Although water is often scarce for human consumption and agriculture, this planet is three-quarters covered by the stuff. The problem is getting the salt out, and this is normally done by the Earth’s water cycle, which produces rain and similar phenomena that replenish the amount of fresh water. Roughly 3% of the water on Earth is fresh water, of which a fraction is potable water.
Over the past decades, the use of desalination has increased year over year, particularly in nations like Saudi Arabia, Israel and the United Arab Emirates, but parched United States states such as California are increasingly looking into desalination technologies. The obvious obstacles that desalination faces – regardless of the exact technology used – involve the energy required to run these systems, and the final cost of the produced potable water relative to importing it from elsewhere.
Other issues that crop up with desalination include the environmental impact, especially from the brine waste and conceivably marine life sucked into the intake pipes. As the need for desalination increases, what are the available options to reduce the power needs and environmental impact?
Heating, Filtering, And Waste
A common type of desalination is distillation, which is essentially what also happens in nature through evaporation of surface waters. As water is heated, it evaporates, with the salts and other dissolved solid matter being left behind. When this process is done using intense heat, and in stages, it is called multi-stage flash distillation (MSF), which is one of the three most common distillation types, together with multi-effect distillation (MED) which uses stages with heated steam that couple into the next stage, effectively reusing the heat. However, by far the most common type of distillation (~69% share) is reverse-osmosis (RO), which uses a pressure differential across a membrane that allows the water molecules to pass, but not salt molecules and many other dissolved solids.
Important to keep in mind is that the output from none of these large-scale desalination processes is a neat separation into water and whatever else is left. Instead there is a fresh water output (~40% for RO), with a concentrate flow that is essentially briny water, allowing with whatever contaminants were in the intake saline or brackish water. This concentrate flow is what is returned to the sea or other body of water from which the intake water was drawn.
In addition to the much higher saline content of this concentrate flow, approximately twice that of seawater, it also has a much higher temperature than the intake water for thermal desalination plants. While an increased temperature of the discharged brine has clear negative effects on the local marine life, the plume of briny water has been reported to persist up to 5 km from the discharge site at some locations. This would render the area unsuitable for a number of species that do not deal well with briny water.
Much of this is highlighted in an August 2021 review by Ihsanullah et al. detailing the known environmental impact of today’s desalination facilities, as well as strategies to make desalination more environmentally friendly. This review also covers the additives that are commonly added to the intake water, and which may end up in the environment include:
- biofouling control additives (e.g. chlorine and sodium hypochlorite).
- anti-foaming additives.
- cleaning chemicals, e.g. for membrane cleaning.
In addition, the waste stream may include various other contaminants, such as copper and nickel as a result of corrosion of heat exchangers and other components of the desalination plant. By the nature of the desalination process, heavy metal concentrations will also be increased. To lessen the environmental impact from this waste stream, the reject streams from desalination plants are increasingly treated before being released back into the environment.
The Energy Benefit Of Membranes
Until the 1980s, the use of thermal desalination was commonplace, which was when RO became commercially available. A massive benefit with RO is its much lower energy requirements per cubic meter of produced fresh water (Elsaid et al., 2020), with MSF (operating at 120°C) requiring the most energy, especially thermally. MED uses significantly less power due to its reusing of heat in its successive stages. As can be seen in the table reproduced here from Ihsanullah et al. (2021), for RO the lack of thermal energy requirements make it significantly more efficient by default, only requiring electrical energy to create the pressure gradient across the membrane.
By requiring electricity rather than electricity and thermal power, essentially any constant source of electrical power can be used, making RO very versatile and suitable for both smaller and larger installations. Considering the rapid decrease in the marketshare of thermal desalination installations, it is likely that RO and similar membrane-based technologies will continue to dominate the market for the foreseeable future.
Capacitive Ionization (CDI) and electrodialysis/electrodialysis reversal (ED and EDR respectively) are some of a number of newer technologies that are seeing some use, though mostly for more brackish water. Along with nanofiltration (NF) and similar filtration technologies, these are held back by material issues as well as higher power usage (especially with CDI and ED/EDR). Other listed technologies are Electro-Deionization, membrane distillation and forward osmosis (FO), according to WNA.
An attractive energy source for powering desalination plants – whether thermal or membrane-based – is a nuclear reactor. These can provide both electrical power and heat, with e.g. Japan’s JAEA demonstrating an MSF desalination plant powered by a high-temperature reactor called the GTHTR300. As MSF can deal more easily with e.g. heavily polluted water better than RO absent pre-treatment of the intake water, the waste heat from nuclear reactors (including today’s existing light-water reactors) may make MSF and MED much more competitive with RO, while preventing the pollution from today’s mostly natural gas-powered MSF and MED desalination plants.
This has been demonstrated over the past decades in e.g. Kazakhstan (BN-350 fast reactor) and Japan, where ten desalination plants have been powered by pressurized water reactors (PWRs), employing mostly MED and RO. In South Korea some of its PWRs also run MED desalination plants that mostly generate water for its own cooling systems. In Egypt and Pakistan, their new nuclear power plants are also used to run MED and RO facilities.
Recycling The Waste
Although it’s generally been the case that the waste stream from desalination plants has been discharged back into the environment, there are good reasons to instead use as much as possible from this concentrated briny water. Especially in the case where seawater is used as the intake water, the concentrate at the output of the desalination process will contain significant amounts of magnesium, gold, uranium, bromine, potassium, cesium, rubidium and lithium, at least some of which may be economically recoverable.
Recently we looked at recovering uranium from seawater, which is challenging due to there being only a few parts per million of uranium dissolved, with the same being the case for the other metals and minerals that may be of interest. Although the oceans contain more uranium and such than can be reasonably mined from the Earth’s crust, fact of the matter is that there’s even more water in which it is diluted.
Since desalination plants massively reduce the amount of water, it logically follows that the resulting ‘waste’ will have much higher concentrations of uranium, lithium and so on, that may make it attractive to filter them out of this concentrated flow. The result of this may be that we can use much if not most of this concentrate, which would reduce the amount of briny, possibly contaminated water that ends up in the environment.
Hope Springs Eternal
If we are to use cheaper, environmentally friendly sources of power for our desalination plants, and use as many resources as possible from the ‘waste’ produced by these plants, we may actually end up saving money and environmental damage from mining elsewhere. Perhaps it is this perspective that is most helpful in any discussion about desalination.
As noted earlier, it is common for nuclear power plants to be involved in desalination. When this process can be performed using MED technology using what amounts to essentially waste heat, and the briny waste is dealt with properly, then it may just provide millions of people with plenty of potable water. One essential part of a desalination part that is hard to underestimate is that it does require access to a sea, ocean or other significant source of brackish or saline water.
When a city is placed in the middle of a desert, then said potable water will always have to be provided by pipeline or similar. But that’s a whole other kettle of fish.