Activated sludge indicates that the sludge is obtained by settling the sewage in presence of abundant oxygen. The sludge is biologically active and it has a number of aerobic bacteria and some other micro-organism having an unusual property of oxidizing the organic matter. A.S.P have the fertilizing constituents which can hold moisture upto 95 – 97 %.

Step 1: the influent is sent to the primary clarifier for proper mixing of the activated sludge and raw of settled sewage.

Step 2: a part of this well mixed sewage is sent to sludge digestion tank for extra recirculation through which it is sent to the drying beds

Step 3: The remaining well mixed sewage is sent to the aeration tank for proper agitation.

Step 4: The agitated or aerated sewage and sludge is sent to the 2nd clarifier. In this secondary clarifier the sludge is allowed to settle down. A part of this activated sludge is sent back to the process of recirculation and through that it goes to the final sludge drying beds and the effluent is sent to natural stream.

Advantages:

a) Lesser land area is required.

b) The head loss in plant is quite low.

c) Capital cost is less.

d) Greater flexibility of operation, permitting control on the quality of effluent is possible.

Disadvantages:

a) High cost of operation with greater power consumption.

b) A lot of machinery to be handled.

c) The bulking of sludge is a common trouble.