Wastewater Calculations - Mathematics and Statistics for the Environment - page 670

24.2.5 g rit C hannel v eloCity C alCulations
The optimum velocity in sewers is approximately 2 ft/sec (fps) at peak flow, because this velocity
normally prevents solids from settling out in the lines; however, when the flow reaches the grit
channel, the velocity should decrease to about 1 fps to permit the heavy inorganic solids to settle. 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 channel dimensions.

■ EXAMPLE 24.9: VELOCITY BY FLOAT AND STOPWATCH

Distance traveled (ft)
Time r

Velocity (fps) = (24.8)

equired(sec)

Problem: A float requires 30 sec to travel 37 ft in a grit channel. What is the velocity of the flow in
the channel?

Solution:

Velocity (fps) = 37 ft/30 sec = 1.2 fps

■ EXAMPLE 24.10: VELOCITY BY FLOW AND CHANNEL DIMENSIONS
This calculation can be used for a single channel or tank or for multiple channels or tanks with the
same dimensions and equal flow. If the flow through each unit of the unit dimensions is unequal, the
velocity for each channel or tank must be computed individually:

Flow (MGD) × 1.55 cfs/MGD

Velocity (fps) = (24.9)

Chann elsinservice × channelwidth (ft)water de × pth (ft)

Problem: The plant is currently using two grit channels. Each channel is 3 ft wide and has a water
depth of 1.3 ft. What is the velocity when the influent flow rate is 4.0 MGD?

Solution:

40 MGD × 1.55 cfs/MGD 6.2 cfs

7.8 ft

Velocity (fps) =

×× =

= 0.79 fps

2

2channel ss3ft 1.3 ft

Because 0.79 is within the 0.7 to 1.4 range, the operator of this unit would not make any adjustments.

Note: The channel dimensions must always be in feet. Convert inches to feet by dividing by 12
inches per foot.

24.2.6 r equired s ettling t ime C alCulations
This calculation can be used to determine the time required for a particle to travel from the surface
of the liquid to the bottom at a given settling velocity. To compute the settling time, settling velocity
in fps must be provided or determined by experiment in a laboratory.

Liquiddepth (ft)
Settli

Settlingtime(sec) = (24.10)

ng velocity (fps)