Treatment system for coal chemical industry wastewater evaporation mother liquor
By introducing a silicon removal and decolorization unit into the coal chemical wastewater evaporation mother liquor treatment system, the problems of silicon impurities clogging the mother liquor and the dark salt color during the drying process were solved, achieving efficient and stable treatment results and a low-cost treatment solution.
Patent Information
- Application Number
- CN202422877392.1
- Authority / Receiving Office
- CN · China
- Patent Type
- Utility models(China)
- Current Assignee / Owner
- Filing Date
- 2024-11-25
- Publication Date
- 2025-11-18
- Estimated Expiration
- 2034-11-25
AI Technical Summary
Traditional evaporation mother liquor has a darker salt color and poorer appearance after drying. It is also prone to clogging the nozzle during the drying process, which affects the stability of the process and increases production costs.
The system employs a combined desiliconization and decolorization unit. The desiliconization agent removes silicon impurities, while the decolorization unit removes organic matter, ensuring the continuity of the drying process and the high quality of the product.
It effectively solves the problems of silicon impurity blockage and dark salt color, improves the stability of the drying process and the appearance quality of the product, and reduces processing costs.
Smart Images
Figure CN223561427U_ABST
Abstract
Description
Technical Field
[0001] This application relates to the field of coal chemical wastewater treatment, and in particular to a treatment system for mother liquor from coal chemical wastewater evaporation. Background Technology
[0002] In the field of zero discharge of coal chemical wastewater, in the evaporation and crystallization section of the coal chemical wastewater reuse and zero discharge process, the wastewater first undergoes pretreatment to remove suspended solids and some organic matter, and then enters the evaporator. By heating and evaporating the water, the salts and other dissolved solids in the wastewater are concentrated to a certain extent, and finally the salts crystallize out to form solid salt substances, which are then separated and treated in subsequent steps. The remaining high-concentration waste liquid is discharged as evaporation mother liquor.
[0003] Treating evaporation mother liquor is a crucial step in the zero-discharge process for coal chemical wastewater. Currently, the most common methods for treating mother liquor are various heating and drying methods. However, the salt remaining after drying is dark in color, of poor quality, and difficult to handle; moreover, the drying process can clog nozzles, causing work interruptions. Summary of the Invention
[0004] To address the aforementioned problems, this utility model provides a treatment system for evaporation mother liquor from coal chemical wastewater, which solves the shortcomings of traditional evaporation mother liquor, such as high salt content and poor color after drying, and easy clogging during the drying process.
[0005] The technical solution of this utility model is:
[0006] A treatment system for mother liquor from coal chemical wastewater evaporation, the system comprising:
[0007] A silicon removal unit is used to connect to the discharge port of the evaporation crystallization system to receive the mother liquor from the evaporation; wherein, the silicon removal unit is filled with a silicon removal agent;
[0008] The decolorization unit is connected to the silicon removal unit and is filled with a decolorizing agent;
[0009] The drying unit, connected to the decolorization unit, is used to dry the evaporated mother liquor after removing silicon-containing impurities and decolorization, to obtain colorless impurity salts.
[0010] As one of the preferred solutions, the system also includes:
[0011] A solid-liquid separator is disposed between the silicon removal unit and the decolorization unit.
[0012] As one of the preferred solutions, the system also includes:
[0013] The sludge storage unit is connected to the silicon removal unit and is used to receive the precipitate discharged by the silicon removal unit.
[0014] As one of the preferred options, the sludge storage unit, the silica removal unit, the solid-liquid separator, and the decolorization unit are connected sequentially from bottom to top.
[0015] As one of the preferred embodiments, the sludge storage unit, the silicon removal unit, the solid-liquid separator, and the decolorization unit are configured as an integrated reaction structure.
[0016] As one preferred embodiment, the cross-section of the sludge storage unit gradually decreases in the direction away from the silicon removal unit.
[0017] As one of the preferred options, the sludge storage unit is cone-shaped, with a larger top and a smaller bottom.
[0018] As one of the preferred embodiments, the silicon removal unit is connected to the discharge port through an inlet pipe, and the silicon removal agent is filled in the inlet pipe.
