During sewage treatment , the microbes are used in which stage of the treatment ?

In secondary treatment.

Primary treatment basically involve physical removal of particles – large and small – from the sewage through filtration and sedimentation. Secondary treatment or  This significantly reduces the BOD (biochemical oxygen demand) of the effluent.

During sewage treatment, microbes are primarily used in the secondary treatment stage. The secondary treatment is a biological treatment process where the sewage is subjected to the action of microorganisms to further break down organic matter and remove dissolved and suspended solids. This process typically involves the use of activated sludge or other biological treatment systems.

Secondary treatment or biological treatment allows vigorous growth of useful aerobic microbes into flocs (masses of bacteria associated with fungal filaments to form mesh like structures). While growing, these microbes consume the major part of the organic matter in the effluent. They consume and metabolize the organic compounds, converting them into simpler form.This helps in reducing the organic load and the concentration of pollutants in the sewage.

On the other hand, the primary treatment stage of sewage treatment involves physical processes such as sedimentation and filtration to remove large particles and solid materials from the sewage. While there may be some presence of naturally occurring microbes in the primary treatment, their role is not as significant as in the secondary treatment.

Therefore, the correct choice among the given options is "In secondary treatment," as that is the stage where microbes are extensively used for sewage treatment.