Class 9 Science Chapter 13 Wastewater Story MCQ with Answers – Complete Quiz Bank

The rationalized CBSE Class 9 Science curriculum has shifted towards objective-type questions, with MCQs forming 40–50% of term assessments and annual board exams. Unlike descriptive answers, MCQs test conceptual precision, quick recall, and the ability to eliminate distractors—skills that reflect real scientific thinking. For Chapter 13: Wastewater Story, the CBSE focuses on application-based MCQs that connect textbook knowledge to everyday sanitation challenges. A single MCQ on sewage treatment might test: (1) identification of biological oxygen demand (BOD) as a sewage quality indicator, (2) understanding the role of aerobic bacteria in activated sludge treatment, or (3) distinguishing between primary and secondary treatment stages. Students who practice varied MCQ formats—single-answer, assertion-reason, and scenario-based—develop the agility to score consistently across different exam papers. The 30 MCQs in this quiz bank are calibrated to NCERT standards and include concept-mapping explanations so you don't just choose the right answer, but understand why other options fail. This is the foundation for 85+ performance in Class 9 Science assessments.

**Question 1:** Which of the following is NOT a source of sewage in a household? (A) Bathrooms and kitchens (B) Laundry water (C) Rainwater from gutters (D) Faecal matter from toilets **Correct Answer:** (C) Rainwater from gutters **Reason:** Rainwater is surface runoff, not domestic sewage; sewage originates from water-using appliances and human waste only. **Question 2:** The primary purpose of a treatment plant is to: (A) Make sewage drinkable (B) Remove harmful substances and make it safe for discharge or reuse (C) Store sewage permanently (D) Convert sewage into fertilizer only **Correct Answer:** (B) Remove harmful substances and make it safe for discharge or reuse **Reason:** Treatment plants reduce contaminants and pathogens to safe levels, enabling environmental discharge or agricultural reuse. **Question 3:** Biological oxygen demand (BOD) measures: (A) The amount of water in sewage (B) The oxygen needed by microorganisms to decompose organic matter in sewage (C) The temperature of treated water (D) The colour of untreated sewage **Correct Answer:** (B) The oxygen needed by microorganisms to decompose organic matter in sewage **Reason:** BOD is a key indicator of organic pollution; higher BOD indicates more biodegradable waste requiring more oxygen for breakdown. **Question 4:** Which microorganism is MOST responsible for breaking down organic matter in activated sludge treatment? (A) Viruses (B) Aerobic bacteria (C) Parasitic worms (D) Algae **Correct Answer:** (B) Aerobic bacteria **Reason:** Aerobic bacteria thrive in aerated tanks and consume organic pollutants, forming the core of secondary treatment. **Question 5:** Grit and gravel are removed in which stage of treatment? (A) Primary treatment (B) Secondary treatment (C) Tertiary treatment (D) Disinfection **Correct Answer:** (A) Primary treatment **Reason:** Grit chambers and screens in primary treatment mechanically separate inert solids before biological treatment begins. **Question 6:** Untreated sewage released into water bodies causes: (A) Increase in dissolved oxygen (B) Eutrophication (algal blooms) and depletion of oxygen (C) Improvement in aquatic life (D) No environmental impact **Correct Answer:** (B) Eutrophication (algal blooms) and depletion of oxygen **Reason:** Excess nutrients in sewage trigger algal overgrowth, which consumes dissolved oxygen, killing aquatic organisms. **Question 7:** Which disease is directly caused by consuming contaminated water? (A) Typhoid (B) Diabetes (C) Asthma (D) Colour blindness **Correct Answer:** (A) Typhoid **Reason:** Typhoid is a waterborne bacterial infection (Salmonella typhi) transmitted through faecal-contaminated water. **Question 8:** The best housekeeping practice to reduce wastewater pollution is: (A) Dumping cooking oil in the sink (B) Disposing of medicines in toilets (C) Using biodegradable soaps and detergents (D) Releasing paint and chemicals into drains **Correct Answer:** (C) Using biodegradable soaps and detergents **Reason:** Biodegradable products break down naturally, reducing persistent pollutants in treated water and receiving ecosystems. **Question 9:** Chlorination in water treatment primarily serves to: (A) Remove all dissolved salts (B) Kill pathogenic bacteria and viruses (C) Increase water hardness (D) Add colour to water **Correct Answer:** (B) Kill pathogenic bacteria and viruses **Reason:** Chlorine is a strong oxidizing agent that disrupts microbial cell membranes, ensuring microbiological safety. **Question 10:** Septic tanks are commonly used in: (A) Large cities with centralized sewage networks (B) Rural areas and small towns without municipal treatment plants (C) Only hospitals (D) Swimming pools **Correct Answer:** (B) Rural areas and small towns without municipal treatment networks **Reason:** Septic tanks serve as decentralized treatment for areas where piped sewage infrastructure is unavailable.

