Study shows majority of ‘home compostable’ plastics do not fully degrade

If you’ve ever turned over your compost pile and found months-old “compostable” plastic items that were mostly intact – well, you’re not alone. New research reports that 60% of these plastics do not fully biodegrade in home composting systems.

The findings are the result of a study carried out by scientists from University College London, which began with ordinary citizens from across the UK completing an online questionnaire about their habits and views on compostable plastics and food waste .

The participants were then invited to take part in the Big Compost Experiment, which involved regularly checking their home compost for traces of previously discarded plastic items, over a 24-month period. These items were placed in non-biodegradable open-mesh bags when they were first placed in the compost so that they would be easier to find when later digging through with a shovel or trowel. Photos and descriptions of the discovered elements were submitted to the scientists.

While a variety of different composting systems were used, the most common was an outdoor closed bin installation, used by 64% of the 1,648 people who took part in the experiment. The supposedly compostable plastic items included things like disposable cutlery, cups, bags and newspaper wrappers.

When the submitted data was analyzed, it was found that approximately 60% of items labeled by manufacturers as “home compostable” were still not fully broken down after 24 months.

Furthermore, based on a random sample of 50 item photos, it was noted that 46% of the items were not was marked as household compostable, while another 14% was marked as special industrial compostable. Needless to say, this suggests that there is definitely some confusion – on the consumer side – about what items can actually be tossed in the home compost pile.

Importantly, 83% of participants stated that they use their compost as a fertilizer in their gardens. This means that uncomposted plastic would seep into the soil and thus into the environment.

“We showed that home composting, being unregulated, is largely inefficient and not a good disposal method for compostable packaging,” said Danielle Purkiss, corresponding author of the study. “The idea that a material can be sustainable is a widespread misconception. Only a system of producing, collecting and reprocessing a material can be sustainable.”

More details about the study are available in an open access paper recently published in the journal Limits to sustainability.

Sources: Frontiers, Big Compost Experiment

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