Question
Describe the stages of sewage treatment — primary, secondary, and tertiary. Explain the role of microorganisms in each stage.
(NEET, CBSE Class 12 — Microbes in Human Welfare)
Solution — Step by Step
Sewage passes through screens and grit chambers to remove large debris (sticks, rags, stones). Then it goes to a sedimentation tank where heavy particles settle as primary sludge, and the liquid portion is called effluent. This stage is purely physical — no microbes involved.
The primary effluent goes to aeration tanks where it is agitated and aerated. Aerobic microbes (bacteria, fungi) form flocs — masses of bacteria associated with fungal filaments. These microbes consume the organic matter in the effluent, reducing BOD (Biochemical Oxygen Demand) significantly.
The settled sludge from secondary treatment is pumped into anaerobic sludge digesters. Here, anaerobic bacteria (methanogens like Methanobacterium) decompose the sludge, producing biogas (a mixture of methane, CO, and HS). The biogas can be used as fuel.
Tertiary treatment removes remaining dissolved solids, nutrients (nitrogen and phosphorus), and pathogens. Methods include chlorination, UV treatment, filtration, and chemical precipitation. This produces water clean enough for release into rivers or even reuse.
graph TD
A["Raw Sewage"] -->|"Screening, Grit removal"| B["Primary Treatment<br/>Sedimentation"]
B -->|"Primary sludge"| C["Sludge Digester<br/>Anaerobic, produces biogas"]
B -->|"Effluent"| D["Secondary Treatment<br/>Aeration tanks, Microbes"]
D -->|"Activated sludge"| C
D -->|"Treated effluent"| E["Tertiary Treatment<br/>Chlorination, UV, Filtration"]
E --> F["Clean water released"]
C --> G["Biogas (CH₄) + Dried sludge"]Why This Works
Sewage is rich in organic matter that depletes oxygen in water bodies (high BOD). The treatment process uses microorganisms to break down this organic matter in a controlled setting, reducing BOD to safe levels before release.
BOD (Biochemical Oxygen Demand) is the amount of oxygen microorganisms need to decompose organic matter. Higher BOD = more pollution. After secondary treatment, BOD is reduced by 85-90%.
Alternative Method — Measuring Treatment Effectiveness
The simplest way to check if treatment worked: measure BOD before and after. Raw sewage has a BOD of 300-400 mg/L. After secondary treatment, it drops to 20-30 mg/L. After tertiary treatment, it approaches 5 mg/L.
For NEET, the key terms: flocs (microbial aggregates in aeration tank), activated sludge (a small part of settled sludge returned to aeration tank as inoculum), and BOD (the measure of organic pollution). Know that secondary treatment is the biological step.
Common Mistake
Students confuse primary and secondary treatment. Primary is physical (settling, filtration — no microbes). Secondary is biological (microbes eat organic matter). If a NEET question asks “which stage uses microorganisms” — the answer is secondary treatment, not primary.