Can the internal turbidity sensor of COD254 be used as the output of standard turbidity?

First, let’s understand the basics of turbidity.

1. Introduction to turbidity

Natural water and wastewater contain a variety of insoluble substances of different particle sizes, such as soil, fine sand, organic matter and microorganisms, which can cause turbidity. The degree of turbidity of the water sample can be expressed by the magnitude of the turbidity. Turbidity indicates the degree to which insoluble substances in water (i.e. suspended solids and colloids) impede the passage of light. That is, turbidity is an optical property of water samples due to the presence of insoluble substances in the water that allow part of the light passing through the water sample to be absorbed or scattered instead of penetrating in a straight line. Turbidity is an important water quality indicator for natural and drinking water. In general, the more insoluble substances in the water, the higher the turbidity, but there is no direct quantitative relationship between the two. Because turbidity is an optical effect, its magnitude is not only related to the amount and concentration of insoluble substances, but also to properties such as particle size, shape, and refractive index of these insoluble substances.

The unit of turbidity varies depending on the method of measurement. At present, the general method of turbidity measurement is the scattering method, which is determined by a turbidity meter.

Table 1 Turbidity units and test methods and related standard documentation

Turbidity unitMeasurement methodWavelength of light sourceDetectorRelevant standards documentation
NTU(Nephelometric Turbidity Unit)Using the turbidity meter method, the degree of scattered light effect of insoluble substances in water on light passing through is analyzed. Often used in reference to USEPA Method 180.1 or Standard Methods for the Testing of Water and WastewaterWhite or broadband: peak spectral output of 400-680nm90 degrees to the incident beam. Single DetectorHJ 1075-2019
FNU (Formazin Nephelometric Unit)FNU is most commonly used to reference the ISO 7027 (European) turbidity method.Infrared,monpchromatic:typical output in 780-900 nm range90 degrees to the incident beam. Single Detector 
FTU(Formazin Turbidity Unit)Based on the turbidity meter method, it is often used for water quality analysis.FTU is consistent with NTU  
FAU(Formazin Attenuation Unit)The turbidity meter method is used to measure turbidity in water bodies. the instrument measures the attenuation of light after it passes through the sample in a direction that is 180° to the incident light. This measurement is usually performed with a spectrophotometer or colorimeter, which is not recognized by most regulatory agencies.
JTUthe Jackson turbidity unit is a unit used in the past, when people used a Jackson turbidity meter to visually measure turbidity: the water sample to be measured was poured into a glass tube placed vertically above the flame until the glass tube could not be seen clearly. until.   

A turbidity meter with an optical system designed to detect 90° scattered light is called a suspension turbidity meter. The measurement results are shown as NTU (Scattered Turbidity Unit). This is also a test method recognized by the U.S. EPA. In addition, the unit of measurement used in the ISO standard is FTU (Formazine Turbidity Unit), 1FTU = 1NTU. EBC (European Beer Turbidity Unit) unit used in the wine industry, 1FTU=4EBC.

2. Turbidity test method

The turbidity of various types of water varies greatly, so the measurement method of turbidity should also be based on different water quality to choose different instruments and methods.

There are three most commonly used methods.

1) Visual method.

  1. Candlelight turbidity meter.

The candle turbidity meter was one of the first standard instruments and standard methods for measuring turbidity that was first used and is still in use today. It consists of three basic components: a standard turbidity glass tube, a stand, and a candle.

Figure 1 Candlelight turbidity meter

Standard turbidity glass tubing should be made of finely ground optical glass and should meet the quality requirements of Nessler’s cuvettes. Initially, the scale on the tube was marked according to a pure standard silica turbid solution (1 mg/L SiO2 = 1 degree).

Table 2: Comparison table of pathlength and turbidity of standard turbidity glass tubes


Later turbidity tubes were engraved according to this table. For the sake of convenience, the standard turbidity glass tube is divided into two types: long tube and short tube: the long tube can measure 25~1000 degrees of water sample, and the short tube can measure 100~1000 degrees of water sample. The candle should be made of beeswax or whale wax, and the burning rate should be 7.3~8.1g per hour.

You may Refer the following video:

Measuring the turbidity of water – YouTube

Note: The pathlength is calculated from the bottom of the glass tube.

A candle turbidity meter can directly measure the turbidity of water above 25 degrees. This turbidity meter is also known as the Jackson candle turbidity meter, and the turbidity measured by this is called Jackson turbidity units (JTU).

b. Turbidimetry. Water samples with turbidity of 0~100 degrees can be determined by turbidimetric method. In this method, a standard turbidity solution of turbidity is formulated into a standard turbidity series according to different turbidity steps. For 10~100 degrees, use 1L of homogeneous colorless glass for turbidiency, and for 0~10 degrees, use Ness’s cuscopire. In the actual measurement, the standard turbid liquid does not need to be configured with pure silica with strict particle size requirements, but can be replaced by kaolin, bleached soil, etc. The configured turbidity solution should be calibrated with a candlelight turbidity meter and then diluted into each standard turbidimetric series. Therefore, turbidity measured by the turbidity method is also the Jackson turbidity unit (JTU).

