When considering whether something is likely to wind up spending years in a landfill, the terms compostable and biodegradable come up. On the surface, the two seem interchangeable. Both imply materials—typically those used in packaging or in products like soap, bags, or cups—can be returned to the earth. But there are actually a few key differences to keep in mind.
If something is biodegradable, it means it will eventually break down in the presence of bacteria, fungi, or other naturally occurring conditions in soil. Typically, this means things derived from plants, animals, or natural minerals. Even plastic, widely believed to be outside of this definition, is biodegradable, though biodegradable-resistant might be a better term. It can take hundreds of years for plastic to succumb to the elements. Any product that will eventually break down to carbon dioxide and water is technically biodegradable, though the idea that it could take centuries certainly affects its environmental profile.
Compostable actually refers to two different things. If something organic is compostable, it means you can add it to a compost pile to use as fertilizer—think food or leaves. But in terms of packaging, compostable means it can become compost under the right conditions. Something labeled compostable needs to be processed in an industrial composting facility to live up to that promise. According to the American Society for Testing and Materials (ASTM), a compostable product needs to biodegrade by 90 percent within 180 days in a dedicated facility. If it’s simply thrown into a landfill, there’s no telling when it might break down.
Ultimately, both biodegradable and compostable items are good for the environment, but it’s a matter of timing. A compostable item can degrade faster, but only under the right conditions. Items labeled biodegradable usually have to break down within a year, but don’t return resources to soil like compostable items can. It's something to keep in mind the next time you make an environmentally sound purchase.