1. Properties of ozone
Ozone is an unstable active gas. It has a special smell at room temperature and the gas will appear light blue. The redox potential of ozone in water is 2.07V, which is currently the second strongest oxidant after fluorine. The application of ozone in wastewater treatment mainly utilizes this feature.
As far as the current situation is concerned, ozone decomposes faster in aqueous solution than in gas phase. The decomposition of ozone in water is mainly affected by temperature and pH value. As the temperature continues to rise, the decomposition rate is also gradually accelerating. When the temperature reaches above 100°C, the decomposition will be very intense. When the temperature reaches above 270°C, it will be directly converted into oxygen. The pH value is also directly related to the decomposition rate. The decomposition half-life in the air at room temperature is 15 to 30 minutes.
2. Analysis of the principle of ozone oxidation
Ozone is a strong oxidant, and its oxidizing ability is much higher than that of chlorine and chlorine dioxide. With the continuous development of society, the requirements for water resources are also getting higher and higher. Some developed countries have used some oxidation technologies such as ozone for sewage treatment to better ensure water quality.
At present, the ozonation process mainly includes two aspects: one is direct ozonation reaction. Two indirect catalytic reactions.
In the process of direct ozonation reaction, two methods are mainly used, namely dipole addition reaction and electrophilic substitution reaction. The main reason for the dipole addition reaction is that ozone has a dipole structure, so during the reaction, it will react with organic matter containing unsaturated bonds to meet the requirements. The electrophilic substitution reaction is mainly because aromatic compounds with electron-withdrawing groups, including -CO OH, -NO 2, -Cl and other groups, are difficult to react with ozone, so when such reactions occur, they will have a certain selectivity. Generally, direct oxidation of organic matter by ozone occurs best under acidic conditions. Although the reaction is slow, it has a good selectivity function, and the oxidation product is also an organic acid. It is difficult to reoxidize, and the response speed of each organic matter is also very different.
Although ozone has strong oxidizing properties, it is difficult to remove sewage during the reaction due to its high selectivity. With the continuous development of science and technology, more and more research has been done in this area, and ozone water treatment has been continuously improved. At present, the purpose of degrading organic matter is achieved by using homogeneous catalysis and heterogeneous catalysis of ozone.
Indirect catalytic reactions are mainly that ozone can directly or through free radicals produced by triggering reactions, proliferation reactions and termination reactions to oxidize a variety of compounds, and each reaction produces different free radicals. The reaction rate of free radicals and organic matter in water is very fast, and no selection is required. The key part is hydroxyl free radicals. Hydroxyl free radicals are the most common oxidants, and their oxidation electrode potential is only lower than that of chlorine. Its advantage is that it can react quickly with organic matter, and it does not need to be selected. It is easy to react with organic matter at different positions in the gas to produce easily oxidized intermediates. For these free genes, the reaction speed is very fast. The current reaction rate has reached 106~109L/mol s, so the catalytic ozone reaction speed of each organic compound is similar, which also causes the low selectivity of free radical reactions.
Ozone oxidation treatment of wastewater uses air or low-concentration ozone containing oxygen. Ozone is an unstable and easily decomposed strong oxidant, so it must be produced on site. The process facilities for ozone oxidation water treatment are mainly composed of ozone generators and gas-water contact equipment. The only way to produce ozone on a large scale is the silent discharge method. The raw gas for producing ozone is air or oxygen. The raw gas must be purified by degreasing, dehumidification, dust removal, etc., otherwise it will affect the ozone production rate and the normal use of the equipment. The concentration of ozone produced from air is generally 10-20 mg/L. The concentration of ozone made from oxygen is 20-40 mg/L. This air or oxygen containing 1% to 4% (weight ratio) of ozone is the ozonated gas used for water treatment.
Ozone oxidation treatment of wastewater uses air or low-concentration ozone containing oxygen. Ozone is an unstable, easily decomposed strong oxidant, so it must be produced on site. The process facilities for ozone oxidation water treatment are mainly composed of ozone generators and gas-water contact equipment. The only way to produce ozone on a large scale is the silent discharge method. The raw gas for producing ozone is air or oxygen. The raw gas must be purified by degreasing, dehumidification, dust removal, etc., otherwise it will affect the ozone production rate and the normal use of the equipment. The concentration of ozone produced from air is generally 10-20 mg/L. The concentration of ozone produced from oxygen is 20-40 mg/L. This air or oxygen containing 1% to 4% (by weight) of ozone is the ozonated gas used for water treatment.
Through the catalytic oxidation treatment of microbubble ozone, the COD removal rate of wastewater can be further improved and the utilization rate of ozone can be fully utilized. In general, when using this method for pretreatment, it will be effectively combined with other methods, such as aerated biological filter (Blackrock Municipal Income Inve), to further improve the removal efficiency of organic matter in wastewater, while ensuring the formation of an effective post-treatment sequence.
