Assertion (A): Activated sludge is partly recycled into aeration tanks.

Reason (R): It serves as an inoculum for aerobic microbes.

Both Assertion (A) and Reason (R) are correct and R is the correct explanation of A.

The correct answer is Option (1) → Both Assertion (A) and Reason (R) are correct and R is the correct explanation of A.

Activated sludge refers to the sediment containing bacterial flocs, which are aggregates of bacteria and other microorganisms that form during the wastewater treatment process. These bacterial flocs play a crucial role in breaking down organic matter in the sewage, aiding in the purification of water.

Activated sludge is formed in the aeration tank during the secondary treatment of sewage, where the sewage is aerated to allow bacteria to grow and form flocs. These flocs help in decomposing the organic waste present in the wastewater.

After the organic content is significantly degraded through biological treatment, the effluent is allowed to settle in a settling tank, and the sediment formed during this process is referred to as "activated sludge." A small part of the activated sludge is pumped back into the aeration tank to serve as the inoculum. The remaining major part of the sludge is pumped into large tanks called anaerobic sludge digesters. Here, other kinds of bacteria, which grow anaerobically, digest the bacteria and the fungi in the sludge. During this digestion, bacteria produce a mixture of gases such as methane, hydrogen sulphide and carbon dioxide. These gases form biogas and can be used as source of energy as it is inflammable.