The Freaky Chemistry of Landfills

The Freaky Chemistry of Landfills
Written by Techbot

We rarely think about what happens to our garbage after it goes into the nearest waste receptacle. Whether it’s a piece of paper, a worn-out pair of flip flops, or leftovers from the back of your refrigerator, trash usually ends up at the same final destination: a landfill. Landfills are home to wild chemistry that can lead to different types of pollution. They’re also sites for clever engineering that seeks to mitigate that pollution.

While the essence of a landfill is a pit full of garbage, landfills are actually highly engineered projects that are filled in phases and managed with intentional architecture, Hinsby Cadillo-Quiroz told Gizmodo. Cadillo-Quiroz is a professor of life science at Arizona State University who has previously studied the microbial processes that lead to the breakdown of garbage in landfills. People may be surprised to learn that there is a large effort to preserve the environment surrounding a landfill, even before the first bit of waste is deposited.

“The landfill starts with a large, large pit,” Cadillo-Quiroz explained. “You have to protect the surrounding watershed, and you put a membrane down through which water cannot cross.”

The landfill liner is usually made of clay, which is bonded to or layered between some type of textile, according to a fact sheet from the U.S. Environmental Protection Agency. Lining is a crucial first step, because no matter how dry the waste or climate may be, water percolation through the landfill is inevitable—whether that be through rainwater infiltration, surface runoff, or water vapor condensing on the garbage through a process Cadillo-Quiroz calls “sweating.”

Knowing that water will eventually find its way to the bottom of the pit, engineers design the landfill so that its floor has a slight slant. The slant forces the incoming water toward a series of wells with suction pumps, which can remove the runoff. With the liner and water collection system in place, operators can begin to fill the landfill, but it’s not simply a matter of tossing garbage into the pit: Waste is placed into the landfill with an intentional plan.

“Operators start filling the landfill in different ways,” Cadillo-Quiroz said. “Some go from east to west or north to south, but they have to have a certain direction to keep filling, so they can drive over [the garbage] to keep filling.”

Cadillo-Quiroz explained that a landfill is filled in phases or “cells,” several of which will make up one layer of garbage. Each layer of waste is approximately 5 feet (1.5 meters) thick before it is capped off by a layer of soil, followed by another layer of garbage, then another layer of soil, and so on. In the layers of soil, operators will dig trenches along the length of the cells and lay a 1-foot diameter pipe in order to control the amount of the gas that is generated by waste decomposition. This type of complex landfill engineering is a relatively new process—Cadillo-Quiroz estimates that, based on the landfills that he has researched, these gas collection pipes became commonplace in the mid-1990s.

Landfill operators will then continue to fill the pit until it is, as Cadillo-Quiroz puts it, “above grade,” meaning the waste and soil layers have passed ground level, which turns the pit into a mountain. This process can take years, depending on how deep the landfill is, but also because the soil and garbage need to stabilize before more garbage is added on top. Landfills are then typically capped off with a layer of grass, according Liz Rodgers, a U.S. Department of Agriculture intern and a PhD student at the University of Missouri. She’s working with Chung-Ho Lin, a professor in the Department of Agriculture, Food & Natural Resources.

The composition of a landfill is highly dependent on its surrounding communities, and Cadillo-Quiroz said that landfills are incredibly heterogeneous, thanks to the different types of garbage that go into landfills at different times from different neighborhoods and population behavior.

“Let’s say that everyone is building at this period of time. That generates a lot of waste. Now you have all of this construction material that comes in big trucks for a whole week,” Cadillo-Quiroz said. “Another example is the seasons. Now we’re in winter, and there are more materials that we use [then] that end up in the trash.”

Rodgers and Lin described a similar phenomenon, in which the pollution emitted from one landfill can be different from the pollution generated from another, since various communities and neighborhoods can have wildly divergent consumption habits.

“If a community is more industrial, we can have a different class of pollutants,” Lin said. Rodgers added: “Rural communities would have a different waste composition than industrial communities, versus urban communities, versus suburban communities, even though they might have common threads that tie them all together, like plastic.”

