“There is a peacefulness you get from natural water that you don’t get from sterilised water,” says Tim Evans. As one of a growing number of people developing chemical-free swimming pools, he would say that – but he’s far from alone. In recent years, fans of wild swimming have been almost evangelical about the restorative powers of propelling yourself through the silky, skin-pricklingly-cold water of seas, rivers and ponds.

And now there are signs that British people are after a more natural experience from our swimming pools, too, after years of counting lengths in bright, heated leisure centres in water treated with chlorine.

In summer 2021, a Georgian lido – Cleveland Pools – in Bath is due to reopen by the banks of the River Avon that will claim to be the world’s first heated, non-chlorinated public swimming pool, its water treated with a filtration system rather than chemicals.

Cleveland Pools, a 200-year-old Georgian lido in Bath, were originally fed by the neighbouring river next to it. The pools are currently being restored. Photographs: Matt Cardy/Getty Images/@multnomahmedia/Ben Birchall/PA

Other cities have already embraced natural pools. In September, a chemical-free pool opened in Edmonton, Canada, on the site of old swimming baths, with its water cleaned by an extensive gravel filtration system. Minneapolis opened a natural swimming pool in 2015; the same year, a natural pool was created by the river in Gothenburg, Sweden.

What is behind the trend? “I think it’s a cultural thing – people are moving towards wanting products which are more natural, have less chemicals, look more aesthetically pleasing,” says Evans, managing director of “swimming pond” company Gartenart. His firm has been involved with a proposal to create a natural, heated public pool in the middle of the River Thames that architecture firm Studio Octopi is behind – they are looking at a site in east London and are working with a developer, as well as investigating two other potential sites for additional pools. There is a similar proposal in New York to create a ‘self-filtering’ floating pool on the East River.

Another reason, says David Nettleton, managing director of Clear Water Revival, which is working on the Cleveland Pools project, is “the rising interest in renovating lidos at the moment”. There is also, he says, a growing unease with pumping chemicals down the drains, as well as concern about the potential negative health effects of chlorine, including skin irritation.

A no swimming area that works as part of the filtration system at the outdoor natural pool in Edmonton, Canada. Photograph: Jason Franson/The Canadian Press/PA Images

From a public health perspective there are advantages to chlorine, which has been used to treat swimming pools for around a century (it was used to disinfect drinking water for the first time in 1897, in Maidstone, to stop a typhoid outbreak).

A 2013 study by researchers at the University of Barcelona found that in three out of four natural pools in Catalonia they looked at, E. coli and faecal enterococci exceeded recommended limits.

Pathogens such as viruses and bacteria can get into a pool in different ways such as faeces from animals and humans, either directly – such as bird droppings – or from runoff water after storms, or “microorganisms associated with the skin or mucous membranes – eyes and mouth”, says Arnau Casanovas-Massana, now an associate research scientist in epidemiology at Yale School of Public Health, and one of the authors of the study.

An artist’s impression of how the a proposed natural, heated public pool in the middle of the River Thames could look. Image: Studio Octopi/Picture Plane Ltd

“Conventional pools containing chlorine are able to inactivate some of these pathogens, although not all. In natural swimming pools, we don’t have that chemical disinfection, and thus the risk for those pathogens persisting in the water is higher.”

Good preventative measures should reduce the risk of potential pathogens entering the system, he says. Natural pools, if properly managed, “can be safe to use from a microbiological point of view [but] the risk is always going to be higher than in properly managed conventional pools”.

Another drawback to natural pools in cities is that they may not be able to serve large populations. “You cannot use natural pools as intensely as chlorinated pools,” says Peter Petrich, director of Biotop, which has created many pools around the world. It built an open-air non-chlorinated pool in King’s Cross, central London, in 2015, along with Slovenian artist Marjetica Potrč and Ooze architects. It was conceived as a temporary living art installation but proved so popular a petition was started to keep it open, although it closed, as planned, in October 2016.

The temporary open-air pool built in King’s Cross, London, in 2015. Photograph: Leon Neal/AFP/Getty Images

While there are private natural pools in the UK, the idea of an urban purpose-built natural public pool – as opposed to public bathing ponds such as the ones on London’s Hampstead Heath, which are fairly common in Europe – is still rare in the UK.

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In smaller private pools, says Evans, one of the most common ways of filtering the water is through a submerged gravel bed. Other natural pools rely heavily on planted areas to filter the water, but the Cleveland Pools project will use filtration technology that, claims Nettleton, can be used indoors. He hopes it will be a game-changer. “The public natural pools in the Germanic countries are very much like ponds – I don’t know if that would really work here,” he says. “We’re not quite so into natural swimming yet that a swimming pond is popular.”

But we were, once. “The Victorians created new places to swim, with floating baths and pontoons, with filtered river water, as well as lidos and bathing islands,” says Caitlin Davies, author of Downstream: A History and Celebration of Swimming the River Thames.

An illustration from 1875 of the floating swimming bath in the Thames at Charing Cross. Image: Chronicle/Alamy

“The Charing Cross floating bath was open for 14 hours at a time, heated, and had capacity for 2,500 people a day.” With a structure built from iron, and moored on the Thames, it was filled with filtered river water. Meanwhile, the Cleveland Pools in Bath were originally fed by the river next to it (though the “ladies’ pool” was filled with spring water).

“By the 1970s,” says Davies, “swimming spots were being closed down for various reasons, such as pollution and safety fears, access got harder and people were encouraged away from natural water and into indoor purpose-built pools.”

Outdoor natural swimming, says Evans, is a chance for urban-dwellers to experience a touch of wildness you don’t get in a chlorinated indoor pool. “We’re increasingly more aware that we do lead busy lives, and we’re looking for ways to get away from that,” he says. “It’s an antidote to city life.”

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