Side View
1.5 m sludge
16 m
Top View
Figure 4. Dimensional sketch of freezing bed for McMurdo.
the concrete panels should be cast with a minimum of
DISCUSSION
three or four channels (one along each side of the bed
and one or two in the middle) to provide a pathway for
the drain pipes. The bottom of the bed, including the
The design features of the McMurdo freezing bed
drain pipes, should be covered with approximately 15
would be essentially the same as a conventional freez-
cm of sand to collect meltwater and convey it to the
ing bed except for the louvered sides. The louvered
drain pipes.
sides allow outside air to freely enter the bed and ac-
To use the bed, the operator would begin applying
celerate the freezing process. However, for this appli-
sludge by early March. Since most of the sludge will
cation, freezing is not a problem. Instead of the lou-
be generated during the summer months, it may nec-
vered sides, it would be beneficial to construct insulated
essary to provide extra capacity in the aerobic digester
solid walls to retain heat during the thawing process.
or to construct a temporary storage tank. Sludge would
The insulated walls will likely slow the freezing pro-
cess during winter but, because of the significant ex-
be applied to the bed directly from the aerobic digester
cess freezing capacity, it should not be enough to re-
or storage tank in sequential 8- to 10-cm layers. Each
duce the freezing design depth to something less than
layer will be frozen completely before the next layer
the thawing design depth.
is applied. CRREL has developed a device called the
A critical component of this freezing bed is the trans-
Automatic Sludge Applicator that automatically ap-
parent roof. Without it, the sludge will not thaw unless
plies a layer when the previous layer is frozen. This
a supplemental heating system is activated. To take full
device counts the number of freezing degree-hours and
advantage of the sun, the bed should be oriented to-
opens a valve or triggers a pump when the prescribed
number of freezing degree-hours is satisfied. This
wards it, and the roof should be pitched to allow entry
sludge application sequence is repeated until all the
of low angle solar radiation.
sludge is applied or the depth of frozen sludge has
Since poured-in-place concrete structures are gen-
erally not feasible at McMurdo, it is likely that the bed
reached the design depth of 1.5 m. At no point during
would be prefabricated in the U.S. or New Zealand.
this phase should it be necessary for the operator to
The bottom could be made from pre-cast concrete pan-
venture outside to perform operation and maintenance.
els and the sides could be made from plates of insu-
By October, all sludge to be dewatered will be fro-
zen and the thawing period will begin. All sludge ap-
lated steel that are bolted together in a way that is simi-
plications would be stopped and the meltwater drain
lar to a water tank. A pipe network for thawing should
be incorporated into the concrete floor panels. Also,
valve would be opened. To prevent odors, it is impor-
6
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