In 1996, Mick Hilleary was asked to design a zoo habitat for a family of otters. The pond water had to be kept clean and crystal-clear, without using chemicals. After studying research from the aquarium industry, he created a pond that relies on friendly bacteria, biological filters, and a UV sterilizer to keep it fresh. “The otters were frolicking and having so much fun, my partner and I looked at each other and said, ‘We’ve got to do this for us,’” he says. By 2000, Hilleary had added a pool sideline to his zoo-exhibit business, Total Habitat, in Bonner Springs, Kan.

Since then, Hilleary has built about 30 natural swimming pools, most of them residential. The pools have pumps that oxygenate the water and draw it through a regeneration zone containing large gravel filters or plants. Beneficial bacteria live on those surfaces and eat the microbes, keeping the water clear enough to see the bottom. “Our filter capacities are adequate for humans, fish, dogs, and cats,” Hilleary says.

Natural swimming pools are foreign to most U.S. pool contractors, but they’re a large industry in Europe, where the concept originated 20-some years ago. Biotop, the Austrian company credited with originating the idea, has installed more than 3,500 natural swimming pools for residential and public use, according to the company. The low operating and energy costs appeal to municipalities overseeing public pools.

“Chlorine is pretty expensive, and there are fewer storage and pumping requirements,” says Morgan Brown, president of Whole Water Systems, Seattle, who lived in Germany and saw natural swimming pools gaining popularity there. “Since you’ve created an ecosystem in balance, the maintenance is less. The only dead body of water on the planet is a chemical pool. That’s like trying to balance on the head of a pin.”

Flora and Filters

Natural swimming pools can cost about the same as or much more than their chemical counterparts, depending on landscaping features and the type of regeneration zone. Rin Robyn Pools in Far Hills, N.J., designs pools that follow the Germany-based BioNova model, which involves a swimming area and a separate, equal-sized regeneration wetland. Owner James G. Robyn, who also builds chlorine pools, says swimming pools cost roughly $100 per square foot, and the need for a biological treatment zone doubles the size and cost. The average cost of his projects is around $250,000.

“Mother Nature knows how to purify water using plants,” Robyn says. “We are mimicking that, building in optimal fashion so it works efficiently with no extraneous input.” The regeneration zone is a rubber or polypropylene-lined overflow pool filled with engineered gravel and native aquatic plants. “There are no standards for natural swimming pools here in the U.S.,” he says. “We’re trying to establish a dialogue with different health departments to set a standard, starting in New Jersey.”

Hilleary’s also trying to police the industry by forming a trade association. So far there are just two members—himself and Michael Littlewood, a pool contractor in England. “Human safety is at the core. We follow every rule the traditional pool industry has developed, plus a few of our own,” says Hilleary, author of Natural Swimming Pools & Ponds: The Total Guide. Instead of plants, he often uses expanded shale, an inert filter medium resembling lava rock, to clean the water.

“We can build the shale into the floor, around the edges, behind walls, or under waterfalls,” Hilleary says. “We’ll often dig pools with a gradual slope so we’re not dealing with retaining walls. The cost is the same as any in-ground pool, except they often beg for additional naturalistic landscaping like boulders and waterfalls.”

Green Lagoon

Natural pools appeal to two types of homeowners—naturalists seeking an alternative to the ubiquitous blue-tiled boxes, and those concerned about health or carbon footprint. In some cases, the technology is used to improve on Mother Nature. Two years ago, Hilleary was asked to rehabilitate a farm pond in Wichita, Kan. The design team diverted the site runoff and installed a rubber liner, pumps, and biofilters, plus plants, expanded shale, and a city water supply to maintain water clarity. “I was out there a week ago, and it’s crystal-clear,” says landscape architect Larry Hoetmer, who collaborated on the design.

Chlorine pools differ mechanically from natural pools. Traditional pool builders struggle with a concrete shell and pipes running through, says Ashland, Ore.–based landscape architect Kerry KenCairn, who is in the process of purchasing a BioNova franchise. BioNova pools, which are fish-free, have an interior wall separating the swimming zone from the regeneration zone to keep swimmers from disturbing the plants’ roots. But everything is contained in a large rubber liner, and the pipes run through gravel. Permitting is straightforward, too. “They have to be called natural ponds, not pools,” KenCairn says. “And you still have to deal with fencing and other health and safety issues.” Establishing a balanced ecosystem takes about a year, she adds, and requires bacteria and pH level testing.

Did we mention algae? It should clear up after the first year, Robyn says, but he counsels clients that this is pond water, and there will be some seasonal algae. That was enough to make a Princeton, N.J., couple hesitate. For them, Robyn built a hybrid design complete with a 60-foot lap pool, dock, beach entry, diving board, spa, and lagoon with a seating ledge. It’s set up so that, if the clients change their minds, it can go natural with water filtered from the planted areas.

One resident, however, didn’t get the message. Says project landscape architect Brian C. Meneghin: “I pulled up one day and there was a great blue heron on the diving board, looking for a little lunch.”