---
title: "Plain sedimentation tank for 3.75 L/s"
subject: "Water Supply Engineering"
chapter: 6
key: "semester_5/water_supply_engineering:sed-as69b"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/sed-as69b
site: BCT Engineering
---

# Plain sedimentation tank for 3.75 L/s

Water Supply Engineering · Chapter 6: Water Treatment

- Asked in: 2069 Asar, 8 marks
- Video: [Design of Continuous Flow Sedimentation Tank - Environmental Engineering Previous Question Paper](https://www.youtube.com/watch?v=Nkgv0e1pkyA) — Splash Environs
- 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-as69b
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-6

## Question

Design a rectangular shape plain sedimentation tank to treat 3.75 l/s of water. Assume effective depth as 2.1 m and detention time as 4 hours.

## Short answer

Flow is 324 m³/day. With 4 h detention and 2.1 m effective depth, required volume is 54 m³, plan area 25.7 m². For L:B=3:1, width is 3.0 m, length 9.0 m. SOR 12.0 m/day, velocity 0.036 m/min. Adopted effective size: 9.0 m × 3.0 m × 2.1 m (2.7 m overall).

## Answer

#### Data and assumption
$Q=3.75$ l/s; effective depth $H=2.1$ m; detention time $t=4$ h. Assume $L:B=3:1$.

#### Step 1: Flow and volume
$$
Q=3.75\times86\,400=324\,000\ \text{l/day}=324\ \text{m}^3/\text{day}
$$
$$
V=Qt=3.75\times10^{-3}\times4\times3600=54\ \text{m}^3
$$

#### Step 2: Plan area and dimensions
$$
A=\frac{V}{H}=\frac{54}{2.1}=25.71\ \text{m}^2,\qquad 3B^2=25.71\Rightarrow B=2.93\ \text{m},\ L=3B=8.78\ \text{m}
$$
Adopt $B=3.0$ m, $L=9.0$ m ($A=27.0$ m²).

#### Step 3: Checks
$$
t=27.0\times2.1/324\times24=4.20\ \text{h},\quad \text{SOR}=\frac{324}{27.0}=12.0\ \text{m}^3/\text{m}^2/\text{day},\quad v=\frac{324}{3.0\times2.1\times1440}=0.036\ \text{m/min}
$$
The velocity is below 0.3 m/min (no scouring). The SOR is below the usual 15 to 30, which means a conservative design for such a small flow. Add 0.3 m free board and 0.3 m sludge zone: overall depth 2.7 m.

**Answer: tank 9.0 m long $\times$ 3.0 m wide $\times$ 2.1 m effective depth (2.7 m overall).**

## Check yourself

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

1. For Q = 3.75 l/s and t = 4 h, what is the tank volume?
   A. 54 m3
   B. 324 m3
   C. 13.5 m3
   D. 5.4 m3
   Answer: A. V = 3.75x10^-3 m3/s x 14400 s = 54 m3. (Key point: Volume from flow)

2. With H = 2.1 m and L:B = 3:1, what are B and L?
   A. B about 2.93 m, L about 8.78 m
   B. B about 5 m, L about 5 m
   C. B about 8.78 m, L about 2.93 m
   D. B about 1.5 m, L about 17 m
   Answer: A. A = 54/2.1 = 25.71 m2 and 3B^2 = 25.71. (Key point: Plan area and size)

3. Why is SOR = 12 m3/m2/day acceptable?
   A. It is below 15 to 30, so conservative for a small flow
   B. It is above the usual range
   C. It exceeds 0.3 m/min
   D. It is within 15 to 30
   Answer: A. A lower SOR only means a conservative design. (Key point: SOR check)

## Related in Water Supply Engineering

- [Rectangular sedimentation tank, 0.2 mm/s, 3 hours](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ased-c71) — Water Supply Engineering, chapter 6, asked 2×
- [Rectangular sedimentation tank for 10 MLD](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ased-s71) — Water Supply Engineering, chapter 6
- [Rectangular sedimentation tank for 3000 persons](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ased-a75) — Water Supply Engineering, chapter 6
- [Plain sedimentation tank for 4 MLD](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ased-b78) — Water Supply Engineering, chapter 6
- [Rectangular sedimentation tank, 9 million L peak demand](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ased-c75) — Water Supply Engineering, chapter 6
- [Rectangular sedimentation tank, 20000 population, 0.4 mm/s](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ased-s73) — Water Supply Engineering, chapter 6

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