Photograph by Lloyd Russell, University of Plymouth
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A plastic shopping bag buried in soil for three years could still hold a full load of groceries.

Photograph by Lloyd Russell, University of Plymouth

Biodegradable shopping bags buried for three years still work

A new study casts doubt on the viability of biodegradable plastics as an answer to plastic pollution.

This article was created in partnership with the National Geographic Society.

Richard Thompson, a British marine biologist who devoted his career to studying plastic waste, has long wondered how well biodegradable shopping bags actually degrade.

So in 2015, he and his graduate students at Plymouth University buried a collection of bags labeled as biodegradable in the school’s garden.

Three years later, when the bags were dug up, they not only had remained intact, they still could carry almost five pounds of groceries.

“It did surprise me that after three years you can still carry shopping home in them,” he said in an interview with National Geographic. “They didn’t have the same strength as they had when they were brand new. But they hadn’t degraded to any meaningful extent.”

The indestructible qualities of biodegradable bags are just one of the findings in a first-of-its-kind study published today in the journal Environmental Science & Technology. The research documents deterioration of five types of shopping bags that were immersed in water, buried in soil, or exposed to outdoor air as if litter. Thompson and his team tested bags commonly dispensed in retail shops around Plymouth and concluded that none of them–including compostable bags­–reliably deteriorated enough over three years to give them any environmental advantages over conventional bags. (Read more about potential problems with bioplastics.)

The study highlights how the term “biodegradable” can confuse consumers, lulling them into thinking the bag will simply disappear if thrown away. If consumers think they are being even more responsible by adding biodegradable bags to their recycling bins, that can destroy efforts to collect conventional plastic bags for remanufacture into new bags, the scientists warn. Chemical additives in biodegradable bags can contaminate the mixture, rendering it unusable.

“If you’ve got bags with a self-destruct function, the recycler doesn’t want that mixed in with other bags,” Thompson said. “They need known and consistent material. So the issue becomes how do you separate biodegradables from conventional plastics? How is the consumer supposed to know how to dispose of it?”

Bag manufacturer disagrees

The study is likely to rekindle a controversy that erupted last summer after the BBC reported preliminary results showing one of the biodegradable bags had not degraded after two years.

Symphony Environmental Technologies, which made the bag that was still intact at two years, criticized the study and called into question Thompson’s credentials, noting “he is not a polymer scientist.”

Thompson, who was awarded the Order of the British Empire by Queen Elizabeth for his research on plastic waste, said he stands behind his group’s research.

“We are entirely confident in the work, as we have been all along,” he said. “And it’s been through full peer review.”

“Biodegradable” controversy

Disposable shopping bags are one of the world’s most widely used plastic products. They are often used for only moments, and the European Union is estimated to use about 100 billion bags every year, with annual use per capita exceeding 450 bags per year in some EU countries. As the world searches for solutions to the growing accumulation of plastic waste on Earth, products advertised as biodegradable have been marketed with increasing frequency, offering the promise of an easy answer to using disposable bags. But in many cases, biodegradability may be just that—only a promise.

“There is no magical degradable material that will breakdown in a very short time in all environments that you expose it to. That does not exist,” says Ramani Narayan, a chemical engineer at Michigan State University and expert on biodegradables. He did not participate in the Plymouth study.

Both the United Nations and the European Union have staked out positions against biodegradables. The UN, in a report published in 2016, flatly declared that biodegradable plastics are not the answer to marine plastic pollution. And the EU last year, with some controversy, recommended banning oxo-biodegradables, which contain additives that are designed to hasten breakdown of polymer molecules, sometimes, according to Britain’s largest manufacturer of oxo products, Symphony, “in the same way as a leaf and leaving nothing behind.”

That process causes the bag to disintegrate into small microplastics, raising concerns that they are adding to the growing collection of microplastics in the world’s oceans.

Testing at three outdoor sites

Thompson and his team tested five types of bags, including one compostable bag, one conventional high-density polyethylene bag, and three kinds of biodegradable bags. Two of the biodegradable bags were oxo-biodegradable. The other biodegradable bag was manufactured in a way that promotes its breakdown differently.

