Secondary sewage treatment is basically which type of treatment process ?

Biological process.

The correct answer is Option (2)- Biological process.

Secondary sewage treatment primarily involves a biological process.

Primary treatment is the initial step in sewage treatment where the removal of suspended solids takes place. During primary treatment, sewage undergoes physical processes to separate large and small particles from the wastewater. This is achieved through the processes of filtration and sedimentation.

During secondary treatment, aerobic microbes, such as bacteria and fungi, multiply rapidly and form flocs. These flocs consist of masses of bacteria entangled with fungal filaments, creating a mesh-like structure. This helps in the breakdown of organic matter in the sewage.

The increased population of aerobic microbes in the sewage consumes the organic matter present in the wastewater. As a result, the biochemical oxygen demand (BOD), which indicates the amount of oxygen required for microbial degradation of organic compounds, is reduced.

Once the BOD of the sewage has been sufficiently reduced, the effluent is directed into a settling tank. In this tank, the bacterial flocs settle down due to gravity, forming a sediment known as activated sludge. The settling process separates the solid components from the liquid effluent.

A portion of the activated sludge collected in the settling tank is returned to the aeration tank, where it serves as an inoculum. The activated sludge contains a high concentration of microorganisms that are capable of breaking down organic matter. This helps maintain a robust microbial population in the aeration tank for efficient treatment.