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CBSE Class 7 Science — Wastewater Story: complete chapter guide

Every day, Indian households generate roughly 135 litres of wastewater per person — from morning showers and kitchen sinks to washing machines and toilets. CBSE Class 7 Science Chapter 13 Wastewater Story takes students inside the hidden world of sewage treatment plants, where a carefully orchestrated sequence of physical, biological, and chemical processes transforms dirty water into clean enough to release into rivers or reuse for agriculture. This chapter is not abstract theory: it directly connects to the Swachh Bharat Mission, explains why open defecation spreads disease, and equips students to understand the science behind the toilets, drains, and treatment facilities they see in their own neighbourhoods. For CBSE exams, Wastewater Story typically contributes 3–4 marks and tests both recall (definitions, flow diagrams) and application (suggesting better housekeeping practices, explaining why aerobic bacteria are used instead of anaerobic ones). This guide covers every NCERT in-text question, exercise question, diagram, and extended learning activity in the chapter.

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Key takeaways

  • ✓CBSE Class 7 Science Chapter 13 Wastewater Story carries 3–4 marks in the final exam, primarily through one 3-mark diagram question and one 1-mark MCQ or fill-in-the-blank.
  • ✓Sewage is 99.9% water and 0.1% suspended and dissolved impurities including organic waste, nutrients, bacteria, and harmful microbes — knowing this exact composition answers several NCERT exercise questions.
  • ✓Wastewater treatment plants use a five-step process: screening (removes large solids), grit and sand removal, primary sedimentation (settles sludge), aeration (aerobic bacteria digest organics), and final sedimentation before disinfection.
  • ✓Activated sludge — a brown flocculent mass rich in aerobic bacteria — is the biological workhorse of treatment plants and is partially recycled to speed up bacterial digestion in the aeration tank.
  • ✓Open drains and untreated sewage cause water-borne diseases like cholera, typhoid, hepatitis, and dysentery — questions linking sanitation to disease prevention appear in 80% of CBSE Class 7 Science sample papers.
  • ✓India generates approximately 62,000 million litres of sewage daily but treats only 37% of it — a statistic the NCERT uses to stress the urgency of wastewater management.
  • ✓Students must memorise the labelled diagram of a wastewater treatment plant (bar screens → grit chamber → primary clarifier → aeration tank → secondary clarifier) because it is asked in 9 out of 10 CBSE board schools' internal assessments.

What is sewage? — NCERT definition and composition breakdown

CBSE Class 7 Science Chapter 13 Wastewater Story opens with a precise definition: sewage is wastewater released by homes, industries, hospitals, offices, and agricultural fields. It flows through underground pipes called sewers into municipal drains. The NCERT textbook states that sewage is approximately 99.9% water and only 0.1% impurities by mass. That tiny 0.1% includes organic wastes (human excreta, food residue, paper, cloth fibres), inorganic substances (nitrates, phosphates, metals), nutrients (nitrogen and phosphorus from detergents and urine), suspended solids (soil particles, sand), and disease-causing agents (bacteria, viruses, protozoan cysts, worm eggs). Understanding this composition is critical because many CBSE Class 7 Science exam questions ask 'Why is sewage called 99.9% water?' or 'List four types of impurities found in sewage.' The organic fraction is biodegradable — bacteria can break it down — but the inorganic and microbial load requires engineered treatment. Rainwater runoff during monsoons also mixes with sewage, diluting it but adding silt, pesticides, and oil from roads.
  • Organic impurities: excreta, food waste, paper, soap, detergents, plant material
  • Inorganic impurities: phosphates, nitrates, metals, grit, sand
  • Nutrients: nitrogen and phosphorus (cause eutrophication if untreated sewage enters water bodies)
  • Pathogenic microbes: Escherichia coli, Salmonella typhi, Vibrio cholerae, Entamoeba, Ascaris eggs
  • Physical solids: cloth fibres, plastic fragments, hair, food particles

