---
title: "Settling tank 30 m x 3 m, 0.02 mm at 25°C; 0.01 mm at 10°C"
subject: "Water Supply Engineering"
chapter: 6
key: "semester_5/water_supply_engineering:sed-as70"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/sed-as70
site: BCT Engineering
---

# Settling tank 30 m x 3 m, 0.02 mm at 25°C; 0.01 mm at 10°C

Water Supply Engineering · Chapter 6: Water Treatment

- Asked in: 2070 Asar, 8 marks
- Video: [Sedimentation tank, design of sedimentation tank, detention time, overflow rate, flow velocity, TSS](https://www.youtube.com/watch?v=43wzunPfwmA) — Civil Engineering Exam
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/sed-as70
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-6

## Question

In a continuous flow settling tank 30 m long and 3 m deep, what detention time would you recommend for effective removal of 0.02 mm particles at 25°C? Assume specific gravity of particles = 2.65. Also determine the percentage of 0.01 mm particles removed in the same tank at 10°C.

## Short answer

vs for 0.02 mm at 25°C = 0.0403 cm/s, giving detention time = 300/0.0403 = 2.07 h. Overflow rate v0 = 0.0403 cm/s. At 10°C, vs for 0.01 mm = 0.00689 cm/s; removal = 0.00689/0.0403 = 17.1%.

## Answer

For 100 % removal a particle must settle the full depth in the detention time: $t=H/v_s$. For smaller particles, the removal is $v_s/v_0$ where $v_0=H/t$.

#### (a) Detention time for 0.02 mm at 25 °C
$d=0.002$ cm, $S_s=2.65$, $\nu_{25}=0.00893$ cm²/s, $H=300$ cm.
$$
v_s=\frac{981\times1.65\times0.002^2}{18\times0.00893}=0.0403\ \text{cm/s}\quad(Re=0.009<1)
$$
$$
t=\frac{300}{0.0403}=7448\ \text{s}=2.07\ \text{h}
$$

#### (b) Removal of 0.01 mm particles at 10 °C
The tank's overflow rate is $v_0=H/t=0.0403$ cm/s. At 10 °C, $\nu=0.01306$ cm²/s, $d=0.001$ cm:
$$
v_s=\frac{981\times1.65\times0.001^2}{18\times0.01306}=0.00689\ \text{cm/s}\quad(Re=0.0005<1)
$$
$$
\text{Removal}=\frac{0.00689}{0.0403}=0.171\Rightarrow17.1\ \%
$$

**Answer: detention time = 2.07 h; removal of 0.01 mm particles at 10 °C = 17.1 %.**

## Check yourself

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

1. With vs = 0.0403 cm/s and H = 300 cm, what is the detention time?
   A. About 2.07 h
   B. About 2.41 h
   C. About 0.57 h
   D. About 20.7 h
   Answer: A. t = 300/0.0403 = 7448 s = 2.07 h. (Key point: Detention time)

2. What is the overflow rate of this tank?
   A. H/t = 0.0403 cm/s
   B. L/t
   C. vs of the small particle
   D. 0.00689 cm/s
   Answer: A. v0 = H/t, which equals the settling velocity of the 0.02 mm particles. (Key point: Tank overflow rate)

3. Why is vs for 0.01 mm particles at 10 C only 0.00689 cm/s?
   A. Smaller diameter and higher viscosity (0.01306 cm2/s)
   B. Lower specific gravity
   C. Higher g
   D. Turbulence
   Answer: A. vs depends on d^2 and 1/nu; both the small d and the cold water reduce it. (Key point: Temperature and viscosity)

4. What percentage of the 0.01 mm particles is removed at 10 C?
   A. 100%
   B. 17.1%
   C. 58.5%
   D. 1.7%
   Answer: B. Removal = vs/v0 = 0.00689/0.0403 = 0.171. (Key point: Percentage removal)

## Close variants set in other exams

- [Settling tank 20 m long, 3.5 m deep; 0.02 and 0.01 mm](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ased-b79) — Water Supply Engineering, chapter 6, asked 3×
- [Settling tank 30 m x 3 m, 0.03 mm at 25°C; 0.025 mm at 20°C](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ased-k72) — Water Supply Engineering, chapter 6, asked 3×

## Related in Water Supply Engineering

- [Settling tank 30 m x 3 m; velocity for 0.02 mm](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ased-bk68) — Water Supply Engineering, chapter 6, asked 2×
- [Settling tank 30 m x 3 m, 0.02 mm, SG 2.50](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ased-bk80) — Water Supply Engineering, chapter 6
- [Settling tank 20 m x 2.5 m; velocity for 0.02 mm](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ased-b66) — Water Supply Engineering, chapter 6, asked 2×
- [Settling tank 15 x 4 x 2.5 m, 96% of 0.015 mm](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ased-k78) — Water Supply Engineering, chapter 6

## Related in other subjects

- [Continuous flow sedimentation tank particle size](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Ased-tank-continuous-4m-60m) — Sanitary Engineering, chapter 7