Design of Freezing Bed for Sludge
Dewatering at McMurdo, Antarctica
C. JAMES MARTEL
INTRODUCTION
cm-thick layer of sand. The bed is covered with a roof
In a report entitled, McMurdo Research Station, Ant-
and surrounded by a snow fence to keep snow from
arctica, Wastewater Treatment Alternatives Evaluation,
accumulating in the bed. Snow should be kept out of
Consoer Townsend Envirodyne (CTE) Engineers
the bed because it insulates the sludge and reduces the
(1999) recommend a belt filter press for sludge thick-
rate of freezing. Without this protection, the predict-
ening and dewatering for McMurdo Station, Antarc-
ability and performance of the process is reduced be-
tica. The belt filter press technology is in common use
cause of the annual variability in snow accumulations.
today at many conventional wastewater treatment
According to Martel and Diener (1991), the freez-
plants. Basically, the belt filter press consists of two
ing bed is capable of dewatering aerobically digested
continuous belts, set one above the other (Fig. 1). Con-
sludge to a 24.5% solids content, which is approxi-
ditioned sludge is fed in between the two belts, which
mately twice the solids content of the sludge produced
transport it through drainage, pressure, and shear zones.
by a typical filter press system. Using the freezing bed
The dewatered sludge is then removed by a scraper.
system could reduce by one-half the volume of sludge
The components of this type of sludge dewatering unit
needing to be returned to the U.S. from McMurdo.
include: a belt and roller press system, a polymer feed
Other advantages of the freeze bed technology include:
system, polymer storage equipment, sludge feed
pumps, an odor scrubbing system, and wash water
Less energy consumption, since nature does the
pumps. Typically, a belt press is capable of producing
freezing and thawing.
a sludge with a 10 to 15% solids content (85 to 90%
No requirement for conditioning chemicals.
liquid).
Minimal odors because the low temperatures limit
Given McMurdo's location at approximately 7750'
biological activity.
south latitude, a treatment plant designed to use the
Simple operation and maintenance.
natural freezethaw process to dewater the sludge may
be a better choice than a belt filter press. Freezing beds
The mechanism that dewaters the sludge is freez-
have proven to be effective at many treatment plants
ing during the winter months and thawing during the
located in Alaska, the northern U.S., and Canada
spring and summer. During the winter freezing period,
(Martel 1998). A conceptual sketch of a freezing bed
ice crystals grow by adding water molecules to their
is shown in Figure 2. An actual freezing bed at Fort
structure. All other impurities, including sludge par-
McCoy, Wisconsin, is shown in Figure 3. It consists of
ticles, are rejected and thus become clumped together
a rectangular in-ground concrete structure with a ramp
into larger particles. During the spring and summer,
at one end and a sump at the other. The ramp is used to
the ice crystals thaw, and the meltwater drains between
provide vehicle access for sludge removal. The sump
and through the joined particles, leaving a solid mate-
is used to collect the meltwater from the bed via the
rial with high solids content.
underdrain system and return it back to the head of the
This report evaluates freezing bed technology, for
plant for further treatment. The underdrain system con-
use at McMurdo, to dewater the aerobically digested
sists of a series of drain pipes covered by an 8- to 10-
sludge produced by the proposed secondary treatment
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