CN110823820A - Turbidity interference elimination method for COD measurement - Google Patents

Turbidity interference elimination method for COD measurement Download PDF

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CN110823820A
CN110823820A CN201910985614.2A CN201910985614A CN110823820A CN 110823820 A CN110823820 A CN 110823820A CN 201910985614 A CN201910985614 A CN 201910985614A CN 110823820 A CN110823820 A CN 110823820A
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胡映天
覃亚丽
赵冬冬
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Zhejiang University of Technology ZJUT
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Abstract

The turbidity interference elimination method for COD measurement includes the following steps: step 1, determining the universal derivation order in the direct derivative spectrum, which specifically comprises the following steps: obtaining a turbidity particle spectral line type, introducing CD and m parameters, and determining a derivative order; step 2, eliminating turbidity interference in COD measurement, specifically comprising: acquiring an n-order derivative spectrum of the water sample, and establishing a regression prediction model and predicting COD. The invention obtains the universal derivative order suitable for a large amount of actual water environments, eliminates turbidity interference by using a method of directly solving the high-order derivative, can simultaneously keep the original spectral characteristics of COD substances, and avoids the operations of obtaining turbidity baseline data, turbidity compensation and the like one by one. The invention avoids acquiring turbidity baseline spectral data one by one in various water environments, and has simple and convenient operation and good universality.

Description

Turbidity interference elimination method for COD measurement
Technical Field
The invention relates to a turbidity interference elimination method used in COD measurement.
Background
The ultraviolet-visible absorption spectrum technology has the advantages of wide application range, simplicity, in-situ, rapidness, no secondary pollution, no reagent, accuracy, stability and the like, and is widely applied to water quality measurement. Chemical Oxygen Demand (COD) is an important parameter that reflects the concentration of organic pollutants in water. To our knowledge, most organic contaminants absorb only uv light and the scattering caused by the turbidity particles causes the transmitted light intensity to decay over the entire spectral range, and thus turbidity particles in the water cause a bias in the COD measurement. The correction of turbidity disturbances is therefore an important task.
The existing COD correction method is realized by establishing a regression model between a prediction error of COD and turbidity, but the method can only obtain a mapping function and has poor universality. In addition, some turbidity compensation techniques are implemented by subtracting the absorbance caused by turbidity particles from the sample absorption spectrum, and there is a technical bottleneck because the true turbidity spectrum in the actual water environment is difficult to obtain and the spectral shape changes greatly with time and space.
In recent years, methods of first or even higher order derivatives of absorbance have been increasingly used to solve problems of sample turbidity, mother liquor background, etc., due to the advantage of derivative spectroscopy techniques that eliminate background while enhancing spectral detail. In the field of water quality monitoring, common treatment methods of derivative spectroscopy include a zero crossing point technology and a ratio derivative spectroscopy. However, these methods also require the true absorption spectrum of the nephelometric particles as a standard baseline in order to find the intersection of all curves or to use as a standard spectral factor. However, due to the existence of technical bottlenecks, the difficulty in obtaining the turbidity spectrum baseline is high, and the significance in obtaining the turbidity spectrum baseline is not great due to the fluctuation of the turbidity spectrum baseline along with time and space. Although the direct derivative method does not require the acquisition of a turbidity baseline, it is currently rarely used due to the lack of relevant theoretical guidance. For example, when the turbidity disturbance correction is performed by using direct derivative spectroscopy, how to determine the derivative number has not been a systematic theoretical guidance method so far. This leads to blind selection of derivation orders or search for an optimal solution through traversal experiments for different orders, which results in huge workload.
Therefore, aiming at eliminating turbidity interference in the COD measurement by the spectrum method, a method for determining the derivative order in the direct derivative spectrum technology needs to be found, the determined order is directly applied to various environments, and turbidity baseline data are prevented from being acquired one by one.
Disclosure of Invention
In order to overcome the defects in the prior art and avoid the operation of acquiring the turbidity spectrum baseline data in the actual water body one by one, the turbidity interference elimination method for COD measurement is provided.
The invention provides a method for determining the universal derivative order in a direct derivative spectrum, and turbidity interference elimination is carried out by utilizing a direct derivative spectrum technology. The method comprises the steps of firstly calculating parameters CD and m related to particle size distribution of turbidity particles in a plurality of typical samples according to absorption spectrum line types, seeking a minimum particle size distribution state according to the parameters, and determining a universal derivative number n under the state; and then establishing a regression model and predicting the COD value by using the nth derivative spectrum of the water sample.
