Sewage treatment and biogas production are two industrial applications where microbes do work that no chemical process can replicate cheaply. NEET questions on this micro-topic test precise recall of organism names, process stages, and the gas composition of biogas — terminology precision is the scoring lever here.
Primary effluent treatment (PET) is physical: screening, grit removal, sedimentation. No microbes yet. The settled solids form primary sludge.
Secondary effluent treatment (SET) is biological. The supernatant from PET enters aeration tanks where aerobic heterotrophic microbes (bacteria, fungi) oxidise dissolved organic matter, forming flocs — masses of bacteria + fungal filaments. This activated-sludge process reduces BOD (Biochemical Oxygen Demand) substantially. A small portion of activated sludge is pumped back as inoculum; the rest passes to an anaerobic sludge digester.
In the anaerobic digester, facultative and obligate anaerobes (notably methanogenic archaebacteria — Methanobacterium) decompose organic matter, producing a mixture of gases: CH₄, CO₂, H₂S. The effluent after secondary treatment meets discharge standards for BOD.
Biogas plants exploit the same methanogenic process on cattle dung (gobar). The design promoted by KVIC uses a floating gas-holder dome. The gas produced — primarily methane (50–70%) with CO₂ (25–45%) — is combustible and used as fuel. Key point: the organisms responsible are archaebacteria (methanogens), NOT ordinary anaerobic bacteria.
A common NEET distractor conflates flocs (aerobic stage) with sludge (anaerobic stage), or misattributes biogas production to Pseudomonas or E. coli instead of methanogens.
(Reference: NCERT Class 12 Biology Chapter 8, page 152)
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
The primary treatment of sewage involves:
Answer: D. Primary treatment is entirely physical — sequential filtration, sedimentation, and removal of floating debris and grit particles. No biological or chemical agents are used at this stage (NCERT Class 12 Biology Chapter 8, page 154).
Why A is wrong: A describes secondary (biological) treatment where aerobic microbes oxidise dissolved organic matter in aeration tanks.
Why B is wrong: B describes tertiary/disinfection treatment (chlorination or UV), which occurs after secondary treatment if required.
Why C is wrong: C describes what happens to settled sludge AFTER secondary treatment — anaerobic digestion is a downstream step.
During secondary sewage treatment, the decrease in BOD of the effluent is achieved by:
Answer: D. The activated sludge process uses aerobic heterotrophs (bacteria and fungi) that consume dissolved organic matter in aeration tanks, reducing BOD significantly (NCERT Class 12 Biology Chapter 8, page 154).
Why A is wrong: A refers to the anaerobic digester which processes sludge — it does not treat the liquid effluent for BOD reduction.
Why B is wrong: B is a physical-chemical method for removing suspended solids/phosphorus, not the biological BOD-reduction step in secondary treatment.
Why C is wrong: C is a disinfection method (tertiary), not a BOD-reduction mechanism. UV kills pathogens but doesn't degrade organic matter.
Flocs formed during activated sludge treatment are composed of:
Answer: A. Flocs are mesh-like aggregations of aerobic bacteria held together by fungal filaments in aeration tanks. Their large surface area aids efficient organic matter decomposition (NCERT Class 12 Biology Chapter 8, page 154).
Why B is wrong: B describes chemical flocculation (water purification), not the biological flocs of activated sludge treatment.
Why C is wrong: C confuses the aerobic floc stage with the downstream anaerobic digester — methanogens operate in oxygen-free conditions, not in aeration tanks.
Why D is wrong: D is not a recognised component of activated sludge flocs — flocs are living microbial masses, not dead debris.
Which group of microorganisms is primarily responsible for the production of biogas from cattle dung?
Answer: C. Biogas (primarily methane) is produced by methanogenic archaebacteria — notably Methanobacterium — which thrive in anaerobic conditions within the biogas digester (NCERT Class 12 Biology Chapter 8, page 155).
Why A is wrong: A is wrong — Pseudomonas is aerobic and functions in bioremediation/soil decomposition, not in the anaerobic methane-generating environment of biogas plants.
Why B is wrong: B is wrong — Trichoderma is an aerobic fungus used as a biocontrol agent, not a methane producer. Also, biogas production is archaeal, not fungal.
Why D is wrong: D is wrong — cyanobacteria are photosynthetic and fix nitrogen. They require light and do not produce methane under anaerobic conditions.
In the biogas plant design promoted by KVIC (Khadi and Village Industries Commission), the gas produced is collected in a:
Answer: B. The KVIC biogas plant uses a floating gas-holder dome that rises as gas accumulates above the slurry in the digester tank. This is the standard design described in NCERT (Class 12 Biology Chapter 8, page 154).
Why A is wrong: A describes a fixed-dome (Janata model) design, not the KVIC floating-holder type. NEET specifically asks about the KVIC design which features a movable dome.
