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Composting vegetable peels, eggshells, trims of fat, and soiled paper towels and turning them into fertilizer is an environmentally friendly way to recycle household organic waste. But a new study has found it’s also a good way to introduce plastic into the environment. The problem is that many people put more than organic material in their food and garden compost bins—they’re also adding elastic bands, bags, films, wraps, and other kinds of packaging.

Nicolas Weithmann and his colleagues at the University of Bayreuth in Germany say the fertilizer produced by industrial composters contains scores of tiny microplastic particles. These tiny plastic fragments, most of which are one to five millimeters long—smaller than a grain of rice—are often washed into the environment, flowing down rivers and out to sea.

Thomas Aspray, a researcher at Heriot-Watt University in Scotland who studies physical contaminants in industrial compost and was not involved in the new study, was not surprised by the discovery. He adds that every fertilizer produced from food waste he has analyzed has contained plastic.

People understand the environmental problems with items like plastic straws and cotton swabs, Aspray says, but they may be less aware of the need to peel a plastic wrapper off a cucumber before putting it in the compost bin. “People don’t necessarily always understand where their food waste is going.”

In Germany, composting facilities process more than 12 million tonnes of organic waste annually. Weithmann and his team analyzed the output of three fertilizer plants, two that handle household organic waste, and one that processes waste from commercial sources. All three plants produced fertilizer containing significant amounts of plastic.

Fertilizer produced from household waste contained 20 to 146 plastic particles per kilogram, while the commercial digester was spitting out 895 particles per kilogram. The commercial waste was so contaminated, the researchers found, because food and drink companies tend to throw out unsold product without removing the packaging.

The net result, the research suggests, is that in Germany alone, plastic in organic waste is adding 35 billion to 2.2 trillion microplastic particles to the environment every year.

Removing all plastic at the fertilizer plant is unviable, as filters fine enough to remove small bits of plastic would also catch larger pieces of food waste. It would be easier to prevent plastic from getting to the plant in the first place.

So why is so much plastic getting mixed in with the organic waste?

At a recent bio-waste conference, representatives from the Italian Composting Council said that in a test of 27 composting plants, five percent of the waste entering the facilities was non-compostable plastic. The Austrian Compost and Biogas Association, likewise, reported that plastic bags make up 80 to 90 percent of the contaminants in organic waste collected from Austrian households. These are most likely bags that people used to line their compost bins to keep them clean.

Another problem may be confusion over terms like “bioplastic.” Bioplastics are produced from renewable organic matter, such as plant material, rather than petrochemicals, so many people believe they are biodegradable or compostable, according to the New York-based Biodegradable Products Institute. But many bioplastics are not biodegradable or compostable. Last year, the Netherlands’ largest waste processing company called for a stop in bioplastic production because too much of it was making it into their organic waste processing stream.

Some plastics, however, are compostable and can be safely thrown in the organic waste bin. Plastics certified as compostable are designed to degrade in an industrial composter. (Confusingly, certified compostable plastics likely won’t break down in your home compost heap—it doesn’t get hot enough.)

The takeaway? Check the packaging label and only put things in your food and garden compost bin that you know are compostable. Unsure? Recycle or trash it.

Charlotte Morton, chief executive of the United Kingdom-based Anaerobic Digestion and Bioresources Association, says the most effective way to tackle the problem of plastic in food waste is for food and drink producers to avoid unnecessary plastic packaging. She says food producers should use plastic suitable for recycling, and “food contact packaging should consist of certified compostable material.”

In the absence of sweeping changes to the way food is manufactured and distributed, the least people can do is to take their rotten tomatoes out of the bag before they chuck them.