Question

BOD test measures
a. Rate of uptake of oxygen by microorganisms.
b. Organic matter present in water.
c. Pollution potential of water.

• A. Only (a) is correct.
• B. Only (b) and (c) are correct.
• C. Only (c) is correct.
• D. All (a), (b) and (c) are correct.

Answer 1

Answer: (D)

Pollution potential is characterised by Biological Oxygen Demand (BOD). BOD is defined as the amount of oxygen (mg) consumed by microbial action when 1000 ml sample of water containing known amount of oxygen is incubated for 5 days at $20^{\circ }C$ in the dark. A high BOD indicates intense level of biodegradable pollutants.

BOD is the amount of oxygen consumed by bacteria and other microorganisms while they decompose organic matter under aerobic conditions. Therefore, the presence of high organic matter in water body indicates high BOD.

Higher BOD indicates a higher pollution potential. If BOD is 1500 mg/L, pollution is considered as low; if BOD is 1500-4000 mg/L, pollution is considered as medium and if BOD is greater than 4000 mg/L, pollution is considered as high. BOD and COD decrease the amount of dissolved oxygen (DO) in water. The value of DO in less polluted water is 8 mg/L and the value of DO in highly polluted water is less than 4 mg/L.