Since the first use of chlorine to disinfect water supply networks in Britain in 1897, chlorine has been used to disinfect tap water for more than 100 years. At present, most cities in my country still use chlorine disinfection. Chlorine is added to the water to generate hypochlorous acid HOCl and hypochlorite. Among them, hypochlorous acid is a small neutral molecule that easily diffuses to the surface of negatively charged bacteria, and through the bacterial wall to the inside of the bacteria, destroying the bacteria due to oxidation Hypochlorite also has bactericidal ability, but because it is negatively charged, it is not easy to access the surface of negatively charged bacteria, and its bactericidal ability is much worse than HOCl.
Use a chlorine bottle to store liquid chlorine for disinfection. At room temperature, when the chlorine valve is opened as required, the liquid chlorine turns into gaseous chlorine. Chlorine is a yellow-green toxic gas with strong oxidizing ability, and it is a special strong pungent gas. Suffocating gas. Principle of chlorine disinfection:
Chlorine is easily soluble in water and combines with water to form hypochlorous acid and hydrochloric acid. Hypochlorous acid plays a major role in the entire disinfection process. For odor-producing inorganic substances, it can be thoroughly oxidized and disinfected. For living natural substances, such as algae and bacteria, it can penetrate cell walls and oxidize its enzyme system (enzymes are biocatalysts) to make them Loss of activity makes the life activities of bacteria obstructed and die. Hypochlorous acid itself is electrically neutral, and it is easily accessible to bacteria and shows good sterilization effect. Hypochlorite ion also has a certain disinfection effect, but it is negatively charged and difficult to access bacteria (bacteria are negatively charged) Therefore, compared with hypochlorous acid, its sterilization effect is much worse, so chlorine disinfection effect is better than using bleaching powder.
When there is no ammonia nitrogen in the water, hypochlorous acid is the main disinfectant; when there is ammonia nitrogen in the water, chloramine will be formed, and as the amount of chlorination continues to increase, the properties of chloramine will change, and monochloramine will be generated respectively. , Dichloramine, Trichloramine. The disinfection effect of chloramine is much slower than that of hypochlorous acid.
(2) Sodium hypochlorite disinfection
The available chlorine in sodium hypochlorite is easily decomposed by the influence of sunlight and temperature, so sodium hypochlorite generator is used to manufacture and apply it on site. Sodium hypochlorite generator uses titanium anode to electrolyze salt water to produce sodium hypochlorite. The reaction formula is:
NaCl+ H2O → NaClO +H2
The reaction formula of sodium hypochlorite in aqueous solution is: NaCIO +H2O → HOCl +Na++0H-
The sodium hypochlorite produced by the generator is a light yellow transparent liquid. The sterilization principle of sodium hypochlorite is mainly hydrolyzed to form hypochlorous acid for disinfection.
(3) Chloramine disinfection
When the raw water has a lot of organic matter and high ammonia content, and the algae and bacteria in the water supply pipe network may re-grow, it is really difficult to achieve that the factory water is all free chlorine, or the chlorophenol odor in the tap water needs to be reduced or avoided , You can consider using chloramine for disinfection. Chloramine disinfection is to add chlorine and ammonia to the water at the same time. Ammonia can be liquid ammonia, ammonium sulfate or ammonium chloride solution; the general weight ratio is controlled at ammonia: chlorine=1:3 or 1:6, and 1 when the water temperature is high ：3, 1:6 when the water temperature is low; in addition to controlling the amount of chlorine and ammonia, the order of dosing should also be paid attention to, generally "chlorine first, then ammonia". Chlorine reacts with ammonia to form chloramine. If natural water contains ammonia nitrogen, it can also react with chlorine to form chloramine. Chloramine disinfection lasts for a long time, so when the water supply pipe and water supply pipe network are very long, it can prevent bacteria from growing again in the pipe network, but chloramine gradually releases hypochlorous acid, and its sterilization ability is far worse than free chlorine. Therefore, a contact time of more than 2 h is required.
(4) Bleaching powder disinfection
Bleaching powder is made of chlorine and lime. Its composition is complex. The chlorine content is 25%~30%. The molecular formula of bleach is Ca(OCl)2 and the chlorine content is 50%~60%. It is easy to be exposed to light, heat and moisture. It is decomposed by the effect, and the chlorine content is reduced. Store in a dry and ventilated place. The reaction formula of bleaching powder and water is:
Ca(OCl) 2 +2H2O- 2HOCl+Ca(OH)2 +CaCl2
The principle of disinfection is the same as that of chlorine.
