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How can paper recyclers know how many times a given input paper has been recycled?

I've read that paper can be recycled "up to 6 times." I'm imagining a paper recycling plant that gets a ton of newspapers, books, cardboard, gift wrapping, and other household and industrial paper waste in their input stream.

If there is a hard limit on "6 recycling" for paper, I imagine the recycling plant would need to reject items that have been recycled too many times.

How can a recycling plant possibly know how many times a given input paper source has already been recycled to ensure the output paper stays within the recycle-no-more-than-6-times limit?

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    I would assume, though I can't be certain, that recycling companies aren't "pre-sorting" recycled paper, but separating the "bad" fibres during the re-pulping process, whether through chemical treatment, 'skimming' or other process.
    – Robotnik
    Sep 14, 2021 at 2:57
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    Separation is done via a "sludge" process - unwanted ooze settles and is extracted before the fibrous mass is re-pressed into new paper. The ooze is often mostly clay from gloss paper, but depending on the input mix will have a lot of short fibre and contaminants in it (plastic fragments are particularly problematic as if they get through they don't accept ink... and that makes the paper much less useful)
    – Móż
    Sep 16, 2021 at 3:07

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