---
title: "Super/break point chlorination; bleaching 20 MLD"
subject: "Water Supply Engineering"
chapter: 6
key: "semester_5/water_supply_engineering:bleaching-a76"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/bleaching-a76
site: BCT Engineering
---

# Super/break point chlorination; bleaching 20 MLD

Water Supply Engineering · Chapter 6: Water Treatment

- Asked in: 2076 Asoj, 8 marks
- 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-a76
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-6

## Question

Differentiate between super chlorination and break point chlorination. What is the significance of residual chlorine? Calculate the daily quantity of bleaching powder required in a treatment plant of capacity 20 million liters per day. Consider chlorine dose of 0.5 mg/l assuming 35% chlorine available in bleaching powder.

## Short answer

Break-point chlorination uses just enough chlorine to pass the break point and leave free residual without dechlorination, unlike super-chlorination’s high dose and required dechlorination. Residual chlorine provides a safety margin against recontamination. For20 MLD at0.5 mg/L and35% chlorine, daily bleach=10.0/0.35=28.57 kg.

## Answer

#### Super-chlorination and break-point chlorination
| Point | Super-chlorination | Break-point chlorination |
|---|---|---|
| Dose | Very high (3 to 15 mg/l), much above the demand | Just enough to pass the break point (typically 1 to 3 mg/l) |
| Aim | Quick kill in an emergency or with short contact time | Destroy ammonia, organics, taste and odour, and obtain a free residual |
| Residual | Large; the excess is removed by de-chlorination | Free residual of 0.2 to 0.5 mg/l |
| De-chlorination | Always required | Not required |
| Use | Epidemics, new or repaired mains, very poor water | Routine for water with ammonia or organic load |
| Cost | Higher (extra chemicals) | Moderate |

#### Significance of residual chlorine
**Residual chlorine** is the chlorine left in water after the demand has been satisfied and the required contact time has passed. A free residual of about 0.2 to 0.5 mg/l is usually maintained at the plant.

**Significance**
- It is a **safety margin** against re-contamination in service reservoirs and distribution pipes.
- Its presence shows that the chlorine demand has been met and that disinfection is complete.
- It is easy to measure (orthotolidine or DPD test) at any point, so it gives a quick check of plant performance.
- It continues to kill bacteria that enter through leaks or back-siphonage.
- Too high a residual gives an unpleasant taste and may produce trihalomethanes, so it is limited (WHO guideline maximum 5 mg/l; Nepal's NDWQS 2062 specifies 0.1 to 0.2 mg/l in chlorinated supplies).

#### Daily quantity of bleaching powder
Flow = 20 MLD $= 20\times10^6$ l/day; dose = 0.5 mg/l; available chlorine = 35 %.

$$
\begin{aligned} \text{Chlorine} &= 0.5\times20\times10^6\ \text{mg/day} = 10.0\ \text{kg/day} \\ \text{Bleaching powder} &= \frac{10.0}{0.35} = 28.57\ \text{kg/day} \end{aligned}
$$

**Answer: 28.57 kg of bleaching powder per day.**

## Check yourself

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

1. Which needs de-chlorination?
   A. Break-point chlorination
   B. Both
   C. Neither
   D. Super-chlorination
   Answer: D. Super leaves a large excess. (Key point: Super vs break-point residual)

2. Why is residual chlorine significant?
   A. Safety margin against re-contamination, and proof that disinfection is complete
   B. It softens water
   C. It removes iron
   D. It clarifies water
   Answer: A. Also easy to measure with DPD. (Key point: Residual significance)

3. Chlorine for 20 MLD at 0.5 mg/l?
   A. 1 kg/day
   B. 0.5 x 20x10^6 mg = 10 kg/day
   C. 100 kg/day
   D. 28.57 kg/day
   Answer: B. 10^7 mg = 10 kg. (Key point: Chlorine mass)

4. Bleaching powder at 35 % chlorine?
   A. 10 x 0.35 = 3.5 kg/day
   B. 10 kg/day
   C. 10 / 0.35 = 28.57 kg/day
   D. 35 kg/day
   Answer: C. Divide by the available fraction. (Key point: Bleaching powder)

## Related in Water Supply Engineering

- [Break point chlorination; bleaching powder for 2 MLD](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ableaching-c69) — 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
- [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
- [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
- [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
- [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