What are biodegradable plastics?

Biodegradable plastic refers to a plastic that can dissolve through biological activity, mostly by microbial organisms. While some biodegradable plastics are compostable, not all of them are. Compostable plastics can break down under regulated circumstances, such as those found in composting or anaerobic digestion facilities. Different types of biodegradable plastics will only decompose under certain conditions that have been proven, such as in soil or wastewater treatment plants. However, most biodegradable plastics require industrial composting facilities as their acceptable end-of-life destination, as we will discuss further later. While these plastics may biodegrade in the open environment, the rate at which this occurs can vary greatly and there are currently no well-defined or widely recognized test procedures or standards. Compostable plastics must also meet certain criteria, including fragmentation, lack of ecotoxicity, and limits on possible pollutants such as heavy metals.

The focus of this page is on compostable biodegradable plastic, with the term "compostable" used extensively and wherever applicable. The emphasis is on the fact that, from the standpoint of sustainable waste management, what matters is the end-of-life behavior of the materials. This includes their ability to be incorporated into composting schemes, enabling support for separate collection of waste (such as compostable bags), or replacing plastic items that would otherwise end up in disposal facilities.

The term "bioplastics" can refer to either microbial plastics (as discussed below) or biodegradable plastics. As a result, it can be a confusing phrase that requires further clarification. Therefore, it is not recommended to use this term without additional explanation.

Bio-based plastic relates to plastic made from renewable sources of energy. For example, sugar cane can be processed to generate ethylene, which can then be used to produce polyethylene (PE). Starch can be converted into lactic acid, which can then be used to create polylactic acid (PLA). The characteristics of bio-based polymers can vary greatly depending on the specific material being used.

Compostable polymers that are appropriate for industrial composting must meet strict specifications. Composability is determined by specific criteria. The US standard for solid material biodegradation (by composting) is ASTM D6400, which is required for the labeling of plastics intended to be aerobically decomposed in municipal or industrial facilities. In the EU, the EN 13432 standard emphasizes the need to recover waste production through industrial composting. The EN 13432 standard is considered a reliable reference for other standards, and comparable requirements to EN 13432 are currently being adopted in the context of other standards. The ASTM standard has recently been amended to conform with EN 13432, and a similar change has been made to the global standard ISO 18606.

When does it make sense to use biodegradable plastic and when does it not?

Important note: Biodegradable plastics, including compostable plastics, cannot and should never be considered a complete replacement for conventional plastics. Reduction of plastic usage must always be the primary focus. If a reusable alternative is appropriate for the operational conditions, it should always be considered first.

It is crucial to note that disposable (and compostable) polymers should not be seen as a solution to the problem of littering since they decompose quickly in the natural environment. Instead, they should be disposed of in separate organic waste collection and composting programs. While compostable plastics have similar composability requirements, concerns remain regarding their biodegradability and related testing methods in fresh/salt water or the natural environment, as previously mentioned.

What are the issues?

Plastics play a vital role in modern society due to their versatility, lightweight nature, and relatively low production costs. However, only about 1% of the world's plastics and plastic goods are currently bio-based, compostable, and/or biodegradable (European Bioplastics e.g., 2020b). The majority of plastics are still produced using fossil fuels, resulting in increased greenhouse gas emissions throughout their supply chain. Plastics pollute the environment at every stage of their life cycle, from production to consumption and eventual disposal.

The recycling rates for plastics are poor, and they enter the ecosystem through means such as littering, incorrect waste disposal, and product wear and tear. They can persist in the environment for several years and even enter the food chain. One challenge is contamination with plastic particles when creating compost from separately collected bio-waste (EEA, 2020).

Biodegradable, compostable, and bio-based plastics are increasingly being promoted as a solution to some of these issues. More and more consumer products, such as plastic containers, packaging, and single-use cups, are labeled as 'compostable,' 'biodegradable,' or 'bio-based.' But what do these terms actually mean? To what extent can biodegradable, compostable, and bio-based plastics help address the sustainability issues posed by plastics?

