UFC 3-280-03
23 JULY 2003
2-2.1
Fixed-Volume Press. The fixed-volume press consists of a number of plates
held rigidly in a frame to ensure alignment. The plates are typically pressed together
hydraulically or electromechanically between fixed and moving ends of the press. The
sludge is typically fed through a large, centralized port in each plate, as shown in Figure
2-2, although some presses are corner fed. Entrained water is then forced out through
area of each plate. As the filter cycle begins, conditioned sludge is fed into the filter
press while the closing device holds the plates firmly together. The pressure in the inlet
sludge feed pump typically ranges from 690 to 1550 kPa (100 to 225 psi). As this
portion of the filtration cycle continues, the solids accumulate on the filter media in the
plate cavity, and filtrate is forced through the plate drainage channels. This portion of
the filtration cycle continues until a maximum pressure is obtained. This maximum
design pressure is then maintained for a period during which more filtrate is removed
and the desired cake solids content is achieved. The filtration cycle is typically ended
when a practical low feed rate is achieved (typically 5 to 7% of the initial or maximum
flow rate). The sludge feed pumping is stopped, and the individual plates are separated,
allowing the sludge cake to be discharged.
2-2.2
Variable-Volume Press. The variable-volume press operates similarly in
principle to the fixed-volume press. However, the variable-volume press incorporates a
flexible membrane across the face of the recess plate. Figure 2-3 shows a schematic of
a variable-volume filter press and filter cycle. The initial step of the filter cycle is similar
to the fixed-volume press, but the pressure of the sludge feed is typically lower and
ranges from 860 to 900 kPa (125 to 130 psi) (EPA 1982a). The filter cake starts to form
when feed pumping is begun. The initial fill time is generally defined as the point when
an instantaneous feed rate, filtrate rate, or cycle time (typically 10 to 20 minutes) is
achieved. After the press is filled, the sludge feed pump is turned off, and the filter cake
starts to form. The membrane is pressurized with compressed air or water to between
1520 to 1920 kPa (220 to 285 psi), thereby compressing the cake. Typically, 15 to 30
minutes of constant pressure is required to dewater the sludge cake to the desired
solids content. When the compression cycle is completed, the air or water is released
from behind the diaphragm, the plates are separated, and the cake is removed. This
compression or squeezing step decreases the overall cycle time required to produce the
sludge cake. In addition, the resultant cake is typically drier than those generated by a
fixed-volume press. However, the variable-volume press typically generates less volume
per cycle, the cakes are much thinner, and the press is typically more automated.
Therefore, it is more expensive than the fixed-volume press (e.g., as much as two to
three times the initial cost based on the same sludge cake volume generated).
2-6
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