A turbidity interference elimination method for COD measurement comprises the following steps:
1) the method for determining the universal derivation order according to the limit state approaching to the minimum particle size distribution so as to ensure the suitability for other practical water environments comprises the following steps:
11) obtaining turbidity particle spectrum line type a (lambda) which represents particle size distribution characteristics; the method comprises the following steps:
filtering a water sample through a Millipore filter membrane of 0.22 mu m, placing filter residues in deionized water with the same volume as that of the original water sample, shaking uniformly, measuring the absorption spectrum of the filter residues, and then normalizing the filter residues as follows:
Figure BDA0002236588520000021
wherein A (λ) is the absorbance of the nephelometric particles at different wavelengths, AmaxIs the maximum absorbance value, a (λ) dimensionless normalized coefficient; when the particle size distribution of the turbidity particles is stable and unchanged, even if the particle concentration is different, the spectral line type can not be changed, and the turbidity particle size distribution can be characterized by a (lambda);
12) introducing and calculating CD and m parameters for reflecting the size and scattering type of turbidity particles; the method comprises the following steps:
a parameter CD is defined to characterize the degree of curvature of the spectral line as follows:
Figure BDA0002236588520000022
substituting a (lambda) in the step 11) into a formula 2 to calculate, wherein CD is the sum of curvature absolute values at different wavelengths;
fitting a (lambda) in step 11) with a polynomial from first order to higher order until the Sum of Squared Errors (SSE) is less than a threshold value,
a(λ)≈cmλm+cm-1λm-1+cm-2λm-2+...+c0(3)
the resulting m is the lowest fitting order satisfying the SSE threshold, C0,C1,…,CmIs the fitting coefficient;
the larger the CD and m values, the steeper the curve, indicating smaller haze particles;
13) determining a universal derivation order according to the limit state to ensure that the method is suitable for other common water samples; the method comprises the following specific steps:
131) respectively searching the maximum values of CD and m in a plurality of typical water samples, and when the maximum values of CD and m are reached simultaneously, the particle size of suspended particles in the corresponding water samples tends to be minimum and approaches to a limit state;
132) when the CD and m cannot reach the maximum simultaneously in the step 131), increasing the samples and repeating the step 1)
133) Under the limit state that the particle size tends to be minimum, n is m and is a universal derivation order, and the absorbance derivative of turbidity can be ensured to be close to 0 for other water samples with larger suspended particles;
2) eliminating turbidity interference in COD measurement by using a direct derivative spectrum method, and avoiding the operation of obtaining and compensating a turbidity baseline; the method comprises the following steps:
21) acquiring an n-order derivative spectrum of a water sample and simplifying a superposition formula; the method specifically comprises the following steps:
turbidity and COD induced light attenuation effects are basically independent of each other, and the superposition property according to Lambert beer law
Figure BDA0002236588520000031
A(λ)＝CL·kCOD(λ)+TL·ktur(λ)＝ACOD(λ)+Atur(λ) (5)
Wherein k isCODAnd kturThe absorption coefficients of COD and turbidity are respectively, C is the COD concentration, T is the turbidity, and L is the optical length; a (. lamda.), ACOD(lambda), and Atur(lambda) is absorbance of the water sample, COD and turbidity respectively;
solving an nth derivative of the formula (5), wherein the spectrum of the nth derivative also meets the superposition property;
Figure BDA0002236588520000032
the absorption spectrum curve of the turbidity particles is generally in a monotonous decreasing trend; organic matters in water have various absorption peaks, so that the absorption spectrum shape is more complicated; according to the mathematical theory, after derivation of the absorption spectrum using the universal derivation order, the right side d of equation (6) in step 1)nAtur(λ)/dλnClose to 0, negligible; at the same time, dnACOD(λ)/dλnWill be retained, and in proportion to the concentration of COD, the original absorption peak will also be sharpened; at this time, the formula is simplified as:
Figure BDA0002236588520000041
thereby eliminating the turbidity interference in the absorption spectrum;
22) establishing a regression prediction model and predicting COD; the method specifically comprises the following steps:
221) establishing a regression prediction model between the nth order absorbance derivative of the modeling water sample and the corresponding real COD value,
COD＝f(A(n) 1,A(n) 2,A(n) 3,…,A(n) k) (8)
wherein f represents a functional relationship, A(n) kIs the nth derivative of the absorbance at the kth feature point;
222) in the actual measurement, the n-order absorbance derivative of the water sample to be measured is input into the formula 8 to predict the COD.
