Question

BOD is --- in polluted water and --- in potable water.
(a) more, less
(b) less, more
(c) less in both
(d) medium in both

Answer 1

Hint: The biochemical oxygen demand is the full form of BOD. A low BOD is an indication of good water quality. As an indicator of the efficiency of wastewater treatment plants, BOD reduction is used.

Complete answer:
The biochemical oxygen demand (BOD) is the amount of dissolved oxygen required (i.e. requested) to break down organic material present over a certain time span in a given water sample at a certain temperature by aerobic biological organisms. Potable water contains less organic compounds than water that is polluted, so the BOD is lower.
Most generally, the BOD value is expressed in milligram of oxygen absorbed per liter of the sample at ${ 20 }℃$ throughout 5 days of incubation and is sometimes used as a proxy for the amount of organic water contamination.
In terms of chemical oxygen demand (COD) analysis, BOD analysis is similar in function, in that both calculate the number of organic compounds in water. However, because it analyses anything that can be chemically oxidized, rather than only levels of biologically oxidized organic matter, COD analysis is less precise.
So, the correct answer is, ‘(a) more, less’.

Note: Oxygen as dissolved oxygen (DO) is determined in its dissolved form. If more oxygen is consumed than is produced, dissolved oxygen levels decrease and some sensitive animals can move further away, weaken, or die.
The BOD is used to assess waste loads at treatment plants and to determine the performance of the BOD-removal of such treatment systems.
To demonstrate the short-term effects on the oxygen levels of the receiving water, BOD of wastewater effluents is used.