Question
Describe the three stages of sewage treatment — primary, secondary, and tertiary. What role do microbes play in secondary treatment?
(NEET 2023, similar pattern)
Solution — Step by Step
This is purely a physical process. No microbes are involved here.
- Screening — large debris (rags, sticks, plastic) is filtered out.
- Grit chamber — sand and gravel settle down due to gravity.
- Primary sedimentation tank — the sewage is held still so that heavier solids settle as primary sludge at the bottom. Floating matter (oils, grease) is skimmed off the top.
The liquid portion that remains is called the effluent. It still contains dissolved organic matter and microbes.
This is the biological stage where microbes do the heavy lifting. The goal is to reduce the BOD (Biochemical Oxygen Demand) of the effluent.
- The primary effluent is passed into aeration tanks where it is constantly agitated and air is pumped in.
- Aerobic microbes (bacteria and fungi) grow rapidly, forming flocs — masses of bacteria associated with fungal filaments. These microbes consume the dissolved organic matter, reducing BOD significantly.
- The effluent then moves to a secondary sedimentation tank (settling tank). The flocs settle down as activated sludge.
- A small portion of activated sludge is recycled back into the aeration tank as inoculum — this maintains the microbial population.
- The remaining sludge is pumped into anaerobic sludge digesters, where anaerobic bacteria (including methanogens) digest the organic matter, producing biogas (CH₄ + CO₂ + H₂S).
This stage removes remaining nutrients, pathogens, and chemicals:
- Chemical treatment — chlorination or UV treatment to kill remaining pathogens
- Nutrient removal — nitrogen and phosphorus are removed to prevent eutrophication in water bodies
- Filtration — through sand or activated carbon for final polishing
After tertiary treatment, the water is safe for release into rivers or for reuse.
BOD is the amount of dissolved oxygen consumed by microbes to decompose organic matter in water. High BOD = highly polluted water. The goal of secondary treatment is to bring BOD down from high levels (300+ mg/L in raw sewage) to acceptable levels (<30 mg/L).
Why This Works
The logic is simple: primary treatment removes what you can see and settle (physical), secondary treatment removes what you can’t see (dissolved organics, using microbes), and tertiary treatment handles what’s left over (pathogens, nutrients, chemicals).
The microbes in the aeration tank are nature’s cleanup crew. By consuming organic matter as their food source, they convert high-BOD sewage into low-BOD effluent. The activated sludge recycling ensures we always have a thriving microbial population ready to work.
Alternative Method — Summary Table
| Stage | Type | What’s Removed | Key Structure |
|---|---|---|---|
| Primary | Physical | Large solids, grit, floating matter | Sedimentation tank |
| Secondary | Biological | Dissolved organics (reduces BOD) | Aeration tank + settling tank |
| Tertiary | Chemical | Pathogens, nutrients | Chlorination/UV unit |
NEET frequently asks about the role of microbes in secondary treatment and the concept of BOD. A 2023 question specifically asked what happens in the aeration tank. Remember: flocs form in the aeration tank, activated sludge settles in the settling tank, and biogas is produced in the anaerobic sludge digester.
Common Mistake
Many students confuse primary sludge with activated sludge. Primary sludge settles in the primary sedimentation tank — it’s mostly solid waste. Activated sludge is the microbial mass (flocs) that settles in the secondary sedimentation tank after biological treatment. They are NOT the same thing. NEET questions specifically test this distinction.