Water treatment process in waterworks (detailed)

As we all know, raw water contains various impurities due to natural and human factors. From the point of view of water treatment, these impurities can be divided into suspended solids, colloids and dissolved substances. The purpose of water purification treatment in urban waterworks is to remove suspended solids, colloidal substances, bacteria and other harmful components that are harmful to human health and industrial production, so that the purified water can meet the needs of domestic drinking and industrial production. The waterworks of the Municipal Water Supply Company adopts conventional water treatment processes, including mixing, reaction, precipitation, filtration and disinfection.

(1) Coagulation reaction treatment

After the raw water is lifted by the water intake pump house, it is first coagulated, namely:

Raw water+water treatment agent → mixing → reaction → alum flower water

The whole coagulation process from the uniform mixing of chemicals and water to the formation of large flocs is called coagulation. Commonly used water treatment agents include polyaluminum chloride, aluminum sulfate and ferric chloride. Basic aluminum chloride is used in Shantou city. According to the chemical properties of aluminum, there are ionized aluminum ions in water after chemical substances are added, and the following reversible reactions occur with water molecules:

Al3+ + 3H2O ←→ Al(OH)3 + 3H+

Aluminum hydroxide has adsorption function, which can make the colloidal particles and tiny suspended solids in water unstable and coalesce with each other, and then be adsorbed and bridged to form larger flocs, which is beneficial to separation and settlement from water.

It is required to complete the mixing process quickly after administration. The purpose of mixing is to make the drug quickly and evenly dispersed in the water through vigorous stirring by hydraulic and mechanical means.

The water treated by coagulation reaction flows into the sedimentation tank through the pipeline and enters the secondary water purification.

(2) precipitation treatment

The process that flocs formed in the coagulation stage are separated from water by gravity is called sedimentation, which is carried out in a sedimentation tank. After the water enters the sedimentation area, it is distributed along the whole water section, enters the sedimentation area, and then slowly flows to the outlet area. Particles in the water sink to the bottom of the pond, and sludge is continuously accumulated and concentrated, and discharged out of the pond regularly.

(3) filtering treatment

Filtration generally refers to the process of intercepting suspended particles in water with porous granular filter materials such as quartz sand through adhesion, so as to further remove fine suspended impurities, organic matter, bacteria, viruses and so on. In the water, and clarify the water.

(4) filtering and disinfecting.

After water is filtered, the turbidity is further reduced, and the remaining bacteria and viruses lose the protection or attachment of turbidity, which creates good conditions for disinfection after filtration. Disinfection does not destroy all microorganisms, but only requires the elimination of pathogenic microorganisms. Although most bacteria and viruses in water can be removed by coagulation, sedimentation and filtration, disinfection plays a role in ensuring that drinking water reaches the bacteriological index, while maintaining a certain amount of residual chlorine at the end of urban water pipes, controlling bacterial reproduction and preventing pollution. The disinfection chlorine dosage (liquid chlorine) is between 1.0-2.5g/m3. Chlorine reacts with water to produce hypochlorous acid, which plays an oxidation role in bacteria, destroys the enzyme system of bacteria and leads to bacterial death. The disinfected water is raised to a certain pressure from the clean water pool through the water supply pump house, and then sent to thousands of households through the water distribution network.

2. Does tap water contain substances harmful to human health?

It can be seen from the production process of tap water above that all kinds of suspended particles and colloidal substances in river water have been separated in the coagulation process. Pathogenic microorganisms in raw water are also eliminated in the disinfection process after filtration. Therefore, in the process of tap water production, the harmful substances contained in raw water have been removed.

So, what about the chemicals added in the production process? In the process of removing the original impurities in water, new impurities will inevitably be added. Will these new impurities endanger our health?

Generally speaking, the water treatment agent added in the coagulation process is precipitated together with the suspended particles and colloids of raw water, so as not to affect the water quality when leaving the factory. Then, only chlorine is left. Chlorine disinfection is the last step in tap water production. Chlorine is added to the water for reaction, and then the water can be transported to citizens' homes for use. So, will chlorine harm our health?

