Yesterday’s daf (50) was talking about leaving uncovered water out and how we’re concerned that a snake could come and drink from the water, thus injecting some of its venom into the water. If you were to want to drink the water, the gemara offers the following solution: filter it, even on Shabbos. The question is asked, but if you filter the water on Shabbos, isn’t that borer? You are specifically trying to separate the bad (the venom) from the good (the water)?
The reason we allow people to filter water is because we’re not sure that there is something bad in the first place. It is only there as a precaution. You might not be filtering out anything bad. In this day and age where filters are commonly found in people’s homes, it is not necessary to disable the filters just for Shabbos because there might not necessarily be bugs or whatnot in the water.
Verbiage 
October 23, 2006 at 2:43 pm
Forget bugs…people filter water because they want all of the extra minerals removed before they drink it. In this instance, we are sure there’s something “bad” in the water: those minerals. In that case, perhaps we would have the question of Borer. The reason I’ve always heard isn’t that we aren’t sure if we’re doing something…if it’s a safek, that sounds like a safek d’oraisa l’chumra case. The reason that I know of is that because the water is already ready for consumption (although it may not be to your liking), we are merely improving the water a little bit.
I remember a couple years back when NY had that whole bug issue, that people couldn’t filter their water on Shabbos, because the water was not ready for consumption.
October 23, 2006 at 11:10 pm
Alan to be fair, the maggid shiur actually mentioned the bug case in particular, so perhaps he was referring to that exclusively.
October 24, 2006 at 10:36 am
Alan,
I’ve heard that if you are filtering because of minerals and stuff then it is permitted on shabbos as long as you would drink it without the filtering as well. A person who would never drink water from the tap unless it was filtered is not allowed to filter it on shabbos.
October 25, 2006 at 7:29 pm
Hey Erica, long time no see! When are you going to update your blog?
October 26, 2006 at 1:40 pm
I learned that you could use a brita filter if you would normally drink the water if it was not filtered.
October 30, 2006 at 11:06 am
My life is too crazy to blog – we have not had our internet working again since we moved so I can’t really do much of anything anyway. You’re not missing much though, don’t worry. Keep up the good blog 🙂