---
title: "Disinfection factors; bleaching powder 5 ML/day"
subject: "Water Supply Engineering"
chapter: 6
key: "semester_5/water_supply_engineering:bleaching-bk79"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/bleaching-bk79
site: BCT Engineering
---

# Disinfection factors; bleaching powder 5 ML/day

Water Supply Engineering · Chapter 6: Water Treatment

- Asked in: 2079 Baisakh, 8 marks
- Video: [pressure filter! Disinfection! chlorination! factory effecting! Numerical! types of aeration!#ioe](https://www.youtube.com/watch?v=Pr8fCTxjjG0) — Sanjay64official
- 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-bk79
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-6

## Question

Discuss the factors affecting disinfection. Calculate the quantity of bleaching powder required per day for disinfecting 5 ML per day. The dose of chlorine has to be 0.6 ppm and the bleaching powder contains 30% of available chlorine.

## Short answer

Key factors include organism type, disinfectant concentration and contact time, temperature, pH, turbidity, and chemical demand. For5 MLD at0.6 mg/L dose and30% chlorine, daily chlorine=3.00 kg/day, so bleaching powder=10.00 kg/day.

## Answer

#### Factors affecting disinfection
1. **Type and number of organisms**: viruses, spores and cysts are more resistant than vegetative bacteria; high numbers need more disinfectant.
2. **Nature and concentration of the disinfectant** (dose): $C^n t$ = constant.
3. **Contact time**: longer contact gives greater kill (at least 30 minutes).
4. **Temperature**: warm water speeds up the reaction; cold water needs more time.
5. **pH**: chlorine is more effective at lower pH, since more HOCl is present.
6. **Turbidity and suspended solids**: they shield microbes and use up the disinfectant, so disinfect after filtration.
7. **Chemical quality**: organic matter, ammonia, iron and manganese consume chlorine (chlorine demand).
8. **Mixing and uniform distribution** of the disinfectant.

#### Bleaching powder required
Flow = 5 ML/day $= 5\times10^6$ l/day; dose = 0.6 ppm; available chlorine = 30 %.

$$
\begin{aligned} \text{Chlorine} &= 0.6\ \text{mg/l}\times5\times10^6\ \text{l/day} = 3\times10^6\ \text{mg/day} = 3.00\ \text{kg/day} \\ \text{Bleaching powder} &= \frac{3.00}{0.30} = 10.00\ \text{kg/day} \end{aligned}
$$

**Answer: 10.00 kg of bleaching powder per day.**

## Check yourself

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

1. Why is chlorine more effective at lower pH?
   A. More hypochlorite ion OCl- is present
   B. More HOCl, the stronger disinfectant, is present
   C. Chlorine gas is released from the water
   D. Turbidity settles faster
   Answer: B. At low pH a larger fraction of chlorine is HOCl, which is a far better disinfectant than OCl-. (Key point: pH and disinfection)

2. Why should water be disinfected after filtration rather than before?
   A. Filters destroy chlorine
   B. Chlorine damages filter media
   C. Suspended solids shield microbes and consume disinfectant
   D. Contact time is shorter after filtration
   Answer: C. Turbidity and suspended solids protect organisms and use up chlorine, so clear water is disinfected more effectively. (Key point: Turbidity effect)

3. According to the relation C^n t = constant, what happens if the disinfectant concentration is increased?
   A. The required contact time decreases
   B. The required contact time increases
   C. The contact time is unchanged
   D. Disinfection stops
   Answer: A. With C^n t constant, a higher C allows a shorter t. (Key point: Contact time and dose)

4. For 5 ML/day at 0.6 ppm and 30% available chlorine, how much bleaching powder is needed per day?
   A. 3.00 kg
   B. 5.00 kg
   C. 6.00 kg
   D. 10.00 kg
   Answer: D. Chlorine = 0.6 x 5 = 3 kg/day; powder = 3/0.30 = 10 kg/day. (Key point: Bleaching powder calculation)

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