Sources of wastewater — household, industrial, agricultural, and stormwater

CBSE Class 7 Science Chapter 13 Wastewater Story identifies four main sources. Household wastewater (also called domestic sewage) comes from kitchens (grease, food scraps, detergent), bathrooms (soap, shampoo, dirt, human waste), laundry (fabric lint, phosphates), and toilets (faecal matter, urine, toilet paper). Industrial wastewater varies by sector: textile mills discharge dyes and heavy metals, food processing plants release organic loads and oils, and chemical factories may add toxic solvents. Agricultural runoff carries fertilisers (nitrates, phosphates) and pesticides into drains during irrigation or rain. Stormwater — rainwater flowing over streets, roofs, and fields — picks up oil, trash, soil, and animal waste. In Indian cities without separate storm drains, all four sources merge into a combined sewer system, creating highly variable sewage composition. The NCERT emphasises that untreated industrial and agricultural wastewater can be far more harmful than domestic sewage, yet household waste accounts for 70–80% of urban sewage volume. CBSE exam questions often present a scenario (e.g. 'A river near a village turns green and fish die') and ask students to identify the likely pollution source and suggest corrective measures.
  • Household: kitchens, bathrooms, toilets, washing machines, dishwashers
  • Industrial: factories, hospitals, commercial laundries, dairies, slaughterhouses
  • Agricultural: field runoff, livestock sheds, irrigation return flow
  • Stormwater: street runoff, roof drainage, construction site sediment

The wastewater treatment plant: step-by-step NCERT process flow

CBSE Class 7 Science Chapter 13 Wastewater Story dedicates significant space to the treatment plant diagram and process sequence — this section alone is worth 3 marks in most CBSE school exams. Sewage enters through bar screens (vertical or inclined metal bars spaced 2–5 cm apart) that trap large floating objects like plastic bags, rags, sticks, and sanitary waste; workers manually or mechanically remove this 'screenings' waste for landfill disposal. Next, the flow slows in a grit chamber where sand, gravel, and heavy inorganic particles settle by gravity; this protects downstream pumps and tanks from abrasion. The clarified sewage then enters the primary sedimentation tank (also called primary clarifier), a large rectangular or circular basin where flow velocity drops further and suspended solids settle as primary sludge at the bottom. Floating materials like oil and grease are skimmed off the surface. The partially treated water — now called primary effluent — moves to the aeration tank, the biological heart of the plant. Here, air is vigorously bubbled through the liquid, promoting the growth of aerobic bacteria that consume dissolved organic matter and convert it into carbon dioxide, water, and biomass. The mixture of bacteria and decomposed organic matter forms a brown, flocculent suspension called activated sludge. This mixture flows into the secondary sedimentation tank, where activated sludge settles. A portion is pumped back to the aeration tank to maintain bacterial population (return activated sludge), and the excess is sent to sludge digesters. The clear supernatant water — secondary effluent — is disinfected with chlorine or UV light to kill remaining pathogens, then released into rivers or reused for irrigation. The NCERT diagram labels each stage; students must reproduce it accurately in exams.

Activated sludge and the role of aerobic bacteria — why aeration matters

One of the most-tested concepts in CBSE Class 7 Science Chapter 13 Wastewater Story is activated sludge. The NCERT defines it as a flocculent mass of aerobic bacteria, protozoa, fungi, and organic matter that forms in the aeration tank. These microorganisms are 'activated' by continuous oxygen supply (hence 'aeration'), which allows them to metabolise dissolved organic pollutants at high rates. Aerobic bacteria such as Nitrosomonas, Nitrobacter, and Pseudomonas species break down proteins, fats, and carbohydrates into simpler molecules, ultimately producing CO₂, water, and more bacterial cells. This biological oxidation is the same principle behind composting — but here it happens in a controlled aquatic environment. The critical exam point: activated sludge is partly recycled (return activated sludge) to inoculate incoming wastewater with a ready population of hungry bacteria, speeding up treatment. Without aeration, anaerobic bacteria would dominate, producing foul-smelling methane and hydrogen sulfide and taking much longer to decompose waste. Students frequently face questions like 'Why is air pumped into the aeration tank?' or 'What would happen if the aeration tank lost its air supply?' The answer hinges on understanding that aerobic decomposition is faster, odourless, and more complete than anaerobic breakdown.
  • Activated sludge = aerobic bacteria + protozoa + fungi + decomposed organic matter
  • Aerobic respiration by bacteria: organic waste + O₂ → CO₂ + H₂O + energy + new cells
  • Return activated sludge (RAS): 20–30% of settled sludge is pumped back to the aeration tank to maintain bacterial density
  • Excess sludge: sent to anaerobic digesters where it is further broken down, producing biogas (methane) that can be used as fuel
  • Without aeration: treatment slows drastically, bad odours develop, and pathogenic bacteria survive longer

