Many objects describe themselves as being recyclable or compostable.
For example, take-away coffee cups, which at first sight seem to look like they can be recycled like paper, but are lined with a layer of plastic.
This coating is often made of compostable plastics. Seeing empty coffee cups littered in parks and roadsides suggests that people think they compost quickly and easily in any conditions.
The issue is how easily such plastics compost, and what conditions they need.
What compostable plastics are used for:
Compostable plastics tend to be single use. You might be familiar with compostable plastic cutlery or carrier-bags. One of the most common is polylactic acid (PLA). It is made from lactic acid, which is often produced from corn, so it is considered a bio-plastic.
The idea behind such plastics is that some functions for plastic are short term. Carrier-bags will rip. Takeaway coffee cups will be discarded after use. Normally single use plastics are a bad thing as they use resources and leave unusable plastic waste. However, with compostable plastics this issue can be avoided. When the item has served its purpose it can be composted and so can be broken down into harmless molecules.
The issue comes when things are either mixed with general rubbish or discarded on the assumption that they will quickly break down outside without causing harm.
How they Break Down in Reality:
For a plastic to biodegrade, rather than just be broken into pieces of microplastic, the bonds between the molecules must break. With many traditional plastics this needs very specific chemicals and temperatures.
In the case of compostable plastics, they can be broken down into small molecules, such as CO2, without the need for specific chemicals. This process is similar to how plant matter composts. However, they still need carefully controlled conditions to ensure they break down fully.
The international standard to describe a plastic as compostable is that up to 90% of the plastic will break down into CO2 in controlled conditions of 70°C after 180 days. This means that in a home compost bin or if discarded into the wider environment, they will not properly biodegrade.
There is little study into how plastics such as PLA break down in such environments. Instead most of the studies have looked at how it breaks down in industrial composting or in marine conditions.
For example, in marine testing environments, made to mimic what might happen if an object made of PLA ended up in the sea, objects only lost 8% of their weight after 6 weeks. So it is vital to ensure compostable plastics are properly disposed of to avoid creating more microplastic pollution.
How Best to Use Them
These plastics can be composted. That is, they can break down without specific chemical treatments. It does not mean they will quickly break down in home compost bins, or if discarded like an apple core.
Compostable plastics must be sent to treatment sites where the right conditions can be provided.
Note that currently many localities do not have suitable sites so political action is needed to ensure all compostable plastics can be easily disposed of properly.
Remember that compostable plastics can be composted efficiently, and so are an improvement on non-compostable plastics.
Compostable plastics should not be seen as a cure-all, instead they can be used for those few applications where non-plastic materials cannot be used. For example, it is better to use a reusable coffee cup than a compostable single-use one.
It is important to ensure they are disposed of properly, as without the right conditions they can take a considerable time to biodegrade and so pose a risk to wildlife if disposed of incorrectly.
For more information contact: firstname.lastname@example.org
Bio-compostable bag by John Cameron on Unsplash
Home composting by Scarab Trust
Industrial Composting by JCCC Sustainability on Flickr