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8.20 CHAPTER EIGHT

in the washwater gullet to prevent a common wall from existing between unfiltered wa-
ter in the gullet and filtered water in the underdrain system. Drainage should be provided
for spaces beneath false floors of this type.
Many designers favor construction of the conduits connecting pretreatment basins and
filters in such a manner that floc destruction because of turbulence and high velocities is
minimized. Drops, bends, and long runs should be minimized or avoided. The need to
avoid turbulence, however, is not universally accepted (Cleasby, 1972). Low velocities
may be dictated by head loss between basins and filters.
In warm climates, filters may be placed outdoors with precautions for controlling al-
gae formation in the filter box. One method is to provide for shock chlorination of filter
influent or washwater. In colder climates, filters are normally housed to prevent ice for-
mation. Filter control consoles are usually provided adjacent to each filter or each set of
filters to allow for local control of the backwash operation. Consoles should be provided
with climate-controlled housing.

Number of Filters. From a cost point of view, one filter is the most ideal. Practically,
however, four filters are the minimum number that should be used to allow for filter wash-
ing and the occasional need for a filter to be out of service for maintenance, without re-
suiting in unreasonable rate increases in the other filters. If self-backwashing filters are
used, allowing for the lowest rate of operation and for one filter out for maintenance, the
remaining filters must be able to produce sufficient water to wash one filter effectively.
Kawamura (1999) recommends the following formula as a guide to determining the re-
quired number of filters:
N = 1.2Q °5

where N is the number of filters and Q is the plant design flow rate in million gallons per
day.

Size of Filters. The size of individual gravity filters is determined by plant capacity, fil-
tration rate, and the number of filters desired. Hydraulic considerations and the effect of
removing a filter from service limit maximum filter size. Additional considerations in-
clude the maximum area to which washwater or air scour can be evenly distributed, the
maximum span length of washwater collection troughs, and available sizes of surface wash
equipment, if used. Single gravity filters of up to 4,500 ft 2 (420 m 2) have been reported
(Clark, Viesman, and Hammer, 1977), but units less than one-half this size are more typ-
ical, even in large plants. Large filters may be divided into two sections by using a cen-
tral gullet, permitting one-half of the filter to be washed at a time, although influent and
effluent piping is usually shared. In general, the practical maximum size of a filter is 1,000
ft 2 (90 m 2) provided the plant is not exceptionally large (Kawamura, 1999).
Pressure filters are usually limited by shipping constraints. The largest standard units
typically available are 12-ft-diameter (3.7-m) tanks. This limits vertical filters to about
113 ft 2 (10 m 2) of filter medium. Horizontal filters are normally not longer than 40 ft (12
m). Larger units of both types can be specially fabricated on site.
Capital cost of filters can generally be minimized by designing for the minimum num-
ber of filters consistent with size limitations. Plant expansions are usually accomplished
by adding filters of the same size as existing units, because installing larger filters may
require extensive changes in the filter wash system.