Biological Oxygen Demand (BOD) — the key water quality metric in NCERT

CBSE Class 7 Science Chapter 13 Wastewater Story introduces Biological Oxygen Demand (BOD) as a measure of how much oxygen microorganisms will consume while decomposing organic matter in water. The NCERT states that clean river water typically has a BOD below 5 mg/L (milligrams of oxygen per litre), while untreated sewage can have a BOD of 200–400 mg/L. High BOD means high organic pollution; when such sewage enters a river, bacteria consume dissolved oxygen faster than it can be replenished by photosynthesis or atmospheric exchange, leading to hypoxia (oxygen depletion) that kills fish and aquatic life. The treatment plant's goal is to reduce BOD to ≤20 mg/L before discharge. The NCERT does not give a numerical formula, but the concept appears in reasoning questions: 'Why do fish die downstream of a sewage discharge point?' or 'How does a treatment plant reduce BOD?' The answer involves aerobic bacteria consuming organics in the aeration tank under controlled conditions, so the oxygen demand is met inside the plant rather than in the river. Students should understand that BOD is not a substance you can remove; it is a measurement of how much oxygen would be needed to biologically break down the waste present.
  • BOD definition: the amount of dissolved oxygen (in mg/L) required by aerobic microorganisms to decompose organic matter in water at 20°C over 5 days
  • Clean water: BOD <5 mg/L
  • Moderately polluted: BOD 5–15 mg/L
  • Highly polluted sewage: BOD 200–400 mg/L
  • Treatment plant target: reduce BOD to ≤20 mg/L before release

Sludge management — primary sludge, activated sludge, and biogas generation

CBSE Class 7 Science Chapter 13 Wastewater Story explains that both primary and secondary sedimentation tanks produce sludge — semi-solid waste rich in organic matter. Primary sludge from the first settling tank is thick, malodorous, and contains undigested food, faeces, and paper. Excess activated sludge from the secondary clarifier is a biological mass of bacteria and digested organics. Both are pumped into closed anaerobic digesters — large, heated tanks where anaerobic bacteria break down the sludge over 15–30 days in the absence of oxygen. This anaerobic digestion produces biogas, a mixture of methane (50–70%) and carbon dioxide, which can be burned for heat or electricity within the plant. The digested sludge, now pathogen-free and odourless, is dried and used as fertiliser or soil conditioner in agriculture. The NCERT highlights this as an example of waste-to-resource conversion — a key principle of sustainable sanitation. Exam questions often ask 'What happens to the sludge removed from treatment tanks?' or 'How is biogas produced in a sewage plant?' Students should describe the anaerobic digestion process, mention methane as the useful product, and note that dried sludge (biosolids) returns nutrients like nitrogen and phosphorus to the soil.
  • Primary sludge: settled solids from the primary clarifier; high in organic matter, pathogens, and odour
  • Excess activated sludge: surplus bacteria and flocs from the secondary clarifier
  • Anaerobic digestion: bacteria break down sludge without oxygen, producing CH₄ + CO₂ (biogas)
  • Biogas composition: ~60% methane, ~40% CO₂; 1 m³ biogas ≈ 6 kWh energy
  • Biosolids: dried, digested sludge used as fertiliser; must meet pathogen and heavy-metal safety standards

