Does tap water contain substances harmful to human health?

From the above tap water production process, it can be seen that the original suspended particles and colloidal substances in river water have been separated in the coagulation process. Pathogenic microorganisms in raw water have also been eliminated in the process of disinfection after filtration. Therefore, the harmful human health substances contained in raw water have been removed in the production process of tap water.

So, what about the medicines added in the production process? New impurities are inevitably added to the process of removing the original impurities in water. Will these new impurities endanger our health?

In general, the water treatment agent added in the coagulation process precipitates with the suspended particles and colloids of raw water, thus not affecting the quality of the water when it leaves the factory. So, there's only chlorine left. Chlorine disinfection is the last step in the production of tap water. After chlorine gas is added to the water, the water can be transported to citizens'families for use. So, will chlorine be harmful to our health? Below we will focus on chlorine gas.

Chlorine (Cl2) is a yellow-green, irritating odor gas, which can dissolve in water. One volume of water can dissolve two volumes of chlorine at room temperature. Under the same conditions, chlorine is heavier than air of the same volume. Under standard conditions, its density is 3.214 g/L. Chlorine gas is easy to liquefy. When the pressure is 101.3 kPa and it is cooled to - 34.6 C, the gaseous chlorine becomes yellow oily liquid chlorine. When liquid chlorine continues to cool to - 101 C, it becomes solid chlorine. Chlorine is a toxic substance, which has strong irritation to human body. Inhalation of a small amount of chlorine can stimulate nasal and laryngeal mucosa, and cause chest pain and cough. Inhalation of more chlorine can cause suffocation and death. When chlorine is added to water, the following reactions occur: Cl2 + H2O = HCl + HClO;

Because the amount of chlorine used in disinfection process is very small (generally only about 0.005g chlorine is introduced into 1L water), it can be said that as long as the tap water from the factory meets the normal national standards, the chlorine gas input in tap water will react with water completely to produce other substances, so it can be considered that the water from the factory does not contain Cl2.

The so-called "keeping a certain amount of residual chlorine at the end of urban water pipes" mentioned above should actually refer to chlorine elements, not chlorine gas.

However, although chlorine has been completely reacted, other substances are produced. Let's start with hypochlorite. Hypochlorite (HClO) is a commonly used disinfectant because of its strong oxidation and strong disinfection ability. Hypochlorite is a weak acid, which is very unstable. It is easy to react under light conditions as follows: 2HClO = 2HCl + O2_;;;;;;;;

In this way, the only impurities that may be contained in the water are HCl.

Hydrogen chloride (HCl) is a colorless and irritating gas with a density about 1.26 times higher than that of air. Hydrogen chloride is very soluble in water (about 500 volumes of hydrogen chloride can be dissolved in 1 volume of water at 0 C). Hydrochloric acid, commonly known as hydrochloric acid, is a strong acid with strong oxidation and corrosiveness.

According to the above equation and the conservation of chlorine atoms, it can be seen that the amount of hydrogen chloride produced by the reaction of chlorine with water is twice as much as that of chlorine. Because the amount of chlorine used in the process of water production is very small, the amount of hydrogen chloride produced is naturally very small. According to the common sense of physiology and health, we know that human gastric juice contains a small amount of hydrochloric acid, so we can think that trace hydrogen chloride does not affect human health, and can almost be neglected. In addition, hydrogen chloride is a volatile gas. Based on this property, it can be inferred that boiled water contains almost no hydrogen chloride.

Thus, we can draw the conclusion that tap water which meets the national standards in production process will not endanger human health.

Finally, we conducted a social questionnaire on the impact of drinking water on human health. According to the survey report, we found that 14.3% of the households drank pure water, 49% drank tap water and 36.7% drank well water. Among the people who drank pure water, about 36.7% thought that pure water was harmless to human body and preferred to drink; 22.4% thought that drinking pure water was harmful to human body and did not like drinking; in addition, about 40.9% did not know whether drinking pure water was harmful, because most people were drinking, so they followed suit. Most people do not drink tap water because of the current serious water pollution situation, indicating that if natural economic conditions permit, they are willing to drink natural river, lake or mineral water. Most people choose which kind of pure water to drink, mostly from the quality, price and other aspects of comprehensive consideration.