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Various methods of desalination (turning salty seawater into drinkable freshwater) have existed for a long time. Thermal desalination is perhaps the simplest method of doing so; heat up the seawater until it evaporates, then condense the pure water on a cool surface. Alternatively, you can do the opposite - freeze the salt water, wait for a while, and then melt it. However, both of these methods have a major problem - it is not cost-efficient to do in significant quantities.

Today, we have several more advanced methods. The most common is reverse osmosis; however, reverse osmosis requires advanced membrane technology that didn't come around until the 1950s and 1960s. The second most common, multi-stage flash distillation, was invented in the late 1950s. MSF is significantly less efficient then RO in several ways (it both requires more energy and has a lower yield), but is still commercially viable.

My question is, given IRL technological pace, what's the earliest point in time when a commercially viable method of desalination could have been invented? Many technologies could have been invented long before they actually were, while others simply could not have. Are there any efficient methods of desalination that fall into the first category, and if so, when could they earliest have plausibly been invented?

I'm asking this question for an alternate history I am writing, if you're wondering why I asked it in the worldbuilding StackExchange. If you think a different StackExchange would be better for answering this question, I'll try asking it there.

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  • $\begingroup$ @JBH I usually understand IRL to mean "in real life". Fair enough to ask for clarification, though :) $\endgroup$
    – Qami
    Apr 28, 2021 at 14:27
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    $\begingroup$ @JBH, that sounds like a frame challenge, better posted as an answer $\endgroup$
    – L.Dutch ♦
    Apr 28, 2021 at 14:42
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    $\begingroup$ Depends very much of what you mean by "commercially viable". Current desalination technology, at less that 1 USD per cubic meter of desalinated water, allows for (high efficiency) agriculture growing expensive crops. In the 19th century they had desalination technology (basically, evaporators and condensers) which was perfectly viable for their needs (supplying fresh water to steam ships -- steam ships absolutely need a source of fresh water for long distance cruises, and they carried a lot of coal anyway). $\endgroup$
    – AlexP
    Apr 28, 2021 at 15:42
  • $\begingroup$ @Qami You're probably right, but the OP's use of other abbreviations makes it unclear. $\endgroup$
    – JBH
    Apr 28, 2021 at 16:29

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What's cost effective depends on what the costs are. If thermal energy is very cheap, thermal desalination becomes much more cost effective.

Cheap thermal energy generally comes in two flavors: solar and geothermal.

Focusing sunlight with mirrors was known as long ago as classical times (or at least is legended to have have been). Archimedes' Death Ray was reputed to have burned a ship with sunlight reflected by polished bronze shields, and though this has since been shown to be unlikely, whenever that legend originated folk clearly knew that concentrating sunlight for heat was possible.

Likewise, geothermal energy has been recognized, in the forms of geysers, fumaroles, hot springs and the like, from ancient times. Heat exchange to evaporate water (to be condensed away from the hot area) could have been as simple as a closed pot immersed in the hot pool.

The ancients were also very good at scaling up individual labor (see Roman heavy equipment powered by men walking in a giant hamster wheel), so scaling this kind of thing up enough to produce commercial quantities doesn't seem out of the question -- and especially with solar heat, expansion isn't limited to a small area as it would be with most surface geothermal sources.

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Multistage flash: Could at latest be built around the time of the first modern age steam engines, essentially you need to build pressure vessels and pipes. Expect as many accidents as with the first steam engines.

RO: difficult to say, but far later - RO requires high pressure so you need decent pumps, but more crucially you need RO membranes - IRL they where invented in the 1950ties, maybe early 20th century at best.

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  • $\begingroup$ Animal intestines can be used as osmotic membranes (a phenomenon that was demonstrated long before the 1950s) -- not sure whether there's a way to support them against pressure to use them for RO, however. $\endgroup$
    – Zeiss Ikon
    Apr 28, 2021 at 14:14
  • $\begingroup$ There is: you could wrap a sturdy net around the intestines to sustain the pressure. $\endgroup$
    – mart
    Apr 28, 2021 at 14:45
  • $\begingroup$ I imagine a cottage industry cleaning pig intestines and turning them inside out, joining them to a stub of bronze pipe, and stuffing them into a fine net bag to go inside a pot... $\endgroup$
    – Zeiss Ikon
    Apr 28, 2021 at 14:54
  • $\begingroup$ I mean people (I'm people) eat sausage it can't be that unhygienic? $\endgroup$
    – mart
    Apr 28, 2021 at 15:03
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    $\begingroup$ Nah, I'm just thinking of how you'd do RO membranes en masse with pre-medieval technology... But this would still requiring knowing about osmosis and the possibility of running it in reverse. $\endgroup$
    – Zeiss Ikon
    Apr 28, 2021 at 15:34
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Our early 1800s technology already had everything that would be needed to build Multi-Stage Flash or Multi-Effect desalinators. Still desalination was possible (and used) much earlier that that (but its yields are lower). Reverse Osmosis is much more difficult, I personally doubt that we can get any practical yields using animal intestines (@Zeiss Ikon), even if the process is refined to perfection.

What makes this question difficult to answer is "commercially viable" requirement. As @AlexP pointed out, there are multiple usage scenarios, and what is perfectly viable in one scenario would be prohibitively expensive in the other.

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In cold climates cryodesalination (salt water freezes to ice, applying pressure to crushed ice makes salt water leave first) would be feasible. If the environment permits this anything that can be made into an decently tight approximation of a piston allows this technology, metal work would help but it could be done solely by people standing on shaped stones made by rubbing them together.

So for it to be commercially viable the invention of "commercialness" seems to be the bottleneck.

I would imagine that a single person could make could make more than water for himself. So surplus water/(unsalty ice) could be traded for goods or stored for ship expeditions in the summer.

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