the bulk freezing method was superior to freeze
cations. Sludge containing 4% solids would be
crystallization because it consistently improved
placed on top of the belt and frozen in 0.5-in. (1.3-
the dewaterability of each sludge tested. In some
cm) sheets. Studies indicated that it took 100 min-
cases, freeze crystallization actually worsened
utes to freeze the 1.3 cm of sludge. The Milwau-
sludge dewaterability.
kee Sewerage Commission's analysis concluded
that energy costs and floor space requirements
Layer freezers
were appreciably higher for this dewatering
As the name implies, a layer freezing device
method than a conventional vacuum filter. As a
freezes sludge in thin layers or sheets. Freezing is
result, no further testing was conducted.
usually accomplished on a refrigerated plate or
In 1973 and again in 1975, Carrier Corporation
belt. After freezing is complete, the layer is re-
obtained patents (no. 3,745,782 and 3,880,756) on
moved and broken into small pieces that are then
a "falling film" freezing device. This device con-
thawed and drained. The layer freezing method
sists of a pair of shell and tube heat exchangers
is similar to bulk freezing in that sludge is frozen
which alternately serve as refrigerant evapora-
indirectly by a refrigerated surface. However, it
tors and refrigerant condensers. The sludge is
avoids the mechanical stresses due to freezing by
pumped to the top of the heat exchangers where
allowing expansion in the opposite direction
it is distributed and allowed to flow down the in-
from the freezing front. Also, it can freeze more
terior of the heat exchange tubes. As it flows
sludge per unit energy input because freezing in
downward, the sludge is frozen in the form of
thin layers is more efficient. The main disadvan-
hollow cylinders by the refrigerant evaporated
tage is the large surface area needed, which can
on the exterior of the tubes. Sludge that is not fro-
translate into a large floor space requirement.
zen by the time it reaches the bottom of the tubes
The first layer freezer for sludge was patented
is collected and recirculated back to the top until
by Downes and Komline (no. 2,174,873) in 1939.
the desired quantity of sludge is frozen. The
Essentially it consists of a flat conveyor belt,
refrigeration cycle is then reversed and the
a freezing chamber, and a thawing chamber.
sludge is melted. The inventors claim that this
Digested sludge from a wastewater treatment
process provides improved refrigeration cycle ef-
plant is applied in a thin layer to the conveyor
ficiency and avoids the structural failure prob-
belt. The belt then transports the sludge into the
lems associated with bulk freezing devices.
freezing chamber where it is frozen. The con-
In 1976, the United Kingdom Atomic Energy
veyor belt continues through a heated section
Authority filed a patent (no. 1,459,175) for a drum
where the sludge is thawed and drained. The en-
freezer whereby sludge is sprayed on a refriger-
ergy required for both freezing and thawing is
ated drum and allowed to freeze. Laboratory
derived from the anaerobic digestion process. No
tests indicated that a 2.0-mm layer of alum sludge
information was found in the literature on
can be frozen within 20 seconds on a stainless
steel drum cooled to 10C. The frozen layer is
whether this device was ever tried full scale.
The Milwaukee Sewerage Commission (1971)
removed by a rotating scraper and transferred to
evaluated two layer freezing methods for condi-
a thaw and settling tank. Supernatant is recircu-
tioning and dewatering waste activated sludge.
lated through a vapor condenser in the refrigera-
The first method was a serpentine conveyor
tion circuit and the heated return flow is sprayed
mounted in a cold air blast freezing unit. A series
over the frozen sludge in the thaw and settling
of pans were attached to the conveyor. Sludge
tank.
The Electric Power Research Institute (EPRI)
was deposited in each pan before it entered the
conducted tests on a layer freezing device con-
freezing unit. The pans entered at the top of the
sisting of a vertical freezing plate. Sludge was
unit and traveled vertically downward in a ser-
sprayed on the top of the plate and allowed to
pentine fashion. This concept was not extensively
flow downward by gravity. Any sludge that was
evaluated because the pan conveyor was not
not frozen was collected and recycled back to the
commercially available and a need was antici-
top of the plate. Brown et al. (1993) found that this
pated for frequent defrosting. The second meth-
technique was only partially successful. The re-
od involved the use of an endless steel belt that
jection of solids by the advancing freezing front
traveled over a brine cooled freezing section. This
caused a buildup of solids in the liquid phase.
concept was judged to be more feasible because
Eventually, the pump was incapacitated because
the belt freezing system was commercially avail-
it was unable to deliver the thickened sludge to
able, and it had been used in other similar appli-
4
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