During the primary treatment of sewage, all the solids that settle af…

The correct answer is Option (1) → primary sludge

Sewage is treated in two stages. Primary treatment is physical: floating debris is removed by filtration, and grit is removed by sedimentation. What settles at the bottom and what stays above it are given different names. The settled solids form the primary sludge, and the liquid above them (the supernatant) forms the effluent.

Option (1) primary sludge — Correct. All the solids that settle during primary treatment form the primary sludge.

Option (2) activated sludge — Incorrect. Activated sludge belongs to secondary treatment. It is the sediment formed when the bacterial flocs settle in the settling tank after aeration; a small part of it is pumped back into the aeration tank to serve as the inoculum.

Option (3) primary effluent — Incorrect. The primary effluent is the liquid (supernatant) left above the settled solids. It is taken on to secondary treatment, so it is not the solids that settle.

Option (4) bacterial flocs — Incorrect. Bacterial flocs are masses of bacteria joined with fungal filaments. They form in the aeration tanks of secondary treatment, not in primary treatment.

NCERT Extract:
Microbes in Sewage Treatment – “… Primary treatment : These treatment steps basically involve physical removal of particles – large and small – from the sewage through filtration and sedimentation. These are removed in stages; initially, floating debris is removed by sequential filtration. Then the grit (soil and small pebbles) are removed by sedimentation. All solids that settle form the primary sludge, and the supernatant forms the effluent. The effluent from the primary settling tank is taken for secondary treatment.”