When sewage is discharged into the river then various bacteria, biological waste and other components and waste material is increased thereby increasing the B.O.D. (i.e. Biological Oxygen Demand ) of the water increases. This increase in B.O.D. is due to the action of bacteria on various components of sewage in the river water. This increase in B.O.D. causes the decrease in the concentration of dissolved oxygen in the river thus representing the fall in the graph but as the bacterial action decreases the oxygen concentration increases thus a rise in the graph showing increase in oxygen concentration in river water back to normal.
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