(a) Water pollution is defined as the contamination of water by foreign substances which make it harmful for health of animals or plants or aquatic life and make it unfit for domestic, industrial and agricultural use.
Oxygen gets into water through two sources:
(i) Dissolution of oxygen at the surface of water from the atmosphere.
(ii) From photosynthesis of aquatic plants.
(b) The measurement of BOD takes 5 days while measurement of COD does not take much time. Hence, COD is preferred over BOD.