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Starting from this broad definition of residual chlorine, the “residual chlorine†control for chlorine dioxide disinfection is mainly focused on solving the composition of “residual chlorine†after mastering chlorine dioxide disinfection and determining the measurement standards and control standards.
6, the "total chlorine residual" as an indicator of "free chlorine" to establish control criteria
Since there is no national standard for chlorine dioxide disinfection drinking water in China, in view of the possible influence of ClO2 in chlorine dioxide disinfection by-products on red blood cells in the blood, we refer to the United States, Spain, the United Kingdom, Germany and Belgium. The maximum limit of the amount of chlorine oxide added and residual, the ClO2 - residual amount of our factory water is controlled within 1.0mg / L, combined with the "remaining total available chlorine" composition, we will be our company according to different seasons The internal control standard of “residual chlorine†is as follows: the minimum total available chlorine in the factory water is 0.6~0.8mg/L, the highest value is 1.2~1.4mg/L, or the factory water free ClO2 ≥0.1mg/L; The total effective chlorine minimum value is 0.2 mg/L. Under this internal control standard, the qualified rate of the total number of factory water bacteria in our company in 2000 is 100%, the qualified rate of total coliform bacteria is 98.7%; the pass rate of internal control standard of “remaining total available chlorine†in pipe network water is 96.5%. The qualified rate of total bacteria in the pipe network was 98.2%, and the qualified rate of total coliform bacteria was 95%.
Chlorine dioxide is one of the new drinking water disinfectants that are mainly introduced and recommended for in-depth study in the “Technology Progress Development Plan for Urban Water Supply Industry 2000†commissioned by the Ministry of Construction of China. In Europe and the United States, chlorine dioxide has been widely used in water treatment, and the application and research of chlorine dioxide in the domestic water industry is also very strong. So far, only 8 water companies in Shenzhen have adopted dioxide. Chlorine is used as a disinfectant for tap water.
Although water supply enterprises using chlorine dioxide disinfection in the country, especially small and medium-sized water supply enterprises, have become more common, due to the lack of corresponding national standards, only one of the chlorites in the “Sanitary Standards for Drinking Water†issued by the Ministry of Health in 2001 There is still a big difference in the determination of the residual amount of salt in the domestic disinfection of drinking water, which leads to great differences in the use and supervision of water supply enterprises and health and epidemic prevention departments.
Shenzhen Guanlan Water Supply Co., Ltd. began to use chlorine dioxide disinfection in a subordinate water plant since the end of 1997. After its success, it has been fully operational in the company's three water plants. After using chlorine dioxide for 3 years, through practice and research, we have initially established a control system for chlorine dioxide disinfection. We will summarize and introduce our practice, and we should learn from the peers to learn and communicate with our peers. 1 Introduction to chlorine dioxide
Chlorine dioxide has a molecular formula of ClO2 and is highly oxidizing. It is a yellow-green gas at normal temperature and pressure and has a similar odor to chlorine (Cl2). ClO2 is easily soluble in water and is not easily hydrolyzed. It exists in a single molecule state in the range of pH 2~10 in water. The solubility in water at 20 ° C is 107.98 g / L, which is 5 times greater than the solubility of chlorine. ClO2 is unstable and may explode when it exceeds 10% in air. The aqueous solution of ClO2 is also unstable, sensitive to light, and photodecomposed to form chlorine, oxygen and chlorine trioxide. Chloride in water can act as a catalytic decomposition. .
The effective chlorine content of ClO2 is 263%. Unlike Cl2, the reaction of ClO2 with organic matter in the oxidation reaction is an oxidation reaction. In addition to the oxidation reaction, Cl2 also has a large number of chlorination substitution reactions. Therefore, Cl2 is easily reacted with organic substances in water during disinfection. An organic halide is formed, while ClO2 does not. 2 Water plant and chlorine dioxide generator overview
Shenzhen Guanlan Water Supply Co., Ltd. has three water plants for chlorine dioxide disinfection. Among them, the water supply capacity of the pit water plant is 30,000 cubic meters, which is the main water purification plant. The water supply accounts for 65-80% of the total water supply of the company. There is also a large sump water plant with a water supply capacity of 20,000 cubic meters and a Niuhu water plant with a capacity of 10,000 cubic meters. The water purification process is as follows:
Chlorine dioxide is generally added after filtration, but there is also a seasonal source water pre-oxidation treatment when the source water manganese content exceeds the standard.
The water sources of the three water plants are from three different reservoirs. The water quality of the water source basically meets the water standards of Class III~IV specified in the National Surface Water Environmental Quality Standard (GHZB1-1999).
Our company chooses a chemical chlorine dioxide generator based on sodium chlorate and hydrochloric acid. The reaction principle is:
The generator has a reaction efficiency of about 80%. The reaction produces a mixed disinfectant with ClO2 as the main component and Cl2 as the auxiliary. The mass ratio of ClO2 to Cl2 is about 1.93 to 1.
