# The clean water should have BOD value:(A)- ${\text{100PPM}}$(B)- Less than ${\text{5PPM}}$(C)- ${\text{20PPM}}$(D)- ${\text{200PPM}}$

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Hint:. BOD stands for ‘Biological Oxygen Demand’, and its value is inversely proportional to the amount of dissolved oxygen present in the water bodies like rivers, streams, lakes, etc.

Biological Oxygen Demand gives us the knowledge about the amount of dissolved oxygen present in the water bodies, and by the use of this dissolved oxygen organic materials gets broken down by the aerobic organism present in that water body through the decomposition process.
-From the relation mentioned in the hint part it is clear that, lower BOD value shows higher amount of dissolved oxygen in the water bodies.
-And high BOD value shows lower amount of dissolved oxygen in the water bodies.
-Clean water should contain a high amount of dissolved oxygen so that organic matter present that gets decomposed by that available oxygen and water quality gets better.
- Hence, the clean water should have BOD value Less than ${\text{5PPM}}$.
So, the correct answer is “Option B”.

Additional information: The whole process involved in Biological Oxygen Demand, usually takes a long period of time, approx. 5 to 7 days. And formula used to count Biological Oxygen Demand is shown as follow:
${\text{BOD = }}\dfrac{{{{\text{D}}_{\text{i}}}{\text{ - }}{{\text{D}}_{\text{f}}}}}{{\text{P}}}$
Where, ${{\text{D}}_{\text{i}}}$ = Initial concentration of dissolved oxygen,
${{\text{D}}_{\text{f}}}$ = Final concentration or 5 to 7 day of dissolved oxygen,
${\text{P}}$ = Volumetric concentration of experimental water.

Note: Here some of may get confused in between BOD and COD because these both parameters are used to decompose organic matters present in the water bodies. But the major difference is that BOD shows decomposition biologically and COD shows decomposition chemically.