Previous Year Question

Biological or Biochemical Oxygen Demand (BOD) is an important water-quality parameter related to the quantity of biodegradable organic matter present in water.

It represents the quantity of dissolved oxygen required by aerobic microorganisms to decompose biodegradable organic matter present in a water sample.

BOD is generally expressed in milligrams of oxygen consumed per litre of water, or mg/L.

In the test, the initial dissolved oxygen of the water sample is measured, and the sample is kept in darkness at a specified temperature for a fixed period.

The remaining dissolved oxygen is measured after the specified period. BOD is determined from the difference between the initial and final dissolved-oxygen values.

The internationally used BOD₅ test is generally conducted for five days at 20°C.

Indian water-quality criteria also widely use BOD measured for three days at 27°C.

High BOD indicates that the water contains a large quantity of biodegradable organic pollutants and microorganisms will consume more oxygen while decomposing them.

This may reduce dissolved oxygen in the water and adversely affect fish and other aquatic organisms.

Under conditions of excessive organic pollution, dissolved oxygen may be exhausted and foul-smelling anaerobic conditions may develop.

Low BOD generally indicates lower biodegradable organic pollution and comparatively better water quality.

BOD does not directly measure total pollution or every type of chemical present in water. It mainly relates to organic matter that microorganisms can decompose.

Chemical Oxygen Demand (COD) measures the oxygen equivalent required for substances that can be oxidised by a strong chemical oxidising agent.

The COD test is comparatively rapid, and its value is generally higher than the BOD of the same sample.

Options (a), (b) and (d) are not related to the use of BOD.