Question

In sewage treatment, secondary treatment is considered highly significant, because

• A. It helps to remove debris from the sewage.
• B. It reduces the BOD level of sewage.
• C. It helps in the production of biogas.
• D. It increases the organic content of sewage.

Answer 1

The Correct Option is B

Secondary treatment is a biological process in sewage treatment that follows primary treatment (physical removal of debris and sedimentation). It primarily involves microbial action (aerobic bacteria, protozoa) to break down dissolved organic matter in sewage. 

BOD measures the amount of oxygen required by microbes to decompose organic waste in water. High BOD = More pollution (depletes oxygen in water bodies, harming aquatic life). Secondary treatment drastically reduces BOD (by ~90%) through:

1. Activated Sludge Process: Aerobic microbes oxidize organic matter.
2. Trickling Filters: Biofilms on media degrade waste.

Answer 2

Secondary treatment uses microbes (activated sludge) to digest organic waste, reducing Biochemical Oxygen Demand (BOD).

BOD (Biochemical Oxygen Demand) is a measure of the amount of oxygen required by microorganisms to break down organic matter in a water sample over a specific period, usually five days at 20°C. It is an important parameter used to assess the pollution level of water and the health of aquatic ecosystems. High BOD levels indicate high amounts of organic pollution, which can lead to oxygen depletion in water, harming aquatic life.

Other options:

• (A) Debris removal: Done in primary treatment.
• (C) Biogas production: Occurs in anaerobic digesters (tertiary treatment).
• (D) Organic content: Decreases, not increases.

Correct Answer: (B) It reduces the BOD level of sewage