Which stage of sewage treatment is responsible for the removal of sus…

The correct answer is Option (3) → Primary treatment

Sewage is treated in two stages. Primary treatment is physical: floating debris is removed by filtration and grit by sedimentation, and the solids settle as primary sludge. Secondary treatment is biological: microbes in aeration tanks eat the dissolved organic matter and lower the BOD (biochemical oxygen demand).

Option (1) Tertiary treatment — Incorrect. The book describes sewage treatment in two stages only, primary and secondary. Suspended solids are removed in the first, primary stage.

Option (2) Secondary treatment — Incorrect. Secondary treatment is biological. Aerobic microbes grow as flocs and consume the organic matter in the primary effluent. Solid particles have already been removed before this stage.

Option (3) Primary treatment — Correct. Primary treatment is the physical removal of particles, large and small, by filtration and sedimentation. This is where the suspended solids are taken out.

Option (4) Sludge treatment — Incorrect. Sludge is a product of treatment, not a separate stage for removing solids. The activated sludge goes to anaerobic sludge digesters, where bacteria digest it and produce biogas.

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.”