[0019] As one of the preferred options, the silicon removal agent includes any one of aluminum salts, iron salts, and magnesium oxide.
[0020] As one of the preferred options, the decolorizing agent includes any one of ozone, hydrogen peroxide, sodium hypochlorite, and potassium permanganate.
[0021] Compared with the prior art, this application has the following advantages:
[0022] This utility model proposes a treatment system for mother liquor from coal chemical wastewater evaporation. The system includes: a silicon removal unit, which is connected to the discharge port of an evaporation crystallization system to receive the mother liquor; wherein the silicon removal unit is filled with a silicon removal agent; a decolorization unit, which is connected to the silicon removal unit and filled with a decolorization agent; and a drying unit, which is connected to the decolorization unit and is used to dry the mother liquor after removing silicon-containing impurities and decolorizing it to obtain colorless impurities.
[0023] By adopting the technical solution of this application, this system combines continuous silicon removal, decolorization, and drying. Silicon removal eliminates the main cause of clogging in the drying equipment, decolorization improves the appearance of impurities, and the drying unit achieves efficient processing of the final product. It addresses various problems in the evaporation mother liquor in stages, effectively solving issues such as silicon impurity clogging, poor salt quality, and dark color in traditional processes. This system has a simple structure, is easy to deploy, and has high application prospects. It can significantly improve the efficiency of coal chemical wastewater treatment and reduce treatment costs. Attached Figure Description
[0024] To more clearly illustrate the technical solution of this application, the drawings used in the description of this application will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.
[0025] Figure 1 This is a flowchart illustrating the framework of a system for treating mother liquor from coal chemical wastewater evaporation, as described in one embodiment of this application.
[0026] Figure 2 This is a schematic diagram of the overall structure of the treatment system for coal chemical wastewater evaporation mother liquor according to another embodiment of this application.
[0027] Explanation of reference numerals in the attached figures:
[0028] 1. Decolorization unit; 2. Solid-liquid separator; 3. Silicon removal unit; 4. Sludge storage unit; 5. Water inlet pipe. Detailed Implementation
[0029] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.
[0030] It should be noted that zero-discharge treatment of coal chemical wastewater is a treatment process for high-concentration, complex organic wastewater generated by the coal chemical industry, aiming to achieve wastewater reuse or zero discharge. Coal chemical wastewater mainly originates from the production processes of coal-to-oil, coal-to-gas, and coal-to-olefins, containing large amounts of toxic and harmful substances such as organic matter, ammonia nitrogen, volatile phenols, cyanides, sulfides, and high concentrations of salt. Commonly used processes for coal chemical wastewater treatment include pretreatment, biochemical treatment, advanced treatment, evaporation and crystallization, and end-of-pipe sludge treatment, ultimately achieving the reuse or zero discharge of the treated coal chemical wastewater.
[0031] This application primarily addresses the issue in the evaporation and crystallization process where, after crystallization, most of the salts in the waste liquid have crystallized, leaving a high-concentration, uncrystallized solution known as the mother liquor. This mother liquor typically contains salts that are difficult to crystallize further, as well as some highly soluble organic compounds. These substances are challenging to treat and require further processing methods such as concentration, solidification, or incineration.
[0032] However, traditional mother liquor treatment methods face a series of technical and operational problems in practice, mainly including the following aspects:
[0033] Because of the complex composition of the mother liquor, the residual salt darkens in color during the drying process, forming dark solid salt that affects its appearance. Furthermore, the dried salt contains not only common inorganic salts but also a large amount of organic impurities, resulting in poor quality, low industrial recycling value, and high disposal costs.
[0034] The evaporation mother liquor has a very high salt concentration and complex composition. Therefore, during the drying process, salts or other organic impurities (such as silicon-containing impurities) in the solution easily crystallize, deposit, or adhere to the area around, inside, or on the surface of the nozzles, causing nozzle blockage. Once the nozzles are blocked, the drying equipment needs to be shut down frequently for cleaning and maintenance, leading to production discontinuity and increased maintenance costs.
