BiologyISC Class 12 · Microbes in Human Welfare

Sewage Treatment and BOD: ISC Class 12 Biology

Updated 15 October 2026 · Fact-checked

Sewage is treated in two stages. Primary treatment is physical: filtration and sedimentation remove debris and grit as primary sludge. Secondary treatment is biological: in aeration tanks aerobic microbes grow as flocs and consume organic matter, sharply reducing BOD. Flocs settle as activated sludge; most goes to anaerobic digesters that produce biogas.

Understand Sewage Treatment and BOD

What sewage is. Municipal waste water, mainly human excreta, containing large amounts of organic matter and microbes, many of them pathogenic. It cannot be discharged directly into rivers, so it is treated in sewage treatment plants (STPs) by the heterotrophic microbes naturally present in sewage.

Primary treatment (physical).
1. Floating debris removed by sequential filtration.
2. Grit (soil, small pebbles) removed by sedimentation.
3. Settled solids form the primary sludge; the liquid above (supernatant) is the primary effluent, sent for secondary treatment.
No microbial action is used; dissolved organic matter remains, so the effluent still has a high BOD.

Secondary treatment (biological).
1. Primary effluent enters large aeration tanks, where it is agitated mechanically and air is pumped in.
2. Useful aerobic microbes grow vigorously into flocs: mesh-like masses of bacteria held together with fungal filaments.
3. The microbes consume the major part of the organic matter, so the BOD falls significantly. Treatment continues until BOD is low enough.
4. The effluent passes to a settling tank, where flocs sediment as activated sludge.
5. A small part of this activated sludge is returned to the aeration tank, where it serves as inoculum.
6. The major part goes to anaerobic sludge digesters, where anaerobic bacteria digest the bacteria and fungi of the sludge, producing biogas (methane, hydrogen sulphide and carbon dioxide), which is inflammable and used as fuel.
7. The treated effluent is released into rivers and streams.

Comparison table.

	Primary treatment	Secondary treatment
Nature	physical	biological (biochemical)
Process	filtration, sedimentation	aeration, growth of flocs, digestion
Role of microbes	none	aerobic microbes (flocs) and anaerobic bacteria (digesters)
Removes	floating debris, grit, settleable solids	dissolved and suspended organic matter
Effect on BOD	not the purpose; BOD stays high	BOD greatly reduced
Product	primary sludge, primary effluent	activated sludge, biogas, clean effluent

BOD (biochemical oxygen demand). The amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria. The BOD test measures the rate of oxygen uptake by micro-organisms in a water sample, so BOD is an indirect measure of organic matter. The greater the BOD, the greater the polluting potential. Untreated sewage in a river raises BOD, depletes dissolved oxygen and kills fish; nutrients also cause algal blooms (eutrophication).

BOD arithmetic used in questions.

Percentage reduction = (BOD in − BOD out)/BOD in × 100.
Daily oxygen-demand load (kg) = BOD (mg/L) × volume (L) ÷ 10⁶.

National effort. Untreated sewage reaching rivers has risen with urbanisation, while STPs have not kept pace. The Ministry of Environment and Forests began the Ganga Action Plan and Yamuna Action Plan to build STPs so that only treated sewage enters these rivers.

Diagram checklist (secondary treatment). Primary effluent in → aeration tank (label: air pumped in, mechanical agitator, flocs) → settling tank (label: activated sludge settling, clear effluent out to river) → arrow back to aeration tank labelled 'part of activated sludge as inoculum' → arrow to anaerobic sludge digester labelled 'biogas'. Name each tank; ISC often letters them A, B, C.

Flow chart: sewage → filtration → sedimentation (primary sludge out) → primary effluent → aeration tank (flocs, BOD falls) → settling tank → activated sludge (inoculum back; rest to digester → biogas) → treated effluent to river.

