Class 9 Science Chapter 13 Wastewater Story: Complete Important Questions & Solutions

Chapter 13 sits at the intersection of biology, chemistry, and public health—making it a favourite for mixed-format question papers. CBSE examiners reward: (1) correct naming and sequencing of treatment stages, (2) understanding of bacterial action in sewage decomposition, (3) ability to link poor sanitation to waterborne diseases like cholera and typhoid, and (4) practical knowledge of individual household measures (composting toilets, greywater recycling). In recent years, 1-mark MCQs have focused on identifying contaminants and primary pollutants; 2-mark questions test definitions of BOD (Biochemical Oxygen Demand) and sludge; 3-mark questions demand process diagrams or multi-step reasoning; and 5-mark questions ask for full treatment plant flow-charts with written explanations. Board examiners also favour scenario-based HOTS questions—for example, 'Why would a village near a river see increased waterborne diseases if the local factory discharges untreated effluent?'—requiring you to connect microbiology, ecology, and public policy. Mastering this chapter unlocks easy marks across the Life Processes and Natural Resources units.

**Question 1:** Which of the following is NOT a source of wastewater? (A) Household drainage (B) Industrial discharge (C) Agricultural runoff (D) Rainwater from clean roofs **Answer:** (D) Rainwater from clean roofs. NCERT defines wastewater as water contaminated with sewage (human waste), industrial chemicals, or excess fertilisers. Clean rainwater is not sewage. (Tip: Sources include homes, factories, hospitals, and farms—anything that adds dissolved solids, pathogens, or nutrients.) **Question 2:** What does BOD stand for in sewage treatment? (A) Bacterial Oxygen Demand (B) Biochemical Oxygen Demand (C) Biological Oxygen Depletion (D) Breakdown Oxygen Determination **Answer:** (B) Biochemical Oxygen Demand. BOD measures how much oxygen bacteria need to decompose organic matter in water. High BOD = polluted water with lots of decomposable waste. **Question 3:** Which microorganism is mainly responsible for breaking down organic matter in secondary treatment? (A) Algae (B) Bacteria (C) Fungi (D) Protozoa **Answer:** (B) Bacteria. Activated sludge (in aeration tanks) contains aerobic bacteria that consume organic compounds in wastewater and convert them to CO₂, water, and biomass. **Question 4:** Primary sludge removed in preliminary treatment contains mostly: (A) Dissolved salts (B) Oil and grease only (C) Sand, gravel, and suspended solids (D) Microbes **Answer:** (C) Sand, gravel, and suspended solids. Primary treatment uses screens and settling tanks to remove large insoluble particles before wastewater enters biological treatment. **Question 5:** Which disease is caused by drinking water contaminated with Vibrio cholerae bacteria? (A) Typhoid (B) Cholera (C) Dysentery (D) Hepatitis A **Answer:** (B) Cholera. The NCERT explicitly names Vibrio cholerae as the causative agent, transmitted through contaminated water in areas with poor sanitation and sewage treatment.

**Question 1:** Define sewage. Name any two sources of wastewater. **Answer:** Sewage is liquid waste produced by households, industries, and institutions—a mixture of human excreta, waste water from kitchens and bathrooms, and other discarded liquids. Two sources: (1) Domestic discharge from homes (toilets, showers, sinks); (2) Industrial effluent from factories and chemical plants. (Alternative: Agricultural runoff rich in fertilisers.) **Question 2:** What is the difference between primary and secondary treatment in a sewage plant? **Answer:** Primary treatment: Physical removal of large insoluble particles (sand, gravel, paper) using screens and settling tanks—does NOT kill pathogens. Secondary treatment: Biological degradation using aerobic bacteria in aeration tanks to decompose dissolved organic matter—reduces BOD by 80–90%. Both are essential; secondary treatment makes water safer for discharge or reuse. **Question 3:** List two waterborne diseases and their causative agents. **Answer:** (1) Typhoid—caused by Salmonella typhi (bacterium) found in faecal matter, spreads through contaminated food/water; (2) Cholera—caused by Vibrio cholerae (bacterium), spreads via contaminated drinking water in unhygienic conditions. [Alternatives: Dysentery (Shigella), Hepatitis A (virus), Polio (virus).] **Question 4:** How does a composting toilet help reduce water pollution? **Answer:** A composting toilet decomposes human faeces using bacteria and fungi without water, producing compost instead of liquid sewage. Advantages: (1) No faecal matter enters groundwater or sewage systems, preventing contamination; (2) Compost can enrich soil instead of burdening treatment plants; (3) Especially useful in areas without piped water or central sewage infrastructure. **Question 5:** What does 'sludge' mean in wastewater treatment? **Answer:** Sludge is the semi-solid material consisting of settled solids, microorganisms, and organic matter removed during treatment. Primary sludge (from settling tanks) is raw and may contain pathogens; secondary sludge (from aeration tanks) contains dead microbes. Sludge is further treated (anaerobic digestion, drying) before safe disposal or reuse as fertiliser.

