New plastics designed to break down naturally have been hailed as environmentally friendly alternatives to traditional plastics. Instead of taking decades or even centuries to decompose, they vanish in a few years.
But new research at North Carolina State University suggests they may not be so green after all.
The study, led by NCSU doctoral student James Levis, found that biodegradable plastics can release large amounts of methane gas when they break down in landfills. Methane is one of the most problematic greenhouse gases, able to trap much more heat than carbon dioxide, making it a major contributor to global warming. It's also flammable.
"Everybody assumes that biodegradable is desirable. This study calls that into question," says Dr. Morton Barlaz, an author on the study and director of the Department of Civil, Construction and Environmental Engineering at NCSU.
Biodegradable plastics are commonly used to make yard-waste bags and disposable drink cups. Unlike plastic bottles that are designed to be recycled, these materials were created with composting in mind. They're also able to break down quickly if they end up as litter.
Though they might have been intended to go into the compost, many are destined to end up in a landfill instead.
"You can say a cup is compostable," Levis says. "But here in Raleigh and most places, there's no separate bin, and it's going to end up going in the garbage."
In a well-managed compost system, Levis says, biodegradable plastics don't release much methane at all. As long as oxygen is present, they will give off mostly carbon dioxide and water.
But that's not the case in landfills, where garbage is starved of oxygen.
Without oxygen, biodegradable plastics are decomposed by bacteria that give off methane. These bacteria are already hard at work in landfills slowly digesting food waste, paper and other biodegradable materials.
When the bacteria destroy plastics, though, they work quickly. Some biodegradable plastics get destroyed even faster than newsprint, according to the team's computer model.
This means that a lot of methane is released in a very short time, and that creates a challenge for landfill operators.
Many landfills try to capture methane from decomposing garbage to prevent it from becoming a hazard or escaping into the atmosphere. This captured methane is often burned to generate electricity or used industrially.
But the speedy decay of biodegradable plastics means that landfill operators would have to begin sucking out methane right away. And that, according to John Roberson, project manager for capital projects like the South Wake Landfill, just isn't possible. To capture methane, operators drill wells throughout the packed garbage and use a system of slotted pipes to suck it out.
"Since it's an active landfill being used, we can't just put wells everywhere," Roberson says. "They'd get destroyed on a daily basis."
As a result, landfill operators usually can't start capturing methane until two to five years after the landfill starts filling with garbage. By then, the methane from biodegradable plastics is long gone and has escaped into the atmosphere.
If biodegradable plastics were designed to decay more slowly in landfills, Barlaz says, operators would be able to capture more of it for fuel.
At the South Wake Landfill, an electricity-generating project is in development. Methane sucked out of the North Wake Landfill is burned at the adjacent Covidien pharmaceutical plant. The Orange County Landfill plans to begin generating electricity next year, in a joint project with UNC.
In this respect, the uncollected gas from biodegradable plastics isn't just an environmental hazard, it's a missed opportunity. In landfills with methane-capture projects, Levis says, the plastics are a tremendous untapped resource.
"It's a big environmental take away that a lot of people haven't focused on yet," Levis said.
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