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Biochemical Oxygen Demand

Biochemical Oxygen Demand


Biochemical Oxygen Demand (BOD)

Biochemical Oxygen Demand (BOD) is one of the most important indicators of the quality of water. It determines the quantity of oxygen needed by the microorganisms to break down the organic material in water. BOD is one of the most significant measures of the level of water pollution especially as it relates to organic contaminants.

  1. Importance of BOD

  • Water Quality Assessment: BOD aids in the assessment of the amount of organic pollution in water, which is very important in water quality control.
  • Ecosystem Health: High BOD levels can cause low oxygen conditions in water bodies which is not good for aquatic life.
  • Treatment Efficiency: It is crucial to control BOD levels in order to evaluate the efficiency of wastewater treatment processes.
  1. Measurement of BOD

  • Principle: BOD is determined by diluting a water sample and placing it in a dark container at a fixed temperature (usually 20°C) for a fixed time (normally five days, referred to as BOD₅). BOD is calculated using the decrease in dissolved oxygen (DO) during this period.
  • Procedure:
    1. Sample Preparation: A fixed volume of the water sample is taken in a BOD bottle.
    2. Initial DO Measurement: The initial dissolved oxygen concentration is measured using a DO meter which is a portable instrument.
    3. Incubation: The sample is kept in the dark to avoid photosynthesis that may otherwise lead to changes in DO levels.
    4. Final DO Measurement: At the end of the incubation period, the final DO concentration is determined.
    5. Calculation: BOD is determined by the difference between the initial and final dissolved oxygen concentrations.
  • Units: BOD is measured in milligrams per litre (mg/L) of oxygen used up by the microorganisms.
  1. Factors Affecting BOD

  • Organic Matter: The extent and type of organic pollutants present in the water sample determine the BOD levels.
  • Temperature: This is because higher temperatures promote microbial growth, and therefore, higher BOD values are expected.
  • Microbial Population: The concentration of microorganisms in the water sample and their level of activity influence the BOD results.
  • Inhibitory Substances: Some of the chemicals that can reduce microbial activity and hence reduce the BOD values include.
  1. Applications of BOD

  • Wastewater Treatment: BOD is used to assess and regulate the effectiveness of biological treatment processes in wastewater treatment facilities.
  • Environmental Monitoring: BOD is used by regulatory agencies to determine the effect of effluent discharges on natural water bodies.
  • Pollution Control: BOD is used by industries to check whether they are meeting their discharge permits and other environmental standards.
  1. BOD vs. COD
  • Chemical Oxygen Demand (COD): COD determines the quantity of oxygen needed to oxidise organic and inorganic compounds in water.
  • Differences:
    • Method: BOD depends on biological oxidation, while COD employs a strong chemical oxidant.
    • Time Frame: BOD normally takes five days to determine and is expressed as BOD5 while COD can be determined within a few hours.
    • Sensitivity: BOD is more specific to biodegradable organic pollution while COD includes both biodegradable and non biodegradable substances.

Conclusion

BOD is an essential factor in determining water quality and controlling wastewater treatment procedures. BOD is the measure of the amount of dissolved oxygen required for the decomposition of the organic matter in water, which helps in determining the pollution level and its effects on the environment. The monitoring of BOD is important in checking compliance with set standards and in the management of water quality.

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