Preferably, the threshold value of the Sum of Squared Errors (SSE) of step 13) is 0.5.
By using the method provided by the invention, the universal derivative order suitable for a large amount of actual water environments is obtained, and then the turbidity interference is eliminated by using the method of directly solving the high-order derivative, so that the original spectral characteristics of COD (chemical oxygen demand) substances can be simultaneously reserved, and operations such as obtaining turbidity baseline data one by one, compensating turbidity and the like are avoided.
The key points and the protection points of the invention are as follows:
1. the direct derivative spectrum method is used for eliminating turbidity interference, and the acquisition and compensation operation of a turbidity baseline is avoided.
2. The parameters CD and m are set forth and defined to reflect the size of the turbidity particles and the type of scattering.
3. A method for determining the derivative order. Theoretically, the nth order absorbance derivative of the turbidity particles will approach zero when n ≧ m. Therefore, the elimination of turbidity interference can be realized as long as the selected derivative order is greater than or equal to m.
4. The application of extreme conditions. For practical purposes, the particle size of the suspended particles cannot be infinitely small due to the dissolution effect, and there must be a limit approaching the minimum particle size distribution. And determining the derivative order n in the state, and applying to other actual water environments.
The invention has the advantages that
1. The invention adopts a direct derivative spectrum method to process turbidity interference in COD measurement, avoids acquiring turbidity baseline spectrum data in various water environments one by one, and has simple and convenient operation.
2. The derivation order determination method provided seeks a limit state by using parameters related to particle size, and determines the derivation parameters according to the limit state so as to ensure the same applicability in other practical water environments, so that the method has good universality.
Drawings
FIG. 1 is a flow chart of the method of the present invention.
Fig. 2 is a simulation of absorbance curves for rayleigh scattering and mie scattering when the power k of the wavelength takes different values.
FIG. 3(A) is an absorption spectrum curve of formalin standard solution with different turbidity.
Fig. 3(B) is an absorption spectrum curve of formalin turbidity solution after normalization.
FIG. 4(A) is an absorption spectrum curve of wastewater from four plants.
FIG. 4(B) is a normalized line of absorption spectra of nephelometric particles in four plant wastewaters and formalin solutions.
FIG. 5(A) is an absorption spectrum before and after filtration of wastewater from four plants.
FIG. 5(B) is a fourth derivative absorption spectrum before and after four-plant wastewater filtration.
Detailed Description
The technical scheme of the invention is further explained by combining the attached drawings.
The invention provides a method for determining the universal derivative order in a direct derivative spectrum, and turbidity interference elimination is carried out by utilizing a direct derivative spectrum technology. The method comprises the steps of firstly calculating parameters CD and m related to particle size distribution of turbidity particles in a plurality of typical samples according to absorption spectrum line types, seeking a minimum particle size distribution state according to the parameters, and determining a universal derivative number n under the state; then, establishing a regression model and predicting a COD value by using the nth derivative spectrum of the water sample, and referring to FIG. 1
The turbidity interference elimination method for COD measurement includes the following steps:
a: determining the universal derivation order in the direct derivative spectrum;
a. spectral line of turbidity particles
Firstly, turbidity reflects the scattering optical properties of suspended particles in water, and rayleigh scattering is caused when the particle size is much smaller than the incident wavelength, while mie scattering theory is met when the particle size is equivalent to the wavelength scale. In practical water environment, the particle size is 0.1-10 μm, and thus the particle size mainly belongs to the Mie scattering category. Theoretically, rayleigh scattering intensity is inversely proportional to the fourth power of the wavelength; the Mie scattering algorithm is more complex due to the fact that the Mie scattering algorithm contains complex variables and infinite series. In short, the mie scattering intensity is inversely proportional to the lower power of the wavelength, with the specific power depending on the size of the particle size. Thus, the absorption spectrum profile of the turbidity particles is determined by the particle size distribution.
