Knowledge of Chemical oxygen demand

Knowledge of Chemical oxygen demand

1. Definition of COD.

COD (Chemical Oxygen Demand) 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 the amount of reducing substances in water. The reducing substances in water include various organic substances, nitrites, sulfides, ferrous salts, etc., but the main ones are organic substances. Therefore, chemical oxygen demand (COD) is often used as an indicator to measure the amount of organic substances in water. The greater the chemical oxygen demand, the more serious the water pollution by organic substances. The determination of chemical oxygen demand (COD) varies with the determination of reducing substances in water samples and the determination method. The most commonly used methods are acid potassium permanganate (KMnO4) oxidation method and potassium dichromate (K2Cr2O7) oxidation method. The potassium permanganate oxidation method has a low oxidation rate, but is relatively simple and can be used when determining the relative comparison value of the organic content in water samples. Potassium dichromate oxidation method has high oxidation rate and good reproducibility, and is suitable for determining the total amount of organic matter in water samples. Organic matter is very harmful to industrial water systems. Strictly speaking, chemical oxygen demand also includes inorganic reducing substances in water. Usually, because the amount of organic matter in wastewater is much greater than the amount of inorganic matter, chemical oxygen demand is generally used to represent the total amount of organic matter in wastewater. Under the measurement conditions, organic matter without nitrogen in water is easily oxidized by potassium permanganate, while organic matter containing nitrogen is more difficult to decompose. Therefore, oxygen demand is suitable for determining natural water or general wastewater containing organic matter that is easily oxidized, while organic industrial wastewater with more complex components is often measured for chemical oxygen demand.

Water containing a large amount of organic matter will contaminate ion exchange resins when passing through the desalination system, especially anion exchange resins, which will reduce the exchange capacity of the resin. Organic matter can be reduced by about 50% after pretreatment (coagulation, clarification and filtration), but it cannot be removed in the desalination system, so it is often brought into the boiler through the feed water to reduce the pH value of the boiler water. Sometimes organic matter may also be brought into the steam system and condensate, causing the pH to decrease and causing system corrosion. High organic matter content in the circulating water system will promote microbial reproduction. Therefore, whether for desalination, boiler water or circulating water system, the lower the COD, the better, but there is no unified limit index. When COD (KMnO4 method) is greater than 5mg/L in the circulating cooling water system, the water quality has begun to deteriorate.

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

2. How is COD produced?

COD (chemical oxygen demand) is mainly derived from substances in the water sample that can be oxidized by strong oxidants, especially organic matter. These organic substances are widely present in wastewater and polluted water, including but not limited to sugars, oils and fats, ammonia nitrogen, etc. The oxidation of these substances consumes the dissolved oxygen in the water, thereby increasing the chemical oxygen demand. Specifically:

1. Sugar substances: such as glucose, fructose, etc., are commonly found in wastewater from the food processing industry and the biopharmaceutical industry, and they will increase the COD content.

2. Oils and fats: Wastewater containing oils and fats discharged during industrial production will also lead to an increase in COD concentration.

3. Ammonia nitrogen: Although it does not directly affect the determination of COD, the oxidation of ammonia nitrogen will also consume oxygen during wastewater treatment, indirectly affecting the COD value.

In addition, there are many types of substances that can produce COD in sewage, including biodegradable organic matter, industrial organic pollutants, reducing inorganic substances, some organic matter that is difficult to biodegrade, and microbial metabolites. The oxidation of these substances consumes the dissolved oxygen in the water, resulting in the generation of COD. Therefore, chemical oxygen demand is an important indicator to measure the degree of pollution of organic matter and reducing inorganic matter in water. It reflects the total amount of substances in water that can be oxidized and decomposed by oxidants (usually potassium dichromate or potassium permanganate) under certain conditions, that is, the degree to which these substances consume oxygen.

1. Organic matter: Organic matter is one of the main sources of COD in sewage, including biodegradable organic matter such as proteins, carbohydrates and fats. These organic matter can be decomposed into carbon dioxide and water under the action of microorganisms.

2. Phenolic substances: Phenolic compounds are often used as pollutants in wastewater in some industrial processes. They can have a serious impact on the water environment and increase the COD content.

3. Alcoholic substances: Alcoholic compounds, such as ethanol and methanol, are also common sources of COD in some industrial wastewater.

4. Sugar substances: Sugar compounds, such as glucose, fructose, etc., are common components in wastewater from some food processing industries and biopharmaceutical industries, and they will also increase the COD content.

5. Grease and fat: Grease and fat-containing wastewater discharged during industrial production will also lead to an increase in COD concentration.

6. Ammonia nitrogen: Although ammonia nitrogen does not directly affect the determination of COD, the oxidation of ammonia nitrogen will also consume oxygen during the wastewater treatment process, indirectly affecting the COD value.

