Chemical Oxygen Demand (COD) 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 industrial wastewater properties 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.

Determination method: dichromate method, potassium permanganate method, spectrophotometry, rapid digestion method, rapid digestion spectrophotometry in line with national standard HJ-T399-2007 water chemical oxygen demand determination.

definition

Under certain conditions, the amount of oxidant consumed by the reducing substance in the oxidation of 1 liter of water sample is used as an index, and is converted into the number of milligrams of oxygen required per liter of water sample after oxidation, expressed in mg/L. It reflects the extent to which water is contaminated with reducing substances. This indicator is also used as one of the comprehensive indicators of the relative content of organic matter.

Generally, the oxidizing agent used for measuring chemical oxygen demand is potassium permanganate or potassium dichromate. The values ​​obtained by using different oxidizing agents are also different, so it is necessary to indicate the detection method. In order to be uniform, countries have certain monitoring standards. According to the different oxidants, it is called potassium dichromate oxygen consumption (commonly known as chemical oxygen demand, chemical oxygen demand, referred to as cod) and potassium permanganate oxygen consumption (commonly called oxygen consumption). , oxygen consumption, referred to as oc, also known as permanganate index).

The chemical oxygen demand can also be compared with the biochemical oxygen demand (BOD), and the ratio of BOD/COD reflects the biodegradability of the sewage. The biochemical oxygen demand analysis takes a long time. Generally, the bio-energy can be basically consumed in more than 20 days. For the convenience, it takes about 95% of the oxygen consumption to be environmental monitoring data for five days. The mark is BOD5.

Detailed

The chemical oxygen demand indicates the amount of oxygen required for the oxidation of organic matter in one liter of sewage by potassium dichromate under strong acidic conditions, which can roughly indicate the amount of organic matter in the sewage. COD is an important indicator of organic pollution in water bodies and can reflect the degree of pollution of water bodies.

The so-called chemical oxygen demand (COD) is the amount of oxidant consumed when a water sample is treated with a certain strong oxidant under certain conditions. It is an indicator of how much reducing substances are present in the water. The reducing substances in water include various organic substances, nitrites, sulfides, ferrous salts, etc., but mainly organic substances. Therefore, chemical oxygen demand (COD) is often used as an indicator to measure the amount of organic matter in water. The greater the chemical oxygen demand, the more serious the water is contaminated by organic matter. The measurement of chemical oxygen demand (COD) varies depending on the amount of reducing substance in the water sample and the measurement method. The most common application at present is the acidic potassium permanganate oxidation method and the potassium dichromate oxidation method. Potassium permanganate (KMnO4) method has a low oxidation rate, but it is relatively simple. When the relative content of organic matter in water samples is relatively large, potassium dichromate (K2Cr2O7) method can be used, and the oxidation rate is high and the reproducibility is good. Suitable for determining the total amount of organic matter in a water sample. Organic matter is very harmful to industrial water systems. Strictly speaking, chemical oxygen demand also includes inorganic reducing substances present in water. Generally, since the amount of organic matter in the wastewater is much larger than the amount of the inorganic substance, the chemical oxygen demand is generally used to represent the total amount of organic substances in the wastewater. Under the conditions of the measurement, the organic substance not containing nitrogen in the water is easily oxidized by potassium permanganate, and the organic substance containing nitrogen is more difficult to decompose. Therefore, the oxygen consumption is suitable for the determination of natural water or general wastewater containing organic matter that is easily oxidized, while the chemically oxygen demand is often determined for organic industrial wastewater with complex composition.

Water containing a large amount of organic matter contaminates the ion exchange resin when passing through the desalination system, and is particularly liable to contaminate the anion exchange resin, resulting in a decrease in resin exchange capacity. When the organic matter is pretreated (coagulation, clarification and filtration), it can be reduced by about 50%, but it cannot be removed in the desalination system, so it is often brought into the boiler by the make-up water to lower the pH of the furnace water. Sometimes organic matter may also be carried into the steam system and condensate to lower the pH and cause system corrosion. High levels of organic matter in the circulating water system promote microbial growth. Therefore, regardless of the desalination, furnace water or circulating water system, the COD is as low as possible, but there is no uniform limit. When COD (KMnO4 method) > 5 mg / L in the circulating cooling water system, the water quality has begun to deteriorate.

In the standard of drinking water, Class I and Class II water chemical oxygen demand (COD) ≤ 15mg / L, Class III water chemical oxygen demand (COD) ≤ 20mg / L, Class IV water chemical oxygen demand (COD) ≤ 30mg/L, Class V water chemical oxygen demand (COD) ≤ 40mg / L. The larger the value of COD, the more serious the pollution of the water body.

Ecological impact

High chemical oxygen demand means that the water contains a large amount of reducing substances, mainly organic pollutants. The higher the chemical oxygen demand, the more serious the organic pollution of the river. The source of these organic pollutants may be pesticides, chemical plants, organic fertilizers, etc. If left untreated, many organic pollutants can be deposited by the sediment at the bottom of the river and cause long-lasting toxic effects on aquatic organisms in the next few years. After a large number of aquatic organisms die, the ecosystem in the river is destroyed. If people feed on the creatures in the water, they will absorb the toxins in these organisms and accumulate in the body. These poisons often cause carcinogenic, teratogenic and mutagenic effects, which are extremely dangerous to humans. In addition, if irrigation is carried out with contaminated river water, plants and crops will be affected, and growth will be poor, and people will not be able to access these crops. However, high chemical oxygen demand does not necessarily mean the above-mentioned hazards. The specific judgments need to be analyzed in detail, such as the analysis of the types of organic matter, and the impact on water quality and ecology. Is it harmful to the human body? If detailed analysis is not possible, the chemical oxygen demand can be measured again in a few days. If the pre-contrast value drops a lot, it means that the reducing substances contained in the water are mainly degradable organic substances, which are relatively harmful to humans and organisms. light.

