---
title: "Break point chlorination; bleaching powder for 2 MLD"
subject: "Water Supply Engineering"
chapter: 6
key: "semester_5/water_supply_engineering:bleaching-c69"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/bleaching-c69
site: BCT Engineering
---

# Break point chlorination; bleaching powder for 2 MLD

Water Supply Engineering · Chapter 6: Water Treatment

- Asked in: 2069 Chaitra, 8 marks
- Video: [Bleaching Powder Problems](https://www.youtube.com/watch?v=CvdSJG9wvqI) — CIVIL_VSMSRKIT NIPANI
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/bleaching-c69
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-6

## Question

What is break point chlorination? How can you obtain the break point? Describe. How much quantity of bleaching powder is to be added in the treatment plant to disinfect 2 MLD of water, if the dose of chlorine is 0.5 ppm?

## Short answer

Break-point chlorination is dosing chlorine enough to satisfy all demand and provide free residual. The break point is obtained by testing increasing chlorine doses, measuring residual after fixed contact time, and plotting; the dose at minimum residual is the break point. For 2 MLD at 0.5 mg/L dose, assuming 30% available chlorine, bleaching powder = 3.33 kg/day.

## Answer

#### Break-point chlorination
**Break-point chlorination** is the addition of chlorine to water in a quantity large enough to oxidise all the chlorine-demanding substances (reducing agents, ammonia, organic matter) so that a **free residual chlorine** is left. The dose at which the residual falls to its minimum and then starts to rise again as free chlorine is the **break point**.

```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

1. **Zone 1 (O to A)**: the chlorine first reacts with reducing compounds (Fe$^{2+}$, Mn$^{2+}$, H$_2$S, organic matter) and is used up. No residual appears.
2. **Zone 2 (A to B)**: once this demand is met, chlorine reacts with ammonia and amines to form **chloramines** (NH$_2$Cl, NHCl$_2$). The combined residual rises to a peak at B.
3. **Zone 3 (B to C)**: further chlorine oxidises the chloramines to nitrogen gas and other products (N$_2$O, NCl$_3$), so the residual **falls** to a minimum at C. Tastes and odours are most noticeable in this zone.
4. **Zone 4 (beyond C)**: all the ammonia and oxidisable organic matter have been destroyed; the added chlorine remains as **free chlorine** (HOCl and OCl$^-$), and the residual rises in direct proportion to the dose.

#### How the break point is obtained
Add a series of increasing chlorine doses to equal samples of the water (say 0.5, 1, 1.5, 2, ... mg/l), allow a fixed contact time (say 30 min) and measure the residual chlorine in each (DPD or orthotolidine test). Plot the residual against the dose. The residual first rises, then falls to a minimum, and then rises again. The dose at the minimum is the **break point**; the working dose is taken at or a little beyond it.

#### Bleaching powder required
The strength of the bleaching powder is not given, so the usual 30 % available chlorine is assumed. Plant flow = 2 MLD $= 2\times10^6$ l/day; dose = 0.5 ppm (mg/l).

$$
\begin{aligned} \text{Chlorine} &= 0.5\ \text{mg/l} \times 2\times10^6\ \text{l/day} = 10^6\ \text{mg/day} = 1.00\ \text{kg/day} \\ \text{Bleaching powder} &= \frac{1.00}{0.30} = 3.33\ \text{kg/day} \end{aligned}
$$

**Answer: 3.33 kg of bleaching powder per day (assuming 30 % available chlorine).**

## Check yourself

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

1. Break-point chlorination: what is the break point?
   A. The dose at which the combined residual (chloramines) is at its peak
   B. The dose at which no chlorine residual is left at all
   C. The dose at which the free chlorine residual first appears in Zone 1
   D. The dose at which the residual falls to its minimum, after which added chlorine stays as free chlorine
   Answer: D. Past the break point, ammonia and organics are oxidised, so extra chlorine stays as free residual that rises with dose. (Key point: Break point definition)

2. How is the break point found in practice?
   A. Dose equal samples with increasing chlorine, measure residuals after fixed contact time, plot, and read the minimum before the final rise
   B. Measure turbidity only
   C. Add the maximum dose and test pH
   D. Count bacteria after boiling
   Answer: A. Residual first rises, falls to a minimum, then rises again. (Key point: Finding the break point)

3. Chlorine required for 2 MLD at 0.5 ppm?
   A. 10 kg/day
   B. 0.5 mg/l x 2x10^6 l = 1.0 kg/day
   C. 0.1 kg/day
   D. 3.33 kg/day
   Answer: B. 10^6 mg = 1 kg. (Key point: Chlorine mass)

4. With 30 % available chlorine assumed, bleaching powder per day?
   A. 1.0 x 0.30 = 0.3 kg/day
   B. 30 kg/day
   C. 1.0 / 0.30 = 3.33 kg/day
   D. 1.0 kg/day
   Answer: C. Divide by available fraction. (Key point: Bleaching powder)

## Related in Water Supply Engineering

- [Annual bleaching powder 10 MLD; break point](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ableaching-s71) — Water Supply Engineering, chapter 6
- [Bleaching powder for 2 MLD, 30% chlorine](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ableaching-b79) — Water Supply Engineering, chapter 6
- [Super/break point chlorination; bleaching 20 MLD](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ableaching-a76) — Water Supply Engineering, chapter 6
- [Break point chlorination; bleaching powder 500000 L](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ableaching-c71) — Water Supply Engineering, chapter 6
- [Bleaching powder for 5 MLD, chlorine demand 0.1](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ableaching-a75) — Water Supply Engineering, chapter 6
- [Alum and chlorine quantity, 12 MLD](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Aalum-chlorine-as69) — Water Supply Engineering, chapter 6