Sewage Treatment and BOD: key facts

BOD definition: Meaning oxygen that bacteria would use to oxidise all the organic matter in 1 L of water
Unit mg/L
High BOD more organic matter, greater polluting potential
Lowered by secondary (biological) treatment; High BOD = high organic pollution.
Percentage reduction: $% reduction = \frac{BOD _{in} - BOD _{o u t}}{BOD _{in}} \times 100$; Used to judge how well secondary treatment works.
Daily load: $load (kg/day) = \frac{BOD (mg/L) \times flow (L/day)}{1 0 ^{6}}$; mg to kg divides by one million.
Fate of activated sludge: Small part pumped back to the aeration tank as inoculum
Major part anaerobic sludge digesters
Product biogas (methane, hydrogen sulphide, carbon dioxide), used as fuel; Write activated sludge, not just sludge.

Meaning	oxygen that bacteria would use to oxidise all the organic matter in 1 L of water
Unit	mg/L
High BOD	more organic matter, greater polluting potential
Lowered by	secondary (biological) treatment

Small part	pumped back to the aeration tank as inoculum
Major part	anaerobic sludge digesters
Product	biogas (methane, hydrogen sulphide, carbon dioxide), used as fuel

How to answer Sewage Treatment and BOD questions

ISC sets labelled sewage-plant diagrams with tanks to identify, primary versus secondary comparisons, flocs and activated sludge, the meaning of BOD and why STPs matter.

1Identify tanks from what happens in them: air and flocs = aeration tank; settling of flocs = settling tank; gas = anaerobic sludge digester.
2Call primary treatment physical and secondary treatment biological; never call secondary 'chemical'.
3Write 'activated sludge', not just 'sludge', for the settled flocs.
4Explain BOD as oxygen demand of organic matter per litre and link high BOD to high pollution.
5In sums, write the formula, substitute with units and give the answer with %, mg/L or kg/day.
6For 'importance', give reduced organic matter, pathogens and BOD, so rivers are not polluted or eutrophic.

Sewage Treatment and BOD in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

BOD as a pollution indicator

Suppose a school eco-club tests a river at two points: upstream of a town drain the BOD is 3 mg/L, and just below the drain it is 60 mg/L, where they also notice dead fish.

Interpret the readings and suggest a solution.

Try it first, then show the solutionSolution

BOD rose 60/3 = 20 times below the drain, so the drain is adding a large load of organic matter (untreated sewage).
Bacteria oxidising this matter use up dissolved oxygen, so fish suffocate.
The drain should pass through an STP: primary treatment to remove solids, then secondary treatment to cut BOD before discharge.
The club can report the readings to the local body and repeat tests after treatment.

Answer: Untreated sewage raised BOD twentyfold and depleted oxygen; treating the drain in an STP would lower BOD and protect fish.

Memory hook: High BOD, low oxygen, dead fish.

Tricks to remember Sewage Treatment and BOD

Short memory aids. Say each one aloud once, then use it on a question the same day.

Primary = Physical, Secondary = Bugs
P for Physical; S for Small organisms (microbes) doing the work.
Use it when: Comparing the two stages.
A-S-D order
Aeration tank, Settling tank, Digester.
Use it when: Naming tanks in a diagram.
BOD is a bill
BOD is the oxygen 'bill' the organic matter will charge the river; a bigger bill means dirtier water.
Use it when: Explaining what high BOD means.

Sewage Treatment and BOD: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Primary versus secondary · 3 marks · Short answerBoard level

Distinguish between primary and secondary treatment of sewage (three points).

Try it first, then show the solutionSolution

Primary is physical; secondary is biological (1 mark).
Primary uses filtration and sedimentation to remove debris and grit; secondary uses aerobic microbes (flocs) in aeration tanks to consume organic matter (1 mark).
Primary does not aim to lower BOD; secondary greatly reduces BOD (1 mark).

Answer: Physical/filtration and sedimentation/BOD high versus biological/flocs/BOD reduced.

Example 2: Flocs and activated sludge · 2 marks · Short answerBoard level

What are flocs? What is activated sludge and what happens to it?

Try it first, then show the solutionSolution

Flocs: masses of aerobic bacteria associated with fungal filaments forming mesh-like structures in the aeration tank (1 mark).
Activated sludge: the flocs that settle in the settling tank; a small part is returned to the aeration tank as inoculum, and the major part goes to anaerobic sludge digesters (1 mark).

Answer: Bacteria-fungal meshes; settled flocs, partly recycled as inoculum, mostly digested anaerobically.

