1. Concept Explanation (Exam-Focused)
Purification of Water (or Water Treatment) is
the process of removing physical, chemical, and biological impurities
from raw water to make it safe for drinking.
In SSC JE / RRB JE, questions are:
✔ Process-sequence based
✔ Numerical on sedimentation & filtration
✔ MCQs on functions of each unit
✔ Statement-based traps (what removes what)
📌 Golden Exam Rule:
Treatment units are arranged in the order of impurity
removal.
Typical Flow of Water Treatment Plant
Raw Water → Screening → Sedimentation → Filtration →
Disinfection → Clear Water Reservoir
2. Major Treatment Units & Their Purpose
1️⃣ Screening
✔ Removes large floating
matter (leaves, rags)
✔ No effect on bacteria or turbidity
2️⃣ Sedimentation
✔ Removes suspended solids
by gravity
✔ Reduces load on filters
✔ Plain sedimentation removes ~60% turbidity
📌 Often combined with coagulation
3️⃣ Coagulation & Flocculation
✔ Chemicals used: Alum (most
common)
✔ Converts colloidal particles → flocs
🧠 Mnemonic:
“Coagulate small → Flocculate big”
4️⃣ Filtration
✔ Removes fine suspended
matter & bacteria
✔ Types asked in JE:
- Slow
Sand Filter (SSF)
- Rapid
Sand Filter (RSF) (more important)
5️⃣ Disinfection
✔ Kills pathogenic organisms
✔ Most common method: Chlorination
3. Important Formulas (Highly Repeated)
1️⃣ Surface Loading Rate
(Sedimentation Tank)
Where:
= Discharge (m³/day)
= Surface area (m²)
2️⃣ Filtration Rate
Unit: m³/m²/day
3️⃣ Detention Time
4️⃣ Standalone Centered Equation
(VERY IMPORTANT)
🧠 Short Formula Trick:
“Q/A appears everywhere”
4. Short Notes / One-Liners (Last-Day Revision)
✔ Sedimentation removes suspended
solids only
✔ Coagulation removes colloidal impurities
✔ Filtration improves clarity &
bacteriological quality
✔ Disinfection ensures pathogen-free water
✔ RSF works on gravity + pressure
5. PYQ Patterns (SSC JE / RRB JE)
🔹 Correct sequence of
treatment units
🔹
Function-based MCQs (which unit removes what)
🔹
Difference between SSF & RSF
🔹
Numerical on detention time & filtration rate
⚠ Typical Trap:
Chlorination does NOT remove turbidity
6. Solved Examples
Example 1 (Sedimentation – Numerical)
Flow = 10,000 m³/day
Surface area = 500 m²
✅ Answer: 20 m³/m²/day
Example 2 (Conceptual)
Which unit removes colloidal particles?
