How green are compostable plastics? (part 1 of 3)

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2020/09/11 How green are compostable plastics? (part 1 of 3)

How green are compostable plastics?

Source from: https://renew.org.au/renew-magazine/reuse-recycling/how-green-are-compostable-plastics/

Plastic rubbish is a common sight in waterways such as the Yarra. Can compostable plastics help fix this waste problem?

Plastic is a miracle of modern science. It is cheap to produce, can be used in nearly endless applications and is incredibly durable. But for those of us who care about protecting our environment, plastic—especially the single-use variety—can feel like the enemy. The proliferation of biodegradable plastics in consumer goods may seem to resolve this tension, providing a guilt-free way to use plastic in our daily lives. But biodegradables also present many challenges in terms of sustainability, and in some cases may exacerbate problems caused by our single-use culture. As responsible consumers, is there a place for these materials in our lives?

Compostable vs biodegradable
To understand the potential benefits and pitfalls of new types of plastic, we first need to dig into the chemistry behind these materials. All types of plastic are polymers—a long chain of repeating carbon-based molecules tightly bound together. This molecular structure means that plastic is extremely durable, taking hundreds of years to break down in the environment. This is both what makes plastic so useful, and—in light of the 3 million tonnes of plastic consumed every year in Australia alone and the 40% of it that is used only once—also what makes it such a problem.

Technically, all plastic is biodegradable—eventually. Plastic, like other materials, will be broken down by microbes or fungi in the environment into CO2 and biomass if given enough time, even if that takes thousands of years. But what we generally call ‘biodegradable plastic’ is designed specifically to give microbes more opportunities to break the material down faster, on time scales of months rather than centuries.

Since biodegradable is a broad concept, ‘compostable plastic’ is a more accurate term for what we want to talk about in this article, according to Mike Williams, a CSIRO researcher.
“Compostable plastic is a better term, because there are rules around it,” Mike says. “If you say a bag is compostable, there’s criteria it must meet. If you say a bag is biodegradable, most people would think it breaks down, but it doesn’t specify how long that will take. It will biodegrade, but maybe the next lot of dinosaurs will see that happen.”

“Biodegradable as a label or as a claim is meaningless because it doesn’t tell you whether the biodegradation actually happens or under what circumstances it happens, it doesn’t tell you the byproducts, it doesn’t tell you anything,” says Australasian Bioplastics Association (ABA) president Rowan Williams.

Most compostable plastic sold in Australia is meant to be broken down in an industrial composter. This plastic generally adheres to Australian Standard 4736, a voluntary standard that is certified by ABA’s third-party auditors, which requires that industrial compostable plastics meet certain requirements:

A minimum of 90% biodegradation of plastic materials within 180 days in compost
A minimum of 90% of plastic materials should disintegrate into less than 2 mm pieces in compost within 12 weeks
There should be no toxic effect from the resulting compost on plants and earthworms
Hazardous substances such as heavy metals should not be present above the maximum allowed levels
Plastic materials should contain more than 50% organic materials.

These logos, indicating a product meets voluntary Australian standards, appear on many home (left) or industrial (right) compostable products.
There are similar requirements for Australian Standard 5810, which governs plastics that can be composted at home. Not all compostable products are marked with the standard number, but most should have a green seedling leaf on them and a symbol indicating whether it’s appropriate to compost them at home or industrially.

What are the drawbacks of compostable plastic?
It’s important to understand the potential issues compostable plastic presents before we can develop ideas about successful applications. It’s already common to encounter compostable plastic in our daily lives, but many of us still don’t know how to use it or dispose of it correctly. This plastic is designed to be industrially composted, and yet there is little information given to consumers who may encounter it about how to make that happen.

Since it is designed for industrial composting, it should be appropriate to put compostable plastic into a green bin with food waste to be composted—however, compostable plastic is still considered a contaminant by the EPA in some states, like New South Wales. In practice, most composters will accept plastic if they can see it’s clearly marked as compostable.

Because of this lack of clarity around disposal, it’s likely that some, if not most compostable plastic in Australia will end up either in landfill or as pollution in our environment. When it makes it to landfill, compostable plastic is just as harmful as traditional plastic. And as litter in our natural environments, it can last years.

A 2019 study by marine biologist Richard Thompson and Imogen Napper at University of Plymouth in England, published in the journal Environmental Science and Technology, exposed biodegradable, oxo-degradable and compostable plastic shopping bags to a variety of environments, including shallow seawater and soil. After three years in soil, the oxo-degradable bags and the biodegradable bag were all able to carry a load of groceries. The compostable bag was unable to carry a load of groceries, but still hadn’t entirely deteriorated.

A UK study found this bag marketed as oxo-biodegradable was still able to carry a load of groceries after three years in the sea. Image: Lloyd Russell, University of Plymouth
“The overarching conclusion was that you should not rely on any of these materials to degrade rapidly across a range of different natural environments,” Richard says.

Compostable plastic can also undermine other waste-reduction efforts. If it ends up mixed with recyclable plastic, it can compromise recycled plastic’s durability, essentially adding a “self-destruct” feature, Richard says. And even if correctly sorted into industrial composting, plastic can take longer to degrade than other compostable materials, leaving microplastics in the compost.

“Large industrial composting facilities usually work to process their material in two to three months. Even if the compostable plastic conforms to the Australian standard, it may not completely degrade in that time,” says Mike. “Within the standard there can be less than 10 per cent residual material left after 180 days.” As a result, composters who accept compostable plastic may find “they have residual plastic in their final product.”

Compostable plastic can also compromise what makes standard plastic so desirable: it is more expensive for consumers, and, by definition, is not as durable.
“If you’re [a supermarket], you don’t want people to take their meat tray home and have it fall apart,” Mike says.

Ultimately, common compostable plastic products like coffee cups still encourage people to use them once and throw them out. And that isn’t sustainable, regardless of how quickly they decompose.

This infographic from the New Zealand Parliamentary Commissioner for the Environment helps explain the sometimes confusing differences between types of degradation in biodegradable and compostable plastics. Image: New Zealand Parliamentary Commissioner for the Environment

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