Question

The sewage is treated by aerobic biological process in ‐‐‐‐‐‐ treatment.

• A. Preliminary
• B. Primary
• C. Secondary
• D. Both 1 and 2

Hint:

Remember the sequence:
- Primary = Physical (Settling).
- Secondary = Biological (Microbes).
- Tertiary = Chemical/Advanced.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
Sewage treatment involves several stages to remove contaminants. The question asks to identify which specific stage utilizes aerobic (oxygen-requiring) microorganisms to degrade organic matter.

Step 2: Detailed Explanation:

• Preliminary (A): Physical removal of large debris (rags, sticks) using screens and grit chambers.

• Primary (B): Physical process involving large tanks where heavier solids settle to the bottom (sludge) and oils float to the top (scum). About 30-50% of organic matter is removed here physically.

• Secondary (C): This is the Biological Treatment stage. Aerobic microorganisms (bacteria and protozoa) are used to break down the remaining dissolved organic matter. Techniques include:
- Activated Sludge Process: Air is pumped into the tank to stimulate aerobic bacteria.
- Trickling Filters: Sewage is sprayed over rocks covered with a biofilm of aerobic microbes.
- This stage removes about 85-90% of the organic matter (BOD).

• Tertiary: Advanced chemical or physical cleaning (filtration, disinfection) to make water safe for discharge into sensitive areas.

Step 3: Final Answer:
Aerobic biological processes take place during the Secondary treatment of sewage.