Biodegradable and compostable plastics waste management

A circular economy aims to preserve as much of a product's value and resources as possible. While biodegradable and compostable polymers are technically recyclable, they are not currently being recycled into new plastic. When conventional plastics are recycled together with biodegradable or compostable plastics, they are considered contaminants. However, increased market share in the future may worsen the problem, but it may also make recycling certain biodegradable or compostable plastics economically viable (Cripps et al., 2019). Nonetheless, more research, innovation, and investment in plastic recycling are necessary.

Biodegradable (and compostable) polymers are often not recyclable with conventional plastics. For example, if biodegradable plastic is mixed and melted with standard plastics such as PET or HDPE, the entire load will be contaminated since the chemical nature of biodegradable plastics is so different. Moreover, deterioration may affect the mechanical qualities of recovered plastics. While evidence from countries that have widely adopted compostable plastics to optimize separate collection indicates only minor pollution of plastics with biodegradable plastics, it is always important to keep the two streams separate in order to optimize both the decomposition of organic waste (with incorporated compostable plastics) and the reusing of traditional plastics.

When disposed of in landfills, biodegradable (and compostable) items are not always more environmentally beneficial (rather, the opposite is true). Biodegradable items, such as plastic containers and utensils, are broken down in landfills by microorganisms (similar to composting, but in an anaerobic condition, i.e., without oxygen), which then produce methane. Methane can be a useful energy source when captured, such as in an anaerobic digestion facility, but it is a potent greenhouse gas when released into the atmosphere, which occurs for a significant portion of methane from landfills, including controlled landfills.

Compostable plastics are not recommended for household composting due to differences in conditions compared to industrial composting. It also makes little sense to use biodegradable bags or cutlery in home composting practices. This is why criteria that apply to degradability in central composting are favored rather than home composting systems.

Advantages of biodegradable plastics:

These polymers not only degrade faster when disposed of but they can also be recycled using organic methods. Additionally, they are non-toxic because they do not contain any chemicals or poisons. Recycling reduces landfill difficulties, and recycled bio-waste can also be used as fertilizer or renewable energy for biogas.

Although the production of biodegradable polymers requires more expenditure, it is ultimately worthwhile. When the costs of clean-up and the negative impact on the environment are taken into account, biodegradable plastic items are undoubtedly the better option.

Compared to regular plastics, biodegradable plastics require less energy to manufacture. For example, a corn-based polymer uses 65% less energy to produce than a petroleum-based polymer. Additionally, the production of bioplastics does not require the process of locating, obtaining and transporting hydrocarbons. This means that fewer fossil fuels will be used, resulting in less environmental damage. Moreover, it emits 68% fewer greenhouse emissions during manufacturing, resulting in significant environmental benefits.

Disadvantages of biodegradable plastics:

Biodegradable plastics are made from crops such as soybeans and maize, which pose a risk of contamination because these crops are often treated with pesticides on the farm, which can be transported or incorporated into the final product.

One disadvantage of biodegradable plastics is the need for expensive processors and composters, especially those that require high temperatures for breakdown. In addition to cost, the availability of equipment can also be a challenge. These types of plastics cannot degrade in cold ocean waters, so they will either float in the sea or degrade into microplastics, posing a health risk to aquatic life. Therefore, the use of certain types of plastics may not fully address all pollution concerns.

References:

Blessing, J. M., et al., 2017, Bevölkerungsrepräsentative Online-Befragung in Deutschland zu Biokunststoffen, Arbeitsbericht Oktober 2017, Hochschule Hannover, Hochschule Weihenstephan-Triesdorf, Hannover, Germany, accessed 20 June 2020.

https://bioplastics.org.au/150-organisations-worldwide-back-ellen-macarthur-call-ban-oxo-degradable-plastic-packaging/

https://www.plasticplace.com/blogs/blog/5-surprising-secrets-of-biodegradable-plastic-bags.

https://www.eea.europa.eu/publications/biodegradable-and-compostable-plastics#:~:text=Although%20biodegradable%20and%20compostable%20plastics,conventional%20plastics%20when%20collected%20together.

https://www2.mst.dk/Udgiv/publications/2020/02/978-87-7038-165-9.pdf

https://en.wikipedia.org/wiki/Biodegradable_plastic.