Biogas

BiologyMicrobes in Human WelfareFor NEET aspirants

Biogas is a fuel gas, mainly methane, made by microbes that work without oxygen. Microbes also clean sewage, and its leftover sludge yields biogas, so this page covers both: sewage, primary and secondary treatment, flocs, BOD, activated sludge, anaerobic sludge digesters, methanogens and the gobar gas plant, as in the NCERT Class 12 chapter Microbes in Human Welfare. NEET often asks the order of the treatment steps, the meaning of BOD and the gases in biogas.

On this page1Sewage2Primary treatment3Secondary treatment4Sewage and rivers5Methanogens6Biogas plant7Exam essentials8Quick revision9Solved examples10Practice

Key Points at a Glance

★ Must learn Sewage: municipal waste water, with human excreta as a major component; rich in organic matter and microbes, many pathogenic.
Sewage is treated in sewage treatment plants (STPs) by heterotrophic microbes, in two stages.
★ Must learn Primary treatment is physical: filtration removes floating debris, sedimentation removes grit; settled solids form primary sludge.
Secondary treatment is biological: aerobic microbes grow as flocs in aeration tanks and consume organic matter.
★ Must learn BOD: oxygen that would be consumed if all organic matter in one litre of water were oxidised by bacteria. Higher BOD, more pollution.
★ Must learn Settled flocs form activated sludge: a small part returns as inoculum; the major part goes to anaerobic sludge digesters.
Sludge digesters release biogas: methane, hydrogen sulphide and $C O_{2}$ .
★ Must learn Methanogens (for example Methanobacterium) grow anaerobically on cellulose and give methane with $C O_{2}$ and $H_{2}$ .
Biogas plant: concrete tank 10-15 feet deep fed with dung slurry; a floating cover rises as gas forms; spent slurry is a fertiliser.
Biogas technology in India: IARI and KVIC. Ganga and Yamuna Action Plans: Ministry of Environment and Forests.

1. Sewage

Cities and towns generate large quantities of waste water every day.
A major component of this waste water is human excreta.

★ Very important Sewage (urban waste water): municipal waste water. It contains large amounts of organic matter and microbes, many of which are pathogenic (disease-causing).

Sewage cannot be discharged directly into natural water bodies such as rivers and streams.
★ Exam imp Before disposal, sewage is treated in sewage treatment plants (STPs) to make it less polluting.
The treatment is done by heterotrophic microbes naturally present in the sewage.
It is carried out in two stages: primary treatment and secondary treatment.

Key idea

Sewage is full of organic matter and pathogens, so microbes in STPs must clean it before it reaches rivers.

2. Primary Treatment

★ Exam imp Primary treatment: physical removal of particles, large and small, from sewage by filtration and sedimentation.
The particles are removed in stages.

Floating debris is removed by sequential filtration (filtering again and again).
Grit (soil and small pebbles) is removed by sedimentation (settling).
All the solids that settle form the primary sludge; the liquid above them (supernatant) forms the effluent.
The effluent from the primary settling tank is taken for secondary treatment.

Key idea

Primary treatment only filters and settles; it removes what can be seen, not the dissolved organic matter.

3. Secondary Treatment (Biological Treatment)

The primary effluent is passed into large aeration tanks.
It is constantly agitated mechanically, and air is pumped into it.
Useful aerobic microbes grow vigorously into flocs.
While growing, the microbes consume the major part of the organic matter in the effluent; this significantly reduces the BOD.
The sewage is treated until its BOD is reduced.
The effluent is then passed into a settling tank, where the bacterial flocs sediment; this sediment is the activated sludge.
A small part of the activated sludge is pumped back into the aeration tank to serve as the inoculum.
The major part is pumped into large tanks called anaerobic sludge digesters.
There, other bacteria that grow anaerobically digest the bacteria and fungi in the sludge.
During this digestion, bacteria produce methane, hydrogen sulphide and $C O_{2}$ , which form biogas.
The effluent from the secondary treatment plant is generally released into rivers and streams.

★ Very important Flocs: masses of bacteria associated with fungal filaments, forming mesh-like structures. Activated sludge: the sediment of flocs in the settling tank.

Biogas from the sludge digesters is inflammable, so it can be used as a source of energy.

Memory Trick Flocs are a living net: fungal filaments form the mesh, and masses of bacteria fill it.

3.1 Biochemical oxygen demand (BOD)

★ Very important 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 uptake of oxygen by micro-organisms in a sample of water.
★ Exam imp Thus, indirectly, BOD is a measure of the organic matter present in the water.
The greater the BOD of waste water, the more is its polluting potential.