Why C is wrong: C is incorrect — biogas plants do not use external pressurised cylinders; gas is collected at low pressure within the dome structure itself.
Why D is wrong: D is nonsensical — a perforated base outlet would release gas into the ground, not collect it. Gas rises and is collected at the top.
The major gaseous components of biogas are:
Answer: A. Biogas is predominantly methane (CH₄, 50–70%) and carbon dioxide (CO₂, 25–45%), with traces of H₂S. This composition makes it combustible and useful as fuel (NCERT Class 12 Biology Chapter 8, pages 154–155).
Why B is wrong: B is wrong — hydrogen is a trace component at best; CO₂ is present but the dominant combustible component is methane, not H₂.
Why C is wrong: C is wrong — nitrogen (N₂) is not a significant component of biogas. The anaerobic digestion of organic matter produces CH₄ + CO₂, not N₂.
Why D is wrong: D is wrong — oxygen cannot be present (anaerobic environment) and H₂S is only a trace impurity, not a major component.
A portion of the activated sludge from the aeration tank is pumped back into the system. The purpose of this step is to:
Answer: C. Recycled activated sludge provides a dense microbial inoculum — the aerobic bacteria and fungi that form flocs. This ensures rapid colonisation and BOD reduction in fresh incoming effluent (NCERT Class 12 Biology Chapter 8, page 154).
Why A is wrong: A is wrong — recycling sludge does not increase COD; the entire purpose is to DECREASE biological oxygen demand. COD increase would be counterproductive.
Why B is wrong: B is wrong — pH neutralisation is a chemical step (lime/acid addition), not achieved by recycling biological sludge.
Why D is wrong: D is wrong — aerobic bacteria cannot convert into archaea; these are entirely different domains of life. Methanogens operate in the separate anaerobic digester, not the aeration tank.
After secondary treatment, the remaining sludge is transferred to an anaerobic digester. Which of the following correctly describes the process occurring in this digester?
Answer: B. In the anaerobic sludge digester, obligate and facultative anaerobes (including methanogens) break down organic matter without oxygen, producing methane, carbon dioxide, and hydrogen sulphide as gaseous products (NCERT Class 12 Biology Chapter 8, page 154).
Why A is wrong: A describes aerobic respiration — the digester is specifically anaerobic; no oxygen is supplied.
Why C is wrong: C is wrong — photosynthesis requires light and occurs in autotrophs. The sealed, dark anaerobic digester has no photosynthetic activity.
Why D is wrong: D describes a nitrification process carried out by chemoautotrophs (like Nitrosomonas) in aerobic soil — not relevant to the sealed anaerobic sludge digester.
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Given
A municipal sewage treatment plant receives raw sewage. The plant uses primary treatment followed by secondary biological treatment with activated sludge, then sends excess sludge to an anaerobic digester.
Required
Identify the correct sequence: (i) type of organisms in the aeration tank, (ii) what the recycled sludge provides, (iii) the organisms responsible for gas production in the digester.
Concept
Sewage treatment proceeds in defined stages with different microbial communities at each stage. The aeration tank (secondary treatment) uses aerobic heterotrophs. The anaerobic digester uses methanogens (archaebacteria).
Formula
No mathematical formula applies — this is a process-recall question.
Substitution
(i) Aeration tank → aerobic heterotrophic bacteria and fungi (forming flocs)
(ii) Recycled sludge → microbial inoculum for fresh effluent
(iii) Anaerobic digester → methanogenic archaebacteria (Methanobacterium)
Calculation
No calculation required. The answer is derived from matching organisms to their correct process stage.
Final answer
(i) Aerobic heterotrophs (bacteria + fungi)
(ii) Inoculum (starter culture of decomposers)
(iii) Methanogenic archaebacteria
Common trap
Confusing the aeration tank organisms (aerobic heterotrophs) with the digester organisms (anaerobic methanogens). A frequent distractor attributes methane production to Pseudomonas or general "anaerobic bacteria" — the specific answer NEET expects is methanogens/archaebacteria.
Similar NEET-style question
"In a sewage treatment plant, the
biological treatment step uses _____ in aeration tanks, while the sludge digester relies on _____. Name the domain of life to which the gas-producing organisms belong."
Expected answer: Aerobic heterotrophic microbes (bacteria + fungi); methanogenic organisms; Domain Archaea.
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Sewage treatment: primary (sedimentation), secondary (aerobic — flocs of microbes; BOD reduces), tertiary. Biogas: methanogens (Methanobacterium) anaerobic digestion of cattle dung → CH4 + CO2 + H2 + H2S.
-- NCERT Class 12 Biology, Chapter 8, p. 154More in Biology and Human Welfare: 4 exam traps and mistakes · 1 question pattern from its other lessons.
1 question from NEET 2020. Answers verified against NTA official keys.
Which of the following is put into Anaerobic sludge digester for further sewage treatment ?
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