Talk about residual chlorine

Residual chlorine refers to the amount of chlorine remaining in water after chlorine disinfection for a certain period of time. There are three forms of residual chlorine in water: total residual chlorine, free residual chlorine and combined residual chlorine. The role of residual chlorine is to ensure continuous sterilization and also to prevent water from being re-polluted. However, if the amount of residual chlorine exceeds the standard, it may aggravate the odor produced by phenols and other organic substances in the water, as well as organic chlorine compounds that may be mutagenic, teratogenic and carcinogenic. Free chlorine is the form of chlorine gas dissolved in water. Like chlorous acid and chloric acid, it has strong oxidizing properties and also has bactericidal properties. The smell and taste threshold concentration of free residual chlorine is 0.2-0.5mg/L. Sterilization effect: Experiments have shown that when the free residual chlorine is above 0.3mg/L within 30 minutes of contact, it can kill intestinal pathogenic bacteria (such as typhoid fever, dysentery, etc.), Leptospira, Brucella, etc. If chloramine is used for disinfection, the content of combined residual chlorine is generally more than twice that of free residual chlorine, and the contact time should not be less than 2 hours. Enteroviruses (infectious hepatitis virus, polio virus, etc.) are more resistant to chlorine disinfectants than enteropathogenic bacteria. According to reports, if the free residual chlorine can be guaranteed to be 0.5mg/L and the contact time is 30-50 minutes, the enterovirus can also be killed. Therefore, when it is suspected that the water source may be contaminated by enteroviruses, the dosage and contact time of chlorine disinfection can be increased to ensure the safety of drinking water. The free residual chlorine in the terminal water of the centralized water supply pipe network can also be used as a signal to indicate whether there is re-pollution. Therefore, the water quality standard also makes corresponding regulations on the free residual chlorine in the terminal water of the pipe network. The standard stipulates that 'after 30 minutes of contact with water, it should not be lower than 0.3mg/L, and the water at the end of the pipe network should not be lower than 0.05mg/L (suitable for chlorine disinfection)'.