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A Break Down of the Break Down

The difference between compostable, biodegradable and every confusing term in between.


Searching for the most sustainable pet waste bags brought me down this rabbit hole. My degree in conservation and environmental science did not prepare me for the pet aisle of the co-op.  After hours of research—and no bag purchase—I instead came up with these quick definitions of all things relating to bio-plastics. 


Organic material that comes from plants and animals. It is a renewable source of energy.


Matter that is decomposed by bacteria or other living organisms within an unspecified timeline. Almost all matter—plastic included— is biodegradable but it will take hundreds of thousands of years for most things to break down.


Natural fertilizer that is safe for gardening and farming.


Material that will break down and can be used safely as compost. More specifically, the matter is broken down by microorganisms and oxygen into carbon dioxide, water and biomass to create compost. This process happens fairly quickly, it is usually measured by a growing seasons but can take up to a year.

For a plastic to be considered compostable it must break down completely within a given timeframe, becoming indistinguishable from the compost and more importantly, cannot leave any toxic material behind.

Certified Compostable (ASTM D6400 or EN 13432)

Materials must break down within 12 weeks and biodegrade at least 90% of the matter within 6 months in an industrial composting facility. By the end of the 6 months, about 10% of solid matter will be left in the form of compost or biomass and water. The end product must be free of toxins so the compost will not be harmful when the facility sells it to gardeners and farmers.

When certified compostable materials are composted in an industrial composting site, they go back to organic matter and can be used safely as compost. The problem arises because certified compostable products must be disposed of in a designated composting facility, not at home and not sent to landfills. This is because certified compostable materials tend to require higher temperatures to biodegrade quickly enough, or in some cases at all. Few areas in the U.S. have curbside collection for industrial composting.

To make everything more confusing, all compostable plastics are biodegradable, but not all biodegradable plastics would be considered compostable.

Bio-Plastic/Plant-Based Plastic

Is a large family of plastics which are 1) sourced from biomass at the beginning of their life —this is known as bio-based—, 2) metabolized into organic biomass at the end of their life—this is known as biodegradable— or 3) both.

Biodegradable Plastic

A type of bio-plastic that is able to be broken down by microorganisms into carbon dioxide, water, and biomass within an unspecified timeline. As stated earlier, almost all matter—plastic included— is biodegradable but it will take hundreds of thousands of years for most things to break down.

This means that just because a plastic can biodegrade, it will not necessarily be safe enough to be used as compost since the time it is given to break down is too long to remove harmful matter.

Oxo-Biodegradable Plastic

Plastic that uses metal salts to speed up degradation, resulting in extremely small fragments of plastic that are not visible to the human eye, also known as micro-plastics. Further decomposition depends on living organisms and bacteria and this plastic typically doesn’t break down fully in landfills.

Oxo-Biodegradation is harmful for two reasons. Some oxo-biodegradable plastics use Cobalt to speed up degradation and cobalt cannot be destroyed once it has entered an environment. Secondly, biodegradable and oxo-biodegradable plastics are only partially broken down into micro-plastics rather than decomposing into organic compostable matter, leaving us with an even bigger, micro-plastic problem.



Sources: National Geographic: Planet or Plastic, EIA US Energy Information Administration, Pela Case

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