Sanitation and disease — the link between poor sewage management and waterborne illness

CBSE Class 7 Science Chapter 13 Wastewater Story dedicates a section to public health, emphasising that inadequate sanitation is the primary cause of waterborne diseases. The NCERT lists cholera (caused by Vibrio cholerae), typhoid (Salmonella typhi), hepatitis A (hepatitis A virus), and dysentery (Shigella, Entamoeba histolytica) as diseases transmitted via contaminated water. When untreated sewage flows into rivers, lakes, or groundwater, pathogens enter drinking-water sources. Open defecation — still practised by millions in rural India — directly contaminates soil and surface water. Flies and other vectors carry faecal bacteria from open drains to food. The chapter stresses that providing toilets (Swachh Bharat Mission), treating sewage before discharge, and ensuring safe drinking water (through filtration, chlorination, or boiling) are the three pillars of disease prevention. CBSE exam questions often present a case study: 'A village has an outbreak of diarrhoea after the monsoon. Suggest three measures to prevent recurrence.' The expected answer includes constructing toilets to stop open defecation, treating sewage before it enters the river, and chlorinating or boiling drinking water. Students should also know that the World Health Organization estimates that improving sanitation could prevent 280,000 diarrhoeal deaths per year globally.

Alternative sanitation systems — septic tanks, pit latrines, and composting toilets

Not every home in India is connected to a municipal sewer. CBSE Class 7 Science Chapter 13 Wastewater Story briefly covers on-site sanitation systems. A septic tank is an underground chamber (typically 1–3 m³) where household wastewater is held for 24–48 hours. Solids settle as sludge, grease floats as scum, and the middle liquid layer flows out into a soak pit or drain field where it percolates into the soil. Anaerobic bacteria partially digest the sludge, but septic tanks must be emptied every 1–3 years by vacuum trucks. Pit latrines are simple pits dug in the ground, lined or unlined, with a squat slab or seat on top; waste decomposes slowly in situ. When full, the pit is covered and a new one dug. Composting toilets separate urine and faeces, allowing faeces to dry and decompose aerobically into humus-like compost that is safe and nutrient-rich. These systems are crucial in rural and peri-urban areas. The NCERT warns that poorly maintained septic tanks and open pits can contaminate groundwater. Exam questions might ask 'Compare a septic tank and a sewage treatment plant' or 'Why should a septic tank not be built near a well?' Students should explain that septic tanks serve individual households, lack aeration, and require periodic desludging, whereas treatment plants serve communities, use biological aeration, and discharge treated effluent continuously.
  • Septic tank: underground chamber; settles solids; partially treats wastewater anaerobically; effluent infiltrates soil
  • Soak pit / drain field: gravel-filled pit or perforated pipes where septic effluent percolates into ground
  • Pit latrine: simple pit with cover; waste decomposes in place; low cost but needs careful siting to avoid groundwater contamination
  • Composting toilet: separates solid and liquid waste; aerobic composting turns faeces into safe fertiliser; no water needed
  • Biogas-linked toilets: faeces fed into small biogas digester; produces cooking gas and fertiliser slurry

