Describe the role of microbes in the treatment of sewage, explaining primary and secondary treatment.

1. 1 · Open with the need. Sewage is full of organic matter and pathogens, so it is treated in STPs by heterotrophic microbes before release into rivers.
2. 2 · Primary treatment. Filtration removes floating debris; sedimentation removes grit; settled solids form primary sludge; the supernatant is the effluent.
3. 3 · Aeration and flocs. In aeration tanks with stirring and air, aerobic microbes form flocs that consume organic matter and reduce BOD.
4. 4 · Settling and digestion.
   • Flocs settle as activated sludge; part is returned as inoculum.
   • The rest goes to anaerobic sludge digesters, where bacteria produce methane, H₂S and CO₂ (biogas).
5. 5 · Draw the flow chart. [Flow chart: raw sewage, primary settling, aeration tank, settling tank, anaerobic digester, effluent to river]
6. 6 · Link to India. Ganga and Yamuna Action Plans build STPs so only treated sewage enters rivers.
7. 7 · Close in one line. So microbes turn polluting sewage into safer water and useful biogas.

Scoring tip: Define BOD in the answer and say that treatment continues until BOD is reduced. Many students leave this out.