Hi there, I found a gravity-fed system that you pour water in through the top, and the water slides through ceramic pieces which act as filters, and the bottom reservoir holds the clean drinking water. The system is called the Berkey water filter if you’d like to see how the model works if it’s ok for Shabbat use. Since it is a gravity-fed system that does not require electricity I’m not worried about it for Shabbat use on the electricity front, but I’m worried if in general it would be problematic to pour water into it on Shabbat. Since everything we do on Shabbat and holidays must be for that day only....I’m concerned if I poured in accidentally too much water into the filter, thereby I won’t finish it on Shabbat and there’s some leftover for the next day...is that ok? And in general, is water pouring through this filter considered chilul Shabbat? Thank you so much for your amazing website,
Shalom, Thank you for your question. Whether one may use a (non electric) water filter on Shabat is a very good question. One of the 39 forbidden labors on Shabat is "borrer" – separating or filtering. It is forbidden on Shabat, for example, to filter out the impurities (dregs) from wine. What is the case with water? In most cases we are dealing with filtering water that looks clean even before the filtering. Even though we know that there are impurities in the water, because they are not visible to the naked eye, it is not considered as a forbidden labour. In general the laws of Shabat (and many other areas of Jewish law) relate to the level of visible phenomena, and not the level of physics smaller than that. Based on this, if the water you are filtering looks clear even before filtering, there is no problem putting it through the filter. There are those who maintain that if a person could not drink the water before the filtering, then it is forbidden to filter the water on Shabat. However, there are certainly opinions to rely upon which allow filtering the water when it is clean enough for most people to drink, and only you yourself are fastidious and don't drink such water. If you are dealing with water that is not able to be drunk at all without filtering, that is cloudy and the sand or dirt can be seen in it, then it is certainly going to be problematic to filter it on Shabat. In connection with your question about filtering more water than you need for the Shabbat itself – this is not such a problem as you might imagine. You are correct that it would be forbidden to pour water into the filterer if one's intent was entirely to prepare water for after Shabat. But, if ones needs some of the water on Shabat itself, then it makes no difference if you pour one cup of water into the filter, or ten cups worth. This is because the act of refilling the filter is one act, and because the action is needed for Shabat itself, it is not forbidden to do the same act with just a greater volume of water. Based on this it would be forbidden to add extra cups of water to filter if each cup required a separate act. But if you take a jug full of tap water, and pour all that into the filter system, it makes no difference if you need only a small amount of that water for Shabat or the entire jug. Blessings.