[0035] The aforementioned problems severely impact the stability and economy of the process in actual operation, directly leading to decreased work efficiency and increased production costs. Therefore, this application aims to reduce the color of the mother liquor, thereby improving the quality of the salt after drying the mother liquor, and reducing the content of easily scaling substances in the wastewater to prevent scaling. To achieve this objective, reference is made to... Figure 1 and Figure 2 As shown, Figure 1 This is a flowchart illustrating the framework of a system for treating mother liquor from coal chemical wastewater evaporation, as shown in this utility model. Figure 2 This is a schematic diagram of the overall structure of a treatment system for evaporation mother liquor from coal chemical wastewater, as shown in this utility model. This utility model provides a treatment system for evaporation mother liquor from coal chemical wastewater, comprising: a silicon removal unit 3, connected to the discharge port of an evaporation crystallization system to receive the evaporation mother liquor; wherein the silicon removal unit 3 is filled with a silicon removal agent; a decolorization unit 1, connected to the silicon removal unit 3 and filled with a decolorization agent; and a drying unit, connected to the decolorization unit 1, for drying the evaporation mother liquor after removing silicon-containing impurities and decolorizing it, to obtain colorless impurities.
[0036] Specifically, the silicon removal unit 3 is connected to the discharge port of the evaporation crystallization system. Therefore, after the evaporation crystallization system discharges the mother liquor, it is introduced into the silicon removal unit 3 for treatment. Specifically, the mother liquor discharged by the evaporation crystallization system has a high temperature and can be discharged directly or pumped to the silicon removal unit 3.
[0037] Coal chemical wastewater often contains high concentrations of silicate and other silicon-containing impurities. Especially after concentration and evaporation, silicates in the mother liquor are prone to crystallization and precipitation, clogging subsequent treatment equipment, such as the nozzles of spray dryers. The silica removal unit 3 is filled with a silica removal agent. This agent precipitates silicates through a chemical reaction, reducing the silicon concentration. Therefore, the silica removal unit 3 effectively solves the nozzle clogging problem. By removing silicates from the mother liquor at an early stage, the frequency of nozzle clogging can be significantly reduced, ensuring the continuity and stability of the drying process.
[0038] In some embodiments, a water inlet pipe 5 is provided in the lower middle part of the silicon removal unit 3, and the mother liquor enters from the lower middle part of the silicon removal unit 3. The silicon removal agent is added to the water inlet pipe 5 of the silicon removal unit 3. The silicon removal agent and the evaporation mother liquor are mixed by hydraulic action. When the mother liquor enters the silicon removal unit 3, the evaporation mother liquor and the silicon removal agent are fully mixed, and the silicon in the evaporation mother liquor precipitates under the action of the agent.
[0039] The evaporation mother liquor, even after desiliconization, remains dark in color and contains a large amount of organic matter, colored metal ions, or impurities remaining from high-temperature treatment. After desiliconization, the mother liquor enters decolorization unit 1 for further processing. The decolorizing agent comes into full contact with the mother liquor in decolorization unit 1, and under the action of the decolorizing agent, the mother liquor changes from reddish-black to pale yellow or even colorless. Because the mother liquor has been essentially decolorized to colorless, the impurities produced during drying are also colorless, resulting in higher quality impurities that are beneficial for subsequent processing. Therefore, decolorization unit 1 effectively reduces the amount of dark-colored salts, resulting in a better appearance for the solid salt obtained after drying.
[0040] The evaporation mother liquor, after desiliconization and decolorization, enters the drying unit. In the drying unit, residual moisture is evaporated through heating or other drying techniques, ultimately yielding dried, light-colored or colorless solid salts. Therefore, compared to traditional methods of drying evaporation mother liquor, the evaporation mother liquor from this application, after desiliconization and decolorization, does not clog equipment during the drying process, especially spray dryers. It also produces light-colored solid salts with fewer impurities, resulting in higher quality and appearance, making it more valuable for recycling. This solves the problem of dark-colored and difficult-to-dispose-of salts in traditional drying processes.
