Why This Topic is Important for ECET
In Environmental Engineering, water treatment questions are common in ECET. Sedimentation and Filtration are two key processes in water purification. Understanding their principles, formulas, and design parameters is important because they are direct scoring areas – mostly formula-based and conceptual. Many ECET & GATE-style MCQs are framed from these.
📘 Concept Notes
1. Sedimentation
- Definition: Process of settling particles under gravity from water.
- Purpose: Removes suspended solids before filtration.
Types of Sedimentation:
- Plain Sedimentation – Removes grit & heavy solids.
- Sedimentation with Coagulation – Uses chemicals like alum to remove colloids.
- Sedimentation with Flocculation – Encourages particle aggregation for better settling.
Design Parameters:
- Surface overflow rate (SOR) or velocity:
where
QQQ = flow rate,
AAA = surface area of tank. - Settling time:
Stokes’ Law (for fine particles):
where vsv_svs = settling velocity, ddd = diameter of particle, ρp\rho_pρp = particle density, ρ\rhoρ = fluid density, μ\muμ = viscosity.
2. Filtration
- Definition: Process of passing water through porous medium to remove suspended & colloidal impurities.
- Types:
- Slow Sand Filter (SSF):
- Filtration rate: 100–200 L/hr/m²
- Simple, effective, needs large area.
- Rapid Sand Filter (RSF):
- Filtration rate: 3000–6000 L/hr/m²
- Requires backwashing.
- Compact design, widely used in cities.
Design Parameters:
- Filtration rate:
Head loss (Darcy’s Law):
where LLL = thickness of filter bed, KKK = coefficient of permeability.
⚙️ Formulas
🔟 10 MCQs
Q1. Sedimentation is mainly used to remove:
a) Dissolved salts
b) Suspended solids
c) Gases
d) Hardness
Q2. The formula represents:
a) Settling velocity
b) Surface overflow rate
c) Filtration rate
d) Head loss
Q3. A sedimentation tank of area 300 m² treats 15,000 m³/day. Find overflow rate.
a) 0.5 m³/m²/day
b) 50 m³/m²/day
c) 100 m³/m²/day
d) 200 m³/m²/day
Q4. Stokes’ law is valid for:
a) Large particles
b) Turbulent flow
c) Very fine particles in laminar flow
d) All particle sizes
Q5. Slow sand filter typical filtration rate is:
a) 100–200 L/hr/m²
b) 1000 L/hr/m²
c) 5000 L/hr/m²
d) 10,000 L/hr/m²
Q6. Rapid sand filter is cleaned by:
a) Scraping top sand layer
b) Backwashing
c) Chemical regeneration
d) Replacement
Q7. A rapid sand filter of 50 m² supplies 3000 m³/hr. Find filtration rate.
a) 30 L/hr/m²
b) 600 L/hr/m²
c) 60,000 L/hr/m²
d) 3000 L/hr/m²
Q8. Head loss through filter follows:
a) Darcy’s law
b) Stokes’ law
c) Bernoulli’s theorem
d) Continuity equation
Q9. Coagulant commonly used in sedimentation is:
a) Sodium chloride
b) Alum
c) Gypsum
d) Silica
Q10. Which is true?
a) SSF requires less area than RSF
b) RSF has higher filtration rate than SSF
c) RSF does not need backwashing
d) SSF gives higher output than RSF
✅ Answer Key
| Q | Answer |
|---|---|
| 1 | b |
| 2 | b |
| 3 | c |
| 4 | c |
| 5 | a |
| 6 | b |
| 7 | c |
| 8 | a |
| 9 | b |
| 10 | b |
🧠 Explanations
- Q1: Sedimentation removes suspended solids → (b). Dissolved salts/hardness need chemical treatment.
- Q2: Formula is surface overflow rate → (b).
- Q3:
→ (c). - Q4: Stokes’ law valid for fine particles in laminar flow → (c).
- Q5: SSF rate is 100–200 L/hr/m² → (a).
- Q6: RSF cleaned by backwashing → (b).
- Q7:
→ (c). - Q8: Head loss in filter beds obeys Darcy’s law → (a).
- Q9: Alum is common coagulant → (b).
- Q10: RSF has higher rate than SSF → (b).
🎯 Motivation / Why Practice Matters
In ECET 2026, Environmental Engineering questions are usually direct, from formulas or design values.
- Practicing sedimentation & filtration helps you save time in the exam.
- These questions are low-hanging fruit – no lengthy derivations, only formula application & memory of standard values.
👉 Mastering this ensures 2–3 sure-shot marks in the competitive exam.
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