Yale Experts Explain Compost

January 10, 2022

Ecosystem scientist Dr. Whendee Silver and compost expert Forest Abbott-Lum walk us through the benefits and challenges associated with the breakdown of food waste.

What is composting and the basic science behind it?

Forest Abbott-Lum (MEM ’21), Composter in Residence at the Yale Sustainable Food Program, describes composting as “the managed process of decomposition.”

“When a leaf falls from a tree onto the forest floor, it turns into this rich organic material called humus when it decomposes,” Abbott-Lum says. “Composting is when we take organic waste produced by the things that we do as humans and we recreate that same process by managing the process of decomposition.”

Abbott-Lum goes on to explain that the process of decomposition requires the perfect ratio of both nitrogenous and carbonaceous materials. Nitrogenous materials, often referred to as ‘greens,’ includes nitrogen-rich things like your food waste and yard waste. Carbonaceous materials, referred to as ‘browns,’ includes carbon-rich things like dead leaves, sticks, twigs, and cardboard.

“The goal here is to lay out a diverse buffet for microorganisms to eat as quickly as possible,” Abbott-Lum says.

Dr. Whendee Silver (MFS ’87, PhD ’92), the Rudy Grah Chair and Professor of Ecosystem Ecology and Biogeochemistry in the Department of Environmental Science, Policy, and Management at U.C. Berkeley, gives us the basic science of compost piles at the microscopic level.

“What happens during the composting process is that the complex community of microbes that are within that organic material, which we call feedstocks, start to break it down in a feeding frenzy,” Silver explains.

The microbes start with what is easy to eat, and as they make their way through the feedstocks, the material leftover becomes more difficult to break down. Silver explains that as the microbial activity increases, they generate heat and the temperature in the compost pile rises dramatically. Some microbes are killed off at these high temperatures.

“When material has reached a point at which the microbes do not have the tools to break it down anymore, the microbial activity decreases and the temperature drops dramatically,” Silver says. “The materials that are resistant to decay include some of the microbial bodies, recombined constituents of the original feedstocks, and wood and straw-like materials. Together these create a slow-release fertilizer that is ready to be added to soil.”

What are the benefits of composting?

Benefits of composting exist on global, local, and individual scales. Perhaps the most prominent benefit of composting is its climate change mitigation potential. Abbott-Lum illustrates this benefit by describing what happens to our food waste in the absence of composting.

“You would take organic waste–let’s say it is a banana peel–put it into a garbage bag, seal it off, and then most likely, it would be taken to a landfill or an incinerator,” Abbot-Lum explains. “If it was taken into a landfill, the banana peel will be buried under hundreds of thousands of tons of other garbage.  Trapped under here, the banana peel begins to decompose anaerobically, or without the presence of oxygen. When organic waste decomposes anaerobically, the process releases methane, which is a greenhouse gas that is 28 to 34 times more potent than carbon dioxide.”

According to the FDA, in the United States around 30-40% of food is wasted from farm to fork. Much of that food waste ends up in landfills, resulting in ever-increasing amounts of heat-trapping methane emissions into the atmosphere.

Composting, however, allows organic waste to decompose in a much more climate-friendly way and releases significantly less methane than landfilling.

“When you compost that food waste, microorganisms are responsible for breaking it down,” Abbott-Lum says. “As they work, these microorganisms exhale carbon dioxide just like you and I, which is a much less potent greenhouse gas.”

Our experts point out that there are numerous other personal and community-wide benefits to composting.

“On top of the climate benefits, through composting processes we are producing a product that improves soil health,” Silver says. “By increasing the organic matter content of soils with compost, we’re adding nutrients back into the soils to try to replenish what had been taken when we harvest material from those soils. This is beneficial to new plant growth.”

Silver explains that as these new, thriving plants grow, they pull carbon dioxide out of the atmosphere while also producing food for animals or people.

“At the end of the day, we’re producing a product that’s valuable for people and for the planet,” Silver says.

Abbott-Lum says that by supporting a local composting business in your community, often referred to as micro-haulers, you are helping pave the way for green economic growth and job creation.

“Additionally, landfills are frequently sited in poor communities, so just by reducing the amount of waste we send to a landfill through composting, there is an important public health and environmental justice benefit,” Abbott-Lum adds.

What are some of the different ways to process organics?

Composting procedures may differ depending upon the scales and strategies used.

“The traditional form of composting involves having open piles of compost outside which are layered with brown and green materials, and turned periodically to accelerate the decomposition process,” Silver says. “This works best for those who have their own backyard.”

Abbot-Lum adds that another at-home strategy for composting is called vermicomposting, wherein red wiggler worms are placed in a confined environment with organic waste to drive forth the decomposition process.

At the commercial level, there are numerous strategies for managing and processing organic waste.

For example, Aerated windrow composting, involves forming organic waste into rows of long piles called “windrows” and aerating them periodically by either manually or mechanically turning the piles.

In-vessel composting keeps all organic waste in a confined container and regulates air flow and temperature using buried tubes and probes to achieve optimal aerobic decomposition. Pyrolysis composting is a thermochemical treatment wherein organic material is exposed to high temperatures, causing molecules to separate and chemically recombine as material beneficial to soils. 

