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One morning this fall, large trucks tossed a wet, muddy substance onto a cornfield just beyond the south Twin Cities suburbs — loading the soil with nutrients for another growing season.

The Metropolitan Council carries out this little-known ritual twice a year in rural Dakota County. The black substance landing on the ground is no ordinary fertilizer. It's human manure, the stuff that's left behind after the agency purifies sewage from several growing cities.

The council incinerates most of the solid material it extracts from the region's wastewater. But flush a toilet in Lakeville, Rosemount, Farmington, Apple Valley or Elko New Market and the waste travels to the council's Empire plant, where the "biosolids" are heated to remove pathogens and later spread on nearby farm fields. The agency is now embarking on a $23 million project to process more of that material as the area grows.

Using human waste as fertilizer is more common outside the metro area. About a quarter of the solid waste that flows down toilets and drains in Minnesota is later applied to land after treatment, largely to corn and soybean fields. Biosolids are an inexpensive, soil-enhancing fertilizer but have also recently attracted scrutiny across the country because some contain industrial "forever chemicals" known as PFAS.

At the Empire plant, the council first removes pathogens from the biosolids in an anaerobic digester. They are then stored on asphalt pads — spanning 8 acres — until farmers in the area are ready to spread it. It still smells when it arrives in the field, but the odor is more akin to fertilizer than human excrement.

"This has a lot of nutrients. So it's really, really good [fertilizer]," said Camila Ciampolini, who manages the land application process at the Met Council, while walking around the field in Castle Rock Township.

The council will expand its biosolids storage capacity by installing a roof over part of the pad, spanning about three football fields. That will improve the drying process, allowing more material to be stacked.

The plant is a relatively small part of the council's massive wastewater system, which spans 600 miles of pipes and nine treatment plants. Most wastewater ends up at the council's Metro plant in St. Paul, which treats 15 times as much as Empire and torches the solids in giant incinerators. The agency also turns waste from cities around Lake Minnetonka into a pelletized fertilizer.

The most prominent Minnesota example of applying biosolids to land is in Duluth, where the Western Lake Superior Sanitary District spreads biosolids on about 2,000 acres a year — some of it to help grass grow at taconite mines. St. Cloud and Rochester also apply their solids to nearby fields.

"It's a manure. It just comes from a different animal," said Dr. Carl Rosen, a soil expert at the University of Minnesota. "It has all of the nutrients that manure has in it."

Biosolids improve soil structure and increase microbial activity, Rosen said.

PFAS concerns

The practice is not without controversy. The presence of PFAS in some biosolids has prompted states across the country to conduct more rigorous testing and take steps to reduce the contamination.

The Minnesota Pollution Control Agency is seeking grant funding from the state to study the issue here. A 2009 study found PFAS in biosolids from Minnesota wastewater treatment plants — including Empire — but the agency wants to know more about how it is absorbed into soil and crops, said Aaron Luckstein, the MPCA's section manager for municipal wastewater. The new study will also outline best practices for managing PFAS-contaminated waste.

"We do not believe that biosolids is a key [PFAS] exposure pathway," Luckstein said. "It's present in everyday products that people use, and there's a greater exposure risk there than what we believe would be present through biosolids."

Biosolids are heavily regulated to protect human health. For the type produced by the Empire plant, state rules dictate how much time must elapse between spreading them on fields and growing crops. That ranges from 30 days for feed crops to more than 20 months for human food that grows in the ground, like carrots and potatoes.

Sherry Bock, an MPCA biosolids expert, said sunlight, time and natural factors such as soil microbes help reduce potential pathogens lingering in the biosolids. The material has also been treated at the plant.

"It's not what you flush down the toilet. It's a processed form of that," Bock said. "And it's really the microbes that have broken down what we put into the wastewater plant."

Rosen said that biosolids are generally safe but can include just about anything in the wastewater stream — from antibiotics to hair-care products and caffeine.

"We need to be aware that when you apply biosolids, there's more than just nutrients that are being applied," said Rosen, who would be part of the MPCA study if funding is approved. "Some of them get broken down, and some of them are very slow to break down."

As for other pollutants in biosolids, Luckstein said, "they're not at levels that we would at this point in time be concerned for human health or other risks."

Plant upgrades

The Met Council project is expected to cost about $23.7 million, about half of which would pay for the new roof and other improvements to the biosolids pads. Other upgrades will remove more moisture from the biogas that is emitted during the digestion process.

The plant has several digesters that heat the solids to about 98 degrees for more than 20 days in an oxygen-free environment.

"What it does, that heat, is it reduces the disease-causing microorganisms," said Rene Heflin, who oversees wastewater plant engineering for the Met Council. "We reduce the pathogens to where its safe for the environment."

The council applies the material to about 600 acres of farmland each year, generally in the fall and the spring to farms in the surrounding area.

The upgrade will bring the plant's capacity to store biosolids in line with how much liquid the plant can handle — about 24 million gallons per day. Because of water conservation, the amount of water flowing into the plant has been stable or decreasing. But there's still plenty of solid material.

"Because we're all eating about the same and going to the restroom about the same … the solids concentration is going up," Heflin said.

Eric Roper • 612-673-1732 Twitter: @StribRoper