First, let’s understand the production process of tap water:
1. How is tap water produced?
As we all know, raw water contains various impurities due to natural factors and man-made factors. From the perspective of water supply treatment, these impurities can be divided into three categories: suspended matter, colloid, and dissolved matter. The purpose of water purification treatment in urban water plants is to remove suspended substances, colloidal substances, bacteria and other harmful components in raw water that will harm human health and industrial production, so that the purified water can meet the needs of domestic drinking and industrial production. . The water plant of the Municipal Water Supply Corporation adopts conventional water treatment processes, which include mixing, reaction, precipitation, filtration and disinfection.
(1) Coagulation reaction treatment
After the raw water is lifted by the water intake pump room, it first undergoes coagulation process treatment, that is:
Raw water + water treatment agent → Mixing → Reaction → Alum flower water
From the time when the agent and water are evenly mixed until large particle flocs are formed, the entire process is called coagulation. Commonly used water treatment agents include polyaluminum chloride, aluminum sulfate, ferric chloride, etc. Shantou City uses basic aluminum chloride. According to the chemical properties of aluminum, there are ionized aluminum ions in the water after the agent is added, and they have the following reversible reaction with water molecules:
Al3+ + 3H2O ←→ Al(OH)3 + 3H+
Al(OH)3 + 3H+
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Aluminum hydroxide has an adsorption effect, which can destabilize colloidal particles and tiny suspended solids that are not easy to settle in the water, coalesce with each other, and then be adsorbed and bridged, thereby forming larger flocs to facilitate separation from the water. , settle down.
The mixing process needs to be completed quickly after adding the medicine. The purpose of mixing is to disperse the agent quickly and evenly in the water through vigorous hydraulic and mechanical stirring.
The water treated by the coagulation reaction flows into the sedimentation tank through the pipe and enters the second stage of water purification.
(2) Sedimentation treatment
The process in which the floc formed in the coagulation stage is separated from the water by gravity is called sedimentation, and this process is carried out in the sedimentation tank. After water flows into the sedimentation zone, it is distributed along the entire cross-section of the water zone, enters the sedimentation zone, and then slowly flows to the outlet zone. The particles in the water sink to the bottom of the pool, and the sludge is continuously accumulated and concentrated, and is regularly discharged out of the pool.
(3) Filtration treatment
Filtration generally refers to using a granular filter material layer with gaps such as quartz sand to intercept suspended particles in the water through adhesion, thereby further removing small suspended impurities and Organic matter, bacteria, viruses, etc., make water clear.
(4) Post-filtration disinfection treatment
After the water is filtered, the turbidity is further reduced, and at the same time, the remaining bacteria, viruses, etc. lose the protection or attachment of turbidity, which is post-filtration disinfection. Create good conditions. Disinfection does not require the elimination of all microorganisms, but only requires the elimination of disease-causing microorganisms. Although water can remove most bacteria and viruses through coagulation, sedimentation and filtration, disinfection plays a role in ensuring that drinking water meets the bacteriological indicators of drinking water. At the same time, it maintains a certain amount of residual chlorine at the end of urban water pipes to control bacterial reproduction. and prevent pollution. The amount of chlorine added for disinfection (liquid chlorine) is between 1.0-2.5g/m3. Hypochlorous acid generated mainly through the reaction of chlorine and water acts as an oxidizing agent inside the bacteria, destroying the bacterial enzyme system and causing the bacteria to die. The disinfected water is raised from the clean water tank to a certain water pressure through the water delivery pump room, and then delivered to thousands of households through the water transmission and distribution pipe network.
2. Does tap water contain substances harmful to human health?
From the above tap water production process, it can be seen that various suspended particles and colloidal substances originally in the river water have been separated during the coagulation process. The pathogenic microorganisms in the raw water have also been eliminated during the post-filtration disinfection process. Therefore, substances harmful to human health contained in raw water have been removed during the tap water production process.
So, what about the chemicals added during the production process? In the process of removing original impurities from water, new impurities are inevitably added. Will these new impurities harm our health?
The water treatment agent added during the coagulation process usually precipitates together with the suspended particles and colloids of the raw water, thus not affecting the quality of the water when it leaves the factory. Then, only chlorine is left. Chlorine disinfection is the last step in producing tap water. After adding chlorine gas to the water, the water can be transported to households for use.
