Pretreatment of tap water raw water by replacing chlorine with chlorine dioxide

Application status of chlorine dioxide disinfection technology

Water is the source of life. The pollution of the water environment and the shortage of water sources have become one of the important challenges currently facing. The quality of drinking water has received extensive attention from people. Unsustainable development has also spurred an international call for the quality of drinking water in China.

In the traditional water purification process, liquid chlorine pre-oxidation and disinfection processes are generally employed. Since the discovery that chlorine disinfection can lead to the production of "tri-" compounds such as trihalomethanes and haloacetic acids, the by-products of the disinfection of chlorine and chlorine derivatives and their hazards have become more and more concerned. Controlling disinfection by-products in drinking water, and actively finding alternatives to chlorine, safer and superior disinfection methods and disinfectants have become one of the top priorities in the water supply industry. Chlorine dioxide is favored by the water treatment industry because of its strong bactericidal ability and bactericidal durability, and does not produce by-products of chlorine disinfection.

Chlorine dioxide has a broad-spectrum bactericidal ability and is a relatively good disinfectant. It kills pathogenic microorganisms in water generally better than chlorine, and its disinfection effect is basically not affected by pH value. At the same time, it can also remove water. A variety of harmful substances (such as iron , manganese , sulfide, cyanide and nitrite). The biggest advantage of drinking water chlorine dioxide disinfection technology is that it does not generate harmful halogenated organic substances in the process of purifying drinking water. The World Health Organization ranks its safety as AI grade. The United States lists chlorine dioxide as an alternative chlorine. Disinfectant.

In developed countries, the use of chlorine dioxide in the purification of drinking water has become widespread. At present, there are about 1,000 water plants in the United States that use chlorine dioxide, making it the third-largest disinfectant in the United States after chlorine and chloramine. In Europe, the use of chlorine dioxide disinfection is more common, such as more than 70% of drinking water plants in Germany use chlorine dioxide for secondary disinfection. Since the 1990s, China has begun testing and productive applications of chlorine dioxide in some small and medium-sized water plants. At present, it has been used in many water plants, and it has gradually become one of the preferred disinfection methods in railway water supply. Hengshui and Renqiu on the Beijing-Kowloon Railway, which meets the Yellow Railway, use chlorine dioxide disinfection in the water supply. With the intensification of water pollution in China's water sources and the improvement of people's requirements for water quality, chlorine dioxide purification will have a wider market.

Second, the characteristics of chlorine dioxide

(1) Physical and chemical properties

Chlorine dioxide is a yellow-green to orange gas at room temperature, the color change depends on its concentration; there is a pungent odor similar to chlorine and ozone; the boiling point is 11 ° C, the melting point is -59 ° C, and the vapor at 0 ° C The pressure is 6.53 × 10 4 Pa, its solubility in water is 8.3 g / L, and the dissociation constant under pH neutral conditions is 1.2 × 10 -7 , that is, it remains substantially in a state of no dissociation. Chlorine dioxide has a high volatility and escapes from the solution with a slight aeration.

Chlorine dioxide is an easily explosive gas. When the content of chlorine dioxide in the air is more than 10% or the content of the aqueous solution is more than 30%, it is easy to explode; when it is heated and exposed to light or organic matter, it can accelerate decomposition and cause explosion.

Chlorine dioxide is easily soluble in water, soluble in alkaline solution, sulfuric acid. It is easy to be volatilized when dissolved in water. It decomposes into hypochlorous acid, chlorine gas and oxygen when it is heated. It is easily decomposed by light.

Chlorine dioxide is a strong oxidant. Its effective chlorine is 2.6 times that of chlorine. It reacts violently with many substances and is highly corrosive.

(2) Oxidation characteristics

Chlorine dioxide has a strong oxidizing power, and its oxidation-reduction potential E 0 = -1.50V. Its theoretical oxidizing power is 2.63 times that of chlorine, but weaker than ozone. Based on its strong oxidation, it is capable of rapidly oxidizing and decomposing bacteria, viruses, amino acids in proteins, and many reducing substances. Chlorine dioxide can interact with a variety of inorganic ions and organic matter. Therefore, chlorine dioxide can remove a variety of harmful substances in water while disinfecting.

(3) Disinfection characteristics

Chlorine dioxide has a wide range of inactivation of microorganisms and strong inactivation ability. In addition to killing common bacteria, it also has autotrophic bacteria such as Escherichia coli, iron bacteria, sulfate-reducing bacteria, etc., polio. Virus, hepatitis virus, Giardia cysts, etc. also have a good killing effect, and the disinfection effect is basically not affected by the pH value, does not produce drug resistance, and the sustained sterilization ability is 2 times longer than chlorine. In addition, there are significant advantages that do not substantially interfere with organic substances in water to produce harmful halogenated organic substances. The organic by-products in the disinfection are mainly low molecular weight acetamidine, carboxylic acid, etc., and the content is much lower than the ozone oxidation product. The effect on reducing cations and cations is mainly from the harmful state to the harmless state. Chlorine dioxide is less expensive to disinfect than ozone. The use of it as a disinfectant for micro-contaminated tap water is the most worthwhile consideration. Since bacteria, viruses, fungi, etc. are all single-cell low-level microorganisms, their enzymes are distributed on the surface of the cell membrane and are easily inactivated by chlorine dioxide attack. In human and animal cells, the enzyme system is distributed in the cytoplasm and protected by external systems. Chlorine dioxide is difficult to contact with the enzyme, so it is less harmful to humans and animals. Compared to chlorine, it diffuses faster in water, so that microorganisms can be inactivated more quickly and efficiently at lower concentrations.

(4) Other characteristics

Chlorine dioxide also has the characteristics of deodorization, decolorization, and removal of iron and manganese.