In addition, it is worth noting that COD not only reacts to organic matter in water, but also represents inorganic substances with reducing properties in water, such as sulfide, ferrous ions, sodium sulfite, etc. Therefore, when treating sewage, it is necessary to comprehensively consider the contribution of various pollutants to COD and take appropriate treatment measures to reduce the COD value.

Organic matter is the main source of COD. They include various organic matter, suspended matter, and difficult-to-decompose substances in sewage. The high COD content in sewage will pose a great threat to the water environment. The treatment and monitoring of COD is one of the important measures to prevent and control pollution. Therefore, COD determination is one of the commonly used test methods in sewage treatment and environmental monitoring.

The determination of COD is an easy-to-operate process with high analytical sensitivity. The determination of COD can be completed by directly observing the color change of the sample or the current or other signals after the chemical reagent is titrated to generate oxidation products. When the COD value exceeds the standard, it is necessary to carry out corresponding treatment to avoid environmental pollution. In short, understanding what COD means plays a vital role in protecting the water environment and conducting pollution control.

3. The impact of high COD.

‌COD (chemical oxygen demand) is an important indicator for measuring the degree of organic pollution in water bodies. Excessive content will have a serious impact on river water quality. ‌

The measurement of COD is based on the amount of oxidant consumed when reducing substances (mainly organic matter) are oxidized and decomposed in 1 liter of water under certain conditions. These reducing substances will consume a large amount of dissolved oxygen during the decomposition process, causing aquatic organisms to lack oxygen, which in turn affects their normal growth and survival, and may cause a large number of deaths in severe cases. In addition, the reduction of dissolved oxygen will accelerate the deterioration of water quality, promote the corruption and decomposition of organic matter, and produce more toxic and harmful substances, such as ammonia nitrogen, which will cause greater harm to aquatic organisms and water quality. Long-term exposure to sewage containing high concentrations of organic matter may also cause serious harm to human health, such as causing gastrointestinal diseases, skin diseases, etc. Therefore, the excessive COD not only poses a threat to aquatic organisms, but also poses a potential risk to human health.

In order to protect the water environment and human health, effective measures must be taken to prevent and control the excessive COD. This includes reducing the discharge of organic matter in industrial and agricultural activities, as well as strengthening wastewater treatment and monitoring to ensure that the discharged water quality meets the standards, thereby maintaining a good water ecological environment.

COD is an indicator of the content of organic matter in water. The higher the COD, the more seriously the water body is polluted by organic matter. When toxic organic matter enters the water body, it not only harms organisms in the water body such as fish, but can also be enriched in the food chain and enter the human body, causing chronic poisoning. .

COD has a great impact on water quality and ecological environment. Once organic pollutants with elevated COD content enter rivers, lakes and reservoirs, if they are not treated in time, many organic matter may be adsorbed by the soil at the bottom of the water and accumulate for many years. These organisms will cause damage to various organisms in the water, and may continue to be toxic for several years. This toxic effect has two effects:

On the one hand, it will cause the death of a large number of aquatic organisms, destroy the ecological balance of the water body, and even directly destroy the entire river ecosystem.

On the other hand, toxins will slowly accumulate in aquatic organisms such as fish and shrimp. Once humans consume these toxic aquatic organisms, the toxins will enter the human body and accumulate for many years, leading to unpredictable serious consequences such as cancer, deformities, and gene mutations. In the same way, if people use polluted water for irrigation, crops will also be affected, and people will also inhale a large amount of harmful substances in the process of eating.

When COD is very high, it will cause the deterioration of natural water quality. The reason is that the self-purification of water requires the degradation of these organic matter. The degradation of COD necessarily requires oxygen consumption, and the reoxygenation capacity in the water does not meet the requirements. DO will drop directly to 0 and become anaerobic. In the anaerobic state, it will continue to decompose (anaerobic treatment of microorganisms), and the water will become black and smelly (anaerobic microorganisms look very black and contain hydrogen sulfide gas).

4. Methods for treating COD

The first point

Physical method: It uses physical action to separate suspended matter or turbidity in wastewater, which can remove COD in wastewater. Common methods include pre-treating sewage through sedimentation tanks, filter grids, filters, grease traps, oil-water separators, etc., to simply remove COD of particulate matter in sewage.

Second point

Chemical method: It uses chemical reactions to remove dissolved substances or colloidal substances in wastewater, and can remove COD in wastewater. Common methods include neutralization, precipitation, oxidation-reduction, catalytic oxidation, photocatalytic oxidation, micro-electrolysis, electrolytic flocculation, incineration, etc.