**Question 11:** In the activated sludge process, the term 'sludge' refers to: (A) The inert sand and grit collected in primary treatment (B) A concentrated mass of microorganisms and organic particles that consume pollutants (C) The liquid effluent discharged from the plant (D) Chlorine-treated water ready for reuse **Correct Answer:** (B) A concentrated mass of microorganisms and organic particles that consume pollutants **Reason:** Activated sludge is a biologically active suspension; aeration keeps it in constant contact with incoming sewage, ensuring efficient degradation. **Question 12:** Which statement correctly describes the relationship between sewage composition and treatment method selection? (A) All sewage requires identical treatment regardless of origin (B) Treatment method is independent of pollutant composition (C) High BOD sewage requires extended aeration or additional secondary treatment stages (D) Industrial sewage needs no treatment before mixing with domestic sewage **Correct Answer:** (C) High BOD sewage requires extended aeration or additional secondary treatment stages **Reason:** BOD indicates organic loading; higher concentrations demand longer biological treatment to achieve discharge standards. **Question 13:** Tertiary treatment (polishing) removes: (A) All suspended solids and grit only (B) Residual nutrients, pathogens, and sometimes specific chemicals not addressed in primary and secondary stages (C) All dissolved salts from treated water (D) Chlorine added during disinfection **Correct Answer:** (B) Residual nutrients, pathogens, and sometimes specific chemicals not addressed in primary and secondary stages **Reason:** Tertiary treatment includes sand filters, membrane systems, or nutrient-removal technologies to meet stringent discharge or reuse standards. **Question 14:** The sludge produced during treatment must be handled carefully because: (A) It contains only inert minerals (B) It may harbour pathogens and heavy metals accumulated from sewage (C) It is sterile and safe to dispose anywhere (D) It has no agricultural value **Correct Answer:** (B) It may harbour pathogens and heavy metals accumulated from sewage **Reason:** Sludge concentrates contaminants; improper disposal risks soil and water contamination; proper composting or incineration is essential. **Question 15:** A waterborne disease outbreak in a town suggests: (A) The municipal water supply is untreated (B) Treatment plants are functioning optimally (C) Possible contamination of water due to inadequate treatment, distribution leaks, or sewage cross-connections (D) The town has no sanitation system **Correct Answer:** (C) Possible contamination of water due to inadequate treatment, distribution leaks, or sewage cross-connections **Reason:** Waterborne outbreaks indicate breach of the water safety chain—treatment failure, pipe cracks allowing sewage infiltration, or cross-connections between sewer and water lines. **Question 16:** Which practice is MOST effective in reducing disease transmission in areas with poor sewage infrastructure? (A) Boiling drinking water before consumption (B) Open defecation (C) Using hand pumps directly above pit latrines (D) Storing untreated sewage in open tanks **Correct Answer:** (A) Boiling drinking water before consumption **Reason:** Boiling kills most waterborne pathogens; this is a practical household intervention when municipal treatment is unavailable, until infrastructure improves. **Question 17:** The primary advantage of constructed wetlands for wastewater treatment is: (A) They require high energy input (B) They use natural biological and chemical processes, are cost-effective, and provide ecological benefits (C) They treat sewage in 1 hour only (D) They eliminate the need for any monitoring **Correct Answer:** (B) They use natural biological and chemical processes, are cost-effective, and provide ecological benefits **Reason:** Constructed wetlands employ wetland plants, soil microbes, and sediment to treat sewage naturally, reducing operational costs and creating wildlife