Researchers race to track the mysterious path of microplastics after they seep into the ground

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Jesse Vermaire, an associate professor at Carleton University’s biology department, is on a mission to solve a mystery that is simultaneously microscopic and global in scale.

He’s trying to figure out what becomes of the countless tiny fragments of plastic that enter sewage systems every day.

It’s a question without a simple answer, and one that has driven him to do strange things. He has, among other activities, taken tweezers to samples of treated sewage, waded through muck in farmers’ fields and been part of a team that feeds plastic to a swarm of crickets by sprinkling small fibres of it over their food like Parmesan cheese.

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All of it is in service of a goal that scientists increasingly recognize as an important one: keeping small plastic particles from becoming so ubiquitous in the environment that it is impossible to stop them from accumulating in the bodies of living things, including humans, where their health effects are unknown. Sewage is one of the most crucial fronts in that effort. “The waste water stream is the link between human use of plastic and the rest of the environment,” Dr. Vermaire said during an interview in his basement lab in Carleton’s Loeb Building.

He pulled out several tubs of biosolids, a gooey substance left over after sewage is treated. The waste product, which Dr. Vermaire’s team has collected from 22 waste water treatment plants across Canada, comes in three forms: pellets, cake and slurry. They look, respectively, like rabbit food, molten chocolate lava cake and chocolate slushy.

Dr. Vermaire’s research started with a 10-gram batch of biosolids. His team examined it under a microscope, pulling out tiny pieces of plastic. But it was impossible; there were too many to count. They had to scale down to a single gram. Eventually, they found 600 particles of plastic per gram, six times what Dr. Vermaire had expected.

Tiny pieces of plastic like these, known as microplastics, have been detected all around the world: in oceans, in the Great Lakes, in Europe’s drinking-water reservoirs and even in the intestines of Arctic seabirds. They come from a variety of sources, including the deterioration of larger plastic products. Plastic doesn’t readily biodegrade, meaning microplastics persist in the environment indefinitely.

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According to Dr. Vermaire, the most common way for plastic fragments to enter the sewage stream is through washing machines. Fibres from clothing made with polyester or other synthetic materials mix with water, then enter household drains and make their way to sewage treatment plants. There, waste water is separated from biosolids and treated.

The treated water is recirculated, and the biosolids – which are rich with nitrogen, phosphorus, sulphur and calcium – are often applied to farmers’ fields as fertilizer. But biosolids carry more than just plant nutrients. They still harbour those synthetic fibres from people’s jeans, T-shirts and fleeces. “Biosolids are probably the most concentrated material with microplastics that gets released into the environment,” Dr. Vermaire said.

And that, for the time being, is where the trail ends. Nobody knows exactly where those microplastics go after they land on fields. That’s what Dr. Vermaire and other researchers are racing to discover.

Where do microplastics go?

Microplastic fibres break off clothing in washing machines. Any clothing with polyester can produce microfibres, from fleece to blue jeans.

Water from washing machines mixes with other wastewater and goes to sewage treatment plants.

Slurry

Cake

Pellets

Treatment plants treat the water, removing all waste. The waste is left over, coating the tanks and becoming a thick sediment. This is scooped up and turned into biosolids. Biosolids come in three forms: cake (thickest), slurry (middle-thick) and pellets (think rabbit food).

b

c

a

The biosolids are applied to fields. Months later, two-thirds of the plastic is gone. Where did it go? There are a few possibilities:

a

The plastic seeps down earthworm tunnels and into tile drains, pipes that run underneath farmers' fields.

b

The plastic is blown off the top of the soil by rain or wind.

c

The plastic is eaten by insects and mechanically broken down into even smaller pieces that could be undetectable.

MURAT YÜKSELIR AND KATHRYN HELMORE

/ THE GLOBE AND MAIL

Where do microplastics go?

Microplastic fibres break off clothing in washing machines. Any clothing with polyester can produce microfibres, from fleece to blue jeans.

Water from washing machines mixes with other wastewater and goes to sewage treatment plants.

Slurry

Cake

Pellets

Treatment plants treat the water, removing all waste. The waste is left over, coating the tanks and becoming a thick sediment. This is scooped up and turned into biosolids. Biosolids come in three forms: cake (thickest), slurry (middle-thick) and pellets (think rabbit food).

b

c

a

The biosolids are applied to fields. Months later, two-thirds of the plastic is gone. Where did it go? There are a few possibilities:

a

The plastic seeps down earthworm tunnels and into tile drains, pipes that run underneath farmers' fields.

b

The plastic is blown off the top of the soil by rain or wind.

c

The plastic is eaten by insects and mechanically broken down into even smaller pieces that could be undetectable.

MURAT YÜKSELIR AND KATHRYN HELMORE

/ THE GLOBE AND MAIL

Where do microplastics go?

Microplastic fibres break off clothing in washing machines. Any clothing with polyester can produce microfibres, from fleece to blue jeans.

Water from washing machines mixes with other wastewater and goes to sewage treatment plants.

b

Slurry

c

a

Cake

Pellets

Treatment plants treat the water, removing all waste. The waste is left over, coating the tanks and becoming a thick sediment. This is scooped up and turned into biosolids. Biosolids come in three forms: cake (thickest), slurry (middle-thick) and pellets (think rabbit food).

The biosolids are applied to fields. Months later, two-thirds of the plastic is gone. Where did it go? There are a few possibilities:

b

a

c

The plastic seeps down earthworm tunnels and into tile drains, pipes that run underneath farmers' fields.