Third point

Physical and chemical method: It uses physical and chemical reactions to remove dissolved substances or colloidal substances in wastewater. It can remove COD in wastewater. Common methods include grid, filtration, centrifugation, clarification, filtration, oil separation, etc.

Fourth point

Biological treatment method: It uses microbial metabolism to convert organic pollutants and inorganic microbial nutrients in wastewater into stable and harmless substances. Common methods include activated sludge method, biofilm method, anaerobic biological digestion method, stabilization pond and wetland treatment, etc.

5. COD analysis method.

Dichromate method

The standard method for determining chemical oxygen demand is represented by the Chinese standard GB 11914 "Determination of Chemical Oxygen Demand of Water Quality by Dichromate Method" and the international standard ISO6060 "Determination of Chemical Oxygen Demand of Water Quality". This method has high oxidation rate, good reproducibility, accuracy and reliability, and has become a classic standard method generally recognized by the international community.

The determination principle is: in sulfuric acid acid medium, potassium dichromate is used as an oxidant, silver sulfate is used as a catalyst, and mercuric sulfate is used as a masking agent for chloride ions. The sulfuric acid acidity of the digestion reaction liquid is 9 mol/L. The digestion reaction liquid is heated to boil, and the boiling point temperature of 148℃±2℃ is the digestion temperature. The reaction is cooled by water and refluxed for 2h. After the digestion liquid is cooled naturally, it is diluted to about 140ml with water. Ferrochlorine is used as an indicator, and the remaining potassium dichromate is titrated with ammonium ferrous sulfate solution. The COD value of the water sample is calculated based on the consumption of ammonium ferrous sulfate solution. The oxidant used is potassium dichromate, and the oxidizing agent is hexavalent chromium, so it is called the dichromate method.

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 measure quickly in large quantities.

Potassium permanganate method

COD is measured using potassium permanganate as an oxidant, and the measured result is called potassium permanganate index.

Spectrophotometry

Based on the classic standard method, potassium dichromate oxidizes organic matter, and hexavalent chromium generates trivalent chromium. The COD value of the water sample is determined by establishing a relationship between the absorbance value of hexavalent chromium or trivalent chromium and the COD value of the water sample. Using the above principle, the most representative methods abroad are EPA.Method 0410.4 "Automatic Manual Colorimetry", ASTM: D1252-2000 "Method B for the determination of chemical oxygen demand of water-sealed digestion spectrophotometry" and ISO15705-2002 "Small Sealed Tube Method for the Determination of Chemical Oxygen Demand (COD) of Water Quality". my country's unified method is the "Rapid Sealed Catalytic Digestion Method (Including Spectrophotometry)" of the State Environmental Protection Administration.

Rapid Digestion Method

The classic standard method is the 2h reflux method. In order to increase the analysis speed, people have proposed various rapid analysis methods. There are two main methods: one is to increase the concentration of the oxidant in the digestion reaction system, increase the acidity of sulfuric acid, increase the reaction temperature, and increase the catalyst to increase the reaction speed. The domestic method is represented by GB/T14420-1993 "Analysis of Boiler Water and Cooling Water Chemical Oxygen Demand Determination Potassium Dichromate Rapid Method" and the unified methods recommended by the State Environmental Protection Administration "Coulometric Method" and "Rapid Closed Catalytic Digestion Method (Including Photometric Method)". The foreign method is represented by the German standard method DIN38049 T.43 "Rapid Method for Determination of Chemical Oxygen Demand of Water".

Compared with the classic standard method, the above method increases the sulfuric acid acidity of the digestion system from 9.0 mg/L to 10.2 mg/L, the reaction temperature from 150℃ to 165℃, and the digestion time from 2h to 10min~15min. The second is to change the traditional method of digestion by heating with thermal radiation, and use 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 unified power and time in order to achieve the best digestion effect. The price of microwave ovens is also very high, and it is difficult to formulate a unified standard method.

Lianhua Technology developed a rapid digestion spectrophotometric method for chemical oxygen demand (COD) in 1982, which achieved the rapid determination of COD in sewage with the method of "10 minutes digestion, 20 minutes value". In 1992, this research and development result was included in the American "CHEMICAL ABSTRACTS" as a new contribution to the world's chemical field. This method became the testing standard of the environmental protection industry of the People's Republic of China in 2007 (HJ/T399-2007). This method successfully achieved an accurate COD value within 20 minutes. It is simple to operate, convenient and fast, requires a small amount of reagents, greatly reduces the pollution generated in the experiment and reduces various costs. The principle of this method is to digest the water sample added with Lianhua Technology's COD reagent at 165 degrees for 10 minutes at a wavelength of 420 or 610nm, then cool it for 2 minutes, and then add 2.5ml of distilled water. The COD result can be obtained using Lianhua Technology's COD rapid determination instrument.