Blog Post

What's the Difference Between Biodegradable and Compostable?

David Platt • Jul 26, 2019

Both are great forms of environmental practice, but what's the difference?

We recently came across a statement by a plastics manufacturer which said they had produced a biodegradable polythene bag through the use of additives in the manufacturing process. A little scientific understanding would suggest this could be a misleading statement aimed at capturing the upswing in interest in environmentally friendly materials. This begs the question, what is the main difference between biodegrable materials and compostable materials?

Biodegradable Materials

Biodegradable materials will break down to water, carbon dioxide and other naturally occurring materials. There is no formal time limit on this definition but an expectation that this will occur within a 12 month timescale. The problem, of course, is that the bacterial action required depends on the amount of oxygen present. The temperature and oxygen conditions of a landfill can vary enormously.

By putting additives in to some plastics they can become what may be called oxo-degradable. If the plastic is left out in the open air then the sheet of plastic will degrade and fall apart. However, at a chemical level, the long chain polymer molecules that make up the plastic remain intact.

Compostable Materials

"Compostable" refers to biodegradable materials which break down to ‘useful’ nutrients components. There are two strict British Standards definitions of what is Industrial Composting and Home Composting . These define the temperature and atmospheric conditions and the amount of material left after a period of time.

The systems also provide recognisable logos for manufactures to put on their products.

	Industrial Composting: BS EN 13432	Home Composting: Vinçotte Certification Programme
Biodegradation	Test performed at 58 °C +/- 2 °C, carbon dioxide at least 90 % compared with control within 6 months (approx 182 days)	Test performed at ambient temperature (20 - 30 °C), carbon dioxide at least 90 % compared with control within 365 days
Disintegration	Test performed at whatever temperatures are achieved in vessels, each at least 140 litre capacity. At maximum of 12 weeks (approx 84 days) no more than 10 % of original dry weight of test material > 2 mm.	Test performed at 20 - 30°C in vessels each at least 140 litre capacity. At maximum of 26 weeks (182 days) no more than 10 % of original dry weight of test material > 2 mm.
Current certification and logos	AFOR / Din Certco	Vinçotte

Source: https://www.bpf.co.uk/topics/standards_for_compostability.aspx

Final Words

So, the plastic bag with an additive is oxo-biodegradable and not really biodegradable. The ‘plastic’ bag made from corn starch is fully home compostable and truly biodegradable.

Only one word of caution, is that the corn starch ‘plastic’ should not be added to the plastic recycling because it contaminates the plastic recycling process and has to be removed before the product is extruded.

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