Don’t Be Salty: How To Make Desalination Work In Tomorrow’s World

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

Water salinity diagram. (Credit: Peter Summerlin)
Water salinity diagram. (Credit: Peter Summerlin)

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 salts 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.

 Number and capacity of operational desalination facilities by (a) technology and (b) feedwater type. (Ihsanullah et al., 2021)
Number and capacity of operational desalination facilities by (a) technology and (b) feedwater type.
(Ihsanullah et al., 2021)

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:

  • antiscalants.
  • 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

Ihsanullah et al., 2021
Ihsanullah et al. (2021)

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.

60 thoughts on “Don’t Be Salty: How To Make Desalination Work In Tomorrow’s World

  1. “As the need for desalination increases, what are the available options to reduce the power needs and environmental impact?”

    Cutting back on the need for water in places where there is no water ?
    have we not yet learnt that trying to bend the planet to our will is/has reached a tipping point where nature is now fighting back and trying to kill us off ?

    I wonder if figuring out fusion will be a blessing or a curse.
    The fact that money isn’t tied to fixing the planet but only consuming it’s resources does not bode well for so called “unmeterable” power.
    Take desalination for example, the article gives good insight into the waste products and the problems with them. But if power was cheap/free no one would be worrying about it, just how fast and big you can build the plants.

    1. Most water is used for power generation and agriculture (both growing food and watering lawns). Honestly the solution nobody wants to discuss is ending farm subsidies or ending subsidies for water beyond a threshold. Like most tech solutions, this one is fixing a problem that could more easily be fixed by using less resources (but that doesn’t generate jobs or money so no one wants to discuss that).

      1. While I agree about useless watering of lawns, I completely disagree with the former. Fresh water is not only useful for agriculture, it’s also useful for the whole ecosystem. When some farmer is pumping 1 liter of water uphill, this liter will be missing downhill to a variety of ecosystem. Sure, we need to feed the ever-growing mankind (until it figures out that it would be best to limit its population by itself that wait for nature to do it), but we are also responsible not to destruct the few ecosystem left. So a water treatment plant that can bring that liter downhill from the seashore might help to sustain a healthy ecosystem anyway. Don’t forget that water doesn’t disappear once spread on a field. It evaporate, increase the air humidity, it fills the reservoir… and so on.

        1. When crops get irrigated, it is not the case that 100% or even most of that ends up back in reservoirs anyway. That would be awesome if it was the case, because then we wouldn’t need to stop at lawns, we could happily irrigate the entire planet and finally destroy cacti and all their thorny desert brethren once and for all

      2. It isn’t jobs or money. We are the smart monkeys. Some of us make places habitable because we feel like it. There is no global “we” involved. Make fresh water from sea water wherever you want. The nuclear option seems ideal.

        There are other ways to get fresh water where you need it, like change the local climate. For example California used to have the largest lake West of the Mississippi in the Southern part of the state. It moderated the climate into Nevada and beyond. They drained it. So recreate it. Pump sea water from the San Francisco Bay and put a drain out to the ocean near Los Angeles. Get lower temperatures and rain in the mountains.

        Don’t think small.

    2. Well, we’re expanding. We take away existing living spaces to build our cities on.
      It’s just fair that we welcome nature on cities, giving animals and plants, trees a home.
      – There’s enough space on top of buildings, on balconies, etc.
      But for this, we need more water. Much more water. We could collect rain water, we could turn saltwater into normal water with the help if the sun, it doesn’t need to be “fresh”. The nature we invited to our city may be able to assist in the filtering process, at least in parts.

    3. Increasing the energy available to humanity will always be a curse. It will always result in an uncontrollable tropism on our part to expand to the ceiling of that new envelope and destroy our finite habitat in increasingly potent and surprising ways. I know, it’s a bummer, I’m fun at parties, etc.

        1. Native population growth in nearly all developed countries is below replacement levels and they’re growing their populations via immigration. Only some developing countries still have positive native population growth.

    1. It’s not the number, it’s the mass, strictly speaking.
      But even that’s not quite correct, either.
      Let’s stop being so misanthropic. Destroying is easy, self-destruction, too.
      We must take responsibility. Let’s stop blaming ourselves. The more people working together, the better.

      We must find new resources, without being a burden to earth.
      We could use plankton, seaweed, algae etc.

      Or find resources in space. On dead planets or asteroids in our reach.

      People often complain that we shouldn’t mess up space, but forget, that except us, no one cares about beauty, music or science.

      If we’re gone, there’s no one left to value and admire beautiful things.

