Treatment using UV light a possible alternative for chlorine in swimming pools

23 November 2016 by Webredactie Communication

Swimming pool water is usually disinfected using a chlorine product such as hypochlorite to kill all micro-organisms present. Unfortunately, hypochlorite reacts to other impurities in the water, such as sweat and urine, to form disinfection by-products. These disinfection by-products are irritants. Marjolein Peters studied an alternative method of purification and examined its effect on micro-organisms. On Friday 25 November, she will be awarded her PhD at TU Delft for her research.

Waste in the water

During swimming we leave anthropogenic (human) pollutants in the water; these can include sweat, urine, skin fragments, hair and sebum, as well as micro-organisms. Active swimmers, for instance, release on average 100-200 ml of sweat per m2 skin surface per hour while swimming (water temperatures below 29 ˚C). Both the water temperature and the level of exertion of the swimmer affect the degree to which we release pollutants in the swimming pool. 

Film on tiles

At present swimming pools often use hypochlorite products to kill micro-organisms (such as bacteria and viruses). If hypochlorite is no longer used in the swimming pool water, the micro-organisms are no longer killed and they can grow on the walls of the pool, resulting in a slimy layer of biofilm. Marjolein Peters studied how to prevent the growth of biofilm in swimming pools by making smart use of alternative materials. Peters: “Polypropylene (a polymer) is a good material to use, because it has the least growth of biofilm. Concrete and stainless steel showed the most growth of biofilm.” Removal of the biofilm was also studied using UV light and scrubbing.

Treatment using UV light

Micro-organisms that remain in the swimming pool water but do not form a biofilm can be removed and deactivated using a UV-based treatment while the water is circulating. Here a combination of filter and disinfection methods is used. The water is treated in three steps: first a biological filter containing micro-organisms is used to 'eat up' the dissolved pollutants. Then, an ultra-filtration process comprised of a very fine-meshed sieve is used to remove hair, skin fragments and micro-organisms up to 0.2 micrometres in size. And finally the remaining micro-organisms are irradiated and inactivated using ultra violet (UV) light.

Water quality

The advantage of this method is that as chlorine products are no longer used, there are no irritant disinfection by-products. Because the micro-organisms are now only inactivated in the treatment, it is important to analyse the microbiological quality of the water after the UV-based treatment process. For this reason, Peters examined the extent to which pathogens, bacteria such as E. coli, Salmonella and Campylobacter, were still present in the water. Peters: “The result was that 1.8 out of 100,000 people have a chance of being infected by the pathogen E. coli bacteria which remain in the water. This is a very small chance, and the risk of actually becoming sick from this is even smaller.”

Implementing the UV treatment process

According to Peters, to thoroughly research the potential of a UV-based treatment process as an alternative to hypochlorite requires testing at a larger scale. “The UV-based treatment process is more expensive than using hypochlorite. This is in part because the water needs to be circulated past the UV lights in a short time to be effective. This uses a lot of energy for a large leisure or competition pool. For this reason the UV treatment process is probably better suited for use in smaller pools, such as paddling pools and therapy pools, where the water circulation is smaller. Actually implementing a UV-based treatment process in swimming pools will require a change to the law, which currently prescribes chlorine products.”

More information
PhD candidate Marjolein Peters, M.C.F.M.Peters@remove-this.tudelft.nl, +31 (0)6 45 26 10 88
Science Communication Adviser, Wendy Dallinga, W.M.Dallinga@remove-this.tudelft.nl, 015 278 7538

Read more about the research into chlorine-free swimming pools in the Stories of Science on the website of the Faculty of Civil Engineering and Geosciences.

© 2017 TU Delft

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