Why Is Desalination So Difficult?

Why Is Desalination So Difficult?

An overview of seawater desalination: removing salt to make drinkable water from the ocean.
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It might surprise you to learn that there are more than 18,000 desalination plants operating across the globe. But, those plants provide less than a percent of global water needs even though they consume a quarter of all the energy used by the water industry. The oceans are a nearly unlimited resource of water with this seemingly trivial caveat, which is that the water is just a little bit salty. It’s totally understandable to wonder why that little bit of salt is such an enormous obstacle.

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55 Responses

  1. Practical Engineering says:

    ✉ Want to keep up with everything I’m working on? I have a mailing list that isn’t annoying! https://practical.engineering/email-list
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    • Permaculture Ecuador says:

      bruh literally never quote the WHO or WEF ever again.

    • paintedwings74 says:

      I don’t understand why a flash evaporator would reduce scale build up. Probably because I can’t visualize it; do you know of an image or paper I could read to clear up this idea? Thanks for all you contribute, Grady. I love it.

    • Discover Your Origins says:

      Or just build satellites that can boil ocean water 🌊 💧 🌀 but that would be too cheap, the world just wants to make money.

    • Vera Fleck says:

      The brine is also a potential source for rare minerals, etc.

    • Discover Your Origins says:

      @verafleck6936  Greed and building things is the least of people’s worries when they will have fancy phones and computers but no water.

  2. Rexford L says:

    I’ve drank literally thousands of gallons of desalinated water over 20 years while I was in the US Navy. First ship used 7 stage evaporators and the last two used reverse osmosis. You couldn’t tell the difference between them since it was pure water that came out of them and the engineers added minerals back into them to make them drinkable.

    • John Micheal says:

      How does the engine add the minerals back?

    • Nikko the Goblin says:

      Ship scale is a lot easier than city scale, glad it works

    • MeMad Max says:

      @John Micheal Its not an engine. Its a desalinization plant. The chemicals are put in the water using a metering device before the water goes into the storage tank on the ship. We only had bromine injection on our ship. Bromine is used to kill any bacteria in the water.

    • Robert Lewis says:

      Same here. The water got interesting on one of the big decks I was deployed on when it tried to distill saltwater fouled by fuel dumped by a Harrier. The “drinking” water tasted and stank of JP5 for weeks.

    • MeMad Max says:

      On our ship(USS Camden) we were fine with just a 5 stage evap. One was in the forward MMR and one back aft. The one back aft always made more water as my evap had a vacumn leak somewhere in the aux exhaust preheater that I could never find(but made a great shower during shutdown when the vacuum was broken lol) Sounds like you were on a carrier lol.

  3. Josh Pulliam says:

    As a 15 year water / wastewater professional I very much appreciate and enjoy this type of content. Thanks Grady

  4. Impending Joker says:

    Here in Tampa they tried to build a RO desal plant near the Apollo Beach Power Plant. The biggest issue was not any of what you outlined here. The problem was zebra mussels. They are a non native invasive species that would collect on the intake pipes for the desal plant and they were spending 100’s of thousands of dollars each month just to keep the pipes clean, and that is what killed the project in the long run.

    • Cowa Bunga says:

      Imagine living in Florida out of all places

    • sneedium miner says:

      Not sure why one of the wettest states in the US needed a desal plant anyway. There’s no way they would make any money when their competitors would just be collecting and treating the readily available freshwater for basically free by comparison.

    • Cheles Bells says:

      _Shells have been used to lime, or raise the pH, of soils for years, so it wasn’t so far fetched to think that a useful compost could be made from Zebra Mussels._
      (googled them)

    • Mr. Cool says:

      ​@Cowa Bungaconsidering the amount of people who move down here, I don’t think your opinion is the popular one

    • hedgehog3180 says:

      @Cheles Bells You could but that doesn’t mean you’d have made any money from it, lime isn’t exactly in short supply.

