Which of the following is not included in primary treatment of sewage?

The correct answer is Option (4) → Use of microbes

Sewage is treated in two stages. Primary treatment is physical: particles, large and small, are removed from the sewage by filtration and sedimentation. Secondary treatment is biological: the primary effluent is passed into aeration tanks, where aerobic microbes grow and eat up most of the organic matter.

(1) Filtration — Included in primary treatment. Floating debris is removed by repeated filtration.

(2) Sedimentation — Included in primary treatment. Grit such as soil and small pebbles settles down.

(3) Physical separation of faecal matter — Included in primary treatment. The solids that settle form the primary sludge; this is a physical removal.

(4) Use of microbes — Not included in primary treatment. Microbes are used only in secondary (biological) 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.”
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).”