Which of the following is pumped into anaerobic sludge digesters during the sewage treatment?

The correct option is D Activated sludge
Sewage treatment plants (STPs) are specifically designed to treat sewage to make it less polluting. The treatment is carried out in two stages, namely:

Primary treatment of sewage: It includes removal of solid particles by using methods like sequential filtration (to remove floating debris) and sedimentation (to remove heavier insoluble particles).

The sediment which is obtained at the end of the primary treatment is known as primary sludge. After sedimentation, the fluid that remains above the sediment is collected as primary effluent and is subjected to secondary treatment.

Secondary treatment of sewage: It 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 primary effluent is poured into huge aeration tanks where it is continuously agitated (disturbed) by stirring and also air is pumped into it. This leads to rapid growth of heterotrophic aerobic microbial flocs (masses of bacteria associated with 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 pumped back into aeration tanks and is used as an inoculum during secondary treatment. The rest is fed into anaerobic sludge digesters and as the name suggests, here bacteria grow anaerobically to digest the flocs. During the process of digestion, a mixture of gases is produced such as methane, hydrogen sulphide and carbon dioxide. These gases can be used as a source of energy.

Hence, activated sludge is pumped into anaerobic sludge digesters during sewage treatment.