Australia & New Zealand

New Zealand Questions Compostable Plastics

The surging consumer anxiety around the downstream effects of plastics has led to a huge increase in single-use plastic items or packaging on display labelled as "biodegradable", "compostable" or "home compostable".

But what do those labels actually mean when it comes to putting these products in your home compost?

The truth is we don’t really know how those products will behave in a range of composting environments – or how many Kiwis compost at home, or what the conditions in a “typical” compost system in New Zealand might be.

So NZ Gardener, in association with WasteMINZ and the Magazine Publishers Association NZ, is driving a citizen science trial, to run alongside a larger piece of Scion research, to discover how home compostable packaging breaks down in a range of different composting systems all over the country. Register to be part of the citizen science compost trial here.

According to the WasteMINZ guidelines, to be labelled as biodegradable, plastic would need to break down through the actions of living organisms such as bacteria and fungi which are active under certain conditions.

But if biodegradable plastic breaks down under some other set of conditions – say, in landfill or the sea – it can still fragment into those highly problematic microplastics.

To be labelled as compostable, plastic would be designed to break down in a commercial compost but it might not break down in your home system (which is unlikely to reach the same temperatures).

Then there are a few plastics which are labelled as home compostable and which are designed to breakdown in a so-called “typical” compost bin. But what are the range of conditions in composting bins in New Zealand?

We don’t actually know; so it’s impossible to say if “home compostable” packaging would actually break down in all of them.

And what if you don’t have a compost bin and these products end up in landfill? Well, composting microbes require oxygen, and for the most part waste in landfills breaks down anaerobically, without oxygen.

That means compostable plastic will either break down and emit methane, or stay inert so the biomass within isn’t released.

Compostable and home compostable plastics also cannot be recycled and cause problems if they end up mixed with plastics destined for recycling because they can render them unrecyclable.

Scion has received waste minimisation funding to investigate both commercial and home compostable packaging. Tests will be done in the laboratory and in a few compost bins to see how well different sorts of plastics and packaging actually disintigrates.

But due to the high cost of this kind of detailed testing, Scion will be able to test only a very small range of home composting environments within a limited range of ambient temperatures. And, as gardeners already know, items in compost break down differently depending on lots of factors, especially the temperature (as in the weather but also the internal temperature of your heap).

To register for the NZ Gardener citizen science trial, you first complete a survey to determine your composting habits and expertise (but we need every kind of composter, from expert to beginner, to take part).

You’ll then be sent the same items that are being tested as part of the Scion research and asked to place them in your home compost in a mesh bag attached to a non-biodegradable plastic clip and continue composting as normal.

After a set period of time, you will be asked to send a photo of the contents of the bag to WasteMINZ who will determine how well the samples have broken down.

The citizen science project will enable the manufacturers of packaging to determine how well it breaks down in real life home composts.

But it should also determine what types of home composting systems are more effective at breaking down compostable packaging and what level of skill or ability is needed.

Responses will also be used as part of a wider piece of research to help us understand Kiwis’ composting and recycling habits.



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Does compostable plastic break down in a home compost system?




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