In sewage treatment, flocs are:

The correct answer is Option (3) → the masses of bacteria associated with fungal filaments.

Option (1) the solids that settle during sedimentation. — Incorrect. In primary treatment, grit and other solids settle down and form the primary sludge. They are settled solids, not flocs.

Option (2) the supernatant that is formed above the primary sludge — Incorrect. The liquid above the primary sludge is the primary effluent. It is taken for secondary treatment, and the flocs grow in it only later, in the aeration tanks.

Option (3) the masses of bacteria associated with fungal filaments. — Correct. These microbes eat up most of the organic matter of the effluent.

Option (4) the bacteria which grow anaerobically and are also called anaerobic sludge digesters. — Incorrect. After the flocs settle as activated sludge, a large part of it is pumped into tanks called anaerobic sludge digesters. There, other bacteria that grow without oxygen digest the bacteria and fungi of the sludge and give biogas. The digesters are tanks, and these bacteria are not flocs.

Note: A similar question was also asked in CUET (UG) 2026 on 25-05-2026 (Slot 1).

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.”