Method for online monitoring and turbidity compensation of chemical oxygen demand water quality

Abstract

The invention relates to a method for on-line monitoring and turbidity compensation of chemical oxygen demand water quality, which comprises the following steps: S1, respectively measuring absorbance of a sample at 610nm + / -10nm and 760nm + / -10nm after adding an oxidizing agent, a masking agent and an acid solution by taking pure water as a reference, and marking the absorbance as A1 and A2; s2, calculating the compensated chemical oxygen demand absorbance according to a formula 1 or a formula 2; s3, the absorbance is converted into the concentration of the chemical oxygen demand according to a formula 3 or a formula 4, by means of the method, the measurement precision of a turbidity water sample can be remarkably improved, especially under the high turbidity condition, the reliability and consistency of the CODcr measurement result are ensured, and therefore various complex water quality environments can be effectively handled, more accurate and stable online monitoring data are provided, and the method is suitable for popularization and application. The problems that existing CODcr online monitoring equipment is weak in turbidity interference resistance, low in measurement accuracy, poor in stability and the like are solved.

Description

Technical Field

[0001] The invention relates to the field of water quality online analysis, in particular to a method for turbidity compensation for chemical oxygen demand water quality online monitoring. Background Art

[0002] Chemical oxygen demand (CODcr) is an important indicator for measuring the total amount of organic pollutants in water bodies, reflecting the content of reducing substances in water bodies that can be oxidized by strong oxidants. At present, the traditional method for determining CODcr mainly uses the potassium dichromate method to measure the oxidation amount of organic matter in water samples by spectrophotometry. However, spectrophotometry is susceptible to turbidity interference caused by sediment, suspended particles and colloids in water samples, which will affect the final measurement results. Because these substances scatter and absorb incident light, the measured absorbance not only reflects the concentration of organic matter, but also contains additional signals caused by turbidity, making the measured value of CODcr higher. Especially when there are turbidity components in the water sample that are difficult to remove by digestion, this interference becomes more significant.

[0003] Existing technical solutions:

[0004] Pollution source area: After removing larger floating objects and particulate matter from the sample, measure the absorbance of the sample at 610nm±10nm, and then calculate the CODcr concentration by the slope and intercept of the standard curve.

[0005] Surface water area: Use a 63μm mesh to filter the sample. After natural sedimentation for half an hour, take the supernatant and measure the absorbance at 610nm±10nm. Similarly, calculate the CODcr concentration using the standard curve.

[0006] Disadvantages of existing technology:

[0007] Low accuracy of measurement results: Organic matter in natural water bodies is usually attached to particulate matter. Although pre-treatment methods such as filtration or sedimentation can reduce the impact of turbidity, they may also reduce the CODcr concentration in the water sample. This treatment method may excessively remove turbidity and fail to fully reflect the true characteristics of the water body. In addition, if fine sediment and suspended particles cannot be effectively oxidized and digested, the absorbance will be high, which will affect the calculation of CODcr concentration and reduce the accuracy of the measurement.

[0008] Poor stability of measurement results: The turbidity conditions of different water samples vary greatly, especially in natural water bodies, where turbidity fluctuates with time and environmental conditions. This uncontrollability causes large fluctuations in measurement results under different water quality conditions, making it difficult to ensure the stability of the results, affecting the continuity and accuracy of online monitoring equipment.

[0009] Therefore, it is particularly necessary to develop a detection method that can effectively compensate for turbidity interference and improve the accuracy of CODcr measurement, especially a detection method used in an online water quality monitoring system. To this end, the present invention proposes a dual-wavelength CODcr turbidity compensation technology based on spectrophotometry, which realizes the correction of CODcr measurement value by selecting appropriate wavelengths and combining algorithm compensation, thereby effectively solving the turbidity interference problem. Summary of the invention

[0010] The purpose of the present invention is to overcome the shortcomings of the prior art, meet the actual needs, provide a method for turbidity compensation for online monitoring of chemical oxygen demand water quality, so as to solve the problem that the measurement result is high due to turbidity interference in the current CODcr determination method, and provide a CODcr detection method based on dual-wavelength turbidity compensation technology, which can effectively monitor the CODcr concentration within 400NTU, and the error meets ±4mg / L or ±10% (whichever is greater). Through this technology, the accuracy of water sample determination can be improved, ensuring that reliable CODcr determination results can be obtained under various water quality conditions, especially under high turbidity conditions.

