A clean swimming pool without chlorine

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The use of chlorine in swimming pools is required by law. Chlorine quickly kills bacteria and comes with fewer disadvantages than other disinfectants. But not everyone reacts well to chlorine. Some people develop a rash, red eyes or irritated airways. You could question whether chlorine is in fact the healthiest way of fighting bacteria. That’s why we look for technologies to develop chlorine-free swimming pools. An alternative that is healthier and more pleasant for swimmers.

 Swimming pools are mainly polluted by visitors’ unhygienic behaviour

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How dirty are our current swimming pools? And what compounds does the swimming water contain? There is hardly any research on this. That surprised me. And what was the outcome of my research? Swimming pools are mainly polluted by visitors’ unhygienic behaviour – the source of 70 to 80 percent of all pollution. For example, people don’t shower before taking a swim. This allows unnecessary amounts of sweat, bacteria and skin flakes to end up in the water. And there are people who urinate in the pool. If people were more hygienic it would make a huge difference to the level of pollution.

Together with behavioural scientists we looked at how you can change people’s current behaviour. It’s far too expensive to post a pool attendant by the showers all day but you can hang up certain stickers telling people to shower first. Nevertheless, not any old sticker will do – there is a philosophy behind it, just as with traffic signs. Two of the stickers we designed didn’t have the desired effect. The third one did. The number of people who took a shower before swimming rose from 25 to 30 percent. On top of that, you can teach young children who are taking swimming lessons about hygienic swimming behaviour. These are all small steps you can take to increase people’s awareness.

Besides changing swimmers’ behaviour, we also look for other technologies to keep the water clean. We studied different water treatment techniques and biological filters. You can disinfect water with the use of UV-light, for instance. If you radiate water with UV-light, the bacteria are inactivated. However, it’s not easy to treat the entire pool with UV-light. Direct exposure to UV-light is unhealthy for swimmers. Therefore, you have to circulate the water so that it passes the UV-light several times a day. But in that case, how fast should the water flow? And how do you do this without using too much electricity? We’re still working on this. 


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I am Maarten Keuten and I am researcher at the section Sanitary Engineering at TU Delft. I want to develop cleaner swimming pools, without the use of chlorine. That’s why I’m studying various purification systems and biological filters that keep the water clean. Besides this I also look at how people could behave in a more hygienic way in the water. This could make up to a 70% difference in the levels of pollution! 

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Social and Commercial opportunities

In the end, our research is suited to different types of swimming pools, all over the world. It will provide cleaner water in which it’s pleasant to swim. However, the research isn’t suitable for recreational pools with water slides and whirlpools. These recreational elements increase the risk of Legionella. You need a residual disinfectant like chlorine for that.

As soon as the study is concluded there will be a discussion with the medical world. How clean does a non-chlorinated swimming pool really need to be? As clean as surface water, for example, or cleaner still? There are all kinds of gradations. And will it be possible to change the law so that not every swimming pool has to use chlorine? There is still a long way to go. But in the end we want everyone to be able to go for a pleasant and healthy swim. This includes the people who are currently staying away because of the use of chlorine. 

My expertise

Pool water treatment, heating, ventilation, air-conditioning, sanitary engineering, process automation, legionella control, corrosion, swimming pools