**Question 1:** Draw a simple labelled diagram of a sewage treatment plant showing primary and secondary treatment stages. **Answer:** [Diagram: Box labelled 'Sewage Input' → 'Screen & Grit Removal' → 'Primary Settling Tank' (sludge removed at bottom) → 'Aeration Tank with Bacteria' (secondary treatment) → 'Secondary Settling Tank' → 'Treated Water Output' with 'Sludge to Digestion'. Labels: screens remove large objects; settling tanks separate insoluble solids; aeration tank provides oxygen for bacteria; secondary tank removes dead biomass.] **Key point:** Primary treatment is physical/chemical; secondary is biological (bacteria + oxygen break down dissolved organics). Marks awarded for correct sequence and labelling of both stages. **Question 2:** Explain why aeration (supply of oxygen) is essential in the secondary treatment of sewage. **Answer:** Aeration in secondary treatment provides oxygen for aerobic bacteria to thrive and break down dissolved organic matter (proteins, carbohydrates, fats) in wastewater. Without sufficient oxygen: (1) bacteria cannot respire efficiently, slowing decomposition; (2) anaerobic (oxygen-free) conditions favour growth of foul-smelling gas-producing organisms; (3) BOD remains high, making treated water unsafe for discharge. Aeration tanks with diffusers or mechanical stirring ensure bacteria remain active and wastewater is purified to acceptable standards (BOD reduced to <5 mg/L). **Question 3:** How does poor sanitation in a village lead to the spread of waterborne diseases? Explain with an example. **Answer:** Poor sanitation allows human faecal matter containing pathogens to contaminate water sources. Example: If pit latrines or open defecation is practised near a well or stream, bacteria like Salmonella typhi (typhoid) and protozoa like Entamoeba histolytica (dysentery) seep into groundwater. People drinking from the well then ingest pathogens, causing diarreal disease and fever. Overcrowding without proper sewage treatment multiplies this risk. Solution: Built toilets, waste treatment plants, and clean water supply break the cycle. (Examiners reward: clear cause–effect chain + organism name + disease outcome.) **Question 4:** What are three household practices that reduce wastewater pollution? **Answer:** (1) **Use of composting toilets:** No water is needed; faeces decompose naturally to form compost, avoiding sewage system overload and groundwater contamination. (2) **Segregation of greywater:** Wastewater from showers and kitchens (greywater) can be treated on-site and reused for gardening or toilet flushing, reducing fresh water demand and sewage volume. (3) **Safe disposal of hazardous household waste:** Medicines, oils, and chemicals should not be poured down drains; instead, they should be collected and disposed of through municipal hazardous waste schemes to prevent contamination of treatment plants and receiving waters. These practices are especially valuable in areas with inadequate central sewage infrastructure.