Figure 2 The principle difference between transmission turbidity detection and scattered turbidity detection

2) Scattering method. The turbidity is less than 25 degrees, especially for water samples below 5 degrees, which is sometimes inconvenient to use the turbidity method. The scattering method is a turbidity meter made by applying the principle of light scattering. According to the Tindall effect, the intensity of scattered light is proportional to the size and total number of suspended particles, i.e., to the turbidity. The greater the intensity of the scattered light, the higher the turbidity. Therefore, the turbidity of a water sample can be determined according to this scattering turbidity meter. The turbidity measured on a scattering turbidity meter is called the scattering turbidity unit (NTU). When determined with a scattering turbidity meter, the commonly used standard reference turbidity solution is formulated from formalhydrazine polymer. Therefore, the unit of scattered turbidity is also called the formalmazid turbidity unit (FTU). It is specified that 1.25 mg of hydrazine sulfate and 12.5 mg of hexamethylenetetramine in 1 L of water produce a turbidity of 1 degree.

The difference between a scattering turbidity meter and a candle turbidity meter is that the former measures the intensity of the scattered light of the turbid material in a specific direction (mainly at a 90° angle to the incident light), while the latter is the total degree of obstruction of the passage of light by the turbid substance, including the effect of absorption and scattering.

Turbidity in water is an important indicator of water that may be polluted, and it is also an important parameter for the selection and design of treatment equipment in waterworks, and an important control index for the operation and dosage of waterworks, especially when using chemical methods to treat drinking water or wastewater, turbidity is sometimes used to control the dosage of chemicals.

China’s drinking water standard stipulates that the turbidity does not exceed 1NTU, and in special cases, it does not exceed 3NTU. In order to ensure that there is no scaling and clogging and does not affect the quality of the product, some industrial water also has certain requirements for turbidity, for example, the cooling water shall not exceed 50~100NTU, the papermaking water shall not exceed 2~5NTU, the water for textiles and bleaching and dyeing shall be less than 5NTU, and the water for semiconductor integrated circuits shall be zero.

3) Transmission method

A calibrated scattering turbidity meter or a transmission turbidity meter can be used to measure NTU values. However, data acquisition with a transmission turbidity meter is time-consuming, involves many steps, and requires a large sample volume.

4) Turbidity meter determination method

The turbidity meter is a special instrument for measuring the turbidity of water made according to the principle of scattering or transmitting light by turbid liquid, and is generally used for continuous automatic determination of water turbidity.

The measurement principle of the turbidity meter refers to the conversion of the signal generated by the scattering and absorption of light by impurities such as suspended matter, colloidal matter, plankton and microorganisms in the water substance. For the water body with sedimentation substances, that is, suspended particles, the solution should be fully shaken to read the highest value, and for the stable water body, the solution should also be shaken and shaken before measurement.

Figure 3 The measurement principle of Scattering turbidity meter

3.Test calibration of COD

COD is the main indicator to characterize the degree of water pollution, and the main detection methods at present are acid permanganate oxidation, alkaline permanganate oxidation and dichromate oxidationThe first two methods mainly detect the COD content of groundwater and less polluted water, and the latter mainly detect the COD content in sewage. The advantage of these two methods is that the oxidation is complete and the determination is accurate. However, these methods require strict detection conditions, long test time, and a large number of chemical reagents, some reagents need to be treated, otherwise it will cause secondary pollution of the environment. Therefore, a simple and rapid UV spectrophotometric detection of COD in water can solve these problems. COD does not use ultraviolet measurement, but uses visible light, as detailed in “Determination of Chemical Oxygen Demand of Water Quality Rapid Digestion Spectrophotometry” (HJ/T 399 2007).

Although various organic substances have their own absorbance, and the absorption of each wavelength of light is different, we found that most of the organic matter has a strong absorption at the wavelength of 254 nm in the ultraviolet spectral region, so it is very accurate to measure COD in the ultraviolet region with a wavelength of 254 nm. However, in fact, most of the sewage samples we usually do are turbid, which contain a large amount of suspension and some colloids, which will have a certain impact on the determination of COD content in the process of measurement, and will make the measurement result larger, so we must correct the amount of COD measured at this time. In other words, we should also measure the turbidity of the water at this time, which can be measured in the visible region at a wavelength of 546 nm. In this way, the difference between the two measurements is the amount of COD in the water.

4. From the perspective of turbidity test principle, whether turbidity compensation can be used as standard turbidity output

The measurement principle of the turbidity detection system is mainly based on the law of absorption of light and the law of scattering of light, so the turbidity is usually determined by using a photodetector to detect the size of the transmitted beam or the scattered beam, and when the turbidity is measured by the transmission method, the weakening of the transmitted light intensity follows the law of absorption by Lambobil, according to which the light intensity after passing through the liquid I0 can be obtained:

I0=MIieKTL (1)

where: M—the geometric parameters of the instrument:Ii– incident light intensity,T– aqueous turbidity;K— coefficients;L– the thickness of the water sample;

The intensity of the incident light is assumed Ii No change; Convert the form of Eq. (1).

