Secondary treatment of sewage involves:

Mechanical agitation coupled with addition of aerobic microbes

The correct answer is Option (2) - Mechanical agitation coupled with addition of aerobic microbes

The description you provided outlines the two stages of sewage treatment:

1. Primary Treatment: This stage involves the physical removal of solid particles from sewage through processes such as filtration and sedimentation. During this stage, larger particles settle out, and smaller particles are filtered out, resulting in a reduction of the suspended solids in the sewage.
2. Secondary Treatment/Biological Treatment: In this stage, the primary effluent (partially treated sewage) is subjected to biological treatment. It is treated with aerobic microbes in an aerobic tank, where the microbes break down organic matter present in the sewage. This process involves continuous agitation for proper mixing and aeration, which provides the necessary oxygen for the microbes to thrive and decompose organic substances.

After the organic content is significantly degraded through biological treatment, the effluent is allowed to settle in a settling tank, and the sediment formed during this process is referred to as "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.