[0011] In order to achieve the purpose of the present invention, the technical solution adopted by the present invention is: designing a method for turbidity compensation for online monitoring of chemical oxygen demand water quality, comprising the following steps:

[0012] S1. Using pure water as a reference, measure the absorbance of the sample at 610nm±10nm and 760nm±10nm after adding masking agent, oxidant and acid solution, digestion and cooling, and mark them as A1 and A2;

[0013] S2. When A2 is a non-negative number, use Formula 1 to calculate the compensated chemical oxygen demand absorbance:

[0014] A=A1-f×A2-aFormula 1

[0015] When A2 is negative, no compensation is required because the effect of turbidity is negligible. Use Equation 2 to calculate the absorbance of chemical oxygen demand:

[0016] A=A1 formula 2

[0017] Further: After obtaining the corrected absorbance A, the present invention further converts the absorbance into the concentration of chemical oxygen demand:

[0018] When A2 is a non-negative number: Use Formula 3 to calculate:

[0019] C=K×(A1-f×A2-a)+BFormula 3

[0020] When A2 is a negative number, use Formula 4 to calculate:

[0021] C=K×A1+B Formula 4

[0022] Wherein, A1 is the absorbance measured at 610nm±10nm;

[0023] A2 is the absorbance measured at 760nm±10nm;

[0024] f and a are turbidity correction parameters, which are determined by actual measurements;

[0025] K and B are the slope and intercept of the working curve, respectively, which are obtained by curve calibration;

[0026] C is the final calculated CODcr concentration.

[0027] Preferably, A1 represents the sample absorbance measured at a wavelength of 610nm±10nm with pure water as reference after digestion and cooling, which is used to measure the absorbance of organic matter and some reducing substances in the water sample after digestion. A2 represents the turbidity absorbance measured at a wavelength of 760nm±10nm with pure water as reference after digestion and cooling, which is used to measure the turbidity absorbance in the water sample after digestion.

[0028] Preferably, when the digested sample has turbidity, A2 is a non-negative value, the compensated sample absorbance is calculated according to formula (1), and the compensated CODcr concentration is obtained by formula (3). When the digested sample has no turbidity, A2 is a negative value, the compensated sample absorbance is calculated according to formula (2), and the compensated CODcr concentration is obtained by formula (4).

[0029] Preferably, 8g / L to 18g / L potassium dichromate is used as an oxidant, 20g / L to 300g / L mercuric sulfate is used as a masking agent, and 5g / L to 20g / L silver sulfate is dissolved in 98% concentrated sulfuric acid as a mixed acid solution.

[0030] Preferably, the digestion solution is obtained by mixing the sample to be tested, the oxidant, the masking agent and the acid solution and digesting them at 160° C. to 175° C. for 10 minutes to 25 minutes.

[0031] Preferably, the volume ratio of the water sample to be tested, the oxidant, the masking agent and the acid solution in the digested sample is 5:1:1:2.5.

[0032] Preferably, the turbidity correction parameter f is 0.9-1.3, and a is 0.005-0.035.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] 1. Through this method, the present invention can significantly improve the measurement accuracy of turbidity water samples, especially under high turbidity conditions, ensure the reliability and consistency of CODcr measurement results, thereby effectively coping with various complex water quality environments, providing more accurate and stable online monitoring data, and overcoming the problems of existing CODcr online monitoring equipment such as weak anti-turbidity interference ability, low measurement accuracy, and poor stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a schematic diagram of the process of the present invention. DETAILED DESCRIPTION

[0036] The present invention is further described below in conjunction with the accompanying drawings and embodiments:

[0037] A method for turbidity compensation for online monitoring of chemical oxygen demand water quality, see Figure 1 , including the following steps:

[0038] S1. Using pure water as a reference, measure the absorbance of the sample at 610nm±10nm and 760nm±10nm after adding masking agent, oxidant and acid solution, digestion and cooling, and mark them as A1 and A2;

[0039] S2. When A2 is a non-negative number, use Formula 1 to calculate the compensated chemical oxygen demand absorbance:

[0040] A=A1-f×A2-aFormula 1

[0041] When A2 is negative, no compensation is required because the effect of turbidity is negligible. Use Equation 2 to calculate the absorbance of chemical oxygen demand:

[0042] A=A1 formula 2

[0043] Further: After obtaining the corrected absorbance A, the present invention further converts the absorbance into the concentration of chemical oxygen demand:

[0044] When A2 is a non-negative number: Use Formula 3 to calculate:

[0045] C=K×(A1-f×A2-a)+BFormula 3

[0046] When A2 is a negative number, use Formula 4 to calculate:

[0047] C=K×A1+B Formula 4

[0048] Wherein, A1 is the absorbance measured at 610nm±10nm;

[0049] A2 is the absorbance measured at 760nm±10nm;

[0050] f and a are turbidity correction parameters, which are determined by actual measurements;

[0051] K and B are the slope and intercept of the working curve, respectively, which are obtained by curve calibration;

[0052] C is the final calculated CODcr concentration.

[0053] Furthermore, in certain embodiments of the present invention, the volume ratio of the sample, the masking agent, the oxidant and the acid solution is 2.5:1:1:2.5.

[0054] In some embodiments of the present invention, the oxidant is potassium dichromate. In some embodiments of the present invention, the concentration of the oxidant is 8 g / L to 18 g / L. In some embodiments, the concentration of the oxidant is 11 g / L.

[0055] In some embodiments of the present invention, the masking agent is mercuric sulfate. In some embodiments of the present invention, the concentration of the masking agent is 50 g / L to 300 g / L. In some embodiments, the concentration of the masking agent is 100 g / L.

[0056] In some embodiments of the present invention, silver sulfate is selected as a catalyst in the acid solution. In some embodiments of the present invention, the concentration of silver sulfate in the acid solution is 5 g / L to 20 g / L. In some embodiments, the concentration of silver sulfate in the acid solution is 10 g / L.

[0057] In certain embodiments of the present invention, the volume of the sample to be tested does not exceed 5 ml. In certain embodiments, the volume of the sample to be tested is 2.5 ml.

[0058] In some embodiments of the present invention, the temperature of high temperature digestion is 160°C to 175°C, and the digestion time is 10 minutes to 25 minutes. In some embodiments, the temperature of high temperature digestion is 165°C, and the digestion time is 15 minutes.

[0059] In some embodiments of the present invention, the cooling temperature is 40° C. to 55° C. In some embodiments, the cooling temperature is 50° C.

[0060] In the present invention, f and a are turbidity correction parameters. In certain embodiments of the present invention, f is 0.9 to 1.3, and a is 0.005 to 0.035. In certain embodiments, f is 1.1891, and a is 0.0081.

[0061] The present invention needs to judge the A2 and then apply different formulas for calculation.

[0062] The effectiveness and accuracy of this method are verified by multiple examples below:

[0063] Embodiment 1

[0064] In order to verify the rationality of the turbidity correction parameters f and a, kaolin was selected to simulate the turbidity components of the actual water sample. First, the main component of kaolin is aluminosilicate, which does not contain organic matter and reducing substances. Second, kaolin has a certain adsorption capacity, and organic matter in water is easily adsorbed on the particulate matter, so that it can better represent the impact of particulate matter on turbidity in the simulated water sample, which fits the characteristics of suspended particulate matter in the actual water body. Third, kaolin is insoluble in water and acid and alkali solutions, and is not easily oxidized and digested to lose its interference significance.

