The sludge which is obtained by setting sewage in presence plentiful oxygen is termed as active sludge. It is biologically active contains a great number of aerobic bacteria and other microorganisms. It possesses an unusual property to oxidize the organic matter.
Active Sludge Process
When the active sludge is mixed with sewage, the microorganisms multiply rapidly. As a result of solids present in sewage are rapidly oxidized. It also converts the suspended and colloidal matter to settleable precipitate.
Operational Features: The flow diagram of the activated sludge process is given below
The activated sludge process consists of the following operations:
a) Mixing of activated sludge
Some portion of the activated sludge is added to the effluent of the primary clarifier and properly mixed.
The mixed liquor containing activated sludge and effluent is aerated properly in the aeration tank.
c) Setting in secondary clarifier
After aeration, the mixed liquior is taken to the secondary clarifier. Sludge is allowed to settle in this tank. Settled sludge is the activated sludge. A portion of it is sent for re-circulation. The excess activated sludge is sent to the sludge digestion tank and then to sludge drying beds for further treatment and disposal.
Advantages of activated sludge process
Following are the advantages of active sludge process:
1) Cost of installation is low.
2) This process requires a small area of land.
3) Loss of head through the treatment plant is comparatively low.
4) There is no fly and odour nuisance.
Disadvantages of activated sludge process
Following are the disadvantages of the activated sludge process:
1) If there is a sudden increase in the volume of sewage or if there is a sudden change in the character of sewage, there are adverse effects on the working of the process.
2) Cost of operating the process is relatively high.
3) This process is sensitive to certain types of industrial wastes.
4) The wet sludge obtained at the end of the process requires a suitable method for its disposal.