Arrange the given products formed during sewage treatment in correct…

The correct answer is Option (2) → (D), (C), (B), (A)

(D) Primary sludge — first. In primary treatment floating debris is filtered out and grit settles. All the solids that settle form the primary sludge.

(C) Flocs — second. In secondary (biological) treatment the effluent is aerated in large tanks. Aerobic microbes grow into flocs, masses of bacteria held together by fungal filaments, and they eat up the organic matter, lowering the BOD.

(B) Activated sludge — third. The effluent goes to a settling tank, where the flocs settle as activated sludge. A small part is sent back to the aeration tank as inoculum.

(A) Biogas — fourth. The major part of the sludge is pumped into anaerobic sludge digesters. There anaerobic bacteria digest it and release methane, hydrogen sulphide and carbon dioxide, which make up biogas.

Note: The same question was also asked in CUET (UG) 2025 on 28-05-2025 (Slot 1).

NCERT Extract:
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. A small part of the activated sludge is pumped back into the aeration tank to serve as the inoculum. 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. These gases form biogas and can be used as source of energy as it is inflammable.”