Referring to fig. 2, the simulation of the absorbance curves of rayleigh scattering and mie scattering under different values of k, which is the power of wavelength, is shown. It can be seen that the smaller the particle size, the larger the value of k, and the steeper the curve of the absorbance curve.
In order to characterize the spectral profile of the turbidity particles in a certain water environment, a water sample can be filtered through a Millipore filter membrane (0.22 μm), filter residues are placed in deionized water with the same volume as that of the original water sample, the absorption spectrum of the filter residues is measured after shaking up, and then normalization is carried out, as follows:
Figure BDA0002236588520000061
wherein A (λ) is the absorbance of the nephelometric particles at different wavelengths, AmaxIs the maximum absorbance value, a (λ), a dimensionless normalized coefficient.
Fig. 3(a) shows absorption spectra of formalin standard solutions with different turbidities, and fig. 3(B) shows a normalized formalin turbidity absorption spectrum. It can be seen that when the particle size distribution of the turbidity particles is stable and constant, the spectral line pattern does not change even if the particle concentration is different.
b. Introducing CD and m parameters;
the present invention defines a parameter, CD, for characterizing the degree of curvature of a spectral line as follows:
Figure BDA0002236588520000062
CD is the sum of the absolute values of curvature at different wavelengths.
The turbidimetric profile is then fitted with a polynomial from first order to higher order until the Sum of Squared Error (SSE) is less than a certain threshold (0.5 is chosen here),
a(λ)≈cmλm+cm-1λm-1+cm-2λm-2+...+c0(3)
the resulting m is the lowest fitting order satisfying the SSE threshold, C0,C1,…,CmAre fitting coefficients.
The larger the CD and m values, the more steeply curved the curve, indicating smaller haze particles. In summary, the two parameters of the turbidity line type, the bending degree CD and the polynomial fitting order m, can reflect the overall size of suspended particles in different water environments and the scattering type of the suspended particles.
c. Determining the derivative order;
in order to eliminate turbidity interference signals in absorption spectrum, the invention adopts a direct derivative spectrum method. Assuming the derivative order is n, mathematically the nth order absorbance derivative of turbidity will approach zero when n ≧ m. For larger particles, the absorbance curve is closer to the horizontal or inclined line when m is 0 or 1, at which time the scattering intensity no longer has a significant relationship with the wavelength, and the requirement for eliminating turbidity interference can be satisfied as long as the derivative number n is greater than 1. For smaller particle size particles, a higher derivative number n is required to bring the nth order absorbance derivative of turbidity close to 0.
In a practical aqueous environment, the particle size of the suspended particles cannot be infinitesimal due to dissolution, so that CD and m cannot be infinitely large. Respectively searching the maximum values of CD and m in a plurality of typical water samples, and when the maximum values of CD and m are reached simultaneously, the particle size of suspended particles in the corresponding water samples tends to be minimum and approaches to a limit state; if CD and m cannot reach maximum simultaneously, the sample is increased. In a limit state that the particle size tends to be minimum, n is m, and n is a universal derivation order, in other words, the derivation order obtained in the state that the particle size is minimum can ensure that the absorbance derivative of turbidity is close to 0 for other water samples with larger suspended particles, so that the elimination of turbidity interference is realized.
B, eliminating turbidity interference in COD measurement;
a. acquiring an n-order derivative spectrum of a water sample;
for the scattering effect of turbidity particles, because the contribution of the turbidity particles in the absorption spectrum is obviously larger than other influencing factors, the turbidity and the light attenuation effect caused by COD are basically independent of each other, and the turbidity and the COD satisfy the superposition property of the Lambert beer law. At this time, the process of the present invention,
Figure BDA0002236588520000071
A(λ)＝CL·kCOD(λ)+TL·ktur(λ)＝ACOD(λ)+Atur(λ) (5)
wherein k isCODAnd kturThe absorption coefficients of COD and turbidity, respectively, C is the COD concentration, T is the turbidity, and L is the optical length. A (. lamda.), ACOD(lambda), and Atur(lambda) absorbance of water sample, COD and turbidity, respectively.
And (4) solving an nth derivative according to the determined universality derivative order in the last step and the formula (5), wherein the spectrum of the nth derivative also meets the superposition property.
