How bad is not sorting your plastics for recycling?
How bad is not sorting your plastics for recycling?
Illuminating answers to environmental questions.
May 6 2008 7:34 AM

Thou Shalt Sort Thy Plastics

How bad is it to mix your soda bottles with your yogurt cups?

I've been tossing my used yogurt cups in the recycling bin for years. So imagine my horror when I recently got around to reading the fine print on my city's sanitation guidelines—yogurt cups, it turns out, are supposed to go in the regular trash. Has my inadvertent sorting error ruined many tons' worth of recyclable plastics?

Illustration by Robert Neubecker. Click image to expand.

No, there's hardly a need to flagellate yourself over such a minor environmental sin. Sure, you've been making life ever-so-slightly less pleasant for the hardworking employees of your local recycling facility—they exert considerable effort picking through incoming refuse. But your yogurt cups, which are probably made of polypropylene, won't cause much damage to the recycling stream itself. The same can't be said for items made of polyvinyl chloride, such as certain kinds of pipes and food containers. Mix those in with your empty soda bottles and you could be wreaking some serious havoc.


Your recycling center's distaste for yogurt cups is par for the course throughout the United States. Of the seven types of numbered plastics, only No. 1 (polyethylene terephthalate, or PET) and No. 2 (high-density polyethylene, or HDPE) are commonly recycled. Even in those rare municipalities that ask residents to throw all plastics in the same recycling bin, it's mostly just the PET (mostly in the form of beverage bottles) and HDPE (detergent bottles) that get processed. While it's technically feasible to recycle other plastics, the process is expensive and results in plastic that's widely deemed inferior. Products made from plastics No. 3 through No. 7—a range that includes food trays, grocery bags, six-pack rings, and your yogurt cups (designated No. 5)—are typically either landfilled or shipped overseas for incineration. (There's great interest in the developing world in burning plastics to recover the fossil fuels [PDF] from which they're made.)

When loads of plastic are dumped on a recycling facility's floor, the sorting fun begins. Workers often start by picking through the piles in search of obviously discordant items—kiddie play sets, lawn furniture, clothing mannequins. They also scan for plastic mounds that are drenched in nonrecyclable trash, such as food slurries or medical waste. While a little caked-on tomato sauce isn't going to ruin a batch of PET bottles, a Dumpster's worth of nonrecyclable garbage will; if a large apartment building was careless about separating its rubbish, then hundreds of pounds of plastics may have to be sent to the landfill. According to a 2005 Environmental Protection Agency study in the Pacific Northwest, 24 percent of plastic bottles were rejected as too contaminated for recycling. (By comparison, 14 percent of metal goods were rejected, and just 1 percent of newspapers.)

The remaining plastics are then sent along a conveyor belt, where they're sorted by hand—a hazardous task, given the prevalence of syringes and other dangerous surprises in the deluge. Workers mostly look for empty beverage bottles, which are the industry's version of gold nuggets—such bottles are almost always made out of PET, the most easily recycled plastic. This is likely the step in the process at which your erroneously sorted yogurt cups are picked out.

If your misplaced polypropylene slips past the human inspectors, however, it may get caught during the ensuing phase, when the machines go to work. Most use either X-rays or near infrared spectroscopy to analyze the chemical properties of passing plastics. Items that register as either non-PET or non-HDPE are ejected from the sorting belt with jets of air. The best machines claim an accuracy rate of 98 percent; they are occasionally stymied when bottles are stuck together or excessively flattened. As a result, a final manual inspection is often necessary to verify that a load is free of any meaningful contaminants.

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