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Lately, postal cardboard packages have been arriving with an increasing level of exterior contamination with generic black dirt. Although the composition of this dirt is unknown, this can be a concern in manufacturing certain recycled products, such as food packaging. Since it is generally infeasible to separate the contaminated portions, this has also become an issue for me in maintaining a recycling stream of the highest possible quality.

My question is then twofold. From the perspective of the recycler, in particular in the US, to what extent can such contamination be tolerated or mitigated? From the perspective of the consumer, what should be the threshold for sending such cardboard instead to the landfill stream?

  • @LShaver Not a good duplicate. From what I understand contamination with oil/fat is really difficult (don't recycle your dripping pizza boxes), but other substances may be less of a problem. It does not help of course that the OP is unable to determine what the generic black dirt is. – user2451 Mar 6 '17 at 19:51
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    Do you a better description of the dirt? Maybe a photo? – user2451 Mar 6 '17 at 19:52
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    Cardboard doesn't start with being 'of the highest possible quality' I doubt that cardboard is processed into food grade anything. Also the repulping process is pretty robust. Look at what goes into wood the first round: bark, dirt on the logs, rocks. I wouldn't worry about ordinary quanities of dirt. – Sherwood Botsford Mar 12 '17 at 19:52
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The dirt that you are seeing is probably just marks made by rubber conveyer belts that are very common in the shipping industry. That's not a problem at all for the recycler. That all comes out in the wash. By the time it gets made into new packaging material it's usually pretty clean. If the final product isn't clean, it won't be used for food. If it's enough dirt to change the weight of the cardboard, then it's a lot bigger concern, and you should consider composting it instead.

If it looks like it might be grease or oil, then it's less useful for high quality paper products. You can tell it's oil if it has absorbed into the carboard and spread out in somewhat round patterns, much like how grease from your pizza soaks into the pizza box. You can identify heavy automotive grease if it spreads to another surface wiped against it. Grease and oil can also be identified by how a drop of water is prevented from absorbing into the contaminated area. It's usually recommended not to put that into the recycle bin, even though it might be recycled for other uses.

What's more of a problem is when there is a lot of tape and plastic labels, so you can help by removing most of that. Either way, the cardboard is probably not going to be rejected. To be certain, you should contact the organization that runs your recycle program and ask them. They usually make a good effort to be clear on what they can and cannot use, and the answer is usually just a phone call away.

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