Biodegradable bags may not biodegrade

How much do plastic bags break down in three years?
02 May 2019


Plastic in the ocean


Biodegradable plastic bags are still sturdy enough to carry shopping after three years buried in soil or in the sea.

Research from the University of Plymouth into how carrier bags break down in the environment has shown that biodegradable bags don’t necessarily break down any faster than conventional plastic bags.

Conventional plastic can take decades or centuries to break down, and large amounts of discarded plastic bags are sent to landfill or enter the environment as litter. Alternative forms of plastic have been developed to try and ease this waste burden.

Compostable and biodegradable plastic bags are capable of breaking down into their natural components, with compostable plastics breaking down more easily than biodegradable plastics. Oxo-biodegradable plastic bags have an additive that should make the process a bit faster than standard biodegradable bags.

The study, pulished in the journal Environmental Science and Technology, took samples of biodegradable, oxo-biodegradable, compostable and conventional plastic bags from high street shops. The bags were kept in open air, in the sea, and buried in soil, and the scientists monitored how they broke down - or didn’t break down.

The biodegradable, oxo-biodegradable and conventional bags in the soil and sea all still had enough strength to carry full loads of groceries three years later.

“Comparing oxo-biodegradable, biodegradable and conventional carrier bags, they all performed in the same way,” says Imogen Napper, the lead author of the study.

The compostable bag had completely vanished in the sea after three months. It was still present in the soil after 27 months, although it wasn’t strong enough to carry shopping.

All of the bag types had fragmented within nine months in the open air. Because they disintegrated rather than decomposing, this isn’t necessarily good news, as Napper explains. “It doesn’t mean it’s completely breaking down into natural components, it’s breaking down potentially into microplastics, which you could argue is even worse.”

Microplastics are fragments of plastic that are smaller than 5mm. They can cause environmental issues, such as being eaten by animals or clogging the oceans, and due to their tiny size it is very difficult to clean them up.

Why are bags labelled as biodegradable if they show no signs of biodegrading? “They can be completely degraded, if the bags were designed to go to a specific place such as an industrial composter,” says Napper.

Industrial composters can provide the right conditions for breakdown, which might not be present in a home composter, a landfill site, or the environments where litter can end up. If a biodegradable bag isn’t sent to the right place at the end of its life, it may offer no advantage over a conventional bag.

But many people think that plastics labelled “biodegradable” will break down without such help, and don’t know that they need to be disposed of in a particular way. Even those bags which aren’t littered will usually end up in landfill sites, where they’ll linger along with the rest of the waste.

Napper believes correcting the public's confusion is the best way to tackle plastic bag build-up in the environment. “In day-to-day life it can be quite difficult knowing where a bag is designed to go, or how it gets to that industrial composter. We really need to address how they’re labelled so we can properly inform the general public what we should be doing with this bag.”


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