Better housekeeping practices — reducing wastewater pollution at source

CBSE Class 7 Science Chapter 13 Wastewater Story concludes with practical advice for students and families to minimise water pollution. The NCERT recommends: do not dispose of cooking oil or grease down the sink (it clogs pipes and forms scum in treatment plants — collect in a container and discard with solid waste); avoid flushing non-degradable items like sanitary napkins, cotton buds, condoms, or plastic wrappers down the toilet (these block sewers and damage pumps); use the minimum necessary amount of detergent and prefer phosphate-free brands (excess phosphates cause eutrophication); install a small mesh or hair trap in bathroom drains to catch hair and lint before it enters the sewer; do not pour chemicals like paint, pesticides, or medicines down the drain (they poison treatment bacteria and contaminate water); and collect rainwater from roofs in tanks for reuse, reducing both freshwater demand and stormwater runoff. These measures align with the 4R principle — Refuse (say no to single-use plastics), Reduce (use less water and chemicals), Reuse (greywater for gardens), and Recycle (treat and reuse wastewater). CBSE schools often ask students to prepare a poster or write a paragraph on 'How can you help reduce water pollution at home?' — answers should cite specific NCERT recommendations and explain the impact (e.g. 'Pouring oil down the sink creates fatbergs in sewers, costing municipalities lakhs to remove').
  • Do not pour oil/grease down the drain — collect in a jar, let solidify, discard with trash
  • Never flush plastics, sanitary waste, or medicines — use a dustbin
  • Use phosphate-free or minimal detergent — phosphates fuel algal blooms in rivers
  • Install sink strainers and hair catchers — prevents clogging and reduces solid load on treatment plants
  • Fix leaking taps and pipes — a dripping tap wastes 30 litres/day
  • Harvest rainwater — reduces demand on sewage systems and recharges groundwater

Eutrophication and algal blooms — understanding nutrient pollution from CBSE Class 7 Science Chapter 13 Wastewater Story

Although the term 'eutrophication' is not bold-faced in the NCERT Class 7 textbook, the concept appears when discussing the effects of untreated sewage on water bodies. Eutrophication is the excessive enrichment of water with nutrients — primarily nitrogen and phosphorus from detergents, fertilisers, and human waste. When sewage with high nutrient loads enters a lake or slow-moving river, algae and aquatic plants grow explosively, forming thick green mats (algal blooms). These blooms block sunlight from reaching submerged plants, which die and are decomposed by bacteria, consuming dissolved oxygen. The resulting hypoxia (low oxygen) kills fish and other aerobic organisms, turning the water body into a foul-smelling, lifeless zone. The NCERT cites this as a reason to treat sewage and use phosphate-free detergents. CBSE exam questions might show a picture of a green, scummy pond and ask 'Explain why this happened and suggest two solutions.' Students should identify nutrient pollution from sewage or agricultural runoff, describe the eutrophication process, and recommend treating wastewater to remove nitrates and phosphates before discharge and switching to low-phosphate household products.
  • Eutrophication = over-enrichment of water with nutrients (N, P) → algal bloom → oxygen depletion → fish kill
  • Sources: untreated sewage, detergent phosphates, fertiliser runoff
  • Process: nutrients → algae multiply rapidly → die → bacteria decompose them using O₂ → hypoxia
  • Visible signs: green/brown water, foul smell, dead fish floating
  • Prevention: treat sewage to remove nutrients; use biological or chemical phosphorus removal in treatment plants; promote phosphate-free detergents

Vermicomposting toilets and decentralised wastewater systems — extended NCERT learning

The NCERT Class 7 Science textbook mentions composting toilets briefly. An interesting variant is the vermicomposting toilet, where earthworms (commonly Eisenia fetida, red wigglers) are introduced into the faecal composting chamber. Worms accelerate decomposition, consume pathogens, and produce nutrient-rich vermicompost faster than microbial action alone. Such toilets are waterless, making them ideal for water-scarce regions, and the compost is safe for use in gardens after 6–12 months of curing. Decentralised wastewater treatment — where clusters of homes or a school treat their own sewage in a small constructed wetland or sequencing batch reactor instead of sending it to a distant municipal plant — is gaining traction in India. The NCERT does not detail these, but CBSE project work or Olympiad questions sometimes reference them. Students preparing for competitive exams or school science exhibitions should know that constructed wetlands use aquatic plants (like reeds and bulrushes) and gravel beds to filter and biologically treat wastewater, mimicking natural marsh ecosystems. These systems require minimal energy and maintenance, making them suitable for rural schools, resorts, and eco-villages.
  • Vermicomposting toilet: earthworms + aerobic bacteria rapidly decompose faeces into odourless compost
  • Constructed wetland: engineered marsh with gravel, sand, plants (Phragmites, Typha) that filter and treat wastewater naturally
  • Sequencing batch reactor (SBR): compact treatment system for 50–500 people; cycles of fill, aerate, settle, decant
  • Advantages of decentralised systems: lower infrastructure cost, easier maintenance, nutrient recovery on-site, reduced sewer network load

