Chemical Oxygen Demand (COD): Importance, Advantages, Disadvantages

Chemical oxygen demand (COD) is a measure of how much oxygen can be utilized by reactions in a given volume of a solution. Common units of measurement include milligrams per liter (mg/L), which stands for milligrams of oxygen consumed per liter of solution. Measurement of oxidizable contaminants in surface water (such as lakes and rivers) and wastewater is COD’s most prevalent use. Since it gives a metric for determining the influence of effluent on the receiving body, Chemical Oxygen Demand is helpful for analyzing water quality.

Chemical Oxygen Demand (COD)
Chemical Oxygen Demand (COD)

Typically, wastewater is subjected to a Chemical Oxygen Demand test. Water contamination is quantified by analyzing the concentration of biodegradable substances. Discharges of wastewater that contain excessive amounts of organic material might harm the surrounding ecosystem.

Both the biological oxygen demand and the chemical oxygen demand can be used to determine how much oxygen a given water sample requires. Biochemical oxygen demand only accounts for the oxygen required by living things, while chemical oxygen demand accounts for everything that can be oxidized.

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What is Chemical Oxygen Demand (COD)?

Chemical oxygen demand (COD) is a quantitative measure of the oxygen consumption necessary for the chemical oxidation of both organic and inorganic constituents found in wastewater. This process involves the utilization of oxidizing agents, such as potassium permanganate and potassium dichromate, to facilitate the oxidation reactions.

  • The presence of COD facilitates the acceleration of chemical oxidation of organic matter, obviating the requirement for additional equipment. The sole method for ascertaining the organic load in highly toxic sewage is through this approach.
  • In order to quantify the chemical oxygen demand (COD), the water sample is enclosed within a sealed vessel and exposed to a strong oxidizing agent, such as potassium dichromate and sulfuric acid, under specific conditions of duration and temperature.
  • The chemical oxygen demand (COD) refers to the amount of oxygen needed by both the inorganic and organic constituents present in a given sample of wastewater.
  • The reintroduction of treated wastewater into the environment has the potential to introduce organic materials into adjacent water bodies.
  • Elevated concentrations of chemical oxygen demand (COD) in wastewater are indicative of the existence of organic compounds, which have the potential to reduce the levels of dissolved oxygen.
  • Generally, this phenomenon tends to have adverse effects on the natural environment and encounters challenges related to regulatory compliance.
  • The measurement of oxygen demand is a crucial parameter in the assessment of the effects of organic pollution on water systems, with the ultimate goal of mitigating its presence.

Importance of Chemical Oxygen Demand

Chemical Oxygen Demand (COD) is a significant parameter for assessing water quality and finds extensive utilization across various applications, which encompass:

  • The measurement of Chemical Oxygen Demand (COD) holds significant importance in the field of wastewater treatment as it quantifies the quantity of oxygen necessary for the degradation of organic pollutants present in water.
  • The measurement of chemical oxygen demand (COD) can be conducted through various methods, namely direct or indirect approaches. The determination of COD value is typically conducted through the analysis of samples using laboratory test methods. The utilization of offline methods can be a time-intensive endeavor, given that the Chemical Oxygen Demand (COD) value holds significant importance in the context of biological treatment procedures.
  • Fortunately, in addition to conventional laboratory testing procedures, options such as automated COD analyzer devices are now available. These real-time devices are installed in a wastewater treatment plant’s sample stream and continuously report a variety of metrics. This method provides a simpler and faster means to analyze the chemical oxygen demand compared to offline monitoring. Indirect measurement is achieved through the employment of online analyzers because they are typically installed in a bypass of the process stream.

Strategy to reduce COD in wastewater

Numerous strategies have been demonstrated to effectively mitigate chemical oxygen demand (COD) in wastewater treatment practices. Wastewater treatment methods such as coagulation and flocculation, as well as microbial-mediated processes for chemical oxygen demand (COD) removal, are widely employed techniques for the mitigation of COD in wastewater.

Wastewater separation

  1. Colloidal particles are effectively eliminated from wastewater through the implementation of coagulation and flocculation processes in wastewater treatment. In the process of coagulation, a harmless agglomerating agent is introduced into the water in order to facilitate the aggregation of all the suspended particles, thereby enabling their efficient removal through filtration from the wastewater. Examples of such agents include ferric chloride (FeCl) and alum, among other substances.
  2. Flocculation is a process that involves the formation of larger particles, known as flocs, in order to eliminate aggregated particles from water. This is achieved through the utilization of a chemical polymer, commonly referred to as a flocculating agent.
  3. Following their placement in a sedimentation tank for additional treatment before disposal, flocs undergo sedimentation as part of the wastewater treatment process.
  4. The reduction of organic substances in wastewater through the application of coagulants and flocculants leads to a decrease in the competition for dissolved oxygen between marine organisms and bacteria. This is due to the deprivation of essential nutrients for microorganisms, which inhibits their growth and survival.

