A method for screening carbon source types and determining appropriate dosage based on denitrification performance

Abstract

The present invention discloses a method for screening carbon source types based on denitrification performance and determining the appropriate dosage. Based on the existing carbon source pilot test method, an activated sludge acclimation step is added to achieve a more practical denitrification effect. At the same time, traditional carbon sources and new carbon sources, as well as new carbon sources themselves, can be compared to evaluate the denitrification capacity and acclimation time of different carbon sources, thereby making a more scientific choice of carbon source. The method of the present invention can be used to conduct experiments on a single new carbon source or to conduct experiments and comparisons on multiple different types of carbon sources simultaneously.

Description

Technical Field

[0001] The present invention relates to the technical field of water treatment and denitrification, and in particular to a method for screening carbon source types based on denitrification performance and determining appropriate dosage. Background Art

[0002] Current wastewater treatment plants generally face the problem of insufficient carbon sources for denitrification, which has become a major factor restricting the efficiency of biological denitrification. To achieve ideal denitrification results, it is necessary to consider the requirement of adding external carbon sources to supplement the electron donors for denitrification.

[0003] Methanol and sodium acetate are ideal supplemental carbon sources for denitrification. However, methanol is highly toxic and has a low flash point, making it a hazardous chemical. While sodium acetate is relatively stable, it is relatively expensive. These challenges have led researchers to explore and screen for new, low-cost, high-denitrification carbon sources. Designing a method to efficiently and accurately screen and determine the appropriate dosage of supplemental carbon sources suitable for long-term wastewater treatment plant operation is crucial for long-term plant operation.

[0004] Patent specification CN109231503A discloses a method for screening enhanced carbon sources for biofilm anaerobic denitrification in pond wastewater treatment, comprising the following steps: (1) isolation and cultivation of biofilm anaerobic microorganisms; (2) preparation of an anaerobic microbial suspension; (3) determination of anaerobic microbial metabolic characteristics; and (4) determination and analysis of anaerobic microbial carbon source utilization. This patented technology uses a Biolog-ECO ecological panel to screen the carbon sources required by anaerobic microorganisms.

[0005] Conventional carbon source pilot tests usually take the activated sludge mixture from the biochemical pool as the water sample, add different concentrations of carbon source and nitrate nitrogen (NO3-N) to the water sample, and measure the removal effect of the carbon source on nitrate nitrogen.

[0006] For example, patent specification CN112047462A discloses a method for screening carbon sources for sewage treatment plants. The method includes the following steps: Step 1: Collect several aliquots of the sludge-water mixture from the end of the biological pool and aerobic pool; Step 2: Pour the sludge-water mixture into a container, stir, and measure the DO of the sludge-water mixture; Step 3: After the DO drops below 0.5 mg / L, add potassium nitrate and a carbon source to the container to achieve an initial nitrate-nitrogen content of 35-45 mg / L and a COD of 175-225 mg / L; Step 4: 0-120 minutes after adding the potassium nitrate and carbon source, sample the container, filter it, and then measure the nitrate-nitrogen concentration; Step 5: Plot a response curve and determine the denitrification rate; and select a carbon source based on the denitrification rate.

[0007] There are many new carbon sources on the market. The different water quality of different sewage treatment plants leads to different properties of activated sludge. These new carbon sources have not been used in sewage treatment plants. If they are directly tested on the activated sludge in the biochemical pool of the sewage treatment plant, it is often impossible to reflect the actual denitrification performance of the carbon source during the actual long-term operation of the sewage treatment plant, and it is also impossible to accurately determine the appropriate dosage of the carbon source. It is difficult to provide practical guidance on the selection of carbon sources and the corresponding dosage of the sewage treatment plant. Summary of the Invention

[0008] The present invention provides a method for screening carbon source types based on denitrification performance and determining appropriate dosages. On the basis of the original carbon source pilot test method, an activated sludge acclimation step is added to achieve a more practical denitrification effect. At the same time, traditional carbon sources and new carbon sources, as well as new carbon sources themselves, can be compared to evaluate the denitrification capacity and acclimation time of different carbon sources, thereby making a more scientific selection of carbon sources.

