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While conventional pool systems rely on disinfection using chlorine, natural pools and swimming ponds utilise the self-cleaning effect of nature. But how thorough and hygienic is natural pool water? Dr Jakob Schelker, technology and R&D expert at Biotop, has the answer.

Conventional pool systems are based on the idea that the water is continuously disinfected. Such systems were built from 1910 onwards, just when a large number of dangerous pathogens had been newly discovered. In the spirit of the times, any form of microbial life was seen as a threat and chlorine could reliably combat this. Disinfection prevents all life in the water, as chlorine attacks all cells. Sufficient disinfectant must always be present in chlorine pools to prevent the formation of harmful bacteria. Corners of a chlorinated swimming pool that do not have a flow through them represent a high risk for the development of bacterial colonies, as there are plenty of nutrients available here due to the many destroyed cells. In a biologically purified pool, on the other hand, areas with less flow are not a problem. The aim here is to achieve an ecological balance as in a natural body of water. For example, in the biofilters we have developed, adapted biofilms are formed that specialise in the decomposition of organic material and cells of all kinds. Bacteria and viruses introduced from outside are efficiently broken down there. At the same time, nutrients such as phosphates are reduced in biologically purified ponds and pools in order to limit the formation of algae.

What water quality for bathing facilities?

The water quality for swimming pools and bathing facilities is defined by three main aspects:

The bathing water must fulfil certain chemical-physical parameters - e.g. have a pH value like drinking water, i.e. be neither too acidic nor too alkaline. A value outside this range can lead to skin intolerance. At Biotop, we always analyse the filling water during the planning phase of a new system.

The water must be hygienically suitable. For this purpose, faecal bacteria indicators (such as E. coli, enterococci and P. aeruginosa) are measured in the laboratory in most countries. The strictest limits for these indicators apply to drinking water. In most countries, a small number of indicators may be present in biologically purified bathing facilities - e.g. up to 100 CFU E. coli/100ml according to the German FLL guideline (Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau e.V.). The limit values for bathing areas in natural bodies of water (bathing lakes, sea, etc.) are significantly higher than for swimming ponds and natural pools. For example, the European Bathing Water Directive (2006) allows up to 500 CFU E. coli/100ml for lakes and bathing areas by the sea.

Certain substances should not be present in the water or only in very small quantities. Examples include toxic heavy metals such as lead or arsenic, but also nitrite. This substance is part of the natural nitrogen cycle and is usually quickly broken down into less harmful nitrate. Nitrite can also occur in swimming facilities as a result of heavy fertiliser or nutrient loads - often together with high turbidity and a bad odour.

Statutory provisions

The FLL guideline (2017 for private facilities[1], 2011 for public facilities[2]), which was developed in Germany and is currently valid, plays a central role throughout Europe. It defines the precise specifications for the construction, operation and, in particular, hygiene of swimming ponds and natural pools. Many aspects of the directive have been incorporated into national regulations in other European countries - such as the Bathing Pool Hygiene Ordinance applicable in Austria[3] . In Switzerland, the so-called technical recommendation for the private sector has been in force since 2017[4], as has a separate standard for publicly operated facilities[5]. In any case, the national requirements must be complied with when building and operating biologically purified bathing facilities. While regular monitoring of water quality is not mandatory in the private sector, it is mandatory in most national guidelines and standards for public facilities (e.g. public outdoor pools, campsite pools and hotel pools).

[1] Guidelines for the planning, construction and maintenance of private swimming and bathing ponds, second edition, 2017, FLL-Verlag Bonn

[2] Guidelines for the planning, construction, maintenance and operation of outdoor swimming pools with biological water treatment (swimming and bathing ponds), 2011, FLL-Verlag Bonn

[3] Regulation on hygiene in baths, hot tubs (whirlpools), sauna facilities, hot air and steam baths and small bathing ponds (Bäderhygieneverordnung 2012 – BHygV 2012)

[4] Specialist recommendations for the planning and construction of private, artificial bathing ponds, swimming ponds and natural pools, 2017, Swiss Association for Near-Natural Bathing Waters and Constructed Wetlands (SVBP)

[5] Standard for biologically treated communal pools, 2024, Swiss Association for Near-Natural Bathing Waters and Constructed Wetlands (SVBP)

Biofilter vs. chlorine disinfection

The filter technology used in swimming ponds consists on the one hand of a biofilter whose large surfaces are covered with biofilms and degrading bacteria. The microbes obtain their energy by breaking down organic materials and cells. The rapid and extensive decomposition is due to the biofilm's great demand for energy. In the nutrient-poor tank, this simply receives little biomass for degradation. Comparable self-cleaning processes also take place in natural flowing waters, for example in gravel banks of rivers. When the water leaves the biofilter, it is enriched with nutrients. In ponds, aquatic plants quickly absorb these substances and bind them. In natural pools without plants, we connect our PhosTec filter stage downstream of the biofilter, which binds and removes nutrients using minerals. For some of our systems, we also recommend the installation of a UVC unit, which breaks up cells (especially single-celled algae) and enables rapid degradation by the biofilter. However, UVC units are only switched on temporarily and after increased bathing activity, as they would otherwise disrupt bacterial development in the biofilter.

