Passive Desalination Discovers How To Avoid Salt-Clogging

Saltwater is plentiful, but no good for drinking. Desalinization is the obvious solution, but a big problem isn’t taking the salt out, it’s where all that leftover salt goes. Excess salt accumulates, crystallizes, collects, and clogs a system. Dealing with this means maintenance, which means higher costs, which ultimately limits scalability.

The good news is that engineers at MIT and in China have succeeded in creating a desalination system that avoids this problem by intrinsically flushing accumulated salt as it is created, keeping the system clean. And what’s more, the whole thing is both scalable and entirely passive. The required energy all comes from gravity and the sun’s heat.

To do this, the device is constructed in such a way that it mimics the thermohaline circulation of the ocean on a small scale. This is a process in which temperature and density differentials drive a constant circulation and exchange. In the team’s system, this ultimately flushes concentrations of salt out of the system before it has a chance to collect.

The entirely passive nature of the device, its scalability, and the fact that it could desalinate water without accumulating salt for years means an extremely low cost to operate. The operating principle makes sense, but of course, it is careful engineering that shows it is actually possible. We have seen projects leveraging the passive heating and circulation of water before, but this is a whole new angle on letting the sun do the work.

15 thoughts on “Passive Desalination Discovers How To Avoid Salt-Clogging

  1. The real question is if the design improvement on this passive system and be adapted to other active systems. Passive desalination isn’t going to produce enough water for crops.

    1. Passive solar desalinization doesn’t have to be just a source of drinking water. It also represents a kind of cooling, since pure water can evaporate to cool a space and concentrated brine can absorb water vapor to lower humidity. Or to store heat for later without a conventional thermal mass or battery. (Using e.g. CaCl2, you produce heat by dilution with water, but a cycle can form only with a stronger device that can remove water from nearly saturated brines rather than this sort of thing.)

  2. That doesn’t solve what to do with the salt. Even small cities use millions of gallons of water per day. If you scale this up to be the sole source for a city you’re concentrating hundreds of thousands of pounds of salts per day. If you do the cheap solution & dump it in the ocean you’re going to destroy the local in habitats with brine.

    1. Right? Keeping the desalination device from clogging is certainly *a* problem, but it’s far from *the* problem that most needs to be addressed. “Where all that leftover salt goes” isn’t addressed by this.

      “A big problem isn’t taking the salt out” is also a false assertion. Doing so currently takes massive amounts of energy. A passive solution leveraging thermal solar energy, which this purports to be, is a much bigger deal than any of its self-cleaning properties (but, still, leaves “where all the leftover salt goes” an open question).

    2. Lots of salt from long dry seas is actively mined and shipped around the globe anyway – and folks like salt rather more than is good for them. I don’t think finding a use for the salt produced in the few places globally such a system might make sense would matter at all. Though as this system sounds like it doesn’t actually gather any salt, as gathering the salt gums up the works…

      And as oceans are fully of dynamic moving water as long as you are only ever removing a small amount of fresh water compared to the volume of the nearby salt water source it is basically not going to notice at the local level, and with the relatively rapidly wider cycle of water with the tides and the scale of the worlds oceans they won’t notice either.

      1. All of the salt deposits we mine at an industrial scale are somewhat pure. When an inland sea dries up you get different salts concentrated in different locations. This creates a brine which is still a mix of every salt in the ocean along with microplastics and other pollutants. So your artisinal sea salt still needs processing

        “As long as you’re only removing a small volume of water…” that’s the problem, they won’t be. Again, were talking millions, or tens of millions of gallons per day all day every day. For a city like San Diego or Dubai you’re going to create a hypersaline plume around the plant. Compounded by the fact that these installations will be built in protected bays and lagoons where the natural water flow is slow and wave action is minimal.

        1. I fail to see how enough water could possibly be removed at once to make a dent on the salinity of say an ocean. Doesn’t the water eventually get back to the sea? It’s not like the water destroyed, for example it might be consumed, circulated, expelled, disposed, treated, and eventually evaporated or dumped. The water cycle will continue even if there are a few more steps in this case.

    3. This is not as big a problem as you might think. Back of the napkin math says 1.43e+12 tonnes of water evaporate from the oceans per day. This is more a matter of concentrating it in a single place, which can be managed.

    4. Why can’t the treated waste water that would otherwise be released back in to the ocean be mixed with the concentrated salt water to dilute it?
      That is if it’s not being recycled back into the system to reduce the need for desalination in the first place (which it should).

  3. When I first saw this last week I thought that I’d worry less about salt clogging than I would about bio-clogging.

    You’re using seawater as a raw material, which is full of microscopic life. Then you’re putting that into a sunny environment, sheltered from predators, with a constant warm stream of nutrient-filled water trickling though.

    Maybe I don’t understand the mitigation, but I just don’t see how this doesn’t clog up with algae inside of the first week.

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