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Research status of on-line TOC analyzer for water quality

2022-12-15 15:25:22

This article introduces the working principle of online total organic carbon (TOC) analyzer for water quality, expounds in detail the oxidation method and detection method of online TOC analyzer for water quality, and comments on the structure, technical indicators, method standards and main model numbers of the main international online TOC analyzer for water quality.


Total organic carbon (TOC) in water refers to the total carbon content of dissolved (DOC) and suspended (POC) organic matter in water. There are many kinds of organics in water, including natural organics and synthetic organics, which cannot be separated and identified by current technology. However, because it cannot reflect the type and composition of organic matter in water, it cannot reflect the different pollution consequences caused by the same total amount of total organic carbon. In water quality detection, TOC is often used to represent the total organic carbon content in water. It can directly represent the total amount of organic matter more than BOD5 or COD, and is usually used as an important basis for evaluating the degree of organic matter pollution in water bodies.

Total organic carbon (TOC) analysis is widely used in many industries, including not only the management of water inflow, sewage treatment and final drainage during wastewater treatment, but also the monitoring of organic substances in surface water, tap water, cooling water, reclaimed water, cleaning water and other water bodies. It has increasingly become an extremely widely used monitoring method.

Working principle of online TOC analyzer

At present, there are many brands of instruments on the market for online monitoring water quality TOC, but the design idea of these online analyzers is the same, that is, first oxidize organics in water into carbon dioxide in various ways, and then determine the concentration of TOC by measuring the amount of carbon dioxide.

Oxidation method of online water quality TOC analyzer

According to different oxidation principles, online water quality TOC analyzer can be divided into dry oxidation method and wet oxidation method. The dry oxidation method is mainly high temperature combustion oxidation. This method requires the use of acid, carrier gas and standard solution. The diluted water sample is acidified and aerated to remove the carbon dioxide generated from the decomposition of inorganic carbonate in the water sample. Then the acidified sample is injected into a high temperature combustion furnace for combustion. The control temperature is generally 680-1000 ℃. However, not all organic carbon can be completely oxidized. Therefore, under this condition, Catalyst such as platinum or copper oxide must be used to directly measure the TOC content.

This analytical method is a mature and widely used test method because of its relatively thorough high-temperature combustion, high oxidation efficiency and high measurement limit, and it can oxidize particulate organic compounds. However, this method is large in volume, high in energy consumption, and the catalyst will be poisoned if it is used for a long time. The catalyst needs to be replaced regularly. Because the aeration of the water sample after acidification will cause the loss of volatile organic compounds in the water sample, the direct measurement results reflect the content of organic carbon that cannot be blown out. Alternatively, 1200 ℃ high temperature (HT) oxidation method can be used, so that all carbon compounds can be completely oxidized without catalyst. This method can measure TOC concentration up to 50,000 mg/l without dilution. The water sample is gasified in a special heat-resistant ceramic reactor, and all carbon is completely oxidized into carbon dioxide gas.

At present, the water quality oxidation technology of online TOC analyzer is relatively mature and reliable. The wet oxidation methods include ultraviolet oxidation, persulfate oxidation, ultraviolet light+persulfate oxidation, etc. Ultraviolet oxidation method is to remove inorganic carbon from the water sample before entering the ultraviolet reactor, and use ultraviolet light (185 nm or dual wavelength 185 nm+254 nm) to directly irradiate the water to produce strong oxidizing hydroxyl groups. Organic carbon in the water can be oxidized to carbon dioxide under the action of catalyst, but granular and colloidal organic carbon cannot be completely oxidized. Persulfate oxidation method is to remove inorganic carbon in water quality through phosphoric acid, heat it to 100 ℃ in a closed container, and use persulfate as catalyst, such as Na2S2O8 or (NH4) 2S2O8, to oxidize organic carbon. When the working temperature of persulfate is close to 100 ℃, the recovery rate of organic molecules cannot be guaranteed to be completely recovered, and the oxidant will be infected in water containing high concentration of chloride ions.

Although it can be solved by increasing the concentration of persulfate and prolonging the reaction time, with the increase of temperature, the decomposition of persulfate will lead to the loss of oxidant, and the temperature must be controlled. The principle of ultraviolet light+persulfate oxidation method is to remove inorganic carbon in water sample first, and then convert organic carbon into carbon dioxide in water sample under the combined action of ultraviolet lamp and persulfate oxidant. Because the efficiency of persulfate oxidation is not very high, and ultraviolet radiation can not be used for high concentration TOC water, but it can activate its oxidation capacity, so that the organic matter in it can be completely decomposed. The cooperation of the two can measure the heavily polluted water. If auxiliary heating and pressure are also applied, the oxidation will be sufficient. Because of its strong applicability and wide measurement range, it is highly popular and the technology is mature, so the ultraviolet/potassium persulfate oxidation method has been widely used.

Detection method of online water quality TOC analyzer

Online TOC detection of water quality is essentially the detection of carbon dioxide concentration. There are many methods to detect carbon dioxide, including conductivity generation, non dispersive infrared detection (NDIR), electrochemical method, gas chromatography, etc. At present, non dispersive infrared detection (NDIR) and conductivity detection are widely used in online water quality TOC analyzer internationally. Conductivity generation is divided into direct conductivity generation and thin film conductivity generation, which has the advantages of simple structure, sensitive response, low cost, etc., and has become a widely used method for detecting total organic carbon. Non dispersive infrared detection (NDIR) has high accuracy, fast response, long service life and good selectivity. NDIR method is widely used internationally to measure CO2 concentration.

Structure, main types and parameters of online water quality TOC analyzer

Structure and composition of automatic online water quality TOC analyzer

Automatic online water quality TOC mainly consists of water sample collection unit, pretreatment unit, water sample introduction unit, inorganic carbon (IC) removal unit, total organic carbon oxidation reaction unit, post-processing unit and data display, recording, storage and transmission unit.

Main types and parameters of automatic online water quality TOC analyzer

With the continuous application of various new methods and technologies, according to different water quality, different oxidation technologies, different detection methods, various international forms of automatic online total organic carbon analyzers for water quality have emerged, which are widely used in various fields of production and life. Table 1 lists the well-known automatic online total organic carbon analyzer for water quality in the international and domestic markets.

At present, there are many kinds of automatic online total organic carbon analyzers for water quality in the world, but the products of foreign brands are complete. The main features are as follows:
① The product segmentation is very comprehensive. There are special online TOC detectors for different types of water quality, such as ultra pure water and pharmaceutical water, drinking water industry and environmental water quality monitoring, especially for pharmaceutical and semiconductor wastewater, industrial process water and wastewater to concentrated brine.
② In terms of TOC detection principle, although the fundamental oxidation principle and detection method of TOC have not changed, many more efficient oxidation methods and more advanced and sensitive sensors have been produced. For example, the patented 1200 ° C high temperature (HT) oxidation method of German LAR Company can completely oxidize all carbon compounds without catalyst. American Hach Company adopts the patented secondary advanced oxidation method (TSAO) of hydroxyl radical to realize complete and thorough oxidation of samples.
③ Thanks to the development of detection and communication technology, online water quality TOC has made great progress in real-time, accuracy and intelligence. Some advanced sensors are used for automatic diagnosis, and high-precision conductivity sensors are used for accurate measurement and repeatability. With the development of communication technology, online monitoring is more real-time, and remote control is convenient and fast.

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