Wastewater Calculations - Mathematics and Statistics for the Environment - page 671

■ EXAMPLE 24.11
Problem: The plant's grit channel is designed to remove sand, which has a settling velocity of 0.080
fps. The channel is currently operating at a depth of 2.3 ft. How many seconds will it take for a sand
particle to reach the channel bottom?

Solution:

2.3 ft
0.080 fps

Settlingtime(sec) = = 28.7 se c

24.2.7 r equired g rit C hannel l ength C alCulations
This calculation can be used to determine the length of channel required to remove an object with
a specified settling velocity:

Channeldepth (ft) lowvelo × city (fps)

Channellength = (2 4.11)

0.080 fps

■ EXAMPLE 24.12
Problem: The plant's grit channel is designed to remove sand, which has a settling velocity of 0.080
fps. The channel is currently operating at a depth of 3 ft. The calculated velocity of flow through
the channel is 0.85 fps. The channel is 36 ft long. Is the channel long enough to remove the desired
sand particle size?

Solution:

3ft 0.85 fps
0.080 fps

×

Channellength = = 31.6 ft

Yes, the channel is long enough to ensure that all of the sand will be removed.

24.2.8 v eloCity oF s Cour C alCulations
The Camp-Shields equation (Camp, 1942) is used to estimate the velocity of scour necessary to
resuspend settled organics:

8 ρρ
ρ

−

kgd
f

p

v =

(24.12)

s

where
v s = Velocity of scour.
k = Empirically determined constant.
d = Nominal diameter of the particle.
f = Darcy-Weisbach friction factor.
ρ p = Particle density.
ρ = Fluid density.

If the channel is rectangular and discharges over a rectangular weir, the discharge relation based on
Bernoulli's equation is