So, will chlorine harm our health?
Let’s focus on chlorine below.
Chlorine (Cl2) is a yellow-green gas with a pungent smell that can be dissolved in water. At room temperature, 1 volume of water can dissolve 2 volumes of chlorine gas. Under the same conditions, chlorine is heavier than the same volume of air. Under standard conditions, its density is 3.214g/L. Chlorine gas is easily liquefied. When the pressure is 101.3kPa and cooled to -34.6°C, the gaseous chlorine turns into yellow oily liquid chlorine. Liquid chlorine continues to cool to -101°C and becomes solid chlorine. Chlorine is a toxic substance that is highly irritating to the human body. Inhaling a small amount of chlorine can irritate the mucous membranes of the nose and throat, and cause chest pain and cough. Inhaling more chlorine can cause suffocation and death.
When chlorine is added to water, the following reaction will occur:
Cl2 + H2O = HCl + HClO
Because the amount of chlorine used in the disinfection process is very small (usually 1L Only about 0.005g of chlorine is introduced into the water). It can be said that as long as the tap water from the factory meets the normal national standards, the chlorine added to the tap water will completely react with the water to form other substances, so it can be considered that the water from the factory does not contain Cl2. The so-called "maintaining a certain amount of residual chlorine at the ends of urban water pipes" above actually refers to chlorine element, not chlorine gas.
However, although the chlorine gas has completely reacted, other substances have been produced. Let’s look at hypochlorous acid first. Hypochlorous acid (HClO) has strong oxidizing properties, so it has strong sterilization and disinfection capabilities and is a commonly used disinfectant. Hypochlorous acid is a weak acid and very unstable. It is prone to the following reaction under light conditions:
2HClO = 2HCl + O2↑
In this way, the impurities that may be contained in the water are Only HCl remains.
Hydrogen chloride (HCl) is a colorless gas with a pungent odor. Its density is greater than that of air, about 1.26 times that of air. Hydrogen chloride is very soluble in water (at 0°C, 1 volume of water can dissolve approximately 500 volumes of hydrogen chloride). The aqueous solution of hydrogen chloride is called hydrochloric acid, commonly known as hydrochloric acid. It is a strong acid with strong oxidizing and corrosive properties.
From the above equation, according to the conservation of chlorine atoms, it can be seen that the amount of hydrogen chloride finally generated after a certain amount of chlorine gas reacts with water is twice the original chlorine gas. Since very small amounts of chlorine are used in the production of water, the amount of hydrogen chloride produced is naturally negligible. According to common sense of physiological hygiene, we know that human gastric juice contains a small amount of hydrochloric acid, so it can be considered that trace amounts of hydrogen chloride do not affect human health and can be almost ignored. In addition, hydrogen chloride is a volatile gas. Based on this property, it can be inferred that boiled water contains almost no hydrogen chloride.
From this, we can draw the conclusion that tap water whose production process meets national standards will not harm human health.
Finally, we conducted a social questionnaire on the issue of "the impact of drinking water on human health." Through survey reports, we found that 14.3% of people drink pure water at home, 49% drink tap water, and 36.7% drink well water. Among those who drink purified water: about 36.7% think that purified water is harmless to the human body and prefer to drink it; 22.4% think that drinking purified water is harmful to the human body and do not like to drink it; in addition, about 40.9% People are not sure whether drinking pure water is harmful or not, because most people are drinking it, so they just drink it. Most people do not drink tap water because of the current serious water pollution situation. They express their willingness to drink natural river, lake or mineral water if natural and economic conditions permit. Most people choose what kind of purified water to drink based on comprehensive considerations such as quality and price.
Because chlorine is generally used to purify tap water, and the residual chlorine will generate carcinogens such as trihalomethanes during the heating process. Long-term drinking will cause great harm to the human body. Especially in recent years, water source pollution has become more and more serious, which directly leads to an increase in the residual chlorine content in tap water. Therefore, drinking boiled water directly from tap water is not a healthy choice. Re-purification of tap water is the key. Tap water purified by water purification equipment is called direct drinking water. It absorbs residual chlorine and other harmful substances in tap water while retaining minerals that are beneficial to the human body. It is truly healthy water. Water health cannot be ignored. We advise everyone to pay attention to their health and drink more boiled water.