Secondary treatment of sewage involves

The correct answer is Option (4) → Mechanical agitation coupled with addition of aerobic microbes

Note: Option (2) Anaerobic sludge digestion can also be argued, as NCERT describes the anaerobic sludge digesters under secondary treatment. Option (4) is the most appropriate answer because it is the step that treats the sewage effluent itself.

Sewage is treated in two stages. Primary treatment is physical: solid particles are removed. Secondary treatment is biological: microbes eat up the organic matter.

Option (1) Physical removal of large and small particles from sewage through filtration — Incorrect. Floating debris is removed by sequential filtration. This is the first step of primary treatment.

Option (2) Anaerobic sludge digestion — Also part of it. The activated sludge left after aeration is pumped into anaerobic sludge digesters. There, bacteria that live without oxygen digest the bacteria and fungi of the sludge and give off methane, hydrogen sulphide and carbon dioxide (biogas). It treats the sludge, not the effluent.

Option (3) Sedimentation to remove smaller and larger particles — Incorrect. Grit (soil and small pebbles) is removed by sedimentation. This is also primary treatment; the settled solids form the primary sludge.

Option (4) Mechanical agitation coupled with addition of aerobic microbes — Correct. The primary effluent goes into aeration tanks. Air is pumped in while it is stirred (the mechanical agitation of the option), so aerobic microbes grow into flocs. They consume most of the organic matter and so lower the BOD of the water.

NCERT Extract:
Microbes in Sewage Treatment – “… Secondary treatment or Biological treatment : The primary effluent is passed into large aeration tanks (Figure 8.6) where it is constantly agitated mechanically and air is pumped into it. This allows vigorous growth of useful aerobic microbes into flocs (masses of bacteria associated with fungal filaments to form mesh like structures). While growing, these microbes consume the major part of the organic matter in the effluent. This significantly reduces the BOD (biochemical oxygen demand) of the effluent. … 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.”