There are two main types of pollution that are generated by landfills—air and water pollution—and the specific chemistry of that pollution is highly dependent on what goes into a landfill. That’s especially true for leachate, or the water that infiltrated a landfill and then leached chemicals and pollutants from the decomposing garbage. The chemicals that got into leachate can include heavy metals from electronic waste, bits of plastic that are breaking down, and decomposed organic matter. The trouble is that, even though landfills are engineered to collect as much leachate as possible, some is bound to escape, permeating through soil and aquifers into drinking water and natural water reservoirs.

“In most of these systems, it would be rainwater that we’re concerned with that would be percolating through the waste, or runoff that could be picking up things from the surface,” Rogers said. “Even though these landfills have liners, nothing is 100% perfect. Things will be getting through them. We can’t totally prevent contaminants from leaking out of landfills, no matter how good a liner is.”

The air pollution, meanwhile, consists of gases that are generated by the breakdown of waste within the landfills, some of which can be nitrous oxide, carbon dioxide, or hydrogen sulfide. But most of the air pollution generated by landfills is methane.

When the landfill is capped, organic material like paper, vegetable scraps, and even cooking grease will break down in an oxygen-rich environment. As the microorganisms present in garbage feed on the organic material, they consume whatever oxygen is left over inside the landfill after it’s been capped. Once the microorganisms use all of the oxygen available to them, they will then continue to break down the waste in an anaerobic environment, which produces methane. Landfill operators are aware of this chemistry and can use the pipes that run through the landfill’s layers of soil to help suck out some of the methane before it is emitted to the atmosphere.

“When you apply high vacuum pressure, then the physics of diffusion is going to pull air from outside, and that air brings some oxygen,” Cadillo-Quiroz said. “When [operators] start seeing some oxygen that is starting to appear in the gas that they’re extracting, they will reduce the pressure.”

Even though the landfill operators are able to manage some of the methane emission, methane is a potent greenhouse gas, and landfills are a particularly noteworthy source of the pollutant. The EPA estimates that 14.5% of methane emissions in 2020 were from landfills, making them the third largest emitter of the gas.

Landfills in their current state may feel like a lose/lose—short of completely revolutionizing the way we create and dispose of garbage, it would appear that we’re stuck with these pollution-creating pits of trash. However, Lin and Rodgers are researching the multitude of chemicals that can exist in leachate in order to update outdated regulatory lists of what contaminants exist in landfill runoff and what their impact on human health could be. Additionally, the duo is studying a method to minimize the impact of landfill leachate on watersheds and aquifers through a process called phytoremediation. By planting a barrier of trees around the landfill after it is capped, the roots of these trees could pull leachate out of the soil before it reaches a source of water.

Rodgers explained: “We established these buffers of trees, these phytoremediation systems, that use poplars and willows, which are specialized, fast-growing trees, to take up some of the contaminants and remediate these sites. They’re located all throughout Michigan and Wisconsin at landfills, because these sites we’ve identified as being within the Great Lakes watershed, so potentially the groundwater flowing through these sites could reach the Great Lakes.”

Cadillo-Quiroz, meanwhile, argues that, while landfills are a potent source of methane, the gas could be a huge opportunity for energy. If we can harness the methane that’s emitted from a landfill, Cadillo-Quiroz says, it could be used to power generators, meaning that this waste gas could be repurposed into a source of energy—but only if we design landfills more intentionally and with better management systems.

“I think that’s the outlook to it. Instrumenting the landfills and manipulating the microbes may actually give us an economic opportunity and ecological opportunity to use these places beyond just being the pit where we dump things,” Cadillo-Quiroz said.

Landfills, with all their diversity and active chemistry, are so much more than pits of garbage. They are ever-changing sources of pollution that live and breathe—but they don’t have to stay that way. For now, landfills contribute to the growing climate crisis, but with some more research and engineering, they could become our ally.

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