Tips and Tricks BOD rises with organic matter. Untreated sewage has a very high BOD; treated effluent has a lower BOD; relatively clean river water is usually lowest. In a BOD question, the largest value marks the most polluted sample.

Memory Trick Filter, Settle, Aerate, Settle, Digest: settling comes twice, once on each side of aeration. The first settling gives primary sludge; the second gives activated sludge, which is then digested.

Primary treatment

Physical. Filtration and sedimentation. Removes debris and grit. Gives primary sludge and effluent.

Secondary treatment

Biological. Aerobic microbes in aeration tanks. Removes organic matter, cuts BOD. Gives activated sludge.

NEET Focus

Three traps that statement questions set on sewage treatment.

A high BOD means more organic matter and more pollution; treatment lowers the BOD.
Only a small part of the activated sludge returns as inoculum; the major part goes to the anaerobic sludge digesters.
Primary treatment is physical; secondary treatment is biological and aerobic; the sludge digester is anaerobic.

Read 'small part' and 'major part' carefully before marking a statement.

Feature	Aeration tank	Anaerobic sludge digester
Oxygen	Air pumped in (aerobic)	No oxygen (anaerobic)
Microbes	Aerobic microbes as flocs	Bacteria that grow anaerobically
What happens	Microbes consume organic matter; BOD falls	Bacteria digest the bacteria and fungi of the sludge
Product	Effluent with low BOD; flocs for activated sludge	Biogas: methane, hydrogen sulphide, $C O_{2}$

Key idea

Secondary treatment lets aerobic flocs eat the organic matter; the leftover sludge is digested anaerobically to give biogas.

4. Sewage Treatment and Our Rivers

Microbes treat millions of gallons of waste water every day across the globe.
This method has been practised for more than a century, in almost all parts of the world.
★ Exam imp Till date, no man-made technology has been able to rival the microbial treatment of sewage.
The activated sludge process also helps in recycling water in nature.

Increasing urbanisation produces sewage in much larger quantities than ever before.
The number of STPs has not increased enough to treat such large quantities.
So untreated sewage is often discharged directly into rivers, polluting them and increasing water-borne diseases.
The Ministry of Environment and Forests has started the Ganga Action Plan and the Yamuna Action Plan to save these rivers from pollution.
Under these plans, many STPs are to be built, so that only treated sewage is discharged into the rivers.

Key idea

Microbial sewage treatment has no man-made rival; the Ganga and Yamuna Action Plans aim to send only treated sewage into rivers.

5. Biogas and Methanogens

★ Very important Biogas: a mixture of gases, predominantly methane, produced by microbial activity; it may be used as fuel.

Microbes produce different gaseous end-products during growth and metabolism.
The gas produced depends on the microbes and on the organic substrates they use.
In dough fermentation, cheese-making and beverage production, the main gas is $C O_{2}$ .
★ Exam imp Methanogens: bacteria that grow anaerobically on cellulosic material and produce large amounts of methane along with $C O_{2}$ and $H_{2}$ .
A common methanogen is Methanobacterium.
Methanogens are commonly found in the anaerobic sludge during sewage treatment.
They are also present in the rumen (a part of the stomach) of cattle, where much cellulosic food collects.
In the rumen, they help break down cellulose and play an important role in the nutrition of cattle.
So cattle excreta (dung), commonly called gobar, is rich in methanogens.
Dung can be used to generate biogas, commonly called gobar gas.

Source	Main gases produced
Dough, cheese and beverage fermentations	$C O_{2}$
Methanogens on cellulosic material	Methane, with $C O_{2}$ and $H_{2}$
Anaerobic sludge digesters	Methane, hydrogen sulphide and $C O_{2}$

Memory Trick Methanogens make methane where there is no oxygen. Find them in three places: Sludge, Rumen and Dung.

Quick Recall: tap to check

Can human beings digest the cellulose in their food?

No. Humans have no rumen and do not make the enzyme that breaks down cellulose; cattle digest it with the help of rumen microbes.

Name a common methanogen.

Methanobacterium.

What is the main gas produced when dough is fermented?

C O_{2}

, not methane.

Key idea

Methanogens of sludge, rumen and dung turn cellulose into methane-rich biogas.

6. The Biogas Plant

Bio-wastes are collected, and a slurry of dung (dung mixed with water) is fed into a concrete tank 10-15 feet deep, the digester.
Microbes in the slurry produce gas.
A floating cover (the gas holder) placed over the slurry keeps rising as gas is produced.
An outlet connected to a pipe supplies biogas to nearby houses.
The spent slurry (sludge) is removed through another outlet and may be used as fertiliser.

