This MCQ module is based on: Sewage Treatment Biogas
TOPIC 37 OF 64
Sewage Treatment Biogas
🎓 Class 12
Biology
CBSE
Theory
Ch 8 – Microbes in Human Welfare
⏱ ~14 min
🌐 Language:
🧠 AI-Powered MCQ Assessment
▲
📝 Worksheet / Assessment
▲
This assessment will be based on: Sewage Treatment Biogas
Upload images, PDFs, or Word documents to include their content in assessment generation.
Sewage Treatment Biogas
10.7 Microbes in Sewage Treatment
Every Indian city produces enormous quantities of waste water — called sewage. It is loaded with organic matter and pathogenic microbes. If discharged untreated, sewage devastates rivers, lakes and oceans — kills fish, contaminates drinking water, spreads cholera and typhoid. Microbes themselves help treat the very sewage they live in — a beautifully ironic solution.
Modern Sewage Treatment Plants (STPs) use two principal stages — primary treatment (physical) and secondary treatment (biological, involving microbes). Many advanced STPs add a tertiary (chemical/UV) polishing stage too.
Primary Treatment — Physical Removal
Primary treatment involves the physical removal of large and small particles from sewage:
- Bar screens / sequential filtration: Remove floating debris — paper, plastic, twigs.
- Grit chamber: Allows grit, sand, pebbles to settle.
- Primary sedimentation tank (PST): Remaining particles settle as primary sludge; oils/fats float as scum. Both are removed.
- The supernatant (clarified water with dissolved organics) is now called primary effluent and is passed to secondary treatment.
Secondary Treatment — Biological (the Microbe Stage)
This is the microbe-driven stage. Primary effluent is passed into large aeration tanks, where:
- Vigorous mechanical agitation and air pumping introduce oxygen.
- Naturally occurring aerobic microbes form masses called flocs — bacterial+fungal masses.
- Flocs feed on dissolved organic matter, breaking it down into CO₂, H₂O and biomass. BOD (biological oxygen demand) drops dramatically.
BOD (Biochemical Oxygen Demand): The amount of oxygen consumed by bacteria in 5 days while oxidising organic matter in 1 L of water at 20°C. High BOD = lots of organic pollution. After secondary treatment, BOD must fall to acceptable levels before discharge.
Once BOD is reduced, the effluent is passed to a settling tank, where the flocs (now called activated sludge) settle. A part of this sludge is pumped back to the aeration tank as inoculum. The rest is pumped into large anaerobic sludge digesters.
Anaerobic Sludge Digestion
In the digesters, anaerobic bacteria (including methanogens) digest the bacterial flocs and remaining organic matter, producing a mixture of gases — methane (CH₄), hydrogen sulphide (H₂S), and CO₂ — collectively called biogas. Thus, even the waste sludge yields valuable fuel.
The treated effluent (clean water) is generally released into rivers/streams. The Ganga Action Plan and Yamuna Action Plan were launched to install adequate STPs along Indian rivers; large progress has been made under the Namami Gange Mission.
10.8 Microbes in Biogas Production
Biogas is a mixture of gases (predominantly methane CH₄) produced by microbial activity in the absence of oxygen. Biogas can be used as fuel.
Methanogens — the Gas Producers
The bacteria that produce methane gas are collectively called methanogens. The most notable is Methanobacterium. They are also found in:
- Rumen of cattle — where they help digest cellulose. This is why cattle dung is rich in methanogens and produces biogas easily.
- Anaerobic sewage-sludge digesters — as seen in section 10.7.
- Paddy fields, marshes, swamps — sources of "marsh gas".
Fixed-Dome Biogas Plant (KVIC / Deenbandhu Models)
India's biogas plants are based on the constant availability of cattle dung in villages. The technology was developed by the Indian Agricultural Research Institute (IARI) and Khadi and Village Industries Commission (KVIC). The fixed-dome (Deenbandhu) design is most popular.
Components of a biogas plant:
- Inlet pipe / mixing tank: Cattle dung mixed with water in 1:1 ratio is poured in.
- Digester / dome: An underground brick chamber where anaerobic digestion occurs. Methanogens produce gas; the gas collects in the dome.
- Gas outlet: Pipe at the top of the dome leads gas to nearby kitchens.
- Outlet chamber (slurry tank): Spent slurry is pushed out as the dome fills with gas. Slurry is a rich manure (NPK).
Composition and Use of Biogas
| Component | % | Notes |
|---|---|---|
| Methane (CH₄) | 50-75% | The combustible/useful gas — clean blue flame |
| Carbon dioxide (CO₂) | 25-50% | Non-combustible — dilutes the gas |
| Hydrogen sulphide (H₂S) | 0-3% | Trace — gives rotten-egg smell; removed for high-end use |
| Hydrogen, nitrogen, oxygen | 0-3% | Traces |
Uses: Cooking fuel (replaces firewood, kerosene, LPG); electricity generation (rural micro-grids); vehicle fuel after purification (Bio-CNG); industrial heating.
