Composting and feeding food waste to animals have big climate benefits, study finds

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Millions of tons of food waste end up in landfills each year. But alternatives like composting, anaerobic digestion and even feeding food waste to animals are much better for the climate, according to a study by researchers at the University of Pennsylvania’s School of Veterinary Medicine and other institutions.

The study, published in the journal Nature Food, found that diverting all food waste away from landfills in the United States would avoid around 10% of the planet-warming carbon emissions that come from the country’s agriculture sector.

“It is really huge,” said Zhengxia Dou, a professor of agricultural systems at Penn Vet who led the study.

The results are a call to action, said Sintana Vergara, an environmental engineer who studies climate change and waste management at Swarthmore College and was not involved in the study.

“We know that there are these very big environmental burdens posed by food waste,” Vergara said. “Here’s this paper saying, look, if we reuse [it], here are all these benefits.”

Big climate benefits to reusing and composting food waste

The study investigated three alternative disposal options for food waste: composting, repurposing food waste as animal feed and anaerobic digestion — a process in which bacteria break down food waste in the absence of oxygen and produce a gas that can be used for energy.

The study synthesized data from 91 previous studies conducted around the world to estimate the carbon footprint of these three disposal options. The researchers used statistical tools to average across these studies’ differences, said Whendee Silver, an ecologist who studies climate change and compost at the University of California, Berkeley and was not involved in the study.

“I think that the approach is really solid,” Silver said. “It’s pretty creative.”

Compared to landfilling, composting emits a small amount of climate pollution, according to the researchers. Anaerobic digestion is carbon-negative, while turning food waste into animal feed is roughly carbon-neutral. Creating animal feed has the added benefit of replacing new feed that would be produced by growing soybeans or corn, Dou said.

“We won’t need to use the land and water and fertilizer and herbicide to produce those conventional feeding ingredients,” Dou said. “So that’s a huge benefit for the resources and for the environment.”

When used together, these three alternative disposal options even out to be roughly carbon-neutral, Dou said. In contrast, every ton of food waste sent to a landfill emits roughly 1 ton of carbon dioxide equivalent, she said.

The study imagines a world in which no food waste is sent to landfills. Instead, one-third would be composted, one-third processed through anaerobic digestion and the last third fed to animals.

In the U.S., where food waste is the leading source of municipal solid waste that heads to landfills, the study found that diverting all food waste toward these three alternatives would avoid close to 40 million metric tons of carbon dioxide equivalents per year.

That’s a small fraction of the 6,343 million metric tons of carbon dioxide equivalents the U.S. emitted in 2022. But it’s still significant, Dou said.

When it comes to tackling climate change, she said, “even the small, everyday things … all count.”

What you can do to reduce the climate impact of your food waste

Food waste is a problem that everyone can tackle inside their own kitchens, Dou said.

“As average consumers, we are all part of the problem, and we can all be part of the solutions,” she said.

Dou recommends thinking about avoiding food waste when planning meals and shopping for food. If you do end up with inedible food, it’s better to compost it, rather than throw it in the trash, she said.

Some places — like New York City — offer municipal compost pickup. In Philadelphia, residents can drop off food waste for composting at some community sites. Private companies like Bennett Compost and Circle Compost also pick up compost on the curbside for a fee. You can also make your own compost in a backyard or community garden.

“If possible, I encourage everyone to participate,” Dou said.

The study could be useful for policymakers looking to create larger scale change because it quantifies the benefits of potential waste diversion policies, said Tibisay Pérez, a research scientist studying composting at the University of California, Berkeley who was not involved in the study.

“You’re providing a framework,” she said.

The study’s results offer an opportunity to rethink agricultural systems in countries like the U.S., Vergara said. Some of the solutions outlined in the study are already happening in places such as Colombia, she said, where people often feed food scraps to backyard chickens or pigs and compost whatever’s left.

“It’s already being done,” Vergara said. “How do we encourage or support what’s already happening, where it’s already happening? Then in places where it’s not happening, the industrialized world, … how can we rethink the scale at which we operate our agricultural systems, or think about large-scale ways of rerouting food waste?”