What instrument is used to measure COD

1.Why Measure Chemical Oxygen Demand (COD)?

COD water analysis is critical in wastewater for determining the amount of waste (contamination) in the water. Waste that’s high in organic matter requires treatment to reduce the amount of organic waste before discharging into receiving waters.

Why does this matter?

If wastewater treatment facilities do not reduce organic content of the wastewater before it reaches natural waters, microbes in the receiving water will consume the organic matter. As a result, microbes will consume the oxygen in the receiving water to breakdown the organic waste. This oxygen depletion is called eutrophication and can lead to the death of animal life.

Chemical oxygen demand is defined as the oxidation of organic substances contained in waste water with a strong oxidant (silver heavy electroate) and the experimental data. During the COD test, it reacts oxidatively due to the assistance of strong acids, catalytic agents and high temperatures

The effect should be far more complete than the biochemical response of the BOD experiment.

2.Measure Chemical Oxygen Demand (COD)

At first, we think such a problem: how to measure Chemical Oxygen Demand (COD)?

We know, Chemical oxygen demand (COD) is one of the control items of environmental water quality standards and sewage and wastewater discharge standards. Generally, under constant conditions, the content of pollutant substances in water is indirectly obtained from the oxidation capacity and oxidation rate of the compound, that is, according to the amount of compounds consumed, the chemical oxygen demand – COD is obtained.

A chemical oxygen demand (COD) test is used to measure the amount of organic compounds in a water sample. It measures the capacity of water to consume oxygen during the decomposition of organic matter and the oxidation of inorganic chemicals such as Ammonia and nitrate.

The compounds used to measure COD are substances with strong oxidizing capacity and high oxidation rate, usually potassium permanganate or potassium dichromate. The test method of using this oxidation reaction requires certain oxidation conditions, such as the capacity of the sample, the type and quantity of compounds used, the heating method, temperature, time, container shape, the determination of the characteristics of the liquid for oxidation and the end point of the reaction, and the method of data presentation.

The specific measurement process of COD is as follows:

a certain volume of water sample is placed in the flask, and a certain amount of potassium dichromate solution, sulfuric acid, and silver sulfate are added. The mixture is heated and refluxed for 2h. Most organic matter is oxidized under acidic conditions.

organic matter+Cr₂O₇^2-+H⁺CO₂+H₂O+2Cr³⁺

After the mixture has cooled (the concentrate has been washed off the distillation flask) it is diluted with deionized water and then titrated with standard ammonium ferrous sulfate with ferrifine as an indicator. The ferrous ions react with the dichromate ions, and the titration end point is the color of the solution changing from blue-green to reddish-brown.

6Fe₂₊+Cr₂O₇^2-+14H⁺6Fe³⁺+2Cr³⁺+7H₂O

At the same time as measuring the COD of the sample, the whole blank experiment was performed with distilled water. The purpose of the blank experiment is to eliminate errors caused by additional organic contaminants in the agent. The COD is calculated as follows.

COD=(t_blank-t_sample)/V_sample ×Concentration of ammonium ferrous sulfate×8000

In the formula, t_blank and t_sample are titration volumes, mL, V_sample is sample volume, mL, 8000 is the coefficient, and 1/4 O₂ molar mass is converted in mg/L.

Under the specific conditions of the above-mentioned oxidation reaction rate, according to the (GB 11914-89) method, the determination method of COD is the potassium dichromate method. However, the oxidation of all pollutants present in water is different. Therefore, due to the different types and quantities of oxidized substances, the adoption of the potassium dichromate method is limited. For the determination of COD, researchers at home and abroad have proposed a variety of methods, and some of these reasonable results have been adopted in the detection of environmental water. The GB method is a manual operation, which is quite complex and prone to human error. In order to unify the various conditions of oxidation operation, there is a great need for automated instruments to perform a test method that uses the oxidation reaction speed to perform manual operation. The automatic determination method of COD varies according to the type of oxidizable compound. The current development trend is dominated by the potassium dichromate method.

However，the laboratory method is very cumbersome and inconvenient, which is not conducive to long-term automatic on-line monitoring in the field.

