small particles to a granular material that resem-
Scope
bles coffee grounds (see Fig. 1). These grains do not
Objective 1 was accomplished by thoroughly
dissolve or suspend upon rewetting, even with
reviewing the literature and evaluating past
vigorous agitation. This granular material can be
attempts at building a device. The main part of this
easily handled and transported with conventional
study was devoted to developing the preliminary
earth moving equipment. No other sludge condi-
design criteria (Objective 2). These criteria were
tioning or dewatering process can produce this
developed by bench scale studies conducted at
dramatic transformation.
CRREL in Hanover, New Hampshire.
Because freezethaw conditioned sludge is in a
granular form, land application becomes much
HISTORY OF MECHANICAL
more practical and economical. In areas where
FREEZETHAW CONDITIONING
landfill costs are high, the land application option
offers a significant savings in disposal costs. New
Literature review
Jersey American Water Company estimated that it
Generally, mechanical sludge freezing devices
could avoid a 0/ton tipping fee if land applica-
can be categorized into bulk freezers, freeze crystal-
tion was possible (Brown et al. 1993). However, as
lizers and layer freezers. A conceptual sketch of
pointed out by DeWolfe and Dempsey (1992), care
each freezer is shown in Figure 2.
must be taken to control a potential phosphorus
deficiency in soils, which may impact plant
Bulk freezers
growth.
The bulk freezer was the first mechanical device
The mechanism responsible for converting
used to freeze sludge. It consists of a large container
alum sludge into granular particles is the ice crys-
with refrigeration coils. Sludge is pumped into the
tal formation process. It is well known that ice
tank where it remains until completely frozen.
crystals form from water molecules only. All other
Thawing is also accomplished in the same contain-
substances including alum floc particles are
er by reversing the refrigeration cycle. The meltwa-
pushed away from the growing ice crystal. This
ter is drained out of the container, leaving the gran-
forces the floc particles to become consolidated at
ular sludge particles at the bottom. The container is
the boundaries between ice crystals. After freezing
then drained and emptied and the process is re-
peated. This method was used for several years at
is complete, the sludge is no longer a suspension of
the Stocks Filtration Plant in Yorkshire, England
fine floc particles but a matrix of ice crystals and
(Benn and Doe 1969). It worked very well except
consolidated floc particles or grains. When the ice
that the tanks had a tendency to rupture after rela-
crystals thaw, the grains remain consolidated and
tively few cycles due to the expansion force of ice
do not redissolve. These grains are large enough to
(Farrell 1971).
easily settle by gravity in the clear meltwater. The
A recent report by AWWA (1990) indicates that
sludge is dewatered by decanting or draining the
there are a few operational bulk freezethaw plants
meltwater.
in Germany. The Wuppertal Municipal Works in
In cold climates, freezethaw conditioning is
West Germany reportedly uses a bulk freezing de-
easily accomplished during the winter months in
vice similar to that described in Doe et al. (1969).
outdoor freezing beds (Martel and Diener 1991).
Alum sludge containing approximately 2% solids
However, this technology is not usable at water
is pumped into one of two sequentially operated
treatment plants located in warm climates or at
freezing tanks. However, to avoid damaging the
plants without the available land area. For these situ-
tank by the expansion of ice, only 40% of the tank
ations, a mechanical freezing device is needed. This
contents are frozen.
report is the result of several years of study and
Kawamura and Trussell (1991) report the use of
experimentation in developing such a device.
freezethaw conditioning at the Kashiwai Water
Purification Plant near Tokyo, Japan. Freezethaw
Objectives
is used because sludge from this plant is difficult to
The objectives of this study are:
dewater by conventional methods. The sludge is
1. To develop a conceptual mechanical freeze
gravity thickened and centrifuged prior to freezing.
thaw conditioning device based on lessons
Twenty-five tanks are used to freeze and thaw the
learned from previous attempts.
sludge. After thawing is complete, it is filtered by
2. To develop preliminary design criteria for the
vertical belt presses to achieve 4045% solids.
new device.
2
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