Hello,
First: good mod, congrats.
Now to the point, since I upgraded to .21 the mod doesn't work.
When I try to put a unused filter inside a machine it says: "Unused air filter cannot be smelted".
(I'm using the game version from the factorio page)

I hope that this info helps. Sorry if I made any mistakes with my english.

I am having the same issue and its two years later :s

Hey,

I'd need more info for this one. Which Factorio version and mod version are you running? Did you upgrade from an older mod version? Was your save an old one?
Thanks!

Greetings,
Schorty

The newest game version (1.1.80) and the newest mod version (0.8.3) in my case.
This problem appeared just now.