Sludge compost is an increasing source of microplastics, researchers say

Heavy machinery and piles of compost at a sprawling facility.
Compost piles are prepped for screening at the Tulare Lake Compost plant. Researchers say that natural fertilizers created from sewage sludge are an increasing source of wind-blown microplastics.
(Robert Gauthier / Los Angeles Times)

A team of UCLA researchers has put a new spin on the 1970s rock classic “Dust in the Wind” — only this one is grimmer and grimier than the original hit by Kansas.

They found that wind picks up microplastics from human-sewage-based fertilizers at higher concentrations than previously known, and may be an “underappreciated” source of airborne plastic bits, flakes and threads.

“If you blow wind on soil with microplastics, you’d expect a similar amount of soil and microplastic to be present in the dust,” said Sanjay Mohanty, a UCLA professor of civil and environmental engineering. “But here we found less soil in the dust and a lot more microplastics. That means every time the wind blows, the particles leave the soil preferentially.”

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That’s problematic, he said, because it means people are potentially inhaling these particles, which measure between 1 and 5,000 micrometers, or 5 millimeters, in size. Most of these particles are likely to be coated with harmful chemicals such as plastic additives, heavy metals, pesticides and other chemicals that have been poured down the drain or have trickled off streets into storm drains.


It’s these “passenger” chemicals that are the biggest health concern, Mohanty said, citing a growing body of research into known and potential harms.

“The more plastics we put out there, the more they are going to end up in soil, in water and in people’s food and bodies,” said Avi Kar, a senior attorney and director with the Natural Resources Defense Council’s Health and Food, People & Communities Program. “And they will bring along with them the many harmful chemicals that are used to make plastic.”

In many major cities across the world, including Los Angeles, municipal sewage systems process human waste by separating wastewater from biosolids. The wastewater is filtered and often injected into the soil, where it is refiltered to be used for irrigation and in some cases, drinking water. The biosolids — the residue left behind — can be dried out, repackaged and sold as fertilizer.

Globally, the annual production of biosolids is estimated to be 100 million tons, according to Mohanty and his colleagues. That is projected to increase to 175 million tons per year by 2050. According to estimates by the U.S. Environmental Protection Agency, more than 2 million dry metric tons of biosolids are applied to land each year in the United States.

In 2011, 51% of the nation’s biosolids was applied to land and agricultural fields; 22% was disposed of in landfills; 16% was incinerated; and 11% was subjected to other management practices.

Sludge-based fertilizers are appealing for environmental and economic reasons. They provide farmers and gardeners with a natural and sustainable means of reintroducing nutrients to the soil and of extending the life of municipal landfills.


The Los Angeles County Sanitation Districts, for instance, treat sewage from 5.6 million people. In 2022, they produced roughly 110,000 tons of fertilizer, of which 20,000 were processed at the Tulare Lake Compost site, southeast of Kettleman City.

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In 2021, Mohanty and his colleagues discovered that sludge-based fertilizers contained far more plastic particles than were previously suspected.

More recently, however, the team set out to determine what happened to the particles in compost. To do this, they spread fertilizer on fields in Lind, Wash., about 75 miles southwest of Spokane. They then placed portable wind tunnels over the plots, and turned on the fans. They found that microplastics were roughly 2.5 times more likely to become airborne than other soil particles.

The reason for this, Mohanty said, is that microplastics aren’t “sticky” like the other elements. He said farmers and gardeners in California often use water on their fields and lawns in summer months not only to irrigate crops but to reduce dust. Because plastic does not absorb water, it doesn’t stick like the minerals and organic material in the soil.

“The findings aren’t surprising in light of the ubiquitous nature of plastics in agriculture and the low densities and weights of microplastics,” said Mark Gold, director of the Natural Resources Defense Council’s Water Scarcity Solutions Program, who was not involved in the research.

Shelly Walther, an environmental scientist with the Los Angeles County Sanitation Districts, said she was a little wary about how the researchers went about identifying and counting microplastics in their samples.


She said that while microplastic research has been growing, there are concerns in the scientific community about the methods and technologies used, as well as the reliability and reproducibility of some of the research.

But, she said, it’s clear that there are microplastics in biosolids, and she’s not surprised some could become airborne. However, she questioned whether all plastics can be blown away, saying that some are denser and less likely to get picked up by a breeze.

Nevertheless, she said, plastic is a real and growing problem for the waste stream.

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The L.A. sanitation districts have supported laws that would help reduce the amount of plastic getting into our waste at the source, including those that would require filters in washing machines and driers.

Mohanty said one of the biggest sources of microplastics in wastewater was washing machines.

“Every time we wash our clothes, we are producing millions and millions of fibers” that get drained directly into our wastewater, he said.

Bryan Langpap, the sanitation districts’ spokesman, said “Hallelujah” when asked about laws designed to reduce plastic — and other contaminants — at their source.


“We just have to stop producing so much of this stuff. Because once it’s in our consumer products, and it’s in our homes, you can’t stop it from getting into the air and moving around,” he said, noting research that has found plastic particles in the Arctic, in the deepest trenches of the ocean, and in just about every single place scientists have looked.

He said the problem may be even bigger than we know, citing recent research that found hundreds of thousands of nanoplastic particles in single-use water bottles — an issue that Mohanty, too, has considered when it comes to plastic in fertilizer.

He said unlike microplastics, his models show that nanoplastics — which range in size from 1 nanometer to 1 micrometer — are more likely to cling to soil, because of stronger attractive force on the surfaces. And while that may keep them from blowing into the air and into our lungs, it’s possible they are also being taken up by the crops and plants.