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Last updated date: 04th Dec 2023
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MVSAT Dec 2023

Relationship between DO and BOD is that they
(a) Are directly proportional
(b) Are inversely proportional
(c) Are not related
(d) Always remain equal to each other

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Hint: DO are dissolved oxygen and BOD are biochemical oxygen are certain parameters by which one can find out the degree of water pollution or the concentration of readily biodegradable organic matter in the water bodies.

Complete answer:
DO or dissolved oxygen is the amount of oxygen dissolved in water bodies and it is usually measured in parts per million or ppm. The amount of dissolved oxygen used up by aerobic microorganisms to decompose the organic matters present in a sample of water is termed Biochemical Oxygen Demand or Biological Oxygen Demand (BOD). BOD and DO are inversely proportional to each other i.e a decline in DO levels reflects a high level of BOD.

Additional Information:
-BOD test evaluates the dissolved oxygen used up by aerobic microorganisms over a 5-days period at 20℃. BOD concentration of wastewater is expressed in mg/L. BOD of clean freshwater is 2mg/L whereas a value exceeding 5 mg/L indicates contamination.
- Since aerobic microorganisms live by decomposing organic matters, the amount of oxygen used is proportional to their number and metabolic rate. Thus, the more the number of organisms in the water body, the more would be the requirement of dissolved oxygen or DO in the water body.
-DO is influenced by various factors such as temperature and type of water body. A river that is continuously streaming dissolves more oxygen than a still water body like ponds, reservoirs, etc. Also, cold water can hold more oxygen than a warm water body.
So, the correct answer is ‘Are inversely proportional.’

Note: Sewage water with its high concentration of organic matter increases the BOD of the water body significantly. This is due to the requirement of more aerobic bacteria to decompose the organic matter. The abnormal increase of aerobic microbial activity causes a shortage of dissolved oxygen for other aquatic species like fishes and they ultimately die.
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