Compostable plastic that has not fully disintegrated in a compost bin

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Most compostable plastics labelled as suitable for home composting don’t break down properly in garden bins, leaving scraps of residual plastic polluting garden soils and even entering the food chain.

The findings have prompted researchers to call for an overhaul of certification standards and have led at least one company to scrap the use of compostable plastics altogether.

More than 1600 members of the public took part in the Big Compost Experiment, a UK-wide study to test how well compostable plastic packaging decomposes in home composting bins.

They composted packaging such as newspaper and magazine wrappers, food caddy liners and shopping bags for three to 12 months, then sifted through the resulting compost to spot any remaining plastic.

Most of the items were judged to still be clearly visible to the naked eye, often in large intact pieces. Only around one-third of items were reported as having fully composted, while some 60 per cent of certified “home compostable” plastics didn’t break down properly.

Danielle Purkiss at University College London (UCL) says the results expose a major problem with “home compostable” certifications.

These involve controlled lab experiments that use a fixed type of compost, a specific set of microorganisms and small swatches of compostable materials, she says.

But home composting comes in all shapes and sizes, with conditions varying according to the type of compost bin used, the kinds of soil in the garden or allotment and where composters live.

“We’re showing that some of the underlying standards and testing to prove the performance of these materials – they don’t actually reflect the real world that they are going into,” she says. “And that’s a real red flag for us – it sort of means that, in fact, standards and certification aren’t fit for purpose.”

Entering the food chain

Purkiss says compost produced in gardens and allotments around the UK is used to grow food, vegetables and culinary herbs, and that therefore there is a high risk that residues of plastics are entering the food chain. “We know there’s a route there into the soil, and therefore into the food chain, and into the things we eat or other organisms eat,” she says.

Compostable plastics have surged in popularity as brands hunt for more sustainable alternatives to oil-based plastic packaging.

But most compostable plastic ends up being burned or sent to landfill in the UK because there is no dedicated collection route to take the waste to industrial composters.

Some people will put compostable packaging into food waste collections, but it is usually treated as a contaminant and fished out.

Home composting is therefore one of the only reliable routes for customers to ensure their compostable plastic is actually composted.

However, Purkiss says packaging producers should now think twice before they instruct the public to home compost such plastic.

“You shouldn’t be using the term ‘home compostable’ if it doesn’t represent the actual variety of environments that consist of home composting,” she says.

Phasing out compostables

This week, organic food company Abel & Cole said it is phasing out compostable plastics in its packaging in light of the evidence from UCL.

Hugo Lynch, the firm’s sustainability project manager, says the findings from the study chime with the feedback from Abel & Cole’s own customers, a “noticeable number” of whom wrote in to complain that packaging wasn’t composting properly in their home bins.

“There are a number of products that are certified to [degrade] within a set time frame,” he says. “However, based on our experience, our customers’ experience, conversations that we have had independently with waste processers and also the results of the UCL reports, it’s clear that a lot of things that are certified under those conditions – it just doesn’t happen in real-world conditions.”

Lynch says Abel & Cole has now switched to using paper alternatives where possible and is working with suppliers to ensure compostable plastic is phased out completely from its range by the end of next year.

There is a risk the debate over home compostables is being blown out of proportion, says David Newman from the Bio-based and Biodegradable Industries Association (BBIA), which represents producers of biodegradable and compostable packaging.

Newman describes home composting as a “hobby activity that takes place in the back garden” that many people are not very good at. “It’s a very small part of the population that actually does it, and an even smaller part of the population which does it properly,” he says. “To say that home compostables don’t work, 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 fish out any other compostable plastic wrapping. Routing compostable packaging into industrial composting, an industry he says is “governed by professionals”, would ensure items fully break down. “The moral of the story is we need to sort out our food waste collection systems,” he says.

Journal reference: Frontiers in Sustainability, DOI: 10.3389/frsus.2022.942724

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