As China's environmental management continues to put forward higher requirements, in order to better strengthen the monitoring of pollution source emissions, the implementation of total pollutant control, automatic online monitoring of pollution sources is gradually unfolding. As the monitoring of water pollution with human health, its importance is self-evident. Understanding the relevant water quality indicators is the basis of water pollution monitoring, which must be mixed with our "old friends" COD, BOD, ammonia nitrogen. Today, Xiaobian will lead everyone to understand the intermediate power of sewage treatment - COD digestion.
Chemical Oxygen Demand COD (Chemical Oxygen Demand) is a chemical method for measuring the amount of reducing substances in a water sample that need to be oxidized. The oxygen equivalent of a substance (usually organic matter) that can be oxidized by a strong oxidant in wastewater, wastewater treatment plant effluent and contaminated water. In the study of river pollution and the nature of industrial wastewater and the operation and management of wastewater treatment plants, it is an important and fast-measured organic pollution parameter, often expressed by the symbol COD.
Since the automatic online monitoring of pollution sources is continuous real-time monitoring on site, it has visibility, checkability, impartiality and objectivity, so it can provide good services for environmental management and decision-making. The COD automatic online monitor for pollution source wastewater is one of the most widely installed automatic online monitors. It is completely different from manual field sampling and laboratory analysis in the technical route. In order to control the monitoring error within the allowable limits and ensure the accuracy and precision of the monitoring results, COD automatic online monitoring also needs to follow certain quality control procedures to evaluate the quality of the data obtained.
In the industry, COD digestion devices are most commonly used in several ways, such as dichromate method, potassium permanganate method, spectrophotometry, rapid digestion method, and rapid digestion spectrophotometry.
Dichromate method
The standard method for the determination of chemical oxygen demand is represented by the Chinese standard GB11914 "Deterinary Determination of Chemical Oxygen Demand in Water Quality" and the International Standard ISO6060 "Determination of Chemical Oxygen Demand in Water", which has high oxidation rate and reproducibility. Good, accurate and reliable, it has become a universally accepted standard method in the international community. However, this classic standard method still has some shortcomings: the reflow device occupies a large experimental space, the water and electricity consumption is large, the reagent dosage is large, the operation is inconvenient, and it is difficult to quickly measure in large quantities.
Potassium permanganate method
The COD was determined by using potassium permanganate as an oxidant, and the measured potassium permanganate index was measured. The potassium permanganate index refers to the amount of oxygen consumed when treating a water sample with potassium permanganate as an oxidizing agent under certain conditions, expressed in mg/L of oxygen. Some organic substances and reducing inorganic substances in the water can consume potassium permanganate. Therefore, the potassium permanganate index is often used as a comprehensive indicator of the degree of contamination of water by organic matter. After the water sample is added with sulfuric acid to make it acidic, a certain amount of potassium permanganate solution is added, and the reaction is heated in a boiling water bath for a certain period of time. The remaining potassium permanganate was added to an excess of sodium oxalate solution for reduction, and then excess sodium oxalate was dropped back with potassium permanganate solution, and the permanganate index was calculated by calculation.
Spectrophotometry
Based on the classical standard method, potassium dichromate oxidizes organic matter, hexavalent chromium forms trivalent chromium, and the COD value of the water sample is determined by the relationship between the absorbance of hexavalent chromium or trivalent chromium and the COD value of the water sample. Using the above principles, the most important representative method in foreign countries is the US Environmental Protection Agency EPA. Method0410.4 "Automatic Manual Colorimetric Method", American Society for Testing and Materials ASTM: D1252-2000 "Method for Determination of Chemical Oxygen Demand in Water B-Seal Dissolution Spectrophotometry and International Standard ISO15705-2002 "Determination of Water Oxygen Demand (COD) by Small Sealed Tube Method". China is the unified method of the State Environmental Protection Administration, "Fast closed catalytic digestion method (including spectrophotometry)."
Rapid digestion
The classic standard method is the 2h reflux method. In order to improve the speed of analysis, various rapid analysis methods are proposed. Compared with the classical standard method, the acidity of the digestion system is increased from 9.0 mg/l to 10.2 mg/l, the reaction temperature is increased from 150 ° C to 165 ° C, and the digestion time is reduced from 2 h to 10 min to 15 min. The second is to change the traditional way of heating and dissipating by heat conduction radiation, and using microwave digestion technology to improve the digestion reaction speed. Due to the wide variety of microwave ovens and different powers, it is difficult to test the uniform power and time in order to achieve the best digestion effect. The price of microwave ovens is also very high, making it difficult to develop a uniform standard method.
Rapid digestion spectrophotometry
Determination of chemical oxygen demand (COD), whether it is reflux volume method, rapid method or photometric method, is the masking agent with potassium dichromate as oxidant, silver sulfate as catalyst and mercury sulfate as chloride ion. A method based on COD digestion system. On this basis, a lot of research work has been carried out for the purpose of saving reagents, reducing energy consumption, and being simple, fast, accurate and reliable. The method has the advantages of small occupied space, small energy consumption, small reagent dosage, minimum waste liquid, low energy consumption, simple operation, safety and stability, accuracy and reliability, and is suitable for large-scale determination, and makes up for the deficiency of the classical standard method.
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