Filtering Water on Shabbos

Ask the Rov: Can I use a water filter on Shabbos?

By Rabbi Chaim Hillel Raskin – Rov of Anash in Petach Tikvah

Halacha states that filtering a liquid from visible impurities transgresses a tolda of borer — selecting the waste from the good — unless the water is drinkable for most people without filtering, in which case there is no real waste.¹ While Shulchan Aruch warns that using a cloth may constitute the melacha of melabein, laundering, or may lead to sechita, squeezing out the water afterward, this is not of concern with a designated filter.

What if the liquid is only barely drinkable without filtering? While some acharonim write that if the liquid is at all drinkable, it’s not true borer and may be done with a shinuy, others prohibit it, and the Alter Rebbe writes to be machmir.²

Regular tap water that is fit to drink may be filtered.³ Even if people don’t like its taste, poskim posit that the “waste” in this case — the unpleasant taste — isn’t apparent to the eye and, therefore, may be removed.⁴

What if there are minuscule bugs in the water that require filtering for kashrus concerns?

If one follows a view that the water is forbidden and undrinkable as is, some poskim would prohibit it. Others are still lenient if the majority of the local population (i.e., non-Jews) drink it unfiltered since the bugs are few and not easily seen.⁵ There is additional room for leniency when it isn’t certain that there are bugs in that cup.⁶ Some under-the-sink filters don’t remove and segregate the bugs from the water, but instead cause them to remain behind in the pipe. This leaves even more room for leniency.⁷

While some contemporary poskim prohibit the use of water machines since the water isn’t drinkable while it passes through the filter, the consensus of poskim is to be lenient.⁸

See Sources (open PDF)

From The Weekly Farbrengen by Merkaz Anash