Slurry as bonus fertiliser: The spent slurry coming out of the outlet is much richer in plant nutrients (N, P, K) than the original dung, because methanogens have broken down complex molecules. Many farmers actually keep biogas plants as much for the manure as for the gas.
🎯 Interactive: STP Stage Explainer
Pick a sewage-treatment stage and see what happens:
What happens: —
Microbes: —
Output: —
📐 Activity 10.3 — Estimate Biogas Output
Setup: A dairy farmer has 8 cows producing 80 kg dung per day combined. Each kg of dung yields ~40 L of biogas.
Predict: How much biogas will be produced per day? How many hours of cooking does this allow (a kitchen stove burns ~400 L/hr)?
- Calculate daily biogas yield in litres.
- Calculate hours of cooking gas available.
- Estimate annual savings if LPG cylinder costs Rs 900 and lasts a month (16 kg).
(a) Daily biogas = 80 × 40 = 3200 L/day.
(b) Cooking hours = 3200 ÷ 400 = 8 hours/day — easily covers all cooking + boiling water + some electricity generation.
(c) Annual LPG saved ≈ 12 cylinders × Rs 900 = Rs 10,800/year. Plus, the slurry manure (~25 kg/day) is worth additional Rs 5000-8000/yr in saved fertiliser. Total saving ~Rs 18,000/year.
This explains why the National Biogas Programme has been actively promoting domestic biogas plants in rural India since the 1980s.
10.9 Worked Examples
Worked Example 10.9.1
Define BOD. How does its value change before and after secondary sewage treatment?
BOD (Biochemical Oxygen Demand) = the amount of oxygen consumed by bacteria when oxidising organic matter in 1 L of water at 20°C for 5 days. High BOD = lots of organic pollution; bacteria use up dissolved O₂, suffocating fish.
Before treatment: Raw sewage has BOD of 200-600 mg/L. After secondary treatment: BOD drops to <30 mg/L (often 10-20 mg/L). This is because aerobic microbes in the activated sludge oxidise organic matter to CO₂ + H₂O + biomass, removing the dissolved load that bacteria would otherwise feed on.
Worked Example 10.9.2
Why is anaerobic sludge digestion both pollution-control and energy-generation?
Activated sludge from settling tanks would otherwise need disposal — landfill or incineration, both costly and polluting. In anaerobic digesters, methanogens (e.g., Methanobacterium) ferment the sludge to biogas (methane + CO₂). This achieves two goals at once:
- Pollution control — sludge volume reduces by 50-60%; the residue is stable, non-smelly manure.
- Energy generation — methane is captured and used as cooking gas or fuel for electricity generators powering the STP itself.
Many modern STPs are energy-positive — they generate more energy than they consume.
Worked Example 10.9.3
Why are biogas plants located near cattle sheds in India?
Three reasons:
- Steady feedstock: Cattle produce dung daily, providing reliable fresh substrate. Each cow ≈ 10 kg dung/day.
- Methanogens already present: Cattle rumen is rich in Methanobacterium; their dung contains the live inoculum needed to seed the biogas digester.
- Convenience: No transport cost; the gas pipe runs directly to the kitchen; slurry output goes straight to fields.
That's why India's National Biogas Programme has built >4 million plants, mostly attached to dairy units.
Competency-Based Questions
Q1. Flocs of bacteria and fungi found in aeration tanks of STPs are called: L1 Remember
(b) Activated sludge. The settled flocs of microbes are called activated sludge — partly recycled as inoculum, partly digested anaerobically.
Q2. The major combustible component of biogas is: L1 Remember
(c) Methane (CH₄). Constitutes 50-75% of biogas; the only major combustible component. CO₂ dilutes; H₂S is corrosive trace.
Q3. Why are methanogens described as 'anaerobic'? Give two natural habitats. L2 Understand
Methanogens cannot tolerate oxygen — they die or stop functioning in its presence. Their metabolism produces methane only under strictly anaerobic conditions. Natural habitats: (i) Rumen of cattle (no O₂, digesting cellulose); (ii) Marshes, swamps, paddy fields (waterlogged soil); (iii) Deep-sea sediments; (iv) Anaerobic sludge digesters in STPs and biogas plants.
Q4. Analyse: A sewage outlet from a small town has BOD of 350 mg/L. Local fishermen report dying fish downstream. Trace the chain of events. L4 Analyse
Chain of events:
- High organic load → aerobic bacteria in river bloom rapidly.
- These bacteria consume dissolved O₂ → DO drops from ~8 mg/L to <2 mg/L.
- Fish need 5+ mg/L O₂ to survive; below 2 mg/L they suffocate and die.
