People around the world are increasingly dependent on drinking water from seawater desalination plants. Water is a scarce resource. And it will probably stay that way because while in some regions of the world per capita consumption is steadily increasing, there is a lack of water elsewhere. The increasing world population and the effects of climate change are also exacerbating the problem. According to the United Nations World Water Report, global demand will increase by 20 to 30 percent by 2050.
The most common method is what is known as reverse osmosis with which drinking water is obtained on ships or in the large desalination plants, for example in the Middle East. To do this, you need a membrane that lets water molecules through but holds back the salt components. The water is pressed through them so that you have salt-free water on one side and the remaining brine on the other. However, you cannot press the water all the way through, because otherwise, the concentration in the brine would keep increasing, and at some point, the pores in the membrane would be clogged with salt crystals.
It is called “reverse osmosis” because such membranes normally cause salt concentrations to equalize. This happens all the time in nature. With reverse osmosis, the process goes in the other direction in that the water is pressed through the membrane with a lot of pressure against its natural direction. Depending on the salt concentration, energy consumption between 2 and 3 kWh per cubic meter of drinking water must be expected due to the high pressures, even in optimal systems.
The seawater is heated and evaporated, leaving the salt behind. The water vapor is captured and cooled, whereby it condenses and liquid water, which is free of minerals, is created. However, this method is used less often due to the more complex process management.
The disadvantages of the two methods
Distilled water is not recommended as drinking water and it increases the corrosion of the pipes in the systems. That is why minerals are added to the desalinated water afterward. The second problem is high energy consumption. Because regardless of whether water is pressed through a membrane with a lot of pressure or heated to let it evaporate, both take a lot of energy. However, the regions where such systems are needed usually have great potential for renewable energies. In other words, there is a lot of wind on the coasts and a lot of sun in the Middle East where such processes are needed most. The trend is increasingly going in this direction.
Saudi Arabia as a pioneer
However, if the energy required for heating comes from the sun, distillation makes a lot of sense. Saudi Arabia plans to use solar technology to desalinate seawater at Neom, a linear city that it is developing along the country’s northern Red Sea coast. (details: https://www.neom.com/en-us). The technology uses a dome constructed from glass and steel into which seawater flows. The energy to heat the continuous inflow and outflow of water is produced by concentrating solar radiation, from a large number of parabolic mirrors surrounding the dome. Through this process, the seawater evaporates, condenses, and is precipitated as freshwater. The remaining brine is extracted and sold commercially for industrial use in batteries, roads, fertilizer, and detergents.
The technology generates no carbon emissions, produces less brine than facilities using conventional reverse osmosis, and will process drinking water cheaper.
For many people, seawater desalination is becoming more and more important and the demand will increase sharply in the future. Research, development, and implementation of efficient desalination plants is the basis for life in many regions and can also prevent future conflicts over drinking water.