**Question 1:** Describe the complete process of sewage treatment in a municipal treatment plant, including all major stages and the purpose of each. **Full Solution:** A municipal sewage treatment plant operates in three main stages: **1. Primary Treatment (Physical Removal):** Wastewater enters through screens that remove large insoluble objects (plastic, paper, rags). It then flows into a grit chamber where sand and gravel settle. Finally, in primary settling tanks, remaining suspended solids settle as 'primary sludge' (removed at the base); grease floats and is skimmed off. Purpose: Remove 40–50% of suspended solids, reduce load on biological treatment. Sludge is sent to digestion. **2. Secondary Treatment (Biological Degradation):** Clarified water from primary settlement enters aeration tanks containing 'activated sludge'—a mixture of aerobic bacteria and protozoa. Air is pumped in (via diffusers or mechanical stirring) to provide oxygen. Bacteria consume dissolved organic matter (proteins, carbohydrates, fats), converting them to CO₂, water, and biomass over 4–8 hours. This reduces BOD by 80–90%. The mixture then flows into secondary settling tanks where dead biomass (secondary sludge) settles and is partly recycled back to aeration tanks (to maintain bacterial population) and partly sent to digestion. **3. Sludge Treatment (Disposal):** Primary and secondary sludges are combined and sent to anaerobic digesters (oxygen-free tanks). Anaerobic bacteria break down remaining organics over 20–30 days, producing biogas (methane + CO₂—used for plant energy) and sterile, nutrient-rich solid digestate. Digestate is dried and used as fertiliser or landfilled. **Final Output:** Treated water (with BOD <5 mg/L and <200 CFU/100 mL faecal coliforms) is discharged safely into rivers or reused for irrigation/toilet flushing. **Why each stage matters:** (1) Primary removes large solids that would clog pipes and biological reactors. (2) Secondary removes dissolved pollutants and kills many pathogens through bacterial action and retention time. (3) Sludge treatment prevents hazardous waste disposal and recovers energy/nutrients. Together, they reduce pollution by >95%. **Question 2:** A coastal town suffers frequent cholera outbreaks. Analyse the likely causes and propose solutions linking sewage, water supply, and household practices. **Full Solution:** **Likely Causes:** 1. **Untreated sewage discharge into sea:** If the town discharges primary-treated or untreated sewage into the ocean, Vibrio cholerae bacteria (which live naturally in marine environments) multiply, especially during warm seasons. 2. **Contamination of drinking water supply:** Faecal matter from open defecation or leaking sewage pipes contaminates wells or communal water taps. Chlorination failures or absence of treatment allow pathogens to reach consumers. 3. **Poor hygiene practices:** Inadequate handwashing after toilet use, eating food contaminated with sewage-tainted water, and poor kitchen sanitation spread the pathogen. **Proposed Solutions:** 1. **Upgrade sewage treatment:** Install/improve secondary treatment plants to reduce BOD and eliminate >99.9% of faecal bacteria before discharge. Use UV or chlorination as tertiary treatment for pathogen kill. 2. **Secure water supply:** Pipe clean water from protected sources; chlorinate and test regularly (weekly) for faecal coliforms (target: <1 CFU/100 mL). Install household water storage tanks with lids to prevent re-contamination. 3. **Promote household sanitation:** Build individual flush toilets or composting toilets for all households to prevent open defecation. Provide soap and handwashing facilities at schools and markets. Educate on boiling drinking water in endemic seasons. 4. **Monitoring:** Set up surveillance to track cholera cases and test water sources forVibrio cholerae weekly during high-risk seasons. **Expected outcome:** Within 6–12 months, combined infrastructure + behaviour change should reduce incidence by >80%, as demonstrated in cities like Dhaka and Kolkata. **Question 3:** Explain how bacterial action in activated sludge achieves wastewater purification. What conditions must be maintained, and why do plants fail if these conditions are not met? **Full Solution:** **Mechanism of Bacterial Purification:** Activated sludge is a consortium of aerobic heterotrophic bacteria (e.g., Zoogloea, Bacillus) and protozoa (e.g., Tetrahymena). These organisms respire using dissolved organic matter in sewage as an energy source: Organic matter + O₂ → CO₂ + H₂O + Energy (for bacterial growth) + Dead biomass Examples of substrates broken down: - **Proteins** → amino acids → NH₄⁺ (ammonia, nitrified to NO₃⁻) - **Carbohydrates** → glucose → CO₂ + H₂O - **Lipids** → fatty acids → oxidised to CO₂ Protozoa (ciliates) graze on suspended bacteria, further clarifying water and stabilising the biomass. Over 4–8 hours of aeration, BOD drops from ~300 mg/L (raw sewage) to ~5 mg/L (treated water). **Essential Conditions:** 1. **Dissolved oxygen ≥2 mg/L:** Aeration tanks must maintain minimum oxygen concentration using diffusers, surface aerators, or bubble columns. If O₂ falls <1 mg/L, aerobic bacteria cannot respire; anaerobic organisms take over, producing H₂S (rotten-egg smell) and methane instead of CO₂—wastewater remains foul and unpurified. 