[0009] The specific technical solutions are as follows:

[0010] A method for screening carbon source types and determining appropriate dosage based on denitrification performance includes:

[0011] The acclimation phase includes the following steps:

[0012] S01, take the activated sludge and water mixture from the biochemical pool of the sewage treatment plant as the test water sample, stir it in a closed manner to remove oxygen, and make the dissolved oxygen (DO) less than 0.3 mg / L, and take the supernatant of the water sample to test nitrate nitrogen;

[0013] S02, adding a nitrate nitrogen source according to the nitrate nitrogen detection value so that the nitrate nitrogen concentration in the water sample reaches a set value, and adding an experimental carbon source according to a first set amount;

[0014] S03, the water sample is stirred in a closed manner, and the stirring is stopped after the set time, and the supernatant of the water sample is taken to detect nitrate nitrogen;

[0015] S04, after the sampling is completed, the sample is allowed to settle, the supernatant is removed, and a volume of sewage treatment plant influent equal to the removed supernatant is added, and the experimental carbon source is added according to the second set amount. After aeration for a period of time, the sample is stopped, and the mixture is stirred and deoxygenated until the dissolved oxygen (DO) is less than 0.3 mg / L. The supernatant of the water sample is collected and tested for nitrate nitrogen;

[0016] S05, repeating steps S02 to S04 one or more times until the nitrate nitrogen removal rate at the set time in step S03 remains stable, and the acclimation is completed;

[0017] The gradient experiment stage includes the following steps:

[0018] S11, after the acclimated activated sludge water sample has consumed all the degradable COD, it is allowed to settle. The supernatant is replaced with an equal volume of deoxygenated sewage treatment plant effluent. After mixing, the sample is evenly distributed into multiple experimental containers for a gradient experiment. Before the gradient experiment begins, all water samples are deoxygenated to a dissolved oxygen DO of <0.3 mg / L.

[0019] S12, according to the carbon source concentration added during the daily operation of the sewage treatment plant (specifically COD Cr Design a gradient experiment and add a concentration gradient of carbon source to each experimental container (specifically COD Cr A group without adding carbon source was designed as blank sample;

[0020] S13, measuring the initial COD and nitrate nitrogen of the water sample in each experimental container, and adding an appropriate amount of nitrate nitrogen source in combination with the designed carbon source concentration gradient;

[0021] S14, adding the experimental carbon source to each experimental container according to the designed carbon source concentration gradient, stirring for a period of time, and taking the supernatant of the water sample to detect nitrate nitrogen;

[0022] Based on the nitrate nitrogen removal results corresponding to different types of experimental carbon sources in the acclimation stage and the different carbon source concentrations in the gradient experiment stage (specifically COD Cr Based on the corresponding nitrate nitrogen removal results, the carbon source suitable for the sewage treatment plant was screened and the appropriate dosage of the corresponding carbon source was determined.

[0023] The method of the present invention adds a second addition process of the experimental carbon source and the corresponding aeration and stirring process to remove oxygen to a dissolved oxygen DO of less than 0.3 mg / L during the acclimation stage to simulate the aerobic-anoxic switching operation process of the biochemical pool. The specific duration of each stage can be determined according to the actual situation of the sewage treatment plant to better fit the actual operation status of the sewage treatment plant.

[0024] In step S01, the MLSS (suspended solids concentration) of the activated sludge-water mixture may be 2500-5000 mg / L.

[0025] When the method of the present invention is used to conduct parallel experiments on different types of experimental carbon sources, the difference in MLSS of the activated sludge-water mixture of the effluent from the biochemical pool of the sewage treatment plant taken in step S01 in each group of experiments is preferably within ±1%, or the MLSS is converted to a uniform value when performing calculations to improve the comparability between different types of experimental carbon sources. In addition, the MLVSS (volatile suspended solids concentration) and SV of the activated sludge-water mixture of the effluent from the biochemical pool of the sewage treatment plant taken in step S01 in each group of experiments can also be tested. 30 (sludge settling ratio), etc.

[0026] In a preferred embodiment, after the nitrate nitrogen removal rate remains stable at the time set in step S03 in step S05, the MLSS and SV of the water sample are measured. 30 .

[0027] Changes in MLSS of water samples before and after acclimation and SV based on water samples after acclimation 30 The sludge index (SVI) calculated from the MLSS and the sludge volume index (MLSS) is used as the basis for screening carbon sources. These indicators can reflect the degree of sludge proliferation before and after acclimation, indicating whether the experimental carbon source will significantly increase the sludge volume. If the sludge volume increases significantly, the subsequent sludge treatment and disposal costs will increase.

