Transforming food waste into energy

The U.S. produces loads of food waste per year. This means that all of the energy required to produce and process these materials goes to pot, too. That’s like tossing enough juice to power more than 50 million American homes for a whole year right in the trash.

But we can reverse the equation, at least a bit, by turning this trashed grub back into energy. Around the country, specialized machines at farms, wastewater treatment plants, and other facilities can take food waste and use a process called anaerobic digestion to turn it into fuel. “A lot of time it’s considered a real win-win because not only are you keeping this wasted food out of a landfill, you’re also generating electricity that might otherwise have been produced by combusting fossil fuels,” says Callie Babbitt, a sustainability researcher at Rochester Institute of Technology.

What is anaerobic digestion?

Anaerobic digestion is the process of harnessing microorganisms to turn decaying organic materials into two things: a type of fuel called biogas, and a nutrient-rich substance called digestate that can be used as a plant fertilizer. The technique has existed for thousands of years and has been utilized around the world for decades, but it’s only recently caught on in the U.S.¹

Anaerobic digestion occurs naturally in the wild but can be sped up and optimized in an environment with carefully controlled temperatures and acidity and moisture levels.² In the case of food waste, the material is added to an oxygen-free chamber at temperatures as high as 100 degrees Fahrenheit, where tiny organisms break it down into a form of energy called biogas. This product is mostly made of methane and carbon dioxide. Biogas can be converted into electricity, heat, or fuels through processing methods like burning or liquefaction. Typically, it takes between two and six weeks for this process to go down.

The byproduct digestate—essentially the leftover undigested materials and dead microorganisms—proves helpful for agriculture because it contains nutrients like nitrogen, phosphorus, and potassium that plants use to grow and reproduce.³ It can also be burned into a charcoal-like substance called biochar and put into soil, where it can remove pollutants like metals and even suck carbon from the atmosphere.

In 2019, anaerobic digesters processed more than 17 million tons of food waste in the U.S.⁴ Most of these systems operate at wastewater treatment facilities, where both food waste and sewage sludge are fed into digestion tanks. The sludge helps digest food waste because leftover grub is too acidic to digest on its own, explains Shakira Hobbs, an environmental engineer at the University of California, Irvine. Sludge offers a natural solution.

Is it possible to turn food into electricity?

After the anaerobic digestion process, the gas produced from food waste can be burned to generate electricity. About one-third of U.S. wastewater treatment plants use this biogas to power their own facility, and around 10% of the facilities sell electricity from biogas for use in electrical grids. A small amount also inject biogas into natural gas pipelines after cleaning it to remove carbon dioxide, water vapor, and other unneeded substances. 

What is biogas used for?

Besides providing heat and electricity for treatment facilities and power for broader electrical grids, biogas can also be turned into renewable natural gas. This gas can then be sent into pipelines and used as a substitute for plain-old natural gas (that’s methane), which must be extracted from the ground. Natural gas contributes nearly one-third of the country’s energy and is often burned to make electricity, along with heating and cooking.

Can food waste make biofuel?

Biogas from anaerobic digestion is just one type of fuel that’s made from food waste. Our scraps can also be turned into biofuels, including ethanol and biodiesel. The former is often produced by unleashing enzymes, or proteins that help speed up chemical reactions, onto starchy foods like corn. The resulting sugars are then fermented with yeast to create ethanol.

While bioethanol often comes from plants grown specifically for this purpose, which take up millions of acres of land, researchers suggest that redirecting more food waste (such as excess supermarket products like rice, bread, and pasta) can reduce land, fertilizer, and energy use.^5,6,7

Biodiesel, on the other hand, can be made from food waste like used cooking oils and animal fats that may otherwise clog up pipes. These materials undergo a reaction with the help of an alcohol and a chemical catalyst that turns them into biodiesel and glycerin—a sugar alcohol that can be put into cosmetics. Biodiesel is typically mixed with refined petroleum products such as diesel fuel and gasoline and put into vehicles that run on conventional diesel. While biodiesel can also make electricity and heat, it has proven controversial because it has a tendency to clog pipes and degrade over time. Biodesel is also associated with negative environmental impacts due to fertilizer use, the greenhouse gas emissions released during crop farming and fuel conversion.⁸

What are the benefits of turning food waste into energy?

Given the U.S.’s goal to phase out fossil fuels from energy generation by 2035, turning more food waste into clean power could help reach that milestone. Biofuels have already made a dent worldwide: In 2022, they replaced global consumption of around 2 million barrels’ worth of oil per day, or 4% of the transport sector’s oil demand. Since these fuels can be made domestically, they can also reduce the country’s reliance on imported fossil fuels.