Bags were exposed to environmental conditions in three different sites. For the experiment, some of the bags were cut into strips and placed in mesh pouches that exposed them to outdoor elements at each of three different test sites. Whole bags were also used in each of the test locations.

For the soil test in the university garden, samples were buried almost ten inches deep. For the outdoor exposure test, samples were placed on a wall in the garden with a southern exposure. For the marine test, samples were submerged more than three feet beneath the surface of Plymouth Harbor. A fourth laboratory test site was set up as a control.

Samples were set out July 10, 2015, and inspected regularly for signs of surface loss, holes, or disintegration. Samples were also measured for tensile strength, meaning how easily they broke under tension.

Compostable bag disappears

At the harbor site, all the bags and test strips had acquired a microbial biofilm on the surface after a month. The compostable bag disappeared after three months.

At the open-air site in the garden, all of the bags and test strips had become too brittle to test further or had disintegrated into microplastics after nine months. They could not be tested further.

At the soil site in the garden, the bags remained intact. Although the compostable bag survived in its original shape for 27 months, it was unable to hold any weight without tearing.

Plastics 101

Narayan, the chemist, says the study provides real world data revalidating the limitations of biodegradables. But he questioned the inclusion of a compostable bag in tests that compostable bags are not designed to endure. Compostable bags, he says, are intended to be disposed of in industrial composters and regulations in the United States and most other nations require label instructions that clearly describe that.

“That is where the confusion is,” he says. “The compostable bag is biodegradable only in the industrial composting environment and is meant to be disposed of in that environment.”

Vegware, the maker of the compostable bags used in the study, said in a statement to the Guardian newspaper it has updated its description of its bag labels to say: “commercially compostable where accepted.”

Likewise, Symphony says oxo-biodegradables are not intended to degrade in landfills or submerged at depth in the seas. Rather, oxo-biodegradable bags are intended to degrade if they become litter on the open landscape or ocean surface, said Michael Stephen, Symphony's deputy chairman, in an interview.

Oxo-biodegradables contain stabilizers to give the bags a “useful service life,” Stephen said, and keep them from falling apart while the customer is carrying groceries to their car. Usually, he said, bag companies want the stabilizers to function 18 months.

“When the stabilizer has been exhausted and the product has come to the end of its useful life, then the catalyst kicks in and it starts to degrade,” Stephen said.

The timeline for degradation varies. “In a warm environment, it will degrade within a year. In a damp, cold environment, maybe two or three years, but it will be a whole lot quicker than conventional plastic,” Stephen said. "Do you want two years or 100 years?"

Imogen Napper, who led the study as part of her PhD and a National Geographic Explorer, said she noticed “very little change” in the samples in the soil over the three years. Still, she was skeptical they would hold groceries. But she loaded a bag with box of cereal, cans of Coke, bananas and oranges, crackers, and pasta. “The bags were discolored and disgusting, but they were still usable,” she said.

Thompson said the study should not be read as an argument against development of biodegradables or compostables. Instead, he says, the study argues for rethinking which products work best as biodegradables. “We’ve got to link these products to appropriate uses,” he said.

Contained environments, such as football stadiums, may be better suited to use biodegradable or compostable products than retail shops. Shopping bags can end up anywhere at the end of their lives. But in a stadium, disposable food containers and wrappers, even those containing uneaten food, can be gathered in one place, and all of the waste can be processed in an industrial composter. “The idea that all of that can go into the same waste stream makes sense,” Thompson said.

A better future for plastic bags, he suggested, may be to reverse course and stick with the property that made them so popular in the first place–durability. That is the study’s final conclusion: “A bag that can and is reused many times presents a better alternative to degradability.”

National Geographic is committed to reducing plastics pollution. Learn more about our non-profit activities at natgeo.org/plastics. Learn what you can do to reduce your own single-use plastics, and take your pledge.

The National Geographic Society and Sky Ocean Ventures have launched the Ocean Plastic Innovation Challenge, which asks problem solvers around the globe to develop novel solutions to tackle the world’s plastic waste crisis. Have an idea? Submit your solution by June 11 at oceanplastic-challenge.org.

This story was updated at 2 pm ET on April 29, 2019, with information from Symphony.