[0028] The nitrate nitrogen source in the method of the present invention can be potassium nitrate.

[0029] In a preferred embodiment, after the nitrate nitrogen removal rate remains stable at the time set in step S03 in step S05, sufficient aeration is performed and the supernatant of the water sample is taken to detect COD Cr , reflecting the refractory organic matter in the experimental carbon source, and based on this, screening the carbon source suitable for the sewage treatment plant. When the method of the present invention is used to screen different experimental carbon sources, if the COD Cr It is significantly higher than other experimental carbon sources, indicating that the experimental carbon source contains a large amount of difficult-to-degrade organic matter, which is not conducive to sewage treatment. Therefore, it can also be used as one of the important bases for screening carbon sources.

[0030] In step S02, the set value may be 15-20 mg / L.

[0031] In one embodiment, in step S02, the dosage of the experimental carbon source is based on COD Cr The ratio of the meter to the set value is greater than 4.

[0032] In one embodiment, the first set amount in step S02 and the second set amount in step S04 are not less than twice the carbon source dosage during daily operation of the sewage treatment plant. For example, they can be 2 to 4 times the carbon source dosage during daily operation of the sewage treatment plant.

[0033] In one embodiment, the first set amount in step S02 is the same as the second set amount in step S04.

[0034] The set time in step S03 can be 30 minutes and / or 2 hours. 30 minutes is primarily for the hydraulic retention time of the denitrification deep bed filter, while 2 hours is primarily for the hydraulic retention time of the anoxic section of the biochemical tank. These two times are relatively typical. Of course, additional sampling time points such as 1 hour and 3 hours can be added as needed.

[0035] In step S04 , the volume of the supernatant removed may be 50% to 70% of the total volume of the water sample, preferably 60% of the total volume of the water sample.

[0036] In step S05, the set value of the nitrate-nitrogen concentration may be increased or not changed each time steps S02 to S04 are repeated. For example, if the denitrification effect during the acclimation process is good and the measured NO3-N concentration is less than 1 mg / L, the set value of the NO3-N concentration in the water sample may be increased, for example, to 25 mg / L, each time a nitrate-nitrogen source is added to introduce NO3-N during the repeated execution of steps S02 to S04.

[0037] In step S05, steps S02 to S04 may be repeated 3 to 5 times. The number of repetitions may also be adjusted based on the results of each measurement. If there is no significant increase in nitrate nitrogen removal, the acclimation phase may be terminated. When comparing different experimental carbon sources, the number of repetitions in the acclimation phase may be standardized to ensure that the comparison is conducted under the same conditions.

[0038] The method of the present invention consumes all degradable COD before the gradient experiment stage (specifically, continuous stirring and / or aeration can be used to consume the degradable COD), which is beneficial for minimizing the impact of COD added during the acclimation stage during the gradient experiment. Because the COD concentration during the gradient experiment stage is lower than that during the acclimation stage, if the COD during the acclimation stage is not completely consumed, the denitrification effect of the gradient experiment will be significantly affected.

[0039] In step S11 , the volume of the replaced supernatant may be 50% to 70% of the total volume of the water sample, preferably 60% of the total volume of the water sample.

[0040] In step S11, the number of the experimental containers may be 4 to 6, and a blank sample may be included as a control group.

[0041] In step S12, the carbon source is added at a concentration gradient of COD Cr The concentration can be 10mg / L, 20mg / L, 30mg / L, 40mg / L, 50mg / L, etc.

[0042] In one embodiment, in step S13, an appropriate amount of nitrate nitrogen source is added to make the nitrate nitrogen concentration in the water sample be between 10 and 15 mg / L.

[0043] In step S14, the sealed stirring time can be 30 minutes and / or 2 hours. 30 minutes is primarily for the hydraulic retention time in the denitrification deep bed filter, while 2 hours is primarily for the hydraulic retention time in the anoxic section of the biochemical tank. These two times are relatively typical. Of course, additional sampling time points such as 1 hour and 3 hours can be added as needed.

[0044] Compared with the prior art, the present invention has the following beneficial effects:

[0045] The present invention provides a method for screening carbon source types based on denitrification performance and determining appropriate dosage. On the basis of the original carbon source pilot test method, an activated sludge acclimation step is added to achieve a more practical denitrification effect. At the same time, traditional carbon sources and new carbon sources, as well as new carbon sources themselves, can be compared to evaluate the denitrification capacity and acclimation time of different carbon sources, thereby making a more scientific selection of carbon sources.