In chlorine-operated systems (also known as salt electrolysis), the highly reactive chlorine directly oxidises the membranes of all living cells and destroys them. Solids and inorganic particles are removed mechanically in a sand filter system. The sand filter is then regularly and usually automatically backwashed to wash the particles out of the sand again. Large quantities of dissolved nutrients from the destroyed cells remain in the pool water, as do many free DNA components. The effect of the chlorine lasts for a few days, but then it must be topped up. If this is not done, the many nutrients provide ideal conditions for the rapid growth of bacteria and pathogens.

Natural water is healthier

Aktives Chlor hat viele negative Eigenschaften, darunter seine zellzerstörende Wirkung. Diese ist auch problematisch für Haut, Zähne, Atemwege und Lunge. Wissenschaftliche Untersuchungen haben gezeigt, dass bestimmte Erkrankungen bei Kindern, die einer Chlorbelastung ausgesetzt sind, wie auch bei Profi-Schwimmern:innen und Schwimmlehrern:innen vermehrt auftreten. Dazu gehören Asthma, Infektionen der Atemwege, wie auch Reizungen der Haut und Augen[1]. Zudem ist bekannt, dass Chlor mit weitgehend ungefährlichen organischen Substanzen (z.B. UV-Blocker aus Sonnencremes) neue Verbindungen eingeht[2]. Viele der dabei neu entstehenden Stoffe sind als krebserregend oder endokrin wirksam klassifiziert (z.B. Trihalomethan und Haloketone), sodass aktuell in der Wissenschaft eine weitreichendere Gefährdung der Badegäste durch Chlor als bisher angenommen vermutet wird[3].

[1] https://doi.org/10.18192/riss-ijhs.v2i1.1526

[2] https://doi.org/10.1021/es062367v and https://doi.org/10.1016/j.heliyon.2023.e13673

[3] https://www.jstor.org/stable/20485906 and https://doi.org/10.1021/es062367v

Keeping natural pool and pond water clean

The key to clean, natural bathing water is a functioning filter system, which should be checked regularly to ensure it is working properly. A minor service includes, for example, washing out the filter mats and correcting the flow through the filter. Secondly, inputs from outside in the form of dangerous germs, nutrients and especially phosphorus must be prevented. Just a few grams of lawn fertiliser can represent a critical phosphorus load in a biologically cleaned swimming system and lead to massive algae growth and inadequate hygienic cleaning of the pool water. Animals, such as dogs or waterfowl, can also cause direct nutrient and germ inputs.

Hygienically tested

Biotop filter systems have been tested for their ability to break down typical faecal bacteria and pathogens. The filters render the majority of dangerous bacteria harmless with each pass. The majority of this process is carried out by protozoa and a small percentage by specialised viruses. In addition, UV radiation from sunlight and microalgae provide natural disinfection directly in the pool. Microalgae release substances to attack bacteria and protect themselves against them. Our biologically purified swimming systems are hygienically tested and therefore flawless. Exceptions are systems that have been acutely contaminated by direct contamination or where the filters are not correctly dimensioned. In the case of public facilities, there is also a risk that the filter systems will be overloaded if the pool is constantly busy.

Sustainable and resource-friendly

A biologically purified pond also conserves resources. Due to the natural purification of the water, it never needs to be changed and is constantly self-sustaining. The pond or natural pool water also does not need to be drained in winter. Only water used for cleaning or lost through evaporation needs to be topped up. A pond or natural pool can also score points in terms of electricity consumption. As not every corner of the pool needs to be constantly flooded, smaller pumps can be used for the filter and skimmer. This means that electricity consumption is significantly lower than with a conventional pool system. Another positive effect is that the "waste water" from biologically purified pool systems is of excellent quality and can simply seep away into the garden, contributing to natural groundwater recharge. In contrast, chlorinated water must be disposed of and/or treated. Swimming ponds also have a positive effect on the environment. As a recent study shows, ponds create new habitats for frogs, toads and newts, which are among the most endangered animal species in the world. So if you don't mind the occasional croak, a swimming pond is the right choice in many respects.

[1] https://doi.org/10.1073/pnas.2123070119

 

Dr Jakob Schelker is an authorised signatory and head of the Technology, Research & Development departments at Biotop P&P International GmbH. He has many years of expertise in researching the self-purification capacity of streams, rivers and lakes. His doctoral thesis focussed on the influence of intensive forestry on the water quality of streams. Before joining Biotop, he was head of a research group at the University of Vienna.