Arrange the given products formed during sewage treatment in correct sequence:

(A) Biogas

(B) Activated sludge

(C) Flocs

(D) Primary sludge

The correct option is (C), (D), (B), (A).

Sewage treatment is carried out in distinct stages to purify wastewater before it is released back into the environment. The products formed during this process occur in a specific sequential order:
1. Flocs (C) and Primary sludge (D): During the initial phases of sewage treatment, physical and biological processes begin. Primary treatment involves physical removal of particles through filtration and sedimentation, forming the Primary sludge (D). Concurrently, in secondary (biological) treatment, the primary effluent is aerated, prompting the growth of useful aerobic microbes into mesh-like structures called Flocs (C).
2. Activated sludge (B): Once the biological oxygen demand (BOD) of the sewage is significantly reduced, the effluent is passed into a settling tank. Here, the bacterial flocs are allowed to sediment. This sedimented mass is known as Activated sludge (B).
3. Biogas (A): A major portion of the activated sludge is then pumped into large anaerobic sludge digesters. Here, anaerobic bacteria digest the organic matter, bacteria, and fungi present in the sludge. This digestion process releases a mixture of gases (primarily methane, carbon dioxide, and hydrogen sulfide) known as Biogas (A).

Therefore, tracking the progression from the initial physical/biological stages to the final anaerobic digestion, the correct sequential order of the products formed is (C), (D), (B), (A).