Environmental Water-Treatment Knowledge: The Meaning and Role of Chemical Oxygen Demand (COD)
Under certain conditions, the amount of oxidant consumed to oxidize the reducing substances in 1 L of water sample is taken as an indicator, converted to the milligrams of oxygen required if the entire water sample were oxidized, expressed in mg/L. It reflects the degree of pollution by reducing substances in the water. This indicator is also one of the comprehensive indicators of the relative content of organic matter.
Generally, the oxidants used to measure chemical oxygen demand are potassium permanganate or potassium dichromate, and the values obtained using different oxidants also differ, so the detection method must be stated. For comparability, each country has certain monitoring standards. According to the different strong oxidants added, they are respectively called potassium-dichromate oxygen consumption (conventionally called chemical oxygen demand, Chemical Oxygen Demand, abbreviated COD) and potassium-permanganate oxygen consumption (conventionally called oxygen consumption, Oxygen Consumption, abbreviated OC, also called permanganate index).
Chemical oxygen demand can also be compared with biochemical oxygen demand (BOD); the BOD/COD ratio reflects the biodegradability of the sewage. Biochemical oxygen demand analysis takes a long time—generally more than 20 days for the organisms in water to basically consume it all; for convenience, the value of about 95% oxygen consumed after five days is usually taken as environmental-monitoring data, marked as BOD5.
Its determination principle is: in a sulfuric-acid acidic medium, using potassium dichromate as the oxidant, silver sulfate as the catalyst, and mercuric sulfate as the masking agent for chloride ions, the acidity of the digestion reaction liquid is 9 mol/L sulfuric acid, heated to boil the digestion reaction liquid, with a boiling-point temperature of 148°C ±2°C as the digestion temperature. Reflux heating and reaction for 2 h with water cooling, after the digestion liquid naturally cools, using ferroin as the indicator and ammonium ferrous sulfate solution to titrate the remaining potassium dichromate, the COD value of the water sample is calculated from the consumption of ammonium ferrous sulfate solution. The oxidant used is potassium dichromate, and the oxidizing species is hexavalent chromium, hence the name dichromate method.
The so-called chemical oxygen demand (COD) is the amount of oxidant consumed when a certain strong oxidant is used to treat a water sample under certain conditions. It is an indicator of the amount of reducing substances in water. The reducing substances in water include various organic matters, nitrites, sulfides, ferrous salts, etc., but mainly organic matter. Therefore, chemical oxygen demand (COD) is often used as an indicator of the amount of organic matter in water. The larger the chemical oxygen demand (COD), the more serious the pollution of the water body by organic matter. The determination of chemical oxygen demand (COD) varies with the reducing substances in the measured water sample and the determination method. At present, the most commonly used are the acidic potassium-permanganate oxidation method and the potassium-dichromate oxidation method. The potassium permanganate (KMnO4) method has a lower oxidation rate but is relatively simple, and can be used when relatively comparing the organic-matter content of water samples. The potassium dichromate (K2Cr2O7) method has a high oxidation rate and good reproducibility, suitable for determining the total amount of organic matter in water samples. Organic matter causes great harm to industrial water systems. Strictly speaking, chemical oxygen demand also includes inorganic reducing substances present in water. Usually, because the amount of organic matter in wastewater is far greater than that of inorganic matter, chemical oxygen demand is generally used to represent the total amount of organic matter in wastewater. Under the determination conditions, nitrogen-free organic matter in water is easily oxidized by potassium permanganate, while nitrogen-containing organic matter is more difficult to decompose. Therefore, oxygen consumption is suitable for determining natural water or general wastewater containing easily oxidized organic matter, while the more complex organic industrial wastewater often determines chemical oxygen demand.
Water containing large amounts of organic matter will pollute ion-exchange resins when passing through the desalination system, especially easily polluting anion-exchange resins and reducing their exchange capacity. Organic matter can be reduced by about 50% during pretreatment (coagulation, clarification, and filtration), but cannot be removed in the desalination system, so it often enters the boiler through make-up water, lowering the boiler-water pH. Sometimes organic matter can also enter the steam system and condensate, lowering the pH and causing system corrosion. In circulating-water systems, high organic-matter content promotes microbial reproduction. Therefore, for desalination, boiler water, or circulating-water systems, the lower the COD the better, but there is no unified limit indicator. In circulating cooling-water systems, when COD (KMnO4 method) > 5 mg/L, the water quality has begun to deteriorate.