    2. “ya we could institute reforms which could easily solve these problems, but I prefer not having to change my lifestyle in even the smallest way, so let’s just let several billion people in the poorest parts of the world die”

  2. I think I’m missing something on this point:
    “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.”

    I understand the first sentence. My confusion is with the second sentence. Ok, so the outflows from the desalinization process are going to be sent to a second process where uranium et al will be removed. Ok, great, but after this second filtering process, ALL of the concentrated, briny++ water still needs to go *somewhere* – presumably back to the place from which it came, no? What am I not understanding?

    Or is the assumption that after the uranium freaks agree to take the briny++ water, someone jumps up and yells “No backsies!” :-)

    1. The brine is typically sent to evaporation ponds. Solar heating puts the remaining water up into the air, and what remains is sea salt. This salt can be used for food, road deiceing, or the minerals can be processed out. All the minerals can have some industrial use. So, there is no need to dispose of waste.

      The key to make this economical is to locate desalination plants on coastlines with low value land, like desert land. If the land is valuable, like southern California. Then it makes more sense to discharge the brine back into the sea.

    1. Nuclear has a lot to offer right now. Los Angeles is dealing with rolling blackouts in part because coal and natural gas plants can’t make up the hole left by shutting down SONGS

    1. Yes, but how to capture it? You’d need large areas of collectors (ie, clear domes to condense and collect the vapor). If you don’t have collectors, then your water vapor will be subject to the whims of air currents, and won’t always go where you want it to (ref: weather or climate).

  3. People really need to stop farming in desert areas. Living in a desert areas is dumb enough but farming in a desert is absolute stupid. Also, this talk of getting more fresh water ignores that too much water is already being wasted. On top of that, we aren’t even using water recycling which would actually capture the water from storm drains. Water isn’t expensive enough for businesses to conserve it.

      1. There is also in many cases a much more controlled, pest free environment – as nothing that normally lives with or upon arable crops exists there unless you brought it in – which makes it in many ways ideal for farming especially experimental farming!

        Though I also agree entirely on the general water waste and grey water waste being rather on the money…

          1. Sadly, estrogen is a tough compound to destroy.
            It survives sewage treatment plants and get discharged into rivers. People downstream then drink that “fresh” water, and pass it into their local sewage plants along with some more estrogen that compensates for the amounts they drank and was retained by their bodies, and people wonder why the amount of testosterone in men has dropped dramatically in the past 50 years or more.
            Plus, BPA plastics and soy products also release estrogen compounds.
            Enjoy the bottled water!

    1. What’s even more interesting is the California aqueduct extracts water from the desert. Cedar Springs Dam flows water that once flowed through the desert. Also Owens Lake is dryer than ever and flowing through the desert into the cities. I don’t think aqueducts are the answer. People bound to the coast should be desalinizing and develop policies that work with the environment.

    2. Los Angeles area and storm drain runoff….the gigatons of water I saw on a trip there in a rainy season…my goodness….if only we could put that into the reservoirs instead of into the Pacific. How would one do that….HMMMM???

      But lets blame their water issues on climate change. It has nothing to do with them, it’s the rest of the world that did it.

    3. The mention of research by Saudi Arabia, a kingdom run by a single family sustained by slavery and oil wealth, should be reason enough to give people pause. These are the same people spending half a trillion dollars on The Line, the city which isn’t afraid of the laws of physics. A decade or so ago the Kingdom decided they would become a grain exporting country, irrigating the crops with a massive aquifer which had supplied drinking water. Within a few years, they went back to being a grain importing country as well as a water importing country.

  4. Why can’t large scale high intensity focused sunlight( using lenses or parabolic mirrors) be used to evaporate seawater. The water would be collected as fresh distilled water. Minerals could be added simply.

    1. I recall a low-tech survival method of getting fresh water. Dig a large hole in moist soil/sand, doesn’t need to be very deep. Container at the center of the hole. Hole covered with clear plastic, edges held down with sand/dirt/rocks, and a rock in the center, over the container.

      Evaporation condenses water on the underside of the plastic. Water drains down to the center, where it drips into the container. Either collect the water periodically, or (if one’s survival package has a small hose) set up with a hose in the container, from which water can be drunk periodically.

      On a large scale one might have a trench instead of a round hole, with a trough running down the center. Panes of glass angled toward the trough replace the plastic, for better efficiency (transparency) and reduced maintenance. Rather little external power should be required compared to other methods.

      Is it feasible to scale up that sort of scheme? What are the limitations re. climate, location, maintenance? Can it be made more efficient? An applied research problem.