Let's focus on chlorine.

Chlorine (Cl2) is a yellow-green gas, which has a pungent smell and is soluble in water. At room temperature, 1 volume of water can dissolve 2 volumes of chlorine. Under the same conditions, chlorine gas is heavier than air with the same volume, and its density is 3.2 14g/L under standard conditions. Chlorine gas liquefies easily. When the pressure is 10 1.3kPa and cooled to -34.6℃, gaseous chlorine will become yellow oily liquid chlorine. Liquid chlorine is continuously cooled to-10 1℃ and becomes solid chlorine. Chlorine is a toxic substance, which has strong irritation to human body. Inhaling a small amount of chlorine can irritate nasal cavity and throat mucosa, causing chest pain and cough. Inhaling more chlorine will suffocate you.

When chlorine gas is added to water, the following reactions will occur:

Cl2 + H2O = HCl + HClO

Because the amount of chlorine used in the disinfection process is very small (generally, only about 0.005g of chlorine is introduced into 1L water), it can be said that as long as the tap water meets the normal national standards, the chlorine put into tap water will completely react with water to generate other substances, so it can be considered that the tap water does not contain Cl2. The so-called "maintaining a certain amount of residual chlorine at the end of urban tap water pipes" actually refers to chlorine, not chlorine.

However, although chlorine gas has completely reacted, other substances will be produced. Let's look at hypochlorous acid first. Hypochloric acid (HClO) has strong oxidizing property, so it has strong sterilization and disinfection ability, and it is a commonly used disinfectant. Hypochloric acid is a weak acid, which is very unstable, and the following reactions are easy to occur under light conditions:

2HClO = 2HCl + O2↑

In this way, the only impurity that may be contained in water is HCl.

Hydrogen chloride (HCl) is a colorless and irritating gas, and its density is higher than that of air, about 1.26 times that of air. Hydrogen chloride is very soluble in water (at 0℃, 1 volume of water can dissolve about 500 volumes of hydrogen chloride). The aqueous solution of hydrogen chloride is called hydrochloric acid, commonly known as hydrochloric acid, which is a strong acid with strong oxidizing and corrosive properties.

As can be seen from the above formula, according to the conservation of chlorine atoms, the amount of chlorine in a substance reacts with water to finally generate hydrogen chloride, which is twice as much as the original chlorine. Because the amount of chlorine used in the process of producing water is very small, the amount of hydrogen chloride produced is naturally very small. According to physiology and health knowledge, we know that human gastric juice contains a small amount of hydrochloric acid, so it can be considered that a small amount of hydrochloric acid does not affect human health, which is almost negligible. In addition, hydrogen chloride is a volatile gas. According to this property, it can be inferred that there is almost no hydrogen chloride in boiling water.

From this, it can be concluded that the tap water whose production process meets the national standard will not endanger human health.

Finally, we conducted a social questionnaire survey on the issue of "the influence of drinking water on human health". Through the investigation report, we found that 14.3% people drank pure water, 49% people drank tap water and 36.7% people drank well water. Among the people who drink pure water: about 36.7% people think that pure water is harmless to human body and prefer to drink it; 22.4% people think that drinking pure water is harmful to human body and don't like drinking it; In addition, about 40.9% people don't know whether drinking pure water is harmful, because most people are drinking it, so they will follow it. Most people don't drink tap water because of the serious water pollution at present, which shows that they are willing to drink natural rivers, lakes or mineral water if natural and economic conditions permit. Most people choose what kind of pure water to drink, mostly considering quality and price.

Water inlet pump-reservoir-clarifier-filter tank-metering box-filter tank-clarifier-water outlet pump

Firstly, the water is pumped from the pump house to the pool, and after preliminary filtration, it is added with water precipitant for polymerization and filtration to obtain clean water, then chlorine gas is added for disinfection (chlorine dioxide is added in small water plants), and the water is stored in the clean water pool for later use, and then pumped out by a high-pressure pump to supply water.

Process flow chart of waterworks