Unfortunately, there are different disposal regulations all over, so there is no one answer of what to do when you’re done with filters. There are, however, a few things to consider. The filters themselves are usually made of relatively harmless material (e.g. metal, sealant, cellulous, glass fiber), so the main factor is what you have been filtering. If you process cardstock or paper, then the filters are likely to have collected soot, which is quite easy to dispose of. If that cardstock has a plastic coating, then it may have emitted formaldehyde, which has much more stringent regulations.