Miljøprojekt, 1153 – Alternativer til klor som desinfektionsmiddel i offentlige svømmebade

Alternativer til klor som desinfektionsmiddel i offentlige svømmebade

Summary

The recreation industry is growing rapidly and it is expected that swimming pools and water theme parks, etc. are going to experience increased pressure from the bathers and more stringent demands from the users as regards documentation and ensuring satisfactory water quality so that bathers can be confident regarding the cleanliness of the facilities.

Historically, chlorine has always been the preferred disinfectant for protection of the hygienic water quality in swimming pools. Chlorine has the advantage of being effective against a considerable number of microorganisms. Chlorine reacts quickly and when dosed sufficiently it secures a lasting disinfection effect in the water.

When adding free chlorine to swimming pool water, however, a very large number of unwanted chlorinated disinfection by-products (DBP) are formed during chlorine’s reaction with the contamination deriving from the bathers, and from substances leaching from materials in contact with the pool water. The nature and especially the amounts of these contaminated substances are hardly known. The best known disinfection by-products are chloramines, trihalomethane (THM), halogen acetic acid (HAA) and haloacetonitriles (HAN). A considerable number of other chlorinated organic substances are known to be formed, but these are never, or only rarely identified. The existence of such chlorine disinfected by-products is a potential problem for the both bathers and pool staff with regard to convenience as well as to health.

Swimming pool water is a very complex system, where the water quality in the pool is determined by the load, the processes in the pool and the water treatment system.

Many chemical interrelationships have not been adequately investigated, but one fixed point seems to be the fact that the development of unwanted by-products is reduced with lower content of chlorine.

Based on the chemistry of chlorine in aqueous systems, it can be shown that unchanged disinfection efficiency can be maintained with lower content of free chlorine by lowering pH compared to the current pH interval applied in Danish swimming pools. This practice is applied in a number of European countries without having problems with the microbiological water quality. Thus, it seems obvious to suggest a similar practice in Denmark. A most likely companion effect will be the reduction of the content of disinfected by-products and thereby a reduction in chlorine problems both with regard to convenience as well as to health. A reduced content of chlorine demands strict control of the chlorine content and the hydraulics in swimming pools to avoid periods or areas with too low chlorine content causing reduced disinfection effect. Therefore, it is necessary to be aware of this situation at any change to lower chloride content in Danish swimming pools.

Traditionally, the chemical water quality in swimming pools is controlled by using online measurements of the concentration of free chlorine, pH and temperature. Furthermore, THM is measured twice a year as an indicator of the content of organic disinfection by-products.

It ought to be considered whether a better picture of the content of organic DBP could be achieved by supplementing the THM measurements with measurements of AOX that express the sum of the adsorbable chlorinated organic compounds, which in swimming pool water practically corresponds to the total content of chlorinated organic compounds.

Research into the development of DBP in swimming pools has led to much more understanding of the processes behind the most significant parameters governing DBP formation. Based on this knowledge it is possible to point out a number of new parameters, which with a limited effort could be used in order to obtain a considerably better picture of the physical-chemical water quality as well as to control the treatment processes in the water treatment plant. One example is measurement of turbidity, which gives a picture of the impacts from particles, including microorganisms. Another example is measurement of NVOC using Membrane Inlet Mass Spectrometry (MIMS). MIMS makes it possible to measure (online) the content of specific THMs both with regard to the pool water and the air in the swimming baths, and possibly also – by adaptation/development – a number of other DBPs.

In swimming pools the bathers more or less constantly affect the pool quality with microorganisms – either due to insufficient cleaning before entering the pool or because they accidentally defecate (faecal incidents) or urinate in the pool. Furthermore, the bathers can affect the pool quality with saliva or slime, e.g. from nose or throat.

This contamination of the pool water poses a risk of infecting other bathers if the microorganisms are pathogens. The route of contamination from one bather to another bather through the pool water may thus be very short in time and distance. To reduce the risk of infection, the pool water is continuously recycled through a water treatment facility. However, due to the retention time of the pool water it is necessary to add a disinfectant to the pool water in order to avoid cross infection between bathers.

The generally applied demands for disinfectants in swimming pools are very broad and inaccurate and they are, therefore, difficult to make use of in the evaluation of new and alternative disinfectants. Based on the evaluation of a number of alternative disinfectants no alternatives have been identified, which could work with the same effectiveness as chlorine.

However, a more differentiated view of the risk scenario for various pool applications can open the way for application of other disinfectants, if other risk scenarios than the very fast disinfection ensured by chlorine are given higher priority. If this is the case, testing new disinfectants should be made under controlled conditions using reference data for chlorine’s effect on selected microorganisms.

