Question

High BOD indicates
A. Highly polluted water
B. Less pollution in water
C. Less sewage
D. Less microorganisms

Answer 1

Hint: BOD or the Biological Oxygen Demand is the oxygen required by decomposing bacteria to decompose the organic mass present in the water. The concentration of organic mass present in the water indicated the pollution in water. So, BOD also indicates the pollution in water.

Complete answer:
BOD is Biological Oxygen Demand. BOD is the amount of dissolved oxygen needed by aerobic organisms to decompose the organic material present in a given water sample at a certain temperature over a specific time period.

When any organic matter (organic pollutants) are present in water, the decomposer or the aerobic microorganisms present in that water body, decompose that organic matter by oxidizing it. This process requires oxygen and bacteria take oxygen from the dissolved oxygen in the water. So, the higher the amount of organic pollutants in the water, the higher oxygen is required by the bacteria and this oxygen required by the bacteria is known as BOD.

So, the amount of pollution in a water body is directly proportional to the BOD. BOD depends on time and temperature. Suppose there are 200 bacteria in a water sample, but they are taking more time to decompose or they decompose the pollutants slowly, so they will use less oxygen but, if these 200 bacteria are recomposing rapidly, they will take less time and BOD will be high. So, whenever we calculate BOD, we calculate it for a particular time.

Temperature also affects the process of decomposition. As we know the rate of reaction increases when temperature increases. So, the rate of decomposition also increases with increase in temperature.

We can see that BOD is directly proportional to the amount of pollution. So, High BOD indicates highly polluted water.

Hence, the correct answer is option (A).

Note: The BOD is commonly expressed in milligrams of oxygen used per liter of water sample during 5 days of incubation at 20 °C and is often used as the degree of organic pollution present in the water.