Which of the following statements accurately describes secondary sewage treatment?

I. During secondary treatment, aerobic microbes proliferate rapidly and form flocs, which are clusters of bacteria intertwined with fungal filaments to create mesh-like structures.

II. The growing microbes utilize organic matter, leading to a reduction in the biochemical oxygen demand (BOD) of the sewage.

III. Once the BOD of the sewage has been decreased, the effluent is directed into a settling tank. Within this tank, the bacterial flocs settle down, forming sediment known as activated sludge.

IV. A portion of the activated sludge is used as an inoculum in the aeration tank, while the remaining sludge is transferred to anaerobic sludge digesters, which are large tanks.

V. Within the anaerobic sludge digesters, heterotrophic microbes decompose the bacteria and fungi present in the sludge through anaerobic digestion, resulting in the production of a gas mixture that includes carbon dioxide, nitrogen, and carbon monoxide. This gas mixture is commonly referred to as biogas.

Which of the statements given above are correct?

 I, II, III and IV

The correct answer is Option (1)- I, II, III and IV

The given statements provide an overview of the secondary sewage treatment process. Here is an explanation of each statement:

I. 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.

II. 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.

III. 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.

IV. 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.

The remaining portion of the activated sludge is transferred to large tanks called anaerobic sludge digesters. These digesters create an oxygen-free environment suitable for the growth of anaerobic bacteria. 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.

Overall, the secondary sewage treatment process utilizes aerobic and anaerobic microbes to break down organic matter, reduce the BOD of the wastewater, and generate biogas from the sludge.