Assessment of combined ozone\chlorine and chlorine disinfection processes on the microbial and physico-chemical qualities of swimming pools

Abstract

Intruduction: Application of proper disinfection methods and disinfectants can control pathogen growth and its transmission to swimmers by contaminated water. The aim of this study was to investigate the effect of ozone-chlorine and chlorine disinfection processes on physicochemical and microbial quality of swimming pool water. Material and methods: In this descriptive- analytical study, one chlorine disinfected and one ozone-chlorine disinfected pool were selected from 60 active pools in Tabriz city for sampling. Samples were obtained from each pool in three seasons namely summer, autumn and winter under two conditions of low (7:00 AM) and high swimmer load (5:00 PM) between 2018 and 2019. Physicochemical parameters and microbiological indicators were analysed based standard methods for the examination of water and wastewater. In this study Polymerase chain reaction (PCR) assay to the detection of 8 important and pathogenic bacterial species. In addition, bacterial community structure and diversity was compared in two swimming pools by using next generation sequences (NGS) of the 16S rRNA gene. Results: The results of physicochemical analysis showed that pH, residual free chlorine and turbidity in 41.6%, temperature in 50%, alkalinity in 100% and hardness in 91.6% of samples were above national standards and WHO guidelines. Ozone-chlorine disinfected pool status was more desirable than chlorinated pool for all physicochemical parameters except residual free chlorine. In 83.3% of ozone-chlorine disinfected pool samples, the value of free chlorine was higher than the maximum standard limit. Bacterial tests showed that Pseudomonas aeruginosa was not detected in any of the water samples. Fecal coliforms and fecal streptococci were present in only one sample (chlorine disinfected pool, maximum swimmer load, summer) and the rest of the samples were negative for these bacteria. HPC and Staphylococcus aureus in 100% and 83.3% of samples of chlorine and ozone-chlorine disinfected pools did not meet the national standards, respectively. According to the results, in all seasons, the microbial quality of ozone-chlorine disinfected pool water was more favorable than the chlorinated pool. Also, the microbial load of both swimming pools in summer was higher than other seasons, and maximum swimmers load (evening) than the before use (morning). PCR results indicated the presence of two Pseudomonas aeruginosa and Enterobacteriaceae in some samples. NGS finding proved that, 410 and 406 bacterial species were identified in the sample of chlorine disinfected pool water and ozone-chlorine disinfected pool, respectively. Of the 8 dominant bacterial species in each swimming pool, Pseudomonas alcaliphila, Pseudomonas stutzeri and Propionibacterium acnes were common species between the two pools. Oleomonas sagaranensis (350 read /18593), Staphylococcus caprae (302 read /18593) and Anaerococcus Octavius (110 read /18593) were among the dominant bacteria in the chlorine disinfected pool. Bacterial diversity in the ozone-chlorine disinfected pool was lower than chlorinated pool and the highest bacterial sequencing belonged to the genus Pseudomonas (37507 read /43720, 85.79%). Conclusion: Results of physicochemical and microbial tests, PCR assay and NGS analysis showed that water quality of ozone-chlorine disinfected pool was more desirable than chlorinated pool. The high value of free chlorine in pools, especially ozone-chlorine disinfected pool indicating improper and unscientific adjustment of disinfectant dosage, lack of management awareness and improper operation of the pool which due to the health effects caused by the high chlorine dose and disinfection by products, it is necessary to control and reduce this parameter. It is determined by comparing culture and molecular methods, old techniques such as culture media are insufficient for accurate monitoring of microbiological quality of pools due to lack of detection of viable but nonculturable bacteria (VBNC) and reporting false negative result. Therefore, the use of both molecular methods and conventional culturing for microbial analyzes can enhance the monitoring of swimming pools water quality. Keywords: Swimming pool, Microbial quality, PCR, NGS, Tabriz