As China continues to impose higher requirements on environmental management, in order to better strengthen the monitoring of the emission of pollutants, the total amount of pollutants is controlled, and the automatic on-line monitoring of pollution sources is gradually being carried out. As the monitoring of water pollution with human health, it is important Sex is even more self-evident. Understanding the relevant water quality indicators is the basis for water pollution monitoring. This must be familiar with our 'old friends' COD, BOD, and ammonia nitrogen horses. Today, Xiaobian leads everyone to understand The intermediate force of sewage treatment - COD digestion.
COD (Chemical Oxygen Demand) is a chemical method for measuring the amount of reducing substances that need to be oxidized in a water sample. Wastewater, Effluent from wastewater treatment plants and contaminated water, Substances that can be oxidized by strong oxidants (usually organic) Oxygen equivalents. In the study of river pollution and industrial wastewater properties and in the operation and management of wastewater treatment plants, it is an important and relatively rapid determination of organic pollution parameters, often denoted by the symbol COD.
Because the automatic online monitoring of pollution sources is continuous real-time monitoring on the site, with visibility, accountability, fairness and objectivity, it can provide good services for environmental management and decision-making. The pollution source wastewater COD automatic online monitoring instrument is an installation application. One of the most widely used automatic on-line monitors is completely different from the manual on-site sampling in the technical route. Laboratory analysis. In order to control the monitoring error within the allowable limits, to 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 mainly use dichromate method, potassium permanganate method, spectrophotometry, rapid digestion method, rapid digestion spectrophotometry and other methods are most commonly used.
Dichromate method
The standard method for the determination of chemical oxygen demand is represented by the Chinese Standard GB11914 "Dichromate Method for Determination of Chemical Oxygen Demand of Water Quality" and the International Standard ISO6060 "Determination of Water Quality Chemical Oxygen Demand", which has a high oxidation rate and reproducibility. Good, accurate and reliable, has become the universally recognized standard method of the international community. However, this classical standard method still has its shortcomings: The back-flow device occupies a large experimental space, consumes large amounts of water and electricity, requires large amounts of reagents, and is inconvenient for operation. Mass rapid measurement.
Potassium permanganate method
The COD was measured using potassium permanganate as the oxidant. The measured potassium permanganate index was determined. The potassium permanganate index refers to the oxygen consumed when the water sample was treated with potassium permanganate as an oxidant under certain conditions. The quantity is expressed in mg/L of oxygen. Potassium permanganate can be consumed by some organic and reducing inorganic substances in water. Therefore, the potassium permanganate index is often used as a comprehensive indicator of the degree of contamination of water bodies with organic matter. After being acidic, add a certain amount of potassium permanganate solution and heat the reaction in a boiling water bath for a certain period of time. The remaining potassium permanganate is added in excess of sodium oxalate solution to reduce, and excess potassium oxalate solution is used to drip excess oxalic acid. Sodium, Calculate the permanganate index 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 value of hexavalent chromium or trivalent chromium and the COD value of the water sample. Using the above principles, the most representative method in foreign countries is the US Environmental Protection Agency EPA.Method 0410.4 “Automatic Manual Colorimetry”, American Society for Testing and Materials ASTM: D1252-2000 “Method for Determining Chemical Oxygen Demand in Water B—Seal Dissolution Spectrophotometric method and international standard ISO15705-2002 "Determination of chemical oxygen demand (COD) for small water seals". China is a unified method of the State Environmental Protection Administration "rapidly closed catalytic decomposition (including fractional photometry)".
Fast digestion
The classical standard method is the reflux 2h method. To improve the analysis speed, various rapid analysis methods have been proposed. Compared with the classical standard method, the acidity of the digestion system for the digestion system is increased from 9.0 mg/l to 10.2 mg/l, and the reaction temperature is determined by From 150°C to 165°C, the digestion time is reduced from 2h to 10min to 15min. The second is to change the traditional method of heating and digestion by thermal radiation, and the microwave digestion technology is used to increase the digestion reaction rate. Due to the wide range of microwave ovens, the power is not uniform. , It is difficult to test out the unified power and time, in order to achieve the best effect of digestion. The price of microwave ovens is also very high, it is difficult to formulate a unified standard method.
Rapid digestion spectrophotometry
The chemical oxygen demand (COD) determination method is based on potassium dichromate as the oxidant, silver sulfate as the catalyst, and mercury sulfate as the masking agent for chloride ions, both in the reflux capacity method, the rapid method, and the photometric method. Based on the COD digestion system, based on this method, a large amount of research has been carried out to achieve the goal of saving reagents and reducing energy consumption, ease of operation, speed, accuracy and reliability. This method has a small footprint, low energy consumption, and reagent consumption. Small, liquid waste is minimized, energy consumption is small, operation is simple, safe and stable, accurate and reliable, suitable for large-scale measurement and other characteristics, to make up for the lack of classic standard methods.
