Question

BOD is_________ in polluted water and __________ in portable water.
A. More, less
B. Less, medium
C. Medium, more
D. Less, more

Answer 1

Hint: Diseases spread by unsafe or polluted water includes cholera, giardia, typhoid, dysentery, Amoebiasis. Malaria is also a water borne disease as mosquitoes stay in stagnant water and lay eggs and these increase their growth and population. BOD is biochemical oxygen demand also known as biological oxygen demand, which is required by the aquatic plants and organisms to break down the complex inorganic substituents into organic simpler units.

Complete answer:
Option A more, less: the water present in rivers and ponds are likely to be polluted more than the water we drink. Hence it is obvious that water in ponds has more Biochemical oxygen demand and less amount of BOD is present in potable water.
Option A is correct.
Option B less, medium: the BOD is more in polluted water than the portable water. So, less BOD is not correct and medium, there is nothing like medium BOD, it's either less or more.
Option B is correct.
Option C medium, more: the polluted water has more BOD, biodegradable organisms will indeed have more in the river than portable.
Option C is not correct.
Option D less, more: as they convert the organic wastes water to inorganic, it will be a greater number of BOD.
Option D is not correct.

So, Option B is the correct answer.

Note:
The purpose of having a greater number of BOD in polluted water as the microorganisms act as natural cleaner for the water body and portable water is already cleaned and purified by adding alum to it as alum is a natural cleaning agent for the water and make it suitable for drinking. As we already discussed about the BOD, COD known as chemical oxygen demand is also like BOD but instead of microorganisms chemicals are used to clean the water.