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 9, page 206)
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:
During secondary sewage treatment, the decrease in BOD of the effluent is achieved by:
Flocs formed during activated sludge treatment are composed of:
Which group of microorganisms is primarily responsible for the production of biogas from cattle dung?
In the biogas plant design promoted by KVIC (Khadi and Village Industries Commission), the gas produced is collected in a:
The major gaseous components of biogas are:
A portion of the activated sludge from the aeration tank is pumped back into the system. The purpose of this step is to:
After secondary treatment, the remaining sludge is transferred to an anaerobic digester. Which of the following correctly describes the process occurring in this digester?
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. ---
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, Ch. 9, p. 206These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Similar Terms
ELISA = screening (cheap, sensitive); Western blot = confirmation. CD4+ T cell count tracks disease progression.
Question on which test confirms HIV vs which monitors progression.
ELISA first (screening); Western blot confirms; CD4+ count = monitoring marker (<200/µL = AIDS).
Category: Similar Terms
Active: own immune response (vaccination, infection). Passive: pre-formed antibodies received (colostrum, antiserum injection).
Question on immunity type from vaccination, antiserum, mother's milk.
Active = body produces; passive = body receives. Vaccination → active. Mother's milk → passive (IgA).
Root cause: term confusion
Vaccination = ACTIVE (body produces antibodies in response to antigen). Antiserum = passive. Maternal IgA in milk = passive.
Root cause: term confusion
Anopheles = vector (transmits). Plasmodium = causative agent (causes disease). Female Anopheles bite is the transmission route.
29 questions from NEET 2020, 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.
Streptokinase produced by bacterium Streptococcus is used for
The capacity to generate a whole plant from any cell of the plant is called:
Which one of the following is NOT an advantage of inbreeding?
Identify the asexual reproductive structure associated with Penicillium :
Select the incorrect statement with respect to acquired immunity.
Which of the following plants is monoecious? (b) Plant tissue culture (ii) Pomato
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
active vs passive immunity confusion
Vaccination triggers the body to produce its own antibodies (active immunity), but students interpret 'given externally' as passive. Passive immunity (antiserum, colostrum IgA from mother's milk) provides preformed antibodies with no immune response from the recipient. Questions that pair vaccination, antiserum, and colostrum expose students who cannot classify by mechanism rather than by route of administration.
vector vs causative agent confusion
Questions name both the vector (organism that transmits) and the causative agent (organism that causes disease) as answer options. Students who associate 'Anopheles = malaria' select Anopheles as the causative agent rather than recognising it as the vector; Plasmodium is the causative agent. The distinction -- vector transmits, causative agent infects -- is lost when both options are plausible from partial recall.
microbe product pair swap
Match-list questions pair 10 or more microbe names with industrial or household products. Students confuse Lactobacillus (lactic acid fermentation, curd) with Saccharomyces cerevisiae (ethanol, bread), or Aspergillus niger (citric acid) with Clostridium butylicum (butyric acid). Partial recall of one-to-one mappings produces swapped pairs, especially when two products share the same chemical class.
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