Question

Choose the correct option indicating decrease in the BOD of given samples of water.

• A. Untreated sewage $\to$ Treated sewage $\to$ Drinking water $\to$ River water
• B. Untreated sewage $\to$ Treated sewage $\to$ River water $\to$ Drinking water
• C. Untreated sewage $\to$ Treated sewage $\to$ River water $\to$ Drinking water
• D. Untreated sewage = Treated sewage $\to$ River water $\to$ Drinking water

Answer 1

The correct option is B Untreated sewage $\to$ Treated sewage $\to$ River water $\to$ Drinking water

Biological Oxygen Demand (BOD) is a measure of the oxygen used by microorganisms to decompose waste in water.
If there is a large quantity of organic waste in the water supply, there will also be a lot of bacteria present working to decompose the waste. In this case, the demand for oxygen will be high (due to all the bacteria) so the BOD level will be high. As the waste is consumed or dispersed through the water, BOD levels will begin to decline.

So untreated sewage has the most waste materials and this would have highest BOD level. While drinking water is the least polluted water and hence have lowest BOD. And sewage even after treatment has more waste than normal river water. Thus the correct sequence will be:
Untreated sewage $\to$ Treated sewage $\to$ River water $\to$ Drinking water