Microbial action

  1. Another effective technique for the removal of chemical oxygen demand (COD) involves the introduction of bacteria or other microorganisms capable of decomposing organic constituents present in wastewater. Sewage treatment encompasses the presence of both aerobic and anaerobic bacteria.
  2. Aerobic chemical oxygen demand (COD) removal refers to the process of introducing bacteria or other microorganisms that facilitate the decomposition of organic waste-derived compounds into carbon dioxide and water in the presence of air.
  3. The recommended treatment method for wastewater containing chemical oxygen demand (COD) levels below 3000 mg/L is aerobic COD treatment. In the context of anaerobic chemical oxygen demand (COD) removal, microorganisms are employed to convert organic waste constituents into biomass in an oxygen-deprived environment.
  4. This methodology yields ethanol that can be employed as a viable alternative energy resource for various applications such as power generation, heating, and drying, rendering it a highly advantageous approach.
  5. Anaerobic COD treatment is deemed appropriate for COD levels exceeding 2,000 mg/L.

Measurement of COD

Various methodologies can be employed to determine the chemical oxygen demand (COD). Examples of testing methods include online testing and offline laboratory approaches that utilize environmental analyzers.

According to the COD testing method, in an acidic environment and in the presence of a potent oxidizing agent, it is observed that nearly all organic constituents undergo oxidation to carbon dioxide. The COD analysis is a method used to ascertain the necessary concentration of oxygen required for the chemical oxidation of organic compounds present in water.

Environmental analyzers are advanced scientific instruments that are employed in modern chemical oxygen demand (COD) testing procedures. The EasyPREP COD-200 is considered a groundbreaking innovation in this particular domain. During the course of this procedure, the COD sample mixes undergo a series of heating, cooling, and evaluation steps.

Difficulties with COD monitoring

Despite the fact that the test is firmly established in legislation, there exist a multitude of issues and obstacles that are inherently linked to its utilization.

  • A delay exists between the transportation of samples to the laboratory and the subsequent two-hour testing period, resulting in the potential for environmental harm to occur prior to the availability of data.
  • The cost and inconvenience of the test.
  • Hazardous chemicals are used in the test, which must be disposed of properly to avoid harm to the operators.
  • The experimental procedure falls short in replicating natural processes, as it entails an artificial incubation utilizing a potent oxidizing agent.
  • The measurement technique exhibits a lack of precision and possesses a relatively elevated minimum detection threshold. Therefore, this method cannot be applied to river samples that are clean and uncontaminated.

Determination of COD

Potassium dichromate, when combined with sulfuric acid, silver sulfate, and mercury sulfate, can oxidize any organic matter that may be present in a sample of water, resulting in the production of carbon dioxide (CO2) and water (H2O). During the blank and sample titrations, the volume of ferrous ammonium sulfate that was eaten can be used as a comparison point for determining the amount of potassium dichromate that was utilized. The amount of potassium dichromate that was utilized in the reaction was proportional to the amount of oxygen (O2) that was needed to oxidize the organic material that was present in the wastewater.

A. Required reagents

  • Preparation of Potassium dichromate Solution: To prepare the solution, measure precisely 6.13 grams of potassium dichromate that has been dried at a temperature of 105°C for a minimum of two hours. Then, add this measured amount of potassium dichromate to 800 milliliters of distilled water. The flask should be agitated with vigor in order to facilitate the dissolution of its contents. Subsequently, 1000 ml of water should be added and the mixture should be thoroughly combined.
  • Preparation of Silver sulfate-Sulfuric acid Solution: To prepare the solution, 10 grams of silver sulfate (Ag2SO4) should be dissolved in 500 milliliters of concentrated sulfuric acid. The resulting solution should then be diluted to a total volume of 1000 milliliters. It is recommended to allow the solution to remain undisturbed for a duration of 24 hours prior to utilization.
  • Preparation of Mercury sulfate Solution: Take care when dissolving 0.1 grams of HgSO4 in 5 milliliters of sulfuric acid.
  • Preparation of Ferrous ammonium sulfate Solution (0.025 M): The procedure involves the dissolution of 9.8 grams of ferrous ammonium sulfate in 100 milliliters of distilled water, along with the addition of 20 milliliters of sulfuric acid. The solution should be cooled and subsequently diluted with 1,000 milliliters of distilled water. In order to calculate the chemical oxygen demand, it is necessary to standardize the solution by determining its actual concentration.
  • Preparation of Ferroin Indicator: A solution can be prepared by combining 400 ml of purified water with 3.5 grams of Iron Sulfate heptahydrate and 7.5 grams of Phenanthroline monohydrate. Thoroughly agitate the mixture to achieve dissolution, subsequently incorporating a sufficient quantity of distilled water to attain a final volume of 500 milliliters.