  5. Why not use atomic plants the size they use in ships for the desalination plants instead of building plants that take years to build. Build under ground water storage to prevent evaporation . The Romans and other societies did it and they did not have the technical ability we have today.
    In south America the install nets in the mountain areas and capture the morning due low tech no moving parts . every drop of water collected helps

  6. Ok, I just want to take you out and beat you all with a stick. Learn something about thermodynamics. My technology has done more work with less energy outside of a nuclear reaction. From this I built this pump.

    It is the pump of last resort for Nuclear Reactors. It will change EVERYTHING!!!

    The basic premise of this pump is the “SAFE AND CONTROLLED” hypersonic detonation of Oxyhydrogen gas resulting in a hypersonic shock wave traveling at near Mach 4.5. It harnesses three work potentials to produce pressure and two work potential to produce vacuum.

    1. The Hypersonic shock wave transfers its kinetic energy to the momentum of water traveling to the exit.
    2. The 5100°F detonation temperature expands 1000 times a small portion of the water to steam.
    3. Thermal energy and hypersonic flame front combine to achieve Thermolysis, splitting water into additional reactant hydrogen and oxygen components to carry the detonation further.
    1. Oxyhydrogen gas charge has to implode to roughly 1/2000 of its initial volume. This includes the additional reactants from item 3 above.
    2. Condensation of the steam. Since the process normally below the standard boiling point the mass of the system quickly condenses this steam component in less than 2 seconds.

    In the pulse detonation form of the pump, water is easily evacuated from the combustion chamber by the shock wave and superheated steam ejecting water past the check valve. Followed by the implosion and condensation phase which leaves a vacuum in the chamber at about 2.2 PSIA in line with the vapor pressure of water. In the rotary detonation form of the pump the Mach 4.5 shock wave is focused as the high pressure along with a slight draw of water which expands to steam and keeps the pump body temperature between the feed water temperature at a predictably low temperature, and can drive the vacuum below 0.5 PSIA. In vacuum applications we will be able to continue to accelerate these gaseous water molecules the same way ion pumps operate. Harnessing the pressure differences between the high velocity and high-pressure steam, the condensed liquid water and the ambient external pressure allows the current invention to produce work at levels of efficiency no other device has attained. No other work model has ever harnessed three potentials or ever produced both pressure and vacuum all in the same footprint, or has attained such a high thermodynamic yield of chemical energy to work below those of nuclear processes. The current prototype has moved more water for less energy than has ever been recorded, and far exceeds what fuel cells technology will ever attain in industrial applications. This one technology is the true key to the utilization of Hydrogen above all others into the future and will replace the Rankine cycle because it is the simplification and improvement of the Rankine cycle to be known as the Turner-Wood cycle.

    Examples presented include but are not limited to.
    1) Moving water and water-based liquids
    2) Clean water by low pressure flash distillation economically.
    3) Extreme cost effective production of vacuum.
    4) High velocity for venturi-based work models. (Steam repressurization, low pressure flash distillation, steam generation, powder metal production, and low temperature aerospace propulsion)

    Total global production of electricity utilizes 33% to move water on the planet, and this process has shown in the current 3rd prototype to move 120% more water over even the best motor driven pump ever built. It is simpler and will last much longer than traditional pumps. It will prove itself out to be the single greenest technology to date, with the greatest global impact to mankind.

    And if any of you take 5 minutes to call me I will explain how this can offer another path other than fusion. One that skips the high temperature conversion step to pressure that fusion will require and that it has to do with MHD and ionized water vapor traveling at Mach 4.5

    Now, I hope one of you PhD fools calls me. I already shamed my PhD Chemist father-in-law and now he has to write my paper because of disbelieving me. It has to do with properly harnessing Gibs Free Energy and extending the transition time in a…NON-LINEAR PROGRESSION…

    Call me…408-431-5595

    Or waste your time on all other forms and still the pump will always improve and exceed them. Its a thermodynamic thing…

  7. One desalination option that is unfortunately being suppressed is Algenol’s proprietary process of growing algae to produce vast amounts of biofuels from reclaimed CO2. Their bioreactors grow the algae in saltwater contained in solar activated bioreactors and a byproduct of their process is that the salt water gets desalinated into fresh water.

    1. OMG why do I waste mt time on you folks..I can clean 1000 Cubic Meters for 72KW. You can’t makewater potable for less than 64KW per cubic meter. I do 1000 times better and this is the result?

      You are all proving I can do better by enlisting monkeys to help me. Even better Sea Monkeys…

  8. Short term nuclear desal seems like way to go. Longer term, we merely desal over and over. Better find a crater on the moon to place salt in, with tarp on top, so moon has chlorine supply and earth lowers its overhead. Meanwhile moon has a sodium tail…

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