Which of following statements are correct regarding sewage treatment?…

The correct answer is Option (4) → B, C and D only

Sewage is treated in two stages. Primary treatment removes particles by physical means. Secondary (biological) treatment uses microbes to lower the BOD.

(A) Lower the BOD of wastewater, greater its polluting potential. — Incorrect. It is the other way round. The greater the BOD, the greater the polluting potential, because more oxygen is needed to break down the organic matter.

(B) Primary treatment involves physical removal of particles through filtration and sedimentation. — Correct. Floating debris is filtered out first, and then the grit settles. The solids that settle form the primary sludge.

(C) Flocs are masses of bacteria associated with fungal filaments. — Correct. In the aeration tanks, aerobic microbes grow as flocs, which are masses of bacteria held together with fungal filaments in a mesh.

(D) Activated sludge is obtained during secondary treatment of sewage. — Correct. When the BOD has fallen, the effluent goes to a settling tank, where the flocs settle as activated sludge.

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 – “… This allows vigorous growth of useful aerobic microbes into flocs (masses of bacteria associated with fungal filaments to form mesh like structures).”
Microbes in Sewage Treatment – “… The greater the BOD of waste water, more is its polluting potential.”
Microbes in Sewage Treatment – “… Once the BOD of sewage or waste water is reduced significantly, the effluent is then passed into a settling tank where the bacterial ‘flocs’ are allowed to sediment. This sediment is called activated sludge.”