(i) Expand BOD.
(ii) At a particular segment of a river near a sugar factory, BOD is much higher than normal level. What is it indicative of? What will happen to living organism in this part of river?
(iii) Under what conditions will BOD be lowered in river? How will it affect aquatic life?

(i) Biological Oxygen Demand
(ii) The higher BOD indicates high organic matter in river. Microbes involved in the biodegradation of organic matter in water body consume a lot of oxygen. Due to this, a sharp decline occur in the amount of dissolved oxygen. This leads to killing of fish and other microorganisms in that part of river.
(iii) BOD of water body decrease when the amount of organic matter decreases. Thus, microbes do not need oxygen for its decomposition.
Due to the decreases BOD, aquatic life will start flourishing.