Class 7 Science • Chapter 13

Wastewater Story

Vardaan Learning Institute • Detailed Chapter Notes

💧 1. What is Wastewater?

Wastewater is water that has been used and contaminated. It comes from homes, industries, hospitals, and farms. Also called sewage.

            Sources of Wastewater / Sewage:
            🏠 Domestic sources: Used water from toilets (black water),
                    kitchens, bathrooms, washing machines (grey water)
🏭 Industrial sources: Chemicals, metals, oils from factories and manufacturing
                    plants
🌾 Agricultural sources: Pesticides, fertilisers, animal waste washed into water
                    bodies during rains
🏥 Hospital sources: Microbes, pharmaceutical chemicals, hazardous waste

        

⚠️ What Is Sewage Made Of? Sewage contains: water (99.9%) + organic impurities (food waste, human/animal excreta, plant matter) + inorganic impurities (salts, metals) + disease-causing microorganisms (pathogens like cholera bacteria, typhoid bacteria, hepatitis virus) + nitrates, phosphates (from fertilisers)

🏭 2. Sewage Treatment — WWTP (Wastewater Treatment Plant)

Untreated sewage released into rivers/lakes is extremely dangerous. Wastewater Treatment Plants (WWTP) clean sewage before releasing it. The treatment has THREE stages:

RAW SEWAGE (collected from homes/industries via underground sewers)

⬇

🔩 STAGE 1: PRIMARY TREATMENT (Physical Removal)

Bar Screens / Mesh Screens: Large debris (plastic, sticks, rags, stones, leaves) scooped out mechanically
→ Grit Chamber: Water slows down → heavy grit (sand, gravel, small stones) sinks to bottom and is removed
→ Sedimentation Tank / Clarifier: Water is held still → solid organic matter sinks to bottom as sludge. Lighter matter (oils, grease) floats as scum → both removed. Remaining water = clarified/primary effluent

⬇

🦠 STAGE 2: SECONDARY TREATMENT (Biological Removal)

Aeration Tank: Air is pumped into the clarified water → aerobic bacteria (added) consume dissolved organic matter → bacteria break it down into CO₂ + water + more bacteria
→ Secondary Sedimentation: Bacterial mass (activated sludge) settles out. Water above is much cleaner = secondary effluent

⬇

☣️ STAGE 3: TERTIARY/ADVANCED TREATMENT (Chemical + Disinfection)

Filtration: Through sand beds → removes remaining particles
→ UV / Chlorination / Ozonation: Kills remaining disease-causing microorganisms (pathogens)
→ Water is now clean enough to be safely released into rivers/sea, or even reused for irrigation

⬇

✅ TREATED WATER — Safe for release into environment ♻️

📸 AI Image Prompt

A clear, step-by-step infographic showing a Wastewater Treatment Plant (WWTP) layout. The infographic flows left to right with labeled numbered stages: Stage 1 "Primary Treatment" (shown as: brown/murky arrow entering → Bar Screens (metal grid removing sticks/rags) → Grit Chamber (sand settling) → Sedimentation Tank showing sludge settling at bottom and scum floating at top, clean water in middle). Stage 2 "Secondary Treatment" (yellow/grey: aeration tank with air bubbles rising, bacteria symbols breaking down organic matter → secondary settling tank). Stage 3 "Tertiary Treatment" (light blue: chlorine chemical being added, UV light lamp, sand filter bed). Final output: clean, clear blue water being released into a clean river with fish and plants visible. Show sludge (brown) being separately processed (sludge drying beds) → become biogas or fertiliser. Color code: dirty/brown for raw sewage → progressively lighter blue as treatment proceeds. Include labels for: "Influent" (input) and "Effluent" (output). Educational infographic style, white background, clear labels.

Fig. 13.1 — Three-stage wastewater treatment process at a WWTP

🪣 3. What Happens to the Sludge?

            The sludge collected from sedimentation tanks is NOT wasted! It is processed separately:
            🔄 Anaerobic digestion in large closed tanks (digesters) → bacteria break it down
                    without oxygen → produce biogas (methane) which is used as fuel for energy
🌱 After digestion, the remaining sludge becomes a nutrient-rich fertiliser for
                    agriculture
So — even "waste" has value! A WWTP converts waste into biogas + fertiliser + clean water

        

🚽 4. Sanitation — Individual and Public Responsibility

            
                    Safe Sanitation Practice
                    Why It Matters
                
                    Using and maintaining proper toilets/latrines
                    Prevents spread of diseases like cholera, typhoid, polio, hepatitis
                
                    Not defecating in open areas or near water bodies
                    Prevents contamination of groundwater and soil; prevents epidemics
                
                    Treating domestic wastewater before discharge
                    Prevents water body pollution; protects aquatic life
                
                    Handwashing with soap after toilet use and before food
                    Kills pathogens; simple and highly effective disease prevention
                
                    Not disposing chemicals/oils/medicines in drains
                    Toxic chemicals are not removed by standard WWTP; harm aquatic life

Safe Sanitation Practice	Why It Matters
Using and maintaining proper toilets/latrines	Prevents spread of diseases like cholera, typhoid, polio, hepatitis
Not defecating in open areas or near water bodies	Prevents contamination of groundwater and soil; prevents epidemics
Treating domestic wastewater before discharge	Prevents water body pollution; protects aquatic life
Handwashing with soap after toilet use and before food	Kills pathogens; simple and highly effective disease prevention
Not disposing chemicals/oils/medicines in drains	Toxic chemicals are not removed by standard WWTP; harm aquatic life

💡 Open Drains and Open Defecation — Biggest Threats In India and many developing countries, open drains carry sewage, breeding grounds for mosquitoes (malaria, dengue) and flies (typhoid, cholera). Swachh Bharat Mission (2014) aimed to eliminate open defecation by building millions of toilets across India. Access to toilets = basic human right AND essential for public health!

📝 5. Quick Revision