Here’s what happens to ‘biodegradable’ bags after 3 years in the sea or soil

Apr 29, 2019 by


Plastic bag
CC BY 2.0 Biodegradable plastic bag after being submerged in sea water for 3 years. (Photo: Lloyd Russell, University of Plymouth)

After 3 years of being buried and submerged, ‘biodegradable’ and ‘compostable’ single-use plastic bags could still hold a full load of groceries.

Single-use plastic must be one of humankind’s most ironic follies, an oxymoron that is proving disastrous for the planet – a material that is used just once, but lasts forever(ish). And it’s a disaster that many people think they have a right to partake in, as evidenced by the outrage spewed forth whenever legislators start talking about single-use plastic levies and bans.

In a perfect world, we would change our habits and give up single-use plastics and that would be the end of the story. Alas, we are an imperfect species, and instead of giving up things like plastic shopping bags, we just argue about them. In the meantime, material scientists are working on single-use plastics that are ostensibly less harmful for the environment. And while that’s great, it’s not so simple. For example, are compostable and biodegradable bags really compostable and biodegradable?

This is what researchers from the University of Plymouth set out to discover.

Led by Research Fellow Imogen Napper, the team took five kinds of plastic bags (all of them widely available from retailers in the UK) and exposed them to air, buried them in the ground, and submerged them in the sea, for three years.

The team monitored the bags regularly and recorded visible loss in surface area and disintegration as well as assessments of more subtle changes in tensile strength, surface texture and chemical structure.

According to the University, after nine months in the open air, all the plastic had disintegrated into fragments.

Yet the biodegradable, oxo-biodegradable and conventional plastic formulations were still strong enough to carry groceries after being in the soil or the marine environment for over three years.

The compostable bag disappeared from the experimental test rig in the marine environment within three months – but survived being buried in soil for 27 months.

Napper, who did the work as part of her PhD, said, “After three years, I was really amazed that any of the bags could still hold a load of shopping. For a biodegradable bag to be able to do that was the most surprising. When you see something labelled in that way, I think you automatically assume it will degrade more quickly than conventional bags. But, after three years at least, our research shows that might not be the case.”

While I am sure the intention behind creating more sustainable materials is good, the findings of this research suggest that it’s not nearly as simple as eco-aspirational people may want to believe. A plastic bag that says “compostable” should not necessarily assuage guilt over using single-use plastic, especially if consumers do not have the information about how best to dispose of these items to hasten their degradation.

In their conclusion, one of the more compelling questions that the researchers wonder is this: Can biodegradable formulations be relied upon to offer a sufficiently advanced rate of degradation to offer any realistic solution to the problem of plastic litter?

Professor Richard Thompson OBE, Head of the International Marine Litter Research Unit (and involved with the study) says, “This research raises a number of questions about what the public might expect when they see something labelled as biodegradable. We demonstrate here that the materials tested did not present any consistent, reliable and relevant advantage in the context of marine litter. It concerns me that these novel materials also present challenges in recycling. Our study emphasises the need for standards relating to degradable materials, clearly outlining the appropriate disposal pathway and rates of degradation that can be expected.”

Or better yet, just ban the things already.

You can see more about the study in the video below.

The study was published in Environmental Science and Technology.

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