[0041] In summary, this system combines continuous silicon removal, decolorization, and drying. Silicon removal eliminates the main cause of drying equipment blockage, decolorization improves the appearance of impurities, and the drying unit achieves efficient processing of the final product. It addresses various issues in the evaporation mother liquor in stages, effectively solving problems such as silicon impurity blockage, inconsistent salt quality, and dark color in traditional processes. This system has a simple structure, is easy to deploy, and has high application prospects. It can significantly improve the efficiency of coal chemical wastewater treatment and reduce treatment costs.
[0042] It is worth mentioning that setting up a decolorization unit 1 in the evaporation mother liquor treatment process of the evaporation crystallization section, and / or setting the decolorization unit 1 after the silicon removal unit 3, centralizes the mother liquor decolorization process, avoids the complexity of dispersed decolorization in the pre-treatment stage of wastewater treatment, and allows the decolorizing agent to react with more colored impurities in a targeted manner, thereby improving the utilization rate of the decolorizing agent and reducing the amount of decolorizing agent used, making it more economical and efficient in practical applications.
[0043] Different wastewater compositions require different types of reagents, which can be flexibly adjusted during operation.
[0044] In some embodiments, the silica removal agent may include any one of aluminum salts, iron salts, and magnesium oxide. By selecting aluminum salts, iron salts, or magnesium oxide, appropriate agents can be chosen for different silica concentrations, ensuring efficient silicate precipitation and reducing the risk of subsequent equipment blockage.
[0045] In some embodiments, the decolorizing agent includes any one of ozone, hydrogen peroxide, sodium hypochlorite, and potassium permanganate. The selection of ozone, hydrogen peroxide, sodium hypochlorite, and potassium permanganate allows the system to flexibly handle different types of colored impurities, ensuring that the final dried salt is colorless and improving product quality.
[0046] To further facilitate the entry of the desiliconized evaporation mother liquor into the decolorization unit 1, a solid-liquid separator 2 is installed between the desiliconization unit 3 and the decolorization unit 1. Therefore, in the desiliconization unit 3, the silicon in the evaporation mother liquor precipitates under the action of the reagent, and the silicon-containing impurities form precipitated suspensions or precipitates. Some of the precipitates settle directly, while some precipitated suspensions rise with the mother liquor. Upon reaching the solid-liquid separator 2, most of the suspended solids are retained, and the solids ultimately precipitate, preventing them from entering the decolorization unit 1. Therefore, the evaporation mother liquor with removed solid particles allows the decolorizing reagent to act directly on the colored substances in the liquid, improving decolorization efficiency; it also avoids nozzle clogging problems in the subsequent drying unit, improving the overall operating efficiency of the processing system.
[0047] In some embodiments, the solid-liquid separator 2 may be a centrifugal separator, a cyclone separator, or a filter, etc.
[0048] In another embodiment, the system further includes a sludge storage unit 4, connected to the silicon removal unit 3, for receiving the precipitate discharged from the silicon removal unit 3. Within the silicon removal unit 3, reagents such as aluminum salts, iron salts, or magnesium oxide react with silicon in the wastewater to form solid precipitate (sludge). By designing the sludge storage unit 4 to provide a dedicated container or space, and by centrally collecting the sludge in the storage unit, it is possible to effectively prevent precipitate from clogging pipes or affecting the operating efficiency of subsequent treatment equipment.
[0049] The sludge storage unit 4 is equipped with an openable outlet, so that sludge can be periodically transferred out of the storage unit.
[0050] Preferably, the sludge storage unit 4 is used in conjunction with the solid-liquid separator 2. The silicon in the evaporation mother liquor precipitates under the action of the reagent. Some of the precipitated suspended solids rise with the mother liquor. When they reach the solid-liquid separator 2, most of the suspended solids are intercepted by the solid-liquid separator 2. The solids eventually fall back to the sludge storage unit 4. The sludge stored in the sludge storage unit 4 is discharged from the bottom outlet at regular intervals every day.
[0051] Specifically, the sludge storage unit 4, the silica removal unit 3, the solid-liquid separator 2, and the decolorization unit 1 are sequentially connected from bottom to top. In this embodiment, the water inlet pipe 5 of the silica removal unit 3 is located in the lower middle part, the top outlet of the silica removal unit 3 is connected to the bottom inlet of the solid-liquid separator 2, the top outlet of the solid-liquid separator 2 is connected to the bottom inlet of the decolorization unit 1, and the top outlet of the decolorization unit 1 is connected to the drying unit. The decolorization unit 1 has a decolorizing agent inlet at its bottom, and the treated evaporation mother liquor is finally discharged from the top to the drying unit.
