We have been discussing the laws of borer. A commonly asked question is whether it is permissible to use a water filter on Shabbos. The answer to this question would depend on the individual person and the situation. The Shulchan Aruch and Rama in 319:10 say that one may filter water from any sediment floating inside. The reason is that most people would drink the water in this state. The Mishna Berura brings an exception to this rule. If someone would not drink the water with this sediment, then for him it is forbidden to use a filter. There is a controversy amongst poskim if this ruling applies nowadays to our water. Being that the unwanted parts of the water are not noticeable, and the water looks the same before and after the filtering process, some say that there is no problem of borer. Therefore, even someone that would not drink the unfiltered water, may filter the water on Shabbos.
Can one use a filter if he is worried that there may be bugs in the water? If you know that there are bugs in the water, and need to filter the water to make the water permissible to drink, it would be forbidden to filter this water on Shabbos for drinking purposes. If you assume, however, that there are not bugs in the water, and just want to use a filter to be extra careful, then some poskim allow the use of a filter. Each individual should ask a competent halachic authority.