The term 'activated sludge' is used to denote:

The correct answer is Option (3) → Sediment containing bacterial flocs

Option (1) All solids that settle as sediment — Incorrect. Not all settling solids are activated sludge. The solids that settle in primary treatment form the primary sludge; activated sludge is the bacterial flocs that settle after secondary treatment.

Option (2) Sediment containing all type of organic matter — Incorrect. Activated sludge is not any organic sediment. It is the sediment of bacterial flocs (bacteria with fungal filaments), part of which is pumped back into the aeration tank as inoculum.

Option (3) Sediment containing bacterial flocs — Correct. In the aeration tank, bacteria grow into masses with fungal filaments called flocs, which digest the organic matter. In the settling tank these bacterial flocs settle as sediment; this sediment is the activated sludge, part of which is sent back to the aeration tank.

Option (4) Sediment containing soil and small pebbles only — Incorrect. Soil and small pebbles are grit, removed by sedimentation in primary treatment. Activated sludge is made of bacterial flocs.

NCERT Extract:
Microbes in Sewage Treatment – “… Once the BOD of sewage or waste water is reduced significantly, the effluent is then passed into a settling tank where the bacterial ‘flocs’ are allowed to sediment. This sediment is called activated sludge. A small part of the activated sludge is pumped back into the aeration tank to serve as the inoculum.”