Example 3: Percentage reduction · 2 marks · Short answerHarder board level

The effluent entering an aeration tank has a BOD of 200 mg/L, and the effluent leaving the settling tank has a BOD of 20 mg/L. Calculate the percentage reduction in BOD and explain how it was achieved.

Try it first, then show the solutionSolution

Reduction = (200 − 20)/200 × 100 = 90% (1 mark).
Aerobic microbes growing as flocs in the aerated tank consumed most of the organic matter, which is what BOD measures (1 mark).

Answer: 90%, by microbial consumption of organic matter in the aeration tank.

Example 4: Why air is pumped · 2 marks · Short answerHarder board level

Why is the effluent in the aeration tank agitated and supplied with air? What would happen if aeration stopped?

Try it first, then show the solutionSolution

The flocs are aerobic; agitation and air give them oxygen and mix them with organic matter, so they grow vigorously and oxidise it (1 mark).
Without air, aerobic microbes would slow down and BOD would stay high; anaerobic conditions would give foul gases and poorly treated effluent (1 mark).

Answer: Oxygen for aerobic flocs; without it treatment fails and BOD stays high.

Example 5: Daily oxygen-demand load · 3 marks · Short answerHarder board level

An STP treats 5 million litres of sewage a day. The BOD falls from 200 mg/L before secondary treatment to 20 mg/L after it. Calculate the daily oxygen-demand load (in kg) entering and leaving secondary treatment, and the load removed.

Try it first, then show the solutionSolution

Entering: 200 mg/L × 5 000 000 L = 1 000 000 000 mg = 1000 kg/day (1 mark).
Leaving: 20 mg/L × 5 000 000 L = 100 000 000 mg = 100 kg/day (1 mark).
Removed: 1000 − 100 = 900 kg of oxygen demand per day, kept out of the river (1 mark).

Answer: 1000 kg/day in, 100 kg/day out, 900 kg/day removed.

Example 6: A plant that fails · 4 marks · Short answerHOTS / competency

Suppose a town's STP is overloaded, so effluent leaves the aeration tank after half the usual time. (a) Predict the effect on BOD of the discharged effluent. (b) Explain two effects on the river. (c) Suggest two remedies.

Try it first, then show the solutionSolution

(a) Flocs have less time to consume organic matter, so the BOD of the effluent stays high (1 mark).
(b) River bacteria oxidise the extra organic matter and use up dissolved oxygen, killing fish; nutrients cause algal blooms (eutrophication); pathogens spread water-borne disease (any two) (2 marks).
(c) Build more STP capacity or more aeration tanks; reduce inflow by separating storm water; recycle enough activated sludge as inoculum (1 mark).

Answer: BOD stays high; oxygen depletion, eutrophication, disease; expand and manage the STP.

Where students lose marks in Sewage Treatment and BOD

The slips examiners see most often, and what to write instead.

Mistake: Calling secondary treatment chemical
In 2025 examiners reported sedimentation and filtration listed for both stages, and secondary treatment called a chemical rather than a biochemical process.
Do this: Primary: physical; secondary: biological.
Usually costs: 1 mark.
Mistake: Writing 'sludge' for activated sludge
Examiners (2025) noted 'sludge' used for 'activated sludge'.
Do this: Primary sludge from primary treatment; activated sludge from the settling tank.
Usually costs: 1 mark.
Mistake: Misidentifying tanks
Examiners (2025) found Tanks A and B identified the wrong way round, and the first sedimentation tank or digester named as the flocs tank.
Do this: Flocs grow only in the aeration tank, where air is pumped in.
Usually costs: 1 mark.
Mistake: Vague role of microbes
Examiners (2025) saw answers such as 'microbes are used to treat microbes'.
Do this: Say microbes consume organic matter and reduce BOD, preventing pollution and eutrophication.
Usually costs: 1 mark.

Exam tips

Draw the secondary treatment as boxes with arrows, labelling air in, flocs, activated sludge, inoculum and digester.
Define BOD with 'one litre' and 'oxidised by bacteria'.
Give units in BOD sums: mg/L, % or kg/day.
Mention Ganga Action Plan and Yamuna Action Plan for national measures.
Link biogas from digesters to methanogens when asked.

