What happens to composting at Carnegie Mellon?

Did you know that the food you drop into the compost bins at the University Center sometimes becomes soil for Carnegie Mellon's gardens? The green bins next to the trash and recycling ones are one way Carnegie Mellon reduces waste and our carbon footprint.

Carnegie Mellon's composting system has been around since 2008, when the University started collecting food prep, coffee grounds, and leftover food from the Schatz Dining Room. Now, the green bins are in various parts of campus, including the Resnik Food Hall and departmental buildings.

Two types of waste are composted: pre-consumer waste and post-consumer waste. Pre-consumer waste refers to uncooked food left over from meal preparation, such as banana peels, broccoli ends, or a corn cob without its kernels. Post-consumer waste refers to food that has been cooked, leftovers after consumers are done eating, and also includes used compostable utensils. While not mandated, the University encourages dining vendors to use compostable serving products such as plates and napkins so consumers can easily drop their entire meal into the compost bin.

So what happens when you drop food off in the compost bin? First, the compost bags are collected by Facilities Management Services (FMS) and thrown into the compost dumpster. It is important that the compost contains only compostable waste, as items in bags are not sorted, and any non-compostable waste can lead to lower quality compost. If FMS finds that a bag has more than about five percent contamination, the entire bag is thrown into the landfill dumpster.

The compost dumpster is picked up by AgRecycle, Carnegie Mellon's compost partner. They take the waste to their commercial compost facility, where they add carbon-heavy materials, or "browns," like newspaper, leaves, and cardboard.

The compost waste brought from the University is high in nitrogen from vegetables and is often called "greens." AgRecycle creates a pile with a higher proportion of browns than greens and waters the pile. Microbes then begin to break down the compost, warming up the pile.

In a backyard compost pile, the mixture doesn't get hot enough to break down oils and meats, but in a commercial facility like AgRecycle, the larger scale allows the heat to do so while killing harmful bacteria. Once the microbes have finished breaking down the compost, what's left is humus, a organic material rich in nutrients, perfect for adding to soil.

Carnegie Mellon often buys this humus to add to their own soil in the gardens outside of campus buildings. In this way, they're "completing the circle," as Green Practices Manager Deborah Steinberg put it in an interview with The Tartan.

Because compost can be so easily contaminated, green bins are not given willy-nilly. This is where Steinberg comes in.

"Part of what I do is different programs to talk about how we can treat campus better or ... giving information behind decision-making, how can you choose the more sustainable thing," Steinberg said. "We do a lot of work with zero waste, so you know, trying to get people to compost more so that we're sending less to the landfills."

While composting does take additional cost and resources in terms of training and paying another vendor to process the compost, "we find that it's like a value added benefit." Steinberg said. "We have ambitions of being closer to zero waste."

For Carnegie Mellon, zero waste is considered a 90 percent diversion, which means that 90 percent of waste is being sent to places other than the landfill. This includes recycling and composting. About 28 percent of landfill waste is organic material, meaning waste that can be composted.

"It's the largest percentage of things that we throw out, and it's so easy to compost it, and the compost turns into soil which is only benefiting our farmlands and our gardens," Steinberg said. "So we're taking that away from landfill space, which is gonna fill up. ... Soon it will be closer and closer to where we live and play. And it's just rotting away and making methane which is adding to climate change."

But why compost rather than just recycle? In addition to diverting organic waste from landfills, composting also helps reduce the amount of plastic in recycling processes. While plastic is touted as recyclable, the Environmental Protection Agency estimates that only 8.3 percent of plastic is actually recycled. Plastic is actually extremely difficult to recycle into a material that manufacturers would want to use in their products. Plastic is often thin, flimsy, and made differently by each manufacturer, which is not ideal for recycling. In contrast, aluminum cans and cardboard are very easy to recycle, so it is still important to have both compost and recycling bins.

So how much does Carnegie Mellon compost? In the 2021-2022 academic year, Carnegie Mellon composted 180.19 tons of waste. In comparison, the highest amount of waste they have composted is 653.7 tons during the 2018-2019. These numbers do not necessarily mean that Carnegie Mellon has become more wasteful, but are rather to be expected — during the pandemic, there were fewer people on campus, so there was less waste being produced in general across trash, recycling, and compost bins.

Steinberg says meeting numerical targets for recycling, composting, or waste is counterintuitive. "Wouldn't we just encourage people to use plastic water bottles and recycle them? What we actually want is all of those numbers to go down. It's hard to focus on ... because if we actually did a really good thing, which was [to] encourage people to use less plastic water bottles, our numbers would look worse."

Steinberg says that they are not pressured to meet numerical targets, but it is certainly something that the University considers when thinking about environmental impact.