BiologyCBSE Class 12 · Microbes in Human Welfare

Sewage Treatment and Biochemical Oxygen Demand (BOD)

Updated 15 October 2026 · Fact-checked

Sewage is treated in two stages. Primary treatment physically removes debris and grit by filtration and sedimentation. Secondary (biological) treatment uses aerobic microbes that form flocs in aeration tanks and consume organic matter, lowering BOD. The settled activated sludge is partly recycled and partly digested by anaerobic bacteria that release biogas.

Understand Sewage Treatment and BOD

What sewage is. Cities generate huge amounts of waste water every day; its main component is human excreta. This municipal waste water, sewage, contains a lot of organic matter and microbes, many of them pathogenic. Discharging it untreated pollutes rivers, so it is treated in sewage treatment plants (STPs) by the heterotrophic microbes naturally present in it.

Primary treatment (physical).
1. Sequential filtration removes floating debris.
2. Sedimentation removes grit (soil and small pebbles).
3. All settled solids form the primary sludge; the liquid above (supernatant) is the primary effluent, which goes on for secondary treatment.

Secondary treatment (biological).
1. The primary effluent goes into large aeration tanks, where it is constantly agitated mechanically and air is pumped in.
2. Useful aerobic microbes multiply vigorously and form flocs: mesh-like masses of bacteria held together by fungal filaments.
3. As they grow, the microbes consume the major part of the organic matter, which significantly reduces the BOD of the effluent. Treatment continues until BOD is reduced.
4. The effluent passes into a settling tank, where the flocs settle as activated sludge.
5. Some of the activated sludge (a small part) is returned to the aeration tank as inoculum (a ready supply of the right microbes).
6. The major part goes to large anaerobic sludge digesters, where anaerobic bacteria digest the bacteria and fungi in the sludge, producing methane, hydrogen sulphide and carbon dioxide: biogas, which is inflammable and can be used as fuel.
7. The effluent from secondary treatment is released into rivers and streams.

BOD (biochemical oxygen demand) is 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 microorganisms in a water sample, so BOD is an indirect measure of the organic matter in water. The greater the BOD, the greater the polluting potential of the waste water. The aim of secondary treatment is to reduce BOD (not to remove every trace of organic matter).

Why it matters. The Ministry of Environment and Forests began the Ganga Action Plan and Yamuna Action Plan to save these rivers from pollution, proposing many STPs so that only treated sewage is discharged into them.

Flow chart:
raw sewage → [primary: filtration + sedimentation] → primary sludge (solids) + primary effluent → [aeration tank: aerobic microbes form flocs; BOD falls] → [settling tank] → activated sludge + treated effluent (to rivers) → activated sludge: small part back to aeration tank (inoculum); major part → anaerobic sludge digester → biogas (CH₄, H₂S, CO₂).

Draw and label: flow diagram of an STP. Checklist: boxes connected by arrows in order: inlet with screens, primary settling tank (with sludge at bottom), aeration tank (with air bubbles and a stirrer), secondary settling tank, outlet to river; a return arrow from the settling tank to the aeration tank; a side arrow to an anaerobic sludge digester with a gas outlet. Labels: primary treatment, primary sludge, primary effluent, aeration tank, flocs, settling tank, activated sludge, return (inoculum), anaerobic sludge digester, biogas, treated effluent.

Sewage Treatment and BOD: key facts

BOD: $B O D = O_{2} consumed if all organic matter in 1 L were oxidised by bacteria$; Higher BOD = more organic matter = more polluting.
BOD reduction: $% reduction = \frac{B O D _{in} - B O D _{o u t}}{B O D _{in}} \times 100$; Secondary treatment brings the largest fall.
Flocs: $flocs = bacteria + fungal filaments (mesh)$; Aerobic; consume organic matter in aeration tanks.
Fate of activated sludge: $small part \to aeration tank (inoculum); major part \to anaerobic digester$; Digester bacteria produce biogas.
Biogas from digesters: $C H_{4} + H_{2} S + C O_{2}$; Inflammable; used as a source of energy.