3. Thinking about "residual chlorine"
The disinfection control of water plants inevitably encounters the problem of “residual chlorineâ€. In a narrow sense, according to the definition in China's "Standards for the Examination of Drinking Water Standards" (GB5750-85): Residual chlorine refers to chlorine that has been disinfected by chlorination and left in contact with water for a certain period of time. Residual chlorine is one of the important indicators in the "Sanitary Standard for Drinking Water" (GB5749-85). The purpose of retaining residual chlorine in tap water is to maintain a sustained antibacterial power to a certain extent and prevent secondary pollution of tap water. However, this provision in the national standard is formulated for chlorine and chlorine disinfectants. If it is used for non-chlorinated disinfectants, it is also clear that the free residual chlorine in contact with 30-minute factory water is not less than 0.3 mg/L. It is not suitable. In today's increasingly diversified disinfectant selection, we believe that we should understand the "residual chlorine" in a broad sense from the role and purpose of residual chlorine, that is, we should understand "residual chlorine" as: "contact After a certain period of time, leave the remaining disinfectant in the water. â€
4 "Residual chlorine" control process
At the beginning of our company's use of chlorine dioxide generators, we also used the o-tolidine visual colorimetry (OT method) to detect and control the dosage of chlorine dioxide. To learn more about the actual situation of “residual chlorineâ€, we also adopted the fifteenth edition of the Standard Test Method for Water and Wastewater, published by the American Public Health Association and the American Waterworks Association and the United States Water Pollution Control Association in 1983. The DPD spectrophotometric method recommended in the 20th edition published in 1998 was used as a control test. After more than a year of practice, we found that the use of ortho-toluidine visual colorimetry to detect and control chlorine dioxide has many drawbacks: in terms of actual control, the coloration is low due to the OT method used to detect chlorine dioxide. Many, from the control of the water plant investment control, the dosage is too high, and the display value of the “remaining chlorine†of the factory water is very low. The color of the “remaining chlorine†of the pipe network is lower, or even no color at all. From the detection point of view, the OT method used to detect the chlorine dioxide display value does not mean the content of chlorine dioxide, nor the effective chlorine content of the synthetic, it does not have the meaning of chemical detection.
Although o-tolidine visual colorimetry is one of the preferred methods for detecting residual chlorine in China's national standard GB5750-85 "Standard Test Method for Drinking Water", OT method can detect total residual chlorine, and can also roughly distinguish free residual chlorine. And combined residual chlorine, but this method is only suitable for the detection of residual chlorine content in the traditional disinfection mode using chlorine or bleach as a disinfectant. In the disinfection system with chlorine dioxide as the main disinfectant, after the chlorine dioxide is added to the water, the main component of the "residual chlorine" which has bactericidal or bacteriostatic ability in the water is chlorine dioxide monomer and chlorite. And a small amount of free residual chlorine and compound residual chlorine, etc., the content of free residual chlorine and compound residual chlorine can be all developed by OT method, and the "main residual chlorine" is the main component of sterilization and bacteriostasis - - Chlorine dioxide monomer and chlorite can not fully participate in the reaction and all color development, which is the reason for the low value of "residual chlorine" after chlorine dioxide disinfection by OT method. And we found that the OT method shows the low value and the water
There is also no stable correspondence between the actual "residual chlorine" (remaining available chlorine, including ClO2 monomer, ClO 2 - , free residual chlorine and compound residual chlorine), which is roughly only 1 4 to 3 2 of the actual remaining available chlorine. The degree of lowness varies with the water temperature and the content of "residual chlorine" and composition.
The same, so the use of the value of the OT method to estimate the "residual chlorine" of chlorine dioxide disinfection is extremely unreliable. 5 disinfection of chlorine dioxide
5.1 The sterilization effect under several “total available chlorine†levels. We eliminated the filtered water before the poisoning and the factory water after disinfection. The total number of bacteria was tested. The results are shown in Table 1:
Table 1 sterilization rate test
Total number of bacteria in filtered water (/ml)
522
536
426
1926
The total number of bacteria in the factory water (pieces / ml)
17
13
13
8
Total available chlorine in the factory water (mg/L)
0.93
0.61
0.89
0.90
Sterilization rate (%)
96.7
97.6
96.9
99.6
5.2 Study on the composition of "residual chlorine" after chlorine dioxide disinfection
We used DPD spectrophotometry to determine the water content of several water plants (Figure 1) and pipe network water (Figure 2) under the normal dosage of chlorine dioxide (1~2 kg/Km 3 ). The composition of chlorine shows that the approximate proportion of each component is as follows:
Figure 1: Proportion of the “remaining chlorine†components of the factory water in terms of available chlorine as a percentage of the total available chlorine
Figure 2: Proportion of each “remaining chlorine†component of the network water expressed as available chlorine in the total available chlorine
It is indicated that the "residual chlorine" remaining in water is mainly present in the form of chlorite when disinfected with chlorine dioxide under normal dosage.
