• When any type of sewage is discharged into the natural water into the sea or into surface water stream, the oxygen demand by the organic matter in the sewage is completed.
• This process is represented by the deoxygenated curve.
• As the process of deoxygenation starts the deficit of oxygen is made up by the process of re-oxygenation which is represented by the re-oxygenation curve.
• The combined curve of these two curve is called as oxygen sag curve in which the minimum deficit of D.O. is indicated, which is also known as the critical point of the oxygen sag curve.
• After this lowest point the reoxygenation starts to make the D.O. to reach 100%

Diagram:-