Old Tzitzis

What should be done with old pairs of tzitzis that cannot be worn anymore? (torn and/or possul)

Since the Tzitzit is not a holy article (Tashmish Kedushah) but only a Mitzvah article (Tashmish Mitzvah); a Tzitzit that wore out should not be thrown or used in a disrespectful way but does not have to be placed in Genizah; it can be disposed in a plastic bag in a respectful way. Best will be to recycle the strings to be used as bookmarks in Sifrei Kodesh. (Kitzur Shulchan Aruch 9; 19)