---
title: "Treatment process; sedimentation factors; coagulants"
subject: "Water Supply Engineering"
chapter: 6
key: "semester_5/water_supply_engineering:treatment-impurities-coagulants"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/treatment-impurities-coagulants
site: BCT Engineering
---

# Treatment process; sedimentation factors; coagulants

Water Supply Engineering · Chapter 6: Water Treatment

- Asked in: 2075 Asoj, 8 marks
- Video: [Sedimentation & Coagulation | Purification of Water |  Part - 08 | Environmental Engineering](https://www.youtube.com/watch?v=Iy03sjbtK5o) — Study Site
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/treatment-impurities-coagulants
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-6

## Question

Discuss the treatment process and impurities removal. Explain the affecting factors in a sedimentation tank. Why do we use coagulants in the water treatment process?

## Short answer

Treatment steps remove specific impurities: screening removes debris, sedimentation heavy silt, coagulation colloids, filtration fine particles, disinfection pathogens. Sedimentation is affected by particle size, specific gravity, temperature, overflow rate, and detention time. Coagulants neutralize colloid charges and form flocs to remove turbidity, colour, and bacteria.

## Answer

#### Treatment processes and impurities removed
| Treatment unit | Impurity removed |
|---|---|
| Screening | Floating matter, debris |
| Aeration | Gases (CO$_2$, H$_2$S), taste, odour, Fe, Mn |
| Plain sedimentation | Heavy silt and sand |
| Coagulation + flocculation + sedimentation | Colloids, turbidity, colour |
| Filtration | Fine suspended particles, floc, bacteria |
| Disinfection | Pathogens |
| Softening | Hardness |

#### Factors affecting sedimentation in a tank
1. **Size of particle**: $v_s \propto d^2$; bigger particles settle quicker.
2. **Specific gravity of particle**: heavier particles settle faster ($v_s \propto S_s-1$).
3. **Temperature (viscosity)**: warmer water has lower viscosity, so settling is faster; $v_s \propto 1/\nu$.
4. **Surface overflow rate / tank area**: larger surface area for the flow reduces the overflow rate and improves removal.
5. **Detention time**: the longer, the better the removal.
6. **Velocity of flow**: should be low (below about 0.3 m/min) so that the settled sludge is not scoured.
7. **Tank shape and depth**, inlet and outlet arrangement (short-circuiting, density currents, wind).
8. **Flocculation**: coagulated particles are bigger and settle faster.
9. **Hindered settling** at high concentration and tank turbulence.

#### Why coagulants are used
- Fine silt and colloids (0.001 to 1 µm) have a negative charge and do not settle by themselves in a reasonable time.
- A coagulant such as alum supplies positive ions (Al$^{3+}$) that **neutralise the charge**, and forms a jelly-like Al(OH)$_3$ floc which **enmeshes** the particles:
$$
Al_2(SO_4)_3 + 3Ca(HCO_3)_2 \rightarrow 2Al(OH)_3\downarrow + 3CaSO_4 + 6CO_2
$$
- The large flocs settle quickly, which removes **turbidity, colour, bacteria** and organic matter, reduces tank size and detention time, and increases the length of filter runs.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why are coagulants used in water treatment?
   A. To soften water
   B. To add oxygen
   C. To neutralise the charge of colloids and form flocs that settle quickly
   D. To kill all bacteria
   Answer: C. Alum supplies Al3+ that neutralises negative colloids and forms Al(OH)3 floc enmeshing particles. (Key point: Why coagulants)

2. A benefit of using coagulants is:
   A. Larger tanks needed
   B. Removal of turbidity, colour and bacteria, smaller tanks and longer filter runs
   C. No need for filtration
   D. Higher detention time
   Answer: B. Large flocs settle fast, so tanks can be smaller and filters carry less load. (Key point: Benefits of coagulation)

3. Why should flow velocity in a sedimentation tank be low (below about 0.3 m/min)?
   A. To increase turbulence
   B. So that settled sludge is not scoured
   C. To reduce detention time
   D. To increase overflow rate
   Answer: B. High velocity re-suspends deposited sludge. (Key point: Flow velocity)

4. How does particle specific gravity affect settling?
   A. Heavier particles settle faster, vs proportional to (Ss - 1)
   B. No effect
   C. Heavier particles settle slower
   D. vs is proportional to 1/Ss
   Answer: A. Stokes' law has vs proportional to (Ss - 1). (Key point: Specific gravity)

## Related in Water Supply Engineering

- [Treatment processes, impurities removal, settlement](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Atreatment-impurities-sedimentation-factors) — Water Supply Engineering, chapter 6, asked 2×
- [Factors affecting water demand; variation](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Afactors-affecting-demand) — Water Supply Engineering, chapter 3, asked 5×
- [Rectangular sedimentation tank, 9 million L peak demand](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ased-c75) — Water Supply Engineering, chapter 6
- [Rectangular sedimentation tank, 20000 population, 0.4 mm/s](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ased-s73) — Water Supply Engineering, chapter 6
- [Types of impurities in water](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Atypes-of-impurities) — Water Supply Engineering, chapter 4, asked 3×
- [Rectangular sedimentation tank, 0.2 mm/s, 3 hours](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ased-c71) — Water Supply Engineering, chapter 6, asked 2×

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