How to answer Sewage Treatment and BOD questions

STP questions follow the plant's sequence. Name each stage, say what happens there and what it produces.

1Split the answer into primary (physical) and secondary (biological) treatment; marks are often split this way.
2Primary: filtration (floating debris), sedimentation (grit); products: primary sludge and primary effluent.
3Secondary: aeration with agitation → aerobic flocs → organic matter consumed → BOD falls; then settling → activated sludge.
4State both fates of the activated sludge (inoculum and anaerobic digester) and the gases produced.
5Define BOD in one line and say what high or low BOD means.
6For numericals, compute percentage reduction or compare BOD values to decide which sample is more polluted.

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 measure

Suppose a town's STP tests its water: raw sewage has a BOD of 320 mg/L, primary effluent 210 mg/L, and the final treated effluent 24 mg/L. A nearby lake receiving untreated drain water shows fish deaths.

Calculate the percentage BOD reduction in primary treatment, in secondary treatment, and overall. Explain why untreated drain water kills fish.

Try it first, then show the solutionSolution

Primary: (320 − 210)/320 × 100 = 34.4%.
Secondary: (210 − 24)/210 × 100 = 88.6%.
Overall: (320 − 24)/320 × 100 = 92.5%.
Untreated sewage has high BOD: bacteria decomposing its organic matter use up dissolved oxygen in the lake, so fish suffocate.

Answer: Primary 34.4%, secondary 88.6%, overall 92.5% reduction; high-BOD sewage depletes dissolved oxygen and kills fish.

Memory hook: High BOD steals the fish's oxygen.

Tricks to remember Sewage Treatment and BOD

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

P for Physical, S for Swallowing
Primary = physical removal; Secondary = microbes swallow organic matter.
Use it when: Distinguishing the two stages.
Flocs = Fungi-Linked Organised Colonies
Bacteria held together by fungal filaments.
Use it when: Defining flocs.
Air in, gas out
Aeration tank needs air (aerobic); digester makes gas without air (anaerobic).
Use it when: Where aerobic and anaerobic microbes work.

Sewage Treatment and BOD: solved examples

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

Example 1: Flocs · 1 mark · Very short answerBoard level

During biological treatment of sewage, what are the masses of bacteria held together by fungal filaments called, and what do they do?

Try it first, then show the solutionSolution

They are flocs.
Flocs consume the major part of the organic matter in the effluent, reducing BOD.

Answer: Flocs; they digest organic matter and lower BOD.

Example 2: Primary, activated and anaerobic · 3 marks · Short answerBoard level

Explain with reference to sewage treatment: (a) primary sludge (b) activated sludge (c) anaerobic sludge digesters.

Try it first, then show the solutionSolution

(a) Solids that settle during primary treatment (debris and grit removed by filtration and sedimentation) (1 mark).
(b) Sediment of bacterial flocs in the settling tank after secondary treatment; a small part is returned to the aeration tank as inoculum (1 mark).
(c) Large tanks where anaerobic bacteria digest the bacteria and fungi of the activated sludge, producing biogas (methane, hydrogen sulphide, CO₂) (1 mark).

Answer: Primary sludge = settled solids; activated sludge = settled flocs; digesters = anaerobic digestion producing biogas.

Example 3: Reducing BOD · 3 marks · Short answerHarder board level

The BOD of the primary effluent is reduced during sewage treatment. List the steps by which this is achieved.

Try it first, then show the solutionSolution

Primary effluent is passed into large aeration tanks, constantly agitated and supplied with air (1 mark).
Aerobic microbes grow vigorously as flocs (bacteria with fungal filaments) (1 mark).
The flocs consume most of the organic matter, so oxygen demand of the water (BOD) falls; treatment continues until BOD is sufficiently reduced (1 mark).

Answer: Aeration + agitation → flocs → organic matter consumed → BOD reduced.

