---
title: "Horizontal flow grit chamber, Dang"
subject: "Sanitary Engineering"
chapter: 7
key: "semester_6/sanitary_engineering:grit-dang-273000"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/grit-dang-273000
site: BCT Engineering
---

# Horizontal flow grit chamber, Dang

Sanitary Engineering · Chapter 7: Wastewater Treatment

- Asked in: 2079 Chaitra, 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-dang-273000
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-7

## Question

Design a horizontal flow rectangular grit chamber to operate at 22°C for a city located at Dang district having population of 273000. Assume any other data suitably.

## Short answer

Settling velocity by Hazen 0.0375 m/s, scour velocity 0.228 m/s. Adopt v_h=0.20 m/s. A_c=4.27 m², B=4.3 m, H=1.0 m. L_theo=5.33 m, final L=max(1.25×,30 s)=7.0 m. Chamber: 7.0 m × 4.3 m × 1.0 m water depth, total depth 1.55 m.

## Answer

**Assumptions** (Nepal practice): water supply 135 lpcd, 80% of it reaches the sewer, peak factor 2.5 (assumed, on the safe side; CPHEEO suggests about 2.25 for this population) (so $Q_{avg}=273000\times135\times0.8/1000 = 29484$ m³/day); grit of $d=0.2$ mm, $G=2.65$; $k=0.06$, $f=0.03$; the temperature is 22°C (Dang). Two horizontal-flow units are provided, each able to take the full peak flow.

#### Step 1: Design flow

$$
Q = \frac{273000\times 135\times 0.8\times 2.5}{1000\times 86400} = 0.8531\ \text{m}^3/\text{s}
$$

#### Step 2: Settling velocity of the grit

Hazen's formula (with temperature correction), $d$ in mm:

$$
v_s = 418\,(G-1)\,d^2\left(\frac{3T+70}{100}\right)\ \text{mm/s} = 418\times (2.65-1)\times 0.2^2\times\frac{3\times 22+70}{100} = 37.52\ \text{mm/s} = 0.0375\ \text{m/s}
$$

#### Step 3: Horizontal (flow-through) velocity

The velocity must not scour settled grit. Scour (critical) velocity (Camp-Shields):

$$
v_c = \sqrt{\frac{8k}{f}(G-1)\,g\,d} = \sqrt{\frac{8\times 0.06}{0.03}(2.65-1)\times 9.81\times 0.2\times 10^{-3}} = 0.228\ \text{m/s}
$$

Adopt $v_h = 0.20$ m/s (the scour velocity rounded down; the usual range is 0.2-0.3 m/s). This is high enough to keep the light organic matter moving, so it passes on instead of settling.

#### Step 4: Cross-section and dimensions

$$
\begin{aligned}
A_c &= \frac{Q}{v_h} = \frac{0.8531}{0.20} = 4.266\ \text{m}^2\\
\text{Adopt depth } H &= 1.0\ \text{m} \Rightarrow B = \frac{A_c}{H} = 4.27\ \text{m (adopt 4.3 m)}\\
L_{theo} &= \frac{v_h H}{v_s} = \frac{0.20\times 1.0}{0.0375} = 5.33\ \text{m}\\
L &= \max(1.25\,L_{theo},\ 30\,v_h) = \max(6.66,\ 6.00) = 6.66\ \text{m (adopt 7.0 m)}
\end{aligned}
$$

(25% is added for inlet and outlet turbulence, and the length is not allowed to give less than 30 s detention.)

#### Step 5: Checks

$$
\begin{aligned}
\text{Detention time} &= \frac{L}{v_h} = \frac{7.0}{0.20} = 35\ \text{s}\ (\text{within 30-90 s})\\
\text{Surface loading} &= \frac{Q}{B\,L} = \frac{0.8531\times 86400}{4.3\times 7.0} = 2449\ \text{m}^3/\text{m}^2\text{/day}
\end{aligned}
$$


**Total depth** $= H + \text{grit storage } 0.25 + \text{free board } 0.30 = 1.55$ m.

```text
 Plan:   inlet ->[ <-- L = 7.0 m --> ]-> outlet (weir)
                 |  B = 4.3 m          |
 Section: water level ~~~~~~~~~~~~~~~~~~~  H = 1.0 m
          grit storage 0.25 m  ______________
```
**Answer: provide a grit chamber (at least two units in parallel, each for the full flow so that one can be cleaned) of size $L\times B\times H = 7.0\ \text{m}\times 4.3\ \text{m}\times 1.0\ \text{m}$ (water depth), total depth about 1.55 m, with flow velocity 0.20 m/s.**

## Check yourself

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

1. How is the design flow found for the city?
   A. Population x peak factor only
   B. Average flow with no peak factor
   C. Population x 135 only
   D. Population x 135 lpcd x 0.8 sewage fraction x peak factor 2.5, giving 0.853 m3/s
   Answer: D. Grit chambers are designed for peak flow. (Key point: Design flow)

2. Why is 22°C used in Hazen's formula?
   A. It sets the population
   B. It sets the width
   C. It sets scour velocity
   D. Settling velocity depends on temperature through the factor (3T+70)/100
   Answer: D. Temperature changes water viscosity and therefore settling velocity. (Key point: Temperature)

3. What length results after adding 25% to the theoretical length 5.33 m?
   A. 9.0 m
   B. 5.33 m
   C. 4.0 m
   D. 6.66 m, adopted as 7.0 m
   Answer: D. 1.25 x 5.33 = 6.66 m; 30 v_h = 6.0 m is smaller, so 6.66 governs. (Key point: Dimensions)

4. Why provide two units in parallel?
   A. Each takes the full flow so one can be cleaned while the other works
   B. To reduce BOD
   C. To halve the flow velocity
   D. To double the length
   Answer: A. Parallel units permit cleaning without stopping treatment. (Key point: Units)

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