Disposal Method Matters: The Truth Behind Biodegradable Plastics

Source: Tanja Narancic, Steven Verstichel, Srinivasa Reddy Chaganti, Laura Morales-Gamez, Shane T. Kenny, Bruno De Wilde, Ramesh Babu Padamati, and Kevin E. O’Connor. (2018) Biodegradable plastic blends create new possibilities for end-of-life management of plastics but they are not a panacea for plastic pollution. Environmental Science & Technology 52: (18) 10441-10452. https://pubs.acs.org/doi/10.1021/acs.est.8b02963

What is the problem with plastics?

Plastics exhibit many advantages over other materials in terms of processability, affordability, and desirable properties. These advantages enable their widespread use in countless applications. Unfortunately, plastics often come with a major side effect: many do not break down in the environment, contributing to the large buildup of plastic waste around the globe. This buildup is large enough to essentially create large islands of plastic in our oceans!

Why do traditional plastics stick around for so long in the environment? All plastics are made of long molecules called polymers, and it’s largely the types of polymers used that give plastics their unique properties and determine their applications. Traditional plastics are made of polymers that were designed for their in-use properties (strength, durability, water resistance, etc.) rather than their end-use properties (how they behave after being thrown away), and most of these polymers are very hard for microbes (great decomposers) to break down. To help fix the problem of plastic waste accumulation, end-use properties should be a major focus when designing new plastics.

Plastic garbage, pictured here on a beach, takes a long time to break down in the environment. Source: pixabay.com

 

How can biodegradable plastics help?

One potential solution to this problem is designing plastics made from biodegradable polymers, which microbes can break down. Many biodegradable plastics are already used commercially in some single-use items like packaging, cups, straws, and plastic-ware. Some, such as TPS and PLA, are bio-sourced and derived from starchy plants, like corn and potatoes. Biodegradable plastics can also be made from traditional, petrochemical sources (think fossil fuels). For example, it was recently found that PET (commonly used to make soda bottles) can be broken down by a particular bacterium. However, this bacterium isn’t common, so recycling is still your best bet for disposing of PET.

(left) A biodegradable plastic cup made of PLA. Source: goodstartingpackaging.com. (right) Biodegradable plastic-ware. Source: Wikipedia.org

A big challenge in designing biodegradable plastics is making their in-use properties as good as those of traditional plastics. One approach is to mix different polymers to obtain a blend of properties in the final plastic. For example, PLA, which is brittle, can be mixed with softer polymers to increase its flexibility for packaging applications.

Are biodegradable plastics a simple solution to the plastic problem? How do they break down in the environment? How should they be disposed? A group of scientists addressed these questions in their recent study, published in Environmental Science & Technology.

Disposal Methods Matter… A lot!

In their study, Tanja Narancic and colleagues, used plastics made from biodegradable polymers and blends, with properties appropriate for use in packaging. To test how disposal impacted the breakdown of these plastics, the researchers exposed them to conditions consistent with managed and unmanaged environments. It is important to note that biodegradable polymers largely break down into carbon dioxide and methane. Reducing the release of these greenhouse gas emissions is a large factor in determining the proper disposal of biodegradable plastics.

Managed environments represent proper disposal scenarios: industrial composting, anaerobic (oxygen-free) digestion, and home composting. Composting reduces methane emissions and returns organic carbon to the soil. Industrial composting conditions mainly differ from home composting conditions based on temperature (industrial is ~122°F while home is ~82°F). Anaerobic digestion will produce more methane, but it collects it for use as alternative fuel. Didn’t see recycling mentioned here? Biodegradable plastics (except for PET) should NOT be recycled, because they contaminate the process and result in low-quality recycled products.

Unmanaged environments represent systems where plastic litter could end up: marine, fresh water, aquatic anaerobic digestion (water treatment plants), and soil. Landfills are often anaerobic, and you would likely see a slower breakdown of biodegradable plastics into carbon dioxide and methane. However, managing greenhouse gas emissions from landfills is an ongoing challenge.

The figure below shows whether the different plastics met the required standards for biodegradation in each environment.

Results of biodegradation tests for bio-degradable plastics in different environments. Reprinted (adapted) with permission from Narancic et al. (2018) Copyright 2018 American Chemical Society.

What’s the secret to successful biodegradation? Oxygen, the right microbes, and a little bit of heat! All the tested plastics degraded with industrial composting—which is often used to define a plastic as biodegradable. However, many did not pass the home composting test. PLA, for example, needs higher temperatures to break down. Interestingly, the researchers found that if you add 20% of PCL (a polymer that will break down in home composting conditions) to PLA, the blended plastic will be home-compostable and more flexible—improving both its end-of-life and in-use properties! As home composting can be as simple as tossing plastic in your backyard compost bin with your food waste, while industrial composting is not available in most communities, blending could make proper disposal much more accessible to consumers.

Biodegradation was very limited in unmanaged environments, with only two plastics passing in marine and fresh water (where you see so much plastic accumulation). In the cases where biodegradation does occur, carbon dioxide and methane byproducts are released into the atmosphere… not a happy outcome.

What does this mean for us?

A compostable or biodegradable plastic cup can be just as bad as a traditional one if you don’t dispose of it properly. Many biodegradable plastics need industrial scale composting to degrade properly, and very few break down in home composting or unmanaged environments. Plus, they shouldn’t be recycled! There is some fear that biodegradable labeling might lead to more littering or mismanaged waste because this concept is not well understood. Next time you’re shopping for environmentally responsible plastic, read labels carefully! Some products will break down in home compost, but others will not. For examples of what labels to look for, see here.

In their current state, biodegradable plastics are not a cure-all. The design and blending of biodegradable polymers could someday result in all packaging and single-use plastic waste being able to break down in a backyard compost bin or in unmanaged environments like oceans, but the field is not there yet. At present, the best solution to slowing the build-up of plastics in the environment continues to be reducing consumption and properly disposing of waste. Reduce and Reuse all plastics, Recycle traditional plastics, and Compost biodegradable plastics where you can!

 

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Mary Davis

I earned my PhD in Chemical Engineering from Princeton University in 2018, where my research focused on nanoscale polymer systems and how their properties change with geometry. I am now applying my background in polymers to environmental systems as a postdoctoral research associate at the U.S. EPA. This involves studying the breakdown of plastics and the generation of microplastics in the environment, as well as their interactions with other pollutants. When I’m not working in the lab, I enjoy crafting, cooking, and being outside.

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