource quantity was 2,354.9 m 3 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. ). Furthermore, the majority of the water resources is concentrated in southern China, leaving the northern and western regions to experience drought (Yi et al. ; Lyu et al. ). 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. ). 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. ; Lyu et al. ). These measures include implementing a very stringent water management system that specifies water efficiency objectives (Wang et al. ), adjusting the inner structures of primary, second and tertiary industries for a series of water-saving targets (Chao et al. ), and implementing trans-basin water diversion projects across the country (such as the South-to-North Water Diversion Project) (Zhao et al. ).
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. ).
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. ). Householdlevel rainwater collection can provide only limited additional water resources because of the complex climate characteristics in China (Gu et al. ). 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. ; Zheng et al. ).
As an overview, 43% of China's population lives in its 11 coastal provinces with a 1,800 km coastline occupying 13.7% 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. ). MED was the primary technique for desalinating seawater before the 1960s, and this technology has been applied in more large desalination plants (Feng & Xie     6.5 × 10 5 m 3 /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 × 10 5 m 3 /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.  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 ; Shi et al. ). It is possible that more petrochemical and iron and steel industries could adopt seawater desalination technology in order to provide more water resources. 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 × 10 5 m 3 /d, accounting for approximately 63.9% of the 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.

Geographic distribution
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. ).  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  Actively advance the utilization of alternative water resources, including rainwater, reclaimed water, seawater and brackish water Implementation of an outline regarding the protection of water safety Mar. 7, 2015 Encourage local authorities to construct reclaimed water projects, enhance the utilization of brackish water, and speed up the construction of seawater desalination projects Electricity use for seawater desalination should apply the electricity price for agricultural production from December 1, 2013 Implementing scheme regarding the infrastructure construction of the special seawater industry in Hangzhou, Zhejiang Province Oct. 14, 2014 Put forward the base construction of seawater desalination technology and equipment manufacturing, and gradually improve the industry system of seawater desalination Augment the utilization of alternative water resources, such as reclaimed water, rainwater and seawater; construct sewage treatment facilities when constructing municipal water supply facilities serious situation of the imbalance between water supply and water demand within coastal provinces.

EXISTING PROBLEMS AND SUGGESTIONS REGARDING SEAWATER DESALINATION Existing problems
Lack of legal security and insufficient policy support At present, a rational market mechanism for the development and utilization of water resources including seawater desalination is not formed.
Relatively high cost of desalinated seawater compared with tap water The cost of seawater desalination is comprised of the investment, operation and maintenance, and energy consumption costs. The operation and maintenance cost  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

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

Setting up a complete legal system related to seawater desalination
It is necessary to set up a complete legal system related to

Improving public involvement and receptivity towards the desalinated seawater
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,   , the phase of industrialization of seawater desalination (1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005),  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  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.