---
title: "Sedimentation with coagulation"
subject: "Water Supply Engineering"
chapter: 6
key: "semester_5/water_supply_engineering:sedimentation-with-coagulation"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/sedimentation-with-coagulation
site: BCT Engineering
---

# Sedimentation with coagulation

Water Supply Engineering · Chapter 6: Water Treatment

- Asked in: 2067 Asar (old course); 2066 Bhadra (old course); 2070 Chaitra (old course) (asked 3 times)
- 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/sedimentation-with-coagulation
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-6

## Question

What do you understand by sedimentation with coagulation? Why is it necessary? Explain the different processes involved in sedimentation with coagulation.

## Short answer

Sedimentation with coagulation adds chemicals like alum to neutralize colloid charges, forming heavy flocs that settle quickly. It is necessary because colloids remain suspended due to repulsion and extremely low settling velocity. The process involves rapid mixing of coagulant, slow mixing for flocculation, and settling of flocs in a tank.

## Answer

**Sedimentation with coagulation** is the settling of fine suspended and colloidal impurities after chemicals (coagulants such as alum, ferric chloride or ferrous sulphate) have been added, so that the particles join into large, heavy flocs which settle quickly. It is also called chemical or aided sedimentation.

#### Why it is necessary
- Colloidal particles (0.001 to 1 µm) and very fine silt carry a negative charge and repel one another, so they stay suspended and plain sedimentation cannot remove them. By Stokes' law, their settling velocity is extremely small.
- It removes turbidity, colour, bacteria and some organic matter, so the filters are loaded less and run longer.
- It allows a much smaller and cheaper tank (higher overflow rate, 30 to 40 m³/m²/d, and detention of 2 to 4 h).

#### Processes involved
```text
 Raw    +-------+   +-----------+   +---------+   +--------+
 water->| Rapid |-->|Flocculator|-->|Settling |-->| Filter |
        |  mix  |   |slow stir  |   |  tank   |   +--------+
        +-------+   +-----------+   +---------+
        coagulant     flocs form     flocs settle
```
1. **Coagulation (rapid mixing)**: the coagulant is added and violently mixed for 30 s to 2 min. The positive metal ions neutralise the negative charge of the colloids (destabilisation), and metal hydroxide forms.
$$
Al_2(SO_4)_3 + 3Ca(HCO_3)_2 \rightarrow 2Al(OH)_3\downarrow + 3CaSO_4 + 6CO_2
$$
Alkalinity is needed (about 0.5 mg/l as CaCO$_3$ per 1 mg/l of alum); lime or soda ash is added if the water lacks it.
2. **Flocculation (slow mixing)**: gentle stirring for 20 to 40 min brings the micro-flocs together to form large flocs by inter-particle collision (bridging and sweep floc).
3. **Sedimentation**: the large flocs settle in the settling tank under gravity; the clarified water is collected at the outlet weir and sludge is withdrawn from the hopper.
4. **Filtration** follows to remove the remaining fine flocs, then disinfection.

## Check yourself

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

1. Why does plain sedimentation fail to remove colloidal particles?
   A. They are too heavy
   B. They are positively charged and attract the tank walls
   C. They carry a negative charge and repel one another, and are so small that their settling velocity is extremely low
   D. They dissolve in water
   Answer: C. Colloids stay suspended because of repulsive charge and tiny size, so Stokes' settling is negligible. (Key point: Why coagulation is needed)

2. What is the correct order of processes in sedimentation with coagulation?
   A. Flocculation, rapid mixing, sedimentation
   B. Sedimentation, rapid mixing, flocculation
   C. Rapid mixing, flocculation, sedimentation, then filtration
   D. Filtration, sedimentation, rapid mixing
   Answer: C. The coagulant is mixed rapidly, gentle stirring builds flocs, flocs settle, and filtration follows. (Key point: Process order)

3. What happens in the rapid mixing (coagulation) step?
   A. Metal ions neutralise the negative charge of colloids and metal hydroxide forms
   B. Large flocs settle
   C. Bacteria are killed by chlorine
   D. Dissolved gases are removed
   Answer: A. Rapid mixing for 30 s to 2 min destabilises the colloids by charge neutralisation. (Key point: Coagulation mechanism)

4. What is needed for alum to form Al(OH)3 floc?
   A. Acidity in the water
   B. Alkalinity, with lime or soda ash added if the water lacks it
   C. High temperature only
   D. Dissolved oxygen
   Answer: B. Alum reacts with bicarbonate alkalinity; about 0.5 mg/l alkalinity is used per mg/l alum. (Key point: Alkalinity)

## Related in Water Supply Engineering

- [Coagulant and factors affecting coagulation](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Acoagulant-factors) — Water Supply Engineering, chapter 6
- [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×
- [Plain sedimentation tank for 4 MLD](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ased-b78) — Water Supply Engineering, chapter 6

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