Determination Of Biological Oxygen Demand (BOD) Of Water

BOD is the index of water pollution. In the presence of organic and inorganic waste products numerous microorganisms grow . These act as food bases for the microorganisms, hence they decompose and utilise the substrates. However, the amount of oxygen required by bacteria during aerobic decomposition of organic compounds in sewage is called BOD. It is also represented by the amount of organic matter present in water or effluents. For more organic matter, more oxygen is required by bacteria for its decomposition. This results in release of organic nutrients in water bodies such as lakes, rivers, ponds, etc. resulting in death of fish (asphyxiation). For measuring BOD, water samples are incubated at 20oC for 5 days in dark under aerobic conditions, whereas the same can be incubated at 27o C for 3 days in tropical and subtropical regions where metabolic activities are higher. Oxygen is also consumed during nitrification, therefore, 1 ml of 0.05 % allylthiourea should be added to water samples to check over estimation of BOD.


  • BOD-free water (deionised distilled water passed through activated carbon
  • and re-distilled)
  • BOD bottles
  • Erlenmeyer flask
  • Pipette
  • BOD incubator
  • pH meter
  • Phosphate buffer solution (pH 7.4)
  • H2SO4 1 N (2.8 ml in 100 ml BOD-free distilled water)
  • Allylthiourea solution (0.5%)
  • All reagents required for estimation of dissolved oxygen as given in previous experiment


  1. Add 1N acid/1N alkali in the water sample to adjust the pH to 7.0
  2. Gently transfer this water into BOD bottles so as bubbles should not come out.
  3. Add 1 ml of allylthiourea to each bottle to avoid nitrification
  4. Measure dissolved oxygen following the steps as described for dissolved oxygen
  5. Incubate the other BOD bottle at 27oC for 3 days in a BOD incubator.
  6. Measure the amount of oxygen as done earlier.


Calculate the BOD of water by using the following formula:

Biological oxygen demand of water


Practical Microbiology

Gaurav Singh

Editor in Chief Medical Microbiology & RDT Labs - RDT Labs Magazine | BSc Medical Microbiology | MSc Microbiology

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