Katherine Martinko is an expert in sustainable living. She holds a degree in English Literature and History from the University of Toronto.

Composting is a bit like alchemy. You put food scraps into a bin and, with the help of some busy microorganisms and insects, you end up with rich, usable soil. Converting trash to treasure is an ideal way of dealing with waste, so it's not surprising that materials scientists have been working for years to develop compostable plastics. As the logic goes, if plastic could be composted in the same way that food scraps are, then we would solve much of the world's plastic pollution problem.

Unfortunately, the resulting engineered plastics labeled as "home compostable" do not break down nearly as well as one might hope. Recent findings from a U.K.-based citizen science project called "The Big Compost Experiment" found that the public is highly confused about what's compostable and what isn't, and that a majority of items labeled as home compostable (coffee cups and pods, tea bags, magazine wrappers, wipes, cling film, shopping bags, etc.) did not disintegrate fully in private compost bins.

The research, published in the journal Frontiers in Sustainability and led by Danielle Purkiss of University College London's Plastic Waste Innovation Hub, is groundbreaking in its reliance on citizen science to get a broad view of what actually happens in private households.

Despite a lack of infrastructure to deal with compostable plastics, consumers are wildly enthusiastic about them, with 85% of study participants believing them to be a better option for the planet than conventional fossil fuel-based plastics. Manufacturers are churning them out at ever-increasing rates while promising they'll break down in backyard composters (and hoping the infrastructure will catch up). 

"Biodegradability" refers to the capability of a particular material to be degraded by biological activity, but it does not describe under what conditions it occurs, nor how long it will take. "Compostable" is more specific, describing a material that can undergo biological degradation in a compost site, "at a rate consistent with other known compostable materials, leaving no visibly distinguishable or toxic residues." Compostable can be considered a subcategory within biodegradable.

The Big Compost Experiment was designed to test these beliefs. It ran from November 2019 to November 2021, with 9,701 participants from across the U.K. participating in a short survey about perceptions and behaviors surrounding biodegradable and compostable plastics. Following that, 1,648 people participated in a real-life composting experiment, placing one or two items in a healthy, functioning compost bin (inside a non-biodegradable plastic net bag for easier tracking), and then following up when contacted by the researchers to see what degradation had occurred. In total, 902 individuals completed the experiment.

Participants selected items that they had identified as home compostable based on labels. This revealed to researchers that most people are quite confused about how to identify home compostable plastics. From the study: 

Of items that were properly identified as home compostable, 59% did not fully degrade in outdoor closed-bin composters (the most common variety), 11% still showed small pieces, and 30% reported no remnants (or were unable to be located). Different kinds of composters had similar results. Curiously, varying lengths of composting time (anywhere from 3 to 21 months) did not affect results all that much; the "health and dynamics" of an individual's composter seemed to matter more.

Purkiss, Allison, Lorencatto, Michie, Miodownik

Home compostable plastics do not break down successfully in real-life composting scenarios. Manufacturers may claim that people are "doing it wrong," but this argument fails to take into account "the real behavior of normal people, who have a range of abilities in regard to managing composting." If something is designed to break down, it should do so for the average person with limited knowledge of the process.

There really is no good way to deal with compostable plastics at this point. Anaerobic digesters (AD), which convert food scraps to soil, are not optimized to take compostable plastics; these are typically removed at the facility to prevent contamination and sent to landfill or incinerated, which increases costs and creates delays. From the study:

Other research has shown that plastics labeled as biodegradable and/or compostable fail to break down under simulated compost, sea, or soil environment experiments—with some shopping bags emerging still strong enough to carry a load of groceries after more than three years.

The researchers would like to see labeling improved and clarified. For starters, it should be regulated, which it currently is not. People need help figuring out "the nature of the compostable material, their environmental impact and how to correctly dispose of them."

Beyond that, the materials science itself has a long way to go. The researchers write that "the current mix of compostable polymers on the market will not reliably compost in the wide range of conditions found in home composting," which shows that it's likely premature to be making claims of compostability. Yes, it's difficult to recreate "natural experiments" in a lab, but scientists need to do a better job of understanding real world conditions—especially since compost is often used to enrich garden soil and grow food, leading to concerns about contamination.

The researchers acknowledge the immensity of this challenge. A material has to be stable and inert long enough for a product to be transported and used, and then suddenly switch to being food for microorganisms when the time for disposal rolls around. It's a tall order.

In the meantime, separate routes for industrial composting might be the best environmental outcome, but it would rely on proper labeling and economic viability.

The Big Compost Experiment's conclusion is rather discouraging: "Home composting is not at present a viable, effective or environmentally beneficial waste processing method for compostable or biodegradable plastics." It may refer only to the U.K., but it's safe to apply the same conclusion to the United States and elsewhere. 

You can read the full study here.

Purkiss, Danielle et al. "The Big Compost Experiment: Using citizen science to assess the impact and effectiveness of biodegradable and compostable plastics in UK home composting." Front. Sustain., vol. 3, 2022. DOI:10.3389/frsus.2022.942724

"Fracking moratorium; Bloodhound; Big Compost Experiment; transit of Mercury." BBC Inside Science. 7 Nov. 2019.

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