2. **Temperature 20–30 °C:** Bacterial enzymes work optimally in this range. Winter shutdown in cold climates slows decomposition (doubling treatment time); summer peaks require more aeration. 3. **pH 6.5–8.0:** Extreme pH inhibits enzyme activity. Industrial effluent (acids or alkalis) must be neutralised before reaching the plant. 4. **Nutrients (N, P):** Bacteria need nitrogen and phosphorus at ~C:N:P = 100:5:1 (in terms of organic substrate). Nitrogen-poor sewage (e.g., from homes with little protein waste) may need supplementation. 5. **Adequate retention time (4–8 hours):** Plants that rush sewage through too quickly don't give bacteria time to consume dissolved organics; treated water remains high in BOD. 6. **Stable mixed liquor:** The activated sludge concentration (typically 1500–4000 mg/L) must be maintained by balancing return of settled sludge to aeration tanks and removal of excess sludge for treatment. **Why Plants Fail If Conditions Are Not Met:** - **Low oxygen:** Anaerobic sludge becomes foamy, floats, escapes treatment tanks, and pollutes receiving waters with raw organic matter and foul gases. - **Cold temperature:** Bacteria become dormant; BOD removal drops to 20–30% instead of 80–90%. Winter sewage in northern India may be discharged inadequately treated. - **Wrong pH:** Acid rain or industrial discharge can kill beneficial bacteria, collapsing the microbial ecosystem and halting purification. - **Low nutrients:** Nitrogen-deficient sewage limits bacterial growth; decomposition slows, treatment fails. - **Short retention time:** Overloading a plant (increasing flow without adding more aeration tanks) leaves sewage inadequately treated. - **Sludge washout:** If return-sludge is not properly controlled, the bacterial population declines; the plant 'washes out' and stops working. **Real example:** During the 2015 monsoon, heavy rainfall diluted sewage and cooled aeration tanks in Delhi; BOD removal fell from 85% to 50%, and downstream water quality deteriorated. Quick response: reduced inflow, increased aeration, allowed warm weather to recover bacterial activity. **Conclusion:** A sewage plant is a living system; it must be operated within design parameters. Any failure to maintain O₂, temperature, pH, or sludge balance cascades into ecosystem collapse and loss of purification—making operation and monitoring as critical as infrastructure itself.

**Case Study: Monsoon Contamination in Rural Wells** A village of 5,000 people in rural Madhya Pradesh relies on hand-pumped tube wells for drinking water. During the monsoon, three wells simultaneously showed high turbidity (cloudy water) and positive faecal coliform tests (>100 CFU/100 mL). Health authorities reported a spike in acute diarrhoeal disease affecting 120 children <5 years in the following week. Investigation revealed a recently constructed community pit latrine 15 metres upslope from the wells, and a nearby small dairy farm with uncontained animal waste. **Questions:** (a) Explain how monsoon rains could have contaminated the tube wells with faecal coliforms despite the wells being 15 m away from the latrine. (b) Why are faecal coliforms used as an indicator of sewage contamination rather than counting all bacteria? (c) The health team proposed three interventions: (i) disinfection of wells, (ii) construction of a piped water network with treatment, (iii) promotion of household-level treatment (boiling, solar disinfection). Rank these by immediate effectiveness vs. long-term sustainability. (d) What role does wastewater treatment in this scenario—is it sufficient to solve the problem? Why or why not? **Detailed Solution with Steps:** **(a) Mechanism of Groundwater Contamination During Monsoon:** **Step 1:** Understand groundwater flow. Monsoon rainfall raises the water table. The tube wells draw water from the uppermost aquifer layer (~10–20 m depth). **Step 2:** Pit latrine hydraulics. The pit latrine at 15 m upslope and 5–10 m higher elevation (sloped terrain) acts as a contamination source. During monsoon, the water table rises and approaches the pit; percolating rainwater creates a hydraulic gradient that forces faecal matter (containing Vibrio, Salmonella, Entamoeba cysts, and indicator bacteria like E. coli and Faecal Streptococcus) downward and laterally into the aquifer. **Step 3:** Microbial transport. Faecal coliforms are small enough (~0.5 μm) and can survive in soil water for weeks to months. They travel through sandy/silty soil 15 m in 2–4 weeks under high hydraulic gradient (monsoon conditions). The dairy farm's runoff (rich in animal faeces) accelerates this via preferential flow paths (cracks, macropores in the soil). **Conclusion:** Groundwater contamination is not prevented by horizontal distance alone; vertical separation (depth), hydrogeological properties (permeability), and water table dynamics matter critically. Monsoon elevated the water table, allowing pathogens to reach the well zone. **(b) Why Use