Sewage is dirty city waste water full of organic matter and microbes, many of them harmful. In a sewage treatment plant (STP), heterotrophic microbes that are naturally present in the sewage clean it in two stages: primary treatment (physical removal of solids) and secondary or biological treatment (aerobic microbes eat the organic matter and lower BOD). Memory hook: "Nature's own cleaners" — we do not add microbes, they are already there in the sewage.
Overview of sewage treatment: raw sewage passes through primary (physical) treatment to remove solids, then secondary (biological) treatment where naturally present aerobic microbes form flocs and reduce BOD.
Your doubts, answered
Do we add microbes to sewage, or are they already there?
We do NOT add microbes. NCERT clearly says the treatment is carried out by the heterotrophic microbes naturally present in the sewage. This is a common NEET trap. The plant only gives these existing microbes the right conditions (like air in the aeration tank) to grow and work faster.
What are the two stages of sewage treatment?
Stage 1 is Primary treatment (physical removal of large and small particles by filtration and sedimentation). Stage 2 is Secondary or Biological treatment (aerobic microbes grow into flocs and eat the organic matter, lowering BOD). Remember: Primary = Physical, Secondary = Biological.
Are the sewage-treating microbes autotrophic or heterotrophic?
They are heterotrophic. They cannot make their own food, so they feed on the organic matter present in the sewage. This is exactly why they clean the water — as they eat the organic waste, the water becomes less polluting.
Why can't sewage be discharged directly into rivers?
Sewage has large amounts of organic matter and pathogenic (disease-causing) microbes. If dumped directly, microbes in the river use up dissolved oxygen to break down this organic waste. Fish and other aquatic life then die from lack of oxygen. So sewage is first treated in STPs to make it less polluting.
What is BOD in one line?
BOD (Biochemical Oxygen Demand) is the amount of oxygen that would be used if all the organic matter in one litre of water were oxidised by bacteria. More organic matter means higher BOD. Secondary treatment reduces BOD, so low treated-water BOD means clean water.
⚠️ The NEET trap ✗ Special microbes are added from outside to treat the sewage in the STP. ✓ No microbes are added. The heterotrophic microbes are already naturally present in the sewage; the plant only helps them grow (for example by pumping air in secondary treatment). 🧠 Read the source of the microbes carefully.
Real NEET questions
2017
Which of the following in sewage treatment removes suspended solids?
A · Tertiary treatment
B · Secondary treatment
C · Primary treatment ✓
D · Sludge treatment
Solution: Primary treatment involves physical removal of particles by filtration and sedimentation. Floating debris is removed by sequential filtration and grit (soil, small pebbles) by sedimentation, so suspended solids are removed here. Secondary treatment is biological (microbes lower BOD), not physical removal.
2016
A river with an inflow of domestic sewage rich in organic waste may result in:
A · Drying of the river very soon due to algal bloom.
B · Increased population of aquatic food web organisms.
C · An increased production of fish due to biodegradable nutrients.
D · Death of fish due to lack of oxygen. ✓
Solution: Organic-rich sewage lets microbes multiply and use up the dissolved oxygen while breaking down the waste (high BOD). With oxygen gone, fish and other aquatic animals die. This is exactly why sewage must be treated in STPs before release.
Solved Microbes in Human Welfare NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
