Bruce Kania built his large home atop a bluff with an expansive view of the Yellowstone River, which flows past on its 670-mile journey from Yellowstone National Park to its confluence with the Missouri River.

Mr. Kania moved here a decade ago to enjoy the natural setting. But he soon began to worry about the surface water draining into the Yellowstone. Kania’s black dog went for a swim one day in a pond on his property, and the dog came out of the water covered in slime.

“You could smell him from 50 feet away,” Kania said.

The experienced inventor got to thinking about water quality. Irrigation ditches divert clean water from the Yellowstone. The ditches feed pastures and fields, and what is left drains back into the river. Beef cattle roam the land. In the nearby town of Shepherd, a giant feedlot produced tons of manure every day.

Where some saw pollution, Kania saw nutrients.

“It became apparent that this represented a terrific opportunity for somebody,” he said. “What can be done with those nutrients?”

The answer is floating bio-islands.

In his college days, Kania had been a fishing guide in northern Wisconsin. There, he saw giant natural floating islands that had grown over generations, big enough to support trees. These areas had huge muskellunge, prized sport fish, and Kania thought the islands were one reason. They scrubbed the water clean and provided the forage that grew big fish.

In Montana, Kania decided to produce man-made floating islands that could do the same. They’re constructed of layers of mesh and insulating foam. The mesh feels like the rough side of a sponge: The bigger islands are buoyant enough to hold people.

Most are at least 25 square feet, and the least expensive costs around $600. They’re topped with soil and seeds and then launched in lakes and ponds. The islands act like magnets for microbes, and the seeds sprout into vegetation that pulls nitrogen, phosphorus, and other nutrients that cause slime from the surrounding water, cleaning it and allowing aquatic life to flourish.

Kania shows off pictures of giant perch that his employees have caught in test ponds on his property. The perch’s bellies were full of snails, which Kania thinks consume the vegetation growing from the islands, which he calls “floating supermarkets.”

Kania, a tall, bearded man who moved to Montana in 1976, has turned his invention into a business called Floating Islands International. He has about a half-dozen employees and is now licensing the islands to companies that will make them full time.

That leaves time for Kania to get back to inventing. “Our license holders do the hard work of manufacturing,” he says. “We do the fun work of invention.”

The mesh for the islands is made from recycled plastic bottles and comes in wide black rolls. Each island consists of several layers, which are stacked before a worker shoots expanding foam between the layers. When the foam dries, the layers are locked together.

Holes are cut into the mesh and filled with plants. Grasses and flowers grow above the water, while their roots grow down through the mesh and into the water. Insects and minnows hide and forage in the roots underneath the islands, and bigger fish are attracted to the smaller fish. The islands expand over time.

The floating islands allow water to circulate, and they provide surface area for nutrient-hungry microbes, which form a film that is loosely connected to the islands. As the plants use more nitrogen and phosphorus to grow, there is less available in the water for algae blooms, which give off an unpleasant odor and form a suffocating shadow on top of the water.

“We’re taking a big enough bite out of the nutrient load that we’re not getting algae,” says Damien Austin, Kania’s staff biologist.

Evidence is mounting that the islands can remove ammonia, heavy metals, and other chemicals from the water, too. They act as a sponge on a wet countertop: Water is drawn in, and the bad stuff sticks. “It looks like a good idea, and I’m kind of surprised no one else has thought of this before, at least in a marketable way,” says Joe Gathman, a biology professor at the University of Wisconsin at River Falls.

“Ideally, problems like [excess nutrients] should be stopped at their source: Mop it up before it gets into the lake. But these islands could be helpful little mops,” he says.

The main office for Floating Islands International is just a few feet from Kania’s house. A shop and laboratory are a mile down a dirt road. Twice a week, Kania and his staff – which includes his wife, Anne – meet for lunch at the Feedlot Steakhouse outside Shepherd. They talk floating islands, from buoyancy calculations to which type of soil works best.

Floating islands are being used from downtown Chicago to Singapore. In Billings, Mont., a city of 100,000, city engineers are using floating islands to filter storm water. The local zoo uses an island in its otter habitat and no longer has to chemically treat the water, says Tim Mulholland, an engineer and one of eight license-holders who build the islands.

Kania’s licensees recently built a 22,000-square-foot island for the US Army Corps of Engineers, which launched it in a lake in Oregon. The island provides habitat for the Caspian tern. The shore birds are being relocated away from the Columbia River estuary, where they eat 5 million salmon fry a year.

“It’s floating great,” says Kitia Chambers, an Army Corps engineer. “We’ll definitely monitor this one, and if all goes well, we’d like to use this design again.”

Ideally, the licenses will fund other inventions. Kania is planning to launch an island this summer with a wind turbine on it, and he’s trying shredded carpet in the islands.

“We’re not a marketing company, and we’re not a manufacturing company,” he says. “My job as CEO is to maintain the perspective of an inventor.”