Drafting a future with droughts
Water desalination
Need to know
Green energies and more efficient plants will spread water desalination infrastructure more evenly around the world. The challenge, though, remains in brine.
Day Zero, the day when there’ll be no more water coming out of a tab, is fast approaching for various cities in the world.
In 2018 it was almost there for Cape Town, in South Africa.
In 2022 the drought reached unprecedented levels throughout Europe and water consumption dropped to its minimum.
There is still a drier landscape ahead as the gap between water supply and demand narrows down.
This is partly due to the increase in demand (growing world population; more crops, mines, and industries), but also to a reduction in supply (due to climate change, which causes droughts in larger areas and for larger periods of time).
Desalination technologies
With 97% of the planet’s water being salty, desalination seems the most natural solution to water shortage.
The Middle East and the developed nations took note and applied technology to obtain fresh water out of the oceans through one of the two known processes:
- Thermal desalination, which uses heat. Salt’s boiling point is considerably higher than water’s (1465°C compared to 100°C). Therefore, if you boil ocean water, only the water part will evaporate, leaving all the salt behind.
- Membrane desalination, also called reverse osmosis, which uses pressure. Salty water is pressed through a membrane that is only partially permeable, thus it lets water pass through and keeps the salt on the other side.
Nowadays there are around 16,000 desalination plants in 173 countries, but this amount is unevenly distributed.
71% is produced in high-income countries with a clear leader: Saudi Arabia.
There’s a reason for the list order: desalination is very costly. Not only the equipment requires a considerable investment, but thermal desalination also needs an impressive influx of energy to work its magic.
In countries where fossil fuels are widely available, the cost is certainly not an issue, but carbon dioxide production leaves a long, ugly tail in its wake.
Desalination plants are responsible for a combined 76 million tons of CO2 per year, an amount expected to grow to around 218 million tons by 240 if no action is taken.
Membrane desalination, although still costly, requires less energy, and if we combine it with green energy, the results are more promising.
Specific energy consumption (SEC) in kWh/m3
Pumping new ideas
Saudi Arabia, in its revolutionary NEOM futuristic city, is working on a new desalination program under Solar Water technology: a so-called “solar dome”, entirely green.
Sea water will be pumped through glass aqueducts to a solar dome, where an array of parabolic mirrors will concentrate the solar radiation into the dome to heat the water. High-pressurized steam will be released and preserved in reservoirs and irrigation channels. It’s 100% carbon neutral.
But this is not the only revolutionary idea.
Seawater Reverse Osmosis (SWRO) is applying technology to achieve SEC reduction by isobaric energy recovery devices (ERDs).
Furthermore, the membranes currently used for desalination under SWRO are mainly thin polyamide films rolled into a hollow tube through which the water wicks. This material might well be part of the past soon, as research shows that nanomaterials like graphene -100,000 times smaller than the diameter of a human hair- could work just as well and require less pressure to pump water through.
The problem, however, lies elsewhere. A byproduct of water desalination is brine, a highly salty liquid that, if thrown back into the sea, sinks down because it’s denser than water. It kills ocean life at the bottom because it depletes the oxygen levels. Brine can also contain chemicals and rise water temperature.
How can this brine be used? Tomatoes, seaweed, certain fish can tolerate high salinity. But we’re talking about a small demand. In theory, brine could be used in lithium batteries, grit for roads, fertilizers, and detergents. But salt recovery technology is not currently operational in high scale.
Now, there are some plants that are naturally tolerant of salty soil, such as barley, rye, safflower, and sugar beets. Just as increased nitrogen fixation is a result of a symbiotic relationship between bacteria and roots, researchers have determined that the roots of plants that are naturally salt tolerant contain certain bacteria that play a role in salt tolerance.
This is a huge breakthrough, because a laboratory study has shown that alfalfa plants grown from seeds inoculated with the bacteria are more tolerant of salt. Perhaps it may now be possible to grow crops in soils with high salinity by inoculating the seeds with bacteria from salt-tolerant plants.
