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(Being asked as a theoretical question. Obviously everyone should ask their LOR.)

There is a water filter (or purifier) that works in the following way: The water flows into a part where there is some sort of "chemical" (in the shape of round white balls) that help to "clean the water". The water then pours from there out going through a normal sort of filtering part. If we look at the water is the "ochel" (food) and the white balls as the "pasules" (not good part) is there a problem of borer in the fact that the water (the "ochel" in this case) passes through this filter, "separating" it from the white balls (the "pasoles" in this case.) Perhaps this is "borer b'cli" (separating with a vessel) and would be prohibited on Shabbos.

I'm pointing this out because it's not a "normal case" of using a filter where the water simply passes through a sort of sifting part (which could be muter on Shabbos if a person would drink this water without having used the filter.) The water is getting mixed together with some sort of chemical then "separated" from that so in regards to the chemical we have a "mixture" of "ochel and pasoles".)

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Can you link to or show this filter? I've never heard of a filter like this. To filter water, the water has to flow through the filter, not just near it. –  Ariel Apr 23 '13 at 1:14
    
@Ariel of course it flows through the filter! The "purifying process" happens when it passes through and mixes with this "chomer" or whatever it is. Then the water exists through a point on the cli where there is a normal "filter". The information I gathered from someone who installs many types of filters in peoples homes. –  Yehoshua Apr 23 '13 at 6:01
    
So what is this "chomer"? I've never heard of a filter with such a thing. Can you link to any examples? –  Ariel Apr 23 '13 at 6:11
    
BTW I'm not asking to be annoying - understanding the construction of the filter can help understand what exactly is happening to the water. –  Ariel Apr 23 '13 at 6:16
    
@Ariel Many filters work like this actually. In some way or another. How else is the water "cleaned"? I will try and find an example online. The point is the water gets to a stage where one may not be able to drink it when it's running through the part with this "chomer". –  Yehoshua Apr 23 '13 at 9:03
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2 Answers

Rav Nissan Kaplan just discussed this aspect in his weekly Halacha Shiur in the Mir. Download the shiur at Using Water Filter on Shabbos

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summarizing the content of the shiur would be helpful otherwise this would be better served as a comment –  not-allowed to change my name Apr 24 '13 at 0:55
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What's the outcome from the shiur? –  Yehoshua May 1 '13 at 13:51
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Mori WeRabi, Harav Mansour Shelit"a holds that it is Mutar to use a water filter on Shabbat providing that the water is drinkable without the filter.

source

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