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

# Various forms of chlorination

Water Supply Engineering · Chapter 6: Water Treatment

- Asked in: 2070 Asar, 8 marks
- Video: [General Chemistry Lecture 225 - Various Forms of Chlorination of Natural Water-IV](https://www.youtube.com/watch?v=RgniSVjAYWU) — Chemistry Lectures By Prof. I. Z. Khan
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/forms-of-chlorination
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-6

## Question

Describe in detail the various forms of chlorination.

## Short answer

Forms include plain chlorination (chlorine only, dose 0.5-1 mg/L), pre-chlorination before filters, post-chlorination after filtration leaving residual, double (pre+post), break-point to achieve free residual, super-chlorination with high dose and dechlorination, de-chlorination to remove excess, and chloramine process using combined residual for long distribution.

## Answer

Chlorination is the addition of chlorine, as gas, bleaching powder (CaOCl$_2$, 25 to 35 % available Cl), high-test hypochlorite (HTH) or sodium hypochlorite, to destroy pathogenic organisms. The following forms are used.

1. **Plain chlorination**: chlorine is the only treatment given to the water. It is used for fairly clear raw water from protected sources (for example lakes or springs with low turbidity). Dose about 0.5 to 1 mg/l.
2. **Pre-chlorination**: chlorine is added before the filters (or before sedimentation) to reduce the load of organisms and algae, control taste and odour, oxidise iron, and help coagulation.
3. **Post-chlorination**: the normal disinfection step, applied after filtration, just before the water goes to the clear water tank. The dose is set to leave a residual of 0.2 to 0.5 mg/l.
4. **Double chlorination**: pre- and post-chlorination used together; also called two-stage chlorination.
5. **Break-point chlorination**: chlorine is added beyond the break point so that a free residual is present (see the curve below).
6. **Super-chlorination**: the dose is much higher than the demand (3 to 15 mg/l or more), followed by **de-chlorination** (using sulphur dioxide, sodium thiosulphate, sodium sulphite or activated carbon). Used during epidemics, after repair of mains, when the contact time is short or when the water is of very poor quality.
7. **De-chlorination**: removal of the excess chlorine after super-chlorination.
8. **Chloramine process**: ammonia is added with the chlorine to form chloramines, which give a longer-lasting but weaker residual; used in long distribution systems.

**Reaction in water**:
$$
Cl_2 + H_2O \rightarrow HOCl + HCl, \qquad HOCl \rightleftharpoons H^+ + OCl^-
$$

**Break-point curve**:
```text
 Residual
 chlorine
   ^
   |                  B
   |                 / \                     /
   |                /   \                  /
   |               /     \               /    free residual
   |              /       \            /      (HOCl + OCl-)
   | combined    /         \         /
   | residual   /           \      /
   |           /             \   /
   |          /               \/
   +----------+-------+-------+----------------->  Chlorine dose
   O          A       B       C
   |  Zone 1  | Zone 2| Zone 3|     Zone 4
```
A = demand of reducing agents met, B = peak of combined residual (chloramines), C = break point

## Check yourself

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

1. Where are pre- and post-chlorination applied?
   A. Pre after filtration; post before sedimentation
   B. Both at the intake only
   C. Both in the distribution pipes
   D. Pre before filters or sedimentation; post after filtration before the clear water tank
   Answer: D. Double chlorination uses both. (Key point: Pre- vs post-chlorination)

2. What follows super-chlorination?
   A. De-chlorination with SO2, sodium thiosulphate, sodium sulphite or activated carbon
   B. Coagulation with alum
   C. Aeration
   D. Nothing
   Answer: A. Excess chlorine must be removed before supply. (Key point: Super-chlorination)

3. What does the chloramine process give?
   A. A stronger, quicker kill than free chlorine
   B. A more lasting but weaker residual, suited to long distribution systems
   C. No residual
   D. Removal of hardness
   Answer: B. Ammonia added with chlorine forms chloramines. (Key point: Chloramine process)

4. When is plain chlorination used?
   A. For highly polluted water in epidemics
   B. Only after super-chlorination
   C. For already good or well-treated water, with dose 0.5 to 1 mg/l and at least 30 min contact
   D. For softening
   Answer: C. No pre-treatment is involved; just disinfection. (Key point: Plain chlorination)

## 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×
- [Super vs break point chlorination; residual](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Asuper-vs-break-point-chlorination) — Water Supply Engineering, chapter 6
- [Break point chlorination; pH and temperature effects](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Abpc-ph-temperature-hocl) — Water Supply Engineering, chapter 6, asked 2×