After a century of renovations and updates, a Minneapolis park’s swimming hole will return to its freshwater origins and become the nation’s first “natural” public swimming pool.

The $4 million pool project, which is part of a new master plan for the 22-acre Webber Park in north Minneapolis, is currently in the design stage. Construction is expected to begin next year, with a 2014 opening. “We really wanted to create a natural place where people will want to gather,” says Robert Schunicht, vice president of Landform, the Minneapolis planning and engineering firm handling the park’s renovation.

The plans for Webber Park’s new pool are drawing local and national attention. “The community is really excited about this,” says Jon Olson, who represents the neighborhoods near the new pool on the Minneapolis Park and Recreation Board. At 28,000 square feet of swimming area, the pool will offer triple the recreation space of the park’s aging concrete pool and wading pool — without any of the chlorine. The preliminary design includes lap lanes, a diving area, water slide, and a non-swimmers’ area with a maximum depth of 39 inches.

But instead of the traditional pool chemicals, the Webber Park pool will rely on a European approach that uses a biological, plant-based filtration system. As designed, the water in a natural pool flows from the swimming area to a biological filter and from there it percolates through a regeneration pond filled with hydroponic plants, gravel, and other aggregates.

Unlike some chlorine-free pools, “there are no plants in the swimming pool, and there is no soil,” explains James Robyn, CEO and president of BioNova Natural Pools, a New-Jersey-based branch of the German firm BioNova Global, which will be working with Landform on the project.

The filtered water then returns to the swimming pool via standard pumps and pipes. Depending on the climate where a natural pool is built, those plants can vary.  For this installation, the designers are talking with Minnesota growers to develop the right mix of non-invasive, native plants for the filtering, says Brady Halverson, Landform’s project designer.

In terms of materials, the pool’s floor will be lined with a commercial PVC membrane, which less flexible and thicker (60 mil vs. 30 mil) than a residential pool’s vinyl liner. “It’s also field-seamable,” Robyn said. “It comes in rolls. We cut it and heat-weld the pieces together. It doesn’t have to be preconfigured.”

Unlike many traditional swimming pools, there will be no main drains at the Webber pool and theoretically a lower risk of entrapment for users. “We will have Virginia Graeme Baker Act considerations for the [proposed] water slide, because the water used to operate the slide will need high-performance pumps,” Robyn said. “But the biological flow is done with smaller pumps.”

In some ways, the Webber Park project sounds more like a lake than a swimming pool, but that was part of its appeal to the community. “We don’t have lakes on the north side of Minneapolis,” said Olson. “People wanted to be able to go to the beach.”

Fittingly for this project, Webber Park’s very first pool, built in 1910, was a spring-fed swimming pond, according to a city parks history. That original pool was converted to chlorinated city water in the 1920s. The existing pool, scheduled to be demolished in 2012, was built in 1979.

The hybrid nature of the new Webber Park pool required some special considerations. The Minneapolis Park and Recreation Board operates 60 wading pools, two water parks, and 12 beaches on the city’s lakes, but this project didn’t fall into any of those categories. As a result, the board last spring had to get special state legislature approval to build the Webber pool and regulate it not as a pool, but a public bathing beach. The city only monitors E. coli levels at its beaches, closing them as necessary for health reasons.

“From a microbiological standpoint, our water quality standards are based on the European standards for public natural swimming pools, which are roughly twice as strict as what are required at many public bathing beaches,” Robyn said.

Webber’s plant-based filtering system is expected to cost less to operate than a traditional chemical pool, according to those involved in the project, although specific figures weren’t available. (The park board’s budget includes more than $414,000 in 2012 for aquatic recreation services, which includes management, staffing, and lessons for swimmers and sailors at Minneapolis lakes, pools, and water parks.) Due to the expected cost savings, Olson said the city would no longer charge visitors to use the Webber Pool when it reopens.