Eloise Gibson is Newsroom's environment and science editor. She's written for the New Zealand Herald, Stuff.co.nz, The Listener, and BBC Future.com. Twitter: @eloise_gibson.

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Does ‘compostable’ plastic really break down?

Does 'compostable' plastic really break down in your home compost bin, and what does it leave behind in the soil? A two-year long experiment looks for answers.

There's a conundrum faced by backyard composters around New Zealand: does 'compostable' plastic really break down in an ordinary home compost bin, and what does it leave behind when it does?

As concerns grow about micro-plastics ending up in our bodies, on our coasts, and in our food, there is a push to understand what packaging will – and won’t – break down fully in our garden composts, and whether it leaves behind any toxic contaminants.

Those slimy mung beans that nobody ate may still be entombed in their proudly ‘plant-based, compostable’ plastic packaging after six months in the worm bin. This is despite their 'environmentally friendly' claims, raising questions about what the remnants are doing to the soil, microbes, plants and worms.

But home composters may feel they have little choice but to run an uncontrolled experiment: chuck an item in, and see what happens. Since compostable plastics can't be put in the recycling system, lest they contaminate other plastics, trying to deal with them at home may seem preferable to sending them to landfill.

Now a rigorous experiment is planned in Rotorua that should reveal how thoroughly supposedly compostable packaging breaks down in New Zealand conditions, and whether the process leaves behind problem residues that could end up in your vegetable garden.

Crown-owned research institute Scion has won a grant of just under half a million-dollars from the Government’s Waste Minimisation Fund, some of which it will spend running a two-year experiment studying how various kinds of 'compostable' plastic and paper degrade - both in home-like composting environments, and fancier, higher-heat commercial facilities.

Associate Environment Minister Eugenie Sage announced the funding earlier in July, saying compostable packaging could be part of the answer to New Zealand's plastics problem, but only “when the materials are actually compostable”.

“We want to ensure that products labelled as “compostable” don’t contain elements such as micro-plastics which cause contamination issues in the final compost product,” said Sage.

The end results of Scion’s experiment will be used to create labelling standards for New Zealand-made compostable plastic - though people still won’t necessarily know whether overseas-made products labelled ‘compostable’ or ‘home compostable’ will break down completely in New Zealand’s climate.

The experiment comes amid increasing concern about the environmental and health effects of micro-plastics - tiny, invisible pieces of plastic that can be left behind when packaging or other plastic products break apart. A separate study led by Scion found that micro-plastics were ubiquitous in Auckland's waterways and coastlines - especially on the West Coast - though their major source was apparently wastewater from people washing polar-fleece type clothing, rather than plastic packaging.

While it may not be the source of most of Auckland's coastal particles, packaging is source of most of the plastic that ends up in landfill, where it remains without degrading.

Plant-based, 'biodegradable' and 'compostable' plastics are on the rise, but they don't always disappear from the environment the way people might hope for.

Biodegradable plastic samples. Photo: Supplied

Just because plastic is plant-based rather than petroleum-based does not mean it will automatically breaks down better in the environment than traditional plastic, says Scion. Whether it breaks down cleanly or not comes down to its chemical structure, rather than whether it was made from corn, sugarcane or fossil fuels in the first place. The question of whether any residue is toxic depends on what additives were mixed in to give the plastic certain handy properties - again, not the source of the plastic itself.

Often bioplastic is made by extracting sugar from plants, creating the Polylactic acid plastic (PLA) that is used in an increasing amount of food packaging, bottles, takeaway coffee cup lids, and other items. But this plastic can't always be relied on to break down in home composts because of the temperatures needed for microbes to attack it.

Worse, compostable plastics that mistakenly end up in the recycling system can contaminate the stockpile and prevent ordinary petroleum plastics - even ones with a high recycling value - from being recycled as well.

Meanwhile, some council compost collection schemes – like Wellington's -- have stopped accepting compostable packaging, because it prevents the resulting compost from being certified as organic.

Little wonder that people are willing to try putting things in their home composts, whether or not they are strictly labelled as being suitable to do that.

Elspeth MacRae, Scion’s chief innovation and science officer, says home composters needn't be too alarmed about poisoning their families, if they have already been having a go at composting supposedly 'compostable' packaging at home.

Putting an item that was not truly compostable, or one that was only suitable for industrial, higher-temperature composting facilities, into your home compost bin might result in particles being left behind in the soil, she says.

But it's very unlikely people would eat these particles or micro-plastics when eating vegetables grown in their garden, using the resulting compost, says MacRae.

“It’s possible (to get micro-plastics in your garden), depending what kind of plastic you put in there. You might want to watch if it's industrially or home compostable [because] industrially compostable packaging needs higher temperatures,” she says.

"But they can’t actually pass into the roots (of your vegetables) as micro-plastic particles, the only thing that could pass into the roots is some toxins. But if [your vegetables] are growing really well, it’s unlikely," says MacRae.

So people running home experiments in composting packaging are probably not poisoning themselves with plastic-laced carrots, though they might not get the tidy, clean results they want. "I would say your likelihood (of eating the plastic) is extremely low,” she says.