Making fuel from food waste rather than from rows of crops can take things a step further, cutting down the amount of land, fertilizer, and energy needed to produce it.⁹ In fact, some data show that turning all U.S. food waste into renewable diesel could lower the country’s greenhouse gas emissions by up to 2.6% annually.¹⁰ And if all of the world’s wasted food was turned into usable energy, it could power the U.S. for two whole years.

Is food waste to energy reliable?

Food waste–derived energy has lots of potential. By processing 100 tons of food waste every day, anaerobic digestion can make enough energy to power 800 to 1,400 homes annually. Given the tens of millions of tons of food waste created each year in the U.S., our rotting junk could go far. But right now, organic waste only creates about 3% of all energy consumed nationwide.

That’s partly because this type of energy hasn’t received as much attention and investment as other forms of renewable power, says U.C. Irvine’s Hobbs, so it’s currently relatively expensive to produce. “It’s not as cool or attractive as other technologies … it doesn’t have the ‘wow’ factor like solar or wind,” she says.

A lack of local infrastructure may prevent this type of power generation, says RIT’s Babbitt. For example, if a facility produces electricity from anaerobic digestion and wants to sell it back to the grid, nearby solar farms might be competing to use the same transmission lines. 

Is food waste to energy a real solution?

Even if it never reaches prime time like solar or wind energy in the U.S., Hobbs says food waste–derived fuel could work well on smaller grids or as a supplemental form of energy.

Still, sourcing energy from food waste comes with significant downsides. If biogas isn’t properly treated, digestion and energy conversion processes can release substances such as sulfur dioxide, ammonia, and nitrogen oxides—all of which are harmful to the environment and human health.^11,12 In fact, biogas may prove 10 times more toxic to humans than natural gas and emit three times more nitrogen oxides when burned in internal combustion engines.¹³

Even if these dangerous compounds don’t escape, biomethane still regularly leaks into surrounding air from inefficient anaerobic digesters—this gas has nearly 30 times the atmospheric warming potential of carbon dioxide. “It’s not a slam dunk environmental benefit, especially if we look at the realities of methane leakage from biogas production,” says Jonathan Krones, an industrial ecologist at Boston College who studies solid waste processing.

Bioethanol and biodiesel also pose risks. Ethanol is most commonly blended with petroleum for fuel, which still emits greenhouse gasses, pollutants, and hazardous chemicals.¹⁴  

Are there other countries turning food waste into energy?

Overall, Europe, China, and the United States make up 90% of the world’s biogas production. Europe leads the globe, and Germany claims two-thirds of the continent’s capacity. While Germany used to rely mostly on crops for biogas production, the nation has shifted toward using agricultural waste and livestock waste and capturing methane from landfills. 

In China, the government has promoted the use of household-scale anaerobic digesters since the 1970s. Today, millions of households use their digesters to turn agricultural and food waste into energy and fertilizer.¹⁵ China’s commercial biogas projects skyrocketed between 2005 and 2016.¹⁶ By 2025, the country’s biogas production could replace 25 million tons of coal and reduce carbon dioxide emissions by 46 million tons annually.¹⁷ 

What are some other solutions for food waste?

Excess food has plenty of uses beyond power. You can even get in on the action at home.

Composting

Microorganisms and insects break down organic matter including food into a soil-like substance that can return nutrients back to the ground—compost. You can carry out this process in your backyard or sign up for a local composting program that accepts food scraps.

Fertilizer

Animal byproducts left over from slaughter facilities can help plants thrive. Companies turn leftover blood from meatpacking plants, commonly from cows, into a fertilizer because it has a high concentration of nitrogen. Steamed animal bones can also be turned into a helpful fertilizer.

Animal feed 

Even if we don’t make a meal of it, our leftover food can feed livestock. This practice has been around for hundreds of years, but it has been carefully regulated in the U.S. since the 1980s when disease outbreaks in people were linked to livestock feed containing animal products. 

The EPA recommends giving extra meals to people in need before feeding it to animals. But if you want to learn more about donating or selling food scraps for livestock meals, you can check out this guide for more information on state and federal laws.

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13. A Human Health Toxicity Assessment of Biogas Engines Regulated and Unregulated Emissions, Applied Sciences, Oct. 2020 ↩︎
14. Bioethanol Production, Reference Module in Earth Systems and Environmental Sciences, 2022 ↩︎
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16. China’s Biogas Industry’s Sustainable Transition to a Low-Carbon Plan—A Socio-Technical Perspective, Sustainability, Mar. 2023 ↩︎
17. Prospects of China’s Biogas: Fundamentals, Challenges and Considerations, Energy Reports, Nov. 2020 ↩︎