Bathers release bacteria in swimming pool water, but little is known about the fate of these bacteria and potential risks they might cause. Therefore, shower water was characterized and subjected to chlorination to identify the more chlorine-resistant bacteria that might survive in a chlorinated swimming pool and therefore could form a potential health risk. The total community before and after chlorination (1 mg Cl₂ L¹ for 30 s) was characterized. More than 99% of the bacteria in the shower water were Gram-negative. The dominant bacterial families with a relative abundance of 10% of the total (non-chlorinated and chlorinated) communities were Flavobacteriaceae (24–21%), Xanthomonadaceae (23–24%), Moraxellaceae (12–11%) and Pseudomonadaceae (10–22%). The relative abundance of Pseudomonadaceae increased after chlorination and increased even more with longer contact times at 1 mg Cl₂ L¹. Therefore, Pseudomonadaceae were suggested to be relatively more chlorine resistant than the other identified bacteria. To determine which bacteria could survive chlorination causing a potential health risk, the relative abundance of the intact cell community was characterized before and after chlorination. The dominant bacterial families in the intact community (non-chlorinated and chlorinated) were Xanthomonadaceae (21–17%) and Moraxellaceae (48–57%). Moraxellaceae were therefore more chlorine resistant than the other identified intact bacteria present.

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Aims Most swimming pools use residual disinfectants like chlorine for disinfection. The use of chlorine has several drawbacks: some waterborne-pathogens are chlorine resistant and disinfection by-products (DBPs) may be formed which are associated with various health risks. Therefore, an alternative treatment was developed which consists of biological sand filtration, ultra-filtration and UV-disinfection. The goal of this study was to compare the microbial risks for bathers in a UV-disinfected pool compared to a chlorinated pool with the use of a quantitative microbial risk assessment. Methods In this microbial risk assessment, the microbial release was calculated from multiple factors such as pool content, number of simultaneous swimmers, duration and frequency of swimming, ingestion of pool water, the hygienic behaviour of swimmers and the actual release of microbial cells per swimmer. The concentration of faecal bacteria was calculated from shedding experiments and known pathogen concentrations in faeces. The Dutch illness probability (283/1000) was used to calculate the number of infected bathers, which were all assumed to sheds 108 faecal pathogens per g faeces. The used reference pathogens were Campylobacter jejuni, E. coli O157:H7 and Salmonella enterica. The removal of pathogens by treatment with UV-disinfection was set to 5-log units, every 4 hours, while during chlorination these 5-log units are known to be achieved in 30 seconds. The dose-response relationship for E. coli and S. enterica was simulated with a beta-Poisson distribution and for C. jejuni a hypergeometric function was used. The yearly risk of infection was calculated separately for each of the bacterial pathogens and a normal range sensitivity analysis was done. Results The average bacterial cell concentration during opening hours in a UV-based swimming pool were the highest for C. jejuni (3.1×10-3 cells L-1) > S. enterica (9.5×10-4 cells L-1) > E. coli (7.2×10-4 cells L-1). These calculated pathogen concentrations were about 180 times higher than calculated pathogen concentrations in a chlorinated swimming pool in which the averaged concentration was 4.0×10-6 cells L-1 for pathogenic E. coli cells. Based on the average pathogen concentration during opening hours, the yearly risk of infection was calculated to be 9.8×10-8 for the chlorinated swimming pool and 1.8×10-5 for the UV-based swimming pool treatment. The yearly risks of infection of a UV-based treated swimming pool were the highest for C. jejuni (1.7×10-3) > E. coli (1.8×10-5) > S. enterica (3.5×10-7). The simulated yearly risk of infection was found to be most sensitive for the number of bathers releasing pathogens. Conclusions The yearly risks of infection of E. coli and S. enterica in a UV-based treated swimming pool were lower than the drinking water guidelines (10-4), but for C. jejuni it was higher. For a more complete picture of the health risks, the effects of disinfection by-products should also be taken into account in future risk assessments, as is the effect of other pathogens like Pseudomonas, Cryptosporidium and Giardia. UV-based treatment might be a good alternative for chlorination for some specific types of swimming pools. @en

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Swimming pools are traditionally disinfected with a residual disinfectant such as sodium hypochlorite. Nowadays, swimming water without a residual disinfectant is increasingly popular, as can be seen by the growing number of (natural) swimming ponds (Weilandt 2015), but health risks for bathers do raise concerns for these type of pools, so some form of disinfection is needed (Giampaoli et al. 2014). The combination of ultra-filtration and UV-disinfection for pool water treatment, without a residual disinfectant might be an interesting alternative. The Dutch Innovative Pool project (DIPool) was initiated to explore this new treatment concept, to study its applicability for swimming pools and to verify first system design specifications. The goals of this study were to compare the microbial water quality during treatment without disinfection, treatment with ultra-filtration, treatment with UV-disinfection, treatment with chlorination, and the influence of single treatment steps. Methods All treatment concepts were studied in a pilot plant equipped with a pool basin and simulated bathing load, containing only chemical components. Adenosine triphosphate (ATP) and intact cell count measurements were used to monitor the microbial water quality before and after each treatment step at regular intervals in several experiments during 23 days of operation. High pollutant/nutrient conditions without recirculation were used to study the effect of single treatment steps, and recirculation conditions were used to study the effect of accumulation of pollutants/nutrients. A chlorinated pilot plant was used as reference for the alternative disinfection. The influence of a biological activated carbon filtration during chlorinated conditions was also investigated. Both pilot plants were operated at 1 m3/h, with a turnover time of 30 minutes during the recirculation conditions. Results The results showed that the microbial quality of pool water with UV-disinfection was similar to that of chlorinated pool water, while pool water treatment without disinfection resulted in a higher microbial number. After 23 days, most of the results during chlorination and UV-disinfection experiments with recirculation were within 103-105 intact cells/mL and 0.2-13 ng intracellular ATP /L which is similar to both bottled and tap water. Treatment steps with the highest reduction in microbial quality were ultrafiltration (UF) in the treatment setup with UV-disinfection and chlorination in the treatment setup with chlorination. The largest increase in microbial quality was observed during residence in the pool basin, which was obvious, and during biological activated carbon filtration. Most important conclusions were that i) the microbial water quality of pool water with a UV-based treatment can be similar to that of pool water with chlorination ii) UF plays an important role in maintaining a low number of micro-organisms during UV-based treatment and iii) P-limitation is an additional method to limit microbial growth in swimming pool water. @en