Environmental Engineering Reference
In-Depth Information
5.3.7 Preliminary Treatment Process Control Calculations
5.3.7.1 Channel Velocity
The desired velocity in sewers in approximately 2 fps at peak flow,
because this velocity normally prevents solids from settling from the
lines; however, when the flow reaches the grit channel, the velocity
should decrease to about 1 fps to allow the heavy inorganic solids to set-
tle. In the example calculations that follow, we describe how the velocity
of the flow in a channel can be determined by the float and stopwatch
method and by flow and channel dimensions.
5.3.7.1.1 velocity by float and Stopwatch
Distance Traveled (ft)
Time R
Velocity (fps)
=
(5.7)
equired(s)
Example 5.8
Problem:
A float takes 25 s to travel 34 ft in a grit channel. What is the
velocity of the flow in the channel?
Solution:
34 ft
25 s
Velocity
=
=
1.4fps
Example 5.9
Problem:
A float takes 30 s to travel 37 ft in a grit channel. What is the
velocity of the flow in the channel?
Solution:
37 ft
30 s
Velocity
=
=
1.2fps
5.3.7.1.2 velocity by flow and Channel dimensions
×
f Channels in Service
Channel Width (ft)
Flow (MGD)
1.55 cfs/MGD
Velocity (fps)
=
⎡
⎣
⎢
No. o
⎤
⎦
⎥
(5.8)
×
× ater Depth (ft)
W
This calculation can be used for a single channel or tank or mul-
tiple channels or tanks with the same dimensions and equal flow. If the
flows through each unit of the unit dimensions are unequal, the velocity
for each channel or tank must be computed individually.