The plastic is blown off the top of the soil by rain or wind.

The plastic is eaten by insects and mechanically broken down into even smaller pieces that could be undetectable.

MURAT YÜKSELIR AND KATHRYN HELMORE / THE GLOBE AND MAIL

Applying biosolids to fields has been common practice in Canada for more than four decades.

The country produces around 666,000 dry tonnes of biosolids annually and recycles about 50 per cent of that on land, depending on the municipality. Some recycle more.

Ottawa produced 50,000 tonnes in 2022. Of that, 33,000 tonnes were directly applied to agricultural land, and another 14,000 were applied later in the form of enriched fertilizer products.

Toronto produces 195,000 tonnes of biosolids each year. If not put on fields, all that material would end up in landfills.

Canada is not the only country that does this. More than half of the biosolids produced in the United States and the European Union are applied to farms.

For researchers, the process of tracing the movement of microplastics in biosolids begins when all that waste material hits the ground.

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David Lapen, a research scientist with the Ottawa Research Development Centre, part of Agriculture and Agri-Food Canada, stood recently among waist-high wheat in a field about one hour away from Carleton University.

In spring 2021, Dr. Lapen and Dr. Vermaire applied 13 tonnes of cake biosolids to a portion of this field. When they tested the field months later, they could find only one-third of the microplastics originally contained in the biosolids. This complemented research done around Ontario’s Lake Simcoe by Jill Crossman, from the University of Windsor. She found that 99 per cent of microplastics were missing from fields one year after biosolid application.

Where are these microplastics going? There are a few possibilities, Dr. Lapen said.

He picked up a shovel and dug it into the soil. Bending down, he pointed out circular tubes that ran through the dirt. They were earthworm burrows, small pathways through which water – and anything else – can seep deep into the ground.

In addition to this natural drainage system, many fields have a man-made one: large perforated pipes called tile drains, which run below the surface. These pipes, especially important in the spring, drain excess water from fields and release it into the ecosystem.

“When we apply liquid slurry biosolids on these fields, within 10 minutes we have black water coming out of the tiles,” Dr. Lapen said. “This pipe is a rapid transport pathway to the environment.” The missing microplastics could have been carried away by water into these drainage channels. Or sun and rain could have broken them down into particles too small to detect.

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Another possibility is that the plastic is being eaten and metabolized by insects.

At another lab at Carleton, a refrigerator buzzes with live crickets. A team of scientists mix small plastic fibres with cricket food, in an attempt to find out if the insects will eat the plastic – and, if so, what their digestive systems do to it.

“What happens to those microplastics in the body? Are they broken down? Do they pass by untouched?” said Mahmoud El-Saadi, a doctoral student who is part of the cricket team.

Their findings so far suggest that crickets do eat plastic, happily. The group has also found that a cricket’s gut will break down microplastics mechanically, but not chemically.

The plastic remains complete in cricket feces. Outside the laboratory, it would be released back into the environment, only in smaller pieces – perhaps too small for researchers to notice.

There is currently no way of removing microplastics from biosolids, and preventing plastic from entering sewage treatment plants in the first place would require drastic changes, such as eliminating clothing that contains polyester, nylon, acrylic or other synthetic fibres. These materials account for about 60 per cent of clothing, worldwide.

The only tool we have for dealing with the proliferation of microplastics is containing the spread, Dr. Lapen said.

The first step in what Dr. Lapen called a “multibarrier approach” to containing microplastics in biosolids is immediately raking the soil after the biosolids are applied to fields. This incorporates the material into the top layer of the soil, preventing runoff. It also stops wind from picking up plastic particles. This practice is regulated under Canadian law.

The second step involves boxes about one-foot wide and six-feet deep, partly buried in the ends of fields and connected to tile drains. The boxes collect water and contain it until farmers release it. This allows farmers to control the flow of water from their fields into the ecosystem.

The third step relies on what are known as bioreactors: plant beds of wood chips combined with other substances that either physically sequester plastics or absorb them and chemically transform them.

This step is also important for dealing with the other contaminants included in biosolids and fertilizers, such as per- and polyfluoroalkyl substances, also known as “forever chemicals,” or PFAS – a grouping of around 4,700 chemicals found in everyday items, such as takeout containers, makeup and non-stick cookware. Evidence suggests that PFAS have harmful health effects and negatively impact the environment.

This type of barrier can be mimicked by bordering fields with ditches, trees and shrubs, which trap and transform contaminants before they hit the ecosystem at large.

“This natural capital, natural infrastructure, is bar none going to get you the best bang for buck,” Dr. Lapen said.

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But the latter two solutions are hard to implement, and are not widely practised or required.

Installing and maintaining the boxes is time-consuming and expensive. Dr. Lapen estimated that each one costs around $500. In large-scale operations, farmers would need hundreds or thousands. And if something went wrong with the boxes and a crop failed, the use of the technique could void a farmer’s insurance claims.

As for ditches, they use up viable land, cutting down on crop yields and profits. They also segment fields, making harvesting and sowing with tractors much more cumbersome.

But these steps are essential, Dr. Lapen said. While the details of the movement of plastic from biosolids to the ecosystem are still mysterious, the idea that contamination must be contained is not up for debate.

“We look at practices that are going to keep things in the topsoil, no matter what it is,” Dr. Lapen said. “That is a priority.”

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