Critical dissolved oxygen (D.O.) deficit occurs in which one of the following zones of pollution of ‘oxygen sag curve’ in case of self-purification of natural streams?

Zone

Main Characteristics

Zone of Clear Water (1 & 5)

Clear water,  presence of fishes.

Zone of Degradation (2)

Dissolved oxygen level falls due to decomposition of Organic Matter in this Zone.

Dark and turbid water.

Fishes may present

Algae’s are absent.

Zone of Active Decomposition (3)

Zone of heavy pollution.

Water is very dark and turbid

Dissolved oxygen level fall to Zero.

Critical Oxygen Deficient occurs.

Formation of dirty scum layer.

Both fishes and algae absents

Zone of recovery (4)

Dissolved oxygen level increases and may reach to saturated value.

Decomposition of organic matter takes place up to Nitrate level.

Both fishes and Algae reappear.

BOD falls down.