Question

Hello Rabbi
 
At home we use a “Brita” device for drinking water. It’s a pitcher with a filter that looks like a sponge; you fill it with water and it flows through the filter into the lower section, from which it can be poured. I personally am completely fine with tap water, but my wife prefers filtered water (for herself and for the children).
 
On the Sabbath, a couple of friends were staying with us and drew my attention to halakhic problems that I didn’t know about and hadn’t thought of.
 
1. Selecting. Is it permitted on the Sabbath to fill water into the pitcher? What you can see is that all the water goes down through the filter. The sponge is supposed to absorb tiny bits of dirt.
 
2. Preparation from sacred to weekday. Is it permitted to fill the pitcher completely on the Sabbath, or do you always have to estimate whether the filtered water will be used on the Sabbath or only afterward? What about putting water into the refrigerator on the Sabbath (for example, putting the pitcher in)? If someone doesn’t care about filtered water (like me), is it preferable for him to avoid using the Brita this way?
 
3. I’m not sure exactly how to ask this, but “how severe is this transgression” if it is indeed forbidden and I didn’t know, because I hadn’t learned and hadn’t thought enough about every single thing? I’m a simple Jew, and I know the laws of the Sabbath in a practical way from what I saw in my father’s home and from lessons I heard and laws I read in various places. I have not had the privilege of studying the laws of the Sabbath in an organized way, and I need to decide whether it is reasonable on my part to go on with life without learning all the laws (it is very hard for me!).
 
Thank you very much