[0052] Therefore, the bottom-up sequential arrangement utilizes gravity, allowing sediment (sludge) to naturally sink to the sludge storage unit 4 at the bottom of the system, eliminating the need for additional power equipment, thus simplifying system design and reducing energy consumption. The axially sequential arrangement makes the treatment system more compact, with simple and efficient material flow. Steps such as silica removal, solid-liquid separation, decolorization, and drying are performed sequentially, allowing materials to flow smoothly through each unit, ensuring the continuity and stability of the entire wastewater treatment process.
[0053] In some embodiments, the residence time of the mother liquor in the desiliconization unit 3 is 15 min-60 min; and the residence time in the decolorization unit 1 is 30 min-240 min.
[0054] More specifically, the sludge storage unit 4, the desiliconization unit 3, the solid-liquid separator 2, and the decolorization unit 1 are configured as an integrated reaction structure. In this embodiment, each unit is integrated into an integrated reactor, meaning that each unit can share an external enclosed shell and is internally interconnected, without the need for separate pipe connections. The lower part has the desiliconization unit 3, which includes the sludge storage unit 4, and the upper part has the decolorization unit 1. The solid-liquid separator 2 connects the two units, making the overall system more compact and integrated, reducing the number of pipes, reducing the system's footprint, and optimizing space utilization.
[0055] In some embodiments, the cross-section of the sludge storage unit 4 gradually decreases in the direction away from the desiliconization unit 3. In this embodiment, the sludge storage unit 4 is located below the desiliconization unit 3, and therefore gradually decreases in size from top to bottom, allowing the sludge to be gradually guided and concentrated within the storage unit along the flow direction. This utilizes gravity to guide the sediment from a wider top area to a narrower bottom area, helping the sludge to settle naturally and accumulate, facilitating further processing or cleaning.
[0056] The discharge port is located at the bottom of the sludge storage unit 4, which helps to centrally discharge sludge through the bottom discharge port, thereby reducing the cleaning frequency and improving the ease of operation.
[0057] For example, the bottom portion or the entire sludge storage unit 4 is designed as a funnel or cone-shaped structure, allowing the sludge to naturally collect along the slope to the sludge discharge outlet at the bottom.
[0058] For example, the sludge storage unit 4 can be stepped or inverted convex.
[0059] Preferably, the sludge storage unit 4 is cone-shaped, wider at the top and narrower at the bottom. The gradually narrowing cross-section allows the sludge to settle sufficiently within the storage unit, reducing the content of solid particles in the liquid and improving the efficiency of sludge collection and discharge.
[0060] In summary, the following provides an exemplary embodiment of this application.
[0061] The evaporation mother liquor discharged from a zero-discharge system for coal chemical wastewater is pumped into the desiliconization unit 3. Sodium aluminate, a desiliconizing agent, is simultaneously added to the inlet pipe 5 of the desiliconization unit 3. The desiliconizing agent and the evaporation mother liquor are mixed in the inlet pipe 5 and enter the desiliconization unit 3 together. In the desiliconization unit 3, the desiliconizing agent reacts with the evaporation mother liquor for 30 minutes to generate a large amount of precipitate, adsorbing silicon and precipitating it together to the sludge storage unit 4. The evaporation mother liquor passes through the solid-liquid separator 2 and enters the decolorization unit 1. Ozone, a decolorizing agent, is added to the decolorization unit 1. The ozone flow rate is 5 kg / h and the ozone concentration is 120 mg / L. The ozone reacts with the evaporation mother liquor in the decolorization unit 1 for 2 hours.
[0062] The mother liquor after desiliconization and decolorization was transported to the drying unit for drying without any equipment blockage, and white impurities were obtained.
