Strategies for chemically healthy public swimming pools

Abstract

Swimming pools are used around the world for recreational, rehabilitation and physical activity and therefore it is imperative that the water and air quality are safe for the health of the bathers. Chlorination is by far the most widely applied method to control pool water quality and to prevent spreading of pathogens between swimmers because of its residual disinfection effect. In addition to potential contamination of pathogenic microorganisms, swimming pool water is polluted by organic matter deposited from the bathers such as saliva, urine, sweat, hair and personal care products. Since chlorine is a strong oxidant it oxidizes the organic matter in the pool water and forms disinfection byproducts (DBPs). More than 100 different DBPs have been identified. Some of these have been found to be genotoxic and may pose an increased cancer risk for the bathers. The aim of this thesis was to give an overview of the strategies which can be used to achieve microbiological safe water with low levels of DBPs to ensure healthy environment for bathers. There are different approaches to achieve healthy environment in public swimming pools which in this thesis are divided into three strategies: alternatives to chlorination, removal of precursors and DBPs, and inhibition of the DBP formation. None of the alternative disinfection agents which are used for private swimming pools are applicable for public swimming pools. Thus chlorine is the most likely future disinfectant in public swimming pools. The strategy with removal of precursors and DBPs includes several methods: pre-swim showering, filtration, ozonation, activated carbon, stripping, and UV treatment. In general, decreasing the load of precursors by requiring pre-swim showering would decrease the formation of DBPs. However, addition of precursors cannot be completely avoided. Hair and skin cells are precursors for DBPs so good filtration with fast removal of particles could also be an option to obtain lower DBPs formation. Another way to remove precursors is to ozonate the pool water, since ozonation of the precursors leads to organic compound which is less reactive towards chlorine. Ozone is also able to remove combined chlorine and other DBPs but the reaction is slow. Activated carbon is able to adsorb precursors and DBPs except chloramines which are removed by catalytic reaction. Formation of DBPs is unavoidable. However, the volatile DBPs can be removed by stripping while UV treatment is used for control of combined chlorine levels. The last strategy with inhibition of the DBP formation mainly focused on pH since change in temperature and chlorine level are very limited due to comfort and safety of the bathers. The aim of the PhD study was to investigate the possibility of UV used for combined chlorine removal to remove other DBPs and to investigate the effect of pH on the formation of selected DBPs. The investigations were carried out in laboratory setups in order to have controlled experimental conditions. The UV treatment can remove a range of DBPs but with varying efficiency. In general, the photolysis efficiency increased with bromine substitution of chlorine in the structure of the DBPs. Combined chlorine was used as actinometer to estimate the removal which could be expected at actual UV treatment applied in swimming pools if no other formation or removal pathways are considered. For very volatile DBPs, the removal by UV treatment will be relative low for the fate of the DBP unless it is very easily photolysed (such as trichloronitromethane). For non-volatile DBPs, the sensitivity for photolysis is important to achieve significant removal of the DBPs by UV treatment. The investigation suggested a significant removal of trichloronitromethane, chloral hydrate and the bromine containing haloacetonitriles and trihalomethanes may occur as a beneficial side-effect of chloramine control by UV in swimming pools. Changing the pH value of the pool water affected the investigated groups of DBPs differently. An analogue consisting of the main component in urine and sweat and particles consisting of skin cells and hair were used as precursor material and in both cases the formation of THMs decreased with decreasing pH while HAN formation increased. The effect of pH on the formation of HAAs depended on the precursor type. The particles did not form trichloramine during chlorination whereas the body fluid analogue formed trichloramine. The trichloramine formation showed strong pH dependency with increasing formation at pH below 7.0. The presence of bromide did not change the impact of pH on the DBP formation, but it did increase the total amount of formed DBPs. The estimated toxicity increased with decreasing pH similar to the HAN formation. From evaluation of which DBPs were formed, theirs extent and theirs toxicity, an optimal pH range for pool waters was identified to pH 7.0 – 7.2. It is estimated that in the wider pH range (pH 6.8 – 7.5) the pH effect on DBP formation was minimal compared to other factors which may affect the formation of DBPs in swimming pools. The future swimming pool will make use of several methods to minimise the level of DBPs in the pool. The different methods have to be optimised together to ensure the best water quality. The aim should not be only to minimise level of trihalomethanes since other and more toxic DBPs are formed and these does not always follow the same tendency as trihalomethanes (e.g. when changing pH).