Aqueous turbidity T can be obtained, Correspondence with the logarithm of transmitted light intensity I0


Where: T—aqueous turbidity; Ii– incident light intensity;K1— coefficients;

From Eq. (2), it can be seen that when the incident light intensity II is constant, the turbidity T of the water sample is proportional to the logarithm of the transmitted light intensity IoTherefore, the turbidity value can be obtained by measuring the magnitude of the transmitted light intensity Io.

According to the different angles formed by the incident beam and the scattered beam, the scattering method can be divided into forward scattering, vertical scattering and backward scattering, International Standard ISO7027-1984 “Determination of Turbidity in Water Quality” specifies the angle at which the scattering method measures turbidity at 90° ± 2.5°.

When measuring turbidity with scattering Io, the intensity of scattered light in the 90° direction is Io according to Rayleigh’s scattering law


During the ceremony: – incident light intensity;V— Particle volume;

λ– wavelength of incident light;N– the total number of particles per unit volume;K2— coefficients;Under certain conditions,Suppose λ and V are constants;Then the ratio of Io and II is proportional to the total number of particles per unit N, that is, the turbidity of the water sample to be measured:

IoIi= K3T

During the ceremony:T– aqueous turbidity;K3— coefficients;

Therefore, under the premise that the incident light intensity is Ii, the scattered light intensity Io is proportional to the turbidity T of the liquid to be measured, and the turbidity can be obtained by measuring the scattered light intensity Io.

The characteristic relationship between the natural logarithm S of transmitted light intensity Io and the turbidity T of the aqueous sample is shown in the figure.

Figure4 The relationship between the natural logarithm of transmitted light intensity and the turbidity of the aqueous sample

When the turbidity T<1000 NTU, the linearity between the two is better. When the turbidity T> 1000 NTU, there is a nonlinear attenuation between the two, and the transmission method has a wide range of measurement, but when measuring low turbidity liquids, the small change of turbidity has a weak influence on the change of the transmitted light signal, and the measurement accuracy is low In terms of circuit design, there are high requirements for the stability and resolution of photoelectric receiving devices and amplifier devices, so the transmission method is not suitable for the measurement of low-turbidity liquids, and  is more suitable for the measurement of medium and high turbidity water samples.

Turbidity is related to, but not a direct measure of, suspended solids in solution. Turbidity measures the intensity of light scattered by a sample. There are many factors that affect light scattering, including the size of particles in water, the shape of particles in water, the refractive index of particles in water, the color of particles and fluids, the concentration of particulate matter, etc., particles of different particle sizes in a certain range, the light scattering intensity is the same in the direction of 90 ° (light scattering in this direction is not sensitive to particle size), so the turbidity meter measures the scattered light intensity in the direction of 90 °, which can minimize the size, shape, and size of the particles. The color interferes with the measurement results to ensure the accuracy of the measurement results.

Figure 5 Relationship between 90° scattered light intensity and turbidity

When T<200NTU, there is a strong linear relationship between the scattered light intensity and the turbidity of the water sample, when the T > 200NTU, there is an obvious nonlinear relationship between the two, if the scattered light measurement method is still used, the software must be used for nonlinear correction, but this will reduce the measurement accuracy, when the T>2000NTU, the particles of the water sample will scatter the incident light many times, so that the scattered light intensity is greatly reduced. It can be seen that the scattering method is suitable for the detection of low turbidity liquids, and has high sensitivity and measurement accuracy, so the 90° scattering method can be used in the turbidity measurement within 0 ~ 200NTU, and the transmission method can be used in 200NTU ~ 1000NTU.

Attachment: The relationship and influence of turbidity with other indicators

Briefly introduce the relationship between turbidity, chromaticity, and transparency.

Chromaticity: Water bodies can also show different colors when they are polluted by industrial wastewater. These colors are divided into true colors and surface colors. Chromaticity is an important indicator to evaluate sensory quality, and some countries’ water quality standards require chromaticity to be between 5~20 degrees. The drinking water quality standard stipulates that the color should not be greater than 15 degrees.

Although turbidity and chromaticity are both optical properties of water, they are different. Chromaticity is caused by dissolved substances in water, while turbidity is caused by insoluble substances in water.  Therefore, if the water sample is left for a long time, there will be more precipitation, and if the optical path is in the middle and lower layers of the water sample, it will lead to an increase in turbidity.

In water quality analysis, the determination of turbidity is usually only used for natural and water use. As for secondary domestic sewage and industrial wastewater, most of them are quite turbid because they contain a large number of suspended pollutants, and this kind of water sample is generally only used for the determination of suspended solids but not for the determination of turbidity.

Transparency refers to the clarity of the water sample, and clean water is transparent. The more suspended solids and colloidal particles in the water, the less transparent it is. Usually groundwater is more transparent. Transparency is a water quality indicator related to the combined effects of both color and turbidity of water.

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