Example 4: Comparing water samples · 2 marks · Very short answerHarder board level

Sample A has a BOD of 8 mg/L and sample B 180 mg/L. Which is more polluted? Explain what the BOD test actually measures.

Try it first, then show the solutionSolution

Sample B: higher BOD means more organic matter and greater polluting potential (1 mark).
The BOD test measures the rate of oxygen uptake by microorganisms in the sample; this indirectly measures the organic matter present (1 mark).

Answer: B is more polluted; BOD indirectly measures organic matter via oxygen uptake by microbes.

Example 5: Why recycle sludge? · 2 marks · Very short answerHOTS / competency

Why is a small part of the activated sludge pumped back into the aeration tank? Predict what would happen to treatment if this were stopped.

Try it first, then show the solutionSolution

It serves as an inoculum: it supplies a large population of the right aerobic microbes (flocs) to the fresh effluent (1 mark).
Without it, new effluent would have fewer active microbes, flocs would form slowly, organic matter would be digested more slowly and BOD would remain higher for longer, lowering the plant's efficiency (1 mark).

Answer: It acts as inoculum; without it, digestion slows and BOD stays high.

Example 6: Full STP · 5 marks · Long answerHarder board level

Describe the treatment of sewage in an STP from raw sewage to the release of effluent. Name the gases produced and state one use.

Try it first, then show the solutionSolution

Primary treatment: floating debris removed by sequential filtration, grit by sedimentation; settled solids form primary sludge; supernatant is primary effluent (1 mark).
Secondary treatment: primary effluent in aeration tanks with agitation and air; aerobic microbes form flocs and consume organic matter, reducing BOD (1 mark).
Effluent passes to settling tanks; flocs settle as activated sludge; small part returned as inoculum (1 mark).
Major part of sludge goes to anaerobic sludge digesters, where anaerobic bacteria digest it, producing methane, hydrogen sulphide and CO₂ (biogas), used as fuel (1 mark).
Treated effluent is released into rivers and streams; such plants are central to the Ganga and Yamuna Action Plans (1 mark).

Answer: Primary (filtration, sedimentation) → secondary (aeration, flocs, BOD fall) → settling (activated sludge) → digesters (biogas: CH₄, H₂S, CO₂) → effluent to rivers.

Where students lose marks in Sewage Treatment and BOD

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

Mistake: Microbes in primary treatment
Students think all stages are biological.
Wrong working
In primary treatment, bacteria digest the organic matter.
Right working
Primary treatment is physical: filtration and sedimentation.
Do this: P for physical.
Usually costs: 1 mark.
Mistake: Anaerobic flocs
Aerobic and anaerobic stages are mixed.
Wrong working
Anaerobic bacteria form flocs in the aeration tank.
Right working
Aerobic microbes form flocs; anaerobic bacteria work in the sludge digester.
Do this: Air in the aeration tank = aerobic.
Usually costs: 1 mark.
Mistake: High BOD means clean water
Students think 'more oxygen' is good.
Wrong working
High BOD shows the water has a lot of oxygen.
Right working
High BOD means much organic matter that will consume oxygen: highly polluted.
Do this: BOD is oxygen demand, not oxygen content.
Usually costs: 1 mark.
Mistake: Omitting the fate of sludge
Students stop at the settling tank.
Wrong working
Activated sludge is removed.
Right working
Small part returned as inoculum; major part to anaerobic digesters producing biogas.
Do this: Always give both fates.
Usually costs: 1 mark in 3-5 mark answers.

Exam tips

Draw a neat flow chart of the STP; it can earn full marks on description questions.
Use the exact words: sequential filtration, sedimentation, primary sludge, aeration tank, flocs, activated sludge, inoculum, anaerobic sludge digesters, biogas.
Define BOD exactly: oxygen consumed if all organic matter in one litre were oxidised by bacteria.
Name the gases of the digester: methane, hydrogen sulphide, carbon dioxide.
Mention the Ganga and Yamuna Action Plans for questions on importance.

