---
title: "Disinfection methods; alum and bleaching powder"
subject: "Water Supply Engineering"
chapter: 6
key: "semester_5/water_supply_engineering:bleaching-k72"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/bleaching-k72
site: BCT Engineering
---

# Disinfection methods; alum and bleaching powder

Water Supply Engineering · Chapter 6: Water Treatment

- Asked in: 2072 Kartik, 8 marks
- Video: [How to do disinfection of drinking water by using bleaching powder?](https://www.youtube.com/watch?v=uF5ejX72bUQ) — DR. GURU'S BIO LAB
- 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-k72
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-6

## Question

Mention the common methods of disinfection. Calculate the daily quantity of alum and bleaching powder required in a treatment plant of capacity 25 million liters per day. Consider optimum dose of alum as 15 mg/l and chlorine dose of 0.5 mg/l assuming 30% chlorine available in bleaching powder.

## Short answer

Common disinfection methods include boiling, chlorination, chloramine, ozonation, UV, and iodine. For25 MLD, alum at15 mg/L needs375 kg/day; chlorine dose0.5 mg/L requires12.5 kg/day chlorine, thus bleaching powder with30% chlorine=41.67 kg/day.

## Answer

#### Common methods of disinfection
1. **Boiling** (for small household quantities, 5 to 10 min).
2. **Chlorination** (gas, bleaching powder, hypochlorite): the most common method.
3. **Chloramine** and **chlorine dioxide** treatment.
4. **Ozonation**: a strong oxidant, no residual.
5. **Ultraviolet (UV) radiation**: for clear water, no residual.
6. **Iodine and bromine**, **potassium permanganate**, **silver (Katadyn)** for small supplies.
7. **Lime** and excess lime treatment for softening and disinfection.

#### Quantities required for 25 MLD
Flow = 25 MLD $= 25\times10^6$ l/day. 1 mg/l $\times$ 1 MLD = 1 kg/day.

**Alum** (dose 15 mg/l):
$$
15\times25 = 375\ \text{kg/day}
$$

**Chlorine** (dose 0.5 mg/l):
$$
0.5\times25 = 12.5\ \text{kg/day}
$$
**Bleaching powder** (30 % available chlorine):
$$
\frac{12.5}{0.30} = 41.67\ \text{kg/day}
$$

**Answer: alum = 375 kg/day; bleaching powder = 41.67 kg/day.**

## Check yourself

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

1. Which disinfection methods leave no residual in the water?
   A. Chlorination and chloramine
   B. Ozonation and UV radiation
   C. Bleaching powder and hypochlorite
   D. Boiling and chlorine dioxide
   Answer: B. Ozone and UV are effective but give no residual protection in the distribution system. (Key point: Methods without residual)

2. What does 1 mg/l x 1 MLD equal?
   A. 1 kg/day
   B. 10 kg/day
   C. 0.1 kg/day
   D. 1 g/day
   Answer: A. 1 mg/l x 10^6 l/day = 10^6 mg/day = 1 kg/day. (Key point: Quick dose-flow shortcut)

3. What daily alum quantity is needed for 25 MLD at 15 mg/l?
   A. 15 kg/day
   B. 41.67 kg/day
   C. 375 kg/day
   D. 3750 kg/day
   Answer: C. 15 x 25 = 375 kg/day. (Key point: Alum quantity)

4. For 25 MLD, 0.5 mg/l chlorine and 30% available chlorine, what is the bleaching powder per day?
   A. 12.5 kg
   B. 37.5 kg
   C. 41.67 kg
   D. 125 kg
   Answer: C. Chlorine = 0.5 x 25 = 12.5 kg/day; powder = 12.5/0.30 = 41.67 kg/day. (Key point: Bleaching powder quantity)

## Related in Water Supply Engineering

- [Disinfection factors; bleaching powder 5 ML/day](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ableaching-bk79) — 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
- [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
- [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
- [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
- [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