---
title: "Settling tank 30 m x 3 m, 0.03 mm at 25°C; 0.025 mm at 20°C"
subject: "Water Supply Engineering"
chapter: 6
key: "semester_5/water_supply_engineering:sed-k72"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/sed-k72
site: BCT Engineering
---

# Settling tank 30 m x 3 m, 0.03 mm at 25°C; 0.025 mm at 20°C

Water Supply Engineering · Chapter 6: Water Treatment

- Asked in: 2072 Kartik, 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-k72
- 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 0.03 mm particles at 25°C. Assume specific gravity of particles = 2.65. Also determine the percentage of 0.025 mm particles removed in the same tank at 20°C.

## Short answer

Detention time is 0.92 hours (55 minutes) using Stokes' law for the settling velocity of 0.03 mm particles at 25°C. For 0.025 mm particles at 20°C, the removal efficiency in the same tank is 61.6%.

## Answer

For complete removal in an ideal continuous-flow tank, $t=H/v_s$; for smaller particles the removal is $v_s/v_0$ with $v_0=H/t$.

#### (a) Detention time for 0.03 mm at 25 °C
$d=0.003$ cm, $S_s=2.65$, $\nu_{25}=0.00893$ cm²/s, $H=300$ cm.
$$
v_s=\frac{981\times1.65\times0.003^2}{18\times0.00893}=0.0906\ \text{cm/s}\quad(Re=0.030<1)
$$
$$
t=\frac{300}{0.0906}=3310\ \text{s}=55.2\ \text{min}=0.92\ \text{h}
$$

#### (b) Removal of 0.025 mm particles at 20 °C
Overflow rate of the tank: $v_0=H/t=0.0906$ cm/s. At 20 °C, $\nu=0.01007$ cm²/s, $d=0.0025$ cm:
$$
v_s=\frac{981\times1.65\times0.0025^2}{18\times0.01007}=0.0558\ \text{cm/s}\quad(Re=0.014<1)
$$
$$
\text{Removal}=\frac{0.0558}{0.0906}=0.616\Rightarrow61.6\ \%
$$

**Answer: detention time = 0.92 h (55 min); removal of 0.025 mm particles at 20 °C = 61.6 %.**

## Check yourself

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

1. What is the detention time for complete removal of 0.03 mm particles at 25 C?
   A. About 0.92 h
   B. About 9.2 h
   C. About 3.3 h
   D. About 0.092 h
   Answer: A. t = 300/0.0906 = 3310 s = 0.92 h. (Key point: Detention time for full removal)

2. Which value is the overflow rate used in part (b)?
   A. 0.0906 cm/s, from H/t
   B. 0.0558 cm/s
   C. 0.0030 cm/s
   D. 0.616 cm/s
   Answer: A. The tank is fixed by part (a), so v0 = H/t = 0.0906 cm/s. (Key point: Tank overflow rate)

3. What percentage of 0.025 mm particles is removed at 20 C?
   A. 100%
   B. 61.6%
   C. 38.4%
   D. 15.6%
   Answer: B. vs = 0.0558 cm/s, so removal = 0.0558/0.0906 = 61.6%. (Key point: Removal of smaller particles)

## 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, 0.02 mm at 25°C; 0.01 mm at 10°C](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ased-as70) — Water Supply Engineering, chapter 6, asked 3×

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- [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×
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- [Settling tank 15 m x 7 m x 3 m, 95% removal](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ased-b81) — 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