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
During secondary treatment of sewage, flocs mainly cause:
(a) a rise in BOD (b) a fall in BOD (c) removal of grit (d) production of methane
Hint
They consume organic matter.
Answer
(b) a fall in BOD.
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): The main aim of secondary treatment is to remove every trace of organic matter from the effluent.
Reason (R): A high BOD indicates a high polluting potential of water.
(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 true but R is false.
(d) A is false but R is true.
Hint
Treatment continues until BOD is reduced, not to zero.
Answer
(d). The aim is to reduce BOD significantly, not to remove all organic matter; R is true.
Q3 · 2 marks · Very short answerBoard level
Why is the secondary treatment of sewage called biological treatment? Which type of microbes act in the aeration tank and which in the digester?
Hint
Aerobic vs anaerobic.
Answer
Because microbes, not physical processes, remove the organic matter. Aerobic microbes (flocs of bacteria and fungal filaments) act in the aeration tank; anaerobic bacteria act in the sludge digester.
Q4 · 3 marks · Short answerHOTS / competency
Suppose the air pumps of an STP's aeration tank fail for a week, though sewage keeps flowing in. Predict the effect on (a) floc formation, (b) the BOD of the effluent released, (c) the river downstream.
Hint
Flocs are aerobic.
Answer
(a) Without oxygen, the aerobic microbes cannot grow vigorously, so floc formation drops. (b) Organic matter is not consumed, so the effluent leaves with high BOD. (c) In the river, bacteria decomposing the extra organic matter use up dissolved oxygen, harming fish and other aquatic life.
Q5 · 4 marks · Case-basedHarder board level
Read and answer.
Suppose a flow chart of an STP shows: Tank P receives raw sewage and has solids collecting at the bottom; Tank Q has stirrers and air bubbles; Tank R has a sediment, part of which is piped back to Q; Tank S is sealed and has a gas pipe.
(a) Name the treatment stages represented by P and Q. (1)
(b) What is the sediment in R called, and why is part of it piped back? (1)
(c) Name the microbes and the products in S. Why is S sealed? (2)
Hint
Anaerobic.
Answer
(a) P: primary treatment (sedimentation; primary sludge); Q: secondary (biological) treatment in the aeration tank. (b) Activated sludge; part is returned as inoculum to seed the aeration tank with flocs. (c) Anaerobic bacteria digest the sludge, producing methane, hydrogen sulphide and CO₂ (biogas). S is sealed to keep air out (anaerobic conditions) and to collect the gas.
Q6 · 1 mark · Very short answerHarder board level
In a sample, BOD fell from 250 mg/L to 35 mg/L after secondary treatment. Calculate the percentage reduction.
Hint
(in − out)/in × 100.
Answer
(250 − 35)/250 × 100 = 215/250 × 100 = 86%.

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 removed by sedimentation in primary treatment?Show answer
Grit (soil and small pebbles).
2.What are flocs?Show answer
Masses of bacteria associated with fungal filaments forming mesh-like structures.
3.What does a high BOD indicate?Show answer
High organic matter and high polluting potential.
4.Where does activated sludge go?Show answer
A small part back to the aeration tank as inoculum; most to anaerobic sludge digesters.
5.Name the gases produced in sludge digesters.Show answer
Methane, hydrogen sulphide and carbon dioxide.
6.Name two river action plans.Show answer
Ganga Action Plan and Yamuna Action Plan.

Sewage Treatment and BOD: frequently asked questions

What is the difference between primary and secondary treatment of sewage?

Primary treatment physically removes solids by filtration and sedimentation; secondary treatment uses aerobic microbes (flocs) to consume organic matter and reduce BOD.

What is 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. It indicates how polluted water is.

What is activated sludge?

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

How is biogas produced in an STP?

Anaerobic bacteria in sludge digesters digest the bacteria and fungi of the activated sludge, releasing methane, hydrogen sulphide and carbon dioxide.

Why must sewage be treated before release into rivers?

Untreated sewage has high organic matter and pathogens; its high BOD depletes oxygen in rivers and harms aquatic life, and it spreads disease.