In drinking-water standards, Class I and II water have chemical oxygen demand (COD) ≤ 15 mg/L, Class III water COD ≤ 20 mg/L, Class IV water COD ≤ 30 mg/L, and Class V water COD ≤ 40 mg/L. The larger the COD value, the more serious the pollution of the water body.
High chemical oxygen demand means the water contains large amounts of reducing substances, mainly organic pollutants. The higher the chemical oxygen demand, the more serious the organic pollution of the river water; the sources of these organic pollutants may be pesticides, chemical plants, organic fertilizers, etc. If not treated, many organic pollutants can be adsorbed by bottom sediment and deposit at the bottom of the river, causing persistent toxic effects on aquatic organisms for years to come. After large numbers of aquatic organisms die, the river's ecosystem is destroyed. If humans eat the organisms in the water, they will absorb large amounts of the toxins accumulated in these organisms, which often have carcinogenic, teratogenic, and mutagenic effects and are extremely dangerous to humans. In addition, if polluted river water is used for irrigation, plants and crops will also be affected, easily growing poorly, and humans cannot eat these crops. But high chemical oxygen demand does not necessarily mean the aforementioned hazards; a detailed analysis is needed to make a specific judgment, such as analyzing the kinds of organic matter and their actual impact on water quality and ecology, and whether they are harmful to humans. If a detailed analysis cannot be performed, the chemical oxygen demand can also be re-measured after a few days; if it drops significantly compared with the previous value, it indicates that the reducing substances in the water are mainly easily degradable organic matter, with relatively mild harm to humans and organisms.
Policy and measure suggestions for emission-reduction projects: 1. Treat the sewage-treatment plant, sewage pipe network, sludge treatment, and reclaimed-water utilization as indispensable components of sewage-treatment engineering, and implement systematic construction. 2. Give priority to the operational effectiveness of sewage-treatment plants, shifting from construction-focused to operation-and-maintenance-focused. [1]
The standard method for chemical oxygen demand determination is represented by China's standard GB11914 'Water Quality—Determination of Chemical Oxygen Demand—Dichromate Method' and the international standard ISO6060 'Water Quality—Determination of the Chemical Oxygen Demand,' which has high oxidation rate, good reproducibility, accuracy, and reliability, and has become a universally recognized classic standard method in the international community.
Chemical oxygen demand represents the oxygen required to oxidize the organic matter in 1 L of sewage under strongly acidic conditions with potassium dichromate, and roughly indicates the amount of organic matter in the sewage. COD is an important indicator of organic pollution in water bodies and can reflect the pollution degree of the water body.
However, this classic standard method still has shortcomings: the reflux device occupies a large experimental space, consumes a lot of water and electricity, uses a large amount of reagents, is inconvenient to operate, and is difficult to determine in large batches quickly.
Using potassium permanganate as the oxidant to determine COD, the result is called the permanganate index.
Based on the classic standard method, potassium dichromate oxidizes organic matter, hexavalent chromium becomes trivalent chromium, and the COD value of the water sample is determined by establishing a relationship between the absorbance values of hexavalent or trivalent chromium and the water-sample COD value. Using the above principle, the main foreign representative methods are the U.S. EPA Method 0410.4 'Automated and Manual Colorimetric Method,' ASTM D1252—2000 'Standard Test Method for Chemical Oxygen Demand (Dichromate Reactor Closed Reflux Titrimetric Method),' and the international standard ISO15705—2002 'Water Quality—Determination of the Chemical Oxygen Demand (COD)—Small-Scale Sealed-Tube Method.' China uses the unified method of the State Environmental Protection Administration, 'Rapid Sealed Catalytic Digestion Method (including spectrophotometry).'
Compared with the classic standard method, the above methods increase the digestion-system sulfuric-acid acidity from 9.0 mg/L to 10.2 mg/L, the reaction temperature from 150°C to 165°C, and reduce the digestion time from 2 h to 10–15 min. The second is to change the traditional heat-radiation heating digestion method and adopt microwave digestion technology to increase digestion speed. Because there are many types of microwave ovens with different powers, it is difficult to test a unified power and time to achieve the best digestion effect. Microwave ovens are also expensive, making it difficult to formulate a unified standard method.
