■Where to collect water? According to two national standards, for rectangular pools, the principle is to collect water from three or more points that are almost evenly spaced along the diagonal, at a depth of 20 cm below the water surface. Free residual chlorine is also measured near the intake of the circulation filtration system. Checking the concentration near the intake is to prevent bacteria from proliferating inside the filtration system. ■What is in the bottle for testing bacteria? Samples for E. coli and general bacteria are collected in sterilized bottles that have been pre-filled with sodium thiosulfate to neutralize residual chlorine. After collection, testing should be conducted promptly, and if it cannot be done immediately, the samples should be stored in a cool, dark place and tested within 12 hours of collection. ■Differences in the frequency of testing between schools and the general public 1. Schools: The main water quality parameters are tested once every 30 days within the cumulative usage days. Free residual chlorine is checked at least once before use and once every hour during use as part of daily inspections. 2. General public: Parameters such as pH and bacteria are tested at least once a month. Free residual chlorine is tested at least once every morning and at least twice in the afternoon. ■Timing for measuring total trihalomethanes Total trihalomethanes are measured at least once a year during warmer periods for general pools, and at least once during an appropriate time within the usage period for schools.

■Why is there no reading even when chlorine is added? The reasons for not detecting free residual chlorine even after adding chlorine agents include the possibility that chlorine is being used to break down contaminants, degradation of the color reagent in the measuring device, or contamination of the colorimetric tube. When in doubt about the reagent, you can verify by measuring tap water. This is because tap water is maintained at a residual chlorine level of 0.1 mg/L or higher according to the Water Supply Act. ■What problems arise if the pH is off? When the pH shifts towards acidity, the disinfection effect is enhanced, but it can lead to concrete deterioration, pipe corrosion, and decreased purification ability. If it shifts towards alkalinity, the disinfection effect decreases. It is necessary to monitor not only the amount of chlorine but also the pH. ■Handling of chlorine According to general pool standards, swimming should be temporarily halted if free residual chlorine falls below 0.4 mg/L, and chlorine agents should be added to bring it back to 0.4 mg/L or higher before resuming. At Torii, we handle easily transportable sodium hypochlorite, solid chlorine agents, pH adjusters, and other chemicals used for pool water quality management. Please feel free to consult us first.