Application of chlorine dioxide in water treatment
Sep 30, 2019
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Application of chlorine dioxide in water treatment
At present, the oxidizing disinfectant used in water supply in China is mainly liquid chlorine. However, with the change of source water pollution, the content of various organic substances in the wastewater has increased. The use of liquid chlorine disinfection will produce chlorinated organic matter, and some of the products have mutagenic effects. In order to meet people's continuous improvement of water quality requirements, it is a safer and more economical new type of oxidative disinfectant that can replace chlorine, which has become a development direction for future water treatment. Among them, chlorine dioxide (ClO2) and ozone (O3) are more promising. In this chapter we talk about chlorine dioxide.
1. Chlorine dioxide (ClO2)
1.1 Application of chlorine dioxide
At the beginning of the 19th century, American scientist Dary H. discovered ClO2 gas. In the 1940s, chlorine dioxide began to be used in the sterilization of food processing, bleaching of paper and purification of water. Since chlorine dioxide does not react with organic matter to form THMs, it is increasingly used in drinking water treatment. In 1983, the US Environmental Protection Agency (EPA) proposed that the content of chloroform in drinking water must be less than 0.1 mg/L, and it is recommended to use ClO2 for disinfection. The safety of chlorine dioxide disinfection has been classified as Class A1 by the World Health Organization (WHO) and has been identified as the most ideal replacement for chlorine disinfectants. At present, there are thousands of water plants in the United States and Europe that use ClO2 for disinfection; China is mostly used in papermaking, textile and other industries, and is gradually applied to waterworks.
In the feed water treatment, ClO2 can be used not only as a highly effective disinfectant, but also in the raw water, before the sedimentation tank or before the filter tank, for pre-oxidation or intermediate oxidation to control the smell (especially chlorophenol or algae by-products) Smell, etc.), prevent microbial growth, enhance coagulation filtration; can also be used to remove iron, manganese and color in water. In addition, some countries in Europe have combined ClO2 and O3, that is, Cl2, for drinking water treatment, and achieved good results.
1.2 Physical properties of chlorine dioxide
Chlorine dioxide (ClO2) is a yellow-green gas at room temperature (20 ° C). It has a pungent odor similar to chlorine and ozone. It has a molecular weight of 67.45, is heavier than air, has a melting point of -59 ° C and a boiling point of 11 ° C.
ClO2 is very soluble in water and does not react with water. The solubility at 22 ° C is about 5 times that of chlorine, reaching 2.9 g / L. The solubility of ClO2 in water decreases with increasing temperature. At the same time, although the electronic structure of the chlorine dioxide molecule is unsaturated, it does not exist in the polymerization state in water, which is very advantageous for the rapid diffusion of ClO2 in water. However, the ClO2 aqueous solution is volatile, and ClO2- and ClO3- are formed under higher temperature and illumination, and should be stored at low temperature in the dark.
According to reports, ClO2 can be compressed into a deep red liquid at normal temperature, which is extremely volatile and extremely unstable. It will explode when exposed to light, mechanical collision or contact with organic matter; when the volume concentration in air exceeds 10% or when the concentration in water exceeds 30% There will also be an explosion. However, there is almost no risk of explosion when the concentration of ClO2 solution is below 10g/L.
Because ClO2 is sensitive to pressure, temperature and light, it cannot be compressed for liquefaction storage and transportation. It can only be prepared on site at the time of use.
1.3 Oxidation and disinfection mechanism of chlorine dioxide
As a strong oxidant, ClO2 has strong oxidizing properties under acidic conditions:
ClO2 + 4H+ + 5e = Cl- + 2H2O
Under medium conditions at a water plant pH ≈7,
ClO2 + e = ClO2-
ClO2- + 2H2O + 4e = Cl - + 4OH-
ClO2 can oxidize and remove a small amount of reducing acid groups such as S2-, SO32-, and NO2- in water, and remove Fe2+, Mn2+ and heavy metal ions in water. In addition, for the oxidation of organic matter in water, Cl2 is mainly electrophilic, while ClO2 is mainly redox, which can degrade humic acid, fulvic acid, etc., and the degradation products are not in the form of chloroform.
