Difference between COD & BOD

COD & BOD – Similarities and Differences

Introduction –

  • COD & BOD are the two main characteristic properties water that indicate the degree of pollution of water.
  •  Both the parameters test the oxygen-demanding strength of the wastewater via different methods and properties.
  • The basic features of COD (Chemical Oxygen Demand) & BOD (Biochemical Oxygen Demand) and their basic differences are explained in this article.
Definition  COD  measures the oxygen demand for the decomposition of both inorganic and organic matter present in wastewater  BOD  measures the oxygen demand for decomposition of organic material by microorganisms present in wastewater
Oxidation        By chemical process    By biological process
Oxidant used  Chemical Agent – Potassium Dichromate      Aerobic microorganisms
Conditions maintained for test         Acidic conditions      Aerobic conditions
Matter considered for decomposition   Both Organic and Inorganic matter  Only organic matter
Required Time                     Takes about 2 to 3  hours,   is  quick method   Takes 5 days(for standard BOD test), is  a slow method
Required Temperature  At  high temperature of 150°C  At  low temperature of 20°C
Values  COD  values are higher than BOD  BOD  values are lower than COD
Permissible Limits  250 – 500 ppm30 mg/l
Preferable waste treated   Domestic and industrial waste.  Mainly sewage and wastewater from different water bodies.
Limitation of the test  Can’t oxidize compounds like Acetate, Toluene, Benzene, etc. by the oxidizing agent.   Only measures biodegradable matter, can’t be preferred for industrial waste.

What is COD/BOD Ratio?

  1. It is generally said that COD & BOD shares a theoretical  relationship.
  2. The COD/BOD ratio is an indicator  for the presence of organic and inorganic matter present in the wastewater, compost and other natural and man-made material .
  3. COD is always greater than BOD, hence the ratio will be always greater than 1.
  4. To find COD and BOD ratio, perform COD and BOD protocol for given water sample.
  5. Divide the COD obtained value by BOD value for given water samples.

Note –

  1. COD/BOD ratios can be highly variable and depends on source of water.
  2. If high COD/BOD indicates that the water contains greater percentage of slowly biodegradable and non-biodegradable material in the sample.
  3. The ratio of BOD/COD (reverse) would change the value of sample water sample.

Standard Ratio –

BOD/COD Ratio is used in –

  • In domestic waste the ratio between BOD/COD is 1:2.
  • For untreated wastewater the BOD/COD ratio is between 0.3 to 0.8.

COD/BOD Ratio is used in –

  • The food processing wastewater will have the COD/BOD ratio of 2:1.
  • The textiles wastewater may contain dyes will have higher ratio of COD/BOD about 5:1.

Takeaways –

  1. If BOD/COD is 0.92 to 1, waste water is fully biodegradable.
  2. If BOD/COD > 0.6 ,  biological treatment is acceptable.
  3. If BOD/COD is between 0.3 to 0.6, then some adjustment is required.
  4. If BOD/COD < 0.3, then the biological treatment is not possible.

References –





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