- Dead fish add more organic matter → vicious cycle.
- Anaerobic zone develops → H₂S, CH₄ produced → river smells; even bottom invertebrates die.
- Eventually river becomes a 'dead zone'.
Q5. HOT (Create): Design a community biogas plant for a village of 200 cattle producing 2 tonnes of dung/day. Compute output and suggest end-uses. L6 Create
Computation: 2000 kg × 40 L/kg = 80,000 L/day of biogas ≈ 50 kg LPG-equivalent.
End-uses:
- Domestic cooking — pipe gas to 100 households (1 hr cooking each).
- Village electricity — power a 5-kW biogas generator → run 200 LED bulbs, school computers.
- Bio-CNG — purify a portion for use in panchayat-owned tractors and water pumps.
- Slurry fertiliser — 1.5 tonnes nutrient-rich manure/day → distributed to 50-acre village fields.
Assertion–Reason Questions
Choose: (A) Both true, R explains A. (B) Both true, R doesn't explain A. (C) A true, R false. (D) A false, R true.
A: Aerobic flocs in STPs greatly reduce the BOD of sewage.
R: The flocs metabolise dissolved organic matter using oxygen, converting it to CO₂ and biomass.
Answer: (A). Both true and R explains A. Removing dissolved organic matter directly lowers the oxygen demand of downstream bacteria.
A: Cattle dung makes excellent biogas feedstock.
R: The rumen of cattle naturally hosts methanogens that pass into the dung.
Answer: (A). Both true and R explains A. Dung is pre-seeded with the right anaerobes — biogas starts within 1-2 days.
A: Biogas plants operate well in both summer and winter.
R: Methanogens are tolerant of a wide temperature range from 4°C to 60°C.
Answer: (D). A is FALSE — biogas production drops sharply in cold winters because methanogen activity declines below ~20°C. R is also FALSE; methanogens prefer 30-40°C. Hence A=false. (Best fit: D as written — R is partially true that they tolerate range, but methane output is temperature-sensitive.)
Frequently Asked Questions - Sewage Treatment Biogas
What is the main concept covered in Sewage Treatment Biogas?
In NCERT Class 12 Biology Chapter on Microbes in Human Welfare, "Sewage Treatment Biogas" covers the core biological structures, processes, and pathways students need for board exam success. The MyAiSchool lesson explains the topic with definitions, labelled diagrams, comparison tables, and interactive simulations. Scientific terminology and physiological/genetic significance are highlighted throughout to build conceptual depth aligned with CBSE 2025-26 syllabus.
How is Sewage Treatment Biogas useful in real-life or applied biology?
Real-life applications of "Sewage Treatment Biogas" from NCERT Class 12 Biology Microbes in Human Welfare include medical diagnostics, agriculture, biotechnology, public health, evolutionary insights, and ecological monitoring. The MyAiSchool lesson links every biological concept to a tangible application so students see biology as a problem-solving framework for living systems and real-world challenges.
What are the key terms students should memorize for Sewage Treatment Biogas?
Key terms in "Sewage Treatment Biogas" (NCERT Class 12 Biology Microbes in Human Welfare) are tabulated in the MyAiSchool key-terms grid. Students should memorize each term with its precise definition, function, and example. Terminology is high-yield in CBSE board exams — 1-mark MCQs and 2-mark short answers test definitions directly. The Summary section provides a printable quick-reference card.
How does this part connect to other parts of the chapter?
NCERT Class 12 Biology Microbes in Human Welfare is structured so each part builds biological understanding sequentially. "Sewage Treatment Biogas" connects to neighbouring parts via shared mechanisms, structural hierarchies, and physiological processes. The MyAiSchool lesson cross-references related concepts with internal links so students can navigate the whole chapter as one connected biological story rather than disconnected fragments.
What types of CBSE board questions come from Sewage Treatment Biogas?
CBSE board questions from "Sewage Treatment Biogas" typically include: (1) 1-mark MCQs on definitions and processes, (2) 2-mark short-answer differences/comparisons, (3) 3-mark labelled-diagram questions, (4) 5-mark long-answer essays combining mechanism + diagram + significance. The MyAiSchool lesson tags each Competency-Based Question (CBQ) with Bloom level (L1-L6) so students know how to study for each weight.
How can students use the interactive simulation effectively?
The interactive simulation in the "Sewage Treatment Biogas" lesson allows students to explore biological processes, classifications, or pathways using selectors and sliders, with live visual feedback. To use it effectively: (1) explore each option/state, (2) compare with textbook diagrams, (3) note the function/outcome changes, (4) try the integrated practice quiz. The simulation reinforces visual-spatial understanding that pure text-based study cannot.
AI Tutor
Biology Class 12 – NCERT (2025-26)
Ready
🎁 Join our community and get free AI credits!