[0065] Experimental method: First, prepare four groups of chemical oxygen demand (CODcr) standard solutions with concentrations of 0 mg / L, 20 mg / L, 40 mg / L and 60 mg / L as the basic solutions for the experiment. Secondly, accurately weigh kaolin of different masses and add them to these basic solutions to simulate different turbidity conditions. By adjusting the amount of kaolin added, the turbidity range of the mixed solution is controlled between 0 NTU and 500 NTU. Then, add 5 ml of the mixed solution, 1 ml of the masking agent, 1 ml of the oxidant and 2.5 ml of the acid solution in sequence, and digest at a high temperature of 165 ° C for 15 minutes. After the digestion is completed, the solution is cooled to 50 ° C, and the absorbance A1 and A2 of the digestion solution are measured at wavelengths of 610 nm ± 10 nm and 760 nm ± 10 nm, respectively, with pure water as the reference. Subsequently, the correction parameters f and a are used to perform turbidity compensation according to formula (1) or formula (2). According to the known working curve slope K and intercept B, the compensated absorbance is calculated according to formula (3) or formula (4) to obtain the compensated chemical oxygen demand (CODcr) concentration. Finally, the measurement result is compared with the theoretical value to calculate the measurement error. The error is required to be controlled within ±4mg / L or ±10% (the larger one) to verify the rationality of the correction parameters. The test results are shown in Table 1:

[0066] Table 1 Measurement results of turbidity mixed standard samples of different concentrations

[0067]

[0068] Through data analysis and comparison, it can be seen that when f is 1.1891 and a is 0.0081, first, in the range of sample turbidity of 0 to 500 NTU, except for individual cases, the measurement errors of most samples meet the preset error control standards (±4 mg / L or ±10%, whichever is greater). Specifically, the error of the 60 mg / L standard solution under the turbidity condition of 510 NTU exceeds the set error range, and a large negative deviation occurs, indicating that the high turbidity sample at this concentration has a greater impact on the measurement accuracy. This may be due to the excessive interference of excessive turbidity on optical measurement, resulting in the inability of compensation parameters to completely correct the error. Secondly, when the turbidity of the sample does not exceed 400 NTU, the absolute error of the experimental data is generally controlled within ±3 mg / L. For samples with concentrations of 20 mg / L, 40 mg / L and 60 mg / L, within this turbidity range, the difference between the compensated chemical oxygen demand (CODcr) concentration and the theoretical value is small, indicating that the correction method performs well under medium and low turbidity conditions.

[0069] In general, the rationality of the correction parameters f and a was well verified, and the compensated chemical oxygen demand measurement results were close to the theoretical value, with an absolute error of less than 3 mg / L, indicating that the compensation method has good applicability and stability under turbidity conditions within 400 NTU.

[0070] Embodiment 2

[0071] Verifying the accuracy and stability of the turbidity compensation method of the present invention

[0072] Experimental method: Prepare chemical oxygen demand (CODcr) standard solutions with concentrations of 40 mg / L and 80 mg / L respectively, measure once every 1 hour, and compare the changes in routine data and quality control verification data to verify the accuracy and stability of the compensation algorithm, as shown in Table 2:

[0073] Table 2 Routine and quality control data of standard samples

[0074]

[0075]

[0076] The data results show that A2 is a negative number when testing pure water or standard solution. According to formula (2), the compensated absorbance A=A1, and then the compensated data is calculated by formula (4). The routine measurement data of 40mg / L standard solution within 24 hours is relatively stable, with absolute errors within ±2mg / L and relative errors within ±4%, which shows that the compensation algorithm has high accuracy and stability in long-term measurement of medium-concentration samples. The 24-hour zero drift in the quality control data is 0.06%, while the 24-hour span drift result of the 80mg / L standard solution is 1.33%, which shows that the instrument has high stability and small data drift, ensuring the reliability of the measurement results.

[0077] Embodiment 3

[0078] Water samples from six water sampling points in Shandong, Shaanxi and Sichuan were selected and compared with the laboratory standard method to investigate the accuracy and anti-interference ability of the compensation algorithm of the present invention. The specific data are shown in Table 3:

[0079] Table 3 Water sample comparison data after using compensation algorithm

[0080]

[0081]

[0082] From the above data, it can be seen that the test results of surface water samples meet the set requirements, that is, within the range of ±4mg / L or ±10% (the larger value). For pollution source water samples, due to their high concentration, although the absolute error exceeds ±4mg / L, it still meets the technical requirement of ±10%. The compensation algorithm effectively compensates for water samples with different turbidity and shows good adaptability. However, the turbidity material composition in different water bodies is different, so in specific applications, the correction parameters f and a can be appropriately adjusted to optimize the compensation effect.

