Water Quality Q&A

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• [Q1]How do I find out the results of water quality testing?
• [Q2]Is residual chlorine different from chloride ions?
• [Q3]I’ve heard that chlorine produces carcinogenic substances like trihalomethanes, so you need to install a water filter. Is that true?
• [Q4]The concentration of residual chlorine has recently become weaker. Is there any reason for this?

[Q1]How do I find out the results of water quality testing?

The results of water quality testing for drinking water are given on the Water Quality Testing Results page

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[Q2] Is residual chlorine different from chloride ion?

Chlorine is added to mains water to disinfect it. This is to protect tap water against disease-causing bacteria, and by law tap water must contain at least 0.1 mg/L of residual chlorine. “Residual chlorine Erefers to chlorine in mains water which is still in a form that retains its disinfecting effect. Chloride ions, however, are formed when table salt is dissolved in water. Unlike residual chlorine, they have no oxidizing or disinfecting effect.

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[Q3]I’ve heard that chlorine produces carcinogenic substances like trihalomethanes, so you need to install a domestic water filter. Is that true?

Trihalomethanes are produced when chlorine added as a disinfectant reacts with humic substances, some of the organic substances present in water from the Yodo River. By law, the basic level of trihalomethanes must be less than 0.1 mg/l, a level determined on the basis that it will have no effect on human health if ingested continuously over the course of a lifetime. Advanced water treatment has been introduced on the basis of research into water purification methods that reduce the trace amounts of organic substances such as trihalomethanes. The current water purification system has reduced the concentration of trihalomethanes in tap water to less than a tenth of the basic level determined according to safety considerations. The low concentration in Osaka’s mains water supply means there is no need to use a domestic water filter to remove it.
Salespeople will frequently add reagents that turn mains water a yellow color, telling people that this shows poisons are present and they need to install a water filter. This is a misuse of the tolidine test for confirming the presence of residual chlorine, which actually shows that the normal amount required for disinfection is present to meet the legal requirements.
When a domestic water filter is used, be sure to carry out maintenance procedures in line with the manufacturer’s manual, such as changing the cartridge. In addition, as water filters remove the residual chlorine used for disinfection, treated water is vulnerable to contamination by bacteria and should be used quickly.

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[Q4]The concentration of residual chlorine has recently become weaker. Is there any reason for this?

To ensure the correct concentration of residual chlorine in tap water, we monitor the concentration at purification plant outlets, taking into account changes in the data from monitoring devices installed at water faucets around the city. When large amounts of organic material are present the chlorine is used up before it reaches the taps, and so regulation of the amount of chlorine at the purification plant outlet would formerly result in a concentration of over 1.5 mg/l on occasion. Advanced water treatment has greatly reduced the amount of organic material and hence the consumption of chlorine, enabling the level to be adjusted down to 0.6 mg/l. People living near purification plants used to complain about the smell of chlorine, but both the smell and the amount of residual chlorine have now lessened. The concentration of residual chlorine is still the legally required level of 0.1 mg/l even for people living in areas far from purification plants.

[Page Water Quality Q&A under Industrial Waterworks under Water Quality Management on the Osaka City Waterworks Bureau Website ends here.]