✅
Answer: Coagulation
7. MCQs (Exam Level – 30 Questions)
1. Which process removes colloidal impurities?
A) Sedimentation
B) Filtration
C) Coagulation
D) Disinfection
✅
Ans: C
2. Most commonly used coagulant is:
A) Lime
B) Alum
C) Chlorine
D) Bleaching powder
✅
Ans: B
3. Which unit kills pathogenic bacteria?
A) Screening
B) Sedimentation
C) Filtration
D) Disinfection
✅
Ans: D
4. Rapid sand filters are cleaned by:
A) Scraping
B) Backwashing
C) Chemical washing
D) Sedimentation
✅
Ans: B
5. Detention time is highest in:
A) Filter
B) Sedimentation tank
C) Chlorination tank
D) Screen chamber
✅
Ans: B
6. Screening in a water treatment plant removes:
A) Dissolved salts
B) Colloidal impurities
C) Large floating matter
D) Bacteria
✅
Answer: C
📝
Explanation: Screens remove rags, leaves, debris only
7. Plain sedimentation mainly removes:
A) Dissolved solids
B) Colloidal matter
C) Suspended solids
D) Bacteria
✅
Answer: C
8. Efficiency of plain sedimentation in turbidity removal
is about:
A) 20%
B) 40%
C) 60%
D) 90%
✅
Answer: C
9. Coagulation is used to remove:
A) Suspended solids
B) Dissolved salts
C) Colloidal impurities
D) Bacteria only
✅
Answer: C
10. Most commonly used coagulant in water treatment is:
A) Lime
B) Bleaching powder
C) Alum
D) Chlorine
✅
Answer: C
11. The process of gentle stirring after coagulation is
called:
A) Mixing
B) Sedimentation
C) Flocculation
D) Filtration
✅
Answer: C
12. Flocculation helps in:
A) Killing bacteria
B) Increasing particle size
C) Dissolving impurities
D) Removing colour
✅
Answer: B
13. Which unit reduces the load on filters the most?
A) Screening
B) Sedimentation
C) Disinfection
D) Storage
✅
Answer: B
14. Rapid sand filters are cleaned by:
A) Scraping
B) Backwashing
C) Hand raking
D) Chemical treatment
✅
Answer: B
15. Slow sand filters are cleaned by:
A) Backwashing
B) Air scouring
C) Scraping the top layer
D) Chemical washing
✅
Answer: C
16. Filtration rate is highest in:
A) Slow sand filter
B) Rapid sand filter
C) Pressure filter
D) Sedimentation tank
✅
Answer: B
17. Which filter is more effective bacteriologically?
A) Rapid sand filter
B) Pressure filter
C) Slow sand filter
D) Mechanical filter
✅
Answer: C
18. Schmutzdecke layer is formed in:
A) Rapid sand filter
B) Slow sand filter
C) Sedimentation tank
D) Chlorination tank
✅
Answer: B
19. Detention time is maximum in:
A) Filter
B) Sedimentation tank
C) Screen chamber
D) Chlorination tank
✅
Answer: B
20. Which process does NOT remove turbidity?
A) Sedimentation
B) Filtration
C) Coagulation
D) Chlorination
✅
Answer: D
📝
Trap: Chlorination is only for disinfection
21. Filtration mainly removes:
A) Dissolved salts
B) Colour only
C) Fine suspended matter & bacteria
D) Large debris
✅
Answer: C
22. The main purpose of disinfection is to:
A) Improve taste
B) Remove hardness
C) Kill pathogenic organisms
D) Remove turbidity
✅
Answer: C
23. Most commonly used disinfectant in water treatment
plants is:
A) Ozone
B) UV rays
C) Chlorine
D) Potassium permanganate
✅
Answer: C
24. Residual chlorine ensures:
A) Removal of hardness
B) Continued disinfection
C) Colour removal
D) pH control
✅
Answer: B
25. Filtration rate is expressed in:
A) m/day
B) m³/day
C) m³/m²/day
D) L/min
✅
Answer: C
26. Which sequence is CORRECT?
A) Filtration → Sedimentation → Disinfection
B) Screening → Sedimentation → Filtration → Disinfection
C) Disinfection → Filtration → Sedimentation
D) Screening → Filtration → Sedimentation
✅
Answer: B
27. Rapid sand filters require:
A) Large area
B) Skilled operation
C) No chemicals
D) No backwashing
✅
Answer: B
28. Which statement is TRUE?
A) RSF gives better bacterial removal than SSF
B) SSF requires frequent backwashing
C) Coagulation is essential before RSF
D) Chlorination removes turbidity
✅
Answer: C
29. Which unit is designed based on surface overflow
rate?
A) Filter
B) Sedimentation tank
C) Chlorination tank
D) Screen chamber
✅
Answer: B
30. Which of the following is INCORRECT?
A) Alum helps in floc formation
B) Filtration improves clarity
C) Chlorination kills pathogens
D) Sedimentation removes dissolved solids
✅
Answer: D
Last-Minute JE Super Memory Trick 🧠
“Large → Screen | Heavy → Settle | Tiny → Coagulate |
Fine → Filter | Germs → Chlorine”