Wastewater Story - Class 7 Science (CBSE)
Based on the 2025-26 NCERT syllabus for Class 7 Science. This chapter educates students about wastewater, its treatment, and the importance of proper sanitation for health and the environment.
1. Why this chapter matters
Access to clean water and proper sanitation is a basic human right. Understanding what happens to wastewater after we flush or drain it helps students appreciate sanitation infrastructure. In CBSE exams, this chapter contributes 4-6 marks.
2. What is sewage?
Sewage is wastewater containing human waste, household detergents, food scraps, and other waste materials.
Composition of sewage
- Used water from baths, showers, and sinks (greywater).
- Human excreta (faeces and urine) from toilets (blackwater).
- Kitchen waste (food scraps, oils, detergents).
- Soaps and cleaning chemicals.
- Industrial waste (if connected to sewer system).
Why sewage is harmful
- Contains disease-causing microorganisms (bacteria, viruses, parasites).
- Decomposes to produce foul-smelling gases.
- Can contaminate drinking water sources.
- Spreads diseases like cholera, typhoid, dysentery, hepatitis.
3. The sewer system
A sewer is an underground pipe that carries sewage from homes and buildings to a treatment plant.
Septic tank
In areas without a central sewer system, homes use septic tanks. Solid waste settles at the bottom, and the liquid is partially treated before being released into the soil.
4. Sewage treatment
Sewage treatment happens at a Sewage Treatment Plant (STP). The treatment occurs in multiple stages:
Stage 1: Physical treatment (primary treatment)
- Wastewater is passed through screens and grit chambers to remove large objects (sticks, rags, plastic).
- This stage removes about 30-40 percent of the solid waste.
- Solid waste (sludge) settles at the bottom.
Stage 2: Biological treatment (secondary treatment)
- Air is pumped into the water to encourage aerobic bacteria to grow.
- These bacteria break down the organic matter in the wastewater.
- The process is called aeration.
- After aeration, the water is allowed to settle again.
Stage 3: Tertiary treatment (polishing)
- The water is disinfected (using chlorine or UV light) to kill remaining microorganisms.
- The treated water can be released into rivers or reused for irrigation.
What happens to the sludge?
Sludge (solid waste from treatment) is sent to digesters where anaerobic bacteria break it down. The biogas produced can be used as fuel.
5. Sanitation and health
Poor sanitation is a major cause of disease in many parts of the world.
Problems caused by poor sanitation
- Open defecation leads to contamination of soil and water.
- Flies and insects carry disease-causing organisms from waste to food.
- Stagnant sewage breeds mosquitoes (causing malaria and dengue).
Improving sanitation
- Proper toilets connected to a sewer system or septic tank.
- Hand washing with soap after using the toilet.
- Safe disposal of household waste.
- Community awareness programmes.
6. Alternative toilets
Composting toilets (dry toilets)
These toilets do not use water. Human waste is collected and mixed with carbon-rich material (sawdust, leaves). Microorganisms decompose the waste, producing compost that can be used as fertiliser.
Advantages of composting toilets
- Save water (no flushing required).
- Produce useful compost.
- Reduce the load on sewage treatment plants.
- Suitable for areas with water scarcity.
7. Water conservation
- Fix leaking taps immediately -- a dripping tap can waste thousands of litres per year.
- Use a bucket instead of a hose for washing cars.
- Collect rainwater (rainwater harvesting) for non-drinking uses.
- Do not pour cooking oil or chemicals down the drain.
8. Worked examples
Example 1: What is the difference between greywater and blackwater?
Greywater is wastewater from baths, showers, and sinks (without toilet waste). Blackwater is wastewater from toilets containing human excreta. Blackwater is more dangerous and requires more treatment.
Example 2: Why should we not throw cooking oil down the drain?
Cooking oil solidifies and blocks the sewer pipes. It also interferes with the biological treatment process at the STP, reducing treatment efficiency.
Example 3: How do aerobic bacteria help in sewage treatment?
Aerobic bacteria feed on the organic matter in wastewater, breaking it down into simpler, less harmful substances. This is the key biological treatment step.
9. Common mistakes and how to fix them
| Mistake | Fix |
|---|---|
| Thinking all wastewater is equally hazardous | Blackwater is more hazardous than greywater |
| Believing sewage treatment removes all chemicals | Some chemicals (pharmaceuticals, microplastics) may remain |
| Confusing primary and secondary treatment | Primary = physical screening. Secondary = biological breakdown |
| Using too much detergent | Excess detergent is hard to treat and pollutes water bodies |
| Pouring chemicals in drains thinking they will be treated | Many chemicals pass through treatment and contaminate rivers |
10. CBSE exam focus
| Question type | Marks | Frequency |
|---|---|---|
| Define sewage and its composition | 2 marks | 1 question |
| Stages of sewage treatment | 3 marks | 1 question |
| Sanitation and disease prevention | 2 marks | 1 question |
| Alternative toilets (composting) | 2 marks | Occasional |
| Water conservation methods | 2 marks | 1 question |
11. Self-test
- What is sewage? Give its main components.
- List the stages of sewage treatment in order.
- Why is open defecation harmful?
- What is a composting toilet? State two advantages.
- Differentiate between aerobic and anaerobic treatment.
- Why should we not pour used cooking oil down the kitchen sink?
12. Answer key
- Sewage is wastewater from homes containing human waste, detergents, food scraps, and chemicals.
- Three stages: Primary (physical screening), Secondary (biological aeration), Tertiary (disinfection).
- It contaminates soil and water, spreads diseases like cholera and typhoid, and harms the environment.
- A composting toilet uses no water. Waste decomposes into compost. Advantages: saves water, produces fertiliser.
- Aerobic treatment uses oxygen-breathing bacteria. Anaerobic treatment (for sludge) uses bacteria that do not need oxygen.
- Oil blocks pipes and interferes with sewage treatment. It should be disposed of separately.
13. Quick revision
- Sewage = wastewater from homes and businesses.
- Sewer system = underground pipes carrying sewage.
- Primary treatment: removes large solids.
- Secondary treatment: aerobic bacteria break down organic matter.
- Tertiary treatment: disinfection with chlorine or UV.
- Poor sanitation causes waterborne diseases.
- Composting toilets: waterless, produce compost.
- Never pour oil, chemicals, or medicines down the drain.
- Treated water can be reused for irrigation.
