Page 460 - Water and wastewater engineering

GRANULAR FILTRATION 11-33

e. The volume of backwash water for 15 minutes (0.25 h) of backwash is

.
)
(36 m/h )(2 cells )(2 6 m )(55 m )(025 h 257..4 m 3
.
.
3
Provide two times this volume or 514.8 or about 520 m .
f. Assuming a gravity feed, the lowest level of water the backwash tank should be 11 m
above the lip of the wash troughs (Kawamura, 2000).

Gullet
There are two gullets. The lower gullet carries the filtered water to the piping that leads to the
clear well. It will not be discussed. The upper gullet is the one generally recognized and the one
that is the focus of this discussion. It carries pretreated water to the filter and carries backwash
water and filter-to-waste water away for treatment and recycling through the plant.
Flat-bottomed gullets ( Figure 11-14 ) may be designed with the following equation (Camp,
1970; Hudson, 1981):

⎛ 2( Q ww ) ⎞ 12 /
2
2
H ⎜ h 2 ⎟ (11-20)
⎝ gb h ⎠

where H depth at upstream end, m
h depth at distance x in Figure 11-14 , m
3
Q ww wash water discharge, m /s
x length of gullet, m
2
g acceleration due to gravity, 9.81 m/s
b width of gullet, m
The upper elevation of H, the depth of wash water at the upstream end, is set below the bot-
tom of the wash troughs discharging into the gullet so that the wash trough discharge is a free fall.
The elevation of the bottom of the gullet (the lower elevation of H ) is highly variable. Various
authors show it at the bottom of the underdrain, at the bottom of the media, at some elevation
between the top and bottom of the media, and at the top of the media (Amirtharajah, 1978; Castro
et al., 2005; Kawamura, 2000; MWH, 2005, Reynolds and Richards, 1996).

Backwash Gullet
troughs crest

v 2
>1.7
2g
H
h
Backwash
v
effluent
x
FIGURE 11-14
Illustration of terms used for gullet design ( Equation 11-20 ).