---
title: "Grit chamber for 200 l/s, SOR 2"
subject: "Sanitary Engineering"
chapter: 7
key: "semester_6/sanitary_engineering:grit-200-ls-sor-2"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/grit-200-ls-sor-2
site: BCT Engineering
---

# Grit chamber for 200 l/s, SOR 2

Sanitary Engineering · Chapter 7: Wastewater Treatment

- Asked in: 2071 Magh, 8 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/grit-200-ls-sor-2
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-7

## Question

Design a grit chamber for a sewage flow of 200 liter/sec with SOR = 2 cm/sec and detention time = 1 min. Specific gravity of organic and inorganic particles are 1.2 and 2.7 respectively. Assume size of both organic and inorganic materials as 0.21 mm. Take k = 0.06 and f = 0.03.

## Short answer

For200 L\/ s, SOR=2 cm\/ s, detention 60 s: surface area=10 m\u00b2, volume=12 m\u00b3, depth=1.2 m. Scour: organic0.081 m\/ s, grit0.237 m\/ s, adopt0.20 m\/ s. Length=12 m, width=0.9 m. Chamber: L=12.0 m, B=0.9 m, water depth1.20 m (total ~1.75 m), velocity0.20 m\/ s; two units.

## Answer

$Q=200$ l/s $=0.2$ m³/s, SOR $=2$ cm/s, detention $=1$ min $=60$ s, particle size 0.21 mm, $G_{inorganic}=2.7$, $k=0.06$, $f=0.03$.

#### Step 1: Surface area and depth

$$
\begin{aligned}
Q &= 0.200\ \text{m}^3/\text{s}\\
\text{Surface area } A &= \frac{Q}{SOR} = \frac{0.200}{0.020} = 10.00\ \text{m}^2\\
\text{Volume } V &= Q\,t = 0.200\times 60 = 12.00\ \text{m}^3\\
\text{Depth } H &= \frac{V}{A} = 1.20\ \text{m}
\end{aligned}
$$

#### Step 2: Scour velocity for the two kinds of particles

$$
v_c=\sqrt{\frac{8k}{f}(G-1)\,g\,d}
$$

| Particle | $G$ | $d$ (mm) | $v_c$ (m/s) |
|---|---|---|---|
| Organic | 1.2 | 0.21 | 0.081 |
| Inorganic (grit) | 2.7 | 0.21 | 0.237 |

The horizontal velocity must be **above** the organic scour velocity (organic matter stays suspended and is carried through) and **below** the grit scour velocity (settled grit is not re-suspended). Adopt $v_h = 0.20$ m/s, which satisfies 0.081 < 0.20 < 0.237 m/s. (The flow velocity should also be near 0.2-0.3 m/s in practice.)

#### Step 3: Dimensions

$$
\begin{aligned}
L &= v_h\,t = 0.20\times 60 = 12.00\ \text{m}\\
B &= \frac{A}{L} = \frac{10.00}{12.00} = 0.83\ \text{m}\\
\text{Check: } A_c &= B\,H = 0.83\times 1.20 = 1.000\ \text{m}^2 \approx \frac{Q}{v_h} = 1.000\ \text{m}^2
\end{aligned}
$$

Adopt $L = 12.0$ m, $B = 0.9$ m, water depth $H = 1.20$ m, plus 0.25 m grit storage and 0.30 m free board (total depth about 1.75 m). Provide two units in parallel.

**Answer: grit chamber $L\times B\times H = 12.0\times 0.9\times 1.20$ m, flow velocity 0.20 m/s, detention 60 s, surface overflow rate 2 cm/s.**

## Check yourself

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

1. For Q = 0.2 m^3/s and SOR = 2 cm/s, what is the surface area?
   A. A = 0.2 x 0.02 = 0.004 m^2
   B. A = 0.2 / 0.02 = 10 m^2
   C. A = 0.2 x 60 = 12 m^2
   D. A = 0.02 / 0.2 = 0.1 m^2
   Answer: B. A = Q/SOR with SOR converted to 0.02 m/s gives 10 m^2. (Key point: Surface area from SOR)

2. Scour velocities are 0.081 m/s (organic) and 0.237 m/s (grit). Which velocity is acceptable and why?
   A. 0.30 m/s, because faster flow carries organics better
   B. 0.05 m/s, because slow flow settles everything
   C. 0.20 m/s, since it is above the organic and below the grit scour velocity
   D. 0.081 m/s, equal to the organic scour velocity
   Answer: C. Between the two scour velocities, organics pass through while settled grit stays down. (Key point: Scour velocity window)

3. Why is L = 12 m for v_h = 0.20 m/s?
   A. L = A / B
   B. L = v_h x t = 0.20 x 60
   C. L = H x t
   D. L = Q / H
   Answer: B. A particle moves at v_h for the detention time, so L = v_h t = 12 m. (Key point: Length of chamber)

## Close variants set in other exams

- [Grit chamber for 190 l/s](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Agrit-190-ls) — Sanitary Engineering, chapter 7, asked 2×

## Related in Sanitary Engineering

- [Grit chamber for 180 l/s, SOR 1.5](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Agrit-180-ls-sor-1p5) — Sanitary Engineering, chapter 7
- [Grit chamber for 60 MLD, 0.23 mm](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Agrit-60mld-0p23) — Sanitary Engineering, chapter 7, asked 2×
- [Grit chamber for 58 MLD at 25°C](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Agrit-58mld-25c) — Sanitary Engineering, chapter 7
- [Grit chamber for 50 MLD](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Agrit-50mld-0p2mm) — Sanitary Engineering, chapter 7, asked 2×
- [Grit chamber for 55×10⁶ l/d at 27°C](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Agrit-55e6-ld-27c) — Sanitary Engineering, chapter 7
- [Horizontal flow grit chamber, Dang](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Agrit-dang-273000) — Sanitary Engineering, chapter 7

## Related in other subjects

- [Organic/inorganic water pollutants, effects, remedies](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Awater-pollution-substances-effects-remedies) — Engineering Chemistry, chapter 3, asked 2×
- [Tractive force design for 15 cumecs, 5 cm particles](https://www.bctengineer.com/question.md?key=semester_4%2Fhydraulics%3Atractive-force-15-cumecs-5cm-particles) — Hydraulics, chapter 10, asked 2×
- [Tractive force design for 10 cumecs, 3 cm particles](https://www.bctengineer.com/question.md?key=semester_4%2Fhydraulics%3Atractive-force-10-cumecs-3cm-particles) — Hydraulics, chapter 10, asked 2×