How biodegradable are bags?

Research helped destroy the thinking that a plastic bag with a label ‘biodegradable’ is safe for the environment.

Plastic bags are currently public enemy number one, but are biodegradable bags any better? The Science Media Centre comments:

Writing in the latest issue of Royal Society Open Science, European researchers have argued that the existing industry standards and testing methods are insufficient to predict the biodegradability of single-use plastic carrier bags within lakes, rivers and oceans.

AUT Professor of Engineering Thomas Neitzert said the research helped “destroy the thinking that a plastic bag with a label ‘biodegradable’ is safe for the environment”.

“The co-existence of conventional plastic bags and so-called biodegradable plastic bags of compostable materials is also upsetting current recycling operations and is confusing the general public.

“Biodegradable plastic bags are in many cases made from crude oil, requiring carbon-based production processes and are emitting CO2 or methane when degrading. On the way to a low carbon economy, we should, therefore, carry a reusable bag made from cloth or jute, like our parents did.”

University of Auckland senior lecturer Dr Ivanhoe Leung, co-deputy director of the Centre for Green Chemical Science, said plastic pollution was “one of the most challenging environmental issues that is facing the world today”.

The science behind biodegradable or compostable bags had gone a long way since they were first developed, he said, and given the right conditions they can break down into harmless materials within a few months.

“The challenge, however, is to separate these biodegradable plastic materials from the waste stream so that they can be broken down under the right conditions.

“For example, undesirable substances like acids or methane gas can be produced from biodegradable plastics if they are broken down in places that lack oxygen. These could be landfill sites, or anaerobic marine habitats like saltmarshes or brackish waters.”

University of Waikato’s Professor Kim Pickering said it was important to assess how long things take to degrade in real life situations, and what they break into.

“If it is to be assumed that we cannot prevent some plastic products getting into the environment, then biodegradable plastics could be a step in the right direction (depending on the product), but it shows that there are great uncertainties regarding the impact these could have on the environment and so we should still assume responsibility of waste plastic and consider its disposal, whether biodegradable or not.”