habitats. **Question 18:** Reclamation of treated wastewater for agriculture requires: (A) No additional treatment (B) Primary treatment only (C) Tertiary treatment and pathogen removal to meet irrigation safety standards (D) Chlorine addition without any other process **Correct Answer:** (C) Tertiary treatment and pathogen removal to meet irrigation safety standards **Reason:** Reuse for crop irrigation demands stringent pathogen reduction (log reduction values for E. coli and viruses) to prevent crop and consumer contamination. **Question 19:** Which indicator best reflects the overall pollution level and biodegradability of sewage in a single test? (A) Colour of water (B) Odour intensity (C) Biological oxygen demand (BOD) (D) Temperature **Correct Answer:** (C) Biological oxygen demand (BOD) **Reason:** BOD quantifies organic pollution with precision; typical domestic sewage BOD is 200–300 mg/L; efficient treatment reduces it to <5 mg/L before discharge. **Question 20:** Sanitation coverage refers to: (A) The percentage of the population with access to basic sewage, toilet facilities, and safe waste disposal (B) Only the number of water treatment plants (C) The total volume of sewage produced annually (D) The colour and appearance of public toilets **Correct Answer:** (A) The percentage of the population with access to basic sewage, toilet facilities, and safe waste disposal **Reason:** Sanitation coverage is a UN Sustainable Development Goal (SDG-6); incomplete coverage in India remains a public health challenge linked to open defecation and disease prevalence.

**Question 21:** **Assertion (A):** Aeration in the activated sludge process is essential for treatment efficiency. **Reason (R):** Aeration provides dissolved oxygen required by heterotrophic bacteria to oxidize organic matter and generate ATP for their metabolic activities. (A) Both A and R are true, and R is the correct explanation of A (B) Both A and R are true, but R is NOT the correct explanation of A (C) A is true, but R is false (D) A is false, but R is true **Correct Answer:** (A) Both A and R are true, and R is the correct explanation of A **Reason:** Aeration directly enables aerobic biodegradation; without sufficient dissolved oxygen (typically maintained at 2–4 mg/L), heterotrophic bacteria cannot efficiently metabolize pollutants. **Question 22:** **Assertion (A):** Secondary treatment is more effective at removing pathogens than primary treatment alone. **Reason (R):** Secondary treatment employs biological processes (microbial competition and predation) that reduce pathogenic organism numbers, whereas primary treatment removes only settleable solids. (A) Both A and R are true, and R is the correct explanation of A (B) Both A and R are true, but R is NOT the correct explanation of A (C) A is true, but R is false (D) A is false, but R is true **Correct Answer:** (A) Both A and R are true, and R is the correct explanation of A **Reason:** Secondary treatment achieves 99–99.9% pathogen reduction through biological competition and environmental stress; primary treatment alone reduces pathogens by only 20–30%. **Question 23:** **Assertion (A):** Untreated sewage discharged into a river causes eutrophication, leading to fish kills. **Reason (R):** Sewage contains nitrogen and phosphorus; excessive nutrient input triggers algal blooms that consume dissolved oxygen during decomposition, creating anoxic (oxygen-free) zones. (A) Both A and R are true, and R is the correct explanation of A (B) Both A and R are true, but R is NOT the correct explanation of A (C) A is true, but R is false (D) A is false, but R is true **Correct Answer:** (A) Both A and R are true, and R is the correct explanation of A **Reason:** This is the classic eutrophication mechanism: nutrient enrichment → rapid algal growth → algal decay and oxygen depletion (biochemical oxygen demand spike) → fish mortality. **Question 24:** **Assertion (A):** Disposing of medicines and chemical cleaners down household drains is hazardous despite wastewater treatment. **Reason (R):** Many