Honestly I think nobody's invented a good water desalination device because nobody knows the scientific definition of clean drinking water.

So scientifically, what is the definition of clean fresh water?

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    $\begingroup$ Fresh water for what purpose? $\endgroup$
    – Jan Doggen
    Jun 27 '19 at 8:23
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    $\begingroup$ I'm sure some government(s) somewhere have a definition and a process for testing if water is 'fresh" by that definition. That's where I'd look first. Generally, it's the quantity of non-water things in the water, but some non-water things make water less fresh in smaller quantities. Interestingly, even ads/specs for water filtration devices (like BRITA) will tell what particulates they remove that presumably make water non-fresh. epa.gov/dwstandardsregulations may help $\endgroup$
    – user967
    Jun 27 '19 at 15:07
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    $\begingroup$ I dispute the idea that nobody has invented a good water desalination device. Hundreds of thousands of people live off desalinated water. I also dispute the idea that nobody knows a scientific definition of "clean fresh water" - for example, every government and public water authority will have a definition of this. Some other people who use water will also have definitions of this, and they will probably be different. $\endgroup$ Jun 27 '19 at 18:35
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    $\begingroup$ I don't think there is a definition for fresh water, but there is for potable water $\endgroup$
    – Fred
    Jun 28 '19 at 4:43
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    $\begingroup$ Sorry folks, you've all made me realize I should have defined my question differently. $\endgroup$
    – leeand00
    Jun 28 '19 at 15:13

I think everyone would agree that distilled water is clean fresh water, but it wouldn't be practical to supply a large population with a water supply made up entirely of distilled water. Good water desalination devices using semi-permeable membranes have been invented and are used in some drought stricken countries. They can filter out things as small as sodium and chlorine ions, so produce very high quality water, but it tends to be expensive. When you are talking about a water supply for millions of people, these expensive purification methods are not practical for everyday purposes.

The basis of a mass supply of clean, fresh water suitable for drinking is filtration, so water from reservoirs is run through filtration beds of sand to remove particulate matter and most micro-organisms. It then has to be sterilised, which is done with chlorine. The result is a product which although containing a number of impurities in trace amounts (chlorine, calcium, iron etc) which are not harmful, fulfils the scientific definition of clean, fresh water suitable for drinking. Water from deep boreholes is also suitable despite trace amounts of mineral impurities, and is free of harmful micro-organisms so needs no sterilisation. Shallower wells and boreholes may be contaminated by pathogenic microbes, so the water sometimes needs sterilisation.


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