[0046] The method of the present invention can be used to experiment on a single new carbon source, or to conduct experiments and comparisons on multiple different types of carbon sources at the same time, for example, by comparing conventional carbon sources such as sodium acetate, glucose, and acetic acid, or by comparing the denitrification effects of different types of new carbon sources. DETAILED DESCRIPTION

[0047] The present invention will be further described below with reference to specific examples. It should be understood that these examples are only intended to illustrate the present invention and are not intended to limit the scope of the present invention.

[0048] In the following examples, the operating methods without specifying specific conditions are generally carried out under conventional conditions or conditions recommended by the manufacturer.

[0049] The nitrate nitrogen source used in this embodiment is potassium nitrate.

[0050] A method for screening carbon source types and determining appropriate dosages based on denitrification performance was developed. Four experimental carbon sources provided by different manufacturers were compared and screened using sodium acetate as a control experimental carbon source commonly used in sewage treatment plants. The following steps were performed for each experimental carbon source:

[0051] First, the domestication stage, including the following steps:

[0052] S01: Take the activated sludge and water mixture from the biochemical pool of the sewage treatment plant (the initial MLSS of the acclimation is 2891 mg / L) as the test water sample, stir it in a closed manner to remove oxygen, and reduce the dissolved oxygen (DO) to <0.3 mg / L. Take the supernatant of the water sample and filter it for nitrate nitrogen detection;

[0053] S02, add nitrate nitrogen source according to the nitrate nitrogen detection value, so that the nitrate nitrogen concentration in the water sample reaches the set value of 20mg / L, according to 100mg COD Cr The experimental carbon source was added at the target concentration of / L;

[0054] S03, the water sample was stirred in a closed manner for 2 hours, and the stirring was stopped after settling for 5 minutes. The supernatant of the water sample was filtered and tested for nitrate nitrogen;

[0055] S04, after sampling, let it settle for half an hour, remove the supernatant, the volume of the removed supernatant is 60% of the total volume of the water sample, add the same volume of sewage treatment plant water as the removed supernatant, according to 100mg COD Cr The experimental carbon source was added at a target concentration of / L, and aeration was stopped after overnight. Stirring was continued to remove oxygen until the dissolved oxygen DO was less than 0.3mg / L, and the supernatant of the water sample was taken to detect nitrate nitrogen;

[0056] S05, repeat steps S02 to S04, and repeat steps S02 to S04 five times in total during the whole experiment process to complete the acclimation, and measure the MLSS and SV of the water sample at this time. 30 and COD after 3 hours of aeration Cr , calculate the MLSS growth rate and SVI during the acclimation stage.

[0057] Table 1 shows the experimental data measured and calculated using the experimental carbon sources A to D provided by four different manufacturers and sodium acetate during the above-mentioned acclimation process.

[0058] Table 1

[0059]

[0060] Note: SVI = SV after completion of acclimation 30 / After taming is completed MLSS×10 6 .

[0061] You can see:

[0062] 1. According to the results of nitrate nitrogen removal in the first experiment over 2 hours, experimental carbon sources A to D were not as effective as sodium acetate in denitrification. However, according to the results of nitrate nitrogen removal in the fifth experiment over 2 hours, i.e., the denitrification results after acclimation, experimental carbon source A had the best denitrification performance, followed by sodium acetate. Carbon source D, like sodium acetate, showed no significant change in denitrification performance before and after acclimation. This shows that the activated sludge in this sewage treatment plant can quickly adapt to carbon source D and achieve its denitrification effect without acclimation, while carbon sources A to C require a period of acclimation to achieve their denitrification effect. Based on the preliminary judgment of the denitrification trend, experimental carbon source D can be used as a substitute for sodium acetate in emergency addition to the sewage treatment plant (when sodium acetate is out of stock or the price of carbon source D is low). Under the condition of long-term operation of the sewage treatment plant, experimental carbon source A gradually replacing sodium acetate can achieve better denitrification effect than the other experimental carbon sources, providing more scientific and practical guidance for the screening and use of carbon sources in sewage treatment plants.

[0063] 2. COD of experimental carbon sources A~D after acclimation Cr The residual values ​​are close to those of sodium acetate, indicating that the experimental carbon sources A to D contain less refractory organic matter, have little impact on effluent quality, and are suitable for use in sewage treatment plants.

