Class 9 Biology Chapter 8: Microbes in Human Welfare — Important Questions with Full Solutions

Chapter 8: Microbes in Human Welfare consistently appears in board exams because it tests both conceptual understanding and application skills. Examiners ask questions on: (1) identification of beneficial microbes by their functions; (2) industrial fermentation processes (yogurt, bread, cheese, alcohol); (3) sewage treatment plant operations and stages; (4) biogas production chemistry and practical advantages; (5) household uses of microbes. The 2024–25 rationalized NCERT has retained all core topics, making this chapter high-weightage for 2026 exams. Most board papers dedicate 8–12 marks to this chapter across different question types. Key patterns include: definition-based 1-mark MCQs, process-flow 2-mark answers (e.g., steps in yogurt making), diagram-labelling 3-mark questions (sewage treatment stages), and 5-mark case scenarios (e.g., a city's biogas plant efficiency problem). Students who master these 18 questions build the exact confidence and vocabulary needed for board success.

These single-mark questions test quick recall and concept clarity. Practise these under timed conditions to sharpen your confidence. **Q1. Which microbe is commonly used in the production of yogurt?** a) Escherichia coli b) Lactobacillus c) Bacillus subtilis d) Clostridium botulinum **Answer: b) Lactobacillus** — This bacterium ferments lactose in milk, producing lactic acid, which coagulates casein and gives yogurt its characteristic taste and texture. **Q2. Biogas is primarily composed of:** a) CO₂ and H₂ b) CH₄ and CO₂ c) N₂ and O₂ d) H₂S and NH₃ **Answer: b) CH₄ and CO₂** — Methane (CH₄) forms 50–70% and carbon dioxide (CO₂) forms 30–40% of biogas; small traces of H₂S and NH₃ may be present. **Q3. The process by which sewage is treated biologically in tanks is called:** a) Chlorination b) Sedimentation c) Activated sludge process d) Filtration **Answer: c) Activated sludge process** — Aeration and microbes (bacteria, protozoa) break down organic matter in activated sludge tanks. **Q4. Penicillin is an antibiotic produced by:** a) Aspergillus flavus b) Penicillium notatum c) Streptomyces d) Bacillus polymyxa **Answer: b) Penicillium notatum** — This fungus was discovered by Fleming and is the primary source of the penicillin antibiotic family. **Q5. Which of the following is a thermophilic microbe used in biogas production?** a) Methanobacterium b) Escherichia coli c) Lactobacillus lactis d) Saccharomyces cerevisiae **Answer: a) Methanobacterium** — This archaeon thrives at 50–60°C and converts acetic acid and hydrogen into methane (CH₄).

These questions require concise explanations and deserve 2–3 sentences. Focus on process steps, definitions, and one relevant example. **Q1. Explain how lactobacillus helps in the production of curd. Why is the temperature maintained at 40°C?** **Answer:** Lactobacillus bacteria ferment lactose (milk sugar) into lactic acid. This acid causes milk proteins (casein) to denature and coagulate, forming curds. Temperature is maintained at 40°C because this is the optimal growth temperature for lactobacillus; below this, fermentation slows; above 45°C, bacteria die. This controlled warmth ensures rapid and efficient curd formation. **Q2. Name two microbes used in household fermentation. State one product made from each.** **Answer:** (1) Saccharomyces cerevisiae — used in bread-making; yeast ferments sugars anaerobically, producing CO₂ gas which causes dough to rise. (2) Lactobacillus — used in yogurt production; bacteria ferments lactose into lactic acid, coagulating milk proteins to form thick, tangy yogurt. **Q3. What is the activated sludge process? Name one microbe involved.** **Answer:** The activated sludge process is a secondary (biological) stage of sewage treatment where aerated tanks containing activated sludge (a mixture of bacteria and protozoa) break down dissolved organic matter. Bacteria like Bacillus and Pseudomonas oxidise organic compounds; protozoa consume bacteria, further purifying water. This process reduces BOD (Biological Oxygen Demand) significantly. **Q4. List two advantages of biogas as a fuel source.** **Answer:** (1) It is renewable and produced from organic waste (dung, agricultural residue, food waste), making it sustainable and economically viable for rural households. (2) Biogas combustion produces fewer pollutants than fossil fuels; burning 1 m³ of biogas releases ~20 MJ energy with minimal CO₂ and no sulphur compounds. **Q5. Why are probiotics added to certain food products? Give one example.** **Answer:** Probiotics (beneficial microbes like Lactobacillus and Bifidobacterium) are added to foods to improve gut health and digestion. They inhibit pathogenic bacteria and strengthen intestinal immunity. Example: buttermilk and fermented yogurts contain live lactobacillus cultures that aid lactose digestion and restore gut microflora.

