Explanation [ alt="F3 Killi 21.9.20 Pallavi D2" src="//storage.googleapis.com/tb-img/production/20/09/F3_Killi_21.9.20_Pallavi_D2.png" style="width: 450px; height: 275px;"> Correct order is Zone A – Zone of clear water Zone B – Zone of Degradation Zone C – Zone of Active decomposition Zone D – Zone of Recovery Zone E – Zone of clear water ZONES OF POLLUTION: A polluted stream undergoing self-purification presents the following four distinct zones of pollution (1) ZONES OF DEGRADATION: (i) This usually occurs below the output sewer when discharging its contents into the stream. The zone is characterized by water becoming dark and turbid with the formation of sludge deposits on the bottom. (ii) Dissolved oxygen gets reduced to 40%. (2) ZONES OF ACTIVE DECOMPOSITION: (i) This is marked by heavy pollution. It is characterized by the absence of dissolved oxygen; water is grayish and darker with active anaerobic organic decomposition accompanying and with the evolution of methane (CH4), Hydrogen sulphide (H2S), Carbon dioxide (CO2), and Nitrogen (N2), bubbling to the surface with masses of sludge forming black scum. (ii) Fish life is practically absent, fungi and bacteria disappear. As the organic decomposition slackens, reaction sets in, and D.O. again rises to its original level (i.e. 40%). (3) ZONE OF RECOVERY: (i) In this zone, the stream tries to recover its former appearance. Most of the organic matter has been settled as sludge, B.O.D. falls and the D.O. content rises above 40% microscopic aquatic life reappears. (ii) Water becomes clearer, fungi decrease and algae reappear. Mineralization is active and products such as nitrates, sulphates, and carbonates are formed. (4) CLEAR WATER ZONE: (i) In this, the natural stream condition is restored, the D.O. is higher than the BOD. Oxygen balance is attained and recovery is said to be complete. Water becomes attractive in appearance. Some pathogenic organisms may, however, be present.