How many of the following statements are correct with respect to the activated sludge?

I - It is composed of soil and small pebbles that are removed during the primary treatment of the sewage
II - It can be used for biogas production
III - It is fed into anaerobic sludge digesters
IV - It is obtained at the end of the oxidation of organic wastes during the secondary treatment of the sewage

The correct option is A 3
Sewage is the municipal waste water that is generated every day in cities and towns. Due to the high concentration of organic wastes and other microbial content, sewage has to be treated before being poured into water bodies.

Primary treatment of sewage includes removal of solid particles from the sewage by using methods like filtration (to remove suspended particles) and sedimentation (to remove heavier insoluble particles).
The sediment which is obtained at the end of the primary treatment consists of particles such as soil and small pebbles, etc and is known as primary sludge.
Thus, statement I is incorrect.

After sedimentation, the fluid that remains above the sediment is collected as effluent and is subjected to secondary treatment. Secondary treatment involves oxidation of the organic matter present in sewage by the enzymatic actions of aerobic microbes, in the presence of oxygen. In this process the effluent is poured into huge aeration tanks where it is continuously agitated (disturbed) by stirring and pumping it with air. This leads to rapid growth of heterotrophic aerobic microbial flocs (masses of bacteria and filamentous fungi) that feed on the organic wastes present in the sewage and degrade it.

Once all the organic matter in sewage is degraded, the sewage is allowed to sit undisturbed in settling tanks. This causes the microbial flocs to settle down as sediment and this is referred to as activated sludge. The liquid effluent is separated out and released into water bodies.

Some of the activated sludge is used as an inoculum in the next operation of secondary treatment of sewage to initiate the growth of the heterotrophic aerobic microbes. The rest is fed into anaerobic sludge digesters in which anaerobic microbes feed on the organisms present in the activated sludge and produce biogas (mixture of gases like methane, hydrogen sulphide and carbon dioxide)

Thus 3 statements. i.e., statements II, III and IV are correct.