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Chemical Oxygen Demand

Chemical Oxygen Demand


chemical oxygen demand

Chemical Oxygen Demand (COD)

COD is one of the most significant parameters of water quality. It defines the amount of oxygen required to chemically oxidise organic and inorganic materials in water. COD is a valuable parameter in determining the ability of wastewater to pollute the organic content of the receiving environment.

  1. Importance of COD

  • Water Quality Assessment: COD is used in determining the level of organic pollution in water and this is very important in water quality management.
  • Treatment Efficiency: COD is a key factor that is employed in the determination of the effectiveness of the wastewater treatment processes.
  • Environmental Impact: High COD levels also lower the dissolved oxygen in water bodies and this affects the aquatic life and the environment.
  1. Measurement of COD

  • Principle: COD is calculated by using a titration method in which the water sample containing the organic and inorganic substances is treated with a strong chemical oxidant like potassium dichromate (K₂Cr₂O₇) in an acidic solution.
  • Procedure:
    1. Sample Preparation: A fixed volume of the water sample is taken and is added with potassium dichromate solution and sulfuric acid.
    2. Digestion: The mixture is then heated to enhance the rate of the oxidation reaction, which may be done in a closed reflux system.
    3. Titration: The remaining dichromate is then titrated with a reducing agent, which is usually ferrous ammonium sulfate (FAS) to determine the amount of dichromate that has been consumed.
    4. Calculation: COD is calculated by the difference between the initial concentration of dichromate and the final concentration of dichromate after the reaction.
  • Units: COD is expressed in milligrams per litre (mg/L) of oxygen required to oxidise the sample.
  1. Factors Affecting COD

  • Sample Composition: This is because there are substances that can be easily oxidized and this results to high COD values.
  • Interferences: Some of the interferences are the presence of inorganic substances such as chlorides and nitrites which interfere with the COD determination.
  • Sample Handling: The samples must be stored and treated in a proper way so that the COD levels are not affected before the test is conducted.
  1. Applications of COD

  • Wastewater Treatment: COD is used to assess and control the efficiency of the biological treatment and chemical oxidation of wastewater.
  • Regulatory Compliance: Almost all environmental standards set the maximum concentration of COD in the wastewater effluent to reduce water pollution.
  • Industrial Processes: COD measurements assist industries in establishing the level of organic content in their effluents and in designing appropriate treatment regimes.
  1. COD vs. BOD

  • Biochemical Oxygen Demand (BOD): BOD measures the amount of dissolved oxygen required by the microorganisms for the decomposition of the organic matter in water within a period of five days.
  • Differences:
    • Time Frame: COD is found within a few hours while BOD takes several days to determine.
    • Oxidation Method: COD uses a chemical oxidant while BOD uses biological action.
    • Sensitivity: COD is more rapid and comprehensive in assessing the extent of organic pollution, including those that are hardly biodegradable.

Conclusion

COD is one of the most significant water quality parameters and is used in the assessment of the efficiency of wastewater treatment. COD is the amount of oxygen required by the organic and inorganic compounds in water and is used to assess the pollution level and impact on the environment. It is important to regularly track COD in order to ensure that the water does not violate any regulatory standards and to control the quality of water.

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