Whether the chemical oxygen demand (COD) determination method is the reflux-volume method, the rapid method, or the spectrophotometric method, all are based on the determination method using potassium dichromate as the oxidant, silver sulfate as the catalyst, mercuric sulfate as the masking agent for chloride ions, and measuring COD under sulfuric-acid acidic conditions. On this basis, a large amount of research has been carried out to save reagents, reduce energy consumption, and achieve easy, rapid, accurate, and reliable operation. The rapid-digestion spectrophotometric method combines the advantages of the above methods: it uses a sealed tube as the digestion tube, puts a small amount of water sample and reagent into the sealed tube, places it in a small constant-temperature heating bath, heats and digests at constant temperature, and determines the COD value by spectrophotometry; the sealed tube specification is f16 mm, length 100–150 mm, wall thickness 1.0–1.2 mm, with a screw opening and a screw sealing cap. The sealed tube is acid-resistant, high-temperature-resistant, and pressure- and explosion-proof. One type of sealed tube can be used for digestion, called a digestion tube. Another type can be used both for digestion and as a colorimetric tube, called a digestion-colorimetric tube. The small heating digester uses an aluminum block as the heating body, with evenly distributed heating holes. Aperture f16.1 mm, hole depth 50–100 mm, set heating temperature is the digestion reaction temperature. At the same time, due to the suitable size of the sealed tube, the digestion reaction liquid occupies a suitable space ratio in the sealed tube. The sealed tube containing the digestion reaction liquid is partly inserted into the heater heating hole, the bottom of the sealed tube is heated at a constant 165°C; the upper part of the sealed tube is higher than the heating hole and exposed to space, naturally cooled by air so that the top of the tube drops to about 85°C; the temperature difference ensures that the reaction liquid in the small sealed tube is in a micro-boiling reflux state at this constant temperature. A compact COD reactor can hold 25 sealed tubes. After sealed-tube digestion, the digestion liquid is transferred to a cuvette and can be measured on a general spectrophotometer, or after sealed colorimetric tube digestion it can be measured directly on a COD-specific spectrophotometer. At a wavelength of 600 nm, the COD value of samples of 100–1000 mg/L can be measured; at 440 nm, the COD value of samples of 15–250 mg/L can be measured. This method has the characteristics of small space occupation, low energy consumption, small reagent dosage, minimal waste liquid, low energy consumption, simple operation, safety and stability, accuracy and reliability, and is suitable for large-batch determination, making up for the shortcomings of the classic standard method.
Chemical Oxygen Demand (COD) determination methods—whether the reflux volumetric method, the rapid method, or the photometric method—all use potassium dichromate as the oxidant, silver sulfate as the catalyst, and mercuric sulfate as a masking agent for chloride ions, and are based on a COD digestion system measured under sulfuric-acid acidic conditions. Building on this, extensive research has been conducted to save reagents, reduce energy consumption, and achieve simple operation, speed, and accuracy and reliability. The rapid digestion spectrophotometric method combines the advantages of the above methods: it uses a sealed tube as the digestion tube, placing a small measured sample of water and reagent into the sealed tube, which is then placed in a small constant-temperature heating bath for constant-temperature digestion, and the COD value is determined by spectrophotometry. The sealed tube specifications are: 16 mm OD, length 100 mm–150 mm, wall thickness 1.0 mm–1.2 mm, with a screw-open mouth and a screw-on sealing cap. This sealed tube is acid-resistant, high-temperature-resistant, and compression- and explosion-proof. One type of sealed tube is used for digestion and is called a digestion tube. Another type can be used both for digestion and as a colorimetric tube, and is called a digestion-colorimetric tube. The small heating digester uses an aluminum block as the heating element, with evenly distributed heating holes. The hole diameter is 16.1 mm and the hole depth is 50 mm–100 mm, with the set heating temperature being the digestion reaction temperature. Because of the suitable tube dimensions, the digestion liquid occupies an appropriate proportion of the tube volume. The sealed tube containing the digestion liquid is partially inserted into the heater hole, with its bottom heated at a constant 165°C; the upper part protrudes above the heating hole and is exposed to air, where natural cooling lowers the tube-mouth top to about 85°C; this temperature difference ensures the reaction liquid remains in a slightly boiling reflux state at that constant temperature. The compact COD reactor can hold 25 sealed tubes. After digestion, the liquid is transferred to a cuvette for measurement on a general spectrophotometer; alternatively, after digestion in a sealed colorimetric tube, it can be measured directly in that tube on a COD-specific spectrophotometer. At 600 nm, samples with a COD value of 100–1000 mg/L can be measured; at 440 nm, samples with a COD value of 15–250 mg/L can be measured. This method features a small footprint, low energy and reagent consumption, minimal waste liquid, simple and safe operation, accuracy, reliability, and suitability for high-throughput determination, thereby compensating for the shortcomings of the classical standard method.
