In-situ Online Monitoring System of Field Water Quality
author: Susan
2022-01-04
Water pollution originates from the industrial fast development and life waste can threaten human being living standards. It is more and more important to monitor water quality in the field. In the past the water quality monitoring by chemical tools need to collect water samples in the field and send back to analyze in the lab with low efficiency and expensive instrument cost, how can self-build a simple spectroscopy system to perform in-situ and online analysis? This topic has become environmental protection demands. Research on green, clean, accurate and effective water quality monitoring instrument has become popular in recent year.
Principle
1. Water quality COD ultraviolet spectroscopy detection principle
2. The current methods for measuring COD mainly include: potassium dichromate standard method, permanganate index method, spectrophotometry, coulometric method, etc. Due to the fast detection speed and pollution-free advantages of the spectrophotometric method, this method has become one of the popular methods explored by most scholars. The principle of measuring COD is shown in Figure 1.
This constant is affected by factors such as the temperature of the water sample and the characteristic wavelength. When the same sample cell is used for detection, that is, the optical path is unchanged, and when the substance to be detected and the characteristic wavelength is the same, the molar absorption constant does not change. Then the absorbance and the concentration of the substance to be measured increase, and the transmittance and the concentration of the substance to be measured are not linearly proportional. Lambert-Beer law is the basic principle of COD measurement by ultraviolet spectroscopy, and it is also the theoretical basis of quantitative analysis.
Figure 2 Schematic diagram of Lambert-Beer law
Figure 3 System architecture diagram
Figure 4 The overall structure of the water quality inspection system
Principle
1. Water quality COD ultraviolet spectroscopy detection principle
2. The current methods for measuring COD mainly include: potassium dichromate standard method, permanganate index method, spectrophotometry, coulometric method, etc. Due to the fast detection speed and pollution-free advantages of the spectrophotometric method, this method has become one of the popular methods explored by most scholars. The principle of measuring COD is shown in Figure 1.
Figure 1 Schematic diagram of COD measurement by UV spectrum
3. Lambert-Beer's LawThis constant is affected by factors such as the temperature of the water sample and the characteristic wavelength. When the same sample cell is used for detection, that is, the optical path is unchanged, and when the substance to be detected and the characteristic wavelength is the same, the molar absorption constant does not change. Then the absorbance and the concentration of the substance to be measured increase, and the transmittance and the concentration of the substance to be measured are not linearly proportional. Lambert-Beer law is the basic principle of COD measurement by ultraviolet spectroscopy, and it is also the theoretical basis of quantitative analysis.
Figure 2 Schematic diagram of Lambert-Beer law
4. COD, pH, dissolved oxygen, temperature detection principles and methods, focusing on UV spectroscopy and potentiometric analysis
Spectroscopy Solution
Mainly composed of water quality monitoring system and water quality monitoring system management platform：
The detection method of the water quality monitoring system is in-situ detection, which is responsible for the collection and upload of various data of water quality parameters, and mainly monitors 4 indicators such as COD, dissolved oxygen, pH, and temperature.
When monitoring, the water quality sensor is directly immersed in the monitored rivers and lakes, and the water quality monitor system management platform is responsible for receiving, analyzing, storing, and displaying various data on water quality indicators. The network channel between the water quality monitoring system and the water quality monitor system management platform is relatively complicated. The water quality monitoring system sends the collected water quality index data to the network server in real time through the 4G network. The water quality monitor system management platform transmits the server through the transparent cloud Real-time data received on the show. The system architecture is shown in Figure 3：
Spectroscopy Solution
Mainly composed of water quality monitoring system and water quality monitoring system management platform：
The detection method of the water quality monitoring system is in-situ detection, which is responsible for the collection and upload of various data of water quality parameters, and mainly monitors 4 indicators such as COD, dissolved oxygen, pH, and temperature.
When monitoring, the water quality sensor is directly immersed in the monitored rivers and lakes, and the water quality monitor system management platform is responsible for receiving, analyzing, storing, and displaying various data on water quality indicators. The network channel between the water quality monitoring system and the water quality monitor system management platform is relatively complicated. The water quality monitoring system sends the collected water quality index data to the network server in real time through the 4G network. The water quality monitor system management platform transmits the server through the transparent cloud Real-time data received on the show. The system architecture is shown in Figure 3：
Figure 3 System architecture diagram
The mechanical structure design is shown in Figure 4：
Figure 4 The overall structure of the water quality inspection system
The optical method with good development prospects, pollution-free, and fast response time is adopted for detection. The optical system is shown in Figure 5 and consists of a Pulse Xenon Light Source ATG1030, a fiber sensor probe, a full-spectrum miniature spectrometer ATP2000P, and a central controller. The light source emits ultraviolet-visible wavelengths. The light in the range is transmitted to the optical fiber sensing probe through the optical fiber. The optical fiber sensing probe is designed based on the laboratory sample pool. It is used as a carrier for the spectral absorption experiment of the water sample to be tested. It is suitable for in-situ detection in the field. A spectrometer is an instrument that decomposes complex light into a spectrum. The spectrometer can collect and analyze the light passing through the object, and convert the optical signal into an outputable electrical signal through a built-in photodetector.
Figure 5 Optical system design drawing
The circuit part of the field water quality in-situ monitoring system is controlled by the smallest system of the STM32F407VET6 single-chip microcomputer
The core of the system is equipped with AD module, GPS module, power supply module, water quality monitoring module, signal conditioning circuit, voltage amplifier module, 4G communication module, etc.
