Inhibition of Legionella Growth in Circulating Bathing Water by a Filter Refreshment Method using a High Concentration of Chlorine

In most of hot spring baths in Japan, bathing water is circulated for extended use to conserve hot spring water. In recent years, massive outbreaks of Legionnaires’ disease among hot spring bath users have been reported in many districts in Japan. In the present chapter, the authors used a bath model to investigate the effectiveness of backwashing the filtering medium using a high concentration of chlorine for disinfection and growth inhibition of Legionella. They then assessed the usefulness of this method from the perspective of hygiene control of circulating bathing water. Chlorine backwashing by the filter refreshment method (5 to 10 mg/liter) was performed once a day for 9 days, and the bathing water and the water from the filter unit were collected every day prior to backwashing to determine the presence of Legionella and amoebae. The number of Legionella in both bathing water and filter water was maintained at a level lower than 10 to 70 CFU/100 ml by repeated backwashing with chlorinated water alone once a day; the Legionella growth was greatly inhibited compared to that under non-disinfection conditions. On the basis of these results, daily backwashing by the filter refreshment method is considered to be effective for growth inhibition of both Legionella and host amoebae in circulating bathing water. Together with daily use of the filter refreshment method, addition of chlorine into the bathing water to a minimum concentration of 0.2 to 0.4 mg/liter may ensure the supply of circulating bathing water with increased microbial safety.