Another popular commercial strategy for managing and recycling organic waste is through use of an anaerobic digestor.

“Anaerobic digesting is taking the organic waste and placing it in a closed container without any oxygen,” Silver says. “This drives activity from microbes that are really good at producing methane. However, anaerobic digesters have the ability to keep the methane from escaping into the atmosphere and capture it to burn directly as fuel or convert it to another fuel source called methanol.”

The material left over at the end of the process, called digestate, is partially composted.

Anaerobic digestion facilities therefore have significant greenhouse gas reduction potential. In addition to the generation of local renewable energy and displacement of fossil fuel use, these facilities also create local job opportunities and often tax credits for communities.

What are the key differences between composting at home and composting at a large commercial facility?

“There are really big differences between commercial scale composting and backyard composting,” Silver says. “Commercial facilities have a large enough mass of material and a large enough surface area to grow the microbes that you need to create the best product at the end from the perspective of low greenhouse gas emissions, higher benefit to the plants, and lower pollution potential.”

Abbott-Lum adds that commercial facilities can reach much higher temperatures that both sterilize and break down organic waste in a way that a backyard pile cannot.

Commercial facilities also have the advantage of access to a wide range of feedstocks.

“These facilities can put together recipes of material that create a more optimum condition and a more optimal product at the end,” Silver says. “When we’re composting at home, we’re pretty much just dumping in whatever food waste we produce, a little bit of our yard waste, and maybe some of the paper waste. That’s not necessarily the optimal recipe to create a nutrient-rich compost with the best climate benefit.”

In fact, Silver encourages people to send their organic waste to a commercial facility–if they have access to it– over backyard composting.

What are some of the current challenges associated with composting at different scales?

A current challenge to community-wide composting is a lack of access to services.

“If you’re not living in the handful of major municipalities like New York City or San Francisco that has curbside composting pickup, you will have to put extra time and effort into what you do with a stream of organic waste coming from your household,” Abbott-Lum says. 

Our experts point out that composting in urban areas can also prove more challenging, as individuals may not have backyards or access to the “brown” materials like leaves, twigs, and sticks needed to supplement food waste in a successful compost pile.

When there is access to composting resources or services, contamination of compost collection raises a huge concern.

“We need to become better at not putting things in the compost bin that do not fall into the simple categories of food waste, green waste, or things that used to be alive,” Silver says. “Farmers and ranchers who use the majority of the compost that we as a society produce really don’t want plastic, utensils, and all the other garbage that doesn’t break down in a composting system sitting on their beautiful landscapes.”

What about compostable ware and bioplastics?

It is commonplace to see single-use utensils, cups, and plates labeled as ‘compostable,’ but our experts agree that they are not always suitable for your compost bin.

“A lot of these bioplastics are made of tough polymers that require extremely high heat and moisture to break down,” Abbott-Lum says. “There is a possibility that the conditions required to break them down will be achieved in an industrial compost site, but you are never going to achieve that in your backyard compost pile.”

Silver recently observed compostable utensils left in a compost pile for 11 weeks. She explains that while the piles reached temperatures that were too hot to touch, the compostable utensils came out relatively intact.

“In theory, compostable ware is great, but in our research, things like compostable forks and potato or corn plastic break do not break down well in compost piles,” Silver says. “These items are likely to break down in a landfill eventually, but at that point, they are likely to breakdown anaerobically and produce methane, the potent greenhouse gas we are trying to avoid.”

Silver recommends that as individuals, we avoid compostable ware and instead switch to reusable utensils, cups, and plates, and simply wash them when we are done.

What can the average person do if they are concerned about food waste and curious about composting?

Our experts agree that as individuals, we can all take more responsibility for both reducing our food waste and for keeping our organic waste out of landfills.

“Priority number one is reducing your upstream food waste,” Abbott-Lum says. “Think critically about how you buy and consume food. Are you making a shopping list? Do you have a plan for eating your leftovers throughout the week? Try to find and fill gaps in your everyday habits that will eventually limit the amount that you would actually need to compost.”

Silver suggests that anyone interested in composting should do a little bit of online research to determine which tools and resources they need, and to give it a go.

“I like to think of composting as a gateway drug to being more aware of your carbon footprint,” Silver says. “Once you get started, you might find that you’ll start looking around your home and your place of work for the rest of your lives for ways in which you can contribute to solving the climate crisis.”

Abbot-Lum agrees.

“Composting is a daily practice of a tangible environmental good that you can literally measure by the pound,” Abbott-Lum says. “It is easy to do, it’s fun, it’s satisfying, and it all adds up.”

What is Yale Doing?

Yale has programs in place to donate all unopened, leftover food and beverages to a local food bank. The Downtown Evening Soup Kitchen (DESK) and The Yale Hunger and Homeless Action Project (YHHAP) both provide food for those in need. All non-perishable food and food refrigerated in a timely manner is accepted. 

All food waste from Yale College Dining Halls goes to an anaerobic digestion facility; options for an on-campus composting facility are being studied. Please explore this comprehensive list of materials that are acceptable and unacceptable in our compost streams as of 2021.

Yale community members living off-campus can explore opportunities to send their compost to a New Haven micro-hauler like Peels & Wheels for a small monthly fee.