Figure 7 Absorbance fitting curve diagram corresponding to different concentrations a) λ=256nm b) λ=266nm c) λ=276nm d) λ=286nm e) λ=296nm
The core of the system is equipped with AD module, GPS module, power supply module, water quality monitoring module, signal conditioning circuit, voltage amplifier module, 4G communication module, etc.
1. The influence of wavelength on COD detection
However, the composition of the wild water is complex, and the maximum absorption peak is not all at 254nm. The clay, microorganisms, some inorganic substances and interfering ions in the water also have light absorption characteristics at 254nm. Therefore, we are exploring the best detection wavelength of COD. The collected concentrations are 20mg/L, 50mg/L, 125mg/L, 160mg/L, 250mg/L, 350mg/L.
The UV absorption spectrum of the COD standard solution is shown in Figure 6.
However, the composition of the wild water is complex, and the maximum absorption peak is not all at 254nm. The clay, microorganisms, some inorganic substances and interfering ions in the water also have light absorption characteristics at 254nm. Therefore, we are exploring the best detection wavelength of COD. The collected concentrations are 20mg/L, 50mg/L, 125mg/L, 160mg/L, 250mg/L, 350mg/L.
The UV absorption spectrum of the COD standard solution is shown in Figure 6.
Figure 6 COD absorption spectra of different concentrations
In the spectral region where λ is 250nm-300nm, the spectrum is relatively stable.
Determined, the absorption peak is more obvious spectral curve. Collect the UV absorption spectra of COD standard solutions of different concentrations, collect the absorbance values at 256nm, 266nm, 276nm, 286nm, 296nm λ respectively, and establish a one-variable linear regression model. The absorbance fitting curves corresponding to different wavelengths are shown in Figure 7.
Determined, the absorption peak is more obvious spectral curve. Collect the UV absorption spectra of COD standard solutions of different concentrations, collect the absorbance values at 256nm, 266nm, 276nm, 286nm, 296nm λ respectively, and establish a one-variable linear regression model. The absorbance fitting curves corresponding to different wavelengths are shown in Figure 7.
Figure 7 Absorbance fitting curve diagram corresponding to different concentrations a) λ=256nm b) λ=266nm c) λ=276nm d) λ=286nm e) λ=296nm
To sum up: The COD concentration at the wavelength of 286nm, 276nm, the absorption at 296nm is the best and has a strong reference value. Among them, λ is 286nm, which is very representative and the best detection wavelength.
2. The influence of temperature on COD detection
2. The influence of temperature on COD detection
The absorbance of the COD standard solution is obviously different at different temperatures. As the temperature gradually increases, the absorbance of COD gradually increases.
Figure 10 The absorbance of different turbidity standard solutions at 286 nm and 560 nm
Figure 14 Comparison of COD before compensation Figure 15 pH fitting curve after compensation and actual value Figure 16 Dissolved oxygen fitting curve
Conclusion
Taking actual water samples as the research object, the interference factors that affect COD detection accuracy and the prediction models of water quality index detection are verified by actual water samples. The detection results meet the requirements of national environmental protection standards and have universal application value. Four different locations of the Yitong River in Changchun were selected for field testing of the system prototype. The test results show that the field water quality in-situ monitoring system developed in this paper is accurate and meets the monitoring requirements of actual water samples in the field. It has certain feasibility and applicability.
Optosky ATE2000 full spectrum water quality monitor self-developed based on this market demand. It's a new generation of user-friendly instrument, easy-to-install and operate can measure multi-parameters in real time. It has no chemical reagents for water pollution again, non-expert can operate with less maintenance work.
Related Products
ATP2000P Pulse Xenon Light Source ATG1030 UV-Visible Fibers Water Quality Monitor ATE2000
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Figure 8 The spectrum of 70 mg/L COD solution with temperature
3. The effect of turbidity on COD detection
Figure 9 Spectral curves of different turbidity standard solutions under COD of 20mg/L
When the turbidity is 0, the potassium hydrogen phthalate solution has two large peaks near 235nm and 280nm at λ. With the increase of the concentration of the turbidity solution, the absorption peak also fluctuates to varying degrees.
Figure 10 The absorbance of different turbidity standard solutions at 286 nm and 560 nm
4. Interfering ion research
Figure 11 a) Na2SO3 spectrogram b)The absorbance curve of the actual water sample before and after filtration
Test Result
Figure 12 COD standard solution concentration and absorbance fitting diagram Figure 13 The spectrum of COD actual water sample and standard water sample (20℃)
Figure 14 Comparison of COD before compensation Figure 15 pH fitting curve after compensation and actual value Figure 16 Dissolved oxygen fitting curve
Taking actual water samples as the research object, the interference factors that affect COD detection accuracy and the prediction models of water quality index detection are verified by actual water samples. The detection results meet the requirements of national environmental protection standards and have universal application value. Four different locations of the Yitong River in Changchun were selected for field testing of the system prototype. The test results show that the field water quality in-situ monitoring system developed in this paper is accurate and meets the monitoring requirements of actual water samples in the field. It has certain feasibility and applicability.
Optosky ATE2000 full spectrum water quality monitor self-developed based on this market demand. It's a new generation of user-friendly instrument, easy-to-install and operate can measure multi-parameters in real time. It has no chemical reagents for water pollution again, non-expert can operate with less maintenance work.
Related Products
ATP2000P Pulse Xenon Light Source ATG1030 UV-Visible Fibers Water Quality Monitor ATE2000
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