---
title: "Alum and chlorine quantity, 12 MLD"
subject: "Water Supply Engineering"
chapter: 6
key: "semester_5/water_supply_engineering:alum-chlorine-as69"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/alum-chlorine-as69
site: BCT Engineering
---

# Alum and chlorine quantity, 12 MLD

Water Supply Engineering · Chapter 6: Water Treatment

- Asked in: 2069 Asar, 8 marks
- Video: [Water Treatment Math | Chlorine Dose Calculation](https://www.youtube.com/watch?v=eavbtRNxhHM) — American Water College
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/alum-chlorine-as69
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-6

## Question

Find the quantity of alum and chlorine required in a treatment plant of capacity 12 million lit/day. If optimum dose of alum is 3 mg/l and residual chlorine is expected in the distribution pipe is at the concentration of 0.2 mg/l.

## Short answer

With12 MLD flow and alum dose3 mg/L, alum required=36 kg/day. Assuming negligible chlorine demand, the dose equals desired residual0.2 mg/L, giving chlorine=2.4 kg/day.

## Answer

Plant capacity = 12 MLD $= 12\times10^6$ l/day. Using $1\ \text{mg/l}\times1\ \text{MLD} = 1\ \text{kg/day}$:

**Alum** (optimum dose 3 mg/l):
$$
\text{Alum} = 3\times12 = 36\ \text{kg/day}
$$

**Chlorine**: the residual chlorine wanted in the distribution pipe is 0.2 mg/l. No chlorine demand is given, so the demand of water is assumed to be nil and the dose is taken as equal to the residual (if there were a demand, the dose = demand + residual):
$$
\text{Chlorine} = 0.2\times12 = 2.4\ \text{kg/day}
$$
If bleaching powder with 30 % available chlorine is used, the quantity would be $\dfrac{2.4}{0.30} = 8.0$ kg/day.

**Answer: alum = 36 kg/day; chlorine = 2.4 kg/day.**

## Check yourself

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

1. How much alum is needed for 12 MLD at an optimum dose of 3 mg/l?
   A. 4 kg/day
   B. 36 kg/day
   C. 360 kg/day
   D. 15 kg/day
   Answer: B. 3 mg/l x 12 MLD = 36 kg/day. (Key point: Alum quantity)

2. Why is the chlorine dose taken equal to the 0.2 mg/l residual?
   A. Residual is always the same as the dose
   B. No chlorine demand is given, so demand is assumed nil
   C. Chlorine is not consumed in water
   D. Residual is the only part that disinfects
   Answer: B. Dose = demand + residual, and with demand assumed nil the dose equals the residual. (Key point: Dose when demand is not given)

3. What chlorine quantity per day is needed for 12 MLD?
   A. 0.24 kg/day
   B. 2.4 kg/day
   C. 8.0 kg/day
   D. 24 kg/day
   Answer: B. 0.2 x 12 = 2.4 kg/day. (Key point: Chlorine quantity)

## Related in Water Supply Engineering

- [Chlorine dose and contact time at pH 8 (0.5 mg/l)](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Achlorine-dose-contact-c72) — Water Supply Engineering, chapter 6, asked 2×
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- [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
- [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

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