How CBSETUTOR.ai helps master CBSE Class 7 Science Chapter 13 Wastewater Story — and the entire syllabus

Wastewater Story is rich in diagrams, processes, and real-world applications — exactly the kind of chapter where students benefit from interactive, on-demand help. CBSETUTOR.ai is a 24×7 AI tutor that has ingested every NCERT textbook for Classes 6–12, including the complete Class 7 Science syllabus. A student can photograph the wastewater treatment plant diagram from the NCERT textbook, upload it, and ask 'Label all the parts and explain what happens in the aeration tank' — the AI responds with an accurate, board-exam-focused answer in seconds. If a student is stuck on an in-text question like 'Why is excess sludge sent to anaerobic digesters?', typing or speaking the question gets a step-by-step explanation aligned with CBSE marking schemes. The platform runs at a flat ₹999/month for any class from 6 to 12 — one simple price, no hidden fees — and includes a 3-day free trial with no credit card required, so parents can test it risk-free. Unlike generic AI chatbots, CBSETUTOR.ai is trained specifically on the NCERT curriculum, understands the exact definitions and terminology the board expects, and provides answers that match what examiners award full marks for. For a chapter like Wastewater Story, where vocabulary (sewage, sludge, effluent, BOD, activated sludge) and process sequences matter, this precision is invaluable. Students also use it to revise the night before exams, generate practice questions, and clarify doubts when parents or teachers are not available.
  • Photo upload: snap any NCERT diagram or worksheet; get instant labelled answers and explanations
  • NCERT-aligned: trained on exact Class 7 Science textbook content, definitions, and question patterns
  • Available 24×7: no waiting for tutor appointments or school hours
  • One price for all classes: ₹999/month covers Class 6 to Class 12, all subjects
  • 3-day free trial: no credit card, no commitment — parents can evaluate before paying

Common mistakes in CBSE Class 7 Science Chapter 13 Wastewater Story exams — and how to avoid them

Students lose marks on predictable errors. First, confusing 'sewage' and 'sludge': sewage is liquid wastewater; sludge is the solid/semi-solid sediment that settles out. Second, drawing the treatment plant flow in the wrong sequence — it must go bar screen → grit chamber → primary sedimentation → aeration → secondary sedimentation. Mixing up the order (e.g. putting aeration before primary sedimentation) costs the full 3 marks. Third, stating that 'bacteria are killed in the aeration tank' — wrong; aerobic bacteria thrive and multiply there; pathogens are killed later by chlorination. Fourth, writing that BOD is a chemical you remove — BOD is a measurement, not a substance; the treatment removes organic matter, which lowers BOD. Fifth, using vague language: instead of 'bad things are removed,' write 'suspended solids, organic matter, and pathogens are removed.' Sixth, forgetting to label diagrams fully — every arrow, tank, and pipe should have a clear label. Seventh, not linking sanitation to disease in application questions — if asked 'Why should sewage be treated?', include both environmental reasons (prevent river pollution, eutrophication) and health reasons (prevent cholera, typhoid). Practising past-year CBSE and school question papers with these pitfalls in mind dramatically improves scores.
  • Mistake: calling sludge 'sewage' or vice versa → use precise terms
  • Mistake: wrong treatment sequence → memorise the exact NCERT flow
  • Mistake: saying bacteria are removed in aeration → they are cultivated there; pathogens are killed by chlorine later
  • Mistake: 'BOD is removed' → correct phrasing: 'organic matter is removed, reducing BOD'
  • Mistake: incomplete diagram labels → every component must be named
  • Mistake: ignoring the health angle in answers → always connect sanitation to disease prevention when relevant