Practice questions on Sewage Treatment and BOD

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
In the aeration tank, mesh-like clumps of bacteria held together by fungal filaments are called:
(a) primary sludge (b) flocs (c) effluent (d) grit
Hint
Mesh-like.
Answer
(b) flocs.
Q2 · 2 marks · Short answerBoard level
Define BOD. What does a high BOD of a water sample indicate?
Hint
One litre.
Answer
BOD is the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria. A high BOD indicates a large amount of organic matter, i.e. high polluting potential.
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Primary treatment of sewage significantly reduces its BOD.
Reason (R): Primary treatment removes floating debris and grit by filtration and sedimentation.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is false but R is true.
(d) Both A and R are false.
Hint
Which stage cuts BOD?
Answer
(c). BOD is reduced significantly by secondary (biological) treatment.
Q4 · 2 marks · Short answerHarder board level
Name the gases produced in an anaerobic sludge digester and state one use of the mixture.
Hint
Biogas.
Answer
Methane, hydrogen sulphide and carbon dioxide; together they form biogas, which is inflammable and used as fuel (energy source).
Q5 · 2 marks · Short answerHarder board level
The raw effluent of a dairy has a BOD of 300 mg/L. The pollution board requires an 85% reduction before discharge. What maximum BOD may the treated effluent have?
Hint
15% may remain.
Answer
Remaining = 300 × (1 − 0.85) = 300 × 0.15 = 45 mg/L. So the treated effluent may have a BOD of at most 45 mg/L.
Q6 · 2 marks · Short answerHarder board level
Why is part of the activated sludge pumped back into the aeration tank?
Hint
Starter.
Answer
It serves as inoculum: it adds a large population of active aerobic microbes (flocs) to the fresh effluent, so organic matter is broken down quickly and BOD falls faster.
Q7 · 3 marks · Short answerHOTS / competency
Suppose a city decides to save money by skipping secondary treatment and releasing primary effluent into a lake. Predict three consequences for the lake over a few months.
Hint
Organic load, nutrients, pathogens.
Answer
BOD of the lake rises as bacteria oxidise the dissolved organic matter, depleting oxygen and killing fish; nutrients cause algal blooms (eutrophication), which later decay and worsen oxygen loss; pathogens remain, spreading water-borne diseases to people who use the water.
Q8 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose a diagram shows three tanks: Tank X receives effluent and has air pumped through it; Tank Y receives the output of X and has a layer of settled material at the bottom; Tank Z receives most of that settled material and gives off gas.
(a) Name tanks X, Y and Z. (1)
(b) Name the settled material in Y and the gas from Z. (1)
(c) Why is the treatment in X called biological? (1)
Hint
Aeration, settling, digester.
Answer
(a) X: aeration tank; Y: settling tank; Z: anaerobic sludge digester. (b) Activated sludge; biogas (mainly methane, with H₂S and CO₂). (c) Living aerobic microbes (flocs) break down the organic matter, lowering BOD.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What is the supernatant of primary treatment called?Show answer
Primary effluent.
2.Where do flocs form?Show answer
In the aeration tank.
3.What happens to most of the activated sludge?Show answer
It is pumped to anaerobic sludge digesters.
4.What does BOD indirectly measure?Show answer
The amount of organic matter in water.
5.Name the river action plans of the Ministry of Environment and Forests.Show answer
Ganga Action Plan and Yamuna Action Plan.
6.BOD falls from 250 to 30 mg/L. Percentage reduction?Show answer
88%.

Sewage Treatment and BOD: frequently asked questions

What is the difference between primary and secondary sewage treatment?

Primary treatment is physical (filtration and sedimentation of solids); secondary treatment is biological, using aerobic microbes in aeration tanks to consume organic matter and reduce BOD.

What is BOD?

The amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria; it indicates organic pollution.

Why does secondary treatment lower BOD?

Aerobic microbes in the flocs use up most of the organic matter, which is what creates the oxygen demand.

What is activated sludge?

The sediment of bacterial flocs in the settling tank after aeration; part is reused as inoculum and the rest is digested anaerobically.

Is biogas produced during sewage treatment?

Yes. Anaerobic bacteria in sludge digesters produce methane, hydrogen sulphide and carbon dioxide, which form biogas.