Most certified “home-compostable” plastics do not fully break down in home compost bins, a UK citizen science project has found.

The project also showed that while 85 per cent of people surveyed were “enthusiastic” about buying compostable packaging, many were confused by what “compostable” meant.

The study was published in Frontiers in Sustainability.

Danielle Purkiss from University College London, who ran the study, said even when people correctly identified certified home-compostable plastic and placed it in their home composter, most of those plastics remained in large fragments after a year, and many lingered as microplastics.

Some barely broke down at all.

“We have photos [of home compost-certified plastic] that people have submitted after 12 months where you can still read the home compost certification label on it, which is ironic,” Ms Purkiss said.

“There were plastic bags that were still so intact you’d probably be able to hold your shopping in them.”

Plastic not so fantastic

The world’s appetite for plastic has increased dramatically in the past couple of decades. Since the turn of the millennium, global plastics production — and ensuing waste — doubled.

Around half of the 353 million tonnes of plastic waste generated in 2019 ended up in landfill, while just under a quarter was “mismanaged”, meaning it was burned in open pits or ended up in waterways, oceans or dumped on land.

And while plastics that make their way into the environment do eventually break down, the process can take hundreds of years, and involves them first eroding into microplastics that permeate every corner of the globe.

In an attempt to tackle plastic pollution, many countries, including Australia, are phasing out single-use plastics and replacing them with recyclable, reusable or compostable options.

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Compostable plastics are often made of polylactic acid or PLA, and come in two types: industrial and home compostable.

Industrial compostable plastics, which in Australia meet requirements of Australian Standard AS 4736–2006, need the hot, controlled conditions of a commercial facility to fully biodegrade.

Plastics that are certified home-compostable are required to completely biodegrade in a home compost after 12 months. 

Complete biodegradation means the plastic is broken down into carbon dioxide, water, and a small amount of biomass, which can be used as compost.

To get the “home-compostable” stamp of approval in Australia (AS 5810-2010), the plastic is tested according to standards — such as specific temperature, moisture content, airflow and microbe mix — to see if it biodegrades within a year. 

But if a home compost bin is, say, a little cooler or drier than the standard testing lab conditions, that same plastic might take longer to break down.

A green compostable bag
The Australian certified home-compostable logo (bottom left) and industrial-compostable logo (bottom right).(Wikimedia Commons)

Home-grown experiment

Compostable plastics in the UK undergo similar tests to Australian standards.

To see how they fare in the real world, Ms Purkiss and her colleagues set up the Big Compost Experiment.

Over two years, more than 900 people in the UK placed a total of 1,307 home-compostable-certified plastic items, such as food trays and newspaper wrapping, in a loose non-biodegradable net bag before adding it to their home compost.

“The bags don’t affect the way plastic breaks down — the mesh gaps are big enough that worms and bacteria can get through,” Ms Purkiss said.

“The mesh bag just helps people find what’s left of the plastic material at the end of the experiment.”

Participants checked the state of the home-compostable plastic at three-month intervals.

A dirty white plastic bag that's beginning to disintegrate
Study participants were asked to provide photographs of how well their plastics degraded (or not).(Supplied:

As expected, longer composting times usually meant more biodegradation, but by the end of the experiment, 60 per cent of certified home-compostable plastics remained as fragments bigger than 2 millimetres.

Another 12 per cent disintegrated into pieces smaller than 2 mm, but were still visible.

“These are the most-keen people [when it comes to composting] and even they couldn’t get the materials to fully break down consistently,” Ms Purkiss said.

“That’s really telling.”

A random selection of photos also showed some people put industrial compostable plastic in their compost bin.

Time to shift from home composting (for now)?

CSIRO polymer chemist Pete Cass, who was not involved with the study, wasn’t surprised that many plastics that claim to be suitable for home composting don’t fully break down within a reasonable time.

If the UK study was conducted in Australia, the results would likely be similar, and maybe slightly better, Dr Cass said.

“Warmer climates tend to speed things up a bit.”

Ms Purkiss and her crew didn’t dig into what made some home composters more successful at breaking down plastics than others, but a spin-off study is examining the microbial make-up of those efficient bins to see if they share a common mix of bugs.

Many study participants reported feeling “disgusted” by how much plastic remained in their compost bin.

“We’ve had emails from people [in the study] saying, ‘this is horrible. I had to spend two hours picking out pieces of plastic before I put the compost on my flowerbeds,'” Ms Purkiss said.

And there’s a lot we don’t yet know about how microplastics in compost interact with, for instance, worms when they’re spread on gardens, she added.

So for now, instead of designing plastics for the home compost bin, Ms Purkiss suggested more focus be placed on plastics destined for industrial facilities, where the environment can be tightly controlled.

There’s simply too much variation in factors such as moisture and microbial make-up when it comes to home composting, and designing plastics that can break down in a wide range of environments is a hard ask.

Just because the UK study suggests many “home-compostable” plastics don’t break down quickly enough in home compost, it doesn’t mean we should give up on them completely, Dr Cass said.

“Ideally, the best plastic would be very fast to disintegrate and biodegrade, it would be recyclable, renewable and have good barrier properties, which is a limitation of [current] compostable plastics — they tend to let oxygen and moisture through which can affect food shelf life.

“At the state of everything at the moment, [home-compostable plastic] won’t solve the plastic pollution problem alone, but technology is heading in the right direction.”

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