Frequently asked questions

Which topics from CBSE Class 7 Science Chapter 13 Wastewater Story are most frequently asked in exams?+
The wastewater treatment plant diagram (labelling and flow sequence) appears in 90% of school exams and carries 3 marks. The second most common question is 'What is sewage and what does it contain?' (2 marks). The link between poor sanitation and waterborne diseases (cholera, typhoid) is asked in 70% of sample papers, usually as a 2-mark or 3-mark application question. Better housekeeping practices — like not pouring oil down the drain — appear as 1-mark fill-in-the-blanks or match-the-following. BOD definition and its significance is tested in 40% of CBSE Class 7 Science Chapter 13 Wastewater Story questions.
How many marks does CBSE Class 7 Science Chapter 13 Wastewater Story typically carry in the final exam?+
Wastewater Story contributes 3–4 marks out of the 80-mark theory paper in most CBSE schools. The standard pattern is one 3-mark question (usually the treatment plant diagram or a process-explanation question) and one 1-mark objective question (MCQ, fill-in-the-blank, or true/false). Occasionally, a 2-mark question on sanitation and disease or better housekeeping practices replaces the 3-mark question. In internal assessments and periodic tests, the chapter can carry 5–6 marks, including short-answer and diagram-based questions.
What is the exact definition of sewage according to NCERT Class 7 Science Chapter 13?+
The NCERT defines sewage as wastewater released by homes, industries, hospitals, offices, and agricultural activities. It is a mixture of 99.9% water and 0.1% suspended and dissolved impurities, including organic waste (human excreta, food waste, paper), inorganic substances (phosphates, nitrates, metals), nutrients, suspended solids, and disease-causing microorganisms (bacteria, viruses, protozoa, worm eggs). This precise composition should be memorised because CBSE Class 7 Science Chapter 13 Wastewater Story questions often ask 'Why is sewage 99.9% water?' or 'List four impurities in sewage.'
Why is air bubbled through the aeration tank in a sewage treatment plant?+
Air is pumped into the aeration tank to provide oxygen for aerobic bacteria. These bacteria require oxygen to rapidly decompose dissolved organic matter in the sewage through aerobic respiration, converting it into carbon dioxide, water, and new bacterial cells. This process dramatically reduces the Biological Oxygen Demand (BOD) of the wastewater. Without aeration, anaerobic bacteria would dominate, decomposition would be much slower, foul odours (methane, hydrogen sulfide) would develop, and pathogenic bacteria would survive longer. The vigorous aeration also keeps the activated sludge suspended and well-mixed with the incoming wastewater.
What happens to the sludge removed from the sedimentation tanks in CBSE Class 7 Science Chapter 13 Wastewater Story?+
Both primary sludge (from the primary clarifier) and excess activated sludge (from the secondary clarifier) are pumped into closed anaerobic digesters. There, anaerobic bacteria break down the organic matter over 15–30 days in the absence of oxygen, producing biogas — a mixture of about 60% methane and 40% carbon dioxide. This biogas can be used as fuel to generate electricity or heat for the treatment plant. The digested sludge, now largely pathogen-free and odourless, is dried and used as fertiliser or soil conditioner in agriculture, returning nutrients like nitrogen and phosphorus to the land.
How does untreated sewage cause fish to die in rivers, according to the NCERT?+
Untreated sewage is rich in organic matter, which has a high Biological Oxygen Demand (BOD). When it enters a river, aerobic bacteria in the water begin decomposing the organic waste, consuming large amounts of dissolved oxygen in the process. This depletes the oxygen available for fish and other aquatic animals. Additionally, nutrients (nitrates and phosphates) in the sewage can cause eutrophication — explosive algal growth that blocks sunlight, kills submerged plants, and further depletes oxygen when the algae die and decompose. The resulting hypoxic (low-oxygen) conditions suffocate fish, leading to mass die-offs. CBSE Class 7 Science Chapter 13 Wastewater Story emphasises treating sewage to reduce BOD and nutrients before discharge.
What are the main waterborne diseases linked to poor sanitation in CBSE Class 7 Science Chapter 13 Wastewater Story?+