[0083] In summary, the turbidity compensation method provided by the present invention can be effectively applied to the detection of water samples with turbidity not exceeding 400NTU, and ensures that the measurement error is controlled within ±4mg / L or ±10% (the larger value). The application of this technology significantly improves the accuracy of water sample measurement, especially when facing high turbidity water bodies, the compensation algorithm can well reduce interference factors and ensure reliable chemical oxygen demand (CODcr) measurement results. Through the introduction of the compensation method, not only the scope of application of the measurement system is expanded, but also the anti-interference ability under complex water quality conditions is greatly enhanced. Whether it is surface water or pollution source water samples, this method has shown good adaptability and improved the overall reliability and accuracy of the water sample analysis process.

[0084] In addition, the components designed in the present invention are all universal standard parts or components known to technical personnel in this field. Their structures and principles can be known to technical personnel through technical manuals or through conventional experimental methods. They can be fully implemented by technical personnel in this field. Needless to say, the content protected by the present invention does not involve improvements to internal structures and methods.

[0085] The embodiments of the present invention disclose preferred embodiments, but are not limited thereto. A person skilled in the art can easily understand the spirit of the present invention based on the above embodiments and make different extensions and changes. However, as long as they do not deviate from the spirit of the present invention, they are all within the protection scope of the present invention.

Claims

1. A method for turbidity compensation for online monitoring of chemical oxygen demand water quality, characterized in that: The following steps are involved: S1. Using pure water as a reference, measure the absorbance of the sample at 610nm±10nm and 760nm±10nm after adding masking agent, oxidant and acid solution, digestion and cooling, and mark them as A1 and A2; S2. When A2 is a non-negative number, use Formula 1 to calculate the compensated chemical oxygen demand absorbance: A=A1-f×A2-aFormula 1 When A2 is negative, no compensation is required because the effect of turbidity is negligible. Use Equation 2 to calculate the absorbance of chemical oxygen demand: A=A1 formula 2 Further: After obtaining the corrected absorbance A, the present invention further converts the absorbance into the concentration of chemical oxygen demand: When A2 is a non-negative number: Use Formula 3 to calculate: C=K×(A1-f×A2-a)+BFormula 3 When A2 is a negative number, use Formula 4 to calculate: C=K×A1+B Formula 4 Wherein, A1 is the absorbance measured at 610nm±10nm; A2 is the absorbance measured at 760nm±10nm; f and a are turbidity correction parameters, which are determined by actual measurements; K and B are the slope and intercept of the working curve, respectively, which are obtained by curve calibration; C is the final calculated CODcr concentration.

2. The method for turbidity compensation for online monitoring of chemical oxygen demand water quality according to claim 1, characterized in that: The selected detection wavelengths are 610nm±10nm and 760nm±10nm respectively.

3. The method for turbidity compensation for online monitoring of chemical oxygen demand water quality according to claim 1, characterized in that: After identifying the value of A2, the system adaptively selects Formula 1 or Formula 2 for absorbance compensation, and further calculates the concentration of chemical oxygen demand through Formula 3 or Formula 4.

4. The method for turbidity compensation for online monitoring of chemical oxygen demand water quality according to claim 1, characterized in that: The turbidity correction parameter f is 0.9-1.3, and a is 0.005-0.

035.

5. The method for turbidity compensation for online monitoring of chemical oxygen demand water quality according to claim 1, characterized in that: It is necessary to judge the A2 and then apply different formulas for calculation.

6. The method for turbidity compensation for online monitoring of chemical oxygen demand water quality according to claim 1, characterized in that: The oxidant is 8g / L to 18g / L potassium dichromate, the masking agent is 20g / L to 300g / L mercuric sulfate, and the acid solution is 5g / L to 20g / L silver sulfate dissolved in concentrated sulfuric acid with a mass fraction of 98%.

7. The method for turbidity compensation for online monitoring of chemical oxygen demand water quality according to claim 1, characterized in that: The digestion solution is obtained by mixing the sample to be tested, an oxidant, a masking agent and an acid solution and digesting them at 160° C. to 175° C. for 10 min to 25 min.

8. The method for turbidity compensation for online monitoring of chemical oxygen demand water quality according to claim 1, characterized in that: The volume ratio of the water sample to be tested, the oxidant, the masking agent and the acid solution in the digested sample is 5:1:1:2.5.

Images