This story was originally published by The Guardian and appears here as part of the Climate Desk collaboration.

Most plastics marketed as “home compostable” don’t actually work, with as much as 60 per cent failing to disintegrate after six months, according to research.

An estimated 10 per cent of people can effectively compost at home, but for the remaining 90 per cent of the population, the best place to dispose of compostable plastics is in the landfill, where they slowly break down, releasing methane, researchers say. If compostable plastic ends up among food waste, it contaminates it and blocks the recycling process, the study finds. The only solution is to use less plastic.

“The bottom line is that home compostable plastics don’t work,” said Prof. Mark Miodownik, an author of the paper published in the journal Frontiers in Sustainability. “Let’s just stop. Let’s not pretend to ourselves that it’s going to be some sort of panacea, and you can sell people stuff without really having the infrastructure to deal with the waste and hope that it’s all going to go away.”

Recent research shows that most compostable plastic people put in their home compost will still be there after six months. Photo by Geo Lightspeed7/ Wikimedia Commons (CC BY-SA 4.0)

The study showed that most of the plastic that people put in their home compost shouldn’t be there anyway. Researchers found 14 per cent of plastic packaging items were certified “industrially compostable” and 46 per cent have no compostable certification (for example, they could be “100% biodegradable,” which typically means they cannot be composted).

People are confused by the labels and struggle to work out what goes where, yet 85 per cent of people remain enthusiastic about buying compostable plastics, the report said.

“People want them to work,” said Miodownik, who is part of University College London’s Plastic Waste Innovation Hub. “People are really trying to do the right thing, mostly, so I feel bad for them that it’s turned out this way. But actually, home composting just doesn’t work,” he said.

Researchers based their findings on data from 9,700 people across the U.K. who completed a survey called the Big Compost Experiment about their understanding of plastic waste; 1,600 of whom took part in a home composting experiment and 900 completed it. Those that took part had a range of composters, from indoor wormeries to outdoor trenches. Participants used spades, trowels and sieves to go through their compost and look for traces of plastic before recording results online. If 90 per cent of the carbon in the test materials had disappeared within six months, it was considered compostable.

Most plastics marketed as “home compostable” don’t actually work, with as much as 60 per cent failing to disintegrate after six months, according to new research. #PlasticWaste #ClimateCrisis #CompostablePlastic #Recycling

The results showed there was no specification that was reliably home-compostable. The study also suggests that the laboratory tests for these materials don’t work, which is a wider issue for the plastics sector, and calls into question whether these product standards really protect the environment.

“I think if people continue to market home compostables, it’s greenwash,” said Miodownik. “Before it was unclear, but now we have the evidence. People are making claims for material without much understanding of what has to happen in order for it to actually be 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 packaging, magazine wraps, bags, cups, plates and cutlery.

The term biodegradable refers to a material being degraded by biological activity but is not specific about how long that might take and under what conditions. In 2019, another team of researchers found plastic bags that claimed to be biodegradable were still able to carry shopping items three years after being buried in the soil and sea.

The growth in recyclable, compostable and reusable plastics is a result of attempts to tackle plastic pollution but there are few places to dispose of them. There is, for example, no U.K.-wide system of collection for compostable and biodegradable plastics. “In-vessel” composters, where the composting takes place in an enclosed environment, are best at breaking down industrially compostable materials, but food waste is generally sent to anaerobic digesters, which cannot process them.

“Reduce and reuse are often big money savers for everyone, and yet it seems the strategy that is least intuitive to people,” said Miodownik.

If your local authority uses industrial composting to process food waste, then you should use compostable bags, but this is rare in the U.K. Most food waste is processed using anaerobic digesters, which turn the waste into biogas. All bags are removed as part of this process — which takes time and energy — if they are compostable or not. If possible, putting local authority food waste in newsprint would be better.

However, the bottom line is that recycling food waste is a big win environmentally and should be encouraged, even if people do use plastic bags to do it. For home composting, using no bag or paper is the best option.

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Sending food waste through anaerobic processing is stupid on several levels. Not only does it take longer and not kill anaerobic pathogens, but it takes multiple stages of treatment, generally requiring physical labor.
Nothing composts as quickly over the winter months, so simply naming a time period, regardless of season, has got to badly skew results.
The other thing is that the "compostable" plastic bags around 15 years or so ago broke down readily in sunlight. For a while, I tossed bagsful of compostable material into a trench, without bothering about snow, etc. Come spring, the bags had deteriorated enough that grabbing them by the (tied off) "handles" and giving each one a sharp shake spilled the contents into the summer composter. I typically save bags to reuse one way or another: those compostable ones in the bottom drawer decomposed in plain air in the space of 5 years, to the point that they crumbled to powder with any method of gathering the bits other than a vacuum cleaner.

Probably what's actually needed is standards. And the standard should be that all bags & film wrap are actually compostable and biodegradable.

"Compostable plastics should degrade into compost at a similar rate to naturally compostable materials, leaving no visible residue."

Some compostables take a lot longer than others. Avocado pits and eggshells come to mind.