We are beginning siman 13! There is an age-old question regarding borer and filters. Historically, people would make a bag out of a tight-knit cloth, and eventually the liquid would seep through. This bag is a problem, because the bag is considered a kli for the purpose of filtering. It is either borer or merakeid (sifting).
The one exception is when the liquid is clear, and people would use the liquid as-is. Therefore, filtering the water is not considered borer, because any pesoles in question is not considered halachic pesoles. It is considered that the person is filtering their water for extra vigilance or the like, and it is muttar.
Another exception: The Rosh also discusses even filtering actual pesoles while one is drinking, such as by placing a filter over one’s mouth while drinking, and writes that it will be muttar.
Although there is not an issue with borer, the question as it relates to dosh is whether there is an issur derabanan in absorbing the liquid through the cloth bag because one may then possibly squeeze it out. By doing so, one may transgress dosh and/or melabein, so maybe Chazal established a gezeira against using the filter at all on Shabbos, out of concern that one may squeeze it. The Chayei Adam will discuss this question in this siman.
The Chayei Adam writes that there is no concern of melabein in liquids other than water and white wine.Therefore, it is muttar to filter a clear liquid which is not water, and there is no gezeira out of concern for dosh or melabein. The Chayei Adam will explain why there is no issue of dosh in a moment, but first gives another example.
Although there are limitations when it comes to refuah on Shabbos, one situation in which refuah would be muttar is choli kol haguf, where the entire body is affected by the issue. He writes that if one deliberately wets a cloth to use as a compress for a headache, it would certainly be an issue to wet it with water, as it would be melabein. However, it is muttar to use other liquids, such as wine or milk, for the same reason above, because there is no gezeira in this case.
The Chayei Adam now explains why there is no issue of schita/dosh. He writes that Chazal did not extend their gezeira to these types of cases, because the liquid in these cases (filtering, compress) is not normally retrieved in these cases, so we are not concerned that a person will suddenly forget himself and retrieve the liquid. Additionally, there is no concern of laundering the item, because it remains dirty when using other liquids. However it would be assur to use water, since water would actually cleanse the item, both in terms of soaking the item and in terms of squeezing it.
Summary
Using a cloth to filter a liquid will be assur many times because of borer or merakeid.
There are cases were borer or merakeid will not apply. Two examples were given.
a) clear liquids which people use as is.
b) drinking directly through a filter.
Filtering water would be assur, even when borer is not an issue, because of melabein which occurs when the water soaks through the cloth.
There is no issue with filtering when one uses liquids other than water.
a) Melabein doesn’t apply because these liquids dirty the cloth rather than cleansing it.
b) Dosh would only apply when one squeezes the liquid out.
c) People in general do not squeeze the cloth to retrieve the liquid when used in these types of amounts and circumstances, so Chazal did not prohibit soaking because of a fear that one may subsequently squeeze the item.
d) In situations where squeezing to retrieve the liquid is normal, Chazal prohibited soaking the cloth whether with water or other liquids, because of a concern that it may lead to squeezing.
