23 JULY 2003
2-18.104.22.168 Process Factors. The process factors consist primarily of sludge
characteristics, including particle sizes, specific gravity, sludge conditioning, and sludge
a. Although no specific data are available on the particle size distribution for
different sludge dewatering applications, the general effects of particle sizes on filtration
are best illustrated by the following examples (WPCF 1983). First, if particles are of
equal size, the resultant cake will be loosely packed and relatively unstable, especially if
the filtration cycle incurred large pressure drops. Second, if the particles are relatively
flat, the resultant cake may generate a relatively impervious envelope characterized by
a high moisture content or fluid-like center. Ideally, a wide variety of particle sizes is
desirable to keep an open matrix of particles that allows free drainage of entrained wa-
ter. This effect is common for biological sludges, because their gelatinous nature allows
small void spaces to be filled. Most sludge requires the use of conditioning chemicals or
filter aids (i.e., fly ash) to generate the desired particle range or to provide additional
structural integrity to allow for open drainage and water release. Subparagraph 2-4.4.5
provides a detailed discussion of chemical conditioning and filter aids. In addition to the
use of filter aids, mixing of chemical sludge, such as alum or metal hydroxide sludge,
with biological sludge may add structural integrity and aid in the dewatering of the
b. The specific gravity of particles can also affect the cake formation and
filtration pressures. If the sludge contains a wide range of specific gravities, particles
can settle in the lower chambers of the press and cause poor cake formation and
unbalanced pressure in the cake. This effect keeps the larger particles from settling out.
This effect is less noticeable for sludge feeds containing finely sized particles.
c. To make filtration more effective, sludge can be conditioned in several
ways. Using more than one conditioning chemical, regulating the mixing energy,
allowing the sludge to age, and using heat can all be combined. The treatability tests
described in Paragraph 2-6 can help determine the effectiveness of sludge conditioning.
Additional details on sludge conditioning effects are presented in Paragraph 2-4.4.
d. Sludge storage may also have an effect on filtration performance. Storage
time can either be the period in which the sludge is stored before conditioning, or the
period after initial mixing with the conditioning chemicals before filtration. Generally,
prolonged storage is detrimental to filterability in either case. Additional details on
sludge storage are presented in Paragraph 2-4.2.
2-22.214.171.124 Equipment/Auxiliary System Factors. Equipment and auxiliary system fac-
tors that typically affect sludge dewatering performance include pressure, number of
plates, feed method, and mixing systems.
a. Pressure in a filter press is the overall driving force of the filtration process.
Filter presses are typically designed for operating pressures of 690 to 1550 kPa (100 psi
or 225 psi). In general, the higher pressure will yield higher percentages of sludge cake