Faecal Coliforms as Indicator Microorganisms:** **Step 1:** Definition. Faecal coliforms are thermotolerant bacteria (mainly E. coli and Klebsiella pneumoniae) that can survive at 44–45 °C—a temperature that specifically selects for organisms from warm-blooded animal intestines (humans, livestock). **Step 2:** Advantages over total bacterial count: - **Specificity:** Total bacteria (millions of species, mostly harmless soil organisms) are not diagnostic of sewage. Faecal coliforms are nearly always sewage-associated. - **Correlation with pathogens:** Presence of faecal coliforms strongly correlates (>90%) with presence of enteric pathogens (Salmonella, Shigella, Vibrio, viruses, parasites). Total bacteria count is unreliable. - **Easy, rapid testing:** Culture at 44 °C in 24 hours; cheaper and faster than culturing specific pathogens (which may take weeks). - **Standards:** WHO and Indian Standards (IS 10500) set faecal coliform limit at **0 CFU/100 mL** for drinking water—a simple, measurable threshold. **Practical relevance:** In the village scenario, finding >100 CFU/100 mL faecal coliforms immediately signals sewage contamination and justifies intervention (boiling, chlorination, well closure) without waiting for months to culture Salmonella or Vibrio cholerae. **(c) Ranking of Three Interventions:** | Intervention | Immediate Effectiveness | Long-Term Sustainability | Ranking | |---|---|---|---| | (i) Disinfection of wells (shock chlorination) | **Very high** — kills faecal coliforms in 24 hrs; water becomes safe in 48 hrs. Diarrhoea cases decline immediately. | **Very low** — source (latrine, dairy) is not fixed; recontamination occurs within days–weeks after chlorine decays. Requires repeated disinfection (unsustainable for a poor village). | **2nd (immediate)** | | (ii) Piped water network with central treatment plant | **Medium** (2–4 months to construct) — once functional, provides treated, chlorinated water with >99% pathogen removal; safe distribution via pipes prevents re-contamination. | **High** — capital cost recovered over 10–15 years; government subsidies, user fees make it financially sustainable. But requires trained operators and spare parts supply. | **1st (medium-to-long term)** | | (iii) Household-level treatment (boiling, solar disinfection) | **Medium** (immediate education, but behaviour change takes 3–6 months) — effective if adopted consistently (>98% pathogen kill via boiling); reduces disease burden quickly in early adopters. | **Medium** — depends on continuous household compliance and fuel availability (firewood cost, time burden). Solar disinfection is free but weather-dependent (fails in monsoon). | **2nd (behaviour-dependent)** | **Optimal strategy:** Combine (ii) + (iii). Implement piped water network (2–4 years timeline) as long-term solution while promoting boiling and solar disinfection immediately to prevent acute outbreaks. Meanwhile, (i) disinfection serves as emergency short-term bridge. **(d) Role of Wastewater Treatment—Is It Sufficient?** **Step 1:** Define the problem. The contamination pathway is: Pit latrine → groundwater → tube well. This is **point-source pollution** at origin (latrine waste in groundwater), not end-of-pipe pollution in a river or municipal network. **Step 2:** Why conventional wastewater treatment is insufficient: - Wastewater treatment plants (primary + secondary) treat sewage that has *already been collected and piped* to a central facility. - The village has **no sewerage network**—faeces enter the environment directly via pit latrines, never reaching a treatment plant. - Even if a treatment plant existed 50 km away (in a district town), it would not treat the contaminated groundwater aquifer already affected. **Step 3:** What is actually needed: 1. **Source control:** Upgrade pit latrines to pour-flush toilets with sealed septic tanks (prevent seepage). Design septic tanks with 1.5–2 m vertical separation from water table and >15 m horizontal distance from wells (ISI 2030). 2. **Aquifer protection:** Seal/relocate tube wells away from upslope contamination sources. Install protected hand pumps with sanitary platforms. 3. **Decentralised treatment (if sewage collection is eventual goal):** Small community-scale septic systems or constructed wetlands can treat wastewater on-site before it reaches groundwater. **Step 4:** Conclusion: Wastewater treatment is **not sufficient** for this scenario because the problem is uncontrolled wastewater disposal (open latrines) entering groundwater directly. Treatment plants are relevant *only if* a sewerage network is built to collect faeces and transport them to a treatment facility (a 10–15 year project and luxury in rural areas). The immediate and realistic solution is source protection (sealed toilets, well relocation, aquifer monitoring) and household-level disinfection. **Key lesson for examiners:** This question tests whether students understand that **treatment is downstream medicine**; upstream prevention (sanitation, hygiene, source control) is always cheaper and more effective in rural/low-income contexts.

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