COD measurement can help us assess the amount of organic matter in the water body, and then judge the quality of the water. In practice, there are a variety of instruments that can be used to measure COD, and a few of the common ones are described below. The main measurement methods of COD are potassium dichromate method and spectrophotometry.

3.Introduce of several COD test methods

UV-Vis Spectrophotometer: This is a commonly used instrument for measuring COD. It uses the absorption properties of UV-Vis light to measure the concentration of organic matter in a solution. In COD measurements, the commonly used reagent is potassium permanganate (KMnO4), which has a distinct absorption peak in the UV-visible region. By measuring the absorbance of the solution at a specific wavelength, the concentration of organic matter in the solution can be calculated, and thus the COD value can be obtained.

Figure 1 Schematic diagram of UV-Vis Spectrophotometer

Rapid COD Analyzer: This is a portable instrument that can quickly measure COD values in water samples. It uses the chemical oxidation method, which oxidizes and decomposes the organic matter in the water sample by adding an oxidant, and then measures the difference in the concentration of dissolved oxygen in the solution before and after the oxidation reaction to calculate the COD value. This instrument is easy to operate and has a fast measurement speed, making it suitable for rapid inspection in the field.

Flow Injection Analyzer: This is an automated instrument that enables high-throughput COD measurements. It uses flow injection technology to measure COD by mixing samples and reagents in a certain proportion and then continuously filling and analyzing them. This instrument has the advantages of high sensitivity, high precision, and high repeatability, and is suitable for rapid analysis of large batches of samples.

Figure 2 Schematic diagram of Flow Injection Analyzer

Gas Chromatography-Mass Spectrometry (GC-MS): This is an advanced instrument that can be used for the qualitative and quantitative analysis of organics in COD measurements. It combines gas chromatography and mass spectrometry to separate and identify complex organic samples. In COD measurements, GC-MS analysis can be performed to determine the composition and content of various organic compounds in water samples, allowing for a more accurate assessment of water quality.

Figure 3 Schematic diagram of Gas Chromatography-Mass Spectrometry (GC-MS)

Atomic Absorption Spectrophotometer: An atomic absorption spectrometer is an instrument commonly used for COD measurements. It uses metal ions to measure the oxidation of organic matter. In COD measurement, a commonly used reagent is potassium permanganate (KMnO4), which oxidizes with organic matter in solution to form metal ions. By measuring the absorption spectrum of metal ions in solution, the concentration of organic matter in the solution can be calculated, and thus the COD value can be obtained.

Figure 4 Schematic diagram of Atomic Absorption Spectrophotometer

Electrochemical Analyzer: An electrochemical analyzer is an instrument commonly used for COD measurements. It uses electrochemical methods to measure changes in current or potential in a solution to assess the amount of organic matter. Commonly used electrochemical methods in COD measurement include potentiometric titration and potentiometric scanning. By measuring the change in current or electric potential in the solution, the concentration of organic matter in the solution can be calculated, and thus the COD value can be obtained.

The above instruments are just a few of the common methods used in COD measurement, and with the continuous advancement of science and technology, more new instruments and methods will be developed. Choosing the right instrument for COD measurement can improve the accuracy and efficiency of the measurement, and provide reliable data support for environmental monitoring and water quality analysis.

4.A specific introduction to an instrument for online monitoring of COD.

Structural features:

The whole measurement system is mainly composed of two instruments: an instrument (secondary instrument) and an online UV-COD sensor (primary meter).

Part of the composition, the sensor is in contact with the measured aqueous solution, and the instrument displays the concentration value and temperature value of the aqueous solution and the work state.

How it works?

The light waves sent by the UV light sensor are absorbed, reflected, and scattered by the analyte during transmission

A small fraction of the light hits the receiver, and the transmittance of the transmitted light is proportional to the concentration of the COD

The concentration of COD is calculated by measuring the transmittance of transmitted light. Automatic compensation for turbidity disturbances, yes

Combined with an air cleaner, contamination can be eliminated and on-site maintenance work can be reduced, even for long-term monitoring, It have Excellent stability.

