What does Biodegradable and Compostable mean?
Awareness of the problem of plastic pollution and the impact on the environment has led to a growth in so-called biodegradable and compostable options.
In basic terms, the definition is simple: If something is biodegradable, then, given the right conditions and presence of microorganisms, fungi, or bacteria, it will eventually break down to its basic components and blend back in with the earth. Ideally, but not always, these substances degrade without leaving any toxins behind.
For example, when a plant-based product might break down into carbon dioxide, water, and other naturally occurring minerals, the substance seamlessly mixes back into the earth, leaving no toxins behind. Unfortunately, many materials—even ones with a biodegradable label—do break down in a more harmful manner, leaving chemicals or other damaging substances in the soil.
In terms of environmental benefits, the best biodegradable material will break down quickly rather than taking years. It leaves nothing harmful behind and saves landfill space. Unfortunately, not everything that's advertised as "biodegradable" meets these criteria.
Compostable products are all biodegradable, but they are specifically intended for a composting environment. In the right setting, these products break down even more quickly, usually within 90 days, and they leave behind a nutrient-rich organic material called humus, which creates a healthy soil environment for new plant growth.
Whether an item is compostable or simply biodegradable, it needs to be placed in an environment that facilitates its breakdown. Compostable products require composting environments. Most biodegradable items need to be degraded in a controlled composting environment or facility—and very few of these facilities exist in the world. These large facilities are designed to keep materials at 140 degrees Fahrenheit for 10 consecutive days.
For example, PLA, a popular biodegradable material for green companies, will only decompose into carbon dioxide and water in a controlled composting environment, not in a backyard composting arrangement, according to standards developed by the Biodegradable Products Institute.
Biodegradable plastics are one set of materials that are becoming a popular replacement as consumers demand green alternatives. Rather than remaining stable for hundreds of years biodegradable plastics can be broken down by microbes, chewed up and turned into biomass, water and carbon dioxide (or in the absence of oxygen, methane rather than CO2).
A subset of them are compostable, which means that not only are they broken down by microbes, but they can be turned – alongside food and other organic waste – into compost. Only a minority of these plastics are home compostable, so, the label “compostable” most often means industrially compostable. That coffee cup with a Seedling logo you’re drinking from won’t decompose very quickly, if at all, on your home compost heap, but will break down inside the right kind of industrial equipment.
There’s a European standard for compostable packaging: EN 13432. It requires that the packaging break down under industrial-scale composting conditions within 12 weeks, leaving no more than 10% of the original material in pieces bigger than 2mm, and doing no harm to the soil itself through heavy metals or worsening its structure.
But while most of these bioplastics require industrial composters to break them down after use, they are far from guaranteed to make it to one. Given humanity’s track record, it makes sense to ask what happens if they end up where they shouldn’t.
To test how different kinds of plastic bag fare in different environments, Imogen Napper at the University of Plymouth collected carrier bags with various claims about biodegradability, and put them in three different natural environments over a period of three years: buried in soil, left in the sea, and hung up in the open air. She tested bags labelled as biodegradable, compostable, and oxo-biodegradable, as well as conventional high density polyethylene (HDPE) bags. (The European Commission has recently recommended a ban on oxo-biodegradable plastics, because of fears that they break down into microplastics.)
In Napper’s experiment, the bag labelled “compostable” (which stated it adhered to standard EN 13432) disappeared entirely within three months when it was left in seawater. In soil it remained intact for two years, but disintegrated when the researchers loaded it with shopping. The rest of the bags – including the one labelled “biodegradable” – were still present in both soil and sea water after three years, and could even hold shopping.
After nine months in the open air, all of the bags had disintegrated or were beginning to come apart, mostly breaking down into microplastics. That’s because sunlight helps break down plastics through a process called photo-oxidation, in which the plastic becomes weathered and brittle, eventually fragmenting rather than breaking down to its organic components.
“That doesn't actually mean it's breaking down into its most natural counterparts of carbon and hydrogen, it just means they're becoming smaller pieces,” says Napper. “Which you could argue is more problematic because you can't clean up – it's like trying to pick up Smarties with chopsticks.”
“It’s important to understand the differences between terms like compostable, biodegradable and (oxo)-degradable,” a spokesperson said. “Discarding a product in the environment is still littering, compostable or otherwise. Burying isn’t composting. Compostable materials can compost with five key conditions – microbes, oxygen, moisture, warmth and time.”
In another study five different types of plastic carrier bag were compared. These included two types of oxo-biodegradable bag, one biodegradable bag, one compostable bag, and a high-density polyethylene bag – a conventional plastic bag.
The study found a lack of clear evidence that biodegradable, oxo-biodegradable and compostable materials offered an environmental advantage over conventional plastics, and the potential for fragmentation into microplastics caused additional concern.
Prof Richard Thompson, head of the unit, said the research raised questions about whether the public was being misled.
“We demonstrate here that the materials tested did not present any consistent, reliable and relevant advantage in the context of marine litter,” he said. “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.”