3.3 Effect of temperature.
Through the Arrhenius formula (Equation 3), the overall temperature can be further increased, the reaction rate can be further increased, and the ozone catalytic oxidation reaction can be effectively carried out. As the temperature continues to rise in this process, the solubility of ozone is also decreasing, which will further reduce the driving force of gas-liquid mass transfer and reduce the rate. It can be seen that the increase in temperature and the reaction rate are inversely proportional to the gas-liquid mass transfer rate. In practical applications, it is necessary to effectively adjust the temperature of wastewater, which will further increase the consumption rate. Therefore, for each catalytic reaction system, it is necessary to operate in combination with the actual situation.
4. Application of ozone oxidation technology in wastewater treatment
4.1 Application in refinery wastewater treatment
Using ozone catalytic oxidation technology for experimental research, the overall decontamination effect can be further regulated.
As far as the current situation is concerned, the double membrane process has been widely used and applied in deep treatment, but due to the increasing salt content of its concentrated water, the treatment difficulty has been further increased. The homemade catalyst and its ozone catalytic oxidation reactor and double membrane device combination have achieved good results in deep treatment and refining, and the industrial device has been determined in combination with the actual situation to enable it to meet the relevant requirements.
4.2 Application in industrial wastewater treatment
4.2.1 Application in organic wastewater treatment
At present, we will choose to study fine chemical organic wastewater mainly containing dye intermediates and pharmaceutical intermediates, mainly for their low treatment efficiency. Through experimental control, the reaction conditions are effectively selected, and the removal of COD rate, decolorization rate, BOD5/COD and other indicators are studied. From the results, it can be seen that Mn/C synergistic ozone has the best effect and takes less time. In this process, the pH value reaches 9, and the removal efficiency of the port also reaches 91.6% and 34.9%. Through analysis, it can be seen that since ozone oxidation will destroy the unsaturated groups in the wastewater, the compounds are converted. The gold-jade ratio after treatment will increase the BOD5/COD of the raw water, which plays a very important role in the subsequent biological treatment.
4.2.2 Application in the treatment of printing and dyeing wastewater
The overall experiment uses cordierite honeycomb ceramics, diatomaceous earth, activated alumina and activated carbon as carriers, and its main components are FexOy, CuO, NiO, MnxOy, BaO, and then compares the experiments. At the same time, the iron-loaded activated carbon catalyst focuses on the catalytic oxidation of printing and dyeing wastewater by ozone. From the research results, it can be seen that the iron-loaded catalyst has strong activity, and its catalytic performance reaches the best state when the roasting temperature reaches 750ºC.
4.3 Application in food industry wastewater treatment
Through the research and analysis of food industry wastewater, it can be seen that its water quality changes greatly, so the "hydrolysis acidification-contact oxidation-ozone catalytic oxidation-aerated biological filter (BAF)" combined process is proposed. The COD of the wastewater gradually decreased from 2000-7000 mg/L to 100 mg/L, and it has been in a declining state until it reaches the relevant standards. From the experiment, it can be seen that the hydrolysis acidification system and the ozone catalytic oxidation (ceramic particles loaded with MnO2 as a catalyst)-aerated biological filter deep treatment system are very critical parts to ensure the operation of the system, and it is necessary to focus on strengthening research.
In short, based on the current situation, the advantages of ozone are high efficiency and no secondary pollution, but it also has disadvantages, mainly large dosage and high cost. The degradation of organic matter can only play a role in transformation, and cannot be fully optimized. Therefore, it is still very necessary to strengthen research in this area, and it needs to attract our attention.
5. Combined application with other technologies
Since the application of ozone in water treatment, due to the high equipment and operation costs of ozone treatment technology, although extensive research has been conducted, there are few practical applications except for drinking water disinfection. In recent years, due to the lack of effective methods in water treatment practice, such as chlorine disinfection byproducts, difficult to biodegrade or toxic and hazardous organic wastewater treatment, and with the improvement of the performance of ozone generating equipment, ozone technology has once again received attention, and ozone water treatment technology has been improved and developed.
Ozone/activated carbon technology Activated carbon in the reaction, it may be the role of ·OH in alkaline solution, can trigger an ozone-based chain reaction, accelerate the decomposition of ozone, and generate free radicals such as ·OH. As a catalyst, the reaction of activated carbon with ozone to degrade trace organic pollutants and other reactions involving ozone to generate ·OH (such as increasing pH, adding H2O2, UV radiation) belong to advanced oxidation technology. In addition, activated carbon has the characteristics of large specific surface area and easy use, and is a type 1 catalyst with great practical application potential. The removal of organic matter by ozone biological activated carbon includes three processes: ozone oxidation, activated carbon adsorption and biodegradation.
Photocatalytic ozonation Photocatalytic ozone oxidation (O3/UV) is a type of photocatalysis. That is, when ozone is added, it is accompanied by light (generally ultraviolet light). This method does not use ozone to react directly with organic matter, but uses active secondary oxidants produced by the decomposition of ozone under ultraviolet light to oxidize organic matter. Ozone can oxidize many organic matter in water, but the reaction between ozone and organic matter is selective, and organic matter cannot be completely decomposed into CO2 and H2O. The ozonation products are often carboxylic acid organic matter. In order to improve the oxidation rate and efficiency of ozone, other measures must be taken to promote the decomposition of ozone and produce active OH radicals.
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