Question

In a sewage plant, BOD is indicated as high, after some time it shows a decrease then
A. There is an increase in bacterial growth but a decrease in oxygen.
B. There is a decrease in bacterial growth but an increase in oxygen.
C. There is an increase in bacterial growth but increase in oxygen.
D. There is a decrease in bacterial growth but decrease in oxygen.

Answer 1

Hint: Wastewater released from the sewage treatment plants contains organic matter. This organic matter is decomposed by microorganisms using oxygen. The amount of oxygen consumed by these organisms in breaking down the waste is known as the biochemical oxygen demand (BOD).

Complete answer:
The amount of oxygen used to consume or oxidise the entire organic matter present in one litre of water by aerobic microorganisms is called biological oxygen demand (BOD). The BOD test measures the rate of consumption of oxygen by microorganisms in a sample of water at a specific temperature over a period. The biological oxygen demand is an indicator of the presence of the organic matter in the water. If more oxygen is consumed and less oxygen is produced, the dissolved oxygen level will decrease and some sensitive animals may move away from that place, weaken or may die.
As per the question, initially, BOD is indicated as high, this means at that particular time more microorganisms were engaged in the oxidation of organic matter, more bacterial growth. But after some time BOD decreases, this indicates that there is a decrease in bacterial growth but an increase in dissolved oxygen is given a water sample.

Thus, option B. There is a decrease in bacterial growth but increase in oxygen is the correct answer.

Additional information:
BOD may be used to gauge the effectiveness of wastewater treatment plants. It is listed as a conventional pollutant in the U.S. Clean Water Act.

Note:
BOD is a measure of the organic matter existing in water.The greater is a measure of BOD of waste water; more is its polluting potential.