Global water use has reached six times what it was a century ago and is expected to increase by 20 to 50 percent by 2050, according to the United Nations' new World Water Development Report. At present, more than a quarter of the population lives in areas with severe water scarcity, and possible water scarcity poses a serious threat to human survival.

70% of the earth's surface is covered by water, but the freshwater resources necessary for human survival are extremely limited, accounting for only 2.5% of the total water in the world, and the remaining 97.5% is undrinkable sea water. Desalination has become an important way to solve the shortage of water resources. According to the United Nations University, there are now more than 160,000 seawater filtration plants around the world, producing 95 million cubic meters of fresh water a day, nearly half of which comes from the Middle East and North Africa. In addition, eight countries, including the Maldives, Singapore and Qatar, produce more desalinated water than comes from natural sources. By the end of 2017, China had completed 136 desalination projects, producing nearly 1.2 million tons of water per day.

There are more than 20 kinds of seawater desalination technologies, which are mainly divided into two categories: distillation process and membrane process. The distillation method has a long history, and the multi-stage flash technology widely used in Middle East countries is of this kind. The technology was invented in the 1950s, the basic principle is to heat the seawater successively through the temperature, pressure step by step reduction of the flash chamber, so that it evaporates and condenses to obtain fresh water. This technology has greatly improved the scale problem caused by distillation, but the energy consumption is high. The newly developed low-temperature multi-effect distillation technology can operate several evaporators in series under the condition of no more than 70 degrees Celsius, which reduces energy consumption, and has less scale and high water quality, and has more promotion potential.

The reverse osmosis method in the membrane method is also a very widely used technology. The main principle is to pressurize the seawater by a high pressure pump to produce a pressure greater than the osmotic pressure of the seawater, so that the fresh water seeps through the anti-permeable membrane, and the concentrated salt water is discharged from the other end. With the improvement of membrane performance and the decline of price, reverse osmosis method has become the least investment, the lowest cost of seawater desalination means. At this stage, the energy consumption of producing 1 cubic meter of desalinated water has been reduced from more than 20 degrees of electricity to less than 4 degrees of electricity, and it is possible to further reduce in the future. Thanks to technological advances, Singapore has brought the cost of desalting water down to $0.49 per cubic meter.

Although the supply of seawater is endless, the cost of desalination is decreasing, but the cost is still high. Most existing desalination facilities are located in high-income countries with severe water shortages. Still, technological innovation is always to be expected. Researchers at Rice University in the US have been able to increase the efficiency of solar desalination systems by more than 50 per cent by using cheap plastic lenses to focus sunlight on "hot spots" of nanophotonics.

The progress of desalination technology is encouraging, but the resulting environmental problems can not be ignored. High temperature, high salt and chemically doped wastewater is usually discharged directly into the ocean, affecting the Marine ecological environment to some extent, and even altering biological populations. Therefore, while using seawater to "quench one's thirst", human beings should also strengthen the consideration of the Marine ecological environment and pay attention to the treatment of wastewater.

(Lin Rui Shangkaiyuan)

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