---
title: "Coagulant and factors affecting coagulation"
subject: "Water Supply Engineering"
chapter: 6
key: "semester_5/water_supply_engineering:coagulant-factors"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/coagulant-factors
site: BCT Engineering
---

# Coagulant and factors affecting coagulation

Water Supply Engineering · Chapter 6: Water Treatment

- Asked in: 2074 Chaitra, 8 marks
- Video: [Factors affecting coagulation and flocculation Process || Failure reason of Water treatment process](https://www.youtube.com/watch?v=2EX3bI-Om3A) — Environmental experts
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/coagulant-factors
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-6

## Question

What do you mean by coagulant? What are the affecting factors in coagulation? Briefly describe.

## Short answer

A coagulant (e.g. alum) destabilizes colloidal particles to form settleable flocs. Key factors are coagulant dose (jar test), pH (alum best 6.5–8.5), alkalinity, turbidity, temperature, rapid and slow mixing, detention time, and coagulant aids.

## Answer

#### Coagulant
A **coagulant** is a chemical added to water to destabilise the fine suspended and colloidal particles and make them join into large, settleable flocs. Common coagulants are **alum** (aluminium sulphate), **ferrous sulphate (copperas)** with lime, **ferric chloride**, **ferric sulphate**, **sodium aluminate** and **polyelectrolytes** (as coagulant aids).

Alum reaction:
$$
Al_2(SO_4)_3\cdot 18H_2O + 3Ca(HCO_3)_2 \rightarrow 2Al(OH)_3\downarrow + 3CaSO_4 + 18H_2O + 6CO_2
$$

#### Factors affecting coagulation
1. **Type and dose of coagulant**: an under- or over-dose gives poor floc; the optimum is found by the jar test.
2. **pH of water**: each coagulant works best in a range (alum about pH 6.5 to 8.5; ferrous sulphate with lime pH above 8.5).
3. **Alkalinity**: alum needs about 0.5 mg/l alkalinity (as CaCO$_3$) for each mg/l of alum.
4. **Turbidity and nature of particles**: low turbidity needs more coagulant or a coagulant aid, since there are fewer particles to collide.
5. **Temperature**: low temperature slows the reaction and makes weak floc, increasing the dose and time.
6. **Mixing**: rapid mixing for 30 s to 2 min ensures instant dispersal; slow stirring then builds floc. Too fast mixing breaks the floc.
7. **Detention time** in the flocculator (20 to 40 min).
8. **Presence of ions and organic matter** (e.g. sulphates, colour compounds) alters the dose.
9. **Coagulant aids** (activated silica, polyelectrolytes, bentonite) help form stronger and heavier floc.

## Check yourself

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

1. What is a coagulant?
   A. A chemical that destabilises fine particles so they join into large settleable flocs
   B. A chemical that kills germs
   C. A gas used in aeration
   D. A filter medium
   Answer: A. Alum, ferric chloride, ferrous sulphate with lime and similar chemicals cause flocs to form. (Key point: Coagulant definition)

2. How is the optimum coagulant dose found?
   A. By guessing from turbidity only
   B. By the jar test
   C. By using the maximum dose
   D. By measuring chlorine demand
   Answer: B. Under- or over-dosing gives poor floc; the jar test finds the optimum. (Key point: Optimum dose)

3. How should mixing be done for good floc?
   A. Slow stirring first, then violent mixing
   B. Violent mixing throughout
   C. Rapid mixing for 30 s to 2 min, then slow stirring; too fast mixing breaks floc
   D. No mixing is needed
   Answer: C. Rapid mixing disperses the chemical instantly, slow stirring builds flocs. (Key point: Mixing)

4. Which is correct for alum?
   A. Works best at pH below 4
   B. Needs no alkalinity
   C. Works best at pH above 10
   D. Works best at about pH 6.5 to 8.5 and needs about 0.5 mg/l alkalinity per mg/l
   Answer: D. Each coagulant has an effective pH range, and alum consumes alkalinity. (Key point: Alkalinity and pH)

## Related in Water Supply Engineering

- [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×
- [Forms of chlorination and affecting factors](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Aforms-of-chlorination-factors) — Water Supply Engineering, chapter 6, asked 3×
- [Factors affecting chlorination](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Afactors-affecting-chlorination) — Water Supply Engineering, chapter 6
- [Sedimentation with coagulation](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Asedimentation-with-coagulation) — Water Supply Engineering, chapter 6, asked 3×
- [Intake, its functions and site factors](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Aintake-functions-site-factors) — Water Supply Engineering, chapter 5
- [Treatment process; sedimentation factors; coagulants](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Atreatment-impurities-coagulants) — Water Supply Engineering, chapter 6

## Related in other subjects

- [Incentives; factors affecting wage/salary](https://www.bctengineer.com/question.md?key=semester_7%2Forganization_and_management%3Aincentives-wage-factors) — Organization and Management, chapter 7, asked 3×
- [Stability limits and factors affecting transient stability](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Astability-limits-factors) — Power System Analysis II, chapter 6, asked 2×
- [Factors affecting sewage; calculating discharge](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Afactors-sewage-calculate-discharge) — Sanitary Engineering, chapter 2
- [Factors affecting bearing capacity; water table](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Abearing-capacity-factors-water-table) — Foundation Engineering, chapter 6, asked 5×