[0063] During this process, various indicators of the evaporation mother liquor were obtained through corresponding analytical techniques. The table below shows the changes in these indicators:
[0064] Table 1 shows the changes in the indicators of the evaporated mother liquor:
[0065]
[0066] Table 1
[0067] Table 1 shows that the reduction in COD indicates a significant decrease in the concentration of organic matter in the evaporation mother liquor after treatment by the silicon removal unit 3 and the decolorization unit 1, effectively removing a large amount of organic pollutants and improving water quality. The substantial reduction in color demonstrates that the decolorization unit 1 effectively removed pigments and turbidity from the water. The reduced silica content decreases the formation of precipitates, preventing blockages in equipment and pipes, and contributing to improved overall system efficiency and lifespan. Therefore, this invention solves the problem of scaling and blockage during the drying process of mother liquor generated from zero-discharge coal chemical wastewater and improves the quality of the dried mixed salts, which are now white.
[0068] It should be noted that the various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.
[0069] It should also be noted that, in this document, the terms "upper," "lower," "left," "right," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the invention. Furthermore, relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations, nor should they be construed as indicating or implying relative importance. Moreover, the term "comprising" or any other variation thereof is intended to cover non-exclusive inclusion, such that a process, method, article, or terminal device that comprises a list of elements includes not only those elements, but also other elements not expressly listed, or elements inherent to such a process, method, article, or terminal device.
[0070] The above provides a detailed description of a treatment system for mother liquor from coal chemical wastewater evaporation, as provided in this application. Specific examples have been used to illustrate the principles and implementation methods of this application. The descriptions of these embodiments are merely for the purpose of aiding understanding this application, and the content of this specification should not be construed as limiting this application. Furthermore, those skilled in the art will recognize that various modifications and variations in the specific implementation methods and application scope may occur based on this application. It is neither necessary nor possible to exhaustively list all possible implementation methods here, but any obvious variations or modifications derived therefrom remain within the protection scope of this application.
Claims
1. A treatment system for coal chemical wastewater evaporation mother liquor, characterized in that, The system comprises: a silicon removal unit, which is in communication with a discharge port of the evaporative crystallization system to receive the evaporation mother liquor; wherein the silicon removal unit is filled with a silicon removal agent; a decolorization unit, which is in communication with the silicon removal unit and is filled with a decolorization agent; a drying unit, which is in communication with the decolorization unit to dry the evaporation mother liquor after removal of silicon-containing impurities and decolorization, to obtain a colorless impure salt.
2. The treatment system for coal chemical wastewater evaporation mother liquor according to claim 1, characterized in that, The system further comprises: a solid-liquid separator, which is arranged between the silicon removal unit and the decolorization unit.
3. The system for treating coal chemical wastewater evaporation mother liquor according to claim 1, characterized in that, The system further comprises: a sludge storage unit, which is in communication with the silicon removal unit to receive the precipitate discharged from the silicon removal unit.
4. The system for treating coal chemical wastewater evaporation mother liquor according to any one of claims 1-3, characterized in that, The sludge storage unit, the silicon removal unit, the solid-liquid separator and the decolorization unit are sequentially arranged in communication from bottom to top.
5. The system for treating coal chemical wastewater evaporation mother liquor according to claim 4, characterized in that, The sludge storage unit, the silicon removal unit, the solid-liquid separator and the decolorization unit are configured as an integrated reaction structure.
6. The system for treating coal chemical wastewater evaporation mother liquor according to claim 3, characterized in that, The cross section of the sludge storage unit gradually decreases in a direction away from the silicon removal unit.
7. The system for treating coal chemical wastewater evaporation mother liquor according to claim 6, characterized in that, The sludge storage unit is in a conical shape with a large upper part and a small lower part.
8. The system for treating coal chemical wastewater evaporation mother liquor according to claim 1, characterized in that, The silicon removal unit is in communication with the discharge port through a water inlet pipe, and the silicon removal agent is filled in the water inlet pipe.
9. The system for treating coal chemical wastewater evaporation mother liquor according to claim 1, characterized in that, The silicon removal agent comprises any one of an aluminum salt, an iron salt and magnesium oxide.
10. The system for treating coal chemical wastewater evaporation mother liquor according to claim 1, characterized in that, The decolorization agent comprises any one of ozone, hydrogen peroxide, sodium hypochlorite and potassium permanganate.