pharmaceuticals and synthetic chemicals are resistant to biological degradation in treatment plants and persist in treated effluent, contaminating receiving water bodies. (A) Both A and R are true, and R is the correct explanation of A (B) Both A and R are true, but R is NOT the correct explanation of A (C) A is true, but R is false (D) A is false, but R is true **Correct Answer:** (A) Both A and R are true, and R is the correct explanation of A **Reason:** Refractory compounds (drugs, antibiotics, endocrine disruptors) survive standard wastewater treatment, creating emerging contaminants in aquatic ecosystems; better housekeeping practices mandate proper medicine disposal centres. **Question 25:** **Assertion (A):** Sludge produced in treatment plants must undergo further treatment (composting or incineration) before disposal or agricultural use. **Reason (R):** Sludge accumulates pathogens, heavy metals, and persistent organic pollutants concentrated from the entire sewage stream, posing direct health and environmental risks if applied untreated to soil. (A) Both A and R are true, and R is the correct explanation of A (B) Both A and R are true, but R is NOT the correct explanation of A (C) A is true, but R is false (D) A is false, but R is true **Correct Answer:** (A) Both A and R are true, and R is the correct explanation of A **Reason:** Sludge is a concentration step; pathogens and contaminants are removed from liquid effluent and deposited in solids; improper sludge management transfers risks from water to soil and groundwater. **Question 26:** **Assertion (A):** Rural areas relying on groundwater without piped sewage systems face higher risk of waterborne diseases compared to urban areas with centralized treatment. **Reason (R):** Absence of adequate sanitation allows septic tank effluent and open defecation waste to seep into shallow aquifers; individual septic tanks lack the treatment capacity of municipal plants, and pathogen breakthrough is common. (A) Both A and R are true, and R is the correct explanation of A (B) Both A and R are true, but R is NOT the correct explanation of A (C) A is true, but R is false (D) A is false, but R is true **Correct Answer:** (A) Both A and R are true, and R is the correct explanation of A **Reason:** Rural contamination occurs because individual septic tanks (typical treatment depth ~1 m) allow pathogens to survive travel to groundwater within 30–50 m; municipal plants achieve multi-log pathogen removal before environmental discharge. **Question 27:** **Assertion (A):** Treated wastewater can be safely reused for agricultural irrigation without any tertiary treatment if the receiving soil is well-drained. **Reason (R):** Soil does not filter pathogens effectively; even well-drained soil allows pathogenic bacteria and viruses to contaminate crops and groundwater unless treated effluent meets WHO/national microbial standards (e.g., <10³ CFU/100 mL faecal coliforms). (A) Both A and R are true, and R is the correct explanation of A (B) Both A and R are true, but R is NOT the correct explanation of A (C) A is true, but R is false (D) Both A and R are false **Correct Answer:** (D) Both A and R are false **Reason:** Assertion is false; secondary-treated effluent without tertiary treatment and disinfection poses pathogen risks. Reason is partially true, but soil filtration depends on soil type and pathogen characteristics; tertiary treatment + disinfection is mandatory for irrigation reuse. **Question 28:** **Assertion (A):** Chlorination of treated wastewater is the final step to ensure microbial safety before discharge or reuse. **Reason (R):** Chlorine (Cl₂) oxidizes bacterial cell membranes and viral proteins, inactivating most pathogens, but residual chlorine (0.2–0.5 mg/L) persists to protect water during distribution, preventing post-treatment contamination. (A) Both A and R are true, and R is the correct explanation of A (B) Both A and R are true, but R is NOT the correct explanation of A (C) A is true, but R is false (D) A is false, but R is true **Correct Answer:** (A) Both