Civil Engineering Reference
In-Depth Information
30
1.2
1.70
4.0
1.55
1.65
25
1.0
4.1
2.65
20
0.8
2.60
15
0.6
10
0.4
5
0.2
0
0
0
0.5
1.0
1.5
2.0
2.5
3.0
60 percent weight grain size calculated as effective size times uniformity coefficient
Fig. 12-12.
Backwash rates based on filter media grain size (Reprinted from
Water Treatment
Plant Design,
3d ed., by permission. Copyright
1998, American Water Works Association and
American Society of Civil Engineers)
v
v
t
(12-3)
t
/3
b
t
b
20
in which
t
is the temperature in
C,
v
b
t
is the backwash rate at temperature
t
,
v
b
20
is the backwash rate at 20
C, and
is the water viscocity in centipoise at temperature
t.
Figure 12-12 applies only for a media with a uniformity coefficient
1.5 and only
if fluidization of the filter bed is required (fluidization air-scouring filter wash). A
backwash rate of at least 18 gpm / sq ft (45 m / h) is necessary to purge air bubbles
trapped in coarse deep filter beds after air scouring.
56
Media characteristics impact the wash water flow, increasing with larger media size
and higher media density. If more than one type of media is employed, the backwash
rate must provide for proper stratification of the filter media. Each method of filter
washing has characteristic upflow rates and durations. The design of the underdrain
and auxiliary scour systems largely affect the flow rate required.
The required head for backwashing gravity filters typically ranges from 4 to 7 feet,
with additional head required to compensate for headlosses in the piping system be-
tween the backwash pump and the filters and the required static lift.