  5. Simple But Expensive says:

    I worked in a power plamt that used a multistage RO to clean up produced water from an oilfield. The oil was separated, and the water was run through softeners, but it was still in the part per thousand range. We ran the RO at 75% permeate and 25% reject in the winter. We had to run it at only 70% permeate in the summer due to the water being much hotter. Input temperature and pressure have a high effect on the process. We got <1 ppm hardness, and around 16 ppm total dissolved solids. This was necessary because the next step was deionization, yeilding 17.8 mmho water. Theoretically pure water (which doesn’t exist) is 18 mmho. Water that clean will actually make you sick with extended use. It basically reverses the osmosis in your intestines, pulling nutrients out of your blood instead of vise versa.

    As far as seawater desalination, I took a contract in Saudi Arabia in the mid eighties. Theentire eastern half of the country was supplied freshwater by a huge RO plant just south of Al Jubail on the Persian Gulf coast.

    • Seal Piercing says:

      What did you use the DI water for?

    • Simple But Expensive says:

      @Seal Piercing DI water was used for injection into a GE frame 6 gas turbine for NOx control. Some of the RO water was used in the engine inlet evap coolers to increase air density and improve power output (more air, more fuel).

      One plant that my company operated started with well water. The sequence was sand filter, carbon filter, softener, another carbon filter, RO, deionization. At each stage some of the water got sent to the next stage and some got used in a process such as the majority of the soft water being used as makup water for the boiler. All effluent streams (incuding rainwater) ended up in a flock tank and then a press. No water was allowed to leave the plant except as vapor from the inlet coolers and the cooling tower. Even black water was run through a mini sewage digester plant. The water that came off the press went back into the cooling tower. The solid “cake” that came off the press was chemically and mechanically identical to limestone, but was still classified as hazardous waste and had to be disposed of accordingly. We bagged it, and shipped it to a facility that stored it. The facility was, appropriately enough, an abandoned limestone quarry. That was the only zero discharge plant I have ever seen. Even then, the government wanted to dispute that classification since the bagged limestone left the plant. There was a tax break involved for being a zero discharge demonstration plant. Our lawyers pointed out that under their criteria it was impossible to have a zero discharge plant because some evaporation is required in any power plant. In the end, both sides decided to nullify any contracts and abandon the project. As far as I know, the plant is still rusting away, and nobody is attempting that kind of usage efficiency today because when they look at doing it that plant is held up as an example of a failure, even though it really succeeded. All because a government bureaucrat wanted to avoid giving a tax break.

    • sneedium miner says:

      @Seal Piercing what do you NOT use DI water for? it causes far less corrosion and leaves next to no scale/fouling

    • Tösel Treps says:

      ​@Simple But Expensivewow, that’s a really good story. are you still in the business?

    • Roger Olander says:

      The theoretical limit of water purity is 18.2 Mohm. In the semiconductor industry we meet a water purity requirement of 18 Mohm continually.

  6. Morgan Medrano says:

    I’m a Navy veteran and I served on a Nuclear Aircraft Carrier. We had a desalination system built into the Reactor system using the excess heat from the steam powerd turbines. It was actually very efficient.

    • andybaldman says:

      Nobody cares, bud.

    • Adriel Torres-Zhivkov says:

      nice, i was wondering exactly this. as nuclear and other powerplants have to boil water anyways, why aren’t those systems combined?!

    • Morgan Medrano says:

      @adrieltc  sadly nuclear doesn’t have large profit margins. So big energy used the fear of radiation and meltdowns to justify closing plants… then went back to fossil fuels.

    • Bradley says:

      @andybaldman I care.

    • TacticusPrime says:

      @Adriel Torres-Zhivkov They are combined at times, but the designs have to be tested thoroughly.

  7. Paul says:

    Edit: I sometimes forget that 27 years of combined experience between water and wastewater doesn’t compete with people on the internet who are certified Google engineers

    As a lead operator of a R.O. desal plant that produces 22.5 million gallons per day, this is a great video. I do want to point out that the post treatment process of adding minerals is also done for more than taste. It’s to prevent dehydration. When water approaches a low TDS, it lacks Sodium, Calcium, and Magnesium. What happens is there’s nothing for the body to absorb from the water. This can lead to increased urination which leads to an electrolyte imbalance in your body. Water and your body wants to be balanced. If your water lacks minerals; it will pull them from your body such as sodium from your blood and calcium from your bones.

