Biodegradability, compostability and bioplastics
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.
Industrial compost facility
Conditions in an industrial compost facility are tightly controlled to ensure rapid degradation of materials to less than 10% of their original dry weight.
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
Australian compostability standard symbol
Any material or product bearing this symbol must meet the specific requirements of the Australian compostability standard AS4736-2006.
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.
Plastic types and biodegradation
Some plastics produced from fossil fuels or biomass can biodegrade under the right conditions. Just because a plastic is made from a biomass does not mean it is automatically biodegradable. This can be confusing for consumers who are trying to make good decisions around reducing plastic pollution.
Image from Plastics in the Environment: Te Ao Hurihuri – The Changing World. Royal Society Te Apārangi
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.
Domestic compost system
The conditions in domestic composting systems are more variable than industrial facilities.
Potatopak (now rebranded as earthpac) 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.
Related content
Read more about Bioplastics and Microplastics and then explore How harmful are microplastics? Some microorganisms degrade plastic. Discover more about research to correlate numbers of marine microorganisms that degrade plastic with levels of plastic in the ocean.
Learn about Oceans of rubbish, Plastics and recycling, Biodegradability and The future of plastics: reusing the bad and encouraging the good.
Read the Connected article Down the drain to see how students in Petone, Lower Hutt, took action to prevent rubbish from entering their local marine environment.
Plastic is a wicked problem. It’s incredibly useful, but it’s also a huge environmental issue. Helpful PLD resources are Thinking about plastic – planning pathways, which includes our interactive planning pathway, and Material World – Recycling and biodegradability.
When we throw something away, how do we know where ‘away’ is? The Sustainable Seas National Science Challenge developed online tools to help us find out. Ocean Plastic Simulator is an interactive computer simulation that shows where plastic is likely to end up when it is dropped in the ocean.
New Zealand science organisations Royal Society Te Apārangi and the Office of the Prime Minister’s Chief Science Advisor created reports and resources to help us rethink plastic. Check out the interactive timeline. It gives a short history of plastic – including innovations and some of the impacts.
Explore alternatives to fossil fuel-based plastics in The ZESPRI biospife, Starch-based disposable plates and trays, Skateboards made out of harakeke? and Turning old into new.
Activity ideas
Explore biodegradability in the Biodegradability experiment or Testing the biodegradability of potato plates – where students can compare the rate of degradation of disposable plates using three different disposal methods.
In Make a composite material container students can develop their understanding of composite materials and how that a composite material has new and different properties when compared to the properties of the individual materials.
For activities based around reuse and recycling try: Plastic – reuse, recycle or rubbish game, DIY plastic recycling plant, Waste – a growing challenge! and Determining the properties of plastic and glass.
The What happens to our plastic bottles? activity is perfect for NZC levels 1 and 2. It uses the New Zealand Ready to Read books At the Beach and What Does the Tide Bring In? to introduce the PET plastic recycling process.
Citizen science projects
Bring some citizen science into the classroom with:
Backyard Battle and Mizuiku Upstream Battle – two citizen science projects run by Keep New Zealand Beautiful. The aim is to collect and audit litter to help provide a better understanding of the pathways and root causes of pollution in Aotearoa.
This case study details how a similar project was used with students in year 2–4.
Sustainable Coastlines is running a long-term beach litter citizen science project Litter Intelligence.
Litterati – an international project that allows participants to photograph, upload and tag litter in their own area.
Global Earth Challenge is another international project – it has a section on plastic pollution.
Useful links
The Ministry for the Environment has information about biodegradable and compostable plastics on their website.
To find out more about biodegradable plastics in New Zealand on the Plastics New Zealand website.
