Water Filters on Shabbos

Can a water filter be used on Shabbos? One of the 39 forbidden labors of Shabbos is borer - separating - and a water filter separates the water from dirt particles and bugs (which is precisely the whole reason for using it). Can one separate water from offensive particles?

This is not a new question. People have been using water filters for a long time, particularly on Passover. The Kaf Ha-Hayim (319:120) records that the practice in Jerusalem was to use a crude water filter to keep out the bugs. If such great scholars - as many of the residents of Jerusalem at that time were - would do such a thing, then it is evidently permissible. Similarly, Kinyan Torah (2:66) writes that the practice in Hungary was to use a water filter. The question is why this is permissible.

The Hazon Ish (Hilkhos Shabbos, no. 53) adresses this issue. If there is only a preference for filtered water but not a requirement, then the water is really drinkable both before and after filtering. Therefore, the filter is not really serving the function of changing the water because the water was perfectly good before the filtering. The Hazon Ish also recommends allowing a little filtered water to flow into the sink, and not only into a cup, to show that one has no specific desire for filtered water and is willing to waste it a little.

R. Yitzhak Ya'akov Weiss (Minhas Yitzhak 7:23), who was for many years a rabbinic judge in Manchester, England, dealt with exactly this issue (in a 1976 responsum). There were small bugs in the water system and many were using filters to keep out the bugs. R. Weiss relates that there are three situations:

1. When most people do not necessarily desire filtering, in which case filtering is entirely permissible.
2. When most people would want filtering, in which case filtering is permissible only in a different manner (with a shinui).
3. When everyone would need filtering, in which case it is entirely prohibited. Although, some would say that even in this case it is permissible with a shinui.

The Hayei Adam (16:8, Nishmas Adam) discusses the case of water that people would, in theory, drink except is prohibited to be drunk. This is exactly our case (if one accepts that the water is prohibited). The majority of New Yorkers are not observant Jews and do not think twice about drinking unfiltered water. The Hayei Adam brings proofs back and forth but leaves the question unanswered.

R. Weiss suggests that since once the filter is attached to the sink, the normal (and only) way to get water is through the filter, getting water in that way is considered using one's hand. Therefore, taking water with one's hand to use shortly is a permissible form of separation. However, he adds that one should follow the Hazon Ish's suggestion of allowing a little water to go to waste. He also writes that one who puts some sort of shinui on the filter for Shabbos is praiseworthy.

The rabbinic authorities in the Breuers community were evidently not convinced by this argument, if we accept the notice I received as authentic. So ask you rabbi for guidance on this issue.

See also: Az Nidberu 8:8; Shemiras Shabbos Ke-Hilkhasah 3:56 and n. 163; Ketzos Ha-Shulhan 125:37