Question

Limit of BOD prescribed by the Central Pollution Control Board for discharge of industrial and municipal waste waters into natural surface waters is -
(a) $< 3 \ ppm$
(b) $< 10 \ ppm$
(c) $< 30 \ ppm$
(d) $< 100 \ ppm$

Answer 1

Hint: Biological Oxygen Demand or Biochemical Oxygen Demand is the amount of dissolved oxygen required by aerobic, microscopic, biological organisms in a given water sample to break down organic matter. A higher BOD indicates a higher level of microorganisms, hence, more pollution. Hence, the CPCB has prescribed a limit in order to keep the waste waters in check.

Complete step by step answer:
- The BOD of water obtained from a water body helps in determining the amount of oxygen required by aerobic organisms present in it to carry out their metabolic reactions.

- It is defined as the number of milligrams of ${ O }_{ 2 }$ required for decomposition of one litre of waste or water by decomposing microorganisms.
- The CPCB prescribes the limit of discharge of industrial and municipal wastewaters into natural water bodies as $< 10 \ ppm$.
- The higher the BOD, the higher is the oxygen present in the water that may be formed due to the presence of organic waste and effluents, and the more fertile environment for microorganisms. A value of less than $< 10 \ ppm$ indicates low pollution of water.
So, the correct answer is ‘$< 10 \ ppm$’.

Additional Information:
- BOD is similar to chemical oxygen demand, or COD. The difference is that COD is a less specific method of calculation as it measures all reactions that require the consumption of oxygen.
- The technique was first put forward by Winkler.
- The two methods used for measurement are the dilution and manometric method.

Note:
- The samples for BOD analysis cannot be preserved and they need to be transported to the laboratory shortly after taking the sample.
- CPCB suggests testing the BOD of seasonal and perennial rivers and lakes 3-4 times a year.
- Alternatively, biosensors can be used instead of BOD apparatus.