For decades the city of Cold Spring, Minn., has wrestled with pollution from farms fouling its drinking water. Sealing contaminated wells and digging new ones didn't fix it. Changes to farm practices haven't come fast enough.

The levels of toxic nitrate just kept creeping up.

Now the central Minnesota town of 4,000 is pioneering a new way to cut the nitrate in tap water: getting hordes of bacteria to gobble it up.

That's what happening inside Cold Spring's new $6.5 million nitrate removal plant. It's the state's first biological treatment plant for the common pollutant and one of a handful in the country.

"We're excited," said Cold Spring Public Works Director Jon Stueve. "We knew that this is the first ever in Minnesota."

Bacteria, one of the oldest life forms on the planet, have long been used to treat dirty wastewater piped from toilets and industry. Treatment plants in Europe have used the enterprising bacteria for decades to treat drinking water, too, and it's used in a limited way by some U.S. treatment plants to remove minerals like manganese. But the biological approach to cleaning drinking water has never become widespread in the U.S. For one, there's an ick factor — people don't like the idea of bacteria swimming in their tap water.

That's changing.

The "bugs," as water professionals sometimes call the microbes, can destroy nitrate without creating the problematic stream of concentrated waste that conventional treatments do.

Nitrate is one of Minnesota's top water contaminants, a chemical salt that creates algae blooms, harms fish, taints drinking water and contributes to the massive dead zone in the Gulf of Mexico. Its main source is the fertilizer spread on crops, both synthetic nitrogen fertilizer and manure.

Cold Spring's new 5,000-square-foot treatment plant with its copper-colored metal roof sits on the edge of town under a water tower, near a giant Cold Spring Brewing plant that churns out White Claw hard seltzers and other drinks. Six other cities in the state have resorted to adding nitrate removal systems.

Inside looms the heart of the operation: two huge, blue steel vats about 10 feet wide and 12 feet tall. These bioreactors are full of granular activated carbon, some sand — and zillions of bacteria.

They are native microbes. The indigenous bacteria live in the aquifer Cold Spring draws its water from, and probably in the human gut, too. They are not added.

Give them a meal of acetic acid — vinegar essentially — and they proliferate. They feast on the nitrate, stripping the oxygen from the molecules and leaving a harmless nitrogen gas that floats away.

Disinfectants such as chlorine then easily kill off all the bacteria before the cleaned water is piped to homes and businesses. For now, the plant treats water from the city's four wells. The brewery has its own wells and a reverse osmosis system to reduce nitrate, Stueve said.

Water users shouldn't notice any change, Stueve said.

But they may have noticed the 5% bump in their water bill last year. The state's 2018 bonding bill paid for about $4 million for the project. Customer water bills pay the remaining $2 million. A federal grant took care of about $468,000.

The treatment will cut Cold Spring's nitrate to 5 milligrams per liter. While the bacteria eat it to nearly zero, the city blends it with other water that raises the level. That's half the 10 milligrams per liter state and federal standard for nitrate in drinking water. State health officials want the water treated for nitrate to be at half the enforced limit or less, which meets the federal Safe Drinking Water Act requirements.

"We could get zero," said Ryan Capelle, the drinking water treatment lead at Stantec in Minneapolis, the engineering company that designed the plant. "But because the cost of the treatment is high, we wanted to be reasonable."

There's growing concern the federal safe drinking water limit should be lowered. It was set decades ago to guard against the potentially fatal blue baby syndrome that starves infants of oxygen. Newer research links exposure to lower nitrate levels to other health effects, such as increased risk of colorectal cancer, thyroid disease and neural tube defects in preterm babies, and brain and spinal cancers in children. The Washington-based nonprofit Environmental Working Group is pushing to dramatically lower the standard.

"There's this gap between what's legal and what's actually safe," said Sydney Evans, senior science analyst at the organization.

Treatment is a back-end fix, she said. It's crucial for farmers to reduce pollution on the front end, she said. "Otherwise the costs are going to keep going up and up and up."

Stueve said he's confident Cold Spring's water is safe. If the state decides to lower nitrate limits, the city left plenty of room to add more vessels and expand treatment.

Cold Spring's plant uses a system developed by AdEdge Water Technologies in Duluth, Ga., called biotta. There's only about a dozen or so plants in the U.S. treating drinking water with bacteria, according to Martin Lawrence, AdEdge manager of applications engineering.

"The biggest hurdle we have to get over is convincing people this is safe," Lawrence said.

The Minnesota Department of Health says it is. It approved the approach in 2013 after vetting it and reviewing results of a pilot project at Lincoln-Pipestone Rural Water System which supplies about 50,000 people across southwest Minnesota. That utility just received $4 million in state bonding funds for half the cost of building a permanent biotta system.

More may follow. The approach could be sustainable because it can cut chemical use and create a more biologically stable water system, said Brian Noma, a public health engineer at the Department of Health. Conventional reverse osmosis and ion exchange treatments remove nitrate but they generate a brine polluted with concentrated nitrate and salts that has to be dealt with.

The brine predicament was the reason Lincoln-Pipestone was forced to invest in a new nitrate treatment.

Biological treatment is now an option in the manual that sets design standards for drinking water treatment in the 10-state Great Lakes region, Noma said. He said he hopes the Cold Spring plant "opens the door."

Jess Brown, R&D practice director at Carollo Engineers in California, called the biotta treatment a game changer in the U.S. European utilities have been doing biological de-nitrification since the early 1980s, he said.

Under a microscope the nitrate-loving bacteria look like tiny rods, he said. They are proteobacteria — hundreds of types from three different classes of the microbes.

Since they are living organisms, treatments must be managed correctly to maintain the right environment. If bacteria multiply too fast they can generate bio-slime or biofilm — a goo that is flushed off, said Stantec's Capelle.

"Bug poop," Capelle said. "If the bugs aren't happy there will be more bio-slime created."

Stueve said the bugs at the Cold Spring plant will be happy.

"I think we're on the right path," he said.

Staff writer Jeff Hargarten contributed to this story.