Would it be possible to add a minimal threshold for the filters to be able to absorb/ pull pollution? I find them OP if they can reduce pollution all the way to 0.

I'm not sure how this threshold would make them less OP. The numbers haven't been tweaked or balanced, so I'm always curious about input on which absorption rates sound more fair.

I use next gen evolution, and one of it's settings is that it can slowly increase evolution as trees get damaged by pollution. This happens at 60 units in the chunk.

I've been hot rodding my personal copy of this mod to add such a limit for last few versions, and it served me well so far. Can even just leave it up to a mod setting, which would allow players to simply set it to 0 to continue the current behavior.

The use case you mention sounds good.

If you already have the code, you can make a pull request on the Github page: https://github.com/JoeyDP/Factorio-Better-Air-Filtering.

My code is pretty jury rigged, I just added a check to see if pollution is above a constant in a few functions.
It still had some odd behaviors to it.

I can try to work it out later, though. I haven't updated in a few versions, so it's possibly a bit out of date.