A monitoring programme related to testing new disinfectants and technologies should encompass:

Indicator organisms for faecal contamination (e.g. E. coli). Even though E. coli is highly sensitive to chlorine and is inactivated much faster than many other organisms, the presence of E. coli will show a faecal contamination with insufficient disinfection. With no E. coli present, however, there is no proof that the disinfection is sufficiently effective.
Well characterised organisms with moderate sensitivity to the tested disinfectant or technology – e.g. Pseudomonas aeruginosa or Staphylococcus aureus – to give a consistent basis for comparison between different disinfection methods.
The general microbial population (e.g. total count 37°C) that will reflect a broad spectrum of organisms and, thus, the general efficiency of the disinfection. A distinct increase will indicate a loss in disinfection.
Organisms that are especially relevant to the disinfection method in question or to the application of the pool, e.g. Legionella in relation to warm water pools or aerosol formation, e.g. from spa pools.
Problem organisms which are especially resistant towards a disinfection method in question; e.g. Cryptosporidium which is resistant to chlorine.
Pathogens actually present that pose a real infection threat.
Model organisms, e.g. to virus.

In general it is recommended to prepare a risk assessment of the microorganisms actually present in pool water. The risk assessment should be undertaken taking into account the specific application of the pool (elitist swimmers, baby swimming, play pool, swimming education, etc).

There is an urgent need for development of new microbiological analytical methods with faster response time allowing for an application in the operation of swimming pools, and methods for quantitative detection of a number of microorganisms, which are only rarely measured today, e.g. vira.

In comparison with the procedures and standards applied to control the microbiological as well as the chemical water quality in swimming pools in Central European countries, the conclusion is that there are very big differences in the procedures applied. Some countries apply generally accepted technical standards, some apply officially recommended guidelines, and others have regulations defined by the local authorities.

With regard to the microbiological water quality there seems to be relatively good agreement on the generally accepted microbiological parameters: total count 37°C, coliform bacteria, and Pseudomonas aeruginosa. For other microorganisms like Legionella and Staphylococcus no uniformity exists in the regulation.

In all countries chlorine is the disinfectant, while the accepted level of chlorine varies significantly from country to country. In Germany the chlorine concentration is maintained in the range 0.3-0.6 mg/l, while in England and Denmark the range is 1-3 mg/l. Also the accepted level for bound chlorine varies considerably with the lowest values in Germany – less than 0.2 mg/l – and the highest values in England and Holland – less than 1 mg/l. The level of THM is regulated only in Germany, Denmark and Switzerland with acceptable concentrations in the range 20-50 µg/l. Also the standards for pH vary significantly, covering the range 6.5-8.3.

From the investigation, the immediate perspective in Danish swimming pools is a continued use of chlorine as disinfectant with an improved control of DBP-production induced by a decrease in the free chlorine concentration. Efficient disinfection is ensured by a combined reduction of pH to the interval 6.5-7.0 and a chlorine content of e.g. 0.3-0.6 mg/l. For both parameters the mentioned ranges lie outside the existing regulations.

Further, an improved water quality with reduced DBP-content can be achieved by using supplementary technology in the recycled water loop of the swimming pool. A number of relevant technologies are available, which are on an acceptable level of experience for implementing and which can contribute to lower the DBP-content either by direct removal of the unwanted DBPs or by removal of those precursors that are the prerequisite for the development of the DBPs. In general the actual technologies are not very well described in relation to swimming pools, and there is a need for experimental work in preparation for establishing qualified documentation to secure accurate design and to get maximal benefit from investment in new water treatment technology – including estimation of the economics.

Besides the contribution to cut down the level of DBP in swimming pool water, a number of technologies might contribute to the support of the disinfection and among other things secure disinfection of chlorine-resistant pathogens.

For the time being there is no tradition/regulation that Danish swimming pools should continuously monitor the performance of the technology installed for water treatment. This is, however, practice in for example Germany, and it is assumed that routine sampling and analyses for documentation of technologies’ effectiveness could give valuable contributions to improved water quality in pools both directly by optimising poorly performing technology and indirectly by providing improved knowledge about technologies’ function including possibilities for optimisation.

Implementation of monitoring programmes for routine performance checks of the current technology installed at Danish swimming pools is very likely to contribute to improved pool water quality. With an aim towards identification of possibilities for optimisation of the current technology installed at Danish swimming pools – mainly sand filters and activated carbon filters – it is recommended to undertake well documented tests of relevant optimisation scenarios.