Test: Zones of Pollution in River - Civil Engineering (CE) MCQ

The growth of algae reappear in the zone of recovery.The growth of fungi decreases.

There are 4 types of pollution zones in the river, namely zone of degradation, zone of active decomposition, zone of recovery and zone of clear water.

The zone of recovery is the third zone of pollution in the river where river tries to recover the polluted water through the self purification process.

The dissolved oxygen, reduced to 0 in the zone of active decomposition. The water is highly polluted in this zone.

It is the zone of degradation as dissolved oxygen has reduced to 40%. The fishes will die when dissolved oxygen is less than 4ppm.

The water retains its original dissolved oxygen concentration in the zone of clean water. It has a clear appearance.

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.

• ZONES OF DEGRADATION:
  • 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.
  • Dissolved oxygen gets reduced to 40%.
• ZONES OF ACTIVE DECOMPOSITION: 
  • 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.
  • 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%).
• ZONE OF RECOVERY: 
  • 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.
  • Water becomes clearer, fungi decrease and algae reappear. Mineralization is active and products such as nitrates, sulphates, and carbonates are formed.
• CLEAR WATER ZONE: 
  • 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.

In the zone of active decomposition, the concentration of D.O reduced to 40%. It is the second zone of pollution in the river.