Oxygen deficit:-

• The oxygen deficit D at any time in a polluted river stream is the difference between the actual DO content of water at that time and the saturation DO content at the water temperature.

  Oxygen Deficit = $DO_{sat} – DO_{actual}$

• In order to maintain clean conditions in a river stream, the oxygen deficit must be nil and this can be found out by knowing the rates of deoxygenation and reoxygenation.

Deoxygenation curve:

• In a polluted stream, the DO content goes on reducing due to decomposition of volatile organic matter.

• The rate of deoxygenation depends upon the amount of organic matter remaining to be oxidized at a given time as well as on the temperature of reaction, hence at given temperature, the curve showing depletion of DO with time i.e. deoxygenation curve is similar to the first stage BOD-curve.

Reoxygenation curve:

• Inorder to counter the balance of the consumption of DO due to deoxygenation, atmosphere supplies O2 to water and the process is called reoxygenation.

• The rate at which the oxygen is supplied by atmosphere to the polluted water depends upon:-

  • The depth of receiving water

  • Condition of flow

  • Oxygen deficit

  • Temperature

Oxygen deficit curve:

• In a running polluted stream exposed to the atmosphere, the deoxygenation as well as reoxygenation go hand in hand, if deoxygenation is more rapid than the reoxygenation an oxygen deficit results.

• The amount of resultant oxygen deficit can be obtained by algebraically adding the deoxygenation and reoxygenation curve.

• The resultant curve is known as oxygen sag.