The NCERT lists cholera (caused by Vibrio cholerae bacterium), typhoid (Salmonella typhi bacterium), hepatitis A (hepatitis A virus), and dysentery (Shigella bacteria or Entamoeba histolytica protozoa) as diseases transmitted through contaminated water and poor sanitation. These pathogens are present in human faeces and enter water supplies when sewage is not properly treated or when open defecation occurs. Drinking or using this contaminated water for cooking spreads the diseases. The chapter stresses that providing toilets, treating sewage, and ensuring safe drinking water are essential to prevent these illnesses.
Why should cooking oil not be poured down the kitchen sink, as per NCERT better housekeeping practices?+
Cooking oil and grease should not go down the drain because they solidify and stick to the inside of sewer pipes, gradually narrowing the passage and eventually causing blockages (fatbergs). In treatment plants, oil floats on the surface of sedimentation tanks as scum, interfering with the settling process and requiring extra effort to remove. Additionally, a small amount of oil can contaminate large volumes of water — one litre of oil can pollute up to one million litres of water. The NCERT recommends collecting used oil in a container, letting it solidify, and discarding it with solid waste instead.
What is activated sludge and why is some of it recycled in the treatment plant?+
Activated sludge is a brown, flocculent mixture of aerobic bacteria, protozoa, fungi, and partially decomposed organic matter that forms in the aeration tank. It is 'activated' by continuous oxygen supply, which keeps the bacteria highly active and efficient at breaking down dissolved organic pollutants. After secondary sedimentation, 20–30% of the settled activated sludge is pumped back to the aeration tank (called return activated sludge or RAS) to maintain a high population of bacteria ready to digest incoming sewage. This recycling accelerates treatment and ensures consistent performance. The remaining excess sludge is sent to digesters for biogas production.
Can students use CBSETUTOR.ai to get help with diagram-based questions from CBSE Class 7 Science Chapter 13 Wastewater Story?+
Absolutely. Students can photograph the wastewater treatment plant diagram directly from their NCERT textbook or any worksheet, upload it to CBSETUTOR.ai, and ask questions like 'Label all the parts of this diagram' or 'Explain what happens in the aeration tank.' The AI, trained on the full Class 7 Science syllabus, provides accurate labels, descriptions, and explanations that match CBSE marking schemes. This feature is especially helpful for visual learners and for last-minute exam revision. The service costs ₹999/month for all classes 6–12, with a 3-day free trial and no credit card required to start.
What is Biological Oxygen Demand (BOD) and why is it important in CBSE Class 7 Science Chapter 13 Wastewater Story?+
Biological Oxygen Demand (BOD) is the amount of dissolved oxygen (measured in milligrams per litre, mg/L) that aerobic microorganisms need to decompose organic matter present in water. It is a key indicator of water pollution: clean river water has BOD below 5 mg/L, while untreated sewage can have BOD of 200–400 mg/L. High BOD means high organic pollution. Wastewater treatment plants aim to reduce BOD to around 20 mg/L or less before releasing effluent into rivers, ensuring that the water body's oxygen levels remain sufficient to support aquatic life. The NCERT uses BOD to explain why untreated sewage kills fish and why aeration is critical in treatment.
How can a septic tank contaminate groundwater if not properly designed or maintained?+
A septic tank releases partially treated effluent into the surrounding soil via a soak pit or drain field. If the tank is poorly constructed (cracked walls, no proper settling compartment), located too close to a well or borewell (less than 10–15 metres), or not desludged regularly (allowing sludge to overflow), pathogens and nutrients can seep into groundwater. Shallow water tables and permeable sandy soils increase this risk. Contaminated groundwater used for drinking can spread diseases like cholera and typhoid. The NCERT advises maintaining septic tanks properly, siting them away from water sources, and using them only where municipal sewers are unavailable.

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