A and R are true, and R is the correct explanation of A **Reason:** Chlorination achieves 2–3 log reduction of bacteria and 1–2 log reduction of viruses; residual chlorine (free chlorine 0.2–0.5 mg/L) is maintained during distribution to prevent secondary contamination through pipe cracks. **Question 29:** **Assertion (A):** Countries with lower sanitation coverage typically report higher incidence of diarrhoeal diseases, particularly among children under 5. **Reason (R):** Inadequate sewage infrastructure and open defecation allow faecal pathogens (bacteria: E. coli, Vibrio; protozoa: Cryptosporidium; viruses: Rotavirus, Norovirus) to contaminate water and food, causing infectious diarrhoea, which is especially dangerous for young children with developing immune systems. (A) Both A and R are true, and R is the correct explanation of A (B) Both A and R are true, but R is NOT the correct explanation of A (C) A is true, but R is false (D) A is false, but R is true **Correct Answer:** (A) Both A and R are true, and R is the correct explanation of A **Reason:** This is epidemiologically well-established; WHO data confirm that improving sanitation reduces diarrhoeal mortality by ~39%; mechanistic explanation correctly identifies pathogen transmission routes. **Question 30:** **Assertion (A):** Biodegradable detergents reduce water pollution more effectively than conventional (non-biodegradable) detergents in wastewater treatment systems. **Reason (R):** Biodegradable detergents are broken down by microorganisms in treatment plants and in the environment, whereas non-biodegradable (persistent) detergents resist microbial degradation, remain in treated effluent, persist in aquatic ecosystems, and cause foam and toxicity to aquatic life. (A) Both A and R are true, and R is the correct explanation of A (B) Both A and R are true, but R is NOT the correct explanation of A (C) A is true, but R is false (D) A is false, but R is true **Correct Answer:** (A) Both A and R are true, and R is the correct explanation of A **Reason:** Biodegradable detergents (linear alkyl sulphonates, LAS) are 90%+ degraded in treatment plants; non-biodegradable branched-chain detergents resist degradation and cause persistent foaming in rivers, reducing oxygen transfer and harming aquatic organisms.

**Trap 1: Confusing Primary and Secondary Treatment Roles** Students often think primary treatment removes all solids and secondary removes all dissolved pollutants. Reality: Primary (screening, grit removal, sedimentation) removes only 30–40% of BOD and settable solids; secondary (activated sludge, trickling filters) removes 80–95% of dissolved/colloidal organic matter. A question asking "Which stage removes most BOD?" has the answer **secondary**, not primary. Trap option: "Primary treatment removes all pollutants, so secondary is redundant." **Trap 2: Mistaking Pathogen Reduction Percentages** Each treatment stage has different log-reduction capacity. Primary: 1–2 log (90–99% reduction). Secondary: 2–3 log (99–99.9% reduction). Disinfection: adds 2–4 log. A trap option claims "Primary treatment alone achieves 99.9% pathogen removal"—false. Always verify: the question asks for total removals or individual stage performance? Start a 3-day free trial at cbsetutor.ai to practice tier-wise MCQs that clarify these distinctions. **Trap 3: Misunderstanding BOD Levels** A trap option states "Treated wastewater has BOD of 200 mg/L" (similar to raw sewage). Reality: Raw domestic sewage: 200–300 mg/L. Secondary-treated: <10 mg/L. Discharge standard: <5 mg/L. If a question shows BOD of 150 mg/L for treated water, it's inadequately treated. **Trap 4: Confusing Reuse and Discharge Standards** Treated water for discharge to rivers has different microbial limits than water for irrigation. Discharge: 10³ CFU/100 mL faecal coliforms acceptable in some regions. Irrigation: <10² CFU/100 mL (stricter). A question mixing these standards is testing your knowledge of guidelines. **Trap 5: Assuming All Sewage Requires Identical Treatment** Industrial sewage (high