A municipal wastewater plant is to be designed to treat maximum flow rate
of
6
0
0
0
0
m
^
3
/
d
.
Target particle for settling has the following characteristics
-
D
₁
2
0
0
x
1
0
-
6
m
,
k
=
0
.
0
5
,
f
=
0
.
0
2
5
,
pp
=
1
.
2
5
x
1
0
3
kg
/
m
^
3
.
For a rectangular
classifier having ratio of length to width
>
6
,
overflow rate is at least
4
times
the settling v
é
locity and horizontal velocity at
-
most one
-
third of the scour
velocity.
(
i
)
Find the dimensions of the rectangular tank
(
ii
)
Determine detention time
This is AI generated Response.
Asked by: Md Israrul Haque
Created at: 2025-09-11 22:44:53
Final Answer:
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Sure! Let's break down the problem step-by-step to find the dimensions of the rectangular tank and the detention time for the wastewater treatment plant.
---
## Given Data
- **Maximum flow rate**: \( Q = 6000 \, \text{m}^3/\text{d} \)
- **Particle characteristics**:
- Diameter: \( D_1 = 200 \times 10^{-6} \, \text{m} \)
- Settling velocity constant: \( k = 0.05 \)
- Friction factor: \( f = 0.025 \)
- Particle density: \( \rho_p = 1.25 \times 10^3 \, \text{kg/m}^3 \)
- **Classifier dimensions**:
- Length to Width ratio: \( L/W > 6 \)
- **Overflow rate**: At least 4 times the settling velocity.
- **Horizontal velocity**: At most one-third of the scour velocity.
## Step 1: Calculate Settling Velocity
The settling velocity \( V_s \) for a particle can be calculated using the formula:
\[
V_s = k \cdot D_1^2 \cdot (\rho_p - \rho_f) \cdot g
\]
Where:
- \( \rho_f \) is the density of water, approximately \( 1000 \, \text{kg/m}^3 \)
- \( g \) is the acceleration due to gravity, approximately \( 9.81 \, \text{m/s}^2 \)
### Substituting the Values
1. Calculate \( \rho_p - \rho_f \):
\[
\rho_p - \rho_f = 1.25 \times 10^3 \, \text{kg/m}^3 - 1000 \, \text{kg/m}^3 = 250 \, \text{kg/m}^3
\]
2. Calculate \( V_s \):
\[
V_s = 0.05 \cdot (200 \times 10^{-6})^2 \cdot (250) \cdot (9.81)
\]
\[
= 0.05 \cdot 4 \times 10^{-8} \cdot 250 \cdot 9.81
\]
\[
= 0.05 \cdot 4 \times 10^{-8} \cdot 2452.5
\]
\[
= 0.05 \cdot 9.81 \times 10^{-5} = 4.905 \times 10^{-7} \, \text{m/s}
\]
### Settling Velocity
So, the settling velocity \( V_s \) is approximately:
\[
V_s \approx 4.905 \times 10^{-7} \, \text{m/s}
\]
## Step 2: Calculate Overflow Rate
Given that the overflow rate is at least 4 times the settling velocity:
\[
Q_{overflow} = 4 \cdot V_s
\]
### Calculating Overflow Rate
\[
Q_{overflow} = 4 \cdot 4.905 \times 10^{-7} \, \text{m/s} = 1.962 \times 10^{-6} \, \text{m/s}
\]
## Step 3: Calculate Dimensions of Rectangular Tank
### Flow Rate in m³/s
Convert the maximum flow rate from m³/d to m³/s:
\[
Q = \frac{6000 \, \text{m}^3/\text{d}}{86400 \, \text{s/d}} \approx 0.0694 \, \text{m}^3/\text{s}
\]
### Area Calculation
The area \( A \) of the tank can be calculated from the flow rate and the overflow rate:
\[
A = \frac{Q}{Q_{overflow}}
\]
Substituting the values:
\[
A = \frac{0.0694}{1.962 \times 10^{-6}} \approx 35323.12 \, \text{m}^2
\]
### Dimensions
Let \( W \) be the width. Then, the length \( L \) can be expressed as:
\[
L = 6W
\]
Substituting in the area equation:
\[
A = L \cdot W = 6W \cdot W = 6W^2
\]
Setting the two equations equal:
\[
6W^2 = 35323.12
\]
\[
W^2 = \frac{35323.12}{6} \approx 5887.19
\]
\[
W \approx \sqrt{5887.19} \approx 76.7 \, \text{m}
\]
Then,
\[
L = 6W \approx 6 \cdot 76.7 \approx 460.2 \, \text{m}
\]
## Step 4: Determine Detention Time
The detention time \( t \) can be calculated using the volume of the tank and the flow rate:
\[
V = L \cdot W \cdot h
\]
Where \( h \) is the height of the tank. Assuming an average height \( h \):
\[
V = 35323.12 \cdot h
\]
The detention time \( t \) can be calculated as:
\[
t = \frac{V}{Q}
\]
Substituting gives:
\[
t = \frac{35323.12 \cdot h}{0.0694}
\]
Assuming a typical height \( h \) (e.g., \( 3 \, \text{m} \)):
\[
t = \frac{35323.12 \cdot 3}{0.0694} \approx 1525629.36 \, \text{s} \approx 17.65 \, \text{days}
\]
---
## Final Answers
1. **Dimensions of the rectangular tank**:
- Width \( W \approx 76.7 \, \text{m} \)
- Length \( L \approx 460.2 \, \text{m} \)
2. **Detention time**:
- \( t \approx 17.65 \, \text{days} \)
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