Why is desalination waste usually disposed back into the sea instead of converting it into chemical products?

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?




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](https://en.wikipedia.org/wiki/Reverse_osmosis)) or ions(like in [electrodialysis](https://en.wikipedia.org/wiki/Electrodialysis)) between 2 separated volumes. All numbers are just illustrative.


It is technologically/economically advantageous in [desalination](https://en.wikipedia.org/wiki/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](https://en.wikipedia.org/wiki/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.




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For curiosity, here is the [Wikipedia table](https://en.wikipedia.org/wiki/Desalination#Energy_consumption) of energy consuption of chosen methods:





| Desalination Method | [Multi-stage flash  distillation-MSF](https://en.wikipedia.org/wiki/Multi-stage_flash_distillation) | [Multiple-effect distillation-MED](https://en.wikipedia.org/wiki/Multiple-effect_distillation) | [Mechanical Vapor  Compression MVC](https://en.wikipedia.org/wiki/Vapor-compression_desalination) | [Reverse  osmosis-RO](https://en.wikipedia.org/wiki/Reverse_osmosis) |
| --- | --- | --- | --- | --- |
| 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](https://en.wikipedia.org/wiki/Desalination#Technologies) for the extensive list.


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