Bioplastics are a form of plastic that can be made from renewable bio-based resources. Many bioplastic materials are designed to be biodegradable, and some are designed to be compostable. These properties are important for the functionality of the end product and for its disposal. When considering the environmental impact of disposing of a bioplastic product, the difference in meaning between biodegradable and compostable is important.

Biodegradable or compostable?

Many bioplastics are designed to be biodegradable but not all of these are compostable. So, what do these two terms tell us?

A biodegradable product breaks down into smaller compounds with the help of biological organisms, such as fungi and bacteria. In aerobic conditions, biodegradable products will break down to produce carbon dioxide, water and biomass. In anaerobic conditions, they produce carbon dioxide, methane, water and biomass.

A compostable product also breaks down into smaller compounds with the help of biological organisms, but it does so in specific conditions to a defined outcome. In general, a compostable product breaks down in a specific timeframe in a controlled moist, warm, aerobic environment to produce compost that is non-toxic and can enhance soil and support plant life.

Differences in meaning

Technically, all plastics (bioplastic and traditional) will biodegrade to a certain extent under suitable conditions, but many take hundreds of years and produce harmful residues. On the other hand, compostable products are biodegradable products that break down quickly under defined conditions into raw materials that can enhance soil quality and support plant growth.

A compostable product must meet specific criteria in terms of:

  • time
  • environmental conditions
  • quality of compost produced.

Whether a bioplastic product is made from a biodegradable material or a compostable material is critical for minimising environmental impact and increasing sustainability. It is an important element for claiming greater sustainability of bioplastics over conventional plastics or for claiming enhanced functionality of one bioplastic over another. It’s also important in choosing the most appropriate way to dispose of an item or material.

Industrial versus domestic composting

Many bioplastic products are designed to be compostable. However, in many cases, this compostability will only occur in the tightly controlled conditions of industrial composting facilities.

There are now internationally recognised and measurable standards for identifying and labelling products or materials as compostable. The standards define the required degree of degradation of a material within a specific time period, as well as the size and toxicity of the resulting compost. For example, the Australian standard AS 4736 states: “The material must compost to no more than 10% of its original dry weight retained on a 2 mm sieve within 12 weeks. The resulting compost must have no toxic effect on plants or earthworms.”

Home composting systems generally operate at lower temperatures than industrial facilities and conditions are more variable and less managed. Many bioplastics will not degrade in home compost systems, and those that do may take considerably longer.

Why definitions are important

A good understanding of definitions helps the consumer make informed choices about the products they buy and how to dispose of them.

Standards help avoid false and misleading claims for bioplastic materials and help consumers and manufacturers identify appropriate disposal options for products. However, making or labelling a product or material compostable has little value if the product doesn’t end up in the appropriate waste management system after use or if the appropriate waste management system is not readily available.

In New Zealand, most large-scale industrial composting facilities have not investigated the option of compostable plastic disposal, so we are unable to take full advantage of the compostability of these materials. Currently, our industrial composting facilities are designed to process food and garden organics, untreated timber waste and agricultural and sewage waste.

Potatopak is an innovative New Zealand company that makes disposable food ware from recycled potato starch. The products are designed to compost in a domestic compost system within 30 days. Use this article to find out more about these products.

Activity idea

Try this experiment to explore the degradability of different materials with your students.

Useful links

To find out more about degradable plastics in New Zealand download the PDF Managing the Transition: Degradable Plastics in New Zealand created by Plastics New Zealand, from the EP TECH website.

    Published 21 May 2013