Solid/Liquid Separation
By Capillary Filtration
P. LEVIN, Research Engineer
General American Research Division (GATX)
Niles, Illinois
J.W. PATTERSON, Assistant Professor
Department of Environmental Engineering
Illinois Institute of Technology
Chicago, Illinois
APPLICATION TO SLUDGE DEWATERING
The typical wastwater treatment system has two principal products: the treated water
which is generally discharged to rivers or streams; and a sludge which is a dilute mixture
containing 1 to 4 percent solids in water. The sludge must be processed to reduce its high
water content, and to improve its handling characteristics. This is the function of the sludge
processing system described in this report.
The type of system best suited to separate, process, and dispose of the sludge depends
on such factors as the type and quantity of sludge, disposal opportunities, and economic
feasibility. In many regions of the country, relatively simple procedures (such as lagooning)
can be employed. There are other, more complex processes employing vacuum filters,
centrifuges, and other equipment. No one approach is clearly superior, even in situations
that appear to be similar, and 25 to 50 percent of the cost of waste treatment may be due to
sludge disposal m.
A substantial portion of the costs of ultimate disposal of sludge is in dewatering the
sludge prior to ultimate disposal. Existing techniques for dewatering sludges, including
vacuum filtration and centrifuging, give satisfactory performance for sludges obtained by
sedimentation, i.e., primary sludges; however, when applied to biological sludges,
coagulant requirements substantially increase, and recoveries and cake solids decrease.
Dewatering performance is often unsatisfactory, and better techniques are necessary to
dewater biological sludges.
The primary objective of the study was to monitor the operation of a pilot apparatus
designed to continuously dewater sludge, taking advantage of the unique properties of
cellulose sponge. A detailed review of the literature revealed no previous use of cellulose
sponge in a liquid-solids separation process; sponge properties revealed in this study were
not previously known to any of the cellulose sponge manufacturers mentioned herein.
The ability of the materials to dewater waste activated sludge on a continuous basis
over extended periods of time without any deterioration of product quality or performance
was to be demonstrated. Additionally, economic data on coagulant requirements, which
represent approximately 1/3 of the costs of biological sludge dewatering, were to be
obtained '". Finally relationships between sludge feed parameters including solids content,
volatile content, and sludge volume index and resultant cake solids obtained and coagulant
requirements were to be established and appraised on a statistical basis.
CONCEPT OF CAPILLARY FILTRATION
The principle of capillary filtration is illustrated in Figure 1. The capillary forces of a
hydrophilic material are utilized to draw water through a microporous screen, generally
with openings of 30-50 microns. The solids in the wastewater are trapped by the screen and
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