Figure 5 Schematic diagram of the UV measurement system

Advantage of our COD analyzer：

Fully intelligent, multi-functional, high measurement performance, strong environmental adaptability;
Multi-parameter display at the same time, concentration value, output current, time, relay status;
Two 4-20mA transmission outputs, relay high and low alarm control outputs, RS485 communication outputs and other variables
RS485 upload function;
Immersion, plug-in, flow-through and other installation methods;
Cleaning and maintenance is very simple, calibrated once every three months;
The two-point correction method is used to ensure the accuracy of measurement;
Menus are available in both English and Chinese
Self-set password: Users can set or change their own password to avoid misoperation caused by unrelated personnel;

5.Research status of COD online monitor

When the incident light propagates in the solution, the medium has an absorption effect on it, and the light intensity in the original propagation direction is attenuated, so the type and quantitative analysis of the substances in the solution can be known by detecting the degree of light intensity attenuation. Dissolved organic matter (COD) in water absorbs ultraviolet light and can therefore be determined or converted to COD concentration by absorbance.

Shimadzu’s UVM-4020 uses a new weighted multi-wavelength absorbance detection method (WMW), which improves the shortcomings of the previous UV COD measuring instrument. The UV absorption spectrum of 5 representative organic matter is detected between 243nm~290nm, and many organic substances can be detected by multiple wavelengths, so it is easy to detect changes in water quality.

S::CAN Full Spectrum Detection Method: Austria is the inventor of the UV-Vis spectroscopy sensor and the only company in the world to successfully apply spectrometry to the measurement of water quality parameters, which can be used directly in liquid media for continuous measurement, and is the industry leader in this technology. This unique technology allows us to determine a wide range of parameters in different applications, such as pure water, drinking water, environmental monitoring, wastewater treatment, and industrial wastewater treatment processes. In addition, the S::CAN full spectrum method can not only measure COD, but also realize the measurement of multiple parameters such as BOD, TOC, BOD, TSS, nitrate nitrogen, ammonia nitrogen, turbidity and so on.

In 2005, the Anhui Institute of Optics and Mechanics of the Chinese Academy of Sciences invented the “spectroscopic non-contact COD/DOC water quality online detection method and device”, which uses optical fiber introduction and holographic concave grating for dispersive elements.

6.The distinguish of COD, BOD and TOC

6.1 COD and BOD

Compared with BOD tests, COD tests have the advantage of saving time and high accuracy (by COD tests.)

It takes about 4 hours compared to 5 days for BOD, and the average difference for COD is about 5%, while the average difference for BOD is about 5%.

up to about 20%), so if the COD value can be used as the operation control of the treatment plant

The system will be able to achieve the effect of “striving for timeliness and seeing immediate results.”

Compared with the BOD test, the COD test has a drawback, that is, it is not a one-size-fits-all approach Standardized experiments. Theoretically, if the oxidants used are different, the measured COD values will also be different (due to Different oxidants have different oxidizing capacities). The U.S. Standard Methods stipulate the use of potassium dichromate, It is used as an oxidant for COD experiments, but potassium permanganate is commonly used as an oxidant in Japan. generally speaking The oxidation capacity of potassium dichromate on organic substances is about 3~5 times that of potassium permanganate (depending on different organic substances different multiples). Therefore, for the same waste water sample, the measured COD value in Taiwan is generally about in Japan This measurement is 2~3 times. In other words, the COD discharge standard in Japan is set at 100mg/L,The discharge standard of the bay should be set at 200~300mg/L in order to stand at the same level as Japan.

The reaction mechanism of BOD test is biological oxidation, and the reaction mechanism of COD test is chemical oxidation, the degree of common chemical oxidation is much stronger than the degree of biological oxidation, not to mention the COD test, in order to enhance the ability of oxidation, in addition to adding the strong oxidant potassium dichromate, in addition to adding the catalyst silver sulfate-sulfuric acid solution, and boiling at high temperature for 2 hours, therefore, the COD value measured under such intense oxidation conditions must be greater than the BOD value, so (COD – BOD) >>0, that is, the COD/BOD ratio is Evergrande 1.0.

