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

# Factors affecting chlorination

Water Supply Engineering · Chapter 6: Water Treatment

- Asked in: 2070 Chaitra (old course)
- Video: [Environmental Engineering 1 | Module 5 - Lecture 3 | Factors affecting Chlorination | CE 405](https://www.youtube.com/watch?v=1mp6SwSSM94) — Alex -E Academy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/factors-affecting-chlorination
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-6

## Question

What are the factors affecting chlorination?

## Short answer

Key factors: chlorine dose must exceed demand; contact time (Ct constant, at least 30 min); pH 6-7 for more HOCl; higher temperature speeds kill; low turbidity avoids shielding; organism type (viruses/cysts more resistant); free chlorine more effective than combined; proper mixing; and presence of ammonia/organics that consume chlorine.

## Answer

The efficiency of chlorination (kill of micro-organisms) depends on the following factors.

1. **Chlorine dose and demand**: dose must exceed the demand (organic matter, ammonia, iron) to leave a residual.
2. **Contact time**: the longer the contact, the better the kill ($C^n t$ = constant); usually at least 30 min.
3. **pH**: hypochlorous acid (HOCl) is a much stronger disinfectant than OCl$^-$; at low pH more HOCl is present, so chlorination is best at pH 6 to 7.
4. **Temperature**: higher temperature speeds up the reaction and increases the kill rate; cold water needs a longer contact time or a higher dose.
5. **Turbidity and suspended matter**: particles shield bacteria and consume chlorine; clear water is disinfected better.
6. **Kind and number of organisms**: viruses, cysts and spores are more resistant than bacteria.
7. **Nature of chlorine compound**: free chlorine is a faster disinfectant than combined (chloramines).
8. **Mixing**: proper, quick mixing ensures uniform contact.
9. **Ammonia, organic nitrogen and reducing substances** in the water.

The overall relationship used in design is $C^n\,t = \text{constant}$ for a given percentage kill, where $C$ is the concentration of the disinfectant, $t$ the contact time and $n$ a constant (about 1 for chlorine).

## Check yourself

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

1. What does the relation C^n t = constant mean?
   A. For a given kill, higher concentration needs less contact time and vice versa
   B. Contact time has no effect
   C. Concentration is independent of kill
   D. Only temperature matters
   Answer: A. C is concentration, t contact time, n about 1 for chlorine. (Key point: Contact time)

2. How does temperature affect chlorination?
   A. Higher temperature reduces kill
   B. Higher temperature speeds the reaction and kill; cold water needs more time or dose
   C. No effect
   D. Cold water kills faster
   Answer: B. Reaction rates rise with temperature. (Key point: Temperature)

3. Why does turbidity reduce chlorination efficiency?
   A. Turbid water has lower pH
   B. Particles increase HOCl
   C. Particles shield bacteria and consume chlorine
   D. Turbidity speeds up the reaction
   Answer: C. Clear water is disinfected better. (Key point: Turbidity)

4. Which are more resistant than bacteria?
   A. Coliforms
   B. E. coli only
   C. None
   D. Viruses, cysts and spores
   Answer: D. They need a higher dose or longer contact. (Key point: Organism resistance)

## Related in Water Supply Engineering

- [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×
- [Break point chlorination with sketch; factors](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Abpc-sketch-factors) — Water Supply Engineering, chapter 6
- [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×
- [Coagulant and factors affecting coagulation](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Acoagulant-factors) — Water Supply Engineering, chapter 6

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