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I read this and still don't fully understand the difference. Which decomposes faster? Do they mean biodegradables don't need "certain" conditions so they are good to go into a bin (or a hole in your backyard)? Or do they mean biodegradable plastic leave some non-soil residue behind?

Plastic that is compostable is biodegradable, but not every plastic that is biodegradable is compostable. Whereas biodegradable plastic may be engineered to biodegrade in soil or water, compostable plastic refers to biodegradation into soil conditioning material (i.e., compost) under a certain set of conditions. In order for a plastic to be labeled as commercially “compostable” it must able to be broken down by biological treatment at a commercial or industrial composting facility. Composting utilizes microorganisms, heat and humidity to yield carbon dioxide, water, inorganic compounds, and biomass that is similar in characteristic to the rest of the finished compost product. Decomposition of the plastic must occur at a rate similar to the other elements of the material being composted (within 6 months) and leave no toxic residue that would adversely impact the ability of the finished compost to support plant growth. ASTM Standards D6400 and D6868 outline the specifications that must be met in order to label a plastic as commercially “compostable”. There are currently no ASTM standard test methods in place for evaluating the ability of a plastic to compost in a home environment.

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  • My understanding is that compostable plastics are both biodegradable & they can be composted. Given that compost is used to benefit gardens, some biodegradable plastics are not suitable for composting due to the potential for soil of food contamination.
    – Fred
    Commented Nov 25, 2022 at 10:29

4 Answers 4

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The problem is that different people use different definitions for biodegradable and compostable. What people usually agree on is the following:

Biodegradable means that a plastic can disintegrate into basic elements via a natural process. With basic elements we mean things like water, carbon dioxide, methane and biomass.

Compostable plastic takes things one step further and says that it should disintegrate under conditions achieved during composting.

The trouble begins when people try to refine these definitions;

  • most people say that with composting the resulting basic elements should be harmless to the environment (so no toxic residues), but there is compostable plastics on the market that releases a toxin called PFAS

  • some say that biodegradable means "with the use of microorganisms", so that would exclude dissolving into water which is regularly considered as biodegradable by others.

  • some say that biodegradable means "no human intervention", as opposed to composting that is regarded as a human-driven process. However most biodegradable plastics need specific conditions (moist and 50°C+) to start disintegrating, conditions which you won't easily find in nature.

  • some say that anaerobic composting (composting without oxygen) doesn't count as compostable, others disagree

  • some say compostable plastics can only be made from biobased materials, but researchers found a way to create home compostable plastics using a mixture of PLA (biobased) and PCL (fossil-fuel based) plastics.

  • many people say that full decomposition should be achieved within a reasonable timeframe and usually a shorter one for composting than for biodegradation. Then again there are also people that don't consider time (which IMHO is incorrect as it makes everything biodegradable, theoretically speaking)

  • some consider microplastics to qualify as basic elements for degradation (personally I disagree because all plastics degrade under the influence of sun light so that would make any plastic biodegradable)

For what it's worth; compostable plastics are generally considered to be a bit more environmentally friendly than biodegradable plastics because they break down faster, should not contain any harmful residues and because they are almost always made from biobased materials such as starch or cellulose. But as the points above show you, you need to really understand people's definitions to be able to validate that.

There are country-specific regulations and standards saying when you can call something compostable, home compostable or biodegradable. I wrote a bit about that in this answer.

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Compostable plastic can be added to a compost heap, to be degraded and transformed into compost along with the rest of the compostable materials, to be turned into a soil improver within 6 months. The municipal council in my town gives out compostable plastic bags for food waste, which are then collected in the weekly pick-up, and composted in huge heaps at the council's waste facility. People can go purchase bales and truckloads for their gardens.

Biodegradable plastic, it is said, will break down if exposed to the workings of heat (sun) and wind and water. Like the weather. You might have seen someone storing something in a biodegradable one-use plastic bag, and that bag fragmenting after a few weeks, and them having to pick up the pieces. Trouble is with this sort, it just fragments down to molecules of plastic.

But, "More than 50 species of plastivore – small organisms that consume plastic – have been discovered, which are mostly bacteria and fungi but include some insects capable of turning plastic waste into energy." And these critters don't even need the plastic to be biodegradable.

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Same. Biodegradable means disposal reintroduction into the environment where it dissolves fue to biological and outdoor activactivity...

Compostable implies it can be added to compost as a soil amendment.

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Biodegradable and compostable plastics are both designed to break down over time, but they differ in how they decompose and the environmental impact of their breakdown process.

Biodegradable plastics break down through natural processes into smaller molecules, eventually returning to nature. However, the breakdown process may leave behind residues or microplastics that can persist in the environment and cause harm.

Compostable plastics, on the other hand, not only break down into smaller components but also undergo a specific process called composting. During composting, organic materials decompose aerobically, producing nutrient-rich compost that can be used to enrich soil. Compostable plastics fully degrade into natural elements, leaving no harmful residues behind.

Compostable plastics are typically made from renewable resources like plant-based materials and are certified to meet specific compostability standards. They provide a more sustainable alternative to traditional plastics because they reduce the amount of waste sent to landfills and contribute to the production of valuable compost that improves soil health.

In summary, while both biodegradable and compostable plastics break down over time, compostable plastics undergo a more controlled decomposition process and result in the production of beneficial compost. Therefore, compostable plastics offer a superior environmental solution for reducing plastic waste and promoting sustainability.

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