Scion's experiment will test how well products break down in New Zealand conditions - although MacRae warns that the temperature, moisture and other conditions they test will not necessarily match what any particular household has in their garden at home.

“What we’re planning to do is take a number of products (that claim to be compostable) and test them in a range of compostable environments that are present in New Zealand, so that includes home composting and industrial composting," says MacRae.

But: “Everyone will have different home compost environments. You can’t cover every sort of environment that home people will have. People just putting it in their garden without following a protocol, will be unlikely to (get the same results)."

One thing Scion should be able to reliably tell people is whether products break down fully as claimed, or merely into micro-plastics, because that isn't dependent on the exact composting environment, she says. "Some plastic will only break down to micro-plastics and some will degrade fully and that will happen either in industrial or home composts, so we will be testing that and those materials should be capable of breaking down to the point they are supposed to.”

The results will be "worm-tested" and "plant-tested" to make sure they're suitable and safe to go in the garden. “We'll be testing to see if worms are okay with it and if plants can grow in it, so there’s no toxins, things like that,” MacRae says.

Scion’s standards for the worm test adhere to Australian edicts, requiring worms weigh no more than 10 percent less when living in the composted former plastic, versus normal soil.

The composted material is also tested for toxins such as lead, mercury and arsenic.

The industrial composting involves hotter temperatures of up to 75C, while the home composting test is done at between 23C and 27C – still likely a balmier climate than some people’s backyard bins in winter (bearing in mind that temperatures inside the bin can rise as various materials break down in there).

Scion’s chief innovation and science officer Elspeth MacRae. Photo: Supplied

Number one worry

Plastics are top of mind for a large majority of New Zealanders, according to a recent Colmar Brunton poll, with the issue easily topping climate change as the most-cited issue for the environment.

A recent Royal Society report on plastic says New Zealand’s daily plastic waste per person is 159g a day - compared to the United States' 286g, Australia's 117g and Norway’s 26g.

Packaging is the biggest source of disposable plastic by far, according to the Royal Society’s Plastics in the Environment report. (The Royal Society’s helpful infographics on plastic are here.)

In the past 20 years, the amount of plastic produced in the world has more than doubled, the report says, and the vast majority of plastic ever produced is still around, in one form or another.

Only a quarter has ever been reused or recycled, with most of it being taken to landfills.

The lack of recycling got worse – or at least more visible – when China said it would no longer take low-grade plastics for recycling from Western countries. It later emerged that much of the 'recyclable' plastic exported offshore was going to landfill, anyway.

Scion’s study of Auckland beaches and waterways with other New Zealand university researchers found that New Zealand shares a micro-plastic pollution problem with much of the rest of the world - namely invisible particles showing up around the coast and mostly microfibres associated with polar-fleece-type material. The tiny fibres end up in wastewater when people wash dressing gowns, blankets, hoodies and other items containing plastic fibres.

Micro-plastic particles can also end up in our bodies. In fact, globally, almost all adults in developed countries have detectable levels of Bisphenol A - a type of plastic used in drink and food containers - in their tissues, says the Royal Society report.

Macrae says it's unclear whether finding plastic particles in our bodies is really evidence of a growing problem, or whether it's merely coming to people's attention because of increasingly sensitive tests allowing minuscule traces of micro-plastics to be detected when they previously couldn’t.

“You can detect BPA in most people, but [that may be because] we can detect tinier and tinier amounts. You have to look at what are the harmful levels," she says. (A major research programme by the US Department of Health into health effects of BPA is reportedly due to report later in 2019.)

The question of what levels of exposure are harmful is being explored in a raft of studies, mostly overseas - and the evidence is still developing. But there's reason to be cautious about the risk of plastic particles around us, and to try to fill the knowledge gaps about its effects on our health, the Royal Society concluded. One possible risk is that micro-plastics can accumulate toxins, which are then inhaled or eaten by people, fish or animals who end up ingesting the plastic.

While research on safe levels of these particles continues, people are facing a growing conundrum of what to do with growing mountains of plastic waste.

Some companies are developing their own stewardship schemes to take back their plastic or compostable packaging and re-use, recycle or compost it, while others are working on new plastics that can break down harmlessly, or at least get closer to being harmless.

There's more work to come on New Zealand's plastics problem.

Macrae is one of 11 experts on a panel set up by Prime Minister's Science Advisor Juliet Gerrard for her major Rethinking Plastics project, which she says will build on the Royal Society’s work and delve into the area of plastic-cutting policy. “Change is urgent, but the evidence base to guide change is lacking,” says a note published by Gerrard's office on the Royal Society's plastics report.

Both Gerrard and the Royal Society have noted we'll need to keep making use of plastic's good points, like reduced transport loads and lower fuel emissions (because plastic packaging is so much lighter to carry than glass, metal or even paper).

A case in point: the Boeing 787 Dreamliner is half-comprised of plastic, "allowing for significant weight reductions and fuel savings", according to the report.