Compostable plastic packaging: the perfect solution for plastic waste?

To achieve net zero and limit the worst of climate change, we have to rethink our reliance on plastic.

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This statement is more urgent than ever, but the problem with plastics is unfortunately much bigger than just climate change. In reality, plastics also directly threaten human health and endanger habitats when leaked into the natural environment. Paradoxically, one of the main culprits of this damage is also claimed by some as the next green solution  — compostable plastics.

To unpack this new material, we conducted a study on the environmental impact of compostable plastic bags, to find out if they truly offer a more sustainable alternative to traditional materials like paper, fossil plastics and recycled plastic. In this article, we’ll focus on one part of the study: the damage that leaked compostable plastics have on our ecosystems.

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The consensus on compostable packaging is unclear

The biodegradable plastics market is projected to expand 2-3x between 2021 and 2026 [1]. And at Sourceful, we’ve seen firsthand the escalating demand for compostable packaging. But whilst many brands are running towards compostable plastics, the consensus is still murky, with other companies (like Tesco and Abel & Cole) publicly distancing themselves from them. We wanted to use our research to help fill the vital knowledge gap and build consensus.

Compostable plastics are often considered a green alternative because they degrade and so are often (incorrectly) assumed to effectively disappear in the natural environment. The theory is that this reduces the amount of plastic in the ocean and the risk of microplastics. But as with most things — it’s not that simple. Life cycle assessments (LCAs) have historically struggled to account for leaked waste and microplastics because of a lack of data, even though both play a major part in a material’s overall environmental impact.

To tackle this, we partnered with the Sustainable Materials Innovation Hub at the University of Manchester. This gave us access to the latest labs, data on new and innovative materials and their in-house expertise — all invaluable to our study. Together, we investigated how traditional fossil fuels and compostable plastics behave when leaked. Here’s what we found.

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The impact of leaked waste is twofold

One of the major environmental impacts of plastics (fossil and compostable), besides their carbon footprint, is their effect on the natural environment when leaked.

Leaked plastic waste generates both physical (e.g. animals ingesting microplastics or being entangled in larger pieces) and chemical risks (e.g. the leaching of toxic additives like plasticisers and flame retardants) to wildlife from the breakdown of plastic into microplastics and nano-plastics. Not only does the breakdown of plastics directly leach toxic elements but they can also act as a magnet for other environmentally harmful pollutants.

To make matters worse, leaked plastic waste has also been found to be directly connected to climate change. Researchers at the Ocean University of China found that microplastics reduced the growth of microalgae and the efficiency of photosynthesis, in turn degrading plankton’s ability to remove carbon dioxide from the atmosphere [2]. The knock-on effect is that the ocean itself cannot capture carbon as efficiently; an essential resource in our fight against climate change, given that it sequesters 30-50% of total CO2 emissions from human activity.

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Compostable plastics and leaked waste

The longer a plastic takes to break down, the more likely it will be ingested or cause entanglement. Put another way, the risk of adverse effects increases the longer a plastic persists. To account for this, we assessed each material for its degradation time in freshwater, marine, and soil environments, and used that data to identify a leaked waste impact rating for each material.

Our study found that whilst compostable plastics do reduce the risk of some adverse effects (less risk of entanglement and a shorter period of microplastics), they are not a cure-all for plastic pollution. Compostable plastics can persist in the natural environment for over half a century, which puts into question the popular claim that these plastics are the next green solution. This matches up with the conclusions that Narancic et al. made in their study [3]. Here’s an overview of degradation times for fossil plastics and common compostable plastics:

1. Fossil plastics take around 4-5,500 years to degrade in soil (with some studies suggesting this is even higher, at around 10,000 years); the worst and longest degradation time amongst all plastics. This is made worse by the common use of harmful additives [4].
2. Compostable plastics like PLA take on average 1-63 years in soil[3] to degrade completely. In water, PLA does not degrade at all.
3. Other compostable plastics like TPS and PHB take on average 4-6 months to degrade completely.

So compostable plastics do have a tighter degradation window than fossil plastics, and they also typically contain fewer toxic additives (such as flame retardants and stabilisers). But they still can have a significant degradation window, especially and unfortunately PLA, one of the most common materials used in compostable packaging (including coffee lids and bags).

Admittedly, it is still hard to know the exact time it takes for a plastic to decay; the field of estimating polymer lifetimes is still relatively new. But we do have enough comparative data to give us an indicative hierarchy of materials that we can use to assess performance and inform decisions.

Compostable plastics do slightly reduce the risk of microplastics because of their shorter degradation times. But our larger study showed that compostable bags emit 1.5-2x more greenhouse gas emissions over their full life cycle than virgin fossil plastics. In addition, given they are still relatively new, there are uncertainties about the unintended consequences that could come from their use. ****This begs the question: are the reduced risks from leaked waste enough to offset the increase in carbon footprint? For now, we don’t think so.

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What does this mean for my packaging?

It’s clear that we need to move away from fossil plastics. And in their current state, compostable plastics are not the next green solution. So, what’s the answer?

First, brands should follow the waste hierarchy. Can this packaging component be removed? Can we use less materials without compromising function? How can I design this product so it’s easy to recycle?

Second, brands should prioritise responsibly sourced paper if possible, which typically has the lowest impact of any material. Its full life cycle emissions are low, and there’s no risk of microplastics if leaked. That’s not to say it’s perfect; forests are often mismanaged and producing and recycling paper still generates emissions, uses large amounts of water and potentially also harmful chemicals. This is why certifications like the Forest Stewardship Council (FSC) and Programme for the Endorsement of Forest Certification (PEFC) are so important.

Paper isn’t also appropriate for every use-case and product, like liquids. This is why we stress prioritising paper if possible. Packaging should always be carefully matched with the product, and blanket rules rarely result in success.

For more information about this study, email climate@sourceful.com

Notes

Thanks to Dr. Guilhem de Hoe and Dr. Chloe Loveless from the University of Manchester for leading the collaboration.

Our study focused on the typical compostable plastics currently seen on the market (PLA, PBAT, PHA and TPS). Our study did not include a nascent group of materials classed as unmodified natural polymers, which we’re interested in exploring in the future.

References

1. MarketsandMarkets. (2021). Biodegradable Plastics Market - Global Forecast to 2026.
2. Zhang, C., Chen, X., Wang, J., & Tan, L. (2017). Toxic effects of microplastic on marine microalgae Skeletonema costatum: Interactions between microplastic and algae. Environmental Pollution, 220(B), 1282-1288. [Link] https://doi.org/10.1016/j.envpol.2016.11.005
3. Narancic, T., Verstichel, S., Chaganti, S. R., Morales-Gamez, L., Kenny, S. T., De Wilde, B., Padamati, R. B., & O’Connor, K. E. (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. [Link] https://doi.org/10.1021/acs.est.8b02963
4. Chamas, A., Moon, H., Zheng, J., Qiu, Y., Tabassum, T., Jang, J. H., Abu-Omar, M., Scott, S. L., & Suh, S. (2020). Degradation Rates of Plastics in the Environment. ACS Sustainable Chemistry & Engineering, 8(9), 3494-3511. [Link] https://doi.org/10.1021/acssuschemeng.9b06635