Most plastics marketed as “home compostable” don’t actually work, with up to 60% not disintegrating after six months, according to research.
An estimated 10% of people can compost effectively at home, but for the remaining 90% of the population, the best place to dispose of compostable plastics is the landfill, where they slowly decompose, releasing methane, researchers say. . If compostable plastic ends up in food waste, it contaminates it and blocks the recycling process, according to the study. The only solution is to use less plastic.
“The bottom line is that home compostable plastics don’t work,” said Professor Mark Miodownik, author of the paper, published in the journal Frontiers in Sustainability. ” Let’s stop. Let’s not pretend it’s going to be some kind of panacea, and you can sell stuff to people without really having the infrastructure to deal with the waste and hope it all goes away.
The study showed that most of the plastics people put in their home compost shouldn’t be there anyway. The researchers found that 14% of plastic packaging was certified as “industrially compostable” and 46% had no compostable certification at all (for example, it could be “100% biodegradable”, which generally means that it cannot be composted).
People are confused by labels and struggle to know what goes where, but 85% of people remain enthusiastic about buying compostable plastics, according to the report.
“People want them to work,” said Miodownik, who is part of University College London’s Plastics Waste Innovation hub. “People really try to do the right thing most of the time, so I feel bad for them that it turned out that way. But in reality, backyard composting just doesn’t work,” he said.
The researchers based their findings on data from 9,700 people across the UK who completed a survey called the great compost experience on their understanding of plastic waste, of which 1,600 participated in a home composting experiment and 900 completed it. Those who participated had a range of composters, ranging from indoor vermicomposters to outdoor trenches. Participants used shovels, trowels and sieves to scour their compost and look for traces of plastic before recording the results online. If 90% of the carbon in the materials tested was gone within six months, it was considered compostable.
The results showed that there was no specification that was reliably home compostable. The study also suggests that laboratory testing for these materials is not working, which is a wider problem for the plastics sector, and calls into question whether these product standards are truly protecting the environment.
“I think if people continue to market home compostable products, that’s greenwashing,” Miodownik said. “Before it wasn’t clear, but now we have the evidence. People are making claims about material without really understanding what needs to happen for it to be truly biodegradable.
Compostable plastics should degrade into compost at a similar rate to naturally compostable materials, leaving no visible residue. Common uses for compostable plastic include food wraps, magazine wrappers, bags, cups, plates and cutlery.
The term biodegradable refers to a material degraded by biological activity, but does not specify how long this may take and under what conditions. In 2019, another team of researchers plastic bags found that claimed to be biodegradable could still carry groceries three years after being buried in soil and sea.
The growth of recyclable, compostable and reusable plastics is the result of attempts to tackle plastic pollution, but there are few places to dispose of them. There is, for example, no UK-wide collection scheme for compostable and biodegradable plastics. “Vat” composters, where composting takes place in an enclosed environment, are best for breaking down industrially compostable materials, but food waste is usually sent to anaerobic digesters, which cannot process it.
“Reduce and reuse often saves everyone a lot of money, and yet it seems like the strategy is the least intuitive for people,” Miodownik said.
If your local authority uses industrial composting to process food waste, you should use compostable bags, but this is rare in the UK. Most food waste is treated using anaerobic digesters, which turn the waste into biogas. All bags are removed as part of this process – which takes time and energy – whether they are compostable or not. If possible, putting local authority food waste in the newspaper would be best.
However, the bottom line is that recycling food waste is a big win for the environment and should be encouraged, even if people use plastic bags to do it. For home composting, not using a bag or paper is the best option.