MICROBES IN HUMAN WELFARE:: TOPIC 4NEET & CBSE Class 12 Biology

What is Sewage?

• Sewage = Municipal wastewater containing:
  • Human excreta
  • Organic matter
  • Microorganisms
  • Disease-causing (pathogenic) microbes

Why is sewage treated?

Untreated sewage:

• Pollutes rivers and lakes.
• Decreases dissolved oxygen.
• Spreads water-borne diseases.
• Harms aquatic organisms.

Therefore, sewage is treated in Sewage Treatment Plants (STPs).

Sewage Treatment – Flow Chart

Sewage
   ↓
Primary Treatment (Physical)
   ↓
Primary Effluent
   ↓
Secondary Treatment (Biological)
   ↓
Activated Sludge
   ↓
Anaerobic Sludge Digester
   ↓
Biogas Produced
   ↓
Treated Water Released into Rivers

STEP 1 : Primary Treatment (Physical Treatment)

Purpose

Removes large and small solid particles.

Processes

(A) Filtration

Removes:

• Plastic
• Cloth
• Wood pieces
• Floating debris

↓

(B) Sedimentation

Heavy particles settle down.

Removes:

• Sand
• Soil
• Pebbles
• Grit

Products formed

Settled solids
→ Primary Sludge

Upper liquid
→ Primary Effluent

Primary effluent is sent for biological treatment.

STEP 2 : Secondary Treatment (Biological Treatment)

This is the most important NEET topic.

The primary effluent enters large aeration tanks.

What happens?

✔ Air is continuously pumped.

✔ Mechanical agitation is done.

This provides oxygen.

↓

Useful aerobic bacteria multiply rapidly.

Formation of Flocs

Bacteria combine with fungal filaments.

They form mesh-like structures called

Flocs

Definition

Flocs are masses of bacteria associated with fungal filaments.

Function of Flocs

The microbes consume organic matter present in sewage.

As organic matter decreases,

↓

BOD decreases.

What is BOD?

BOD = Biochemical Oxygen Demand

Definition

Amount of oxygen required by bacteria to oxidize organic matter present in 1 litre of water.

Important Points

High BOD

More organic matter

More pollution

Low BOD

Cleaner water

Less pollution

NEET Line

Greater the BOD, greater the pollution.

Activated Sludge

After BOD decreases,

The sewage enters a settling tank.

Flocs settle at the bottom.

This settled material is called

Activated Sludge

What happens to Activated Sludge?

Small Portion

Sent back into aeration tank.

Purpose:

Acts as inoculum (starter culture).

Remaining Portion

Transferred to

Anaerobic Sludge Digester

STEP 3 : Anaerobic Digestion

Inside sludge digester,

Anaerobic bacteria digest

• Dead bacteria
• Fungi
• Organic matter

Gases Produced

During digestion,

Biogas is produced.

Contains:

• Methane (CH₄)
• Hydrogen sulphide (H₂S)
• Carbon dioxide (CO₂)

Uses of Biogas

• Fuel
• Electricity
• Cooking
• Heating

STEP 4 : Final Effluent

After treatment,

Water becomes much cleaner.

It is discharged into

• Rivers
• Lakes
• Streams

Importance of Sewage Treatment

✔ Reduces pollution

✔ Removes pathogens

✔ Reduces BOD

✔ Produces biogas

✔ Protects aquatic life

✔ Prevents water-borne diseases

Ganga Action Plan (GAP)

Purpose:

Clean River Ganga.

Method:

Construction of Sewage Treatment Plants (STPs).

Yamuna Action Plan (YAP)

Purpose:

Reduce pollution in River Yamuna.

Method:

Treat sewage before discharge.

Complete Process (Easy Flow)

Raw Sewage
      ↓
Filtration
      ↓
Sedimentation
      ↓
Primary Sludge + Primary Effluent
                     ↓
              Aeration Tank
                     ↓
     Aerobic bacteria + Fungi
                     ↓
                Flocs formed
                     ↓
     Organic matter consumed
                     ↓
             BOD decreases
                     ↓
            Settling Tank
                     ↓
           Activated Sludge
            ↙             ↘
 Small part          Large part
 (Inoculum)              ↓
                    Anaerobic
                 Sludge Digester
                        ↓
              Methane + CO₂ + H₂S
                        ↓
                     Biogas
                        ↓
               Treated Water Released

NEET Important Definitions

Sewage

Municipal wastewater containing human excreta, organic matter, and microbes.

Primary Sludge

Solid particles settled during primary treatment.

Primary Effluent

Liquid remaining after primary treatment.

Flocs

Masses of aerobic bacteria associated with fungal filaments.

Activated Sludge

Settled bacterial flocs obtained after secondary treatment.

BOD

Amount of oxygen required by bacteria to oxidize organic matter present in 1 litre of water.

Biogas

Mixture of methane, carbon dioxide, and hydrogen sulphide produced during anaerobic digestion.

NCERT-Based NEET Questions

1. Primary treatment is: Physical treatment.
2. Secondary treatment is: Biological treatment.
3. Flocs are formed by: Bacteria + fungal filaments.
4. Activated sludge contains: Settled bacterial flocs.
5. BOD stands for: Biochemical Oxygen Demand.
6. High BOD indicates: High pollution.
7. Biogas mainly contains: Methane.
8. Biogas is produced in: Anaerobic sludge digester.
9. Small amount of activated sludge is used as: Inoculum.
10. Large amount of activated sludge is sent to: Anaerobic sludge digester.

Memory Trick

“Filter → Settle → Air → Flocs → BOD ↓ → Activated Sludge → Biogas → River”

This sequence is the complete sewage treatment process and is one of the most frequently tested NCERT topics in NEET Biology.