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How does biodegradable plastic break down?

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Quick answer

Biodegradable plastic breaks down when bacteria and fungi secrete enzymes that chop its long polymer chains into small molecules, which the microbes then digest. This process needs moisture, oxygen, and warmth, so it happens quickly in industrial composters but very slowly in landfills or oceans.

Biodegradable plastic breaks down when microorganisms such as bacteria and fungi secrete enzymes that cut its polymer chains into smaller molecules, which they then consume as food. This process requires the right conditions of moisture, oxygen, and temperature, and it can take weeks to years depending on the material and environment.

Step by step

  1. Microorganisms like bacteria and fungi land on the surface of the plastic.
  2. They release enzymes that break the long polymer chains into shorter fragments.
  3. The microbes absorb these fragments and use them as a source of energy and carbon.
  4. Inside the cells, the fragments are metabolised into water, carbon dioxide, and new microbial biomass.
  5. The plastic gradually disintegrates as more microbes colonise and continue the process.

Key things to understand

  1. Biodegradable plastics are designed so that naturally occurring microbes can use them as a food source.
  2. Microbes attach to the plastic surface and release enzymes that break long polymer chains into shorter fragments.
  3. These fragments are absorbed into microbial cells and converted into water, carbon dioxide, and biomass.
  4. The breakdown speed depends on the plastic's chemical structure and the surrounding conditions.
  5. Industrial composting facilities provide ideal heat, moisture, and oxygen, so breakdown there is much faster than in a home compost bin or landfill.

Labelled diagram

  1. Plastic surface

    Long polymer chains form the solid plastic item.

  2. Microbes attach

    Bacteria and fungi colonise the surface and secrete enzymes.

  3. Enzyme action

    Enzymes cut the polymer chains into smaller fragments.

  4. Microbial digestion

    Fragments are absorbed and metabolised inside microbial cells.

  5. End products

    Water, carbon dioxide, and biomass are released.

How microbes break down biodegradable plastic

Real-world examples

  • A biodegradable plastic bag in an industrial composting facility can break down in a few months at high temperatures.
  • A biodegradable fork buried in a home garden may take over a year to decompose because temperatures are lower.
  • A biodegradable plastic bottle lost at sea breaks down extremely slowly because cold water slows microbial activity.
  • In a sealed landfill, even a certified biodegradable bag can remain intact for decades due to lack of oxygen.

Common mistake

✗ Myth: All biodegradable plastics break down quickly and harmlessly in any environment, including landfills and oceans.

✓ Fact: Biodegradable plastics only break down efficiently under specific conditions of heat, moisture, and oxygen, which are often absent in landfills and oceans. Without those conditions, they can persist for years and cause the same environmental problems as conventional plastic.

People also ask

What causes biodegradable plastic to break down?
Microorganisms such as bacteria and fungi cause the breakdown. They attach to the plastic, release enzymes that split the polymer chains into smaller pieces, and then consume those pieces as food. The process also requires moisture, oxygen, and warmth to proceed at a useful speed.
Why doesn't biodegradable plastic break down in the ocean?
Ocean water is often too cold and lacks the high concentrations of specific microbes needed for rapid breakdown. Many biodegradable plastics are designed for industrial composters, not marine conditions, so they degrade extremely slowly in seawater and can still harm marine life.
Is biodegradable plastic better for the environment?
It can be better if it is collected and processed in an industrial composting facility, where it breaks down quickly. But if it ends up in a landfill or as litter, it offers little advantage over ordinary plastic and can still pollute ecosystems for a long time.
Do biodegradable plastics break down in a landfill?
Usually very slowly, because landfills are often sealed and lack the oxygen and moisture that most biodegradable plastics need. Without those conditions, microbes cannot work efficiently, so the plastic may persist for many years.
What is the difference between biodegradable and compostable plastic?
Compostable plastic is a subset of biodegradable plastic that must break down within a specific time frame, leave no toxic residue, and support plant growth. Biodegradable plastic only needs to break down eventually, with no guaranteed timeline or safety standard.
How long does biodegradable plastic take to decompose?
It varies widely: in an industrial composter it can break down in a few weeks to a few months, while in soil or seawater it may take a year or more. In a landfill, it might last as long as ordinary plastic.
Can biodegradable plastic be recycled?
Generally no, because it can contaminate the recycling stream for conventional plastics. Even small amounts can weaken recycled products, so most recycling programs ask you to put biodegradable plastics in the trash or a compost collection instead.

Part of the guide: Everyday Science: Light, Heat & Matter →