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Another Place Plastics Are Turning Up: Organic Fertilizer From Food Waste

A prep cook at a San Francisco restaurant drops fish skin into a food scrap recycling container. Turning food waste into fertilizer is popular in parts of Europe and is catching on in the U.S. But tiny plastics are also making their way into that fertilizer — and into the food chain.

Tiny particles of plastic are showing up all over the world, floating in the ocean, buried in soil, in food and even in beer. Now there's new research that's found microplastics in fertilizer — organic fertilizer from food waste, in fact.

Collecting food waste to make fertilizer is a big deal in parts of Europe and is catching on in the U.S. But Ruth Freitag, a chemist at the University of Bayreuth in Germany, says there's a problem.

"What happens most of the time is that people don't like to put garbage into the bin as it is. They like to wrap it up," she says — usually in a plastic bag. Freitag says some of the contamination also comes from plastic food wrappers as well; she can tell by the type of plastic they find.

Writing in the journal Science Advances, the team reports finding plastic in fertilizer made from food waste from both households and commercial sources. These are small particles, fractions of an inch, that result from the composting or "biodigesting" processes that turn organic waste into fertilizer.

Freitag says the takeaway message here is that even an environmentally friendly idea like using food waste for fertilizer can go awry in unexpected ways.

"Some good ideas work, but only when people are responsible," she says, noting that German laws for recycling organic waste are pretty clear — and strict. She notes that communities or businesses planning to recycle food waste should keep in mind how easily it can get contaminated with plastic.

Eventually, she says, the plastic pieces get washed out of the fertilizer that's spread on land and washes into waterways.

That's where researcher Chelsea Rochman at the University of Toronto has been finding tiny pieces of plastic. "If we move away from the ocean and go upstream," she explains, "there's evidence of microplastics in rivers and lakes and other freshwater bodies."

In a perspective published this week in the journal Science, Rochman notes that she's found tiny bits of plastic in what comes out of sewage treatment plants. That "sludge" is sometimes used for fertilizer. "The sewage sludge, for example, that we're spreading on the earth [is] a source of plastic out into the environment," says Rothman, who studies aquatic ecology. "How is that interacting with animals and soils?"

Rochman says there hasn't been much research on tracking microplastics on land. Most people have been focusing on where it usually ends up: the oceans. But it's clear that microplastics are making their way into the food chain. "We find it in our seafood," says Rochman, "we find it in our sea salt. There's now evidence of it in drinking water."

Rochman says there's good news here, though. As people track the myriad pathways that plastic waste takes, the closer they get to cutting it off at the source.

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