---
title: "Settling tank 30 m x 3 m; velocity for 0.02 mm"
subject: "Water Supply Engineering"
chapter: 6
key: "semester_5/water_supply_engineering:sed-bk68"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/sed-bk68
site: BCT Engineering
---

# Settling tank 30 m x 3 m; velocity for 0.02 mm

Water Supply Engineering · Chapter 6: Water Treatment

- Asked in: 2068 Baisakh (old course), 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-bk68
- 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 velocity of water would you recommend for effective removal of 0.02 mm particles. Express the velocity in mm/min. Assume specific gravity of particles = 2.65 and kinematic viscosity of water = 0.01 cm$^2$/sec.

## Short answer

Settling velocity by Stokes is 0.036 cm/s (21.58 mm/min). Horizontal velocity = v_s × L/H = 21.58 × 10 = 215.8 mm/min, so recommend ≤216 mm/min.

## Answer

For complete removal in an ideal horizontal-flow tank, a particle must settle through depth $H$ while the water flows through length $L$:
$$
v_h=v_s\,\frac{L}{H}
$$

#### Step 1: Settling velocity of the 0.02 mm particle
$d=0.002$ cm, $S_s=2.65$, $\nu=0.01$ cm²/s:
$$
v_s=\frac{981\times1.65\times0.002^2}{18\times0.01}=0.03597\ \text{cm/s}=0.3597\ \text{mm/s}=21.58\ \text{mm/min}\quad(Re=0.007<1)
$$

#### Step 2: Velocity of flow
$L=30$ m, $H=3$ m, $L/H=10$:
$$
v_h=21.58\times10=215.8\ \text{mm/min}
$$
(= 0.216 m/min, which is below the usual maximum of 0.3 m/min.)

**Answer: velocity of flow $\approx216$ mm/min.**

## Check yourself

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

1. Which relation gives the flow velocity for complete removal?
   A. vh = vs × H/L
   B. vh = vs × L/H
   C. vh = vs × L × H
   D. vh = vs/(L/H) squared
   Answer: B. A particle must settle H while the water travels L, so vh/vs = L/H. (Key point: Basic relation)

2. What is vs for the 0.02 mm particle, using Stokes' law?
   A. 0.03597 cm/s from g(Ss-1)d^2/(18ν)
   B. 0.3597 cm/s from g d/(18ν)
   C. 3.597 cm/s from g(Ss-1)d/ν
   D. 0.03597 cm/s from 18ν/(g(Ss-1)d^2)
   Answer: A. Stokes' law is vs = g(Ss-1)d^2/(18ν) with d = 0.002 cm, giving 0.03597 cm/s. (Key point: Settling velocity)

3. 0.03597 cm/s is expressed in mm/min as about
   A. 2.16 mm/min
   B. 216 mm/min
   C. 21.6 mm/min
   D. 0.36 mm/min
   Answer: C. 0.03597 cm/s = 0.3597 mm/s, and multiplying by 60 gives 21.58 mm/min. (Key point: Unit conversion)

4. With L/H = 10, the recommended velocity is about
   A. 2.16 mm/min
   B. 216 mm/min
   C. 21.6 mm/min
   D. 2160 mm/min
   Answer: B. vh = 21.58 × 10 = 215.8 mm/min (0.216 m/min), below the 0.3 m/min limit. (Key point: Final velocity)

## Close variants set in other exams

- [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×

## Related in Water Supply Engineering

- [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×
- [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 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×
- [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
- [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