For the same wastewater, the COD/BOD ratio of the raw wastewater is usually about a constant, so if a wastewater treatment plant has accumulated a considerable amount of COD/BOD ratio data (e.g. with a record of more than six months), it is not necessary to take the time to measure the BOD every day, because the BOD value can be estimated by dividing the daily measured COD by the average of the COD/BOD ratio of the plant. However, it should be noted that the environmental protection agency does not recognize the BOD value obtained by COD conversion.

The organic matter measured by the BOD test is only the part that can be degraded by microorganisms, while the organic matter measured by COD includes organic matter that is degradable by microorganisms, difficult to decomposable by microorganisms, and non-degradable by microorganisms (except for the five organic substances listed in Section 12 of this paper). Therefore, BOD theoretically refers to 100% biodegradable organic matter, while (COD – BOD) refers to the part of organic matter in wastewater that microorganisms cannot decompose or are difficult to decompose.

Therefore, the COD/BOD ratio of wastewater can be used to evaluate the biodegradability of wastewater. If the COD/BOD ratio of the wastewater is larger, it means that the biodegradability of the wastewater is lower, or that the wastewater may contain toxic substances, so it is not suitable for biological treatment, and conversely, if the ratio is smaller, it means that the biodegradability of the wastewater is higher, and the more suitable it is for biological treatment.

Generally speaking, if the untreated wastewater COD/BOD<2.0, it means that the biodegradability of the wastewater is quite high, and it is suitable for biological treatment method; if the untreated wastewater COD/BOD>3.0, it means that the biodegradability of the wastewater is quite high, and it is not suitable for the treatment of the wastewater by the biological treatment method, but it may also indicate that the wastewater contains more or less toxic substances.

The COD/BOD ratio of wastewater increases due to biological treatment. For example, if the raw wastewater COD/BOD = 2.0 (biologically degradable), the COD/BOD ratio will increase to more than 5.0 or even 10.0 after activated sludge treatment.

After the wastewater is treated by biological treatment method, it still cannot be removed (that is, it is biodegradable COD), it can be said that it is no longer likely to consume dissolved oxygen in the river water. Therefore, the river is the main one From the standpoint that the goal is to prevent the dissolved oxygen of the river water from dropping to zero and smelling, it is theoretically a place for biological treatment There is really no need to set COD discharge water standards. However, for waste that may contain toxic substances water, it must be required to pass a toxicity test, such as a bioassay, before it can be discharged Safe.

6.2 COD and TOC

COD is the content of organic matter that potassium dichromate can oxidize, some organic matter can not be oxidized by potassium dichromate, and TOC can completely measure the organic matter in the water sample, they are a related and unnecessary condition, that is to say, COD is high TOC must be high, TOC high COD is not necessarily high. For specific wastewaters, multiple groups of TOC and COD should be determined to determine their correlation. To put it simply, TOC is measured in carbon 12, COD is measured in oxygen 16, roughly calculated, it should be COD > TOC, and the regression coefficient of the relevant equation should be about 16/12 = 1.3, which is also limited to carbon-containing organic compounds, such as organic matter containing nitrogen, sulfur, phosphorus and other elements, COD will be larger than TOC. If everyone from the molecular level, the essence of redox reactions. TOC measures the total organic carbon in the water, but there is no way to point out what valence state the carbon of organic matter is, and the process is theoretically to oxidize all the carbon in different valence states into carbon dioxide. COD chromium measurement is the oxidation of samples with potassium dichromate under specific conditions, in which the organic matter in carbon, nitrogen and other elements, but also other reducing substances such as nitrous nitrogen. Ferrous iron and other partial oxidation, their oxidation can only be expressed relatively completely, oxidation rates from 0% to 120% are possible. This is all related to the composition of the water sample, so they are all comprehensive indicators to measure the level of organic pollution in the water body. Let’s be specific, such as benzene and cyclohexane, the two organic substances, TOC is theoretically the same, but COD, in theory, benzene is lower, because from the average valence state of carbon, it is a little higher is minus one, and the latter is minus two, and it has to lose more electrons to reach the positive tetravalent of CO2. In fact, it is not completely confirmed, maybe benzene because of the existence of a stable conjugated system, the conditional oxidation potential of potassium dichromate Monet, its COD is zero. It should be added that there is no way to quantify the other elements in organic matter with TOC.

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