Figure BDA0002236588520000072
According to scattering theory, the shorter the wavelength, the stronger the scattering and the greater the absorbance value. That is, the absorption spectrum curve of the turbidity particles generally has a monotonically decreasing trend. And the organic matters in water have various absorption peaks, so the absorption spectrum shape is more complicated. According to the mathematical theory, the absorbance derivative d of the second term turbidity on the right in equation (6) is guided by the determination of the universal derivative order in the previous stepnAtur(λ)/dλnClose to 0, negligible; at the same time, the first COD absorbance derivative on the rightnACOD(λ)/dλnWill be retained and in proportion to the concentration of COD, the original absorption peak will also be sharpened. At this time, the formula can be simplified as:
Figure BDA0002236588520000081
thereby eliminating the turbidity interference in the absorption spectrum.
b. Establishing a regression prediction model and predicting COD;
establishing a regression prediction model between the nth order absorbance derivative of the modeling water sample and the corresponding real COD value,
COD＝f(A(n) 1,A(n) 2,A(n) 3,…,A(n) k) (8)
wherein f represents a functional relationship, A(n) kThe nth derivative of the absorbance at the kth feature point.
In actual measurement, the nth order absorbance derivative of the water sample to be measured is input into a regression prediction model (formula 8) to predict the COD.
Four typical plant effluents were selected as the study subjects, and the absorption spectra thereof are shown in FIG. 4 (A). The filter residue of each water sample after being filtered through a Millipore filter membrane (0.22 μm) was put into the same volume of deionized water to obtain the corresponding turbidity particle absorption spectrum. The turbidity profiles of the water samples were obtained by normalization, as shown in fig. 4 (B). The results show that the turbidity particles in different water environments have different optical properties due to different particle size distributions, and the absorption spectra have larger difference. Compared with four actual water samples, the absorbance curve of the formalin turbidity liquid is closer to the absorbance curve of Rayleigh scattering. Since the formalin turbidity particles generally follow a normal distribution centered at 1.2 μm, the particle size is small, close to the colloidal state, and the absorbance curve is more curved and steep. The particles existing in a real water environment are often larger and have poorer uniformity, and the absorbance curve of the particles is flatter. That is, formalin standard nephelometric liquid can be regarded as a limiting state with minimal particle size distribution for the determination of the universal derivation parameters.
According to the method of the present invention, CD and m parameters of the spectral line of turbidity particles in each water sample were calculated separately as shown in Table 1.
TABLE 1 CD and m parameters of normalized spectral curves for turbidity particles in various water samples (where SSE is the sum of squares of error)
It can be seen from the table that the formalin turbidity solution has larger CD and m parameters, which is consistent with the above analysis, i.e. the absorbance curve of the formalin turbidity solution is more curved and contains smaller particles, which can be regarded as "limit" state. At this time, as long as the derivative order n is greater than or equal to the m value of the formalin turbidity liquid, the order n can be universally used for eliminating turbidity interference of other actual water samples. Generally, a derivative order that is too high will result in too low signal intensity and noise amplification, and therefore, generally, zero to fourth derivative spectra are more widely used in spectrophotometry-chemometrics. According to table 1, the m value of the formalin turbidity solution is 4, so that the derivative order can be determined to be 4, so as to eliminate the interference of turbidity particles in different water samples.
As shown in fig. 5(a), the absorption spectrum curves of the four real water samples before and after filtration have a certain difference due to the presence of turbidity particles. Four-order derivative spectra were calculated for them, respectively, and as shown in fig. 5(B), the four-order absorbance derivatives of each water sample were substantially coincident before and after filtration, and the spectral peak characteristics were more distinct. The turbidity interference is inhibited and eliminated by a direct derivative spectroscopy method, and the universality derivative order determined by the method is practical and effective.
The embodiments described in this specification are merely illustrative of implementations of the inventive concept and the scope of the present invention should not be considered limited to the specific forms set forth in the embodiments but rather by the equivalents thereof as may occur to those skilled in the art upon consideration of the present inventive concept.