These questions demand detailed explanation with examples, process steps, or comparative analysis. Answer in 4–6 sentences with one worked diagram concept if applicable. **Q1. Describe the main stages of sewage treatment and the role of microbes in each stage.** **Answer:** Sewage treatment involves three stages: (1) *Primary treatment*: physical screening and sedimentation remove large particles and settle organic matter as sludge; microbes are not active here. (2) *Secondary treatment (activated sludge process)*: aerated tanks with microbes (bacteria like Bacillus, Pseudomonas, and protozoa) oxidise dissolved and colloidal organic matter; bacteria use organic carbon as energy source (C + O₂ → CO₂ + H₂O), while protozoa consume bacteria, reducing pathogen load. (3) *Tertiary treatment*: remaining dissolved salts and nutrients are removed by filtration and chemical precipitation; chlorination or UV disinfection kills remaining pathogens. Microbes are most active in secondary stage, reducing BOD from ~300 ppm to <30 ppm. The sludge produced is either incinerated or anaerobically digested to produce biogas. **Q2. Write the overall equation for anaerobic decomposition of glucose and explain how biogas is produced from cow dung.** **Answer:** Overall equation for glucose fermentation in biogas digesters: C₆H₁₂O₆ → 3 CH₄ + 3 CO₂ (under anaerobic conditions at 50–60°C) In biogas production from cow dung: (1) Cow dung (containing cellulose, proteins, and fats) is mixed with water in anaerobic digester tanks. (2) *Hydrolytic bacteria* break down complex polymers into simple sugars and fatty acids. (3) *Acidogenic bacteria* ferment these into volatile fatty acids and hydrogen. (4) *Methanogenic archaea* (especially Methanobacterium) convert acetic acid and H₂ into CH₄ and CO₂. The process takes 40–60 days at 50–60°C. Approximately 1 kg dry dung yields ~200 litres of biogas (containing ~65% CH₄ and ~35% CO₂), sufficient to cook meals for a small household for 2–3 hours. **Q3. Compare aerobic and anaerobic processes in microbes. Which is used for biogas production and why?** **Answer:** *Aerobic process*: occurs in presence of oxygen; microbes completely oxidise organic matter to CO₂ and H₂O; used in sewage treatment secondary stage; yields high energy (~2800 kJ/mol glucose) but no usable fuel gas; rate is fast (hours to days). *Anaerobic process*: occurs without free oxygen; microbes incompletely oxidise organic matter to CH₄, CO₂, H₂, and organic acids; used in biogas digesters; yields methane (usable fuel) and energy (~1000 kJ/mol); slower process (40–60 days). Biogas production uses anaerobic decomposition because: (1) produces methane, a valuable renewable fuel; (2) handles waste (dung, agricultural residue) sustainably; (3) requires no aeration infrastructure (lower cost). Sewage treatment uses aerobic because it is faster and more complete, reducing pathogens and BOD in short timescales.

