Ques 2) consider the following information for a rectangular primary clarifier a
ID: 287989 • Letter: Q
Question
Ques 2)
consider the following information for a rectangular primary clarifier
a) determine the required surface area(m^2),tank volume (m^3) and detention time (hours) under both average flow conditions and peak flow conditions
b) calculate and compare the scour velocity (m/s) and peak flow horizontal velocity (m/s)
Based on these two values,do you expect the particles to re-suspend during the clarification process.
number of clarifiers = 2
Qaverage=20,000m^3/d
Qpeak=50,000 m^3/d
channel width= 6m
sidewater depth = 4m
overflow rate at average flow =40m^3/m^2.d
density of water =1000kg/m^3
beta=0.05
diameter of particle =200 micrometer
g=9.81m/s^2
darcy-weisbach friction factor (f)=0.03
Explanation / Answer
Calculate the required surface area
A = Q/OR
= (20,000 m3 /d) / (40 m3 / m2 . d)
= 500 m2
Determine tank length
L = A/W
= 500 m2 / 2* 6 m = 41.7 m
However for the sake of convenience, the surface dimensions will be rounded off to 6 m by 42 m
Tank Volume = 4 m * 2(42m * 6m) = 2016 m2
Overflow rate = Q/A = (20,000 m3 /d) / 2(6m * 42 m)
= 39.7 m3 / m2.d
Detention time = Vol/Q
= (2016 m3) (24h/d) / (20,000 m3 /d)
= 2.42 h
Determine the detention time and overflow rate at peak flow
Overflow rate = Q/A = (50,000 m3/d) / 2(6m * 42 m)
= 99.2 m3 / m2.d
Detention time = Vol / Q
= (2016 m3) (24h/d) / (50,000 m3 /d)
= 0.97 h
Scour Velocity
Cohesion constant (k) = 0.05
Specific gravity (s) = 1.00
Acceleration due to gravity (g) = 9.81 m/s2
Diameter of particle (d) = 200 * 10-6 m
Darcy Weisbach friction factor (f) = 0.03
VH = 8k (s-1)gd
-------------- ………………………………(1)
f
= (8)(0.03)*(0.25)*(9.81)*(200*10-6)
----------------------------------------------
0.03
Compare the scour velocity with the peak flow horizontal velocity
V = Q/Ax
= ((50,000 m3/d) / 2 * (6m *4 m)) ((1/ (24 h/d) * (3600 s/h))
= 0.012 m/s