ST. PAUL, Minn. — A growing body of research has been documenting an emerging threat to Minnesota waters — tiny plastic particles found in everything from shampoo to fleece jackets.

But even as we learn more about how prevalent these pollutants are, there’s still much we don’t know about their dangers or how to respond.

So-called microplastics are tiny — less than 5 millimeters across. They can come from litter or plastic bags that break down over time. In some cases, they start out small, as microbeads added to products like facial soap and toothpaste.

One of the biggest sources are plastic fibers from fleece jackets, athletic clothing, carpet and other synthetic fabric.

“They are breaking apart in the washing machine, we believe,” said Timothy Hoellin, associate biology professor at Loyola University in Chicago. “These small little threads, they find their way into the wastewater treatment system and then, into our aquatic environment.”

Austin Baldwin, a hydrologist with the U.S. Geological Survey, studied the St. Croix, Namekagon and Mississippi rivers in 2015. The results were published earlier this year in a brief issued by the National Park Service.

Baldwin’s team found microplastics in all of the samples they took of water, sediment, fish and mussels. The level of concentration was surprising: They found as many as 111 microscopic pieces of plastic in a single fish.

Scientists worry that microplastics might clog the digestive systems of fish and make them feel full, so they end up starving. Baldwin said there need to be more study of the biological impacts.

“We still don’t really know, is 100 pieces of microplastic in a fish high?” he asked. “It sounds high, but what’s the risk associated with that?”

Researchers also are looking into whether chemicals in the water attach themselves more readily to a piece of plastic than something else, like a piece of sand.

“As the plastic is floating in the water, other chemicals basically see it as a sponge,” Baldwin said. “They latch onto the plastic and they concentrate on the surface. Then the question is, if a fish eats that piece of plastic, do those contaminants get passed onto the fish or not?”

Amid growing concern about plastic pollution, Congress easily passed a ban on one of the sources in 2015. The Microbead-Free Waters Act barred manufacturers from adding the tiny spheres to products beginning in July.

But Hoellin says microbeads were an easy target, and they’re not the biggest source of microplastics in the environment.

“Trying to change how we make fabrics and wash our fabrics — that is a much more challenging and really widespread source of pollution, so it’s not going to be as simple,” Hoellin said.

Microplastic fibers in the wastewater are so small they slip through filters and screens designed to capture larger particles. Hoellin’s team sampled Chicago rivers and found higher concentrations of microplastics downstream of sewer plants.

“What I’ve seen is that some wastewater treatment plants are really effective at retaining 99 percent of the microplastic that comes in as raw sewage,” Hoellin said. “But even that 1 percent, when it’s added up on a daily, yearly basis, is amounting to a lot of plastic pollution.”

Hoellin noted there’s no legal requirement for wastewater plants to treat for microplastics.

“They’re effective at doing what they’re designed to do, which is to make our water safe,” he said. “But they weren’t designed to remove these type of pollutants, and we don’t have any regulations that they should.”

Some treatment plants have fairly sophisticated methods of filtering, while others are more rudimentary, Baldwin said. Also, heavier types of plastics probably settle out during treatment into the sludge, which is often applied to land.

“What happens to all these plastic fibers that are in the sludge?” Baldwin asked. “Do they just make their way back into the river? Do they get blown off the landscape and into the atmosphere? That’s a question that really hasn’t been answered either.”

At the Metropolitan Wastewater Treatment Plant in St. Paul, the plant’s assistant general manager, Larry Rogacki, explained the various steps the raw sewage goes through to treat for pathogens and remove pollutants like phosphorus.

He points to the bar screens that catch larger items, which are transferred to a conveyor belt and end up in an incinerator. Later, the wastewater goes into tanks where solids settle to the bottom and other stuff floats to the top, where it can be scraped off.

“It’s mostly grease, fats, plastics, big particles, small particles, anything that’s lighter than water,” Rogacki said. By the time the treated wastewater is discharged into the Mississippi River, Rogacki estimates that 96-98 percent of all microplastics have been removed.

Retrofitting the plant to eliminate 100 percent of microplastics would require installing sand filters that could capture smaller particles, he said. It would be costly — close to $1 billion. Rogacki said the plant has land set aside in case the treatment is added in the future.

What scientists say might be more effective — and less expensive — is to figure out how to keep plastic out of the wastewater stream in the first place.

“Because it’s such a complicated mix of different kinds of particles, trying to legislate it or set universal or nationwide or even statewide regulations is going to be a real challenge,” Hoellin said.

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