Test yourself: hide labels

Figure 3: A typical biogas plant. Dung and water enter the digester, methane-rich gas collects in the floating gas-holder, and sludge leaves by a separate outlet.

Cattle dung is available in large quantities in rural areas, where cattle are used for many purposes.
So biogas plants are more often built in rural areas.
★ Exam imp Biogas is used for cooking and lighting.
Biogas technology in India was developed mainly through the Indian Agricultural Research Institute (IARI) and the Khadi and Village Industries Commission (KVIC).

Memory Trick Farm research and village industry: a village fuel came from the Indian Agricultural Research Institute (IARI) and the Khadi and Village Industries Commission (KVIC).

Memory Trick In, up and out: dung and water go in, the gas-holder goes up with the gas, and the sludge comes out.

Quick Recall: tap to check

In a biogas plant, which part rises as gas is produced?

The floating cover, that is, the gas-holder.

Which two inputs are shown entering the inlet of a biogas plant?

Dung and water.

What leaves through the separate outlet, and how is it used?

Sludge (spent slurry); it is used as fertiliser.

What is the deep tank holding the slurry called?

The digester.

Key idea

A biogas plant is a sealed digester for dung slurry, with a rising gas-holder above and an outlet for fertiliser sludge.

7. Exam Essentials

Pairs to Match

Term or body	Matches with
Primary treatment	Physical removal by filtration and sedimentation
Sequential filtration	Floating debris
Sedimentation	Grit (soil and small pebbles)
Primary sludge	Solids that settle in primary treatment
Flocs	Bacteria with fungal filaments in mesh-like masses
Aeration tank	Aerobic microbes; BOD falls
Activated sludge	Sediment of flocs in the settling tank
Anaerobic sludge digesters	Digestion of sludge; biogas
BOD	Indirect measure of organic matter in water
Methanobacterium	A common methanogen
Rumen	Part of cattle stomach holding methanogens
Gobar	Cattle dung
Spent slurry	Fertiliser
IARI and KVIC	Biogas technology in India
Ministry of Environment and Forests	Ganga and Yamuna Action Plans

Exceptions

Primary treatment is physical, not biological; microbial action is the secondary stage.
The aeration tank is aerobic; the sludge digester is anaerobic, not aerobic.
Only a small part of the activated sludge returns as inoculum; the major part goes to the digesters.
In dough, cheese and beverages the main gas is $C O_{2}$ , not methane.
Biogas is not pure methane; it is a mixture, predominantly methane.
Unlike cattle, humans cannot digest cellulose.
Untreated sewage must not go into rivers; only the secondary effluent is released.

Numbers to Remember

2 stages of sewage treatment: primary (physical) and secondary (biological).
One litre: the volume of water in the definition of BOD.
More than a century: how long microbial sewage treatment has been practised.
Millions of gallons: waste water treated by microbes every day across the globe.
10-15 feet: depth of the concrete tank of a biogas plant.

8. Quick Revision

Sewage (urban waste water) is municipal waste water; human excreta is a major part; it holds organic matter and pathogens.
STPs use heterotrophic microbes; treatment has two stages.
Primary: sequential filtration removes floating debris; sedimentation removes grit; settled solids form primary sludge.
The supernatant of primary treatment is the effluent, sent to secondary treatment.
Secondary: aeration tanks with agitation and air; aerobic microbes grow as flocs and consume organic matter.
Flocs are bacteria with fungal filaments in mesh-like masses.
BOD is the oxygen needed to oxidise all organic matter in one litre of water; high BOD means high pollution.
Settled flocs form activated sludge; a small part is the inoculum, the rest goes to anaerobic sludge digesters.
Digesters give biogas: methane, hydrogen sulphide and $C O_{2}$ ; the effluent goes to rivers.
Ganga and Yamuna Action Plans (Ministry of Environment and Forests) aim to build more STPs.
Biogas is mainly methane; methanogens such as Methanobacterium make it anaerobically from cellulose.
Methanogens live in anaerobic sludge and the cattle rumen, so dung (gobar) is rich in them.
Biogas plant: 10-15 feet concrete tank, dung slurry, rising floating cover, gas pipe, spent slurry as fertiliser.
Biogas is used for cooking and lighting; IARI and KVIC developed the technology.