These extended-response questions test comprehensive understanding. Answer in 8–10 sentences with diagrams concepts, equations, and detailed reasoning. **Q1. Explain the industrial production of bread and the roles of yeast (Saccharomyces cerevisiae) and bacteria in this process. How does fermentation cause bread to rise?** **Answer:** *Bread production* is one of humanity's oldest fermentation processes. Bread dough consists of flour (wheat starch + gluten proteins), water, salt, and yeast. When Saccharomyces cerevisiae (baker's yeast) is added to dough at 37–40°C, it begins anaerobic fermentation of glucose (from flour starch hydrolysis by flour enzymes): C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂ The CO₂ gas produced forms countless tiny bubbles trapped within gluten network (a protein matrix formed by hydrating gluten proteins). These bubbles expand during proofing (1–2 hours at 35°C) and further during baking (60–65°C for 30–40 minutes); this expansion makes dough rise, creating bread's spongy, airy texture. Ethanol (C₂H₅OH) produced during fermentation evaporates during baking, contributing to bread's characteristic flavour and aroma. Simultaneously, lactic acid bacteria (LAB) like Lactobacillus and Pediococcus naturally present in flour or added as starter cultures ferment sugars slightly, producing lactic acid; this lowers dough pH (to ~4.0), inhibits pathogenic bacteria, improves dough handling (elasticity and extensibility), and enhances flavour complexity and shelf-life. Temperature control is critical: below 20°C, fermentation is too slow; above 45°C, yeast dies. Industrial bakeries use compressed yeast (dried or fresh cultures) and control fermentation time precisely. This process demonstrates how microbes transform simple ingredients into a nutritious, flavourful staple food. **Q2. A municipal sewage treatment plant receives 10,000 litres of sewage daily with BOD = 400 ppm. After primary treatment, BOD = 350 ppm. After secondary (activated sludge) treatment, BOD = 25 ppm. Calculate the percentage reduction in BOD at each stage and explain why secondary treatment is more effective.** **Answer:** *Percentage reduction in BOD:* Primary treatment: BOD reduction = (400 − 350) / 400 × 100 = 50/400 × 100 = **12.5%** Secondary treatment: BOD reduction = (350 − 25) / 350 × 100 = 325/350 × 100 = **92.9%** Total reduction (primary + secondary): BOD reduction = (400 − 25) / 400 × 100 = 375/400 × 100 = **93.75%** *Why secondary treatment is more effective:* Primary treatment (screening and sedimentation) removes only large, settleable solids and grit—hence only ~12.5% BOD reduction. Secondary (activated sludge) treatment introduces *living microbes* (bacteria and protozoa) in aerated tanks. Heterotrophic bacteria (Bacillus, Pseudomonas, Arthrobacter) oxidise dissolved organic carbon, amino acids, and complex polymers, converting them to CO₂, H₂O, and bacterial biomass: C + O₂ → CO₂ + H₂O (oxidation) Protozoa (ciliates like Tetrahymena, flagellates) consume these bacteria, reducing bacterial population and further stabilising organic matter. Continuous aeration provides dissolved oxygen (DO ≥ 2 mg/L) essential for aerobic microbial respiration. This biological process reduces soluble BOD from 350 ppm to ~25 ppm (92.9% reduction) in 6–8 hours of residence time. The microbial community adapts to waste composition, making secondary treatment highly efficient. Final effluent with BOD ≤ 30 ppm is safe for river discharge or tertiary treatment (if stringent standards required). **Q3. A biogas digester produces 50 m³ of biogas per day from cow dung with a methane content of 65%. If 1 m³ CH₄ releases 20 MJ energy, calculate daily energy output and compare with equivalent fossil fuel (assume 1 litre diesel = 45 MJ). What are the environmental and economic benefits of this biogas system?** **Answer:** *Energy calculation:* Biogas produced = 50 m³ per day Methane content = 65% Volume of CH₄ = 50 × 0.65 = **32.5 m³** Energy released = 32.5 m³ × 20 MJ/m³ = **650 MJ per day** Equivalent diesel required = 650 MJ ÷ 45 MJ/litre = **14.4 litres diesel per day** Annual equivalent = 14.4 × 365 = **5,256 litres diesel per year** *Environmental benefits:* (1) Replaces fossil fuel combustion, reducing greenhouse gas emissions (CO₂ equivalent); methane is captured for controlled combustion rather than released as potent greenhouse gas from manure piles. (2) Anaerobic digestion destroys pathogenic bacteria and viruses, reducing disease transmission. (3) Digestate (residual sludge) is nutrient-rich fertilizer, eliminating need for chemical fertilizers, thus reducing agricultural pollution. (4) Reduces pressure on forests for firewood and coal mining. (5) One 50 m³/day biogas plant replaces ~5,256 litres diesel annually—equivalent to ~14 tonnes CO₂ reduction. *Economic benefits:* (1) Renewable energy independence—rural farmers avoid diesel price fluctuations (diesel cost ~₹100/litre in India; annual fuel cost saved = ₹5,25,600). (2) Zero-cost feedstock: cow dung is waste byproduct. (3) Income generation: excess digestate sold as organic manure (₹500–₹1000/tonne), generating additional revenue. (4) Initial capital cost (~₹2–₹3 lakhs for 50 m³ plant) recovered in 3–4 years through fuel savings. (5) Government subsidies (MNRE, state schemes) reduce initial investment. (6) Creates local employment for plant operation and maintenance. This model is especially valuable for dairy farms and agricultural communities, supporting livelihoods while addressing energy poverty.

