Filtering on Shabbos

Q. May one filter water on Shabbos using a special filtering device?

A. Shulchan Aruch (OC 319:10) writes that one may pour clear wine or clear water through a filter on Shabbos to remove impurities, provided the liquid is drinkable without filtration. Since the particles do not prevent one from drinking the water, removing these particles is not treated as “borer” (separating “pesoles” from ochel” -bad from good). This is not the position of the Rambam, who does not allow the use of a filter on Shabbos under all circumstances, but the halacha follows the lenient view. The Mishnah Berurah (319:34) writes that poskim dispute to what degree must the liquid be potable to allow filtration. The Bach requires that it be suitable for everyone, but most poskim maintain that filtration is permissible if most people would drink the water or beverage. The latter opinion is generally followed. The Mishnah Berurah (Beiur Halachah s.v. ho’il) adds that a fastidious person who would not drink the liquid without filtration, may not do so on Shabbos, even though the majority of people would not care. Rav Elyashiv (cited in Hilchos Shabbos B’Shabbos 1:9, note 32) writes that a fastidious person only refers to one who would not drink the liquid even under extenuating circumstances (such as when they are very thirsty).

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The Gerald & Karin Feldhamer OU Kosher Halacha Yomis is dedicated to the memory of Rav Yisroel Belsky, zt"l, who served as halachic consultant for OU Kosher for more than 28 years; many of the responses in Halacha Yomis are based on the rulings of Rabbi Belsky. Subscribe to the Halacha Yomis daily email here.