    • Luis Sierra says:

      Interesting, was thinking about whether it was just because of the taste, thanks for the exmplanation

    • Dean Lawson says:

      Wow that’s fascinating. I suspected there was likely more to this than just taste. Thanks for this explanation!

    • aboriginal silkworm rancher says:

      your body absorbs the water from the water

    • HeRo TuRtle says:

      yes, that was exactly my thought as well! thank you for putting this out here. it’s not “deadly” (as a few claim), but definitely not healthy in the long run.

      the simple explanation for this: it’s not the type of water we’re used to. we drink water than ran over stone first (and absorbed those minerals) as opposed to plants that naturally get rained on directly and therefore fare less well when watered with mineralized water (like from a tap; again, they’ll survive; but do grow better with desalinated water.)

  8. Sean O'Brien says:

    An idea I’ve long wondered about in areas where seaside land is fairly cheap, such as the Middle East: seawater canals with an arched greenhouse over the top with collection gutters on the sides. Water from the canal evaporates, condenses on the greenhouse, and runs into the collection troughs. I’m sure this would be more complicated in practice, but it seems like a good way to use solar energy in a passive manner.

    • BigKatz says:

      that’s the best idea in the comment section (I’m a scientist in this field)

    • yeahitskimmel says:

      Dang that’s pretty smart and simple

    • Tech Heck says:

      water like that would start growing organisms

    • filonin2 says:

      @Tech Heck So? They can be filtered out easily. The hard part is removing the salt, which this does.

    • Mira says:

      Armchair science time. This could run into issues with creating flow. There are no rivers flowing through Dubai (other than a creek with the sea on both ends), according to a cursory check of Google Maps. So you’d need to induce flow to prevent salt and organism buildup. Flow could also reduce the efficiency because the new water would need to heat up.

      Also, the space vs. output might be an issue for certain areas, although if you live in a desert there’s probably a lot of available space.

      I’d bet you could get more out of a solution like this with a bunch of (pretty cheap) mirrors too. Cool idea

  9. Daniel Clarke says:

    I’d like to know why the “Ocean Vapor Towers” approach doesnt make more sense than these methods, it seems a logical way to get fresh-water from salt-water and it leverages cycles so nature is doing a significant chunk of the work

    • glasslinger says:

      It’s slow per liter of water treated compared to other methods. Thus for a given amount of water the equipment has to be MUCH larger. (more expensive)

    • nahkaimurrao says:

      Basically scale, the number of towers you would need is astronomical

    • sneedium miner says:

      construction cost vs output. running cost isn’t everything

    • hedgehog3180 says:

      At that point you’re basically engaged in a small geoengineering project since you’re trying to change the local climate and that is basically impossible to predict because you have to factor in the entire global climate to figure out what will happen. No one in their right mind would sign off on spending millions on a project that you can’t even guarantee the outcome of. If a region doesn’t get a lot of rainfall then there’s some pretty powerful natural forces behind it and it’s fairly easy to conclude that humans can’t easily influence that. Like climate change is only happening because our entire global civilization is engaged in the same activity and has been doing so for about two centuries so it really isn’t easy to influence the climate of our planet.

      Also there’s the fact that you can’t necessarily directly use rainwater, we usually use groundwater which has spent thousands of years filtering through the ground to make it clean. Plants that draw directly from sources such as lakes and rivers still do need some amount of cleaning before it becomes safe to drink and it’s susceptible to ground pollution, which an area that experiences little rainfall that would normally wash it out probably has a lot of.

    • seneca983 says:

      Based on this video it seems that reverse osmosis is a *lot* more energy efficient than distillation. It may seem attractive to use sunlight directly for evaporation instead of electricity. However, if sunlight is plentiful it might still be more efficient to convert that into electricity and use it to run reverse osmosis.

  10. Orson Peters says:

    Grady, it might be interesting to do a video on how drinkwater is made in the west coast of the Netherlands. Here the regular groundwater is too brackish, so we combine rainwater with river water by pumping it into our sand dunes, where the sand naturally filters it before we pump it out of the reservoir underneath the dunes.

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