B. Procedure for COD

  • Put ten milliliters of the sample into a flask that has a round bottom.
  • In order to prevent the solution from coming into direct contact with the flask during the heating process, it is advisable to introduce glass beads into the mixture.
  • Shake the flask containing 1 ml of mercury sulfate (HgSO4) solution to combine the contents.
  • Include 5 milliliters (ml) of a potassium dichromate (K2CrO7) solution.
  • Now, gently and carefully add 15 milliliters of the solution made of silver sulfate and sulfuric acid.
  • The reflex condenser should be connected, and the contents should be subjected to digestion for a duration of two hours on a heated plate.
  • Following the process of digestion, it is recommended to cleanse the condenser by utilizing a quantity of 25 milliliters of distilled water, which should be collected within the identical flask.
  • Incorporate 2-4 drops of ferroin indicator into the flask and proceed to titrate until reaching the endpoint using a solution of ferrous ammonium sulfate with a concentration of 0.025 M.
  • The blank should be prepared using the same procedure as the sample, with the substitution of distilled water for the sample.

C. Calculation of COD

COD = 8x1000xDFxMx(VB – VS)/Volume of the sample (in mL)

where, DF – Factor of Dilution (if applicable), M  – Molarity of a standard solution of Ferrous Ammonium Sulfate, VB – Volume consumed during titration with blank solution, and VS –  Volume utilized during titration sample preparation

Advantages of COD

  • The primary benefits of COD testing include its rapid nature and adherence to rigorous standards. The importance of speed is undeniable; however, it is crucial to ensure that accuracy is not sacrificed in the process. The key advantage of COD analyzers lies in their combination of speed and precision, which proves beneficial in various contexts.
  • The chemical oxygen demand (COD) typically exceeds the biological oxygen demand (BOD) due to the greater capacity for chemical oxidation of organic compounds compared to biological oxidation. This encompasses a variety of chemical substances that exhibit toxicity toward living organisms. COD tests are highly valuable for the assessment of industrial sewage, as they can detect substances that may not be detected by BOD testing methods.

Disadvantages of COD

One potential drawback of this method is its inability to distinguish between inorganic and organic carbons, as well as its susceptibility to interference from halides, nitrates, and peroxide. Under specific conditions, variations in results may occur when measuring Chemical Oxygen Demand (COD) in warmer environments or at room temperature. Therefore, it is crucial to closely monitor the temperature during COD measurements.

Application of COD

  • The chemical oxygen demand (COD) serves as an indicator for assessing the quality of water and wastewater. The Chemical Oxygen Demand (COD) test is frequently employed as a means of assessing the effectiveness of water treatment plants. The basis of this test lies in the observation that when subjected to acidic conditions, a potent oxidizing agent has the ability to completely oxidize nearly all organic compounds into carbon dioxide. The chemical oxygen demand (COD) refers to the quantity of oxygen that is utilized in the process of chemically oxidizing organic contaminants present in water, resulting in the formation of inorganic end products.
  • The chemical oxygen demand (COD) is commonly determined by employing a potent oxidizing agent, such as potassium dichromate, potassium iodate, or potassium permanganate, in an acidic environment. An excessive quantity of the oxidant is intentionally introduced into the sample. Upon the completion of oxidation, the determination of organic concentration in the sample is achieved through the quantification of the remaining oxidizing agent within the solution. Typically, this procedure is carried out through titration, employing an indicator solution. The concentration of chemical oxygen demand (COD) is typically denoted in milligrams per liter (mg/L), representing the quantity of oxygen consumed per unit volume of solution.
  • COD tests take 2-3 hours, while BOD tests take 5 days. It measures all organic pollutants, including non-biodegradable ones. The link between BOD and COD must be empirically determined for each sample. The BOD of a sample can be estimated using COD test results. In contrast to the BOD test, hazardous substances including heavy metals and cyanides in the samples do not affect the COD test oxidants. Therefore, the COD test can measure the strength of wastes too dangerous for the BOD test. Organic compounds like benzene and pyridine resist dichromate oxidation and may provide an erroneously low COD.



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Kabita Sharma

Kabita Sharma, a Central Department of Chemistry graduate, is a young enthusiast interested in exploring nature's intricate chemistry. Her focus areas include organic chemistry, drug design, chemical biology, computational chemistry, and natural products. Her goal is to improve the comprehension of chemistry among a diverse audience through writing.

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