(5) Chlorine dioxide
In recent years, water plants have successively adopted chlorine dioxide generators. The chlorine dioxide solution produced by the generators disinfects the tap water. Chlorine dioxide is a yellow-green gas with a pungent odor. It is quite stable in aqueous solution and can be used as Disinfectants are used safely. The color of its aqueous solution is related to the concentration. It is yellow-green at low concentration and orange-red at high concentration. It is a high-efficiency oxidant with powerful decoloring and bleaching ability. Chlorine dioxide generator is a kind of equipment with very low operating cost, accurate drug dosing and excellent disinfection effect. It does not contain the complicated or harmful components produced by the chlorine plant. It uses sodium chlorate and hydrochloric acid for quantitative titration and control The total reaction is expressed as follows:
NaClO + 2HCl- NaCl+Cl2 + 0.5ClO2+ 2H2O
It has a good disinfection effect on pathogenic microorganisms transmitted by water, including viruses, tooth spores and heterotrophic bacteria, sulfate reducing bacteria and fungi in the waterway system. Its main function is to have better adsorption and permeability to the cell wall, can effectively oxidize the enzymes in the cell, and can quickly control the synthesis of microbial protein. Chlorine dioxide first reacts with water to produce chlorous acid HClO2. Chlorous acid is a rather weak weak acid with oxidative bleaching effect.
Chlorine dioxide is considered an ideal substitute for chlorine disinfectant. The disinfection mechanism of chlorine dioxide is mainly through adsorption and osmosis, enters the cell body, oxidizes the enzyme system and biological macromolecules in the cell, and kills bacteria and viruses well, and does not damage animals and plants. The sterilization effect lasts for a long time. , Not sensitive to PH. The reaction between chlorine dioxide and organic matter in water is oxidation, while chlorine is mainly a substitution reaction.
Ozone is a strong oxidant, which can automatically decompose into oxygen, which is inflammable and explosive. Ozone is often prepared with air as a raw material. The air must be purified and dried to increase ozone productivity, prevent equipment fouling and reduce energy consumption. The solubility of ozone in water is limited, and a contact room between ozone and water, as well as exhaust gas recycling and disposal equipment are necessary to improve the efficiency of use. However, due to the complexity of ozone production equipment, large investment and high operating costs, it has not been widely promoted.
The disinfection mechanism of ozone includes direct oxidation and indirect oxidation that produces free radicals. Like chlorine and chlorine dioxide, the structure of microorganisms is destroyed by oxidation to achieve the purpose of disinfection. Therefore, the disinfection effect is directly related to its redox potential. Ozone molecules are unstable and easily decompose on their own. The retention time in the water is very short, so the continuous disinfection efficiency of the pipe network cannot be maintained. Moreover, ozone disinfection produces bromate, aldehydes, ketones and other by-products. Among them, bromate is in the water quality standard. There are regulations that some of the by-products such as aldehydes and ketones are harmful compounds, and some of them reduce the biological stability of the pipe network. Therefore, ozone disinfection is subject to certain restrictions in use. For large and medium-sized pipe network systems, chlorine must be used for ozone disinfection to maintain the continuous disinfection effect in the pipe network.
Chlorine, chlorine dioxide, and ozone disinfection are chemical disinfection, while ultraviolet light is physical disinfection. Ultraviolet light refers to light waves with electromagnetic wave wavelengths between 200 and 380 nm. The mechanism of ultraviolet disinfection is different from other oxidants. It uses wavelengths of 254 nm and its vicinity. The damage of the area to the microbial DNA prevents protein synthesis and prevents bacteria from reproducing. Due to the high efficiency of ultraviolet rays to kill cryptosporidium and no by-products, ultraviolet disinfection shows good market potential in water supply treatment.
Because chlorine disinfection cannot effectively kill Cryptosporidium oocysts, while ultraviolet rays have a good effect on Cryptosporidium oocysts. Moreover, ultraviolet disinfection does not produce harmful by-products within the range of conventional disinfection doses, but ultraviolet disinfection cannot maintain the continuous disinfection effect in the pipe network. The application in large water plants must be combined with chlorine, and its use is currently restricted.
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