ClO2 is a kind of spectrum and high-efficiency disinfectant. It has been proved by experiments that it has good killing effect on bacteria, spores, algae, fungi and viruses. Regarding the disinfection mechanism of ClO2, it is generally believed that ClO2 has a good adsorption and penetration effect on the microbial cell wall, and can penetrate into the interior of the cell and react with the thiol-containing (-SH) enzyme to rapidly inactivate and inhibit it. The synthesis of intracellular proteins to achieve the purpose of inactivating microorganisms.
Since bacteria, viruses, and fungi are all single-cell low-level microorganisms, their enzymes are distributed on the surface of cell membranes and are easily inactivated by ClO2 attack. In human and animal cells, the enzyme system is protected by the system in the cytoplasm, and ClO2 is difficult to directly contact with the enzyme, so it is less harmful to humans and animals.
1.4 Oxidation and disinfection characteristics of chlorine dioxide
ClO2 is a strong oxidant, and the organic matter in the oxidized water is selective.
(1) ClO2 has strong oxidizing ability, and its oxidizing ability is 2.5 times that of chlorine. It can quickly kill pathogenic bacteria, viruses and algae (including spores, viruses and worms) in water.
(2) Unlike chlorine, the disinfection performance of ClO2 is not affected by pH. This is mainly because chlorine disinfection is sterilized by hypochlorous acid and chlorine dioxide is sterilized by itself.
(3) ClO2 does not react with ammonia or chloramine, and it can also exert a good bactericidal effect in water containing high ammonia, and disinfection with chlorine is greatly affected.
(4) The inactivation ability of ClO2 increases with the increase of water temperature, thus making up for the decrease of solubility of ClO2 in water due to the increase of water temperature.
(5) The residual amount of ClO2 can last for a long time in the pipe network, so the inactivation effect on viruses and bacteria is more effective than ozone and chlorine.
(6) ClO2 has strong decolorization, deodorization and iron and manganese removal effects.
(7) ClO2 disinfection is only selective oxidation reaction with certain organic substances, which will be degraded to oxygen-containing groups. No chlorinated organic matter is produced. The dosage is small, about 40% of the chlorine dosage. %, and is not affected by ammonia nitrogen in water. Therefore, the use of ClO2 instead of chlorine disinfection can reduce the production of chloroform in water by 90%.
1.5 Preparation and economic comparison of chlorine dioxide
The preparation method of ClO2¬ includes a chemical reaction method, an electrolytic salt method, an ion exchange method, and the like. Among them, chemical methods and electrolysis methods are widely used in production.
1.5.1 Chemical Law
The chemical reaction to prepare ClO2 is as follows:
(1) Hydrochloric acid reacts with sodium chlorite
5NaClO2 + 4HCl = 5NaCl + 4HCl + 2H2O
(2) Hydrochloric acid reacts with sodium chlorate
2NaClO3 + 4HCl = 2NaCl + 2ClO2 + 2H2O
(3) Liquid-gas mixture reaction
2NaClO2 + Cl2 = 2NaCl + 2ClO2
According to the ClO2 generator developed by the method (3), solid sodium chlorite is used in the reactor, and chlorine gas diluted with air is passed through the reactor, so that excess sodium chlorite can be maintained during the reaction to make all chlorine gas Both participate in the reaction to avoid mixing chlorine into the product. However, due to the high price of NaClO2, the cost and operating cost of this method are high and it is difficult to promote it in drinking water treatment.
At present, the general method of chemical preparation of ClO2 refers to the method (1). This method has a small production scale, simple equipment, and is convenient for automatic operation, and is suitable for production applications in water treatment; but the problem encountered is also that NaClO2 is expensive, and the theoretical yield of the method ClO2 is only 80%. To this end, a company has developed a chlorine dioxide generator that uses NaClO3 and H2SO4 to produce ClO2. The reaction principle is:
2NaClO3 + 2NaCl + 2H2SO4 = 2ClO2 + Cl2 + 2H2O + 2Na2SO4
Chlorine gas is generated in the reaction, and the user purifies the gas as needed.
