How effective are the alternatives to plastic bags?

A three-year experiment that exposed compostable and biodegradable bags to the elements came back with some concerning results.

30 April 2019
packagingplasticsupermarketssustainability
image credit: Imogen Napper

The silver bullet for solving the plastic problem doesn’t appear to be biodegradable or compostable alternatives, particularly when it comes to shopping bags, results of a new study have revealed.

Researchers from the University of Plymouth’s International Marine Litter Research Unit conducted a trial where five bag materials were exposed to the natural environment for three years. All were left out in conditions they could potentially encounter if discarded as litter, including the air, soil and sea.

Shockingly, both the biodegradable and compostable plastic bags were still capable of carrying full loads of shopping after being in the soil or the marine environment.

Five plastic bag materials widely available from high-street retailers in the UK, were tested including one compostable bag, two oxo-biodegradable bags, one biodegradable bag and conventional carriers made from high-density polyethylene.

The bags were monitored at regular intervals and deterioration was measured in terms of visible loss in surface area and disintegration, as well as assessments of more subtle changes in strength, surface texture and chemical structure.

Plastic bags contribute a large amount of waste to the environment, according to a Greenpeace survey from last year. It found the top 10 supermarkets in the UK, including Sainsbury’s, Tesco, Morrisons, Waitrose, Co-op and Aldi, were producing billions of single-use plastic bags each year.

Its study revealed that supermarkets were putting 810,000 tonnes of single-use plastic packaging on the market every year, including 1.1bn shopping bags, 1.2bn plastic produce bags for fruit and vegetables and 958m reusable ‘bags for life’ a year.

Compostable fared best but bad marks for biodegradable

While the University of Plymouth’s research results varied depending on the environmental conditions, none of the bags decomposed fully in all environments.

After nine months in the open air, all the materials had completely disintegrated, but only into fragments. This raised questions as to whether plastic alternatives actually offer an environmental benefit, particularly as the potential for fragmentation into microplastics caused additional concerns, according to researchers.

Offering some hope was the compostable bag, which completely disappeared from the experimental test rig in the marine environment within three months – worth nothing considering the consumer response to last year’s Blue Planet. However, while showing some signs of deterioration in the soil, it was still present after 27 months.

“After three years, I was really amazed that any of the bags could still hold a load of shopping,” said research fellow Imogen Napper, who led the study as part of her PhD. “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.”

Recycling issues

Consumer concerns over plastic pollution have led to an increase in biodegradable and compostable options, some of which have been marketed as breaking down more readily than ordinary plastic.

“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,” commented professor Richard Thompson, head of the International Marine Litter Research Unit, who was also involved in the study.

“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."

The research also found that compostable bags needed to be disposed of via a managed composting process that uses naturally occurring micro-organisms. However, this type of recycling facility is currently not publicly available in the UK.

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