Most compostable plastics labeled as suitable for home composting do not break down properly in garden bins, leaving residual plastic remains polluting garden soils and even entering the food chain.
The findings prompted researchers to call for a review of certification standards and led to at least one company abandoning the use of compostable plastics altogether.
Over 1,600 members of the public took part in the Big Compost Experiment, a UK-wide study to test how well compostable plastic packaging breaks down in home compost bins.
They composted packaging such as newspaper and magazine wrappers, food box liners and shopping bags for three to 12 months, then sifted through the resulting compost to spot any remaining plastic.
Most of the objects were found to still be clearly visible to the naked eye, often in large, intact pieces. Only about a third of items were declared fully composted, while some 60% of plastics certified as “home compostable” do not decompose properly.
Danielle Purkiss of University College London (UCL) says the results reveal a major problem with “compostable at home” certifications.
These involve controlled laboratory experiments that use a fixed type of compost, a specific set of microorganisms and small samples of compostable materials, she says.
But home composting comes in all shapes and sizes, with requirements varying depending on the type of compost bin used, the types of soil in the garden or housing estate and where the composters live.
“We show that some of the underlying standards and tests to prove the performance of these materials – they don’t actually reflect the real world they come in,” she says. “And that’s a real red flag for us – it kind of means that actually standards and certification are not fit for purpose.”
Enter the food chain
Purkiss says compost produced in gardens and gardens across the UK is used to grow food, vegetables and culinary herbs, so there is a high risk of plastic residue entering the food chain. “We know there’s a pathway in the soil, and therefore in the food chain, and in the things that we eat or that other organisms eat,” she says.
Compostable plastics have grown in popularity as brands look for more sustainable alternatives to petroleum-based plastic packaging.
But most compostable plastics end up being burned or sent to landfill in the UK as there is no dedicated collection route to take the waste to industrial composters.
Some people will put compostable wrappers in food waste collections, but they are usually treated as contaminants and fished out.
Home composting is therefore one of the only reliable ways for customers to ensure that their compostable plastic is actually composted.
However, Purkiss says packaging producers should now think twice before asking the public to compost this plastic at home.
“You shouldn’t use the term ‘home compostable’ if it doesn’t represent the actual variety of environments that consist of home composting,” she says.
Phase out compostables
This week, organic food company Abel & Cole said it was phasing out compostable plastics in its packaging in light of evidence from UCL.
Hugo Lynch, the company’s sustainability project manager, says the study’s findings match feedback from Abel & Cole’s own customers, a “notable number” of whom have written to complain that the packaging does not were not composting properly in their household trash cans.
“There are a number of products that are certified [degrade] within a given period of time,” he said. “However, based on our experience, the experience of our customers, the conversations we have had independently with waste processors and also the results of the UCL reports, it is clear that a lot that are certified under these conditions – it’s just not happening in real world conditions.
Lynch says Abel & Cole has now switched to using paper alternatives wherever possible and is working with suppliers to ensure compostable plastic is completely eliminated from its range by the end of the year. ‘next year.
There’s a risk that the debate over home compostables will be overblown, says David Newman of the Bio-based and Biodegradable Industries Association (BBIA), which represents producers of biodegradable and compostable packaging.
Newman describes home composting as a “back yard hobby activity” that many people aren’t very good at. “It’s a very small part of the population that actually does it, and an even smaller part of the population that does it right,” he says. “Saying home compostables aren’t working also means your composting isn’t working.”
A better solution, he says, would be for people to be allowed to put compostable materials in their food waste bin. Most councils allow the inclusion of caddy liners for food waste, but will opt out of any other compostable plastic packaging. Sending compostable packaging to industrial composting, an industry he says is “professionally governed”, would ensure that the items decompose completely. “The moral of the story is that we need to sort our food waste collection systems,” he says.
Journal reference: Frontiers of sustainabilityDOI: 10.3389/frsus.2022.942724
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