5.3 Stability test of “residual total available chlorineâ€
We took two water samples, stored in a brown glass bottle sealed and placed in a cabinet and stored at room temperature. Then we made the remaining total available chlorine and repeated measurements of each component for 24 hours. The results are shown in Table 2: Total effective chlorine stability test unit: mg/L (in terms of chlorine)
Project sample
Free ClO2
Free residual chlorine
- ClO 2
Compound residual chlorine
Total available chlorine
Sample one
first
<0.03
<0.03
0.50
0.11
0.61
After 24 hours
<0.03
<0.03
0.46
0.13
0.59
Sample two
first
<0.03
<0.03
0.58
0.20
0.78
After 24 hours
<0.03
<0.03
0.54
0.22
0.76
Tests have shown that the "residual total available chlorine" in the form of combined residual chlorine and chlorite is stable under closed light conditions.
6.1 "Residual chlorine" detection method selection
In order to scientifically and reasonably detect and evaluate the "residual chlorine" of chlorine dioxide, and facilitate the scientific addition and management of chlorine dioxide, we have explored the detection method of "residual chlorine" and the control index of "residual chlorine". Since 2000, we have abandoned the OT method and adopted the DPD spectrophotometry method in the US Standard Test Method for Water and Wastewater as the internal standard method for detecting “residual chlorine†throughout the company. The DPD method is chosen primarily for the following reasons:
1. The advantage of the DPD method is that it can separate chlorine dioxide and other various forms of chlorine (including free residual chlorine, total residual chlorine and chlorite), and is easier to perform colorimetric examination.
2. Due to the use of potassium iodate as a standard solution, the standard curve is simple to manufacture, which avoids the need for frequent calibration with unstable ClO2 or chlorine.
3. Accuracy can meet the requirements.
6.2 Selection of “Residual Chlorine†Control Indicators
The core issue of controlling “residual total available chlorine†is to ensure the microbiological indicators of drinking water, and to ensure the safety of disinfectant residues to the human body. Therefore, in terms of the selection of the control indicators for “residual chlorineâ€, we are not constrained by the requirement of “free residual chlorine†in the national standard, but according to the characteristics of the “remaining chlorine†component of chlorine dioxide disinfection. The total residual chlorine measurement standard for chloramine disinfection, all the residual chlorination or bacteriostatic ClO2 monomer, ClO2 -, free chlorine and chemical residual chlorine are converted into effective chlorine, which we call "the remaining total effective Chlorine, with the “main residual available chlorine†as the main control index, established a water quality internal control standard system. We did not choose a single control indicator of “free ClO2†as “residual chlorineâ€, mainly considering:
1. The disinfectant produced by the chlorine dioxide generator is actually a mixed gas of ClO2 and Cl2. The composition of "residual chlorine" is complicated. It is not reasonable to use the ClO2 monomer as the "residual chlorine".
2. From the composition of "residual chlorine", ClO2 is mostly converted into ClO2 - after being added to water. ClO2 - is also a strong oxidant. Although its bactericidal ability is much weaker than that of ClO2 monomer, it still has a certain antibacterial ability and should not be completely ignored.
3. Only the free ClO2 as the control object easily leads to the neglect of a large amount of ClO2 -, which is not conducive to monitoring the content of ClO2 - while controlling the "residual chlorine".
4. Since the content of free ClO2 in water is too low under the normal dosage, it is basically in the vicinity of the detection limit of the detection method, which is not conducive to accurate determination. On the other hand, because the value is small, it leads to The sensitivity of the change in dosage is not strong. These unfavorable factors will reduce the operability of the internal control standards, which is not conducive to the "chlorine" adjustment control of the grassroots water producers. The choice of "remaining total available chlorine" can be avoided.
6.3 The establishment of the “residual chlorine†internal control system based on the “residual total available chlorine†and the “remaining chlorine†as a supplement to free ClO2.
7 Conclusion and analysis
7.1 DPD method has simple, rapid and accurate method, and can distinguish chlorine dioxide and other various forms of chlorine. It is very suitable for the water industry to check chlorine dioxide and its residual chlorine residue in water.
7.2 Practice shows that the use of “residual total available chlorine†as the main indicator and free ClO 2 as the auxiliary index, the “residual chlorine†control system combined with the DPD method can not only become the “residual chlorine†control for controlling and guiding the chlorine dioxide disinfection drinking water. One of the methods, and has the characteristics of strong operability.
7.3 Although there are 5 electron transfer during the reduction of Cl+4 filC -1 in ClO2, the theoretical effective chlorine is 2.5 times that of Cl2, but in fact the process of Cl+4 filC +3 has the greatest effect, that is, ClO2 The process of conversion to ClO2 -, once formed into ClO2 - is relatively stable, and is not easy to be further reduced, so it is not possible to simply equate the effective chlorine of ClO2 to 2.5 times that of Cl2. The bactericidal ability of ClO2 is 2.5 times that of Cl2.
7.4 How to control and reduce the amount of ClO2 - produced by ClO2 disinfection remains to be further studied.
Author: Mission Hills Shenzhen City Water Co., Ltd. Huang Xiaoping / bell smile