I hear the terms compostable and biodegradable sometimes used interchangeably when discussions are taking place about 'good' plastics, so I thought it may be an idea to see if we can understand the distinctions between these terms.
The ASTM (The American Society of Testing and Measurements) defines compostable as a material capable of undergoing biological decomposition in a compost site as part of an available program. It breaks down such that the plastic is no longer visible as it fully breaks down into carbon dioxide, water, inorganic compounds, and biomass, at a rate consistent with known compostable materials (e.g. starch), and leaves no toxic residue. So basically, what they are saying is that the compostable material is like the trunk of a tree that has fallen. Over a period of time it decomposes to raw materials that can then be reabsorbed to generate organic matter.
How long does it take to break down? The standards require 60% (90% in Europe) conversion of the 'plastic' into carbon dioxide within 180 days for resins made from a single polymer and 90% conversion of the item into carbon dioxide for co-polymers or polymer mixes.
Also important is the quality and non-toxicity of the compost produced as the compostable plastic bag breaks down. For a product to be labelled compostable the standard requires that any resulting concentration of heavy metals is below certain limits. They also actively test the rate of plant growth in soil mixed with the composted 'plastic' against plants grown in controlled compost samples.
So you can generally feel pretty good about using bags and other packaging that is marked as being compostable.
Biodegradable means that a product will degrade from the action of naturally occurring microorganisms, such as bacteria and fungi, over a period of time. However – unlike compostable products – there is no requirement for products labelled `biodegradable’ to meet any eco-toxicity or break-down timeframe requirements. So this time frame could be thousands of years which obviously does not sit well with where we are heading in our quest to help the environment.
It is also possible that the biodegradable bag will just fall apart and produce micro plastics which then fall into the food chain of various organisms.
So be wary of biodegradable claims as they can do more harm than good and only really give us a false sense of security that we are 'doing the right thing' about minimising our use of plastics.