heavy metals, recalcitrant chemicals) and domestic sewage (high BOD, pathogens) require different treatment sequences. A trap says "Standard activated sludge treats all types equally." False. Industrial sewage may need chemical pretreatment or extended biological stages. **Trap 6: Ignoring the Sludge Treatment Chain** Raw sludge from treatment is NOT ready for land application. It must undergo composting, incineration, or anaerobic digestion to reduce pathogens and stabilize organic matter. A question stating "Sludge from secondary clarifiers is safe for direct agricultural use" is false and tests this nuance. **Trap 7: Confusing Tertiary Treatment with Disinfection** Tertiary (sand filters, membrane filtration, nutrient removal) focuses on residual organic/nutrient pollutants. Disinfection (chlorine, UV, ozonation) targets microbes. They are complementary, not interchangeable. A trap option claims "Tertiary treatment kills all pathogens"—incorrect; disinfection is the primary pathogen-removal step. **Trap 8: Overlooking Anaerobic vs. Aerobic Processes** Activated sludge is aerobic; anaerobic digesters are separate units for sludge stabilization. A question asking "Which process in activated sludge produces biogas?" is misleading; biogas comes from anaerobic digestion of sludge, not from the main aeration basin. The trap is conflating two distinct treatment stages.

**Pre-Exam Preparation (2–3 weeks before):** Set a daily MCQ target: 5–7 questions from this bank daily, focusing on one difficulty tier per day (Week 1: Easy, Week 2: Medium, Week 3: Hard/Assertion-Reason). Time yourself: 1 minute per question (average). Note recurring trap options and concept gaps. Create a personal error log (e.g., "I always confuse primary vs. secondary treatment") and revisit those sections. **During the Exam (60-minute Science paper with ~40 MCQs):** **Minute 0–2:** Skim all MCQs. Identify easy ones (usually definitions, direct facts) and hard ones (assertion-reason, multi-step reasoning). Do NOT start reading hard questions yet. **Minute 2–30:** Answer all easy questions (typically 10–12 questions) at 1 minute per question. Build confidence and secure 40–50% marks. **Minute 30–50:** Tackle medium-difficulty questions (12–15 questions). For each: read the question → eliminate 2 obviously wrong options → choose between remaining 2. Spend max 1.5 minutes per question. **Minute 50–58:** Attempt assertion-reason and complex scenario MCQs (5–8 questions). Read carefully. Ask: "Is A true? Is R true? Does R explain A?" Spend 2 minutes per question. **Minute 58–60:** Review skipped questions or flagged uncertain answers. Do NOT change answers unless you have strong reason; first instinct is often correct. **Strategy for Assertion-Reason MCQs:** 1. **Evaluate A independently:** Is this statement factually correct per NCERT? 2. **Evaluate R independently:** Is this reasoning/explanation correct? 3. **Check causality:** Does R logically explain why A is true? Only select (A) if all three are yes. If A is true but R is false or doesn't explain, mark (B). If A is false, mark (C) or (D) depending on R. **Strategy for Scenario-Based MCQs:** Read the scenario first (e.g., "A town experiences a diarrhea outbreak after heavy rainfall"). Identify the hidden question: "What is the most likely cause?" (sewage contamination of water). Then match to options. Eliminate options that don't connect to the scenario. **Marking Scheme and Self-Assessment:** Each correct MCQ in CBSE papers typically carries 1 mark. No negative marking in most Class 9 term assessments (verify with your school). Aim for 35+/40 on this quiz bank (87.5% accuracy) before the exam. Use this benchmark: Scoring <75% = revisit medium MCQs; 75–85% = focus on trap options; 85%+ = attempt mock exams under timed conditions. **Day-of-Exam Mindset:** Remember: You are not expected to answer every question. Skipping 2–3 hard questions and scoring 95% on the rest (37–38/40) is a strong board-exam performance. Manage anxiety by breathing deeply before hard questions; move on if stuck >2 minutes.