9. Solved Examples

Solved Example 1

Match List I with List II.
List I: A. Primary sludge, B. Flocs, C. Activated sludge, D. Biogas
List II: I. Mixture of methane, hydrogen sulphide and

C O_{2}

, II. Solids that settle during primary treatment, III. Masses of bacteria with fungal filaments, IV. Sediment of flocs in the settling tank
Choose the correct answer:
(A) A-II, B-III, C-IV, D-I
(B) A-III, B-II, C-IV, D-I
(C) A-II, B-IV, C-III, D-I
(D) A-IV, B-III, C-II, D-I

Solution:

Answer: (A). Primary sludge is the settled solids (II), flocs are bacteria with fungal filaments (III), activated sludge is settled flocs (IV), and biogas is the gas mixture (I).

Solved Example 2

Read the statements.
A. The BOD test measures the rate of uptake of oxygen by micro-organisms.
B. A higher BOD means a lower polluting potential.
C. Secondary treatment reduces the BOD of the effluent.
D. Primary treatment is a biological process.
E. The effluent from secondary treatment is released into rivers.
Choose the correct answer:
(A) A, B and C only
(B) B, D and E only
(C) A, C and E only
(D) A, D and E only

Solution:

Answer: (C). B is wrong: higher BOD means more pollution. D is wrong: primary treatment is physical.

Solved Example 3

Arrange the steps of sewage treatment in the correct order.
A. Digestion in anaerobic sludge digesters
B. Removal of floating debris by sequential filtration
C. Growth of flocs in aeration tanks
D. Removal of grit by sedimentation
E. Formation of activated sludge in the settling tank
Choose the correct answer:
(A) B, C, D, E, A
(B) D, B, C, E, A
(C) B, D, E, C, A
(D) B, D, C, E, A

Solution:

Answer: (D). Filtration (B) and sedimentation (D) are primary; aeration (C) and settling (E) are secondary; the sludge is then digested (A).

Solved Example 4

Which of the following is NOT correct about methanogens?
(A) They grow anaerobically on cellulosic material
(B) They produce methane along with

C O_{2}

and

H_{2}

Solution:

Answer: (D). Methanogens are anaerobic. The flocs of aeration tanks are made by aerobic microbes.

Solved Example 5

Statement I: A small part of the activated sludge is pumped back into the aeration tank as inoculum.
Statement II: The major part of the activated sludge is released directly into rivers.
Choose the correct answer:
(A) Both Statement I and Statement II are correct
(B) Both Statement I and Statement II are incorrect
(C) Statement I is correct but Statement II is incorrect
(D) Statement I is incorrect but Statement II is correct

Solution:

Answer: (C). The major part of the activated sludge goes to anaerobic sludge digesters, not to rivers.

Solved Example 6

Biogas technology in India was developed mainly through the efforts of:
(A) ICMR and IARI
(B) IARI and KVIC
(C) KVIC and CSIR
(D) ICAR and NDDB

Solution:

Answer: (B). The Indian Agricultural Research Institute and the Khadi and Village Industries Commission.

Practice Questions

Give examples to prove that microbes release gases during metabolism.Answer: Bacteria release $C O_{2}$ that puffs up dosa and idli dough; yeast releases $C O_{2}$ in bread dough; Propionibacterium sharmanii makes the $C O_{2}$ holes of Swiss cheese; methanogens release methane with $C O_{2}$ and $H_{2}$ ; sludge digesters give methane, hydrogen sulphide and $C O_{2}$ .
What is sewage? In which way can sewage be harmful to us?Answer: Sewage is municipal waste water, with human excreta as a major component. It is rich in organic matter and pathogenic microbes, so untreated sewage pollutes rivers and spreads water-borne diseases.
What is the key difference between primary and secondary sewage treatment?Answer: Primary treatment is physical: filtration and sedimentation remove particles. Secondary treatment is biological: aerobic microbes consume the organic matter and reduce the BOD.
Do you think microbes can also be used as a source of energy? If yes, how?Answer: Yes. Methanogens digest dung, plant waste and sewage sludge anaerobically and produce biogas, mainly methane. Being inflammable, it is used for cooking and lighting.
Three samples, river water, untreated sewage and secondary effluent from an STP, were labelled A, B and C without a record. Their BOD values were 20 mg/L, 8 mg/L and 400 mg/L. Which is most polluted? Assign each label, taking the river water as relatively clean.Answer: C (400 mg/L) is the most polluted: untreated sewage. A (20 mg/L) is the secondary effluent. B (8 mg/L) is the river water. BOD falls as organic matter is removed.
Match the parts of a biogas plant.
List I: A. Digester, B. Gas-holder, C. Gas outlet pipe, D. Spent slurry
List II: I. Supplies gas to nearby houses, II. Used as fertiliser, III. Rises as gas collects, IV. Tank where dung slurry is fed
(A) A-IV, B-I, C-III, D-II
(B) A-IV, B-III, C-I, D-II
(C) A-III, B-IV, C-I, D-II
(D) A-IV, B-III, C-II, D-IAnswer: (B). The digester holds the slurry, the gas-holder rises, the pipe supplies gas, and the spent slurry is a fertiliser.
Read the statements.
A. Methanogens are common in anaerobic sludge.
B. Cattle dung is rich in methanogens.
C. Biogas plants are mostly built in cities.
D. Biogas is used for cooking and lighting.
Choose the correct answer:
(A) A, B and C only
(B) A, B and D only
(C) B, C and D only
(D) A and C onlyAnswer: (B). Biogas plants are built more often in rural areas, where dung is plentiful.
Arrange the working of a biogas plant in order.
A. Gas is supplied through a pipe
B. Dung slurry is fed into the tank
C. The floating cover rises
D. Microbes produce gas from the slurry
(A) B, D, C, A
(B) B, C, D, A
(C) D, B, C, A
(D) B, D, A, CAnswer: (A). Feeding, gas production, rising cover, then supply.
Which of the following is NOT a component of the biogas formed in anaerobic sludge digesters?
(A) Methane
(B) Hydrogen sulphide
(C) Carbon dioxide
(D) OxygenAnswer: (D). The digesters work without oxygen; biogas there is methane, hydrogen sulphide and $C O_{2}$ .

