UFC 3-280-03
23 JULY 2003
solids, slightly greater cake densities, and slightly shorter cycle times. High pressures
are generally necessary for biological sludge. However, higher pressures do not provide
increased benefits for very dense material (i.e., dense minerals, carbon, dirt, sand) or if
final moisture content is not an issue (e.g., polishing applications). Typical pressure
requirements for both fixed-volume and variable-volume filter press sludge dewatering
applications are presented in Subparagraph 2-3.3.2, and Tables 2-4 and 2-5,
respectively. Although many types of sludge can be successfully treated at either of
these terminal pressures, using the higher pressure for some sludges (e.g., metal
hydroxide sludge) can increase cake resistance and decrease porosity because of
compression, resulting in decreased filtration flow rates. To avoid this problem, evaluate
how well the lower pressure worked for dewatering a similar sludge. In addition to
evaluating operating pressures, selecting proper filter media and sludge conditioning
can alleviate pressure effects.
b. The number of plates in the press can also affect the overall efficiency of
the filtration process and sludge cake moisture content. The effect of increasing the
number of plates that is most often observed is poor distribution of sludge throughout
the filter chamber. This happens especially in larger filter presses (>630 mm, 24 inches)
that are fed at one end because the chambers nearest the feed entry point begin filling
with sludge and filtering, while the chambers at the center or end of the press have not
yet started to fill. As a result, unequal pressures develop in the press, resulting in cakes
with various solids yield and moisture contents. In addition, equipment can be damaged
(plates can warp and eventually break). This effect may be alleviated by using a lower
pressure filling cycle or filling the press from both ends. In general, when 80 or more
plates are used, feeding from both ends of the press should be considered.
c. The sludge feed method and sludge transport method are critically
important to filter performance. After conditioning, it is important not to allow the floc that
has formed to deteriorate. Therefore, a positive displacement pump that minimizes floc
shearing, such as a plunger, piston, or progressive capacity pump, should be used to
transfer conditioned sludge into the press. Centrifugal-type pumps should not be used
to feed the press because the high shear force of the impeller can cause floc shearing
and deterioration or can destabilize it. Additional details on sludge transport and feed
pumps are presented in Paragraph 2-4.3.
d. The type and amount of mixing in the auxiliary systems that add chemicals
for conditioning are important performance factors. During conditioning, the type and
amount of mixing should be sufficient to ensure that the feed properly flocculates and to
prevent particles from segregating because of varying size and density. High mixing or
long agitation periods may increase the potential of floc shearing and further reduce the
overall sludge filterability. Additional chemical conditioning considerations are presented
in Subparagraph 2-4.4.5.
2-3.3.2 Typical Performance Data. Typical performance data for various types of
sludge (including municipal wastewater, industrial waste, and various other sludges)
from both fixed-volume and variable-volume presses are presented in Tables 2-4 and 2-
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