Drinking water for daily life is closely related to everyone and is a necessity in all our activities and production. Therefore, many people are very concerned about the quality of drinking water, especially the drinking water in the residential area. At present, the low level of domestic residential areas is directly supplied by water plants, while the high level is supplied by secondary water. In order to ensure the health of water quality, some disinfectants are usually added to the drinking water before leaving the factory. In order to regulate the use of these disinfectants, the residual chlorine and chlorine dioxide are clearly specified in some drinking water standards. The detection standard of residual chlorine in secondary water supply can be found in the old version of Code and Standard for Sanitary Installations of Secondary Water Supply and the new version of GB 5749-2022 Sanitary Standard for Drinking Water.
If you want to know the detection standard of residual chlorine in secondary water supply, you should first understand why disinfectants should be added to our tap water, and this will further explain what health risks exist in drinking water or secondary water supply.
The drinking water used in our community comes from local water plants, and most of the water sources of the water plants come from protected surface water and some groundwater. These water sources can not be directly drunk at the beginning, but should be delivered to thousands of households after coagulation, clarification, filtration, disinfection and other steps in the waterworks. Although the water body is basically clean after so many processes, there are many links in real life that will affect the change of water quality. For example, the drinking water will contact with the pipeline during transportation, which has a certain probability of being polluted by microorganisms. If the water tank is not cleaned and maintained as required during the secondary water supply, microorganisms and bacteria will grow for a long time. In addition, if the secondary water supply in the community is lack of management, it is easy to cause secondary pollution to drinking water. The retention of chloride ions in water will solve these problems.
Residual chlorine is also known as free chlorine in drinking water. Its role is mainly to form hypochlorite through hydrolysis, and change the protein of bacteria or viruses by using the strong oxidation of hypochlorite, thus killing these microorganisms and viruses in the water. For example, coliform and salmonella are common in secondary water supply.
The existing drinking water standards have made clear provisions on the detection of residual chlorine in secondary water supply, among which the GB 5749-2022 Sanitary Standard for Drinking Water stipulates that free chlorine (residual chlorine) should be in contact with the water body for more than 30 minutes, the residual chlorine content in drinking water should be less than or equal to 2mg/L and more than or equal to 0.3mg/L when it leaves the factory, and the water at the end of the pipe network, that is, the water reaching the home of residential users, should be less than or equal to 2mg/L, ≥ 0.05mg/L. This standard must be followed when detecting the residual chlorine content in the secondary water supply. As long as the residual chlorine obtained is within the above range, it meets the standard.
If the residual chlorine content in the secondary water supply is found to be too low during the detection, it needs to be adjusted in time to increase the amount of digestion agent, otherwise the too low residual chlorine cannot play a role in sterilization and disinfection, thus affecting the water safety of residents. When the residual chlorine content in the water is too high, it is easy to cause irritation, and it is also likely to react with other organic substances to generate carcinogens such as chloroform.
Therefore, the residual chlorine in the secondary water supply shall be detected regularly according to the standard, and the problems encountered shall be handled in a timely manner.
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