We use a Brita filter for our tap water. Those filters can be bought almost everywhere: at the supermarkets that are big enough to have a non-food department, at the Mediamarkt, at some household stores.
But you can’t recycle them in the store where you buy those filters.
I gave up on asking in the stores and emailed the Brita office. They have a really good service: I can download a special coupon, pack at least 6 filters together and bring them to MRW. From there I can send them free of charge to the Barcelona office.
MRW is some kind of Spanish DHL. They deliver parcels all over Spain. And they are in Mazarrón, almost within walking distance from our house. What more does an environmentally-conscious consumer want?