clear filtrate. Also, we were surprised by the rela-
by the phase change operation. Theoretically it
takes 93 watt-hours to convert 1 kilogram of
tive cleanliness of the filter cloths after each use.
water (or sludge) to ice. Also, it takes 1.16 W-h/C
This observation and the tests we conducted sug-
to cool the liquid sludge to the freezing point and
gest that washing requirements would be mini-
0.58 W-h/C to cool the frozen sludge below
mal, but a longer term study is needed to confirm
this finding.
the freezing point. If we assume an initial temper-
ature of 10 C and a final temperature of 2C, the
The freezing tests indicate that the initial solids
content is an important parameter in determining
energy required to freeze one metric ton (1,000 kg)
the effective grain size. Sludges that contained a
of sludge is 105.8 kW-h. Assuming 50% losses due
to air infiltration, the estimated energy required is
greater solids content had larger effective grain
sizes. For example, at a freezing rate of 6 kg/h-m2
158.6 kW-h/metric ton. If the electrical cost is as-
the effective grain sizes were 0.16, 0.15, and 0.08
sumed to be
||content||
.07/kilowatt-hour, the cost of freez-mm for the sludges containing the 1213%, 67%,
ing one metric ton of sludge is .10. In terms of
water used, the cost is
||content||
.004/m3 since it takes ap-and 23% solids, respectively. The reason for the
proximately 3000 m3 of treated water to produce
larger grain size in the thicker freezethaw condi-
tioned sludges may be the availability of more solids
one metric ton of sludge. This is a relatively small
to fill the space between crystal boundaries. An im-
cost compared to the total cost of water treatment
which typically ranges from
||content||
.25 to
||content||
.50/m3.portant conclusion from this finding is that filter-
ing alum sludge before freezing not only reduces
the amount of sludge to be frozen, but it increases
CONCLUSIONS
the grain size of the product. Therefore it may be
more advantageous to use a belt press rather than
The bench-scale tests conducted in this study
a vacuum filter prior to freezing. As mentioned
indicate that the proposed freeze separator con-
previously, a vacuum filter will dewater alum
cept is technically and economically feasible. The
sludge to 6% solids, while a belt presses can typi-
filter leaf tests show that the vacuum filter should
cally dewater alum sludge to 12% solids.
be able to reduce the sludge volume by at least
The estimated size of the surface area needed
one-third. The optimum operational vacuum was
for the horizontal belt freezer serving a 3,785 m3/
100 mm of Hg and the filtration time was 5.0 min-
day (1,000,000 gal./day) plant is 10.2 m2. This esti-
utes. Any low porosity cloth appears to be suitable
from a filtrate quality point of view. However, the
mate is based on the following assumptions: 1) the
durability of these filter cloths after many freeze
sludge production rate is 1% of the water pro-
thaw cycles is still unknown.
cessed (Sanks 1978) and the specific gravity is
The results of the freezing studies show that
approximately equal to water, i.e. 1.0, 2) the vacu-
both freezing rate and initial solids content will af-
um filter reduces the volume of sludge by 67%,
3) the freezing rate is 15.5 kg/h-m2, and 4) the
fect the grain size of the freezethaw conditioned
sludge. In general, faster freezing rates will cause a
operational period is 8 hours/day. If the belt is 1.2
decrease in grain size. Conversely, sludges con-
m wide, the length of the freezing chamber would
taining a greater initial solids content will produce
be 8.5 m. This is about the same size as the blast
larger grains. The maximum freezing rate for the
freezers used by the food industry. A greater capac-
proposed freeze separator is projected to be 15.5
ity could be achieved by operating for longer peri-
kg/h-m2. Faster freezing will reduce the effective
ods. For example, the unit could handle the sludge
produced by a 11,355 m3/day plant if it were oper-
grain size below that of fine sand. This could result
ated 24 hours/day.
in a decease in the hydraulic conductivity of the
If a belt press were used instead of a vacuum
granular material, which is important for remov-
filter, the surface area of the horizontal belt freezer
ing meltwater.
could be reduced from 10.2 m2 to 4.0 m2. This re-
Based on the excellent results obtained from
duction is based on the assumption that the belt
freezing alum sludge containing 1213% solids,
press will reduce the sludge volume by 83% and
we concluded that it may be more effective to use a
the freezing rate can be increased to 19.8 kg/
belt filter rather than a vacuum filter prior to freez-
h-m2, as indicated by our tests. This configuration
ing. A belt press would remove up to 50% more
would be particularly beneficial to large plants
water from the sludge and thus reduce the cost of
freezing. Also, the freezing rate could be increased
where floor space is limited.
to 19.8 kg/h-m2.
One of the major concerns about the use of
freezethaw conditioning is the energy required
Curing time had no significant effect on grain
14
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