**Case Study: Rural Biogas and Sewage Treatment Integration** A small village (population 5,000) faces two problems: (1) inadequate waste management—daily sewage discharge into a nearby water body; (2) energy scarcity—frequent power cuts affecting households and schools. The village council proposes a dual-system solution: an anaerobic biogas plant processing cow dung and organic waste, plus a municipal sewage treatment plant with biogas co-digestion. **Scenario Details:** - Daily cow dung available: 2,000 kg (500 dairy cattle) - Daily sewage produced: 10,000 litres (2 litres per person) - Current sewage BOD: 400 ppm - Biogas yield from cow dung: 200 litres/kg dry dung (~10% dry matter in fresh dung = 20,000 litres raw dung ÷ 10 × 200 = ~400 m³ biogas/day) - Methane content: 65% - Energy from 1 m³ CH₄: 20 MJ **Step 1: Analyze the biogas production potential from cattle waste alone.** Daily fresh dung = 2,000 kg Dry matter (DM) in dung ≈ 20% (assumption for cattle dung) = 2,000 × 0.20 = 400 kg DM Biogas yield = 400 kg DM × 200 litres/kg = 80,000 litres = 80 m³ Methane volume = 80 × 0.65 = 52 m³ Daily energy from methane = 52 × 20 MJ = **1,040 MJ** **Step 2: Calculate additional biogas from sewage sludge co-digestion.** Daily sewage = 10,000 litres = 10 m³ Organic solids in sewage ≈ 200 kg/day (typical municipal sewage ~20 g/litre) Biogas from sludge ≈ 200 kg × 0.15 m³/kg (lower yield than pure dung) = 30 m³ Methane from sludge = 30 × 0.65 = 19.5 m³ Energy from sludge methane = 19.5 × 20 MJ = 390 MJ **Step 3: Estimate total energy output and household benefit.** Total daily biogas = 80 + 30 = 110 m³ Total daily methane = 52 + 19.5 = 71.5 m³ Total daily energy = 1,040 + 390 = **1,430 MJ** Assuming average household cooking energy need ≈ 100 MJ/day (LPG equivalent ~2.5 kg): Households powered = 1,430 ÷ 100 = **~14 households fully supplied daily** With population 5,000 (~1,000 households), system provides 1.4% of total energy—modest but significant for schools, health centers, street lighting. **Step 4: Evaluate sewage treatment efficiency and microbial roles.** Sewage treatment stages with microbial breakdown: - *Primary (physical)*: BOD remains ~400 ppm (settling only) - *Secondary (biogas co-digestion tank)*: Anaerobic microbes (Bacteroides, Clostridium, Methanobacterium) break down sludge for biogas; BOD in liquid phase: bacterial decomposition reduces soluble BOD by ~40% = remaining BOD ~240 ppm - *Tertiary (polishing lagoon or constructed wetland)*: Aerobic bacteria and plants further reduce BOD to ~30 ppm; final effluent safe for irrigation or discharge. **Step 5: Assess environmental and socioeconomic impact.** *Environmental:* - Daily methane capture (71.5 m³) prevents equivalent release from manure decomposition; methane global warming potential 28–34× CO₂, so ~1.5–2 tonnes CO₂e avoided daily - Sewage treatment removes pathogenic bacteria (E. coli, Vibrio, Salmonella), preventing waterborne disease outbreaks - Nutrient recovery: digestate contains N (2%), P (1%), K (0.5%), reducing chemical fertilizer demand *Socioeconomic:* - Diesel replacement: ~1,430 MJ ÷ 45 MJ/litre ≈ 32 litres diesel-equivalent daily; annual diesel cost saved = 32 × 365 × ₹100 = ~₹11.68 lakhs - Health: clean water reduces diarrhoea, typhoid, hepatitis cases; lower health costs - Employment: plant operation, maintenance, digestate marketing creates 5–10 jobs - Women's dignity: toilets connected to treatment plant improve sanitation access - Farmer income: digestate sale (~200 tonnes/year at ₹800/tonne) generates ₹1.6 lakhs additional revenue **Step 6: Identify challenges and propose solutions.** *Challenges:* - Seasonal variation: cattle numbers and dung availability fluctuate - Methane slip: gas losses reduce energy recovery - Toxic compounds: if agricultural runoff enters plant, ammonia/H₂S buildup inhibits microbes *Solutions:* - Co-substrate mixing: add food waste, agricultural residue to stabilise feed - Gas capture: sealed digesters with condensate recovery; flare excess gas safely - Pretreatment: screening and primary settling to remove toxic, non-degradable matter - Microbe adaptation: seed digester with consortia adapted to local feedstock **Conclusion:** The integrated biogas-sewage treatment system transforms waste into energy and clean water, demonstrating circular economy principles. While 1.4% energy supply seems modest, the ancillary benefits—disease prevention, improved soil health, livelihood creation—justify investment. This model is replicable in 50,000+ villages across India lacking grid electricity and sanitation infrastructure.