Common Mistakes to Avoid

Watch out

Calling primary treatment biological. Correct: it is physical (filtration and sedimentation).
Reading a high BOD as clean water. Correct: high BOD means more organic matter and more pollution.
Mixing up primary sludge and activated sludge. Correct: primary sludge is settled solids; activated sludge is settled flocs.
Sending all the activated sludge back to the aeration tank. Correct: only a small part returns; the major part goes to digesters.
Calling the sludge digester aerobic. Correct: it is anaerobic; the aeration tank is aerobic.
Writing that biogas is pure methane. Correct: it is a mixture, predominantly methane.
Placing methanogens in the aerobic flocs. Correct: methanogens are anaerobic, found in anaerobic sludge and the rumen.
Crediting the Ganga Action Plan to the wrong body. Correct: the Ministry of Environment and Forests started it.

Frequently Asked Questions

What is sewage and why must it be treated?

Sewage is the municipal waste water of cities and towns; human excreta is a major part of it. It carries large amounts of organic matter and microbes, many of them pathogenic. Discharged untreated into rivers, it pollutes them and spreads water-borne diseases, so it is first treated in sewage treatment plants.

What is the difference between primary and secondary sewage treatment?

Primary treatment is physical. Sequential filtration removes floating debris and sedimentation removes grit, giving primary sludge and an effluent. Secondary treatment is biological. The effluent is aerated so that aerobic microbes grow as flocs and consume most of the organic matter, which lowers the BOD.

What is BOD and what does it tell us?

BOD, or 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 test measures the rate of oxygen uptake by microbes, so it indirectly measures organic matter. A higher BOD means a higher polluting potential.

What are flocs and activated sludge?

Flocs are masses of bacteria associated with fungal filaments, forming mesh-like structures in the aeration tank. They consume organic matter in the effluent. When the BOD has fallen, the effluent goes to a settling tank, where the flocs sediment. This sediment is activated sludge; a small part of it is reused as inoculum.

What happens in an anaerobic sludge digester?

The major part of the activated sludge is pumped into large tanks called anaerobic sludge digesters. Bacteria that grow without oxygen digest the bacteria and fungi in the sludge. During digestion they produce methane, hydrogen sulphide and carbon dioxide, which together form biogas, an inflammable gas that can be used as a source of energy.

What are methanogens and where are they found?

Methanogens are bacteria that grow anaerobically on cellulosic material and produce large amounts of methane, along with carbon dioxide and hydrogen. Methanobacterium is a common example. They occur in the anaerobic sludge of sewage treatment and in the rumen of cattle, where they help digest cellulose, so cattle dung is rich in them.

How does a biogas plant work?

Dung slurry and other bio-wastes are fed into a concrete tank 10 to 15 feet deep. Microbes in the slurry produce biogas, and a floating cover over the slurry rises as gas collects. A pipe from an outlet supplies the gas to nearby houses for cooking and lighting, and the spent slurry, removed through another outlet, is used as fertiliser.

Which points on sewage treatment and biogas does NEET ask most?

NEET most often asks the order of treatment steps, physical versus biological treatment, the definition and meaning of BOD, flocs and activated sludge, the gases in biogas, methanogens and Methanobacterium, and the parts of a biogas plant. The questions follow NCERT wording closely, so learn each term exactly.

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