Third, the preparation technology of chlorine dioxide

There are dozens of methods for preparing chlorine dioxide, which can be divided into three major categories: chemical method, electrolysis method and stable chlorine dioxide activation method.

(1) Chemical law

Chemical methods are currently the most widely used methods in drinking water purification, including the chlorate method, the chloric acid method, and the chlorite method.

The chlorate method is to prepare chlorine dioxide by reducing sodium chlorate with a reducing agent in a highly acidic medium, and the efficiency varies depending on the reducing agent. The main reducing agent currently has hydrochloric acid, methanol, sodium oxide and the like. The purity of chlorine dioxide in the chlorate hydrochloride method is below 70%, and its production cost and initial investment are low. Domestically produced generators generally adopt this method.

Chlorine dioxide prepared by the chlorite method has high purity and is classified into an acidification method and an oxidation method. Oxidation is the reaction of chlorite with chlorine or hypochlorous acid to produce chlorine dioxide. This method is widely used by foreign large water plants. The acidification method is to react chlorite with an acid (mainly hydrochloric acid) to produce chlorine dioxide, but the reaction rate is slow, and the amount of hydrochloric acid is large, which is only suitable for small-scale production. The chlorite method is costly in this method because of the higher chlorite content than the chlorate. In addition, chlorite is a strong oxidant and is susceptible to explosions due to external conditions.

The chloric acid method is to prepare chloric acid first, and then catalyze and reduce to prepare chlorine dioxide, which is mainly used for industrial professional production.

(2) Electrolysis

The electrolytic solution is prepared by electrolysis to produce chlorine dioxide by electrolysis using sodium chloride solution and sodium chlorate solution. However, the diaphragm and the electrode have a limited life, and the chlorine dioxide concentration is low, the equipment is complicated, and the operation and maintenance are difficult.

(3) Chlorine dioxide activation method

Chlorine dioxide is unstable and generally needs to occur on site. For this reason, people produce high-purity chlorine dioxide, which is stabilized by a stabilizer such as carbonate to facilitate storage and transportation. In use, an activator such as hydrochloric acid is used. It is activated to produce chlorine dioxide. The stable chlorine dioxide is easy to use, but the price is high, and it is only suitable for small-scale water disinfection in areas with good economic conditions.

Economic analysis of chlorine dioxide disinfection

(1) The dosage of chlorine dioxide

The amount of chlorine dioxide added is related to the water quality being treated. According to the data, when the concentration of bacteria is 105-106/mL, 0.5mg/L of chlorine dioxide can kill more than 99% of the oxygen-oxidizing bacteria after 5 minutes, and the effect is relatively long, 0.5mg/L of two. The killing ability of oxidized chlorine to isoxic bacteria was maintained above 99% within 12 hours, and the bactericidal capacity was still 86.3% after 24 hours of action. In order to ensure the microbiological safety of drinking water, the dosage C of chlorine dioxide must meet three requirements: the residual amount of plant water R F , the consumption of microbial and reducing substances C 1 and the water supply directly in contact with water. Facility consumption C 2 :

C=R F +C 1 +C 2

The residual amount R F of the factory water is in the range of 0.05-0.1 mg/L, and the sum of the consumption C 1 of the microorganisms and reducing substances and the consumption amount C 2 of the water supply facilities directly in contact with water, and the actual application situation Relevant, generally need to be determined by experiments, for micro-polluted water, between 0.1 ~ 0.6mg / L can be. In addition, when the chlorine dioxide dosage exceeds 0.5 mg/L, it may cause the chlorite to exceed the standard. Therefore, the dosage of chlorine dioxide should be determined for different applications.

(2) Economic analysis

In the current common water supply disinfection method, according to the cost analysis, the liquid chlorine disinfection cost is the lowest, followed by chloramine, chlorine dioxide is in the middle, and ozone is the highest.

The cost of chlorine dioxide disinfection mainly includes three parts: civil construction costs, equipment purchase costs, and operating costs. Operating costs mainly include depreciation, maintenance, pharmacy, utilities, and labor.

Among the several methods commonly used in chlorine dioxide disinfection, the dosage of chlorine dioxide is 0.5mg/L, the amount of treated water is calculated according to 10000m 3 /d, and other disinfection methods are calculated according to the lower limit of dosage, according to the current domestic market and The cost statistics of various disinfection methods after calculation of raw material prices (see Table 1) are shown in Table 2.

It should be noted that economic analysis only reflects the cost of applying a certain condition. Due to the different equipment used, the amount of treated water and the different application environmental conditions, the access is very large, and some equipment and treatment methods are applied to each. The investment and operating costs of different scales and conditions of water plants are different, and even some equipment is sometimes difficult to use in some units.

Chlorite and chlorine dioxide are strong oxidants. The safety issues in use, storage and transportation should be considered comprehensively. In the design and operation plan, the relevant regulations of the “Safety Management Regulations for Hazardous Chemicals” should be strictly implemented.

In view of the application conditions and characteristics of railway water supply points, line length and small water volume, it is necessary to consider those schemes and equipments that are safe in equipment, high in automation, easy to operate, repair, maintenance and use.

V. Some problems exist

(1) During the treatment of water with chlorine dioxide, by-products such as chlorite and chlorate are produced due to the action of substances contained in the water. When the chlorine dioxide generator is not operating well, hydrochloric acid, chlorite, chlorate, etc. are easily caused to enter the water body, thereby causing more by-products and causing secondary pollution.

(2) Chlorine dioxide must be prepared on site and is inconvenient to use.

(3) Some harmful gases are generated in some systems.

(4) Regardless of the method and method used for water disinfection, water quality testing and monitoring should be strengthened.

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