How sewage treatment plants could handle food waste, sparing landfills and climate

Treatment plants can capture over 95% of methane from food waste, compared to about 50% at landfills. 

Ahmed Ibrahim Yunus, Georgia Institute of Technology and Joe Frank Bozeman III, Georgia Institute of Technology

Every day, food scraps disappear into trash bags, are hauled away and forgotten. But that waste could be turned into something productive.

Across the United States, about 97 million metric tons of food waste are discarded each year, of which about 37 million metric tons end up buried in landfills.

Once underground, that organic material breaks down without oxygen and releases methane, a short-lived yet powerful greenhouse gas.

At the same time, the nutrients and energy stored in that food are permanently lost. But there is a better way. Research my colleagues and I conducted found that communities across the country already operate facilities designed to handle organic matter: wastewater treatment plants. Many larger, well-funded plants already have the infrastructure to process food waste, though not every plant is ready to do so today.

Landfills are not designed for food waste

Food waste is fundamentally different from plastics, metals or glass. It’s organic and can decompose naturally. But when it’s placed in a landfill, its decomposition emits significant greenhouse gases.

Modern landfills are designed to capture the methane emitted, but even the most efficient systems still allow almost 58% to escape into the atmosphere. That food waste could be turned into energy or fertilizer, but instead it contributes to global warming.

By contrast, wastewater treatment plants process sewage using microbial communities that naturally break down organic matter. Many also capture methane produced during treatment and convert it into usable energy. Others recover nutrients such as phosphorus that can be turned into agricultural fertilizer. Over time, many plants have evolved from simple sanitation systems into resource-recovery facilities that generate power, reclaim materials and reduce environmental pollution.

These existing systems already process organic matter and could handle food waste, too.

What happens when food waste goes to a treatment plant

Our research examined what would happen if food waste were sent to wastewater treatment plants rather than landfills. We used real data from a full-scale plant that handles food waste along with sewage.

When we compared greenhouse gas emissions for the same food waste composition, we found that sending food to a landfill would emit 58.2 kilograms (129 pounds) of carbon dioxide equivalent per ton of food waste.

In comparison, we looked at a conventional wastewater treatment plant, the type of plant most common in the U.S. It achieved net-negative emissions of –0.03 kilograms (about 1 ounce) of carbon dioxide equivalent per ton of food waste treated. The plant captures over 95% of methane, compared to roughly 50% at landfills, saving the atmosphere from additional greenhouse gases.

But we found that the advanced treatment plant we studied reduced emissions further. In our analysis, the advanced facility achieved net-negative emissions of –0.19 kilograms (about 7 ounces) of carbon dioxide equivalent per ton of food waste treated.

Both conventional and advanced plants achieve these benefits in similar ways. Treating food waste at either type of plant prevents the 58.2 kilograms of carbon dioxide equivalent per ton that would otherwise escape from landfills. The plants capture biogas to generate renewable electricity, reducing the need to purchase power from the grid. They also recover enough nutrients to fertilize about 23 acres of farmland annually, reducing the need for synthetic fertilizers, which require energy-intensive mining and processing.

How the logistics work

Getting the food waste to a wastewater plant doesn’t mean people put their food scraps in the drain or grind them up with an in-sink disposal. At the plant we studied, food waste was collected separately, much like recycling or yard waste, and transported by truck to treatment plants. Our emissions calculations don’t include truck emissions, because trucks are used in the other methods of food waste disposal as well.

Some cities already collect food waste by truck to go to composting facilities. San Francisco has done so since 1996. And New York City has the nation’s largest curbside organics collection, which composts food waste from 3.4 million households.

At the southeastern U.S. treatment plant we studied, trucks deliver food waste to a receiving station, where it’s processed to remove plastics, metals and other nonorganic materials before being blended into a slurry with the sewage solids. This mixture is then added to anaerobic digesters – sealed tanks where microorganisms break down organic material.

The methane that is produced is captured to generate electricity and heat. The remaining solid material is rich in nutrients and can be used to produce useful material, such as fertilizer.

We also found that adding food waste did not overload the plant or cause problems in its operation. The facility processed all of the county’s landfilled food waste – 107,320 tons annually, representing 38% of the county’s total food waste generation. Because of food waste’s lower density compared to wastewater, this added only 0.43% to the plant’s daily capacity. The plant consistently met effluent water regulatory standards. And at certain points, treatment efficiency improved as a result of the additional organic material, which supported the system’s biological processes.

The economics may surprise cities

Local officials, as well as taxpayers, are often worried about the potential costs of a project like this. Wastewater treatment is already expensive, and communities’ existing plants may be nearing capacity.

But the economic results from our analysis suggest that handling food waste in wastewater treatment plants can be financially viable. Towns already pay landfills and incinerators what are called “tipping fees,” based on the weight of the waste delivered. Wastewater treatment plants can also charge these fees.

They can also sell, or use themselves, the methane produced and sell the fertilizer. That additional income means plants can make money even if they charge lower tipping fees than landfills.

Not every wastewater plant is ready to accept food waste immediately. The facility we analyzed is large and well equipped. Smaller operations would likely require new or upgraded equipment, which would involve planning and local investment.

The overall finding of our research is that the limitation isn’t technological or financial. The core systems already exist to transform food waste into a recoverable resource: Cities already handle organic material every day. And they operate complex biological treatment systems. Our evidence suggests these facilities could, in fact, handle food waste in ways that are environmentally beneficial and economically realistic.

Ahmed Ibrahim Yunus, Ph.D. Candidate in Environmental Engineering, Georgia Institute of Technology and Joe Frank Bozeman III, Assistant Professor of Civil and Environmental Engineering and Public Policy, Georgia Institute of Technology