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I was reading about the desalination processes such as reverse osmosis and capacitive deionization and their alternatives. They all work on removing ions from water and disposing them back to sea/ocean in the form of brine.

This got me thinking why do they have to move the previously dissolved ions to another liquid, forming brine? Why don't they dispose these ions in their “normal” form without dissolving them again? Does it has anything to do with their reactivity/danger?

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  • $\begingroup$ @BelalBahaa - Do you mean something like why isn't the ionic aqueous sodium removed as sodium metal, and the chloride removed as gas? $\endgroup$
    – Todd Minehardt
    Jun 6 at 23:08
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    $\begingroup$ It costs far more to completely separate ions from seawater than it does to partition it into less-concentrated (not perfectly pure, but drinkable and usable in agriculture and industry), and more-concentrated waste (i.e., brine, containing the unwanted ions). Water is used in massive amounts, and the most-economical process wins. $\endgroup$ Jun 7 at 0:03
  • $\begingroup$ @ToddMinehardt I mean why aren’t they removed as a sodium ion and chlorine ions separately without dissolving them in another electrolyte for disposal? $\endgroup$ Jun 7 at 0:45
  • $\begingroup$ @DrMoishePippik Is cost the only reason or are there other reasons for it? $\endgroup$ Jun 7 at 0:46
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    $\begingroup$ @BelalBahaa, Sodium and Chloride ion cannot exist separately in different solutions. $\endgroup$
    – AChem
    Jun 7 at 0:58

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There is no dissolving them again. Ions remain dissolved all the time.

It is like taking

  • $\pu{11 m3}$ of sea water with salinity e.g. $\ce{30 g/L}$,

producing

  • $\pu{1 m3}$ of (about) pure water (with residual salt)
  • $\pu{10 m3}$ of sea water with salinity $\pu{33 g/L}$

by active redistribution of water (like in reverse osmosis) or ions(like in electrodialysis) between 2 separated volumes. All numbers are just illustrative.

It is technologically/economically advantageous in desalination to use big excess of input sea water to keep the salinity difference between the input water and the waste water small. With big difference, the process would be less efficient and more demanding.

This is kind of analogical to efficiency of heat pumps, used for building heating. Maximal efficiency is reached at the minimal difference between interior and exterior temperature.

There would be just minor difference between processing of the waste water from desalination and raw sea water, if we wanted to get minerals from the waste water. Additionally, there would be large water volumes, questioning capacity to process it and demand for its output.


For curiosity, here is the Wikipedia table of energy consuption of chosen methods:

Desalination Method Multi-stage flash
distillation-MSF
Multiple-effect
distillation-MED
Mechanical Vapor
Compression MVC
Reverse
osmosis-RO
Electrical energy (kWh/m3) 4–6 1.5–2.5 7–12 3–5.5
Thermal energy (kWh/m3) 50–110 60–110 None None
Electrical equivalent of thermal energy (kWh/m3) 9.5–19.5 5–8.5 None None
Total equivalent electrical energy (kWh/m3) 13.5–25.5 6.5–11 7–12 3–5.5

See Desalination Technologies for the extensive list.

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  • $\begingroup$ Also, by not making waste brine too brackish, it washes away waste which might precipitate and clog filters and plumbing. $\endgroup$ Jun 7 at 12:05
  • $\begingroup$ You seem to know your water separation so can you explain why they do electrodialysis or RO when in hot environments, they can use vacuum distillation for it and then reroute that steam using a closed system in some form of vapor recirculation. The incessant insolation will supply enough energy I'd guess and then its just vacuum pump's operating cost and you can use water to cool its internals so that heat isn't wasted either. Route the facilities cooling system's heat to the brine as well. Total heat capture and then reuse. $\endgroup$ Jun 7 at 16:14
  • $\begingroup$ Distillation is energetically less favourable. $\endgroup$
    – Poutnik
    Jun 7 at 16:32

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