Page 404 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Rapid Filtration 359

12.4.2.7 Control Systems 12.4.3.3 Net Water Production
In former times, each individual filter was operated through a An expression to calculate total filter area that takes into
console for that filter located on the operating floor. The console account the time used for backwash and the water used for
was fitted with valves that controlled air flow to open and close backwash was given by Letterman (1977, 1982), that is,
valves and switches for any pumps and motors. The end of a
filter run could be initiated by headloss or time. A manual over- Q(design) ¼ A(filter) [number of filter runsper unit time
ride was possible. For the plant as a whole, a wall was fitted with
net water production per unit area of bed
various switches and gauges for control. During the 1980s,
per run] (12:40)
computer control became more common, that is, SCADA sys-
tems. The open=closed position of actuated valves and sensor
The number of filter runs per day is
data were input to an ‘‘interface board,’’ which in turn provided
data that may be interpreted by computer software. Motors were
controlled by micro-relays on the same board. The valves and N filter runs ¼ 1440 min =d (12:41)
motors could be controlled by an operator at the computer or d (WP=v) þ TB
through ‘‘set-points’’ with value monitored by the software.
and the ‘‘net water production’’ per filter run per unit area of
filter bed is
12.4.3 FILTER BOX
Area and depth are the main issues of filter box design. Both NWP ¼ WP VB (12:42)
are based largely on practice, except the pilot plant data that
may provide the basis for exploring alternatives, for example, Therefore, Equation 12.40 is
higher than normal filtration velocities, deep media bed, back-

wash alternatives, etc. filter runs
Q(design) ¼ A(filter) N NWP (12:43)
d
12.4.3.1 Filtration Rate

The filtration rate selected, that is, superficial filtration velocity, 1440 min=d
(12:44)
¼ A(filter)
Q=A, may be based either on practice or on a pilot plant study. (WP=v þ TB) (WP VB)
2
The traditional HLR was 4.88 m=h(2gpm=ft ) from about 1900
2
to about 1950. By 1970, the norm was 12.2 m=h(5 gpm=ft ), where
3
which is current, albeit there is no absolute upper limit. As noted, Q(design) is the design flow to a given filter (m =day)
2
the Los Angeles Aqueduct Plant at Sylmar, placed in operation A(filter) is the plan area of filter bed (m )
2
in 1987, demonstrated v 32.3 m=h (13.3 gpm=ft ). WP is the volume of water production per unit area of filter
3 2
bed per filter run (m water produced=m filter plan
12.4.3.2 Area of Filters area=filter run)
2
3
A first estimate of total filter area is determined by the selected v is the filtration rate (m =m =min)
filtration rate, that is, TB is the filter down time per backwash(min downtime=
filter run)
Q(peak day) VB is the volume of water required per unit of filter bed
(12:39)
A(total filter area) ¼ 3 2
v(selected) area per backwash(m backwash water=m filter bed
area=filter run)
where NWP is the net water production unit area of filter bed per
3
A(total filter area) is the calculated area of all filter beds to filter run (m water produced=unit area of filter bed=run)
2
be constructed (m )
Q(peak day) is the average flow for the estimated peak day ValuesgivenbyLetterman(1977,p.24)forsomeparameters
3
during the design period (m =s) are as follows: 10 TB 30 min, 2.037 VB 20.37 m 3
2
2
v(selected) is the superficial filtration velocity selected by backwash water=m filter bed area (50 VB 500 gal=ft ).
3
2
practice or pilot plant (m =m =s)
12.4.3.4 Depth of Filter Box
The filter bed area requires a uniform backwash, which is Guidelines from Kawamura (1991, p. 210) for the overall
the major limitation for bed area (as filter bed area increases depth of the filter box are as follows:
the difficulty of achieving a uniform backwash rate increases
due to the need for longer lateral pipes and thus increased
headloss). A rule of thumb for American practice is that the Type of Filter Depth (m) Range (ft) Depth (m) Average (ft)
2
2
area of a single filter bed should be less than 93 m (1000 ft ). Ordinary gravity 3.7–6.1 12.20 5.2 17
Usually, filters are much smaller; for example, the ones in Self-backwash 5.5–7.6 18–25 6.7 22
2
2
Figure 12.25 are each 26 m (280 ft ).