2NaClO2 + Cl2 = 2NaCl + 2ClO2
It is said that the mixed gas produced by the generator accounts for 70% of ClO2, and the remaining 30% is Cl2. The ClO2 content can reach 95% after using the purifier. The generator price is less than 1/2 of that of the same size electrolytic generator, which is lower than the price of the generator using NaClO2. The equipment can be operated continuously or intermittently, and the generator can be adjusted in a wide range. At the same time, the price of NaClO3 is low, only one tenth of the price of NaClO2, the operating cost is low, and it has certain competitiveness.
1.5.2 Electrolysis
Electrolytic NaCl solution is used to produce ClO2. Using salt as raw material, a diaphragm electrolysis process is used to inject saturated brine into the anode chamber, tap water is added to the cathode chamber, and ions are directed to migrate after the power is turned on, thereby generating ClO2 and O3 around the anode chamber and the neutral electrode. , H2O2, Cl2 and other mixed gases. In production, the ClO2 yield can be improved by lowering the electrolysis temperature, controlling the flow rate of the brine, and increasing the ClO3- content of the anode chamber. The mixed gas ClO2 produced is only about 10%, and most of it is chlorine gas except O3 and H2O2. This cannot avoid the disadvantages of liquid chlorine disinfection. At the same time, the ClO2 content is difficult to calculate accurately, the equipment is complicated, the parts that are easily damaged are expensive, and the operation and maintenance are difficult. However, this method is still widely used in China.
It has also been reported that electrolysis can produce a composite disinfectant based on ClO2, which accounts for 37% of ClO2, 27% of Cl2, 15% of O3, 10% of H2O2, and 11% of others. O3 and ClO2 are first oxidatively decomposed and sterilized by the organic matter in the precipitation due to the oxidation rate of O3>ClO2>Cl2, and 27% of Cl2 can ensure sufficient residual chlorine in the water. This has a good effect on rapidly oxidizing and killing microorganisms in water and stabilizing water quality.
In addition, there have been reports on liquid stability of ClO2 and solid stability of ClO2. According to the relevant information, 10 mg/L of liquid chlorine is added for disinfection, the cost of the medicament is about 0.022 yuan/ton of water; the ClO2 made by using HCl and NaClO2 is added at 0.5 mg/L, and the cost of disinfecting tons of water is about 0.02 yuan. The use of liquid-stable chlorine dioxide and solid-stable chlorine dioxide disinfection, the cost of the drug is 0.35 yuan and 0.12 yuan per ton of water (0.5mg / L dosage), obviously economical.
1.6 Problems with the use of chlorine dioxide
When ClO2 is added to water, 50% to 70% is converted to ClO2- and ClO3-. Many experiments have shown that ClO2-, ClO3- damages red blood cells, interferes with the absorption and metabolism of iodine, and raises blood cholesterol. The US EPA recommends that the total amount of residual oxidants (ClO2+ClO2-+ClO3-) <1.0 mg/l="" during="" chlorine="" dioxide="" disinfection="" will="" not="" affect="" the="" health="" of="" normal="" people.="" in="" practice,="" the="" dose="" of="" clo2="" is="" controlled="" below="" 0.5="">
ClO2 oxidizes and decomposes organic matter with strong selectivity. It can oxidize and remove Fe2+, Mn2+, cyanide, phenol, etc. in water; it can oxidize thiol, secondary and tertiary amines, eliminate unpleasant odor in water, but it is not easy to oxidize organic substances such as alcohol, aldehyde, ketone and primary amine, resulting in removal. thorough.
Chlorine dioxide is more active, explosive, and inherently toxic. Therefore, pay great attention to safety when using ClO2. Generally, the dilution concentration of raw materials should be strictly controlled in the ClO2 preparation system to prevent misoperation and corresponding safety measures should be established. ClO2 storage should be kept away from light at low temperature; ClO2 workshop should disable fire and set up good ventilation and ventilation equipment.
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