[0064] 3. The sludge index (SVI) of the five experimental carbon sources was within the normal range.

[0065] 4. When the acclimation was completed, the MLSS growth rate of experimental carbon sources A and D was lower than that of sodium acetate, while the MLSS growth rate of experimental carbon sources B and C was higher than that of sodium acetate. This indicates that sodium acetate and experimental carbon sources B and C will cause a large amount of sludge when used in the long-term operation of the sewage treatment plant, thereby increasing the sludge treatment cost, while the experimental carbon sources A and D have relatively less sludge production and will not increase the sludge treatment cost of the sewage treatment plant too much.

[0066] According to the experimental results of the acclimation stage, a gradient experiment was conducted on the experimental carbon sources A to D and sodium acetate based on the conventional carbon source addition concentration of the sewage treatment plant. For any carbon source, the following steps were performed:

[0067] S11. After the acclimated activated sludge water sample has consumed all the degradable COD, it is allowed to settle. The supernatant is replaced with an equal volume of deoxygenated sewage treatment plant effluent. The volume of the replaced supernatant is 60% of the total volume of the water sample. After mixing, the supernatant is evenly distributed into 4 experimental containers (one of which is a blank sample control group) to carry out a gradient experiment. Before the gradient experiment begins, all water samples are deoxygenated to a dissolved oxygen DO of <0.3 mg / L.

[0068] S12, design the carbon source concentration gradient in each experimental container of the gradient experiment according to the carbon source concentration added during the daily operation of the sewage treatment plant. Specifically, COD Cr The concentrations were 10 mg / L, 20 mg / L, and 30 mg / L, respectively, and a group without adding carbon source was designed as a blank sample.

[0069] S13, measuring the initial COD and nitrate nitrogen of the water sample in each experimental container, and adding an appropriate amount of nitrate nitrogen source in combination with the designed carbon source addition concentration gradient to make the nitrate nitrogen concentration in the water sample 15 mg / L.

[0070] S14, adding the experimental carbon source to each experimental container according to the designed carbon source concentration gradient, stirring for 2 hours in a closed manner, settling for 5 minutes, taking water samples, filtering the supernatant, and detecting nitrate nitrogen.

[0071] Table 2 shows the experimental data measured and calculated for the experimental carbon sources A to D and sodium acetate during the above gradient experiment.

[0072] Table 2 Unit: mg / L

[0073]

[0074] Adding COD to the carbon source Cr The average COD of the experimental carbon source was obtained by averaging the 2h nitrate nitrogen removal with a concentration gradient of 10mg / L, 20mg / L, and 30mg / L.Cr The amount of nitrate nitrogen removed when the dosage is 20 mg / L (last column). Comparing the denitrification effects of experimental carbon sources A to D and sodium acetate, experimental carbon source A has the best denitrification effect, followed by sodium acetate. Although the denitrification effect of experimental carbon source C is not as good as sodium acetate, it is also relatively good, which is consistent with the denitrification effect trend at the end of the acclimation stage.

[0075] In summary, experimental carbon source A is the most suitable carbon source with high-efficiency denitrification performance required for the long-term operation of the sewage treatment plant. At the same time, it will not excessively increase the sludge treatment cost of the sewage treatment plant, and it also contains less difficult-to-degrade organic matter and has little impact on the effluent quality of the sewage treatment plant.

[0076] Based on the nitrate nitrogen removal results corresponding to different concentrations of experimental carbon source A in the gradient experiment phase, the appropriate dosage of experimental carbon source A during the actual operation of the sewage treatment plant can be determined.

[0077] In addition, it should be understood that after reading the above description of the present invention, those skilled in the art may make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the claims attached to this application.

