Is there a problem on Shabbos of using a water filter on one’s sink in New York City where there are supposedly small bugs in the water? By Turning on the faucet the water goes through a filter? Is this the same halacha if the filter out side of the sink in a portable jug as well?
It is permitted to use the filter on the faucet (those who wish to be stringent may allow the water to flow into the drain for a short time after opening the faucet, before the water is collected). It is better not to use an external filter.
Sources: The question involves the prohibition of borer. Although the water is drunk by most people, those who are wary of drinking it on account of the (supposed) bugs in the water would be considered istenis with regard to the water, and the prohibition of borer would seemingly apply (Biur Halachah, 319, quoting from Peri Megadim). However, there are a number of reasons for which it might be permitted to use the filter, even on Shabbos:
1) Chazon Ish (Orach Chaim 53) writes (as a possibility) that it is permitted to use a teapot on Shabbos, even though a net is present at the spout that prevents the leaves from coming out. Rabbi Chaim Kanievsky has testified that the custom in the Chazon Ish’s house was to use such a teapot. One possible reason for which this is not considered borer is (presumably) that the teapot is not a utensil for separation and filtering, and it is used even when no filtering is required. The same would apply to the filter on a faucet.
2) Rabbi Shlomo Zalman Auerbach (Shemiras Shabbos chap. 3, note 125) explained that it is permitted to use a teapot (with a net/filter) because the utensil is made solely for use immediately before drinking, and therefore its use is not considered derech bereirah but rather derech achilah. This is also the reason for which it is permitted to use a salt-shaker with rice inside, as many poskim have agreed (see Ayil Meshulash, chap. 7, note 110; Shemiras Shabbos 3:60).
3) Minchas Yitzchak (vol. 7, no. 23) relies on the same ruling of Chazon Ish (above) with regard to the question of a filter on a faucet, explaining further that because there is no other way to get the water from the faucet (the filter is on the whole time) it is considered derech achilah (see Rema, end of 321, and Biur Halachah).
4) A number of poskim dispute the ruling of the Chazon Ish concerning a teapot, and there is room to argue that the status of a salt-shaker, which does not include any utensil or parts that are made for the purpose of borer, is different to a filter on a faucet. However, even according to these authorities, there is room to argue that the NY water is not considered pesoles, because the water is in drinkable in principle, and is only avoided on halachic grounds. According to Shulchan Aruch Harav (500:18, based on Magen Avraham 500:12) it would appear that “halachic pesoles” is not considered pesoles for borer purposes. Concerning this idea, see at length Chayei Adam, kelal 16, no. 8, and Nishmas Adam no. 5, who ends the issue with tzarich iyun. On an external (jug) filter, it is therefore better to be stringent.
5) To my knowledge, the entire issue of bugs in NY water has not been fully clarified, and if it remains a safek this might be another reason for leniency (though it can be argued that for those who are stringent, it remains a full problem). For those who wish to be stringent, the water can be allowed to flow into the drain for a couple of seconds after the faucet is opened (and only then collected in a cup or jug), so that there is no question of borer de’oraisa.
[See Yeshurun, vol. 17, p. 535, from which some of the above was taken.]