Claims (2)

1. A turbidity interference elimination method used in COD measurement is characterized in that: the method comprises the following steps:
1) the method for determining the universal derivation order according to the limit state approaching to the minimum particle size distribution so as to ensure the suitability for other practical water environments comprises the following steps:
11) obtaining turbidity particle spectrum line type a (lambda) which represents particle size distribution characteristics; the method comprises the following steps:
filtering a water sample through a Millipore filter membrane of 0.22 mu m, placing filter residues in deionized water with the same volume as that of the original water sample, shaking uniformly, measuring the absorption spectrum of the filter residues, and then normalizing the filter residues as follows:
wherein A (λ) is the absorbance of the nephelometric particles at different wavelengths, AmaxIs the maximum absorbance value, a (λ) dimensionless normalized coefficient; when the particle size distribution of the turbidity particles is stable and unchanged, even if the particle concentration is different, the spectral line type can not be changed, and the turbidity particle size distribution can be characterized by a (lambda);
12) introducing and calculating CD and m parameters for reflecting the size and scattering type of turbidity particles; the method comprises the following steps:
a parameter CD is defined to characterize the degree of curvature of the spectral line as follows:
substituting a (lambda) in the step 11) into a formula 2 to calculate, wherein CD is the sum of curvature absolute values at different wavelengths;
fitting a (lambda) in step 11) with a polynomial from first order to higher order until the Sum of Squared Errors (SSE) is less than a threshold value,
a(λ)≈cmλm+cm-1λm-1+cm-2λm-2+...+c0(3)
the resulting m is the lowest fitting order satisfying the SSE threshold, C0,C1,…,CmIs the fitting coefficient;
the larger the CD and m values, the steeper the curve, indicating smaller haze particles;
13) determining a universal derivation order according to the limit state to ensure that the method is suitable for other common water samples; the method comprises the following specific steps:
131) respectively searching the maximum values of CD and m in a plurality of typical water samples, and when the maximum values of CD and m are reached simultaneously, the particle size of suspended particles in the corresponding water samples tends to be minimum and approaches to a limit state;
132) when the CD and m cannot reach the maximum simultaneously in the step 131), increasing the samples and repeating the step 1)
133) Under the limit state that the particle size tends to be minimum, n is m and is a universal derivation order, and the absorbance derivative of turbidity can be ensured to be close to 0 for other water samples with larger suspended particles;
2) eliminating turbidity interference in COD measurement by using a direct derivative spectrum method, and avoiding the operation of obtaining and compensating a turbidity baseline; the method comprises the following steps:
21) acquiring an n-order derivative spectrum of a water sample and simplifying a superposition formula; the method specifically comprises the following steps:
turbidity and COD induced light attenuation effects are basically independent of each other, and the superposition property according to Lambert beer law
Figure FDA0002236588510000021
A(λ)＝CL·kCOD(λ)+TL·ktur(λ)＝ACOD(λ)+Atur(λ) (5)
Wherein k isCODAnd kturThe absorption coefficients of COD and turbidity are respectively, C is the COD concentration, T is the turbidity, and L is the optical length; a (. lamda.), ACOD(lambda), and Atur(lambda) is absorbance of the water sample, COD and turbidity respectively;
solving an nth derivative of the formula (5), wherein the spectrum of the nth derivative also meets the superposition property;
Figure FDA0002236588510000022
the absorption spectrum curve of the turbidity particles is generally in a monotonous decreasing trend; and organic matter in water is composed ofDue to the existence of various absorption peaks, the shapes of absorption spectra are more complicated; according to the mathematical theory, after derivation of the absorption spectrum using the universal derivation order, the right side d of equation (6) in step 1)nAtur(λ)/dλnClose to 0, negligible; at the same time, dnACOD(λ)/dλnWill be retained, and in proportion to the concentration of COD, the original absorption peak will also be sharpened; at this time, the formula is simplified as:
Figure FDA0002236588510000023
thereby eliminating the turbidity interference in the absorption spectrum;
22) establishing a regression prediction model and predicting COD; the method specifically comprises the following steps:
221) establishing a regression prediction model between the nth order absorbance derivative of the modeling water sample and the corresponding real COD value,
COD＝f(A(n) 1,A(n) 2,A(n) 3,…,A(n) k) (8)
wherein f represents a functional relationship, A(n) kIs the nth derivative of the absorbance at the kth feature point;
222) in the actual measurement, the n-order absorbance derivative of the water sample to be measured is input into the formula 8 to predict the COD.
2. The turbidity interference cancellation method for use in COD measurement according to claim 1, characterized by: step 13) the threshold value of the Sum of Squared Errors (SSE) is 0.5.
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CN111929265A (en) * 2020-10-09 2020-11-13 天津市赛普新锐仪器科技有限公司 Accurate compensation determination method for COD and/or BOD of domestic sewage
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