Claims

1. A method for screening carbon source types and determining appropriate dosage based on denitrification performance, characterized in that: include: The acclimation phase includes the following steps: S01, take the activated sludge and water mixture from the biochemical pool of the sewage treatment plant as the test water sample, stir it in a closed manner to remove oxygen, and make the dissolved oxygen (DO) less than 0.3 mg / L, and take the supernatant of the water sample to test nitrate nitrogen; S02, adding a nitrate nitrogen source according to the nitrate nitrogen detection value so that the nitrate nitrogen concentration in the water sample reaches a set value, and adding an experimental carbon source according to a first set amount; S03, the water sample is stirred in a closed manner, and the stirring is stopped after the set time, and the supernatant of the water sample is taken to detect nitrate nitrogen; S04, after the sampling is completed, the sample is allowed to settle, the supernatant is removed, and a volume of sewage treatment plant influent equal to the removed supernatant is added, and the experimental carbon source is added according to the second set amount. After aeration for a period of time, the sample is stopped, and the mixture is stirred and deoxygenated until the dissolved oxygen (DO) is less than 0.3 mg / L. The supernatant of the water sample is collected and tested for nitrate nitrogen; S05, repeating steps S02 to S04 one or more times until the nitrate nitrogen removal rate at the set time in step S03 remains stable, and the acclimation is completed; The gradient experiment stage includes the following steps: S11, after the acclimated activated sludge water sample has consumed all the degradable COD, it is allowed to settle. The supernatant is replaced with an equal volume of deoxygenated sewage treatment plant effluent. After mixing, the sample is evenly distributed into multiple experimental containers for a gradient experiment. Before the gradient experiment begins, all water samples are deoxygenated to a dissolved oxygen DO of <0.3 mg / L. S12, designing a gradient experiment based on the carbon source concentration added during routine operation of the sewage treatment plant to add a carbon source concentration gradient to each experimental container, and designing a group without adding a carbon source as a blank sample; S13, measuring the initial COD and nitrate nitrogen of the water sample in each experimental container, and adding an appropriate amount of nitrate nitrogen source in combination with the designed carbon source concentration gradient; S14, adding the experimental carbon source to each experimental container according to the designed carbon source concentration gradient, stirring for a period of time, and taking the water sample supernatant to detect nitrate nitrogen; Based on the nitrate-nitrogen removal results corresponding to different types of experimental carbon sources in the acclimation stage and the nitrate-nitrogen removal results corresponding to different carbon source concentrations in the gradient experimental stage, the carbon sources suitable for the sewage treatment plant were screened and the appropriate dosage of the corresponding carbon sources was determined.

2. The method according to claim 1, characterized in that In step S01, the MLSS of the activated sludge-water mixture is 2500-5000 mg / L; For different types of experimental carbon sources, the MLSS of the activated sludge-water mixture obtained from the effluent of the biochemical pool of the sewage treatment plant in step S01 fluctuates within ±1%, or the MLSS is converted to a uniform value during calculation.

3. The method according to claim 2, characterized in that In step S05, after the nitrate nitrogen removal rate remains stable at the time set in step S03, the MLSS and SV of the water sample are measured. 30 ; Changes in MLSS of water samples before and after acclimation and SV based on water samples after acclimation 30 The sludge index calculated by MLSS is used as the basis for screening carbon sources.

4. The method according to claim 1, wherein In step S05, after the nitrate nitrogen removal rate remains stable at the time set in step S03, the water is fully aerated and the supernatant of the water sample is taken to detect COD. Cr , reflecting the situation of difficult-to-degrade organic matter in the experimental carbon source, and using this as one of the bases to screen carbon sources suitable for sewage treatment plants.

5. The method according to claim 1, wherein In step S02, the set value is 15-20 mg / L.

6. The method according to claim 1, characterized in that In step S02, the dosage of the experimental carbon source is based on COD Cr The ratio of the meter to the set value is greater than 4.

7. The method according to claim 1, characterized in that The first set amount in step S02 and the second set amount in step S04 are both not less than 2 times the amount of carbon source added during daily operation of the sewage treatment plant; The first set amount in step S02 is the same as the second set amount in step S04.

8. The method according to claim 1, characterized in that The set time in step S03 is 30 minutes or 2 hours.

9. The method according to claim 1, characterized in that In step S04 , the volume of the supernatant removed is 50% to 70% of the total volume of the water sample.

10. The method according to claim 9, characterized in that In step S04 , the volume of the supernatant removed is 60% of the total volume of the water sample.

11. The method according to claim 1, wherein In step S11, the volume of the replaced supernatant is 50% to 70% of the total volume of the water sample.

12. The method according to claim 11, characterized in that In step S11, the volume of the replaced supernatant is 60% of the total volume of the water sample.

13. The method according to claim 1, wherein In step S05 , each time the process of steps S02 to S04 is repeated, the set value of the nitrate nitrogen concentration is increased or not changed.

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