Removal method

Policy recommendations for emission reduction projects: 1. Implement sewage treatment plants, sewage pipe networks, sludge treatment, and reclaimed water as an integral part of wastewater treatment projects. 2. The actual effect of the operation of the sewage treatment plant will be taken as a priority area, and the transformation from construction to operation and maintenance will be realized.

test methods

Dichromate method

The standard method for the determination of chemical oxygen demand is represented by China's standard GB11914 "Deterinary Determination of Chemical Oxygen Demand in Water Quality" and International Standard ISO6060 "Determination of Chemical Oxygen Demand in Water", which has high oxidation rate and reproducibility. Good, accurate and reliable, has become a classic standard method generally recognized by the international community.

The measuring principle is as follows: in the acidic medium of sulfuric acid, potassium dichromate is used as the oxidant, silver sulfate is used as the catalyst, mercury sulfate is the masking agent of chloride ion, and the acidity of the reaction solution is 9 mol/L, and the digestion reaction liquid is boiled by heating. The boiling temperature of 148 ° C ± 2 ° C is the digestion temperature. The reaction reaction was heated under reflux with water for 2 h. After the digestion solution was naturally cooled, it was diluted with water to about 140 ml to test the ferrous iron as an indicator, and the remaining potassium dichromate was titrated with ammonium ferrous sulfate solution according to the ammonium ferrous sulfate solution. The consumption of the COD value of the water sample is calculated. The oxidizing agent used is potassium dichromate, and the oxidizing property is hexavalent chromium, so it is called the dichromate method.

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 amount is large, the operation is inconvenient, and it is difficult to quickly measure in large quantities.

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. Method 0410.4 "Automatic Manual Colorimetric Method", American Society for Testing and Materials ASTM: D1252-2000 "Method for Determination of Chemical Oxygen Demand in Water B-Seal Digestion Spectrometry Photometric Method 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 analysis speed, various rapid analysis methods are proposed. There are two main methods: one is to increase the concentration of oxidant in the digestion reaction system, increase the acidity of sulfuric acid, increase the reaction temperature, and increase the conditions of the promoter to increase the reaction rate. The domestic method is GB/T14420-1993 "Boiler water and cooling water analysis method for chemical oxygen demand determination of potassium dichromate fast method" and the unified method recommended by the State Environmental Protection Administration "Coulomb method" and "rapid closed catalytic digestion method (including Spectrophotometry) is the representative of this method. Foreign countries are represented by the German standard method DIN38049 T.43 "Fast method for the determination of chemical oxygen demand in water".

Compared with the classical standard method, the acidity of the digestion system was increased from 9.0 mg/l to 10.2 mg/l, the reaction temperature was increased from 150 °C to 165 °C, and the digestion time was 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 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 the COD digestion system. On this basis, people have carried out a lot of research work for the purpose of saving reagents, reducing energy consumption, and being simple, fast, accurate and reliable. The rapid digestion spectrophotometry combines the advantages of the above various methods. It refers to the use of a sealed tube as a digestion tube, taking a small meter of water sample and reagent in a sealed tube, placing it in a small constant temperature heating dish, heating and dissolving at a constant temperature, and using spectroscopic light. The COD value was determined by spectrophotometry; the sealed tube was φ16mm and the length was 100mm~150mm. The opening with a wall thickness of 1.0mm~1.2mm was a screw mouth and a spiral sealing cover was added. The sealed tube has acid resistance, high temperature resistance and pressure crack resistance. A sealed tube can be used as a digestion, called a digestion tube. Another type of sealed tube can be used as a digestion method, and can also be used as a colorimetric tube for colorimetry, which is called a digestion colorimetric tube. The small heating digestion device uses an aluminum block as a heating body, and the heating holes are evenly distributed. The pore diameter is φ16.1 mm, the pore depth is 50 mm to 100 mm, and the set heating temperature is the digestion reaction temperature. At the same time, due to the proper size of the sealed tube, the digestion reaction liquid occupies a suitable proportion of the space of the sealed tube. A part of the sealing tube containing the digestion reaction liquid is inserted into the heating hole of the heater, and the bottom of the sealing tube is heated at a constant temperature of 165 ° C; the upper part of the sealing tube is heated above the heating hole and exposed to the space, and the top of the nozzle is lowered to 85 ° C under natural cooling of the air. Left and right; the difference in temperature ensures that the reaction liquid in the small sealed tube is in a micro-boiling state at this constant temperature. The compact COD reactor holds 25 sealed tubes. After digesting the reaction with a sealed tube, the digested solution can be measured on a general luminometer by diverting it into a cuvette. After digestion with a sealed colorimetric tube, it can be directly measured on a COD dedicated photometer with a sealed colorimetric tube. A sample with a COD value of 100 mg/L to 1000 mg/L can be measured at a wavelength of 600 nm, and a sample having a COD value of 15 mg/L to 250 mg/L can be measured at a wavelength of 440 nm. 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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