At cbsetutor.ai, we've engineered an AI tutor designed specifically for Class 9 CBSE Biology mastery. Here's how we drill these 18 Microbes in Human Welfare questions and patterns daily: **Adaptive MCQ Drills:** Each session starts with 5–10 randomised 1-mark MCQs on Chapter 8 topics. The AI tracks your response time and accuracy; if you answer <80%, it immediately presents similar questions with scaffolded hints (e.g., "Think about temperature optima for lactobacillus") until you build fluency. Spaced repetition ensures questions resurface every 3–5 days to cement recall. **Guided Short-Answer Practice:** For 2-mark questions, our AI presents a question, allows you 3 minutes to type an answer, then compares your response against NCERT-aligned model answers using semantic analysis. It identifies gaps (missing process steps, vague explanations) and asks follow-up probes: "You mentioned activated sludge—which microbes are most active here?" This Socratic approach builds conceptual depth beyond rote memorization. **Worked Problem Solving:** For 3-mark and 5-mark questions, the AI breaks down each question into steps: (1) identify key concepts (e.g., fermentation, anaerobic conditions); (2) recall relevant equations (C₆H₁₂O₆ → 3 CH₄ + 3 CO₂); (3) apply logic to the specific context; (4) articulate full solution with reasoning. You solve alongside hints; the AI reveals each step gradually, teaching problem decomposition. **Case-Study Simulations:** Weekly, the AI presents Chapter 8 case scenarios (biogas plant efficiency, sewage treatment troubleshooting, food fermentation contamination) requiring multi-step reasoning. You propose solutions; the AI evaluates against board-standard rubrics and provides corrective feedback—exactly mirroring board examiner expectations. **Real-Time Error Correction:** Common mistakes are caught instantly: confusing aerobic vs. anaerobic processes, misremembering BOD reduction percentages, or forgetting microbial roles in sewage stages. The AI explains *why* your answer is incorrect and re-teaches the concept with fresh examples. **Board-Exam Simulation:** Monthly, you take a full Chapter 8 mock exam (18 questions mirroring 1-mark, 2-mark, 3-mark, 5-mark distribution) under 90-minute timed conditions. The AI auto-grades, generates a diagnostic report (strengths, weaknesses, time-management gaps), and prescribes targeted drills to shore up weak areas before the real 2026 board exam. **Parent-Transparent Progress:** Your parents receive weekly reports showing mastery levels (% correct, average response time, concept gaps) in simple dashboard format. This ensures accountability and allows timely intervention if you're struggling. **Start a 3-day free trial at cbsetutor.ai** to experience this personalized AI drilling—no credit card needed, full access to Chapter 8 and all Class 9 Biology chapters.

Chapter 8: Microbes in Human Welfare is high-weightage (8–12 marks) and tests both memory and application. The 18 questions in this guide cover all expected question types and difficulty levels. *Key concepts to lock in:* (1) **Household fermentation:** lactobacillus (yogurt, curd), Saccharomyces cerevisiae (bread), LAB (pickles)—know optimal temperatures and chemical products. (2) **Industrial scale:** penicillin from Penicillium notatum, antibiotics from Streptomyces, vaccine production. (3) **Sewage treatment:** three stages (primary, secondary, tertiary), microbial roles (bacteria oxidise organic matter, protozoa control bacteria), BOD reduction %, activated sludge composition. (4) **Biogas:** anaerobic decomposition equation (C₆H₁₂O₆ → 3 CH₄ + 3 CO₂), methanogenic archaea (Methanobacterium), composition (65% CH₄, 35% CO₂), energy yield, advantages (renewable, reduces emissions, fertilizer byproduct). Practice these 18 questions until you can answer each without hesitation, explain reasoning for wrong options, and apply concepts to novel scenarios. Board examiners often ask unfamiliar cases (e.g., "A biogas plant's methane yield dropped—suggest causes"); mastering fundamentals and worked examples ensures you can navigate such questions confidently. Aim for 90%+ accuracy on 1-mark and 2-mark questions, and full-mark solutions on 3-mark and 5-mark extended responses.